CARBON PERGAMON Carbon37(1999)1785-1796 Surface characterization of electrochemically oxidized carbon fibers Z.R. Yue, W. Jiang, L. Wang, S.D. Gardner, C U. Pittman Jr . k Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA Department of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, US Received 7 October 1998; accepted 5 February 1999 Abstract High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current to the fibers serving as an anode in 1% wt aqueous KNO,. Progressive fiber loss occurred with increasing extents of electrochemical oxidation. XPS studies(C Is and o Is) indicated that the oxygen/carbon atomic ratio rose rapidly to 0. 24 as the extent of electrochemical oxidation was increased from 0 to 133 C/g and then remained almost constant as the extent of electrochemical oxidation rose to 10 600 C/g. Fitting the C Is spectra demonstrated that the rise in surface oxygenated unctional groups was mainly due to an increase in carboxyl(COoH)or ester(COOR) groups. An increase in the intensity of the O Is peak (534.6-535.4 ev) after electrochemical oxidation corresponded to chemisorbed oxygen and/or adsorbed water. Electrochemical oxidation increased surface activity by generating more surface area via the formation of ultramicropore, and by introducing polar oxygen-containing groups over this extended porous surface. FT-IR spectra showed a broad peak at about 1727 cm from C=O stretching vibrations of carboxyl and/or ketone groups, the relative intensity of which increased significantly with the extent of electrochemical oxidation. Post-oxidation heat-treatments in flowing nitrogen at 550C for 30 min caused further weight losses due to decarboxylation of carboxyl groups and other reactions in which oxygenated functions decomposed. These weight losses increased with the extent of electrochemical oxidation. This demonstrated that more oxygenated groups formed on the internal pore surfaces as pores increasingly penetrated deeper into the fibers with increased electrochemical treatment. Weight loss depended on the heat treatment temperature since different types of carbon-oxygen surface groups were formed during the electrochemical oxidations Different functions have different abilities to decarboxylate or decarbonylate. The amount of Ag and NaoH uptake by electrochemically oxidized fibers rapidly decreased as the temperature of the post heat treatment increased to 550C. beyond 550C the progressive decrease in Ag adsorption and Naoh uptake continued at a slower rate and approached 0 umol/g fter heating to 850C. Conversely, after heat treatment I, adsorption showed a marked increase as the treatment temperature was raised. Thermal decomposition of carbon-oxygen complexes within the pore structure leads to a lower hydrophilicity of the pore surface. The extensive micropore surface area generated by electrochemical oxidation becomes more accessible to I2 as CO, and CO evolve. Very narrow pores(<10 a diameter) blocked by hydrogen bonding and oxygenated functions become more open. XPS analyses illustrated that the surface oxygen content decreased significantly after heat-treating to 550 or 850%C and was lowest after the 850 treatment. c 1999 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon fibers; B. Electrochemical treatment; Heat treatment; C. X-ray photoelectron spectroscopy (XPS);D. Surface 1. Introduction of carbon fiber-reinforced resin composites depend on the The mechanical properties and environmental stability fiber and the matrix [1-4]. Previous studies have attempted to generate strong adhesion between the fiber surface and sponding author. Tel. +1-601-325-7616: fax: + 1-601 matrix 5-8 to improve stress transfer from the relatively weak and compliant matrix to the strong and stiff reinforc- ddress: pittman @ra. msstate edu(C U. Pittman Jr) ing fibers, Therefore surface treatment of carbon fibers is -6223/99/S-see front matter 1999 Elsevier Science Ltd. All rights reserved
PERGAMON Carbon 37 (1999) 1785–1796 Surface characterization of electrochemically oxidized carbon fibers b a b b a, Z.R. Yue , W. Jiang , L. Wang , S.D. Gardner , C.U. Pittman Jr. * a Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA b Department of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, USA Received 7 October 1998; accepted 5 February 1999 Abstract High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current to the fibers serving as an anode in 1% wt aqueous KNO . Progressive fiber weight loss occurred with increasing extents of 3 electrochemical oxidation. XPS studies (C 1s and O 1s) indicated that the oxygen/carbon atomic ratio rose rapidly to 0.24 as the extent of electrochemical oxidation was increased from 0 to 133 C/g and then remained almost constant as the extent of electrochemical oxidation rose to 10 600 C/g. Fitting the C 1s spectra demonstrated that the rise in surface oxygenated functional groups was mainly due to an increase in carboxyl (COOH) or ester (COOR) groups. An increase in the intensity of the O 1s peak (534.6–535.4 eV) after electrochemical oxidation corresponded to chemisorbed oxygen and/or adsorbed water. Electrochemical oxidation increased surface activity by generating more surface area via the formation of ultramicropores, and by introducing polar oxygen-containing groups over this extended porous surface. FT-IR spectra 21 showed a broad peak at about 1727 cm from C5O stretching vibrations of carboxyl and/or ketone groups, the relative intensity of which increased significantly with the extent of electrochemical oxidation. Post-oxidation heat-treatments in flowing nitrogen at 5508C for 30 min. caused further weight losses due to decarboxylation of carboxyl groups and other reactions in which oxygenated functions decomposed. These weight losses increased with the extent of electrochemical oxidation. This demonstrated that more oxygenated groups formed on the internal pore surfaces as pores increasingly penetrated deeper into the fibers with increased electrochemical treatment. Weight loss depended on the heat treatment temperature since different types of carbon–oxygen surface groups were formed during the electrochemical oxidations. 