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Cite this: DOI: 10.1039/c4nr03145h
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Covalently coupled hybrid of graphitic carbon nitride with reduced graphene oxide as a superior performance lithium-ion battery anode† Yongsheng Fu,‡ Junwu Zhu,‡ Chong Hu, Xiaodong Wu and Xin Wang* An in situ chemical synthetic approach has been designed for the fabrication of a covalently coupled hybrid consisting of graphitic carbon nitride (g-C3N4) with reduced graphene oxide (rGO) with differing g-C3N4/rGO ratio. The epoxy groups of graphene oxide (GO) undergo a nucleophilic substitution reaction with dicyandiamide (C2H4N4) to form the C2H4N4–GO composite via a covalent C–N bond, and then both the in situ polymerization of C2H4N4 and the thermal reduction of GO can be achieved at higher temperatures, forming the covalently coupled g-C3N4–rGO. FT-IR, CP-MAS NMR and XPS analyses, clearly revealed a covalent interaction between the g-C3N4 and rGO sheets. The g-C3N4–rGO exhibits an unprecedented high, stable and reversible capacity of 1525 mA h g−1 at a current density of 100 mA g−1 after 50 cycles. Even at a large current density of 1000 mA g−1, a reversible capacity of 943 mA h g−1 can still be retained. The superior electrochemical performance of g-C3N4–rGO is attributed to the specific
Received 9th June 2014, Accepted 8th August 2014
characteristics of the unique nanostructure of g-C3N4–rGO and the concerted effects of g-C3N4 and
DOI: 10.1039/c4nr03145h
rGO, including covalent interactions between the two moieties, the good conductivity and high special surface area of the nanocomposite, as well as the template effect of the planar amino group of g-C3N4
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for the dispersed decoration of Li+ ions.
Introduction The increasing energy demands along with the depletion of conventional fossil fuel reserves have stimulated intense research on alternative energy conversion and storage systems. Technological improvements in rechargeable solid-state batteries are being driven by the demands for portable electronic devices.1–3 Among the rechargeable batteries, lithium ion batteries (LIBs) have become the predominant battery technology due to their characteristics of high-energy density, flexible and lightweight design, and long lifespan in comparison to competing battery technologies.4 Generally, the active materials employed in LIBs for the consumer electronics market are made up of a lithium cobalt
Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, 210094 Nanjing, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: N species content (at%) of g-C3N4–rGO-1 based on XPS analysis; the first cycle discharge capacity, charge capacity and coulombic efficiency for the g-C3N4, rGO, and g-C3N4–rGO-n (n = 0.25, 0.5, 1, 2, 3) electrodes; nitrogen adsorption/desorption isotherm of g-C3N4; cycle performance of the g-C3N4–rGO-n (n = 0.25, 0.5, 3) electrodes at a current rate of 100 mA g−1 between 3.0 and 0.01 V versus Li+/Li. See DOI: 10.1039/ c4nr03145h ‡ These authors contributed equally to this work.
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oxide cathode paired with a graphite anode. Graphite has many advantages, such as low and flat voltage range, high Coulombic efficiency, good cyclability and low cost, however, bulk graphite can react with Li to form LiC6 structures with a lower specific capacity of graphite of 372 mA h g−1, significantly limiting the specific energy of LIBs.5 Unlike bulk graphite, graphene sheets can react with Li to give LiC3 structures by the effect of Li being stored on both sides of each nanosheet. Theoretically, such a structure can double the specific capacity of graphite. Moreover, graphene sheets have a large specific surface area, remarkable electrical conductivity, excellent adsorptivity, and high chemical and thermal stability.6–10 Therefore, diverse graphene-based materials for anodes have been developed to make more active spaces for Li storage.11,12 Recently, it has been found that the electronic and chemical properties of graphene can be modified by chemical doping heteroatoms, such as nitrogen atoms.13–16 For instance, the pyridinic N atoms can effectively improve the reversible capacity of a nitrogen-doped graphene electrode due to the stronger electronegativity of nitrogen compared to that of carbon.17 Cheng et al. prepared the N-doped graphene electrodes with a high capacity of 1043 mA h g−1 at a low current density of 50 mA g−1, much improved Coulombic efficiency and cycle performance in comparison with undoped graphene.15 Cai et al. reported that the high-level N-doped
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graphene exhibits a high initial reversible capacity of 1123 mA h g−1 at a low current density of 50 mA g−1 and the high N-doping level is conducive to improve the rate capability and reversible capacity.16 Unfortunately, N-doping levels in graphene are usually lower than 6 at% because of the lack of available synthetic methods via the substitution of carbon by nitrogen atoms.18 As an analogue of graphite, graphitic carbon nitride (g-C3N4) is composed of ordered tri-s-triazine subunits connected through planar tertiary amino groups in a layer and also has a stacked two-dimensional structure, which can be regarded as N-substituted graphite with the highest nitrogen-doping level.19 Recently, g-C3N4 has attracted considerable attention owing to its unique electronic and optical properties, which show promising potential applications in optoelectronic conversion, water-splitting photocatalysis and the degradation of organic pollutants.20,21 However, the low electrical conductivity of g-C3N4 derived from the intrinsic porous microstructure largely reduces the electrochemical properties, becoming a bottleneck in its application in the area of electrochemistry.22,23 If g-C3N4 is hybridized with highly-conductive graphene sheets via covalent bonds, then it may be possible to obtain a stable and high capacity anode material for LIBs as a result of the interaction of individual components as well as the concerted effect. Herein, we have developed an in situ approach towards the fabrication of a covalently coupled hybrid of g-C3N4 with rGO as an anode material for lithium storage. Graphene oxide (GO) was prepared from powdered flake graphite by a modified Hummer’s method.24 The epoxy groups of GO undergo a nucleophilic substitution reaction with dicyandiamide (C2H4N4) to form the C2H4N4–GO composite via a covalent C–N bond. At higher temperatures, both the in situ polymerization of C2H4N4 and thermal reduction of GO can be achieved, forming the covalently coupled g-C3N4–rGO nanohybrid. The nanohybrid exhibits an unprecedented high, stable and reversible capacity of 1525 mA h g−1 at a current density of 100 mA
Fig. 1
g−1 after 50 cycles, suggesting a remarkably promising candidate for energy storage.
Results and discussion Structure and morphology In order to clarify the formation process of g-C3N4–rGO, a variety of techniques, such as Fourier transform infrared (FT-IR) spectroscopy, 13C magic-angle spinning (MAS) NMR spectroscopy, powder X-ray diffraction (XRD) and X-ray photoelectron (XPS) spectroscopy were employed for detailed characterization. As shown in Fig. 1(a), GO exhibits the following IR features in the range of 1000–2000 cm−1: CvO stretching vibrations of the COOH groups (1726 cm−1), O–H deformation vibrations of the COOH groups (1620 cm−1), O–H deformation vibrations of tertiary C–OH (1396 cm−1) and C–O stretching vibrations of the epoxy groups (1050 cm−1).25 It is noted that the epoxy group peak of C2H4N4–GO at 1050 cm−1 of GO is largely reduced. This may be ascribed to the fact that the epoxy groups of GO have undergone ring-opening with nucleophiles via an SN2 mechanism.26 For pure g-C3N4, the absorption peaks at about 1635, 1534, 1406, 1318 and 1238 cm−1 in the range of 1000–2000 cm−1 (Fig. 1(b)) may be assigned to the typical stretching modes of CN heterocycles.27–30 The absorption peaks at 807 cm−1 is attributed to the characteristic breathing mode of triazine units, which is in agreement with published work on bulk g-C3N4.27–30 For g-C3N4–rGO, in addition to the above peaks of g-C3N4, the adsorption around 1570 cm−1 is assigned to the stretching vibrations of the graphene nanosheets because of the thermal reduction of GO.27 Magic angle spinning solid-state NMR (MAS NMR) spectroscopy is one of the most powerful techniques for addressing issues of molecular structure, especially for insoluble or poorly crystalline materials. Fig. 2(a) shows the 13C MAS NMR spectra of C2H4N4, GO and C2H4N4–GO. Three distinct signals
FT-IR spectra of C2H4N4, GO, C2H4N4–GO, pure g-C3N4 and g-C3N4–rGO.
