Disordered Mesoporous TiO2-xNx + Nano Au: An Electronically Integrated
Nanocomposite for Solar H2 Generation
Kumarsrinivasan Sivaranjani,a Sivaraman RajaAmbal,a Tanmay Das,bKanak Roy,a Somnath
Bhattacharyya,b Chinnakonda S. Gopinatha,c,d,*
a
Catalysis Division, CSIR - National Chemical Laboratory, Dr.Homi Bhabha Road,
Pune 411 008, India.
b
Department of Condensed Matter Physics and Materials Science, Tata Institute of
Fundamental Research, 1 Homi Bhabha Road, Colaba, Mumbai 400 005.
c
CSIR-Network of Institutes for Solar Energy (NISE), CSIR-NCL Campus,
Dr. Homi Bhabha Road, Pune 411 008, India.
d
Centre of Excellence on Surface Science, CSIR-National Chemical Laboratory,
Dr.Homi Bhabha Road, Pune 411 008, India.
Phone: +0091-20-2590 2043; Fax: +0091-20-2590 2633
E-mail: cs.gopinath@ncl.res.in; Website:http://nclwebapps.ncl.res.in/csgopinath/
2
Abstract:
We report on H2 generation by visible light driven photocatalysis by electronically
integrated nano-Au-particles with multifunctional, disordered mesoporous TiO2-xNx (Au-NT)
nanocomposites. Solar H2 generation (1.5 mmol h-1 g-1) from aqueous methanol has been
demonstrated with Au-NT nanocomposites. Water splitting activity of Au-NT is attributed to
the single most important factor of 21.1 ps lifetime of charge carriers observed from
fluoresence lifetime measurements, indicating high electron injection efficiency from nano Au
to conduction band of titania, and hence charge separation as well as utilization. This is
directly supported by the observation of high photoluminiscence emission intensity with AuNT highlighting the energy transfer from nano gold to titania. p-n heterojunction observed
between Au (001) and TiO2 (101) facet helps towards higher charge separation and their
utilisation. Low meso channel depth (<10 nm) associated with disordered mesoporous TiO2xNx
helps the charge carriers to move towards the surface for redox reactions and hence
charge utilization.Visible light absorption, due to surface plasmon resonance of nano Au,is
observed in a broad range between 500 and 750 nm helps in harvesting visible light
photons.Finally, an electronically integrated nano Au with TiO2-xNxin Au-NT is evident from
XPS and Raman spectroscopy measurements. All the above factors help to achieve high rate
of H2 production. It is likely possible that higher rate of H2 production, than reported here, is
feasible by strategically locating Au-clusters in porous titania to generate hot spots through
electronic integration.
Keywords:Hydrogen, Photocatalysis, Water splitting, Mesoporous materials, Au-TiO2,
Nanocomposite
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1. Introduction
Visible light driven photocatalytic water splitting reaction (WSR) is projected to be an
indispensable solution to meet a significant part of clean and sustainable energy demand at
global level [1]. Producing hydrogen, a clean fuel from water and sun light, by overall water
splitting reaction (OWSR) is an ultimate goal to sustain the globe in a better way [1-3]. In
spite of many efforts in the past, OWSR by visible light driven photocatalysis is yet to
produce a breakthrough in terms of quantum efficiency of ≥ 10 %, which is the minimum
bench mark to think about commercialization. From the rate of slow progress of OWSR
witnessed in the last few decades, in our opinion, achieving 10 % efficiency might be
considered as a long term solution for energy demands. Nonetheless, H2 production through
WSR with a sacrificial agent, such as methanol, glycerol, might be considered as a short term
goal, as it avoids the four electron process to produce molecular oxygen in OWSR [4-5].
There are significant numbers of reports available on WSR and OWSR, and much more
concerted efforts are necessary to produce a sustainable photocatalyst system [1-5].
Among the available materials, TiO2 is considered to be the best candidate for
photocatalysis due to various reasons, such as photostability, non-toxicity, large abundance,
cost effectiveness and high oxidation potential [6]. However, large band gap and fast charge
carrier recombination are considered to be the major drawbacks of TiO2. There were many
efforts in the past to make titania as a visible light active photocatalyst through metal ion
doping, anion doping, surface sensitization etc [7-12]. Nitride anion doping is considered to
be the most suitable path to reduce the band gap of titania and bring more visible light
absorption [9,10]. It has been demonstrated that nitride or anionic nitrogen is responsible for
band gap reduction and visible light absorption in TiO2-xNx [9,11]. Recently TiO2-xNx was
successfully prepared by various methods [9,11-16].
Along with visible light absorption, charge carrier recombination probability should be
suppressed or minimized to increase the quantum efficiency. Introducing mesoporosity in
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titania increases the rate of diffusion of charge carriers from bulk to the surface [14,17].
