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Preparation and Characterization of Ultrathin Films Layer-by-Layer Self-Assembled from Graphite Oxide Nanoplatelets and Polymers Thierry Cassagneau, Fre´de´ric Gue´rin, and Janos H. Fendler* Center for Advanced Materials Processing, Clarkson University, PO Box 5814, Potsdam, New York 13699-5814 Received March 23, 2000. In Final Form: June 12, 2000 Graphite, oxidized by HNO3 and NaClO3 to 25% (GO-25), 42% (GO-42), and 33% (GO-33), has been dispersed to partially exfoliated nanoplatelets and, along with poly(diallyldimethylammonium chloride) (PDDA) and poly(ethylene oxide) (PEO), layer-by-layer self-assembled to ultrathin S-(PDDA/GO)10, S-PDDA/(GO/PEO)10, and S-(PDDA/GO/PEO)10 films (where S is the indium tin oxide- or chromium/ gold-coated glass or quartz substrate and GO refers to platelets of oxidized graphite). The GO powders have been characterized by X-ray diffraction prior and by transmission electron microscopy after their dispersions. Thicknesses of each of the successively adsorbed layers were comparatively determined by absorption spectrophotometry, surface plasmon resonance spectroscopy, quartz crystal microbalance, and scanning force microscopy. Good agreements were obtained by these three different techniques.
Introduction There is an increasing interest in the layer-by-layer self-assembly of oppositely charged polyelectrolytes and nanoparticles (or nanoplatelets).1 That a large variety of different materials can be layered in controllable thickness and in desired order renders this approach eminently suitable for the fabrication of ultrathin electronic, electrooptical, and charge storage devices and sensors.2 Recently we have preliminarily reported the layer-bylayer self-assembly of ultrathin films composed of 2.0 ( 0.5 nm thick layers of poly(diallyldimethylammonium chloride), PDDA, and exfoliated graphite oxide (GO) nanoplatelets3 and of PDDA, GO, and poly(ethylene oxide) (PEO).4 The demonstrated outstanding physical, chemical, and mechanical properties of polymer-GO composites5,6 and the opportunity for controlling lateral conductivities (1) For reviews, see: (a) Mallouk, T. E.; Kim, H.-N.; Ollivier, P. J.; Keller, S. W. In Comprehensive Supramolecular Chemistry; Alberti, G.; Bein, T., Eds; Pergamon Press: Oxford, U.K., 1996; pp 189-217. (b) Decher, G. In Comprehensive Supramolecular Chemistry; Sauvage, J.P., Ed.; Pergamon Press: Oxford, U.K., 1996; pp 507-528. (c) Fendler, J. H. Chem. Mater. 1996, 8, 1616. (d) Decher, G. Science 1997, 277, 1232. (e) Protein Architecture; Interfacing molecular Assemblies and Immobilization Biotechnology; Lvov, Y., Mo¨hwald, H., Eds; Marcel Dekker: New York, 1999. Polyelectrolyte Multilayers Symposium, Colloid Division, 219th American Chemical Society National Meeting, San Francisco, CA, March 26-30, 2000. (2) Decher, G.; Lehr, B.; Lowack, K.; Lvov, Y.; Schmitt, J. Biosens. Bioelectron.1994, 9, 677. Lee J.-K.; Yoo, D. S.; Handy, E. S.; Rubner, M. F. Appl. Phys. Lett. 1996, 69, 1686. Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640. Hong, H.; Davidov, D.; Tarabia, M.; Chayet, H.; Benjamin, I.; Faraggi, E. Z.; Avny, Y.; Neumann, R. Synth. Met. 1997, 85, 1265. Tarabia, M.; Hong, H.; Davidov, D.; Kirstein, S.; Steitz, R.; Neumann, R.; Avny, Y. J. Appl. Phys. 1998, 83, 725. Moriguchi, I.; Fendler, J. H. Chem. Mater. 1998, 10, 2205. Ichinose, I.; Tagawa, H.; Mizuki, S.; Lvov, Y.; Kunitake, T. Langmuir 1998, 14, 187. Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848. Caruso, F.; Furlong, D. N.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 4559. Sweryda-Kraviec, B.; Cassagneau, T.; Fendler, J. H. Adv. Mater. 1999, 11, 659. Handy, E. S.; Pal, A. J.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 3525. Wu, A.; Yoo, D.; Lee, J.-K.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 4883. Kaschak, D. M.; Lean, J. T.; Waraksa, C. C.; Saupe, G. B.; Usami, H.; Mallouk, T. E. J. Am. Chem. Soc. 1999, 121, 3435. Lvov, Y. M.; Kamau, G. N.; Zhou, D. L.; Rusling, J. F. J. Colloid Interface Sci. 1999, 212, 570. Danilowicz, C.; Manrique, J. M. Electrochem. Commun. 1999, 1, 22. (3) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637. (4) Cassagneau, T.; Fendler, J. H. Adv. Mater. 1998, 10, 877.
