Supporting Information Synthesis and Characterization of Monodisperse Metallodielectric SiO 2 @Pt@SiO 2 Core-Shell-Shell Particles Alexey Petrov 1, Hauke Lehmann 1, Maik Finsel 1, Christian Klinke 1,2, Horst Weller 1,2,3 and Tobias Vossmeyer* 1 1 Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, D-20146 Hamburg 2 The Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, D-22761 Hamburg 3 Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia *corresponding author: tobias.vossmeyer@chemie.uni-hamburg.de 1
PARTICLE SYNTHESIS Materials. (3-Aminopropyl)trimethoxysilane (APS, Aldrich, 97%), tetraethyl orthosilicate (TEOS, Aldrich, 99.99%), ammonium hydroxide (NH 4 OH, Aldrich, 28%, 14.5 M), chloroauric acid trihydrate (HAuCl 4 3H 2 O, Aldrich, 99.99%), sodium hydroxide (NaOH, Merck, >98%), tetrakis(hydroxymethyl)phosphonium chloride (THPC, Aldrich, 80%), chloroplatinic acid hydrate (H 2 PtCl 6 H 2 O, Aldrich, 99.9%), ascorbic acid (C 6 H 8 O 6, Aldrich, >99%), polyvinylpyrrolidon (PVP40, average mol wt 40 000, Aldrich), ethanol (EtOH, absolute, Merck, 99.8%), tetrahydrofurane (THF, anhydrous, dried under Schlenk conditions, Aldrich, 99.9%). Chemicals were used as supplied. Throughout the experiments, Millipore water (Milli-Q, 18.2 MOhm cm) was used. Synthesis of SiO 2 particles. The SiO 2 submicron particles with a diameter of 349 nm were produced by a seeded growth method. 1 Briefly, 100 ml EtOH were heated to 45 C, followed by the addition of 3 ml Millipore water, 13.6 ml NH 4 OH (28%) and 5.6 ml TEOS. The reaction suspension was stirred for four hours. This was followed by four further additions of 7 ml Millipore water and 3.5 ml TEOS in intervals of four hours. During the whole reaction (incl. growth steps) the temperature was kept constantly at 45 C. The obtained particles were purified by repeated centrifugation (4000 rzb, 10 min., 5 C) and finally redispersed and stored in EtOH at room temperature. The concentration of the SiO 2 particles stock suspension with a value of 1.28 10-9 mol/l was determined gravimetrically. NH 2 -functionalization of SiO 2 particles. The functionalization of the SiO 2 particles with primary amino groups was carried out under inert gas in dry THF. The particles were first transferred into THF by repeated centrifugation (at least 4 times) and resuspension in THF. The 2
suspension of the SiO 2 particles in 200 ml THF was then heated to 60 C. This was followed by the injection of 10 ml APS into the hot suspension. After 24 h stirring at 60 C the particles were purified by repeated centrifugation (4000 rzb, 10 min., 5 C) and resuspension in EtOH. The functionalized particles were finally dispersed and stored in EtOH at room temperature. The particle concentration was adjusted to the value of 1.28 10-9 mol/l by adding EtOH to the purified stock solution (confirmed by gravimetric analysis). Synthesis of the SiO 2 @Au core-seed particles. In the first step, GNPs were prepared following the procedure of Duff et al. 2 Briefly, 540 ml Millipore water were mixed with 6 ml 1 M NaOH. Next, 144 µl THPC were added. Under vigorous stirring 24 ml HAuCl 4 solution (1 wt% in H 2 O) were quickly added (color change after about 1 second from yellow to dark brown) and then stirred for further 30 minutes at room temperature. Subsequently, the solution was aged for 24 hours at 4 C and finally used without further purification. Assuming 100% conversion of the gold salt and a GNP diameter of 2 nm, the particle concentration of GNP solution was calculated to 2.05 10-5 mol/l. SiO 2 @Au particles were synthesized by a modified method based on the protocol of Oldenburg et al. 3 For the coupling of the GNPs to the SiO 2 surface, 500 ml of the aqueous GNP solution were placed into an ultrasonic bath (Bandelin Sonorex RK103 H). Next, 30 ml of the ethanolic solution of the SiO 2 -NH 2 particles were injected quickly under sonication. The suspension was sonicated for 30 min, followed by repeated centrifugation (3000 rzb, 10 min., 5 C) and redispersion in Millipore water until the supernatant was completely colorless. The SiO 2 @Au seed particles were finally resuspended and stored in 30 ml Millipore water, while maintaining the particle concentration of 1.28 10-9 mol/l. 3
