A comprehensive review of oxygen vacancy modified photocatalysts: synthesis, characterization, and applications.

Photocatalytic technology is a novel approach that harnesses solar energy for efficient energy conversion and effective pollution abatement, representing a rapidly advancing field in recent years. The development and synthesis of high-performance semiconductor photocatalysts constitute the pivotal focal point. Oxygen vacancies, being intrinsic defects commonly found in metal oxides, are extensively present within the lattice of semiconductor photocatalytic materials exhibiting non-stoichiometric ratios. Consequently, they have garnered significant attention in the field of photocatalysis as an exceptionally effective means for modulating the performance of photocatalysts. This paper provides a comprehensive review on the concept, preparation, and characterization methods of oxygen vacancies, along with their diverse applications in nitrogen fixation, solar water splitting, CO2 photoreduction, pollutant degradation, and biomedicine. Currently, remarkable progress has been made in the synthesis of high-performance oxygen vacancy photocatalysts and the regulation of their catalytic performance. In the future, it will be imperative to develop more advanced in situ characterization techniques, conduct further investigations into the regulation and stabilization of oxygen vacancies in photocatalysts, and comprehensively comprehend the mechanism underlying the influence of oxygen vacancies on photocatalysis. The engineering of oxygen vacancies will assume a pivotal role in the realm of semiconductor photocatalysis.

Unveiling the effect of the structural transformation of CoZn-MOF on BiVO4 photoanode for efficient photoelectrochemical water oxidation.

Photoelectrochemical (PEC) water splitting has been widely investigated for solar-to-hydrogen conversion. However, issues like high charge recombination rate and slow surface water oxidation kinetics severely hinder its (PEC) conversion efficiency. Herein, we constructed MOF-derived CoOOH cocatalyst on BiVO4 photoanode, using a feasible electrochemical activation strategy. The BiVO4-based photoanode obtained shows a high photocurrent density of 3.15 mA/cm2 at 1.23 VRHE and low onset potential. Detailed experiments and theoretical calculations show that during the activation of CoZn-MOFs, there was a partial breakage of 2-methylimidazole (mIM) linker, an increase in the oxidation state of Cobalt ion (Co), and increased O2-. The high PEC performance is mainly attributed to the MOF-derived CoOOH, which provides rich active sites for hole extraction and reduces the overpotential for oxygen evolution reaction. Furthermore, when CoZnNiFe-LDHs were decorated on BiVO4 using the ions exchange method, the photocurrent density of BiVO4/CoZnNiFe-LDHs photoanode got to 4.0 mA/cm2 at 1.23 VRHE, accompanied with high stability. This study provides insights into understanding the key role played by the structural transformation of MOF cocatalyst in PEC water splitting processes.

Modulating Charge Transfer Efficiency of Hematite Photoanode with Hybrid Dual-Metal-Organic Frameworks for Boosting Photoelectrochemical Water Oxidation.

The glorious charge transfer efficiency of photoanode is an important factor for efficient photoelectrochemical (PEC) water oxidation. However, it is often limited by slow kinetics of oxygen evolution reaction. Herein, a dual transition metal-based metal-organic frameworks (MOF) cocatalyst, Fe@Ni-MOF, is introduced into a titanium-doped hematite (Fe2O3:Ti) photoanode. The combination of Ni and Fe can optimize the filling of 3d orbitals. Moreover, the introduction of Fe donates electrons to Ni in the MOF structure, thus, suppressing the irreversible (long-life-time) oxidation of Ni2+ into Ni3+. The resulting Fe@Ni-MOF/Fe2O3:Ti photoanode exhibits ∼threefold enhancement in the photocurrent density at 1.23 V versus the reversible hydrogen electrode. Kinetic analysis of the PEC water oxidation processes indicates that this performance improvement is primarily due to modulating the charge transfer efficiency of hematite photoanode. Further results show that a single transition metal-based MOF cocatalyst, Ni-MOF, exhibits slow charge transfer in spite of a reduction in surface charge recombination, resulting in a smaller charge transfer efficiency. These findings provide new insights for the development of photoelectrodes decorated with MOFs.

γ-ray induced formation of oxygen vacancies and Ti3+ defects in anatase TiO2 for efficient photocatalytic organic pollutant degradation.

