Intercalation of Organic Ammonium Ions Into Graphite Oxide

Intercalation of Organic Ammonium Ions into LayeredGraphite Oxide

Zong-huai Liu, Zheng-Ming Wang, Xiaojing Yang, and Kenta Ooi*

Marine Resources and Environment Research Institute, National Institute of AdvancedIndustrial Science and Technology, 2217-14 Hayashi, Takamatsu, 761-0395 Japan

Received November 15, 2001. In Final Form: March 18, 2002

The intercalation of large organic ammonium ions (tetramethylammonium ions (TMA+), tetraethyl-ammonium ions (TEA+), tetrapropylammonium ions (TPA+), and tetrabutylammonium ions (TBA+)) intolayered graphite oxide (GO) was systematically investigated. The intercalation reactions were completedat 25 C after 3 days, and stable colloidal suspensions were obtained at TAAl/Hs ) 5 (molar ratio oftetraalkylammonium ions (TAA+) over exchangeable protons in GO). The sediments after centrifuging thecolloidal suspensions showed amorphous phase X-ray diffraction patterns, indicating that exfoliation ofthe layered structure into nanosheets took place in the suspension. When the sediments were dried at 70C for 3 days, layered structures of TAA+-intercalated GO materials with basal spacings of 1.56, 1.67, 1.84,and 2.37 nm, respectively, appeared. The basal spacing of the layered compounds decreased with a decreaseof relative humidity during drying. When the dried TAA+-intercalated GO compounds were exposed to ahumid saturated atmosphere, the basal spacing increased gradually, finally becoming an amorphousstructure. The maximum saturation of intercalated TAA+ ions into GO decreased with the increase in alkylchain length. When the TAA+-intercalated materials were washed with distilled water and acid-treated,a process of deintercalation of TAA+ ions from the interlayer occurred. A schematic model for thedeintercalation-intercalation involving a exfoliation process is proposed. The layered structure of TAA+-intercalated GO materials is discussed in terms of the dimension of the GO layer and the sizes of H2Omolecules and TAA+ ions.

Introduction

Intercalation into layered compounds has drawn muchattention from both the fundamental and practicalviewpoints.1-4 Graphite oxide (GO) is a typical two-dimensional solid in bulk form, with strong covalentbonding within the layers. Weaker interlayer contact ismade by hydrogen bonds between intercalated watermolecules.5-7 Some functional groups, such as hydroxyl,carbonyl, and other groups, embedded in carbon sheets inGO lamellae make graphite oxide hydrophilic and exhibita rich intercalation chemistry. So far, many intercalatedGO materials have been synthesized and their physico-chemical properties investigated.8-17 To control the spaceoccurring between intercalated surfactant ions, Matsuoet al. have used various kinds of surfactants with different

chain lengths to synthesize surfactant-intercalated GOmaterials. These materials can be used as hosts formolecular recognition.8 The exfoliation of GO particlesoccurs easily in dilute (alkaline) aqueous solution; poly-aniline-intercalated GO material and nanometer com-pounds with particle properties have been synthesized byan exfoliation/adsorption process.9-11 Using the hydro-philicity of GO, some polar organic molecules and poly-mers, such as alcohol,12 poly(ethylene oxide) (PEO),13,14poly(vinyl alcohol) (PVA),15 poly(diallyldimethylammo-nium chloride) (PDDA),16 poly(furfuryl alcohol) (PFA),17and even others,18 can be easily inserted into its lamellaeto form intercalated GO nanocomposites by differentmethods with different c-axis repeat distances. Theexistence of these polymers enables the physicochemicalproperties of GO to be greatly changed. Liu et al.19 havesynthesized a poly(vinyl acetate) intercalated GO nano-composite using poly(vinyl acetate) (PVAc), with an oil-soluble polymer as guest molecules. The compound has ahigh stability against extraction by organic solvent dueto a strong interaction between the GO layers and PVAcchains.

In the present work, systematic studies on theintercalation reaction of large organic ions into lay-ered GO were conducted using tetraalkylammonium ionswith different methylene chain lengths. The TAA+-intercalated GO materials were obtained by an exfoliation/reassembling process, and a structural model for theexfoliation/reassembling process involving the intercala-tion reaction is proposed.

* Address correspondence to this author. Telephone: +81-87-869-3511. Fax: +81-87-869-3551. E-mail: [email protected].

