Disentangling surface and bulk properties of Ta₃N₅ photoanodes

Understanding how material and defect characteristics govern photoelectrochemical performance is essential for developing efficient and stable photoelectrodes. Although bulk properties are often emphasized, surfaces and interfaces can equally determine activity and stability under operation. Here, we use depth-sensitive characterization to disentangle surface and bulk properties of Ta₃N₅ thin films. By preparing photoelectrodes from TaO_x, TaN_x, and Ta precursors, we systematically vary shallow and deep-level defect concentrations. Structural, compositional, and optoelectronic analyses show that the surfaces consistently exhibit oxygen enrichment, increased structural disorder, and higher deep-level defect densities than the bulk. However, the specific surface structure and its spatial extent depend strongly on precursor chemistry. Ta₃N₅ derived from TaO_x forms an extended, amorphous, oxide-rich surface with fewer deep-level defects, whereas TaN_x and Ta-derived Ta₃N₅ films exhibit thinner, more crystalline surfaces with increased mid-gap defect densities. A brief hydrofluoric acid treatment removes the disordered surface layer, improving crystallinity and hydrophilicity and enhancing photoelectrochemical performance and stability for ferrocyanide oxidation. These results highlight interfacial and defect engineering as routes toward durable, high-efficiency Ta₃N₅ photoelectrodes.