Critical roles of co-catalysts for molecular hydrogen formation in photocatalysis

Surface hydrophobization of zeolite enables mass transfer matching in gas-liquid-solid three-phase hydrogenation under ambient pressure

Attaining high hydrogenation performance under mild conditions, especially at ambient pressure, remains a considerable challenge due to the difficulty in achieving efficient mass transfer at the gas-liquid-solid three-phase interface. Here, we present a zeolite nanoreactor with joint gas-solid-liquid interfaces for boosting H2 gas and substrates to involve reactions. Specifically, the Pt active sites are encapsulated within zeolite crystals, followed by modifying the external zeolite surface with organosilanes. The silane sheath with aerophilic/hydrophobic properties can promote the diffusion of H2 and the mass transfer of reactant/product molecules. In aqueous solutions, the gaseous H2 molecules can rapidly diffuse into the zeolite channels, thereby augmenting H2 concentration surround Pt sites. Simultaneously, the silane sheath with lipophilicity nature promotes the enrichment of the aldehydes/ketones on the catalyst and facilitates the hydrophilia products of alcohol rediffusion back to the aqueous phase. By modifying the wettability of the catalyst, the hydrogenation of aldehydes/ketones can be operated in water at ambient H2 pressure, resulting in a noteworthy turnover frequency up to 92.3 h-1 and a 4.3-fold increase in reaction rate compared to the unmodified catalyst.

Fermi Level Pinning in Concentrated Light-Induced Band Edge Tuning Maximizes Photocatalytic Solar-to-Hydrogen Efficiency

Here, we demonstrate a concentrated light-induced band edge tuning effect in photocatalytic hydrogen production. This band movement along with Femi level pinning leads to two distinct catalytic behaviors upon irradiation flux increase. Specifically, the concentration of the light promotes more long-lived carriers bound to the surface electronic states, progressively boosting energy conversion efficiency to a maximum value. Afterward, efficiency diminishes gradually due to poor carrier transfer. This work offers critical insights into efficient and economical photocatalytic hydrogen production.

Quantum yield enhancement in the photocatalytic HCOOH decomposition to H2 under periodic illumination

Despite numerous studies on controlled periodic illumination to improve the quantum yield of photocatalytic reactions, debates still exist on the nature of such effect. In our system, we proposed that enhanced electron transfer is the promotion mechanism.

Chemical design of nanozymes for biomedical applications

With the advancement of nanochemistry, artificial nanozymes with high catalytic stability, low manufacturing and storage cost, and greater design flexibility over natural enzymes, have emerged as a next-generation nanomedicine. The catalytic activity and selectivity of nanozymes can be readily controlled and optimized by the rational chemical design of nanomaterials. This review summarizes the various chemical approaches to regulate the catalytic activity and selectivity of nanozymes for biomedical applications. We focus on the in-depth correlation between the physicochemical characteristics and catalytic activities of nanozymes from several aspects, including regulating chemical composition, controlling morphology, altering the size, surface modification and self-assembly. Furthermore, the chemically designed nanozymes for various biomedical applications such as biosensing, infectious disease therapy, cancer therapy, neurodegenerative disease therapy and injury therapy, are briefly summarized. Finally, the current challenges and future perspectives of nanozymes are discussed from a chemistry point of view. STATEMENT OF SIGNIFICANCE: As a kind of nanomaterials that performs enzyme-like properties, nanozymes perform high catalytic stability, low manufacturing and storage cost, attracting the attention of researchers from various fields. Notably, chemically designed nanozymes with robust catalytic activity, tunable specificity and multi-functionalities are promising for biomedical applications. It's crucial to define the correlation between the physicochemical characteristics and catalytic activities of nanozymes. To help readers understand this rapidly expanding field, in this review, we summarize various chemical approaches that regulate the catalytic activity and selectivity of nanozymes together with the discussion of related mechanisms, followed by the introduction of diverse biomedical applications using these chemically well-designed nanozymes. Hopefully our review will bridge the chemical design and biomedical applications of nanozymes, supporting the extensive research on high-performance nanozymes.

