Suitability of $\beta$-Mn$_2$V$_2$O$_7$/$\beta$-Cu$_2$V$_2$O$_7$ solid solutions for photocatalytic water-splitting

TL;DR

This study uses density functional theory to predict the electronic properties of solid solutions of $eta$-Mn$_2$V$_2$O$_7$ and $eta$-Cu$_2$V$_2$O$_7$, aiming to improve their suitability for photocatalytic water splitting.

Contribution

It provides theoretical predictions on how substituting Cu or Mn affects the band gap and synthesis feasibility of these pyrovanadate solid solutions.

Findings

Substitution narrows the band gap up to 20% Cu or Mn.

Experimental UV-vis confirms band gap narrowing for Mn substitution.

Higher substitution levels are harder to synthesize.

Abstract

The pyrovanadates $\beta$-Mn$_2$V$_2$O$_7$ and $\beta$-Cu$_2$V$_2$O$_7$ were previously investigated as photoanode materials for water splitting. Neither of them, however, was found to be sufficiently active. In this work we predict the properties of solid solutions of these two structurally similar pyrovanadates via density functional theory calculations to explore the suitability of their band structure for water splitting and to assess their ease of synthesis. We predict that substitution of up to 20\% Cu or Mn into $\beta$-Mn$_2$V$_2$O$_7$ and $\beta$-Mn$_2$V$_2$O$_7$ respectively leads to a narrowing of the band gap, which in the former case is experimentally confirmed by UV-vis spectroscopy. Calculations for solid solutions in the intermediate composition range, however, yield nearly constant band gaps. Moreover, we predict solid-solutions with higher substitution levels to be increasingly difficult to synthesize, implying that solid solutions with low substitution levels are most relevant in terms of band gaps and ease of synthesis.