A simple one-step synthetic route to access a range of metal-doped polyoxovanadate clusters

Abstract

VO(OⁱPr)₃ is a useful precursor for the synthesis of a range of metal-doped polyoxovanadate (POV) cage compounds, its reactions with hydrated metal salts providing a route to arrangements containing Bi and other main group metals, transition metals and lanthanides. The new POV compounds [Bi₂(DMSO)₆V₁₂O₃₃Br]₂[M(DMSO)₆] (2^Br-M, M = Co^II, Ni^II, Cu^II, Zn^II) [Bi₂(DMSO)₆V₁₂O₃₃Cl]₂[Ca(DMSO)_x]·yDMSO (2^Cl-Ca), [Bi₂(DMSO)₆V₁₂O₃₃Cl]₂[LnCl(DMSO)₇] (2^Cl-Ln, Ln = La^III, Ce^III, Eu^III), [Bi₂(DMSO)₆V₁₀O₂₈F₂]₃[Bi(DMSO)₅]₂ (3), [V₁₂O₃₂(DMSO)][Gd(NO₃)(DMSO)_5.5]₂ (4) and [Ln(DMSO)₄V₁₂O₃₂Cl][LnCl(DMSO)₇] (5^Cl-Ln, Ln = Ce^III, Eu^III) have been structurally characterised, and their properties studied using UV-Vis spectroscopy and cyclic voltammetry. Drop-casting these compounds onto fluorine-doped tin oxide followed by calcination provides a simple approach to thin films of metal-doped BiVO₄ or LnVO₄, depending on the composition of the cage precursor. The applications of the BiVO₄ films as photoanodes for water oxidation is explored, with transition metal doping of BiVO₄ improving the activity (∼1.8–2.4 times the photocurrent density of undoped BiVO₄ at 1.23 V vs. RHE) while lanthanide or Ca-doping is detrimental.