E-mail: [email protected].(1) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and

Molecular Sieves; Academic Press: London, 1978; p 407.(2) Clearfield, A., Ed. Inorganic Ion Exchange Materials; CRC Press:

Boca Raton, FL, 1982.(3) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593.(4) Schollhorn, R. Chem. Mater. 1996, 8, 1747.(5) Nakajima, T.; Mabuchi, A.; Hagiwara, R. Carbon 1988, 26, 357.(6) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998,

102, 4477.(7) Mermoux, M.; Chabre, Y.; Rousseau, A. Carbon 1991, 29, 469.(8) Matsuo, Y.; Niwa, T.; Sugie, Y. Carbon 1999, 37, 897.(9) Liu, P.; Gong, K. Carbon 1999, 37, 701.(10) Inagaki, M.; Suwa, T. Carbon 2001, 39, 915.(11) Cassagneau, T.; Fendler, J. H. J. Phys. Chem. B 1999, 103, 1789.(12) Matsuo, Y.; Tahara, K.; Sugie, Y. Carbon 1997, 35, 113.(13) Cassagneau, T.; Cuerin, F.; Fendler, J. H. Langmuir 2000, 16,

7318.(14) Matsuo, Y.; Tahara, K.; Sugie, Y. Carbon 1996, 34, 672.(15) Matsuo, Y.; Hatase, K.; Sugie, Y. Chem. Mater. 1998, 10, 2266.(16) Kotov, N. V.; Dekany, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637.(17) Kyotani, T.; Moriyama, H.; Tomita, A. Carbon 1997, 35, 1185.

(18) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.;Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999,11, 771.

(19) Liu, P.; Gong, K.; Xiao, P.; Xiao, M. J. Mater. Chem. 2000, 10,933.

4926 Langmuir 2002, 18, 4926-4932

10.1021/la011677i CCC: $22.00 2002 American Chemical SocietyPublished on Web 05/15/2002

Experimental Section

Materials. The starting material, GO, was synthesized fromnatural graphite by the Staudenmaier method.20 The pH titrationtoward Na+ ions was obtained in (0.1 M NaCl + NaOH) solutionswith different NaOH concentrations at 25 C. The ion-exchangecapacity was evaluated from the titration curve.

Tetraalkylammonium hydroxides were purchased from WakoPure Chemical Co. They included tetramethylammonium hy-droxide (15 wt % solution), tetraethylammonium hydroxide (10wt %), tetrapropylammonium hydroxide (10 wt %), and tetra-butylammonium hydroxide (10 wt %).

Intercalation Reaction. Intercalation reactions of tetra-methylammonium ions (TMA+) were studied batchwise. Weighedsamples (0.1 g) of GO were soaked in TMA+ hydroxide solutions(20 cm3) with different concentrations at 25 C for 3 days. Theamount of tetramethylammonium hydroxide added ranged from1 to 25-fold that of the exchangeable capacity of GO (1 e TMAl/Hs e 25). After soaking, the solutions were separated bycentrifugation, and the resultant colloids were subjected to X-raydiffraction (XRD) analyses in their wet state, followed by air-drying at a relative humidity of 40% at 25 C or heating at 70C for 3 days. The influence of relative humidity on the basalspacing was studied by equilibrating the sample with anatmosphere at a different humidity. The hydroxide ion concen-trations in the supernatant solutions were determined by acidtitration.

Intercalation reactions of TEA+, TPA+, and TBA+ ions werecarried out at TAAl/Hs ) 5, and the colloids obtained were treatedby the same procedure as that for TMA+-intercalated colloids.The colloids in the wet state were washed with 30 cm3 of waterthree times, and the gels obtained were air-dried at a relativehumidity of 40% at 25 C for 2 days.

Deintercalation of TAA+ ions from the interlayer of GO wascarried out by mixing the TAA+-intercalated GO materials with0.1 M HCl solution at 25 C for 2 days. The extractability ofTAA+ ions was calculated from the difference between TNconcentrations in the samples before and after the acid treatment.

Chemical Analyses. The TN (total nitrogen) and TC (totalcarbon) contents of the TAA+-intercalated GO compounds, theacid-treated compounds, and the water-washed compounds weredetermined by a Sumigraph type NCH-21 NCH analyzer.

