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Monoclinic Tungsten Oxide with {100} Facet Orientation and Tuned Electronic Band Structure for Enhanced Photocatalytic Oxidations Ning Zhang, Chen Chen, Zongwei Mei, Xiaohe Liu, Xiaolei Qu, Yunxiang Li, Siqi Li, Weihong Qi, Yuanjian Zhang, Jinhua Ye, Vellaisamy.A.L. Roy, and Renzhi Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02275 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 7, 2016

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ACS Applied Materials & Interfaces

Monoclinic Tungsten Oxide with {100} Facet Orientation and Tuned Electronic Band Structure for Enhanced Photocatalytic Oxidations Ning Zhang, *, †, ¶ Chen Chen, † Zongwei Mei, ‡ Xiaohe Liu, *, † Xiaolei Qu, † Yunxiang Li, ∥ Siqi Li, † Weihong Qi, † Yuanjian Zhang, # Jinhua Ye, §, ∥ Vellaisamy A. L. Roy, ¶ and Renzhi Ma *, †, §

† School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China §

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials

Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡

Peking University Shenzhen Graduate School, School of Advanced Materials, University

Town, Shenzhen, Guangdong 518055, China ∥

TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin

University, 92 Weijin Road, Nankai District, Tianjin 300072, China #

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

ACS Paragon Plus Environment

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Department of Physics and Materials Science, City University of Hong Kong, Tat Chee

Avenue, Kowloon, Hong Kong SAR, China KEYWORDS Tungsten oxide, Crystal face, Photocatalysis, Water splitting, Energy band

ABSTRACT Exploring surface exposed highly active crystal facets for photocatalytic oxidations is promising in utilizing monoclinic WO3 semiconductor. However, the previously reported high active facets for monoclinic WO3 were mainly toward enhancing photocatalytic reductions. Here we report that the WO3 with {100} facet orientation and tuned surface electronic band structure can effectively enhance photocatalytic oxidation properties. The {100} faceted WO3 single crystals are synthesized via a facile hydrothermal method. The UV-visible diffuse reflectance, X-ray photoelectron spectroscopy valence band spectra, and photoelectrochemical measurements suggest that the {100} faceted WO3 has a much higher energy level of valence band maximum in compared with the normal WO3 crystals without preferred orientation of crystal face. The density functional theory calculations reveal that the shift of O 2p and W 5d states in {100} face induce such a unique band structure. In comparison with the normal WO3, the {100} faceted WO3 exhibits an O2 evolution rate about 5.1 times in water splitting, and also shows an acetone evolution rate of 4.2 times as well as CO2 evolution rate of 3.8 times in gaseous degradation of 2propanol. This study demonstrates an efficient crystal face engineering route to tune the surface electronic band structure for enhanced photocatalytic oxidations.


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INTRODUCTION Nowadays, exploration of efficient semiconductor photocatalysts has generated immense interest in the fields of energy conversion and environmental remediation.1-9 As the surface properties of semiconductor photocatalysts are crucial to the photocatalytic performances, exposed crystal facets influence much on the photocatalytic reactions.10-15 In designing and tailoring photocatalysts with high-active crystal facets exposed, several factors should be considered. Firstly, the surface energy, which influence the surface active site, defects and adsorption of reactive molecules; 10, 12 high surface energy leading to a high reactivity is widely accepted in photocatalysis.16, 17 Secondly, the surface atomic coordination, which affects the adsorption of surface molecules and number of active sites for the photoreactions.18, 19 Thirdly, the surface electronic band structure (e. g. the positons of conduction band, valence band, and bandgap), which finally determine the redox power of photoexcited electron-hole pairs and influences on the reactivity and feasibility of photocatalytic reactions directly.18-22 Up to now, {001} and {111} in TiO2, {111} in Ag3PO4, {010} and {110} in BiVO4, and so on were proved to be more efficient than other facets in photocatalytic reductions or oxidations and tailoring the surface crystal facets for a single crystal photocatalyst provides an effective way to tune the photocatalytic reactions.10, 11, 18, 21, 22 Monoclinic phase tungsten trioxide (m-WO3) is an efficient visible light sensitive photocatalysts in decomposing organic compounds, splitting water, and CO2 photoreduction.23-30 Up to now, some crystal faces of WO3 were proved to be efficient for photocatalysis. For example, WO3 octahydrates with tungstic acid stabilized {111} facets showed enhanced photocatalytic reduction ability to recover Ag+ into Ag.31 Moreover, WO3 nanoplates with predominant {001} facet exhibits superior CO2 photo-reduction performance than the quasicubic-like WO3 crystals with


