Plasmonic p–n Junction for Infrared Light to Chemical Energy Conversion

Hollow cubic CuSe@CdS with tunable size for plasmon-induced Vis-NIR driven photocatalytic properties

The rational design of the dimension and geometry of a plasmonic semiconductor cocatalyst is vitally important for efficient utilization of near-infrared (NIR) light and superior photocatalytic hydrogen generation. Herein, hollow cubic CuSe@CdS composites with different sizes and strong localized surface plasmon resonance (LSPR) were prepared by selenizing size-tunable Cu2O templates and loading CdS nanoparticles. The size of hollow cubic CuSe can affect the surface area and the conduction band potential through the size effect, regulating the carrier behavior of the CuSe@CdS heterojunction. The CuSe@CdS composites show enhanced and wide absorption in the full spectrum due to the LSPR effect of CuSe. Meanwhile, the composites show excellent photocatalytic hydrogen capacity in the full spectrum in a 0.35 M Na2S/0.25 M Na2SO3 sacrificial reagent solution. The best hydrogen production rate of CSCE2 is 1.518 mmol g-1 h-1 (5.54 times higher than that of CdS) under Vis light (780 > λ > 420 nm) irradiation and 0.28 mmol g-1 h-1 under NIR light (λ > 780 nm) illumination. Interestingly, the photocatalytic activity for H2 under Vis-NIR light (λ > 420 nm) is about 3 times (up to 4.45 mmol g-1 h-1) higher than that without NIR light assistance, due to the photothermal effect. Various analyses and DFT calculations demonstrate that the p-n heterojunction formed in the composites consists of p-type CuSe and n-type CdS, which achieves efficient carrier transfer and separation under the synergistic effect of the size effect and the photothermal effect. In addition, the expansion of the photocatalytic performance to the NIR range is mainly due to the "hot-electron" injection mechanism induced by the LSPR effect of CuSe. The reasonable design coupled with the plasmonic materials offers a new path to achieving the highly efficient conversion of solar energy to hydrogen energy.

P-N Heterojunction Embedded CuS/TiO2 Bifunctional Photocatalyst for Synchronous Hydrogen Production and Benzylamine Conversion

The coupling of photocatalytic hydrogen production and selective oxidation of benzylamine is a topic of significant research interest. However, enhancing the bifunctional photocatalytic activity in this context is still a major challenge. The construction of Z-scheme heterojunctions is an effective strategy to enhance the activity of bifunctional photocatalysts. Herein, a p-n type direct Z-scheme heterojunction CuS/TiO2 is constructed using metal-organic framework (MOF)-derived TiO2 as a substrate. The carrier density is measured by Mott-Schottky under photoexcitation, which confirms that the Z-scheme electron transfer mode of CuS/TiO2 is driven by the diffusion effect caused by the carrier concentration difference. Benefiting from efficient charge separation and transfer, photogenerated electrons, and holes are directedly transferred to active oxidation and reduction sites. CuS/TiO2 also exhibits excellent bifunctional photocatalytic activity without noble metal cocatalysts. Among them, the H2 evolution activity of the CuS/TiO2 is found to be 17.1 and 29.5 times higher than that of TiO2 and CuS, respectively. Additionally, the yields of N-Benzylidenebenzylamine (NBB) are 14.3 and 47.4 times higher than those of TiO2 and CuS, respectively.

Mid-IR Light-Activatable Full Spectrum LaB6 Plasmonic Photocatalyst

Photocatalysts as long-lasting, benign reagents for disinfection of bacteria in hospitals and public areas/facilities/transportation vehicles are strongly needed. A common limitation for all existing semiconductor photocatalysts is the requirement of activation by external UV-vis-near-infrared (NIR) light with wavelengths shorter than ≈1265 nm. None of the existing photocatalysts can function during nighttime in the absence of external light. Herein, an unprecedented LaB6 plasmonic photocatalyst is reported, which can absorb UV-vis-NIR light and mid-IR (3900 nm) light to split water and generate hydrogen and hydroxyl radicals for the decomposition of organic pollutants, as well as kill multidrug-resistant Escherichia coli bacteria. Mid-IR light (≈2-50 µm) is readily available from the natural environments via thermal radiation of warm/hot objects on the earth including human bodies, animals, furnances, hot/warm electrical devices, and buildings.

Potassium permanganate oxidation enhanced by infrared light and its application to natural water

Photocoupled permanganate (PM) is an effective way to enhance the oxidation efficiency of PM, however, the activation of PM by infrared has received little attention. This study aimed to investigate the ability of infrared light to activate PM. When coupled with infrared, the degradation rate of 4-chlorophenol (4-CP) is increased to 3.54 times of PM oxidation alone. The accelerated reaction was due to the formation of vibrationally excited PM by absorbing 3.1 kJ mol-1 infrared energy, which also leads to the primary reactive intermediates Mn(V/IV) in the reaction system. The infrared coupled PM system also showed 1.14-2.34 times promotion effect on other organic pollutants. Furthermore, solar composed of 45% infrared, coupled PM system showed excellent degradation performance, where the degradation of 4-CP in 10 L of tap water and river water was 68 and 23 times faster than in ultrapure water, respectively. The faster-increased degradation rate in natural waters is mainly due to the abundant inorganic ions, which can stabilize the manganese species, and then has a positive effect on 4-CP degradation. In summary, this work develops a energy-efficient photoactivated PM technology that utilizes infrared and provides new insights into the design of novel sunlight-powered oxidation processes for water treatment.

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.

