Development of a Software to Analyze the Dispersion State of Particles on a Flat Substrate
Authors: Ooya Kayato , Shiroishi Hidenobu , Harada Yuya 
Published: February 2020

Functional particles such as TiO2 and Pt/C are often fixed on a flat substrate for practical uses. rotating ring disk electrode (RRDE) technique, in which Pt/C nanoparticles were dispersed on a glassy-carbon (GC) disk electrode, is known as a method capable of evaluating a catalyst activity. The technique was often applied to the evaluation of oxygen reduction catalysts used in fuel cells or metal-air batteries, because the reaction intermediates such as hydrogen peroxide can be detected by the ring electrode. However, the accuracy of the technique was significantly affected by the dispersivity of the catalyst particles on the electrode. Thus, the catalysts on the disk electrode are required to be uniformly dispersed and many researchers have sought the best dispersion method. Although microscope photographic images were used for the estimation of dispersivity on the electrode, no quantitative analyses were performed by using the images.

In this study, we have developed a program called “Roughness Analyzer” which can quantitatively analyze the dispersiveness of particles on a flat substrate.

This software can load a three dimensional data of particles fixed on a substrate measured by a surface roughness meter (SV-C4500 CNC, Mitutoyo) and display a 2D contour map of the loaded data. It should be noted that, the height of each point is expressed by the brightness of each dot. If necessary, a user can change the range of height represented by the brightness and the number of level. This software is equipped with an auxiliary function to estimate only a substrate surface from a three-dimensional data of the particles fixed on a substrate. This function can automatically determine the points of each cross-section of the 2D contour map that is likely to be surface of the substrate itself, and curves connecting these points was obtained by a spline interpolation method. Thus, only particle-derived height data are calculated by subtracting the height of only the substrate surface data from that of experimental raw data. Analytical functions are “Cluster Analysis” and “Distribution Analysis.” The Cluster Analysis can detect each secondary particle agglomerated on the substrate from the height data. Each secondary particle is filled with a different color and the area, the volume, and the average height of each particle are calculated respectively. The Distribution Analysis can divide the substrate surface in any number from the center of the substrate surface in a radial direction and an angular direction. Further, it can automatically calculate the area, the volume and the percentages of area occupancy of particles in each section.

For example, the dispersiveness of a modified GC electrode was evaluated by the software. The electrode was prepared as follows: A 2.0 mg of Pt/MWCNTs was suspended in 1 mL of 0.1 wt% Nafion-EtOH by ultrasonication for 10 min. Then, 10 μL of the suspension was cast onto a glassy-carbon disk (6 mm φ, 0.283 cm2) – Pt ring electrode (Nikko Keisoku) and dried for 1 h at 60°C by using an infrared lamp. Fig. 1 shows the percentage of the area occupancy for the secondary particles in each section. Each cumulative bar represents the percentage of area occupancy in the same radial range, thus, it means good dispersivity of the particles when each cumulative bar has similar height, namely, the standard deviation of the heights is small. Moreover, each element in each cumulative bar means the percentage of area occupancy for the particles at a different angular direction in the same radial range. The desirable dispersivity can be achieved when the standard deviation of the height of the bars is small. However, the small standard deviations are not a sufficient condition for good dispersivity. Criteria for dispersivity are the average height of the particles as well as the standard deviations.

Figure 1