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triangular Pd nanocrystal composites
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Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites Chan Zheng, Wenzhe Chen, Shuguang Cai, Xueqing Xiao, Xiaoyun Ye College of Materials Science and Engineering, Fujian University of Technology, 3 Xuefu Road, Fuzhou 350118, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 May 2014 Accepted 31 May 2014
Novel graphene oxide (GO)/triangular Pd nanocrytal composites were prepared by a simple method involving the direct reduction of palladium acetate on the surface of GO. Transmission electron microscopy images confirmed that the triangular Pd nanoparticles (NPs) were successfully and homogenously deposited onto the surface of the two-dimensional GO sheet. Raman spectroscopy revealed that the loading of Pd NPs resulted in a lesser-ordered and more defective graphitic structure attributed to the increased defects and interaction between Pd NPs and GO sheets. The corresponding optical limiting (OL) behavior was investigated at 532 nm using the open aperture Z-scan technique with nanosecond pulses. Results showed that the OL property was significantly enhanced in GO/triangular Pd composites compared with its individual counterparts, indicating the usability of these hybrids in optoelectronic applications. Furthermore, the synthetic OL properties in GO/triangular Pd nanocrytal composites were found to originate from two-photon absorption in the GO sheet and nonlinear scattering in the triangular Pd NPs. & 2014 Published by Elsevier B.V.
Keywords: Carbon materials Nanocomposites Optical materials and properties Optical limiting Z-scan technique
1. Introduction Graphene, a monolayer of carbon atoms packed into a dense honeycomb crystal structure, is attracting considerable attention from both the experimental and theoretical research bodies since its discovery in 2004 [1–3]. Given the unique nanostructure and fascinating properties of graphene, it has been shown to have promising applications in electronics, optics, magnetic, biomedicine, catalysis, sensors, energy storage, among others [1–3]. Recent studies have revealed that graphene exhibits strong optical limiting (OL) properties [4–9]. Such properties are crucial in attenuating intense and potentially dangerous laser beams, allowing only reduced transmission to the target area while exhibiting high-transmittance for low ambient light. Thus, graphene can protect human eyes and optical sensors from damage. The excellent OL properties of graphene mainly originate from nonlinear scattering (NLS), reverse saturable absorption (RSA), two-photon absorption (TPA), and multi-photon absorption, and may differ from materials and laser pulses [4–9]. The assembly and immobilization of metal nanoparticles (NPs) on material surfaces is a primary step in developing new hybrid materials [10,11]. When nanoparticles are anchored in 2D or 3D networks, new collective optical, electronic, and magnetic properties that deviate from those of their isolated counterparts are obtained by the loading of nanoparticles. These synthetic properties have potential applications in biological and chemical sensing, as well as in electronic and optoeletronic use [10,11]. Most recently,
interest has centered on hybrid materials based on graphene coated with metal NPs. Significant effort has been focused on growing metal NPs, such as Pt, Pd, Au, and Ag, onto graphene oxide (GO) sheets [12]. Graphene-based metal NP composites, which are dependent on the type of anchored metal NPs, have demonstrated potential applications in catalysis, surface-enhanced Raman scattering, electrochemical sensing, medicine, and biology [12]. Experimental results have proven that graphene-loaded metal NP composites could be applied in nonlinear optics [13–15]. Furthermore, such composites have showed improved nonlinear optical or OL properties as compared to single component because such multifunctional hybrid materials take advantage of both the superior properties of graphene and metal NPs, which promote the efforts to explore the possibility of graphene based hybrids as optical limiters. For example, the OL efficiency of functionalized hydrogen-exfoliated graphene was improved upon the infusion of Pt and Pd NPs because of the enhanced nonlinear absorption, which originated from interband transitions and charge transfers [13]. However, most metal NPs are spherical in shape, and depositing metal NPs with varied morphologies onto the surfaces of GO is important because many properties of metal NPs strongly depend on their morphologies. Growing other shapes of metal NPs onto GO would promote the applications of GO–metal nanocomposites in the field of nonlinear optics. Here we report our studies on the deposition of novel triangular Pd NPs onto the GO sheet surface for the reason that Pd NPs
http://dx.doi.org/10.1016/j.matlet.2014.05.199 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Zheng C, et al. Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.199i
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are not only of interest for NLO; however, they may also be promising for the acoustically induced nonlinear optics [16]. Transmission electron microscopy (TEM) image and Raman spectra were applied to characterize the GO/triangular Pd hybrids. The OL behaviors were investigated by the open aperture (OA) Z-scan technique using 8 ns laser pulses at 532 nm.
