Viable Photocatalysts under Solar-Spectrum Irradiation: Nonplasmonic Metal Nanoparticles

Charge separation and molecule activation promoted by Pd/MIL-125-NH2hybrid structures for selective oxidation reactions

The Pd/MIL-125-NH₂hybrid photocatalyst exhibits great advantages in charge separation and molecule activation, with sufficient generation of both superoxide radical and singlet oxygen toward selective oxidation of organic molecules.

Encapsulating low-coordinated Pt clusters within a metal–organic framework induces spatial charge separation boosting photocatalytic hydrogen evolution

A strategy is developed to synthesize MOF photocatalyst encapsulating low-coordinate Pt clusters with high photocatalytic hydrogen-evolution performance under visible light.

Synergistic Effect over Sub-nm Pt Nanocluster@MOFs Significantly Boosts Photo-oxidation of N-alkyl(iso)quinolinium Salts

Quinolones and isoquinolones are of interest to pharmaceutical industry owing to their potent biological activities. Herein, we first encapsulated sub-nm Pt nanoclusters into Zr-porphyrin frameworks to afford an efficient photocatalyst Pt_0.9@PCN-221. This catalyst can dramatically promote electron-hole separation and ¹O₂ generation to achieve synergistic effect first in the metal-organic framework (MOF) system, leading to the highest activity in photosynthesis of (iso)quinolones in >90.0% yields without any electronic sacrificial agents. Impressively, Pt_0.9@PCN-221 was reused 10 times without loss of activity and can catalyze gram-scale synthesis of 1-methyl-5-nitroisoquinolinone at an activity of 175.8 g·g_cat⁻¹, 22 times higher than that of PCN-221. Systematic investigations reveal the contribution of synergistic effect of photogenerated electron, photogenerated hole, and ¹O₂ generation for efficient photo-oxidation, thus highlighting a new strategy to integrate multiple functional components into MOFs to synergistically catalyze complex photoreactions for exploring biologically active heterocyclic molecules.

•

A state-of-the-art photocatalyst for preparation of bioactive (iso)quinolones

•

Synergistic catalysis of photogenerated e⁻/h⁺ and ¹O₂

•

Sub-nm Pt_0.9@PCN-221 with a high efficiency of e⁻-h⁺ separation and ¹O₂ generation

Reduction of carbon dioxide on photoexcited nanoparticles of VIII group metals

The photocatalytic reduction of carbon dioxide on nanoparticles of group VIII transition metals represents an emerging research area in recent years because of their promise in transforming carbon dioxide, a greenhouse gas, into value-added chemicals and fuels with the energy input of light. This mini review summarizes the fundamentals of the reduction of carbon dioxide and addresses how the photoexcitation of the metal nanoparticles can influence the reactions. The important roles of non-thermal hot electrons and photothermal effect in the photocatalytic reduction of carbon dioxide are highlighted, and the recent research reported in the literature are overviewed. There are still challenges in characterizing the photocatalytic reactions to distinguish the contributions of non-thermal and photothermal effects.

Nanostructured metals for light-based technologies

The basic theoretical understanding of light interacting with nanostructured metals that has existed since the early 1900s has become more relevant in the last two decades, largely because of new approaches to structure metals down to the nanometer scale or smaller. Here, a broad overview of the concepts and applications of nanostructuring metals for light-based technologies is given. The theory of the response of metals to an applied oscillating field is given, including a discussion of nonlocal, nonlinear and quantum effects. Using this metal response, the guiding of electromagnetic (light) waves using metals is given, with a particular emphasis on the impact of nanostructured metals for tighter confinement and slower propagation. Similarly, the influence of metal nanostructures on light scattering by isolated metal structures, like nanoparticles and nanoantennas, is described, with basic results presented including plasmonic/circuit resonances, the single channel limit, directivity enhancement, the maximum power transfer theorem, limits on the magnetic response from kinetic inductance and the scaling of gap plasmons to the nanometer scale and smaller. A brief overview of nanofabrication approaches to creating metal nanostructures is given. Finally, existing and emerging light-based applications are presented, including those for sensing, spectroscopy (including local refractive index, Raman, IR absorption), detection (including Schottky detectors), switching (including terahertz photoconductive antennas), modulation, energy harvesting and photocatalysis, light emission (including lasers and tunneling based light emission), optical tweezing, nonlinear optics, subwavelength imaging and lithography and high density data storage.

