Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications.

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

Dual-plasmonic Au@Cu7S4 yolk@shell nanocrystals for photocatalytic hydrogen production across visible to near infrared spectral region.

Near infrared energy remains untapped toward the maneuvering of entire solar spectrum harvesting for fulfilling the nuts and bolts of solar hydrogen production. We report the use of Au@Cu7S4 yolk@shell nanocrystals as dual-plasmonic photocatalysts to achieve remarkable hydrogen production under visible and near infrared illumination. Ultrafast spectroscopic data reveal the prevalence of long-lived charge separation states for Au@Cu7S4 under both visible and near infrared excitation. Combined with the advantageous features of yolk@shell nanostructures, Au@Cu7S4 achieves a peak quantum yield of 9.4% at 500 nm and a record-breaking quantum yield of 7.3% at 2200 nm for hydrogen production in the absence of additional co-catalysts. The design of a sustainable visible- and near infrared-responsive photocatalytic system is expected to inspire further widespread applications in solar fuel generation. In this work, the feasibility of exploiting the localized surface plasmon resonance property of self-doped, nonstoichiometric semiconductor nanocrystals for the realization of wide-spectrum-driven photocatalysis is highlighted.

Spatial Separation of Cocatalysts on Z-Scheme Organic/Inorganic Heterostructure Hollow Spheres for Enhanced Photocatalytic H2 Evolution and In-Depth Analysis of the Charge-Transfer Mechanism.

A Z-scheme heterojunction with spatially separated cocatalysts is proposed for overcoming fundamental issues in photocatalytic water splitting, such as inefficient light absorption, charge recombination, and sluggish reaction kinetics. For efficient light absorption and interfacial charge separation, Z-scheme organic/inorganic heterojunction photocatalysts are synthesized by firmly immobilizing ultrathin g-C3 N4 on the surface of TiO2 hollow spheres via electrostatic interactions. Additionally, two cocatalysts, Pt and IrOx , are spatially separated along the Z-scheme charge-transfer pathway to enhance surface charge separation and reaction kinetics. The as-prepared Pt/g-C3 N4 /TiO2 /IrOx (PCTI) hollow sphere photocatalyst exhibits an exceptional H2 evolution rate of 8.15 mmol h-1 g-1 and a remarkable apparent quantum yield of 24.3% at 330 nm in the presence of 0.5 wt% Pt and 1.2 wt% IrOx cocatalysts on g-C3 N4 and TiO2 , respectively. Photoassisted Kelvin probe force microscopy is used to systematically analyze the Z-scheme charge-transfer mechanism within PCTI. Furthermore, the benefits of spatially separating cocatalysts in the PCTI system are methodically investigated in comparison to randomly depositing them. This work adequately demonstrates that the combination of a Z-scheme heterojunction and spatially separated cocatalysts can be a promising strategy for designing high-performance photocatalytic platforms for solar fuel production.

A Sensor-Integrated Face Mask Using Au@SnO2 Nanoparticle Modified Fibers and Augmented Reality Technology.

In this work, we develop a wireless sensor-integrated face mask using Au@SnO2 nanoparticle-modified conductive fibers based on augmented reality (AR) technology. AR technology enables the overlay of real objects and environments with virtual 3D objects and allows virtual interactions with real objects to create desired meanings. With the help of the AR system, the size of the mask could be precisely estimated and then manufactured using 3D printing technology. The body temperature sensor and respiratory sensor were integrated into the mask so that vital parameters of the human body could be continuously monitored without removing the personal protective equipment. Furthermore, the outer part of the mask consists of conductive fabric modified with Au@SnO2 core-shell nanoparticle additives, which enhanced the filtration efficiency of airborne aerosols. A significant improvement in the filtration efficiency of particulate matter 2.5 was observed after applying an external voltage to the conductive textiles. A smartwatch with a heart rate sensor was paired with the mask to display sensor data on the mask through wireless transmission. Therefore, this sensor-integrated mask system with AR technology provides the first line of defense to combat global threats from pathogens and air pollutants.

Au@Cu2O Core-Shell and Au@Cu2Se Yolk-Shell Nanocrystals as Promising Photocatalysts in Photoelectrochemical Water Splitting and Photocatalytic Hydrogen Production.

