A Stable, Narrow-Gap Oxyfluoride Photocatalyst for Visible-Light Hydrogen Evolution and Carbon Dioxide Reduction

Rational Design of Conjugated Polymers for Photocatalytic CO2 Reduction: Towards Localized CO Production and Macrophage Polarization

Light presents substantial potential in disease treatment, where the development of efficient photocatalysts could enhance the utilization of photocatalytic systems in biomedicine. Here, we devised a novel approach to designing and synthesizing photocatalysts of conjugated polymers for photocatalytic CO2 reduction, relying on a multiple linear regression model built with theoretically calculated descriptors. We established a logarithmic relationship between molecular structure and CO yield and identified the poly(fluorene-co-thiophene) deviant (PFT) as the optimal one. PFT excited a CO regeneration ratio of 231 nmol h-1 in acetonitrile and 46 nmol h-1 in an aqueous solution with a reaction selectivity of 88%. Further advancements were made through the development of liposomes encapsulating PFT for targeted macrophage delivery. By distributing PFT on the liposome membranes, our constructed photocatalytic system efficiently generated CO in situ from surrounding CO2. This localized CO production served as an endogenous signaling molecule, promoting the desirable polarization of macrophages from the M1 to M2 phenotype. Consequently, the M2 cells reduced the secretion of pro-inflammatory cytokines (TNF-α, IL-6, and IL-1β). We also demonstrated the efficacy of our system in treating lipopolysaccharide-induced inflammation of cardiomyocytes under white light irradiation. Moreover, our research provides a comprehensive understanding of the intricate processes involved in CO2 reduction by a combination of theoretical calculations and experimental techniques including transient absorption, femtosecond ultrafast spectroscopy, and in situ infrared spectroscopy. These findings pave the way for further advancements of conjugated polymers and photocatalytic systems in biomedical investigation.

Internal strain-driven bond manipulation and band engineering in Bi2-x Sb x YO4Cl photocatalysts with triple fluorite layers

In extended solid-state materials, the manipulation of chemical bonds through redox reactions often leads to the emergence of interesting properties, such as unconventional superconductivity, which can be achieved by adjusting the Fermi level through, e.g., intercalation and pressure. Here, we demonstrate that the internal 'biaxial strain' in tri-layered fluorite oxychloride photocatalysts can regulate bond formation and cleavage without redox processes. We achieve this by synthesizing the isovalent solid solution Bi2-x Sb x YO4Cl, which undergoes a structural phase transition from the ideal Bi2YO4Cl structure to the Sb2YO4Cl structure with (Bi,Sb)4O8 rings. Initially, substitution of smaller Sb induces expected lattice contraction, but further substitution beyond x > 0.6 triggers an unusual lattice expansion before the phase transition at x = 1.5. Detailed analysis reveals structural instability at high x values, characterized by Sb-O underbonding, which is attributed to tensile strain exerted from the inner Y sublayer to the outer (Bi,Sb)O sublayer within the triple fluorite block - a concept well-recognized in thin film studies. This concept also explains the formation of zigzag Bi-O chains in Bi2MO4Cl (M = Bi, La). The Sb substitution in Bi2-x Sb x YO4Cl elevates the valence band maximum, resulting in a minimized bandgap of 2.1 eV around x = 0.6, which is significantly smaller than those typically observed in oxychlorides, allowing the absorption of a wider range of light wavelengths. Given the predominance of materials with a double fluorite layer in previous studies, our findings highlight the potential of compounds endowed with triple or thicker fluorite layers as a novel platform for band engineering that utilizes biaxial strain from the inner layer(s) to finely control their electronic structures.

Molecular Terpyridine-Lanthanide Complexes Modified Carbon Nitride for Enhanced Photocatalytic CO2 Reduction

Molecule/semiconductor hybrid catalysts, which combine molecular metal complexes with semiconductors, have shown outstanding performances in photocatalytic CO2 reduction. In this work, we report two hybrid catalysts for the selective photoreduction of CO2 to CO. One is composed of carbon nitride and a terpyridine-Lu complex (denoted as LutpyCN), and the other is composed of carbon nitride and a terpyridine-Ce complex (denoted as CetpyCN). Compared with pristine carbon nitride, the hybrid catalysts LutpyCN and CetpyCN display a noteworthy increase in CO generation, boosting the yield by approximately 176 times and 106 times, respectively. Mechanistic studies demonstrate that such significant enhancement in photocatalysis is primarily due to more efficient separation of photogenerated carriers for hybrid catalysts after modifying CN with molecular terpyridine-lanthanide species.

