Controlling Anisotropic Nanoparticle Growth Through Plasmon Excitation. Rongchao Jin, Y. Charles Cao, Encai Hao, Gabriella S.

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1 1 Controlling Anisotropic Nanoparticle Growth Through Plasmon Excitation Rongchao Jin, Y. Charles Cao, Encai Hao, Gabriella S. Métraux, George C. Schatz, and Chad A. Mirkin Department of Chemistry and Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208, USA Outline of Supplementary Information 1. Experimental 2. TEM and Spectroscopic Characterization 3. Calculation of Temperature Increase under 550 ± 20 nm Beam Excitation 4. Supporting Figures Referred to in Main Text Figure S1 Electron diffraction and high-resolution TEM images of a Type 2 nanoprism. Figure S2 Laser (532.8 nm) induced conversion of silver nanospheres to nanoprisms. Figure S3 TEM image and theoretical modeling of dimer and trimer intermediates. Figure S4 Optical spectrum of Ag colloid after 550-nm/395-nm dual beam excitation. Figure S5 The emission spectrum of a fluorescent tube. Figure S6 Nanoprism growth when spherical Ag particles (4.8 ± 1.1 nm) are added to an existing colloid of nanoprisms (edge length = 38 ± 7 nm).

2 2 Supplementary Information 1. Experimental Chemicals. AgNO 3 (99.998%), trisodium citrate dihydrate (99.9%), and NaBH 4 (99%) were purchased from Aldrich. Bis (p-sulfonatophenyl) phenylphosphine dihydrate dipotassium (BSPP) was purchased from Strem Chemicals, Inc. All H 2 O was purified by a Barnstead Nanopure H 2 O purification system (resistance = 18.1 MSAcm). Synthesis of Ag Colloids. The photoinduced synthesis of Ag nanoprisms involves two general steps, including preparation of a colloidal suspension of Ag nanospheres (diameter < 10 nm) and subsequent conversion to larger prism structures with visible light. In our previous work, an Ag colloid (8 ± 1.7 nm) was prepared in air at room temperature (referred to as protocol 1) 23. Since that report, we have found that the initial particle size, monodispersity, and surface properties are critical for subsequent high-yield conversion to nanoprisms. In particular, large Ag particles (i.e. > 15 nm in diameter) are not efficiently converted to nanoprisms under the conditions explored. In this work, we modified protocol 1 for the preparation of the Ag colloid by adopting a low temperature method (see, for example, Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740). This method (referred to as protocol 2) allows for better control over Ag particle size. A typical experiment is as follows. Nanopure H 2 O (95 ml), aqueous trisodium citrate (1 ml, 30 mm, freshly prepared), and aqueous AgNO 3 (2 ml, 5 mm, freshly prepared) were mixed in a 250 ml three-neck flask. The flask was immersed in an ice bath, and the solution was bubbled with Ar under constant stirring for ~30 min. Aqueous NaBH 4 (1 ml, 50 mm, freshly made prior to injection by adding ice-cold nanopure water to NaBH 4 ) was quickly injected into the vigorously stirred, ice-cold solution. The clear solution immediately turned light yellow. The reaction was allowed to proceed for ~15 min, and during this time, 3-5 drops of NaBH 4 solution were added every two minutes to the solution to ensure complete reduction of the Ag + ions in the solution.

3 3 Then, BSPP solution (1 ml, 5 mm, freshly made) and NaBH 4 (0.5 ml, 50 mm) were added to the solution in a dropwise fashion over a 5 min time period. The resulting Ag colloid was aged overnight under stirring in the dark. TEM analysis showed that the as-prepared particles have an average diameter of 4.8 ± 1.1 nm. Note that the BSPP solution gradually degrades in air, which is accompanied by a solution color change from clear to yellow, and this degradation process is accelerated under light. Since trisodium citrate is not an efficient stabilizer of silver colloids, we add BSPP as a second ligand to further stablize the Ag nanoparticles. BSPP was selected because of its moderate binding affinity to silver, and because it allows one to gain better control over the initial Ag particle size and dispersity. Note that strongly binding ligands (such as amines or thiols) do not work as well for the photoinduced conversion of Ag spheres to prisms. It has been reported that citrate stablized Ag nanoparticles undergo particle aggregation under intense light or laser excitation (see, for example, Karpov S. V., et al. Colloid Journal 1997, 59, ). In contrast, we found that Ag particles passivated with citrate/bspp show sufficient stability and allow for photoinduced conversion of spheres to prism structures. Ag colloids stablized with citrate (no BSPP) with broad size distributions of particles (a few nms to several tens of nms) can be converted to nanoprisms but only after removing particles (>15 nm) by centrifuging the suspension at 18,000 rpm for 30 min. Photoinduced Conversion Experiments. In our previous work 23, we used a conventional fluorescent light tube (e.g. 40 W, white daylight type, General Electric, see Supplementary Figure S5 for its emission spectrum) to effect the sphere to prism transformation. To investigate the effect of wavelength on the conversion reaction, we utilized a xenon lamp as the light source (Novalight system, 150 W, light output ~ 12 W, Photon Technology, Inc.). The xenon lamp source shows nearly flat emission in the visible range and is thus ideal for experiments that involve different wavelengths. Optical band filters (diameter = 25 mm, band width = 10 nm or 40 nm) were obtained

