Deposição e caracterização de filmes finos do sistema Zn(1-x)CoxO obtidos por spray pirólise para aplicação em sensor de ozônio

Abstract

The current concern about reducing the harmful effects of air pollution on environment and human health requires adequate gas sensors that allow an efficient control and monitoring on a wide range of pollutants. This encourages the research on new materials and on the modulation of features in order to allow the optimization of gas sensing properties of the current devices. Herein we investigate the effect of the cobalt concentration and the annealing, under controlled atmosphere, on the structural, microstructural, surface chemistry, optical, and gas sensing properties of Zn(1-x)CoxO thin films prepared by spray pyrolysis technique. The films were deposited on Si (100) substrates using a acetate route. The X-ray diffraction (XRD) results indicated that the films have the hexagonal wurtzite ZnO phase and that a loss of crystallinity occurs with Co addition. Scanning Electron Microscopy (SEM) micrographs revealed the films surface are porous and formed by grains, whose average size decreases with Co addition. X-ray photoelectron (XPS) and photoluminescence (PL) spectroscopies results of Co-doped thin films confirmed the presence of Co in the oxidation state +2. The XPS measurements also revealed a higher concentration of oxygen vacancy (Vo) for 2.5 mol% Co film and the effect of annealing in reducing the Vo concentration in films. The pure ZnO film exhibits two broad emission bands in the ultraviolet and green energy range, associated with excitonic transitions NBE and defects related to zinc vacancy (VZn)/ interstitial oxygen (Oi), respectively. Incorporation of Co in ZnO matrix leads to an attenuation of NBE band emission and favored defect-band emissions in the red and the blue energy range, which are respectively attributed to the Co2+ ion, and to the interstitial zinc (Zni)-related defect. The annealing of pure ZnO film promoted the formation of Oi-related defect in the red energy range, and VZn-related defect in infrared energy range when carried out on oxygen and nitrogen atmosphere, respectively. For Co-doped films, the annealing increased the concentration of Zni or Oi/VZn-related defects depending on Co concentration and atmosphere. The DC electrical resistance measurements performed to evaluate the ozone gas sensing of Zn(1-x)CoxO films showed an improvement of the ozone sensor response with the reduction of film thickness and with the Co-doping, where the x= 2.5 mol% is the optimal Co concentration. Moreover, cobalt-doped zinc oxide films (ZnCoO) showed selectivity toward ozone gas in a large concentration range (21-696ppb) and then emerging as a promising material for practical application in ozone gas sensing. The conclusions derived from the average grain size, the point defects and the effect of annealing on the gas sensing suggest that the overall improvement of the ozone sensor response could be attributed to the presence of Zni and Vo-related defects, that act as active sites on the films surface favoring the adsorption of oxygen species.