Article pubs.acs.org/JPCC
Silver Nanoparticles Stabilized with Thiols: A Close Look at the Local Chemistry and Chemical Structure Chiara Battocchio,*,† Carlo Meneghini,‡ Ilaria Fratoddi,§,∥ Iole Venditti,§ Maria Vittoria Russo,§ Giuliana Aquilanti,⊥ Chiara Maurizio,# Federica Bondino,∇ Roberto Matassa,○ Marco Rossi,∥,○ Settimio Mobilio,‡ and Giovanni Polzonetti† †
Department of Physics, INSTM, CNISM and CISDiC, University Roma Tre, Via della Vasca Navale 84, 00146 Rome, Italy Department of Physics, University Roma Tre, Via della Vasca Navale 84, 00146 Rome, Italy, and OGG GILDA, c\o ESRF, Grenoble, France § Department of Chemistry, and ∥Center for Nanotechnology for Engineering (CNIS), University of Rome “Sapienza”, P.le A.Moro 5, 00185 Rome, Italy ⊥ Sincrotrone Trieste S.C.p.A s.s. 14, km 163.5, I-34149 Basovizza, Trieste, Italy # Department of Physics and Astronomy, University of Padova, via Marzolo 8, 35131 Padova, Italy ∇ IOM-CNR, Laboratorio TASC, S.S.14, Km. 163.5, I-34149 Basovizza, Italy ○ Department of Basic and Applied Sciences for Engineering, University of Rome “Sapienza”, Via A. Scarpa 14, 00161 Roma, Italy ‡
ABSTRACT: The local atomic structure and chemical nature of newly synthesized silver nanoparticles (AgNPs) functionalized with the organic thiol allylmercaptane (AM) have been probed combining synchrotron radiation-based techniques: Xray photoelectron spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (XAFS). Complementary information about the chemical and electronic structure is obtained combining XAFS and XPS data. These results coherently suggest a core shell morphology of the NPs resulting in metallic Ag cores surrounded by Ag2S-like phase. The external layer of AM molecules is grafted to the NPs surface through Ag−S chemical bonds. NP size and composition were found as a function of the chemical synthetic route (i.e., Ag/AM molar ratio). It was observed that by increasing the Ag/AM ratio, larger AgNPs were obtained. It was found that a higher Ag/AM molar ratio leads to an increasing of the Ag2S layer thickness, while the external AM layer remains unvaried. TEM analysis showed well-separated and dispersed nanoparticles, and ED pattern allowed one to identify two different phases of single crystal corresponding to the presence of Ag face-center-cubic single-crystal symmetry, together with weak diffraction spots in agreement with Ag2S cubic symmetry in Im3m space groups. alized NPs can be widely modified to fit target applications, acting on the synthesis parameters and/or opportunely selecting the capping agents. As a matter of fact, the interactions with the capping molecules at the NPs surface add additional degrees of freedom, which may produce unexpected properties, such as the magnetic behavior observed for thiol-capped NPs of metals that do not exhibit magnetic feature in bulk form, such as Au, Ag, or Cu.2,6,7 By decreasing the NP size, the electronic and chemical structures of the surfaces/interfaces become relevant in determining their physical-chemical response; nevertheless, these details may be quite elusive to standard probes, whereby nominally similar systems may depict widely different average
1. INTRODUCTION Metallic nanoparticles (NPs) of noble elements may show peculiar chemical activity and unique physical properties such as photoluminescence, optical chirality, and even ferromagnetic behavior,1,2 making them suitable for innovative applications in catalysis, nanoelectronics, sensing, and bioanalysis.3 These special properties originate from a complex interplay among many competing factors such as the NP atomic and electronic structure, chemical composition, and morphology. Dealing with real systems, the NPs are usually embedded in matrixes or capped with opportune molecules (functionalization) to avoid coalescence and preserve their properties for applicative purposes. Functionalization of metallic NPs with organic ligands (molecule-capping method)4,5 is a synthesis route specifically addressed to the massive NPs production, thus providing a reliable control of particle composition, shape, and size distribution. Moreover, the overall properties of function© 2012 American Chemical Society
Received: June 12, 2012 Revised: August 7, 2012 Published: August 7, 2012 19571
dx.doi.org/10.1021/jp305748a | J. Phys. Chem. C 2012, 116, 19571−19578
The Journal of Physical Chemistry C
Article
Figure 1. HR-XPS spectra of AgNP/AM samples prepared with four different Ag/thiol ratios. All of the reported spectra have been normalized to the same counts value. (a) S2p spectra; (b) Ag3d spectra.