and ion intercalation by the extent of graphite oxidation (to GO) and by the judicious organization of the selfassembled layers have prompted our work. The realization of the full potential of these films requires the more precise evaluation of the properties of the GO building blocks in a given environment. For example, the quantitative evaluation of GO intercalation requires the precise determination of the GO content (and/or thickness) within the entire film. We have, therefore, controllably oxidized graphite to different extents to facilitate its electrostatic attraction to the oppositely charged PDDA3,4 and carefully monitored the thickness of each self-assembled layer by atomic force microscopy (AFM), absorption spectroscopy, surface plasmon resonance spectroscopy (SPR), and quartz crystal microbalance (QCM) measurements. The combination of SPR and QCM has been recently employed to examine the adsorption of surfactants at solid-liquid interfaces,7 and QCM has been extensively used to (5) Chuiko, A. A.; Ogenko, V. M.; Ganyuk, L. N.; Dubrovina, L. V. Vysokomol. Soedin. Ser. B 1990, 32, 903. Schedelniedrig, T.; Sotobayashi, H.; Ortegavillamil, A.; Bradshaw, A. M. Surf. Sci. 1991, 247, 83. Ezquerra, T. A.; Kulescza, M.; Baltacalleja, F. J. Synth. Met. 1991, 41, 915. Choi, H. Y.; Chang, F. K. Pol. Eng. Sci. 1991, 31, 1294. Baranovskij, V. M.; Ponomarenko, A. T.; Khomik, A. A.; Shevchenko, V. G.; Kestelman, V. N. Acta Polym. 1992, 43, 148. Bakhshi, A. K. Superlattices and Microstruct. 1992, 11, 465. Sano, M.; Sasaki, D. Y.; Kunitake, T. J. Chem Soc., Chem. Commun. 1992, 1326. Sano, M.; Sasaki, D. Y.; Kunitake, T. Macromolecules 1992, 25, 6961. Sano, M.; Sasaki, D. Y.; Kunitake, T. Science 1992, 258, 441. Don, R. C.; Gillespie, J. W.; Lambing, C. L. T. Polym. Eng. Sci. 1992, 32, 620. Dufour, P. R.; Gee, A. W. J.; Kingma, J. A.; Mens, J. W. M. Wear 1992, 156, 85. Frazier, A. B.; Allen, M. G. J. Appl. Phys. 1993, 73, 4428. Vovchemko, L. L.; Semko, L. S.; Chernish, I. G.; Matsui, L. Y. Inorg. Mater. 1993, 29, 1062. Lambin, G.; Delvaux, M. H.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L.; Clarke, T. C.; Rabe, J. P. Synth. Met. 1993, 57, 4365. Chen, E. J. H.; Croman, R. B. Compos. Sci. Technol. 1993, 48, 173. Saunders, D. S.; Galea, S. C.; Deirmendjian, G. K. Composites 1993, 24, 309. Iroh, J. O.; Bell, J. P.; Scola, D. A.; Wesson, J. P. Polymer 1994, 35, 1306. Sano, M. O.; Sandberg, S.; Yoshimura. Langmuir 1994, 10, 3815. Celzard, A.; Furdin, G.; Mareche, J. F.; Morae, E.; Dufort, M.; Deleuze, C. Solid State Commun. 1994, 92, 377. Calderone, A.; Parente, V.; Bre´das, J. L. Synth. Met. 1994, 67, 151. (6) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. J.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771. (7) (a) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546-1552. (b) Caruso, F.; Rinia, H. A.; Furlong, D. N. Langmuir 1996, 12, 2145-2152. (c) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422-3426.
