Geometric Effect of Single or Double Metal-Tipped CdSe Nanorods on Photocatalytic H2 Generation

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Pt cocatalyst morphology on semiconductor nanorod photocatalysts enhances charge trapping and water reduction

In photocatalysis, metal-semiconductor hybrid structures have been proposed for ideal photocatalytic systems. In this study, we investigate the effect of morphology and surface nature of Pt cocatalysts on photocatalytic hydrogen evolution activity in Pt-tipped CdSe nanorods. Three distinct morphologies of Pt cocatalysts were synthesized and employed as visible light photocatalysts. The rough tips exhibit the highest activity, followed by the round and cubic tips. Kinetic investigations using transient absorption spectroscopy reveal that the cubic tips exhibit lower charge-separated states feasible for reacting with water and water reduction rates due to their defectless surface facets. In contrast, the rough tips show a similar charge-separation value but a two-fold higher surface reaction rate than the round tips, resulting in a significant enhancement of hydrogen evolution. These findings highlight the importance of rational design on metal cocatalysts in addition to the main semiconductor bodies for maximizing photocatalytic activities.

Unravelling Dynamics Involving Multiple Charge Carriers in Semiconductor Nanocrystals

The use of colloidal nanocrystals as part of artificial photosynthetic systems has recently gained significant attention, owing to their strong light absorption and highly reproducible, tunable electronic and optical properties. The complete photocatalytic conversion of water to its components is yet to be achieved in a practically suitable and commercially viable manner. To complete this challenging task, we are required to fully understand the mechanistic aspects of the underlying light-driven processes involving not just single charge carriers but also multiple charge carriers in detail. This review focuses on recent progress in understanding charge carrier dynamics in semiconductor nanocrystals and the influence of various parameters such as dimension, composition, and cocatalysts. Transient absorption spectroscopic studies involving single and multiple charge carriers, and the challenges associated with the need for accumulation of multiple charge carriers to drive the targeted chemical reactions, are discussed.

Lateral charge migration in 1D semiconductor-metal hybrid photocatalytic systems

Colloidal nanorods based on CdS or CdSe, functionalized with metal particles, have proven to be efficient catalysts for light-driven hydrogen evolution. Seeded CdSe@CdS nanorods have shown increasing performance with increasing rod length. This observation was rationalized by the increasing lifetime of the separated charges, as a large distance between holes localized in the CdSe seed and electrons localized at the metal tip decreases their recombination rate. However, the impact of nanorod length on the electron-to-tip localization efficiency or pathway remained an open question. Therefore, we investigated the photo-induced electron transfer to the metal in a series of Ni-tipped CdSe@CdS nanorods with varying length. We find that the transfer processes occurring from the region close to the semiconductor-metal interface, the rod region, and the CdSe seed region depend in different ways on the rods' length. The rate of the fastest process from excitonic states generated directly at the interface is independent of the rod length, but the relative amplitude decreases with increasing rod length, as the weight of the interface region is decreasing. The transfer of electrons to the metal tip from excitons generated in the CdS rod region depends strongly on the length of the nanorods, which indicates an electron transport-limited process, i.e., electron diffusion toward the interface region, followed by fast interface crossing. The transfer originating from the CdSe excitonic states again shows no significant length dependence in its time constant, as it is probably limited by the rate of overcoming the shallow confinement in the CdSe seed.

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.

