Nitrogen Doping and Reduction of Graphene

Simultaneous Nitrogen Doping and Reduction of Graphene

Oxide

Xiaolin Li,Hailiang Wang,Joshua T.Robinson,Hernan Sanchez,Georgi Diankov,

and Hongjie Dai*

Department of Chemistry,Stanford Uni V ersity,Stanford,California94305

Received August21,2009;E-mail:hdai@https://www.wendangku.net/doc/a52969893.html,

Abstract:We developed a simple chemical method to obtain bulk quantities of N-doped,reduced graphene oxide(GO)sheets through thermal annealing of GO in ammonia.X-ray photoelectron spectroscopy(XPS) study of GO sheets annealed at various reaction temperatures reveals that N-doping occurs at a temperature as low as300°C,while the highest doping level of～5%N is achieved at500°C.N-doping is accompanied by the reduction of GO with decreases in oxygen levels from～28%in as-made GO down to～2%in1100°C NH3reacted GO.XPS analysis of the N binding con?gurations of doped GO?nds pyridinic N in the doped samples,with increased quaternary N(N that replaced the carbon atoms in the graphene plane)in GO annealed at higher temperatures(g900°C).Oxygen groups in GO were found responsible for reactions with NH3and C-N bond formation.Prereduced GO with fewer oxygen groups by thermal annealing in H2 exhibits greatly reduced reactivity with NH3and a lower N-doping level.Electrical measurements of individual GO sheet devices demonstrate that GO annealed in NH3exhibits higher conductivity than those annealed in H2,suggesting more effective reduction of GO by annealing in NH3than in H2,consistent with XPS data. The N-doped reduced GO shows clearly n-type electron doping behavior with the Dirac point(DP)at negative gate voltages in three terminal devices.Our method could lead to the synthesis of bulk amounts of N-doped, reduced GO sheets useful for various practical applications.

Introduction

Graphene exhibits various interesting physical properties,1 large surface areas(～2600m2/g),and high chemical stability, all of which could be utilized for potential applications including graphene nanoribbon?eld effect transistors,2,3graphene sheet supercapacitors,4and lithium secondary batteries.5Chemical doping is important to modulate the electrical properties of graphene.Devising doping methods for this two-dimensional material will be key to its future applications and requires drastically different approaches from conventional methods for bulk materials.6-11Recently,we reported N-doping of individual graphene nanoribbons through electrical joule heating in NH3and suggested reactions occurring mostly at the edges and defect sites on graphene.6Substitutional N-doped multiplayer graphene sheets were synthesized by Liu et al.by adding NH3gas during chemical vapor deposition(CVD)growth of graphene.11N-doped graphite was also prepared by arc discharge of carbon electrodes in the presence of H2/pyridine or H2/ammonia.12 Currently,systematic investigations of graphene doping are still needed.

Here we use GO as a starting material to investigate the reaction with NH3at elevated temperatures.We investigate the N-doping and reduction effect of annealing of GO in NH3by XPS characterization and electrical measurements,in side-by-side comparisons to GO annealed in H2.The results lead to insights to the degree of doping and reduction effects at various temperatures,the roles played by oxygen groups at graphene edges and defect sites in the reaction,and electrical properties of the resulting graphene sheets.Our method also provides an effective method for the synthesis of gram-scale N-doped reduced GO sheets,which could lead to useful properties unattainable by undoped graphene.

Results and Discussion

Graphene oxide(GO)sheets were synthesized from graphite powder using a modi?ed Hummers method(detailed synthesis procedure in Supporting Information).13,14Due to harsh oxi-

(1)Geim,A.K.;Novoselov,K.S.Nat.Mater.2007,6,183–191.

(2)Li,X.L.;Wang,X.R.;Zhang,L.;Lee,S.W.;Dai,H.J.Science

2008,319,1229–1232.

(3)Wang,X.R.;Ouyang,Y.J.;Li,X.L.;Wang,H.L.;Guo,J.;Dai,

H.J.Phys.Re V.Lett.2008,100,206803.

