Spontaneous formation and efficient stabilization of gold nanoparticles with an average diameter of 7 approximately 20 nm from hydrogen tetrachloroaureate(III) hydrate (HAuCl4.3H2O) were achieved in air-saturated aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer solutions at ambient temperature in the absence of any other reducing agent. The particle formation mechanism is considered here on the basis of the block copolymer concentration dependence of absorption spectra, the time dependence (kinetics) of AuCl4- reduction, and the block copolymer concentration dependence of particle size. The effects of block copolymer characteristics such as molecular weight (MW), PEO block length, PPO block length, and critical micelle concentration (cmc) are explored by examining several PEO-PPO-PEO block copolymers. Our observations suggest that the formation of gold nanoparticles from AuCl4- comprises three main steps: (1) reduction of metal ions by block copolymer in solution, (2) absorption of block copolymer on gold clusters and reduction of metal ions on the surface of these gold clusters, and (3) growth of metal particles stabilized by block copolymers. While both PEO and PPO blocks contribute to the AuCl4- reduction (step 1), the PEO contribution appears to be dominant. In step 2, the adsorption of block copolymers on the surface of gold clusters takes place because of the amphiphilic character of the block copolymer (hydrophobicity of PPO). The much higher efficiency of particle formation attained in the PEO-PPO-PEO block copolymer systems as compared to PEO homopolymer systems can be attributed to the adsorption and growth processes (steps 2 and 3) facilitated by the block copolymers. The size of the gold nanoparticles produced is dictated by the above mechanism; the size increases with increasing reaction activity induced by the block copolymer overall molecular weight and is limited by adsorption due to the amphiphilic character of the block copolymers.

A single-step synthesis of gold nanoparticles with an average diameter of approximately 10 nm from hydrogen tetrachloroaureate(III) hydrate (HAuCl4.3H2O) has been achieved in air-saturated aqueous solutions that contain poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers but not any other reducing agent. These amphiphilic block copolymers act as both reductants and colloidal stabilizers and prove very efficient in both functions. The formation of gold nanoparticles is controlled by the overall molecular weight and relative block length of the block copolymer. The synthesis procedure reported here is environmentally benign and economic, as it involves the minimum possible number of components: it uses water as the solvent, it uses commercially available polymers, it proceeds fast to completion, and it results in a "ready-to-use" product.

Highly crystalline TiO2 nanostructures were prepared through a facile inorganic acid-assisted hydrothermal treatment of hexagonal-structured assemblies of nanocrystalline titiania templated by cetyltrimethylammonium bromide (Hex-ncTiO2/CTAB Nanoskeleton) as starting materials. All samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The influence of hydrochloric acid concentration on the morphology, crystalline and the formation of the nanostructures were investigated. We found that the morphology and crystalline phase strongly depended on the hydrochloric acid concentrations. More importantly, crystalline phase was closely related to the morphology of TiO2 nanostructure. Nanoparticles were polycrystalline anatase phase, and aligned nanorods were single crystalline rutile phase. Possible formation mechanisms of TiO2 nanostructures with various crystalline phases and morphologies were proposed.

Gold nanoparticles with an average diameter in the range 5-20 nm have been synthesized from hydrogen tetrachloroaureate (III) hydrate (HAuCl(4)·3H(2)O) in air-saturated aqueous PEO-PPO-PEO block copolymer solutions at ambient temperature in the absence of any other reducing agent (PEO: poly(ethylene oxide), PPO: poly(propylene oxide)). The particle size was controlled by the block copolymer concentration and PEO and PPO block lengths. Our findings indicate that longer PEO blocks lead to an increase in particle size because of an increase in reaction activity. Adsorption of PO segments on gold nanoparticles seems to prevent particle growth from aggregation, and results in small particle size and high colloidal stability. An increase of the HAuCl(4) concentration causes a change in the particle shape from spherical to triangular or hexagonal nanoplates.

We report here on the effects that the solution properties of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers have on the reduction of hydrogen tetrachloroaurate(III) hydrate (HAuCl4.3H2O) and the size of gold nanoparticles produced. The amphiphilic block copolymer solution properties were modulated by varying the temperature and solvent quality (water, formamide, and their mixtures). We identified two main factors, (i) block copolymer conformation or structure (e.g., loops vs entanglements, nonassociated polymers vs micelles) and (ii) interactions between AuCl4- ions and block copolymers (attractive ion-dipole interactions vs repulsive interactions due to hydrophobicity), to be important for controlling the competition between the reactivities of AuCl4- reduction in the bulk solution to form gold seeds and on the surface of gold seeds (particles) and the particle size determination. The particle size increase observed with increased temperature in aqueous solutions is attributed to enhanced hydrophobicity of the block copolymer, which favors AuCl4- reduction on the surface of seeds. The lower reactivity and higher particle sizes observed in formamide solutions are attributed to the shielding of ion-dipole interaction between AuCl4- ions and block copolymers by formamide, which overcomes the beneficial effects of formamide on the block copolymer conformation (lower micelle concentration).

Growth processes of hydrocarbon droplets (C6-C16: n-hexane, cyclohexane, n-octane, n-decane, n-dodecane, n-tetradecane, and n-hexadecane) in oil/water emulsions under surfactant-free conditions were examined by dynamic light scattering (DLS) and freeze-fracture electron microscopy (FFEM). DLS results showed that the growth rate of droplet size decreased with increase in hydrocarbon chain length. For example, n-hexane droplets grew within 1 h from submicrometer to micrometer droplets, while n-hexadecane droplets with sizes of several tens of nanometers kept their dispersibility for 24 h. We determined the growth processes as coalescence and molecular diffusion (Ostwald ripening) in terms of the Lifshitz-Slyozov-Wagner theory and the Smoluchowski equation. Furthermore, FFEM was used to examine the growth mechanism in detail. Direct imaging of n-hexane and cyclohexane droplets by FFEM allowed us to observe very fine oil droplets (∼10 nm in diameter) though DLS could not detect these droplets, suggesting that fine droplets of shorter hydrocarbons such as n-hexane and cyclohexane grow via both molecular diffusion and coalescence processes in a very short time after preparation.

We report here the surfactant-and reducer-free synthesis of gold nanoparticles from an aqueous hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl 4 ·4H 2 O) solution using a high-frequency (950 kHz) ultrasound (in the absence of any stabilizing, capping and reducing agents). In particular, size, shape and stability of gold nanoparticles prepared by the 950 kHz ultrasound irradiation (sonication) for 8 min in the surfactant-free aqueous solutions were examined in terms of AuCl 4 -concentration (in the range of 0.01-0.1 mM), additional salts (NaCl, HCl and NaOH) and temperature (in the range of 4-60 ˚C). We found that higher AuCl 4 -concentration promoted particle growth (size increase) and plate formation.In addition, the plate formation was enhanced with the addition of NaCl or HCl (but not NaOH). This is most likely due to the AuCl 4 -reduction on a certain crystal facets (e.g., (110) facets) caused by the adsorption of Cl -ions on specific crystal facets (e.g., (111) facets).Furthermore, we revealed that the temperature elevated above 50 ˚C led to the formation of spherical gold nanoparticles with the diameter of 20-60 nm from a 0.1 mM AuCl 4 -aqueous solution while triangular plates formed coexisting with spherical nanoparticles below 50 ˚C.

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