Mechanics of Deformation under Traction and Friction of a Micrometric Monolithic MoS2 Particle in Comparison with those of an Agglomerate of Nanometric MoS2 Particles

Tribology of small inorganic nanoparticles in suspension in a liquid lubricant is often impaired because these particles agglomerate even when organic dispersants are used. In this paper we use lateral force microscopy to study the deformation mechanism and dissipation under traction of two extreme configurations (1) a large MoS2 particle (similar to 20 mu m width) of about 1 mu m height and (2) an agglomerate (similar to 20 mu m width), constituting 50 nm MoS2 crystallites, of about 1 mu m height. The agglomerate records a friction coefficient which is about 5-7 times that of monolithic particle. The paper examines the mechanisms of material removal for both the particles using continuum modeling and microscopy and infers that while the agglomerate response to traction can be accounted for by the bulk mechanical properties of the material, intralayer and interlayer basal planar slips determine the friction and wear of monolithic particles. The results provide a rationale for selection of layered particles, for suspension in liquid lubricants.

Growth of Nanometric Gold Particles in Solution Phase

The time evolution of colloidal gold particles in the nanometric regime has been investigated by employing electron microscopy and electronic absorption spectroscopy. The particle size distributions are essentially Gaussian and show the same time dependence for both the mean and the standard deviation, enabling us to obtain a time-independent universal curve for the particle size. Temperature dependent studies show the growth to be an activated process with a barrier of about 18 kJ mol(-1). We present a phenomenological equation for the evolution of particle size and suggest that the growth process is stochastic.

Indium Nitride Nanometric-Objects on c-Sapphire Grown by Plasma-Assisted Molecular Beam Epitaxy

The indium nitride (InN)-based nanometric-objects were grown directly on a c-sapphire substrate by using plasma-assisted molecular beam epitaxy (PAMBE) at different substrate temperatures. High resolution X-ray diffraction (HRXRD) reveals the InN (0002) reflection and full width at half maximum (FWHM) found to be decreased with increasing the growth temperature. The size, height and density of the grown nanometric-objects studied by scanning electron microscopy (SEM) has remarkable differences, evidencing the decisive role of substrate temperature. Photoluminescence (PL) studies revealed that the emission energy is shifted towards the higher side from the bulk value, i.e., a blue shift in the PL spectra was observed. The temperature dependence of the PL peak position shows an ``S-shaped'' emission energy shift, which can be attributed to the localization of carriers in the nanometric-objects.

Photoluminescence of Sr(2-x)Ln(x)CeO(4+x/2) (Ln = Eu, Sm or Yb) prepared by a wet chemical method

A wet chemical route is developed for the preparation of Sr2CeO4 denoted the carbonate-gel composite technique. This involves the coprecipitation of strontium as fine particles of carbonates within hydrated gels of ceria (CeO2.xH(2)O, 40CeO2.xH(2)O dehydroxylation is followed by the reaction with SrCO3 to form Sr2CeO4 with complete phase purity. Doping of other rare-earths is carried out at the co-precipitation stage. The photoluminescence (PL) observed for Sr2CeO4 originates from the Ce4+-O2- charge-transfer (CT) transition resulting from the interaction of Ce4+ ion with the neighboring oxide ions. The effect of next-nearest-neighbor (NNN) environment on the Ce4+-O2- CT emission is studied by doping with Eu3+, Sm3+ or Yb3+ which in turn, have unique charge-transfer associated energy levels in the excited states in oxides. Efficient energy transfer occurs from Ce4+-O2- CT state to trivalent lanthanide ions (Ln(3+)) if the latter has CT excited states, leading to sensitizer-activator relation, through non-resonance process involving exchange interaction. Yb3+-substituted Sr2CeO4 does not show any line emission because the energy of Yb3+-O2- CT level is higher than that of the Ce4+-O2- CT level. Sr2-xEuxCeO4+x/2 shows white emission at xless than or equal to0.02 because of the dominant intensities of D-5(2)-F-7(0-3) transitions in blue-green region whereas the intensities of D-5(0)-F-7(0-3) transitions in orange-red regions dominate at concentrations xgreater than or equal to0.03 and give red emission. The appearance of all the emissions from D-5(2), D-5(1) and D-5(0) excited states to the F-7(0-3) ground multiplets of Eu3+ is explained on the basis of the shift from the hypersensitive electric-dipole to magnetic-dipole related transitions with the variation in site symmetry with increasing concentration of Eu3+. White emission of Sr2-x SmxCeO4+x/2 at xless than or equal to0.02 is due the co-existence of Ce4+-O2- CT emission and (4)G(4)(5/2)-H-6(J) Sm3+ transitions whereas only the Sm3+ red emission prevails for xgreater than or equal to0.03. The above unique changes in PL emission features are explained in terms of the changes in NNN environments of Ce4+. Quenching of Ce4+-O2- CT emission by other Ln(3+) is due to the ground state crossover arising out of the NNN interactions.