Specifically, disordered mesoporosity is known to reduce the diffusion length of charge
carriers, since the depth of mesopores are few nanometers (< 10 nm) [14,18], unlike several
hundred nanometers in conventional mesoporous materials, like MCM-41 and SBA-15 [1921].These types of mesopores are known as pseudo three dimensional (p3D) mesopores [22].
This disordered mesoporous framework provides an easy route for the diffusion of reactants
as well as products due to low diffusion barriers.
On the other hand, noble metal cluster deposition on titania acts as an electron sink by
selectively storing electrons [23].Interestingly nano Au or Ag deposition on titania brings
more visible light absorption through surface plasmon resonance (SPR) bands [24]. It is a
known fact that the final properties of Au/TiO2 system, especially WSR activity, mainly
depends on the preparation procedure. Bamwenda et al prepared Au/TiO2 and
Pt/TiO2photocatalysts by different preparation routes and showed its WSR activity from
aqueous ethanol solution depends on the preparation procedure [25]. Recently Murdoch et al.
has shown that the photocatalytic activity is independent of the gold cluster size between 3
and 12 nm, and Au/anatase gives two orders of magnitude higher rate of H2 production than
on rutile titania [26]. Subramanian et al [27] has demonstrated the negative shift of Fermi
level (EF) of Au/TiO2 nanocomposite; higher negative potential shift has been observed with
EF of Au/TiO2 with decreasing gold particle size. Hence the resulting composite materials are
more reductive than the pure TiO2; further the life time measurements indicate the charge
injection occurs from titania to gold. However, exact opposition to the above, electron
injection from Au to the conduction band (CB) of titania was suggested due to strong
localization of plasmonic near fields close to the Au – TiO2 interface [28-29]. It is to be noted
that preparation methods are different in the above reports and it is very likely that the same
influences the electron injection mechanism. Apart from the above controversies, the
mechanistic aspects of Au/TiO2 system and integration of nano Au with titania is less
5
understood [27].However, H2 production with Au-TiO2 composite through WSR demonstrates
its high potential[25-29], and a better understanding would lead to tap the same with better
activity.
A tandem approach has been employed to synthesize an electronically integrated
composite of nano Au and disordered mesoporous TiO2-xNx (Au-NT) in a single step by
simple solution combustion method (SCM). Au-NT composite material possesses some of the
most desired properties, such as nanogold clusters with SPR in broad visible light range, TiO2xNx
with disordered mesoporosity, low meso-channel depth (≤ 10 nm) and high surface area,
electrically interconnected nanoparticles (EINP) [18], and significantly large lifetime of
electrons (21 ps). These composite materials show decent activity towards H2 generation from
aqueous methanol solution under visible light irradiation. This work is a part of ongoing study
from our laboratory on the materials development for visible light driven photocatalysis [1114,18,30-34].
2. Experimental Section
2.1 Synthesis of xAu-NT materials:
We employed SCM which involves a simple synthesis protocol, and requires a short
reaction time (< 10 minutes) and cheap starting materials. All the chemicals employed were of
analytical grade and used as such without any further purification. Titanium nitrate (SigmaAldrich) as Ti precursor, gold (III) chloride (Sigma-Aldrich) as gold precursor and urea
(Merck) as fuel were used. Required quantity of aqueous titanyl nitrate, gold chloride and urea
were taken in a 250 ml beaker and introduced into a muffle furnace preheated at 400 0C.
Water evaporation takes place in the first few minutes, followed by smoldering type
combustion that occurs in the next 1-2 min. Immediately aftercombustion process was
completed, xAu-NT materials was removed from the furnace. Series of nano gold on nitrogen
doped mesoporous titania (xAu-NT) composite materials were prepared by changing the Au
atom % from 0 to 0.3 with fixed (urea/Ti4+) ratio equal to 10. In xAu-NT, x stands for nominal
6
Au atom %. We employed urea as fuel, to avoid any carbon impurities, so that no further
calcination is required. During combustion process, in situ generation of NH3 occurs due to
urea decomposition, and this acts as nitrogen source as well as creates a reduction atmosphere
to reduce Au3+ to Au clusters.
Au-NT nanocomposites have been subjected to detailed characterization and
photocatalytic WSR measurements (see supplementary material for detailed characterization
and procedure for photocatalytic activity measurements).
