Facile Fabrication of Sandwich Structured WO3 Nanoplate Arrays for

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Facile Fabrication of Sandwich Structured WO Nanoplate Arrays for Efficient Photoelectrochemical Water Splitting Xiaoyang Feng, Yubin Chen, Zhixiao Qin, Menglong Wang, and Liejin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04887 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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ACS Applied Materials & Interfaces

Facile Fabrication of Sandwich Structured WO3 Nanoplate Arrays for Efficient Photoelectrochemical Water Splitting Xiaoyang Feng, Yubin Chen*, Zhixiao Qin, Menglong Wang and Liejin Guo* International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, P. R. China. ABSTRACT: Herein, sandwich structured tungsten trioxide (WO3) nanoplate arrays were firstly synthesized for photoelectrochemical (PEC) water splitting via a facile hydrothermal method followed by an annealing treatment. It was demonstrated that the annealing temperature played an important role in determining the morphology and crystal phase of the WO3 film. Only when the hydrothermally prepared precursor was annealed at 500 °C, the sandwich structured WO3 nanoplates could be achieved, probably due to the crystalline phase transition and increased thermal stress during the annealing process. The sandwich structured WO3 photoanode exhibited a photocurrent density of 1.88 mA·cm-2 and an incident photon-to-current conversion efficiency (IPCE) as high as 65% at 400 nm in neutral Na2SO4 solution under AM 1.5G illumination. To our knowledge, this value is one of the best PEC performances for WO3 photoanodes. Meanwhile, a simultaneous hydrogen and oxygen evolution was demonstrated for the PEC water splitting. It was summarized that the high PEC performance should be attributed to the large electrochemically active surface area and active monoclinic phase. The present study can provide a guidance to develop

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highly efficient nanostructured photoelectrodes with the favorable morphology. KEYWORDS: WO3, sandwich structure, photoelectrode, water splitting, hydrogen INTRODUCTION With the approaching depletion of fossil fuels and increasing concern on the global warming, enormous efforts have been devoted to developing sustainable and clean energy.1-5 Solar energy is renewable and green, which can supply sufficient power to meet humanity’s needs. However, its intermittent nature and low energy density restrict the practical application. Seeking an efficient way to convert solar energy to chemical fuels should be attractive to overcome the shortages. Photoelectrochemical (PEC) water splitting for hydrogen production has been considered to be a potential approach for efficient solar energy conversion.6-10 A number of semiconductor photoelectrodes have been examined for PEC water splitting in the last decades. In particular, metal oxides are superior candidates due to their good stability. The n-type metal oxide semiconductors, such as TiO2,11,12 WO313-18 and Fe2O319,20 have attracted great attentions for solar water oxidation, which is considered to be the rate-limiting step in PEC water splitting. WO3 is regarded as a promising photoanode material, because it presents visible-light response (Eg = 2.5-2.8 eV), inherently good electron transport property (∼ 12 cm2 V-1 s-1) compared with TiO2 (∼ 0.3 cm2 V-1 s-1),21 and a moderate hole diffusion length (∼ 150 nm) compared with α-Fe2O3 (2-4 nm). However, WO3 also shows some disadvantages, such as sluggish kinetics of photogenerated holes, rapid electron-hole

recombination,

and

slow

charge


transfer

at

the

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semiconductor/electrolyte interface.21 To improve the PEC performance of WO3 photoelectrodes, one of the effective strategies is to develop nanostructured photoelectrodes with controllable morphology and structure. Compared to the planar bulk architecture, nanostructured configurations (such as nanowire, nanorod, nanosheet) offer advantages in PEC applications due to the efficient light absorption, decreased electron-hole recombination, and high electrochemically active surface area.22-26 In the past several years, the apparent progress has been achieved for nanostructured WO3 photoanodes for PEC water splitting. Su et al. prepared vertically aligned WO3 nanowire and nanoflake arrays, which were beneficial to the efficient charge transfer.27 The highest saturation photocurrent was about 1.43 mA·cm-2 using a two-electrode setup. Kalanur et al. found that oriented WO3 nanorods could show a photocurrent as high as 2.26 mA·cm-2 at 1.23 V vs. RHE, which was attributed to the direct pathway of charge carriers provided by the vertically aligned nanorod arrays.28 WO3 nanomultilayers exposed with highly reactive (002) facets were demonstrated to show a photocurrent density of 1.62 mA·cm-2 at 1.25 V vs. Ag/AgCl.4 However, the sandwich structured WO3 photoanode has never been studied. To our knowledge, even the reports about sandwich structured morphologies for other semiconductors are scarce. Sumanta et al. reported a green synthesis of sandwich-structured CuO for efficient

non-enzymatic

sensing

of

glucose,

which

possessed monomodal

channel-type pores with largely improved surface area and pore volume.29 It is expected that the unique microstructure and suitable architecture could also be



