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Nanostructured powders xFe/nano-Al2O3 with the Fe loading of x = 0.0 – 5.0 wt% were obtained using laser vaporization by CO2 laser. XRF, XRD, HRTEM, PL and UV–Vis DRS techniques were employed to investigate physicochemical, structural and optical properties of the synthesized nanopowders with the average particle size of 9 nm. Nanopowders xFe/nano-Al2O3 as model catalysts were tested in isobutane dehydrogenation reaction. The results obtained were compared with similar data for the xFe/γPb-Al2O3 systems synthesized by the conventional sol–gel method. According to XRD and UV–Vis DRS data, in the series of xFe/nano-Al2O3 samples a great part of Fe3+ ions is in the disordered environment of subsurface layers of Al2O3 nanocrystallites, predominantly in the tetrahedral coordination. In distinction to samples of the xFe/γPb-Al2O3 series, in the case of nanostructured xFe/nano-Al2O3 powders the formation of Fe2O3 phase does not occur at any concentrations of iron or conditions of testing. The analysis of the PL spectra of xFe/nano-Al2O3 powders also showed the presence of surface sites of Fe3+ ions, which were not detected for xFe/γPb-Al2O3. Catalytic testing of the xFe/nano-Al2O3 series samples in isobutane dehydrogenation revealed the formation of the iron active sites that ensure catalytic activity of the samples. Differences in the catalytic properties of FeOx/Al2O3 samples obtained by the sol–gel method and laser vaporization are related to different states of Fe3+ ions. Thus, the xFe/nano-Al2O3 nanopowders, in contrast to xFe/γPb-Al2O3, contain a large amount of active Fe3+ sites. These sites, being involved in the dehydrogenation reaction, are present predominantly on the surface of the nanopowders. [ABSTRACT FROM AUTHOR]

In this work, the oxide nanopowders of Al 2 O 3 , ZrO 2 , Y 2 O 3 , Gd 2 O 3 , CeO 2 , and SiO 2 were synthesized by CW CO 2 laser vaporization technique with controlled gas condensation in an inert atmosphere. Methods for controlling the size of the resulting nanoparticles by adjusting the gas composition and pressure during the vaporization process have been demonstrated. The potential for producing ultrasmall oxide nanoparticles with dimensions less than 5 nm has been shown. The size distribution of nanoparticles taken from different parts of the evaporation-condensation tract was studied using scanning (SEM) and transmission (TEM) electron microscopy methods. The effect of synthesis conditions (pressure and composition of the inert gas) on characteristics of the nanoparticles is discussed. Using a wide class of simple oxides as the example, it is shown that the powders synthesized by the laser method consist of three types of particles: target spherical particles with a diameter of 3–20 nm (more than 98%), larger spherical particles with a diameter of 50–200 nm, and shapeless large particles with sizes more than 200 nm. The possibility of separating large particles from the main particles using the original labyrinth system for gas pumping is shown. The obtained particles with controlled sizes can be effectively used in various applications, in particular, for the preparation of catalysts and adsorbents. • Laser-synthesized Al 2 O 3 , ZrO 2 , Y 2 O 3 , Gd 2 O 3 , CeO 2 , SiO 2 nanopowders were obtained. • The CW CO 2 laser vaporization technique with controlled gas condensation was used. • The effect of synthesis conditions on nanopowders characteristics was established. • The possibility of separating large particles from the main particles was shown. [ABSTRACT FROM AUTHOR]

