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Cite this: DOI: 10.1039/c4cc06567k Received 21st August 2014, Accepted 23rd October 2014
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Fabrication of graphene quantum dot-decorated graphene sheets via chemical surface modification† Jaehoon Ryu, Eunwoo Lee, Seungae Lee and Jyongsik Jang*
DOI: 10.1039/c4cc06567k www.rsc.org/chemcomm
A novel approach to link graphene quantum dots (GQDs) with graphene was explored via chemical surface modification of graphene using 1,5-diaminonapthalene (DAN).
Environmental pollution has become one of the greatest issues in society, and photocatalysts have attracted a great deal of attention as one solution to this issue.1 Graphene quantum dots (GQDs) are of particular interest because they possess properties that make them suitable as efficient photocatalysts. These properties include upconversion photoluminescence, a relatively narrow band gap, non-toxicity, high aqueous solubility, and biocompatibility. Additionally, photoexcited GQDs are excellent electron acceptors and donors.2 Hence, many studies have focused on the synthesis of GQD/semi-conductor composites and have demonstrated that GQDs increase visible-light absorbance and accept photoexcited electrons from semiconductors.3 On the other hand, a recent study showed that GQDs only serve as visible-light sensitizers in the photocatalytic process, providing a new possibility for GQDs to be used as a sensitizer.4 However, high efficiency has only been shown at elevated pH levels, which could hinder practical applications. Additionally, GQD photocatalysts have drawbacks such as a high recombination rate and low adsorption on organic pollutants. Therefore, it is essential to resolve these problems. The unique properties of graphene, including its high conductivity, large specific surface area, good chemical and thermal stabilities, and unique mechanical properties, make graphene a promising material for composites.5 Graphene has four outstanding attributes in graphene composites: it acts as an electron mediator, increases visible-light absorbance, provides numerous reaction sites, and induces the adsorption of pollutants via p–p interactions.5b Hence, the combination of GQDs and graphene can show synergetic effects.
School of Chemical and Biological Engineering, College of Engineering, Seoul National University (SNU), Seoul, Korea. E-mail: [email protected]; Fax: +82 2 888 1604; Tel: +82 2880 7069 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc06567k
This journal is © The Royal Society of Chemistry 2014
Herein, we propose a facile and reliable synthetic route for the fabrication of GQD-decorated graphene sheets (GDGS) via chemical surface modification. This novel and simple approach describes the combination of GQDs with graphene modified by amide bonds. To the best of our knowledge, this is the first attempt to link graphene and GQDs. Furthermore, the GDGS are demonstrated as photocatalysts having high efficiencies and durabilities, suggesting its potential applications. Scheme 1 describes the overall experimental synthetic process for the fabrication of the GDGS. Monodisperse graphene oxide (GO) was spin-coated onto a polyethylene terephthalate (PET) film. The linker 1,5-diaminonaphthalene (DAN) was stacked onto the surface of the GO by p–p stacking between the DAN naphthalene group and the sp2-carbon plane of the GO.5a The GQDs were bonded to the surface of the GO via an amidation reaction involving the thermal dehydration of the ammonium salt.6 Finally, the composite was reduced using a pressure-assisted reduction method.7 The GDGS were fabricated by adjusting the concentration of the DAN solution (2, 6, 20, and 60 mM), referred to herein as GDGS-2, GDGS-6, GDGS-20, and GDGS-60, respectively. Transmission electron microscopy (TEM) was used to detail the morphologies of the reduced graphene oxide (RGO) and GDGS-x (x = 2, 6, and 20) (Fig. 1).
Scheme 1
Schematic diagram of the GDGS fabrication procedure.
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Fig. 1 TEM images of (a) GDGS-2, (b) GDGS-6, and (c) GDGS-20; (d) variation of the average number of GQDs per unit area as a function of the molar concentration of DAN.
Fig. 1a–c shows that the GQDs were well dispersed on the surface of the graphene. Moreover, it is evident that the GQD population on the graphene increased with increasing concentration of the DAN solution, indicating that DAN acts as a linker between the GQDs and graphene. However, when the surface of the GO was modified with 420 mM DAN solution, there was no significant increase in the population over that of GDGS-20 (Fig. 1d). These data indicate that the sp2-carbon planes of the GO available for p–p interactions with DAN were entirely saturated by the DAN naphthalene group when the concentration of the DAN solution exceeded 20 mM. The FTIR spectra of the GQDs, RGO, and GDGS are shown in Fig. 2a. Compared with RGO, three new peaks appeared for the
Fig. 2 (a) FTIR spectra, (b) XRD spectroscopy data and (c) Raman spectra of GQDs, RGO, and GDGS; (d) conductivity of RGO and GDGS-x (x = 2, 6, 20) before and after pressure-assisted reduction.