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Fig. 2 rGO.
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(a) Solid-state 13C MAS NMR spectra of C2H4N4, GO and C2H4N4–GO. (b) Solid-state 13C CP-NMR spectra of pure g-C3N4, rGO and g-C3N4–
dominate the spectrum of GO: signals at 58.9 and 69.1 ppm are assigned to the 13C nuclei in the epoxy and hydroxyl groups, respectively; the resonance at 129.2 ppm corresponds to the un-oxidized sp2 carbons of the graphene network. Besides, there is a weaker signal at 228.1 ppm, which presumably arises from the carbonyl groups.31–33 For C2H4N4, the signals at 158.8 and 167.5 ppm can be assigned to sp and sp2 carbons, respectively. In comparison with GO and C2H4N4, the solid-state 13C MAS NMR spectrum of C2H4N4–GO exhibits two additional resonances, the shoulder at 119.9 ppm can be assigned to sp3 hybridized carbon atoms, giving important evidence for the C–N covalent interaction between C2H4N4 and GO. Another signal centred at 19.8 ppm is assigned to the sp3 hybridized carbon of CH3 in the GO sheets.33 Whereas, no CH3 signal is observable for GO alone most likely due to the steric effect caused by small interlayer spacings, which may hinder the motion of CH3, enhancing NMR residual dipolar
couplings. Fig. 2(b) shows the solid-state 13C cross-polarisation nuclear magnetic resonance (CP-NMR) spectra of g-C3N4, rGO and g-C3N4–rGO. The spectrum of g-C3N4 gives two peaks at 155.6 and 164.3 ppm corresponding to the sp2-hybridized carbon atoms of CN2(NHx) and CN3 in the g-C3N4 networks, respectively.34,35 The spectrum of rGO shows a broad resonance in the range of 70–150 ppm, which is the feature of chemical shift anisotropy of the 13C nucleus.32 Besides, there are no typical resonances of oxygenated carbons, which can further confirm the thermal reduction of GO and the formation of the graphene structure at higher temperatures. When compared with rGO, the solid-state 13C MAS NMR spectrum of g-C3N4–rGO exhibits a much broader resonance in the range of 100–200 ppm, implying the in situ covalent doping of g-C3N4 nanosheets onto the rGO sheets. The XRD diffraction patterns of g-C3N4–rGO, rGO, g-C3N4 and GO are shown in Fig. 3(a). The diffraction pattern of
Fig. 3 (a) XRD diffraction patterns of GO, pure g-C3N4, rGO and g-C3N4–rGO in the range of 5–80°. (b) Raman spectra of pure g-C3N4, GO and g-C3N4–rGO.
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g-C3N4 is in good agreement with the recent reports on g-C3N4 prepared by polymerization of dicyandiamide and melamine.19,27,30,36 The peak at a 2θ value of 27.51° can be indexed to the (002) crystal plane of the stacking of the conjugated aromatic system with an interplanar distance of 0.326 nm. The peak at 13.10° can be indexed to the (100) crystal plane of in-planar ordering of tri-s-triazine units with a distance of 0.675 nm. While GO exhibits a sharp peak at 10.80° for the (001) facets corresponding to 0.87 nm interlayer spacing.25 The rGO shows a broad diffraction peak at 24.54°, corresponding to the (002) interlayer stacking with a distance of 0.388 nm.36 It is interesting that g-C3N4–rGO gives a diffraction peak at 26.04° with a layer-to-layer distance of 0.362 nm, which may be assigned to the interlayer stacking between g-C3N4 and rGO through covalent interactions. Besides, no typical diffraction peak of GO (001), rGO (002) or g-C3N4 (002) is observable. It is speculated that the rGO in the g-C3N4–rGO was fully exfoliated due to the in situ growth of g-C3N4 nanosheets on both sides of rGO nanosheets. The intercalation of rGO and g-C3N4 each other can effectively prevent the g-C3N4 stacking and the rGO restacking. This is why g-C3N4–rGO shows a diffraction pattern different from g-C3N4, GO or rGO. Raman spectroscopy is one of the most sensitive and informative techniques to characterize the disorder in sp2 carbon materials. When compared with GO (Fig. 3(b)), the G-band of
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g-C3N4–rGO remarkably shifted from 1565 to 1588 cm−1 and the D-band shifted from 1336 to 1351 cm−1 caused by the covalent doping of g-C3N4, which are similar to the results reported on N-doped graphene systems.15,37,38 While the intensity ratio of the D- to G-band (ID-band/IG-band) is 1.12, significantly larger than that of the GO (0.70), indicating that the GO has been reduced to rGO, which has much higher conductivity. Raman spectroscopy can also be used to investigate the single-, bi-, and multilayer characteristics of the graphene and graphene oxide layers. It can be clearly seen that the 2D peak position of g-C3N4–rGO is observed at 2680 cm−1 with a symmetrical shape, indicating rGO has been fully exfoliated.39,40 X-ray photoelectron spectroscopy (XPS) is an important technique for the characterization of composites, which can provide information about elemental composition and their chemical state. As shown in Fig. 4(a), the XPS survey spectrum of GO shows only C and O elements. While the survey spectra of C2H4N4–GO and g-C3N4–rGO show that there is the element N besides C and O. Table 1 lists the C, O and N content of the samples according to the XPS analyses. It can be seen that the O content of g-C3N4–rGO is 8.7 at%, which is much lower than that of the GO (58.9 at%), indicating the deoxygenation of GO and the formation of the graphene structure during the in situ construction of g-C3N4–rGO. The lower the oxygen content, the higher the electrical conductivity. The N content of g-C3N4–
Fig. 4 (a) XPS survey spectra of the synthesized GO, C2H4N4–GO and g-C3N4–rGO samples. High resolution N1s (b) and C1s (c) spectra of the g-C3N4–rGO. The inset shows the high resolution C1s XPS spectra of GO. (d) High resolution C1s spectra of C2H4N4–GO.
