Characterization by electrolyte electroreflectance and X-ray photoelectron spectroscopy of amorphous Ni 59Nb 40Pt 1− xSn x alloys and their activation by HF solutions

Abstract

The influence of chemical etching with HF on the nature of the surface of amorphous Ni 59Nb 40Pt 1− x Sn x alloys has been studied in situ by electrolyte electroreflectance (EER) and ex situ by X-ray photoelectron spectroscopy (XPS). The EER spectrum of the untreated alloy in 0.5 M H 2SO 4 shows a bipolar band, which disappears after the HF treatment yielding a structureless EER spectrum similar to that of Pt, but reappears after several hours in the 0.5 M H 2SO 4 electrolyte. This process of dissolution by HF of an oxide species and its reappearance after a few hours cannot be followed by XPS, since the time interval between sample withdrawal from the electrolyte and actual measurement is of a few hours as well. XPS spectra showed the presence of metallic Nb before and after the HF treatment, and that niobium pentoxide was the main species in the as-quenched alloy, but that after treatment with HF it became a minor component, the main one being NbO. The main effect of the HF treatment is to produce a platinum enrichment of the surface, as unequivocally determined by cyclic voltammetry, XPS and EER. After Ar sputtering for 9 min the XPS spectrum of the untreated alloy showed metallic Nb only, while in the HF-treated alloy the peaks of metallic Nb were swamped by those of NbO and some Nb 2O 5. We interpret this difference as being due to the formation by the HF attack of a porous Nb film which becomes oxidized in the electrolyte and/or during transfer to the spectrometer, and so thick that it is not eliminated by Ar sputtering for 9 min.