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equally exposure of {001} {010}, and {100} faces.32 However, although above discovered {111} and {001} facets for WO3 are interesting and crucial to the improvement of photocatalytic reactions, they only promoted the photoreductive reactions.31, 32 Generally, WO3 photocatalyst is more suitable for the oxidations in that the positions of valence band maximum (VBM) locates at relatively high energy level but the conduction band minimum (CBM) locates at low energy level in comparison with TiO2 and the chemical property is unstable in some photoreductive reactions because the valence of W tend to be reduced from +6 into +5.24, 26 Therefore, finding high active crystal face for oxidative reactions is more promising in utilizing the unique property of m-WO3 semiconductor. Unfortunately, such high active facet for photocatalytic oxidations is seldom reported. In this work, we report m-WO3 crystals with dominated {100} face and tuned surface band structures for high photocatalytic oxidations. The {100} faceted WO3 was synthesized by a simple one step hydrothermal method, avoiding the damage of the surface structure and the decrease of active sites caused by high temperature calcination to remove the covered stabilizer of tungstic acid in previous methods.31, 32 The analysis on the band structure suggested that the {100} faceted WO3 nanocrystals yielded much high level of VBM in comparison with the normal WO3 crystals without preferred crystal face growth. The density functional theory calculations revealed that the positive shift of O 2p and W 5d states in {100} face induced such a unique band structure. The {100} faceted WO3 showed much enhanced photocatalytic oxidations activity in water splitting to produce O2 and degradation of 2-propanol in air. EXPERIMENTAL SECTION Preparation of Tungsten Oxides: The WO3 materials were synthesized by hydrothermal


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method. In a typical process, tungsten acid (H2WO4, 0.002 mol, AR grade, Wako Pure Chemical Industries, Ltd.), and triethylamine (Et3NH, 15 mL, AR grade, Sinopharm Chemical Reagent Co., Ltd) were mixed together and dissolved in 400 mL of distilled water under vigorous magnetic stirring at room temperature. After the H2WO4 dissolved completely, HCl solution was added dropwise into the solution to adjust the pH value to 2.1 - 3.0. Then the white precipitates were generated promptly. The resulting suspension was transferred into a Teflon-lined stainless steel autoclave (500 mL), which was subsequently heated at 180 °C for 32 h. After natural cooling to room temperature, the product was filtered and washed for several times with deionized water and ethanol and then dried at 180 °C for about 10 h. Photocatalytic and Photoelectrochemical Evaluation: For water splitting to produce O2, the WO3 powders (0.1 g) were dispersed in 100 ml of H2O in a quartz plate covered glass reaction cell. AgNO3 (0.5 g) was added in the cell as sacrificial reagent. A 300W Xe arc lamp with a L42 UV-light cutoff filter (λ > 400 nm) was employed as the light source for the photocatalytic reactions. The amount of evolved O2 was determined from a gas chromatograph (GC-8A, Shimadzu, Japan). For decomposition of gaseous 2-propanol, the photocatalysts (0.2 g) were spread uniformly in a glass plate with area of 8 cm2, which was set in the bottom of a quartzmade vessel. Then the vessel was washed by artificial air [V(N2): V(O2) = 4:1] for 10 min to remove adsorbed gaseous impurities. Proper amount of gaseous 2-propanol was injected into the vessel and maintained in dark for 1 h to establish an adsorption-desorption equilibrium. The visible light was produced from 300 W Xe lamp with a L42 filter and a water filter. The gas phase products during the photocatalytic reactions were analyzed with a gas chromatograph system (GC-2014, Shimadzu, Japan), using a flame ionization detector (FID) for organic compounds determination. For photoelectrochemical measurements, the detailed information


