Journal of the Korean Physical Society, Vol. 65, No. 3, August 2014, pp. 0∼0
Metal Oxide Semiconductors as Visible Light Photocatalysts
George Kiriakidis∗
Institute of Electronic Structure and Laser (IESL),
FORTH, Vasilika Vouton, GR-70013 Heraklion, Greece
and Department of Physics, University of Crete, GR-70013 Heraklion, Greece
Vassilios Binas†
Institute of Electronic Structure and Laser (IESL),
FORTH, Vasilika Vouton, GR-70013 Heraklion, Greece
and Quantum Complexity & Nanotechnology Center (QCN),
Department of Physics, University of Crete, GR-71003 Heraklion, Greece
(Received 14 January 2014, in final form 25 February 2014)
Mn-, Co-, and Mn-Co doped TiO2 ternary and quaternary semiconducting powder materials
prepared by using the co-precipitation method and fully characterized by using x-ray diffraction
(XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM – EDS) and UV- visible
diffuse reflection spectroscopy were utilized for the photocatalytic degradation of methylene blue
under visible light irradiation. These materials show a red shift and absorption in the visible region
depending on the dopant type and concentration. These materials have proven to be effective as
visible light photocatalysts.
PACS numbers: 82.65.+r, 77.84.Cg, 68.47.Gh
Keywords: Visible light photocatalysts, Titanium dioxide, Manganese, Cobalt, Doping
DOI: 10.3938/jkps.65.0
parent thin film transistors (TTFTs) and as flat panel
displays while they have already found use in a number
of additional applications such as photocatalysis. Photocatalysis is being considered as a more advanced oxidation process to resolve environmental problems such
as air pollution [8] and textile industrial wastewater [9].
The application in textile industries, in particular, is
important as they produce large volumes of toxic, nonbiodegradable and potentially carcinogenic dyes. In the
past decades, TiO2 has been extensively studied as a
photocatalyst for the degradation of different forms of
pollutants such as inorganic and/or VOC (volatile organic compound) gases and liquids (organic dyes) [10]
due to its low cost, innoxiousness, chemical inertness,
and high photocatalytic performance under UV light.
The energy required for the activation of TiO2 as a photocatalyst is larger than its band gap (i.e., > 3.1 eV for
anatase TiO2 ) a fact that limits its use to outdoor applications using solar radiation despite the fact that UV
represents only a small amount, about 3%, of the solar
spectrum. Recently, intensive efforts related to the development of active visible light photocatalysts to harvest
the additional harmless part (∼45%) of the visible part
of the solar spectrum, leading to effective outdoor applications that may also expand to safe indoor use, have
been made.
I. INTRODUCTION
In an evolving world market, metal oxide (MO)-based
semiconductor materials have taken on an exceptionally significant role due to their wide range of properties and corresponding applications [1]. Over the last
two decades, binary (along with ternary and quaternary) transition-metal oxides have been attracting fundamental scientific interest [2]. Their strong electronelectron interaction, combined with their compositional
simplicity and stoichiometric diversity, makes them important model compounds for understanding the effects
of strong correlations in physical (i.e., electron transport [3], transparency [4] and magnetism [5]) and chemical (i.e., gas sensing [6], photocatalysis [7]) phenomena. Novel properties are predicted both for thin films
of these oxides on hard, soft and flexible substrates, and
for materials in nanostructured powder form. Recent
advances in materials science have demonstrated in oxides some effects hitherto confined to high-purity and
defected (amorphous) semiconductors. Currently, transition MOs are increasingly allowing the incorporation of
new functionalities into conventional electronics as trans∗ E-mail:
† E-mail:
kiriakid@iesl.forth.gr
binasbill@iesl.forth.gr; Fax: +32810391295
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Journal of the Korean Physical Society, Vol. 65, No. 3, August 2014
Doping with transition-metal oxides is a promising
way to improve photocatalytic efficiency in the visible
range by taking into account that transition-metal atoms
can exist in various oxidation states. A number of research groups have used different methods (such as solgel, co-precipitation, impregnation, spattering, etc) to
introduce transition metals such as V, Cr, Mn, Fe, Co,
Ni, Cu, Zr, Nb, Mo, Ru, Ag, Au and Pt into TiO2 [7,
10–12]. The introduction of transition-metal cations into
TiO2 is believed to be able to change the coordination
environment of Ti in the lattice and to modify the electronic structure of TiO2 , which may lead to a shift in the
light absorption edge from UV towards the visible light
region.
