Synergy between Plasmonic and Electrocatalytic Activation of Methanol Oxidation on Palladium–Silver Alloy Nanotubes

Localized surface plasmon resonance (LSPR) excitation of noble metal nanoparticles has been shown to accelerate and drive photochemical reactions. Here, LSPR excitation is shown to enhance the electrocatalysis of a fuel‐cell‐relevant reaction. The electrocatalyst consists of Pd_xAg alloy nanotubes (NTs), which combine the catalytic activity of Pd toward the methanol oxidation reaction (MOR) and the visible‐light plasmonic response of Ag. The alloy electrocatalyst exhibits enhanced MOR activity under LSPR excitation with significantly higher current densities and a shift to more positive potentials. The modulation of MOR activity is ascribed primarily to hot holes generated by LSPR excitation of the Pd_xAg NTs.     

The Chemical Potential of Plasmonic Excitations

By the photoexcitation of localized surface plasmon resonances of metal nanoparticles, one can generate reaction equivalents for driving redox reactions. We show that, in such cases, there is a chemical potential contributed by the plasmonic excitation. This chemical potential is a function of the concentration of light, as we determine from the light‐intensity‐dependent activity in the plasmon‐excitation‐driven reduction of CO₂ on Au nanoparticles. Our finding allows the treatment of plasmonic excitation as a reagent in chemical reactions; the chemical potential of this reagent is tunable by the light intensity.     

Direct Photocatalysis for Organic Synthesis by Using Plasmonic‐Metal Nanoparticles Irradiated with Visible Light

Recent advances in direct‐use plasmonic‐metal nanoparticles (NPs) as photocatalysts to drive organic synthesis reactions under visible‐light irradiation have attracted great interest. Plasmonic‐metal NPs are characterized by their strong interaction with visible light through excitation of the localized surface plasmon resonance (LSPR). Herein, we review recent developments in direct photocatalysis using plasmonic‐metal NPs and their applications. We focus on the role played by the LSPR of the metal NPs in catalyzing organic transformations and, more broadly, the role that light irradiation plays in catalyzing the reactions. Through this, the reaction mechanisms that these light‐excited energetic electrons promote will be highlighted. This review will be of particular interest to researchers who are designing and fabricating new plasmonic‐metal NP photocatalysts by identifying important reaction mechanisms that occur through light irradiation.     

Converting Ag2SCdS and Ag2SZnS into AgCdS and AgZnS Nanoheterostructures by Selective Extraction of Sulfur

A mild three‐step solution strategy is developed to prepare Ag? MS (M=Zn, Cd) nanoheterostructures composed of MS nanorods with silver tips. First, Ag₂S? MS heterostructures are synthesized by following a solution–liquid–solid mechanism with Ag₂S nanoparticles as catalysts, then the Ag₂S sections of the heterostructures are converted into silver nanoparticles by selective extraction of sulfur. Notably, for the prepared Ag? CdS heterostructures, the localized surface plasmon resonance of silver remarkably intensifies the photoluminescence of CdS by enhancing the excitation light absorption, which is beneficial for potential applications of CdS nanoparticles in the fields of biolabeling, light‐emitting diodes, and so forth. The strategy reported herein would be useful for designing and fabricating other metal–semiconductor hybrid nanostructures with desirable performances.     

基于局域表面等离子体共振效应的光学生物传感器*

贵金属纳米粒子表现出许多常规块体材料所不具备的优异性能,其中局域表面等离子体共振 (LSPR) 特性是研究热点之一。LSPR 的形状和位置与纳米粒子的组成、大小、形状、介电性质以及局域介质环境密切相关。基于这一特性,贵金属纳米粒子已广泛应用于光学生物传感器、光过滤器和表面增强光谱等领域。本文对各种结构的贵金属纳米粒子的制备方法及其在光学生物传感器中的应用进行了综述,并对 LSPR 纳米传感器的未来发展前景做了展望。     

Nanoplasmonic Photoluminescence Spectroscopy at Single‐Particle Level: Sensing for Ethanol Oxidation

Surface plasmon resonances of metal nanoparticles have shown significant promise for the use of solar energy to drive catalytic chemical reactions. More importantly, understanding and monitoring such catalytic reactions at single‐nanoparticle level is crucial for the study of local reaction processes. Herein, using plasmonic photoluminescence (PL) spectroscopy, we describe a novel sensing method for catalytic ethanol oxidation reactions at the single‐nanoparticle level. The Au nanorod monitors the interfacial interaction with ethanol during the catalytic reaction through the PL intensity changes in the single‐particle PL spectra. The analysis of energy relaxation of excited electron–hole pairs indicates the relationship between the PL quenching and ethanol oxidation reaction on the single Au nanorod.     

