Gold Nanoparticles Stabilized with Aromatic Thiols ... - ACS Publications

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Gold Nanoparticles Stabilized with Aromatic Thiols: Interaction at the Molecule−Metal Interface and Ligand Arrangement in the Molecular Shell Investigated by SR-XPS and NEXAFS Chiara Battocchio,*,† Francesco Porcaro,† Subhrangsu Mukherjee,‡,§,⊥ Elena Magnano,§ Silvia Nappini,§ Ilaria Fratoddi,∥ Maurizio Quintiliani,∥ Maria Vittoria Russo,∥ and Giovanni Polzonetti† †

Department of Sciences and CISDiC, Roma Tre University, via della Vasca Navale 79, 00146 Rome, Italy International Centre for Theoretical Physics, Strada Costiera 11, 34151 Trieste, Italy § IOM-CNR Laboratorio TASC, SS 14, Km 163,5 Basovizza, 34149 Trieste, Italy ∥ Department of Chemistry, Sapienza University, P.le A. Moro 5, 00185 Rome, Italy ‡

S Supporting Information *

ABSTRACT: Small gold nanoparticles capped with 4trimethylsilylethynyl-1-acetylthiobenzene (SEB) were prepared with spherical shape and different mean sizes (5−8 nm). The functionalized gold nanoparticles (AuNPs-SEB) were deposited onto TiO2 substrates, and the interaction at the molecule−gold interface, the molecular layer thickness, and the ligand organization on the surface of Au nanospheres were investigated by means of synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS) and angular dependent near edge X-ray absorption spectroscopy (NEXAFS) at the C K-edge. In order to obtain better insight into the molecular shell features, the same measurements were also carried out on a self-assembling monolayer (SAM) of SEB anchored on a “flat” gold surface (Au/Si(111) wafer). The comparison between angular dependent NEXAFS spectra collected on the self-assembling monolayer and AuNPs-SEB allowed for successfully probing the molecular arrangement of SEB molecules on the gold nanospheres surface. Furthermore, DFT calculations on the free SEB molecule as well as bonded to a small cluster of gold atoms were developed. The comparison with experimental results allowed better understanding of the spectroscopic signatures in the experimental absorption spectra and rationalization of the molecular organization in the SAM, NPs having a thin molecular shell, and NPs covered by a thick layer of ligands.

1. INTRODUCTION

strongly dependent on nanoparticle interactions: for example, coupled-particle localized surface plasmon resonance (LSPR)5 occurs at a frequency that is shifted from the single particle one, and the magnitude of this assembly induced plasmon shift depends on the strength of the interparticle coupling that, in turn, depends on the distance between adjacent particles.6 Gold particles organized into complex three-dimensional architectures give rise to a broad, red-shifted LSPR feature, and the morphology of the plasmonic particle assemblies has a strong impact on their optical response.7 Since the plasmonic field coupling decays exponentially with interparticle distance, it is of primary importance, from a technological point of view, to ensure that the noble metal nanoparticles are conveniently spaced.8 The dispersion and aggregation of metal nanoparticles (MNPs) critically depends on their surface chemistry, and for

There is a considerable amount of literature on metal nanoparticles and their technological applications. This is due to the surprising chemical−physical behavior of the nanomaterials: they show peculiar properties noticeably different from bulk materials with the same composition, and their optical and chemical properties vary with mean sizes, dispersion, and order. This makes metal-based nanoparticles outstanding candidates for technological applications in a number of fields, from nanoelectronics to biomedicine and catalysis.1,2 Among others, gold nanoparticles (AuNPs) have been found to improve the performance of DSSC (dye sensitized solar cells),3 and AuNPs embedded into proton-conducting membranes of polymer electrolytes open new perspectives in fuel cell applications.4 Although an impressive amount of research work has been done and reported about fundamental and applicative properties of noble metal nanoparticles, some aspects are still open for investigation. Among others, the topic of the influence of nanoparticle organization on their optical properties is extremely interesting. In fact, the plasmonic properties are © 2014 American Chemical Society

