Electrochemically synthesized tungsten trioxide nanostructures for photoelectrochemical water splitting: Influence of heat treatment on physicochemical properties, photocurrent densities and electron shuttling

Tao Zhu, Meng Nan Chong, Yi Wen Phuan, Eng Seng Chan

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    Abstract

    The primary aim of this study was to investigate the influence of heat treatment on the physicochemical properties, photocurrent densities and capacity for electron shuttling of nanostructured tungsten trioxide (WO3) thin films prepared via an electrochemical deposition method. The as-deposited amorphous WO3 films were further subjected to heat treatment at various annealing temperatures to transform the amorphous material into polycrystalline WO3 nanostructures. X-ray diffraction (XRD) spectra indicated the existence of polycrystalline WO3 nanostructures on the surfaces of the fluorine-doped tin oxide (FTO) electrodes. Through XRD analysis, a clear and distinctive phase transition from amorphous to monoclinic WO3 was observed with elevated heat treatment. This WO3 phase transition was further examined by field emission-scanning electron microscopy (FE-SEM) imaging whereby the surface morphologies of the nanostructured WO3 thin films were observed to progress through four major physical transformation stages during the heat treatment process. Further Fourier transform infrared (FTIR) spectroscopy analysis reaffirmed that the appearance and disappearance of key functional groups during heat treatment coincided with the thermally induced phase transitions of the polycrystalline WO3 nanostructures. In addition, the influence of heat treatment on the intrinsic electron shuttling ability of nanostructured WO3 thin films synthesised at different annealing temperatures was studied using electrochemical impedance spectroscopy (EIS). EIS results indicated that the separation of photogenerated charge carriers in the nanostructured WO3 thin films was greatly enhanced when the films were annealed at 600?C, exhibiting recombination times of 3.65ms. This was thermodynamically linked to the increase in average WO3 crystallite size and the reduction of WO3 grain boundaries during the thermally induced phase transition, which led to the suppression of the electron-hole pair recombination rate.
    Original languageEnglish
    Pages (from-to)297 - 303
    Number of pages7
    JournalColloids and Surfaces A: Physicochemical and Engineering Aspects
    Volume484
    Issue numberNovember 2015
    DOIs
    Publication statusPublished - 2015

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