The influence of chitin nanocrystals on structural evolution of ultra-high molecular weight polyethylene/chitin nanocrystal fibers in hot-drawing process

Ultra-high molecular weight polyethylene (UHMWPE)/chitin nanocrystal (CNC) fibers were prepared. Compared with the pure UHMWPE fibers, the ultimate tensile strength and Young’s modulus of UHMWPE/CNC fibers are improved by 15.7% and 49.6%, respectively, with the addition of chitin nanocrystals (CNCs) of 1 wt%. The melting temperature (T _m) of UHMWPE/CNC fibers was higher than that of pure UHMWPE fibers. Pure UHMWPE fibers and UHMWPE/CNC fibers were characterized with respect to crystallinity, orientation and kebab structure by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM). It is found that the CNCs act as the shish structure in UHMWPE/CNC fibers and the kebab crystals are grown around the CNCs. There was almost no difference between pure UHMWPE fibers and UHMWPE/CNC fibers in orientation. But the degree of crystallinity of various stages of UHMWPE/CNC fibers was respectively higher than the corresponding stage of pure UHMWPE fibers. Moreover, the addition of 1 wt% CNCs improved the thickness of kebab crystals and accelerated the transformation of kebab to shish.     

On the evolution of particle size average and size distribution in suspension polymerization processes

The dispersion of methyl methacrylate (MMA) and its suspension polymerization were used as models to elaborate the evolution of particle size average and size distribution in the course of suspension polymerization. The underlying mechanisms for the occurrence of the dynamic and static steady states in the population of drops were defined and their effects on the evolution of drop/particle size average and size distributions were examined. The characteristic intervals of suspension polymerizations (transition, steady-state, growth, and identification) were elaborated. The formation of satellite droplets and their evolution in the course of polymerization were also discussed.     

The influence of metal nanoparticle size distribution in photoelectron spectroscopy

We present an innovative approach to characterize small metal nanoclusters embedded in an amorphous carbon matrix. A simple mathematical relation linking binding energy shifts to the mean nanoparticle (NP) dimensions allows determination of the distribution of NP sizes by fitting the Au 4f X-ray photoemission spectrum. The NP size distributions obtained using our method are compared with those obtained from X-ray diffraction spectra.     

Effect of Pt particle size and distribution on photoelectrochemical hydrogen evolution by p-Si photocathodes

In this work, we investigate the effect of the average size and density of Pt clusters on silicon on the photoelectrochemical production of hydrogen. The metallization of Si is performed via electroless deposition from aqueous HF solutions and from water-in-oil microemulsions. The first method enables control of the average diameter and density of Pt clusters by properly changing the deposition parameters like HF concentration and immersion times. However, on one hand, size dispersion is relatively wide and particles agglomeration may occur with this deposition technique. On the other hand, Pt islands with smaller dimensions at the nanoscale as well as with a narrower size distribution are deposited from reversed micellar solutions. Photoelectrochemical experiments show that the effect of Pt morphology on photoconversion efficiency strongly depends on light intensity. At low power of illumination (10 mW/cm2), Pt islands with a mean diameter of 100 nm and a density of 15 particles/microm2, which can be obtained via electroless deposition from a HF-based solution, provide the best photoelectrochemical performance. Nevertheless, this configuration of Pt clusters yields an abrupt collapse of photoconversion efficiency from 31% to 11.8% when the light power is increased up to 100 mW/cm2. At this light intensity, Pt islands with a mean size and density of approximately 40 nm and 75 particles/microm2, respectively, obtained via the microemulsion method, allow photoconversion efficiency as high as 20% to be achieved.     