responses.8,9 Therefore, it is noticeable that understanding the relationship among structure, chemistry, and average properties of nanosized particles is a challenging task that requires advanced complementary probes to monitor the range of factors involved. The aim of this work is to provide detailed chemical and structural characterization of newly synthesized hybrid systems made by silver NPs functionalized with allyl mercaptan (AM) molecules to better understand the nature of the NPs and shed light on the grafting mode of AM molecules to silver at the NP surface. In this framework, complementary information has been obtained by exploiting state-of-the-art synchrotron radiation methodologies: high-resolution X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectroscopy at both Ag and S K-edges. The analysis of the XAFS data provides details about the local chemistry and atomic distribution and then the molecular structure around Ag and S sites. The analysis of XPS spectra allows us to understand the electronic nature of Ag and S moieties. By combining the information achieved by these two methods, it is possible to get a deep insight on the complex nature of the NPs likely having a core shell structure in which the metallic Ag core is surrounded by Ag2S-like phase. At the external NP surface, AM thiols are bridged to the NP forming Ag−S−C links. NP size increases by raising the Ag/AM ratio; moreover, by varying the Ag/AM stoichiometry, we obtained Ag@Ag2S@AM core−shell NPs with different shell thicknesses. Several studies are reported in the literature concerning the preparation and structural investigation of Ag@Ag2S core−shell systems, but they are usually prepared by means of different preparative routes as, for example, ion implantation, sol−gel synthesis,10 or a sacrificial templating route, that is, a first step of AgNPs preparation followed by sacrificial oxidation by reaction with atomic S.11,12 In this context, our study about the AM reactivity at the AgNPs surface and then Ag@Ag 2 S@AM hybrid formation is completely new.
Brust−Schriffin procedure13−16 with Ag/S molar ratios of 0.25/1, 0.5/1, 1/1, and 2/1, in maintaining the AgNO3 concentration constant and always using the same molar ratio between reactants, TOAB and NaBH4. The typical procedure used for the 0.25/1 sample is herein reported as an example: a toluene solution of AM (0.375 mL, 4.71 mmol, toluene 40 mL) is mixed with a solution of AgNO3 (200 mg, 1.18 mmol, in 20 mL of deionized H2O). Next, 360 mg (0.658 mmol) of TOAB in 40 mL of toluene was added, and the suspension was bubbled under argon at room temperature for 15 min. 260 mg (6.87 mmol) of NaBH4 dissolved in 15 mL of deionized water was added dropwise, and the reaction mixture turned from yellow to brown color. The mixture was allowed to react for 2 h at room temperature under magnetic stirring. The obtained product was extracted with H2O, and the obtained brown solid was isolated by evaporation of the organic layer. The solid was resuspended in absolute ethanol, and separated by centrifugation with water five times at 5000 rpm. The brown product was recovered with a total yield of 32%. High-resolution XPS experiments (HR-XPS) were carried out at the BACH (Beamline for Advanced DiCHroism) line at the ELETTRA synchrotron facility.17 XPS data were collected in fixed analyzer transmission mode (pass energy = 30 eV). Photon energies of 380 and 460 eV were used for C1s, S2p, and Ag3d spectral regions with energy resolution ΔE = 0.29 eV. The Au Fermi energy and metallic Ag3d5/2 signals were used for the energy scale calibration. XPS data analysis was performed via curve-fitting of S2p and Ag3d experimental spectra, by using a combination of Voigt shaped peaks. The S2p 3/2−S2p 1/2 and Ag3d5/2−Ag3d 3/2 doublets were fitted by using the same fwhm for the two spin−orbit components of the same signal, with a spin−orbit splitting of 1.2 eV for S2p and 6.0 eV for Ag3d, and branching ratios S2p3/2/S2p1/2 = 2, Ag3d5/2/Ag3d3/2 = 3/2, respectively, have been detected. In case of identifying the presence of many chemically different species of the same element, the same fwhm (full width half-maximum) value was used for all individual photoemission bands to reduce the number of refinement parameters, thus improving the reliability of the results.
2. EXPERIMENTAL SECTION AM-stabilized AgNPs were prepared at room temperature in a two-phase system under argon atmosphere, via modified 19572
dx.doi.org/10.1021/jp305748a | J. Phys. Chem. C 2012, 116, 19571−19578
The Journal of Physical Chemistry C
Article
Table 1. HR-XPS Data Collected on Four AgNP/AM Samples Prepared with Different Ag/Thiol Ratiosa S2p3/2 Ag/AM 2/1
1/1
0.5/1
0.25/1
BE (eV) xratiosa fwhm (eV) BE (eV) xratiosa fwhm (eV) BE (eV) xratiosa fwhm (eV) BE (eV) xratiosa fwhm (eV)
Ag3d5/2
Ag2S-like
Ag−S−Rb
physisorbed AM
Ag(0) + Ag2S
Ag−S−Rb
161.00 0.60 1.02 160.59 0.50 1.26 160.91 0.45 0.97 160.98 0.37 0.90
161.94 0.22
163.22 0.18
369.03 0.92 0.97 368.20 >0.95 1.07 368.20 >0.90 1.01 368.20 >0.92 1.01
369.23