(4)Stoller,M.D.;Park,S.J.;Zhu,Y.W.;An,J.H.;Ruoff,R.S.Nano

Lett.2008,8,3498–3502.

(5)Yoo,E.J.;Kim,J.;Hosono,E.;Zhou,H.S.;Kudo,T.;Honma,I.

Nano Lett.2008,8,2277–2282.

(6)Wang,X.R.;Li,X.L.;Zhang,L.;Yoon,Y.K.;Weber,P.K.;Wang,

H.L.;Guo,J.;Dai,H.J.Science2009,324,768–771.

(7)Gunlycke,D.;Li,J.;Mintmire,J.W.;White,C.T.Appl.Phys.Lett.

2007,91,112108.

(8)Cervantes-Sodi,F.;Csanyi,G.;Piscanec,S.;Ferrari,A.C.Phys.Re V.

B2008,77,165427.

(9)Wu,Y.P.;Fang,S.B.;Jiang,Y.Y.Solid State Ionics1999,120,

117–123.

(10)Hulicova,D.;Kodama,M.;Hatori,H.Chem.Mater.2006,18,2318–

2326.

(11)Wei,D.C.;Liu,Y.Q.;Wang,Y.;Zhang,H.L.;Huang,L.P.;Yu,G.

Nano Lett.2009,9,1752–1758.(12)Panchakarla,L.S.;Subrahmanyam,K.S.;Saha,S.K.;Govindaraj,

A.;Krishnamurthy,H.R.;Waghmare,U.V.;Rao,C.N.R.2009,

https://www.wendangku.net/doc/a52969893.html,/abs/0902.3077.

(13)Hummers,W.S.;Offeman,R.E.J.Am.Chem.Soc.1958,80,1339–

1339

.

Published on Web10/09/2009

10.1021/ja907098f CCC:$40.75 2009American Chemical Society J.AM.CHEM.SOC.2009,131,15939–159********

dization,GO sheets have limited sizes (from several hundred nanometers to 1or 2μm,Figure 1a)and disrupted conjugation in the plane.There are vacancies of carbon atoms in the plane,and abundant functional groups such as epoxide,hydroxyl,phenol,carbonyl,carboxyl,lactone,and quinone are present at both the edges and defects in the plane (Figure 1a).15-21We used atomic force microscopy (AFM)to characterize GO sheets deposited on the substrate from a suspension and observed that GO was mostly single-layer sheets of various shapes and sizes

with an apparent thickness of ～1nm (Figure 1a),corresponding to single layer GO.15

Reaction with ammonia was done by annealing GO samples in a 2Torr NH 3/Ar (10%NH 3)atmosphere.For sample preparation,as-made GO sheets were deposited on SiO 2substrates from solution and dried to form thick ?lms or lyophilized to obtain ?uffy powders for reactions and subsequent XPS characterization.We carried out GO annealing in NH 3with the samples heated in an NH 3?ow from room temperature to various temperatures up to 1100°C (detailed doping process in Supporting Information).Control experiments were done by annealing GO samples in 2Torr of H 2at the same temperatures.We used XPS to characterize the elemental composition of GO sheets reacted under various conditions (Figure 1b and 1c).As-made GO sheets showed more than ～28%oxygen and no nitrogen signal in the XPS spectrum (Figures 1b,c and 2).The high-resolution C1s XPS spectrum of as-made GO sheets showed a second peak at higher binding energies (Figure 3a and 3b),corresponding to large amounts of sp 3carbon with C s O bonds,carbonyls (C d O),and carboxylates (O s C d O),15-21resulted from harsh oxidation and destruction of the sp 2atomic structure of graphene (Figure 1a).

(14)Sun,X.M.;Liu,Z.;Welsher,K.;Robinson,J.T.;Goodwin,A.;Zaric,

S.;Dai,H.J.Nano Res.2008,1,203–212.

