彭鵬，劉洪濤，武斌，湯慶鑫，劉云圻

1東北師范大學(xué)先進(jìn)光電子功能材料研究中心，紫外光發(fā)射材料與技術(shù)教育部重點(diǎn)實(shí)驗(yàn)室，長(zhǎng)春 130024

2中國(guó)科學(xué)院化學(xué)研究所有機(jī)固體院重點(diǎn)實(shí)驗(yàn)室，北京分子科學(xué)國(guó)家研究中心，北京 100190

1 lntroduction

Graphene, a two-dimensional crystal formed by covalent connection of sp2hybridized carbon atoms, is well known for its fascinating physical properties such as quantum electronic transport, extremely high mobility, high elasticity and outstanding optical properties1–5. Specially, a family of functionalized graphene including graphene oxide (GO),reduced graphene oxide (RGO) and functionalized RGO that are readily scaled up provides a much broader horizon for various applications. As a highly hydrophilic layered material due to the vast number of various oxygen-containing functional groups,GO can be exfoliated to stable dispersions in polar solution such as water or ethanol, thereby facilitating the conversion of GO back to graphene by chemical reduction6–9. Although the exact chemical structure of GO is still ambiguous, the most probable structural models are often simplistically assumed to be graphene sheet bonded with hydroxyl and epoxy groups at the upper and lower graphene basal plane, while carboxyl and carbonyl groups at the defect and the edge10. The oxygencontaining functional groups provides a convenient knob for the chemical doping of graphene with foreign atoms such as nitrogen to tailor its chemical and electronic properties11,12. Up to now, while several approaches were demonstrated for the preparation of nitrogen doped graphene, suitable functionalization method for introducing different configuration of nitrogen into GO lattice is still lacking. In this sense, the fundamental open question, i.e., the effects of nitrogen configuration and chemical environment around nitrogen on the resulting graphene properties, have not been addressed, which is important for studying the electrical properties and electrocatalytic activity of nitrogen doped graphene13,14.

Here, we successfully synthesized pyrazine and pyridine nitrogen doped graphene by Schiff base condensation reaction between the amino groups of the o-aryl diamine compound and carbonyl groups of GO, and the effects of nitrogen configuration and electron-withdrawing groups on its electrical properties of nitrogen doped RGO were studied. When GO is doped by the oaryl diamine compound with strong electron-withdrawing groups such as trifluoromethyl groups, the field-effect transistor(FET) devices exhibit p-type doping rather than generally considered n-type doping of nitrogen doped graphene15,16. The symmetry of the transfer curve is significantly reduced, and the Dirac point shifts towards the positive gate voltage. The electron transport characteristic apparently weakens, and the devices show hole-transport-dominated ambipolar behavior. When the trifluoromethyl groups were eliminated and the electronwithdrawing ability of o-aryl diamine compound decreases, the devices gradually shift to ambipolar field effect, or even turning to weak n-type doping effect. Furthermore, the introduction of pyridine nitrogen makes the devices shift to weak p-type doping,indicating the different doping effect of different configuration of nitrogen. This study elucidates the fundamental roles of nitrogen configuration and its chemical environment on graphene electrical properties.

2 Experimental

2.1 Synthesis of graphene oxide

Graphene oxide (GO) was prepared using a modified Hummers method reported by ourselves and other researchers17,18.

2.2 Preparation of BTF-GO/RGO

0.24 g (1 mmol) bis(trifluoromethyl)-1,2-diaminobenzene(BTFMDAB) (97%, Alfa Aesar, USA) was added into 100 mL of absolute ethanol. After ultrasonication, the dispersion turned to be transparent orange-yellow. Then 0.07 g (0.5 mmol) sodium bisulfate monohydrate (NaHSO4·H2O, 99%, Acros) was added into the dispersion and sonicated for 5 min, after which, the reaction system gradually became turbid. 5 mL of concentrated graphene oxide solution was added into the dispersion. The reaction system was stirred at room temperature for three days.Afterwards, the dispersion was washed several times by centrifugation with absolute ethanol and water successively several times to obtain BTFMDAB modified graphene oxide denoted as BTF-GO. Thin film of BTF-GO on SiO2/Si substrate was fabricated by spin-coating at a speed of 2500 r·min-1for 60 s. BTF-RGO was obtained by thermal annealing of BTF-GO at 300 °C for 30 min in a tube resistance furnace under Ar atmosphere.

