The surface of electrodeposited n-Cu2O thin films were modified by sulphidation using aqueous Na2S followed by (NH4)2S vapor treatment. Compared to untreated thin films, the resultant films showed an enhanced response to liquefied petroleum (LP) gas at a relatively low operating temperature (~45 °C), one of the lowest reported for a cuprous oxide based material. X-ray diffraction spectra confirmed that the films were of single phase. Observed by contact angle measurements and energy dispersive x-ray spectroscopy, (NH4)2S vapor treatment converted the highly wetting sulphided films containing both Na and S to a partially wetting surface by forming sulphur on the film surface. This modification on the film surface, enabled the sensor response to recover to an ambient level after stoppage of LP gas flow, which sulphidation alone was inapt. Scanning electron micrographs complemented roughness measurements made by atomic force microscopy and showed a transformation of polycrystalline morphology of bare n-Cu2O film to one having highly porous structures, which thereby increased the surface area of the surface modified films. Therefore, this work demonstrates that the surface of the n-type Cu2O thin films modified by (NH4)2S vapor treatment and sulphidation can alter the surface wetting nature and increase the surface area to enhance LP gas sensing at a relatively low temperature.

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Cuprous oxide thin films were electrodeposited in an acetate bath (∼60°C) using FTO, ITO, Ti, and Cu substrates to understand their influence on the surface morphology of the films and their wetting nature against liquid petroleum gas (LPG). According to spectral response and capacitance-voltage analysis, n-Cu2O films resulted in the electrodeposition bath at pH range of 5.7-6.5. In addition, SEM data revealed the substrate effect on crystal morphologies of Cu2O thin films with unique porous and tetrahedron structures of Cu2O having different grain sizes. XRD analysis gives evidence in support of these grain size variations which emphasize that Ti and ITO have the lowest grain sizes of 24 and 30.9 nm, respectively. Following the contact angle measurements and their wetting natures, characterized films were subjected to LP gas sensing evaluations in which, proved that deposited n-Cu2O films on FTO, ITO, and Ti can improve gas sensor performances with their moderate wetting behaviors. Under stable sensing conditions, ITO exhibits the most enhanced LPG response, recovery, and stability over time. In comparison, all the experimental results unveiled that substrates do have a major effect, in optimizing interface properties of Cu2O films to achieve better response in many fields of applications.

We report a novel mechanism to effectively detect LP gas based on surface modification through sulphidation followed by passivation of electrodeposited n-type cuprous oxide (Cu2O), forming a thin film n-Cu2O/p-CuxS semiconducting heterostructure. Electrochemically deposited n-type cuprous oxide (n-Cu2O) thin films on Ti substrates in acetate bath were sulphided using Na2S to fabricate n-Cu2O/p-CuxS heterostructures. Subsequent passivation of these thin film structures using (NH4)2S vapor enhanced the sensitivity (fractional change in thin film resistance) when exposed to liquefied petroleum (LP) gas. Scanning electron micrographs (SEM) confirmed that typical unsulphided n-type Cu2O thin films exhibit polycrystalline surface morphology, while SEM of both sulphided and passivated thin films revealed micro/nano-crystalline surface morphological features with porous structures. As expected, the thin film structures obtained through sulphidation followed by passivation of n-type Cu2O films decreased the resistance (100 kΩ) in comparison to the resistance (1 MΩ) of the unsulphided n-type Cu2O thin films. Upon exposure to LP gas, the resistance of these thin film structures increased while, sensitivity to LP gas depended on the sensing temperature. Exposure of thin film structures fabricated by electrodepositing n-* Correspondance should be addressed to Ms.