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46904-AC7
Ordered Hybrid Polymer-Nanorod Composites for Renewable Energy

Stephen G. Boyes, Colorado School of Mines

Recently metallic and semiconducting nanorods have generated a great deal of interest, as they exhibit very different properties to nanospheres, primarily due to the ability to tune both the optical and electronic properties by controlling the size and aspect ratio of the nanorods. Control over these properties suggests nanorods have potential application in the next generation of electronic and photonic devices. However, realization of these applications ultimately requires manipulation and rational assembly of the nanorods. The goal of this program is to develop ordered assemblies of polymer modified metallic and semiconducting nanorods in phase separated diblock copolymer films. This research presents a novel and versatile method for the surface modification of metallic and semi-conducting nanorods using any thiol-terminated polymer prepared by reversible addition fragmentation chain transfer (RAFT) polymerization. The use of RAFT polymerization to surface modify nanorods and for the successful preparation of hybrid polymer-nanorod thin films will potentially have an impact on the fields of photonics, solar energy storage, and nanoelectronics.

During the first year of this project two major advances were made. The first advance involves the development of a templated polymerization technique for oxidative polymerization of pyrrole or thiophene using polymer modified gold nanorods as the template. Within this work, gold nanorods were surface-modified with poly(acrylic acid) (PAA) and utilized as a doping agent for the templated oxidative polymerization of pyrrole and thiophene.  Molecular weights of 10,880 g/mol and 1,120 g/mol of PAA were used in the surface modification of the gold nanorods.  The pyrrole hydrogen bonds with the acid functionality of the PAA and then undergoes oxidative polymerization via the addition of ammonium persulfate, yielding a 6-9 nm thick PAA-polypyrrole (PAA-PPy) coating around the gold nanorods depending on the molecular weight of PAA.  The PAA modified gold nanorods were also used in an alternative approach to the oxidative polymerization of pyrrole and thiophene.  In this case, Fe³⁺ cations were coordinated within ionized PAA chains, with molecular weights of 10,880 g/mol and 1,120 g/mol, on the surface of the gold nanorods.  The Fe³⁺ acts as an oxidizing agent to polymerize both pyrrole and thiophene which produces a 4-6 nm thick coating of PAA-PPy and a 4-6 nm coating of PAA-polythiophene around the gold nanorods, depending on the molecular weight of PAA. Following the preparation of the PAA-PPy coated nanorods, dissolution of the gold nanorods via a solution of KCN was utilized to yield a hollow network of PAA-PPy nanostructures with a diameter of 45-54 nm depending on the amount of pyrrole added to the system.

The second advance made with in this project involves the formation of hybrid nanostructures consisting of metallic nanospheres surrounding polymer modified gold nanorods. Within this work, PAA surface modified gold nanorods were placed into a deionized ultra filtered (DIUF) water solution of gold (Au), palladium (Pd), or platinum (Pt) salts and upon reduction with NaBH₄, zero valent metal nanoparticles were produced in the PAA. With a DIUF water system utilizing NaBH₄ as a reducing agent, Au nanoparticles of 7 ± 0.5 nm, Pd nanoparticles of 4 ± 0.25 nm, and Pt nanoparticles of 5 ± 0.4 nm were synthesized on PAA surface modified gold nanorods. Control over nanoparticle diameter is beneficial for many applications, therefore the concentration of Pt metal salts available for particle formation was varied in both an aqueous system of DIUF water and a solution of ethylene glycol (EG).  EG, which acts as both a solvent and a reducing agent, was used with varying amounts of Pt metal salt, ranging from 0.1 mg/mL, 0.2 mg/mL, and 1.0 mg/mL, in which the particle diameters increase from 2.0 ± 0.5 nm, to 4.0 ± 0.5 nm, and 6.0 ± 2.0 nm, respectively.  This shows that the diameter of Pt nanoparticle is dependant on the amount of Pt salt added to the system for reduction.  In order to verify if particle diameter could also be controlled in an aqueous environment of DIUF water, varying amounts of Pt metal salt ranging from 0.1 mg/mL, 0.2 mg/mL, and 1.0 mg/mL produced Pt particle diameters that increased from 2.5 ± 0.5 nm, to 4.0 ± 0.5 nm, and 6.0 ± 0.5 nm respectively.

 This program had an impact on both graduate and undergraduate students involved in the research. The project supported one graduate student for the 12 months of the reporting period and contributed to the student successfully defending his dissertation and graduating with his Ph.D. In conjunction to this, a senior undergraduate student was involved in the research as part of his senior research program. Both students presented oral presentations at the National ACS Meeting in New Orleans on this research, with the undergraduate student winning a travel award to attend the meeting and was also named outstanding senior research student within the chemistry department at the Colorado School of Mines.

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