Z.R. Yue et al. /Carbon 37(1999)1785-1796 793 4000 I value of solution before adsorption o Ladsorpeon 800900 Fig. 9. Effect of treatment temperature on Ag adsorption and the Fig. 10. Effect of Heat-treatment temperature on the Naoh pH of the Ag solution after adsorption when electrochemically uptake and I, adsorption by electrochemically oxidized oxidized (5300 C/g)carbon fibers were heat treated after oxida- C/g)carbon fiber NaoH uptake was calculated by measurin tion. Heat-treatment of fiber was carried out at the specified change in pH of the NaoH 11 temperature for 30 min, under a nitrogen flow. Ag adsorptio volume of solution was 25-50 ml: fiber added was about 0.035 g was calculated by measuring the change in concentration of Ag. Adsorption was carried out at r 24 h to insure equilibriu Initial Ag concentration was 5 mM in 50 ml of solution; pH was reached. I, adsorption culated by measuring the adjusted with NH; H, O to 855, Fiber added was about 0.04 g, change in concentration of I, with a starch Adsorption was carried out at 25C for 24 h. to insure equilibrium indicator. The adsorption solution was aqueous I,/KI with concentration of 0.01 M. About 35 mg of fibers and 25 ml of solution were added into a flask which contribute to Ag adsorption have mostly been 25"C for 24 h in the dark thermally decomposed at 550oC. Since Ag is primarily adsorbed as Ag by carboxyl groups and Ag c by ortho and para hydroquinone(and other readily oxidized sites), it density on hydrophilic(oxygenated) versus hydroph is clear most of these functions had decomposed on (increasingly heat-treated) pore surfaces may also heating at 550C for 30 min. These functions, when present, react with Ag to generate H 3.2.3. Post-heating studies by X-ray photoelectron A decrease in the amount of surface acidic groups (carboxyl and phenolic hydroxyl groups), after heat treat- The XPS survey re shown in Fig. l1 for the ment was also measured by NaoH titrations. a plot of the dized(5300 C/g)and Naoh uptake onto fibers oxidized to 5300 C/g, versus the post heat-treated(at and 850C under nitroge heating temperature is shown in Fig. 10. This plot exhibits fibers. Major peaks in the spectra are due to the c Is and o the same trend as the a curve versus Is photoelectrons. A smaller N Is peak is also discernible treatment temperature(Fig. 9). The quantity of acidic The O/C and N/C atomic ratios, calculated from high functions decreased from 1767 umol/g to 189 umol/g resolution C Is, o Is and n Is XPs spectra, are shown in upon post-heating at 550C. Clearly, acidic carboxyl and Table 2. These data further demonstrate that the electro- phenolic hydroxyl functions were progressively destroyed chemically oxidized carbon fibers contain a higher oxygen at higher temperatures. After heating to 750 and 850C content in the outer 50A than the as-received fibers almost all the acidic functions had been decomposed However, the oxygen content in the outer 50 A surface In contrast to the Ag adsorption and NaoH uptake region decreases significantly after heating at 550 or curves, the amount of 1, adsorption by electrochemically 850C. The higher the treatment temperature, the lower the oxidized fibers increased as the heating temperature in- oxygen contents become. This is consistent with the data creased(see Fig. 10).I, adsorption is known to occur by a in Fig. 8, where the fiber weight loss increased with physical adsorption instead of chemical adsorption [62] increasing heat treatment temperature lodine adsorption mainly depends on factors such as the gh resolution C Is spectra(Fig. 12)of both the specific surface area and the micropore structure. From the as-received carbon fibers and the electrochemically oxi- data in Fig. 10, it can be postulated that more I, adsorption dized (5300 C/g) fibers were compared to the corr an occur because more surface area may be available for sponding spectra of the heat treated fibers(550 and 850.C) physical adsorption of iodine. Decomposition of oxygen- to further understand the trends noted above. The C ls ated functions within narrow micropores may increase pore spectra(Fig. 12)have each been resolved into the five surface area for 4 pinch points"allowing more internal individual component peaks discussed in Fig. 3.The iameters and open dsorption. The relative adsorption relative concentration of carbon-oxygen complexes in-Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 1793 1 Fig. 9. Effect of treatment temperature on Ag adsorption and