Article pubs.acs.org/JPCC
Fabrication of Covalently Functionalized Graphene Oxide Incorporated Solid-State Hybrid Silica Gel Glasses and Their Improved Nonlinear Optical Response Lili Tao,† Bo Zhou,†,# Gongxun Bai,† Yonggang Wang,† Siu Fung Yu,† Shu Ping Lau,† Yuen Hong Tsang,*,† Jianquan Yao,‡,§ and Degang Xu*,‡,§ †
Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong ‡ The Institute of Laser & Optoelectronics, The College of Precision Instruments and Opto-Electronic Engineering, Tianjin University, Tianjin, 300072, China § Key Laboratory of Opto-Electronics Information and Technical Science (Ministry of Education), Tianjin University, Tianjin 300072, China
ABSTRACT: Solid-state graphene oxide (GO) homogeneously incorporated silica gel glasses have been successfully fabricated, and the good nonlinear optical (NLO) response shown indicates its high potential as the optical limiting material to protect optical detectors and human eyes from damage caused by high power lasers. The GO sheets are covalently functionalized with 3aminopropyltriethoxysilane and connected to the silica glass matrix through Si−O bonding, resulting in an increase in GO incorporation concentration up to 20.2 mol % (C/SiO2) without reaching the saturation limit. Digital images and element mapping analysis of carbon have confirmed the homogeneous dispersion of GO in the silica gel glass. Because of the higher twophoton absorption coefficient, lower NLO starting threshold, and optical limiting threshold shown at 532 nm wavelength, the NLO performance at this wavelength is better than at 1064 nm. At an input fluence as low as 1.3 J cm−2 at 532 nm and 2.3 J cm−2 at 1064 nm, the glass incorporated with 5.1 mol % GO starts to exhibit an NLO response, suggesting its potential as a laser optical limiting absorber. In addition to the discussion of the probable reasons behind the phenomenon, it has been demonstrated for the first time that the NLO performance of GO in solid-state silica gel glass is stronger than in deionized water.
I. INTRODUCTION
which is more simple and cost-effective than the carbon nanotube (CNT),10 another extensively studied nonlinear material. GO has covalently bounded epoxide and hydroxyl functional groups on either side of its basal plane with carboxyl groups only at the edge sites.13−15 These groups make GO hydrophilic, different from hydrophobic graphene, CNT, C60, etc., and much more attractively, they provide handles for the connection of GO to nanoparticles, DNA, special substrates, or other functional materials.16−20 Normally, it is difficult to
Graphene oxide (GO) has shown many applications in the fields of lasers, optics, and photonics acting as, for example, polarizer, saturable absorber, optical limiters, light emitting diode (LED), and photovoltaic as well as terahertz devices.1−7 Nowadays, high power lasers with various operational wavelengths are widely available for industry and military applications. Therefore, the study of nonlinear optical (NLO) materials which have a percentage of transmitted light decreased with incident light intensity has become much more significant because of their capability in protecting human eyes, sensors, and cameras from the high power lasers. GO has been demonstrated to be a good broadband NLO material8−11 and can be mass fabricated through the Hummers’ method, © 2013 American Chemical Society
Received: May 6, 2013 Revised: September 22, 2013 Published: October 8, 2013 23108
dx.doi.org/10.1021/jp404463g | J. Phys. Chem. C 2013, 117, 23108−23116
The Journal of Physical Chemistry C
Article
silica matrix via triethoxysilyl groups in the GO-APTES, which are hydrolyzed and polycondensed with tetraethoxysilane [Si(OC2H5)4, TEOS] during the sol−gel process.34 Finally, the GO sheet is covalently connected to the silica matrix through Si−O bond, effectively preventing the agglomeration of GO and greatly improving the loading concentration. The NLO properties of the homogeneous GO incorporated silica gel glass obtained using this method have been studied.