Selective-Epitaxial Hybrid of Tripartite Semiconducting Sulfides for Enhanced Solar-to-Hydrogen Conversion

The design and synthesis of advanced semiconductors is crucial for the full utilization of solar energy. Herein, colloidal selective-epitaxial hybrid of tripartite semiconducting sulfides CuInS₂ Cd(In)SMoS₂ heteronanostructures (HNs) via lateral- and vertical-epitaxial growths, followed by cation exchange reactions, are reported. The lateral-epitaxial CuInS₂ and Cd(In)S enable effective visible to near-infrared (NIR) solar spectrum absorption, and the vertical-epitaxial ultrathin MoS₂ offer sufficient edge sulfur sites for the hydrogen evolution reaction (HER). Furthermore, the integrated structures exhibit unique epitaxial-staggered type II band alignments for continuous charge separation. They achieve the H₂ evolution rate up to 8 mmol h^-1 g^-1 , which is ≈35 times higher than bare CdS and show no deactivation after long-term cycling, representing one of the most efficient and robust noble-metal-free photocatalysts. This design principle and transformation protocol open a new way for creating all-in-one multifunctional catalysts in a predictable manner.

New insights into the degradation of synthetic pollutants in contaminated environments

The environment is contaminated by synthetic contaminants owing to their extensive applications globally. Hence, the removal of synthetic pollutants (SPs) from the environment has received widespread attention. Different remediation technologies have been investigated for their abilities to eliminate SPs from the ecosystem; these include photocatalysis, sonochemical techniques, nanoremediation, and bioremediation. SPs, which can be organic or inorganic, can be degraded by microbial metabolism at contaminated sites. Owing to their diverse metabolisms, microbes can adapt to a wide variety of environments. Several microbial strains have been reported for their bioremediation potential concerning synthetic chemical compounds. The selection of potential strains for large-scale removal of organic pollutants is an important research priority. Additionally, novel microbial consortia have been found to be capable of efficient degradation owing to their combined and co-metabolic activities. Microbial engineering is one of the most prominent and promising techniques for providing new opportunities to develop proficient microorganisms for various biological processes; here, we have targeted the SP-degrading mechanisms of microorganisms. This review provides an in-depth discussion of microbial engineering techniques that are used to enhance the removal of both organic and inorganic pollutants from different contaminated environments and under different conditions. The degradation of these pollutants is investigated using abiotic and biotic approaches; interestingly, biotic approaches based on microbial methods are preferable owing to their high potential for pollutant removal and cost-effectiveness.

Synthesis of Metal-Capped Semiconductor Nanowires from Heterodimer Nanoparticle Catalysts

Semiconductor nanowires (NWs) capped with metal nanoparticles (NPs) show multifunctional and synergistic properties, which are important for applications in the fields of catalysis, photonics, and electronics. Conventional colloidal syntheses of this class of hybrid structures require complex sequential seeded growth, where each section requires its own set of growth conditions, and methods for preparing such wires are not universal. Here, we report a new and general method for synthesizing metal-semiconductor nanohybrids based on particle catalysts, prepared by scanning probe block copolymer lithography, and chemical vapor deposition. In this process, metallic heterodimer NPs were used as catalysts for NW growth to form semiconductor NWs capped with metallic particles (Au, Ag, Co, Ni). Interestingly, the growth processes for NWs on NPs are regioselective and controlled by the chemical composition of the metallic heterodimer used. Using a systematic experimental approach, paired with density functional theory calculations, we were able to postulate three different growth modes, one without precedent.

Enhanced Light-Driven Charge Separation and H2 Generation Efficiency in WSe2 Nanosheet-Semiconductor Nanocrystal Heterostructures

Semiconductor-catalyst heterostructures have shown promising performances for light-driven H₂ generation, although further development of these materials is hindered by the lack of cost-effective and efficient catalysts. In this paper, we adopt a colloidal method to prepare few-layer WSe₂ nanosheets without exfoliation and apply them as catalysts for forming heterostructures with a wide range of semiconductor absorbers (CdS nanorods, CdSe/CdS dot-in-rods, TiO₂ nanoparticles, g-C₃N₄ nanosheets). These WSe₂-semiconductor heterostructures show enhanced solar-to-hydrogen conversion efficiencies compared to semiconductors without WSe₂. The detailed mechanism of this enhancement has been investigated using WSe₂ nanosheet-decorated CdSe/CdS dot-in-rods as a model system, which display ∼5.5-fold higher hydrogen generation apparent quantum efficiency compared to free CdSe/CdS dot-in-rods. Transient absorption spectroscopic studies reveal efficient charge separation in WSe₂-decorated CdSe/CdS dot-in-rods, suggesting its key role in enhancing the H₂ generation efficiency of WSe₂-semiconductor heterostructures. This work demonstrates the great potentials of WSe₂ nanosheets as catalysts for light-driven hydrogen production and the important effect of forming WSe₂-semiconductor heterostructures in facilitating charge separation and photocatalysis.