Received: December 24, 2013 Revised: March 17, 2014 Published: March 25, 2014 8159

dx.doi.org/10.1021/jp4126057 | J. Phys. Chem. C 2014, 118, 8159−8168

The Journal of Physical Chemistry C

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assembling monolayer of SEB anchored on a “flat” gold surface (Au/Si(111) wafer): the comparison between data collected onto the self-assembling monolayer and AuNPs-SEB was carried out with the aim to probe the molecular arrangement of SEB molecules on the gold nanoparticle “core” surface, and to demonstrate that angular dependent NEXAFS analysis, extensively used up to now to investigate self-assembling monolayers on “flat” surfaces, is a reliable and useful technique for the study of the local molecular arrangement also in the overlayer adsorbed onto the nanoparticle surface. For the purpose of comparison, and to obtain a deeper level of interpretation of the experimental data, density functional theory (DFT) calculations were carried out on a SEB molecule, both free and bonded to an Au11 cluster.

a number of materials, a surface passivation layer is required to insulate the core for the environment and stabilize the MNPs. Recently, much attention has been devoted to the development of new strategies for MNP stabilization. The chemical stabilization of MNPs by means of capping metallic clusters with appropriate organic ligands, i.e., the so-called moleculecapping method, presents several advantages over other preparative methods such as low preparation costs and better NP size, monodispersity, and shape control. The capping molecules play a double role: on one hand, they stabilize the MNPs; on the other hand, they can be opportunely selected to functionalize the metal cluster, making the resulting system better suited for different applications. Small (diameter of less than 5 nm) MNPs of noble metals are very good candidates for technological applications such as catalysis, nanoelectronics, sensing, and bioanalysis.9 The enhanced stability of noble metal nanoparticles capped with ligands has been attributed mainly to the relatively strong chemical bond at the interface between the capping agent and the transition metal. Among other techniques that are appropriate to address this topic, synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS) has already been extensively used to investigate the sulfur−metal bonds of self-assembled monolayers of organometallic thiols on flat surfaces10,11 as well as on noble metal nanoparticles.12,13 In particular, the high surface to bulk ratio of metal nanoparticles makes them ideally suited to sensitive SR-XPS measurements. Furthermore, XPS is an appropriate and widely used method to quantitatively measure the amount of material at the NP surface.14 On the other hand, the angular dependent NEXAFS technique is extremely well suited to investigate small aromatic molecules’ self-assembling behavior on metal or oxide surfaces15 and was applied to the investigation of functionalized NPs, showing a correlation between the NP size and the molecular order.16 Hereafter, a study about the ability of appropriate ligands to attain an ordered arrangement on the surface of metal nanoparticles is reported, in view of identify a new chemical strategy to induce nanoparticles to self-assemble on surfaces. 4Trimethylsilylethynyl-1-acetylthiobenzene (SEB), an aromatic ligand of simple chemical structure and well-ascertained solubility and stability, was selected as capping agent and on purpose synthesized. This substrate offers the potential to be easily converted in the thiol derivative and linked to the gold surface,17 and due to the silyl group, it can be used for subsequent synthetic steps, for example, for C−C coupling reactions.18 In fact, the silyl protection can be easily removed to achieve an alkyne-terminal functionality19 and be allowed to react and obtain nanoparticles with modified external functionalities.20,21 Gold nanoparticles stabilized with SEB (AuNPs-SEB) have been prepared with four different Au/thiol stoichiometric ratios, in order to modulate their size; AuNPs-SEB have been deposited onto a TiO2 surface. The chemical bond between capping molecules and AuNP, as well as the thiol/Au interface and thickness of the molecular overlayer, was investigated by means of synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS). The molecular organization of the functionalizing thiol layer was studied by means of angular dependent near edge X-ray absorption spectroscopy (NEXAFS) at the C K-edge. In order to obtain a better insight into the “metallic core”/“molecular overlayer” interface and SEB molecule organization in the overlayer shell, both SR-XPS and angular dependent NEXAFS data were also collected on a self-

