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

MAXphases with the general formula M (n+1) AX (n) are layeredcarbides, nitrides, and carbonitrides with varying stackingsequence 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 & GE; 3, have hardly been prepared.This work addresses open questions regarding the synthesis conditions,structure, and chemical composition of the "514" MAXphase. In contrast to literature reports, no oxide is needed to formthe MAX phase, yet multiple heating steps at 1,600 & DEG;C are required.Using high-resolution X-ray diffraction, the structure of (Mo1-x V (x) )(5)AlC4 is thoroughly investigated, and Rietveld refinementsuggests P-6c2 as the most fittingspace group. SEM/EDS and XPS show that the chemical composition ofthe MAX phase is (Mo0.75V0.25)(5)AlC4. It was also exfoliated into its MXene sibling (Mo0.75V0.25)(5)C-4 using two different techniques(using HF and an HF/HCl mixture) that lead to different surface terminationsas shown by XPS/HAXPES measurements. Initial investigations of theelectrocatalytic properties of both MXene versions show that, dependingon the etchant, (Mo0.75V0.25)(5)C-4 can reduce hydrogen at 10 mA cm(-2) withan overpotential of 166 mV (HF only) or 425 mV (HF/HCl) after cyclingthe samples, which makes them a potential candidate as an HER catalyst.