A Ternary Dumbbell Structure with Spatially Separated Catalytic Sites for Photocatalytic Overall Water Splitting

Solar-driven overall water splitting based on metal sulfide semiconductor photocatalysts remains as a challenge owing to the strong charge recombination and deficient catalytic active sites. Additionally, significant inhibition of back reactions, especially the oxidation of sulfide ions during the photocatalytic water oxidation catalysis, is an arduous task that requires an efficient photogenerated hole transfer dynamics. Here, a ternary dumbbell-shaped catalyst based on RuO2/CdS/MoS2 with spatially separated catalytic sites is developed to achieve simultaneous production of hydrogen and oxygen under simulated solar-light without any sacrificial agents. Particularly, MoS2 nanosheets anchored on the two ends of CdS nanowires are identified as a reduction cocatalyst to accelerate hydrogen evolution, while RuO2 nanoparticles as an oxidation cocatalyst are deposited onto the sidewalls of CdS nanowires to facilitate oxygen evolution kinetics. The density functional theory simulations and ultrafast spectroscopic results reveal that photogenerated electrons and holes directionally migrate to MoS2 and RuO2 catalytic sites, respectively, thus achieving efficient charge carrier separation. The design of ternary dumbbell structure guarantees metal sulfides against photocorrosion and thus extends their range in solar water splitting.

Nonepitaxial Gold-Tipped ZnSe Hybrid Nanorods for Efficient Photocatalytic Hydrogen Production

For the first time, colloidal gold (Au)-ZnSe hybrid nanorods (NRs) with controlled size and location of Au domains are synthesized and used for hydrogen production by photocatalytic water splitting. Au tips are found to grow on the apices of ZnSe NRs nonepitaxially to form an interface with no preference of orientation between Au(111) and ZnSe(001). Density functional theory calculations reveal that the Au tips on ZnSe hybrid NRs gain enhanced adsorption of H compared to pristine Au, which favors the hydrogen evolution reaction. Photocatalytic tests reveal that the Au tips on ZnSe NRs effectively enhance the photocatalytic performance in hydrogen generation, in which the single Au-tipped ZnSe hybrid NRs show the highest photocatalytic hydrogen production rate of 437.8 µmol h^-1 g^-1 in comparison with a rate of 51.5 µmol h^-1 g^-1 for pristine ZnSe NRs. An apparent quantum efficiency of 1.3% for hydrogen evolution reaction for single Au-tipped ZnSe hybrid NRs is obtained, showing the potential application of this type of cadmium (Cd)-free metal-semiconductor hybrid nanoparticles (NPs) in solar hydrogen production. This work opens an avenue toward Cd-free hybrid NP-based photocatalysis for clean fuel production.

Light-Induced Formation of MoOxSy Clusters on CdS Nanorods as Cocatalyst for Enhanced Hydrogen Evolution

