CuWO₄ with CuO and Cu(OH)₂ Native Surface Layers for H₂S Detection under in-Field Conditions

The paper presents the possibility of detecting low H₂S concentrations using CuWO₄. The applicative challenge was to obtain sensitivity, selectivity, short response time, and full recovery at a low operating temperature under in-field atmosphere, which means variable relative humidity (%RH). Three different chemical synthesis routes were used for obtaining the samples labeled as: CuW1, CuW2, and CuW3. The materials have been fully characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). While CuWO₄ is the common main phase with triclinic symmetry, different native layers of CuO and Cu(OH)₂ have been identified on top of the surfaces. The differences induced into their structural, morphological, and surface chemistry revealed different degrees of surface hydroxylation. Knowing the poisonous effect of H₂S, the sensing properties evaluation allowed the CuW2 selection based on its specific surface recovery upon gas exposure. Simultaneous electrical resistance and work function measurements confirmed the weak influence of moisture over the sensing properties of CuW2, due to the pronounced Cu(OH)₂ native surface layer, as shown by XPS investigations. Moreover, the experimental results obtained at 150 °C highlight the linear sensor signal for CuW2 in the range of 1 to 10 ppm H₂S concentrations and a pronounced selectivity towards CO, CH₄, NH₃, SO₂, and NO₂. Therefore, the applicative potential deserves to be noted. The study has been completed by a theoretical approach aiming to link the experimental findings with the CuW2 intrinsic properties.