PERGAMON Carbon 37 (1999) 1607–1618

Adsorption of precious metal ions onto electrochemicallyoxidized carbon fibers

b a b a b a ,*Z.R. Yue , W. Jiang , L. Wang , H. Toghiani , S.D. Gardner , C.U. Pittman Jr.aDepartment of Chemistry, Mississippi State University, University /Industry Chemical Research Center, Mississippi State, MS 39762,

USAbDepartment of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, USA

Received 25 November 1998; accepted 23 January 1999

Abstract

1Electrochemically oxidized carbon fibers (ECF) adsorbed a prodigious amount of Ag in contrast to oxygen plasma and1nitric acid treated carbon fibers. The amount of adsorbed Ag reached 3700 mmol /g after 5652 C/g of electrochemical

1oxidation. This value approaches the 4050 mmol /g of Ag which adsorbed onto steam-activated Kenaf-based carbon (with a2surface area of 1284 m /g determined by N /BET) under the same adsorption conditions. ECF oxidized to 9540 C/g2

1 2adsorbed more than its own weight of Ag (12 608 mmol /g). These fibers exhibited a surface area of 115 m /g (CO –DR).21 1Two different reactions occurred during Ag adsorption. These reactions were ion exchange adsorption between Ag and

1acidic functions (carboxyl) and redox adsorption between Ag and reducing functions such as catechol groups on theseelectrochemically oxidized fibers (ECF). The redox capability was expressed by the reaction electric potential (E) using the

1Nernst equation. High resolution XPS C 1s spectra of ECFs (level of oxidation 5300 C/g), before and after Ag adsorption,showed that the carbon atoms present in phenolic, alcohol or ether groups and those present in carbonyl or quinone groups

1increased after Ag adsorption. X-ray diffraction and X-ray photoelectron spectroscopy (XPS) Ag 3d spectra of the ECF1 0 1showed that adsorbed Ag was reduced to Ag after both Ag adsorption and subsequent post-heat treatment of the fibers

31under N at 5508C for 30 min. Only about one-third as much Au adsorption occurred versus the extent of electrochemical21 31oxidation as was observed for Ag . This ratio matches the requirement that three electrons are required to convert Au to

0 1Au versus one to convert Ag to Ag. High resolution angle resolved XPS (ARXPS) Pd 3d and Pt 4f spectra show that there21 21are two different Pd oxidation states and three different Pt oxidation states present after adsorption of Pd and Pt onto

0 0ECF. The peak areas as a function of take off angle showed that substantial amounts of Pd and Pt are present in addition to21 21 41Pd and Pt and Pt on the outermost surface regions of oxidized fibers. 1999 Elsevier Science Ltd. All rights

reserved.

Keywords: A. Carbon fibers; B. Electrochemical treatment; C. Adsorption, X-ray photoelectron spectroscopy (XPS); D. Adsorptionproperties

1. Introduction functional groups on their surface, strong corrosion resist-ance and, especially, good electrical conductivity.

Adsorption of precious metallic ions onto carbonaceous We have previously investigated the surface area andmaterials is of interest for the preparation of carbon pore size distribution of electrochemically oxidized carbonsupported catalysts [1,2], and for the removal of metallic fibers [4]. These oxidized carbon fiber surface areas,ions from water effluents [3]. This is, in part, because derived by applying the DR method to CO adsorption2

2carbonaceous materials have unique properties, such as data, increased almost linearly from ¯1 to 132 m /g as thehigh specific surface area, the existence of various forms of extent of electrochemical oxidation increased from 0 to

10600 C/g [4]. At 6360 C/g levels of oxidation about1000 mmol /g of acidic functions were formed [5,6] and in*Corresponding author. Tel.: 11-601-3257616; fax: 11-601-other studies we have achieved up to 2475 mmol /g of3257611.

E-mail address: cpittman@ra.msstate.edu (C.U. Pittman Jr.) acidic functions (10 600 C/g of oxidation) [7]. Further-

0008-6223/99/$ – see front matter 1999 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 99 )00041-X

1608 Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618

1more, a larger amount of Ag adsorbed onto electro- 2.3. Adsorption of precious metallic ionschemically oxidized carbon fibers upon adjusting the pH of

1the Ag solution to 10.5 [4]. These previous studies 12.3.1. Ag adsorptionshowed that extensive electrochemical oxidation of carbon

Oxidized carbon fiber samples (0.04–0.5 g) were im-fibers produced a porous carbon fiber with interesting

mersed in 50–100 ml of AgNO solution (|5 mM) and3adsorbent capabilities.shaken at 258C for 24 h in the dark. The initial pH value of

In the present investigation, PAN-based carbon fibersthe AgNO solution was adjusted with aqueous NH OH to3 4were electrochemically oxidized from 0 to 10 600 C/g. 1|10.5 generating [Ag(NH ) ] complexed ions. The3 2The adsorption properties of electrochemically oxidized 1change in Ag concentration after adsorption was de-1 31 21 21carbon fibers towards Ag , Au , Pd and Pt weretermined by KSCN titration using Fe(NH )(SO ) as the4 4 2characterized by chemical analysis, X-ray photoelectronindicator [8]. Before titration, the pH of all of the

spectroscopy and X-ray diffraction.adsorbates was adjusted to an acidic state with HNO (pH3

2–4). When highly oxidized fiber samples were used, thefiber weight gain could be used to follow the amount of

2. Experimental 1Ag adsorbed. The fibers were carefully washed withdistilled water followed by drying at 1208C for 2 days

2.1. Materialsprior to weighing.

The carbon fiber consisted of high strength, type II,31PAN-based fibers (Thornel T-300) manufactured by 2.3.2. Au adsorption

Amoco Performance Products, Inc. with 3000 filaments per Gold foil (Aldrich, 199.9%, |0.5 g) was dissolved intotow. All other chemicals were of analytical purity from a 3:1 weight ratio of HNO /HCl. This solution was3

Aldrich and used as received. concentrated by heating followed by dilution with distilledwater to 500 ml. The concentration of AuCl in this3

2.2. Electrochemical oxidation and other surface solution was |5 mM and pH was 2–2.2. Oxidized carbontreatments fiber samples (|0.04g) were immersed in 25 ml of AuCl3

31solution and shaken at 258C for 24 h. The change in AuContinuous electrochemical treatments were carried out concentration after adsorption was determined by 1,2-

in a U-tube apparatus. An aqueous 1%wt KNO solution benzoquinone titration using aminobiphenyl as the in-3

(no pH adjustment) was used as the electrolyte. The carbon dicator [9].fibers were fed continuously and served as the anode. A254-cm long U-shaped stainless steel bar inside the U-tube 21 212.3.3. Pd and Pt adsorptionacts as the cathode. A gear system allowed variation of the

PdCl (99%) and K PtCl (98%) (Aldrich) were used as2 2 4fiber residence time in the oxidation reaction and theprecursors to prepare aqueous stock solutions of PdCl2voltage could be varied from 30 to 45 V to change the rate(6.0 mM) and K PtCl (2.8 mM) with distilled water. The2 4of current flow. The schematic diagram of this apparatuspH of the K PtCl solution was 6.2. To prevent Pd(OH)2 4 2and the details of the treatment methods have previouslyprecipitation of PdO?(H O) , the pH of the PdCl solution2 x 2been described [4]. After electrochemical oxidation, allwas adjusted to 2.3 with HCl. Qualitative adsorptions of

samples were thoroughly washed with distilled water, and 21 21Pd and Pt were conducted by immersing oxidizeddried at 1108C. 21fiber samples (|0.1g) into 50 ml of either of the Pd or

For comparison, nitric acid and oxygen plasma treated 21Pt stock solutions, respectively. These solutions werefibers were also considered.

shaken at 258C for 24 h.The nitric acid surface treatments involved submerging

the as-received carbon fibers in 70% nitric acid (Aldrich,A.C.S. reagent grade) at 1158C for different times, then 2.4. X-ray photoelectron spectroscopy (XPS)rinsing with distilled water and drying at 1108C [12–17].

Oxygen plasma treatment was carried out with a 40-kHz All samples (after adsorption) analyzed by XPS wereplasma system (Advanced Plasma System, Inc., Model first washed repeatedly with distilled water and then driedB-12) consisting of an aluminum vacuum chamber (123 in a vacuum at 708C/10–20 mmHg for 24h.12316 in) with two interior high-voltage plates. The Several 2.5-cm fiber sections (each of which containedcarbon fiber tows were spread and wound onto a copper approximately 3000 fiber filaments) were carefully cutwire frame and then hung between the two high-voltage from the carbon fiber tow. These fiber sections wereplates. Plasma power was adjusted to 200 W and the positioned on top of a custom stainless steel sample holder.exposure time varied from 1 to 10 min. The oxygen flow The fibers were held firmly in place by a gold foil maskrate was controlled (about 1–2 standard l per min) in order secured to the sample holder with screws. The gold foilto set the plasma pressure at the desired values. contained a machined oval opening in its center that

Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618 1609

exposed a 1.530.8-cm area of the underlying carbonfibers.

XPS experiments were performed on a Physical Elec-tronics PHI Model 1600 surface analysis system. Theinstrument is equipped with a PHI 10-360 sphericalcapacitor energy analyzer (SCA) fitted with an OmniFocus III small-area (800-mm diameter analysis area) lensand a high-performance multi-channel detector. Sampleswere oriented such that the axial direction of the carbonfibers was in the plane of the X-ray source and theanalyzer entrance. Angle-resolved XPS (ARXPS) spectrawere recorded at electron take-off angles of 10, 30 and 908.

XPS spectra were obtained using an achromatic MgKa

(1253.6 eV) X-ray source operated at 200 W. Survey scanswere collected from 0–1100 eV with the pass energy equal

1Fig. 1. Ag adsorption onto O plasma treated carbon fibers2to 46.95 eV. High-resolution scans were performed with theversus plasma exposure. pH value of AgNO initial solution was3pass energy adjusted to 23.50 eV. The pressure inside the adjusted with NH ?H O to 10.5; volume of solution was 50 ml;3 229vacuum system was maintained at approximately 10 weight of fibers was about 0.17 g; initial concentration AgNO3

Torr during all XPS experiments. was 5 mM; T5258C; t524 h.A non-linear least-squares curve fitting program

(XPSPEAK95 software, version 2.0) with a Gaussian-Lorent-zian mix function and Shirley background subtraction was onto oxidized fibers decreased with progressive oxidationused to deconvolute the XPS peaks. The carbon 1s electron by oxygen plasma at 200 W [12]. The fiber surface areabinding energy corresponding to graphitic carbon was (N BET) remained approximately constant as plasma2

referenced at 284.6 eV for calibration [10]. The higher oxidation continued [12]. This implies that oxygen plasmaenergy C 1s peaks were found at 286.2, 287.5, 288.6 and treatment might preferentially cut away some high surface290.7 eV, respectively. Atomic ratios were calculated from area roughened regions resulting in a somewhat smootherthe XPS spectra after correcting the relative peak areas by surface [18–20]. Therefore, this surface smoothing might

1sensitivity factors based on the transmission characteristics lead to a slight decrease in Ag adsorption.1of the Physical Electronics SCA [11]. Nitric acid oxidized fibers exhibited increased Ag

adsorption as the oxidation time was prolonged in 70%2.5. X-ray diffraction HNO solution at 1158C. These results are shown in Fig. 2.3

1The quantity of Ag adsorbed onto oxidized fibers was upX-ray diffraction was performed with a Philips ana- to 625 mmol /g after 105 min of oxidation.

lytical X-ray B.V., X’Pert-MPD multi-purpose diffractome- In contrast to oxygen plasma and nitric acid treatments,ter. Selected carbon fiber samples (|50 mg) were pow- electrochemically oxidized carbon fibers adsorbed adered, and the diffraction lines were measured with CuKa1

radiation (l50.15405 nm).

3. Results and Discussion

13.1. Adsorption of Ag

13.1.1. Quantity of Ag adsorption by oxidized carbonfibers

To explore the ability of oxidized carbon fibers to1adsorb Ag , three different surface-treated carbon fibers

1were employed. The amount of Ag adsorbed onto oxygenplasma-treated carbon fibers as a function of the exposuretime to the oxygen plasma at 200 W is shown in Fig. 1.

1The amount of Ag adsorption did not increase with 1Fig. 2. Ag adsorption onto carbon fibers oxidized in 70% HNO3treatment time up to 10 min of plasma exposure. Instead, at 1158C versus treating time. pH value of AgNO initial solution31Ag adsorption decreased slightly from |38 to 23 mmol / was adjusted with NH ?H O to 10.5; volume of solution was 503 2

g. This result is in accord with previous experiments, ml; weight of fibers was about 0.04 g; initial concentration AgNO3

where the absolute values of methylene blue adsorption was 5 mM; T5258C; t524 h.

1610 Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618

1 1Fig. 3. Ag adsorption onto electrochemically oxidized carbon fibers versus the extent of oxidation at different ratios of initial Agpresent /weight of fibers immersed. pH value of AgNO initial solution was adjusted with NH ?H O to 10.5; initial concentration AgNO3 3 2 3

was 5 mM; T5258C; t524 h.

1prodigious amount of Ag with progressive electrochemi- oxidations produced special, highly porous carbon fibers1cal oxidation as shown in Fig. 3, (see square symbols). with high Ag adsorption capacities. Such fibers might

This adsorption curve was obtained from the same aqueous have potential uses in precious metal recovery and aAgNO adsorption solution used with the oxygen plasma precious metal catalyst-supports.3

and nitric acid treated carbon fibers described above. The1amount of Ag adsorbed reached 3700 mmol /g after 5652 13.2. Adsorption properties of Ag onto1C/g of electrochemical oxidation when the ratio of Ag in electrochemically oxidized fibers

the absorption solution to the weight of fibers immersed1was 12 500 mmmol Ag /g fiber. This value approached 1Two kinds of reactions might occur during Ag ad-1the 4050 mmol /g of Ag which adsorbed onto steam 1sorption. First, ion exchange between Ag and acidic

activated Kenaf-based carbon (which exhibited a surface carboxyl functions present on the pore surfaces can occur2area of 1284 m /g by N /BET [4]) under the same2 as follows:adsorption conditions. When fibers subjected to 10 600 and

1 19540 C/g of electrochemical oxidation were employed, Ag 1 –COOHá–COOAg 1 H (1)1very large amounts of Ag could be adsorbed. For

1example, as shown in Fig. 4, total of 12 608 mmoles of The amount of Ag adsorbed by this exchange depends1 1Ag were absorbed per gram of ECF (oxidized to 9540 on, (1) the initial concentration of Ag , (2) the amount of

C/g) after 30 days. These fibers exhibited a surface area of carboxyl groups present, (3) the pH of solution, and (4)2 1115 m /g in CO –DR measurements. This amounts to a the ability of the Ag ions to penetrate the existing2

1weight of Ag adsorbed that was 1.36 times the fiber microporosity of the fiber.1 1weight! This is three times the amount of Ag adsorbed by Redox adsorption of Ag onto the electrochemically

1the Kenaf fibers described above. This large Ag ad- oxidized carbon fibers (ECF) also occurs. The reduction ofsorption value is due to the high surface area (although precious-metal ions by various carbon surfaces has beenonly one-tenth the surface area of the Kenaf-based fibers) repeatedly described in the literature [21–26]. Previousand the large quantity of both acidic functions and work showed that there are stronger metallic silver X-ray

1reducing functions present on the electrochemically oxi- diffraction peaks in the ECF after Ag adsorption [5],1 1dized fibers [5,6]. Fig. 4 shows the plots of Ag adsorbed indicating that some portion of the Ag was reduced to

0 0versus the fiber oxidation level for long adsorption periods. Ag by the ECF and that some of the resulting Ag formedThese results demonstrate that extensive electrochemical Ag crystals. In addition to carboxyl groups a substantial

Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618 1611

1Fig. 4. Ag adsorption onto electrochemically oxidized carbon fibers versus the extent of oxidation. (h) 0.2 g of fibers were immersed in 4l of aqueous AfNO solution (pH 10.5), kept in dark at room temp. (s) 1.25 g samples immersed in 6 l of AgNO solution (conc.52003 3

mM, pH 10.5), kept in dark at room temp.

0 0 0quantity of phenolic hydroxyls exists on the pore surfaces DE is the difference between E and E ;1 0Ag /Ag ECF / ECFr 01of ECF (reactions (4–6)). Functions which can reduce Ag R and F are the molar gas law constant and the Faraday

0to Ag on the ECF include catechol and hydroquinone constant, respectively; n is number of electrons (heren51) and T is the temperature (K). E increases asfunctions generated during electrochemical oxidations on

0 1E , [Ag ], [ECF ] and pH get larger, and decreases1 0the oxidized surfaces of lateral planes exposed within the Ag /Ag r0as either E or [ECF ] increase. In the redoxpore structure [5]. These reductions can be represented as ECF / ECF or 0

1follows: adsorption of Ag by ECF, the extent of reaction (4)1depended mainly on [Ag ] and pH. The dependence on2 1 0

1ECF áECF 1 e 1 H E (2)r 0 ECF / ECF0 r the [Ag ] can clearly be observed in Fig. 3 where pH andT are constant.

1 2 0 0Ag 1 e áAg E 5 0.7996 V (3)1 0Ag /Ag A series of ECF with increasing extents of oxidation1were immersed in solutions where the ratio of Ag present

1 0 1ECF 1 Ag áECF 1 Ag 1 H (4) in the solution to the weight of the fiber immersed wasr 0

held constant (see Fig. 3). This was done for the following0 1

1Ag precipitation can also occur because Ag solutions four different ratios of the initial amounts of Ag /g ECF:1can oxidize readily oxidized surface sites on high surface 818, 1404, 3247 and 12 500 mmol of Ag /g of ECF. Each

area carbons. Such oxidations can create surface oxides. curve in Fig. 3 represents the series of experimentsECF and ECF represent reductive and oxidativer o performed at one of these ratios. As the extent of electro-

01groups, respectively, present on the pore surfaces. E is the chemical oxidation increased, the amount of Ag adsorbed

standard reduction potential (Volts). The extent to which went up at all four ratios. An initial straight line increment1reaction (4) occurs can be expressed by the reaction’s in Ag adsorption occurred versus the extent of fiber

electric potential (E) using the Nernst equation: oxidation in each set of experiments. Then the plotscurved, becoming almost horizontal as the extent of fiber0E 5 DE oxidation increased further. The linear portion of these

1[ECF ]2.303 RT r1 curves extended to higher amounts of Ag adsorbed per g]]] ]]1 n log[Ag ] 1 log 1 n pHF G 1n 3 F [ECF ] of ECF as the ratio of the initial amount of Ag /g of fiber0

was increased. This increase in the amount of adsorbed(5)1Ag with an increase in the extent of electrochemical

0 0 0where DE 5 E 2 E .1 0 oxidation indicated that the amount of –COOH andAg /Ag ECF / ECFr 0

The terms [ECF ] and [ECF ], of course, do not refer to reducing functions [ECF ] increased as the extent ofr 0 r

freely dissolved species so the term log([ECF ] / [ECF ]) electrochemical oxidation went up (this was independentlyr 0

signifies surface redox properties. known from NaOH uptake experiments).

1612 Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618

1The ratio of the amount of initial Ag present in tion has been presented. A substantial increase in peak II1solution to the weight of fibers used was adjusted by and peak III occurred after Ag adsorption. The rise in

increasing the volume of the AgNO solution and/or carbonyl /quinone groups (peak III) after silver adsorption3

decreasing the weight of the fiber used in the adsorption is due to the oxidation of surface phenolic groups present1experiments (Fig. 3). The amount of Ag adsorbed in catechol- and hydroquinone-type functions during the

1 1 0increased as the ratio (initial moles of Ag present to the reduction of Ag to Ag [5]. (Eq. (6)).1weight of the fibers used) was increased. The adsorption The increase in peak II is expected if [Ag(NH ) ]3 2

1solutions underwent a smaller decrease in both the [Ag ] solutions directly oxidize carbon surfaces. Net oxidative1and the pH when larger initial amounts of Ag were hydroxylation could be expected to compete if water

employed while holding the weight of ECF constant (e.g. attacks intermediate radical cation species formed at1as the initial amount of Ag present to weight of fiber locations where ortho or para dihydroxy groups are not

1increased). Thus, the amount of Ag adsorbed increased. present (Eq. (7)).1The linear portion of the curves in Fig. 3, which occurred Reduction of Ag to Ag can readily occur at locations

at lower electrochemical oxidation levels, can be consid- remote from the site of functional group oxidation occur-ered to be the adsorption capacities of those ECFs towards ring within the pore structure of the fibers. This is clear

1Ag . The carbon fibers shown in Fig. 4 exhibited long from the buildup of metallic silver on the fiber surfaces,linear plots to very high oxidation levels when immersed easily observed when highly oxidized fibers have been

1 1in large excesses of Ag ions. exposed to solutions with a large excess of Ag present.Fig. 5 shows the high resolution XPS C 1s spectra of Thus, as functional group oxidations reduce silver, silver

1ECF oxidized to 5300 C/g. The spectrum of these same atoms build up blocking access of other large Ag ions1fibers after Ag adsorption is also shown for comparison. from reaching the active sites in the pores. However, both

Deconvolution of the C 1s spectra [10,27] gives five peaks silver metal and graphitic structures are electrical conduc-1including, (1) the graphitic carbon (peak I, 284.6 eV), (2) tors. Thus, Ag reduction at a silver crystallite surface can

carbon present in phenolic, alcohol, ether or C=N groups induce remote oxidation within the pores. The deposition(peak II, 286.1–286.3 eV), (3) carbon present in carbonyl of silver outside the pores onto the fiber surface willor quinone groups (peak III, 287.3–287.6 eV), (4) carbox- inevitably change the probe depth of XPS as the amount ofyl or ester groups (peak IV, 288.4–288.9 eV), and (5) silver deposited increases. Thus, with enhanced deposition,carbon present in carbonate groups and/or adsorbed CO XPS will, on average, increasingly be sampling the outer

˚and CO (peak V, 290.4–290.8 eV). For clarity, only the carbon surface relative to depths of 50–100 A (90 electron21curve-resolved spectrum of the ECF before Ag adsorp- take-off angle).

1Fig. 5. High resolution XPS C 1s spectra of electrochemically oxidized (5300 C/g) carbon fibers before and after Ag adsorption

Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618 1613

1The XPS Ag 3d spectrum of ECF after Ag adsorption this peak to broaden when both oxidation states areis shown in Fig. 6 together with the spectrum of these present. The FWHM of the Ag 3d peak found in Fig. 65 / 2

same fibers after they have been heat-treated in N at was used here to attempt to characterize the existence of215508C for 30 min. Both spectra exhibited the same two Ag Ag on the oxidized fibers by comparing ECF fibers after

0 13d peaks. Ag is known to exhibit a Ag 3d spectrum with Ag adsorption with those same fibers after the subsequent0 0 1 1peaks at 374.3 eV (Ag ) and 368.3 eV (Ag ) [11]. Ag 5508C heat treatment. During heating, Ag in compounds3 / 2 5 / 2

0in silver acetate also exhibits a 3d peak at 368.3–368.8 can be reduced to Ag [28,29]. However, the values of5 / 20eV [11]. Therefore it is difficult to differentiate Ag and FWHM in Fig. 6 provide no specific evidence for the

1 1 1Ag using XPS. The Ag 3d peak of the ECF, after Ag existence of Ag . X-ray diffraction patterns (see Fig. 7b)5 / 20adsorption, at 368.1 eV is assigned to Ag based on the exhibit a strong diffraction peak for metallic silver on the

1literature [11]. However, small differences in energy fibers after Ag adsorption. Furthermore, a large amount0 1between the Ag 3d peaks of Ag and Ag should cause of metallic silver particles on large fractions of the surface

1614 Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618

1Fig. 6. High resolution XPS Ag 3d spectra of electrochemically oxidized carbon fibers after Ag adsorption and after these fibers werepost-heat-treated under N at 5508C for 30 min.2

31 0of electrochemically oxidized carbon fibers can also be Redox adsorption of Au onto the ECF as Au wasseen by the naked eye or under a microscope. Therefore, it confirmed by X-ray diffraction Fig. 7c. The ARXPS Au 4fis possible to prepare a carbon fiber-supported silver spectra of these fibers (Fig. 9) did not change significantly

1catalyst by the redox adsorption of Ag onto electro- (peak position and FWHM) as the electron take-off anglechemically oxidized carbon fibers without further treat- was varied from 90 to 108. The binding energy of the Au

0ment. At higher extents of electrooxidation, the coating of 4f peak at 84 eV was assigned to Au [11]. The XPS7 / 20Ag can completely encase the fiber. Au/C atomic ratio (Fig. 10) on the surface of the fibers

was about 0.02 at a 308 electron take-off angle. This ratio313.3. Au adsorption onto electrochemically oxidized increased from 0.0072 to 0.018 and 0.019 as the electron

carbon fibers take-off angle decreased from 90 to 30 and 108, respective-ly (Table 1), indicating that the outermost surface of the

31The amount of Au adsorption onto electrochemically ECF is gold-enriched relative to locations deeper into theoxidized carbon fibers versus the extent of electrochemical pore system. Both the O 1s and C 1s spectra show the

31 31oxidation is shown in Fig. 8. The amount of Au concentration of oxygenated species increased after Au31adsorption increased as the extent of oxidation went up. adsorption due to the redox reaction between Au and

The pH of the AuCl solution used in the adsorption ECF. The O/C ratio was 0.26 before, and 0.29 after, the32studies was 2.0. Thus, the AuCl anion was actually adsorption (308 electron take-off angle).4

31present in the solution instead of the Au cation. The ion2exchange reaction between AuCl and the ECF might not431 21 21occur. The molar amounts of Au adsorbed per gram of 3.4. Pd and Pt adsorption onto electrochemically

1fibers were only one of third that of adsorbed Ag . This is oxidized carbon fibersreadily observed by comparing the straight line portions of

21 21the adsorption plots versus extent of electrooxidation (at Qualitative adsorption of Pd and Pt was alsothe lower level of oxidation) shown in Figs. 3 and 8. At carried out in this research. The Pd/C and Pt /C atomic

1equivalent levels of oxidation, the moles of Ag adsorbed ratios from APXPS measurements are summarized in (Fig.13(Fig. 3) are always about three times that of the Au 10). Using a 908 electron take-off angle, the Pd/C atomic

moles adsorbed (Fig. 8). This corresponds to the fact that, ratio was 0.029 while that for Pt /C was 0.011. ARXPS31based on stoichiometry, Au needs three electrons to spectra (see Figs. 11 and 12) show that the depth dis-

0 1 0produce Au but Ag needs only one to give Ag . tribution of the two metals is different. Palladium (Fig. 10)

Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618 1615

atomic ratio (Fig. 10) increased significantly (from 0.26321to 0.365 at 308 take-off angle) after Pd adsorption but

21only very slightly increased after Pt adsorption.High resolution ARXPS Pd 3d and Pt 4f spectra of these

ECF are shown in Figs. 11 and 12, respectively. The Pd 3dspectra at each take off angle can be fitted with two pairsof peaks while each of the Pt 4f spectra can be fitted withthree pairs of peaks. This indicates that there are twodifferent oxidation states present for Pd and three differentoxidation states of Pt on the fiber surface after adsorption.The Pd 3d peaks at 335.1 and 337.5 eV (Fig. 11) may5 / 2

0 21readily be assigned to Pd and Pd , respectively, accord-ing to the literature [1]. The Pt 4f peaks at 71.1, 72.57 / 2

0 21 41and 74.2 eV are also easily assigned to Pt , Pt and Pt ,41respectively [1]. The generation of Pt may be due to the

21 0 41reaction 2Pt →Pt 1Pt .0 21The percentages of Pd and Pt calculated at each of

the three electron take-off angles are also shown in Fig. 110and 12. The relative amount of Pd is greatest closer to the

0 21surface. For example, the Pd and Pt percentages were˚28 and 72% in the outer |20 A (108 take-off angle) versus

˚13 and 87%, respectively, in the outer |100 A (908

take-off angle). A similar phenomenon is observed with Pt.˚The amount of Pt8 falls from |54% in the outer |20 A

˚(108 take-off angle) to |23% in the outer |100 A (9080 0take-off angle). However, the overall Pd and Pt content

in the oxidized fibers is relatively small. No significant0 0X-ray diffraction patterns of Pd and Pt can be observed

21 21Fig. 7. X-ray diffraction patterns of electrochemically oxidized from the oxidized fibers after adsorption in Pd and Pt1 31 21carbon fibers before and after adsorption of Ag , Au , Pd and solutions.

21Pt .

˚is fairly evenly distributed within the outer 100 A while 4. Conclusionsthe distribution of Pt (Fig. 10) is more concentrated on the

˚outermost surface (|20 A) and becomes depleted more In contrast to oxygen plasma and nitric acid-treatedrapidly with increasing depth. Moreover, the O 1s/C 1s carbon fibers, electrochemically oxidized carbon fibers

1adsorb a large amount of Ag . The amount of adsorbed1Ag reached 3700 mmol /g on fibers oxidized to 5652

1C/g. This value approached the 4050 mmol /g of Agwhich adsorbed onto steam activated Kenaf-based carbonunder the same conditions. The Kenaf carbon was shown

2to have 1284 m /g of N /BET surface area [4]. More than21three times that much Ag was adsorbed by ECF oxidized

2to 9540 C/g despite the fact its surface area of 115 m /gwas only one-tenth that of the Kenaf carbon. Two kinds of

1adsorption processes occur during Ag adsorption. One is1ion exchange adsorption between Ag and acidic functions

(carboxyl groups). The second is the redox adsorption of0 1Ag when Ag is reduced by multiple hydroxyl functions

(e.g. such as catechol-like structures) on the electrochemi-cally oxidized carbon fibers (ECF). The extent of redox

31 reaction can be approximately expressed by the reaction’sFig. 8. Au adsorption onto electrochemically oxidized carbonelectric potential (E) using the Nernst equation. For afibers as a function of the extent of oxidation pH value of AuCl3given level of fiber oxidation, the extent of this redoxinitial solution was 2.0; volume of solution was 25 ml; weight of

1reaction depended mainly on the [Ag ] and the pH of thefibers was about 0.04 g; initial concentration of AuCl was 5 mM;3

T5258C; t524 h. solution in which the ECF was immersed.

1616 Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618

31Fig. 9. High resolution Au 4f ARXPS spectra of electrochemically oxidized (5300 C/g) carbon fibers after Au adsorption.

High resolution C 1s spectra of ECF (5300 C/g) and the heat-treating these silver-containing fibers under N at21same fibers after Ag adsorption showed that the carbon 5508C for 30 min., showed that a substantial amount of

1 0present in phenolic groups and the carbon present in adsorbed Ag is reduced to Ag . However, XPS was notcarbonyl or quinone groups increased substantially after able to determine the specific ratio of the two silver

1 1Ag adsorption. X-ray diffraction patterns and XPS Ag 3d oxidation states on the fibers after Ag adsorption.1 31spectra of ECF, after Ag adsorption and also after post- Au adsorption onto electrochemically oxidized carbon

Fig. 10. The ARXPS O 1s/C 1s, Au 4f /C 1s, Pd 3d/C 1s and Pt 4f /C 1s atomic ratios of carbon fibers electrochemically oxidized to 530031 21 21C/g and these same fibers after adsorption of Au , Pd and Pt .

Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618 1617

Table 1Atomic ratios obtained from ARXPS spectra of electrically

13 13oxidized carbon fiber before and after adsorption of Au , Pd ,12and Pt

Fiber Take-off angle (8)

Oxygen/carbon Metal /carbona batomic ratio atomic ratio

90 30 10 90 30 10

ECF – 0.26 – – – –(5300 C/g)

13ECF/Au 0.30 0.29 0.28 0.0072 0.018 0.01912ECF/Pd 0.35 0.36 0.39 0.029 0.036 0.031

12ECF/Pt 0.28 0.28 0.25 0.011 0.014 0.031a ARXPS O 1s/C 1s spectral results.b ARXPS Ag 3d/C 1s, Au 4F/C 1s, Pd 3d/C 1s and Pt 4f /C 1s

spectral results.

fibers versus the extent of oxidation indicated that the31amount of Au adsorption was only one third of the

1 31adsorbed Ag due to the fact that Au needs three0 1electrons to be reduced to Au , but Ag needs only one to

0generate Ag .21 21Adsorptions of Pd and Pt were carried out quali-

Fig. 12. High resolution ARXPS Pt 4f spectra of electrochemical-tatively. High resolution ARXPS Pd 3d and Pt 4f spectra 21ly oxidized (5300 C/g) carbon fibers after Pt adsorptionshow that each Pd 3d spectrum can be fitted with two pairof peaks, and each Pt 4f spectra can be fitted with threepair of peaks, suggesting that there are two different showed a significant increase in the concentration of both

0 12 0 0valences for Pd (Pd , Pd ) and three different valences Pd and Pt with decreasing electron take-off angle0 12 14 0 0for Pt (Pt , Pt , Pt ) in the fibers’ surface after indicating that Pd and Pt are present primarily in the

adsorption. The calculated percentages of peak areas outermost surface of oxidized fibers.

21Fig. 11. High resolution ARXPS Pd 3d spectra of electrochemically oxidized (5300 C/g) carbon fibers after Pd adsorption

1618 Z.R. Yue et al. / Carbon 37 (1999) 1607 –1618

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