Susceptible Surface Sulfide Regulates Catalytic Activity of CdSe Quantum Dots for Hydrogen Photogeneration

Heterogeneous CuS QDs/BiVO4@Y2O2S Nanoreactor for Monitorable Photocatalysis

Exploration of multifunctional integrated catalysts is of great significance for photocatalysis toward practical application. Herein, a 1D confined nanoreactor with a heterogeneous core-shell structure is designed for synergies of efficient catalysis and temperature monitoring by custom encapsulation of Z-scheme heterojunction CuS quantum dots/BiVO4 (CuS QDs/BiVO4) and Y2O2S-Er, Yb. The dispersed active sites created by the QDs with high surface energy improve the mass transfer efficiency, and the efficient electron transport channels at the heterogeneous interface extend the carrier lifetime, which endows the nanoreactor with excellent catalytic performance. Meanwhile, real-time temperature monitoring is realized based on the thermally coupled levels 2H11/2/4S3/2→4I15/2 of Er3+ using fluorescence intensity ratio, which enables the monitorable photocatalysis. Furthermore, the nanoreactor with a multidimensional structure increases effective intermolecular collisions to facilitate the catalytic process by restricting the reaction within distinct enclosed spaces and circumvents potential unknown interaction effects. The design of multi-space nanoconfined reactors opens up a new avenue to modulate catalyst function, providing a unique perspective for photocatalytic applications in the mineralization of organic pollutants, hydrogen production, and nitrogen fixation.

Liquid-Liquid Phase Separation-Mediated Photocatalytic Subcellular Hybrid System for Highly Efficient Hydrogen Production

Plant chloroplasts have a highly compartmentalized interior, essential for executing photocatalytic functions. However, the construction of a photocatalytic reaction compartment similar to chloroplasts in inorganic-biological hybrid systems (IBS) has not been reported. Drawing inspiration from the compartmentalized chloroplast and the phenomenon of liquid-liquid phase separation, herein, a new strategy is first developed for constructing a photocatalytic subcellular hybrid system through liquid-liquid phase separation technology in living cells. Photosensitizers and in vivo expressed hydrogenases are designed to coassemble within the cell to create subcellular compartments for synergetic photocatalysis. This compartmentalization facilitates efficient electron transfer and light energy utilization, resulting in highly effective H2 production. The subcellular compartments hybrid system (HM/IBSCS) exhibits a nearly 87-fold increase in H2 production compared to the bare bacteria/hybrid system. Furthermore, the intracellular compartments of the photocatalytic reactor enhance the system's stability obviously, with the bacteria maintaining approximately 81% of their H2 production activity even after undergoing five cycles of photocatalytic hydrogen production. The research brings forward visionary prospects for the field of semi-artificial photosynthesis, offering new possibilities for advancements in areas such as renewable energy, biomanufacturing, and genetic engineering.

Ligand-Induced Symmetry Breaking as the Origin of Multiexponential Photoluminescence Decay in CdSe Quantum Dots

The bright photoluminescence (PL) of colloidal CdSe quantum dots (QDs) makes them interesting for optical applications. For most of them, well-defined PL properties, dominated by a single excitonic state, are required. However, in many PL experiments with QD ensembles, multiexponential decay was observed. On the basis of spin-orbit density functional theory and screened configuration interaction calculations, we show that highly symmetric and defect-free CdSe QDs with diameters of 1.7 and 2.0 nm possess a multiexponential low-temperature PL at the single-dot level. This is a consequence of ligand-induced symmetry breaking with a subsequent rearrangement of the lowest eight excitonic states in two sets of four singly degenerate excitonic states. For each set, the lowest state is dark and the other three are bright. We find that the splitting between the sets can be modified by the coverage and choice of the ligand, which facilitates the engineering of the PL properties of CdSe QDs.

Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II-VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Se_x S_{1- x} )₄ -(CdS)₂ core/shell QDs with a proper S_surface /Cd_surface ratio exhibits a remarkably improved photocurrent density (≈18.4 mA cm^-2 under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO₂ and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.

Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor-metal hybrid nanoparticles (HNPs) focusing on metal-chalcogenides-metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor-metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.

