Monodispersed silver-palladium nanoparticles for ethanol oxidation reaction achieved by controllable electrochemical synthesis from ionic liquid microemulsions

Journal of Colloid and Interface Science 557 (2019) 450–457

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Monodispersed silver-palladium nanoparticles for ethanol oxidation reaction achieved by controllable electrochemical synthesis from ionic liquid microemulsions Xian Sun a, Qi Qiang a, Zhiguang Yin a, Zenglin Wang a,⇑, Yi Ma ⇑,a, Chuan Zhao a,b,⇑ a b

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, and School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, China School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 1 April 2019 Revised 23 August 2019 Accepted 12 September 2019 Available online 13 September 2019 Keywords: Ionic liquid Electrodeposition AgPd nanoparticles Efficiency electrocatalytic activity Monodispersed deposition

a b s t r a c t Alloyed nanoparticles are promising electrocatalysts for electrochemical energy storage and conversion devices. However, syntheses of alloyed nanoparticles with controllable size and stoichiometry remain challenging. In this study, continuous, uniform and monodispersed bimetallic AgPd nanoparticles (NPs) with diameters
10 nm are achieved by electrochemical synthesis from quaternary ionic liquid microemulsion (ILM) for use as electrocatalysts for ethanol oxidation reaction (EOR). It is found that the ionic liquid, 1-butyl-3-methyl-imidazolium chloride ([BMIM]Cl), acts not only as a soft template and co-surfactant for the formation of micro-reactors, but also as an electrolyte for enhancing conductivity. The stoichiometry (AgxPdy), size and size distribution of AgPd NPs can be accurately tuned by varying electrolyte composition, electrodeposition conditions, and ionic liquids concentrations. Attributed to the high surface area, optimal stoichiometric ratio, and strong attachment onto substrates without using organic binders, the as-deposited AgxPdy NPs exhibit extraordinary electrocatalytic activity and stability for EOR. It is found that the mass activity of Ag49Pd51 NPs/Pt electrode reaches 3360 mA mg1 for EOR in 1.0 M ethanol and 1.0 M KOH aqueous solution, which is much higher than commercial Pd/C catalyst (210.5 mA mg1) and also the highest among state-of-the-art AgPd NPs electrocatalysts reported to date for EOR in alkaline media. Ó 2019 Elsevier Inc. All rights reserved.

⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Wang), [email protected] (Y. Ma), [email protected] (C. Zhao). https://doi.org/10.1016/j.jcis.2019.09.043 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