4 4 from Intor, Inc. The photoconversion of nanospheres to nanoprisms was performed in either a glass or quartz cell (the latter is used in double beam experiments when light (< 400 nm) is introduced). The Ag colloid was sealed in the reactor, which was wrapped with aluminum foil. For the single beam excitation experiment, the 550 ± 20 nm beam (green, ~100,000 Lux, measured with a digital light meter, Model LM-1, Family Defense Products) was introduced to the Ag colloid through a hole (ca. 20 mm in diameter) in the aluminum wrap; the distance between the reactor and the light output window was 8~ 10 cm. For the laser excitation experiment, a laser beam (532.8 nm, CW, light output ~ 0.2 W, Nd:YAG) was introduced to a glass bottle containing the Ag colloid (~10 ml, approx. 20 cm away from the laser output window). The laser beam was back-scattered with diffusing glass chips to illuminate the entire colloidal solution. For the double beam excitation experiment, two holes (~ 20 mm in diameter) were made in the aluminum wrap of the quartz cell, and two beams (the 550 ± 20 nm primary beam with irradiance of approx. 80,000 Lux, and the 450 ± 5 nm beam with irradiance of approx. 1,000 Lux) from two Xe lamps were simultaneously introduced to the Ag colloid. The double beam cooperativity also was tested by exposing the Ag colloid (with or without the 550 ± 20 nm optical filter) to a conventional fluorescent light tube (tube ~1m, sample-to-tube distance ~10 cm). To prepare different sized (edge length) Ag nanoprisms, a primary beam (450 nm (ca. 14,000 Lux, without correction for wavelength sensitivity factor, same below), 490 nm (ca. 30,000 Lux), 520 nm (ca. 50,000 Lux), 550 nm (ca. 80,000 Lux), 650 nm (ca Lux), and 750 nm (beyond the spectral range of the light meter Model LM-1), respectively, width = 40 nm) coupled with a secondary beam (340 nm, width = 10 nm) from two xenon lamps, respectively, was used to photolyze the Ag colloid. The primary beam was used to control the nanoprism size (edge length) while the second beam was used to effect unimodal growth. For primary excitation wavelengths >500 nm, the nanoprism size distribution could be improved by first aging the Ag colloid under

5 5 fluorescent light until the 670 nm peak begins to appear and then initiating the two beam experiment. All of the photosynthetic reactions were monitored by UV-VIS-NIR spectroscopy by sampling aliquots of the reaction mixture every 2-3 hours. The reaction was stopped after no apparent changes were observed in the spectrum, which takes approx. 20~ 50 hours (Note that the conversion time may change significantly with different excitation wavelengths and intensities, initial particle size, and the chemical history of the initial Ag colloid (e.g. aging time). Finally, qualitatively comparable results (e.g. bimodal and unimodal growth) were observed for spherical Ag particles prepared via protocol 1 and TEM and Spectroscopic Characterizations TEM imaging was performed with a 200 kv Hitachi H8100. Approximately 400 particles were used for the particle size statistical analyses. High-resolution TEM imaging was carried out with a 200 kv field-emission Hitachi HF 2000 electron microscope equipped with a Gatan Imaging System. UV-VIS-NIR spectroscopic measurements of colloids were performed with a Cary 500 spectrometer. The emission spectrum of a fluorescent tube (white daylight type, Philips TLD 36W/865 or General Electric 40 W) was measured with a HP 8453 diode array spectrophotometer, and is given in arbitrary units for the nm range. 3. Calculation of Temperature Rise under 550 ± 20 nm Beam Excitation (~ 0.2 Watt) The parameters for the Ag colloid (100 ml, Ag atomic concentration = 0.1 mm); The volume of a Type 1 prism (edge length = 70 nm, thickness = 10 nm): cm 3 ; The mass of a Type 1 prism = (cm 3 ) 10.5 (g/cm 3 ) = g; The number of Type 1 prisms in 100 ml of colloid = ; The energy of a 550-nm photon = 1240 (evanm)/550 (nm) = 2.25 ev = J The 550-nm photon flux = 0.2 (J/s)/ (J/photon) = photons/sec;

