Bottom-Up Methods for Synthesis of Nanostructured Solids

The major advantage of precipitation synthesis such as the type described above is that large quantities of nanoparticles can be made. However, it can be difficult to tailor the size since only kinetic factors are available to arrest growth. The addition of complexing agents or surfactants can help control particle size. Following the example of gold nanoparticles, it is possible to synthesized 

Hydrothermal reactions typically produce nanometer-sized particles that can be quenched to form a nanoparticle powder or cross-linked to produce nanocrystalline stmctures (Feng and Xu, 2001). Hydrothermal conditions allow for reduction in solubilities of ionic materials and thus more rapid nucleation and increased ion mobility, resulting in faster growth. Via judicious choice of the hydrothermal conditions, a measure of control can be exerted over the size and morphology of the materials. As mentioned earlier, the viscosity and ionic strength of solvents is a function of the temperature and pressure at which the reaction is carried out. Other experimental parameters, such as the precursor material and the pH, have 

Given the choice of starting materials in hydrothermal reactions, it is worth noting that the larger the spectator ions, the poorer the crystallinity. This is explainable in terms of the electrostatic potential of the metal hydroxides. With lower electrostatic potentials comes a greater likelihood that the metal sol will undergo oleation and cross-link. 

An interesting phenomenon in water-oil-amphiphile systems is the presence of self-assembled arrays of amphiphiles (surfactants) called micelles. From 1948 to 1950, Philip Alan Winsor reported that upon simple mixing (i.e., without the need for high shear conditions), oil, water, and amphiphiles yielded clear, macro-scopically homogeneous single phases which he termed type IV systems (Winsor, 1948, 1950). The term microemulsion was introduced later by Jack H. Shulman, a Columbia University chemistry professor, to denote these thermodynamically stable optically isotropic, transparent oil-water-amphiphile dispersions (Shulman et al., 1959). Type IV systems contain small droplets of one liquid dispersed within the other, with a self-assembled layer of surfactant molecules (micelles) along the interface between the two phases. The spontaneous self-assembly of the micelle is driven by the thermodynamic tendency to minimize the surface tension between the water and the oil in the presence of the amphiphile (Hoar and Shulman, 1943). 

Micelle solutions were originally characterized with a bulk aqueous phase where the hydrophobic carbon chains were turned inward to help stabilize the oil phase. Later, reverse micelles were also characterized, where the conditions were reversed. A bulk oil phase was used with the hydrophilic head groups turned inward to help stabilize the aqueous phase. Micelles require very stringent conditions, dictated by the molar proportions of oil, water, and surfactant. However, the formation of micelle solutions is driven by the differences in the polarity of the two groups any factor that affects the polarity, such as temperature, 

---