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Unique and facile solvothermal synthesis of mesoporous WO3 using a solid precursor and a surfactant template as a photoanode for visible-light-driven water oxidation

Mesoporous tungsten trioxide (WO3) was prepared from tungstic acid (H2WO4) as a tungsten precursor with dodecylamine (DDA) as a template to guide porosity of the nanostructure by a solvothermal technique. The WO3 sample (denoted as WO3-DDA) prepared with DDA was moulded on an electrode to yield efficient performance for visible-light-driven photoelectrochemical (PEC) water oxidation. Powder X-ray diffraction (XRD) data of the WO3-DDA sample calcined at 400°C indicate a crystalline framework of the mesoporous structure with disordered arrangement of pores. N2 physisorption studies show a Brunauer-Emmett-Teller (BET) surface area up to 57 m2 g-1 together with type IV isotherms and uniform distribution of a nanoscale pore size in the mesopore region. Scanning electron microscopy (SEM) images exhibit well-connected tiny spherical WO3 particles with a diameter of ca. 5 to 20 nm composing the mesoporous network. The WO3-DDA electrode generated photoanodic current density of 1.1 mA cm-2 at 1.0 V versus Ag/AgCl under visible light irradiation, which is about three times higher than that of the untemplated WO3. O2 (1.49 μmol; Faraday efficiency, 65.2%) was evolved during the 1-h photoelectrolysis for the WO3-DDA electrode under the conditions employed. The mesoporous electrode turned out to work more efficiently for visible-light-driven water oxidation relative to the untemplated WO3 electrode.

Porous material design, which has been developed employing template-directed approaches using small organic compounds [25], supramolecular assembly [26], and polymer beads [27], is of great importance in many research fields because of the high porosity, large area per unit volume, and favorable design of a porous structure [25,28,29]. So far, several efforts in nanostructural and porosity controls of WO 3 have been provided to increase the contact area between an electrode and an electrolyte solution and to make electron transport in WO 3 films more efficient, enhancing performance of PEC water oxidation at WO 3 electrodes [3,11,[30][31][32][33]. For example, Santato et al. have reported that crystalline WO 3 photoanodes with interconnected nanoparticulate structures improved photoelectrochemical properties [30][31][32]. Berger et al. have demonstrated that random porous layers of WO 3 produced significantly higher photocurrent efficiency than a compact layer [33]. Our group recently demonstrated a crystalline small mesoporous network of a WO 3 photoanode for high improvement in performance of PEC water oxidation [3].
Numerous methods have been employed to control the dimension, morphology, and crystal structure of WO 3 , e.g., vacuum evaporation [34], chemical vapor deposition [35,36], sol-gel precipitation [22,[30][31][32], hydrothermal/ solvothermal [37][38][39][40], surfactant/hard template techniques [3,41,42], and so on. Among the abundant methods, hydrothermal/solvothermal techniques can provide a cost-effective and one-step route synthesis of WO 3 [37][38][39][40]. Although the surfactant template techniques require a liquid tungsten precursor to utilize interaction with a surfactant in principle, we have focused on the interaction between a solid tungsten precursor and a surfactant under solvothermal conditions to yield mesoporous WO 3 . Herein, we report the unique and facile synthesis of mesoporous WO 3 utilizing solid H 2 WO 4 as a tungsten precursor with an organic amphiphilic molecule, dodecylamine (DDA), as a surfactant template for porosity of the nanostructure. The mesoporous WO 3 exhibited high surface area and improved the performance of PEC water oxidation compared to the corresponding materials prepared without a template.

Synthesis of mesoporous WO 3
In a typical synthesis, 1.7 g of DDA (9.0 mmol) was dissolved in 15 mL ethanol under stirring at room temperature. Tungstic acid (0.9 g; 3.6 mmol) was added to the DDA solution with stirring for 30 min to yield a suspension. It was transferred to a Teflon-lined stainless steel autoclave and then placed in an oil bath at 150°C for 24 h. After the autoclave was cooled down to room temperature, the solid product was recovered by centrifugation, then washed repeatedly by ethanol and air-dried. The solid product was calcined at 400°C with a rate of 1°C min −1 and then maintained at 400°C for 1 h in flowing N 2 , followed by changing to O 2 flow (at 400°C) for 2 h to result in a WO 3 sample (denoted as WO 3 -DDA). A control sample (denoted as WO 3 -bulk) was prepared in the same manner except for the addition of DDA.