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Table 1 C, O and N content (at%) of the samples according to XPS analysis
Samples
C
O
N
GO g-C3N4–rGO
41.1 77.1
58.9 8.7
— 14.2
rGO is up to 14.2 at%, which is much higher than that previously reported on N-doped graphene for LIBs. As will be seen later, the high-level nitrogen doping can further improve the electrochemical activity and provide more lithium insertion/ extraction sites, resulting in a significant increase in reversible capacity and rate capability. Based on high resolution N1s XPS analysis (Fig. 4(b)), there are four types of N species in the g-C3N4–rGO. The peaks centered at 398.8, 400.1 and 402.2 eV may be assigned to the pyridinic nitrogen (C–NvC), pyrrolic nitrogen (N–(C)3) and amino functional groups (–NH2), respectively.27,36,41 The peak centred at 401.4 eV can be assigned to secondary amine (C–N– H), which plays a role in the covalent link between g-C3N4 and the rGO. From Table 1S,† it can be seen that among the nitrogen-containing species, the nitrogen atom of the pyridine moiety is predominant (49.8%), which is the most suitable for Li storage. The C1s spectra can provide further information regarding the C–N covalent interaction between C2H4N4 and GO, and the formation of g-C3N4–rGO. It can be clearly seen from Table 2 and Fig. 4(c, d) that there is a remarkable decrease in the epoxy group content of C2H4N4–GO compared with that of GO, whereas the hydroxyl group content is considerably increased. The increase of hydroxyl groups may be ascribed to the fact that the nucleophile substitution reaction of C2H4N4 and GO can give rise to ring opening of the epoxy groups, yielding hydroxyl groups. In comparison with GO and C2H4N4–GO, the graphitic carbon content of g-C3N4–rGO is significantly increased due to the thermal reduction of GO. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) are valuable techniques for characterizing the morphology and nanostructures of materials. As shown in Fig. 5(a and b), it can be clearly seen that g-C3N4–rGO has a two-dimensional structure consisting of sheets with chiffon-like ripples and wrinkles, which make the g-C3N4–rGO possess a large specific surface area of 311.85 m2 g−1 compared to that of rGO (35.21 m2 g−1) and g-C3N4 (55.70 m2 g−1) (Fig. 1S†). As shown in Fig. 5(c), the g-C3N4–rGO exhibits a typical IUPAC type IV pattern, implying the existence of mesopores. The high special surface area of mesoporous g-C3N4–rGO can provide a large electrode–electrolyte interface
Table 2
C species content (at%) of the samples based on XPS results
Samples
CvC
C–OH
C–O–C
–CvN
CvO
GO C2H4N4–GO g-C3N4–rGO
31.7 31.8 60.8
6.8 29.1 18.9
57.2 19.7 7.1
— 16.1 8.7
4.3 0.3 4.5
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Fig. 5 (a) Typical TEM and (b) FE-SEM images of g-C3N4–rGO; (c) nitrogen adsorption/desorption isotherm of g-C3N4–rGO-1; (d) the energydispersive X-ray spectrum of g-C3N4–rGO-1.
area and more lithium insertion/extraction sites, and therefore, it is expected to offer an enhanced electrochemical performance for LIBs. The energy-dispersive X-ray spectroscopy (EDS) of g-C3N4–rGO from the FE-SEM images are shown in Fig. 5(d), there are four elements in the product, C, N, O, and Si (the high Si signals come from the silicon wafer), and the nitrogen content of the g-C3N4–rGO was calculated to be 14.5 at%, which is consistent with the XPS results. The formation mechanism of covalently coupled g-C3N4–rGO On the basis of FT-IR, solid state 13C NMR and XPS results, the formation mechanism of the covalently coupled g-C3N4–rGO is illustrated in Scheme 1. Firstly, purified natural graphite was chemically exfoliated to create graphene oxide (GO) containing hydroxyl, epoxy and carboxyl oxygenated functional groups on their basal planes and edges. Among the oxygenated functional groups, the epoxy group has high reactivity, for instance, ring-opening reactions can proceed by either an SN1 or SN2 mechanism, depending on nature of the epoxide and the reaction conditions. In a basic solution, the ring-opening reaction occurs (via SN2 attack) from the less hindered side.26 Dicyandiamide can be considered as a base because the lone pair of electrons on the nitrogen atom is available for donation. Therefore, the epoxy groups of GO can readily undergo a nucleophilic substitution reaction (SN2) with C2H4N4 to form C2H4N4 chemically modified GO sheets (C2H4N4–GO) via a covalent C–N bond. At higher temperatures, g-C3N4 nanosheets can be formed via self-condensation of dicyandiamide,19 while the thermal reduction of GO can be accompanied by the elimination of oxygen-containing groups,42 consequently forming the covalently coupled g-C3N4–rGO nanohybrid. Superior lithium-storage capacity and cyclability of g-N3C4-rGO The charge–discharge characteristics of g-C3N4–rGO were investigated for use as an anode in LIBs. Fig. 6 shows the first
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Scheme 1
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Illustration of the formation process of g-C3N4–rGO.
and second charge–discharge profiles of pure g-C3N4 and g-C3N4–rGO-n (n = 0.25, 0.5, 1, 2, 3) electrodes at a current density of 100 mA g−1 between 0.01 and 3 V. The g-C3N4 electrode exhibits a very low reversible capacity of 68 mA h g−1 with a Coulombic efficiency of 45% (Fig. 6(f )). While the rGO electrode delivered a specific capacity of 1200 mA h g−1 in the initial discharging and a reversible capacity of 426 mA h g−1 in the first charging, with a Coulombic efficiency of only 33%.43 Interestingly, the combination of g-C3N4 and rGO via covalent interactions can lead to a significantly enhanced electrochemical performance. As shown in Fig. 6(a–e) and Table 2S,† with an increase in the nitrogen-doping levels of g-C3N4–rGO-n, much higher reversible capacity of the electrode can be delivered, and g-C3N4–rGO with 14.2 at% N-doping exhibits the highest reversible capacity (Fig. 6(c)). In the first cycle, g-C3N4– rGO delivered a lithium insertion capacity 3002 mA h g−1 and a reversible charging capacity of 1705 mA h g−1, which can be attributed to the specific characteristics of the unique nano-
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structure of g-C3N4–rGO and the concerted effects of g-C3N4 and rGO, including the covalent interactions between the two moieties. The g-C3N4–rGO-1 shows a high Coulombic efficiency of 57%, which is much higher than that of undoped rGO. It is speculated that the combination of g-C3N4 and rGO via covalent interactions and in situ growth of g-C3N4 nanosheets on both sides of rGO nanosheets can suppress the electrolyte decomposition and surface side reactions of rGO with the electrolyte to form a solid electrolyte interphase (SEI) film.15,44 It is noted that the g-C3N4–rGO with higher g-C3N4 contents gave lower reversible capacities due to the fact that excessive g-C3N4 in the hybrid may result in lower conductivity (Fig. 6(e)). Fig. 7(a) and 2S† demonstrate the discharge capacity changes of g-C3N4–rGO-n (n = 0.25, 0.5, 1, 3) electrodes at a current density of 100 mA g−1 during 50 cycles, together with their Coulombic efficiencies. It can be clearly seen that all the four electrodes exhibit excellent cycle performance and the
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Fig. 6 Charge/discharge curves of (a) g-C3N4–rGO-0.25, (b) g-C3N4–rGO-0.5, (c) g-C3N4–rGO-1, (d) g-C3N4–rGO-2, (e) g-C3N4–rGO-3 and (f ) pure g-C3N4 between 0.01 and 3 V at a current density of 100 mA g−1.
Fig. 7 (a) Cycle performance and Coulombic efficiency of the g-C3N4–rGO-1 electrode at a current rate of 100 mA g−1 between 3.0 and 0.01 V versus Li/Li+. (b) Comparison of rate performance of g-C3N4–rGO-n (n = 0.5, 1) electrodes.
