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Preparation and optical limiting properties of carbon nanotubes coated with Au nanoparticle composites embedded in silica gel-glass
Materials Letters 65 (2011) 150–152
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation and optical limiting properties of carbon nanotubes coated with Au nanoparticle composites embedded in silica gel-glass Chan Zheng ⁎, Wenzhe Chen, Xiaoyun Ye, Shuguang Cai, Xueqing Xiao, Mingjie Wang Department of Materials Science and Engineering, Fujian University of Technology, 3 Xueyuan Road, Fuzhou 350108, PR China
a r t i c l e
i n f o
Article history: Received 5 August 2010 Accepted 30 September 2010 Available online 30 October 2010 Keywords: Carbon nanotubes Au nanoparticles Nanocomposites Sol–gel preparation Optical limiting
a b s t r a c t Au nanoparticle (NP) coated carbon nanotubes (CNTs) (CNTs@AuNPs) embedded in silica gel-glass (CNTs@AuNPs/silica gel-glass) were prepared by the sol–gel technique. Subsequent analysis confirmed the successful introduction of the CNTs@AuNPs to the silica gel-glass. Coating with AuNPs dramatically improved the mechanical properties of the silica gel-glass matrix, despite the higher Brunauer–Emmett–Teller (BET) specific surface area and pore volume of the CNTs@AuNPs/silica gel-glass compared with CNT/silica gel-glass. The optical limiting (OL) properties of the CNTs@AuNPs measured at a laser wavelength of 532 nm were slightly weaker after introduction into the solid-state matrix. This reduction is probably attributed to the synergistic nonlinear optical effects of reverse saturable absorption arising from the CNTs and saturable absorption from the AuNPs embedded in the silica gel-glass. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are a class of promising one-dimensional nanomaterials that have attracted tremendous interest in recent years because of their superior and broadband optical limiting (OL) properties [1–3]. However, their OL responses are not ideal as they have a high limiting threshold compared to other OL materials, such as phthalocyanines [4], and are not instantaneous, which means that the OL effect of CNTs only become obvious for nanosecond or longer laser pulses. Therefore, research has focused on the modification of CNTs to make them more suitable for OL applications. Experimentally, the OL performance of CNTs can be improved by functionalization with nanoparticles (NPs) [5–7], organic molecules [8] and polymers [9]. Functionalization with NPs, such as Au and Ag NPs [5], ZnO NPs [6] and CdSe NPs [7], on the sidewall of CNTs, provides a facile and convenient method to obtain modified CNTs with enhanced OL effects. In these coated nanostructure materials, NPs are attached to the surface of the CNTs through covalent or noncovalent bonds. The interaction between the CNTs and attached NPs may produce synergistic OL effects and novel and interesting properties. Consequently, several studies have investigated these materials [5–7]. However, to date, most investigations have concentrated on suspensions/solutions. To the best of our knowledge, there are no reports on the OL behavior of solid-state NP coated CNTs. Studies focusing on the
⁎ Corresponding author. Tel./fax: + 86 591 22863181. E-mail address: [email protected] (C. Zheng). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.09.086
solid state will be of fundamental importance to development of practical applications in the field of nonlinear optics. In this study, CNTs coated with Au NPs (CNTs@AuNPs) were embedded in silica gel-glass. The CNTs@AuNPs/silica gel-glass was prepared by the sol–gel technique and analyzed by scanning electron microscopy (SEM), UV/Vis spectroscopy and pore structure measurements. In addition, the OL properties were investigated at the laser wavelength of 532 nm using the Z-scan method. 2. Experimental 2.1. Preparation of CNTs@AuNPs/silica gel-glass Preparation of the CNTs@AuNPs has been reported previously [10]. The CNTs@AuNPs/silica gel-glass was fabricated by the hydrolysis and polycondensation of tetraethoxysilane (Si(OC2H5)4, TEOS) and methyltriethoxysilane (CH3Si(OC2H5)3, MTES) in the presence of CNTs@AuNPs. For comparison, CNTs/silica gel-glass was also prepared. Specifically, TEOS (7.0 mL), MTES (2.2 mL), ethanol (10.5 mL) and DMF (7 mL) were mixed ultrasonically for 30 min. A small amount of hydrofluoric acid was then added dropwise to promote hydrolysis (pH = 4). Next, 3.2 mL of a CNTs@AuNPs water suspension was added gradually and the mixture sonicated continuously for 3 h. After sonication, the mixture was divided into several aliquots of equal volume, cast into individual polystyrene cells, sealed, and left to age and dry for several weeks. The final samples were transparent silica gel-glasses with optical quality and a uniform thickness of 1.2 mm. These samples were used for optical measurements without further processing.
