Efficient and stable oxygen evolution of zirconium nitride thin film electrodes prepared by the reactive RF sputtering technique

369

J. Electroanal. Chem., 220 (1987) 369-372 Elsevier Sequoia S.A., Lausanne- Printedin The Netherlands

Preliminarynote

EFFICIENT AND STABLE OXYGEN EVOLUTION ON ZIRCONIUM NITRIDE THIN FILM ELECTRODES PREPARED BY THE REACTIVE RF SPUTTERING TECHNIQUE

MASASHIAZUMA, YOSHIHIRONAKATO and HIROSHI‘IXJBOMURA Department of Chemistq Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560 (Japan)

(Received 14th January1987)

Studies

have been made on the electrochemical

properties

of transition

metal

nitrides such as TiN [l-4], VN [3], NbN [5], and TaN [5], but these transition metal nitrides were either passivated (TiN [3], NbN [5] and TaN [5]) or dissolved in aqueous solutions (VN [3]) under anodic polarization, like the respective transition metals. Very recently, we have reported [6] that niobium nitride (NbN) amorphous thin films, prepared by the reactive RF sputtering technique, can be used as a stable electrode and can cause oxygen evolution in aqueous solutions in a wide region of pH, although an electrode of niobium metal is rapidly passivated. In the present note we report that zirconium nitride amorphous thin films, prepared similarly by the reactive RF sputtering technique, are also quite stable and allow oxygen evolution at various pHs, contrary to easily passivated zirconium metal. The catalytic activity of the zirconium nitride films for the oxygen evolution in alkaline solutions was higher than that reported for NbN [6] and even somewhat higher than that of nickel metal, which is known as an efficient electrode material for oxygen evolution. The zirconium nitride thin films were prepared in an ULVAC Model SBR-1104E RF-sputtering apparatus. Mirror-finished carbon plates (Kyowa Carbon, 10 mm x 10 mm X 2 mm) used as substrates, were cleaned chemically and sputter-etched by nitrogen plasma, and then zirconium nitride thin films were sputter-deposited at an RF power of 2.5 W cme2 using a zirconium plate (99.9%) as a target under 6.5 X 10v3 Torr (1 Torr = 133.3 Pa) nitrogen (99.9995%) atmosphere. The deposition rate was calculated to be 4.0 f 0.4 run min-’ from the weight measurement of the deposited film with the density assumed to be 8.0 g cmm3. In all experiments the films were deposited for 3 h and the thickness of the deposited films was about 0.7 pm. The chemical composition of the deposited films was investigated using a Shimadzu ESCA 750 spectrometer and the Ar+-ion etching technique. The (N/Zr) 0022-0728/87/$03.50

8 1987 Elsevier Sequoia S.A.

E I V vs. SCE

Fig. 1. Cyclic voltammograms for a zirconium nitride (ZrN,) fii electrode (solid line) and a zirconium metal electrode (dotted line) in 0.5 M H2S04. Sweep rate 50 mV/s.

atomic ratio, as calculated from the relative intensities of the Nls and Zbd peaks corrected for the ionization cross sections, was 0.50 f 0.02 at the surface (i.e., for no Ar+-ion sputter-etching) and 0.70 f 0.05 in the bulk (i.e., in a region of the Ar+-ion etching time longer than 60 s). The latter value is not corrected for the difference in the etching rate between Zr and N. Fihns of such a composition are designated as ZrN, for convenience in the present paper. The binding energy of the Zr3d peak in the bu& was 182.3 eV, which is a value intermediate between that of Zr metal (179.5 eV) and ZrG, (187.3 eV). At the surface, the Zr3d peak was at 187.2 eV. This suggests that the surface of the films was oxidized to some extent. X-ray diffraction analysis of the films showed no diffraction peak, indicating an amorphous structure. The electrical resistivity of the deposited films was less than 10m2 61 cm. The electrochemical properties of the ZrN, film were investigated in an aqueous solution of 0.5 M H,SO,, 0.5 M NarSO,, or 1.0 M KOH (M= mol dm-3) at room temperature using a saturated calomel electrode (SCE) as the reference electrode. Figure 1 shows a typical cyclic voltammogram of a ZrN, film electrode (solid line) in 0.5 M H,SO,, as compared with that of a zirconium metal electrode (dotted line). The anodic current for the latter electrode was observed only in the first forward scan and became negligible in later scans, showing clearly that the electrode is passivated by the flow of the anodic current. The cyclic voltammogram for the ZrN, fihn electrode was, on the contrary, quite stable during repeated scans for more than 30 min. Gas evolution was observed in a potential region more positive than 1.2 V, suggesting oxygen evolution. Gas evolution was also observed in another potential region, more negative than -0.8 V, due to hydrogen evolution. Similar results were obtained in 0.5 M Na,SO, and 1.0 M KOH solutions. The electrocatalytic activity of the ZrN, film electrode for the hydrogen evolution was not high, like that of a zirconium metal electrode. For the oxygen

371

1.6

0 0

-5

-4

-2 -3 log (j ! Acm-2)

-1

Fig. 2. Tafel plots for the oxygen evolution currents at a zirconium nitride (ZrN,) fii electrode (0, solid line), smooth platinum (0, broken line), and smooth nickel (A, dotted line) in 1.0 M KOH, measured under quasi-steady-state galvanostatic conditions. The ZR drop is not corrected.

evolution, the ZrN, film electrode showed a fairly high electrocatalytic activity in alkaline solutions such as 1.0 A4 KOH. Figure 2 shows a Tafel plot for the oxygen evolution current at the ZrN, film electrode (solid line), compared with those at a smooth platinum electrode (broken line) and at a smooth nickel electrode (dotted line). These plots were obtained under galvanostatic conditions, by first holding the current density at 100 mA cm-* for 10 min, and then lowering the current density stepwise and holding it for 2 min at each step. The potential at each current density for the ZrN, film electrode for the oxygen evolution can be seen to be much lower than that of smooth platinum, and even somewhat lower than that of smooth nickel, which is known to be a good electrode material for oxygen evolution in alkaline solutions. The catalytic activity of ZrN, is also higher than that of NbN reported before [6]. Detailed comparison of the catalytic activity should, however, be made by taking account of the roughness factors of the ZrN, and NbN films, which are not determined yet. The zirconium nitride (ZrN,) films sometimes peeled off the carbon substrates by prolonged passage of the oxygen-evolution current at a density of about 100 mA cm-* for more than 1 h. It is likely that, since the carbon substrate has a rather rough surface on a microscopic scale, as seen by scanning electron microscopy, the ZrN, fihn has a number of pinholes or cracks. The electrolyte solution can thus penetrate through the pinholes or cracks to the substrate, and oxygen evolution may occur inside the film and at the ZrN,-substrate interface, finally causing the film to peel off. In conclusion, the present work has revealed that zirconium nitride films have a fairly good electrochemical stability irrespective of the solution pH and can cause oxygen evolution efficiently in alkaline solutions, while zirconium metal is passivated rapidly. The present result is encouraging in that the nitridation of transition metals might be effective for finding new, low-cost electrode materials of high stability and catalytic activity.

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