7 research outputs found

    Generation of Pronounced Resonance Profile of Charge-Transfer Contributions to Surface-Enhanced Raman Scattering

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    A chemically enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a series of metal-charge-transfer (CT) complex systems fabricated by a self-assembly method. The developed Ag/4-mercaptophenols (MPH)/<i>n</i>-TiO<sub>2</sub> system presented layer number-dependent SERS spectra. By using the electron density values of the Ag<sub>13</sub>/MPH and Ag<sub>13</sub>/MPH/TiO<sub>2</sub> system calculated using the density functional theory (DFT) and by using these values in combination with the results of our previous investigations on the mechanism of the Ag/MPH/TiO<sub>2</sub> system, the absorption threshold of the CT complexes was clearly defined. The degree of CT was selected to study the layer number-dependent SERS spectra. Based on the layer number-dependent SERS data, it has been inferred that the degree of CT represents a resonance phenomenon. In addition, the CT resonance occurs at higher energy in the Ag/MPH/<i>n</i>-TiO<sub>2</sub> system than in the monolayer TiO<sub>2</sub> system owing to the blue-shift of CT states with the continuous introduction of TiO<sub>2</sub>. Thus, we provide a good example of the use of a CT complex system to investigate the chemical mechanism of SERS

    Heterogeneous Photochemical Conversion of NO<sub>2</sub> to HONO on the Humic Acid Surface under Simulated Sunlight

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    The poor understanding of HONO sources in the daytime highlights the importance of the heterogeneous photochemical reaction of NO<sub>2</sub> with aerosol or soil surfaces. The conversion of NO<sub>2</sub> to HONO on humic acid (HA) under simulated sunlight was investigated using a flow tube reactor at ambient pressure. The uptake coefficient (γ) of NO<sub>2</sub> linearly increased with irradiation intensity and HA mass in the range of 0–2.0 μg/cm<sup>2</sup>, while it decreased with the NO<sub>2</sub> concentration. The HONO yield was found to be independent of irradiation intensity, HA mass, and NO<sub>2</sub> concentration. The temperature (278–308 K) had little influence on both γ and HONO yield. Additionally, γ increased continuously with relative humidity (RH, 7–70%), and a maximum HONO yield was observed at 40% RH. The heterogeneous photochemical reaction of NO<sub>2</sub> with HA was explained by the Langmuir–Hinshelwood mechanism

    pH-Dependent SERS by Semiconductor-Controlled Charge-Transfer Contribution

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    With the explosive development of analysis and detecting techniques based on SERS, the further understanding and exploit of the chemical mechanism becomes particularly important. We investigated the contribution of semiconductor to surface-enhanced Raman scattering (SERS) in metal and semiconductor heterostructure consisting of Ag/4-mercaptophenol (MPH)/TiO<sub>2</sub>. For this, we used the distinctive property, where the band edge position of an oxide semiconductor such as TiO<sub>2</sub> is sensitive to the pH value, to control the charge-transfer (CT) contribution. It was found that increasing the pH of the buffer solution negatively shifts the conduction band edge of TiO<sub>2</sub>, thereby increasing the conductive band electron density at an equilibrium state. Thus, the relative band intensities of Ag/MPH/TiO<sub>2</sub> increase in the SERS spectrum, which is attributed to the Herzberg–Teller contribution that occurs via CT. Moreover, because of the slower transport of cations from the pH buffer solution to the surface of TiO<sub>2</sub>, which results from the space-charge limitation, there is a decay time that is associated with the pH-response process

    Controllable Synthesis of SERS-Active Magnetic Metal–Organic Framework-Based Nanocatalysts and Their Application in Photoinduced Enhanced Catalytic Oxidation

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    Fabrication of multifunctional nanocatalysts with surface-enhanced Raman scattering (SERS) activity is of vital importance for monitoring catalytic courses in situ and studying the reaction mechanisms. Herein, SERS-active magnetic metal–organic framework (MOF)-based nanocatalysts were successfully prepared via a three-step method, including a solvothermal reaction, an Au seed-induced growth process, and a low-temperature cycling self-assembly technique. The as-synthesized magnetic MOF-based nanocatalysts not only exhibit outstanding peroxidase-like activity, but can also be applied as a SERS substrate. Owing to these features, they can be used for monitoring in situ catalytic oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H<sub>2</sub>O<sub>2</sub> via a SERS technique, and the concentration of H<sub>2</sub>O<sub>2</sub> was determined. Owing to the intrinsic character of the Fe-based MOF material (MIL-100­(Fe)), a novel photoinduced enhanced catalytic oxidation effect was demonstrated, in which the catalytic oxidation of TMB and <i>o</i>-phenylenediamine was accelerated. This study provides a versatile approach for the fabrication of functional MOF-based nanocomposites as a promising SERS substrate with a unique photoinduced enhanced peroxidase-like activity for potential applications in ultrasensitive monitoring, biomedical treatment, and environmental evaluation

