Synthetic Opals Based on Silica-Coated Gold Nanoparticles

Langmuir 2002, 18, 4519-4522

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Synthetic Opals Based on Silica-Coated Gold Nanoparticles Florencio Garcı´a-Santamarı´a,† Vero´nica Salgueirin˜o-Maceira,‡ Cefe Lo´pez,† and Luis M. Liz-Marza´n*,‡ Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco 28049, Madrid, Spain, and Departamento de Quı´mica Fı´sica, Universidade de Vigo, E-36200, Vigo, Spain Received January 31, 2002. In Final Form: March 15, 2002 High-quality solid colloidal crystalline assemblies have been prepared through the sedimentation of monodisperse silica spheres, each containing a single gold nanoparticle in its center. The metal centers provide a strong absorption at a well-defined wavelength that complements the Bragg reflection arising from the periodic structure of the opal. The optical properties of these systems were characterized through transmission and specular reflectance measurements. It is clearly shown that absorption and diffraction are basically independent of each other and their relative influence can be separated through infiltration with liquids of varying refractive index.

Introduction The use of composite, nanostructured materials is attracting great interest both from a fundamental point of view and for an increasing number of potential practical applications.1 The incorporation of several components with markedly different properties opens up a whole range of possibilities that cannot be achieved with conventional single-component systems. Among the different properties that are interesting in nanostructured systems, maybe the most studied and best understood are optical properties, and still new systems with new or improved optical properties continue to be developed. Within the broad range of nanoparticle compositions currently under study, noble metals have received a special interest because of the coupled oscillation of conduction electrons when interacting with an external electromagnetic wave of a certain wavelength, which is termed surface plasmon.2,3 The frequency of the plasmon resonance mainly depends on the nature of the metal, the size and shape of the nanoparticle, and the nature of the surrounding medium,4 so that it can be conveniently tuned. Another vastly studied optical property of nanostructured systems is the diffraction of visible and near-infrared light when a periodic modulation of the dielectric constant is achieved in the length scale of several hundreds of nanometers. Such systems are commonly termed photonic crystals.5 It has been predicted that face-centered-cubic (fcc) structures made of metallic spheres (coated or not) could lead to important improvements in optical features over those made of dielectric particles.6,7 Photonic crystals comprising metals in their composition have been prepared previously, by infiltration of nanoparticles,8,9 electroless deposition,10 or electrochemical † ‡

CSIC. Universidade de Vigo.

(1) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature 1997, 386, 143. (2) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (3) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (4) Mulvaney, P. Langmuir 1996, 12, 788. Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (5) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. John, S. Phys. Rev. Lett. 1987, 58, 2486. Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals; Princeton University Press: Princeton, NJ, 1995. (6) Moroz, A. Phys. Rev. Lett. 1999, 83, 5274. (7) Moroz, A. Europhys. Lett. 2000, 50, 466.

deposition.11 Preliminary results were also recently shown on the use of gold nanoshells as units for opal formation.12 All these methods end up with a system in which bulklike gold is obtained, since there is direct physical contact between neighboring units. We present here the preparation and optical properties of a novel system that joins together the optical properties of metal nanoparticles and photonic crystals. The incorporation of metal nanoparticles within colloidal crystalline arrangements is made through the synthesis of composite, core-shell colloid spheres comprising a metallic nanoparticle as the core and amorphous silica as the shell.13 The size of the core is uniform enough that it displays a well-defined plasmon band, while the thickness of the shell is large enough to enable the formation of a crystalline arrangement with a suitable lattice constant to yield photonic band gap effects in the visible. After the synthesis of such composite spheres, synthetic opals are made by means of the simple natural sedimentation method,14 and the robustness and mechanical stability of the crystals are enhanced through a sintering process.15 Since metal cores are far apart from each other, the properties of single nanoparticles are fully preserved in the opal. The study of the optical properties is described, not only for dry opals, but also after infiltration with liquids of varying composition (which implies varying refractive index), so that a more complete description of the system is achieved. Experimental Section Chemicals. Tetrachloroauric acid (HAuCl4‚3H2O), trisodium citrate, 3-aminopropyltrimethoxysilane (APS), tetraethoxysilane (TEOS), and sodium silicate solution (Na2O(SiO2)3-5, 27 wt % (8) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature 1999, 401, 548. (9) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Lenhoff, A. M.; Robolt, J. F.; Kaler, E. W. Adv. Mater. 2001, 13, 396. (10) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957. (11) Wijnhoven, J. E. G. J.; Zevenhuizen, S. J. M.; Hendriks, M. A.; Vanmaekelbergh, D.; Kelly, J. J.; Vos, W. L. Adv. Mater. 2000, 12, 888. (12) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (13) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (14) Sacks, M. D.; Tseng, T.-Y. J. Am. Ceram. Soc. 1984, 67, 526. Mayoral, R.; Requena, J.; Moya, J. S.; Lo´pez, C.; Cintas, A.; Mı´guez, H.; Meseguer, F.; Va´zquez, L.; Holgado, M.; Blanco, A. Adv. Mater. 1997, 9, 257. (15) Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Blanco, A.; Moya, J. S.; Requena, J.; Mifsud, A.; Forne´s, V. Adv. Mater. 1998, 10, 480.

