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    Layer-by-Layer Assembly of Ultrathin Composite Filmsfrom Micron-Sized Graphite Oxide Sheets and

    Polycations

    Nina I . Kovtyukhova,*, P a tricia J . Ollivier , B e njam in R . Mar t in ,

    Thomas E. Mallouk,*, Sergey A. Chizhik, Eug e nia V. B uz a ne va, | a n dAlexand r D. G orchinskiy|

    I n st i tu te o f S u r fa ce Ch emi str y, N a ti o n a l A ca d emy of S ci en ces of U kr a i n e, 3 1 , P r . N a u ky,

    2 5 20 2 2 K yi v, U kr a i n e; D ep a r tm en t of Ch emi str y, Th e P en n syl va n i a S ta te Un i ver si ty,U n i ver si ty P a r k, P en n syl va n i a 1 6 80 2 ; M eta l -Polymer Research Institute, 32A Kirov Street,

    G omel , 2 4 66 5 2, B el a r u s; a n d N a ti o n a l T. S h evch en ko U n i ver si ty, 6 4 , V l a d i m i r ska ya S tr .,2 52 03 3 K y i v , U k r a i n e

    Receiv ed N ovem ber 24, 1998. Revised M anu scr ip t R eceiv ed D ecem ber 28, 1998

    U nilam ellar colloids of gra phite oxide (G O) w ere prepared from na tur a l graphit e and w ereg ro wn a s mo n o la ye r a n d mu lt i la ye r t h in f i lms o n ca t io n ic su rfa ce s by e le ct ro st a t ic se l f-assembly. The multilayer films were grown by alternate adsorption of anionic GO sheetsa nd cat ionic poly(a llyla mine hy drochloride) (P AH). The monolay er films consist ed of 11-14 thick GO sheets, with lateral dimensions between 150 nm and 9 m. Silicon substrates

    primed with a mine monolay ers gave part ial G O monolayers, but sur faces primed with Al13O4-(OH)24(H 2O)127+ ions gave densely tiled films tha t covered a pproximat ely 90%of the sur face.When alkaline GO colloids were used, the monolayer assembly process selected the largestsheets (from 900 nm to 9 m) from the suspension. In this case, many of the flexible sheetsa ppea red folded in AFM ima ges. Multila yer (GO/P AH)n f ilms were invariably thicker thanexpected from the individual thicknesses of the sheets and the polymer monolayers, andthis behavior is a lso at t ributed to folding of the sheets. Multilayer (GO/P AH)n a n d (GO/polyaniline)n f ilms g rown bet w e en in diu m-t in ox ide a n d P t e le ct ro des sh ow diode likebehavior, a nd higher current s a re observed w ith t he conductive polya niline-conta ining films.The resisitivity of these films is decreased, a s expected, by part ial r eduction of GO t o ca rbon.

    Introduction

    The assembly of composite thin films from lamellarinorga nic part icles a nd organic macromolecules is aninexpensive and versa tile route to functional na nometer-scale structures.1 Th es e f il ms a r e g r ow n b y a w e tchemical, layer-by-layer adsorption method, which iss i m i l ar t o t hat dev el oped earl i er b y D ec her and c o-workers for organic polyelectrolytes.2 The addition ofinorganic components offers the possibi l i ty of addedoptical, electronic, magnetic, mechanical, and thermalpropert i es t hat m ay b e di f f i c ul t t o ac hi ev e b y usi ngorganic polymers alone. Additionally, the continuousinorga nic sheets provide a barrier to interpenetra t ionof sequent ially deposited polymer or nan opa rt icle la yers,a feature tha t can be importa nt in thin fi lms intended

    to act as current recti fiers, photodiodes, or Coulombblockade devices. Several electronic and photonic ap-

    plications a long t hese lines ha ve established the ut ilityof this method.3

    The layer-by-layer assembly method rel ies on theexfolia tion of solids t o produce colloids of sh eets, w hichtypical ly have nanometer thicknesses and lateral di-mensions of tens of na nometers to microns. Exfoliat ionprocedures based on ion exchange and redox reactionshave been developed for a variety of lamellar sol ids.4

    Thus, multi lay er inorgan ic/organic f i lms have beengrown from lamellar metal phospha tes, t i tana tes, nio-bates,5 silicates,1b an d meta l chalcogenides,6 compoundswhich in bulk form possess transport properties rangingfrom insulating to semimetallic. Recently, Fendler andc o- w orkers hav e show n t hat suc h f i l m s c an a l so b e

    National Academy of Sciences of Ukraine. The Pennsylvania State Universi ty. M e t a l-Polymer Research Inst i tute.| National T. Shevchenko University.(1) (a) Iler, R. K. J. Colloid I nterfaceSci. 1966, 21, 569. (b) Klein feld,

    E . R . ; F e r g u s o n , G . S . Science 1994, 265, 370. (c) Fendler, J . H. ;Meldrum, F. Ad v . M a t er . 1995, 7, 607. (d) Mallouk, T. E.; Kim, H.-N.;O ll iv i er , P . J . ; K e ll er , S . W. I n Comprehensive Supr amolecul arChemistry; Alberti , G., Bein, T., Eds.; Elsevier Science; Oxford, UK,1996; Vol. 7, pp 189-218.

    (2) Decher, G. Science1997, 277, 1232.

    (3) (a) Ka schak, D. M.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118,4222. (b) Feldheim, D. L.; Grabar, K. C.; Natan, M. J . ; Mallouk, T. E.J. Am. Chem. Soc. 1996, 118, 7640. (c) Keller, S. W.; J ohnson, S. A.;Yonemoto, E. H. ; B righam, E . S . ; Mal louk, T. E . J. Am. Chem. Soc.1995, 117, 12879. (d) Ca ssag neau , T.; Ma llouk, T. E .; Fendler, J . H. J .Am. Chem. Soc. 1998, 120, 7848.

    (4) (a) J acobson, A. J . M a t e r . Sc i . F o r u m 1994, 152-153, 1. (b)J acobson, A. J . In Comprehensive Supram olecular Ch emi stry; Alberti,G., Bein, T., Eds.; Elsevier Science; Oxford, UK, 1996; Vol. 7, pp 315-335.

