Laurent Servant

Nanostructured SnO2–ZnO Heterojunction Photocatalysts Showing Enhanced Photocatalytic Activity for the Degradation of Organic Dyes

Nanoporous SnO(2)-ZnO heterojunction nanocatalyst was prepared by a straightforward two-step procedure involving, first, the synthesis of nanosized SnO(2) particles by homogeneous precipitation combined with a hydrothermal treatment and, second, the reaction of the as-prepared SnO(2) particles with zinc acetate followed by calcination at 500 °C. The resulting nanocatalysts were characterized by X-ray diffraction (XRD), FTIR, Raman, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption analyses, transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectroscopy. The SnO(2)-ZnO photocatalyst was made of a mesoporous network of aggregated wurtzite ZnO and cassiterite SnO(2) nanocrystallites, the size of which was estimated to be 27 and 4.5 nm, respectively, after calcination. According to UV-visible diffuse reflectance spectroscopy, the evident energy band gap value of the SnO(2)-ZnO photocatalyst was estimated to be 3.23 eV to be compared with those of pure SnO(2), that is, 3.7 eV, and ZnO, that is, 3.2 eV, analogues. The energy band diagram of the SnO(2)-ZnO heterostructure was directly determined by combining XPS and the energy band gap values. The valence band and conduction band offsets were calculated to be 0.70 ± 0.05 eV and 0.20 ± 0.05 eV, respectively, which revealed a type-II band alignment. Moreover, the heterostructure SnO(2)-ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than those of individual SnO(2) and ZnO nanomaterials. This behavior was rationalized in terms of better charge separation and the suppression of charge recombination in the SnO(2)-ZnO photocatalyst because of the energy difference between the conduction band edges of SnO(2) and ZnO as evidenced by the band alignment determination. Finally, this mesoporous SnO(2)-ZnO heterojunction nanocatalyst was stable and could be easily recycled several times opening new avenues for potential industrial applications.

Raman Spectroelectrochemistry of a Carbon Supercapacitor

Confocal Raman microspectrometry has been applied to the in situ study of an electrical double-layer capacitor involving activated carbon cloth electrodes and an organic electrolyte. The Raman spectra of the carbon electrodes have been recorded during the charge and discharge. The intensity variations of the G and D bands have been correlated with carbon electronic conductivity changes. At the same time, the salt concentration has been measured in the electrolyte diffusion layer at about 10 μm from the carbon electrodes and significant ionic depletions have been observed during the charges and discharges. The whole results indicate that the two double-layer interfaces do not work in a symmetrical way and a mechanism is proposed to explain the observed effects.

Raman enhancement of azobenzene monolayers on substrates prepared by Langmuir-Blodgett deposition and electron-beam lithography techniques.

Nanostructured metallic platforms for Raman enhancement were fabricated using Langmuir-Blodgett and electron beam (e-beam) lithography techniques. The gold platforms were inscribed on thin glass slides with the purpose of using them in a transmission geometry experimental setup under a confocal microscope. The plasmon frequency of the gold nanostructures was determined in the visible-near-infrared range for various pattern sizes prepared by Langmuir-Blodgett transfer and e-beam lithography. The surface Raman enhancement factors were determined for a monolayer of azobenzene molecules adsorbed on gold through thiol bonding and compared for both LB transfer and e-beam samples for nanostructures of comparable geometries.

In situ Raman imaging of interdiffusion in a microchannel

This letter presents an experimental setup to probe the interdiffusion of various miscible and nonreacting liquids. A Raman confocal microscope allows us to image the local concentrations of two coflowing liquids in a microchannel. These steady-state measurements provide precise quantitative information about the kinetics of the interdiffusion process. For all the pairs of fluids investigated, we found this process to follow a classical diffusive scaling law with a interdiffusion coefficient D that depends on both liquids.

Raman Spectroelectrochemistry of a Lithium/Polymer Electrolyte Symmetric Cell

A symmetric lithium cell Li/P(EO) 20 LiTFSI/Li has been studied at 80°C by confocal Raman microspectrometry while current densities of up to 0.5 mA cm -2 are passed through the cell. The Raman observation is performed on the edge of the cell along a line of points extending from one electrode to the other at a depth of about 20 μm within the electrolyte. Local salt concentration is measured in the electrolyte as a function of time, current density I, and electrolyte thickness L (90 and 160 μm). When the steady-state regime is reached, linear and symmetric salt concentration gradients are observed. They are proportional to I and L as expected from the theoretical predictions for a binary electrolyte containing a fully dissociated salt with negligible ionic associations. In addition, preliminary results have been obtained concerning the establishment and relaxation of the steady state. From these data, it is shown that salt diffusion coefficient and ionic transport numbers can be determined with a reasonable precision. Confocal Raman microspectrometry can therefore be considered as a new and powerful spectroelectrochemical method to study transport properties in polymer electrolytes.

