Communication Cite This: J. Am. Chem. Soc. 2018, 140, 864−867
pubs.acs.org/JACS
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*,‡
Downloaded via KAOHSIUNG MEDICAL UNIV on July 11, 2018 at 09:47:31 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
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 ‡ Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ⊥ 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 S Supporting Information *
One-dimensional (1D) plasmonic nanostructures assembled with the noble metal NPs would be very promising as direct plasmonic photocatalysts, which can offer a unique strategy to tune the LSPRs in and around the plasmonic nanostructure.6a,7 Moreover, the plasmonic coupling resulting from interparticle interactions can further amplify the EM energy in the nearfields centered on the nanostructures.8 However, the assembly of NPs in a controlled fashion is still challenging. Most reported methods for self-assembly require the use of templates, polymers, ligands or molecular linkers as well as intricate postassembly separation processes, which are difficult to reproduce and scale up.7b,8,9 Obviously, these shortcomings makes it difficult to efficiently prepare plasmonic photocatalysts. Recently, the design of a hybrid plasmonic metal nanostructures, in which the core of the plasmonic NP is covered with a very thin shell, has received wide attention as a promising strategy for plasmon-driven chemical conversions.6b,10 However, compared to single component plasmonic Au or Ag NP-driven photocatalytic reactions,1b the Au−Ag hybrid nanostructures are rarely investigated, such as Au@Ag core−shell, which have LSPRs that can be finely tuned by varying the size/shape of the core and the thickness of the shell.11 Moreover, if the Au−Ag hybrid NPs were assembled into 1D nanochains (NCs), the coupling of the plasmonic bimetallic compound with a broader range of resonance frequencies in the visible light region would enhance the catalytic performance of the materials in the light-harvesting photocatalytic systems. Herein, we report a facile and auxiliary agent-free selfassembly strategy for the construction of well-defined hybrid Au−Ag NCs via the introduction of NaCl and ammonia into ethanol/H2O solutions of the NPs. The obtained structures show finely modulated surface plasmon coupling of the Au/Ag bicomponent system. As a proof-of-concept for the direct
ABSTRACT: Herein, we report the successful application of hybrid Au−Ag nanoparticles (NPs) and nanochains (NCs) in the harvesting of visible light energy for selective hydrogenation reactions. For individual Au@Ag NPs with Au25 cores, the conversion and turnover frequency (TOF) are approximately 8 and 10 times higher than those of Au25 NPs, respectively. Notably, after the self-assembly of the Au@Ag NPs, the conversion and TOF of 1D NCs were approximately 2.5 and 2 times higher than those of isolated Au@Ag NPs, respectively, owing to the coupling of surface plasmon and the increase in the rate at which hot (energetic) electrons are generated with the formation of plasmonic hot spots between NPs. Furthermore, the surface-enhanced Raman scattering (SERS) activity of 1D Au@Ag NCs was strengthened by nearly 2 orders of magnitude.
T
he utilization of solar energy to drive catalytic chemical transformations is one of the most promising strategies to substantially reduce the consumption of fossil fuels in the future.1 In recent years, plasmonic catalysis as a new method of chemical conversion has received much attention for the performance of photochemical transformations with plasmonic metal nanoparticles (NPs) under relative low-intensity light.1b,2 Plasmon-driven chemical transformations can proceed on plasmonic metal NPs (Au, Ag) due to their unique optical properties, i.e., localized surface plasmon resonance (LSPR).3 LSPR can generate strong electromagnetic (EM) fields localized around particles and confine or focus electromagnetic energy in their near-fields; this consequently produces various near-field enhancement effects,4 which lead to widespread applications, such as catalysis and surface-enhanced Raman scattering (SERS).2b,5 Thus, the generation and tuning of the LSPR is a crucial element for broadening the applications of plasmonic metal nanostructures.1b,6 © 2018 American Chemical Society
Received: October 23, 2017 Published: January 5, 2018 864
DOI: 10.1021/jacs.7b11293 J. Am. Chem. Soc. 2018, 140, 864−867
Communication