    (5) (a) Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. A m. Ch em. Soc.1994, 11 6, 8817. (b) Fang, M., Kim, C.-H.; Saupe, G. B.; Kim, H.-N.;Waraksa, C. C.; Miwa, T.; Fujishima, A.; Mallouk, T. E. Submitted toChem. M ater.

    (6) Ollivier, P . J . ; Kovtyukhova, N. I .; Keller, S. W.; Mallouk, T. E.J. Chem. Soc., Chem. Comm un. 1998, 1563.

    771Chem. M ater. 1999, 11 , 771-778

    10.1021/cm981085u CCC : $18.00 1999 America n Chemica l SocietyP ubl ish ed on Web 01/28/1999

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    grown from graphite oxide (GO), 7 which can subse-quently be reduced electrochemically to make electroni-cal l y conduct i ng graphit i c f i lm s. I n t heir w ork, GOnanoparticles were prepared from synthetic graphite.They not ed t hat t he m ult i layer f il m s c onsis t ed ofi ncom plet el y exfol ia t ed plat el et s t hat w ere t ens ofnan ometers in t heir latera l dimensions. In this paper,we revisi t the assembly of GO/polycat ion thin fi lms,using GO prepared from natu ra l crysta ls. We show th at

    exfoliat ed GO d erived from these crysta ls is a mechan i-cal l y rob ust m at eri a l t hat deposi t s conform al l y oncationic surfa ces a s micron-sized, na nometer-thick sheets.

    G ra phite oxide is a pseudo-tw o-dimensional solid inb ul k form , w i t h s t rong c ov al ent b ondi ng w i t hi n t helayers. Weak interlayer contacts are made by hydrogenbonds between intercalated water molecules. 8-11 Thecarbon sheets in GO contain embedded hydroxyl andcarbonyl groups, as well as carboxyl groups si tuatedmainly at the edges of the sheets.8,9,12 While there is noc onsensus as t o t he prec i se s t ruc t ure of GO l ayers ,different structural models,9a,b,10 wh ich correspond t o ani deal form ula of C 8O2(OH)2, hav e b een adv anc ed. Arecent st udy of the structure of GO a rgues from 13C and

    1H NMR evidence for the presence of epoxy groups. 13Nakajima and co-workers have proposed that the carbonlayers in GO are l inked together in pairs by sp 3 C-Cbonds perpendicular to the sheets. 10 According to Kli-nowski et al . ,13 t he c arb on l ayers i n GO c ont ai n t w okinds of domains, aromatic regions with unoxidizedbenzene rings an d a lipha tic regions w ith six-memberedcarbon rings. The r elat ive size of the domains, wh ichare random l y di s t r i b ut ed, depends on t he degree ofoxidat ion. I n both models, th e hyd roxyl groups projectabove and below the carbon grid. The phenolic hydroxylgroups are acidic and, together with the carboxyl groups,are responsible for the negative charge on the GO sheetsin aqueous suspensions.9,13 The surfa ce cha rge density

    of colloida l G O pa rt icles (degree of oxidat ion 85%) wa smea sured by F endler a nd co-worker s a s 0.4 per 100 2.7

    Th e G O i nt e rl a y er d is t a n ce i s n ot con s t a n t a n ddepends strongly on the GO:H 2O rat i o .8-10,15 I n v erydilute a queous suspensions, t he interlayer distance islarge, so intera ction between t he la yers is sufficientlyweak that exfol iat ion occurs. 8 Our previous researchshowed that the number of layers in the GO colloidalpar ticles ca n be contr olled by th e dilution of the suspen-sions.16,17 The a dsorption capa city for Cu(II) ions, w hichw a s for b ot h aqueous suspensi ons16 a n d t h in f il ms

    deposited from these suspensions on powder supports(ZnO, Al2O3, fumed SiO2),17 increased with decreasingGO concentrat ion in the start ing suspension. For ex-am pl e, t he adsorpt i on c apac i t y of GO f i l m s on Si O 2increased by a factor of 2.4 when th e GO concentra tionin the start ing suspension was decreased from 1.0 to0.3 g/L. This r esult s hows th a t m ore complete exfoliat ionprovides increased a ccess to th e GO functional groups.For sa mples prepar ed from t he col loids w ith G O con-

    centrations of 0.02-0.3 g/L, the ma ximum a dsorptioncapacity, 22-24 mmol/g, w a s obta ined. This va lue isclose to the total quantity of oxygen-containing groupsin G O (25 m mol/g9) and gives indirect evidence thatdilute GO colloids are exfoliated as monolayers.

    We report here a detai led study of the preparationa nd cha ra cterizat ion of G O/polycation films grown onp l a n a r S i a n d A l2O3/Al supports. U sing a tomic forcemicroscopy (AFM), ellipsometry , a nd electrical mea-surements, t he following q uestions ha ve been ad dressed:

    What does the first layer of the sheets adsorbed on asubstra te look like microscopica lly?

    C an w e af fect t he q ual it y of m ono- and m ult i layerfilms by varying the conditions of their deposition and

    the chemical composition of the substrate surface?Wha t a re the electronic properties of G O/polycation

    films, and how are they influenced by the nature of thepolycation?

