Keywords: Noble metal Nanocomposites, nanoelectrocatalyst, phosphonium ionic liquid, MWCNTs

Transcription

1 Advanced Materials Research Online: ISSN: , Vol. 829, pp doi: / Trans Tech Publications, Switzerland Ionic Liquid aided chemical synthesis of noble metal nanocomposites as efficient nanoelectrocatalysts DJafar Vatan Khah Dowlat Sara* 1,a, Mojtaba Shamsipur 2,b, Ahmad Rouhollahi 3,c 1 College of Sciences, Faculty of Chemistry, University of Tehran, Tehran-Iran 2 Faculty of Basic Sciences, University of Razi, Kermanshah-Iran 3 Faculty of Basic Sciences, K.N.Toosi University of Technology, Tehran-Iran a djafarvatankhah@ut.ac.ir, b mshamsipur@yahoo.com, c rouhollahi@kntu.ac.ir Keywords: Noble metal Nanocomposites, nanoelectrocatalyst, phosphonium ionic liquid, MWCNTs Abstract. Platinum, Palladium and Gold nanocomposite using non-functionalized Multiwalled Carbon Nanotube (MWCNTs) has been prepared in the presence of phosphonium-based ionic liquid (Trihexyl Tetradecyl-phosphonium bis (trifluoromethylsulfonyl) amide) TTP(TFMS) 2 A. In this research, the presence of ascorbic acid (AA) in the quaternary phosphonium ionic liquid and MWCNTs without any modification, different noble metal nanocomposites of Pt, Pd and Au are formed. The mixture of MWCNT and IL has already been well-ultrasonicated until the best possible long-term stable dispersion to be formed. Under the similar time durations and similar concentrations of IL, the synthesis has been carefully performed. Working in these conditions leads to platinum-ionic liquid-mwnts nanocomposite as black viscose powders. The nanostructures of the as-prepared nanopowders was checked with SEM. Using palladium nanocomposites as the modification agent on the glassy carbon electrode (GCE), the electrocatalytic effect of nanocomposites has been electrochemically investigated. Comparing three nanocomposites show that noble metal nanoparticles well supported on the bundles of MWCNTs but palladium NPs particle density is more than platinum NPs and PtNPs particle density is more than gold one. Anyway, the electrocatalytic activity of nanocomposites changes as PdNPs>PtNPs>AuNPs. Introduction Carbon nanotube (CNT)/metal hybrid materials have been paid much attention, due to their new applications in catalysis, electrocatalysis, fuel cells, nanoelectronics, nanomedicine, nanodiagnostics and nanobiosensors. Among various CNT/metal hybrid materials, the noble metal (NM) nanocomposites are considered to be the most efficient catalysts for cathode materials in a fuel cell. The distribution and crystallite size of noble metal nanoparticles supported on CNTs are the main factors to influence their properties; therefore, synthesis of highly dispersed and nanosized NM catalysts supported on CNTs is particularly critical. For the sake of increasing the efficiency of these nanoelectrocatalysts, appropriate linking agents between MWCNTs and NMNPs are the key for the successful preparation of NM nanocomposites [1]. Generally, modified CNTs with special functional group by wet chemical oxidation are usually used for the preparation of CNTs/NM nanocomposites so that the CNTs could bind NM nanoparticles (NMNPs) through their functional group with the aid of the linking agents. However, the wet chemical oxidation process might damage the MWCNT walls, which would influence their electrical and thermal properties. Therefore, it is of great interest to develop facile routes to prepare CNTs/NM nanocomposites with the unmodified CNT. Ionic liquids (ILs) are very interesting green solvents for organic chemical reactions, separations, and electrochemical applications [2]. The advantages of ILs in inorganic nanomaterials synthetic processes have received more and more attention due to their unique physical and chemical properties, such as large electrochemical window, polar but low interface tension, high thermal stability, and extended hydrogen bond systems; [3, 4]. In the synthesis of inorganic materials, the ILs could serve as solvents, reactants, or templates. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-05/03/16,22:31:10)

