Unraveling a Single-Step Simultaneous Two-Electron Transfer Process from Semiconductor to Molecular Catalyst in a CoPy/CdS Hybrid System for Photocatalytic H2 Evolution under Strong Alkaline Conditions

Carrier confinement activated explicit solvent dynamic of CdS/BiVO4/H2O and optimized photocatalytic hydrogen evolution performances

Herein, using an electrophoretic deposition strategy, a S-scheme CdS (cubic)/BiVO4 (monoclinic) heterostructured photocatalyst is fabricated. The as-synthesized photocatalysts exhibit high carrier separation efficiency, prominent hydrogen evolution ability and high stability. The results of the detailed density functional theory (DFT) prove that the photogenerated electrons and holes are located in BiVO4 and CdS components, respectively. Besides, an explicit solvent model based on the electron-enriched region in CdS/BiVO4 heterojunction is designed deliberately to investigate the solid/liquid interface issues. Intriguing findings demonstrate that the surface hydrogen diffusing rate in CdS/BiVO4/H2O is faster than that of BiVO4/H2O and is highly associated with the electron-enrich effect, which has a greater capacity to promote water decomposition, the possibility of proton collision and photocatalytic hydrogen evolution. Notably, the H p orbital can participate in the electron-enrich effect during solvation, thus reforming the orbital energy level and activating the HER of the BiVO4 component in the CdS/BiVO4 system.

Stabilization and Utilization of Pyrite under Light Irradiation: Discussion of Photocorrosion Resistance

The control of pyrite (FeS₂) oxidation from a source is a problem of great concern on treatment of acid mine drainage (AMD). Compared with air and water, the effect of light on pyrite oxidation has not attracted enough attention. However, we found that pyrite photocorrosion in the light promoted the oxidation of pyrite. Herein, we introduce a method of coating pyrite with graphene oxide (GO), which can inhibit the oxidation and photocorrosion of pyrite while it can also degrade organic pollutants. The characterization results show that a covalent bond forms between the GO and pyrite. The stable and uniform GO coating prevents the permeation of O₂ and H₂O and promotes the transfer of photogenerated electrons. Moreover, it changes the conduction band (CB) and valence band (VB) levels of GO-pyrite. All of these are vital for preventing the corrosion of pyrite and promoting its photocatalytic ability. More importantly, the effect of CB and VB levels on the oxidized species was discussed. The inhibition of photocorrosion is achieved by the reaction of GO with the photoinduced h⁺, ^•OH, and ^•O₂ ^-. The study provides insights for source treatment of AMD under light and the reuse of massive abandoned pyrite.

Amorphous TiO2 as a multifunctional interlayer for boosting the efficiency and stability of the CdS/cobaloxime hybrid system for photocatalytic hydrogen production

The construction of both highly efficient and stable hybrid artificial photosynthetic systems comprising semiconductors as photosensitizers and abundant metal-based molecular complexes as cocatalysts for photocatalytic H2 generation remains challenging. Herein, we report an effective and stable CdS/cobaloxime hybrid system prepared by inserting an amorphous TiO2 (a-TiO2) interlayer with adjustable thickness and by covalently-surface-attaching molecular cobaloxime catalysts. This hybrid system displayed outstanding photocatalytic H2 production and reached a maximum rate of ∼25 mmol g-1 h-1, which was ∼20.8 times that of pure CdS and 1.7 times that of the CdS/cobaloxime system without an a-TiO2 interlayer (CdS/Co). More importantly, 6 nm a-TiO2 uniformly coated CdS nanorods (CdS NRs) exhibited exceptional 200 h long-term catalytic behaviour under ≥420 nm visible light irradiation. However, the H2 production performance of the CdS/Co hybrid system decreased significantly over 10 h. Density functional theory (DFT) calculations indicated that the a-TiO2 surface can provide abundant bonding sites for the effective immobilization of molecular catalysts. Moreover, Mott-Schottky electrochemical measurements and femtosecond transient absorption spectroscopy revealed that the a-TiO2 interlayer had favourable band levels that could fasten the photoexcited electron transfer from CdS to molecular cobaloxime and could extract holes with intraband electronic states generated by defects, thus prohibiting CdS photocorrosion and improving the stability of the hybrid system. This study proposes a strategy for designing multifunctional interlayers for the effective immobilization of molecular catalysts, beneficial regulation of photoinduced charge carriers, and improvement of the stability as well as facilitation of the construction of artificial photosynthetic hybrid systems with high efficiency and durability.

