Spectroscopic studies of the corrosion of model iron electrodes in carbonate and phosphate buffer solutions

We have extended our unenhanced (non-SERS) Raman spectroscopic investigations to include a study of the corrosion of an iron electrode in carbonate and phosphate buffer solutions. The measurements have been supported by electrochemical investigations (via cyclic voltammetry), enabling oxidation and reduction reactions to be systematically followed at variable applied potentials. In a carbonate buffer (pH = 9.4) the surface oxidation led to the formation of a ‘green rust’ (a hydrated hydroxy-carbonate), followed by the α- and β-forms of FeOOH and an underlying magnetite layer formed on the cathodic (reduction) cycle. In a phosphate buffer (pH = 7.7) the surface was passivated by hydrated phosphates of iron [identified as FePO₄ · xH₂O and Fe₃(PO₄)₂·8H₂O]. The formation of oxides (Fe₂O₃ and Fe₃O₄) were inferred from voltammetry, but spectral identification was more difficult because of broad, ill-defined spectra. Despite the challenges of using unenhanced Raman spectroscopy, we believe that the effort was worthwhile, the reactions identified being more likely to be relevant to real electrochemical environments. Copyright © 2008 John Wiley & Sons, Ltd.     

Deterioration of iron archaeological artefacts: micro‐Raman investigation on Cl‐containing corrosion products

Deterioration after excavation of archaeological iron artefacts buried in soil is often associated with the presence of chlorinated phases in corrosion products, leading to serious problems for conservation of metallic objects of cultural heritage. Therefore, in order to better understand the corrosion process related to the presence of chlorine, some high‐resolution techniques of material characterisation are implemented. The analyses are realised on cross sections of corroded iron objects excavated from archaeological sites dated from the 12th to the 16th century A.D. Cl‐containing phases appear even when the Cl level in the water is low. In addition to the common oxy‐hydroxide‐containing chloride, akaganeite (β–FeOOH) often mentioned in the literature, a ferrous hydroxychloride β–Fe₂(OH)₃Cl was also found in the corrosion layers. Moreover, part of the study is based on the preparation and desalinisation of pure akaganeite powder in order to characterise the influence of the chloride content on its crystalline structure as characterised by Raman spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.     

Low‐temperature Raman spectroscopic studies in NaNbO3

Raman spectroscopic measurements were carried out in the temperature range 10–300 K to understand the low‐temperature antiferroelectric (AFE)–ferroelectric (FE) phase transition in NaNbO₃. Several modes in the low wavenumber range were found to disappear, while some new modes appeared across the transition. The temperature dependence of mode wavenumbers suggests that, during cooling, the AFE–FE phase transition begins to occur at 180 K, while the reverse transition starts at 260 K during heating. During cooling, the two phases were found to coexist in the temperature range of 220–160 K. Upon heating, the FE phase is retained up to 240 K and both FE and AFE phases coexist in the temperature range 240–300 K. In contrast to the earlier reports, the present results suggest a different coexistence region and the reverse transition temperature. The reported relaxor‐type FE behaviour over a broad temperature is consistent with the observed coexistence of phases during cooling and heating cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic studies of ZnSe/GaAs interfaces

ZnSe/semi‐insulating GaAs interfaces were studied by observing photogenerated plasmon–LO (PPL) coupled modes by nonresonant micro‐Raman spectroscopy. The effect of the carriers generated by the focused laser beam was investigated for a series of different thicknesses of ZnSe epitaxial layers. The PPL mode in GaAs was observed in the micro‐Raman spectra for all samples, but with different magnitude. The plasma is believed to be an electron gas as a result of the negative nature of the interfacial region that contains predominantly hole traps. The free carrier concentration is estimated to be > 10¹⁸ cm⁻³ and their lifetime ∼0.1 ns. This relatively long lifetime suggests that the ZnSe/GaAs interface has to be of high structural quality leading to a low recombination velocity. ZnSe/GaAs heterostructures of less crystalline quality (as determined by resonant Raman measurements) shows the effect of photogenerated carriers only to lesser extent. Copyright © 2007 John Wiley & Sons, Ltd.     