C. Zheng et al. / Materials Letters 65 (2011) 150–152
151
2.2. Characterization The morphologies of the CNTs@AuNPs before and after introduction into the silica gel-glass were observed by transmission electron microscope (TEM, JEM-2010, JEOL Ltd.) with an accelerating voltage of 200 kV and a field emission SEM (JSM-6700, JEOL Ltd.), respectively. In the SEM investigation, gold was deposited on the freshly fractured surface of the sample by sputtering for clearness of the images. UV/Vis transmittance spectra were collected on Varian Cary 50 UV/Vis spectrophotometer. Pore structure was measured at 77 K by nitrogen absorption isotherm using a Micromeritics ASAP 2010 M Surface Area and Porosimetry analyzer. The hardness and modulus were characterized on a Nanoindenter XP (Nano instruments), and determined on the basis of the load-displacement curves.
Transmittance (%)
(b)
(a)
200
300
400
500
600
700
800
Wavelength (nm) 2.3. Z-scan measurements Fig. 2. UV/Vis spectra of (a) CNTs/silica gel-glass and (b) CNTs@AuNPs/silica gel-glass.
OL analysis was performed using an open-aperture Z-scan technique [11]. We used 8-ns pulses generated by a Q-switched Nd:YAG laser operating with a repetition rate of 1 Hz and at a wavelength of 532 nm in the experiments. Front-scattering light as a function of angular position was collected from 10° to 90°. The energy of a single pulse was approximately 200 μJ. For direct comparison, the linear transmittance of the samples was adjusted to approximately 70%. 3. Results and discussion Fig. 1a presents a representative TEM image of CNTs@AuNPs before introduction into the silica gel-glass. The AuNPs with diameters of approximately 5 nm were uniformly and tightly attached over the surfaces of the CNTs. Only a few AuNPs were located in areas without CNTs. The SEM morphology of a fracture surface of the CNTs@AuNPs/ silica gel-glass (Fig. 1b) indicates the successful introduction and homogeneous dispersion of CNTs@AuNPs in the silica gel-glass matrix. No AuNPs could be distinguished because the CNTs@AuNPs were completely encased by the dielectric silica gel-glass matrix. The successful introduction of CNTs@AuNPs was also supported by UV/Vis transmittance spectra (Fig. 2). For CNTs/silica gel-glass, no absorption was detected at the observed wavelength. In contrast, for CNTs@AuNPs/silica gel-glass a broad surface plasmon band centered at approximately 550 nm occurred due to the AuNPs that are coated on CNTs. The relatively broad band and weak intensity could be attributed to interaction between the Au NPs and silica gel matrix. Pore structure measurements were used to investigate the microstructure of the silica gel-glasses. The Barrett–Joyner–Halenda (BJH) pore size, Brunauer–Emmett–Teller (BET) specific surface area
and pore volume are summarized in Table 1. The pore size distributions were similar in both types of silica gel-glasses, and correspond to the interspaces of silica granules formed by the hydrolysis and condensation of TEOS. In contrast, the specific surface area and pore volume of the CNTs@AuNPs/silica gel-glass were greater than those of the CNTs/silica gel-glass. We previously found that the introduction of CNTs might influence the gel formation process [12], in which CNTs act as inhomogeneous crystalline sources and induce growth of the silica network around them. For the CNTs@AuNPs/silica gel-glass, the attached isolated AuNPs can also serve as sites of inhomogeneous nucleation. As a result, small pores consisting of the interspaces of silica granules formed around the AuNPs may exist but be too small to be detected under our experimental conditions. However, these could have contributed to the increased surface area and pore volume. This assumption is confirmed by the reduced gelation time of the CNTs@AuNPs/silica gelglass compared to the CNTs/silica gel-glass at the same doping level. Interestingly, it is worth noting that the hardness and modulus (Table 1) of the CNTs@AuNPs/silica gel-glass were largely enhanced compared to the CNTs/silica gel-glass, despite the higher specific surface area and pore volume of the CNTs@AuNPs/silica gel-glass. CNTs disperse homogenously both in CNTs/silica gel-glass [13] and CNTs@AuNPs/silica gel-glass (Fig. 1b), however, the CNTs@AuNPs tend to arrange orderly in certain direction owing to the interaction between CNTs and AuNPs, which would make CNTs work more effectively in the matrix, and hence enhanced hardness and modulus. Based on this result, we can conclude that coating AuNPs onto the
Fig. 1. Morphology of CNTs@AuNPs (a) before and (b) after introduction into silica gel-glass.