GdFeO3 Perovskite Oxide Decorated by Group X Heterometal Oxides and Bifunctional Oxygen Electrocatalysis

Bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are necessary in the renewable energy systems. However, the kinetically slow and large energy-demanding procedures of oxygen electrocatalysis make the preparation of bifunctional catalysts difficult. In this work, we report a novel hierarchical GdFeO₃ perovskite oxide of a spherelike nanostructure and surface modification with the group X heterometal oxides. The nanostructured GdFeO₃ layer behaved as a bifunctional electrocatalyst in the oxygen electrocatalysis of OER and ORR. Moreover, the surface decoration with catalytically active PtO_x + Ni/NiO nanoparticles enhanced the electrocatalytic performances substantially. Incorporation of mesoporous PtO_x + Ni/NiO nanoparticles into the porous GdFeO₃ nanostructure enlarged the electrochemically active surface area and provided the interconnected nanostructures to facilitate the OER/ORR. The nanostructures were visualized by scanning electron microscopy and transmission electron microscopy images, and the surface area and pore size of nanoparticles were analyzed from N₂ adsorption/desorption isotherms. Tafel analysis indicates that surface modification effectively improves the kinetics of oxygen reactions and accordingly increases the electrocatalytic efficiency. Finally, the 2 wt % PtO_x + NiO|GdFeO₃ (x = 0, 1, and 2) electrode achieved the enhanced OER performance with an overpotential of 0.19 V at 10 mA/cm² in an alkaline solution and a high turnover frequency of 0.28 s^-1 at η = 0.5 V. Furthermore, the ORR activity is observed with an onset potential of 0.80 V and a half-wave potential (E_1/2) of 0.40 V versus reversible hydrogen electrode.

Noble metal-free 0D-1D NiCoP/Mn0.3Cd0.7S nanocomposites for highly efficient photocatalytic H2 evolution under visible-light irradiation

Efficient and noble metal-free co-catalyst loading is an effective solution for separating and transferring photo-generated carriers and lowering the overpotential in photocatalytic H₂ evolution activity. In this work, we designed and prepared a series of novel NiCoP/Mn_0.3Cd_0.7S (NCP/MCS) composites by modifying MCS nanorods with the co-catalyst NCP using a simple calcination method. Notably, the 10-NCP/MCS composite displays the optimum photocatalytic H₂ evolution rate of 118.5 mmol g^-1 h^-1 under visible-light irradiation. This is approximately 3.39 times higher than that of pure MCS. The corresponding apparent quantum efficiency is 10.2% at 420 nm. The superior photocatalytic activity of the NCP/MCS composites can be attributed to the efficient separation of photogenerated carriers caused by the intimate heterojunction interface between NCP and MCS, smaller transfer resistance, and lower overpotential of NCP. Moreover, the NCP/MCS composites exhibit remarkable photostability. A plausible mechanism is proposed.

Direct Observation of Dynamic Bond Evolution in Single-Atom Pt/C3 N4 Catalysts

Single-atom catalysts are promising platforms for heterogeneous catalysis, especially for clean energy conversion, storage, and utilization. Although great efforts have been made to examine the bonding and oxidation state of single-atom catalysts before and/or after catalytic reactions, when information about dynamic evolution is not sufficient, the underlying mechanisms are often overlooked. Herein, we report the direct observation of the charge transfer and bond evolution of a single-atom Pt/C₃ N₄ catalyst in photocatalytic water splitting by synchronous illumination X-ray photoelectron spectroscopy. Specifically, under light excitation, we observed Pt-N bond cleavage to form a Pt⁰ species and the corresponding C=N bond reconstruction; these features could not be detected on the metallic platinum-decorated C₃ N₄ catalyst. As expected, H₂ production activity (14.7 mmol h^-1  g^-1 ) was enhanced significantly with the single-atom Pt/C₃ N₄ catalyst as compared to metallic Pt-C₃ N₄ (0.74 mmol h^-1  g^-1 ).