(15)Stankovich,S.;Dikin, D. A.;Piner,R. D.;Kohlhaas,K. A.;

Kleinhammes,A.;Jia,Y.Y.;Wu,Y.;Nguyen,S.T.;Ruoff,R.S.Carbon 2007,45,1558–1565.

(16)Szabo,T.;Berkesi,O.;Forgo,P.;Josepovits,K.;Sanakis,Y.;Petridis,

D.;Dekany,I.Chem.Mater.2006,18,2740–2749.

(17)Hontoria-Lucas,C.;Lopez-Peinado,A.J.;Lopez-Gonzalez,J.de D.;

Rojas-Cervantes,M.L.;Martin-Aranda,R.M.Carbon 1995,33,1585–1592.

(18)He,H.Y.;Klinowski,J.;Forster,M.;Lerf,A.Chem.Phys.Lett.1998,

287,53–56.

(19)Lerf,A.;He,H.Y.;Forster,M.;Klinowski,J.J.Phys.Chem.B 1998,

102,4477–4482.

(20)Gao,W.;Alemany,L.B.;Ci,L.J.;Ajayan,P.M.Nat.Chem.2009,

1,403–408

.

Figure 1.Annealing of GO in NH 3and H 2.(a)Schematic structure and AFM image of GO sheets.Left panel:Schematic structure of GO sheets.The

conjugated plane is disrupted.There are missing carbon atoms in the plane,and functional groups like epoxide (1),hydroxyl (2),phenol (3),carbonyl (4),carboxyl (5),lactone (6),and quinone (7)are present at both the edges and in the plane.15-21Right panel:A representative AFM image of GO sheets.(b)XPS spectra of GO sheets annealed in 2Torr of NH 3/Ar (10%NH 3)at various temperatures.(c)XPS spectra of GO sheets annealed in 2Torr of H 2at various temperatures.

15940

J.AM.CHEM.SOC.

9

VOL.131,NO.43,2009

A R T I C L E S Li et al.

XPS revealed that N-doping occurred at a temperature as low as 300°C for GO annealed in NH 3,with ～3.2%N detected in the sample (Figures 1b,2a).N levels in GO sheets annealed in NH 3between 300and 1100°C were in a range of ～3-5%,with 500°C annealing affording the highest N-doping level of ～5%(Figure 2a).In addition to N-doping,NH 3annealing of GO also showed an obvious reduction https://www.wendangku.net/doc/a52969893.html,paring the oxygen levels of GO samples annealed in NH 3and H 2at various temperatures,we found that the oxygen levels in GO annealed in NH 3were lower than in those annealed in H 2at the same temperatures except for 1100°C (Figure 2b).This indicated more effective reduction effects of thermal annealing in NH 3than in H 2below ～1100°C.

GO samples annealed in NH 3and H 2both showed much lower signals at the higher binding energy end of the C1s peak than as-made GO,indicating thermal annealing in NH 3and H 2removed functional groups and sp 3carbon (Figure 3a and 3b).Detailed analysis of the full width at half-maximum of the C1s peak at 284.5eV (graphite-like sp 2C)showed that GO samples annealed in NH 3exhibited wider C1s peaks than those annealed in H 2(Figure 3c).This was likely due to N incorporation into the sp 2network of reduced GO samples upon annealing in NH 3.It is known that,rather than a single symmetry peak with a constant width,the C1s peak of sp 2carbon at 284.5eV becomes asymmetric and broadened toward the high binding energy side as the amount of functional groups increases.15-21GO annealing in NH 3led to ～3to 5%N incorporation into the sheets to afford C -N bonded groups.Thus,GO annealed in NH 3exhibited broader C1s peaks than GO annealed in H 2(Figure 3c),due to N-doping and C -N species.15,21-23