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C. Zheng et al. / Materials Letters 65 (2011) 150–152
Table 1 BJH pore size, BET specific surface values, pore volume, hardness and modulus of CNTs and CNTs@AuNPs/silica gel-glass. Sample
BJH pore BET surface Pore Hardness/ Modulus/ size/nm area/cm2 g− 1 volume/cm3 g− 1 MPa MPa
CNTs 3.4398 CNTs@AuNPs 3.5695
723.81 789.52
0.6931 0.8951
0.204 0.675
2.659 6.264
CNTs surfaces can improve the mechanical properties of the silica gelglass matrix. These enhanced properties could facilitate practical application of sol–gel derived nanocomposites. The 532 nm open-aperture Z-scan results of CNTs@AuNPs dispersed in aqueous solution and embedded in the silica gel-glass are presented in Fig. 3a. Both samples exhibited decreased transmittance near the laser beam focus, suggesting that the OL properties of the CNTs@AuNPs are maintained after being introduced into the solidstate gel-glass matrix. However, the aqueous CNTs@AuNPs had slightly lower transmittance than those embedded in the silica gelglass, implying that the aqueous CNTs@AuNPs have better nonlinear optical properties. Generally, pronounced and symmetrical valleys of open Z-scan curves are indicative of either nonlinear absorption (NLA) or nonlinear scattering (NLS). To further investigate the mechanisms, we measured the front-scattering signal as a function of angular position from 10° to 90° for both samples (Fig. 3b). The nonlinear optical properties of the CNTs@AuNPs clearly originated from NLS due to the strong scattering signal. A similar observation
(a) Normalized transmittance
1.1
1.0
was made in an earlier investigation of the OL behavior of Au and Ag NPs coated CNT composites [5]. In contrast, for the CNTs@AuNPs/silica gel-glass, the scattering signal was very weak, suggesting that the OL performance might not arise from NLS. Accordingly, it is reasonable to attribute the OL behavior to NLA, just like the CNTs/silica gel-glass. Unfortunately, unlike the enhanced OL effect observed for CNTs [13], the CNTs@AuNPs had slightly weaker OL properties after introduction into the silica gel-glass. We believe that these differences are due to the coating AuNPs. Ryansnyanskiy et al. reported saturable absorption (SA) in AuNPs embedded in Al2O3, SiO2, and ZrO matrices, while Qu et al. observed a transition from SA to reverse saturable absorption (RSA) in AuNPs precipitated in silicate glasses [14,15]. The results of both these studies indicate that AuNPs should present SA when embedded or precipitated in solid-state matrices. Therefore, the observed weakened OL behavior might be a synergistic result of RSA arising from the CNTs and SA resulting from the AuNPs. These synergistic effects are the subject of further study in our laboratory. 4. Conclusions In summary, CNTs@AuNPs silica gel-glasses were prepared by the sol–gel technique. SEM and UV/Vis spectra confirmed the successful introduction of CNTs@AuNPs to the silica gel-glasses. Coating with Au NPs dramatically improved the mechanical properties of the silica gelglass matrix, although the Au NPs coated silica had higher BET specific surface area and pore volume than the CNTs/silica gel-glass. The OL properties (at λ = 532 nm) of the CNTs@AuNPs were slightly weaker after introduction into the solid-state matrix. This result could probably be attributed to the synergistic nonlinear optical effects of RSA arising from the CNTs and SA resulting from the Au NPs embedded in the silica gel-glass. Despite this, the method we propose is a facile and effective way to encapsulate functionalized CNTs in a solid-state matrix, which could lead to future practical applications in the field of optics.
0.9
Acknowledgments
0.8
This work was supported by the Natural Science Foundation of Fujian Province (2010J05101).
0.7
References Aqueous solution Gel-glass
0.6
-40
-30
-20
-10
0
10
20
30
40
Z (mm)
(b) Scattering signal (Arb. Units)
Aqueous solution Gel-glass
0
20
40
60
80
100
Angular position (Deg) Fig. 3. (a) Results of the open-aperture Z-scan for a CNTs@AuNPs in aqueous solution and CNTs@AuNPs/silica gel-glass at 532 nm. (b) Plot of the front-scattered signal as a function of angular position from 10° to 90°. The scattered signal is plotted in arbitrary units.
