Raman spectroscopic study of the uranyl tellurite mineral moctezumite PbUO2(TeO3)2

The uranyl tellurite mineral moctezumite, Pb(UO₂)(TeO₃)₂, was studied by Raman spectroscopy and complemented with infrared spectroscopy. The presence of the stretching and bending vibrations of uranyl (UO₂)²⁺ and tellurite (TeO₃)²⁻ ions was inferred, and the observed bands were assigned to uranyl and tellurite units vibrations. U O bond lengths calculated from the spectra with two empirical relations are close to those inferred from the X‐ray single‐crystal structure of moctezumite. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman scattering of L‐tryptophan enhanced by surface plasmon of silver nanoparticles: vibrational assignment and structural determination

Vibrational bands of L ‐tryptophan which was adsorbed on Ag nanoparticles (∼10 nm in diameter) have been investigated in the spectral range of 200–1700 cm⁻¹ using surface‐enhanced Raman scattering (SERS) spectroscopy. Compared with the normal Raman scattering (NRS) of L ‐tryptophan in either 0.5 M aqueous solution (NRS‐AS) or solid powder (NRS‐SP), the intensified signals by SERS have made the SERS investigation at a lower molecular concentration (5 × 10⁻⁴ M ) possible. Ab initio calculations at the B3LYP/6‐311G level have been carried out to predict the optimal structure and vibrational wavenumbers for the zwitterionic form of L ‐tryptophan. Facilitated with the theoretical prediction, the observed vibrational modes of L ‐tryptophan in the NRS‐AS, NRS‐SP, and SERS spectra have been analyzed. In the spectroscopic observations, there are no significant changes for the vibrational bands of the indole ring in either NRS‐AS, NRS‐SP, or SERS. In contrast, spectral intensities involving the vibrations of carboxylate and amino groups are weak in NRS‐AS and NRS‐SP, but strong in SERS. The intensity enhancement in the SERS spectrum can reach 10³–10⁴‐fold magnification. On the basis of spectroscopic analysis, the carboxylate and amino groups of L ‐tryptophan are determined to be the preferential terminal groups to attach onto the surfaces of Ag nanoparticles in the SERS measurement. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman and mid‐IR spectroscopic study of the magnesium carbonate minerals—brugnatellite and coalingite

Two hydrated hydroxy magnesium carbonate minerals brugnatellite and coalingite with a hydrotalcite‐like structure were studied by Raman spectroscopy. Intense bands are observed at 1094 cm⁻¹ for brugnatellite and at 1093 cm⁻¹ for coalingite attributed to the CO₃²⁻ν₁ symmetric stretching mode. Additional low intensity bands are observed at 1064 cm⁻¹. The existence of two symmetric stretching modes is accounted for in terms of different anion structural arrangements. Very low intensity bands at 1377 and 1451 cm⁻¹ are observed for brugnatellite, and the Raman spectrum of coalingite displays two bands at 1420 and 1465 cm⁻¹ attributed to the (CO₃)²⁻ν₃ antisymmetric stretching modes. Very low intensity bands at 792 cm⁻¹ for brugnatellite and 797 cm⁻¹ for coalingite are assigned to the CO₃²⁻ out‐of‐plane bend (ν₂). X‐ray diffraction studies by other researchers have shown that these minerals are disordered. This is reflected in the difficulty of obtaining Raman spectra of reasonable quality and explains why the Raman spectra of these minerals have not been previously or sufficiently described. A comparison is made with the Raman spectra of other hydrated magnesium carbonate minerals. Copyright © 2008 John Wiley & Sons, Ltd.     

Brillouin scattering of light by spin waves in ferromagnetic nanorods

We report the investigations of spin wave modes of arrays of Ni and Co nanorods using Brillouin light scattering. We have revealed the significant influence of spin wave modes along the nanorod axis in contrast to infinite magnetic nanowires. Unusual optical properties featuring an inverted Stokes/anti-Stokes asymmetry of the Brillouin scattering spectra have been observed. The spectrum of spin wave modes in the nanorod array has been calculated and compared with the experiment. Experimental observations are explained in terms of a combined numerical–analytical approach taking into account both the low aspect ratio of individual magnetic nanorods and dipolar magnetic coupling between the nanorods in the array. The optical studies of spin-wave modes in nanorod metamaterials with low aspect ratio nanorods have revealed new magnetic and magneto-optical properties compared to continuous magnetic films or infinite magnetic nanowires. Such magnetic artificial materials are important class of active metamaterials needed for prospective data storage and signal processing applications.     

Dielectric behavior of nano-structured and bulk Li Ni Zn ferrite samples

The ac conductivity and dielectric properties of spinel ferrite nanoparticles of Li_0.1(Ni_1−xZn_x)_0.8Fe_2.1O₄ (x=0.0–1.0) prepared by the chemical co-precipitation method were investigated as functions of frequency and temperature by using a complex impedance technique. Parts of the precipitated powders were pressed into a disk-shape and were sintered at 1473 K for 2 h to increase the particle size to the bulk scale (dimensions >100 nm). The ac conductivity of the samples increases with increasing temperature, ensuring the semiconducting behavior of both nano and bulk samples, in agreement with the Koops model to describe heterogeneous structures. The significant decrease in ac conductivity σ′_ac, dielectric constant, and dielectric loss of the as-prepared nanosamples compared to their bulk counterparts is correlated to the small size of the grain compared to the grain boundary size. This might be useful for many applications requiring the reduction of eddy current effects.     

