Optical properties of α-Fe2O3microcrystals in the infrared

Optimal Spectral Resolution for Infrared Studies of Solids and Liquids

Due to a legacy originating in the limited capability of early computers, the spectroscopic resolution used in Fourier transform infrared spectroscopy and other systems has largely been implemented using only powers of two for more than 50 years. In this study, we investigate debunking the spectroscopic lore of, e.g., using only 2, 4, 8, or 16 cm-1 resolution and determine the optimal resolution in terms of both (i) a desired signal-to-noise ratio and (ii) efficient use of acquisition time. The study is facilitated by the availability of solids and liquids reference spectral data recorded at 2.0 cm-1 resolution and is based on an examination in the 4000-400 cm-1 range of 61 liquids and 70 solids spectra, with a total analysis of 4237 peaks, each of which was also examined for being singlet/multiplet in nature. Of the 1765 liquid bands examined, only 27 had widths <5 cm-1. Of the 2472 solid bands examined, only 39 peaks have widths <5 cm-1. For both the liquid and solid bands, a skewed distribution of peak widths was observed: For liquids, the mean peak width was 24.7 cm-1 but the median peak width was 13.7 cm-1, and, similarly, for solids, the mean peak width was 22.2 cm-1 but the median peak width was 11.2 cm-1. While recognizing other studies may differ in scope and limiting the analysis to only room temperature data, we have found that a resolution to resolve 95% of all bands is 5.7 cm-1 for liquids and 5.3 cm-1 for solids; such a resolution would capture the native linewidth (not accounting for instrumental broadening) for 95% of all the solids and liquid bands, respectively. After decades of measuring liquids and solids at 4, 8, or 16 cm-1 resolution, we suggest that, when accounting only for intrinsic linewidths, an optimized resolution of 6.0 cm-1 will capture 91% of all condensed-phase bands, i.e., broadening of only 9% of the narrowest of bands, but yielding a large gain in signal-to-noise with minimal loss of specificity.

Tailoring Fe2O3-Al2O3 catalyst structure and activity via hydrothermal synthesis for carbon nanotubes and hydrogen production from polyolefin plastics

Fe₂O₃-Al₂O₃ catalysts applied for conversion of polyolefin plastic waste into multi-walled carbon nanotubes (MWCNTs) and H₂ are typically produced by impregnation, co-precipitation or sol-gel synthesis at atmospheric pressure and temperatures below 100 °C. This study utilized hydrothermal conditions and established the role of precipitating agents (urea, N-methylurea and N,N'-dimethylurea) on properties and catalytic activity of Fe₂O₃-Al₂O₃ catalysts (Fe-u, Fe-mu and Fe-dmu, respectively). The precipitating agent played a key role in tailoring the properties, such as crystallization degree, surface area and reducibility. The precipitating agents influenced the yield and outer diameters of MWCNTs but did not affect graphitization degree. Among the synthesized catalysts, Fe-u had the largest surface area and preferential formation of the highly reducible α-Fe₂O₃ crystalline phase. As a result, Fe-u had the highest activity during conversion of pyrolysis gas from low-density polyethylene (LDPE) into MWCNTs, yielding 0.91 g·g^-1-catalyst MWCNTs at 800 °C as compared to 0.42 and 0.14 g·g^-1-catalyst using Fe-dmu and Fe-mu, respectively. Fe-dmu favored the growth of MWCNTs with smaller outer diameters. Fe-u demonstrated high efficiency during operation using a continuous flow of pyrolysis gas from a mixture of polyolefins (70 wt% polypropylene, 6 wt% LDPE and 24 wt% high density polyethylene) producing 4.28 g·g^-1-catalyst MWCNTs at 3.2% plastic conversion efficiency and a stable H₂ flow for 155 min (25-32 vol%). The obtained data demonstrate that the selection of an appropriate precipitating agent for hydrothermal synthesis allows for the production of highly active Fe₂O₃-Al₂O₃ catalysts for the upcycling of polyolefin plastic waste into MWCNTs and H₂.

Hematite nanoparticle clusters with remarkably high magnetization synthesized from water-treatment waste by one-step “sharp high-temperature dehydration”

Hematite (α-Fe₂O₃) nanoparticle clusters with an exceptionally high magnetization of 51 emu g⁻¹ were synthesized for the first time. This material was prepared from water-treatment waste by a new “sharp high-temperature dehydration” process.