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1. Introduction Alloy catalysts have garnered considerable interest because they often exhibit distinctly higher activity for many chemical reactions than monometallic systems [1–3]. Alloy catalysts can be designed efficiently to achieve high synergy between various metal atoms. Furthermore, the size and shape of nanoparticles can be tuned to allow for further improvement of catalytic activities [4–9]. Rational design and synthesis of alloy nanostructures with favourable size, shape, and composition offer enormous opportunities for construction of advanced electrocatalysts for energy devices such as fuel cells with enhanced activity and stability [10,11]. Consequently, facile yet powerful synthetic methods are highly desirable for controlling the composition, morphology, and architecture of alloy nanocatalysts. Fuel cells are crucial energy systems for stationary and portable applications. Recently, there is an increasing interest to use of ethanol as a fuel in oxidation reaction in the fuel cells, as it can be obtained in a great deal from the renewable biomass resources with less toxic than methanol. Therefore, a large number of EOR catalysts have been investigated [12–15]. Koper et al. found that the modification of low-index Pt single crystals with a submonolayer coverage of Sn can enhance the EOR in perchloric acid solution [16]. Rizo et al. described a cubic Pt–Sn nanoparticles with a Pt-rich core, a Sn-rich subsurface layer, and a Pt-skin surface structure, which showed a six times higher EOR activity than Pt nanocubes. The authors claimed that the particular atomic composition was responsible for the high activity [17]. RocaAyats and coworkers reported a new support of titanium carbonitride for Pt-Sn nanoparticulated catalysts for the EOR, which showed an enhanced EOR and CO tolerance [18]. However, as can be seen, most of the efficient EOR catalysts belonged to the Ptbased catalysts, the high costs in the cathode have hampered the commercialization of fuel cells [19]. A number of approaches/techniques have been developed to resolve these critical issues by using relatively cheaper Pd as an alternative electrocatalyst. However, the cost of Pd is still very high and significant loading of Pd catalysts is requires for high catalytic performance [20–22]. Lessexpensive metals such as Ag also exhibit good activity, and are more stable in acid [23–25]. It would be of economical benefit to utilize AgPd alloys to improve catalyst activity and stability and lower the catalyst cost [26–28]. Pd-Ag alloy nanoparticles (NPs) with enhanced electrocatalytic activity and stability toward EOR in alkaline solutions have been reported due to nanoporous structuring [29], although the aggregation of Pd-Ag alloy NPs was still inevitable when such Pd-Ag NPs were coated onto the substrate surface, subsequently diminishing the mass activity of the catalyst. Electrodeposition is a versatile method and can possibly be used for controllable deposition of Pd-Ag alloy NPs directly on the surface of electrodes without further coating treatment. Nevertheless, direct electrodeposition of AgPd alloy nanoparticle smaller than 10 nm in aqueous solution remains challenging to date [30]. Here we reported a novel quaternary ionic liquid microemulsion for electrodeposition of uniform and monodispersed AgPd nanoparticles of different sizes and compositions on the surface of a substrate electrode. The ionic liquids based microemulsions combine the advantages of conventional microemulsions for limiting the nucleation, growth and agglomeration of NPs, with advantages of ionic liquids such as intrinsic ionic conductivity (faster deposition rate), high dissolution properties and wide electrochemical window [31–34]. The size and size distribution of AgPd NPs can be tuned by varying electrodeposition conditions, the deposition current density and ionic liquids concentrations. The as-deposited AgPd NPs were monodispersed on the surface of electrode and exhibited remarkable, electrocatalytic activity

and stability for ethanol oxidation, thus can be potentially used for direct alcohol fuel cells. 2. Experimental section 2.1. Chemicals and reagents All chemicals reagents were analytical grade and used as received without any further purification. Deionized water was used throughout. Silver nitrate (AgNO3, 99.95%) potassium hydroxide (KOH) and ethanol (AR) were purchase from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Potassium palladochloride (K2PdCl4, 99.95%), 1-Butyl-3-methylimidazolium chloride ([BMIM][Cl], 99%) and Hexadecyl trimethyl ammonium bromide (CTAB, 99.0%) were obtained from Shanghai Aladdin Co., Ltd. (Shanghai, China). 2.2. Syntheses of AgPd nanoparticles (NPs) The ionic liquid microemulsion (ILM) was prepared by the stepwise addition of n-butanol, CTAB, [BMIM][Cl] and aqueous solution of AgNO3 (0.01 M) and K2PdCl4 (0.01 M) in different weight ratio to make the total weight 7.0 g proportions [35]. The typical ILM contains 8.6 wt% CTAB, 7.2 wt% [BMIM][Cl], 77.1 wt% n-butanol, 7.1 wt % aqueous solution of AgNO3 (0.01 M) and K2PdCl4 (0.01 M). A three-electrode electrochemical system (Pyrex glass) was used with a clean and bright Pt plate substrate (10  10 mm) as the working electrode, a Pt foil (10  10 mm) as the counter electrode, and a Pt spiral as the quasi reference electrode. Electrodeposition of AgPd nanoparticles was performed with galvanostatic system in a two-electrode cell at 298 K for 120 s. 2.3. Electrochemical experiments The cyclic voltammetry (CV) measurements were carried out with a CHI660C electrochemical workstation in ILM systems at 298 K. The electrocatalytic oxidation of ethanol was studied by linear sweep voltammetry (LSV) in 1.0 M ethanol and 1.0 M KOH at a scan rate of 50 mV s1 under continuously N2 bubbling. Cyclic voltammetry in deaerated supporting electrolyte were also performed in the same conditions without the presence of ethanol (blank reaction). All electrochemical experiments used were three-electrode electrochemical system, AgPd NPs/Pt electrode as the working electrode, a Pt counter electrode and a Ag/AgCl (3.0 M KCl) reference electrode with a Luggin capillary. The mass current density (mass activity) was normalized by the loaded AgPd NPs mass of each catalyst. The weight of AgPd NPs deposited on the substrate were obtained as follows: The total number (n) and the average size (d, nm) of AgPd NPs in an area of 400 nm  400 nm were measured by the statistic analysis of SEM images obtained for the monodispersed AgPd NPs electrodeposited on the surface of a Pt foil electrode (1  1 cm); Then the weight (W, mg) of AgPd NPs deposited on both sides of the Pt foil was calculated as follow:

WðmgÞ ¼

 3 4 d 107 x107 p n 2D 6 3 400  400 2  10

where ‘‘43  p



d 2106

3

” is the volume of single nanoparticle accord7

7

10 x10 ing to the sphere volume formula, ‘‘400400 ” is an area transfer to make the value from ‘‘400 nm  400 nm” to ‘‘1  1 cm”; ‘‘2” refers to two sides of the electrodes; and D represents specific mass of AgxPdy alloy (mg/mm3) and is the sum of DAgx% and DPdy%, DPd and DAg are the specific mass of Pd metal and Ag metal, respectively.

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The mass activity of Pt electrode was normalized by the mass of Pt calculated by mono-dispersed Pt atoms at the electrode.

and square root of scan rate (see Fig. 1c), indicates the electrodeposition process of AgPd NPs from ILM was controlled by the diffusion [36].

2.4. Physical characterizations The AgPd NPs deposited on the Pt substrate were examined by field emission environment scanning electron microscopy. X-ray Photoelectron Spectroscopy (Kratos Analytical Ltd) (XPS) and Energy Dispersive Spectroscopy (EDS) measurements was used to determine elemental composition. 3. Results and discussion 3.1. The electrochemical behavior of metal ions in the quaternary ionic liquid microemulsion (ILM) The deposition mechanism of AgPd alloy from quaternary ILM and the function of IL in the ILM were investigated using CV measurements, which were carried out in quaternary ILM containing different metal ions in the aqueous phase. As shown in Fig. 1a, the reduction peaks of single Ag(I) and Pd(II) in the ILM appeared at 0.52 V and 0.41 V, respectively, while only one peak located at 0.46 V was observed for AgPd suggesting the formation of AgPd alloy. Furthermore, it is noted that the AgPd reduction peak on the cathodic scan increased markedly with the scan rate, and the AgPd reduction peak potentials gradually shifted towards more negative values (Fig. 1b). The results suggest that AgPd NPs are electrodeposited on the electrode surface from the ILM system as expected. The high intensity of the reduction peak reflects to an energetically deposition behavior of the metal ions. Besides, the linear relationship observed between the reduction peak current

3.2. Electrochemical deposition of AgPd NPs in ILM Electrodeposition of AgPd NPs was performed in galvanostatic mode at various deposition currents at 298 K for 120 s, and the SEM images of the obtained AgPd NPs were shown in Fig. 2. The distinctly different morphology for each sample indicated that deposition current densities have significant effect on the morphology of the AgPd NPs. When the deposition current density was lower than 0.2 mA cm2, sparse AgPd NPs were deposited on the Pt surface (Fig. 2a) and the particle size was around 8 nm. With an increase of the deposition current density to 0.3 mA cm2, numerous crystal nucleus of the AgPd NPs appeared on the electrode surface and the particle size slightly increased to 10 nm with a narrow size distribution of 4–16 nm (Fig. 2b). With a further increase of deposition current density, the particle size gradually increases to 12, 14, 16 and 18 nm for the current density at 0.5, 0.8, 1.0 and 1.5 mA cm2, respectively. It is shown that AgPd NPs became even larger and displayed irregular shapes under the higher current density (over 1.0 mA cm2), which was due to the fast nucleus growth and demulsification of ILMs during the electrodeposition process under a high current density [37]. The effects of [BMIM][Cl] concentration on the size and size distribution of AgPd NPs were investigated on the current density kept at 0.3 mA cm2 (Fig. 3). In the absent of [BMIM][Cl], the average size of as-deposited AgPd NPs is about 35 nm with a broad size distribution (Fig. S1). When 1.43 wt% [BMIM][Cl] is added to the microemulsion, more uniform AgPd NPs are deposited on the elec-

Fig. 1. (a) CV curves of ILM containing Ag(I), Pd(II) and Ag(I) + Pd(II) in aqueous phase and (b) CVs of ILM containing Ag(I) + Pd(II) in aqueous phase obtained at scan rates (mV s1): 10, 30, 50, 70, and 90; (c) Linear relationship between the reduction peak current density against the square root of scanning rate.