For GO exposed to 2Torr of NH 3/Ar (10%NH 3)at room temperature,XPS revealed no N signal after pumping the sample in vacuum.The clear N signals in our high temperature N-doped samples were not due to physisorbed NH 3but covalent C -N species formed during NH 3annealing.We also dispersed N-doped GO samples in various solvents like dichloroethane,alcohols,and H 2O by sonication and then deposited the GO on substrates for further XPS analysis.XPS showed no change in N-doping level before and after sonication,suggesting formation of covalent C -N bonds in the samples instead of physisorbed NH 3(XPS data not shown).We also tried to further anneal N-doped GO sheets (made by 700°C annealing in NH 3)in H 2up to 900°C and found the N levels in the samples were stable.We observed no signi?cant decrease of the N level up to 900°C annealing in H 2(Figure S2).Heat treatment of

N-doped

Figure 2.N-doping and reduction effects of GO.(a)Nitrogen percentage in the GO sheets annealed in NH 3at various temperatures detected by XPS.(b)Oxygen percentage in the GO sheets annealed in NH 3and H 2respectively at various temperatures detected by XPS.The error bars are based on more than three different spots measured over the

samples.

Figure 3.XPS spectra of C (1s)peaks.(a)High resolution C (1s)spectra of as-made GO and GO annealed in NH 3at different temperatures.(b)High resolution C (1s)spectra of as-made GO and GO annealed in H 2at different temperatures.(c)The C (1s)peak widths of as-made GO and NH 3and H 2annealed GO sheets respectively,measured from the full width at half-maximum of the peak at 284.5eV.Dashed line is the corresponding C (1s)peak width of pristine HOPG (highly oriented pyrolytic graphite)as a reference.

J.AM.CHEM.SOC.

9

VOL.131,NO.43,200915941

Nitrogen Doping and Reduction of Graphene Oxide A R T I C L E S

graphite in the range 900to 1200°C is necessary to break C -N bonds and remove nitrogen.21

We investigated the bonding con?gurations of N atoms in the NH 3annealed GO sheets based on high-resolution N1s XPS spectra.The N1s peaks in the XPS spectra of GO annealed at 300to 1100°C were ?tted into two peaks,a lower energy peak A and a higher energy peak B (Figure 4a -c and Figure S5).In all the samples,peak A is near 398.3eV,corresponding to pyridinic N (Figure 4d).11,21-23The binding energy of the high energy peak B increased with annealing temperature (Figure 4a -c and Figure S5),indicating different N bonding con?gura-tions in GO sheets reacted with NH 3at different temperatures.In GO annealed at 300-500°C,the N bonding con?gurations of component B could be indexed to amide,amine,or pyrrolic N.11,21-23For samples annealed at high temperatures,i.e.g ～900°C,the peak position of component B was near 401.1eV (Figure 4d),which could be indexed to quaternary N,i.e.N that replaced the carbon atom in the graphene sheets and bonded to three carbon atoms (Figure 4d inset).11,21-23Our data suggested that higher temperature annealing of GO in NH 3above ～900°C afforded more quaternary N incorporated into the carbon network of graphene.Raman spectra were used to characterize the N-doped GO.The G peak position of GO annealed in NH 3at 1100°C showed an obvious downshift (Figure S6).

To glean the reaction pathway between NH 3and GO,we carried out control experiments by performing 900°C NH 3annealing of prereduced GO sheets made by annealing in H 2at various temperatures ranging from 300to 1100°C.A detectable amount of N was only observed by XPS in GO prereduced in H 2below ～500°C (Figure S1).The N levels were below the

