Thermal Analysis and Raman Spectra of Different Phases of the Ionic Liquid Butyltrimethylammonium Bis(trifluoromethylsulfonyl)imide

The ionic liquid butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, [C4C1C1C1N][Tf2N], is a glass-forming liquid that exhibits partial crystallization depending on the cooling rate. Differential scanning calorimetry (DSC) indicates crystallization at T-c = 227 K, melting at T-m = 258 K, glass transition at T-g similar to 191 K, and also cold crystallization at T-cc similar to 219 K. Raman spectroscopy shows that the crystalline structure obtained by slow cooling is formed with [Tf2N](-) in cisoid conformation, whereas [Tf2N](-) in transoid conformation results from fast cooling. No preferred conformation of the butyl chain of the [C4C1C1C1N](+) cation is favored by slow or fast cooling of [C4C1C1C1N][Tf2N]. Low-frequency Raman spectroscopy shows that crystalline domains developing in the supercooled liquid result in a glacial state made of a mixture of crystallites and amorphous phase. However, these crystalline structures obtained by slow cooling or cold crystallization are not the same because anion-cation interactions promote local structures with distinct conformations of the [Tf2N](-) anion.

Temperature-dependent Raman study of thermal parameters in CdS quantum dots

We report on the strong temperature-dependent thermal expansion, alpha(D), in CdS quantum dots (QDs) embedded in a glass template. We have performed a systematic study by using the temperature-dependent first-order Raman spectra, in CdS bulk and in dot samples, in order to assess the size dependence of alpha(D), and where the role of the compressive strain provoked by the glass host matrix on the dot response is discussed. We report the Gruneisen mode parameters and the anharmonic coupling constants for small CdS dots with mean radius R similar to 2.0 nm. We found that gamma parameters change, with respect to the bulk CdS, in a range between 20 and 50%, while the anharmonicity contribution from two-phonon decay channel becomes the most important process to the temperature-shift properties.

Surface-enhanced Raman spectroscopy studies of organophosphorous model molecules and pesticides

This work reports the analytical application of surface-enhanced Raman spectroscopy (SERS) in the trace analysis of organophosphorous pesticides (trichlorfon and glyphosate) and model organophosphorous compounds (dimethyl methylphosphonate and o-ethyl methylphosphonothioate) bearing different functional groups. SERS measurements were carried out using Ag nanocubes with an edge square dimension of ca. 100 nm as substrates. Density functional theory (DFT) with the B3LYP functional was used for the optimization of ground state geometries and simulation of Raman spectra of the organophosphorous compounds and their silver complexes. Adsorption geometries and marker bands were identified for each of the investigated compound. Results indicate the usefulness of SERS methodology for the sensitive analyses of organophosphorous compounds through the use of vibrational spectroscopy.

High Viscosity of Imidazolium Ionic Liquids with the Hydrogen Sulfate Anion: A Raman Spectroscopy Study

Ionic liquids based on 1-alkyl-3-methylimidazolium cations and the hydrogen sulfate (or bisulfate) anion, HSO4-, are much more viscous than ionic liquids with alkyl sulfates, RSO4-. The structural origin of the high viscosity of HSO4- ionic liquids is unraveled from detailed comparison of the anion Raman bands in 1-ethyl-3-methylimidazolium hydrogen sulfate and 1-butyl-3-methylimidazolium hydrogen sulfate with available data for simple HSO(4)(-) salts in crystalline phase, molten phase, and aqueous solution. Two Raman bands at 1046 and 1010 cm(-1) have been assigned as symmetric stretching modes nu(s)(S = O) of HSO4-, the latter being characteristic of chains of hydrogen-bonded anions. The intensity of this component increases in the supercooled liquid phase. For comparison purposes, Raman spectra of 1-ethyl-3-methylimidazolium ethyl sulfate and 1-butyl-3-methylimidazolium methyl sulfate have been also obtained. There is no indication of difference in the strength of hydrogen bond interactions of imidazolium cations with HSO4- or RSO4- anions. Raman spectra at high pressures, up to 2.6 GPa, are also discussed. Raman spectroscopy provides evidence that hydrogen-bonded anions resulting in anion-anion interaction is the reason for the high viscosity of imidazolium ionic liquids with HSO4-. If the ionic liquid is exposed to moisture, these structures are disrupted upon absorption of water from the atmosphere.

