Advances in Nanoparticles, 2013, 2, 223-228 http://dx.doi.org/10.4236/anp.2013.23031 Published Online August 2013 (http://www.scirp.org/journal/anp) Nonlinear Coefficient Determination of Au/Pd Bimetallic Nanoparticles Using Z-Scan José Luis Jiménez Pérez1*, Rubén Gutiérrez-Fuentes2, José Francisco Sánchez Ramírez1, Omar Uriel García Vidal1, Daniel Erick Téllez-Sánchez1, Zormy Nacary Correa Pacheco1, Alfredo Cruz Orea3, Jesús Antonio Fuentes García1 1Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas del IPN, México, D.F., México 2Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, México D.F., México 3Departamento de Física, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, México, D.F., México Email: *jimenezp@fis.cinvestav.mx Received March 22, 2013; revised April 22, 2013; accepted April 30, 2013 Copyright © 2013 José Luis Jiménez Pérez et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT In this paper we present the nonlinear optical characterization of Au/Pd nanoparticles in order to obtain the nonlinear refractive indices using the Z-scan technique. The experiments were performed using a 514 nm laser beam Ar+, with 14 Hz of modulation frequency, as excitation source. By using a lens the excitation beam was focused to a small spot and the sample was moved across the focal region along the z-axis by a motorized translation stage. Seven samples with different concentration ratio of Au/Pd nanoparticles were prepared by simultaneous reduction of gold and palladium ions in presence of poly (N-vinyl-2-pirrolidone) (PVP) using ethanol as a reducing agent. In this work, we report the application of the Z-scan technique, to generate optical transmission of laser light as a function of the z position for so- lutions containing bimetallic nanoparticles of Au (core)/Pd (shell) with average sizes ranging from 3 to 5 nm. The mag- nitude of the obtained nonlinear refractive index was in the order of 10−8 cm2/W. Our results show that the nonlinear refractive index has a nonlinear behavior when the (Au/Pd) ratio was increased. Keywords: Nanofluids; Nanoparticles; Nonlinear Refractive Indices; Z-Scan 1. Introduction Bimetallic nanoparticles (NPs) have attracted great in- terest among the scientific and technological community since they lead to many interesting size dependent elec- trical, chemical, and optical properties [1-3]. Specifically they often exhibit enhanced catalytic properties which differ from that of their monometallic counterparts, in thin films [4]. On the other hand, the studies for new nonlinear optical materials have increasing interest in the past years, due to the numerous applications in various fields such as optical communication devices and elec- tronics [5]. Nanoparticles colloids in solution have been studied extensively because of their large third-order nonlinear susceptibilities and nonlinear optical response [6,7], chemical and biological sensors [8,9], optical en- ergy transport [10,11], nonlinear thermal material [12,13], thermal therapy [14,15] and medicine [16]. Z-scan technique is one of the simplest and effective tools for measuring third order of nonlinear optics such as nonlinear refraction coefficient and absorption. It has been widely used in material characterization [17,18]. This method utilizes a focused laser beam that is intense enough to access nonlinearities in a sample. As the sam- ple passes through the focal point of the beam, changes in its transmittance, due to nonlinear absorption and nonlinear refraction, can be measured by using an open aperture and close aperture experimental setup, respectively. In the open aperture technique the beam is focused into a detector, after passed through the sample. As the sample travels through the focus of the initial beam, the trans- mittance either increase or decrease (depending on the nonlinearity of the sample) and the detector receives more or less light than the linear transmittance, yielding a hump or dip in the curve of transmittance as a function of the sample position. For nonlinear refraction, after the beam passes through the sample, it is attenuated by a semi- closed aperture that usually allows less than 50% of the initial beam to be detected by the detector. The converg- ing and diverging of the beam (allowing more and less of the beam to pass through the aperture, respectively) due *Corresponding author. C opyright © 2013 SciRes. ANP
