1790 Z.R. Yue et al. / Carbon 37(1999)1785-1796 ore surfaces goes up. These carbon atoms are where The O 2s-C 2s peak separation in Fig. 5 gradually oxidation occurs, so the relative amount of oxygen i increases throughout the series from the as-received to the creases Fig 4 shows the O Is spectra fitted to three component Xie and Sherwood [52] suggested that the o 2s region peaks. Peak I(531.2-531 6 ev) corresponds to C=o should show a greater sensitivity to the oxygen environ- groups(ketone, lactone, carbonyl); peak Il(532.2-533.4 ment than the O Is region. The O 2s-C 2s separation ev) to C-OH and/or C-O-C groups and peak Ill(534.6- should be intermediate between those of -C=O and-C- 535.4 ev) is of low intensity (probably due to chemisorbed OH groups in the oxidized fibers and is predicted to follow oxygen and perhaps some adsorbed water [28, 52]). The the series: -C-0-C->-C=0>-C-OH. Thus, the C=O contribution to the o Is profile(peak I) increases smallest separation, observed in the as-received fiber, may significantly from 42%(as-received fiber) to 53-54%(after be attributed to the prevalence of phenolic hydroxyls over 133 C/g of electrochemical oxidation). The peak I/ peak II arboxyl functions. The increase in the O 2s-C 2s peak area ratios for as-received fiber and oxidized (133 C/g) separation with the increasing fiber oxidation is due to fibers are <I and >I respectively. Clearly, the o Is more contribution from -C=O groups( mostly in COOH), consistent with the C ls profiles(Fig. consistent with Figs. 3 and 4 3), corroborating the increase in carboxyl groups on oxidized fibers. Furthermore Fig. 4 also reveals that the hemisorbed oxygen or adsorbed water (peak In) in- 3.1.3. FT-IR studie. creased obviously after electrochemical oxidation Since XPs can only probe num sampling depth Valence band spectra have also been recorded for the of approximately 100 a (and only about 50 A at a 30 take as-received and electrochemically oxidized fibers(Fig. 5). off angle), FT-iR was also employed to explore the change Two distinct peaks due to the o 2s electrons(near 27eV in functional groups induced by electrochemical oxidation and the C 2s electrons(near 17 ev)are present. a shoulder Oxidized fibers were ground into powders and FT-IR near 10 ev corresponds to the O 2p electrons. The relative spectra(see Fig. 6) were obtained on KBr pellets of these intensity of O 2s and o 2p peaks increased after 133 C/g powders which represent the entire fiber mass and not just of electrochemical oxidation versus the C 2s peak, but they the outer concentric shell. The relative intensity of the did not change much versus the C 2s peak upon higher broad peak at about 1727 cm (the C=O stretching levels of oxidation vibrations of ketones and/or carboxyl groups)increased 10600cg) dor ethe Peak Ill rbed oxygen or adsorbed water 975% 5378 Binding Energy(ev) Fig. 4. High-resolution XPS O Is spectra of electrooxidized carbon fibers with different extents of electrochemical oxidation.1790 Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 pore surfaces goes up. These carbon atoms are where The O 2s–C 2s peak separation in Fig. 5 gradually oxidation occurs, so the relative amount of oxygen in- increases throughout the series from the as-received to the creases. most highly electrochemically oxidized (10 600 C/g) fiber. Fig. 4 shows the O 1s spectra fitted to three component Xie and Sherwood [52] suggested that the O 2s region peaks. Peak I (531.2–531.6 eV) corresponds to C5O should show a greater sensitivity to the oxygen environ￾groups (ketone, lactone, carbonyl); peak II (532.2–533.4 ment than the O 1s region. The O 2s–C 2s separation eV) to C-OH and/or C-O-C groups and peak III (534.6– should be intermediate between those of –C5O and –C– 535.4 eV) is of low intensity (probably due to chemisorbed OH groups in the oxidized fibers and is predicted to follow oxygen and perhaps some adsorbed water [28,52]). The the series: –C–O–C–.–C5O.–C–OH. Thus, the C5O contribution to the O 1s profile (peak I) increases smallest separation, observed in the as-received fiber, may significantly from 42% (as-received fiber) to 53–54% (after be attributed to the prevalence of phenolic hydroxyls over 133 C/g of electrochemical oxidation). The peak I/peak II carboxyl functions. The increase in the O 2s–C 2s peak area ratios for as-received fiber and oxidized (133 C/g) separation with the increasing fiber oxidation is due to fibers are ,1 and .1 respectively. Clearly, the O 1s more contribution from –C5O groups (mostly in COOH), spectra in Fig. 4 are consistent with the C 1s profiles (Fig. consistent with Figs. 3 and 4. 3), corroborating the increase in carboxyl groups on oxidized fibers. Furthermore, Fig. 4 also reveals that the chemisorbed oxygen or adsorbed water (peak III) in- 3.1.3. FT-IR studies creased obviously after electrochemical oxidation. Since XPS can only probe a maximum sampling depth Valence band spectra have also been recorded for the of approximately 100 A (and only about 50 A at a 30 ˚ ˚ 8 take as-received and electrochemically oxidized fibers (Fig. 5). off angle), FT-IR was also employed to explore the change Two distinct peaks due to the O 2s electrons (near 27 eV) in functional groups induced by electrochemical oxidation. and the C 2s electrons (near 17 eV) are present. A shoulder Oxidized fibers were ground into powders and FT-IR near 10 eV corresponds to the O 2p electrons. The relative spectra (see Fig. 6) were obtained on KBr pellets of these intensity of O 2s and O 2p peaks increased after 133 C/g powders which represent the entire fiber mass and not just of electrochemical oxidation versus the C 2s peak, but they the outer concentric shell. The relative intensity of the 21 did not change much versus the C 2s peak upon higher broad peak at about 1727 cm (the C5O stretching levels of oxidation. vibrations of ketones and/or carboxyl groups) increased Fig. 4. High-resolution XPS O 1s spectra of electrooxidized carbon fibers with different extents of electrochemical oxidation