Raman spectroscopy and DFT calculations of para-coumaric acid and its deprotonated species

An electronic and vibrational spectroscopic analysis of p-coumaric acid (HCou) and its deprotonated species was performed by UV-vis and Raman, respectively, and the results were supported by density functional theory (OFT) calculations. Electronic UV-vis spectral data of HCou solutions show that the deprotonation of the carboxyl group (Cou(-)) leads to a blue shift of the lowest energy electronic transition in comparison to the neutral species, whereas the subsequent deprotonation of the phenolic moiety (Cou(2-)) carries out to a more delocalized chromophore. The DFT geometric parameters calculations suggest that the variation in the electronic delocalization for the three organic species is due to different contribution of a quinoid structure that is significantly distorted in the case of Cou(2-). The Raman data of HCou and its sodium salts show that the main spectral features that allow to differentiate the three organic species are those involving the styrene nu(C=C)(sty) vibration at 1600cm(-1) region. Even though the Raman spectra of the sodium salts of Cou(-) and Cou(2-) anions show subtle differences, the appearing of a band at ca. 1598cm(-1) in the Na(2)Cou spectrum, assigned to a mode involving the carboxylate asymmetric stretching, nu(as)(COO), and the styrene stretching, nu(C=C)(sty), is quite characteristic, as confirmed by the theoretical Raman spectrum. Considering that p-coumaric acid is an archetypical phenolic compound with several biological activities that essentially depend upon the medium pH, Raman spectroscopy results reported in this work can provide a proper way to characterize such important phytochemical compound in different protonation states. In order to complement the characterization of the sodium salts, X-ray diffraction (XRD) and thermal analysis were performed. (C) 2011 Elsevier B.V. All rights reserved.

Graphene modification with gold nanoparticles using the gas aggregation technique

Gold nanoparticles (Au-NPs) were deposited on single layer graphene (SLG) and few layers graphene (FLG) by applying the gas aggregation technique, previously adapted to a 4-gun commercial magnetron sputtering system. The samples were supported on SiO2 (280 nm)/Si substrates, and the influence of the applied DC power and deposition times on the nanoparticle-graphene system was investigated by Confocal Raman Microscopy. Analysis of the G and 2D bands of the Raman spectra shows that the integrated intensity ratio (I-2D/I-G) was higher for SLG than for FLG. For the samples produced using a sputtering power of 30W, the intensity (peak height) of the G and 2D bands increased with the deposition time, whereas for those produced applying 60W the peak heights of the G and 2D bands decreased with the deposition time. This behaviour was ascribed to the formation of larger Au-NPs aggregates in the last case. A significant increase of the Full Width Half Maximum (FWHM) of the G band for SLG and FLG was also observed as a function of the DC power and deposition time. Surprisingly, the fine details of the Raman spectra revealed an unintentional doping of SLG and FLG accompanying the increase of size and aggregation of the Au-NPs. (C) 2011 Elsevier B.V. All rights reserved.

USE OF THE LINEAR LIGHT SENSOR ILX554 IN OPTICAL SPECTROSCOPY

USE OF THE LINEAR LIGHT SENSOR ILX554 IN OPTICAL SPECTROSCOPY. This technical note describes the construction of a low-cost optical detector. This device is composed by a high-sensitive linear light sensor (model ILX554) and a microcontroller. The performance or the detector was demonstrated by the detection of emission and Raman spectra of the several atomic systems and the results reproduce those found in the literature.

Spectroscopic Study on the Structural Differences of Thermally Induced Cross-Linking Segments in Emeraldine Salt and Base Forms of Polyaniline

This paper reports the spectroscopic study on the structural differences of thermally induced cross-linking segments in polyaniline in its emeraldine salt (PANI-ES) and base (PANI-EB) forms. Casting films of PANI-ES (ES-film) and PANI-EB (EB-film) were prepared and heated at 150 degrees C under atmospheric air for 30 min. Raman spectra excited at 632.8 nm of heated ES-film presented the characteristic bands of phenazine-like structures at 1638, 1392, and 575 cm(-1), whereas EB-film showed lower relative intensities for these bands. The lower content of phenazine-like segments in heated EB-film is related to residual polaronic segments from preparation procedures, as revealed by Raman. This statement was confirmed by a sequence of thermal and doping experiments in both films. Quantum-chemical calculations by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) showed that the phenazine-like structure presents the intense Raman band at 1350 cm(-1) due to heterocycle breathing mode, and the non-phenazine-like structure (substituted hydrophenazine-type) presents higher energy for HOMO-LUMO transition, indicating the lack of conjugation in the heterocycle compared with the phenazine-like structure. According to experimental and theoretical data reported here, it is proposed that only thermally treated PANI-ES presents phenazine-like rings, whereas PANI-EB presents heterocyclic non-aromatic structures.

