Your browser doesn't support javascript.
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 72
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Am Chem Soc ; 142(3): 1341-1347, 2020 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-31893500

RESUMO

Active oxygen species (AOS) play key roles in many important catalytic reactions relevant to clean energy and environment. However, it remains challenging to characterize the active sites for producing AOS and to image the surface properties of AOS, especially on multicomponent metallic catalyst surfaces. Herein, we utilize tip-enhanced Raman spectroscopy (TERS) to probe the local generation and diffusion of OH radicals on a Pd/Au(111) bimetallic catalyst surface. The reactive OH radicals can be catalytically generated from hydrogen peroxide (H2O2) at the metal surface, which then oxidizes the surface adsorbed thiolate, a reactant that is used as the TERS probe. By TERS imaging of the spatial distribution of unreacted thiolate molecules, we demonstrate that the Pd surface is active for generation of OH radicals and the Pd step edge shows much higher activity than the Pd terrace, whereas the Au surface is inactive. Furthermore, we find that the locally generated OH radicals at the Pd step edge could diffuse to both the Au and the Pd surface sites to induce oxidative reactions, with a diffusion length estimated to be about 5.4 nm. Our TERS imaging with few-nanometer spatial resolution not only unravels the active sites but also characterizes in real space the diffusion behavior of OH radicals. The results are highly valuable to understand AOS-triggered catalytic reactions. The strategy of using reactants with large Raman cross sections as TERS probes may broaden the application of TERS for studying catalysis with reactive small molecules.

2.
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.

3.
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.

4.
Anal Chem ; 91(17): 11092-11097, 2019 Sep 03.
Artigo em Inglês | MEDLINE | ID: mdl-31361476

RESUMO

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) appears as a promising in situ nanospectroscopic tool for characterization and understanding of the electrochemical interfacial processes at the nanometer scale and molecular level. However, the wide application of EC-TERS is hampered by its low sensitivity as a result of the optical path distortion due to the refractive index mismatch of the multilayer media (air, glass, and electrolyte). Here, we propose a new side-illumination EC-TERS setup by coupling a water immersion objective with a high numerical aperture to a scanning tunneling microscope scanning head customized with a large open space and a compact spectroelectrochemical cell. It not only effectively eliminates the optical distortion but also increases the sensitivity remarkably, which allows sensitive monitoring of the electrochemical redox processes of anthraquinone molecules. More importantly, EC-TERS is able to independently control the tip position and laser illumination position. By utilizing this feature, we reveal that the irreversible reduction reaction of anthraquinone observed in EC-TERS is induced by the synergistic effect of the negative potential and laser illumination rather than the localized surface plasmon. The highly improved sensitivity and the flexibility to control the tip and laser illumination position on the nanometer scale endows EC-TERS as an important tool for the fundamental understanding of the photo- or plasmon electrochemistry and the interfacial structure-activity relationship of important electrochemical systems.

5.
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.

6.
Nat Commun ; 10(1): 892, 2019 02 21.
Artigo em Inglês | MEDLINE | ID: mdl-30792388

RESUMO

Electrocatalytic reduction of CO2 to fuels and chemicals is one of the most attractive routes for CO2 utilization. Current catalysts suffer from low faradaic efficiency of a CO2-reduction product at high current density (or reaction rate). Here, we report that a sulfur-doped indium catalyst exhibits high faradaic efficiency of formate (>85%) in a broad range of current density (25-100 mA cm-2) for electrocatalytic CO2 reduction in aqueous media. The formation rate of formate reaches 1449 µmol h-1 cm-2 with 93% faradaic efficiency, the highest value reported to date. Our studies suggest that sulfur accelerates CO2 reduction by a unique mechanism. Sulfur enhances the activation of water, forming hydrogen species that can readily react with CO2 to produce formate. The promoting effect of chalcogen modifiers can be extended to other metal catalysts. This work offers a simple and useful strategy for designing both active and selective electrocatalysts for CO2 reduction.

