Nonisothermal nucleation in the CuCl solid solution in glass: Formation of two distributions of nanoparticles of a new phase in the solid solution with a negative jump of the nucleation temperature

The processes of nonisothermal nucleation in the CuCl solid solution in glass with a negative jump of the nucleation temperature have been investigated using optical spectroscopy and exciton-thermal analysis. Two distributions of CuCl nanoparticles differing in the average radii R (2.3 and 14.0 nm) have been obtained for the temperature drop from T ₁ = 700°C to T ₂ = 500°C. The formation of two distributions of CuCl particles has been numerically simulated and the agreement between the experimental and calculated radius distributions of CuCl particles has been obtained.     

Formation of two distributions of nanoparticles of a new phase in a solid solution

Processes of the formation of the double distribution of CuCl nanoparticles in glass in two-stage annealing have been analyzed. The distribution of CuCl nanoparticles with the average radius R ₁ = 14 and 18 nm appears in 20 and 40 min at temperature T ₁ = 700°C in samples 1 and 2, respectively. By this time, the formation of new clusters ends, and only the growth of previously formed clusters occurs (the second stage of nucleation). Then, the formation of the CuCl phase continues at T ₂ = 500°C. A decrease in the temperature is accompanied by a decrease in the critical radius of particles of a new phase. For this reason, the formation and growth of new CuCl nuclei begin again and the second distribution with a mean radius of 1 nm or larger is formed. As a result, double distributions of CuCl nanoparticles with significantly different mean radii are formed in samples 1 and 2. The concentration and mean radius of CuCl particles in distributions have been determined from the optical absorption spectra of CuCl nanocrystals at 80 K in the wavelength range of 300–500 nm.     

Nonisothermal nucleation in the CuCl solid solution in glass: Dissolution of subcritical CuCl nuclei with a positive jump of the nucleation temperature

The processes of nucleation in a CuCl solid solution in glass with a positive temperature jump from T ₁ = 500°C to T ₂ = 550, 600, and 650°C have been investigated using optical spectroscopy and exciton-thermal analysis. The dissolution of a part of the particles of the CuCl nanomelt formed previously at T ₁ has been observed at T ₂. Variations in the shape of the radius distribution curve of the CuCl particles due to the dissolution of initial nuclei have been determined from the melting kinetics of CuCl particles during linear heating of the sample. The nonisothermal nucleation of CuCl in glass under conditions of the temperature jump has been simulated numerically. The results of calculations of the variations in the radius distribution of CuCl particles are in agreement with the experiment. The calculated data on the variation in time of the critical radius r _c and the concentration of CuCl monomers in glass after the temperature jump have been obtained.     

Production and decay of subcritical nuclei in a solid solution

The decay of “subcritical” nuclei in a solid solution has been revealed in the investigation of the CuCl phase nucleation kinetics in glass. As soon as “supercritical” nuclei with an average radius R = 1.1 nm are created at 500°C, a sharp temperature increase up to 650°C transforms most nuclei created in the first annealing stage into “subcritical” ones, and this results in the decay of 80% of the nuclei in 5 min, while the remaining 20% of the nuclei grow in size to 2.4 nm. Their growth provides a sixfold increase in the CuCl phase growth rate against that in conventional annealing at 650°C. The kinetic dependences of the nucleation parameters—the amount of the phase and the average radius and concentration of the particles—were determined by the intrinsic absorption spectra of the CuCl nanocrystals. The critical radius of the CuCl nanomelt at 650°C has been estimated as 1.3 nm and the evaporation heat of the CuCl phase molecules in glass, as 13 kJ/mol. It is shown that multistage annealing makes it possible not only to control the parameters of the particles of the new phase, but also to determine the critical parameters of the initial nucleation stage.     

