Interface Engineering of a CoOx/Ta3N5Photocatalyst for Unprecedented Water Oxidation Performance under Visible-Light-Irradiation

A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface, and Interface

Solar energy is a renewable resource that can supply our energy needs in the long term. A semiconductor photocatalysis that is capable of utilizing solar energy has appealed to considerable interests for recent decades, owing to the ability to aim at environmental problems and produce renewal energy. Much effort has been put into the synthesis of a highly efficient semiconductor photocatalyst to promote its real application potential. Hence, we reviewed the most advanced methods and strategies in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites, and (iii) control of the electron-hole (e^--h⁺) recombination, while these three processes could be influenced by remodeling the crystal lattice, surface, and interface. Additionally, we individually examined their current applications in energy conversion (i.e., hydrogen evolution, CO₂ reduction, nitrogen fixation, and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment). Overall, in this review, we particularly focused on advanced photocatalytic activity with simultaneous wastewater decontamination and energy conversion and further enriched the mechanism by proposing the electron flow and substance conversion. Finally, this review offers the prospects of semiconductor photocatalysts in the following three vital (distinct) aspects: (i) the large-scale preparation of highly efficient photocatalysts, (ii) the development of sustainable photocatalysis systems, and (iii) the optimization of the photocatalytic process for practical application.

Hollow Nanostructures for Photocatalysis: Advantages and Challenges

Photocatalysis for solar-driven reactions promises a bright future in addressing energy and environmental challenges. The performance of photocatalysis is highly dependent on the design of photocatalysts, which can be rationally tailored to achieve efficient light harvesting, promoted charge separation and transport, and accelerated surface reactions. Due to its unique feature, semiconductors with hollow structure offer many advantages in photocatalyst design including improved light scattering and harvesting, reduced distance for charge migration and directed charge separation, and abundant surface reactive sites of the shells. Herein, the relationship between hollow nanostructures and their photocatalytic performance are discussed. The advantages of hollow nanostructures are summarized as: 1) enhancement in the light harvesting through light scattering and slow photon effects; 2) suppression of charge recombination by reducing charge transfer distance and directing separation of charge carriers; and 3) acceleration of the surface reactions by increasing accessible surface areas for separating the redox reactions spatially. Toward the end of the review, some insights into the key challenges and perspectives of hollow structured photocatalysts are also discussed, with a good hope to shed light on further promoting the rapid progress of this dynamic research field.

Multiple Doped Carbon Nitrides with Accelerated Interfacial Charge/Mass Transportation for Boosting Photocatalytic Hydrogen Evolution

The interaction of water molecule with catalysts is crucial to photocatalysis, but the surface property manipulation still remains a great challenge. In this study, we report an in situ multiple heteroelement (sodium, oxygen, and iodide) doping strategy based on a molten salt-assisted route to prepare a green-colored carbon nitride (GCN). The as-prepared GCN yields 25.5 times higher H₂ evolution rate than that of bulk polymeric carbon nitride under visible light. Experimental characterization data demonstrate that the GCN delivers upshift of the conduction band and increased specific surface area and hydrophilicity. As confirmed by time-resolved PL spectra, DMPO spin-trapping EPR analysis, and so on, the excellent activity is dominantly ascribed to the greatly enhanced hydrophilicity and, subsequently, efficient interfacial charge transfer and hydrogen liberation. Moreover, through surface charge modification, the GCN also shows an increased degradation activity of rhodamine B. This work highlights the importance of surface modulation through multiple earth-abundant element incorporation for designing of advanced and practical photocatalysts.

Preparation of Nanostructured Ta3N5 Electrodes by Alkaline Hydrothermal Treatment Followed by NH3 Annealing and Their Improved Water Oxidation Performance