Metal and metal-oxide particles are commonly photodeposited on photocatalysts by reduction and oxidation reactions, respectively, consuming charges that are generated under illumination. This study reveals that amorphous MoO_xS_y clusters can be easily photodeposited at the tips of CdS nanorods (NRs) by in situ photodeposition for the first time. The as-prepared MoO_xS_y-decorated CdS samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma (ICP) to determine the composition and the possible formation pathways of the amorphous MoO_xS_y clusters. The MoO_xS_y-tipped CdS samples exhibited better hydrogen evolution performance than pure CdS under visible-light illumination. The enhanced activity is attributed to the formation of intimate interfacial contact between CdS and the amorphous MoO_xS_y clusters, which facilitates the charge separation and transfer. Through time-resolved photoluminescence (TRPL) measurements, it was clearly observed that all MoO_xS_y-decorated CdS samples with different loadings of MoO_xS_y showed a faster PL decay when compared to pure CdS, resulting from the effective trapping of photogenerated electrons by the MoO_xS_y clusters. Kelvin probe force microscopy (KPFM) was further used to study the surface potentials of pure CdS NRs and MoO_xS_y-decorated CdS NRs. A higher surface potential on MoO_xS_y-decorated CdS NRs was observed in the dark, indicating that the loading of MoO_xS_y resulted in a lower surface work function compared to pure CdS NRs. This contributed to the effective electron trapping and separation, which was also reflected by the increased photoelectrochemical response. Thus, this study demonstrates the design and facile synthesis of MoO_xS_y-tipped CdS NRs photocatalysts for efficient solar hydrogen production.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Photocatalytic Hybrid Semiconductor-Metal Nanoparticles; from Synergistic Properties to Emerging Applications

Hybrid semiconductor-metal nanoparticles (HNPs) manifest unique combined and often synergetic properties stemming from the materials combination. These structures exhibit spatial charge separation across the semiconductor-metal junction upon light absorption, enabling their use as photocatalysts. So far, the main impetus of photocatalysis research in HNPs addresses their functionality in solar fuel generation. Recently, it was discovered that HNPs are functional in efficient photocatalytic generation of reactive oxygen species (ROS). This has opened the path for their implementation in diverse biomedical and industrial applications where high spatially temporally resolved ROS formation is essential. Here, the latest studies on the synergistic characteristics of HNPs are summarized, including their optical, electrical, and chemical properties and their photocatalytic function in the field of solar fuel generation is briefly discussed. Recent studies are then focused concerning photocatalytic ROS formation with HNPs under aerobic conditions. The emergent applications of this capacity are then highlighted, including light-induced modulation of enzymatic activity, photodynamic therapy, antifouling, wound healing, and as novel photoinitiators for 3D-printing. The superb photophysical and photocatalytic properties of HNPs offer already clear advantages for their utility in scenarios requiring on-demand light-induced radical formation and the full potential of HNPs in this context is yet to be revealed.

Two-Dimensional Morphology Enhances Light-Driven H2 Generation Efficiency in CdS Nanoplatelet-Pt Heterostructures

Light-driven H₂ generation using semiconductor nanocrystal heterostructures has attracted intense recent interest because of the ability to rationally improve their performance by tailoring their size, composition, and morphology. In zero- and one-dimensional nanomaterials, the lifetime of the photoinduced charge-separated state is still too short for H₂ evolution reaction, limiting the solar-to-H₂ conversion efficiency. Here we report that using two-dimensional (2D) CdS nanoplatelet (NPL)-Pt heterostructures, H₂ generation internal quantum efficiency (IQE) can exceed 40% at pH 8.8-13 and approach unity at pH 14.7. The near unity IQE at pH 14.7 is similar to those reported for 1D nanorods and can be attributed to the irreversible hole removal by OH^-. At pH < 13, the IQE of 2D NPL-Pt is significantly higher than those in 1D nanorods. Detailed time-resolved spectroscopic studies and modeling of the elementary charge separation and recombination processes show that, compared to 1D nanorods, 2D morphology extends charge-separated state lifetime and may play a dominant role in enhancing the H₂ generation efficiency. This work provides a new approach for designing nanostructures for efficient light-driven H₂ generation.