7.
Adv Mater ; 31(16): e1807495, 2019 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-30811702

RESUMO

Sodium metal anodes are poor due to the reversibility of Na plating/stripping, which hinders their practical applications. A strategy to form a sodiophilic Au-Na alloy interphase on a Cu current collector, involving a sputtered Au thin layer, is shown to enable efficient Na plating/stripping for a certain period of time. Herein, electrochemical behaviors of Na plating on different substrates are explored, and it is revealed that the sodiophilic interphase can be achieved universally by in situ formation of M-Na (M = Au, Sn, and Sb) alloys during Na plating prior to Na bulk deposition in the initial cycle. Moreover, it is found that repetitive alloying-dealloying leads to falling-off of thin film sodiophilic materials and thus limits the lifespan of efficient Na cycling. Therefore, an approach is further developed by employing particles of sodiophilic materials combined with the control over the cutoff potential, which significantly improves the stability of Na plating/stripping process. Especially, the low-cost Cu@Sn-NPs and Cu@Sb-MPs composite current collectors allow Na plating and stripping to cycle for 2000 and 1700 times with the average efficiency of 99.9% at 2 mA cm-2 .

8.
Molecules ; 24(2)2019 Jan 10.
Artigo em Inglês | MEDLINE | ID: mdl-30634644

RESUMO

Five ferrocene alkymethylimidazolium cations 1a⁻1d and 2 with different alkyl spacer lengths were reinvestigated using voltammetry and density functional theory (DFT) calculations. The voltammetric responses of ligand 2 toward various anions are described in detail. An interesting and unprecedented finding from both experimental and theoretical studies is that coupled electron and intramolecular anion (F-) transfer may be present in these molecules. In addition, it was also observed that, in these studied molecules, the electrostatic attraction interaction toward F- would effectively vanish beyond 1 nm, which was previously reported only for cations.


Assuntos
Compostos Ferrosos/química , Imidazóis/química , Metalocenos/química , Ânions/química , Cristalografia por Raios X , Teoria da Densidade Funcional , Técnicas Eletroquímicas , Estrutura Molecular
9.
Angew Chem Int Ed Engl ; 58(10): 3092-3096, 2019 Mar 04.
Artigo em Inglês | MEDLINE | ID: mdl-30589160

RESUMO

Lithium metal anodes suffer from poor cycling stability and potential safety hazards. To alleviate these problems, Li thin-film anodes prepared on current collectors (CCs) and Li-free types of anodes that involve direct Li plating on CCs have received increasing attention. In this study, the atomic-scale design of Cu-CC surface lithiophilicity based on surface lattice matching of the bcc Li(110) and fcc Cu(100) faces as well as electrochemical achievement of Cu(100)-preferred surfaces for smooth Li deposition with a low nucleation barrier is reported. Additionally, a purposely designed solid-electrolyte interphase is created for Li anodes prepared on CCs. Not only is a smooth planar Li thin film prepared, but a uniform Li plating/stripping on the skeleton of 3D CCs is achieved as well by high utilization of the surface and cavities of the 3D CCs. This work demonstrates surface electrochemistry approaches to construct stable Li metal-electrolyte interphases towards practical applications of Li anodes prepared on CCs.

10.
J Am Chem Soc ; 140(42): 13680-13686, 2018 10 24.
Artigo em Inglês | MEDLINE | ID: mdl-30280886

RESUMO

Precise measurement of the temperature right at the surface of thermoplasmonic nanostructures is a grand challenge but extremely important for the photochemical reaction and photothermal therapy. We present here a method capable of measuring the surface temperature of plasmonic nanostructures with surface-enhanced Raman spectroscopy, which is not achievable by existing methods. We observe a sensitive shift of stretching vibration of a phenyl isocyanide molecule with temperature (0.232 cm-1/°C) as a result of the temperature-dependent molecular orientation change. We develop this phenomenon into a method capable of measuring the surface temperature of Au nanoparticles (NPs) during plasmonic excitation, which is validated by monitoring the laser-induced desorption process of the adsorbed CO on Au NP surface. We further extend the method into a more demanding single living cell thermometry that requires a high spatial resolution, which allows us to successfully monitor the extracellular temperature distribution of a single living cell experiencing cold resistance and the intracellular temperature change during the calcium ion transport process.