Modification of Visible-Light-Responsive Pb2Ti2O5.4F1.2 with Metal Oxide Cocatalysts to Improve Photocatalytic O2 Evolution toward Z-Scheme Overall Water Splitting

The development of a highly active photocatalyst for visible-light water splitting requires a high-quality semiconductor material and a cocatalyst, which promote both the migration of photogenerated charge carriers and surface redox reactions. In this work, a cocatalyst was loaded onto an oxyfluoride photocatalyst, Pb2Ti2O5.4F1.2, to improve the water oxidation activity. Among the metal oxides examined as cocatalysts, RuO2 was found to be the most suitable, and the O2 evolution activity depended on the preparation conditions for Ru/Pb2Ti2O5.4F1.2. The highest activity was obtained with RuCl3-impregnated Pb2Ti2O5.4F1.2 heated under a flow of H2 at 523 K. The H2-treated Ru/Pb2Ti2O5.4F1.2 showed an O2 evolution rate an order of magnitude higher than those for the analogues without the H2 treatment (e. g., RuO2/Pb2Ti2O5.4F1.2). Physicochemical analyses by X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and time-resolved microwave conductivity measurements indicated that the optimized photocatalyst contained partially reduced RuO2 species with a particle size of ~5 nm. These partially reduced species effectively trapped the photogenerated charge carriers and promoted the oxidation of water into O2. The optimized Ru/Pb2Ti2O5.4F1.2 could function as an O2-evolving photocatalyst in Z-scheme overall water splitting, in combination with an Ru-loaded, Rh-doped SrTiO3 photocatalyst.

Interfacial Electronic States of GeC/g-C3N4 van der Waal Heterostructure with Promising Photocatalytic Activity via Hydrogenation

The bandgap of most known two-dimensional materials can be tuned by hydrogenation, although certain 2D materials lack a sufficient wide bandgap. Currently, it would be perfect to design non-toxic, low-cost, and high-performance photocatalysts for photocatalytic water splitting via hydrogenation. We systematically examine the impact of hydrogenation on the optical and electronic characteristics of GeC/g-C3N4 vdW heterostructures (vdWHs) with four different stacking patterns using first-principles calculations. The phonon spectra, interlayer distance, binding energies and ab initio molecular dynamics calculations show the kinetic, mechanical, and thermal stability of GeC/g-C3N4 vdWH after hydrogenation at 300, 500 and 800 K and possesses anisotropic Poisson's ratio, Young's and bulk modulus, suggesting that it's a promising candidate for experimental fabrication. According to an investigation of its electronic properties, GeC/g-C3N4 vdWH has a bandgap of 1.28 eV, but hydrogenation dramatically increases it to 2.47 eV. As a result of interface-induced electronic doping, the electronic states in g-C3N4 might be significantly adjusted by coming into contact with hydrogenated GeC sheets. The vdWH exhibits a type-II semiconductor, which can enhance the spatial separation of electron-hole pairs and has a strong red-shift of absorption coefficient than those of the constituent monolayers. The high potential drop caused by the significant valence and conduction band offsets effectively separated the charge carriers. The absorption coefficient of GeCH2/g-C3N4 vdWH is highly influenced by a biaxial compressive strain more than the biaxial tensile strain. Our theoretical research implies that the hydrogenated GeCH2/g-C3N4 vdWH possesses tunable optical and electronic behaviour for use as a hole-transport material in solar energy harvesting, nanoelectronic and optoelectronic devices.

Tetrazine-Linked Covalent Organic Frameworks With Acid Sensing and Photocatalytic Activity

The first synthesis and comprehensive characterization of two vinyl tetrazine-linked covalent organic frameworks (COF), TA-COF-1 and TA-COF-2, are reported. These materials exhibit high crystallinity and high specific surface areas of 1323 and 1114 m2 g-1. The COFs demonstrate favorable band positions and narrow band gaps suitable for light-driven applications. These advantages enable TA-COFs to act as reusable metal-free photocatalysts in the arylboronic acids oxidation and light-induced coupling of benzylamines. In addition, these TA-COFs show acid sensing capabilities, exhibiting visible and reversible color changes upon exposure to HCl solution, HCl vapor, and NH3 vapor. Further, the TA-COFs outperform a wide range of previously reported COF photocathodes. The tetrazine linker in the COF skeleton represents a significant advancement in the field of COF synthesis, enhancing the separation efficiency of charge carriers during the photoreaction and contributing to their photocathodic properties. TA-COFs can also degrade 5-nitro-1,2,4-triazol-3-one (NTO), an insensitive explosive present in industrial wastewater, in 20 min in a sunlight-driven photocatalytic process; thus, revealing dual functionality of the protonated TA-COFs as both photodegradation and Brønsted acid catalysts. This pioneering work opens new avenues for harnessing the potential of the tetrazine linker in COF-based materials, facilitating advances in catalysis, sensing, and other related fields.