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Fig. 8
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Schematic model of lithium ion storage in g-C3N4–rGO.
g-C3N4–rGO-1 electrode shows a much larger reversible capacity than the other three electrodes. After 50 cycles, the g-C3N4–rGO-1 electrode can deliver a quite stable and reversible capacity of 1525 mA h g−1. Besides high capacity, the rate capability is also very important for high performance LIBs. At various discharge/charge rates, reversible capacities are retained at 1614, 1445, 1002 mA h g−1 at 100, 200, and 500 mA g−1, respectively. Even at a high current density of 1000 mA g−1, the g-C3N4–rGO-1 electrode can still deliver a reversible capacity as high as 943 mA h g−1, which can be recovered to its initial value when the current density reduces back to 100 mA g−1 after 40 cycles, demonstrating its excellent rate performance and structural stability. It is worth mentioning that the superior lithium-storage capacity and excellent cyclability observed here for g-N3C4–rGO can be attributed to the following reasons. First of all, the porous g-C3N4 with graphitic planes constructed from tri-striazine units connected by planar amino groups can be used as an excellent template for highly dispersed decoration of lithium ions,45 which is beneficial to increase the reversible capacity (Fig. 8). The covalent doping of g-C3N4 can induce a large number of topological defects on the rGO nanosheet, lithium ions can also diffuse from the g-C3N4 framework to the rGO nanosheet through the defects, thus further enhancing the capacity.15,46,47 Secondly, the rGO in the g-C3N4–rGO was fully exfoliated due to the in situ growth of g-C3N4 nanosheets on both sides of the rGO nanosheets, which facilitates lithium ions electrochemical adsorption on and intercalation into both sides of the thin rGO nanosheets, as well as the mesopores inside the graphene planes, therefore providing a higher reversible lithium ion storage capacity.7 Thirdly, the high special surface area of the mesoporous g-C3N4–rGO can provide more lithium insertion/extraction sites to give higher reversible lithium ion storage capacities.44,48 Fourthly, in addition to non-covalent π–π stacking interactions between g-C3N4 and rGO, the structure of g-C3N4–rGO can be more effectively stabilized by covalent interactions between the two moieties via a C–N bond, demonstrating the excellent cyclability. Finally, it can be clearly seen from Fig. 9, the covalently coupled hybrid of g-C3N4 with rGO possesses the good electrical conductivity compared to g-C3N4, which ensures an efficient and continuous charge carrier transport back and
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Fig. 9
EIS of rGO, g-C3N4 and g-C3N4–rGO-n.
forth between the g-C3N4–rGO and current collector via graphene networking, leading to a very high rate capability.
Conclusions In summary, a covalently coupled g-C3N4–rGO hybrid has been successfully synthesized using a two-step approach. Based on the FT-IR, XPS and 13C NMR results, the formation mechanism is described as: the epoxy groups of GO undergo a nucleophilic substitution reaction with C2H4N4 to form an C2H4N4–GO intermediate, and at 550 °C, both the in situ polymerization of C2H4N4 and thermal reduction of GO can be achieved, forming the covalently coupled g-C3N4–rGO. The g-C3N4–rGO exhibits an unprecedented high, stable and reversible capacity of 1525 mA h g−1 at a current density of 100 mA g−1 after 50 cycles. Even at a large current density of 1000 mA g−1, the reversible capacity can be well retained as high as 943 mA h g−1. The superior electrochemical performance of g-C3N4–rGO is attributed to the specific characteristics of the unique nanostructure of g-C3N4–rGO and the concerted effects of g-C3N4 and rGO, including covalent interactions between the two moieties, the good conductivity and high special surface area of g-C3N4–rGO, as well as the template of planar amino groups of g-C3N4 for dispersed decoration of Li+ ions.
Experimental Preparation of the covalently functionalized based g-C3N4 doped graphene sheets All chemicals were of analytical reagent grade and distilled water was used for the experiments. Graphene oxide (GO) was synthesized from purified natural graphite bought from Qingdao Zhongtian Company with a mean particle size of 44 μm according to the method reported by Hummers and Offeman.24 The typical experimental procedure for the synthesis of g-C3N4–rGO (the raw materials weight ratio of C2H4N4 to GO is 1) is as follows: 200 mL of an 0.5 mg mL−1 aqueous suspension of GO was mixed with 100 mg of dicyandiamide
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(C2H4N4). The mixture was stirred and heated at reflux for 2 hours. Once the water of the mixture was removed via freeze drying, the dried grey sponge-like product (labeled as C2H4N4– GO) was placed into a crucible and calcined at 550 °C for a total time of 8 hours under the protection of an N2 atmosphere. Finally, the sample was allowed to cool to room temperature in the oven and labelled as g-C3N4–rGO. When preparing g-C3N4–rGO with differing g-C3N4 content, the product was labelled as g-C3N4–rGO-n, where n is the raw materials weight ratio of C2H4N4 to GO. The yield of the g-C3N4–rGO-1 composites relative to the total mass of GO and carbon-nitride source was calculated to be 41.84%. For comparison, the same method was used to synthesize pure g-C3N4 without GO. Characterization Fourier transform infrared (FT-IR) spectra were recorded on a Bruker VECTOR 22 spectrometer using the KBr pellet technique. X-ray photoelectron spectroscopy (XPS) analyses were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV). The peak positions were internally referenced to the C1s peak at 284.6 eV. Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advanced diffractometer with Cu Kα radiation and the scanning angle ranged from 5° to 70° of 2θ. Transmission electron microscopy (TEM) images were taken with a JEOL JEM2100 microscope. Field-emission scanning electron microscopy (FESEM) was performed with a LEO1550 microscope, equipped with an EDAX 7593-H (Horiba, England). In order to determine the nitrogen content of g-C3N4–rGO, diluted suspensions of g-C3N4–rGO were deposited on silicon wafers and examined using EDS. The Brunauer– Emmett–Teller (BET) surface area was obtained by nitrogen sorption experiments conducted at 77 K using a Micromeritics TriStar II 3020 automated gas adsorption analyzer. Magic angle spinning (MAS) solid state nuclear magnetic resonance (NMR) spectra were recorded using a Bruker Avance 400 MHz spectrometer with a 90° pulse width of 2.5 μs and relaxation delay of 4 s and referenced to the 13C chemical shift of TMS at 0 ppm. Electrochemical measurements The working electrodes were made by coating an electrode slurry containing 80 wt% active material (rGO, g-C3N4–rGO, or g-C3N4), 10 wt% acetylene black (Super-P), 10 wt% polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidinone (NMP) on a copper foil substrate. The electrode film was dried at 120 °C for 2 hours to remove the solvent. Subsequently, the dried Cu foils with electrode materials were pressed and cut into small disks (10 mm in diameter). The small disks were further dried at 80 °C in a vacuum oven for 12 hours before testing. Half cells using Li foil as both the counter and reference electrodes were assembled with Lab-made Swagelok cells for electrochemical measurement. The electrolyte was 1 M LiPF6 dissolved ethylene carbonate and diethyl carbonate (EC–DEC, v/v = 1 : 1). A Celgard 2400 porous membrane was
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used as the separator. Galvanostatic charge and discharge measurements were carried out in the voltage range between 0.01 and 3.0 V vs. Li/Li+ at different current densities using a LAND CT2001A electrochemical workstation at room temperature. Electrochemical impedance spectroscopy (EIS) of the electrodes was carried out by applying a perturbation voltage of 5 mV in a frequency range of 100 kHz to 0.005 Hz using a CHI 660D (Chenhua Shanghai, China) electrochemical workstation.
Acknowledgements This work was supported by NSAF (No. U1230125), RFDP (No.20123219130003), NNSF of China (No. 51322212), the Fundamental Research Funds for the Central Universities (No. 30920130122002, No. 30920140122003, No. 30920140122008), STPP of Jiangsu (No. BE 2012151), Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1401003B), the Opening Project of the Jiangsu Key Laboratory for Environment Functional Materials (No. SJHG1303), the Jiangsu Province Key Laboratory of Fine Petrochemical Engineering (No. KF1206), the Zijin Intelligent Program of NUST (2014) and PAPD of Jiangsu.
