Black Titania for Superior Photocatalytic Hydrogen Production and Photoelectrochemical Water Splitting

Prévia do material em texto

Black Titania for Superior Photocatalytic Hydrogen
Production and Photoelectrochemical Water Splitting
Guilian Zhu,[a] Hao Yin,[a] Chongyin Yang,[a] Houlei Cui,[a] Zhou Wang,[a] Jijian Xu,[a, c]
Tianquan Lin,*[a] and Fuqiang Huang*[a, b]
Introduction
Photocatalytic water splitting has attracted enormous attention
in recent decades for low-cost, environmentally friendly solar-
hydrogen production to support the future hydrogen econo-
my.[1] As perhaps the most important material in photocataly-
sis, TiO2 is the most attractive d-block transition metal func-
tional oxide for photocatalysis.[2] However, the light absorption
ratio and solar-to-hydrogen energy conversion efficiency is too
low for the technology to be economically sound. The main
barriers are the rapid recombination of photogenerated elec-
tron–hole pairs as well as the backward reaction and the poor
activation of TiO2 by visible light. Recently, some hydrogena-
tion methods were demonstrated to enable efficient visible-
light absorption, which result in the formation of black TiO2
and improved photocatalytic activity.[3] The improved solar ab-
sorption attributed to additional intermediate electronic states
induced by hydrogen insertion into the lattice of TiO2.
[3a] If we
consider the danger and difficulty to control a typical hydroge-
nation process (high temperature, high pressure, pure hydro-
gen, etc.), an alternative facile and effective method should be
developed.[4] Most previous studies have explored applications
in photocatalytic degradation,[3a–d, 5] and some materials exhibit
significantly worse photocatalytic activity because of their pro-
pensity to form bulk vacancy defects.[6] The photocatalytic ac-
tivities of black TiO2 reported previously are highly controver-
sial, especially in water decontamination.
Herein, we demonstrate a facile CaH2 reduction method to
synthesize uniform black TiO2 with improved photocatalytic
water decontamination and water-splitting performances at
low temperatures. The reducing agent, CaH2, has been used to
reduce TiO2-based materials to obtain TiAl alloy or Ti2O3.
[7] It is
active at considerably lower temperatures than that required
by conventional techniques. A reduction of transition metal
oxides by metal hydride at low temperatures has become
more useful to generate oxygen deficiencies than that ach-
ieved by standard methods.[8] CaH2 decomposes and releases
highly active hydrogen atoms at a low temperature. The re-
duced nanoparticles have crystalline-core/amorphous-shell
structures and exhibit a 2.4 times higher efficiency in photoca-
talytic water decontamination and a 4.5 times higher H2 pro-
duction rate in photoelectrochemical (PEC) water splitting than
commercial P25 TiO2.
Results and Discussion
The morphologies of reduced TiO2 were verified by TEM. The
pristine TiO2 nanocrystals are highly crystallized as illustrated
by the well-resolved lattice features shown in the high-resolu-
To utilize visible-light solar energy to meet environmental and
energy crises, black TiO2 as a photocatalyst is an excellent solu-
tion to clean polluted air and water and to produce H2. Herein,
black TiO2 with a crystalline core–amorphous shell structure re-
duced easily by CaH2 at 400 8C is demonstrated to harvest over
80 % solar absorption, whereas white TiO2 harvests only 7 %,
and possesses superior photocatalytic performances in the
degradation of organics and H2 production. Its water decon-
tamination is 2.4 times faster and its H2 production was
1.7 times higher than that of pristine TiO2. Photoelectrochemi-
cal measurements reveal that the reduced samples exhibit
greatly improved carrier densities, charge separation, and pho-
tocurrent (a 4.5-fold increase) compared with the original TiO2.
Consequently, this facile and versatile method could provide
a promising and cost-effective approach to improve the visi-
ble-light absorption and performance of TiO2 in photocatalysis.
[a] G. Zhu,+ Dr. H. Yin,+ Dr. C. Yang, H. Cui, Dr. Z. Wang, J. Xu, Dr. T. Lin,
Prof. Dr. F. Huang
CAS Key Laboratory of Materials for Energy Conversion
and State Key Laboratory of High Performance Ceramics and
Superfine Microstructure
Shanghai Institute of Ceramics, Chinese Academy of Sciences
1295 Dingxi Road, Shanghai, 200050 (P.R. China)
E-mail : huangfq@mail.sic.ac.cn
[b] Prof. Dr. F. Huang
National Laboratory for Molecular Sciences
and State Key Laboratory of Rare Earth Materials Chemistry and
Applications
College of Chemistry and Molecular Engineering
Peking University
Beijing 100871 (P.R. China)
[c] J. Xu
Key Laboratory of Silicon Materials
School of Materials Science and Engineering
Zhejiang University
Hangzhou 310027 (P.R. China)
[++] These authors contributed equally to this work.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500488.
ChemCatChem 2015, 7, 2614 – 2619 Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2614
Full PapersDOI: 10.1002/cctc.201500488
http://dx.doi.org/10.1002/cctc.201500488
tion transmission electron microscopy (HRTEM) image (Fig-
ure S1). The lattice distances of pristine and reduced TiO2 are
both 0.35 nm, which is typical for anatase (1 0 1). However,
after CaH2 reduction, the titania nanocrystal (TiO2¢x) consist of
a crystalline TiO2 as a core and a highly disordered surface
layer (�2 nm thick) as shown in Figure 1 a.