CrOx/Al2O3 model catalysts in the form of nanopowders with the chromium content of 0.96 wt% and the mean particle size 5.5, 7.3, 11.7, 13.1, and 20.5 nm were synthesized by a laser vaporization technique. CrOx/Al2O3 nanocatalysts are mainly represented by softly agglomerated particles with a sphere-like morphology. It was shown that a considerable increase in the mean particle size from 5.5 to 20.5 nm with the corresponding decrease in specific surface area from 297 to 80 m2/g of the freshly prepared catalysts decreased isobutane conversion and selectivity to isobutylene at 550 °C by 2–4% and 10–15 mol.%, respectively. The laser synthesis was found to produce Al2O3 nanoparticles with stable highly dispersed Cr2O3 clusters on their surface, which serves as the active catalytic centers. The presence of such highly dispersed clusters with a size of 2–4 nm was confirmed by the HRTEM method. The catalytic properties of the tested nanocatalysts with the established steady state were stable in isobutane dehydrogenation over 25–30 dehydrogenation-regeneration cycles. On the nanocatalysts that reached a steady state, an increase in the mean particle size led to a decrease in isobutane conversion by 7 and 6%; selectivity to isobutene decreased by 11 and 10 mol.% at the reaction temperatures 560 and 580 °C, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • Al 2 O 3 NPs with the particle size from 2 to 21 nm were synthesized by CO 2 laser vaporization. • The structure, morphology, thermal, and optical properties of Al 2 O 3 NPs were studied. • Photoluminescence is determined by 3d elements, surface hydroxyl groups, and F-type centers. • The size effect for Al 2 O 3 NPs has a boundary corresponding to 6–7 nm. A series of Al 2 O 3 nanoparticles with the sizes ranging from ~ 2 to 21 nm, according to XRD, HRTEM and BET, was obtained by vaporization of α-Al 2 O 3 in flowing helium and argon at different pressures under the action of a cw CO 2 laser. The particle size was changed by varying the composition of ambient gas (He, Ar) and its pressure in a vaporization chamber from 0.034 to 0.9 bar. The effect of the synthesis conditions on the properties of Al 2 O 3 nanopowders is presented and discussed. Particles with the size smaller than 6–7 nm have a faceted shape, whereas the shape of larger particles is close to spherical. It was found that the resulting nanopowders consist of a mixture of transition aluminas, among which the γ -polymorph is dominant. Thermal analysis showed that for smaller nanoparticles the phase transition to α-Al 2 O 3 occurs at a lower temperature. In nominally pure Al 2 O 3 nanoparticles with different sizes, photoluminescence at RT and 80 K is determined by trace level concentrations (≤10–3 wt%) of uncontrolled impurities of 3d elements (Cr3+(O h) and Fe3+(T d)), surface hydroxyl groups, and point defects represented mostly by F-type centers. The acquired experimental data indicate that manifestation of the size effect for Al 2 O 3 nanoparticles has a boundary corresponding to 6–7 nm. Al 2 O 3 nanoparticles with the dominant γ-Al 2 O 3 structure obtained by the laser method can be promising in many chemical reactions, particularly as the supports of catalysts and sorbents, for which the high specific surface area is of special importance. [ABSTRACT FROM AUTHOR]

Europium doped Gd 2 O 3 sphere-like nanoparticles with d m = 9.3 ± 3.5 nm were synthesized by cw CO 2 laser vaporization technique in a flowing mixture of argon and oxygen. According to XRD data, the Eu:Gd 2 O 3 nanoparticles crystallize in the monoclinic symmetry class (C2/m space group). High-resolution luminescence spectroscopy study showed that the ultra-narrow 5D 0 → 7F 0 transition of Eu3+ demonstrates only two peaks corresponding to two inequivalent C s positions of Eu3+ ion in monoclinic Gd 2 O 3 lattice that is explained by the peculiarities of local environment of Eu3+ ion at these sites. The hypersensitive transition 5D 0 → 7F 2 dominates in the spectrum and is expanded to the red part of the spectrum in comparison with cubic Eu:Gd 2 O 3 due to intense transitions terminating at higher-lying components of the crystal-field-split 7F 2 state. In the luminescence spectrum, an additional weak band with the maximum at 407 nm corresponding to the electronic transitions 4f65 d1(7F J) → 4f7(8S 7/2) of Eu2+ was detected. The obtained values of chromaticity coordinates and absolute quantum yield are (0.644; 0.325) and ca. 1%, respectively. The phase transformations have been investigated using differential scanning calorimetry and thermogravimetry (50–1400 °C). After annealing in air at 700 °C, the monoclinic symmetry class of the Eu:Gd 2 O 3 nanoparticles is preserved and the particle size increases to d m = 17.8 ± 6.1 nm. After annealing, the chromaticity coordinates (0.659; 0.334) and absolute quantum yield (ca. 4%) can be obtained using red phosphor based on monoclinic Gd 2 O 3 :Eu3+. The lifetime of the excited 5D 0 state of Eu3+ in the annealed nanoparticles is longer than that in the as-synthesized nanoparticles, due to the suppression of nonradiative decay after annealing. [Display omitted] • Monoclinic Eu doped Gd 2 O 3 nanoparticles are obtained by laser vaporization technique. • Simultaneous observation of Eu3+ and Eu2+ emissions through PL spectroscopy is reported. • CIE coordinates are suitable for using monoclinic Gd 2 O 3 :Eu3+ as a red phosphor. [ABSTRACT FROM AUTHOR]

