Formation of internal p-n junctions in Ta₃N₅ photoanodes for water splitting

Tantalum nitride photoanodes for water splitting are fabricated by anodizing tantalum foils, with subsequent nitridation of the foils in NH₃. The as-synthesized Ta₃N₅ film has n-type conductivity. Loading Co ions during and after the anodization process forms a Ta₃N₅:Co film consisting of p- and n-type Ta₃N₅ domains. Both the Ta₃N₅ and Ta₃N₅:Co electrodes have a band gap of 2.0 eV. The p-n junctions in the Ta₃N₅:Co electrode create an internal electrical field favorable for hole transfer from the n-type domains to the p-type domains. When the photoanodes are immersed in a 0.5 M KOH aqueous solution for water splitting with one-sun illumination, the Ta₃N₅:Co photoanode exhibits photocurrents an order of magnitude higher than those of the bare Ta₃N₅ photoanode. AC impedance spectroscopy analysis reveals that the p-n junctions formed by Co-doping reduce the interfacial charge transfer resistance by an order of magnitude. The diffuse reflectance spectra of the electrodes indicate that Co incorporation minimizes the defect states in the bulk Ta₃N₅. Intensity-modulated photocurrent spectroscopy analysis reveals that the high electron transit rate of the Ta₃N₅:Co electrode can be attributed to its fewer defect states. A photoelectrochemical reaction using the Ta₃N₅:Co photoanode produces H₂ and O₂ at a ratio close to 2 : 1, and N₂ evolution from the reaction is negligible. The present study demonstrates the establishment of a p-n junction configuration that considerably enhances the performance of nitride anodes in photocatalyzed water splitting.