In this work, we demonstrated the practical use of Au@Cu2O core-shell and Au@Cu2Se yolk-shell nanocrystals as photocatalysts in photoelectrochemical (PEC) water splitting and photocatalytic hydrogen (H2) production. The samples were prepared by conducting a sequential ion-exchange reaction on a Au@Cu2O core-shell nanocrystal template. Au@Cu2O and Au@Cu2Se displayed enhanced charge separation as the Au core and yolk can attract photoexcited electrons from the Cu2O and Cu2Se shells. The localized surface plasmon resonance (LSPR) of Au, on the other hand, can facilitate additional charge carrier generation for Cu2O and Cu2Se. Finite-difference time-domain simulations were carried out to explore the amplification of the localized electromagnetic field induced by the LSPR of Au. The charge transfer dynamics and band alignment of the samples were examined with time-resolved photoluminescence and ultraviolet photoelectron spectroscopy. As a result of the improved interfacial charge transfer, Au@Cu2O and Au@Cu2Se exhibited a substantially larger photocurrent of water reduction and higher photocatalytic activity of H2 production than the corresponding pure counterpart samples. Incident photon-to-current efficiency measurements were conducted to evaluate the contribution of the plasmonic effect of Au to the enhanced photoactivity. Relative to Au@Cu2O, Au@Cu2Se was more suited for PEC water splitting and photocatalytic H2 production by virtue of the structural advantages of yolk-shell architectures. The demonstrations from the present work may shed light on the rational design of sophisticated metal-semiconductor yolk-shell nanocrystals, especially those comprising metal selenides, for superior photocatalytic applications.

Crystal Facet Dependent Energy Band Structures of Polyhedral Cu2O Nanocrystals and Their Application in Solar Fuel Production.

We demonstrated a facile hydrothermal method to synthesize the (100)-, (110)- and (111)-oriented Cu2O nanocrystals (NCs) by controlling the concentration of the incorporated anions (CO32- and SO32-). The crystal facet dependent activity of the orientation controlled Cu2O NCs in the rhodamine B (RhB) photodegradation and photocatalytic hydrogen (H2) evolution was found to follow the trend: (111) > (110) > (100). The mechanism was investigated by characterizing the optical property, energy band structure, interfacial charge carrier dynamics and reducing ability. The results indicated that the (111)-oriented Cu2O NCs exhibit the higher conduction band (CB) potential as compared with the (110)-oriented and (100)-oriented Cu2O NCs, which resulted in the largest driving force of interfacial electron transfer for (111)-oriented Cu2O NCs to carry out solar fuel generation. The current study offers an easy strategy for crystal facet engineering of semiconductors and provides important physical insights into their electronic properties for the desired solar energy conversions.

Free-Standing, Interwoven Tubular Graphene Mesh-Supported Binary AuPt Nanocatalysts: An Innovative and High-Performance Anode Methanol Oxidation Catalyst.

Pt-based alloy or bimetallic anode catalysts have been developed to reduce the carbon monoxide (CO) poisoning effect and the usage of Pt in direct methanol fuel cells (DMFCs), where the second metal plays a role as CO poisoning inhibitor on Pt. Furthermore, better performance in DMFCs can be achieved by improving the catalytic dispersion and using high-performance supporting materials. In this work, we introduced a free-standing, macroscopic, interwoven tubular graphene (TG) mesh as a supporting material because of its high surface area, favorable chemical inertness, and excellent conductivity. Particularly, binary AuPt nanoparticles (NPs) can be easily immobilized on both outer and inner walls of the TG mesh with a highly dispersive distribution by a simple and efficient chemical reduction method. The TG mesh, whose outer and inner walls were decorated with optimized loading of binary AuPt NPs, exhibited a remarkably catalytic performance in DMFCs. Its methanol oxidation reaction (MOR) activity was 10.09 and 2.20 times higher than those of the TG electrodes with only outer wall immobilized with pure Pt NPs and binary AuPt NPs, respectively. Furthermore, the catalyst also displayed a great stability in methanol oxidation after 200 scanning cycles, implying the excellent tolerance toward the CO poisoning effect.

Triple-Channel Charge Transfer over W18O49/Au/g-C3N4 Z-Scheme Photocatalysts for Achieving Broad-Spectrum Solar Hydrogen Production.