    Experimental Section

    Materials. G O w a s s y n t h es i ze d f r om n a t u r a l g r a p hi t epowder (325 mesh, G AK-2, U kraine) by t he method of Hum-m e r s a n d O f f e m a n . 18 I t w a s f o u n d t h a t , p r i o r t o t h e G Oprepar at ion a ccording to ref 18, an a ddit ional gra phite oxida-tion procedure was needed. Otherwise, incompletely oxidizedgra phite-core/G O-shell par ticles were a lwa ys observed in thefinal product. The gra phite powder (20 g) wa s put into a n 80C solution of concentrated H 2S O4 (30 mL), K 2S 2O8 (10 g), and

    P 2O5 (10 g). The resultant dark blue mixture was thermallyisolated a nd a llowed to cool to room temperat ure over a periodof 6 h. The mixture was then carefully diluted with dist illedwater, f iltered, and washed on the filter until the rinse waterpH became neutral. The product was dried in air at ambienttemperature overnight. This preoxidized graphite was thensubjected to oxidation by Hummers method. The oxidizedgraphite powder (20 g) wa s put into cold (0 C) concentra tedH 2S O4 (460 mL). KMnO 4 (6 0 g ) w a s a d d ed g r a d u a ll y w i t hst ir r ing and cooling, so t ha t t he t e mp e rat u r e of t he mixt u r ewa s not allowed to reach 20 C. The mixture wa s then st irredat 35 C for 2 h , and d is t il led w at e r (920 mL ) wa s a d d ed . I n15 min, the reaction wa s terminat ed by the addit ion of a lar geamount of dist illed water (2.8 L) and 30%H 2O2 solution (50mL), after which the color of the mixture changed to brighty e llow. The mixt u r e wa s f i l t e r ed a nd w ashe d w it h 1: 10 H C lsolution (5 L) in order to remove metal ions. The GO productwa s suspended in dist illed wa ter to give a viscous, brown, 2%d ispe r sion, w hich wa s su b je ct e d t o d ia ly sis t o c omp let e lyr e move me t al ions and ac id s. The r e sult ing 0.5% w/v GOdispersion, which is stable for a period of years, was used toprepare exfoliated G O.

    Exfoliation was achieved by dilution of the 0.5%GO disper-sion (1 mL) with deionized water (24 mL), followed by 15 minsonicat ion. The result ing homogeneous yellow-brown sol,wh ich conta ined 0.2 g/L G O, wa s sta ble for a period of mont hsand wa s u se d for f i lm p r ep ar at ion.

    An aq ueous solution (0.01 M) of poly(allyla mine h ydrochlo-ride), PAH, (Aldrich, MW ) 50 000-65 000) was adjusted t op H 7 wit h N H 3 and was used for growth of polycation layers.

    (7) (a) Kotov, N. A.; Dekany, I.; Fendler, J . H. Ad v . M a t er . 1996, 8,637. (b) Ca ssa gnea u, T.; Fend ler, J . H . Ad v . M a t er . 1998, 10, 877.

    (8) (a) Thiele, H. Ko l l o i d - Z 1948, 11 1, 15. (b) Croft, R. C. Q u a r t .Rev. 1960, 14, 1.(9) (a) Scholz, W.; Boehm, H. P. Z. Anorg. Allg.Chem. 1969, 36 9,

    327. (b) Clauss, A.; Boehm, H. P.; Hofmann, U. Z. Anorg. Allg.Chem.1957, 291, 205.

    (10) Naka jima, T.; Mabuchi, A.; Hagiw ara , R. Carbon 1988, 26, 357.(11) Ka rpenko, G.; Turov, V.; Kovtyukhova, N.; Ba kai, E.; Ch uiko,

    A. Theor. Exp. Chem. (Russ.) 1990, 1, 102.(12) Hadzi, D.; Novak, A. Trans. Faraday. Soc. 1955, 51, 1614.(13) Lerf, A.; He, H.; Forster, M.; Klinowski, J . J. Phys. Chem. B

    1998, 102, 4477.(14) Hennig, Z. Progr. I norg.Chem. 1959, 1, 125.(15) La gow, R. J . ; Ba dachha pe, R. B.; Wood, J . L.; Marg ra ve, J . L.

    J. Chem. Soc. Dalton 1974, 1268.(16) Kovtjukhova, N. I.; Karpenko, G. A. M ater. Sci. Forum 1992,

    91-93, 219.(17) (a) Kovtyukh ova, N. I .; Chu iko, A. A. Abstracts; Fal l Meeting

    of the Ma teria ls Research Society, 1994, Boston, C9.6. (b) Kovtyukh ova,N. I . ; B uzaneva, E. V. ; Senkevich, A. Carbon 1998, 36, 549. (18) H ummers, W.; Offema n, R. J. A m. Chem. Soc. 1958, 80, 1339.

    772 Ch em . M at er ., V ol . 11, N o. 3, 1999 K ovt yu k h ova et al .

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    An aqueous solution of doped polyaniline (PAN) was madefr om a sat u r a t e d solu t ion of t he e me r ald ine b ase for m indimethylforma mide. A 3 mL portion of this solution wa s slowlyadded w ith stirr ing to 26 mL of wa ter, wh ich had been acidifiedto pH 3.5 with aqueous HCl. The pH of the final PAN solutionwa s then a djusted to 2.5 by addit ion of aqueous HCl. The alu-minum Keggin ion Al13O4(OH)24(H 2O)127+ wa s p r e par e d t r omAl13O4(OH)25(H 2O)11(S O4)3x(H 2O ), w hich wa s a vai lab le f r oma previous study.5 Briefly, 0.102 g of the sulfate salt was addedto a solution of 0.042 g of Ba Cl 2 in 200 mL of wat er and st irredovernight. The resulting 0.3 mM solution of the chloride salt

    of the aluminum K eggin ion w as filtered using a 0.2 m filter.P olished (100) Si w afers w ere sonicated in C Cl 4 for 15 min

    and then rinsed wit h 2-propanol and wa ter. Their surface wasthen hydr oxylat ed by 30 min sonication in piranha solution(4:1 concentra ted H 2S O4:30% H 2O2) (CAUTION: p ir a n h asolution reacts violently with organic compounds!) and wasrinsed sequentially with water, methanol, and 1:1 methanol/toluene before the surface derivit ization st eps began.

    Aluminum foil, Al-coa ted gla ss, both bearin g a n at ive oxide,and I TO g lass we r e c le ane d b y w ashing w it h he xane for 15min prior to GO adsorption.

