Hollow Cobalt-Based Bimetallic Sulfide Polyhedra for Efficient All-pH-Value Electrochemical and Photocatalytic Hydrogen Evolution

Operando Spectroscopic Monitoring of Metal Chalcogenides for Overall Water Splitting: New Views of Active Species and Sites

Metal-based chalcogenides exhibit great promise for overall water splitting, yet their intrinsic catalytic reaction mechanisms remain to be fully understood. In this work, we employed operando X-ray absorption (XAS) and in situ Raman spectroscopy to elucidate the structure-activity relationships of low-crystalline cobalt sulfide (L-CoS) catalysts toward overall water splitting. The operando results for L-CoS catalyzing the alkaline hydrogen evolution reaction (HER) demonstrate that the cobalt centers in the bulk are predominantly coordinated by sulfur atoms, which undergo a kinetic structural rearrangement to generate metallic cobalt in S-Co-Co-S moieties as the true catalytically active species. In comparison, during the acidic HER, L-CoS undergoes local structural optimization of Co centers, and H2 production proceeds with adsorption/desorption of key intermediates atop the Co-S-Co configurations. Further operando characterizations highlight the crucial formation of high-valent Co4+ species in L-CoS for the alkaline oxygen evolution reaction (OER), and the formation of such active species was found to be far more facile than in crystalline Co3O4 and Co-LDH references. These insights offer a clear picture of the complexity of active species and site formation in different media, and demonstrate how their restructuring influences the catalytic activity.

Highly efficient nitrogen fixation over S-scheme heterojunction photocatalysts with enhanced active hydrogen supply

Photocatalytic N2 fixation is a promising strategy for ammonia (NH3) synthesis; however, it suffers from relatively low ammonia yield due to the difficulty in the design of photocatalysts with both high charge transfer efficiency and desirable N2 adsorption/activation capability. Herein, an S-scheme CoSx/ZnS heterojunction with dual active sites is designed as an efficient N2 fixation photocatalyst. The CoSx/ZnS heterojunction exhibits a unique pocket-like nanostructure with small ZnS nanocrystals adhered on a single-hole CoSx hollow dodecahedron. Within the heterojunction, the electronic interaction between ZnS and CoSx creates electron-deficient Zn sites with enhanced N2 chemisorption and electron-sufficient Co sites with active hydrogen supply for N2 hydrogenation, cooperatively reducing the energy barrier for N2 activation. In combination with the promoted photogenerated electron-hole separation of the S-scheme heterojunction and facilitated mass transfer by the pocket-like nanostructure, an excellent N2 fixation performance with a high NH3 yield of 1175.37 μmol g-1 h-1 is achieved. This study provides new insights into the design of heterojunction photocatalysts for N2 fixation.

Donor-acceptor moiety functionalized covalent organic frameworks for boosting charge separation and H2 photogeneration

Covalent organic frameworks (COFs) have a broad prospect to be used as a photocatalytic platform to convert solar energy into valuable chemicals due to their tunable structures and rich active catalytic sites. However, constructing COFs with tuned sp2-carbon donor-acceptor moiety remains an enormous challenge. Herein, we synthesized two new fully π-conjugated cyano-ethylene-linked COFs containing benzotrithiophene as functional group by Knoevenagel polycondensation reaction. The accetpor 2,2'-bipyridine unit in BTT-BpyDAN-COF skeleton favored the formation of a intermolecular specific electron transport pathway with the donor benzotrithiophene, and thereby promoted charge separation and transfer efficiency. Specifically, a donor-acceptor (D-A) type BTT-BpyDAN-COF exhibited high hydrogen evolution rate of 10.1 mmol g-1h-1 and an excellent apparent quantum efficiency of 4.83 % under visible light irradiation.

Sulfur vacancy-enhanced In2S3-x hollow microtubes for photocatalytic Cr (VI) and tetracycline removal

Morphological regulation and defect engineering are efficient methods for photocatalytic technology by improving photon absorption and electron dissociation. Herein, In2S3-x hollow microtubes with S-vacancies (MIS) were fabricated via a simple solvothermal reaction using In-based metal-organic frameworks (In-MOFs) as a precursor. Experimental results demonstrate that the hollow structure and optimal S-vacancies can jointly accelerate the photocatalytic reaction, attributed to a larger specific surface area, more active sites, and faster electron transfer efficiency. The champion MIS(2) displayed significantly better photocatalytic activity for Cr(VI) reduction and tetracycline (TC) degradation. The Cr(VI) reduction rate by MIS(2) is 3.67 and 2.82 times higher than those of optimal In2S3 template-free (HIS(2)) and MIS(1) with poor S-vacancies, respectively. The removal efficiency of TC by MIS(2) is 1.37 and 1.15 times higher than those of HIS(2) and MIS(1). Further integration of MIS(2) with aerogel simplifies the recovery process significantly.