11.
Angew Chem Int Ed Engl ; 57(40): 13177-13181, 2018 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-30133087

RESUMO

Resolving atomic site-specific electronic properties and correlated substrate-molecule interactions is challenging in real space. Now, mapping of sub-10 nm sized Pt nanoislands on a Au(111) surface was achieved by tip-enhanced Raman spectroscopy, using the distinct Raman fingerprints of adsorbed 4-chlorophenyl isocyanide molecules. A spatial resolution better than 2.5 nm allows the electronic properties of the terrace, step edge, kink, and corner sites with varying coordination environments to be resolved in real space in one Pt nanoisland. Calculations suggest that low-coordinate atomic sites have a higher d-band electronic profile and thus stronger metal-molecule interactions, leading to the observed blue-shift of Raman frequency of the N≡C bond of adsorbed molecules. An experimental and theoretical study on Pt(111) and mono- and bi-atomic layer Pt nanoislands on a Au(111) surface reveals the bimetallic effect that weakens with the increasing number of deposited Pt adlayer.

12.
Nat Commun ; 9(1): 1339, 2018 04 09.
Artigo em Inglês | MEDLINE | ID: mdl-29632301

RESUMO

Dendrite growth of alkali metal anodes limited their lifetime for charge/discharge cycling. Here, we report near-perfect anodes of lithium, sodium, and potassium metals achieved by electrochemical polishing, which removes microscopic defects and creates ultra-smooth ultra-thin solid-electrolyte interphase layers at metal surfaces for providing a homogeneous environment. Precise characterizations by AFM force probing with corroborative in-depth XPS profile analysis reveal that the ultra-smooth ultra-thin solid-electrolyte interphase can be designed to have alternating inorganic-rich and organic-rich/mixed multi-layered structure, which offers mechanical property of coupled rigidity and elasticity. The polished metal anodes exhibit significantly enhanced cycling stability, specifically the lithium anodes can cycle for over 200 times at a real current density of 2 mA cm-2 with 100% depth of discharge. Our work illustrates that an ultra-smooth ultra-thin solid-electrolyte interphase may be robust enough to suppress dendrite growth and thus serve as an initial layer for further improved protection of alkali metal anodes.

13.
J Am Chem Soc ; 140(8): 2880-2889, 2018 02 28.
Artigo em Inglês | MEDLINE | ID: mdl-29409320

RESUMO

Facile interconversion between CO2 and formate/formic acid (FA) is of broad interest in energy storage and conversion and neutral carbon emission. Historically, electrochemical CO2 reduction reaction to formate on Pd surfaces was limited to a narrow potential range positive of -0.25 V (vs RHE). Herein, a boron-doped Pd catalyst (Pd-B/C), with a high CO tolerance to facilitate dehydrogenation of FA/formate to CO2, is initially explored for electrochemical CO2 reduction over the potential range of -0.2 V to -1.0 V (vs RHE), with reference to Pd/C. The experimental results demonstrate that the faradaic efficiency for formate (ηHCOO-) reaches ca. 70% over 2 h of electrolysis in CO2-saturated 0.1 M KHCO3 at -0.5 V (vs RHE) on Pd-B/C, that is ca. 12 times as high as that on homemade or commercial Pd/C, leading to a formate concentration of ca. 234 mM mg-1 Pd, or ca. 18 times as high as that on Pd/C, without optimization of the catalyst layer and the electrolyte. Furthermore, the competitive selectivity ηHCOO-/ηCO on Pd-B/C is always significantly higher than that on Pd/C despite a decreases of ηHCOO- and an increases of the CO faradaic efficiency (ηCO) at potentials negative of -0.5 V. The density functional theory (DFT) calculations on energetic aspects of CO2 reduction reaction on modeled Pd(111) surfaces with and without H-adsorbate reveal that the B-doping in the Pd subsurface favors the formation of the adsorbed HCOO*, an intermediate for the FA pathway, more than that of *COOH, an intermediate for the CO pathway. The present study confers Pd-B/C a unique dual functional catalyst for the HCOOH ↔ CO2 interconversion.