Physical Properties. The sediment from each differenthumidity state was subjected to XRD analysis using a Rigakutype RINT 1200 X-ray diffractometer with a graphite mono-chromator at 25 C. TG-DTA curves were obtained on a MACScience thermal analyzer (System 001, TG-DTA 2000) at aheating rate of 10 C/min. SEM observation was carried out witha Hitachi S-246N scanning electron microscope.

Results and Discussion

Characterization of GO. An SEM image of synthe-sized GO is given in Figure 1. The sample shows platelikeforms without any amorphous or other kinds of crystallizedphase particles. XRD analysis shows diffraction patternscorresponding to a layered structure with a basal spacingof 0.88 nm.

The pH titration curve of GO toward Na+ ions is givenin Figure 2. The total exchange capacity of GO is evaluatedas 4.4 mmol/g from the ion-exchange reaction in a 0.1 MNaOH solution. The titration curve shows a multi-baseacid character, suggesting that the synthesized GOcontains more than one kind of exchangeable protons.The dispersibility of GO in solution depends on the degreeof neutralization (R). Colloidal suspensions were notobserved, and the supernatant solutions were transparentat R < 0.4. The peptization of GO takes place at R > 0.4,and completely dispersed colloidal suspensions are ob-tained at R > 0.9. This shows that the neutralization ofmost of the acid sites is necessary in order to have a

sufficient electrostatic repulsion between each sheet toform stable colloidal suspensions.

Exfoliation by the Intercalation of TAA+ Ions. Allof the intercalation reactions were carried out at 25 C for3 days to make sure the intercalation was complete. Stablecolloidal suspensions were obtained by treating GO withTAA hydroxide solutions at TAAl/Hs ) 5. The suspensionswere centrifuged, and the colloidal sediments weresubjected to XRD measurement in the wet state. The watercontents, roughly evaluated from the weight loss bymaintaining at 150 C, were around 80% for all the TAA+-intercalated GO samples. The XRD patterns of all sedi-ments obtained are shown in Figure 3. The patterns gaveno clear peaks but only a broad diffraction halo in a 2range of 20-40, in striking contrast to that of startingGO (Figure 3a). The broad halo is most likely related toscattering from dispersed single sheets of GO and wateras a solvent, similar to the case of layered titanic acid.21,22This indicates that the stacked sheets of GO are exfoliatedwhen GO is soaked in TAA hydroxide solution. BecauseGO contains acidic groups, negatively charged sites are

(20) Standenmaier, L. Ber. Dtsch. Chem. Ges. 1989, 31, 1481.

(21) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Naka-zawa, H. J. Am. Chem. Soc. 1996, 118, 8329.

(22) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682.

Figure 1. SEM image of starting GO.

Figure 2. The pH titration curve of GO toward Na+.

Intercalation of Organic Ammonium Ions into GO Langmuir, Vol. 18, No. 12, 2002 4927

formed in every GO layer when GO is neutralized by diluteTAA hydroxide solution. The electrostatic repulsive forceoperating between negatively charged sites between GOlayers causes the formation of exfoliated GO.12 The humpof the broad diffraction halo differs in shape dependingon the size of TAA+ ions; it decreases with an increase inthe length of the methylene chain. From detailed analysisof the diffraction envelope, Sasaki et al. have concludedthat their exfoliated nanosheets of titanic acid werearranged in a nearly amorphous manner except for anearest-neighbor correlation.22 In the TPA-GO and TBA-GO systems, which have smaller humps in their diffractionhalos, the nearest-neighbor correlation of the exfoliatedGO nanosheet may be smaller and the nanosheets maybehave more freely in the aggregates.

The OH- concentrations of the supernatant solutionswere determined by acid titration and the amounts ofOH- consumed, and the R values were evaluated from thedecrease of OH- concentrations relative to the initialsolutions. The R values evaluated are 0.94, 0.94, 0.62, and0.51 for GO-TMA, GO-TEA, GO-TPA, and GO-TBAsystems, respectively (Table 1). The latter two systemsshow very low R values although the supernatant solutioncontains an excess of OH- ions. The TPA+ and TBA+ ionsmay be too large to neutralize all the acidic sites in GOsheets freely without any steric influence. Since exfoliationtakes place in all the systems, the volume fraction of guest

ions in the interlayer may play an important role forexfoliation, inaddition to thatof thesurfacechargedensity,similar to the case of layered manganese oxide.23

