Structural Properties of NdTiO₂₊N_1- and Its Application as Photoanode

Mixed-anion inorganic compounds offer diverse functionalities as a function of the different physicochemical characteristics of the secondary anion. The quaternary metal oxynitrides, which originate from substituting oxygen anions (O^2-) in a parent oxide by nitrogen (N^3-), are encouraging candidates for photoelectrochemical (PEC) water splitting because of their suitable and adjustable narrow band gap and relative negative conduction band (CB) edge. Given the known photochemical activity of LaTiO₂N, we investigated the paramagnetic counterpart NdTiO₂₊N_1-. The electronic structure was explored both experimentally and theoretically at the density functional theory (DFT) level. A band gap (E_g) of 2.17 eV was determined by means of ultraviolet-visible (UV-vis) spectroscopy, and a relative negative flat band potential of -0.33 V vs reversible hydrogen electrode (RHE) was proposed via Mott-Schottky measurements. ¹⁴N solid state nuclear magnetic resonance (NMR) signals from NdTiO₂₊N_1- could not be detected, which indicates that NdTiO₂₊N_1- is berthollide, in contrast to other structurally related metal oxynitrides. Although the bare particle-based photoanode did not exhibit a noticeable photocurrent, Nb₂O₅ and CoO overlayers were deposited to extract holes and activate NdTiO₂₊N_1-. Multiple electrochemical methods were employed to understand the key features required for this metal oxynitride to fabricate photoanodes.