Recent developments, advances and strategies in heterogeneous photocatalysts for water splitting

Photocatalytic water splitting (PWS) is an up-and-coming technology for generating sustainable fuel using light energy. Significant progress has been made in the developing of PWS innovations over recent years. In addition to various water-splitting (WS) systems, the focus has primarily been on one- and two-steps-excitation WS systems. These systems utilize singular or composite photocatalysts for WS, which is a simple, feasible, and cost-effective method for efficiently converting prevalent green energy into sustainable H2 energy on a large commercial scale. The proposed principle of charge confinement and transformation should be implemented dynamically by conjugating and stimulating the photocatalytic process while ensuring no unintentional connection at the interface. This study focuses on overall water splitting (OWS) using one/two-steps excitation and various techniques. It also discusses the current advancements in the development of new light-absorbing materials and provides perspectives and approaches for isolating photoinduced charges. This article explores multiple aspects of advancement, encompassing both chemical and physical changes, environmental factors, different photocatalyst types, and distinct parameters affecting PWS. Significant factors for achieving an efficient photocatalytic process under detrimental conditions, (e.g., strong light absorption, and synthesis of structures with a nanometer scale. Future research will focus on developing novel materials, investigating potential synthesis techniques, and improving existing high-energy raw materials. The endeavors aim is to enhance the efficiency of energy conversion, the absorption of radiation, and the coherence of physiochemical processes.

NiRu-Mo2Ti2C3O2 as an efficient catalyst for alkaline hydrogen evolution reactions: the role of bimetallic site interactions in promoting Volmer-step kinetics

The Volmer step in alkaline hydrogen evolution reactions (HERs), which supplies H* to the following steps by cleaving H-O-H bonds, is considered the rate-determining step of the overall reaction. The Volmer step involves water dissociation and adsorbed hydroxyl (*OH) desorption; Ru-based catalysts display a compelling water dissociation process in an alkaline HER. Unfortunately, the strong affinity of Ru for *OH blocks the active sites, resulting in unsatisfactory performance during HER processes. Hence, this study investigates a series of key descriptors (ΔG*H2O, ΔG*H-OH, ΔG*H, and ΔG*OH) of TM (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, or Pt)-Ru/Mo2Ti2C3O2 to systematically explore the effects of bimetallic site interactions on the kinetics of the Volmer step. The results indicate that bimetallic catalysts effectively reduced the strong adsorption of *OH on Ru sites; especially, the NiRu diatomic state shows the highest electron-donating ability, which promoted the smooth migration of *OH from Ru sites to Ni sites. Therefore, Ru, Ni and MXenes are suitable to serve as water adsorption and dissociation sites, *OH desorption sites, and H2 release sites, respectively. Ultimately, NiRu/Mo2Ti2C3O2 promotes Volmer kinetics and has the potential to improve alkaline HERs. This work provides theoretical support for the construction of synergistic MXene-based diatomic catalysts and their wide application in the field of alkaline HERs.

Boosted hydrogen evolution kinetics of heteroatom-doped carbons with isolated Zn as an accelerant

Carbon-based single-atom catalysts, a promising candidate in electrocatalysis, offer insights into electron-donating effects of metal center on adjacent atoms. Herein, we present a practical strategy to rationally design a model catalyst with a single zinc (Zn) atom coordinated with nitrogen and sulfur atoms in a multilevel carbon matrix. The Zn site exhibits an atomic interface configuration of ZnN4S1, where Zn's electron injection effect enables thermal-neutral hydrogen adsorption on neighboring atoms, pushing the activity boundaries of carbon electrocatalysts toward electrochemical hydrogen evolution to an unprecedented level. Experimental and theoretical analyses confirm the low-barrier Volmer-Tafel mechanism of proton reduction, while the multishell hollow structures facilitate the hydrogen evolution even at high current intensities. This work provides insights for understanding the actual active species during hydrogen evolution reaction and paves the way for designing high-performance electrocatalysts.

RuSe2/CeO2heterostructure as a novel electrocatalyst for highly efficient alkaline hydrogen evolution

Constructing heterojunction to adjust the electronic structure of catalysts is a promising strategy for synergistically improving electrocatalytic activity. In addition, RuSe2is recognized as an effective alternative to Pt for boosting alkaline hydrogen evolution reaction (HER) on account of its outstanding catalytic properties. Herein, novel RuSe2/CeO2heterojunction electrocatalysts are fabricated through hydrothermal and thermal treatment methods. The optimal 50% RuSe2/CeO2heterojunction electrocatalyst exhibits a low HER overpotential of 16 mV to attain 10 mA cm-2current density and Tafel slope of 66.1 mV dec-1for hydrogen evolution in 1.0 M KOH. At the same time, the 50% RuSe2/CeO2heterojunction electrocatalyst also maintains a stable HER activity for 50 h or 3000 CV cycles. The experimental results show that formation of heterogeneous interface between RuSe2and CeO2results in the redistribution of electrons at the RuSe2/CeO2interface, thereby changing the electronic structure of RuSe2and enhancing the performance of the RuSe2/CeO2electrocatalyst. This work may provide a feasible way to design efficient hydrogen evolution heterojunction electrocatalysts by modulating the electronic structure in alkaline electrolytes.