Reductive Carbon-Carbon Coupling on Metal Sites Regulates Photocatalytic CO2 Reduction in Water Using ZnSe Quantum Dots

Colloidal quantum dots (QDs) consisting of precious-metal-free elements show attractive potentials towards solar-driven CO₂ reduction. However, the inhibition of hydrogen (H₂ ) production in aqueous solution remains a challenge. Here, we describe the first example of a carbon-carbon (C-C) coupling reaction to block the competing H₂ evolution in photocatalytic CO₂ reduction in water. In a specific system taking ZnSe QDs as photocatalysts, the introduction of furfural can significantly suppress H₂ evolution leading to CO evolution with a rate of ≈5.3 mmol g^-1  h^-1 and a turnover number (TON) of >7500 under 24 h visible light. Meanwhile, furfural is upgraded to the self-coupling product with a yield of 99.8 % based on the consumption of furfural. Mechanistic insights show that the reductive furfural coupling reaction occurs on surface Zn-sites to consume electrons and protons originally used for H₂ production, while the CO formation pathway at surface anion vacancies from CO₂ remains.

Sulfide Ligands in Hybrid Semiconductor-Metal Nanocrystal Photocatalysts: Improved Hole Extraction and Altered Catalysis

Hybrid semiconductor-metal nanocrystals manifest efficient photocatalytic activity related to the metal domain promoting charge carrier separation and providing an active catalytic site. The surface properties of such nanoparticles are also of paramount importance in determining their photocatalytic activity. Addressing the combination of surface effects in catalysis on metals, with the electronic properties of hybrid nanoparticles, we examined the effect of coating CdS-Au hybrid nanoparticles with sulfide, an anion that is expected to bind strongly to both domains, on the photocatalytic functionality. Upon sulfide coating, one-electron processes - namely the oxidative production of hydroxyl radicals and the reductive production of superoxide - were increased, whereas the activity for two-electron reduction processes - H₂ and hydrogen peroxide generation - was hampered. These findings indicate a double-edged sword effect for sulfide coating that on one side relieves the hole extraction bottleneck from the semiconductor segment and, on the other hand, poisons the metal domain restricting its reductive capacity for the two-electron processes requiring a chemisorption step on the metal surface. The work further demonstrates the importance of surface properties for the photocatalytic action of such hybrid nanoparticle systems.

Ag-In-Zn-S Quantum Dot-Dominated Interface Kinetics in Ag-In-Zn-S/NiFe LDH Composites toward Efficient Photoassisted Electrocatalytic Water Splitting

Photoassisted electrocatalysis (P-EC) emerges as a rising star for hydrogen production by embedding photoactive species in electrocatalysts, for which the interfacial structure design and charge transfer kinetics of the multifunctional catalysts remain a great challenge. Herein, Zn-AgIn₅S₈ quantum dots (ZAIS QDs) were embedded into 2D NiFe layered double hydroxide nanosheets through a simple hydrothermal treatment to form 0D/2D composite catalysts for P-EC. With evidence from transient photovoltage spectroscopy, we acquired a clear and fundamental understanding on the kinetics of charge extraction time and extraction amount in the 0D/2D heterojunctions that was proved to play a key role in P-EC. Upon light illumination, for HER, the optimized NiFe-ZAIS exhibits obviously reduced overpotentials of 129 and 242 mV at current densities of 10 and 50 mA cm^-2, which are 22 and 33 mV lower than those of dark electrocatalysis, respectively. For OER, the NiFe-ZAIS electrode also shows low overpotentials of 220 and 268 mV at current densities of 10 and 50 mA cm^-2, respectively, under light illumination, which were able to almost double the intrinsic activity. Finally, with NF@NiFe-ZAIS as both the cathode and the anode, the assembled electrolyzer only requires 1.62 V to reach the overall water splitting current density of 10 mA cm^-2 under P-EC. This work provides a useful example for the profound understanding of the design and the kinetics study of multifunctional P-EC catalysts.

