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