The fabrication of Au/Pd plasmonic alloys on UiO-66-NH2: an efficient visible light-induced photocatalyst towards the Suzuki Miyaura coupling reaction under ambient conditions

The current study on Au/Pd@UiO-66-NH₂ explores a novel approach towards photocatalytic SMC coupling reaction. This investigation highlights a well studied mechanistic pathway towards the formation of biphenyl as the target product.

Visible light-driven oxidation of vanillyl alcohol in air with Au–Pd bimetallic nanoparticles on phosphorylated hydrotalcite

This catalyst could recurrently realize the production of vanillin from vanilla alcohol under mild reaction conditions due to the synergistically double dehydrogenative oxidation (SDDO).

Synergistic photothermal and photochemical partial oxidation of methane over noble metals incorporated in mesoporous silica

We establish a hot-carrier-driven photothermal and photochemical system for partial oxidation of methane using noble metals incorporated in mesoporous silica.

A Promising Application of Optical Hexagonal TaN in Photocatalytic Reactions

Searching for highly active and efficient photocatalysts for photo-induced/photo-assisted reactions remains the most challenging task for solar energy utilization. In previous studies, the search for such materials has mainly focused on precious plasmonic metals (for example, Au, Ag, and Cu) and semiconductor oxides (for example, TiO₂ , ZnO, and WO₃ ). Herein, we report the application of hexagonal tantalum mononitride (TaN) as an optical support in photocatalytic reactions, which could harness visible light to assist CO₂ conversion and decompose organic pollutants. Theoretical studies indicated that the improved electron-hole separation in polar TaN under visible-light illumination was critical for its use in photocatalysis. This study could guide the use of TaN in various photocatalytic reactions and wider optical applications.

Visible-to-NIR Photon Harvesting: Progressive Engineering of Catalysts for Solar-Powered Environmental Purification and Fuel Production

Utilization of diffusive solar energy through photocatalytic processes for environmental purification and fuel production has long been pursued. However, efficient capture of visible-near-infrared (NIR) photons, especially for those with wavelengths longer than 600 nm, is a demanding quest in photocatalysis owing to their relatively low energy. In recent years, benefiting from the advances in photoactive material design, photocatalytic reaction system optimization, and new emerging mechanisms for long-wavelength photon activation, increasing numbers of studies on the harnessing of visible-NIR light for solar-to-chemical energy conversion have been reported. Here, the aim is to comprehensively summarize the progress in this area. The main strategies of the long-wavelength visible-NIR photon capture and the explicitly engineered material systems, i.e., narrow optical gap, photosensitizers, upconversion, and photothermal materials, are elaborated. In addition, the advances in long-wavelength light-driven photo- and photothermal-catalytic environmental remediation and fuel production are discussed. It is anticipated that this review presents the forefront achievements in visible-NIR photon capture and at the same time promotes the development of novel visible-NIR photon harnessing catalysts toward efficient solar energy utilization.

Lattice Engineering on Metal Cocatalysts for Enhanced Photocatalytic Reduction of CO2 into CH4

Photocatalytic conversion of CO₂ into CH₄ represents an appealing approach to alleviate the world's continued reliance on fossil fuels and global warming resulting from increasing CO₂ concentrations in the atmosphere. However, its practical application is greatly limited by serious electron-hole recombination in the photocatalysts and the production of CO and H₂ as side reactions. Herein, for the first time, it is demonstrated that the photocatalytic reduction of CO₂ to CH₄ can be significantly improved through the simultaneous alloying and hydriding of metal cocatalysts. The isolation of Cu and H atoms in Pd lattices play three roles in the enhancement of CO₂ to CH₄ conversion: 1) Cu atoms provide catalytic sites to reduce CO₂ into CO and then to CH₄ to suppress H₂ evolution; 2) H atoms improve the electron-trapping ability of cocatalysts; and 3) H atoms accelerate the reduction of CO to CH₄ , which is the rate-limiting procedure in the conversion of CO₂ into CH₄ . Arising from the synergistic interplay between Pd-H and Cu-CO sites, C₃ N₄ -Pd₉ Cu₁ H_x (15 mg) achieves 100 % selectivity for CH₄ production with an average rate of 0.018 μmol h^-1 under visible-light irradiation. This work provides insights into the design of a cocatalyst for highly selective CO₂ conversion through lattice engineering at atomic precision.