The surface density of the exchange site can becalculated from the total ion exchange capacity (4.4 mmol/g) by assuming that the density of the carbon skeleton ofGO is equal to that (2.3 g/cm3) of graphite and the thicknessof GO is 0.6 nm. The specific surface area of nanosheetscan be evaluated as 103 m2/g from these values. Thenumber of exchange sites per unit area can be derived as2.6/nm2, which corresponds to an area per exchange siteof 0.38 nm2. The area per TAA+ ion can be calculated as0.15, 0.33, 0.57, and 0.92 nm2 for TMA+, TEA+, TPA+, andTBA+ ions, respectively, assuming spherical ions. TheTMA+ and TEA+ ions are small enough to exchange fullywith the protons of the exchange sites, while the TPA+and TBA+ ions are too large to exchange freely withoutsteric influence. The maximum exchange capacities arecalculated as 3 and 1.8 mmol/g for TPA-GO and TBA-GO systems, respectively, which agreed comparativelywell with the experimentally determined exchange ca-pacities (Table 1). These calculations suggest that theexchange sites are distributed homogeneously throughoutthe surface of GO layers.

Reassembling by Drying. When the colloidal sedi-ments were dried at 70 C for 3 days, the layered structurewith different basal spacings reappeared as shown inFigure 4. The XRD patterns of TMA+-intercalated GOmaterial showed peaks at 1.56, 0.79, and 0.52 nmcorresponding to the (001), (002), and (003) reflections,respectively. Since the d-values of the (001) crystal spacingreflections correspond to the interlayer distances, theinterlayer distance is 1.56 nm for the TMA+-intercalatedGO material. The XRD pattern analyses for the otherTEA+-, TPA+-, and TBA+-intercalated GO materials showinterlayer distances of 1.67, 1.84, and 2.37 nm, respec-tively. Sharp diffraction lines up to the fourth order forthese samples indicate a highly ordered hydrate structure,similar to the cases of TBA+-intercalated protonic titanateand TMA+-intercalated birnessite.22,23 The individualsheets of exfoliated GO are reassembled together by dryingat 70 C; the interlayer distance depends on the size of theintercalated TAA+ ions.

The rate of intercalation reaction was studied bychanging the soaking time for the TMA-GO system atTMAl/Hs ) 5. The colloidal suspension could be obtainedafter only 2 h of soaking. The sediment obtained bycentrifugation had an interlayer distance similar to thatof the TMA+- intercalated GO materials obtained by drying

(23) Liu, Z.-h.; Ooi, K.; Kanoh, H.; Tang, W.; Tomida, T. Langmiur2000, 16, 4154.

Figure 3. Wet-state XRD patterns of GO treated with TAAOHsolutions at TAAl/Hs ) 5. (a) Starting GO; (b) GO treated withTMAOH; (c) GO treated with TEAOH; (d) GO treated withTPAOH; (e) GO treated with TBAOH.

Table 1. Amounts of TAA+ Intercalated at DifferentMolar Ratios of TAAOH in Solution over Exchangeable

Protons in GO

intercalatedTAA+ TAA+/H+

OH- consumed(mmol/g)

degree ofneutralization (R)

TMA+ 1 3.12 0.715 4.11 0.94

10 4.35 0.9925 - -

TEA+ 5 4.12 0.94TPA+ 5 2.72 0.62TBA+ 5 2.24 0.51

Table 2. Structural Models of Samples ofAmmonium-Intercalated GOa Dried at 70 C for 3 days

cationcation size,b

nm model of sample

observed interlayerspacing of sample,

nm

TMA 0.50-0.60 one layer of TMA 1.56one layer of water

TEA 0.65-0.75 one layer of TEA 1.67one layer of water

TPA 0.80-0.90 one layer of TPA 1.84one layer of water

TBA 0.95-1.05 one layer of TBA 2.37two layers of water

a Based on GO slab thickness of 0.60 nm and a van der Waalsradius of water of 0.28 nm. b Szostak, R. Molecular Sieves: Principalof Synthesis and Identification; Van Nostrand Reinhold: New York,1989; p 95.

4928 Langmuir, Vol. 18, No. 12, 2002 Liu et al.

at 70 C. The intensity of the XRD peaks was hardlychanged by soaking for 1 day. This indicates that theintercalation reaction of TAA+ into GO is fast. GO maybe easily dispersed in TAA hydroxide solutions, and issubject to rapid exfoliation.