[1] Chin KC, Gohel A, Elim HI, Chen WZ, Ji W, Chong GL, et al. J Mater Res 2006;21: 2758–66. [2] Chen Y, Lin Y, Liu Y, Doyle J, He N, Zhuang XD, et al. J Nanosci Nanotechnol 2007;7: 1268–83. [3] Wang J, Chen Y, Blau WJ. J Mater Chem 2009;19:7425–43. [4] Chen Y, Hanack M, Araki Y, Ito O. Chem Soc Rev 2005;34:517–29. [5] Chin KC, Gohel A, Chen WZ, Elim HI, Ji W, Chong GL, et al. Chem Phys Lett 2005;409:85–8. [6] Zhu YW, Elim HI, Foo Y-L, Yu T, Liu YJ, Ji W, et al. Adv Mater 2006;18:587–92. [7] Feng M, Sun RQ, Zhan HB, Chen Y. Carbon 2010;48:1177–85. [8] Liu ZB, Tian JG, Guo Z, Ren ZG, Du T, Zheng YJ, et al. Adv Mater 2008;20:511–5. [9] Wu HX, Qiu XQ, Cao WM, Lin YH, Cai RF, Qian SX. Carbon 2007;45:2866–72. [10] Zheng C, Feng M, Zhan HB. Carbon 2010;48:3750–9. [11] Sheik-Bahae M, Said AA, Wei T-H, Hagan DJ, Stryland EWV. IEEE J Quantum Electron 1990;26:760–9. [12] Zheng C, Feng M, Zhen X, Huang J, Zhan HB. Mater Lett 2007;61:644–7. [13] Zhan HB, Zheng C, Chen WZ, Wang MQ. Chem Phys Lett 2003;382:313–7. [14] Ryasnyanskiy AI, Palpanta B, Debrusa S, Palc U, Stepanov A. J Lumin 2007;127: 181–5. [15] Qu SL, Gao YC, Jiang XW, Zeng HD, Song YL, Qiu JR, et al. Opt Commun 2003;224: 321–7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation and optical limiting properties of carbon nanotubes coated with Au nanoparticle composites embedded in silica gel-glass Chan Zheng ⁎, Wenzhe Chen, Xiaoyun Ye, Shuguang Cai, Xueqing Xiao, Mingjie Wang Department of Materials Science and Engineering, Fujian University of Technology, 3 Xueyuan Road, Fuzhou 350108, PR China
a r t i c l e
i n f o
Article history: Received 5 August 2010 Accepted 30 September 2010 Available online 30 October 2010 Keywords: Carbon nanotubes Au nanoparticles Nanocomposites Sol–gel preparation Optical limiting
a b s t r a c t Au nanoparticle (NP) coated carbon nanotubes (CNTs) (CNTs@AuNPs) embedded in silica gel-glass (CNTs@AuNPs/silica gel-glass) were prepared by the sol–gel technique. Subsequent analysis confirmed the successful introduction of the CNTs@AuNPs to the silica gel-glass. Coating with AuNPs dramatically improved the mechanical properties of the silica gel-glass matrix, despite the higher Brunauer–Emmett–Teller (BET) specific surface area and pore volume of the CNTs@AuNPs/silica gel-glass compared with CNT/silica gel-glass. The optical limiting (OL) properties of the CNTs@AuNPs measured at a laser wavelength of 532 nm were slightly weaker after introduction into the solid-state matrix. This reduction is probably attributed to the synergistic nonlinear optical effects of reverse saturable absorption arising from the CNTs and saturable absorption from the AuNPs embedded in the silica gel-glass. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) are a class of promising one-dimensional nanomaterials that have attracted tremendous interest in recent years because of their superior and broadband optical limiting (OL) properties [1–3]. However, their OL responses are not ideal as they have a high limiting threshold compared to other OL materials, such as phthalocyanines [4], and are not instantaneous, which means that the OL effect of CNTs only become obvious for nanosecond or longer laser pulses. Therefore, research has focused on the modification of CNTs to make them more suitable for OL applications. Experimentally, the OL performance of CNTs can be improved by functionalization with nanoparticles (NPs) [5–7], organic molecules [8] and polymers [9]. Functionalization with NPs, such as Au and Ag NPs [5], ZnO NPs [6] and CdSe NPs [7], on the sidewall of CNTs, provides a facile and convenient method to obtain modified CNTs with enhanced OL effects. In these coated nanostructure materials, NPs are attached to the surface of the CNTs through covalent or noncovalent bonds. The interaction between the CNTs and attached NPs may produce synergistic OL effects and novel and interesting properties. Consequently, several studies have investigated these materials [5–7]. However, to date, most investigations have concentrated on suspensions/solutions. To the best of our knowledge, there are no reports on the OL behavior of solid-state NP coated CNTs. Studies focusing on the
⁎ Corresponding author. Tel./fax: + 86 591 22863181. E-mail address: [email protected] (C. Zheng). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.09.086
solid state will be of fundamental importance to development of practical applications in the field of nonlinear optics. In this study, CNTs coated with Au NPs (CNTs@AuNPs) were embedded in silica gel-glass. The CNTs@AuNPs/silica gel-glass was prepared by the sol–gel technique and analyzed by scanning electron microscopy (SEM), UV/Vis spectroscopy and pore structure measurements. In addition, the OL properties were investigated at the laser wavelength of 532 nm using the Z-scan method. 2. Experimental 2.1. Preparation of CNTs@AuNPs/silica gel-glass Preparation of the CNTs@AuNPs has been reported previously [10]. The CNTs@AuNPs/silica gel-glass was fabricated by the hydrolysis and polycondensation of tetraethoxysilane (Si(OC2H5)4, TEOS) and methyltriethoxysilane (CH3Si(OC2H5)3, MTES) in the presence of CNTs@AuNPs. For comparison, CNTs/silica gel-glass was also prepared. Specifically, TEOS (7.0 mL), MTES (2.2 mL), ethanol (10.5 mL) and DMF (7 mL) were mixed ultrasonically for 30 min. A small amount of hydrofluoric acid was then added dropwise to promote hydrolysis (pH = 4). Next, 3.2 mL of a CNTs@AuNPs water suspension was added gradually and the mixture sonicated continuously for 3 h. After sonication, the mixture was divided into several aliquots of equal volume, cast into individual polystyrene cells, sealed, and left to age and dry for several weeks. The final samples were transparent silica gel-glasses with optical quality and a uniform thickness of 1.2 mm. These samples were used for optical measurements without further processing.
C. Zheng et al. / Materials Letters 65 (2011) 150–152
151
2.2. Characterization The morphologies of the CNTs@AuNPs before and after introduction into the silica gel-glass were observed by transmission electron microscope (TEM, JEM-2010, JEOL Ltd.) with an accelerating voltage of 200 kV and a field emission SEM (JSM-6700, JEOL Ltd.), respectively. In the SEM investigation, gold was deposited on the freshly fractured surface of the sample by sputtering for clearness of the images. UV/Vis transmittance spectra were collected on Varian Cary 50 UV/Vis spectrophotometer. Pore structure was measured at 77 K by nitrogen absorption isotherm using a Micromeritics ASAP 2010 M Surface Area and Porosimetry analyzer. The hardness and modulus were characterized on a Nanoindenter XP (Nano instruments), and determined on the basis of the load-displacement curves.
Transmittance (%)
(b)
(a)
200
300
400
500
600
700
800
Wavelength (nm) 2.3. Z-scan measurements Fig. 2. UV/Vis spectra of (a) CNTs/silica gel-glass and (b) CNTs@AuNPs/silica gel-glass.