10.1021/la000442o CCC: $19.00 © 2000 American Chemical Society Published on Web 08/03/2000
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determine the thickness of self-assembled polyions with a variety of building blocks (octamolybdate, SiO2 particles, water-soluble proteins, for example).8-10 Here we show that assumptions about the density of the layers can be corroborated with SPR and absorption spectroscopy experiments when properly done. Electrochemical characterizations of the self-assembled PDDA/GO/PEO films will be reported subsequently.11 Experimental Section Materials. Poly(ethylene oxide), PEO, MW ) 300 000 (Aldrich), 20 wt % poly(diallyldimethylammonium chloride), PDDA (Aldrich), graphite powder, 1-2 µm (Aldrich), sodium chloride (Aldrich), and 15.8 N nitric acid (Fisher) were used as received. All aqueous solutions were prepared in 18 MΩ water obtained by purification of distilled water with a Millipore Milli-Q system. Soda lime float glass, coated with SiOx and ITO, with R < 125 Ω/0 and >87% transmission were a gift from Photran. Quartz slides were purchased from Chemglass, Inc. The 6-MHz AT-cut quartz piezoelectric crystals (1.32 cm in diameter) were purchased from International Crystals Manufacturing Co. (Oklahoma City, OK). Silver electrodes, in a keyhole pattern with an overlapped area of 0.35 cm2, had been applied to their opposite faces by the manufacturer using a thin layer of chromium (10 nm) to provide good adhesion. Synthesis and Characterization of Graphite Oxide. Graphite was oxidized by the Brodie’s method, to different extents, according to the reported procedures.12 Typically, 30 mL of 15.8 N aqueous HNO3 was added dropwise to a stirred flask, containing 5 g of graphite and 42.5 g of NaClO3, while the flask was allowed to cool. The resulting mixture was heated to 60-80 °C under stirring for 20-24 h. The gas liberated was directed through a cooler to an aqueous basic solution. The solid was separated by centrifugation (6000 rpm) and washed with 150 mL of 1 N HCl and 300 mL of deionized water. The resulting solid 25%-oxidized graphite (designed as GO-25) was dried in a oven at 80 °C for 1 day or further oxidized (in a second oxidative treatment by following the same procedure as used initially). The twice oxidized graphite powder was determined to be 42% oxidized and was, therefore, referred to as GO-42. A third and identical oxidative cycle was performed on GO-42, leading to GO-33 (33%-oxidized graphite) after elemental analysis. Oxidation of graphite was ascertained by FTIR measurements of the powders in diffuse reflectance mode. Absorption bands at 3362 cm-1, ν(OH), 1707 cm-1 (keto group stretching, strong), 1370 cm-1 (very strong), which can be assigned to νs(CdO) and ν(OH) bending, and 827 cm-1 (weak) possibly due to νsc(O-CdO) were observed. Additionally, absorption bands at 1646 cm-1 (strong) and 1560 cm-1 were observed which can be attributed to νas(CdO), including those from COO- moieties, and ν(CdC) and CO- stretching modes, respectively.13 The bare graphite was characterized by absorption bands at 1546 cm-1 owing to CdC bonds, and 1086 cm-1. The extent of graphite oxidation was estimated (see Supporting Information) by taking into account the elemental composition of each GO and the amount of exchangeable protons (see Table 1). Anal. (Galbraith Laboratories, Inc.). Found for GO-25: C, 81.45; H, (PEO/GO)n (see the lower part of Figure 2). For all the systems considered in this study, the extinction