Controlling the Structural and Optical Properties of Ta₃N₅ Films through Nitridation Temperature and the Nature of the Ta Metal

The development of a reliable synthetic route to produce high performance Ta₃N₅ photoanodes has been complicated by the large number of synthetic parameters, notably nitridation conditions. A systematic study of nitridation from 850 °C–1000 °C reveals that, contrary to common knowledge, nitridation temperature has little effect on the quality of the Ta₃N₅ produced. Rather, it is the nature of the tantalum starting material and substrate that play a key role. Ta₃N₅ films synthesized by thermal oxidation and subsequent nitridation of Ta thin films on inert fused silica substrates exhibit identical structural and optical properties, regardless of preparation temperature. The optical spectra collected on these samples reveal clear, distinct features that give insight into the electronic band structure. Films grown in the same manner on Ta foils, however, reveal that textured Ta₂N is formed at the Ta₃N₅/Ta interface even at low temperature, as shown by grazing incidence X-ray scattering. Ta₃N₅ on Ta foils is converted to bulk Ta₅N₆ at 1000 °C, and the possible mechanisms for these phase transitions are discussed