Enhancement in redox and electrocatalytic activity observed on Si ion-implanted Ni
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Ni was surface-alloyed with Si by ion implantation. The material was examined for its redox and electrocatalytic behavior in NaOH by cyclic voltammetry. The surface was characterized by x-ray photoelectron spectroscopy, x-ray and electron diffraction, and electron and atomic force microscopy. The ion implantation enhanced activity toward the redox conversion of Ni(OH)2 ↔ NiOOH and the anodic oxidation of glucose reached about 3.5 times and about 2.8 times, respectively. The material is an amorphous mixed oxide of Ni and Si. The effect is discussed considering the true surface area and the generation of active surface sites in relation to the oxygen evolution.
I. INTRODUCTION
Ni oxides/hydroxides have industrial importance in the applications of batteries,1,2 catalysts,3,4 fuel cells,5,6 recording media,7 and electrochromic8 and sensor9–12 devices. The surface-bound redox couple Ni2+/Ni3+ is known to catalyse the electrooxidation of many small organic molecules in alkaline media.9–15 Ni-based alloys are favorable starting electrode materials in view of the potential improvement in catalytic activity by the alloying components and of the simplicity in technology. The electrochemistry of bulk alloys fabricated by melting9–11,16,17 and surface alloys prepared by electrodeposition12–14 have extensively been investigated. In the current work, Ni surface was alloyed with Si by ion implantation. In this process, the Ni surface was irradiated in vacuum with a beam of Si ions having defined energy and ion dose. Si is known to be one of the components in Ni bulk alloys and serves here as a reference point. The ion implantation may provide new aspects regarding the material dispersity, amorphization, and nonequilibrium composition which may be relevant for the electrochemical behavior. The redox conversion Ni(OH)2 ↔ NiOOH and the anodic oxidation of glucose were studied here for this type of materials.
II. EXPERIMENTAL
Samples under study were specimens of commercially pure Ni (10 × 10 × 1 mm3, Goodfellow, Cambridge, UK). One face was lapped using SiC paper in successive grades from 600 to 4000 grit and polished to a mirrorlike finish with aqueous silica suspensions, 0.1, 0.06, and 0.02 m, successively (Buehler GmbH). A cleaning was 616
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J. Mater. Res., Vol. 19, No. 2, Feb 2004 Downloaded: 15 Mar 2015
made in acetone, ethanol, and water before use. Sample surfaces were implanted with Si to ion doses of 5 × 1016 and 2 × 1017 ions cm−2 using the facilities at the Forschungszentrum Rossendorf. The low ion dose was implanted at two energy steps (40, 50 keV), the high ion dose at three energy steps (35, 45, 50 keV). The x-ray diffraction measurements were conducted on step scan diffractometers in grazing incidence geometry (URD6 and D5000, Siemens) using Goebel mirror and detector scan at fixed incidence angles (0.5° and 1.0°). The depth profiles were determined by Auger spectroscopy (Microlab 310F, Fisons) at intervals of argon sputtering (3 keV Ar ions, current density of 2 A cm−2). The Auger
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