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Fig. 2. SEM images of AgPd NPs electrodeposited at various current densities (a) 0.2 mA/cm2, (b) 0.3 mA/cm2, (c) 0.5 mA/cm2, (d) 0.8 mA/cm2, (e) 1.0 mA/cm2, (f) 1.5 mA/cm2 on the Pt electrode surface at 298 K for 120 s.

trode surface and the average size of AgPd NPs decreases to 18 nm. With the increase of [BMIM][Cl] content, the density of AgPd NPs increases and the size distribution decreases. When [BMIM][Cl] content increases to 7.14 wt%, the size and size distribution of AgPd NPs reach the minimum values (average size
10 nm, size distribution 4–16 nm) (Fig. 3c). The AgPd NPs were monodispersed on the surface and form close-packed arrays. With a further increase of [BMIM][Cl] content from 7.14 to 15.71 wt%, the size of AgPd NPs increased significantly, and the size distribution also became broad. This behavior is because that [BMIM][Cl] acts as a co-surfactant in the construction of polar micelles, the size of polar micelles decreases significantly with the increase of [BMIM][Cl] content in the ILM, leading to a decrease in the size of AgPd NPs. Furthermore, the molecular weight of IL is much smaller than that of the surfactant, therefore the influence of IL content on the size of

AgPd NPs is very significant. When IL content exceed 7.14 wt%, the polar micelles became unstable owing to the difference in polarity between the surfactant and [BMIM][Cl], leading to a demulsification of ILMs during the electrodeposition process [38]. When the soft template function is lost, irregular and large-sized AgPd NPs are formed. The as-deposited AgPd NPs were characterized by X-ray photoelectron spectroscopy (XPS). Fig. 4a and 4b display the high resolution Ag 3d and Pd 3d XPS spectra of AgPd NPs. The two major peaks appeared at binding energies of 368.2 and 374.3 eV with a spinenergy separation of 6.1 eV correspond to Ag3d5/2 and Ag3d3/2, respectively (Fig. 4a), which is assigned to metal Ag and is in good agreement with previous reports [39]. The observation of Pd 3d5/2 and Pd 3d3/2 at 335.2 and 340.6 eV confirms the presence of metallic Pd in AgPd NPs [40]. The composition (Table 1) and the element

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Fig. 3. SEM images of AgPd NPs electrodeposited with various [BMIM][Cl] concentration (a) 1.43 wt%, (b) 4.29 wt%, (c) 7.14 wt%, (d) 10.00 wt%, (e) 12.86 wt%, (f) 15.71 wt% on the Pt electrode surface at 0.3 mA/cm2 for 120 s.

distribution (Fig. 4c) of AgPd NPs were measured by Energy Dispersive Spectroscopy (EDS). The Pd and Ag are uniformly distributed on the electrode surface and the elemental mappings of Pd and Ag match well with the NP shape (Fig. 4c1). Fig. S2 shows the XRD pattern of AgPd NPs deposited from the ILM. with AgNP/Pt, PdNP/Pt and Pt as references. For AgNP/Pt and PdNP/Pt, all diffraction peaks can be assigned to metal Ag and Pd, respectively. Enlarged figure (inset in Fig. S1b) was introduced to show the angle shift of AgPdNP. It is shown that an alloy peak (38.8°) located between the Ag (38.2°) and Pd (39.8°) can be observed clearly, which is the direct evidence of the formation of AgPd alloy. In ILM systems, we further investigated the correlation between the composition of microemulsion droplets and the stoichiometry of the electrodeposited AgPd NPs. It is found that the size of AgPd NPs doesn’t change significantly with the molar ratio between Ag

