The effect of the post-annealing temperature on the nano-structure and energy band gap of SnO2 semiconducting oxide nano-particles synthesized by polymerizing-complexing sol-gel method

Nano-crystalline SnO2 particles have been synthesized by sol-gel process using a simple starting hydro-alcoholic solution consisting of SnCl4, 5H(2)O and citric acid as complexing and ethylene glycol as polymerization agents. The structural properties of the prepared tin oxide nano-powders annealed at different temperatures (300-700 degrees C) have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. The XRD patterns show SnO2-cassiterite phase in the nano-powders, and size of crystals increases by increasing the annealing temperatures. The TEM images show nano-particles as clusters with size in the range of 5-25 nm. Electron diffraction pattern of nano-powders annealed at different temperatures shows a homogeneous distribution of spherical particles due to the effect of ethylene glycol as polymerizing agent in sol-gel process. The optical direct band gap values of SnO2 nano-particles were calculated to be about 4.05-4.11 eV in the temperature range 300-700 degrees C by optical absorption measurements. These values exibit nearly a 0.5 eV blue shift from that of bulk SnO2 (3.6 eV), which is related to size decrease of the particles and reaching to the quantum confinement limit of nano-particles. (C) 2008 Elsevier B.V. All rights reserved.