- Email: [email protected]
Preparation of Ag(Shell)–Au(Core) nanoparticles by anti-Galvanic reactions: Are capping agents the “real heroes” of reduction?
Accepted Manuscript Title: Preparation of Ag(Shell) Au(Core) nanoparticles by anti Galvanic Reactions: Are capping agents the “real heroes” of reduction? Author: Puspanjali Sahu B.L.V. Prasad PII: DOI: Reference:
S0927-7757(15)00249-6 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.03.028 COLSUA 19819
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
10-2-2015 18-3-2015 20-3-2015
Please cite this article as: P. Sahu, B.L.V. Prasad, Preparation of Ag(Shell) Au(Core) nanoparticles by anti Galvanic Reactions: Are capping agents the “real heroes” of reduction?, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2015), http://dx.doi.org/10.1016/j.colsurfa.2015.03.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of Ag(Shell) - Au(Core) nanoparticles by anti Galvanic Reactions: Are capping agents the “real heroes” of reduction?
Physical/Materials Chemistry Division CSIR-National Chemical Laboratory *Email: [email protected]
cr
Dr. Homi Bhabha Road, Pune 411008, India
ip t
Puspanjali Sahu and B. L. V. Prasad*
us
Tel. No. +91-20-25902013. Fax: +91-20-25902636
Abstract
an
The formation of Ag(shell)-Au(core) nanoparticles by the reduction of Ag+ ions on preformed Au NPs, mediated by the capping agents like dodecylamine is described. We clearly established that
M
while monodisperse Ag(shell)-Au(core) structures can be formed in presence of weak surface binding ligands such as amines, strong binding ligands such as thiols, lead to formation of separate monometallic Au and Ag particles. Apart from highlighting the influence of metal-ligand binding
d
strength on the formation of core-shell nanoparticle architectures, our study also provides an easy
te
means to produce Ag(shell)-Au(core) structures.
Ac ce p
Keywords: Ag(shell)-Au(core) nanoparticles, anti Galvanic reactions
Page 1 of 15
1. Introduction Recent years gave birth to eloquent ways of colloidal nanostructure preparations and ligands (organic molecules/polymers etc.) evolved as central components in manipulating several morphological features of nanoparticles (NPs).[1-3] Among the different nano-architectures, core-shell type
ip t
structures are receiving immense interest because of their improved physical and chemical properties over their single-component counterparts.[4-8] The synthesis and characterization of bimetallic core-
cr
shell systems is well studied.[9-12] Among the plethora of synthetic strategies developed for metalmetal core-shell NP preparation, sequential reduction method stands out as the most efficient and hence commonly used. This involves use of any pre-synthesized metal NP (this would become the
us
core ultimately) as seeds, on which second metal could be deposited as shell assisted by an external reducing agent. A precondition for this procedure to work efficiently is that the lattice constant of both
an
the metals should match to a certain extent. For example it is easy to generate conformational coating of Au on surface of Ag NPs or vice versa due to their close match of lattice constant. Though this method is easy and convenient, it suffers from many drawbacks such as stepwise preparation of metal
M
NPs, requirement of stringent reaction environment and strong dependence of the final product on the type of seed used (i.e. size, shape etc), pH, the reducing agent, solvent, etc.[13] Also the formation of out on large scale.
d
individual monometallic NPs by this method cannot be avoided, especially if the reaction is carried
te
Many other methods were proposed to overcome these drawbacks. For example formation of monometallic NPs can be avoided by use of Galvanic displacement method. In this procedure, the metal ions having less positive reduction potential form the core and can act as the reducing agent for
Ac ce p
the metal having more positive reduction potential. This results in the growth of second metal as shell. But this protocol suffers from the restriction that only a more noble metal can form the shell on a less noble metal. For example it is easy to generate Au shell on Ag cores by this method owing to high reduction potential of Au than Ag but the inverse (Ag shell on Au core) is not possible by this method. While few protocols for preparation of Ag(shell)-Au(core) particles have been developed, the complicated synthesis procedure involved in these protocols prevents their general use.[14] In this background some recent reports claim that small NPs of more noble metals such as Au and Pt can reduce less noble metal ions like Ag+ which is a deviation from the conventional free energy based arguments used for Galvanic displacement.[15-17] This phenomenon has been termed as Anti Galvanic Reduction (AGR). In some of these methods ligand protected Au NPs were used to which a silver precursor is added subsequently. This raises a question whether the ligand shell present on the nanoparticle surface is acting as a reducing agent.[15,16] In some other where the Au NPs were not coated with any ligand it is not clear whether the initial reducing agent, used to prepare the Au NPs has been completely removed.[16] Also in this method the shell formation was deduced from indirect
Page 2 of 15
evidence as direct imaging did not provide evidence for shell formation, raising question about the effectiveness of the procedure. Thus we felt the need for a systematic study of this phenomenon. Accordingly, we developed a simple method for synthesis of Ag(shell) on Au(core) (these are termed as Ag@Au NPs in the rest of the paper following the normal convention) without addition of any external reducing agent. This was achieved by addition of AgOBz to the dispersion of pre synthesized
ip t
dodecylamine (DDA) capped Au NPs. The formation of Ag@Au NPs by this simple way can take place via any of the following two pathways.
1. The surface atoms (Au0) of Au NPs can act as reducing agents (AGR reaction).
cr
2. The excess DDA molecules present in the dispersion can reduce Ag+ ions to Ag0.
In order to understand which one of the above causes the formation of Ag@Au structures, the reaction
us
was repeated with Au NPs capped by other ligands such as dodecanethiol (DDT). When DDT capped Au NPs were used, no core-shell type architecture was formed; instead separate Au and Ag particles were formed. This failure in the synthesis of Ag@Au particles when DDT capped Au NPs were used
an
as seeds, clearly demonstrated that the reduction of Ag+ on Au0 is mediated by DDA and that the nature of ligand binding with the nanoparticle surface plays a significant role in determining the final
M
nanoparticle architecture (core-shell or individual particles).
2. Material and Method
d
2.1. Materials Toluene was purchased from Merck, India and was degassed before use. Other chemicals were
2.2. Experimental Procedure
te
purchased from Sigma Aldrich and used as received.
