Andrei Kanaev

Near band-gap photoluminescence properties of hexagonal boron nitride

Near band-gap luminescence (hν⩾5eV) of hexagonal boron nitride has been studied by means of the time- and energy-resolved photoluminescence spectroscopy method. Two emissions have been observed at 5.5 and 5.3eV. The high-energy emission at 5.5eV is composed of fixed subbands assigned to bound excitons at 5.47, 5.56, and 5.61eV. The nonstructured low-energy emission at 5.3eV undergoes a large blueshift (up to 120meV) with a linear slope ΔElum∕ΔEexc<1 with increasing excitation energy Eexc. At Eexc⩾5.7eV, the band position is fixed and marks the transition from the Raman to the photoluminescence regime. We assign the 5.3eV band to quasi-donor-acceptor pair (q-DAP) states due to electrostatic band fluctuations induced by charged defects. The shift is explained by photoinduced neutralization of charged defect states. The absence of contribution to the q-DAP luminescence from exciton suggests the existence of a large exciton binding energy, which is qualitatively consistent with theoretical predictions.

From nanoparticles to bulk crystalline solid: nucleation, growth kinetics and crystallisation of mixed oxide ZrxTi1−xO2 nanoparticles

We describe the preparation of mixed metal oxide nanoparticles of a desirable composition and their transformation to the crystalline solids ZrxTi1−xO2 (0.0 ≤ x ≤ 1.0) after heat treatment. The correlation analysis between the size of the nucleus, the crystalline phase and the elemental composition of the solid is presented. Mixed metal oxide zirconium–titanium-oxo-alkoxy (ZTOA) nanoparticles of different elemental compositions 0 ≤ x = CZr/(CZr + CTi) ≤ 1 were prepared via the sol–gel method in a reactor by rapid micromixing of n-propanol fluids containing the precursors and water. The structural transformation of the nanoparticles takes place in two temperature ranges, 210–250 °C and 380–680 °C, which sensitively depends on the elemental composition. In the range 0.3 ≤ x ≤ 0.6, stable ZTOA nanoparticles with a radius of 2.1 ± 0.05 nm appeared at the hydrolysis ratio H ≤ 1.5. The heat treatment results in a single orthorhombic ZrxTi1−xO2 phase. The crystallisation onset temperature was the highest in this range of x, attaining 680 °C at x = 0.5. In the range 0 ≤ x ≤ 0.2, the particle radius decreased to 1.6 nm for pure titanium-oxo-alkoxy nuclei (TOA, x = 0); their heat treatment resulted in a single TiO2 anatase phase. In the range 0.7 ≤ x ≤ 1, the particle radius decreased to 1.8 nm for pure zirconium-oxo-alkoxy nuclei (ZOA, x = 1); their heat treatment resulted in mixed monoclinic and tetragonal ZrO2 phases. The crystalline cell parameters of the observed phases underwent a continuous variation with x. TEM images evidenced the nanoporous structure of submicronic orthorhombic ZrxTi1−xO2 monocrystals with a mean pore size of about that of the ZTOA nanoparticles.

Modification of ZnO thin films induced by high-density electronic excitation of femtosecond KrF laser

The effect of laser irradiation on nanoscale morphology of ZnO thin films has been studied in the regime of high-density electronic excitation. A femtosecond KrF laser (248xa0nm, 450xa0fs) was used to irradiate the films prepared by solgel (SG) and pulsed laser deposition (PLD) methods with fluences below ablation threshold and above that of Mott density. This regime corresponds to electron-hole plasma (EHP) creation, resulting in a specific nanostructuring of monocrystalline ZnO surfaces. A strong nanoscale modification of thin films in the domain of laser fluences 105u2009u2009mJ/cm2<E<180u2009u2009mJ/cm2 has been evidenced. While PLD films maintain a smooth surface with nanoholes arranged in straight and zig–zag lines (similar to ZnO monocrystals) surrounded by domains of ∼100u2009u2009nm size, SG films show the flake-like structure with an open porosity. This behavior can be related to the density of the prepared films.

Agitated reactor with in situ nanoparticle size control by light scattering photon correlation spectroscopy

We propose a simple design of the precipitation reactor vessels permitting measurements of the mean size of nanoparticles by light-scattering photon correlation spectroscopy. The measurements are conducted in connecting transparent capillary array in conditions free of turbulence, which otherwise perturb such measurements. A satisfactory quality of the autocorrelation function has been obtained in test measurements of 100 nm latex nanoparticles.