Synthesis of oriented J type ZnIn2S4@CdIn2S4 heterojunction by controllable cation exchange for enhancing photocatalytic hydrogen evolution

Construction of semiconductor heterojunctions which promote the separation and transport of photogenerated carriers is an effective strategy for improving photocatalytic reaction efficiency. Based on the anisotropic electrical conductivity of layered ZnIn2S4 (ZIS) photocatalyst, an efficient heterojunction should be constructed along the layer plane of ZIS, that is, a J type heterojunction. However, achieving controllable synthesis of the oriented heterojunction of ZIS faces challenges. Herein, we develop a facile, cost-effective and spatially-selective cation exchange synthesis approach to construct J type ZnIn2S4@CdIn2S4 (J-ZIS@CIS) heterojunction using a flower-like hexagonal ZIS as the parent material. The developed synthesis approach can also control crystal structure of the heterojunction component CIS. This work presents a facile and controllable synthesis strategy to construct oriented anisotropic heterojunctions that are otherwise inaccessible. The as-prepared J-ZIS@CIS heterojunction displays a greatly enhanced photocatalytic hydrogen evolution activity with a rate of 183 μmol h-1, 2.77 times higher than that of pristine ZIS. Furthermore, the possible photocatalytic reaction mechanism is presented for the heterojunction.

Infrared Light-Induced Anomalous Defect-Mediated Plasmonic Hot Electron Transfer for Enhanced Photocatalytic Hydrogen Evolution

Efficient utilization of infrared (IR) light, which occupies almost half of the solar energy, is an important but challenging task in solar-to-fuel transformation. Herein, we report the discovery of CuS@ZnS core@shell nanocrystals (CSNCs) with strong localized surface plasmon resonance (LSPR) characteristics in the IR light region showing enhanced photocatalytic activity in hydrogen evolution reaction (HER). A unique "plasmon-induced defect-mediated carrier transfer" (PIDCT) at the heterointerfaces of the CSNCs divulged by time-resolved transient spectroscopy enables producing a high quantum yield of 29.2%. The CuS@ZnS CSNCs exhibit high activity and stability in H₂ evolution under near-IR light irradiation. The HER rate of CuS@ZnS CSNCs at 26.9 μmol h^-1 g^-1 is significantly higher than those of CuS NCs (0.4 μmol h^-1 g^-1) and CuS/ZnS core/satellite heterostructured NCs (15.6 μmol h^-1 g^-1). The PIDCT may provide a viable strategy for the tuning of LSPR-generated carrier kinetics through controlling the defect engineering to improve photocatalytic performance.

Observation of Charge Separation Enhancement in Plasmonic Photocatalysts under Coupling Conditions

Surface plasmon resonance-induced charge separation plays key roles in plasmon-related applications, especially in photocatalysis and photovoltaics. Plasmon coupling nanostructures exhibit extraordinary behaviors in hybrid states, phonon scattering, and ultrafast plasmon dephasing, but plasmon-induced charge separation in these materials remains unknown. Here, we design Schottky-free Au nanoparticle (NP)/NiO/Au nanoparticles-on-a-mirror plasmonic photocatalysts to support plasmon-induced interfacial hole transfer, evidenced by surface photovoltage microscopy at the single-particle level. In particular, we observe a nonlinear increase in charge density and photocatalytic performance with an increase in excitation intensity in plasmonic photocatalysts containing hot spots as a result of varying the geometry. Such charge separation increased the internal quantum efficiency by 14 times at 600 nm in catalytic reactions as compared to that of the Au NP/NiO without a coupling effect. These observations provide an improved understanding of charge transfer management and utilization by geometric engineering and interface electronic structure for plasmonic photocatalysis.

Strong synergy between gold nanoparticles and cobalt porphyrin induces highly efficient photocatalytic hydrogen evolution

The reaction efficiency of reactants near plasmonic nanostructures can be enhanced significantly because of plasmonic effects. Herein, we propose that the catalytic activity of molecular catalysts near plasmonic nanostructures may also be enhanced dramatically. Based on this proposal, we develop a highly efficient and stable photocatalytic system for the hydrogen evolution reaction (HER) by compositing a molecular catalyst of cobalt porphyrin together with plasmonic gold nanoparticles, around which plasmonic effects of localized electromagnetic field, local heating, and enhanced hot carrier excitation exist. After optimization, the HER rate and turn-over frequency (TOF) reach 3.21 mol g^-1 h^-1 and 4650 h^-1, respectively. In addition, the catalytic system remains stable after 45-hour catalytic cycles, and the system is catalytically stable after being illuminated for two weeks. The enhanced reaction efficiency is attributed to the excitation of localized surface plasmon resonance, particularly plasmon-generated hot carriers. These findings may pave a new and convenient way for developing plasmon-based photocatalysts with high efficiency and stability.

Hot hole transfer at the plasmonic semiconductor/semiconductor interface

Localized surface plasmon resonance (LSPR)-induced hot-carrier transfer provides an attractive alternative for light-harvesting using the full solar spectrum. This defect-mediated hot-carrier transfer is identical at the plasmonic semiconductor/semiconductor interface and can overcome the low efficiency of plasmonic energy conversion, thus boosting the efficiency of IR-light towards energy conversion. Here, using femtosecond transient absorption (TA) measurements, we directly observe the ultrafast non-radiative carrier dynamics of LSPR-driven hot holes created in CuS nanocrystals (NCs) and CuS/CdS hetero nanocrystals (HNCs). We demonstrate that in the CuS NCs, the relaxation dynamics follows multiple relaxation pathways. Two trap states are populated by the LSPR-induced hot holes in times (100-500 fs) that efficiently compete with the conventional LSPR mechanism (250 fs). The trapped hot holes intrinsically relax in 20-40 ps and then decay in 80 ns and 700 ns. In the CuS/CdS HNCs, once the CuS trap states have been populated by the LSPR-generated hot holes, the holes get transferred through plasmon induced transit hole transfer (PITCT) mechanism in 200-300 ps to the CdS acceptor phase and relax in 1-8 and 40-50 μs. The LSPR-recovery shows a weak excitation wavelength and fluence dependence, while the dynamics of the trap states remains largely unaffected. The direct observation of formation and decay processes of trap states and hole transfer from trap states provides important insight into controlling the LSPR-induced relaxation of degenerate semiconductors.

