The shape of (nano)islands is among significant factors of the catalytic activity of supported catalysts. A lattice model of the reshaping under reaction conditions is suggested and studied by means ...of kinetic Monte Carlo simulations. It is rooted in experimental findings and is simplified as far as possible to still demonstrate reversible compact—ramified shape transitions. This simple model with complex behavior demonstrates several reshaping regimes and is considered as a possible sub-network of more realistic networks of heterogeneous catalytic reactions.
The spin probe technique was used to study graphite oxide (GO) powders swelled in polar liquids (CH3CN, CH3OH, and H2O) and liquid-free GO membranes (GOM). The nitroxide radicals TEMPO ...(2,2,6,6-tetramethylpiperidine-N-oxyl) and TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) readily penetrated into the interplane space of GO from the solution. Electron paramagnetic resonance (EPR) spectra of these radical probes were sensitive to molecular mobility and orientation ordering within the internal space of GO. The radicals embedded in swelled GO were in two states with different rotational mobilities. The small fraction of radicals located in the interplane space of GO and detected in the broad range of temperatures was in the state of fast rotation, similar to the same radicals dissolved in bulk liquids, thus providing experimental evidence of formation of a liquid-like media within the interplane space of GO. Such mobile media may be responsible for the unusual permeation properties of GOM, which is reported in the literature. Second, less-mobile fraction of radicals was found to be immobilized at the internal surface of GO and was sensitive to phase transformations in the swelled GO structures. The transformations were detected as anomalies at temperature dependences of rotational mobility of radicals. The detected dependence of EPR spectra of probe radicals on orientation of GOM, relative to the direction of magnetic field in the EPR spectrometer, was used for quantitative characterization of orientation alignment of GO planes within the membranes. Such an approach may serve as an elegant method to estimate the relative quality of membranes and other GO-layered structures.
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•Rapid ultrasound-probe preparation of aqueous fullerene dispersions (AFDs).•Sonication probe is more effective for AFD preparation than a bath.•AFDs have up to 80 ppm concentrations, ...180 nm mean diameter, –32 mV ζ-potential.•AFDs of C60, C70, Gd@C82, etc. showed long-term storage and aggregate stability.•Complete characterization and metal/organic impurities control of AFDs is proposed.
A green, scalable, and sustainable approach to prepare aqueous fullerene dispersions (AFD) C60, C70, endohedral metallofullerene Gd@C82, and their derivatives C60Cl6, C70Cl10, and supramolecular and ester-like derivatives, 10 fullerene species total, is proposed. For the first time, an immersed ultrasonic probe was used to preparing dispersions for pristine fullerenes without addends. Both ultrasound-assisted solvent-exchange and direct sonication techniques for AFD preparation using an immersed probe were tested. The average time for AFD preparation decreases 10–15 times compared to an ultrasound-bath-assisted technique, while final fullerene concentrations in AFDs remained at tens of ppm (up to 80 ppm). The aqueous dispersions showed long-term stability, a negatively charged surface with a zeta potential up to –32 mV with an average nanocluster diameter of no more than 180 nm. The total anionic and cationic compositions of samples were found by inductively coupled plasma atomic emission spectroscopy and chromatographic techniques. The highlights and challenges of using an ultrasound probe for AFD production are discussed.
Characteristic size and heat-transfer parameters of aqueous detonation-nanodiamond dispersions of various brands as carbon-based nanomaterials for nanofluidic tasks were assessed by confocal ...photothermal microscopy and transient (time-resolved) photothermal-lens modalities. It was shown that only the part of the transient thermal-lens heating curve of the dispersed sample with light-absorbing particles when the photothermally induced thermal field becomes homogeneous should be used to evaluate the bulk thermal diffusivity. For nanodiamond concentrations of 1–4 mg/mL, a 1–5% increase in thermal diffusivity and thermal conductivity compared with water is observed. A slowdown in the dissipation of the thermal lens effect due to prolonged heat accumulation by nanodiamonds is shown, which is confirmed by significant photothermal signals from nanodiamond clusters according to confocal photothermal microscopy. The size estimation of separate nanodiamond clusters in solution by nonlinear far-field confocal photothermal microscopy is shown; for the selected nanodiamond brands, photothermal microscopy reveals local signals (200–1000 nm) from nanodiamonds that can be assigned to aggregates of ca. 40–70 nm correlated with calculations from the Gibbs–Kelvin equation by differential scanning calorimetry. The discrimination of nonaggregated and aggregated nanodiamonds by photothermal microscopy is demonstrated.