homogeneously disperse GO sheets in a solid state material because of the harsh preparation conditions (e.g., high temperature), poor solubility, and inevitable agglomeration. Thus, NLO performance of GO has been most investigated in the liquid form.8,16,21−23 However, the liquid-based GO has very limited practical applications due to its high rate of evaporation and contamination, leakage by container damage, the mismatch of refractive index between glass and the liquid, easy aggregation of GO sheet, as well as the lack of flexibility and stability. Therefore, it would be of great importance to incorporate GO into the solid-state host, which could overcome the above shortcomings that exist in the liquid state and enable more postprocessing engineering.24 In this case, being isolated from the external environment (e.g., oxidation under high pumping intensity), the GO sheets can be more stable and thus exhibit more advantages for commercial use. Despite the fact that GO has been successfully dispersed into polymer films such as polyvinyl alcohol (PVA) and polymethyl methacrylate (PMMA), the polymer-based host materials have many shortcomings such as low melting point, low optical laser damage threshold, and low transparency in the UV region, and taking PMMA as an example, its glass transition temperature (Tg) is as low as 100 °C27 and nearly nontransparent in the deep UV (99.0%), concentrated H2SO4 (95−97%), KMnO4 (≥99.0%), H2O2 (∼35%), ethanol absolute (≥99.8%), and HCl (37%) are from Sigma-Aldrich. P2O5 (Analytical Reagent), N,N-dimethylformamide (DMF), N,N′-dicyclohexylcarbodiimide (DCC), 3-aminopropyl triethoxysilane (APTES), and tetraethoxysilane (TEOS) are from Aladdin in Shanghai. B. Preparation of GO. GO was prepared from graphite using a modified Hummers’ method.12 In a typical process, graphite powder, K2S 2O 8, and P 2O5 were added into concentrated H2SO4 under vigorously stirring, followed by slowly adding KMnO4 into this suspension with ice bath. H2O2 was added to stop the process after the reaction. The obtained suspension was centrifuged, washed with DI water several times until the suspension was neutral, and finally collected for use. C. Preparation of GO-APTES. GO-APTES was prepared by adding APTES with DCC into GO powder was first dispersed well in DMF, APTES together with DCC were then added, and the formed solution was kept at 323 K for 48 h with continuous stirring. After centrifugation and washing with ethanol three times, the finally obtained GO-APTES was kept in ethanol. D. Preparation of GO-APTES Incorporated Gel Glass. GO-APTES incorporated gel glass was fabricated by the hydrolysis and polycondensation of TEOS in the presence of GO-APTES. The incorporating levels of GO in the gel glasses were adjusted to be 0, 1.3, 2.5, 5.1, 10.1, 15.3, and 20.2 mol % (C/SiO2) by tuning the volume of GO-APTES solution introduced, respectively. In detail, a certain amount of GOAPTES solution was added into the mixture of TEOS, ethanol, and DI water, followed by ultrasonic treatment for 30 min to form a homogeneous and transparent solution. Hydrochloric acid was added as the catalyst during the synthesis so as to promote the hydrolysis process of TEOS, and then it was stirred at room temperature until it was divided into several equal volume parts and then it was cast into polystyrene cells individually, sealed, and left to age and dry for 5 weeks. E. Characterization. After the sample was prepared in the form of KBr pellets, Fourier transform infrared (FTIR) spectra of GO and GO-APTES were recorded using a NICOLET 460 FT-IR spectroscopy. UV−vis absorption spectra of GO aqueous solution and gel glasses were recorded using a SHIMADZU UV-2550 UV−vis spectrophotometer. Raman spectra of GO and GO-APTES incorporated gel glasses were recorded with a HORIBA HR-800 Raman spectrometer with an argon ion laser emitted at 488 nm. Atomic force microscopy (AFM) images were recorded using a Bruker Nanoscope 8 atomic force microscope. TEM/HRTEM images and electron diffraction pattern of GO were observed by a JEOL model JEM-2100F transmission electron microscope. The structure and composition of GO-APTES incorporated gel glasses were investigated using a JEOL model JSM-6490 scanning electron
Scheme 1. Schematic Diagram for the Preparation of Hybrid Silica Gel Glass Incorporated with APTES Functionalized GO Sheets
f u n c t i o n a l i z e d w i t h 3 - a m i n o p r o py l t r i e t h o x y s i l a ne [NH2(CH2)3Si(OC2H5)3, APTES] through the chemical bond -CO-NH- formed between the -COOH contained in the GO sheet and the amino group of APTES in the appearance of N,N′-dicyclohexylcarbodiimide (DCC),32,33 followed by accomplishing the bonding of GO-APTES to the 23109
dx.doi.org/10.1021/jp404463g | J. Phys. Chem. C 2013, 117, 23108−23116
The Journal of Physical Chemistry C
Article
Figure 1. (a) Absorption spectrum of GO. Inset top shows the digital photograph of GO aqueous solution (left) and TEM image of GO sheet (right); inset bottom shows Raman spectrum of GO. (b) HRTEM of GO. Inset shows the electron diffraction pattern of GO. (c) AFM image of asprepared GO sheets and (d) Height profile of a large GO sheet overlapped with another small one.
microscope (SEM) equipped with an energy dispersive X-ray spectroscopy detector. Gold was coated in advance on the sample surface to create the clear image. The NLO properties of GO-APTES incorporated glasses and GO aqueous solution (use a 0.2 cm quartz cell) were investigated using a 8 ns laser pulses from a Nd:YAG laser at a repetition of 10 Hz. The pulse energies in front of and behind the sample were measured by an energy detector. All the measurements were operated at room temperature.