Platinum nanoparticles incorporated in silsesquioxane for use in LbL films for the simultaneous detection of dopamine and ascorbic acid

We describe the preparation of platinum nanoparticles (PtNPs) using the 3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-) as a nanoreactor and stabilizer. The formation of PtNPs was monitored by UV-Vis spectroscopy by measuring the decrease in the intensity of the band at 375 nm, which is attributed to the electronic absorption of PtCl62- ions. TEM images of Pt-SiPy+Cl- nanohybrid indicated an average size of 3-40 nm for PtNPs. The Pt-SiPy+Cl- was used as a polycation in the preparation of layer-by-layer films (LbL) on a glass substrate coated with fluorine-doped tin oxide (FTO) alternating with the polyanion poly(vinyl sulfonic acid) (PVS). The films were electrochemically tested in sulfuric acid to confirm the deposition of Pt-SiPy+Cl- onto the LbL films, observing the adsorption and desorption of hydrogen (E-pa = 0.1 V) and by the redox process of formation for PtO with E-pa = 1.3 V and E-pc = 0.65 V. FTIR and Raman spectra confirmed the presence of the PVS and Pt-SiPy+Cl- in the LbL films. A linear increase in the absorbance in the UV-Vis spectra of the Pt-SiPy+Cl- at 258 nm (pi -> pi* transition of the pyridine groups) with a number of Pt-SiPy+Cl-/PVS or PVS/SiPy+Cl- bilayers (R = 0.992) was observed. These LbL films were tested for the determination of dopamine (DA) in the presence of ascorbic acid (AA) with a detection limit (DL) on the order of 2.6 x 10(-6) mol L-1 and a quantification limit (QL) of 8.6 x 10(-6) mol L-1. The films exhibited a good repeatability and reproducibility, providing a potential difference of 550 mV for the oxidation of DA with AA interferent.

Correlations between Conjugation Length, Macromolecular Dynamics, and Photophysics of Phenylene-Vinylene/Aliphatic Multiblock Copolymers

This work reports a detailed spectroscopy study of a series of multiblock conjugated nonconjugated copolymers built by p-phenylene vinylene type units (PV) and octamethylene spacers, namely, poly(1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-ethenylene) (LaPPS18). The relative proportions of the PV and aliphatic segments were estimated on the basis of solid-state NMR and Raman spectroscopy. The overall structure was characterized by wide angle X-ray diffraction; H-1 wide-line dipolar chemical shift correlation (DIPSHIFT), and centerband-only detection of exchange (CODEX) NMR data, that together with glass transition temperatures allowed us to identify the groups involved in the molecular dynamics. These different structural properties were used to explain the photoluminescence properties in terms of peak position and spectral profile

Aniline-1,4-benzoquinone as a model system for the characterization of products from aniline oligomerization in low acidic media

Increase in the pH medium of aniline polymerization is used for giving products of different morphologies, which are often wrongly attributed to PANI chains. Infrared and Raman spectroscopic data, supported by quantum chemical calculations, show that aniline-1,4-benzoquinone (AnBzq) is a model system for the characterization of the products of aniline oligomerization in low acidic media. The Raman spectra excited at different laser lines reveal the bichromophoric nature of AnBzq, whose absorption bands at 550 nm and 440 nm can be attributed to pi-pi* transitions of the delocalized benzoquinone and amino-phenyl moieties, respectively. (C) 2012 Elsevier B.V. All rights reserved.

Self-supported bacterial cellulose/boehmite organic-inorganic hybrid films

Self-supported organic-inorganic hybrid transparent films have been prepared from bacterial cellulose and boehmite. SEM results indicate that the BC membranes are covered by Boehmite and XRD patterns suggest structural changes on cellulose due to Boehmite addition. Thermal stability is accessed through TG curves and is dependent on Boehmite content. Transparency, as evaluated by UV-Vis absorption, increases with increasing content of boehmite suggesting application of these materials as transparent substrates for opto-electronic devices.

