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

Band gap engineering and transport properties of Ba2In2O5: effect of fluorine doping and hydration

Two types of fluorine doped barium indate solid solutions were prepared by a solid state method: Ba_2-0.5xIn₂O_5-xF_x (x = 0, 0.1, 0.2) and Ba₂In₂O_5-0.5yF_y (y = 0, 0.1, 0.25). Good agreement between the theoretical and experimental values of the pycnometric densities for the studied solid solutions confirms these two distinctly different models of solid solution formation. According to analysis of the XRD data introduction of fluorine ions into the oxygen sublattice of barium indate leads to a decrease in the a lattice parameter and unit cell volume for both types of solid solutions. The effect of fluorine doping and hydration on the band gap of barium indate was studied by means of diffuse reflectance spectroscopy. The estimated value of the band gap width E_g for undoped Ba₂In₂O₅ is 2.94 eV and is in good agreement with literature data. Introduction of fluorine results in a slight increase of E_g and emergence of an additional absorption band in the region of 2.56-2.65 eV, near the fundamental absorption edge, which can be attributed to the -defects appearing upon fluorine doping. The increase of E_g with fluorine introduction correlates well with the decrease of the electronic transport numbers and can be explained by two competing effects: (1) lowering of the top of the valence band due to replacement of O^2- ions by F^-; and (2) changes in the local structure, i.e., lattice contraction and tilting of the InO(F)_x framework. Hydration of the barium indate also leads to an increase of E_g, which was attributed to structural transformation from orthorhombic symmetry to tetragonal. However, with an increase in the fluorine concentration such changes of E_g become less pronounced because of the decrease of the hydration degree due to formation of -defects.

A zinc-based oxysulfide photocatalyst SrZn2S2O capable of reducing and oxidizing water

Although Zn-based binary semiconductors such as ZnO and ZnS are photocatalytically unstable toward water oxidation, we found that mixed-anionization successfully addressed this issue. This report shows that an oxysulfide SrZn₂S₂O functions as a photocatalyst to reduce and oxidize water under band-gap irradiation without noticeable decomposition of the material.

Supporting Ultrathin ZnIn2 S4 Nanosheets on Co/N-Doped Graphitic Carbon Nanocages for Efficient Photocatalytic H2 Generation

Ultrathin ZnIn₂ S₄ nanosheets (NSs) are grown on Co/N-doped graphitic carbon (NGC) nanocages, composed of Co nanoparticles surrounded by few-layered NGC, to obtain hierarchical Co/NGC@ZnIn₂ S₄ hollow heterostructures for photocatalytic H₂ generation with visible light. The photoredox functions of discrete Co, conductive NGC, and ZnIn₂ S₄ NSs are precisely combined into hierarchical composite cages possessing strongly hybridized shell and ultrathin layered substructures. Such structural and compositional virtues can expedite charge separation and mobility, offer large surface area and abundant reactive sites for water photosplitting. The Co/NGC@ZnIn₂ S₄ photocatalyst exhibits outstanding H₂ evolution activity (e.g., 11270 µmol h^-1 g^-1 ) and high stability without engaging any cocatalyst.

A symbiotic hetero-nanocomposite that stabilizes unprecedented CaCl2-type TiO2 for enhanced solar-driven hydrogen evolution reaction

Symbiotic hetero-nanocomposites prevail in many classes of minerals, functional substances and/or devices. However, design and development of a symbiotic hetero-nanocomposite that contains unachievable phases remain a significant challenge owing to the tedious formation conditions and the need for precise control over atomic nucleation in synthetic chemistry. Herein, we report a solution chemistry approach for a symbiotic hetero-nanocomposite that contains an unprecedented CaCl2-type titania phase inter-grown with rutile TiO2. CaCl2 structured TiO2, usually occurring when bulk rutile-TiO2 is compressed at an extreme pressure of several GPa, is identified to be a distorted structure with a tilt of adjacent ribbons of the c-axis of rutile. The structural specificity of the symbiotic CaCl2/rutile TiO2 hetero-nanocomposite was confirmed by Rietveld refinement, HRTEM, EXAFS, and Raman spectra, and the formation region (TiCl4 concentration vs. reaction temperature) was obtained by mapping the phase diagram. Due to the symbiotic relationship, this CaCl2-type TiO2 maintained a high stability via tight connection by edge dislocations with rutile TiO2, thus forming a CaCl2/rutile TiO2 heterojunction with a higher reduction capacity and enhanced charge separation efficiency. These merits endow symbiotic CaCl2/rutile TiO2 with a water splitting activity far superior to that of the commercial benchmark photocatalyst, P25 under simulated sunlight without the assistance of a cocatalyst. Our findings reported here may offer several useful understandings of the mechanical intergrowth process in functional symbiotic hetero-nanocomposites for super interfacial charge separation, where interfacial dislocation appears to be a universal cause.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Encapsulating a Ni(II) molecular catalyst in photoactive metal-organic framework for highly efficient photoreduction of CO2

