石墨烯与卟啉的共价杂化功能化材料：合成和光限幅特性

A Graphene Hybrid Material Covalently Functionalized with Porphyrin: Synthesis and Optical Limiting Property By Yanfei Xu, Zhibo Liu, Xiaoliang Zhang, Yan Wang, Jianguo Tian,* Yi Huang, Yanfeng Ma, Xiaoyan Zhang, and Yongsheng Chen* Graphene, a very recent rising star in material science, with an atomically thin, 2D structure that consists of sp2-hybridized with optoelectronically active porphyrin moelecules, multifunc- tional nanometer-scale materials for optical and/or optoelectronic applications may be generated. In this paper, we report the first organic-solution-processable functionalized-graphene (SPFGra- phene) hybrid material with porphyrins, and its photophysical properties including optical-limiting properties. C O M M U N IC A T IO N www.advmat.de Ministry of Education and Teda Applied Physics School DOI: 10.1002/adma.200801617 Institute of Polymer Chemistry College of Chemistry Nankai University, Tianjin 300071 (P.R. China) E-mail: yschen99@nankai.edu.cn Scheme 1. Synthesis scheme of TPP-NHCO-SPFGraphene. Nankai University, Tianjin 300457 (P.R. China) E-mail: jjtian@nankai.edu.cn Prof. Y. Chen, Y. Xu, X. Zhang, Y. Wang, Prof. Y. Huang, Prof. Y. Ma Key Laboratory for Functional Polymer Materials and Centre for Nanoscale Science and Technology Adv. Mater. 2009, 21, 1275–1279 � 2009 WILEY-VCH Verlag G carbons, exhibits remarkable electronic and mechanical proper- ties.[1–4] Theoretically, the molecules of other allotropic carbon forms can be built from graphene. For example, 1D carbon nanotubes (CNTs) can be built by rolling up graphene with different layers, and 0D fullerenes can be built by wrapping up a single layer of graphene. Graphene (or ‘2D graphite’) is widely used to describe the properties of various carbon-based materials. With the numerous reports of the many exceptional properties and applications of carbon nanotubes[5] and fullerenes,[6] the intensive research of graphene and its use in many nanoelec- tronic and optoelectronic devices, and as a nanometer-scale building block for new nanomaterials, is expected. So far, different device applications, such as field-effect transistors,[7] resonators,[3] transparent anodes,[8] and organic photovoltaic devices have been reported.[9] It is known that perfect graphene itself does not exist, and the solubility and/or processability are the first issues for many perspective applications of graphene- based materials. So far, chemical functionalization of graphene has focused on improving its solubility/processability in both water and organic solvents using different soluble groups.[10–14] However, multifunctional hybrid materials that take advantage of both the superior properties of graphene and a functionalizing material have been largely unexplored. The presence of oxygen-containing groups in graphene oxide renders it strongly hydrophilic and water soluble,[12] and also provides a handle for the chemical modification of graphene using known carbon surface chemistry. Porphyrins are ‘the pigments of life’,[15] with a large extinction coefficient in the visible-light region, predictable rigid structures, and prospective photochemical electron-transfer ability.[16] The extensive 2D 18 p-electron porphyrins and porphyrin-modified acceptor nano- particles exhibit good optoelectronic properties.[17–22] Therefore, it is expected that, by combining 2D nanometer-scale graphene [*] Prof. J. Tian, Dr. Z. Liu, X. L. Zhang Key Laboratory of Weak Light Non-linear Photonics The synthesis of the porphyrin–graphene nanohybrid, 5-4 (aminophenyl)-10, 15, 20-triphenyl porphyrin (TPP) and gra- phene oxide molecules covalently bonded together via an amide bond (TPP-NHCO-SPFGraphene, Scheme 1 and 2) was carried out using an amine-functionalized prophyrin (TPP-NH2) and graphene oxide in N,N-dimethylformamide (DMF), following standard chemistry. Large-scale and water-soluble graphene oxide was prepared by themodified Hummers method.