2.3 Preparation of OPD-GO/RGO and 23DAPGO/RGO

To obtain OPD-GO/RGO and 23DAP-GO/RGO, 0.54 g (5 mmol) o-phenylenediamine (OPD) (98%, Alfa Aesar, USA) and 0.58 g (5 mmol) 2,3-diaminopyridine (23DAP) (98%, Alfa Aesar, USA) were added in 100 mL of absolute ethanol,respectively. The subsequent procedures are the same as the preparation of BTF-GO/RGO.

2.4 Fabrication of the field-effect transistors

About 30 nm thick gold electrode is vacuum evaporated on the RGO films using a shadow mask method. All devices were tested in a glove box filled with nitrogen.

2.5 Characterization

Fourier transform infrared spectra (FT-IR) were obtained with Bruker Tensor 27 Fourier transform infrared spectrometer(Bruker Corporation, Germany) using KBr tablets. TGA measurements were carried out on a Perkin-Elmer thermogravimeter (Model TGA7, PerkinElmer, Inc., USA)under a dry nitrogen flow, heating from room temperature to 800°C, with a heating rate of 10 °C·min-1. X-ray diffraction (XRD)patterns were recorded with RIGAKU D/MAX-TTRIII (CBO,Rigaku Corporation, Japan) with Cu-Kα radiation (λ = 0.154 nm).Raman spectra were measured using a Renishaw inVia Raman Microscope (Renishaw plc, UK), with an excitation wavelength of 514 nm, an exposure time of 10–30 s, and a laser spot size of about 2 μm. Atomic Force Microscopy (AFM) was obtained with NanoMan Dimension V (Veeco Instruments, Inc., USA)with the tapping mode. X-ray photoelectron spectroscopy (XPS)was performed on ESCALab 220I-XL (VG Scientific, USA).The FETs were measured using Keithley 4200 SCS (Keithley Instruments, USA).

3 Results and discussion

A series of nitrogen doped graphene were synthesized via acid-catalyzed Schiff base condensation reaction between aromatic ketone and primary amine groups as illustrated in Fig.1.

The condensation reaction between ketone (C＝O) and amine(―NH2) gives an imine (―C＝N―) linkage, while the reaction of α-diketone (O＝C―C＝O) with 1,2-diamine (ortho-diamine)produces an aromatic pyrazine ring (Fig. 1a). Due to the harsh oxidation process of the modified Hummers’ method, GO contains large amount of oxygen-containing functional groups such as carboxylic, hydroxyl, epoxy and carbonyl groups, which allows the pyrazine-like and pyridine-like nitrogen to be readily introduced into graphitic networks through the pyrazine linkages by the condensation reaction (Fig. 1b). Moreover, the GO compounds with different properties and structures can be synthesized by replacing the hydrogen atoms on the aromatic odiamine with different functional groups. Fig. 1c shows the condensation reactions between GO and bis(trifluoromethyl)-1,2-diaminobenzene (BTFMDAB) to obtain the GO with pyrazine nitrogen and trifluoromethyl groups (BTF-GO),followed by thermal annealing under argon (Ar) atmosphere at 300 °C to obtain nitrogen doped reduced graphene oxide with electron-withdrawing groups (BTF-RGO). Thermal annealing at relatively low temperature rather than high temperature is not only to remove oxygen-containing functional groups in GO, but also to leave the introduced nitrogen atoms and the other functional groups intact.

