Unveiling Catalytic Sites in a Typical Hydrogen Photogeneration System Consisting of Semiconductor Quantum Dots and 3d-Metal Ions

Accelerated water dissociation kinetics by nickel-nickelous hydroxide epitaxial interfaces for superior alkaline hydrogen generation

The intrinsic performance of an electrocatalyst can be reinforced by constructing appropriate epitaxial interfaces, where the modulated electronic states and adsorption/desorption behaviors are conductive to enhancing electrocatalytic activity. Herein, nickel-nickelous hydroxide epitaxial interface supported on nickel foam (Ni-Ni(OH)2/NF) with epitaxial growth of nickel nanoparticles on the surface of nickelous hydroxide nanoribbons is devised for alkaline hydrogen evolution reaction (HER). Notably, the Ni-Ni(OH)2/NF reveals excellent electrocatalytic activity of alkaline HER (158 mV @ 100 mA cm-2), along with robust stability (90 % activity retention after 150 h continuous test at 200 mA cm-2). Theoretical simulations disclose the tuned interface electronic structure and accelerated water dissociation around the epitaxial nickel-nickelous hydroxide interface result in efficient electrochemical activity toward alkaline hydrogen evolution.

Interfacial Charge Transfer Regulates Photoredox Catalysis

Photoredox catalytic processes offer the potential for precise chemical reactions using light and materials. The central determinant is identified as interfacial charge transfer, which simultaneously engenders distinctive behavior in the overall reaction. An in-depth elucidation of the main mechanism and highlighting of the complexity of interfacial charge transfer can occur through both diffusive and direct transfer models, revealing its potential for sophisticated design in complex transformations. The fundamental photophysics uncover these comprehensive applications and offer a clue for future development. This research contributes to the growing body of knowledge on interfacial charge transfer in photoredox catalysis and sets the stage for further exploration of this fascinating area of research.

Flexible nanosheets for plasmonic photocatalysis: microwave-assisted organic synthesis of Ni-NiO@Ni2CO3(OH)2 core-shell@sheet hybrid nanostructures

Visible light-active nickel-based plasmonic photocatalysts provide a cost-effective alternative to noble metals. However, their rarity, fragility, and limited understanding pose challenges. This work presents a microwave-assisted organic synthesis of a Ni-NiO@Ni2CO3(OH)2 core-shell@sheet plasmonic photocatalyst. By employing time and power dependent synthesis, this catalyst exhibits flexible Ni2CO3(OH)2 nanosheets enveloping the Ni-NiO structure, surpassing the pristine Ni@NiO/NiCO3 core-shell counterpart. Chemical reaction mechanisms suggest that irradiation of pristine Ni-NiO/NiCO3 nano structures leads to breakage of amorphous NiCO3 to Ni2+ and CO32-, which further, in the presence of water solvent, interacts with OH- ions leading to the formation of Ni(CO3)·Ni(OH)2. With enhanced light absorption and photocatalytic properties, the resulting core-shell@sheet photocatalyst demonstrates double the hydrogen evolution reaction yield (40 μmol g-1 h-1) compared to the pristine catalyst (20 μmol g-1 h-1). The enhanced H2 yield is attributed to the flexible sheets, cross-dimensional photocatalyst structure, increased surface area for surface reactions, and higher H2 activity of Ni2CO3(OH)2. This research showcases the potential of microwave-assisted synthesis in developing flexible nanosheets with superior solar water splitting performance.

Suppressed charge recombination via defect engineering of confined semiconducting quantum dots for photoelectrocatalysis

Confined semiconducting CuSe quantum dots with abundant Se vacancies are synthesized by pulsed laser deposition with in situ vacuum annealing. With the presence of Se vacancies, the photogenerated charge recombination is suppressed by the self-introduced in-gap trapping states, thus enhancing the photoelectrocatalytic activity under solar illumination.

