Gas sensitive ZnO thin films with desired (002) or (100) orientation obtained by ultrasonic spray pyrolysis

Highly oriented ZnO thin films were obtained by ultrasonic spray pyrolysis. The precursor used was zinc oxide, dissolved in aqueous ammonia solution and acidified with different acids to achieve control over the growing morphology together with both substrate and annealing temperatures. Thus growth along c axis was promoted by adding CH3COOH in the solution, while a axis orientation occurs when HCl or HNO3 acid was used. The size of the crystallites of (002) ZnO layers is smaller than those of (100) textured films. Secondary particles are growing on top of the resulting surfaces, their density higher and ordering more pronounced on the (100) films. The dark electrical conductivity and its changes upon exposure to ammonia, ethanol, acetone and water vapours were also investigated. It has been obtained that the conductivity of the (100) oriented films is significantly lower and its vapour-induced changes are higher than those of the (002) films.