UV Raman Spectroscopy Research Articles

The BALDER Beamline at the MAX IV Laboratory

X-ray absorption spectroscopy (XAS) includes well-established methods to study the local structure around the absorbing element - extended X-ray absorption fine structure (EXAFS), and the effective oxidation number or to quantitatively determine the speciation of an element in a complex matrix - X-ray absorption near-edge structure (XANES). The increased brilliance and intensities available at the new generation of synchrotron light sources makes it possible to study, in-situ and in-operando, much more dilute systems with relevance for natural systems, as well as the micro-scale variability and dynamics of chemical reactions on the millisecond time-scale. The design of the BALDER beamline at the MAX IV Laboratory 3 GeV ring has focused on a high flux of photons in a wide energy range, 2.4-40 keV, where the K-edge is covered for the elements S to La, and the L3-edge for all elements heavier than Sb. The overall design of the beamline will allow large flexibility in energy range, beam size and data collection time. The other focus of the beamline design is the possibility to perform multi-technique analyses on samples. Development of sample environment requires focus on implementation of auxiliary methods in such a way that techniques like Fourier transform infrared (FTIR) spectroscopy, UV-Raman spectroscopy, X-ray diffraction and/or mass spectrometry can be performed simultaneously as the XAS study. It will be a flexible system where different instruments can be plugged in and out depending on the needs for the particular investigation. Many research areas will benefit from the properties of the wiggler based light source and the capabilities to perform in-situ and in-operando measurements, for example environmental and geochemical sciences, nuclear chemistry, catalysis, materials sciences, and cultural heritage.

Promoted catalytic performances of highly dispersed V-doped SBA-16 catalysts for oxidative dehydrogenation of ethane to ethylene

V-doped SBA-16 catalysts (V-SBA-16) with 3D nanocage mesopores have been successfully synthesized by a modified one-pot method under weak acid condition. The obtained materials were characterized by means of small angle XRD, N2 adsorption–desorption, TEM, UV–Vis and UV-Raman spectroscopy. These characterization results indicated that well-order mesoporous structures were maintained even at higher vanadium loadings and high concentration of VOx species were incorporated into the framework of SBA-16 support. The catalytic performances of V-SBA-16, V/SBA-16 and V/SiO2 catalysts were comparatively investigated for the oxidative dehydrogenation of ethane to ethylene. The highest selectivity to ethylene of 63.3% and ethylene yield of 25.6% were obtained over 1.0V-SBA-16 catalyst. The superior catalytic performance of V-SBA-16 catalysts could be attributed to the presence of isolated framework VOx species, the unique structure of SBA-16 support and weak acidity. Moreover, V/SiO2 catalyst exhibited relatively poor catalytic activity duo to the formation of V2O5 nanoparticles on the surface of SiO2 support and the low dispersion of VOx species. These results indicated that the catalytic performances of the studied catalysts were strongly dependent on the vanadium loading, the nature and neighboring environment of VOx species and the structure of support.

In situ loading of CuS nanoflowers on rutile TiO2 surface and their improved photocatalytic performance

CuS nanoflowers, fabricated by an element-direct-reaction route using copper and sulfur powder, were loaded on rutile TiO2 (CuS/TiO2) at low temperature. CuS/TiO2 composites were utilized as the photocatalysts for the degradation of Methylene Blue (MB) and 4-chlorophenol (4-CP). X-ray diffraction (XRD), UV Raman spectroscopy, transmission electron microscopy (TEM), XPS, and UV-visible diffuse reflectance spectra were used to characterize the crystalline phase, morphology, particle size, and the optical properties of CuS/TiO2 samples. It is found that CuS/TiO2 photocatalyst, which CuS are loaded on the surface of rutile TiO2, exhibited enhanced photocatalytic degradation of MB (or 4-CP) than TiO2 or CuS. This indicates that CuS can enhance effectively the photocatalytic activity of rutile TiO2 by forming heterojunction between CuS and rutile TiO2, which is confirmed by photoluminescence (PL) spectra and TEM. Moreover, CuS content has a significant influence on photocatalytic activity and 2wt% CuS/TiO2 showed the maximum photocatalytic activity for degradation of MB.

