Supporting Information for Facile Synthesis of Gold Wavy Nanowires and Investigation of Their Growth Mechanism Cun Zhu,, Hsin-Chieh Peng, Jie Zeng, Jingyue Liu, Zhongze Gu and Younan Xia,,* The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States State Key Laboratory of Bioelectronics, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, P. R. China School of Chemistry and Biochemistry, School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China Department of Physics, Arizona State University, Tempe, Arizona 85287, United States * To whom correspondence should be addressed. E-mail: younan.xia@bme.gatech.edu S1
Experimental Section Chemicals and Materials. Gold(III) chloride hydrate (HCl 4 H 2 O), L-ascorbic acid (AA), cetyltrimethylammonium bromide (), cetyltrimethylammonium chloride (CTAC), sodium hydroxide (NaOH), sodium borohydride (NaBH 4 ), and silver nitrate (AgNO 3 ) were all purchased from Sigma-Aldrich. Deionized water with a resistivity of 18.2 MΩ cm was used for all the syntheses. All chemicals were used as received. Synthesis of Wavy Nanowires. In a standard synthesis, 10 ml of aqueous AA solution (8 mm) was slowly added into 190 ml of an aqueous solution containing HCl 4 (0.21 mm) and (31.6 mm) with a pipette, followed by the introduction of 200 ml of aqueous NaOH solution (0.2 mm). The resultant solution was kept undisturbed (no stirring was applied) at room temperature for 24 h. Synthesis of Nanorods. The nanorods with an average aspect ratio of 3 were synthesized using a two-step procedure according to a previous report with some minor modifications. 15b First, seeds were synthesized by injecting 0.6 ml of ice-cooled aqueous NaBH 4 solution (10 mm) into 10 ml of an aqueous solution containing HCl 4 (0.25 mm) and (100 mm) with a pipette. The seed solution was kept undisturbed at room temperature for 3 h. The resultant seeds were used for a subsequent synthesis of nanorods. In a typical synthesis, a reaction solution was prepared in a 25 ml flask by adding solutions in the following order: 4.75 ml of aqueous solution (100 mm), 0.2 ml of aqueous HCl 4 solution (10 mm), 30 µl of aqueous AgNO 3 solution (10 mm), 32 µl of aqueous AA solution (100 mm), and finally the introduction of 10 µl of the as-prepared seeds. The resultant mixture was kept undisturbed at room temperature for 3 h. Characterizations. Scanning electron microscopy (SEM) images were taken using a Nova NanoSEM 230 microscopy (FEI, Hillsboro, OR) operated at 10 kv. Transmission electron microscopy (TEM) images were taken using a Tecnai G2 Spirit Twin microscope (FEI, Hillsboro, OR) operated at 120 kv. High-resolution TEM images were performed using a JEOL 2100F microscope (JEOL, Tokyo, Japan) operated at 200 kv. XPS survey spectra were taken using a K-Alpha X-ray Photoelectron Spectrometer (Thermo Scientific, USA). S2
TGA was performed using a Q500 thermo-gravimetric analyzer (TA Instruments, USA). Photographs of the reaction solutions were taken using a digital camera (Canon S95). Calculation of the Number of Layers on the Surface of a Wavy Nanowire. Take the nanowires synthesized using the standard protocol as an example, their average radius (r ) and length (L ) were estimate to be 7.5 nm and 75 µm, respectively. The area occupied by a single headgroup (S,0 ) on the surface was estimated to be 0.64 nm 2. 13c If the molecules assembled into a perfect monolayer on the surface of wavy nanowires, the weight percentage of should be: m = = = ρ ρ ρ % = m S * V * πr S * r 2 S,0 * N A S + S 2πr S * L 2 *,0,0,0 + S m + m * N * L * N A 2πr * L + S * N N A 2 * N,0 =,0 n * V + n 2 *364.45 18 23 = 0.64*10 *6.02*10 6 9 2 19.3*10 * 7.5*10 + 18 0.64 *10 *6.02 *10 A A ρ A 23 = 1.29% *364.45 Here N A is to the Avogadro's number, S is the side surface area of a nanowire (the end surface areas can be neglected owing to the relatively large aspect ratio), V is the volume of the nanowire. Both S and V were calculated using a column model. The weight loss of measured from TGA was about 0.82% (Fig. S10), indicating that the molecules only assembled into a monolayer or even a sub-monolayer on the surface of the wavy nanowires. S3
Figure S1. (A, B) SEM images of wavy nanowires with low aspect ratios that were prepared by changing the concentrations of and precursor to 75.8 mm and 0.105 mm, respectively, while the rest of the reaction conditions was kept the same as the standard procedure. The average diameter of the products was 25 nm, with aspect ratios in the range of 10 2-10 3. S4