coefficient of GO at 230 nm was taken to be 230 ) 23412 M‚cm-1, as previously reported.4 Substituting PEO for PDDA (film S-PDDA/(GO-42/ PEO)6) decreased the film thickness. It was calculated that the PEO layer afforded the deposition of about three times less thick GO layers than the PDDA layer, by dividing the thickness found for S-(PDDA/GO-42)6 by the one found for S-PDDA/(GO-42/PEO)6 (S-(PDDA/ GO-42)6/S-PDDA/(GO-42/PEO)6 ) 653 Å/194 Å ) 3.4) or by dividing the thickness found for S-(PDDA/GO-33)6 by the one found for S-PDDA/(PEO/GO-33)6 (S-(PDDA/ GO-33)6/S-(PDDA/PEO/GO-33)6 ) 480 Å/160 Å ) 3, see Table 2). Note that, for the system S-PDDA(GO-42/PEO)6, the higher absorbance of GO measured for the first deposited layer is simply due to the presence of PDDA, allowing the adsorption of a first thicker GO layer and AFM imaging (see next section) confirmed that both the thickness and the coverage of a GO layer were affected by the nature of the polymer layer underneath. Since the determination of the thickness of the different layers is essential for ensuring reliable estimate of the specific capacities of our cathodes, several other techniques have been used for determining these thicknesses (see below). 5. Characterization of the Self-Assembled Films by Atomic Force Microscopy (AFM). Topologies and thicknesses of the GO-42 layers were investigated by AFM. Typical images obtained for a GO-42 layer, in selfassembled films, of S-PDDA/GO-33 and S-PDDA/GO-
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Table 2. Thickness Determination of the Different Organic Layers Using SPR, UV-Visible Spectrophometry, and QCM surface plasmon spectroscopy layers
r
i
d (Å)
Cr Au Ag mercaptoethylamine (MEA) PEI poly(allylamine hydrochloride), PAH (onto MEA) PAH (onto GO-25) poly(diallyldimethylammonium chloride), PDDAb PDDA (onto PEO) poly(ethylene oxide), PEO (onto PDDA) PEO (onto GO-25) GO-25 (onto PAH or PDDA) GO-25 (onto PEI) GO-25 (onto PEO) GO-42 (in the film S-(PDDA/GO-42)10) GO-42 (in the film S-PDDA/(GO-42/PEO)10) GO-42 (in the film S-(PDDA/GO-42/PEO)10) GO-42 (in the film S-PDDA/(PEO/GO-42)10) GO-33 (in the film S-(PDDA/GO-33)10)
-30.3456 -12.3090 -15.2202 2.1740 2.2440 2.1800 2.1800 2.2420 2.2420 2.0600 2.1200 2.3410 2.3410 2.3410
31.1220 1.3890 2.2572 0.0000 0.5080 0.0080 0.0080 0.1380 0.1380 0.070 0.052 0.078 0.1130 0.082
7(1 18 ( 2a 4(1 9(2 17 ( 2 6(1 5(1 5(1 36 ( 3 22 ( 2 9(2
a
abs
QCM
d (Å)
d (Å)
17 ( 1.5
8(1 7(1 36 19 11 65.3c 19.4c 55.5c 16.0c 48.0c
20 ( 1
At pH ) 6.6. b At pH ) 6.5, on quartz, PEO or GO. c Average thickness of one layer of GO in the film.
Figure 3. Typical 1 µm × 1 µm AFM images obtained for a GO-42 layer, self-assembled onto a layer of PDDA (A) and onto a film PDDA/GO-42/PEO (B) layered on quartz substrates in both cases.
42/PEO/GO-42 (where S ) quartz) are illustrated in Figure 3a,b, respectively. The GO-42 nanoplatelets are seen to form a thicker layer on PDDA (Figure 3a) than on PEO (Figure 3b) because the attachment of GO onto PEO is likely to be governed by epitaxial-like and/or hydrophobic interactions (see below). A maximum thickness of 72 Å was measured between the PDDA polyelectrolyte layer and the top of the GO-42 layer, with a significant coverage evaluated to be 80% (see Figure 3a). In contrast only 45% of the available area on PEO was found to be covered by GO nanoplatelets with an average thickness of 20 Å (Figure 3b). Note that these values are less precise than those reported in Table 1, though close within 4-7 Å.