Synthesis of the SiO 2 @Pt CS particles. The electroless deposition of platinum on the SiO 2 @Au core-seed particles was performed using a method described by Lu et al. 4 Under vigorous stirring 6 ml, 2 ml and 0.5 ml of the core-seed SiO 2 @Au particles were added to 66 ml 4.62 mm H 2 PtCl 6 solution at room temperature to obtain the SiO 2 @Pt CS particles with the Pt shell thicknesses of 3 nm, 8 nm and 24 nm respectively. Other shell thicknesses could be obtained by adjusting the volume of the core-seed SiO 2 @Au suspension added to the Pt precursor solution. Next 40 ml of 0.1 M aqueous solution of ascorbic acid were injected quickly. The reaction was complete within one hour, while stirring at room temperature. Synthesis of the SiO 2 @Pt@SiO 2 CSS particles. The complete reaction solution containing the SiO 2 @Pt CS particles with the 3 nm thick Pt shell was used in this step without further purification following the method described by Rodriguez-Fernandez et al. 5 First, 10 g PVP40 were added under sonication and the resulting SiO 2 @Pt particle suspension was sonicated for 1 hour at room temperature (Bandelin Sonorex RK103 H). Next, the suspension was washed two times by centrifugation (3000 rzb, 10 min., 5 C) and redispersion in Millipore water to remove the excess PVP. Finally, the particles were resuspended in 6 ml Millipore water in order to maintain the concentration of 1.28 10-9 mol/l. To the 6 ml aqueous solution of the PVP40 functionalized SiO 2 @Pt CS particles 33 ml EtOH, 0.6 ml NH 4 OH (28%) and 60 µl (for a silica shell thickness d SiO2 = 2 nm) TEOS were added. The silica precursor was added gradually (20 μl steps in 15 min intervals). In order to obtain thicker shells, the amount of added TEOS was increased. For purification, the particles were centrifuged 3 times (3000 rzb, 10 min., 5 C) and redispersed in ethanol. The SiO 2 @Pt@SiO 2 CSS particles were finally resuspended and stored in 6 ml EtOH. The suspension had a particle concentration of 1.28 10-9 mol/l. 4
CHARACTERIZATION Functionalization of the SiO 2 particles with APS. The functionalization of the particles was carried out in dry THF under inert gas conditions to avoid uncontrolled self-reaction of the functionalization agent. The APS molecules bind covalently to the SiO 2 spheres, extending their amine groups outwards as a new termination of the particle surface. For the detection of the NH 2 - functionalization single particle Raman spectra were recorded (Figure S1a). The nonfunctionalized SiO 2 particles were used as reference. After the functionalization a broad peak at 2930 cm -1 could be detected by Raman spectroscopy for the amine functionalized particles. The Raman bands in the 2800-3000 cm -1 region are assigned to the -CH 2 stretching vibrations of the propyl segment of APS. 6,7 Due to the repeated purification of the particles by centrifugation, we assume that predominantly covalently bound APS molecules are responsible for the appearance of the peak. The diameter of the SiO 2 particles remained unchanged after the amine functionalization, because under the reaction conditions only a very thin layer of APS molecules were bound to the particle surface. 5
Figure S1. Single particle Raman spectra of the APS functionalized (red line) and nonfunctionalized (black line) SiO 2 particles (a). Size distribution histogram of the functionalized SiO 2 -NH 2 particles with a mean diameter of (349 ± 9) nm (b). 6
DLS Measurements of the SiO 2 @Au core-seed particles. Figure S2. DLS measurements of the SiO 2 @Au particles prepared with stirring alone (red line) and with stirring and sonication (blue line). 7
SEM images of SiO 2 @Pt particles with different Pt shell thicknesses. Figure S3. SEM images of SiO 2 @Pt CS particles at different magnifications. The SiO 2 core had a diameter of 349 nm and the Pt shell thickness was 6 nm (a), 18 nm (b) and 32 nm (c). UV/Vis analysis of the Au, SiO 2 @Au core-seed and SiO 2 @Pt CS particles. While the pure GNPs have a weak plasmon band, visible by a shoulder at 513 nm, 2 the UV/Vis spectrum of the SiO 2 @Au core-seed particle did barely show this signature due to the comparatively low amount of the GNPs on the SiO 2 surface. 8 The non-normalized UV/vis spectra of the SiO 2 @Pt CS particles exhibited a broad-band absorbance in the visible range of the spectrum (Figure S4). 8