Structural and electrical studies of Ba5RTi3V7O30 (R=Ho,Gd, La) compounds

The ferroelectric ceramics Ba₅RTi₃V₇O₃₀ (R=Ho, Gd, La) have been synthesized by solid-state reaction technique. Preliminary X-ray structural analysis confirmed a single-phase formation of the compound in orthorhombic structure. Surface morphology of the compounds was studied by scanning electron microscopy (SEM). Detailed studies of electrical properties (i.e., dielectric constant, loss tangent, ac conductivity) as a function of temperature (RT-773 K) and at four different frequencies (1 kHz, 10 kHz, 100 kHz and 1 MHz) show ferroelectric–paraelectric phase transition of the compounds of diffuse-type. The activation energy has been evaluated from ac conductivity following Arrhenius equation. The conductivity pattern shows that it is strongly frequency dependent and obeys Jonscher's power relation.     

Molecular dynamics simulation of the (0 0 0 1) α-Al2O3 and α-Cr2O3 surfaces

A simple, rigid pair-potential model is applied to investigate the dynamics of the (0 0 0 1) α-Al₂O₃ and α-Cr₂O₃ surfaces using the molecular dynamics technique. The simulations employ a two-stage equilibration process: in the first stage the simulation-cell size is determined via the constant-stress ensemble, and in the second stage the equilibration of the size-corrected simulation cell is continued in the canonical ensemble. The thermal expansion coefficients of bulk alumina and chromia are evaluated as a function of temperature. Furthermore, the surface relaxation and mean-square displacement of the atoms versus depth into the slab are calculated, and their behaviour in the surface region analysed in detail. The calculations show that even moderate temperatures (∼400 °C) give rise to displacements of the atoms at the surface which are similar to the lattice mismatch between α-alumina and chromia. This will help in the initial nucleation stage during thin film growth, and thus facilitate the deposition of α-Al₂O₃ on (0 0 0 1) α-Cr₂O₃ templates.     

Thermal and photochemical reactivity of oxygen atoms on gold nanocluster surfaces

Reduction of oxidized gold nanoclusters by exposures to foreign gases and irradiation of UV photons has been investigated using X-ray photoelectron spectroscopy. Gold nanoclusters with narrow size distributions protected by alkanethiolate ligands were deposited on a TiO₂(1 1 0) surface with dip coating. Oxygen plasma etching was used for removal of alkanethiolate ligands and oxidization of gold clusters. The oxidized gold clusters were exposed to CO, C₂H₂, C₂H₄, H₂, and hydrogen atoms. Although, C₂H₄ and H₂ did not show any indications of reduction of oxidized gold clusters, CO, C₂H₂, and hydrogen atoms reduced the oxides on gold cluster surfaces. Among them, hydrogen atoms were most effective for reduction. Irradiation of UV photons around 400 nm could also reduce the oxidized gold clusters. The photochemical reduction mechanism was proposed as follows. The photo-reduction was initiated by electronic excitation of gold clusters and oxygen atoms activated reacted with carbon atoms at the surfaces of gold clusters. Carbon species were likely absorbed in gold clusters or remained at the boundaries between gold clusters when gold clusters agglomerated during oxygen plasma exposures. As the photochemical reduction progressed, carbon atoms segregated to the surfaces of gold clusters.     

Adsorption kinetics of alkanes on TiO2 nanotubesarray - structure-activity relationship

Presented are thermal desorption spectroscopy (TDS) measurements of iso-/n-butane adsorption on a variety of TiO₂ nanotubes (TiNTs) samples which are characterized by different crystal structures. The results are compared with a prior study on anatase(0 0 1) thin films grown on SrTiO₃(0 0 1). A distinct kinetic structure-activity relationship was present, i.e., the binding energies of the alkanes depend on the polymorph (anatase vs. mixed anatase/rutile) of TiO₂. A direct-fitting procedure of the TDS data has been applied to extract the kinetics parameters. The binding energies in the limit of zero coverage decrease as anatase thin film > amorphous-TiNTs ∼ polycrystalline anatase TiNTs > polycrystalline mixed anatase/rutile TiNTs.     

Thermodynamics of nitrogen adsorption on the zeolite H-FER

Adsorption (at a low temperature) of nitrogen on the protonic zeolite H-FER results in hydrogen bonding of the adsorbed N₂ molecules with the zeolite Si(OH)Al Brønsted acid groups. This hydrogen bonding interaction leads to activation, in the IR, of the fundamental NN stretching mode, which appears at 2331 cm⁻¹. From the infrared spectra taken over a temperature range, while simultaneously recording integrated IR absorbance, temperature and nitrogen equilibrium pressure, the thermodynamics of the adsorption process was studied. The standard adsorption enthalpy and entropy resulted to be ΔH° = −20(±1) kJ mol⁻¹ and ΔS° = −131(±10) J mol⁻¹ K⁻¹, respectively.