Fig. 2a–c shows the SEM morphology of GO, BTF-GO and BTF-RGO. All samples showed a typical fully exfoliated structure, and the lateral sizes ranging from hundreds of nanometers to tens of micrometers. The morphology and height of GO and BTF-GO can be directly characterized by atomic force microscopy (AFM) as shown in Fig. 2d–f. The GO sheets have an average thickness of about 1.0 nm, corresponding to about three times of the theoretical thickness of a single-layer graphene (～0.34 nm)19,20. This is due to the fact that the functional groups such as hydroxyl, epoxy and carbonyl groups are present in GO, introducing an additional height to the graphene basal plane. Since the introduction of large size trifluoromethyl groups, the height of BTF-GO increases to 1.2 nm, while the height of BTF-RGO further reduces to 1.1 nm due to the removal of oxygen-containing groups during thermal annealing.

The reaction between GO and BTFMDAB was studied by FTIR spectroscopy as shown in Fig. 3a. The spectrum of GO displays characteristic peaks corresponding to oxygencontaining functional groups, including C＝O (1730 cm-1),C＝C (1622 cm-1), O―H (1400 cm-1), C―OH (1226 cm-1) and C―O (1060 cm-1), which is consistent with literatures21–24. It can be seen that BTF-GO shows a new peak at 2906 cm-1corresponding to C―H stretching mode of the BTFMDAB,while the carbonyl peak at 1730 cm-1disappeared, confirming that only the carbonyl groups are involved in the condensation reaction, and the other oxygen-containing functional groups remain intact. Furthermore, there is a significant increase in the peak at around 1622 cm-1corresponding to in-plane vibrations of aromatic C＝C bond and stretching vibrations of C＝N bond,indicating the formation of C＝N bonds.

Fig. 1 Schiff base condensation reaction of GO and diamines. (a) The formation of C＝N bonds via the Schiff base condensation reaction.(b) The reaction between GO and o-phenylenediamine (OPD) and 2,3-diaminopyridine (23DAP) to obtain the OPD-GO and 23DAP-GO,respectively. Subsequent thermal annealing yielding reduced graphene oxide OPD-RGO and 23DAP-RGO respectively. (c) The reaction between GO and bis(trifluoromethyl)-1,2-diaminobenzene (BTFMDAB) to yield BTF-GO, followed by thermal annealing in Ar atmosphere to obtain nitrogen doped graphene with electron-withdrawing groups (BTF-RGO).

Fig. 2 SEM images of (a) GO, (b) BTF-GO and (c) BTF-RGO. AFM images of (d) GO, (e) BTF-GO and (f) BTF-RGO.

Fig. 3b shows the thermogravimetric analysis (TGA) curves of GO and BTF-GO. As expected, GO is thermally unstable and displays a mass loss starting from even below 100 °C, which is attributed to the elimination of bound water between layers and hydroscopic functional groups25,26. One major mass loss near 200–500 °C is related to the removal of the oxygen-containing functional groups. Almost no weight left at about 700 °C,indicating the removal of all the oxygen-containing functional groups and carbon atoms. On the other hand, due to the formation of more stable pyrazine linkages, BTF-GO displays enhanced thermal stability. The oxygen-containing functional groups in GO completely decompose at around 500 °C, while the mass of BTF-GO gradually decreases due to the decomposition of the trifluoromethyl groups. Therefore, after low temperature (300 °C) thermal annealing, the oxygencontaining functional groups can be effectively removed and the intentionally introduced functional groups can be maintained.

Fig. 3 (a) FT-IR spectra and (b) TGA curves of GO and BTF-GO. (c) XRD diffraction patterns and(d) Raman spectra of Graphite, GO, RGO, BTF-GO and BTF-RGO.