As the unique crystalline core–disordered shell structure was
formed, the TiO2¢x samples were gray colored after reduction
at 300 8C (denoted as T300) and black colored after reduction
at 400 8C (denoted as T400). Diffusive reflectance and absorb-
ance spectroscopy (Figure 1 b) reveal that the band gap of the
unmodified pristine TiO2 nanocrystals was approximately
3.2 eV. After CaH2 reduction, the TiO2¢x samples exhibit signifi-
cantly enhanced absorption of visible and near-infrared light.
The absorption edge of black TiO2¢x near l= 406 nm is derived
from the core of crystalline TiO2 that corresponds to �3.05 eV
(Figure S2). We calculated the solar absorption ratios from the
standard AM 1.5G solar spectrum and the absorption spectra
of our samples (Table S1). The solar absorption of T400 is
�81 % of the solar energy, much larger than that reported for
black TiO2 (<30 %)
[3a] and pristine TiO2 (�7 %).[9] The solar ab-
sorption of black TiO2 reduced at 500 8C (T500) is further im-
proved up to �90 % (Figure S3). These results demonstrate
that the reduced samples capture much more sunlight than
pristine P25 and a higher annealing temperature results in the
higher absorption of sunlight.
Interestingly, CaH2 can effectively reduce the rutile phase of
Degussa P25 samples that are mixed anatase and rutile
phases, which is confirmed by XRD. The ratio of the intensity
of the rutile (11 0) peak and the anatase (1 0 1) peak, IR(110)/IA(101),
decreases after CaH2 reduction (Figure 1 c). The ratios of the
rutile phase in pristine P25, T300, and T400, calculated by a nor-
malized RIR (Reference Intensity Ratio) method, are 15, 13, and
8 %, respectively. If the reduction temperature further increases
to 500 8C, the reduced sample consists of anatase TiO2 and
Magneli Ti4O7 without the rutile phase (Figure S4 a), which was
confirmed by HRTEM. Two sets of lattice fringes with spacings
of 0.28 and 0.37 nm are observed (Figure S4b), which corre-
spond to the (0 2 2) and (0¢1 3) atomic planes, respectively,
with an angle 89.48 for Magneli Ti4O7. Furthermore, there is no
amorphous layer on the surface of the crystalline particle of
T500. In other words, the samples maintain the mixed phase of
anatase and rutile below 500 8C, although the ratio of rutile de-
creases. However, if the reduction temperature reaches 500 8C,
the sample becomes a mixture of anatase and Ti4O7, which
means that Ti3++ appears as a result of the reduction.
Black TiO2 with a similar crystalline core–amorphous shell
structure has been synthesized by annealing TiO2 in H2 or by
H-plasma enhanced chemical vapor deposition(CVD) previous-
ly.[7c, 10] It was confirmed that the coloration and strong sunlight
absorption result from the presence of amorphous layers. Ad-
ditionally, the theoretical calculations and experimental studies
reported previously indicate that Ti¢H bonds that exist in the
amorphous layers could form the intermediate states in the
band gap.[4c, 10] To realize the transformation of the surface
chemical states during the CaH2 reduction process, X-ray pho-
toelectron spectroscopy (XPS) was conducted. No impurities
(such as Ca) are observed, except for the adventitious carbon
contaminant. The XPS Ti 2p spec-
tra of the three samples are
shown in Figure 1 d. There are
no Ti3++ signals observed on the
surface of the T300 and T400
samples. This is similar to black
TiO2 reduced by H2, which
showed no detectable signal of
Ti3++.[4a, b] Clearly, the coloration
and strong sunlight absorption
of the reduced samples are not
attributed to the Ti3++ on the sur-
face.
EPR spectra were recorded to
investigate the change of the
crystal lattice structure. The
three samples show different
EPR signals (Figure 2 a). Pristine
P25 does not show a distinct
EPR signal. In contrast, the re-
duced titania nanocrystals dis-
play a single symmetrical signal
with g = 2.003, which could be
assigned to superoxide radicals
(O2
¢) attached to the oxygen va-
cancies on the TiO2 surface.
[11] It
is well known that many oxygen
vacancies are introduced by an-
Figure 1. a) HRTEM images of T400 and b) UV/Vis absorption spectra, c) XRD patterns, and d) Ti 2p XPS spectra of
P25, T300, and T400. The insets in (b) are the standard AM 1.5G solar spectrum (ASTM G173-03) and a photograph
of the samples.
ChemCatChem 2015, 7, 2614 – 2619 www.chemcatchem.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2615
Full Papers
http://www.chemcatchem.org
nealing with ultra-high purity H2.
[12] The EPR signal increases
with elevated annealing temperature, which means that the
oxygen vacancies become denser as the annealing tempera-
ture increases. The superoxide radicals attached to the oxygen
vacancies form Ti4++¢O2¢ species, which facilitate the oxidative
processes and stabilize the separated e¢ and h++.[13] This sug-
gests that T400 has more Ti4++¢O2¢ species and is more strong-
ly oxidizing than T300.
