Letter
pubs.acs.org/NanoLett
IR-Driven Photocatalytic Water Splitting with WO2−NaxWO3 Hybrid
Conductor Material
Guanwei Cui, Wen Wang, Mingyue Ma, Junfeng Xie, Xifeng Shi, Ning Deng, Jianping Xin, and Bo Tang*
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for
Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong
Provincial Key Laboratory of Clean Production of Fine Chemicals, Shandong Normal University, Shandong 250014, China
S Supporting Information
*
ABSTRACT: An IR-driven photocatalytic water splitting system based on WO2−
NaxWO3 (x > 0.25) hybrid conductor materials was established for the first time; this
system can be directly applied in seawater. The WO2−NaxWO3 (x > 0.25) hybrid
conductor material was readily prepared by a high-temperature reduction process of
semiconductor NaxWO3 (x < 0.25) nanowire bundles. A novel ladder-type carrier
transfer process is suggested for the established IR-driven photocatalytic water splitting
system.
KEYWORDS: Photocatalysis, water splitting, infrared light, tungsten dioxide, sodium tungsten bronzes
H
such materials are seldom studied in photocatalysis,20 and
direct water splitting using IR radiation has not been achieved.
Among the oxide conductor materials, WO2 and NaxWO3 (x
> 2.5) are two key tungstic metallic oxides.21,22 Their energy
band structures are shown in Figure 1 (the conduction band
ydrogen generated by photocatalytic water splitting is
considered a clean and renewable energy source to
fundamentally replace traditional fossil-fuel energy sources.1 In
this research field, one of the key issues is to develop
photocatalysts with high solar energy utilization efficiency.
Since the first reported TiO2 semiconductor material that
performed water splitting under ultraviolet light was reported in
1972,2 various semiconductor photocatalysts that function
under visible-light irradiation have been developed through
using narrow-gap semiconductors,3 reducing the band gap,4,5
decorating with dyes,6−8 and constructing heterostructure
materials or Z-scheme photocatalytic systems.9−14 Compared
with UV and visible light, infrared light has a strong penetration
ability, which guarantees sufficient contact with substrates for a
solid−liquid photocatalytic reaction system. However, the
utilization of IR light with low photonic energy, which
constitutes almost half of solar energy, is still a challenge in
the solar energy conversion research field15−17 and especially in
the photocatalytic water splitting research field. This challenge
stems from the difficulty in simultaneously achieving sufficient
IR response and suitable redox potentials for water splitting
using the aforementioned methods. The direct utilization of IR
light has not been achieved with a semiconductor material. To
overcome these restrictions, new kinds of materials or methods
are needed.18,19
Compared with semiconductor materials, metallic oxide
conductor materials have special partially filled band structures,
strong light-absorbing ability, and fast carrier-transfer characteristics. Particularly noteworthy are their gapless energy band
structures and high carrier mobility, which make them potential
materials for utilizing infrared light in photocatalysis. However,
© 2015 American Chemical Society
Figure 1. Energy-level diagrams of WO2−NaxWO3.
and valence band of WO2 are denoted as CB1 and VB1,
respectively; the conduction band and the intrinsic valence
band of NaxWO3 are denoted as CB2 and VB2−0, respectively;
the new energy level produced by Na doping of NaxWO3 is
denoted as VB2−1). According to the energy band structure of
WO2,23 its valence-band top and conduction-band minimum
are located at −0.1 eV and −0.7 eV, respectively. Thus, the
energy band structure of WO2 is suitable for water reduction
but not for water oxidation. The energy band structure of
Received: April 23, 2015
Revised: September 19, 2015
Published: October 5, 2015
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DOI: 10.1021/acs.nanolett.5b01581
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Nano Letters
WO2−NaxWO3 hybrid conductor material at 900 °C; this
material was desired for use in the IR-driven photocatalytic
water splitting process. Here, the raw semiconductor NaxWO3
(x ≈ 0.18) material is denoted as NaHTB-180, and the asprepared samples obtained at 500 °C, 600 °C, 700 °C, 800 °C,
900 °C, and 1000 °C are denoted as NaHTB-C500, NaHTBC600, NaHTB-C700, NaHTB-C800, NaHTB-C900, and
NaHTB-C1000, respectively.
