Supporting Information Hybrid Au-Ag Nanostructures for Enhanced Plasmon-Driven Catalytic Selective Hydrogenation through Visible Light Irradiation and Surface-Enhanced Raman Scattering Zhen Yin,*, Ye Wang, Chuqiao Song, Liheng Zheng, Na Ma, Xi Liu, Siwei Li, Lili Lin, Mengzhu Li, Yao Xu, Weizhen Li, Gang Hu, Zheyu Fang, and Ding Ma*, State Key Laboratory of Separation Membranes and Membrane Processes, School of Environmental and Chemical Engineering, Tianjin Polytechnic University, 399 Binshui West Road, Tianjin 300387, China, Email: yinzhen@tjpu.edu.cn Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China, Email: dma@pku.edu.cn School of Physics, State Key Laboratory for Mesoscopic Physics, Peking University, Beijing,100871, China School of Materials Science and Engineering, Tianjin Polytechnic University, 399 Binshui West Road, Tianjin 300387, China SynCat@Beijing, Synfuels China Technology Co., Ltd, Beijing, 101407, China Chemicals. Hydrogen tetrachloroaurate(iii) trihydrate (HAuCl 4 3H 2 O, 99.9%) and 1,4-benzenedithiol (1,4-BDT, 97%) were ordered from Alfa Aesar. Other chemicals used here were purchased from Beijing Chemicals and Sino-pharm. All chemical regents were of analytical grade. All solutions were prepared in deionized water (DI water, 18.2 MΩ cm) from a Thermo Scientific Nanopure system. Experimental Section Synthesis of Au NPs with different average size. For the synthesis of Au NPs (d av = 13 and 25 nm), the traditional citrate reduction method was adopted. 1 In a typical procedure for Au13 NPs, HAuCl 4 solution (25.4 mm) was added to 50 ml H 2 O in a 100 ml round bottle flask equipped with a condenser. Then, 5 ml sodium citrate solution (38.8 mm) was added into the boiling HAuCl 4 solution. The color of the resulting solution changed from red to dark gray to black and then to purple. About 3-5 min later, the solution changed to red. The reaction mixture was refluxed for 30 1
min in an oil bath. The following procedures were carried out using this seed solution without isolation step. Synthesis of Au@Ag core-shell NPs. Au@Ag core-shell NPs were synthesized by using seed mediated approach adapted from known literature procedures. 2 The synthesis begins with the preparation of citrate-stabilized Au-seed NPs. In a representative synthesis for Au(13)@Ag(9S) NPs with Au13 NPs as core, 50 ml solution of AgNO 3 (10 mg) and sodium citrate (40 mg) were prepared. The seed solution (5 ml) was added quickly with vigorous stirring. Then, a solution of ascorbic acid (20 mg in 10 ml water) was added dropwise in about 5 min, and the mixture was stirred for an additional 1 h. A brownish-yellow Au@Ag core-shell NPs solution was obtained. The average particle diameter was measured to be 30.5 ± 1.5 nm from TEM pictures. The similar procedure was taken to prepare the core-satellites Au^Ag NPs with less amount of the AgNO 3. 1D NCs of Au@Ag NPs via self-assembly. For the assembly of Au(13)@Ag(9S) NPs (Au13 core) as a typical process, 1.5 ml NPs solution was taken and then centrifugated at 10 000 rpm (about 6,200 g) for 10 min. And then, the supernatant was removed carefully with the pipette. (One point must be mentioned that the centrifugation speed was very important for the dispersion and assembly of the hybrid Au-Ag NPs. If the sediment of NPs were difficult to disperse without severe ultrasonic treatment, the assembly process was hard to realize.) The NaCl aqueous solution (5.0 mm, ca. 40-200 μl) and water was added to the precipitate (ca. 1-2 μl) before the addition of ethanol (1.8 ml). After the introduction of ethanol, the mixture solution was allowed to keep still for 30-90 min. Subsequently, a solution of ammonia in ethanol (0.5 ml, 4.2 vol % ammonia in ethanol) was added quickly. The final NaCl concentration in the mixed solution can be controlled from 0.1-0.5 mm according to the necessary amount of salt based on the different NPs size. The self-assembly process could be observed through the color change clearly. The similar procedure was taken to get the 1D nanochains with other core-shell Au@Ag and core-satellites Au25^Ag NPs. 2