Ac ce p
Au NPs were prepared by well known digestive ripening method.[18] In this method, AuCl3 is dispersed in an organic solvent (toluene in the present case) in presence of a long chain surfactant i.e. didodecyldimethylammonium bromide (DDAB). The Au (III) is then reduced to Au (0) with the help of NaBH4. Excess ligand (1:30 metal:ligand ratio) was added to the as prepared Au NPs in order to prevent aggregation. These ligand capped NPs are then separated from excess ligand and other side products by precipitating them with excess ethanol addition. The precipitate was then re-dispersed in toluene and heated in the presence of excess ligand in order to obtain monodisperse NPs. Herein, Au NPs were synthesized in presence of two ligands i.e. DDT and DDA. Detailed synthetic procedure is given in supplementary material (ESM-1). To these Au NPs dispersion in toluene, silver benzoate (AgOBz) was added keeping the precursor molar ratio of Au: Ag 1:10 and the system was heated at 90 ⁰C for 5h. No additional reducing agent was added. 2.3. Instrumentation Formation of NPs and other nanostructures are characterized by UV-Visible (UV-Vis) spectroscopy and Transmission Electron Microscopy (TEM). UV–Visible absorption spectra of NP dispersions
Page 3 of 15
were measured using a Cary-300 UV–Visible spectrophotometer. Samples for TEM analysis were prepared by drop casting the NP dispersions on carbon coated copper grids (200 mesh). TEM analysis was performed on FEI, TECNAI G2 TF 30 and FEI, TECNAI G2 TF 20 instruments. Particle size distribution histograms were plotted taking 300 particles into account.
ip t
3. Results Since the digestive ripening involved repeated precipitation and redispersion we can rule out any remnants of the reducing agent in the final dispersion. Formation of Au NPs was initially confirmed
cr
by the localized surface plasmon resonance (LSPR) peak appeared at 526 nm for DDA and at 528 nm for DDT capped Au NPs[19]. Particle size and size distribution was calculated from transmission
us
electron microscopy (TEM) images, which confirmed that the Au NPs synthesized with both the ligands are monodisperse (ESM-2).
Silver benzoate (AgOBz) was added to these Au NPs dispersion in toluene, keeping the precursor
an
molar ratio of Au: Ag 1:10 and the system was heated at 90 ⁰C for 5h. When AgOBz was added to wine red dispersion of Au NPs capped with DDA, the color of the dispersion became yellow. DDA capped Au NPs, before addition of AgOBz showed a LSPR band at 526 nm characteristic of Au NPs.
M
Upon addition of AgOBz, an increase in intensity of the LSPR band was observed and the band position shifted to lower wavelength with time. The extent of blue shift was initially high i.e. max shifted from 526 nm to 520 nm within 1h of reaction and from 520 nm to 516 nm after 2h of reaction.
d
Negligible shift was observed on further heating i.e. max shifted from 516 nm to 514 nm upon
te
continuing heating for 5h. The same trend was also observed for absorbance intensity. Very strong enhancement in absorbance intensity was noticed up to 2h of heating and only slight enhancement
Ac ce p
was observed afterwards (Fig-1a).
TEM gives further insight into the change occurred in the NPs structure. It was found that the initially monodisperse Au NPs got converted to a core-shell type architecture and the particles formed are reasonably monodisperse with average diameter of 12-14 nm and shell thickness of 2-4 nm (fig 1B). These core-shell structures were further analyzed by dark field mode TEM imaging. The core of the particles appears brighter compared to shell (Fig 1C). Presence of both Au and Ag was further confirmed by Energy-dispersive X-ray Spectral (EDS) analysis (fig-1D). EDS line scan confirmed that both Au and Ag were co-located in the same NPs with higher intensity of Ag at the periphery (inset of Fig. 1D) and no trace of any separated monometallic NPs could be seen. These core-shell particles formed are highly crystalline with exposed 111 planes (inset of Fig. 1B). More TEM images are provided in ESM-3. Thus the above data unequivocally confirm the formation of Ag@Au particles when DDA capped Au NPs are reacted with AgOBz. On the other hand when AgOBz was added to Au NPs capped by DDT, the Au LSPR peak shifted to higher wavelength (555 nm) and another peak appeared at 350 nm. No significant change in peak position was observed upon heating the dispersion for long time (5h) expect slight broadening of the
Page 4 of 15
peak at 350 nm (Fig. 2A). TEM was used for further inspection of the samples prepared from DDT capped Au NPs (Fig. 2B). It was observed that bigger NPs of dark contrast are anchored over a layer of smaller NPs. As the bigger NPs appeared darker in the TEM images obtained in the bright field mode (Fig. 2C) and brighter in dark field mode TEM images, we deduce that these belong to the Au NPs and smaller ones can be claimed as Ag NPs. Presence of both Au and Ag was confirmed by area
ip t
mapping and EDS analysis (Fig. 2D-2F). More TEM images are provided in ESM-4.
4. Discussion
cr
The results obtained are schematically depicted in Fig-3. Addition of AgOBz to amine capped Au NPs, resulted in the formation of Ag@Au NPs. The core-shell formation was confirmed by both UV-
us
Vis spectroscopy and TEM. The Ag shell formation on Au core was associated with an initial increase in SPR band intensity and blue shift of band position.[20, 21] It is well documented that SPR maxima shifts towards lower wavelength with increase in thickness of silver shell. As the shift in peak position
an
decreased significantly after 2h and become negligible at 5h, the reaction was stopped at 5h. TEM gives further confirmation of Ag@Au formation as mentioned in result section.
M
In contrary to the above observation, when AgOBz was added to Au NPs capped by DDT, both Au and Ag NPs formed separately. Formation of separate Au and Ag particles is confirmed by appearance of two separate peaks in UV-Vis spectra at 550 nm and 350 nm, characteristic of Au NPs
d
Ag NPs. [22, 23] Appearance of two distinct peaks for Ag and Au clearly indicate non-interacting nature of both these metals, when DDT was used as capping agent. It was also observed from TEM
te
that Au NPs were anchored over the layer formed by smaller Ag NPs. Both the above mentioned preparation procedure differ only in the nature of anchoring group of the
Ac ce p
ligand. This leads to the question, why the simple change in anchoring group of the ligands completely changes the resultant nanostructures? Both of these ligands have same chain length; hence the steric influence of the ligands on formation of different nanostructures can be overruled. We therefore reckoned that the binding nature of the ligands towards Au and Ag NPs may have a significant impact on the resultant these nanostructures. To understand the effect more clearly, a reaction was carried out, where AgOBz dissolved in toluene was heated at 90 0C with DDT or DDA. With both the ligands reduction of Ag (I) to Ag (0) was seen to start immediately. Reduction of Ag(I) to Ag(0) by amine is well known. It is postulated that while reducing the silver ions, amines undergo involving the formation of an intermediate, involving both the silver carboxylate and the amine [24,25]. We believe that in the present case also similar products could have formed. After the reaction between DDA and AgoBz two peaks (at 474 nm and 494 nm) appeared in the UV-Vis spectra indicating formation of Ag NPs [26]. Continuation of heating caused merging of both the peaks and a new broad peak to appear at 470 nm. Broadening of the peak and substantial red shift of peak position compared to Ag NPs (420 nm) indicate formation of bigger particles or aggregation of small Ag NPs formed initially (Fig-4A). TEM confirms aggregation of small NPs as the cause for the broad peak
Page 5 of 15
appeared at 470 nm when AgOBz was heated in presence of DDA for longer time (Fig-4B and 4C). Highly crystalline nature of DDA capped Ag NPs is proved by selected area electron diffraction pattern (SAED) (inset of Fig. 4B) and HRTEM image (inset of Fig 4C). When the same reaction of AgOBz was carried out with DDT, small Ag NPs formed immediately as indicated by the peak at 350 nm in UV-Vis spectroscopy.[22] Thiols are susceptible to easy oxidation
ip t
forming products like disulfides, sulfones, sulfoxides and sulfonic acids and hence could act as reducing agents as far as noble metals like silver are concerned[26-28]. While the reduction of silver ions could be accomplished both by DDA and DDT there are some interesting differences in the type
cr
of nanoparticles formed. We would like to remind that when in AgOBz is reacted with DDA, initially in the UV-Vis spectrum two peaks are formed which merge into one as the reaction is continued.
us
Interestingly, the reaction between DDT and AgoBz resulted the formation of only one peak could be seen and that also underwent a negligible shift (from 350 to 354 nm) when the heating was prolonged for 5h. This slight change can be attributed, if any, to only a small increment in particle size of Ag
an
NPs[29]. Formation of Ag NPs are further confirmed by TEM (Fig-4E and 4F). As expected, unlike the amine case, the thiol capped Ag NPs formed are small in size and well separated.