Raman scattering was explored to further examine the struc-
tural properties of P25 and the reduced samples. All samples
exhibit similar peaks of typical anatase TiO2 (Figure 2 b). The
anatase structure is tetragonal and belongs to the space group
of I41/amd. Its six active Raman transitions (1A1g, 2B1g, and 3Eg)
lead to five peaks in the Raman spectra because the peaks of
the Alg (n1) and B1g (n2) modes located at approximately ñ=
519 cm¢1 are not distinguishable at room temperature. The in-
tensities and locations of the different Raman vibrational
modes are summarized in Table S2. Clearly, the Eg (n6) peak lo-
cated at ñ= 147 cm¢1 (Figure 2 b inset and Table S2) blueshifts
to ñ= 148, 152, and 157 cm¢1 in T300, T400, and T500, respec-
tively. The full width at half maximum intensity (FWHM) of the
Eg (n6) peaks broaden from 12 to 13, 17, and 22 cm
¢1. As is
well known, the blueshift and broadening of the Eg peak are
either because of nano-sized grains (<10 nm) or the shorten-
ing of the correlation length because of the presence of de-
fects.[14] In this case, the size of the crystallites from TEM analy-
sis are 20–30 nm, which rules out the occurrence of finite-size
effects and indicates the formation of defects. It is easy to un-
derstand that T500 possesses
Ti3++ defects because of the pres-
ence of Ti4O7. With regard to
T300 and T400, the amorphous
layers with an enormous
number of oxygen vacancies
lose lattice periodicity and break
the octahedral symmetry of TiO6
to result in the blueshift and
broadening in the Raman spec-
tra. This is consistent with the
conclusion drawn from the EPR
investigation.
The role of hydrogenation was
examined by NMR spectroscopy
(Figure 2 c). Both the reduced
and pristine P25 show a large
peak at d=++5.1 ppm, which
corresponds to the main type of
bridging proton.[15] The larger
line width in reduced samples
may be caused by the incorpora-
tion of hydrogen atoms at bridg-
ing sites produced in the amor-
phous layer during the reduction
process.[15, 16] Additionally, the
stronger peak at d=++5.1 ppm
suggests stronger hydrogen-
bonded bridging hydroxyl
groups in the reduced TiO2. In contrast to pristine P25, T300
and T400 show an additional sharp resonance at d =
++0.74 ppm on top of a broad base. This peak corresponds to
dynamic exchange between 1H (fast motion) in different envi-
ronments, which means rapid isotropic diffusion and rapid ex-
change.[4c, 17] The rapid exchange may enhance the transport of
photogenerated electrons, which reduces recombination in the
nanocrystal.
The measurement of photoluminescence (PL) emission was
performed to understand the efficiency of charge carrier trap-
ping, migration, transfer, and separation in the semiconductor
as PL emission results from the recombination of free carri-
ers.[18] A trend in the PL emission intensity is observed in the
following order: P25>T300>T400, which indicates that the re-
combination rate of photogenerated electrons and holes is
suppressed in the reduced samples. The hydrogenation of TiO2
can improve their conductivity and carrier density,[19] which fa-
cilitates the carrier transfer and separation and is the reason
why the PL intensities of the reduced samples decrease drasti-
cally. Generally, a low recombination rate of electrons and
holes is a prerequisite for high photocatalytic activity.
The photocatalytic activities of the reduced TiO2 were first
evaluated by the decomposition of a standard model contami-
nant methyl orange (MO) under simulated solar-light irradia-
tion in an aqueous solution. The direct decomposition of MO
in the absence of photocatalysts is not detected under light ir-
radiation in a control experiment. The degradation curves of
MO catalyzed by pristine and reduced TiO2 under simulated
Figure 2. a) EPR spectra, b) Raman spectra, c) 1H NMR spectra and d) PL spectra of the reference P25 and the re-
duced samples. The insets in (b) and (c) are partial magnifications of the region marked with a red border.
ChemCatChem 2015, 7, 2614 – 2619 www.chemcatchem.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2616
Full Papers
http://www.chemcatchem.org
solar-light irradiation are shown in Figure 3 a. For the T400
sample, the photodegradation is complete after 6 min. A trend
in the photocatalytic activity was observed in the following
order: T400>T300>P25, which is consistent with the trend of
enhanced solar absorption. The photodegradation rate con-
stants, calculated by a pseudo-first-order kinetics model, are
0.28, 0.33, and 0.68 min¢1 for P25, T300, and T400, respectively
(Figure S5 a). Correspondingly, the rates of T300 and T400 are
1.2 and 2.4 times higher than that of P25, respectively. The visi-
ble-light catalytic activity was further investigated to evaluate
the visible-light photocatalysis of the reduced samples. T400 is
demonstrated to deliver a greatly enhanced visible-light pho-
tocatalytic performance compared with pristine P25; and T300
also shows an increased activity in MO degradation under visi-
ble light (Figure S5 b). These results coincide well with the
characterization. Compared with the photocatalytic activity of
pristine TiO2, T500 demonstrates a slightly reduced activity be-
cause of the absence of the amorphous layer and the exis-
tence of the Ti4O7 phase (Figure S6 a).