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GDGS at 1650, 1350, and 730 cm 1. All of these peaks are ascribed to the secondary amide function.8 The absorption band at 1650 cm 1 is called the amide I band. The peak at 1350 cm 1 is due to the C–N stretching vibration and is referred to as the amide III band. The peak at 730 cm 1 corresponds to the N–H wagging vibration. The GDGS also showed a peak at 3350 cm 1, which was assigned to the N–H stretching vibration. These findings confirm that the GQDs were successfully combined with the modified graphene through amide bonds. Fig. 2b shows the XRD patterns of the GQDs, RGO, and GDGS. The GDGS showed a strong peak centered at 25.71, corresponding to a 3.5 Å spacing, which was the most compact interlayer among the three specimens. The d-spacing of the GDGS was consistent with that observed in high-resolution TEM (HR-TEM) images (Fig. S2, ESI†). The reduced interlayer distance suggested not only that the GQDs were reduced through pressure-assisted reduction, but also that the reduced GQDs and RGO were strongly bonded and stacked.9 Fig. 2c shows typical Raman spectra for the GQDs, RGO, and GDGS. The relative intensity of the D-band to the G-band (ID/IG) can be used to estimate the extent of structural defects.10 The ID/IG values of the RGO and GDGS were 0.8 and 1.01, respectively. The higher ID/IG value for the GDGS suggests that the GQDs, which have massively defective structures (ID/IG E 1.05), were bonded onto the graphene, and that the graphene sheets were successfully decorated by the GQDs. Conductivity measurements were used to characterize the electrical properties of the GDGS. The reduction step caused the conductivity to increase by three orders of magnitude.11 While the conductivity of RGO was 2.57 S cm 1, that of GDGS-2, GDGS-6, and GDGS-20 were 1.42, 1.0, and 0.7 S cm 1, respectively. Importantly, the conductivity of the GDGS decreased with increasing population of GQDs, because the GQDs have a significantly lower conductivity than the RGO.12 These results demonstrate that the GQDs were bonded onto the surface of the RGO. The ultraviolet-visible (UV-Vis) absorbance spectra of the RGO, GQDs and the GDGS are shown in Fig. 3a. All of the GDGS exhibited increased absorption in the visible-light region compared with pristine GQDs, indicating that the linking of graphene with the GQDs enhanced visible-light absorbance and would likely enable efficient use of solar energy. Moreover, the absorption of visible light increased with increasing GQD population. Furthermore, all GDGS showed a decrease in the intensity of emission peaks compared with GQDs, suggesting that the graphene inhibited charge recombination in the GQDs by accepting electrons from the conduction band of the GQDs (Fig. 3b).13 The photocatalytic activities of the RGO, GQDs and the GDGS-x (x = 2, 6, and 20) were evaluated by following the photodegradation of methylene blue (MB) under visible-light irradiation (l 4 400 nm). For the measurement, the quantities of the as-prepared samples were adjusted to equal the intensity of absorbance at 400 nm for comparing the quantum yield during the photocatalytic process (Table S4, ESI†). Fig. 3c displays the degradation curves of the MB as a function of irradiation time. The C/C0 ratio is the concentration of the MB at a certain reaction time normalized by its initial
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Fig. 3 (a) UV-Visible absorbance, (b) emission spectra with an excitation wavelength of 350 nm, and (c) photodegradation of MB over RGO, GQD and GDGS-x (x = 2, 6, 20) under visible light; experiments performed with samples at constant absorbance at 400 nm (d) photodegradation of MB over P25, GQDs and GDGS-20 under visible light; experiments performed with a constant weight (50 mg) of the samples under visible light.