Evaluation of the plasmonic effect of Au and Ag on Ti-based photocatalysts in the reduction of CO_2 to CH_4

Crystalline TiO_2(P25) and isolated titanate species in a ZSM-5 structure(TS-1) were modified with Au and Ag, respectively, and tested in the gas-phase photocatalytic CO_2 reduction under high purity conditions. The noble metal modification was performed by photodeposition. Light absorbance properties of the catalysts are examined with UV–Vis spectroscopy before and after the activity test. In the gas-phase photocatalytic CO_2 reduction, it was observed that the catalysts with Ag nanostructures are more active than those with Au nanostructures. It is thus found that the energetic difference between the band gap energy of the semiconductor and the position of the plasmon is influencing the photocatalytic activity.Potentially, plasmon excitation due to visible light absorption results in plasmon resonance energy, which affects the excitation of the semiconductor positively. Therefore, an overlap between band gap energy of the semiconductor and metal plasmon is needed.     

Galvanic Replacement Reactions of Active‐Metal Nanoparticles

We present a systemic investigation of a galvanic replacement technique in which active‐metal nanoparticles are used as sacrificial seeds. We found that different nanostructures can be controllably synthesized by varying the type of more noble‐metal ions and liquid medium. Specifically, nano‐heterostructures of noble metal (Ag, Au) or Cu nanocrystals on active‐metal (Mg, Zn) cores were obtained by the reaction of active‐metal nanoparticles with more noble‐metal ions in ethanol; Ag nanocrystal arrays were produced by the reaction of active‐metal nanoparticles with Ag⁺ ions in water; spongy Au nanospheres were generated by the reaction of active‐metal nanoparticles with AuCl₄^? ions in water; and SnO₂ nanoparticles were prepared when Sn²⁺ were used as the oxidant ions. The key factors determining the product morphology are shown to be the reactivity of the liquid medium and the nature of the oxidant–reductant couple, whereas Mg and Zn nanoparticles played similar roles in achieving various nanostructures. When microsized Mg and Zn particles were used as seeds in similar reactions, the products were mainly noble‐metal dendrites. The new approach proposed in this study expands the capability of the conventional nanoscale galvanic replacement method and provides new avenues to various structures, which are expected to have many potential applications in catalysis, optoelectronics, and biomedicine.     

Single‐Molecule Study of a Plasmon‐Induced Reaction for a Strongly Chemisorbed Molecule

Chemical reactions induced by plasmons achieve effective solar‐to‐chemical energy conversion. However, the mechanism of these reactions, which generate a strong electric field, hot carriers, and heat through the excitation and decay processes, is still controversial. In addition, it is not fully understood which factor governs the mechanism. To obtain mechanistic knowledge, we investigated the plasmon‐induced dissociation of a single‐molecule strongly chemisorbed on a metal surface, two O₂ species chemisorbed on Ag(110) with different orientations and electronic structures, using a scanning tunneling microscope (STM) combined with light irradiation at 5 K. A combination of quantitative analysis by the STM and density functional theory calculations revealed that the hot carriers are transferred to the antibonding (π*) orbitals of O₂ strongly hybridized with the metal states and that the dominant pathway and reaction yield are determined by the electronic structures formed by the molecule–metal chemical interaction.     

A facile synthesis and characterization of Ag, Au and Pt nanoparticles using a natural hydrocolloid gum kondagogu (Cochlospermum gossypium)

An environmentally benign method for the synthesis of noble metal nanoparticles has been reported using aqueous solution of gum kondagogu (Cochlospermum gossypium). Both the synthesis, as well as stabilization of colloidal Ag, Au and Pt nanoparticles has been accomplished in an aqueous medium containing gum kondagogu. The colloidal suspensions so obtained were found to be highly stable for prolonged period, without undergoing any oxidation. SEM-EDXA, UV-vis spectroscopy, XRD, FTIR and TEM techniques were used to characterize the Ag, Au and Pt nanoparticles. FTIR analysis indicates that -OH groups present in the gum matrix were responsible for the reduction of metal cations into nanoparticles. UV-vis studies showed a distinct surface plasmon resonance at 412 and 525 nm due to the formation of Au and Ag nanoparticles, respectively, within the gum network. XRD studies indicated that the nanoparticles were crystalline in nature with face centered cubic geometry. The noble metal nanoparticles prepared in the present study appears to be homogeneous with the particle size ranging between 2 and 10 nm, as evidenced by TEM analysis. The Ag and Au nanoparticles formed were in the average size range of 5.5±2.5 nm and 7.8±2.3 nm; while Pt nanoparticles were in the size range of 2.4±0.7 nm, which were considerably smaller than Ag and Au nanoparticles. The present approach exemplifies a totally green synthesis using the plant derived natural product (gum kondagogu) for the production of noble metal nanoparticles and the process can also be extended to the synthesis of other metal oxide nanoparticles.     

