Raman Spectroscopy Observations Research Articles

Reaction pathway, mechanism and kinetics of thermochemical sulfate reduction: insights from in situ Raman spectroscopic observations at elevated temperatures and pressures

In Earth’s crust, thermochemical sulfate reduction (TSR) is a common organic–inorganic interaction, which is in close association with the carbon cycle in sedimentary basins and metal sulfide precipitation in Mississippi Valley-type deposits. However, the reaction pathway and mechanism of TSR need further investigation, mainly due to the complex sulfur species involved in this reaction. Here we applied in situ Raman spectroscopy to disclose the stepwise transformation from sulfate to sulfide and the reaction kinetics in the CH4-Na2SO4/MgSO4/H2SO4-H2O systems at elevated temperatures and pressures. Our results indicate that sulfate is transformed to H2S through a series of reduction and disproportionation reactions. Once dissolved H2S reaches a certain concentration, it reacts with sulfate to form intermediate valence sulfur species (S0 and S3−), which quickly oxidize methane to generate CO2 and H2S. The formation of S3− and S0 is responsible for the autocatalytic effect in TSR. The sulfur speciation significantly affects the reaction kinetics, which could be divided into three stages based on the in situ observations. Extrapolated kinetic data show that TSR could proceed fast under the catalysis of H2S, and the half-life of methane is ∼7.10 million years at 200 °C. Subsequent numerical modeling strongly supports that in situ sulfate reduction could provide sufficient reduced sulfur for the formation of giant Mississippi Valley-type deposits, thus serving as an efficient mineralizing mechanism.

Kinetic Studies of WO3-Based Photochromism in Polyvinyl Alcohol Film.

Tungsten oxide (WO3) has been extensively studied for various photochromic applications. Blue coloration of WO3 is explained in terms of the intervalence charge transfer (IVCT) transition of electrons between W6+ and W5+. However, various absorption spectra with different shapes have been reported. Herein, a transparent film was prepared by drying aqueous solutions containing polyvinyl alcohol, WO3 nanoparticles and ethylene glycol (EG). For comparison, the photochromic behavior of an aqueous WO3 colloidal solution containing EG was also investigated. Under UV irradiation, a single intense peak was always observed at ca. 777 nm in the colloidal solution, but the absorption spectra of the film changed from a peak at 770 nm to two distinct peaks at 654 and 1003 nm. All absorption spectra observed with the film and the colloidal solution were deconvoluted into five peaks at 540, 640, 775, 984, and 1265 nm. Kinetic studies using the colloidal solution indicated that the coloration rates (r0) estimated at the deconvoluted peaks of 640, 775, and 984 nm followed the same rate law. On the other hand, in the case of the film, r0 evaluated at 640 or 984 nm was independent of the water amounts but increased proportionally to the EG amounts and the light intensity, although r0 at 775 nm significantly increased with the increasing amounts of water and EG. Raman and electron spin resonance spectroscopic observations of the film revealed that the photogenerated electrons migrated toward the terminal W═O moiety to accumulate and then a small anisotropic electron spin resonance signal appeared. Our study demonstrates that the absorption at 775 nm is due to IVCT between W6+ and W5+, which is stabilized with water in the bulk and the absorption peaks at 640 and 984 nm are attributable to IVCT on the WO3 surface.

The Possible Role of Anoxic Alkaline High Subcritical Water in the Formation of Ferric Minerals, Methane and Disordered Graphitic Carbon in a BARB3 Drilled Sample of the 3.4Ga Buck Reef Chert.