concentration and is derived from the change in the absorption intensity at l = 660 nm. Before irradiation, the prepared solution was stirred for 30 min in the dark to achieve an adsorption–desorption equilibrium and the GDGS showed enhanced adsorptivity by graphene. After irradiation, the equilibrium was broken as the degradation of MB occurred, and subsequently more MB in the solution would be adsorbed and degraded. Though there is small number of GQDs present as sensitizers in GDGS compared to pristine GQDs, the GDGS-6 and GDGS-20 displayed a fairly enhanced photocatalytic performance. Additionally, the rate of photodegradation improved with the increasing population of decorating GQDs. However, GDGS-2 presented lower photocatalytic efficiency than pristine GQDs, because GDGS-2 has an extremely tiny number of sensitizers in comparison with pristine GQDs. The GDGS-20 had the highest quantum yield and degraded about 96% of the MB solution after 40 min. Clearly, graphene in the prepared GDGS played an essential role in the photocatalysis mechanism. Fig. S18 (ESI†) illustrates the photo-degradation process of GDGS. Under visible-light irradiation, photogenerated electrons from the GQDs transfer to the graphene with high electron conductivity, which enables good charge separation, and subsequently enhances quantum yields. Moreover, graphene enhances adsorption of dyes by inducing p–p stacking with the MB molecules, which facilitates access of photogenerated carriers to the nearby MB.5b The ability to photodegrade MB of GQD was investigated compared with that of P25 as a commercial
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product (Fig. 3d). Experiments were performed with a constant weight (50 mg) of the samples. The GQDs exhibited a higher photocatalytic performance than P25 under visible light irradiation, demonstrating that GQDs were much better visible light-sensitizers than P25. In addition, GDGS represents higher photocatalytic efficiency under visible light irradiation than the P25-graphene composites which has been reported in previous research studies.5b,14 This is attributed to the difference in photocatalytic ability of sensitizers. In conclusion, a facile and reliable route for the preparation of GDGS based on chemical surface modification has been reported. DAN played a key role as a linker material for the connection of GQDs with graphene. The GDGS had a 1.6 times greater photocatalytic performance toward MB photodegradation compared with pristine GQDs because of the influence of the graphene. This novel synthetic approach, which connects graphene to GQDs for the first time, is expected to promote the application of GQDs in photocatalysis and also in many other areas. This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2011-0031573).
Notes and references 1 J. Song, J. Roh, I. Lee and J. Jang, Dalton Trans., 2013, 42, 13897–13904. 2 (a) E. Lee, J. Ryu and J. Jang, Chem. Commun., 2013, 49, 9995–9997; (b) X. Wang, L. Cao, F. Lu, M. J. Meziani, H. Li, G. Qi, B. Zhou, B. A. Harruff, F. Kermarrec and Y.-P. Sun, Chem. Commun., 2009, 3774–3776. 3 H. Zhang, H. Huang, H. Ming, H. Li, L. Zhang, Y. Liu and Z. Kang, J. Mater. Chem., 2012, 22, 10501–10506. 4 S. Hu, R. Tian, L. Wu, Q. Zhao, J. Yang, J. Liu and S. Cao, Chem. – Asian J., 2013, 8, 1035–1041. 5 (a) J. H. An, S. J. Park, O. S. Kwon, J. Bae and J. Jang, ACS Nano, 2013, 7, 10563–10571; (b) H. Zhang, X. Lv, Y. Li, Y. Wang and J. Li, ACS Nano, 2009, 4, 380–386; (c) J. S. Lee, K.-Y. Shin, C. Kim and J. Jang, Chem. Commun., 2013, 49, 11047–11049. 6 C. A. G. N. Montalbetti and V. Falque, Tetrahedron, 2005, 61, 10827–10852. 7 K.-H. Shin, Y. Jang, B.-S. Kim, J. Jang and S. H. Kim, Chem. Commun., 2013, 49, 4887–4889. 8 (a) B. C. Smith, Infrared spectral interpretation: a systematic approach, CRC press, 1998; (b) A. Barth and C. Zscherp, Q. Rev. Biophys., 2002, 35, 369–430. 9 Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai and L. Qu, J. Am. Chem. Soc., 2011, 134, 15–18. 10 L. Jiao, L. Zhang, X. Wang, G. Diankov and H. Dai, Nature, 2009, 458, 877–880. 11 S. Pei and H.-M. Cheng, Carbon, 2012, 50, 3210–3228. 12 X. Zhou, Z. Tian, J. Li, H. Ruan, Y. Ma, Z. Yang and Y. Qu, Nanoscale, 2014, 6, 2603–2607. 13 Y.-S. Chen, H. Choi and P. V. Kamat, J. Am. Chem. Soc., 2013, 135, 8822–8825. 14 J. Sun, Y. Fu, P. Xiong, X. Sun, B. Xu and X. Wang, RSC Adv., 2013, 3, 22490–22497.
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Published on 31 October 2014. Downloaded by Dokuz Eylul University on 02/11/2014 20:12:03.
