Oxygen photoevolution on a tantalum oxynitride photocatalyst under visible-light irradiation: how does water photooxidation proceed on a metal-oxynitride surface?

Ensuring Stability of Anode Catalysts in PEMWE: From Material Design to Practical Application

Proton Exchange Membrane Water Electrolysis (PEMWE) has emerged as a clean and effective approach for the conversion and storage of renewable electricity, particularly due to its compatibility with fluctuating photovoltaic and wind power. However, the high cost and limited performance of iridium oxide catalysts (i. e. IrO2) used as anode catalyst in industrial PEM electrolyzers remain significant obstacles to widespread application. Although numerous low-cost and efficient alternative catalysts have been developed in laboratory research, comprehensive stability studies critical for industrial use are often overlooked. This leads to the failure of performance transfer from catalysts tested in liquid half-cell systems to those employed in PEM electrolyzers. This concept presents a thorough overview for the stability issues of anode catalysts in PEMWE, and discuss their degradation mechanisms in both liquid half-cell systems and PEM electrolyzers. We summarize the comprehensive protocols for assessment and characterization, analyze the effective strategies for stability optimization, and explore the opportunities for designing viable anode catalysts for PEM electrolyzers.

Applications of Vanadium, Niobium, and Tantalum Complexes in Organic and Inorganic Synthesis

Organometallic catalysis is a powerful strategy in chemical synthesis, especially with the cheap and low toxic metals based on green chemistry principle. Thus, the selection of the metal is particularly important to plan relevant and applicable processes. The group VB metals have been the subject of exciting and significant advances in both organic and inorganic synthesis. In this Review, we have summarized some reports from recent decades, which are about the development of group VB metals utilized in various types of reactions, such as oxidation, reduction, alkylation, dealkylation, polymerization, aromatization, protein synthesis, and practical water splitting.

Improvement of Visible‐Light H 2 Evolution Activity of Pb 2 Ti 2 O 5.4 F 1.2 Photocatalyst by Coloading of Rh and Pd Cocatalysts

Pb₂Ti₂O_5.4F_1.2 modified with various metal cocatalysts was studied as a photocatalyst for visible‐light H₂ evolution. Although unmodified Pb₂Ti₂O_5.4F_1.2 showed negligible activity, modification of its surface with Rh led to the best observed promotional effect among the Pb₂Ti₂O_5.4F_1.2 samples modified with a single metal cocatalyst. The H₂ evolution activity was further enhanced by coloading with Pd; the Rh−Pd/Pb₂Ti₂O_5.4F_1.2 photocatalyst showed 3.2 times greater activity than the previously reported Pt/Pb₂Ti₂O_5.4F_1.2. X‐ray absorption fine‐structure spectroscopy, photoelectrochemical, and transient absorption spectroscopy measurements indicated that the coloaded Rh and Pd species, which were partially alloyed on the Pb₂Ti₂O_5.4F_1.2 surface, improved the electron‐capturing ability, thereby explaining the high activity of the coloaded Rh−Pd/Pb₂Ti₂O_5.4F_1.2 catalyst toward H₂ evolution.

Photocatalytic Z-Scheme Overall Water Splitting: Recent Advances in Theory and Experiments

Photocatalytic water splitting is considered one of the most important and appealing approaches for the production of green H₂ to address the global energy demand. The utmost possible form of artificial photosynthesis is a two-step photoexcitation known as "Z-scheme", which mimics the natural photosystem. This process solely relies on the effective coupling and suitable band positions of semiconductors (SCs) and redox mediators for the purpose to catalyze the surface chemical reactions and significantly deter the backward reaction. In recent years, the Z-scheme strategies and their key role have been studied progressively through experimental approaches. In addition, theoretical studies based on density functional theory have provided detailed insight into the mechanistic aspects of some breathtakingly complex problems associated with hydrogen evolution reaction and oxygen evolution reaction. In this context, this critical review gives an overview of the fundamentals of Z-scheme photocatalysis, including both theoretical and experimental advancements in the field of photocatalytic water splitting, and suggests future perspectives.

Durable Metal Heteroanionic Photocatalysts

Heterogeneous photocatalysis provides a promising strategy to generate renewable fuels by harnessing solar energy. Metal heteroanionic photocatalysts have gained attention for their visible-light absorption; however, they are also plagued by photocorrosion, which limits their long-term use. Such photocorrosion occurs from photooxidation of the less electronegative nonoxide ions, leading to decomposition of the material's lattice. In this Perspective, we highlight emerging strategies to develop durable metal heteroanionic photocatalysts. We devote attention to the approaches taken for model metal oxynitrides, oxysulfides, and oxyhalide photocatalysts to provide a holistic framework. This analysis emphasizes the vital roles that interface engineering, charge carrier extraction, and crystal and electronic structure play in providing photodurability. We believe that through these approaches, durable and visible-light-absorbing artificial photosynthetic systems can be developed for a sustainable future.

