Influence of External Gaseous Environments on the Electrical Properties of ZnO Nanostructures Obtained by a Hydrothermal Method

This paper deals with experimental investigations of ZnO nanostructures, consisting of a mixture of nanoparticles and nanowires, obtained by the chemical (hydrothermal) method. The influences of both oxidizing (NO₂) and reducing gases (H₂, NH₃), as well as relative humidity (RH) on the physical and chemical properties of ZnO nanostructures were tested. Carrier gas effect on the structure interaction with gases was also tested; experiments were conducted in air and nitrogen (N₂) atmospheres. The effect of investigated gases on the resistance of the ZnO nanostructures was tested over a wide range of concentrations at room temperature (RT) and at 200 °C. The impact of near- ultraviolet (UV) excitation (λ = 390 nm) at RT was also studied. These investigations indicated a high response of ZnO nanostructures to small concentrations of NO₂. The structure responses to 1 ppm of NO₂ amounted to about: 600% in N₂/230% in air at 200 °C (in dark conditions) and 430% in N₂/340% in air at RT (with UV excitation). The response of the structure to the effect of NO₂ at 200 °C is more than 10⁵ times greater than the response to NH₃, and more than 10⁶ times greater than that to H₂ in the relation of 1 ppm. Thus the selectivity of the structure for NO₂ is very good. What is more, the selectivity to NO₂ at RT with UV excitation increases in comparison at elevated temperature. This paper presents a great potential for practical applications of ZnO nanostructures (including nanoparticles) in resistive NO₂ sensors.