Facile synthesis of Zn0.5Cd0.5S nanosheets with tunable S vacancies for highly efficient photocatalytic hydrogen evolution

In order to effectively improve the separation efficiency of photogenerated charge carriers and thus the photocatalytic activity, in this work, porous Zn0.5Cd0.5S nanosheets with a controlled amount of S vacancies were prepared by a multistep chemical transformation strategy using the inorganic-organic hybrid ZnS-ethylenediamine (denoted as ZnS(en)0.5) as a hard template. The amount of S vacancies and the morphology of the Zn0.5Cd0.5S nanostructures were tailored by adjusting the hydrolysis time. Furthermore, we report the observation of S vacancies in porous Zn0.5Cd0.5S nanosheets at the atomic level using spherical aberration-corrected (Cs-aberrated) transmission electron microscopy (Cs-corrected-TEM). The results revealed that Zn0.5Cd0.5S nanosheets with S vacancies absorb more visible light and generate more electron-hole carriers due to their porous nanosheet structure. At the same time, sulfur vacancies are introduced into the Zn0.5Cd0.5S nanosheets to capture the electrons generated by the light and further extend the lifetime of the carriers. As expected, the photocatalytic activity of Zn0.5Cd0.5S nanosheets prepared by 4 h hydrolysis is 20.5 times higher than that of Zn0.5Cd0.5S(en)x intermediates. Moreover, Zn0.5Cd0.5S-4h showed excellent cycling stability. This work provides a new strategy for the optimization of Zn0.5Cd0.5S photocatalysts to improve photocatalytic hydrogen evolution.

Band Gap Narrowing by Suppressed Lone-Pair Activity of Bi3

Post-transition metal cations with a lone pair (ns2np0) electronic configuration such as Pb2+ and Bi3+ are important components of materials for solar-to-energy conversion. As in molecules like NH3, the lone pair is often stereochemically active in crystals, associated with distorted coordination environments of these cations. In the present study, we demonstrate that suppressed lone pair stereochemical activity can be used as a tool to enhance visible light absorption. Based on an orbital interaction model, we predict that a centrosymmetric environment of the cations limits the orbital interactions with anions, deactivates the lone pair, and narrows the band gap. A high-symmetry Bi3+ site is realized by isovalent substitutions with Y3+ by considering its similar ionic radius and absence of a lone pair. The quaternary photocatalyst Bi2YO4X is singled out as a candidate for Bi substitution from a survey of the coordination environments in Y-O compounds. The introduction of Bi3+ to the undistorted Y3+ site in Bi2YO4X results in a narrowed band gap, as predicted theoretically and confirmed experimentally. The orbital interaction controlled by site symmetry engineering offers a pathway for the further development of post-transition metal compounds for optoelectronic applications.

Topochemical Fluorination of LaBaInO4 to LaBaInO3F2, Their Optical Characterization, and Photocatalytic Activities for Hydrogen Evolution

We report on a nonoxidative topochemical route for the synthesis of a novel indate-based oxyfluoride, LaBaInO3F2, using a low-temperature reaction of Ruddlesden-Popper-type LaBaInO4 with polyvinylidene difluoride as a fluorinating agent. The reaction involves the replacement of oxide ions with fluoride ions as well as the insertion of fluoride ions into the interstitial sites. From the characterization via powder X-ray diffraction (PXRD) and Rietveld analysis as well as automated electron diffraction tomography (ADT), it is deduced that the fluorination results in a symmetry lowering from I4/mmm (139) to monoclinic C2/c (15) with an expansion perpendicular to the perovskite layers and a strong tilting of the octahedra in the ab plane. Disorder of the anions on the apical and interstitial sites seems to be favored. The most stable configuration for the anion ordering is estimated based on an evaluation of bond distances from the ADT measurements via bond valence sums (BVSs). The observed disordering of the anions in the oxyfluoride results in changes in the optical properties and thus shows that the topochemical anion modification can present a viable route to alter the optical properties. Partial densities of states (PDOSs) obtained from ab initio density functional theory (DFT) calculations reveal a bandgap modification upon fluoride-ion introduction which originates from the presence of the oxide anions on the interstitial sites. The photocatalytic performance of the oxide and oxyfluoride shows that both materials are photocatalytically active for hydrogen (H2) evolution.