10.1021/la025594t CCC: $22.00 © 2002 American Chemical Society Published on Web 05/02/2002

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Figure 1. (a) TEM micrograph showing the core-shell morphology of the composite colloid spheres used for opal formation. (b) Representative SEM micrograph showing the high degree of order obtained with the natural sedimentation method. SiO2) were purchased from Aldrich and used as received. Milli-Q water with a resistivity higher than 18.2 MΩ cm and absolute ethanol (Scharlab) were used in all the preparations. Different refractive index liquids were prepared with mixtures of redistilled glycerol (Prolabo) and doubly distilled water. Silica-Coated Metal Nanoparticle Synthesis. Silica-coated Au nanoparticles (Au@SiO2) were prepared by the method previously reported.13 Typically, a gold sol is prepared by boiling 5 × 10-4 M HAuCl4 in the presence of 1.6 × 10-3 M sodium citrate. This results in a stable dispersion of gold particles with an average diameter of around 15 nm, and 10% polydispersity.16 A freshly prepared aqueous solution of APS (2.5 mL, 1 mM) is added to 500 mL of the gold sol under vigorous magnetic stirring. The mixture of APS and gold dispersion is allowed to stand for 15 min, and then 20 mL of a 0.54 wt % sodium silicate solution at pH 10-11 (adjusted with a cation-exchange resin) is added, again under vigorous magnetic stirring. The resulting dispersion (pH ≈ 8.5) is then allowed to stand for 1 week, so that the active silica polymerizes onto the primed gold particle surface. The silica shell thickness is then about 5-7 nm thick. Subsequently, excess of silicate is removed by several centrifugation/redispersion steps, and the core-shell particles formed are stable in both water and ethanol. After redispersion in ethanol, the thinly coated nanoparticles are used as seeds for the formation of Au@SiO2 with thick shells, through the well-known Sto¨ber method. For this work, stepwise addition of TEOS was continued until a total diameter of 225 nm was achieved. Sedimentation of Opals. The preparation of opals was performed by means of the natural sedimentation method.14 This method simply consists of redispersing the particles in distilled water with a concentration of 1 wt % and allowing them to sediment, following Stokes law, which results in the formation (16) Enu¨stu¨n, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317.

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Figure 2. Specular reflectance spectra at different incidence angles for opals made of Au@SiO2 particles with 15 nm core diameter and 225 nm total diameter. The upper plot (a) corresponds to the as-grown opal, while the lower plot (b) shows the spectra measured from the sintered opal. Incidence angles are indicated by the labels. The insets show fits with Bragg’s Law (eq 1). of large, face-centered-cubic (fcc) ordered compacts.17 An example of the quality of the opals is shown in Figure 1. After sedimentation, the supernatant was removed and the opals dried at 45 °C and subsequently sintered at 950 °C for 3 h. This treatment provides mechanical stability and removes the remaining water from the samples.15 Instruments and Characterization. Transmission electron microscopy (TEM) was performed with a Philips CM20 microscope operating at 200 kV. Samples for TEM were prepared by depositing a drop of the suspension on a carbon-coated TEM copper grid. Scanning electron microscopy (SEM) was performed with a Philips XL-30 microscope operating at 30 kV. Prior to the measurements, the samples were coated with a 2 nm thick gold film by means of a vacuum sputterer, to improve electrical conductivity. Transmission and reflectance spectra were obtained with an IFS 66/S Bruker Fourier transform spectrometer at room temperature, with a scan rate of 10 kHz. Reflectance spectra with angle variations were performed with a photomultiplier from Hamamatsu attached to a TM300 Bentham.

