Transition-metal oxide and nitride semiconductors offer considerable promise for photoelectrochemical energy conversion. Yet their efficiency and stability are often limited by surface and interface processes, where atomic structure, electronic defects, and chemical reactions are strongly coupled. In such systems, disorder, point defects, and dynamic surface transformations govern charge transport, interfacial charge transfer, and degradation. Here, we investigate Ta₃N₅ thin film photoanodes as a model system to determine how defect chemistry, structural disorder, and surface composition influence photoelectrochemical performance and stability.

Using controlled synthesis from different precursors, we prepared Ta₃N₅ photoelectrodes with varied oxygen contents, structural order, and shallow- and deep-level defect concentrations. Reduced oxygen incorporation improves long-range structural order but increases deep-level defect densities, whereas higher oxygen concentrations introduce shallow donor states and passivate detrimental mid-gap defects. Depth-sensitive characterization reveals oxygen-enriched, structurally disordered surface regions with elevated defect densities relative to the bulk. The extent and nature of these surface layers depend strongly on precursor chemistry: Ta₃N₅ derived from tantalum oxide forms an extended amorphous oxide-rich surface, while Ta₃N₅ from tantalum nitride or metallic tantalum exhibits thinner, more crystalline surfaces with higher mid-gap state densities.

Photoelectrochemical stability measurements show that under water oxidation conditions, Ta₃N₅ photoanodes degrade through formation of a surface oxide layer that suppresses hole injection and enhances recombination. In contrast, under ferrocyanide oxidation conditions, oxygen-rich films exhibit improved long-term stability, whereas oxygen-poor films with high deep-level defect densities degrade rapidly. Specifically, shallow oxygen donors can kinetically stabilize the interface, whereas deep-level defects facilitate rapid photocarrier trapping and surface oxidation[1]. Brief hydrofluoric acid treatment removes the disordered surface layer, improves crystallinity and hydrophilicity, and enhances performance and stability. Overall, this work highlights the importance of defect, surface, and interface engineering for improving charge transport, corrosion resistance, and catalyst integration in durable photoelectrodes for solar fuel generation.