Solar water splitting is a clean and sustainable process for green hydrogen production. It can reduce the fossil fuel consumption. Tantalum nitride (Ta₃N₅) is one of the limited candidates of semiconductors, which absorb a broad range of visible light and are thermodynamically able to split water without external bias potential. In the present work, we introduce a facile method to prepare a nanostructured Ta₃N₅ photoanode in a two-step process: hydrothermal deposition of perovskite-type NaTaO₃ in a hydrofluoric acid-free NaOH aqueous solution followed by heat treatment in NH₃ atmosphere. The resulted bare Ta₃N₅ electrode was subsequently modified with a Ni-doped CoFeO _x (Ni:CoFeO _x ) as a water oxidation catalyst. After the cocatalyst loading, the electrode shows a photocurrent of about 5.3 mA cm^-2 at 1.23 V vs reversible hydrogen electrode. The electrode maintained about 82% of its initial photocurrent after 7 h irradiation. In addition, a continuous oxygen evolution occurred for 3 h at Faraday efficiency of 96%. This performance is superior to that of the single-layer-modified Ta₃N₅ photoanodes reported so far. This remarkable improvement on the photochemical performance could be due to the uniform nanostructured surface morphology of the present Ta₃N₅ photoanode. Other alkaline salt treatments, such as LiOH and KOH, do not give such nanostructured morphology and accordingly exhibit lower performance than the one treated in NaOH.

Heterostructure of 1D Ta3 N5 Nanorod/BaTaO2 N Nanoparticle Fabricated by a One-Step Ammonia Thermal Route for Remarkably Promoted Solar Hydrogen Production

Heterostructures are widely fabricated for promotion of photogenerated charge separation and solar cell/fuel production. (Oxy)nitrides are extremely promising for solar energy conversion, but the fabrication of heterostructures based on nitrogen-containing semiconductors is still challenging. Here, a simple ammonia thermal synthesis of a heterostructure (denoted as Ta₃ N₅ /BTON) composed of 1D Ta₃ N₅ nanorods and BaTaO₂ N (BTON) nanoparticles (0D), which is demonstrated to result in a remarkable increase in photogenerated charge separation and solar hydrogen production from water, is introduced. As analyzed and discussed, the Ta₃ N₅ /BTON heterostructure is type II and tends to create intimate interfaces between the 1D nanorods and 0D nanoparticles. The 1D Ta₃ N₅ nanorods are demonstrated to transfer electrons along the rod orientation direction. Furthermore, the intimate interfaces of the heterostructure are believed to originate from the similar Ta-based octahedron units of Ta₃ N₅ and BTON. All of the above features are expected to integrally endow increased photoinduced charge separation and one order of magnitude higher solar overall water splitting activity with respect to counterpart systems. These results may open a new avenue to fabricate heterostructures on the basis of nitrogen-containing semiconductors that is extremely promising for solar energy conversion.

Superwettability-Based Interfacial Chemical Reactions

Superwetting interfaces arising from the cooperation of surface energy and multiscale micro/nanostructures are extensively studied in biological systems. Fundamental understandings gained from biological interfaces boost the control of wettability under different dimensionalities, such as 2D surfaces, 1D fibers and channels, and 3D architectures, thus permitting manipulation of the transport physics of liquids, gases, and ions, which profoundly impacts chemical reactions and material fabrication. In this context, the progress of new chemistry based on superwetting interfaces is highlighted, beginning with mass transport dynamics, including liquid, gas, and ion transport. In the following sections, the impacts of the superwettability-mediated transport dynamics on chemical reactions and material fabrication is discussed. Superwettability science has greatly enhanced the efficiency of chemical reactions, including photocatalytic, bioelectronic, electrochemical, and organic catalytic reactions, by realizing efficient mass transport. For material fabrication, superwetting interfaces are pivotal in the manipulation of the transport and microfluidic dynamics of liquids on solid surfaces, leading to the spatially regulated growth of low-dimensional single-crystalline arrays and high-quality polymer films. Finally, a perspective on future directions is presented.

One-step synthesis of single crystalline wedge-shaped Ta3N5 nanoflakes with ultrathin top ends

Wedge-shaped Ta₃N₅ nanoflakes with {010} preferentially exposed facets were fabricated with a one-step flux synthetic method.

Double perovskite compounds A2CuWO6 (A = Sr and Ba) with p-type semiconductivity for photocatalytic water oxidation under visible light illumination

Sr₂CuWO₆ and Ba₂CuWO₆ are novel p-type semiconductors that work well for photocatalytic water oxidation under visible light illumination.