M
Based on the above observation we propose the following sequence of reactions during the formation of Ag@Au NPs. Before proceeding, however, we would like to re-emphasize two facts: (i) both DDA and DDT reduce Ag+ ions to Ag0 and (ii) The binding of DDA to Au and Ag NP surface is weaker as
d
compared to the binding of DDT binding to these nanoparticle surfaces. This second feature has been explained qualitatively based on Hard Soft Acid Base (HSAB) principle and has been validated
te
theoretically [30] and experimentally.[31]
Owing to the procedure used for the preparation, the Au NP dispersions in toluene retain a lot of
Ac ce p
excess ligands in solution, which are one of the reagents in current study. Since refluxing AgOBz with DDA or DDT alone leads to the formation of Ag nanostructures, we reckon that when AgOBz was added to DDA capped Au NPs, the excess ligand present in the Au NP dispersion reduces the Ag+ to Ag0. However, as DDA does bind very strongly to the Au NP surface, the naked NP surface gets exposed when the DDA molecules detach from the Au NP especially under the reflux conditions. At the same time because DDA does not bind strongly to Ag0 surface, nascent uncapped Ag0 atoms/clusters got generated in the solution. These prefer to attach on the surface of Au NPs those have been formed due to ligand detachment as described above, leading to formation of Ag@Au NPs. Our observation that the silver nanostructures formed by the reaction of DDA with AgOBz are not stable and keep growing to form bigger particles as the reaction is allowed to proceed for longer duration accords well with the above discussion. The formation of core-shell architecture is also facilitated by the fact that attachment of Ag on Au NPs surface is thermodynamically favourable, as both Ag and Au possess almost same lattice constant value (differ by 0.2%). [32] On the other hand DDT has strong affinity towards both Ag and Au NPs surface. Hence once Ag+ ions are reduced to Ag0 nanostructures by DDT, they get capped/stabilized by it immediately. This
Page 6 of 15
contention is asserted by the observation that even after prolonged heating there is no big difference in the optical spectra and sizes of the small Ag NPs formed by the reduction of Ag+ ions independently with DDT. So when the reaction is carried out in presence of preformed DDT capped Au NPs we could see the formation of similar small Ag NPs (reduced and capped by the excess DDT present in solution). But these are very stable. Such stable Ag NPs do not attach on the Au NPs surface.
ip t
Secondly as thiols bind to Au NPs surface very strongly, there is no exposed surface on Au NPs also on which these small Ag NPs can attach. Thus the strong affinity of thiols towards both the metal surfaces does not allow the growth of Ag@Au NPs but result in the formation individual Au and Ag
cr
NPs.
us
5. Conclusions
By using ligands possessing different binding affinity toward metal surfaces, we have developed a simple and convenient approach for the preparation of bimetallic Ag@Au NPs. In
an
particular, we demonstrated that reduction of Ag ions in presence of preformed dodecylamine capped Au NPs leads to Ag(shell)-Au(core) nanoparticle formation. On the other hand carrying out the same preparation procedure with dodecanethiol capped Au NPs results in the
M
formation of individual monometallic NPs. So this study not only provides an easy means to synthesize Ag@Au NPs, but also sheds light on the role of ligands in the formation of
d
bimetallic core-shell structures. We hope such a better understanding of the role of ligand will provide greater impetus for the preparation of designer ligands that can provide fine control
te
over multi metallic naostructure synthesis. Our studies also highlight the necessity to study the proposed anti Galvanic reactions in a more careful manner as in those reports also the
Ac ce p
ligands may have actually played the role of reducing agents.
Acknowledgement
P.S. thanks the Council of Scientific and Industrial Research for a fellowship. We gratefully acknowledge CSIR, New Delhi for financial support through XII five year plan network project BSC0112 (Nano-SHE). We thank the IPC Department IISc for helping us with TEM analysis. We also acknowledge the Indo-US Science and Technology Forum (IUSSTF) for support through the Joint Virtual Centre “From Fundamentals to Applications of Nanoparticle Assemblies”.
Page 7 of 15
References
N. Ortiz, S. E. Skrabalak, On the Dual Roles of Ligands in the Synthesis of Colloidal Metal Nanostructures. Langmuir 30 (2014) 6649-59. J. D. A. Iii, R. G. Finke, A review of modern transition-metal nanoclusters: their synthesis, characterization, and applications in catalysis. J. Mol. Catal. A: Chem. 145 (1999) 1-44. M-C. Daniel, D. Astruc, Gold nanoparticles: assembly, supramolecular chemistry, quantumsize-related properties, and applications toward biology, catalysis, and nanotechnology. Chem. rev. 104 (2004) 293-346. P. Migowski, J. Dupont: Catalytic Applications of Metal Nanoparticles in Imidazolium Ionic Liquids. Chem. Eur. J. 13 (2007) 32-39. K. Zhang, Y. Xiang, X. Wu, L. Feng, W. He, J. Liu, W. Zhou, S. Xie: Enhanced Optical Responses of Au@Pd Core/Shell Nanobars. Langmuir 25 (2008) 1162-68. R. G. Chaudhuri, S. Paria, Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization and Applications. Chem. Rev. 112 (2012) 2373-433. A. K. Samal, L. Polavarapu, S. Rodal-Cedeira, L. M. Liz-Marzan, J. Perez-Juste, I. PastarizaSantos, Size Tunable Au@Ag Core–Shell Nanoparticles: Synthesis and Surface-Enhanced Raman Scattering Properties, Langmuir 29 (2013) 15076-15082. Z.-B. Zeng, Y.-Y. Cao, J-J. Chen, X.-D. Wang, J.-F. Yan, X. Chen, Au@Ag core/shell nanoparticles as colorimetric probes for cyanide sensing, Nanoscale 6 (2014) 9939-9943. A. Sarkar, A. Manthiram, Synthesis of Pt@Cu Core-Shell Nanoparticles by Galvanic Displacement of Cu by Pt4+ Ions and Their Application as Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells. J. Phys. Chem. C 114 (2010) 4725-32. Q. Zhang, J. Xie, J.Y. Lee, J. Zhang, C. Boothroyd, Synthesis of Ag@AgAu Metal Core/Alloy Shell Bimetallic Nanoparticles with Tunable Shell Compositions by a Galvanic Replacement Reaction. Small 4 (2008) 1067-71. Y. Mizukoshi, T. Fujimoto, Y. Nagata, R Oshima, Y Maeda: Characterization and Catalytic Activity of Core-Shell Structured Gold/Palladium Bimetallic Nanoparticles Synthesized by the Sonochemical Method. J. Phys. Chem. B 104 (2000) 6028-32. K. Mallik, M. Mandal, N. Pradhan, T. Pal: Seed Mediated Formation of Bimetallic Nanoparticles by UV Irradiation:A Photochemical Approach for the Preparation of "CoreShell" Type Structures. Nano Lett. 1 (2001) 319-22. S. U. Son, Y. Jang, J. Park, H. B. Na, H. M. Park, H. J. Yun, J. Lee, T. Hyeon, Designed Synthesis of Atom-Economical Pd/Ni Bimetallic Nanoparticle-Based Catalysts for Sonogashira Coupling Reactions. J. Am. Chem. Soc. 126 (2004) 5026-27. C. J. Serpell, J. Cookson, D Ozkaya, PD Beer: Core@shell bimetallic nanoparticle synthesis via anion coordination. Nat. Chem. 3 (2011) 478-83. Z. Wu: Anti-Galvanic Reduction of Thiolate-Protected Gold and Silver Nanoparticles. Angew. Chem. Int. Ed. 51 (2012) 2934-38. G. Liu, D-Q. Feng, W. Zheng, T. Chen, D. Li: An anti-galvanic replacement reaction of DNA templated silver nanoclusters monitored by the light-scattering technique. Chem. Comm. 49 (2013) 7941-43. M. Wang, Z. Wu, Z. Chu, J. Yang, C. Yao, Chemico-Physical Synthesis of Surfactant- and Ligand-Free Gold Nanoparticles and Their Anti-Galvanic Reduction Property. Chem. Asian J. 9 (2014) 1006-10. D. S Sidhaye, B. L. V Prasad: Many manifestations of digestive ripening: monodispersity, superlattices and nanomachining. N. J. Chem. 35 (2011) 755-63. P. Mulvaney: Surface Plasmon Spectroscopy of Nanosized Metal Particles: Langmuir 3 (1996) 788-800.
