Nanogold plasmonic photocatalysis for organic synthesis and clean energy conversion

Photocatalytic Recovery of Gold from a Non-Cyanide Gold Plating Solution as Au Nanoparticle-Decorated Semiconductors

In this work, a photocatalytic process was carried out to recover gold (Au) from the simulated non-cyanide plating bath solution. Effects of semiconductor types (TiO₂, WO₃, Nb₂O₃, CeO₂, and Bi₂O₃), initial pH of the solution (3-10), and type of complexing agents (Na₂S₂O₃ and Na₂SO₃) and their concentrations (1-4 mM each) on Au recovery were explored. Among all employed semiconductors, TiO₂ exhibited the highest photocatalytic activity to recover Au from the simulated spent plating bath solution both in the absence and presence of complexing agents, in which Au was completely recovered within 15 min at a pH of 6.5. The presence of complexing agents remarkably affected the size of deposited Au on the TiO₂ surface, the localized surface plasmon effect (LSPR) behavior, and the valence band (VB) edge position of the obtained Au/TiO₂, without a significant change in the textural properties or the band gap energy. The photocatalytic activity of the obtained Au/TiO₂ tested via two photocatalytic processes depended on the common reduction mechanism rather than the textural or optical properties. As a result, the Au/TiO₂ NPs obtained from the proposed recovery process are recommended for use as a photocatalyst for the reactions occurring at the conduction band rather than at the valence band. Notably, they exhibited good stability after the fifth photocatalytic cycle for Au recovery from the actual cyanide plating bath solution.

Rational design for gold nanoparticle-based plasmonic catalysts and electrodes for water oxidation towards artificial photosynthesis

The oxygen evolution reaction (OER) with a large overpotential is the key step common to artificial photosynthesis. In semiconductor photocatalysts, the light available to the reactions is usually limited to UV or visible with wavelengths shorter than the absorption edge of the semiconductors. On the other hand, gold nanoparticle (Au NP)-based plasmonic photocatalysts, particularly hot-electron transfer (HET)-type plasmonic photocatalysts, have the capability to utilize visible-to-near infrared light that makes up most sunlight as a driving force for the energetically uphill reactions. In recent years, experimental and theoretical studies on HET-type plasmonic photocatalysts consisting of Au NPs and a semiconductor have been intensively pursued. This perspective article highlights the fundamentals and recent progress of Au NP-based HET-type plasmonic photocatalysts for OER. After the introduction, the basics for the rational design of plasmonic photocatalysts are treated first. Secondly, the concrete design for the plasmonic photocatalysts is dealt with in the order of semiconductors, Au NPs, and their interface. Thirdly, recent advanced studies on plasmonic photocatalysts for OER are described. Finally, the conclusions are summarized with a direction for future research on plasmonic photocatalysts.

In Situ Investigation of Ultrafast Dynamics of Hot Electron-Driven Photocatalysis in Plasmon-Resonant Grating Structures

Understanding the relaxation and injection dynamics of hot electrons is crucial to utilizing them in photocatalytic applications. While most studies have focused on hot carrier dynamics at metal/semiconductor interfaces, we study the in situ dynamics of direct hot electron injection from metal to adsorbates. Here, we report a hot electron-driven hydrogen evolution reaction (HER) by exciting the localized surface plasmon resonance (LSPR) in Au grating photoelectrodes. In situ ultrafast transient absorption (TA) measurements show a depletion peak resulting from hot electrons. When the sample is immersed in solution under -1 V applied potential, the extracted electron-phonon interaction time decreases from 0.94 to 0.67 ps because of additional energy dissipation channels. The LSPR TA signal is redshifted with delay time because of charge transfer and subsequent change in the dielectric constant of nearby solution. Plateau-like photocurrent peaks appear when exciting a 266 nm linewidth grating with p-polarized (on resonance) light, accompanied by a similar profile in the measured absorptance. Double peaks in the photocurrent measurement are observed when irradiating a 300 nm linewidth grating. The enhancement factor (i.e., reaction rate) is 15.6× between p-polarized and s-polarized light for the 300 nm linewidth grating and 4.4× for the 266 nm linewidth grating. Finite-difference time domain (FDTD) simulations show two resonant modes for both grating structures, corresponding to dipolar LSPR modes at the metal/fused silica and metal/water interfaces. To our knowledge, this is the first work in which LSPR-induced hot electron-driven photochemistry and in situ photoexcited carrier dynamics are studied on the same plasmon resonance structure with and without adsorbates.

All-State Switching of the Mie Resonance of Conductive Polyaniline Nanospheres

Polyaniline (PANI), a conductive polymer, is a promising active material for optical switching. In most studies, active switching has so far been realized only between two states, whereas PANI has a total of six states. The optical properties of nanoscale PANI in all six states have remained unclear. Herein we report on all-state switching of the Mie resonance on PANI nanospheres (NSs) and active plasmon switching on PANI-coated Au nanodisks (NDs). All-state switching of differently sized PANI NSs is achieved by proton doping/dedoping and electrochemical methods. Theoretical studies show that the scattering peaks of the individual PANI NSs originate from Mie resonances. All-state switching is further demonstrated on PANI-coated circular Au NDs, where an unprecedentedly large plasmon peak shift of ∼200 nm is realized. Our study not only provides a fundamental understanding of the optical properties of PANI but also opens the probability for developing high-performance dynamic media for active plasmonics.