To investigate the interlayer distance variation due to the introduction of functional groups of all samples, X-ray diffraction (XRD) patterns were collected as shown in Fig. 3c.The XRD spectrum of pristine graphite exhibits a sharp strong peak at 2θ = 26.4° corresponding to a d-spacing of 3.37 ? (1 ? = 0.1 nm), while GO shows a broad peak at 9.42°corresponding to a d-spacing of 9.34 ?, which is consistent with the AFM height mentioned above. The oxygen-containing functional groups attached on both sides of graphene basal plane and hydrogen bonds between layers both result in the increase of d-spacing of GO compared with graphite.13Due to the slight reduction of oxygen-containing groups during the condensation dehydration reaction, the peak of BTF-GO shifts to 11.5° (dspacing ～7.70 ?), while the peak of BTF-RGO increases to 21.9°(d-spacing ～4.05 ?). The larger 2θ of BTF-RGO than BTF-GO means the smaller d-spacing of BTF-RGO due to the removal of additional oxygen-containing functional groups during thermal annealing. The d-spacing of BTF-RGO (4.05 ?) is slightly larger than that of RGO (3.85 ?), probably due to the large size of the trifluoromethyl group covalently bonded to the basal plane of graphene, which further confirms that the condensation reaction occurred and the functional groups were successfully introduced.

Raman spectroscopy was used to investigate the structural change of GO before and after the reaction. As shown in Fig. 3d,characteristic D and G peaks can be detected in the range from 1200 to 1800 cm-1in the Raman spectra of all samples. The D peaks (1345 cm-1) originate from the lattice defects, while the G peaks at 1580 cm-1correspond to the in-plane stretching of graphite lattice. Compared with pristine graphite, in which the ratio of the D to G band intensities (ID/IG) is ～0.21, GO shows a broad and strong D peak at 1342 cm-1with a larger ID/IGratio of 0.90, indicating the small size of sp2domains and the increase in structural defects in GO27. Moreover, the G peak of GO at 1604 cm-1shifts upward from that of pristine graphite at 1580 cm-1,due to the isolated double bonds in graphitic networks, which has higher resonance frequency28. Similar to GO, the D and G peaks of BTF-GO are around 1348 cm-1and 1604 cm-1,respectively. The almost identical ID/IGratio of 0.83 indicates that no more topological defects are introduced during the reaction between GO and BTFMDAB. After reduction, the full width at half maximum (FWHM) of the D peak of BTF-GO shows a slight decrease, which means an increase in average size of the sp2clusters in BTF-RGO29. The G peak at 1596 cm-1blue-shifts from that of BTF-GO at 1602 cm-1, due to the recovery of hexagonal network of carbon atoms with defects30,31.The ID/IGratio of BTF-RGO reduces to 0.73, further indicating the efficient structural restoration of graphitic framework and the increase of average size of the sp2clusters.

The chemical composition changes during the condensation reaction and thermal annealing are further studied by X-ray photoelectron spectroscopy (XPS) characterization. XPS survey spectra of GO, BTF-GO and BTF-RGO are presented in Fig. S1(Supporting Information). Due to the C 1s peak changes with the doping and annealing process, the high resolution XPS of the C 1s provides the more detail on the chemical composition of GO.As shown in Fig. 4a, solely C 1s peak at 285.6 eV and O 1s peak at 533.5 eV are observed in GO, while the N 1s at peak 397.8 eV is clearly detected in BTF-GO and BTF-RGO, which indicates that nitrogen atoms have been successfully incorporated into GO even after low temperature thermal annealing. Fig. 4b shows the deconvolution of C 1s peak of GO corresponding to sp2carbon with C＝C bond at 284.7 eV and other oxygen-containing functional carbons such as C―O, C＝O and O＝C―O bonds at around 286.5, 287.5 and 288.6 eV, respectively32–34. It can be seen from Fig. 4a–c that the C 1s peak of BTF-GO is similar to that of GO, while the C 1s peak of BTF-RGO changes significantly. After thermal annealing, the peaks ascribed to C―O bond almost disappear and a new peak at 285.8 ascribed to C＝N bonds can be observed as shown in Fig. 4c. Similar to the high resolution C 1s, the N 1s signal also shows significant changes after the condensation reaction and subsequent thermal annealing (Fig. 4d–f). GO has no detectable nitrogen peaks,while BTF-GO displays N 1s peak at 398.7 eV，which can be deconvoluted into three peaks at 398.3, 399.7 and 400.5 eV,attributed to pyrazine-N, imine-N and amine-N, respectively35,36.The appearance of imine-N and amine-N peaks is due to the reaction between monoketone in GO and o-diamine in BTFMDAB, while the stable aromatic pyrazine rings forms when the reaction between the diketone and o-diamine occurs.Interestingly, after thermal annealing at 300 °C, the N 1s peak of BTF-RGO at 398.4 eV is significantly enhanced compared with other two peaks. It is because that aromatic pyrazine rings are much thermally stable than imine and amines that can be removed during the thermal annealing process, eventually leading to pyrazine-dominated nitrogen doped graphene.