ip t
[1] [2]
cr
[3]
us
[4] [5]
an
[6]
[9] [10]
Ac ce p
[11]
te
[8]
d
M
[7]
[12] [13] [14] [15] [16] [17] [18] [19]
Page 8 of 15
[24] [25] [26] [27]
ip t
[23]
cr
[22]
us
[21]
F. Pincella, Y. Song, T. Ochiai, K. Isozaki, K. Sakamoto, K. Miki: Square-centimeter-scale 2D-arrays of Au@Ag core-shell nanoparticles towards practical SERS substrates with enhancement factor of 107. Chem. Phys. Lett. 605-606 115-20. L. Guerrini, J. V. Garcia-Ramos, C. Domingo, S. Sanchez-Cortes, Ultrathin silver-coated gold nanoparticles as suitable substrate for surface-enhanced Raman scattering. J. Raman Spectrosc. 41 (2010) 508-15. T. Zhou, M. Rong, Z. Cai, C. J. Yang, X. Chen: Sonochemical synthesis of highly fluorescent glutathione-stabilized Ag nanoclusters and S2- sensing. Nanoscale 4 (2012) 4103-06. X. Yuan, T. J. Yeow, Q. Zhang, J. Y. Lee, J. Xie: Highly luminescent Ag+ nanoclusters for Hg2+ ion detection. Nanoscale 4 (2012) 1968-71. M. Yamamoto, Y. Kashiwagi, M. Nakamoto: Size-Controlled Synthesis of Monodispersed Silver Nanoparticles Capped by Long-Chain Alkyl Carboxylates from Silver Carboxylate and Tertiary Amine. Langmuir 22 (2006) 8581-86. M. Yamamoto, M. Nakamoto: Novel preparation of monodispersed silver nanoparticles via amine adducts derived from insoluble silver myristate in tertiary alkylamine. J. Mater. Chem. 13 (2003) 2064–65. M.J. Rosemary, T. Pradeep: Solvothermal synthesis of silver nanoparticles from thiolates. J. Colloid Interface Sci. 268 (2003) 81–84. G. Carotenuto, F. Nicolais: Reversible Thermochromic Nanocomposites Based on ThiolateCapped Silver Nanoparticles Embedded in Amorphous Polystyrene. Materials, 2 (2009)
an
[20]
1323-40
[30] [31]
M
Ac ce p
[32]
d
[29]
A. Dhakshinamoorthy, M. Alvaro, H. Garcia Aerobic oxidation of thiols to disulfides using iron metal–organic frameworks as solid redox catalysts, Chem. Commun., 46 (2010) 6476– 78. J Sharma, H-C Yeh, H Yoo, JH Werner, JS Martinez: A complementary palette of fluorescent silver nanoclusters. Chem. Comm. 46 (2010) 3280-82. J. E. Huheey, Inorganic chemistry: principles of structure and reactivity, Harper & Row, 1983. P. Sahu, B. L. V Prasad: Effect of digestive ripening agent on nanoparticle size in the digestive ripening process. Chemical Physics Letters 525-526 (2012) 101-04. Y. Ma, W. Li, E. C. Cho, Z. Li, T. Yu, J. Zeng, Z. Xie, Y. Xie, Au@Ag Core−Shell Nanocubes with Finely Tuned and Well-Controlled Sizes, Shell Thicknesses, and Optical Properties, ACS Nano 4 (2010) 6725-6734
te
[28]
Page 9 of 15
ip t cr us an M d te
Figure 1: A) the UV-Vis absorbance spectra and TEM images of Au@Ag NPs obtained in the B)
Ac ce p
bright field and C) dark field mode. D represent the EDS analysis. Line scan of Ag@Au NPs is given as inset of D.
Page 10 of 15
ip t cr us an M d te
Ac ce p
Figure 2: A) the UV-Vis absorbance spectra of pure DDT capped Au NPs. TEM images of the product when AgOBz was reacted with DDT capped Au NPs obtained in the B) bright field mode and C) dark field mode. The area mapping and EDS (D-F) indicate presence of individual Au and Ag NPs.
Page 11 of 15
ip t cr us an M d te
Figure 3: Schematic of the different types of nanostructure formation in this study. While addition of
Ac ce p
AgOBz to amine capped Au NPs resulted in Ag@Au particles, addition of AgOBz to thiol capped Au NPs resulted in separate Au and Ag NPs.
Page 12 of 15
ip t cr us an M d
te
Figure 4: A) the UV-Vis absorbance spectra B and C) TEM images of DDA capped Ag NPs. Inset of B and C represent SAED pattern and HRTEM images of Ag NPs. D-F) the UV-Vis absorbance
Ac ce p
spectra and TEM images of DDT capped Ag NPs. Inset of B represents SAED pattern for Ag NPs.
Page 13 of 15
Highlights Unconventional Aucore-Agshell NPs were prepared by a simple and convenient method.
ip t
Ag+ ions were reacted with pre-formed dodecylamine or dodecanethiol capped Au NPs. Reaction with dodecylamine capped Au NPs resulted in Aucore-Agshell structures.
cr
Reaction with dodecanethiol capped Au NPs resulted in individual Ag and Au NPs.