Structural changes in perylene from UV Raman spectroscopy up to 1 GPa

Vibrational properties and structural changes under pressure of a highly luminescent molecular organic crystal have been investigated by ultraviolet resonant Raman spectroscopy with a 244-nm excitation. Resonant Raman modes of α-perylene crystal up to 1GPa were followed under hydrostatic pressure in an anvil cell with a sapphire window transparent to ultraviolet light. Nonlinear evolution of intra-molecular modes is induced by pressure. Abrupt shifts of Raman wavenumbers suggest structural and planar modifications of the molecules in the crystal. We interpret these shifts as a first-order phase transition to a lower volume of unit cell. The luminescence of perylene crystal is gradually modified as a consequence of these structural changes. The present experimental setup allows investigating with Raman spectroscopy very luminescent molecules involved in chemical reactions and molecular organic crystals under relatively high pressure (up to 1GPa). Copyright © 2016 John Wiley & Sons, Ltd.

UV Raman spectroscopy of segregated carbon in silicon oxycarbides

Polymer-derived silicon oxycarbides exhibiting ≤1 and 10 vol.% of segregated carbon finely dispersed within a glassy SixOyCz matrix have been investigated by UV Raman spectroscopy using a laser excitation of 4.8 eV (λ = 256.7 nm). Carbon exists as amorphous sp2–sp3 bonded component in SiOC/C (≤1 vol.%) pyrolyzed at 1100°C in H2, including C–C single bonds, polymeric chains and small polycyclic aromatic hydrocarbons (PAHs). The formation of nanocrystalline carbon at T > 1400°C is seen in the Raman spectra of SiOC/C (≤1 vol.%) and SiOC/C (10 vol.%) by the appearance of the G band of graphite. Tempering at 1600°C increases the degree of order within the carbon phase. However, the slight narrowing of the G peak with processing temperature (by about 5%) indicates still not well-crystallized carbon: the Raman results can be best explained by turbostratic carbon (with a lateral size La of ≈2 nm) and do not support the model description in literature as a network of single layer graphene.

Cooper-Carbon Nanostructured Composite Coatings with Controlled Structure

Samples of the Cu-C composite coatings were produced by high-speed ion-plasma magnetron sputtering (HiPMS) with mosaic target (MT). Nanostructure, roughness, mechanical properties as well as electric properties of coatings depend on carbon-cooper ratio. So 10% - 15% cooper con centration (atomic) results in to dry friction coefficient decrease up to 0.1 and wear resistance increase up to 10-17 m3/N·m. The HiPMS method allowed us to effectively sputter very different materials like carbon and cooper at the same velocity with required proportion. At the same time, one could obtain a uniform mixture of ingredients on the atomic level. The coating corresponds to a mixture of nano-sized phases of carbon and cooper. Crystalline dispersion of samples depends on carbon concentration. The obtained products are characterized by XRD, UV (257 nm) and visible (514 nm) Raman spectroscopy. The morphologies are studied with TEM and AFM. Mechanical properties were investigated with friction coefficient and micro hardness tests. There was no chemical interaction during Cu-C precipitation on hot substrate (625 - 725 K). The mechanical properties can be explained by free electrons capture with carbon nanostructures and space charge formation.

In-line ATR-UV and Raman Spectroscopy for Monitoring API Dissolution Process During Liquid-Filled Soft-Gelatin Capsule Manufacturing

Complete dissolution of the active pharmaceutical ingredient (API) is critical in the manufacturing of liquid-filled soft-gelatin capsules (SGC). Attenuated total reflectance UV spectroscopy (ATR-UV) and Raman spectroscopy have been investigated for in-line monitoring of API dissolution during manufacturing of an SGC product. Calibration models have been developed with both techniques for in-line determination of API potency. Performance of both techniques was evaluated and compared. The ATR-UV methodology was found to be able to monitor the dissolution process and determine the endpoint, but was sensitive to temperature variations. The Raman technique was also capable of effectively monitoring the process and was more robust to the temperature variation and process perturbations by using an excipient peak for internal correction. Different data preprocessing methodologies were explored in an attempt to improve method performance.

Beyond double-resonant Raman scattering: Ultraviolet Raman spectroscopy on graphene, graphite, and carbon nanotubes

The study of Raman scattering processes in the ultraviolet excitation energy range is not common in carbon-based materials. This paper reports that the widely known double-resonant Raman scattering, which is dominant in the visible excitation energy in these materials, is suppressed in the UV range. This is an important development because one can now measure two-phonon densities of states that were not accessible before. The results demonstrate the high potential of UV Raman spectroscopy for research on graphitic nanocarbons.