Journal of the American Chemical Society plasmonic photocatalysis, we first confirmed that the hybrid Au−Ag NPs can utilize solar energy to catalyze selective hydrogenations. More importantly, we demonstrated that the Au@Ag NCs are more efficient catalysts than isolated hybrid NPs under visible light. In addition, the application of 1D NCs as nanoprobes for SERS, which generated much stronger SERS signals (by nearly 2 orders of magnitude) than that of the individual NPs, was also investigated. Scheme 1 shows the self-assembly process of hybrid Au−Ag NPs. Different sizes of Au NPs were synthesized by using the
Ag(9S), Figure 1D,E), respectively. A clear difference in contrast (Figure 1C,F and S7−8) in the HR-TEM images can be observed between the Au core and Ag shell. In addition, the high-angle annular dark-field (HAADF) STEM and energy dispersive X-ray spectroscopy (EDX) maps (Figure S9) of the Au(13)@Ag(2S) NCs further confirmed a clear separation of the two elements, indicating existence of the core−shell structure. In the as-prepared product, most of the Au(13)@ Ag(9S) NCs contain 50−100 NPs whereas the Au(13)@ Ag(2S) NCs only contain approximately 20 NPs. As shown in Figure 1B,E, some small gaps between neighboring particles can be observed (mostly in the range of 0.3−1.2 nm with only a few at ca. 2−4 nm), whereas other NPs are closely packed or even merged. In addition, the branched structures were observed in these two NCs. To investigate the effect of the LSPR coupling, the plasmonic property of the core−shell NPs was characterized via UV−vis absorption spectra. For the Au(13)@Ag(2S) NPs (Figure 1G and S10), the UV−vis spectra exhibited two characteristic bands, one at approximately 500 nm and the other at approximately 375 nm, from the Au cores and Ag shells, respectively. For the Au(13)@Ag(9S) NPs (Figure 1H and S10), only dominant Ag absorption band was present at approximately 405 nm, whereas the Au absorption at approximately 520 nm disappeared, suggesting the plasmon excitation of the Au cores was completely shielded by the Ag shells.12 For the 1D NCs, the LSPR spectra displayed distinct bands located at approximately 600 nm for Au(13)@Ag(2S) NCs and at approximately 660 nm for NC Au(13)@Ag(9S) NCs, respectively. Obviously, this new absorption band was the result of the 1D longitudinal plasmon coupling between the Au@Ag NPs, further confirming the successful assembly of core−shell NPs and the formation of the 1D NCs.6a Meanwhile, the color of the NP solution turned from orange red to slate gray for the Au(13)@Ag(2S) and from orange to dark green for the Au(13)@Ag(9S), respectively (Figure S11− 12). The LSPR absorbance can be finely tuned in the visible light region (400−700 nm) due to the plasmon coupling effect. Therefore, the combination of the TEM images and the UV− vis absorption spectra suggest the formation of 1D NCs via linear assembly under the role of salt and ammonia (Figure S13−18).7c,d Meanwhile, we also investigated the assembly of core− satellite NPs, i.e., different numbers of Ag NPs on the Au25 (dav = 25 nm) surface (referred to as Au25^Ag), to obtain the 1D Au25^Ag NCs (Figure 2A−C, S19−20). The LSPR spectra of the Au25^Ag NCs displayed one distinct band located at approximately 660 nm, indicating 1D longitudinal plasmon coupling between NPs was present and the 1D NCs had been prepared (Figure 2G). Furthermore, the NPs of Au25 core with shells ∼4 nm thick (denoted as Au(25)@Ag(4S)) can be used to construct the 1D Au(25)@Ag(4S) NCs, which displayed one new signal located at ∼680 nm (Figure 2D-F, H, S10 and S21−23). Table 1 shows the results of light-promoted reactions using the Au−Ag hybrid catalyst for the selective hydrogenation of ochloronitrobenzene. The results in the dark at same temperature and H2 pressure are also provided for comparison. As shown in Table 1, visible light irradiation (Figure S24) dramatically enhanced the conversion and selectivity of the hydrogenation. For the Au(13)@Ag(2S) NPs, the conversion and TOF reached 13.1% (from 6.2% in the dark) and 2.9 h−1 (from 1.5 h−1 in the dark), approximately 4 and 3 times higher
Scheme 1. Formation of 1D NCs via Self-Assembly Process of Hybrid Au−Ag NPs