This phase-change process at different calcination temperatures was demonstrated by XRD (Figure S1), XPS (Figure S2)
and Raman spectroscopy (Figure S3); the results are detailed in
the Supporting Information. The hexagonal-phase semiconductor NaHTB-180 was completely reconstructed into the
cubic-phase structure at 900 °C for NaHTB-C900 (JCPDS
Card: 75-0238), with the sodium cations migrating from the
tunnel of the initial hexagonal phase into the center of the cube
matrix.28 Meanwhile, a metallic WO2 phase with a monoclinic
structure (JCPDS Card: 86-0134) emerged when the temperature exceeded 800 °C. During the calcination process, the
nanowire bundles (Figure 2A) were reconstructed into micronsized crystals (Figure 2B). The distribution of Na, W, and the
residual C elements in the prepared hybrid material is shown in
the EDS mapping images (Figure 2C,D,E). The residual
reductive agents and carbon particles mostly exist independently or on the surface of the micron-sized crystals. A distinct
phases edge ascribed to the WO2 and NaxWO3 crystal phase
with different crystal lattice distances was observed in the
HRTEM images of the NaHTB-C900 sample and was further
confirmed by the different SAED patterns (Figure 2F).29
The as-prepared products exhibited a color change from
yellowish-green to gray, blue, deep-violet and dark-red with
increasing calcination temperature (Figure 3A), which co-
metallic NaxWO3,24 in contrast to that of semiconductor WO3,
has the same valence-band top located at 2.7 eV, which is
suitable for water oxidation. A new energy level located in the
range from 0.7 to 2.0 eV, produced by Na doping, can be
signed as another valence band, which is partially filled and
greatly decreases the energy band gaps. We assume that, if these
two kinds of conductor materials can be coupled, the resulting
hybrid conductor material will exhibit excellent photocatalytic
water splitting ability under low-energy-photon irradiation
because of the special narrow energy band structures and high
carrier mobility of the conductor materials. In this paper, a
WO2−NaxWO3 hybrid conductor material was prepared via the
high-temperature reduction of semiconductor NaxWO3 (x <
0.25) nanowire bundles (Scheme 1). The hybrid material
Scheme 1. Illustration of the Process Used To Prepare
WO2−NaxWO3
exhibits amazing IR-driven photocatalytic water splitting ability
and can be applied to the splitting of seawater. A novel laddertype carrier transfer process is suggested for the IR-driven
photocatalytic water splitting system.
Sodium tungsten bronzes (NaxWO3) are semiconducting at
low values of x but metallic for x values greater than
approximately 0.25.25 In our previous work, semiconducting,
hexagonal-phase, microcrystalline sodium tungsten bronze
nanowire bundles were prepared via a facile hydrothermal
synthesis (Figure 2A).26 The sodium cations are situated in the
hexagonal channel, but they are too small to stabilize the
sodium tungsten bronze framework at higher temperatures.27
We observed that, when the material was calcined under high
temperatures in the presence of a reducing agent such as carbon
or H2, chemical reduction and an electronic conductivity
transfer process both occurred as a result of the formation of a
Figure 3. (A) Color change for the different samples obtained under
increased calcination temperatures. (B) UV−vis-IR diffuse reflectance
spectra of NaHTB-180 and NaHTB-C900. (C) Photocurrent density
of NaHTB-C900 irradiated by 980 nm laser light.
incides with the color changes related to the increase in the xvalue of NaxWO3 from zero to unity.28 A strong-colored
material usually indicates high light absorbance ability. The
UV−vis−IR absorbance spectrum shows that WO2−NaxWO3
exhibits broad spectral absorption that includes the adsorption
of far-IR light (Figure 3B). The absorption energy gap
estimated from an absorption tail to 1129 nm is approximately
1.10 eV, which indicates that the material has potential
photocatalytic activity under IR irradiation. The electronic
conductivity tests showed that the as-prepared WO2−NaxWO3
hybrid material still exhibited the same expected high electronic
Figure 2. (A) SEM images of the raw material, NaHTB-180. (B) SEM
images of NaHTB-C900. (C, D, E) EDS mapping images of NaHTBC900 for Na, W, and the residual C elements, respectively. (F)
HRTEM and SAED patterns of NaHTB-C900.
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absorbed onto the surface of the photocatalysts to form steady
peroxide complexes, preventing O2 liberation. This phenomenon has been observed in many water splitting systems.32−34
Tungsten oxides suffer heavily from this problem.35−37 Herein,
although no O2 was detected in this photocatalytic water
splitting system, intermediate ·OH directly photogenerated
from water or from the decomposition of the attached H2O2
was detected by the fluorescence probe terephthalic acid
(TA).38 The fluorescence spectra of TAOH, which is produced
through the oxidation of TA by ·OH, exhibited a peak at 426
nm; a linear relationship was observed between the
fluorescence intensity and irradiation time, indicating the
presence of ·OH (Figure S7). We further used an isotopic
tracing method to verify that the detected ·OH was generated
from water splitting. When we used H218O as the reagent under
the same water splitting conditions, TA18OH (mass 183) was
detected by mass spectrometry (MS; Figure S8), which
indicated that the intermediate oxidation product ·OH was
generated from water splitting, not from the residual O2 in the
reaction system.
