Understanding the Role of Vanadium Vacancies in BiVO₄ for Efficient Photoelectrochemical Water Oxidation

Abstract

The understanding of recombination of photogenerated electron/hole pairs at defect sites is a key enabler to develop bismuth vanadate (BiVO 4 ) photoanodes at scale and low cost for photoelectrochemical water splitting. Here, we report a systematic investigation of the impact of vanadium vacancies on the efficiency of BiVO 4 photoanodes for water photooxidation. X-ray photoelectron and photoluminescence spectroscopies reveal that the surfaces of nanostructured BiVO 4 photoanodes obtained by high-temperature synthesis, here used as the model system, suffer from vanadium deficiency and display an increased recombination rate of photoexcited electron/hole pairs. Our simulation indicates that these vanadium vacancies (V V ) create a new sub-band gap level in the proximity of the Fermi level of BiVO 4 . These levels act as recombination centers, explaining the subpar onset potentials and photocurrent densities for water photooxidation observed with these vanadium-deficient BiVO 4 photoanodes. We show that once the V V are eliminated, by a facile post-treatment of the BiVO 4 photoanodes, the photoluminescence lifetimes of the photogenerated carriers are significantly prolonged and the number of catalytically accessible sites is increased. As a result, the photocurrent during water oxidation is increased twofold, achieving 2 mA cm −2 against the standard hydrogen electrode in a 1 M potassium borate buffer electrolyte. These findings provide insights into the critical role played by the vanadium vacancies on the optoelectronic properties of BiVO 4 and a scalable approach for its effective fabrication on large-scale surfaces.

From Stochastic Self‐Assembly of Nanoparticles to Nanostructured (Photo)Electrocatalysts for Renewable Power‐to‐X Applications via Scalable Flame Synthesis

“…Depending on the synthesis parameters such as atomization pressure, nature of materials, high temperature residence time, or temperature of substrates, the variety of physiochemical properties and morphologies of target nanomaterial are obtained. For example, highly porous films with fractal dimensions of ≈1.75 are obtained when materials have high melting/sintering points, and the temperature of substrates are retained low such as fractal ZnO, [49,50] TiO 2 , [25,51] or SnO 2 , [52] while larger stubby, oblong or columnar structures are achieved when the substrate temperature is high, and material has relatively low melting/ sintering temperatures such as BiVO 4 , [53] Bi 2 O 3 , [54,55] Co 3 O 4 [56] or WO 3 . [57] Fabrication of multicomponent/multiphases might be limited with the use of a single nozzle FSP as a uniformed distribution of each component in a multicomponent structure, rather than discrete individual component, is often obtained, i.e., the flexibility in tuning grain boundaries in a singlenozzle FSP system is limited.…”

“…For example, introducing metal or nonmetal elements into (photo)electrocatalysts was reported to increase the conductivity and thus enhance the photocurrent density, [98,[164][165][166][167] while the vacancies could act as recombination sites for electron-hole pairs, lowering the overall efficiencies. [56,[168][169][170] To generate defects, a perfect structure can be modified via in situ treatment such as doping or electrochemical treatments, or via postsynthetic routes from chemical, thermal, electrical, plasma, or irradiation treatments. While multistep and timeconsuming procedures are often required to create defects using conventional wet-phase synthesis approaches, FSP can be utilized to in situ engineer vacancies/defects.…”

“…While some studies show that the catalytic activities of a photocatalyst are improved with the generation of the defects/vacancies, [183,[193][194][195][196][197] other reports demonstrate the culprit of the vacancies as they could be potential recombination sites for photoexcited carriers. [56,170,198,199] Flame-made BiVO 4 photoanodes have been reported as the active photoanode. [53,200,201] However, less information of the surface chemistry of this attractive material made from FSP was reported.…”

“…For example, introducing metal or nonmetal elements into (photo)electrocatalysts was reported to increase the conductivity and thus enhance the photocurrent density, [ 98,164–167 ] while the vacancies could act as recombination sites for electron–hole pairs, lowering the overall efficiencies. [ 56,168–170 ]…”

Photocatalytic Degradation of Palm Oil Mill Effluent (POME) Waste Using BiVO4 Based Catalysts

“…In addition, Lamers et al reported that by purging as-synthesized BiVO 4 photocatalyst with air and inert gas, such as argon and nitrogen, it could generate different types of defect sites, such as vanadium and oxygen vacancies [ 32 ]. Consequently, vanadium vacancies (V v ) generated a new sub-band-gap level in the proximity of the Fermi level of BiVO 4 which can affect photocatalytic efficiency [ 33 ]. It is known that the oxygen vacancy-related V 4+ is paramagnetic.…”