1 Different functions have different abilities to decarboxylate or decarbonylate. The amount of Ag and NaOH uptake by electrochemically oxidized fibers rapidly decreased as the temperature of the post heat treatment increased to 5508C. Beyond 1 5508C the progressive decrease in Ag adsorption and NaOH uptake continued at a slower rate and approached 0 mmol/g after heating to 8508C. Conversely, after heat treatment I adsorption showed a marked increase as the treatment temperature 2 was raised. Thermal decomposition of carbon–oxygen complexes within the pore structure leads to a lower hydrophilicity of the pore surface. The extensive micropore surface area generated by electrochemical oxidation becomes more accessible to I2 ˚ as CO and CO evolve. Very narrow pores (,10 A diameter) blocked by hydrogen bonding and oxygenated functions 2 become more open. XPS analyses illustrated that the surface oxygen content decreased significantly after heat-treating to 550 or 8508C and was lowest after the 8508C treatment. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; B. Electrochemical treatment; Heat treatment; C. X-ray photoelectron spectroscopy (XPS); D. Surface properties 1. Introduction of carbon fiber-reinforced resin composites depend on the effectiveness of the interfacial bond between the carbon The mechanical properties and environmental stability fiber and the matrix [1–4]. Previous studies have attempted to generate strong adhesion between the fiber surface and *Coresponding author. Tel.: 11-601-325-7616; fax: 11-601- matrix [5–8] to improve stress transfer from the relatively 325-7611. weak and compliant matrix to the strong and stiff reinforcE-mail address: cpittman@ra.msstate.edu (C.U. Pittman Jr.) ing fibers. Therefore surface treatment of carbon fibers is 0008-6223/99/$ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(99)00047-0
1786 Z.R. Yue et al. /Carbon 37(1999)1785-1796 an important step in composite manufacture. Interfacial 2. 2. Electrochemical oxidation and heat treatment bonding in composites has been enhanced by fiber surface treatments such as electrochemical oxidation [3, 9-23] and Continuous electrochemical treatments were carried out oxidation in concentrated nitric acid [10, 14, 15, 24-33], in a U-tube apparatus. An aqueous 1% wt KNO, solution potassium permanganate [34], sodium hypochlorite [35], was used as the electrolyte. The carbon fibers were fed hydrogen peroxide and potassium persulfate [36,37]. Gase- continuously and served as the anode. A 254 cm long ous oxidations include air [38], oxygen [34], and ozone 39, 40 oxidation as well as plasma treatments 31, 41-46 B shaped stainless steel bar inside the U-tube acts as the thode. A gear system allowed variation of the fiber Fiber/matrix adhesion is improved through a combination residence time in the oxidation reaction and the voltage of increased acid-base interactions, chemical-bonding [47 could be varied from 30 to 45 V to change the current or by enhanced mechanical interlocking [48] flow. The schematic diagram of this apparatus and the Continuous surface electrochemical oxidation has been details of the treatment methods have previously been preferred. Electrochemical treatments have been carried described [19]. After electrochemical oxidation, all sam- out in acid and alkaline aqueous solutions of ammonium ples were thoroughly washed with distilled water, and ulfate [17], ammonium bicarbonate [21], sodium hydrox- dried at 110°C. ide [22], diammonium hydrogen phosphate [23] and nitric To further explore surface chemistry, oxidized fibers acid [49 Anodic oxidation of fibers in electrolytes can were heated for 30 min in flowing N, at constant tempera- produce a variety of chemical and physical changes in the tures between 150 and 850C fiber surface [11]. Most investigations have been done at low levels of oxidation. previous anodic oxidations 2. 3. Titration and adsorption in aqueous solutions proceeding to higher levels of oxidation, conducted in neutral aqueous potassium nitrate, greatly increased the Both Naoh uptake and the adsorption capacity of fibers quantity of surface acidic functions and the specific surface for silver ion and iodine were determined by the change in rea of PAN-based carbon fibers [19]. Over I mmol/g of concentration from before to after immersing a weighed total titratable acidic functional groups per gram of carbon amount of the fibers in the respective solutions fiber and 67 m /g of specific surface area were achieved by 6360 C/g of electrochemical oxidation in 1% wt KNO 2.3. I. NaOH up In the present investigation, X-ray photoelectron spec- NaoH solutions(4-5 mM) were prepared with boiled distilled water to remove dissolved carbon dioxide. Ap- troscopy, FT-IR, aqueous NaoH titration, the weight loss proximately 0.035 gram of carbon fiber was immersed for measurements upon heat treating oxidized fibers and 24 h in 25-50 ml of NaoH solution in a plastic vial. The adsorption of Ag, and I, were used to characterize the NaoH concentration changes were measured with a ph effects of electrochemical oxidation on the fiber surface meter (lon Analyzer 250, Corning chemical composition and acidic functions(carboxyl and phenolic hydroxyl groups) 2.3.2. Ag adsorption onto the fiber surface and by changing the surface rough- A weighed amount of carbon fiber (0.04 g)was ness and morphology might increase fiber/matrix adhesion immersed in 50 ml of AgNO, solution (+5 mM) and ith reactive epoxy or polyurethane resin matrices. How- shaken at 25"C for 24 h in the dark. The initial pH value of ever, if extensive new ultramicroporosity is generated the AgNO, solution was adjusted with NH3/H,o to 8.55 below the outer surface in the form of micropores lined The change in Ag concentration after adsorption was with acidic functions, matrix resins will be unable to determined by KsCN titration using Fe(NH)(SO,)2as effectively penetrate the pores to enhance adhesion. How- the indicator [ 50]. Before titration, the ph of all of the ever,small gaseous or solution molecules could. Thus, adsorbates was adjusted to an acidic state(pH=2-4) highly oxidized fibers could play a role as adsorbents with useful structural properties 2.3.3.lode Aqueous 1, /KI solutions with an I, concentration of 0. 01M were used in adsorption experiments. Fibers(-35 2. Experimental mg) were added into 25 ml of this solution and shaken at 25C for 24 h in the dark. The I, concentration remaining 2. Materials was determined by Na, S,O, titration with a starch in- dicator 51 The carbon fiber employed consisted of high strength, type Il, PAN-based fibers(Thornel T-300)manufactured 2. 4. X-ray photoelectron spectroscopy (XPS) by Amoco Performance Products, Inc. with 3 000 filaments per tow. All other chemicals were of analytical purity from All samples analyzed by XPs were first dried in a Aldrich Chemical Co. and used as received vacuum at 100°for6h