Phase transitions associated with competing order parameters in compressively strainedSrTiO3thin films

(100)-epitaxial ${\mathrm{SrTiO}}_{3}$ thin films having biaxial compressive strains up to $\ensuremath{-}1.60%$ were grown on lattice mismatched substrates. Two phase transitions induced by the coupled instabilities (antiferrodistortive and ferroelectric) in ${\mathrm{SrTiO}}_{3}$ were revealed in a common set of samples investigated at temperatures from 8 to 600 K by $\mathrm{CuK}\ensuremath{\alpha}$ and synchrotron x-ray diffractions and by UV-Raman spectroscopy. It is shown that the in-plane compressive strain in ${\mathrm{SrTiO}}_{3}$ significantly increases the transition temperature between cubic and tetragonal-antiferrodistortive phases and furthermore precipitates a ferroelectric polar phase at lower temperatures. The strain-temperature phase diagram based on the measurements shows a cubic-to-antiferrodistortive transition temperature that is substantially higher than predicted within a phenomenological Landau-Ginsburg-Devonshire treatment. In contrast the transition to the ferroelectric polar phase is found to be close to the temperature predicted, with the consequence that there is no crossing of the two phase transitions for temperatures up to 600 K.

UV Raman spectroscopy for the characterization of strongly fluorescing beverages

UV Raman spectroscopy with excitation wavelengths below 270 nm is a very promising technique for obtaining fluorescence-free Raman spectra, since fluorescence usually can be found only at wavelengths longer than 300 nm. Moreover, using an excitation source in the UV range leads to the resonance enhancement of the Raman signals for many molecules. Here, we introduce UV Raman spectroscopy as a tool for investigating various colored food samples, which exhibit strong fluorescence using the VIS or NIR Raman excitation. In this context, brewed coffee, green and black tea, cacao and cola beverage and dark edible oil (pumpkin seed oil) have been investigated. Using an excitation source at 266 nm yielded well-resolved, fluorescence-free spectra of all investigated samples.

In Situ Resonant UV-Raman Spectroscopy of Polycyclic Aromatic Hydrocarbons

PAHs are hazardous and persistent pollutants, also found as byproducts of some petrolchemical reaction (e.g., MTH) in relation to the catalyst deactivation, i.e., to the formation of coke species. The analysis of such deactivation products is typically performed by means of chromatographic techniques, with some drawbacks: the extraction and separation of the molecules from the matrix (the catalyst) is always required, and the solubility of the larger ones is often very low also in nonpolar solvents, so that their analysis is not possible with a standard approach. Spectroscopies can represent an interesting alternative for the qualitative analysis of PAHs: in particular Raman spectroscopy has been demonstrated to be a powerful tool in the characterization of carbonaceous materials and PAHs, and the possibility to exploit the resonance effect (allowing the selective enhancement of vibrational features of the resonant species) can be a considerable advantage in the analysis of very diluted species. In the present work, PAHs have been characterized by means of UV Raman spectroscopy: the 244 nm excitation wavelength allowed to exploit the resonance effect and in the meantime to avoid interference due to the visible fluorescence typical of these molecules. A pool of representative PAHs have been analyzed in their pure form, in diluted solution, and dispersed on high surface area microporous supports: the collected data constitute a reference database to be compared with the deactivation products observed during petrolchemical reactions, offering the possibility to univocally identify them.

Lattice distortion mechanism study of TiO2 nanoparticles during photocatalysis degradation and reactivation

In this paper, the photocatalytic process of TiO2 (P25) is directly characterized by using a positron annihilation lifetime spectroscopy (PALS), high-resolution transmission electron microscopy (HRTEM), Photoluminescence spectroscopy (PL) and UV Raman spectroscopy (Raman). The experimental results reveal that: 1) From PALS measurements, because τ1 and τ2 values and their intensity (I1 and I2) assigned to the different size and amounts of defects, respectively, their variations indicate the formation of different types and amounts of defects during the absorption and degradation. 2) HRTEM observations show that the lattice images become partly blurring when the methylene blue is fully degradated, and clear again after exposed in the air for 30 days. According to the results, we propose a mechanism that the lattice distortion induces the defects as electron capture sites and provides energy for improving photocatalytic process. Meanwhile, the lattice distortion relaxation after exposing in the air for 30 days perfectly explains the gradual deactivation of TiO2, because the smaller vacancy defects grow and agglomerate through the several photocatalytic processes. The instrumental PL and Raman are also used to analyze the samples and approved the results of PALS and HRTEM.

