Supporting Information for: Spectroelectrochemistry of. Silver Deposition on Single Gold Nanocrystals

Transcription

1 Supporting Information for: Spectroelectrochemistry of Silver Deposition on Single Gold Nanocrystals Mariana Chirea, * Sean S. E. Collins, Xingzhan Wei, Paul Mulvaney * CIQ-L4, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, , Porto, Portugal School of Chemistry and Bio21 Institute, University of Melbourne, Parkville, VIC, 3010, Australia AUTHOR INFORMATION Corresponding Author * mariana.chirea@uvigo.es, mulvaney@unimelb.edu.au Present Addresses Author present address: Departamento de Química Física, Universidade de Vigo, Vigo, Spain 1

2 1. Materials and Methods: 1.1 Chemicals. Hydrogen tetrachloroaurate (III) trihydrate (HAuCl 4 3H 2 O, 99,999 %, Sigma Aldrich), sodium citrate (99%, Sigma Aldrich), poly (N-vinylpyrrolidone) (Sigma Aldrich, analytical grade, MW 10000), N,N-dimethylformamide (DMF, 99.8%, Merck) silver nitrate (ACS reagent, 99%, Sigma- Aldrich), sodium nitrate, (NaNO 3, pa quality) were used as received. Millipore filtered water (resistivity > 18 MΩ cm) was used for preparation of all aqueous solutions. Prior use, all glassware were cleaned with freshly prepared aqua regia (HNO 3 :HCl = 1:3, % v/v), rinsed abundantly with Millipore water and dried. 1.2 Nanostar synthesis: Gold nanostars (AuNSs) were synthesized following the seed-mediated method developed by Barbosa et. al. (reference 25 in the main manuscript). Gold seeds were prepared by addition under continuous stirring of 5 ml (1 wt %) sodium citrate aqueous solution to a boiling aqueous solution of HAuCl 4 (100 ml, 0.5 mm) allowing to react for 15 min. The Au seeds were transferred into ethanol through PVP modification. Thus, 5 ml of a PVP (MW ) aqueous solution, containing 0.2g of PVP was added drop wise to the Au colloid and allowed to react overnight. The final solution was centrifuged at 4000 rpm for 90 min, the supernatant removed and the spherical nanoparticles redispersed in ethanol. For the synthesis of gold nanostars, 82 μl of an aqueous solution of 50 mm HAuCl 4 was mixed with 15 ml of 10 mm PVP (MW ) solution in DMF. To this solution a certain amount of preformedseed dispersion was added under continuous stirring and allowed to react until completion of the reaction (no further changes in the UV-Vis-NIR spectra). For this specific synthesis a ratio [HAuCl 4 ] to [seed] of 90 was used. The resulted gold nanostars (AuNSs) were purified by sonication for 1 minute, centrifugation at 7000 rpm for 15 minutes and removal of 90% of the supernatant, and replacing the same volume removed with water. The final solution was sonicated for a further 1 minute. 1.3 Spectroelectrochemical experiments: The Dark Field Microscope used for measuring all scattering spectra was a Nikon TE2000-S Eclipse inverted microscope (with 100 W halogen lamp, dry dark field condenser and 40x/0.60 NA dry objective lens) coupled to an IsoPlane SCT 320 imaging spectrometer (Princeton Instruments (150 lines/mm, 800 nm blaze) equipped with a PIXIS 1024 CCD camera, front illuminated and cooled in air at 55 C. Background spectra were taken simultaneously with single particle spectra. The background spectra were selected in a dark area where no nanoparticles or contaminants were present, i.e. bare ITO, and then were subtracted from and used to normalize the sample scattering spectra. To capture video of the electrochemical reactions, a high resolution colour CMOS camera (Thorlabs DCC1645C) was used. ITO coated glass slides (Delta Technologies, R s = 8-12 Ω) were used as working electrodes in all experiments. Samples were marked by using a focused ion beam (FIB) registration technique described elsewhere (reference 1 in this file) enabling the nanostars to be imaged in the scanning electron microscope (SEM) before and after experiments. Individual marked AuNSs modified ITO-coated glass 2

