1792 Z.R. Yue et al. /Carbon 37(1999)1785-1796 hydrophilicity of these pores should decrease on continued Heat-treating temperature: 550C g and weight loss Hold ng time: 30 min, The magnitude of the measured weight losses can not be Protection gas: N2 accounted for even when all the oxygen in the outer 50 A concentric cylindrical region of the fibers( determined by XPS O/C atomic ratios, Figs. 2 and 3)is evolved as CO and H,O from the fibers oxidized to more than 133 C/g Thus, weight loss experiments prove that carbon-oxygen functions of these oxidized fibers are present at depths considerably greater than that probed by XPS. Further- more, the more highly oxidized the fibers become, the deeper the micropore structure penetrates into the fibers. 12000 The walls of micropores, slits and voids which are covered Extent of electrochemical with oxygenated functions extend far below the outer Fig. 7. Weight loss after heating electrochemically oxidized surface and constitute an interconnected, porous, oxyger carbon fibers at 550 C under N, for 30 min versus the extent of rich network CO,, CO and water are evolved via thermal electrochemical oxidation decomposition from throughout the depth of this micropor- diaryl ethers and phenolic hydroxyls>quinoid carbonyls. 3.22 Post heating ads The weight loss increased about 2.3% per 100C incre- Our previous studies [20] have shown that the surface ment in temperature up to about 600C. The amount of properties of electrochemically oxidized carbon fibers can additional weight loss per unit increase in temperature fell be probed by Ag adsorption occurring via ion-exchange off above 600C but was still headed upward at 850C. It is between Ag and carboxyl groups (COOH+ well known that oxidized carbon surfaces decompose Ag*+CO0Ag+H ) and by redox reactions such as that above 200%C to produce CO, and H,O and that CO begins portrayed in Eq(2). to evolve at higher temperatures [61 and references therein]. Phenolic hydroxyls and ketone carbonyl groups can produce CO. Carboxylic anhydrides evolve both CO, and CO. Water evolves upon dehydration reactions occur- ing between phenolic hydroxyls, carboxyls or a hydroxyl agt function reacting with carboxyl group. This evolution of OH H, O, CO, and CO results in significant fiber weight loss H OH rage diameters of the micropores from which these gases evolve as their surface oxygenated groups decompose. The surface activity and 8° The effects of post-oxidation heat treatment temperature on the Ag adsorption and the ph of the Ag solutions after adsorption are shown in Fig. 9. Fibers electrochemi- cally oxidized to 5300 C/ g exhibited a rapid decrease in the amount of Ag adsorbed from 3600 umol/g(no post oxidation heat treatment)to 388.4 pmol/g where treatment temperature of 550C (30 min) was employed eat treatment temperature cC) After temperatures of 750 and 850C almost Fig 8. Weight loss from electrochemically oxidized(5300 C/g) Ag was adsorbed. The change in solution pH from before carbon fibers after heating for 30 min in flowing nitrogen versu to after adsorption presents the same trend exhibited by heat treatment temperature Ag adsorption. This means that the oxygenated functions1792 Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 hydrophilicity of these pores should decrease on continued heating and weight loss. The magnitude of the measured weight losses can not be ˚ accounted for even when all the oxygen in the outer 50 A concentric cylindrical region of the fibers (determined by XPS O/C atomic ratios, Figs. 2 and 3) is evolved as CO2 and H O from the fibers oxidized to more than 133 C/g. 2 Thus, weight loss experiments prove that carbon–oxygen functions of these oxidized fibers are present at depths considerably greater than that probed by XPS. Further￾more, the more highly oxidized the fibers become, the deeper the micropore structure penetrates into the fibers. The walls of micropores, slits and voids which are covered with oxygenated functions extend far below the outer Fig. 7. Weight loss after heating electrochemically oxidized surface and constitute an interconnected, porous, oxygen￾carbon fibers at 5508C under N for 30 min versus the extent of rich network. CO , CO and water are evolved via thermal 2 2 electrochemical oxidation. decomposition from throughout the depth of this micropor￾ous structure. composition propensities: carboxylic acids, anhydrides. diaryl ethers and phenolic hydroxyls.quinoid carbonyls. 3.2.2. Post heating adsorption characterization The weight loss increased about 2.3% per 1008C incre- Our previous studies [20] have shown that the surface ment in temperature up to about 6008C. The amount of properties of electrochemically oxidized carbon fibers can additional weight loss per unit increase in temperature fell 1 be probed by Ag adsorption occurring via ion-exchange off above 6008C but was still headed upward at 8508C. It is 1 between Ag and carboxyl groups (COOH1 well known that oxidized carbon surfaces decompose 1 1 Ag →COOAg1H ) and by redox reactions such as that above 2008C to produce CO and H O and that CO begins 2 2 portrayed in Eq. (2). to evolve at higher temperatures [61 and references therein]. Phenolic hydroxyls and ketone carbonyl groups can produce CO. Carboxylic anhydrides evolve both CO2 and CO. Water evolves upon dehydration reactions occur￾ring between phenolic hydroxyls, carboxyls or a hydroxyl function reacting with carboxyl group. This evolution of H O, CO and CO results in significant fiber weight loss 2 2 and it should increase the average diameters of the micropores from which these gases evolve as their surface oxygenated groups decompose. The surface activity and (2) The effects of post-oxidation heat treatment temperature 1 1 on the Ag adsorption and the pH of the Ag solutions after adsorption are shown in Fig. 9. Fibers electrochemi￾cally oxidized to 5300 C/g exhibited a rapid decrease in 1 the amount of Ag adsorbed from |3600 mmol/g (no post oxidation heat treatment) to 388.4 mmol/g where a heat treatment temperature of 5508C (30 min) was employed. After treatment temperatures of 750 and 8508C almost no 1 Fig. 8. Weight loss from electrochemically oxidized (5300 C/g) Ag was adsorbed. The change in solution pH from before carbon fibers after heating for 30 min in flowing nitrogen versus to after adsorption presents the same trend exhibited by 1 heat treatment temperature. Ag adsorption. This means that the oxygenated functions