Watching Visible Light-Driven CO2 Reduction on a Plasmonic Nanoparticle Catalyst

Photocatalytic reduction of carbon dioxide (CO₂) by visible light has the potential to mimic plant photosynthesis and facilitate the renewable production of storable fuels. Accomplishing desirable efficiency and selectivity in artificial photosynthesis requires an understanding of light-driven pathways on photocatalyst surfaces. Here, we probe with single-nanoparticle spatial resolution the dynamics of a plasmonic silver (Ag) photocatalyst under conditions of visible light-driven CO₂ reduction. In situ surface-enhanced Raman spectroscopy captures discrete adsorbates and products formed dynamically on single photocatalytic nanoparticles, most prominent among which is a surface-adsorbed hydrocarboxyl (HOCO*) intermediate critical to further reduction of CO₂ to carbon monoxide (CO) and formic acid (HCOOH). Density functional theory simulations of the captured adsorbates reveal the mechanism by which plasmonic excitation activates physisorbed CO₂ leading to the formation of HOCO*, indicating close interplay between photoexcited states and adsorbate/metal interactions.

Energy-efficient CO2 hydrogenation with fast response using photoexcitation of CO2 adsorbed on metal catalysts

Many heterogeneous catalytic reactions occur at high temperatures, which may cause large energy costs, poor safety, and thermal degradation of catalysts. Here, we propose a light-assisted surface reaction, which catalyze the surface reaction using both light and heat as an energy source. Conventional metal catalysts such as ruthenium, rhodium, platinum, nickel, and copper were tested for CO2 hydrogenation, and ruthenium showed the most distinct change upon light irradiation. CO2 was strongly adsorbed onto ruthenium surface, forming hybrid orbitals. The band gap energy was reduced significantly upon hybridization, enhancing CO2 dissociation. The light-assisted CO2 hydrogenation used only 37% of the total energy with which the CO2 hydrogenation occurred using only thermal energy. The CO2 conversion could be turned on and off completely with a response time of only 3 min, whereas conventional thermal reaction required hours. These unique features can be potentially used for on-demand fuel production with minimal energy input.

Oxygen-Deficient Black Titania for Synergistic/Enhanced Sonodynamic and Photoinduced Cancer Therapy at Near Infrared-II Biowindow

The conventional inorganic semiconductors are not suitable for in vivo therapeutic nanomedicine because of the lack of an adequate and safe irradiation source to activate them. This work reports on the rational design of titania (TiO₂)-based semiconductors for enhanced and synergistic sono-/photoinduced tumor eradication by creating an oxygen-deficient TiO_{2- x} layer onto the surface of TiO₂ nanocrystals, which can create a crystalline-disordered core/shell structure (TiO₂@TiO_{2- x}) with black color. As found in the lessons from traditional photocatalysis, such an oxygen-deficient TiO_{2- x} layer with abundant oxygen defects facilitates and enhances the separation of electrons (e^-) and holes (h⁺) from the energy-band structure upon external ultrasound irradiation, which can significantly improve the efficacy of sono-triggered sonocatalytic tumor therapy. Such an oxygen-deficient TiO_{2- x} layer can also endow black titania nanoparticles with high photothermal-conversion efficiency (39.8%) at the NIR-II biowindow (1064 nm) for enhanced photothermal hyperthermia. Both in vitro cell level and systematic in vivo tumor-bearing mice xenograft evaluations have demonstrated the high synergistic efficacy of combined and enhanced sonodynamic therapy and photothermal ablation as assisted by oxygen-deficient black titania, which has achieved complete tumor eradication with high therapeutic biosafety and without obvious reoccurrence. This work not only provides the paradigm of high therapeutic efficacy of a combined sono-/photoinduced tumor-treatment protocol but also significantly broadens the nanomedical applications of semiconductor-based nanoplatforms by rational design of their nanostructures and control of their physiochemical properties.