Photocatalytic reduction of CO₂ to CO is a promising strategy for reducing atmospheric CO₂ levels and storing solar radiation as chemical energy. Here, we demonstrate that a molecular catalyst [Ni^II(bpet)(H₂O)₂] successfully encapsulated into a highly robust and visible-light responsive metal-organic framework (Ru-UiO-67) to fabricate composite catalysts for photocatalytic CO₂ reduction. The composite Ni@Ru-UiO-67 photocatalysts show efficient visible-light-driven CO₂ reduction to CO with a TON of 581 and a selectivity of 99% after 20-h illumination, because of the facile electron transfer from Ru-photosensitizer to Ni(II) active sites in Ni@Ru-UiO-67 system. The mechanistic insights into photoreduction of CO₂ have been studied based on thermodynamical, electrochemical, and spectroscopic investigation, together with density functional theory (DFT) calculations. This work shows that encapsulating molecular catalyst into photoactive MOF highlights opportunities for designing efficient, stable and recyclable photocatalysts.

Highly Selective Photoreduction of CO2 with Suppressing H2 Evolution over Monolayer Layered Double Hydroxide under Irradiation above 600 nm

Although progress has been made to improve photocatalytic CO₂ reduction under visible light (λ>400 nm), the development of photocatalysts that can work under a longer wavelength (λ>600 nm) remains a challenge. Now, a heterogeneous photocatalyst system consisting of a ruthenium complex and a monolayer nickel-alumina layered double hydroxide (NiAl-LDH), which act as light-harvesting and catalytic units for selective photoreduction of CO₂ and H₂ O into CH₄ and CO under irradiation with λ>400 nm. By precisely tuning the irradiation wavelength, the selectivity of CH₄ can be improved to 70.3 %, and the H₂ evolution reaction can be completely suppressed under irradiation with λ>600 nm. The photogenerated electrons matching the energy levels of photosensitizer and m-NiAl-LDH only localized at the defect state, providing a driving force of 0.313 eV to overcome the Gibbs free energy barrier of CO₂ reduction to CH₄ (0.127 eV), rather than that for H₂ evolution (0.425 eV).

Metal-Complex/Semiconductor Hybrid Photocatalysts and Photoelectrodes for CO2 Reduction Driven by Visible Light

CO₂ reduction to carbon feedstocks using heterogeneous photocatalysts is an attractive means of addressing both climate change and the depletion of fossil fuels. Of particular importance is the development of a photosystem capable of functioning in response to visible light, which accounts for the majority of the solar spectrum, representing a kind of artificial photosynthesis. Hybrid systems comprising a metal complex and a semiconductor are promising because of the excellent electrochemical (and/or photocatalytic) activity of metal complexes during CO₂ reduction and the ability of semiconductors to efficiently oxidize water to molecular O₂ . Here, the development of hybrid photocatalysts and photoelectrodes for CO₂ reduction in combination with water oxidation is described.

Development of Mixed-Anion Photocatalysts with Wide Visible-Light Absorption Bands for Solar Water Splitting

Rapid fossil-fuel consumption, severe environmental concerns, and growing energy demands call for the exploitation of environmentally friendly, recyclable, new energy sources. Fuel-producing artificial systems that directly convert solar energy into fuels by mimicking natural photosynthesis are expected to achieve this goal. Among them, the conversion of solar energy into hydrogen energy through the photocatalytic water-splitting process over a particulate semiconductor is one of the most promising routes due to advantages such as simplicity, cheapness, and ease of large-scale production. Abundant metal oxide photocatalysts have been developed in the last century, but most are only active under UV-light irradiation. To harvest a much wider range of the solar spectrum, the development of photocatalysts with wide visible-light absorption bands has become increasingly popular this century. Herein, a brief overview of materials developed for promising solar water splitting, with an emphasis on a mixed-anion structure and wide visible-light absorption bands, is presented, with some basic information on the principles, approaches, and research progress on the photocatalytic water-splitting reaction with particulate semiconductors. Typical progress on research into one- and two-step (Z-scheme) overall water-splitting systems by utilizing mixed-anion photocatalysts is highlighted, together with research strategies and modification methods.

Reactive solid phase epitaxy of layered aurivillius-type oxyfluorides Bi2TiO4F2 using polyvinylidene fluoride

Aurivillius-type oxyfluorides are promising ferroelectric and photocatalytic materials. However, their thin films have yet to be fabricated because of the difficulty of synthesis when using both conventional high-temperature gas-phase processes and low-temperature topotactic methods. Here, we present reactive solid phase epitaxy of a layered Aurivillius-type oxyfluoride Bi2TiO4F2 from room-temperature fabricated Bi2TiOx using polyvinylidene fluoride (PVDF) as a fluorine source. Bi2TiO4F2 epitaxial films are obtained by reacting a room-temperature fabricated precursor with PVDF at 330 °C under an Ar flow. However, crystallization does not proceed through PVDF treatment in air, indicating that a reduced atmosphere is crucial to removing oxide ions from the precursor and incorporating fluoride ions. The Bi2TiO4F2 film shows a peak at 240 K in the dielectric constant-versus-temperature curve, which originates from the tilting of Ti(O,F)6 octahedra. This peak temperature is lower than that of the bulk (284 K), suggesting that the local structural distortion is suppressed because of the epitaxial strain from the substrate. Reactive solid phase epitaxy using PVDF as described in this paper should provide a new means of synthesizing transition-metal oxyfluorides in the epitaxial thin-film form.

