Population balance models and Monte Carlo simulation for nanoparticle formation in water-in-oil microemulsions: implications for CdS synthesis

We address controlled CdS nanoparticle formation by tuning experimental synthesis conditions. To this end, a bivariate population balance equation (PBE) model has been developed based on time scale analysis, to explain the mechanism of nanoparticle formation in self-assembled templates. It addresses the process of mixing two water-in-oil (w/o) microemulsions, each containing a pre-dissolved reactant in the microemulsion drops. Brownian collision and coalescence of two water drops of nanometer size results in mixing and exchange of reactant molecules, leading to chemical reaction. The water insoluble reaction product nucleates to form a nanoparticle in an individual drop, which subsequently grows internally by consuming the excess product and by coalescence-exchange with other drops. Finite rates of nucleation and coalescence-exchange are accounted for in the PBE, while the rates of reaction and internal growth of nanoparticles are found to be instantaneous. Experimentally proven binomial redistribution of reactant and product molecules upon drop coalescence is implemented in the present work. This results in a very good prediction of experimental data of the mean aggregate number (MAN) and hence size of CdS nanoparticles. Both our model and Monte Carlo (MC) simulation quantitatively capture the reported variation of MAN with molar excess of Cd2+ concentration and microemulsion drop size. Our results together with previous experimental data establish that usage of stoichiometrically five times or more of excess Cd2+ concentration can cause surface adsorption and desirable enhanced emission intensity of CdS nanoparticles, without altering particle size. We also propose a simplified and computationally efficient univariate PBE model. The univariate model gives very fast (in minutes) and accurate estimates (for low reactant concentrations) of the number and mean size of CdS nanoparticles. Time-scale analysis offers a good a priori choice of the appropriate model based on range of reactant concentrations.     

Synthesis, photoluminescence, and adsorption of CdS/dendrimer nanocomposites

CdS/dendrimer nanocomposites can be synthesized from methanolic Cd(2+) and S(2-) with amine-terminated polyamidoamine dendrimers of generation 8 (G8NH(2)) as stabilizers. By controlling the preparation conditions, nanoparticles with diameters < or = 2 nm can be obtained with a narrow size distribution. They show blue photoluminescence at approximately 450 nm. We studied the effects of various additives on the photoluminescence and elucidated its mechanism. Stable aggregates of two to three G8NH(2) molecules with several CdS nanoparticles form; the particles are located at the surface of the G8NH(2) molecules. The adsorption of the CdS/G8NH(2) nanocomposites on flat substrate surfaces is determined by the substrate chemistry. The hydrophilic nature of G8NH(2) results in weak affinity to graphite but strong affinity to hydroxy-terminated substrates such as mica, oxidized silicon wafers, and carboxylate-terminated monolayers. Patterning of nanocomposites on these hydrophilic substrates is achieved by the microcontact printing method. We propose to use only one molecule, a large dendrimer, to control the nanoparticle formation and also the immobilization of the synthesized nanoparticle/dendrimer composites.     

Preparation of novel silica-cadmium sulfide composite nanoparticles having adjustable void space by size-selective photoetching

The size-selective photoetching technique was used to control the size of a CdS nanoparticle inside a silica shell. With monochromatic light irradiation, the diffuse reflectance spectra of silica-coated CdS nanoparticles were blue-shifted, and the size of the resulting CdS nanoparticles incorporated in the silica shells was adjustable by varying the wavelength of irradiated light. TEM observation revealed that the original CdS nanoparticle seemed to be in close contact with the amorphous silica shell to leave almost no clearance, while the monochromatic light irradiation caused the decrease in the size of CdS particles, resulting in the formation of a void space between the photoetched CdS core particle and the silica shell. The average void spaces available in the shells were calculated to be ca. 1.4 and 2.4 nm with the irradiation at 514 and 458 nm, respectively. These results indicated that the size-selective photoetching technique enables the regulation of void space formed in the core-shell structure by choosing the wavelength of irradiation light.     