2. MATERIALS AND METHODS 2.1. Materials. 4-Iodo-1-acetylthiobenzene and 4-trimethylsilylethynyl-1-acetylthiobenzene (SEB) were prepared according to procedures reported in the literature.22,23 Details on the synthetic procedures and main characterizations are reported in the Supporting Information (instruments and materials details, reaction schemes, and Figures S1−S9). 2.1.1. Synthesis of Gold Nanoparticles. Gold nanoparticles stabilized with the thiol ligand in situ generated from SEB have been prepared by mixing HAuCl4·3H2O and SEB with Au/S molar ratios 1.00/1 (NP(1)), 0.70/1 (NP(2)), 0.50/1 (NP(3)), and 0.25/1 (NP(4)). One of these syntheses (Au/S = 1.00/1) is reported as a typical procedure: an aqueous solution of HAuCl4·3H2O (0.0546 g, 0.138 mmol) in deionized water (5 mL) was mixed with a solution of tetraoctylammonium bromide (TOAB) (0.0995 g, 0.182 mmol) in toluene (5 mL). The two-phase mixture was vigorously stirred until all the tetrachloroaurate was transferred into the organic layer, and a solution of thiol ester SEB (0.0334 g, 0.135 mmol) in toluene (5 mL) was then added. A freshly prepared aqueous solution of sodium borohydride (0.0840 g, 2.178 mmol) in deionized water (5 mL) was rapidly added with vigorous stirring. After further stirring for 3 h, the organic phase was separated, washed with water, and then reduced to 2 mL in a rotary evaporator. After addition of 40 mL of ethanol, the mixture was kept overnight at −18 °C and then centrifuged at 1500 rpm for 15 min. The supernatant, containing excess thiol and TOAB, was separated and the precipitate was washed by centrifugation with ethanol in the same way for an additional 10 times. After removal of the supernatant, a ruby-red solution of NP(1) was then obtained by dissolving the brown precipitate with 10 mL of toluene; the yield was 28%. UV−vis (CHCl3), λmax (nm): 526. FT-IR (film, ν, cm−1): 2955, 2923, 2853, 2154 (CC), 1471, 1378, 1249, 1012, 886, 759. DLS mean diameter (CHCl3): 7.9 ± 0.3 nm NP(1), 7.0 ± 0.3 nm NP(2), 5.8 ± 0.3 nm NP(3), 4.9 ± 0.3 nm NP(4). 2.1.2. SAM Preparation. Gold coated silica wafers prepared by growing Au film 4000 Å thick onto Si(111) substrates were cut into slices (ca. 1 cm2) and washed with several organic solvents, i.e., acetone, ethanol, and chloroform, and blown dry with nitrogen. For the preparation of SEB self-assembled monolayer, the terminal thiol compound was in situ obtained by a deacylation procedure carried out from the precursor acetyl-protected thiol by allowing SEB (15 mg in THF, c = 6.3 × 10−3 M) to react with NH3 (30%, 100 μL, 2.5 mmol) in the molar ratio S/N = 0.03. The solution was stirred at 30 °C for 2 h and filtered on Celite, and a freshly washed gold substrate was dipped into the solution for 4 h. The obtained multilayer was 8160



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calculate the equilibrium geometry and the valence band properties, we used all-electron triple-ζ valence plus polarization (TZVP) atomic Gaussian basis sets for sulfur and carbon centers, while the hydrogen basis sets were chosen to be of the (311/1) type.33 The X-ray absorption spectra were calculated using the Slater transition state method34,35 following the procedures described in refs 28 and 29. In this case the geometry-optimized molecular structure obtained for the ground state was kept fixed and dipole transitions and angular dependent NEXAFS spectra were calculated at all nonequivalent C atomic centers. An IGLO-III basis was used on each of the excitation centers in order to better describe relaxation effects, while for the remaining carbon atoms we used effective core potentials describing the core and the appropriate valence basis.30 A diffuse even-tempered basis set was finally included at the excitation center to account for transitions to unbound resonances. The dipole-excitation spectra thus obtained were then convoluted with an energy dependent Gaussian broadening before comparison with the experimental spectra. Calculations were also performed with a SEB−Au system assuming chemisorption of the SEB molecule through the S atom on the [111] surface of an Au cluster.36,37 A geometry-optimized model for the molecule bonded to the Au substrate was obtained by limiting the degrees of freedom of the molecule. Effective core potentials were used for the Au cluster atoms describing the core and the appropriate valence basis. During the geometry optimization the Au atoms were kept fixed. Translation of the molecule was allowed in the outof-plane direction with respect to the substrate as well as in the in-plane direction. The minimum total energy was obtained for chemisorption on the “on-top” position. This is in agreement with earlier results in similar systems.31 It may be noted here that, as NEXAFS probes the local bonding environment,38 the signal is mainly determined by the simple molecule. Thus a free molecule calculation is in general sufficient to account for all main spectral features and their general angular behavior. Interactions with substrate and molecule−molecule interactions, on the other hand, give minor contributions to the spectra.36 Here we consider bonding to Au in order to see how this affects the NEXAFS line shape, and which transitions are mostly influenced.

