The role of surface stoichiometry in NO2 gas sensing using single and multiple nanobelts of tin oxide

Abstract

Typically used semiconducting metal oxides (SMOs) consist of several varying factors that affect gas sensor response, including film thickness, grain size, and notably the grain–grain junctions within the active device volume, which complicates the analysis and optimisation of sensor response. In comparison, devices containing a single nanostructured element do not present grain–grain junctions, and therefore present an excellent platform to comprehend the correlation between nanostructure surface stoichiometry and sensor response to the depletion layer (Debye length, L_D) variation after the analyte gas adsorption/chemisorption. In this work, nanofabricated devices containing SnO₂ and Sn₃O₄ individual nanobelts of different thicknesses were used to estimate their L_D after NO₂ exposure. In the presence of 40 ppm of NO₂ at 150 °C, L_D of 12 nm and 8 nm were obtained for SnO₂ and Sn₃O₄, respectively. These values were associated to the sensor signals measured using multiple nanobelts onto interdigitated electrodes, outlining that the higher sensor signal of the Sn⁴⁺ surface (up to 708 for 100 ppm NO₂ at 150°) in comparison with the Sn²⁺ (up to 185) can be explained based on a less depleted initial state and a lower surface electron affinity caused by the Lewis acid/base interactions with oxygen species from the baseline gas. To support the proposed mechanisms, we investigated the gas sensor response of SnO₂ nanobelts with a higher quantity of oxygen vacancies and correlated the results to the SnO system.