    Multilayer GO/PAH Film Growth. Hydroxylat ed siliconwa fers were primed wit h one of three different types of cationicmonolay e r s in or d e r t o ini t ia t e t he g r owt h of t he GO f i lms.This was achieved either by (1) reacting with 4-((dimethyl-methoxy)silyl)butyla mine (15 h treatm ent w ith a 5%toluenesolution under dry Ar, over KOH at am bient temperature) orby (2) adsorbing a monolayer of aluminum Keggin ions (5 minadsorption from aq ueous solution of the chloride salt at 80 C 19)or by (3) adsorbing PAH (15 min adsorption from a 0.01 Maq u e ous solu t ion a t p H 7 a nd amb ie nt t e mp e r at u r e ). Theprimed Si substra tes (1, 2, and 3) ar e designat ed hereaft er asSi(NH 2), Si(OH )/Al-Kg , a nd S i(OH)/P AH, r espectively .

    The primed substrates were immersed in an aqueous (pH5) or a queous a mm onia (pH 9) GO sol (0.2 g/L) for 15 min a ndthen rinsed w ith deionized w at er and dried in flowing Ar. Thesamples were then immersed in aqueous PAH solutions for15 min, r insed w ith deionized w at er, and dr ied in flowing Ar.Mu lt ilay e r GO /P AH f i lms we r e g r own b y r e pe at ing t he seadsorption cycles. Preliminary experiments had shown thatthe thickness of a deposited layer (estimated by ellipsometry)does not depend on th e substra te/solution conta ct time in th er a n g e f r om 2 m i n t o 2 h . F or e le ct r on i c m e a s u r em e nt s ,mult ilay er (GO/P AH)14 a nd (G O/P AN)30 films were grown onITO in similar adsorption cycles. For comparison purposes, aGO colloid film (ca. 90 nm thick) was prepared by dip-coatingthe I TO/gla ss in the G O colloida l dispersion.

    Characterization. Atomic force microscopy (AFM) imagesof the layers deposited on Si substrates were obtained with aDigital Instruments Nanoscope IIIa in tapping mode, using a3045 J VW piezo t ube sca nner. The 125 m etched Si cantile-vers had a resonant frequency between 250 and 325 kHz, a ndthe oscillat ion frequency for scan ning w as set to 0.1-3 kHzbelow resonance. Typical images were obtained with line scanr at e s of 2 H z w hile 256 256 pixel samples were collected.

    E ll ip some t r ic me asu r e me nt s we r e mad e wit h a Gae r t ne rmodel L2W26D ellipsometer. An analyzing wavelength of 632nm was u se d , b e c au se GO ab sor b s minimally a t t his wave -length. The incident angle was 70 and the polarizer was setat 45 . Ellipsometric para meters were measur ed following eachGO or P AH ad sor p t ion s t e p . Si su b st r at e s we r e d r ie d in anAr str eam before each mea surement. The film th ickness of theGO /P AH mu lt i lay e r s wa s calc u lat e d u sing t he Si r e fr ac t iveindices, ns ) 3.875 an d ks ) -0.018, determined from a blanksam ple. The refractive index of G O/PAH films w as estimat eda s nf ) 1.540, kf ) 0.

    Transmission electron microscope (TEM) images were ob-t aine d wit h a J E O L 1200 E XI I micr oscope a t 120 kV a c -ce le r at ing volt ag e . S amp le s w e r e p r ep ar e d b y imme r sing acopper grid in the GO sol and drying in air .

    The elemental composition of GO was determined by usinga home-built mass spectrometer with laser probe (LMS). Thediameter of the crat er for single laser impulse wa s 0.42-0.48mm, and t he d e pt h w as 1 m. IR spectra were recorded usinga Perkin-Elmer 325 instrument. XPS spectra were obtainedusing a Kratos Series 800 spectrometer with h)1253.6 eVand an analy zing wind ow of 4 6 mm 2. The accuracy of themeasured core level binding energies (Eb) wa s 0.1 eV. For LMS,IR, an d XPS experiments, GO sam ples were prepared a s filmsby drying a droplet of the sol in air at ambient temperature.X-ray powder diffraction (XRD) patterns were recorded with

    a D RO N -1 inst r u me nt u sing C u KR r a d i a t i on . P r i or t o t h emeasurement, the GO sample was dried in a vacuum over P 2O5for 24 h.

    Electrical measurements of f ilms deposited on ITO glasswere carried out using top Pt electrode conta cts tha t w ere 10m in diam eter and m echan ically pressed into the film, usinga home-built para metric an alyzer. The sensit ivity of currentmeasurements was 0.01 nA. All measurements were carriedout in air at ambient temperature. The turn-on potential foral l t hin f i lm d e vic e s s t u d ie d was t ake n as t he p ot e nt ia l a twhic h a c u r r e nt of 1 . 0 nA was ob se r ve d . I n r e g ions whe r ec ur r e nt w as mor e t han 3 nA, e ve r y s t e p of volt ag e inc r ease(typically 0.1 mV in both the forward and reversed directions)was followed by repeating the measurement cycle to ensuret he r e pr odu cib il i t y of t he cu r r e nt me asu r e d a t t he lowe rvoltage. Measurements were considered irreversible (i.e. , a

    permanent change to a more conductive sta te occurred) whenthe current recorded the second t ime at the lower voltage w asnoticeably higher than that measured in the previous cycle.

    Results and Discussion

    Characterization of the GO Colloid. The XRDpat tern of GO prepa red by preoxidat ion with persulfat efol low ed b y oxidat i on w i t h perm angana t e reveal s asha rp 002 reflection a t 2 ) 12.80, which correspondst o a c-a xis spacing of 6.91 This va lue falls w ithin t hera nge of 6.3-7.7 reported in t he literat ure9a,10,20,21 forGO prepared from nat ura l graphite according to Hum-mers method.18 No 002 diffraction peak from unreactedgraphite (d ) 3.35 ) is observable in t he XRD pat tern.

    The IR spectrum of G O prepared by this method isessent ia l l y i dent ical t o t hat report ed i n t he l it era-ture.9a,12,21 A b a n d a t 3 4 2 0 c m-1 a n d a b r o a d b a n dcentered ar ound 3220 cm-1 a r e a t t r ibu t ed t o O-Hstretching vibrat ions of th e C-OH groups and w at er ,respectively; a band at 1730 cm-1 is assigned to CdOst ret chi ng v ib rat i ons of t he carb onyl and carb oxylg r ou ps . B a n d s a t 1365, 1425, a n d 1615 cm-1 a reassigned to the O-H deformations of C-OH groups a ndwa ter, respectively. A ban d a t 1080 cm-1 is due to C-Ostretching vibrations.