Understanding supported noble metal catalysts using first-principles calculations

Heterogeneous catalysis on supported and nonsupported nanoparticles is of fundamental importance in the energy and chemical conversion industries. Rather than laboratory analysis, first-principles calculations give us an atomic-level understanding of the structure and reactivity of nanoparticles and supports, greatly reducing the efforts of screening and design. However, unlike catalysis on low index single crystalline surfaces, nanoparticle catalysis relies on the tandem properties of a support material as well as the metal cluster itself, often with charge transfer processes being of key importance. In this perspective, we examine current state-of-the-art quantum-chemical research for the modeling of reactions that utilize small transition metal clusters on metal oxide supports. This should provide readers with useful insights when dealing with chemical reactions on such systems, before discussing the possibilities and challenges in the field.

Dual Cocatalysts in TiO2 Photocatalysis

Semiconductor photocatalysis is recognized as a promising strategy to simultaneously address energy needs and environmental pollution. Titanium dioxide (TiO₂ ) has been investigated for such applications due to its low cost, nontoxicity, and high chemical stability. However, pristine TiO₂ still suffers from low utilization of visible light and high photogenerated-charge-carrier recombination rate. Recently, TiO₂ photocatalysts modified by dual cocatalysts with different functions have attracted much attention due to the extended light absorption, enhanced reactant adsorption, and promoted charge-carrier-separation efficiency granted by various cocatalysts. Recent progress on the component and structural design of dual cocatalysts in TiO₂ photocatalysts is summarized. Depending on their components, dual cocatalysts decorated on TiO₂ photocatalysts can be divided into the following categories: bimetallic cocatalysts, metal-metal oxide/sulfide cocatalysts, metal-graphene cocatalysts, and metal oxide/sulfide-graphene cocatalysts. Depending on their architecture, they can be categorized into randomly deposited binary cocatalysts, facet-dependent selective-deposition binary cocatalysts, and core-shell structural binary cocatalysts. Concluding perspectives on the challenges and opportunities for the further exploration of dual cocatalyst-modified TiO₂ photocatalysts are presented.

Subnano-Sized Pt-Au Alloyed Clusters as Enhanced Cocatalyst for Photocatalytic Hydrogen Evolution

Photocatalytic water splitting for H₂ evolution is regarded as the most promising way to overcome the energy and environmental crisis. Pt clusters as a cocatalyst can efficiently enhance the performance of H₂ generation in most photocatalysts, but the activity is still unsatisfied. By tuning the electronic structures of materials, one can develop catalysts with enhanced activity. Here we synthesize a Pt-Au alloy with subnano size as cocatalyst on TiO₂ nanosheets for photocatalytic H₂ generation that shows an outstanding activity with a H₂ generation rate of 80.1 μmol h^-1 for at least 100 h. The activity is twice than the pure Pt cocatalyst, mainly because the optimized hydrogen adsorption energy on Pt cluster is tuned by Au atoms.

TiO2-supported Pt single atoms by surface organometallic chemistry for photocatalytic hydrogen evolution

Platinum single atoms are grafted by SOMC on morphology-controlled TiO₂. Their structure is characterized by EXAFS and other techniques, and their activity and stability in HER and backwards reaction are studied and compared to Pt nanoparticles.