Octahedral Pt-MOF with Au deposition for plasmonic effect and Schottky junction enhanced hydrogenothermal therapy of rheumatoid arthritis

Hydrogen (H₂) therapy is a novel and rapidly developing strategy utilized to treat inflammatory diseases. However, the therapeutic efficacy of H₂ is largely limited with on-target off-synovium toxic effect, nonpolarity and low solubility. Herein, an intelligent H₂ nanogenerator based upon the metal-organic framework (MOF) loaded with polydopamine and Perovskite quantum dots is constructed for the actualization of hydrogenothermal therapy. The biodegradable polydopamine with excellent photothermal conversion efficiencies is used for photothermal therapy (PTT) of rheumatoid arthritis (RA) and perovskite quantum dots (QDs) with unique photophysical properties are used as fluorescent signals for positioning Pt-MOF@Au@QDs/PDA nanoparticles. In addition, the Pt-MOF@Au@QDs/PDA catalyzer combines Au's surface plasmon resonance excitation with Pt-MOF Schottky junction, and exhibits extremely efficient photocatalytic H₂ production under visible light irradiation. The Pt-MOF@Au@QDs/PDA achieves the aggregation of rheumatoid synovial cells by the extravasation through “ELVIS” effect (extravasation through leaky vasculature and subsequent inflammatory cell-mediated sequestration) and extremely efficient photocatalytic H₂ production. By combining PTT and H₂ therapy, the Pt-MOF@Au@QDs/PDA relieves the oxidative stress of RA, and shows significant improvement in joint damage and inhibition of the overall arthritis severity of collagen-induced RA mouse models. Therefore, the Pt-MOF@Au@QDs/PDA shows great potential in the treatment of RA and further clinical transformation.

Metallic Copper-Containing Composite Photocatalysts: Fundamental, Materials Design, and Photoredox Applications

Semiconductor photocatalysis has long been regarded as a potential solution to tackle the energy and environmental challenges since the first discovery of water splitting by TiO₂ almost 50 years ago. The past few years have seen a tremendous flurry of research interest in the modification of semiconductors because of their shortcomings in the aspects of solar harvesting, electron-hole pairs separation, and utilization of photogenerated carriers. Among the various strategies, the introduction of metallic copper into the photocatalysis system can not only enhance the absorption of sunlight and the separation efficiency of photogenerated electrons and holes, but also increase the adsorption ability of substrate and the number of active sites, so as to realize the high solar to chemical energy conversion efficiency. This review focuses on the rational design of copper-based composites and their applications in photoredox catalysis. First, the preparation methods of metallic copper-containing composites are discussed. Then, the applications of different types of copper-based composites in the photocatalytic removal of pollutants, splitting of water to hydrogen production, reduction of carbon dioxide, and conversion of organic matter are introduced. Finally, the opportunities and challenges in the design and synthesis of copper-based composites and their applications in the photocatalysis are prospected.

Recent progress of noble metals with tailored features in catalytic oxidation for organic pollutants degradation

With the increasing serious water pollutions, an increasing interest has given for the nanocomposites as environmental catalysts. To date, noble metals-based nanocomposites have been extensively studied by researchers in environmental catalysis. In detail, serving as key functional parts, noble metals are usually combined with other nanomaterials for rationally designing nanocomposites, which exhibit enhanced catalytic properties in pollutants removal. Noble metals in the nanocomposites possess tailored properties, thus playing different important roles in catalytic oxidation reactions for pollutants removal. To motivate the research and elaborate the progress of noble metals, this review (i) summarizes advanced characterization techniques and rising technology of theoretical calculation for evaluating noble metal, and (ii) classifies the roles according to their disparate mechanism in different catalytic oxidation reactions. Meanwhile, the enhanced mechanism and influence factors are discussed. (iii) The conclusions, facing challenges and perspectives are proposed for further development of noble metals-based nanocomposites as environmental catalysts.

Robust direct Z-scheme exciton transfer dynamics by architecting 3D BiOI MF-supported non-stoichiometric Cu0.75In0.25S NC nanocomposite for co-catalyst-free photocatalytic hydrogen evolution

Designing promising photocatalytic systems with wide photon absorption and better exciton separation ability is a cutting-edge technology for enhanced solar-light-driven hydrogen production. In this context, non-stoichiometric Cu0.75In0.25S nanocrystals (CIS NCs) coupled with three-dimensional (3D) BiOI micro-flowers (BOI MFs) were synthesized through an ultra-sonication strategy forming a CIS-BOI heterojunction, which was well supported by XRD, photocurrent, XPS and Mott-Schottky analyses. Further, the co-catalyst-free CIS-BOI binary hybrid shows improved hydrogen evolution, i.e., 588.72 μmol h-1, which is 3.2 times greater than the pristine CIS NC (183.97 μmol h-1). Additionally, the binary composite confers an apparent conversion efficiency (ACE) of 9.44% (8.90 × 1016 number of H2 molecule per sec), which is extensively attributed to the robust charge carrier separation and transfer efficiency via the direct Z-scheme mechanism (proved through superoxide and H2 evolution activity). Moreover, the broad photon absorption range and productive exciton separation over the CIS-BOI composite are substantially justified by UV-Vis DRS, PL, EIS and photocurrent measurements.