J. L. JIMÉNEZ PÉREZ ET AL. 224 to the changes in the refractive index, a pre-focal valley and post-focal peak are observed for a positive change in refractive index. While a pre-focal peak and a post-focal valley is observed for a negative change in refraction index. In this paper we studied the effect of the concentration of Au/Pd ratio on their nonlinear refraction coefficient. Therefore, we report this effect using a single beam Z- scan technique. The results of this method are found to be in good agreement with the values of other works. 2. Methods 2.1. Sample Preparation Colloidal dispersion of Au/Pd bimetallic nanoparticles was prepared by simultaneous reduction of gold and pal- ladium ions in presence of poly (N-vinyl-2-pirrolidone) (PVP) using ethanol as a reducing agent [19-22]. In a typical synthesis process, ethanol solutions of palladium chloride (0.033 mmol in 25 ml of ethanol) were prepared in advance by stirring dispersions of PdCl2 powder in ethanol for 48 hrs. Solutions of tetrachloroauric acid (0.033 mmol in 25 ml of water) were prepared by dis- solving the HAuCl4·3H2O crystals in water. For prepar- ing bimetallic nanoparticles, solutions containing two metal ions were mixed in 50 ml of pure ethanol/water (1/1 v/v). 151 mg of PVP (Aldrich, average molecular weight 10,000) was added to the total metal ion content of 6.66 × 10−5 mol. The mixture solution was stirred and refluxed at about 100˚C for 1 hr. For the preparation of bimetallic nanoparticles with different Au/Pd ratios (10/1, 5/1, 2/1, 1/1,1/2, 1/5 and 1/10), metal ion solutions of corresponding molar ratios were mixed and refluxed at 100˚C for 1 hr under agitation. The ethanol/water volu- metric ratio of 1:1 and total ion content of 6.6 × 10−5 mol was maintained in the final 50 ml solution. The same procedure was followed to prepare the monometallic palladium (Pd) particles. For the preparation of mono- metallic gold (Au) particles, 23.5 ml of PVP solution (75.5 g in 23.5 ml of water) was added to the gold ion solution, and then an aqueous solution of NaBH4 (0.066 mmol in 1.5 ml of water) was added to the resulting solu- tion at 25˚C. The colloidal dispersions thus prepared are stable, with 3 - 5 nm in average diameter and narrow size distributions. In order to obtain the aqueous nanofluids containing Au/Pd bimetallic nanoparticles, the colloidal dispersions were subsequently dried in vacuum and the metallic particles were dissolved in water maintaining the same initial concentrations of metal content and were placed in a quartz cuvette of 1 mm thick for the optical and thermal optical measurements. All the experiments were performed at room temperature. 2.2. Z-Scan Technique Among the techniques used to characterize the complex susceptibility, the most popular is the Z-scan method. Since its introduction in 1989 [23], this technique has gained importance due to its simplicity compared to other techniques used to measure the nonlinear refraction and absorption of optical materials. In addition to the sim- plicity of the experimental setup, a Z-scan measurement provides a sensitive method for the determination of the signal on values of the real and imaginary parts of x. The technique relies on the basic idea of relating the beam center intensity variation to the refractive index variation. This can be done by monitoring the normalized transmit- tance as a function of the sample path along the incident beam, for a positive nonlinearity. In the case of a nega- tive nonlinearity, the curve is inverted. The transmittance variation between peak and valley positions is proportional to the induced phase shift, ΔΦo and therefore to the nonlinear refractive index (n2) is calculated using the standard relations by means of the following equations [24,25]: 22 00 00 ,14 1T zzzzzzz9 (1) 020 nI Leffk (2) where z is the position, z0 is the Rayleigh length, ΔΦ0 is the phase change due to the nonlinear refraction, n2 is the nonlinear refractive index, k = 2π/λ is the wave vector, I0 = 1.8 × 103 W/cm2 is the on-axis irradiance at focus (i.e., z = 0), and 00 Leff1 expL is the effective length of nonlinear medium, α0 is the linear absorption coefficient of the samples (L denotes the sample thick- ness). The nonlinear refractive index, n2 was calculated from v T being this parameter the value of peak to valley of data transmittance from the closed aperture Z- scan measurement which can be described as [24]: 0.25 0 0.4061 pv TS (3) Here S is the linear transmittance of the aperture. Fig- ure 1 shows the schematic diagram of a single beam Z- scan experimental setup used in the present measurement. The experiments were performed using a CW Ar+ laser beam, 514 nm wavelength (model Cyonics, Uniphase). The beam was focused to a small spot using a lens and Figure 1. Experimental setup for the nonlinear laser spec- troscopy. The closed and open aperture signals are propor- tional to the real and imaginary parts of n2, respectively. Copyright © 2013 SciRes. ANP