Eu2+ and Eu3+ Doubly Doped ZnWO₄ Nanoplates with Superior Photocatalytic Performance for Dye Degradation

Eu²⁺ and Eu³⁺ doubly doped ZnWO₄ nanoplates with highly exposed {100} facets were synthesized via a facile hydrothermal route in the presence of surfactant cetyltrimethyl ammonium bromide. These ZnWO₄ nanoplates were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectrometry, diffuse UV-vis reflectance spectroscopy, photoluminescence spectrophotometry, and photoluminescence lifetime spectroscopy to determine their morphological, structural, chemical, and optical characteristics. It is found that Eu-doped ZnWO₄ nanoplates exhibit superior photo-oxidative capability to completely mineralize the methyl orange into CO₂ and H₂O, whereas undoped ZnWO₄ nanoparticles can only cleave the organic molecules into fragments. The superior photocatalytic performance of Eu-doped ZnWO₄ nanoplates can be attributed to the cooperative effects of crystal facet engineering and defect engineering. This is a valuable report on crystal facet engineering in combination with defect engineering for the development of highly efficient photocatalysts.

Unique Trapped Dimer State of the Photogenerated Hole in Hybrid Orthorhombic CH3NH3PbI3 Perovskite: Identification, Origin, and Implications

Revealing the innate character and transport of the photogenerated hole is essential to boost the high photovoltaic performance in the lead-based organohalide perovskite. However, knowledge at the atomic level is currently very limited. In this work, we systematically investigate the properties of the photogenerated hole in the orthorhombic CH₃NH₃PbI₃ using hybrid functional PBE0 calculations with spin-orbit coupling included. An unexpected trapping state of the hole, localized as I₂^- (I dimer), is uncovered, which was never reported in photovoltaic materials. It is shown that this localized configuration is energetically more favorable than that of the delocalized hole state by 191 meV and that it can highly promote the diffusion of the hole with an energy barrier as low as 131 meV. Furthermore, the origin of I dimer formation upon trapping of the hole is rationalized in terms of electronic and geometric effects, and a good linear correlation is found between the hole trapping capacity and the accompanying structural deformation in CH₃NH₃PbX₃ (X = Cl, Br, and I). It is demonstrated that good CH₃NH₃PbX₃ materials for the hole diffusion should have small structural deformation energy and weak hole trapping capacity, which may facilitate the rational screening of superior photovoltaic perovskites.

Visualizing the Nano Cocatalyst Aligned Electric Fields on Single Photocatalyst Particles

The cocatalysts or dual cocatalysts of photocatalysts are indispensable for high efficiency in artificial photosynthesis for solar fuel production. However, the reaction activity increased by cocatalysts cannot be directly ascribed to the accelerated catalytic kinetics, since photogenerated charges are involved in the elementary steps of photocatalytic reactions. To date, diverging views about cocatalysts show that their exact role for photocatalysis is not well understood yet. Herein, we image directly the local separation of photogenerated charge carriers across single crystals of the BiVO₄ photocatalyst which loaded locally with nanoparticles of a MnO_x single cocatalyst or with nanoparticles of a spatially separated MnO_x and Pt dual cocatalyst. The deposition of the single cocatalyst resulted not only in a strong increase of the interfacial charge transfer but also, surprisingly, in a change of the direction of built-in electric fields beneath the uncovered surface of the photocatalyst. The additive electric fields caused a strong increase of local surface photovoltage signals (up to 80 times) and correlated with the increase of the photocatalytic performance. The local electric fields were further increased (up to 2.5 kV·cm^-1) by a synergetic effect of the spatially separated dual cocatalysts. The results reveal that cocatalyst has a conclusive effect on charge separation in photocatalyst particle by aligning the vectors of built-in electric fields in the photocatalyst particle. This effect is beyond its catalytic function in thermal catalysis.