Z-scheme heterojunctions are fundamentally promising yet practically appealing for photocatalytic hydrogen (H2) production owing to the enhanced redox power, spatial separation of charge carriers, and broad-spectrum solar light harvesting. The charge-transfer dynamics at Z-scheme heterojunctions can be accelerated by inserting charge-transfer mediators at the heterojunction interfaces. In this study, we introduce Au nanoparticle mediators in the Z-scheme W18O49/g-C3N4 heterostructure, which enables an improved H2 production rate of 3465 µmol/g·h compared with the direct Z-scheme W18O49/g-C3N4 (1785 µmol/g·h) under 1 sun irradiation. The apparent quantum yields of H2 production with W18O49/Au/g-C3N4 are 3.9% and 9.3% at 420 and 1200 nm, respectively. The improved photocatalytic H2 production activity of W18O49/Au/g-C3N4 is attributable to the triple-channel charge-transfer mechanism: channel I─Z-scheme charge transfer facilitates charge separation and increased redox power of the photoexcited electrons; channels II and III─the localized surface plasmon resonances from Au (channel II) and W18O49 (channel III) enable light harvesting extension from visible to near-infrared wavelengths.

Flexible BiVO4/WO3/ITO/Muscovite Heterostructure for Visible-Light Photoelectrochemical Photoelectrode.

Flexible electronics has recently captured extensive attention due to its intriguing functionalities and great potential for influencing our daily life. In addition, with the increasing demand for green energy, photoelectrochemical (PEC) water splitting is a clean process that directly converts solar energy to chemical energy in the form of hydrogen. Thus the development of flexible green energy electronics represents a new domain in the research field of energy harvesting. In this work, we demonstrate the BiVO4 (BVO)/WO3/ITO/muscovite heterostructure photoelectrode for water splitting with flexible characteristics. The performance of BVO was modified by specific crystal facets, and the BVO/WO3 bilayer exhibited superior performance of 33% enhanced PEC activity at 1 V vs Ag/AgCl compared with pure BVO due to the proper staggered band alignment. Moreover, excellent mechanical stability was verified by a series of bending modes. This study demonstrates a pathway to a flexible photoelectrode for developing innovative devices for solar fuel generation.

Distinct Carrier Transport Properties Across Horizontally vs Vertically Oriented Heterostructures of 2D/3D Perovskites.

Two-dimensional-on-three-dimensional (2D/3D) halide perovskite heterostructures have been extensively utilized in optoelectronic devices. However, the labile nature of halide perovskites makes it difficult to form such heterostructures with well-defined compositions, orientations, and interfaces, which inhibits understanding of the carrier transfer properties across these heterostructures. Here, we report solution growth of both horizontally and vertically aligned 2D perovskite (PEA)2PbBr4 (PEA = phenylethylammonium) microplates onto 3D CsPbBr3 single crystal thin films, with well-defined heterojunctions. Time-resolved photoluminescence (TRPL) transients of the heterostructures exhibit the monomolecular and bimolecular dynamics expected from exciton annihilation, dissociation, and recombination, as well as evidence for carrier transfer in these heterostructures. Two kinetic models based on Type-I and Type-II band alignments at the interface of horizontal 2D/3D heterostructures are applied to reveal a shift in balance between carrier transfer and recombination: Type-I band alignment better describes the behaviors of heterostructures with thin 2D perovskite microplates but Type-II band alignment better describes those with thick 2D microplates (>150 nm). TRPL of vertically aligned 2D microplates is dominated by directly excited PL and is independent of the height above the 3D film. Electrical measurements reveal current rectification behaviors in both heterostructures with vertical heterostructures showing better electrical transport. As the first systematic study on comparing models of 2D/3D perovskite heterostructures with controlled orientations and compositions, this work provides insights on the charge transfer mechanisms in these perovskite heterostructures and guidelines for designing better optoelectronic devices.

Yolk-shell nanostructures: synthesis, photocatalysis and interfacial charge dynamics.

Solar energy has long been regarded as a promising alternative and sustainable energy source. In this regard, photocatalysts emerge as a versatile paradigm that can practically transform solar energy into chemical energy. At present, unsatisfactory conversion efficiency is a major obstacle to the widespread deployment of photocatalysis technology. Many structural engineering strategies have been proposed to address the issue of insufficient activity for semiconductor photocatalysts. Among them, creation of yolk-shell nanostructures which possess many beneficial features, such as large surface area, efficient light harvesting, homogeneous catalytic environment and enhanced molecular diffusion kinetics, has attracted particular attention. This review summarizes the developments that have been made for the preparation and photocatalytic applications of yolk-shell nanostructures. Additional focus is placed on the realization of interfacial charge dynamics and the possibility of achieving spatial separation of charge carriers for this unique nanoarchitecture as charge transfer is the most critical factor determining the overall photocatalytic efficiency. A future perspective that can facilitate the advancement of using yolk-shell nanostructures in sophisticated photocatalytic systems is also presented.

