Correlation between annealing conditions, microstructure, and photoelectrochemical activity of electrodeposited WO₃ thin films

Abstract

Among various photoanodes, tungsten oxide (WO₃) stands out due to its visible light absorption, long hole diffusion length, and chemical stability in acidic electrolytes. In this study, WO₃ thin films were fabricated onto FTO substrates by using pulsed voltage electrodeposition (PVE). The structural (XRD), morphological and compositional (SEM–EDX), optical (UV–Vis), electronic (Mott–Schottky), surface-chemical (XPS), and LSV measurements were analyzed in detail to understand their PEC properties. A two-stage thermal optimization was then applied to establish an efficient photoanode protocol. In the first stage, the annealing temperature was varied from 250 °C to 400 °C for 2 h. Characterization results showed that 300 °C is a critical crystallization threshold. Below this point, films remain amorphous with negligible photocurrent. Above it, excessive grain growth reduces the active surface area. Specifically, pore density decreased from 22.3 % at 250 °C to 3.82 % at 400 °C. In the second stage, annealing durations between 0.5 and 6 h were tested at 300 °C. Crystallization effectively began after 2 h of annealing. Across the investigated range, average crystallite size increased from 1.22 nm to 18.9 nm. A 4-hour treatment yielded the highest photocurrent density. This sample also showed a donor density of 7.17 × 10¹⁹ cm⁻³ and the optimal Fermi level alignment (E_c−E_f = 0.00679 eV). In contrast, 6 h of annealing induced structural overdevelopment and grain boundary defects, reducing the photocurrent. At 300 °C, an annealing time of 2 h marked the onset of effective crystallization, while 4 h yielded the highest photocurrent density, identifying this condition as the optimal protocol for electrodeposited WO₃ photoanodes. This condition balances crystallinity, morphology, and charge carrier dynamics. XPS further confirmed that oxygen vacancies are the main donors in the films, and the optimized 4 h film sustained a stable photocurrent over 5 h of continuous illumination. The optimized films provide a reproducible baseline for next-generation development. Furthermore, they serve as a foundation for heterojunction architectures. Coupling WO₃ with complementary semiconductors is expected to further suppress recombination and extend light harvesting for efficient hydrogen production.