The photocatalytic activities of our samples were further
evaluated by monitoring H2 evolution from water splitting (Fig-
ure 3 b). The H2 generation of T300 (5.2 mmol h
¢1 g¢1) is
1.7 times higher than that of pristine TiO2 (3.0 mmol h
¢1 g¢1)
and is in the group of the best semiconductors (5–
10 mmol h¢1 g¢1) for the photogeneration of H2. For the sample
reducedat an increased reduction temperature of 500 8C
(T500), the H2 generation decreases to nearly that of P25 be-
cause of its poor crystallinity and the absence of the amor-
phous layer (Figure S6 b). Clearly, low-temperature reduction
by CaH2 is an efficient way to improve the photocatalytic per-
formance of TiO2. T300 is the best catalyst for water splitting,
whereas T400 is the best for the photocatalytic degradation of
MO. H2 production is derived from the reducibility of the pho-
togenerated electrons in the conduction band of black TiO2,
whereas the degradation of MO is based on the oxidative
effect of the photogenerated holes in the valence band. From
our EPR spectroscopy results, it is clear that T400 has more
Ti4++¢O2¢ species, which facilitate the trapping of holes for oxi-
dation reactions on the surface. Combined with its better sun-
light absorption and low recombination, T400 presents
a higher efficiency in photocata-
lytic degradation than T300 and
P25. However, the formation of
molecular H2 and H2O are com-
petitive on the TiO2 surface.
[20] H2
production on T400 is less effi-
cient than H2O production as
O2
¢ on the surface can react
with H++ to form H2O. H2 produc-
tion is the result of a compromise
between the amount of O2
¢ spe-
cies and photogenerated elec-
trons, which is why T300 is
better than T400 and P25 for H2
production.
The excellent photocatalytic
activities of the CaH2-reduced
samples were further demonstrated by H2 generation by using
a PEC cell (Figure 4). To investigate the PEC properties, the pris-
tine and reduced TiO2 films were prepared by a spin-coating
method. The photocurrents of these films were recorded in
the dark and under illumination from a Xe lamp. The potential
was swept linearly at a scan rate of 10 mV s¢1 between ¢1.3
and 0.6 V vs. Ag/AgCl for a full scan and between ¢0.7 and
0.6 VAg/AgCl for visible light in 1 m NaOH electrolyte. Both photo-
electrodes show significantly low dark currents with respect to
their photocurrents (Figure 4 a), which indicates that no drastic
electrocatalytic water splitting occurs. Apparently, the photo-
current of T400 under illumination is 4.5 times higher than that
of pristine P25. Furthermore, the onset potential of the photo-
current shifts from ¢1.05 VAg/AgCl for P25 to ¢1.12 VAg/AgCl for
T400. The higher photocurrent density and the lower onset po-
tential for T400 indicate a more efficient charge separation and
transport compared to that of pristine P25. The efficiencies of
T300 and T500 in PEC H2 production are lower than that of
T400 because T400 possesses a higher crystallinity and a thicker
amorphous layer outside the nanoparticle (Figure S7). The im-
provement of the photocurrent density of T400 is shown in
linear-sweep voltammograms under visible light (Figure 4 b).
This enhancement is attributed to the improved visible-light
absorption of T400 and contributes to its overall photoelectro-
chemical performance.
To further investigate the photoresponses of our samples,
the PEC measurement was performed under illumination for
several 40 s light on/off cycles at 0.23 VAg/AgCl (Figure 4 c). Both
samples exhibit excellent photoresponses in chopped light
cycles for several on/off cycles. The observed steady-state pho-
tocurrent of T400 (1.99 mA cm¢2) is 4.6 times higher than that
of P25 (0.43 mA cm¢2), which indicates the higher amount of
photoinduced carriers and more efficient electron–hole separa-
tion in T400.
Electrochemical impedance spectroscopy (EIS) was conduct-
ed for P25 and T400. Mott–Schottky plots of both samples
show positive slopes characteristic of n-type semiconductor
behavior (Figure 4 d). T400 displays a smaller slope than P25,
which implies a much higher free charge carrier density in
T400. The carrier density can be calculated from the slope of
Figure 3. a) Photocatalytic decolorization of aqueous MO and b) photocatalytic water splitting in an aqueous solu-
tion of 20 % methanol in the presence of P25, T300, and T400 under simulated sunlight.
ChemCatChem 2015, 7, 2614 – 2619 www.chemcatchem.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2617
Full Papers
http://www.chemcatchem.org
Mott–Schottky plots using Equation (1).
Nd ¼ ð2=e0ee0Þ½dð1=C2Þ=dV¤¢1 ð1Þ
in which e0, e, e0, Nd, and d(1/C
2)/dV represent the electron
charge, the dielectric constant of TiO2 (31 for anatase),
[21] the
permittivity of a vacuum, the donor density, and the straight
slope, respectively. The calculated electron concentrations of
T400 and P25 are 1.54 Õ 1021 and 1.38 Õ 1019, respectively, which
indicates an exponential increase of the electron density and,
consequently, a large upward shift of the Fermi level in T400.
This shift can result in a significant bending of the band edge
at the surface of T400, which facilitates charge separation at
the shell–electrolyte interface. The improved charge transport
combined with the facilitated charge separation is responsible
for the much more efficient PEC water splitting and the greatly
enhanced H2 production of T400.
