Preparation of Pt Nanoparticle-Adsorbed Carbon Nanotubes Using Room Temperature Ionic Liquid and Their Use as Electrocatalyst for Oxygen Reduction

Carbon nanotube (CNT) possesses unique characteristics with extremely high mechanical resistance. This conducting material is suitable to fabricate electrode materials. Also it is expected to utilize CNT as an ideal supporting material because of its large surface area-t-volume ratio. However, it is quite difficult to equip functional materials including catalysts with CNT having surface inertness. To immobilize materials on CNT, some modification of the CNT surfaces, such as partial oxidation, is required to generate functional groups, to which functional materials are anchored. In our previous studies, we found that metal sputtering onto room temperature ionic liquid (RTIL) under reduced pressure produces metal nanoparticles suspended in the ionic liquid.^1-3 By this way, several kinds of nanoparticles including alloy⁴ and hollow nanoparticles⁵ have been synthesized, so far. In addition, putting Au nanoparticles suspended IL onto the inert surface of a highly oriented pyrolytic graphite (HOPG) followed by heating was found to immobilize firmly the nanoparticles on the HOPG surface.⁶ In the present research, attempts have been made to preparation of Pt nanoparticle-adsorbed carbon nanotubes (Pt-CNT) by the aforementioned method and electrocatalytic activities of the prepared Pt-CNT toward O₂reduction was examined.

      RTILs used for the preparation of Pt nanoparticles were dried in vacuum prior to use.  A glass plate (2.5 ×2.5 cm), on which RTIL (0.4 mL) was spread, was set in a Cressington 108 auto SE sputter coater.  A Pt foil target (Φ5.7 cm, 99.98%) was placed on 4.5 cm above the glass plate.  Sputter deposition onto RTIL was conducted with 40 mA of sputter current in dry Ar (99.999 %) atmosphere whose pressure did not exceed 7 ± 1 Pa.  The sputtering was conducted at room temperature (298 ± 2 K). Preparation of Pt-CNT was attempted by agitating the Pt sputtered RTIL (0.4 mL) with untreated CNTs (1 mg) at 573 K for 5 h.  The resultant mixture was rinsed by dry acetonitrile several times to remove RTIL, followed by drying in vacuo. A transmission electron microscope (TEM) image of the resulting CNT is given in Figure 1, which shows adsorption of numerous Pt nanoparticles on the surface of CNT. Chemical analysis of Pt-CNT revealed that RTIL worked as a paste to stick Pt nanoparticles on the CNT surface. The mean particle size of Pt nanoparticles on SWCNT was ca. 3.5 nm and the amount of Pt nanoparticle on the Pt-SWCNT composite was 32.9 wt%.

      We prepared three Pt-SWCNT composites whose amounts of Pt on SWCNT were 7.1, 16.0, 24.1 and 32.9 wt% in order to examine their electrocatalytic activities toward oxygen reduction. A rotating ring-disk electrode (RRDE) with glassy carbon disk and Pt ring electrodes was used. Pt-CNT (1 mg) was dispersed in 0.2 mL iso-propanol and 5 μL of the dispersion liquid was applied to the disk electrode and fixed by putting 5 μL of a Nafion iso-propanol solution. The prepared electrode was pretreated with multiple potential scans between 0.5 and 1.25 V vs. RHE in 0.1 M HClO₄ aqueous solution under N₂. The electrochemical surface areas (ECSAs) were measured from cyclic voltammograms, giving 34.4, 30.3, 40.2, and 48.7 m²g^-1of ECSAs for Pt-CNTs with 7.1, 16.0, 24.1 and 32.9 wt% Pt.

     Figure 2 shows voltammograms of disk electrodes modified with four kinds Pt-CNTs and the corresponding voltammograms for the ring electrodes. The onset potential of oxygen reduction became positive with increment of the Pt loading amount. The ring electrode detects H₂O₂produced in oxygen reduction at the disk electrode, however its amount was quite low; generation rate was below 5 % for Pt-CNT with 24.1 wt% and 32.9 wt% Pt even at potentials negative than 0.4 V vs. RHE, and also it was close to 0% for all cases at potentials between 0.7 to 0.85 V. These results showed that Pt-CNT has a favorable electrocatalytic activities for oxygen reduction.

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