Effect of the Support on the Photocatalytic Water Oxidation Activity of Cobalt Oxide Nanoclusters

Recent advances of modification effect in Co3O4-based catalyst towards highly efficient photocatalysis

Tricobalt tetroxide (Co3O4) has been developed as a promising photocatalyst material for various applications. Several reports have been published on the self-modification of Co3O4 to achieve optimal photocatalytic performance. The pristine Co3O4 alone is inadequate for photocatalysis due to the rapid recombination process of photogenerated (PG) charge carriers. The modification of Co3O4 can be extended through the introduction of doping elements, incorporation of supporting materials, surface functionalization, metal loading, and combination with other photocatalysts. The addition of doping elements and support materials may enhance the photocatalysis process, although these modifications have a slight effect on decreasing the recombination process of PG charge carriers. On the other hand, combining Co3O4 with other semiconductors results in a different PG charge carrier mechanism, leading to a decrease in the recombination process and an increase in photocatalytic activity. Therefore, this work discusses recent modifications of Co3O4 and their effects on its photocatalytic performance. Additionally, the modification effects, such as enhanced surface area, generation of oxygen vacancies, tuning the band gap, and formation of heterojunctions, are reviewed to demonstrate the feasibility of separating PG charge carriers. Finally, the formation and mechanism of these modification effects are also reviewed based on theoretical and experimental approaches to validate their formation and the transfer process of charge carriers.

Trends and progress in application of cobalt-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water splitting

There has been a growing interest in water oxidation in recent two decades. Along with that, remarkable discovery of formation of a mysterious catalyst layer upon application of an anodic potential of 1.13 V vs. standard hydrogen electrode (SHE) to an inert indium tin oxide electrode immersed in phosphate buffer containing Co^(II) ions by Nocera et.al, has greatly attracted researchers interest. These researches have oriented in two directions; one focuses on obtaining better understanding of the reported mysterious catalyst layer, further modification, and improved performance, and the second approach is about designing coordination complexes of cobalt and investigating their properties toward the application in water splitting. Although there have been critical debates on true catalysts that are responsible for water oxidation in homogeneous systems of coordination complexes of cobalt, and the case is not totally closed, in this short review, our focus will be mainly on recent major progress and developments in the design and the application of cobalt oxide-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water oxidation reaction, which have been reported since pioneering report of Nocera in 2008 (Kanan Matthew and Nocera Daniel in Science 321:1072-1075, 2008).

Light-Driven Water Oxidation with Ligand-Engineered Prussian Blue Analogues

The elucidation of the ideal coordination environment of a catalytic site has been at the heart of catalytic applications. Herein, we show that the water oxidation activities of catalytic cobalt sites in a Prussian blue (PB) structure could be tuned systematically by decorating its coordination sphere with a combination of cyanide and bidentate pyridyl groups.  K_0.1[Co(bpy)]_2.9[Fe(CN)₆]₂ ([Cobpy–Fe]), K_0.2[Co(phen)]_2.8[Fe(CN)₆]₂ ([Cophen–Fe]), {[Co(bpy)₂]₃[Fe(CN)₆]₂}[Fe(CN)₆]_1/3 ([Cobpy2–Fe]), and {[Co(phen)₂]₃[Fe(CN)₆]₂}[Fe(CN)₆]_1/3 Cl_0.11 ([Cophen2–Fe]) were prepared by introducing bidentate pyridyl groups (phen: 1,10-phenanthroline, bpy: 2,2′-bipyridine) to the common synthetic protocol of Co–Fe Prussian blue analogues. Characterization studies indicate that [Cobpy2–Fe] and [Cophen2–Fe] adopt a pentanuclear molecular structure, while [Cobpy–Fe] and [Cophen–Fe] could be described as cyanide-based coordination polymers with lower-dimensionality and less crystalline nature compared to the regular Co–Fe Prussian blue analogue (PBA), K_0.1Co_2.9[Fe(CN)₆]₂ ([Co–Fe]). Photocatalytic studies reveal that the activities of [Cobpy–Fe] and [Cophen–Fe] are significantly enhanced compared to those of [Co–Fe], while molecular [Cobpy2–Fe] and [Cophen2–Fe] are inactive toward water oxidation. [Cobpy–Fe] and [Cophen–Fe] exhibit upper-bound turnover frequencies (TOFs) of 1.3 and 0.7 s^–1, respectively, which are ∼50 times higher than that of [Co–Fe] (1.8 × 10^–2 s^–1). The complete inactivity of [Cobpy2–Fe] and [Cophen2–Fe] confirms the critical role of aqua coordination to the catalytic cobalt sites for oxygen evolution reaction (OER). Computational studies show that bidentate pyridyl groups enhance the susceptibility of the rate-determining Co(IV)-oxo species to the nucleophilic water attack during the critical O–O bond formation. This study opens a new route toward increasing the intrinsic water oxidation activity of the catalytic sites in PB coordination polymers.