Structural Refinement and Photoluminescence Properties of MnWO4 Nanorods Obtained by Microwave-Hydrothermal Synthesis

Manganese tungstate (MnWO4) nanorods were prepared at room temperature by the co-precipitation method and synthesized after processing in a microwave-hydrothermal (MH) system at 140 degrees C for 6-96 min. These nanorods were structurally characterized by X-ray diffraction (XRD), Rietveld refinements and Fourier transform (FT)-Raman spectroscopy. The growth direction, shape and average size distribution of nanorods were observed by means of transmission electron microscopy (TEM) and high resolution TEM (HR-TEM). The optical properties of the nanorods were investigated by ultraviolet visible (UV-vis) absorption and photoluminescence (PL) measurements. XRD patterns, Rietveld refinement data and FT-Raman spectroscopy indicate that the MnWO4 precipitate is not a single phase structure while the nanorods synthesized by MH processing have a wolframite-type monoclinic structure without deleterious phases. FT-Raman spectra exhibited the presence of 17 Raman-active modes from 50 to 1,000 cm(-1). TEM and HR-TEM micrographs indicated that the nanorods are aggregated due to surface energy by Van der Waals forces and grow along the [100] direction. UV-vis absorption measurements confirmed non-linear values for the optical band gap (from 3.2 to 2.72 eV), which increased as the MH processing time increased. The structural characterizations indicated that the presence of defects in the MnWO4 precipitate promotes a significant contribution to maximum PL emission, while MnWO4 nanorods obtained by MH processing decrease the PL emission due to the reduction of defects in the lattice.

Structural refinement, growth process, photoluminescence and photocatalytic properties of (Ba1-xPr2x/3)WO4 crystals synthesized by the coprecipitation method

Barium praseodymium tungstate (Ba1-xPr2x/3)WO4 crystals with (x = 0, 0.01, and 0.02) were prepared by the coprecipitation method. These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier-transform Raman (FT-Raman) and Fourier-transform infrared (FT-IR) spectroscopies. The shape and size of these crystals were observed by field emission scanning electron microcopy (FE-SEM). Their optical properties were investigated by ultraviolet visible (UV-vis) absorption and photoluminescence (PL) measurements. Moreover, we have studied the photocatalytic (PC) activity of crystals for degradation of rhodamine B (RhB) dye. XRD patterns, Rietveld refinements data, FT-Raman and FT-IR spectroscopies indicate that all crystals exhibit a tetragonal structure without deleterious phases. FT-Raman spectra exhibited 13 Raman-active modes in a range from 50 to 1000 cm(-1), while FT-IR spectra have 8 infrared active modes in a range from 200 to 1050 cm(-1). FE-SEM images showed different shapes (bonbon-, spindle-, rice-and flake-like) as well as a reduction in the crystal size with an increase in Pr3+ ions. A possible growth process was proposed for these crystals. UV-vis absorption measurements revealed a decrease in optical band gap values with an increase of Pr3+ into the matrix. An intense green PL emission was noted for (Ba1-xPr2x/3)WO4 crystals (x = 0), while crystals with (x = 0.01 and 0.02) produced a reduction in the wide band PL emission and the narrow band PL emission which is related to f-f transitions from Pr3+ ions. High photocatalytic efficiency was verified for the bonbon-like BaWO4 crystals as a catalyst in the degradation of the RhB dye after 25 min under UV-light. Finally, we discuss possible mechanisms for PL and PC properties of these crystals.

SnO2 nanocrystals synthesized by microwave-assisted hydrothermal method: towards a relationship between structural and optical properties

The exploration of novel synthetic methodologies that control both size and shape of functional nanostructure opens new avenues for the functional application of nanomaterials. Here, we report a new and versatile approach to synthesize SnO2 nanocrystals (rutile-type structure) using microwave-assisted hydrothermal method. Broad peaks in the X-ray diffraction spectra indicate the nanosized nature of the samples which were indexed as a pure cassiterite tetragonal phase. Chemically and physically adsorbed water was estimated by TGA data and FT-Raman spectra to account for a new broad peak around 560 cm(-1) which is related to defective surface modes. In addition, the spherical-like morphology and low dispersed distribution size around 3-5 nm were investigated by HR-TEM and FE-SEM microscopies. Room temperature PL emission presents two broad bands at 438 and 764 nm, indicating the existence of different recombination centers. When the size of the nanospheres decreases, the relative intensity of 513 nm emission increases and the 393 nm one decreases. UV-Visible spectra show substantial changes in the optical absorbance of crystalline SnO2 nanoparticles while the existence of a small tail points out the presence of localized levels inside the forbidden band gap and supplies the necessary condition for the PL emission.