Boosting Visible-Light-Driven Photocatalytic Hydrogen Production through Sensitizing TiO2 via Novel Nanoclusters

Improving the light utilization and electron-hole separation efficiency plays a central role in photocatalysis for converting light energy to hydrogen energy. Herein, for the first time, a stable, highly dispersible discrete T4 [Cd₃In₁₇Se₃₁]^5- cluster is developed as a novel photosensitizer to sensitize TiO₂ for photocatalytic hydrogen production. Compared with pristine TiO₂ (near zero) and the T4 clusters (19.5 μmol g^-1 h^-1) that exhibit low hydrogen evolution activities, the T4/TiO₂ composite, constructed from traces of 0.127 mol % T4 cluster-sensitized TiO₂, exhibits a dramatically improved photocatalytic activity of 328.2 μmol g^-1 h^-1, highlighting that the photocatalytic efficiency strongly correlates with that of the T4 cluster. In the meantime, the T4/TiO₂ composites are highly stable, remaining robust in a long-time test of 50 h for photocatalytic hydrogen production. Ultrafast transient absorption spectroscopy, in combination with electrochemical analyses, steady-state and time-resolved photoluminescence, and density functional theory calculations, indicates that the T4 cluster not only serve as a photosensitizer to absorb visible light but also form a heterojunction between the interface of the T4 cluster and TiO₂ to accelerate electron injection. This work highlights the great potential of the stable and highly dispersed discrete metal chalcogenide clusters as high-efficiency photosensitizers for converting solar energy to chemical energy.

Cooperative Coupling of Oxidative Organic Synthesis and Hydrogen Production over Semiconductor-Based Photocatalysts

Merging hydrogen (H₂) evolution with oxidative organic synthesis in a semiconductor-mediated photoredox reaction is extremely attractive because the clean H₂ fuel and high-value chemicals can be coproduced under mild conditions using light as the sole energy input. Following this dual-functional photocatalytic strategy, a dreamlike reaction pathway for constructing C-C/C-X (X = C, N, O, S) bonds from abundant and readily available X-H bond-containing compounds with concomitant release of H₂ can be readily fulfilled without the need of external chemical reagents, thus offering a green and fascinating organic synthetic strategy. In this review, we begin by presenting a concise overview on the general background of traditional photocatalytic H₂ production and then focus on the fundamental principles of cooperative photoredox coupling of selective organic synthesis and H₂ production by simultaneous utilization of photoexcited electrons and holes over semiconductor-based catalysts to meet the economic and sustainability goal. Thereafter, we put dedicated emphasis on recent key progress of cooperative photoredox coupling of H₂ production and various selective organic transformations, including selective alcohol oxidation, selective methane conversion, amines oxidative coupling, oxidative cross-coupling, cyclic alkanes dehydrogenation, reforming of lignocellulosic biomass, and so on. Finally, the remaining challenges and future perspectives in this flourishing area have been critically discussed. It is anticipated that this review will provide enlightening guidance on the rational design of such dual-functional photoredox reaction system, thereby stimulating the development of economical and environmentally benign solar fuel generation and organic synthesis of value-added fine chemicals.

Controlled Photoinduced Electron Transfer from InP/ZnS Quantum Dots through Cu Doping: A New Prototype for the Visible-Light Photocatalytic Hydrogen Evolution Reaction

Photoexcited electron extraction from semiconductors can be useful for converting solar energy into useful forms of energy. Although InP quantum dots (QDs) are considered alternative materials for solar energy conversion, the inherent instability of bare InP QDs demands the use of passivation layers such as ZnS for practical applications, which impedes carrier extraction from the QDs. Here, we demonstrate that Cu-doped InP/ZnS (InP/Cu:ZnS) QDs improve the electron transfer ability due to hole capture by Cu dopants. Steady-state and time-resolved photoluminescence studies confirmed that electrons were effectively transferred from the InP/Cu:ZnS QDs to a benzoquinone acceptor by retarding the electron-hole recombination within the QD. We evaluated the photocatalytic H₂ evolution performance of InP/Cu:ZnS QDs under visible light, which showed outstanding photocatalytic H₂ evolution activity and stability under visible light illumination. The photocatalytic activity was preserved even in the absence of a cocatalyst.

