The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

The shapes and sizes of platinum nanoparticles were controlled by changes in the ratio of the concentration of the capping polymer material to the concentration of the platinum cations used in the reductive synthesis of colloidal particles in solution at room temperature. Tetrahedral, cubic, irregular-prismatic, icosahedral, and cubo-octahedral particle shapes were observed, whose distribution was dependent on the concentration ratio of the capping polymer material to the platinum cation. Controlling the shape of platinum nanoparticles is potentially important in the field of catalysis.

https://science.sciencemag.org/content/sci/272/5270/1924.full.pdf

Shape-Controlled Synthesis of Colloidal Platinum Nanoparticles

Microemulsion-based media as novel drug delivery systems

The synthesis and study of so-called “nanoparticles”, particles with diameters in the range of 1−20 nm, has become a major interdisciplinary area of research over the past 10 years. Semiconductor nanoparticles promise to play a major role in several new technologies. The intense interest in this area derives from their unique chemical and electronic properties, which gives rise to their potential use in the fields of nonlinear optics, luminescence, electronics, catalysis, solar energy conversion, and optoelectronics, as well as other areas. The small dimensions of these particles result in different physical properties from those observed in the corresponding macrocrystalline, “bulk”, material. As particle sizes become smaller, the ratio of surface atoms to those in the interior increase, leading to the surface properties playing an important role in the properties of the material. Semiconductor nanoparticles also exhibit a change in their electronic properties relative to that of the bulk material; as th...

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Nanocrystalline semiconductors: Synthesis, properties, and perspectives

While the nanocatalysis field has undergone an explosive growth during the past decade, there have been very few studies in the area of shape-dependent catalysis and the effect of the catalytic process on the shape and size of transition metal nanoparticles as well as their recycling potential. Metal nanoparticles of different shapes have different crystallographic facets and have different fraction of surface atoms on their corners and edges, which makes it interesting to study the effect of metal nanoparticle shape on the catalytic activity of various organic and inorganic reactions. Transition metal nanoparticles are attractive to use as catalysts due to their high surface-to-volume ratio compared to bulk catalytic materials, but their surface atoms could be so active that changes in the size and shape of the nanoparticles could occur during the course of their catalytic function, which could also affect their recycling potential. In this Feature Article, we review our work on the effect of the shape of the colloidal nanocatalyst on the catalytic activity as well as the effect of the catalytic process on the shape and size of the colloidal transition metal nanocatalysts and their recycling potential. These studies provide important clues on the mechanism of the reactions we studied and also can be very useful in the process of designing better catalysts in the future.

Catalysis with transition metal nanoparticles in colloidal solution: nanoparticle shape dependence and stability.

Quantum-sized cadmium sulphide particles have been prepared in water-in-oil microemulsions stabilised by Aerosol-OT. The equilibrium particle size has been varied by changing the droplet size of the parent microemulsion in which the particles are prepared. The UV–VIS spectrum of such particles is sensitive to particle size and hence the kinetics of particle growth can be readily followed using UV–VIS spectrophotometric detection. Using the stopped-flow method, particle growth has been monitored at 280 nm as a function of droplet size, temperature and the nature of the oil phase. The rates are consistent with inter-droplet exchange of solubilizates being the rate-determining step. The apparent activation enthalpies calculated for the growth process agree well with those previously found for the exchange of small ions between microemulsion droplets. To interpret the kinetics, a relatively simple model is proposed based on the rapid coagulation mechanism first proposed by Smoluchowski. In the treatment the usual diffusion-controlled rate constant is replaced with the rate constant for inter-droplet communication.

