One-Step Synthesis, Structure, and Band Gap Properties of SnO ₂ Nanoparticles Made by a Low Temperature Nonaqueous Sol–Gel Technique

Abstract

Because of its electrically conducting properties combined with excellent thermal stability and transparency throughout the visible spectrum, tin oxide (SnO ₂) is extremely attractive as a transparent conducting material for applications in low-emission window coatings and solar cells, as well as in lithium-ion batteries and gas sensors. It is also an important catalyst and catalyst support for oxidation reactions. Here, we describe a novel nonaqueous sol–gel synthesis approach to produce tin oxide nanoparticles (NPs) with a low NP size dispersion. The success of this method lies in the nonhydrolytic pathway that involves the reaction between tin chloride and an oxygen donor, 1-hexanol, without the need for a surfactant or subsequent thermal treatment. This one-pot procedure is carried out at relatively low temperatures in the 160–260 °C range, compatible with coating processes on flexible plastic supports. The NP size distribution, shape, and dislocation density were studied by powder X-ray powder diffraction analyzed using the method of whole powder pattern modeling, as well as high-resolution transmission electron microscopy. The SnO ₂ NPs were determined to have particle sizes between 3.4 and 7.7 nm. The reaction products were characterized using liquid-state ¹³C and ¹H nuclear magnetic resonance (NMR) that confirmed the formation of dihexyl ether and 1-chlorohexane. The NPs were studied by a combination of ¹³C, ¹H, and ¹¹⁹Sn solid-state NMR as well as Fourier transform infrared (FTIR) and Raman spectroscopy. The ¹³C SSNMR, FTIR, and Raman data showed the presence of organic species derived from the 1-hexanol reactant remaining within the samples. The optical absorption, studied using UV–visible spectroscopy, indicated that the band gap ( E _g) shifted systematically to lower energy with decreasing NP sizes. This unusual result could be due to mechanical strains present within the smallest NPs perhaps associated with the organic ligands decorating the NP surface. As the size increased, we observed a correlation with an increased density of screw dislocations present within the NPs that could indicate relaxation of the stress. We suggest that this could provide a useful method for band gap control within SnO ₂ NPs in the absence of chemical dopants.