Silver Coated Platinum Core–Shell Nanostructures on Etched Si Nanowires: Atomic Layer Deposition (ALD) Processing and Application in SERS

A new method to prepare plasmonically active noble metal nanostructures on large surface area silicon nanowires (SiNWs) mediated by atomic layer deposition (ALD) technology has successfully been demonstrated for applications of surface‐enhanced Raman spectroscopy (SERS)‐based sensing. As host material for the plasmonically active nanostructures we use dense single‐crystalline SiNWs with diameters of less than 100 nm as obtained by a wet chemical etching method based on silver nitrate and hydrofluoric acid solutions. The SERS active metal nanoparticles/islands are made from silver (Ag) shells as deposited by autometallography on the core nanoislands made from platinum (Pt) that can easily be deposited by ALD in the form of nanoislands covering the SiNW surfaces in a controlled way. The density of the plasmonically inactive Pt islands as well as the thickness of noble metal Ag shell are two key factors determining the magnitude of the SERS signal enhancement and sensitivity of detection. The optimized Ag coated Pt islands on SiNWs exhibit great potential for ultrasensitive molecular sensing in terms of high SERS signal enhancement ability, good stability and reproducibility. The plasmonic activity of the core‐shell Pt//Ag system that will be experimentally realized in this paper as an example was demonstrated in numerical finite element simulations as well as experimentally in Raman measurements of SERS activity of a highly diluted model dye molecule. The morphology and structure of the core‐shell Pt//Ag nanoparticles on SiNW surfaces were investigated by scanning‐ and transmission electron microscopy. Optimized core–shell nanoparticle geometries for maximum Raman signal enhancement is discussed essentially based on the finite element modeling.     

The Polarization Effect in Surface‐Plasmon‐Induced Photocatalysis on Au/TiO2 Nanoparticles

Controlling the interaction of polarization light with an asymmetric nanostructure such as a metal/semiconductor heterostructure provides opportunities for tuning surface plasmon excitation and near‐field spatial distribution. However, light polarization effects on interfacial charge transport and the photocatalysis of plasmonic metal/semiconductor photocatalysts are unclear. Herein, we reveal the polarization dependence of plasmonic charge separation and spatial distribution in Au/TiO₂ nanoparticles under 45° incident light illumination at the single‐particle level using a combination of photon‐irradiated Kelvin probe force microscopy (KPFM) and electromagnetic field simulation. We quantitatively uncover the relationship between the local charge density and polarization angle by investigating the polarization‐dependent surface photovoltage (SPV). The plasmon‐induced photocatalytic activity is enhanced when the polarization direction is perpendicular to the Au/TiO₂ interface.     

Highly Efficient Visible Light Plasmonic Photocatalyst Ag@Ag(Br,I)

The new plasmonic photocatalyst Ag@Ag(Br,I) was synthesized by the ion‐exchange process between the silver bromide and potassium iodide, then by reducing some Ag⁺ ions in the surface region of Ag(Br,I) particles to Ag⁰ species. Ag nanoparticles are formed from Ag(Br,I) by the light‐induced chemical reduction reaction. The Ag@Ag(Br,I) particles have irregular shapes with their sizes varying from 83 nm to 1 μm. The as‐grown plasmonic photocatalyst shows strong absorption in the visible light region because of the plasmon resonance of Ag nanoparticles. The ability of this compound to reduce Cr^VI under visible light was compared with those of other reference photocatalyst. The plasmonic photocatalyst is shown to be highly efficient under visible light. The stability of the photocatalyst was examined by X‐ray diffraction and X‐ray photoelectron spectroscopy. The XRD pattern and XPS spectra prove the stability of the plasmonic photocatalyst Ag@Ag(Br,I).     

Nanostructured materials with localized surface plasmon resonance for photocatalysis

Localized surface plasmon resonance（LSPR） enhanced photocatalysis has fascinated much interest and considerable efforts have been devoted toward the development of plasmonic photocatalysts.In the past decades,noble metal nanoparticles（Au and Ag） with LSPR feature have found wide applications in solar energy conversion.Numerous metal-based photocatalysts have been proposed including metal/semiconductor heterostructures and plasmonic bimetallic or multimetallic nanostructures.However,high cost and...     

Plasmonic photoelectrochemical reactions on noble metal electrodes of nanostructures

Plasmonic noble metal nanostructures have been targeted due to their strong surface plasmon resonance at photoelectrochemical interfaces. Recently, it has been concluded that, the plasmonic noble metal nanostructures on photoexcitation permit the transfer of effective hot carriers (hot electron/hole pair) to nearby adsorbed molecules where, the transformed hot carriers can efficiently decrease the activation barrier of a reaction. In this review, our recent achievements in the plasmon-mediated chemical reactions of organic molecules such as para-aminothiophenol, substituted para-aminothiophenol and para-nitrothiophenol at nanostructures modified noble metal electrodes using surface enhanced Raman spectroscopy, electrochemical methods, and theoretical calculations will be discussed.     

