Cobalt doped BiVO4 with rich oxygen vacancies for efficient photoelectrochemical water oxidation

Bismuth Vanadate and 3D Graphene Composite Photoanodes for Enhanced Photoelectrochemical Oxidation of Water

Bismuth vanadate (BiVO4) has been one of the most promising photoanodes for the photoelectrochemical (PEC) water oxidation process. Efforts are still on to overcome the drawbacks of this photoanode to enhance the catalytic efficiency and improve the stability. In the present work, three-dimensional graphene (3D-G) was incorporated inside the BiVO4 matrix, primarily to improve the conductivity of the material. The photoanodes are fabricated with the incorporation of a SnO2 heterojunction and application of cobalt borate (Co-Bi) as a cocatalyst. The incorporation of 3D-G has enhanced the photocurrent from 0.72 o 1.21 mA cm-2 in ITO/SnO2/BiVO4 and ITO/SnO2/3D-G-BiVO4 materials; the photocurrent has been improved from 0.89 to 1.52 mA cm-2 in ITO/SnO2/BiVO4/Co-Bi and ITO/SnO2/3D-G-BiVO4. Semiconductor properties are evaluated from the Mott-Schottky measurements, and the charge transfer and transport kinetics of the PEC process are measured from several photoelectrochemical investigations. Both the charge transport and the charge transfer efficiencies are enhanced upon inclusion of 3D-G into the catalyst system. The lifetime of the charge carrier is observed to be increased. The decrease in the decay kinetics of the holes, enhancement in the open-circuit photovoltage (OCPV), and the resulting modulation of the surface states are responsible for the enhancement in the surface charge transfer process due to the inclusion of 3D-G into the catalytic system. Therefore, the additional role of 3D-G in the modulation of the surface states and release of the Fermi level pinning has made the band alignment between the semiconductor and the analyte better, which resulted in enhanced catalytic performance in the photoelectrochemical oxidation of water.

Nanostructured Co-doped BiVO4 for efficient and sustainable photoelectrochemical chlorine evolution from simulated sea-water

The co-production of hydrogen and chlorine from sea-water splitting could be a potential, sustainable and attractive route by any method. However, challenges to overcome are many, and critically, the sustainability and operating potential of the electrocatalyst are important. In this work, we report on Co-doping in the BiVO₄ (Co-BV) crystal lattice and employed the same as the photoanode; Co-BV exhibits a photocurrent of 190 μA cm^-2 at 1.1 V vs. RHE (the reversible hydrogen electrode) in the acidic sodium chloride solution (pH 2.3) under one sun illumination. The best-performing photoanode, with 0.05 mol% of Co doping (0.05 Co-BV), selectively produced active chlorine with 92% faradaic efficiency at 1.1 V vs. RHE by successfully suppressing the kinetically sluggish oxygen evolution reaction (OER) and the stability of the catalyst was demonstrated for up to 20 h. This is the lowest operating potential reported for the chlorine evolution reaction (CER), thus far. The overpotential required for CER with 0.05 Co-BV is lower than that of OER, which leads to selective CER at 1.1 V (vs. RHE). Co-doping into the BiVO₄ lattice decreases the charge transfer resistance and enhances the CER kinetics due to its structural and electronic integration with the BV lattice. We demonstrate that Co-doping also improves the lifetime of the charge carrier and enhances the current density of CER and sustainability of the catalyst.

Suppressing photoinduced charge recombination at the BiVO4||NiOOH junction by sandwiching an oxygen vacancy layer for efficient photoelectrochemical water oxidation

Nickel oxyhydroxide (NiOOH) is regarded as one of the promising cocatalysts to enhance the catalytic performance of photoanodes but suffers from serious interfacial charge-carrier recombination at the photoanode||NiOOH interface. In this work, surface-engineered BiVO₄ photoanodes are fabricated by sandwiching an oxygen vacancy (O_vac) interlayer between BiVO₄ and NiOOH. The surface O_vac interlayer is introduced on BiVO₄ by a chemical reduction treatment using a mild reducing agent, sodium hypophosphite. The induced O_vac can alleviate the interfacial charge-carrier recombination at the BiVO4||NiOOH junction, resulting in efficient charge separation and transfer efficiencies, while an outer NiOOH layer is coated to prevent the O_vac layer from degradation. As a result, the as-prepared NiOOH-P-BiVO₄ photoanode exhibits a high photocurrent density of 3.2 mA cm^-2 at 1.23 V vs. RHE under the irradiation of 100 mW/cm² AM 1.5G simulated sunlight, in comparison to those of bare BiVO₄, P-BiVO_4, and NiOOH-BiVO₄ photoanodes (1.1, 2.1 and 2.3 mA cm^-2, respectively). In addition to the superior photoactivity, the 5-h amperometric measurements illustrate improved stability of the surface-engineered NiOOH-P-BiVO₄ photoanode. Our work showcases the feasibility of combining cocatalysts with O_vac, for improved photoactivity and stability of photoelectrodes.

Effect of Mn2+ incorporation on the photoelectrochemical properties of BiVO4

Mn2+–BiVO4 photoanodes (Mn2+ = 0.2–1%) to improve the charge-carrier separation and electrical conductivity of BiVO4 are reported.