Effect of preliminary low-temperature annealing on the kinetics of nucleation

The changes observed in the kinetics of CuCl nucleation in glass due to preliminary low-temperature annealing have been investigated using optical spectroscopy. The influence of the number of nuclei formed at a temperature of 500°C on the growth rate of the CuCl phase at 650°C has been examined. The amount of the CuCl phase in the sample is determined from the optical absorption coefficient in the range of band-to-band transitions in the CuCl nanocrystals. The average radius of CuCl particles is calculated from the position of the maximum of the exciton band. It has been demonstrated that the preliminary formation of CuCl nuclei at 500°C for 3 h makes it possible to increase the growth rate of the CuCl phase by a factor of 6. In the sample with preliminarily formed nuclei, there occur two opposite processes: thermal decomposition of part of the small nuclei, which have become subcritical at 650°C; and the growth of larger nuclei, which have become supercritical at 650°C due to the diffusive inflow of the components of the new phase. When the equilibrium concentration is reached, the new phase contains particles with a smaller radius but at a higher concentration as compared to those in the case of the conventional nucleation at 650°C.     

Initial nucleation stages and properties of CuCl nanoparticles in glasses

The time variation and temperature dependence of a CuCl phase nucleation in a glass was studied by exciton spectroscopy. The phase formation kinetics at three temperatures was measured. A time delay in attaining a stationary rate of the new phase growth was observed at all temperatures, in agreement with the Zeldovich theory. The kinetic parameters of the CuCl phase formation were determined in the initial stage, when the critical nuclei possessing a zero surface energy (and an effective radius below 1.3 nm) appear in the glass matrix. The first-order phase transition in the new phase is 200 K below the melting temperature of CuCl single crystals. The temperature dependence of the CuCl phase nucleation rate reveals the second and third stages of the new phase formation. The activation energies for diffusion of the CuCl phase components in the glass matrix are determined.     

Size effects in the exciton energy and first-order phase transitions in CuCl nanocrystals in glass

The absorption spectra and the melting and crystallization kinetics of CuCl nanocrystals in glass are investigated in the range of particle radii 1–30 nm. Three discontinuities are found on the curves representing the size dependence of the melting point T _m(R) and the crystallization point T _c(R). As the particle radius gradually decreases from 30 nm in the range R⩽12.4 nm there is a sudden 60° drop in the temperature T _c in connection with the radius of the critical CuCl nucleus in the melt. A 30° drop in T _m is observed at R=2.1 nm, and a second drop of 16° in the temperature T _c is observed for CuCl particles of radius 1.8 nm. The last two drops are associated with changes in the equilibrium shape of the nanoparticles. In the range of smaller particles, R⩽1.34 nm the T _c(R) curve is observed to merge with the T _m(R) curve, owing to the disappearance of the work of formation of the crystal surface during crystallization of the melt as a result of the zero surface tension of CuCl particles of radii commensurate with the thickness of the effective surface layer. An increase in the size shift of the exciton energy is observed in this same range of CuCl particle radii (1–1.8 nm). The size dependence of the melting and crystallization temperatures of the nanoparticles is attributed to variation of the free energy in the surface layer of a particle. Fiz. Tverd. Tela (St. Petersburg) 41, 310–318 (February 1999)     

Molecular dynamics simulations of the formation for NaCl cluster at the interface between the supersaturated solution and the substrate

Molecular dynamics simulations of supersaturated aqueous NaCl solution including the Pt(100) or NaCl(100) crystal surfaces have been performed at an average temperature of 298 K. The behavior of the NaCl cluster produced in the solution have been studied through the consideration of the water dielectric property near the crystalline surfaces for understanding the role of crystal growth on the surface. The surfaces in the solutions greatly influence heterogeneous nucleation in crystallization process. Density profile of the supersaturated solution and polarization of water molecules was calculated in order to describe the effect of the surfaces on the solution structure at the solid–liquid interfaces. The formation levels of NaCl clusters heavily depended on the water orientation at the interfaces. NaCl clusters were easily formed near the Pt(100) surface compared with the NaCl(100) surface owing to a different construction of water molecules between the platinum and NaCl surface.     