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Cite this: DOI: 10.1039/c4cc06567k Received 21st August 2014, Accepted 23rd October 2014
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Fabrication of graphene quantum dot-decorated graphene sheets via chemical surface modification† Jaehoon Ryu, Eunwoo Lee, Seungae Lee and Jyongsik Jang*
DOI: 10.1039/c4cc06567k www.rsc.org/chemcomm
A novel approach to link graphene quantum dots (GQDs) with graphene was explored via chemical surface modification of graphene using 1,5-diaminonapthalene (DAN).
Environmental pollution has become one of the greatest issues in society, and photocatalysts have attracted a great deal of attention as one solution to this issue.1 Graphene quantum dots (GQDs) are of particular interest because they possess properties that make them suitable as efficient photocatalysts. These properties include upconversion photoluminescence, a relatively narrow band gap, non-toxicity, high aqueous solubility, and biocompatibility. Additionally, photoexcited GQDs are excellent electron acceptors and donors.2 Hence, many studies have focused on the synthesis of GQD/semi-conductor composites and have demonstrated that GQDs increase visible-light absorbance and accept photoexcited electrons from semiconductors.3 On the other hand, a recent study showed that GQDs only serve as visible-light sensitizers in the photocatalytic process, providing a new possibility for GQDs to be used as a sensitizer.4 However, high efficiency has only been shown at elevated pH levels, which could hinder practical applications. Additionally, GQD photocatalysts have drawbacks such as a high recombination rate and low adsorption on organic pollutants. Therefore, it is essential to resolve these problems. The unique properties of graphene, including its high conductivity, large specific surface area, good chemical and thermal stabilities, and unique mechanical properties, make graphene a promising material for composites.5 Graphene has four outstanding attributes in graphene composites: it acts as an electron mediator, increases visible-light absorbance, provides numerous reaction sites, and induces the adsorption of pollutants via p–p interactions.5b Hence, the combination of GQDs and graphene can show synergetic effects.
School of Chemical and Biological Engineering, College of Engineering, Seoul National University (SNU), Seoul, Korea. E-mail: [email protected]; Fax: +82 2 888 1604; Tel: +82 2880 7069 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc06567k
This journal is © The Royal Society of Chemistry 2014
Herein, we propose a facile and reliable synthetic route for the fabrication of GQD-decorated graphene sheets (GDGS) via chemical surface modification. This novel and simple approach describes the combination of GQDs with graphene modified by amide bonds. To the best of our knowledge, this is the first attempt to link graphene and GQDs. Furthermore, the GDGS are demonstrated as photocatalysts having high efficiencies and durabilities, suggesting its potential applications. Scheme 1 describes the overall experimental synthetic process for the fabrication of the GDGS. Monodisperse graphene oxide (GO) was spin-coated onto a polyethylene terephthalate (PET) film. The linker 1,5-diaminonaphthalene (DAN) was stacked onto the surface of the GO by p–p stacking between the DAN naphthalene group and the sp2-carbon plane of the GO.5a The GQDs were bonded to the surface of the GO via an amidation reaction involving the thermal dehydration of the ammonium salt.6 Finally, the composite was reduced using a pressure-assisted reduction method.7 The GDGS were fabricated by adjusting the concentration of the DAN solution (2, 6, 20, and 60 mM), referred to herein as GDGS-2, GDGS-6, GDGS-20, and GDGS-60, respectively. Transmission electron microscopy (TEM) was used to detail the morphologies of the reduced graphene oxide (RGO) and GDGS-x (x = 2, 6, and 20) (Fig. 1).
Scheme 1
Schematic diagram of the GDGS fabrication procedure.
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Fig. 1 TEM images of (a) GDGS-2, (b) GDGS-6, and (c) GDGS-20; (d) variation of the average number of GQDs per unit area as a function of the molar concentration of DAN.
Fig. 1a–c shows that the GQDs were well dispersed on the surface of the graphene. Moreover, it is evident that the GQD population on the graphene increased with increasing concentration of the DAN solution, indicating that DAN acts as a linker between the GQDs and graphene. However, when the surface of the GO was modified with 420 mM DAN solution, there was no significant increase in the population over that of GDGS-20 (Fig. 1d). These data indicate that the sp2-carbon planes of the GO available for p–p interactions with DAN were entirely saturated by the DAN naphthalene group when the concentration of the DAN solution exceeded 20 mM. The FTIR spectra of the GQDs, RGO, and GDGS are shown in Fig. 2a. Compared with RGO, three new peaks appeared for the
Fig. 2 (a) FTIR spectra, (b) XRD spectroscopy data and (c) Raman spectra of GQDs, RGO, and GDGS; (d) conductivity of RGO and GDGS-x (x = 2, 6, 20) before and after pressure-assisted reduction.