3 slides were inserted in an electrochemical cell specially built to fit in the Dark Field Microscope and connected to a PGSTAT 302N potentiostat (Metrohm AutoLab BV). The cell, described elsewhere (reference 8 in the main manuscript) was composed of a steel cage and a PTFE ring. The electrochemical cell configurations and experimental conditions varied slightly between the NaBH 4 treated and non-treated nanostar-modified electrodes: Non-treated nanostar-modified electrode: Prior to use, the ITO coated glass slides were washed by sonication in absolute iso-propanol, Millipore water and dried under a stream of nitrogen. 150 μl of diluted (10 ) washed gold nanostars were spin coated at 3000 rpm for 1 minute on freshly cleaned 25 mm 25 mm ITO-coated glass slides. The ITO slide was then placed inside the cell. Through the PTFE ring, a coiled Pt wire and a straight Ag wire were inserted and used as counter and quasi-reference electrodes, respectively. Another Ag wire, protected from contacting the electrolyte solution by PTFE tape was used to connect the particlecontaining ITO-coated glass slide, the working electrode, to the potentiostat. The Pt counter electrode was cleaned by polishing on micro cloth pads with diamond suspension (0.3 µm), sonicated in Millipore water for 5 minutes and dry under a stream of N 2.The cell was filled with a filtered (0.22 μm PES membrane) 0.1 M NaNO 3 electrolyte aqueous solution (approximately 1.3 ml) with a AgNO 3 concentration of M and covered with a cover slip to minimize evaporation and eliminate meniscus lensing. Spectroelectrochemical measurements were carried out on nanostars in the middle of the working electrode NaBH 4 treated nanostar-modified electrode: A standard photolithography step was carried out on a 25 mm 25 mm ITO-coated microscope slide using AZ4562 photoresist. Enough photoresist to cover the slide was spin coated on the ITO surface and then heated face-up on a hot plate at 100 C for 5 minutes. The slide was then placed in a UVexposure box on top of a mask and turned on for 90 seconds. A mask of the following dimensions was used: Figure S1. UV-lithography mask dimensions for patterning ITO working and counter electrodes. 3

4 Following the exposure the slide was dipped into a 40 ml of 0.15 M sodium hydroxide aqueous solution for 1 minute. After that, a wet etching step was carried out to remove the UV-exposed ITO in HCl acid (37%), the slide was etched in 2:1 solution of HCl (37%): H 2 O for 8 minutes. The remaining AZ4562 photoresist was then removed by sonicating the slide in acetone for 15 seconds. The patterned ITO glass slides were then washed further by sonication in absolute iso-propanol, then water and dried under a stream of nitrogen. 20 μl of diluted (10 ) aqueous solution of washed gold nanostars were spin coated at 500 rpm for 10 minutes on the end of the 12 mm T-shape protrusion which makes up the working electrode. The other side of the pattern, the U-shape, was the counter (auxiliary) electrode. Before mounting in the electrochemical cell the slide was immersed in 40 ml of 0.04 M NaBH 4 solution for 15 minutes to remove PVP (ref. 21 in the manuscript). The slide was then rinsed repeatedly with water and dried in a stream of N 2. Then the slide was placed inside the cell. Through the PTFE ring, a 0.5 mm diameter platinum/polypyrrole (Pt/PPy quasi-reference electrode (preparation reported in reference 2 of this file) was inserted so that the end of it was positioned < 1 mm from the end of the working electrode. Another Ag wire and Pt wire, protected from contacting the electrolyte solution by PTFE tape, were used to connect the particle-containing working electrode (T-shaped ITO) and counter-electrode (U-shape ITO) to the potentiostat, respectively. The cell was filled with a filtered (0.22 μm PES membrane) 0.1 M NaNO 3 electrolyte aqueous solution (approximately 1.3 ml) with a AgNO 3 concentration of M and covered with a cover slip to minimize evaporation and eliminate meniscus lensing. Spectroelectrochemical measurements were carried out on nanostars near the end of the working electrode. Changes made to the electrochemical cell configuration in the NaBH 4 treated stars experiment have only been made to reduce the area of the working electrode and to use smaller amounts of nanostars. Also a more stable quasi-re was used. 1.4 Scanning electron microscopy: Scanning electron microscopy (FEI NOVA Dual Beam 200) was carried out on single particles before and after the spectroelectrochemical experiments described above. The particles were only exposed once before and once afterwards with the same settings to ensure a consistent electron exposure. The SEM beam setting were 10 kv, 0.54 na, with a 10 μs dwell time. The magnification used was 250,000 in immersion mode with a 5 mm working distance. 1.5 Theoretical modelling: Finite-element method (COMSOL) was used to simulate the nanostar structure. In modelling, the particles were delimited by the perfectly matched layers, which prevented any unwanted reflections through absorbing the scattering off the particles. All calculations were performed in an effective embedding medium n = 1.33, to match experiment. The optical constants of gold were taken from Johnson and Christy (reference 26 in main text). For theoretical modelling a nanostar was chosen with features very close to the highest number of stars in the sample. The average size of a nanostar estimated from the SEM images was: 57 nm core diameter, 27 nm tip length, 23 nm width at the base of tip, 2.5 nm radius of tip apex and 8 tips. These parameters were used for the theoretical modelling of the optical spectrum of a nanostar spin coated on ITO and immersed in aqueous solution of 0.1M NaNO 3. The same parameters were used for the simulation of silver electrodeposition. 4

