Sensors and Actuators B 202 (2014) 1270–1280 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Role of various interfaces of CuO/ZnO random nanowire networks in H 2 S sensing: An impedance and Kelvin probe analysis Niyanta Datta, Niranjan S. Ramgir ∗ , Suresh Kumar, P. Veerender, M. Kaur, S. Kailasaganapathi, A.K. Debnath, D.K. Aswal, S.K. Gupta Thin Film Devices Section, Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India article info Article history: Received 29 January 2014 Received in revised form 17 June 2014 Accepted 18 June 2014 Available online 26 June 2014 Keywords: ZnO Nanowires Impedance measurements Work function Band bending 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In recent years, a great deal of research has been focused on the synthesis of metal oxide-based nanomaterials because of their superior and enhanced functional properties for realizing func- tional nanodevices. Among different nanostructures, nanowires (NWs), in particular, are looked upon as a favourable candi- date for realizing next-generation gas sensors. They offer various advantages including high surface area-to-volume ratio, effective pathway for electron transfer (length of NWs), dimensions com- parable to the extension of the surface charge region, enhanced and tunable surface reactivity implying possible room-temperature operation, faster response and recovery time, relatively simple preparation methods allowing large-scale production, convenient to use, ease of fabrication and manipulation, high integration ∗ Corresponding author. Tel.: +91 22 2559 5839. E-mail address: niranjanpr@yahoo.com (N.S. Ramgir). density, and low power consumption [1]. In order to harness the complete advantage of nano-dimension, i.e., high surface area- to-volume ratio, it is desirable to use the single nanostructure. However, the problems associated with the use of single nano- structure, viz. sample to sample variation, complexity of the sensor fabrication approach, and the in-built issue of randomness raise a major concern over the important parameters, namely repro- ducibility and repeatability of the sensor. Use of NWs in thin-film form wherein the average properties of multiple NWs is mea- sured circumvents the above-measured problems to a great extent. Herein NWs can be selectively grown between the predefined elec- trodes or electrical contacts can be provided by depositing the electrodes with known dimensions on the NW network itself [2]. Accordingly, NW-based sensors in thin form have been investi- gated widely for possible sensor device applications. Among these, ZnO NWs, in particular, provide the advantages of ease of synthe- sis using physical/chemical processes, wide bandgap, high thermal stability, and easy control over morphology [3]. Besides, the ability to manipulate the wide bandgap provides the opportunity to tailor http://dx.doi.org/10.1016/j.snb.2014.06.072 0925-4005/© 2014 Elsevier B.V. All rights reserved.