Synthesis of Nitrogen-Doped Graphene on Pt(111) by Chemical Vapor

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Synthesis of Nitrogen-Doped Graphene on Pt(111) by Chemical Vapor Deposition Gaku Imamura† and Koichiro Saiki*,†,‡ † ‡

Department of Chemistry, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8561, Japan Department of Complexity Science & Engineering, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8561, Japan ABSTRACT: Chemical doping of graphene with foreign atoms is one of the most promising ways to modify the electronic structure of graphene. We fabricated nitrogen (N)-doped graphene on a Pt(111) surface through a chemical vapor deposition method; the heated substrate was exposed to such N-containing organic molecules as pyridine and acrylonitrile. Analysis by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed that N-doped graphene was formed on a Pt(111) surface from pyridine at the substrate temperature (TS) higher than 500 °C, while nitrogen was not doped at TS higher than 700 °C. Exposing the heated substrate to acrylonitrile also led to formation of graphene but nitrogen was not incorporated at any TS. On the basis of the experimental results, we discuss the growth mechanisms of N-doped graphene at low and high TS.

1. INTRODUCTION Graphene, a single layer of sp2-bonded carbon atoms arranged in a honeycomb crystal lattice, has been attracting much attention due to its fascinating properties since it was first isolated by Geim and Novoselov in 2004.1,2 The intriguing properties of graphene reported so far are anomalous quantum Hall effect, ultrahigh carrier mobility, over micrometer-scale spin coherence length, etc.3
5 In addition to these physical properties, the interest of graphene is driven by a promise of device applications such as field effect transistors (FET), transparent electrodes, supercapacitors, and lithium ion second batteries. For these purposes, modification of electronic structure is necessary. For instance, a bandgap opening is crucial for achieving the high on/ off ratio of FET, and the increase of conductivity is necessary for application to transparent electrodes, etc. Chemical doping is one of the most promising ways to change the electronic structure by carrier injection or extraction. Substitutional doping, which replaces some of carbon atoms in the sp2 network with foreign atoms such as nitrogen or boron, is more favorable than molecular doping from the viewpoint of structural robustness. Many theoretical studies have revealed doped graphene would present peculiar properties which cannot be expected in nondoped one.6
9 One of such works indicates that the transport spin anisotropy could be induced by doping heteroatoms into a graphene nanoribbon; hence, doped graphene nanoribbons would be used as spin filter devices.10,11 Another computational study shows the graphene which contains nitrogen atoms at specific sites can exhibit metal-free catalytic activity.12 In contrast with these theoretical or computational studies of doped graphene, only a limited number of experimental studies have been reported so far. Among those, Wei et al. synthesized nitrogen-doped graphene by chemical r 2011 American Chemical Society

vapor deposition and measured its electrical properties.13 Wang et al. functionalized single layer graphene nanoribbons through electrothermal reactions with ammonia.14 Moreover, bilayer boron-doped graphene as well as nitrogen-doped one was synthesized by Panchakarla et al. by using arc discharge.15 Even though several methods have been carried out to form doped graphene, there seem no solid ways to dope impurities into graphene. Thus, development of a substantial way to fabricate doped-graphene is strongly required. Here we report growth of nitrogen (N)-doped graphene on a Pt(111) surface by the chemical vapor deposition process. The heated Pt substrate was exposed to nitrogen-containing molecules, and the products were characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results showed the formation of N-doped graphene depends strongly on the temperature of the substrate and the choice of source material.

2. EXPERIMENTAL SECTION The Pt(111) substrate was first mechanically polished in air with alumina powder. After the rinsing, the substrate was transferred into an ultrahigh vacuum (UHV) chamber with a base pressure of 10
8 Pa. The surface of the substrate was cleaned by repeated cycles of Arþ sputtering and annealing at 827 °C. The cleanness of the surface was checked by XPS (X-ray source, Thermo VG Scientific). Graphene was synthesized by exposing the substrate to such nitrogen-containing organic molecules as pyridine (C5H5N) and acrylonitrile (C3H3N). Received: March 6, 2011 Revised: April 21, 2011 Published: May 04, 2011 10000

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Figure 1. XPS spectrum of the Pt(111) substrate exposed to ethylene in the region of C 1s.