Trimetallic MOF-derived CoFeNi/Z-P NC nanocomposites as efficient catalysts for oxygen evolution reaction

We used sodium hydroxide-mediated approach and tannic acid etching to prepare hollow structured trimetallic MOF-derived CoFeNi/Z-P NC nanocomposites. Remarkably, the resulting CoFeNi/Z-P NC nanocomposites have large specific surface area and mesoporous structure, making their active sites more accessible and mass transfer more effective. More complex trimetallic components provide greater possibilities for further improving electrocatalytic performance. The CoFeNi/Z-P NC nanocomposites demonstrate notable enhancements for the OER, and 10 mA cm-2 current density is achieved at a low overpotential of 244 mV, with a low Tafel slope of 66.2 mV dec-1 and have good stability in alkaline solutions. In addition, as a cathode material for overall alkaline water splitting, CoFeNi/Z-P NC is better than RuO2 with longer cycling stability.

MOF-derived S-doped NiCo2O4 hollow cubic nanocage for highly efficient electrocatalytic oxygen evolution

Inducing the surface reconstruction of spinels is critical for improving the electrocatalytic oxygen evolution reaction (OER) activity. Herein, S-doped NiCo2O4 hollow cubic nanocage was synthesized by anion etching Metal-Organic Frameworks (MOFs) template and air annealing strategies. The hollow structure possesses a large specific surface area and pore size, facilitating active site exposure and mass transport. S2- doping regulates the electronic structure, reducing the oxidation potential of Ni sites during the OER process, thus promoting the surface reconstruction into γ-NiOOH active species. Meanwhile, S2- doping enhances conductivity, accelerating interfacial charge transfer. As a result, S-NiCo2O4-6 exhibits superior OER activity (262 mV overpotential @ 10 mA cm-2) and stability in 1.0 M KOH solution. Furthermore, 20 % Pt/C‖S-NiCo2O4-6 only needs 1.832 V to achieve 50 mA (the electrochemical active area is 4 cm2) in a homemade anion exchange membrane (AEM) electrolyzer. This work proposes a novel approach for preparing efficient anion-doped spinel-based OER electrocatalysts.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Metal organic framework-derived transition metal-doped CoSx nanocage for enhanced visible light-assisted methanol electrocatalytic oxidation

Designing noble metal-free anode catalysts for visible light-assisted direct methanol fuel cells still remains a significant challenge. In this study, combining the photocatalytic and electrocatalytic properties of CoSx, a visible light-assisted methanol electrocatalytic oxidation strategy was provided. Doping engineering was employed to adjust the electronic structure of CoSx and improve their photoassisted methanol electrocatalytic oxidation activity. Using ZIF-67 as precursor, transition metal-doped CoSx (M-CoSx, M = Zn, Cu, Ni, and Cd) nanocage was synthesized by cation exchange and L-cysteine-controlled etching. Cd doping not only widens the light adsorption to the visible region but also enhances the separation efficiency of photogenerated electron-hole pairs. The electrochemical and photochemical results indicated that the strong oxidative photogenerated hole, OH˙, and O2˙- are beneficial for methanol electrocatalytic oxidation. The synergistic electrocatalytic and photocatalytic effect will be a practical strategy for improving the methanol electrocatalytic oxidation activity of noble metal-free semiconductor catalysts.

Principles of Design and Synthesis of Metal Derivatives from MOFs

Materials derived from metal-organic frameworks (MOFs) have demonstrated exceptional structural variety and complexity and can be synthesized using low-cost scalable methods. Although the inherent instability and low electrical conductivity of MOFs are largely responsible for their low uptake for catalysis and energy storage, a superior alternative is MOF-derived metal-based derivatives (MDs) as these can retain the complex nanostructures of MOFs while exhibiting stability and electrical conductivities of several orders of magnitude higher. The present work comprehensively reviews MDs in terms of synthesis and their nanostructural design, including oxides, sulfides, phosphides, nitrides, carbides, transition metals, and other minor species. The focal point of the approach is the identification and rationalization of the design parameters that lead to the generation of optimal compositions, structures, nanostructures, and resultant performance parameters. The aim of this approach is to provide an inclusive platform for the strategies to design and process these materials for specific applications. This work is complemented by detailed figures that both summarize the design and processing approaches that have been reported and indicate potential trajectories for development. The work is also supported by comprehensive and up-to-date tabular coverage of the reported studies.

Ethanol-Induced Ni2+ -Intercalated Cobalt Organic Frameworks on Vanadium Pentoxide for Synergistically Enhancing the Performance of 3D-Printed Micro-Supercapacitors

The synthesis of metal-organic framework (MOF) nanocomposites with high energy density and excellent mechanical strength is limited by the degree of lattice matching and crystal surface structure. In this study, dodecahedral ZIF-67 is synthesized uniformly on vanadium pentoxide nanowires. The influence of the coordination mode on the surface of ZIF-67 in ethanol is also investigated. Benefitting from the different coordination abilities of Ni²⁺ , Co²⁺ , and N atoms, spatially separated surface-active sites are created through metal-ion exchange. Furthermore, the incompatibility between the d⁸ electronic configuration of Ni²⁺ and the three-dimensional (3D) structure of ZIF-67 afforded the synthesis of hollow structures by controlling the amount of Ni doping. The formation of NiCo-MOF@CoOOH@V₂ O₅ nanocomposites is confirmed using X-ray absorption fine structure analysis. The high performance of the obtained composite is illustrated by fabricating a 3D-printed micro-supercapacitor, exhibiting a high area specific capacitance of 585 mF cm^-2 and energy density of 159.23 µWh cm^-2 (at power density = 0.34 mW cm^-2 ). The solvent/coordination tuning strategy demonstrated in this study provides a new direction for the synthesis of high-performance nanomaterials for electrochemical energy storage applications.