Synthesis of Fluoride-Substituted Layered Perovskites ZnMoO4 with an Enhanced Photocatalytic Activity

Oxyfluorides possess considerable attention for their multiple excellent properties, but the conventional high-temperature solid-state syntheses have seen bottlenecks in the synthesis of new compounds. Herein, we report a novel layered oxyfluoride ZnMoO4:F, which is prepared by a facile hydrothermal method using ZnF2 as the fluoride source. The fluoride anions are successfully introduced into the oxygen sublattice, which is confirmed by a combined analysis using XRD, STEM, and TGA techniques. The as-synthesized ZnMoO4:F has an absorption edge at around 550 nm, indicating a red shift of Eg to the visible region compared to the oxide counterpart. The layered oxyfluoride exhibits an enhanced photocatalytic active for hydrogen evolution under simulated sunlight (λ > 350 nm), and the activity of ZnMoO4:F (651.9 μmol g-1) was 2 times higher than that of ZnMoO4 (309.7 μmol g-1). Further electrochemical analysis has shown that the conduction band position plays a critical role in the high performances of ZnMoO4:F. This work sheds new light on the future design and synthesis of novel fluoride-doped materials for photocatalysis applications.

Orbital Engineering in Sillén-Aurivillius Phase Bismuth Oxyiodide Photocatalysts through Interlayer Interactions

Multicomponent inorganic compounds containing post-transition-metal cations such as Sn, Pb, and Bi are a promising class of photocatalysts, but their structure-property relationships remain difficult to decipher. Here, we report three novel bismuth-based layered oxyiodides, the Sillén-Aurivillius phase Bi₄NbO₈I, Bi₅BaTi₃O₁₄I, and Bi₆NbWO₁₄I. We show that the interlayer Bi-Bi interaction is a key to controlling the electronic structure. The replacement of the halide layer from Cl to I negatively shifts not only the valence band but also the conduction band, thus providing lower electron affinity without sacrificing photoabsorption. The suppressed interlayer chemical interaction between the 6p orbitals of the Bi lone-pair cations reduces the conduction bandwidth. These oxyiodides have narrower band gaps and show much higher water oxidation activities under visible light than their chloride counterparts. The design strategy has not only provided three novel Bi-based photocatalysts for water splitting but also offers a pathway to control the optoelectronic properties of a wider class of lone-pair (ns²np⁰) semiconductors.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO2 reduction reaction (CRR) is an effective strategy because it affords the conversion of CO2 into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

Layered β-ZrNBr Nitro-Halide as Multifunctional Photocatalyst for Water Splitting and CO2 Reduction

Developing mixed-anion semiconductors for solar fuel production has inspired extensive interest, but the nitrohalide-based photocatalyst is still in shortage. Here we report a layered nitro-halide β-ZrNBr with a narrow band gap of ca. 2.3 eV and low defect density to exhibit multifunctionalities for photocatalytic water reduction, water oxidation and CO₂ reduction under visible-light irradiation. As confirmed by the results of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations, the formation of anion vacancies in the nitro-halide photocatalyst was inhibited due to its relatively high formation energy. Furthermore, performance of β-ZrNBr can be effectively promoted by a simple exfoliation into nanosheets to shorten the carrier transfer distance as well as to promote charge separation. Our work extends the territory of functional photocatalysts into the nitro-halide, which opens a new avenue for fabricating efficient artificial photosynthesis.

Two-dimensional SiC/AlN based type-II van der Waals heterobilayer as a promising photocatalyst for overall water disassociation

Two-dimensional (2D) van der Waals (vdW) heterostructures made by vertical assembling of two different layers have drawn immense attention in the photocatalytic water disassociation process. Herein, we suggest a novel 2D/2D vdW heterobilayer consisting of silicon carbide (SiC) and aluminum nitride (AlN) as an exciting photocatalyst for solar-to-hydrogen conversion reactions using first-principles calculations. Notably, the heterostructure presents an inherent type-II band orientation wherein the photogenic holes and electrons are spatially separated in the SiC layer and the AlN layer, respectively. Our results indicate that the SiC/AlN heterostructure occupies a suitable band-gap of 2.97 eV which straddles the kinetic overpotentials of the hydrogen production reaction and oxygen production reaction. Importantly, the built-in electric field at the interface created by substantial charge transfer prohibits carrier recombination and further improves the photocatalytic performance. The heterostructure has an ample absorption profile ranging from the ultraviolet to the near-infrared regime, while the intensity of the absorption reaches up to 2.16 × 10⁵ cm^-1. In addition, external strain modulates the optical absorption of the heterostructure effectively. This work provides an intriguing insight into the important features of the SiC/AlN heterostructure and renders useful information on the experimental design of a novel vdW heterostructure for solar energy-driven water disassociation with superior efficiency.