Preparation of electrodes
The WO 3 film-coated ITO electrodes (ITO/WO 3 ) were prepared employing a doctor-blade technique. Before coating, ITO glass substrates (1.0 cm −2 area) were cleaned up by a UV-ozone treatment (photo surface processor PL16-110, Sen Lights Co., Osaka, Japan) for 15 min. In a typical procedure, WO 3 powder (200 mg), PEG (100 mg), and Marpolose (20 mg) were mixed in 300 μL of water. The mixture suspension was stirred for approximately 2 to 4 h until a smooth paste was formed. The resulting paste was squeezed over an ITO glass substrate by a doctor-blade coater and dried at 80°C for 15 min. After repeating the procedure for two times, the electrodes were calcined at 400°C and maintained at 400°C in flowing N 2 for 1 h, followed by changing to O 2 flow (at 400°C) for 2 h.

Structural characterization
Characterization of the morphological features and the crystalline phase was conducted by field-emission scanning electron microscopy (FESEM; JSM-6500 F, JEOL Ltd., Akishima, Tokyo, Japan) and powder X-ray diffraction (XRD; MiniFlexII, Rigaku Corporation, Tokyo, Japan) using monochromated Cu Kα (λ = 1.54 Å) radiation. Nitrogen adsorption-desorption isotherms were measured using a BELSORP-miniII (BEL Japan, Inc., Osaka, Japan) at 77 K. Prior to gas adsorption, samples were degassed in vacuum for 4 h at 150°C. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the surface areas. The pore size distributions were obtained from analysis of the adsorption branches of the isotherms by the Barrett-Joyner-Halenda (BJH) method. Fourier transform infrared spectra were recorded on a Jasco FT/IR-4200 spectrophotometer (Jasco Inc., Tokyo, Japan).

Photoelectrochemical measurements
Photoelectrochemical measurement was carried out in a two-compartment photoelectrochemical cell separated by a Nafion membrane using an electrochemical analyzer (HZ-3000, Hokuto Denko Co. Ltd., Tokyo, Japan). A threeelectrode system has been employed by using ITO/WO 3 and Ag/AgCl electrodes in one compartment as the working and reference electrodes, respectively, and a Pt wire in the other compartment as the counter electrode. An aqueous 0.1 M phosphate solution was used as an electrolyte in both compartments of the cell, which was saturated with Ar gas prior to the measurement. The cyclic voltammogram (CV) was recorded at a scan rate of 50 mV s −1 at 25°C. Light (λ > 390 nm) was irradiated from the backside of the working electrode using a 500-W xenon lamp (Optical ModuleX; Ushio Inc., Tokyo, Japan) with a UV-cut filter (L39) and liquid filter (0.2 M CuSO 4 ) for cutting of heat ray. The output of light intensity was calibrated as 100 mW cm −2 using a spectroradiometer (USR-40; Ushio Inc., Tokyo, Japan). Photoelectrocatalysis was conducted under the potentiostatic conditions of 0.5 V versus Ag/AgCl at 25°C under illumination of light (λ > 390 nm, 100 mW cm −2 ) for 1 h. The amounts of H 2 and O 2 evolved were determined from the analysis of the gas phase (headspace volume: 87.3 mL) of counter and working electrode compartments, respectively, using gas chromatography (GC-8A with a TCD detector and molecular sieve 5A column and Ar carrier gas; Shimadzu Corporation, Kyoto, Japan).