Size and temperature dependence of photoluminescence of hybrid perovskite nanocrystals.

In this work, we studied the effects of particles' size and temperature on the photoluminescence (PL) of CH3NH3PbBr3 perovskite nanocrystals (PNCs), with the PNC size controlled by varying the surface passivating ligands. The structural and optical properties of the PNCs were investigated using UV-Vis and PL spectroscopy, revealing strong quantum confinement effects. Temperature dependent PL measurements showed the spectral blue shift of the PL peak for the small PNCs (3.1 ± 0.2 nm) with decreasing temperature from 300 K to 20 K, which is opposite to the red shift with decreasing temperature observed for large- (9.2 ± 0.5 nm) and middle-sized (5.1 ± 0.3 nm) PNCs. The PL lifetime also increased with increasing temperature for the larger PNCs, while it remained about the same for the small and middle-sized PNCs. This increase in lifetime with temperature is attributed to exciton dissociation to free carriers at higher temperatures and to the formation of polar domains in the PNCs. However, the small and middle-sized PNCs did not show such a trend, which may be due to efficient defect passivation as higher concentration of 3-aminopropyl trimethoxysilane (APTMS) was used and to the role of particle size in surface state delocalization. Cryo-X-ray diffraction showed no new peak formation or peak splitting as temperature was varied, which suggests efficient crystal phase stabilization in PNCs of all three sizes controlled by the concentration of APTMS. These results emphasize the importance of size and surface properties of PNCs in their optical properties such as PL quantum yield, PL lifetime, and crystal phase stability.

In Situ Analysis of Growth Behaviors of Cu2O Nanocubes in Liquid Cell Transmission Electron Microscopy.

Metal oxides have attracted substantial attention over the years and are commonly used in the semiconductor industry because of their excellent physical and chemical properties. Among the various metal oxides, cuprous oxide (Cu2O) is regarded as a promising material. It is inexpensive, earth-abundant, and nontoxic; therefore, it can be used in catalysis, sensors, solar cells, and p-type semiconductors. However, the redox reaction of Cu2O is still uncertain. The size, morphology, and structure of Cu2O strongly influence its properties. In this work, we developed a new synthesis method of Cu2O that involves reducing the precursor by an electron beam without reducing agent. The growth process of Cu2O nanocubes was observed via in situ liquid cell transmission electron microscopy (in situ LCTEM). The nucleation kinetics, oscillating growth behavior, and redox reaction of the Cu2O nanocubes in the liquid phase were systematically studied. Cu2O exhibited a round shape at the beginning and transformed into a cubic shape afterward. Interestingly, the Cu2O nanocubes grew clearly under long-term observation; however, their diameters increased and fluctuated during the short-term observation. The electron beam not only stimulated the solution to reduce the nanocubes but also caused electron radiation effect to the nanocubes. During the Cu2O growth and dissolution, the cubic shape evolved with specific planes in the {100} family. Our direct observation sheds light on the preparation of Cu2O by a reduction method, extending the study of reaction kinetics and providing a new way to synthesize metal oxides.

In situ TEM observation of Au-Cu2O core-shell growth in liquids.

Heterogeneous nanoparticles are widely used in catalysis, sensors and biology due to their versatile functions. Among the various heterogeneous nanoparticles, Au-Cu2O core-shell nanoparticles show high stability and short response times for use as sensors and catalysts and have thus attracted much attention. Previous studies show that the properties of Au-Cu2O are mainly related to the shape and size of the Au-Cu2O nanoparticles. However, the forming behavior of heterostructures and the mechanism have not been fully explored. In this work, liquid cell transmission electron microscopy (LCTEM) was used to investigate the formation of these interesting Au-Cu2O nanoparticles and their process of aggregation. The electron beam and dispersion of gold nanoparticles are both important parameters for the reduction reaction in in situ LCTEM. The Au-Cu2O core-shell nanoparticles can be synthesized to have two morphologies, multifaceted and cubic. The nanoparticles grew into these different morphologies due to the amount of remaining citrate ligands on the surface of the gold nanoparticles. For the multifaceted nanoparticles, the epitaxy of the two components is confirmed from high-resolution TEM images and electron diffraction patterns with an epitaxial relationship of Au (020)//Cu2O (020) and Au [101]//Cu2O [101]. The growth rate is approximately 210 nm2 s-1. On the other hand, the cubic nanoparticles nucleate and grow independently. The growth kinetics and elemental distributions have been systematically studied. In addition, the nanoclusters would float, rotate, and finally aggregate with the surrounding clusters. This in situ experiment sheds light on the growth mechanisms of nanostructures and will improve the applicability and controllability of heterostructure synthesis.

