Experimental And Theoretical Investigations On The Effect Of Magnetic Field On The Growth Of Co₃O₄ Nanoparticles

As rapid advances in nanoscience and nanotechnology, recent years our demands for nanomaterials no longer limit in the simply synthesis of them. Due to their decreased dimensions and elevated specific surface areas, behaviors of nanomaterials are often determined by their surface properties rather than bulk properties, and this otherwise requires us to be able to realize the controlled synthesis of nanomaterials, for instance, to control their size distribution and the morphology. Common methods for the controlled synthesis of nanomaterials always involve the addition of surfactants, and have distinct disadvantages of possibly being toxic for environment and human bodies. Thus other surfactant-free synthetic methods still need to be further developed.Magnetic-field-induced hydrothermal methods have unique advantages compared to those traditional routes, such as that the reaction systems are simple and environmental-friendly. Hitherto, researchers have already obtained a series of plausible findings in this field, however, most of these findings are experimental, and theoretical works which are essential for the further exploration of this field are very rare, for instance, we have not achieved a persuasive explanation for why and how magnetic fields influence the growth process and physical properties of nanomaterials. Moreover, current research in this field only focuses on the synthesis of ferro- or ferri-magnetic materials. Thus magnetic-field-induced hydrothermal methods still deserve us to further explorer, from both theoretically and experimentally.In this dissertation, we select a kind of non-ferromagnetic material as the researching object, and then both theoretically and experimentally investigate the effect of external magnetic fields on the morphology of nanomaterials during their growth, in order to find the exact mechanism how magnetic fields work, and to further expand it to the magnetic-field-induced synthesis of other materials. The main achievements and innovations in this paper are summarized as follows:1. Successfully realized the controlled synthesis of Co₃O₄ nanocubes by applying magnetic fields to hydrothermal systems.We find a simple, surfactant-free hydrothermal system to synthesis Co₃O₄ nanoparticles, and investigate the effect of external magnetic fields on the morphology of final products. We found that the as-prepared Co₃O₄ nanoparticles are irregular polehydra when external magnetic fields are absent; however, when external magnetic fields are applied to the reaction system, the obtained Co3O4 nanoparticles are prefect cubes. By tuning other reaction parameters such as the amount of reactants and the composition of solvents, prefect Co3O4 nanocubes with size ranging from 8 nm to 120 nm were successfully prepared, which indicated that the effect of external magnetic fields is applicable for particles with different size.2. Successfully validated the issue of"Magnetic fields can vary the energy of different crystal surfaces of materials"from a theoretical aspect.We used a kind of software (VASP) which based on density function theory to calculate the energy of different crystal surfaces of Co3O4. By establishing surface models with different magnetic configurations, we simulated the influence of external magnetic fields and detailedly investigate the variation of energy of Co3O4 (100) and (111) surfaces. The surface energy of (111) and (100) are 1.849 J/m2 and 1.64 J/m2 respectively when magnetic fields are absent; but when magnetic fields are applied, these values change to 1.326 J/m2 and 1.44 J/m2, respectively. Under the influence of external magnetic fields, the energy of (100) decreases rapidly and thus replaces (111) to be the surface with lowest energy. According to the thermodynamic theory of crystal growth, surface with lowest energy will be exposed in the final morphology of crystal. Therefore, when external magnetic fields are applied, (100) surfaces are exposed, leading to the preferred formation of nanocubes, which is in good agreement with our experimental results. When external magnetic fields are absent, there may be a competitive growth between (100) and (111) surfaces; and the final morphology of particles are irregular polyhedra.In summary, we both theoretically and experimentally examine the issue of"Magnetic fields can vary the energy of different crystal surfaces of materials". Based on this, we can utilize magnetic fields to control the growth of materials. Magnetic-field-induced hydrothermal methods, as simple, surfactant-free and scalable methods, have extraordinary broad prospect in future applications. Our findings not only help to understand the interactions between magnetic fields and materials more deeply, but also have instructional meanings for future investigations on the synthesis of materials under magnetic fields.