(I) and Pd(II), rather the total concentration of these metal ions in the aqueous phase (Fig. S3 and Fig. S4). But increases the deposition time, the particle size increases (Fig. S5). The stoichiometric ratio of the deposited AgPd NPs could be controlled by tuning the molar ratio of Ag(I) and Pd(II) inside the droplets, provided that the other electrodeposition conditions and total concentration of metal ions in the aqueous solution remain unchanged (Table 1). When the molar ratio of Ag(I) and Pd(II) in the aqueous solution was between 3:7 and 7:3, the stoichiometric ratios between metallic Ag and Pd in the AgPd NPs match well with the molar ratios of Ag(I) and Pd(II) in the aqueous solution (Table 1). This is attributed to the fact that the ILMs are extremely small (diameter
20 nm) (Fig. S6), almost all the metal ions in nanodrops were reduced on the electrode surface via the so-called ‘‘exhaustive deposition” process [36]. These results demonstrate a significant advantage of the

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Fig. 4. XPS spectra of (a) Ag NPs and (b) Pd NPs for AgPd NPs electrodeposited from ILM. (c) EDS image of related element mapping Ag (c2) and Pd (c3) for AgPd NPs (c1) on the Pt electrode surface.

Table 1 Composition of AgPd NPs with the molar ratio of Ag(I):Pd(II) in the solution. Molar ratio of Ag+ and Pd2+ in aqueous solution

Composition of the nanoparticles

10:90 30:70 50:50 70:30 90:10

Ag13Pd87 Ag29Pd71 Ag49Pd51 Ag69Pd31 Ag88Pd12

ILM system for electrodeposition of alloy NPs over traditional electrodeposition processes where the stoichiometric ratio of metals in an alloy is difficult to control because of the various reduction potentials for different metal ions. 3.3. Electrocatalytic performance of AgPd-NPs/Pt electrode for ethanol oxidation reactions Fig. 5a displays the voltammetric profiles for the EOR of Ag49Pd51 NPs deposited onto a Pt electrode with different size. All the catalysts show the similar profile with one oxidation peak located around 0.23 V, which may suggest one type active site for ethanol oxidation in these catalysts [16]. The blank reaction results shown in Fig. S7 reveal similar electrochemical profiles for all catalysts employed in the electrolyte solution in the potential range depicted, thus the oxidation peak in Fig. 5a can be totally ascribed

to EOR. For bare Pt electrode, only a small ethanol oxidation peak was observed, indicating the substrate Pt electrode is able to catalyze ethanol oxidation, but shows the lowest catalytic activity. When Ag49Pd51 NPs were deposited on the Pt electrode, the activity of EOR increased and it closely related to the particle size of the AgPd NPs. When the average size of Ag49Pd51NPs was 20 nm, the mass activity for EOR was only 1290 mA mg1. When the size decreases to 10 nm, the mass activity of Ag49Pd51NPs increases about 3-fold and reaches to 3359 mA mg1. Furthermore, the peak position shifts to more negative potentials (from 0.21 to 0.24 V) with increasing the particle size of the catalysts. The results indicated that the electrocatalytic activity of Ag49Pd51 NPs for ethanol oxidation is strongly size-dependent, and the small size is favorable for high catalytic activity owing to high specific surface area. Remarkably, the obtained mass activity for Ag49Pd51 NPs/Pt electrode for EOR in 1.0 M ethanol and 1.0 M KOH aqueous solution is much higher than commercial Pd/C catalyst (210.5 mA mg1) [41], and is also the highest among the state-of-the-art AgPd NPs reported for EOR under similar conditions (Table S1), including Pd80Ag20/C (691.6 mA mg1) [41], Pd50Ag50 (1970 mA mg1) [29], [email protected] NPs (1160 mA mg1) [42]. The EOR activity of AgPd NPs with different Ag:Pd ratios were also evaluated under same conditions with Ag-NPs/Pt, Pd-NPs/Pt and bare Pt electrodes as references (Fig. 5b). All AgxPdy NPs catalysts exhibit significantly improved catalytic performance and also more negative peak positions compared to pure Ag and Pd NPs. The highest activity was obtained on Ag49Pd51 NPs with the Ag:Pd

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Fig. 5. LSV curves of the AgPd-NPs/Pt catalysts in 1.0 M ethanol and 1.0 M KOH aqueous solution for EOR at scan rate of 50 mV s1. (a) different size of AgPd NPs; (b) different stoichiometry AgPd NPs; (c) the mass activity as a function of Pd content in AgPd NPs for EOR, (d) chronoamperometric curves of Ag-NPs/Pt, Pd-NPs/Pt and AgPd-NPs/Pt catalysts in 1.0 M ethanol and 1.0 M KOH aqueous solution at a potential of 0.3 V.

