Advances in electrolytic and gas sensing technologies continue to be driven by careful selection and engineering of materials. Copper oxides—both cuprous oxide, Cu2O, and cupric oxide, CuO—are abundant, environmentally friendly, and highly versatile. An attractive feature unique to both copper oxides is the ease of synthesis through a one-step thermal oxidation of copper foil in ambient environment, yielding various oxide compositions and morphologies according to the oxidation temperature and time. There are many possible applications for the copper oxide materials, including pigments in ceramics, catalysts, sensors, solar cells, and batteries, to name a few. This work presents applications in electrochemical cells, more specifically photocatalytic water splitting and CO2 reduction, and gas sensors.
The synthesis processes of copper oxides are characterized in terms of processing parameters and inspected with X-ray diffraction measurements and scanning electron microscope (SEM). Three kinds of copper oxides were investigated for photocatalytic testing: 1 μm-thick, and 5 μm-thick Cu2O films via 0.5 hr and 10 hr oxidation at 300 °C, respectively, and a 10 μm thick film Cu2O with 8 μm tall vertically-aligned CuO nanowire array on top via a 2 hr oxidation at 500 °C. Under AM 1.5 illumination, photocurrents of 0.8, 1.3, and 1.7 mA/cm2, respectively, were recorded for these samples, exceeding the performance of previously reported as-synthesized, co-catalyst-free copper oxide photocathodes. Possible explanations for the observed performance based on increased minority carrier diffusion length and enhanced surface electric field are discussed. Future prospects of highly photoactive and stable copper oxide-based photocathodes are also explored. The effectiveness of surface passivation for the copper oxide photocathodes using pristine and hydrogenated TiO2 thin films are quantified through prolonged photoelectrochemical testing. Photocathodes protected with TiO2 films of 50 nm thickness deposited by atomic layer deposition exhibited excellent stability, but the photocurrent dropped to ~0.06 mA/cm2. The results of CO2 reduction using electrochemically reduced Cu sites from copper oxide electrodes as precursors for CO2 reduction is also demonstrated. The proportion of reaction products H2, CO, HCOOH, and CH3COOH is shown to be tunable according to the surface morphology and composition of the original oxide electrode. Therefore, these electrodes exhibit the potential for highly selective liquid fuel production, including a measured H2/CO product ratio of ~2.6 for maximized production of liquid fuels using the Fischer-Tropsch process.
A simple gas sensing architecture taking advantage of the vertically-aligned growth of CuO nanowires is demonstrated. Complete devices are formed instantly following CuO nanowire synthesis by affixing a pair of electrode pads of a second substrate on top of the nanowire array to form a complete electrical circuit. This device architecture offers simple and facile integration of nanowires into a working device. A resistance change R/R0 of ~6 was observed for 8.1% H2 concentration increasing to ~26 for 25.5% H2 concentration. Recovery time is excellent at ~0.5 min or less. A description for the formation of facile microheater-integrated devices is outlined as a promising next step. A process flow to fabricate this device along with heat transfer analysis to predict the temperature distribution in the device is provided and the power consumption may be further minimized with a proposed pulsed heating strategy.