Spectroscopic and kinetic characterization of photogenerated charge carriers in photocatalysts

The catastrophic consequences of increased power consumption, such as drastically rising CO₂ levels, natural disasters, environmental pollution and dependence on fossil fuels supplied by countries with totalitarian regimes, illustrate the urge to develop sustainable technologies for energy generation. Photocatalysis presents eco-friendly means for fuels production via solar-to-chemical energy conversion. The conversion efficiency of a photocatalyst critically depends on charge carrier processes taking place in the ultrafast time regime. Transient absorption spectroscopy (TAS) serves as a perfect tool to track those processes. The spectral and kinetic characterization of charge carriers is indispensable for the elucidation of photocatalytic mechanisms and for the development of new materials. Hence, in this review, we will first present the basics of TAS and subsequently discuss the procedure required for the interpretation of the transient absorption spectra and transient kinetics. The discussion will include specific examples for charge carrier processes occurring in conventional and plasmonic semiconductors.

Dual-plasmon-enhanced nitrophenol hydrogenation over W18O49–Au heterostructures studied at the single-particle level

The dual-plasmonic W18O49–Au heterostructure exhibited enhanced catalytic performance in nitrophenol hydrogenation. The HEI process and coupling effect were demonstrated by single-particle spectroscopy and FDTD simulation.

Enhanced Charge Separation for Efficient Photocatalytic H2 Production by Long-Lived Trap-State-Induced Interfacial Charge Transfer

Photogeneration of charge carriers in semiconductors provides the scientific fundamental for photocatalytic water splitting. However, an ongoing challenge is the development of a new mechanism promoting charge carrier separation. Here we propose a trap-state-induced interfacial charge-transfer transition mechanism (TSICTT), in which electrons in long-lived trap states recombine with holes on the valence band (VB) of the semiconductor, thus prolonging the electron lifetime. We demonstrate this concept in the Sr₄Al₁₄O₂₅:Eu²⁺, Dy³⁺/CdS (SAO/CdS) heterostructure, where trapped electrons with a lifetime of up to several hours in the SAO persistent luminescence phosphor (PLP) can continuously consume holes on the VB of CdS nanoparticles (NPs). We discover that the interfacial interaction and the work function difference between SAO and CdS are crucial for the TSICTT, which finally contributes to the improved H₂ production from 34.4 to 1212.9 μmol g_CdS^-1 h^-1 under visible-light irradiation. This model introduces a new strategy to manipulate charge carrier transport for the effective utilization of solar energy.

Photogenerated hole traps in metal-organic-framework photocatalysts for visible-light-driven hydrogen evolution

Efficient electron-hole separation and carrier utilization are key factors in photocatalytic systems. Here, we use a metal-organic framework (NH₂-UiO-66) modified with inner platinum nanoparticles and outer cadmium sulfide (CdS) nanoparticles to construct the ternary composite Pt@NH₂-UiO-66/CdS, which has a spatially separated, hierarchical structure for enhanced visible-light-driven hydrogen evolution. Relative to pure NH₂-UiO-66, Pt@NH₂-UiO-66, and NH₂-UiO-66/CdS samples, the Pt@NH₂-UiO-66/CdS composite exhibits much higher hydrogen yields with an apparent quantum efficiency of 40.3% at 400 nm irradiation and stability over the most MOF-based photocatalysts. Transient absorption measurements reveal spatial charge-separation dynamics in the composites. The catalyst's high activity and durability are attributed to charge separation following an efficient photogenerated hole-transfer band-trap pathway. This work holds promise for enhanced MOF-based photocatalysis using efficient hole-transfer routes.

Non-Noble Plasmonic Metal-Based Photocatalysts

Solar-to-chemical energy conversion via heterogeneous photocatalysis is one of the sustainable approaches to tackle the growing environmental and energy challenges. Among various promising photocatalytic materials, plasmonic-driven photocatalysts feature prominent solar-driven surface plasmon resonance (SPR). Non-noble plasmonic metals (NNPMs)-based photocatalysts have been identified as a unique alternative to noble metal-based ones due to their advantages like earth-abundance, cost-effectiveness, and large-scale application capability. This review comprehensively summarizes the most recent advances in the synthesis, characterization, and properties of NNPMs-based photocatalysts. After introducing the fundamental principles of SPR, the attributes and functionalities of NNPMs in governing surface/interfacial photocatalytic processes are presented. Next, the utilization of NNPMs-based photocatalytic materials for the removal of pollutants, water splitting, CO₂ reduction, and organic transformations is discussed. The review concludes with current challenges and perspectives in advancing the NNPMs-based photocatalysts, which are timely and important to plasmon-based photocatalysis, a truly interdisciplinary field across materials science, chemistry, and physics.

Direct Electron Transfer from Upconversion Graphene Quantum Dots to TiO2 Enabling Infrared Light-Driven Overall Water Splitting

Utilization of infrared light in photocatalytic water splitting is highly important yet challenging given its large proportion in sunlight. Although upconversion material may photogenerate electrons with sufficient energy, the electron transfer between upconversion material and semiconductor is inefficient limiting overall photocatalytic performance. In this work, a TiO₂/graphene quantum dot (GQD) hybrid system has been designed with intimate interface, which enables highly efficient transfer of photogenerated electrons from GQDs to TiO₂. The designed hybrid material with high photogenerated electron density displays photocatalytic activity under infrared light (20 mW cm⁻²) for overall water splitting (H₂: 60.4 μmol g_cat.⁻¹ h⁻¹ and O₂: 30.0 μmol g_cat.⁻¹ h⁻¹). With infrared light well harnessed, the system offers a solar-to-hydrogen (STH) efficiency of 0.80% in full solar spectrum. This work provides new insight into harnessing charge transfer between upconversion materials and semiconductor photocatalysts and opens a new avenue for designing photocatalysts toward working under infrared light.