Anionic ordering in Pb2Ti4O9F2 revisited by nuclear magnetic resonance and density functional theory

A combination of ¹⁹F magic angle spinning (MAS) nuclear magnetic resonance (NMR) and density functional theory (DFT) were used to study the ordering of F atoms in Pb₂Ti₄O₉F₂. This analysis revealed that F atoms predominantly occupy two of the six available inequivalent sites in a ratio of 73 : 27. DFT-based calculations explained the preference of F occupation on these sites and quantitatively reproduced the experimental occupation ratio, independent of the choice of functional. We concluded that the Pb atom's 6s² lone pair may play a role (∼0.1 eV per f.u.) in determining the majority and minority F occupation sites with partial density of states and crystal orbital Hamiltonian population analyses applied to the DFT wave functions.

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.

Importance of steric bulkiness of iridium photocatalysts with PNNP tetradentate ligands for CO2 reduction

A series of Ir complexes has been developed as multifunctional photocatalysts for CO₂ reduction to give HCO₂H selectively. The catalytic activities and photophysical properties vary widely across the series, and the bulky group insertion resulted in the formation of HCO₂H and CO with the catalyst turnover number of >10 400.

Manipulation of charge carrier flow in Bi4NbO8Cl nanoplate photocatalyst with metal loading

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi4NbO8Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds.

An unusual O2-/F- distribution in the new pyrochlore oxyfluorides: Na2B2O5F2 (B = Nb, Ta)

Two new oxyfluorides with a pyrochlore-type structure, Na₂Nb₂O₅F₂ and Na₂Ta₂O₅F₂, have been obtained, for which the XRD crystallographic study coupled with ¹⁹F solid state NMR reveals an unusual O/F distribution. Both materials are n-type semiconductors exhibiting photoconductive properties.

Structural Dimension Modulation in the New Oxysulfide System of Ae2Sb2O2S3 (Ae = Ca, Ba)

Inspired by the abundant structural diversity and potential applications of Sb-based oxysulfides, two new compounds with the same stoichiometry, Ae2Sb2O2S3 (Ae = Ca, Ba), were successfully synthesized via solid state...

Pyrochlore-structural Nd2Ta2O5N2 Photocatalyst with Absorption Edge of over 600 nm for Z-scheme Overall Water Splitting

A pyrochlore-structural oxynitride Nd2Ta2O5N2 with a visible light absorption edge of ca. 620 nm was explored for photocatalytic water splitting. Dual functions of Nd2Ta2O5N2 were confirmed by proton reduction and...

Effect of Micropores of a Porous Coordination Polymer on the Product Selectivity in RuII Complex-catalyzed CO2 Reduction

To develop an efficient CO2 reduction catalyst, hybridizing a molecular catalyst and a porous coordination polymer (PCP) is a promising strategy because it can combine both advantages of the precise reactivity control of the former and the CO2 adsorption property of the latter. Although several PCP hybrid catalysts have been reported to date, the CO2 sorption behavior and the CO2 reduction reactivity have been investigated separately, and the CO2 enrichment during the catalysis is still unclear. We report CO2 photoreduction under different temperatures and pressures using a PCP-RuII complex hybrid catalyst. The product selectivity (CO or HCOOH) varied depending on the reaction conditions. The altered selectivity could be interpreted in terms of the CO2 capture in the micropores of a PCP.

Bi4AO6Cl2 (A = Ba, Sr, Ca) with Double and Triple Fluorite Layers for Visible-Light Water Splitting

Layered oxyhalides containing double or triple fluorite layers are promising visible-light-responsive water-splitting photocatalysts with unique band structures. Herein, we report on the synthesis, structure, and photocatalytic property of Bi₄BaO₆Cl₂ (I4/mmm) with alternating double (Bi₂O₂) and triple (Bi₂BaO₄) fluorite layers, which was extracted from the crystallographic database on the basis of Madelung potential calculations. Rietveld refinements from powder X-ray and neutron diffraction data revealed the presence of cationic disorder between Bi₂O₂ and Bi₂BaO₄ layers, leading to electrostatic stabilization. DFT calculations suggested that photogenerated electrons and holes flow through the double and triple layers, respectively, which may suppress electron-hole recombination. We expanded this double-triple system to include Bi₄CaO₆Cl₂ and Bi₄SrO₆Cl₂ with orthorhombic distortions and different degrees of cationic disorder, which allow band gap tuning. All the double-triple compounds Bi₄AO₆Cl₂ showed stable water-splitting photocatalysis in the presence of a sacrificial reagent.

