V.A. Lebedev

Origin of long-range orientational pore ordering in anodic films on aluminium

Porous anodic aluminium oxide has a long history of practical application for corrosion protection and coloring. In the last few decades a lot of hi-tech applications of this material have been found owing to the discovery of anodization conditions leading to the formation of highly ordered porous structures with a narrow pore size distribution. Here we show that in-plane orientation of the porous system in anodic films on aluminium is fully determined by the intrinsic crystallographic orientation of the Al substrate. The anisotropy of aluminium oxidation rates on a scalloped metal–oxide interface leads to reorientation of Al spikes in certain directions, which builds up an in-plane orientational order on a macroscopic scale restricted by a crystallite size. This is a unique example of the inheritance of the substrate crystal structure by an amorphous film through a size difference of three orders of magnitude.

Polarization reversal induced by heating-cooling cycles in MgO doped lithium niobate crystals

Polarization reversal during heating-cooling cycles was investigated in MgO doped lithium niobate (MgO:LN) crystal using piezoresponse force microscopy. The essential dependence of the domain structure evolution scenario on the maximal temperature in the cycle has been revealed experimentally. It has been shown that the heating of the engineered domain matrix from room temperature to 85 °C leads to light size reduction of the isolated domains at the matrix edges, whereas the heating to 170 °C leads to essential reduction of the domain size. The opposite strong effect of the domain formation and growth during cooling after pulse heating have been revealed in single domain MgO:LN. The simulation of the time dependence of the pyroelectric field during heating-cooling cycle allowed to reveal the temperature hysteresis and to explain all observed effects taking into account the temperature dependence of the bulk conductivity.

Photocatalytic properties of nanocrystalline TiO2 modified with CuO and WO3

Much attention is paid to studies of the processes occurring in heterogeneous catalysis with the use of nanomaterials, in particular, photocatalysis. Titanium dioxide and zinc oxide are among the most widespread photocatalysts due to their high chemical stability, suitable band gap, and rather high lifetime of nonequilibrium electron–hole pairs. In order to increase the photocatalytic activity (PCA) of these materials, including the case of a visible light action, various approaches are applied, in particular, the development of a composition material with a metal–semiconductor or semiconductor–semiconductor contact. We have chosen semiconductors of n-type (WO3) and p-type (CuO) as components of such composites because of their potential positive influence on the PCA of titanium dioxide at the expense of spatial separation of nonequilibrium charge carriers and, respectively, increase in their lifetime. In order for the obtained composites to be commercially valuable, we have chosen a method of synthesis by modifying the ready TiO2 preparations, including those that are commercially available (Degussa P25). It has been shown that the modification with CuO lowers PCA of titanium dioxide, while the modification with WO3 enhances it by 25%. It has been demonstrated that WO3/TiO2 composite manifests PCA under visible light illumination.

Temperature Effect on the Stability of the Polarized State Created by Local Electric Fields in Strontium Barium Niobate Single Crystals

The stability of ferroelectric domain patterns at the nanoscale has been a topic of much interest for many years. We investigated the relaxation of the polarized state created by application of a local electric field using a conductive tip of a scanning probe microscope for the model uniaxial relaxor system SrxBa1−xNb2O6 (SBN) in its pure and Ce-doped form. The temporal relaxation of the induced PFM contrast was measured at various temperatures. The average value of the induced contrast decreases during heating for all investigated crystals. Below the freezing temperature the induced state remains stable after an initial relaxation. Above the freezing temperature the induced state is unstable and gradually decays with time. The stability of the induced state is strongly affected by the measuring conditions, so continuous scanning results in a faster decay of the poled domain. The obtained effects are attributed to a decrease of the induced polarization and backswitching of the polarized area under the action of the depolarization field.

Non-classical growth of water-redispersible spheroidal gold nanoparticles assisted by leonardite humate

The growth of gold nanoparticles (AuNPs) assisted by humate – a natural hyperbranched polyelectrolyte – was studied using in situ and ex situ techniques. The conditions for formation of almost monodisperse gold nanospheres of 13 ± 3 nm diameter with pronounced plasmonic properties were defined. A striking similarity was found between humate- and citrate-mediated growth of AuNPs: the formation of gold nanospheres involved rapid nucleation of gold seeds, slow growth within the intermediate agglomerates (visible as nanoworms after drying on a TEM grid), and rapid peptization into the final gold nanospheres. Both humate and citrate syntheses produced ultrastable gold sols with pronounced plasmonic properties. The substantial difference was slower kinetics of humate synthesis, 240 min versus 15 min for citrate, and the water redispersible properties of the humate-capped gold nanospheres after freeze drying, which was not seen with the citrate AuNPs. Theoretical calculations revealed a leading role of steric factors in the formation of intermediate aggregates of capped AuNPs at the stage of their slow growth in the case of both citrate and humate. We suggested that it was the polyelectrolyte nature of humate which enabled the water-redispersibility of humate- versus citrate-capped gold nanoparticles.