    Deconvolution of the C1s peak in the XPS spectrumshows th e presence of four t ypes of carbon bonds: C-C(284.8 eV), C-O (286.2 eV), CdO (287.7 eV), and O-Cd

    O (288.5 eV). B y int egra ting the a rea of the deconvolu-tion peaks, the following approxima te percenta ges wereob t ai ned: C-C, 49.5; C-O, 31.4; CdO, 9.1; O-CdO,2.9.

    The LMS spectrum of GO prepared by this methodga ve th e following a tomic composition (wt %): H, 2.3;C, 45.2; O, 46.5; P , 3.3; K, 2.7; C:O r a tio ) 1.3. The sameor nearly t he sam e C:O rat io ha s been found previouslyfor G O samples prepared from na tura l graphite.21,22 I tis generally accepted that the conversion of graphite to

    (19) Schoenherr, S.; Goerz, G.; Mueller, D.; Gessner, W. Z. Anorg.Allg. Chem. 1981, 47 6, 188.

    (20) Carr, K. E. Carbon 1970, 8, 245.(21) Kyotan i, T.; Suzuki, K.; Ya mash ita, H .; Tomita, A. Tanso1993,

    160, 255.(22) Slabaugh, W. H.; Sei ler, B . C. J. Phys. Chem. 1962, 66, 396.

    A ssem bl y of U lt r at h i n Com posi t e F i l m s Ch em . M at er ., V ol . 11, N o. 3, 1999 773

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    GO is complete when the C:O ratio becomes 2.0. Theobserved ratio C:O:H ) 4:3.1:2.5 is richer in O and Ht han t hat cal cul at ed for C 8O2(OH)2: 4:2:1; this can beexplained by the presence of intercala ted/adsorbedwa ter and car boxyl groups, as shown by IR and XP S. I tshould be noted th at the ideal formulation does not ta ke

    into a ccount t he presence of car boxyl groups, which a remainly si tua ted on the edges of the sheets,8 or interca-l a t e d w a t e r , s o m e a m o u n t o f w h i c h i s p r o b a b l y a nintegral part of the GO structure. 3 Allowing that 2.9%of the car bon at oms are present a s carboxyl groups (fromXPS) and assuming tha t t he pota ssium ions are incor-porat ed by ion-exchange, we calculate a formula ofC 3.77O2.05H 0.92K 0.070.73H 2O, or C 8O2.25(OH)1.95(OK)0.151.55H 2O, which gives a C:O:H ratio of 4:2.95:2.5. Therema ining oxygen (2.05 w t %) is most probably boundto phosphorus, an impurity introduced by the graphitepreoxidat ion step.

    TEM imag es of th e G O sol (Figure 1) reveal flexible,

    wrinkled sheets of di fferent latera l sizes ra nging fromhundreds t o t housands of nanom et ers . Fl exi bl e G Oparticles were also observed by Hennig and Carr. 14,20

    No graphite par t icles a re observed in t hese images.

    AFM Images of the First Adsorbed GO Layer. SiSubstrates. AFM imag es (Figure 2a-c) of the G O filmsdeposited in one adsorpt ion cycle from aq ueous solutionshow sur face covera ges of about 30%for S i(NH 2), 85%for S i(OH )/P AH, a nd 90%for S i(OH)/Al-Kg subst ra tes.The ma in feat ures a re 150-900-nm-wide islands, whosesi z e i s c l ose t o t hat det erm i ned b y TEM for t he GOsheets. In some cases, corruga tions a nd the turned-inedges of the sheets are seen. For the PAH-primed Sisubstrat e, the avera ge roughness of t he sheet-covered

    part of the surfa ce is 4.5 By comparison, the rough-ness of th e Si(OH)/P AH su bstr a te w a s 6.2-, indicat ingsl ight smoothing of th e surface by t he G O monolayer.The height of the islan ds on S i(NH 2) a nd Si(OH)/P AH ,10.6-14.1 , an d the a verag e roughness of their sur face,3.9-4.5 , are consistent with exposure of GO basalplanes cov ered b y adsorbed H 2O. 8-10 Ellipsometricmeasurements gave 11 and 14 thicknesses for GOmonolayers on Si(NH 2) and Si(OH)/P AH. Consid ering

    that the thickess of the priming layer is about 7 , andthat the GO sheets cover only part of the surface, theseresul t s are i n reasonab l e agreem ent w i t h t he i s l andheights measured by AFM.

    According to Na kajimas str uctura l model,10 the thick-ness of a GO m onol ayer depends on t he c ont ent ofhydroxyl groups on i ts basa l planes an d can reach 8.2 for the completely hyd roxylat ed monolayer. Assumingthe presence of completely hydroxylated GO carbonlayers in very dilute colloids, one can take the thicknessof a GO monolayer as 8.2 . By comparing this valuew i t h t he hei ght of t he i s l ands i n Fi gure 2, one c anconclude tha t the islands consist of a single G O sheetcovered by a layer of adsorbed water molecules. The

    thickness of doubled GO layers can be estimated fromthe thickness of two GO monolayers, 2 8.2 , plusthe distance between the layers, which is determinedby the thickness of the layer of weakly bound mobilewater molecules.11,13 This interlayer distance can beestimated a t a bout 3 , from th e repeat distance alongt he c-axis of well-hydra ted GO samples, Ic )11 ,9,14

    minus the thickness of a GO monolayer, 8.2 . Thismeans the thickness of doubled GO layers should beabout 20 or more, if wa ter a dsorbed onto the top basa lplane is considered. This value is significantly greaterthan the height of adsorbed islands (10.6-14.1 ). Theheight, 20.1 , of the G O island s a dsorbed onto the Al-Keggin-primed Si surface (Figure 2b) is roughly consis-

    tent wit h th e thickness of the Al-Keggin a nchoring layer(7 23) a n d a m on ol a y er of G O s h ee t s c ov er ed b yadsorbed H 2O (10.6-14.1 ).

