Effects of electrodeposition synthesis parameters on the photoactivity of nanostructured tungsten trioxide thin films: optimisation study using response surface methodology

The main aim of this study was to synthesize and characterise nanostructured tungsten trioxide (WO₃) thin films via electrodeposition and subsequently, optimise the electrodeposition synthesis parameters using response surface methodology (RSM). Statistical Box-Behnken RSM design was used to investigate and optimise the effects of four independent electrodeposition synthesis parameters, namely: deposition time, precursor tungsten (W) concentration, annealing temperature and pH. In addition, the synergistic interaction between different electrodeposition synthesis parameters was identified and quantified in enabling a higher photoactivity achievable by nanostructured WO₃ thin films. Resultant nanostructured WO₃ thin films were characterised using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and photocurrent density measurements under one-Sun irradiation. From the electrodeposition synthesis process, it was found that there was a gradual increase in the nanocrystallites WO₃ size from 30 nm to 70 nm when the annealing temperature was varied between 400 °C and 600 °C. XRD results verified the existence of the same photoactive phase of monoclinic WO₃ with increasing annealing temperature with the preferred growth orientation along the (002) planar. Whilst from the Box-Behnken RSM design, it was found that the optimum deposition time, precursor W concentration, annealing temperature and pH were: 60 min, 0.15 mol/L, 600 °C, and pH 1.0, respectively. The highest photocurrent density of 120 μA/cm² was measured at 1 V (versus Ag/AgCl) for nanostructured WO₃ thin film synthesized at the optimum conditions as informed by the Box-Behnken RSM. Further analysis and validation of the Box-Behnken RSM model using analysis of variance (ANOVA) revealed that the RSM-derived statistical predictive model was robust, adequate and representative to correlate the various electrodeposition synthesis parameters to photocurrent density. This study highlights the importance to systematically optimise the electrodeposition synthesis parameters in order to achieve a higher photocurrent density on nanostructured WO₃ thin film for sustainable hydrogen production from photoelectrochemical water splitting reaction.