    Multiphonon Resonant Raman Scattering and Photoinduced Charge-Transfer Effects at ZnO–Molecule Interfaces

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    Detailed understanding of the underlying mechanisms of surface-enhanced Raman scattering (SERS) remains challenging for different experimental conditions. In this study, a novel laser-driven photoinduced interfacial charge transfer (CT) was observed based on UV–visible–infrared excitation wavelengths (325, 488, 514, 633, and 785 nm) through surface modification of ZnO nanorods by 4-aminothiophenol (PATP). SERS spectra combined with well-characterized surface morphology and optical spectroscopy indicate that the chemical enhancement occurs at visible-infrared excitation but at ultraviolet excitation (325 nm) multiphonon resonant Raman Scattering (MRRS) results in additional strong enhancements of particular Raman transitions through Cu–ZnO–PATP model. The relationships between the excitation photon energies (3.82, 2.54, 2.41, 1.96, and 1.58 eV), and its Raman shift were discussed. We found the strong dependence of the Raman shifts with the exchanges of excitation photon energies. These results highlight the role of excitation energy in determining the interface enhanced Raman scattering for semiconductor-molecule models. This implies that copper sheet under the ZnO improve the interfacial CT in ZnO-molecule and act as an effective donor for inhibiting reversible CT, and there was a strong interaction, which might be regarded as CT resonance process, between PATP molecules and the ZnO surface. This work not only shows a possibility for further understanding the origin of the SERS mechanism from semiconductor substrates but also for exhibits a situ characterization technique for probing the photoinduced interfacial charge-transfer processes

    Raman Investigation of Nanosized TiO<sub>2</sub>: Effect of Crystallite Size and Quantum Confinement

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    The influence of the TiO<sub>2</sub> particle size on the enhanced Raman spectroscopy properties was systematically investigated on the nanometer-size scale. We report on the enhanced Raman spectrum of 4-mercaptobenzoic acid adsorbed on TiO<sub>2</sub> nanoparticles. The results presented in this study highlight the major findings that the intensities of both the molecular lines and the phonon modes of TiO<sub>2</sub> are strongly size-dependent. The TiO<sub>2</sub> crystallite size estimated using the Scherrer equation varied from 6.8 to 14.2 nm; as a function of crystal size, a large increase in intensity is observed, with a maximum near 10.9 nm and a subsequent decline at larger sizes. Moreover, we have investigated quantum confinement effects between TiO<sub>2</sub> and the adsorbed molecules and attribute this to a charge-transfer resonance, which is responsible for the Raman enhancement

    Interfacial Charge-Transfer Effects in Semiconductor–Molecule–Metal Structures: Influence of Contact Variation

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    The charge-transfer resonance of Raman measurements in nanosized semiconductor–molecule–metal interfaces as a function of the excitation energy with four models (Cu–ZnO–PATP–Ag, Cu–Ag–PATP–ZnO, Cu–ZnO–Ag–PATP, and Cu–Ag–ZnO–PATP assemblies) to describe this dependence provides a powerful tool to study the chemical mechanism of surface enhanced Raman scattering (SERS). We measured the SERS spectra of self-assembled p-aminothiophenol (PATP) molecule junctions at 488, 514, 633, and 785 nm excitation wavelengths. We followed changes at the molecule junctions during the conditioning and eventually effect of charge-transfer (CT) through molecule–ZnO interfaces. Our results demonstrate that the interaction between the semiconductor bands and molecular energy levels can lead to novel charge behavior. The typical ZnO-PATP interfacial electron–hole recombination causes an increase in the CT resonance enhancement of Raman scattering, which is mainly responsible for the drastic change in molecular polarizability. We also proposed a complementary interpretation of the mechanism responsible for the highly variable enhancement observed in SERS
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