Shape evolution of single-crystalline iron oxide nanocrystals

Shape- and size-controlled synthesis of single-crystalline maghemite (gamma-Fe2O3) nanocrystals are performed by utilizing a solution-based one-step thermolysis method. Modulating the growth parameters, such as the type and amount of capping ligands as well as the growth time, is shown to have a significant effect on the overall shape and size of the obtained nanocrystals and on the ripening process itself. The resulting shapes of the novel structures are diverse, including slightly faceted spheres, diamonds, prisms, and hexagons, all of which are in fact truncated dodecahedron structures with different degrees of truncation along the {111}, {110}, or {100} faces. Spherical nanocrystals are easily assembled into the three-dimensional superlattices, demonstrating the uniformity of these nanocrystals. The size-dependent magnetic properties are examined, and large hexagon-shaped gamma-Fe2O3 nanocrystals are shown to be ferrimagnetic at room temperature.     

Phase-controlled one-dimensional shape evolution of InSe nanocrystals

Cubic-phase InSe nanowires were synthesized. Obtaining an appropriate high reaction temperature for formation of nanocrystals using poor solubility of Se powder in oleylamine induced the generation of unusual cubic phase of InSe to form nanowires. In comparison, hexagonal-phase nanoplates were formed by dissolving Se powder in oleylamine before increase of reaction temperature. The diameter of highly monodisperse wires was controlled by varying the amount of Se.     

Symmetry of LaAlO3 nanocrystals as a function of crystallite size

Properties of LaAlO₃ nanocrystallites obtained by the precipitation method, doped with praseodymium and chromium ions were examined by spectroscopic means. Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and electronic spectroscopy proved that symmetry of LaAlO₃ crystallites depends on their size. At room temperature the smallest obtained crystals (average size ∼5 nm) have cubic symmetry, while the largest ones (average size over 110 nm) exhibit rhombohedral symmetry. Possible explanations for this phenomenon are discussed.     

反应时间对GaP纳米材料粒度的影响

以GaCl3和Na3P为原料,利用苯热合成方法,在相同的温度、不同的反应时间下制备GaP纳米粒子。由分散在苯中的GaP纳米粒子的吸收光谱和透射电子显微镜测试结果可知,反应时间延长时GaP苯溶液的光吸收谱发生明显的蓝移。本文分析了出现这一现象的原因,并从实验上进行了验证。     

Aggregate size distribution evolution for Brownian coagulation-sensitivity to an improved rate constant

Brownian motion causes small aggregates to encounter one another and grow in gaseous environments, often under conditions in which the coalescence rate (say, spheroidization by "sintering") cannot compete. The polydisperse nature of the aerosol population formed by this mechanism is typically accounted for by formulating an evolution equation for the joint PDF of the state variables needed for describing individual particles. In the simple case of fractal-like aggregates (prescribed morphology and state, characterized just by the number of aggregated spherules, or total aggregate volume), we use the quadrature method of moments and Monte Carlo simulations to show that recent improvements in the laws governing free molecule regime coagulation frequency (rate "constant") of these aggregates cause systematic changes in the shape of the asymptotic aggregate size distribution, with significant implications for the light-scattering power and inertial impaction behavior of such aggregate populations.     

Heterodimers based on CoPt3-Au nanocrystals with tunable domain size

We describe an approach to synthesize colloidal nanocrystal heterodimers composed of CoPt(3) and Au. The growth is based on the nucleation of gold domains on preformed CoPt(3) nanocrystals. It is a highly versatile methodology which allows us to tune independently the size of the two domains in each dimer by varying several reaction parameters. The statistical analysis of the distribution of the domain sizes in the dimers and the compositional mapping achieved by dark field imaging and energy dispersive spectroscopy confirm that the two domains in each dimer are indeed made of CoPt(3) and Au, respectively. Structural characterization by high-resolution transmission electron microscopy shows that the two domains, both having cubic fcc Bravais lattice, can share a common {111}, {100}, or {110} facet, depending on the size of the initial CoPt(3) seeds. The magnetization measurements evidence a ferromagnetic CoPt(3) phase with a relatively low anisotropy as a consequence of their disordered crystalline structure, regardless of the presence of a Au tip. We believe that this prototype of nanocrystal dimer, which can be manipulated under air, can find several applications in nanoscience, as the Au section can be exploited as the preferential anchor point for various molecules, while the CoPt(3) domain can be used for magnetic detection.     