1786 Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 an important step in composite manufacture. Interfacial 2.2. Electrochemical oxidation and heat treatment bonding in composites has been enhanced by fiber surface treatments such as electrochemical oxidation [3,9–23] and Continuous electrochemical treatments were carried out oxidation in concentrated nitric acid [10,14,15,24–33], in a U-tube apparatus. An aqueous 1% wt KNO solution 3 potassium permanganate [34], sodium hypochlorite [35], was used as the electrolyte. The carbon fibers were fed hydrogen peroxide and potassium persulfate [36,37]. Gase- continuously and served as the anode. A 254 cm long ous oxidations include air [38], oxygen [34], and ozone U-shaped stainless steel bar inside the U-tube acts as the [39,40] oxidation as well as plasma treatments [31,41–46]. cathode. A gear system allowed variation of the fiber Fiber/matrix adhesion is improved through a combination residence time in the oxidation reaction and the voltage of increased acid–base interactions, chemical-bonding [47] could be varied from 30 to 45 V to change the current or by enhanced mechanical interlocking [48]. flow. The schematic diagram of this apparatus and the Continuous surface electrochemical oxidation has been details of the treatment methods have previously been preferred. Electrochemical treatments have been carried described [19]. After electrochemical oxidation, all samout in acid and alkaline aqueous solutions of ammonium ples were thoroughly washed with distilled water, and sulfate [17], ammonium bicarbonate [21], sodium hydrox- dried at 1108C. ide [22], diammonium hydrogen phosphate [23] and nitric To further explore surface chemistry, oxidized fibers acid [49]. Anodic oxidation of fibers in electrolytes can were heated for 30 min in flowing N at constant tempera- 2 produce a variety of chemical and physical changes in the tures between 150 and 8508C. fiber surface [11]. Most investigations have been done at low levels of oxidation. Previous anodic oxidations 2.3. Titration and adsorption in aqueous solutions proceeding to higher levels of oxidation, conducted in neutral aqueous potassium nitrate, greatly increased the Both NaOH uptake and the adsorption capacity of fibers quantity of surface acidic functions and the specific surface for silver ion and iodine were determined by the change in area of PAN-based carbon fibers [19]. Over 1 mmol/g of concentration from before to after immersing a weighed total titratable acidic functional groups per gram of carbon amount of the fibers in the respective solutions. 2 fiber and 67 m /g of specific surface area were achieved by 6360 C/g of electrochemical oxidation in 1% wt KNO 2.3.1. NaOH uptake 3 solutions [19,20]. NaOH solutions (4–5 mM) were prepared with boiled In the present investigation, X-ray photoelectron spec- distilled water to remove dissolved carbon dioxide. Aptroscopy, FT-IR, aqueous NaOH titration, the weight loss proximately 0.035 gram of carbon fiber was immersed for measurements upon heat treating oxidized fibers and 24 h in 25–50 ml of NaOH solution in a plastic vial. The 1 adsorption of Ag , and I were used to characterize the NaOH concentration changes were measured with a pH 2 effects of electrochemical oxidation on the fiber surface meter (Ion Analyzer 250, Corning). chemical composition and morphology. Introducing more 1 acidic functions (carboxyl and phenolic hydroxyl groups) 2.3.2. Ag adsorption onto the fiber surface and by changing the surface rough- A weighed amount of carbon fiber (|0.04 g) was ness and morphology might increase fiber/matrix adhesion immersed in 50 ml of AgNO solution (|5 mM) and 3 with reactive epoxy or polyurethane resin matrices. How- shaken at 258C for 24 h in the dark. The initial pH value of ever, if extensive new ultramicroporosity is generated the AgNO solution was adjusted with NH /H O to 8.55. 3 32 1 below the outer surface in the form of micropores lined The change in Ag concentration after adsorption was with acidic functions, matrix resins will be unable to determined by KSCN titration using Fe(NH )(SO ) as 4 42 effectively penetrate the pores to enhance adhesion. How- the indicator [50]. Before titration, the pH of all of the ever, small gaseous or solution molecules could. Thus, adsorbates was adjusted to an acidic state (pH52–4). highly oxidized fibers could play a role as adsorbents with useful structural properties. 2.3.3. Iodine adsorption Aqueous I /KI solutions with an I concentration of 2 2 0.01M were used in adsorption experiments. Fibers (|35 2. Experimental mg) were added into 25 ml of this solution and shaken at 258C for 24 h in the dark. The I concentration remaining 2 2.1. Materials was determined by Na S O titration with a starch in- 22 3 dicator [51]. The carbon fiber employed consisted of high strength, type II, PAN-based fibers (Thornel T-300) manufactured 2.4. X-ray photoelectron spectroscopy (XPS) by Amoco Performance Products, Inc. with 3 000 filaments per tow. All other chemicals were of analytical purity from All samples analyzed by XPS were first dried in a Aldrich Chemical Co. and used as received. vacuum at 1008C for 6 h