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achieved for the sandwich structured WO3, leading to the improved PEC property. Herein, sandwich structured WO3 nanoplate arrays were prepared via a simple hydrothermal method followed by an annealing treatment. It was demonstrated that the annealing process has an important influence on the physicochemical and PEC properties of WO3 photoanodes. A sandwich structured morphology could be achieved at an annealing temperature of 500 °C. The sandwich structured WO3 photoanode showed a photocurrent density of 1.88 mA·cm-2 at 1.3 V vs. Ag/AgCl, which was among the best values for WO3 photoanodes.3,4,30 EXPERIMENTAL SECTION Chemicals Ammonium wolframate (H40N10O41W12·xH2O, 85-90%), citric acid (C6H8O7·H2O, 99.5%), hydrogen peroxide (H2O2, 30 wt%), hydrochloric acid (HCl, 36-38 wt%) and sodium sulfate (Na2SO4, 99.0%) were purchased from the Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. Milli-Q ultra-pure water (18.2 MΩ·cm at 25 °C) was used in the present study. Material Synthesis In a typical synthesis of the WO3 film, 1 g of ammonium wolframate was firstly dissolved into 95 mL of ultra-pure water. Subsequently, 700 µL of concentrated HCl was added to the above aqueous solution and stirred vigorously for 0.5 h to obtain H2WO4 precipitate (yellowish gel). Afterwards, 2 mL of H2O2 and 0.5 g of citric acid were added and stirred for 1 h to give a transparent solution. The as-prepared precursor was transferred to a Teflon-lined autoclave. The fluorine-doped tin oxide


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(FTO) coated glass (TCO-15, Nippon Sheet Glass, 14 Ω/□),9 which was previously cleaned with deionized water, acetone, and ethanol in sequence, was placed at an angle against the wall of the Teflon-lined autoclave with the conducting side facing down. The hydrothermal synthesis was conducted at 160 °C for 5 h. After the synthesis, the autoclave was allowed to cool down to room temperature naturally. The prepared electrode was taken out and rinsed with de-ionized water and ethanol several times, and then annealed under different temperatures for 1 h in a muffle furnace. Characterization X-ray diffraction (XRD) analysis was conducted on a PANalytical X’pert PRO diffractometer using Cu Kα irradiation (λ = 0.154 nm). The morphology and structure were characterized using transmission electron microscopy (TEM, FEI Tecnai G2 F30) and scanning electron microscopy (SEM, JEOL JSM-7800F). Thermogravimetric (TG) analysis was performed with a STA 449C thermal analyzer from NETZSCH. The optical properties were analyzed by a Hitachi double-beam U-4100 UV-vis-NIR spectrophotometer. The absorbance was calculated from

the

experimental

transmittance and reflectance data by the following equation, where A is the absorbance, T is the transmittance, and R is the reflectance.31 A% =100%–T%–R%

(1)

Photoelectrochemical Measurement The PEC measurements were carried out in a three-electrode system. The as-prepared sample, a platinum foil, and an Ag/AgCl electrode were used as the work, counter, and reference electrodes, respectively. A 0.5 M Na2SO4 solution was used as the



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electrolyte. A 500 W xenon lamp coupled with an AM 1.5G filter was used as the light source, and the light intensity was measured to be 100 mW·cm-2. The photocurrent densities were measured by a CHI 760D scanning potentiostat (CH Instruments). The incident photon-to-current conversion efficiency (IPCE) tests were carried out by measuring the photocurrent produced under chopped monochromatic light irradiation at a fixed potential of 1.0 V vs. Ag/AgCl. The value could be calculated from equation (2): IPCE ሺλሻ=