Ac ce p
te
d
M
an
us
Aucore-Agshell NPs formation by this route critically depends on the ligand nature.
Page 14 of 15
Ac
ce
pt
ed
M
an
us
cr
i
Graphical Abstract (for review)
Page 15 of 15
S0927-7757(15)00249-6 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.03.028 COLSUA 19819
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
10-2-2015 18-3-2015 20-3-2015
Please cite this article as: P. Sahu, B.L.V. Prasad, Preparation of Ag(Shell) Au(Core) nanoparticles by anti Galvanic Reactions: Are capping agents the “real heroes” of reduction?, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2015), http://dx.doi.org/10.1016/j.colsurfa.2015.03.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of Ag(Shell) - Au(Core) nanoparticles by anti Galvanic Reactions: Are capping agents the “real heroes” of reduction?
Physical/Materials Chemistry Division CSIR-National Chemical Laboratory *Email: [email protected]
cr
Dr. Homi Bhabha Road, Pune 411008, India
ip t
Puspanjali Sahu and B. L. V. Prasad*
us
Tel. No. +91-20-25902013. Fax: +91-20-25902636
Abstract
an
The formation of Ag(shell)-Au(core) nanoparticles by the reduction of Ag+ ions on preformed Au NPs, mediated by the capping agents like dodecylamine is described. We clearly established that
M
while monodisperse Ag(shell)-Au(core) structures can be formed in presence of weak surface binding ligands such as amines, strong binding ligands such as thiols, lead to formation of separate monometallic Au and Ag particles. Apart from highlighting the influence of metal-ligand binding
d
strength on the formation of core-shell nanoparticle architectures, our study also provides an easy
te
means to produce Ag(shell)-Au(core) structures.
Ac ce p
Keywords: Ag(shell)-Au(core) nanoparticles, anti Galvanic reactions
Page 1 of 15
1. Introduction Recent years gave birth to eloquent ways of colloidal nanostructure preparations and ligands (organic molecules/polymers etc.) evolved as central components in manipulating several morphological features of nanoparticles (NPs).[1-3] Among the different nano-architectures, core-shell type
ip t
structures are receiving immense interest because of their improved physical and chemical properties over their single-component counterparts.[4-8] The synthesis and characterization of bimetallic core-
cr
shell systems is well studied.[9-12] Among the plethora of synthetic strategies developed for metalmetal core-shell NP preparation, sequential reduction method stands out as the most efficient and hence commonly used. This involves use of any pre-synthesized metal NP (this would become the
us
core ultimately) as seeds, on which second metal could be deposited as shell assisted by an external reducing agent. A precondition for this procedure to work efficiently is that the lattice constant of both
an
the metals should match to a certain extent. For example it is easy to generate conformational coating of Au on surface of Ag NPs or vice versa due to their close match of lattice constant. Though this method is easy and convenient, it suffers from many drawbacks such as stepwise preparation of metal
M
NPs, requirement of stringent reaction environment and strong dependence of the final product on the type of seed used (i.e. size, shape etc), pH, the reducing agent, solvent, etc.[13] Also the formation of out on large scale.
d
individual monometallic NPs by this method cannot be avoided, especially if the reaction is carried
te
Many other methods were proposed to overcome these drawbacks. For example formation of monometallic NPs can be avoided by use of Galvanic displacement method. In this procedure, the metal ions having less positive reduction potential form the core and can act as the reducing agent for
Ac ce p
the metal having more positive reduction potential. This results in the growth of second metal as shell. But this protocol suffers from the restriction that only a more noble metal can form the shell on a less noble metal. For example it is easy to generate Au shell on Ag cores by this method owing to high reduction potential of Au than Ag but the inverse (Ag shell on Au core) is not possible by this method. While few protocols for preparation of Ag(shell)-Au(core) particles have been developed, the complicated synthesis procedure involved in these protocols prevents their general use.[14] In this background some recent reports claim that small NPs of more noble metals such as Au and Pt can reduce less noble metal ions like Ag+ which is a deviation from the conventional free energy based arguments used for Galvanic displacement.[15-17] This phenomenon has been termed as Anti Galvanic Reduction (AGR). In some of these methods ligand protected Au NPs were used to which a silver precursor is added subsequently. This raises a question whether the ligand shell present on the nanoparticle surface is acting as a reducing agent.[15,16] In some other where the Au NPs were not coated with any ligand it is not clear whether the initial reducing agent, used to prepare the Au NPs has been completely removed.[16] Also in this method the shell formation was deduced from indirect
Page 2 of 15
evidence as direct imaging did not provide evidence for shell formation, raising question about the effectiveness of the procedure. Thus we felt the need for a systematic study of this phenomenon. Accordingly, we developed a simple method for synthesis of Ag(shell) on Au(core) (these are termed as Ag@Au NPs in the rest of the paper following the normal convention) without addition of any external reducing agent. This was achieved by addition of AgOBz to the dispersion of pre synthesized
ip t
dodecylamine (DDA) capped Au NPs. The formation of Ag@Au NPs by this simple way can take place via any of the following two pathways.
1. The surface atoms (Au0) of Au NPs can act as reducing agents (AGR reaction).
cr
2. The excess DDA molecules present in the dispersion can reduce Ag+ ions to Ag0.
In order to understand which one of the above causes the formation of Ag@Au structures, the reaction
us
was repeated with Au NPs capped by other ligands such as dodecanethiol (DDT). When DDT capped Au NPs were used, no core-shell type architecture was formed; instead separate Au and Ag particles were formed. This failure in the synthesis of Ag@Au particles when DDT capped Au NPs were used
an
as seeds, clearly demonstrated that the reduction of Ag+ on Au0 is mediated by DDA and that the nature of ligand binding with the nanoparticle surface plays a significant role in determining the final
M
nanoparticle architecture (core-shell or individual particles).
2. Material and Method
d
2.1. Materials Toluene was purchased from Merck, India and was degassed before use. Other chemicals were
2.2. Experimental Procedure
te
purchased from Sigma Aldrich and used as received.
Ac ce p
Au NPs were prepared by well known digestive ripening method.[18] In this method, AuCl3 is dispersed in an organic solvent (toluene in the present case) in presence of a long chain surfactant i.e. didodecyldimethylammonium bromide (DDAB). The Au (III) is then reduced to Au (0) with the help of NaBH4. Excess ligand (1:30 metal:ligand ratio) was added to the as prepared Au NPs in order to prevent aggregation. These ligand capped NPs are then separated from excess ligand and other side products by precipitating them with excess ethanol addition. The precipitate was then re-dispersed in toluene and heated in the presence of excess ligand in order to obtain monodisperse NPs. Herein, Au NPs were synthesized in presence of two ligands i.e. DDT and DDA. Detailed synthetic procedure is given in supplementary material (ESM-1). To these Au NPs dispersion in toluene, silver benzoate (AgOBz) was added keeping the precursor molar ratio of Au: Ag 1:10 and the system was heated at 90 ⁰C for 5h. No additional reducing agent was added. 2.3. Instrumentation Formation of NPs and other nanostructures are characterized by UV-Visible (UV-Vis) spectroscopy and Transmission Electron Microscopy (TEM). UV–Visible absorption spectra of NP dispersions
Page 3 of 15
were measured using a Cary-300 UV–Visible spectrophotometer. Samples for TEM analysis were prepared by drop casting the NP dispersions on carbon coated copper grids (200 mesh). TEM analysis was performed on FEI, TECNAI G2 TF 30 and FEI, TECNAI G2 TF 20 instruments. Particle size distribution histograms were plotted taking 300 particles into account.
ip t
3. Results Since the digestive ripening involved repeated precipitation and redispersion we can rule out any remnants of the reducing agent in the final dispersion. Formation of Au NPs was initially confirmed
cr
by the localized surface plasmon resonance (LSPR) peak appeared at 526 nm for DDA and at 528 nm for DDT capped Au NPs[19]. Particle size and size distribution was calculated from transmission
us
electron microscopy (TEM) images, which confirmed that the Au NPs synthesized with both the ligands are monodisperse (ESM-2).
