Detailed Analysis of the Synthesis and Structure of MAX Phase (Mo_0.75V_0.25)₅AlC₄ and Its MXene Sibling (Mo_0.75V_0.25)₅C₄

Abstract

MAX phases with the general formula M n+1 AX n are layered carbides, nitrides, and carbonitrides with varying stacking sequence of layers of M 6 X octahedra and the A element depending on n. While "211" MAXphases (n = 1) are very common, MAX phases with higher n, especially n ≥ 3, have hardly been prepared. This work addresses open questions regarding the synthesis conditions, structure, and chemical composition of the "514" MAX phase. In contrast to literature reports, no oxide is needed to form the MAX phase, yet multiple heating steps at 1,600 °C are required. Using high-resolution X-ray diffraction, the structure of (Mo 1-x V x ) 5 AlC 4 is thoroughly investigated, and Rietveld refinement suggests P-6c2 as the most fitting space group. SEM/EDS and XPS show that the chemical composition of the MAX phase is (Mo 0.75 V 0.25 ) 5 AlC 4 . It was also exfoliated into its MXene sibling (Mo 0.75 V 0.25 ) 5 C 4 using two different techniques (using HF and an HF/HCl mixture) that lead to different surface terminations as shown by XPS/HAXPES measurements. Initial investigations of the electrocatalytic properties of both MXene versions show that, depending on the etchant, (Mo 0.75 V 0.25 ) 5 C 4 can reduce hydrogen at 10 mA cm −2 with an overpotential of 166 mV (HF only) or 425 mV (HF/HCl) after cycling the samples, which makes them a potential candidate as an HER catalyst.

M5X4: A Family of MXenes

“…The herringbone-type structure seen in Mo 4 VAlC 4 was also observed in Ti 2.5 Ta 2.5 AlC 4 (Figure S23), but Rietveld refinement of the XRD patterns reveals that both Ti 2.5 Ta 2.5 AlC 4 and Ti 2.675 Nb 2.325 AlC 4 fit the P 6 3 /mmc structure better than the herringbone-type P 6 ̅c2 and P 6 ̅m2 structures (Figures S29, S30). Further analysis is required to fully resolve the structure of these M 5 AX 4 MAX phases. As the second example of this structure, the atomic arrangement may be characteristic of the M 5 X 4 family as a more stable configuration.…”