Influence of morphology on photoanodic behaviour of WO₃ films in chloride and sulphate electrolytes

Photoelectrochemical (PEC) synthesis is gaining an increasing interest amongst the advanced oxidation processes, which can convert solar energy to chemical energy in the form of value-added oxidants suitable for water treatment and disinfection. To ensure the efficient generation of desired product, careful engineering of semiconductor/electrolyte interface is required. In this study the effect of surface morphology on the competition between the photoanodic oxidation of water molecules and anions occurring on the surface of tungsten (VI) oxide electrodes in sulphate and chloride electrolytes is analysed. Four WO₃ coatings composed of plate-like particles, similar in shape but of different sizes, were prepared using chemical solution deposition technique and methanol, ethanol, isopropanol or butanol as reductants. Crystalline structure, morphology, surface chemical composition and optical properties of the coatings were characterised applying X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and photoluminescence spectroscopy techniques. The photoelectrochemical performance of WO₃ layers was studied using the methods of electrochemical impedance spectroscopy, cyclic voltammetry and chronoamperometry. Possible photoanodic processes are analysed on the basis of the proposed potential-assisted photochemical approach, which takes into consideration high oxidizing power of photogenerated holes in WO₃ and formation of radicals as intermediates. Significant role of specific adsorption of chloride ions as well as intermediate products of their oxidation (Cl·, Cl₂˙⁻) in lowering the photocurrent onset potential, trapping the photogenerated charge carriers as well as protecting WO₃ surface from accumulation of hydroxo species is revealed. Influence of morphology on the competition between certain photoanodic reactions occurring at the semiconductor/electrolyte interface in the PEC systems investigated is explained considering the effects of steric hindrance as well as electrostatic interactions between adsorbing/reacting species at rough electrified interfaces.