J. Phys. Chem. C 2008, 112, 8767–8772
8767
ARTICLES Optical Properties of Ag@Polypyrrole Nanoparticles Calculated by Mie Theory Sunjie Ye and Yun Lu* Department of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed: September 25, 2007; ReVised Manuscript ReceiVed: April 3, 2008
The unique optical properties of Ag@polypyrrole composite nanoparticles were systematically investigated in theory and experiment for the first time. On the basis of the classical Mie theory, the extinction, absorption, and scattering efficiencies of the Ag@polypyrrole nanoparticles were theoretically calculated. In addition, their theoretical UV-vis extinction spectra were plotted and compared with the corresponding experimental results. For Ag@polypyrrole nanoparticles, the influence of the microstructure factors, including Ag core size, the thickness and oxidation degree of the polypyrrole shell, and homogeneity of the particle size and shape, on the optical properties of the composite nanoparticle were qualitatively analyzed and discussed in detail. The understanding to the optical properties of Ag@polypyrrole composite nanoparticles could help us to control either parameters in order to design the optimal functional composite particles and to match the practical applications in common or special. Introduction Optical properties of nanometer-sized metal particles embedded in solid dielectric materials have been of increasing interest for both fundamental and practical reasons, largely because of their novel applications as photonics and electronics devices based on quantum size effects.1–6 Most previous work has been done on different insulator matrices, whereas the investigation on conductive matrices has been scarcely reported due to the complexity of the system and the difficulty with the study of the microstructure and the optical properties. On the other hand, the conducing polymers with the unique π-conjugated system could provide an effective route for the flow of electronic charges7 and could thus be switched between two states of optical absorption: highly absorbing in the oxidizing state and almost nonabsorbing in the reducing state. Among the conducting polymers, polypyrrole (PPy) is especially promising for commercial applications because of its good environmental stability, proper redox properties, easier synthesis, and higher conductivity than many other conducting polymers. Our present work focuses on the optical properties of the composites with nanosized Ag particles embedded in PPy (Ag@PPy nanoparticles). Ag particles exhibit a strong surface resonance, the wavelength of which covers the visible range, depending on size, shape, and volume fraction of the particle.5,6 This feature may be used to enhance the optical properties of the matrix in which Ag particles are embedded. We studied the optical properties of Ag@PPy nanoparticles with diverse core size, shell thickness and oxidizing or reducing state of the PPy shell. The optical extinction spectra were theoretically plotted, and the extinction, absorption and scattering efficiencies were calculated using Mie theory.8 According to our results, the intrinsic properties of Ag@PPy nanoparticles are mainly determined by the microstructure factors, namely, * Corresponding author e-mail:
[email protected].
homogeneity of the particle size distribution, particle shape, Ag core size, and the thickness and the oxidation degree of the PPy shell. In principle, accurate control of any of the parameters could fine-tune the optical properties of the composite nanoparticles, thereby making possible the design and exploitation of the composite nanoparticles most adequate for particular applications. Experimental Section The silver colloids were prepared by the polyol process.9a A 1.5 g portion of polyvinylpyrrolidone (PVP) was dissolved in 75 mL of ethylene glycol, to which 0.05 g of AgNO3 was then added. After the silver nitrate was completely dissolved , the reaction was allowed to proceed for 22 h at 120° to obtain Ag colloid A. The similar procedure was taken with PVP (7.0 g) and AgNO3 (0.2 g) to synthesize Ag colloid B. Ag colloids A and B are different only in their particle radius. To understand the effect of homogeneity of the particle size and shape on the optical properties of Ag@PPy nanoparticles, we used a different procedure to synthesize the Ag colloid.9b A 0.30 g portion of AgNO3 was dissolved in 75 mL of ethylene glycol with 13.5 g of PVP. The solution was stirred at room temperature for 1 h to get Ag colloid C, in which the particles have a little inhomogeneity in size and shape. A 0.082 g portion of FeCl3 and 35 µL of pyrrole (Py) were added to Ag colloids A and C, separately, under vigorous magnetic stirring. The reaction system was allowed to stand for 24 h at room temperature to synthesize the Ag@PPy (in oxidation state) nanoparticles A and D. The proper amounts of FeCl3 (0.115 g) and Py (50µL) were added to Ag colloid A to get Ag@PPy nanoparticles B with different PPy thickness compared to Ag@PPy nanoparticles A. The mixture of Ag colloid B with.0.150 g FeCl3 and 65µL Py was stirred for 24 h to obtain Ag@PPy nanoparticles C, which have the same shell