[8,9,23] Results of atomic force microscopy (AFM, see Supporting Information, Fig. S1), thermogravimetry analysis (TGA), and X-ray diffraction (XRD) characterization have confirmed that this graphene material can be easily dispersed at the state of complete exfoliation, which consists of almost entire single-layered graphene sheets in H2O. [8,9] TPP-NH2 and graphene oxide molecules are covalently bonded together by an amide bond. Much care has been taken to make sure all the unreacted TPP-NH2 has been removed using extensive solvent washing, sonication, and membrane filtration. Details are given in the Experimental part. The attachment of organic molecules to graphene oxide has made TPP-NHCO- SPFGraphene soluble in DMF and other polar solvents. Figure 1 shows FTIR spectra of TPP-NHCO-SPFGraphene, TPP-NH2, and graphene oxide. In the spectrum of graphene oxide, the peak at 1730 cm�1 is characteristic of the C––O stretch of the carboxylic group on the graphene oxide. In the spectrum mbH & Co. KGaA, Weinheim 1275 C O M M U N IC A T IO N www.advmat.de blended sample of graphene oxide with TPP-NH2 (as a control sample) in DMF. Graphene oxide shows a strong absorption band at 268 nm. The TPP-NH2 spectrum exhibits a strong Soret absorption at 419 nm, and weak Q-bands between 500 and 700 nm, which are consistent with that of TPP-NH2 analogues.[24] The control sample exhibits a broad absorption at 274 nm, while the hybrid TPP-NHCO- SPFGraphene exhibits a broad absorption at 280 nm, which should be the corresponding graphene oxide peak at 268 nm with a red-shift of 12 nm. A similar band is also observed for TPP-NHCO-SPFGraphene and the control sample at 419 nm, which corre- sponds to the Soret band of the TPP-NH2 moiety, and no obvious shift is observed for either samples. These results indicate that in the ground state attachment of the TPP-NH2 moiety has perturbed the electronic state of the graphene oxide, but no significant effect is observed on the TPP-NH2 part.Scheme 2. Schematic representation of part of the structure of the covalent 1276 of TPP-NHCO-SPFGraphene, the peak at 1730 cm�1 almost disappears, and a new broad band emerges at 1640 cm�1, which corresponds to the C––O characteristic stretching band of the amide group.[10] The stretching band of the amide C–N peak appears at 1260 cm�1. These results clearly indicate that the TPP-NH2 molecules had been covalently bonded to the graphene oxide by the amide linkage. Transmission electron microscopy (TEM) was used to further characterize the TPP-NHCO- SPFGraphene (see Supporting Information, Fig. S2). TPP-NHCO-SPFGraphene. Figure 2 shows UV-vis absorption spectra of TPP-NHCO- SPFGraphene, TPP-NH2, graphene oxide, and a physically concentration to g the inset Fig. 3A). Figure 1. FTIR spectra of TPP-NHCO-SPFGraphene, TPP-NH2, and gra- phene oxide. A band emerges at 1640 cm�1 that corresponds to the C––O stretch of the amide group, indicating that the TPP-NH2 molecules have been covalently bonded to the graphene oxide by an amide linkage. Figure 2. UV absorp oxide, and the c NHCO-SPFGraphen sample (graphene o 1.4mg L�1. (Differe 0.3–0.9 were used fo � 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhe enerate a standard curve (in mg L�1, Fig. 3 and On the basis of the applicability of Beer’s law, tion of TPP-NHCO-SPFGraphene, TPP-NH2, graphene ontrol sample in DMF. Concentrations: TPP- e, 27mg L�1; graphene oxide, 30mg L�1; the control xide 31mg L�1, TPP-NH2 1.4mg L �1); TPP-NH2, nt concentrations with a maximum absorption of r a better comparison.) The prevention of aggregation is of parti- cular importance for graphene processability and applications, because most of their attractive properties are only associated with individual graphene sheets. Solution-phase UV-vis-NIR spectroscopy has been reported to demonstrate a linear relationship between the absorbance and the relative concentrations of single-walled carbon nanotubes (SWNTs) in different solvents, which obey Beer’s law at low concentrations, and has been used to determine the solubility of SWNTs.[25] Figure 3 shows the absorption spectra of solutions of TPP-NHCO-SPFGraphene with different con- centrations. The absorption values at 419 nmwere plotted against im Adv. Mater. 2009, 21, 1275–1279 C O M M U N IC A T IO N www.advmat.de Figure 3. Concentration dependence of the UV absorption of TPP-NHCO-SPFGraphene in DMF (concentrations are 40, 35, 32, 27, 21, 14, and 12mg L�1, from a to g, respectively). The the plot of optical density at 419 nm versus concentration is shown in inset A) is, and inset B) we estimated the effective extinction coefficient of the TPP- NHCO-SPFGraphene from the slope of the linear least- squares fit to be 0.024 L mg�1 cm�1, with an R value of 0.992. The absorbance of solutions of TPP-NHCO-SPFGraphene at other wavelengths was also in line with Beer’s law. For example, the inset B) in Figure 3 shows that a linear relationship exists between the absorption and the concentrations measured at the maximal absorption position for the graphene moiety in the