Published: January 24, 2011 r2011 American Chemical Society 2656 dx.doi.org/10.1021/jp1104196 | J. Phys. Chem. C 2011, 115, 2656–2664 ARTICLE pubs.acs.org/JPCC Preparation of Elemental Cu and Ni Nanoparticles by the Polyol Method: An Experimental and Theoretical Approach Kyler J. Carroll, † J. Ulises Reveles, ‡ Michael D. Shultz, † Shiv N. Khanna, ‡ and Everett E. Carpenter* ,† † Department of Chemistry and ‡ Department of Physics, Virginia Commonwealth University, Richmond, Virginia, 23284-2000, United States b S Supporting Information ABSTRACT: This paper discusses the relationship between synthesis conditions, crystal morphology, and theoretical modeling of copper and nickel nanoparticles prepared by a modified polyol process. The polyol serves as a solvent, a reducing agent, and a capping agent, and we investigate the role several polyol types play in the nucleation and growth of metallic nanoparticles. The nanoparticles are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Our results demonstrate that changing the solvent system from a short chain polyol (ethylene glycol) to a long chain polyol (tetraethylene glycol) greatly affects the resulting morphology of copper nanoparticles. These results suggest that the polyol is playing a major role as an in situ capping agent and that the various polyol chain lengths in-turn result in various particle morphologies by directly altering the nucleation and growth steps. We were also able to use theoretical modeling to investigate the mechanism for growth to better understand the intermediate structure stability. This work presents an alternative approach in investigating the polyol mechanism by using both theoretical and experimental results and opens new insight for the synthesis of metals and alloys by the polyol process. ’ INTRODUCTION There has been extensive research into the synthesis of metal particles by the polyol process, and it continues to generate numerous publications. 1 Over the past decade the polyol process has been used to prepare elemental Co, Ni, Cu, Ag, Au, Pt, Pd, Cd, and Fe and also bimetallic alloys of CoNi, AgPd, AuPt, and FePt. 2 Several papers also highlight the formation of metal oxides such as Fe 3 O 4 , CoFe 2 O 4 , CuFe 2 O 4 , and ZnFe 2 O 4 . 3 More recently, the polyol process has been used to prepare aqueous ferrofluids as MRI contrast agents, bimetallic core/shell nano- particles for catalysis, TiO 2 nanocomposites for monolithic dye- sensitized solar cells, cobalt carbide nanoparticles for permanent magnet research, and Ag and Ag@Au nanoparticles for surface- enhanced Raman scattering. 1a,4 The polyol process refers to a polyalcohol that acts not only as a solvent but also as a mild reducing agent, and when coupled with a base, it serves as a perfect medium for the reduction of metal salt precursors. In this process, a solid inorganic precursor is sus- pended in a liquid polyol. The solution is then stirred and heated to a given temperature, which can reach the boiling point of the polyol for less easily reducible metals. The starting materials can be either hydroxides (e.g., Cu(OH) 2 ), nitrates (e.g., AgNO 3 ), oxides (e.g., Cu 2 O), chlorides (e.g., FeCl 2 ), or acetates (e.g., Ni(CH 3 COO) 2 ). The reduction to metal can be achieved in various polyols such as ethylene glycol, propylene glycol, diethyl- ene glycol, trimethylene glycol, and butylene glycol (Figure 1). The choice of which polyol is used for the reduction of metal precursors is determined by the boiling point and reduction potential of the glycol; for example, easily reducible metals (Pt, Pd, and Cu) do not require high heat and can be reduced in propylene glycol (bp ∼188 °C), while less easily reducible metals (Co, Fe, and Ni) require higher temperature for which trimethyl- ene glycol may be suitable (bp 327 °C). Although the physical properties such as size, shape, and crystal structure of the particles have been controlled by manip- ulating synthetic conditions influencing the nucleation and growth steps, there are limited attempts to fully understand the polyol reaction mechanism. 5 Consequently, nanoparticles are often synthesized through trial and error or combinatorial methods, running large numbers of experiments and system- atically varying the parameters. Larcher et al. utilized theoretical calculations to prepare a thermodynamic approach to a mecha- nism for the polyol process. 6 In their calculations they assumed that precursor reduction results in the total oxidation of ethylene glycol into CO 2 and H 2 O. In this situation, above the boiling point (200 °C), the ethylene glycol has the maximum reducing power. Experimental research on Cu and Ni shows however that these conditions are not necessary for the reduction. 2b,7 Received: November 1, 2010 Revised: January 3, 2011