Conclusions
We reported a low-temperature reduction route by CaH2 to im-
prove the photocatalytic performance of TiO2. The treatment
decreases the ratio of rutile and induces defects and an amor-
phous shell in reduced TiO2, which enables it to exhibit differ-
ent colors and absorb more sunlight. The photocatalytic organ-
ic degradation and water-splitting performances of the low-
temperature reduced samples are both superior to those of
P25. In addition, the electron concentration and photoelectro-
chemical water-splitting proper-
ties are improved greatly after
CaH2 reduction. Through the al-
teration of the reduction tem-
perature, photocatalysts for spe-
cific applications (pollutant deg-
radation and water splitting,
etc.) can be produced controlla-
bly and easily. The low-tempera-
ture reduction method is prom-
ising for other oxide catalysts to
enhance their photocatalytic ac-
tivity.
Experimental Section
Synthesis
The preparation of the catalysts
was performed using CaH2 as a re-
ducing reagent and commercial
P25 TiO2 (Degussa) as precursor.
P25 (0.2 g) and CaH2 (0.4 g) were
ground finely in an Ar-filled glove-
box, sealed in an evacuated Pyrex
tube, and then reacted at 300–
500 8C for 5 h. After cooling to RT,
the obtained powders were
washed with 0.1 m HCl to remove
the residual CaH2 and the CaO by-
product.
Photocatalysis
H2 production by photocatalytic water splitting was performed by
using a top-irradiation Pyrex reaction cell. Photocatalyst powder
(100 mg) was dispersed by ultrasonication for 2 min into an aque-
ous solution (200 mL) that contained methanol (40 mL) as the sac-
rificial reagent. Pt (0.5 wt %) was loaded in situ by the impregna-
tion of H2PtCl6 (0.05 mL, 10 g L
¢1) in the suspension. Then the sus-
pension was degassed thoroughly with pure N2 and irradiated by
a 300 W Xe lamp. The temperature of the reaction solution was
maintained at RT by a flow of water. The amount of H2 evolved
was determined by GC (Shanghai, GC-7900, thermal conductivity
detector (TCD), N2 carrier). The photocatalytic activities were com-
pared based on the average H2 evolution rate in the first 5 h.
The photodegradation of MO was performed in an aqueous solu-
tion under irradiation from a 300 W iodine gallium lamp. Typically,
the as-prepared samples (100 mg) were dispersed in a Pyrex glass
reactor that contained MO solution (100 mL, 10 mg L¢1). Before irra-
diation, the suspensions were stirred in the dark for 30 min to
reach the adsorption/desorption equilibrium. At given time inter-
vals, portions of the suspension (�5 mL) were taken for analysis
after centrifugation. The visible-light-driven photodegradation of
MO was conducted under the same condition but using a 420 nm
filter to cut off the UV light and allow only visible light (>420 nm)
to pass through.
PEC measurements were performed by using a conventional three-
electrode electrochemical workstation (CHI600B, CH Instruments),
in whichthe TiO2 film on an fluorine-doped tin oxide (FTO) sub-
Figure 4. a) Linear-sweep voltammograms collected under AM 1.5 solar spectrum simulation using a three-elec-
trode setup (P25 or T400 working, Pt counter, Ag/AgCl reference electrodes, scan rate of 10 mV s¢1) in 1 m NaOH
electrolyte (pH 13.6). b) Linear-sweep voltammograms collected under visible light, the UV light was eliminated by
a 420 nm cutoff glass light filter. c) Transient photocurrent responses of P25 or T400 samples at 0.23 V vs. Ag/
AgCl. d) Mott–Schottky plots collected at a frequency of 1 kHz in dark.
ChemCatChem 2015, 7, 2614 – 2619 www.chemcatchem.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2618
Full Papers
http://www.chemcatchem.org
strate, a Pt wire, and an Ag/AgCl (KCl saturated) electrode were
used as the working, counter, and reference electrodes, respective-
ly. A 1.0 m NaOH aqueous solution (pH 13.6) was used as the sup-
porting electrolyte to maintain the stability of the film. A 150 W Xe
lamp was used as the light source to simulate the sunlight irradia-
tion. A set of linear sweeps and transient photocurrent responses
were recorded in the dark and under illumination. Mott–Schottky
plots were derived from impedance–potential tests conducted at
a frequency of 100 Hz in the dark.
Characterization
UV/Vis diffuse reflectance spectra were obtained by using a UV/Vis
spectrometer (Hitachi U-4100) using BaSO4 as a reference. XRD
patterns were recorded by using a Bruker D8 advanced diffractom-
eter with CuKa radiation. The morphology was characterized by
TEM (JEOL JEM-2100). XPS experiments were performed by using
a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with MgKa
radiation (hn= 1253.6 eV). EPR spectra were collected by using
a Bruker EMX-8 spectrometer at 9.0 GHz and at 300 K. Raman spec-
tra were collected by using a Thermal Dispersive Spectrometer
using a laser with an excitation wavelength of 532 nm at a laser
power of 10 mW. Solid-state 1H magic-angle spinning (MAS) NMR
spectra were acquired by using a 600 MHz Bruker spectrometer. PL
spectra were measured at RT by using a fluorescence spectropho-
tometer (F-4600, Hitachi, Japan) with an excitation wavelength of
320 nm.
Acknowledgements
This work was financially supported by NSF of China (Grant Nos.
91122034, 51125006, 61376056, and 51402336), and Science and
Technology Commission of Shanghai (Grants 13JC1405700 and
14YF1406500).