Photoresponsive DNA materials and their applications

Photoresponsive nucleic acids attract growing interest as functional constituents in materials science. Integration of photoisomerizable units into DNA strands provides an ideal handle for the reversible reconfiguration of nucleic acid architectures by light irradiation, triggering changes in the chemical and structural properties of the nanostructures that can be exploited in the development of photoresponsive functional devices such as machines, origami structures and ion channels, as well as environmentally adaptable 'smart' materials including nanoparticle aggregates and hydrogels. Moreover, photoresponsive DNA components allow control over the composition of dynamic supramolecular ensembles that mimic native networks. Beyond this, the modification of nucleic acids with photosensitizer functionality enables these biopolymers to act as scaffolds for spatial organization of electron transfer reactions mimicking natural photosynthesis. This review provides a comprehensive overview of these exciting developments in the design of photoresponsive DNA materials, and showcases a range of applications in catalysis, sensing and drug delivery/release. The key challenges facing the development of the field in the coming years are addressed, and exciting emergent research directions are identified.

Photothermal polymerization of benzoxazines

Visible irradiation of mixtures of benzoxazine monomers and metal salt catalysts leads to extensive photothermal polymerization, which allows the preparation of complex polybenzoxazine features via photolithography.

Crystallographic interface control of the plasmonic photocatalyst consisting of gold nanoparticles and titanium(iv) oxide

A big question in the field of plasmonic photocatalysis is why a typical photocatalyst consisting of gold nanoparticles and rutile titanium(iv) oxide (Au/R-TiO₂) usually exhibits activity much higher than that of Au/anatase TiO₂ (Au/A-TiO₂) under visible-light irradiation. Shedding light on the origin should present important guidelines for the material design of plasmonic photocatalysts. Au nanoparticles (NPs) were loaded on ordinary irregular-shaped TiO₂ particles by the conventional deposition precipitation method. Transmission electron microscopy analyses for the Au/TiO₂ particles ascertain that faceting of Au NPs is induced on R-TiO₂ by using a domain-matching epitaxial junction with the orientation of (111)_Au//(110)_R-TiO2, whereas non-faceted hemispherical Au NPs are exclusively formed on A-TiO₂. The faceting probability of Au NPs (P_f) on R-TiO₂ increases with decreasing Au particle size (d_Au) to reach 14% at d_Au = 3.6 nm. A clear positive correlation between the photocatalytic activity and P_f in several test reactions indicates that the heteroepitaxial junction-induced faceting of Au NPs is the principal factor for governing the plasmonic photocatalytic activity of Au/TiO₂. In light of this finding, R-TiO₂ nanorods with a high percentage (95%) of {110} facets were hydrothermally synthesized and used for the support of Au NPs. Consequently, the P_f value increases to as much as 94% to enhance the photocatalytic activity with respect to that of Au/R-TiO₂ with P_f = 14% by factors of 2.2–4.4 depending on the type of reaction.

In the represented plasmonic photocatalyst consisting of Au nanoparticles (NPs) and TiO₂, the combination of crystal facet engineering of TiO₂ and atom-level-interface control between Au NP and TiO₂ gives rise to a drastic activity enhancement.

Electrospun Ceramic Nanofibers for Photocatalysis

Ceramic fiber photocatalysts fabricated by electrospinning hold great potential in alleviating global environmental and energy issues. However, many challenges remain in improving their photocatalytic efficiencies, such as the limited carrier lifetime and solar energy utilization. To overcome these predicaments, various smart strategies have been invented and realized in ceramic fiber photocatalysts. This review firstly attempts to summarize the fundamental principles and bottlenecks of photocatalytic processes. Subsequently, the approaches of doping, surface plasmon resonance, and up-conversion fluorescent to enlarge the light absorption range realized by precursor composition design, electrospinning parameter control, and proper post heat-treatment process are systematically introduced. Furthermore, methods and achievements of prolonging the lifetime of photogenerated carriers in electrospun ceramic fiber photocatalysts by means of introducing heterostructure and defective composition are reviewed in this article. This review ends with a summary and some perspectives on the future directions of ceramic fiber photocatalysts.

On the Roles of Electron Transfer in Catalysis by Nanoclusters and Nanoparticles

Electron transfer plays a major role in chemical reactions and processes, and this is particularly true of catalysis by nanomaterials. The advent of metal nanoparticle (NP) catalysts, recently including atomically precise nanoclusters (NCs) as parts of nanocatalyst devices has brought increased control of the relationship between NP and NC structures and their catalytic functions. Consequently, the molecular definition of these new nanocatalysts has allowed a better understanding and management of various kinds of electron transfer involved in the catalytic processes. This Minireview brings a chemist's view of several major aspects of electron-transfer functions concerning NPs and NCs in catalytic processes. Particular focus concerns the role of NPs and NCs as electron reservoirs and light-induced antenna in catalytic processes from H₂ generation to more complex reactions and sustainable energy production.

