Preparation of Nanomaterials in Flow at Supercritical Conditions from Coordination Complexes

Abstract

The development of nanosciences and nanotechnologies in the twenty-first century is linked to the progresses made with the nanomaterial synthesis approaches. Control, reproducibility, scalability, and sustainability are the key issues for the design of advanced nanostructured materials. Among the synthesis methods, the supercritical fluid-based flow process presents an efficient alternative for the continuous, controlled, scalable, and sustainable synthesis of nanomaterials, especially from coordination complexes, which is the main topic of this book chapter. First, the supercritical fluids are defined and their specific properties introduced with the possibility to adjust them playing with pressure, temperature, and composition for mixtures. The case of water is also described underlining the remarkable evolution from a polar solvent in normal conditions of pressure and temperature to a nonpolar one at supercritical conditions. After, the typical supercritical flow processes of nanomaterials are technically described in details with the different elements, namely injection, mixers, reactors, and pressure regulators. This allows introducing the main operating parameters giving access to a continuous and control synthesis of nanomaterials by mastering thermodynamics, hydrodynamics, and chemistry. Coupling chemistry of coordination complexes and chemical engineering in supercritical fluids leads to the design of high-quality and unique nanostructures. This is in particular illustrated with the synthesis of nanooxides from flow supercritical sol–gel syntheses. The access to highly crystallized oxides with controlled compositions is discussed with the synthesis of BaTiO3-based materials. The supercritical route is also a versatile method. Beyond the continuous production of nanooxides, it is also possible to prepare in flow nitrides, sulfides, selenides, phosphides, …, nanocrystals (GaN, CdS, CdSe, InP, …). Adding surfactants in situ or ex situ playing with the process offers the possibility to design hybrid organic/inorganic nanoparticles with a control of the strength of the bond at the interface between the inorganic core and the organic shell. This chapter is ended with the description of supercritical coflow reactors, which allow a high level of control of the synthesis operating conditions. All the bricks are now available from a chemical engineering and coordination complex chemistry point of view to go towards multisteps and one pot processes for the continuous and sustainable design of advanced and multifunctional nanomaterials.