Why Surface Modification?The surface modification of colloid particles of various nature and size has become a very common practice during the last few decades for a number of reasons. This section describes the most relevant objectives that lead to the design of procedures aiming to a controlled surface modification.
7.2.1
Chemical and Colloidal StabilityMajor problems to be overcome in nanoparticle dispersions are the degradation of the material through chemical etching and the agglomeration caused by strong van der Waals attractive forces.Chemical etching is not an important limitation in the case of noble metal nanoparticles, since these particles are stable in most environments, but it will be shown below that even they can be dissolved if not properly protected. The problem is much worse in the case of other transition metals, such as iron or nickel, since they are readily oxidized, so that their magnetic properties dramatically change. Similarly, many semiconductor nanoparticles, such as metal chalcogenides, are air sensitive, so that they can be completely dissolved in the presence of oxygen and light. These processes will be discussed below, as well as possible means to prevent them.On the other hand, colloidal stability is always a relevant issue when inorganic (lyophobic) nanoparticles are synthesized in liquid solvents. The main forces affecting colloidal stability are (attractive) van der Waals forces and (repulsive) double layer and steric interactions. Such forces have been extensively discussed in the literature. In the case of aqueous dispersions (and, in general, in polar solvents), double-layer interactions will be primarily involved in the separation of colloid particles from each other, while for dispersions in non-polar solvents, steric effects due to adsorbed organic chains will be of major relevance.Particle size has significant effects upon colloid stability. As a consequence, compared to micron-sized particles, nanosized particles exhibit different flocculation and coagulation behavior. Hence, they require different mechanisms to achieve colloid stability. Deryagin and Landau [1] and Verwey and Overbeek [2] independently developed a theory (known as DLVO theory) to calculate the energy (V T ) of interaction between charged spherical particles as a function of interparticle separation and other parameters. V T is considered to be the sum of the double-layer repulsion V R and van der Waals attraction V A . For small, nanosized particles (radius a, separation D) this gives:where e 0 is the permittivity of vacuum, e r is the relative permittivity of the medium in the diffuse layer, w 0 the surface potential, and j ±1 the double layer thickness. 7.2 Why Surface Modification? 217 7 Metal and Semiconductor Nanoparticle Modification via Chemical Reactions 218 Fig. 7.1 DLVO total interaction energy vs interparticle separation curves as the particle size is varied from the nanosized to micronsized domains. The profiles are calculated from Eq.(1), based on the following parameters: A = 2´10 9 J, w ...