Kinetics and mechanism of formation of quantum-sized cadmium sulphide particles in water–aerosol-OT–oil microemulsions

The sodium salt of the di-chained anionic surfactant bis-2-ethylhexylsulfosuccinate [Aerosol-OT or Na(AOT)] stabilises essentially monodisperse, spherical water-in-oil microemulsion (w/o) droplets in alkanes over a wide range of pressure, temperature and composition. In order to investigate the effect of change in counterion charge and size on the microemulsion properties, we have replaced the Na+ counterion by doubly charged ions. The surfactant is then M2+(AOT)2·nH2O: M is from the series Mg2+, Ca2+, Co2+, Ni2+, Cu2+ and Zn2+, and n is the number of water ligands associated with the surfactant molecule. The value of n was determined by FTIR and depends on M2+, but can be between 2 and 8 per molecule of M2+(AOT)2 depending on the nature of M. The ion replacement, assessed by UV–VIS spectrophotometry, is ca. 100% efficient. The effect of temperature on the phase stability of the single-phase M2+(AOT)2 water-in-oil (w/o) microemulsion systems is negligible, in contrast to that observed for the corresponding Na(AOT) system. The structure and properties of the microemulsion are found to be dependent on the counterion identity. Small-angle neutron scattering (SANS) and viscosity measurements provide evidence for the existence of rod-shaped aggregates for Co2+, Ni2+, Cu2+ and Zn2+ at low water constants given by w=[H2O]/[AOT]≈ 5, whilst for Mg2+ and Ca2+ spherical aggregates are present as for Na+. On further addition of water at constant surfactant concentration (w > 10) with Co2+, Ni2+, Cu2+ and Zn2+ the aggregates undergo a shape change, and a more spherical structure is favoured. The results may be explained in terms of the interaction of the different counterions with the SO–3 head group of the surfactant.

Variation of surfactant counterion and its effect on the structure and properties of Aerosol-OT-based water-in-oil microemulsions

The preparation of colloidal particles of platinum ‘in situ’ in water-in-oil microemulsions formed by the aerosol-OT (AOT)–water–heptane system and the hexadecyltrimethylammonium bromide (CTAB)–water–heptane–chloroform system is described. In both systems, particle growth is controlled and particle sizes of the order of a nm or so are obtained. In the AOT system, there is only a weak correlation between the particle size and droplet size. To obtain further insight into the mechanism of particle growth, the kinetics of the process were investigated. Nucleation particles (Pt°) are produced in an initial step by the reduction of PtCl2–6(PtIV) using hydrazine as reducing agent. The kinetics of this stage were found to be erratic and this was attributed to a reaction of AOT with hydrazine. This also had the effect of flocculating the microemulsion system (forming networks) which drastically perturbed the size measurements made using the sedimentation velocity (ultracentrifuge) method. Fractal growth was also indicated by light-scattering measurements.In CTAB microemulsions, the hydrazine was stable but a gradual replacement of the platinum complex ion PtIVCl2–6 by Br– was indicated. The kinetics of nucleation were affected by internal partitioning within droplets, but for both systems small particles in the 1–2 nm radius size range were obtained. The sizes were dependent on droplet size but again the particle sizes did not equate with the droplet sizes.The catalytic ability of these particles in microemulsions has been demonstrated.

Synthesis and characterisation of colloidal metal and semiconductor particles prepared in microemulsions

Characterisation of cadmium sulphide colloids in reverse micelles

In this article we desribe the preparation of microparticles of platinum and cadmium sulphide in reverse micellar media. The equilibrium particle size is rationalised in terms of the kinetics of the rate determining step. In the case of platinum this is the reduction step PtIV → Pt0. For CdS semiconductor microparticles the kinetics of microparticle formation has been followed as a function of wavelength using stopped-flow kinetics. The important step is shown to be the inter-droplet reagent transfer.

Thomas F. Towey

Papers

Kinetics and mechanism of formation of quantum-sized cadmium sulphide particles in water–aerosol-OT–oil microemulsions

Variation of surfactant counterion and its effect on the structure and properties of Aerosol-OT-based water-in-oil microemulsions

Synthesis and characterisation of colloidal metal and semiconductor particles prepared in microemulsions

Characterisation of cadmium sulphide colloids in reverse micelles

Microparticle Formation in Reverse Micelles