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was described in supporting information. Computational details: The density of states (DOS) plots of WO3 were obtained by using the first principles density functional theory (DFT) calculations to specify the electronic structure of WO3 and {100} facet in these models. CASTEP (Cambridge Sequential Total Energy Package) was used to perform DFT calculations with the GGA-PBE adopted. The Brilloiouin zone k-point sampling was performed using a 3 × 3 × 3 Monkhorst - Pack (MP) mesh for whole WO3 unit cell and 3 × 3 × 1 for {100} faceted WO3, and the cutoff energy of plane waves is set to 380 eV. The convergence criteria for structural optimization were set to be Ultra-fine quality with the tolerance for self-consistent field (SCF), energy, maximum force, and maximum displacement of 5.0×10-6 eV per atom, 5.0×10-6 eV per atom, 0.01 eVA-1, and 5.0×10-4 A, respectively. Characterizations: X-ray diffraction were performed by a RIGAKU Rint-2000 X-ray diffractometer with graphite monochromatized Cu-Kα radiation (λ=1.54184 Å) equipped. Scanning electron microscopy images were recorded with a FEI Helios Nanolab 600i field emission scanning electron microscopy. Transmission electron microscopy and high-resolution were performed with a FEI Tecnai G2 F20 field emission transmission electron microscopy operated at 200 kV. UV-visible diffuse reflectance spectra were recorded with a Shimadzu UV2600 Spectrophotometer. The X-ray photoelectron spectroscopy spectra were recorded on a Thermo Fisher ESCALAB 250Xi spectrophotometer. The Brunauer-Emmett-Teller surface areas measurements were carried out in an ASAP 2020 HD88 Surface Area Analyzer. Time-resolved photofluorescent spectra were recorded from a Fluorolog 3-22 steady state & time resolved fluorescence spectrometer.


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RESULTS AND DISCUSSION Materials Synthesis and Characterizations To fabricate the WO3 products, tungsten acid was dissolved in alkaline solution firstly, and then the solution was transformed into autoclave for hydrothermal reactions after adding proper amount of acid to adjust the pH value. Figure 1 shows the X-ray diffraction (XRD) pattern of WO3 product prepared at pH value of 2.5 by using triethylamine (Et3NH) to dissolve H2WO4. All diffraction peaks are indexed to monoclinic phase of WO3 with JCPDS cards No. 43-1035, indicating that the product has high purity. In comparison with the standard XRD profile, the relative strength of (200) diffraction peak in as-papered product is much stronger than other diffraction peaks such as (002) and (020). For instance, the diffraction intensity ratio of (200) / (002) (abbreviated as R (200) / (002)) is about 6.0 for as prepared product, which is much larger than the R

(200) / (002)

= 0.99 in standard XRD profile. This phenomenon suggests that the WO3

synthesized in this condition have a preferred crystal face growth of {100} facets.

Figure 1 Standard XRD profile of JCPDS cards No. 43-1035 and XRD profile of the prepared WO3 products prepared in pH value of 2.5 by using Et3NH as base to dissolve the H2WO4.


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Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and structure of the product. Figure 2a illustrates a typical SEM image of WO3 product prepared in pH value of 2.5 by using Et3NH to dissolve H2WO4. Most of the products exhibit plate-like morphology with wide size distributions (length is from 1 to 3 µm and width is from 100 to 500 nm). The higher magnified SEM image in Figure 2b displays that these plates have a nanoscale thickness of 50-100 nm with rectangular cross section and most of the plates have well-developed flat facets and sharp facet edges, implying a preferred crystal face would be exposed. A TEM image in Figure 3c shows some nanoplate with width of 100 - 600 nm and the constant contrast in each nanoplate indicates a uniform thickness in a single nanoplate. The selected area electron diffraction (SAED) pattern taken from a nanoplate (marked by the arrow in Figure 2c) is shown in Figure 2d. The diffraction spots are indexed to [100] zone axis of m-WO3, which indicates that this nanoplate is single crystal phase and the exposed crystal face is {100} facet. The directions of (001) and (010) diffraction spots suggest that the long dimension is along [001] whereas the lateral dimension is along [010]. The high resolution TEM (HRTEM) image in Figure 2e taken under the [100] displays two clear interplanars distance measured as 0.386 and 0.377 nm (also see in Figure S1 of supporting information), corresponding to (002) and (020) facets, respectively. All these characterizations serve as strong evidence that the single crystal WO3 nanoplates with preferred growth of {100} facet is successfully fabricated. Base on above analysis, the structural model of the product is illustrated in Figure 2f. In a perfect nanoplate, the major exposed face in a nanoplate are {100} facets. Such a face distribution caused the much stronger (100) diffraction peak than other peaks in XRD profile including the (010) and (001) peaks. Figure 2g is the atomic structure of monoclinic WO3, displaying that every W atom is 6-fold coordinated and O atom is 2-fold coordinated.


