Helical Nanostructure Formation By Nanoconfinement of Spatiotemporal Patterns in Electrochemical Reactions
Authors: Fukami Kazuhiro , Yasuda Takumi , Nakata Masahiro , Maeda Yuki , Kitada Atsushi , Murase Kuniaki 
Published: July 2021

Spatiotemporal pattern formation in anodization and electrodeposition is of interest since the patterns can be encoded as structures. Such materials having unique periodic structures can show a variety of properties, and thus the strategy of materials production based on dynamic self-organization has become one of the hot topics. Among the production of materials based on spatiotemporal pattern formation, we have recently focused on spatial symmetry breaking which leads to the formation of chiral materials such as nanohelices. In the present paper, we will show two examples of helical nanostructure formation based on spatiotemporal pattern formation in electrochemical reactions.
As the first example, we discuss the helical nanopore formation in a silicon wafer by platinum(Pt)-assisted chemical etching (PacEtch). When a silicon wafer modified with metallic nanoparticles is immersed in an etchant containing HF and H2O2, the local corrosion of silicon is highly enhanced at the interface between metal and silicon. This local corrosion results in the formation of track-pores, the diameter of which is determined by the size of the metallic nanoparticles. The track-pores are generally cylindrical, but they show helical shapes especially when using Pt nanoparticles for local corrosion. Since the etching of silicon is local corrosion, the oxidation reaction must be balanced with the reduction reaction which is the reduction of H2O2. We measured the corrosion potential during PacEtch and found that it is located in the potential region where a negative differential resistance is observed in the reduction of H2O2 on a Pt electrode. We have also confirmed that the reduction of H2O2 spontaneously oscillates when we apply the same potential as the corrosion potential using both a Pt disk electrode and the etchant. These results suggest that the periodic shapes of helical nanopores formed by PacEtch originate from the periodic reduction of H2O2 on the Pt nanoparticles.
As another example, we show the co-electrodeposition of palladium (Pd) and copper (Cu). When the co-electrodeposition of Pd-Cu was conducted in an aqueous solution containing PdCl2 and CuCl2, a 2-dimensional target micropattern was spontaneously formed. The spatial pattern formation resulted in the spontaneous formation of a multilayer of Pd-rich and Cu-rich layers. The characteristic size of the target micropattern is ~2 µm. As the next step, we utilized a porous alumina membrane with 200 nm in diameter, whose one of the top surfaces were covered with a gold thin film to be utilized as the working electrode. When Pd-Cu was deposited within the nanopores of the membrane, Pd-Cu rods were obtained. Interestingly, the Pd-rich region grew helically in the rod, which was clearly observed by selective chemical dissolution of the Cu-rich region. This result suggests that spontaneous helical pattern formation is achieved by confining the 2-dimensional spatiotemporal pattern within cylindrical nanopores which is much smaller than the characteristic 2-dimensional pattern.
These two examples strongly suggest that the nanoconfinement of spatiotemporal pattern formation leads to spatial symmetry breaking and results in spontaneous helical pattern formation. We believe that the production of helical materials by nanoconfinement of spatiotemporal patterns is a new class of materials science in chirality.