In situ electron microscopy study of the thermal motion of liquid lead nanoparticles in thin aluminum foils

The kinetics of thermal motion of liquid lead nanoparticles in thin aluminum foils has been investigated in situ by transmission electron microscopy. Dependences of the diffusion coefficient of particles on temperature and particle size have been obtained. The results of the investigations can be interpreted on the assumption that the mobility of particles is controlled by the nucleation of steps on {111} faces of their surface.     

Assessment of growth of silver nanoparticles synthesized from an ethylene glycol–silver nitrate–polyvinylpyrrolidone solution

The growth of silver nanoparticles synthesized at 27±1 °C from an ethylene glycol–silver nitrate–polyvinylpyrrolidone solution has been assessed using a methodology that combines theoretical calculations based on the Mie Theory with experimental UV/VIS spectra and average particle size determinations from TEM micrographs. A plot of experimental maximum absorbance times bandwidth as a function of the corresponding average particle radius cube gives a curve with two linear portions of significantly different slopes, suggesting that formation of silver particles takes place during two distinct periods. These results and theoretical calculations seem to indicate that particle formation involves a long nucleation-growth period (about 13 h) during which the number of particles increases, followed by growth only, with a constant number of particles. The ratio of theoretical and experimental maximum absorbance indicates that even after 67 h of reaction, only 45% of the initial Ag(I) species has been transformed into silver nanoparticles.     

Ion Beam "Photography": Decoupling Nucleation and Growth of Metal Clusters in Glass

We demonstrate that room temperature MeV ion irradiation of a glass containing copper oxide initiates nucleation of pure Cu clusters via the inelastic "electronic" component of the ion energy loss, when the latter is above a threshold value. The clusters grow under subsequent thermal annealing, following Lifshitz-Slyozov-Wagner kinetics. The decoupling of nucleation and growth is analogous to that occurring in the photographic process. It allows total control over the cluster density, average size, and size distribution.     

Electronic excitations and optical response of metal nanocomposites under heavy ion implantation

The optical transmission and ion-induced luminescence under implantation of copper ions into quartz glass (a-SiO₂) have been measured to study the processes of formation of copper nanoparticles. It is shown that in situ measurements are more informative in comparison with the ordinary approach—investigation of the properties of ion-implanted nanocomposites only after implantation. A series of experiments was performed to prove that the ion-induced luminescence band at 545–550 nm is due to Cu⁺ ions dissolved in a-SiO₂. The combined use of in situ optical techniques makes it possible to monitor the states of implanted copper (metal nanoparticles and dissolved atoms) by the change in the optical absorption near the surface plasmon resonance of nanoparticles and by the intensity of ion-induced luminescence of Cu⁺ states in solid solution. It is shown that the optical bands of defects, dissolved copper, and nanoparticles can be separated within a simple linear approximation. Near the surface plasmon resonance and defect bands, ion-induced transient optical absorption has been revealed. The transient optical absorption near the surface plasmon resonance is explained by the temperature effect. The relationship between the electronic excitation, radiation-induced optical response, and the kinetics of nanoparticle formation is analyzed. Several stages of nanoparticle formation have been established: accumulation of implanted copper in solid solution, nucleation of nanoparticles, coalescence, growth of nanoparticles, and saturation of nanocomposites.     

Precipitation Kinetics in Binary Alloys near Grain Boundaries

Based on the method of the free energy density functional, the effect of grain boundaries on the precipitation process in binary alloys is considered. A comparative analysis of precipitation kinetics has been carried out for a single-crystal fragment of a solid solution and for a fragment containing a part of the boundary between two grains. We have found the influence of grain boundaries on the kinetics of the average radius of precipitates, their concentration, and nucleation rate for several compositions of the alloy.

     

Model of a nanoparticle in the theory of inhomogeneous medium

Basic thermodynamic characteristics (density distribution profile near the surface of a nanoparticle, adsorption, and interfacial surface tension) of the structureless nanoparticle-vapor/liquid equilibrium system are calculated using a unified approach. A joint solution of the basic equation in the Van der Waals theory of an inhomogeneous medium for the density distribution profile in the spherical system of coordinates and the Gibbs equation for the interfacial tension and absorption is obtained. The features of nucleation of nanoparticles are considered. The results generalize some familiar formulas and provide a more adequate interpretation of experimental results.     