Facet Engineered Interface Design of Plasmonic Metal and Cocatalyst on BiOCl Nanoplates for Enhanced Visible Photocatalytic Oxygen Evolution

Integration of plasmonic metal and cocatalyst with semiconductor is a promising approach to simultaneously optimize the generation, transfer, and consumption of photoinduced charge carriers for high-performance photocatalysis. The photocatalytic activities of the designed hybrid structures are greatly determined by the efficiencies of charge transfer across the interfaces between different components. In this paper, interface design of Ag-BiOCl-PdO_x hybrid photocatalysts is demonstrated based on the choice of suitable BiOCl facets in depositing plasmonic Ag and PdO_x cocatalyst, respectively. It is found that the selective deposition of Ag and PdO_x on BiOCl(110) planes realizes the superior photocatalytic activity in O₂ evolution compared with the samples with other Ag and PdO_x deposition locations. The reason was the superior hole transfer abilities of Ag-(110)BiOCl and BiOCl(110)-PdO_x interfaces in comparison with those of Ag-(001)BiOCl and BiOCl(001)-PdO_x interfaces. Two effects are proposed to contribute to this enhancement: (1) stronger electronic coupling at the BiOCl(110)-based interfaces resulted from the thinner contact barrier layer and (2) the shortest average hole diffuse distance realized by Ag and PdO_x on BiOCl(110) planes. This work represents a step toward the interface design of high-performance photocatalyst through facet engineering.

Recent Progress in Metal-Organic Frameworks for Applications in Electrocatalytic and Photocatalytic Water Splitting

The development of clean and renewable energy materials as alternatives to fossil fuels is foreseen as a potential solution to the crucial problems of environmental pollution and energy shortages. Hydrogen is an ideal energy material for the future, and water splitting using solar/electrical energy is one way to generate hydrogen. Metal-organic frameworks (MOFs) are a class of porous materials with unique properties that have received rapidly growing attention in recent years for applications in water splitting due to their remarkable design flexibility, ultra-large surface-to-volume ratios and tunable pore channels. This review focuses on recent progress in the application of MOFs in electrocatalytic and photocatalytic water splitting for hydrogen generation, including both oxygen and hydrogen evolution. It starts with the fundamentals of electrocatalytic and photocatalytic water splitting and the related factors to determine the catalytic activity. The recent progress in the exploitation of MOFs for water splitting is then summarized, and strategies for designing MOF-based catalysts for electrocatalytic and photocatalytic water splitting are presented. Finally, major challenges in the field of water splitting are highlighted, and some perspectives of MOF-based catalysts for water splitting are proposed.

Facet-selective interface design of a BiOI(110)/Br-Bi2O2CO3(110) p–n heterojunction photocatalyst

A BiOI₍₁₁₀₎/Br-BOC₍₁₁₀₎ p–n heterostructure photocatalyst with a high interface quality was designed and synthesized by facet-dependent selective adsorption.

Controllable nanothorns on TiO2mesocrystals for efficient charge separation in hydrogen evolution

Herein, we investigated that sheet-like TiO₂mesocrystals with controllable nanothorns on the {101} facet during the topotactic transformation exhibit facet-induced charge separation and anisotropic electron flow, realizing the superior facet-dependent photocatalysis in solar energy conversion.

Facet-Engineered Surface and Interface Design of Photocatalytic Materials

The facet-engineered surface and interface design for photocatalytic materials has been proven as a versatile approach to enhance their photocatalytic performance. This review article encompasses some recent advances in the facet engineering that has been performed to control the surface of mono-component semiconductor systems and to design the surface and interface structures of multi-component heterostructures toward photocatalytic applications. The review begins with some key points which should receive attention in the facet engineering on photocatalytic materials. We then discuss the synthetic approaches to achieve the facet control associated with the surface and interface design. In the following section, the facet-engineered surface design on mono-component photocatalytic materials is introduced, which forms a basis for the discussion on more complex systems. Subsequently, we elucidate the facet-engineered surface and interface design of multi-component photocatalytic materials. Finally, the existing challenges and future prospects are discussed.

Surface and interface design in cocatalysts for photocatalytic water splitting and CO2reduction

This review outlines the recent progress on designing the surface and interface of cocatalysts to create highly efficient photocatalysts for water splitting and CO₂reduction.