Results and discussion
The powder XRD patterns of the WO 3 samples calcined at 400°C and 500°C are shown in Figure 1. Small-angle XRD patterns (Figure 1 (a)) of WO 3 -DDA at 400°C showed a single diffraction peak at low 2θ, being a sign of formation of mesoporous structures, but the weak intensity and broadness of the peak are possibly due to disordered mesoporous structures. The d-spacing, calculated from the XRD peak at 2θ = 2.3°is 3.78 nm. Weakening of the intensity of the diffraction peak for WO 3 -DDA at 500°C (Figure 1 (b)) suggests degradation of the mesostructure at higher temperature. The wideangle XRD patterns of both the WO 3 -DDA and WO 3bulk samples revealed crystallization of the framework after calcination at 400°C and higher degree of crystallization at 500°C, though crystallinity of WO 3bulk seems to be higher than that of WO 3 -DDA at both calcination temperatures. The d-spacings calculated from the XRD peaks of both WO 3 -DDA and WO 3 -bulk were in good agreement with phase-pure monoclinic WO 3 (JCPDS number: 43-1305). Average crystallite sizes for WO 3 -DDA, estimated using [002] reflections were 5.7 and 11.6 nm at 400°C and 500°C, respectively, which suggests that progressive growth of the WO 3 nanocrystal in the porous network is responsible for degradation of the mesostructure at 500°C. N 2 adsorption/desorption isotherms of the WO 3 samples calcined at 400°C and 500°C are shown in Figure 2. The isotherm (Figure 2 (a)) of WO 3 -DDA calcined at 400°C could be classified as type IV, characteristic of mesoporous materials [26,43]. In this isotherm, the adsorption amount gradually increased in a range of P/P 0 = 0.4 to 0.85, which could be explained by the classical capillary condensation observed for mesopores. The H2 hysteresis loop in the isotherm (Figure 2 (a)) may be caused by roughness of the pore and particle surface [44]. The BET surface area and mesopore volume for WO 3 -DDA calcined at 400°C were 57 m 2 g −1 and 0.08 cm 3 g −1 , respectively, as summarized in Table 1. The pore size distribution (Figure 2) by the BJH method shows narrow distribution with a peak pore width at 4.9 nm. Isotherm of WO 3 -DDA calcined at 500°C shows a predominantly type II nature, and the BET surface area was drastically reduced to 12 m 2 g −1 . The pore size distribution of WO 3 -DDA calcined at 500°C gives a wider peak at approximately 50 nm due to large interparticle pores. These results are in accordance with the degradation of the mesoporous structure of WO 3 -DDA due to progressive growth of WO 3 nanocrystals at higher temperature of 500°C, as observed in the XRD measurement. The WO 3 -bulk sample synthesized without DDA exhibited typical type II isotherms, characteristic of nonporous solids. The BET surface area is 24 m 2 g −1 at 400°C, which is noticeably low compared to the mesoporous WO 3 -DDA.
The Fourier transform infrared (FTIR) spectra of asmade (before calcination) and calcined (400°C and 500°C ) WO 3 -DDA samples are shown in Figure 3. C-H stretching vibration bands of the hydrocarbon chains at 2,919 cm −1 (asymmetric) and 2,844 cm −1 (symmetric) along with C-H bending vibration bands at 1,469 cm −1 of CH 2 groups were clearly observed in the as-made sample. Comparing the FTIR spectra of the as-made WO 3 -DDA with calcined WO 3 -DDA samples, we could see that peaks due to C-H vibration diminished completely for the calcined samples. This indicates complete removal of DDA during calcination at 400°C and 500°C, which is very much necessary to generate high porosity for these mesoporous materials.
The scanning electron microscopy (SEM) images of the calcined WO 3 -DDA samples are shown in Figure 4. The SEM images of the top view (Figure 4a,b) exhibit that a mesoporous network is composed of tiny spherical WO 3 particles of ca. 5 to 20 nm in diameter, being well connected to each other. In a few places, the spherical particles agglomerate to form large particles. A close look into these images suggests that the average dimension of particles increases with calcination temperature from 400°C to 500°C due to sintering of WO 3 nanocrystals at higher calcination temperature. After preparation of a  mesoporous WO 3 film on an ITO electrode, the film thickness was measured to be ca. 12 μm from the crosssectional SEM image (Figure 4c). This crystalline mesoporous structure of the connected WO 3 particles is important to yield a large interface between the electrolyte and film as well as efficient electron transport through the film, which are consequently expected to work efficiently for PEC water oxidation since the electron and hole pairs generated