FET devices were fabricated to study the electrical properties of nitrogen doped graphene, and the schematic and SEM image of a typical FET device are shown in Fig. S2 (Supporting Information). In order to confirm that the data is not from the artifacts in the test, the devices were studied in both directions of VDS. In this case, the curve in positive VDSis denoted as n-type transfer, while negative VDSas p-type transfer (Fig. S3(Supporting Information)). FET devices based on RGO usually display p-type characteristics in air, in which drain-source current (IDS) increases with the negative gate voltage increase.This is because the electron carriers on the surface of RGO are trapped by moisture and oxygen in air, resulting in p-type field effect. To remove the doping effect from air, all devices were tested in the glove box filled with nitrogen. Fig. 5a–c shows the transfer curves (n-type, source-drain voltage (VDS) is 1 V) and the corresponding output curves of RGO. The device based on RGO exhibits a symmetrical ambipolar field effect, the p-type and n-type carriers are balanced and the Dirac point is at the 2 V gate voltage. The hole and electron mobilities are calculated by the following equation: μ = (ΔIDS)/(ΔVG)·L/(WCiVDS), where Ciis the specific capacitance of the dielectric, L and W are the channel length (50 μm) and width (1450 μm), respectively, VDS is the drain-source voltage (1 V). The hole and electron mobilities are calculated to be 0.33 and 0.28 cm2?V-1?s-1respectively from the transfer curve in Fig. 5a.

Fig. 5 (a) n-Channel transfer curve and its corresponding (b) p-channel output and (c) n-channel output curves of the field-effect transistor (FET) based on RGO. (d) n-Channel transfer curve and its corresponding (e) p-channel output and(f) n-channel output curves of the FET based on BTF-RGO.

It can be seen from the Fig. 5d–f that the transfer curve of the device based on BTF-RGO shows hole-transport-dominated ambipolar behavior, and the electron transport characteristic weakens significantly. The symmetry of the transfer curve is obviously reduced, and the Dirac point shifts towards the positive gate voltage at about 9 V, clearly indicating a p-type doping effect. The hole and electron mobilities are calculated to be 0.82 and 0.31 cm2?V-1?s-1. The BTF-RGO devices have resulted in a higher hole mobilities (about 2.5 times than that of RGO) and p-doping effect due to the strong electronwithdrawing ability of the trifluoromethyl groups, which increases the hole concentration and improves the hole transport37.

Fig. 6 (a) n-Channel transfer curve, and its corresponding (b) p-channel output and (c) n-channel output curves of the FET based on OPD-RGO.(d) n-Channel transfer curve and its corresponding (e) p-channel output and (f) n-channel output curves of the FET based on 23DAP-RGO.