Interparticle interactions in agglomerates of alpha-Fe2O3 nanoparticles: influence of grinding

We have chemically prepared a sample of antiferromagnetic alpha-Fe2O3 nanoparticles by a gel-sol technique. M?ssbauer spectra of the as-prepared sample showed that superparamagnetic relaxation was suppressed due to strong magnetic interparticle interactions even at room temperature. However, subsequent grinding of the sample by hand in a mortar for some minutes resulted in fast superparamagnetic relaxation of some of the particles. The effect was even more dramatic if the alpha-Fe2O3 powder was ground for a longer time or together with nonmagnetic eta-Al2O3 nanoparticles. Similar effects were found after low-energy ball milling. Thus it is found that the agglomeration of the nanoparticles during preparation under wet conditions results in strong magnetic interparticle interaction, but a relatively gentle mechanical treatment is sufficient to break up the agglomerates, resulting in much weaker interactions. We show that these effects can also be seen when a soil sample containing magnetic nanoparticles is ground.     

Shape evolution and self assembly of monodisperse PbTe nanocrystals

In this communication, we report our recent achievement in synthesis and self-assembly of both spherical and cubic PbTe nanocrystals using a high-temperature solution-phase synthesis approach. The possible mechanism of nanocrystalline evolution from spherical to cubic structure has also been discussed. It is possible to use the highly orientated PbTe nanocrystals as building blocks to achieve thickness-controlled film for further manipulation into thermoelectric devices.     

The influence of oxygen on the fluorescence enhancement of fatty-acid-capped CdS nanocrystals

CdS nanocrystals were synthesized in 1-octadecene (ODE) solution with oleic acid (OA) as a capping agent. Freshly prepared CdS nanocrystals showed a weak orangelike fluorescence due to defects on the particle surface. After several weeks' storage, the bulk fluorescence of CdS nanocrystals was dramatically enhanced, which gave the nanocrystals a blue or violet appearance. UV and photoluminescence (PL) measurements were employed to investigate this process. It has been found that the oxygen in the air played the most important role. The oxygen atoms could absorb on the surface of particles and a layer of oxide was gradually formed, which effectively passivated the surface of CdS nanocrystals. Interestingly, this oxidative process had no relation to UV illumination.     

Interparticle Charge-Transport-Enhanced Electrochemiluminescence of Quantum-Dot Aerogels

Electrochemiluminescence (ECL) represents a widely explored technique to generate light, in which the emission intensity relies critically on the charge-transfer reactions between electrogenerated radicals. Two types of charge-transfer mechanisms have been postulated for ECL generation, but the manipulation and effective probing of these routes remain a fundamental challenge. Here, we demonstrate the design of quantum dot (QD) aerogels as novel ECL luminophores via a versatile water-induced gelation strategy. The strong electronic coupling between adjacent QDs enables efficient charge transport within the aerogel network, leading to the generation of highly efficient ECL based on the selectively improved interparticle charge-transfer route. This mechanism is further verified by designing CdSe-CdTe mixed QD aerogels, where the two mechanistic routes are clearly decoupled for ECL generation. We anticipate our work will advance the fundamental understanding of ECL and prove useful for designing next-generation QD-based devices.     

Interparticle diffusion of aromatic hydrocarbons on aerosil surface

It has been demonstrated by the photobleaching method that the photostability of pyrene molecules on the surface of pyrogenic nonporous SiO₂ (aerosil) is independent of the surface concentration of the fluophor. Through a kinetic analysis of the recovery of fluorescence of the luminophore and solution of the two-dimensional diffusion equation, coefficients of interparticle diffusion were estimated for the molecules of pyrene and N,N-dimethylaniline. Institute of Surface Chemistry, National Academy of Sciences of Ukraine, 31 Nauki Prospect, Kiev, Ukraine 252039. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 1, pp. 23–26, January–February, 1998.