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Figure 2 (a) SEM image, (b) enlarged SEM image, (c) TEM image, (d) SAED pattern under [100] zone axis, (e) HRTEM image, and (f, g) structural models of WO3 product prepared at pH value of 2.5 by using Et3NH as base to dissolve the H2WO4. During the formation of WO3 crystals, it was suggested that H2WO4 dissolved in the alkali solution formed by Et3NH and water firstly: Et3NH + H2O → Et3NH2+ + OH-, H2WO4 + 2OH→ WO42-+ 2H2O.27 Then, droppings of HCl solution provided an acid environment and tungstic acid precipitation was produced: WO42- + 2H+ + nH2O → WO3·(n+1)H2O. Finally, WO3·nH2O transformed into m-WO3 under the hydrothermal condition and grew into WO3 crystals: WO3·nH2O → WO3 + nH2O.32 To study the key factors for the formation of {100} faceted WO3, some control experiments were carried out. Figure 3a is the XRD profile for the product prepared at a controlled synthetic condition without Et3NH but kept other synthetic condition unchanged (in the presence of H2WO4, water, and HCl under hydrothermal environment of 180 °C for 32 h


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in pH value of 2.5). It is observed that the preferred growth of {100} is diminished (the R (002)

(200) /

decreased to 0.96) and the relative intensity ratio of {100} to other diffraction peaks is close

to the standard PDF card in Figure 1, indicating there is no obvious preferred growth of crystal face appeared. The SEM and TEM images in Figure S2a-b of supporting information show that the product is composed of quasi-cubic-like nanocrystals with sizes from 50-200 nm. The HRTEM and SAED pattern Figure S2c-d of supporting information display that the (010) face is exposed on a face of cube (detailed analysis can be seen in Figure S2 of supporting information). All characterizations implies a uniform distribution of {100}, {010}, and {001} on the surface, which is different to the as prepared {100} faceted WO3 nanoplates. Furthermore, the Et3NH was replaced same amount of NaOH and NH3 solutions were also carried out. The XRD profiles of the products in Figure 3b and c show that only pure phase hexagonal phase of tungsten oxide hydrates (WO3·0.33H2O, indexes to JCPDS cards No. 35-1001) are generated. The SEM images in Figure S3a and b of supporting information display that nanorods are formed when the products are prepared by NaOH and NH3, which are much different to the product prepared by Et3NH. All these results strongly suggest that the addition of Et3NH in synthetic process played unique function in tuning the growth of monoclinic phase WO3 with exposed {100} facets, which may be attributed to that the Et3NH2+ generated from Et3NH solution serves as capping agent to change the growth rate of some crystal faces as the case in our previous reports.33-35 More importantly, the formation of m-WO3 crystal growth was sensitive to the pH values during the synthesis. When the product was synthesized at pH value of 3.0 and kept other condition unchanged, the preferred orientation of {100} face was weakened with R

(200) / (002)

decreased to

3.0 as shown in the XRD profile of Figure 3d. Moreover, when pH value was decreased to 2.1, the {100} face preferred growth was almost diminished with R (200) / (002) decreased to 1.5 and the


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impurity of orthorhombic WO3·0.33H2O is generated as shown in Figure 3e. Hence, the pH value of 2.5 was a relatively optimal condition for the formation of m-WO3 with strongly preferred growth of {100} facet. Corresponding SEM images of WO3 prepared at pH value of 3 and 2.1 are illustrated in Figure S3a and b of supporting information, respectively. Different from the product prepared at pH value of 2.5, WO3 products obtained at pH value of 3.0 and 2.1 is mainly composed of nanorods with rectangle like cross section. The changes in morphology may be caused by the increase of other facets such as (010) and (001) in the surface.