Coagulation of nanoparticles in reverse micellar systems: a Monte Carlo model

The process of formation of nanoparticles obtained by mixing two micellized, aqueous solutions has been simulated using the Monte Carlo technique. The model includes the phenomena of finite nucleation, growth via intermicellar exchange, and coagulation of nanoparticles after their formation. Using the model, an exploratory study has been conducted to analyze whether the coagulation of nanoparticles is the reason for the formation of nanoparticles whose sizes are comparable to the size of the reverse micelles. The model explains the possible mechanism of coagulation of semiconductor nanoparticles formed within reverse micelles and its effect on the evolution of their size with time. The model is predictive in nature, and the simulation results compare well with those observed experimentally.     

Effect of the Spherical Silica Surface on the Photoluminescence of the Loaded CdS Nanoparticles

CdS nanoparticles were formed on the surface of silica microspheres by the improved layer‐by‐layer self‐assembled technique. High‐resolution electron microscope (HRTEM) image and energy dispersive x‐ray analysis (EDX) confirmed formation of a quasi‐continuous CdS nanoparticles film on the silica microspheres. The results of UV‐vis and fluorescence spectra display that the spherical silica surface has a great effect on the photoluminescence of the loaded CdS nanoparticles. In contrast to the CdS nanoparticles powder, the composite can exhibit the emission ascribed to the band gap transition when the CdS nanoparticles film is relatively thick. This phenomenon is probably due to an enhancement of the crystallinity of CdS nanoparticles induced by the silica spheres.     

Synthesis of CdS, ZnS and Ag2S nanoparticles stabilized by sodium bis(2-ethylhexyl)sulfosuccinate and polyoxyethylenesorbitan monooleate in aqueous medium

The effect of synthesis conditions (molar ratio between precursors, concentration of surfactants, synthesis temperature) on the size of CdS, ZnS and Ag₂S nanoparticles (NPs) stabilized by sodium bis(2-ethylhexyl)succinate and polyoxyethylenesorbitan monooleate was studied. It was established that stabilization by polyoxyethylenesorbitan results in formation of smaller NPs (～8 nm) as compared to that in the presence of sodium bis(2-ethylhexyl)sulfosuccinate (14–60 nm), which is due to the difference between the adsorption rates of these surfactants onto the surface of synthesized NPs. The resulting aqueous dispersions of CdS, ZnS and Ag₂S NPs exhibit long-term stability to sedimentation. The nanoparticle size increases insignificantly with temperature increasing to 65–70°C and rises abruptly at higher temperatures. The increase in the ratio between concentrations of precursors (sulfide and metal ions) also results in an increase in NP size, allowing one to synthesize nanoparticles of prescribed sizes. The optical properties of the resulting nanoparticles were studied. The positions of the exciton peaks and the luminescence intensity peaks of the dispersions of synthesized CdS and ZnS NPs were determined.     

Mechanism of nanoparticle formation in self-assembled colloidal templates: population balance model and Monte Carlo simulation

Modeling the nanoparticle formation mechanism in water-in-oil microemulsion, a self-assembled colloidal template, has been addressed in this paper by two formalisms: the deterministic population balance equation (PBE) model and stochastic Monte Carlo (MC) simulation. These are based on time-scale analysis of elementary events consisting of reactant mass transport, solid solubilization, reaction, coalescence-exchange of drops, and finally nucleation and growth of nanoparticles. For the first time in such a PBE model, realistic binomial redistribution of molecules in the daughter drops (after coalescence-exchange of two drops) has been explicitly implemented. This has resulted in a very general model, applicable to processes with arbitrary relative rates of coalescence-exchange and nucleation. Both the deterministic and stochastic routes could account for the inherent randomness in the elementary events and successfully explained temporal evolution of mean and variance of nanoparticle size distribution. This has been illustrated by comparison with different yet broadly similar experiments, operating either under coalescence (lime carbonation to make CaCO(3) nanoparticles) or nucleation (hydride hydrolysis to make Ca(OH)(2) nanoparticles) dominant regimes. Our calculations are robust in being able to predict for very diverse process operation times: up to 26 min and 5 h for carbonation and hydrolysis experiments, respectively. Model predictions show that an increase in the external reactant addition rate to microemulsion solution is beneficial under certain general conditions, increasing the nanoparticle production rate significantly without any undesirable and perceptible change in particle size.     