Mechanistic Insights into Photocatalyzed Hydrogen Desorption from Palladium Surfaces Assisted by Localized Surface Plasmon Resonances

Nanoparticles synthesized from plasmonic metals can absorb low-energy light, producing an oscillation/excitation of their valence electron density that can be utilized in chemical conversions. For example, heterogeneous photocatalysis can be achieved within heterometallic antenna-reactor complexes (HMARCs), by coupling a reactive center at which a chemical reaction occurs to a plasmonic nanoparticle that acts as a light-absorbing antenna. For example, HMARCs composed of aluminum antennae and palladium (Pd) reactive centers have been demonstrated recently to catalyze selective hydrogenation of acetylene to ethylene. Here, we explore within a theoretical framework the rate-limiting step of hydrogen photodesorption from a Pd surface-crucial to achieving partial rather than full hydrogenation of acetylene-to understand the mechanism behind the photodesorption process within the HMARC assembly. To properly describe electronic excited states of the metal-molecule system, we employ embedded complete active space self-consistent field and n-electron valence state perturbation theory to second order within density functional embedding theory. The results of these calculations reveal that the photodesorption mechanism does not create a frequently invoked transient negative ion species but instead enhances population of available excited-state, low-barrier pathways that exhibit negligible charge-transfer character.

Unravelling the nucleation mechanism of bimetallic nanoparticles with composition-tunable core-shell arrangement

The structure and atomic ordering of Au-Ag nanoparticles grown in the gas phase are determined by a combination of HAADF-STEM, XPS and Refl-XAFS techniques as a function of composition. It is shown consistently from all the techniques that an inversion of chemical ordering takes place by going from Au-rich to Ag-rich compositions, with the minority element always occupying the nanoparticle core, and the majority element enriching the shell. With the aid of DFT calculations, this composition-tunable chemical arrangement is rationalized in terms of a four-step growth process in which the very first stage of cluster nucleation plays a crucial role. The four-step growth mechanism is based on mechanisms of a general character, likely to be applicable to a variety of binary systems besides Au-Ag.

Enhanced visible-light photocatalytic activity to volatile organic compounds degradation and deactivation resistance mechanism of titania confined inside a metal-organic framework

Poor visible-light-driven activity and deactivation property as well as wide band gap of the most common TiO₂ photocatalyst significantly limits its practical application in volatile organic compounds (VOCs) purification. In this study, tiny TiO₂ nanoparticles incorporated into a typical metal-organic framework (MOF), NH₂-UiO-66, with controllable TiO₂ content and size, were synthesized based on the hard-soft acid-base (HSAB) principle and applied to VOCs purification. Compared to bare TiO₂, the TiO₂@NH₂-UiO-66 composites could extend the optical absorption to the visible light range and accelerate the photogenerated electrons-holes separation, due to the excellent interface contact between TiO₂ and NH₂-UiO-66. Moreover, the abundant interconnected 3D cavities of the outer MOF allowed for VOCs to easily diffuse into the pores, producing a concentration microenvironment around the encapsulated TiO₂. The TiO₂@NH₂-UiO-66 composites exhibited a markedly improved photocatalytic efficiency and a good resistance to deactivation during the photocatalytic degradation of gaseous styrene under visible light illumination, which were associated with the synergetic effects between the TiO₂ and MOF. The TiO₂@NH₂-UiO-66 with 5 wt% TiO₂ could efficiently mineralize styrene to CO₂ to some extent companying with the removal ratio >99% within 600 min, whereas the removal efficiency over the bare TiO₂ only 32.5%.

Light-Enhanced Carbon Dioxide Activation and Conversion by Effective Plasmonic Coupling Effect of Pt and Au Nanoparticles

Photocatalytic reduction of carbon dioxide (CO₂) is attractive for the production of valuable fuels and mitigating the influence of greenhouse gas emission. However, the extreme inertness of CO₂ and the sluggish kinetics of photoexcited charge carrier transfer process greatly limit the conversion efficiency of CO₂ photoreduction. Herein, we report that the plasmonic coupling effect of Pt and Au nanoparticles (NPs) profoundly enhances the efficiency of CO₂ reduction through dry reforming of methane reaction assisted by light illumination, reducing activation energies for CO₂ reduction ∼30% below thermal activation energies and achieving a reaction rate 2.4 times higher than that of the thermocatalytic reaction. UV-visible (vis) absorption spectra and wavelength-dependent performances show that not only UV but also visible light play important roles in promoting CO₂ reduction due to effective localized surface plasmon resonance (LSPR) coupling between Pt and Au NPs. Finite-difference time-domain simulations and in situ diffuse reflectance infrared Fourier transform spectroscopy further reveal that effective coupling LSPR effect generates strong local electric fields and excites high concentration of hot electrons to activate the reactants and intermediate species, reduce the activation energies, and increase the reaction rate. This work provides a new pathway toward the efficient plasmon-enhanced chemical reactions via reducing the activation energies by utilizing solar energy.