then rinsed with different solvents (ethanol, THF, and acetone) in order to achieve the formation of a SEB film in the monolayer thickness regime. 2.2. Instrumentation. UV−vis spectra were run in CHCl3 solution by using quartz cells with a Varian Cary 100 Scan spectrophotometer. Size and size distribution of AuNPs in CHCl3 solution have been investigated by means of the dynamic light scattering (DLS) technique using a Brookhaven instrument (Brookhaven, NY) equipped with a 10 mW HeNe laser at a 632.8 nm wavelength at a temperature of 25.0 ± 0.2 °C. Correlation data have been acquired and fitted in analogy to our previous work.24 2.3. SR-Induced Spectroscopic Techniques. 2.3.1. Synchrotron Radiation Induced X-ray Photoelectron Spectroscopy. Experiments (SR-XPS) were carried out at the BACH (Beamline for Advanced diCHroism) line at the ELETTRA synchrotron facility.25 XPS data were collected in fixed analyzer transmission mode (pass energy = 30 eV), with the monochromator entrance and exit slits fixed at 30 μm. Photon energies of 380 and 600 eV were used for the C 1s, S 2p, Au 4f, and O 1s spectral regions, respectively, with energy resolution NE = 0.22 eV. Calibration of the energy scale was made referencing all the spectra to the gold Fermi edge of an Au reference sample, and the Au 4f7/2 signal was always found at 83.96 eV. Curve-fitting analysis of the C 1s, O 1s, S 2p, and Au 4f spectra was performed using Gaussian curves as fitting functions. The S 2p3/2,1/2 doublet was fitted by using the same full width at half-maximum (fwhm) for both components, a spin−orbit splitting of 1.2 eV, and a branching ratio (S 2p3/2/S 2p1/2) of 2. For the Au 4f7/2,5/2 doublet, a splitting of 3.6 eV, a branch ratio Au 4f7/2/Au 4f5/2 of 4/3, and the same fwhm values for both spin−orbit components were used. When several different species were identified in a spectrum, the same fwhm value was used for all individual photoemission bands. 2.3.2. Angular Dependent Synchrotron Induced NEXAFS. Measurements were performed at the ELETTRA storage ring at the BEAR (Bending magnet for Emission Absorption and Reflectivity) beamline.26,27 The available spans for the energy range from the visible to soft X-rays (3−1600 eV), and the UHV end station offers a large variety of geometries well suited for polarization dependent experiments.28,29 To examine the molecular orientation, the incident angle of the linearly polarized synchrotron radiation was varied from normal (90°) to grazing (20°) incidence with respect to the sample surface. The photon energy and resolution were calibrated and experimentally tested at the K absorption edges of Ar, N2, and Ne. In addition, our carbon K-edge spectra have been further calibrated using the resonance at 285.50 eV assigned to the C 1s−π* ring transition. The raw C K-edge NEXAFS spectra were normalized to the incident photon flux by dividing the sample spectrum by the spectrum collected on a freshly sputtered gold surface. The spectra were then normalized subtracting a straight line that fits the part of the spectrum below the edge and assessing as 1 the value at 320.00 eV. Measurement reproducibility was checked: the same spectra were collected in different points for each sample, and different samples of the same material were prepared and tested (at least two for each system). 2.4. Computational Methods. The equilibrium geometry and C 1s X-ray absorption spectra of the free molecule were calculated by DFT with the computer code StoBe30 following the method outlined in refs 28, 29, and 31. A gradient corrected RPBE exchange/correlation functional was applied.32 To