In the present work, we focus on the photocatalytic efficiency of metal-oxide semiconductors doped with transition metals, which are capable of absorbing and being
activated under visible light irradiation. We synthesized
Mn-, Co- and Mn-Co doped TiO2 nanoparticles using the
co-precipitation method and report on their photocatalytic efficiencies. The morphology, the structure and the
optical and the physical properties of the prepared metaloxide semiconductors (MOSs) were investigated by x-ray
diffraction (XRD), scanning electron microscopy-energy
dispersive spectroscopy (SEM – EDS), and UV-Vis spectroscopy. The photocatalytic efficiencies of the prepared
doped MOs were evaluated based on the degradation of
the organic dye methylene blue (MB) in water as a model
reaction under black and visible light irradiation.
II. EXPERIMENTS
A co-precipitation method was used to prepare Mn-,
Co- and Mn/Co (1:1) -doped TiO2 ternaries and quaternaries with dopant concentrations in solution of 0.1 wt%.
Doped titanium dioxide was precipitated at pH ˜7 from
an aqueous solution of TiOSO4 [titanium (IV) oxysulfate
hydrate] by using as dopant Mn or Co or Mn/Co and by
the addition of ammonia. After the suspension had been
aged overnight, the precipitate was filtered and dried in
air at 373 K. The residue was crushed to a fine powder
and calcinated in a furnace at 973 K for 3 h.
Powder X-ray diffraction patterns were collected on
a Rigaku D/MAX-2000H rotating anode diffractometer
(Cu Kα radiation) equipped with a secondary pyrolytic
graphite monochromator operating at 40 kV and 80 mA
over the 2θ collection range of 10 – 80◦ . The scan rate
was 0.05◦ s−1 . The average crystal size (D in nm) of the
nanoparticles was calculated from the line broadening of
the X-ray diffraction peak according to the Scherrer formula as follows: D = kλ/βcosθ, where k is the Scherrer
constant (∼0.9), λ is the wavelength of the X-ray radiation (1.54 Å for Cu Kα), β is the full width at half
maximum (FWHM) of the diffraction peak measured at
2θ, and θ is the Bragg angle. The UV – visible diffuse
reflectance spectra of the final powders were measured
Fig. 1. (Color online) XRD patterns of Mn-, Co- and
Mn/Co- doped TiO2 all in concentrations of 0.1 wt%.
on a Perkin Elmer LAMBDA 950 system with BaSO4 as
a reference standard. The band gaps were determined
by plotting the Kubelka – Munk function. The surface
morphology and an elemental analysis of the MOs were
carried out using SEM and EDS spectroscopy on a JSM6390LV instrument. The powders were dispersed on a
sample holder, gold-coated and viewed through an electron microscope.
Photocatalytic degradation of MB was carried out at
room temperature in the presence of black and visible
light irradiation. Illumination was provided by two 18W Osram blue-black lights (Osram blue black light 18W L/W 73) or four 18-W visible lamps (Philips Master
TL-D 18-W/840) placed on the top of a homemade environmental photocatalytic box at a distance of ∼25 cm
from the suspension. The environmental box was also
further equipped with two fans to keep the temperature
stable.
In a typical experiment, 100 mg of fabricated powder
material was suspended in 100 ml of aqueous MB solution (10 ppm) in the presence of atmospheric oxygen
at two different pH values (6 and 10). Prior to irradiation, the powders were well dispersed; the suspension was
treated ultrasonically for 10 min, followed by stirring in
the dark for 30 min to achieve adsorption equilibrium.
Dye degradation was monitored using an Ultraviolet–
Visible spectrophotometer (VARIAN Cary 50) by measuring the absorbance of the MB 664-nm (λmax for MB)
characteristic line.
III. RESULTS AND DISCUSSION
The XRD patterns of Mn-, Co- and Mn-Co doped
TiO2 (total dopant concentration in the solution was 0.1
wt %) calcinated at 700 ◦ C for 3 h are shown in Fig. 1.