Engineering cuboctahedral N-doped C-coated p-CuO/n-TiO2 heterojunctions toward high-performance photocatalytic cross-dehydrogenative coupling

The low separation efficiency of photogenerated electron-hole (e-h) pairs severely limits the activation of photocatalyts. One brilliant strategy is to construct a p-n type semiconductor heterojunction, which can establish an inner electric field to separate the e-h pairs with high efficiency. Here, for the first time, a cuboctahedral N-doped carbon-coated CuO/TiO2 p-n heterojunction (CuO-TiO2@N-C) was designed and fabricated successfully via direct calcination of a benzimidazole-modulated cuboctahedral HKUST-Cu with titanium-tetraisopropanolate absorbed inside concomitantly. Full structural characterizations incorporating DFT computations demonstrate that the CuO/TiO2 p-n heterostructure can greatly boost the transport and separation of photoinduced e-h pairs. The nitrogen-doped carbon coating, with its excellent conductivity, porosity, stability and surface reaction activity, plays a pivotal role in promoting the overall performance and effectiveness of the reaction. The CuO-TiO2@N-C displays significantly higher photocurrent density (0.042 μA cm-2) than the CuO@N-C (0.014 μA cm-2) and TiO2@N-C (0.03 μA cm-2) electrodes, proving that the p-n heterojunction can improve the e-h generation efficiency. This unique photocatalyst affords superior photocatalytic efficiency, cycle stability and substrate scope towards cross-dehydrogenative coupling reactions.

The Emerging Layered Hydroxide Plates with Record Thickness for Enhanced High-Mass-Loading Energy Storage

The past decade has witnessed the development of layered-hydroxide-based self-supporting electrodes, but the low active mass ratio impedes its all-around energy-storage applications. Herein, the intrinsic limit of layered hydroxides is broken by engineering F-substituted β-Ni(OH)₂ (Ni-F-OH) plates with a sub-micrometer thickness (over 700 nm), producing a superhigh mass loading of 29.8 mg cm^-2 on the carbon substrate. Theoretical calculation and X-ray absorption spectroscopy analysis demonstrate that Ni-F-OH shares the β-Ni(OH)₂ -like structure with slightly tuned lattice parameters. More interestingly, the synergy modulation of NH₄ ⁺ and F^- is found to serve as the key enabler to tailor these sub-micrometer-thickness 2D plates thanks to the modification effects on the (001) plane surface energy and local OH^- concentration. Guided by this mechanism, the superstructures of bimetallic hydroxides and their derivatives are further developed, revealing they are a versatile family with great promise. The tailored ultrathick phosphide superstructure achieves a superhigh specific capacity of 7144 mC cm^-2 and a superior rate capability (79% at 50 mA cm^-2 ). This work highlights a multiscale understanding of how exceptional structure modulation happens in low-dimensional layered materials. The as-built unique methodology and mechanisms will boost the development of advanced materials to better meet future energy demands.

Integrating electronic structure regulation and dynamic active sites construction on NixCd1-xS-Ni0 photocatalyst for efficient hydrogen evolution

Regulating electronic structure and enriching active sites of photocatalysts are effective strategies to promote hydrogen evolution. Herein, a unique NixCd1-xS-Ni0 photocatalyst, including the surface nickel (Ni) doping and atomic Ni0 anchoring sites, is successfully prepared by Ni2+ ions exchange reaction (Ni2++ CdS → NixCd1-xS) and in-situ photo-induction of Ni0(Ni2++NixCd1-xS→hνNixCd1-xS-Ni0), respectively. As to Ni doping, the Ni replaced cadmium (Cd) atoms introduce hybridized states around the Fermi level, modulating the electronic structure of adjacent S atoms and optimizing the photocatalytic activity of sulfur (S) atoms. Besides, photogenerated Ni0 atoms, anchored on unsaturated S atoms, act as charge transfer bridges to reduce Ni2+ ions in the solution to Ni clusters (NixCd1-xS-Ni0→ne-NixCd1-xS-Ni). Subsequently, the displacement reaction of Ni clusters with protons (H+) spontaneously proceeds to produce hydrogen (H2) in an acidic solution (NixCd1-xS-Ni→2H+H2↑+Ni2++NixCd1-xS-Ni0). The equilibrium of photo-deposition/dissolution of Ni clusters realizes the construction of dynamic active sites, providing sustainable reaction centers and enhancing surface redox kinetics. The NixCd1-xS-Ni0 exhibits a high hydrogen evolution rate of 428 mmol·h-1·g-1 with a quantum efficiency of 75.6 % at 420 nm. This work provides the optimal S electronic structure for photocatalytic H2 evolution and constructs dynamic Ni clusters for chemical replacement reaction. This work provides the optimal S electronic structure for photocatalytic H2 evolution and constructs dynamic Ni clusters for displacement reaction, opening a dual pathway for efficient water reduction.