2. Experimental Synthesis of GO/ triangular Pd composites: Synthesis of GO: Graphene was prepared from the oxidation of natural graphite powder via modified Hummers' method [17]. Ten and five grams of graphite powder and sodium nitrate were respectively stirred in concentrated sulfuric acid (230 mL) while being cooled in an ice bath. Subsequently, 30 g potassium permanganate was gradually added to form a new mixture. The mixture was allowed to warm to room temperature and was heated to 35 1C using a water bath. The mixture was then gently stirred for 2 h. The reaction mixture was cooled with an ice bath, and distilled water was added in excess to the mixture. Aqueous hydrogen peroxide solution (30 wt%) was added to the mixture, which was stirred for 2 h to reduce the excess KMnO4. The resultant suspension was intensively washed first with a diluted HCl solution and followed by distilled water via filtration. Subsequently, the suspension was centrifuged at 3000 rpm to remove the residual unexfoliated graphite and oxidative agents. The resulting material was dried by lyophilization to yield the GO powder. Synthesis of GO/triangular composites: Thirty milliliters of GO were dispersed into a 45 ml aqueous suspension by ultrasonication. Then, 0.27 g sodium citrate was added to the above GO aqueous graphene suspension, and the solution was boiled at 100 1C. Subsequently, 8.6 mg palladium acetate and 2.3 ml (0.1 mol L 1) ascorbic acid (AA) solution was successively added to the system rapidly, and the latter was refluxed for 2.5 h. The resultant nanocomposite was washed with distilled water using centrifugation (3000 rpm) to remove the free palladium NPs that had formed in the solution. The final nanocomposite was dried by lyophilization. Characterization: The geometry of the obtained GO/triangular Pd composites was investigated by transmission electron microscopy (TEM; JEM-2010; accelerating voltage, 200 kV). For the TEM observations, the samples were ultrasonicated in ethanol to ensure dispersion. A drop of the dispersed sample was left to dry on a
commercial, carbon-coated Cu TEM grid. Raman spectra of the GO and GO/triangular Pd composites were recorded using a Raman spectrometer (Renishaw Invia) at an ambient temperature and excitation wavelength of 785 nm. Z-scan measurement: The OL behavior of GO and GO/ triangular Pd composites were examined by the OA Z-scan technique [18]. The excitation light source was an Nd:YAG laser (Brio 640, Quantel, Les Ulis, France) with a repetition rate of 1 Hz. The laser pulses (period, 4 ns; wavelength, 532 nm) were split into two beams by a mirror. The pulse energies in front and at the back of the sample were monitored by energy detectors D1 and D2 (PE25, Ophir Optronics Solutions Ltd., Jerusalem, Israel). The laser beam waist was approximately 14.5 μm, and the energy of a single pulse was set at 200 μJ. All measurements were conducted at room temperature. Hybrid gel glasses were vertically fixed with a clamp. Each sample was mounted on a computer-controlled translation stage that shifts the sample along the z-axis.
3. Results and discussion Fig. 1 displays the representative images for GO/triangular Pd composites at low (Fig.1(a) and high (b)) magnifications. The Pd triangular NPs appeared to be homogeneously deposited on the GO surface with an average particle size of approximately 5 nm. The HRTEM image (Fig. 1(b)) further confirms the triangular morphology of the loaded Pd NPs and shows that the lattice spacing of the Pd NPs are about 0.23 nm, which correspond to single crystals of Pd with several (111) facets. The results suggest that triangular Pd NPs have been successfully attached on the GO surface. Raman spectroscopy is a versatile and non-destructive characterization technique used to investigate the nature and structure of carbon materials. Fig. 2 illustrates the Raman spectra of GO and GO/ triangular Pd composites, in which the characteristic bands are located at 1320 (G band) and 1596 cm 1 (D band), respectively. The G band corresponds to the in-plane optical vibrations of graphene atoms, whereas the D band is attributed to the intrinsic defects in the graphitic plane, including the edge effects and graphene ripple. D band exhibits a sharper intensity than the G band (Fig. 2), which indicates lesser-ordered and more defective graphitic structure of our as-prepared GO compared with those previously reported [12]. Loading of Pd NPs increases the D band
Fig. 1. TEM images of GO/triangular Pd composites.