Localized surface plasmon resonance for enhanced electrocatalysis

Electrocatalysis plays a vital role in energy conversion and storage in modern society. Localized surface plasmon resonance (LSPR) is a highly attractive approach to enhance the electrocatalytic activity and selectivity with solar energy. LSPR excitation can induce the transfer of hot electrons and holes, electromagnetic field enhancement, lattice heating, resonant energy transfer and scattering, in turn boosting a variety of electrocatalytic reactions. Although the LSPR-mediated electrocatalysis has been investigated, the underlying mechanism has not been well explained. Moreover, the efficiency is strongly dependent on the structure and composition of plasmonic metals. In this review, the currently proposed mechanisms for plasmon-mediated electrocatalysis are introduced and the preparation methods to design supported plasmonic nanostructures and related electrodes are summarized. In addition, we focus on the characterization strategies used for verifying and differentiating LSPR mechanisms involved at the electrochemical interface. Following that are highlights of representative examples of direct plasmonic metal-driven and indirect plasmon-enhanced electrocatalytic reactions. Finally, this review concludes with a discussion on the remaining challenges and future opportunities for coupling LSPR with electrocatalysis.

Advancing Plasmon-Induced Selectivity in Chemical Transformations with Optically Coupled Transmission Electron Microscopy

Nanoparticle photocatalysts are essential to processes ranging from chemical production and water purification to air filtration and surgical instrument sterilization. Photochemical reactions are generally mediated by the illumination of metallic and/or semiconducting nanomaterials, which provide the necessary optical absorption, electronic band structure, and surface faceting to drive molecular reactions. However, with reaction efficiency and selectivity dictated by atomic and molecular interactions, imaging and controlling photochemistry at the atomic scale are necessary to both understand reaction mechanisms and to improve nanomaterials for next-generation catalysts. Here, we describe how advances in plasmonics, combined with advances in electron microscopy, particularly optically coupled transmission electron microscopy (OTEM), can be used to image and control light-induced chemical transformations at the nanoscale. We focus on our group's research investigating the interaction between hydrogen gas and Pd nanoparticles, which presents an important model system for understanding both hydrogenation catalysis and hydrogen storage. The studies described in this Account primarily rely on an environmental transmission electron microscope, a tool capable of circumventing traditional TEM's high-vacuum requirements, outfitted with optical sources and detectors to couple light into and out of the microscope. First, we describe the H₂ loading kinetics of individual Pd nanoparticles. When confined to sizes of less than ∼100 nm, single-crystalline Pd nanoparticles exhibit coherent phase transformations between the hydrogen-poor α-phase and hydrogen-rich β-phase, as revealed through monitoring the bulk plasmon resonance with electron energy loss spectroscopy. Next, we describe how contrast imaging techniques, such as phase contrast STEM and displaced-aperture dark field, can be employed as real-time techniques to image phase transformations with 100 ms temporal resolution. Studies of multiply twinned Pd nanoparticles and high aspect ratio Pd nanorods demonstrate that internal strain and grain boundaries can lead to partial hydrogenation within individual nanoparticles. Finally, we describe how OTEM can be used to locally probe nanoparticle dynamics under optical excitation and in reactive chemical environments. Under illumination, multicomponent plasmonic photocatalysts consisting of a gold nanoparticle "antenna" and a Pd "reactor" show clear α-phase nucleation in regions close to electromagnetic "hot spots" when near plasmonic antennas. Importantly, these hot spots need not correspond to the traditionally active, energetically preferred sites of catalytic nanoparticles. Nonthermal effects imparted by plasmonic nanoparticles, including electromagnetic field enhancement and plasmon-derived hot carriers, are crucial to explaining the site selectivity observed in PdH_x phase transformations under illumination. This Account demonstrates how light can contribute to selective chemical phenomena in plasmonic heterostructures, en route to sustainable, solar-driven chemical production.

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

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

Light-Mediated Polymerization Induced by Semiconducting Nanomaterials: State-of-the-Art and Future Perspectives

Direct capture of solar energy for chemical transformation via photocatalysis proves to be a cost-effective and energy-saving approach to construct organic compounds. With the recent growth in photosynthesis, photopolymerization has been established as a robust strategy for the production of specialty polymers with complex structures, precise molecular weight, and narrow dispersity. A key challenge in photopolymerization is the scarcity of effective photomediators (photoinitiators, photocatalysts, etc.) that can provide polymerization with high yield and well-defined polymer products. Current efforts on developing photomediators have mainly focused on organic dyes and metal complexes. On the other hand, nanomaterials (NMs), particularly semiconducting nanomaterials (SNMs), are suitable candidates for photochemical reactions due to their unique optical and electrical properties, such as high absorption coefficients, large charge diffusion lengths, and broad absorption spectra. This review provides a comprehensive insight into SNMs' photomediated polymerizations and highlights the roles SNMs play in photopolymerizations, types of polymerizations, applications in producing advanced materials, and the future directions.

