Exploring Platinum Thin Films as Electrodes for High-Temperature and -Pressure Electrochemical Studies in Aqueous Systems

This work reports a methodology for the fabrication of Pt thin-film electrodes for electrochemical studies in hydrothermal systems. The research process was meticulous, with particular attention paid to the multilayer Ti/Pt/Ti/Al₂O₃ film structure and annealing conditions that are expected to impact the morphology of the films, surface composition and electrochemical response. The findings of this study are significant, as they provide valuable insights into the behaviour of Pt thin-film electrodes in various media and conditions. Two different approaches were adopted for the preparation of the electrodes: in one case, the Ti/Pt/Ti/Al₂O₃ films deposited on sapphire wafers were exposed to rapid thermal annealing at 900°C under argon for 5 min, followed by argon ion milling to etch the final electrode pattern (Pt-RA(900)), while in the other case, uncapped Pt films were annealed, after etching, in a tubular oven under argon at specific temperatures between 200°C and 900°C (Pt-TO). Rapid annealing at 900°C on capped films resulted in the formation of a Pt₃Ti intermetallic alloy with remarkable mechanical and chemical stability even after 10 h of immersion in deionised water, acid (0.1 M H₂SO₄) and alkaline media (0.1 M KOH) conditions at temperatures up to 150°C, despite the dissolution of the Al₂O₃ top layer at 150°C and long immersion times (> 10 h). In the case of uncapped Pt films, diffusion and oxidation of Ti through the Pt film at high temperature resulted in the formation of TiO₂ on the surface of Pt. The results were confirmed by using a comprehensive suite of ex-situ characterisation techniques to follow changes in the Pt electrode surface morphology and composition before and after immersion in H₂O, 0.1 M H₂SO₄ and 0.1 M KOH solutions under argon. Ex-situ electrochemical characterisation studies were also conducted to correlate the changes in the electrode surface properties, including the electrochemical surface area, with different annealing conditions and after various hydrothermal treatments in neutral, alkaline and acidic media.