Silver benzoate (AgOBz) was added to these Au NPs dispersion in toluene, keeping the precursor
an
molar ratio of Au: Ag 1:10 and the system was heated at 90 ⁰C for 5h. When AgOBz was added to wine red dispersion of Au NPs capped with DDA, the color of the dispersion became yellow. DDA capped Au NPs, before addition of AgOBz showed a LSPR band at 526 nm characteristic of Au NPs.
M
Upon addition of AgOBz, an increase in intensity of the LSPR band was observed and the band position shifted to lower wavelength with time. The extent of blue shift was initially high i.e. max shifted from 526 nm to 520 nm within 1h of reaction and from 520 nm to 516 nm after 2h of reaction.
d
Negligible shift was observed on further heating i.e. max shifted from 516 nm to 514 nm upon
te
continuing heating for 5h. The same trend was also observed for absorbance intensity. Very strong enhancement in absorbance intensity was noticed up to 2h of heating and only slight enhancement
Ac ce p
was observed afterwards (Fig-1a).
TEM gives further insight into the change occurred in the NPs structure. It was found that the initially monodisperse Au NPs got converted to a core-shell type architecture and the particles formed are reasonably monodisperse with average diameter of 12-14 nm and shell thickness of 2-4 nm (fig 1B). These core-shell structures were further analyzed by dark field mode TEM imaging. The core of the particles appears brighter compared to shell (Fig 1C). Presence of both Au and Ag was further confirmed by Energy-dispersive X-ray Spectral (EDS) analysis (fig-1D). EDS line scan confirmed that both Au and Ag were co-located in the same NPs with higher intensity of Ag at the periphery (inset of Fig. 1D) and no trace of any separated monometallic NPs could be seen. These core-shell particles formed are highly crystalline with exposed 111 planes (inset of Fig. 1B). More TEM images are provided in ESM-3. Thus the above data unequivocally confirm the formation of Ag@Au particles when DDA capped Au NPs are reacted with AgOBz. On the other hand when AgOBz was added to Au NPs capped by DDT, the Au LSPR peak shifted to higher wavelength (555 nm) and another peak appeared at 350 nm. No significant change in peak position was observed upon heating the dispersion for long time (5h) expect slight broadening of the
Page 4 of 15
peak at 350 nm (Fig. 2A). TEM was used for further inspection of the samples prepared from DDT capped Au NPs (Fig. 2B). It was observed that bigger NPs of dark contrast are anchored over a layer of smaller NPs. As the bigger NPs appeared darker in the TEM images obtained in the bright field mode (Fig. 2C) and brighter in dark field mode TEM images, we deduce that these belong to the Au NPs and smaller ones can be claimed as Ag NPs. Presence of both Au and Ag was confirmed by area
ip t
mapping and EDS analysis (Fig. 2D-2F). More TEM images are provided in ESM-4.
4. Discussion
cr
The results obtained are schematically depicted in Fig-3. Addition of AgOBz to amine capped Au NPs, resulted in the formation of Ag@Au NPs. The core-shell formation was confirmed by both UV-
us
Vis spectroscopy and TEM. The Ag shell formation on Au core was associated with an initial increase in SPR band intensity and blue shift of band position.[20, 21] It is well documented that SPR maxima shifts towards lower wavelength with increase in thickness of silver shell. As the shift in peak position
an
decreased significantly after 2h and become negligible at 5h, the reaction was stopped at 5h. TEM gives further confirmation of Ag@Au formation as mentioned in result section.
M
In contrary to the above observation, when AgOBz was added to Au NPs capped by DDT, both Au and Ag NPs formed separately. Formation of separate Au and Ag particles is confirmed by appearance of two separate peaks in UV-Vis spectra at 550 nm and 350 nm, characteristic of Au NPs
d
Ag NPs. [22, 23] Appearance of two distinct peaks for Ag and Au clearly indicate non-interacting nature of both these metals, when DDT was used as capping agent. It was also observed from TEM
te
that Au NPs were anchored over the layer formed by smaller Ag NPs. Both the above mentioned preparation procedure differ only in the nature of anchoring group of the
Ac ce p
ligand. This leads to the question, why the simple change in anchoring group of the ligands completely changes the resultant nanostructures? Both of these ligands have same chain length; hence the steric influence of the ligands on formation of different nanostructures can be overruled. We therefore reckoned that the binding nature of the ligands towards Au and Ag NPs may have a significant impact on the resultant these nanostructures. To understand the effect more clearly, a reaction was carried out, where AgOBz dissolved in toluene was heated at 90 0C with DDT or DDA. With both the ligands reduction of Ag (I) to Ag (0) was seen to start immediately. Reduction of Ag(I) to Ag(0) by amine is well known. It is postulated that while reducing the silver ions, amines undergo involving the formation of an intermediate, involving both the silver carboxylate and the amine [24,25]. We believe that in the present case also similar products could have formed. After the reaction between DDA and AgoBz two peaks (at 474 nm and 494 nm) appeared in the UV-Vis spectra indicating formation of Ag NPs [26]. Continuation of heating caused merging of both the peaks and a new broad peak to appear at 470 nm. Broadening of the peak and substantial red shift of peak position compared to Ag NPs (420 nm) indicate formation of bigger particles or aggregation of small Ag NPs formed initially (Fig-4A). TEM confirms aggregation of small NPs as the cause for the broad peak
Page 5 of 15
appeared at 470 nm when AgOBz was heated in presence of DDA for longer time (Fig-4B and 4C). Highly crystalline nature of DDA capped Ag NPs is proved by selected area electron diffraction pattern (SAED) (inset of Fig. 4B) and HRTEM image (inset of Fig 4C). When the same reaction of AgOBz was carried out with DDT, small Ag NPs formed immediately as indicated by the peak at 350 nm in UV-Vis spectroscopy.[22] Thiols are susceptible to easy oxidation
ip t
forming products like disulfides, sulfones, sulfoxides and sulfonic acids and hence could act as reducing agents as far as noble metals like silver are concerned[26-28]. While the reduction of silver ions could be accomplished both by DDA and DDT there are some interesting differences in the type
cr
of nanoparticles formed. We would like to remind that when in AgOBz is reacted with DDA, initially in the UV-Vis spectrum two peaks are formed which merge into one as the reaction is continued.
us
Interestingly, the reaction between DDT and AgoBz resulted the formation of only one peak could be seen and that also underwent a negligible shift (from 350 to 354 nm) when the heating was prolonged for 5h. This slight change can be attributed, if any, to only a small increment in particle size of Ag
an
NPs[29]. Formation of Ag NPs are further confirmed by TEM (Fig-4E and 4F). As expected, unlike the amine case, the thiol capped Ag NPs formed are small in size and well separated.