5 2. Additional results Effect of Pre-Imaging with SEM and NaBH4 Treatment Figure S2. SEM images of PVP stabilized nanostars imaged before and after the chronoamperogram. These particles, which had been exposed to the electron beam prior to electrochemical silver deposition, did not undergo silver shelling and surface plasmon shifts were not observed on these particles. All scale bars are 100 nm. 5

6 Figure S3. (a,b) Conversely, these two gold nanostars, previously unexposed to an electron beam, have been imaged with a scanning electron microscope after silver deposition. The sharp needles of the nanostars are much blunter due to preferential deposition onto the tips. Scale bars are 100 nm. (c) Dark field image of the resultant AuNS-PVP@Ag core shell nanoparticles. The spectra are strongly blueshifted. However, as noted already, while the blob-like nature of the silver deposition is evident, these particles could not be imaged beforehand to provide references, since the imaging prevented silver deposition from occurring. 6

7 (a) (b) Figure S4. (a) Shift of the surface plasmon band peak position as a function of time and applied potential of a PVP stabilized gold nanostar (unexposed to an electron beam prior reaction) measured during electrodeposition of metallic silver from M AgNO 3 and 0.1 M NaNO 3 aqueous solution at selected potentials varying from 0 mv to -100 mv. (b) Selected full scattering spectra of the same nanostar throughout the deposition process. The147 nm blue shift corresponded to a 6-7 nm Ag shell deposited on this nanostar. 7

8 Electrochemistry on Bare- NaBH 4 Treated Gold Nanostars Figure S5. Full spectroelectrochemical data from the NaBH 4 treated nanostar-modified electrode experiment in the main text in Figure 3 (red plot) accompanied by data obtained under the same conditions except without the addition of any silver to the electrolyte solution (green plot). The slight blue shift (~3 nm) can be attributed to non-faradaic charging. NaBH 4 was found to remove the PVP ligand layer from the gold nanostars. 8

9 Figure S6. SEM images of NaBH4 treated Au nanostars (not previously imaged by the electron beam) after Ag deposition, obtained using the same potential sequence carried out in Fig S5. This figure demonstrates that particles exposed to, and not exposed to the e-beam prior to Ag deposition, undergo almost identical Ag deposition on NaBH4 treated particles. This was not the case for untreated particles. 9

10 Furthermore, note the absence of any indiscriminate Ag nucleation on the surrounding ITO. Silver deposition occurs exclusively on the nanostars. Figure S7. SEM images of a NaBH 4 treated Au nanostar before (a) and after (b) Ag deposition, obtained using the same potential sequence carried out in Fig S5. (c) Full scattering spectra of the above particle at various stages of the deposition process. This figure highlights the complex spectra of irregular Au nanostars by the multiple peaks visible in the spectral window. 10

11 Figure S8. SEM images of a NaBH 4 treated Au nanostar before (a) and after (b) Ag deposition, obtained using the same potential sequence carried out in Fig S5. (c) Full scattering spectra of the above particle at various stages of the deposition process. This figure shows highlights the irregularity of the shifts induced by the shelling process because this particle underwent a decrease in scattering intensity with deposited Ag. 11

12 References: (1) Novo, C.; Funston, A. M. ; Pastoriza-Santos, I.; Liz-Marzán, L. M.; Mulvaney, P. Spectroscopy and High-Resolution Microscopy of Single Nanocrystals by a Focused Ion Beam Registration Method. Angew. Chem. Int. Ed. Engl. 2007, 19, (2) Ghilane, J.; Hapiot, P.; Bard, A. J. Metal/Polypyrrole Quasi-Reference Electrode for Voltammetry in Nonaqueous and Aqueous Solutions. Anal. Chem. 2006, 78,