Characterization. UV-visible (UV-vis) spectra were recorded using a Perkin-Elmer Lambda 35. TEM and HR-TEM imaging was performed on the FEI Tecnai G2 F30 operated at 300 kv equipped with the energy dispersive X-ray (EDX) system from EDAX Inc. that has a point resolution of 0.20 nm. Electron microscope specimens were prepared by dispersing the particles solution in water or ethanol and drop-casting it onto TEM copper-grids. The particle size and shell thickness were obtained by analyzing at least 200 particles on the TEM images using the Gatan Digital micrograph software. High-angle annular dark-field scanning TEM (HAADF-STEM), and energy dispersive X-ray (EDX) analyses were collected on a FEI Talos 200A field-emission transmission electron microscope operating at 200 kv accelerating voltage using Cu TEM grids. The particle concentrations were quantified by ICP atomic emission spectrometer (ICP-AES, Profile Spec, Leeman, USA). General procedure for photocatalytic hydrogenation of o-chloronitrobenzene. In a typical catalytic reaction, o-chloronitrobenzene, the NPs or NCs suspension in water (10.0 ml) and ethanol (10.0 ml) were mixed together and placed into photocatalytic autoclave. In order to remove the air from the reaction system, the autoclave was purged with H 2 three times before the catalytic reaction. Then, the autoclave was pressurized with 3 MPa. The catalytic reactions were carried out under light irradiation. The light irradiation was carried out using a Xe lamp (Perfect, PLS-SXE300, China) at 2.5 W cm 2 with a 420 nm cutoff filter (wavelengths of 420 1100 nm). During the catalytic experiment, the pressure of H 2 was maintained constant and kept stirring. After the reaction, the mixture was extracted with dichloromethane and analyzed by GC-MS (Agilent 5975C with triple-axis detector) and GC (Agilent, 7820A) equipped with a flame ionization detector (FID). In addition, dodecane was used as an internal standard. We used illumination by wavelengths in the range of 450 700 nm (450 nm, 475 nm, 500 nm, 520 nm, 550 nm, 600 nm, 650 nm and 700 nm) with the spectrally filtered bands with 15 nm full-width at half-maximum in order to investigate the relationship between the reaction efficiency and the wavelengths of irradiated light. Reaction conditions: 0.5 mmol o-chloronitrobenzene, water 10 ml and ethanol 10 ml 3
as solvent, the amount of Au and Ag were maintained at 0.3 mg, temperature 65, H 2 pressure 3 MPa, time 10-24 h. Turnover frequency (TOF) calculations. TOF of the catalyst = (mole of o-chloronitrobenzene conversion) / (mole of surface atoms of Au or Ag reaction time). The surface atoms can be calculated based on the metal dispersion. 3 The metal dispersion, D M, or fraction exposed of a metal catalyst is the ratio of the number of surface metal atoms to the total number of metal atoms: D M = N surface atoms /N total atoms, i.e., the fraction of metal atoms at the surface. Hence, we calculated the surface atoms according to the equation given by Vannice 3a,4 : where V M is the bulk atomic volume of the metals (cm 3 ), A M is the area of an atom (cm 2 ), and d is the metal particle size in nm. The radius of Au and Ag atom is r Au = r Ag = 0.144 nm. The average size of NPs from the TEM was used for the d value. For the Au@Ag NPs, the surface atoms of Ag were calculated and used for TOF. For the core-satellite NPs, the total surface atoms were based on the surface atoms of Au and Ag (4 nm size) and then used for TOF. Theoretical calculation. FDTD simulations were performed to calculate the extinction spectra and the near-field enhancement by using the commercial software FDTD Solutions (Lumerical Solutions Inc.). For all simulated nanostructures excitation with 532 nm was assumed. The hybrid NPs and NCs of Au@Ag were modelled as follows: 4.0 nm Ag shell coating on the 25 nm Au NPs, the gap in the NCs was 0.5 nm and 1 nm, respectively. Raman spectroscopy and characterization. The Raman spectra were collected from the sample solution in a liquid cell with Horiba Labram Aramis spectrometer using a red light-emitting diode (LED) laser (λ=785 nm) and a 10 objective lens. The SERS activities were recorded from the solutions of NCs or NPs in order to average out inhomogeneity in the SERS of adsorbed molecules and minimize the photodamage of probe molecules. The SERS spectra were recorded using 785 nm excitation in order to 4
avoid plasmon dephasing associated with the interband transition of Au at 2.5 ev ( 500 nm), in conjunction with a grating of 1,200 lines/mm. The Raman spectra were recorded at 19 mw with a collection time of 30 s for all samples. The glass coverslips (0.2-0.15 mm) were carefully placed on top of the liquid vessel to eliminate solvent evaporation and to act as a reference point from which the focal volume was lowered to a depth of 200 μm into the sample. Surface-enhanced Raman scattering measurements and calculation of enhancement factor (EF): The NPs or NCs were incubated with a 0.5 mm 1,4-BDT solution in ethanol for 1~2 h. The nanostructures were then washed with DI water twice and re-dispersed in water at a particle number concentration of ca. 0.5-1 nm. The calculation of an average EF followed other previous reports. 