M
Based on the above observation we propose the following sequence of reactions during the formation of Ag@Au NPs. Before proceeding, however, we would like to re-emphasize two facts: (i) both DDA and DDT reduce Ag+ ions to Ag0 and (ii) The binding of DDA to Au and Ag NP surface is weaker as
d
compared to the binding of DDT binding to these nanoparticle surfaces. This second feature has been explained qualitatively based on Hard Soft Acid Base (HSAB) principle and has been validated
te
theoretically [30] and experimentally.[31]
Owing to the procedure used for the preparation, the Au NP dispersions in toluene retain a lot of
Ac ce p
excess ligands in solution, which are one of the reagents in current study. Since refluxing AgOBz with DDA or DDT alone leads to the formation of Ag nanostructures, we reckon that when AgOBz was added to DDA capped Au NPs, the excess ligand present in the Au NP dispersion reduces the Ag+ to Ag0. However, as DDA does bind very strongly to the Au NP surface, the naked NP surface gets exposed when the DDA molecules detach from the Au NP especially under the reflux conditions. At the same time because DDA does not bind strongly to Ag0 surface, nascent uncapped Ag0 atoms/clusters got generated in the solution. These prefer to attach on the surface of Au NPs those have been formed due to ligand detachment as described above, leading to formation of Ag@Au NPs. Our observation that the silver nanostructures formed by the reaction of DDA with AgOBz are not stable and keep growing to form bigger particles as the reaction is allowed to proceed for longer duration accords well with the above discussion. The formation of core-shell architecture is also facilitated by the fact that attachment of Ag on Au NPs surface is thermodynamically favourable, as both Ag and Au possess almost same lattice constant value (differ by 0.2%). [32] On the other hand DDT has strong affinity towards both Ag and Au NPs surface. Hence once Ag+ ions are reduced to Ag0 nanostructures by DDT, they get capped/stabilized by it immediately. This
Page 6 of 15
contention is asserted by the observation that even after prolonged heating there is no big difference in the optical spectra and sizes of the small Ag NPs formed by the reduction of Ag+ ions independently with DDT. So when the reaction is carried out in presence of preformed DDT capped Au NPs we could see the formation of similar small Ag NPs (reduced and capped by the excess DDT present in solution). But these are very stable. Such stable Ag NPs do not attach on the Au NPs surface.
ip t
Secondly as thiols bind to Au NPs surface very strongly, there is no exposed surface on Au NPs also on which these small Ag NPs can attach. Thus the strong affinity of thiols towards both the metal surfaces does not allow the growth of Ag@Au NPs but result in the formation individual Au and Ag
cr
NPs.
us
5. Conclusions
By using ligands possessing different binding affinity toward metal surfaces, we have developed a simple and convenient approach for the preparation of bimetallic Ag@Au NPs. In
an
particular, we demonstrated that reduction of Ag ions in presence of preformed dodecylamine capped Au NPs leads to Ag(shell)-Au(core) nanoparticle formation. On the other hand carrying out the same preparation procedure with dodecanethiol capped Au NPs results in the
M
formation of individual monometallic NPs. So this study not only provides an easy means to synthesize Ag@Au NPs, but also sheds light on the role of ligands in the formation of
d
bimetallic core-shell structures. We hope such a better understanding of the role of ligand will provide greater impetus for the preparation of designer ligands that can provide fine control
te
over multi metallic naostructure synthesis. Our studies also highlight the necessity to study the proposed anti Galvanic reactions in a more careful manner as in those reports also the
Ac ce p
ligands may have actually played the role of reducing agents.
Acknowledgement
P.S. thanks the Council of Scientific and Industrial Research for a fellowship. We gratefully acknowledge CSIR, New Delhi for financial support through XII five year plan network project BSC0112 (Nano-SHE). We thank the IPC Department IISc for helping us with TEM analysis. We also acknowledge the Indo-US Science and Technology Forum (IUSSTF) for support through the Joint Virtual Centre “From Fundamentals to Applications of Nanoparticle Assemblies”.
Page 7 of 15
References
N. Ortiz, S. E. Skrabalak, On the Dual Roles of Ligands in the Synthesis of Colloidal Metal Nanostructures. Langmuir 30 (2014) 6649-59. J. D. A. Iii, R. G. Finke, A review of modern transition-metal nanoclusters: their synthesis, characterization, and applications in catalysis. J. Mol. Catal. A: Chem. 145 (1999) 1-44. M-C. Daniel, D. Astruc, Gold nanoparticles: assembly, supramolecular chemistry, quantumsize-related properties, and applications toward biology, catalysis, and nanotechnology. Chem. rev. 104 (2004) 293-346. P. Migowski, J. Dupont: Catalytic Applications of Metal Nanoparticles in Imidazolium Ionic Liquids. Chem. Eur. J. 13 (2007) 32-39. K. Zhang, Y. Xiang, X. Wu, L. Feng, W. He, J. Liu, W. Zhou, S. Xie: Enhanced Optical Responses of Au@Pd Core/Shell Nanobars. Langmuir 25 (2008) 1162-68. R. G. Chaudhuri, S. Paria, Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization and Applications. Chem. Rev. 112 (2012) 2373-433. A. K. Samal, L. Polavarapu, S. Rodal-Cedeira, L. M. Liz-Marzan, J. Perez-Juste, I. PastarizaSantos, Size Tunable Au@Ag Core–Shell Nanoparticles: Synthesis and Surface-Enhanced Raman Scattering Properties, Langmuir 29 (2013) 15076-15082. Z.-B. Zeng, Y.-Y. Cao, J-J. Chen, X.-D. Wang, J.-F. Yan, X. Chen, Au@Ag core/shell nanoparticles as colorimetric probes for cyanide sensing, Nanoscale 6 (2014) 9939-9943. A. Sarkar, A. Manthiram, Synthesis of Pt@Cu Core-Shell Nanoparticles by Galvanic Displacement of Cu by Pt4+ Ions and Their Application as Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells. J. Phys. Chem. C 114 (2010) 4725-32. Q. Zhang, J. Xie, J.Y. Lee, J. Zhang, C. Boothroyd, Synthesis of Ag@AgAu Metal Core/Alloy Shell Bimetallic Nanoparticles with Tunable Shell Compositions by a Galvanic Replacement Reaction. Small 4 (2008) 1067-71. Y. Mizukoshi, T. Fujimoto, Y. Nagata, R Oshima, Y Maeda: Characterization and Catalytic Activity of Core-Shell Structured Gold/Palladium Bimetallic Nanoparticles Synthesized by the Sonochemical Method. J. Phys. Chem. B 104 (2000) 6028-32. K. Mallik, M. Mandal, N. Pradhan, T. Pal: Seed Mediated Formation of Bimetallic Nanoparticles by UV Irradiation:A Photochemical Approach for the Preparation of "CoreShell" Type Structures. Nano Lett. 1 (2001) 319-22. S. U. Son, Y. Jang, J. Park, H. B. Na, H. M. Park, H. J. Yun, J. Lee, T. Hyeon, Designed Synthesis of Atom-Economical Pd/Ni Bimetallic Nanoparticle-Based Catalysts for Sonogashira Coupling Reactions. J. Am. Chem. Soc. 126 (2004) 5026-27. C. J. Serpell, J. Cookson, D Ozkaya, PD Beer: Core@shell bimetallic nanoparticle synthesis via anion coordination. Nat. Chem. 3 (2011) 478-83. Z. Wu: Anti-Galvanic Reduction of Thiolate-Protected Gold and Silver Nanoparticles. Angew. Chem. Int. Ed. 51 (2012) 2934-38. G. Liu, D-Q. Feng, W. Zheng, T. Chen, D. Li: An anti-galvanic replacement reaction of DNA templated silver nanoclusters monitored by the light-scattering technique. Chem. Comm. 49 (2013) 7941-43. M. Wang, Z. Wu, Z. Chu, J. Yang, C. Yao, Chemico-Physical Synthesis of Surfactant- and Ligand-Free Gold Nanoparticles and Their Anti-Galvanic Reduction Property. Chem. Asian J. 9 (2014) 1006-10. D. S Sidhaye, B. L. V Prasad: Many manifestations of digestive ripening: monodispersity, superlattices and nanomachining. N. J. Chem. 35 (2011) 755-63. P. Mulvaney: Surface Plasmon Spectroscopy of Nanosized Metal Particles: Langmuir 3 (1996) 788-800.
