Computational and Photoelectrochemical Study of Hydrogenated Bismuth Vanadate

Abstract

We demonstrate hydrogenation as a facile method to significantly enhance the performance of BiVO 4 films for photoelectrochemical water oxidation. Hydrogenation was performed for BiVO 4 films by annealing them in hydrogen atmosphere at elevated temperatures between 200 and 400 °C. Hydrogen gas can reduce BiVO 4 to form oxygen vacancies as well as hydrogen impurities. DFT calculation predicted that both oxygen vacancies and hydrogen impurities are shallow donors for BiVO 4 with low formation energies. These defects could increase the donor densities of BiVO 4 without introducing deep trap states. Electrochemical impedance measurements showed that the donor densities of BiVO 4 films were significantly enhanced upon hydrogenation. Hydrogen-treated BiVO 4 (H-BiVO 4 ) photoanodes achieved a maximum photocurrent density of 3.5 mA/cm 2 at 1.0 V vs Ag/AgCl, which is 1 order of magnitude higher than that of air-annealed BiVO 4 obtained at the same potential. The enhanced photoactivities were attributed to increased donor densities of H-BiVO 4 , which facilitates the charge transport and collection.

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

“…Wang and co-workers have previously calculated formation energies for hydrogen interstitials (H int ) and substitutional hydrogen on oxygen lattice sites (H O ), as well as for native defects, in BiVO 4 . 23 They found that the formation energy for H int is lower than for all other defects under nearly all conditions. In addition, the formation energy of H O is low compared to native defects under the O-poor conditions encountered during H 2 annealing.…”

“…36 Both hydrogen defect types act as donors in BiVO 4 and can contribute to the n-type conductivity of the material. 23 This is particularly important for synthesis of BiVO 4 . As described above, native defect theory would predict that material synthesized under Orich conditions, which is the typical environment used for formation of BiVO 4 , should exhibit p-type character.…”

“…[10][11][12] Such electron small polarons localize at V sites, leading to a change of formal oxidation state from V 5+ (3d 0 ) to V 4+ (3d 1 ), the concentration of which will increase with increasing electron polaron concentration. Given that H int and H O are both predicted to be donors in BiVO 4 , 23 the concentration of V 4+ is expected to increase with increasing hydrogen impurity concentration. Quantitative comparison of solid-state NMR performed on powders with PDS data obtained from thin films cannot be established with high accuracy.…”

“…For the case of BiVO 4 , annealing in hydrogen has been shown to improve its PEC performance by increasing its n-type conductivity. [22][23][24] However, the mechanisms by which H 2 annealing does this are not yet clear and the role of residual hydrogen during synthesis has not been addressed.…”

Enhancing Charge Carrier Lifetime in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment

“…[23] It is also found to dramatically improve the PEC water splitting performance of other metal oxides, including the currently highest performing metal oxide photoanode, BiVO4. [24][25][26][27][28] The improved performance was initially assumed to originate from the increased electrical conductivity due to oxygen vacancy formation and/or hydrogen insertion (i.e., an increase in carrier density through donor doping). [23][24][25][26][27][28][29] However, to the best of our knowledge, no report has yet provided direct evidence for the influence of hydrogen treatment on the carrier transport properties of BiVO4.…”