Characterization of optical, chemical, and structural changes upon reduction of sol–gel deposited SnO2 thin films for optical gas sensing at high temperatures

Abstract

Sensor technologies that can operate under extreme conditions including high temperatures, high pressures, highly reducing and oxidizing environments, and corrosive gases are needed for process monitoring and control in advanced fossil energy applications. Optical sensors based on metal oxide thin films are important candidates and an improved understanding of the fundamental link between structural/chemical changes in thin films and measured optical properties in the presence of various gas species under relevant conditions is needed to aid in sensor development efforts. In support of this need, the manuscript describes the synthesis and structural/optical characterization of SnO2 thin films through standard sol–gel techniques that are subsequently reduced in an inert atmosphere containing 4–10% hydrogen. Early stage film reduction occurs through formation of metallic liquid Sn nanoparticles upon exposure to hydrogen at elevated temperatures (>~500°C) that solidify upon cooling to room temperature. Simulated sensing measurements illustrate that a significant change in transmission of films deposited on planar substrates were observed under conditions where significant film reduction occurred. Because film reduction is an irreversible process in the absence of an oxidizing species, these results suggest that SnO2 may have limited practical use as a high temperature material for optical sensing in reducing atmospheres. Our results also highlight that caution must be employed in the interpretation of high temperature sensing results based upon standard surface interaction models for temperatures and gas environments where bulk film reduction may occur. This work serves as a basis for similar investigations in other standard metal oxide systems for gas sensing applications such as TiO2, ZnO, and WO3.