XPS detection limit in GO samples reduced in H 2at higher temperatures (>500°C)and then reacted with NH 3at 900°C (Figure S1).These experimental ?ndings suggested that certain oxygen functional groups in the as-made GO were responsible for reactions with NH 3to form C -N bonds and afford N-doping.These oxygen functional groups were mostly removed from GO by heating to ～500°C,resulting in much reduced reactivity with NH 3and little N-doping in graphene.It is known in graphite oxide that carboxylic and lactone groups begin to decompose at ～250°C and -COOH and carbonyl groups are reduced by 450°C heat treatment.21Phenol and quinone groups decompose almost entirely between 500and 900°C.Above 900°C,-OH groups start to decrease and oxygen-containing groups in graphite oxide are completely eliminated at ～1100°C.21Taken together,we suggest that oxygen functional groups in GO including carbonyl,carboxylic,lactone,and quinone groups are responsible for reacting with NH 3to form C -N bonds.For GO reduced in H 2by annealing above ～500°C,these reactive oxygen groups are decomposed and removed,thus leading to much reduced reactivity with NH 3seen in our experiments.For higher quality and lower defect density graphene sheets 24and graphene nanoribbons 2produced by mild oxidization,we expect lower reactivity with NH 3at elevated temperatures and thus lower N-doping levels than GO.Oxygen groups existing

(21)Kinoshita,K.Carbon:electrochemical and physicochemical properties ;

Wiley:New York,1988.

(22)Pietrzak,R.Fuel 2009,88,1871–1877.

(23)Kelemen,S.R.;Afeworki,M.;Gorbaty,M.L.;Kwiatek,P.J.;Solum,

M.S.;Hu,J.Z.;Pugmire,R.J.Energy Fuel 2002,16,1507–1515.(24)Li,X.L.;Zhang,G.Y.;Bai,X.D.;Sun,X.M.;Wang,X.R.;Wang,

E.G.;Dai,H.J.Nat.Nanotechnol.2008,3,538–542

.

Figure 4.XPS spectra of N dopants in graphene.(a)High resolution N (1s)spectra of GO annealed in NH 3at 300°C.The peak is ?tted into a low and high energy A and B two components centered at 398.5and 399.9eV respectively.(b)High resolution N (1s)spectra of GO annealed in NH 3at 700°C.The peak is ?tted into low and high energy A and B components centered at 398.3and 400.8eV respectively.(c)High resolution N (1s)spectra of GO annealed in NH 3at 900°C.The peak is ?tted into low and high energy A and B components centered at 398.2and 401.1eV respectively.(d)Positions of peaks A and B for different NH 3annealing temperatures.Inset is a schematic structure showing the two predominant binding conditions of nitrogen in graphene annealed at high temperatures g 900°C.

15942

J.AM.CHEM.SOC.

9

VOL.131,NO.43,2009

A R T I C L E S Li et al.

at the edges and defect sites in the plane of high quality graphene could react with NH 3in a similar manner as in GO,giving rise to N-doping effects.6In our current work,we used GO as a model system to investigate the reaction with NH 3,since GO contained large numbers of functional groups at defect and edge sites,which gave suf?ciently high N-doping levels easily detected by spectroscopy.

To investigate how N-doping affects the electronic properties of graphene,we made single-sheet,back-gated electrical devices of GO (with Pd source -drain (S -D)contacts,Figure 5a)after annealing in NH 3and H 2at high temperatures (500-900°C)(Figure 5and Figures S3,S4).Figure 5showed typical electrical devices of GO annealed in NH 3and H 2respectively at 700and 900°C.GO annealed in NH 3and H 2both showed p-type behavior in air,which was due to doping by physisorbed molecular oxygen and polymers involved in device fabrication.6To avoid the complication from physisorbed oxygen,we measured the devices in a vacuum (～5×10-6Torr).The Dirac point (DP)of the GO sheet annealed in NH 3at 500(Figure S3),700(Figure 5b),and 900°C (Figure 5c)was at negative gate voltages of V gs <～-20V in vacuum,an indication of n-type electron doping behavior due to N-dopants in graphene.After high-bias electrical annealing 6to further remove phys-isorbed species,the NH 3-annealed GO sheet showed completely n-type behavior with the DP moved to highly negative gate voltages (Figure 5b,5c).Lower resistance was observed for GO annealed at 900°C compared to those annealed at 700°C due to more effective reduction of GO at higher temperatures.The Dirac point (DP)of the GO sheet annealed in NH 3at 900°C was also at negative gate voltages of V gs <～-20V in vacuum (Figure 5c).In contrast,the DP positions of a H 2annealed GO sheet were near the intrinsic V gs )0V (Figure 5d)and remained so after electrical annealing (Figure 5d).The results con?rmed n-doping by N dopants in graphene afforded by thermal reaction with NH 3.