1240 j ሺλሻ ×100% I0 ሺλሻ × λ

ሺ2 ሻ

where λ corresponds to the wavelength of incident monochromatic light (nm), j (λ) is the photocurrent density (mA·cm-2) under illumination at wavelength λ, and I0 (λ) is the incident light intensity (mW·cm-2) at wavelength λ. Electrochemical impedance spectroscopy (EIS) was conducted using CHI 760D in the frequency range of 0.1-105 Hz at 0.5 V vs. Ag/AgCl and an AC voltage perturbation of 5 mV. Evolved H2 and O2 gases were measured by a gas chromatography equipped with a thermal conductivity detector, a TDX-01 column, and Ar carrier gas. All the measurements were conducted with the work electrode illuminated from the front side. RESULTS AND DISCUSSION In order to examine the crystal phases, XRD patterns of the as-prepared films before and after annealing at different temperatures were studied. As shown in Figure 1a. The diffraction peaks of the hydrothermally prepared sample could be indexed to orthorhombic WO3·0.33H2O (JCPDS No. 72-0199), with two strong diffraction peaks


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of (111) and (220) planes appearing at 18.1 and 28.1o. When the annealing temperature arrived at 400 °C, the diffraction peaks of orthorhombic WO3·0.33H2O completely disappeared. Instead, the XRD pattern of anhydrous hexagonal WO3 (JCPDS No. 75-2187), with two strong diffraction peaks of (100) and (200) planes appearing at 14.0 and 28.2o, could be clearly seen. As the annealing temperature reached 475 °C, the diffraction peaks of hexagonal WO3 were obviously weakened, and anhydrous monoclinic WO3 (JCPDS No. 72-0677) simultaneously existed with the diffraction peak corresponding to (200) plane appearing at 24.3o. Further increasing temperature to 500 and 600 °C led to the generation of pure monoclinic WO3. To study the weight loss of the WO3 film during the heat treatment, TG analysis of the hydrothermally prepared WO3·0.33H2O film was investigated in an air atmosphere. As displayed in Figure 1b, an initial weight loss was found at around 280 °C, which was ascribed to the departure of surface-absorbed and structural water,32,33 while the weight loss in the temperature range of 280-455 °C could be attributed to the release of H2O from WO3·0.33H2O. After the TG test, the sample’s weight remained nearly 97.5% of its origin weight. Meanwhile, the mass loss was calculated to be 2.5% through the chemical formulas of WO3·0.33H2O and WO3. It turned out that the TG result was consistent with the XRD measurement.



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Figure 1. (a) XRD patterns of as-prepared films before and after annealing at different temperatures. (b) TG plot of the hydrothermally prepared WO3·0.33H2O film in air. UV-vis absorption spectra were examined to investigate the light-absorbance properties of the as-prepared samples. As shown in Figure 2, the absorption edge showed continuously red shift as the annealing temperature gradually increased to 475 °C, which should result from the different crystal structures.27 The WO3 samples annealed at 475, 500, and 600 °C exhibited the similar absorption edges at around 465 nm, which was in accordance with the reported band gap of ca. 2.6 eV for WO3.34,35 The inset in Figure 2 exhibits the photographs of WO3 films under different annealing temperature. As the annealing temperature increased, the film color turned from


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greyish to dusty blue, then yellow green and finally yellow.

Figure 2. UV-visible absorption spectra of as-prepared films before and after annealing at different temperatures. The inset is the corresponding photographs. SEM images of as-prepared films annealed at different temperatures were shown in Figure 3a-f. It was noticed that multilayer nanoplates were prepared for the hydrothermally fabricated WO3·0.33H2O. The average length and thickness of WO3·0.33H2O nanoplates were about 1.5 µm and 250 nm (Figure 3a and Figure S1a). Morphology changes started to appear after the WO3·0.33H2O nanoplate arrays were annealed. The surface of the nanoplates annealed at 400 °C seemed much rougher (Figure 3b). As the annealing temperature reached 475 °C, the convex morphology at the two edges of origin nanoplates was emerged, and then the morphology of WO3 nanoplates was changed significantly at 500 °C. The integrated WO3 nanoplates were noticed to be split into three parts, showing a distinct sandwich structured morphology (Figure 3d and Figure S1b). This sandwich-like morphology included a thick layer in the middle and two thin layers on either hand. As the annealing temperature was improved to 600 °C, the sandwich structured morphology disappeared, and the large