2.2Characterization of Methods:
Powder X-ray diffraction (PXRD) data of TiO2-xNx andxAu-NT materials was
collected from PANalytical X’pert Pro dual goniometer X-ray diffractometer. A propotional
counter detector was used for low angle experiments. The data were collected with a step size
of 0.020 and a scan rate of 0.50/min. The sample was rotated throughout the scan for better
counting statistics. The radiation used was Cu Kα (1.5418 Å) with Ni filter and the data
collection was carried out using a flat holder in Bragg-Brentango geometry (0.20). Energy
dispersive X-ray (EDX) analysis and scanning electron microscopy (SEM) measurements
were performed on a SEM system equipped with EDX attachment (FEI, Model Quanta 200
3D). EDX spectra were recorded in the spot-profile mode by focusing the electron beam onto
specific regions of the sample. Calibration of the experiment for nitrogen estimation was
measured with several mixtures of gallium nitride and alumina powder mainly to ensure the
reliability of nitrogen estimation. Nitrogen adsorption/desorption isotherms for the materials
were collected from Quantachrome autosorb automated gas sorption system (NOVA 1200).
The Brunauer-Emmett-Teller (BET) equation was used to calculate the surface area from the
adsorption branch. The pore size distribution was calculated by analyzing the adsorption
branch of the nitrogen sorption isotherm using Barret-Joyner-Halenda (BJH) method. A FEI
TECNAI 3010 electron microscope operating at 300 kV (Cs = 0.6 mm, resolution 1.7 Å) was
employed for high resolution transmission electron microscopy (HRTEM) measurements of
7
Au-NT materials. Samples were crushed and dispersed in isopropanol by sonication before
depositing it onto a holy carbon grid. A FEI-TITAN microscope operated at 300 kV equipped
with FEG source, Cs (spherical aberration coefficient) corrector for condenser lens systems
and a high angle annular dark field (HAADF) detector was used to perform
scanning
transmission electron microscopic (STEM) experiments. Semi convergence angle of electron
probe incident of the specimen and camera length were maintained 17.8 mrad and 128 mm
respectively during the experiments and all images were taken with HAADF detector. All
energy dispersive x-ray (EDX) spectral data were taken in spot mode for 180 seconds keeping
other experimental parameters same as high resolution Z contrast imaging which was acquired
for a dwell time of 20 microseconds. The specimen for STEM experiments was prepared by
dispersing the material (0.10 Au-NT) in water and placing a drop of the dispersion on a Cu
TEM grid covered with carbon film, which was then dried.
Diffuse reflectance UV-vis measurements were performed on a spectrophotometer
(Shimadzu, Model UV-2550) with spectral-grade BaSO4 as the reference material. Raman
spectra were recorded on a Horiba JY LabRAM HR 800 Raman spectrometer coupled with
microscope in reflectance mode with 633 nm excitation laser source and a spectral resolution
of 0.3 cm-1. PL measurements were performed using Horiba Jobin Yuon Fluorolog 3
spectrophotometer equipped with 450 W xenon lamp at room temperature under the excitation
light of 280 nm. The conditions are fixed as far as possible in order to compare the
photoluminescence signals directly. XPS measurement has been made using a custom built
ambient pressure XPS system from Prevac, Poland, and equipped with VG Scienta SAX 100
emission controller monochromator using AlKα anode (1486.6 eV) in transmission lens
mode. The photoelectrons are energy analyzed using VG Scienta’s R3000 differentially
pumped analyzer. The spectra were recorded at a pass energy of 50 eV.
Fluorescence lifetime measurements were performed using Horiba Jobin Yvon
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Fluorolog 3 spectrophotometer having a 450 W xenon lamp. Fluorescence lifetime decays
were collected by a time-correlated single photon counting (TCSPC) setup from IBH Horiba
Jobin Yvon (U.S.) using a 375 nm diode laser (IBH, U.K., NanoLED-375 L, with a λmax =375
nm) having a FWHM of 89 ps as a sample excitation source.
The photoelectrochemical properties of the Au/NT materials were measured by linear
sweep voltametry (LSV) using Autolab PGSTAT30 (Eco-Chemie) instrument in a
conventional three-electrode test cell, with Ag/AgCl as the reference electrode, in 1 M NaOH
solution at ambient conditions. For preparing the working electrode, the FTO plates were
washed thoroughly with acetone and IPA and thereafter the catalyst slurry made by dispersing
5 mg catalyst in 1 ml of isopropanol was drop casted on the electrode surface and dried at
room temperature for overnight. This electrode was used as the working electrode for all the
electrochemical studies. Lamp used - 400 W medium pressure Hg vapor UV lamp.
2.3Photocatalytic activity measurements:
The photocatalytic activity was measured for the H2O splitting under visible light. The
reaction was carried out at ambient conditions using a borosilphotoreactor of ca. 50 ml
capacity, equipped with a port for the withdrawal of gas samples at regular intervals. For each
experiment, 100 mg of fresh catalyst was dispersed in 32 ml water and 8 ml methanol to serve
as sacrificial reagent. 125 W simulated white light source or Newport’s solar simulator with
AM1.5 filter was used as irradiation source. The experiments were conducted at around pH =
7. Hydrogen evolved was sampled and analyzed periodically on a gas chromatograph
(Chemito, model-8610, Porapak-Q column, thermal conductivity detector at 353 K).