Bidentate pyridyl groups are coordinated to the catalytic cobalt sites in a cyanide-based Co−Fe structure to afford well-tuned extended network structures, which exhibit an outstanding photocatalytic performance compared to the regular Co−Fe PBA.

Promotion of the water oxidation activity of iridium oxide by a nitrogen coordination strategy

The water oxidation reaction is the pivotal half-reaction for photo-/electro-catalytic water splitting. Fabrication of high-efficiency and robust water oxidation is essential to realize wide-scale artificial photosynthesis. Here, we report an efficient strategy to improve the water oxidation activity of iridium oxide by a nitrogen-coordination method. Due to the coordination effect, the iridium oxide can be well dispersed to generate ultra-small nanoparticles and the intrinsic activity can be improved for the water oxidation reaction. This study suggests that high-performance water oxidation catalysts can be constructed based on a nitrogen-coordination strategy.

Polyphenol-metal network derived nanocomposite to catalyze peroxymonosulfate decomposition for dye degradation

Persulfate based advanced oxidation process is a promising technology for refractory contaminants removal. Cobalt is considered as the most efficient metal in catalyzing peroxymonosulfate decomposition. Although different cobalt based nanomaterials have been developed, easy aggregation and metal ion leaching during catalytic reaction would result in its deficiency. To address the above issue, in this work, carbon supported Co/CoO core-shell nanocomposite was in-situ fabricated by using polyphenol-metal coordinate as precursor. Results indicated that cobalt nanoparticle with size of 10 nm was successfully prepared and well dispersed within the carbon matrix. By using as-prepared material as catalyst, 50 mg/L orange II was completely removed under the condition of 0.2 g/L peroxymonosulfate, 0.05 g/L catalyst, pH = 4.0-10.0. Both sulfate and hydroxyl radicals were formed during peroxymonosulfate decomposition, while sulfate radical dominated the pollutant removal. Mechanism study revealed that the cobalt was the key site for catalyzing peroxymonosulfate decomposition. This work might provide valuable information in designing and fabricating metal anchored carbon composite catalyst for efficiently and cost-effectively activate peroxymonosulfate.

Effect of Zinc Acetate Concentration on Optimization of Photocatalytic Activity of p-Co3O4/n-ZnO Heterostructures

In this work, p-Co₃O₄/n-ZnO heterostructures were fabricated on Ni substrate by hydrothermal-decomposition method using cobaltous nitrate hexahydrate (Co(NO₃)₂·6H₂O) and zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) as precursors with zinc acetate concentration varying from 5.0 to 55.0 mM. Structure and morphology of the developed samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). Effect of zinc acetate concentration on the photocatalytic activity of p-Co₃O₄/n-ZnO heterostructures was investigated by degradation of methyl orange (MO) under the UV light irradiation. The fabricated p-Co₃O₄/n-ZnO heterostructures exhibited higher photocatalytic activity than pure Co₃O₄ particles. In order to obtain the maximum photocatalytic activity, zinc acetate concentration was optimized. Specifically, at 35 mM of zinc acetate, the p-Co₃O₄/n-ZnO showed the highest photocatalytic activity with the degradation efficiency of MO reaching 89.38% after 72 h irradiation. The improvement of photocatalytic performance of p-Co₃O₄/n-ZnO heterostructures is due to the increased concentration of photo-generated holes on Co₃O₄ surface and the higher surface-to-volume ratio in the hierarchical structure formed by nano-lamellas.