6 6 Bulk silver specific heat capacity = J/g/; The heat capacity of a Type 1 prism = (J/g/K) (g/particle) = J/K In the calculation, it is assumed that the absorbed photon energy is rapidly equilibrated among the conduction electrons, resulting in hot electron gas 29. The hot electrons equilibrate with the phonons on a time scale of a few picoseconds, which leads to a temperature increase in the Ag lattice. The temperature increase of Ag particles under 550 ± 20 nm beam excitation can be estimated by the equation, )T = )H/C p, where, )H = the total absorbed energy, and C p = heat capacity for Ag nanoparticles (assumed to be the bulk value C p = 235 J/(KgOK)). If one photon is absorbed by a Type 1 nanoprism, then )T = photon energy/heat capacity = K. In a second calculation, we assume that the beam energy (beam power = 0.2 W, measured by a light meter) is 100% absorbed (for estimation of maximum temperature increase), and that there is a 1 picosecond time scale for heat transfer from the surface plasmon excited state to the Ag lattice (electron-phonon coupling time = a few picosecond, see, for example: Link, S., Burda, C., Wang, Z. L., El-Sayed, M. A. J. Chem. Phys. 1999, 111, 1255), and during this time there is no heat dissipation to the surroundings. In this case, )H = 0.2 (W) (s), C p = (JOK -1 Og -1 ) 0.1 (L) (molol -1 ) 108 (gomol -1 ), thus, )T = 10-9 K. Note that once the electrons and lattice have reached equilibrium, the heat is finally dissipated into the surroundings (water and air) by phonon-phonon couplings. Energy storing by the Ag lattice as temperature increases is negligible, because the photon flux in our experiments is extremely low, and the Ag lattice can efficiently dissipate heat to the surroundings. In addition, multiple photon absorption is statistically negligible due to the extremely low photon flux. 4. Supporting Figures Referred to in Main Text (arranged in the order of appearance in main text)

7 A 7 [111] zone Electron beam 220 (422) (101) (111) (011) B o d 220 = 1.44 A (110) [110] zone C o d 111 = 2.36 A 2 nm Figure S1 (A) A representative diffraction pattern for a Type 2 nanoprism (the [111] zone axis). (B) A high-resolution TEM image of a Type 2 nanoprism (the [111] zone). The hexagonal lattice image shows a spacing of 1.44 D, indexed as {220} of fcc Ag. (C) A high-resolution image of Ag nanoprism stacks showing lattice images with 2.36 D spacing, indexed as {111} of fcc Ag (JCPDS card number: ).

8 8 Extinction Initial Ag colloid After nm laser induced conversion A Wavelength (nm) B 200 nm Figure S2 (A) UV-VIS-NIR spectra of a silver colloid (4.8 ± 1.1 nm) before (dash line) and after (solid line) excitation with a nm laser beam (Nd:YAG, ~ 0.2 W). (B) TEM image of the resulting nanoprisms after laser induced conversion shows a bimodal size distribution.

9 9 A 200 nm 10 Model for DDA calculation B 20 Model for DDA calculation C Extinction 5 70 nm (Thickness =10 nm) nm (Thickness =10 nm) Wavelength (nm) Wavelength (nm) Figure S3 (A) TEM image shows dimer and trimer intermediate species (as shown in Scheme 1) observed at the early stages of the Type 2 particle growth. These species are mostly likely to be the intermediates in the edge-selective fusion growth process. (B) and (C) Theoretical modeling of the optical spectra of dimer and trimer species.

10 Ag colloid 550 ± 20 nm 395 ± 5nm Extinction Wavelength (nm) Figure S4 Optical spectrum of Ag colloid after dual beam 550-nm/395-nm excitation. The 395-nm corresponds to the dipole plasmon of Ag nanospheres. Coupled beam excitation does not result in unimodal growth process.

11 11 Conventional fluorescent tube (cool white type) 546 nm Irradiance (a.u.) 440 nm 610 nm Wavelength (nm) Figure S5 The emission spectrum of a fluorescent tube (cool white type). The 546-nm and 440-nm bands have the appropriate intensity ratio (100% : 40%) to effect photosynthetic cooperativity and hence unimodal growth. The 610-nm band does not contribute to the photocooperativity.

12 12 A) Ag nanospheres (4.8 ± 1.1 nm) Nanoprism growth increase in size hν=450/340-nm B) No change in size but increase in number Extinction (a.u.) After addition and converting of Ag nanospheres into nanoprisms Colloid of Ag nanoprisms Wavelength (nm) Figure S6 (A) Scheme for two possible nanoprism growth routes when spherical Ag particles (4.8 ± 1.1 nm) are added to an existing colloid of nanoprisms (edge length = 38 ± 7 nm). In principle, the new spherical particles can be used to grow larger prisms or produce more of the 38 nm nanoprisms. The experimentally observed result is the latter (see B ). (B) The UV-VIS-NIR spectrum of the colloid after the Ag nanospheres described in A have been completely converted into nanoprisms (red line) is almost identical to spectrum for the 38 nm starting prisms (black line). Ag nanospheres (added) : existing nanoprisms = 4 : 1 (atomic ratio).