hybrid. These results demonstrate that the hybrid was homogeneously is the plot of the absorption of the graphene moiety versus concentration. The straight line is a linear least-squares fit to the data, which indicates that the hybrid TPP-NHCO-SPFGraphene is homogeneously dissolved in the solvent. dispersed in DMF. In order to probe the excited-state interactions of TPP-NH2 and graphene in the hybrid, fluorescence spectra of TPP-NH2, the control sample, and TPP-NHCO-SPFGraphene are compared in Figure 4. Fluorescence spectroscopic changes observed for TPP-NH2, the control sample, and TPP-NHCO-SPFGraphene in DMF, with the normal- ization of the absorbance of the Soret band excitation wavelength (419 nm) to the same value (0.24). Adv. Mater. 2009, 21, 1275–1279 � 2009 WILEY-VCH Verlag G Figure 4. Upon excitation of TPP-NH2 at a Soret band of 419 nm, with the absorbance of TPP-NH2, the control sample, and TPP-NHCO-SPFGraphene being the same value (0.24) at the excitation wavelength, the solution of the control sample Exhibits 14% quenching of the fluorescence emission, while a much stronger 56% quenching is observed for the hybrid TPP-NHCO- SPFGraphene. Excitation of TPP-NHCO-SPFGraphene and the control sample at other excitation wavelengths (400, 450, and 500 nm) shows a much stronger quenching (see Supporting information, Fig. S3–S5). The observed luminescence quenching indicates that there is a strong interaction between the excited state of TPP-NH2 and graphene moieties in the hybrid. Possible pathways for the fluorescence quenching of the excited TPP-NH2 may be attributed to two possible competitive processes: photoinduced electron transfer (PET) and energy transfer (ET). Similar luminescence quenching has been observed for the hybrids of CNTs with porphyrins, and a PETmechanism has been demonstrated for these hybrids.[26] Molecular-orbital theory and experimental results have shown that closed-cage carbon structures, such as fullerenes and carbon nanotubes, are favorable electron acceptors, because of their unique p-electron system when the two moieties are connected directly.[27] Thus, after photoexcitation, the intramolecular donor–acceptor inter- action between the twomoieties of TPP-NH2 and graphene in our TPP-NHCO-SPFGraphene nanohybrid may have a charge transfer from the photoexcited singlet TPP-NH2 to the graphene moiety, and this results in the observed fluorescence quenching and energy release. In this TPP-NHCO-SPFGraphene nanohy- brid, the effective intramolecular energy quenching may also be facilitated by a through-bond mechanism, as a result of the direct linkage mode of the two moieties by the amide bond.[25] With the efficient energy and/or electron transfer upon photoexcitation, and the reported excellent optical limiting properties of C60, carbon nanotubes and their functionalized materials,[25,28,29] it would be both interesting and important to investigate the optical limiting properties of the TPP-NHCO- SPFGraphene. Optical limiting materials are materials that exhibit high transmittance of low-intensity light and attenuate intense optical beams.[30] They can be used to protect optical sensors, for example, eyes or charge-coupled device (CCD) cameras, from possible damage caused by intense laser pulses, and have potential applications in the field of optical switching and other areas. Figure 5 shows open-apertureZ-scan[31] results of TPP-NHCO- SPFGraphene, TPP-NH2, graphene oxide, a control blend sample of TPP-NH2 with graphene oxide (1: 1 weight ratio), and C60. The optical limiting properties of the solutions of these materials were investigated using 532 nm pulsed laser irradiation, and C60 was employed as a standard. The details of the measurement are described in the Supporting Information. To compare the optical limiting effect, all of the sample concentrations were adjusted to have same linear transmittance of 75% at 532 nm in cells1mm thick. The open-aperture Z-scan measures the transmittance of the sample as it translates through the focal plane of a tightly focused beam. As the sample is brought closer to focus, the beam intensity increases, and the nonlinear effect increases, which will lead to a decreasing transmittance for reverse saturatable absorption (RSA), two-photon absorption (TPA), and nonlinear mbH & Co. KGaA, Weinheim 1277 C O M M U N IC A T IO N www.advmat.de The authors gratefully acknowledge the financial support from MoST (#2006CB0N0702), MoE (#20040055020), the NSF (#20774047, 1278 scattering. As