Morphology Engineering in Multicomponent Hollow Metal Chalcogenide Nanoparticles

Hollow metal chalcogenide nanoparticles are widely applicable in environmental and energy-related processes. Herein, we synthesized such particles with large compositional and morphological diversity by combining scanning probe block copolymer lithography with a Kirkendall effect-based sulfidation process. We explored the influence of temperature-dependent diffusion kinetics, elemental composition and miscibility, and phase boundaries on the resulting particle morphologies. Specifically, CoNi alloys form single-shell sulfides for the synthetic conditions explored because Co and Ni exhibit similar diffusion rates, while CuNi alloys form sulfides with various types of morphologies (yolk-shell, double-shell, and single-shell) because Cu and Ni have different diffusion rates. In contrast, Co-Cu heterodimers form hollow heterostructured sulfides with varying void numbers and locations depending on synthesis temperature and phase boundary. At higher temperatures, the increased miscibility of CoS₂ and CuS makes it energetically favorable for the heterostructure to adopt a single alloy shell morphology, which is rationalized using density functional theory-based calculations.

Ensemble Effect of Ruthenium Single-Atom and Nanoparticle Catalysts for Efficient Hydrogen Evolution in Neutral Media

Hydrogen evolution reaction (HER) plays a key role in electrochemical water splitting, which is a sustainable way for hydrogen production. The kinetics of HER is sluggish in neutral media that requires noble metal catalysts to alleviate energy consumption during the HER process. Here, we present a catalyst comprising a ruthenium single atom (Ru₁) and nanoparticle (Ru_n) loaded on the nitrogen-doped carbon substrate (Ru₁-Ru_n/CN), which exhibits excellent activity and superior durability for neutral HER. Benefiting from the synergistic effect between single atoms and nanoparticles in the Ru₁-Ru_n/CN, the catalyst exhibits a very low overpotential down to 32 mV at a current density of 10 mA cm^-2 while maintaining excellent stability up to 700 h at a current density of 20 mA cm^-2 during the long-term test. Computational calculations reveal that, in the Ru₁-Ru_n/CN catalyst, the existence of Ru nanoparticles affects the interactions between Ru single-atom sites and reactants and thus improves the catalytic activity of HER. This work highlights the ensemble effect of electrocatalysts for HER and could shed light on the rational design of efficient catalysts for other multistep electrochemical reactions.

Heterointerface and Tensile Strain Effects Synergistically Enhances Overall Water-Splitting in Ru/RuO2 Aerogels

Designing robust electrocatalysts for water-splitting is essential for sustainable hydrogen generation, yet difficult to accomplish. In this study, a fast and facile two-step technique to synthesize Ru/RuO₂ aerogels for catalyzing overall water-splitting under alkaline conditions is reported. Benefiting from the synergistic combination of high porosity, heterointerface, and tensile strain effects, the Ru/RuO₂ aerogel exhibits low overpotential for oxygen evolution reaction (189 mV) and hydrogen evolution reaction (34 mV) at 10 mA cm^-2 , surpassing RuO₂ (338 mV) and Pt/C (53 mV), respectively. Notably, when the Ru/RuO₂ aerogels are applied at the anode and cathode, the resultant water-splitting cell reflected a low potential of 1.47 V at 10 mA cm^-2 , exceeding the commercial Pt/C||RuO₂ standard (1.63 V). X-ray adsorption spectroscopy and theoretical studies demonstrate that the heterointerface of Ru/RuO₂ optimizes charge redistribution, which reduces the energy barriers for hydrogen and oxygen intermediates, thereby enhancing oxygen and hydrogen evolution reaction kinetics.

Collaborative hydrothermal and calcination fabrication of ZnOS heterostructures for visible-light-driven H2 production

Photocatalytic water splitting is forecasted as a promising strategy for H₂ production. In this work, novel zinc oxide/zinc sulfide (ZnOS-x) (x = 1, 2, 3 and 4) heterostructures were fabricated by a collaborative hydrothermal and calcination method with different amounts of trithiocyanuric acid. The formation of ZnOS-x heterostructures was confirmed by PXRD, XPS, and HRTEM. Moreover, ZnOS-3 nanoparticles exhibited homogeneous and smooth surface morphology structure. ZnOS-3 displayed efficient charge separation and transfer efficiency upon photoinduction. ZnOS-3 showed the highest average H₂ evolution reaction rate (78.87 μmol h^-1) under visible-light irradiation, which increased with increase in the ratio of trithiocyanuric acid in the ZnOS-x series. This work provides a new insight to prepare uniformly integrated heterostructures of metal oxides/sulfides for visible-light-driven H₂ generation.