10.1021/jp077710c CCC: $40.75 2008 American Chemical Society Published on Web 05/27/2008
8768 J. Phys. Chem. C, Vol. 112, No. 24, 2008 thickness and different Ag core radius compared to Ag@PPy nanoparticles A. The Ag@PPy (in reduction state) nanoparticles were obtained by adding Ag@PPy (in oxidation state) particles to NH3 · H2O (25%). The reaction lasted 24 h to ensure the complete reduction of the PPy shell. Micrographs of Ag@PPy nanoparticles were observed with TEM JEM-4000EX (JEOL, Japan). UV-vis spectra of Ag@PPy nanoparticles dispersions were recorded with a UV 240 spectrophotometer (Shimadzu, Japan). Quartz cells with a path length of 1 cm were used. The blank solution was prepared by diluting 0.1 mL of ethylene glycol-PVP solution with 10 mL of ethylene glycol. Ag@PPy nanoparticle dispersoid (0.1 mL) was also dispersed by dilution with 10 mL of ethylene glycol for UV-vis spectroscopy analysis. When the particle size is tens of nanometers, the scattering of free electrons with the particle surface becomes extraordinarily important for the response of the electrons to optical excitation. In this situation, the UV-vis spetrum was thereupon recorded as the extinction spectrum.10 The relative permittivities of PPy (either in oxidation or reduction state) were measured on an impedance/gain-phase analyzer (HP, American). Results and Discussion TEM micrographs of Ag@PPy nanoparticles with different core size or shell thickness are presented in Figure 1. It can be seen from Figure 1, panels A-C, that the composite nanoparticles are homogeneous, whereas in Figure 1D the nanoparticles show a little inhomogeneity in both size and shape. We employ a generalized Mie expression to calculate the combined absorption and scattering properties of Ag@PPy nanoparticles. Mie theory is used to categorize the significant electromagnetic modes inside and outside the particle and to acquire some intuitive feeling for how a particle of given size and optical properties absorbs and scatters light. For the
Ye and Lu calculation of theoretical spectra of nanoparticles, the refractive index of materials is indispensable and important. In our case, the refractive indexes (nre and nim) of the Ag core were obtained from the reported literature.11 The relative permittivities (′ + i′′) were experimentally measured, according to which the corresponding refractive indexes (n + ki) of PPy (either in oxidation state or reduction state) were calculated by the following equation and Kramers-Kronig relations, as shown in Figure 2.
n)
√ε′2 + ε′′2 + ε′ 2
k)
√ε′2 + ε′′2 - ε′ 2
(1)
The theoretical UV-vis spectra of Ag@PPy nanoparticles, plotted in Figure 3, panels Aa and Ba, were calculated by using a modified version of the Mie computer program developed by Kreibing and Vollmer1 and compiled in Fortran 77. As a comparison, the corresponding experimental spectra of nanosized Ag@PPy (in either the oxidation state or the reduction state) particles were recorded in Figure 3, panels Ab and Bb. In the case of the Ag@PPy nanoparticles in the oxidation state, there was a maximum extinction located at 420 nm in both experimental and calculated spectra. When the PPy shell was in the reduction state, the maximum extinction of Ag@PPy nanoparticles decreased and shifted to longer wavelength. Overall, the calculated curves were consistent with the experimental spectra, implying that the Mie theory calculation could be an applicable way to explore the influence of different factors on the extinction spectra of Ag@PPy nanoparticles. When Ag@PPy nanoparticles were inhomogeneous in size distribution and irregular shape, some disagreement emerged between the calculated and experimental spectra, as shown in Figure 4. In the case of the PPy shell in the oxidation state, the calculated extinction decreased in the wavelength range of 600-800 nm, where the experimental one rose. In the case of
Figure 1. TEM images of Ag@PPy nanoparticles, (A) R1 ) 15 ( 0.5 nm, R2 ) 20 ( 0.5 nm; (B) R1 ) 15 ( 0.5 nm, R2 ) 22.5 ( 0.5 nm; (C) R1 ) 20 ( 0.5 nm, R2 ) 25 ( 0.5 nm; (D) R1 ) 10nm (average), R2 ) 25nm (average).
