One-step synthesis of tunable nitrogen-doped graphene from graphene oxide and its high performance field emission properties

Vacuum 168 (2019) 108817

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One-step synthesis of tunable nitrogen-doped graphene from graphene oxide and its high performance field emission properties

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Shuxian Yua,1, Renjie Tanga,1, Kun Zhanga, Siyu Wua, Xinliang Yanga, Wenjie Wua, Yijun Chena, Yan Shenb, Xiaolei Zhangc, Junchao Qiand, Yenan Songa,*, Zhuo Suna a Engineering Research Center for Nanophotonics and Advanced Instrument, Ministry of Education, School of Physics and Materials Science, East China Normal University, Shanghai, 200062, China b State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou, 510275, China c State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200062, China d Jiangsu Key Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, Suzhou, 215009, China

A B S T R A C T

Simultaneous reduction, repairing and doping of graphene oxide has been realized by the chemical vapor deposition method, using acetonitrile as a nitrogen source. A step-wise increase of acetonitrile partial pressure from 15 to 90 Pa results in nitrogen doped graphene (NG) with gradually tuning N-doping concentration from 0.38 to 0.66 at% and systematically rising graphite-N ratio from 23.25 to 45.39%, which in turn modulates field emission performance and enhance the stability. Raman spectroscopy and X-ray photoelectron spectroscopy suggest pyrrolic-N and pyridinic-N doping bring defects, while defects decrease as N-doping concentration increases due to the rising of graphite-N ratio. Proper defects may increase emission site density and N-doping can reduce work function. When N-doping concentration is controlled at 0.42 at%, NG exhibits the considerable decreasing of turn-on field from 3.35 to 2.18 V/μm at the emission current of 10 μA/cm2 and increasing of field enhancement factor from 1835 to 2967. It also reveals a good field emission stability with no degradation, which is superior to pristine reduced GO emitters. It is suggested the NG with tuning concentration of three type nitrogen emitter is a widely candidate for various field emission devices and applications.

1. Introduction Since its first exfoliation from graphite in 2004 [1], graphene has attracted much attentions in material science and industry [2–4]. Graphene is considered as a promising electron emitter due to its electrical conductivity, carrier mobility, high thermal conductivity, and chemical stability [5–8]. Furthermore, the presence of numerous edges may render graphene superior to CNTs for electron tunneling [9–12]. For carbon nanomaterials, previous studies have shown that nitrogendoping is an effective method to reduce tunneling potential barriers, which can reduce the turn-on field and significantly increase the electron emission current [13–21]. Nitrogen atoms can provide more valence electrons to carbon nanomaterials, thereby increasing the state density of electrons and reducing the Fermi level. In the case of graphene, theoretical researches have also indicated that doping is one of the most feasible methods to control the semiconducting properties of graphene, which can modify the electronic band structure of graphene and modulate work function [22,23]. Since the characteristic of field emission has a close relationship to emission site density and work function of the graphene, doping is a practicable way to enhance its

field emission performance. To date, many studies have shown that N can be introduced into graphene and graphene oxide (GO) during synthesis or by post treatments [24–35]. Direct synthesis methods mainly include arc-discharge of a graphite electrode [24], chemical vapor deposition (CVD) with methane (CH4) and ammonia (NH3) [25,26] and solvothermal synthesis [27]. Post treatment methods mainly include thermal annealing GO with NH3 [28–30] and graphene treated with nitrogen plasma [14,31,32]. Of them, arc-discharge and plasma methods may result in many lattices defects and special instruments or rigorous conditions are certainly required. Solvothermal method requires a large number of organic solvents, which is not eco-friendly. CVD is the common approach for the synthesis of nitrogen doped graphene (NG). On the other hand, few experiments have reported the effects of N-doping concentration and nitrogen doping types on the field emission properties. In this work, we demonstrated a one-step method to simultaneously realize reduction, repairing and doping of GO by CVD. Acetonitrile (C2H3N) was used as nitrogen source and different N-doping concentration was controlled by changing partial pressure. We used pointtyped graphene field emitters, namely graphene emitters with a small

*

Corresponding author. E-mail address: [email protected] (Y. Song). 1 Shuxian Yu and Renjie Tang contributed equally. https://doi.org/10.1016/j.vacuum.2019.108817 Received 23 June 2019; Received in revised form 12 July 2019; Accepted 12 July 2019 Available online 17 July 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.