3. Results and Discussion
Fig. 1a shows powder x-ray diffraction (XRD) pattern of all xAu-NT materials and x
in xAu-NT indicates a nominal amount of gold loaded in atom percent. All the peaks in XRD
pattern could be indexed to anatase phase of TiO2 (JCPDS File 21-1272) [34,35]. All the
9
peaks in XRD are broad indicating the nanocrystalline nature of materials. Crystallite size was
calculated by Debye-Scherrer equation and shown in Table 1. Up to 0.10Au-NT, there is no
Table 1:Physicochemical properties of xAu-NT materials.
Material
Pore
volume
(cc/g)
0.44
Crystallite
size (nm)
Bulk Au
(Atom %)§
N-TiO2 (NT)
BET
Pore Dia.
Surface
(nm)
2
area (m /g)
234
7
6.8
0
0.01Au-NT
218
8.2
0.37
6.0
0.009
0.03Au-NT
200
8.1
0.4
6.2
0.026
0.05Au-NT
213
8
0.43
6.9
0.045
0.10Au-NT
171
9.6
0.41
8.0
0.11
0.30Au-NT
157
6.8
0.27
8.8
0.28
§
Measured from EDX
Figure1:(a) Wide angle powder XRD pattern, and (b) UV-Visible absorption spectra of xAuNT nanocomposites. Inset in (a) shows the low angle XRD pattern. Inset in (b) shows a
photograph for the color variation with increasing Au-content of xAu-NT, with sample labels
as A = N-TiO2, B = 0.01Au-NT, C= 0.03Au-NT, D=0.05AuNT, E=0.10AuNT and F =
0.30Au-NT.
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feature observed in XRD that corresponds to metallic gold. However, 0.30Au-NT shows a
peak at 2θ = 44° corresponds to (200) plane of metallic gold [35].This supports the well
dispersed nano Au clusters on xAu-NT (x ≤ 0.1).
Inset in Fig. 1a shows the low angle XRD pattern of all xAu-NT materials.
Mesoporous nature of Au-NT materials was confirmed by the above feature around 2 = 10.
Only one peak was observed indicating the disordered mesoporosity [14,34]. Unlike the
ordered hexagonal mesoporous materials, like MCM-41, SBA-15, no other low angle features
was observed [19-21] highlighting the disordered mesoporous nature and it is due to an
intergrowth of fundamental particles. High Au loading (x = 0.3) significantly affects the
mesoporosity of the materials. A gradual decrease in the intensity of low angle peak at 2θ =1°
indicating an increasing pore blockage by nanogold clusters. However, all xAu-NT possess
disordered mesoporosity along with nanocrystallinity.
Fig. 1b shows the UV-visible spectra of xAu-NT nanocomposites. The absorption
edge of titania remains at around 380 nm, in spite of nitrogen doping [14]. Inset in Fig. 1b
shows a photograph for color associated with xAu-NT nanocomposites. N-TiO2 shows a pale
yellow color, and upon Au introduction blue to grayish blue color develops gradually. On
increasing Au to 0.1 atom % and above, the color changes increasingly towards dark blue.
SPR features of nano gold in these composite materials bring more visible light absorption
between 500 and 750 nm [26,27].Above light absorption is due to collective oscillation of
electrons of the gold particles in response to optical excitation [27].Visible light absorption
between 500 and 750 nm demonstrates surface plasmon state energy, at least, between 1.65
and 2.5 eV above the EF of gold and indicating production of energised electrons. Above SPR
peak intensity increases with increased gold loading and the position of this peak depends on
the gold particle size or aggregation, and the surrounding environment [36,37]. Hence visible
light photons can be harvested by using xAu-NT nanocomposites. Indeed, TEM studies of
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0.1Au-NT shows size distribution of gold clusters (Fig. 3) suggesting the different size
particles leads to broad absorption in the visible light regime. However, pore volume and
surface area decreases with increasing Au-content, especially at x ≥ 0.1, (Table 1) indicating
an increased pore blocking due to gold clusters in pores. Apart from SPR, nano gold clusters
act as electron sink by forming Schottky barriers with TiO2, and the active sites for H2
evolution. Thus nano Au in xAu-NT increases the visible light absorption significantly.
Figure 2:(a) N2 adsorption-desorption isotherms, and (b) BJH pore-size distribution of xAuNT composite materials. (c) HRTEM image of 0.05Au-NT, composite material. SAED is
shown in the inset. No distinct gold particles are directly observed indicating the possibility of
good dispersion.