Z.R. Yue et al. /Carbon 37(1999)1785-1796 each of which contained chemical oxidation was defined in terms of coulombs approximately 3000 1 vere cut from the carbon fiber (AXn) per gram(C/g) tow and positioned of a custom stainless steel sample holder. The vere held firmly in place by a gold foil mask secured to the sample holder with screws 3.1.1. Weight loss of carbon fibers The gold foil contained a machined oval opening in its There was a continual loss of weight of the carbon fibers center that exposed a 1.5 cm by 0.8 cm area of the as the extent of oxidation increased. The weight loss was underlying carbon fibers KPS experiments were performed on a Physical Elec- oxidation(Fig. 1)from the onset of oxidation up to 4000 tronics PHI Model 1600 surface analysis system equipped C/g. At this point 17-18% of the initial fiber weight had with a PHI 10-360 spherical capacitor energy analyzer been lost. a slower loss of weight occurred as the extent of (SCA)fitted with an Omni Focus Ill small-area lens(800 electrochemical oxidation increased from 4000 to 8000 um diameter analysis area)and a high-performance multi- C/g At 8000 C/g 20-21% of the mass was gone. Then a channel detector. Samples were oriented such that the axial arp increase in the extent of weight loss occurred with direction of carbon fibers was in the plane of the X-ray continued oxidation over 8000 C/g. A 30% weight loss angle was at 30 Progressive weight loss occurs with CO, evolution. KPS spectra were obtained using an achromatic Mg k Active site atoms on the fiber surface were oxidized to (1253.6 eV)X-ray source operated at 200 W. Survey scans form such oxygen-containing surface groups as C-Oh were collected from 0-1100 ev with a pass energy equal to C=O, COOH and finally CO,. The types of oxygen 46.95 eV. High-resolution scans were performed with the functions and the simplified step-wise progression mecha- ass energy adjusted to 23.50 ev. The vacuum system nism for carbon surface oxidation in Eq (1) have been pressure was maintained at approximately 10 Torr widely studied [54-59 during all XPS experiments A non-linear least squares curve fitting program XPSPEAK95 software, Version 2.0) with a Gaussian- Lorentzian mix function and Shirley background subtrac- tion was used to deconvolve the xPs peaks. The Lorentz Gaussian mix was 60%. The carbon Is electron binding energy corresponding to graphitic carbon was referenced at 284.6 ev for calibration [52]. The peak constraints for fitting were used. All the higher energy C Is peaks fitted were shifted to higher binding energies by about 1.55, 3.0, 4.0 and 6. 1 ev, respectively. Atomic ratios were calculated from the XPs spectra after correcting the relative peak areas by sensitivity factors based on the transmissio characteristics of the Physical Electronics SCA [53 25. Fourier transform infrared spectroscopy 20 FT-IR spectroscopy was used for analyzing functional groups formed on the electrochemically oxidized carbon fibers. Treated fibers were cut and mixed with KBr. the mixture was analyzed with a Bruker Instruments Inc odel IFS 25 FT-IR spectrometer. 3. Results and discussion 3.I. Influence of the extent of electrochemical oxidation 20004000600080001000012000 High strength PAN-based carbon fibers were continuous- ly electrochemically oxidized by applying current (A)for Extent of electro-oxidation(C/g) specific residence times(n)to the fibers which served as an Fig. 1. eight loss of carbon fiber as a function of the extent of anode in 1% wt KNo solution. The extent of electro- electrochemical oxidation
Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 1787 Several 2.5 cm sections (each of which contained chemical oxidation was defined in terms of Coulombs approximately 3000 fibers) were cut from the carbon fiber (A3t) per gram (C/g). tow and positioned on top of a custom stainless steel sample holder. The fibers were held firmly in place by a 3.1.1. Weight loss of carbon fibers gold foil mask secured to the sample holder with screws. There was a continual loss of weight of the carbon fibers The gold foil contained a machined oval opening in its as the extent of oxidation increased. The weight loss was center that exposed a 1.5 cm by 0.8 cm area of the approximately proportional to the extent of electrochemical underlying carbon fibers. XPS experiments were performed on a Physical Elec- oxidation (Fig. 1) from the onset of oxidation up to 4000 C/g. At this point 17–18% of the initial fiber weight had tronics PHI Model 1600 surface analysis system equipped with a PHI 10-360 spherical capacitor energy analyzer been lost. A slower loss of weight occurred as the extent of electrochemical oxidation increased from 4000 to 8000 (SCA) fitted with an Omni Focus III small-area lens (800 mm diameter analysis area) and a high-performance multi- C/g. At 8000 C/g 20–21% of the mass was gone. Then a sharp increase in the extent of weight loss occurred with channel detector. Samples were oriented such that the axial continued oxidation over 8000 C/g. A 30% weight loss direction of carbon fibers was in the plane of the X-ray had occurred at about 10 600 C/g. source and the analyzer detection slit. The electron take-off Progressive weight loss occurs with CO evolution. angle was at 308. 2 XPS spectra were obtained using an achromatic Mg K Active site atoms on the fiber surface were oxidized to a form such oxygen-containing surface groups as C–OH, (1253.6 eV) X-ray source operated at 200 W. Survey scans were collected from 0–1100 eV with a pass energy equal to C5O, COOH and finally CO . The types of oxygen 2 functions and the simplified step-wise progression mecha- 46.95 eV. High-resolution scans were performed with the nism for carbon surface oxidation in Eq. (1) have been pass energy adjusted to 23.50 eV. The vacuum system 29 widely studied [54–59]. pressure was maintained at approximately 10 Torr during all XPS experiments. A non-linear least squares curve fitting program (XPSPEAK95 software, Version 2.0) with a GaussianLorentzian mix function and Shirley background subtraction was used to deconvolve the XPS peaks. The Lorentz/ Gaussian mix was 60%. The carbon 1s electron binding (1) energy corresponding to graphitic carbon was referenced at 284.6 eV for calibration [52]. The peak constraints for fitting were used. All the higher energy C 1s peaks fitted were shifted to higher binding energies by about 1.55, 3.0, 4.0 and 6.1 eV, respectively. Atomic ratios were calculated from the XPS spectra after correcting the relative peak areas by sensitivity factors based on the transmission characteristics of the Physical Electronics SCA [53]. 2.5. Fourier transform infrared spectroscopy FT-IR spectroscopy was used for analyzing functional groups formed on the electrochemically oxidized carbon fibers. Treated fibers were cut and mixed with KBr. The mixture was analyzed with a Bruker Instruments Inc. Model IFS 25 FT-IR spectrometer. 3. Results and discussion 3.1. Influence of the extent of electrochemical oxidation High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current (A) for specific residence times (t) to the fibers which served as an Fig. 1. Weight loss of carbon fiber as a function of the extent of anode in 1% wt KNO solution. The extent of electro- electrochemical oxidation. 3
