C H A P T E R
4
(Photo)catalyst Characterization
Techniques: Adsorption Isotherms and
BET, SEM, FTIR, UVVis,
Photoluminescence, and Electrochemical
Characterizations
Sedat Yurdakal1, Corrado Garlisi2, Levent Özcan3,
Marianna Bellardita4 and Giovanni Palmisano2
1
Department of Chemistry, Faculty of Science and Literature, Afyon Kocatepe University, Ahmet
Necdet Sezer Campus, Afyonkarahisar, Turkey, 2Department of Chemical Engineering, Khalifa
University of Science and Technology, Abu Dhabi, United Arab Emirates, 3Department of Biomedical
Engineering, Faculty of Engineering, Afyon Kocatepe University, Ahmet Necdet Sezer Campus,
Afyonkarahisar, Turkey, 4“Schiavello-Grillone” Photocatalysis Group, Dipartimento
di Energia, Ingegneria dell’informazione, e modelli Matematici (DEIM), University of Palermo,
Palermo, Italy
4.1 ADSORPTION ISOTHERMS AND
BRUNAUEREMMETTTELLER
SURFACE AREA DETERMINATION
or light (photocatalysis), coming from a ultraviolet (UV), visible (Vis), UVVis, or solar
source, with the suitable energy needed
according to the band-gap of the used semiconductor [1,2]. Therefore (photo)catalyst surface area, pore size, particle size (and their
distribution), and adsorptiondesorption phenomena of the species (i.e., molecules, ions,
radicals) on the (photo)catalyst surface are very
important parameters for the (photo)catalytic
4.1.1 Introduction
The catalyst’s surface is the place in which
heterogeneous catalytic or photocatalytic reactions occur. On the catalyst’s surface there are
sites that could be activated by heat (catalysis)
Heterogeneous Photocatalysis
DOI: https://doi.org/10.1016/B978-0-444-64015-4.00004-3
87
© 2019 Elsevier B.V. All rights reserved.
88
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
reactions to proceed with an assessment of the
(photo)catalytic activity and to investigate the
reaction mechanisms. For a (photo)catalytic
reaction not only adsorption, but also desorption processes taking place after the reaction on
the adsorbed state, are very relevant to ensure the
efficient turnover of active sites. Otherwise, poisoning of active sites can occur.
The actual number of active sites on the
(photo)catalyst surface is generally unknown,
therefore specific surface area of (photo)catalysts (surface area per unit mass) is used as a
basis for the determination of specific activity
of the (photo)catalyst [3]. For kinetic modeling
in heterogeneous (photo)catalysis, reaction
rates should be determined per unit surface
area of (photo)catalyst. For instance, the initial
reaction rates (2r0) can be estimated by the following Eq. (4.1) [4]:
1 dn
V dC
ð 2 r0 Þ 5 2
5 2
(4.1)
S dt
S dt
where n represents the substrate moles, t the
irradiation time, S the specific surface area, V
the suspension volume, and C the substrate
concentration. It must be noted that (2r0)
value is normalized to the catalyst surface area
and, therefore, it is a reliable parameter for
evaluating the intrinsic activity of a catalyst.
Generally, higher surface area due to smaller particles corresponds to higher activity.
However, as the activity mainly depends on the
type and density of the active sites present on
the exposed surface, this relationship is not
straightforward. Although the (photo)catalyst
particle size can influence the (photo)catalyst
activity, the crystallinity extent and the crystal
size are other important parameters. The thermal
treatments are often used to increase the crystallinity in the catalyst preparation and they give
rise to a decrease of the specific surface area due
to particles sintering [5,6]. In conclusion, crystalline photocatalysts, which are prepared at low
temperature, have high surface area and can
present a high photocatalytic activity.
Liquid phase (photo)catalysis at a laboratory
or pilot scale generally employs catalyst powders in liquid suspensions in batch reactors. In
this form, the photocatalyst particles are well
dispersed in the reacting media and the available catalyst surface area for adsorption of the
substrates is maximum [7]. To optimize catalyst dispersion, ultrasonic treatment is very
helpful before performing the (photo)catalytic
experiments. However, supported (photo)catalysts are ideal for large-scale (photo)catalytic
applications because they allow to operate in
continuous mode and to avoid the difficult
recovery and separation of the finely suspended
catalyst particles both for liquid and gas phase
reactions [8,9]. Although the (photo)catalyst specific surface area is the same for both freely
suspended and supported (photo)catalysts, the
surface available for the reaction in the latter
case is greatly limited, mainly depending on the
geometry and size of supporting material, for
example, Pyrex or ceramics.
Photocatalytic systems are more complex
than the catalytic ones since irradiation efficiency must be optimized, whereas in the catalytic systems all the active sites on the surface
are always available. Moreover, the irradiation
efficiency of photocatalytic systems could be
easily optimized in supported systems rather
than the powders. Indeed, penetration depth
of UV, UVVis, or solar light in a slurry is
limited, while absorption of thin photoactive
layers [8,9] may be better controlled. For these
reasons, transparent supports, such as Pyrex,
are desirable for photocatalytic system, while
in catalytic systems many different kinds of
supports can be used.
4.1.2 AdsorptionDesorption
Phenomena
The adsorption occurs by a physical or
chemical interaction of a species on the (photo)
catalyst surface. The surface is called adsorbent and the adsorbed species are called
HETEROGENEOUS PHOTOCATALYSIS
89
4.1 ADSORPTION ISOTHERMS AND BRUNAUEREMMETTTELLER SURFACE AREA DETERMINATION
θ5
number of adsorption sites occupied
number of adsorption sites available
(4.2)
The fractional coverage (Eq. (4.3)) is often
expressed in terms of the volume of adsorbate:
θ5
V
Vmon
(4.3)
where Vmon is the volume of adsorbate corresponding to complete monolayer coverage.
The species could be adsorbed on the
surface in two main fashions: physically
(physical adsorption or physisorption) and
chemically (chemisorption).
In physical
adsorption, there are van der Waals forces or
dipoledipole interactions between adsorbent
and adsorbate [11]. These forces are very weak
and the energy released when a species is
physically adsorbed is of the same magnitude
as the condensation enthalpy (c. negative
2040 kJ/mol). The adsorbate can easily
diffuse on the surface and rotate. Moreover,
the adsorptiondesorption equilibrium time is
very short.
On the other hand, in chemisorption the
species are chemically adsorbed on the surface
and the concerned enthalpy (c. negative
100400 kJ/mol) is much higher than physical
adsorption enthalpy. Moreover, the distance
between adsorbent and adsorbate is smaller
with respect to the case of physical adsorption,
comparable to chemical bond lengths. Before
chemical adsorption, physical adsorption typically occurs. The enthalpy of the latter can be
measured by following the temperature rise of
a surface whose heat capacity is known. This
low energy is not enough to break the bonds
of adsorbate molecules.
Physical adsorption can result in the formation of single or multimolecular layers
and it is reversible as the attraction forces are
weak, while chemical adsorption forms a
monomolecular layer and it is irreversible. In
addition, physical adsorption usually takes
place at low temperature and decreases with
increasing temperature, while chemical
adsorption is highly specific and takes place
at high temperature. Furthermore, physical
adsorption is not selective: for instance N2
can be adsorbed on any surface physically at
its boiling point. On the other hand, N2 can
be adsorbed chemically on Fe, W, Ca, and Ti
surfaces at room temperature, while it cannot
be adsorbed on Ni, Ag, Cu, and Pb surfaces.
Nitrogen adsorption on silica gel at 77 K and
oxygen adsorption on charcoal at 150 K
could be given as examples for physical and
chemical
adsorption, respectively
(see
Fig. 4.1) [12].
Adsorbed volume
adsorbates [10,11]. Adsorbate is referred to as
adsorptive before adsorbing the surface.
Desorption is the detachment of the species
from the surface, that is, the reverse process of
adsorption.
The fractional coverage θ (Eq. (4.2)) (ranging
from 0 to 1) is:
N2 on silica gel
O2 on charcoal
0
P/P 0
1
FIGURE 4.1 Nitrogen adsorption on silica gel at 77 K
and oxygen adsorption on charcoal at 150 K. Source:
Adapted from W.J. Moore, Physical Chemistry, Practice-Hall,
Inc., New Jersey, USA, 1972.
HETEROGENEOUS PHOTOCATALYSIS
90
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
4.1.3 Classification of Adsorption
Isotherms
Adsorption process is usually studied
through isotherms representing the amount of
adsorbate on the adsorbent as a function of its
partial pressure or concentration at constant
temperature. The adsorbed quantity is nearly
always normalized by the mass of the adsorbent to allow comparison of different materials
[13]. The observation of the adsorption and
desorption branches of an isotherm can provide plenty of information.
Isotherm shape depends on physicochemical conditions and solid porous texture.
According to IUPAC classification six types
can be distinguished [11,1417], but only I, II,
IV, and VI types are usually found in catalyst
characterizations (Fig. 4.2) [14].
FIGURE 4.2 Six types of adsorption isotherm classified
by IUPAC [17].
The pores are classified as micropores
(size , 2 nm), mesopores (2 nm , size ,50 nm),
and macropores (size . 50 nm), depending on
their size [11,14,17].
4.1.3.1 Type I
This isotherm signifies microporous solids
and gas molecules adsorb just as a monolayer
[1417]. The adsorption takes place also at very
low relative pressures because of strong interaction between pore walls and adsorbate.
Therefore, at low pressure, a gas molecule can
find many free binding sites on the surface. By
increasing pressure, both the available sites
are occupied and the adsorption probability
decrease. Pore filling takes place without capillary condensation in the low relative pressure
region (c. ,0.3). Once micropores are filled, the
adsorption continues on the external surface, following the behavior described for meso or
macroporous solids. Once all pores have been
filled, the isotherm saturates, that is a further
increase in pressure does not cause any difference in adsorption process (see Fig. 4.2). Typical
examples of microporous solids are active carbons, zeolites, and zeolite-like crystalline solids.
Type 1 isotherms are also characteristic for
chemisorption [18]. Oxygen adsorption on charcoal could be given as an example (see Fig. 4.1).
4.1.3.2 Type II
This isotherm is suitable for nonporous or
macroporous solids [1417]. At low relative
pressure, available sites could be filled as
monolayer and at point B (see Fig. 4.2) the
monolayer coverage is complete. At high relative pressure the monolayer surface adsorbs
more layers (multilayer adsorption), therefore
the adsorbate thickness continuously increases
until condensation pressure has been reached.
If the interaction between the adsorbed gas
and the sample is stronger, the pressure at
which the monolayer formation is complete
becomes lower. Nitrogen adsorption on silicate
at 77 K could be given as an example for this
HETEROGENEOUS PHOTOCATALYSIS
4.1 ADSORPTION ISOTHERMS AND BRUNAUEREMMETTTELLER SURFACE AREA DETERMINATION
isotherm type (see Fig. 4.1). Type II isotherms
are characteristic for physisorption [18].
4.1.3.3 Type III
Solids with low adsorption capacity show
this type of isotherm [1517]. It is not possible
to extrapolate the monolayer capacity by using
the curve. Bromine adsorption on silicate at
352 K and nitrogen adsorption on polyethylene
could be given as examples for this type of isotherm [17]. This type of isotherm can arise also
when using a nonpolar surface with polar
molecules: in such cases at low partial pressure
a reduced uptake can be observed due to the
repulsive interactions, whereas an increased
presence of adsorbate molecules promote the
adsorption at higher partial pressures.
4.1.3.4 Type IV
This isotherm fits mesoporous solids
[1417]. It is characterized by a hysteresis loop
and a saturation plateau at p/p0 5 0.60.95.
The hysteresis loop in the isotherm is due to
capillary condensation taking place in mesopores by increasing relative pressure. At low
relative pressures, the curve resembles that of
macroporous solids, with the surface covered
by a monolayer. At intermediate pressures
multilayer adsorption occurs and, by increasing pressure above a certain level, capillary
condensation takes place with a steep increase
of the adsorbed volume. After the filling of
mesopores is complete, adsorption continues
on the low external surface. Most catalysts
show this type of isotherm. N2 adsorption on
CdIn2S4 [19], N2 adsorption on TiO2 [20] could
be given as examples for this type of isotherm.
Other examples are benzene adsorption on
Fe2O3 or on silica gel at 500 C [21].
4.1.3.5 Type V
These types of isotherms are observed for
adsorption heat of monolayer lower than that
of condensation heat and in the presence of
capillary condensation [1517]. Initially, the
91
solid surface is covered as monolayer and
multilayer, then capillary condensation starts.
Mesoporous solids with low adsorption capacity show this type of isotherm. Water adsorption on active carbon at 273 K could be given
as an example for this type of isotherm [22].
4.1.3.6 Type VI
This isotherm can be referred to as stepwise
multilayer adsorption and appears only when
the sample surface contains different types of
adsorption sites with energetically different
characteristics [1417]. Unless a (photo)catalyst surface shows very clearly distinguished
kinds of sites, it will not show a stepwise
isotherm. Isotherms of well crystallized zeolites like X (one step corresponding to cavities
filling) or silicate (two steps, corresponding,
respectively, to channel filling and to an adsorbentadsorbate transition) show type VI isotherm [14]. Argon or krypton adsorption on
graphite at 77 K could be also given as an
example of this isotherm [17]. Each step in
height corresponds to the adsorbed gas volume of the corresponding zone.
4.1.4 Adsorption Hysteresis
Hysteresis appears in the multilayer range
of physisorption isotherms [1723]. It is usually related to condensation inside capillaries
of mesoporous structures. These hysteresis
loops can appear in different shapes. Fig. 4.3
shows two extreme loop types: H1 and H4. In
H1, the two branches are almost vertical and
almost parallel over a wide range of gas
uptake. While in the H4 type, the branches
stay nearly horizontal and parallel over a wide
p/p range. Other hysteresis types, H2 and H3,
can be considered as intermediate cases
between these two limit forms.
Rigorously, the desorption branch of hysteresis loops takes place at a pressure related
only to the features of the adsorbate and not to
HETEROGENEOUS PHOTOCATALYSIS
92
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
very low relative pressures. These hysteresis
shapes are associated with the deformation of
nonrigid pores’ walls or with chemical adsorption. In such cases, one way to try to remove
the residual adsorbate from the surface is high
temperature outgassing.
4.1.5 Adsorption Models
FIGURE 4.3 Hysteresis loop types classified by IUPAC
[17].
the type of adsorbent at a fixed temperature
(e.g., for N2 at 77 K at p/p 5 0.42 and for benzene at 298 K at p/p 5 0.28).
The shapes of hysteresis loops have often
been correlated to specific pore morphologies.
Type H1 is often associated with porous materials, which consist of almost uniform agglomerated spheres. Therefore, these porous
materials have a narrow pore-size distribution.
Many porous materials, such as inorganic
oxide gels and porous glasses, give rise to type
H2 loops, however their pore-size and poreshape distribution is not well-defined.
The type H3 loop is observed with aggregates of plate-like particles giving rise to slitshaped pores. On the other side, a type H4
loop can be correlated to narrow slit-like
pores.
Low pressure hysteresis is indicated by
dashed lines in Fig. 4.3. This kind of hysteresis
can be observed in microporous materials at
Adsorption isotherm is the variation of the
fractional coverage (θ) with pressure at a working temperature [10,12,13,24]. The amount of
adsorbate on the adsorbent surface and
adsorptiondesorption equilibrium depend on
adsorbent and adsorbate properties, pressure,
and temperature. Adsorption isotherms provide much information, such as surface area,
pore-size, and pore-volume distributions, and
the catalystsubstrate interactions.
Theoretically and experimentally derived
isotherms can be represented by simple equations that correlate directly the concentration
of the adsorbed species to the pressure.
4.1.5.1 Langmuir Isotherm
The Langmuir isotherm is the simplest one;
it is valid for monolayer physical adsorption of
gases or liquids and applies to ideal conditions. Therefore it has an importance in
adsorption theory similar to that of the ideal
gas equation [25]. The other isotherms derive
from the Langmuir isotherm. The assumptions
of the Langmuir isotherm are listed below
[10,12,13]:
1. the adsorption is completed when all the
active sites are covered by a monolayer;
2. each site can host one adsorbed molecule,
all active sites are equal, and the surface is
uniform;
3. the adsorption of a molecule at a site is not
affected by the occupation of neighboring
sites.
HETEROGENEOUS PHOTOCATALYSIS
4.1 ADSORPTION ISOTHERMS AND BRUNAUEREMMETTTELLER SURFACE AREA DETERMINATION
At the dynamic equilibrium it can be written (Eq. (4.4)):
SðgÞ 1 MðsurfaceÞ "SMðsurfaceÞ
(4.4)
in which S is adsorptive and M is adsorbent.
The adsorption and desorption rate constants
are ka and kd, respectively. The rate of change
of surface coverage (Eq. (4.5)) due to adsorption is proportional to the partial pressure (p)
of S and the number of vacant sites [N(1 2 θ)],
where N is the total number of available sites:
dθ
5 ka pNð1 2 θÞ
dt
(4.5)
Similarly, the rate of θ change by desorption
(Eq. (4.6)) is proportional to the adsorbed species number, Nθ:
dθ
5 2 kd Nθ
dt
(4.6)
At equilibrium adsorption and desorption
rates are equal and, by solving, θ (Eq. (4.7)) is
found according to the Langmuir model:
θ5
Kp
;
1 1 Kp
(4.7)
K5
ka
kd
93
(4.8)
The Langmuir isotherm (p vs θ) curves for
different K values are shown in Fig. 4.4, in
which K (Eq. 4.8) is the (photo)adsorption constant. When the K value is very low, such as
0.01, this equation can be approximated as
θ 5 Kp. In addition, θ 5 Kp, with a first order
dependence on p, is valid for all K values at
low pressures (see Fig. 4.4). However, when K
is very high, catalyst surface can be considered
covered by a monolayer of adsorbate (θ 5 1)
even at moderate pressure. The fractional coverage increases by increasing pressure and, for
all available sites of the surface to be occupied
(θ 5 1), the pressure must be very high especially for low K values. Depending on the
considered temperatures, different K values
can be obtained, and the temperature dependence of K can be used to determine the
isosteric enthalpy of adsorption, ΔadH , the
standard enthalpy of adsorption at a fixed surface coverage. To determine this quantity we
recognize that K is essentially an equilibrium
FIGURE 4.4
Langmuir isotherms
of adsorption for different K values
from 0.01 to 10.