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nanoplates were broken into some small nanoplates, producing thick blocks with a mesoporous morphology (Figure 3e). Moreover, the sandwich structured WO3 nanoplate arrays were nearly perpendicular to the FTO substrate, and the thickness was about 2.3 µm (Figure 3f). Low and high resolution TEM images of a partial WO3 nanoplate were shown in Figure 3g and h. The lattice fringes in the HRTEM micrograph indicated the crystalline nature of the product. Lattice fringes of 0.384 and 0.365 nm were clearly observed, corresponding to the d-spacing values of the (002) and (200) planes of monoclinic WO3, respectively. The inset of Figure 3h depicts the selected area electron diffraction (SAED) image of the partial WO3 nanoplate. Regular diffraction spots proved the single crystalline nature of the WO3 nanoplate.36


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Figure 3. SEM images of as-prepared films (a) before and after annealing at (b) 400 °C, (c) 475 °C, (d) 500 °C, (e) 600 °C, respectively. (f) Cross-section SEM image of the WO3 film annealed at 500 °C. (g) TEM and (h) HRTEM images of the WO3 film annealed at 500 °C. The inset is the electron diffraction pattern. Subsequently, we investigated the effect of the hydrothermal time on the morphology. The hydrothermal time was varied from 2 to 6 h, with the WO3·0.33H2O films all annealed at 500 °C for 1 h. As shown in Figure S1 and S2, WO3·0.33H2O nanoplates continued growing with the hydrothermal time prolonging from 2 to 6 h. The nanoplates gradually became larger, and the film got much thicker. Interestingly, the sandwich structured WO3 nanoplates always appeared after annealed at 500 °C regardless of the hydrothermal time. To make a comparison, the WO3·0.33H2O powder scratched from the hydrothermally prepared film was annealed at 500 °C for 1 h. As shown in Figure S3, the sandwich-like morphology could be also achieved for the annealed powder, indicating that the annealing process played a key role in generating the sandwich structured morphology. To better understand the influence of the annealing treatment on the morphology, WO3·0.33H2O nanoplates were annealed at 500 °C for different time. As displayed in Figure S4, the sandwich structured morphology began to show up after annealing for 0.5 h. However, the side layers in the sandwich structure seemed a little thinner than those annealed for 1 h. When the annealing time was extended to 3 h, the side layers became much thicker, and tended to be broken. When the annealing time was further increased to 5 h, the sandwich structured nanoplates were broken into small fragments. Therefore, the careful control of the annealing time is important for generating the



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sandwich-like morphology. In addition, the influence of the heating and cooling rates on the formation of the sandwich structures has been examined. As shown in Figure S5 and S6, the sandwich structures could be obtained for all samples, indicating that the heating and cooling rates had no apparent effect on the morphology. As summarized from the XRD results, the varied annealing temperature accounted for the different crystal structures. As displayed in Figure 4a, the crystalline structure transformed from orthorhombic to hexagonal further to monoclinic phase with the increased annealing temperature. Firstly, the basic structural element of orthorhombic WO3·0.33H2O consists of an infinite plane of [WO6] octahedra sharing their corners and forming six-membered rings along the (001) plane. Each (001) plane is made up of two types of [WO6] octahedra: type I [WO6] and type II [WO5(H2O)]. Subsequently, when orthorhombic phase transforms to hexagonal WO3 due to the removal of water, the W-O bonds of the original phase are broken and a new hexagonal W-O network appears.28 Finally, further annealing at high temperature (up to 500 °C) totally transforms the metastable phase of hexagonal WO3 to the thermodynamically stable monoclinic WO3. The monoclinic WO3 is a ReO3-type structure with the corner sharing WO6 octahedra connected in the a-, b- and c-directions.28 According to the crystal phase transformation and XRD results, the lattice parameters, cell volumes, and calculated cell volumes per W atom of the samples were shown in Table 1. The cell volume per W atom is equal to the cell volume divided by the number of W atoms in a unit cell. It was noted that the orthorhombic and hexagonal phases owned the similar cell volumes per W atom. However, when the crystal phase was further


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transformed to the monoclinic one, the cell volume per W atom decreased significantly. As shown in Figure 3f and S7, the thickness of annealed films was apparently decreased compared to the hydrothermally prepared WO3·0.33H2O film, which could result from the removal of water in WO3·0.33H2O. Meanwhile, increasing the annealing temperature from 400 to 600 °C could further lead to the gradually decreased thickness of as-prepared WO3 films, which should be ascribed to the morphology change and decreased cell volumes.