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Figure 2 shows N2 adsorption-desorption isotherms and Barrett-Joyner-Hollenda
(BJH) pore size distribution of xAu-NT. All materials show type IV isotherm with H2
hysteresis loop, which is typical for disordered mesoporous materials [14,22].xAu-NT
nanocomposites show narrow pore size distribution with an optimum pore diameter around
8±1 nm. Compared to TiO2-xNx, xAu-NT exhibits a significant decrease in BET surface area.
Among these composite materials, 0.05Au-NT shows high surface area around 213 m2/g.
Surface area decreases with more Au loading and 0.30Au-NT exhibit a surface area of 157
m2/g indicating an increasing pore blockage.
HRTEM result for 0.05Au-NT nanocomposite is shown in Figure 2c (for more
HRTEM images refer Fig. SI-1). All xAu-NT particles exhibits spherical or near spherical
shape morphology. A disordered mesoporous structure is observed for all xAu-NT materials.
Above disordered mesoporosity arises due to an intergrowth of fundamental particles and the
same leads to aggregates with significant extra framework void space. Presence of meso and
macro pores is clearly visible in Figure 2c, which assists for faster diffusion of reactants and
products inheterogeneous catalysis. Selected-area electron diffraction (SAED) pattern
confirms the crystalline nature of xAu-NT nanocomposites with anatase phase TiO2. HRTEM
image shows the majority of lattice fringes corresponds to (101) crystallographic planes of
anatase phase (d(101) = 0.352 nm). These observations are in excellent agreement with XRD
results. Present disordered mesoporous structure has an additional advantages due to smal
mesochannle depth of ≤ 10 nm, and hence the same is referred as p3D mesopores [18,22];this
is in contrast to micron size long mesochannels in ordered mesoporous materials [19].
Finally, nanocrystallites in xAu-NT are electrically well connected with each other,
and this connectivity extends upto few μm (Figs. 2c and Fig S1). Due to EINP nature [14,18],
the excited electroncan move effectively from one end to the other and from the bulk to the
surface due to small diffusion length under light harvesting conditions. Advantageous EINP
13
feature associated with xAu-NT composite material enhances the diffusion of charge carriers
to the surface.
Figure 3: (a) Z-contrast image of 0.10 Au-NT at low magnification. (b) High resolution Zcontrast image of the nanoparticle, shown within the white square in panel a. EDX spectrum
recorded (c) on the Au-nanoparticle shown in panel b, and, (d) on the TiO2 matrix.
Unlike many reports cited in the introduction, we were unable to find any gold
particles on Au-NT in the above HRTEM studies, likely due to similar contrasts of Au and
titania. To explore various aspects of gold, Z-contrast image analysis was carried out with
FEI-TITAN microscope operated at 300 KV. Fig. 3a shows a representative Z-contrast image
of 0.10Au-NT at low magnification, where small bright regions indicate the possibility of
accumulating higher atomic number elements than the TiO2 matrix. On an average, particle
size varies from 5 to 21 nm (Fig. 3a). Fig 3b is the high resolution Z-contrast image of the
bright region within the white square shown in Fig. 3a where the lattice (~0.2 nm) spacing
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matches with the projected distance between Au(001) planes. The EDX spectrum recorded on
the same above particle (Fig. 3c) confirms it as Au nanoparticle embedded in a matrix
containing Ti and O. The EDX spectrum taken from TiO2 matrix region shown in Fig. 3d
proves that the matrix does not contain any Au. Considering all this results presented in Fig. 3
it can be stated that Au nanoparticles were grown within TiO2 matrix, and Au(001) and TiO2
(101) planes makes a p-n heterojunctions, which is expected to help for charge separation and
hence its utilisation to a better extent. It is also to be noted that the gold particles are isolated
with a minimum average inter-particle distance of 200 nm.
Figure 4:(a) Raman spectra of xAu-NT composite materials. Raman active Eg mode on NTiO2 at 145 cm-1 shows a blue shift to 154 cm-1 with increased line broadening and
asymmetry and decreased intensity with increasing Au-content (b) Photoluminescence spectra
for xAu-NT materials. Dotted lines are guide to eye.
Raman spectroscopy is a versatile tool to determine the structural features of the
composites. Fig. 4a shows the Raman spectra for all xAu-NT composite materials. All six
Raman active fundamental modes are observed at 145 (Eg), 198 (Eg), 398 (B1g), 516 (A1g +
B1g), 640 (Eg) cm-1 for the anatase phase [38]. Raman active Eg mode of TiO2 at 145 cm-1
exhibits a systematic blue shift to 154 cm-1 upon nanocomposite formation with Au. Further,
15
full-width at half maximum (FWHM) measured for Eg (as well as other features) changes
from 15.1 cm-1 on N-TiO2 to 25.2 cm-1 on 0.1Au-NT indicating a line broadening. Above line
broadening is accompanied with asymmetry for all xAu-NT composites and particularly it is
evident with Eg feature at 640 cm-1. It is to be underscored here that anatase (101) facet is the
most abundant plane (Figs. 1a and 2c) and corrugated with alternating rows of 5- coordinated
Ti (likely Ti3+) and bridging oxygen at the edges of corrugation [39]. This makes (101) an
ideal facet for interaction with deposited Au-clusters through charge transfer and leads to a
blue shift [39]. Indeed this electronic interaction on polar surface is likely to assist for fast
electron transfer.
