Electrochemical and XPS Study of the Nickel-Titanium Electrode Surface

A Ni-Ti alloy with a 50:50 atomic composition has shown exceptional properties as a fixed potential LCEC detector for carbohydrates and related substances. It exhibited excellent sensitivity and superior long-term stability compared to pure Ni. A study was therefore undertaken by means of cyclic voltammetry (CV), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) to understand the role of Ti and the respective surface oxides of Ni and Ti in the catalytic stability of the detector. CV results showed that Ti is initially oxidized, most likely to TiO(2) in 0.1 M NaOH solution. The oxidation of Ni to nickel(II) oxide also occurs at potentials close to that of Ti. At higher potentials in the range of +0.4 to +0.5 V vs Ag/AgCl reference, nickel(II) oxide undergoes further oxidation to the Ni(III) oxidation state. This state is responsible for the catalysis of carbohydrates, amino acids and other biosubstances. When Ni-Ti and Ni are repetitively CV cycled in the potential range of 0.0 to +0.6 V, a second wave appears at more negative potentials during the reverse cathodic scan for Ni but not for Ni-Ti. SEM images of these two electrodes in the oxidized form show the Ni-Ti surface remains smoother in appearance. This smoothness is consistent with the fact that the thickness of the surface "oxide" layer increases less rapidly, as Ni-Ti is repetitively CV cycled, compared to pure Ni. XPS results for the nature of the surface oxides are consistent with oxidized Ti as TiO(2), Ni(II) predominantly as Ni(OH)(2), and Ni(III) possibly as NiOOH. Possible reasons for Ti stabilizing the Ni-Ti alloy as a LCEC detector are discussed.