The present article reports Raman spectroscopic observations of siderite, hematite, disordered graphitic carbon and possibly greenalite inside the quartz matrix of a banded iron sample from the BARB3 core drilled inside the 3.4Ga Buck Reef Chert of the Barberton Greenstone Belt in South Africa. The article also reports Raman spectroscopic observations of quartz cavities, concluding in the presence of water, methane and sodium hydroxide at high concentration leading to pH ~ 15 inside the inclusion, suggesting an Archean water which was strongly basic. FeIII-greenalite may also be present inside the inclusion. The possible role of anoxic alkaline high subcritical water in the formation of ferric minerals and the CO required for the synthesis of molecules of biological interest has been demonstrated theoretically since 2013 and summarized in the concept of Geobiotropy. The present article experimentally confirms the importance of considering water in its anoxic strongly alkaline high subcritical domain for the formation of quartz, hematite, FeIII-greenalite, methane and disordered graphitic carbon. Methane is proposed to form locally when the carbon dioxide that is dissolved in the Archean anoxic alkaline high subcritical water, interacts with the molecular hydrogen that is emitted during the anoxic alkaline oxidation of ferrous silicates. The carbon matter is proposed to form as deposition from the anoxic methane-rich fluid. A detailed study of carbon matter from diverse origins is presented in a supplementary file. The study shows that the BARB3_23B sample has been submitted to ~ 335°C, a temperature of the high subcritical domain, and that the graphitic structure contains very low amounts of oxygen and no hydroxyl functional groups. The importance of considering the structure of water is applied to the constructions of the Neoproterozoic and Archean banded iron formations. It is proposed that their minerals are produced inside chemical reaction chambers containing ferrous silicates, and ejected from the Earth's oceanic crust or upper mantle, during processes involving subduction events or not.

Cadmium oxide as a substitute for sodium phosphate structural units: structure and properties

Amorphous glasses in the system xCdO-(100-x)(yNa2O + zP2O5), (x = 0, 10, 20, 30, 40, 42.5, 45, y = z) mol% are prepared by a fast quenching technique. Addition of CdO at the expense of equal amounts of both P2O5 and Na2O causes a substantial influence on the distinctive feature of the measured Raman spectra. The concentration of nonbridging oxygen atoms (NBO) rises with increasing CdO content. The numerical data from the deconvolution of Raman spectra revealed that Q3 (branching group) and Q2 (metaphosphate group) decline while Q1 (pyrophosphate group or end group) increases. The rise in the number of NBO in terms of Q1 is supported by the change in molar volume, free volume, and packing density. This result is compatible with Raman and NMR spectroscopic observations. Increased CdO content causes a rise in the O/P ratio, which causes an increase in NBO in the glass network which lowers the glass’s hardness number.

Photobiomodulation reduces hippocampal apoptotic cell death and produces a Raman spectroscopic "signature".

Apoptotic cell death within the brain represents a significant contributing factor to impaired post-traumatic tissue function and poor clinical outcome after traumatic brain injury. After irradiation with light in the wavelength range of 600-1200 nm (photobiomodulation), previous investigations have reported a reduction in apoptosis in various tissues. This study investigates the effect of 660 nm photobiomodulation on organotypic slice cultured hippocampal tissue of rats, examining the effect on apoptotic cell loss. Tissue optical Raman spectroscopic changes were evaluated. A significantly higher proportion of apoptotic cells 62.8±12.2% vs 48.6±13.7% (P<0.0001) per region were observed in the control group compared with the photobiomodulation group. After photobiomodulation, Raman spectroscopic observations demonstrated 1440/1660 cm-1 spectral shift. Photobiomodulation has the potential for therapeutic utility, reducing cell loss to apoptosis in injured neurological tissue, as demonstrated in this in vitro model. A clear Raman spectroscopic signal was observed after apparent optimal irradiation, potentially integrable into therapeutic light delivery apparatus for real-time dose metering.

In Situ Spectroelectrochemical Detection of Oxygen Evolution Reaction Intermediates with a Carboxylated Graphene-MnO2 Electrocatalyst.

In electrocatalyst-assisted water splitting, the oxygen evolution reaction (OER) imposes a performance limit due to the formation of different catalyst-bound intermediates and the scaling relationship of their adsorption energies. To break this scaling relationship in OER, a bifunctional mechanism was proposed recently, in which the energetically demanding step of forming the *OOH intermediate, through the attack of a water molecule on the oxo unit (*O, with * representing a reactive metal center), is facilitated by proton transfer to the second catalytic site. This mechanism was supported theoretically but so far by only very few experiments with a proton-transfer agent in basic media. However, active metal-containing catalysts could be destroyed in alkaline media, raising questions on practical applications. To date, this mechanism still lacks a systematic spectroscopic support by observing the short-lived and limited amount of reactive intermediates. Here, we report an operando Raman spectroscopic observation of the OER intermediates in neutral media, for the first time, via a bifunctional mechanism using a carboxylated graphene-MnO2 (represented by Gr-C-MnO2) electrocatalyst. The formation of the Mn-OOH intermediate after the attack of a water molecule on the Mn═O complex is followed by a proton transfer from Mn-OOH to the functionalized carboxylates. The role of the functionalized carboxylates to improve the catalytic efficiency was further confirmed by both pH-dependent and isotope (H/D)-labeling experiments. Furthermore, with a unique strategy of using a hybrid aqueous/nonaqueous electrolyte, the OER was alleviated, allowing sufficient Mn-OH and Mn-OOH intermediates for in situ Raman spectroscopic observation.

