Synthesis and photoelectrochemical characterization of (ammonolyzed) Nb2O5 thin films

Abstract

Growing energy demand and the need to reduce CO₂ emissions have driven research into solar energy conversion technologies, such as water splitting by photoelectrochemical processes (PEC). In this context, niobium pentoxide (Nb₂O₅) is a promising material due to its stability and semiconducting properties, although its wide bandgap limits its absorption in the visible spectrum. To improve its performance, ammonolysis has been explored as a method to modify its electronic structure and enhance its photoelectrochemical response. Nb₂O₅ thin films were synthesized by dip-coating on silicon and quartz glass substrates, followed by heat treatment and ammonolysis at temperatures of 750 and 800 °C in an ammonia atmosphere. Structural characterization confirmed the formation of Nb₂O₅ with a pseudohexagonal structure, while X-ray photoelectron spectroscopy (XPS) analysis demonstrated the incorporation of nitrogen in the ammonia-treated samples. The performance of the photoelectrodes was evaluated through PEC tests under UV (500 W/m²) and visible (1000 W/m²) light illumination. On silicon substrates, films ammonolyzed at 800 °C showed a significant increase in photogenerated current under UV irradiation (~1 mA), indicating an improvement in charge separation efficiency. However, the response in the visible spectrum remained low (~100 μA), suggesting that although ammonolysis slightly reduces the bandgap, absorption is still insufficient for efficient visible light conversion. In contrast, films on quartz glass exhibited negligible photoelectrochemical activity, indicating that this substrate does not favor charge transport under the evaluated conditions. Post-PEC XPS analysis revealed that the ammonolyzed samples suffered from charging effects, indicating a loss of conductivity. The survey spectrum showed an increase in oxygen and carbon content, likely due to contamination from the electrolyte or reaction intermediates. Additionally, the emergence of more pronounced silicon peaks suggests degradation or partial detachment of the thin film during electrochemical testing. The chemical stability evaluation was hindered by these charging effects, preventing a conclusive assessment of post-PEC oxidation states and composition. In conclusion, ammonolysis at 800 °C enhances the PEC activity of Nb₂O₅ in the UV range but does not significantly improve its efficiency in the visible spectrum. Furthermore, the post-PEC analysis indicates that the thin films may degrade or detach over time under electrochemical conditions. These findings highlight the need for additional strategies, such as doping, heterostructure formation, or improved stabilization techniques, to optimize the material for long-term PEC applications.