Fig. 4b shows the photoluminescence (PL) emission spectra of Degussa P25 (P25) and
xAu-NT nanocomposites. All xAu-NT composites and P25 show peaks at 419, 442 and 470
nm. Emission peak due to the band gap transition appears at 380 nm for all materials (not
shown) [40]. The emission band at 419 nm is due to free exciton emission of TiO2 [41].
Appearance of shoulder around 470 nm is due to the surface state, such as Ti4+−OH [42].The
emission feature at 440 nm originates from the charge-transfer transition from Ti3+ to oxygen
anion in a 𝑻𝒊𝑶𝟖−
𝟔 complex [43] present in the material. All the above three emission features
show the lowest intensity for P25. N-TiO2 shows a significant increase in the intensity of all
the features, compared to P25. Upon Au deposition on N-TiO2, intensity of all emission
features picks up and the maximum intensity is observed with 0.05Au- NT. Particularly the
intensity of the feature, at 442 nm, increases significantly than other features suggesting a
better interaction of Au with NT through unsaturated (101) facet. In fact, the above
observations reiterates the presence of p-n heterojunctions in xAu-NT nanocomposites, which
helps for electron injection into the CB of TiO2. On further Au loading, intensity of all
emission features start to decrease. Nonetheless, xAu-TiO2 exhibits higher intensity emission
features (than P25 and N-TiO2) underscore the energy transfer from nano Au to the CB of
16
titania. Indeed this is the critical observation that highlights the electronic interaction between
nano Au and TiO2. The maximum intensity emission features observed with 0.05Au-NT
indicating the lifetime of charge carriers are likely to be higher, and hence the possibility of
electron injection from nano Au to the CB of TiO2. Indeed the lifetime measurements fully
support the above conclusions (vide infra). Surface plasmon state energy measured (1.65-2.5
eV) from UV-vis absorption spectroscopy (Fig. 1b) indicating the production of energetic
electrons (~3.3 eV) due to PL excitation at λ = 375 nm. This ensures injection of electrons
from surface plasmon states of Au to CB of TiO2 under visible light irradiation as well as
under PL measurement conditions. It is to be noted that the EF of gold and CBMin of TiO2
appears around 0 and -0.5eVon the normal hydrogen electrode (NHE) scale [2,3,28]. Electron
injection from surface plasmon states of Au to CB of TiO2 is in accordance with the above
energy levels. On increasing the Au-content ≥0.1, emission features decreased in intensity and
it is attributed to bigger size or agglomeration of gold particles and hence the possibility of
lower electron injection from Au to TiO2.Bigger size Au particles, as observed in XRD (Fig.
1a), decreases the interaction with TiO2-xNx lattice, and thereby the electronic interaction also
decreases. Indeed aggregation of gold particle at x = 0.3 re-exposes the TiO2-xNx surface sites,
and hence the PL emission intensity decreases dramatically. An optimum quantity of gold
around x = 0.05 in the xAu-NT composite materials maximizes the interaction between titania
and gold clusters.
To know more about the electronic integration aspects of xAu-NT, XPS studies have been
carried out with custom-built high pressure XPS[44].A very interesting observation was made
with Ti 2p core level results (Figure 5). As prepared xAu-NT nanocomposites show three
features for Ti 2p core level (thin traces in Fig. 5); this is in contrast to the typical two core
level spin-orbit coupling peaks due to Ti 2p3/2 and 2p1/2 as observed for NT and TiO2
[11,12,14,18]. A glance at the Ti 2p core level spectra obtained from xAu-NT indicates a
possible non-uniform static-charge buildup upon photoelectron emission. Non-uniform static-
17
Figure 5:XPS core level spectra recorded for Ti 2p core level for NT and xAu-NT (thin
traces) with low energy electron flood gun (bold traces). Low energy electron flood gun
employed to neutralize the non-uniform static charge buildup on xAu-NT.
charge buildup occurs due to the presence of differentially conducting areas on the
surface layers, especially due to deposition of small amount of gold (or any metal
nanoclusters) on titania. Particulate nature of xAu-NT is a reason for the above observation.