HETEROGENEOUS PHOTOCATALYSIS
94
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
constant, and then by using the van’t Hoff
equation it is possible to write (Eq. (4.9)):
@lnK
Δad H
5
(4.9)
@T θ
RT 2
4.1.5.2 Temkin and Freundlich Isotherms
An assumption of the Langmuir isotherm
is the independence and equivalence of the
adsorption sites [10,13,26]. Deviations from
the isotherm can often be traced due to the
failure of these assumptions. For example,
the enthalpy of adsorption often becomes
less negative as θ increases, which suggests
that the energetically most favorable sites are
occupied first. Various attempts have been
made to take these variations into account.
One of them is the Temkin isotherm
(Eq. (4.10)).
The Temkin isotherm reads
θ 5 c1 lnðc2 pÞ
(4.10)
where c1 and c2 are constants, and it is based
on the assumption that the adsorption
enthalpy changes linearly with pressure.
The Freundlich isotherm (Eq. (4.11))
θ 5 c1 p1=c2
(4.11)
describes a logarithmic change of the adsorption enthalpy and attempts to incorporate the
role of substratesubstrate interactions on the
surface.
The different isotherms are typically in
agreement with experimental data over
restricted ranges of pressure, but they remain
largely empirical. Empirical, however, does
not mean useless; if the parameters of a reasonably reliable isotherm are known, useful
results can be collected on the extent of surface
coverage under various conditions. This kind
of information is essential for any discussion
on heterogeneous (photo)catalysis.
4.1.5.3 BrunauerEmmettTeller Isotherm
Langmuir adsorption isotherms can be used
only to characterize a monolayer uptake and
in the ideal conditions described by this model’s assumptions. For instance, according to
Langmuir’s isotherm, all binding sites saturate
at high pressure [11]. However, this is untrue
in many cases.
The most common method to assess isotherms describing a multilayer adsorption is
the BrunauerEmmettTeller (BET) model
(Eq. (4.12)), which was derived by Stephan
Brunauer, Paul Emmett, and Edward Teller
[27]:
V
cðp=p0 Þ
5
0
Vmon
1 2 ðp=p Þ 1 2 ð1 2 cÞðp=p0 Þ
(4.12)
The linearized equation, which is useful to
obtain the volume of a monolayer (Vmon) and
the model’s parameter c, reads as follows
(Eq. (4.13)):
p
1
c21 p
5
1
2 pÞ Vmon c Vmon c p0
Vðp0
(4.13)
In these equations, p0 is the vapor pressure
above a layer of adsorbate that is more than
one molecule thick and that resembles a pure
bulk liquid, Vmon is the monolayer coverage
volume of adsorbate, and c is a constant which
is large when the desorption enthalpy ðΔdes H Þ
of the monolayer is high compared with the
vaporization enthalpy (Δvap H Þ of the liquid
adsorbate (Eq. (4.14)):
c 5 eðΔdes H
2Δvap H Þ=RT
(4.14)
For high values of c, the interaction of vapor
molecules with surface is higher than intermolecular interaction [11]. Therefore, at least at
low pressures, a Langmuir type of adsorption
is obtained for high c values. Multilayer
adsorption starts at higher pressures. For low
values of c, the molecules prefer binding to
themselves. Therefore, the first monolayer only
HETEROGENEOUS PHOTOCATALYSIS
4.1 ADSORPTION ISOTHERMS AND BRUNAUEREMMETTTELLER SURFACE AREA DETERMINATION
95
FIGURE 4.5 BET isotherm curves for different c values.
forms at relatively high pressures for low c
values and, when it has formed, it is easier for
the next molecules to adsorb.
The BET surface area could be determined
(Eq. (4.15));
Vmon
(4.15)
Surface area 5
Na σ
22; 414
In which Na is Avogadro number and σ is
the area occupied by an adsorbate molecule
(i.e., c. 0.162 nm2 for a nitrogen molecule)
[15,28,29]. The obtained result should be
divided by the used mass of catalyst to obtain
this value in m2 per g of catalyst.
Fig. 4.5 illustrates the shape of BET isotherms for different c values. They rise continuously by increasing partial pressure because
there is no limit to the amount of adsorbate
that may condense when multilayer coverage
takes place.
When c .. 1, the BET isotherm takes the
simpler form (Eq. (4.16)):
V
1
5
Vmon
1 2 ðp=p0 Þ
(4.16)
This expression is applicable to unreactive
gases on polar surfaces, for which c 102
because ΔHdes is significantly greater than
ΔHvap . The BET isotherm fits experimental
observations moderately well over restricted
pressure ranges, but it typically underestimates the extent of adsorption at low pressures and overestimates the same at high
pressures.
4.1.6 BrunauerEmmettTeller Surface
Area Determination
Surface area of materials can be determined
by following concentration (or pressure) of
adsorptive or by measuring the adsorbed gas
amount [11,12,30]. There are many methods
to determine surface area, that is, vacuumvolumetric, flow, and gravimetric methods. In
HETEROGENEOUS PHOTOCATALYSIS
96
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
the vacuum-volumetric method, used in BET,
pressure transducers display pressure changes
with high accuracy during the adsorptiondesorption process and p/p0 values are
determined by creating partial vacuum conditions. Commonly nitrogen (adsorptive) and
helium (nonadsorptive) gases are used in the
flow apparatus of the BET instrument and
the sample is cooled with liquid nitrogen. The
adsorption and subsequent desorption extents
are monitored by a thermal conductivity detector. BET instruments are commonly used in
laboratories or in the industry to determine the
surface area, pore size, and pore volume of
materials. The BET instrument needs to be calibrated by a known volume of pure nitrogen in
the absence of any sample before starting the
analysis.
4.1.6.1 Preparation of Sample
To measure the real surface area of the sample, all the impurities and water, which block
the pores, should be removed from the surface.
For this aim, the samples need to be cleaned
from adsorbed contaminants through what is
called a degassing process by using vacuum or
flow of an inert gas possibly at high temperature (typically in the range of 250 C400 C)
[17]. The catalyst with a known mass is placed
in suitable glass cells and the glass cells are
placed into heating mantles and connected to
the outgas port of the machine during the
degassing process. The used temperature
should be high enough to efficiently remove
surface contaminant species without changing
the surface morphology. In the case of (photo)
catalysts prepared at low temperature, containing a high degree of amorphous phase, the
high temperature degassing process could
increase the crystallinity degree, changing the
properties of the catalysts. Recently, it has
been shown that even (photo)catalysts prepared at high temperature can undergo subtle
changes during the degassing step, even in
medium vacuum conditions: this is the case of
graphene-doped brookiterutile nanostructured samples prepared via solgel, which
drastically change their ability to absorb visible
radiation due to the introduction of Ti31 states,
promoting their photoactivity under visible
light [31]. The suitability of the degassing conditions for a certain (photo)catalyst should
then be checked by testing the (photo)catalysts
before and after this treatment.
4.1.6.2 Used Gases for
BrunauerEmmettTeller Analysis
To obtain a measurable adsorption in the
BET instrument by proper interaction between
surface and gas molecules, a low temperature
must be maintained [32]. Nitrogen adsorption
at 77 K represents the most widely used technique to determine catalyst surface area and to
characterize its porous texture [14,17]. The
starting point is the determination of the
adsorption isotherm, that is, the nitrogen
adsorbed volume versus its relative pressure.
Argon, carbon dioxide, krypton, and hydrogen
gases could be also used as adsorptive in case
of specific needs. Argon is a noble gas, therefore it is monoatomic and with a spherical
shape. Consequently, this form eliminates the
orientation problem in adsorption. Moreover,
since argon is nonpolar and inert, it is not
affected by surface charges. Its adsorptiondesorption equilibrium occurs in a short
time at 87 K. However, by using liquid nitrogen during the cooling process, a full isotherm
cannot be obtained, because 77 K is below the
argon’s triple point. Krypton at 77 K is frequently used for lowsurface-area measurements as well as small pores of thin films. Its
saturation pressure is 2.63 torr at 77 K, therefore the void volume error is small because
fewer molecules stay in the void volume.
4.1.6.3 BrunauerEmmettTeller
Instrument and its Working Principle
Fig. 4.6 shows a BET diagram to measure
surface area determination through the
HETEROGENEOUS PHOTOCATALYSIS
4.1 ADSORPTION ISOTHERMS AND BRUNAUEREMMETTTELLER SURFACE AREA DETERMINATION
97
FIGURE 4.6 Schematic diagram of volumetric
method apparatus.
Instrument manifold
Data
collector
Nitrogen
P
P
Helium
Sample cell
Injection port
Liquid nitrogen
dewar
TABLE 4.1 Information Available on BET Instruments and Models to Fit Data [33]
Measurement
Models
Notes
Surface area
BET, Langmuir, Temkin, Freundlich
Can be calculated from section of isotherm
(generally p/p 5 0.050.35)
Total pore volume Kelvin equation
Generally carried out at p/p 5 0.9900.998,
although theoretically all pores should be full at
p/p 5 0.995
Mesopore volume, BJH, DollimoreHeal
area, and
distribution
Requires full adsorption and desorption
isotherm
Micropore
distribution
DubininRadushkevich and DubininAstakhov,
HorvathKawazoe, SaitoFoley, ChengYang,
MP method
Requires full adsorption isotherm
Pore-size
modeling
Density functional theory
Requires full adsorption isotherm
Surface energy
Density functional theory
Requires full adsorption isotherm
volumetric method [32]. Once degassing, the
cell containing the catalyst is moved to the
analysis port, where liquid nitrogen in a dewar
is used to cool the sample and maintain it at a
constant temperature. Nitrogen and helium
gases are injected into the sample cell with a
calibrated piston. When the analysis starts, the
adsorbed vapor amount on the sample can be
determined by changing pressure after equilibrium. These measurements can be repeated at
different pressures to obtain an adsorption
isotherm through which surface area, poresize, and pore-volume distribution can be
determined. Suitable reference materials with
known surface area, such as α-alumina, should
be used periodically to verify the correct functioning of the apparatus [12,30].
It is not just BET surface area that can be
measured by using this instrument. For
instance pore-volume, pore-size and their distribution, and surface energy could be also
determined. Table 4.1 shows a selection of
HETEROGENEOUS PHOTOCATALYSIS
98
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
other models, such as Temkin, Barrett-JoynerHalenda (BJH), and density functional theory,
which can be applied to fit an isotherm [33].
The adsorption of a gas on the surfaces of
powders and solids is also used to determine
the distribution of the pore size [14]. In fact,
the pore size can be determined by the data of
adsorption or desorption branches by means
of iterative calculation procedures. The numerical integration BJH is the most commonly
used in experimental data processing software.
The BJH method allows to obtain the pore
diameter distribution curve starting from the
Kelvin equation, where the radius rk is related
to the relative pressure p/po, at which the capillary condensation of N2 inside the pores of
that determined size can be verified
(Eq. (4.17)):
ln
p
2γVm
52
0
p
rk RT
(4.17)
where γ is the surface tension of adsorbate, Vm
is the molar volume of adsorbate, rk is the
Kelvin radius.
The hypotheses of the model are as follows:
pores are open and cylindrical, and there is no
intercommunication among them. Through the
Kelvin equation and the mathematical method
BJH the integral pore volume curve f(d) is
built, where d 5 2rk; in such a way the distribution curve of the pores is achieved. This procedure is performed automatically by the
software connected to the instrumentation
used.
Two case studies on nitrogen adsorptiondesorption isotherms with analysis of
pore-size distribution for different catalysts are
presented below.
Ling et al. [19] reported solvothermal preparation of CdIn2S4 photocatalysts for selective
photocatalytic oxidation of organic aromatic
compounds under visible irradiation. The catalysts are named according to solvothermal
treatment time (such as CdIn2S4-12h). Fig. 4.7
shows the nitrogen adsorptiondesorption isotherms of CdIn2S4 photocatalysts. These curves
coincide with type IV with a typical H3 hysteresis loop, therefore the samples have mesoporous structures, since the typical H3 loop is
FIGURE 4.7 Nitrogen adsorptiondesorption isotherms and corresponding pore-size distribution
curves (inset) of the CdIn2S4 photocatalysts. Source: Reproduced from C.
Ling, X. Ye, J. Zhang, J. Zhang, S.
Zhang, S. Meng, et al., Solvothermal
synthesis of CdIn2S4 photocatalyst for
selective photosynthesis of organic aromatic compounds under visible light,
Sci. Rep. 7 (2017) 27, with permission,
Copyright 2017 Nature Publishing
Group.
HETEROGENEOUS PHOTOCATALYSIS
4.1 ADSORPTION ISOTHERMS AND BRUNAUEREMMETTTELLER SURFACE AREA DETERMINATION
900
200
HP0.5 Ads
Vads [cm3/g]
800
700
Pore area [m2/g]
99
600
150
HP0.5 Des
HP2 Ads
HP2 Des
100
50
500
0
400
300
0.5
p/p0
0
1
HP2
HP0.5
200
100
0
1
Pore width [nm]
10
FIGURE 4.8 Pores distributions of HP0.5 and HP2 samples. Adsorptiondesorption isotherm of HP0.5 and HP2 samples are reported in the inset. Source: Reproduced from J. Sanz, I. Sobrados, J. Soria, S. Yurdakal, V. Augugliaro, Anatase nanoparticles boundaries resulting from titanium tetrachloride hydrolysis, Catal. Today 281 (2017) 198204 with permission, Copyright
2017 Elsevier Publishing.
derived from aggregation of plate-like particles
into slit-shaped pores. N2 adsorptiondesorption isotherms of all the prepared samples are
similar.
The article also reports the pore-size distribution obtained for the different isotherms
(inset in Fig. 4.7) at different synthesis times.
They are all characterized by a rather wide
pore-size distribution, which can grant efficient transport routes for reactants and products. The BET surface areas of the samples
are c. 67.2, 73.0, 82.0, and 71.3 m2/g and the
pore volumes of the samples 0.2047, 0.1846,
0.2268, and 0.2251 cm3/g for catalyst preparation times of 12, 18, 24, and 32 h, respectively.
Fig. 4.8 shows nitrogen adsorption
desorption isotherms of HP0.5 and HP2
photocatalysts, mainly amorphous homeprepared anatase TiO2, prepared by boiling of
TiCl4 solution in water (1:10, v/v) for 0.5 and
2 h [20]. Their BET specific surface area is
196 m2/g for both catalysts (inset of Fig. 4.8).
Approximately, 143 and 195 m2/g of this
figure are due to the exterior surface of HP0.5
and HP2, whereas 53 and 1 m2/g correspond
to their microporosity. Micropores (,4 nm
size) and mesopores (#13 nm size) are exhibited by the pore-size distribution curves of
HP0.5 sample, whereas in HP2 micropores are
low and mesopores are smaller (c. 7 nm)
(Fig. 4.8).
In both samples, amorphous titania, existing
in the form of defective and short titania
chains smaller than 1.5 nm, is responsible for
the microporosity. The significant decrease in
microporosity observed in HP2, along with the
improved crystallinity in the anatase phase is
generated by a prolonged ageing treatment,
suggesting that the amorphous titania chains
are converted into anatase crystals through
condensation on the surface of anatase nanoparticles, thus eliminating the structural
defects of the latter, and also promoting
agglomerates densification.
HETEROGENEOUS PHOTOCATALYSIS
100
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
4.2 SCANNING ELECTRON
MICROSCOPY
4.2.1 Introduction
Scanning electron microscopy (SEM) is one
of the most versatile techniques used for the
observation and analysis of the microstructure
morphology of catalysts. The basic principles
of SEM were established in the 1930s and early
1940s by Knoll along with other pioneers in
the field of electron optics; the first SEM microscope debuted in 1938 (Von Ardenne) with the
first commercial instruments released by
Siemens-Schuckertwerke. The first SEM used
to examine the surface of a solid specimen was
described by Zworykin et al. (1942), working
at RCA Laboratories in the United States [34].
From that point on, SEM technology has
shown remarkable progress and become
almost routine, being used in any application
in industry and science in which compositional, morphological, and topographical features affect the functional properties of the
materials. In the field of (photo)catalysis, the
study of morphology, chemical composition,
surface and internal microstructure of (photo)
catalysts plays a key role in the preparation of
materials with increasing selectivity, conversion rate, and lifetime.
The sample is irradiated with a finely
focused electron beam systematically rastered
across the surface of the specimen, resulting in
a wide range of signals that reveal information
about the sample including morphology and
chemical composition. The main reason for
SEM’s efficacy lies in the much higher resolution that can be obtained as compared with
light-optical instruments; values on the order
of 1.55 nm are standard for commercial SEM
(c. 200 nm of resolution for optical microscopes) and more advanced research instruments are also available with resolutions better
than 1 nm. Moreover, SEM microscope has a
larger depth of field, which allows a large
amount of the sample to be in focus at a time,
yielding a characteristic three-dimensional
appearance crucial for appreciating the surface
structure of (photo)catalysts. The high lateral
resolution of an advanced SEM is comparable
to the scanning tunneling microscope (STM) or
atomic force microscope (AFM), and despite
the fact that the vertical resolution is much
lower than that of the STM or AFM, SEM is
preferred when dealing with pronounced
topography (i.e., high roughness), where STM
or AFM experience difficulties [35]. Compared
with the transmission electron microscope
(TEM), the SEM delivers 3D images, rather
than 2D as in TEM, and allows for a larger
area of sample to be analyzed; on the other
hand, TEM has a higher magnifying power
and resolution, which are essential requirements when studying particle crystallinity as
well as lattice structure and defects, which
play a primary role in many catalytic
processes.
This section will focus first on the main
principles of SEM technology and then will
offer an overview of the main uses of SEM
microscopy in the investigation of different
catalytic materials in the form of various
nanostructures, membranes, and thin films.
