TiO2/Cu2O/CuO Multi-Nanolayers as Sensors for H2 and VOCs: An Experimental and Theoretical Investigation

Abstract

Highly sensitive TiO2/Cu2O/CuO multi-nanolayers have been grown in various thicknesses by a cost-effective and reproducible combined spray-sputtering-annealing approach. The ultra-thin TiO2 films were deposited by spray pyrolysis on top of sputtered-annealed Cu2O/CuO nanolayers to enhance their gas sensing performance and to improve their protection against corrosion at high operating temperatures. The prepared heterostructures have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet visible (UV-Vis) and microRaman spectroscopy. The gas sensing properties were measured at several operating temperatures, where the nanolayered sensors with oxide thicknesses of between 20 and 30 nm (Cu2O/CuO nanolayers) exhibited a high response and excellent selectivity to ethanol vapour only after thermal annealing at 420C. The results obtained at an operating temperature of 350 C demonstrate that the CuO/Cu2O nanolayers with a thickness between 20 and 30 nm are sensitive mainly to ethanol vapour, with a response of ~150. The response changes from ethanol vapors to hydrogen gas as CuO/Cu2O nanolayers thickness changes from 50 nm to 20 nm. Density functional theory-based calculations were carried out of the geometries of the CuO(1̅11)/Cu2O(111) and TiO2(111)/CuO(1̅11)/Cu2O(111) heterostructures and their sensing mechanism towards alcohols of different chain lengths and molecular hydrogen. The reconstructed hexagonal Cu2O(111) surface and the reconstructed monoclinic CuO(1̅11) and TiO2(111) facets, all terminated in an O layer, lead to the lowest surface energies for each isolated material. We studied the formation of the binary and ternary heteroepitaxial interfaces for the surface planes with the best matching lattices. Despite the impact of the Cu2O(111) substrate in lowering the atomic charges of the CuO(1̅11) adlayer in the binary sensor, we found that it is the different surface structures of the CuO(1̅11)/Cu2O(111) and TiO2(111)/CuO(1̅11)/Cu2O(111) devices that are fundamental in driving the change in the sensitivity response observed experimentally. The experimental data presented here, supported by the computational results, promote the use of the multi-nanolayered films tested in this work as reliable, accurate and selective sensor structures for the tracking of gases at low concentrations.