Recent advances and perspectives for solar-driven water splitting using particulate photocatalysts

The conversion and storage of solar energy to chemical energy via artificial photosynthesis holds significant potential for optimizing the energy situation and mitigating the global warming effect. Photocatalytic water splitting utilizing particulate semiconductors offers great potential for the production of renewable hydrogen, while this cross-road among biology, chemistry, and physics features a topic with fascinating interdisciplinary challenges. Progress in photocatalytic water splitting has been achieved in recent years, ranging from fundamental scientific research to pioneering scalable practical applications. In this review, we focus mainly on the recent advancements in terms of the development of new light-absorption materials, insights and strategies for photogenerated charge separation, and studies towards surface catalytic reactions and mechanisms. In particular, we emphasize several efficient charge separation strategies such as surface-phase junction, spatial charge separation between facets, and polarity-induced charge separation, and also discuss their unique properties including ferroelectric and photo-Dember effects on spatial charge separation. By integrating time- and space-resolved characterization techniques, critical issues in photocatalytic water splitting including photoinduced charge generation, separation and transfer, and catalytic reactions are analyzed and reviewed. In addition, photocatalysts with state-of-art efficiencies in the laboratory stage and pioneering scalable solar water splitting systems for hydrogen production using particulate photocatalysts are presented. Finally, some perspectives and outlooks on the future development of photocatalytic water splitting using particulate photocatalysts are proposed.

Crystal structure dependent cation exchange reactions in Cu2-xS nanoparticles

Because of high mobility of Cu⁺ in crystal lattice, Cu_2-xS nanoparticles (NPs) utilized as cation exchange (CE) templates to produce complicated nanomaterials has been extensively investigated. Nevertheless, the structural similarity of commonly used Cu_2-xS somewhat limits the exploration of crystal structure dependent CE reactions, since it may dramatically affect the reaction dynamics and pathways. Herein, we select djurleite Cu_1.94S and covellite CuS nanodisks (NDs) as starting templates and show that the crystal structure has a strong effect on their CE reactions. In the case of djurleite Cu_1.94S NDs, the Cu⁺ was immediately substituted by Cd²⁺ and solid wurtzite CdS NDs were produced. At a lower reaction temperature, these NDs were partially substituted, giving rise to the formation of Janus-type Cu_1.94S/CdS NDs, and this process is kinetically and thermodynamically favorable. For covellite CuS NDs, they were transformed into hollow CdS NDs under a more aggressive reaction condition due to the unique disulfide covalent bonds. These disulfide bonds distributed along [0 0 1] direction were gradually ruptured/reduced and CuS@CdS core-shell NDs could be obtained. Our findings suggest that not only the CE reaction kinetics and thermodynamics, but also the intermediates and final products are intimately correlated to the crystal structure of the host material.

Synergistically Modulating Geometry and Electronic Structures of a Chalcogenide Photocatalyst via an Ion-Exchange Strategy

Maneuvering the architecture and composition of semiconductors is essential to optimizing their performance in photocatalytic solar-to-fuel conversion. Here, we show that ion exchange, having a disparate mechanism with direct nucleation and growth of semiconductor crystals, can provide a new platform for rational control over the geometry and electronic structures of chalcogenide semiconductor photocatalysts. As a demonstration, the ZnSe nanocubes possessing a hollowed architecture and doped with a controllable amount of Ag⁺ ions are accessed via sequential ion exchange. The kinetics of the exchange reaction offers a knob for regulating the electronic structures of the Ag-doped ZnSe hollow cubes and, hence, their functions in light harvesting and photogenerated charge separation. Such synergistically geometric and optoelectronic modulation of ZnSe brings an order of magnitude enhancement in photocatalytic H₂ evolution activity relative to commercial ZnSe powders. Our study corroborates that ion exchange may open up new horizons for judicious fabrication and engineering of semiconductor-based photocatalyst materials.

Enhanced Charge Carrier Separation and Improved Biexciton Yield at the p-n Junction of SnSe/CdSe Heterostructures: A Detailed Electrochemical and Ultrafast Spectroscopic Investigation

Tin chalcogenides (SnX, X = S, Se)-based heterostructures (HSs) are promising materials for the construction of low-cost optoelectronic devices. Here, we report the synthesis of a SnSe/CdSe HS using the controlled cation exchange reaction. The (400) plane of SnSe and the (111) plane of CdSe confirm the formation of an interface between SnSe and CdSe. The Type I band alignment is estimated for the SnSe/CdSe HS with a small conduction band offset (CBO) of 0.72 eV through cyclic voltammetry measurements. Transient absorption (TA) studies demonstrate a drastic enhancement of the CdSe biexciton signal that points toward the hot carrier transfer from SnSe to CdSe in a short time scale. The fast growth and recovery of CdSe bleach in the presence of SnSe indicate charge transfer back to SnSe. The observed delocalization of carriers in these two systems is crucial for an optoelectronic device. Our findings provide new insights into the fabrication of cost-effective photovoltaic devices based on SnSe-based heterostructures.

Homogeneous Carbon/Potassium-Incorporation Strategy for Synthesizing Red Polymeric Carbon Nitride Capable of Near-Infrared Photocatalytic H2 Production

The efficient utilization of near-infrared (NIR) light for photocatalytic hydrogen generation is vitally important to both solar hydrogen energy and hydrogen medicine, but remains a challenge at present, owing to the strict requirement of the semiconductor for high NIR responsiveness, narrow bandgap, and suitable redox potentials. Here, an NIR-active carbon/potassium-doped red polymeric carbon nitride (RPCN) is achieved for by using a similar-structure dopant as the melamine (C₃ H₆ N₆ ) precursor with the solid KCl. The homogeneous and high incorporation of carbon and potassium remarkably narrows the bandgap of carbon nitride (1.7 eV) and endows RPCN with a high NIR-photocatalytic activity for H₂ evolution from water at the rate of 140 µmol h^-1 g^-1 under NIR irradiation (700 nm ≤ λ ≤ 780 nm), and the apparent quantum efficiency is high as 0.84% at 700 ± 10 nm (and 13% at 500 ± 10 nm). A proof-of-concept experiment on a tumor-bearing mouse model verifies RPCN as being capable of intratumoral NIR-photocatalytic hydrogen generation and simultaneous glutathione deprivation for safe and high-efficacy drug-free cancer therapy. The results shed light on designing efficient photocatalysts to capture the full spectrum of solar energy, and also pioneer a new pathway to develop NIR photocatalysts for hydrogen therapy of major diseases.