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emission area, to increase edge emission. Meanwhile, the effect of Ndoping concentration and types on the field emission properties have been researched by characterization of microstructure. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy revealed appropriate N-doping introduced defects, which can enhance emission site density and reduce work function. Compared with reduced graphene oxide (RGO), the NG showed significant improvement in field emission characteristics by lowering the turn-on field and increasing the field enhancement factor (β). We considered point-typed NG has a broad application prospect, especially for X-ray sources or microwave devices [36]. 2. Experimental details 2.1. Synthesis of GO GO was synthesized from graphite powder by a modified Hummers’ method. In a three-necked flask, 0.6 g graphite powder and 0.6 g sodium nitrate were mixed with 35 ml concentrated H2SO4. The mixture was vigorously stirred in an ice bath below 20 °C for 1 h. Then 3.6 g KMnO4 was added within 1 h followed by 2 h of stirring. Afterwards, the obtained solution was transferred to a 35 °C water bath and stirred for another 30 min. Next, 150 ml deionized water was added into the solution within 1.5–3 h. The solution was heated up to 98 °C followed by 15 min of stirring and then became brown. It was then transferred into the deionized water of 60 °C. When the solution was cooled, 10 wt % H2O2 was introduced, changing the solution from brown to yellow till it stayed invariant. Several hours later, the settled contents were washed with 5% HCl aqueous solution and then washed five times with deionized water. Finally, the cleaned contents were dried using a freeze-drying equipment to obtain the GO.

Fig. 1. Fabrication process of the NG emitter and schematics of the field emission measurement: (a)–(b) fabrication process of the GO emitter; (c)–(d) fabrication process of the NG emitter by the CVD method and (e) a schematic of the field emission measurement.

2.2. Emitter fabrication The GO emitter, which has a small tip-typed emission area, was fabricated by attaching GO paste onto the graphite rod. The organic adhesive was fabricated by mixing ethyl cellulose and terpineol at a mass ratio of 1:20 in a water bath at 80 °C for 4 h, and stirring with a magneton. The GO paste was fabricated by thoroughly mixing GO and organic adhesive paste in proper proportion. Fig. 1a and b showed the fabrication process of the GO emitter. The GO paste was attached to the graphite rod with an inner diameter of 0.5 mm. Then it was placed into a constant temperature box at 60 °C for 30 min to remove the residual solvent existing on the surface of GO. Fig. 1c and d showed the RGO and NG emitters were fabricated by reducing and doping GO with CVD method. RGO was obtained after reducing by hydrogen without nitrogen source. C2H3N was used as nitrogen source to fabricate NG and vapor concentration was modulated by real-time pressure control system. The GO emitter was placed into the CVD furnace and the vacuum degree was evacuated below 1 Pa. With a flow of 10 Pa hydrogen, the furnace was heated up to 1000 °C within 1.5 h and kept for another 30 min. Certain acetonitrile (15, 30, 45, 90 Pa) was introduced into the flow for preparation of NG emitters. The emitters were then cooled to room temperature under hydrogen ambient after 1 h growth. Fig. 1e showed the schematic of the field emission measurement. The emitter was fixed on the stannum holder, which was stuck on the ITO glass with the conductive adhesive. The distance between the anode and the emitter was controlled at 150 μm.