Optical Properties of Ag @Polypyrrole Nanoparticles
Figure 2. The refractive indexes of PPy at different wavelengths, (A) in the oxidation state and (B) in the reduction state.
the PPy shell in the reduction state, the extinction decreased with the longer wavelength in both experimental and calculated spectra, although the calculated extinction decreased more sharply. Such a disagreement might well reflect imperfection of experimental samples, such as particle size polydispersity, nonuniformity of shells, or aspherical particles.12 In particular, polydispersity of the particle size and deformation of the particle shape from an ideal sphere could cause the change of the peak position and spectral shape. We calculated and compared the extinction (Qext), absorption (Qabs), and scattering (Qsca) properties of Ag@PPy (in either the oxidation state or the reduction state) nanoparticles using Mie theory. Obvious differences can be seen in Figure 5. Oxidized Ag@PPy nanoparticles presented a peak at 420 nm and higher extinction intensity at all wavelengths. Such changes after oxidation should be the result of an alteration in the imaginary part of the refractive index of the PPy shell on the Ag nanoparticles.13 In general, the refractive index of particles that scatter but do not absorb light in a given wavelength range is always real (nim ) 0), whereas the refractive index of particles that both scatter and absorb light is complex (nim * 0). Thus it can be seen that it is the imaginary part (nim) of the refractive index that accounts for light absorption. In the case of PPy in the oxidation state, the imaginary part of the refractive index of the PPy shell covered on the Ag particle is not equal to zero, indicating the enhanced absorption properties, the intensified
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8769
Figure 3. The calculated (a) and experimental (b) UV-vis extinction spectra of Ag@PPy nanoparticles A with a core radius of 15 nm and a shell thickness of 5 nm, (A) in the oxidation state and (B) in the reduction state.
extinction (the sum of absorption and scattering), and the decreased scattering ratio (Φs) of Ag@PPy particle. The reversible switching behavior between the two states of oxidation and reduction is a unique characteristic of PPy and other conductive polymers. These conductive polymers thus exhibit a novel optical property: highly absorbing in the oxidizing state and almost nonabsorbing in the reducing state.14,15 The different absorption properties of oxidized and reduced PPy can be explained in terms of the energy band model. In PPy, the chemical bonding leads to one unpaired electron (the π electron) per carbon atom. The π-electron density increases between every alternate pair of carbon atoms to form a π bond, resulting in electron delocalization along the backbone of the polymer. Usually, the π-electron band could be divided into the filled π band and the empty π/ band. The appearance of the π-π/ energy gap, the energy difference between the highest occupying state in the π band and the lowest unoccupying state in the π/ band, would make the electron transition between the π band and the π/ band easier or more possible. For the reduced PPy, its energy gap is higher than the energy of UV-visible light, that is, UV-visible light can not excite the electron transition. As a result, the reduced PPy nearly does not absorb UV-visible light. The presence of conjugated bonds indicates the possibility of electron transition if there exists a mechanism, such as by
8770 J. Phys. Chem. C, Vol. 112, No. 24, 2008
Ye and Lu
Figure 6. Band diagram for PPy (a) in the reduction state and (b) in the oxidation state. A.O., additional orbit.
Figure 4. The calculated (a) and experimental (b) UV-vis extinction spectra of the synthesized Ag@PPy nanoparticles D, which are inhomogeneous in both particle size and particle shape, (A) in the oxidation state and (B) in the reduction state.
Figure 5. The UV-vis extinction spectra for synthesized Ag@PPy nanoparticles A with a core radius of 15 nm and a shell thickness of 5 nm, (a) in the oxidation state and (b) in the reduction state.