Modeling shell formation in core-shell nanocrystals in reverse micelle systems

The mechanisms responsible for the formation of the shell in core-shell nanocrystals are ion-displacement and heterogeneous nucleation. In the ion-displacement mechanism, the shell is formed by the displacement reaction at the surface of the core nanoparticle whereas in heterogeneous nucleation the core particle induces the nucleation (or direct deposition) of shell material on its surface. The formation of core-shell nanocrystals via the post-core route has been examined in the current investigation. A purely probabilistic Monte Carlo scheme for the formation of the shell has been developed to predict the experimental results of Hota et al. (Hota, G.; Jain, S.; Khilar, K. C. Colloids Surf., A 2004, 232, 119) for the precipitation of Ag2S-coated CdS (Ag2S@CdS) nanoparticles. The simulation procedure involves two stages. In the first stage, shell formation takes place as a result of the consumption of supersaturation, ion displacement, and reaction between Ag+ and excess sulfide ions. The growth in the second stage is driven by the coagulation of nanoparticles. The results indicate that the fraction of shell deposited by the ion-displacement mechanism increases with increasing ion ratio and decreases with increasing water-to-surfactant molar ratio.     

Stability and optical properties of dispersions of CdS,ZnS, and Ag<Subscript>2</Subscript>S nanoparticles synthesized in microemulsion

CdS, ZnS, and Ag₂S nanoparticles have been synthesized in microemulsion. The synthesis of nanoparticles depend on the composition of the reaction medium and on the proportions of the precursors. The phase diagram for sedimentation stable dispersion of the nanoparticles synthesized in microemulsion has been determined. The region of the stable nanoparticle dispersion is much smaller than the region of the inverse microemulsion in the n-heptane-AOT-H₂O system. The UV-vis absorption and photoluminescence spectra of the CdS, ZnS, and Ag₂S nanoparticles have been investigated. The size of the nanoparticles increases with an increase in droplet size in the microemulsion, and this shifts the exciton peaks.     

Fabrication of Luminescent CdS Nanoparticles on Short‐Peptide‐Based Hydrogel Nanofibers: Tuning of Optoelectronic Properties

The pH‐induced self‐assembly of three synthetic tripeptides in water medium is used to immobilize luminescent CdS nanoparticles. These peptides form a nanofibrillar network structure upon gelation in aqueous medium at basic pH values (pH 11.0–13.0), and the fabrication of CdS nanoparticles on the gel nanofiber confers the luminescent property to these gels. Atomic force microscopy, field‐emission scanning electron microscopy, and high‐resolution transmission electron microscopy clearly reveal the presence of CdS nanoparticles in a well‐defined array on the gel nanofibers. This is a convenient way to make organic nanofiber–inorganic nanoparticle hybrid nanocomposite systems. The size of the CdS nanoparticles remains almost same before and after deposition on the gel nanofiber. Photoluminescence (PL) measurement of the CdS nanoparticles upon deposition on the gel nanofibers shows a significant blue shift in the emission spectrum of the nanoparticles, and there is a considerable change in the PL gap energy of the CdS nanoparticles after immobilization on different gel nanofibrils. This finding suggests that the optoelectronic properties of CdS nanoparticles can be tuned upon deposition on gel nanofibers without changing the size of the nanoparticles.     

CdS/TiO2复合纳米微粒的原位合成及性质研究

采用一种新方法,在TiO₂表面原位合成CdS纳米微粒,并用红外光谱跟踪了CdS/TiO₂复合纳米微粒的形成过程.紫外吸收光谱研究表明TiO₂对CdS纳米微粒的形成有很好的稳定作用,荧光光谱研究结果表明,这种纳米异质结构有着良好的电荷分离.     