Morphology Engineering of BiVO4 with CoOx Derived from Cobalt-containing Polyoxometalate as Co-catalyst for Oxygen Evolution

Bismuth vanadate (BiVO₄ ) as a metal oxidation semiconductor has stimulated extensive attention in the photocatalytic water splitting field. However, the poor transport ability and easy recombination of charge carriers limit photocatalytic water oxidation activity of pure BiVO₄ . Herein, the photocatalytic activity of BiVO₄ is enhanced via adjusting its morphology and combination co-catalyst. First, the Cu-BiVO₄ was synthesized by copper doping to control the growth of {110} facet of BiVO₄ , which is regarded for the separation of photo-generated charge carriers. Then the CoO_x in-situ generated from K₆ [SiCo^II (H₂ O)W₁₁ O₃₉ ] ⋅ 16H₂ O was photo-deposited on Cu-BiVO₄ surface as co-catalyst to speed up reaction kinetics. Cu-BiVO₄ @CoO_x hybrid catalyst shows highest photocatalytic activity and best stability among all the prepared catalysts. Oxygen evolution is about 34.6 μmol in pH 4 acetic acid buffer under 420 nm LED irradiation, which is nearly 20 times higher than that of pure BiVO₄ . Apparent quantum efficiency (AQE) in 1 h and O₂ yield are 1.83% and 23.1%, respectively. O₂ evolution amount nearly maintains the original value even after 5 cycles.

Step-Scheme Heterojunction between CdS Nanowires and Facet-Selective Assembly of MnOx-BiVO4 for an Efficient Visible-Light-Driven Overall Water Splitting

The spatial separation and transport of photogenerated charge carriers is crucial in building an efficient photocatalyst for solar energy conversion into chemical energy. A step-scheme CdS/MnO_x-BiVO₄ photocatalyst was synthesized by spatial deposition of MnO_x and one-dimensional (1D) CdS nanowires on a three-dimensional (3D) decahedron BiVO₄ surface. The photocatalytic activity of CdS/MnO_x-BiVO₄ for the overall water-splitting reaction was investigated without sacrificial reagent under visible light irradiation. The synthesized photocatalysts were thoroughly analyzed using high-end characterization techniques. The 5CdS/MnO_x-BiVO₄ exhibited the highest H₂ and O₂ production rates of 1.01 and 0.51 mmol g^-1 h^-1, respectively, with an apparent quantum yield of 11.3% in the absence of any sacrificial reagent. The excellent photoactivity is due to the presence of oxygen vacancies along with effective charge separation/transfer properties and strong interaction of cocatalysts (MnO_x and Pt) with the photocatalysts (BiVO₄ and CdS) in the 5CdS/MnO_x-BiVO₄ heterojunction. The significance of the presence of MnO_x and Pt cocatalysts on the selective facets of BiVO₄ for efficient overall water splitting reaction is highlighted in this work.

Effect of Cu doping on the photocatalytic activity of InVO4 for hazardous dye photodegradation under LED light and its mechanism

Cu doped InVO₄ (xCu-InVO₄ (x = 0.06-0.15 wt %) was synthesized by a facile one-pot hydrothermal method for the removal of methylene blue (MB) under LED light irradiation. The X-ray photoelectron spectroscopy (XPS) analysis indicated the coexistence of V⁵⁺ and V⁴⁺ species due to the O-deficient nature of the xCu-InVO₄. The synthesized photocatalysts displayed a morphology of spherical and square shaped particles (20-40 nm) and micro-sized rectangle rods with a length range of 100-200 μm. The xCu-InVO₄ exhibited superior adsorption and photodegradation efficiency compared to pristine InVO₄ and TiO₂ due to the presence of O₂ vacancies, V⁴⁺/V⁵⁺ species, and Cu dopant. The optimum reaction conditions were found to be 5 mg L^-1 (MB concentration), pH 6, and 100 mg of photocatalyst mass with a removal efficiency and mineralization degree of 100% and 96.67%, respectively. The main active species responsible for the degradation of MB were ^•OH radicals and h⁺. Reusability studies indicated that the 0.13Cu-InVO₄ was deactivated after a single cycle of photocatalytic reaction due to significant leaching of V⁴⁺ and Cu²⁺ species.

Lattice Defects Engineering in W-, Zr-doped BiVO4 by Flame Spray Pyrolysis: Enhancing Photocatalytic O2 Evolution

A flame spray pyrolysis (FSP) method has been developed, for controlled doping of BiVO4 nanoparticles with W and Zr in tandem with the oxygen vacancies (Vo) of the BiVO4 lattice. Based on XPS and Raman data, we show that the nanolattice of W-BiVO4 and Zr-BiO4 can be controlled to achieve optimal O2 evolution from H2O photocatalysis. A synergistic effect is found between the W- and Zr-doping level in correlation with the Vo-concentration. FSP- made W-BiVO4 show optimal photocatalytic O2-production from H2O, up to 1020 μmol/(g × h) for 5%W-BiVO4, while the best performing Zr-doped achieved 970 μmol/(g × h) for 5%Zr-BiVO4. Higher W-or Zr-doping resulted in deterioration in photocatalytic O2-production from H2O. Thus, engineering of FSP-made BiVO4 nanoparticles by precise control of the lattice and doping-level, allows significant enhancement of the photocatalytic O2-evolution efficiency. Technology-wise, the present work demonstrates that flame spray pyrolysis as an inherently scalable technology, allows precise control of the BiVO4 nanolattice, to achieve significant improvement of its photocatalytic efficiency.