The recent development of efficient Earth-abundant transition-metal nanocatalysts

Whereas noble metal compounds have long been central in catalysis, Earth-abundant metal-based catalysts have in the same time remained undeveloped. Yet the efficacy of Earth-abundant metal catalysts was already shown at the very beginning of the 20th century with the Fe-catalyzed Haber-Bosch process of ammonia synthesis and later in the Fischer-Tropsch reaction. Nanoscience has revolutionized the world of catalysis since it was observed that very small Au nanoparticles (NPs) and other noble metal NPs are extraordinarily efficient. Therefore the development of Earth-abundant metals NPs is more recent, but it has appeared necessary due to their "greenness". This review highlights catalysis by NPs of Earth-abundant transition metals that include Mn, Fe, Co, Ni, Cu, early transition metals (Ti, V, Cr, Zr, Nb and W) and their nanocomposites with emphasis on basic principles and literature reported during the last 5 years. A very large spectrum of catalytic reactions has been successfully disclosed, and catalysis has been examined for each metal starting with zero-valent metal NPs followed by oxides and other nanocomposites. The last section highlights the catalytic activities of bi- and trimetallic NPs. Indeed this later family is very promising and simultaneously benefits from increased stability, efficiency and selectivity, compared to monometallic NPs, due to synergistic substrate activation.

Au nanoparticle-doped Co3O4–CoFe2O4@SiO2 as a catalyst for visible-light-driven water oxidation

Composites of low dielectric constant SiO₂, Au and metal oxide was obtained, it showed higher O₂ evolution performance due to enhancing the electron transfer rate.

Nitrogen doped RGO-Co3O4 nanograin cookies: highly porous and robust catalyst for removing nitrophenol from waste water

The fabrication of nanograins with a uniform morphology wrapped with reduced graphene oxide (RGO) in a designed manner is critical for obtaining a large surface, high porosity and efficient catalytic ability at mild conditions. Hybrid structures of metal oxides decorated on two-dimensional (2D) RGO lacked an interface and channels between the individual grains and RGO. The present work focuses on the synthesis of RGO-wrapped Co₃O₄ nanograin architecture in micron-sized polyhedrons and the ability to reduce aromatic nitro compounds. Doping N in the designed microstructure polyhedrons resulted in very large surface area (1085.6 m² g^-1) and pore density (0.47 m³ g^-1) microcages. Binding energies from x-ray photoelectron spectroscopy (XPS) and Raman intensities confirmed the presence of doped N and RGO-wrapped around Co₃O₄ nanograins. However, the morphology and microstructure was supported by FESEM and HRTEM images revealing the fabrication of high integrity RGO-Co₃O₄ microstructure hybrids composed of a 10 nm grain size with narrower grain size distribution. Ammonia treatment produced interconnected channels and dumbbell pores that facilitated ion exchange between the catalyst surface and the liquid medium at the grain boundary interfaces, and offered less mass transport resistance providing fast adsorption of reactants and desorption of the product causing surface renewal. Prepared N-RGO-Co₃O₄ shows the largest percentage reduction (96%) of p-nitrophenol (p-NP) at room temperature as compared to pure Co₃O₄ and RGO-Co₃O₄ nanograin microstructures over 10 min. Fabricated architectures can be applied effectively for fast and facile treatment of industrial waste streams with complex organic molecules.

Water Oxidation Mechanisms of Metal Oxide Catalysts by Vibrational Spectroscopy of Transient Intermediates

Water oxidation is an essential reaction of an artificial photosystem for solar fuel generation because it provides electrons needed to reduce carbon dioxide or protons to a fuel. Earth-abundant metal oxides are among the most attractive catalytic materials for this reaction because of their robustness and scalability, but their efficiency poses a challenge. Knowledge of catalytic surface intermediates gained by vibrational spectroscopy under reaction conditions plays a key role in uncovering kinetic bottlenecks and provides a basis for catalyst design improvements. Recent dynamic infrared and Raman studies reveal the molecular identity of transient surface intermediates of water oxidation on metal oxides. Combined with ultrafast infrared observations of how charges are delivered to active sites of the metal oxide catalyst and drive the multielectron reaction, spectroscopic advances are poised to play a key role in accelerating progress toward improved catalysts for artificial photosynthesis.

The recent development of efficient Earth-abundant transition-metal nanocatalysts

This review presents the recent remarkable developments of efficient Earth-abundant transition-metal nanocatalysts.

