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1.
Acc Chem Res ; 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-32031367

RESUMO

The rational design of highly efficient catalysts relies on understanding their structure-activity relationships and reaction mechanisms at a molecular level. Such an understanding can be obtained by in situ monitoring of dynamic reaction processes using surface-sensitive techniques. Surface-enhanced Raman spectroscopy (SERS) can provide rich structural information with ultrahigh surface sensitivity, even down to the single-molecule level, which makes it a promising tool for the in situ study of catalysis. However, only a few metals (like Au, Ag, and Cu) with particular nanostructures can generate strong SERS effects. Thus, it is almost impossible to employ SERS to study transition metals (like Pt, Pd, Ru, etc.) and other nonmetal materials that are usually used in catalysis (material limitation). Furthermore, SERS is also unable to study model single crystals with atomically flat surface structures or practical nanocatalysts (morphology limitation). These limitations have significantly hindered the applications of SERS in catalysis over the past four decades since its discovery, preventing SERS from becoming a widely used technique in catalysis. In this Account, we summarize the extensive efforts done by our group since the 1980s, particularly in the past decade, to overcome the material and morphology limitations in SERS. Particular attention has been paid to the work using core-shell nanostructures as SERS substrates, because they provide high Raman enhancement and are highly versatile for application on different catalytic materials. Different SERS methodologies for catalysis developed by our group, including the "borrowing" strategy, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), and SHINERS-satellite strategy, are discussed in this account, with an emphasis on their principles and applications. These methodologies have successfully overcome the long-standing limitations of traditional SERS, enabling in situ tracking of catalysis at model single-crystal surfaces and practical nanocatalysts that can hardly be studied by SERS. Using these methodologies, we systematically studied a series of fundamentally important reactions, such as oxygen reduction reaction, hydrogen evolution reaction, electrooxidation, CO oxidation, and selective hydrogenation. As such, direct spectroscopic evidence of key intermediates that can hardly be detected by other traditional techniques was obtained. Combined with density functional theory and other in situ techniques, the reaction mechanisms and structure-activity relationships of these catalytic reactions were revealed at a molecular level. Furthermore, the future of SERS in catalysis has also been proposed in this work, which we believe should be focused on the in situ dynamic studies at the single-molecule, or even single-atom, level using techniques with ultrahigh sensitivity or spatial resolution, for example, single-molecule SERS or tip-enhanced Raman spectroscopy. In summary, core-shell nanostructure-enhanced Raman spectroscopies are shown to greatly boost the application of SERS in catalysis, from model systems like single-crystal surfaces to practical nanocatalysts, liquid-solid interfaces to gas-solid interfaces, and electrocatalysis to heterogeneous catalysis to photocatalysis. Thus, we believe this Account would attract increasing attention to SERS in catalysis and opens new avenues for catalytic studies.

2.
Biosens Bioelectron ; 154: 112067, 2020 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-32056962

RESUMO

The creatinine concentration of human urine is closely related to human kidney health and its rapid, quantitative, and low-cost detection has always been demanded. Herein, a surface-enhanced Raman spectroscopic (SERS) method for rapid and cost-effective quantification of creatinine concentrations in human urine was developed. A Au nanoparticle solution (Au sol) was used as a SERS substrate and the influence of different agglomerating salts on its sensitivity toward detecting creatinine concentrations was studied and optimized, as well as the effect of both the salt and Au sol concentrations. The variation in creatinine spectra over time on different substrates was also examined, demonstrating reproducible quantitative analysis of creatinine concentrations in solution. By adjusting the pH, a simple liquid-liquid solvent extraction procedure, which extracted creatinine from human urine, was used to increase the SERS detection selectivity toward creatinine in complex matrices. The quantitative results were compared to those obtained with a clinically validated enzymatic "creatinine kit (CK)." The limit of detection (LOD) for the SERS technique was 1.45 mg L-1, compared with 3.4 mg L-1 for the CK method. Furthermore, cross-comparing the results from the two methods, the average difference was 5.84% and the whole SERS detection process could be completed within 2 min compared with 11 min for the CK, indicating the practicality of the quantitative SERS technique. This novel quantitative technique shows promises as a high-throughput platform for relevant clinical and forensic analysis.