The peaks at 2θ values of 25.3, 37.6, and 48.2 correspond
Metal Oxide Semiconductors as Visible Light Photocatalysts· · · – George Kiriakidis and Vassilios Binas
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Table 1. Main Characterization results of Ti − M metal oxides.
Sample
Undoped TiO2
Mn − TiO2
Co − TiO2
Mn/Co − TiO2
dopant
concentration
wt % OR mole
fraction
−
0.1
0.1
0.1
d(101)
2θ
Anatase crystal
Size (D, nm)
Indirect band gap
(eV)
25.25
25.20
25.20
24.95
35.5
38.7
40.1
40.1
3.1
2.8
2.8 [1.55*]
2.8 [1.5*]
*sub-band gap
to the (101), (004), and (200) planes, respectively, and
they are all anatase signature peaks. Doping with Mn
and Co cations led to a small shift of the main (101) peak
to a lower angle in comparison with the undoped anatase
TiO2 , an indication of an increase in the lattice constant
and the presence of dopant-induced lattice stress. In the
case of Mn/Co-doped TiO2 , the positions of the (101),
(004), and (200) diffractions were also shifted to smaller
θ. In Table 1, these differences are listed. It should
be noted that no obvious manganese- or cobalt-oxiderelated diffraction peaks were observed. The dependence
of the anatase crystal size on the dopant incorporation
is also depicted in Table 1. The anatase crystal size was
found to be increased significantly by the incorporation
of dopant ions into the lattice of TiO2 .
Figure 2 shows the UV-VIS absorption spectra as a
function of wavelength for the Mn-, Co- and Mn/Codoped TiO2 semiconductors. All the doped semiconductors exhibited an absorption edge in the visible light
range (400 – 800 nm), Fig. 2(a). The incorporation of
dopants led to a distinct decrease in the band-gap energy to below the value of 3.1 eV of the undoped TiO2
(Table 1). The absorption curves of the doped semiconductors were clearly shifted to lower energies and showed
a significant sub-band-gap absorption with two absorption humps. The first was between 370 – 500 nm and the
second between 470 – 800 nm. These two broad absorption humps are normally associated with d-d electronic
transitions of Mn2+ and Co2+ in the octahedral coordination of Ti in the lattice [13, 14]. The band-gap energies (Eg) for all the samples were determined by using
Kubenka-Munk plots as shown in Fig. 2(b) and Table 1.
In the case of Co- and Mn/Co-doped TiO2 , a new subband-gap appears with a value of 1.6 eV while the broad
peak around 650 nm is usually attributed to the Co2+
and the Co3+ species [15]. The new absorption shoulders of the doped TiO2 in the range of 400 – 800 nm
are believed to be the reason for the enhanced overall
photocatalytic activity within the visible range shown
below. The textural features of the MOs were investigated with SEM, and representative images are shown
in Figs. 3(a,b,c). No specific morphology changes were
Fig. 2. (Color online) (a) UV- VIS absorption spectra as a
function of wavelength for Mn-, Co-, and Mn/Co- doped TiO2
semiconductors, (b) Energy gaps calculated from a KubenkaMunk plots for Mn-doped TiO2 , Co-doped TiO2 and Mn/Co
co-doped TiO2 in concentrations of 0.1 wt%.
detected, but the spherically shaped particles of all MOs
demonstrated some degree of agglomeration with diameters ranging from 0.1 to 40 µm.
The photocatalytic efficiencies of Mn-, Co- and Mn-Co
doped TiO2 were studied based on photo-degradation at
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Journal of the Korean Physical Society, Vol. 65, No. 3, August 2014
Fig. 4. (Color online) Photocatalytic efficiency of MB
degradation under black light irradiation of Mn-, Co-, and
Mn/Co- doped photocatalysts for pH values (a) 10 and (b)
6.