Figure 4. (a) The unit cells of orthorhombic WO3·0.33H2O, hexagonal WO3, and monoclinic WO3. (b) Schematic illustration of the forming process for sandwich structured WO3 nanoplates. (Ⅰ) self-assembly; (Ⅱ) ripening growth; (Ⅲ) oriented attachment; (Ⅳ) annealing at 475 °C; (Ⅴ) annealing at 500 °C. On the basis of the above analysis, a possible formation mechanism is proposed for the sandwich structured WO3 nanoplates. As shown in Figure 4b, in the hydrothermal process, the growth began with colloidal seeds followed by thermodynamically controlled nucleation and generation of nanocrystals.37,38 The nanocrystals then underwent two dimensional self-assembly under



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selective crystal-facet adhesion to form three dimensional crystallite layers, which went through ripening and further growth to form nanoplates.39 Subsequently, the original nanoplates were covered by several thin layers during the oriented attachment, which provided the lowest possible energy configuration to the geometrically arranged nanostructure,39 and the multilayer WO3·0.33H2O nanoplates appeared. Herein, citric acid40-42 and H2O243 acted as the lamellar nanoreactor for the formation of the multilayer WO3·0.33H2O nanoplates. As the annealing temperature reached 475 °C, the convex morphology at the two edges of multilayer nanoplates was emerged. When annealed at 500 °C, sandwich structured WO3 was obtained probably due to the decreased cell volumes per W atom from the crystalline phase transition, as well as the continuously increased thermal stress during the annealing process. Table 1. Summary of the lattice parameters, cell volumes and calculated cell volumes per W atom of the different samples according to their corresponding JCPDS cards from XRD results. Sample

Orthorhombic

a (Å)

b (Å)

c (Å)

α

β

γ

V (cell)

N

V (W atom)

(deg)

(deg)

(deg)

(Å3)

(W atom)

(Å3)

7.359

12.513

7.704

90

90

90

709.4

12

59.12

7.298

7.298

3.899

90

90

120

179.8

3

59.93

7.306

7.540

7.692

90

90.88

90

423.7

8

52.96

WO3·0.33H2O Hexagonal WO3 Monoclinic WO3

V

(cell):

cell volume; N(W atom): number of W atoms in a unit cell; V (W atom): cell volume per W

atom.

Linear sweep voltammetry of the WO3 nanoplate arrays annealed at different temperatures was shown in Figure 5a. The photocurrent of as-prepared


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WO3·0.33H2O film was negligible, as displayed in Figure S8. With the increased annealing temperature, the photocurrent of WO3 nanoplate arrays first increased and then decreased. When annealed at 500 °C, the sandwich structured WO3 nanoplate arrays exhibited the highest photocurrent density of 1.88 mA·cm-2 at 1.3 V vs. Ag/AgCl.

Figure 5. (a) Linear sweep voltammetry of WO3 photoanodes annealed at different temperatures under chopped incident light. (b) IPCE plot of WO3 nanoplate arrays annealed at 500 °C measured at 1.0 V vs. Ag/AgCl.

Figure 5b depicts the IPCE values of the sandwich structured WO3 photoanode annealed at 500 °C. The onset was observed at 460 - 470 nm,