4.2.2 Principle of Scanning Electron
Microscopy
The main constituent parts of a typical SEM
are electron column, scanning system, detector
(s), display, vacuum system, and electronics
controls (Fig. 4.9A). The electron gun at the
top of the column generates an electron beam,
whose path is controlled by a series of electromagnetic lenses: the condenser determines the
size of the electron beam (and thus the resolution), whereas the objective lens moves the
smallest spot formed by the beam up and
down in space (working distance) to meet the
specimen surface. The scanning coils defect
HETEROGENEOUS PHOTOCATALYSIS
4.2 SCANNING ELECTRON MICROSCOPY
101
FIGURE 4.9 (A) Scheme of SEM column showing electron gun, lenses, deflection system, and electron detector [35].
(B) Illustration of several signals generated by the interaction between the electron beam and the sample with the regions
from which the various signals are detected [37].
and “raster” the beam in the x- and y-axes
over a rectangular area on the sample surface.
The interaction between the incoming primary
electrons and the sample results in a number
of signals in the form of electromagnetic radiation (Fig. 4.9B). Certain portions of this radiation, generally secondary electrons (SEs) and
backscattered electrons (BSEs), are collected by
appropriate detectors whose output signal is
amplified and displayed on a computer monitor [36]. SEs are ejected from the shells of
constituent atoms in the sample following
inelastic scattering phenomena. Since the
energy of such electrons is very small (typically an average of around 35 eV), only those
generated within a few nanometers of the
material surface are emitted outside the specimen providing detailed surface and topographic information with good resolution.
Conversely, BSEs are those elastically scattered
backward and emitted out of the specimen.
These electrons undergo single or multiple
scattering events and escape from the surface
with an energy greater than 50 eV. Given their
higher energy as compared with SEs, BSEs
bring information from a relatively deep
region and are sensitive to the composition of
the sample; as a consequence, heavier elements
that backscatter more efficiently appear brighter than lighter elements in a BSE image.
Other important signals generated due to
the electron beamsample interaction are characteristic X-rays, which are used for elemental
analysis by energy-dispersive X-ray spectroscopy (EDS) or wavelength-dispersive X-ray
spectroscopy (WDS). When an inner-shell electron is ejected from a constituent atom in the
sample, the vacant orbital is filled with an
outer-shell electron resulting in the emission of
an X-ray with an energy corresponding to the
energy gaps between the two different shells
of the excited atomic element. Along with
HETEROGENEOUS PHOTOCATALYSIS
102
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
characteristic X-rays, which produce welldefined lines in the EDS spectrum, continuum
X-rays generated as primary beam electrons
are slowed down to varying degrees because
of the electromagnetic field of atomic nuclei.
The distribution of this energy loss is continuous in the EDS spectrum and not characteristic
of the specimen atomic number. One of the
major limitations of EDS is the impossibility of
detecting the lightest elements such as H and
He, while all the other elements can usually be
studied and the composition of the sample
determined in a semiqualitative way. Heavy
elements can be accurately detected with percent errors of c. 0.1%, while low atomic numbers such as C, N, O have errors of 1%5%
depending on the sample preparation and an
appropriate calibration for quantitative calculations [35]. Many of the limitations of the EDS
technique, such as low energy resolution and
low peak-to-background ratio, which make it
difficult to identify and quantify trace elements, are overcome by WDS. EDS and WDS
are usually used in conjunction with each
other, with EDS providing a qualitative overview and WDS, with its higher resolution and
sensitivity, successively refining the details,
aiming for trace elements and performing
quantitative analysis.
4.2.3 Applications of Scanning Electron
Microscopy
4.2.3.1 Nanostructures: Nanoparticles,
Nanotube, Nanowire, and Nanorods
SEM is extensively used for the characterization of nanostructures in terms of
dimensionality, size, shapes, particle agglomeration, aspect ratio, and porosity. Fig. 4.10
shows some of the different morphologies that
can be encountered in terms of nanostructured
catalysts. In particular, Fig. 4.10A displays
platinum/iron nanoparticles supported on
reduced graphene oxide powder used as
anode catalyst for the methanol electrooxidation [38]. Nanoparticles are spherical in shape
with size lower than 100 nm and are distributed homogeneously on the reduced graphene
oxide. Fig. 4.10B shows copper(II) oxide
nanosheets annealed at 700 C for electrocatalytic
oxygen evolution reaction [39]. The nanosheets
thickness is about 40 nm. Fig. 4.10C depicts
SnO2 nanorods doped by indium for catalytic
toluene oxidation [40]. Nanorods present a
cross-sectional side length of c. 125 nm and their
clean and smooth surfaces provide evidence of
the fact that nanorods have a compact structure
without mesopores.
4.2.3.2 Membranes
The investigation of membrane morphology
is fundamental to obtain information on their
microstructure and specifically on swelling,
asymmetry, mechanical strength, pore size and
shape, rugosity, catalyst dispersion and stability over the membrane, all these being important factors affecting the catalytic performance
and the lifetime of membranes [4143].
Fig. 4.11 shows cross-sectional SEM micrographs of PES/OGCN-LSMM membranes consisting of oxygenated graphitic carbon nitride
(OGCN) used as photocatalyst and poly(ether
sulfone) (PES) as base polymer modified with
hydrophilic surface modifying macromolecules
(LSMM) [44]. Specifically, SEM images display
the influence on the membrane structure of the
solvent evaporation time during the casting
step. The asymmetric membranes depicted in
Fig. 4.11AC with solvent evaporation times
of 0, 3, and 4 min, respectively, consist of the
following main layers: a dense skin layer on
the top and a porous sublayer formed by
finger-like shapes, becoming more irregular in
the middle of the cross-section, linked to
macrovoids structures beneath. The skin layer
is the active layer for the membrane performance, whereas the porous bottom layer
imparts mechanical stability to the PES/
OGCN-LSMM membrane. The occurrence of
HETEROGENEOUS PHOTOCATALYSIS
4.2 SCANNING ELECTRON MICROSCOPY
103
FIGURE 4.10
SEM images of various nanostructures: (A) platinum/iron nanoparticles. (B) CuO nanosheets. (C) SnO2
nanorods doped by indium. Source: (A) Reproduced from A. Eshghi, M.M. Sabzehmeidani, Platinumiron nanoparticles supported on reduced graphene oxide as an improved catalyst for methanol electro oxidation, Int. J. Hydrogen Energy 43 (2018)
61076116 with permission, Copyright 2018 Elsevier Publishing. (B) Reproduced from M. Qian, X. Liu, S. Cui, H. Jia, P. Du,
Copper oxide nanosheets prepared by molten salt method for efficient electrocatalytic oxygen evolution reaction with low catalyst loading, Electrochim. Acta 263 (2018) 318327 with permission, Copyright 2018 Elsevier Publishing. (C) Reproduced from Y. Liu, Y.
Guo, Y. Liu, X. Xu, H. Peng, X. Fang, et al., SnO2 nano-rods promoted by In, Cr and Al cations for toluene total oxidation: the
impact of oxygen property and surface acidity on the catalytic activity, Appl. Surf. Sci. 420 (2017) 186195 with permission,
Copyright 2017 Elsevier Publishing.
interconnected pores can be observed at 4 min
(Fig. 4.11C), which form both the skin layer
and finger-like layer. After an evaporation
time of 5 min (Fig. 4.11D), the membrane
(PES/OGCN-LSMM5min) was characterized
by an intrinsically interconnected pore topography with two main ranges of pore size:
c. 73 μm for large pores and 3040 μm for
small pores located below the large pores, as
shown in Fig. 4.11D. Thereby, a longer evaporation
time led not only to sponge-like microvoid
shapes, with a corresponding reduction in the
finger-like macrovoids, but also resulted in a
distinct and dense skin layer with higher thickness compared with the PES/OGCN-LSMM
membranes fabricated at lower solvent evaporation time. The formation of a thicker selective layer was one of the reasons for the best
filtration and photocatalytic performance of
PES/OGCN-LSMM5min, which showed the
HETEROGENEOUS PHOTOCATALYSIS
104
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE
4.11 Cross-sectional
SEM
images
of
(A)
PES/
OGCNLSMM0min;
(B)
PES/
OGCNLSMM3min; (C) PES/OGCNLSMM4min; and (D) PES/OGCNLSMM5min. The scale bar is 50 μm
for the images on the left, 5 μm for
the images on the right. Source:
Reproduced from N.E. Salim, J. Jaafar,
A. Ismail, M. Othman, M.A. Rahman,
N. Yusof, et al., Preparation and characterization of hydrophilic surface modifier macromolecule modified poly
(ether sulfone) photocatalytic membrane for phenol removal, Chem. Eng.
J. 335 (2018) 236247 with permission,
Copyright
2018
Elsevier
Publishing.
highest phenol degradation under UV
irradiation.
Fig. 4.12 displays the top and cross view of
catalytic pervaporation membranes prepared
by two different methods and used for
esterification reaction of n-butanol and acetic
acid [45]. Three layers stand out clearly: (1) the
top layer consists of a porous catalytic layer
made of an ion-exchange resin; (2) the middle
layer is a dense poly(vinyl alcohol) selective
HETEROGENEOUS PHOTOCATALYSIS
4.2 SCANNING ELECTRON MICROSCOPY
105
FIGURE 4.12 Top view and crosssectional SEM images of membranes: fabricated by blending (A and B); fabricated by
immersion phase inversion before esterification reaction of acetic acid (C and D);
the latter membrane are also displayed
after esterification reaction (E and F).
Source: Reproduced from W. Zhang, W.
Qing, N. Chen, Z. Ren, J. Chen, W. Sun,
Enhancement of esterification conversion using
novel composite catalytically active pervaporation membranes, J. Membr. Sci. 451 (2014)
285292 with permission, Copyright 2014
Elsevier Publishing.
layer; (3) the bottom layer made of PES serves
as a support layer. The membrane with the
catalytic layer fabricated by immersion phase
inversion (Fig. 4.12C and D) presents a more
porous structure as compared to the catalytic
membrane prepared by blending method
(Fig. 4.12A and B), instead showing a more
dense catalytic layer. An enhanced porous
structure reduces the diffusion resistance, facilitating the transport of components from the
bulk to the active sites for the reaction and, at
the same time, enhances the exposure of the
catalyst and boosts the number of active sites
available for the reaction. Moreover, no visible
changes in the membrane morphology could
be noticed before (Fig. 4.12C and D) and after
esterification (Fig. 4.12E and F) carried out in
pervaporation membrane reactors, giving evidence of the good structure stability of the
membrane.
4.2.3.3 Thin Films
SEM is a routine characterization for the
investigation of porosity, thickness, uniformity,
and composition of catalytic thin films
[4648]. An example of SEM analysis carried
out on such materials is provided in Fig. 4.13,
displaying the morphology of electrodeposited
Cu2O thin films, modified by the introduction
of different contents of Eu31, and EDS spectra
HETEROGENEOUS PHOTOCATALYSIS
106
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.13 SEM micrographs of electrodeposited Cu2O thin films displaying some precipitates along with cubic Cu2O
grains (indicated with black circles). Images in the insets are obtained in backscattering mode and show the occurrence of another
phase indicated by arrows. Eu31 concentration in the electrolyte is 0% (A), 2.5% (B), 10% (C). EDS spectra from (D) cubic crystals
and (E) precipitates for the Cu2O film obtained with Eu31 concentration of 2.5% in the electrolyte. SEM micrographs of corresponding areas are given in (a) and (b). Source: Reproduced from S. Shyamal, P. Hajra, H. Mandal, A. Bera, D. Sariket, A.K.
Satpati, et al., Eu modified Cu2O thin films: significant enhancement in efficiency of photoelectrochemical processes through suppression of charge carrier recombination, Chem. Eng. J. 335 (2018) 676684 with permission, Copyright 2018 Elsevier Publishing.
of the catalysts prepared with a concentration
of Eu31 in the electrolyte of 2.5% with respect
to the Cu21 concentration [49]. The films consist of cubic grains with different dimensions
and the average grain sizes become gradually
larger with increasing concentration of Eu31.
Moreover, the materials prepared in the presence of Eu31 show some precipitates, which
are marked with black circles in Fig. 4.13B and
C. Such precipitates probably originate from
the growth of europium hydroxide or
hydrated europium oxide as a result of the
alkaline pH (1213) of the electrodeposition
solution. The presence of a new phase
attributable to these precipitates in the Cu2O
matrix is corroborated by the black spots in
the SEM micrographs displayed as insets,
which were obtained in the backscattering
mode. These black spots are not present in the
pure film (Fig. 4.13A) and their amount
increases with the doping (Fig. 4.13B and C).
EDS spectra show that cubic shaped grains
HETEROGENEOUS PHOTOCATALYSIS
4.3 FOURIER-TRANSFORM INFRARED SPECTROSCOPY
mainly consist of Cu and O without Eu
(Fig. 4.13D), while Eu is present, along with
Cu, O, and Sn (from fluorine-doped SnO2 glass
substrate (FTO)), in the areas corresponding to
the new phase inclusions. The authors concluded that the larger ionic radius of Eu31,
compared with that of Cu1, is responsible for
its precipitation in the form of inclusions of
another phase, this acting as getter centers
resulting in the purification of host material
from detrimental impurities and, consequently,
in the upsurge in lifetime of nonequilibrium
photocarriers.
4.3 FOURIER-TRANSFORM
INFRARED SPECTROSCOPY
4.3.1 Introduction
Infrared (IR) spectroscopy has been one of
the earliest characterizations used for the investigation of catalytic systems and adsorption processes and it is still one of the most common
[50]. IR spectroscopy is based on the vibrations
of the atoms of a molecule, which result in a
characteristic IR spectrum attained by sending
an IR beam through a sample and determining
what aliquot of this beam is absorbed at a specific energy. The resulting peaks in an absorption spectrum occur at frequencies characteristic
of vibrations from functional units in sample
molecules. IR light usually does not have
enough energy to excite electrons, but it may
indeed result in vibrational excitation of covalently bonded atoms and group.
The key advantage of this technique is the
huge amount of information that can be
inferred from the direct monitoring of the
interaction between (photo)catalysts and
adsorbed molecules in the IR range. Indeed,
most of the energies associated with most significant molecular vibrations in catalysts occur
in the mid-IR (typically 2004000 cm21), and
the localized nature of these vibrations,
107
depending on the type of the bonds involved
and local environment, gives rise to a unique
IR “fingerprint” spectrum characteristic of the
investigated (photo)catalytic system. In this
context, IR spectroscopy may provide fundamental information about surface Lewis and
Brønsted acid sites, surface hydroxyl chemistry, poisoning of catalytic sites, molecular
structure of the surface metal oxide species
and their location on supported (photo)catalysts, as well as surface coverage of the metal
oxide overlayer [51]. In addition, this technique may be also used to investigate the
(photo)catalystsubstrate interactions, namely:
(1) active centers on which molecules are
adsorbed and react; (2) the constraint of molecular motion in the adsorbed state, namely the
rotation hindrance; (3) nature and geometry of
adsorption complex on the catalyst surface; (4)
bond rearrangements between catalyst and
substrate upon adsorption phenomena; and (5)
kinetic data of surface reaction through the
acquisition of time-resolved spectra [52].
IR spectroscopy is an old technique that has
been commercially available since the 1940s.
Primitive instruments were equipped with
prisms serving as dispersive elements, which
were then replaced in the mid-1950s by diffraction gratings employed in dispersive
machines. An important qualitative improvement came about in the late 1950s with the
emergence of Fourier-transform infrared spectrophotometer (FTIR), which, unlike dispersive
instruments, makes it possible to collect all
wavelengths simultaneously, allowing for
faster analysis, enhanced sensitivity and
optical throughput. Moreover, with continued
advancements in computer technology, IR
spectroscopy has made further progress. As a
result, FTIR has attracted growing attention
over the past decades for its potential in a
plethora of applications, including (photo)catalytic studies. FTIR spectrophotometers have
ended up belonging to the standard equipment in scientific laboratories, not least due to
HETEROGENEOUS PHOTOCATALYSIS
108
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
their relatively low costs as compared with
other modern instruments for physicochemical
characterization of the surface properties [53].
This section will cover the main operating
principles of FTIR spectroscopy, followed by
the main approaches pursued in the field of
(photo)catalysis. We will focus on recent in
situ FTIR studies of some important catalytic
reactions, namely reforming reactions, CO2
reduction, NOx reduction, and alcohol oxidation, which have been the subject of intensive
research work over the last years. In doing so,
we will point out how FTIR spectroscopy is a
crucial characterization to develop groundbreaking catalysts and gain more insight into
the mechanism of these important reactions.
4.3.2 Operating Principle and Main
Setups for Fourier-Transform Infrared
Spectroscopy of Catalysts
Fig. 4.14 shows a block diagram describing
an FTIR spectrophotometer. FTIR spectroscopy
is based on the principle that the interference
of radiation between two beams results in a
signal called interferogram. This is usually
generated by a Michelson interferometer,
which relies on a beamsplitter to split the
incoming IR beam into two optical beams, one
reflecting off a fixed mirror and the second
reflecting off a movable mirror. The path that
one beam travels is a fixed length and the
other is continually changing as its mirror
moves. The two beams are thus recombined as
they meet back at the beamsplitter and the
radiation emerging from the interferometer
reaches the sample compartment and finally
the detector. After amplification of the signal,
the data are translated into digital form by an
analog-to-digital converter and eventually
transferred to a computer in which Fourier
transform is carried out to obtain the desired
IR spectrum [54].
The interactions between the matter and IR
light can be seen in terms of alterations of
molecular dipoles linked to vibrations and
rotations; a molecule indeed absorbs IR light
only if the rotations or vibrations inside a molecule lead to a net change in the dipole
FIGURE 4.14 Block diagram of an FTIR
spectrophotometer.
Fixed mirror
Moving mirror
IR source
Beam splitter
Sample compartment
Detector
Amplifier
Analog-to-digital converter
Computer
HETEROGENEOUS PHOTOCATALYSIS
4.3 FOURIER-TRANSFORM INFRARED SPECTROSCOPY
moment of the molecule itself. The interactions
between the fluctuations in the dipole moment
and the alternating electrical field of the radiation thus play a key role since the IR light will
be absorbed only if the frequency of the radiation corresponds to the vibrational frequency
of the molecule, resulting in a variation of the
amplitude of molecular vibration. Such vibrations are usually of two kinds: stretching and
bending. The first is associated with an alteration in the interatomic distance along bond
axis, whereas the second is associated with a
variation in the angle between two bonds.
