p-Type Doping of Graphene with Cationic Nitrogen - ACS Publications

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P-type Doping of Graphene with Cationic Nitrogen Sangwoo Chae, Gasidit Panomsuwan, Maria Antoaneta Bratescu, Katsuya Teshima, and Nagahiro Saito ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02237 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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ACS Applied Nano Materials

P-type Doping of Graphene with Cationic Nitrogen Sangwoo Chae,† Gasidit Panomsuwan,‡ Maria Antoaneta Bratescu,† Katsuya Teshima,§ Nagahiro Saito*,†,∥ †Department of Chemical Systems Engineering, Graduate School of Engineering, Nagoya University, Furo-cho,

Chikusa-ku, Nagoya 464-8603, Japan ‡Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Chatuchak, Bangkok 10900,

Thailand §Department of Materials Chemistry, Faculty of Engineering, Shinshu University, Wakasato, Nagano 380-8553, Japan ∥Conjoint

Research Laboratory in Nagoya University, Shinshu University, Furo-cho, Chikusa-ku, Nagoya 464-8603,

Japan

Supporting Information Placeholder ABSTRACT: Tailoring electrical properties of graphene by nitrogen doping is currently of great significance in a broad area of advanced applications. Bonding configuration of nitrogen atoms in graphene plays the vital role in controlling its electrical, chemical and optical properties. Here, we report for the first time a simple bottom-up synthesis of a novel cationic nitrogen-doped graphene (CNG) by a solution plasma (SP). A mixture of ionic liquid and organic solvent was used as starting precursor. CNG exhibited an orthorhombic structure possibly due to the presence cationic nitrogen in hexagonal graphene lattice. Nitrogen doping content was found as high as 13.4 at%. Electrical characterization demonstrated that the CNG exhibited a p-type semiconducting behavior with superior electrical conductivity and carrier concentration. Such unique electrical characteristics of CNG are mainly attributed to the presence of cationic nitrogen with preserved planar structure. KEYWORDS: cationic nitrogen, solution plasma, graphene, ptype, high electrical conductivity, high carrier concentration Graphene is currently considered as the most attractive 2D material for a broad range of potential applications in electronic, sensor, and electrochemical energy devices1-5 due to its fascinating properties, such as planar structure, large surface area, high thermal/electrical conductivity, and tunable electrical properties. Chemical doping of graphene by heteroatoms can effectively tailor its electronic, chemical, and optical properties to realize graphene with desired functional properties. Among several types of heteroatoms studied, nitrogen can easily be incorporated into the hexagonal lattice of graphene, thus creating redistribution of charge and spin density state in the graphene.6 In general, when a nitrogen atom is doped into graphene, three typical bonding configurations are detectable: (1) quaternary N (or graphitic N) at the basal plane of graphene,7 (2) pyridinic N, and (3) pyrrolic N at the edge site.8 Bonding configuration of N atom doping plays an essential role in influencing electron and hole transport behaviors of graphene. Over the past decade, a significant progress has been paid on the control of N bonding configuration in nitrogen-doped graphene (NG) at specific position by altering processing temperature, synthetic method, and starting precursor.9 Graphitic N could result in n-type semiconducting graphene with high mobility of charge carriers since the excess valence electrons are delocalized.10 In contrast,