Direct conjugation of semiconductor nanoparticles with proteins

Nanocrystalline CdS particles directly conjugated with bovine serum albumin (BSA) protein were prepared by applying the supercritical fluid processing technique, rapid expansion of a supercritical solution into a liquid solvent. The direct conjugation takes advantage of the unique features of the process for nanoparticle formation. The BSA-conjugated CdS nanoparticles in stable aqueous suspension or in the solid state were characterized by using microscopy, X-ray diffraction, and optical spectroscopy methods. The results show that well-dispersed CdS nanoparticles are coated with BSA in a core-shell-like arrangement and that the protein species associated with the nanoparticles remain functional according to the modified Lowry assay. These BSA-conjugated CdS nanoparticles are also strongly luminescent, with the luminescence spectrum contributed to primarily by the exciton emission.     

Effect of chelating functional polymer on the size of CdS nanocluster formation

Chelating poly(acrylates-co-2-methylacrylic acid 3-(bis-carboxymethylamino)-2-hydroxy-propyl ester) microspheres of diameter 250-310 nm were prepared by the soap-free emulsion polymerization method for varying amounts of GMA-IDA. Then CdS/copolymer composite was generated by chemical deposition on the surface of the copolymer microspheres. By XRD analysis it is found that the chelated CdS nanoparticles are a pure cubic zinc blende structure. The CdS/copolymer composite is examined by UV-vis absorbance, photoluminescence, and TEM observation. Average CdS nanoparticle size calculated from Henglein's empirical curve is in the range of 3.0-8.0 nm and varies according to the GMA-IDA molar ratio during polymerization, pH value during chelation, and postchelation annealing temperature. Higher ratio of chelating group, pH value, and annealing temperature produce larger CdS nanoparticles. As GMA-IDA ratio increases, photoluminescence exhibits a red shift from 510 to 520 nm, photoluminescence increases, and bandwidth decreases. Photoluminescence of the CdS nanoparticle becomes negligible when diameter exceeds 5 nm.     

Preparation, characterization, surface modification and redox reactions of silver nanoparticles in the presence of tryptophan

The synthesis and characterization of water-soluble dispersions of Ag nanoparticles by the reduction of AgNO(3) using tryptophan under alkaline synthesis conditions are reported. The Ag nanoparticle formation was very slow at low concentration and rapid at extremes. For surface modification and redox reactions, manipulating the interparticles interaction controlled the size of Ag nanoparticles aggregates. Our results suggest that the replacement of the BH(4)(-) ions adsorbed on the nanoparticle surface by tryptophan destabilizes the particles and further caused aggregation. A mechanism is proposed for the formation of silver nanoparticles by tryptophan. The experimental results are supported by theoretical calculations. The Ag nanoparticles were characterized by UV-vis absorption, dynamic light scattering and transmission electron microscopy techniques.     

Silica nanoparticle formation in the TPAOH-TEOS-H2O system: a population balance model

A model describing the kinetics of silica nanoparticle formation in the TPAOH-TEOS-H(2)O system is presented. These nanoparticles are an important intermediate in the clear-solution synthesis of silicalite-1, so understanding the mechanisms by which they are formed and stabilized is a key step in determining the crystallization behavior of pure-silica zeolites. The model presented here is based on the mass-conserving form of the Becker-D?ring population balance equations, describing growth and fragmentation by addition or removal of monomeric units, and modified to account for rapid equilibration of small silicate species and electrostatic and/or template stabilization of nanoparticles. The model predictions compare favorably with the experimental results. It is found that nanoparticle evolution exhibits distinct time regimes consisting of TEOS hydrolysis, condensation, nanoparticle formation, Ostwald ripening, and a self-sharpening mechanism in particle size distribution toward equilibrium due to stabilization during which no apparent changes in average particle size and pH are observed. Finally, the model provides an alternative, to a recent hypothesis, kinetics point of view to explain the enhanced stability of nanoparticles over extended periods of time.     

