Study On Properties And Microstructures Of Na-doped Zno Thin Films Prepared By Sol-gel Method

ZnO is an important semiconductor, with wide direct band gap, which has a broad range of applications in gas sensor, solar cell, laser diode, water treatment, air purification, antifogging materials, and self-cleaning. Therefore, perfecting photoelectric properties, enhancing photocatalytic activity and improving range of reversible transition by means of doping and optimization of preparation, have become important research subject in the areas such as photochemistry and material science.In this paper, undoped and Na-doped ZnO thin films have been prepared by sol-gel method on silicon and quartz substrates. Microstructure, composition, surface topography, transmission spectra, photoluminescence spectra and contact angle have been characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), scanning electron microscope (SEM), ultraviolet-visible (UV-Vis) spectrophotometer, fluorophotometer (FL) and contact angle apparatus. Methyl orange (C14H14N3NaO3S), a widely used dye, was employed as a representative dye pollutant to evaluate the photocatalytic activity of thin films. The effects of preheating temperature, annealing temperature and Na doping on microstructure, surface chemical composition and valence state, surface topography, transmission spectra and optical band gap, photoluminescence zone, surface wettability and photocatalytic activity were studied in detail. The main results of the study are as follows:The effect of preheating temperature and annealing time on microstructure, surface morphology, transmission spectra, optical band gap and photoluminescence spectra of Na-doped ZnO (Na:Zn=0.08) thin films were studied by XRD, AFM, UV-vis and FL. XRD and AFM results show that the thin films preheated at 150℃and 250℃have a high preferred c-axis orientation, but the thin film preheated at 200℃without preferred c-axis orientation. Surface rms roughness and average grain size of the samples annealed at the same temperature decreases with an increase of preheating temperature. When the annealing temperature is 400℃, no peaks corresponding to either Na metal or any of its oxides are observed in XRD patterns for the thin films with different annealing time. The diffraction peaks intensity of the thin films decreases with increases annealing time. As annealing time increase to 90min and 120min, the thin films annealed at 600℃and 800℃show NaO2(101) peak. UV-Vis results indicate that the optical band gap Eg decreases with increasing annealing time. Eg decreases with increasing annealing temperature. PL spectra results indicate that the annealing temperature has large effect on PL spectra when the preheating temperature is lower. The higher annealing temperature thin films have violet-blue emission and broad green-yellow emission, which are connected with film defects. However, the annealing temperature has little effect on PL spectra when the preheating temperature is higher. The results also indicate that the annealing time have little effect on PL spectra for the Na-doped ZnO thin films with the same annealing temperature, which have the same photoluminescence zone with different intensity and peak position.Microstructure, composition, surface topography, optical band gap, photoluminescence spectra, contact angle and photocatalytic activity of undoped and Na-doped ZnO thin films (preheating temperature and annealing time are 150℃and 60min, respectively)were investigated by XRD, XPS, AFM, SEM, UV-Vis, FL and contact angle apparatus. XRD results show that all the peaks in the pattern correspond to ZnO hexagonal wurtzite structure. The Na-doped ZnO thin film (Na:Zn=0.08) have significant (002) preferential orientation which indicates that moderate Na-doping can enhance the ZnO (002) preferential orientation, but excessive Na-doping will deteriorate it. Surface topography results indicated that as Na content is lower, smaller particles distributed homogeneously and compactly on the surface of the thin films annealed at the same temperature. As Na content increases, average particles size and gap of particles increase and density of the thin film decreases. XPS results show that as Na:Zn increase from 0.0 to 0.10, full width of half maximum of Nals increases at first and then decreases and peak position decreases. These indicate that Na+ions may be occupied substitutional sites and generate acceptor Nazn.Zn2p3 and ZnLMM Auger spectrum indicate that Zn element exist mainly in form of Zn2+ ions but slightly in form of Zn elementary substance. The optical band gaps of all the thin films are smaller than the direct band gap of ZnO bulk (3.37eV) at room temperature. This result may be explained by the band tail which is composed of defect localized states both at the bottom of conduction band and the top of the valance band. For Na-doped ZnO thin films annealed at the same temperature, with the increasing of Na doping, Eg fluctuate in a certain range. For the Na-doped ZnO thin films annealed at 400℃, UV-violet emission band around 380-420nm, blue emission band and green emission band are observed. The UV-violet peak redshifts first, and then blueshifts with the increases of Na content. For the thin films annealed at 800℃, a strong green-yellow emission band fluctuate in a range of 540-570nm with increasing Na content.The effect of Na content on surface wettability and reversible transition of undoped and Na-doped ZnO thin films annealed at 600℃and 800℃was studied in detail. The results show that the contact angel of the un-irradiated thin films is larger. Under UV irradiation all samples exhibit a light induced transition. The contact angle reduction rate is closely connected with Na content. It must be pointed out that the changes in wettability of the thin films are reversible. After the UV irradiated films were placed in dark for seven days (or annealed at 200℃for 60min) in ambient conditions, a new water droplet was used to measure the surface wettability, and the initial wetting state was obtained again. The effect of annealing temperature on photocatalytic activity of ZnO thin films was investigated. The results show that photocatalytic degradation efficiency of the films increases from 73.4% to 88.0% with increasing annealing temperature. When the annealing temperature increase from 400 to 800℃, the apparent reaction rate increase from 0.0056min-1 to 0.0071min-1 for the reacting process in the range of 0-60min and increase from 0.0084 min-1 to 0.0149 min-1 for the reacting process in the range of 80-180min. The effect of Na content on photocatalytic activity of undoped and Na-doped ZnO thin films annealed at 600℃was studied in detail. The results indicate that photocatalytic degradation efficiency of the thin films increases first and then decrease with increasing Na:Zn. The maximum value is 89.1% for the thin film with Na:Zn=0.08.The main innovations of this paper are as follows:1. The effect of Na doping, preheating temperature, annealing temperature and time on microstructure, surface topography and optical properties are studied in detail. The possible mechanisms of photoluminescence of the thin films have been investigated. This research provides experimental basis for mechanism study of photoluminescence of ZnO.2. Surface wettability and reversible transition of undoped and Na-doped ZnO thin films have been studied, and the corresponding mechanisms have been analyzed.3. For the photocatalytic activity of undoped and Na-doped ZnO thin films have been investigated, and the corresponding mechanisms have been suggested.4. For the undoped and Na-doped ZnO thin films, reversible transition of surface wettability is obvious. Photocatalytic degradation of contaminated water is effective. UV emission, blue emission and green-yellow emission are observed. Therefore, the researches of this paper have potential applications in optoelectronic device, water treatment, and self-cleaning.