2 590 Ultrafine Grained and Nano-Structured Materials IV It is speculated that there is strong coulombic electrostatic interaction within phosphonium-based ILs composed of bulky cations and anions as an ion pair. And therefore, it is expected that MWCNTs can bind phosphonium group of the IL directly with electrostatic coulombic interaction. As a result, it might synthesize of CNTs/NM nanocomposites directly with the unmodified MWCNTs and the phosphonium-based IL. The lack of pretreatment procedure for functional groups of MWCNTs and the special designed ILs could significantly simplify the synthesis process and aid to safety of surface activity of CNTs. However, to date, to the best of our knowledge, the synthesis of MWCNTs/NM nanocomposites has been little explored with the unmodified MWCNTs and especially with the phosphonium-based ILs. Inspired by this, herein, we investigate the preparation of CNTs/NM nanocomposites with the assistance of cation-π interaction between unmodified CNTs and the phosphonium-based IL. Here, TTP(TFMS) 2 A as a little-used IL is used as a linking agent or linker to bind noble metal salt and MWCNTs with the assistance of electrostatic cation-π interaction. Ascorbic acid (AA) is also used as a mild reducing agent to result in the direct formation of NMNPs on the MWCNTs. Furthermore, cyclic voltammetry tests are used to characterize the electroactivities of the asprepared MWCNTs/NM nanocomposites. It is observed that the as-prepared MWCNTs/NM nanocomposites have an improved desired electroactivity. Experimental Synthesis route. In our synthesis procedure, approximately 2.4 to 2.7 mmol TTP(TFMS) 2 A and 8 mg unmodified MWCNT (Iranian research Institute for Oil Industry) were added into the Milli-Q deionized water (Younglin Korea Aquamax, 18.2 MΩ.cm) to make a 10 ml solution in a round flask. The solution was ultrasonicated for 5 min to result in a homogeneous dispersion and then to mmol Noble metal crystalline salts K2PtCl6, PdCl 2, HAuCl 4.3H 2 O was added separately in the above hot solution, respectively. Then, the flask was placed in an oil bath which is maintained at C. After 15 min, 0.25 mmol AA was added into the hot solution. The solution was kept at C in oil bath for 6 hrs. to complete the reaction. Finally, the products were recovered by repeated centrifugation and washing with deionized water and dried at 70 0 C for 5 h. The procedure for synthesis of different nanocomposites at other conditions is similar to the above. The mixing program will certainly affect on the quality of final product. Characterization Scanning electron microscopy (SEM) was done on a KYKY-EM200 scanning electron microscope at an accelerating voltage of 26 kv. The electrochemical measurements were performed at room temperature using an Electrochemical Workstation (µ-autolab TYPEIII model 70783). Table1. Different amounts of used reactants in the synthesis of noble metal nanocomposites IL (gr) MWCNT(gr) Noble Metal salt (gr) Ascorbic Acid (gr) Time (Min.) Pt/MWCNT/IL Pd/MWCNT/IL Au/MWCNT/IL Electrochemical measurements. Cyclic voltammetry experiments were performed at room temperature. The working electrode was a very thin film of product spreading on a glassy carbon electrode (GCE) with 3 mm in diameter. The product layer was prepared by the following procedures: (a) making a slurry by sonicating 1 mg product in 1 ml Mili-Q deionized water; (b) spreading 15 µl of the slurry on the GCE by micropipet; and (c) then drying the electrode surface at room temperature for 7 h. Pt wire and a Ag/AgCl electrode were used as the counter and

