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

Abstract

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 m(2) 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.

Keywords: Mesoporous structure; Photoelectrocatalysis; Tungsten trioxide; Water oxidation.

Figure 1

Small-angle and wide-angle X-ray diffraction patterns. (A) Small-angle and (B) wide-angle XRD patterns of WO₃-DDA and WO₃-bulk samples after being calcined at 400°C and 500°C. (a) WO₃-DDA calcined at 400°C, (b) WO₃-DDA calcined at 500°C, (c) WO₃-bulk calcined at 400°C, and (d) WO₃-bulk calcined at 500°C.

Figure 2

N₂ sorption isotherms and pore size distribution. (A) N₂ sorption isotherms and (B) pore size distribution of WO₃-DDA and WO₃-bulk samples after being calcined at 400°C and 500°C. In N₂ sorption, isotherm adsorption and desorption points are marked by filled and empty symbols, respectively. (a) WO₃-DDA calcined at 400°C, (b) WO₃-DDA calcined at 500°C, and (c) WO₃-bulk calcined at 400°C.

Figure 3

FTIR spectra of WO₃-DDA samples. (a) The as-made sample and samples after being calcined at (b) 400°C and (c) 500°C.

Figure 4

Scanning electron microscopic (SEM) images. Top view of WO₃-DDA samples calcined at (a) 400°C and (b) 500°C. (c) Cross-sectional view of the ITO/WO₃-DDA electrode after being calcined at 500°C.

Figure 5

Cyclic voltammograms (CVs) for samples calcined at 400°C (left) and 500°C (right). (a) ITO/WO₃-DDA and (b) ITO/WO₃-bulk electrodes in a 0.1 M phosphate buffer solution with pH = 6.0. The dashed and solid lines represent CVs measured in the dark and upon irradiation of visible light (λ > 390 nm, 100 mW cm^-2), respectively.

Figure 6

Photocurrent density versus time profiles during PEC water oxidation using samples calcined at 500°C. (a) ITO/WO₃-DDA and (b) ITO/WO₃-bulk electrodes in 0.1 M phosphate buffer solution of pH = 6.0 at 0.5 V versus Ag/AgCl with visible light irradiation (λ > 390 nm, 100 mW cm^-2).