Role of Oxygen Defects on the Photocatalytic Properties of Mg-Doped Mesoporous Ta3 N5

Decoupling light absorption and carrier transport via heterogeneous doping in Ta3N5 thin film photoanode

The trade-off between light absorption and carrier transport in semiconductor thin film photoelectrodes is a major limiting factor of their solar-to-hydrogen efficiency for photoelectrochemical water splitting. Herein, we develop a heterogeneous doping strategy that combines surface doping with bulk gradient doping to decouple light absorption and carrier transport in a thin film photoelectrode. Taking La and Mg doped Ta₃N₅ thin film photoanode as an example, enhanced light absorption is achieved by surface La doping through alleviating anisotropic optical absorption, while efficient carrier transport in the bulk is maintained by the gradient band structure induced by gradient Mg doping. Moreover, the homojunction formed between the La-doped layer and the gradient Mg-doped layer further promotes charge separation. As a result, the heterogeneously doped photoanode yields a half-cell solar-to-hydrogen conversion efficiency of 4.07%, which establishes Ta₃N₅ as a leading performer among visible-light-responsive photoanodes. The heterogeneous doping strategy could be extended to other semiconductor thin film light absorbers to break performance trade-offs by decoupling light absorption and carrier transport.

Indirect bandgap, optoelectronic properties, and photoelectrochemical characteristics of high-purity Ta3N5 photoelectrodes

The (opto)electronic properties of Ta3N5 photoelectrodes are often dominated by defects, such as oxygen impurities, nitrogen vacancies, and low-valent Ta cations, impeding fundamental studies of its electronic structure, chemical stability, and photocarrier transport. Here, we explore the role of ammonia annealing following direct reactive magnetron sputtering of tantalum nitride thin films, achieving near-ideal stoichiometry, with significantly reduced native defect and oxygen impurity concentrations. By analyzing structural, optical, and photoelectrochemical properties as a function of ammonia annealing temperature, we provide new insights into the basic semiconductor properties of Ta3N5, as well as the role of defects on its optoelectronic characteristics. Both the crystallinity and material quality improve up to 940 °C, due to elimination of oxygen impurities. Even higher annealing temperatures cause material decomposition and introduce additional disorder within the Ta3N5 lattice, leading to reduced photoelectrochemical performance. Overall, the high material quality enables us to unambiguously identify the nature of the Ta3N5 bandgap as indirect, thereby resolving a long-standing controversy regarding the most fundamental characteristic of this material as a semiconductor. The compact morphology, low defect content, and high optoelectronic quality of these films provide a basis for further optimization of photoanodes and may open up further application opportunities beyond photoelectrochemical energy conversion.

Defect Engineering of Photocatalysts towards Elevated CO2 Reduction Performance

Photocatalytic CO₂ reduction provides a promising solution to address the crises of massive CO₂ emissions and fossil energy shortages. As one of the most effective strategies to promote CO₂ photoconversion, defect engineering shows great potential in modulating the electronic structure and light absorption properties of photocatalysts while increasing surface active sites for CO₂ activation and conversion. This Review summarizes the recent progress in defect engineering of photocatalysts to promote CO₂ reduction performances from the following four aspects: 1) Approaches to defect (mainly vacancy and dopant) generation in photocatalysts; 2) defect structure characterization techniques; 3) physical and chemical properties of defect-engineered photocatalysts; 4) CO₂ reduction performance enhancements in activity, selectivity, and stability of photocatalysts by defect engineering. This Review is expected to present readers with a comprehensive view of progress in the field of photocatalytic CO₂ reduction through defect engineering for elevated CO₂ -to-fuels conversion efficiency.

Nanostructure Engineering and Modulation of (Oxy)Nitrides for Application in Visible-Light-Driven Water Splitting

(Oxy)nitride-based nanophotocatalysts have been extensively investigated for solar-to-chemical conversion, and not only allow wide spectral utilization to achieve high theoretical energy conversion efficiency but also exhibit suitable conduction and valence band positions for robust reduction and oxidation of water. During the past decades, a few reviews on the research progress in designing and synthesizing new visible-light-responsive semiconductors for various applications in solar-to-chemical conversion have been published. However, those on the effects of their bulk and composite (surface/interface) nanostructures on basic processes as well as solar water splitting performances to produce hydrogen are still limited. In this review, a brief introduction on the relationship between the nanostructure photocatalytic properties is included. Three main processes of solar water splitting are involved, allowing the elucidation of the correlation with the nanostructural properties of the photocatalyst such as surface/interface, size, morphology, and bulk structure. Subsequently, the development of methodologies and strategies for modulating the bulk and composite structures to improve the efficiencies of the basic processes, particularly charge separation, is summarized in detail. Finally, the prospects of (oxy)nitride-based photocatalysts such as controlled synthesis, modulation of 1D/2D morphology, exposed facet regulation, heterostructure formation, theoretical simulation, and time- and space-resolved spectroscopy are discussed.