oxidation, to eliminate or reduce the band gap between the filled and empty bands.16 In our case, the oxidized PPy was achieved by using FeCl3 acting as the oxidant as well as the doping agent. Consequently, two additional bonding and antibonding orbits were formed, respectively, between the π band and the π/ band (as shown in Figure 6). Here, the energy gap was narrowed to
the energy range of UV-visible light, implying the possible electron transition excited by the light. Therefore, the oxidized PPy showed a high absorption property. Table 1 and Figure 7 give the spectral information of Ag@PPy nanoparticles with a determinate core/shell structure. The Ag@PPy nanoparticles (R1 ) 15 nm, R2 ) 20 nm) with an oxidized shell have a lower scattering ratio (Φs), showing a maximum at the same wavelength as the maximum extinction. Such a property could be interpreted by the resonance effect. On the basis of the quantum mechanism, the particle can be viewed as a collection of polarizable molecules. The scattering intensity of such species could be enhanced by several orders of magnitude due to their strong electron coupling.17 Notably, the resonance aroused by the interaction between the photons and the electrons at the interface of the nanopaticles could result in a direct increase of the scattering.18 When the energy of incident photons is equal or close to the energy gap of the Ag@PPy nanoparticles, the incident photons (as a vibration system) resonate with the electrons at the surface (another vibration system), thereby emitting the resonance scattering photons with the same magnitude of energy absorbed from incident photons. Then the scattering photons are further absorbed by similar interface electrons in the ground-state to show a resonance effect. In an ideal condition, the photons of resonance scattering light have the same energy as the photons of resonance extinction, so that the peaks of resonance scattering and extinction locate at the same wavelength. Thus, we consider that resonance is the key factor affecting the extinction property of Ag@PPy nanoparticles. The classical theory considers the particles as obstacles with a refractive index different from that of the surrounding medium.13,19 The extent to which a particle absorbs and scatters light depends on its size, shape, and refractive index relative to the surrounding medium (both real and imaginary parts). In a region near the extinction maximum, the real part of m (npar/ nmed) varies, and the imaginary part of m increases, causing not only the extinction but also the scattering cross section to increase. The important point to note is that although the extinction and scattering cross sections depend on both the real and imaginary parts of m, the scattering cross section is a much stronger function of m than the absorption cross section and can be enhanced to a greater extent. Therefore, under certain conditions, Φs has a maximum near the extinction peak. Figure 8A displayed the experimental optical spectra for Ag@PPy (in the oxidation state) nanoparticles with different thickness of the PPy shell of (a) 5 and (b) 7.5 nm and with the same radius of the Ag core (15 nm). The spectra have an extinction peak at 420 and 425 nm, respectively. Figure 8B shows the calculated optical spectra for Ag@PPy (in the oxidation state) nanoparticles with the same radius of the Ag core of 15 nm and with different thicknesses of the PPy shell of (a) 5, (b) 7.5, and (c) 10 nm, respectively. Corresponding to
Optical Properties of Ag @Polypyrrole Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8771
TABLE 1: Calculated Optical Extinction and Scattering Properties for Ag@PPy (in the Oxidation State) Nanoparticles (R1 ) 15 nm, R2 ) 20 nm) at Different Wavelengths
three kinds of different shell thicknesses, the spectra displayed an extinction peak at 420, 425, and 430 nm, respectively. Here, a quite good agreement between calculated and experimental optical spectra confirmed the predictability of optical spectra for Ag@PPy nanoparticles with different shell thickness calculated using Mie theory. The experimental and calculated spectra for Ag@PPy nanoparticles with different shell thickeness all showed a red shift of the peak position with the increase of PPy shell thickness. Explanations could be associated with the surface resonance effect, namely, collective oscillations of the conduction electrons under the action of the electromagnetic field. In addition, both the band position and the width depended on intrinsic properties of the core (metal nature, size, and geometry) and the optical behavior of the host matrix. When the Ag particle was embedded in the matrix of PPy, its wave function would decrease compared to that in vacuum, lowering the energetic position of the surface resonance state.20,21 As the thickness of the PPy shell increases, the Ag core function becomes weaker. Therefore, the peak in the spectra shifts to longer wavelength with the increasing thickness of the PPy shell for the same Ag core (as shown in Figure 8). The experimental optical behaviors for the composite nanoparticles with the same shell thickness of 5 nm and varied Ag core of radius (a) 15 nm or (b) 20 nm are shown in Figure 9A. The spectra presented a peak with broader width at 420 and 430 nm, respectively. Figure 9B shows the calculated extinction spectra for nanoparticles with the same shell thickness of 10 nm and Ag cores of different radii of (a) 15, (b) 20, (c) 30, (d) 40, and (e) 50 nm. For Figure 9B, traces a-c, the spectra displayed a peak with broader width at 430, 450, and 500 nm, respectively. Here, calculated optical spectra are consistent with experimental ones, demonstrating that the optical spectra for
Figure 7. The calculated Φs for Ag@PPy nanoparticles (R1 ) 15 nm, R2 ) 20nm), (a) in the oxidation state and (b) in the reduction state, as a function of wavelength.
Ag@PPy nanoparticles with different core radius are predictable using Mie theory. Whereas with the radius of the Ag core increasing to 40 and 50 nm, the spectra revealed two peaks that shift to the longer wavelength with the increasing radius of the Ag core (Figure 9B, traces d and e). The first peak near 450 nm should be attributed to an electric dipole of Ag, and the second peak that appeared for the larger particle should be assigned to an electric quadruple of Ag particles.22 Under the excitation of photons, the electron in the filled band could absorb enough energy and be transferred to the empty band. With decreasing Ag particle size, the energy gap between the filled band and the empty band would increase, which would reduce the number of electrons transferred to the empty band. Hence,
Figure 8. Experimental (A) and calculated (B) optical spectra for Ag@PPy (in the oxidation state) nanoparticles with different shell thicknesses of (a) 5, (b) 7.5, and (c) 10 nm and with the same radius of the Ag core (15 nm).