    An AFM image of the first GO layer adsorbed froma n a queous am monia suspension (pH 9) onto a S i(NH 2)s u bs t r a t e i s s h ow n i n F i g u re 2d . I n t h is ca s e , t h eadsorption process selects much larger sheets (900-9000 nm) wh ich cover a bout 60-65%ar ea of the surfa ce.The dissociat ion of the GO hydroxyl groups (si tuatedm ai nly on t he b a sal planes) occurs a round pH 9 a ndsignificant ly increases the negative charge density onthe G O sheets. The increased at tra ct ion of the sheetsfor the cationic surface results in higher coverage thanthat observed at lower pH. This interaction is appar-

    ently more effective for the larger sheets, which canbridge over neutr a l regions of the incompletely primedsurface. Previous studies have shown that the aminepriming lay er does not completely cover th e surfa ce, a ndtha t it is only partia lly protona ted at pH 9.5 The averageroughness of the sheets on the surface is 4.4 . Theheight of the sheets, which are corrugated and some-times ha ve turn ed-in edges, is in the ra nge of 19-23 .The increased t hickness may be due to a hydra ted lay erof char ge-compensat ing NH 4+ ions, which cover thebasa l plane surfa ce. The a dsorption of a bila yer of sheets

    (23) J ohansson, G.; Lund gren, G.; Sillen, L. G.; Soderquist, R. ActaChem. Scand. 1960, 14, 769.

    Figure 1. Tran smission electron micrograph of colloidalgraphite oxide pa rticles.

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    seems unlikely, since in basic media the exfoliation ofGO occurs more readily.8

    A l2O3/ A l Su bst r at es. A FM i m ages of t he f i rs t GOlayer grown on Al-coated glass (Figure 3a) and alumi-num foil (not shown) resemble those of the substrates,except that corrugations similar to those seen in theTEM ima ge of GO a nd AFM imag es of the GO/Si ha veappeared. Because both Al2O3/Al subst ra tes a re veryrough and because the flexible GO sheets conform tothe surfa ce, i t is not possible to determine the latera land vert ical dimensions of the sheets. However, the

    av erage roughness of t he b right er a rea i n t he i m age,which is presumed to have an adsorbed GO sheet , is2.7 nm. B y compar ison, the substra te roughness is 3.5-nm, a gain indicating a sl ight smoothing of the surfa ceby the GO sheets.

    Characterization of GO/PAH Multilayers.Ellipsometry.Figure 4 show s plots of film t hickness,

    det erm i ned b y ell ipsom et ry , v ersus t he num ber ofa dsorption cycles for G O/P AH multilay er films. Thefilms were grown on Si(NH 2), S i(OH)/P AH, a nd Si(OH )/Al-Kg substrates. The l ineari ty of the fi lm thickness

    Figure 2. Tapping-mode AFM images of the first graphite oxide layer deposited from aqueous and aqueous ammonia sols onprimed Si substrates: (a) Si(NH 2), G O sol pH 5, with the linescan sh owing t he a pparent h eight of sheet (14 ) above the background;(b) Si (OH)/Al-Kg , G O sol pH 5; (c) Si(OH)/P AH, G O sol pH 5; a nd (d) S i(NH 2), G O sol pH 9.

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    pl ot s i ndi c at es t hat on av erage t he sam e am ount ofmaterial is deposited in each adsorption cycle. However,for each t he sa m ple, t he av erage i ncrease i n l ayerthickness per PAH/G O bilayer is different a nd ra ngesfrom 29 to 50 (Table 1). The lowest value, 29 perP AH/G O bilayer, is found for GO grown fr om aq ueousam monia solution (pH 9). The sma ller la yer pa ir th ick-ness in this case ma y a rise from par t ial deprotonationof t he underl yi ng P A H l ayer a t t h i s pH. B ecause t he

    surface charge density is lower, fewer a nionic sheets a rebound by t he polymer per unit ar ea.

    It sh ould be noted th at the mea sured thickness of theGO/P AH layer pair (40-50 ) i s m or e t h a n t h a texpected from the thickness of monolayer GO sheets(10.6-14.1 , a s estima ted by AFM for t he first depos-ited GO layer, Figure 2a-c) a nd t he th ickness of singleP A H l ayer ( 5 24). This can be explained in part byfolding of the flexible GO sheets, which is apparent in

    the AFM images of al l G O monolayers except the lowcoverage layer grown on Si(NH 2). The multilayer ad-sorption model proposed by Kleinfeld and Ferguson forclay /polycat ion films 1b m ay al so b e operat i v e for GO/PAH. In their model , each adsorption cycle depositsabout t wo layers of polyelectrolyte, but they r earra nge(possibly by folding in the present case) into alternatingsingle sheet /polycat ion films.

    A plot of th e thickness of a G O/P AH film grown fromaqueous solution onto Si(NH 2) is l inear only after thethird cycle. The a verage increase in thickness per P AH/G O bilayer is 23 in the first tw o adsorption cycles an d49 in the following three adsorption cycles (Figure4). This behavior is reminiscent of that observed by

    Kleinfeld a nd F ergus on for cla y/polyca tion films, wh ichnucleate in islands and completely cover the surfaceonly aft er several a dsorption cycles.25 We conclude t ha tthe G O/P AH film coverag e is r elatively complete a fteradsorption of the second bi layer (the first adsorptioncycle covers 30%of the su rfa ce; Figure 2a ). In subse-quent adsorption cycles, the layer pair thickness is closeto th a t found for G O/P AH m ultila yers on S i(OH)/P AH.In the lat ter case, the surface coverage is 85%afterthe first adsorption cycle, as shown in Figure 2c.

    AFM. AFM images of four or five bila yer G O/P AHfilms on al l three substrates were similar and did notclearly resolve the sheet edges or the voids betweensheets. From these images, one can only conclude thatthe m ultilayer films completely cover t he surfa ce. Figure3b show s a t ypic al i m age of a Si (OH)/Al -K g/(GO/PAH)3GO fi lm. The average roughness of this f i lm is20 , wh ich is consistent w ith a surfa ce of loosely tiledan d folded sheets. Multilayer film roughness an d t hick-ness para meters, determined by AFM a nd ellipsometry,respectively, are summarized in Table 1.