Please cite this article as: Zheng C, et al. Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.199i
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intensity and increases the ID/IG from 1.39 to 1.51. The broadening is attributed to the increased defects and the interaction between Pd NPs and graphene sheets. The OA Z-scan technique was employed to measure the OL properties of the obtained GO and GO/triangular Pd composites, and the results are shown in Fig. 3. The OA curve exhibits a symmetric valley of the normalized transmittance about the focal point (z¼0), which implies the presence of nonlinear absorption (NLA) or NLS. This nonlinear behavior is a typical characteristic for the OL phenomena. The depth of the valley of OA Z-scan curve is a direct measurement of the extent of OL. Fig. 3 reveals that OL is significantly enhanced in GO/Pd NPs compared with its companions. In materials, OL behavior arises mainly from NLA (two-photon absorption (2PA), free-carrier absorption, and reverse saturable absorption (RSA)), nonlinear refraction, and NLS [18]. To identify the OL mechanisms of the resulting GO/triangular Pd composites, we tried fitting the Z-scan data numerically to different nonlinear transmission equations, including RSA and 2PA [18]. By simulating the nonlinear transmittance using the appropriate pulse propagation equations, we found that the best numerical fit was to a 2PAtype process, and the results are displayed in Fig. 3. The total nonlinear absorption coefficient (β) calculated from the numerical fits are 0.50 and 0.59 cm/GW for GO and GO/triangular Pd composites, respectively. The theory of TPA process fitted well
Fig. 4. Typical nonlinear scattering results for GO and GO/triangular Pd composites at 532 nm. The energy detector was located 451 from the axis.
with the experimental curves, which confirms that TPA is a major mechanism responsible for the OL of GO and GO/triangular Pd composites. Given that the valleys in the open aperture Z-scan curves could be signatures of either NLA or NLS, an experiment involving NLS measurements was performed to determine if NLS contributes to the observed OL effects in GO nanostructures. Typical nonlinear scattering results of GO and GO/triangular Pd composites are presented in Fig. 4. For the GO/triangular Pd composites, a significant scattering peak that is symmetric about the focus was evident, indicating NLS. However, no NLS peak for GO is observed, indicating that no NLS contribution exists. This finding suggests a significant enhancement in NLS, which is due to the presence of triangular Pd NPs in the nanocomposites. In NLS, it is well accepted that the light absorption by metal NPs induces a very high temperature rise, which leads to the formation of rapidly expanding microplasmas. The formation and rapid expansion of these microplasmas in turn transfer the thermal energy to the surrounding medium, forming solvent microbubbles. These microplasmas and microbubbles strongly scatter light from the transmitted beam direction, leading to a decrease in the measured transmitted light energy and hence OL occurs [19]. In GO/triangular Pd composites, we believe that the conducting twodimensional GO facilitates heat transfer between the Pd NPs and surrounding solvent molecules and therefore improves the heatbased OL performance. Meanwhile, as the radiation energy has been absorbed by the composites, the outer Pd NPs coating can rapidly scatter the energy and consequently the scattering energy would be effectively absorb by substrate GO sheet. The energy transfer between the Pd NPs and GO may have a positive influence on the resulting OL response. Therefore, combining 2TA of GO sheets and NLS of triangular Pd NPs contributes to the improved OL property in triangular Pd NPs composites.