Fig. 3. (Color online) SEM images of Mn-, Co- and
Mn/Co- doped TiO2 in concentration of 0.1 wt%.
room temperature of the standard organic dye MB in a
test reaction chamber under black and visible light irradiation. MB alone did not show any photo-degradation
when irradiated by black or visible light. The photocatalytic efficiencies of Mn-, Co- and Mn/Co-doped TiO2
for MB photo-degradation under black light irradiation
at two different pH conditions (10 and 6) are shown in
Figs. 4(a) and (b) respectively. In both cases, the doped
TiO2 showed a significant photocatalytic efficiency on
the order of 60% under black light irradiation with its
value remaining almost unchanged when the pH was increased from 6 to 10. When the suspension (doped TiO2
and MB) was irradiated with visible light (Figs. 5(a) and
(b)), all doped TiO2 shown significant photocatalytic efficiency (about 60%) at pH = 6. However, when the pH
was increased to 10, all doped TiO2 showed an improved
photocatalytic efficiency of about 70%. Particularly for
the case of Mn-doped TiO2 while at pH = 6 it had shown
a photocatalytic efficiency of about 18% at pH = 10 the
efficiency was increased to 56% (a significant 66% increase). Herrmann et al. [16] showed a corresponding
increase in the adsorption of MB when the pH was increased from 3 to 9, a result that is in agreement with the
influence of pH on the ionization state of TiO2 , because
the adsorption of MB (cationic configuration) is favored
in alkaline solutions.
The kinetics of the photocatalytic degradation of MB
was evaluated by applying the Langmuir – Hinshelwood
model in which the photocatalytic activity of a semiconductor can be estimated through the equation lnCo /Ct
= kt, where Co and Ct represent the concentrations of
the organic pollutant at zero time and time t, and k is
a rate constant for the reaction. Figures 6(a) and (b)
show the kinetics (photo-degradation rate) of MB (pH
= 10 and 6) under visible light irradiation in the presence of Mn-, Co- and Mn/Co-doped TiO2 . The photocatalytic degradation rate of MB was found to follow the
Metal Oxide Semiconductors as Visible Light Photocatalysts· · · – George Kiriakidis and Vassilios Binas
Fig. 5. (Color online) Photocatalytic efficiency of MB
degradation under visible light irradiation of Mn-, Co-, and
Mn/Co- doped photocatalysts for pH values (a) 10 and (b)
6.
Table 2. Experimental values of the rate constant and
the photocatalytic efficiency for Mn-, Co- and Mn/Co-doped
TiO2 for the degradation of MB under visible light irradiation.
Sample
Mn − TiO2
Co − TiO2
Mn/Co − TiO2
pH
6
10
6
10
6
10
Visible light irradiation
Photocatalytic
Rate constant
efficiency (%) (k, ×10−3 min−1 )
18
1.27
54
3.84
50
3.59
69
5.92
58
4.68
62
4.89
first-order kinetic model. The first-order rate constants,
k, were calculated from the slopes of the graphs of ln
Co /Ct versus t for Mn-, Co- and Mn/Co-doped TiO2
and the results are listed in Table 2. As shown in this
table, the rate constants for all materials increased with
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Fig. 6. (Color online) Kinetic study of the oxidation of
MB under visible irradiation of Mn-, Co- and Mn/Co- doped
photocatalysts for pH values (a) 10 and (b) 6.
increasing pH from 6 to 10, with corresponding values
for of kM n = 3.84 × 10−3 min−1 , kCo = 5.92 × 10−3
min−1 and kM n/Co = 4.89 × 10−3 min−1 under visible
light at pH = 10.
IV. CONCLUSION
In this work, anatase Mn-, Co and Mn/Codoped TiO2
semiconductor powder materials were synthesized using
a co-precipitation method with particle sizes in the range
between 35 – 40 nm. Optical measurements indicated a
red shift and an absorption in the visible region that
depended on the doping material. In the case of Co and
Mn/Co dopants an extra sub-band gap was found to be
present. The photocatalytic efficiencies of Mn, Co and
Mn/Codoped TiO2 were found to be significantly higher
under visible light as compared to black light irradiation.
We conclude that these semiconductors can be applied
to the degradation of a wide range of organic pollutants,
thus contributing to an enhanced industrial wastewater
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Journal of the Korean Physical Society, Vol. 65, No. 3, August 2014
treatment process with the distinct advantage of visiblelight activation over the conventional undoped TiO2 .
ACKNOWLEDGMENTS
This work was partially supported by the EU FP7
Programmes (FP7-REGPOT-2012-2013-1) under grant
agreement no 316165 and FP7 IP “ORAMA” no 246334.
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