3.
J Phys Chem Lett ; 2020 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-32079400

RESUMO

Plasmonic optical antennas (POAs), often constructed from gold or silver nanostructures, can enhance the radiation efficiency of emitters coupled to POAs and are applied in surface-enhanced Raman spectroscopy (SERS), and light-emitting devices. Over the past four decades, radiation enhancement factors (REFs) of POA-emitter systems were considered to be difficult to calculate directly, and have been predicted indirectly and approximately, assuming POAs are illuminated by electromagnetic planewaves without emitters. The validity of this approximation remains a significant open problem in the SERS theory. Here, we develop a method based on the rigorous optical reciprocity theorem, for accurately calculating the REFs of emitters in nanoparticle-substrate nanogaps for single-molecule SERS, and scanning probe-substrate nanogaps for tip-enhanced Raman spectroscopy. We show that the validity of the planewave approximation breaks down if high-order plasmonic modes are excited. The as-developed method paves the way towards designing high-REF POA-nanostructures for luminescence-related devices.

4.
Angew Chem Int Ed Engl ; 59(1): 154-160, 2020 Jan 02.
Artigo em Inglês | MEDLINE | ID: mdl-31697030

RESUMO

Conversion of carbon monoxide to high value-added ethylene with high selectivity by traditional syngas conversion process is challenging because of the limitation of Anderson-Schulz-Flory distribution. Herein we report a direct electrocatalytic process for highly selective ethylene production from CO reduction with water over Cu catalysts at room temperature and ambient pressure. An unprecedented 52.7 % Faradaic efficiency of ethylene formation is achieved through optimization of cathode structure to facilitate CO diffusion at the surface of the electrode and Cu catalysts to enhance the C-C bond coupling. The highly selective ethylene production is almost without other carbon-based byproducts (e.g. C1 -C4 hydrocarbons and CO2 ) and avoids the drawbacks of the traditional Fischer-Tropsch process that always delivers undesired products. This study provides a new and promising strategy for highly selective production of ethylene from the abundant industrial CO.

5.
J Am Chem Soc ; 142(2): 715-719, 2020 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-31887023

RESUMO

The study of the oxygen reduction reaction (ORR) at high-index Pt(hkl) single crystal surfaces has received considerable interest due to their well-ordered, typical atomic structures and superior catalytic activities. However, it is difficult to obtain direct spectral evidence of ORR intermediates during reaction processes, especially at high-index Pt(hkl) surfaces. Herein, in situ Raman spectroscopy has been employed to investigate ORR processes at high-index Pt(hkl) surfaces containing the [011̅] crystal zone-i.e., Pt(211) and Pt(311). Through control and isotope substitution experiments, in situ spectroscopic evidence of OH and OOH intermediates at Pt(211) and Pt(311) surfaces was successfully obtained. After detailed analysis based on the Raman spectra and theoretical simulation, it was deduced that the difference in adsorption of OOH at high-index surfaces has a significant effect on the ORR activity. This research illuminates and deepens the understanding of the ORR mechanism on high-index Pt(hkl) surfaces and provides theoretical guidance for the rational design of high activity ORR catalysts.

6.
Chem Commun (Camb) ; 56(2): 253-256, 2019 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-31803874

RESUMO

The interfacial electron transfer capability of Si/SiO2 wafer supported single layer graphene is optimized by thermal annealing in an inert gas environment, which facilitates its applications in both electrochemical and electronic devices.

7.
Anal Chem ; 2019 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-31682106

RESUMO

Surface enhanced Raman spectroscopy (SERS) is an ultrasensitive label-free analytical technique that can provide unique chemical and structural fingerprint information. However, gaining reliable quantitative analysis with SERS remains a huge challenge because of poor reproducibility and the instability of nanostructured SERS active surfaces. Herein, an effective strategy of coating Au nanoparticles (NPs) with ultrathin and uniform Prussian blue (PB) shell (Au@PB NPs) was developed for quantitative detection of dopamine (DA) concentrations in blood serum and crystal violet (CV) contaminants in lake water. The only intense PB Raman signal at 2155 cm-1 served as an ideal and interference-free internal standard (IS) for correcting fluctuations in the Raman intensities of analytes. Also, the stability of Au@PB NPs was investigated, exhibiting good functionality in strong acid solutions and thermal stability at 100 °C. This work demonstrates a convenient and fast quantitative SERS technique for detecting analyte concentrations in complex systems and has a great number of potential applications for use in analytical chemistry.