Water Oxidation by Size-Selected Co27 Clusters Supported on Fe2 O3

The complexity of the water oxidation reaction makes understanding the role of individual catalytic sites critical to improving the process. Here, size-selected 27-atom cobalt clusters (Co₂₇ ) deposited on hematite (Fe₂ O₃ ) anodes were tested for water oxidation activity. The uniformity of these anodes allows measurement of the activity of catalytic sites of well-defined nuclearity and known density. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) characterization of the anodes before and after electrochemical cycling demonstrates that these Co₂₇ clusters are stable to dissolution even in the harsh water oxidation electrochemical environment. They are also stable under illumination at the equivalent of 0.4 suns irradiation. The clusters show turnover rates for water oxidation that are comparable or higher than those reported for Pd- and Co-based materials or for hematite. The support for the Co₂₇ clusters is Fe₂ O₃ grown by atomic layer deposition on a Si chip. We have chosen to deposit a Fe₂ O₃ layer that is only a few unit cells thick (2 nm), to remove complications related to exciton diffusion. We find that the electrocatalytic and the photoelectrocatalytic activity of the Co₂₇ /Fe₂ O₃ material is significantly improved when the samples are annealed (with the clusters already deposited). Given that the support is thin and that the cluster deposition density is equivalent to approximately 5 % of an atomic monolayer, we suggest that annealing may significantly improve the exciton diffusion from the support to the catalytic moiety.

Synthesis of Nanoporous Metals, Oxides, Carbides, and Sulfides: Beyond Nanocasting

Nanoporous metal-based solids are of particular interest because they combine a large quantity of surface metal sites, interconnected porous networks, and nanosized crystalline walls, thus exhibiting unique physical and chemical properties compared to other nanostructures and bulk counterparts. Among all of the synthetic approaches, nanocasting has proven to be a highly effective method for the syntheses of metal oxides with three-dimensionally ordered porous structures and crystalline walls. A typical procedure involves a thermal annealing process of a porous silica template filled with an inorganic precursor (often a metal nitrate salt), which converts the precursor into a desired phase within the silica pores. The final step is the selective removal of the silica template in either a strong base or a hydrofluoric acid solution. In the past decade, nanocasting has become a popular synthetic approach and has enabled the syntheses of a variety of nanoporous metal oxides. However, there is still a lack of synthetic methods to fabricate nanoporous materials beyond simple metal oxides. Therefore, the development of new synthetic strategies beyond nanocasting has become an important direction. This Account describes new progress in the preparation of novel nanoporous metal-based solids for heterogeneous catalysis. The discussion begins with a method called dealloying, an effective method to synthesize nanoporous metals. The starting material is a metallic alloy containing two or more elements followed by a selective chemical or electrochemical leaching process that removes one of the preferential elements, resulting in a highly porous structure. Nanoporous metals, such as Cu, Ag, and CuTi, exhibit remarkable electrocatalytic properties in carbon dioxide reduction, oxygen reduction, and hydrogen evolution reactions. In addition, the syntheses of metal oxides with hierarchical porous structures are also discussed. On the basis of the choice of hard template, nanoporous metal oxides with bimodal pore size distributions can be obtained. Combining nanocasting with chemical etching, a cobalt oxide with a hierarchical porous structure was synthesized, which possessed a surface area up to 250 m(2) g(-1), representing the highest surface area reported to date for nanoporous cobalt oxides. Lastly, this Account also covers the syntheses of nanoporous metal carbides and sulfides. The combination of in situ carburization and nanocasting enabled the syntheses of two ordered nanoporous metal carbides, Mo2C and W2C. For nanoporous metal sulfides, an "oxide-to-sulfide" synthetic strategy was proposed to address the large volume change issue of converting metal nitrate precursors to metal sulfide products in nanocasting. The successful syntheses of ordered nanoporous FeS2, CoS2, and NiS2 demonstrated the feasibility of the "oxide-to-sulfide" method. Concluding remarks include a summary of recent advances in the syntheses of nanoporous metal-based solids and a brief discussion of future opportunities in the hope of stimulating new interests and ideas.

Dual-Templated Cobalt Oxide for Photochemical Water Oxidation

Mesoporous Co3 O4 was prepared using a dual templating approach whereby mesopores inside SiO2 nanospheres, as well as the void spaces between the nanospheres, were used as templates. The effect of calcination temperature on the crystallinity, morphology, and textural parameters of the Co3 O4 replica was investigated. The catalytic activity of Co3 O4 for photochemical water oxidation in a [Ru(bpy)3 ](2+) [S2 O8 ](2-) system was evaluated. The Co3 O4 replica calcined at the lowest temperature (150 °C) exhibited the best performance as a result of the unique nanostructure and high surface area arising from the dual templating. The performance of Co3 O4 with highest surface area was further examined in electrochemical water oxidation. Superior activity over high temperature counterpart and decent stability was observed. Furthermore, CoO with identical morphology was prepared from Co3 O4 using an ethanol reduction method and a higher turnover-frequency number for photochemical water oxidation was obtained.