Typical topologies of the PDDA and PEO in the selfassembled systems of S-PDDA/GO-42/PDDA and S-PDDA/GO-42/PEO (where S ) quartz) are shown in Figure 4a,b, respectively. The resulting films appear to be smooth, having rms values of 0.9 and 1.1 nm, respectively. 6. Characterization of the Self-Assembled Films by Surface Plasmon Spectroscopy. Surface plasmon spectroscopy (SPR) was also used to establish the thicknesses of the PDDA and PEO layers deposited onto the corresponding electrodes. A glass slide was first coated by a 5-10 Å thick Cr layer and a subsequent gold layer of 360-450 Å. The substrate was derivatized by selfassembling a layer of mercaptoethylamine (7 ( 1 Å), therefore exposing amine groups in the upper part of the layer. These amine groups are able to strongly interact with poly(allylamine hydrochloride), PAH,22 leading to the adhesion of a 4 ( 1 Å thick positively charged layer. It is possible that some GO is removed from the selfassembled film during the adsorption of the next polyelectrolyte layer. Such situation has, in fact, been encountered for poly(styrene sulfonate) (PSS) and Ru(bpy)32+ multilayers.23 Thus the thickness determination (by SPR for example) of the GO layer may suffer from uncertainties that cannot be detected unless using a technique that really probes the total amount of GO in the entire film (for example, optical absorption or fluorescence). This problem was addressed by critically comparing SPR and absorption spectroscopic data (Figure 5). We choose a test GO compound (GO-25). GO-25 was deposited onto the systems Au-MEA/PAH (for SPR) and onto quartz-PDDA/PSS/ PAH (for spectrophotometry). Table 2 summarizes the main results. The thickness of a GO-25 layer deposited onto a polycationic layer (PAH or PDDA) was found to be 36 ( 3 Å by SPR and 36 Å by absorption. It should be noted that this good agreement was also found with the systems S-(GO-25/PEO/PDDA)n, S-(GO-25/PDDA)n, and S-(GO-25/PEO)n (where S ) Au-MEA/PAH, for SPR, and S ) quartz-PDDA, for UV-visible absorption). This result clearly indicated that no GO platelets could be desorbed upon adsorption of the next polycationic layer and revealed the existence of strong electrostatic interactions. It should be noted that the slight decrease of absorbance during the self-assembly of S-PDDA/(GO(22) Cassagneau, T.; Fendler, J. H.; Mallouk, T. E. Langmuir 2000, 16, 241. (23) Gue´rin, F.; Cassagneau, T.; Fendler, J. H. Unpublished results.
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Figure 4. Typical 1 µm × 1 µm AFM topological images obtained after depositing a PEO layer onto S-PDDA/GO-42 (A) and a PDDA layer laid down onto S-PDDA/GO-42 (B), S standing for quartz substrate.
Figure 5. Quantitative comparison between surface plasmon spectroscopy and spectrophotometry. In both cases, the appropriate substrates were exposed to the same GO colloidal suspension for a same dipping time and washing procedure. Here the samples consisted of a quartz substrate covered with an increasing number of bilayers (PDDA/GO-25). Note that the thicknesses of the first (GO1) and second (GO2) layers were found to be 35.6 and 39.9 Å, respectively. A gold substrate precoated by a layer of mercaptoethylamine (MEA) was used for surface plasmon spectroscopy; the thicknesses of the first (GO1) and second (GO2) layers were found to be 36.0 and 39.0 Å, respectively. In that latter case, the GO layer was electrostatically built up using poly(allylamine hydrochloride), PAH, as the polycation.