9.
Angew Chem Int Ed Engl ; 58(45): 16062-16066, 2019 Nov 04.
Artigo em Inglês | MEDLINE | ID: mdl-31513325

RESUMO

It is vital to understand the oxygen reduction reaction (ORR) mechanism at the molecular level for the rational design and synthesis of high activity fuel-cell catalysts. Surface enhanced Raman spectroscopy (SERS) is a powerful technique capable of detecting the bond vibrations of surface species in the low wavenumber range, however, using it to probe practical nanocatalysts remains extremely challenging. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to investigate ORR processes on the surface of bimetallic Pt3 Co nanocatalyst structures. Direct spectroscopic evidence of *OOH suggests that ORR undergoes an associative mechanism on Pt3 Co in both acidic and basic environments. Density functional theory (DFT) calculations show that the weak *O adsorption arise from electronic effect on the Pt3 Co surface accounts for enhanced ORR activity. This work shows SHINERS is a promising technique for the real-time observation of catalytic processes.

10.
Acc Chem Res ; 52(10): 2784-2792, 2019 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-31532621

RESUMO

Surface plasmons (SPs) originating from the collective oscillation of conduction electrons in nanostructured metals (Au, Ag, Cu, etc.) can redistribute not only the electromagnetic fields but also the excited carriers (electrons and holes) and heat energy in time and space. Therefore, SPs can engage in a variety of processes, such as molecular spectroscopy and chemical reaction. Recently, plenty of demonstrations have made plasmon-mediated chemical reactions (PMCRs) a very active research field and make it as a promising approach to facilitate light-driven chemical reactions under mild conditions. Concurrently, making use of the same SPs, surface-enhanced Raman spectroscopy (SERS) with a high surface sensitivity and energy resolution becomes a powerful and commonly used technique for the in situ study of PMCRs. Typically, various effects induced by SPs, including the enhanced electromagnetic field, local heating, excited electrons, and excited holes, can mediate chemical reactions. Herein, we use the para-aminothiophenol (PATP) transformation as an example to elaborate how SERS can be used to study the mechanism of PMCR system combined with theoretical calculations. First, we distinguish the chemical transformation of PATP to 4,4'-dimercaptoazobenzene (DMAB) from the chemical enhancement mechanism of SERS through a series of theoretical and in situ SERS studies. Then, we focus on disentangling the photothermal, hot electrons, and "hot holes" effects in the SPs-induced PATP-to-DMAB conversion. Through varying the key reaction parameters, such as the wavelength and intensity of the incident light, using various core-shell plasmonic nanostructures with different charge transfer properties, we extract the key factors that influence the efficiency and mechanism of this reaction. We confidently prove that the transformation of PATP can occur on account of the oxygen activation induced by the hot electrons or because of the action of hot holes in the absence of oxygen and confirm the critical effect of the interface between the plasmonic nanostructure and reactants. The products of these two process are different. Furthermore, we compare the correlation between PMCRs and SERS, discuss different scenario of PMCRs in situ studied by SERS, and provide some suggestions for the SERS investigation on the PMCRs. Finally, we comment on the mechanism studies on how to distinguish the multieffects of SPs and their influence on the PMCRs, as well as on how to power the chemical reaction and regulate the product selectivity in higher efficiencies.

11.
J Phys Chem A ; 123(42): 9199-9208, 2019 Oct 24.
Artigo em Inglês | MEDLINE | ID: mdl-31549839

RESUMO

Sulfadiazine, as a class of antibiotics, has been widely used in the world for decades; however, its surface-enhanced Raman spectra (SERS) on gold colloids are obviously different from ordinary Raman spectra in the solid powder and liquid solution. To explore the reasons for such significant differences, we used density functional theory calculations and normal-mode analysis to investigate the effects of the configuration, conformation, protonation, hydrogen-bonding interaction, and adsorption configurations of sulfadiazine on gold clusters to check these different effects on the vibrational assignments. Our calculated results can be summarized as two points. First, the Raman spectra strongly depend on the configuration, conformation, protonation, and hydrogen bonding of sulfadiazine. Second, the wagging vibration displays a significant vibrational frequency shift and a very strong SERS peak responsible for the observed SERS signal when sulfadiazine is adsorbed on gold clusters through the terminal amino group. This is different from another adsorption configuration through two oxygen atoms of the -SO2NH- group on gold clusters. Finally, we further investigate the potential energy surfaces along the wagging vibration and the binding interaction of -NH2 adsorbed on different sites of gold surfaces.