Ru(4+) ion in CeO(2) (Ce(0.95)Ru(0.05)O(2-delta)): A non-deactivating, non-platinum catalyst for water gas shift reaction

Hydrogen is a clean energy carrier and highest energy density fuel. Water gas shift (WGS) reaction is an important reaction to generate hydrogen from steam reforming of CO. A new WGS catalyst, Ce(1-x)Ru(x)O(2-delta) (0 <= x <= 0.1) was prepared by hydrothermal method using melamine as a complexing agent. The Catalyst does not require any pre-treatment. Among the several compositions prepared and tested, Ce(0.95)Ru(0.05)O(2-delta) (5% Ru(4+) ion substituted in CeO(2)) showed very high WGS activity in terms of high conversion rate (20.5 mu mol.g(-1).s(-1) at 275 degrees C) and low activation energy (12.1 kcal/mol). Over 99% conversion of CO to CO(2) by H(2)O is observed with 100% H(2) selectivity at >= 275 degrees C. In presence of externally fed CO(2) and H(2) also, complete conversion of CO to CO(2) was observed with 100% H(2) selectivity in the temperature range of 305-385 degrees C. Catalyst does not deactivate in long duration on/off WGS reaction cycle due to absence of surface carbon and carbonate formation and sintering of Ru. Due to highly acidic nature of Ru(4+) ion, surface carbonate formation is also inhibited. Sintering of noble metal (Ru) is avoided in this catalyst because Ru remains in Ru(4+) ionic state in the Ce(1-x)Ru(x)O(2-delta) catalyst.

Electrocatalysis and redox behavior of Pt(2+) ion in CeO(2) and Ce(0.85)Ti(0.15)O(2): XPS evidence of participation of lattice oxygen for high activity

Electronic states of CeO(2), Ce(1 -aEuro parts per thousand x) Pt (x) O(2 -aEuro parts per thousand delta) , and Ce(1 -aEuro parts per thousand x -aEuro parts per thousand y) Ti (y) Pt (x) O(2 -aEuro parts per thousand delta) electrodes have been investigated by X-ray photoelectron spectroscopy as a function of applied potential for oxygen evolution and formic acid and methanol oxidation. Ionically dispersed platinum in Ce(1 -aEuro parts per thousand x) Pt (x) O(2 -aEuro parts per thousand delta) and Ce(1 -aEuro parts per thousand x -aEuro parts per thousand y) Ti (y) Pt (x) O(2 -aEuro parts per thousand delta) is active toward these reactions compared with CeO(2) alone. Higher electrocatalytic activity of Pt(2+) ions in CeO(2) and Ce(1 -aEuro parts per thousand x) Ti (x) O(2) compared with the same amount of Pt(0) in Pt/C is attributed to Pt(2+) ion interaction with CeO(2) and Ce(1 -aEuro parts per thousand x) Ti (x) O(2) to activate the lattice oxygen of the support oxide. Utilization of this activated lattice oxygen has been demonstrated in terms of high oxygen evolution in acid medium with these catalysts. Further, ionic platinum in CeO(2) and Ce(1 -aEuro parts per thousand x) Ti (x) O(2) does not suffer from CO poisoning effect unlike Pt(0) in Pt/C due to participation of activated lattice oxygen which oxidizes the intermediate CO to CO(2). Hence, higher activity is observed toward formic acid and methanol oxidation compared with same amount of Pt metal in Pt/C.

CO oxidation by CeO(2)-Al(2)O(3)-CeAlO(3) hybrid oxides

A modified solution combustion technique was successfully used to synthesize sub-10 nm crystallites of hybrid CeO(2)-Al(2)O(3)-CeAlO(3). The fuel in the solution combustion was tuned to obtain mixed oxides and solid solutions of the compound. The compounds were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. XRD and TEM analysis showed the substitution of Al(3+) ions in the CeO(2) matrix when a combination of glycine, urea, hexamine and oxalyl dihydrazide was used as fuel for the synthesis. The compounds showed high activity for CO oxidation and the activity of the compounds was dependent upon the composition of the oxide.

Electrochemical studies on Zn/nano-CeO 2 electrodeposited composite coatings

The Zn-CeO 2 composite coatings through electrodeposition technique were successfully fabricated on mild steel substrate. As a comparison pure zinc coating was also prepared. The concentration of CeO 2 nanoparticles was varied in the electrolytic bath and the composites were electrodeposited both in the presence and absence of cetyltriammonium bromide (CTAB). The performance of the CeO 2 nanoparticles towards the deposition, crystal structure, texture, surface morphology and electrochemical corrosion behavior was studied. For characterizations of the electrodeposits, the techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) were used. Both the additives ceria and surfactant polarize the reduction processes and thus influence the deposition process, surface nature and the electrochemical properties. The electrochemical experiments like potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) studies carried out in 3.5 wt. NaCl solution explicit higher corrosion resistance by CeO 2 incorporated coating in the presence of surfactant. © 2012 Elsevier B.V. All rights reserved.

