Controlled Growth of WO₃ Photoanode under Various pH Conditions for Efficient Photoelectrochemical Performance

Herein, the effects of the precursor solution's acidity level on the crystal structure, morphology, nucleation, and growth of WO₃·nH₂O and WO₃ nanostructures are reported. Structural investigations on WO₃·nH₂O using X-ray diffraction and Fourier-transform infrared spectroscopy confirm that the quantity of hydrate groups increases due to the interaction between H⁺ and water molecules with increasing HCl volume. Surface analysis results support our claim that the evolution of grain size, surface roughness, and growth direction on WO₃·nH₂O and WO₃ nanostructures rely on the precursor solution's pH value. Consequently, the photocurrent density of a WO₃ photoanode using a HCl-5 mL sample achieves the best results with 0.9 mA/cm² at 1.23 V vs. a reversible hydrogen electrode (RHE). We suggest that the improved photocurrent density of the HCl-5 mL sample is due to the efficient light absorption from the densely grown WO₃ nanoplates on a substrate and that its excellent charge transport kinetics originate from the large surface area, low charge transfer resistance, and fast ion diffusion through the photoanode/electrolyte interface.