Observation of visible light activated photocatalytic degradation of stearic acid on thin films of tantalum oxynitride synthesized by aerosol assisted chemical vapour deposition

UV activated photocatalysts deposited using chemical vapour deposition have found commercial success as self-cleaning coatings. However, only limited work has been conducted on the use of the more recently discovered visible light activated photocatalysis for this application. Tantalum oxynitride is an established visible light photocatalyst, and in this paper we have investigated the ability of thin films of tantalum oxynitride to photocatalytically degrade a model organic pollutant, stearic acid, and therefore assess the coatings potential for self-cleaning applications. Thin films of tantalum oxide were formed using aerosol assisted chemical vapour deposition (AACVD) of tantalum ethoxide, and then converted into tantalum oxynitride through ammonolysis at temperatures between 550 °C and 750 °C. Investigation of the films using XRD, UV-vis spectroscopy and XAFS identify that amorphous tantalum oxynitride is formed during the ammonolysis, with complete conversion to TaON under conditions of 700 °C for 24 hours. The self-cleaning ability of this film was assessed using stearic acid as the model pollutant, with a degradation rate of 2.5(2) × 1013 molecules per min per cm2 when exposed to a 5-sun solar simulator, equipped with a UV cut-off filter. We therefore conclude that tantalum oxynitride thin films are able to act as self-cleaning coatings through visible light photocatalysis and that films of tantalum oxynitride can be synthesized using a scalable chemical vapour deposition route.

Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems

Visible light-driven water splitting using cheap and robust photocatalysts is one of the most exciting ways to produce clean and renewable energy for future generations. Cutting edge research within the field focuses on so-called "Z-scheme" systems, which are inspired by the photosystem II-photosystem I (PSII/PSI) coupling from natural photosynthesis. A Z-scheme system comprises two photocatalysts and generates two sets of charge carriers, splitting water into its constituent parts, hydrogen and oxygen, at separate locations. This is not only more efficient than using a single photocatalyst, but practically it could also be safer. Researchers within the field are constantly aiming to bring systems toward industrial level efficiencies by maximizing light absorption of the materials, engineering more stable redox couples, and also searching for new hydrogen and oxygen evolution cocatalysts. This review provides an in-depth survey of relevant Z-schemes from past to present, with particular focus on mechanistic breakthroughs, and highlights current state of the art systems which are at the forefront of the field.

The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation

This Review addresses the technical challenges, scientific basis, recent progress, and outlook with respect to the stability and degradation of catalysts for the oxygen evolution reaction (OER) operating at electrolyzer anodes in acidic environments with an emphasis on ion exchange membrane applications. First, the term "catalyst stability" is clarified, as well as current performance targets, major catalyst degradation mechanisms, and their mitigation strategies. Suitable in situ experimental methods are then evaluated to give insight into catalyst degradation and possible pathways to tune OER catalyst stability. Finally, the importance of identifying universal figures of merit for stability is highlighted, leading to a comprehensive accelerated lifetime test that could yield comparable performance data across different laboratories and catalyst types. The aim of this Review is to help disseminate and stress the important relationships between structure, composition, and stability of OER catalysts under different operating conditions.

Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure-Activity Relationship

The structure of cobalt oxide (CoO_x) nanoparticles dispersed on rutile TiO₂ (R-TiO₂) was characterized by X-ray diffraction, UV-vis-NIR diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray absorption fine-structure spectroscopy, and X-ray photoelectron spectroscopy. The CoO_x nanoparticles were loaded onto R-TiO₂ by an impregnation method from an aqueous solution containing Co(NO₃)₂·6H₂O followed by heating in air. Modification of the R-TiO₂ with 2.0 wt % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with good reproducibility. Structural analyses revealed that the activity of this photocatalyst depended strongly on the generation of Co₃O₄ nanoclusters with an optimal distribution. These nanoclusters are thought to interact with the R-TiO₂ surface, resulting in visible light absorption and active sites for water oxidation.

Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy

Semiconductor electrodes capable of using solar photons to drive water-splitting reactions, such as haematite (α-Fe2O3), have been the subject of tremendous interest over recent decades. The surface has been found to play a significant role in determining the efficiency of water oxidation with haematite; however, previous works have only allowed hypotheses to be formulated regarding the identity of relevant surface species. Here we investigate the water-oxidation reaction on haematite using infrared spectroscopy under photoelectrochemical (PEC) water-oxidation conditions. A potential- and light-dependent absorption peak at 898 cm(-1) is assigned to a Fe(IV)=O group, which is an intermediate in the PEC water-oxidation reaction. These results provide direct evidence of high-valent iron-oxo intermediates as the product of the first hole-transfer reaction on the haematite surface and represent an important step in establishing the mechanism of PEC water oxidation on semiconductor electrodes.