Electrochemical Crystal Growth of Titanium Oxyfluorides-A Strategy for Development of Electron-Doped Materials

We report on the growth of single crystals of an electron-doped titanium oxyfluoride, Li₂Ti(O,F)₃, employing high-temperature electrolysis of TiO₂ with a eutectic Li₂MoO₄-LiF melt. Greenish octahedral-shaped crystals (∼30 μm in size) with a cubic rocksalt-type structure were successfully obtained by precisely tuning the applied voltage. The temperature-dependent magnetic susceptibility data revealed a paramagnetic behavior at low temperatures, ensuring the presence of Ti³⁺ ions (mean valence number of +3.78; F/Ti ∼ 0.15). The crystals exhibited clear visible-light absorption and produced H₂ from water in the presence of a sacrificial reagent under UV-light irradiation. Li₂Ti(O,F)₃ more efficiently produced H₂ compared with a nondoped oxyfluoride Li₅Ti₂O₆F, likely due to the doped electrons for the former. This work highlights a promising electrochemical approach toward growing electron-doped oxyfluoride crystals.

Controllable Syntheses, Crystal Structure Evolution, and Photoluminescence of Polymorphic Zirconium Oxyfluorides

Precise synthesis of polymorphic phases with similar components but distinct crystal structures is one of the key problems in inorganic chemistry. In this work, we report a fluorination method adopting ZrO₂ as the starting material and NH₄F as the fluoridation agent that can afford multiphases in the Zr-O-F system, including Zr₇O₉F₁₀, Zr₃O₂F₈, ZrO_0.46F_3.08, ZrO_0.33F_3.33, β-ZrF₄, NH₄Zr₂F₉, and NH₄ZrF₅. A preliminary phase formation diagram was established as a function of the fluorination temperature (T), reaction time (t), and F/Zr ratio after systematic optimization of the preparation conditions. Among the as-obtained phases, the detailed crystal structures of Zr₇O₉F₁₀ and ZrO_0.33F_3.33 were refined based on the powder X-ray diffraction patterns. As the F/O ratio increases, the crystal structures of Zr-O-F phases transform gradually from an anion-deficient α-UO₃-related structure of Zr₇O₉F₁₀ to an anion-excess ReO₃-related structure of ZrO_0.33F_3.33. At last, we also prepared Ti-doped ZrO₂, Zr₇O₉F₁₀, ZrO_0.46F_3.08, and ZrO_0.33F_3.33 to study the host-lattice-dependent photoluminescence properties of zirconium oxyfluorides. The four materials show distinct photoluminescence in the UV and visible regions due to different local coordination environments of Zr/Ti. This work demonstrates the low-temperature fluorination method as an efficient route to phase-selective polymorphic metal oxyfluorides, which can be employed in further structure-property relationship studies.

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.

A bifunctional lead-iron oxyfluoride, PbFeO2F, that functions as a visible-light-responsive photoanode and an electrocatalyst for water oxidation

The oxyfluoride PbFeO₂F was investigated as a photoanode material and as an electrocatalyst for water oxidation. PbFeO₂F powder, which was synthesized by a high-pressure method and had an estimated bandgap of 2.1 eV, was deposited onto a fluorine-doped tin oxide (FTO) substrate. Mott–Schottky plot measurements for the PbFeO₂F/FTO electrode showed n-type semiconductivity of PbFeO₂F, with a flat-band potential of +0.53 ± 0.05 V vs. reversible hydrogen electrode (RHE). The PbFeO₂F/FTO electrode, which was modified with a conductive TiO₂ layer and a cobalt phosphate water-oxidation cocatalyst, showed a clear anodic photocurrent in aqueous K₃PO₄ solution under visible-light irradiation (λ < 600 nm). The PbFeO₂F/FTO electrode without any modification functioned as a stable water-oxidation electrocatalyst to form O₂ with a faradaic efficiency of close to unity. This study demonstrates that PbFeO₂F is a bifunctional material, serving as a water-oxidation photoanode under a wide range of visible-light wavelengths and as an electrocatalyst that operates at a relatively low overpotential for water oxidation.

The PbFeO₂F serves as a bifunctional material for a water-oxidation photoanode workable under a wide range of visible light and a water-oxidation electrocatalyst operatable at a relatively low overpotential.

Water Splitting with a Single-Atom Cu/TiO2 Photocatalyst: Atomistic Origin of High Efficiency and Proposed Enhancement by Spin Selection

Anatase TiO2 is an intensely investigated photocatalytic material due to its abundance and chemical stability. However, it suffers from weak light harvesting and low photocatalytic efficiency. Experiments show that light absorption and photocatalytic properties can be enhanced simultaneously by TiO2 doping with well-dispersed Cu atoms, forming a single-atom catalyst (Cu/TiO2) that can be used for solar water splitting and other applications. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrate that Cu/TiO2 is inactive before light irradiation due to rapid electron-hole recombination via both shallow and deep traps. Surprisingly, the shallow trap is more detrimental to the Cu/TiO2 performance than the deep trap because it couples better to free carriers. After light irradiation, leading to electron transfer and Cu/TiO2 protonation, the shallow trap is eliminated, and a local distortion around the Cu atom stabilizes the deep trap state on the Cu d-orbital, decoupling it from free charges and giving rise to high photocatalytic hydrogen generation activity. We further demonstrate that the photocatalytic performance of Cu/TiO2 can be enhanced by spin selection, achievable experimentally via optical intersite spin transfer or chiral semiconductor coating. Both H adsorption and spin selection enhance charge carrier lifetimes by an order of magnitude. The spin selection mechanism does not require formation of the H species, which necessitates concurrent sources of electrons and protons and which is intrinsically unstable because water splitting involves frequent proton shuffling. Our results rationalize the experimental observations at the atomistic level, provide mechanistic insights into operation of single atom photocatalysis, and demonstrate that spin selection can be used to develop advanced and efficient systems for solar energy conversion.