Copper-Based Plasmonic Catalysis: Recent Advances and Future Perspectives

With the capability of inducing intense electromagnetic field, energetic charge carriers, and photothermal effect, plasmonic metals provide a unique opportunity for efficient light utilization and chemical transformation. Earth-abundant low-cost Cu possesses intense and tunable localized surface plasmon resonance from ultraviolet-visible to near infrared region. Moreover, Cu essentially exhibits remarkable catalytic performance toward various reactions owing to its intriguing physical and chemical properties. Coupling with light-harvesting ability and catalytic function, plasmonic Cu serves as a promising platform for efficient light-driven chemical reaction. Herein, recent advancements of Cu-based plasmonic photocatalysis are systematically summarized, including designing and synthetic strategies for Cu-based catalysts, plasmonic catalytic performance, and mechanistic understanding over Cu-based plasmonic catalysts. What's more, approaches for the enhancement of light utilization efficiency and construction of active centers on Cu-based plasmonic catalysts are highlighted and discussed in detail, such as morphology and size control, regulation of electronic structure, defect and strain engineering, etc. Remaining challenges and future perspectives for further development of Cu-based plasmonic catalysis are also proposed.

Main Avenues in Gold Coordination Chemistry

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.

In-situ nanospectroscopic imaging of plasmon-induced two-dimensional [4+4]-cycloaddition polymerization on Au(111)

Plasmon-induced chemical reactions (PICRs) have recently become promising approaches for highly efficient light-chemical energy conversion. However, an in-depth understanding of their mechanisms at the nanoscale still remains challenging. Here, we present an in-situ investigation by tip-enhanced Raman spectroscopy (TERS) imaging of the plasmon-induced [4+4]-cycloaddition polymerization within anthracene-based monomer monolayers physisorbed on Au(111), and complement the experimental results with density functional theory (DFT) calculations. This two-dimensional (2D) polymerization can be flexibly triggered and manipulated by the hot carriers, and be monitored simultaneously by TERS in real time and space. TERS imaging provides direct evidence for covalent bond formation with ca. 3.7 nm spatial resolution under ambient conditions. Combined with DFT calculations, the TERS results demonstrate that the lateral polymerization on Au(111) occurs by a hot electron tunneling mechanism, and crosslinks form via a self-stimulating growth mechanism. We show that TERS is promising to be plasmon-induced nanolithography for organic 2D materials.

Continuous-Flow Photocatalytic Microfluidic-Reactor for the Treatment of Aqueous Contaminants, Simplicity, and Complexity: A Mini-Review

Water pollution is a growing global issue; there are many approaches to treating wastewater, including chemical coagulation, physical adsorption, and chemical oxidation. The photocatalysis process has provided a solution for removing pollutants from wastewater, where the pair of the photoelectron and hole works through an asymmetric way to degrade the contaminants under UV irradiation. This method offers an alternative route for treating the pollutant with a lower energy cost, high efficiency, and fewer byproducts. A continuous-flow microfluidic reactor has a channel size from tens to thousands of micrometers, providing uniform irradiation and short diffusion length. It can enhance the conversion efficiency of photocatalysis due to the simple spatial symmetry inside the microreactor channel and among the individual channels. In addition, the bandgap of TiO2, ZnO, or other photocatalyst nanoparticles with symmetric crystal structure can be modified through doping or embedding. In this mini-review, a review of the reported continuous-flow photocatalytic microfluidic reactor is discussed from the perspective of both microreactor design and material engineering.

In Situ Investigation of Charge Performance in Anatase TiO2 Powder for Methane Conversion by Vis-NIR Spectroscopy

The intrinsic behavior of photogenerated charges and reactions with chemicals are key for a photocatalytic process. To observe these basic steps is of great importance. Here we present a reliable and robust system to monitor these basic steps in powder photocatalysts, and more importantly to elucidate the key issue in photocatalytic methane conversion over the benchmark catalyst TiO₂. Under constant excitation, the absorption signal across the NIR region was demonstrated to be dominated by photoexcited electrons, the absorption of photoexcited holes increases toward shorter wavelengths in the visible region, and the overall shapes of the photoinduced absorption spectra obtained using the system demonstrated in the present work are consistent with widely accepted transient absorption results. Next, in situ measurements provide direct experimental evidence that the initial step of methane activation over TiO₂ involves oxidation by photoexcited holes. It is calculated that 90 ± 6% of photoexcited electrons are scavenged by O₂ (in dry air), 61 ± 9% of photoexcited holes are scavenged by methane (10% in argon), and a similar amount of photoexcited electrons can be scavenged by O₂ even when the O₂ concentration is reduced by a factor of 10. The present results suggest that O₂ is much more easily activated in comparison to methane over anatase TiO₂, which rationalizes the much higher methane/O₂ ratio frequently used in practice in comparison to that required stoichiometrically for photocatalytic production of value-added chemicals via methane oxidation with oxygen. In addition, methanol (a preferable product of methane oxidation) is much more readily oxidized than methane over anatase TiO₂.

