Electrochemical Reduction of Ozone Dissolved in Perchloric Acid Solutions at Boron-doped Diamond Electrodes Tsuyoshi Ochiai, 1;2 Kazuki Arihara, 1 Chiaki Terashima, 1 and Akira Fujishima 1 1 Technology Research and Development Department, General Technology Division, Central Japan Railway Company, 1545-33 Ohyama, Komaki 485-0801 2 Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555 (Received June 19, 2006; CL-060695; E-mail: arihara@jr-central.co.jp) With boron-doped diamond electrodes, an apparent cathodic peak was observed at 0 V in cyclic voltammograms obtained in 0.1 M HClO 4 solution containing ozone. This peak was attribut- ed to the reduction of dissolved ozone. A calibration curve ob- tained from the cathodic peak current, which has a slope of 6.7 mA cm 2 (mg L 1 ) 1 , can be applied to determining the dis- solved ozone concentration. This result will lead to the develop- ment of an electrochemical ozone sensor employing a diamond electrode. Ozone is a strong oxidant, close to fluorine in strength, and has been applied to sterilization, deodorization, and decoloriza- tion. 1 Additionally, the activity is remarkably enhanced in water owing to the generation of reactive oxygen species. However, in order to gain satisfactory effects, ozone concentration in water must be monitored and controlled strictly, usually in a range of 0.1–20 mg L 1 . Electrochemical detection of dissolved ozone provides some advantages in producing more compact device and lowering cost. However, since the measuring atmosphere is extremely oxidative due to the presence of ozone, the detect- ing electrodes have been limited to a few noble metals. 2,3 Recently, conductive diamond has attracted increasing at- tention as a promising electrode material for electrochemical sensors due to chemical inertness, dimensional stability, low re- sidual current, and wide potential window. 4 Many research works have been mainly dedicated to developing biosensors and dissolved trace-metal detectors. 4,5 On the other hand, Yano and co-workers reported that an ideal diamond electrode inhib- ited the oxygen-reduction reaction and retained a relatively wide potential window even in the presence of dissolved oxygen. 6 This character is tremendously advantageous in the applications to practical analysis under normal atmospheric conditions. Ac- cordingly, we have tried for the first time to apply the diamond electrodes to the electrochemical detection of dissolved ozone. A polycrystalline boron-doped diamond (BDD) thin film was synthesized on an n-Si(111) substrate with a high-pressure microwave plasma-assisted chemical vapor deposition (CVD) system (AX6500, Seki Technotron Corp). The substrate was pre-treated by polishing with 0.5–1.0 mm diamond powder, fol- lowed by ultrasonication in acetone. The carbon source was an acetone/methanol mixture (9:1, v/v) containing B 2 O 3 as the boron source (B/C atomic ratio 10 4 ppm). Deposition for 6 h at a microwave power of 8 kW produced a diamond electrode with a thickness of ca. 5 mm, resistivity of 8.3 mcm, and a typi- cal doping level of ca. 2 10 21 atoms cm 3 that was confirmed by SIMS. The diamond electrode was characterized by SEM observation and Raman spectroscopy. A 0.1 M HClO 4 solution was prepared with an analytical grade reagent and ultra pure water. Ozone solutions were pre- pared by bubbling ozone gas generated by a silent electric dis- charge system (ED-OG-R3Lt, EcoDesign Inc.) into the electro- lyte solution. The concentration of dissolved ozone was calculat- ed from UV absorbance at 258 nm with the molar absorption co- efficient of 2900 M 1 cm 1 . 7 Electrochemical measurements were performed in a two- compartment glass cell. Geometric area of the working electrode was 0.38 cm 2 . Pt wire was employed as the counter electrode, which was settled in a compartment separated by a glass-filter. A KCl saturated AgjAgCl electrode was employed as the refer- ence. Before each experiment, the BDD electrode was electro- chemically oxidized at þ3:2 V in a 0.1 M H 2 SO 4 solution in or- der to normalize the electrode condition. Cyclic voltammetry with BDD electrodes was performed in the presence of ozone in electrolyte solutions. Cyclic voltammo- grams (CVs) in Figure 1 demonstrate that ozone dissolved in a 0.1 M HClO 4 solution can be detected as a clear cathodic wave at 0 V, through the following reaction: 2,3 O 3 þ 2H þ þ 2e ! H 2 O þ O 2 : ð1Þ No discernable cathodic responses were observed in the poten- tial range from þ1:0 to 0:5 V under the Ar and O 2 atmosphere. Ideal BDD electrodes generally represent a considerably high overpotential toward the hydrogen evolution reaction, up to 1:0 V, as well as the low and flat background response. 6,8 In addition, the O 2 -reduction reaction is inhibited in the positive potential region from 0:4 V. Defining the potential inducing the current density of 5 mA cm 2 as the negative limit of the potential window, those under Ar and O 2 atmosphere were 0:97 and 0:8 V, respectively. Such a wide and flat potential -1 -0.5 0 0.5 1 -20 -15 -10 -5 0 E / V vs. Ag|AgCl i / μA cm -2 Figure 1. CVs obtained at an BDD electrode in 0.1 M HClO 4 under Ar (dotted line), O 2 (dashed line), and O 3 (2.6 mg L 1 , solid line) atmosphere. Scan rate was 100 mV s 1 . 1018 Chemistry Letters Vol.35, No.9 (2006) Copyright Ó 2006 The Chemical Society of Japan