Superfine palladium nanocrystals on a polyphenylene framework for photocatalysis

To enable the recycling and recovery of these nanosized noble metals, various carriers are adopted for catalyst design.

Visible light-driven selective hydrogenation of unsaturated aromatics in an aqueous solution by direct photocatalysis of Au nanoparticles

Selective hydrogenation of various chemical bonds, such as CC, CC, CO, NO, and CN, is efficiently driven by visible light over a supported gold nanoparticle (AuNP) photocatalyst under mild reaction conditions.

Light-assisted surface reactions on metal nanoparticles

Light-assisted surface reaction can lower reaction temperature, potentially reducing the energy use by providing light together with heat.

Twin defects engineered Pd cocatalyst on C3N4 nanosheets for enhanced photocatalytic performance in CO2 reduction reaction

Photocatalytic conversion of CO₂ to value-added chemicals, a potential route to addressing the depletion of fossil fuels and anthropogenic climate change, is greatly limited by the low-efficient semiconductor photocatalyst. The integration of cocatalyst with light-harvesting semiconductor is a promising approach to enhancing the photocatalytic performance in CO₂ reduction reaction. The enhancement is greatly determined by the catalytic active sites on the surface of cocatalyst. Herein, we demonstrate that the photocatalytic performance in the CO₂ reduction reaction is greatly promoted by twin defects engineered Pd cocatalyst. In this work, Pd nanoicosahedrons with twin defects were in situ grown on C₃N₄ nanosheets, which effectively improve the photocatalytic performance in reduction of CO₂ to CO and CH₄ in comparison with Pd nanotetrahedrons without twin defects. It is proposed that the twin boundary (TB) terminations on the surface of Pd cocatalysts are highly catalytic active sites for CO₂ reduction reaction. Based on the proposed mechanism, the photocatalytic activity and selectivity in CO₂ reduction were further advanced through reducing the size of Pd icosahedral cocatalyst resulted from the increased surface density of TB terminations. The defect engineering on the surface of cocatalyst represents a novel route in realizing high-performance photocatalytic applications.

Facet Engineered Interface Design of Plasmonic Metal and Cocatalyst on BiOCl Nanoplates for Enhanced Visible Photocatalytic Oxygen Evolution

Integration of plasmonic metal and cocatalyst with semiconductor is a promising approach to simultaneously optimize the generation, transfer, and consumption of photoinduced charge carriers for high-performance photocatalysis. The photocatalytic activities of the designed hybrid structures are greatly determined by the efficiencies of charge transfer across the interfaces between different components. In this paper, interface design of Ag-BiOCl-PdO_x hybrid photocatalysts is demonstrated based on the choice of suitable BiOCl facets in depositing plasmonic Ag and PdO_x cocatalyst, respectively. It is found that the selective deposition of Ag and PdO_x on BiOCl(110) planes realizes the superior photocatalytic activity in O₂ evolution compared with the samples with other Ag and PdO_x deposition locations. The reason was the superior hole transfer abilities of Ag-(110)BiOCl and BiOCl(110)-PdO_x interfaces in comparison with those of Ag-(001)BiOCl and BiOCl(001)-PdO_x interfaces. Two effects are proposed to contribute to this enhancement: (1) stronger electronic coupling at the BiOCl(110)-based interfaces resulted from the thinner contact barrier layer and (2) the shortest average hole diffuse distance realized by Ag and PdO_x on BiOCl(110) planes. This work represents a step toward the interface design of high-performance photocatalyst through facet engineering.

