Surface modification of ZnO nanowires with CuO: a tool to realize highly-sensitive H$_2$S sensor

Hydrothermally grown ZnO nanowires (NWs) have been successfully synthesized and surface modified by an ultrathin layer of CuO using dip coating technique to achieve a highly sensitive H$_2$S sensor. XRD analysis confirmed the hexagonal structure of ZnO without any Cu sub-oxide peaks. After CuO modification, the peak shift was observed in the electronic states of O and assigned to the defects and increase in adsorbed oxygen species. Similarly, a red shift was also observed in the band edge absorption after CuO modification arising due to defects. The sensor film showed an overall $n$-type character as confirmed using I(V) characteristics. Interestingly, sensor response kinetics towards H$_2$S were enhanced after CuO modification. The highest sensor response value of 298 was measured towards 10 ppm H$_2$S at 150$^\circ$C for CuO : ZnO NWs sample having 1.26 at.% of Cu. This improved sensor response has been attributed mainly to the formation of randomly distributed $p$–$n$ nano-hetero-junctions between $p$-type CuO and $n$-type ZnO over the sensor surface. In particular, the $p$–$n$ nano-hetero-junctions collapsed due to conversion of semiconducting CuO into metallic CuS after the unique interaction with H$_2$S.