8772 J. Phys. Chem. C, Vol. 112, No. 24, 2008
Ye and Lu particles with oxidized or reduced PPy shell display different optical properties. For the Ag@PPy nanoparticles with definite core size and shell thickness, the particles with the oxidized shell have enhanced absorption and lower scattering efficiency (Φs), which result from the change of the energy gap after oxidation. Moreover, the Φs value has the maximum near the extinction peak due to the resonance effect. For Ag@PPy (in the reduction state) nanoparticles, its Φs value increases with the longer wavelength because of the nonabsorbing property of the reduced PPy. Furthermore, for the Ag@PPy nanoparticles, inhomogeneity of the particle size and deformation of the particle shape can cause the change of the peak position and spectral shape in UV-vis extinction spectra. The significance of the present work is that, by forecasting the optical property of the Ag@PPy nanoparticle, one could presume the corresponding information regarding different metal@conductive polymer composite nanoparticles and thus exploit their potential in diversified applications by controlling their composition, size, and morphology of both the core and the shell. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20574034) and the Testing Foundation of Nanjing University. References and Notes
Figure 9. Experimental (A) and calculated (B) optical spectra for Ag@PPy (in the oxidation state) nanoparticles with different radii of the Ag core of (a) 15, (b) 20, (c) 30, (d) 40, and (e) 50 nm and with the same shell thickness.
the intensity of the surface extinction of Ag@PPy nanoparticles would be reduced, and the extinction peak would blue-shift to higher energy. Conclusions The scattering, absorption and extinction properties of Ag@PPy nanoparticle were systematically studied for the first time using Mie theory. The calculated UV-vis extinction spectra show a blue shift of the extinction peak with the decreasing size of Ag core and a red shift with the increasing thickness of the PPy shell. Interestingly, the Ag@PPy nano-
(1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Averitt, R. D.; Westcott, S.; Halas, N. J. J. Opt. Soc. Am. 1999, 16, 1824. (3) Sershen, S. R.; Westcott, S. L.; Halas, N. J. J. Biomed Mater Res 2000, 51, 293. (4) Jackson, J. B.; Halas, N. J. J. Phy. Chem B 2000, 105, 2743. (5) Mandal, T. K.; Flemin, M. S.; Walt, D. R. Nano Lett. 2002, 2, 3. (6) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (7) Brian, R.; Keith, S. Chem. Mater. 1995, 7, 1082. (8) Mie, G. Ann. Phys. 1908, 25, 377. (9) (a) Silvert, P. Y.; Tekaia-Elhsissen, K. J. Mater. Chem. 1997, 7, 293. (b) Slistan-Grijalvaa, A.; Herrera-Urbina, R. Phys. E, 2005, 25, 438. (10) Yguerabide, J.; Yguerabide, E. E. Anal. Biochem. 1998, 262, 137. (11) Johnson, P. B.; Christy, R. W. Phys. ReV. B. 1972, 6, 4370. (12) Persson, B. J. Phys. ReV. B. 1991, 44, 3277. (13) (a) Mulvaney, Langmuir 1996, 12, 788. (b) Georgiadis, R.; Peterlinz, K. Langmuir, 2000, 16, 6759. (14) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. ReV. 1988, 88, 183. (15) Kaiser, A. B. Rep. Prog. Phys. 2001, 64, 1. (16) Chiang, C. K.; Fincher, C. R.; Park, Y. W. Phys.ReV. Lett 1977, 39, 1098. (17) Bohren, C. F.; Huffman, D. R. Extinction and Scattering of Light by Small Particles; Wiley: New York, 1983. (18) Collings, J. P.; Gibbs, E. J. Phys. Chem. B. 1999, 103, 8474. (19) Robert, F. P.; Bustamante, C. J. Am. Chem. Sot. 1993, 115, 5393. (20) Hertel, T.; Knoesel, E. Phys. ReV. B. 1996, 54, R5295. (21) Hertel, T.; Knoesel, E. Phys. ReV. Lett. 1996, 76, 535. (22) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969.
JP077710C
