Sub-ppm H2S sensing at room temperature using CuO thin films

We have investigated room temperature sensing characteristics of CuO films to various gases i.e. Cl2, H2S, NH3, CH4, CO and NO. CuO films were prepared by oxidation of Cu films, which were deposited on polycrystalline alumina substrates by thermal evaporation technique. CuO films have been found to be highly selective towards H2S. We demonstrate that the H2S response of CuO films can be divided in to three regions: (a) low concentrations (100–400 ppb), (b) intermediate concentrations (500 ppb to 50 ppm), and (c) high concentrations (>50 ppm). For low concentrations (100–400 ppb), the response curves have been found to be highly reversible with very small response (∼60 s) and recovery (∼90 s) times, indicating suitability of CuO films for sub-ppm sensing of H2S. Oxidation of H2S by adsorbed oxygen is found to be the responsible sensing mechanism. However, at very high H2S concentrations (>50 ppm), surface of CuO grains is found to convert into CuS, resulting in an irreversible response curve. For intermediate concentrations (500 ppb to 50 ppm), the response curve is governed by both H2S oxidation and CuS formation mechanisms.