S = 1/2 Chain in BiVO3F: Spin Dimers versus Photoanodic Properties

BiVO₃F was prepared, characterized, and identified as a unique example of bismuth vanadyl oxyhalide with paramagnetic V⁴⁺ centers. Its crystal structure shows 1D magnetic units with rare alternation of edge-sharing O-O and F-F μ₂ bridges along the octahedral chains. Structural pairing across the O₂ edges induces antiferromagnetic spin dimers (S = 0) with J/K_b ≈ 300 K, ∼15 times greater than the exchange across the F₂ bridges, within a non-ordered magnetic ground state. Despite multiple compositional, structural, and electronic analogies with the BiVO₄ scheelite compound, one of the most promising photoanodes for solar water splitting, the photoactivity of BiVO₃F is relatively modest, partially due to this electronic pairing benefitting fast electron-hole recombination. Similar to monoclinic VO₂, the V⁴⁺ spin dimerization deters the singlet → triplet electronic photoexcitation, but results in potential carrier lifetime benefits. The reduction of the bandgap from an E_g of ∼2.4 eV to ∼1.7 eV after incorporation of d¹ cations in BiVO₄ makes BiVO₃F an inspiring compound for local modifications toward an enhanced photoactive material. The direct d → d transition provides a significant enhancement of the visible light capture range and opens a prospective route for the chemical design of performant photoanodes with a mixed anionic sublattice.

Pyrochlore oxides as visible light-responsive photocatalysts

This perspective describes the use of pyrochlore oxides in photocatalysis with focus on the strategies to enhance their activity.

The InSe/SiH type-II van der Waals heterostructure as a promising water splitting photocatalyst: a first-principles study

Photocatalytic water splitting for hydrogen production has attracted increasing research attention in recent years, and great efforts have been made in order to find the ideal photocatalyst. In this work, we proposed a two-dimensional material-based van der Waals (vdW) heterostructure constructed by vertically stacked indium selenide (InSe) and silicane (SiH) and studied the feasibility of using it as a possible photocatalyst for water splitting by using first-principles methods. The results show that the InSe/SiH is a direct band gap semiconductor with appropriate gap value and band edge position for photocatalysts in water splitting. Importantly, this heterostructure presents type-II band alignment at the equilibrium configuration, which supports the effective separation of photoexcited electrons and holes. A built-in electric field set up within the interface of the heterostructure will further hinder the electron-hole recombination and thus improve the photocatalytic efficiency. In addition, compared with separated InSe and SiH monolayers, the heterostructure exhibits enhanced light absorption capabilities in ultraviolet and visible light regions. These findings indicate that the InSe/SiH vdW heterostructure is a promising candidate for photocatalysts for solar water splitting.

Synthesis of NaMoO3F and Na5W3O9F5 with Morphological Controllability in Non-Aqueous Solvents

NaMoO₃F and Na₅W₃O₉F₅ were synthesized by solvothermal reaction of MoO₃ and WO₃, respectively, with NaF in nonaqueous solvents. These reactions were realized at low temperatures (150-200 °C) without the use of HF. This synthesis method is much more facile and safe procedure compared with general synthesis methods for oxyfluorides which includes hydrothermal reaction under a presence of HF or solid-state reaction at high temperatures in vacuum sealed tube or under high pressure. In the case of the reaction of MoO₃ with NaF, the kind of solvent largely affected the obtained morphologies of NaMoO₃F. The morphology in the case of acetonitrile as a solvent was rodlike with a micrometer-scale size, while that in the case of ethanol was polyhedral with a size of several hundred nanometers. In addition, the solvothermal reaction of WO₃ with NaF led to the formation of Na₅W₃O₉F₅. Also, the difference of solvents for the solvothermal reaction affected the obtained particle sizes. The effect of the solvents on the morphologies of the obtained oxyfluorides probably resulted from the difference of the solubility of NaF and the subsequent dissolution ratio of MoO₃ or WO₃ in the used solvents. Our synthesis method can expand the applicability of oxyfluorides by providing a new phase and/or unique morphology.