Hantzsch ester as hole relay significantly enhanced photocatalytic hydrogen production

Hantzsch ester (DHPE) retards the recombination of electron–hole pairs through extracting holes from g-C₃N₄, thus dramatically improving visible photocatalytic hydrogen production.

Controllable synthesis of graphitic carbon nitride nanomaterials for solar energy conversion and environmental remediation: the road travelled and the way forward

This review discusses advances in the synthesis and design of g-C₃N₄-based nanomaterials and their various photocatalytic and photoredox applications.

Molten salt-assisted a-axis-oriented growth of Ta3N5 nanorod arrays with enhanced charge transport for efficient photoelectrochemical water oxidation

Molten salt-assisted a-axis-oriented growth of Ta₃N₅ nanorod arrays with enhanced charge transport for efficient photoelectrochemical water oxidation.

Surface-Modified Ta3N5 Nanocrystals with Boron for Enhanced Visible-Light-Driven Photoelectrochemical Water Splitting

Photocatalysts for water splitting are the core of renewable energy technologies, such as hydrogen fuel cells. The development of photoelectrode materials with high efficiency and low corrosivity has great challenges. In this study, we report new strategy to improve performance of tantalum nitride (Ta₃N₅) nanocrystals as promising photoanode materials for visible-light-driven photoelectrochemical (PEC) water splitting cells. The surface of Ta₃N₅ nanocrystals was modified with boron whose content was controlled, with up to 30% substitution of Ta. X-ray photoelectron spectroscopy revealed that boron was mainly incorporated into the surface oxide layers of the Ta₃N₅ nanocrystals. The surface modification with boron increases significantly the solar energy conversion efficiency of the water-splitting PEC cells by shifting the onset potential cathodically and increasing the photocurrents. It reduces the interfacial charge-transfer resistance and increases the electrical conductivity, which could cause the higher photocurrents at lower potential. The onset potential shift of the PEC cell with the boron incorporation can be attributed to the negative shift of the flat band potential. We suggest that the boron-modified surface acts as a protection layer for the Ta₃N₅ nanocrystals, by catalyzing effectively the water splitting reaction.

CoO x nanoparticle anchored on sulfonated-graphite as efficient water oxidation catalyst

Ultrasmall CoO_x nanoparticles on sulfonated graphite exhibit highly efficient water oxidation activity and can be used for electrochemical and solar water oxidation.

Development of efficient, robust and earth-abundant water oxidation catalysts (WOCs) is extremely desirable for water splitting by electrolysis or photocatalysis. Herein, we report cobalt oxide nanoparticles anchored on the surface of sulfonated graphite (denoted as “CoO_x@G-Ph-SN”) to exhibit unexpectedly efficient water oxidation activity with a turnover frequency (TOF) of 1.2 s^–1; two or three orders of magnitude higher than most cobalt-based oxide WOCs reported so far. The CoO_x@G-Ph-SN nanocomposite can be easily prepared by a soft hydrothermal route to have an average CoO_x size below 2 nm. Additionally, the loading of CoO_x@G-Ph-SN catalyst on the surface of a BiVO₄ or Fe₂O₃ photoanode can boost remarkably the photoanode currents for robust photocatalytic water oxidation under visible light irradiation. Its excellent activity and photochemical stability for water oxidation suggest that this ultrasmall cobalt-based composite is a promising candidate for solar fuel production.

A Redox Shuttle Accelerates O2 Evolution of Photocatalysts Formed In Situ under Visible Light

A redox shuttle strategy is demonstrated to be a promising approach to accelerate hole removal for efficient O₂ production with mesoporous graphitic carbon nitride, WO₃ , BiVO₄ , NiTi-LDH, and Ag₃ PO₄ water-oxidation catalysts under visible-light irradiation.

