Synthesis and characterization of Zno/Sno2 nanocomposite via a two step spray pyrolysis route

Abstract

Zinc oxide (ZnO), tin oxide (SnO2) and bilayer ZnO/SnO2 thin films have been prepared by spray pyrolysis method on a glass substrate. The surface morphological, structural, optical and electrical properties of the as-deposited pure ZnO, pure SnO2 and bilayer ZnO/SnO2 thin films are studied in details. Tin oxide (SnO2) and Zinc oxide (ZnO) thin films have been deposited at substrate temperature 450°C and 300°C respectively. Bilayer ZnO/SnO2 thin films have been synthesized with four different thicknesses of SnO2 layer (200 nm, 250 nm, 300 nm and 350 nm) but constant thickness of ZnO. The SEM micrographs of as-deposited films show uniform surface with comprised of dense nanoparticles. It is also observed that the surface morphology of the films are strongly dependent upon the thickness of the SnO2 bottom layer. Fibrous and non-fibrous shaped observed on the whole substrate of the as-deposited bilayer ZnO/SnO2 thin film at 400 nm thickness. When the thickness is increasing then fibrous shape of the films disappear gradually. The crystalline structure of the as-deposited pure ZnO, pure SnO2 and bilayer ZnO/SnO2 thin films were studied by X-ray diffraction (XRD). From the XRD data it is clear that the pure ZnO, pure SnO2 and bilayer ZnO/SnO2 thin films are polycrystalline nature. Where pure ZnO shown hexagonal wurtzite type crystal structure, pure SnO2 shown tetragonal rutile crystal structure and bilayer ZnO/SnO2 thin films have shown mixed faces of hexagonal wurtzite of ZnO and tetragonal rutile crystal structure of SnO2. For bilayer ZnO/SnO2 thin films the crystalline size is increasing with inreasing the thickness of SnO2 layer, the intensity of (110) peak of SnO2 substantially increases indicating the good crystalline property of the double layered films. It is also observed from AFM that the three dimensional view of the films shown the growth of the film takes place with closely placed sharp peaks and valleys. The surface roughness of the bilayer ZnO/SnO2 film increases with increasing thickness of SnO2 and the behavior is strongly reflected in the dependence of RMS values on the film thickness. The optical transmittance for pure ZnO is 92% and pure SnO2 is 83%. In the case of bilayer ZnO/SnO2 thin films the transmittance decrease with increasing the thickness of SnO2 layer. The optical direct band gap for pure ZnO is 3.38, for pure SnO2 is 3.68 and for bilayer thin films bandgap varies from 3.47 to 3.64 eV. Hall effect measurement has revealed that all of the films have n-type carrier concentration and in the case of bilayer thin films carrier concentration increase with increasing thickness of SnO2 layer. The electrical study of the deposited films confirms that the temperature dependent resistivity’s of the deposited bilayer ZnO/SnO2 films are decreasing with with increasing thickness of SnO2 layer and also showed its semiconducting nature in all the films. Moreover, the sheet resistance of the deposited bilayer ZnO/SnO2 thin films are also decreasing with with increasing thickness of SnO2 layer. This study reveals that bilayer ZnO/SnO2 thin film can be used for gas sensing application and other opto-electronic devices more effectively.