Insights on Carbon Neutrality by Photocatalytic Conversion of Small Molecules into Value-Added Chemicals or Fuels

Photocatalytic conversion of small molecules (including H₂O, CO₂, N₂, CH₄, and benzene) into value-added chemicals or fuels (e.g., H₂, NH₃, C₂ ⁺, etc.) is a promising strategy to cope with both the worldwide increasing energy demand and greenhouse gas emission in both energy sectors and chemical industry, thus paving an effective way to carbon neutrality. On the other hand, compared with conventionally thermo- or electrocatalytic processes, photoactivation can convert these very stable small molecules by the unexhausted solar energy, so leading to store solar energy in chemical bonds. Thus, it can effectively reduce the reliance on the nonrenewable fossil fuels and avoid the substantial emission of hazardous gases such as CO₂, NO _x , and so on while producing valued-added chemicals. For example, semiconductors can absorb solar light to split H₂O into H₂ and O₂ or convert CO₂ to alcohols, which can then be used as zero or neutral carbon energy sources. Although many efforts have already been made on photocatalytic small molecule activation, the light-energy conversion efficiency is still rather moderate and the yield of aimed value-added chemicals cannot meet the requirement of large-scale application. The core for these artificial photocatalytic processes is to discover a novel photocatalyst with high efficiency, low cost, and excellent durability. Over the past two decades, the Tang group has discovered a few benchmark photocatalysts (such as dual-metal-loaded metal oxides, atomic photocatalysts, carbon-doped TiO₂, and polymer heterojunctions, etc.) and investigated them for photocatalytic conversion of the above-mentioned five robust molecules into value-added chemicals or liquid fuels. Besides, advanced photocatalytic reaction systems including batch and continuous flow membrane reactors have been studied. More importantly, the underlying reaction mechanism of these processes has been thoroughly analyzed using the state-of-the-art static and time-resolved spectroscopies. In this Account, we present the group's recent research progress in search of efficient photocatalysts for these small molecules' photoactivation. First, the strategies used in the group with respect to three key factors in photocatalysis, including light harvesting, charge separation, and reactant adsorption/product desorption, are comprehensively analyzed with the aim to provide a clear strategy for efficient photocatalyst design toward small and robust molecule photoactivation under ambient conditions. The application of in situ and operando techniques on charge carrier dynamics and reaction pathway analysis used in the group are next discussed. Finally, we point out the key challenges and future research directions toward each specific small molecule's photoactivation process.

Efficient Charge Transfers in Highly Conductive Copper Selenide Quantum Dot-Confined Catalysts for Robust Oxygen Evolution Reaction

Defective quantum dots (QDs) are the emerging materials for catalysis by virtue of their atomic-scale size, high monodispersity, and ultra-high specific surface area. However, the dispersed nature of QDs fundamentally prohibits the efficient charge transfer in various catalytic processes. Here, we report efficient and robust electrocatalytic oxygen evolution based on defective and highly conductive copper selenide (CuSe) QDs confined in an amorphous carbon matrix with good uniformity (average diameter 4.25 nm) and a high areal density (1.8 × 10¹² cm^-2). The CuSe QD-confined catalysts with abundant selenide vacancies were prepared by using a pulsed laser deposition system benefitted by high substrate temperature and ultrahigh vacuum growth conditions, as evidenced by electron paramagnetic resonance characterizations. An ultra-low charge transfer resistance (about 7 Ω) determined by electrochemical impedance spectroscopy measurement indicates the efficient charge transfer of CuSe quantum-confined catalysts, which is guaranteed by its high conductivity (with a low resistivity of 2.33 μΩ·m), as revealed by electrical transport measurements. Our work provides a universal design scheme of the dispersed QD-based composite catalysts and demonstrates the CuSe QD-confined catalysts as an efficient and robust electrocatalyst for oxygen evolution reaction.

Mechanistic Insights Into Iron(II) Bis(pyridyl)amine-Bipyridine Skeleton for Selective CO2 Photoreduction

A bis(pyridyl)amine-bipyridine-iron(II) framework (Fe(BPAbipy)) of complexes 1-3 is reported to shed light on the multistep nature of CO₂ reduction. Herein, photocatalytic conversion of CO₂ to CO even at low CO₂ concentration (1 %), together with detailed mechanistic study and DFT calculations, reveal that 1 first undergoes two sequential one-electron transfer affording an intermediate with electron density on both Fe and ligand for CO₂ binding over proton. The following 2 H⁺ -assisted Fe-CO formation is rate-determining for selective CO₂ -to-CO reduction. A pendant, proton-shuttling α-OH group (2) initiates PCET for predominant H₂ evolution, while an α-OMe group (3) cancels the selectivity control for either CO or H₂ . The near-unity selectivity of 1 and 2 enables self-sorting syngas production at flexible CO/H₂ ratios. The unprecedented results from one kind of molecular catalyst skeleton encourage insight into the beauty of advanced multi-electron and multi-proton transfer processes for robust CO₂ RR by photocatalysis.