42/PEO)10 (middle plot in Figure 2) corresponds to a GO thickness decrease of 0.6 Å/layer, a values below the range of uncertainties found by SPR and QCM. PEO formed a very thin (typically 4-6 Å) film onto GO25 or PDDA. The values obtained by SPR were in good (24) (a) Serizawa, T.; Hashiguchi, S.; Akashi, M. Langmuir 1999, 15, 5363. (b) Serizawa, T.; Yamamoto, K.; Akashi, M. Langmuir 1999, 15, 4684.
agreement with QCM measurements (Table 2). Its importance was crucial both for allowing further adsorption of GO or PDDA in the films incorporating it and for enhancing the charging properties of the constructed cathode.4,11 7. Formation of Adsorbed Bilayers PEO/GO and PEO/PDDA. Even though PEO is a neutral polymer both anionic GO nanoplatelets and cationic PDDA could be adsorbed onto it, suggesting that nonelectrostatic interactions could also drive the self-assembly (Figure 3b). Imaging the surface of a PEO layer deposited onto a S-PDDA/GO-42 fim (Figure 4a) unambiguously indicated that the surface was smooth with no visible platelets, while the thickness of the film increased. A simple procedure of repetitive adsorption and drying cycles has been recently reported to be sufficient for the deposition of water-soluble neutral polymers onto gold substrates.24,25 The amount of polymer adsorbed was found to depend on the ionic strength of the solution and the molecular weight of the polymer. Two different polymers could also be assembled by this technique. For example, poly(vinyl alcohol) (PVA) could be layered with PEO, poly(N-vinylformamide) (PNVF), poly(vinylamine) (PVAm), or poly(glucosyloxyethyl methacrylate) (PGEMA). The hydrophobic effect, enhanced by NaCl which could release water hydration from the polymer (PVA), was proposed to be responsible for the observed physisorption.24 Importantly, the thickness of a PEO layer deposited onto PVA, 2 ( 4 Å (in the presence of 2 M NaCl), was comparable to that observed for the deposition of PEO onto PDDA at zero ionic strength (5 ( 1 Å). The presence of charges at the surface of the GO nanoplatelets (-COO-H+, -COONa+) and/or at the PDDA polymer chains (-NH3+Cl-) is likely to play an effect similar to NaCl in expelling the water solvating the PEO chains and promoting thereby hydrophobic interactions. Graphitic aromatic domains in graphite oxide may reinforce the hydrophobic effect responsible for the attraction between neutral PEO and anionic GO nanoplatelets. It is also possible that this type of hydrophobic interaction may favor an epitaxial-like interaction as it has been observed between pyrolytic graphite and common monomers (ethylene oxide and propylene for example).26 Finally, it should be noted that layers of PDDA or GO adsorbed onto a PEO layer were always thinner than those self-assembled onto each other in the absence of PEO (Table 2). Thus, the thickness of a PDDA layer, adsorbed onto GO, decreased from 17 to 8 Å when adsorbed onto PEO; the thickness of a GO layer, (25) Serizawa, T.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1903. (26) (a) Sano, M.; Sasaki, D. Y.; Kunitake, T. J. Chem. Soc., Chem. Commun. 1992, 1326. (b) Sano, M.; Sasaki, D. Y.; Kunitake, T. Macromolecules 1992, 25, 6961. (c) Sano, M. Adv. Mater. 1996, 8, 521.
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Figure 6. Cumulated frequency shift as measured from dried QCM covered with a silver film after successive deposition of PEI and GO-25. The frequency shift of the first layer of PEI adhering to the silver oxide surface of the electrode was not taken into account in the calculation of the average shift associated with the deposition of one layer of PEI onto GO-25, ∆fh(PEI), and was arbitrarily set at the origin.