On the deactivation of Mo/HZSM-5 in the methane dehydroaromatization reaction

The deactivation of Mo/HZSM-5 during the non-oxidative methane aromatization (MDA) reaction that yields benzene and hydrogen was investigated. Catalysts were recovered from the reactor after pre-activation and after increasing time on stream in methane. The physico-chemical properties of the spent catalysts were characterized in detail by Ar physisorption, 27Al MAS NMR and X-ray photoelectron spectroscopy. The nature of the carbon deposits was determined by UV Raman spectroscopy and TGA, and the size and location of the Mo-carbide particles by TEM and STEM-HAADF. The results show that the main cause for catalyst deactivation is the formation of a carbonaceous layer at the external zeolite surface. This layer is made up from polyaromatic hydrocarbons and decreases the accessibility of the Brønsted acid sites in the micropores. At the same time, the decreased interaction of the Mo-carbide particles with the external zeolite surface results in their sintering. The lower Mo-carbide dispersion decreases methane conversion rates. The decreased accessibility of the Brønsted acid sites shifts the selectivity from benzene to unsaturated intermediates formed on the Mo-carbide particles. Silylation of the external surface mainly results in lower rate of coke formation at the external surface, slowing down catalyst deactivation.

Industrial development of long chain paraffin (n-C100–C130) dehydrogenation catalysts and the deactivation characterization

Pt–Sn–K–Mg/Al2O3 catalysts for the dehydrogenation of long chain paraffins (n-C100–C130) were successfully developed and applied in the industry. The catalysts were tested on both the industrial side fixed bed reactor and the industrial PACOL dehydrogenation plant. The industrially deactivated Pt–Sn–K–Mg/Al2O3 catalysts were characterized by TG–DTA, elemental analysis, UV Raman spectroscopy, TPO, SEM, XRD, BET, MIP and CO-chemisorption. The industrial 72-day-run dada indicated that Pt–Sn–K–Mg/Al2O3 catalysts performed excellent dehydrogenation conversion of 11.6% and high mono-olefin selectivity of 94.1%. The daily output of alkylbenzene over Pt–Sn–K–Mg/Al2O3 catalysts was as high as 333.6ton/day. The deactivated catalysts were deposited by the amorphous and graphitized coke, of which the weight percentage was 3.78% and the H/C mole ratio was 1.19. The coke covered parts of the Pt and support surface, while the pore and crystalline structure of the catalysts were not destroyed.

Enhanced photocatalytic activity for the degradation of rhodamine B by TiO2 modified with Gd2O3 calcined at high temperature

TiO2 modified with Gd2O3 (Gd/TiO2) nanoparticles were prepared by an impregnation method using TiO2-500°C support (anantase phase) or TiO2-650°C support (mixed phases of anatase and rutile), respectively. The surface and bulk crystalline phases of Gd/TiO2 have been characterized by UV Raman spectroscopy and XRD. Besides, the morphology, particle size, and the optical properties of Gd/TiO2 have been studied by TEM, BET, and UV–visible diffuse reflectance spectra. It is interesting to note that the surface crystalline phase (surface anatase phase or surface mixed phases) of TiO2 which has been impregnated with Gd2O3 can be stabilized after calcination at 800°C. This work provides a promising method for controlling the surface phase, crystallinity, and particle size of TiO2 at high temperatures. Moreover, photocatalytic activity of Gd/TiO2 has been evaluated by degradation of rhodamine B and 4-chlorophenol in aqueous solution under UV irradiation. It is found that Gd/TiO2 exhibits good photocatalytic activity compared to pure TiO2. The higher photocatalytic activity of Gd/TiO2 is mainly attributed to an increase in the anatase crystallinity, an increase in photoinduced charge separation, which induced by surface phase junction, and to an increase in the extent of its optical absorption spectrum, as compared to TiO2.