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GDGS at 1650, 1350, and 730 cm 1. All of these peaks are ascribed to the secondary amide function.8 The absorption band at 1650 cm 1 is called the amide I band. The peak at 1350 cm 1 is due to the C–N stretching vibration and is referred to as the amide III band. The peak at 730 cm 1 corresponds to the N–H wagging vibration. The GDGS also showed a peak at 3350 cm 1, which was assigned to the N–H stretching vibration. These findings confirm that the GQDs were successfully combined with the modified graphene through amide bonds. Fig. 2b shows the XRD patterns of the GQDs, RGO, and GDGS. The GDGS showed a strong peak centered at 25.71, corresponding to a 3.5 Å spacing, which was the most compact interlayer among the three specimens. The d-spacing of the GDGS was consistent with that observed in high-resolution TEM (HR-TEM) images (Fig. S2, ESI†). The reduced interlayer distance suggested not only that the GQDs were reduced through pressure-assisted reduction, but also that the reduced GQDs and RGO were strongly bonded and stacked.9 Fig. 2c shows typical Raman spectra for the GQDs, RGO, and GDGS. The relative intensity of the D-band to the G-band (ID/IG) can be used to estimate the extent of structural defects.10 The ID/IG values of the RGO and GDGS were 0.8 and 1.01, respectively. The higher ID/IG value for the GDGS suggests that the GQDs, which have massively defective structures (ID/IG E 1.05), were bonded onto the graphene, and that the graphene sheets were successfully decorated by the GQDs. Conductivity measurements were used to characterize the electrical properties of the GDGS. The reduction step caused the conductivity to increase by three orders of magnitude.11 While the conductivity of RGO was 2.57 S cm 1, that of GDGS-2, GDGS-6, and GDGS-20 were 1.42, 1.0, and 0.7 S cm 1, respectively. Importantly, the conductivity of the GDGS decreased with increasing population of GQDs, because the GQDs have a significantly lower conductivity than the RGO.12 These results demonstrate that the GQDs were bonded onto the surface of the RGO. The ultraviolet-visible (UV-Vis) absorbance spectra of the RGO, GQDs and the GDGS are shown in Fig. 3a. All of the GDGS exhibited increased absorption in the visible-light region compared with pristine GQDs, indicating that the linking of graphene with the GQDs enhanced visible-light absorbance and would likely enable efficient use of solar energy. Moreover, the absorption of visible light increased with increasing GQD population. Furthermore, all GDGS showed a decrease in the intensity of emission peaks compared with GQDs, suggesting that the graphene inhibited charge recombination in the GQDs by accepting electrons from the conduction band of the GQDs (Fig. 3b).13 The photocatalytic activities of the RGO, GQDs and the GDGS-x (x = 2, 6, and 20) were evaluated by following the photodegradation of methylene blue (MB) under visible-light irradiation (l 4 400 nm). For the measurement, the quantities of the as-prepared samples were adjusted to equal the intensity of absorbance at 400 nm for comparing the quantum yield during the photocatalytic process (Table S4, ESI†). Fig. 3c displays the degradation curves of the MB as a function of irradiation time. The C/C0 ratio is the concentration of the MB at a certain reaction time normalized by its initial
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Fig. 3 (a) UV-Visible absorbance, (b) emission spectra with an excitation wavelength of 350 nm, and (c) photodegradation of MB over RGO, GQD and GDGS-x (x = 2, 6, 20) under visible light; experiments performed with samples at constant absorbance at 400 nm (d) photodegradation of MB over P25, GQDs and GDGS-20 under visible light; experiments performed with a constant weight (50 mg) of the samples under visible light.
