Synthesizing molybdenum-doped bismuth vanadate nanoneedle array as photocatalyst for water oxidation using bifunctional molybdenum as dopant and structure directing agent

Facile Combination of Bismuth Vanadate with Nickel Tellurium Oxide for Efficient Photoelectrochemical Catalysis of Water Oxidation Reactions

Bismuth vanadate (BVO) having suitable band edges is one of the effective photocatalysts for water oxidation, which is the rate-determining step in the water splitting process. Incorporating cocatalysts can reduce activation energy, create hole sinks, and improve photocatalytic ability of BVO. In this work, the visible light active nickel tellurium oxide (NTO) is used as the cocatalyst on the BVO photoanode to improve photocatalytic properties. Different NTO amounts are deposited on the BVO to balance optical and electrical contributions. Higher visible light absorbance and effective charge cascades are developed in the NTO and BVO composite (NTO/BVO). The highest photocurrent density of 6.05 mA/cm2 at 1.23 V versus reversible hydrogen electrode (VRHE) and the largest applied bias photon-to-current efficiency (ABPE) of 2.13% are achieved for NTO/BVO, while BVO shows a photocurrent density of 4.19 mA/cm2 at 1.23 VRHE and ABPE of 1.54%. Excellent long-term stability under light illumination is obtained for NTO/BVO with photocurrent retention of 91.31% after 10,000 s. The photoelectrochemical catalytic mechanism of NTO/BVO is also proposed based on measured band structures and possible interactions between NTO and BVO. This work has depicted a novel cocatalytic BVO system with a new photocharging material and successfully achieves high photocurrent densities for catalyzing water oxidation.

Enhancement of charge separation and hole utilization in a Ni2P2O7-Nd-BiVO4 photoanode for efficient photoelectrochemical water oxidation

It is necessary for photoelectrochemical (PEC) water splitting to reduce the electron-hole recombination rate and enhance the water oxidation reaction kinetics. Here, we prepared Ni₂P₂O₇-Nd-BiVO₄ composite photoanodes by coupling Ni₂P₂O₇ co-catalysts to neodymium (Nd)-doped BiVO₄ surfaces through photo-assisted electrodeposition. The Ni₂P₂O₇-Nd-BiVO₄ photoanode exhibits a high photocurrent density of 3.6 mA cm^-2 at 1.23 V vs reversible hydrogen electrode (RHE), which is three times higher than that of the bare BiVO₄ (1.2 mA cm^-2). Detailed characterizations demonstrate that Nd doping reduces the band gap, significantly increases the carrier density and effectively reduces the charge transfer resistance. More importantly, the Ni₂P₂O₇ co-catalyst has multiple roles. Specifically, it can act as a hole extraction layer to accelerate hole migration and inhibit hole-electron recombination. At the same time, it significantly improves the water oxidation reaction kinetics. In addition, it also provides more water oxidation active sites. This work provides ideas for the design and study of efficient BiVO₄-based photoanodes.

Heteroatom Doping Strategy for Establishing Hematite Homojunction as Efficient Photocatalyst for Accelerating Water Splitting

Hematite (α-Fe₂ O₃ ) is one of the promising photocatalysts for water oxidation, owing to its stable, abundant and visible-light responsive features. Enhancing electrical conductivity and accelerating oxidation evolution kinetics are expected to improve photocatalytic ability of hematite toward water oxidation. In this work, strategies of doping heteroatoms and developing pn homojunction are adopted to enhance the photocatalytic ability of hematite electrodes. The Ti and Mg dopants are separately incorporated in two layers of hematite electrodes via two-step hydrothermal reaction and one-step annealing process. The effect of regrowth time for synthesizing Mg-doped hematite on the photoelectrochemical performance of Mg-doped and Ti-doped hematite (Mg-Fe₂ O₃ /Ti-Fe₂ O₃ ) electrode is studied. The size of rod-like structure and gaps in-between play important roles on the photocatalytic ability of Mg-Fe₂ O₃ /Ti-Fe₂ O₃ . The optimized Mg-Fe₂ O₃ /Ti-Fe₂ O₃ electrode is prepared by using merely 10 min for synthesizing the Mg-doped hematite top layer, which shows the highest photocurrent density of 2.83 mA/cm² at 1.60 V_RHE along with the highest carrier density of 5.89×10¹⁶  cm^-3 and the smallest charge-transfer resistance. This largely improved photoelectrochemical performance is attributed to the more donor generation with heteroatom-doping and more efficient charge cascade with homojunction establishment. Other p-type metals are encouraged to dope in hematite as the second layer to couple with the n-type Ti-doped hematite for developing efficient pn homojunction and improve the photocatalytic ability of hematite in the near future.