88 Z.R. Yue et al. /Carbon 37(1999)1785-179 Partial decarboxylation with the resultant weight loss led to increased fiber surface fac roughness. The shape of the weight loss versus the extent of electrochemical oxidation curve(Fig. 1)shows that the morphology/pore structure continually changes during 020 oxidation and CO, evolution. The number and type of active sites change with increasing extent of electrochemi- cal el keygen- containing functions per gram of fiber and a higher surface area due to continually developing ultramicroporosity below the outer fiber surface. Our previous studies [20] demonstrated that a large increase in acidic functions (to 1-1. 1 mmol/g of fiber), measured by NaoH uptake occurred as electrochemical oxidation proceeded to 6360 C/g. To accommodate this number of titratable groups Fig. 2. XPS O Is/C Is and N Is/c Is atomic ratios of (a)the there must be a large increase in surface area even if every as-received carbon fiber and fibers electrochemically oxidized in o wt KNO, solution at levels of(b) 133 C/g;(c)1060 C/g;(d) surface carbon atom is oxygenated. The only way that such 4240Clg(e)5300c/g(f6360c/gand(g)10600C/g a large surface could form is by the generation of a small diameter pore/slit interconnected network below the outer surface of the fibers. However, nitrogen BET measure- appears in Table 1. Fig. 2 shows the O Is/c Is and n ments were only able to detect a small fraction of this new ls/c Is atomic ratios obtained from high resolution XPS porosity. Thus, the majority of the pores/slits etc. must be The as-received fibers display a smaller O Is/C Is ratio very small. Such very small pores require thermal activa-(0. 15)while electrochemically oxidized samples show tion to effect nitrogen filling. Specific surface areas were higher O Is/C Is ratios (0.23-0.27). The N 1s/c Is more effectively measured by DR/CO2 adsorption at 273K atomic ratios remained below 0.04 at all levels of oxidation and interpreted with the aid of density functional theory indicating no specific nitrogen incorporation occurred from [191. DR/CO, measurements were able to account for KNO, or dissolved nitrogen during electrochemical oxida most of the surface area which had to exist based on tion. These values reflect the integrated o/c and N/c existing titratable acidic functions. A large fraction of the ratios only over the sampling depth of-50 A from which pores were found to be very small(diameters of 4-6A). ejected electrons are able to escape when probed by XPS Thus, the 1 mmol/g of total acidic groups per gram of using a 30 electron take off angle. The surface oxygen carbon fiber( formed after 6360 C/g of electrochemical concentration rose rapidly to 24% after initial electro- oxidation) were occupying 67 m /g of surface area mainly chemical oxidation at 133 C/g and then remained at this composed of 4, 5 and 6 A average diameter ultramicro- level (or rose somewhat)with an increase in the extent of oxidation up to 10 600 C/g. The total amount of acidic functions(detected by NaoH titration) increased from 3 3. 1. 2. Studies by X-ray photoelectro umol/g(as-received )to 2476 umol/g(10 600 C/g)[601 XPS experiments were performed on both as-received This large (838-fold) increase in acidic functions and selected electrochemically oxidized carbon fibers. A COOH and phenolic -OH) which accompanies fiber weight summary of the fiber treatments and their designations loss(e.g, loss of carbon and nitrogen from oxidized fibers) Table I Treatments of carbon fibers used in XPS analysis Applied current Residence time Extent of electro (Amps) xidation(C/g) b)ECF-40-0.05 44.2 (c)ECF-10-0.1 d)ECF-15-0.6 117.8 (e)ECF-10-0.5 (fECF-10-0.6 (g)ECF-10-1.0 10,600 All electrochemical oxidations are referenced eceived fiber
1788 Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 Partial decarboxylation with the resultant weight loss led to increased fiber surface area and increased surface roughness. The shape of the weight loss versus the extent of electrochemical oxidation curve (Fig. 1) shows that the morphology/pore structure continually changes during oxidation and CO evolution. The number and type of 2 active sites change with increasing extent of electrochemical oxidation, giving a higher total number of oxygencontaining functions per gram of fiber and a higher surface area due to continually developing ultramicroporosity below the outer fiber surface. Our previous studies [20] demonstrated that a large increase in acidic functions (to 1–1.1 mmol/g of fiber), measured by NaOH uptake, occurred as electrochemical oxidation proceeded to 6360 Fig. 2. XPS O 1s/C 1s and N 1s/C 1s atomic ratios of (a) the C/g. To accommodate this number of titratable groups as-received carbon fiber and fibers electrochemically oxidized in there must be a large increase in surface area even if every 1% wt KNO solution at levels of (b) 133 C/g; (c) 1060 C/g; (d) 3 surface carbon atom is oxygenated. The only way that such 4240 C/g; (e) 5300 C/g; (f) 6360 C/g and (g) 10 600 C/g. a large surface could form is by the generation of a small diameter pore/slit interconnected network below the outer surface of the fibers. However, nitrogen BET measure- appears in Table 1. Fig. 2 shows the O 1s/C 1s and N ments were only able to detect a small fraction of this new 1s/C 1s atomic ratios obtained from high resolution XPS. porosity. Thus, the majority of the pores/slits etc. must be The as-received fibers display a smaller O 1s/C 1s ratio very small. Such very small pores require thermal activa- (0.15) while electrochemically oxidized samples show tion to effect nitrogen filling. Specific surface areas were higher O 1s/C 1s ratios (0.23–0.27). The N 1s/C 1s more effectively measured by DR/CO