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Amorphous and graphitized nitrogen-doped (N-doped) carbon spheres are investigated as structurally well-defined model systems to gain a deeper understanding of the relationship between synthesis, structure, and their activity in the oxygen reduction reaction (ORR). N-doped carbon spheres were synthesized by hydrothermal treatment of a glucose solution yielding carbon spheres with sizes of 330 ± 50 nm, followed by nitrogen doping via heat treatment in ammonia atmosphere. The influence of a) varying the nitrogen doping temperature (550–1000 °C) and b) of a catalytic graphitization prior to nitrogen doping on the carbon sphere morphology, structure, elemental composition, N bonding configuration as well as porosity is investigated in detail. For the N-doped carbon spheres, the maximum nitrogen content was found at a doping temperature of 700 °C, with a decrease of the N content for higher temperatures. The overall nitrogen content of the graphitized N-doped carbon spheres is lower than that of the amorphous carbon spheres, however, also the microporosity decreases strongly with graphitization. Comparison with the electrocatalytic behavior in the ORR shows that in addition to the N-doping, the microporosity of the materials is critical for an efficient ORR.

Poly(methyl methacrylate) (PMMA) is a very versatile polymer which is used as a glass substitute or as an economical alternative to polycarbonate for many types of important applications, due to its particular physical properties. In this study we deal with the Raman spectroscopic characterization of the glass transition of PMMA, the value of the glass transition temperature being generally a decisive parameter for determining the application of polymers. The information obtained by two-dimensional correlation spectroscopy (2DCOS) analysis and perturbation-correlation moving-windows spectroscopy (PCMW2D) analysis of the temperature dependent depolarized Raman spectra enabled us to recognize that the glass transition of PMMA is ruled by intermolecular interactions which influence the vibrational modes of the molecular groups associated with ν(C[double bond, length as m-dash]O), δa(C-H) of α-CH3 and/or O-CH3, ν(C-O-C), ν(C-COO), and ν(C-C-O). This information was employed for the temperature dependent study of the Raman shift and of the full width at half maximum of the Raman peaks obtained through anisotropic and isotropic Raman spectra, of the depolarization ratio, of the Raman spectroscopic noncoincidence effect, and of the Raman peak intensities represented by Arrhenius-type plots, all results supporting the outcomes of this work. The comparison with results obtained by differential scanning calorimetry and with published results in molecular dynamics studies was also part of this work. As the main result, one can highlight the peak associated with the ν(C-O-C) stretching mode at around 812 cm-1 as the one which presents the better outcome for explaining the glass transition from the molecular point of view.

The actual chemical structure of polyfurfuryl alcohol (PFA) is still uncertain in spite of several studies on the topic, variations during the polymerization processes being one reason that must be addressed. The use of a limited set of analytical techniques is often insufficient to provide an exhaustive chemical characterization. Moreover, it is still not possible to exactly determine presence and amount of each specific functional group in the polymeric structure. We employed both Fourier Transform Infrared Spectroscopy (FTIR) and Resonant Raman spectroscopy (RR), corroborated by quantum mechanically aided analysis of the experimental spectra, to infer about the chemical structure of two samples of PFAs, synthetized in different ways and appearing macroscopically different, the first one being a liquid and viscous commercial sample, the second one being a self-prepared solid and rigid sample produced following a thermosetting procedure. The vibrational spectroscopic analysis confirms the presence of differences in their chemical structures. The viscous form of PFA is mainly composed by short polymeric chains, and is characterized by the presence of isolated furfuryl alcohol and furfural residues similar to 5-hydroxymethylfurfural; the thermosetted PFA is formed by more cross-linked structures, characterized by several ketones and alkene double bonds, as well as a significant presence of Diels-Alder structures. In summary, the present study evidences how the use of both FTIR and RR spectroscopy, the latter carried out at several laser excitation wavelengths, indicates an accurate way to spectroscopically investigate complex polymers enabling to satisfactorily infer about their peculiar chemical structure.

Here, we present a comprehensive study that encompasses changes within the crystal and magnetic structure in the brownmillerite-type phase Ca2Fe2O5 induced by the substitution of Fe3+ with Ga3+. 61 synthetic single-crystal samples of Ca2Fe2−xGaxO5 0.00 ≤ x ≤ 1.328 have been investigated by single-crystal X-ray diffraction at 25 °C. We find that pure Ca2Fe2O5 and samples up to x ~ 1.0 have space group Pnma, Z = 4, whereas samples with x > 1.0 show I2mb symmetry, Z = 4. The Raman spectroscopic measurements exhibit that the change from Pnma to I2mb space group symmetry is reflected by a significant shift of two Raman modes below 150 cm−1. These Raman modes are obviously linked to changes in the Ca–O bond lengths at the phase transition. 57Fe Mossbauer spectroscopy was used to characterize the cation distribution and magnetic structure as a function of composition and temperature. Thereby, the strong preference of Ga3+ for the tetrahedral site is verified, as an independent method besides XRD. At room-temperature, Ca2Fe2−xGaxO5 solid solution compounds with 0 ≤ x ≤ 1.0 are antiferromagnetic ordered, as revealed by the appearance of magnetically split sextets in the Mossbauer spectra; samples with higher Ga3+ contents are paramagnetic. Over and above, the substitution of Fe3+ by Ga3+ results in the appearance of sharp, additional magnetic hyperfine split sextets, which can be attributed to cluster configurations within the individual tetrahedral chains. The temperature-dependent (20–720 K) Mossbauer study reveals a transition from the magnetically ordered to the paramagnetic state at a temperature of about 710 K for the Ca2Fe2O5 end-member.

