Sizable Excitonic Effects Undermining the Photocatalytic Efficiency of β-Cu₂V₂O₇

Abstract

Copper vanadates have been proposed as promising photoanodes for water-splitting photoelectrochemical cells, but their performance has recently been shown to be severely limited. To understand this behavior, we study the electronic structure and the optical properties of β-CuVO both experimentally and computationally. The measured absorption spectrum shows an absorption peak at 1.5 eV followed by the onset of an apparent continuum at 2.26 eV, as generally found for this class of materials. We perform calculations within the framework of the QS GW̃ method and the Bethe-Salpeter equation while including effects of magnetic ordering, nuclear quantum motion, and thermal vibrations. We demonstrate the occurrence of two kinds of excitons with high binding energies upon optical excitation in β-CuVO, which account for the first absorption peak and the lower edge of the apparent continuum. The results are confirmed by photoluminescence measurements, where sub-band-gap emissions are found for both excitons. These results provide an explanation for the low photocatalytic efficiencies of copper vanadates, despite the favorable size of their optical band gaps.

Enhanced Bulk Transport in Copper Vanadate Photoanodes Identified by Combinatorial Alloying

“…However, the low photocurrent generation from absorption of photons below~2.3 eV in energy has limited the development of these materials. The recent work on strong excitonic features in b-Cu 2 V 2 O 7 by Wiktor et al 31 offers an explanation for this observation. We hypothesize that relatively high alloying concentration of elements without energetically accessible empty d orbitals in some way disrupts the formation of excitons on neighboring Cu 2+ cations, although further exploration hinges upon identification of the crystallographic location of these alloying elements.…”

“…Recently, theoretical assessment of b-Cu 2 V 2 O 7 via calculations including effects of magnetic order, nuclear quantum motion, and thermal vibrations, combined with experimental optical absorption measurements, has indicated the importance of Cu-centered excitons in both the high-visible absorption and the limited photoactivity of b-Cu 2 V 2 O 7 . 31 A subgap optical absorption peak observed for most copper vanadate phases has previously been ascribed to ligand field allowed d-d localized electronic transitions on the Cu 2+ cations. 19 Wiktor et al 31 identify this transition around 1.5 eV as a localized electron-hole exciton on the same Cu 2+ site, and additionally identify a higher-energy excitonic feature at 2.1 eV corresponding to excitation of the electron to the unoccupied d orbitals of a neighboring Cu 2+ cation.…”

“…31 A subgap optical absorption peak observed for most copper vanadate phases has previously been ascribed to ligand field allowed d-d localized electronic transitions on the Cu 2+ cations. 19 Wiktor et al 31 identify this transition around 1.5 eV as a localized electron-hole exciton on the same Cu 2+ site, and additionally identify a higher-energy excitonic feature at 2.1 eV corresponding to excitation of the electron to the unoccupied d orbitals of a neighboring Cu 2+ cation. They further identify the electronic band-gap energy to be 2.5 eV.…”

Evaluation of Copper Vanadate (β-Cu2V2O7) as a Photoanode Material for Photoelectrochemical Water Oxidation

“…47 Since the TRMC shows that there is only minor wavelength dependency on the carrier mobility and lifetime in Cu 2 V 2 O 7 , we attribute the sharp increase in the IPCE of Cu 2 V 2 O 7 below 350 nm to a wavelength-dependent photogeneration yield in this material. Indeed, the first electronic transition in Cu 2 V 2 O 7 is a Laporte-forbidden Cu dd transition, 31,35 like in hematite. It has been suggested that such transitions do not significantly contribute to the photocurrent.…”

Successes and Opportunities for Discovery of Metal Oxide Photoanodes for Solar Fuels Generators

“…17 The breadth of these materials development strategies far exceeds the breadth of candidate photoanodes for which they have been deployed. The general bias in scientific research towards continued investigation of well-researched materials is both broadly known and recently evaluated as a limitation on creativity and discovery, 18 21 and computational investigation of β-Cu 2 V 2 O 7 22 to elucidate performance-limiting properties. Of these, only the copper vanadates exhibit a photon energy onset of photoactivity near 2 eV, the desired upper-limit described above.…”