Coupling carbon dioxide reduction with water oxidation in nanoscale photocatalytic assemblies

Closing the photosynthetic cycle on the nanometer scale under membrane separation of the half reactions for developing scalable artificial photosystems.

Hollow Fluffy Co3O4 Cages as Efficient Electroactive Materials for Supercapacitors and Oxygen Evolution Reaction

Nano-/micrometer multiscale hierarchical structures not only provide large surface areas for surface redox reactions but also ensure efficient charge conductivity, which is of benefit for utilization in areas of electrochemical energy conversion and storage. Herein, hollow fluffy cages (HFC) of Co3O4, constructed of ultrathin nanosheets, were synthesized by the formation of Co(OH)2 hollow cages and subsequent calcination at 250 °C. The large surface area (245.5 m2 g(-1)) of HFC Co3O4 annealed at 250 °C ensures the efficient interaction between electrolytes and electroactive components and provides more active sites for the surface redox reactions. The hierarchical structures minimize amount of the grain boundaries and facilitate the charge transfer process. Thin thickness of nanosheets (2-3 nm) ensures the highly active sites for the surface redox reactions. As a consequence, HFC Co3O4 as the supercapacitor electrode exhibits a superior rate capability, shows an excellent cycliability of 10,000 cycles at 10 A g(-1), and delivers large specific capacitances of 948.9 and 536.8 F g(-1) at 1 and 40 A g(-1), respectively. Catalytic studies of HFC Co3O4 for oxygen evolution reaction display a much higher turnover frequency of 1.67×10(-2) s(-1) in pH 14.0 KOH electrolyte at 400 mV overpotential and a lower Tafel slope of 70 mV dec(-1). HFC Co3O4 with the efficient electrochemical activity and good stability can remain a promising candidate for the electrochemical energy conversion and storage.

Facile synthesis of monodisperse Co3O4 quantum dots with efficient oxygen evolution activity

Monodisperse, water-dispersible Co3O4 quantum dots with sizes of around 4.5 nm are prepared through a simple solution method. The resultant cobalt oxide quantum dots exhibit excellent visible-light-driven oxygen evolution activities in the [Ru(bpy)3](2+)-persulfate system under mild pH conditions.

A general approach towards multi-faceted hollow oxide composites using zeolitic imidazolate frameworks

To realize the various functionalities and maximize the structural advantages, hollow particles with multiple compositions and complex structures are highly desirable. However, the development of a convenient and scalable method for the synthesis of such multi-compositionally complex hollow structures remains a big challenge. Herein, we report an efficient and universal strategy to fabricate a variety of porous hollow oxide nanocomposites (Co3O4/SiO2, Co3O4/TiO2, ZnO/SiO2, and ZnO/TiO2) composed of nanosized subunits, which involves a sol-gel process to form a shell onto the dodecahedral zeolitic imidazolate framework (ZIF) template and a subsequent thermolysis-induced transformation of the template at appropriate temperatures. Such a multi-compositional hollow structure with a large surface area endows the materials with exceptional properties and performances. As an example, we demonstrated that these complex hollow oxide composites, especially the Co3O4/SiO2 hollow dodecahedra, exhibit a significantly enhanced photocatalytic performance as oxygen evolution photocatalysts.

Cobalt-manganese-based spinels as multifunctional materials that unify catalytic water oxidation and oxygen reduction reactions

Recently, there has been much interest in the design and development of affordable and highly efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts that can resolve the pivotal issues that concern solar fuels, fuel cells, and rechargeable metal-air batteries. Here we present the synthesis and application of porous CoMn2 O4 and MnCo2 O4 spinel microspheres as highly efficient multifunctional catalysts that unify the electrochemical OER with oxidant-driven and photocatalytic water oxidation as well as the ORR. The porous materials were prepared by the thermal degradation of the respective carbonate precursors at 400 °C. The as-prepared spinels display excellent performances in electrochemical OER for the cubic MnCo2 O4 phase in comparison to the tetragonal CoMn2 O4 material in an alkaline medium. Moreover, the oxidant-driven and photocatalytic water oxidations were performed and they exhibited a similar trend in activity to that of the electrochemical OER. Remarkably, the situation is reversed in ORR catalysis, that is, the oxygen reduction activity and stability of the tetragonal CoMn2 O4 catalyst outperformed that of cubic MnCo2 O4 and rivals that of benchmark Pt catalysts. The superior catalytic performance and the remarkable stability of the unifying materials are attributed to their unique porous and robust microspherical morphology and the intrinsic structural features of the spinels. Moreover, the facile access to these high-performance materials enables a reliable and cost-effective production on a large scale for industrial applications.