classic citrate method (Au NPs with an average size of dav = 13 nm in diameter are referred to as Au13, dav = 13 nm as Au13, and dav = 25 nm as Au25). (See Figure S1 in the Supporting Information.) Then, core−shell Au@Ag or core−satellite Au^Ag NPs were prepared through a seed-mediated growth process. Major advantages of the seed-growth method include that the structure of the resultant Au−Ag NPs can easily be controlled and the average size (dav) can be tuned from nearly 10 to 50 nm by varying the amount of either AgNO3 or Au seeds in the reaction solution (Figure S2−6).11a Figure 1 shows the transmission electron microscopy (TEM) images of 1D NCs assembled from two types of core−shell structures: Au@Ag NPs with ∼2 nm shell (dav = 17 nm, denoted as Au(13)@Ag(2S), Figure 1A,B) and the Au@Ag NPs with ∼9 nm shell (dav = 30 nm, denoted as Au(13)@
Figure 1. Typical TEM, HR-TEM images and UV−vis absorption spectra of the 1D NCs: (A−C, G) Au(13)@Ag(2S) NCs and (D−F, H) Au(13)@Ag(9S) NCs. 865
DOI: 10.1021/jacs.7b11293 J. Am. Chem. Soc. 2018, 140, 864−867
Communication
Journal of the American Chemical Society
irradiation (Figure S25). More importantly, the 1D NCs can significantly improve the activity toward hydrogenation due to the existence of plasmonic hot spots.13 Apparently, the hydrogenation reactions occur on the Au and Au−Ag hybrid nanostructures due to photoexcitation of the LSPR. For the Au NPs, the plasmon-induced hot electrons were generated due to the surface plasmon decay process in Au NPs, which can be transferred into a Feshbach resonance of an H2 molecule adsorbed on the Au NPs, thus triggering the dissociation of a H2 molecule.3b,14 For the Au−Ag hybrid NPs, the Au core acts as a light antenna, inducing near-field enhancements at the interface of Ag atoms and adsorbates that can induce a direct interfacial electronic transition.15 The surface plasmons excited in the Ag shell due to the LSPR coupling of Au and Ag decay into hot electrons with energies between the vacuum level and the work function of Ag, and then the hot electrons can transfer into the orbitals of the adsorbates, thus driving the adsorbates over the activation barriers of hydrogenation. The Au25 core size for the Au@Ag core−shell NPs resulted in better activity due to the balance of the field enhancements and absorbed photon fractions.15 Moreover, decreasing the thickness of the Ag shell can enhance the photocatalytic activity.15 Once the NCs are formed, the EM enhancements and the nonconservation of linear momentum in the gap or hot spot between the NPs can significantly increase the rate at which hot (energetic) electrons are generated, leading to distinct improvements in the photoactivity.1b,13 Moreover, the formation of plasmonic hot spots in the NCs can be confirmed by finite-difference time-domain (FDTD) simulations (Figure S26). In addition, the extended absorption properties of coupled particles in the NCs would be another reason for the enhancement of photocatalytic activity. The SERS spectra of 1,4-benzenedithiol (1,4-BDT) adsorbed on the surfaces of the NCs and NPs of Au25^Ag and Au(25)@ Ag(4S) are shown in Figure S27−28. Compared with individual NPs, the NCs displayed much stronger SERS responses for both the Au(25)@Ag(4S) and Au25^Ag. Due to the existence of NPs junctions, the NCs can provide a number of plasmonic hot spots, thereby providing extremely intense local EM field at the conjunctions, which result in more intense SERS signals.16 The enhancement factor (EF) of the Au(25)@Ag(4S) NCs was roughly calculated to be 2.4 × 107 based on the SERS peak at 1565 cm−1 (the benzene ring mode, 8a);17 this value is nearly 2 orders of magnitude higher than the EF of the original Au(25)@Ag(4S) NPs (4.9 × 105), suggesting a 28-fold and a 51-fold increase of the values previously reported for Ag@Au core−shell nanocubes17a and Ag/Ag homojunction NPs18 in solution phase, respectively. The EF of the Au25^Ag NCs was roughly estimated to be 5.2 × 106, which is nearly 2 orders of magnitude higher than that of individual Au25^Ag NPs (7.8 × 104). The SERS EF of Au(25)@Ag(4S) NCs and Au25^Ag NCs were found to be approximately 1−2 orders of magnitude higher than those reported for hybrid Au−Ag nanostructures.17a,19 For the Au(25)@Ag(4S) NPs, the very thin Ag shell enhanced the SERS activity due to the strong EM enhancement from the Au core. Hence, the Au(25)@Ag(4S) NCs can exhibit the highest SERS EF value since the 1,4-BDT molecules attached to the shell could also be influenced by the strong EM enhancement from the Au core due to thin Ag layers.17a In summary, we have demonstrated the harvesting of visible light energy for chemical reactions using hybrid Au−Ag NPs and NCs assembled by the individual hybrid NPs. Compared to the pristine Au NPs, the 1D hybrid Au−Ag NCs displayed
Figure 2. Typical TEM, HR-TEM images and UV−vis absorption spectra of the 1D NCs: (A-C, G) Au25^Ag NCs; (D-F, H) Au(25)@ Ag(4S) NCs.