Conjugated nanoporous polycarbazole bearing a cobalt complex for efficient visible-light driven hydrogen evolution

A conjugated nanoporous polycarbazole (CNP) cross-linked by pyridine and coordinated to Co(iii) displays high catalytic performance for visible light-driven H₂ generation.

A biomimetic self-assembled cobaloxime@CdS/rGO hybrid for boosting photocatalytic H2 production

A biomimetic CoPe@CdS/rGO hybrid that self-assembles via the integration of a molecular cobalt catalyst and CdS nano-semiconductor on reduced graphene oxide was constructed for boosting photocatalytic H2 production. Photoinduced electron transfer from CdS/rGO to the molecular catalyst occurs and a long-lived charge-separation state forms for high H2 production.

A bio-inspired strategy for enhanced hydrogen evolution: carbonate ions as hole vehicles to promote carrier separation

Natural photosynthesis involves a subtle electron transfer mechanism in which freely-moving electron transfer intermediates (plastoquinone and plastocyanin) are capable of effectively separating the photo-generated carriers, and therefore, it has high quantum efficiency. Inspired by this mechanism, in this study, carbonate (CO32-) ions were employed as hole vehicles to promote photo-generated carrier separation, and greatly improved the photocatalytic hydrogen evolution activity of K4Nb6O17 nanosheets. The hydrogen evolution rate at the optimal concentration of CO32- ions reached 2018 μmol h-1 g-1, which was 16.3 times that of the blank sample (124 μmol h-1 g-1). This marked enhancement was based on the transfer of holes from the photocatalyst to the sacrificial reagent (methanol) via CO32- ions; this process is faster than direct hole transfer between the photocatalyst and sacrificial reagent. This bio-inspired strategy provides a facile and cost-effective approach to improve the solar-to-fuel conversion efficiency of photocatalysts.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Ultrathin Visible-Light-Driven Mo Incorporating In2 O3 -ZnIn2 Se4 Z-Scheme Nanosheet Photocatalysts

Inspired by natural photosynthesis, the design of new Z-scheme photocatalytic systems is very promising for boosting the photocatalytic performance of H₂ production and CO₂ reduction; however, until now, the direct synthesis of efficient Z-scheme photocatalysts remains a grand challenge. Herein, it is demonstrated that an interesting Z-scheme photocatalyst can be constructed by coupling In₂ O₃ and ZnIn₂ Se₄ semiconductors based on theoretical calculations. Experimentally, a class of ultrathin In₂ O₃ -ZnIn₂ Se₄ (denoted as In₂ O₃ -ZISe) spontaneous Z-scheme nanosheet photocatalysts for greatly enhancing photocatalytic H₂ production is made. Furthermore, Mo atoms are incorporated in the Z-scheme In₂ O₃ -ZISe nanosheet photocatalyst by forming the MoSe bond, confirmed by X-ray photoelectron spectroscopy, in which the formed MoSe₂ works as cocatalyst of the Z-scheme photocatalyst. As a consequence, such a unique structure of In₂ O₃ -ZISe-Mo makes it exhibit 21.7 and 232.6 times higher photocatalytic H₂ evolution activity than those of In₂ O₃ -ZnIn₂ Se₄ and In₂ O₃ nanosheets, respectively. Moreover, In₂ O₃ -ZISe-Mo is also very stable for photocatalytic H₂ production by showing almost no activity decay for 16 h test. Ultraviolet-visible diffuse reflectance spectra, photoluminescence spectroscopy, transient photocurrent spectra, and electrochemical impedance spectroscopy reveal that the enhanced photocatalytic performance of In₂ O₃ -ZISe-Mo is mainly attributed to its widened photoresponse range and effective carrier separation because of its special structure.