The addition of Pt nanoparticles or an O2-evolution
cocatalyst (OEC) such as carbon dots, manganese, and cobalt
based cocatalysts to the semiconductors has been reported to
facilitate the release of H2O2 or O2.31,33,39 Herein, we observed
that when we loaded the photocatalyst with Pt nanoparticles
(Figure S9), we detected H2O2 produced from water splitting
in the reaction solutions by UV−vis spectroscopy using otolidine as a special peroxide indicator (Figure S10).33 In the
control experiments, no H2 or water oxidation species (·OH or
H2O2) was detected in the aforementioned test, which indicates
that the water splitting process occurred during the photocatalytic process. We will study the improvement of the
photocatalytic water splitting efficiency induced by the addition
of Pt nanoparticles or other O2-evolution cocatalysts in our
future work.
Although tungsten oxides suffer from the produced
intermediate active oxygen, the WO2−Na xWO3 hybrid
conductor material exhibited a long lifetime, maintaining the
same photoactivity after 90 h (Figure S11). This long lifetime
may be due to the low reaction rates and the presence of special
inner redox processes under light irradiation, as discussed in the
following section.
The IR-driven photocatalytic water splitting ability of the
WO2−NaxWO3 hybrid conductor material was suggested to
owe to the formation of special ladder-type narrow energy band
structures originating from the overlapping bands between the
metallic WO 2 and Na xWO 3 (Figure 1),23,24 which is
demonstrated by the valence-band energy XPS spectrum and
upconversion luminescence. As shown in Figure 5, the valence-
conductivity character as WO2 (Figure S4). To determine
whether the electrons of the conductive metallic oxides could
be excited between energy bands as semiconductors, photocurrent measurements were performed under irradiation by a
Xe lamp and a 980 nm laser (wavelength range 808−1070 nm).
As shown in Figure 3C and Figure S5, a distinct photocurrent
was observed for the WO2−NaxWO3 hybrid conductor
materials, even under IR irradiation, which demonstrates that,
although metallic oxide conductors different from semiconductors, their electrons can still be excited and applied to
photocatalysis.
The photocatalytic properties of these metallic oxides were
investigated for the photocatalytic water splitting process.
Among the hybrid materials prepared under different temperatures, NaHTB-C900, which exhibited a violet color and was
composed of 37.5% WO2 and 62.5% NaxWO3 (x ≈ 0.54, as
estimated from the XRD data), exhibited the strongest H2
evolution ability under solar light mimicked by a 1000-W Xe
lamp (Figure 4A), including the IR light from a 980 nm laser
Figure 4. (A) Photocatalytic activity of different samples for water
splitting. (B) Photocatalytic H2 evolution rates for NaHTB-C900.
(Figure 4B). Moreover, only with the appearance of the WO2
phase at temperatures beyond 800 °C did the material begin to
exhibit H2 evolution ability. For comparison, little H2 evolution
was observed when metallic WO2 was used alone. The possible
deep reductive species such as W and WC were not beneficial
for H2 evolution. Therefore, the photocatalytic water splitting
by the as-prepared WO2−NaxWO3 was due to the synergistic
effect of WO2 and NaxWO3. The solar-to-hydrogen efficiency
obtained from Xe-lamp irradiation and 980 nm laser irradiation,
under the assumption that all incident light was absorbed by
the photocatalyst, was 0.25% and 0.17% per gram of NaHTBC900, respectively.30 The TON of the 12 h photocatalytic
reaction is 2771 and 27 762 for 980 nm laser irradiation and for
the solar spectrum mimicked by a 1000-W Xe lamp,
respectively.
Pure water is the ideal reaction conditions for the practical
application of the photocatalytic water splitting process. Herein,
the optimal pH range was between 6 and 8 for the water
splitting process by NaHTB-C900 (Figure S6), which is
suitable for pure water. Sea water (pH = 6.50), Earth’s largest
water source, can be directly decomposed for the first time,
showing almost the same H2 evolution efficiency order as that
obtained from the optimal buffer solution conditions (pH =
6.0), which is significance for the future final practical
application of this technology.
