Igor N. Leontyev

Size dependence of the lattice parameters of carbon supported platinum nanoparticles: X-ray diffraction analysis and theoretical considerations

Carbon supported Pt nanoparticles with diameters ranging from 2 to 28 nm have been studied using X-ray diffraction. The unit cell parameter of synthesized Pt/C nanoparticles is always lower than that of bulk Pt. By decreasing the average particle size D to approximately 2 nm, the unit cell parameter a nonlinearly decreases by about 0.03 A which corresponds to a variation of 0.7% in comparison to bulk Pt, and the size effect is predominant for sizes ranging from 2 to 10 nm. The dependence a(1/D) is approximated well using a straight line with a slope of −0.0555 ± 0.0067 nm−1 and an intercept of −3.9230 ± 0.0017 A. For interpreting the obtained experimental dependence of the unit cell parameter of Pt/C nanoparticles, four different theoretical approaches such as the thermal vacancy mechanism, continuous-medium model, Laplace pressure, and bond order–length–strength correlation mechanism, were used. Comparison of the calculated dependencies, based on the above models, with the experimental ones, shows that the continuous-medium model agrees best with the experimentally found unit cell parameter dependence of our carbon supported Pt nanoparticles.

The compressibility of nanocrystalline Pt

High-pressure behavior of carbon supported Pt nanoparticles (Pt/C) with an average particle size of 10.6u2009nm was investigated by in situ high-pressure synchrotron radiation x-ray diffraction up to 14u2009GPa at ambient temperature. Our results show that the compressibility of Pt/C nanoparticles decreases substantially as the particle size decreases. An interpretation based upon the available mechanisms of structural compliance in nanoscale vs bulk materials was proposed.

Ferroelectric BaTiO3 single crystal under superstrong electric fields up to 55 MV/m: A comparative experimental and theoretical study

We investigated the effect of huge electric field up to E=55u2002MV/m on tetragonal ferroelectric phase of BaTiO3 single crystal. Both lattice parameters and the dielectric susceptibility along the polar axis were determined. The induced polarization shows a continuous increase up to 20% of the spontaneous polarization. The longitudinal d33 and transverse d31 piezoelectric coefficients were also determined. The experimental data are described by a phenomenological approach. Interestingly, an intermediate critical electric field value of E∼25u2002MV/m was evidenced suggesting that the physics below and above this value are different.

Electrochemically Synthesized Pt/Al2O3 Oxidation Catalysts

Pt/γ–Al2O3 catalysts made by fast and simple electrochemical dispersion method were characterized using X-ray absorption spectroscopy, CO chemisorption, transmission electron microscopy and X-ray diffraction, and compared with an impregnated catalyst with respect to oxidation of CO and NO. A combination of techniques revealed average particle sizes of 3–4xa0nm for 0.81–3.8xa0wt% Pt/γ–Al2O3 catalysts. Electrochemically prepared materials demonstrated catalytic activity comparable to that of conventional impregnated catalyst and reasonable stability.Graphical Abstract

Influence of Carbon Support on Catalytic Layer Performance of Proton Exchange Membrane Fuel Cells

AbstractPt-based catalysts supported onto various carbon nanotubes (CNTs) and carbon black Vulcan XC-72 have been studied in this work. The samples have been prepared via electrochemical dispersion technique. The carbon nanotubes exhibit the mesoporous morphology with the pore size of 5—30xa0nm, while Vulcan XC-72 support exhibits a microporous structure. The agglomeration of Pt particles (5—7xa0nm) is found to be more extended at the carbon nanotubes compared to the carbon black. The examination of the synthesized catalysts in a catalytic membrane electrode assembly layer reveals that CNTs favor a power density higher than in the carbon black support owing to a more suitable porous structure of the catalyst layer. Moreover, the maximum characteristics of membrane electrode assembly are obtained for few-walled carbon nanotubes with a broad carbon having size distribution.n Graphical Abstract

Domain structures and correlated out-of-plane and in-plane polarization reorientations in Pb(Zr0.96Ti0.04)O3 single crystal via piezoresponse force microscopy

Pb(Zr1-xTix)O3 single crystal with a low titanium content (x = 4%) was studied by the piezoresponse force microscopy (PFM) and X-ray diffraction (XRD). The XRD studies showed that the crystal faces are orthogonal to the principal cubic axes and confirmed the existence of an intermediate phase between the high-temperature paraelectric (PE) phase and the low-temperature antiferroelectric (AFE) one. A significant temperature hysteresis of phase transitions was observed by the XRD: On heating, the AFE state transforms into the intermediate one at about 373 K and the PE phase appears at 508 K, whereas on cooling the intermediate phase forms at 503 K and persists down to at least 313 K. The PFM investigation was focused on the intermediate phase and involved measurements of both out-of-plane and in-plane electromechanical responses of the (001)-oriented crystal face. The PFM images revealed the presence of polarization patterns switchable by an applied electric field, which confirms the ferroelectric character ...

Electrochemical Synthesis and Photocatalytic Activity of Differently Shaped CuOx Particles

CuOx powders with diff erently shaped particles were firstly prepared via an electrochemical method by oxidation and dispersion of copper electrodes in an electrolyte solution under pulse alternating current (PAC). By means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) the current density is found to have an influence on the morphology and composition of CuOx particles. Photocatalytic efficiency of CuOx towards methyl orange (MO) degradation under visible light was investigated. The prepared polyhedral particles show the best photocatalytic activity of 81 % towards MO comparing to octahedral and spherical particles with 70 and 61 %, respectively.

Thin ferroelectric Nd-doped BiFeO3 films with orthorhombic structure

X-ray diffraction and Raman spectroscopy of epitaxial Nd-doped bismuth ferrite films on MgO substrates reveal their orthorhombic symmetry Fmm2 (a = 7.914 Å, b = 7.913 Å, and c = 7.937 Å).

Stress alteration in heteroepitaxial (Ba,Sr)TiO3/(0 0 1) MgO thin films via growth mechanism

We have studied stress effects in heteroepitaxial (Ba0.7Sr0.3)TiO3 thin films deposited by rf sputtering on (0 0 1) MgO substrates. By varying the deposition conditions, three-dimensional (3D) nucleation growth and layer-by-layer growth mechanisms were realized. X-ray diffraction (XRD) measurements revealed that tetragonal distortion and Curie temperature (Tc) depend on the growth mechanism. The frequency of the E(TO) soft mode in the polarized Raman spectra correlated with the value of two-dimensional stress, determined from XRD experiments. Temperature-dependent lattice parameters and Raman spectra revealed an upward shift of the Curie temperature. Two-dimensional compressive stress was found to be larger in the film grown by the 3D nucleation mechanism.