4. Conclusion Novel GO/triangular Pd composites were prepared by the simple method, which involved the direct reduction of palladium acetate on the surface of GO. TEM images confirmed that the triangular Pd NPs have been successfully and homogenously deposited onto the surface of the two-dimensional GO sheet. Raman spectrum revealed that loading of Pd NPs resulted in lesser-ordered and more defective graphitic structure, which is attributed to the increased defects and the interaction between Pd NPs and GO sheets. The corresponding OL behavior was investigated using the open aperture Z-scan
Please cite this article as: Zheng C, et al. Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.199i
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technique with nanosecond pulses. Results show that the OL property is significantly enhanced in GO/triangular Pd composites compared with its individual counterparts owing to the addition NLS effects. The synthetic OL properties in GO/triangular Pd nanocrytal composites were found to originate from TPA in GO sheet and NLS in triangular Pd NPs. Meanwhile, the energy transfer between the Pd NPs and GO may have a positive influence on the resulting OL response. All results imply the usability of these hybrids in optoelectronic applications.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 61108056). References [1] Rao CNR, Sood AK, Subrahmanyam KS, Govindaraj A. Angew Chem Int Ed 2009;48:7752–77. [2] Geimand AK, Novoselov KS. Nat Mater 2007;6:183–91.
[3] Zhu YW, Murali S, Cai WW, Li XS, Suk JW, Potts JR, et al. Adv Mater 2010;22:3906–24. [4] Liu ZB, Zhang XL, Xu YF, Chen YS, Tian JG. Appl Phys Lett 2009;94:021902. [5] Feng M, Zhan HB, Chen Yu. Appl Phys Lett 2010;96:033107. [6] Liaros N, Aloukou P, Kolokithas-Ntoukas A, Bakandritsos A, Szabo T, Zboril R, et al. J Phys Chem 2013;117:6842–50. [7] Zheng XQ, Feng M, Zhan HB. J Mater Chem C 2013;1:6759–66. [8] Tao LL, Zhou B, Bai GX, Wang YG, Yu SF, Lau SP, et al. J Phys Chem C 2013;117:23108–16. [9] Jiang X-F, Polavarapu L, Neo ST, Venkatesan T, Xu QH. J Phys Chem Lett 2012;3:785–90. [10] Curri ML, Comparelli R, Striccoli M, Agostiano A. Phys Chem Chem Phys 2010;12:11197–207. [11] Wang L, Luo J, Schadt MJ, Zhong CJ. Langmuir 2009;26:618–32. [12] Huang X, Yin ZY, Wu SX, Qi XY, He QY, Zhang QC, et al. Small 2011;7: 1876–902. [13] Anand B, Kaniyoor A, Sai SSS, Philip RJ, Ramaprabhu S. J Phys Chem C 2013;1:2773–80. [14] Sun ZY, Dong NN, Wang KP, Kőnig DN, Nagaiah TC, Sánchez MD. Carbon 2013;62:182–92. [15] Lee YH, Polavarapu L, Gao NY, Yuan PY, Xu Q-H. Langmuir 2012;28:321–6. [16] Ebothe J, Kityk IV, Chang G, Oyama M, Plucinski KJ. Physica E: Low-Dimens Syst Nanostruct 2006;35:121–5. [17] Hummers WS, Offeman RE. J Am Chem Soc 1958;80:1339. [18] Sheik-Bahae M, Said AA, Wei T-H, Hagan DJ, Stryland EWV. IEEE J Quantum Electron 1990;26:760–9. [19] Pan H, Chen WZ, Feng YP, Ji Wei. Appl Phys Lett 2006;88:223106.
Please cite this article as: Zheng C, et al. Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.199i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites Chan Zheng, Wenzhe Chen, Shuguang Cai, Xueqing Xiao, Xiaoyun Ye College of Materials Science and Engineering, Fujian University of Technology, 3 Xuefu Road, Fuzhou 350118, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 May 2014 Accepted 31 May 2014
Novel graphene oxide (GO)/triangular Pd nanocrytal composites were prepared by a simple method involving the direct reduction of palladium acetate on the surface of GO. Transmission electron microscopy images confirmed that the triangular Pd nanoparticles (NPs) were successfully and homogenously deposited onto the surface of the two-dimensional GO sheet. Raman spectroscopy revealed that the loading of Pd NPs resulted in a lesser-ordered and more defective graphitic structure attributed to the increased defects and interaction between Pd NPs and GO sheets. The corresponding optical limiting (OL) behavior was investigated at 532 nm using the open aperture Z-scan technique with nanosecond pulses. Results showed that the OL property was significantly enhanced in GO/triangular Pd composites compared with its individual counterparts, indicating the usability of these hybrids in optoelectronic applications. Furthermore, the synthetic OL properties in GO/triangular Pd nanocrytal composites were found to originate from two-photon absorption in the GO sheet and nonlinear scattering in the triangular Pd NPs. & 2014 Published by Elsevier B.V.