To unveil the origin for the p-type doping effect of nitrogen doped graphene, we further fabricated FET devices based on RGO modified by o-phenylenediamine (OPD). Similar to BTFMDAB, OPD can also form pyrazine rings by the condensation reaction between α-diketone and o-diamine to obtain nitrogen doped graphene as shown in Fig. 1b. Compared with BTFMDAB, OPD cannot provide strong electronwithdrawing groups into the graphitic framework, and the doping effect mainly originates from the pyrazine-nitrogen. The morphology and height of OPD-GO and OPD-RGO were also characterized by SEM and AFM as shown in Fig. S4 (Supporting Information). The successful condensation reaction between GO and OPD was fully characterized by FTIR, TGA, XPS and Raman (Fig. S5 (Supporting Information)). Fig. 6a–c shows the transfer curve and the corresponding output curves of FET devices based on OPD-RGO. The device exhibits an almost symmetrical ambipolar field effect, the p-type and n-type carriers are almost balanced and the Dirac point is at the -6 V gate voltage, indicating a weak n-type doping effect coming from the pyrazine nitrogen introduced by OPD. This also confirms that the p-type doping effect in BTF-RGO comes from the electron-withdrawing trifluoromethyl groups in BTFMDAB.

To further investigate the doping effect of pyridine nitrogen,2,3-diaminopyridine (23DAP) was obtained by introducing pyridine-nitrogen on the basis of OPD, and the doping effect of 23DAP-GO comes from pyridine and pyrazine nitrogen. As shown in Figs. S6 and S7 (Supporting Information), similar characterizations were performed on 23DAP-GO/RGO to confirm that the nitrogen was successfully introduced into graphitic networks. The devices based on 23DAP-RGO display a hole-transport-dominated ambipolar field effect. The p-type carriers dominate and the curve exhibits poor symmetry. The Dirac point is at around 8 V, indicating a weak p-doping effect on GO. (Fig. 6d–f) This is because the lone pair electrons of pyridine nitrogen atoms in 23DAP cannot participate in the conjugation, and the electronegative nitrogen exhibits an electron-withdrawing effect that counteracts the n-type doping effect from pyrazine nitrogen, eventually making the devices exhibit a weak p-type doping effect. The p-type doping effect of 23DAP-RGO originating from the electron-withdrawing effect of pyridine nitrogen is analogue to that of BTF-RGO with strong electron-withdrawing groups. Those above experiments demonstrate that besides the nitrogen content38,39, the nitrogen configuration and its chemical environment are also important for the doping effect of graphene. The pyrazine nitrogen provides weak n-type doping effect, while pyridine nitrogen exhibits p-type doping effect due to the electron-withdrawing ability.

4 Conclusions

In summary, we successfully synthesized nitrogen doped graphene by Schiff base condensation reaction, and further investigated the effect of nitrogen configuration and chemical environment on its electrical properties. Pyrazine nitrogen was successfully introduced into graphitic framework, which are confirmed by the characterization of XRD, FTIR, TGA,Raman and XPS. FETs based on the BTF-RGO exhibit significantly hole-dominated ambipolar field-effect behavior with the Dirac point at 9 V gate voltage, and the hole mobilities up to 2.5 times of RGO. The weak p-type doping effect originates from the strong electron-withdrawing trifluoromethyl groups. By studying the FETs based on OPDRGO and 23DAP-RGO, in which pyrazine nitrogen, and mixed pyrazine and pyridine nitrogen are contained respectively, we found that pyrazine nitrogen provides weak n-type doping effect, while pyridine nitrogen exhibits weak ptype doping effect due to the electron-withdrawing ability of pyridine nitrogen. The enhancement of p-type doping effect is accompanied by the introduction of groups with stronger electron-withdrawing ability into the graphitic framework. In short, we can achieve fine modulation of doping effect of graphene by introducing different configurations of nitrogen,which would expedite their research and practical applications.

Supporting lnformation： available free of charge via the internet at http://www.whxb.pku.edu.cn.