Figure 3 XRD profiles of WO3 products prepared with (a) no base, (b) NaOH, and (c) NH3 as base in the pH value of 2.5; XRD profiles of the prepared WO3 products prepared at pH value of (d) 3.0 and (e) 2.1 by using Et3NH as base to dissolve H2WO4. To determine the possible existed defect of W5+ generated in synthetic process, the charge states of the products were characterized by X-ray photoelectron spectrometer (XPS). Figure 4 a and b present the XPS patterns of the {100} faceted WO3 prepared in the presence of Et3NH at pH value of 2.5 as well as normal WO3 synthesized only at pH value of 2.5 for comparison purpose (as mentioned in Figure 3a and Figure S2 in supporting information). As shown from the O 1s


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spectra in Figure 4a, the fitted peaks at 530.2 eV are attributed to the W-O bond in WO3, whereas the peaks at 531.5 is attributed to the surface hydroxides. The fitted W 4f peaks are shown in Figure 4b, the W 4f7/2 and W 4f5/2 locate at binding energy 35.2 eV and 37.4 eV, respectively. The oxidation states of the W in all sample identified as + 6. All the W 4f and O 1s regions agree well with reported tungsten trioxide XPS spectra and both the two samples exhibit similar XPS spectra, indicating that there is only W6+ in as synthesized {100} faceted WO3 and normal WO3.36, 37

Figure 4 XPS spectra of (a) O1s and (b) W 4f for {100} faceted WO3 and normal WO3. Electronic band structure


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In the following study, the bandgaps of the as synthesized products were evaluated. Figure 5a presents the UV-visible diffuse reflectance spectrum for {100} faceted WO3 as well as the normal WO3 for comparison purpose. The optical absorbance edges for both {100} faceted WO3 and normal WO3 are about 464 nm, exhibiting a good absorption ability in visible light region. The bandgaps are determined by equation (αhυ) n/2=A(hυ - Eg), where the α, υ, A, Eg, and n are the absorption coefficient, incident light frequency, constant, band gap, and an integer, respectively.23, 38 As an indirect band transition semiconductor, the value of n for m-WO3 is 4. The band gap energy for the WO3 products are obtained for the plot of (ahυ)2 versus hυ. As shown in the inset of Figure 5a, these bandgaps are calculated to be 2.84 eV for {100} faceted WO3 and 2.85 eV for normal WO3. After that, XPS valence band (VB) spectra were recorded to detect the relative VBM positions of WO3 crystals. As illustrated in Figure 5b, {100} faceted WO3 shows VBM about 2.36 eV, which is about 0.31 eV larger than 2.05 eV for normal WO3. Based on the bandgaps and VBM determined from Figure 5a and b, the relative band positions of {100} faceted WO3 and normal WO3 versus normal hydrogen electrode (NHE, pH = 0) can be proposed. As shown in Figure 5c, the VBM in {100} faceted WO3 is about 0.31 eV higher than that of the normal WO3. Correspondingly, the CBM of {100} faceted WO3 is positioned at a lower energy level than that of the normal WO3 as the bandgaps between the two products are similar. To further prove this band structure, especially the directly evidence for the difference in proposed CBM between the two products, a photochemical method was employed for determining the possibility of a shift in the flat-band positions between {100} faceted WO3 and normal WO3.39-41 The obtained results were given in Figure S5 of supporting information, which presented the dark and photo-currents vs. applied electrode potential curves. The flat-band potential for {100} faceted WO3 was detected as about -0.24 V (vs. Ag / AgCl, pH = 7).


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Otherwise, the flat-band potential for the normal WO3 was approximately -0.27 V (vs. Ag / AgCl, pH = 7), which was about -0.03 V smaller than that of {100} faceted WO3. These measurements suggested that the energy level of CBM for {100} faceted WO3 was indeed lower than that of the normal WO3, consistent with the trend of proposed band structure in Figure 5c. Based on above analysis, tailoring the preferred orientation crystal facets of WO3 to {100} appears to shift VBM toward higher energy level whereas CBM to lower energy level.