Structure and Photocatalytic Activity of PdCrOx Cocatalyst on SrTiO3 for Overall Water Splitting

The mechanism of PdCrOx multi-component cocatalyst formation on SrTiO3 was investigated using transmission electron microscopy, X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy. The PdCrOx/SrTiO3 samples were synthesized by a photodeposition method under UV light irradiation (λ > 300 nm) for various time periods (0–5 h). The fine structure and valence state of the Pd species of PdCrOx nanoparticles were varied from Pd oxide to a mixture of metallic Pd and oxidized Pd species with an increase in the irradiation time. The overall water-splitting activity of PdCrOx was strongly dependent on the photoirradiation time during deposition. Although longer photoirradiation time during preparation did not influence the H2 evolution activity of PdCrOx/SrTiO3 from aqueous methanol solution, it was effective in suppressing the O2 photoreduction activity, which is one of the backward reactions during overall water splitting.

Hetero-metallic active sites coupled with strongly reductive polyoxometalate for selective photocatalytic CO2-to-CH4 conversion in water

The photocatalytic reduction of CO2 to value-added methane (CH4) has been a promising strategy for sustainable energy development, but it is challenging to trigger this reaction because of its necessary eight-electron transfer process. In this work, an efficient photocatalytic CO2-to-CH4 reduction reaction was achieved for the first time in aqueous solution by using two crystalline heterogeneous catalysts, H{[Na2K4Mn4(PO4) (H2O)4]⊂{[Mo6O12(OH)3(HPO4)3(PO4)]4[Mn6(H2O)4]}·16H2O (NENU-605) and H{[Na6CoMn3(PO4)(H2O)4]⊂{[Mo6O12(OH)3(HPO4)3(PO4)]4[Co1.5Mn4.5]}·21H2O (NENU-606). Both compounds have similar host inorganic polyoxometalate (POM) structures constructed with strong reductive {P4Mo6 V} units, homo/hetero transition metal ions (MnII/CoIIMnII) and alkali metal ions (K+ and/or Na+). It is noted that the {P4Mo6 V} cluster including the six MoV atoms served as a multi-electron donor in the case of a photocatalytic reaction, while the transition metal ions functioned as catalytically active sites for adsorbing and activating CO2 molecules. Additionally, the presence of alkali metal ions was believed to assist in the capture of more CO2 for the photocatalytic reaction. The synergistic combination of the above-mentioned components in NENU-605 and NENU-606 effectively facilitates the accomplishment of the required eight-electron transfer process for CH4 evolution. Furthermore, NENU-606 containing hetero-metallic active sites finally exhibited higher CH4 generation selectivity (85.5%) than NENU-605 (76.6%).

Enhanced charge separation and transport efficiency induced by vertical slices on the surface of carbon nitride for visible-light-driven hydrogen evolution

Numerous vertical slices with thicknesses in the range of 100–200 nm were generated from pristine bulk carbon nitride (BCN) via an ammonium nitrate (NH₄NO₃)-assisted hydrothermal treatment. Compared with the structure of BCN, the obtained novel hierarchical structure consisted of more uniform mesopores (2–14 nm) and possessed enlarged specific surface area of 64.1 m² g⁻¹. It was elucidated that both NH₄⁺ and NO₃⁻ play important roles in the formation of the vertical slices, which could not only create an acidic environment for the hydrothermal system but also form hydrogen bonds with the surface tri-s-triazine units of BCN simultaneously. It was found that the hierarchical structure exhibited enhanced crystallinity, reduced photoluminescence emission, and increased photocurrent response. Consequently, a hydrogen evolution rate of 1817.9 μmol h⁻¹ g⁻¹ was achieved by the hierarchical structure, which was 4.1 times higher than that of BCN. The hydrothermal post-treatment strategy explored in this work provides a new insight into the design and modification of polymeric carbon nitride for generating a hierarchical porous microstructure.

A hierarchical g-C₃N₄ structure with vertical slices and enhanced efficiency of charge separation is achieved via an NH₄NO₃-aided hydrothermal post-treatment.

Preparation of a magnetic mesoporous Fe3O4–Pd@TiO2 photocatalyst for the efficient selective reduction of aromatic cyanides

Fe₃O₄–Pd@TiO₂ exhibited extremely superior photocatalytic activity for the selective reduction of aromatic cyanides to aromatic primary amines.