Size effect of bifunctional gold in hierarchical titanium oxide-gold-cadmium sulfide with slow photon effect for unprecedented visible-light hydrogen production

Gold nanoparticles (Au NPs) with surface plasmonic resonance (SPR) effect and excellent internal electron transfer ability have widely been combined with semiconductors for photocatalysis. However, the in-depth effects of Au NPs in multicomponent photocatalysts have not been completely understood. Herein, ternary titanium oxide-gold-cadmium sulfide (TiO₂-Au-CdS, TAC) photocatalysts, based on hierarchical TiO₂ inverse opal photonic crystal structure with different Au NPs sizes have been designed to reveal the SPR effect and internal electron transfer of Au NPs in the presence of slow photon effect. It appears that the SPR effect and internal electron transfer ability of Au NPs, depending on their sizes, play a synergistic effect on the photocatalytic enhancement. The ternary TAC-10 photocatalyst with ~ 10 nm Au NPs demonstrates an unprecedented hydrogen evolution rate of 47.6 mmolh^-1g^-1 under visible-light, demonstrating ~ 48% enhancement comparing to the sample without slow photon effect. In particular, a 9.83% apparent quantum yield under 450 nm monochromatic light is achieved for TAC-10. A model is proposed and finite-difference time-domain (FDTD) simulations reveal the size influence of Au NPs in ternary TAC photocatalysts. This work suggests that the rational design of bifunctional Au NPs coupling with slow photon effect could largely promote hydrogen production from visible-light driven water splitting.

An NIR dual-emitting/absorbing inorganic compact pair: A self-calibrating LRET system for homogeneous virus detection

Many conventional optical biosensing systems use a single responsive signal in the visible light region. This limits their practical applications, as the signal can be readily perturbed by various external environmental factors. Herein, a near-infrared (NIR)-based self-calibrating luminescence resonance energy transfer (LRET) system was developed for background-free detection of analytes in homogeneous sandwich-immunoassays. The inorganic LRET pair was comprised of NIR dual-emitting lanthanide-doped nanoparticles (LnNPs) as donors and NIR-absorbing LnNPs as acceptors, which showed a narrow absorption peak (800 nm) and long-term stability, enabling stable LRET with a built-in self-calibrating signal. Screened single-chain variable fragments (scFvs) were used as target avian influenza virus (AIV)-binding antibodies to increase the LRET efficiency in sandwich-immunoassays. The compact sensor platform successfully detected AIV nucleoproteins with a 0.38 pM limit of detection in buffer solution and 64 clinical samples. Hence, inorganic LnNP pairs may be effective for self-calibrating LRET systems in the background-free NIR region.

Control of Chemical Reaction Pathways by Light-Matter Coupling

Because plasmonic metal nanostructures combine strong light absorption with catalytically active surfaces, they have become platforms for the light-assisted catalysis of chemical reactions. The enhancement of reaction rates by plasmonic excitation has been extensively discussed. This review focuses on a less discussed aspect: the induction of new reaction pathways by light excitation. Through commentary on seminal reports, we describe the principles behind the optical modulation of chemical reactivity and selectivity on plasmonic metal nanostructures. Central to these phenomena are excited charge carriers generated by plasmonic excitation, which modify the energy landscape available to surface reactive species and unlock pathways not conventionally available in thermal catalysis. Photogenerated carriers can trigger bond dissociation or desorption in an adsorbate-selective manner, drive charge transfer and multielectron redox reactions, and generate radical intermediates. Through one or more of these mechanisms, a specific pathway becomes favored under light. By improved control over these mechanisms, light-assisted catalysis can be transformational for chemical synthesis and energy conversion.

Plasmonic Photocatalysts

Plasmonic photocatalysts, i [...]

Synthesis of coralloid carbon nitride polymers and photocatalytic selective oxidation of benzyl alcohol

Polymeric carbon nitride (C₃N₄) is currently the most potential nonmetallic photocatalyst, but it suffers from low catalytic activity due to rapid electron-hole recombination behavior and low specific surface area. The morphology control of C₃N₄is one of the effective methods used to achieve higher photocatalytic performance. Here, bulk, lamellar and coralloid C₃N₄were synthesized using different chemical methods. The as-prepared coralloid C₃N₄has a higher specific surface area (123.7 m² · g^-1) than bulk (5.4 m² · g^-1) and lamellar C₃N₄(2.8 m² · g^-1), thus exhibiting a 3.15- and 2.59-fold higher photocatalytic efficiency for the selective oxidation of benzyl alcohol than bulk and lamellar C₃N₄, respectively. Optical characterizations of the photocatalysts suggest that coralloid C₃N₄can effectively capture electrons and accelerate carrier separation, which is caused by the presence of more nitrogen vacancies. Furthermore, it is demonstrated that superoxide radicals (·O₂^-) and holes (h⁺) play major roles in the photocatalytic selective oxidation of benzyl alcohol using C₃N₄as a photocatalyst.

Advances in nanomaterials for heterogeneous photocatalysis

Photocatalysis method for environmental applications has been using for a long time. This review article traces back the origin of catalysis, its classification and journey of development to heterogeneous photocatalysis and the article’s novelty is in the simplicity, and easily understandable language, designed for the beginners. These heterogeneous photocatalysts are grouped into eleven different categories. As the paper is focused on photocatalysis, an insight on fundamental principles and mechanisms of photocatalysis are explained systematically with schematic illustrations and reactions that take place during redox- oxidation and reduction reactions in photocatalysis. With an approach towards utilizing green energy and expanding the photocatalyst’ absorption wavelength range towards the visible regime, bandgap engineering techniques by adopting doping and hetero-structures are explained with examples of different materials. In addition, dominating factors of photocatalysis reaction viz. composition of a heterogeneous photocatalyst, doping, hetero-structures, pH, surface defects on photocatalysis reaction are explored, focussing on variable charge transfer mechanisms. The main influencing factor in generating reactive oxygen species is pH of the photocatalysis reaction and are studied indetail. The effect of alkalinity or acidity in catalyst surfaces and molecular interaction depending upon the point zero charges of the photocatalyst are discussed. For the better study of catalyst properties, careful analysis and study is a much-needed field as a scope for further improvement. Hence, this article will guide a beginner to understand the photocatalysis topic with ease.