Keywords: Carbon materials Nanocomposites Optical materials and properties Optical limiting Z-scan technique
1. Introduction Graphene, a monolayer of carbon atoms packed into a dense honeycomb crystal structure, is attracting considerable attention from both the experimental and theoretical research bodies since its discovery in 2004 [1–3]. Given the unique nanostructure and fascinating properties of graphene, it has been shown to have promising applications in electronics, optics, magnetic, biomedicine, catalysis, sensors, energy storage, among others [1–3]. Recent studies have revealed that graphene exhibits strong optical limiting (OL) properties [4–9]. Such properties are crucial in attenuating intense and potentially dangerous laser beams, allowing only reduced transmission to the target area while exhibiting high-transmittance for low ambient light. Thus, graphene can protect human eyes and optical sensors from damage. The excellent OL properties of graphene mainly originate from nonlinear scattering (NLS), reverse saturable absorption (RSA), two-photon absorption (TPA), and multi-photon absorption, and may differ from materials and laser pulses [4–9]. The assembly and immobilization of metal nanoparticles (NPs) on material surfaces is a primary step in developing new hybrid materials [10,11]. When nanoparticles are anchored in 2D or 3D networks, new collective optical, electronic, and magnetic properties that deviate from those of their isolated counterparts are obtained by the loading of nanoparticles. These synthetic properties have potential applications in biological and chemical sensing, as well as in electronic and optoeletronic use [10,11]. Most recently,
interest has centered on hybrid materials based on graphene coated with metal NPs. Significant effort has been focused on growing metal NPs, such as Pt, Pd, Au, and Ag, onto graphene oxide (GO) sheets [12]. Graphene-based metal NP composites, which are dependent on the type of anchored metal NPs, have demonstrated potential applications in catalysis, surface-enhanced Raman scattering, electrochemical sensing, medicine, and biology [12]. Experimental results have proven that graphene-loaded metal NP composites could be applied in nonlinear optics [13–15]. Furthermore, such composites have showed improved nonlinear optical or OL properties as compared to single component because such multifunctional hybrid materials take advantage of both the superior properties of graphene and metal NPs, which promote the efforts to explore the possibility of graphene based hybrids as optical limiters. For example, the OL efficiency of functionalized hydrogen-exfoliated graphene was improved upon the infusion of Pt and Pd NPs because of the enhanced nonlinear absorption, which originated from interband transitions and charge transfers [13]. However, most metal NPs are spherical in shape, and depositing metal NPs with varied morphologies onto the surfaces of GO is important because many properties of metal NPs strongly depend on their morphologies. Growing other shapes of metal NPs onto GO would promote the applications of GO–metal nanocomposites in the field of nonlinear optics. Here we report our studies on the deposition of novel triangular Pd NPs onto the GO sheet surface for the reason that Pd NPs
http://dx.doi.org/10.1016/j.matlet.2014.05.199 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Zheng C, et al. Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.199i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98
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are not only of interest for NLO; however, they may also be promising for the acoustically induced nonlinear optics [16]. Transmission electron microscopy (TEM) image and Raman spectra were applied to characterize the GO/triangular Pd hybrids. The OL behaviors were investigated by the open aperture (OA) Z-scan technique using 8 ns laser pulses at 532 nm.