3 Advanced Materials Research Vol reference electrodes, respectively. An aqueous solution with a concentration of 1mM K 3 Fe(CN) 6 and 0.1M KCl was used as the electrolyte solution. Cyclic voltammetry experiment of methanol oxidation on MWCNTs/Pd-modified GCE was performed in the solution of CH 3 OH (1 mol/l) and H 2 SO 4 (1 mol/l). Results and discussion CNTs are allotropes of carbon with a cylindrical nanostructure. The bonding in CNTs is sp 2, with each atom joined to three neighbors as in graphite. The CNTs can therefore be considered as rolledup grapheme sheets. Aida et al. have reported that ILs could interact with CNTs by means of cationπ interactions between the phosphonium ion component and the π-electronic CNT surface (Fukushima et al. 2003; Fukushima and Aida 2007). Based on the special molecular structure of the TTP(TFMS) 2 A IL and MWCNTs, a cation-π interaction-assisted formation mechanism for our synthesized CNTs/NM nanocomposites is proposed. In the beginning of the synthesis process, the IL could adsorb on the surface of the CNTs through the cation-π interaction between CNTs and phosphonium cation of the IL. This interaction could contribute to suppress the agglomeration of MWCNTs. After that, noble metal salt is added into the solution. As the phosphonium cations have positive charges, Coulombic force interaction between phosphonium cations and noble metal anions like PtCl 6 2 -, PdCl 4 2-, AuCl 4 - will be formed. That is, the anions are coupled with the phosphonium group of the IL which is linked with surface of the CNTs through the cation-π electrostatic coulombic interaction. After the AA reducing agent is added to the solution, the PtCl 6 2- will be reduced to zerovalent noble metal nanoparticles. The XPS and SAXS studies indicate that there is effective interaction of the IL with the noble metal surface, which is responsible for the stabilization of the NMNPs in IL. Therefore, our in situ synthesized NMNPs could be stabilized on the surface of MWCNTs by the effective interaction of the IL and noble metal surface, which results in the MWCNT composites. The electrochemical response of the synthesized MWCNTs/Pd nanocomposites is studied by cyclic voltammetry measurements. Figure 6 shows the electrochemical response of 1 mmol/l K 3 Fe(CN) 6 at the modified GCEs with the synthesized MWCNTs/Pd nanocomposites. The results indicate that the electrochemical response of MWCNT/Pd nanocomposites-modified electrodes is much stronger than that of simply the bare GCE that confirms the electrocatalytic effect of the nanocomposite at the surface of electrode. SEM images in figure 2 to 4 shows the noble metals well-supported on MWCNT bundles. But still there are obvious differences on the distribution of NMNPs and paricle density in the SEM images. It is observed that, the distribution of PdNPs >PtNPs>AuNPs. Anyway, the NPs tend to be agglomerated and aggregated nevertheless using ionic liquid as linkers. Also in some case the bundles are disentangled as we can see in the SEM of AuNPs. To sum up, we can conclude that using IL cause the interaction between the NMNPs and the surface of MWCNTs to be strengthened and prevents the bundles to be twisted more. Fig1. Trihexyl Tetradecyl-phosphonium bis (trifluoromethylsulfonyl) imide

4 i / A i / A i / A 592 Ultrafine Grained and Nano-Structured Materials IV Fig2. Au/MWCNT/IL Fig3. Pd/MWCNT/IL Fig4. Pt/MWCNT/IL Glass Electrode in K3Fe(CN)6 + KCl GC Pd Nanocomposi in K3Fe(CN)6 + KCl x x x x x x x x x x x x x x x E / V x E / V Fig5. 1mM K 3 Fe(CN) 6 and 0.1M KCl Bare GCE Fig6. Pd/MWCNT/IL modified GCE Methanol oxidation on Pd CNT modified GC 0.100x x x x x x x E / V Fig 7. Methanol Oxidation on Pd Summary Noble metal nanoparticles (Au, Pt and Pd) has been synthesized in presence of little used phosphonium based IL in similar quantities and routes. They are well supported on the bundles MWCNTs and creat the best nanoelectrocatalyst. SEM images analysis shows the particle size distribution and particle density is high and CV confirms the electrocatalytic improved behavior. At future, we are trying to electrochemically synthesis of noble metal nanocomposites with the highest possible purity.

5 Advanced Materials Research Vol References [1] Karousis N, Tagmatarchis N, Tasis D (2010) Current progress on the chemical modification of carbon nanotubes. Chem Rev 110(9): [2] Endres F, Abedin SZE (2006) Air and water stable ionic liquids in physical chemistry. Phys Chem Chem Phys 8(18): [3] Guo SJ, Dong SJ, Wang EK (2010) Constructing carbon nanotube/pt nanoparticle hybrids using an imidazolium-saltbased ionic liquid as a linker. Adv Mater 22(11): [4] Zhou Y, Schattka JH, Antonietti M (2004) Room-temperature ionic liquids as template to monolithic mesoporous silica with wormlike pores via a sol gel nanocasting technique. Nano Lett 4(3):

6 Ultrafine Grained and Nano-Structured Materials IV / Ionic Liquid Aided Chemical Synthesis of Noble Metal Nanocomposites as Efficient Nanoelectrocatalysts /