Magnetron sputter deposition and nitridation of Ta3N5 thin films for photo-electrochemical water splitting

Abstract

The reliance of human society on fossil fuels has created significant environmental consequences, as such it is imperative that renewable and sustainable sources of energy generation are developed. Hydrogen generation via solar water splitting is a compelling method of renewable energy production and could form the basis for a future hydrogen economy. Tantalum nitride (Ta3N5) is a compelling solar water splitting material due to its appreciable band gap of 2.1 eV and redox potentials that straddle those of water, which enables Ta3N5 to absorb light in the visible spectrum and perform the overall splitting of water into oxygen and hydrogen. However, Ta3N5 self-oxidizes under water splitting conditions which suppresses its performance. This can be avoided by separating Ta3N5 from H2O under water splitting conditions, or by performing the reduction of water with Ta3N5 instead of the oxidation reaction. This thesis describes the synthesis and characterisation of Ta3N5 thin films via sputter deposition and thermal nitridation, with the aim of developing Ta3N5 as a high performance photo-electrode material through the incorporation of chromium and aluminium as acceptor dopants to drive p-type properties. Films are characterised using SEM/EDS, XRD, UV-Vis, and SIMS to determine phase, elemental composition and opto-electric properties. Photo-response and majority carrier concentration were measured using Hall effect apparatus and electrochemical techniques. In this work, films were synthesized with the goal of directly depositing Ta3N5. Several phases within the tantalum nitride phase series were observed, with the metastable TaN group of phases being the most common. Ta3N5 was synthesized from sputtered tantalum oxide films and successfully annealed to produce Ta3N5. The difficulty in depositing Ta3N5 directly is addressed by describing a mechanism for the ammonolysis of Ta2O5 to Ta3N5. Acceptor doped Ta3N5 photo-electrodes were produced and characterised using this method. Chromium doped Ta3N5 films exhibited an exsolved chromium nitride phase in XRD results, and a solubility limit for chromium in Ta3N5 was determined to be ~6 at. %. For aluminium doped films, no exsolved phase was observed and no solubility limit determined under the conditions presented. Aluminium doped Ta3N5 films demonstrated improved photocathode response relative to the standard Ta3N5 films.

Date of Award	2023
Original language	English