Photoinduced Defect and Surface Chemistry of Niobium Tellurium Oxides ANbTeO6 (A = K, Rb, Cs) with Defect-Pyrochlore Structure

The niobium tellurium oxides ANbTeO₆ with varying A cations (A = K, Rb, Cs) and defect-pyrochlore crystal structure were used to investigate the effect of A on crystal-structure deformation and defect-chemistry. We show that the light absorption of these compounds in visible light is due to defects and not the effect of a low band gap. Using the materials in photocatalytic hydrogen generation, the prevailing defects and the surface composition change significantly during photocatalytic hydrogen evolution.

Constructing Fe-MOF-Derived Z-Scheme Photocatalysts with Enhanced Charge Transport: Nanointerface and Carbon Sheath Synergistic Effect

Creatively constructing Z-scheme composites is a promising and common strategy for designing effective photocatalyst systems. Herein, we synthesized Z-scheme Fe₂O₃@Ag-ZnO@C heterostructures from the Fe-MOFs and applied it to photodegradation of tetracycline and methylene blue pollutants in wastewater. The optimized sample exhibits a remarkable performance as well as stability under visible light irradiation. The calculating and experimental results demonstrate that the Fe₂O₃@ZnO nanointerface and carbon sheath together boost the transfer efficiency of photogenerated carriers and absorption ability, thereby improving the photocatalytic activity. Furthermore, detailed mechanism investigation reveals the pivotal role of reactive oxygen species (^•OH and ^•O₂^-) generated, resulting in remarkable performance. In addition, cell biology experiments reveal that the wastewater after photocatalytic treatment has good biological compatibility, which is important for applications. This work provides valuable information for constructing high-performance Z-scheme photocatalysts from MOFs for environmental treatment.

Oxysulfide Ba5(VO2S2)2(S2)2 Combining Disulfide Channels and Mixed-Anion Tetrahedra and Its Third-Harmonic-Generation Properties

Mixed-anion compounds are among the most promising systems to design functional materials with enhanced properties. In particular, heteroleptic environments around transition metals allow tuning of the polarity or band-gap engineering for instance. We present the original oxysulfide Ba₅(VO₂S₂)₂(S₂)₂, the fifth member in the quaternary system Ba-V-S-O. It exhibits the mixed-anion building units V⁵⁺O₂S₂ and isolated disulfide pairs (S₂)^2-. The structure is solved by combining single-crystal and powder X-ray diffraction and transmission electron microscopy. First-principles calculations were combined in order to highlight the anion roles. In particular, our density functional theory study shows that the 3p states of the disulfide pairs dictate the band gap. In this study, we point out anionic tools for band-gap engineering that can be useful for the design of phases for numerous applications. Finally, third harmonic generation (THG) was measured and compared to the large THG observed for Cu₂O, which reveals the potential for nonlinear-optical properties that should be further investigated.

Light Absorption Properties and Electronic Band Structures of Lead-Vanadium Oxyhalide Apatites Pb5 (VO4 )3 X (X=F, Cl, Br, I)

The Pb-V oxyhalide apatite compounds Pb₅ (VO₄ )₃ X (X=F, Cl, Br, I) were successfully synthesized using a facile solution method and studied with respect to their structural/optical characteristics and electronic band structures. UV-visible diffuse reflectance spectroscopy, electrochemical analysis and first-principles calculations showed that the synthesized apatites behaved as n-type semiconductors, with absorption bands in the UV-visible region that could be assigned to electron transitions from the valence band to a conduction band formed by hybridized V 3d and Pb 6p orbitals. Among the apatites examined, Pb₅ (VO₄ )₃ I had the smallest band gap of 2.7 eV, due to an obvious contribution of I 5p orbitals to the valence band maximum. Based on its visible light absorption capability, Pb₅ (VO₄ )₃ I generated a continuous anodic photocurrent under visible light (λ>420 nm) in a solution of 0.1 m NaI in acetonitrile.

Structure of the water-splitting photocatalyst oxysulfide α-LaOInS2 and ab initio prediction of new polymorphs

We unveil the structure and investigate the visible light water-splitting of the photocatalyst α-LaOInS₂, the second polymorph in this composition. This remarkable oxysulfide exhibits rare mixed anion InS₅O octahedra leading to both O-2p and S-3p hybridized with indium states in the vicinity of the Fermi level. Ab initio structure prediction shows the stability of such heteroleptic environments and points to other hypothetical polymorphs.

Formation of breathing pyrochlore lattices: structural, thermodynamic and crystal chemical aspects

The structural diversity of breathing pyrochlore lattices was investigated on the example of ordered pyrochlores in terms of group-theoretical analysis, Landau thermodynamics and crystal chemistry.