Visible-Light-Driven Catalytic Deracemization of Secondary Alcohols

Deracemization of racemic chiral compounds is an attractive approach in asymmetric synthesis, but its development has been hindered by energetic and kinetic challenges. Here we describe a catalytic deracemization method for secondary benzylic alcohols which are important synthetic intermediates and end products for many industries. Driven by visible light only, this method is based on sequential photochemical dehydrogenation followed by enantioselective thermal hydrogenation. The combination of a heterogeneous dehydrogenation photocatalyst and a chiral molecular hydrogenation catalyst is essential to ensure two distinct pathways for the forward and reverse reactions. These reactions convert a large number of racemic aryl alkyl alcohols into their enantiomerically enriched forms in good yields and enantioselectivities.

Electrocatalytic upcycling of polyethylene terephthalate to commodity chemicals and H2 fuel

Plastic wastes represent a largely untapped resource for manufacturing chemicals and fuels, particularly considering their environmental and biological threats. Here we report electrocatalytic upcycling of polyethylene terephthalate (PET) plastic to valuable commodity chemicals (potassium diformate and terephthalic acid) and H2 fuel. Preliminary techno-economic analysis suggests the profitability of this process when the ethylene glycol (EG) component of PET is selectively electrooxidized to formate (>80% selectivity) at high current density (>100 mA cm-2). A nickel-modified cobalt phosphide (CoNi0.25P) electrocatalyst is developed to achieve a current density of 500 mA cm-2 at 1.8 V in a membrane-electrode assembly reactor with >80% of Faradaic efficiency and selectivity to formate. Detailed characterizations reveal the in-situ evolution of CoNi0.25P catalyst into a low-crystalline metal oxy(hydroxide) as an active state during EG oxidation, which might be responsible for its advantageous performances. This work demonstrates a sustainable way to implement waste PET upcycling to value-added products.

State-of-the-art progress in tracking plasmon-mediated photoredox catalysis

Metal nanocrystals (NCs), particularly for plasmonic metal NCs with specific morphology and size, can strongly interact with ultraviolet-visible or even near-infrared photons to generate energetic charge carriers, localized heating, and electric field enhancement. These unique properties offer a promising opportunity for maneuvering solar-to-chemical energy conversion through different mechanisms. As distinct from previous works, in this review, recent advances of various characterization techniques in probing and monitoring the photophysical/photochemical processes, as well as the reaction mechanisms of plasmon-mediated photoredox catalysis are thoroughly summarized. Understanding how to distinguish and track these reaction mechanisms would furnish basic guidelines to design next-generation photocatalysts for plasmon-enhanced catalysis.

Simultaneous Conduction and Valence Band Regulation of Indium-Based Quantum Dots for Efficient H2 Photogeneration

Indium-based chalcogenide semiconductors have been served as the promising candidates for solar H₂ evolution reaction, however, the related studies are still in its infancy and the enhancement of efficiency remains a grand challenge. Here, we report that the photocatalytic H₂ evolution activity of quantized indium chalcogenide semiconductors could be dramatically aroused by the co-decoration of transition metal Zn and Cu. Different from the traditional metal ion doping strategies which only focus on narrowing bandgap for robust visible light harvesting, the conduction and valence band are coordinately regulated to realize the bandgap narrowing and the raising of thermodynamic driving force for proton reduction, simultaneously. Therefore, the as-prepared noble metal-free Cu_0.4-ZnIn₂S₄ quantum dots (QDs) exhibits extraordinary activity for photocatalytic H₂ evolution. Under optimal conditions, the Cu_0.4-ZnIn₂S₄ QDs could produce H₂ with the rate of 144.4 μmol h⁻¹ mg⁻¹, 480-fold and 6-fold higher than that of pristine In₂S₃ QDs and Cu-doped In₂S₃ QDs counterparts respectively, which is even comparable with the state-of-the-art cadmium chalcogenides QDs.