3. RESULTS AND DISCUSSION The gold nanoparticles stabilized with 4-trimethylsilylethynyl-1thiobenzene, in situ obtained from 4-trimethylsilylethynyl-1acetylthiobenzene, SEB, were prepared with different Au/S molar ratios by using a modified Brust’s two-phase procedure,39 consisting of a chemical reduction of HAuCl4 in the presence of the ligand, with NaBH4 as reducing agent and tetraoctylammonium bromide (TOAB) as phase transfer agent. The thioacetyl deprotection occurs in the reaction mixture and the thiol ligand in situ generated from SEB is linked to gold, in analogy with procedures described in the literature.17 UV−vis spectra evidenced the plasmon resonance at about 526 nm, and as for nanoparticle dimensions, DLS data indicated that upon increase of the thiol amount the capped NP mean size decreases, with hydrodynamic diameters in the range 5−8 nm. A similar trend has already been observed for small gold nanoparticles capped with allylmercaptan (AM), as discussed by some of us in previous works.11,40 3.1. SR-XPS: Interaction at the Molecule−Gold Interface in SAMs and AuNPs-SEB. The high spectral resolution and photon flux of synchrotron radiation induced XPS are 8161



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Au(0) atoms, is usually taken as reference (and reported in Table 1). Although for SAM a single Au 4f spin−orbit pair can be observed, with the main Au 4f7/2 signal centered at 83.96 eV as expected for metallic gold, AuNPs-SEB show two couples of spin−orbit peaks. In fact, in SAMs usually only the signal related to metallic-like gold atoms can be observed due to the predominance of bulk atoms with respect to surface atoms that are involved in the S−Au bond. Otherwise, in small nanoparticles as NP(1), NP(2), NP(3), and NP(4) the surface to volume ratio increases, allowing us to observe also the Au 4f signal related to substrate−thiol interface atoms. The bulk atom signal is still more intense, but the spin−orbit pair at higher BE values (Au 4f7/2 = 84.8 eV) clearly appears as a pronounced shoulder on the measured spectra.10 As for S 2p spectra (Figure 1b), the BE position of the S 2p3/2 signal, taken as reference for the S 2p3/2−1/2 spin−orbit pair, indicates whether the sulfur atom is covalently bonded to the metal surface. For thiols chemisorbed on metals, as well as for thiols covalently bonded to metal nanoparticle surface, an S 2p3/2 BE value of nearly 162 eV is expected;11,41 S 2p3/2 signals around 163.5 eV are usually assigned to physisorbed thiols or thiolates.42,43 For SAM, as expected for a monolayer sample, only the spin−orbit pair at low BE values can be observed (S 2p3/2 at 162.16 eV). Otherwise, all AuNPs-SEB show two S 2p spin−orbit doublets, with the S 2p3/2 component occurring at about 162.2 eV and respectively 163.4 eV. The peak at the lower BE value is ascribed to sulfur covalently bonded to gold, subsequent to the anchoring of the thiol on the metal atoms on the AuNP surface; the component at higher BE values is assigned to sulfur in the free thiol terminal group of physisorbed molecules on the external AuNP surface.11 The relative contributions of the different S 2p and Au 4f components, corresponding to relative amounts of S and Au atoms in different configurations (i.e., bulk Au atoms, interface Au atoms interacting with S, S atoms covalently bonded to Au and free thiolate moieties), can be estimated from the ratio between the respective signal intensities (peak areas, individuated by following a peak-fitting procedure). The calculated intensity ratios, reported as percent of the single species in Table 1, evidence that the amount of gold atoms chemically bonding thiols on the NP surface is about 10% for all NPs, suggesting that a thin layer of chemically bonded thiol of similar thickness covers the AuNP surface in all samples. The observed S 2p3/2 signal at about 163.4 eV indicates that this first layer is covered by a second one of physisorbed SEB molecules; looking at the atomic percent of sulfur species, we observe that

