From Self‐Assembly of Platinum Nanoparticles to Nanostructured Materials

Abstract

One-dimensional (1D) nanostructures of metals, semiconductors and conductive polymers, such as nanowires and nanorods, are very attractive because of their unique electrical, optical, magnetic, and mechanical properties, as well as their potential applications in nanodevices. [1] The synthesis of these nanostructures has been explored by many processes, which are all based on point-initiated uniaxial growth of the crystal or the polymer. One-dimensional arrangements of metal nanoparticles using templates, such as DNA or polymers, have been generated successfully. [2,3] Nickel nanochains were, for instance, synthesized in a solution of poly-(vinyl pyrrolidone). [4] Because of dipolar magnetic interactions, 1D self-assembly is typical for magnetic nanoparticles. [5] We recently reported that Chini clusters (of general formula [Pt 3 (CO) 6 ] m 2À , m = 6) when deposited on flat sup-ports, self-assemble into nanowires. [6] On the other hand, porous hard templates and liquid crystalline phases have been widely used to obtain nanostructured materials. [7][8][9][10][11][12][13][14][15][16][17][18][19][20] In this communication, we first report the 1D self-assembly of platinum nanoparticles in very dilute solution and then show that the organization of much more concentrated nanoparticles into larger nanostructures (fibers or lamellae) can be controlled by using liquid crystals as templates.Platinum nanoparticles are very attractive materials for catalytic and electrocatalytic applications. Ichikawa et al. synthesized platinum and palladium nanowires in mesoporous silica FSM-16. [10,11] These platinum nanowires exhibit unique properties in carbon monoxide (CO) chemisorption and catalysis as well as in magnetism, due to the specific morphology provided by the confinement in the mesoporous channels of the mesostructured silica.To synthesize small Pt nanoparticles, CO reduction of platinum salt (H 2 PtCl 6 ) in aqueous solutions was conducted. At low Pt concentration (up to 10 À3 m) during the slow reduction process the yellow solution turned to pink and the UV-visible (UV/Vis) spectrum displayed a maximum absorption at 537 nm, which increased in intensity until the reduction was complete. Quite unexpectedly, at low Pt concentration, we found that self-assembly of 1.5-2 nm Pt particles into nanofibers (with diameters in the range 5-20 nm) occurred (Figure 1 a). The maximum absorption at 537 nm ( Figure 2) is different from the known plasmon band of individual spherical Pt nanoparticles located around 215 nm. [21,22] The peculiar spectrum shape is probably due to the rodlike assembly of the Pt nanoparticles and their mutual interaction. The spectrum evolution associated with TEM observations at different steps of the reduction process showed that the nanoparticles spontaneously self-as-Figure 1. TEM images of Pt self-assemblies and nanostructured materials obtained by CO reduction of platinum salts in solution or in swollen liquid crystals (SLCs): a,b) in water at concentrations of 5 10 À4 m and 2.5 10 À3 m, respectively, c)...

Shape Control of Platinum Nanoparticles

“…In contrast, under CO, dba is only displaced from the metal coordination sphere and can interact with the particles surface as well as CO which is known to help the particles stabilization. [18][19][20], in the same conditions as the precedent, but with a lower metal concentration of 9.12 × 10 -4 mol L -1 instead of 3.43 × 10 -3 mol L -1 . These experiments indicate there is little effect of the reaction mixture dilution on the nanowires formation whatever the solvent nature and HDA content: their characteristics (dispersion, size and diameter) are similar (see Supporting Information S1).…”

“…Very recently, 1D self-assembly of platinum nanoparticles into nanofibers was reported, upon reduction of H 2 PtCl 6 in aqueous solution under CO atmosphere without a template. [19] In the presence of a liquid crystal template, well-defined straight fibers constituted of individual self-assembled particles with high shape factors were observed. These results were explained by a "soft" template behavior of the swollen liquid crystal allowing a confinement of the particles and then directing their self-assembly into controlled shapes.…”

Carbon Monoxide Assisted Self‐Assembled Platinum Nanoparticles for Catalytic Asymmetric Hydrogenation

“…The properties of self‐assembled NSs are significantly different from individual NPs and bulk solid. The synthesis of metal NPs and their subsequent growth into NSs material using templates like surfactants, polymers, or biological molecules have been well studied 2. Porous hard materials and liquid crystalline phases are also used as templates to generate nanostructured materials 2.…”

“…The synthesis of metal NPs and their subsequent growth into NSs material using templates like surfactants, polymers, or biological molecules have been well studied 2. Porous hard materials and liquid crystalline phases are also used as templates to generate nanostructured materials 2. Self‐assembly of NPs to NSs depends on several intriguing factors such as the shape and size of the particles, and also the nature of the media or solvent in which they are dispersed 3…”

Activating Molecules, Ions, and Solid Particles with Acoustic Cavitation

“…However bubbling CO into a Pt(IV) solution without ultrasonic irradiation at room temperature doesn't lead to the formation of Pt nanoparticles but to a pink colloidal suspension of platinum carbonyl species 20 . Thus, the carbon monoxide ability to reduce platinum ions at room temperature is enhanced due to the local heating in the liquid shell surrounding the bubble during its collapse 7 .…”