Results and Discussion Figure 1a shows a representative transmission electron micrograph of the core-shell particles used for this study, where the monodispersity both in the core size and in the total diameter can be easily observed. Additionally, an SEM image (Figure 1b) is included to show the crystalline arrangement of the same nanoparticles after sedimentation and sintering. Initially, the quality of the opals was assessed through specular reflectance. Figure 2 shows the spectra at different angles for the same opal “as grown” and after sintering. Spectra measured at the same angle are blue(17) Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Mifsud, A.; Moya, J. S.; Va´zquez, L. Langmuir 1997, 13, 6009.

Synthetic Opals Based on Gold Nanoparticles

Langmuir, Vol. 18, No. 11, 2002 4521

shifted for the sintered opal, as a result of the lattice constant reduction due to the structure and sphere contraction.18 The peak positions can be fitted with Bragg equation:where θ is the incident angle with respect to the

λ ) 2d(〈〉 - sin2 θ)1/2

(1)

growth surface, d is the distance between the crystalline planes responsible for that reflection, and 〈〉 is the average dielectric constant of the sample, defined as

〈〉 ) 1φ + 2(1 - φ)

(2)

1 and 2 are the dielectric constants of silica and opal interstices, respectively, and φ is the filling factor (0.74 for fcc close-packed structures). The Au cores have not been included in the calculation, since they occupy less than 0.1% of each particle, and therefore their contribution to diffraction should be negligible. However, there is still a contribution from absorption, as will be shown below. The reflectance spectra shown in Figure 2 are basically identical to what would be expected for opals made of 225 nm pure silica spheres, and no sign of the gold cores is found. However, two (in principle) independent phenomena are expected to govern the optical properties of these composite systems. On one hand, we have the plasmon resonance associated with the Au nanoparticle cores embedded in a medium with the refractive index of silica (ca. 1.46). For these systems, the band is expected11 to be around 525 nm and it must be clearly observable, since their concentration is relatively high, as compared to standard dispersions.19 On the other hand, we expect Bragg reflection arising from the regular arrangement of the spheres in an fcc lattice,20 and whose precise position strongly depends on the lattice constant, as well as on the refractive indexes of the spheres and the voids. The surface plasmon resonance is mainly an absorbance when the nanoparticle concentration is low enough and hardly contributes to reflectance for interparticle separations larger than 50 nm.21 Conversely, the Bragg diffraction is observed better as a reflection, due to a forbidden conduction of electromagnetic radiation within certain wavelengths through the crystalline network. Therefore, an optical study of the opals was performed through measurements of both transmittance and reflectance. To investigate the possible coupling between plasmon absorption and Bragg reflection, we performed an index-matching experiment, based on the infiltration of the voids of a sintered opal with liquids of varying refractive index. To cover a range that includes the refractive index of silica, mixtures of water (n ) 1.333) and glycerol (n ) 1.475) were used. The results are shown in Figures 3 and 4 for reflectance and transmittance, respectively. The specular reflectance spectra simply show a red shift of the maximum, due to an increase of the opal interstices refractive index, and a progressive decrease of intensity, due to a reduction of the optical contrast between the spheres and the background. The match point would be placed at an intermediate composition between 75% and 100% glycerol, but closer to pure glycerol. This shows again that the presence of the metal cores does not visibly alter the diffraction properties of the opals. (18) Garcı´a-Santamarı´a, F.; Mı´guez, H.; Ibisate, M.; Meseguer, F.; Lo´pez, C. Langmuir 2002, 18, 1942. (19) Simple calculations performed assuming a perfect fcc packing yield an estimate for the gold concentration in the sintered opals of ca. 0.03 mol L-1. (20) Busch, K.; John, S. Phys. Rev. E 1998, 58, 3896. (21) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. Colloids Surf. A 2002, 202, 119.

Figure 3. Specular reflectance spectra in different media (air, water, and water-glycerol mixtures) for sintered opals made of Au@SiO2 particles with 15 nm core diameter and 225 nm total diameter. The corresponding void refractive indexes are indicated.

Figure 4. Transmittance spectra in different media (air, water, and water-glycerol mixtures) for sintered opals made of Au@SiO2 particles with 15 nm core diameter and 225 nm total diameter. The corresponding refractive indexes are indicated.

