As shown in Fig. 4 and Table 3, the

As shown in Fig. 4 and Table 3, the C 1s high resolution spectra ofthe untreated UHMCFs showed the characteristic of graphitic structurewhich had the highest percentage of graphitic carbon (-C-C). In comparison with untreated UHMCFs, decreases in the relative content ofgraphitic carbon (-C-C) occurred and there was an obvious rise in therelative contents of C-O and C=O (peak III) groups. This was possiblydue to oxidative degradation of the graphitic structure at the surface ofthe fibers [43]. The relative content of carbonate groups (R1-OCOO-R2)also gradually decreased with the elevation of the current densities,which indicated that step-wise oxidation from –C-C to –C-O and –C=Ohappened to UHMCF surfaces and few carboxyl and carbonate groupswere generated.3.4. XRD analysisXRD patterns of UHMCFs in the form of equatorial scan are shownin Fig. 5. As can be seen, the 002 peak, 100 peak and 004 peak werefound in XRD spectrum of all the fibers. These peaks, especially the 002peak, were a reflection of the imperfect graphite structure. In the preparation of CFs, typical turbostratic carbon phase well-oriented in fiberdirection are formed, and many tetrahedral carbon-type crosslinks alsoshow up between the graphite-type carbon layers [44]. The existence ofthe 002 peak confirmed that UHMCFs still had turbostratic graphitestructure. The average interlayer spacing d002 indicative for the degreeof the turbostratic structure could be estimated from the 002 peak, andthe crystallite thickness Lc was also calculated according to Scherrer'sformula through equatorial pattern scanning [45,46]. XRD structuralparameters of UHMCFs are shown in Table 4. As an important parameter of the crystallite structure, the content of void Vp can be calculated through the following formula: Vp = 1-ρfd002/(ρgdg), where ρfmeans the density of carbon fiber, d002 represents the average interlayerspacing of carbon fiber, ρg is the density of ideal single graphite, and dgmeans the average interlayer spacing of ideal single graphite. The Vpvalues of UHMCFs with and without surface oxidation are also shown inTable 4.From a comparison of untreated and surface oxidized UHMCFs, thevalue of the average interlayer spacing d002 and the value of void Vpslightly increased, which was possibly due to chemical etching in surface oxidation and the contaminants removing from fiber surfaces. Thecrystallite thickness Lc went through an initial increase followed by adecrease in the process of electrochemical oxidation, which was consistent with surface roughness results by AFM. The results of XRDanalysis by powder method were a reflection of the integral structuralparameter rather than only surface structure. As a result, changes in thevalues of d002, Vp, and Lc were not so obvious which indicated theelectrochemical surface oxidation possibly couldn't affect the internalgraphite structure of UHMCFs.3.5. Raman spectroscopy analysisRaman spectroscopy has been demonstrated to be a useful techniqueto follow the micromechanics of deformation of CFs [47–49], and it canbe used to investigate the structure within a few tens of nanometers[50]. The Raman spectrum of CFs in the range of 1000–2000 cm−1mainly shows two characteristic bands. G-line (located at about