Nd-Fe-B永磁合金腐蚀产物的拉曼光谱研究

本文应用拉曼光谱法,研究了Nd—Fe—B永磁合金在NaCl,NaHSO_4水溶液及大气环境中的腐蚀产物组成及其变化。测定结果表明,Nd—Fe—B合金大气腐蚀产物的主要组成为:Nd(OH)_3,γ—FeOOH,α—FeOOH,Fe_3O_4及B_2O_3,在NaCl溶液中则有NdCl_3生成。在不同PH值的NaHSO_4溶液中,锈层的组成有所变化。据此,对Nd—Fe—B合金的腐蚀过程与机理进行了探讨。     

Using Raman spectroscopy as a tool for the detection of iron in glass

Raman spectroscopy has been used to identify iron‐containing glasses. This nondestructive technique offers a fast method to obtain qualitative information about the presence of iron oxides in glass. The effect of the iron content in glass samples is reflected on the topology of the Raman spectra: A strong link between the ratio of the Q₂/Q₃ vibration units of the silica tetrahedral structure is seen. If matrix effects are taken into account, also (semi)quantitative results can be obtained from the calibration lines. The linear calibration is based on the normalized band intensity at 980 cm⁻¹ (I₉₈₀/I₁₀₉₀) and the iron oxide concentration for similar glasses. In amber and dark colored glasses, an extra peak in the spectrum indicates the presence of a FeS chromophore. Different series of glasses of various origins (ancient and modern/industrial glass) have been considered. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman spectroscopic determination of the acidity constants of salicylaldoxime in aqueous solution

A sensitive and efficient method for the determination of acidity constants of salicylaldoxime (SALO) (2‐hydroxybenzaldehyde oxime), using both Raman spectroscopic and potentiometric methods, at 30 °C in 50% (wt/wt) of ethanol–water mixture and at the constant ionic strength I = 0.1 M is proposed. The effect of pH on the Raman spectra has also been studied. The Raman spectroscopic technique allows the identification of compounds in different molecular and molecular ionic structures. The limit of detection (LOD) was determined to be 0.05 mol dm⁻³ for SALO by means of Raman peak area. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic studies of LixSiy compounds

The phase diagram of the Li–Si system contains several phases with Li and Si in well defined ratios. So far, only the Raman spectrum of LiSi has been reported. In this work, we present experimental Raman scattering results for the crystalline lithium silicide phases Li₁₂Si₇, Li₇Si₃, Li₁₃Si₄, and Li₂₁Si₅/Li₂₂Si₅, which show clearly distinguishable Raman modes. The experimental results are compared with theoretical data obtained by density functional theory calculations. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman spectra of selected mineral phases of the Morasko iron meteorite

Selected minerals from oval inclusions (nodules) in the Morasko IAB iron meteorite have been investigated by electron microprobe (EMP) and micro‐Raman spectroscopy. Collected spectra of quartz, forsterite‐rich olivine, chromite, kosmochlor (Na–Cr‐rich clinopyroxene) and Na‐feldspar (albite) were compared with corresponding measurements of earth minerals and used for fast identification of meteorite fragments not EMP characterized. Some structural conclusions concerning particular minerals, like solid‐state form or average crystallite size, were also drawn from the measured spectra. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterization of corrosion products formed on the surface of carbon steel by Raman spectroscopy

Corrosion of carbon steel in seashore salty soils containing 10, 20, and 34 wt% (saturated) water was investigated. The corrosion rate was measured and corrosion products were analyzed using Raman spectroscopy. It was found that carbon steel in the soil with 10 wt% water content had the largest corrosion rate and the corrosion was dominated by localized corrosion. The corrosion rate drops dramatically and turns to be general corrosion with increase of water content. The corrosion products in the soil with 20 and 34 wt% water content are mainly composed of α‐FeOOH, while in the soil with 10 wt% water content, the products show a delaminated structure of two layers with the inner layer mainly consisting of α‐FeOOH and the outer layer composed of Fe₂O₃ and Fe₃O₄. Copyright © 2008 John Wiley & Sons, Ltd.     

A resonance Raman spectroscopic study of the protonation of Disperse Orange 25 in solution and adsorbed on oxide surfaces

The protonation of Disperse Orange 25 (DO25) in aqueous solution, and upon adsorption on oxide surfaces, was studied by resonance Raman (RR) spectroscopy. The neutral and protonated forms of DO25 were modelled by DFT calculations of the isolated molecules in the gas phase at the B3‐LYP/DZ level, enabling calculation of the vibrational spectra of these species, together with vibrational assignments. RR spectra show that DO25 is physisorbed on the SiO₂ surface, but its adsorption on SiO₂ Al₂O₃ or H‐mordenite results in protonation. This observation indicates the presence of Brønsted acidic sites on these oxide surfaces with pK_a values ⩽2.5. RR studies of the adsorption of DO25 can therefore provide useful information on the nature of surface acidity on oxides, which is complementary to that obtained from other probe molecules. Copyright © 2007 John Wiley & Sons, Ltd.     