12.
ACS Nano ; 2019 Oct 08.
Artigo em Inglês | MEDLINE | ID: mdl-31478375

RESUMO

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

13.
J Am Chem Soc ; 141(35): 13977-13986, 2019 Sep 04.
Artigo em Inglês | MEDLINE | ID: mdl-31436416

RESUMO

The chemical reactivity and/or the diffusion of Ag atoms or ions during thermal processing can cause irreversible structural damage, hindering the application of Ag nanowires (NWs) in transparent conducting films and other applications that make use of the material's nanoscale properties. Here, we describe a simple and effective method for growing monolayer SnO2 on the surface of Ag nanowires under ambient conditions, which protects the Ag nanowires from chemical and structural damage. Our results show that Sn2+ and Ag atoms undergo a redox reaction in the presence of water. First-principle simulations suggest a reasonable mechanism for SnO2 formation, showing that the interfacial polarization of the silver by the SnO2 can significantly reduce the affinity of Ag to O2, thereby greatly reducing the oxidation of the silver. The corresponding values (for example, before coating: 17.2 Ω/sq at 86.4%, after coating: 19.0 Ω/sq at 86.6%) show that the deposition of monolayer SnO2 enables the preservation of high transparency and conductivity of Ag. In sharp contrast to the large-scale degradation of pure Ag-NW films including the significant reduction of its electrical conductivity when subjected to a series of harsh corrosion environments, monolayer SnO2 coated Ag-NW films survive structurally and retain their electrical conductivity. Consequently, the thermal, electrical, and chemical stability properties we report here, and the simplicity of the technology used to achieve them, are among the very best reported for transparent conductor materials to date.

14.
Angew Chem Int Ed Engl ; 58(41): 14534-14538, 2019 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-31373130

RESUMO

Adsorption plays a critical role in surface and interface processes. Fractional surface coverage and adsorption free energy are two essential parameters of molecular adsorption. However, although adsorption at the solid-gas interface has been well-studied, and some adsorption models were proposed more than a century ago, challenges remain for the experimental investigation of molecular adsorption at the solid-liquid interface. Herein, we report the statistical and quantitative single-molecule measurement of adsorption at the solid-liquid interface by using the single-molecule break junction technique. The fractional surface coverage was extracted from the analysis of junction formation probability so that the adsorption free energy could be calculated by referring to the Langmuir isotherm. In the case of three prototypical molecules with terminal methylthio, pyridyl, and amino groups, the adsorption free energies were found to be 32.5, 33.9, and 28.3 kJ mol-1 , respectively, which are consistent with DFT calculations.

15.
J Am Chem Soc ; 141(31): 12192-12196, 2019 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-31328527

RESUMO

Investigating the chemical nature of the adsorbed intermediate species on well-defined Cu single crystal substrates is crucial in understanding many electrocatalytic reactions. Herein, we systematically study the early stages of electrochemical oxidation of Cu(111) and polycrystalline Cu surfaces in different pH electrolytes using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). On Cu(111), for the first time, we identified surface OH species which convert to chemisorbed "O" before forming Cu2O in alkaline (0.01 M KOH) and neutral (0.1 M Na2SO4) electrolytes; while at the Cu(poly) surface, we only detected the presence of surface hydroxide. Whereas, in a strongly acidic solution (0.1 M H2SO4), sulfate replaces the hydroxyl/oxy species. This results improves the understanding of the reaction mechanisms of various electrocatalytic reactions.

16.
Angew Chem Int Ed Engl ; 58(35): 12133-12137, 2019 Aug 26.
Artigo em Inglês | MEDLINE | ID: mdl-31268611

RESUMO

The fundamental understanding of the subtle interactions between molecules and plasmons is of great significance for the development of plasmon-enhanced spectroscopy (PES) techniques with ultrahigh sensitivity. However, this information has been elusive due to the complex mechanisms and difficulty in reliably constructing and precisely controlling interactions in well-defined plasmonic systems. Herein, the interactions in plasmonic nanocavities of film-coupled metallic nanocubes (NCs) are investigated. Through engineering the spacer layer, molecule-plasmon interactions were precisely controlled and resolved within 2 nm. Efficient energy exchange interactions between the NCs and the surface within the 1-2 nm range are demonstrated. Additionally, optical dressed molecular excited states with a huge Lamb shift of ≈7 meV at the single-molecule (SM) level were observed. This work provides a basis for understanding the underlying molecule-plasmon interaction, paving the way for fully manipulating light-matter interactions at the nanoscale.