This work, intended for advanced undergraduate students, explains the basic concepts and an actual scientific use of Terahertz Raman spectroscopy, displaying some of the benefits of this versatile ...

Simple substrates for surface enhanced Raman spectroscopy (SERS), producible in a cost-efficient way, are of growing interest both for scientific and for environmental applications. In this study, we demonstrate the use of three types of bio-based tannin-furanic rigid foams as precursor materials for SERS substrates. Coated with a silver layer, these substrates allowed the detection of several well-known analytes in the mM regime by Raman spectroscopy. Specific optimization of the standard tannin-furanic foam morphology by tuning the chemical synthesis led to a smaller and more homogeneously distributed pore structure, supplying more active hot spot areas. Thus, we obtained a significant increase and a lower relative standard deviation (RSD) of the SERS signal recorded over the mapped SERS substrate area, for several analytes, in particular for Malachite Green dye. This work represents a feasibility study opening several potential applications of this biopolymers in fields such as the detection of water pollutants, virtually combining filtration and SERS capabilities driven by a controlled porosity.

Pyroxene-type germanate compounds with the composition CaCuGe2O6–CaZnGe2O6have been synthesizedviaa solid-state ceramic sintering route. Phase-pure polycrystalline and small single-crystal material was obtained all over the series, representing a complete solid-solution series. Differential thermal analysis, single-crystal X-ray diffraction and Raman spectroscopy were used to characterize phase stability, phase changes and structural alterations induced by the substitution of Cu2+with Zn2+. Whereas pure CaCuGe2O6exhibitsP21/csymmetry with a strong distortion of theM1 octahedra and two different Ge sites, one of them with an unusual fivefold coordination, the replacement of Cu2+by Zn2+induces a chemically driven phase change to theC2/csymmetry. The phase change takes place around Zn2+contents of 0.12 formula units and is associated with large changes in the unit-cell parameters. Here, the increase ofcby as much as 3.2% is remarkable and it is mainly controlled by an expansion of the tetrahedral chains. Further differences between theP21/candC2/cstructures are a more regular chain of edge-sharingM1 octahedra as a consequence of more and more reduced Jahn–Teller distortion and a less kinked, symmetry-equivalent tetrahedral chain. The coordination of the Ca site increases from sevenfold to eightfold with large changes in the Ca—O bond lengths during the phase change. Raman spectroscopy was mainly used to monitor theP21/ctoC2/cphase change as a function of composition, but also as a function of temperature and to follow changes in specific Raman modes throughout the solid-solution series.

γ, β and α relaxation processes in nylon 6,6 were studied by means of Raman spectroscopy, being a vibrational spectroscopic technique sensitive to chemical groups in the polymeric matrix and therefore sensitive to temperature dependent changes in the molecular configuration of the polymeric sample probed. Through a temperature-dependent Raman spectroscopic experiment within the temperature range from -120 °C to +120 °C we determined temperature-dependent changes of peak parameters of the Raman bands associated with the C O, C C, and N H stretching vibrations, being able to determine the temperature ranges where changes occur which can be associated with relaxation processes, which result to be in accordance with published experimental results obtained by thermodynamic and mechanical techniques, i.e. macroscopic techniques.

Here it is reported a multidisciplinary approach based on tomography and infrared techniques applied to the characterization of tannin porous rigid foams, potentially usable as new insulating materials in green building technology. With conventional x-ray tomography it was possible to preliminary evaluate the homogeneity of the samples at low resolution, while then, thanks to the synchrotron source, it was possible to obtain more detailed information at a micro-scale level. At the same time chemical characterization was done through Fourier Transform infrared (FTIR) imaging. Conventionally, FTIR imaging is limited to a planar projection, not considering the 3D structure of the material. To avoid this limitation, a FTIR 3D-tomography setup was built and the foams characterized by a chemical point of view. The idea is to directly correlate these data with the 3D-structural information obtained with the x-ray computed tomography exploiting the synchrotron radiation as source, allowing a complete characterization of the material morphology and chemistry at the microscale.