These source materials were introduced into the chamber through variable leak valves. The substrate was heated by flowing electric current through the tantalum wire on which the Pt substrate was fixed. The relationship between the electric current and the temperature of the substrate (TS) was calibrated with an infrared pyrometer in the high temperature range and with a thermocouple in the low temperature range. The amount of exposure was controlled by the pressure of the source gases and the exposure time. In situ XPS measurement was carried out after the gas was evacuated, and the substrate was cooled to room temperature (RT). An Mg KR excitation source (hν = 1253.6 eV) was used for XPS measurement, while an Al KR excitation source (hν = 1486.6 eV) was also used in the case when the innershell photoelectrons and the Auger electrons interfered with each other. Photoelectrons were analyzed with a hemispherical analyzer (PHOIBOS-100, SPECS). The energy of the spectrometer was calibrated using the Ag 3d5/2 line. Raman spectroscopy was performed with a spectrometer (NRS-3100, JASCO Corp.) ex situ after the XPS measurement.

Figure 2. (a) XPS spectra of the samples exposed to pyridine ambient in the region of C 1s. Short solid lines are the peak positions. (b) Schematic illustration of a pyridine molecule adsorbed on Pt(111) surface in Rpyridyl structure.

3. RESULTS 3.1. Formation of Graphene from Ethylene. We first fabricated nondoped graphene on Pt(111) to obtain the typical XPS spectrum of graphene. It is well-known that graphene could be formed by exposing a heated Pt(111) surface to ethylene gas.16,17 A cleaned Pt(111) surface was exposed to 200 langmuirs (1 langmuir = 1  10
6 Torr 3 s) ethylene at TS of 700 °C. The XPS spectrum of graphene thus prepared is shown in Figure 1. Since the graphene produced by this method is thought to be monolayer,17,18 the observed single C 1s peak can be assigned to the graphene grown on a Pt(111) surface. The C 1s peak appears at 283.7 eV, which is slightly lower than 284.2 eV observed typically for highly oriented pyrolytic graphite (HOPG). The decrease in binding energy of C 1s peak might be caused by an interaction between the graphene and the Pt(111) surface, although a chemical bond is not formed between graphene and Pt.19 A theoretical study indicates that electrons are transferred from graphene to Pt in order to equilibrate the Fermi levels.20 As the work function of Pt is higher than that of graphene, the Fermi level of the graphene is lowered, resulting in a lower shift of the C 1s binding energy. We grew graphene on Pt(111) from benzene,21,22 which also showed a lower binding energy than HOPG.23 Oshima and Nagashima also reported a chemical shift

Figure 3. Raman spectra of the samples exposed to pyridine ambient. Peaks in the shaded area are caused by adsorbed O2 molecules, which are not informative in this study. Inset: 2D band region spectrum of the 500 °C sample.

of the C 1s peak to lower binding energy for the graphene on Pt(111).24 Therefore, it is appropriate to assign the C 1s peak at 283.7 eV to the graphene on a Pt(111) surface. 3.2. Formation of Graphene from Pyridine. The heated substrate was exposed to pyridine gas. TS was changed from RT to 700 °C. The pressure of pyridine was set at 0.1 Pa, and the time of exposure was fixed at 1 h in order to achieve saturated adsorption. After the exposure, the samples were annealed at 100 °C for 1 h under UHV conditions to eliminate the adsorbed pyridine molecules. Figure 2a shows the spectra of the C 1s region. In the case of deposition at RT, the peak has a broad shoulder at higher binding energies. This asymmetry arises from 10001

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Figure 4. (a) XPS spectra of the samples exposed to pyridine ambient in the region of N 1s. Two dashed lines are two components, 398.0 and 399.8 eV, respectively. (b) Schematic image of the N-doped graphene. The positions of graphitic N and pyridinic N are represented.

the configuration of pyridine molecules on a Pt(111) surface. A pyridine molecule adsorbs often with its molecular plane perpendicular to the Pt(111) surface as shown in Figure 2b. This configuration which accompanies the C
H bond cleavage is named R-pyridyl structure.25,26 In this structure three different types of carbon atoms would appear in the XPS spectrum: a carbon atom bonded to the Pt surface, a carbon atom bonded to the nitrogen atom, and other three carbon atoms. The RT spectrum shown in Figure 2a was fitted by three Gaussian components with the area ratio of 3:1:1, which is in good agreement with the R-pyridyl structure. With increasing TS, the shoulder at higher binding energies becomes weakened and finally disappeared at high TS. Simultaneously, the main peak shifts down to 283.7 eV. As we have considered that the peak at 283.7 eV can be assigned to the graphene on a Pt(111) surface, it can be judged that graphene was formed from pyridine on the heated Pt substrate. Figure 3 shows the Raman spectra of the samples fabricated from pyridine. The spectra of a Pt(111) substrate and the sample deposited at 250 °C show no distinct peak in this region, while the spectrum of the sample deposited at 500 °C has a clear D peak (1312 cm
1) and a G peak (1592 cm
1). As the peaks characteristic of graphite-like carbon materials are observed,27 we can confirm that sp2 carbon network is formed for the sample deposited at TS = 500 °C. Since the width of 2D peak (2620 cm
1) is as sharp as 36 cm
1, the graphene formed at 500 °C is considered to be monolayer.28 The relationship between the peak intensity ratio of D peak to G peak (ID/IG) and the width of G peak (WG) can be used as an indicator for

