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High-throughput chemical vapor deposition (CVD) growth of carbon or graphene shells encapsulating gold nanoparticles (AuNPs) is a challenge due to limited solubility of carbon in AuNPs. Such a growth is only possible by utilizing surface-oxidized AuNPs. There is a lack of fundamental understanding regarding the role of morphology and surface oxidation of AuNPs in the formation of graphene shells. Here, we studied a simple wet-chemical synthesis of AuNPs with hexadecyltrimethylammonium bromide as a surfactant and the effect of postsynthesis quenching on size, shape, and defect density of AuNPs. In the next step, plasma oxidation kinetics of AuNPs to form surface gold oxide (AuO x ) was studied. The surface gold oxide shell thickness was found to be independent of stoichiometry of AuO x and followed the Cabrera−Mott model for oxidation kinetics. The surface-oxidized AuNPs were further utilized as catalysts for the growth of graphene shell encapsulated AuNPs (GNPs) in a xylene CVD process. The unstable surface gold oxide played a major role in the CVD process by accepting electrons from the incoming carbon feed, resulting in graphene shells around AuNPs. On the other hand, surface oxidized AuNPs with lattice defects, in a similar xylene CVD process, resulted in amorphous carbon and distorted graphene shells encapsulating AuNPs. Overall, this study reveals the mechanisms and critical factors relevant to the growth of GNPs. Such an approach for hybridizing graphene shells with AuNPs is promising for nanoelectronics and sensing.

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.

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