Surface Treatment of Boron Doped Diamond Using Atmospheric-Pressure Nitrogen Plasma Jet
Authors: Tomita Toru , Kinumoto Taro , Ichiki Ryuta , Tsumura Tomoki , Toyoda Masahiro 
Published: February 2020

Conductive boron-doped diamond (BDD) exhibits high physical and chemical stability, a wide potential window, a low background current. Owing to the typical characters, BDD is expected for use as an electrode material in the electrochemical applications such as electrolysis, waste water treatment, pH sensing, and electroanalysis. In order to further functionalize BDD, the surface treatment technique, e.g., enlargement the surface (reaction) area, control the morphology, chemical functionalization, is required and several kinds of the treatment method have been attempted including catalytic etching, oxygen treatment and so-on.
We attempted to use atmospheric-pressure nitrogen plasma jet (APN2PJ) for the surface treatment of BDD for the first time. APN2PJ is a unique technique and can generate NH and nitrogen radicals at high density even under atmospheric pressure. In this presentation, the effects of APN2PJ on the chemical as well as electrochemical properties of BDD, especially the electrochemical double layer capacitance and the behavior at high potential region up to 2.5 V vs. RHE, are reported. In addition, the effect of following ultrasonic treatment is also reported.
BDD electrode (De Nora Permelec Ltd., Nb substrate, thickness of BDD layer = 2.5 mm, boron content = 9.000 ppm) was used as purchased. APN2PJ treatment was carried out original apparatus. The treatment time was restricted at 3, 5 and 7 minutes. Mixture gas of nitrogen and hydrogen was used as the atmospheric gas. The applied voltage and pulse current of APN2PJ was set at 5 kV and 1 A, respectively. Hereafter, BDD without APN2PJ treatment is denoted as BDD-0 and BDD after plasma treatment as BDD-(APN2PJ treatment time).
After APN2PJ treatment, the morphology of BDD was observed by FE-SEM and the chemical analysis was carried out by XPS. Cyclic voltammetry was carried out in a three-electrode electrochemical cell under nitrogen atmosphere. A gold mesh and Ag|AgCl electrode was used as the counter electrode and the reference electrode. 0.5 mol dm-3 H2SO4 aqueous solution was used as the electrolyte. The electrochemical double layer capacitance of each BDD electrodes was estimated in the potential region between 0.5 V and 0.6 V vs. RHE.
Figure 1 a) and b) shows FE-SEM images of BDD-0 and BDD-7. The surface of BDD-0 was consisted by tiny single crystals of BDD of several micrometer sizes. On the other hand, many boundary (grooves), pits and the granular precipitates were clearly observed on the surface of BDD-7, which shows APN2PJ treatment gives morphology change of the surface of BDD.
From XPS analysis, the peaks were observed in the spectra of C1s, O1s and N1s for each BDD electrodes after APN2PJ treatment, although only C1s peak was observed for BDD-0. In particular, in N1s XP spectra of BDD electrodes after APN2PJ treatment, four peak components assigned to Nitride, N-C sp3 bond, N-C sp2 bond and N-O bond were observed, showing that nitrogen was introduced into BDD surface and simultaneously, the morphology change as shown in Fig 1 b) was taken place.
We also investigated the effects of following ultrasonic treatment. This was carried out in pure water. Figure 1 c) shows FE-SEM image of BDD-7 following ultrasonic treatment. Here, thus BDD was named as U-BDD-(APN2PJ treatment time). The granular precipitates, which are observed on the surface of BDD-7 (Fig. 1 b)), were disappeared and the grooves and holes became to be clearly observed.
The electrochemical double layer capacitance was increased with an increase in APN2PJ treatment time and was further increased by following ultrasonic treatment. If we assumed that the capacitance relates to the geometric surface area of the electrode, the surface area of BDD would be increased with appearance of the grooves and the pits by APN2PJ. Furthermore, it was reasonably considered that the ultrasonic treatment gives disappearance of the precipitates as well as cleaning of the surface, which would lead to increase in the capacitance (surface area). The behavior at high potential region will be also presented.

Figure 1