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Figure 5. XPS spectra of the samples exposed to acrylonitrile ambient. (a) In the region of C 1s. Short solid lines are the peak positions. (b) In the region of N 1s.

characterizing the crystallinity of carbon materials.29 According to this relationship, the value (WG = 10.6 cm
1, ID/IG = 0.43) of the sample deposited at TS of 500 °C is located in the region similar to that of graphite powder. We have estimated the crystalline size (La) of the graphene from the ID/IG ratio using the equation described in ref 30. The crystallite size of the graphene deposited at 500 °C was ∼45 nm. An absence of characteristic Raman peaks for the sample deposited at 250 °C means that graphene network was not formed, although the C 1s peak was observed at 284.0 eV. Figure 4a shows the N 1s XPS spectra of the samples deposited at various TS. At RT deposition, a single peak appears at 399.0 eV, which can be assigned to nitrogen atoms in the adsorbed pyridine molecules. The peak becomes broadened and splits into two components, 398.0 and 399.8 eV with increasing TS. A peak at around 398.0 eV is generally assigned to pyridinic N as shown in Figure 4b, and a peak at around 399.8 eV is assigned to the nitrogen in a disordered state.13,31,32 For the sample deposited at 250 °C, the disordered nitrogen component is more intense than the pyridinic one. But the situation changes for the sample deposited at 500 °C, where the pyridinic peak at 398.0 eV becomes prominent. This result may come from the decomposition of adsorbed pyridine molecules. At TS of 250 °C pyridine molecules are thought to decompose and fragments of pyridine exist on Pt(111). At TS of 500 °C the graphene network is formed, into which nitrogen atoms are thought to be incorporated at a pyridinic site. For the sample deposited at 700 °C, the N 1s signal was not observed at all (Figure 4a). The intensity ratio of N 1s peak to C 1s peak is evaluated to be 0.34, 0.15, and 0.08 10002

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Figure 6. Raman spectra of the samples exposed to acrylonitrile ambient. Inset: 2D band region spectrum of the 500 °C sample.

for TS of RT, 250 °C, and 500 °C, respectively. The higher TS becomes, the smaller IN1s/IC1s becomes. From the intensity ratio, we estimated the nitrogen content of the TS = 500 °C sample to be ∼4.0 at. %. On the basis of the XPS and Raman results, we conclude that N-doped graphene was formed on a Pt(111) surface at 500 °C. The crystallinity of the N-doped graphene is as good as graphite powder, and most of the residual nitrogen atoms are incorporated into the graphene at pyridinic site. 3.3. Acrylonitrile on Pt(111). A heated Pt(111) substrate was exposed to acrylonitrile atmosphere. The total amount of exposure was set at 50 langmuirs, and TS was changed from RT to 600 °C. After the exposure, the samples were annealed at 100 °C under UHV conditions for 1 h. The XPS measurement was carried out in situ, and the Raman spectrum was measured ex situ. Figure 5a,b shows the C 1s and N 1s XPS spectra of the samples deposited at various TS. The C 1s peak of the sample deposited at RT appears at 284.6 eV, which corresponds to acrylonitrile molecules physisorbed on the Pt(111) surface. With increasing TS, the C 1s peak shifts to lower binding energy as is the case of the pyridine-exposed samples. The C 1s peak is always observed at 283.7 eV, corresponding to the graphene on Pt(111). Although the N 1s peak appears for the sample deposited at RT, it disappears completely for the samples deposited at elevated TS (Figure 5b). Figure 6 shows the result of Raman measurement. The 500 °C sample has both clear D and G peaks which are characteristic of graphite. Moreover, it has a single sharp 2D peak at 2617 cm
1 with a fwhm of 40 cm
1. On the basis of the Raman spectrum, we can confirm that monolayer graphene was formed from acrylonitrile molecules on the Pt(111) surface at 500 °C. The value of WG (12.0 cm
1) and ID/IG (0.41) indicates that the acrylonitriledeposited sample could be located in the graphite powder region like the pyridine-deposited sample. The crystalline size of the graphene is estimated to be 47 nm.30 From the results of XPS and Raman spectra, it can be said that deposition of acrylonitrile molecules onto the heated Pt(111) substrates gives monolayer graphene with high crystallinity. Nitrogen, however, could not be incorporated into graphene, in contrast to the pyridine-deposited samples.