Stretching vibrations can occur in-phase (symmetric) or out-of-phase (asymmetric). When
different terminal groups are present in the
molecule, the stretching modes will have varying proportions depending on the stretching of
each group, and the coupling will change. On
the other hand there are four types of bending
vibrations: scissoring, rocking, wagging, and
109
twisting [55]. All of these characteristic vibrations contribute to the IR spectrum and, given
that molecules are often characterized by a
number of bonds with many possible vibrations, an IR spectrum can have many absorption signals. A detailed explanation of
molecular vibrations is beyond the scope of
this section, but this topic is discussed in detail
in other textbooks [54,56].
FTIR spectroscopy is a popular technique
for the characterization of solid catalysts, not
least because it offers a wide variety of setups
and configurations that can be arranged to
adapt the experiment to the nature of the sample under investigation [57]. Nowadays FTIR
spectrophotometers are indeed equipped with
a number of accessories, which allow the
instrument to operate in different modes such
as transmission (TIR), diffuse reflectance
(DRIFTS), attenuated total reflection (ATR),
and reflection-absorption (RAIRS) (Fig. 4.15)
FIGURE 4.15 Most common setups used for the investigation of catalysts by IR spectroscopy. Source: Reproduced from
F. Zaera, New advances in the use of infrared absorption spectroscopy for the characterization of heterogeneous catalytic reactions,
Chem. Soc. Rev. 43 (2014) 76247663 with permission, Copyright 2014 RSC Publishing.
HETEROGENEOUS PHOTOCATALYSIS
110
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
[58]. Most of the early studies were conducted
in TIR mode, where a self-sustaining form of
the sample is positioned inside a cell and
exposed to the IR beam, which is collected and
analyzed after passing through the catalyst
[59,60]. TIR setups are quite straightforward
but have some constraints since they require
highly transparent and sturdy catalysts necessary
for collecting enough IR intensity, while ensuring
the preparation of a self-sustaining sample.
DRIFTS has been shown to be more sensitive to surface species than TIR mode and to
be an excellent in situ technique. Light incident
onto a powder sample can partially undergo
specular reflection by the catalyst surface, be
partly scattered diffusely, and partly penetrate
into the sample. The IR radiation reflected by
the roughened surfaces is collected by a higharea parabolic mirror and analyzed. Since regular reflection distorts the resulting spectra,
the DRIFTS accessory is designed to remove
the specularly reflected component. DRIFTS is
more suitable for the investigation of highly
absorbing catalysts, which generally present
very low signal and sloping baselines when
studied in transmission [61]. On the other
hand, the reproducibility of DRIFTS intensities
can be low due to differences in catalyst loading procedure and in scattering coefficients,
which vary with cell geometry.
In ATR-IR mode, the IR ray is focused into
a crystal of relatively high refractive index
with an angle exceeding the critical angle for
internal reflection. The IR beam is then
reflected from the internal surface of the crystal producing an evanescent wave being projected orthogonally into the catalyst in close
contact with the crystal. Part of the radiation
of the evanescent wave is absorbed by the catalyst while the reflected portion reaches the
detector. The resultant attenuated radiation is
eventually measured giving rise to the IR spectral characteristics of the catalyst. The main
benefits of ATR techniques derive from easy
sample preparation, unlike traditional FTIR
sampling by transmission; the catalyst can
indeed be analyzed in its natural state, with no
need to be heated or pressed into pellets to collect reliable spectra [54,62].
RAIRS spectroscopy is a specialized
approach that relies on the absorption and
reflection of low incident angle IR radiation by
the surface molecules of a highly reflective or
polished sample. In this technique, metals are
usually used as substrates to accompany the
absorption process and results are provided in
terms of the change in the reflectance spectrum
of the substrate. On the used metallic substrates, only the vibrational modes having a
component of their dipole change perpendicular to the surface can be detected, providing
important information on adsorption geometry
[63]. RAIRS has demonstrated to be very effective for the exploration of low-surface-area systems and adsorption processes at either solid/
liquid or solid/gas interfaces [58].
4.3.3 Applications of Fourier-Transform
Infrared Spectroscopy in Key Catalytic
Reactions
4.3.3.1 Reforming Reactions
Hydrogen global production has so far been
dominated by fossil fuels, with the most
important contemporary technologies being
the steam reforming of hydrocarbons. The
increasing concerns about energy supply and
environmental concerns have led to a wide utilization of alternative energy with the aim of
replacing carbon-intensive energy sources and
reducing global warming emissions. In the
past decades, growing attention has been paid
to the H2 generation by steam reforming of
biomass-derived ethanol and methanol, which
are particularly appropriate for on-board H2
production since they are easy to store, transport, and handle [64,65]. In this context, IR
spectroscopy is a powerful tool for the investigation of activity/selectivity of innovative
HETEROGENEOUS PHOTOCATALYSIS
4.3 FOURIER-TRANSFORM INFRARED SPECTROSCOPY
111
FIGURE 4.16 (A) In situ DRIFT spectra obtained for CeOx/npAu and TiOx/npAu catalysts upon exposure to CH3OH.
(B) Reaction mechanism of methanol steam reforming on an oxide functionalized npAu surface. Source: Reproduced from J.
Shi, C. Mahr, M. Murshed, T. Gesing, A. Rosenauer, M. Bäumer, et al., Steam reforming of methanol over oxide decorated nanoporous gold catalysts: a combined in situ FTIR and flow reactor study, Phys. Chem. Chem. Phys. 19 (2017) 88808888 with permission,
Copyright 2017 RSC Publishing.
catalysts towards H2 generation and determination of crucial reaction intermediates and
preferential reaction pathways, which occur
over the surface of such catalysts.
To this aim, in situ DRIFTS spectroscopy
has been recently employed to study the steam
reforming of methanol on a number of catalysts [6466] such as TiOx and CeOx deposited
inside a bulk nanoporous gold (npAu) [67].
The spectra obtained during exposure of
CeOx/npAu and TiOx/npAu to CH3OH are
shown in Fig. 4.16A. Upon exposure to methanol, CO2 was formed starting from 150 C and
the intensity of the formate band (1346 cm21)
decreased due to the dehydrogenation of the
surface bonded formate. The occurrence of
negative band at c. 1338 cm21 in CeOx/npAu
was indicative of the desorption/consumption
of surface OH, whereas methyl formate
formed over TiOx/npAu surface (1770 cm21
and 1190 cm21). Based on these observations,
the proposed mechanism, shown in Fig. 4.16B,
involves a first step that is the deprotonation
of methanol with the generation of a methoxy
group, then dehydrogenated to formaldehyde.
Reactive OH groups control whether (1) formate reacts with an adjacent methoxy group
giving rise to methyl formate, or (2) formate
directly reacts with OH giving rise to formic
acid. The availability of reactive OHads therefore plays a key role in the catalytic performance. While methyl formate forms on the
surface of the less active TiOx/npAu catalyst,
in the other case, formate is directly oxidized
with OHads to formic acid, which is decomposed to CO2 and H2, this being the dominating reaction pathway on the more active
CeOx/npAu catalysts.
In situ DRIFTS spectroscopy has been also
employed to study the adsorbed intermediates
on multicomponent Ni/Fe/Cu-based catalysts
active for ethanol reforming reactions [68].
Fig. 4.17A shows the spectra of Ni1Fe0.5Cu1
catalyst recorded at different temperature
HETEROGENEOUS PHOTOCATALYSIS
112
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.17 (A) DRIFT spectra for ethanol decomposition on
Ni1Fe0.5Cu1 catalyst. (B) Proposed
reaction mechanism on the same
catalyst. Source: Reproduced from
A. Kumar, J.T. Miller, A.S.
Mukasyan, E.E. Wolf, In situ XAS
and FTIR studies of a multicomponent Ni/Fe/Cu catalyst for
hydrogen production from ethanol,
Appl. Catal. A 467 (2013) 593603
with permission, Copyright 2013
Elsevier Publishing.
under a continuous flow of helium saturated
with ethanol. The bands at 720785 cm21 were
assigned to (CH)n rocking vibrations for
n 5 14, the CH stretching bands were located
between 2800 and 3000 cm21 for CH2 and CH3,
whereas the CH2 and CH3 bending vibrations
occurred at 13501470 cm21. The presence of
bands at 860880 cm21 was indicative of ethoxy species; those at 15501560 cm21 and
1505 cm21 were attributed to acetate and
HETEROGENEOUS PHOTOCATALYSIS
4.3 FOURIER-TRANSFORM INFRARED SPECTROSCOPY
carbonate species, respectively. Acetaldehyde
formation was more evident at temperature
above 200 C through the IR bands at
17201740 cm21 and at 14001450 cm21 corresponding to CO and to CH2 bending in aldehydes, while the extra peaks occurring at 2700
and 2750 cm21 at 350 C were ascribed to the
absorption of formyl CHO group. Finally, the
band at 1640 cm21 was related to the bending
vibration mode of adsorbed molecular water,
while bands at around 3700 cm21 to the OH
bond stretching in ethanol and water molecules, being the intensities greatly affected by
temperature. Based on the DRIFTS observations and other characterizations, the proposed
reaction pathway in Fig. 4.17B starts with the
adsorption of ethanol, which binds to the
active site through its OH group. The second
step is the adsorption of ethoxy species, which
can either give rise to acetaldehyde losing one
hydrogen or progressively losing more hydrogen atoms by sequential CH bond scission.
Depending on the metal in the multicomponent catalyst interacting with the adsorbed
ethanol, different pathways are possible: (1)
ethoxy on the Cu surface can further decompose to form acetaldehyde and hydrogen; (2)
ethoxy on Ni surface enters an unstable state
and decomposes to give methane, carbon monoxide, and carbon; or (3) a higher alkane forms
on Fe surface, this being an active catalyst for
FischerTropsch synthesis.
4.3.3.2 CO2 Reduction
The capture and utilization of CO2 has been
recognized as a potential route to decrease the
level of generated CO2 emissions [6971]. The
abundant CO2 is thus a suitable raw material
for a closed energy loop, where generated CO2
emissions can be catalytically transformed into
CO, CH4 and CH3OH. FTIR spectroscopy here
is a crucial technique to monitor the adsorbed
species formed during catalyst exposure to
CO2 and CO as well as for the identification of
the chemical states of the used catalysts
113
[71,72]. With regard to the monitoring of the
reactant and product species, the IR absorbance intensities of the characteristic vibrational modes such as ν(C 5 O) at 2357 cm21 for
CO2, ν(C 5 O) at 2172 cm21 for CO, and
δ(CH) at 3016 cm21 for CH4, can be monitored over time and translated to concentrations by calibrating for each compound in a
way that vibrational response of the pure species is linked to measured pressures within the
pressure range of the experiments [73]. This
strategy has been used to assess the performance of Fe-modified Ni/CeO2 catalysts
towards CO2 reduction in a batch reactor [74]
(Fig. 4.18). Specifically, as shown in Fig. 4.18A,
pure Fe catalyst was found to be 100% selective towards CO, whereas pure Ni shows the
lowest CO selectivity, with the CO/CH4 product ratio increasing with Fe content (product
ratios for 10% CO2 conversion are specified in
parentheses). However, the pure Fe sample
was the least active with a final CO2 conversion of 8.3% and the performance of the
bimetallic catalysts improved by decreasing
the Fe content (Fig. 4.18B). The equimolar
Ni3Fe3 showed an enhanced production of
CO (Fig. 4.18C), while pure nickel catalyst
had the highest generation of CH 4
(Fig. 4.18D).
In situ FTIR spectroscopy can be also used
to assess the oxidation states of catalysts such
as copper species using CO as a probe molecule. Indeed, it has been found that CO
adsorbed onto Cu1 sites exhibits a vibration
peak at 2128 cm21, whereas the characteristic
vibrations for Cu21CO sites and free CO
occur at 2200 and 2142 cm21, respectively [75].
This has been useful to identify the oxidation
state of copper in CuTiO2 composites for
CO2 reduction [72]. The authors of this work
found that only Cu1CO peak came out in the
spectrum when injecting increasing concentrations of CO in an IR cell containing the catalyst, giving evidence of the fact that no Cu21
species is present (Fig. 4.19A). Furthermore,
HETEROGENEOUS PHOTOCATALYSIS
114
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.18 Catalytic reduction of CO2 by H2 at 623 K for a set of Ni3Fex/CeO2 catalysts with increasing loading
amounts x of Fe. (A) Ratio of CO/CH4 (the value at 10% CO2 conversion is shown in brackets). (B) Conversion of CO2,
(C) generation of CO, and (D) generation of CH4 over time. Source: Reproduced from L.R. Winter, E. Gomez, B. Yan, S. Yao, J.
G. Chen, Tuning Ni-catalyzed CO2 hydrogenation selectivity via Ni-ceria support interactions and NiFe bimetallic formation, Appl.
Catal. B 224 (2018) 442450 with permission, Copyright 2018 Elsevier Publishing.
the intensity of the Cu1CO peak increased
by increasing the loading of Cu, further demonstrating the presence of Cu1 (Fig. 4.19B).
These photocatalysts were tested for CO2
reduction in the presence of H2O under solar
light radiation and the spectrum of the sample
prepared with Cu weight percentage of 1% is
displayed in Fig. 4.19C. In dark conditions,
water (1640 cm21) and carboxylate CO2
2
(1267 cm21) were detected, the presence of the
latter indicating that electrons could be spontaneously transferred to CO2 from the defective
HETEROGENEOUS PHOTOCATALYSIS
4.3 FOURIER-TRANSFORM INFRARED SPECTROSCOPY
115
FIGURE 4.19 (A) IR spectra of CO adsorption on the CuTiO2 catalyst with Cu weight percentage of 1% with various
CO contents. (B) IR spectra after adsorption of CO for 35 min at room temperature on catalysts with different Cu content.
(C) FTIR spectra of CuTiO2-1.0 in the presence of CO2 and water: curve (a) raw CuTiO2-1.0 sample; curve (b) after
adsorption for 30 min in dark conditions; curve (c) after light irradiation for 20 min; curve (d) after light irradiation for
50 min. Source: Reproduced from S. Zhu, S. Liang, Y. Tong, X. An, J. Long, X. Fu, et al., Photocatalytic reduction of CO2 with
H2O to CH4 on Cu(I) supported TiO2 nanosheets with defective {001} facets, Phys. Chem. Chem. Phys. 17 (2015) 97619770 with
permission, Copyright 2015 the RSC Publishing.
anatase facets even in the dark [69]. Under
solar simulated irradiation, the intensity of
H2O peak decreased and that of CO2 species
increased, pointing out that CO2 species are
the dominant intermediates on the catalyst’s
surface during the photoreduction, while
water is gradually consumed by donating electrons or scavenging holes. The enhanced activity of these catalysts may be attributed to the
key role of Cu1, which improves CO2 adsorption, increases the migration rate of the generated photocarriers, and acts as an active site
for the formation of CH4.
4.3.3.3 Nitrogen Oxides Reduction
Nitrogen oxides (NOx) are known to be
extremely dangerous to human beings and the
environment, since they contribute to acid
HETEROGENEOUS PHOTOCATALYSIS
116
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
rain, photochemical smog, ozone depletion,
and greenhouse effects [76]. One of the most
efficient process to reduce NOx is selective catalytic reduction (SCR) with ammonia. In this
area, FTIR is used to investigate the extent of
adsorption of NO and NH3 on catalysts, the
reduction step and acid sites of the catalysts,
which play a primary role in the NH3SCR
performance [77,78]. One of the several studies
performed in this field is shown in Fig. 4.20
[79]. By means of in situ DRIFTS, the authors
explored the change in acid sites of CeZr2Ox
catalysts exposed to NH3 at 200 C upon the
introduction of Nb (Fig. 4.20A). The bands at
1603 cm21 and 1209, 1263, 1193, and 1182 cm21
were assigned to asymmetric and symmetric
bending vibrations of the NH bonds in coordinated NH3 adsorbed on Lewis acid sites. On
the other hand, the bands at 1668 cm21 and
1434, 1444, and 1414 cm21 were attributed to
symmetric and asymmetric bending vibrations
of NH41 species on Brønsted acid sites. The
negative bands around 3700 cm21 were due to
the hydroxyl consumption as a result of the
reaction between NH3 and hydroxyls. The
highest content of Brønsted acid sites was
found in pure Nb2O5. The addition of Nb to
CeZr2Ox increased the catalyst acidity and
CeNb3.0Zr2Ox showed the highest amount of
NH41 bound to Brønsted acid sites, which
explained the best NH3SCR performance. In
another experiment, NH3 was preadsorbed at
FIGURE 4.20 (A) IR spectra of Nb2O5 and CeNbaZr2Ox catalysts following adsorption of 500 ppm NH3 with 300 mL/
min flow rate at 200 C. (B) In situ DRIFTS of NO 1 O2 reacted with preadsorbed NH3 species at 200 C on CeNb3.0Zr2Ox
catalyst. Source: Reproduced from S. Ding, F. Liu, X. Shi, H. He, Promotional effect of Nb additive on the activity and hydrothermal
stability for the selective catalytic reduction of NOx with NH3 over CeZrOx catalyst, Appl. Catal. B 180 (2016) 766774 with permission, Copyright 2016 Elsevier Publishing.
HETEROGENEOUS PHOTOCATALYSIS
4.3 FOURIER-TRANSFORM INFRARED SPECTROSCOPY
200 C on this catalyst which was then exposed
to a mixture NO2 1 O2 to perform the catalytic
reduction. The resulting IR spectra in
Fig. 4.20B shows how the catalyst surface was
covered by ionic NH41 (1668 and 1434 cm21)
and coordinated NH3 (3360, 3260, 3155, 1603,
and 1209 cm21). After 3 min, all bands belonging to NH3 species were replaced by nitrate
species, namely bidentate nitrate at 1583 cm21,
bridging nitrate at 1622 and 1217 cm21. This
confirmed the high activity of CeNb3.0Zr2Ox
catalyst and the beneficial effect of Nb.