Crucial Role of Donor Density in the Performance of Oxynitride Perovskite LaTiO2 N for Photocatalytic Water Oxidation

LaTiO₂ N photocatalysts were prepared by thermal ammonolysis of flux-synthesized La₂ Ti₂ O₇ and La₂ TiO₅ , and were investigated for water oxidation. Though LaTiO₂ N derived from La₂ TiO₅ appears defect-free by UV/Vis/near-IR and electron paramagnetic resonance (EPR) spectroscopy, its performance is much lower than that of conventional La₂ Ti₂ O₇ -derived LaTiO₂ N with defects. It is shown by Mott-Schottky analysis that La₂ TiO₅ -derived LaTiO₂ N has significantly lower donor density; this can result in insufficient built-in electric field for the separation of photogenerated electrons and holes. The lower donor density is also consistent with the smaller difference between the Fermi level and the valence-band maximum, which accounts for a lower oxidative power of the holes. In light of this discovery, the donor density was increased substantially by introducing anion vacancies through annealing in Ar. This resulted in improved performance. The CoO_x -assisted La₂ TiO₅ -derived LaTiO₂ N annealed at 713 °C has a higher quantum efficiency (25 %) at 450 nm than high-performance conventional CoO_x /LaTiO₂ N (21 %).

Enhanced photocatalytic oxygen evolution over Mo-doped Ca2NiWO6 perovskite photocatalyst under visible light irradiation

A series of Mo-doped Ca₂NiWO₆ (Ca₂NiW_1−xMo_xO₆, x = 0–0.05) was synthesized by a solid-state reaction.

Actualizing efficient photocatalytic water oxidation over SrTaO2N by Na modification

Introducing Na into the B site of SrTaO₂N enhances the local Ta–O(N) bond strength and prohibits defect formation and photocatalytic self-decomposition.

Band-gap engineering of porous BiVO4 nanoshuttles by Fe and Mo co-doping for efficient photocatalytic water oxidation

We demonstrate the first synthesis of uniform Fe and Mo co-doped BiVO₄ (Fe/Mo-BVO) porous nanoshuttles (PNSs) through a simple solvothermal method combined with a subsequent impregnation thermal treatment.

Synthesis of novel CoOx decorated CeO2 hollow structures with an enhanced photocatalytic water oxidation performance under visible light irradiation

Cobalt oxide decorated octahedral ceria hollow structures (CoO_x/CeO₂) with various contents of CoO_x nanoparticles were prepared via a simple chemical impregnation method.

Liberation of three dihydrogens from two ethene molecules as mediated by the tantalum nitride anion cluster Ta3N2− at room temperature

The full dehydrogenation of C₂H₄ by gas-phase anions Ta₃N₂⁻ as well as the structure and reactivity of the M–N–C cluster is reported for the first time.

A robust and efficient catalyst of CdxZn1−xSe motivated by CoP for photocatalytic hydrogen evolution under sunlight irradiation

Cd_xZn_1−xSe/CoP composites have a high efficiency of 45.1 mmol h⁻¹ g⁻¹ and a high quantum yield of 11.8% at ∼520 nm.

Effects of the SrTiO3 support on visible-light water oxidation with Co3O4 nanoparticles

SrTiO₃ particle as a support material had a strong influence on the formation of Co₃O₄ nanoparticles, resulting in different photocatalytic activities for visible-light water oxidation (λ > 480 nm).

Graphene in Photocatalysis: A Review

In recent years, heterogeneous photocatalysis has received much research interest because of its powerful potential applications in tackling many important energy and environmental challenges at a global level in an economically sustainable manner. Due to their unique optical, electrical, and physicochemical properties, various 2D graphene nanosheets-supported semiconductor composite photocatalysts have been widely constructed and applied in different photocatalytic fields. In this review, fundamental mechanisms of heterogeneous photocatalysis, including thermodynamic and kinetics requirements, are first systematically summarized. Then, the photocatalysis-related properties of graphene and its derivatives, and design rules and synthesis methods of graphene-based composites are highlighted. Importantly, different design strategies, including doping and sensitization of semiconductors by graphene, improving electrical conductivity of graphene, increasing eloectrocatalytic active sites on graphene, strengthening interface coupling between semiconductors and graphene, fabricating micro/nano architectures, constructing multi-junction nanocomposites, enhancing photostability of semiconductors, and utilizing the synergistic effect of various modification strategies, are thoroughly summarized. The important applications including photocatalytic pollutant degradation, H₂ production, and CO₂ reduction are also addressed. Through reviewing the significant advances on this topic, it may provide new opportunities for designing highly efficient 2D graphene-based photocatalysts for various applications in photocatalysis and other fields, such as solar cells, thermal catalysis, separation, and purification.