In situ probing of interfacial processes in the electrodeposition of copper by confocal Raman microspectroscopy

Confocal Raman microspectroscopy is applied to the in situ probing of interfacial processes in pulsed-current copper electrodeposition. This technique provides time-resolved characterization of the vibrational spectra of sulphate ions whenever in solution or adsorbed on the growing electrode. It also confirms the formation of cuprous oxide in the reduction process as the solution pH is increased by proton reduction. Moreover, when the passage of current is terminated, this technique provides evidence for the recombination of copper ions with copper metal to produce cuprous oxide on the outermost branches of the deposit.

Raman and FTIR Spectroscopy Investigations of Carbon-Coated Li x FePO4 Materials

Raman and Fourier transform infrared (FTIR) spectroscopy investigations were performed on carbon-coated LiFePO 4 materials differing by the temperature of their thermal treatments (575 and 800°C) and by their electrochemical performance, with that obtained at a higher temperature showing larger reversible capacity and better capacity retention at high rates. Raman spectra gave information on the carbon located at the surface of the LiFePO 4 particles, which was shown for the two samples to be highly disordered with small in-plane correlation lengths (<3 nm). A UV Raman study has shown that these carbon coatings contain almost no sp 3 -type carbon hybridization. This study has also highlighted again that the sp 3 -type C/sp 2 -type C ratio cannot be determined straightforwardly from Raman spectra recorded with visible excitation (such as 632.8 nm), and thus that no direct correlation can be done between the Raman band intensity ratio I D /I G and the sp 3 -type C/sp 2 -type C ratio; a UV Raman study is necessary to get the true information on the sp 3 -type C contribution. The baseline and absolute intensity of the FTIR spectra were shown to be sensitive to changes in the electronic conductivity of the C-LiFePO 4 samples. Furthermore, good crystallinity was maintained for Li x FePO 4 materials upon cycling, showing good reversibility of the lithium deintercalation/intercalation reaction.

Mapping dynamic concentration profiles with micrometric resolution near an active microscopic surface by confocal resonance Raman microscopy. Application to diffusion near ultramicroelectrodes : first direct evidence for a conproportionation reaction

Abstract Confocal Raman microspectroscopy is a very efficient means for probing the molecular composition of micrometric-sized samples. Its coupling with Raman resonance spectroscopy allows the specific tracking of very dilute species by considerably enhancing its Raman bands. Thus, spatially resolved information on the chemical composition of diffusion layers, which build up spontaneously near an active surface placed in a solution, can be obtained with a micrometric resolution. In this work, the applicability of the method for imaging diffusional transport towards ultramicroelectrodes with a micrometric resolution is examined. The efficiency and versatility of confocal resonance Raman microspectroscopy have been tested by probing the composition of the two different diffusion layers which build up in the vicinity of an ultramicroelectrode during the reduction of tetracyanoquinodimethane (TCNQ) on its first or second electrochemical wave. Besides the establishment of the method, this work affords the first direct experimental evidence of the existence and role of conproportionation reactions, which take place on the second reduction wave of EE electrochemical systems. In both cases, the concentration profiles of the radical anion TCNQ − agree extremely well with the theoretical predictions.

Chemical Reaction Imaging within Microfluidic Devices Using Confocal Raman Spectroscopy: The Case of Water and Deuterium Oxide as a Model System

Microfluidic devices face presently a tremendous interest, especially for the development of labs-on-a-chip systems. One of the primary challenges for such applications is the ability to perform local chemical detection and analysis from various species. In this paper, we investigate the use of confocal Raman spectroscopy from both qualitative and quantitative sides, to obtain spatially resolved concentration maps of chemically reactive fluids flowing in different channels networks. As a model chemical reaction, we used the isotopic exchange reaction between D(2)O and H(2)O, which is diffusion-controlled and whose equilibrium states exhibit distinct Raman signatures depending on the composition. Two types of chip technologies were studied, which are typical of those used for chemical kinetics investigations. In the first one, reagent mixing occurs by molecular interdiffusion of the two streams (H(2)O and D(2)O) flowing side by side in the same channel; in the second one, reagents are hosted in droplets moving in winding channels that enhance the mixing. In the first series of experiments, we were able to extract Raman images of H(2)O, D(2)O, and HOD concentrations in the main channel together with an estimate of an interdiffusion coefficient, and in the second one, we evidenced the influence of channel wiggles on mixing efficiency.

Quantitative label-free RNA detection using surface-enhanced Raman spectroscopy

Surface-Enhanced Raman Spectroscopy (SERS) was performed to detect label-free RNA. We defined conditions which make it possible to probe the four bases of RNA, in single strands of polyadenosine (pA), polyuridine (pU), polycytosine (pC) and polyguanosine (pG). We therefore present below a quantitative analysis of mixtures of non-hybridized single strands, based on the deconvolution of the SERS mixture spectrum into the relative contributions of the SERS spectra of each constituent.