Permeable Silica Shell through Surface-Protected Etching Qiao Zhang, Tierui Zhang, Jianping Ge, Yadong Yin* University of California, Department of Chemistry, Riverside, California 92521 Experimental Chemicals: Tetraethylorthosilicate (TEOS), poly(vinyl pyrrolidone) (PVP, Mw ~ 10,000), sodium hydroxide (NaOH), HAuCl 4, sodium citrate, and ammonium hydroxide (NH 3 H 2 O, 28% by weight in water) were purchased from Sigma-Aldrich. Ethanol and isopropanol were of analytical grade and obtained from Fisher. All chemicals were directly used as received without further purification. Synthesis of SiO 2 solid spheres: In a typical synthesis, 1 ml TEOS was injected into a mixture of 4 ml of deionized H 2 O, 1 ml of NH 3 H 2 O and 20 ml 2-propanol at room temperature under magnetic stirring. After reacting for 2 hours, the colloidal spheres were collected by centrifugation, re-dispersed in 30 ml deionized water. Synthesis of Au@SiO 2 core-shell structures: A gold sol (30 ml) is prepared according to the standard sodium citrate reduction method. Briefly, a solution of hydrogen tetrachloroaurate trihydrate was prepared in water and heated to reflux with magnetic stirring, followed immediately by addition of 1 ml of 3 wt% freshly prepared trisodium citrate-water solution, which initiated the reduction of the hydrogen tetrachloroaurate trihydrate. The aurate-citrate solution was allowed to reflux for approximately 30 min. or until completion of the redox reaction as indicated by a change in solution color from faint yellow to dark red. This method produces a stable, deep-red dispersion of gold particles with an average diameter of around 15 nm and 10% polydispersity. An aqueous solution
of PVP (0.235 ml, 12.8 mg/ml) was added to the sol. The mixture was allowed to stand at room temperature for 24 hours under magnetic stirring, allowing PVP molecules to attach to Au nanoparticles, which were then separated from solution by centrifuging at 11,000 rpm for 45 min and re-dispersed in 92 ml ethanol. The as-prepared solution was then mixed with 13.2 ml of deionized H 2 O and 2.48 ml of NH 3 H 2 O. TEOS (3.45 ml) was injected into the mixture at room temperature under stirring. After reacting for 4 hours, the spheres were collected by centrifugation, and finally re-dispersed in 24 ml of deionized water. Etching SiO 2 spheres: PVP K15 (Mw ~ 10,000) was added to the as-prepared solid SiO 2 solution (molar ratio of PVP/SiO 2 is 10). The mixture was heated up to 100 C for 3 hours to load PVP, and then cooled to room temperature. Under magnetic stirring, 1 ml of 0.1 g/ml sodium hydroxide (NaOH) dissolved in water was added to 4 ml of the as-prepared solution at room temperature. As more material is etched away from the silica particles, the solution becomes less and less opaque. Therefore, we were able to monitor the progress of the etching process by measuring transmission through the solution using UV-Vis spectrometry. For the etching completion times reported in Fig 1b, the criterion was 75% transmittance at the wavelength of 700 nm. The solution was cleaned with repeated actions of water dilution and centrifugation. The colloidal spheres were finally dispersed in polar solvents such as deionized water or ethanol. Au@SiO 2 core-shell particles can be etched using a similar procedure. Catalytic reduction of 4-nitrophenol: The reduction of 4-NP by NaBH 4 was chosen as a model reaction to test the catalytic activity and stability of the Au@SiO 2 nanocatalysts. Aqueous solutions of 4-NP (0.03 ml, 0.01M) and NaBH 4 (0.20 ml, 0.1M) were added to deionized water (2.5 ml) in a quartz cuvette under magnetic stirring. After adding yolk-shell Au@SiO 2 catalyst particles (0.02 ml), the bright yellow solution gradually faded as the reaction proceeded. UV-Vis spectra were recorded at regular interval to monitor the progress of the reaction. Characterization: Morphology of the products was characterized by using a Tecnai T12 transmission electron microscope (TEM). The hollow spheres dispersed in water were cast onto a carbon-coated copper grid, followed by evaporation under vacuum at
room temperature. Fourier transform infrared (FT-IR) spectra were collected with a Bruker Equinox 55 spectrophotometer scanning from 400-4000 cm -1 with a resolution of 4 cm -1 for 64 scans. Measurements were performed with pressed pellets, which were made by using KBr powder as diluents. Thermo-gravimetric analyses (TGA) were carried out on a Mettler Toledo TGA/SDTA 851e under N 2 atmosphere in the temperature range of 40-750 C at a rate of 10 C/min. A probe-type Ocean Optics HR2000CG-UV-NIR spectrometer was used to measure the UV-Vis spectra of the solution to monitor the real-time variation of concentration of 4-NP. The integration time was set to be 3 msec. Figure S1. Fourier transform infrared (FTIR) spectra of pure silica (blue line); silica with PVP refluxed at 100 C for 3 hours (red line); the PVP-coated silica spheres after etching (green line); and pure PVP (black line). For as-synthesized solid silica colloids, weak absorption bands between 1350 and 1650 cm -1 are attributed to C-H bending vibration in unhydrolyzed -OEt groups. Both the PVP treated and the etched sample clearly show additional bands centered at about 2968 and 2950 cm -1 which can be assigned to the CH 2 stretching modes of PVP: ν as (CH 2 ) for the pyrrolidone ring, ν as (CH 2 ) for the polymer backbone, respectively. In consistent to previous observations [Saegusa, T., Pure and Applied Chemistry 1995, 67, 1965-1970], the C=O stretching band of PVP originally at ~1678 cm -1 shifts to a lower frequency to ~1653 cm -1 upon forming a hydrogen bond with silica surface.
Figure S2. TEM images showing the structure evolution of Au@SiO 2 core-shell particles: (a) the original samples; and (b-f) the samples after etching by NaOH for (b) 1 hr 30 min, (c) 2 hrs, (d) 2 hr 30 min, (e) 2hr 45min, and (f) 3hrs. All scale bars are 200 nm. Figure S3. UV-vis spectra of Au@SiO 2 yolk-shell structures before (red) and after (blue) twelve successive cycles of reduction/separation.
Figure S4. (a) Digital photo showing 4-NP and NaBH 4 solution before adding catalyst (left) and after completing catalytic reaction (right). (b) TEM image of Au@SiO 2 yolk-shell catalyst particles after twelve successive cycles of reduction/separation.