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Abstract
WO3 thin-film photoanodes were electrodeposited from peroxotungstic acid (PTA) solutions onto
fluorine-doped tin oxide substrates and annealed at ≤500ºC for times up to 18 h. The effects of
processing parameters, including deposition potentials and times, compositions of the electrolyte
solutions, and post-deposition annealing conditions, on the physical, mineralogical, optical, and
photoelectrochemical properties of the films were studied.
The amorphous films transformed to the polycrystalline monoclinic phase upon annealing ≥275ºC.
Higher deposition rates were obtained by increasing the deposition potential (–0.3 to –0.6 V versus
Ag/AgCl) and W concentration of PTA (0.05-0.20 mol L-1), which determined the flux of PTA ions
toward the working electrode. The deposition rates also depended on the types (oxalic, formic, and
citric acids) and amounts (0.01-0.10 mol L-1) of carboxylic acids added to PTA as dispersing agents.
The hydronium ions and conjugate bases formed upon dissolution of carboxylic acids participated in
solute dispersion and separation, which altered the mechanisms of nucleation and grain growth. The
deposition rates and times (≤3 h) resulted in films of varied thicknesses (168-1400 nm). The grain
sizes of the films differed slightly (77-122 nm) with the W concentration in PTA and they differed
significantly (42-132 nm) with the sizes of the conjugate bases of the carboxylic acids. Reduced
optical indirect band gaps (Eg; 2.5-3.3 eV) were obtained by increasing the thickness and grain
(particle and/or agglomerate) size of the films, both of which affected the film crystallinity. The
predominance of (002) and (200) X-ray diffraction peaks from planes parallel to the substrate of the
films depended on the nucleation density and the thermally induced recrystallisation kinetics, which
were controlled by the W concentration of PTA and the annealing conditions, respectively. This
resulted in films with three types of preferred orientation: (002), both (002) and (200), and (200).
Enhanced photoelectrochemical properties were obtained from (i) the lower Eg and improved light
absorption associated with films consisting of greater thickness; (ii) the beneficial balances of (a)
surface reaction and recombination sites and (b) hole and electron transports in the film associated
with small agglomerates (~93 nm); and (iii) the low Eg, low valence band maximum, and suppressed
charge-carrier recombination in films associated with (002) orientation.