Figure S4. UV/Vis spectra of the seed GNPs, SiO 2 @Au core-seed particles, and SiO 2 @Pt CS particles with Pt shell thicknesses of 6 nm, 13 nm, 18 nm and 32 nm dispersed in water. 9
HRSEM and TEM images of SiO 2 @Pt particles with a Pt shell thickness of 3 nm. Figure S5. HRSEM (a) and TEM (b) image of SiO 2 @Pt CS particles with a Pt shell thickness of ~3 nm. 10
Influence of NH 4 OH concentration on the SiO 2 -shell formation. Figure S6. TEM images of SiO 2 @Pt@SiO 2 CSS particles at different magnifications with a SiO 2 core particle of 349 nm in diameter and a Pt shell thickness of 6 nm. The reaction solutions contained 8.4 M H 2 O, 21 mm TEOS, 0.10 M (a), 1.31 M (b) and 3.25 M (c) NH 4 OH in ethanol. 11
Influence of H 2 O concentration on the SiO 2 -shell formation. Figure S7. TEM images of SiO 2 @Pt@SiO 2 CSS particles at different magnifications with a SiO 2 core particle of 349 nm in diameter and a Pt shell thickness of 6 nm. The reaction solutions contained 0.2 M NH 4 OH, 34 mm TEOS, 3.7 M (a), 14.6 M (b) and 23.1 M (c) H 2 O in ethanol. 12
Colloidal stability of SiO 2 @Pt CS and SiO 2 @Pt@SiO 2 CSS particles. Figure S8. DLS data of as synthesized and 14 months aged SiO 2 @Pt CS particles (d core = 349 nm, d Pt-shell = 5 nm) in water (a). DLS data of as synthesized and 14 months aged SiO 2 @Pt@SiO 2 CSS particles (d core = 349 nm, d Pt-shell = 5 nm, d SiO2-shell = 3 nm) in ethanol (b). SEM images of the SiO 2 @Pt CS particles deposited on interdigitated electrodes. Figure S9. SEM images of films prepared of SiO 2 @Pt CS particles with Pt shell thicknesses of 3 nm (a), 8 nm (b) and 24 nm (c). 13
Resistance of films as a function of the reciprocal Pt shell thickness. Figure S10. Representative SEM images of cross-sections of films consisting of SiO 2 @Pt CS particles with a Pt shell thickness of 32 nm (a), 18 nm (b) and 13 nm (c). Plot of resistance versus the reciprocal Pt shell thickness measured at room temperature (d). The electrodes used for these measurements had a spacing of 400 µm. The measured resistances were normalized to a common film thickness of 7.5 µm and a common width of 5000 µm covering the electrode gap. Four samples were prepared and measured for each Pt shell thickness. 14
REFERENCES (1) Bogush, G. H.; Tracy, M. A.; Zukoski, C. F. Preparation Of Monodisperse Silica Particles: Control Of Size And Mass Fraction. J. Non. Cryst. Solids 1988, 104, 95 106. (2) Duff, D. G.; Edwardsb, P. P. A New Hydrosol of Gold Clusters. J. Chem. Soc., Chem. Commun. 1993, 96 98. (3) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Nanoengineering of Optical Resonances. Chem. Phys. Lett. 1998, 288 (2-4), 243 247. (4) Lu, L.; Sun, G.; Xi, S.; Wang, H.; Zhang, H.; Wang, T.; Zhou, X. A Colloidal Templating Method to Hollow Bimetallic Nanostructures. Langmuir 2003, 19 (11), 3074 3077. (5) Rodríguez-Fernández, J.; Pastoriza-Santos, I.; Pérez-Juste, J.; García De Abajo, F. J.; Liz- Marzán, L. M. The Effect of Silica Coating on the Optical Response of Sub-Micrometer Gold Spheres. J. Phys. Chem. C 2007, 111, 13361 13366. (6) Volovšek, V.; Furić, K.; Bistričić, L.; Leskovac, M. Micro Raman Spectroscopy of Silica Nanoparticles Treated with Aminopropylsilanetriol. Macromol. Symp. 2008, 265 (1), 178 182. (7) Okabayashi, H.; Izawa, K.; Yamamoto, T.; Masuda, H.; Nishio, E.; O Connor, C. J. Surface Structure of Silica Gel Reacted with 3-Mercaptopropyltriethoxysilane and 3- Aminopropyltriethoxysilane: Formation of the S-S Bridge Structure and Its Characterization by Raman Scattering and Diffuse Reflectance Fourier Transform Spectroscopic Studi. Colloid Polym. Sci. 2002, 280 (2), 135 145. (8) Westcott, S. Lou; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Formation and Adsorption of Clusters of Gold Nanoparticles onto Functionalized Silica Nanoparticle Surfaces. Langmuir 1998, 14 (12), 5396 5401. 15