Simultaneous enhancements in photon absorption and charge transport of bismuth vanadate photoanodes for solar water splitting

Abstract

n-Type bismuth vanadate has been identified as one of the most promising photoanodes for use in a water-splitting photoelectrochemical cell. The major limitation of BiVO4 is its relatively wide bandgap (∼2.5 eV), which fundamentally limits its solar-to-hydrogen conversion efficiency. Here we show that annealing nanoporous bismuth vanadate electrodes at 350 °C under nitrogen flow can result in nitrogen doping and generation of oxygen vacancies. This gentle nitrogen treatment not only effectively reduces the bandgap by ∼0.2 eV but also increases the majority carrier density and mobility, enhancing electron–hole separation. The effect of nitrogen incorporation and oxygen vacancies on the electronic band structure and charge transport of bismuth vanadate are systematically elucidated by ab initio calculations. Owing to simultaneous enhancements in photon absorption and charge transport, the applied bias photon-to-current efficiency of nitrogen-treated BiVO4 for solar water splitting exceeds 2%, a record for a single oxide photon absorber, to the best of our knowledge.

Determination of the Intrinsic Defect at the Origin of Poor H2 Evolution Performance of the Monoclinic BiVO4 Photocatalyst Using Density Functional Theory

“…In contrast, ε xx was slightly different from ε zz in the case of intrinsic defective Bi 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18 pristine and the intrinsic defective materials were 38.3 and 41.2, respectively. Those values are close to the experimentally reported one of 52, 48 and also very close to that obtained of 45.7 for anatase TiO 2 . 49 Here, let us simply remind that several studies mentioned that dielectric constant value greater than 10 is sufficient for good exciton dissociation to free c...…”

Efficient Water Splitting Cascade Photoanodes with Ligand‐Engineered MnO Cocatalysts

“…The BF 4 ‐treated MnO/BiVO 4 /WO 3 anode achieved 6.25 mA cm −2 at the 1.23 V versus RHE. We emphasize that the photocurrent of BF 4 ‐treated MnO attached to the BiVO 4 /WO 3 anode is the highest of the previously reported BiVO 4 ‐based photoanodes with hole scavenger for solar water splitting, as shown in Table S2 and Figure S4 (Supporting Information) 13, 14, 16, 17, 21, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69. Otherwise, Ca–EDTA‐treated MnO NPs/BiVO 4 /WO 3 recorded the lowest photocurrent density of about 2.5 mA cm −2 .…”

“…Therefore, most of the previously reported results related to BiVO 4 ‐based photoanodes13, 14, 16, 19, 47, 51, 52, 53 were measured in sulfite oxidation condition to show photo‐electrochemical properties of BiVO 4 ‐based electrodes independently of its poor water oxidation kinetics, as shown in Table S2 (Supporting Information). The photo‐electrochemical current densities of the BiVO 4 ‐based photoanodes4, 13, 14, 16, 17, 19, 20, 21, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 64, 65, 66, 67, 68, 69, 77 were plotted as a function of potential versus RHE. Thus, we measured PEC properties of BiVO 4 ‐based anodes under sulfite oxidation to figure out the effect of ligand engineering.…”

Role of Hydrogen in Defining the n-Type Character of BiVO4 Photoanodes

“…This was recently addressed by Kim et al, whose DFT PBE+U calculations also indicated the presence of deep level states associated with O vac defects in Bi-VO 4 . 49 In that work, they found that the negative ionization energy of O vac centers leads to spontaneous ionization to a 2+ charge state. However, it was determined that it is energetically favourable for these donated electrons to form small electron polarons via self-trapping at vanadium sites.…”