Recent developments and applications of hybrid surface plasmon resonance interfaces in optical sensing

Nanostructured noble metals exhibit an intense optical near field due to surface plasmon resonance, therefore promising widespread applications and being of interest to a broad spectrum of scientists, ranging from physicists, chemists, and materials scientists to biologists. A wealth of research is available discussing the synthesis, characterization, and application of noble metal nanoparticles in optical sensing. However, with respect to the sensitivity of the frequency and width of these surface plasmon resonance modes to the particle’s shape, size, and environment, in nearly every case, success strongly depends on the availability of highly stable, adhesive, and sensitive nanoparticles. This undoubtedly presents a challenging task to nanofabrication. The past decade has witnessed fascinating advances in this field, in particular, the construction of oxide-based hybrid plasmonic interfaces to overcome the problem addressed above by (1) coating the metallic nanostructures with thin overlayers to form sandwiched structures or (2) embedding metallic nanostructures in a dielectric matrix to obtain metal/dielectric matrix nanocomposite films. In this critical review, we focus on recent work related to this field, beginning with a presentation of hybrid films with enhanced structural and optical stability, readily and selectively designed using chemical and physical techniques. We then illustrate their interesting optical properties and demonstrate exciting evidence for the postulated application in surface plasmon sensing fields. Finally, we survey the work remaining to be done for that potential to be realized.     

Prolonged Hot Electron Dynamics in Plasmonic‐Metal/Semiconductor Heterostructures with Implications for Solar Photocatalysis

Ideal solar‐to‐fuel photocatalysts must effectively harvest sunlight to generate significant quantities of long‐lived charge carriers necessary for chemical reactions. Here we demonstrate the merits of augmenting traditional photoelectrochemical cells with plasmonic nanoparticles to satisfy these daunting photocatalytic requirements. Electrochemical techniques were employed to elucidate the mechanics of plasmon‐mediated electron transfer within Au/TiO₂ heterostructures under visible‐light (λ>515 nm) irradiation in solution. Significantly, we discovered that these transferred electrons displayed excited‐state lifetimes two orders of magnitude longer than those of electrons photogenerated directly within TiO₂ via UV excitation. These long‐lived electrons further enable visible‐light‐driven H₂ evolution from water, heralding a new photocatalytic paradigm for solar energy conversion.     

Nanoplasmonic zirconium nitride photocatalyst for direct overall water splitting

The ability of plasmonic nanostructures to efficiently harvest light energy and generate energetic hot carriers makes them promising materials for utilization in photocatalytic water spitting.Apart from the traditional Au and Ag based plasmonic photocatalysts,more recently the noble-metal-free alternative plasmonic materials have attracted ever-increasing interest.Here we report the first use of plasmonic zirconium nitride（ZrN） nanoparticles as a promising photocatalyst for water splitting.Highl...     

Supported noble metal nanoparticles as photo/sono-catalysts for synthesis of chemicals and degradation of pollutants

This review summarizes the utilization of supported noble metal nanoparticles (such as Au/TiO2, Au/ZrO2, Ag/AgCl) as efficient photo/sono-catalysts for the selective synthesis of chemicals and degradation of environmental pollutants. Supported noble metal nanoparticles could efficiently catalyze the conversion of solar energy into chemical energy. Under UV/visible light irradiation, important chemical transformations such as the oxidation of alcohols to carbonyl compounds, the oxidation of thiol to disulfid...     

Activation of Oxygen on Gold and Silver Nanoparticles Assisted by Surface Plasmon Resonances

Surface plasmon resonances (SPRs) have been found to promote chemical reactions. In most oxidative chemical reactions oxygen molecules participate and understanding of the activation mechanism of oxygen molecules is highly important. For this purpose, we applied surface‐enhanced Raman spectroscopy (SERS) to find out the mechanism of SPR‐assisted activation of oxygen, by using p‐aminothiophenol (PATP), which undergoes a SPR‐assisted selective oxidation, as a probe molecule. In this way, SPR has the dual function of activating the chemical reaction and enhancing the Raman signal of surface species. Both experiments and DFT calculations reveal that oxygen molecules were activated by accepting an electron from a metal nanoparticle under the excitation of SPR to form a strongly adsorbed oxygen molecule anion. The anion was then transformed to Au or Ag oxides or hydroxides on the surface to oxidize the surface species, which was also supported by the heating effect of the SPR. This work points to a promising new era of SPR‐assisted catalytic reactions.