The microstructure effect on the Au/TiO2 and Ag/TiO2 nanocomposites photocatalytic activity

D. A. Kozlov, V. A. Lebedev, A. Yu. Polyakov, K. M. Khazova, A. V. Garshev Faculty of Materials Science, Lomonosov Moscow State University, 1-73 Leninskiye gory, Moscow, 119991, Russia Faculty of Chemistry, Lomonosov Moscow State University, 1-3 Leninskiye gory, Moscow, 119991, Russia Baikov Institute of Metallurgy and Material Science RAS, 49 Leninskiy prospekt, Moscow, 119334, Russia kozlov@inorg.chem.msu.ru, vasya lebedev@mail.ru, a.yu.polyakov@gmail.com, khazovakm@gmail.com, garshev@inorg.chem.msu.ru

Gold Decoration and Photoresistive Response to Nitrogen Dioxide of WS2 Nanotubes

Composites of WS2 nanotubes (NT-WS2 ) and gold nanoparticles (AuNPs) were prepared using aqueous HAuCl4 solutions and subjected to surface analysis. The obtained materials were jointly characterized by X-ray photoelectron (XPS), Raman scattering (RSS), and ultraviolet photoelectron (UPS) spectroscopies. Optical extinction spectroscopy and electron energy loss spectroscopy in the scanning transmission electron microscopy regime (STEM-EELS) were also employed to study plasmon features of the nanocomposite. It was found that AuNPs deposition is accompanied by a partial oxidative dissolution of WS2 , whereas Au-S interfacial species could be responsible for the tight contact of metal nanoparticles and the disulfide. A remarkable sensitivity of n-type resistance of NT-WS2 and Au-NT-WS2 to the adsorption of NO2 gas was also demonstrated at room temperature using periodical illumination by a 530 nm light-emitting diode. Au-NT-WS2 nanocomposites are found to possess a higher photoresponse and enhanced sensitivity in the 0.25-2.0 ppm range of NO2 concentration, as compared to the pristine NT-WS2 . This behaviour is discussed within the physisorption-charge transfer model to explore sensing properties of the nanocomposites.

Targeted synthesis ultrafine α- and γ-Bi2O3 having different morphologies

Reactions of nitric acid solutions of bismuth with alkalis have been studied. The effect caused by coprecipitation conditions in the presence of additional agents (EG, PEG 400, or PEG 8000) and without on the phase composition and morphology of α- and γ-Bi2O3 has been determined.

Research data supporting the publication: 'Structural effects in UO

Project: PhD work by A.J. Popel: ‘The effect of radiation damage by fission fragments on the structural stability and dissolution of the UO2 fuel matrix’. The Excel file ‘XRD_UO2_films_LSAT’ with raw XRD data supporting Figure 10 and XRD results in the publication: A.J. Popel, V.A. Lebedev, P.G. Martin, A.A. Shiryaev, G.I. Lampronti, R. Springell, S.N. Kalmykov, T.B. Scott, I. Monnet, C. Grygiel, I. Farnan, Structural effects in UO2 thin films irradiated with fission-energy Xe ions, J. Nucl. Mater. 482 (2016) 210-217, https://doi.org/10.1016/j.jnucmat.2016.10.024. The XRD analysis was performed to assess crystallographic structure of the as-produced and irradiated thin films of UO2 on LSAT substrates. The data were generated in June-July 2014 at the Department of Earth Sciences, University of Cambridge, Cambridge, UK. A D8 Bruker diffractometer equipped with a primary Ge monochromator for Cu Ka1 and a Sol-X solid state detector operating in standard Bragg-Brentano geometry was used for the analysis. The samples were spun during signal collection and a zero-background sample holder was used in all cases. The data can be accessed through the University of Cambridge Data Repository.

_2

Project: PhD work by A.J. Popel: ‘The effect of radiation damage by fission fragments on the structural stability and dissolution of the UO2 fuel matrix’. The Excel file ‘U_ICP-MS data’ with output ICP-MS data and calculations for water and acid dilutions supporting Figures 2 and 3 and ICP-MS results in the publication: A.J. Popel, V.G. Petrov, V.A. Lebedev, J. Day, S.N. Kalmykov, R. Springell, T.B. Scott, I. Farnan, The effect of fission-energy Xe ion irradiation on dissolution of UO2 thin films, J. Alloys Compd. 721 (2017) 586-592, https://doi.org/10.1016/j.jallcom.2017.05.084. The ICP-MS analysis was performed to measure 238U concentration in the extracted solutions. The data were generated on the 13-14th of October 2014 at the Department of Earth Sciences, University of Cambridge, Cambridge, UK, on a Perkin Elmer SCIEX Elan DRC II quadrupole ICP-MS. The data can be accessed through the University of Cambridge Data Repository.