Microwave-Assisted Polyol Process for Synthesis of Ni Nanoparticles Dongsheng Li and Sridhar Komarneni w Materials Research Institute, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 Ni nanoparticles of controlled size in the range 30–100 nm were synthesized with polyvinyl pyrrolidone (PVP) and dodecylamine (DDA) as protecting agents through a microwave-assisted tech- nique using ethylene glycol both as a reducing agent and a sol- vent. The morphology of Ni nanoparticles was controlled by the amount of DDA added in these systems. Particle size and size distribution were controlled by the concentration of metal source and the DDA to PVP ratio. The synthesized Ni nanoparticles were characterized by transmission electron microscopy for par- ticle size and morphology, and size distribution was calculated by Image J TM software. Experimental UV-Vis spectra of Ni nanoparticles matched with the calculated spectrum based on Mie’s theory. The microwave-assisted polyol process was found to be faster than the conventional-polyol process in Ni nanopar- ticle synthesis and the former technique is expected to be cost- effective compared with the latter. I. Introduction M ETAL nanoparticles are of great interest in the modern materials world because of their special electronic, cata- lytic, optical, and magnetic properties as compared with their bulk counterparts. These special properties have been applied in various fields ranging from microelectronics to biomedical sci- ence. Magnetic nanoparticles, such as Fe, Co, and Ni, have at- tracted much attention because of their unique properties and applications in various fields. Ni nanoparticles could be used as electrode materials in multilayer ceramic capacitors and cata- lysts. 1,2 Ni nanoparticles have been synthesized through many methods, such as chemical/electrochemical methods, sonochem- ical method, 3 microwave plasma deposition, 4 conventional po- lyol process, 5 and spray-pyrolysis method 4–6 with various reducing agents. In order to obtain dispersed nanoparticles, Ni nanoparticles have also been synthesized in the Al 2 O 3 matrix, the Al-MCM41 host, or in the polymer matrix. 7–9 However, Ni nanoparticles with controlled morphology were obtained with limited success. Binary protecting agent systems were used to control particle shape in the synthesis of CdSe and Co nano- particles. Manna et al. 10 reported the synthesis of shape-con- trolled CdSe nanoparticles with a binary surfactant system containing tri-n-octylphosphine oxide (TOPO) and hex- ylphosphonic acid (HPA). Puntes et al. 11 synthesized magnetic cobalt nanorods and nanospheres with narrow size distributions and a high degree of shape control by the injection of an or- ganometallic precursor into a hot surfactant mixture of TOPO and oleic acid under an inert atmosphere. In this paper, the microwave-assisted polyol method (M-P process) was reported for the synthesis of Ni nanoparticles. Re- ducing reaction was carried out in a binary protecting agent system including polyvinyl pyrrolidone (PVP) and dodecylamine (DDA). Ni nanoparticles with controlled morphology were syn- thesized using this method. This technique is expected to be more cost-effective for large-scale production because it is sim- pler, for example, compared with the procedure of injection of an organometallic precursor into a hot surfactant mixture. 11 II. Experimental Procedure Nickel acetate tetrahydrate was used as a Ni metal precursor in all experiments. Ethylene glycol and PVP were used as a reduc- ing agent and a protecting agent, respectively. DDA was used as a second protecting agent, which could coordinate with the Ni ion precursor to affect the morphology of nanoparticles. Pro- tecting agents, PVP and DDA, were dissolved in ethylene glycol. The metal precursor was mixed with reducing agent and pro- tecting agents and a small amount of H 2 PtCl 6 was added in some experiments to form the seeds for the synthesis of Ni nanoparticles. The mixed solution was treated using a micro- wave digestion system (MARS-5, CEM Corp., Matthews, NC); the details of this equipment were given in several of our pre- vious publications. 1216 Conventional-polyol (C-P) experiments were carried out in Parr bombs heated in an oven. The micro- wave-polyol (M-P) reactions were carried out at 1951C for 45 min in all cases while the C-P reactions were carried out for 2–17 h. The synthesized Ni particles could be kept in the reaction so- lution with the protecting agent for months without oxidation. Before characterization, the synthesized particles were centri- fuged and washed in alcohol to remove most of the protecting agents. The characterization of Ni particles was conducted quickly afterward to avoid the oxidation of the Ni particles. The size and morphology of synthesized particles were deter- mined using a transmission electron microscope (TEM, Philips 420, Tungsten-based 120 Kev, Eindhoven, the Netherlands). The energy-dispersive spectrometry (EDS) analysis was carried out with a Cresham Sirius 30 Si (Li) X-ray detector (Cupertino, CA) attached to the Philips 420 TEM. Synthesized Ni nanopar- ticles were characterized by powder X-ray diffraction (XRD). The XRD patterns were recorded on a scintag diffractometer (Cupertino, CA) operated at 35 kV and 30 current with CuK a radiation. Particle size distribution was determined by Image J TM software based on the assumption that particles are spher- ical in size (particle size was calculated based on 95% confidence interval). For the optical property characterization, the reaction solutions were diluted with alcohol. The UV-Vis spectra were recorded with samples in 1 cm cuvettes using Agilent 8453 UV- Visible (UV-Vis) Spectrophotometer (Palo Alto, CA) at 251C, in the spectral range of 250–1100 nm. III. Results and Discussion (1) Theoretical Basis For the synthesis of magnetic metal nanoparticles such as Ni and Co nanoparticles, it is difficult to prepare isolated magnetic nanoparticles, partially because the forces between the particles are large. These forces are due to the high electron affinity and the high surface tension arising from the partially filled d-orbital, from the van der Waals forces between polarizable metal par- ticles, and from magnetic dipole interactions. With PVP as a J ournal J. Am. Ceram. Soc., 89 [5] 1510–1517 (2006) DOI: 10.1111/j.1551-2916.2006.00925.x r 2006 The American Ceramic Society 1510 N. Dudney—contributing editor This research was supported by the Metals Program, Division of Materials Research, National Science Foundation, under Grant No. DMR-0096527. w Author to whom correspondence should be addressed. e-mail: komarneni@psu.edu Manuscript No. 20644. Received June 7, 2005; approved December 15, 2005.