Effects of TMA+ Concentration and Drying Con-ditions. The influence of TMA+ concentration on theintercalation reaction was investigated by changing TMAl/Hs to 1, 5, 10, and 25. Seventy percent of the acidic siteswere occupied by TMA+ ions at TMAl/Hs ) 1. The XRDanalysis of the sediment in wet state shows a new layeredstructure with a basal spacing of 0.98 nm (Figure 5a).This shows that short-range swelling takes place to formstable layers having a basal spacing a little larger thanstarting GO. At TMAl/Hs g 5, the R values are above 0.9and the XRD patterns show only amorphous halos in wetstate, corresponding to an exfoliated state.

The sediments at different TMA+ concentrations weredried at 70 C for 3 days and subjected to XRD analysis(Figure 5b-e). The layered structure with a basal spacingof 0.98 nm was retained for the sample at TMAl/Hs ) 1.The other dried samples show a layered structure withbasal spacing of 1.56 nm, independent of the TMA+concentration. The exfoliation by TMA+ intercalation andthe reassembling of GO sheets by drying take placeindependent of TMA+ concentration in the region TMAl/Hs g 5.

The TMA+-intercalated GO material obtained at TMAl/Hs ) 5 was dried under different conditions. The XRDpatterns of the dried samples show that the layeredstructure is retained after drying, but the basal spacingdecreases slightly with a decrease of the relative humidityof the atmosphere (Figure 6). The basal spacing around1.54 nm hardly changes even after vacuum-drying at 70C for 1 day. This indicates that the free water moleculesexisting in the interlayer of GO dehydrate easily, but thereare some hydrated water molecules that are stronglyattached between TMA+ ions and GO layers. The dryingbehavior is different from that in TMA+-intercalatedlayered manganese oxide, in which most of the watermolecules are easily dissipated from the interlayer byvacuum-drying for 1 day.23

When the vacuum-dried sample was exposed to anatmosphere with saturated humidity, the basal spacingof the layered compound increased again from 1.54 to 1.82nm after 1 day of exposure (Figure 6f), and finally returnedto an amorphous phase after 5 days (Figure 6g). Thisindicates that the dehydration-rehydration of TMA+-intercalated GO material involving an exfoliation process

Figure 4. Changes of XRD patterns of TAA+-intercalatedsamples at TAAl /Hs ) 5 after drying at 70 C for 3 days. (a)TMA+-intercalated GO; (b) TEA+-intercalated GO; (c) TPA+-intercalated GO; (d) TBA+-intercalated GO.

Figure 5. Changes of XRD patterns of TMA+-intercalatedsamples at different concentrations dried at 70 C for 3 days.(a) TMAl/Hs ) 1 in wet state; (b) TMAl/Hs ) 1; (c) TMAl/Hs )5; (d) TMAl/Hs ) 10; (e) TMAl/Hs ) 25.

Figure 6. XRD patterns of TMA+-intercalated samples underdifferent drying conditions. (a) TMA+-intercalated sample atTMAl/Hs ) 5 in wet state; (b) sample a dried at 25 C for 1 day;(c) sample b dried on silica gel for 1 day; (d) sample c dried at70 C for 1 day; (e) sample d vacuum-dried at 70 C for 1 day;(f) sample e exposed to saturated humidity at 25 C for 1 day;(g) sample e exposed to saturated humidity at 25 C for 5 days.


is a reversible reaction. This is similar to those of clayminerals such as montmorillonite and smectite whichswell and exfoliate in wet state by some soft-chemicalprocedures, returning to a layered structure in a driedstate.24,25

Effect of Water Washing and Acid Treatment. Thecolloidal suspension prepared at TAAl/Hs ) 5 was cen-trifuged and then washed 3 times with 30 mL of distilledwater, and the sediments were dried at 25 C for 2 days.The XRD patterns of the dried samples show that thebasal spacings of all compounds decrease to around 1 nmby water washing, due to the partial deintercalation ofthe TAA+ ions from the interlayers. The basal spacing ofthe TMA+-intercalated layer is equal to that obtaineddirectly from the solution with TMAl/Hs ) 1. The totalnitrogen analysis showed that 1.2 mmol/g TMA+ ionsremained in the compound, which means that 70% ofTMA+ ions were deintercalated by water washing.