Recent advances in understanding and design of efficient hydrogen evolution electrocatalysts for water splitting: A comprehensive review

An unsustainable reliance on fossil fuels is the primary cause of the vast majority of greenhouse gas emissions, which in turn lead to climate change. Green hydrogen (H₂), which may be generated by electrolyzing water with renewable power sources, is a possible substitute for fossil fuels. On the other hand, the increasing intricacy of hydrogen evolution electrocatalysts that are presently being explored makes it more challenging to integrate catalytic theories, catalytic fabrication procedures, and characterization techniques. This review will initially present the thermodynamics, kinetics, and associated electrical and structural characteristics for HER electrocatalysts before highlighting design approaches for the electrocatalysts. Secondly, an in-depth discussion regarding the rational design, synthesis, mechanistic insight, and performance improvement of electrocatalysts is centered on both the intrinsic and extrinsic influences. Thirdly, the most recent technological advances in electrocatalytic water-splitting approaches are described. Finally, the difficulties and possibilities associated with generating extremely effective HER electrocatalysts for water-splitting applications are discussed.

Hollow CoFe-based hybrid composites derived from unique S-modulated coordinated transition bimetal complexes for efficient oxygen evolution from water splitting under alkaline conditions

The oxygen evolution reaction (OER) is an important reaction in water splitting. However, the high cost and slow-rate catalysts hinder commercial applications. Although an important process for manufacturing of hollow structures, it is difficult to construct complicated hollow structures with an excellent and regulable shape for multi-component materials. In this study, we demonstrate that sulfur-Co,Fe bimetallic nitrogen carbon hollow composite hybrids (x-S-CoFe@NC) can be synthesized by regulating the amount of sulfur and using the hydrothermal method. For OER, 32-S-CoFe@NC exhibits excellent electrocatalytic activity with a low overpotential of 232 mV, which is higher than those of 0-S-CoFe@NC (270 mV), 23-S-CoFe@NC (247 mV), and RuO₂ (243 mV) catalysts at 10 mA cm^-2. In addition, with air as the cathode, a rechargeable Zn-air battery with outstanding long-life cycling stability for 80 hours based on 32-CoFe@NC + Pt/C is proposed. The advanced technique described here supplies a new route for preparing hollow transition bimetal carbon hybrids with an adjustable composite arrangement for electrocatalysis and water splitting.

A hollow Co3-xCuxS4 with glutathione depleting and photothermal properties for synergistic dual-enhanced chemodynamic/photothermal cancer therapy

Chemodynamic therapy has become an emerging cancer treatment strategy, in which tumor cells are killed through toxic reactive oxygen species (ROS), especially hydroxyl radicals (˙OH) produced by the Fenton reaction. Nevertheless, low ROS generation efficiency and ROS depletion by cellular antioxidant systems are still the main obstacles in chemodynamic therapy. In the present work, we propose a dually enhanced chemodynamic therapy obtained by inhibiting ˙OH consumption and promoting ˙OH production based on the administration of bimetallic sulfide Co_3-xCu_xS₄ nanoparticles functionalized by polyethylene glycol. These bimetallic nanoparticles display glutathione depleting and photothermal properties. The nanoparticles are gradually degraded in a tumor microenvironment, resulting in Co²⁺ and Cu²⁺ release. The released Co²⁺ triggers a Fenton-like reaction that turns endogenous hydrogen peroxide into highly toxic ˙OH. In the cellular environment, Cu²⁺ ions are reduced to Cu⁺ by endogenous GSH, which decreases the intracellular antioxidant capacity and additionally up-regulates ˙OH production via the Cu⁺-induced Fenton-like reaction. Moreover, under near-infrared light irradiation, the bimetallic nanoparticles display a photothermal conversion efficacy of 46.7%, which not only improves chemodynamic therapy via boosting a Fenton-like reaction but results in photothermal therapy through hyperthermia. Both in vitro cancer cell killing and in vivo tumor ablation experiments show that the bimetallic nanoparticles display outstanding therapeutic efficacy and negligible systemic toxicity, indicating their anticancer potential.

Metal-organic framework nanocrystal-derived hollow porous materials: Synthetic strategies and emerging applications

Metal-organic frameworks (MOFs) have garnered multidisciplinary attention due to their structural tailorability, controlled pore size, and physicochemical functions, and their inherent properties can be exploited by applying them as precursors and/or templates for fabricating derived hollow porous nanomaterials. The fascinating, functional properties and applications of MOF-derived hollow porous materials primarily lie in their chemical composition, hollow character, and unique porous structure. Herein, a comprehensive overview of the synthetic strategies and emerging applications of hollow porous materials derived from MOF-based templates and/or precursors is given. Based on the role of MOFs in the preparation of hollow porous materials, the synthetic strategies are described in detail, including (1) MOFs as removable templates, (2) MOF nanocrystals as both self-sacrificing templates and precursors, (3) MOF@secondary-component core-shell composites as precursors, and (4) hollow MOF nanocrystals and their composites as precursors. Subsequently, the applications of these hollow porous materials for chemical catalysis, electrocatalysis, energy storage and conversion, and environmental management are presented. Finally, a perspective on the research challenges and future opportunities and prospects for MOF-derived hollow materials is provided.

