Effect of post-annealing temperature on nano-structure and energy band gap of indium tin oxide (ITO) nano-particles synthesized by polymerizing–complexing sol–gel method

In this paper, we report on the structural, microstructural and optical properties of nano-crystalline indium tin oxide (ITO) particle, which has been synthesized by sol–gel process using a simple starting hydro-alcoholic solution consisting of In(NO3)3.5H2O, SnCl4.5H2O, citric acid as complexing and ethylene glycol as polymerization agents. The structural properties of indium tin oxide nano-powders annealed at different temperatures (T=350–650 °C) have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. The XRD patterns show In2O3-cubic phase in nano-powders without any indication of crystalline SnOx as an additional phase. The TEM images show the nano-particles as nearly spherical shaped with size in the range of 10–45 nm as the size of grains increases by increasing the annealing temperatures. The optical direct band gap of ITO nano-particles was calculated to be about 3.39–4.02 eV in the temperature range 350–650 °C by optical absorption measurements. The optical band gap shifts to a higher energy with increasing annealing temperature is due to the improvement of the crystallinity, thereby increasing the carrier concentration as determined from the Burstein–Moss effect.