Characterization of Miscanthus pyrolysis by DRIFTs, UV Raman spectroscopy and mass spectrometry

The aim of this work was to identify and quantify the chemical and structural modifications occurring during the pyrolysis of Miscanthus × giganteus. Miscanthus was pyrolyzed at slow heating rate (5K/min) in the temperature range 200–400°C. The biochars were then characterized by combining diffuse reflectance infrared Fourier transform spectrometry (DRIFTs) and Ultraviolet Raman spectrometry (UV Raman). In addition, the quantification of the functional groups remained on the biochars was conducted by temperature programmed desorption-mass spectrometer (TPD-MS) analysis.DRIFT analysis showed that the decrease in the hydroxyl and CH stretching bands and the evolution of the carboxyl vibration peak mainly occur between 300 and 350°C. The evolution of the main markers (H2O, CO, CO2, CH4, and H2) of the functional groups presents in the biochars as analyzed by TPD-MS is consistent with the spectroscopic analysis. It shows two peaks. The first one in the temperature range of 200–380°C was explained by the loss of the main O and H containing groups and to the main primary volatile compounds. The second step (above 380°C) can be interpreted by the rearrangement of the aromatic rings in the polycyclic structure.

ChemInform Abstract: UV Raman Spectroscopic Characterization of Catalysts and Catalytic Active Sites

AbstractReview: 131 refs.

Surface-oxide stress induced band-structure modulation in two-dimensional Si layers

We experimentally studied an impact of the surface oxide layer on quantum confinement effects (QCE) in Si quantum well (SQW) structures (surface-oxide/two-dimensional-Si/buried-oxide) with various surface oxide layer thickness TOX on silicon-on-insulator (SOI) substrate, by UV-Raman spectroscopy, photoluminescence (PL) method, and two-dimensional (2D) stress simulator. UV-Raman data show that tensile strain ε of SQW, stressed by a thermal expansion mismatch between surface oxide and Si layers, decreases with decreasing the TOX. According to the strain behavior in the SQW and strained-Si on strained-SOI, PL results show that bandgap EG of the SQW rapidly expands with decreasing TOX. As a result, we can estimate the EG of the fully relaxed SQW. However, QCE in SQW keep thermally stable, in spite of carrying out a high temperature N2 annealing process.

Combining Raman and infrared spectroscopy as a powerful tool for the structural elucidation of cyclodextrin-based polymeric hydrogels.

A detailed experimental and theoretical vibrational analysis of hydrogels of β-cyclodextrin nanosponges (β-CDNS), obtained by polymerization of β-cyclodextrin (β-CD) with the cross-linking agent ethylenediaminetetraacetic acid (EDTA), is reported here. Thorough structural characterization is achieved by exploiting the complementary selection rules of FTIR-ATR and Raman spectroscopies and by supporting the spectral assignments by DFT calculations of the spectral profiles. The combined analysis of the FTIR-ATR spectra of the polymers hydrated with H2O and D2O allowed us to isolate the HOH bending of water molecules not involved in symmetrical, tetrahedral environments. The analysis of the HOH bending mode was carried out as a function of temperature, showing the existence of a supercooled state of the water molecules. The highest level of cooperativity of the hydrogen bond scheme was reached at a value of the β-CD/EDTA molar ratio n = 6. Finally, the connectivity pattern of "uncoupled" water molecules bound to the nanosponge backbone was found to be weakened by increasing T. The temperature above which the population of non-tetracoordinated water molecules becomes predominant turned out to be independent of the parameter n.

Water and polymer dynamics in a model polysaccharide hydrogel: the role of hydrophobic/hydrophilic balance.

The molecular dynamics of water and a polymer matrix is here explored in a paradigmatic model of a polysaccharide hydrogel, by the combined use of UV Raman scattering and infrared measurements. The case example of cyclodextrin nanosponges (CDNS)/hydrogel is chosen since the simultaneous presence in the structure of the polymer matrix of both hydrophilic and hydrophobic sites mimics the complexity of polysaccharide hydrogels. In this way, the contributions provided by the balance between the hydrophilicity/hydrophobicity and the grade of entanglement of the polymer hydrogel to lead to the formation of the gel phase are separately accounted and evaluated. As main results, we found that the hydrophobic CH groups inserted on the aromatic ring of CDNS experience a more pronounced dynamic perturbation with respect to the carbonyl groups due to the collision between the solvent and vibrating atoms of the polymer. The overall results provide a detailed molecular picture of the swelling phenomena occurring when a chemically cross-linked polymer contacts with water or biological fluids and exploits the potentiality of UV Raman spectroscopy to retrieve dynamic information besides their structural counterpart obtained by the classical analysis of the basic features of vibrational spectra.