OL analysis was performed using an open-aperture Z-scan technique [11]. We used 8-ns pulses generated by a Q-switched Nd:YAG laser operating with a repetition rate of 1 Hz and at a wavelength of 532 nm in the experiments. Front-scattering light as a function of angular position was collected from 10° to 90°. The energy of a single pulse was approximately 200 μJ. For direct comparison, the linear transmittance of the samples was adjusted to approximately 70%. 3. Results and discussion Fig. 1a presents a representative TEM image of CNTs@AuNPs before introduction into the silica gel-glass. The AuNPs with diameters of approximately 5 nm were uniformly and tightly attached over the surfaces of the CNTs. Only a few AuNPs were located in areas without CNTs. The SEM morphology of a fracture surface of the CNTs@AuNPs/ silica gel-glass (Fig. 1b) indicates the successful introduction and homogeneous dispersion of CNTs@AuNPs in the silica gel-glass matrix. No AuNPs could be distinguished because the CNTs@AuNPs were completely encased by the dielectric silica gel-glass matrix. The successful introduction of CNTs@AuNPs was also supported by UV/Vis transmittance spectra (Fig. 2). For CNTs/silica gel-glass, no absorption was detected at the observed wavelength. In contrast, for CNTs@AuNPs/silica gel-glass a broad surface plasmon band centered at approximately 550 nm occurred due to the AuNPs that are coated on CNTs. The relatively broad band and weak intensity could be attributed to interaction between the Au NPs and silica gel matrix. Pore structure measurements were used to investigate the microstructure of the silica gel-glasses. The Barrett–Joyner–Halenda (BJH) pore size, Brunauer–Emmett–Teller (BET) specific surface area
and pore volume are summarized in Table 1. The pore size distributions were similar in both types of silica gel-glasses, and correspond to the interspaces of silica granules formed by the hydrolysis and condensation of TEOS. In contrast, the specific surface area and pore volume of the CNTs@AuNPs/silica gel-glass were greater than those of the CNTs/silica gel-glass. We previously found that the introduction of CNTs might influence the gel formation process [12], in which CNTs act as inhomogeneous crystalline sources and induce growth of the silica network around them. For the CNTs@AuNPs/silica gel-glass, the attached isolated AuNPs can also serve as sites of inhomogeneous nucleation. As a result, small pores consisting of the interspaces of silica granules formed around the AuNPs may exist but be too small to be detected under our experimental conditions. However, these could have contributed to the increased surface area and pore volume. This assumption is confirmed by the reduced gelation time of the CNTs@AuNPs/silica gelglass compared to the CNTs/silica gel-glass at the same doping level. Interestingly, it is worth noting that the hardness and modulus (Table 1) of the CNTs@AuNPs/silica gel-glass were largely enhanced compared to the CNTs/silica gel-glass, despite the higher specific surface area and pore volume of the CNTs@AuNPs/silica gel-glass. CNTs disperse homogenously both in CNTs/silica gel-glass [13] and CNTs@AuNPs/silica gel-glass (Fig. 1b), however, the CNTs@AuNPs tend to arrange orderly in certain direction owing to the interaction between CNTs and AuNPs, which would make CNTs work more effectively in the matrix, and hence enhanced hardness and modulus. Based on this result, we can conclude that coating AuNPs onto the
Fig. 1. Morphology of CNTs@AuNPs (a) before and (b) after introduction into silica gel-glass.
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C. Zheng et al. / Materials Letters 65 (2011) 150–152
Table 1 BJH pore size, BET specific surface values, pore volume, hardness and modulus of CNTs and CNTs@AuNPs/silica gel-glass. Sample
BJH pore BET surface Pore Hardness/ Modulus/ size/nm area/cm2 g− 1 volume/cm3 g− 1 MPa MPa
CNTs 3.4398 CNTs@AuNPs 3.5695
723.81 789.52
0.6931 0.8951
0.204 0.675
2.659 6.264
