Fluorescence quenching in N-doped graphene derived from graphitic nitrogen

Abstract

In this work, N-doped graphene (NG) has been synthesized successfully by calcination of dicyandiamide and glucose mixture in an argon atmosphere. The photoluminescence properties of NG were studied systematically. The results show that N-doping can quench its fluorescence and the quenching efficiency depends on the graphitic N content.

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Graphene, a two-dimensional single layer form of sp²-hybridized carbon atoms, has attracted more and more scientific attention because of its extraordinary properties and potential applications since it was discovered in 2004.^1–7 However, the absence of a band gap in pristine graphene leads to it having no intrinsic semiconducting properties. Thus, opening and tuning the band gap of graphene is essential for improving the semiconducting properties and expanding its applications. In general, chemical doping of N is considered an effective method to modify the properties of graphene. Theoretical studies have shown that nitrogen doping can result in the higher positive charge communication between carbon atoms and nearby nitrogen atoms and a positive shift of Fermi energy at the apex of the brillouin zone in graphene.^8,9 In the experimental research, N-doped graphene (NG) has been synthesized successfully by chemical vapor deposition (CVD),^10,11 solvothermal,^12,13 nitrogen plasma process,^14,15 arc discharge,^16,17 thermal treatment,^18,19 N₂H₄ treatment,²⁰ segregation growth²¹ etc.

Notably, fluorescence quenching has been widely used in the selective detection of biomolecules,²² resonance Raman spectroscopy,²³ detection of mercuric ions,²⁴ etc. For properties of N-doped graphene, most of previous research has focused on its electronic and thermal properties, a few work has been done on the optical properties of NG, especially photoluminescence properties.^25,26 Thus, the mechanism of fluorescence quenching in NG is not clear.

In this work, we report a simple and effective approach to synthesis NG by heating a mixture of dicyandiamide as nitrogen source and monohydrate glucose as carbon source under argon as a productive flow (see ESI,† Experimental section).

The doping concentration of N in NG-X (the definitions of NG-X samples is listed in Table 1) is up to 22.44 at% and can be controlled by regulating the mass ratio of precursors. The photoluminescence (PL) properties of NG at room temperature were also studied systematically. The results show that doping can quench its fluorescence and the quenching efficiency is attributed mainly to the graphitic N.

Table 1 Definitions of UD and NG-X samples

Sample	Mass ratio^a X
a X is the mass ratio of dicyandiamide and glucose.
UD	0
NG-5	5
NG-10	10
NG-15	15
NG-20	20
NG-25	25

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Morphology of the obtained samples is characterized by transmission electron microscopy (TEM). The TEM image (Fig. 1(a)) reveals that UD prepared by only heating monohydrate glucose in the same synthesis condition as NG-20's exhibits massive nanoparticles, while the laminar morphology of NG-20 is shown in Fig. 1(b). N-doped nanosheets usually exhibit a typical wrinkled structure with corrugation and scrolling, which results from thermodynamically stable bending.²⁷ Comparing Fig. 1(a) with 1(b), it can be easily seen that dicyandiamide plays a key role of temporary in formation of the two-dimensional structures of NG. The same function of dicyandiamide as temporary was been reported by Li et al.²⁸ Besides, the morphology of NG-20 also is no significant difference with pure graphene, indicating two-dimensional structure morphology is also well maintained after nitrogen doping.

Fig. 1 TEM images of synthesized (a) UD and (b) NG-20. UD was prepared by only heating monohydrate glucose in the same condition as NG-20's (see ESI,† Experimental section).

Fig. 2(a) shows two remarkable peaks and one weak peak in the Raman spectrum of NG-20, which are predominant features of N-doped graphene.²⁹ The two most intense peaks at approximately 1568 cm⁻¹ and 1370 cm⁻¹ are assigned to the G-band and D-band, respectively. The G-band corresponds to the zone center E_2g vibrational mode present in sp² carbon materials, while the D-band is associated with structural defects and partially ordered structures of the sp² domains. The high intensity of the D-band indicates the existence of many defects in NG-20. It is reported that doping of nitrogen introduces large number of topological defects.¹⁰ As a result; the presence of D-band in the Raman spectrum demonstrates the successful doping of N atoms in graphene sheets. The 2D peak is the most prominent feature of graphene in the Raman spectrum.³⁰ Compared with the single-layer graphene, the broad and weak 2D-band observed at 2738 cm⁻¹ indicates that the sample is few-layer graphene, which is consistent with TEM characterizations. Fig. 2(b) displays the X-ray diffraction (XRD) patterns of NG-20 sample, the weak and broad peak from 20° to 40° indicates a loss of long-range order structure,³¹ which is in excellent agreement with Raman spectra analyses.

Fig. 2 (a) Raman spectra of NG-20, the laser excitation wavelength is 532.08 nm. (b) XRD pattern of NG-20. (c) FTIR transmittance spectra of NG-20 and UD. (d) The content of C, N, and O in NG-X samples at different mass ratio X of dicyandiamide and glucose.

Fourier transform infrared spectroscopy (FTIR) was used to analyze the types of chemical bond in NG-20. Taking UD as a reference, the main peaks of UD centered around 3425 cm⁻¹ and 1082 cm⁻¹, which are ascribed to the O–H stretching vibrations and C–O stretching vibrations. For NG-20, in addition to the mentioned peak at about 3425 cm⁻¹, two new peaks appear at 1550 cm⁻¹ and 1220 cm⁻¹, which result from the introduction of dicyandiamide. The new peak at 1550 cm⁻¹ can be attributed to C=C and/or C=N, and another new peak at 1220 cm⁻¹ can be attributed to C–O and/or C–N, respectively. In the meantime, the wide O–H stretching vibrations peak about 3425 cm⁻¹ is weak, which implies the low oxygen content in NG-20 sample. All of these results confirm that nitrogen-atoms were doped effectively into graphene skeleton after the introduction of dicyandiamide.

