The synthesis mechanism of Mo2C on Ag-Cu alloy substrates by chemical vapor deposition and the impact of substrate choice

Two-dimensional (2D) transition metal carbides have shown promise for numerous applications because of their potential for a large surface area to volume ratio, while maintaining many of the useful properties of transition metal carbides. Recent work has explored the use of chemical vapor deposition on liquid metal substrates to controllably synthesize transition metal carbides. While alloys have been used to lower the required synthesis temperature, further understanding of the impact of these substrates on the synthesis mechanism is needed for future optimization. This work systematically analyzes the effects of using a Ag-Cu alloy as a substrate for Mo2C synthesis by chemical vapor deposition at a temperature below the melting point of Cu. The impact of parameters, such as time, cooling rate, composition of the alloy substrate, and methane partial pressure, on the synthesis and structure of Mo2C is studied. The results conclusively demonstrate that synthesis of Mo2C is controlled by Mo diffusion through the liquid alloy. Utilizing a Ag-Cu alloy as a substrate successfully reduced the necessary synthesis temperature below the melting point of Cu; however, Mo2C coalescence is limited due to the separation of the Ag and Cu components in the alloy upon cooling. The results show that Ag alone is not a suitable substrate for Mo2C synthesis, and Mo2C flake size decreased with increasing Ag concentration in the Ag-Cu alloy substrate. This is most likely due to the inability of Ag to dehydrogenate methane effectively. Thus, an optimal substrate for Mo2C synthesis should be able to dehydrogenate methane, have a low melting temperate, and, if an alloy, demonstrate solid solubility.