A review of metal oxynitrides for photocatalysis

This review highlights the recent progress in the production of hydrogen using a metal oxynitride photocatalyst under visible light irradiation.

High pressure polymorphism of β-TaON

The high pressure behavior of TaON was studied using a combination of Raman scattering, synchrotron X-ray diffraction, and X-ray absorption spectroscopy in diamond anvil cells to 70 GPa at ambient temperature. A Birch-Murnaghan equation of state fit for baddeleyite structured β-TaON indicates a high bulk modulus value Ko = 328 ± 4 GPa with K = 4.3. EXAFS analysis of the high pressure XAS data provides additional information on changes in the Ta-(O,N) and Ta-Ta distances. Changes in the X-ray diffraction patterns and Raman spectra indicate onset of a pressure induced phase transition near 33 GPa. Our analysis indicates that the new phase has an orthorhombic cotunnite-type structure but that the phase transition may not be complete even by 70 GPa. Similar sluggish transformation kinetics are observed for the isostructural ZrO2 phase. Analysis of compressibility data for the new cotunnite-type TaON phase indicate a very high bulk modulus Ko ∼ 370 GPa, close to the theoretically predicted value.

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.

Tantalum-based semiconductors for solar water splitting

This review describes the current status of the design, synthesis, and applications of tantalum-based semiconductors, including tantalum oxides, tantalates and tantalum (oxy)nitrides, for photocatalytic and photoelectrochemical water splitting.

Photocatalytic and photoelectrochemical studies of visible-light active α-Fe2O3–g-C3N4 nanocomposites

Synergistic enhancement in photocatalytic degradation of α-Fe₂O₃–g-C₃N₄ due to an increase in visible-light absorption efficiency and rapid photoinduced charge separation.

Synthesis and photocatalytic activity of perovskite niobium oxynitrides with wide visible-light absorption bands

Photocatalytic activities of perovskite-type niobium oxynitrides (CaNbO₂N, SrNbO₂N, BaNbO₂N, and LaNbON₂) were examined for hydrogen and oxygen evolution from water under visible-light irradiation. These niobium oxynitrides were prepared by heating the corresponding oxide precursors, which were synthesized using the polymerized complex method, for 15 h under a flow of ammonia. They possess visible-light absorption bands between 600-750 nm, depending on the A-site cations in the structures. The oxynitride CaNbO₂N, was found to be active for hydrogen and oxygen evolution from methanol and aqueous AgNO₃, respectively, even under irradiation by light at long wavelengths (λ<560 nm). The nitridation temperature dependence of CaNbO₂N was investigated and 1023 K was found to be the optimal temperature. At lower temperatures, the oxynitride phase is not adequately produced, whereas higher temperatures produce more reduced niobium species (e. g., Nb³(+) and Nb⁴(+)), which can act as electron-hole recombination centers, resulting in a decrease in activity.

First-principles study of the electronic, optical properties and lattice dynamics of tantalum oxynitride

First-principles calculations of the electronic, optical properties and lattice dynamics of tantalum oxynitride are performed with the density functional theory plane-wave pseudopotential method. The analysis of the electronic structure shows a covalent nature in Ta-N bonds and Ta-O bonds. The hybridization of anion 2p and Ta 5d states results in enhanced dispersion of the valence band, raising the top of the valence band and leading to the visible-light response in TaON. It has a high dielectric constant, and the anisotropy is displayed obviously in the lower energy region. Our calculation indicated that TaON has excellent dielectric properties along [010] direction. Various optical properties, including the reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum are derived from the complex dielectric function. We also present phonon dispersion relation, zone-center optical mode frequency, density of phonon states, and some thermodynamic properties. The experimental IR modes (B(u) at 808 cm(-1) and A(u) at 863 cm(-1)) are reproduced well and assigned to a combination of stretching and bending vibrations for the Ta-N bond and Ta-O bond. The thermodynamic properties of TaON, such as heat capacity and Debye temperature, which were important parameters for the measurement of crystal physical properties, were first given for reference. Our investigations provide useful information for the potential application of this material.