2. Experimental Synthesis of GO/ triangular Pd composites: Synthesis of GO: Graphene was prepared from the oxidation of natural graphite powder via modified Hummers' method [17]. Ten and five grams of graphite powder and sodium nitrate were respectively stirred in concentrated sulfuric acid (230 mL) while being cooled in an ice bath. Subsequently, 30 g potassium permanganate was gradually added to form a new mixture. The mixture was allowed to warm to room temperature and was heated to 35 1C using a water bath. The mixture was then gently stirred for 2 h. The reaction mixture was cooled with an ice bath, and distilled water was added in excess to the mixture. Aqueous hydrogen peroxide solution (30 wt%) was added to the mixture, which was stirred for 2 h to reduce the excess KMnO4. The resultant suspension was intensively washed first with a diluted HCl solution and followed by distilled water via filtration. Subsequently, the suspension was centrifuged at 3000 rpm to remove the residual unexfoliated graphite and oxidative agents. The resulting material was dried by lyophilization to yield the GO powder. Synthesis of GO/triangular composites: Thirty milliliters of GO were dispersed into a 45 ml aqueous suspension by ultrasonication. Then, 0.27 g sodium citrate was added to the above GO aqueous graphene suspension, and the solution was boiled at 100 1C. Subsequently, 8.6 mg palladium acetate and 2.3 ml (0.1 mol L 1) ascorbic acid (AA) solution was successively added to the system rapidly, and the latter was refluxed for 2.5 h. The resultant nanocomposite was washed with distilled water using centrifugation (3000 rpm) to remove the free palladium NPs that had formed in the solution. The final nanocomposite was dried by lyophilization. Characterization: The geometry of the obtained GO/triangular Pd composites was investigated by transmission electron microscopy (TEM; JEM-2010; accelerating voltage, 200 kV). For the TEM observations, the samples were ultrasonicated in ethanol to ensure dispersion. A drop of the dispersed sample was left to dry on a
commercial, carbon-coated Cu TEM grid. Raman spectra of the GO and GO/triangular Pd composites were recorded using a Raman spectrometer (Renishaw Invia) at an ambient temperature and excitation wavelength of 785 nm. Z-scan measurement: The OL behavior of GO and GO/ triangular Pd composites were examined by the OA Z-scan technique [18]. The excitation light source was an Nd:YAG laser (Brio 640, Quantel, Les Ulis, France) with a repetition rate of 1 Hz. The laser pulses (period, 4 ns; wavelength, 532 nm) were split into two beams by a mirror. The pulse energies in front and at the back of the sample were monitored by energy detectors D1 and D2 (PE25, Ophir Optronics Solutions Ltd., Jerusalem, Israel). The laser beam waist was approximately 14.5 μm, and the energy of a single pulse was set at 200 μJ. All measurements were conducted at room temperature. Hybrid gel glasses were vertically fixed with a clamp. Each sample was mounted on a computer-controlled translation stage that shifts the sample along the z-axis.
3. Results and discussion Fig. 1 displays the representative images for GO/triangular Pd composites at low (Fig.1(a) and high (b)) magnifications. The Pd triangular NPs appeared to be homogeneously deposited on the GO surface with an average particle size of approximately 5 nm. The HRTEM image (Fig. 1(b)) further confirms the triangular morphology of the loaded Pd NPs and shows that the lattice spacing of the Pd NPs are about 0.23 nm, which correspond to single crystals of Pd with several (111) facets. The results suggest that triangular Pd NPs have been successfully attached on the GO surface. Raman spectroscopy is a versatile and non-destructive characterization technique used to investigate the nature and structure of carbon materials. Fig. 2 illustrates the Raman spectra of GO and GO/ triangular Pd composites, in which the characteristic bands are located at 1320 (G band) and 1596 cm 1 (D band), respectively. The G band corresponds to the in-plane optical vibrations of graphene atoms, whereas the D band is attributed to the intrinsic defects in the graphitic plane, including the edge effects and graphene ripple. D band exhibits a sharper intensity than the G band (Fig. 2), which indicates lesser-ordered and more defective graphitic structure of our as-prepared GO compared with those previously reported [12]. Loading of Pd NPs increases the D band
Fig. 1. TEM images of GO/triangular Pd composites.
