Comparative analysis of physico-chemical and gas sensing characteristics of two different forms of SnO2 films

In this paper the results of comparative studies on the crystallinity, morphology and chemistry combined with the gas sensor response of two different forms of tin dioxide (SnO2) films prepared by the Rheotaxial Growth and Thermal Oxidation (RGTO) and by the Laser-enhanced Chemical Vapour Deposition (L-CVD) methods, respectively, are presented. For this purpose the X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and X-ray Photoelectron spectroscopy (XPS) have been used. XRD studies for both samples show the contribution from the crystalline SnO2 in the cassiterite rutile phase without any evident contribution from the tin oxide (SnO) phase. SEM and AFM studies show that the surface morphology of RGTO and L-CVD SnO2 samples is characterized by grains/nanograins of different size and surface roughness. In turn XPS studies confirm for both SnO2 samples a slight nonstoichiometry with a relative [O]/[Sn] concentration of 1.8, and a slightly different amount of C contamination at the surface of internal grains with relative [C]/[Sn] concentration of 3.5 and 3.2, respectively. This undesired C contamination cannot be ignored because it creates an uncontrolled barrier for the potential gas adsorption at the internal surface of sensor material. This is confirmed by the gas sensor response in NO2 atmosphere of both SnO2 samples because the sensitivity is evidently smaller for RGTO SnO2 with respect to the L-CVD SnO2 samples, whereas the response time showed a completely opposite tendency. It is probably caused by the smaller extension of internal surface and grain dimension with respect to the Debye length, as well as higher C contamination observed for RGTO SnO2 with respect to L-CVD SnO2 samples