Boron Concentration-Dependent Electrochemical Properties of Boron-Doped Diamond Electrodes

Introduction Boron-doped diamond (BDD) electrodes have been attracted considerable attention due to their excellent electrochemical properties, such as wide potential window in aqueous solutions, low background current, and high chemical stability, compared with the conventional electrodes. Hence, BDD electrodes have been investigated for many applications, for example, wastewater treatment, electrochemical sensors, and electroorganic synthesis. On the other hand, it is known that electrochemical properties of BDD can be altered by adjusting boron concentration and/or sp 2 -bonded carbon impurities 1 . Therefore, the investigation of the correlation between synthetic condition and electrode properties is important to synthesize suitable BDD for each application. In this work, we synthesized and characterized several BDD electrodes with different boron concentration. Experimental BDD films were prepared onto p-type (100) silicon wafer substrates using microwave plasma enhanced chemical vapor deposition method. They were synthesized with various B/C raitos in the feed gas: 0.01%, 0.1%, 0.5%, 1%, and 2%. Boron concentration in the synthesized BDD was estimated by secondary ion mass spectroscopy (SIMS) and glow discharge optical emission spectroscopy (GDOES). The morphologies were observed by scanning electron microscopy (SEM). Structures were characterized by X-ray diffraction (XRD) and Raman spectroscopy. Electrochemical measurements were carried out in a single compartment cell using a conventional three-electrode system: BDD as a working electrode, a Pt wire as a counter electrode, and Ag/AgCl (saturated KCl) as a reference electrode. The potential windows were evaluated by cyclic voltammetry (CV) in an aqueous solution of 0.1 M H 2 SO 4 . Before electrochemical measurements, the BDD surface was hydrogenated by hydrogen plasma treatment or oxidized by anodic oxidation treatment. Results and Discussion SIMS and GDOES analysis indicated that actual boron concentration depended almost proportionally on B/C raitos in the feed gas. The SEM images showed that the grain size and thickness of the BDD films gradually decreased with increasing boron concentration. It is assumed that increasing B/C ratios in the feed gas promotes nucleation of diamond but also interferes with the growth of the grains. XRD patterns of all BDD films showed three sharp peaks, corresponding to (111), (220), and (311) diffractions of the diamond cubic structure. The Raman spectra of all BDD films showed the peak of center zone optical phonon of diamond at around 1300 cm −1 . Moreover, in the spectra of more than 0.5%BDD films, the two wide bands were observed at around 470 cm −1 and 1220 cm −1 , which are usually observed in high doped BDD. On the other hand, it could not be observed sp 2 -bonded band at around 1500 cm −1 . Consequently, it was successful to synthesize BDD films with different boron concentration. From CV measurements in 0.1 M H 2 SO 4 , the potential window narrowed with increasing boron concentration after surface oxidation (Fig.). In addition, CV measurements of 1 mM K 3 [Fe(CN) 6 ] showed the peak-to-peak potential separation increased with decreasing boron concentration. It is suggested that the thickness of the space-charge layer at the BDD surface reduces with increasing boron concentration. In order to give further insights, we are currently investigating the electrode property on CO 2 reduction. References (1) T. Watanabe, Y. Honda, K. Kanda, and Y. Einaga, Phys. Status Solidi A 211 , 2709 (2014). Figure 1