The morphological evolution of ZnO thin films by Sn ions doping and its influence on the surface energy and photocatalytic activity

Abstract

Abstract In this work, Sn-doped ZnO thin films were deposited by sol-gel method. The structural, morphological and optical properties of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible spectroscopy, fluorescence spectroscopy, respectively. The composition of the films was examined by energy dispersive X-ray spectroscopy (EDX). The photocatalytic performance was tested using methylene blue (MB) as a simulated organic pollutant. The results showed that all the ZnO thin films were preferentially oriented along the c-axis direction. The Sn-doping led the grains of the ZnO films to decrease. However, a kind of truncated-cone shaped nanostructures occurred on the surface of ZnO thin films when Sn-doping concentration was above 2 at.%, and both the number and the size of the ZnO truncated-cones increased with the further rise of the Sn-doping concentration. As a result, the surface roughness of the films was largely enhanced and the optical absorption was also improved. The wettability of the films was investigated by the water droplet spreading on the ZnO thin film surfaces. It showed that Sn-doping resulted in a smaller contact angle. The photocatalytic test on MB dyes showed that Sn-doping enhanced photodegradation efficiency of ZnO thin films. The improvement of the degradation efficiency was attributed to three main factors: (1) the formation of truncated-cone nanostructures on the surface of ZnO thin films increased the surface roughness and optical absorption; (2) the recombination of photogenerated electrons and hole was suppressed; (3) Sn-doping enhanced the surface energy of the ZnO thin films.