5 Briefly, the TEM analysis was used to determine the average diameter of a sample of NPs and this dimension was then used to calculate the average surface area and volume expected from a single nanoparticle in the sample. Here we assume that all NPs in a given sample had a uniform diameter with a regular spherical shape, although in some cases this was not strictly true. The volume of the NP is used to determine the number of Au atoms and mass of the nanoparticle. This information can be used with the Au concentration of the NPs sample determined from ICP-MS to estimate the concentration of particles in solution. The surface area of the NP is used with the NP concentration to determine the surface-area per volume of the NP solution, which can be used to determine the number of adsorbed 1,4-BDT molecules in the sample. The SERS peak at 1565 cm -1 (the benzene ring mode, 8a) of 1,4-BDT molecule was employed to calculate the SERS EF using the following equation: average EF I I SERS Bulk N N Bulk ads where I SERS and I Bulk are the intensities of the same band for the SERS and ordinary spectra from a bulk sample, N Bulk is the number of bulk molecules probed for a bulk sample, and N ads is the number of adsorbed molecules probed in SERS. I SERS and I Bulk were determined by the intensity of 8a bands. N bulk was determined from the ordinary Raman spectrum of a 0.1 M 1,4-BDT solution in 12 M aqueous NaOH and the focal 5
volume of our Raman system. We have assumed the focal or scattering volume is constant due to the identical acquisition parameters between samples. So, it is effectively cancelled out from these calculations. Hence, the final equation can be expressed with concentrations (mol/l) as the following equation: I average EF I SERS Bulk C C Bulk ads 6
Figure S1. Typical TEM images and UV-vis absorption spectra of Au NPs. (A) Au13 (d av = 13 nm); (B) Au25 (d av = 25 nm); (D) UV-vis absorption spectra of the solution of the Au NPs. Figure S2. Typical TEM images of the Au(13)@Ag(2S) (left) and Au(13)@Ag(9S) (right) NPs with Au13 NPs as core. 7
Figure S3. Typical TEM images of the Au(25)^Ag (left) and Au(25)@Ag(4S) (right) NPs with Au25 NPs as core. Figure S4. EDX elements analysis of the Au(13)@Ag(9S) NPs with ~9 nm Ag shell. 8
Figure S5. Photos of the solution of the isolated Au(13)@Ag(2S) (A) and Au(13)@Ag(9S) (B) NPs. Figure S6. Photos of the solution of isolated Au(25)@Ag(4S) (A) and Au25^Ag (B) NPs. 9
Figure S7. Typical HR-TEM image of the Au(13)@Ag(2S) NPs with Au13 NPs as core and around 2 nm Ag shell. Figure S8. Typical HR-TEM image of the Au(13)@Ag(9S) NPs. 10
Figure S9. Typical TEM (A), HR-TEM (B), HAADF-STEM (C and E) and (D, F-H) 11
EDS mapping images (red color, Au; green color, Ag) of the Au(13)@Ag(2S) NCs. STEM-EDX maps (D and H) shown as an overlay of gold and silver signals, further confirming the core shell structure. Figure S10. Experimental (line) and calculated (symbol) absorption spectra of Au and Au@Ag NPs. The changes to the LSPR peaks are consistent with the results of experimental measurements and FDTD calculations. 12
Figure S11. Photos of the solution of isolated Au(13)@Ag(2S) NPs (Left) and 1D Au(13)@Ag(2S) NCs (Right). Figure S12. Photos of the solution of isolated Au(13)@Ag(9S) NPs and 1D Au(13)@Ag(9S) NCs (Right). 13
Discussion about the role of NaCl and ammonia: As the surface plasmon resonance at the Au/Ag interface is very sensitive to the electronic coupling between NPs, a spectroscopic method was used to monitor the chain formation process in situ. Using the assembly of Au(13)@Ag(9S) NPs as an example, the addition of NaCl (0.5 mm in the final solution) and ammonia into the ethanol/h 2 O solution of the NPs resulted in a gradual shift in the absorption band due to the self-assembly of the NPs. The plasmonic signal originally located at approximately 400 nm decreased in strength, whereas a new shoulder at longer wavelength started to develop and eventually evolved into a distinct feature (approximately 660 nm) after 210 min (Fig. S13). This new signal represented the longitudinal plasmon coupling between the Au(13)@Ag(9S) NPs, indicating the successful construction of the chainlike structure. Similar phenomenon can also be observed in the assembly process of the Au(13)@Ag(2S) NPs (Fig. S14). Actually, the role of salt and ammonia in an ethanolic medium can be discussed based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (Fig. S15-18). The salt concentration plays a pivotal role in the self-assembly process described herein. More importantly, the addition of ammonia can reduce the surface charge density by altering the ionic strength and/or the dielectric constant of the surrounding medium, which allows the controllable propagation of the 1D NCs. 14