14.
Anal Chem ; 90(3): 2018-2022, 2018 02 06.
Artigo em Inglês | MEDLINE | ID: mdl-29275628

RESUMO

Raman scattering and fluorescence spectroscopy permeate analytic science and are featured in the plasmon-enhanced spectroscopy (PES) family. However, the modest enhancement of plasmon-enhanced fluorescence (PEF) significantly limits the sensitivity in surface analysis and material characterization. Herein, we report a Ag nanoantenna platform, which simultaneously fulfills very strong emission (an optimum average enhancement of 105-fold) and an ultrafast emission rate (∼280-fold) in PES. For applications in surface science, this platform has been examined with a diverse array of fluorophores. Meanwhile, we utilized a finite-element method (FEM) and time-dependent density functional theory (TD-DFT) to comprehensively investigate the mechanism of largely enhanced radiative decay. PES with a shell-isolated Ag nanoantenna will open a wealth of advanced scenarios for ultrasensitive surface analysis.

15.
Chem Commun (Camb) ; 54(1): 10-25, 2017 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-29139483

RESUMO

In this feature article, we discuss in detail developmental bottleneck issues in Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades. We divide SERS research into two different directions with different targets. Fundamental research is extending the limits of SERS to single-molecule, sub-nanometer resolution and femtosecond processes. In contrast, practical research is expanding the range of applications with the aim of providing versatile analytical tools for surface, materials, life, environmental, forensic and food sciences and also commercial instruments for use in daily life. In the second direction there have continually been many complex bottlenecks to be overcome. We attempt to enumerate the key issues in detail and also describe the achievements made to overcome the bottlenecks. In the last, but not least important part, we discuss the remaining bottlenecks and possible strategies for overcoming them to enable SERS to be an even more powerful and versatile technique.

16.
J Am Chem Soc ; 139(30): 10339-10346, 2017 08 02.
Artigo em Inglês | MEDLINE | ID: mdl-28700232

RESUMO

Insightful understanding of how interfacial structures and properties affect catalytic processes is one of the most challenging issues in heterogeneous catalysis. Here, the essential roles of Pt-Au and Pt-oxide-Au interfaces on the activation of H2 and the hydrogenation of para-nitrothiophenol (pNTP) were studied at molecular level by in situ surface-enhanced Raman spectroscopy (SERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Pt-Au and Pt-oxide-Au interfaces were fabricated through the synthesis of Pt-on-Au and Pt-on-SHINs nanocomposites. Direct spectroscopic evidence demonstrates that the atomic hydrogen species generated on the Pt nanocatalysts can spill over from Pt to Au via the Pt-Au and Pt-TiO2-Au interfaces, but would be blocked at the Pt-SiO2-Au interfaces, leading to the different reaction pathways and product selectivity on Pt-on-Au and Pt-on-SHINs nanocomposites. Such findings have also been verified by the density functional theory calculation. In addition, it is found that nanocatalysts assembled on pinhole-free shell-isolated nanoparticles (Pt-on-pinhole-free-SHINs) can override the influence of the Au core on the reaction and can be applied as promising platforms for the in situ study of heterogeneous catalysis. This work offers a concrete example of how SERS/SHINERS elucidate details about in situ reaction and helps to dig out the fundamental role of interfaces in catalysis.