Figure S2. (A) TEM and (B) SEM image of a typical sample of wavy nanowires, with arrows indicating the twin boundaries. Multiple crystal domains and twin boundaries could be observed in the straight regions of the wavy nanowires. Due to the existence of a large number of kinks, the nanowires exhibited a wavy appearance. S5
Figure S3. SEM image of the nanostructures obtained after the reaction had proceeded for 35 min (corresponding to the sample in Figure 2D). The rods were aligned more or less along the same trajectories as the wires, indicating that nanowires had probably formed by this time point. However, the nanowires tended to break into short segments owing to melting of the freshly formed connection regions between the rods when the sample was subjected to heating by the electron beam. This observation also provides another evidence to support the proposed growth mechanism based on particle attachment. S6
Figure S4. (A, C) TEM images of wavy nanowires obtained after the reaction had proceeded for (A) 2, and (C) 24 h, respectively. (B, D) SEM images of the corresponding samples in (A) and (C), respectively. Since the nanowires had grown for a sufficiently long period of time, the connection regions became robust enough to sustain electron beam heating. Note that kinks still existed in some regions of the nanowires obtained at t=24 h, giving them a wavy appearance. S7
Figure S5. High-resolution TEM images of the nanostructures obtained in the early stage of a synthesis, detailing the initially formed connection regions : (A) two particles were bridged by a more or less amorphous region formed by additionally deposited atoms; (B) single-crystal connection region likely developed from the attachment of particles with the same orientation; and (C) defected connection region developed from the attachment of particles with different orientations. The insets show the diffraction patterns taken from different sites of the connection regions as marked with red frames. The participation of nanoparticles with different orientations, as well as the involvement of twinned nanoparticles in the attachment, seems to be responsible for the generation of twin boundaries and kinks in the resultant wavy nanowires. S8
Figure S6. TEM images of nanostructures evolved from different welding configurations in the early stage of a synthesis: (A) head to head, and (B) head to side and side to side, respectively. The arrows indicate the attachment and welding points. All these three kinds of welding configurations could be found in the product of a typical synthesis. However, since the density of molecules on the side surface of rod or necklace-like nanostructures was much higher than that on the end surfaces, attachment and welding of nanostructures in the head to head configuration was most favorable owing to the steric effect, leading to the successful generation of 1D nanowires as a major component in the product. S9
Figure S7. TEM images of (A) dendritic nanowires, and (B) nanosheets that were formed due to head to side and side to side attachment, respectively, during the synthesis. These types of nanostructures (in very small quantities) tended to precipitate to the bottom of the solution due to gravity. S10
Figure S8. Photograph of the nanowires floating at the oil/water interface that were synthesized by using the standard protocol and scaling down the amounts of all chemicals by 20 times. Before the synthesis, we used a mixture of concentrated sulfuric acid and hydrogen peroxide to render the inner surface of the glass vial hydrophilic, We also used nitrogen to remove the dissolve oxygen in de-ionized water prior to the preparation of all solutions. Silicon oil (5 ml) was introduced onto the surface of water with a pipette to generate an oil/water interface. S11
Figure S9. Typical TEM image of nanorods with an average aspect ratio of 3 that were synthesized using a procedure reported by Murphy 15b with some minor modifications. S12
Figure S10. (A) XPS survey spectrum of bulk showing the peaks for Br 3d, C 1s, N 1s, and O 1s; (B) XPS spectrum of the Br 3d region collected with a resolution of 0.1 ev. S13
Figure S11. A typical TGA curve of the wavy nanowires shown in Figure 1. S14
Figure S12. (A) Photograph of the nanostructures floating at the air/water interface that were obtained by replacing with CTAC in the standard synthesis and scaling down the amounts of all chemicals by 20 times, and (B) SEM and TEM (inset, where the scale bar is 2 µm) images of the nanostructures. S15