Photothermal Catalyst Engineering: Hydrogenation of Gaseous CO2 with High Activity and Tailored Selectivity

This study has designed and implemented a library of hetero-nanostructured catalysts, denoted as Pd@Nb2O5, comprised of size-controlled Pd nanocrystals interfaced with Nb2O5 nanorods. This study also demonstrates that the catalytic activity and selectivity of CO2 reduction to CO and CH4 products can be systematically tailored by varying the size of the Pd nanocrystals supported on the Nb2O5 nanorods. Using large Pd nanocrystals, this study achieves CO and CH4 production rates as high as 0.75 and 0.11 mol h-1 gPd-1, respectively. By contrast, using small Pd nanocrystals, a CO production rate surpassing 18.8 mol h-1 gPd-1 is observed with 99.5% CO selectivity. These performance metrics establish a new milestone in the champion league of catalytic nanomaterials that can enable solar-powered gas-phase heterogeneous CO2 reduction. The remarkable control over the catalytic performance of Pd@Nb2O5 is demonstrated to stem from a combination of photothermal, electronic and size effects, which is rationally tunable through nanochemistry.

Non-Noble Metal Nanoparticles Supported by Postmodified Porous Organic Semiconductors: Highly Efficient Catalysts for Visible-Light-Driven On-Demand H2 Evolution from Ammonia Borane

From the viewpoint of controlling the visible-light-driven activities of catalysts containing metal nanoparticles (NPs) by tuning the microstructures of semiconducting supports, we employed a postsynthetic thermal modification approach to prepare carbon nitride (C₃N₄) species featuring different microstructures and then we synthesized Co and Ni NPs supported by these C₃N₄ species, which were used to catalyze the room-temperature H₂ evolution from ammonia borane (NH₃BH₃). The systematic investigation showed that the catalysts had different activities under light irradiation. Compared with the pristine C₃N₄-based catalyst, all the modified C₃N₄-based catalysts had enhanced activities. The highest active Co catalyst with a total turnover frequency of 93.8 min^-1 was successfully obtained, which exceeded the values of all the reported heterogeneous noble metal-free catalysts. The structure characterizations indicated that the postmodified porous C₃N₄ species had the different band structures, photoluminescence lifetime, and photocurrent density under visible light irradiation, leading to the different separation efficiency of photogenerated charge carriers. These characteristics helped us regulate the electronic characteristics of Co and Ni NPs in the supported catalysts and then led to the significantly different and enhanced activity in the visible-light-driven H₂ evolution.

The Preparation of a Highly Efficient Ag3PO4/Ag/Bi2O2CO3 Photo-Catalyst and the Study of Its Photo-Catalytic Organic Synthesis Reaction Driven by Visible Light

Ag3PO4/Ag/Bi2O2CO3 composites were prepared by a hydrothermal and precipitation method. The morphology, structure, and valence state of the photo-catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface areas, and UV-vis diffuse reflectance spectra (UV-vis DRS). They were applied as heterogeneous catalysts in the synthesis of esters from aldehydes (or alcohols) and alcohols and the synthesis of imines from alcohols and amines under visible light irradiation. The photo-catalytic activities of the esterification reactions of aldehydes and alcohols were heavily dependent on the loading of Ag3PO4/Ag/Bi2O2CO3 as well as the intensity and wavelength of the visible light. Furthermore, their conversion under visible light irradiation was superior to that in the dark. Herein a reaction mechanism from aldehydes and alcohols to esters was proposed, and the Ag3PO4/Ag/Bi2O2CO3 catalysts could be used six times without a significant decrease in activity. Using these catalysts under visible light could motivate future studies to develop efficient recyclable photo-catalysts and facilitate many synthetic organic reactions.

Hot plasmonic electron-driven catalytic reactions on patterned metal-insulator-metal nanostructures

The smart design of plasmonic nanostructures offers a unique capability for the efficient conversion of solar energy into chemical energy by strong interactions with resonant photons through the excitation of surface plasmon resonance, which increases the prospect of using sunlight in environmental and energy applications. Here, we show that the catalytic activity of CO oxidation can be tuned by using new model systems: two-dimensional (2D) arrays of metal-insulator-metal (MIM) plasmonic nanoislands designed to efficiently shuttle hot plasmonic electrons. Hot plasmonic electrons are generated upon the absorption of photons on noble metals, followed by the injection of these hot electrons into the Pt nanoparticles through tunneling or Schottky emission mechanisms, depending on the energy of the hot electrons. We found that these MIM nanostructures exhibit higher catalytic activity (i.e. by 40-110%) under light irradiation, revealing a significant impact on the catalytic activity for CO oxidation. The thickness dependence of the enhancement of catalytic activity on the oxide layers is consistent with the tunneling mechanism of hot electron flows. The results imply that surface plasmon-induced hot electron flows by light absorption significantly influence the catalytic activity of CO oxidation.