Charge Carrier Dynamics in Photocatalytic Hybrid Semiconductor-Metal Nanorods: Crossover from Auger Recombination to Charge Transfer

Hybrid semiconductor-metal nanoparticles (HNPs) manifest unique, synergistic electronic and optical properties as a result of combining semiconductor and metal physics via a controlled interface. These structures can exhibit spatial charge separation across the semiconductor-metal junction upon light absorption, enabling their use as photocatalysts. The combination of the photocatalytic activity of the metal domain with the ability to generate and accommodate multiple excitons in the semiconducting domain can lead to improved photocatalytic performance because injecting multiple charge carriers into the active catalytic sites can increase the quantum yield. Herein, we show a significant metal domain size dependence of the charge carrier dynamics as well as the photocatalytic hydrogen generation efficiencies under nonlinear excitation conditions. An understanding of this size dependence allows one to control the charge carrier dynamics following the absorption of light. Using a model hybrid semiconductor-metal CdS-Au nanorod system and combining transient absorption and hydrogen evolution kinetics, we reveal faster and more efficient charge separation and transfer under multiexciton excitation conditions for large metal domains compared to small ones. Theoretical modeling uncovers a competition between the kinetics of Auger recombination and charge separation. A crossover in the dominant process from Auger recombination to charge separation as the metal domain size increases allows for effective multiexciton dissociation and harvesting in large metal domain HNPs. This was also found to lead to relative improvement of their photocatalytic activity under nonlinear excitation conditions.

Engineering Reaction Kinetics by Tailoring the Metal Tips of Metal-Semiconductor Nanodumbbells

Semiconductor-metal hybrid nanostructures are one of the best model catalysts for understanding photocatalytic hydrogen generation. To investigate the optimal structure of metal cocatalysts, metal-CdSe-metal nanodumbbells were synthesized with three distinct sets of metal tips, Pt-CdSe-Pt, Au-CdSe-Au, and Au-CdSe-Pt. Photoelectrochemical responses and transient absorption spectra showed that the competition between the charge recombination at the metal-CdSe interface and the water reduction on the metal surface is a detrimental factor for the apparent hydrogen evolution rate. For instance, a large recombination rate (k_rec) at the Pt-CdSe interface limits the quantum yield of hydrogen generation despite a superior water reduction rate (k_WR) on the Pt surface. To suppress the recombination process, Pt was selectively deposited onto the Au tips of Au-CdSe-Au nanodumbbells in which the k_rec was diminished at the Au-CdSe interface, and the large k_WR was maintained on the Pt surface. As a result, the optimal structure of the Pt-coated Au-CdSe-Au nanodumbbells reached a quantum yield of 4.84%. These findings successfully demonstrate that the rational design of a metal cocatalyst and metal-semiconductor interface can additionally enhance the catalytic performance of the photochemical hydrogen generation reactions.

Low-coordinated surface atoms of CuPt alloy cocatalysts on TiO2 for enhanced photocatalytic conversion of CO2

We report the photocatalytic conversion of CO2 to CH4 using CuPt alloy nanoclusters anchored on TiO2. As the size of CuPt alloy nanoclusters decreases, the photocatalytic activity improves significantly. Small CuPt nanoclusters strongly bind CO2 intermediates and have a stronger interaction with the TiO2 support, which also contributes to an increased CH4 generation rate. The alloying and size effects prove to be the key to efficient CO2 reduction, highlighting a strategic platform for the design of photocatalysts for CO2 conversion.

Rational design of semiconductor-based photocatalysts for advanced photocatalytic hydrogen production: the case of cadmium chalcogenides

This review summarizes the recent advances in developing CdX (X = S, Se, Te)-based photocatalyst systems for photocatalytic hydrogen production from water.

Quantum confined colloidal nanorod heterostructures for solar-to-fuel conversion

Colloidal one-dimensional (1D) semiconductor nanorods (NRs) offer the opportunity to simultaneously maintain quantum confinement in radial dimensions for tunable light absorptions and bulk like carrier transport in the axial direction for long-distance charge separations.