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which was consistent with the onset of UV-vis absorption spectrum. The sandwich structured WO3 nanoplate arrays exhibited an IPCE up to 65% at 400 nm, which was among the best values compared to the previously reported WO3 photoanodes with different morphologies (Table S1). The stability test was carried out in 0.5 M Na2SO4 neutral solution. As displayed in Figure 6a, the photocurrent of sandwich structured WO3 nanoplate arrays remained 89% of the initial value after a 1200 s period. However, when the test time was lasted to 2 h, the photocurrent remained 60% of the initial one (Figure S9). The chemical dissolution of the WO3 film in the neutral electrolyte might result in the photocurrent damping. In most cases, the acid solution was widely used as the electrolyte for the PEC stability test to overcome the chemical dissolution of the WO3 film.44-46 However, the neutral solution is much closer to the practical PEC condition. One useful route to improve the stability of WO3 photoanodes is to form the heterojunction to avoid the direct contact of WO3 with the electrolyte. For instance, the WO3/BiVO4 heterostructured photoanodes have been examined, and the pretty good photocurrent and stability have been achieved.9,47 Therefore, improving the stability of the sandwich structured WO3 photoelectrode by constructing high-quality heterojunction will be carried out in our future’s work. Hydrogen and oxygen evolutions for the sandwich structured WO3 were tested in a three-electrode configuration with an applied potential of 0.6 V vs. Ag/AgCl under AM 1.5G irradiation. As shown in Figure 6b, the simultaneous generation of hydrogen and oxygen with the molar ratio of ca. 2:1 was demonstrated for the PEC process. By comparing the generated


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hydrogen amount with the expected amount based on the current, the Faradaic efficiency of the sandwich structured WO3 electrode was estimated to be 70-80%, which indicated the existence of probable side reactions.

Figure 6. (a) Photocurrent-time plot (at 1.0 V vs. Ag/AgCl) and (b) time course curves of H2 and O2 evolution (at 0.6 V vs. Ag/AgCl) over the sandwich structured WO3 photoanode in a three-electrode cell under simulated sunlight. The irradiation area was 1.9 cm2 for determining the amount of generated H2 and O2.

To understand the reasons for the varied PEC properties of different WO3 photoelectrodes, the electron transport properties were assessed using the EIS. Equivalent circuit modelling was based on the impedance experimental data, which was plotted as real admittance vs. imaginary admittance.3,48 As shown in



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the inset of Figure 7, Rs denotes the series resistance, CPE is the constant-phase element for the electrode-electrolyte interface, and Rct is the charge-transfer resistance across the interface between the electrode and electrolyte. Rs is affected by the sheet resistance of the electrode and electrolyte, which was detected at high frequency around 100 kHz. The Rct value is in lieu of the diameter of the circle, which is inversely proportional to the efficiency of charge separation. Moreover, the CPE represents an integrated capacitor associated with all trap states. The Rs and Rct values derived from the impedance data were summarized in Table 2. The Rs values had little change. However, the Rct value firstly decreased and then increased with the increased annealing temperature. The sample annealed at 500 °C owned the smallest Rct. The Nyquist experimental data and fitting plot of as-prepared WO3·0.33H2O photoanode were shown in Figure S10. Obviously, the impedance value of as-prepared WO3·0.33H2O photoanode was far greater than those of the annealed samples. As a consequence, the smallest interface resistances for the sample annealed at 500 °C should be indispensable for its higher PEC performance.


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Figure 7. Nyquist experimental data and fitting plots of as-prepared samples after annealing at different temperatures. The inset shows the proposed equivalent circuit. Table 2. Resistance values of Rs and Rct as obtained from the EIS measurements. Sample WO3-400 °C

Rs (ohm) 54.5

Rct (ohm) 1203.0

WO3-475 °C

54.6

219.6

WO3-500 °C

53.2

188.2

WO3-600 °C

57.0

258.5

To further investigate the origin of the varied PEC performances, the electrochemically active surface area (ECSA) of each film was studied, which distinctly impacted the electrocatalytic performance. The ECSA can be estimated from the double-layer capacitance (CDL) of the electrode surface, and the CDL can be determined by measuring the non-Faradaic capacitive current from the scan-rate dependence of cyclic voltammograms (CVs).49 The non-Faradaic potential region is typically a 0.1 V potential range centered at the open-circuit potential (OCP). As shown in equation (3), the capacitive current (ic) is equal to the product of the scan rate (v), and the electrochemical



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double-layer capacitance. Therefore, a plot of ic as a function of v led to a straight line with a slope corresponding to CDL. The ECSA can be calculated from the double-layer capacitance according to equation (4), where Cs is the intrinsic specific capacitance.49 ic = v CDL ESCA =

Cీై C౩

(3) (4)