J. L. JIMÉNEZ PÉREZ ET AL. 225 the sample was scanned along a Z-axis by a motorized translation stage (Zaber). The transmitted light, in the far field, passed through the closed and open apertures and was recorded by the detectors Dc, Do and Dr (reference detector). The laser beam waist ω0 at the focus length was measured to be 10.9 µm and the Rayleigh length was found to be satisfied the basic criteria of the Z-scan ex- periment. A quartz optical cell, 1 mm thick, containing the speci- men solution was translated across the focal region along the z-axial direction. All the measurements were carried out at room temperature for both closed aperture and open aperture configurations. Optical absorption spectra of the fluid samples were measured using a UV-Vis-NIR double beam spectropho- tometer (Shimadzu UV3101PC). For transmission electron microscopic (TEM) obser- vations, a drop of fluids was spread on a carbon coated copper microgrid. A JEOL JEM200 microscope was used for the low magnification of the samples. 3. Results and Discussion The absorption spectra of the Au/Pd nanofluids at dif- ferent ratios (10/1, 5/1, 2/1, 1/1, 1/2, 1/5, 1/10) were measured by spectrophotometer. The measurements of absorption spectra were carried out at room temperature for visible wavelength, 350 - 800 nm. The spectra are shown in Figure 2. Figure 3 displays a typical TEM images showing the particles with a uniform distribution and uniform shape. The average particle size obtained from TEM images was 3.9 nm. For closed aperture setup normalized transmittance is attributed to the nonlinearity of the refractive index which was considered here [26]. In Figures 4-10 the closed aperture Z-scan curves obtained for Au/Pd nanofluid, at different concentration, at beam intensity of I0 = 1.8 × Figure 2. Optical absorption spectra of bimetallic nanopar- ticles with different Au/Pd mole ratios. Figure 3. TEM image of Au/Pd particles with an average size of 3.9 nm. Figure 4. Closed aperture Z-scan curve for Au/Pd nanopar- ticles at a concentration 10/1. The solid line is the best fit- ting of the standard closed aperture equations to the ex- perimental data. Figure 5. Closed aperture Z-scan curve for Au/Pd nanopar- ticles at a concentration 5/1. The solid line is the best fitting of the standard closed aperture equations to the experi- mental data. 103 W/cm2 are shown. The circle symbols represent the experimental data while the solid lines are theoretical fits Copyright © 2013 SciRes. ANP
J. L. JIMÉNEZ PÉREZ ET AL. 226 Figure 6. Closed aperture Z-scan curve for Au/Pd nanopar- ticles at a concentration 2/1. The solid line is the best fitting of the standard closed aperture equations to the experi- mental data. Figure 7. Closed aperture Z-scan curve for Au/Pd nanopar- ticles at a concentration 1/1. The solid line is the best fitting of the standard closed aperture equations to the experi- mental data. Figure 8. Closed aperture Z-scan curve for Au/Pd nanopar- ticles at a concentration 1/2. The solid line is the best fitting of the standard closed aperture equations to the experi- mental data. Figure 9. Closed aperture Z-scan curve for Au/Pd nanopar- ticles at a concentration 1/5. The solid line is the best fitting of the standard closed aperture equations to the experi- mental data. Figure 10. Closed aperture Z-scan curve for Au/Pd na nopa r- ticles at a concentration 1/10. The solid line is the best fit- ting of the standard closed aperture equations to the ex- perimental data. to the closed aperture. The theoretical transmittance curves presented in Fig- ures 4-10 are fitted very well with the experimental data and they show symmetry curves. The peak follow by valley illustrate a self-focusing effect for a negative change in refraction. The solid line shows the theoretical fitting using a well-known normalized transmittance. The nonlinear refraction coefficients n2 (cm2/W) to- gether with the values of linear absorption of all samples obtained in the present work are listed on Table 1. Figure 11 shows the variation of the nonlinear refrac- tion index coefficient as a function of concentrations for different ratios. The nonlinear refraction coefficient de- crease with increasing of concentration (Au/Pd (10/1) to Au/Pd (2/1)). By other hand, if the concentration is in- verse (Au/Pd (1/1) to Au/Pd (1/10)), the nonlinear refrac- tion coefficient increases with the increasing of concen- tration. However the increase of nonlinear of refractive index coefficient with the concentration did not show a Copyright © 2013 SciRes. ANP