Superoleophobic Surfaces Obtained via Hierarchical Metallic Meshes

Hierarchical metallic surfaces demonstrating pronounced water and oil repellence are reported. The surfaces were manufactured with stainless-steel microporous meshes, which were etched with perfluorononanoic acid. As a result, a hierarchical relief was created, characterized by roughness at micro- and sub-microscales. Pronounced superoleophobicity was registered with regard to canola, castor, sesame, flax, crude (petroleum), and engine oils. Relatively high sliding angles were recorded for 5 μL turpentine, olive, and silicone oil droplets. The stability of the Cassie-like air trapping wetting state, established with water/ethanol solutions, is reported. The omniphobicity of the surfaces is due to the interplay of their hierarchical relief and surface fluorination.

PDielec: The calculation of infrared and terahertz absorption for powdered crystals

The Python package PDielec is described, which calculates the infrared absorption characteristics of a crystalline material supported in a non-absorbing medium. PDielec post processes solid-state quantum mechanical and molecular mechanical calculations of the phonons and dielectric response of the crystalline material. Using an effective medium method, the package calculates the internal electric field arising from different particle morphologies and calculates the resulting shift in absorption frequency and intensity arising from the coupling between a phonon and the internal field. The theory of the approach is described, followed by a description of the implementation within PDielec. Finally, a section providing several examples of its application is given. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Synchrotron far-infrared spectroscopy of corroded steel surfaces using a variable angle of incidence

Far-infrared spectroscopy, using a synchrotron source, has been used to study carbon steel corroded in CO2-saturated brine in the presence and absence of the corrosion inhibitor 2-mercaptopyrimidine (MPY), which allowed the steel surface roughness to be modified. The effect of the angle of incidence (θi, 30-80°) on the band intensity and observed bands of the spectra from these surfaces has been determined. For the MPY-treated steel (low surface roughness) the highest band intensity is observed at high θi (80°) and different bands were observed at different θi. In contrast, for the MPY-free steel (high surface roughness) the highest band intensity is observed at low θi (30°) and spectral content changes were not observed. The results are explained in terms of the roughness of the MPY-treated and MPY-free steels, and their effect on the level of diffusely reflected light of the incident infrared beam.

Spectroscopic signature of the superparamagnetic transition and surface spin disorder in CoFe2O4 nanoparticles

Phonons are exquisitely sensitive to finite length scale effects in a wide variety of materials. To investigate confinement in combination with strong magnetoelastic interactions, we measured the infrared vibrational properties of CoFe(2)O(4) nanoparticles and compared our results to trends in the coercivity over the same size range and to the response of the bulk material. Remarkably, the spectroscopic response is sensitive to the size-induced crossover to the superparamagnetic state, which occurs between 7 and 10 nm. A spin-phonon coupling analysis supports the core-shell model. Moreover, it provides an estimate of the magnetically disordered shell thickness, which increases from 0.4 nm in the 14 nm particles to 0.8 nm in the 5 nm particles, demonstrating that the associated local lattice distortions take place on the length scale of the unit cell. These findings are important for understanding finite length scale effects in this and other magnetic oxides where magnetoelastic interactions are important.

First-order metal-insulator transition and infrared identification of shape-controlled magnetite nanocrystals

The first-order metal-insulator transition (MIT) in magnetite has been known for a long time but is still controversial in its nature. In this study, well-defined magnetite nanocrystals (NCs) with controllable size, shape and terminated surface are first employed to elucidate this important issue, and new discoveries such as a highly suppressed phase transition temperature are identified by monitoring the variable-temperature electric resistance and infrared spectroscopy. Significantly, by carefully comparing the infrared vibrational bands of the as-prepared magnetite NCs with octahedral and cubic shapes, respectively, we found that these two forms of magnetite NCs exhibited different transmittance changes and frequency shifts of the infrared characteristics, presumably due to the differences in the lattice distortions on the corresponding {001} and {111} terminal surfaces. This result produced evidence in support of the charge ordering of Fe atoms along the low dimensionality at octahedral B sites undergoing the MIT. Taken together, infrared identification was proposed to be an available characterization strategy for MIT, which can reflect more information on the elusive lattice distortion of crystallographic structure or exposed surfaces.

