With this method, a metal organic precursor is decomposed in a high boiling point organic solvent at high temperatures in the presence of surfactants. In some cases reducing agents, such as 1,2 – hexadecanediol or oleylamine, are used.67, 81-83 In order to control size, morphology and particle size distribution, many parameters have to be controlled: concentration84-86 and nature of solvent,87 reaction duration,88 temperature,87 ratio of surfactant to precursor,85-86 heating rate,89 ratio between surfactants90 and etc. By adjusting these parameters, monodisperse particles in a broad range of size with a variety of morphologies can be obtained precisely.
The crystallization process at high temperatures yields particles with high crystallinity and magnetic properties. Further advantage of thermal decomposition is the high yield and the scalability of reactions by using inexpensive standard lab equipment. Beside the control of many parameters, however, usually the reaction should be performed in an inert atmosphere and it requires long reaction times in order to obtain well defined particles.
The yielding synthesized nanoparticles possess a core shell structure, where the highly crystalline core is surrounded by an inner shell of disordered iron oxide (dead layer) and an outer shell consisting of a monolayer of organic surfactant (oleate surfactant). The presence of surfactant makes the MNP dispersable in unipolar solvent like hexane directly after synthesis.81 In addition, doping of ferrite MNPs with other metals like cobalt is also directly possible during synthesis by adding the using the combination of both iron and cobalt precursors.
63By comparing the advantages and drawbacks of all aforementioned methods, thermal decomposition turns out to be the superior method for the synthesis of the desired magnetic nanoparticles.54, 91-92 Two routes for conducting thermal decomposition experiments exist: hot-injection and heat-up method.81, 93 For the hot-injection method, a solution which contains the metal-organic precursor is injected into the heated mixture of the remaining reactants.93 In contrast, for the heat-up route all reactants are already combined before starting the reaction.
Despite the slightly different experimental procedure of both methods, the principles of nanoparticle formation are the same. Both routes yield particles of similar monodispersity when synthesis parameters are well adjusted.92 In the following, the basic theoretical background knowledge, i.e.
the classical crystallization theory, for the synthesis of NP will be explained. This includes homogeneous nucleation, growth by kinetically and thermodynamically controlled processes and Ostwald ripening.