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MOFs have garnered multi-disciplinary attention due to their unique inherent properties

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Various synthetic strategies of MOFs-derived hollow porous materials are summarized

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Emerging applications of MOFs-derived hollow porous materials are reviewed

2D Network overtakes 3D for photocatalytic hydrogen evolution

3-Dimensional (3D) cyanide coordination polymers, typically known as Prussian blue Analogues (PBAs), have received great attention in catalysis due to their stability, easily tuned metal sites, and porosity. However, their high crystallinities and relatively low number of surface-active sites significantly hamper their intrinsic catalytic activities. Herein, we report the utilization of a 2-dimensional (2D) layered cobalt tetracyanonickelate, [Co-Ni], for the reduction of protons to H₂. Relying on its exposed facets, layered morphology, and abundant surface-active sites, [Co-Ni] can efficiently convert water and sunlight to H₂ in the presence of a ruthenium photosensitizer (Ru PS) with an optimal evolution rate of 30 029 ± 590 μmol g^-1 h^-1, greatly exceeding that of 3D Co-Fe PBA [Co-Fe] and Co-Co PBA [Co-Co]. Furthermore, [Co-Ni] retains its structural integrity throughout a 6 hour photocatalytic cycle, which is confirmed by XPS, PXRD, and Infrared analysis. This recent work reveals the excellent morphologic properties that promote [Co-Ni] as an attractive catalyst for the hydrogen evolution reaction (HER).

Synergy of heterojunction and interfacial strain for boosting photocatalytic H2 evolution of black phosphorus nanosheets

As an emerging post-graphene two-dimensional material, black phosphorus (BP) has attracted enormous interest as a promising cocatalyst for photocatalytic hydrogen (H₂) evolution, however, the activity of either pristine bulk or BP nanosheets is far from satisfactory. Herein, we present an effective strategy to greatly boost the H₂ evolution performance of BP via applying the synergistic effect of heterojunction and interfacial lattice strain. A multilayered heterostructure coupling BP nanosheets and nickel oxide (NiO) nanosheets with abundant interface P-Ni and PO bonds is synthesized and utilized as a proof-of-concept material for our design. Both the experimental and theoretical results have revealed that the strain is formed in BP-NiO multilayered heterostructure. The generated lattice strain induces the charge redistribution at the interface between BP and NiO, which leads to the improved electron transfer efficiency and favorable H* adsorption kinetics for photocatalytic H₂ evolution reaction. As a result, the BP-NiO heterostructure with strain effect exhibits much enhanced photocatalytic H₂ evolution activity in the presence of Eosin Y (EY) as photosensitizer, exceeding that of zero-strained BP/NiO heterostructure and many other reported noble-metal-free cocatalyst.

Rational Design of Better Hydrogen Evolution Electrocatalysts for Water Splitting: A Review

The excessive dependence on fossil fuels contributes to the majority of CO₂ emissions, influencing on the climate change. One promising alternative to fossil fuels is green hydrogen, which can be produced through water electrolysis from renewable electricity. However, the variety and complexity of hydrogen evolution electrocatalysts currently studied increases the difficulty in the integration of catalytic theory, catalyst design and preparation, and characterization methods. Herein, this review first highlights design principles for hydrogen evolution reaction (HER) electrocatalysts, presenting the thermodynamics, kinetics, and related electronic and structural descriptors for HER. Second, the reasonable design, preparation, mechanistic understanding, and performance enhancement of electrocatalysts are deeply discussed based on intrinsic and extrinsic effects. Third, recent advancements in the electrocatalytic water splitting technology are further discussed briefly. Finally, the challenges and perspectives of the development of highly efficient hydrogen evolution electrocatalysts for water splitting are proposed.

Cyclic voltammetry electrodynamic deposition of Co9-xMnxS8 nanosheet arrays for electrocatalytic hydrogen evolution

Room-temperature cyclic voltammetry (CV) electrodynamic deposition is proposed herein for the first time to deposit Co_9-xMn_xS₈ nanosheet arrays. The incorporation of Mn spin states induces atomic distortion, which contributes to more active edge sites and the fine-tuning of the electronic structure. The Co_9-xMn_xS₈ (x = 4.5) catalyst exhibits enhanced catalytic activity toward the hydrogen evolution reaction (HER) when compared with pristine Co₉S₈. This work offers a promising strategy for the design of highly efficient HER electrocatalysts.