Electrical measurements con?rmed that thermal annealing of GO in NH 3was effective for GO reduction in agreement with XPS data.The normalized resistance (de?ned as R ·W /L ,where R is the measured resistance of the graphene device and W and L are the graphene sheet width and channel length respectively)of GO reduced in NH 3at 500to 900°C were lower at the minimum conductivity DP than GO annealed in H 2at the corresponding temperatures (Figure 5e).This was consistent with thermal annealing in NH 3affording a more

effective

Figure 5.Electrical properties of single GO sheet annealed in NH 3vs H 2.(a)A typical AFM image of a N-doped GO sheet device.The right panel depicts

the device structure with a 300nm thick SiO 2as gate dielectric and heavily doped Si substrate as back-gate.(b)Current -gate voltage (I ds -V gs )curves (recorded at V ds )1V)of a single GO device fabricated with an NH 3-annealed (700°C)GO sheet.Red solid line:device measured in air.Green solid line:device measured in vacuum.Blue solid line:device measured in vacuum after electrical annealing.(c)Current -gate voltage (I ds -V gs )curves of a single GO device fabricated with an NH 3-annealed (900°C)GO sheet.Red solid line:device measured in air.Green solid line:device measured in vacuum.Blue solid line:device measured in vacuum after electrical annealing.(d)Current -gate voltage (I ds -V gs )curves of a single GO device fabricated with a H 2-annealed (900°C)GO sheet.Red solid line:device measured in air.Green solid line:device measured in vacuum.Blue solid line:device measured in vacuum after electrical annealing.6(e)Statistics of normalized sheet resistance of devices fabricated with single GO sheets annealed in NH 3and H 2at different temperatures.The error bars are based on more than 20different devices measured.Normalized resistance is de?ned as R ·W /L ,where R is resistance of device and W and L are the GS width and channel length respectively.

J.AM.CHEM.SOC.

9

VOL.131,NO.43,200915943

Nitrogen Doping and Reduction of Graphene Oxide A R T I C L E S

reduction effect than annealing in H2,as revealed by XPS (Figure2b).For comparison with other reduction methods,GO annealed in NH3at900°C showed higher conductivity than the same GO reduced by a recently reported hydrazine solvo-thermal reduction method at180°C,25although the normalized resistance of the900°C NH3annealed GO was still more than 100times higher than that of pristine graphene due to the irreversible defects such as large vacancies and disrupted conjugation in the plane resulted from harsh oxidization.24-26 Conclusion

In summary,we obtained up to5%N-doped,reduced GO sheets by thermal annealing GO in NH3.The chemical doping and reduction effects were elucidated by XPS characterization and electrical transport measurements.Oxygen groups such as carboxylic,carbonyl,and lactone groups were suggested to be essential for reactions between graphene and NH3for C-N bond formation.For different graphene samples with varying degrees of oxidation,the degree of reaction with ammonia and N-doping will depend on the amounts of these oxygen functional groups at the defect and edge sites of graphene.We expect that N-doped,reduced graphene sheets could be used for further functionalization chemistry and for various potential applications including in the area of clean energy.

Acknowledgment.This work was supported by MARCO-MSD, Intel,and ONR.

Supporting Information Available:Experimental details and supplementary?gures.This material is available free of charge via the Internet at https://www.wendangku.net/doc/a52969893.html,.

JA907098F

(25)Wang,H.L.;Robinson,J.T.;Li,X.L.;Dai,H.J.J.Am.Chem.Soc.

2009,131,9910–9911.

(26)Wang,H.L.;Wang,X.R.;Li,X.L.;Dai,H.J.Nano Res.2009,2,

336–342.

15944J.AM.CHEM.SOC.9VOL.131,NO.43,2009

A R T I C L E S Li et al.