Keywords: electrochemistry · hydrogen · photochemistry ·
titanium · water splitting
[1] a) M. R. Hoffmann, S. T. Martin, W. Y. Choi, D. W. Bahnemann, Chem. Rev.
1995, 95, 69 – 96; b) A. L. Linsebigler, G. Q. Lu, J. T. Yates, Chem. Rev.
1995, 95, 735 – 758; c) Z. Y. Yin, Z. Wang, Y. P. Du, X. Y. Qi, Y. Z. Huang, C.
Xue, H. Zhang, Adv. Mater. 2012, 24, 5374 – 5378.
[2] a) P. V. Kamat, J. Phys. Chem. C 2012, 116, 11849 – 11851; b) A. Kubacka,
M. Fernandez-Garcia, G. Colon, Chem. Rev. 2012, 112, 1555 – 1614.
[3] a) X. B. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331, 746 – 750;
b) G. M. Wang, H. Y. Wang, Y. C. Ling, Y. C. Tang, X. Y. Yang, R. C. Fitzmor-
ris, C. C. Wang, J. Z. Zhang, Y. Li, Nano Lett. 2011, 11, 3026 – 3033; c) H.
Yin, T. Lin, C. Yang, Z. Wang, G. Zhu, T. Xu, X. Xie, F. Huang, M. Jiang,
Chem. Eur. J. 2013, 19, 13313 – 13316; d) G. Zhu, T. Lin, X. Lì, W. Zhao, C.
Yang, Z. Wang, H. Yin, Z. Liu, F. Huang, J. Lin, J. Mater. Chem. A 2013, 1,
9650 – 9653; e) H. Pan, Y.-W. Zhang, V. B. Shenoy, H. Gao, J. Phys. Chem.
C 2011, 115, 12224 – 12231; f) S. Hoang, S. P. Berglund, N. T. Hahn, A. J.
Bard, C. B. Mullins, J. Am. Chem. Soc. 2012, 134, 3659 – 3662; g) C. Sun, Y.
Jia, X.-H. Yang, H.-G. Yang, X. Yao, G. Q. Lu, A. Selloni, S. C. Smith, J.
Phys. Chem. C 2011, 115, 25590 – 25594; h) Z. Wang, C. Yang, T. Lin, H.
Yin, P. Chen, D. Wan, F. Xu, F. Huang, J. Lin, X. Xie, M. Jiang, Adv. Funct.
Mater. 2013, 23, 5444 – 5450.
[4] a) A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli,
C. L. Bianchi, R. Psaro, V. Dal Santo, J. Am. Chem. Soc. 2012, 134, 7600 –
7603; b) T. Xia, C. Zhang, N. A. Oyler, X. B. Chen, Adv. Mater. 2013, 25,
6905 – 6910; c) T. Xia, W. Zhang, W. J. Li, N. A. Oyler, G. Liu, X. B. Chen,
Nano Energy 2013, 2, 826 – 835.
[5] a) W. D. Zhu, C. W. Wang, J. B. Chen, D. S. Li, F. Zhou, H. L. Zhang, Nano-
technology 2012, 23, 455204 – 455209; b) Y. H. Hu, Angew. Chem. Int. Ed.
2012, 51, 12410 – 12412; Angew. Chem. 2012, 124, 12579 – 12581; c) Z. K.
Zheng, B. B. Huang, J. B. Lu, Z. Y. Wang, X. Y. Qin, X. Y. Zhang, Y. Dai,
M. H. Whangbo, Chem. Commun. 2012, 48, 5733 – 5735.
[6] T. Leshuk, R. Parviz, P. Everett, H. Krishnakumar, R. A. Varin, F. Gu, ACS
Appl. Mater. Interfaces 2013, 5, 1892 – 1895.
[7] a) G. Guangsi, C. Yongjun, H. Xiaomei, Rare Metal Mater. Eng. 2008, 37,
1153 – 1156; b) S. Tominaka, Inorg. Chem. 2012, 51, 10136 – 10140; c) X.
Chen, L. Liu, F. Huang, Chem. Soc. Rev. 2015, 44, 1861 – 1885.
[8] a) Y. Tsujimoto, C. Tassel, N. Hayashi, T. Watanabe, H. Kageyama, K. Yoshi-
mura, M. Takano, M. Ceretti, C. Ritter, W. Paulus, Nature 2007, 450,
1062 – 1068; b) C. Tassel, T. Watanabe, Y. Tsujimoto, N. Hayashi, A. Kitada,
Y. Sumida, T. Yamamoto, H. Kageyama, M. Takano, K. Yoshimura, J. Am.
Chem. Soc. 2008, 130, 3764 – 3765; c) M. Hayward, Chem. Mater. 2006,
18, 321 – 327.
[9] X. Chen, S. S. Mao, Chem. Rev. 2007, 107, 2891 – 2959.
[10] T. Xia, X. Chen, J. Mater. Chem. A 2013, 1, 2983 – 2989.
[11] a) T. Xia, W. Zhang, J. B. Murowchick, G. Liu, X. Chen, Adv. Energy Mater.
2013, 3, 1516 – 1523; b) J. Soria, J. Sanz, I. Sobrados, J. M. Coronado, F.
Fresno, M. D. Hern�ndez-Alonso, Catal. Today 2007, 129, 240 – 246; c) S.
Maurelli, M. Vishnuvarthan, G. Berlier, M. Chiesa, Phys. Chem. Chem.
