Role of various interfaces of CuO/ZnO random nanowire networks in H2S sensing: An impedance and Kelvin probe analysis

Abstract CuO-modified ZnO random nanowire networks have been demonstrated to enhance the sensitivity and selectivity towards H 2 S. CuO being p-type and ZnO being n-type semiconductors, modification with CuO results in the formation of random nano p–n junction distributed over the nanowire surface, thereby leading to depleted nanowires. The enhanced response has been attributed mainly to the interaction of CuO with H 2 S forming CuS, a degenerated semiconductor with a metallic conductance behaviour, causing a drastic change in the resistance. The governing sensing mechanism can be envisaged to have contributions from the different regions namely nanowires bulk (depleted), junctions among nanowires and the interface between sensor and Au contact electrode, respectively. To establish the governing sensing mechanism, it becomes critical to isolate the contribution arising from each of them. In the present work, we report the impedance and the Kelvin probe studies of CuO/ZnO random nanowire network sensor films. Impedance studies indicate that the contributions arising from the bulk and the nanowire–electrode contact is negligible. A drastic variation in the resistance of the sample arises mainly due to the band bending. The extent of band bending depends on the ambient oxygen and the interaction with the test gas. Temperature- and gas concentration-dependent studies clearly indicated that the CuS formation is the major cause for such bending. Work function measurements further corroborates the finding of impedance studies.