A similar approach was taken to study the
activity of highly dispersed MnOx/SAPO-34,
where SAPO-34 is microporous molecular
sieve of the chabazite structure with a pore
size of about 4.3 Å [80]. Fig. 4.21A shows the
in situ FTIR spectra of such catalyst after
adsorption of NO 1 O2 at 200 C for 30 min and
after purging with Ar for further 30 min. The
band at 1909 cm21 was ascribed to gas phase
or weakly adsorbed NO and that at 1835 cm21
to nitrosyl species. The bands at 1610, 1518,
117
1389, and 1220 cm21 were attributed to
bridged nitrate, monodentate nitrate, nitrate,
and bidentate nitrate species, respectively. The
intensity of the bands at 1909, 1835, 1610, 1518,
and 1389 cm21 increased after 10 min while
their position was unaffected. On the other
hand, continuous purging at 200 C resulted in
the shift of the band at 1220 cm21 to 1197 cm21
attributable to bridged nitrate species. In
another experiment, NH3 was introduced at
200 C after adsorption of NO 1 O2. As shown
in Fig. 4.21B, the band at 1610 cm21 (bridged
nitrate) disappeared instantaneously after NH3
introduction, while the bands at 1625 and
1525 cm21 corresponding to bridged nitrates
and bidentate nitrate, respectively, appeared at
the same time, indicating that specific chemical
environment of the bridged nitrate species
(1610 cm21) might be involved in the catalytic
process. This was further supported by the
decrease in the intensity of the band at
1197 cm21, this being also attributable to the
bridged nitrate species. However, the band at
FIGURE 4.21 (A) IR spectra of MnOx/SAPO-34 catalyst during NO 1 O2 adsorption at 200 C for various times. (B) In
situ FTIR spectra of MnOx/SAPO-34 exposed to NO 1 O2 followed by exposure to NH3 at 200 C. Source: Reproduced from
C. Yu, B. Huang, L. Dong, F. Chen, X. Liu, In situ FT-IR study of highly dispersed MnOx/SAPO-34 catalyst for low-temperature
selective catalytic reduction of NOx by NH3, Catal. Today 281 (2017) 610620 with permission, Copyright 2017 Elsevier
Publishing.
HETEROGENEOUS PHOTOCATALYSIS
118
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
1610 cm21 was again observed after 20 min of
flowing NH3. Moreover, the intensity of the
band at 1389 cm21 (nitrate species) decreased
and the OH stretching bands at 38003500 cm21
were observed after introducing NH3. These
results suggest that the bridged nitrate species
(1610 cm21) might react with the NH3 ad-species
directly, and other NOx ad-species could be converted into bridged nitrate species (1610 cm21).
4.3.3.4 Alcohol Oxidation
The development of innovative catalysts for
oxidation of alcohols is a hot topic in catalysis.
Two of the most common applications include
direct alcohol fuel cells, by means of electrocatalysis, and photocatalytic conversion of alcohols
into aldehydes [8184]. The first application has
attracted great attention due to the possibility of
using fuel cells as power sources for transportation vehicles and portable electronic devices [85],
while the second application is one of the most
crucial syntheses for laboratory research and
commercial production, because aldehydes are
extensively employed in beverages, drugs, food
industries and as starting molecules for the synthesis of a number of intermediates and fine
chemicals [86].
When investigating new catalysts for alcohol fuel cells, FTIR is a valuable tool for
exploring the process of interfacial electrocatalysis and identifying reaction products in
response to a change in potential. One of the
numerous studies in this area is shown in
Fig. 4.22, displaying the results of in situ FTIR
analysis on Pd/C and Pd2Ru/C catalysts for
alkaline direct ethanol fuel cells [87]. FTIR reference spectra were acquired at 20.86 V, and
L
0.1 V compared with the previous one. (C) Integrated peak
areas of acetaldehyde band at 926 cm21 versus potential
for Pd/C and Pd2Ru/C electrocatalysts. Source: Reproduced
from J. Guo, R. Chen, F.-C. Zhu, S.-G. Sun, H.M. Villullas,
New understandings of ethanol oxidation reaction mechanism on
Pd/C and Pd2Ru/C catalysts in alkaline direct ethanol fuel cells,
Appl. Catal. B 224 (2018) 602611 with permission, Copyright
2018 Elsevier Publishing.
FIGURE 4.22 (A) FTIR spectra of Pd/C electrocatalyst.
(B) FTIR spectra of Pd2Ru/C electrocatalyst. The sample
potential was changed from 20.60 to 0.20 V, whereas the
reference potential was 20.86 V versus Hg/HgO/NaOH
(1.0 M). Each spectrum was recorded with an increase of
HETEROGENEOUS PHOTOCATALYSIS
4.3 FOURIER-TRANSFORM INFRARED SPECTROSCOPY
studied spectra were obtained between 20.6
and 0.2 V versus Hg/HgO/NaOH (1.0 M) in
1.0 M NaOH with 1.0 M ethanol. The IR spectra of both catalysts, depicted in Fig. 4.22A and
B, showed negative bands at 1553 and
1418 cm21 ascribed to symmetric and asymmetric stretching of the OCO bonds that
originated from acetate ions. The two positive
bands at 1085 and 1045 cm21 were due to a
decrease in the amount of ethanol in the thin
layer of electrolyte between the working electrode and the optical window used for the
analysis. The production of acetaldehyde was
confirmed by the band at 926 cm21 accounting
for the CCO asymmetric stretching of the
aldehyde. To better appreciate the effect of Ru
on the improvement of the electrocatalytic performance, the area of acetaldehyde band was
normalized by ethanol oxidation current for
the same amount of Pd loading and results for
the two catalysts are reported in Fig. 4.22C.
Acetaldehyde production was significantly
larger on Pd2Ru/C than on Pd/C almost over
the entire potential range, except for at 0.2 V
because of the Pd oxidation occurring on
Pd2Ru/C. In this study, FTIR analysis gave
evidence that ethanol oxidation in alkaline
solution on such catalysts takes place mainly
through the pathway without CC bond
breaking.
Regarding the photooxidation of alcohols to
aldehydes, FTIR can be used to monitor
adsorbed reagents, intermediates, and products during irradiation, to gain an insight
into the photoinduced reaction mechanism.
One of the recent works in this area reports an
in situ FTIR study on the oxidation of toluene
on Ag3VO4/TiO2 nanorods under visible irradiation [88]. The resulting spectra are shown
in Fig. 4.23. After toluene adsorption and
before light irradiation (t 5 0), the peaks at
3075 and 3038 cm21, ascribed to CH stretching, and the ones at 2937 and 2884 cm21,
assigned to methyl stretching of toluene, were
clearly visible (Fig. 4.23A). The bands at 1610
119
and 1500 cm21 were assigned to the vibrations
of the aromatic ring (Fig. 4.23B). During the
reaction, the intensity of the bands at 3075,
3038, 2937, and 2884 cm21 decreased, proving
that toluene was degraded, and new signals
appeared: (1) bands at 2360 and 2342 cm21
ascribed to CO2 (Fig. 4.23C); (2) bands at 1469
and 1462 cm21 due to benzyl alcohol; (3) bands
at 1548 and 1510 cm21 ascribed to the formation of C 5 O bonding in benzaldehyde; (4)
bands at 1529 and 1564 cm21 due to asymmetric stretching vibration of the carboxylate
group COO2; and (5) bands at 1640 and
1658 cm21 attributed to C 5 C of benzoic acid
(Fig. 4.23B). Moreover, signals at 3695, 3682,
and 3664 cm21 derived from surface hydroxyls, which serve as adsorption sites, while
signals at 3730, 3717, and 3706 cm21 were
linked to sources of hydroxyl radicals originated from the reactions between holes and
hydroxyl groups adsorbed on the catalyst surface (Fig. 4.23D). Based on the FTIR analysis
and other characterizations, the authors concluded that the photodegradation of toluene
on Ag3VO4/TiO2 nanorods started with the
oxidation to benzyl alcohol with a series overoxidation to benzaldehyde, benzoic acid and,
finally, to CO2 and water.
Another example of photocatalytic oxidation
of aromatic alcohols is provided in Fig. 4.24,
showing the time-resolved ATR-FTIR spectra
of a home-prepared anatase TiO2 (HP0.5),
mainly amorphous, used for the oxidation of
benzyl alcohol to benzaldehyde under UV irradiation and in an atmosphere of synthetic air
saturated with water and benzyl alcohol before
the experiment [89]. The bands of benzyl alcohol at 1498, 1454, 1372, 1208, 1078, and
1037 cm21 (not labeled in Fig. 4.24) decreased
under illumination, whereas the bands at
1640 cm21 (bending vibration mode of
adsorbed molecular water) increased in intensity. The high intensity and the continuous
increase of this band proved either that illumination supports an effective photoadsorption
HETEROGENEOUS PHOTOCATALYSIS
120
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
3075
0.12
Absorbance (a.u.)
0h
1h
2h
3h
4h
3038
2937
2884
0.08
(B)
0h
1h
1610
2h
3h
4h
0.12
Absorbance (a.u.)
0.16 (A)
0.08
1500
1510
1529
1548
1564
1462
1469
1658
1640
0.04
0.04
0.00
0.00
3100
3000
2900
Wavenumber (cm–1)
2800
0.04
(C)
2342
Absorbance (a.u.)
1h
0.00
2h
3h
–0.02
(D)
0h
4h
0.02
1h
2h
0.00
3h
3730
3717
–0.04
3706
3682
3695
2380
2340
2360
Wavenumber (cm–1)
2320
2300
3740
3720
3700
4h
3664
3642
3623
3680 3660 3640
Wavenumber (cm–1)
3620
–0.02
2400
1450
1500
0h
2360
0.02
1550
1600
Wavenumber (cm–1)
1650
Absorbance (a.u.)
3200
3600
FIGURE 4.23 IR spectra recorded during the photooxidation of toluene on Ag3VO4/TiO2 nanorods as a function of
irradiation time. IR spectra (A)(D) correspond to different wavenumber ranges. Source: Reproduced from X. Zou, Y. Dong,
X. Zhang, Y. Cui, Synthesize and characterize of Ag3VO4/TiO2 nanorods photocatalysts and its photocatalytic activity under visible
light irradiation, Appl. Surf. Sci. 366 (2016) 173180 with permission, Copyright 2016 Elsevier Publishing.
of water on photocatalyst surface or that water
is generated as a product of the photocatalytic
decomposition of the benzyl alcohol. Due to
the high intensity and broadness of this band,
it was not possible to detect other compounds
that might be generated during the photooxidation process such as benzaldehyde or benzoic acid, as the bands of molecular adsorbed
water overlap the region for the carbonyl
stretching band. New bands appeared (in the
14001500 cm21 range and at 1275 cm21),
which cannot be unambiguously ascribed to specific species, but based on their wavenumber
values, they might indicate the occurrence of
dicarboxylic acid arising from the breakage of
the aromatic ring.
4.4 UVVISIBLE SPECTROSCOPY
4.4.1 Introduction
UVVisible (UVVis) spectroscopy is a useful technique for the determination of the
absorption properties of materials. In particular,
in the case of semiconductors, measurements of
HETEROGENEOUS PHOTOCATALYSIS
121
4.4 UVVISIBLE SPECTROSCOPY
FIGURE 4.24 Time-resolved ATR-FTIR of
HP0.5 thin film under UV irradiation in an atmosphere of synthetic air saturated with water and
benzyl alcohol. Source: Reproduced from V.
Augugliaro, H. Kisch, V. Loddo, M.J. López-Muñoz, C.
Márquez-Álvarez, G. Palmisano, et al., Photocatalytic
oxidation of aromatic alcohols to aldehydes in aqueous
suspension of home-prepared titanium dioxide: 2.
Intrinsic and surface features of catalysts, Appl. Catal.
A 349 (2008) 189197 with permission, Copyright
2008 Elsevier Publishing.
diffuse reflectance allow to estimate the bandgap energy value. This parameter is critical in
the field of photocatalysis since it determines
the light energy to be used to activate the semiconducting solids. As far as catalysts are concerned, the diffused reflectance technique
provides information on the electronic properties of a solid and—in the presence of transition
metals cations—on the coordination and oxidation state of metal.
UV and Vis radiations are part of the electromagnetic spectrum. The study of the interactions of the electromagnetic radiation with
the matter (spectroscopy) allows to gain relevant knowledge on the materials of interest,
such as the absorption, transmission, and
reflectivity properties [90].
Absorbance spectroscopy, or spectrophotometry, is the quantitative determination of
the amount of light absorbed or transmitted by
a given material as a function of wavelengths.
This gives information about the electronic
transitions happening in the studied material.
In the case of isolated atoms only electronic
transitions can occur, while, in the case of
molecules, rotational and vibrational motions
are also generated.
4.4.2 Interaction of Light With the
Matter
When light reaches matter an interaction
occurs and, depending on the structure of the
materials and on the wavelength of the light,
different phenomena can be observed: photon
absorption, scattering, reflection, or refraction
(Fig. 4.25) [91].
The various components in which the incident light is split after the interaction with the
sample are represented by the following
equation:
I0 5 A% 1 T 1 R 1 S
(4.18)
where A% is the absorbance (A% 5 1102A), T
is the transmitted light, R is the reflected light,
and S is the scattered light.
The fraction of the transmitted light can be
calculated
by
the
BeerLambert
law
(Eq. (4.19)), which states that the ratio between
HETEROGENEOUS PHOTOCATALYSIS
122
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.25 Interaction of light with matter.
where ε is the molar absorptivity coefficient of
the material and c is the concentration of the
absorbing species.
From the BeerLambert law, it can be
deduced that the amount of light absorbed by
a medium is independent of the intensity of
the incident light.
FIGURE 4.26
Schematic of the light scattering from a
powder sample.
the fraction of the light measured after interaction with the sample (I) and the incident intensity (I0) depends on the path length of light
through the sample (l) and on the material
properties (kλ is the attenuation coefficient, a
typical constant of the medium crossed by the
light and depends on the wavelength λ):
I
5 e2kλ l 5 T
I0
(4.19)
The ratio I/I0 is defined as transmittance
(T), whereas absorbance (A) can be calculated
from the opposite of the natural logarithm of
T, the expression assumes the following form
[92] (Eq. (4.20)):
A 5 kλ l 5 εcl 5 2 ln
I
I0
(4.20)
4.4.3 Band-Gap Determination
In the case of semiconducting solids,
UVVis spectroscopy allows to calculate the
optical band gap (Eg), namely the minimum
energy that must be supplied to an electron to
promote it from the top of the valence band to
the bottom the conduction band (CB) and the
electronic processes occurring in the material
after the interaction with the light [93]. The Eg
value determination is the first key step when
dealing with semiconductors to be used in all
technologies involving the conversion of solar
to chemical or electric energy. When a monochromatic light impinges the surface of a powder, a part is absorbed, a part is reflected, and
a part is scattered, and after various reflections
it can return to the surface (Fig. 4.26). The diffuse reflected light becomes weaker, with
respect to the incoming one, if it is partially
absorbed by the material. The total reflected
light, measured with respect to a nonabsorbing
HETEROGENEOUS PHOTOCATALYSIS
123
4.4 UVVISIBLE SPECTROSCOPY
material (BaSO4, KBr, KCl, MgO) is used, after
some elaborations, to determine the Eg value.
Moreover, the occurrence of dd transitions,
exciton binding energies, phonon absorptions
and emissions, excitations to or from color
centers and/or defect bands can result in a difficult interpretation of UVVis diffuse reflectance spectra.
The electronic transitions that occur within a
material as a result of energy supply (in this
case in the form of light energy) are of two
types: direct and indirect [94]. Direct transition is
described as interaction between two particles
(an electron and a photon) where only photons
excite electrons, whereas indirect transition is
described as a three-particle interaction (photon,
electron, and phonon) requiring, at the same
time, vibrations and energy transition from the
crystal lattice (phonons). A phonon is defined as
a unit of vibrational energy that rises from the
oscillation of atoms within a crystalline lattice.
The lattice vibration, due to the atoms’ thermal
energy, generates mechanical waves. A packet of
these waves can move inside the crystal with a
fixed energy and momentum, the waves can be
treated as particles, named phonons. Just as a
photon is a quantum of electromagnetic energy
or light, a phonon is a quantum of vibrational
mechanical energy.
From the shape of the diffuse reflectance
spectrum it is possible to distinguish the different transition types by mathematical elaborations based on the Tauc Eq. (4.21):
αhν 5 Aðhν2Eg Þn
(4.21)
where α is the absorption coefficient, h is the
Planck’s constant, A is the absorption constant,
ν is the light frequency, Eg is the band-gap
energy, and n is related to the type of electronic transitions. The exponent value is 1/2
for a direct allowed and 3/2 for a direct forbidden transition, 2 for an indirect allowed,
and 3 for an indirect forbidden transition [95].
From the graph obtained by plotting (αhν)1/n
versus hν it is possible to calculate the bandgap energy by extrapolating the straight line to
(αhν)1/n 5 0 axis (Tauc plot) [9698].
The coefficient α can be acquired from
the diffuse reflectance spectrum by the
KubelkaMunk (KM) function, provided that
this function is able to properly describe the
diffuse reflectance phenomena in a solid
[99103]. When light of determinate energy is
absorbed by a material, the measurement of
the diffuse reflected light at different wavelengths originates the so-called diffuse reflectance spectrum. By considering an infinitely
thick sample, the intensity of the diffuse reflectance spectrum can be expressed by the
KubelkaMunk Eq. (4.22):
FðRN Þ 5
ð12RN Þ2
α
5
S
2RN
(4.22)
where the reflectance of an “infinitely” thick
sample (RN) represents the ratio of the intensity of light reflected from a sample to the
intensity of reflection from a standard sample,
α is the absorption coefficient, and S is the
scattering coefficient. By introducing the coefficients α and S, both the absorption and scattering phenomena are taken into consideration in
Eq. (4.20) [104106]. If the absorption coefficient in not dependent on wavelength, then F
(RN) is proportional to the absorption coefficient α, and this latter can be substituted in
Eq. (4.21) [107,108]:
½FðRN Þhν1=n 5 Aðhν 2 Eg Þ
(4.23)
By plotting [F(RN)hν]1/n versus hν it is possible to calculate the band-gap energy of a material by drawing a tangent line to the point of
inflection of the curve: the hν value at the point
of intersection of the tangent line with the horizontal axis is the Eg value [109]. The unit for hν
is eV (electron volts), and its relationship to
the wavelength λ (nm) is hν 5 1239.7/λ. For
practical applications typical sample thicknesses
of 13 mm are sufficient.
