Effect of Convection on Chirality of Spatiotemporal Patterns Observed in the Reduction of Hydrogen Peroxide
Authors: Yasuda Takumi , Nakata Masahiro , Miyamoto Masayuki , Kitada Atsushi , Murase Kuniaki , Fukami Kazuhiro 
Published: July 2021

Background
The spatiotemporal patterns have been reported in many electrochemical systems. Most of the patterns disappear when the reactions stop. This is because they originate from the spatial distribution of the local current and potential during the reactions. However, it is possible to inscribe the patterns into materials by utilizing the pattern formation in the material processing. This process enables to fabricate materials with unique shapes reflecting the pattern profile and is expected to be a new method for the production of such highly sophisticated materials. As one of the materials formed in this process, we have focused on the helical nanopore formed in the Platinum-assisted chemical etching of Si induced by the oxidation of Si and the reduction of H2O2. In this phenomenon, the helical pores are spontaneously formed by immersing the Si substrate modified with Pt nanoparticles into the etchant. We reported that the formation of helical pores originates from a spatiotemporal pattern emerging in the reduction reaction of H2O2 on the Pt particle.
Our recent paper on helical nanopore formation in the etching of silicon implies that it is possible to fabricate chiral materials by utilizing the spatiotemporal pattern. In order to utilize the chiral material formed by the spatiotemporal pattern formation in the field of chiral chemistry, it is an inevitable task to control the chirality of the material. Because it is determined by the chirality of the pattern, we have to understand the effect of spatial symmetry breaking on the chirality of the spatiotemporal pattern. In this report, we focus on convection as the chiral condition introduced to the system and investigate its effect on the chirality of the spatiotemporal pattern. Among the several spatiotemporal patterns, we focus on the rotating waves (RW), where active and inactive sites rotate on the ring electrode because it is considered that this pattern emerges during the helical pore formation.
Experimental
In order to investigate the effect of the convection on the RW, we simulate the formation of the RW during the electrochemical reduction of H2O2 on the Platinum ring electrode based on the model constructed by Fukushima. In this model, we deal with the local electrode potential (E) and the local concentration of H2O2 (c) on the electrode surface as the variables. The differential equations governing E and c ware obtained by considering the equivalent circuit and the consumption and diffusion of H2O2, respectively. Since RWs have two different modes, which are the right-handed rotating wave (R-RW) and the left-handed rotating wave (L-RW), either may appear preferentially in the system where convection is considered. We conducted the simulation several times in a certain condition with random perturbation, and examined the percentage of R-RW in all RWs formed (R-RW%). In this simulation we always introduced the right-handed convection.
Result and discussion
The R-RW% changed depending on the velocity of the convection (v). When the convection was strong and v was 10 m/s, the R-RW was 0%, meaning that only the L-RW was formed. It should be emphasized that the RWs in the opposite direction to that of the convection, i.e. upstream RWs were formed in this condition. In contrast, when convection was mild and v was 0.1~1 m/s, the direction of the preferential RW was different from that of the RW formed when v was 10 m/s. In other words, the RW whose direction was the same as that of the convection (the downstream RW) was preferentially formed under mild convection. These results suggest that the formation of downstream RWs under mild convection is governed by the nonlinear dynamics.
The observation of the change in the distribution of c and E showed that E in some area of the electrode got out of the potential region, where a negative differential resistance (NDR) appeared in the current-potential curve, at the initial stage of the RW formation. As a result, the double concavities emerged in the distribution of c. Therefore, we concluded that the competition of the effects of the convection on the double concavities and the whole concavity caused the change of the preferential RW with v.