pyridinic N and pyrrolic N atoms resulted in either weak n-type or p-type semiconducting behavior.11 Another possible N bonding configuration in graphene is in a cationic form.12 Graphene doped with cationic N is expected to more preserve its planar structure as compared to other N bonding configurations at high N doping levels.13 However, cationic N has been rarely investigated and not well understood at the present stage. Its electrical and charge transport properties are also still unresolved. Therefore, controlling the N bonding configuration with a cationic form in NGs is very challenging for further development towards this exiting area. In recent experimental studies on NGs, they have extensively been synthesized via either direct synthesis (e.g., chemical vapor deposition, solvothermal, and arc discharge14) or post-synthesis treatment (e.g., thermal treatment in NH3 and plasma treatment15). However, most of these methods are complex and requires high processing temperature under a vacuum system. Precursors used are also expensive and highly toxic. With the aim to scale up production of NGs, development of the simpler synthesis method with balance between cost, time, and controllability is urgently needed. Very recently, our group has demonstrated a powerful strategy to synthesize nitrogen-doped carbons (NCs) by a simple method called “solution plasma (SP)”. SP is non-thermal plasma in liquid phase, which can keep reaction temperature close to room temperature during synthesis at atmospheric pressure. A number of both aliphatic and aromatic organic precursors has been used in the synthesis of NCs by SP (e.g. pyridine, aniline, acrylonitrile, benzonitrile, cyanopyridine, etc.).16–20 The N doping content in NCs varied broadly from 0.5 to 7.8 at%, depending on the synthesis conditions and type of precursors.19–21 All NCs synthesized by SP mentioned above were typically in the form of nanoparticles while formation of graphene nanosheets with high N doping level seemed to be far from success by SP. In a further effort by our group, Hyun et al have successfully synthesized N-doped carbon nanosheets (NCNSs) through SP by using N-methyl-2-pyrrolidone (NMP) at specific SP conditions without post-heat treatment.21,22 However, the N doping level was still limited only 1.3–2.3 at%. This interesting finding has shed light on the potential of SP as an alternative bottom-up method to synthesize sheet-like carbon structure and challenge remains on how to increase N doping level with desired bonding configuration.

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Figure 1. (a) TEM image, (b) HRTEM image and corresponding (c) SAED pattern of CNG. The inset of (b) shows enlarged area in yellow rectangle where revealed hexagonal honey-comb lattice structure of CNG. Here, we report for the first time the successful synthesis of cationic N-doped graphene (CNG) by SP. A mixture of 10% 1Ethyl-3-methylimidazolium dicyanamide (EMIM DCA, C8H11N5) and 90% dimethylformamide (DMF, C3H7NO) was used as a precursor. The EMIM DCA ionic liquid (IL) was selected since its high nitrogen content, environmentally friendly solvent (green solvent), stable liquid solution at high temperatures, and very low vapor pressure.23 A bipolar high voltage pulse with high frequency of 200 kHz was applied to a pair of tungsten electrodes that were submerged in the mixed precursor. After discharge, the black solid product was obtained and hereafter denoted as CNG. Integration of several characterization techniques and crystal lattice calculation were used to verify the presence of cationic nitrogen (N+) in CNG. Moreover, the presumable mechanism of CNG by SP was also proposed and described.

RESULTS AND DISCUSSION We first verified the formation of graphene-like structure of CNG by transmission electron microscopy (TEM). Clearly, TEM image in Figure 1a reveals that CNG has sheet-like morphology with lateral size of about 20–30 m. High resolution TEM images of CNG in Figure 1b displays a hexagonal honey-comb lattice structure of graphene. The corresponding selected area electron diffraction (SAED) pattern of CNG is composed of six spots in asymmetrical hexagonal arrangement (P63mc), which refer to 111, 111, 111, 1 11, 202, and 202 lattice points (Figure 1c). This diffraction feature is different from the pristine graphene, which typically has six spots with symmetrical hexagonal arrangement (a = b = 2.470 Å), as shown in Table 1.24 On the basis of crystal lattice calculation, lattice constants, a and b, of CNG were not identical since cationic nitrogen induced the distortion of six-membered ring structure of graphene lattice plane through contraction along a-axis direction. This lattice distortion led to the change in the crystal structure of CNG to an orthorhombic structure (Cmmm) with a and b of 4.016 and 3.350 Å, respectively.

graphite infer its formation of few-layered graphene and disordered stacking of graphene layers. Table 1. Crystallographic calculation of pristine graphene and CNG. Crystal structure Space group Point group

Pristine graphene

CNG

Hexagonal P63mc D6v4

Orthorhombic Cmmm D2h19

a=b a = 2.470 Å b = 2.470 Å c = 6.790 Å  =  = 90,  = 120

ab a = 4.016 Å b = 3.350 Å c = 6.890 Å  =  =  = 90

3D lattice

Lattice parameters

The structural properties of CNG were examined by X-ray diffraction (XRD), as shown in Figure 2a. The XRD pattern shows two pronounced peaks at 25.9° and 43.1°, corresponding to the (002) and (111) planes of orthorhombic structure of CNG, respectively. The 002 interlayer spacing was calculated to be 0.340 nm, which was slightly larger than that of pristine graphite (0.335 nm). Such a larger 002 interlayer spacing might be induced by the presence of defect and N doping in the carbon framework structure. The broadening of 002 peak and the absence of 004 peak of CNG as compared to