CNTs surfaces can improve the mechanical properties of the silica gelglass matrix. These enhanced properties could facilitate practical application of sol–gel derived nanocomposites. The 532 nm open-aperture Z-scan results of CNTs@AuNPs dispersed in aqueous solution and embedded in the silica gel-glass are presented in Fig. 3a. Both samples exhibited decreased transmittance near the laser beam focus, suggesting that the OL properties of the CNTs@AuNPs are maintained after being introduced into the solidstate gel-glass matrix. However, the aqueous CNTs@AuNPs had slightly lower transmittance than those embedded in the silica gelglass, implying that the aqueous CNTs@AuNPs have better nonlinear optical properties. Generally, pronounced and symmetrical valleys of open Z-scan curves are indicative of either nonlinear absorption (NLA) or nonlinear scattering (NLS). To further investigate the mechanisms, we measured the front-scattering signal as a function of angular position from 10° to 90° for both samples (Fig. 3b). The nonlinear optical properties of the CNTs@AuNPs clearly originated from NLS due to the strong scattering signal. A similar observation
(a) Normalized transmittance
1.1
1.0
was made in an earlier investigation of the OL behavior of Au and Ag NPs coated CNT composites [5]. In contrast, for the CNTs@AuNPs/silica gel-glass, the scattering signal was very weak, suggesting that the OL performance might not arise from NLS. Accordingly, it is reasonable to attribute the OL behavior to NLA, just like the CNTs/silica gel-glass. Unfortunately, unlike the enhanced OL effect observed for CNTs [13], the CNTs@AuNPs had slightly weaker OL properties after introduction into the silica gel-glass. We believe that these differences are due to the coating AuNPs. Ryansnyanskiy et al. reported saturable absorption (SA) in AuNPs embedded in Al2O3, SiO2, and ZrO matrices, while Qu et al. observed a transition from SA to reverse saturable absorption (RSA) in AuNPs precipitated in silicate glasses [14,15]. The results of both these studies indicate that AuNPs should present SA when embedded or precipitated in solid-state matrices. Therefore, the observed weakened OL behavior might be a synergistic result of RSA arising from the CNTs and SA resulting from the AuNPs. These synergistic effects are the subject of further study in our laboratory. 4. Conclusions In summary, CNTs@AuNPs silica gel-glasses were prepared by the sol–gel technique. SEM and UV/Vis spectra confirmed the successful introduction of CNTs@AuNPs to the silica gel-glasses. Coating with Au NPs dramatically improved the mechanical properties of the silica gelglass matrix, although the Au NPs coated silica had higher BET specific surface area and pore volume than the CNTs/silica gel-glass. The OL properties (at λ = 532 nm) of the CNTs@AuNPs were slightly weaker after introduction into the solid-state matrix. This result could probably be attributed to the synergistic nonlinear optical effects of RSA arising from the CNTs and SA resulting from the Au NPs embedded in the silica gel-glass. Despite this, the method we propose is a facile and effective way to encapsulate functionalized CNTs in a solid-state matrix, which could lead to future practical applications in the field of optics.
0.9
Acknowledgments
0.8
This work was supported by the Natural Science Foundation of Fujian Province (2010J05101).
0.7
References Aqueous solution Gel-glass
0.6
-40
-30
-20
-10
0
10
20
30
40
Z (mm)
(b) Scattering signal (Arb. Units)
Aqueous solution Gel-glass
0
20
40
60
80
100
Angular position (Deg) Fig. 3. (a) Results of the open-aperture Z-scan for a CNTs@AuNPs in aqueous solution and CNTs@AuNPs/silica gel-glass at 532 nm. (b) Plot of the front-scattered signal as a function of angular position from 10° to 90°. The scattered signal is plotted in arbitrary units.
[1] Chin KC, Gohel A, Elim HI, Chen WZ, Ji W, Chong GL, et al. J Mater Res 2006;21: 2758–66. [2] Chen Y, Lin Y, Liu Y, Doyle J, He N, Zhuang XD, et al. J Nanosci Nanotechnol 2007;7: 1268–83. [3] Wang J, Chen Y, Blau WJ. J Mater Chem 2009;19:7425–43. [4] Chen Y, Hanack M, Araki Y, Ito O. Chem Soc Rev 2005;34:517–29. [5] Chin KC, Gohel A, Chen WZ, Elim HI, Ji W, Chong GL, et al. Chem Phys Lett 2005;409:85–8. [6] Zhu YW, Elim HI, Foo Y-L, Yu T, Liu YJ, Ji W, et al. Adv Mater 2006;18:587–92. [7] Feng M, Sun RQ, Zhan HB, Chen Y. Carbon 2010;48:1177–85. [8] Liu ZB, Tian JG, Guo Z, Ren ZG, Du T, Zheng YJ, et al. Adv Mater 2008;20:511–5. [9] Wu HX, Qiu XQ, Cao WM, Lin YH, Cai RF, Qian SX. Carbon 2007;45:2866–72. [10] Zheng C, Feng M, Zhan HB. Carbon 2010;48:3750–9. [11] Sheik-Bahae M, Said AA, Wei T-H, Hagan DJ, Stryland EWV. IEEE J Quantum Electron 1990;26:760–9. [12] Zheng C, Feng M, Zhen X, Huang J, Zhan HB. Mater Lett 2007;61:644–7. [13] Zhan HB, Zheng C, Chen WZ, Wang MQ. Chem Phys Lett 2003;382:313–7. [14] Ryasnyanskiy AI, Palpanta B, Debrusa S, Palc U, Stepanov A. J Lumin 2007;127: 181–5. [15] Qu SL, Gao YC, Jiang XW, Zeng HD, Song YL, Qiu JR, et al. Opt Commun 2003;224: 321–7.