Charge carrier dynamics of photoexcited Co3O4 in methanol: extending high harmonic transient absorption spectroscopy to liquid environments

Charge carrier dynamics in Co3O4 thin films are observed using high harmonic generation transient absorption spectroscopy at the Co M2,3 edge. Results reveal that photoexcited Co3O4 decays to the ground state in 600 ± 40 ps in liquid methanol compared to 1.9 ± 0.3 ns in vacuum. Kinetic analysis suggests that surface-mediated relaxation of photoexcited Co3O4 may be the result of hole transfer from Co3O4 followed by carrier recombination at the Co3O4-methanol interface.

Understanding the role of gold nanoparticles in enhancing the catalytic activity of manganese oxides in water oxidation reactions

The Earth-abundant and inexpensive manganese oxides (MnOx) have emerged as an intriguing type of catalysts for the water oxidation reaction. However, the overall turnover frequencies of MnOx catalysts are still much lower than that of nanostructured IrO2 and RuO2 catalysts. Herein, we demonstrate that doping MnOx polymorphs with gold nanoparticles (AuNPs) can result in a strong enhancement of catalytic activity for the water oxidation reaction. It is observed that, for the first time, the catalytic activity of MnOx/AuNPs catalysts correlates strongly with the initial valence of the Mn centers. By promoting the formation of Mn(3+) species, a small amount of AuNPs (<5%) in α-MnO2/AuNP catalysts significantly improved the catalytic activity up to 8.2 times in the photochemical and 6 times in the electrochemical system, compared with the activity of pure α-MnO2.

Self-template construction of hollow Co3O4 microspheres from porous ultrathin nanosheets and efficient noble metal-free water oxidation catalysts

Developing noble metal-free water oxidation catalysts is essential for many energy conversion/storage processes (e.g., water splitting). Herein, we report the facile synthesis of hollow Co3O4 microspheres composed of porous, ultrathin (<5 nm), single-crystal-like nanosheets via a novel "self-template" route. The successful preparation of these hollow Co3O4 nanomaterials includes three main steps: (1) the synthesis of solid cobalt alkoxide microspheres, (2) their subsequent self-template conversion into hollow cobalt hydroxide microspheres composed of ultrathin nanosheets, and finally (3) thermal treatment of hollow cobalt hydroxide microspheres into the hollow Co3O4 material. The as-obtained hollow Co3O4 nanomaterial possesses a high BET surface area (∼180 m(2) g(-1)), and can serve as an active and stable water oxidation catalyst under both electrochemical and photochemical reaction conditions, owing to its unique structural features. In the electrochemical water oxidation, this catalyst affords a current density of 10 mA cm(-2) (a value related to practical relevance) at an overpotential of ∼0.40 V. Moreover, with the assistance of a sensitizer [Ru(bpy)3](2+) (bpy = 2,2'-bipyridine), this nanomaterial can catalyze water oxidation reactions under visible light irradiation with an O2 evolution rate of ∼12 218 μmol g(-1) h(-1). Our results suggest that delicate nanostructuring can offer unique advantages for developing efficient water oxidation catalysts.

Semiconductor photocatalysts for water oxidation: current status and challenges

Artificial photosynthesis is a highly-promising strategy to convert solar energy into hydrogen energy for the relief of the global energy crisis. Water oxidation is the bottleneck for its kinetic and energetic complexity in the further enhancement of the overall efficiency of the artificial photosystem. Developing efficient and cost-effective photocatalysts for water oxidation is a growing desire, and semiconductor photocatalysts have recently attracted more attention due to their stability and simplicity. This article reviews the recent advancement of semiconductor photocatalysts with a focus on the relationship between material optimization and water oxidation efficiency. A brief introduction to artificial photosynthesis and water oxidation is given first, followed by an explanation of the basic rules and mechanisms of semiconductor particulate photocatalysts for water oxidation as theoretical references for discussions of componential, surface structure, and crystal structure modification. O2-evolving photocatalysts in Z-scheme systems are also introduced to demonstrate practical applications of water oxidation photocatalysts in artificial photosystems. The final part proposes some challenges based on the dynamics and energetics of photoholes which are fundamental to the enhancement of water oxidation efficiency, as well as on the simulation of natural water oxidation that will be a trend in future research.