HETEROGENEOUS PHOTOCATALYSIS
124
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
Fig. 4.27 depicts the determination of
band-gap value for Degussa P25 TiO2, the
semiconductor most used in heterogeneous
photocatalysis. Since the latter is an indirect
semiconductor, n 5 2. In the absorption spectrum the band gap corresponds to the point
at which absorption begins to increase from
the baseline, since this defines the minimum
amount of energy that must be supplied to a
photon to excite an electron across the band
gap.
The diffuse reflectance spectra and the Tauc
plot diagram of various laboratory-made TiO2
samples containing two different polymorphic
TiO2 phase (rutile and brookite) are reported
in Fig. 4.28. For some semiconductors different
band-gap values could be measured depending on the phase composition: TiO2 rutile
exhibited a band-gap of 2.95 eV, whilst the calculated band-gap of the brookite polymorphic
form was 3.25 eV [110].
In the case of coupled TiO2BiVO4 systems,
the reflectance spectra (Fig. 4.29) allow to
appreciate, by increasing the BiVO4 amount,
an enhancement in the visible absorption of
the samples with the consequent decrease of
the band-gap energy. Moreover, for the higher
TiO2
2
[F(R’∞)hνν]1/2
amounts of BiVO4, another absorption feature
can be noted in addition to that of TiO2. This
finding confirms the formation of a heterostructure between the two oxides [111].
The presence of mid-band gap states, due
for example to the presence of defects or dopant agents, induces the absorption of light at
lower energies, resulting in a more complex
data interpretation. Fig. 4.30 reports the transformed KM function for a TiO2 samples doped
with nitrogen [112]. In this case two absorption
edges can be observed at 400 and 560 nm corresponding to 3.1 and 2.21 eV, respectively.
The first edge is associated to the band gap of
TiO2 while the second one is tentatively
assigned to energy states (located above the
valence band produced by nitrogen doping)
1.5
1
0.5
Eg = 3.18 eV
0
2.7
2.9
3.1
3.3 3.5
hν [eV]
3.7
3.9
4.1
FIGURE 4.27 Band-gap determination from the Tauc
plot obtained by manipulating diffuse reflectance spectra
of TiO2.
FIGURE 4.28 Diffuse reflectance spectra of different
TiO2 polymorphic forms: (A) brookite, (B) mixture of
brookite and rutile, (C) rutile. Inset: plot of the square root
of the KubelkaMunk function versus the energy of the
absorbed light of (A) rutile and (B) brookite. Source:
Reproduced from A. Di Paola, G. Cufalo, M. Addamo, M.
Bellardita, R. Campostrini, M. Ischia, et al., Photocatalytic
activity of nanocrystalline TiO2 (brookite, rutile and brookitebased) powders prepared by thermohydrolysis, Colloid. Surf. A
317 (2008) 366376 with permission, Copyright 2008 Elsevier
Publishing.
HETEROGENEOUS PHOTOCATALYSIS
125
4.4 UVVISIBLE SPECTROSCOPY
FIGURE
4.29 UVVis
diffuse
reflectance
spectra
of
TiO2BiVO4
systems.
Source:
Reproduced
from
R. Fiorenza, M. Bellardita, S. Scirè,
L. Palmisano, Photocatalytic H2 production
over inverse opal TiO2 catalysts, Catal. Today
321322 (2019) 113119 with permission,
Copyright 2017 Elsevier Publishing.
FIGURE 4.30 Band-gap determination from the Tauc plot obtained from
diffuse reflectance spectra of N-doped
TiO2. Source: Reproduced from R. Amadelli,
L. Samiolo, M. Borsa, M. Bellardita, L.
Palmisano, N-TiO2 photocatalysts highly
active under visible irradiation for NOx
abatement and 2-propanol oxidation, Catal.
Today 206 (2013) 1925 with permission,
Copyright 2013 Elsevier Publishing.
HETEROGENEOUS PHOTOCATALYSIS
126
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.31 UVVis diffuse reflectance
spectra of TiO2CuO systems. Source:
Reproduced from R. Fiorenza, M. Bellardita, S.
Scirè, L. Palmisano, Photocatalytic H2 production
over inverse opal TiO2 catalysts, Catal. Today
321322 (2019) 113119 with permission,
Copyright 2017 Elsevier Publishing.
that shift the absorption of the material toward
the visible light.
Similarly (Fig. 4.31) a mid-gap band located
above the valence band has been identified in
samples of TiO2 doped with CuO, especially at
higher CuO amounts [111].
4.4.4 Determination of Oxidation States
of Metals
With regard to catalysts containing metals,
UVVis spectroscopy is able to provide information on the location of metal cations at the
surface due to charge transfer or dd transitions, which cause a characteristic absorption
in the UVVis region [113].
Weckbuysen et al. [114] examined the
diffuse reflectance spectra of chromium supported on alumina samples and developed a
method to quantify the amount of Cr61, Cr31,
and Cr21 species by considering the
characteristic absorption bands of every ion.
Moreover, as the metal absorption in the
UVVis region due to charge transfer or dd
transitions depends on the surface hydration
or the adsorption of molecules, it is also possible to determine the coordination type of the
cation [114].
4.5 PHOTOLUMINESCENCE
SPECTROSCOPY
The phenomena that implicate absorption of
energy and subsequent emission of light are
classified generically under the term luminescence, which includes fluorescence, phosphorescence, and photoluminescence (PL). When a
sample is excited by photons, light emission at
different wavelengths can occur, and with a
decay time characteristic of the sample environment. The emitted light can be dispersed
HETEROGENEOUS PHOTOCATALYSIS
4.5 PHOTOLUMINESCENCE SPECTROSCOPY
by a spectrograph and the spectrum can give
information about the structure and the electronic properties of the sample [115].
Fluorescence is the absorption and emission
of species from atoms or molecules.
Phosphorescence is similar to fluorescence,
except that the time between absorption and
emission is much longer. Fluorescence differs
from phosphorescence because the electronic
energy transitions responsible for fluorescence
do not involve a change in the electronic spin;
then fluorescence is “fast” (ns time scale). In
phosphorescence emissions, on the other hand,
a change in the spin state of the electrons occurs
and this causes the persistence of the radiation
after the excitation for a detectable time: then
phosphorescence is “slow” (longer time scale).
PL is the term used to describe the absorption
and emission of light by semiconductor devices
and originates from an absorption/emission
process between different electronic energy
levels in the material. The photoexcitation
causes the material to turn into a higher electronic state, with subsequent release of energy
(photons), returning (relaxing) to a lower
energy level because a part of the excitation
energy is converted into heat in the medium.
The emission of light, or luminescence through
this process is the PL.
As illustrated in Fig. 4.32, fluorescence
occurs when a chemical species absorbs a photon and is excited to a singlet electronic state,
relaxes via nonradiative mechanisms, emits a
lower energy photon, and then returns to the
ground electronic state. The spectrum of the
wavelengths emitted can be used to identify
atoms and molecules, as well as to determine
chemical structures, and the intensity of the
photons emitted can be used to determine the
concentration of chemical species. The excited
atoms or ions may first undergo an optical or a
nonradiative transition to some intermediate
level, before emitting fluorescent light in a
transition to the ground state, or to some
higher-lying energy level. It is also possible
127
FIGURE 4.32 Scheme of electronic transitions in a
chemical species after photons absorption.
that a cascade of emission processes occurs,
that is, a cascade of transitions to lower-lying
energy levels; in that case, more than one fluorescence photon can be emitted per each
absorbed photon.
PL has been usually used to investigate the
structure and the properties of the active sites
on the surface of metal oxides and zeolites
[116119], because of its high sensitivity and
nondestructive character. Moreover, PL has
been useful in the field of photocatalysis over
semiconductors for understanding the surface
processes, as the PL spectrum is an effective
way to study the electronic structure and the
optical and photochemical properties of semiconductor materials, giving information on
surface oxygen vacancies and defects and the
efficiency of charge carrier trapping and transfer [120123].
The amount and type of PL depends on the
material under investigation and on wavelength of the laser used to photoexcite it. The
PL spectrum and its dependence on the
HETEROGENEOUS PHOTOCATALYSIS
128
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
irradiation intensity can give important information for the characterization of the materials. In particular, PL spectra and their intensity
dependencies can allow:
• the determination of the band-gap energy,
the wavelength of maximum gain, and the
composition of ternary or quaternary layers;
• the identification of impurity levels;
• the investigation on recombination
mechanisms.
PL consists of radiation emitted by crystalline and amorphous solids or by nanostructures as a consequence of optical excitation
with energy higher or equal to the band-gap
energy; in particular, it derives from the radiative recombination processes of photo-excited
electron/hole pairs (e2/h1 pairs).
Fig. 4.33 represents the main photophysical
processes occurring in a semiconductor excited
by light with energy equal to or greater than
the band gap. Process I represents the photoexcitation of electrons from the valence band
(VB) to the CB, with different energy levels
that can give rise to different excited states,
with a simultaneous generation of holes (h1)
in the VB. However, the electrons promoted in
the CB easily come back to the VB
recombining with the holes because they are
very unstable. During the recombination process a determined quantity of chemical energy
can be released as heat or energy. The light
energy can be dissipated as radiation, resulting
in an emission of luminescence from the material, originating the PL phenomenon. The
excited electrons having different energy levels
in the CB can easily pass to CB via nonradiative transitions; subsequently, processes II, III,
or IV (see Fig. 4.33) will probably take place.
Process II is the bandband PL process by
which electron transitions from the bottom
edge of CB to the upper edge of VB can occur,
with a simultaneous release of radiant energy.
In this case, the photon energy effectively corresponds to the band-gap energy. However,
the photon energy can sometimes be higher
than the band-gap energy, and it principally
arises from the transitions of higher energy of
excited electrons from the CB band directly to
the upper edge of VB. This PL signal can
derive also from a kind of bandband PL phenomenon. Process III is related to the excitonic
PL process in which the nonradiative transitions of excited electrons from the lower edge
of CB to different subbands (or surface states)
take place first, and subsequent radiative
Excitated state
e–
e–
e–
e–
e–
e–
CB
FIGURE 4.33 Main photophysical processes of a semiconductor excited by light
with energy equal to or higher than the band
gap: I, photo-excited process; II, bandband
PL process; III, excitonic PL process; IV, nonradiative transition process.
Intermediate
levels
(I)
(III)
(II)
(IV)
hν ≥ Eg
VB
h+
h+
h+
h+
h+
h+
Ground state
HETEROGENEOUS PHOTOCATALYSIS
4.5 PHOTOLUMINESCENCE SPECTROSCOPY
transitions from the subband to the upper
edge of VB can occur. The energy of the radiative photon, which represents the difference of
energy between the subband and the upper
edge of VB, is lower than the band-gap energy.
Commonly, the excitonic PL signal originates
from surface oxygen vacancies and defects of
semiconductors. In addition, the excited electrons at the lower edge of CB can come back
through process IV to VB straightforwardly or
indirectly by nonradiative transitions.
Only processes II and III can give rise to PL
phenomena. The bandband PL spectrum
(process I) can directly reflect the separation of
the photoinduced charge carriers, i.e., the
stronger the bandband PL signal, the higher
the recombination rate of photoinduced carriers. The excitonic PL spectrum (process III)
cannot directly reflect the separation of the
photoinduced carriers. However, it can reveal
some important information about oxygen
vacancies, surface defects, and surface states,
which can strongly affect photocatalytic reactions. Moreover, the study of the PL spectra
dependence on external parameters, such as
sample temperature, pressure, energy and
intensity of the exciting radiation, along with
applied electric or magnetic fields, helps to
better acquire this kind of information.
Mathew et al. [124] studied the UVVis PL
of TiO2 nanoparticles prepared by a hydrothermal method to confirm the presence of defect
levels due to oxygen vacancies. The PL spectrum (Fig. 4.34A) showed six main emission
peaks. Emission at 387 nm is ascribed to excitonic emission (Fig. 4.34B); the other emission
peaks recorded at higher wavelengths with
respect to the band gap (387 nm) are assigned
to the so-called surface state emissions, i.e.,
recombination processes at different trapping
sites near or far from the CB bottom. The oxygen vacancies and surface hydroxyl groups are
dominant sites for trapped electrons and holes
and contribute to the visible luminescence. In
particular the emissions at 421, 491, and
129
574 nm have been assigned to recombination
of electrons from Ti31 3d states to trapping
sites due to OH2, whilst those at 532 and
612 nm have been attributed to the jump of
electrons from oxygen vacancies to the VB top.
As PL signals of semiconductor materials
result from the recombination of photoinduced
charge carriers, in general, the lower the PL
intensity, the lower the recombination rate of
photoinduced electronhole pairs, and the
higher the photocatalytic activity of semiconductor photocatalysts [123,125,126]. However,
it has been demonstrated also that the stronger
the signal, the greater are the number of
defects and/or oxygen vacancies of the sample
and the higher could be the photocatalytic
activity [127,128].
These results indicate that the relationships
between the PL spectrum and photocatalytic
activity depend on several parameters, which
can lead to conflicting effects. Liqiang et al.
[127] studied the photocatalytic activity of
TiO2 nanoparticles doped with various
amounts of lanthanum. The photocatalytic
activity order of La-doped TiO2 samples was
1 . 1.5 . 3 . 0.5 . 5 . 0 mol%, which was the
same as the intensity observed for their PL
spectra (Fig. 4.35), namely, the stronger the PL
intensity, the higher the photocatalytic activity,
demonstrating that there were certain relationships between PL spectra and photocatalytic
activity. The PL spectra increase was attributed
to the presence of surface oxygen vacancies
and defects, which were beneficial for the
photocatalytic activity by capturing the photoproduced electrons, thus enhancing the charge
separation.
The same research group [128] reported on
the presence of many oxygen vacancies on the
surface of ZnO nanoparticles and, by means of
EPR measurements, they proved the strong
ability of oxygen vacancies to bind electrons.
The results highlighted how a smaller particle
size is correlated to a greater presence of oxygen vacancies with higher probability of
HETEROGENEOUS PHOTOCATALYSIS
130
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.34 (A) PL spectrum
of TiO2 nanocrystals; (B) emission
mechanism for TiO2 nanoparticles.
Excitation
wavelength
274 nm.
Source: Reproduced from S. Mathew,
A.K. Prasad, T. Benoy, P.P. Rakesh,
M. Hari, T.M. Libish, et al.,
UVVisible photoluminescence of
TiO2 nanoparticles prepared by hydrothermal method, J. Fluoresc. 22 (2012)
15631569 with permission, Copyright
2012 Springer Publishing.
(A) 1000
421 nm
Fluorescence intensity (arb.unit)
387 nm
800
493 nm
600
532 nm
400
573 nm
612 nm
200
0
300
400
500
600
700
800
Wavelength (nm)
(B)
Ec
Ti3+ 3d states
274 nm
574 nm
421 nm
Deep donor level
(oxygen vacancies)
491 nm
612 nm
387 nm
532 nm
Deep acceptor level
(OH–)
Ev
Ground state
O2– 2p states
HETEROGENEOUS PHOTOCATALYSIS
4.5 PHOTOLUMINESCENCE SPECTROSCOPY
131
FIGURE 4.35 PL spectra of pure and La-doped TiO2
nanoparticles. Source: Reproduced from J. Liqiang, S. Xiaojun,
X. Baifu, W. Baiqi, C. Weimin, F. Honggang, The preparation
and characterization of La doped TiO2 nanoparticles and their
photocatalytic activity, J. Solid State Chem. 177 (2004)
33753382 with permission, Copyright 2004 Elsevier
Publishing.
FIGURE 4.36 PL spectra of pure ZnO nanoparticles
with different particle size. Source: Reproduced from L. Jing,
Y. Qu, B. Wang, S. Li, B. Jiang, L. Yang, et al., Review of photoluminescence performance of nano-sized semiconductor materials
and its relationships with photocatalytic activity, Sol. Energy
Mater. Sol. Cells 90 (2006) 17731787 with permission,
Copyright 2006 Elsevier Publishing.
formation of excitons, leading to a stronger PL
signal and, eventually, a higher photocatalytic
activity. PL spectra of ZnO nanoparticles with
different particle size are reported in Fig. 4.36.
The PL intensity and the photocatalytic activity
decreased by increasing the particle size.
A nonlinear trend of the intensity with the
CeO2 amount was found in PL spectra of
TiO2CeO2 samples [129], indicating that
various parameters, as for instance particle
size, recombination velocity, and presence of
defects, were responsible in a different way
for the shape of PL spectra (see Fig. 4.37).
This finding demonstrates that the PL
mechanisms of semiconductor nanoparticles
are very complex and it is not possible to
extrapolate the contribution of the single
factors.
HETEROGENEOUS PHOTOCATALYSIS
132
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.37 PL spectra of
TiO2CeO2
samples.
Source:
Reproduced from R. Fiorenza, M.
Bellardita, T. Barakat, S. Scirè, L.
Palmisano, Visible light photocatalytic
activity of macro-mesoporous TiO2CeO2
inverse opals, J. Photochem. Photobiol. A
352 (2018) 2534 with permission,
Copyright 2018 Elsevier Publishing.
4.6 ELECTROCHEMICAL
CHARACTERIZATIONS
4.6.1 Introduction
The most used techniques for electrochemical characterization of electro-, photo- and
photoelectrocatalysts are linear sweep and
cyclic voltammetry (CV), choroamperometry,
and electrochemical impedance spectroscopy
(EIS). The Fermi level of a semiconductor can
be also determined by potentiometric measurements [130].
The used catalysts are generally supported
on glass or metals as metal oxides. The prepared anodes could be characterized directly,
while the (photo)catalysts in powder form
should be immobilized, mostly adsorbed physically, on a transparent conducting surface,
such as indium-doped SnO2 glass substrate
(ITO) and FTO electrodes [131].
These electrochemical characterization techniques are mostly used for the determination
of the current density of (photo)catalysts under
dark and light. The differences of obtained
current density between dark and light show if
(photo)catalysts work under the desired irradiation: UV, UVVis, solar, or Vis. High differences indicate that the material probably is
responsive to different kinds of radiation.
Therefore, these electrochemical techniques are
commonly used to study the performance of
modified (photo)catalysts (i.e., nonmetal doped
or metal loaded) with respect to unmodified
ones [132].