Morphology of InN nanorods using spectroscopic Raman imaging

We report the vibrational properties of vertical and oblique InN nanorods (NRs) grown by molecular beam epitaxy (MBE). Surface optical (SO) Raman mode at 561 cm⁻¹, belonging to E₁ symmetry [SO(E₁)], is identified along with symmetry allowed Raman modes of E₂(low), E₂(high), and E₁(LO) at 87, 489, and 589 cm⁻¹, respectively, corresponding to wurtzite InN phase. Usually, SO phonon modes arise due to breakdown of translational symmetry of surface potential at surface defects, which are attributed by the surface roughness. Intensity distribution of E₁(LO) and SO(E₁) phonon modes over a specified area have been analysed using Raman area mapping with an optical resolution of 400 nm. Imaging with E₁(LO) phonon mode, originating from the bulk of the sample, distinguishes the vertical NRs alone. We are able to resolve NR morphologies in both vertical and oblique cases with additional Raman mapping analysis of SO(E₁) phonon mode, emerging from the surface irregularities, which are confined to the tip of MBE grown NRs. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman study of a deuterated iron hydroxycarbonate to assess long‐term corrosion mechanisms in anoxic soils

In several contexts such as cultural heritage, oil and gas or nuclear waste disposal, the long‐term corrosion mechanisms of iron in anoxic soils are studied. For this purpose, corrosion layers formed on ferrous archaeological artefacts from the site of Glinet (16th century, Normandy, France) were characterised. The main phases identified are siderite (FeCO₃), chukanovite (iron hydroxycarbonate: Fe₂(OH)₂CO₃ and magnetite (Fe₃O₄). In order to provide reliable Raman references for further studies on carbonated systems, the iron hydroxycarbonate (chukanovite) was synthesised on iron discs. The corrosion mechanisms were investigated by re‐corroding the archaeological samples in a deuterated solution. Raman characterisation on cross sections inside the layer revealed the presence of deuterated chukanovite, allowing the deuterium tracing of the spreading of the corrosion. A set of chukanovite samples was synthesised with various D/H ratios. Using these reference data, the proportion of deuterated chukanovite in re‐corroded artefacts was evaluated, and the corrosion rate was estimated as less than 1.6 µm/year. Copyright © 2010 John Wiley & Sons, Ltd.     

Mössbauer spectroscopy of iron meteorite Dronino and products of its corrosion

The results of the first study of iron meteorite Dronino and products of its corrosion by Mössbauer spectroscopy are presented. Mössbauer parameters for iron phase, sulfide inclusions, products of terrestrial corrosion and iron oxides in concretion are determined and analyzed.     

Raman spectroscopic study of various types of tourmalines

Both polarized and unpolarized Raman scattering studies of seven tourmalines from the Lucyen mines in Vietnam are presented. These tourmalines, according to their chemical compositions, can be classified into four groups: G1, liddicoatite; G2, elbaite; G3, uvite; and G4, feruvite. The Raman spectra were recorded in two spectral ranges, i.e. 150–1600 cm⁻¹ and 3000–4000 cm⁻¹. In the lower spectral range, which covers the metal ion‐oxygen bond vibrations, all the observed A₁ and E modes are identified. In the higher spectral range, we investigated the OH stretching vibrations and showed that all the observed OH stretching modes have the A₁ character. In both spectral ranges, we found that the same group classification of tourmalines can be applied, and the grouping characterizations are consistent with the chemical composition results. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the uranyl carbonate mineral voglite