The multispectral modality and technique for optically dense samples of optoacoustic spectroscopy were applied to measure spectra and high absorbances of concentrated aqueous dispersions of undoped ...nanodiamonds. The data from optoacoustic and optical transmission measurements and DSC data of the mean particle size by the Gibbs–Kelvin equation are compared to estimate the difference in composition of various nanodiamond trademarks. Optoacoustic spectra confirm the contribution of surface dimer chains into the absorption of nanodiamonds in the long wavelength range. Optoacoustic and conventional absorption spectra of aqueous solutions of nanodiamond fractions after centrifugation (15300g) and ultracentrifugation (130000g) revealed a separation of a highly absorbing non-diamond sp2 phase. The two-step separation by ultracentrifugation followed by extra centrifugation made it possible to isolate a highly absorbing and soluble nanodiamond phase with the particle size of 3.6 nm, showing a change in spectra compared to the starting nanodiamond material.
We investigate the Diophantine equation of the form
, with
where
and
are odd and
is even, and
is a sufficiently small natural number. We obtain a complete description of the set of solutions to such ...an equation.
Reversible phase transformation in the Brodie graphite oxide-acetonitrile system, which is intercalation or release of part of the sorbed liquid from the interplanar space accompanied by an increase ...or a decrease in interplanar distances, is commonly observed in twice-oxidized materials. We observed this phenomenon for once-, twice- and thrice-oxidized materials using the EPR spin probe technique, DSC, and temperature programmed XRD. It was shown that all materials under study formed similar low temperature (LT) and high temperature (HT) swollen structures with acetonitrile. The phase transformation of these structures is observed for materials with different oxidation levels in the same temperature range (∼20 K). The twice- and thrice-oxidized Brodie graphite oxides form swollen structures with the same parameters, while the once-oxidized material sorbs less acetonitrile at lower temperatures, but shows the same interplanar distances as those in twice- and thrice-oxidized materials. The spin probe technique has proven its sensitivity to the appearance of small amounts of the new forming swollen structures, which makes this method useful in studies of phase transformations.
Once-, twice- and thrice-oxidized Brodie graphite oxide materials form similar low temperature (LT) and high temperature (HT) swollen structures with acetonitrile.
A 68-element survey study was carried out on micro-impurities from 20 commercial nanodiamond samples from eight international manufacturers. The measurements were made using inductively coupled ...plasma atomic emission spectroscopy with detection limits for most elements below 1ppm. Various methods of sample preparation were compared: direct introduction of suspensions, ashing with microwave-assisted acid digestion, and microwave-assisted acid extraction, and their advantages and disadvantages evaluated. The work demonstrates the feasibility of nanodiamond analysis by direct slurry nebulization, which provides a multi-element, rapid, simple, and relatively low-cost route for nanodiamond purity estimation. Most of the samples contain relatively high amounts of Fe, Na, Ca, Si, Cu, Al, S, and Ti (>100ppm), while Pb, Zn, K, Mn, B, Cr, Mg, Mo, Sn, W, Ba, Sb, Co, and Sr are at low but significant amounts. In addition, in several samples, we found some uncommon elements like Ag, Ce, Y, Hf, Zr, and Hg. This study provides new possibilities for certifying nanodiamond purity, understanding their properties and behavior, and advancing their production technology.
The Fe–Cr–C–Ti–Al cored wire layers were cladded by laser, arc and hybrid methods. Mechanical tests have shown that the cladded layers strongly differ in wear resistance. The metallographic, X-ray ...diffraction and durometric studies were carried out to identify the reasons of such difference. The analysis of the study results showed that the main reason is the difference between the cooling rates of the cladded metal in the region of 1100–800 K. This difference provides the formation of the metastable austenitic structure, which significantly impacts the wear resistance of the cladded coatings.
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