Whereas this system readily produces hydrogen under
illumination, the simultaneous liberation of oxygen is not
observed, which is ascribed to the kinetically competitive
reactions of two-electron water oxidation to peroxides such as
H2O2.31 Although H2O2 could decompose into O2, it is readily
Figure 5. Valence-band XPS spectra of NaHTB-180 and NaHTBC900.
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design and preparation of IR-driven photocatalysis systems that
will greatly improve solar-energy utilization efficiency.
band energy XPS spectra of NaHTB-C900 showed three
valence-band peaks (marked as VB1, VB2−1, and VB2−0), located
at approximately 0.0, 1.0, and 3.0 eV (vs Fermi level,
corresponding to −0.1, 2.0, and 2.7 eV vs NHE), respectively,
exhibiting a ladder-type valence-band energy structure.
As shown in Figure 1, the aforementioned ladder-type carrier
transfer process may include three carrier generation processes
(marked as e1−h1, e2−h2, and e3−h3) and two carrier
recombination processes (marked as h1+e2 and h2+e3). The
produced excited e1 electrons in the CB1 of WO2 and h3 holes
in the VB2−0 of NaxWO3 are consumed in the water-splitting
redox reaction. During this process, at least three photons are
necessary to complete the carrier transfer, which may cause
upconversion-induced luminescence. As shown in Figure S12, a
broad upconversion luminescence peak centered at 726 nm was
observed for NaHTB-C900 when it was irradiated with a 980
nm laser. Moreover, pump-power dependence of the
luminescence intensities was observed, with a slope of 3.04 in
the double-logarithmic representation, which indicates that this
emission was generated by three excitation photons.40 The
photonic energy requirement for the three carrier generation
processese1−h1, e2−h2, and e3−h3are 0.6, 0−1.3, and 0.7
eV, respectively, which are all in the IR energy range, ensuring
photocatalytic water-splitting capability under IR irradiation. As
shown in Figure S13, compared with the semiconductor
material NaHTB-180, the hybrid conductor material NaHTBC900 exhibited multiple redox peaks on the cyclic voltammetry
curves, indicating the presence of a complex crystal phase
interface with different W chemical states, which may be helpful
for carrier transfer during the photocatalytic process.
Generally accepted, the metallic oxides will be doped with
carbon atoms under high-temperature calcination conditions,41
which also results in decreasing band gap energy.42 To
determine whether the energy band change originated from
the carbon dopants, the semiconductor NaxWO3 was reduced
by H2 instead of carbon under the same temperature for 30
min. A WO2−NaxWO3 hybrid conductor material (marked as
NaHTB-H900) with lower IR-driven photocatalytic water
splitting ability was obtained. Moreover, no carbon ion peak
centered at 281.5 eV was observed in the XPS spectrum of the
NaHTB-C900 (Figure S14).41 A WO2−WO3 hybrid material
prepared via partial reduction of WO3 by carbon43 exhibited
little photocatalytic activity under the same reaction conditions
(Figure 4A), which indicates that the electronic conductivity of
the components was essential for ensuring the efficient inner
carrier transfer process. Thus, we concluded that the IR-driven
photocatalytic water splitting ability of WO2−NaxWO3 is a
consequence of the special ladder-type energy band and the
barrier-free interface carrier transport due to the electronic
conductivity of WO2 and NaxWO3. It is worth noting that the
platinum-like property of metallic WO2 and the surface residual
carbon particles are proposed to facilitate H2 evolution in the
absence of a noble metal such as platinum.31,44,45
In a summary, a ladder-type, IR-driven, overall water splitting
photocatalytic system based on WO2−NaxWO3 (x > 0.25)
hybrid conductor materials was established; this system was
successfully applied to seawater splitting. In addition to the
tungsten conductive oxides, other hybrid conductive oxides
with the general chemical formula of MO2−AxMO3 (M = V,
Mo, Ta, Re, Nb, Tc, Ru; A = Na, K, Sr) are proposed to have
potential similar photocatalytic properties. The established
ladder-type photocatalytic system affords a new strategy for the
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.nanolett.5b01581.
Synthesis of the photocatalysts, material characterization,
and other catalytic results (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: tangb@sdnu.edu.cn.
Funding
This work was supported by 973 Program (2013CB933800),
National Natural Science Foundation of China (21227005,
21390411, 21535004, 21575082), Development plan of science
and technology for Shandong Province of China
(2013GGX10706), Shandong Province Natural Science
Foundation (ZR2015BM023), and A Project of Shandong
Province Higher Educational Science and Technology Program
(J13LD06).
Notes
The authors declare no competing financial interest.
■
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