(XPS, PHI 5000Versaproblll, with X-ray beam at 200 μm). 3.1.1. Scanning electron microscope (SEM) Fig. 2 showed the SEM images of NG flakes, where the vertical alignments to the substrate could be expressly observed. The NG had many micron-meter sized sharp edges, which were suitable for high emission site density. 3.1.2. Raman spectroscopy and XPS The Raman spectroscopy of graphene has four bands, denoted as D, G, D′, G′ band [37]. D peak (1320-1350 cm−1) has no relationship to the number of graphene layers but depends solely on the level of disorder. G peak (1570-1585 cm−1) is derived from the first-order Raman resonance scattering process of single phonon, reflecting the degree of graphitization. D'peak is at 1602-1625 cm−1, which originates from the symmetry breaking on account of finite sp2 crystallite size and presents as a shoulder of the G band. G′ peak is mainly located at 26402680 cm−1, which is the characteristic peak of graphene [38–40]. Studies have shown that the ratio between the intensity of D and G peak (ID/IG), varies inversely with the size of the crystalline grain or interdefect distance [41–43]. As a consequence, ID/IG can be used to represent the defect concentration. Fig. 3a showed plenty of defects were introduced at 15 Pa partial pressure of C2H3N. As the pressure increased, ID/IG decreased, which meant the concentration of defects decreased. Studies have confirmed that the concentration of defects could neither be too low nor too high. Chen [44] found that defects in carbon materials could emit electrons, so the existence of defects improved the field emission properties of carbon materials. While the defect concentration was excessively high, it greatly increased the content of amorphous carbon in carbon materials, which increased the work function and reduced the β as a

3. Results and discussion 3.1. Characterization The samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800, 5 kV), Raman spectrometer (NERCN-TC-011, with laser excitation at 532 nm) and X-ray photoelectron spectroscopy 2

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Fig. 2. SEM images of NG flakes with 30 Pa partial pressure of acetonitrile: (a) image magnified in 2 k; (b) image magnified in 10 k; (c) image magnified in 20 k and (d) image magnified in 40 k.

XPS was used to research the binding energy of the C1s and N1s photoelectrons of NG. The elemental composition of our samples were examined by XPS. As shown in Table 1, the concentration of oxygen nearly went down to zero from GO to NG, which proved the reducing process. Moreover, the higher partial pressure of C2H3N was, the more nitrogen was doped into the graphene. Thus, successive increment of partial pressure of acetonitrile virtually controlled the N-doping level.

consequence. Besides, a large number of defects became the vulnerable objects of ion bombardment in field emitting, which produced larger defects, resulting in the destabilization of emission current density. Meanwhile, because the N-doping provided excess electron carriers, the work function of graphene decreased with the N-doping level [19]. The combined effect of the two factors led to an enhanced field emission response and stability.

Fig. 3. (a) Raman spectrums of NG with different partial pressure of acetonitrile; (b) XPS N1s of NG with different partial pressure of acetonitrile. 3

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position, filling the original defective position, and contributing to repair the defects, which was confirmed by XPS measurements. When the N atom in hydrogen cyanide was bonded to two carbon atoms forming five-member ring, the pyrrolic-N was formed. The pyridinic-N bonded to two carbon atoms forming hexatomic ring. As shown in Fig. 4c, nitrogen atoms in pyrrolic-N or pyridinic-N only bonded to another two carbon atoms, therefore, the graphene honey-comb structure was destroyed with defects introducing.

Table 1 Compositions of the C, O, N elements and ratios of three doping-typed N content in total nitrogen content of different samples. Samples

C-total

O-total

N-total

Graphitic-N

Pyrrolic-N

Pyridinic-N

GO RGO NG(15 Pa) NG(30 Pa) NG(45 Pa) NG(90 Pa)