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Figure 2. (a) XRD pattern and (b) Raman spectrum of CNG. Furthermore, to gain more structural information, Raman spectroscopic measurement was carried out and is shown in Figure 2b. Raman spectrum of CNG showed the D band at ~1340 cm‒1, G band at ~1580 cm‒1, 2D band at ~2700 cm‒1, and G + D band at ~2940 cm‒1 (a combination scattering peak). These four peaks are typically found for graphene and graphite.25 The D band at about 1340 cm‒1 is typically observed only when the graphene has defects in the crystal structure on the edge of the graphene or has disordered structure.26 Therefore, the emergence of D band of CNG is caused by its internal defects in the graphene structure and the functionalization of the graphene sheet edge. Moreover, the D' band is observed only at N doping, which is not observed in pure graphene.27 The average intensity ratio of 2D band and G band (I2D/IG) obtained from 30 different random spots was found to be about 0.83 ± 0.05 (Figure S4), while full width at half maximum of the 2D band was about 68 ± 3 cm‒1, which corresponded to three-layer graphene.28 These Raman data suggest that CNG synthesized by SP had the uniformity of fewlayer graphene. Another important aspect should be noted here is the shift of G and 2D bands since it can be used to identify type of doping in graphene.29,30 It was found that G and 2D bands shifted about 7 and 4 cm–1 toward lower wavenumber, respectively, as compared to those of pristine graphene. The blue shift of both G and 2D bands indicate the existence of hole doping (p-type) in CNG.

Figure 3. (a) XPS survey spectrum, and (b) high-resolution XPS N 1s spectrum of CNG. (pyridinic N : NP, pyrrolic N : NPR, cationic N : NC, oxidized N : NOX) The X-ray photoelectron spectroscopy (XPS) survey spectrum of CNG was composed of C 1s peak (284.5 eV), O 1s peak (532.5 eV) and N 1s peak (399.5 eV), as shown in Figure 3a. From the quantitative analysis of elemental composition, the content of C, O, and N were 81.4, 5.2, and 13.4 at%, respectively. The N doping content in CNG had a higher level than those reported in previous works (ca. 1–7 at%) (Table S1). Such a high N doping level in CNG is likely due to less decomposition of N atoms under low reaction temperature of SP. The XPS N 1s spectrum can be deconvoluted into four peaks associated with various types of N bonding configurations, including pyridinic N (NP: 398.5 ± 0.1 eV), pyrrolic N (NPR: 400.0 ± 0.1 eV), cationic N (NC: 401.3 ± 0.2 eV), and oxidized-N (NOX: 403.5 ± 0.2 eV),20,31,32 as shown in Figure 3b. The N 1s peak was mainly contributed by NP (48.5%), NC (28.4%), while NPR (20.9%) and NOX (2.2%) were minor contribution. It is important to note that the peak feature of XPS N 1s for CNG was definitely different from that of EMIM DCA reported by Chang et al.33 This result implies that CNG was not physically doped by cationic N from the EMIM molecules, but cationic N was incorporated into the graphene lattice structure. Moreover, the detectable NP and NPR species support Raman spectra results since they induced internal defects of the graphene structure and functionalization of the graphene sheet edge.27 Typically, when a nitrogen atom is doped into graphene lattice, there are three common bonding configurations, including quaternary N (or graphitic N), pyridinic N, and pyrrolic N. However, in this work, the N atoms favored to exist in uncommon form of NC rather than NG. This is likely due to two possible reasons as follows: (i) use of ILs containing cationic N as precursor and (ii) low-temperature reaction occurred in SP preventing its transformation to NG (typically formed at high temperature).



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For the electrical measurements, Hall-effect measurement was performed at 20–290 K on area 4 × 4 mm2 that was patterned on the sample using copper pressure clips in a van der Pauw configuration, at a magnetic field of 0.5 T and current of 70 mA. To test reproducibility, samples were re-measured several times and the measured data were found to be deviated less than 1%. As the temperature was lowered, the mobility and carrier concentration decreased, whereas the resistance increased (Figure 4a–c). This characteristic trend indicates that CNG behave like a semiconductor. Hall mobility and carrier concentration at 290 K were measured to be 3.4 cm2∙V–1∙s–1 and 1019 cm–3, respectively. The carrier concentration is considerably high at the same level of organic semiconductors (see Table S2). The sheet resistance was measured to be 16.0 Ω∙sq–1, which is very low for carbon-based materials ever reported. The result of 4-probe measurement also showed sheet resistance of 15.0 Ω∙sq–1 and resistivity of 0.25 Ω∙cm.