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Cite this: DOI: 10.1039/c4nr03145h
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Covalently coupled hybrid of graphitic carbon nitride with reduced graphene oxide as a superior performance lithium-ion battery anode† Yongsheng Fu,‡ Junwu Zhu,‡ Chong Hu, Xiaodong Wu and Xin Wang* An in situ chemical synthetic approach has been designed for the fabrication of a covalently coupled hybrid consisting of graphitic carbon nitride (g-C3N4) with reduced graphene oxide (rGO) with differing g-C3N4/rGO ratio. The epoxy groups of graphene oxide (GO) undergo a nucleophilic substitution reaction with dicyandiamide (C2H4N4) to form the C2H4N4–GO composite via a covalent C–N bond, and then both the in situ polymerization of C2H4N4 and the thermal reduction of GO can be achieved at higher temperatures, forming the covalently coupled g-C3N4–rGO. FT-IR, CP-MAS NMR and XPS analyses, clearly revealed a covalent interaction between the g-C3N4 and rGO sheets. The g-C3N4–rGO exhibits an unprecedented high, stable and reversible capacity of 1525 mA h g−1 at a current density of 100 mA g−1 after 50 cycles. Even at a large current density of 1000 mA g−1, a reversible capacity of 943 mA h g−1 can still be retained. The superior electrochemical performance of g-C3N4–rGO is attributed to the specific
Received 9th June 2014, Accepted 8th August 2014
characteristics of the unique nanostructure of g-C3N4–rGO and the concerted effects of g-C3N4 and
DOI: 10.1039/c4nr03145h
rGO, including covalent interactions between the two moieties, the good conductivity and high special surface area of the nanocomposite, as well as the template effect of the planar amino group of g-C3N4
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for the dispersed decoration of Li+ ions.
Introduction The increasing energy demands along with the depletion of conventional fossil fuel reserves have stimulated intense research on alternative energy conversion and storage systems. Technological improvements in rechargeable solid-state batteries are being driven by the demands for portable electronic devices.1–3 Among the rechargeable batteries, lithium ion batteries (LIBs) have become the predominant battery technology due to their characteristics of high-energy density, flexible and lightweight design, and long lifespan in comparison to competing battery technologies.4 Generally, the active materials employed in LIBs for the consumer electronics market are made up of a lithium cobalt
Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, 210094 Nanjing, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: N species content (at%) of g-C3N4–rGO-1 based on XPS analysis; the first cycle discharge capacity, charge capacity and coulombic efficiency for the g-C3N4, rGO, and g-C3N4–rGO-n (n = 0.25, 0.5, 1, 2, 3) electrodes; nitrogen adsorption/desorption isotherm of g-C3N4; cycle performance of the g-C3N4–rGO-n (n = 0.25, 0.5, 3) electrodes at a current rate of 100 mA g−1 between 3.0 and 0.01 V versus Li+/Li. See DOI: 10.1039/ c4nr03145h ‡ These authors contributed equally to this work.
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oxide cathode paired with a graphite anode. Graphite has many advantages, such as low and flat voltage range, high Coulombic efficiency, good cyclability and low cost, however, bulk graphite can react with Li to form LiC6 structures with a lower specific capacity of graphite of 372 mA h g−1, significantly limiting the specific energy of LIBs.5 Unlike bulk graphite, graphene sheets can react with Li to give LiC3 structures by the effect of Li being stored on both sides of each nanosheet. Theoretically, such a structure can double the specific capacity of graphite. Moreover, graphene sheets have a large specific surface area, remarkable electrical conductivity, excellent adsorptivity, and high chemical and thermal stability.6–10 Therefore, diverse graphene-based materials for anodes have been developed to make more active spaces for Li storage.11,12 Recently, it has been found that the electronic and chemical properties of graphene can be modified by chemical doping heteroatoms, such as nitrogen atoms.13–16 For instance, the pyridinic N atoms can effectively improve the reversible capacity of a nitrogen-doped graphene electrode due to the stronger electronegativity of nitrogen compared to that of carbon.17 Cheng et al. prepared the N-doped graphene electrodes with a high capacity of 1043 mA h g−1 at a low current density of 50 mA g−1, much improved Coulombic efficiency and cycle performance in comparison with undoped graphene.15 Cai et al. reported that the high-level N-doped
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graphene exhibits a high initial reversible capacity of 1123 mA h g−1 at a low current density of 50 mA g−1 and the high N-doping level is conducive to improve the rate capability and reversible capacity.16 Unfortunately, N-doping levels in graphene are usually lower than 6 at% because of the lack of available synthetic methods via the substitution of carbon by nitrogen atoms.18 As an analogue of graphite, graphitic carbon nitride (g-C3N4) is composed of ordered tri-s-triazine subunits connected through planar tertiary amino groups in a layer and also has a stacked two-dimensional structure, which can be regarded as N-substituted graphite with the highest nitrogen-doping level.19 Recently, g-C3N4 has attracted considerable attention owing to its unique electronic and optical properties, which show promising potential applications in optoelectronic conversion, water-splitting photocatalysis and the degradation of organic pollutants.20,21 However, the low electrical conductivity of g-C3N4 derived from the intrinsic porous microstructure largely reduces the electrochemical properties, becoming a bottleneck in its application in the area of electrochemistry.22,23 If g-C3N4 is hybridized with highly-conductive graphene sheets via covalent bonds, then it may be possible to obtain a stable and high capacity anode material for LIBs as a result of the interaction of individual components as well as the concerted effect. Herein, we have developed an in situ approach towards the fabrication of a covalently coupled hybrid of g-C3N4 with rGO as an anode material for lithium storage. Graphene oxide (GO) was prepared from powdered flake graphite by a modified Hummer’s method.24 The epoxy groups of GO undergo a nucleophilic substitution reaction with dicyandiamide (C2H4N4) to form the C2H4N4–GO composite via a covalent C–N bond. At higher temperatures, both the in situ polymerization of C2H4N4 and thermal reduction of GO can be achieved, forming the covalently coupled g-C3N4–rGO nanohybrid. The nanohybrid exhibits an unprecedented high, stable and reversible capacity of 1525 mA h g−1 at a current density of 100 mA
Fig. 1
g−1 after 50 cycles, suggesting a remarkably promising candidate for energy storage.
Results and discussion Structure and morphology In order to clarify the formation process of g-C3N4–rGO, a variety of techniques, such as Fourier transform infrared (FT-IR) spectroscopy, 13C magic-angle spinning (MAS) NMR spectroscopy, powder X-ray diffraction (XRD) and X-ray photoelectron (XPS) spectroscopy were employed for detailed characterization. As shown in Fig. 1(a), GO exhibits the following IR features in the range of 1000–2000 cm−1: CvO stretching vibrations of the COOH groups (1726 cm−1), O–H deformation vibrations of the COOH groups (1620 cm−1), O–H deformation vibrations of tertiary C–OH (1396 cm−1) and C–O stretching vibrations of the epoxy groups (1050 cm−1).25 It is noted that the epoxy group peak of C2H4N4–GO at 1050 cm−1 of GO is largely reduced. This may be ascribed to the fact that the epoxy groups of GO have undergone ring-opening with nucleophiles via an SN2 mechanism.26 For pure g-C3N4, the absorption peaks at about 1635, 1534, 1406, 1318 and 1238 cm−1 in the range of 1000–2000 cm−1 (Fig. 1(b)) may be assigned to the typical stretching modes of CN heterocycles.27–30 The absorption peaks at 807 cm−1 is attributed to the characteristic breathing mode of triazine units, which is in agreement with published work on bulk g-C3N4.27–30 For g-C3N4–rGO, in addition to the above peaks of g-C3N4, the adsorption around 1570 cm−1 is assigned to the stretching vibrations of the graphene nanosheets because of the thermal reduction of GO.27 Magic angle spinning solid-state NMR (MAS NMR) spectroscopy is one of the most powerful techniques for addressing issues of molecular structure, especially for insoluble or poorly crystalline materials. Fig. 2(a) shows the 13C MAS NMR spectra of C2H4N4, GO and C2H4N4–GO. Three distinct signals
FT-IR spectra of C2H4N4, GO, C2H4N4–GO, pure g-C3N4 and g-C3N4–rGO.