4.6.2 Cyclic and Linear Sweep
Voltammetry
CV, one of the most used techniques in
electroanalytical chemistry, is used to investigate the thermodynamics of redox reactions
and the kinetics of electron-transfer reactions,
as well as to determine adsorption behaviors
of substrates on the electrode surface. The electrochemical transitions can be studied by CV,
whether it is reversible or not, as well as the
number of electrons received (or given), the
rate constants, the reaction mechanism, and
HETEROGENEOUS PHOTOCATALYSIS
133
4.6 ELECTROCHEMICAL CHARACTERIZATIONS
the diffusion coefficient of the electroactive
species in the redox reactions. In the cyclic voltammetric technique, characterization is carried out by utilizing the redox potentials, the
separation of peak potentials and the changing
of the potential scanning speed.
In addition, the catalyst surface must be
modified to improve the performance in electrocatalysis, photocatalysis, and photoelectrocatalysis. By using the cyclic voltammograms
for electrode modification, the thickness of the
surface layers, and the amount of electrodeposited substrate can be also calculated when
the electrode surface area is known.
CV is a technique in which a linearly varying potential is applied to an unstirred solution, where the forward potential scan
increases up to a certain value, at which point
the backward scan is performed. Backward
scanning of the potential is not performed in
the linear sweep voltammetry (LSV) technique.
The CV and LSV are performed experimentally in a three-electrode system consisting of
working, reference, and counter electrodes
(Fig. 4.38A). The working electrode potential is
controlled versus a reference electrode, that is,
a silver/silver chloride electrode (Ag/AgCl) or
a saturated calomel electrode (SCE). Fig. 4.38B
(B)
Working
electrode
Computer
controlled
potentiostat
Counter
electrode
On/Off
Reference
electrode
Inert gas
entry
Top
view
Counter
electrode
Inert gas
outlet
Working
electrode
Reference
electrode
Potential, V versus SCE
(A)
shows the linear potential scan applied in a triangular waveform used in the CV. The voltage
is scanned linearly from 10.80 V to 20.15 V.
After reaching 20.15 V, reverse scanning is
performed until the initial potential value
(10.80 V) is reached. Single or multiple cycles
can be used to study the reversibility of the
processes [133].
The voltammogram is a curve of the potential versus the current. A cyclic voltammogram
is obtained by measuring the current at the
working electrode during the potential scan.
A typical cyclic voltammogram obtained in a
solution of 6.0 mM K3[Fe(CN)6] in aqueous
1.0 M KNO3, using a platinum working electrode, is shown in Fig. 4.39. At the initial
potential value, 0.80 V, the voltammogram
shows a very small anodic current. This very
small negative initial current is due to the oxidation of the water to oxygen. The current
value in the range 0.70.4 V is zero since there
is not any oxidable species in solution. When
the potential value is less positive than 0.4 V,
cathodic current occurs (b point). This current
is due to the reduction of [Feııı(CN)6]3 to
[Feıı(CN)6]4. While the current between b and
d points increases rapidly, the [Feııı(CN)6]3
concentration on the electrode surface
Cycle 1
Cycle 2
b
–0.2
0
a
0.2
c
0.4
Forward
scan
Reverse
scan
E
0.6
0.8
Efinal
initial
0
20
40
Time,s
60
80
FIGURE 4.38 (A) Schematic of electrochemical working system with three-electrode. (B) Typical excitation signal for CV: a
triangular potential waveform with switching potentials at 0.8 and 0.2 V versus SCE. Source: Reproduced from P.T. Kissinger,
W.R. Heineman, Cyclic voltammetry, J. Chem. Educ. 60 (1983) 702706 with permission, Copyright 1983 ACS Publishing.
HETEROGENEOUS PHOTOCATALYSIS
134
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
d
Epc
20
cathodic
c
e
f
ipc
10
Current, μA
g
h
b
0
a
k
ipa
i
anodic
–10
peak, the current decreases by consuming of
[Feıı(CN)6]4 in the anodic reaction [133].
Important parameters in the cyclic voltammogram are cathodic peak potential (Epc),
anodic peak potential (Epa), cathodic peak current (ipc), and anodic peak current (ipa).
Fig. 4.39 shows how these parameters are
measured.
In a reversible electrode reaction, the anodic
and cathodic peak currents are approximately
equal in absolute value and opposite in sign.
The peak potential difference (ΔEp) of a
reversible electrode reaction at 25 C is given
by the following Eq. (4.24):
ΔEp 5 jEpa 2 Epc j 5
j
Epa
–20
0.8
0.6
0.4
0.2
0
Potential, V versus SCE
–0.2
FIGURE 4.39 Cyclic voltammogram of 6.0 mM K3Fe
(CN)6 in 1.0 M KNO3. Scan initiated at 0.8 V versus SCE in
negative direction at 50 mV/s. The area of platinum electrode is 2.54 mm2. Source: Reproduced from P.T. Kissinger,
W.R. Heineman, Cyclic voltammetry, J. Chem. Educ. 60 (1983)
702706 with permission, Copyright 1983 ACS Publishing.
decreases gradually. The initial current drops
rapidly (from d to f) after the maximum,
because the distance between the diffusion
layer and the electrode surface starts to
increase (Fig. 4.39). At point f (20.15 V), the
voltage sweep turns in the opposite direction,
but even towards the more positive potential
values, the current is still cathodic because the
potential is negative enough for [Feııı(CN)6]3
to be reduced. When the potential values reach
the more positive values, [Feııı(CN)6]3 cannot
be reduced anymore and, thus, the current
decreases down to zero, eventually turning
anodic. The anodic current, formed during the
forward scan, relates to the oxidation of
[Feıı(CN)6]4, which accumulated on the electrode surface. After forming this anodic current
0:0592
n
(4.24)
in which n is the electron number used for the
half reaction. In a reversible system, n can be
predicted by measuring ΔEp value. The ΔEp
values at different scan rates are used to
determine if the electron transport kinetics are
slow and to obtain the corresponding rate
constants.
The RandlesSevcik equation is a relationship between the current and the substrate
concentration. This Eq. (4.25) is given below
for a temperature of 25 C.
ip 5 ð2:69 3 105 Þn3=2 AcD1=2 υ1=2
(4.25)
In this equation, ip is the peak current (A), c
the concentration (mol/cm3), A the electrode
surface area (cm2), D the diffusion coefficient
(cm2/s), and v the scan rate (V/s). According
to this equation, the peak current density is
proportional to the concentration of the electroactive species and the square root of the
scanning speed and the diffusion coefficient. If
concentration, electrode surface area, and scan
rate are known, the diffusion coefficient can be
determined by using the CV through the above
equation.
The RandlesSevcik equation is also used
to calculate electrode surface area if n and D
HETEROGENEOUS PHOTOCATALYSIS
135
4.6 ELECTROCHEMICAL CHARACTERIZATIONS
(A)
80
(B)
50
40
60
i/μA
i/μ
μA
30
40
d
20
b
c
a
0
d
20
c
10
b
a
0
–10
–0.2
0.0
0.2
0.4
0.6
0.8
–0.2
0.0
0.2
0.4
0.6
0.8
E/V (vs. SCE)
E/V (vs. SCE)
FIGURE 4.40 Cyclic voltammograms of 5.0 3 1024 M hydrazine (A) and hydroxylamine (B) in 0.1 M phosphate buffer
solutions (pH 7.0) at bare GCE (a), PPy/GCE (b), Au/GCE (c), and Au/PPy/GCE (d). Scan rate: 50 mV/s. Source:
Reproduced from J. Li, X. Lin, Electrocatalytic oxidation of hydrazine and hydroxylamine at gold nanoparticlepolypyrrole nanowire
modified glassy carbon electrode, Sensor. Actuat. B 126 (2007) 527535 with permission, Copyright 2007 Elsevier Publishing.
values of a reversible electroactive species
(such as K3(FeCN)6) are known [134].
Li and Lin [135] used the CV to evaluate
the electrocatalytic activity of the Au nanoparticlepolypyrrole nanowires modified glassy
carbon electrode (Au/PPy/GCE) for hydrazine (Fig. 4.40A) and hydroxylamine oxidation (Fig. 4.40B). A very small oxidation
current at c. 0.65 V is obtained when glassy
carbon electrode (GCE) is used, whereas a
higher current value at c. 0.60 V is obtained
when PPy/GCE is used (Fig. 4.40A). On an
Au/GCE electrode, a sharp catalytic oxidation
peak was determined at 0.18 V and the oxidation current of hydrazine increased five times
as compared with that of PPy/GCE electrode.
When Au/PPy/GCE electrode is used, a
sharp catalytic peak at 0.135 V was reached as
the highest current value (c. 78 μA). The shift
of peak potentials to more negative potentials
is used to demonstrate the catalytic activities
of the modified electrodes. A similar behavior
was observed when modified electrodes were
used for CV of hydroxylamine (Fig. 4.40B).
Since photoelectrochemical applications
require electron transfer at the solid/electrolyte interface, applied potential versus current
density measurements are commonly used for
characterization [136].
Photoelectrocatalytic degradation performance of an electrode towards a molecule can
be predicted by using the LSV. In addition, the
optimum potential for photoelectrocatalytic
degradation can be determined without performing
the
experiments
at
different
potentials.
Ensaldo-Renteriaa et al. [137] used LSV in
the photoelectrochemical characterization of
nanotube TiO2 electrodes on Ti layer surface
(Ti/TiO2-NT) obtained by anodic oxidation on
the Ti plate for 30 min at 55 V in ethylene glycol electrolyte containing NH4F (0.3%, w/w)
and H2O (3%, v/v). The electrode performances were tested for photoelectrocatalytic
degradation of acid green 50 dye (AG50).
Photocurrent performances of Ti/TiO2-NT
electrodes, calcined at different temperatures
(450 C, 600 C, and 800 C), were measured
HETEROGENEOUS PHOTOCATALYSIS
136
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.41 Photocurrent response of
Ti/TiO2-NT calcined at different temperatures upon UV-light illumination of linear
potential sweep voltammetry at a scan rate
of 5 mV/s. Source: Reproduced from M.K.
´
´
450ºC Ensaldo-Renterıaa, G. Ramırez-Robledoa, A.
Sandoval-Gonzálezc, C.A. Pineda Arellanob, A.
600ºC A. Álvarez-Gallegosc, Á. Zamudio-Larac,
et al., Photoelectrocatalytic oxidation of Acid
800ºC
Green 50 dye in aqueous solution using Ti/
TiO2-NT electrode, J. Environ. Chem. Eng. 6
Dark
(2018) 11821188 with permission, Copyright
2018 Elsevier Publishing.
Photocurrent density (A/cm2)
700
560
420
280
140
0
0.00
0.25
0.50
0.75
1.00
1.25
1.50
Potential (V vs SCE)
from 0.0 to 1.50 V in a Na2SO4 electrolyte
(0.05 M, pH 1) under UV irradiation. The
linear sweep voltammograms show that
Ti/TiO2-NT, produced at 600 C, has the
highest photocurrent values (Fig. 4.41). Higher
photocurrents in an aqueous solution containing only inert supporting electrolyte are
explained by the faster production of strong
OH radicals generated by water oxidation
through photogenerated holes. Subsequent
photoelectrocatalytic degradation experiments
were carried out by using Ti/TiO2-NT electrode calcined at 600 C. The photocatalytic
performance of such electrodes has been also
assessed; however, the photoelectrocatalytic
techniques were more effective than the photocatalytic ones for AG50 degradation.
Cyclic voltammetric technique is also
widely used for determination of catalyst
properties in electrocatalytic, photocatalytic, or
photoelectrocatalytic studies.
Kaur et al. [138] investigated a goldfunctionalized iron disulfide (Au@FeS2) nanomaterial prepared through the hydrothermal
method for photodegradation of a dye under
visible irradiation. Owing to the dependence of
their voltammetric characteristic on the electrode/electrolyte surface interface, Fe21/Fe31
redox system was used for electrocharacterization of Au@FeS2 (Fig. 4.42). The CV characteristic
of FeS2 and Au@FeS2-modified electrodes are
shown in Fig. 4.42AC. The peak current
changes and the peak separation of electrodes
could be related to the electrontransfer-rate
constant of the modified electrodes. Au@FeS2modified electrode showed an increase of
37% in the current characteristic with respect to
FeS2-modified electrode. Au nanoparticles have
a conductive character and therefore this composite could have channeled electrons faster
than FeS2. The peak-to-peak separation value
(ΔE) of Au@FeS2 is also lower than FeS2, 0.21
versus 0.25 V, which also confirms the catalytic
role played by Au nanoparticles to facilitate
faster electron transfer towards the electrode in
bulk solution. In addition, in the presence of
the Au@FeS2-modified electrode, an almost
reversible oxidation/reduction process for the
Fe21/Fe31 species was found since the peak currents ratio, ia/ic, was c. 1 [139]. As derived from
voltammograms in Fig. 4.42A, the capacitance,
peak current (ipa) and formal potential values
were 1.17 mF, 1.09 3 1024 A, and 0.22 V, respectively, for the Au@FeS2-modified electrode, and
0.9 mF, 7.97 3 1025 A and 0.24 V for the FeS2modified electrode.
The electron numbers involved in an electrochemical redox reaction are calculated from
ΔE 5 0.059/n. The n value was 0.236 and 0.280
for the FeS2 and the Au@FeS2-modified
HETEROGENEOUS PHOTOCATALYSIS
4.6 ELECTROCHEMICAL CHARACTERIZATIONS
137
FIGURE 4.42 (A) Cyclic voltammogram of FeS2 and Au@FeS2-modified electrodes at 0.1 V/s scan rate, (B) cyclic voltammograms of Au@FeS2 with variation of scan rate from 10 mV/s to 100 mV/s, (C) variation of anodic peak current Ipa
versus square root of potential scan rate, (D) areal capacitance of Au@FeS2 from CV. Source: Reproduced from G. Kaur, D.
Pooja, M. Kumar, A. Thakur, R. Bala, A. Kumar, Electrochemical aspects of photocatalysis: Au@FeS2 nanocomposite for removal of
industrial pollutant, Phys. Chem. Chem. Phys. 19 (2017) 3241232420 with permission, Copyright 2017 RSC Publishing.
electrodes, respectively. The diffusion coefficient for the charge carriers at the Au@FeS2modified electrode was 1.30 3 10212 cm2/s,
value obtained by the RandlesSevcik equation (see Eq. (4.25)) for reversible reaction systems, where n is assumed to be 1 and the
diffusion coefficients of the redox species are
assumed to be equal [140].
The scan rate effect of Au@FeS2-modified
electrode was studied by recording CV as a
function of scan rate (10100 mV/s) and the
results are shown in Fig. 4.42B. The redox
peak current values (ipc and ipa) increase linearly with the square root of scan rate (v1/2) for
the Fe21/Fe31 system (Fig. 4.42C), indicating a
diffusion-controlled process [141]. The slopes
and intercepts can be estimated by using
Eq. (4.26) for anodic peak current:
ipa 5 1:007 3 1025 3 v1=2 1 5:88
(4.26)
The current coefficient ipa/v1/2, another
indicator of the reversible electron transfer, has
HETEROGENEOUS PHOTOCATALYSIS
138
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
also been found constant at different scan
rates,
1.17 3 1025 A/(mV/s)1/2
for
the
Au@FeS2-modified electrode and the reduced
species of the redox couple. Fig. 4.42D shows
the areal capacitance (capacitance per unit
area) characteristic of the Au@FeS2-modified
electrode at different scan rates; it is highest at
10 mV/s and decreases gradually as the rate
increases to 100 mV/s.
4.6.3 Choronoamperometry
The photocurrent values obtained by
chronoamperometric measurements are used
to determine the charge separation degree of
photocatalysts. These measurements are usually carried out in an electrolyte solution
such as aqueous Na2SO4 and under UV, Vis,
or UVVis irradiation. It is possible to compare the current values at a constant voltage
obtained in the dark and under irradiation
by switching the light on and off at certain
time intervals. At the same time, it is also
possible to compare the photocurrent values
of different photocatalysts or photoelectrocatalysts. In particular, it has been reported
that, when the catalysts are modified by doping or loading by nonmetal or metals, the
photocurrent intensity increased along with
photocatalytic activity. Lv et al. [142] investigated the enhancement of photocatalytic
activity of TiO2 by Ag doping (Ag/TiO2)
under different conditions by electrochemical deposition on the FTO surface. The catalysts were tested for methylene blue
degradation under visible irradiation. It was
determined that the higher the photocurrent
values of the photocatalysts, the higher the
photocatalytic activity (Fig. 4.43).
Zheng et al. [143] investigated a novel type
of hierarchical TiO2@MoS2 composites by decorating thin layer MoS2 nanosheets onto 1D
anodic self-ordered anatase TiO2 nanotube
arrays by a facile hydrothermal method.
Photocatalytic degradation results of the prepared catalysts for Rhodamine B (RhB) dye
showed that TiO2@MoS2 composite is the most
effective photocatalyst. Characterization of
nanostructured photoelectrodes was performed both in the dark and under UV irradiation by using linear sweep voltammetric and
chronoamperometric measurements. Linear
sweep voltammetric and chronoamperometric
measurements show that the current density
obtained with a TiO2@MoS2 composite photoelectrode is higher than that of bare TiO2 and
MoS2. The maximum photocurrent density of
TiO2@MoS2 composite is twice as high as that
of TiO2; 7.72 mA/cm2 versus 3.38 mA/cm2 at
0.8 V. The significant photocurrent density
increase gained in the presence of TiO2@MoS2
composite with respect to TiO2 and MoS2 may
be attributed to a faster electron transport from
VB to CB and a high separation efficiency of
the photogenerated electronhole pairs,
attained thanks to the synergy brought about
by the heterojunction between anatase TiO2
with a wide band gap (c. 3.2 eV) and MoS2
with a narrow band gap (c. 1.8 eV). Under irradiation, electrons are excited from the VB of
MoS2 to its CB, leaving behind holes in the VB
of MoS2. Since the CB of TiO2 is lower than
that of MoS2, the photoinduced electrons in
the CB of MoS2 can be transferred to the CB of
TiO2. The photogenerated electrons can be
trapped by oxygen molecules in the aqueous
solution to form superoxide radical anions,
meanwhile, the holes move towards the opposite direction and oxidize water into hydroxyl
radicals, which are highly oxidative and can
react with organic species to produce mainly
carbon dioxide and water. The high interfacial
charge transfer and separation ability suppressed the recombination of electronhole
pairs, which explained the enhanced photoactivity
and
photocurrent
response
in
TiO2@MoS2 composites.