Table 1. Selective Hydrogenation of o-Chloronitrobenzene over Au NPs, Au−Ag Hybrid NPs and NCs Catalyst under Visible Light Irradiation and in the Dark Conversion (%)a Entry 1 2 3 4 5 6 7 8 9 10
Catalyst Au13 NPs Au(13)@Ag(9S) NPs Au(13)@Ag(9S) NCs Au(13)@Ag(2S) NPs Au(13)@Ag(2S) NCs Au25 NPs Au25^Ag NPs Au25^Ag NCs Au(25)@Ag(4S) NPs Au(25)@Ag(4S) NCs
Selectivity (%)b
TOF (h−1)
Dark
Light
Dark
Light
Dark
Light
2.7 3.8
3.3 4.2
62.4 68.3
66.8 72.8
0.7 1.2
0.8 2.5
2.9
5.6
61.2
77.2
1.1
3.4
6.2
13.1
77.3
82.3
1.5
2.9
5.3
21.2
75.2
77.6
0.6
3.7
1.3 3.6 3.1 4.7
4.5 9.2 11.7 34.1
58.2 76.1 81.2 77.3
67.3 81.0 82.8 84.5
0.2 1.1 0.9 1.0
0.6 2.9 3.5 8.1
3.9
82.5
70.7
94.1
0.8
16.5
a
The conversion and selectivity were calculated from the content of product by gas chromatography (GC). bThe byproduct was 1-chloro2-nitrosobenzene. For detailed reaction conditions, see the Experimental Section.
than those of Au13 NPs, respectively. For the Au(13)@Ag(2S) NCs, the conversion and TOF increased to 21.2% (from 5.3% in the dark) and 3.7 h−1 (from 0.6 h−1 in the dark), respectively. For the Au(25)@Ag(4S) NPs, the conversion and TOF reached 34.1% (from 4.7% in the dark) and 8.1 h−1 (from 1.0 h−1 in the dark), about 8 and 10 times higher than those of Au25 NPs, respectively. Furthermore, for the Au(25)@Ag(4S) NCs, the conversion and TOF were up to 82.5% (from 3.9% in the dark) and 16.5 h−1 (from 0.8 h−1 in the dark), around 2.5 and 2 times higher than that of isolated Au(25)@Ag(4S) NPs, respectively. These results indicated that the Au−Ag hybrid NPs with a core−shell structure function as photocatalysts and that the catalytic performance of Au NPs for these organic synthesis reactions is significantly enhanced under visible light 866
DOI: 10.1021/jacs.7b11293 J. Am. Chem. Soc. 2018, 140, 864−867
Communication
Journal of the American Chemical Society
(7) (a) Yoon, J. H.; Lim, J.; Yoon, S. ACS Nano 2012, 6, 7199. (b) Klinkova, A.; Choueiri, R. M.; Kumacheva, E. Chem. Soc. Rev. 2014, 43, 3976. (c) Yin, Z.; Zhang, W.; Fu, Q.; Yue, H.; Wei, W.; Tang, P.; Li, W.; Li, W.; Lin, L.; Ma, G.; Ma, D. Small 2014, 10, 3619. (d) Zhang, H.; Wang, D. Y. Angew. Chem., Int. Ed. 2008, 47, 3984. (8) Jones, M. R.; Osberg, K. D.; Macfarlane, R. J.; Langille, M. R.; Mirkin, C. A. Chem. Rev. 2011, 111, 3736. (9) (a) Chen, T.; Wang, H.; Chen, G.; Wang, Y.; Feng, Y.; Teo, W. S.; Wu, T.; Chen, H. ACS Nano 2010, 4, 3087. (b) Yang, M. X.; Chen, G.; Zhao, Y. F.; Silber, G.; Wang, Y.; Xing, S. X.; Han, Y.; Chen, H. Y. Phys. Chem. Chem. Phys. 2010, 12, 11850. (10) (a) Aslam, U.; Chavez, S.; Linic, S. Nat. Nanotechnol. 2017, 12, 1000. (b) Schlather, A. E.; Manjavacas, A.; Lauchner, A.; Marangoni, V. S.; DeSantis, C. J.; Nordlander, P.; Halas, N. J. J. Phys. Chem. Lett. 2017, 8, 2060. (11) (a) Yoon, J. H.; Zhou, Y.; Blaber, M. G.; Schatz, G. C.; Yoon, S. J. Phys. Chem. Lett. 2013, 4, 1371. (b) Wang, H.; Chen, L.; Feng, Y.; Chen, H. Acc. Chem. Res. 2013, 46, 1636. (c) Li, J.-F.; Zhang, Y.-J.; Ding, S.-Y.; Panneerselvam, R.; Tian, Z.-Q. Chem. Rev. 2017, 117, 5002. (12) Ma, Y.; Li, W.; Cho, E. C.; Li, Z.; Yu, T.; Zeng, J.; Xie, Z.; Xia, Y. ACS Nano 2010, 4, 6725. (13) Besteiro, L. V.; Govorov, A. O. J. Phys. Chem. C 2016, 120, 19329. (14) Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Nano Lett. 2013, 13, 240. (15) Li, K.; Hogan, N. J.; Kale, M. J.; Halas, N. J.; Nordlander, P.; Christopher, P. Nano Lett. 2017, 17, 3710. (16) Chen, G.; Wang, Y.; Yang, M.; Xu, J.; Goh, S. J.; Pan, M.; Chen, H. J. Am. Chem. Soc. 2010, 132, 3644. (17) (a) Yang, Y.; Liu, J.; Fu, Z.