by photoexcitation of WO 3 would have less chance to recombine before participating in a water oxidation reaction at the WO 3 surface. The PEC properties of the ITO/WO 3 electrodes were studied in a 0.1 M phosphate solution. Figure 5 shows the CVs of the ITO/WO 3 electrodes. On CVs of samples  calcined at 400°C for both WO 3 -DDA and WO 3 -bulk ( Figure 5, left), no redox response was observed in the dark in a potential range of 0.4~1.0 V versus Ag/AgCl except for a response based on WO 3 /H x WO 3 below 0.2 V. Upon irradiation of visible light, the anodic current (0.13~0.18 mA cm −2 at 1.0 V versus Ag/AgCl) was hardly generated for both samples. This is ascribed to insufficient crystallinity of both WO 3 -DDA and WO 3 -bulk calcined at 400°C. Crystallinity rather than porosity for samples calcined at 400°C is a dominant factor for the PEC performance of the WO 3 -based photoanode under the conditions employed [20,32]. On CVs of the samples calcined at 500°C (Figure 5, right), the significantly high photoanodic current due to water oxidation was observed upon visible light irradiation above an onset potential of 0.17 V versus Ag/AgCl due to higher crystallinity. The photoanodic current reached 1.1 mA cm −2 at 1.0 V for WO 3 -DDA, which is about three times higher compared to that for the WO 3 -bulk (0.36 mA cm −2 at 1.0 V) electrode in spite of the degradation of mesoporous structure for WO 3 -DDA calcined at 500°C.
The degraded mesoporous structure for WO 3 -DDA might result in favorable conditions for PEC water oxidation compared with the nanoparticle structure of WO 3 -bulk. Otherwise, another important factor might be involved in the higher performance of the WO 3 -DDA electrode. In the present paper, we do not pursue interpretation of the higher performance of the WO 3 -DDA electrode because our attention is on the solvothermal synthesis of a mesoporous structure of WO 3 .
Photoelectrocatalysis over the ITO/WO 3 electrodes was conducted in a 0.1 M phosphate solution (pH = 6.0) under potentiostatic conditions at 0.5 V versus Ag/AgCl for 1 h upon visible light irradiation (λ > 390 nm, 100 mW cm −2 ). The photocurrent-time profiles of both WO 3 -DDA and WO 3 -bulk calcined at 500°C exhibit initial spikes in the photocurrent upon illumination (related with the capacitance component at the solidliquid interface), followed by a photocatalytic current, as shown in Figure 6. The photocurrent density of WO 3 -DDA at 1 min was 0.37 mA cm −2 , which is 2.5 times higher than that of the WO 3 -bulk (0.15 mA cm −2 at 1 min) electrode. The charge amount passed during 1-h photoelectrocatalysis for WO 3 -DDA (0.89 C) was 3.9 times higher than that of WO 3 -bulk (0.23 C). As a consequence of the high charge amount, the markedly high amount (n O2 = 1.5 μmol, Faradaic efficiency (FE O2 ) = 65%) of O 2 evolved for the WO 3 -DDA electrode compared to that (n O2 = 0.4 μmol, FE O2 = 58%) of the WO 3 -bulk electrode, as summarized in Table 2. As compared with performances of PEC water oxidation under the same conditions for WO 3 -based photoanodes reported earlier [3], the performance of the present mesoporous WO 3 -DDA is lower than that of the small mesoporous WO 3 film (n O2 = 4.2 μmol, FE O2 = 79%) [3], but much higher than that of interparticle mesoporous WO 3 (n O2 = 0.9 μmol, FE O2 = 61%) and bulk WO 3 (n O2 = 0.4 μmol, FE O2 = 44%) [3]. The high performance of the mesoporous WO 3 -DDA photoanode is attributed to its high surfaceto-volume ratio which offers a large number of water oxidation sites at the electrolyte-WO 3 interface, and wellconnected WO 3 particles for efficient electron transport through the film.

Conclusions
We have prepared mesoporous WO 3 materials by a unique and facile solvothermal method using solid H 2 WO 4 as a tungsten precursor. DDA was used as a template for the  formation of nanostructure, which generates mesoporosity after removing DDA by calcination. The present surfactant template technique is very unique in terms of use of a solid tungsten precursor in a solvothermal method, compared with a common technique using liquid tungsten precursors for interaction with surfactants in principle. The mesoporous network has a disordered arrangement of pores which is composed of well-connected tiny spherical WO 3 particles with a diameter of ca. 5 to 20 nm. The DDA-templated WO 3 photoanode showed three times higher photoanodic current density upon visible light irradiation and provided the efficient performance of PEC water oxidation compared to the untemplated WO 3 , which is promising as an efficient material for high-performance solar energy conversion.