the Fig. 10. Effect of Heat-treatment temperature on the NaOH 1 pH of the Ag solution after adsorption when electrochemically uptake and I adsorption by electrochemically oxidized (5300 2 oxidized (5300 C/g) carbon fibers were heat treated after oxida- C/g) carbon fiber. NaOH uptake was calculated by measuring the tion. Heat-treatment of fiber was carried out at the specified change in pH of the NaOH solution. Initial pH was 11.688; 1 temperature for 30 min, under a nitrogen flow. Ag adsorption volume of solution was 25–50 ml; fiber added was about 0.035 g; 1 was calculated by measuring the change in concentration of Ag . Adsorption was carried out at 258C for 24 h to insure equilibrium 1 Initial Ag concentration was 5 mM in 50 ml of solution; pH was reached. I adsorption was calculated by measuring the 2 . adjusted with NH H O to 8.55; Fiber added was about 0.04 g; change in concentration of I by Na S O titrations with a starch 3 2 2 22 3 Adsorption was carried out at 258C for 24 h. to insure equilibrium. indicator. The adsorption solution was aqueous I /KI with an I 2 2 concentration of 0.01 M. About 35 mg of fibers and 25 ml of solution were added into a flask. Adsorption was carried out at 1 258C for 24 h in the dark. which contribute to Ag adsorption have mostly been 1 thermally decomposed at 5508C. Since Ag is primarily 1 adsorbed as Ag by carboxyl groups and Ag8C by ortho and para hydroquinone (and other readily oxidized sites), it density on hydrophilic (oxygenated) versus hydrophobic is clear most of these functions had decomposed on (increasingly heat-treated) pore surfaces may also change. heating at 5508C for 30 min. These functions, when 1 1 present, react with Ag to generate H . 3.2.3. Post-heating studies by X-ray photoelectron A decrease in the amount of surface acidic groups spectroscopy (carboxyl and phenolic hydroxyl groups), after heat treat- The XPS survey spectra are shown in Fig. 11 for the ment was also measured by NaOH titrations. A plot of the as-received, electrochemically oxidized (5300 C/g) and NaOH uptake onto fibers oxidized to 5300 C/g, versus the post heat-treated (at 5508C and 8508C under nitrogen) heating temperature is shown in Fig. 10. This plot exhibits fibers. Major peaks in the spectra are due to the C 1s and O 1 the same trend as the Ag adsorption curve versus 1s photoelectrons. A smaller N 1s peak is also discernible. treatment temperature (Fig. 9). The quantity of acidic The O/C and N/C atomic ratios, calculated from high functions decreased from 1767 mmol/g to 189 mmol/g resolution C 1s, O 1s and N 1s XPS spectra, are shown in o upon post-heating at 550 C. Clearly, acidic carboxyl and Table 2. These data further demonstrate that the electro￾phenolic hydroxyl functions were progressively destroyed chemically oxidized carbon fibers contain a higher oxygen ˚ at higher temperatures. After heating to 750 and 8508C content in the outer 50A than the as-received fibers. ˚ almost all the acidic functions had been decomposed. However, the oxygen content in the outer 50 A surface 1 In contrast to the Ag adsorption and NaOH uptake region decreases significantly after heating at 550 or curves, the amount of I adsorption by electrochemically 8508C. The higher the treatment temperature, the lower the 2 oxidized fibers increased as the heating temperature in- oxygen contents become. This is consistent with the data creased (see Fig. 10). I adsorption is known to occur by a in Fig. 8, where the fiber weight loss increased with 2 physical adsorption instead of chemical adsorption [62]. increasing heat treatment temperature. Iodine adsorption mainly depends on factors such as the High resolution C 1s spectra (Fig. 12) of both the specific surface area and the micropore structure. From the as-received carbon fibers and the electrochemically oxi￾data in Fig. 10, it can be postulated that more I adsorption dized (5300 C/g) fibers were compared to the corre- 2 can occur because more surface area may be available for sponding spectra of the heat treated fibers (550 and 8508C) physical adsorption of iodine. Decomposition of oxygen- to further understand the trends noted above. The C 1s ated functions within narrow micropores may increase pore spectra (Fig. 12) have each been resolved into the five diameters and open ‘‘pinch points’’ allowing more internal individual component peaks discussed in Fig. 3. The surface area for I adsorption. The relative adsorption relative concentration of carbon–oxygen complexes in- 2