III. RESULTS AND DISCUSSION A. Structure Characterization of As-Prepared GO. The UV−vis absorption spectrum of GO aqueous solution is shown in Figure 1a. The characteristic absorption peak at 230 nm is ascribed to the π → π* transition of aromatic C−C bond, and the shoulder at around 300 nm to the n → π* transition of C O.11 The corresponding digital photograph of GO aqueous solution provided in Figure 1a inset shows a characteristic brown color, agreeing with other reports.35,36 In the Raman spectrum of the as-prepared GO given in the bottom inset of Figure 1a, there are two characteristic peaks located at 1345 cm−1 (D) and 1595 cm−1 (G), respectively. The peak G being sensitive to the configuration of sp2 sites is assigned to the plane vibrations with E2g symmetry, while the peak D is due to
Figure 2. FTIR spectra of GO and GO-APTES.
the breathing mode of k-point phonons of A1g symmetry, resulting from the defects and disorder of the graphene sheet.37,38 Two other peaks located at 2700 cm−1 (2D) and 2927 cm−1 (3S) are also recorded.39−41 It is reported that the 2D peak position of single-layer graphene is located at 2679 cm−1, with its shift around 20 cm−1 to a higher frequency for 23110
dx.doi.org/10.1021/jp404463g | J. Phys. Chem. C 2013, 117, 23108−23116
The Journal of Physical Chemistry C
Article
layered GO sheets in both HRTEM and AFM images agrees well with the layer information indicated by the Raman spectrum. B. FTIR and Solubility of GO-APTES. FTIR spectra of GO and GO-APTES are shown in Figure 2. The broad absorption peak of GO at 3423 cm−1 is due to the O−H stretching of C− OH. After reaction with APTES, it shows a bathochromic shift to 3402 cm−1 because of overlapping of the N−H stretching vibration.15 The peak at 1641 cm−1 of GO is ascribed to the CO of −COOH, which shows a hypsochromic shift to 1659 cm−1 in the spectrum of GO-APTES due to the ester formation from carboxyl group, indicating the successful chemical bond between −COOH of GO and −NH2 of APTES.44 Moreover, the two strong peaks at 2980 and 2904 cm−1, respectively, correspond to the C−H asymmetric and symmetric stretching vibrations of CH2; both peaks together with the one at 1402 cm−1 due to the bending mode of −CH2 confirm the bonding of APTES to GO,45 which is further evidenced by the observation of the peaks at 1225 cm−1 (C−N of the alkyl chains) and 1049 cm−1 (Si−O−C of APTES) in the FTIR spectrum.17,45 As shown in Figure 3a, GO exhibits excellent solubility in DI water because of hydrophilic groups in it such as −OH and −COOH contrast to its poor solubility in TEOS. In order to obtain GO homogeneously incorporated hybrid silica gel glass, a surface modification of GO was performed to form homogeneous GO solution in TEOS, which was chemically bonded to APTES through chemical reaction between −COOH of GO and −NH2 of APTES, followed by replacing the hydrophilic group −COOH with hydrophobic −CH3, and the GO homogeneously dispersed TEOS were obtained as shown in Figure 3a. Figure 3b shows the digital photos of gel glasses incorporated with GO before (top) and after (bottom) modification with APTES. After modification with APTES, GO
Figure 3. (a) Solubility of GO and GO-APTES in DI water and TEOS, respectively. (b) Digital photos of silica gel glasses incorporated with GO before modification (top) and after modification (bottom).
the multilayer (2−4 layers) graphene.42 In the present work, the 2D peak position shows a shift of 21 cm−1 to higher frequency compared with the reported 2D peak position of single-layer graphene, indicating the obtainment of multilayer (2−4 layers) GO sheets. Figure 1a top inset shows the TEM image of GO sheet, which is so thin and transparent that the copper mesh underneath can be clearly observed. Judging from the edge of the HRTEM image in Figure 1b, a three-layered GO sheet can be observed, agreeing well with the result obtained from the Raman spectrum. The selected area electron diffraction (SAED) pattern of the GO sheet, given in Figure 1b inset, shows a ring-like pattern of dark and light consisting of many bright spots, indicating the low crystallinity of GO. Figure 1c shows the AFM image of as-prepared GO sheets, which are thin (