Evolution of Cationic Vacancy Defects: A Motif for Surface Restructuration of OER Precatalyst

AbstractDefects have been found to enhance the electrocatalytic performance of NiFe‐LDH for oxygen evolution reaction (OER). Nevertheless, their specific configuration and the role played in regulating the surface reconstruction of electrocatalysts remain ambiguous. Herein, cationic vacancy defects are generated via aprotic‐solvent‐solvation‐induced leaking of metal cations from NiFe‐LDH nanosheets. DFT calculation and in situ Raman spectroscopic observation both reveal that the as‐generated cationic vacancy defects tend to exist as VM (M=Ni/Fe); under increasing applied voltage, they tend to assume the configuration VMOH, and eventually transform into VMOH‐H which is the most active yet most difficult to form thermodynamically. Meanwhile, with increasing voltage the surface crystalline Ni(OH)x in the NiFe‐LDH is gradually converted into disordered status; under sufficiently high voltage when oxygen bubbles start to evolve, local NiOOH species become appearing, which is the residual product from the formation of vacancy VMOH‐H. Thus, we demonstrate that the cationic defects evolve along with increasing applied voltage (VM → VMOH → VMOH‐H), and reveal the essential motif for the surface restructuration process of NiFe‐LDH (crystalline Ni(OH)x → disordered Ni(OH)x → NiOOH). Our work provides insight into defect‐induced surface restructuration behaviors of NiFe‐LDH as a typical precatalyst for efficient OER electrocatalysis.

Evolution of Cationic Vacancy Defects: A Motif for Surface Restructuration of OER Precatalyst

Defects have been found to enhance the electrocatalytic performance of NiFe-LDH for oxygen evolution reaction (OER). Nevertheless, their specific configuration and the role played in regulating the surface reconstruction of electrocatalysts remain ambiguous. Herein, cationic vacancy defects are generated via aprotic-solvent-solvation-induced leaking of metal cations from NiFe-LDH nanosheets. DFT calculation and in situ Raman spectroscopic observation both reveal that the as-generated cationic vacancy defects tend to exist as VM (M=Ni/Fe); under increasing applied voltage, they tend to assume the configuration VMOH , and eventually transform into VMOH-H which is the most active yet most difficult to form thermodynamically. Meanwhile, with increasing voltage the surface crystalline Ni(OH)x in the NiFe-LDH is gradually converted into disordered status; under sufficiently high voltage when oxygen bubbles start to evolve, local NiOOH species become appearing, which is the residual product from the formation of vacancy VMOH-H . Thus, we demonstrate that the cationic defects evolve along with increasing applied voltage (VM → VMOH → VMOH-H ), and reveal the essential motif for the surface restructuration process of NiFe-LDH (crystalline Ni(OH)x → disordered Ni(OH)x → NiOOH). Our work provides insight into defect-induced surface restructuration behaviors of NiFe-LDH as a typical precatalyst for efficient OER electrocatalysis.

Halogen Bonding Propensity in Solution: Direct Observation and Computational Prediction.

Halogen-bonded complexes are often designed by consideration of electrostatic potential (ESP) predictions. ESP predictions do not capture the myriad variables associated with halogen bond (XB) donors and acceptors; thus, binding interaction cannot be quantitatively predicted. Here, a discrepancy between predictions based on ESP energy difference (ΔVs ) and computed gas phase binding energy (ΔEbind ) motivated the experimental determination of the relative strength of halogen bonding interactions in solution by Raman spectroscopic observation of complexes formed from interacting five iodobenzene-derived XB donors and four pyridine XB acceptors. Evaluation of ΔEbind coupled with absolutely-localized molecular orbital energy decomposition analysis (ALMO-EDA) deconvolutes halogen bonding energy contributions and reveals a prominent role for charge transfer (CT) interactions. Raman spectra reveal ΔEbind accurately predicts stronger interactions within iodopentafluorobenzene (IPFB) complexes than with 1-iodo-3,5-dinitrobenzene (IDNB) complexes even though IPFB has similar electrostatics to IDNB and contains a smaller σ-hole.