adsorbed onto PDDA, decreased from 36 to 9 Å when adsorbed onto PEO. 8. Characterization of the Self-Assembled Films by Quartz Crystal Microbalance (QCM). QCM has been fruitfully employed for monitoring the layer-by-layer growth of sequentially adsorbed polyelectrolytes and a variety of different building blocks.8-10,27,28 The combined use of QCM and SPR has been mainly motivated by the complementarity of the obtained analytical data precluding the need for operating QCM in solution, which deviates from the Sauerbrey equation in most of the cases.7a Here we aim to establish a critical comparison between the thickness measurements obtained by QCM and SPR. Prior to deposition silver-coated quartz crystals were exposed to a 1.0% KOH solution (w/v) for 20 s at 50 °C in order to generate silver oxide layer on the top of which a polyelectrolyte layer (typically PEI, pH ) 6.6) could be adsorbed. SPR was conducted in parallel using a glass slide covered with a thin layer of Cr (5-10 Å) and a silver film of 650-700 Å thick oxidized in a similar fashion. Note that the silver film must be manipulated very carefully and remained fragile despite the underlayer of Cr. The same dipping solutions and times were used in both the SPR and QCM to allow an accurate comparison of the thickness, determined by these methods. Three bilayers were typically laid down, and the frequency shifts were measured after carefully washing and drying the quartz crystal after each step of adsorption. The thickness d of the film deposited onto the silver electrode can be directly deduced from the frequency shift ∆f, according to
2d (Å) ) -∆f (Hz)/0.810F (g/cm3)
(2)
where r is the density of the adsorbed film. A variation of 1 Hz corresponded to the adsorption of 4.5 ng of materials. Figure 6 indicates that an average shift of 68 Hz was observed after deposition of a GO-25 layer, corresponding to an average thickness of 20 ( 1 Å of GO on both sides of the electrode (assuming a density of 2.1 g/cm3). Oxidizing graphite is expected to decrease the density relative to bulk graphite (2.2 g/cm3), because the (27) Marchi-Artzner, V.; Lehn, J.-M.; Kunitake, T. Langmuir 1998, 14, 6470-6478. (28) Ebara, Y.; Okahata, Y. Langmuir 1993, 9, 574-576.
Figure 7. Surface plasmon resonance curves of a silver substrate prior and after oxidation and after the sequential adsorption of a PEI layer and GO-25 nanoparticles onto the oxidized silver substrate.
sheets are more expanded. It should be noted that decreasing the density of GO by 0.1 causes an increase of only 1 Å in the estimated average thickness. An average frequency shift of 34 Hz was associated with the deposition of a layer of PEI onto GO-25. This value corresponded to a thickness of 17 ( 1.5 Å (taking r ) 1.2 g/cm3 for the density). Comparable frequency shifts, 30-50 Hz, were found for PEI layers sequentially self-assembled with a glucose oxidase8 or colloidal SiO2 particles,5b at pH ) 6.5 and zero ionic strength, but with a resulting thickness of 8 Å. This lower value might be due to the different electrodes used in these studies. Very satisfactory agreements were found for the thicknesses determined by the different methods. Thus, the thickness of a PEI layer was determined to be of 18 ( 2 and 17.0 ( 1.5 Å by SPR and QCM; the thickness of a GO-25 layer, self-assembled onto PEI, was determined to be of 22 ( 2, 19, and 20 ( 1 Å by SPR, absorption spectrophotometry, and QCM, respectively (see Figures 6 and 7 and Table 2). Furthermore, these values overlap well with those previously determined4 and substantiate therefore the validity of our approach for accurate thickness determinations. Conclusions Self-assembly of ultrathin films from anionic nanoplatelets of graphite oxide (prepared by the oxidation of graphite to different extent), cationic poly(diallyldimethylammonium chloride), and neutral poly(ethylene oxide) is reported here. Particularly significant has been the demonstration of the adsorption of charged graphite oxide nanoplatelets onto neutral poly(ethylene oxide)sand vice versasand the unambiguous determination of the thicknesses of each layer deposited by three different methods: absorption spectroscopy, surface plasmon spectroscopy, and quartz crystal microbalance. Properties of these ultrathin films render them potentially useful for the construction of lithium-ion intercalation electrodes. Supporting Information Available: A discussion of the quartz crystal microbalance and Sauerbrey equation, calculation of the formal degree of graphite oxidation platelets, a schematic of the quartz crystal microbalance, surface plasmon resonance curves, and a transmission electron micrograph of GO-42 nanoplatelets. This material is available free of charge via the Internet at http://pubs.acs.org. LA000442O
