♦ Rational Synthesis of Nanoporous Platinum Particles with Multiple Architectures toward Highly Active Electrocatalysts

Platinum (Pt) nanoparticles with desired shape and morphology have attracted a great deal of attention, because of their widespread use in energy conversion/storage devices, sensors, and catalysts. To date, a diverse spectrum of Pt-based nanoparticles, such as nanospheres, nanocubes, multipods, dendrites, have been successfully synthesized so far. Among these morphologies, nanoporous Pt particles are very promising, because they can provide high surface area and a large number of edges and corners, which are important factors for electrocatalytic applications.
Hence, development of a facile and economic method for high yield synthesis of nanoporous Pt particles with hierarchical architectures is still a challenging issue to be solved. For this purpose, I have focused on a direct chemical deposition approach with assistance of surfactants in aqueous solution phases. A wide range of multifunctional nanoporous metals with different morphologies and compositions has been successfully synthesized by a facile wet-chemistry approach.
Ordered mesoporous Pt particles can be synthesized by using direct templating from lyotropic liquid crystals (LLC) made of surfactants at high concentrations. The direct templating process involves a simple replication from well-ordered LLCs by electrochemical processes. Therefore, this process is applicable to a wide variety of metals which are generally known to be deposited by using electrochemical processes. Furthermore, various nanostructures (e.g., lamellar, 2D hexagonal, and 3D bicontinuous cubic, and 3D cage-type structures) can be selectively synthesized by controlling the mesostructures of the original LLC.
Recently, I developed a new approach mediated by block copolymer to produce nanoporous Pt particles with well-defined shape and high surface areas. This novel approach is distinctly different from the above mentioned LLC-template approach.
Importantly, several atomic steps are exposed on the Pt branch surface, which can act as highly electrocatalytic sites. The nanoporous Pt structure maximizes the electrochemically active surface area. The open nanoporous structure is highly beneficial for their use as an electrocatalyst, primarily because of their superior tolerance to undesirable agglomeration of the active sites. Compared to commercially available Pt products (e.g., Pt black and Pt/carbon composite), my sample shows 20 times higher current density in methanol oxidation reaction (MOR) which is an anode reaction in practical direct methanol fuel cell (DMFC).
Currently, I extend the above concept to prepare bimetallic core-shell nanoparticles (Pd@Pt, Au@Pt, Au@Pd@Pt, Ag@Pt) with nanoporous Pt shells. The presence of the core metals not only facilitates the growth of the nanoporous Pt shells but also plays a key role in the enhanced activity. In the case of Pd@Pt nanoparticles, the atoms of Pd and Pt are highly miscible, and the Pd atoms in the core coherently match with the lattice structures of the exterior nanoporous Pt shells, resulting in the formation of the inserted pseudo-Pd-Pt alloy hetero-interface, which is favorable for reducing the electronic binding energy in Pt and facilitating the C-H cleavage reaction in methanol decomposition. Furthermore, many atomic steps exposed on the Pt surface can act as highly active sites for MOR. Thus, superior catalytic activity can be realized by optimizing the core-shell structures.