Simultaneously Tuning the Defects and Surface Properties of Ta3N5 Nanoparticles by Mg-Zr Codoping for Significantly Accelerated Photocatalytic H2 Evolution

The simultaneous control of the defect species and surface properties of semiconducting materials is a crucial aspect of improving photocatalytic performance, yet it remains challenging. Here, we synthesized Mg-Zr-codoped single-crystalline Ta₃N₅ (Ta₃N₅:Mg+Zr) nanoparticles by a brief NH₃ nitridation process, exhibiting photocatalytic water reduction activity 45 times greater than that of pristine Ta₃N₅ under visible light. A coherent picture of the relations between the defect species (comprising reduced Ta, nitrogen vacancies and oxygen impurities), surface properties (associated with dispersion of the Pt cocatalyst), charge carrier dynamics, and photocatalytic activities was drawn. The tuning of defects and simultaneous optimization of surface properties resulting from the codoping evidently resulted in the generation of high concentrations of long-lived electrons in this material as well as the efficient migration of these electrons to evenly distributed surface Pt sites. These effects greatly enhanced the photocatalytic activity. This work highlights the importance and feasibility of improving multiple properties of a catalytic material via a one-step strategy.

Shallow Oxygen Substitution Defect to Deeper Defect Transformation Mechanism in Ta3N5 under Light Irradiation

Defects are ubiquitous in semiconductors and critical to photo(electro)chemical performance, but the change of defect properties under light irradiation remains poorly understood. Herein, we studied defect change properties of Ta₃N₅ with transient absorption (TA) spectroscopy. A broad transient absorption (>650 nm) was observed and attributed to trapped electrons in oxygen impurities (substitution oxygen at nitrogen sites, O_N), and two bleach signals at 510 and 580 nm were obtained and ascribed to free holes of Ta₃N₅. The charge recombination between the trapped electrons and the free holes is sensitively related to O_N defects. The trap-detrapping recombination is retarded by increase of excitation intensity, which is contrary to the normal dependence of charge dynamics on excitation intensity. This abnormal dependence indicates that shallow O_N^• (singly positive charge states) defects of Ta₃N₅ transform to deeper O_N^× (neutral charge states) defects under strong light irradiation. The defect transformation results in long-lived free holes in Ta₃N₅ for photo(electro)catalysis.

Perovskite Oxynitride Solid Solutions of LaTaON2-CaTaO2N with Greatly Enhanced Photogenerated Charge Separation for Solar-Driven Overall Water Splitting

The search for solar-driven photocatalysts for overall water splitting has been actively pursued. Although metal oxynitrides with metal d0/d10-closed shell configuration are very promising candidates in terms of their visible light absorption, they usually suffer from serious photo-generated charge recombination and thus, little photoactivity. Here, by forming their solid solutions of LaTaON2 and CaTaO2N, which are traditionally considered to be inorganic yellow-red pigments but have poor photocatalytic activity, a class of promising solar-driven photocatalysts La1- x Ca x TaO1+yN2- y (0 ≤ x, y ≤ 1) are explored. In particular, the optimal photocatalyst with x = 0.9 has the ability of realizing overall water splitting with stoichiometric H2/O2 ratio under the illumination of both AM1.5 simulated solar light and visible light. The modulated key parameters including band structure, Ta bonding environment, defects concentration, and band edge alignments revealed in La0.1Ca0.9TaO1+ y N2- y have substantially promoted the separation of photogenerated charge carriers with sufficient energetics for water oxidation and reduction reactions. The results obtained in this study provide an important candidate for designing efficient solar-driven photocatalysts for overall water splitting.

Visible-light-driven photocatalytic water oxidation over LaNbON2–LaMg2/3Nb1/3O3 solid solutions

Solid solutions (LaNbON₂)_1−x(LaMg_2/3Nb_1/3O₃)_x (0.0 ≤ x ≤ 1.0) show promising photocatalytic activity for water oxidation into oxygen under visible light illumination (λ ≥ 420 nm).

Prismatic Ta3N5-composed spheres produced by self-sacrificial template-like conversion of Ta particles via Na2CO3 flux

Ta₃N₅ crystals were grown from Na₂CO₃ flux using spherical Ta powders.

Gold nanocrystal anchored In2O3 hollow nanospheres for N2 photofixation to ammonia

Hollow In₂O₃ nanospheres anchored with Au nanocrystals have been fabricated and are active for N₂ photofixation into ammonia. Hollow microstructures and Au nanocrystals improve light absorption and N₂ adsorption and boost charge generation in In₂O₃.

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.