To resolve the above non-uniformity, a low energy electron flood gun was employed to
neutralize the above static charge build-up, which would nullify the same and make the
surface charge-neutral. With charge neutralization, XPS result shows typical Ti 2p features
expected for any TiO2; however, broad core level peaks are observed for xAu-NT (bold traces
in Fig. 5) indicating the presence of mixed valent Ti-ions. Indeed these observations
highlights the following electronic aspects of xAu-NT. (a) Low BE feature observed, after
static charge-neutralization, around 457 eV corresponds to Ti3+ oxidation state on xAu-NT.
(b) Above low BE feature observed, even without static charge-neutralization, demonstrates
the presence of conducting nature of part of xAu-NT material; this is attributed to the
electronic interaction of nano Au clusters with TiO2through p-n heterojunctions. Observation
18
of conducting nature of part of xAu-NT underscores an equilibration of Fermi levels and
electronic integration of Au with NT. (c) Even without charge neutralization, Au-NT shows a
broad Ti 2p photoelectron emission from 456eV till the onset of the next peak at 462 eV. This
is attributed to the interaction of titania with different size gold particles and it is in good
agreement with the nano gold clusters (5-21 nm) shown in Fig. 3. Ti3+oxidation state observed
at correct BE, even without charge neutralization, suggests either Ti3+ is generated upon
deposition of nano gold clusters or nano gold interacts preferentially with Ti3+. Amount of
Ti3+ observed on TiO2-xNxincreases from 0.05 to 0.1AuNT suggesting that more Ti3+is
generated upon gold deposition. Indeed, the above points not only suggest a strong electronic
interaction between Au and titania in Au-NT nanocomposites, but also a polarizing structural
feature on the surface; which helps separate electron from electron-hole pairs, and disordered
mesoporous framework helps towards that. This again indicates the possibility of either EF
equilibration of Au and TiO2or more negative potential for EF of gold than CBMin of TiO2 (0.5 V vsNHE) [28,45]. Indeed sensitization would occur when the EF of gold is more negative
than CBMin of TiO2. Solar H2 production with visible light demonstrates the above situation is,
indeed, present in xAu-NT nanocomposites. Preferentially smaller and uniform size gold
clusters are likely to increase the volume of interaction between gold and titania, which is
likely to increase the electronic integration to a higher level, and it is worth exploring further.
In order to evaluate the efficacy of the above Au-NT nanocomposites, photocatalytic WSR
was carried out under simulated sun light with aqueous methanol solution. Fig. 6a shows the
amount of H2 evolution using 0.05Au-NT nanocomposites for continuous 15 h with
evacuation after every 5 h. Steady rate of H2 evolution was observed for at least another five
such cycles with very similar activity suggests a photostability of the nanocomposites. CO2
was also produced, along with H2, and the same has been confirmed by GC analysis results
(not shown). Molar ratio of H2/CO2 is close to 2.92± 0.1 suggesting a general mechanism of
one molecule each of CH3OH and H2O leading to 3H2 and CO2 on titania surfaces [5]. Fig. 6b
19
Figure 6:Photocatalytic H2 evolution activity of (a) 0.05Au-NT for 15 h, and (b) xAu-NT
nanocomposites in visible light with aqueous methanol solution. Amount of H2 evolution
reported is with 100 mg of catalyst. Dotted lines in (a) indicate evacuation after every 5 h.
shows the photocatalytic H2 evolution activity of all xAu-NT nanocomposites, and
0.05Au-NT shows the highest H2 evolution rate than other compositions. About 150 µmol
H2/h is generated with 100 mg of 0.05Au-NT. Pure TiO2-xNx shows the lowest activity
towards hydrogen generation (14 µmol/h)under similar conditions. Half to one order of
magnitude increase in H2 production observed with xAu-NT composites, compared to TiO2xNx,
demonstrates the critical role of nano Au clusters in solar light harvesting. The high rate of
H2 production is attributed to the electronic integration of nano Au clusters with disordered
mesoporous TiO2-xNx framework.
Charge carrier lifetime measurements were made to explore more on the electron injection
mechanism. We analyzed the emission decay of xAu-NT (x = 0.05 and 0.1), andTiO2-xNxand
the results are shown in Figure 7. Initially the materials were excited with 375 nm LED
source, and emission decay collected at 440 nm with 5000 counts. The emission decay was
best fit into biexponential decay for all materials. For TiO2-xNx, before gold loading, its
lifetime was measured to be 1 = 1.76 ps (1=1)and 2 = 925 ps (2 = 0). The major
contribution is from 1 species with lifetime of 1.76 ps. Upon introduction of nano gold on
20
Figure 7:Emission decay of xAu-NT and TiO2-xNx deposited as thin film on glass substrate.