Hierarchical Photothermal Fabrics with Low Evaporation Enthalpy as Heliotropic Evaporators for Efficient, Continuous, Salt-Free Desalination

Solar-driven seawater evaporation is usually achieved on floating evaporators, but the performances are substantially limited by high evaporation enthalpy, solid salt crystallization, and reduced evaporation due to inclined sunlight. To solve these problems, we fabricated hierarchical polyacrylonitrile@copper sulfide (PAN@CuS) fabrics and proposed a prototype of heliotropic evaporator. Hierarchical PAN@CuS fabrics show significantly decreased water-evaporation enthalpy (1956.32 kJ kg^-1, 40 °C), compared with that of pure water (2406.17 kJ kg^-1, 40 °C), because of the disorganization of the hydrogen bonds at the CuS interfaces. Based on this fabric, a heliotropic evaporation model was developed, where seawater slowly flows from high to low in the fabric. Under solar irradiation (1.0 kW m^-2), this model exhibits a high-rate evaporation (∼2.27 kg m^-2 h^-1) and saturated brine production without solid salt crystallization. In particular, under inclined sunlight (angle range: from -90° to +90°), the heliotropic model retains an almost unchanged solar evaporation rate, whereas the floating model shows severe evaporation reduction (83.9%). Therefore, our study provides a strategy for reducing the evaporation enthalpy, maximally utilizing solar energy and continuous salt-free desalination.

Harvesting Sub-Bandgap IR Photons by Photothermionic Hot Electron Transfer in a Plasmonic p-n Junction

Plasmonic semiconductors are an emerging class of low-cost plasmonic materials, and the presence of a bandgap and band-bending in these materials offer new opportunities to overcome some of the limitations of plasmonic metals. Here, we demonstrate that in a plasmonic p-n heterojunction (Cu_2-xSe-CdSe) the near-IR excitation (1.1 eV) of the hole plasmon in the p-Cu_2-xSe phase results in rapid hot electron transfer to n-CdSe, with an energy 2.2 eV above the Fermi level. This hot electron generation and energy upconversion process can be well-described by a photothermionic mechanism, where the presence of a bandgap in p-Cu_2-xSe facilitates the generation of energetic photothermal electrons. The lifetime of the transferred electrons in Cu_2-xSe-CdSe can reach ∼130 ps, which is nearly 100× longer than that of its metal-semiconductor counterpart. This result demonstrates a novel approach for harvesting the sub-bandgap near IR photons using plasmonic p-n junctions and the potential advantages of plasmonic semiconductors for hot carrier-based devices.

Metal Cation Valency Dependence in Morphology Evolution of Cu2-x S Nanodisk Seeds and Their Pseudomorphic Cation Exchanges

A crucial parameter in the design of semiconductor nanoparticles (NPs) with controllable optical, magnetic, electronic, and catalytic properties is the morphology. Herein, we demonstrate the potential of additive metal cations with variable valency to direct the morphology evolution of copper-deficient Cu_2-x S nanoparticles in the process of seed-mediated growth. In particular, the djurleite Cu_1.94 S seed could evolve from disk into tetradecahedron in the presence of tin(IV) cations, whereas they merely formed sharp hexagonal nanodisks with tin(II) cations. In addition to djurleite Cu_1.94 S, the tin(IV) cations could be generalized to direct the growth of roxbyite Cu_1.8 S and covellite CuS nanodisk seeds into tetradecahedra. We further perform pseudomorphic cation exchanges of Cu_1.94 S tetradecahedra with Zn²⁺ and Cd²⁺ to produce polyhedral zinc sulfide (ZnS) and cadmium sulfide (CdS) NPs. Moreover, we achieve Cu_1.8 S/ZnS and Cu_1.94 S/CdS tetradecahedral heterostructures via partial cation exchange, which are otherwise inaccessible by traditional synthetic approaches.

A NIR-Responsive Phytic Acid Nickel Biomimetic Complex Anchored on Carbon Nitride for Highly Efficient Solar Hydrogen Production

A challenge in photocatalysis consists in improving the efficiency by harnessing a large portion of the solar spectrum. We report the design and realization of a robust molecular-semiconductor photocatalytic system (MSPS) consisting of an earth-abundant phytic acid nickel (PA-Ni) biomimetic complex and polymeric carbon nitride (PCN). The MSPS exhibits an outstanding activity at λ=940 nm with high apparent quantum efficiency (AQE) of 2.8 %, particularly λ>900 nm, as it outperforms all reported state-of-the-art near-infrared (NIR) hybrid photocatalysts without adding any noble metals. The optimum hydrogen (H₂ ) production activity was about 52 and 64 times higher with respect to its pristine counterpart under the AM 1.5 G and visible irradiation, respectively, being equivalent to the platinum-assisted PCN. This work sheds light on feasible avenues to prepare highly active, stable, cheap NIR-harvesting photosystems toward sustainable and scalable solar-to-H₂ production.