Visible-Light-Responsive Photoanodes for Highly Active, Stable Water Oxidation

Solar energy is a natural and effectively permanent resource and so the conversion of solar radiation into chemical or electrical energy is an attractive, although challenging, prospect. Photo-electrochemical (PEC) water splitting is a key aspect of producing hydrogen from solar power. However, practical water oxidation over photoanodes (in combination with water reduction at a photocathode) in PEC cells is currently difficult to achieve because of the large overpotentials in the reaction kinetics and the inefficient photoactivity of the semiconductors. The development of semiconductors that allow high solar-to-hydrogen conversion efficiencies and the utilization of these materials in photoanodes will be a necessary aspect of achieving efficient, stable water oxidation. This Review discusses advances in water oxidation activity over photoanodes of n-type visible-light-responsive (oxy)nitrides and oxides.

Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified by Nitrogen Doping for Efficient Photocatalytic Water Splitting

Ultrathin nitrogen-doped perovskite nanosheets LaTa₂O_6.77N_0.15^- have been fabricated by exfoliating Dion-Jacobson-type layered perovskite RbLaTa₂O_6.77N_0.15. These nanosheets demonstrate superior photocatalytic activities for water splitting into hydrogen and oxygen and remain active with photon wavelengths as far as 600 nm. Their apparent quantum efficiency under visible-light illumination (λ ≥ 420 nm) approaches 1.29% and 3.27% for photocatalytic hydrogen and oxygen production, being almost 4-fold and 8-fold higher than bulk RbLaTa₂O_6.77N_0.15. Their outstanding performance likely stems from their tiny thickness (single perovskite slab) that essentially removes bulk charge diffusion steps and extends the lifetime of photogenerated charges. Theoretical calculations reveal a peculiar 2D charge transportation phenomenon in RbLaTa₂O_6.77N_0.15; thus, exfoliating RbLaTa₂O_6.77N_0.15 into LaTa₂O_6.77N_0.15^- nanosheets has limited impact on charge transportation properties but significantly enhances the surface areas which contributes to more reaction sites.

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.

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.

Zr-Doped Mesoporous Ta3N5 Microspheres for Efficient Photocatalytic Water Oxidation

Tantalum nitride (Ta₃N₅) has been considered as a promising candidate for photocatalytic water splitting because of its strong visible-light absorbance as far as 600 nm. However, its catalytic activity is often hampered by various intrinsic/extrinsic defects. Here, we prepared a series of Zr-doped mesoporous tantalum nitride (Ta₃N₅) via a template-free method and carried out a detailed investigation of the role of Zr doping upon the photocatalytic performance. Various physicochemical properties including crystal structure, optical absorption, and so on were systematically explored. Our results show that doping Zr into Ta₃N₅ induces an enhancement of oxygen content and a suppression of absorption band around 720 nm, indicating an increase of O_N^• defects and a decrease of V_N^••• defects in the structure. Introduction of Zr significantly boosts the photocatalytic oxygen production of Ta₃N₅. The optimized photocatalytic oxygen production rate approaches 105 μmol h^-1 under visible light illumination (λ ≥ 420 nm), corresponding to an apparent quantum efficiency as high as 3.2%. Photoelectrochemical analysis and DFT calculation reveal that the superior photocatalytic activity of Zr-doped Ta₃N₅ originates from a high level of O_N^• defects' concentration, which contributes to a high electron mobility, and a low level of V_N^••• defects' concentration, which often act as charge recombination centers.

Homologous Compounds ZnnIn2O3+n (n = 4, 5, and 7) Containing Laminated Functional Groups as Efficient Photocatalysts for Hydrogen Production

Strong visible light absorption and high charge mobility are desirable properties for an efficient photocatalyst, yet they are hard to be realized simultaneously in a single semiconductor compound. In this work, we demonstrate that these properties coexist in homologous compounds Zn_nIn₂O_3+n (n = 4, 5, and 7) with a peculiar layered structure that combines optical active segment and electrical conductive segment together. Their enhanced visible light absorption originates from tetrahedrally or trigonal-bipyramidally coordinated In atoms in Zn(In)O₄₍₅₎ layers which enable p-d hybridization between In 4d and O 2p orbitals so that valence band minimum (VBM) is uplifted with a reduced band gap. Theoretical calculations reveal their anisotropic features in charge transport and functionality of different constituent segments, i.e., Zn(In)O₄₍₅₎ layers and InO₆ layers as being for charge generation and charge collection, respectively. Efficient photocatalytic hydrogen evolution was observed in these compounds under full range (λ ≥ 250 nm) and visible light irradiation (λ ≥ 420 nm). High apparent quantum efficiency ∼2.79% was achieved for Zn₄In₂O₇ under full range irradiation, which is almost 5-fold higher than their parent oxides ZnO and In₂O₃. Such superior photocatalytic activities of these homologous compounds can be understood as layer-by-layer packing of charge generation/collection functional groups that ensures efficient photocatalytic reactions.