Oriented nanometric aggregates of partially inverted zinc ferrite: One-step processing and tunable high-frequency magnetic properties

In this work, it is demonstrated that the in situ growth of oriented nanometric aggregates of partially inverted zinc ferrite can potentially pave a way to alter and tune magnetocrystalline anisotropy that, in turn, dictates ferromagnetic resonance frequency (f(FMR)) by inducing strain due to aggregation. Furthermore, the influence of interparticle interaction on magnetic properties of the aggregates is investigated. Mono-dispersed zinc ferrite nanoparticles (<5 nm) with various degrees of aggregation were prepared through decomposition of metal-organic compounds of zinc (II) and iron (III) in an alcoholic solution under controlled microwave irradiation, below 200 degrees C. The nanocrystallites were found to possess high degree of inversion (>0.5). With increasing order of aggregation in the samples, saturation magnetization (at 5 K) is found to decrease from 38 emu/g to 24 emu/g, while coercivity is found to increase gradually by up to 100% (525 Oe to 1040 Oe). Anisotropy-mediated shift of f(FMR) has also been measured and discussed. In essence, the result exhibits an easy way to control the magnetic characteristics of nanocrystalline zinc ferrite, boosted with significant degree of inversion, at GHz frequencies. (C) 2015 AIP Publishing LLC.

Structural origin of set-reset processes in Ge15Te83Si2 glass investigated using in situ Raman scattering and transmission electron microscopy

We report here that the structural origin of an easily reversible Ge15Te83Si2 glass can be a promising candidate for phase change random access memories. In situ Raman scattering studies on Ge15Te83Si2 sample, undertaken during the amorphous set and reset processes, indicate that the degree of disorder in the glass is reduced from off to set state. It is also found that the local structure of the sample under reset condition is similar to that in the amorphous off state. Electron microscopic studies on switched samples indicate the formation of nanometric sized particles of c-SiTe2 structure. ©2009 American Institute of Physics

Ce0.78Sn0.2Pt0.02O2-delta: A new non-deactivating catalyst for hydrogen production via water-gas shift reaction

We demonstrate the activity of Ce0.78Sn0.2Pt0.02O2-delta, a new catalyst, towards water-gas shift (WGS) reaction. Over 99.5% CO conversion to H-2 is observed at 300 +/- 25 degrees C. Based on different characterization techniques we found that the present catalyst is resistant to deactivation due to carbonate formation and sintering of Pt on the surface when subjected to longer duration of reaction conditions. The catalyst does not require any pre-treatment or activation between start-up/shut-down reaction operations. Formation of side products such as methane, methanol, formaldehyde, coke etc. was not observed under the WGS reaction conditions indicating the high selectivity of the catalyst for H-2. Temperature programmed reduction of the catalyst in hydrogen (H-2-TPR) shows reversible reduction of Ce4+ to Ce3+, Sn4+ to Sn2+ and Pt4+ to Pt-0 oxidation state with oxygen storage capacity (OSC) of 3500 mu mol g(-1) at 80 degrees C. Such high value of OSC indicates the presence of highly activated lattice oxygen. CO oxidation in presence of stoichiometric O-2 shows 100% conversion to CO2 at room temperature. The catalyst also exhibits 100% selectivity for CO2 at room temperature towards preferential oxidation (PROX) of residual CO in presence of excess hydrogen in the feed. (C) 2010 Elsevier B.V. All rights reserved.

Microwave Assisted Synthesis of Nanostructured Titanium Dioxide with High Photocatalytic Activity

TiO2 (anatase) was synthesized using a microwave-irradiation-assisted chemical method. The reaction conditions were varied to obtain unique nanostructures of TiO2 comprising nanometric spheres giving the materials a very porous morphology. The oxide was characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). The specific surface area and porosity were quantified by the BET method, and the degradation of dyes was carried out using these materials. The photocatalytic activity of the nanometric TiO2 was significantly higher than that of commercially available TiO2 (Degussa P25) for the degradation of the dyes.

Structure evolution and phase change in Ag-5.1 at.% Bi alloy during mechanical alloying

In this paper we explore the enhancement of solubility in a mechanically driven immiscible system experimentally using a mixture of Ag and Bi powders corresponding to a composition of Ag-5.1 at.% Bi. Increase in solubility can be correlated with the combination of sizes of both Ag and Bi at the nanometric scale. It is shown that complete solid solution of Ag-5.1 at.% Bi forms when the respective sizes of :Bi and Ag exceed 13 and 8 nm respectively. We have carried out a thermodynamic analysis of the size- and strain-dependent free energy landscape and compared the results to the initial mixture of microsized particles to rationalize the evolution of Ag solid solution. The agreement indicates that the emerging driving force for the formation of solid solution is primarily due to size reduction rather than the enhanced kinetics of mass transport due to mechanical driving. (c) 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

Nanocomposites by rapid solidification route

Nanometric aggregates of solids can be classified into two types, nanograined or nanophased materials and nanocomposites. In the present paper after a brief review including the relation between size and boundary fraction, the basic principles that can be utilized to synthesize these materials from liquid route has been discussed. We shall present examples to show that with proper choice of systems and conditions it is possible to obtain nanocomposites in systems showing clustering tendencies in liquid as well as the systems exhibiting ordering tendencies leading to compound formation