Efficient nonsacrificial water splitting through two-step photoexcitation by visible light using a modified oxynitride as a hydrogen evolution photocatalyst

A two-step photocatalytic water splitting (Z-scheme) system consisting of a modified ZrO(2)/TaON species (H(2) evolution photocatalyst), an O(2) evolution photocatalyst, and a reversible donor/acceptor pair (i.e., redox mediator) was investigated. Among the O(2) evolution photocatalysts and redox mediators examined, Pt-loaded WO(3) (Pt/WO(3)) and the IO(3)(-)/I(-) pair were respectively found to be the most active components. Combining these two components with Pt-loaded ZrO(2)/TaON achieved stoichiometric water splitting into H(2) and O(2) under visible light, achieving an apparent quantum yield of 6.3% under irradiation by 420.5 nm monochromatic light under optimal conditions, 6 times greater than the yield achieved using a TaON analogue. To the best of our knowledge, this is the highest reported value to date for a nonsacrificial visible-light-driven water splitting system. The high activity of this system is due to the efficient reaction of electron donors (I(-) ions) and acceptors (IO(3)(-) ions) on the Pt/ZrO(2)/TaON and Pt/WO(3) photocatalysts, respectively, which suppresses undesirable reverse reactions involving the redox couple that would otherwise occur on the photocatalysts. Photoluminescence and photoelectrochemical measurements indicated that the high activity of this Z-scheme system results from the moderated n-type semiconducting character of ZrO(2)/TaON, which results in a lower probability of undesirable electron-hole recombination in ZrO(2)/TaON than in TaON.

Molecular Mechanism of Water Oxidation Reaction at Photo-Irradiated TiO2 and Related Metal Oxide Surfaces

Recent studies on the molecular mechanism of water photooxidation (or oxygen photoevolution) reaction on TiO2 and related metal oxides or oxynitrides are reviewed. It is shown that a lot of experimental and theoretical studies give definite support to our recently proposed new mechanism, called “nucleophilic attack of H2O” or “Lewis acid-base” mechanism. The new mechanism has the prominent features that it possesses energetic and kinetics different from the conventional electron-transfer mechanism and can explain water photooxidation reaction on visible-light responsive metal oxides or oxynitrides, contrary to the conventional one. The result indicates that the new mechanism is useful for searching for new efficient visible-light responsive materials for solar water splitting.

Ta3N5 nanotube arrays for visible light water photoelectrolysis

Tantalum nitride (Ta3N5) has a band gap of approximately 2.07 eV, suitable for collecting more than 45% of the incident solar spectrum energy. We describe a simple method for scale fabrication of highly oriented Ta3N5 nanotube array films, by anodization of tantalum foil to achieve vertically oriented tantalum oxide nanotube arrays followed by a 700 degrees C ammonia anneal for sample crystallization and nitridation. The thin walled amorphous nanotube array structure enables transformation from tantalum oxide to Ta3N5 to occur at relatively low temperatures, while high-temperature annealing related structural aggregation that commonly occurs in particle films is avoided. In 1 M KOH solution, under AM 1.5 illumination with 0.5 V dc bias typical sample (nanotube length approximately 240 nm, wall thickness approximately 7 nm) visible light incident photon conversion efficiencies (IPCE) as high as 5.3% were obtained. The enhanced visible light activity in combination with the ordered one-dimensional nanoarchitecture makes Ta3N5 nanotube arrays films a promising candidate for visible light water photoelectrolysis.

Visible-light-Induced photocatalytic oxidation of polycyclic aromatic hydrocarbons over tantalum oxynitride photocatalysts

The photooxidations of five typical polycyclic aromatic hydrocarbons (PAHs) were investigated by using tantalum oxynitride and Pt-tantalum oxynitride as visible light-driven photocatalysts. The electron paramagnetic resonance spin-trap technique and hydrogen peroxide test strip were used to monitor active species formed in these photocatalytic systems. Moreover, the participations of HO*, O2(-*) anions, and holes were further examined by adding their scavengers t-butanol, benzoquinone, and iodine anions, respectively. The reaction intermediates were analyzed by gas chromatography-mass spectrometer (GC-MS). The results show that tantalum oxynitride exhibits good photocatalytic activity for the PAHs photodegradation and the activity is greatly promoted by loading cocatalyst Pt. After 6 h visible light irradiation, phenanthrene, anthracene, benzo[a]anthracene, and acenaphthene can be completely oxidized over Pt-tantalum oxynitride. Under UV light irradiation, the photodegradation rate of PHE over Pt-tantalum oxynitride is 8 times faster than that over titanium dioxide (P25, 80% anatase, 20% rutile). Oxygen plays a crucial role on the photooxidations of PAHs. t-Butanol and benzoquinone almost have no effect on PAHs photodegradations, which indicate that HO* and O2(-*) anions play a negligible role on the photodegradations of PAHs. However, the presence of iodide anions significantly inhibits these degradation reactions, implying the crucial effect of holes on the photocatalytic systems. The PAHs degradations could therefore be attributed to the formation of holes in these systems. Based on the GC-MS analysis, the possible photooxidation pathways of PAHs were also proposed.