Critical Review, Recent Updates on Zeolitic Imidazolate Framework-67 (ZIF-67) and Its Derivatives for Electrochemical Water Splitting

Design and construction of low-cost electrocatalysts with high catalytic activity and long-term stability is a challenging task in the field of catalysis. Metal-organic frameworks (MOF) are promising candidates as precursor materials in the development of highly efficient electrocatalysts for energy conversion and storage applications. This review starts with a summary of basic concepts and key evaluation parameters involved in the electrochemical water-splitting reaction. Then, different synthesis approaches reported for the cobalt-based Zeolitic imidazolate framework (ZIF-67) and its derivatives are critically reviewed. Additionally, several strategies employed to enhance the electrocatalytic activity and stability of ZIF-67-based electrocatalysts are discussed in detail. The present review provides a succinct insight into the ZIF-67 and its derivatives (oxides, hydroxides, sulfides, selenides, phosphide, nitrides, telluride, heteroatom/metal-doped carbon, noble metal-supported ZIF-67 derivatives) reported for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting applications. Finally, this review concludes with the associated challenges and the perspectives on developing the best economic, durable electrocatalytic materials.

Electrocatalytic Water Splitting: From Harsh and Mild Conditions to Natural Seawater

Electrocatalytic water splitting is regarded as the most effective pathway to generate green energy-hydrogen-which is considered as one of the most promising clean energy solutions to the world's energy crisis and climate change mitigation. Although electrocatalytic water splitting has been proposed for decades, large-scale industrial hydrogen production is hindered by high electricity cost, capital investment, and electrolysis media. Harsh conditions (strong acid/alkaline) are widely used in electrocatalytic mechanism studies, and excellent catalytic activities and efficiencies have been achieved. However, the practical application of electrocatalytic water splitting in harsh conditions encounters several obstacles, such as corrosion issues, catalyst stability, and membrane technical difficulties. Thus, the research on water splitting in mild conditions (neutral/near neutral), even in natural seawater, has aroused increasing attention. However, the mechanism in mild conditions or natural seawater is not clear. Herein, different conditions in electrocatalytic water splitting are reviewed and the effects and proposed mechanisms in the three conditions are summarized. Then, a comparison of the reaction process and the effects of the ions in different electrolytes are presented. Finally, the challenges and opportunities associated with direct electrocatalytic natural seawater splitting and the perspective are presented to promote the progress of hydrogen production by water splitting.

Dynamics and control of active sites in hierarchically nanostructured cobalt phosphide/chalcogenide-based electrocatalysts for water splitting

The rational design of efficient electrocatalysts for industrial water splitting is essential to generate sustainable hydrogen fuel. However, a comprehensive understanding of the complex catalytic mechanisms under harsh reaction conditions remains a major challenge. We apply a self-templated strategy to introduce hierarchically nanostructured "all-surface" Fe-doped cobalt phosphide nanoboxes (Co@CoFe-P NBs) as alternative electrocatalysts for industrial-scale applications. Operando Raman spectroscopy and X-ray absorption spectroscopy (XAS) experiments were carried out to track the dynamics of their structural reconstruction and the real catalytically active intermediates during water splitting. Our operando analyses reveal that partial Fe substitution in cobalt phosphides promotes a structural reconstruction into P-Co-O-Fe-P configurations with low-valence metal centers (M⁰/M⁺) during the hydrogen evolution reaction (HER). Results from density functional theory (DFT) demonstrate that these in situ reconstructed configurations significantly enhance the HER performance by lowering the energy barrier for water dissociation and by facilitating the adsorption/desorption of HER intermediates (H*). The competitive activity in the oxygen evolution reaction (OER) arises from the transformation of the reconstructed P-Co-O-Fe-P configurations into oxygen-bridged, high-valence Co^IV-O-Fe^IV moieties as true active intermediates. In sharp contrast, the formation of such Co^III/IV-O-Fe^III/IV moieties in Co-FeOOH is hindered under the same conditions, which outlines the key advantages of phosphide-based electrocatalysts. Ex situ studies of the as-synthesized reference cobalt sulfides (Co-S), Fe doped cobalt selenides (Co@CoFe-Se), and Fe doped cobalt tellurides (Co@CoFe-Te) further corroborate the observed structural transformations. These insights are vital to systematically exploit the intrinsic catalytic mechanisms of non-oxide, low-cost, and robust overall water splitting electrocatalysts for future energy conversion and storage.

Process of metal-organic framework (MOF)/covalent-organic framework (COF) hybrids-based derivatives and their applications on energy transfer and storage

The fossil-fuel shortage and severe environmental issues have posed ever-increasing demands on clean and renewable energy sources, for which the exploration of electrocatalysts has been a big challenge toward energy transfer and storage. Some indispensable features of electrocatalysts, such as large surface area, controlled structure, high porosity, and effective functionalization, have been proved to be critical for the improvement of electrocatalytic activities. Recently, the rapid expansion of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous-organic polymers has provided extensive opportunities for the development of various electrocatalysts. Moreover, combining diverse descriptions of porous-organic frameworks (such as MOFs and COFs) can generate amazing and fantastic properties, affording the formed MOF/COF (including core-shell MOF@MOF and MOF@COF and layer-on-layer MOF-on-MOF or COF-on-MOF) heterostructures wide applications in diverse fields, especially in clean energy and energy transfer. To further boosts electronic conductivity, catalytic performances, and energy storage abilities, these MOF/COF hybrid materials have been widely utilized as versatile precursors for the manufacture of transition metal catalysts embedded within mesoporous carbon nitrides (M@CN_x) and porous carbon nitride frameworks (CN_x) via a facile pyrolysis process. Given that these M@CN_x and CN_x hybrids are composed of abundant catalytic centers, rich functionalities, and large specific surface areas, vast applications in energy transfer and energy storage fields can be realized. In this mini-review, we summarize the preparation strategies of MOF/COF-based hybrids, as well as their derivatives, nanostructure formation mechanism of M@CN_x and CN_x hybrids from MOF/COF-based hybrid materials, and their applications as catalysts for driving diverse reactions and electrode materials for energy storage. Further, current challenges and future prospects of applying these derivatives into energy conversion and storage devices are also discussed.

