62nd Annual Report on Research 2017

In the first year of the project, we focused our research on the synthesis and surface modification of magnetic nano/microstructures with anisotropy in both morphology and magnetic properties. In the second year, while we continued to optimize the synthesis of the magnetic nanostructures with anisotropic morphologies, we also explored their assembly and rheological behaviors in response to external magnetic fields.

Our primary choices of the materials are iron oxide (magnetite (Fe₃O₄) or maghemite (gamma-Fe₂O₃)) as they are earth abundant, easily available in large quantity at a relatively low cost, and imposing low adverse impact to the environment, so they are more relevant than other magnetic materials for future EOR applications. Our synthetic efforts focused on the development of effective methods for producing rod-shaped iron oxide nanostructures with well controllable size, aspect ratio, surface chemistry, and magnetic properties. As it has remained a great challenge in producing magnetic iron oxide with well-defined nano/microrod shapes by direct chemical synthesis, in this project we adopted several indirect methods for the synthesis of iron oxide based magnetic nano/microrods. These methods were proposed based on the fact that there are many effective methods for synthesizing nonmagnetic iron based nano/microrods such as hematite (alpha-Fe₂O₃) and iron(III) oxide-hydroxide (akaganeite, beta-FeOOH). In these indirect synthesis process, the nonmagnetic alpha-Fe₂O₃ or beta-FeOOH nano/microrods with desired dimension and aspect ratio were first prepared based on solution phase precipitation reactions, and then converted into magnetic iron oxide phases through reduction.

In the first synthesis method, we used a thin layer of silica coating to protect the rods from deformation during the reduction process. After the synthesis of the nano/microrods in either alpha-Fe₂O₃ or beta-FeOOH phases, we coated them with a layer of silica through a sol-gel process. The resultant core-shell particles are then reduced in H₂ flow or in a polyol solution at elevated temperatures, producing magnetic nano/microrods. We found that solution phase reduction was effective for reducing thin (tens of nanometers in thickness) beta-FeOOH nano/microrods into magnetite nano/microrods, but less effective for alpha-Fe₂O₃ nano/microrods or if the thickness of the beta-FeOOH rods was a few hundred nanometers or above. Reduction of the dry powder of the nano/microrod precursors under H₂ at an elevated temperature (~ 500 degree C) was found to be very efficient, albeit with the potential safety concerns of heating H₂ at high temperatures for large scale production.

In the second method, we developed a new surface-protection scheme to maintain the nano/microrod structure during the solution phase reduction. Polymeric capping ligands, such as polyacrylic acid, were used to effectively bind to the surface of the nano/microrods and maintain the overall morphology during the conversion process in heated DEG. This method eliminated the need of additional step for silica coating and significantly reduced the time and cost for potential large-scale production. Also, the polyacrylic acid is one of the polymeric surfactants that we proposed in the second step to improve the microscopic displacement efficiency by reducing the oil/water interfacial tension in additional to the proposed effect in enhancing the fluid viscosity. The relevant work was reported recently (Xu et al., Nano Lett., 2017, 17, 2713.) We have extended this surface-protected conversion method to other materials such as Co(OH)₂ and Ni(OH)₂ nanoplates to produce corresponding nanostructured oxide phases with enriched oxygen vacancies, and explored the applications of such structures for oxygen evolution reaction. A manuscript related to this work is currently under review in Nano Energy.

As the solution phase reduction was only limited to thin nano/microrods, we also explored a carbothermal method for replacing H₂ in the gas phase reducing. In this third method, the nano/microrod precursors were coated with a thin layer of polymer such as resorcinol-formaldehyde resin through a sol-gel like process. Calcination of the dry powder of the composite nano/microrods in an inert atmosphere such as nitrogen led to the decomposition of polymer into carbonaceous species which can further reduce the nano/microrods into magnetite phase. We found this method to be very effective for converting both small and large rods into magnetic phases, and the reduction process is highly controllable by the annealing temperature.

We have also carried out systematic studies on the magnetic assembly of the nano/microrods and in water. Although this study is still ongoing, we have already obtained some important results. We first revealed that the rod-shaped magnetic particles have stronger responses to the external fields, as expected in our original proposal. The electrostatic charges on the particle surface has significant influences on the assembly behavior of the magnetic nano/micro- particles. Parameters that can affect the electrostatic interaction such as the ionic strength and the surface charge separation can be used to manipulate the assembly behavior, allow the magnetic particles to assemble or disassemble in a fully reversible manner. We are currently working on the influence of the electrosteric repulsion to the magnetic assembly by introducing polyelectrolyte coating to their surfaces.

We also studied the magnetic assembly and disassembly of the magnetic particles at the air-water interface. Fe₃O₄@C core-shell nanostructures were synthesized using the above carbonization method, and then made hydrophobic so that they can self-assemble at air/water interface to produce a thin floating film on water surface. The floating film of the magnetic particles can be collected from water surface within seconds by applying an external magnetic field, and then rapidly reassemble into a new floating film with the original quality within a second. Such fast separation and reformation are beneficial to many applications that require recycling and enrichment of active materials. As the first demonstration of applications, we used such self-assembled thin films as the solar-thermal conversion materials for steam generation, and found a number of beneficial features including the high solar absorption, fast film separation and reformation, and enhanced water evaporation efficiency. A manuscript relevant to this work is currently under revision in Materials Chemistry Frontiers. While our initial focus was on spherical Fe₃O₄@C core-shell nanostructures, we are currently continuing this research using magnetic nano/micro- structures of anisotropic shapes.