Could Raman spectroscopy distinguish between P. falciparum and P. vivax Infection?

The erythrocytes obtained from patients diagnosed with Plasmodium falciparum or Plasmodium vivax infection (two groups of five patients each) and treated in the Department of Infectious Diseases, University Hospital in Kraków, were measured using the Raman spectroscopy method (1–2 cells of each patient) and then compared with the results from these patients during their convalescence. Principal Component Analysis (PCA) was used to determine the variance between the Raman spectra. Changes in the heme structure were observed by the v4 oxidation state marker and by other heme and hemozoin marker bands, as well as protein side chains marker bands. The recognized Raman bands allow to differentiate changes taking place in the red blood cells during the development of Plasmodium falciparum from Plasmodium vivax. The 1385 cm−1 Asp band along with the 1587 cm−1 Gln vibrations at the beginning of hospitalization specify the invasion of P. falciparum that occurs with the basigin receptor which requires respective protein glycosylation. The 1361 cm−1 and 1544 cm−1 Trp bands indicate the invasion stage of the P. vivax parasite, and the formation of the ligand-receptor complex. To our knowledge, this is the first Raman spectroscopic observation that we can distinguish Plasmodium species.

Indentation densification of fused silica assessed by raman spectroscopy and constitutive finite element analysis

AbstractInelastic deformation of anomalous glasses manifests in shear flow and densification of the glass network; the deformation behavior during indentation testing is linked strongly to both processes. In this paper, the indentation densification field of fused silica is investigated using depth‐resolved Raman spectroscopy and finite element simulations. Through affecting the size of the indent, the normal load and the Raman laser spot size determine the spatial sampling resolution, leading to a certain degree of structural averaging. For appropriate combinations of normal load (indent size) and laser spot diameter, a maximum densification of 18.4% was found at the indent center. The indentation behavior was modeled by extended Drucker‐Prager‐Cap (DPC) plasticity, assuming a sigmoidal hardening behavior of fused silica with a densification saturation of 21%. This procedure significantly improved the reproduction of the experimental densification field, yielding a maximum densification of 18.2% directly below the indenter tip. The degree of densification was found to be strongly linked to the hydrostatic pressure limit below the indenter in accordance to Johnson's expanding cavity model (J. Mech. Phys. Solids, 18 (1970) 115). Based on the good overlap between FEA and Raman, an alternative way to extract the empirical correlation factor m, which scales structural densification to Raman spectroscopic observations, is obtained. This approach does not require the use of intensive hydrostatic compaction experiments.

Cigarette smoke-induced changes in the murine vocal folds: a Raman spectroscopic observation.

Raman spectroscopic methods are being projected as novel tools to study the early invisible molecular level changes in a label-free manner. In the present study, we have used Raman spectroscopy to explore the earliest biochemical changes in murine vocal folds in response to time-bound cigarette smoke exposure. Mice were exposed to cigarette smoke for 2 or 4-weeks through a customized smoke inhalation system. The larynx was collected and initial evaluations using standard methods of analysis such as histopathology and immunofluorescence was performed. Concurrent unstained sections were used for Raman imaging. Two common pathological features of vocal fold disorders including alterations in collagen content and epithelial hypercellularity, or hyperplasia, were observed. The mean spectra, principal component analysis, and Raman mapping also revealed differences in the collagen content and hypercellularity in the smoke exposed tissues. The differences in 2-week exposed tissues were found to be more prominent as compared to 4-week. This was attributed to adaptive responses and the already reported biphasic effects, which suggest that collagen synthesis is significantly reduced at higher cigarette smoke concentrations. Overall findings of the study are supportive of the prospective application of Raman imaging in monitoring changes due to cigarette smoke in the vocal folds.