The excitation and monitored emission wavelength are 375 and 440 nm, respectively. Kinetic
fit using biexponential decay was performed and the results are given in the text.
titania (0.05Au-NT), the lifetime of titania species increases to 1 = 21.1 ps (1 = 0.17)
and 2 = 11.4 ps (2 = 0.83). An order of magnitude increase in the lifetime of 1 from NT
(1.76 ps) to Au- NT (21.1 ps) underscores the importance of gold deposition onNT. This
result corroborates well with observed PL results, where the emission intensity from titania
features increased manifold upon nano gold introduction into titania matrix. Above
observation directly demonstrates the energy transfer from nanogold to titania in an effective
manner. Probably this is the first report to show such high lifetime with Au-TiO2 system.
Photoelectrochemical (PEC) measurements have been made under UV light with xAuNT nanocomposites and the results are shown in Figure 8a. Compared to P25 and TiO2xNx,xAu-NT
displays a four folds increase in photocurrent generation. An increase in
photocurrent under illumination at positive potentials is very typical for n-type conductivity.
21
A careful analysis of the PEC data reveals that the increase in PEC amplitude is the maximum
for 0.05 Au-NT than the other materials. This corroborates well with lifetime and PL
measurements as well as H2 production data.
Figure 8:(a) Photocurrent generated upon UV illumination is shown to demonstrate a four
folds increase in photocurrent density with xAu-NT nanocomposites than without gold
clusters. (b) Chronoamperometry measurements made at 0.5 V with xAu-NT and compared
with that of titania. A slow decay in photocurrent observed on light-off with xAu-NT
indicates a significantly longer life-time of photogenerated electrons.
Chronoamperometry measurements were made to explore the photocurrent generation
at different voltages and representative result is shown in Figure 8b. About five times increase
in photocurrent generation with 0.05 Au-NT, compared to TiO2 (P25), highlights an increase
in photon to current generation due to many factors such as decrease in defects, diffusion of
electrons to nano-Au clusters through mesoporous titania framework.
4. Conclusions
We successfully synthesized visible light active mesoporous nano Au-TiO2xNxcomposite
using a simple combustion synthesis protocol. Solution combustion synthesis at
4000C employed to prepare the present xAu-NT nanocomposite is predominantly kinetic
22
controlled process, due to short time of actual combustion under high flux of ammonia as a
result of urea decomposition. More number of visible light photons could be harvested by
SPR of Au clusters (than TiO2-xNx) and charge carrier mobility is enhanced due to disordered
mesoporosity along with electrically interconnected nanocrystallites ofTiO2-xNx. This aspect
was further enhanced by preparing well dispersed gold particles with p-n heterojunctions ,
which acts as charge separation centers. In contrast to 2 ps lifetime reported for charge
carriers in nanogold in the literature, we observed an order of magnitude higher lifetime (21.1
ps) with xAu-NT nanocomposites. This greatly helps for electron injection from gold to the
CB of titania. Electronic integration of nano gold with titania aspects were supported by XPS,
PL, life time and Raman spectral measurements. With (001) nanogold clusters binding to the
(101) TiO2 facets, a polarized pathway available through p-n heterojunction for charge
separation and utilization. Also the presence of disordered mesoporosity greatly reduces the
diffusion length of charge carriers and EINP helps for fast charge conduction. This added
advantage due to p3D nature of mesopores helps to utilize the charge carriers efficiently for
photocatalysis. Water splitting reaction under visible light with 0.05Au-NT generates
hydrogen at 1.5 mmol/h.g of catalyst, and indeed this is an order of magnitude higher than
that of TiO2-xNx. It is worth noting that a comparison of lifetime of charge carriers and
hydrogen generation under visible light varies linearly between 0.05Au-NT and TiO2-xNx.
We would like to emphasize that an order of magnitude higher lifetime of charge
carriers observed in the present study, in comparison to other reports, is partially attributed to
the material preparation by SCM method. Indeed different preparation procedures adopted by
different groups [25-29,45] could be a reason for the controversies in electron injection
mechanism. This is highly relevant, since surface of nanocomposites, especially Au-TiO2
interface, play a major role in optical properties and the nature of surface and interface
depends on the method of preparation. Indeed, two different preparation methods would lead
23
to significantly different surface characteristics. Nonetheless, different preparation methods
could also bring different electronic integration and hence different characteristics and
capabilities.
Acknowledgments
We thank Dr. K. Sreekumar and Mr. M.U. Sreekuttan for help in PEC measurements. We
acknowledge help from Drs.Jayakannan and ParthaHazra, IISER, Pune to conduct lifetime
measurements at ns and ps levels, respectively, and follow up discussions. KS, SRA and KR
thanks CSIR, New Delhi for senior research fellowship. Financial support from TAPSUN
programme under NWP0056 by CSIR, New Delhi is gratefully acknowledged. Part of the
work is supported by CSISR under CSC-0404.
24
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Supporting Information
(a)
(b)
(c)
Fig. SI-1. HRTEM image of (a) TiO2-xNx and (b and c) 0.10 Au-NT. TiO2 (101) facet was
observed abundantly.