ip t
[1] [2]
cr
[3]
us
[4] [5]
an
[6]
[9] [10]
Ac ce p
[11]
te
[8]
d
M
[7]
[12] [13] [14] [15] [16] [17] [18] [19]
Page 8 of 15
[24] [25] [26] [27]
ip t
[23]
cr
[22]
us
[21]
F. Pincella, Y. Song, T. Ochiai, K. Isozaki, K. Sakamoto, K. Miki: Square-centimeter-scale 2D-arrays of Au@Ag core-shell nanoparticles towards practical SERS substrates with enhancement factor of 107. Chem. Phys. Lett. 605-606 115-20. L. Guerrini, J. V. Garcia-Ramos, C. Domingo, S. Sanchez-Cortes, Ultrathin silver-coated gold nanoparticles as suitable substrate for surface-enhanced Raman scattering. J. Raman Spectrosc. 41 (2010) 508-15. T. Zhou, M. Rong, Z. Cai, C. J. Yang, X. Chen: Sonochemical synthesis of highly fluorescent glutathione-stabilized Ag nanoclusters and S2- sensing. Nanoscale 4 (2012) 4103-06. X. Yuan, T. J. Yeow, Q. Zhang, J. Y. Lee, J. Xie: Highly luminescent Ag+ nanoclusters for Hg2+ ion detection. Nanoscale 4 (2012) 1968-71. M. Yamamoto, Y. Kashiwagi, M. Nakamoto: Size-Controlled Synthesis of Monodispersed Silver Nanoparticles Capped by Long-Chain Alkyl Carboxylates from Silver Carboxylate and Tertiary Amine. Langmuir 22 (2006) 8581-86. M. Yamamoto, M. Nakamoto: Novel preparation of monodispersed silver nanoparticles via amine adducts derived from insoluble silver myristate in tertiary alkylamine. J. Mater. Chem. 13 (2003) 2064–65. M.J. Rosemary, T. Pradeep: Solvothermal synthesis of silver nanoparticles from thiolates. J. Colloid Interface Sci. 268 (2003) 81–84. G. Carotenuto, F. Nicolais: Reversible Thermochromic Nanocomposites Based on ThiolateCapped Silver Nanoparticles Embedded in Amorphous Polystyrene. Materials, 2 (2009)
an
[20]
1323-40
[30] [31]
M
Ac ce p
[32]
d
[29]
A. Dhakshinamoorthy, M. Alvaro, H. Garcia Aerobic oxidation of thiols to disulfides using iron metal–organic frameworks as solid redox catalysts, Chem. Commun., 46 (2010) 6476– 78. J Sharma, H-C Yeh, H Yoo, JH Werner, JS Martinez: A complementary palette of fluorescent silver nanoclusters. Chem. Comm. 46 (2010) 3280-82. J. E. Huheey, Inorganic chemistry: principles of structure and reactivity, Harper & Row, 1983. P. Sahu, B. L. V Prasad: Effect of digestive ripening agent on nanoparticle size in the digestive ripening process. Chemical Physics Letters 525-526 (2012) 101-04. Y. Ma, W. Li, E. C. Cho, Z. Li, T. Yu, J. Zeng, Z. Xie, Y. Xie, Au@Ag Core−Shell Nanocubes with Finely Tuned and Well-Controlled Sizes, Shell Thicknesses, and Optical Properties, ACS Nano 4 (2010) 6725-6734
te
[28]
Page 9 of 15
ip t cr us an M d te
Figure 1: A) the UV-Vis absorbance spectra and TEM images of Au@Ag NPs obtained in the B)
Ac ce p
bright field and C) dark field mode. D represent the EDS analysis. Line scan of Ag@Au NPs is given as inset of D.
Page 10 of 15
ip t cr us an M d te
Ac ce p
Figure 2: A) the UV-Vis absorbance spectra of pure DDT capped Au NPs. TEM images of the product when AgOBz was reacted with DDT capped Au NPs obtained in the B) bright field mode and C) dark field mode. The area mapping and EDS (D-F) indicate presence of individual Au and Ag NPs.
Page 11 of 15
ip t cr us an M d te
Figure 3: Schematic of the different types of nanostructure formation in this study. While addition of
Ac ce p
AgOBz to amine capped Au NPs resulted in Ag@Au particles, addition of AgOBz to thiol capped Au NPs resulted in separate Au and Ag NPs.
Page 12 of 15
ip t cr us an M d
te
Figure 4: A) the UV-Vis absorbance spectra B and C) TEM images of DDA capped Ag NPs. Inset of B and C represent SAED pattern and HRTEM images of Ag NPs. D-F) the UV-Vis absorbance
Ac ce p
spectra and TEM images of DDT capped Ag NPs. Inset of B represents SAED pattern for Ag NPs.
Page 13 of 15
Highlights Unconventional Aucore-Agshell NPs were prepared by a simple and convenient method.
ip t
Ag+ ions were reacted with pre-formed dodecylamine or dodecanethiol capped Au NPs. Reaction with dodecylamine capped Au NPs resulted in Aucore-Agshell structures.
cr
Reaction with dodecanethiol capped Au NPs resulted in individual Ag and Au NPs.
Ac ce p
te
d
M
an
us
Aucore-Agshell NPs formation by this route critically depends on the ligand nature.
Page 14 of 15
Ac
ce
pt
ed
M
an
us
cr
i
Graphical Abstract (for review)
Page 15 of 15