adsorption at 273K atomic ratios remained below 0.04 at all levels of oxidation 2 and interpreted with the aid of density functional theory indicating no specific nitrogen incorporation occurred from [19]. DR/CO measurements were able to account for KNO or dissolved nitrogen during electrochemical oxida- 2 3 most of the surface area which had to exist based on tion. These values reflect the integrated O/C and N/C ˚ existing titratable acidic functions. A large fraction of the ratios only over the sampling depth of |50 A from which pores were found to be very small (diameters of 4–6 A). ejected electrons are able to escape when probed by XPS ˚ Thus, the 1 mmol/g of total acidic groups per gram of using a 308 electron take off angle. The surface oxygen carbon fiber (formed after 6360 C/g of electrochemical concentration rose rapidly to 24% after initial electro- 2 oxidation) were occupying 67 m /g of surface area mainly chemical oxidation at 133 C/g and then remained at this ˚ composed of 4, 5 and 6 A average diameter ultramicro- level (or rose somewhat) with an increase in the extent of pores. oxidation up to 10 600 C/g. The total amount of acidic functions (detected by NaOH titration) increased from 3 3.1.2. Studies by X-ray photoelectron spectroscopy mmol/g (as-received) to 2476 mmol/g (10 600 C/g) [60]. XPS experiments were performed on both as-received This large (838-fold) increase in acidic functions (- and selected electrochemically oxidized carbon fibers. A COOH and phenolic -OH) which accompanies fiber weight summary of the fiber treatments and their designations loss (e.g., loss of carbon and nitrogen from oxidized fibers) Table 1 Treatments of carbon fibers used in XPS analysis a Fiber notation Surface treatment Applied current Residence time Extent of electro- (Amps) (min) oxidation (C/g) (a) as-received 0 0 0 (b) ECF-40-0.05 0.05 44.2 133 (c) ECF-10-0.1 0.1 176.7 1060 (d) ECF-15-0.6 0.6 117.8 4240 (e) ECF-10-0.5 0.5 176.7 5300 (f) ECF-10-0.6 0.6 176.7 6360 (g) ECF-10-1.0 1.0 176.7 10,600 a All electrochemical oxidations are referenced to the as-received fiber
Z.R. Yue et al. /Carbon 37(1999)1785-1796 means that the overall O/C atomic ratios should have decimal places the precision is certainly less. There is a continually increased with progressive oxidation. This nificant decrease in the relative content of graphitic deduction contrasts sharply with the observed O/C ratios carbon(peak I)and a rise in the relative content of carbon from XPS shown in Fig. 2. Therefore, electrochemical bonded to oxygen-containing functions(peaks Il, Ill, IV oxidations are continually generating micropore/void/slit and V)after electrochemical oxidation. This rise comes structures that penetrate increasingly deeper below the mostly from an increase in peak IV assigned to carboxyl outer fiber surface as oxidation progresses. Since XPS (COOH)or ester ( COoR) type groups. The relative nalysis can only sample the outer 50 A of the fiber, concentration of peak IV increased two-fold after oxida- further increases in -COOH and phenolic -OH group tion, from -6.8%(as-received fiber) to 11-14%(electro- generation by oxidation primarily occur below the 50 A chemically oxidized fibers). This is consistent with the data ampling depth of the XPS experiment. Oxygen functions in Fig. 2, where the o Is/C Is atomic ratio increased mainly exist on the internal pore/slit/void surfaces and not 1.5-18-fold after electrochemical oxidation. within the graphitic sheets. Most likely, lateral planes are ight increase in the amount of carbon- oxidized progressively forming pores and slits which oxygen complexes detected by XPs (30 electron take-off interconnect and link as they move increasingly deeper angle) in the outer fiber surface region(50 A depth) as the into the fiber xtent of oxidation increased from 133 C/g to 10 600 C/g High-resolution XPS spectra of the C ls region( Fig 3)(Fig. 3). This contrasts with the increase in the con- show that carbon-based oxides are present on all the entration of acidic functional groups in the fibers mea- samples. Deconvolution of the C Is spectra [28] gives five sured by Naoh uptake which was proportional to the eaks that represent graphitic carbon(peak 1, 284.6 eV), extent of electrochemical oxidation [20]. Clearly, the carbon present in phenolic, alcohol, ether or C=n groups majority of new oxidized functions occur beyond the XPs (peak ll, 286.1-286.3 ev), carbonyl or quinone groups sampling depth of 50 A as the extent of electrochemical (peak Ill, 287.3-287.6 eV), carboxyl or ester groups(peak oxidation goes from 133 C/g to 10 600 C/ IV, 288.4-288.9 eV) and carbon present in carbonate The slight increase in relative concentration of carbon groups and/or adsorbed CO and CO,(peak V, 290.4- oxygen complexes occurring at oxidation levels above 133 290.8eV) The calculated percentages of graphitic and functional ncreasingly porous(e.g, higher void volume). Thus, the carbon atoms are shown in Fig. 3. While listed to two fraction of carbon atoms in this region which exist on the 2.55% 2.42% 84B% 6868% (5300c/g) %舌7%51 Fig. 3. High-resolution XPS C Is spectra of electrooxidized carbon fibers versus the extent of electrochemical oxidation
Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 1789 means that the overall O/C atomic ratios should have decimal places the precision is certainly less. There is a continually increased with progressive oxidation. This significant decrease in the relative content of graphitic deduction contrasts sharply with the observed O/C ratios carbon (peak I) and a rise in the relative content of carbon from XPS shown in Fig. 2. Therefore, electrochemical bonded to oxygen-containing functions (peaks II, III, IV oxidations are continually generating micropore/void/slit and V) after electrochemical oxidation. This rise comes structures that penetrate increasingly deeper below the mostly from an increase in peak IV assigned to carboxyl outer fiber surface as oxidation progresses. Since XPS (COOH) or ester (COOR) type groups. The relative ˚ analysis can only sample the outer 50 A of the fiber, concentration of peak IV increased two-fold after oxidafurther increases in -COOH and phenolic -OH group tion, from |6.8% (as-received fiber) to 