Figure 8a shows the capacitive currents of the WO3 film annealed at 500 °C with different scan rates. Herein, the determined OCP was 0.25 V vs. Ag/AgCl for the WO3 films. The non-Faradaic potential region should be from 0.20 to 0.30 V vs. Ag/AgCl. Assuming that the intrinsic specific capacitance was the same for all WO3 films, the relative surface area could be determined by the ratio of the capacitive currents.50 The relative area was calculated by comparing the capacitive currents at 0.25 V vs. Ag/AgCl under different scan rates and the results were shown in Figure 8b. It was found that the WO3 film annealed at 500 °C had the largest electrochemically active surface area. The larger surface area of the sandwich structured WO3 nanoplate arrays provided more active sites for the surface reaction. Meanwhile, the highest electrochemically active surface area could result in the smallest interface resistance, since the resistance was inversely proportional to the surface area. Hence, the PEC property was gradually enhanced with the increased ECSA for the WO3 films annealed at 400, 475, and 500 °C. However, the WO3-600 °C sample with the pure monoclinic phase exhibited superior activity than the WO3-400 °C sample with


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the hexagonal phase in spite of the smaller surface area, which revealed that the monoclinic WO3 was more active for PEC water splitting.28 As a consequence, the highest electrochemically active surface area and more active monoclinic phase should contribute to the highest PEC performance of the sandwich structured WO3 annealed at 500 °C.

Figure 8. (a) Cyclic voltammograms showing the capacitive currents for the WO3 film annealed at 500 °C at five different scan rates from 0.025 to 0.3 V/s. (b) The linear relationship between the capacitive current at 0.25 V vs. Ag/AgCl and the scan rate for as-prepared films. The relative electrochemically active surface areas (using the value of WO3·0.33H2O film before annealed as the baseline) are compiled in the inset.



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CONCLUSIONS In summary, this work showed a facile approach for the fabrication of oriented sandwich structured WO3 nanoplate arrays on the FTO substrate. The annealing temperature played an important role in determining the morphology and crystal phase of the WO3 film. Only when annealed at 500 °C, the sandwich structured WO3 nanoplates could be achieved. The sandwich structured WO3 film showed a photocurrent density of 1.88 mA·cm-2 at 1.3 V vs. Ag/AgCl and an IPCE value of 65% at 400 nm, which is one of the best PEC performances for reported WO3 photoanodes. Meanwhile, a simultaneous hydrogen and oxygen generation was demonstrated. It was summarized that the efficient PEC performance should be attributed to the large electrochemically active surface area and active monoclinic phase. It is expected that the current study about the sandwich structured WO3 nanoplate arrays can provide a guidance to develop highly efficient photoelectrodes with the favorable morphology and structure. ACKNOWLEDGMENT The authors thank the financial support from the National Natural Science Foundation of China (Nos. 51236007 and 51323011), the grant support from the China Postdoctoral Science Foundation (No. 2014M560768), and the China Fundamental Research Funds for the Central Universities (xjj2015041). ASSOCIATED CONTENT Supporting Information


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SEM images of WO3·0.33H2O films hydrothermally prepared for varied time and the corresponding WO3 annealed at 500 °C; SEM image of the corresponding WO3 by annealing the powder scraped from the as-prepared WO3·0.33H2O film; SEM images of WO3 films annealed at 500 °C for different time; SEM images of WO3 films annealed at 500 °C for 1 h using different heating and cooling rates; Cross-section SEM images of as-prepared films before and after annealing at different temperatures; Linear sweep voltammetry and EIS of the WO3·0.33H2O film; Photocurrent-time plot over the sandwich structured WO3 photoanode; An overview of representative WO3 photoanodes reported for efficient photoelectrochemical water splitting. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Chen); [email protected] (L. Guo). REFERENCES (1) Liu, C. J.; Yang, Y. H.; Li, W. Z.; Li, J.; Li, Y. M.; Shi, Q. L.; Chen, Q. Y., Highly Efficient Photoelectrochemical Hydrogen Generation Using ZnxBi2S3+x Sensitized Platelike WO3 Photoelectrodes. ACS Appl. Mater. Interfaces 2015, 7, 10763-10770. (2) Ping, Y.; Goddard, W. A.; Galli, G. A., Energetics and Solvation Effects at the Photoanode/Catalyst Interface: Ohmic Contact versus Schottky Barrier. J. Am. Chem.

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Facile Fabrication of Sandwich Structured WO3 Nanoplate Arrays for

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