3.2.1.3. Electrode type D. Upon flame oxidation of mechanically
cleaned W disk, we found that a loosely bound yellow layer developed
over a compact dark blue surface layer. This pointed to a formation
of W2O5 and WO3 layer. Preliminary potentiometric
measurements showed that, if yellow layer was kept on the surface,
the pH response at pH > 7 disappeared, but this was not the
case when the yellow layer was removed. In order to characterize
oxide layers formed on the surface, conventional Raman technique
coupled with the optical microscope was applied. Raman spectra of
W rod oxidized in flame points to the formation of a monoclinic
WO3 (see Supplementary data, Figs. S2 and S3). Upon removal of
yellow oxide layer, oxidized surface was found to be uniform with
occasional microscopic cracks. Interestingly, Raman spectra of
dark-blue oxide phase indicated also the behavior of crystalline
WO3 (Fig. 3, p. 2). Raman spectra of microscopic cracks in dark blue
oxide layer (Fig. 3, p. 1) pointed to the presence of low amounts of
oxide. Pair of peaks around 808 and 710 cm2 pointed to WO3
vibrations (see Supplementary data), while another low intensity
peak around 1120 cm2 might point out to a certain amount of
amorphous tungsten oxide.