Metal Domain Size Dependent Electrical Transport in Pt-CdSe Hybrid Nanoparticle Monolayers

Thin films prepared of semiconductor nanoparticles are promising for low-cost electronic applications such as transistors and solar cells. One hurdle for their breakthrough is their notoriously low conductivity. To address this, we precisely decorate CdSe nanoparticles with platinum domains of one to three nanometers in diameter by a facile and robust seeded growth method. We demonstrate the transition from semiconductor to metal dominated conduction in monolayered films. By adjusting the platinum content in such solution-processable hybrid, oligomeric nanoparticles the dark currents through deposited arrays become tunable while maintaining electronic confinement and photoconductivity. Comprehensive electrical measurements allow determining the reigning charge transport mechanisms.

Less Is More: The Case of Metal Cocatalysts

We provide evidence that for a multielectron reaction such as hydrogen reduction, the photocatalyst design should include only a single cocatalytic site per each segment of the semiconductor capable of light excitation. This is to ensure that intermediates are formed at close proximity. These findings are demonstrated by evaluating the efficiency for hydrogen production over a nanoparticle-based photocatalyst consisting of Pt-decorated CdSe@CdS rods. Rods decorated with a single Pt catalyst were found to be the most active for hydrogen production, with QE of 27%, while rods having two reduction sites reached QE of only 18% and rods with multiple sites showed very low activity. The advantage of using a single catalytic site became negligible when the rods were employed in catalyzing a single electron reaction. We believe the implications of this finding are of significance for the proper design of photocatalysts aimed at solar-to-fuel energy conversion.

One-dimensional hybrid nanostructures for heterogeneous photocatalysis and photoelectrocatalysis

Semiconductor-based photocatalysis and photoelectrocatalysis have received considerable attention as alternative approaches for solar energy harvesting and storage. The photocatalytic or photoelectrocatalytic performance of a semiconductor is closely related to the design of the semiconductor at the nanoscale. Among various nanostructures, one-dimensional (1D) nanostructured photocatalysts and photoelectrodes have attracted increasing interest owing to their unique optical, structural, and electronic advantages. In this article, a comprehensive review of the current research efforts towards the development of 1D semiconductor nanomaterials for heterogeneous photocatalysis and photoelectrocatalysis is provided and, in particular, a discussion of how to overcome the challenges for achieving full potential of 1D nanostructures is presented. It is anticipated that this review will afford enriched information on the rational exploration of the structural and electronic properties of 1D semiconductor nanostructures for achieving more efficient 1D nanostructure-based photocatalysts and photoelectrodes for high-efficiency solar energy conversion.

Universal Length Dependence of Rod-to-Seed Exciton Localization Efficiency in Type I and Quasi-Type II CdSe@CdS Nanorods

A critical step involved in many applications of one-dimensional seeded CdSe@CdS nanorods, such as luminescent solar concentrators, optical gains, and photocatalysis, is the localization of excitons from the light-harvesting CdS nanorod antenna into the light-emitting CdSe quantum dot seed. We report that the rod-to-seed exciton localization efficiency decreases with the rod length but is independent of band alignment between the CdSe seed and CdS rod. This universal dependence can be well modeled by the competition between exciton one-dimensional diffusion to the CdSe seed and trapping on the CdS rod. This finding provides a rational approach for optimizing these materials for their various device applications.

Metal hybrid nanoparticles for catalytic organic and photochemical transformations