extremely well suited to investigating the interaction at the interface between thiolate and gold, allowing probing of the chemistry occurring at the headgroup−metal interface in capped nanoparticles as well as in self-assembled monolayers deposited onto “flat” gold surfaces. SR-XPS measurements were carried out at the C 1s, O 1s, S 2p, and Au 4f core levels on NPs with Au/thiol stoichiometric ratios of 1/1 (NP(1)), 0.7/1 (NP(2)), 0.5/1 (NP(3)), and 0.25/1 (NP(4)), and on the thiolate self-assembling monolayer anchored on polycrystalline gold SAM. Au 4f and S 2p spectra collected on SAM, NP(1), NP(2), NP(3), and NP(4) are reported in Figure 1; Au 4f7/2 and S 2p3/2 peak position binding energies (BEs), fwhm values, and relative intensities are reported in Table 1.

Figure 1. (a) Au 4f spectra of SAM, NP(1), NP(2), NP(3), and NP(4). Spin−orbit components indicative for metallic gold atoms are in red (Au 4f7/2) and orange (Au 4f5/2); the signal arising from Au atoms on nanoparticle surface interacting with thiol end groups are in blue (Au 4f7/2) and cyan (Au 4f5/2). (b) SR-XPS S 2p spectra collected on SAM, NP(1), NP(2), NP(3), and NP(4). S 2p3/2 and S 2p1/2 components associated with sulfur atoms covalently bonded to gold are in blue (S 2p3/2) and cyan (S 2p1/2); the signals attributed to free thiol (SAM) and thiolate (NPs) SEB are in red (S 2p3/2) and orange (S 2p1/2).

Au 4f core level spectra are reported in Figure 1a. All spectra, as expected, are composed of spin−orbit doublets (Au 4f7/2, Au 4f5/2); the more intense Au 4f7/2 component, due to metallic

Table 1. Au 4f and S 2p Data (BE, fwhm, Relative Intensity Ratios) for Samples SAM, NP(1), NP(2), NP(3), and NP(4) SR-XPS Au 4f7/2 sample

83.96

fwhm (eV) I ratioa (%)

0.91 100% S−Au

sample BE (eV) fwhm (eV) I ratioa (%) a

SAM

BE (eV)

NP(1) 83.96 84.88 0.91 9.4% Auδ+; 90.6% Au(0)

NP(2)

NP(3)

83.96 83.96 84.86 84.84 0.91 0.91 9.7% Auδ+; 90.3% Au(0) 8.1% Auδ+; 91.9% Au(0) SR-XPS S 2p3/2

NP(4)

attribution

83.96 84.86 0.91 11.0% Auδ+; 89.0% Au(0)

metallic Au Auδ+ (Au−S−)

SAM

NP(1)

NP(2)

NP(3)

NP(4)

attribution

162,16 − 1.30 100% S−Au

162.13 163.35 1.28 19% S−Au; 81% S−H

162.19 163.40 1.30 38% S−Au; 62% S−H

162.24 163.47 1.18 37% S−Au; 63% S−H

162.28 163.38 1.23 32% S−Au; 68% S−H

S−Au S−H

The statistic error in semiquantitative XPS analysis is about 5% of the estimated value.44 8162



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for the calculated λoverlayer, gives the ligand shell thickness expressed in nanometers. For SEB shell on AuNPs a λoverlayer of 0.67 nm is calculated by applying the appropriate equation optimized for organic materials:47

the amount of physisorbed thiol (S−H component) is larger in NP(1) than in NP(2), NP(3), and NP(4), suggesting a thicker molecular overlayer on the NP(1) surface. The hypothesized core−shell structure is coherent with behaviors already reported for analogous systems.11,40 Through XPS data analysis, the thickness of a molecular ligand layer on a substrate surface can be determined. For SAMs, film thickness is routinely estimated by exploiting the correlation between the signal intensity and the coverage thickness; this feature, for an overlayer adsorbed onto a substrate, can be calculated by evaluating the attenuation of the substrate signal (Au 4f7/2 photoelectron peak measured at 83.8 eV of photon energy) assuming a standard exponential attenuation of the signal according to the equation44

