Thermally induced surface oxidation and functional transformation of copper foil for SnO2/Cu2O-based humidity sensing applications

The oxidation level of copper (Cu) foil subjected to various heat treatments and environmental exposure was investigated. The heat treatments were conducted in an ambient air atmosphere. Optical microscope observations revealed a transformation in surface color from reddish-orange (as-received) to bluish and greenish hues at 250 °C. Field emission scanning electron microscopy (FESEM) showed that the Cu foil’s morphology changed from a smooth surface to a particle-like structure. A thorough investigation of the surface properties using X-ray photoelectron spectroscopy (XPS) confirmed increased surface oxidation, both in terms of atomic percentages and oxide layer thickness. Valence band measurements indicated a gradual decrease in the valence band maximum with increasing heat treatment temperature. Interestingly, heat treatments above 150 °C induced the appearance of an additional slope in the valence band spectra, which may be correlated with the emergence of rectifying behavior in the current–voltage (I–V) curves of Cu foils treated above this temperature. Building upon this, an SnO₂ overlayer was deposited onto the 250 °C treated Cu foil, forming a SnO₂/oxidized Cu heterojunction. The resulting hybrid exhibited exceptional humidity sensing performance, with resistance decreasing sharply from 6.25 × 10¹¹ Ω at 40% RH to 8.12 × 10⁷ Ω at 90% RH corresponding to a remarkable sensitivity of 7699.5. This comprehensive study not only clarifies the evolution of Cu surface oxidation and its effect on electronic behavior but also demonstrates the potential of oxidized Cu foils as tunable platforms for electronic device fabrications through controlled surface chemistry and heterostructure engineering.