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
© Li et al.; licensee Springer. 2014
Received: 16 July 2014
Accepted: 11 September 2014
Published: 2 October 2014
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.
The recent advances in nanostructured materials have expanded their potential applications in much-desired materials for efficient solar energy conversion[1–6]. Photoelectrochemical (PEC) water splitting into oxygen and hydrogen is an attractive but challenging way for the conversion of solar energy, following the pioneer work on a TiO2 photoanode for water splitting by Honda and Fujishima. Unfortunately, owing to its wide electronic bandgap (3.0 to 3.2 eV), TiO2 absorbs only an ultraviolet fraction of a solar spectrum (which accounts for just 4% of solar irradiation), being consequently responsible for low efficiency in utilization of solar light[2, 7, 9]. For solar water splitting, intensive researches have been focused on nanostructured materials with narrow bandgaps including WO3[3, 4, 10–19]. WO3, an n-type semiconductor, has attracted immense attention as a photoanode material for water oxidation in PEC cells because of its visible light response (bandgap, Eg = 2.6 to 2.8 eV), a valence band edge position thermodynamically possible for water oxidation (about 3 V versus the normal hydrogen electrode), and good photochemical stability under the acidic conditions[3, 10–12, 20–24].
Porous material design, which has been developed employing template-directed approaches using small organic compounds, supramolecular assembly, and polymer beads, 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 WO3 have been provided to increase the contact area between an electrode and an electrolyte solution and to make electron transport in WO3 films more efficient, enhancing performance of PEC water oxidation at WO3 electrodes[3, 11, 30–33]. For example, Santato et al. have reported that crystalline WO3 photoanodes with interconnected nanoparticulate structures improved photoelectrochemical properties[30–32]. Berger et al. have demonstrated that random porous layers of WO3 produced significantly higher photocurrent efficiency than a compact layer. Our group recently demonstrated a crystalline small mesoporous network of a WO3 photoanode for high improvement in performance of PEC water oxidation.
Numerous methods have been employed to control the dimension, morphology, and crystal structure of WO3, e.g., vacuum evaporation, chemical vapor deposition[35, 36], sol–gel precipitation[22, 30–32], hydrothermal/solvothermal[37–40], 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 WO3[37–40]. 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 WO3. Herein, we report the unique and facile synthesis of mesoporous WO3 utilizing solid H2WO4 as a tungsten precursor with an organic amphiphilic molecule, dodecylamine (DDA), as a surfactant template for porosity of the nanostructure. The mesoporous WO3 exhibited high surface area and improved the performance of PEC water oxidation compared to the corresponding materials prepared without a template.
Tungstic acid (H2WO4) was purchased from Kanto Chemical Co., Inc. (Chuo-ku, Tokyo, Japan). DDA was obtained from Sigma-Aldrich (St. Louis, MO, USA). Polyethylene glycol (PEG, molecular weight = 2,000) was obtained from Wako Chemical Co. (Osaka, Japan). Marpolose (60MP-50) was purchased from Matsumoto Yushi-Seiyaku Co. (Osaka, Japan). An indium tin oxide (ITO)-coated glass substrate was obtained from Asahi Glass Co. (Tokyo, Japan). Millipore water (Merck Ltd., Tokyo, Japan) was used for all the experiments. All other chemicals unless mentioned otherwise were of analytical grade and used as received.
Synthesis of mesoporous WO3
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 N2, followed by changing to O2 flow (at 400°C) for 2 h to result in a WO3 sample (denoted as WO3-DDA). A control sample (denoted as WO3-bulk) was prepared in the same manner except for the addition of DDA.
Preparation of electrodes
The WO3 film-coated ITO electrodes (ITO/WO3) 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, WO3 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 N2 for 1 h, followed by changing to O2 flow (at 400°C) for 2 h.
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 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 three-electrode system has been employed by using ITO/WO3 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 CuSO4) 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 H2 and O2 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
Physicochemical properties of WO 3 samples
Calcination temperature (°C)
Surface area (m2 g-1)
Pore volume (cm3 g-1)
Pore size (nm)
Summary of photoelectrocatalytic water oxidation at different ITO/WO 3 photoanodes calcined at 500°C in 0.1 M phosphate solution
This work was partially supported by the JST PRESTO program and Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Culture, Sports, Science and Technology (No. 24350028). DC thanks JSPS for providing postdoctoral fellowship.
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