Transformations of Ionic Nanocrystals via Full and Partial Ion Exchange Reactions

ConspectusElaborate chemical synthesis methods allow the production of various types of inorganic nanocrystals (NCs) with uniform shape and size distributions. Many single-step synthesis approaches, such as the reduction of metal ions, the decomposition of metal complexes, double replacement reactions, and hydrolysis, have been adapted to promote the generation of monodisperse metal and ionic NCs. However, the question has become, how can we synthesize NCs with thermodynamically metastable phases or very complex structures? The transformation of already-synthesized NCs via elemental substitutions, such as ion exchange reactions for ionic NCs and galvanic replacement reactions for metal NCs, can overcome the difficulties facing conventional one-step syntheses. In particular, NC ion exchange reactions have been studied with numerous combinations of foreign ions and ionic NCs with various shapes. They have been investigated extensively because the reactions proceed under relatively mild conditions thanks to the large surface-to-volume ratio of the NCs relative to their bulk form. The functionality of the resulting ionic NCs, including semiconducting and plasmonic properties, can be easily tuned in a wide range, from the visible to near-infrared. Because anions generally have much larger ionic radii than cations within the frameworks of NCs, the cation exchange reactions proceed much faster than the anion exchange reactions. For ionic NCs above a critical size, the anion framework remains intact, and the original shape of the parent NCs is retained throughout the cation exchange reaction. In contrast, the anion exchange reaction often provides the new NCs with unique structures, such as hollow or anisotropically phase-segregated assemblies.This Account focuses on the full and partial ion exchange reactions involving ionic NCs, which have been thoroughly investigated by our group and others while highlighting important aspects such as the preservation of appearance and dimensions. First, we discuss how each type of ion exchange reaction progresses to understand the morphologies and crystal structures of their final products. This discussion is supported by emphasizing important examples, which help to explore the formation of NCs with thermodynamically metastable phases and complex structures, and other significant features of the ion exchange reactions leading to structure-specific functions. As a special case, we examine how the shape-dependent anionic framework (surface anion sublattice and stacking pattern) of polyhedral Cu₂O NCs determines the crystalline structure of the anion-exchanged products of hollow Cu_xS NCs. In addition, we review the characteristic anion exchange behavior of metal halide perovskite NCs observed in our laboratory and other laboratories. Finally, a general outline of the transformation of NCs via ion exchange reactions and future prospects in this field are provided.

Controlled Photoinduced Generation of "Visual" Partially and Fully Charge Separated States in Viologen Analogues

Charge-separated states with a lifetime scale of seconds or longer not only favor studies using various steady-state analysis techniques but are important for light-energy conversion and other applications. Through a steric-hindrance-induced method, unprecedented photoinduced generation of a partially charge separated (PCS) state with a lifetime of days has been detected in the "visual" mode during the decay of excited states to a commonly observed fully charge separated (FCS) state for viologen analogues. One pale yellow 4,4'-bipyridine-based metalloviologen compound, with an interannular dihedral angle of 1.84° in 4,4'-bipyridine, directly decays to the purple FCS state after photoexcitation. The other pale yellow compound, with a similar coordination framework but a larger interannular dihedral angle (33.74°), changes first to a yellow PCS state and then relaxes slowly (in the dark in Ar, ca. 2 days; 70 °C in Ar, ca. 1 h) to the purple FCS state. The two-step coloration phenomenon is unprecedented for viologen compounds and their analogues and also rather rare for other photochromic species. EPR and Raman data reveal that photoinduced charge separation first generates univalent zinc and radicals and then the received electron in Zn(I) slowly distributes further to 4,4'-bipyridine. Reduction of π-conjugation and a direct to indirect change in band gap account for the prolongation of the relaxation process and the capture of the PCS state. These findings help to understand and control decay processes of excited states and provide a potential design strategy for multicolor photochromism, light-energy conversion with high efficiency, or other applications.

A microfluidic cathodic photoelectrochemical biosensor chip for the targeted detection of cytokeratin 19 fragments 21-1

A microfluidic chip integrated with a microelectrode and a cathodic photoelectrochemical (PEC) biosensor for the ultrasensitive detection of non-small cell lung cancer cytokeratin fragments based on a signal amplification strategy was designed. The mechanism for signal amplification is developed based on the p-n junction of AgI/Bi₂Ga₄O₉, with dissolved O₂ as an electron acceptor to produce the superoxide anion radical (˙O₂^-) as the working microelectrode. By combining this with a novel superoxide-dismutase-loaded honeycomb manganese oxide nanostructure (SOD@hMnO₂) as the co-catalyst signal amplification label, ˙O₂^- can be catalyzed by SOD via a disproportionation reaction to produce O₂ and H₂O₂; then, hMnO₂ is able to trigger the decomposition of H₂O₂ to generate O₂ and H₂O. Therefore, the increased O₂ promotes the separation of electron-hole pairs via consuming more electrons, leading to an effective enhancement of the cathodic PEC behavior. Under optimum conditions, with the cytokeratin 19 fragments 21-1 (CYFRA 21-1) as the targeted detection objects, the microfluidic cathodic PEC biosensor chip exhibited excellent linearity from 0.1 pg mL^-1 to 100 ng mL^-1, with a detection limit of 0.026 pg mL^-1 (S/N = 3). The exciting thing that this work offers is a new strategy for the detection of other important cancer biomarkers for disease diagnosis and prognosis.

Ag2 S-CdS p-n Nanojunction-Enhanced Photocatalytic Oxidation of Alcohols to Aldehydes

Selective oxidation of alcohols to aldehydes under mild conditions is important for the synthesis of high-value-added organic intermediates but still very challenging. For most of the thermal and photocatalytic systems, noble metal catalysts or harsh reaction conditions are required. Herein, the synthesis and use of Ag₂ S-CdS p-n nanojunctions as an efficient photocatalyst for selective oxidation of a series of aromatic alcohols to their corresponding aldehydes is reported. High quantum efficiencies (59.6% and 36.9% under 380 and 420 nm, respectively) are achieved in air atmosphere at room temperature. Photoluminescence and photo-electrochemical tests show that the excellent performance is mainly due to the p-n junction-enhanced charge separation and transfer for the activation of both O₂ (in air) and substrates. This study demonstrates the potential of p-n junction in photocatalytic synthesis under mild conditions.