Enhancing photocatalytic H2 evolution on In2S3/mesoporous TiO2 nanocomposites via one-pot microwave-assisted synthesis using an ionic liquid

Charge transfer and surface photocatalytic reactions are two crucial factors that determine the efficiency of photocatalysts in converting light energy to hydrogen energy. Herein, a fast and green strategy composed of a one-pot microwave-assisted solvothermal synthesis followed by a heat treatment process is developed for synthesizing In₂S₃/mesoporous TiO₂ nanocomposites, using a mixture of tetra-butyl titanate, H₂O, In-S-ionic liquid precursor solution (In-S-ILs) and [Bmim]Cl as the reactants. The ionic liquid cations act not only as a microwave absorbent during the solvothermal process, but also as a morphology-controlling agent via a dissolution-recrystallization process, leading to the formation of nanocomposites consisting of small and uniform sized In₂S₃ and mesoporous TiO₂. The optimum In₂S₃/mesoporous TiO₂ nanocomposite with an In₂S₃ content of 1.01 mol% exhibits a H₂ evolution rate of 637.9 μmol g^-1 h^-1, which is over 10 times that of commercial P25, and is 5.7 and 2.6 times that of neat mesoporous TiO₂ and the milled In₂S₃/mesoporous TiO₂ with the same component, respectively. The improved H₂ evolution activity is predominantly attributed to the small size of In₂S₃, special mesoporous structure of TiO₂, and their intimate heterojunction interface, which guaranteed effective charger transfer between In₂S₃ and mesoporous TiO₂ as well as abundant active sites for photocatalytic reactions. This work provides a novel and simple strategy for obtaining special structures of semiconductor-based nanocomposites for efficient energy conversion.

Atomic Sulfur Passivation Improves the Photoelectrochemical Performance of ZnSe Nanorods

We introduced atomic sulfur passivation to tune the surface sites of heavy metal-free ZnSe nanorods, with a Zn²⁺-rich termination surface, which are initially capped with organic ligands and under-coordinated with Se. The S²⁻ ions from a sodium sulfide solution were used to partially substitute a 3-mercaptopropionic acid ligand, and to combine with under-coordinated Zn termination atoms to form a ZnS monolayer on the ZnSe surface. This treatment removed the surface traps from the ZnSe nanorods, and passivated defects formed during the previous ligand exchange process, without sacrificing the efficient hole transfer. As a result, without using any co-catalysts, the atomic sulfur passivation increased the photocurrent density of TiO₂/ZnSe photoanodes from 273 to 325 μA/cm². Notably, without using any sacrificial agents, the photocurrent density for sulfur-passivated TiO₂/ZnSe nanorod-based photoanodes remained at almost 100% of its initial value after 300 s of continuous operation, while for the post-deposited ZnS passivation layer, or those based on ZnSe/ZnS core–shell nanorods, it declined by 28% and 25%, respectively. This work highlights the advantages of the proper passivation of II-VI semiconductor nanocrystals as an efficient approach to tackle the efficient charge transfer and stability of photoelectrochemical cells based thereon.

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.

Exciton dissociation in an NIR-active triohybrid nanocrystal leading to efficient generation of reactive oxygen species

Lead sulfide (PbS) colloidal quantum dots (QDs) are emerging materials for fundamental studies because of their potential application in near infrared (NIR) light harvesting technologies. However, inefficient electron separation, facile charge recombination and defect state trapping of photoexcited carriers are reported as limitations of the PbS QDs to achieve efficient energy conversion. In the present study, we have synthesized a triohybrid by assembling a semiconductor titanium dioxide (TiO₂), an organic oxidizing molecule phenothiazine (PTZ) and PbS QDs. The triohybrid along with PbS_TiO₂ and PbS_PTZ hybrids has been characterized and the attachment of different components is verified by spectroscopic and microscopic techniques. The interfacial dynamics of the photoexcited carriers in the PbS_TiO₂ and PbS_PTZ hybrids have been investigated separately using steady state and time resolved photoluminescence (TRPL) measurements. The photoinduced electron transfer (PET) from the PbS QD to the conduction band (CB) of TiO₂ and photoinduced hole transfer (PHT) from the valence band (VB) of the QD to the highest occupied molecular orbital (HOMO) of PTZ have been observed and correlated mechanistically to the energy level alignments obtained from cyclic voltammetric (CV) analysis. The PTZ molecule is also found to act as a surface defect passivator of the PbS QD. Finally, simultaneous exciton dissociation and reduced back recombination phenomena have been correlated with a higher reactive oxygen species (ROS) generation activity of the triohybrid than the other two, under IR light irradiation. Thus, a detailed investigation of carrier dynamics and the mechanism of higher NIR light activity for a novel nanohybrid is explored and analyzed which could be beneficial for NIR catalysis or antibacterial activities.