shown in Figure 5, the TPP-NHCO-SPFGraphene has the largest dip among the transmittance curves of the studied materials. Therefore, TPP-NHCO-SPFGraphene demonstrates much better optical limiting properties compared with the benchmark material (C60), the control sample, and the individual components (TPP-NH2 and graphene oxide) of the hybrid. Porphyrins are well known to exhibit RSA in the visible- wavelength range,[28] while graphene oxide has a TPA at 532 nm, which is used in our experiments because the linear absorption peak of graphene oxide is located at 268 nm. Considering the covalent donor–acceptor structure, and the efficient fluorescence quenching of this nanohybrid, we believe that the photoinduced electron and/or energy transfer from the electron donor TPP-NH2 to the acceptor graphene should play an important role for the much-enhanced optical limiting performance.[29] Furthermore, during the Z-scan experiments, as shown in Figure 5, enhanced scattering could also be observed for the Figure 5. Open-aperture Z-scan results of TPP-NHCO-SPFGraphene, TPP-NH2, graphene oxide, control sample, and C60, with the same linear transmittance of 75% to 5 ns, 532 nm optical pulses. sample of TPP-NHCO-SPFGraphene moving towards the focus of the laser. This implies that the observed Z-scan curve is also influenced by nonlinear scattering. Therefore, the much- enhanced optical limiting performance of TPP-NHCO- SPFGraphene should arise from a combination of photoinduced electron and/or energy transfer, RSA, TPA, and nonlinear scattering mechanisms. Similar results have been observed for the hybrid materials of carbon nanotubes with porphyrins.[25,32] In summary, we have reported the first covalently bonded and organic soluble graphene (SPFGraphene) hybrid with porphyrin. FTIR, UV-vis absorption, and TEM studies confirm the covalent functionalization of the graphene. Attachment of TPP-NH2 significantly improves the solubility and dispersion stability of the graphene-based material in organic solvents. In this donor– acceptor nanohybrid, the fluorescence of photoexcited TPP-NH2 is effectively quenched by a possible electron-transfer process. A superior optical limiting effect, better than the benchmark optical limiting material C60 and the control sample, is observed. Photoinduced electron- and/or energy-transfer mechanisms play a significant role in the superior optical limiting performance. With the abundant and highly pure functionalized graphene [8] H. A. Becerril, J. Mao, Z. F. Liu, R. M. Stoltenberg, Z. N. Bao, Y. S. Chen, ACS Nano 2008, 2, 463. � 2009 WILEY-VCH Verlag Gmb [9] Z. F. Liu, Q. Liu, X. Y. Zhang, Y. Huang, Y. F. Ma, S. G. Yin, Y. S. Chen, Adv. Mater. 2008, 20, 3924. [10] S. Niyogi, E. Bekyarova, M. E. Itkis, J. L. McWilliams, M. A. Hamon, R. C. Haddon, J. Am. Chem. Soc. 2006, 128, 7720. [11] Y. X. Xu, H. Bai, G. W. Lu, C. Li, G. Q. Shi, J. Am. Chem. Soc. 2008, 130, 5856. 60708020, and 10574075) of China and the NSF (#07JCYBJC03000) of Tianjin City. Supporting Information is available online at Wiley InterScience or from the author.Published online: Received: June 13, 2008 Revised: August 6, 2008 Published online: Februrary 13, 2009 [1] D. Li, R. B. Kaner, Science 2008, 320, 1170. [2] D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen, R. S. Ruoff, Nature 2007, 448, 457. [3] J. S. Bunch, A. M. van der Zande, S. S. Verbridge, I. W. Frank, D. M. Tanenbaum, J. M. Parpia, H. G. Craighead, P. L. Mceuen, Science 2007, 315, 490. [4] A. K. 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Acknowledgements material readily available, its unique structure, and excellent electronic properties, we expect this organic solution-processable functionalized graphene material to be a competitive entry in the realm of light harvesting and solar-energy conversion materials for optoelectronic devices. Experimental Synthesis of TPP-NHCO-SPFGraphene: The synthesis of TPP-NHCO- SPFGraphene is shown in Scheme 1. TPP-NH2 was synthesized according to the literature [33]. Graphene oxide (30mg) was prepared using our modified Hummers method [8,9,23], and it was then refluxed in SOCl2 (20mL) in the presence of DMF (0.5mL) at 70 8C for 24 h under argon atmosphere. At the end of the reaction, excess SOCl2 and solvent were removed by distillation. In the presence of triethylamine (Et3N, 0.5mL), the above product was allowed to react with TPP-NH2 (30mg) in DMF (10mL) at 130 8C for 72 h under argon. 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