4. DISCUSSION Pyridine and acrylonitrile molecules are found to form graphene when they are deposited on the heated Pt(111) substrate. Nitrogen atoms are incorporated into the graphene lattice for the

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Figure 7. Schematic illustration of models for N-doped graphene formation on a Pt(111) surface fabricated from (a) pyridine and (b) acrylonitrile.

pyridine-deposited sample but not for the acrylonitrile-deposited sample. In the following we will discuss the growth mechanism focusing on the difference between them. 4.1. Growth Mechanism for Pt(111)
Pyridine. To discuss about the mechanism of graphene formation, we should consider the chemical reaction on the Pt(111) surface. The most important factor for the growth of graphene must be surface reaction and diffusion process of decomposed molecules.33 The surface reaction is promoted by catalytic activity of the Pt(111) surface. It has been already known that organic molecules decompose into active carbon species on the heated Pt(111) surface.16,34 Deng et al. reported that exposing acetylene to a Pt(111) surface at 750 K produces dicarbon (C2) molecules, which are stable on the surface.34 Since decomposition of pyridine molecules on a heated Pt surface seems to proceed in a similar way, it is not unreasonable to assume that C2 molecules are produced as carbon fragments on Pt. Deng and Trenary further reported the occurrence of reaction between C2 and NH3 on Pt(111), yielding volatile C2N2 and HCN.35 In the present case of pyridine decomposition, such nitrogen-containing fragments as C2N2 and HCN might be produced and they are likely to evaporate. In contrast, such carbon fragments as C2 which do not contain nitrogen are not volatile over a wide temperature range and can diffuse on the surface.16,34 The fragments assemble and form graphene on the Pt(111) surface.16 Therefore, graphene can be formed on the high-temperature Pt(111) surface, while nitrogen cannot be incorporated into the graphene. This process is schematically illustrated in the first column of Figure 7a. The situation can be changed for lower TS. For the samples deposited at lower TS, the chemical bonds in a pyridine molecule are thought to be partially broken or weakened, but not completely broken. At this temperature, the carbon fragments seem to differ from those produced at higher TS. This suppresses formation of volatile C2N2 or HCN; thus, nitrogen atoms are not likely to evaporate. The weakened chemical bonds can form new bonds with neighboring molecules, and the coalescence among partially decomposed pyridine molecules occurs. The formation of new bonds extends the sp2 carbon network, leading to the graphene growth. At this stage, a small amount of nitrogen atoms can be taken into the sp2 network mainly at the pyridinic sites. Thus, nitrogen-doped graphene can be formed from pyridine molecules, as schematically shown in the second column of Figure 7a. 10003