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Figure 5 (a) UV-Vis spectra of the {100} faceted WO3 and normal WO3; the inset is the calculation diagram of their band gaps; (b) XPS VB spectra of {100} faceted WO3 and normal WO3; (c) Scheme of relative CB and VB positions for {100} faceted WO3 and normal WO3. Theoretical Calculations

Figure 6 DOS plots for {100} face and whole WO3 cell based on the DFT calculation. It was reported that the surface electronic structure of a semiconductor played crucial role in the energy levels of the VBM and CBM. For example, the TiO2 materials with orientated {010}, {100}, {001}, and {101} facets exhibited different CBM or VBM because of the diversity of O 2p states and Ti 3d states.18-21 In this work, the electronic band structure of {100} surface and


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whole WO3 unit cell that without preferred crystal face was compared based on the DFT calculations. The schematic geometrical models of a relaxed m-WO3 thin slab with surface atoms of {100} face and the whole WO3 unit cell are shown in Figure S6 of supporting information. The density of states (DOS) plots based on DFT calculations from these models are shown in Figure 6. In comparison with the whole WO3 unit cell, the total VBM DOS of {100} locates at higher energy mainly due to the negatively shift of O 2p orbitals as illustrated in Figure 6. Correspondingly, the total DOS of CBM in {100} shows a same shift because of the W 5d DOS. Thus, the shift of both O 2p and W 5d states in {100} caused the diversity of VBM and CBM in compared with whole unit cell, which was a main reason why {100} faceted WO3 nanoplates showed a higher energy level of VBM than that of the normal WO3 crystals. Photocatalytic properties According to the theory of photocatalysis, the VBM locates at higher energy level will induce stronger oxidation ability during photocatalytic reactions. To evaluate the photocatalytic oxidation ability, the water splitting to produce O2 over the as synthesized {100} faceted WO3 was studied. In the aqueous solution of AgNO3, the process that photogenerated holes oxidize H2O to produce O2 is expressed as: 2H2O +4h+ → O2 + 4H+ [E(H2O/H2) = + 1.23 V vs. NHE, pH = 0)],42, 43 whereas the electrons are consumed by the Ag+ produced from AgNO3: Ag+ + e- → Ag [E(Ag+/Ag) = + 0.80 V vs. NHE, pH = 0].42, 43 Figure 7a shows a typical result for photocatalytic O2 evolution from aqueous AgNO3 solution under visible light irradiation (λ > 400nm). The produced O2 over {100} facet orientated WO3 and normal WO3 in 4 h are about 33.9 µmol and 17.7 µmol, respectively. Obviously, the {100} faceted WO3 shows higher evolution rate of O2 than normal WO3 in water splitting (8.5 µmolg-1h-1 vs. 4.4 µmolg-1h-1). It is known that the surface area of a photocatalyst greatly affects its catalytic efficiency. The Brunauer-Emmett-


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Teller (BET) surface areas are calculated as 4.7 m2g-1 for {100} faceted WO3 and 12.6 m2g-1 for normal WO3 (the detailed information is shown in Figure S7 of supporting information), the normalized evolution rate to produce O2 in per square meter is thus calculated as 17.9 µmolh-1m2

for {100} facet orientated WO3 and 3.5 µmolh-1m-2 for normal WO3 as shown in Figure 7b; the

{100} faceted WO3 shows about 5.1 times O2 evolution rate as normal WO3. Obviously, the {100} faceted WO3 exhibited much enhanced performance than normal WO3 in water oxidation to produce O2. The apparent quantum yield of {100} faceted WO3 was calculated, which was ~ 0.7 % at wavelength of 400 ± 10 nm.

Figure 7 Photocatalytic water splitting to produce O2 over {100} facet orientated WO3 and normal WO3 under the irradiation of visible light: (a) produced O2, (b) evolution rate of O2 a); λ > 400nm; catalyst: 0.1 g. a)

Normalized by BET surface area of the sample.


