Galvanic Replacement of Liquid Metal Galinstan with Copper for the Synthesis of Core-Shell Cuga-Cu2o Nanomaterials
Authors: Oloye Olawale , Will Geoffrey , O'Mullane Anthony 
Published: February 2020

Anthropogenic carbon dioxide (CO2), resulting from the world’s persistent reliance on fossil fuels as the principal source of energy, has perturbed and induced an imbalance in the natural carbon-cycle. This increased CO2 emission into the environment has been implicated in global warming and other environmental issues1. Therefore, coupling a sustainable energy system with carbon dioxide reduction to produce valuable chemical compounds is being thoroughly investigated1. Among the techniques developed for selective CO2 conversion, electrochemical CO2 reduction is regarded as one of the most appealing due to mild operating conditions and use of renewable energy to power the process. Theoretically, electrochemical CO2 reduction largely depends on the adsorption energies of intermediate species, therefore, metal-based catalysts (Pt, Au, Pd, Ag, Sn, Cu, In, and etc.) are commonly employed which can influence the overall system selectivity. In particular, Cu-based catalysts have shown reasonable activity and potential for selectivity for this reaction owing to moderate adsorption energy for intermediate species on Cu. In addition, Cu is one of the few inexpensive metals that can catalytically convert CO2 to a variety of useful chemicals under environmental conditions (room temperature and atmospheric pressure) via a multi-electron transfer process. Although Cu catalysts show interesting CO2 reduction properties, they still suffer from selectivity issues to generate a desired single product at scale 2.
A recent development is in the area of room temperature liquid metals where the catalytic activity of liquid metal Galinstan has begun to be explored 3. Although in its infancy, we hypothesized that a multi-metallic electrocatalyst of galinstan (GaInSn) and Cu could be active for electrocatalytic and photocatalytic reactions such as CO2 reduction and dye degradation considering that Ga alloys with most metals and should therefore influence the electronic properties of Cu. Previous work has shown that the catalytic activity of multi-metallic electrocatalysts is superior to their mono and bimetallic electrocatalysts counterparts 4. Hence, multi-metallic electrocatalysts exhibit different electronic structures, crystallinity as a result of the interplay of geometric, ligand and electronic effects 5. To date, no report is available in the open literature reporting the alloying of liquid metal GaInSn and Cu via galvanic replacement. Herein, we report the simple synthesis of a multi-metallic nanostructure comprising of a CuGa core with trace In and Sn and a surface layer of Cu2O and Ga2O3. The material was characterized using Scanning Electron Microscopy (SEM), Grazing Incidence X-ray Diffraction (GIXRD), X-ray Photoelectron Spectroscopy (XPS), Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and Transmission Electron Microscopy (TEM). The SAED and TEM images indicate that the core alloy is polycrystalline with well-defined lattice fringes with the presence of crystalline Cu2O and an amorphous region (resulting from gallium oxide). The presence of surface semiconducting oxides with an underlying metal core should in principle be an appropriate system for separating charge carriers under photoexcitation thereby facilitating organic molecule degradation studies.
The multi-metallic nanostructure was therefore engineered towards electrochemical CO2 reduction and photocatalytic pollutant degradation. The preliminary investigation on the photocatalytic activity of this material using Toluidine Blue (TB) under visible light irradiation indicates excellent photocatalytic activity.
References

N. S. Lewis and D. G. Nocera, Proceedings of the National Academy of Sciences of the United States of America, 2006, 103, 15729-15735.

H. Xie, T. Wang, J. Liang, Q. Li and S. Sun, Nano Today, 2018, 21, 41-54.

F. Hoshyargar, H. Khan, K. Kalantar-zadeh and A. P. O'Mullane, Chemical Communications, 2015, 51, 14026-14029.

E. A. Redekop, V. V. Galvita, H. Poelman, V. Bliznuk, C. Detavernier and G. B. Marin, ACS Catalysis, 2014, 4, 1812-1824.

X. L. Tian, L. Wang, P. Deng, Y. Chen and B. Y. Xia, Journal of Energy Chemistry, 2017, 26, 1067-1076.

Figure 1