SEM images of water-washed samples are shown inFigure 8. The particle morphology of the GO nano-composite is dependent on preparation procedure. Theair-dried sample has a distinct lamellar morphology withthin ribbons of about 2 m in width lined up in a regularparallel arrangement (Figure 8a). On the other hand, thefreeze-dried sample has a thin flaky appearance withthickness less than 0.5 m (Figure 8b). These resultsindicate that the significant reassembling of GO nano-sheets progresses during air-drying with each nanosheetstacked regularly to maintain minimum energy conditions.

The TMA+-intercalated compound obtained at TMAl/Hs ) 5 was treated with a 0.1 M HCl solution at 25 C for2 days to extract TMA+ ions from the interlayer. The XRDpattern (Figure 7e) shows that the layered structure

remains after TMA+ extraction, but the intensity of thepeak becomes weaker and broader. In addition, the basalspacing decreases to 0.72 nm, which does not coincidewith that of the starting GO. This suggests that somedestruction or rearrangement of layers takes place duringthe acid treatment.

DTA-TG Analysis. TG-DTA curves of the starting GO,TMA+-intercalated GO, TMA+-intercalated GO afterwater washing, and TMA+-intercalated GO after acidtreatment are shown in Figure 9. Two exothermic peaksat 213 and 521 C are observed for the starting GO, similarto the results by several authors.12,15,19,26 The former isattributed to the destruction of the carbonyl group, andthe latter to the combustion of the carbon skeleton of GO.19For TMA+-intercalated GO, small exothermic peaks areobserved at 163 and 313 C below 400 C. The weight lossbetween 120 and 400 C is 53%, which is 1.5 times largerthan that (34%) for the starting GO. The large weight lossmay be due to the destruction of TMA+ ions in theinterlayer in addition to the destruction of carbonyl groupsof GO. Water washing causes the deintercalation of 70%of TMA+ ions, so the weight loss between 120 and 400 Cis due to the decrease of TMA+ content. The acid-treatedsample shows the smallest weight loss in this temperaturerange; this suggests that some of the carbonyl groups ofGO are destroyed by the acid treatment.(24) Norrish, K. Faraday Discuss. R. Soc. Chem. 1954, 18, 120.

(25) (a) Smalley, M. V. Langmuir 1994, 10, 2884. (b) Smalley, M. V.;Thomas, R. K.; Braganza, L. F.; Matsuo, T. Clay Clays Miner. 1989, 37,474.

(26) Dekany, I.; Kruger-Grasser, R.; Weiss, A. Colloid Polym. Sci.1998, 276, 570.

Figure 7. Changes of XRD patterns of TAA+-intercalatedsamples at TAAl/Hs ) 5 by washing and after drying at 25 Cfor 1 day. (a) TMA+-intercalated GO; (b) TEA+-intercalatedGO; (c) TPA+-intercalated GO; (d) TBA+-intercalated GO; (e)TMA+-intercalated GO acid-treated for 2 days.

Figure 8. SEM images of TMA+-intercalated GO nano-composites: (a) washed with water followed by air-dried; (b)washed with water followed by freeze-dried.


Structural Model for the Intercalation-Deinter-calation. Schematic representation of structural changeduring the intercalation-deintercalation reaction is givenin Figure 10. Pristine GO has a layered structure with a

basal spacing of 0.88 nm. It is well-known that the basalspacing of GO varies widely between 1.1 and 0.6 nmdepending on ambient humidity, which has made itdifficult to determine the ideal structure of GO. Two typesof model have been proposed for the structure of GO: oneis the stage-1 type with Ic ) 0.6 nm7 and the other is thestage-2 type with Ic ) 0.55-0.82 nm.5 The present resultis close to the stage-2 type structural model of wellhydrated GO (basal spacing ) 0.82 nm). Therefore, wemade our structural model based on the stage-2 type. Sincethe thickness of GO is known to be 0.61 nm,26 the galleyheight can be calculated as 0.27 nm, being close to the vander Waals diameter (0.28 nm) of water molecules. Thissuggests the formation of one molecular layer of water inthe interlayer of GO. The intercalation of TMA+ resultsin a short-range swelling in the region 0.3 < R < 0.8; thebasal spacing widens a little to 0.98 nm by the intercala-tion. The galley height can be calculated as 0.37 nm, whichis close to the ionic size (0.44 nm) of TMA+ ions. Sincethere are no water layers between TMA+ ions and GOsheets, it is stable against drying. The stacked GO layersare exfoliated into nanosheets in the region R > 0.9. Thelayered structure disappears in the XRD patterns, andonly halos are observed.