11–14% (electrogeneration by oxidation primarily occur below the 50 A chemically oxidized fibers). This is consistent with the data ˚ sampling depth of the XPS experiment. Oxygen functions in Fig. 2, where the O 1s/C 1s atomic ratio increased mainly exist on the internal pore/slit/void surfaces and not 1.5–1.8-fold after electrochemical oxidation. within the graphitic sheets. Most likely, lateral planes are There is only a slight increase in the amount of carbon– oxidized progressively forming pores and slits which oxygen complexes detected by XPS (308 electron take-off ˚ interconnect and link as they move increasingly deeper angle) in the outer fiber surface region (50 A depth) as the into the fiber. extent of oxidation increased from 133 C/g to 10 600 C/g High-resolution XPS spectra of the C 1s region (Fig. 3) (Fig. 3). This contrasts with the increase in the conshow that carbon-based oxides are present on all the centration of acidic functional groups in the fibers measamples. Deconvolution of the C 1s spectra [28] gives five sured by NaOH uptake which was proportional to the peaks: that represent graphitic carbon (peak I, 284.6 eV), extent of electrochemical oxidation [20]. Clearly, the carbon present in phenolic, alcohol, ether or C5N groups majority of new oxidized functions occur beyond the XPS ˚ (peak II, 286.1–286.3 eV), carbonyl or quinone groups sampling depth of 50 A as the extent of electrochemical (peak III, 287.3–287.6 eV), carboxyl or ester groups (peak oxidation goes from 133 C/g to 10 600 C/g. IV, 288.4–288.9 eV) and carbon present in carbonate The slight increase in relative concentration of carbon– groups and/or adsorbed CO and CO (peak V, 290.4– oxygen complexes occurring at oxidation levels above 133 2 290.8 eV). C/g occurs because the outer |50 A of the fiber becomes ˚ The calculated percentages of graphitic and functional increasingly porous (e.g., higher void volume). Thus, the carbon atoms are shown in Fig. 3. While listed to two fraction of carbon atoms in this region which exist on the Fig. 3. High-resolution XPS C 1s spectra of electrooxidized carbon fibers versus the extent of electrochemical oxidation
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 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 environgroups (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
ZR. Yue et al./ Carbo37(1999)1785-17%6 XPS Valence Band C 2s 172749 160683 4235c/g 5300c/g 4240c/ AS-receivet M carbon fiber 1060c/g Fig. 6. FT-IR spectra of fiber versus extent of electrooxidation treated for 30 min under flowing nitrogen to learn more about the loss functional groups from the fibers by weight loss measurements as a function of temperature. The surface region to a depth of 50 A was probed by XPS after Adsorption of Ag and I, as a function of ting was also used to further probe the pore The weight losses obtained by heating electrochemi- cally-oxidized fibers at 550C under nitrogen for 30 min as Binding Energy(ev) a function of the original extent of oxidation are shown in Fig. 7. No significant weight loss from the as-received Fig. 5. High-resolution XPS valence band spectra of electro- fiber was detected after heating. However, electrochemical- chemically oxidized carbon fibers at different extents of oxidation ly oxidized carbon fibers exhibited weight losses after heating which increased with the extent of oxidation. To a significantly as electrochemical oxidation increased, in- first approximation weight loss was proportional to the dicating the quantity of C=o groups (ketone and/or extent of oxidation showing that the quantity of oxygen carboxyl groups) within the fibers increases with progres- functional groups was approximately proportional to the ive oxidation. The broad peak at about 1630 cm in the extent of electrooxidation. This agrees with NaoH uptake ectrum of the as-received fibers gradually shifts to about which is directly proportional to the extent of oxidation 1600 cm after 5648 C/g of oxidation. This broad peak over 4000 C/g [20] may be associated with a stretching vibrations of aromatics The weight loss of fibers oxidized to 5300 C/g. after (C=C)and/or the bending vibrations of physisorbed H,O. heating for 30 min under nitrogen is shown versus treatment temperature in Fig. 8. The magnitude of weight 3. 2. Heat treatment of electrochemically oxidised carbon loss depended strongly on treatment temperature. As the temperature was increased, the fiber weight loss increased continuously. This illustrates that different types of oxy- Electrochemically oxidized carbon fibers were heat gen-containing functions are present with different de-
Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 1791 Fig. 6. FT-IR spectra of fiber versus extent of electrooxidation. treated for 30 min under flowing nitrogen to learn more about the loss functional groups from the fibers by weight loss measurements as a function of temperature. The ˚ surface region to a depth of 50 A was probed by XPS after 1 heating. Adsorption of Ag and I as a function of 2 heat-treating was also used to further probe the pore surface chemistry and structure. 3.2.1. Weight loss upon heating The weight losses obtained by heating electrochemically-oxidized fibers at 5508C under nitrogen for 30 min as a function of the original extent of oxidation are shown in Fig. 7. No significant weight loss from the as-received Fig. 5. High-resolution XPS valence band spectra of electro- fiber was detected after heating. However, electrochemical- chemically oxidized carbon fibers at different extents of oxidation. ly oxidized carbon fibers exhibited weight losses after heating which increased with the extent of oxidation. To a significantly as electrochemical oxidation increased, in- first approximation weight loss was proportional to the dicating the quantity of C5O groups (ketone and/or extent of oxidation showing that the quantity of oxygen carboxyl groups) within the fibers increases with progres- functional groups was approximately proportional to the 21 sive oxidation. The broad peak at about 1630 cm in the extent of electrooxidation. This agrees with NaOH uptake spectrum of the as-received fibers gradually shifts to about which is directly proportional to the extent of oxidation to 21 1600 cm after 5648 C/g of oxidation. This broad peak over 4000 C/g [20]. may be associated with a stretching vibrations of aromatics The weight loss of fibers oxidized to 5300 C/g, after (C5C) and/or the bending vibrations of physisorbed H O. heating for 30 min under nitrogen is shown versus 2 treatment temperature in Fig. 8. The magnitude of weight 3.2. Heat treatment of electrochemically oxidized carbon loss depended strongly on treatment temperature. As the fibers temperature was increased, the fiber weight loss increased continuously. This illustrates that different types of oxyElectrochemically oxidized carbon fibers were heat- gen-containing functions are present with different de-
