p-n Transition-enhanced Sensing Properties of rGO-SnO2 Heterojunction to NO2 at Room Temperature

RGO-SnO2 heterojunction has been extensively studied as an efficient sensitive material, while its sensing mechanism in surface chemical system is still needed to be concerned. The aim of this work is to evaluate the effects of rGO and materials processing on surface sites of nanocrystalline tin dioxide and room-temperature sensitivity to NO2. Gas-sensitive heterojunction of tin dioxide with 2-dimensional rGO has been synthesized by a one-step hydrothermal method. Contrastive researches about composition, microstructure and surface species of hydrothermal-obtained pristine SnO2, rGO-SnO2 hybrids were performed. Further reduction of rGO in hydrothermal composite with a post-treatment at over 100 °C has been found to affect the hydrophilicity and electrical conduction of rGO-SnO2 composite. Sensitivity of rGO-SnO2 to 2–8 ppm NO2 with fast response was demonstrated at room temperature, while the abrupt change of conductivity type of hydrothermal-obtained rGO-SnO2 heterojunction from $p$ -type to $n$ -type was observed after post-anneal treatment. The conductive type conversion was discussed to be closely related to the higher reduction degree of rGO and enhanced hydrophobic surface of rGO-SnO2 heterojunction.