99.16% 100% 99.62% 99.58% 99.50% 99.34%

0.84% < 0.1% < 0.1% < 0.1% < 0.1% < 0.1%

0.38% 0.42% 0.50% 0.66%

23.25% 36.05% 42.60% 45.39%

43.52% 34.50% 30.61% 28.66%

33.23% 29.45% 26.79% 25.95%

3.2. Field emission properties 3.2.1. The proper mass fractions of GO for field emission 1 wt%, 3 wt%, 5 wt% and 11 wt% GO paste were fabricated and made into RGO emitters respectively. Fig. 5a showed the field emission current density (J) as a function of electric field (E) for RGO emitters with different mass fractions of GO. The turn-on and threshold fields were defined as the macroscopic fields required for emission current densities of 10 μA/cm2 and 1 mA/cm2, which were shown in Table 2. It was clearly observed that the increase of mass fractions of GO (1 wt %-5 wt%) resulted in the turn-on field decreasing from 5.03 to 3.43 V/ μm and threshold field dropping from 8.26 to 5.40 V/μm. While the mass fractions of GO was up to 11 wt%, the turn-on and threshold fields increased to 5.44 V/μm and 9.74 V/μm, respectively. Fig. 5b presented the Fowler–Nordheim (F–N) plots corresponding to the J–E curves shown in Fig. 5 (a). The F–N equation was given by Refs. [54,55].

The N1s spectroscopy was separated into three different peaks at 398.1–399.3 eV (N1), 399.8–401.2 eV (N2) and 401.1–402.7 eV (N3) [45]. Peak N1 was corresponding to a pyridinic structure in which the nitrogen atom was bonded to two carbon atoms. Peak N2 was commensurate with pyrrolic-type nitrogen where the nitrogen atom bonded with a five-member ring. The peak N3 corresponded to the presence of substitutional nitrogen in aromatic graphene [46–48]. N1, N2, N3 were named as pyridinic-N, pyrrolic-N and graphitic-N respectively. The binding energy was corrected by setting the binding energy of the hydrocarbon C 1s feature to 284.8 eV. G. Greczynski and L. Hultman [49,50] found that the bond energy of C 1s peak EBF is closely correlated to the sample work function φSA , such that the sum EFB + φSA is constant. Since the N-doping concentration is from 0.38 to 0.66 at%, the work functions of different samples differ very little [51]. Fig. 3b showed the XPS N1s and bands of pyrrolic-N, pyridinic-N and graphitic-N of our samples by XPS-peak-differentation-imitating analysis. The area under the band represented the content of the kind of nitrogen. The ratios of graphitic-N content in total nitrogen content of different samples were calculated as shown in Table 1. It was suggested that the content of graphitic-N went up as the increase of partial pressure of C2H3N, as a result, the defects decreased. It was completely consistent with Raman spectroscopy results in Fig. 3a. Fig. 4 showed the formation of three doping-types of NG. A study on the thermal decom-position of C2H3N (in the absence of oxygen) was performed [52,53], which proved hydrogen cyanide (HCN) and methane (CH4) were the major decomposition products. The graphitic-N was formed by the N atom in HCN, replacing the original carbon

ln(J/E2)=(-6.44 × 103 × Φ1.5)/βE + C Where Φ was the work function of our samples, which was considered to be same as that of graphene at 5 eV. In single-layer graphene, electrons mimic massless Dirac fermions and follow relativistic carrier dynamics. Thus, their behavior deviates significantly from the nonrelativistic electrons and the electron field emission deviates from the classic F–N equation, which have been extensively studied in recent years [56]. While in our work, graphene was multilayered and plots in the higher electric field region showed linear fit indicating that the emission conformed to the classic F–N law. β could be calculated from the slope (K) of F–N plot and Φ, which was listed in Table 2. Fig. 5c

Fig. 4. N-doping process diagram: (a) pristine GO; (b) the pyrolysis of acetonitrile and the synthesis of NG and (c) three doping-types of NG. 4

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Fig. 5. (a) Field emission J-E curves of different mass fractions of RGO; (b) field emission F–N curves of different mass fractions of RGO and (c) field emission stability of different mass fractions of RGO for 1 h.