17.
Chem Sci ; 10(23): 5893-5897, 2019 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-31360393

RESUMO

Here we report photoelectric-effect-enhanced interfacial charge transfer reactions. The electrochemical corrosion rate of n-type gallium arsenide (n-GaAs) induced by the contact potential at platinum (Pt) and GaAs boundaries can be accelerated by the photoelectric effect of n-GaAs. When a GaAs wafer is illuminated with a xenon light source, the electrons in the valence band of GaAs will be excited to the conduction band and then move to the Pt boundaries due to the different work functions of the two materials. This results in an enhanced contact electric field as well as an enlarged Pt/GaAs contact potential. Consequently, in the presence of electrolyte solution, the polarizations of both the Pt/solution interface and the GaAs/solution interface at the Pt/GaAs/solution 3-phase boundary are enhanced. If the accumulated electrons on the Pt side are removed by electron acceptors in the solution, anodic corrosion of GaAs will be accelerated strictly along the Pt/GaAs/solution 3-phase boundary. This photo-enhanced electrochemical phenomenon can increase the corrosion rate of GaAs and accelerate the process of electrochemical nanoimprint lithography (ECNL) on GaAs. The method opens an innovative, highly efficient, low-cost nanoimprint technique performed directly on semiconductors, and it has prospective applications in the semiconductor industry.

18.
Sci Adv ; 5(6): eaaw3072, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-31245539

RESUMO

Oriented external electric fields (OEEFs) offer a unique chance to tune catalytic selectivity by orienting the alignment of the electric field along the axis of the activated bond for a specific chemical reaction; however, they remain a key experimental challenge. Here, we experimentally and theoretically investigated the OEEF-induced selective catalysis in a two-step cascade reaction of the Diels-Alder addition followed by an aromatization process. Characterized by the mechanically controllable break junction (MCBJ) technique in the nanogap and confirmed by nuclear magnetic resonance (NMR) in bottles, OEEFs are found to selectively catalyze the aromatization reaction by one order of magnitude owing to the alignment of the electric field on the reaction axis. Meanwhile, the Diels-Alder reaction remained unchanged since its reaction axis is orthogonal to the electric fields. This orientation-selective catalytic effect of OEEFs reveals that chemical reactions can be selectively manipulated through the elegant alignment between the electric fields and the reaction axis.

19.
Nat Commun ; 10(1): 2671, 2019 06 17.
Artigo em Inglês | MEDLINE | ID: mdl-31209216

RESUMO

Plasmon-mediated chemical reactions (PMCRs) constitute a vibrant research field, advancing such goals as using sunlight to convert abundant precursors such as CO2 and water to useful fuels and chemicals. A key question in this burgeoning field which has not, as yet, been fully resolved, relates to the precise mechanism through which the energy absorbed through plasmonic excitation, ultimately drives such reactions. Among the multiple processes proposed, two have risen to the forefront: plasmon-increased temperature and generation of energetic charge carriers. However, it is still a great challenge to confidently separate these two effects and quantify their relative contribution to chemical reactions. Here, we describe a strategy based on the construction of a plasmonic electrode coupled with photoelectrochemistry, to quantitatively disentangle increased temperature from energetic charge carriers effects. A clear separation of the two effects facilitates the rational design of plasmonic nanostructures for efficient photochemical applications and solar energy utilization.

20.
J Am Chem Soc ; 141(20): 8053-8057, 2019 May 22.
Artigo em Inglês | MEDLINE | ID: mdl-31070906

RESUMO

Surface plasmons (SPs) are able to promote chemical reactions through the participation of the energetic charge carriers produced following plasmons decay. Using p-aminothiophenol (PATP) as a probe molecule, we used surface-enhanced Raman spectroscopy to follow the progress of its transformation, in situ, to investigate systematically the role of hot electrons and holes. The energetic carrier mediated PATP oxidation was found to occur even in the absence of oxygen, and was greatly influenced by the interface region near the gold surface. The observed reaction, which occurred efficiently on Au@TiO2 nanostructures, did not happen on bare gold nanoparticles (NPs) or core-shell nanostructures when a silicon oxide layer blocked access to the gold. Moreover, the product of the PATP oxidation with oxygen on Au@TiO2 nanostructures differed from what was obtained without oxygen, suggesting that the mechanism through which "hot holes" mediated the oxidation reaction was different from that operating with oxygen activated by hot electrons.

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