Efficient Self-Driving Photoelectrocatalytic Reactor for Synergistic Water Purification and H2 Evolution

The photoelectrocatalytic (PEC) technique has attracted much attention to getting clear energy and environmental purification. Simultaneous reactions of solar energy generation could be used to apply for practical applications to maximize the functionality of reactor systems. Herein, we crafted a self-driving photoelectrocatalytic reactor system, comprising platinum (Pt) modified p-Si nanowires (Pt/Si-NWs) as a photocathode and TiO₂ nanotube arrays (TiO₂-NTAs) as a photoanode for synergistic H₂ evolution and water purification, respectively. Hydrogen evolution in the cathode chamber and environmental remediation in the anode chamber were achieved with the aid of appropriate bandgap illumination and self-built bias voltage. The mismatch of Fermi levels between TiO₂-NTAs and Si-NWs reduced the recombination rates of photoinduced electrons and holes through the formation of Z scheme and inner electric filed. The synergistic PEC reactions exhibited much higher activities than those achieved using other systems so far. This basic principal could be applied for fabricating other PEC reactors in photoelectro conversion devices and be established as design guidelines for reactors to maximize the PEC performance.

Hollow Octahedral Cu2-xS/CdS/Bi2S3 p-n-p Type Tandem Heterojunctions for Efficient Photothermal Effect and Robust Visible-Light-Driven Photocatalytic Performance

Reasonable design of the nanostructure of heterogeneous photocatalysts is of great significance for improving their performance and stability. We report the design and fabrication of hollow sandwich-layered octahedral Cu_2-xS/CdS/Bi₂S₃ p-n-p type tandem heterojunctions constructed via the continuous growth deposition method on the surface of hollow octahedral Cu_2-xS with well-defined structures and interfaces. The unique hollow sandwich nanostructure has a large specific surface area and abundant reaction sites and enhances the separation and transfer of photogenerated carriers. In addition, the formation of a p-n-p heterojunction coupled with the surface plasmon resonance effect of Cu_2-xS could also aid in photocatalytic H₂ evolution performance and photocatalytic degradation efficiency. Under vis-NIR light irradiation, the optimized Cu_2-xS/CdS/Bi₂S₃ photocatalyst displays a notable H₂ production rate of 8012 μmol h^-1 g^-1, and 2,4-dichlorophenol is almost completely photocatalytically degraded in 150 min. This strategy and rational design offer a new path toward the design of specific nanocatalysts with enhanced activity and stability and challenging reactions.

Manipulating the Optoelectronic Properties of Quasi-type II CuInS2/CdS Core/Shell Quantum Dots for Photoelectrochemical Cell Applications

Colloidal core/shell heterostructured quantum dots (QDs) possessing quasi-type II band structure have demonstrated effective surface passivation and prolonged exciton lifetime, leading to enhanced charge separation/transfer efficiencies that are promising for photovoltaic device applications. Herein, we synthesized CuInS₂ (CIS)/CdS core/shell heterostructured QDs and manipulated the optoelectronic properties via controlling the CdS shell thickness. The shell-thickness-dependent optical properties indicate the existence of a quasi-type II band structure in such core/shell QDs, which was verified by ultrafast spectroscopy and theoretical simulations. These quasi-type II core/shell QDs having various shell thicknesses are used as light absorbers for the fabrication of solar-driven QDs-based photoelectrochemical (PEC) devices, exhibiting an optimized photocurrent density of ∼6.0 mA/cm² and excellent stability under simulated AM 1.5G solar illumination. The results demonstrate that quasi-type II CIS/CdS core/shell heterostructured QDs with tailored optoelectronic properties are promising to realize high-performance QDs-based solar energy conversion devices for the production of solar fuels.

Infrared driven hot electron generation and transfer from non-noble metal plasmonic nanocrystals

Non-noble metal plasmonic materials, e.g. doped semiconductor nanocrystals, compared to their noble metal counterparts, have shown unique advantages, including broadly tunable plasmon frequency (from visible to infrared) and rich surface chemistry. However, the fate and harvesting of hot electrons from these non-noble metal plasmons have been much less explored. Here we report plasmon driven hot electron generation and transfer from plasmonic metal oxide nanocrystals to surface adsorbed molecules by ultrafast transient absorption spectroscopy. We show unambiguously that under infrared light excitation, hot electron transfers in ultrafast timescale (<50 fs) with an efficiency of 1.4%. The excitation wavelength and fluence dependent study indicates that hot electron transfers right after Landau damping before electron thermalization. We revealed the efficiency-limiting factors and provided improvement strategies. This study paves the way for designing efficient infrared light absorption and photochemical conversion applications based on non-noble metal plasmonic materials.

Harvesting of hot electrons in non-noble metal plasmonic materials is still little explored. Here the authors investigate plasmon-driven hot electron generation in doped metal oxide nanocrystals and the mechanism of transfer to surface adsorbed molecules by ultrafast transient absorption spectroscopy.

MoS2-Stratified CdS-Cu2-xS Core-Shell Nanorods for Highly Efficient Photocatalytic Hydrogen Production

Heterojunction photocatalysts are widely adopted for efficient water splitting, but ion migration can seriously threaten the stability of heterojunctions, as with the well-known low stability of CdS-Cu_2-xS due to intrinsic Cu⁺ ion migration. Here, we utilize Cu⁺ migration to design a stratified CdS-Cu_2-xS/MoS₂ photocatalyst, in which Cu^I@MoS₂ (Cu^I-intercalated within the MoS₂ basal plane) is created by Cu⁺ migration and intercalation to the adjacent MoS₂ surface. The epitaxial vertical growth of the Cu^I@MoS₂ nanosheets on the surface of one-dimensional core-shell CdS-Cu_2-xS nanorods forms catalytic and protective layers to simultaneously enhance catalytic activity and stability. Charge transfer is verified by kinetics measurements with femtosecond time-resolved transient absorption spectroscopy and direct mapping of the surface charge distribution with a scanning ion conductance microscope. This design strategy demonstrates the potential of utilizing hybridized surface layers as effective catalytic and protective interfaces for photocatalytic hydrogen production.