Figure 4. Hall-effect measurements of CNG: (a) hall mobility, (b) carrier concentration, and (c) resistivity versus 1/T (T = 20–290 K).

In the case of pristine graphene, the carbon atom has 4 valence electrons which are generally arranged in a planar hexagonal structure with sp2 hybridization. However, when a nitrogen atom with five valence electrons is introduced into carbon lattice at high doping level, it causes to a strong repulsive interaction between N dopants, which thus cause destabilization of the C–C bond in graphene. Strain induced by N atoms is released by out-of-plane relaxation of carbon atoms in graphene, thus deteriorating the planar structure. This in turn leads to deterioration of electrical conductivity of NG.34 To avoid this unwanted effect, it requires a dopant with 4 valence electrons (herein cationic nitrogen, N+) to maintain planarity of graphene. In contrast to three common N bonding configurations, the presence of cationic N can stabilize and retain the planar structure of graphene. The presence of cationic N in graphene lattice can also facilitate the electron transfer by moving through the cationic sites on the surface of the CNG. Therefore, the superior carrier concentration and electrical conductivity of CNG is possibly attributed to the synergistic contributions of cationic N and its unique planar structure.

Figure 5. Schematic illustration showing the synthesis of CNG from the mixture of EMIM DCA and DMF by SP.


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Presumable formation mechanism of CNG is schematically illustrated in Figure 5. Under the SP reaction field, each EMIM cation of ILs is first bound to another one through the CH activation. The combined two EMIM cations are subsequently oxidized, which make all of the N contained in the EMIM cations transform into cationic N. Due to instability of π-conjugated bonds in the 5 ring, they transform from the 5 + 5 ring to the 6 + 6 ring through the inverse stone wales (ISW) reaction.35 The ISW reaction of EMIM cations contributes mainly to the formation of CNG. Additional reactions include, during the reaction at the time of plasma generation, the EMIM DCA and DMF can be dissociated into intermediates such as free C=C, C=N and C–N–C, and it is assembled with sp2 hybridized carbon containing N for graphene sheet assembly.36,37 DMF is known as an organic solvent in which graphene is easily dispersed.38 The surface tension of graphene is close to that of the EMIM DCA/DMF mixture solution. This provides shielding to prevent the stacking interactions caused by van der Waals interaction and also facilitates effective dispersion of CNG during synthesis. The addition of DMF as solvent does not only allow graphene dispersion during synthesis but also serve as carbon and nitrogen sources through dissociation during plasma generation. Further growth and formation proceed repeatably via the above reactions, eventually forming a large graphene sheet containng cation N.

CONCLUSIONS We have successfully synthesized a novel type of graphene with cationic N doping by plasma assisted method with use of ILs and DMF as precursor. It is believed that EMIM cation and free radicals of ILs serve as the intermediate molecules for the formation of the CNG via the ISW reaction. The characterization results revealed that CNG exhibited a few layer graphene with high N doping content of 13.4 at%. CNG behave like p-type semiconductors with high carrier concentration and electrical conductivity, which is likely due to the presence of cationic N and its preserved planar structure. The combination of SP and ILs shows a promising strategy in design and synthesis of NGs with cationic N doping, which cannot be achieved in other existing methods. The CNG will be enable to potentially use in many applications in various fields, such as sensor, electronic, and energy devices.

METHODS Chemicals. A mixture of 10% 1-Ethyl-3methylimidazolium dicyanamide (EMIM DCA, C8H11N5, 98.0 % purity, Sigma-Aldrich) and a 90% organic solution was prepared, as shown in Figure S1. The organic solution used, dimethylformamide (DMF, C3H7NO, >99.5% purity,) was purchased from Kanto Chemical. Synthesis of CNG by SP. Figure S1 shows a schematic illustration of experimental setup for the solution plasma process in this study. The discharge was generated at the distance between a pair of tungsten electrode (1.0 mm in diameter, 99.9% purity, Nilaco) that were placed at the center of a glass reactor. A gap distance between two tungsten electrodes shielded with an insulating ceramic tube was fixed at 1.0 mm. Mixture of ionic liquid / organic solvent (EMIM DCA, 10 wt% / DMF, 90 wt%) was inserted in glass reaction. A bipolar high voltage pulse was applied to the tungsten electrodes using a bipolar-DC pulsed power supply (KURITA Seisakusho) set to use a pulse voltage of 2 kV, a pulse frequency of 200 kHz and a pulse width of 1.0 μs. Figure S2 shows the voltage and current waveforms of plasma generated during the