Polycrystalline diamond film fabrication using modulated inductively coupled thermal plasmas at different pressure conditions

This paper investigates pressure influence on polycrystalline diamond formation using sawtooth-waveform modulated induction thermal plasma. Modulated induction thermal plasma was used to promote nucleation of diamond particles in the first stage. The operating pressure was set to 32, 60, and 90 Torr. The deposited diamond films were analyzed by field emission scanning electron microscope and Raman spectroscopic observation. Experimental results indicated that a lower pressure condition provided a higher deposition rate and a qualitatively better diamond film. Spectroscopic and high-speed video camera observation showed that a lower pressure condition expanded the thermal plasma flow axially onto the substrate. Numerical simulation was also made for Ar/CH4/H2 induction thermal plasma to study the influence of pressure on the thermal plasma flow. Calculation results showed that lower pressure involves a higher particle flux of neutral hydrocarbon species on to the substrate surface because of higher convective transport of these species before their ionization. These results imply that neutral hydrocarbon would play a more important role for diamond deposition than hydrocarbon ions in the present thermal plasma diamond film deposition.

Introduction of Room Temperature Ferromagnetism in Nanocrystalline Samarium Oxide by Doping of Co-ion

To enhance the magnetic property as well as to get the magnetic ordering in Sm2O3 which is a good dielectric material, Co-ion is substituted by a chemical route. Rietveld analysis of x-ray diffraction (XRD) pattern of the sample (Sm1.90Co0.10O3) suggests that all the dopants are properly substituted in the monoclinic lattice of Sm2O3. XRD analysis and Raman spectroscopic observation ruled out the segregation of Co-cluster or formation of any impurity in Sm1.90Co0.10O3. Magnetic susceptibility ( $$ \chi $$ ) of the sample measured in the range of 300–14 K by Faraday balance indicates the paramagnetic behavior down to 50 K, below which non-linear magnetic nature with ferromagnetic ordering is found. The onset of ferromagnetism in a doped dielectric host is investigated in detail by observing magnetization versus magnetic field (M–H) data and magnetization versus temperature (M–T) curves recorded under zero field cooled and field cooled conditions. All the data extracted from the magnetic measurements are analyzed to yield useful information of the newly developed diluted magnetic dielectric.

Time-Resolved in situ Raman Spectroscopic Observations of a Biomineralization Model System

Time-Resolved <i>in situ</i> Raman Spectroscopic Observations of a Biomineralization Model System

Microhardness modification of ultrahigh-molecular-weight polyethylene by oxygen plasma: Effect of the polymer crosslinking

This work aims to investigate the effect of the oxygen plasma over the hardness, crystallinity, surface chemistry and structural composition of Ultra High Molecular Weight Polyethylene (UHMWPE) at different times of treatment. The contact angle test was performed to observe the wettability of the material. The crystallinity and crystal size of UHMWPE treated with plasma were studied using X-ray diffraction (XRD). The hardness of the material increased by 70% after 10 min of treatment and subsequently decreased after 20 min of treatment back to the initial microhardness. The increase in the microhardness is caused by the crosslinking of the polymer, which is corroborated by Raman spectroscopic observations.

Optical constants of restored and etched reduced graphene oxide: a spectroscopic ellipsometry study

Structural and defective information in graphene are usually detected by Raman spectroscopy and electrical techniques; however, less attention has been paid to a spectroscopic ellipsometry investigation on graphene. Here, optical responses of graphene grown by chemical vapor deposition (G), reduced graphene oxide (RGO), restored RGO, and etched RGO are characterized by spectroscopic ellipsometry. The Lorentz oscillator model is used to fit the ellipsometric parameters. The resulting optical constants, refractive index and extinction coefficient are higher in less defective graphene, i.e. G and restored RGO. The results can be correlated with the Raman spectroscopic observation – fewer defects leading to higher density and more optical absorption. Our investigation opens a new angle of view to probe the structural information in the defective graphene and thus favors the production of high-quality graphene.