Oxygen vacancies and Ti3+ defects in anatase TiO2 have attracted great attention to address the insufficient optical absorption and photoinduced charge-carrier separation in photocatalysis. In this study, we demonstrate a superficial and innovative approach for synthesizing anatase TiO2 nanoparticles with abundant oxygen vacancies via γ-ray irradiation reduction at room temperature. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) confirm that oxygen vacancies and Ti3+ defects can be quantitatively and extensively obtained by merely regulating the irradiation dosage. Photoelectrochemical measurements suggest that oxygen vacancies and Ti3+ defects promoted the separation of electron-hole pairs and then enhanced the photocatalytic degradation performance for organic pollutant. In comparison with TiO2 (no irradiation), the sample (49.5 kGy irradiation) exhibited a 20.0-fold enhancement in visible-light decomposition of phenol. In addition, the results of scavenge experiments and mechanism analysis revealed that O2- are the dominant active species. The excited electrons generated at the conduction band and oxygen vacancy level of TiO2-x-49.5 conspicuously contributes to generate much more ·O2- species. This novel study shows at room temperature, the γ-ray approach of irradiation leads to faster formation and quantification of oxygen vacancies in the semiconductor materials.

Oxygen-Deficient Nanofiber WO3-x/WO3 Homojunction Photoanodes Synthesized via a Novel Metal Self-Reducing Method.

Defect engineering of semiconductors has been identified as an efficient route toward enhancing the photoelectrochemical performances. There is a security threat in the traditional hydrogen annealing process. In this work, the oxygen-defective nanofiber WO3-x/WO3 homojunction photoanode was in situ-synthesized via a novel metal self-reducing method. The as-prepared photoanode exhibits 1.8 times higher solar water oxidation photocurrent density than that of the bare WO3 film at 1.2 V versus Ag/AgCl. The enhanced photoelectrochemical properties originate from the effective charge separation and injection, attributed to the stronger built-in electric field created by the oxygen-deficient homojunction. Importantly, this novel method is universally applicable to synthesize oxygen-deficient semiconductor materials, including films and powders.

Three-dimensional Composite Catalysts for Al-O2 Batteries Composed of CoMn2O4 Nanoneedles Supported on Nitrogen-Doped Carbon Nanotubes/Graphene.

Great efforts have been focused on studying high-efficiency and stable catalysts toward oxygen reduction reaction (ORR) in metal-air batteries. In view of synergistic effects and improved properties, carbon nanotubes and three-dimensional graphene (CNTs-3D graphene) hybrid catalysts developed via a well-controlled route are urgently required. Herein, a CoMn2O4 (CMO) nanoneedle-supported nitrogen-doped carbon nanotubes/3D graphene (NCNTs/3D graphene) composite was prepared by in situ chemical vapor deposition (CVD) along with hydrothermal methods over a Ni foam substrate. The cyclic voltammetry and linear sweep voltammograms results indicate that the CMO/NCNTs/3D graphene hybrid possesses remarkable electrocatalytic performance toward ORR in alkaline conditions compared with NCNTs/3D graphene, CMO/3D graphene, and 3D graphene catalysts, even outperforming the commercial 20 wt % Pt/C catalyst. Moreover, the Al-air coin cell employing CMO/NCNTs/3D graphene as cathode catalysts obtains an open circuit voltage of 1.55 V and a specific capacity of 312.8 mA h g-1, which are superior to the Al-air coin cell with NCNTs/3D graphene as catalysts. This work supplies new insights to advanced electrocatalysts introducing NCNTs/3D graphene as a catalyst support to develop scalable transition-metal oxide/NCNTs/3D graphene hybrids with excellent catalytic activity toward ORR in Al-air systems.

In Situ Formation of WO3-Based Heterojunction Photoanodes with Abundant Oxygen Vacancies via a Novel Microbattery Method.

Non-stoichiometric ratio semiconductor materials have exhibited excellent performance in energy conversion and storage fields. However, the hydrogen treatment method that is commonly used to introduce oxygen vacancies is expensive and dangerous. In this paper, a novel microbattery method using Zn powder and Fe powder as reductant has been developed to synthesize the oxygen vacancy modified WO3- x films and oxygen-deficient heterojunction films (ZnWO4- x/WO3- x and Fe2O3- x/WO3- x) at room temperature. The as-prepared WO3- x and ZnWO4- x/WO3- x heterojunction films exhibit improved photoelectrochemical performance. It is worth noting that this microbattery method can quickly introduce oxygen vacancies into semiconductor materials, including powders and films at room temperature.