Phys. 2012, 14, 987 – 995.
[12] X. Jiang, Y. Zhang, J. Jiang, Y. Rong, Y. Wang, Y. Wu, C. Pan, J. Phys.
Chem. C 2012, 116, 22619 – 22624.
[13] a) M. D’Arienzo, J. Carbajo, A. Bahamonde, M. Crippa, S. Polizzi, R.
Scotti, L. Wahba, F. Morazzoni, J. Am. Chem. Soc. 2011, 133, 17652 –
17661; b) R. Scotti, I. R. Bellobono, C. Canevali, C. Cannas, M. Catti, M.
D’Arienzo, A. Musinu, S. Polizzi, M. Sommariva, A. Testino, F. Morazzoni,
Chem. Mater. 2008, 20, 4051 – 4061.
[14] a) S. K. Gupta, R. Desai, P. K. Jha, S. Sahoo, D. Kirin, J. Raman Spectrosc.
2010, 41, 350 – 355; b) K.-R. Zhu, M.-S. Zhang, Q. Chen, Z. Yin, Phys. Lett.
A 2005, 340, 220 – 227.
[15] M. Crocker, R. H. M. Herold, A. E. Wilson, M. Mackay, C. A. Emeis, A. M.
Hoogendoorn, J. Chem. Soc. Faraday Trans. 1996, 92, 2791 – 2798
[16] X. B. Chen, L. Liu, Z. Liu, M. A. Marcus, W. C. Wang, N. A. Oyler, M. E.
Grass, B. H. Mao, P. A. Glans, P. Y. Yu, J. H. Guo, S. S. Mao, Sci. Rep. 2013,
3, 1510.
[17] P. Jonsen, Catal. Lett. 1989, 2, 345 – 349.
[18] a) J. G. Yu, L. F. Qi, M. Jaroniec, J. Phys. Chem. C 2010, 114, 13118 –
13125; b) F. B. Li, X. Z. Li, Chemosphere 2002, 48, 1103 – 1111.
[19] W. Q. Han, Y. Zhang, Appl. Phys. Lett. 2008, 92, 203117.
[20] C. B. Xu, W. S. Yang, Q. Guo, D. X. Dai, M. D. Chen, X. M. Yang, J. Am.
Chem. Soc. 2013, 135, 10206 – 10209.
[21] H. Cui, W. Zhao, C. Yang, H. Yin, T. Lin, Y. Shan, Y. Xie, H. Gu, F. Huang, J.
Mater. Chem. A 2014, 2, 8612 – 8616.
Received: May 2, 2015
Published online on August 6, 2015
ChemCatChem 2015, 7, 2614 – 2619 www.chemcatchem.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2619
Full Papers
http://dx.doi.org/10.1021/cr00033a004
http://dx.doi.org/10.1021/cr00033a004
http://dx.doi.org/10.1021/cr00033a004
http://dx.doi.org/10.1021/cr00033a004
http://dx.doi.org/10.1021/cr00035a013
http://dx.doi.org/10.1021/cr00035a013
http://dx.doi.org/10.1021/cr00035a013
http://dx.doi.org/10.1021/cr00035a013
http://dx.doi.org/10.1002/adma.201201474
http://dx.doi.org/10.1002/adma.201201474
http://dx.doi.org/10.1002/adma.201201474
http://dx.doi.org/10.1021/jp305026h
http://dx.doi.org/10.1021/jp305026h
http://dx.doi.org/10.1021/jp305026h
http://dx.doi.org/10.1021/cr100454n
http://dx.doi.org/10.1021/cr100454n
http://dx.doi.org/10.1021/cr100454n
http://dx.doi.org/10.1126/science.1200448
http://dx.doi.org/10.1126/science.1200448http://dx.doi.org/10.1126/science.1200448
http://dx.doi.org/10.1021/nl201766h
http://dx.doi.org/10.1021/nl201766h
http://dx.doi.org/10.1021/nl201766h
http://dx.doi.org/10.1002/chem.201302286
http://dx.doi.org/10.1002/chem.201302286
http://dx.doi.org/10.1002/chem.201302286
http://dx.doi.org/10.1039/c3ta11782k
http://dx.doi.org/10.1039/c3ta11782k
http://dx.doi.org/10.1039/c3ta11782k
http://dx.doi.org/10.1039/c3ta11782k
http://dx.doi.org/10.1021/jp202385q
http://dx.doi.org/10.1021/jp202385q
http://dx.doi.org/10.1021/jp202385q
http://dx.doi.org/10.1021/jp202385q
http://dx.doi.org/10.1021/ja211369s
http://dx.doi.org/10.1021/ja211369s
http://dx.doi.org/10.1021/ja211369s
http://dx.doi.org/10.1021/jp210472p
http://dx.doi.org/10.1021/jp210472p
http://dx.doi.org/10.1021/jp210472p
http://dx.doi.org/10.1021/jp210472p
http://dx.doi.org/10.1002/adfm.201300486
http://dx.doi.org/10.1002/adfm.201300486
http://dx.doi.org/10.1002/adfm.201300486
http://dx.doi.org/10.1002/adfm.201300486
http://dx.doi.org/10.1021/ja3012676
http://dx.doi.org/10.1021/ja3012676
http://dx.doi.org/10.1021/ja3012676
http://dx.doi.org/10.1002/adma.201303088
http://dx.doi.org/10.1002/adma.201303088
http://dx.doi.org/10.1002/adma.201303088
