Gas-phase diagnostics during H2 and H2O plasma treatment of SnO2 nanomaterials: Implications for surface modification

Abstract

Although H2O plasmas are commonly used to enhance the surface oxygen content of various materials, our prior study found that H2O plasma modification of SnO2 nanomaterials resulted in reduction of Sn+4 to Sn0. To further explore this phenomenon and develop a deeper understanding of the mechanism for this behavior, gas-phase species were detected via optical emission spectroscopy during H2O plasma processing (nominally an oxidizing environment), both with and without SnO2 substrates in the reactor. Gas-phase species were also detected in the reducing environment of H2 plasmas, which provided a comparative system without oxygen. Sn* and OH* appear in the gas phase in both plasma systems when SnO2 nanowire or nanoparticle substrates are present, indicative of SnO2 etching. Furthermore, H2 and H2O plasmas reduced the Sn in both nanomaterial morphologies. Differences in H* and OH* emission intensities as a function of plasma parameters show that plasma species interact differently with the two SnO2 morphologies. The H2O plasma gas-phase studies found that under most plasma parameters, the ratio of reducing to oxidizing gas-phase species was ≥1, indicating a reducing environment rather than an oxidizing environment.