Photoelectrochemical Properties of WO₃ Nanoparticulate Thin Films Prepared by Carboxylic Acid-Assisted Electrodeposition

The present work reports the synthesis of transparent (∼60–80% transmission) WO₃ thin films by electrodeposition using peroxotungstic acid (PTA) solutions containing different carboxylic acids (formic, oxalic, and citric) of varying concentrations. The effects of these acids on the deposition kinetics and the morphological, mineralogical, optical, and photoelectrochemical properties of the films were investigated using scanning electron microscopy, X-ray diffraction, UV–vis spectrophotometry, and photoelectrochemical measurements. The deposited film thickness was dependent on the availability of hydronium ions (for hydrogen bonding with PTA ions) and molecular drag in the electrolytes, both of which were controlled through the use of carboxylic acids of different concentrations, degrees of dissociation, molecular weights, and associated sizes of the conjugate bases. The films consisted of agglomerates/particles (42–132 nm), the sizes of which decreased and the true porosities (<6%) of which increased with the amounts and sizes of the conjugate bases. These observations were considered to result from the separation of the PTA ions according to the amounts and sizes of the conjugate bases, the effects of which consequently changed the mechanisms of nucleation and grain growth. A nonlinear relationship between the grain sizes and the photoelectrochemical performances of the films was obtained where a maximum in the photocurrent density was observed for the film consisting of small agglomerates of ∼93 nm (deposited with 0.03 mol L^–1 oxalic acid). This was considered to result from the positive balance of (i) surface reaction and recombination sites and (ii) the hole and electron transports in the film.