Mechanism of Nanoparticle Formation in the Liquid-Phase Thermal Decomposition Method.

A nanoparticle (NP) formation mechanism based on thermal energy equilibration is proposed to explain the formation of NPs in the liquid-phase thermal decomposition route. The prevailing mechanism of monodisperse NP formation in this route limits the nucleation process to be single and instantaneous, followed by growth via monomeric addition. Herein, the extensive investigations carried out in the heat-up method of iron oxide NP synthesis reveal that multiple nucleation events occur in the system when the thermal energy remains constant or varies minimally with time, accompanied by particle growth. In addition, the growth occurs via coagulation and/or monomeric addition depending on the aggregation barrier established by the supplied energy, ligand used, heating rate applied, and the allowed aging period. By fundamentally understanding the influence of these parameters on the precursor decomposition process, a generalized NP formation mechanism in the heat-up approach is proposed and used as a guideline to prepare anisotropic particles without using any shape-directive agents. The mechanistic understandings were further extended to hot-injection and seed-mediated growth protocols which exhibited an analogous NP formation pathway.