Effect of laser polarization and crystalline orientation on ZnO surface nanostructuring in the regime of high-density electronic excitation

The effect of laser polarization and crystalline orientation on nanoscale morphology of ZnO monocrystal has been studied in the regime of high-density electronic excitation related to the saturation of the exciton fluorescence and appearance of the electron-hole plasma continuum. The irradiation with femtosecond KrF laser (248xa0nm, 450xa0fs) was realized in the sub-melting regime of fluences about a half of the ablation threshold. Two types of nanostructures were observed: holes of 10xa0nm diameter arranged in quasi-periodic zigzag and straight lines, which propagate along the crystalline planes a, c, and m. The nanostructuring sensitively depends on the crystalline plane’s orientation to laser electric field polarization (p¯). In crystalline planes c and m, the zigzag and straight lines propagate along the crystalline cell axes with respectively small (≤30°) and large (90°) inclination to p¯. The nanostructuring in the crystalline plane a is rare and of low intensity. This behavior can be related to a stress field due to an accumulation of space charges in the crystalline structure.

Nucleation and fractal growth of zirconium oxo-alkoxy nanoparticles at the induction stage of sol–gel process

Monodispersed zirconium oxo-alkoxy nanoparticles are synthesized via a sol–gel method in a rapid micromixing reactor with in situ particle size measurements. The nucleated nanoparticles of 4.7xa0nm diameter are free from impurities and conserve high chemical activity. They can be associated in 1D fractals. These nanoparticles can form high optical-quality coatings on hydrophilic substrates.

A Photocatalytic Active Adsorbent for Gas Cleaning in a Fixed Bed Reactor

Efficient photocatalysis for gas cleaning purposes requires a large accessible, illuminated active surface in a simple and compact reactor. Conventional concepts use powdered catalysts, which are nontransparent. Hence a uniform distribution of light is difficult to be attained. Our approach is based on a coarse granular, UV-A light transparent, and highly porous adsorbent that can be used in a simple fixed bed reactor. A novel sol-gel process with rapid micro mixing is used to coat a porous silica substrate with -based nanoparticles. The resulting material posses a high adsorption capacity and a photocatalytic activity under UV-A illumination (PCAA = photocatalytic active adsorbent). Its photocatalytic performance was studied on the oxidation of trichloroethylene (TCE) in a fixed bed reactor setup in continuous and discontinuous operation modes. Continuous operation resulted in a higher conversion rate due to less slip while discontinuous operation is superior for a total oxidation to due to a user-defined longer residence time.

Porous monoliths consisting of aluminum oxyhydroxide nanofibrils: 3D structure, chemical composition, and phase transformations in the temperature range 25–1700 °C

AbstractWe present a study on the chemical and structural transformations in highly porous monolitic materials consisting of the nanofibrils of aluminum oxyhydroxides (NOA, Al2O3·nH2O) in the temperature range 20–1700xa0°C. A remarkable property of the NOA material is the preservation of the monolithic state during annealing over the entire temperature range, although the density of the monolith increases from ~0.02 up to ~3xa0g/cm3, the total porosity decreases from 99.3 to 25% and remains open up to 4xa0h annealing at the temperature ~1300xa0°C. The physical parameters of NOA monoliths such as density, porosity, specific area were studied and a simple physical model describing these parameters as the function of the average size of NOA fibrils—the basic element of 3D structure—was proposed. The observed thermally induced changes in composition and structure of NOA were successfully described and two mechanisms of mass transport in NOA materials were revealed. (i) At moderate temperatures (Tu2009≤u2009800xa0°C), the mass transport occurs along a surface of amorphous single fibril, which results in a weak decrease of the length-to-diameter aspect ratio from the initial value ~24 till ~20; the corresponding NOA porosity change is also small: from initial ~99.5 to 98.5%. (ii) At high temperatures (Txa0>u2009800xa0°C), the mass transport occurs in the volume of fibrils, that results in changes of fibrils shape to elliptical and strong decrease of the aspect ratio down to ≤u20092; the porosity of NOA decreases to 25%. These two regimes are characterized by activation energies of 28 and 61xa0kJ/mol respectively, and the transition temperature corresponds to the beginning of γ-phase crystallization at 870xa0°C.n Graphical abstract