Balancing Near-Field Enhancement, Absorption, and Scattering for Effective Antenna-Reactor Plasmonic Photocatalysis

Efficient photocatalysis requires multifunctional materials that absorb photons and generate energetic charge carriers at catalytic active sites to facilitate a desired chemical reaction. Antenna-reactor complexes are an emerging multifunctional photocatalytic structure where the strong, localized near field of the plasmonic metal nanoparticle (e.g., Ag) is coupled to the catalytic properties of the nonplasmonic metal nanoparticle (e.g., Pt) to enable chemical transformations. With an eye toward sustainable solar driven photocatalysis, we investigate how the structure of antenna-reactor complexes governs their photocatalytic activity in the light-limited regime, where all photons need to be effectively utilized. By synthesizing core@shell/satellite (Ag@SiO₂/Pt) antenna-reactor complexes with varying Ag nanoparticle diameters and performing photocatalytic CO oxidation, we observed plasmon-enhanced photocatalysis only for antenna-reactor complexes with antenna components of intermediate sizes (25 and 50 nm). Optimal photocatalytic performance was shown to be determined by a balance between maximized local field enhancements at the catalytically active Pt surface, minimized collective scattering of photons out of the catalyst bed by the complexes, and minimal light absorption in the Ag nanoparticle antenna. These results elucidate the critical aspects of local field enhancement, light scattering, and absorption in plasmonic photocatalyst design, especially under light-limited illumination conditions.

Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles: Key Evidence from the Action Spectrum of the Reaction

By investigating the action spectra (the relationship between the irradiation wavelength and apparent quantum efficiency of reactions under constant irradiance) of a number of reactions catalyzed by nanoparticles including plasmonic metals, nonplasmonic metals, and their alloys at near-ambient temperatures, we found that a photon energy threshold exists in each photocatalytic reaction; only photons with sufficient energy (e.g., higher than the energy level of the lowest unoccupied molecular orbitals) can initiate the reactions. This energy alignment (and the photon energy threshold) is determined by various factors, including the wavelength and intensity of irradiation, molecule structure, reaction temperature, and so forth. Hence, distinct action spectra were observed in the same type of reaction catalyzed by the same catalyst due to a different substituent group, a slightly changed reaction temperature. These results indicate that photon-electron excitations, instead of the photothermal effect, play a dominant role in direct photocatalysis of metal nanoparticles for many reactions.

Metal nanoparticles induced photocatalysis

Photocatalysis induced by light absorption of metal nanoparticles (NPs) has emerged as a promising strategy for exploiting efficient visible-light-responsive composites for solar-energy conversion. In this review, we first introduce the light absorption of metal NPs and the mechanisms proposed in metal-induced photocatalysis (MIP). Then, its applications in water splitting, artificial photosynthesis and inert molecular activation are summarized. To address the challenge of low efficiency in this field, strategies in promoting catalytic activity are reviewed, and particular attention is paid to the particle-size effect of metal. Finally, the challenges and possible development directions of MIP are briefly discussed.

Direct Pathway to Molecular Photodissociation on Metal Surfaces Using Visible Light

We demonstrate molecular photodissociation on single-crystalline metal substrates, driven by visible-light irradiation. The visible-light-induced photodissociation on metal substrates has long been thought to never occur, either because visible-light energy is much smaller than the optical energy gap between the frontier electronic states of the molecule or because the molecular excited states have short lifetimes due to the strong hybridization between the adsorbate molecular orbitals (MOs) and metal substrate. The S-S bond in dimethyl disulfide adsorbed on both Cu(111) and Ag(111) surfaces was dissociated through direct electronic excitation from the HOMO-derived MO (the nonbonding lone-pair type orbitals on the S atoms (n_S)) to the LUMO-derived MO (the antibonding orbital localized on the S-S bond (σ*_SS)) by irradiation with visible light. A combination of scanning tunneling microscopy and density functional theory calculations revealed that visible-light-induced photodissociation becomes possible due to the interfacial electronic structures constructed by the hybridization between molecular orbitals and the metal substrate states. The molecule-metal hybridization decreases the gap between the HOMO- and LUMO-derived MOs into the visible-light energy region and forms LUMO-derived MOs that have less overlap with the metal substrate, which results in longer excited-state lifetimes.