    For a ll the substra tes under invest igat ion (except forvery r ough a luminum foil), G O/P AH m ultila yers d epos-ited from aqueous sols on Al(OH)x-terminated surfaces(Si (OH )/Al-K g a nd Al/Al2O3) are sm oot her t ha n t hosedeposited on NH 2-termina ted surfa ces (Si(NH 2) and Si-(OH )/P AH) (Ta ble 1). C ompar ing th e sur fa ce morphol-

    ogies of the first G O layer a nd th e multilayer films, onecan see that the more densely t i led first layer (90%,Figure 2b), grown on the Keggin-primed surface, yieldsa smoother a nd more compact multilayer film, whereasthe poorly tiled first layer (35%, F igur e 2a ) on S i(NH 2)yi el ds t he roughest m ult i layer surfac e. Agai n , t h i sb ehav i or i s consis t ent w i t h t he m odel proposed b yKleinfeld and Ferguson for multi layer growth on is-land s, wh ich eventua lly coa lesce into smoother films.25

    (24) (a) Lvov, Yu. ; Ha as, H. ; Decher, G. ; Mohwa ld, H. ; Ka lachev,A. J. Phys. Chem. 1993, 97, 12835. (b) Lvov, Yu.; D echer, G .; Mohw ald,H . L a n g m u i r 1993, 9, 481, (c) Decher, G .; H ong, J . ; Schmitt, J . T h i n S ol i d F i l m s 1992, 2 10/211.

    (25) Kleinfeld, E. R.; Ferguson, G. R. Chem. M ater. 1996, 8, 1575.

    Figure 3. (a) Tapping-mode AFM image of the first graphiteoxide la yer deposited from th e a queous sol onto Al-coat ed glassa nd (b)im a ge of a (G O/P AH)3G O film on S i(OH)/Al-Kg . Z r ang eis 15 nm in both images.

    Figure 4. Ellipsometric measurements of the thickness ofmultilayer GO/P AH films vs num ber of adsorption cycles: 1,S i(OH )/P AH(G O/P AH)nG O; 2, S i(OH )/Al-K g(G O/P AH)nG O; 3,Si(NH 2)(G O/P AH)nGO; 4, Si(NH 2)(G O/P AH)nGO (pH 9). Thethicknesses a t an abscissa value of 0.5 correspond to primercationic layers; points on the plots refer to films terminatedb y a GO lay e r .

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    The m ost c om pac t and sm oot h m ul t i l ayer f i l m s aregrown from a queous ammonia solution (pH 9), fromw hi c h v ery l arge ( up t o 9 m w i d e ) G O s h e e t s a r eadsorbed. Each of these sheets covers an area whichexceeds the a rea of voids betw een the sheets (see Figure2d). Multilayer films grown on these large and smoothisland s ha ve the best overal l qua l ity.

    In this connection, it is important to emphasize thatthe lateral dimensions of the GO sheets prepared bypreoxidation a nd oxidation of nat ura l gra phite, followedby proper dispersion of the product G O, ar e significa ntlylarger tha n those previously used for layer-by-layerassembly (50-150 nm)7. While there have now beenmany studies of exfol iat ed la mellar compounds,4 i t isr a r e t h a t on e f in d s h ee t s w i t h l a t er a l d im en s ion sexceeding 1-2 m , ev en w hen t he s t ar t i ng m at eri a lconsists of relatively large single crystals. The fragmen-tation of large individual sheets into submicron piecesma y a rise from mechanical stress during t he exfoliat ionprocess. The factor t ha t limits t heir lat era l dimensionsis probably the density of defects in the sheets, whichapparent ly i s qui t e l ow i n G O prepared from nat ura lgraphit e . The ot her i m port ant poi nt t o not e i s t hatcomplete oxidation is needed to make unilamellar GO

    as opposed t o a suspension of gra phite core/G O sh ellpart icles. This fa ct , an d t he higher di lution of the G Odispersions used in th is study, ma y be the rea son th atthe sheets in our monolayer f i lms a re th inner (10-14) than those reported by Kotov et al . (18-27 ).7

    Multilayer films grown from unilamellar colloids ofzirconium phosphate, layered transition metal oxides,and layered metal dichalcogenides typically have rough-nesses on the order of the individual sheet thickness(0.8-1.6 nm).5,6 In contra st , the r elat ively high rough-ness of the G O/P AH m ultilayers is most probably dueto the flexibility of the G O sheets, w hich allows foldingand turning up of the edges of the sheets. However, inthe ca se of the rela tively rough sur face of the Al-coa ted

    glass, adsorption of the large GO lamellae and multi-lay er (G O/P AH)4GO fi lms results in smoothing of thesur fa ce (Ta ble 1).

    Electronic Properties of GO/Polycation Films.The electrical char acterist ics of G O/polycation multi-layer f i lms were investigated with both electronical lyinsula ting (P AH) an d conducting (PAN) polycat ions. Forcompa rison purposes, a rest a cked GO colloid cont a iningno polycations was also investigated. In al l cases, thefilms were grown on ITO glass, a nd th e top conta ct wa sPt . ITO a nd P t a re respectively intermediate a nd highwork function conta cts, and both G O and P AN ar e goodhole conductors. One w ould therefore expect tha t arectifying conta ct could form betw een the film a nd I TO

    and that the forward bias direction would have the Ptelectrode positve and ITO negative.