concentration and is derived from the change in the absorption intensity at l = 660 nm. Before irradiation, the prepared solution was stirred for 30 min in the dark to achieve an adsorption–desorption equilibrium and the GDGS showed enhanced adsorptivity by graphene. After irradiation, the equilibrium was broken as the degradation of MB occurred, and subsequently more MB in the solution would be adsorbed and degraded. Though there is small number of GQDs present as sensitizers in GDGS compared to pristine GQDs, the GDGS-6 and GDGS-20 displayed a fairly enhanced photocatalytic performance. Additionally, the rate of photodegradation improved with the increasing population of decorating GQDs. However, GDGS-2 presented lower photocatalytic efficiency than pristine GQDs, because GDGS-2 has an extremely tiny number of sensitizers in comparison with pristine GQDs. The GDGS-20 had the highest quantum yield and degraded about 96% of the MB solution after 40 min. Clearly, graphene in the prepared GDGS played an essential role in the photocatalysis mechanism. Fig. S18 (ESI†) illustrates the photo-degradation process of GDGS. Under visible-light irradiation, photogenerated electrons from the GQDs transfer to the graphene with high electron conductivity, which enables good charge separation, and subsequently enhances quantum yields. Moreover, graphene enhances adsorption of dyes by inducing p–p stacking with the MB molecules, which facilitates access of photogenerated carriers to the nearby MB.5b The ability to photodegrade MB of GQD was investigated compared with that of P25 as a commercial
This journal is © The Royal Society of Chemistry 2014
Communication
product (Fig. 3d). Experiments were performed with a constant weight (50 mg) of the samples. The GQDs exhibited a higher photocatalytic performance than P25 under visible light irradiation, demonstrating that GQDs were much better visible light-sensitizers than P25. In addition, GDGS represents higher photocatalytic efficiency under visible light irradiation than the P25-graphene composites which has been reported in previous research studies.5b,14 This is attributed to the difference in photocatalytic ability of sensitizers. In conclusion, a facile and reliable route for the preparation of GDGS based on chemical surface modification has been reported. DAN played a key role as a linker material for the connection of GQDs with graphene. The GDGS had a 1.6 times greater photocatalytic performance toward MB photodegradation compared with pristine GQDs because of the influence of the graphene. This novel synthetic approach, which connects graphene to GQDs for the first time, is expected to promote the application of GQDs in photocatalysis and also in many other areas. This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2011-0031573).
Notes and references 1 J. Song, J. Roh, I. Lee and J. Jang, Dalton Trans., 2013, 42, 13897–13904. 2 (a) E. Lee, J. Ryu and J. Jang, Chem. Commun., 2013, 49, 9995–9997; (b) X. Wang, L. Cao, F. Lu, M. J. Meziani, H. Li, G. Qi, B. Zhou, B. A. Harruff, F. Kermarrec and Y.-P. Sun, Chem. Commun., 2009, 3774–3776. 3 H. Zhang, H. Huang, H. Ming, H. Li, L. Zhang, Y. Liu and Z. Kang, J. Mater. Chem., 2012, 22, 10501–10506. 4 S. Hu, R. Tian, L. Wu, Q. Zhao, J. Yang, J. Liu and S. Cao, Chem. – Asian J., 2013, 8, 1035–1041. 5 (a) J. H. An, S. J. Park, O. S. Kwon, J. Bae and J. Jang, ACS Nano, 2013, 7, 10563–10571; (b) H. Zhang, X. Lv, Y. Li, Y. Wang and J. Li, ACS Nano, 2009, 4, 380–386; (c) J. S. Lee, K.-Y. Shin, C. Kim and J. Jang, Chem. Commun., 2013, 49, 11047–11049. 6 C. A. G. N. Montalbetti and V. Falque, Tetrahedron, 2005, 61, 10827–10852. 7 K.-H. Shin, Y. Jang, B.-S. Kim, J. Jang and S. H. Kim, Chem. Commun., 2013, 49, 4887–4889. 8 (a) B. C. Smith, Infrared spectral interpretation: a systematic approach, CRC press, 1998; (b) A. Barth and C. Zscherp, Q. Rev. Biophys., 2002, 35, 369–430. 9 Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai and L. Qu, J. Am. Chem. Soc., 2011, 134, 15–18. 10 L. Jiao, L. Zhang, X. Wang, G. Diankov and H. Dai, Nature, 2009, 458, 877–880. 11 S. Pei and H.-M. Cheng, Carbon, 2012, 50, 3210–3228. 12 X. Zhou, Z. Tian, J. Li, H. Ruan, Y. Ma, Z. Yang and Y. Qu, Nanoscale, 2014, 6, 2603–2607. 13 Y.-S. Chen, H. Choi and P. V. Kamat, J. Am. Chem. Soc., 2013, 135, 8822–8825. 14 J. Sun, Y. Fu, P. Xiong, X. Sun, B. Xu and X. Wang, RSC Adv., 2013, 3, 22490–22497.
Chem. Commun.
