Two kinds of approaches for preparing surface-modified nanoparticles, such as post-synthesis surface modification and in-situ surface modification, are introduced in this chapter. Post-synthesis surface modification involves the surface modification of manufactured nanoparticles. The in-situ process involves surface modification during the nanoparticle synthesis. For a non-DLVO-type surface interaction, such as a steric or bridge force, a colloid probe Atomic Force Microscope (AFM) is useful for analysing the dispersion behaviour of the nanoparticles. Some examples of the relationship between the surface interaction and the dispersion behaviour of nanoparticles are introduced to develop the interface structure design.
Top1. Introduction
In order to apply nanoparticles and fine particles ranging from 1 nm to 10 µm in diameter in various fields, including applications as functional materials, medicines, cosmetics, and pigments, some of the most important technologies involve controlling the aggregation and adhesion behaviours of such particles. In particular, nanoparticles have different surface structures and surface interactions. Compared to sub-micrometre-sized particles, nanoparticles have an extremely high adhesion and aggregation tendency. Thus, it is quite important to develop techniques to control the dispersion, adhesion, and aggregation phenomena of nanoparticles, in order to apply them to functional materials and products.
Nanoparticles have already become an indispensable material in many industrial fields because of their unique electrical, magnetic, mechanical, optical, and chemical properties, which largely differ from those of their bulk materials (Marignier et al., 1985, Alivisatos 1996, Jun et al., 2005, Kimberly et al., 2002) . However, the formation of aggregates of nanoparticles can hinder the appearance of such unique properties. An example of a nanoparticle aggregate is shown in Figure 1, which was observed by a transmission electron microscope (TEM). An aggregate consists of several hundred to thousands of particles. Because of their non-uniform and large porous structures, these are very difficult to prepare and apply to advanced materials such as ceramics, composites, and other functional materials.
In terms of aqueous media dispersions, the DLVO theory, which was developed by Derjaguin, Landau, and Verwey & Overbeek (1947), is a very useful tool to control and characterise the dispersion phenomena. However, it is still a challenge to control the stability of a suspension with a large load of nanoparticles or suspensions in organic media. The surface modification of nanoparticles is one of the most commonly accepted methods to improve their dispersion stability under these challenging conditions. For example, because the surfaces of inorganic nanoparticles such as metal oxide nitride generally display hydrophilic behaviour, they are rather difficult to disperse in non-polar organic solvents. An organic hydrophobic molecular structure is formed by the adsorption or reaction of an organic surfactant. When modifying the nanoparticle surface, it is very important to design the surface structure based on the type of nanoparticle and the liquid media.
In this chapter, we first introduce some reasons why nanoparticles form aggregates so easily. Then, we introduce two kinds of approaches for preparing surface-modified nanoparticles: post-synthesis surface modification and in situ surface modification (Iijima & Kamiya, 2009, Kamiya & Iijima, 2010). Post-synthesis surface modification involves the surface modification of manufactured nanoparticles. The in situ process involves surface modification during the nanoparticle synthesis. For both methods, various surface modification techniques are discussed for the dispersion of various nanoparticles into various liquid media. For a non-DLVO-type surface interaction such as a steric or bridge force generated by surfactant adsorption and another surface modification in an organic solvent, a colloid probe atomic force microscope (AFM) is useful for analysing the dispersion behaviour of the nanoparticles. Some examples of the relationship between the surface interaction and the dispersion behaviour of nanoparticles are introduced.