The swelling and exfoliation behavior of layered ma-terials can be classified into three categories: short-rangeswelling, long-range swelling, and exfoliation. The short-range swelling is characterized by the formation of hydratelayers in the interlayer. The long-range swelling isassociated with the formation of diffuse double layers andthe consequent change of the electrostatic attractive forceto an osmotic repulsive one. Exfoliation is regarded as thelimitingcaseof long-rangeswelling,where theelectrostaticrepulsive force between neighboring electrical doublelayers occurs even at a very great distance. Long-range

Figure 9. TG-DTA curves of (a) starting GO, (b) TMA+-intercalated GO, (c) TMA+-intercalated GO washed withdistilled water, and (d) TMA+-intercalated GO acid-treated.

Figure 10. Schematic representation of changes of crystal phase and basal spacing of wet and dried TMA+-intercalated GOcompounds.


swelling was not observed in the present system. The sharpbasal diffraction series with intersheet spacing of above2 nm was not observed in a low angular range. Theflexibility of the present sheets may leave the long-rangeswelling incomplete, resulting in a simultaneous ex-foliation with irregularly arranged GO sheets. Similarresults have been observed for TMA+ intercalation intolayered manganese oxide.23 When the exfoliated slurry isdried at room temperature for 1 day, reassembling takesplace and the layered structure with a basal spacing of1.64 nm reappears. The basal spacing corresponds to thedistance that two molecular layers of water occupy in theinterlayer. Drying at 70 C results in a slight decrease ofbasal spacing to 1.56 nm, which corresponds roughly tothe presence of one molecular layer of water. Thedehydration-rehydration reaction on TMA+-intercalatedGO progresses reversibly depending on the relativehumidity of the atmosphere, accompanied by the ex-foliation reaction. These results show that the TMA+-intercalated GO exhibits reversibly short-range swellingand exfoliation, similar to clay minerals.

The water washing caused a deintercalation of TMA+from the interlayer to give a layered compound with abasal spacing of 0.97 nm. This phase can be described asa stable phase since it forms in both the intercalation anddeintercalation reactions. Acid treatment results in thedeintercalation of most of TMA+ ions in the interlayer.The acid-treated sample retains the layered structure,but the intensities of the diffraction peaks become weakerand broader. The resultant basal spacing does not coincidewith that of the starting GO. This suggests that somedestruction or rearrangement of nanosheets takes placeduring the acid treatment.

The schematic models of TAA+-intercalated GO com-pounds are depicted in Figure 11. The intercalation of

TAA+ ions results in the exfoliation of GO nanosheetsindependent of the species of TAA+ ions. The drying of theexfoliated slurry at 70 C results in the reassembling ofnanosheets in stacked layers, with the interlayer distancedepending on the species of TAA+ ions. The structuralmodel was calculated on the basis of the dimension of theGO layer, the van der Waals size of water molecules andthe size of the TAA+ ions. The ionic size of TAA+ ions,evaluated by semiempirical MO calculations with aMOPAC program, are 0.44, 0.65, 0.85, and 1.08 nm forTMA+, TEA+, TPA+, and TBA+ ions, respectively. Thegalley heights are therefore calculated to be 0.95, 1.06,1.23, and 1.76 nm for the TMA-GO, TEA-GO, TPA-GO, and TBA-GO composites, respectively. The numbersof molecular layers of water are calculated as one for thefirst three composites and two for the last. This indicatesthat the basal spacing of the layered compound exhibitingshort-range swelling depends mainly on the ionic size ofthe guest ions.

Conclusion

The GO compound shows swelling and exfoliationbehavior during the intercalation of tetraalkylammoniumions, depending on the neutralization degree of the GOsheet and the species of the guest ammonium ions. Theexfoliated nanosheets in wet state reassemble into stackedlayers following air-drying with the interlayer distancedepending on the size of guest ions. The exfoliation-reassembling process is attractive since it is applicable tothe development of novel nanocomposite materials ornanoporous materials.

LA011677I

Figure 11. Structural model of TAA+-intercalated GO materials.


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Intercalation of Organic Ammonium Ions Into Graphite Oxide