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Fig. 2 rGO.
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(a) Solid-state 13C MAS NMR spectra of C2H4N4, GO and C2H4N4–GO. (b) Solid-state 13C CP-NMR spectra of pure g-C3N4, rGO and g-C3N4–
dominate the spectrum of GO: signals at 58.9 and 69.1 ppm are assigned to the 13C nuclei in the epoxy and hydroxyl groups, respectively; the resonance at 129.2 ppm corresponds to the un-oxidized sp2 carbons of the graphene network. Besides, there is a weaker signal at 228.1 ppm, which presumably arises from the carbonyl groups.31–33 For C2H4N4, the signals at 158.8 and 167.5 ppm can be assigned to sp and sp2 carbons, respectively. In comparison with GO and C2H4N4, the solid-state 13C MAS NMR spectrum of C2H4N4–GO exhibits two additional resonances, the shoulder at 119.9 ppm can be assigned to sp3 hybridized carbon atoms, giving important evidence for the C–N covalent interaction between C2H4N4 and GO. Another signal centred at 19.8 ppm is assigned to the sp3 hybridized carbon of CH3 in the GO sheets.33 Whereas, no CH3 signal is observable for GO alone most likely due to the steric effect caused by small interlayer spacings, which may hinder the motion of CH3, enhancing NMR residual dipolar
couplings. Fig. 2(b) shows the solid-state 13C cross-polarisation nuclear magnetic resonance (CP-NMR) spectra of g-C3N4, rGO and g-C3N4–rGO. The spectrum of g-C3N4 gives two peaks at 155.6 and 164.3 ppm corresponding to the sp2-hybridized carbon atoms of CN2(NHx) and CN3 in the g-C3N4 networks, respectively.34,35 The spectrum of rGO shows a broad resonance in the range of 70–150 ppm, which is the feature of chemical shift anisotropy of the 13C nucleus.32 Besides, there are no typical resonances of oxygenated carbons, which can further confirm the thermal reduction of GO and the formation of the graphene structure at higher temperatures. When compared with rGO, the solid-state 13C MAS NMR spectrum of g-C3N4–rGO exhibits a much broader resonance in the range of 100–200 ppm, implying the in situ covalent doping of g-C3N4 nanosheets onto the rGO sheets. The XRD diffraction patterns of g-C3N4–rGO, rGO, g-C3N4 and GO are shown in Fig. 3(a). The diffraction pattern of
Fig. 3 (a) XRD diffraction patterns of GO, pure g-C3N4, rGO and g-C3N4–rGO in the range of 5–80°. (b) Raman spectra of pure g-C3N4, GO and g-C3N4–rGO.
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g-C3N4 is in good agreement with the recent reports on g-C3N4 prepared by polymerization of dicyandiamide and melamine.19,27,30,36 The peak at a 2θ value of 27.51° can be indexed to the (002) crystal plane of the stacking of the conjugated aromatic system with an interplanar distance of 0.326 nm. The peak at 13.10° can be indexed to the (100) crystal plane of in-planar ordering of tri-s-triazine units with a distance of 0.675 nm. While GO exhibits a sharp peak at 10.80° for the (001) facets corresponding to 0.87 nm interlayer spacing.25 The rGO shows a broad diffraction peak at 24.54°, corresponding to the (002) interlayer stacking with a distance of 0.388 nm.36 It is interesting that g-C3N4–rGO gives a diffraction peak at 26.04° with a layer-to-layer distance of 0.362 nm, which may be assigned to the interlayer stacking between g-C3N4 and rGO through covalent interactions. Besides, no typical diffraction peak of GO (001), rGO (002) or g-C3N4 (002) is observable. It is speculated that the rGO in the g-C3N4–rGO was fully exfoliated due to the in situ growth of g-C3N4 nanosheets on both sides of rGO nanosheets. The intercalation of rGO and g-C3N4 each other can effectively prevent the g-C3N4 stacking and the rGO restacking. This is why g-C3N4–rGO shows a diffraction pattern different from g-C3N4, GO or rGO. Raman spectroscopy is one of the most sensitive and informative techniques to characterize the disorder in sp2 carbon materials. When compared with GO (Fig. 3(b)), the G-band of
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g-C3N4–rGO remarkably shifted from 1565 to 1588 cm−1 and the D-band shifted from 1336 to 1351 cm−1 caused by the covalent doping of g-C3N4, which are similar to the results reported on N-doped graphene systems.15,37,38 While the intensity ratio of the D- to G-band (ID-band/IG-band) is 1.12, significantly larger than that of the GO (0.70), indicating that the GO has been reduced to rGO, which has much higher conductivity. Raman spectroscopy can also be used to investigate the single-, bi-, and multilayer characteristics of the graphene and graphene oxide layers. It can be clearly seen that the 2D peak position of g-C3N4–rGO is observed at 2680 cm−1 with a symmetrical shape, indicating rGO has been fully exfoliated.39,40 X-ray photoelectron spectroscopy (XPS) is an important technique for the characterization of composites, which can provide information about elemental composition and their chemical state. As shown in Fig. 4(a), the XPS survey spectrum of GO shows only C and O elements. While the survey spectra of C2H4N4–GO and g-C3N4–rGO show that there is the element N besides C and O. Table 1 lists the C, O and N content of the samples according to the XPS analyses. It can be seen that the O content of g-C3N4–rGO is 8.7 at%, which is much lower than that of the GO (58.9 at%), indicating the deoxygenation of GO and the formation of the graphene structure during the in situ construction of g-C3N4–rGO. The lower the oxygen content, the higher the electrical conductivity. The N content of g-C3N4–
Fig. 4 (a) XPS survey spectra of the synthesized GO, C2H4N4–GO and g-C3N4–rGO samples. High resolution N1s (b) and C1s (c) spectra of the g-C3N4–rGO. The inset shows the high resolution C1s XPS spectra of GO. (d) High resolution C1s spectra of C2H4N4–GO.
