Electrodeposition of platinum nanoparticles in a room-temperature ionic liquid.

The electrochemistry of the PtCl(6)-PtCl(4)-Pt redox system on a glassy carbon (GC) electrode in a room-temperature ionic liquid (RTIL) [i.e., N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEMEBF(4))] has been examined. The two-step four-electron reduction of PtCl(6) to Pt, i.e., reduction of PtCl(6) to PtCl(4) and further reduction of PtCl(4) to Pt, occurs separately in this RTIL in contrast to the one-step four-electron reduction of PtCl(6) to Pt in aqueous media. The cathodic and anodic peaks corresponding to the PtCl(6)/PtCl(4) redox couple were observed at ca. -1.1 and 0.6 V vs a Pt wire quasi-reference electrode, respectively, while those observed at -2.8 and -0.5 V were found to correspond to the PtCl(4)/Pt redox couple. The disproportionation reaction of the two-electron reduction product of PtCl(6) (i.e., PtCl(4)) to PtCl(6) and Pt metal was also found to occur significantly. The electrodeposition of Pt nanoparticles could be carried out on a GC electrode in DEMEBF(4) containing PtCl(6) by holding the potential at -3.5 or -2.0 V. At -3.5 V, the four-electron reduction of PtCl(6) to Pt can take place, while at -2.0 V the two-electron reduction of PtCl(6) to PtCl(4) occurs. The results obtained demonstrate that the electrodeposition of Pt at -3.5 V may occur via a series of reductions of PtCl(6) to PtCl(4) and further PtCl(4) to Pt and at -2.0 V via a disproportionation reaction of PtCl(4) to PtCl(6) and Pt. Furthermore, the deposition potential of Pt nanoparticles was found to largely influence their size and morphology as well as the relative ratio of Pt(110) and Pt(100) crystalline orientation domains. The sizes of the Pt nanoparticles prepared by holding the electrode potential at -2.0 and -3.5 V are almost the same, in the range of ca. 1-2 nm. These small nanoparticles are "grown" to form bigger particles with different morphologies: In the case of the deposition at -2.0 V, the GC electrode surface is totally, relatively compactly covered with Pt particles of relatively uniform size of ca. 10-50 nm. On the other hand, in the case of the electrodeposition at -3.5 V, small particles of ca. 50-100 nm and the grown-up particles of ca. 100-200 nm cover the GC surface irregularly and coarsely. Interestingly, the Pt nanoparticles prepared by holding the potential at -2.0 and -3.5 V are relatively enriched in Pt(100) and Pt(110) facets, respectively.