HETEROGENEOUS PHOTOCATALYSIS
139
4.6 ELECTROCHEMICAL CHARACTERIZATIONS
FIGURE 4.43
(A) Transient photocurrent responses of TiO2 and Ag/
TiO2 film electrodes under UV. (B)
Photocatalytic degradation efficiencies
of MB with TiO2 and Ag/TiO2 film
electrodes under visible illumination
and TiO2 film under dark. Source:
Reproduced from X. Lv, F. Gao, Y. Yang,
T. Wang, A facile electrochemical
approach to form TiO2/Ag heterostructure
films with enhanced photocatalytic activity, Ind. Eng. Chem. Res. 55 (2016)
107115 with permission, Copyright
2016 ACS Publishing.
4.6.4 Electrochemical Impedance
Spectroscopy
EIS technique is the frequency-dependent
measurement of the total impedance (Z) at an
electrode/solution interface where an alternating voltage is applied. EIS is widely used in
the characterization of semiconductor thin
films. By this technique, the electrode/solution
interface is modeled precisely and accurately,
and kinetic studies on the reactions occurring
at this interface can be performed. Much information could be gained by detailed analysis of
the obtained data from EIS analysis [139,144].
In an electrochemical cell, current flows
through different interfaces, such as the electrode/solution interface and the junctions of
different metals (Fig. 4.44A). At each interface,
HETEROGENEOUS PHOTOCATALYSIS
140
FIGURE 4.44
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
(A) Schematic of a simple electrochemical system and (B) change of potential for a constant frequency
value.
FIGURE 4.45 Modeling of an electrodesolution
interface.
charge accumulation occurs (capacitor behavior) giving rise to a voltage difference. By
designing an appropriate electrochemical cell,
it is possible to separately determine both the
capacitor property and the current flow of the
working electrode/solution interface. In other
words, when the working frequency range is
analyzed by dividing it into regions, the capacitive and faradaic processes related to the
interface can be examined independently of
each other. It is desirable that the changes
upon the application of a potential are related
only to the E1 (working electrode)/solution
interface. For this purpose, to prevent that E2/
solution interface affecting the analysis, the
counter electrode (E2) is selected among nonpolarized materials [145].
Nowadays, there are EIS devices that measure in the frequency range 1025107 Hz. The
frequency is related to the alternating voltage
applied to the cell in the form of sinusoidal
waves, and the value of the potential applied
to the E1/solution interface is time dependent.
Fig. 4.44B shows the time-dependent variation
of the potential for constant amplitude and frequency. If an alternative voltage is applied,
undesirable changes that may occur at the
electrode/solution interface due to external
electrical stimulation can be avoided.
The electrical properties of the electrode/
solution
interface
can
be
physically
explained by means of an equivalent circuit
made of electrical elements such as resistors
and capacitors. Fig. 4.45 shows a simple electrochemical interface and the corresponding
equivalent circuit. The total equivalent resistance corresponding to the equivalent circuit
(interface) is called impedance. The solution
HETEROGENEOUS PHOTOCATALYSIS
141
4.6 ELECTROCHEMICAL CHARACTERIZATIONS
resistance (Rs) and the total resistance
between the working and the counter electrodes (RP) follow Ohm’s law.
The contribution of the capacitor to the total
impedance depends on the frequency. The following Eq. (4.27) is used to compute the total
impedance of the interface defined by the
equivalent circuit in Fig. 4.45.
Z 5 Rs 1
Rp
1 1 ðωCdl Rp Þ2
2j
ωCdl Rp 2
1 1 ðωCdl Rp Þ2
(4.27)
Besides the electrolyte resistance (Rs), the
total impedance consists of two further components: the real part refers to the resistance to
current flow across the interface, while the
imaginary part refers to the charge accumulation on the capacitor, which depends on the
frequency of the alternating current. When
the frequency is close to zero, the real resistance is the sum of Rs and Rp. The latter term
is defined as the polarization resistance and
can be obtained as the sum of Rs and Rct in the
equivalent circuit. When the frequency is infinitely large, the real resistance equals the solution resistance (Eqs. (4.28) and (4.29)).
Rs 1 Rp 5 limω-0 ðZ0 Þ
(4.28)
Rs 5 limω-N ðZ0 Þ
(4.29)
Real and imaginary impedances, reported in
the following, are labeled as Z’ (Eq. (4.30)) and
Zʹʹ (Eq. (4.31)), respectively.
Z0 5 Rs 1
Z00 5 2
Rp
1 1 ðωCdl Rp Þ2
ωCdl Rp 2
1 1 ðωCdl Rp Þ2
(4.30)
(4.31)
If the frequency (ω) term is removed by
solving Eqs. (4.30) and (4.31), Eq. (4.32) is
obtained:
2
Rp 2
Rp
2
Zʹ 2 Rs 1
1 ðZʹʹÞ 5
(4.32)
2
2
Eq. (4.32) is a semicircle equation, and if
imaginary and real impedances are plotted
against each other, Nyquist (ColeCole)
curves are obtained (Fig. 4.46). The total
impedance value (ǀZǀ) (Eq. (4.33)) is the vector
of sum of the real (Z’) and virtual (Z’’) components [145147].
jZj 5 ðZ02 1Zv2 Þ1=2
(4.33)
Z’’ value, referred as the imaginary part, is
related to the capacitor property of the interface and depends on the phase angle (φ). As
shown in Fig. 4.46, φ increases with Z’’ value.
It is also possible to show data in Bode
FIGURE 4.46 Phase angle, real and imaginary impedance relationship. Source: Reproduced from F. Lisdat, D. Schäfer,
The use of electrochemical impedance spectroscopy for biosensing, Anal. Bioanal. Chem. 391 (2008) 15551567; A. Nechache, M.
Cassir, A. Ringuedé, Solid oxide electrolysis cell analysis by means of electrochemical impedance spectroscopy: a review, J. Power
Sources 258 (2014) 164181 with permission, Copyright 2014 Elsevier and 2008 Springer Publishings.
HETEROGENEOUS PHOTOCATALYSIS
142
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
CDL
ZI
ωmax =
1
R⋅C
W
RCT
RSOL
FIGURE 4.47 Randles’ equivalent circuit for an
electrode in contact with an electrolyte. Source:
Reproduced from F. Lisdat, D. Schäfer, The use of electrochemical impedance spectroscopy for biosensing, Anal.
Bioanal. Chem. 391 (2008) 15551567 with permission,
Copyright 2008 Springer Publishing.
ZR
RS
RS + RCT
RS + RCT–2σ⋅CDL
diagrams in which the total impedance and
phase angle versus frequency are plotted on
the graph. Rp and Rs can be determined from
Nyquist diagrams, but also σ can be evaluated
from the Warburg impedance (W) (Fig. 4.47)
[148]. However, the determination of the
capacitance value corresponding to the interface (Cdl) requires a more detailed study.
Modeling and fitting are usually used to evaluate experimental EIS data and to obtain quantitative data.
Ângelo et al. [144] reported on the characterization of thin film photoelectrodes of TiO2
(P25) and a P25/graphene composite by EIS
using a three-electrode configuration. Fig. 4.48
shows the impedance spectra of the catalysts
at 1.0 V reference hydrogen electrode (RHE).
The Nyquist plot (Fig. 4.48A) shows the opposite of the imaginary part of the complex
impedance (2Zimag) versus the real part (ZRe)
when the frequency changes; while the Bode
diagram (Fig. 4.48B) displays the magnitude
impedance (|Z|) and the phase shift (φ) of the
frequency response. Charge transfer and transport phenomena in photoelectrochemical systems are modeled by electrical equivalent
circuits [150] in terms of resistive and capacitive elements.
A number of equivalent circuits have been
proposed for the carrier transport and charge
separation in TiO2 thin films [131]. In this
work some electrical analogues were evaluated
and a Randles circuit (inset of Fig. 4.48A) was
found as the most suitable for modeling the
catalysts.
Furthermore, the Nyquist plot shown in
Fig. 4.48 presents a “full” Randles equivalent
circuit consisting of two separate contributions:
(1) the space-charge capacitance (Cdl), which
takes into account the ability of storing charge
in the double layer at the semiconductor/electrolyte interface; (2) the faradaic contribution,
described by charge-transfer resistance (Rct) at
the same interface and a Warburg impedance
(W) linked to mass transfer (diffusion) resistance [151]. As frequency increases, the faradaic impedance approaches Rct and a
semicircle forms in the Nyquist plot; at lowfrequencies, the faradaic impedance can be
represented as two resistances in series, one
related to electron-transfer at the interface and
the other to mass transport toward the electrode. Rs stands for the series resistance, which
comprises the FTO substrate, the resistance
linked to the ionic conductivity in the electrolyte, and the resistance of external contacts.
The impedance measurements of the TiO2based samples were performed at potential
range of 0.95VRHE to 1.25VRHE and a step of
50 mV under dark. Rct and Cdl parameters
were determined by fitting the proposed electrical circuit to the experimental EIS data.
These parameters are plotted in Fig. 4.49 as a
function of the applied potential and the
HETEROGENEOUS PHOTOCATALYSIS
4.6 ELECTROCHEMICAL CHARACTERIZATIONS
143
FIGURE 4.48 (A) Nyquist diagram and (B) Bode
plots obtained in the dark at 1.00 VRHE in a 3electrode configuration. The solid lines represent the
fittings based on the Randles circuit. Source:
Reproduced from J. Ângelo, P. Magalhães, L. Andrade, A.
Mendes, Characterization of TiO2-based semiconductors
for photocatalysis by electrochemical impedance spectroscopy, Appl. Surf. Sci. 387 (2016) 183189 with permission, Copyright 2016 Elsevier Publishing.
results show that P25 exhibits larger Rct and
Cdl independently of the applied potential.
Conversely, graphene on the catalyst results in
a drop of Rct and Cdl, as shown in the Nyquist
plot (Fig. 4.48A) by the smaller semicircle.
Therefore, in P25 photocatalyst electrons find
it more difficult to reach the semiconductor/
electrolyte interface, where the photocatalytic
process occurs and more electrons have the
possibility of recombining before reacting with
HETEROGENEOUS PHOTOCATALYSIS
144
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
FIGURE 4.49 Impedance results obtained by fitting the experimental data to the equivalent electrical circuit plotted
versus the applied potential with reference to RHE: (A) charge transfer resistance and (B) space-charge capacitance.
Source: Reproduced from J. Ângelo, P. Magalhães, L. Andrade, A. Mendes, Characterization of TiO2-based semiconductors for photocatalysis by electrochemical impedance spectroscopy, Appl. Surf. Sci. 387 (2016) 183189 with permission, Copyright 2016 Elsevier
Publishing.
pollutants. On the other hand, due to the low
photocarrier recombination, P25/graphene
exhibits the highest photocatalytic activity.
4.6.5 Determination of the Valence and
Conduction Band Edge
The Fermi level of a semiconductor can be
measured by the method of Roy et al. [130] by
determining the photopotential obtained during the irradiation of a suspension of catalyst
powders in the presence of an electron
acceptor.
In n-type semiconductors such as TiO2, the
Fermi level (Ef) is located near the CB (Ecb) and
considering that tEcbEfb(e2)t , 0.1 V [152],
by summing the band-gap value to the Fermi
level value, it is possible to find the valence
band edge (Fig. 4.50) [153]. These values are
crucial because they allow to predict the thermodynamic compatibility of semireactions taking place in a system.
Augugliaro et al. [89] used this method to
determine the valence and CB edge of home-
prepared anatase (HPX; X is thermal treatment
time at 100 C) and commercial TiO2 (Degussa
P25, c. 75% anatase and 25% rutile) according
to Roy et al. [130]. 50 mg of catalyst and 10 mg
of methyl viologen (MV) dichloride as electron
acceptor were suspended in a two-necked
flask in 0.1 M KNO3 solution. A platinum wire
was used as working electrode, while saturated Ag/AgCl electrode was used as reference one. The pH values were adjusted by
using HNO3 or NaOH solutions. The suspension was deaerated by bubbling N2 and stirred
magnetically. In general the pH of the starting
suspension was 2.5. The influence of ethanol
on the flat band potential was checked by adding 5 mL of ethanol to the suspension, and
afterwards the same procedure described above
was carried out. Irradiation was performed
with a 150 W xenon arc lamp (OSRAM XBO, I0
(300400 nm) 5 1.4 3 1026 Einstein/(scm2) installed
in a light-condensing lamp housing (PTI,
A1010S) on an optical train. The flask was
mounted at a distance of 30 cm from the lamp.
The quasi-Fermi level of electrons, Efb, of
TiO2 photocatalysts was determined by
HETEROGENEOUS PHOTOCATALYSIS
4.6 ELECTROCHEMICAL CHARACTERIZATIONS
145
FIGURE 4.50 Electrochemical potentials (vs
–1
Potential (V vs NHE)
–0.46 V
–0.45 V
–0.37 V
0
1
2.98 eV
3.05 eV
NHE) of the band edges of TiO2 anatase, brookite,
and rutile at pH 7. Source: Reproduced from A. Di
Paola, M. Bellardita, R. Ceccato, L. Palmisano, F.
Parrino, Highly active photocatalytic TiO2 powders
obtained by thermohydrolysis of TiCl4 in water, J.
Phys. Chem. C 113 (2009) 1516615174 with permission, Copyright 2009 ACS Publishing.
3.26 eV
2
3
Rutile
Anatase
Brookite
HP0.5
HP2
0.4
+
+
HP4
HP6
HP8
+
0.2
FIGURE 4.51 pH versus photovoltage. 50 mg of
catalyst with 10 mg of methyl viologen dichloride are
suspended in 50 mL of 0.1 M KNO3 aqueous solution
at room temperature. Working and reference electrodes are Pt and Ag/AgCl, respectively. Reproduced
from V. Augugliaro, H. Kisch, V. Loddo, M.J. LópezMuñoz, C. Márquez-Álvarez, G. Palmisano, et al.,
Photocatalytic oxidation of aromatic alcohols to aldehydes
in aqueous suspension of home-prepared titanium dioxide:
2. Intrinsic and surface features of catalysts, Appl. Catal.
A 349 (2008) 189197 with permission, Copyright 2008
Elsevier Publishing.
++
0.6
E/V
+
0.0
–0.2
+
+
+
++
+ +
+
+
+
+
–0.4
pH0 = 8.2
pH0 = 5.8
1
2
3
4
5
6
pH
7
8
9
measuring the photovoltage in the presence of
MV as a function of the pH value and the
obtained results are shown in Fig. 4.51. By
increasing the pH of suspension, MV radical
cation
(MV1•)
concentration
increased;
10
therefore, when approaching the inflection
point, pH0, the color turned blue. A lower pH0
value corresponds to a more negative quasiFermi level. The quasi-Fermi level at pH 7 can
be calculated by the following Eq. (4.34) in
HETEROGENEOUS PHOTOCATALYSIS
146
4. (PHOTO)CATALYST CHARACTERIZATION TECHNIQUES
which pH0 value can be obtained from the corresponding titration curve (see Fig. 4.51):
Efb ðpHÞ 5 E0MV21=MV1 1 kðpH0 pHÞ
These results show that the photoelectrochemical characteristics of anatase and
anatase-rutile TiO2 photocatalysts reported in
this paper are very similar [89]. Flatband
potentials of HP0.5 and Degussa P25 in the
presence of 10% (v/v) of ethanol were shifted
cathodically by about 0.06 and 0.09 V, respectively (see Fig. 4.52). These results show that
the reductive power of the reactive electron in
anatase samples in aqueous ethanol solution is
much higher than in pure water.
(4.34)
where E0MV21=MV1 is the redox potential of the
MV21/MV1• couple and k is assumed to be
equal to 0.059 V. The Efb values of TiO2 photocatalysts are reported in Table 4.2. The quasi
Fermi level positions of Degussa P25 (20.55 V)
and HP0.5 to HP6 (20.52 V) samples are very
similar indicating that interfacial electron
transfer to MV proceeds from the CB of anatase TiO2. However, the quasi Fermi level position of HP8 is 20.37 V. This value is
characteristic of the rutile phase of TiO2. By
assuming that the difference between quasi
Fermi level potential and CB edge is negligible,
the valence band edge values of the catalysts
can be obtained by addition of the band-gap
energy, which was computed by diffuse reflectance spectroscopy (DRS). Band edge of HP0.5,
HP2, and HP4 were located at 2.84 V, whereas
it was found to be shifted to 2.74 V for HP6
and HP8 samples. It is worth noting that HP6
contains significant amounts of rutile whereas
HP8 is nearly pure rutile.
0.6
TABLE 4.2 Band Gap, Efb, and VB Edge Values of
TiO2 Catalysts [89]
Catalyst
3.26
2 0.55/ 2 0.64
HP0.5
3.36
2 0.52/ 2 0.58a 2.84
HP2
3.36
2 0.52
2.84
HP4
3.36
2 0.52
2.84
HP6
3.26
2 0.52
2.74
HP8
3.11
2 0.37
2.74
0.4
0.2
2.71
These values were obtained in the presence of ethanol.
0.0
–0.2
pH0 = 5.9
pH0 = 4.7
2
a
FIGURE 4.52 pH versus photovoltage for HP0.5
photocatalyst in the presence (a) and in the absence
(b) of ethanol. Source: Reproduced from V. Augugliaro,
H. Kisch, V. Loddo, M.J. López-Muñoz, C. MárquezÁlvarez, G. Palmisano, et al., Photocatalytic oxidation of
aromatic alcohols to aldehydes in aqueous suspension of
home-prepared titanium dioxide: 2. Intrinsic and surface
features of catalysts, Appl. Catal. A 349 (2008) 189197
with permission, Copyright 2008 Elsevier Publishing.
(a)
1
VB Edge (V)
Degussa
P25
a
(b)
E/V
Band Gap (eV) Efb
3
4
5
pH
6
7
8
9
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