    All GO devices tested in t his ma nner sh owed rectify-

    ing beha vior (Figur e 5). (An I TO/P t cont rol experimentga ve t he expected ohmic response). For t he 60-nm-thickI TO-(GO /P AH)14G O-P t device (curve 2), the forw a rd(ITO(-)/P t (+)) and reverse bias turn-on potentialswere 0.42 and 0.82 V, respectively. These potentialscorrespond to electric fields, E, of 7.0 104 and 1.4 105 V cm -1, respectively. When t he conducting polymerP AN is substitut ed for P AH (curve 1), the forwa rd bia sturn-on potential is basically unaffected, but the reversebias t urn-on potentia l decreases to 0.41 V. In this ca sethe electric fields required to obtain a current densityof 5 mA cm-2 were 4.8 and 4.4 104 V cm-1 i n t h eforwa rd an d reverse directions, respectively. Thesefields are 1-2 orders of magnitude sma ller tha n t hose

    of single and bilayer polymer devices.26 For both devices,the current follows the ln(I) E-1 law, as expected fora tun neling-based char ge injection m echa nism. This issimilar to the behavior observed for most single andbilayer polymer diode devices.

    The resistivity of both the ITO-(G O/P AH)14G O-P ta nd ITO-(GO)x-Pt devices is on the order of 10 6 cm ,and that of the thicker ITO-(G O/P AN)30G O-Pt devicei s approxim at el y 1 order of m agni t ude l ow er. The

    (26) (a) Greenham, N. C . ; Morat t i , S . C. ; B radley, D. D. C. ; Friend,R. H.; Holmes, A. B. N a t u r e 1998, 365, 628. (b) Vestw eber, H.; Oberski,J . ; Greiner, A.; Heitz, W.; Mahrt, R. F.; Baessler, H. Ad v . M a t er . O p t .Electron. 1993, 2, 197.

    Table 1. Characterization of Multilayer GO/PAH F ilms by AFM and Ellipsometry

    sampleaverage roughness of the

    anchored substrate surface, average roughness of

    the film surface, total film

    thickness,

    average increase in filmth ickness per PAH/G O

    bilayer,

    Si(NH 2)/(G O/P AH )3G O 3.8 27 137 23 (first , second)49 (third, fourth)

    Si(NH 2)/(G O/P AH )3G Oa 3.8 13 105 29S i(OH )/P AH/(G O/P AH)3G O 6.2 26 176 51S i(OH )/Al-Kg /(G O/P AH)3G O 4.3 20 130 38G la ss /Al/Al2O3/(G O/P AH)4G O 35.0 22

    Al foil /Al2O3/(G O/P AH )4G O 48.0 58a GO was adsorbed from aqueous ammonia solution (pH 9).

    Figure 5. Current vs voltage chara cteristics of GO colloid an dGO /p oly cat ion mu lt i lay e r f i lms d e posit e d on I TO -c oat e dgla ss: (1) 120-nm-thick ITO-(G O/P AN)30G O-P t device; (2) 60-nm-thick ITO-(G O/P AH)14G O-P t; (3) 90-nm-thick ITO-(GO)x-P t. The positive direction on the volta ge axis is ITO(+)/P t (-).

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    difference arises from the electronic conductivity of thePAN layers, w hich m ediat e electron tra nsfer betweenGO layers. For both the GO/polycat ion an d GO fi lms,an irreversible conversion to a more conductive sta teoccurs when the device is held at high current density.For ITO-(GO/P AH)14G O-Pt and I TO-(GO/P AN)30G O-P t, th is effect became noticea ble at current densities of17 a nd 70mA/cm2, respectively. A similar effect beginsat a current density of a bout 40 mA/cm2 with ITO-

    (GO)x-P t. At cur rent densit ies a bove 470 mA/cm2

    , t hecurrent gradually increases by 3 orders of magnitudeat constant vol tage with the ITO-(G O/P AN)30G O-P tdevi ce. We t ent at i v el y asc rib e t hi s b ehav i or t o aninterna l electrochemica l reaction, which reduces GO t oa m ore conductive ca rbon-rich mat erial. Apparen tly, th eP AN sa mples are more resista nt t o this effect, possiblybecause the PAN itself acts as a reservoir of oxidizingequivalents. Similar electrochemical effects have beenfound with electrodes prepared from bulk GO 27 a ndwere a lso observed by Kotov et al . , in their study of GO/poly(diallyldimethyla mmonium chloride) thin films.7

    Conclusions

    We have shown that the qual i ty and morphology ofGO m onol ayer t hi n f il m s depend on t he source ofgra phite, its degree of oxidat ion, a nd t he conditions usedto prepare the GO colloid, as well as on the nature ofthe surfa ce-priming la yer a nd t he pH of the adsorbingsolutions. By properly controlling these parameters, onecan select intact , unilamellar GO sheets and achievereasonably dense (ca. 90%) coverage on a cat ionics u rf a ce . G O s h ee t s p re pa r e d i n t h is m a n n er f r om

    natural graphite have larger lateral dimensions (1-9m) than an y other lamellar inorga nic colloid studiedto dat e. The thickness of an individual G O layer, w hichcan be ful ly exfol iated by means of preoxidation andoxidat ion st eps, followed by a ppropriat e dilution, is onthe order of 10-14

    Multilayer G O/P AH films deposited in four or fiveadsorption cycles completely cover the surface of cationicsubstrat es. The a verage roughness of these multi layer

    films is lowest when the GO suspension contains largesheets an d wh en the coverage of the first layer of sheetsis highest . However, sequential a dsorption of G O a ndpolycat ions inva riably results in a n a verage thicknesschange per adsorpt i on cycl e t ha t i s 2-3 t i m e s t h emonolayer thickness. This appears to occur because theflexible GO sheets fold and overlap on the surface.

    The electronic chara cteristics of G O multila yer filmscont ac t ed b y I TO and P t e lect rodes are consi st entwith its hole-conducting properties. B y int erleaving G Owith poly(aniline), a hole-conducting polymer, the di-od el ik e b eh a v ior of t h e f il ms i s r et a i n ed , b ut t h econductivity is increased by approximately 1 order ofm agni t ude. Thi n f il m GO devi ces are , how ev er, i r-

    reversibly changed at high current density, most prob-ably because of electrochemical reduction of G O tocarbon.

    Acknowledgment. Thi s w ork w as support ed b yCivilian Research and Development Foundation (UC1-338), and by the National Science Foundation (CHE-9529202). Instrumentation for AFM experiments wasprovided by National Science Foundation grant CHE-9626326.

    CM981085U(27) Touzain, P.; Yazami, R. J. Power Sources 1985, 14, 99.

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