Please cite this article as: Zheng C, et al. Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.199i
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intensity and increases the ID/IG from 1.39 to 1.51. The broadening is attributed to the increased defects and the interaction between Pd NPs and graphene sheets. The OA Z-scan technique was employed to measure the OL properties of the obtained GO and GO/triangular Pd composites, and the results are shown in Fig. 3. The OA curve exhibits a symmetric valley of the normalized transmittance about the focal point (z¼0), which implies the presence of nonlinear absorption (NLA) or NLS. This nonlinear behavior is a typical characteristic for the OL phenomena. The depth of the valley of OA Z-scan curve is a direct measurement of the extent of OL. Fig. 3 reveals that OL is significantly enhanced in GO/Pd NPs compared with its companions. In materials, OL behavior arises mainly from NLA (two-photon absorption (2PA), free-carrier absorption, and reverse saturable absorption (RSA)), nonlinear refraction, and NLS [18]. To identify the OL mechanisms of the resulting GO/triangular Pd composites, we tried fitting the Z-scan data numerically to different nonlinear transmission equations, including RSA and 2PA [18]. By simulating the nonlinear transmittance using the appropriate pulse propagation equations, we found that the best numerical fit was to a 2PAtype process, and the results are displayed in Fig. 3. The total nonlinear absorption coefficient (β) calculated from the numerical fits are 0.50 and 0.59 cm/GW for GO and GO/triangular Pd composites, respectively. The theory of TPA process fitted well
Fig. 4. Typical nonlinear scattering results for GO and GO/triangular Pd composites at 532 nm. The energy detector was located 451 from the axis.
with the experimental curves, which confirms that TPA is a major mechanism responsible for the OL of GO and GO/triangular Pd composites. Given that the valleys in the open aperture Z-scan curves could be signatures of either NLA or NLS, an experiment involving NLS measurements was performed to determine if NLS contributes to the observed OL effects in GO nanostructures. Typical nonlinear scattering results of GO and GO/triangular Pd composites are presented in Fig. 4. For the GO/triangular Pd composites, a significant scattering peak that is symmetric about the focus was evident, indicating NLS. However, no NLS peak for GO is observed, indicating that no NLS contribution exists. This finding suggests a significant enhancement in NLS, which is due to the presence of triangular Pd NPs in the nanocomposites. In NLS, it is well accepted that the light absorption by metal NPs induces a very high temperature rise, which leads to the formation of rapidly expanding microplasmas. The formation and rapid expansion of these microplasmas in turn transfer the thermal energy to the surrounding medium, forming solvent microbubbles. These microplasmas and microbubbles strongly scatter light from the transmitted beam direction, leading to a decrease in the measured transmitted light energy and hence OL occurs [19]. In GO/triangular Pd composites, we believe that the conducting twodimensional GO facilitates heat transfer between the Pd NPs and surrounding solvent molecules and therefore improves the heatbased OL performance. Meanwhile, as the radiation energy has been absorbed by the composites, the outer Pd NPs coating can rapidly scatter the energy and consequently the scattering energy would be effectively absorb by substrate GO sheet. The energy transfer between the Pd NPs and GO may have a positive influence on the resulting OL response. Therefore, combining 2TA of GO sheets and NLS of triangular Pd NPs contributes to the improved OL property in triangular Pd NPs composites.
4. Conclusion Novel GO/triangular Pd composites were prepared by the simple method, which involved the direct reduction of palladium acetate on the surface of GO. TEM images confirmed that the triangular Pd NPs have been successfully and homogenously deposited onto the surface of the two-dimensional GO sheet. Raman spectrum revealed that loading of Pd NPs resulted in lesser-ordered and more defective graphitic structure, which is attributed to the increased defects and the interaction between Pd NPs and GO sheets. The corresponding OL behavior was investigated using the open aperture Z-scan
Please cite this article as: Zheng C, et al. Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.199i
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technique with nanosecond pulses. Results show that the OL property is significantly enhanced in GO/triangular Pd composites compared with its individual counterparts owing to the addition NLS effects. The synthetic OL properties in GO/triangular Pd nanocrytal composites were found to originate from TPA in GO sheet and NLS in triangular Pd NPs. Meanwhile, the energy transfer between the Pd NPs and GO may have a positive influence on the resulting OL response. All results imply the usability of these hybrids in optoelectronic applications.
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Please cite this article as: Zheng C, et al. Enhanced optical limiting properties of graphene oxide/triangular Pd nanocrystal composites. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.05.199i
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