Unlocking photoanode performance through a roadmap for electrophoretic deposition of carbon nitride/tungsten oxide heterojunction thin films

Abstract

The formation of high-quality semiconductor thin films is a significant challenge that requires precise control over various factors, including film thickness, uniformity, crystallinity, and strong adhesion to the underlying substrate. These parameters are particularly critical in photoelectrode fabrication, where the thin film must efficiently interact with incident light to maximise absorption and facilitate effective photogenerated charge separation and transport to improve the overall photoelectrochemical performance. In this work, we systematically investigate the formation of graphitic carbon nitride/tungsten oxide (g-C₃N₄/WO₃) hybrid material thin film using electrophoretic deposition (EPD). Transmission electron microscopy reveals direct contact between g-C₃N₄ and WO₃, while X-ray photoelectron spectroscopy indicates electron transfer from g-C₃N₄ to WO₃, confirming the formation of an effective n–n-heterojunction at the interface. We identified four key EPD coating parameters that affect the thin film quality and photoanode performance, including substrate pretreatment, suspension solvent, deposition voltage and time, and post-annealing. Notably, the dispersion of g-C₃N₄/WO₃ heterojunction particles in acetone, in the presence of iodine, results in an excellent film compared to those prepared in water and isopropanol. The most important parameter is the thickness of the film, which must be optimal for light absorption and charge separation: if the film is too thin, it absorbs insufficient light; conversely, if it's too thick, the photogenerated charge carriers recombine before reaching the electrode or electrolyte. By exploring a range of conditions, we determined the optimal substrate surface chemistry and identified acetone as the best solvent with a suspension concentration of 3 mg mL⁻¹. The ideal deposition parameters were a voltage of 60 V for 10 seconds, and post-annealing at 300 °C for 2 hours in air. These optimised conditions allowed us to maximise the functional characteristics of the resulting photoanode, achieving a photocurrent density of 0.2 mA cm⁻² at 1.23 V vs. RHE, which outperformed other carbon nitride-based materials.