Rational design of isostructural 2D porphyrin-based covalent organic frameworks for tunable photocatalytic hydrogen evolution

Covalent organic frameworks have recently gained increasing attention in photocatalytic hydrogen generation from water. However, their structure-property-activity relationship, which should be beneficial for the structural design, is still far-away explored. Herein, we report the designed synthesis of four isostructural porphyrinic two-dimensional covalent organic frameworks (MPor-DETH-COF, M = H₂, Co, Ni, Zn) and their photocatalytic activity in hydrogen generation. Our results clearly show that all four covalent organic frameworks adopt AA stacking structures, with high crystallinity and large surface area. Interestingly, the incorporation of different transition metals into the porphyrin rings can rationally tune the photocatalytic hydrogen evolution rate of corresponding covalent organic frameworks, with the order of CoPor-DETH-COF < H₂Por-DETH-COF < NiPor-DETH-COF < ZnPor-DETH-COF. Based on the detailed experiments and calculations, this tunable performance can be mainly explained by their tailored charge-carrier dynamics via molecular engineering. This study not only represents a simple and effective way for efficient tuning of the photocatalytic hydrogen evolution activities of covalent organic frameworks at molecular level, but also provides valuable insight on the structure design of covalent organic frameworks for better photocatalysis.

An All-Organic D-A System for Visible-Light-Driven Overall Water Splitting

Direct water splitting over photocatalysts is a prospective strategy to convert solar energy into hydrogen energy. Nevertheless, because of the undesirable electron accumulation at the surface, the overall water-splitting efficiency is seriously restricted by the poor charge separation/transfer ability. Here, an all-organic donor-acceptor (D-A) system through crafting carbon rings units-conjugated tubular graphitic carbon nitride (C-TCN) is proposed. Through a range of characterizations and theoretical calculations, the incorporation of carbon rings units via continuous π-conjugated bond builds a D-A system, which can drive intramolecular charge transfer to realize highly efficient charge separation. More importantly, the tubular structure and the incorporated carbon rings units cause a significant downshift of the valence band, of which the potential is beneficial to the activation for O₂ evolution. When serving as photocatalyst for overall water splitting, C-TCN displays considerable performance with H₂ and O₂ production rates of 204.6 and 100.8 µmol g^-1 h^-1 , respectively. The corresponding external quantum efficiency reaches 2.6% at 405 nm, and still remains 1.7% at 420 nm. This work demonstrates that the all-organic D-A system conceptualized from organic solar cell can offer promotional effect for overall water splitting by addressing the charge accumulation problem rooted in the hydrogen evolution reaction.

Identifying a Real Catalyst of [NiFe]-Hydrogenase Mimic for Exceptional H2 Photogeneration

Inspired by the natural [NiFe]-H₂ ase, we designed mimic 1, (dppe)Ni(μ-pdt)(μ-Cl)Ru(CO)₂ Cl to realize effective H₂ evolution under photocatalytic conditions. However, a new species 2 was captured in the course of photo-, electro-, and chemo- one-electron reduction. Experimental studies of in situ IR spectroscopy, EPR, NMR, X-ray absorption spectroscopy, and DFT calculations corroborated a dimeric structure of 2 as a closed-shell, symmetric structure with a Ru^I center. The isolated dimer 2 showed the real catalytic role in photocatalysis with a benchmark turnover frequency (TOF) of 1936 h^-1 for H₂ evolution, while mimic 1 worked as a pre-catalyst and evolved H₂ only after being reduced to 2. The remarkably catalytic activity and unique dimer structure of 2 operated in photocatalysis unveiled a broad research prospect in hydrogenases mimics for advanced H₂ evolution.

Peptide-mediated porphyrin based hierarchical complexes for light-to-chemical conversion

We present a new strategy for the biomimetic preparation of integrated photoactive complexes consisting of light harvesting and electron separation/transfer components via the hierarchical assembly of porphyrin and platinum nanoparticles on the surface of short-peptide self-assembled structures. The complexes can catalyze the conversion of visible light energy into chemical energy in the absence of an electron mediator and store it as nicotinamide adenine dinucleotide (NADH). This provides a promising step towards artificial photosystems through precisely controlled interactions of light-capturing agents, electron separators and biomolecules.

Semiconductor nanocrystals for small molecule activation via artificial photosynthesis

The protocol of artificial photosynthesis using semiconductor nanocrystals shines light on green, facile and low-cost small molecule activation to produce solar fuels and value-added chemicals.