partial pressure of C2H3N. The rising of surface defects on NG produced more emitter sites, contributing to the field emission current. The field emission properties of NG was dramatically improved as numerous electrons emitted from defects, which could explain the field emission properties of NG with 15Pa and 30 Pa C2H3N were superior to NG with 45Pa and 90Pa C2H3N. However, superabundant defects led to the rise of amorphous carbons in NG, which increased the work function and reduced the β as a consequence. Therefore, there was an optimal defect density in NG when ID/IG was 0.56. In the meantime, N-doping modified the electronic band structure and modulated work function, which enhanced the β. With the combination of the N-doping concentration and defect density, NG exhibited the most enhanced field emission properties when the C2H3N partial pressure was controlled at 30Pa. Fig. 6c showed the field emission stability of each emitter for continued operation of 2 h at a constant applied field with starting current setting at 40–55 mA/cm2. As observed, NG emitters showed superior stability with little degradation rate, compared with RGO emitter. The rising of surface defects of NG produced more emitter sites, contributing to the field emission current. Thus, a stable field emission behavior was attributed to an even distribution of the emitting sites via N-doping.

Table 2 The turn-on fields (V/μm), the threshold fields (V/μm) and β for different mass fractions of RGO. samples

turn-on fields (V/μm)

threshold fields (V/μm)

β

1% 3% 5% 11%

5.03 4.47 3.43 5.44

8.26 7.73 5.40 9.74

1182 1169 1674 882

showed all the RGO emitters had the good emission stability for 1 h with initial current setting at 15–20 mA/cm2 at an constant applied field. There was little degradation rate of the emission current after 1 h. It was clear that the RGO emitter with 5 wt% GO had the minimum turn-on and threshold field and the maximum β. It was considered that when mass fraction of GO was excessively high, the GO could not be fixed well for lack of organic adhesive. Whereas, when mass fraction of GO was excessively low, the emission sites decreased, which impaired field emission properties. The GO was controlled at 5 wt% in later fabricating RGO and NG emitters. 3.2.2. Different N-doping concentration of NG for field emission NG emitters were fabricated with different partial pressure of C2H3N (15, 30, 45, 90 Pa) and their field emission properties were measured. The RGO emitter was used as a comparison. Fig. 6a showed the field emission J-E curves of different NG emitters. The turn-on and threshold fields of each emitter at the emission current density of 10 μA/cm2 and 1 mA/cm2 were shown in Table 3. Compared with RGO, NG had a lower turn-on and threshold field. As shown, there was a considerable decrease in the turn-on and threshold fields of 2.18 V/μm and 4.82 V/μm when partial pressure of C2H3N was 30 Pa. Fig. 6b showed the field emission F–N curves of different NG emitters. Careful consideration of the F–N trend revealed that the increment of partial pressure of C2H3N from 15 to 30 Pa caused the β improve from 2363 to 2907. While β decreased from 2306 to 1866 as partial pressure of acetonitrile increased from 45 to 90 Pa. As shown in Raman and XPS tests, plentiful defects were introduced at 15 and 30 Pa

4. Conclusion The tuning NG was successfully fabricated from GO by the acetonitrile CVD. A step-wise increase of C2H3N partial pressure of 15–90 Pa resulted in NG with gradually increasing N-doping concentration of 0.38–0.66 at% and systematically rising graphite-N ratio of 23.25–45.39%. Raman and XPS spectrums indicated N-doping introduced defects, while the defects decreased as N-doping concentration increased, which was caused by the rising of graphite-N ratio. When the C2H3N partial pressure is controlled at 30Pa, namely the Ndoping concentration is 0.42 at%, NG exhibited the most enhanced field emission properties. Compared with RGO, NG showed significant improvement in field emission characteristics by lowering the turn-on field from 3.35 to 2.18 V/μm and improving β from 1835 to 2967. It was mainly attributed to more emission sites, which was caused by N5

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Fig. 6. (a) Field emission J-E curves of different NG emitters; (b) field emission F–N curves of different NG emitters and (c) field emission stability of different NG emitters. Table 3 The turn-on fields (V/μm), the threshold fields (V/μm) and β for different NG emitters. samples

turn-on fields (V/μm)

threshold fields (V/μm)

β

RGO NG(15 Pa) NG(30 Pa) NG(45 Pa) NG(90 Pa)

3.35 2.56 2.18 2.71 3.15

5.74 4.92 4.82 5.36 5.10

1835 2363 2967 2306 1866

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