Formation of BNC Coordination to Stabilize the Exposed Active Nitrogen Atoms in g-C3 N4 for Dramatically Enhanced Photocatalytic Ammonia Synthesis Performance

It is an important issue that exposed active nitrogen atoms (e.g., edge or amino N atoms) in graphitic carbon nitride (g-C₃ N₄ ) could participate in ammonia (NH₃ ) synthesis during the photocatalytic nitrogen reduction reaction (NRR). Herein, the experimental results in this work demonstrate that the exposed active N atoms in g-C₃ N₄ nanosheets can indeed be hydrogenated and contribute to NH₃ synthesis during the visible-light photocatalytic NRR. However, these exposed N atoms can be firmly stabilized through forming BNC coordination by means of B-doping in g-C₃ N₄ nanosheets (BCN) with a B-doping content of 13.8 wt%. Moreover, the formed BNC coordination in g-C₃ N₄ not only effectively enhances the visible-light harvesting and suppresses the recombination of photogenerated carriers in g-C₃ N₄ , but also acts as the catalytic active site for N₂ adsorption, activation, and hydrogenation. Consequently, the as-synthesized BCN exhibits high visible-light-driven photocatalytic NRR activity, affording an NH₃ yield rate of 313.9 µmol g^-1 h^-1 , nearly 10 times of that for pristine g-C₃ N₄ . This work would be helpful for designing and developing high-efficiency metal-free NRR catalysts for visible-light-driven photocatalytic NH₃ synthesis.

Efficient photocatalytic hydrogen evolution on single-crystalline metal selenide particles with suitable cocatalysts

It is important to improve the apparent quantum yields (AQYs) of narrow bandgap photocatalysts to achieve efficient H₂ production. The present work demonstrates a particulate solid solution of zinc selenide and copper gallium selenide (denoted as ZnSe:CGSe) that evolves H₂ efficiently and is responsive to visible light up to 725 nm. This material was synthesized using a flux-assisted method and was found to comprise single-crystalline tetrahedral particles. The coloading of Ni and Rh, Pt, Pd or Ru as cocatalysts further improved the photocatalytic H₂ evolution rate over this photocatalyst. With the optimal coloading of a Ni–Ru composite cocatalyst, an AQY of 13.7% was obtained at 420 nm during a sacrificial H₂ evolution reaction, representing the highest value yet reported for a photocatalyst with an absorption edge longer than 700 nm. The present study demonstrates that the preparation of single-crystalline particles and the rational assembly of composite cocatalysts are effective strategies that allow the efficient utilization of long wavelengths by metal selenide photocatalysts for solar fuel production.

Coloading of a Ni–Ru composite cocatalyst on a 700 nm-class single-crystalline particulate selenide photocatalyst improves its hydrogen evolution activity.

Phase segregated Cu2-x Se/Ni3Se4 bimetallic selenide nanocrystals formed through the cation exchange reaction for active water oxidation precatalysts

Control over the composition and nanostructure of solid electrocatalysts is quite important for drastic improvement of their performance. The cation exchange reaction of nanocrystals (NCs) has been reported as the way to provide metastable crystal structures and complicated functional nanostructures that are not accessible by conventional synthetic methods. Herein we demonstrate the cation exchange-derived formation of metastable spinel Ni₃Se₄ NCs (sp-Ni₃Se₄) and phase segregated berzelianite Cu_2−xSe (ber-Cu_2−xSe)/sp-Ni₃Se₄ heterostructured NCs as active oxygen evolution reaction (OER) catalysts. A rare sp-Ni₃Se₄ phase was formed by cation exchange of ber-Cu_2−xSe NCs with Ni²⁺ ions, because both phases have the face-centered cubic (fcc) Se anion sublattice. Tuning the Ni : Cu molar ratio leads to the formation of Janus-type ber-Cu_2−xSe/sp-Ni₃Se₄ heterostructured NCs. The NCs of sp-Ni₃Se₄ and ber-Cu_2−xSe/sp-Ni₃Se₄ heterostructures exhibited high catalytic activities in the OER with small overpotentials of 250 and 230 mV at 10 mA cm⁻² in 0.1 M KOH, respectively. They were electrochemically oxidized during the OER to give hydroxides as the real active species. We anticipate that the cation exchange reaction could have enormous potential for the creation of novel heterostructured NCs showing superior catalytic performance.

Bimetallic selenide nanocrystals formed by cation exchange reaction work as a precursor of efficient water oxidation electrocatalyst.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Anomalous Photoinduced Hole Transport in Type I Core/Mesoporous-Shell Nanocrystals for Efficient Photocatalytic H2 Evolution

Controlling the carrier dynamics in a semiconductor nanoparticulate photocatalyst is the key to developing catalytic activity. Generally, type I band alignment is unsuitable for photocatalysts because the photoinduced carriers accumulate in the narrow bandgap semiconductor. To avoid the termination of reactions and/or photocorrosion of materials caused by carrier accumulation, it is common to employ type II band alignment for photoenergy conversion systems instead of type I. However, CdS/ZnS core/mesoporous-shell heterostructures show superior photocatalytic activity despite having type I band alignment that is generally unfavorable for photocatalytic reactions. Transient absorption spectroscopy and time-resolved microwave conductivity revealed efficient photoinduced hole transfer from the CdS phase to the ZnS phase. The defect-mediated hole transfer from the CdS to the ZnS phase resulted in long-lived charge separation (>2.4 ms) leading to high photocatalytic performance. This study provides insight into defect-mediated carrier transfer in nanoparticulate photocatalysts, which could be used as a guideline for the design of highly active and stable nanoparticulate photocatalysts.

Recent advances in copper sulphide-based nanoheterostructures

This tutorial summarizes and integrates recent advances in design and synthesis of copper sulfide-based nanoheterostructures and their applications in energy and healthcare.