Understanding of the finite size effects on lattice vibrations and electronic transitions of nano alpha-Fe2O3

Alpha-Fe(2)O(3) nanocrystals with controlled diameters ranging from 10 to 63 nm were successfully prepared. The finite size effects in alpha-Fe(2)O(3) nanocrystals were probed by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, UV-visible spectrum, and magnetization measurements. With a size reduction, alpha-Fe(2)O(3) nanocrystals showed a lattice expansion and an enlarged axial ratio of c/a that is in apparent contradiction to the previous conjecture of high lattice symmetry for alpha-Fe(2)O(3) nanocrystals at small sizes. The surface terminations of alpha-Fe(2)O(3) nanocrystals were found to be highly hydrated with a size dependence that surprisingly follows the surface hydration chemistry of anatase TiO2 nanocrystals reported recently by us. The lattice vibrations, electronic transitions, and magnetic properties of alpha-Fe(2)O(3) nanocrystals were significantly modified by surface hydration and lattice expansion. The finite size effects that occurred in alpha-Fe(2)O(3) nanocrystals at small sizes were first found to give a red shift in frequencies of perpendicular mode at 540 cm(-1), a blue shift in the electronic transition of double exciton process in visible region, and a significant decrease in the coercive force.

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition

Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (alpha-Fe(2)O(3)) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (gamma-Fe(2)O(3))-like defects in the near surface regions, to which a vibrational mode at 690 cm(-1), active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced alpha-Fe(2)O(3)<-->gamma-Fe(2)O(3) phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates-concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the alpha-->gamma phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the E(u) band at about 460 cm(-1) and the spinel band at 690 cm(-1), instead of the A(2u)/E(u) band at about 550 cm(-1) used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.

Fabrication, characterization, and formation mechanism of hollow spindle-like hematite via a solvothermal process

As a novel hollow nanostructure, hollow spindle-like hematite with uniform size and morphology was solvothermally synthesized. These hollow polycrystalline particles with the length of 220-300 nm, the width of 70-100 nm, and the wall thickness of ca. 18 nm were characterized by TEM, FE-SEM, XRD, FT-IR, TGA, Mössbauer spectrum, and XPS methods. It was found that these hollow-structured particles were transformed from their original solid spindle particles. During the hollow structure formation process, the interiors of solid particles were preferentially dissolved while the retained exteriors were protected by coordinated sulfate ions. The formation mechanism was proposed as a coordination-assisted dissolution process occurred in a reverse microemulsion system.

Uniform nanosized goethite particles obtained by aerial oxidation in the FeSO4-Na2CO3 system

Pure goethite particles in the nanometer size range (from approximately 200 to approximately 80 nm) with an elongated shape (axial ratio from approximately 5 to approximately 8) useful as iron precursors for magnetic recording have been prepared by oxidation of the suspensions resulting from the addition of sodium carbonate to Fe(II) sulfate aqueous solutions under a restrictive set of experimental conditions (Fe(II) concentration, carbonate/Fe(II) mole ratio, temperature, and air flow rate). In all cases, the goethite particles were formed by a dissolution-recrystallization mechanism through an intermediate green-rust phase. The particle size was determined by the carbonate/Fe(II) ratio (which controls the formation pH), the FeSO(4) concentration, and the air flow rate. The smallest particles (length 80 nm) were obtained for a high carbonate/Fe(II) mole ratio (>/=3), a low Fe(II) concentration (0.075 mol dm(-3)), and an air flow rate of 2 dm(3) min(-1). The goethite particles were also characterized by the electron diffraction and high-resolution TEM finding that they were monocrystalline, having the crystalline c axis parallel to the longest particle dimension.

Homogeneous Precipitation of Uniform alpha-Fe2O3 Particles from Iron Salts Solutions in the Presence of Urea

Uniform alpha-Fe2O3 particles within the nanometer range (100-300 nm) have been obtained by precipitation of iron (III) perchlorate in the presence of urea. Different morphology, from spheres to ellipsoidal particles with axial ratio up to approximately 10, was obtained by adding to the initial solution increasing amounts of phosphate anions up to 7 x 10(-3) M. The main targets of this work are the reduction in particle size and precipitation time and the increase of the particles axial ratio, keeping a narrow particle size distribution, in comparison to other methods previously developed to obtain homogenous alpha-Fe2O3 particles. A detailed analysis of the reaction products and a systematic study of the influence of the different precipitation conditions on the characteristics of the resulting particles have been carried out. Finally, some information on the formation mechanism of the ellipsoidal hematite particles in the iron (III) salt-urea-phosphate system is also given. Copyright 1999 Academic Press.