Protonic conduction induced selective room temperature hydrogen response in ZnO/NiO heterojunction surfaces

In this paper, we show that the ionic conduction through surface chemisorbed ambient moisture leads to a remarkably high and selective response towards hydrogen gas at room temperature. The surface adsorbed water molecules acts as surface states, due to porous and granular nature of ZnO nanoparticles of 20 ± 5 nm size. This is depicted as deviation from Arrhenius behavior near room temperatures. The response to hydrogen gas is further enhanced remarkably from 5% to 71% for 1200 ppm when p-type NiO quasi-nanowires (width 15–20 nm) are mixed with these n-type ZnO nanoparticles to form a homogenous NiO/ZnO nano-bulk p-n heterostructures. The maximum response is obtained for about 50–50% composition of NiO/ZnO although it is of still n-type character which signifies the dominance of ZnO in the sensing mechanism. The carrier type reversal from n-type to p-type takes place at a rather high NiO content of about 60–80% NiO in ZnO. The parallel surface ionic current through chemisorbed moisture (surface states) has been identified as a primary factor for high sensitivity to hydrogen gas at room temperature. Further, the presence of heterojunction barriers at the NiO-ZnO interface along with surface ionic conduction synergistically enhanced the selective response to hydrogen at room temperature. [Display omitted] •In this work, we demonstrate the remarkable improvement in sensor response of granular ZnO nanoparticles exhibiting surface ionic conduction.•Their response is drastically enhanced by an order of magnitude through heterostructure formation with NiO quasi nanowires.•The dynamic range as well as response times of the sensor are remarkably enhanced without losing on the selectivity at low temperature (25 °C).•The protonic conduction of ZnO surfaces and their heterostructure at the interface of NiO are the key factors for enhanced sensor performance.