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Table 1 C, O and N content (at%) of the samples according to XPS analysis
Samples
C
O
N
GO g-C3N4–rGO
41.1 77.1
58.9 8.7
— 14.2
rGO is up to 14.2 at%, which is much higher than that previously reported on N-doped graphene for LIBs. As will be seen later, the high-level nitrogen doping can further improve the electrochemical activity and provide more lithium insertion/ extraction sites, resulting in a significant increase in reversible capacity and rate capability. Based on high resolution N1s XPS analysis (Fig. 4(b)), there are four types of N species in the g-C3N4–rGO. The peaks centered at 398.8, 400.1 and 402.2 eV may be assigned to the pyridinic nitrogen (C–NvC), pyrrolic nitrogen (N–(C)3) and amino functional groups (–NH2), respectively.27,36,41 The peak centred at 401.4 eV can be assigned to secondary amine (C–N– H), which plays a role in the covalent link between g-C3N4 and the rGO. From Table 1S,† it can be seen that among the nitrogen-containing species, the nitrogen atom of the pyridine moiety is predominant (49.8%), which is the most suitable for Li storage. The C1s spectra can provide further information regarding the C–N covalent interaction between C2H4N4 and GO, and the formation of g-C3N4–rGO. It can be clearly seen from Table 2 and Fig. 4(c, d) that there is a remarkable decrease in the epoxy group content of C2H4N4–GO compared with that of GO, whereas the hydroxyl group content is considerably increased. The increase of hydroxyl groups may be ascribed to the fact that the nucleophile substitution reaction of C2H4N4 and GO can give rise to ring opening of the epoxy groups, yielding hydroxyl groups. In comparison with GO and C2H4N4–GO, the graphitic carbon content of g-C3N4–rGO is significantly increased due to the thermal reduction of GO. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) are valuable techniques for characterizing the morphology and nanostructures of materials. As shown in Fig. 5(a and b), it can be clearly seen that g-C3N4–rGO has a two-dimensional structure consisting of sheets with chiffon-like ripples and wrinkles, which make the g-C3N4–rGO possess a large specific surface area of 311.85 m2 g−1 compared to that of rGO (35.21 m2 g−1) and g-C3N4 (55.70 m2 g−1) (Fig. 1S†). As shown in Fig. 5(c), the g-C3N4–rGO exhibits a typical IUPAC type IV pattern, implying the existence of mesopores. The high special surface area of mesoporous g-C3N4–rGO can provide a large electrode–electrolyte interface
Table 2
C species content (at%) of the samples based on XPS results
Samples
CvC
C–OH
C–O–C
–CvN
CvO
GO C2H4N4–GO g-C3N4–rGO
31.7 31.8 60.8
6.8 29.1 18.9
57.2 19.7 7.1
— 16.1 8.7
4.3 0.3 4.5
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Fig. 5 (a) Typical TEM and (b) FE-SEM images of g-C3N4–rGO; (c) nitrogen adsorption/desorption isotherm of g-C3N4–rGO-1; (d) the energydispersive X-ray spectrum of g-C3N4–rGO-1.
area and more lithium insertion/extraction sites, and therefore, it is expected to offer an enhanced electrochemical performance for LIBs. The energy-dispersive X-ray spectroscopy (EDS) of g-C3N4–rGO from the FE-SEM images are shown in Fig. 5(d), there are four elements in the product, C, N, O, and Si (the high Si signals come from the silicon wafer), and the nitrogen content of the g-C3N4–rGO was calculated to be 14.5 at%, which is consistent with the XPS results. The formation mechanism of covalently coupled g-C3N4–rGO On the basis of FT-IR, solid state 13C NMR and XPS results, the formation mechanism of the covalently coupled g-C3N4–rGO is illustrated in Scheme 1. Firstly, purified natural graphite was chemically exfoliated to create graphene oxide (GO) containing hydroxyl, epoxy and carboxyl oxygenated functional groups on their basal planes and edges. Among the oxygenated functional groups, the epoxy group has high reactivity, for instance, ring-opening reactions can proceed by either an SN1 or SN2 mechanism, depending on nature of the epoxide and the reaction conditions. In a basic solution, the ring-opening reaction occurs (via SN2 attack) from the less hindered side.26 Dicyandiamide can be considered as a base because the lone pair of electrons on the nitrogen atom is available for donation. Therefore, the epoxy groups of GO can readily undergo a nucleophilic substitution reaction (SN2) with C2H4N4 to form C2H4N4 chemically modified GO sheets (C2H4N4–GO) via a covalent C–N bond. At higher temperatures, g-C3N4 nanosheets can be formed via self-condensation of dicyandiamide,19 while the thermal reduction of GO can be accompanied by the elimination of oxygen-containing groups,42 consequently forming the covalently coupled g-C3N4–rGO nanohybrid. Superior lithium-storage capacity and cyclability of g-N3C4-rGO The charge–discharge characteristics of g-C3N4–rGO were investigated for use as an anode in LIBs. Fig. 6 shows the first
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Scheme 1
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Illustration of the formation process of g-C3N4–rGO.
and second charge–discharge profiles of pure g-C3N4 and g-C3N4–rGO-n (n = 0.25, 0.5, 1, 2, 3) electrodes at a current density of 100 mA g−1 between 0.01 and 3 V. The g-C3N4 electrode exhibits a very low reversible capacity of 68 mA h g−1 with a Coulombic efficiency of 45% (Fig. 6(f )). While the rGO electrode delivered a specific capacity of 1200 mA h g−1 in the initial discharging and a reversible capacity of 426 mA h g−1 in the first charging, with a Coulombic efficiency of only 33%.43 Interestingly, the combination of g-C3N4 and rGO via covalent interactions can lead to a significantly enhanced electrochemical performance. As shown in Fig. 6(a–e) and Table 2S,† with an increase in the nitrogen-doping levels of g-C3N4–rGO-n, much higher reversible capacity of the electrode can be delivered, and g-C3N4–rGO with 14.2 at% N-doping exhibits the highest reversible capacity (Fig. 6(c)). In the first cycle, g-C3N4– rGO delivered a lithium insertion capacity 3002 mA h g−1 and a reversible charging capacity of 1705 mA h g−1, which can be attributed to the specific characteristics of the unique nano-
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structure of g-C3N4–rGO and the concerted effects of g-C3N4 and rGO, including the covalent interactions between the two moieties. The g-C3N4–rGO-1 shows a high Coulombic efficiency of 57%, which is much higher than that of undoped rGO. It is speculated that the combination of g-C3N4 and rGO via covalent interactions and in situ growth of g-C3N4 nanosheets on both sides of rGO nanosheets can suppress the electrolyte decomposition and surface side reactions of rGO with the electrolyte to form a solid electrolyte interphase (SEI) film.15,44 It is noted that the g-C3N4–rGO with higher g-C3N4 contents gave lower reversible capacities due to the fact that excessive g-C3N4 in the hybrid may result in lower conductivity (Fig. 6(e)). Fig. 7(a) and 2S† demonstrate the discharge capacity changes of g-C3N4–rGO-n (n = 0.25, 0.5, 1, 3) electrodes at a current density of 100 mA g−1 during 50 cycles, together with their Coulombic efficiencies. It can be clearly seen that all the four electrodes exhibit excellent cycle performance and the
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Fig. 6 Charge/discharge curves of (a) g-C3N4–rGO-0.25, (b) g-C3N4–rGO-0.5, (c) g-C3N4–rGO-1, (d) g-C3N4–rGO-2, (e) g-C3N4–rGO-3 and (f ) pure g-C3N4 between 0.01 and 3 V at a current density of 100 mA g−1.
Fig. 7 (a) Cycle performance and Coulombic efficiency of the g-C3N4–rGO-1 electrode at a current rate of 100 mA g−1 between 3.0 and 0.01 V versus Li/Li+. (b) Comparison of rate performance of g-C3N4–rGO-n (n = 0.5, 1) electrodes.