The synthesis of Al 2 O 3 :Tb3+ (0.019–0.62 mol%) nanoparticles by laser vaporization of α-Al 2 O 3 :Tb3+ ceramic targets using a cw CO 2 laser in flowing helium at a pressure of 0.1 bar is reported. HRTEM, XRD and BET studies revealed that under the indicated conditions the produced Al 2 O 3 :Tb3+ nanoparticles have faceted and nearly a spherical shape with the average size below 10 nm. As shown by XRD, the phase composition of the synthesized nanoparticles is represented mostly by the low-temperature γ-Al 2 O 3. Photoluminescence (PL) spectra were obtained for Tb3+ ions in γ-Al 2 O 3 nanoparticles upon excitation to the 4f8 → 4f75d (λ ex = 230 nm), 7F 6 → 5H 7 (λ ex = 315 nm) and 5D 3 → 7F 3 (λ ex = 458 nm) transitions, and PL excitation spectra – for the most intense 5D 4 → 7F 5 (λ max = 543 nm) transition. The lifetime of the excited 5D 4 state was found to be ca. 2.9 ms. The obtained values of the chromaticity coordinate and absolute quantum yield were (0.299, 0.509) and 12.6% at λ ex = 315 nm, respectively. • Terbium doped Al 2 O 3 nanoparticles were obtained via laser vaporization. • Al 2 O 3 :Tb3+ nanophosphor with photoluminescent properties was achieved. • CIE coordinates and high QY are suitable for use Al 2 O 3 :Tb3+ as a green phosphor. [ABSTRACT FROM AUTHOR]

Nanosized Eu/γ- and/or η-Al 2 O 3 ([Eu 2 О 3 ] = 0.00, 0.02, 0.45 and 0.92 wt%) powders with the particle size from <10 nm to 21 nm synthesized by cw CO 2 laser vaporization were studied. It was shown that photoluminescence (PL) properties of the obtained samples are caused by luminescence of the Eu3+ and Eu2+ ions incorporated to the Al 2 O 3 matrix in crystallographic positions with the C 3v local symmetry. A scheme based on the formation of Eu2+ ions in the Al 2 O 3 lattice via the charge compensation by oxygen vacancies (F centers) located in the first coordination sphere of europium was proposed. The influence of size effects on PL of Eu/Al 2 O 3 nanoparticles consists first of all in shortening the lifetime of 4f – 4f transitions in Eu3+ ions as a result of increased probability of nonradiative relaxation owing to OH groups and various structural defects. It was found that the conditions of laser vaporization (gas composition and pressure) can change the structure of the resulting Eu/Al 2 O 3 nanoparticles. Image 1 • Nanoparticles of Eu/Al 2 O 3 with the particle size from <10 nm to 21 nm were synthesized by cw CO 2 laser vaporization. • Eu3+ and Eu2+ ions were detected by PL in Eu/Al 2 O 3 nanoparticles. • Eu3+ and Eu2+ ions incorporated to the Al 2 O 3 matrix in crystallographic positions with the C 3v local symmetry. • Size effects on lifetime of 4f – 4f transitions in Eu3+ ions were detected. [ABSTRACT FROM AUTHOR]