A richer description of the optical behavior is found in the transmittance spectra, where both the surface plasmon resonance and the Bragg diffraction are observed. Several features should be noted from the spectra of Figure 4. With respect to the Bragg peaks, their positions basically coincide with those measured in reflectance, though for the samples with a low background refractive index the peaks are close to the lower wavelength detection limit of the instrument, and therefore appear noisy. It is also clear that the intensity of the Bragg peaks decreases when approaching the refractive index matching with silica, exactly as was observed in the reflectance spectra. Regarding the plasmon band, we can see an invariance in its position for all the infiltrated samples. The bands are consistently centered at 523 nm, which agrees with the predictions for noninteracting nanoparticles dispersed in silica, as was recently shown for thin films.22 The silica shells are sufficiently thick, so that gold cores do not “feel” the solvent anymore. However, the scattering gets reduced when the index matching point is approached, and therefore the global intensity of the spectra (including the plasmon band) decreases.13 The only sample that does not stick to this behavior is the dry, non-infiltrated opal, in which the plasmon band is noticeably red-shifted with (22) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 3441.

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Figure 5. Transmittance spectra in air (triangles) and water (circles) for opals (solid symbols) and disordered packings (open symbols) of Au@SiO2 particles.

respect to all the others (543 nm). Also, the transmittance is drastically reduced, which can be again attributed to a much stronger scattering, since the contrast between silica spheres and voids is also much larger than in all the infiltrated samples. Looking at Figure 3, one can easily see that indeed the reflectance spectra show such a difference, which can additionally be the reason for the observed red shift in the plasmon band position. A recent study by Asher and co-workers23 on colloid crystals based on silica spheres loaded with homogeneously distributed silver clusters showed that for such a system (which cannot be directly compared to the one described in the present paper because of the additional complication from the interactions between metal clusters within a single composite sphere), an increased order in the system leads to a blue shift of the silver plasmon band, which was associated with a Borrmann-like phenomenon.24 This effect would imply that a standing wave is produced from the coupling of the incident and diffracted light, so that the long-wavelength side of the band edge should localize the electric field in the higher dielectric constant region of the diffracting array. In Asher’s work, the high metal content within the spheres makes them have an average low real part of the dielectric constant (23) Wang, W.; Asher, S. A. J. Am. Chem. Soc. 2001, 123, 12528. (24) Batterman, B. W.; Cole, H. Rev. Mod. Phys. 1964, 36, 681.

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in the vicinity of the maximum, so that the plasmon absorption would decrease on the red edge. Conversely, in the present case, where a large percentage of each sphere is made of silica, it means that the standing wave will be localized in the particle planes, so that absorption gets enhanced and red-shifted. Nevertheless, one should be cautious when using this reasoning, since for the infiltrated samples, with a noticeably broad range of refractive index differences, this effect is not observed. A further experiment was carried out to test the influence of order on the optical properties. To this end, a piece of the sintered opal was ground, so as to destroy the crystalline structure, and the transmittance was measured, both for the sample in air and after infiltration with water. The spectra obtained are shown in Figure 5, where spectra for the corresponding ordered samples are included for comparison. As expected, the strong scattering from the disordered sample in air renders the gold surface plasmon band hardly visible. In water, there is still a diffuse scattering contribution, but the plasmon band is clearly observed. In the case of the ordered samples, scattering is coherent (diffraction) and the effect on the plasmon band is only observed as a red shift as explained above. Conclusions Artificial opals were prepared by sedimentation of Au@SiO2 core-shell colloid spheres. The optical properties of the opals are determined both by the Bragg diffraction inherent to the ordered structure and by the surface plasmon absorption of the metal cores. The relative contribution of these two optical features can be tuned by infiltration of the opals with liquids of a suitable index of refraction. We found that the position of the plasmon band is red-shifted in the dry opal, as compared to the isolated particles and to infiltrated opals, which seems to be due to a coupling with scattering. Acknowledgment. L.M.L.-M. acknowledges financial support from the Spanish Xunta de Galicia, (Project No. PGIDT01PXI30106PR) and Ministerio de Ciencia y Tecnologı´a (Project No. BQU2001-3799). This work was also partially financed by the Spanish CICyT Project MAT20001670-C04. The authors are indebted to J. B. Rodrı´guez from the CACTI of Vigo University for his assistance with TEM and SEM measurements. LA025594T