The Synthesis Mechanism of Ultrathin Mo₂C on Liquid Metal Substrates by Chemical Vapor Deposition and the Impact of Substrate Choice

Transition metal carbides and nitrides are a particularly valuable group of materials because they combine useful properties found in metals and ceramics.^1-2 These materials have shown high melting temperature, corrosion resistance, high strength and hardness, and good catalytic activity.^1-2 Recently, synthesizing 2D transition metal carbides and nitrides has led to interesting new materials with the same excellent properties, as well as the high surface area to volume ratio of a 2D material. The most common way to synthesize 2D transition metal carbides and nitrides is by a top-down etching method using bulk MAX phases. The transition metal (“M”) is arranged in a nearly hexagonally close packed structure, with C or N (“X”) in the octahedral interstitials.^1-2 These “M-X” layers are held together by a group 13-16 element (“A”), which can be preferentially etched to form 2D “M-X” layers, known as MXenes.¹ However, this etching process causes defects, as well as surface terminations, which can be difficult to control.¹<br/><br/>Recently, ultrathin transition metal carbides have been synthesized by chemical vapor deposition (CVD).² Ultrathin Mo₂C has been synthesized via CVD using a Cu and Mo foil stack as the substrate.² The Cu foil is melted, and the stack is exposed to hydrocarbons at high temperature. It has been proposed that the Mo diffuses through the liquid Cu and reacts with the hydrocarbons, forming ultrathin Mo₂C crystals.^1-2 Unlike MXenes synthesized by MAX phase etching, these crystals are very high quality with low number of defects and no surface terminations.² These high-quality crystals have already been used for applications such as Josephson junctions, hydrogen evolution reaction, photosensors, etc., however, the synthesis mechanism is not completely understood.^1-2<br/><br/>This work systematically analyzes the effects of using different metal alloys (Ag-Cu, In-Cu) as substrates to determine the mechanism of Mo₂C synthesis by CVD. Parameters such as composition of the alloy substrate, synthesis time, cooling rate, CH₄ partial pressure, temperature, and surface energy of the substrate are varied to determine the effects of each parameter on the Mo₂C synthesis mechanism. Ag alone is not suitable for Mo₂C synthesis under these conditions, despite having similar Mo solubility to liquid Cu. However, Mo₂C flakes can be synthesized using Ag-Cu alloys, and the flake size increases with the percentage of Cu in the Ag-Cu alloy substrate. It is determined that the mechanism of Mo₂C synthesis on liquid metals is by Mo diffusion through the liquid alloy, where it eventually reaches the surface and reacts with surface C. This suggests that successful Mo₂C synthesis requires the substrate to dehydrogenate methane and form surface C. Substrate choice can also play an important role in Mo₂C coalescence. Alloys that do not have solid solubility and that segregate at high temperatures prevent Mo₂C coalescence, such as Ag-Cu alloys. However, Mo₂C coalescence occurs on substrates where the alloys remain mixed during cooling, such as In-Cu alloys. Also, graphene and Mo₂C heterostructures can be synthesized by increasing the CH₄ partial pressure. In-Cu alloys reduce the melting temperature, making lower temperature synthesis at 800 °C possible. However, these Mo₂C flakes are much smaller, and there is lower surface coverage. These results may be due to an increased viscosity of the liquid alloy, as well as less efficient pyrolysis of the CH₄ at lower temperatures. This indicates that there is a low-temperature limit for Mo₂C synthesis using CH₄ as a precursor, similar to graphene. Thus, the bottom-up synthesis of high quality, uniform films of transition metal carbides and nitrides is highly dependent on substrate choice, which can be optimized using a comprehensive understanding of the synthesis mechanism.<br/><br/>(1) Anasori, B.; Gogotsi, Y., 2D Metal Carbides and Nitrides (MXenes). Springer: 2019.<br/><br/>(2) Xu, C.; <i>et al</i>.; <i>Nature materials </i><b>2015,</b> <i>14</i> (11), 1135