Synthesizing metal nanoparticles by controlling their structure is an important research theme when attempting to find new properties in nanostructures. In our lab, we are also trying to find new potentials in nanoparticles by controlling their shape.

In many cases, when trying to obtain nanoparticles in a stable dispersion state, the following dispersant-stabilizers, or so-called protective agents, need to be added. However, if stability of dispersion is not required, these stabilizers are not necessary.

Nanoparticles with a consistent particle size are generally synthesized from a consistent solution. However, the reduction reaction in a consistent solution results in a volatile concentration of the source metal salt. In such a case, it is difficult to control the nuclei formation and growth of the particles.

We have been controlling the particle size and atomic arrangement of alloys using PVP in the synthesis of nanoparticles. We have used all types of agents as the reacting agent. A different reacting agent yields a different nanoparticle shape.

The direct reduction of chloroplatinic acid by sodium borohydride provides an interesting example. As this reduction involves an intense chemical reaction, it is highly recommended to use this method under the supervision of an instructor who has expert knowledge in chemistry after reading relevant research papers on the topic thoroughly. When sodium borohydride was introduced to a chloroplatinic acid solution, a sudden reduction reaction occurred, and a platinum nanoparticle with a large number of petal-shaped projections was created. The resulting nanoparticles are often called nano-flowers.

Furthermore, these platinum nanoflowers were observed to be useful as an ionizing reagent for laser desorption ionization mass spectroscopy. We believe that one of the reasons for this finding is that the energy from pulse laser irradiation accumulates at the platinum tips and cause highly effective desorption and evaporation of molecules.

1. Hideya Kawasaki, Tetsu Yonezawa, Takehiro Watanabe, Ryuichi Arakawa, “Platinum nanoflowers for surface-assisted laser desorption/ionization mass spectrometry of biomolecules”, Journal of Physical Chemistry C, 111(44), 16278-16283 (2007).
2. Hideya Kawasaki1, Teruyuki Yao, Takashi Suganuma, Kouji Okumura, Yuichi Iwaki1, Tetsu Yonezawa, Tatsuya Kikuchi, Ryuichi Arakawa, “Platinum Nanoflowers on Scratched Silicon by Galvanic Displacement for an Effective SALDI Substrate”, Angew. Chem. Int. Ed., 16(35), 10832-10843 (2010).
   Kompeito-type nanoparticles can be produced with platinum and wire-shaped nanoparticles can be produced with nickel. We are also interested in the behaviors of these nanoparticles and are currently conducting experiments using high-temperature TEM. We will revisit the topic in the section “In-situ heating TEM”.
3.  Takashi Narushima, Takuya Makino, Tokunaga Tomoharu, Tetsu Yonezawa, “Observation of Microstructural Changes in Polymer-Coated Kompeito-Type Platinum Particles by In Situ Heating TEM”, Journal of Nanoscience and Nanotechnology, 12(3), 2612-2616 (2012).
4. Takashi Narushima, Ren Lu, Tetsu Yonezawa, “Wet Preparation of Organic Stabilizer Free Urchin Structured Nickel Fine Particles and Their In-situ TEM Observation at High Temperature”, Journal of Japan Institute of Metals and Materials, 78(2), 98-102 (2014).
   Furthermore, with respect to the formation of the nanostructure, we are paying attention to porous structures. While the anode oxidization method is well known for the formation of porous structures, dealloying is also an effective method. Dealloying is a method in which only one metal in an alloy is oxidized and dissolved by immersing the alloy in an acid. In some cases, electrochemical methods are also used. We formed a platinum porous structure by dealloying a copper-platinum plate and studied its microstructure.
5. K. Maruya, R. Yamauchi, T. Narushima, S. Miura, T. Yonezawa, “Structure consideration of platinum nanoparticles constructing nanostructures obtained by electrochemical dealloying of a Cu-Pt alloy.”,Journal of Nanoscience and Nanotechnology, 13(4), 2999-3003 (2013).