Tannin-furanic rigid foams are innovative polymeric materials made of inexpensive organic ingredients, and are usually produced via an acid catalyzed polycondensation reaction between furfuryl alcohol and condensed flavonoids (e.g. Mimosa tannin). These bio-friendly foams have already been proposed e.g. as insulating material for eco-sustainable buildings (green building technology), as their most important physical properties are their low thermal conductivity and their high fire resistance. The aim of the present study is the Raman spectroscopic characterization of the tannin-furanic rigid foams in order to establish a tool complementary to infrared spectroscopy for comparing their spectral signature with that of the precursor materials furfuryl alcohol, polymerized furfuryl alcohol, and Mimosa tannin. The discussion deals with similarities and differences to the spectral signatures of sp 2 carbon-based materials, the still preserved organic nature of the tannin-furanic foam, and with similarities and differences to recently reported infrared spectra.

Here, we present a comprehensive study that encompasses changes within the crystal and magnetic structure in the brownmillerite-type phase Ca2Fe2O5 induced by the substitution of Fe3+ with Ga3+. 61 synthetic single-crystal samples of Ca2Fe2xGaxO5 0.00x1.328 have been investigated by single-crystal X-ray diffraction at 25 C. We find that pure Ca2Fe2O5 and samples up to x1.0 have space group Pnma, Z=4, whereas samples with x>1.0 show I2mb symmetry, Z=4. The Raman spectroscopic measurements exhibit that the change from Pnma to I2mb space group symmetry is reflected by a significant shift of two Raman modes below 150 cm1. These Raman modes are obviously linked to changes in the CaO bond lengths at the phase transition. 57Fe Mössbauer spectroscopy was used to characterize the cation distribution and magnetic structure as a function of composition and temperature. Thereby, the strong preference of Ga3+ for the tetrahedral site is verified, as an independent method besides XRD. At room-temperature, Ca2Fe2xGaxO5 solid solution compounds with 0x1.0 are antiferromagnetic ordered, as revealed by the appearance of magnetically split sextets in the Mössbauer spectra; samples with higher Ga3+ contents are paramagnetic. Over and above, the substitution of Fe3+ by Ga3+ results in the appearance of sharp, additional magnetic hyperfine split sextets, which can be attributed to cluster configurations within the individual tetrahedral chains. The temperature-dependent (20720 K) Mössbauer study reveals a transition from the magnetically ordered to the paramagnetic state at a temperature of about 710 K for the Ca2Fe2O5 end-member. (VLID)2956672

The Brill transition is a phase transition process in polyamides related with structural changes between the hydrogen bonds of the lateral functional groups (CO) and (NH). In this study, we have used the potential of Raman spectroscopy for exploring this phase transition in polyamide 6,6 (nylon 6,6), due to the sensitivity of this spectroscopic technique to small intermolecular changes affecting vibrational properties of relevant functional groups. During a step by step heating and cooling process of the sample we collected Raman spectra allowing us from two-dimensional Raman correlation spectroscopy to identify which spectral regions suffered the largest influence during the Brill transition, and from Terahertz Stokes and anti-Stokes Raman spectroscopy to obtain complementary information, e.g. on the temperature of the sample. This allowed us to grasp signatures of the Brill transition from peak parameters of vibrational modes associated with (CC) skeletal stretches and (CNH) bending, and to verify the Brill transition temperature at around 160°C, as well as the reversibility of this phase transition.

Surface-enhanced Raman scattering (SERS) is a versatile spectroscopic technique that suffers from reproducibility issues and usually requires complex substrate fabrication processes. In this article, we report the use of a simple mass production technology based on Blu-ray disc manufacturing technology to prepare large area SERS substrates (∼40 mm2) with a high degree of homogeneity (±7% variation in Raman signal) and enhancement factor of ∼6 × 106. An industrial high throughput injection molding process was used to generate periodic microstructured polymer substrates coated with a thin Ag film. A short chemical etching step produces a highly dense layer of Ag nanoparticles at the polymer surface, which leads to a large and reproducible Raman signal. Finite difference time domain simulations and cathodoluminescence mapping experiments suggest that the sample microstructure is responsible for the generation of SERS active nanostructures around the microwells. Comparison with commercial SERS substrates demo...

In polymeric materials the glass transition (GT) is a well-known and very important relaxation process related to movements of functional groups in the polymeric chain. In this work, we show the potential of Raman spectroscopy for exploring the GT process in the polymer polystyrene. We collected Raman spectra during a step-by-step heating process of the sample, which allowed us to collect signatures of the GT process from peak parameters of specific vibrational modes, and to verify the GT temperature. Results of the latter were in accordance with published values obtained via other methods. We identified the aromatic ring vibrational modes of the phenyl functional groups to be those which, due to steric hindrance, suffer the largest influence during the GT process. This confirms that Raman spectroscopy can be used as a complementary technique to perform GT investigations in polymeric materials due to its sensitivity to small intermolecular changes affecting vibrational properties of relevant functional side groups.