Enhanced photoelectrochemical hydrogen generation in neutral electrolyte using non-vacuum processed CIGS photocathodes with an earth-abundant cobalt sulfide catalyst.

This work reports the novelty of using eco-friendly and cost-effective non-vacuum Electrostatic Spray-Assisted Vapour Deposited Cu(In,Ga)SSe (CIGS) thin films as photocathodes, combined with the earth abundant cobalt sulfide (Co-S) as a catalyst to accelerate the kinetics of photogenerated electron transfer and hydrogen generation for photoelectrochemical water splitting. CdS and ZnO layers were subsequently deposited on top of the selenised CIGS films to increase the charge separation and lower the charge recombination for the photocathodes. In order to improve the lifetime and scalability of the CIGS photocathode and the other cell components, a photoelectrochemical test was conducted in a neutral electrolyte of 0.5 M Na2SO4 under simulated sunlight (AM 1.5G). Both the photocurrent densities and the onset potentials of the photocathodes were significantly improved by the electrodeposition of the low cost and earth-abundant Co-S catalyst, with a photocurrent density as high as 19.1 mA cm-2 at -0.34 V vs. reversible hydrogen electrode (RHE), comparable with and even higher than that of the control photocathode using rare and precious Pt as a catalyst.

Electroless deposition of RuO2-based nanoparticles for energy conversion applications.

This study reports a delicate electroless approach for the deposition of RuO2·nH2O nanoparticles on the VO x ·mH2O nanowires and this method can be extended to deposit RuO2·nH2O nanoparticles on various material surfaces. Electrochemical characterizations, including linear sweep voltammetry (LSV), electrochemical quartz crystal microbalance (QCM) analysis and rotating ring-disc electrode (RRDE) voltammetry, were carried out to investigate the growth mechanism. The deposition involves the catalytic reduction of dissolved oxygen by the V4+ species of VO x ·mH2O, which drives the oxidation of RuCl3 to proceed with the growth of RuO2·nH2O. This core/shell VO x ·mH2O/RuO2·nH2O shows a better catalytic activity of the oxygen reduction reaction (ORR) than RuO2·nH2O, which is ascribed to the pronounced dispersion of RuO2·nH2O. Such an electroless approach was applicable to the preparation of a RuO2-based nanoparticle suspension as well as the deposition of nanocrystalline RuO2·nH2O on other functional supports like TiO2 nanowires. The thus-obtained RuO2-decorated TiO2 nanorods exhibit significantly an enhanced photoactivity toward photoelectrochemical water oxidation. The versatility of the current electroless approach may facilitate the widespread deployment of nanocrystalline RuO2·nH2O in a variety of energy-related applications.

TiO2 Nanowire-Supported Sulfide Hybrid Photocatalysts for Durable Solar Hydrogen Production.

As the feet of clay, photocorrosion induced by hole accumulation has placed serious limitations on the widespread deployment of sulfide nanostructures for photoelectrochemical (PEC) water splitting. Developing sufficiently stable electrodes to construct durable PEC systems is therefore the key to the realization of solar hydrogen production. Here, an innovative charge-transfer manipulation concept based on the aligned hole transport across the interface has been realized to enhance the photostability of In2S3 electrodes toward PEC solar hydrogen production. The concept was realized by conducting compact deposition of In2S3 nanocrystals on the TiO2 nanowire array. Under PEC operation, the supporting TiO2 nanowires functioned as an anisotropic charge-transfer backbone to arouse aligned charge transport across the TiO2-In2S3 interface. Because of the aligned hole transport, the TiO2 nanowire-supported In2S3 hybrid nanostructures (TiO2-In2S3) exhibited improved hole-transfer dynamics at the TiO2-In2S3 interface and enhanced hole injection kinetics at the electrode surface, substantially increasing the long-term photostability toward solar hydrogen production. The PEC durability tests showed that TiO2-In2S3 electrodes can achieve nearly 90.9% retention of initial photocurrent upon continuous irradiation for 6 h, whereas the pure In2S3 merely retained 20.8% of initial photocurrent. This double-gain charge-transfer manipulation concept is expected to convey a viable approach to the intelligent design of highly efficient and sufficiently stable sulfide photocatalysts for sustainable solar fuel generation.

Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O Polyhedra Arising from Tunable Interfacial Band Alignment.

ZnS particles were grown over Cu2O cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu2O cubes stay photocatalytically inactive. ZnS-decorated Cu2O octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photoexcitation. Surprisingly, Cu2O rhombic dodecahedra give greatly suppressed photocatalytic activity after ZnS deposition. Electron paramagnetic resonance spectra agree with these experimental observations. Time-resolved photoluminescence profiles provide charge-transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu2O(110)/ZnS(200) plane interface. A modified Cu2O-ZnS band diagram is presented. Density functional theory calculations generating plane-specific band energy diagrams of Cu2O and ZnS match well with the experimental results, showing that charge transfer across the Cu2O(110)/ZnS(200) plane interface would not happen. This example further illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.

Fully Depleted Ti-Nb-Ta-Zr-O Nanotubes: Interfacial Charge Dynamics and Solar Hydrogen Production.

Poor kinetics of hole transportation at the electrode/electrolyte interface is regarded as a primary cause for the mediocre performance of n-type TiO2 photoelectrodes. By adopting nanotubes as the electrode backbone, light absorption and carrier collection can be spatially decoupled, allowing n-type TiO2, with its short hole diffusion length, to maximize the use of the available photoexcited charge carriers during operation in photoelectrochemical (PEC) water splitting. Here, we presented a delicate electrochemical anodization process for the preparation of quaternary Ti-Nb-Ta-Zr-O mixed-oxide (denoted as TNTZO) nanotube arrays and demonstrated their utility in PEC water splitting. The charge-transfer dynamics for the electrodes was investigated using time-resolved photoluminescence, electrochemical impedance spectroscopy, and the decay of open-circuit voltage analysis. Data reveal that the superior photoactivity of TNTZO over pristine TiO2 originated from the introduction of Nd, Ta, and Zr elements, which enhanced the amount of accessible charge carriers, modified the electronic structure, and improved the hole injection kinetics for expediting water splitting. By modulating the water content of the electrolyte employed in the anodization process, the wall thickness of the grown TNTZO nanotubes can be reduced to a size smaller than that of the depletion layer thickness, realizing a fully depleted state for charge carriers to further advance the PEC performance. Hydrogen evolution tests demonstrate the practical efficacy of TNTZO for realizing solar hydrogen production. Furthermore, with the composition complexity and fully depleted band structure, the present TNTZO nanotube arrays may offer a feasible and universal platform for the loading of other semiconductors to construct a sophisticated heterostructure photoelectrode paradigm, in which the photoexcited charge carriers can be entirely utilized for efficient solar-to-fuel conversion.

Metal-Particle-Decorated ZnO Nanocrystals: Photocatalysis and Charge Dynamics.

Understanding of charge transfer processes is determinant to the performance optimization for semiconductor photocatalysts. As a representative model of composite photocatalysts, metal-particle-decorated ZnO has been widely employed for a great deal of photocatalytic applications; however, the dependence of charge carrier dynamics on the metal content and metal composition and their correlation with the photocatalytic properties have seldom been reported. Here, the interfacial charge dynamics for metal-decorated ZnO nanocrystals were investigated and their correspondence with the photocatalytic properties was evaluated. The samples were prepared with a delicate antisolvent approach, in which ZnO nanocrystals were grown along with metal particle decoration in a deep eutectic solvent. By modulating the experimental conditions, the metal content (from 0.6 to 2.3 at%) and metal composition (including Ag, Au, and Pd) in the resulting metal-decorated ZnO could be readily controlled. Time-resolved photoluminescence spectra showed that an optimal Au content of 1.3 at% could effectuate the largest electron transfer rate constant for Au-decorated ZnO nanocrystals, in conformity with the highest photocatalytic efficiency observed. The relevance of charge carrier dynamics to the metal composition was also inspected and realized in terms of the energy level difference between ZnO and metal. Among the three metal-decorated ZnO samples tested, ZnO-Pd displayed the highest photocatalytic activity, fundamentally according with the largest electron transfer rate constant deduced in carrier dynamics measurements. The current work was the first study to present the correlations among charge carrier dynamics, metal content, metal composition, and the resultant photocatalytic properties for semiconductor/metal heterostructures. The findings not only helped to resolve the standing issues regarding the mechanistic foundation of photocatalysis but also shed light on the intelligent design of semiconductor/metal composite systems to consolidate their utility in photocatalytic fields.