17.
ACS Appl Mater Interfaces ; 9(15): 13564-13570, 2017 Apr 19.
Artigo em Inglês | MEDLINE | ID: mdl-28349691

RESUMO

Because of either thermal/chemical instability or high optical loss in noble metal nanostructures, searching for alternative plasmonic materials is becoming more and more urgent, considering the practical biosensing applications under various extreme conditions. In this work, titanium nitride (TiN), a low-loss metal-like material with both excellent thermal and excellent chemical stabilities, was proposed to composite with Ag hollow nanosphere (HNS) nanostructures as an effective surface-enhanced Raman scattering (SERS) substrate to realize both highly sensitive and highly stable molecular detection. Because of the multiple-mode local surface plasmon resonance around the spherical composite nanospheres and the "gap effect" derived from the ultrasmall nanogaps within the precisely controlled plasmonic arrays, an intensively enhanced local field was successfully induced on this SERS substrate. Combined with the unique charge transferring process between Ag and TiN, a synergistically enhanced SERS sensitivity involving both physical and chemical mechanisms was achieved. Especially, with the isolation of TiN, a time-durable Raman detection on these TiN-Ag HNS arrays was accomplished, showing great potential for practical applications.

18.
Chemphyschem ; 18(3): 281-286, 2017 Feb 02.
Artigo em Inglês | MEDLINE | ID: mdl-27921368

RESUMO

Density functional theory calculations were used to elucidate the mechanism of the addition reaction of acetaldehyde to positively charged hydrogenated silicene (H-silicene). We found that the positively charged H-silicene plane could be partially restructured to form a vacant Si site, which enabled an additional nucleophilic addition reaction. After attachment of the acetaldehyde molecule to the H-silicene plane, two competing pathways were found to be involved in the hydrogen-abstraction process: a random-reaction mechanism and a chain-reaction mechanism. The theoretical results provided detailed information about stable structures and thermodynamic parameters of the reaction pathways, such as equilibrium geometries, Gibbs free energies, and the evolution of the spin densities and atomic charges. Our results reveal that the existence of a positive charge can significantly activate the grafting of unsaturated species on hydrogenated silicene, even if no silicon dangling bond is created proactively. The simulated Raman spectra of the two products were analyzed to elucidate the features of the competing mechanisms.

19.
J Phys Chem A ; 120(42): 8273-8284, 2016 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-27689539

RESUMO

Strong hydrogen-bonding interaction and Raman spectra of hydrated proton have been investigated using hybrid density functional theory method B3LYP. The solvation model of density (SMD) approach is employed in the present calculation to simulate hydrated protons in aqueous solution. Focusing on the different hydrogen-bonded Eigen-water and Zundel-water interactions, we present a better assignment of Raman signals of hydrated proton on the basis of vibrational analysis in different environments. Our results showed that B3LYP calculations could give a good prediction for characteristic vibrational frequencies of Eigen and Zundel isomers in liquid phase. The O-H stretching vibrational frequencies from Eigen and Zundel units are very sensitive to hydrogen-bonding interaction with solvent water molecules. Moreover, the solvation effect and the external electric-field effect lead to the proton deviating from the central position of Zundel structure and finally resulting in a transition to Eigen one in aqueous solution. Furthermore, by combining theoretical prediction and Raman scattering theory, we calculate absolute Raman intensities of characteristic signals based on the polarizability tensor derivatives of hydrated proton clusters. This is very helpful to infer the microstructure of hydrated protons in aqueous solution by using Raman measurements.

20.
Sci Rep ; 6: 31981, 2016 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-27659311

RESUMO

Inter- and intra- molecular hydrogen bonding plays important role in determining molecular structure, physical and chemical properties, which may be easily ignored for molecules with a non-typical hydrogen bonding structure. We demonstrated in this paper that the hydrogen bonding is responsible for the different Raman spectra in solid and solution states of p-Nitrothiophenol (PNTP). The consistence of the theoretical calculation and experiment reveals that the intermolecular hydrogen bonding yields an octatomic ring structure (8) of PNTP in the solid state, confirmed by the characteristic S-H---O stretching vibration mode at 2550 cm-1; when it comes to the solution state, the breakage of hydrogen bond of S-H---O induced the S-H stretching vibration at 2590 cm-1. Our findings may provide a simple and fast method for identifying the intermolecular hydrogen bonding.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA