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Langmuir 2002, 18, 10100-10104
XPS Study of Copper-Doped Carbon Aerogels Ruowen Fu,†,‡ Noriko Yoshizawa,†,§ Mildred S. Dresselhaus,*,† Gene Dresselhaus,† Joe H. Satcher, Jr.,⊥ and Theodore F. Baumann⊥ Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139; PCFM Laboratory, Zhongshan University, Guangzhou, 510275 China; National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569 Japan; and Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551 Received June 14, 2002. In Final Form: August 12, 2002
Copper-doped carbon aerogels were investigated by X-ray photoelectron spectroscopy to determine the chemical nature and distribution of the copper species in the aerogel framework. The Cu2p spectra of both the organic and carbon aerogels show a fairly uniform distribution of copper species in the aerogel network, with a slight increase in copper content going from the edge to the center of the monolith. The O1s spectra of the copper-doped organic aerogel indicate that both the carboxyl and hydroxyl groups of the aerogel framework are involved in chelation of the copper ions. After carbonization, the content of the copper detected by XPS decreases significantly as the copper ions are reduced into metallic copper nanoparticles. These nanoparticles are difficult to detect by XPS because they are coated by a thin carbon layer and migrate into the carbon matrix. The carbon skeleton of the copper-doped carbon aerogels is mainly composed of a uniform micro-graphite-like crystalline network, and no copper-carbon bonds were detectable in the aerogel.
1. Introduction Carbon aerogels are novel mesoporous carbon materials with a low mass density, a large concentration of mesopores, a high surface area, a high electrical conductivity, and other interesting properties.1-5 As a result, these materials are promising candidates for use as electrode materials in supercapacitors and rechargeable batteries, advanced catalyst supports, adsorbent materials, chromatographic packing, and other applications.6-9 Recently, considerable attention has been paid to the possibility of doping metal species into carbon aerogels to control their properties and expand their applicability. Different metalcontaining carbon aerogels have already been prepared and characterized, including Ce- and Zr-doped carbon aerogels,10,11 Cr-, Fe-, Co-, and Ni-containing carbon aerogels,12 ruthenium/carbon aerogels,7,13 and Ag-, Pd-, †
Massachusetts Institute of Technology. Zhongshan University. § National Institute of Advanced Industrial Science and Technology. ⊥ Lawrence Livermore National Laboratory. ‡
(1) Pekala, R. W. Low density, resorcinol-formaldehyde aerogels. US Patent 4,873,218; US Patent 4,997,804, 1988. (2) Pekala, R. W.; Kong, F. M. J. Phys., Colloq. 1989, C4, 33. (3) Fung, A. W. P.; Wang, Z. H.; Lu, K.; Dresselhaus, M. S.; Pekala, R. W. J. Mater. Res. 1993, 8, 1875. (4) Hanzawa, Y.; Kaneko, K.; Yoshizawa, N.; Pekala, R. W.; Dresselhaus, M. S. Adsorption 1998, 4, 187. (5) Hosoya, M.; Reynolds, G.; Dresselhaus, M. S.; Pekala, R. W. J. Mater. Res. 1993, 8, 811. (6) Yang, K. L.; Ying, T. Y.; Yiacoumi, S.; Tsouris, C.; Vittoratos, E. Langmuir 2001, 17, 1961. (7) Pekala, R. W.; Farmer, J. C.; Alviso, C. T.; Tran, T. D.; Mayer, S. T.; Miller, J. M.; Dunn, B. J. Non-Cryst. Solids 1998, 225, 74. (8) Saliger, R.; Fischer, U.; Herta, C.; Fricke, J. J. Non-Cryst. Solids 1998, 225, 81. (9) Moreno-Castilla, C.; Maldonado-Hodar, F. J.; Rivera-Utrilla, J.; Rodriguez-Castellon, E. Appl. Catal., A 1999, 183, 345. (10) Bekyarova, E.; Kaneko, K. Adv. Mater. 2000, 12, 1625. (11) Bekyarova, E.; Kaneko, K. Langmuir 1999, 15, 7119. (12) Maldonado-Hodar, F. J.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hanzawa, Y.; Yamada, Y. Langmuir 2000, 16, 4367.
or Pt-containing carbon aerogels.14 The work associated with these systems has shown that the metal doping of the aerogels can be used to change the pore structure and carbon framework of the aerogels as well as modify their adsorption and electrical properties. Therefore, study of the metal-doped carbon aerogels may lead to significant benefits for the application of carbon aerogels as electrode or catalyst materials. We recently reported the synthesis and characterization of a new class of metal-doped carbon aerogels.15 In this work, our goal was to develop a method that would allow us to control both the content and the distribution of a desired metal species within the carbon framework. Our strategy was the use of a resorcinol derivative containing an ion exchange moiety that could be polymerized using sol-gel techniques to produce metal-doped aerogels. As a result, each repeat unit of the organic polymer would contain a binding site for metal ions, ensuring a uniform dispersion of the dopant. Toward this goal, we developed a new method for the preparation of metal-doped carbon aerogels in which resorcinol is replaced with the potassium salt of 2,4-dihydroxybenzoic acid in the sol-gel process, producing K+-doped hydrogels. The potassium ions in the gel can be replaced with the desired metal ion through an ion exchange process, and the gels can then be dried and carbonized to generate novel metal-doped carbon aerogels. In that report, we presented the synthesis of copper-doped carbon aerogels prepared by this method. One of the unique features of these novel materials is that carbonization of the copper-doped organic aerogels resulted in the formation of spherical copper nanoparticles within the carbon framework. The main objectives of the present work are to use X-ray photoelectron spectroscopy (XPS) (13) Miller, J. M.; Dunn, B. Langmuir 1999, 15, 799. (14) Maldonado-Hodar, F. J.; Ferro-Garcia, M. A.; Rivera-Utrilla, J.; Moreno-Castilla, C. Carbon 1999, 37, 1199. (15) Baumann, T. F.; Fox, G. A.; Satcher, J. H., Jr.; Yoshizawa, N.; Fu, R.; Dresselhaus, M. Langmuir 2002, 18, 7073.
10.1021/la020556v CCC: $22.00 © 2002 American Chemical Society Published on Web 12/02/2002
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Table 1. Elemental Content of Aerogels sample carbon oxygen potassium copper
atomic content % mass content % atomic content % mass content % atomic content % mass content % atomic content % mass content %
DF-K
CA-K
DF-Cu
CA-Cu
70.74 61.15 26.16 30.13 3.10 8.72
94.29 90.37 4.43 5.66 1.27 3.97
70.60 61.22 28.00 32.34
96.73 95.28 3.06 4.02
1.40 6.44
0.14 0.70
to investigate the distribution of the copper ions and copper nanoparticles throughout the aerogel structure and to determine the effects of the copper and potassium ions on the chemical structure of the carbon aerogels. We report here the XPS studies of these metal-doped carbon aerogels, including elemental content of and distribution of dopant in these aerogels as well as the chemical states of the copper, oxygen, and carbon in these materials. The research presented in this report will provide valuable information that can be used to control and modify the structures of metal-doped carbon aerogels. 2. Experimental Section 2.1. Sample Preparation. The preparation of potassiumand copper-doped organic and carbon aerogels has been previously reported.15 In general, the synthesis involves the sol-gel polymerization of formaldehyde with the potassium salt of 2,4dihydroxybenzoic acid, followed by ion exchange with Cu(NO3)2, supercritical drying with liquid CO2, and carbonization at 1050 °C under an N2 atmosphere. The organic aerogels (pre-carbonized) will be referred to in the text as DF-M and the carbon aerogels as CA-M, where M is the doped metal species. 2.2. XPS Determination and Data Processing. XPS characterization of the aerogels was carried out on an AXIS HIS 165 and ULTRA spectrometer made by Kratos Analytical Ltd., England, using Al KR radiation (energy 1486.6 eV) in a vacuum of 5 × 10-9 Torr. X-ray slots of 760 by 350 µm2 and an X-ray power of 150 W (15 kV and 10 mA) were used for all the XPS measurements. The samples were cut into slices by a diamond saw or cut into small particles and then mounted on a sample holder using double-sided adhesive tape. The elemental contents of the various samples were calculated from the areas of the relevant XPS peaks obtained from survey scan spectra (pass energy 160 eV, step height 0.5 eV, and dwell time 50 ms) after fitting of the data to a Shirley background function. High-resolution XPS spectra of Cu2p, O1s, and C1s were recorded with a pass energy of 10 eV, a 0.1 eV step, and a dwell time of 50 ms. The envelopes were curve-fitted using Gaussian curves, after fitting to a Shirley background, using Peakfit software.
3. Results and Discussion 3.1. The Elemental Sample Content. Survey scans were carried out to search for the presence of particular elements in the aerogels. Since these aerogels are prepared from formaldehyde and the potassium salt of 2,4-dihydroxybenzoic acid, the survey scan spectra for all of the samples should show signals for both carbon and oxygen (hydrogen cannot be detected by XPS). As expected, both the potassium- and copper-doped aerogels showed the presence of these elements (Table 1). Survey scans of the DF-K and CA-K showed that these materials contained potassium as well. The potassium content (8.7 wt %) in the DF-K aerogel was lower than would be expected if every repeat unit in the polymer contained a potassium ion (19 wt %). This indicates that some of the carboxylate moieties may have been protonated after gelation during processing. It is interesting to note that, following carbonization, the potassium content decreases from 8.7% to 3.9%. One possible explanation for this observation could
Figure 1. Spot position of XPS determination on the DF-Cu and CA-Cu monoliths. Table 2. Elemental Distribution in the Cross Section of the DF-Cu Aerogel sample carbon oxygen copper
atomic content % mass content % atomic content % mass content % atomic content % mass content %
DF-Cu (0)
DF-Cu (1)
DF-Cu (2)
71.34 62.13 27.33 31.71 1.34 6.16
66.52 60.72 29.38 34.18 1.10 5.10
71.39 63.02 27.65 32.52 0.95 4.46
Table 3. Elemental Distribution in the Cross Section of a CA-Cu Sample sample carbon oxygen copper
atomic content % mass content % atomic content % mass content % atomic content % mass content %
CA-Cu (0)
CA-Cu (1)
CA-Cu (2)
94.81 92.09 4.88 6.32 0.31 1.59
96.84 95.48 3.06 4.02 0.10 0.50
96.33 94.93 3.60 4.73 0.07 0.35
be that the potassium ions reacted with the carbon and volatilized at the 1050 °C carbonization temperature, which may be similar to the reaction of KOH with carbon.16 Following ion exchange, the survey scan of the DF-Cu aerogel indicates that the ion exchange of the potassium ions for copper ions had occurred, based on the disappearance of potassium and the presence of copper (6.44 wt %). Assuming an exchange of two potassium ions for one copper ion, the survey scans show that the incorporation of copper ions was over 90% complete. Interestingly, the copper content for the CA-Cu decreases significantly following carbonization (0.7 wt %). This decrease in copper content can be attributed to a change in oxidation state of the copper in the carbon aerogels. On the basis of TEM observations and XRD diffraction measurements,15 we know that the copper ions in the DF-Cu aerogels are reduced into crystalline copper nanoparticles during pyrolysis. In addition, TEM results show that these copper particles are coated with a thin carbon layer and migrate into the bulk carbon matrix. Therefore, the copper content in the CA-Cu detected by XPS sample is quite low because XPS is only sensitive to the atoms on the surface. To determine the uniformity of the copper distribution in the copper-doped aerogel samples, we recorded the XPS spectra at different positions in the cross section of the monolith (Figure 1). On the basis of the survey scans, it was determined that the copper content increases slightly from the edge to the center of the monolith. In the DF-Cu sample, the copper content increases from 4.46 wt % at the edge to 6.16 wt % at the center and, in the CA-Cu sample, from 0.35 to 1.59 wt % (Tables 2 and 3). This observation can be explained by the fact that the mass density of the aerogels is a little higher in the center than in the edge of the monoliths. Nevertheless, the XPS measurements indicate that the copper has been incor(16) Teng, H.; Wang, S. C. Carbon 2000, 38, 817.
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Fu et al. Table 5. Curve-Fitting Data of C1s Spectra of the DF-K and DF-Cu Aerogels sample DF-K
DF-Cu
Figure 2. Cu2p spectra of DF-Cu and CA-Cu. Table 4. Curve-Fitting Data of the Cu2p Spectra of DF-Cu sample DF-Cu (0) DF-Cu (1) DF-Cu (2)
component peak
position BE (eV)
fwhm (eV)
raw area (cps)
area (%)
1 2 1 2 1 2
932.60 934.40 932.67 934.40 932.70 934.40
1.816 2.238 1.790 2.299 1.810 2.064
8956.22 1910.93 7167.72 1949.41 4446.92 1360.88
82.42 17.58 78.62 21.38 76.57 23.43
porated throughout the aerogel framework in both the DF or CA samples. 3.3. Chemical State of Copper in the Aerogels. The copper 2p spectra of the DF-Cu and CA-Cu samples show two main peaks at around 932 eV (Cu2p3/2) and 953 eV (Cu2p1/2) (Figure 2). The Cu2p3/2 peaks of DF-Cu are composed of two component peaks (∼932.65 and ∼934.50 eV), each of which are related to different copper species (Figure 3). For reference, the binding energy of copper 2p in Cu(NO3)2 is 935.5 eV, and that of Cu(OAc)2 is 935.00 eV.17 Based on these reference spectra, the component peak of Cu2p in the DF-Cu sample at about 934.50 eV was assigned to the normal ion exchanged copper in aerogels. The component peak at about 932.65 eV indicates that a portion of the copper ions is in the state rich in electrons (or the lower valence state). We propose that the shift of the Cu2p peak toward a low binding energy may be related to the chelation of copper ions by the 2,4dihydroxybenzoate unit of the polymer. The benzoate groups and hydroxy moieties can both participate in the coordination of the copper ions, donating electron density to the copper center.18,19 This interaction may be responsible for the shift of the Cu2p peak to lower binding energies in the DF-Cu aerogel. Compared with Tables 2 and 4, it is found that the higher the percentage of the component Cu2p peak for chelation (932.6 eV), the larger is the copper content that the aerogel possesses. Following pyrolysis, the Cu2p spectrum for the CA-Cu aerogel now shows a narrow peak with a binding energy of 932.6 eV (Figure 2). When referenced to standard spectra,17 this peak can be assigned to be metallic copper, in agreement with XRD and TEM results. 3.4. Chemical State of Oxygen in Aerogels. The O1s spectra for both the pre-carbonized potassium and copper-doped aerogels consist of one main peak at centered at 532 eV, with the DF-K aerogel showing a shoulder at lower energy (Figure 4). The O1s spectra for the DF-K aerogel can be fitted as two peaks with the binding energies of about 530.8 and 532.7 eV (Figure 5). The higher energy (17) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corporation: Eden Prairie, MN. (18) Yoshida, T.; Sawada, S. Bull. Chem. Soc. Jpn. 1974, 47, 1024. (19) Wang, D.-B.; Chen, B.-H.; Zhang, B.; Ma, Y.-X. Polyhedron 1997, 16, 2625.
component peak
position BE (eV)
fwhm (eV)
raw area (cps)
area (%)
1 2 3 4 1 2 3 4
284.46 285.90 287.56 290.20 284.50 285.94 287.64 290.33
1.466 1.490 1.843 3.297 1.518 1.664 1.996 3.110
12633.41 5835.39 2503.01 1300.66 22338.88 12118.66 5341.43 1910.81
56.72 26.20 11.24 5.84 53.56 29.05 12.81 4.58
peak is assigned to single bond oxygen, and the lower energy peak is assigned to double bond oxygen, corresponding to the hydroxyl and carboxyl groups of the aerogel.17,19-21 In the O1s spectrum for the DF-Cu aerogel, the lower binding energy peak at about 530.8 eV, assigned to double-bond oxygen, decreases, and the higher binding energy peak is broadened (Figure 4). As a result, the O1s spectrum for the DF-Cu sample becomes a continuous and broadened peak, without a clear double-peak character. This result may be related to the coordination of the carboxyl and hydroxyl groups to the copper ion. Upon chelation of the copper ions, the aromatic carboxyl and hydroxyl groups become part of a larger conjugated structure, causing a uniform electron distribution between the carboxyl and hydroxyl groups. After carbonization, the O1s spectra show that the number of oxygencontaining groups in the metal-doped carbon aerogels is greatly reduced. From the XPS spectra, it appears that almost all of the oxygen-containing groups in the CA-Cu sample are singly bonded oxygen (Figure 6), while most in the CA-K sample are double-bonded oxygen (Figure 7). While the reason for this observation is not clear at this time, one possible explanation might be that the presence of potassium ions in the DF-K material promotes oxidation reactions during pyrolysis, which may be similar to the reaction of KOH with carbon,16 increasing the amount of doubly bonded oxygen atoms (i.e., carbonyl or carbonate groups). 3.5. Chemical State of Carbon in the Aerogels. The XPS spectra for the metal-doped DF aerogels show asymmetric C1s peaks that tail toward high binding energies (see solid lines in Figures 8 and 9). The asymmetry is due to the contribution of oxygen groups in these aerogels to the carbon XPS spectra. The C1s peak for the DF-K aerogel has a shoulder at about 285.9 eV and a component peak at 287.6 eV, corresponding to the hydroxyl and carboxyl groups, as mentioned above. After ion exchange with copper ions, these two peaks become more difficult to resolve in the C1s spectrum for the DF-Cu sample, similar to the change seen in the O1s spectrum. Based on reference spectra,17,19-24 the C1s feature of the DF-K spectrum could be fitted by four component peaks (see dotted lines in Figure 8). The main peak at about 284.5 eV is assigned to aromatic and aliphatic carbon while the peak at about 285.9 eV is assigned to single C-O bonds (hydroxyl group). The peak at about 287.6 eV is assigned to double CdO bonds (carboxyl group), and the peak at about 290.5 eV is the shake-up satellite. We performed similar curve fitting of the C1s feature for the DF-Cu (20) Fu, R.; Zeng, H. Synthetic Fiber Industry (in Chinese) 1990, 13, 19. (21) Cuesta, A.; Martinez-Alonso, A.; Tascon, J. M.; Bradley, R. H. Carbon 1997, 35, 967. (22) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfao, J. J. M. Carbon 1999, 37, 1379. (23) Polovina, M.; Babic, B.; Kaluderovic, B.; Dekanski, A. Carbon 1997, 35, 1047.
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Langmuir, Vol. 18, No. 26, 2002 10103
Figure 3. Curve-fitting results of the Cu2p spectra of DF-Cu.
Figure 4. O1s spectra of DF-K and DF-Cu.
Figure 7. Curve-fitting results of O1s of CA-K.
Figure 8. Curve-fitting results of C1s of DF-K. Figure 5. Curve-fitting results of O1s of DF-K.
Figure 9. Curve-fitting result of C1s of DF-Cu. Figure 6. O1s spectrum of CA-Cu.
aerogel (see dotted lines in Figure 9). The curve fitting results for both systems are listed in Table 5. After carbonization, the metal-doped carbon aerogels lose most of their oxygen-containing groups, so the oxygen (24) Moreno-Castilla, C.; Lopez-Ramon, M. V.; Carrasco-Marin, F. Carbon 2000, 38, 1995.
signal in the high binding energy region of the C1s spectra greatly decreases (Figure 10). Interestingly, the C1s spectra for both the potassium- and copper-doped carbon aerogels are almost identical, even when measuring the spectra at different spots on the samples (Figure 11). This observation indicates that both samples have similar carbon structures and that no copper-carbon bonds are produced during carbonization. The C1s spectra of both carbon aerogels are very narrow (fwhm