Palladium nanoparticles on silica-rich substrates by spontaneous reduction at room temperature

Metallic palladium nanoparticles (<5 nm) have been spontaneously obtained from an acid solution of palladium chloride salt on silica-rich substrates (sepiolite, microfibrous silica, fumed silica, and milled silica glass) without any additional reduction treatment. Variable proportions of metallic palladium were straightforwardly obtained, depending on the nature of the substrate. The presence of a large amount of silanol groups acting as nucleation centers, seem to be a requisite to obtain small Pd nanoparticles. Additionally, an absorbance maximum associated to a surface plasmon resonance corresponding to the spherical particles of metallic palladium has been identified at a wavelength of 238 nm. The large specific surface of the employed substrates with the palladium nanoparticles suggest that these materials could be used for gas catalysis.     

Formation and growth of CuCl phase nuclei in glass

In glass, the CuCl phase starts to form a certain time after the onset of supersaturation. As the temperature is increased, the transient period (stage of formation of critical nuclei) shortens and the growth kinetics of the CuCl phase switches from the first to second stage. The observed pattern of the CuCl phase growth kinetics is fully consistent with the Zel’dovich-Frenkel classical theory of new-phase formation. The delay time is determined by the radius of the critical nucleus (CuCl nanomelt) and the diffusion coefficient of the limiting component, the Cu⁺ ions. The radius of the critical nucleus is about 1 nm and does not vary within a broad temperature range. The activation energy for the CuCl phase growth process does not change in the transition from the formation of critical nuclei to the first and, subsequently, second stage.     

Nonisothermal nucleation in a solid solution of CuCl in glass: Concentration effect upon heating of the solid solution

Physics of the Solid State - The influence of the heating rate of a solid solution of CuCl in glass on the size distribution of the produced CuCl nanoparticles is studied. The distribution curves...     

Fluctuation kinetics of the formation of nanoparticles

The method of stochastic simulations was applied to studying the kinetics of nucleation and growth of nanoparticles from solution in the presence of a ligand-yielding stabilizer. It was established that, at large stabilizer concentrations the process is not described within the framework of the law of mass action, being appreciably dependent on stochastic fluctuations in the number of free vacancies at the surface of the growing particle. This factor is responsible for the limitation of particle growth and for the formation of stable particles with a loose packing and dendrite-like structure. With decreasing stabilizer concentration, the kinetic characteristics of the process approach those typical of classical kinetics, whereas the packing density increases.     

纳米银组装结构上罗丹明B的表面增强荧光效应

采用表面自组装技术,在玻璃表面构筑银纳米粒子的二维组装结构。银纳米粒子组装结构的表面等离子共振光谱中偶极子表面等离子体共振对组装结构更为敏感而表现出较大位移。组装银纳米粒子可极大增强罗丹明B的荧光。荧光的表面增强效应主要来自银纳米粒子对荧光分子所处区域的局部电磁场增强,银纳米粒子的表面分子修饰对其表面增强效应有较大的影响。     

Microphase separation in a cross-linked epoxy phenol polymer

The microphase separation on the surface of samples of a cross-liked epoxy phenol (EP) polymer produced by the curing of a composition containing an epoxy diane oligomer (EO) and a phenol formaldehyde oligomer (PO) has been investigated using electron microscopy with decoration by silver chloride. It has been shown that, in the cross-liked EP polymer, there is a quasi-lattice of mixed-type AgCl nanoparticles, which consists of two sublattices. The formation of simple lattices of particles is caused by the microphase separation according to the type of nucleation and growth of fractal clusters of the cross-liked EO; the spinodal decomposition in the binary system; and the formation of interpenetrating polymer networks represented by the superposition of two lattices of particles or two infinite phase clusters (cross-liked EO and PO, respectively).