Borate melt structure: Temperature‐dependent B–O bond lengths and coordination numbers from high‐energy X‐ray diffraction

AbstractBorate melts containing &lt;20 mol% Na2O have been studied using high‐energy synchrotron X‐ray diffraction. Temperature dependencies of the mean B–O bond lengths are shown to vary strongly with soda content, by comparison to previous measurements on liquid B2O3 and Na2B4O7. Whereas in liquid B2O3 linear thermal expansion of the BØ3 units is observed, with coefficient αBO = 3.7(2) × 10−6 K−1, this expansion is apparently slightly suppressed in melts containing &lt;20 mol% Na2O, and is dramatically reversed at the diborate composition. These effects are interpreted in terms of changes in the mean B–O coordination number, where the reaction BØ4− + BØ3 ⇌ BØ3 + BØ2O− shifts to the right with increasing temperature. The empirical bond‐valence relationship is used to convert measured bond lengths, rBO, to coordination numbers, nBO, including a correction for the expected thermal expansion. This method is more accurate and precise than direct determination of nBO from peak areas in the radial distribution functions. Gradients of ΔnBO/ΔT = −3.4(3) × 10−4 K−1 close to the diborate composition, and ΔnBO/ΔT = −0.3(1) × 10−4 K−1 for a 13(3) mol% Na2O melt are observed, in reasonable agreement with Raman spectroscopic observations and thermodynamic modeling, with some quantitative differences. These observations go toward explaining isothermal viscosity maxima and changes in fragility across the sodium borate system.

In situ optical and Raman spectroscopic observations of the effects of pressure and fluid composition on liquid–liquid phase separation in aqueous cadmium sulfate solutions (≤400 °C, 50 MPa) with geological and geochemical implications

This study presents the first description of liquid–liquid phase separation in a vapor-saturated CdSO4 solution at temperatures above 222.2°C in fused silica capillary tubing, in which a sulfate-rich liquid (Srich) phase separates from an initially homogeneous aqueous solution and coexists with the remaining sulfate-poor liquid (Spoor) phase and a vapor phase. This phase behavior is characterized by a lower critical solution temperature (∼222.2°C), which is a macroscale property of polymer mixtures. In situ Raman spectroscopy shows an increase in Cd2+–SO42− associations with increasing temperature, especially in the immiscible Srich phase. Phase behavior observations and in situ Raman spectroscopic analyses confirm our previous conclusion that strong ion associations are responsible for the liquid–liquid phase separation in inorganic solutions. The effects of pressure and the fluid composition on liquid–liquid phase separation are also investigated. The temperature of liquid–liquid phase separation increases almost linearly with pressure at a rate of ∼0.5°C/MPa and decreases with the addition of methanol. Both decreases in pressure and increases in the methanol concentration decrease the dielectric constant of the solutions, favoring the Cd2+–SO42− interaction and thus liquid–liquid phase separation. The capillary size also exerts a strong influence on liquid–liquid phase separation because the immiscible Srich phase is more stable in fused silica capillary tubing with an inner diameter of ≤300μm. Therefore, liquid–liquid phase separation can occur in hydrothermal fluids hosted in porous rocks/sediments, especially those that are rich in low-dielectric-constant components and/or at low pressures.Liquid–liquid phase separation may play an important role in the formation of Mississippi-valley-type ore deposits because the ore-forming fluids are enriched in low-dielectric-constant components. The occurrence of liquid–liquid phase separation can promote the enrichment of Zn and S in fluids and accelerate thermochemical sulfate reduction as well as the mineralization of sphalerite. In addition, liquid–liquid phase separation may contribute to the enrichment and transport of metals in other hydrothermal systems because it significantly increases the capability of fluids to carry certain components. Finally, we observe crystallization within the Srich droplet, confirming that large clusters and separated dense liquids can act as precursors for crystallization. Therefore, crystallization through liquid–liquid phase separation can represent an important mechanism of nucleation and crystal growth in hydrothermal fluids.

Raman Spectroscopic Observation of Gradual Polymorphic Transition and Phonon Modes in CuPc Nanorod

A Raman scattering investigation of a single α-CuPc nanorod is demonstrated here within the temperature range 300–770 K. From the typical thermal dispersion of Raman shift and phonon line width of in-plane B1g mode at 1526 cm–1, the metastable α polymorph is observed to undergo unambiguous phase transition to thermally stable β phase. The phase transition temperature (456 K) is observed to be significantly lower than that reported for bulk samples. Extensive computation of normal modes associated with both α- and β-CuPc reveal that the “fingerprint” domain across 1300–1600 cm–1 is particularly sensitive to this polymorphic transition where a silent B2g mode appears at 1303 cm–1 in the spectral vicinity of molecular edge vibration sensitive Ag–Bg Davydov doublet.