-W.; Qin, D. J. Am. Chem. Soc. 2014, 136, 8153. (b) Rycenga, M.; Kim, M. H.; Camargo, P. H. C.; Cobley, C.; Li, Z. Y.; Xia, Y. N. J. Phys. Chem. A 2009, 113, 3932. (18) Feng, X. M.; Ruan, F. X.; Hong, R. J.; Ye, J. S.; Hu, J. Q.; Hu, G. Q.; Yang, Z. L. Langmuir 2011, 27, 2204. (19) Zhang, W. Q.; Rahmani, M.; Niu, W. X.; Ravaine, S.; Hong, M. H.; Lu, X. M. Sci. Rep. 2015, 5, 8382.
superior photocatalytic performance in hydrogenation under visible light irradiation due to the coupling of surface plasmon and existence of plasmonic hot spots. In addition, the SERS activities of 1D hybrid Au−Ag NCs were much stronger than those of individual particles especially for the Au(25)@Ag(4S) NCs with the highest EF value. Our findings demonstrating the superior light-driven photocatalytic performance of these materials, shed light on the design of plasmonic-catalytic materials through shell tailoring, and distinctly improve their activity via the assembly process.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b11293. Experimental details and TEM, HR-TEM, HAADFSTEM, EDX map and UV−vis absorption spectra of NPs, FDTD simulations, SERS spectra etc. (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*
[email protected] *
[email protected] ORCID
Zhen Yin: 0000-0002-1252-7809 Xi Liu: 0000-0002-8654-0774 Zheyu Fang: 0000-0001-5780-0728 Ding Ma: 0000-0002-3341-2998 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21725301, 91645115, 21473003, 21673273, 11674012, 61521004, and 21303119) and National Key R&D Program of China (2017YFB0602200). XAFS experiments were performed at the Shanghai Synchrotron Radiation Facility (SSRF).
■
REFERENCES
(1) (a) Lewis, N. S. Science 2016, 351, aad1920. (b) Hartland, G. V.; Besteiro, L. V.; Johns, P.; Govorov, A. O. ACS Energy Lett. 2017, 2, 1641. (2) (a) Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M. Nat. Mater. 2015, 14, 567. (b) Kale, M. J.; Avanesian, T.; Christopher, P. ACS Catal. 2014, 4, 116. (3) (a) Linic, S.; Christopher, P.; Ingram, D. B. Nat. Mater. 2011, 10, 911. (b) Mukherjee, S.; Zhou, L. A.; Goodman, A. M.; Large, N.; Ayala-Orozco, C.; Zhang, Y.; Nordlander, P.; Halas, N. J. J. Am. Chem. Soc. 2014, 136, 64. (4) (a) Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W. Y.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. N. Chem. Rev. 2011, 111, 3669. (b) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (5) (a) Sarina, S.; Zhu, H.; Jaatinen, E.; Xiao, Q.; Liu, H.; Jia, J.; Chen, C.; Zhao, J. J. Am. Chem. Soc. 2013, 135, 5793. (b) Xiao, Q.; Sarina, S.; Waclawik, E. R.; Jia, J.; Chang, J.; Riches, J. D.; Wu, H.; Zheng, Z.; Zhu, H. ACS Catal. 2016, 6, 1744. (c) Dilla, M.; Pougin, A.; Strunk, J. J. Energy Chem. 2017, 26, 277. (6) (a) Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P. Chem. Rev. 2011, 111, 3913. (b) Huang, H.; Zhang, L.; Lv, Z.; Long, R.; Zhang, C.; Lin, Y.; Wei, K.; Wang, C.; Chen, L.; Li, Z.-Y.; Zhang, Q.; Luo, Y.; Xiong, Y. J. Am. Chem. Soc. 2016, 138, 6822. 867
DOI: 10.1021/jacs.7b11293 J. Am. Chem. Soc. 2018, 140, 864−867