Elemental analysis was used to analyze the chemical compositions for NG-X samples. Fig. 2(d) shows the content of C, N and O in different NG-X samples. C atoms take over a large part of total atoms range from 69.85% to 74.83%. The number of N atoms is from 17.77% to 22.44%, which is decreasing with the increase of the mass ratio X. The result suggests that the synthetic approach can easily control the nitrogen doping level by adjusting the mass ratio X of dicyandiamide and glucose. The doping concentrations of nitrogen in samples are much higher than previous obtained N-doped graphene.^10,14,16,19 Besides, the presence of O is possible due to the residual oxygen-containing group from glucose. However, the amount of O atoms is keeping a constant at about 8% in all NG-X samples, which is lower than ones obtained by others chemical approaches.^13,20

Furthermore, X-ray photoelectron spectroscopy (XPS) was further performed to analyze the element bonding configuration for NG-X and UD samples. As the wide XPS spectra in Fig. 3(a), three peaks at 284.7, 400.5 and 533.2 eV can be observed clearly in NG-X samples. They can be attributed to C1s, N1s and O1s, respectively, which means that NG-X samples do not contain any impurities. Moreover, the N1s peak is absent in UD's, indicating that N atoms have been doped into carbon network in NG-X. The high-resolution N1s spectra are given in Fig. 4(b)–(f). All the spectra can be resolved into three evident signals at 398.8, 400.6 and 402.3 eV, corresponding to the pyridinic N, pyrrolic N and graphitic N, respectively.²⁹ The three types of N atoms doped into the graphene network are shown in Fig. 4(g). On the basis of the XPS measurements, the contents of different N types in NG-X samples were calculated quantitatively. Fig. 4(h) reveals the content distributions of the three N types in NG-X samples obtained at different mass ratio X of dicyandiamide and glucose, which agrees well with the result of elemental analysis.

Fig. 3 (a) Wide XPS spectra of NG-X samples and UD. (b–f) High-resolution N1s spectra of NG-X samples. (g) Schematic representation of three N types in N-doped graphene. (h) The content distributions of the three N types in NG-X samples.

Fig. 4 (a) PL spectra of the UD and NG-X samples with excitation wavelength 250 nm. (b) The PL intensity of the UD (X = 0) and NG-X samples. (c) The quenching efficiency of NG-X samples at different mass ratio X of precursors. (d) The quenching efficiency at different graphitic N content.

PL spectra were performed to explore the optical properties of NG-X and UD samples. Fig. 4(a) shows the PL spectra of NG-X and UD with an excitation wavelength of 250 nm. A clear emission peak approximately at 403 nm in blue region can be observed easily in all samples. The PL intensity of samples is displayed in Fig. 4(b). Compared to the UD, it can be found that the PL intensity of NG-X has a dramatic decrease. Especially NG-10 is more apparent than other NG-X. Although oxygen groups can result in fluorescence quenching,³² the O concentrations are keeping constants and the N concentrations increase from 17.77% to 22.44% in all NG-X samples. From this into consideration, it can be confirmed that N doping plays dominant roles in fluorescence quenching in graphene, which may attribute to the effective charge transfer between N and graphene as reported by Schedin et al.³³ According the calculations, the quenching efficiency 100(1 − I_NG-X/I_UD)% was obtained. The relation of the quenching efficiency and the mass ratio X of precursors is shown in Fig. 4(c). The function curve trend is consistent with the trend of graphitic N content in NG-X samples at different mass ratio X in Fig. 3(h). Further, the quenching efficiency depends on graphitic N content is displayed in Fig. 4(d). As shown in Fig. 4(d), the quenching efficiency of NG-5 is 46.86%, while a much higher quenching of 69.55% is observed for NG-10. Moreover, it can be found that the quenching efficiency of NG-X is linear with the graphitic N content. In a word, the quenching efficiency results from graphitic N and is increased greatly with the increase of graphitic N content in NG samples. The high quenching efficiency could be attributed to electron transfer between graphitic N and graphene.

In summary, N-doped graphene was successfully synthesized through thermal decomposition of the mixture of monohydrate glucose and dicyandiamide. The synthetic approach can easily controls the nitrogen doping level by adjusting the mass ratio X of dicyandiamide and glucose. The doping concentration of N in NG-X samples is up to 22.44 at% and the three types of N atoms doped into the graphene network are pyridinic N, pyrrolic N and graphitic N, respectively. The study on PL properties of NG-X samples shows that N-doping can quench its fluorescence and the quenching efficiency depends on the graphitic N content.

Acknowledgements

The authors acknowledge the financial support from Natural Science Foundation for Distinguished Young Scholars of Xinjiang (Grant no. 2013711007), Scientific Research Program of the Higher Education Institution of XinJiang (Grant no. XJEDU2013S25), Key Subject of Theoretical Physics of Xinjiang Uygur Autonomous Region (The graduate innovation Fund: LLWLY201105) and National Natural Science of China (Grant no. 51302240).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and characterizations. See DOI: 10.1039/c5ra03721b

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