Plasmonic photothermal catalysis for solar-to-fuel conversion: current status and prospects

Solar-to-fuel conversion through photocatalytic processes is regarded as promising technology with the potential to reduce reliance on dwindling reserves of fossil fuels and to support the sustainable development of our society. However, conventional semiconductor-based photocatalytic systems suffer from unsatisfactory reaction efficiencies due to limited light harvesting abilities. Recent pioneering work from several groups, including ours, has demonstrated that visible and infrared light can be utilized by plasmonic catalysts not only to induce local heating but also to generate energetic hot carriers for initiating surface catalytic reactions and/or modulating the reaction pathways, resulting in synergistically promoted solar-to-fuel conversion efficiencies. In this perspective, we focus primarily on plasmon-mediated catalysis for thermodynamically uphill reactions converting CO2 and/or H2O into value-added products. We first introduce two types of mechanism and their applications by which reactions on plasmonic nanostructures can be initiated: either by photo-induced hot carriers (plasmonic photocatalysis) or by light-excited phonons (photothermal catalysis). Then, we emphasize examples where the hot carriers and phonon modes act in concert to contribute to the reaction (plasmonic photothermal catalysis), with special attention given to the design concepts and reaction mechanisms of the catalysts. We discuss challenges and future opportunities relating to plasmonic photothermal processes, aiming to promote an understanding of underlying mechanisms and provide guidelines for the rational design and construction of plasmonic catalysts for highly efficient solar-to-fuel conversion.

Plasmonic nano-antimicrobials: properties, mechanisms and applications in microbe inactivation and sensing

Bacterial, viral and fungal infections pose serious threats to human health and well-being. The continuous emergence of acute infectious diseases caused by pathogenic microbes and the rapid development of resistances against conventional antimicrobial drugs necessitates the development of new and effective strategies for the safe elimination of microbes in water, food or on surfaces, as well as for the inactivation of pathogenic microbes in human hosts. The need for new antimicrobials has triggered the development of plasmonic nano-antimicrobials that facilitate both light-dependent and -independent microbe inactivation mechanisms. This review introduces the relevant photophysical mechanisms underlying these plasmonic nano-antimicrobials, and provides an overview of how the photoresponses and materials properties of plasmonic nanostructures can be applied in microbial pathogen inactivation and sensing applications. Through a systematic analysis of the inactivation efficacies of different plasmonic nanostructures, this review outlines the current state-of-the-art in plasmonic nano-antimicrobials and defines the application space for different microbial inactivation strategies. The advantageous optical properties of plasmonic nano-antimicrobials also enhance microbial detection and sensing modalities and thus help to avoid exposure to microbial pathogens. Sensitive and fast plasmonic microbial sensing modalities and their theranostic and targeted therapeutic applications are discussed.

Nanosized CdS as a Reusable Photocatalyst: The Study of Different Reaction Pathways between Tertiary Amines and Aryl Sulfonyl Chlorides through Visible-Light-Induced N-Dealkylation and C-H Activation Processes

It has been found that the final products of the reaction of sulfonyl chlorides and tertiary amines in the presence of cadmium sulfide nanoparticles under visible light irradiation are highly dependent on the applied reaction conditions. Interestingly, with the change of a reaction condition, different pathways were conducted (visible-light-induced N-dealkylation or sp³ and sp² C-H activation) that lead to different products such as secondary amines and various sulfonyl compounds. Remarkably, all of these reactions were performed under visible light irradiation and an air atmosphere without any additive or oxidant in benign solvents or under solvent-free conditions. During this study, the CdS nanoparticles as affordable, heterogeneous, and recyclable photocatalysts were designed, successfully synthesized, and fully characterized and applied for these protocols. During these studies, intermediates resulting from the oxidation of tertiary amines are trapped during the photoinduced electron transfer (PET) process. The reaction was carried out efficiently with a variety of substrates to give the corresponding products at relatively short times in good to excellent yields in parallel with the use of the visible light irradiation as a renewable energy source. Most of these processes are novel or are superior in terms of cost-effectiveness, safety, and simplicity to published reports.

Recent Progress in Plasmonic Hybrid Photocatalysis for CO2 Photoreduction and C–C Coupling Reactions

Plasmonic hybrid nanostructures have been investigated as attractive heterogeneous photocatalysts that can utilize sunlight to produce valuable chemicals. In particular, the efficient photoconversion of CO2 into a stable hydrocarbon with sunlight can be a promising strategy to achieve a sustainable human life on Earth. The next step for hydrocarbons once obtained from CO2 is the carbon–carbon coupling reactions to produce a valuable chemical for energy storage or fine chemicals. For these purposes, plasmonic nanomaterials have been widely investigated as a visible-light-induced photocatalyst to achieve increased efficiency of photochemical reactions with sunlight. In this review, we discuss recent achievements involving plasmonic hybrid photocatalysts that have been investigated for CO and CO2 photoreductions to form multi-carbon products and for C–C coupling reactions, such as the Suzuki–Miyaura coupling reactions.

