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44641-G10
Synthesis of Metal-Filled Carbon Nanoparticle Chains from Polyacrylonitrile Functional Magnetic Nanoparticles

Jeffrey Pyun, University of Arizona

Executive Summary: We report the successful implementation of the proposed research to prepare polymer coated ferromagnetic nanoparticles and organize these colloidal precursors into 1-D carbon mesostructures. Our general approach involved the preparation of polystyrenic (PS) surfactants that were used to synthesize ferromagnetic cobalt nanoparticles (CoNPs). The functional coating around these CoNPs was then ligand exchanged from the initial polystyrene shell to polyacrylonitrile (PAN). Solution deposition of polyacrylonitrile coated cobalt nanoparticles (PAN-CoNPs) onto supporting surfaces in the presence of an external magnetic field resulted in the formation of micron-sized aligned mesoscopic chains. Pyrolysis of these preorganized structures resulted in the thermal conversion of PAN phases into partially graphitic carbon, affording carbon nanowire-like structures with ferromagnetic cobalt colloidal inclusions. Characterization was conducted by atomic force microscopy (AFM), transmission electron microscopy (TEM, field-emission scanning electron microscopy (FE-SEM), x-ray diffraction (XRD), vibrating sample magnetometry (VSM) and Raman spectroscopy. The key accomplishments in the first year of ACS-PRF funding are outline below.

Part 1: Synthesis of Polystyrene Coated Ferromagnetic Cobalt Nanoparticles

An improved synthetic method to prepare the PS-CoNP precursors was developed as a critical first step to enable access to appreciable quantities of ferromagnetic colloidal materials. We previously reported a method to synthesize well-defined PS-CoNPs, however, the yields of these materials were often limited to 100-200 mg per reaction. Attempts to scale up the preparation of PS-CoNP using this method were unsuccessful, affording CoNPs of varying size and morphology. By modifying the polymerization method to scale up the synthesis of polystyrenic surfactants and using a different thermolysis conditions to form CoNPs, the preparation of well-defined ferromagnetic PS-CoNP (particle diameter (D) = 17-22 nm) was achieved with controllable yields of 200, 400 and 820 mg. This result is a significant improvement in the field to prepare ferromagnetic CoNPs. PS-CoNPs isolated as powders were easily redispersed in non-polar solvents (e.g., tetrahydrofuran, toluene, dichloromethane).

Part 2: Functionalization of Ferromagnetic Cobalt Nanoparticles

With appreciable quantities of ferromagnetic CoNPs in hand, functionalization methods were investigated using a ligand exchange process. To facilitate exchange reactions of bound PS-surfactants used in the initial particle synthesis, polymeric surfactants possessing a strongly coordinating carboxylic acid end group was synthesized using controlled radical polymerization and reacted with PS-CoNPs. Facile exchange of these polymers was observed, as confirmed by 1H NMR spectroscopy and size exclusion chromatography (SEC) after acidic degradation of functional CoNPs and recovery of cleaved polymers. A critical piece of this functionalization process was the purification of functional CoNPs from the excess of polymeric surfactants used to promote the ligand exchange process. Magnetic filtration proved effective for isolation of functional CoNPs by passing a colloidal dispersion thru a magnetized column packed with steel wool. After eluting all excess polymers and solvent, the magnetic field is removed and functional CoNPs are eluted off the steel wool enabling rapid purification and isolation of these materials. The preparation of PAN-CoNPs was required to introduce carbon precursors onto ferromagnetic nanoparticles. The nitrile side chain groups on PAN were found to be sufficiently reactive to promote ligand exchange onto CoNPs. Isolation and degradation of PAN-CoNPs after ligand exchange provided conclusive evidence that only PAN surfactants were bound to CoNPs.

Part 3: Magnetic Assembly of Ferromagnetic Nanoparticles

The fundamental aspects of PS-CoNP dipolar orgnanization was conducted to insure tunable control of assembled morphologies of PAN-CoNPs. Investigation of conditions for the solution deposition of ferromagnetic CoNPs onto surfaces was conducted. The ability to access a wide range of morphologies composed of randomly entangled, field-aligned and liquid-crystalline nematic phases of mesoscopic chains was observed by control of particle concentration, external magnetic fields and assembly time. The 1-D dipolar assembly of dispersed ferromagnetic CoNPs into 1-D mesoscopic polymer chains was also confirmed in collaboration with Alamgir Karim at NIST at oil-water interfaces.

Part 4: 1-D Carbon Nanoparticle Chains

The solution deposition of PAN-CoNPs (100 mT) followed by pyrolysis of magnetically assembled mesostructures afforded 1-D carbon nanoparticle chains containing ferromagnetic cobalt colloidal inclusions. FE-SEM before and after pyrolysis (two stage process: 250 °C-air & 600 °C-argon) confirmed that 1-D field aligned morphologies were maintained in the formation of continuous carbon nanowires. Raman spectroscopy confirmed the presence of both disordered and graphitic carbon phases. Conductive tip atomic force microscopy also was conducted and confirmed the formation of semiconducting 1-D carbon after pyrolysis.

Future Work

• Vertical magnetic alignment of PAN-CoNPs to afford highly oriented carbon phases “grown” from surfaces as potential materials for field-effect transitors.

• Electronic characterization of carbon nanocomposites correlated with different pyrolysis conditions.

• Electrochemical characterization of carbon nanocomposites via cyclic voltammetry and impedence spectroscopy to evaluate potential of there materials as electrodes for supercapacitors.

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