Figure S13. UV-vis absorption spectra evolution of the Au(13)@Ag(9S) NPs after addition of ammonia during the self-assembly process. Figure S14. UV-vis absorption spectra evolution of the Au(13)@Ag(2S) NPs during the self-assembly process. 15
1) Role of the NaCl: Systematic investigations were carried out in order to understand the assembly behavior of Au(13)@Ag(9S) NPs with different NaCl concentrations ranging from 0 to 1 mm in the solution of ethanol/h 2 O (Fig. S15-17). Without the presence of NaCl, little difference was visible on the absorption spectra, indicating that no assembly happened even when the time was extended to 3 days (Fig. S15a). As shown in Fig. S16, the color of the solution didn t show any change even after 3 days. If 0.2 mm NaCl or below was added, no obvious change of the absorption spectra and solution color can be shown (Fig. S15b and S16). However, when the concentration of NaCl was 0.3 mm or above, the color of the solution have some change, consistent with the change of UV-vis spectra due to the sluggish aggregation in Fig. S15c. Moreover, when the salt in final solution reached a critical concentration (> 0.3 mm), the NPs became unstable and tended to aggregate. If the salt concentration went up to 1 mm, the dispersing assemblies would precipitate in a short time (Fig. S15d). Similar results can be obtained during the Au(13)@Ag(2S) assembly process (Fig. S18). 2) Role of the ammonia: It can accelerate the assembly process and induce the controllable propagation of the 1D NCs (Fig. S18). 16
Figure S15. UV-vis absorption spectra evolution of the Au(13)@Ag(9S) NPs with different NaCl concentration during the self-assembly process. Figure S16. Photo of the solution of Au(13)@Ag(9S) NPs with different NaCl concentration in the mixture of ethanol/water after 3 days. 17
Figure S17. Photo of the solution of Au(13)@Ag(9S) NPs assemblies with different NaCl concentration in the mixture of ethanol and water. The incubation time: 30min (A); 20 min (B). From the color change of the solution, it can be concluded that the salt concentration plays an important role in the assembly of NPs in the present system. The assembly could not be triggered until there was enough salt in the system. However, the excessive salt would cause the instability of assemblies and then aggregate. Figure S18. Photos of the solution of Au(13)@Ag(2S) NPs assemblies with different NaCl concentration in the mixture of ethanol/water and ammonia after 2 hours. 18
Figure S19. Typical HR-TEM image of the Au25^Ag NPs with Au25 NPs as core and ~ 10 nm or 3 nm Ag NPs as satellite. Figure S20. Photo of the solution of isolated Au25^Ag NPs (Left) and 1D Au25^Ag NCs (Right). 19
Figure S21. Typical HR-TEM image of the Au(25)@Ag(4S) NPs with Au25 NPs as core and around 4 nm Ag shell. 20
Figure S22. Typical TEM (a), HAADF-STEM (b and c) and (d-f) EDS mapping images (red, Au; green, Ag) of the Au(25)@Ag(4S) NCs. STEM-EDX maps (b and f) shown as an overlay of gold and silver signals, further confirming the core-shell structure. 21
Figure S23. Photo of the solution of isolated Au(25)@Ag(4S) NPs and 1D Au(25)@Ag(4S) NCs (Right). 22
Figure S24. Spectrum of the light source used for catalyst illumination with UV-cut filter. Figure S25. Influence of light wavelength on the photocatalytic activity of o-chloronitrobenzene hydrogenation, along with the absorption spectra of the Au(25)@Ag(4S) NCs. 23
Figure S26. Simulated near-field electromagnetic field distributions of the Au@Ag hybrid nanostructures irradiated by an incident plane wave with wavelength of 532 nm: (a) Au25 NPs; (b) Au(25)@Ag(4S) NPs; 1D Au(25)@Ag(4S) NCs with different separation distance: 0.5 nm (c1 and c2); 1 nm (d1 and d2). FDTD calculations displayed a significant increase (10 2 ~10 3 ) of the near-field electromagnetic fields (described by E 2 / E 0 2 ) between NPs upon assembly process, confirming the existence of the plasmonic hot spots. Compared with the isolated Au@Ag NPs, the enhancement of the electromagnetic fields between NPs in the 1D NCs was about 2500, 400 times with the 0.5 and 1 nm gap, respectively. 24
Figure S27. SERS spectra taken from aqueous suspensions of 1,4-BDT-functionalized with the Au25^Ag NCs, Au25@Ag(4S) NCs and individual NPs. 25
Figure S28. The normal Raman spectra taken from 1,4-BDT in aqueous NaOH solution. 26
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