In order to understand heterogeneous catalytic reactions, model catalysts such as a single crystalline surface have been widely studied for many decades. However, catalytic systems that actually advance the reactions are three-dimensional and commonly have multiple components including active metal nanoparticles and metal oxide supports. On the other hand, as nanochemistry has rapidly been developed and been applied to various fields, many researchers have begun to discuss the impact of nanochemistry on heterogeneous catalysis. Metal hybrid nanoparticles bearing multiple components are structurally very close to the actual catalysts, and their uniform and controllable morphology is suitable for investigating the relationship between the structure and the catalytic properties in detail. In this Account, we introduce four typical structures of metal hybrid nanoparticles that can be used to conduct catalytic organic and photochemical reactions. Metal@silica (or metal oxide) yolk-shell nanoparticles, in which metal cores exist in internal voids surrounded by thin silica (or metal oxide) shells, exhibited extremely high thermal and chemical stability due to the geometrical protection of the silica layers against the metal cores. The morphology of the metal cores and the pore density of the hollow shells were precisely adjusted to optimize the reaction activity and diffusion rates of the reactants. Metal@metal oxide core-shell nanoparticles and inverted structures, where the cores supported the shells serving an active surface, exhibited high activity with no diffusion barriers for the reactants and products. These nanostructures were used as effective catalysts for various organic and gas-phase reactions, including hydrogen transfer, Suzuki coupling, and steam methane reforming. In contrast to the yolk- and core-shell structures, an asymmetric arrangement of distinct domains generated acentric dumbbells and tipped rods. A large domain of each component added multiple functions, such as magnetism and light absorption, to the catalytic properties. In particular, metal-semiconductor hybrid nanostructures could behave as effective visible photocatalysts for hydrogen evolution and CO oxidation reactions. Resulting from the large surface area and high local concentration of the reactants, a double-shell hollow structure showed reaction activities higher than those of filled nanoparticles. The introduction of plasmonic Au probes into the Pt-CdS double-shell hollow particles facilitated the monitoring of photocatalytic hydrogen generation that occurred on an individual particle surface by single particle measurements. Further development of catalysis research using well-defined metal hybrid nanocatalysts with various in situ spectroscopic tools provides a means of maximizing catalytic performances until they are comparable to or better than those of homogeneous catalysts, and this would have possibly useful implications for industrial applications.

Flower-like CdSe ultrathin nanosheet assemblies for enhanced visible-light-driven photocatalytic H2 production

Flower-like CdSe ultrathin nanosheet assemblies with excellent visible-light-driven photocatalytic H₂ evolution activity were prepared via a facile solvothermal method.

Uniform decoration of Pt nanoparticles on well-defined CdSe tetrapods and the effect of their Pt cluster size on photocatalytic H2generation

Direct decoration of Pt nanoparticles onto CdSe tetrapods with controlled sizes and their photocatalytic H₂generation efficiency.

Facile fabrication of InSe nanosheets: towards efficient visible-light-driven H2 production by coupling with P25

InSe nanosheets and their coupling with P25 have been successfully synthesized via a hydrothermal–calcining process, and they were for the first time used for visible light photocatalytic H₂ production.

Cu(2)ZnSnS(4)-Pt and Cu(2)ZnSnS(4)-Au heterostructured nanoparticles for photocatalytic water splitting and pollutant degradation

Cu2ZnSnS4, based on abundant and environmental friendly elements and with a direct band gap of 1.5 eV, is a main candidate material for solar energy conversion through both photovoltaics and photocatalysis. We detail here the synthesis of quasi-spherical Cu2ZnSnS4 nanoparticles with unprecedented narrow size distributions. We further detail their use as seeds to produce CZTS-Au and CZTS-Pt heterostructured nanoparticles. Such heterostructured nanoparticles are shown to have excellent photocatalytic properties toward degradation of Rhodamine B and hydrogen generation by water splitting.