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The gaseous photocatalytic oxidation were also studied by degradation of 2-propanol over {100} facet orientated and normal WO3. The degradation of 2-propanol can be summarized as follows: firstly, the 2-propanol photo-oxidized in to acetone in a one-photon reaction: CH3CHOHCH3 + h+ → CH3COCH3 + 2H+ + e-; then, the acetone is oxidized into CO2 by a multiphoton reaction: CH3CHOHCH3 + 5H2O + 18h+ → 3CO2 + 18H+.44-46 Such a clear mechanism and typical intermediate product enable us to compare the photocatalytic oxidation performance over the {100} faceted WO3 and normal WO3 under the gaseous atmosphere. In our experiment, all products are kept in full adsorption of 2-propanol. The produced gaseous acetone in one-photon reaction from oxidation of 2-propanol is illustrated in Figure 8a, which displayed that the generated acetone is linear respect to the irradiation time in 1 h. The evolution rate of acetone over {100} faceted WO3 and normal WO3 are about 17.5 µmolh-1 and 11.2 µmolh-1 respectively. Taking account of the surface area effect, the {100} faceted WO3 showed about 4.2 times activity than the normal WO3 when the evolution rate of acetone is normalized by the BET surface areas (18.6 vs. 4.4 µmolh-1m-2, as shown in Figure 8c). For the multiphoton reaction to produce CO2, the {100} faceted WO3 also shows enhanced activity as shown in Figure 8b. The produced CO2 is 2.2 µmolh-1 over {100} faceted WO3 and 1.7 µmolh-1 over normal WO3. Meanwhile, when the evolution rate of CO2 is normalized by the BET surface areas, the evolution rate CO2 over {100} faceted WO3 shows about 3.8 times than normal WO3 (2.3 vs. 0.7 µmolh-1m-2, as shown in Figure 8c). Taken all results account, the {100} faceted WO3 exhibited much enhanced photocatalytic oxidation activity both in one-photon and multiphoton degradation of gaseous 2propanol in comparison with the normal WO3. In considering that the two samples have almost same bandgaps, the surface atomic coordination states of W and O atom over {100}, {010}, and {001} facets in a monoclinic structure of WO3 are same, the closed lifetime of photogenerated


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electron-hole pairs (as analyzed from time-resolved fluorescent spectra in Figure S8 of supporting information), and the {100} facet even owns relatively slightly lower surface energy than the {010}, {001}, and {111} facets,29, 30 the higher photocatalytic oxidative activities over {100} faceted WO3 than normal WO3 is thus highly depended on its more greater energy level of VBM, which induce stronger oxidization ability. In addition, the {100} faceted WO3 also showed good stability in photocatalytic reactions as illustrated in Figure S9 of supporting information.

Figure 8 The produced (a) acetone, (b) CO2, and (c) evolution rate of acetone and CO2

a)

in the


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photocatalytic degradation of 2-propanol over {100} facet orientated WO3 and normal WO3 under the irradiation of visible light; λ > 400nm; catalyst: 0.2 g. a)

Normalized by BET surface area of the sample.

CONCLUSIONS In summary, the surface electronic band structure could be tuned for enhanced photocatalytic oxidations by constructing m-WO3 with preferred {100} facet orientation. The {100} faceted mWO3 was successfully fabricated by a facile hydrothermal method without high temperature calcinations. The Et3NH and pH value of 2.5 plays crucial role in the preferred growth of {100} facets. The VBM of {100} faceted WO3 shows about 0.31 eV greater energy level than that of the normal WO3 synthesized at similar condition. The DFT calculations revealed that such a unique surface band structure was caused by the shift of O 2p and W 5d states. During the evaluation of photocatalytic performance, the {100} faceted WO3 showed about 5.1 times O2 evolution rate as the normal WO3 in oxidizing water to produce O2. Moreover, the {100} faceted WO3 exhibited 4.2 times acetone evolution rate and 3.8 times CO2 evolution rate as the normal WO3 in oxidizing 2-propanol under air atmosphere. This work reveals a new high active crystal face in WO3 and provides an efficient method to tune the surface electronic band structure for enhancing the photocatalytic oxidations both in liquid and gaseous phase. ASSOCIATED CONTENT Supporting Information Detailed experimental process for photoelectrochemical measurements and additional figures.


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This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author * E-mail: [email protected], [email protected], [email protected] Funding This work is supported by National Natural Science Foundation of China (51402364), Shenghua Lieying project of Central South University, and Hong Kong Scholars Program. Notes The authors declare no competing financial interest. REFERENCES (1)

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Graphical Abstract


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Monoclinic Tungsten Oxide with - ACS Publications - American

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