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 functions
1792 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. Furthermore, 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, oxygencarbon 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 microporous 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 occurring 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 electrochemically 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
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 electrophenolic 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 oxidata 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
1794 ZR. Yue et al./ Carbo37(1999)1785-17%6 o 1s N1s 0009008007 600500400 30020010b0 BE (ev) Fig. 11. XPS survey spectra of (1)as-received carbon fiber, (2) electrochemically oxidized carbon fiber(5300 C/g), and oxidized fiber which was post-heat-treated at, (3)550C and(4)850C under flowing nitrogen. creased after electrochemical oxidation, especially in peak progressive electrochemical oxidation (Fig. 1)and was IV(carboxyl and ester groups). However, the relative approximately proportional to the extent of oxidation. concentration of carbon-oxygen complexes decreased NaoH uptake demonstrated that acidic functions were significantly(especially, the magnitude of peak IV) after generated in direct proportion to the extent of oxidation up post heating at 550C. Peak IV becomes smaller than that to 4000 C/g [19]. Eventually, a huge number of acidic of the as-received fibers after heating the oxidized fibers functions were generated (1060 mol/g and 2476 umol 850C. In contrast, the relat e size of peak Ill /g and 10 600 C/g of electrooxidation, (assigned to carbonyl groups)increases 2-fold after heating respectively)[60]. To accommodate this number of acidic at 550C. Heating at 850C caused the relative amount of functions a very large internal microporeous surface area carbon-oxygen complexes to fall below that found on was generated. XPS studies indicated that the concen- as-received fibers as both oxygen and nitrogen are lost. tration of oxygen within the outer 50 A of the fibers This also can be seen in Table 2 ncreased on oxidation. The o/C atomic ratio determined from O Is and C Is spectra(Fig. 2)rose rapidly with oxidation to 0. 24(133 C/g) and then remained approxi- 4. Conclusions mately constant upon continued oxidation to 10 600 C/g Electrochemical oxidation formed a progressively larg rength pAN-based carbon fibers were continuous proportion of oxygen functional groups as indicated in chemically oxidized in 1% wt KNO,. The fibers reaction (1). XPS C Is spectra( Fig. 3)showed the anode. Fiber weight loss increased with primarily in carboxyl(COOH) or lactone(COOR)groups Table 2 Changes in the XPS atomic ratios upon heat treating electrochemically oxidized fibers under nitrogen Atomic ratIo (1)As-received fiber 0.158 (2)Electrochemically oxidized (5300 C/g)carbon fiber (3)Sample(2)after heating at 550C for 30 min (4)Sample(2)after heating at 850C for 30 min 0.017
1794 Z.R. Yue et al. / Carbon 37 (1999) 1785 –1796 Fig. 11. XPS survey spectra of (1) as-received carbon fiber; (2) electrochemically oxidized carbon fiber (5300 C/g); and oxidized fiber which was post-heat-treated at, (3) 5508C and (4) 8508C under flowing nitrogen. creased after electrochemical oxidation, especially in peak progressive electrochemical oxidation (Fig. 1) and was IV (carboxyl and ester groups). However, the relative approximately proportional to the extent of oxidation. concentration of carbon–oxygen complexes decreased NaOH uptake demonstrated that acidic functions were significantly (especially, the magnitude of peak IV) after generated in direct proportion to the extent of oxidation up post heating at 5508C. Peak IV becomes smaller than that to 4000 C/g [19]. Eventually, a huge number of acidic of the as-received fibers after heating the oxidized fibers to functions were generated (1060 mmol/g and 2476 mmol/g 550 and 8508C. In contrast, the relative size of peak III after 6360 C/g and 10 600 C/g of electrooxidation, (assigned to carbonyl groups) increases 2-fold after heating respectively) [60]. To accommodate this number of acidic at 5508C. Heating at 8508C caused the relative amount of functions a very large internal microporeous surface area carbon–oxygen complexes to fall below that found on was generated. XPS studies indicated that the concen- ˚ as-received fibers as both oxygen and nitrogen are lost. tration of oxygen within the outer 50 A of the fibers This also can be seen in Table 2. increased on oxidation. The O/C atomic ratio determined from O 1s and C 1s spectra (Fig. 2) rose rapidly with oxidation to 0.24 (133 C/g) and then remained approxi- 4. Conclusions mately constant upon continued oxidation to 10 600 C/g. Electrochemical oxidation formed a progressively larger High strength PAN-based carbon fibers were continuous- proportion of oxygen functional groups as indicated in ly electrochemically oxidized in 1% wt KNO . The fibers reaction (1). XPS C 1s spectra (Fig. 3) showed a rise 3 served as the anode. Fiber weight loss increased with primarily in carboxyl (COOH) or lactone (COOR) groups Table 2 Changes in the XPS atomic ratios upon heat treating electrochemically oxidized fibers under nitrogen Samples Atomic ratio O 1s/C 1s N1s/C 1s (1) As-received fiber 0.158 0.022 (2) Electrochemically oxidized (5300 C/g) carbon fiber 0.263 0.025 (3) Sample (2) after heating at 5508C for 30 min 0.194 0.054 (4) Sample (2) after heating at 8508C for 30 min 0.048 0.017