synthesis of CNG. After discharging for 5 min, the synthesized powder was separated by vacuum filtration through a polytetrafluoroethylene (PTFE, Merck Millipore, JVWP04700) membrane filter with a pore size of 0.1 μm. The obtained product was washed excessively with distilled water and ethanol and then dried in a vacuum oven at 200 °C for 3 h to completely remove residual IL and DMF that may be physically bound to CNG. Finally, a total weight of 5 mg of CNG was obtained, indicating that synthesis rate was about 1 mg min–1. Higher amount of CNG can be easily obtained by discharging for a longer time (Figure S3). Characterization. The morphology of sample was observed by transmission electron microscopy (TEM, JEM2500SE, JEOL) at an accelerating voltage of 200 kV, using selected-area electron diffraction (SAED) patterns. The X-ray diffraction (XRD, Smartlab, Rigaku) patterns of the synthesized sample were recorded on XRD instrument with Cu Kα radiation of 0.154 nm, an accelerating voltage of 45 kV and a current of 200 mA. The CNG was uniformly packed with a flat surface into the cavity of the glass slide for the measurement. The Raman spectra of the synthesized sample were recorded on a Raman spectrometer (NRS-100, JASCO) equipped with a laserexcitation wavelength of 532.5 nm at room temperature. The spectra were recorded in the wavenumber range from 1000 to 3000 cm–1. The CNG was dispersed in ethanol and dropped cast on a SiO2/Si wafer. X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe II, ULVAC-PHI) analysis was carried out using a Mg Kα X-ray source, a pass energy of 46.9 eV, and an acquisition step of 0.5 eV. Prior to XPS measurement, the sample surface was etched using an argon ion gun to remove the organic surface contamination layer. The resistivity measurement was accomplished with a four probes device (Loresta-GP MCP-T610, Mitsubishi Chemical).

ASSOCIATED CONTENT Supporting Information Typical current and voltage waveforms of CNG by SP, Statistical analysis of the Raman spectra of I2D/IG of the CNG. Characterization, Hall-effect measurement, electrical conductivity of CNG by SP compared to other carbon materials in literature.

AUTHOR INFORMATION Corresponding Author *Tel.: +81-52-789-3259. Fax: +81-52-789-3259. E-mail: [email protected]

ORCID Sangwoo Chae: 0000-0002-7206-5570 Gasidit Panomsuwan: 0000-0003-4316-5035 Maria Antoaneta Bratescu: 0000-0003-4269-5608 Katsuya Teshima: 0000-0002-5784-5157 Nagahiro Saito: 0000-0001-8757-3933

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was financially supported by Core Research for Evolutional Science and Technology (CREST) of Japan Science



and Technology GJPMJCR12L1).

Agency

(JST)

(JST/CREST

Grant

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Table of Content


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Figure 1. (a) TEM image, (b) HRTEM image and corresponding (c) SAED pattern of CNG. The inset of (b) shows enlarged area in yellow rectangle where revealed hexagonal honey-comb lattice structure of CNG.



Figure 2. (a) XRD pattern and (b) Raman spectrum of CNG. 80x102mm (600 x 600 DPI)


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Figure 3. (a) XPS survey spectrum, and (b) high-resolution XPS N 1s spectrum of CNG. (pyridinic N : NP, pyrrolic N : NPR, cationic N : NC, oxidized N : NOX) 80x102mm (600 x 600 DPI)



Figure 4. Hall-effect measurements of CNG: (a) hall mobility, (b) carrier concentration, (c) resistivity versus 1/T (T = 20–290 K). 80x145mm (600 x 600 DPI)


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Figure 5. Schematic illustration showing the synthesis of CNG from the mixture of EMIM DCA and DMF by SP.




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p-Type Doping of Graphene with Cationic Nitrogen - ACS Publications

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