Improving the rate of the copper-catalyzed Henry reaction by surface plasmon excitation of gold nanoparticles

Green plasmon excitation of colloidal Au nanoparticles, onto which a copper(ii) complex was grafted, in the presence of nitrobenzaldehyde and nitromethane in DMF, lead to the formation of the corresponding nitroaldol with high efficiency.

From 0-dimension to 1-dimensions: Au nanocrystals as versatile plasmonic photocatalyst for broadband light induced RAFT polymerization

Gold nanoparticles and nanorods were utilized as the catalyst for photo-induced RAFT polymerization, because of their strong LSPR performance.

Fundamentals and applications of photo-thermal catalysis

Photo-thermal catalysis has recently emerged as an alternative route to drive chemical reactions using light as an energy source.

Vanadium-based nanomaterials for cancer diagnosis and treatment

In the past few decades, various vanadium compounds have displayed potential in cancer treatment. However, fast clearness in the body and possible toxicity of vanadium compounds has hindered their further development. Vanadium-based nanomaterials not only overcome these limitations, but take advantage of the internal properties of vanadium in photics and magnetics, which enable them as a multimodal platform for cancer diagnosis and treatment. In this paper, we first introduced the basic biological and pharmacological functions of vanadium compounds in treating cancer. Then, the synthesis routes of three vanadium-based nanomaterials were discussed, including vanadium oxides, 2D vanadium sulfides, carbides and nitrides: V_mX_n (X = S, C, N) and water-insoluble vanadium salts. Finally, we highlighted the applications of these vanadium-based nanomaterials as tumor therapeutic and diagnostic agents.

Ring patterns generated by an expanding colloidal meniscus

The drop-and-dry is a common technique allowing for creation of periodic nanoparticle (NP) structures for sensing, photonics, catalysis, etc. However, the reproducibility and scalability of this approach for fabrication of NP-based structures faces serious challenges due to the complexity of the simple, at first glance, evaporation process. In this work we study the effect of the spatial confinement on the NP self-assembly under slow solvent evaporation, when the air-liquid-substrate contact line (CL) expands from the center towards the walls of a cylindrical cell, forming a toroid. Using in situ video monitoring of the stick-slip CL motion, we find regular hydrodynamic perturbations in the meniscus, and reveal fine details of the formation of quasiperiodic rings of close packed NP layers. We report that drying of the toroidal NP droplet has a number of important differences from drying of the classical hemispherical colloidal drops. In toroidal drops we observe linear-in-time average meniscus motion, in contrast to the hemispherical drops where the meniscus moves as a square root of time. While both droplet geometries produce NP ring patterns, the ring width for the toroidal drop decreases with increasing ring radius, while it decreases with decreasing the radius of the hemispherical drop. We suggest that free ligands are the main cause of the Marangoni instabilities driving the periodic vorticity in the meniscus. In addition, we show that the usually ignored contact line tension may yield a considerable contribution to the CL pinning causing the CL slip-stick motion and the ring formation.

Abatement of formaldehyde with photocatalytic and catalytic oxidation: a review

Formaldehyde is one of the vital chemicals produced by industries, transports, and domestic products. Formaldehyde emissions adversely affect human health and it is well known for causing irritation and nasal tumors. The major aim of the modern indoor formaldehyde control study is in view of energy capacity, product selectivity, security, and durability for efficient removal of formaldehyde. The two important methods to control this harmful chemical in the indoor environments are photocatalytic oxidation and catalytic oxidation with noble metals and transition metal oxides. By harmonizing different traditional photocatalytic and catalytic oxidation technologies that have been evolved already, here we give a review of previously developed efforts to degrade indoor formaldehyde. The major concern in this article is based on getting the degradation of formaldehyde at ambient temperature. In this article, different aspects of these two methods with their merits and demerits are discussed. The possible effects of operating parameters like preparation methods, support, the effect of light intensity in photocatalytic oxidation, relative humidity, etc. have been discussed comprehensively.

Recent Advances in Plasmon-Promoted Organic Transformations Using Silver-Based Catalysts

Plasmonics has emerged as a promising methodology to promote chemical reactions and has become a field of intense research effort. Ag nanoparticles (NPs) as plasmonic catalysts have been extensively studied because of their remarkable optical properties. This review analyzes the emergence and development of localized surface plasmon resonance (LSPR) in organic chemistry, mainly focusing on the discovery of novel reactions with new mechanisms on Ag NPs. Initially, the basics of LSPR and LSPR-promoted photocatalytic mechanisms are illustrated. Then, the recent advances in plasmonic nanosilver-mediated photocatalysis in organic transformations are highlighted with an emphasis on the related reaction mechanisms. Finally, a proper perspective on the remaining challenges and future directions in the field of LSPR-promoted organic transformations is proposed.