Raman spectroscopy, complemented with infrared spectroscopy, was used to study the uranyl carbonate mineral voglite. The mineral has the formula Ca₂Cu²⁺ [(UO₂)(CO₃)₃](CO₃)^•6H₂O, and bands attributed to these vibrating units are readily identified in the Raman spectrum. Symmetric stretching modes at 836 and 1094 cm⁻¹ are assigned to ν₁(UO₂)²⁺ and ν₁(CO₃)²⁻ units, respectively. The ν₃ antisymmetric stretching modes of (UO₂)²⁺ are not observed in the Raman spectrum but may be readily observed in the infrared spectrum at 898 cm⁻¹. The ν₃ antisymmetric stretching mode of (CO₃)²⁻ is observed in the Raman spectrum at 1369 cm⁻¹ as a low intensity band as is also the ν₃(CO₃)²⁻ infrared modes at 1362, 1425, 1509 and 1566 cm⁻¹. No ν₂(CO₃)²⁻ Raman bending modes are observed for voglite. The Raman band at 749 cm⁻¹ and the two infrared bands at 747 and 709 cm⁻¹ are assigned to the ν₄(CO₃)²⁻ bending modes. U O bond and O H…O bond lengths in the structure of voglite were inferred from the infrared and Raman spectra. Copyright © 2007 John Wiley & Sons, Ltd.     

Raman and FTIR spectroscopy applied to the conservation report of paleontological collections: identification of Raman and FTIR signatures of several iron sulfate species such as ferrinatrite and sideronatrite

Fossil materials that contain iron sulfide are well known for their instability when exposed to oxygen and humidity. This term however combines a great variety of materials showing different types of damages. Most of them consist of crystal efflorescence appearing on the surface and inside the matrix. In this work, a methodology was determined for the analysis of these damages by the use of Raman and infrared spectroscopy. The infrared and Raman signatures of a large set of iron sulfates were characterized. Specific attention was paid to sideronatrite and ferrinatrite, which are two associated sodium/iron(III) sulfates, and their infrared and Raman bands were partially assigned. Analysis performed on a selection of 11 damaged fossils showed a great variety of degradation products: besides one case that appeared to be a synthetic resin close to polyvinylchloride acetate, which was applied with a brush on the fossil surface, all degradation products belong to the sulfate group. However, many iron‐free sulfates, such as gypsum, halotrichite, epsomite, or pentahydrite were found, often in association with iron sulfates. In one case, despite the presence of iron in the matrix, no iron sulfate could be detected. This shows that the term ‘pyritic fossil’, commonly used by collection managers, is not appropriate as it oversimplifies the reality. A name such as ‘sulfide‐containing fossil’ would be more suitable. Copyright © 2012 John Wiley & Sons, Ltd.     

The Raman spectroscopic studies of aligned MWCNTs treated under high pressure and high temperature

The study of the aligned multiwalled carbon nanotubes (MWCNTs) for interlinking bonding under high pressures and temperatures have been conducted in the diamond anvil cell. The MWCNT samples were analyzed using the Raman spectroscopy, when treated under the combinations of pressure and temperature ranges of 2-20 GPa and 25-500 °C. The analyses show the formation of interlinking bonding at a pressure above 2.5 GPa when treated under the temperature 500 °C, based on the significant change of the relative intensity between D- and G-bands in the Raman spectra. Comparisons of the data obtained before and after the high pressure and high temperature treatments are reported. The result indicates that the aligned MWCNTs may be easier to form the interlinking bonding compared to randomly oriented MWCNTs.     

The growth of the passive film on iron in 0.05 M NaOH studied in situ by Raman micro‐spectroscopy and electrochemical polarisation. Part I: near‐resonance enhancement of the Raman spectra of iron oxide and oxyhydroxide compounds

Raman spectroscopy, in principle, is an excellent technique for the study of molecular species developed on metal surfaces during electrochemical investigations. However, the use of the more common laser wavelengths such as the 514.5‐nm line results in spectra of less than optimal intensity, particularly for iron oxide compounds. In the present work, near‐resonance enhancement of the Raman spectra was investigated for the iron oxide and iron oxyhydroxide compounds previously reported to be present in the passive film on iron, using a tuneable dye laser producing excitation wavelengths between 560 and 637 nm. These compounds were hematite (α‐Fe₂O₃), maghemite (γ‐Fe₂O₃), magnetite (Fe₃O₄), goethite (α‐FeOOH), akaganeite (β‐FeOOH), lepidocrocite (γ‐FeOOH) and feroxyhyte (δ‐FeOOH). Optimum enhancement, when compared to that with the 514.5‐nm line, was obtained for all the iron oxide and oxyhydroxide standard samples in the low wavenumber region (<1000 cm⁻¹) using an excitation wavelength of 636.4 nm. Particularly significant enhancement was obtained for lepidocrocite, hematite and goethite. Copyright © 2010 John Wiley & Sons, Ltd.