http://dx.doi.org/10.1002/adma.201303088
http://dx.doi.org/10.1016/j.nanoen.2013.02.005
http://dx.doi.org/10.1016/j.nanoen.2013.02.005
http://dx.doi.org/10.1016/j.nanoen.2013.02.005
http://dx.doi.org/10.1088/0957-4484/23/45/455204
http://dx.doi.org/10.1088/0957-4484/23/45/455204
http://dx.doi.org/10.1088/0957-4484/23/45/455204
http://dx.doi.org/10.1088/0957-4484/23/45/455204
http://dx.doi.org/10.1002/anie.201206375
http://dx.doi.org/10.1002/anie.201206375
http://dx.doi.org/10.1002/anie.201206375
http://dx.doi.org/10.1002/anie.201206375
http://dx.doi.org/10.1002/ange.201206375
http://dx.doi.org/10.1002/ange.201206375
http://dx.doi.org/10.1002/ange.201206375
http://dx.doi.org/10.1039/c2cc32220j
http://dx.doi.org/10.1039/c2cc32220j
http://dx.doi.org/10.1039/c2cc32220j
http://dx.doi.org/10.1021/am302903n
http://dx.doi.org/10.1021/am302903n
http://dx.doi.org/10.1021/am302903n
http://dx.doi.org/10.1021/am302903n
http://dx.doi.org/10.1021/ic300557u
http://dx.doi.org/10.1021/ic300557u
http://dx.doi.org/10.1021/ic300557u
http://dx.doi.org/10.1039/C4CS00330F
http://dx.doi.org/10.1039/C4CS00330F
http://dx.doi.org/10.1039/C4CS00330F
http://dx.doi.org/10.1038/nature06382
http://dx.doi.org/10.1038/nature06382
http://dx.doi.org/10.1038/nature06382
http://dx.doi.org/10.1038/nature06382
http://dx.doi.org/10.1021/ja800415d
http://dx.doi.org/10.1021/ja800415d
http://dx.doi.org/10.1021/ja800415d
http://dx.doi.org/10.1021/ja800415d
http://dx.doi.org/10.1021/cm051659c
http://dx.doi.org/10.1021/cm051659c
http://dx.doi.org/10.1021/cm051659c
http://dx.doi.org/10.1021/cm051659c
http://dx.doi.org/10.1021/cr0500535
http://dx.doi.org/10.1021/cr0500535
http://dx.doi.org/10.1021/cr0500535
http://dx.doi.org/10.1039/c3ta01589k
http://dx.doi.org/10.1039/c3ta01589k
http://dx.doi.org/10.1039/c3ta01589k
http://dx.doi.org/10.1002/aenm.201300294
http://dx.doi.org/10.1002/aenm.201300294
http://dx.doi.org/10.1002/aenm.201300294
http://dx.doi.org/10.1002/aenm.201300294
http://dx.doi.org/10.1016/j.cattod.2007.08.001
http://dx.doi.org/10.1016/j.cattod.2007.08.001
http://dx.doi.org/10.1016/j.cattod.2007.08.001
http://dx.doi.org/10.1039/C1CP22897H
http://dx.doi.org/10.1039/C1CP22897H
http://dx.doi.org/10.1039/C1CP22897H
http://dx.doi.org/10.1039/C1CP22897H
http://dx.doi.org/10.1021/jp307573c
http://dx.doi.org/10.1021/jp307573c
http://dx.doi.org/10.1021/jp307573c
http://dx.doi.org/10.1021/jp307573c
http://dx.doi.org/10.1021/cm800465n
http://dx.doi.org/10.1021/cm800465n
http://dx.doi.org/10.1021/cm800465n
http://dx.doi.org/10.1016/j.physleta.2005.04.008
http://dx.doi.org/10.1016/j.physleta.2005.04.008
http://dx.doi.org/10.1016/j.physleta.2005.04.008
http://dx.doi.org/10.1016/j.physleta.2005.04.008
http://dx.doi.org/10.1039/ft9969202791
http://dx.doi.org/10.1039/ft9969202791
http://dx.doi.org/10.1039/ft9969202791
http://dx.doi.org/10.1007/BF00768176
http://dx.doi.org/10.1007/BF00768176
http://dx.doi.org/10.1007/BF00768176
http://dx.doi.org/10.1021/jp104488b
http://dx.doi.org/10.1021/jp104488b
http://dx.doi.org/10.1021/jp104488b
http://dx.doi.org/10.1016/S0045-6535(02)00201-1
http://dx.doi.org/10.1016/S0045-6535(02)00201-1
http://dx.doi.org/10.1016/S0045-6535(02)00201-1
http://dx.doi.org/10.1063/1.2937152
http://dx.doi.org/10.1021/ja4030963
http://dx.doi.org/10.1021/ja4030963
http://dx.doi.org/10.1021/ja4030963
http://dx.doi.org/10.1021/ja4030963
http://dx.doi.org/10.1039/c4ta00176a
http://dx.doi.org/10.1039/c4ta00176a
http://dx.doi.org/10.1039/c4ta00176a
http://dx.doi.org/10.1039/c4ta00176a
http://www.chemcatchem.org