The Synthesis Process and Thermal Stability of V2C MXene

Abstract

The effect of etching solution on the synthesis process of two-dimensional vanadium carbide (V2C MXene) was researched. Three etching solutions were used to etch ternary carbide V2AlC at 90 °C. The three solutions were: lithium fluoride + hydrochloric acid (LiF + HCl), sodium fluoride + hydrochloric acid (LiF + HCl), and potassium fluoride + hydrochloric acid (KF + HCl). It was found that only NaF + HCl solution was effective for synthesizing highly pure V2C MXene. The existence of sodium (Na+) and chloridion (Cl−) in etching solution was essential for the synthesis. The thermal stability of the as-prepared V2C MXene in argon or air was studied. From thermogravimetry and differential thermal analysis, V2C MXene was found to be stable in argon atmosphere at a temperature of up to 375 °C. As the temperature increased, V2C MXene was gradually oxidized to form nanoparticles composed of vanadium trioxide (V2O3) and a part of V2C MXene was broken and transformed to vanadium carbide (V8C7) at 1000 °C. In air atmosphere, V2C MXene was stable at 150 °C. At 1000 °C, V2C MXene was oxidized to form vanadium pentoxide (V2O5).

Synthesis and electrochemical properties of V2C MXene by etching in opened/closed environments

“…The pressure is estimated to be 1.5-2 times that of the standard atmospheric press of OE. On one hand, under the conditions of high temperatures and high pressures, the solubility of fluorine salt increases, so the etching capability increases [28]. Especially, LiF, which is almost insoluble in water before.…”

“…More, the etching ions (H + , F -, and Cl -) cannot be lost in CE, and thus exfoliating ability is always kept at a high level. In addition, Table 1 demonstrates that the V 2 C prepared by LiF has a smaller (002) peak, which means that the V 2 C prepared by LiF has a larger interlayer distance according to the Bragg equation, which can be attributed to the larger dry ionic radius of Na + compared to that of Li + [28]. According to the kosmotropic behavior, in the aqueous solution, the smaller Li + has a stronger charge and enables stronger coupling with H 2 O, which results in a hydrated radius of Li + larger than that of Na + .…”

“…Recently, it was reported that pure V 2 C MXene can be prepared by the etching of NaF+HCl solution at 90 ℃ [9,26,27]. Additionally, among NaF, LiF, and KF (potassium fluoride), only NaF can be used with HCl as an etching solution to prepare V 2 C MXene [28]. The mixture of LiF+HCl could not be effective for V 2 C MXene exfoliation in an opened environment (OE) due to the LiF has the lowest solubility in water and the largest hydrated cation radius in the four common fluoride salts (NaF, LiF, KF, and NH 4 F (ammonium fluoride)), which make it difficult to successfully prepare V 2 C MXene.…”

“…In the previous studies to synthesize V 2 C MXene [9,[24][25][26]28], the effects of etching solution, temperature, time, and precursor phase activity were thoroughly studied. However, another important factor, the environment in which the etching was processed, was not considered.…”

“…Thus, the environments to process the etching, opened or closed, must play an important role in the synthesis of MXenes. It is especially important for V 2 C MXene because it needs a higher temperature (90 ℃) and longer time (3-7 d) to synthesize V 2 C [9,25,26,28,32].…”