Anomalies in wide band gap SnO2 nanostructures

Band gaps are important characteristics of materials. Most metal oxide nanomaterials, such as ZnO, exhibit band gap widening, however, SnO2 nanomaterials conversely exhibit band gap narrowing. It is important to understand this phenomenon at a more fundamental level. In this work, the reasons behind the band gap narrowing is probed in terms of crystal structure, electronic charge configuration and quantum mechanical effects of energy discretization in the materials. The samples were calcined at 700 °C for 1 h, 2 h, 5 h, 10 h, 24 h and 48 h for the investigation of the different nanostructured SnO2. The crystallite size of SnO2 nanostructures were found to increase with calcination time. Crystal structural parameters were extracted via the Rietveld method and it was found that changes of both the a and c cell parameters were greatest in the crystallite size range of the nano region (100 nm and below). It was observed that there is a correlation between the crystal dimensions with the band gap and conductivities of the nanomaterials. It is proposed that the band gap narrowing is due to the decrease in the hybridization of the SnO2 nanomaterial energy levels resulting in more discrete energy levels and the narrowing of the band gap. This quantum effect may not be seen in other metal oxides with different electronic configurations and crystal structure.