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The Journal of Physical Chemistry C As we have seen, there are two processes of the graphene growth on Pt(111) for decomposition of pyridine. One is that carbon fragments originating from broken pyridine molecules diffuse on the surface and aggregate to form sp2 carbon network, named “bond breaking process”. The other is the “bond reforming process”, in which chemical bonds of pyridine molecules are weakened and coalescence with neighboring pyridine molecules occurs. The former process is dominant at high TS (over 700 °C), and the latter one is dominant at low TS. Since the intrinsic Raman peaks were not observed for the sample deposited at 250 °C, the formation of graphene network is considered to require a critical temperature near 500 °C. In real experimental conditions, both processes might occur simultaneously. If only either “bond breaking process” or “bonding re-formation process” would proceed, the nitrogen content would have become zero or a certain value, not an intermediate value. The gradual shift of C 1s peak to 283.7 eV, corresponding to the increase of graphene component (283.7 eV), observed in Figure 2a also suggests the coexistence of two processes. The increase of graphene component means the evolution of sp2 network with increasing TS and the decrease of carbon atoms bonded to nitrogen atoms. Thus, the TS dependence of graphene formation can be explained by the gradual change in growth process; namely, the “bond re-forming process” is dominant at lower temperatures and “bond breaking process” becomes dominant with increasing TS. 4.2. Growth Mechanism for Pt(111)
Acrylonitrile. Growth process of graphene from acrylonitrile can be also explained by thermal decomposition on a Pt(111) surface. Acrylonitrile on the heated Pt(111) surface is thought to decompose into some carbon fragments, which were discussed in the pyridine deposition case. But as we have seen in Figure 5b, the N 1s peak was not detected for the samples deposited at elevated TS or the binding energy of C 1s peak shows no gradual change with TS, unlike the pyridine case. These differences might be explained by the structure of an acrylonitrile molecule. There are three types of skeletal bonds in an acrylonitrile molecule: a C
C single bond, a CdC double bond, and a CtN triple bond. In order to consider the stability or reactivity of these bonds, the bond energy is a good measure. The bond energies of each bond are 3.6 eV (C
C), 6.3 eV (CdC), and 9.2 eV (CtN), respectively.36 These values are taken from the typical bonds in organic molecules, so they are not necessarily precise for an acrylonitrile molecule. However, it is reasonable to consider that the C
C bond is relatively weaker than other two bonds. In the pyridine case, we have explained the growth mechanism in terms of “bond re-forming process” and “bond breaking process”. In contrast with the pyridine case, in which the bond energy of skeletal bonds is almost similar to each other, the relatively weak C
C bond exists in an acrylonitrile molecule. When the acrylonitrile molecule decomposes on the heated Pt(111) surface, the C
C single bond is preferentially broken even at low temperatures. This bond cleavage causes CtN and H2CdCH fragments. It is reasonable to consider that H2CdCH species can be converted into C2 molecule because H2CdCH has a C2 skeletal bond. The C2 molecules form a graphene lattice above 500 °C, while the CtN fragment is removed from the Pt surface even at 400 °C (>500 K) by forming volatile C2N2 or HCN molecules. Thus, nitrogen detachment would occur independently of TS, resulting in a single component of C 1s and absence of N 1s signal even at 400 °C. Above the critical temperature graphene network is formed without incorporation of nitrogen, as shown in Figure 7b.

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5. CONCLUSION The heated Pt(111) substrates were exposed to pyridine and acrylonitrile vapors to synthesize nitrogen-doped graphene. Exposure to pyridine leads to formation of graphene, while the quality of N-doped graphene was found to depend on the temperature of the substrate, TS. Graphitization requires high temperature (around 500 °C), while nitrogen tends to evaporate at higher TS. The most optimal TS is found to be 500 °C for growing N-doped graphene from pyridine from the viewpoints of graphene formation and nitrogen incorporation. Exposure to acrylonitrile also produced graphene, while nitrogen signal was not observed at any elevated TS. This result indicates graphene was fabricated from acrylonitrile but nitrogen was not incorporated. With respect to the doping mechanism, we presented a model of two growth processes: “bond breaking process” for high TS and “bonding re-forming process” for low TS. At high temperatures, pyridine molecules adsorbed on the heated Pt(111) surface decompose into carbon fragments. At this stage, the chemical bonds in a pyridine molecule are considerably broken, and nitrogen atoms are converted into volatile molecules and detach from the surface. But when the TS is moderate, the bonds in a pyridine molecule are just weakened but are not completely broken. The weakened chemical bonds can form new bonds with neighboring molecules, and accordingly sp2 carbon network expands with nitrogen atoms. In the case of acrylonitrile, however, the weak single C
C bond in its molecular structure might be preferentially broken, producing CN fragments, which can be easily converted into volatile molecules. Thus, it is hard to form nitrogen-doped graphene from such a molecule as acrylonitrile. ’ AUTHOR INFORMATION Corresponding Author

*Tel: þ81-4-7136-3903. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO). G.I. acknowledges Global COE Program “Chemistry Innovation through Cooperation of Science and Engineering,” MEXT, Japan, for their financial support. ’ REFERENCES (1) Novoselov, K. S.; Geim, A.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (2) Rao, C. N. R.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Angew. Chem., Int. Ed. 2009, 48, 7752–7777. (3) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Solid State Commun. 2008, 146, 351–355. (4) Tombros, N.; Jozsa, C.; Popinciuc, M.; Jonkman, H. T.; Van Wees, B. J. Nature 2007, 448, 571–574. (5) Bunch, J. S.; Van Der Zande, A. M.; Verbridge, S. S.; Frank., I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490–493. (6) Deifallah, M.; McMillan, P. F.; Cora, F. J. Phys. Chem. C 2008, 112, 5447–5453. (7) Peres, N. M. R.; Guinea, F.; Neto, A. H. C. Phys. Rev. B 2005, 72, 174406. 10004

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