Controlled Fabrication of Photoactive Copper Oxide–Cobalt Oxide Nanowire Heterostructures for Efficient Phenol Photodegradation

Abstract

Fabrication of oxide nanowire heterostructures with controlled morphology, interface, and phase purity is critical for high-efficiency and low-cost photocatalysis. Here, we have studied the formation of copper oxide-cobalt nanowire heterostructures by sputtering and subsequent air annealing to result in cobalt oxide (Co(3)O(4))-coated CuO nanowires. This approach allowed fabrication of standing nanowire heterostructures with tunable compositions and morphologies. The vertically standing CuO nanowires were synthesized in a thermal growth method. The shell growth kinetics of Co and Co(3)O(4) on CuO nanowires, morphological evolution of the shell, and nanowire self-shadowing effects were found to be strongly dependent on sputtering duration, air-annealing conditions, and alignment of CuO nanowires. Finite element method (FEM) analysis indicated that alignment and stiffness of CuO-Co nanowire heterostructures greatly influenced the nanomechanical aspects such as von Mises equivalent stress distribution and bending of nanowire heterostructures during the Co deposition process. This fundamental knowledge was critical for the morphological control of Co and Co(3)O(4) on CuO nanowires with desired interfaces and a uniform coating. Band gap energies and phenol photodegradation capability of CuO-Co(3)O(4) nanowire heterostructures were studied as a function of Co(3)O(4) morphology. Multiple absorption edges and band gap tailings were observed for these heterostructures, indicating photoactivity from visible to UV range. A polycrystalline Co(3)O(4) shell on CuO nanowires showed the best photodegradation performance (efficiency ~50-90%) in a low-powered UV or visible light illumination with a sacrificial agent (H(2)O(2)). An anomalously high efficiency (~67.5%) observed under visible light without sacrificial agent for CuO nanowires coated with thin (∼5.6 nm) Co(3)O(4) shell and nanoparticles was especially interesting. Such photoactive heterostructures demonstrate unique sacrificial agent-free, robust, and efficient photocatalysts promising for organic decontamination and environmental remediation.

Structural evolution of cobalt oxide–tungsten oxide nanowire heterostructures for photocatalysis

“…Such heterostructures can exhibit anisotropic light-matter interactions and thus are promising for photocatalysis processes [18,19]. For instance, one-dimensional geometry (e.g., core-shell nanowires) can allow for greater absorption of light in longitudinal direction and result in efficient charge transport (or charge carrier generation) in radial direction [19,20]. However, such merit exhibition shows significant dependence on the structure, morphology, size, and material selection.…”

“…This further narrows down the materials selection criteria for photoactive systems. In this regard, we propose an interesting and novel photocatalyst design through the hybridization of cobalt oxide and tungsten oxide, which can result in one-dimensional multi-component heterostructures with specific structure and tunable band gap energies [19,22,23]. Moreover, tungsten oxide (WO 3 ) is n-type semiconductor with band gap energy of $2.8 eV while cobalt oxide (Co 3 O 4 ) is a p-type semiconductor with band gap energy $1.6 eV for O 2À to Co 3+ charge transfer and $2.2 eV for O 2À to Co 2+ charge transfer [19,24,25].…”

Morphological, Structural and Optical Characterization of Bottom-Up Growth of Ag-WO₃ Core-Shell Nano-Cube Heterostructures

“…Thermal process directly enhances the formation of nano-cube WO3 from the sputtered WO3. This is a bottom-up growth of nanoparticles -the vapor-solid (VS) growth mechanism drives this growth process [33]. The diameter of WO3 nanocubes was varied from ~90nm to ~340 nm (Figure, 2E).…”

“…The band gap tailoring was identified due to the morphology of CSNH and quantum confinement effect [34]. Interfaces of Ag-WO3 nanocubes heterostructures strongly affects the charge transfer and separation mechanisms [33]. This is due to the Ag nano-layer which is act as a localized surface plasmons (LSP).…”

“…Due to their high cost and low throughput, these processes are not accepted widely for general industrial applications. Sputtering deposition, another tradeoff gas phase technique, have been considered because of its potential for assembling heterostructures, comparably lower cost and high throughput than other gas phase methods [33].…”