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Fig. 8
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Schematic model of lithium ion storage in g-C3N4–rGO.
g-C3N4–rGO-1 electrode shows a much larger reversible capacity than the other three electrodes. After 50 cycles, the g-C3N4–rGO-1 electrode can deliver a quite stable and reversible capacity of 1525 mA h g−1. Besides high capacity, the rate capability is also very important for high performance LIBs. At various discharge/charge rates, reversible capacities are retained at 1614, 1445, 1002 mA h g−1 at 100, 200, and 500 mA g−1, respectively. Even at a high current density of 1000 mA g−1, the g-C3N4–rGO-1 electrode can still deliver a reversible capacity as high as 943 mA h g−1, which can be recovered to its initial value when the current density reduces back to 100 mA g−1 after 40 cycles, demonstrating its excellent rate performance and structural stability. It is worth mentioning that the superior lithium-storage capacity and excellent cyclability observed here for g-N3C4–rGO can be attributed to the following reasons. First of all, the porous g-C3N4 with graphitic planes constructed from tri-striazine units connected by planar amino groups can be used as an excellent template for highly dispersed decoration of lithium ions,45 which is beneficial to increase the reversible capacity (Fig. 8). The covalent doping of g-C3N4 can induce a large number of topological defects on the rGO nanosheet, lithium ions can also diffuse from the g-C3N4 framework to the rGO nanosheet through the defects, thus further enhancing the capacity.15,46,47 Secondly, the rGO in the g-C3N4–rGO was fully exfoliated due to the in situ growth of g-C3N4 nanosheets on both sides of the rGO nanosheets, which facilitates lithium ions electrochemical adsorption on and intercalation into both sides of the thin rGO nanosheets, as well as the mesopores inside the graphene planes, therefore providing a higher reversible lithium ion storage capacity.7 Thirdly, the high special surface area of the mesoporous g-C3N4–rGO can provide more lithium insertion/extraction sites to give higher reversible lithium ion storage capacities.44,48 Fourthly, in addition to non-covalent π–π stacking interactions between g-C3N4 and rGO, the structure of g-C3N4–rGO can be more effectively stabilized by covalent interactions between the two moieties via a C–N bond, demonstrating the excellent cyclability. Finally, it can be clearly seen from Fig. 9, the covalently coupled hybrid of g-C3N4 with rGO possesses the good electrical conductivity compared to g-C3N4, which ensures an efficient and continuous charge carrier transport back and
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Fig. 9
EIS of rGO, g-C3N4 and g-C3N4–rGO-n.
forth between the g-C3N4–rGO and current collector via graphene networking, leading to a very high rate capability.
Conclusions In summary, a covalently coupled g-C3N4–rGO hybrid has been successfully synthesized using a two-step approach. Based on the FT-IR, XPS and 13C NMR results, the formation mechanism is described as: the epoxy groups of GO undergo a nucleophilic substitution reaction with C2H4N4 to form an C2H4N4–GO intermediate, and at 550 °C, both the in situ polymerization of C2H4N4 and thermal reduction of GO can be achieved, forming the covalently coupled g-C3N4–rGO. The g-C3N4–rGO exhibits an unprecedented high, stable and reversible capacity of 1525 mA h g−1 at a current density of 100 mA g−1 after 50 cycles. Even at a large current density of 1000 mA g−1, the reversible capacity can be well retained as high as 943 mA h g−1. The superior electrochemical performance of g-C3N4–rGO is attributed to the specific characteristics of the unique nanostructure of g-C3N4–rGO and the concerted effects of g-C3N4 and rGO, including covalent interactions between the two moieties, the good conductivity and high special surface area of g-C3N4–rGO, as well as the template of planar amino groups of g-C3N4 for dispersed decoration of Li+ ions.
Experimental Preparation of the covalently functionalized based g-C3N4 doped graphene sheets All chemicals were of analytical reagent grade and distilled water was used for the experiments. Graphene oxide (GO) was synthesized from purified natural graphite bought from Qingdao Zhongtian Company with a mean particle size of 44 μm according to the method reported by Hummers and Offeman.24 The typical experimental procedure for the synthesis of g-C3N4–rGO (the raw materials weight ratio of C2H4N4 to GO is 1) is as follows: 200 mL of an 0.5 mg mL−1 aqueous suspension of GO was mixed with 100 mg of dicyandiamide
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(C2H4N4). The mixture was stirred and heated at reflux for 2 hours. Once the water of the mixture was removed via freeze drying, the dried grey sponge-like product (labeled as C2H4N4– GO) was placed into a crucible and calcined at 550 °C for a total time of 8 hours under the protection of an N2 atmosphere. Finally, the sample was allowed to cool to room temperature in the oven and labelled as g-C3N4–rGO. When preparing g-C3N4–rGO with differing g-C3N4 content, the product was labelled as g-C3N4–rGO-n, where n is the raw materials weight ratio of C2H4N4 to GO. The yield of the g-C3N4–rGO-1 composites relative to the total mass of GO and carbon-nitride source was calculated to be 41.84%. For comparison, the same method was used to synthesize pure g-C3N4 without GO. Characterization Fourier transform infrared (FT-IR) spectra were recorded on a Bruker VECTOR 22 spectrometer using the KBr pellet technique. X-ray photoelectron spectroscopy (XPS) analyses were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV). The peak positions were internally referenced to the C1s peak at 284.6 eV. Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advanced diffractometer with Cu Kα radiation and the scanning angle ranged from 5° to 70° of 2θ. Transmission electron microscopy (TEM) images were taken with a JEOL JEM2100 microscope. Field-emission scanning electron microscopy (FESEM) was performed with a LEO1550 microscope, equipped with an EDAX 7593-H (Horiba, England). In order to determine the nitrogen content of g-C3N4–rGO, diluted suspensions of g-C3N4–rGO were deposited on silicon wafers and examined using EDS. The Brunauer– Emmett–Teller (BET) surface area was obtained by nitrogen sorption experiments conducted at 77 K using a Micromeritics TriStar II 3020 automated gas adsorption analyzer. Magic angle spinning (MAS) solid state nuclear magnetic resonance (NMR) spectra were recorded using a Bruker Avance 400 MHz spectrometer with a 90° pulse width of 2.5 μs and relaxation delay of 4 s and referenced to the 13C chemical shift of TMS at 0 ppm. Electrochemical measurements The working electrodes were made by coating an electrode slurry containing 80 wt% active material (rGO, g-C3N4–rGO, or g-C3N4), 10 wt% acetylene black (Super-P), 10 wt% polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidinone (NMP) on a copper foil substrate. The electrode film was dried at 120 °C for 2 hours to remove the solvent. Subsequently, the dried Cu foils with electrode materials were pressed and cut into small disks (10 mm in diameter). The small disks were further dried at 80 °C in a vacuum oven for 12 hours before testing. Half cells using Li foil as both the counter and reference electrodes were assembled with Lab-made Swagelok cells for electrochemical measurement. The electrolyte was 1 M LiPF6 dissolved ethylene carbonate and diethyl carbonate (EC–DEC, v/v = 1 : 1). A Celgard 2400 porous membrane was
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used as the separator. Galvanostatic charge and discharge measurements were carried out in the voltage range between 0.01 and 3.0 V vs. Li/Li+ at different current densities using a LAND CT2001A electrochemical workstation at room temperature. Electrochemical impedance spectroscopy (EIS) of the electrodes was carried out by applying a perturbation voltage of 5 mV in a frequency range of 100 kHz to 0.005 Hz using a CHI 660D (Chenhua Shanghai, China) electrochemical workstation.
Acknowledgements This work was supported by NSAF (No. U1230125), RFDP (No.20123219130003), NNSF of China (No. 51322212), the Fundamental Research Funds for the Central Universities (No. 30920130122002, No. 30920140122003, No. 30920140122008), STPP of Jiangsu (No. BE 2012151), Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1401003B), the Opening Project of the Jiangsu Key Laboratory for Environment Functional Materials (No. SJHG1303), the Jiangsu Province Key Laboratory of Fine Petrochemical Engineering (No. KF1206), the Zijin Intelligent Program of NUST (2014) and PAPD of Jiangsu.
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