Heterojunction and Oxygen Vacancy Modification of ZnO Nanorod Array Photoanode for Enhanced Photoelectrochemical Water Splitting

Application of ZnO in the field of photoelectrochemical water splitting is limited because of its wide-band-gap and high recombination rate. Herein is reported the design of an efficient ZnO photoanode deposited with CoO_x nanoparticles to achieve a heterojunction and oxygen vacancies. The CoO_x nanoparticles with abundant oxygen vacancies were anchored onto the nanorod arrays by spin coating and calcination followed by a solvothermal treatment. CoO_x nanoparticles serve the dual function of forming a p-n heterojunction to facilitate the separation of photogenerated carriers, and act as a cocatalyst to decrease water oxidation barrier. Finally, oxygen vacancies increase the number of active redox sites and act as hole traps, enabling their migration to the electrode/electrolyte interface. The composite photoanode exhibits a high incident photon-to-current conversion efficiency (76.7 % at 350 nm), which is twice that of pristine ZnO, and a photoconversion efficiency of 0.68 % (0.73 V versus RHE). The current approach can be expanded to fabricate other efficient photocatalysts.

Coordination chemistry in the design of heterogeneous photocatalysts

Heterogeneous catalysts have been widely used for photocatalysis, which is a highly important process for energy conversion, owing to their merits such as easy separation of catalysts from the reaction products and applicability to continuous chemical industry and recyclability. Yet, homogenous photocatalysis receives tremendous attention as it can offer a higher activity and selectivity with atomically dispersed catalytic sites and tunable light absorption. For this reason, there is a major trend to combine the advantages of both homogeneous and heterogeneous photocatalysts, in which coordination chemistry plays a role as the bridge. In this article, we aim to provide the first systematic review to give a clear picture of the recent progress from taking advantage of coordination chemistry. We specifically summarize the role of coordination chemistry as a versatile tool to engineer catalytically active sites, tune light harvesting and maneuver charge kinetics in heterogeneous photocatalysis. We then elaborate on the common fundamentals behind various materials systems, together with key spectroscopic characterization techniques and remaining challenges in this field. The typical applications of coordination chemistry in heterogeneous photocatalysis, including proton reduction, water oxidation, carbon dioxide reduction and organic reactions, are highlighted.

Cobalt lactate complex as a hole cocatalyst for significantly enhanced photocatalytic H2 production activity over CdS nanorods

A cobalt lactate complex has been prepared in situ, which works as a molecular cocatalyst accelerating hole transfer for the enhanced photocatalytic H₂ evolution activity of CdS.

Oriented Built-in Electric Field Introduced by Surface Gradient Diffusion Doping for Enhanced Photocatalytic H2 Evolution in CdS Nanorods

Element doping has been extensively attempted to develop visible-light-driven photocatalysts, which introduces impurity levels and enhances light absorption. However, the dopants can also become recombination centers for photogenerated electrons and holes. To address the recombination challenge, we report a gradient phosphorus-doped CdS (CdS-P) homojunction nanostructure, creating an oriented built-in electric-field for efficient extraction of carriers from inside to surface of the photocatalyst. The apparent quantum efficiency (AQY) based on the cocatalyst-free photocatalyst is up to 8.2% at 420 nm while the H₂ evolution rate boosts to 194.3 μmol·h^-1·mg^-1, which is 58.3 times higher than that of pristine CdS. This concept of oriented built-in electric field introduced by surface gradient diffusion doping should provide a new approach to design other types of semiconductor photocatalysts for efficient solar-to-chemical conversion.

Spatial separation of the hydrogen evolution center from semiconductors using a freestanding silica-sphere-supported Pt composite

A heterogeneous material based on silica-sphere-supported Pt nanoparticles was designed and used as an efficient freestanding hydrogen evolution cocatalyst for semiconductor photocatalysts.

Construction of molecular rectangles with titanium–oxo clusters and rigid aromatic carboxylate ligands

Four supramolecular rectangles have been successfully constructed using {Ti₅O₇} clusters as vertical edges and bridging aromatic carboxylates as horizontal edges.