61st Annual Report on Research 2016

In the first year of the project, we have focused on the synthesis and surface modification of magnetic nano/microstructures with anisotropy in both morphology and magnetic property. We targeted on the development of effective methods for producing rod-shaped iron oxide (magnetite (Fe₃O₄) or maghemite (gamma-Fe₂O₃)) nanostructures with well controllable size, aspect ratio, surface chemistry, and magnetic properties. We focused on iron oxides as the magnetic materials due to their earth abundance and relatively low production cost and low adverse environmental impact, which make them more relevant than other magnetic materials for EOR applications. In order to enhance the microscopic displacement efficiency, we also explored approaches to enhance the surface charges of the iron oxide nano/microrods or coat them with a layer of polymeric surfactants to help in altering the rock wettability and reducing the water/oil interfacial tension.

As  it has remained a great challenge in producing magnetic iron oxide with well-defined nano/microrod shapes, in this project we adopted an indirect method for the synthesis of iron oxide based magnetic nano/microrods. This method was 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 this indirect synthesis process, 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. We first made efforts in systematically studying the synthesis of alpha-Fe₂O₃ or beta-FeOOH nano/micro rods and established robust recipes for controlling the size and dimension of the products.

In our original plan, we proposed to use a thin layer of silica to protect the rods from deformation during the reduction process. We have successfully confirmed the effectiveness of this approach. 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. During the coating process, a silicon alkoxide precursor was hydrolyzed in the presence of nano/microrods to produce hydrolyzed monomers which can polymerize/crosslink through condensation reactions, producing an inorganic polymer deposited on the nano/microrod surface. The resultant core-shell particles are then reduced in H₂ flow or in a polyol solution such as diethylene glycol (DEG) 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 order to simplify the conversion process as required by the targeted EOR application, we explored two alternative methods for reduction. In the first 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.

We have extended the 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. We are currently exploring the applications of such structures for oxygen evolution reaction.

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 case, the nano/microrod precursors were coated with a thin layer of resorcinol-formaldehyde (RF) resin through a sol-gel like process developed previously in my group. Calcination of the dry powder of the composite nano/microrods in an inert atmosphere such as nitrogen led to the decomposition of RF 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. While the sol-gel like process used for coating the precursor nano/microrods with RF is simpler and more robust than that for silica coating, the reduction process is also highly controllable by temperature, and we were able to fully convert the nano/microrods to magnetite phase at ~ 400 degree C. As the surface of this composite nano/microrods is rich in carboxylate groups, we are now further grafting the surface with monomers containing both a C=C double bond and an amine group through EDC conjugation chemistry. Such surface modification will allow us to produce a robust polymer coating on the core surfaces by copolymerizing the surface double bonds with acrylamide or acrylic acid monomers.