ratios equal to 1, which showed an obvious synergistic effects between the two metals. The mass activity of AgPd NPs for EOR as a function of Pd content is shown in Fig. 5c. Ag49Pd51 NPs shows the best electrochemical performance for ethanol oxidation with the highest ethanol oxidation mass activity. This is in agreement with the literature results [29] and can be attributed to the easier adsorption of OH on the surface [43]. The bimetallic nanoparticle catalysts such as alloy, nanocomposite or hybrid structures were widely investigated for the fuel cells. The introduction of the second component can not only reduced the cost of Pd required in electrocatalysts but also modifies the crystallographic and electronic structures of Pd, which may tune the binding energy between the alloy catalysts and the reactants, resulting in the enhanced activity [44,45]. The stability of the AgPd NPs was further evaluated at a potential of 0.3 V for 2000 s in 1.0 M KOH containing 1.0 M ethanol, and the results are shown in Fig. 5d. For comparison, the stability of Ag NPs and Pd NPs were also tested. All the catalysts show a significant decay at the very beginning and then remain stable. The mass activity of the AgPd NPs at the start and the end of the tests was superior to that of pure metal NPs catalysts. The mass activity of the Ag49Pd51 NPs is higher than that of the Ag29Pd71 NPs and Ag69Pd31 NPs catalysts in the entire time range. These results suggest that the Ag49Pd51 NPs have a long-term high electrocatalytic activity and also greater stability for ethanol oxidation in alkaline media.

4. Conclusion A green, simple, inexpensive approach has been established for electrochemical deposition of AgPd NPs with tunable sizes and compositions from a novel quaternary ILM system. The microemulsions act as soft templates for nanoparticle growth, which allows reliable control over sizes and compositions. The method allows for the preparation of Ag1-xPdx nanoparticles with

a broad size range and a tunable composition by controlling the proportion and composition of the aqueous component. The obtained Ag40Pd51 NPs exhibit extraordinary mass activities towards ethanol oxidation for alcohol fuel cells. It is hoped that this approach can be extended to the preparation of a range of alloy NP catalysts with tunable size and activity onto a substrate electrode, or as powders, for a range of important reactions in electrochemical energy conversion and storage. Acknowledgements The project is financially supported by National Natural Science Foundation of China (No. 21273144, 21603134), the 111 Project (No. B14041), Program for Chang Jiang Scholars and Innovative Research Team in University (IRT-14R33) and an Australian Research Council Discovery Grant (DP160103107). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.09.043. References [1] S. Koh, P. Strasser, J. Am. Chem. Soc. 129 (2007) 12624–12625. [2] K. Sasaki, H. Naohara, Y. Cai, Y.M. Choi, P. Liu, M.B. Vukmirovic, J.X. Wang, R.R. Adzic, Angew. Chem. Int. Ed. 49 (2010) 8602–8607. [3] V.R. Stamenkovic, B. Fowler, B.S. Mun, G.F. Wang, P.N. Ross, C.A. Lucas, N.M. Markovic, Science 315 (2007) 493–497. [4] R. Narayanan, C. Tabor, M.A. El-Sayed, Top. Catal. 48 (2008) 60–74. [5] Y. Tang, M. Ouyang, Nat. Mater. 6 (2007) 754–759. [6] D.L. Feldheim, Science 316 (2007) 699–700. [7] N. Tian, Z.Y. Zhou, S.G. Sun, Y. Ding, Z.L. Wang, Science 316 (2007) 732–735. [8] J.H. Yang, J. Yang, J.Y. Ying, ACS Nano 6 (2012) 9373–9382. [9] Y. Ding, Y. Gao, Z.L. Wanga, N. Tian, Z.Y. Zhou, S.G. Sun, Appl. Phys. Lett. 91 (2007) 121901. [10] S.E. Evarts, I. Kendrick, B.L. Wallstrom, T. Mion, M. Abedi, N. Dimakis, E.S. Smotkin, ACS Catal. 2 (2012) 701–707. [11] M. Li, P. Liu, R.R. Adzic, J. Phys. Chem. Lett. 3 (2012) 3480–3485.

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