Synthesis, surface modification, and multilayer construction of mixed-monolayer-protected CdS nanoparticles

Herein, we describe a study aimed at synthesizing mixed-monolayer-protected CdS nanoparticles and investigating the reactivity of surface-bound functional groups in order to facilitate the immobilization of nanoparticles on a solid substrate as well as the construction of a three-dimensional nanocomposite. CdS nanoparticles initially prepared by the reverse micelle method were used to modify nanoparticle surfaces with 1-decanethiol molecules by ligand exchange. Subsequently, 11-mercapto-1-undecanol was partially incorporated by a place exchange reaction, thereby providing stable, mixed-monolayer-protected CdS nanoparticles. The nanoparticles obtained at each step were characterized by FT-IR and UV-vis spectroscopy, transmission electron microscopy, and elemental analysis. The reactivity of surface hydroxyl groups was verified by a reaction with isocyanate-bearing molecules that provide carbamate bonds in high yields at ambient temperature. The obtained mixed-monolayer-protected nanoparticles were also successfully immobilized on a glass substrate through a carbamate-bond-forming reaction that could be further utilized for multilayer construction in a layer-by-layer fashion.     

Patterning porous matrices and planar substrates with quantum dots

Silica hydrogels and planar substrates were patterned with CdS nanoparticles using a photolithographic method based on the photo dissociation of thiols and cadmium-thiolate complexes. Silica hydrogels were prepared via a standard base-catalyzed route. The solvent was exchanged with an aqueous solution of CdSO₄ and 2-mercaptoethanol, and the samples were then exposed to a focused ultraviolet beam. Planar substrates were patterned by illuminating a precursor solution spin coated on the substrates. CdS nanoparticles formed in the illuminated spots, and had a diameter below about 2 nm. The diameter of the spots illuminated by the UV beam could be varied from a few hundred to a few μm, on both hydrogels and planar substrates. Samples were characterized with transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and optical absorption, photoluminescence and Raman spectroscopies. All these techniques confirmed the chemical identity of the CdS nanoparticles. To investigate the mechanism of nanoparticle formation, we took absorption spectra of the precursor solution as a function of irradiation time. In unirradiated solutions, we noticed a maximum at 250 nm, characteristic of Cd-thiolate complexes. The absorption at 250 nm decreased with increasing irradiation time. A new band appeared at 265 nm for exposures around 5 min, and that band shifted to 290 nm in samples exposed for 10 min. A yellow precipitate formed after about 30 min. XRD showed that the precipitate was cubic CdS, with a mean particle size of 1.4 nm. We attribute formation of CdS to the photodissociation of the thiols and of the Cd-thiolates. UV irradiation of these precursors yields a series of species that can react with Cd²⁺, such as RS^·, S²⁻ and H₂S. Small CdS nanoparticles form in the initial stages of illumination, and present absorption bands in the 265–290 nm region. These CdS aggregates grow, coalesce and precipitate for longer irradiation times.     

利用鲑鱼精DNA为模板构建CdS纳米线

以鲑鱼精DNA为模板合成了CdS纳米粒子，透射电子显微观察表明所生成的CdS是一种直径约为3nm的线形结构，紫外-可见吸收光谱，拉曼光谱和X-射线光电子能谱结果表明，CdS纳米粒子的生长点为DNA中的磷酸根，并且在这一体系中可能存在从CdS到DNA中碱基的电子转移。     

足球状CdS微米球的合成与发光性能的初步研究

应用化学水浴沉淀法,合成了具有特殊形貌的由纳米颗粒组成的Cds微米球,通过改变反应温度实现了对产物尺寸的控制．XRD测试结果表明生成产物为结晶良好的六角结构CdS,PL谱测试说明了产物有较好的光致发光性能．利用TEM,SAED,FEG-SEM和紫外吸收谱分析等测试手段对其结构和形成机理进行了研究．     

水溶性CdSe/CdS核壳纳米粒子制备的影响因素及其对CdSe/CdS光谱特性的影响

以半胱氨酸镉配合物为前体, 在水溶液中合成CdSe纳米粒子, 以CdS对其表面进行修饰, 得到具有核壳结构的CdSe/CdS 纳米粒子. 采用XRD, TEM表征其结构及形貌; 以荧光光谱研究了时间、pH值、壳量、壳前体加入方式、稳定剂用量等因素对CdSe/CdS光谱特性的影响.