Electrochimica Acta 143 (2014) 357–365 Contents lists available at ScienceDirect Electrochimica Acta j ourna l ho me page: www.elsevier.com/locate/electacta Plasma-ionic liquid reduction for synthesizing platinum nanoparticles with size dependent crystallinity Quoc Chinh Tran a , Van-Duong Dao a , Kwang-Deog Jung b , Ho-Suk Choi a,∗ a Department of Chemical Engineering, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon, 305-764, Korea b Clean Energy Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea a r t i c l e i n f o Article history: Received 11 July 2014 Received in revised form 8 August 2014 Accepted 9 August 2014 Available online 21 August 2014 Keywords: Platinum nanoparticles Ionic liquid Liquid plasma Methanol oxidation. a b s t r a c t Platinum nanoparticles (PtNPs) were successfully synthesized using a liquid plasma system with 1-butyl- 3-methylimidazolium tetrafluoroborate under atmospheric pressure. The PtNPs with a size of 3 nm were uniformly distributed on the surface of a copper grid. The particle size, shape and crystallinity of the Pt were further controlled by a mixture of hydrogen and argon gases, stabilizers and plasma reduction time. Furthermore, a smaller particle size had a better forward sweep of catalytic performance in oxidation of methanol while the cube-like structure was better for backward sweep of the electrochemical catalyst. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Owing to their unique size-related structure and properties such as quantized electronic structure and high specific area, nanoparti- cles (NPs) have been attracting much attention as electrochemical catalysts as hydrogen oxidation, methanol oxidation, and oxygen reduction [1–4]. Both an active area and stability during operation are required for the material to be used as a catalyst [4]. Thus, Pt is traditionally the most popular material for electrochemical catalysts. So far, there have been numerous methods producing and utilizing PtNPs with high specific surface area [4–16]. Recent reports show that the catalytic activity of the metal depends on the particle size [17] and particle shape [18]. Until now, Pt has been mostly prepared from conventional methods based on colloidal techniques [5–19]. In such methods, PtNPs can be synthesized from the reduction of metal salt dis- solved in a solution [6,7,19] or a hydrolysis/condensation route of forming the particles and their aggregates [10,11]. A stabilizer or stabilizing agents were also used in another route during the phase of particle formation [4]. There are, however, some drawbacks to the conventional methods: for example, long aging and reduction times, involvement of organic solvents in some processes, neces- sity of removing the stabilizers at the final stage, and preparation of complex molecular precursors [4]. Additionally, the use of chemi- cal reducing agents, such as hydrogen gas, LiBH 4 , NaBH 4 , ethylene ∗ Corresponding author. E-mail address: hchoi@cnu.ac.kr (H.-S. Choi). glycol, alcohol, etc. [4], can make additional disadvantages. There- fore, the development of a new method which overcomes the unfa- vorable process restrictions, such as organic solvents, stabilizing agents, long aging and reduction times and high chemical toxicity and so on, is still a challenge in the current development of catalysts. Recently, microwave irradiation and plasma have been used to synthesize PtNPs with a short reduction time [4,20–27]. However, the stabilizers or hydrogen have still been used to control at the particle size of the PtNPs [4,20–27]. In addition, the complex sys- tems are necessary to use a Pt electrode in liquid solution [4,20–25]. Very recently, we reported a new strategy to chemically bond PtNPs on multi-walled carbon nanotubes (MWNTs) in room temperature ionic liquid (RTIL) under atmospheric pressure plasma and to physi- cally stabilize them through the film of ionic liquid supramolecules (ILSMs) on the surface of the PtNPs/MWNT [28]. The developed technology is able to overcome all the disadvantages. Here, we report a systematic synthesis of PtNPs through a sim- ple strategy of reducing Pt ions via ionic liquid-plasma reduction and also illustrate how effectively this process can control particle size and achieve narrow size distribution. We further electrochem- ically investigate the influence of particle size on the oxidation of methanol. 2. Experimental section 2.1. Materials Chloroplatinic acid hydrate (H 2 PtCl 6 .xH 2 O, 99.9% trace metals basis), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMI]BF 4 ), http://dx.doi.org/10.1016/j.electacta.2014.08.022 0013-4686/© 2014 Elsevier Ltd. All rights reserved.