λoverlayer (nm) = [0.65 + 0.007(KE)0.93 ]/Z 0.38

(2)

where KE is the kinetic energy of the selected photoelectrons arising from the overlayer; Z is the mean atomic number, which can be taken as equal to 4 for organic materials.47 By applying eq 1 to NP(1), NP(2), NP(3), and NP(4), and after multiplication by λoverlayer, the approximated TR→∞ values reported in Table 2 are obtained. The implemented calculation Table 2. Overlayer Thickness Values Estimated for SAM and NPsa

0

Is = Is exp( −T /λAu)

where Is is the intensity of the substrate signal in the presence of the overlayer, Is0 is the intensity of the same signal for the clean surface, T is the overlayer thickness, and λAu is the inelastic mean free path for gold. For alkanethiol SAMs on Au, in the literature it is reported that λAu is correlated to the kinetic energy (KE) of the photoemitted electron by the equation10

sample

SAM

SAM thickness (nm) TR→∞ (nm) TNP (nm) RDLS (nm)

0.89

NP(1)

NP(2)

NP(3)

NP(4)

2.06 0.93 3.95

0.86 0.40 3.50

0.92 0.39 2.90

0.98 0.37 2.45

a

For NPs, TR→∞, TNP, and DLS hydrodynamic mean radius RDLS (calculated as measured DDLS/2) are reported.

λAu = 0.3(KE)0.64

for NPs in the range 2−160 nm diameter46 requires that the core mean radius be known; using the DLS mean radius values (RDLS, fourth row of Table 2) as the sum of core radius and shell thickness, we calculated the NP overlayer thickness values TNP with the equation46

Within the limits of the method, we find that the thickness value calculated in this way for SAM is 8.96 Å. This value can be roughly compared with thiol molecular dimensions (8.12 Å) evaluated by the chemical structure, allowing then exclusion of the presence of a thick multilayer, in excellent agreement with the expectations for a monolayer absorption regime. A more accurate comparison between the expected SAM thickness and experimental results will be discussed at the end of section 3.3, considering the SEB molecular dimensions, the S−Au bond length calculated by DFT molecular geometry optimization, and the tilt angle of SEB molecules on the Au “flat” surface calculated by AD-NEXAFS data analysis. As for AuNPs-SEB, it is still possible to estimate the thickness of the molecular ligand shell (overlayer) around the metallic core (substrate), at least in a first approximation by introducing a geometrical correction term that accounts for the spherical shape of the substrate surface.45 In the approximation that it is strictly appropriate for microscopic spherical particles, but rough for nanoparticles, the overlayer thickness TR→∞, expressed in units of λoverlayer (attenuation length of photoelectrons arising from the overlayer and traveling through the overlayer itself), is46

TNP = (TR ≈ 1 + βT0)/(1 + β)

(3)

where TR≈1 = (TR→∞R)/(R + α); α = 1.8/(A B C ); β = 0.13α2.5/R1.5; C = (Zs/Zo)0.3; and considering that TNP + R = RDLS. The calculated TNP values (multiplied by λoverlayer to obtain values in nanometers), reported in Table 2, are indeed still roughly approximated, since the DLS mean radius is referred to the hydrodynamic radius, thus overestimating NP sizes. Anyway, within the limits of the approximated method, a clear trend in T values that reflects extremely well the semiquantitative XPS predictions is observed. NP(1), which showed the most intense S 2p component associated with physisorbed molecules, has the thicker ligand shell. On the other hand, NP(2), NP(3), and NP(4) are found to have thinner overlayers, corresponding to a lower physisorbed SEB percent in XPS and to an approximated thickness value lower than that observed for the SAM. 3.2. Near Edge X-ray Absorption Fine Structure Spectroscopy. In order to identify the different transitions in comparison with the experiment, near edge X-ray absorption spectra were simulated through DFT. The theoretical angle integrated spectrum of the “free” thiolate molecule is shown in Figure 2b. The angle integrated spectra relative to the individual carbon sites are shown under the integrated spectra. Looking at the two calculated spectra, it is noteworthy that the main difference between the two simulated spectra is the presence of the sharp peak at 287.5 eV in the free molecule Xray absorption spectrum (Figure 2b) and associated with the C 1s atomic orbital to the π* CO molecular orbital (absorption site C2 in Figure 2b). The subtracted result of the contribution of C2 from the total absorption spectrum of the free molecule results in a good match with the NEXAFS spectrum of the SEB−Au system (results are shown in Figure S10 in the 0.1 0.5 0.4