Fluorine Engineered Self-Supported Ultrathin 2D Nickel Hydroxide Nanosheets as Highly Robust and Stable Bifunctional Electrocatalysts for Oxygen Evolution and Urea Oxidation Reactions

Developing highly efficient noble-metal-free electrocatalysts with a scalable and environmentally friendly synthesis approach remains a challenge in the field of electrocatalytic water splitting. To overcome this problem, self-supported fluorine-modified 2D ultrathin nickel hydroxide (F-Ni(OH)₂ ) nanosheets (NSs) for the oxygen evolution reaction (OER) and urea oxidation reaction (UOR) are prepared with a scalable and ascendant one-step synthesis route. The enhanced redox activity, electrical conductivity and a great number of exposed active sites of the heterogeneous catalysts improve charge migration for the electrocatalytic reactions. The density of states of the d orbitals of the Ni atoms significantly increases near the Fermi level, thereby indicating that the Ni atoms near the F-dopants promote electrical conduction in the Ni(OH)₂ monolayer. The F-Ni(OH)₂ electrocatalyst exhibits notable OER and UOR activity with onset potentials of 1.43 and 1.16 V versus RHE, respectively required to reach 10 mA cm^-2 , which are comparable to those of commercial noble-metal-based electrocatalysts. With RuCo-OH nanospheres, the settled F-Ni(OH)₂ ||RuCo-OH cell requires merely 1.55 and 1.37 V to reach 10 mA cm^-2 with superb durability for 24 h in overall water and urea electrolysis, respectively. Overall, high-quality, and efficient noble-metal-free electrocatalysts for overall water and urea electrolysis can be prepared with a simple, scalable, and reproducible preparation method.

Micro/macrostructure and multicomponent design of catalysts by MOF-derived strategy: Opportunities for the application of nanomaterials-based advanced oxidation processes in wastewater treatment

Advanced oxidation processes (AOPs) have demonstrated an effective wastewater treatment method. But the application of AOPs using nanomaterials as catalysts is challenged with a series of problems, including limited mass transfer, surface fouling, poor stability, and difficult recycling. Recently, metal-organic frameworks (MOFs) with high tunability and ultrahigh porosity are emerging as excellent precursors for the delicate design of the structure/composition of catalysts and many MOF-derived catalysts with distinct physicochemical characteristics have shown optimized performance in various AOPs. Herein, to elucidate the structure-composition-performance relationship, a review on the performance optimization of MOF-derived catalysts to overcome the existing problems in AOPs by micro/macrostructure and multicomponent design is given. Impressively, MOF-derived strategy for the design of catalyst materials from the aspects of microstructure, macrostructure, and multicomponent (polymetallic, heteroatom doping, M/C hybrids, etc.) is firstly presented. Moreover, important advances of MOF-derived catalysts in the application of various AOPs (Fenton, persulfate-based AOPs, photocatalysis, electrochemical processes, hybrid AOPs) are summarized. The relationship between the unique micro/macrostructure and/or multicomponent features and performance optimization in mass transfer, catalytic efficiency, stability, and recyclability is clarified. Furthermore, the challenges and future work directions for the practical application of MOF-derived catalysts in AOPs for wastewater treatment are provided.

A Pacman-Like Titanium-Doped Cobalt Sulfide Hollow Superstructure for Electrocatalytic Oxygen Evolution

Transition-metal sulfides (TMSs) are attractive oxygen evolution reaction (OER) electrocatalysts. Developing new strategies to improve their electrochemical performance of TMSs is of great significance. Herein, a unique pacman-like titanium-doped cobalt sulfide hollow superstructure (Ti-CoSx HSS) is fabricated as an OER electrocatalyst. Using a prearranged metal-organic framework (MOF)-on-MOF heterostructure as a precursor treated by one-pot sulfidation, a sequential structural conversion process leads to the formation of Ti-CoSx HSS, which is assembled by interconnected Ti-doped CoSx nanocages around a cake-like cavity. Benefiting from the architecture and compositional advantages, Ti-CoSx HSS exhibits excellent OER performance with an overpotential of 249 mV at 10 mA cm^-2 and Tafel slope of 45.5 mV dec^-1 due to increased active site exposure, enhanced electron and mass transfer. This strategy enabled by MOF-on-MOF paves the way toward innovative MOF derivatives for various applications.