Double heterojunction nanowire photocatalysts for hydrogen generation

Charge separation and charge transfer across interfaces are key aspects in the design of efficient photocatalysts for solar energy conversion. In this study, we investigate the hydrogen generating capabilities and underlying photophysics of nanostructured photocatalysts based on CdSe nanowires (NWs). Systems studied include CdSe, CdSe/CdS core/shell nanowires and their Pt nanoparticle-decorated counterparts. Femtosecond transient differential absorption measurements reveal how semiconductor/semiconductor and metal/semiconductor heterojunctions affect the charge separation and hydrogen generation efficiencies of these hybrid photocatalysts. In turn, we unravel the role of surface passivation, charge separation at semiconductor interfaces and charge transfer to metal co-catalysts in determining photocatalytic H2 generation efficiencies. This allows us to rationalize why Pt nanoparticle decorated CdSe/CdS NWs, a double heterojunction system, performs best with H2 generation rates of ∼434.29 ± 27.40 μmol h(-1) g(-1) under UV/Visible irradiation. In particular, we conclude that the CdS shell of this double heterojunction system serves two purposes. The first is to passivate CdSe NW surface defects, leading to long-lived charges at the CdSe/CdS interface capable of carrying out reduction chemistries. Upon photoexcitation, we also find that CdS selectively injects charges into Pt NPs, enabling simultaneous reduction chemistries at the Pt NP/solvent interface. Pt nanoparticle decorated CdSe/CdS NWs thus enable reduction chemistries at not one, but rather two interfaces, taking advantage of each junction's optimal catalytic activities.

Electron transfer kinetics in CdS nanorod-[FeFe]-hydrogenase complexes and implications for photochemical H₂ generation

This Article describes the electron transfer (ET) kinetics in complexes of CdS nanorods (CdS NRs) and [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI). In the presence of an electron donor, these complexes produce H2 photochemically with quantum yields of up to 20%. Kinetics of ET from CdS NRs to CaI play a critical role in the overall photochemical reactivity, as the quantum efficiency of ET defines the upper limit on the quantum yield of H2 generation. We investigated the competitiveness of ET with the electron relaxation pathways in CdS NRs by directly measuring the rate and quantum efficiency of ET from photoexcited CdS NRs to CaI using transient absorption spectroscopy. This technique is uniquely suited to decouple CdS→CaI ET from the processes occurring in the enzyme during H2 production. We found that the ET rate constant (k(ET)) and the electron relaxation rate constant in CdS NRs (k(CdS)) were comparable, with values of 10(7) s(-1), resulting in a quantum efficiency of ET of 42% for complexes with the average CaI:CdS NR molar ratio of 1:1. Given the direct competition between the two processes that occur with similar rates, we propose that gains in efficiencies of H2 production could be achieved by increasing k(ET) and/or decreasing k(CdS) through structural modifications of the nanocrystals. When catalytically inactive forms of CaI were used in CdS-CaI complexes, ET behavior was akin to that observed with active CaI, demonstrating that electron injection occurs at a distal iron-sulfur cluster and is followed by transport through a series of accessory iron-sulfur clusters to the active site of CaI. Using insights from this time-resolved spectroscopic study, we discuss the intricate kinetic pathways involved in photochemical H2 generation in CdS-CaI complexes, and we examine how the relationship between the electron injection rate and the other kinetic processes relates to the overall H2 production efficiency.

Beyond band alignment: hole localization driven formation of three spatially separated long-lived exciton states in CdSe/CdS nanorods

Colloidal one-dimensional semiconductor nanoheterostructures have emerged as an important family of functional materials for solar energy conversion, although the nature of the long-lived exciton state and their formation and dissociation dynamics remain poorly understood. In this paper we study these dynamics in CdSe/CdS dot-in-rod (DIR) NRs, a representative of 1D heterostructures, and DIR-electron-acceptor complexes by transient absorption spectroscopy. Because of a quasi-type II band alignment of CdSe and CdS, it is often assumed that there exists one long-lived exciton state with holes localized in the CdSe seed and electrons delocalized among CdSe and CdS. We show that excitation into the CdS rod forms three distinct types of long-lived excitons that are spatially localized in the CdS rod, in and near the CdSe seed and in the CdS shell surrounding the seed. The branching ratio of forming these exciton states is controlled by the competition between the band offset driven hole localization to the CdSe seed and hole trapping to the CdS surface. Because of dielectric contrast induced strong electron-hole interaction in 1D materials, the competing hole localization pathways lead to spatially separated long-lived excitons. Their distinct spatial locations affect their dissociation rates in the presence of electron acceptors, which has important implications for the application of 1D heterostructures as light-harvesting materials.