J. L. JIMÉNEZ PÉREZ ET AL. 227 Table 1. Nonlinear optical properties of Au/Pd nanofluid at different concentrations. Molar ratios of 10/1, 5/1, 2/1, 1/1, 1/2, 1/5, 1/10 were measured at 514 nm laser beam. Au/Pd ΔTpv Δφ0 α (cm−1) n2 (cm2/W) 10/1 1.48 3.99 13.5 −13.0 ×10−8 5/1 1.78 5.21 10 −8.59 ×10−8 2/1 1.03 3.02 7.9 −3.93 ×10−8 1/1 1.27 3.72 6.9 −4.23 ×10−8 1/2 1.45 4.25 8.9 −6.23 ×10−8 1/5 1.24 3.63 6.28 −3.76 ×10−8 1/10 1.84 5.39 6.84 −6.08 ×10−8 Figure 11. Variation of the nonlinear refraction index coef- ficient as a function of Au/Pd ratio. linear relationship as displayed in Figure 11. Shahriari et al. [26] studied the effect of concentration and particle size on nonlinearity of Au nanofluid prepared by γ (60Co) radiation. From the experimental results [26], the authors obtained values of refractive index coefficient nonlinear of the same order found in this work. 4. Conclusion The nonlinearity properties of Au/Pd nanofluid prepared at different concentrations have been successfully inves- tigated using a single beam Z-scan method. A CW, Ar+ laser beam, at wavelength 514 nm, was used as excitation source. The Au/Pd nanofluid shows a good nonlinear response. The sign of the nonlinear refractive index is found to be negative and the magnitude is in the order of 10−8 cm2/W. A nonlinear relationship was obtained for nonlinear refractive index as a function of Au/Pd ratio. Until our knowledge these results are reported for the first time in the literature. These results showed that Au/Pd nanofluid has significant values of nonlinear re- fractive index, thus it could be good candidate for nonlin- ear thermal material. 5. Acknowledgements The authors are thankful to the Mexican Agencies, CONACYT, CGPI-IPN and COFAA-IPN for financial supports. REFERENCES [1] H. Kumarakuru, D. Cherns, M. G. Montes de Oca and D. J. Fermin, “TEM Studies of Stress Relaxation in Catalytic Au-Pd Core-Shell Nanoparticles,” Journal of Physics: Con- ference Series, Vol. 371, 2012, Article ID: 012025. [2] C. Hsu, C. Huang, Y. Hao and F. Liu, “Au/Pd Core-Shell Nanoparticles for Enhanced Electrocatalytic Activity and Durability,” Electrochemistry Communication, Vol. 23, No. 1, 2012, pp. 133-136. doi:10.1016/j.elecom.2012.07.027 [3] M. A. Montero, M. R. Gennero de Chialvo and A. C. Chialvo, “Electrocatalytic Activity of Core-Shell Au@Pt Nanoparticles for the Hydrogen Oxidation Reaction,” In- ternational Journal of Hydrogen Energy, Vol. 36, No. 6, 2011, pp. 3811-3816. [4] F. Liu, D. Wechsler and P. Zhang, “Alloy-Structure-De- pendent Electronic Behavior and Surface Properties of Au-Pd Nanoparticles,” Chemical Physics Letters, Vol. 461, No. 4-6, 2008, pp. 254-259. doi:10.1016/j.cplett.2008.07.029 [5] Y. Dirix, C. Bastiaansen, W. Caseri and P. Smith, “Ori- ented Pearl-Necklace Arrays of Metallic Nanoparticles in Polymers: A New Route toward Polarization-Dependent Color Filters,” Advanced Materials, Vol. 11, No. 3, 1999, pp. 223-227. doi:10.1002/(SICI)1521-4095(199903)11:3<223::AID-A DMA223>3.0.CO;2-J [6] L. L. Beecroft and C. K. Ober, “Nanocomposite Materials for Optical Applications,” Chemical Materials, Vol. 9, No. 6, 1997, pp. 1302-1317. doi:10.1021/cm960441a [7] Y. Wang, “Nonlinear Optical Properties of Nanometer- Sized Semiconductor Clusters,” Accounts of Chemical Research, Vol. 24, No. 5, 1991, pp. 133-139. doi:10.1021/ar00005a002 [8] R. Elghanian, J. J. Storhoff, R. C. Mucic, R. L. Letsinger and C. A. Mirkin, “Selective Colorimetric Detection of Po- lynucleotides Based on the Distance-Dependent Optical Properties of Gold Nanoparticles,” Sciences, Vol. 227, No. 5329,1997, pp. 1078-1081. doi:10.1126/science.277.5329.1078 [9] C. Loo, A. Lin, L. Hirsch, M. Lee, J. Barton, N. Halas, J. West and R. Drezek, “Nanoshell-Enabled Photonics-Based Imaging and Therapy of Cancer,” Technology in Cancer Research and Treatment, Vol. 3, No. 1, 2004, pp. 33-40. [10] M. Quinten, A. Leitner, J. R. Krenn and F. R. Aussenegg, “Electromagnetic Energy Transport via Linear Chains of Silver Nanoparticles,” Optics Letters, Vol. 23. No. 17, 1998, pp. 1331-1333. doi:10.1364/OL.23.001331 [11] M. L. Brongersma, J. W. Hartman and H. A. Atwater; “Electromagnetic Energy Transfer and Switching in Na- noparticle Chain Arrays below the Diffraction Limit,” Physical Review B, Vol. 62, No. 24, 2000, pp. R16356- Copyright © 2013 SciRes. ANP
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