TR →∞ = (0.74A0.36 ln(A)B−0.9 + 4.2AB−4.1)/(A3.6 + 8.9) (1)

where A is the ratio of the normalized integrated intensities of a unique signal from the AuNPs-SEB shell (overlayer) to that of a unique signal from the core (substrate), i.e., A = (Io/Is)(Is∞/ Io∞); Io is the measured overlayer XPS signal intensity (C 1s intensity for our SEB molecules), Is is the measured substrate intensity (Au 4f7/2), Io∞ is the XPS intensity from a pure, infinitely thick sample of overlayer, and Is∞ is the intensity from a pure, infinitely thick substrate. The ratio for pure materials Io∞/Is∞ was calculated from reference values, as for the SAM thickness calculation. B is the ratio of effective attenuation lengths between overlayer and substrate electrons traveling within the overlayer material (estimated as (KEo/KEs)0.872); B = 0.371 in our case.45,46 The resulting thickness value, multiplied 8163



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NEXAFS C K-edge spectra of SAM, NP(1), and NP(2) (taken as examples of “thick” and “thin” SEB shells on AuNP core) recorded at an X-ray incidence angle of 54.7° on the substrate (magic incidence), for which the measured intensity distribution is independent of the molecular orientation, are displayed in Figure 3.

Figure 3. NEXAFS C K-edge spectra collected on SAM, NP(1), and NP(2) at an X-ray incidence angle of 54.7° on the substrate (magic incidence: no angular dependent effects38).

The peak assignments of the relevant spectral features can be made by comparison with calculated spectra and literature data. As a first approach, NEXAFS C K-edge spectra were calibrated (rigid shift of photon energy by about 1.10 eV for all samples) using literature data on benzene-like analogous molecules as a reference (see Table S1 in the Supporting Information for complete feature list and assignment). Comparing the magic angle spectra shown in Figure 3 with the calculated spectra of Figure 2b, it can be clearly observed that a good agreement could be obtained between the experiment and simulations. The simulation results indicate that coverage of the molecule is high enough that molecule−substrate interaction does not have a dramatic influence on the spectral line shape. The sharp feature at about 287.5 eV that appears in the calculated free acetyl-protected thiol spectrum is not observed in either SAM or NP spectra, thus indicating that the hypothesized deacylation reaction successfully and completely arises during NP functionalization. On the basis of the calculations, the peak around 285.5 eV could be associated with the nearly unaltered C 1s → π* transition of the benzene ring, due to the ring carbons not bonded to the alkyne group; the signal at about 286.0 eV originates from the excitation of the alkyne carbons (C 1s → π* CC), and includes a contribution from the ring carbon bonded to the acetylene functional group.48 A shoulder associated with C 1s → σ* transitions of C−S bonds is observable at nearly 286.5 eV. All spectra show a band at 288.5 eV associated with C 1s → σ* C−H transitions due to the

Figure 2. (a) Optimized geometry for the “free” SEB molecule used for calculations. (b) Calculated C K-edge NEXAFS spectra for the “free” acetyl-protected thiol molecule. The different carbon absorption sites are labeled. (c) Optimized geometry for SEB molecule bonded to Au cluster. It is to be noted that for the bound molecule the C(O)CH3 end group is absent. To align the theoretical curves with the experimental spectra, a rigid shift of 1 eV was applied.

Supporting Information). We can thus infer that the free molecule NEXAFS peak positions and line shape are not dramatically altered by the substrate introduction; this implies that the calculations carried out for the free molecule can be, to a first approximation, directly extended to the self-assembling monolayer and capped nanoparticles. The feature related to the C2 atom contribution is expected to disappear in the bonded system, since the hypothesized synthetic path involves a deacylation reaction, with subsequent loss of −C(O)CH3 endgroup (molecular structure reported in Figure 2c). 8164



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Figure 4. Angular dependent NEXAFS C K-edge spectra collected with normal (black line) and grazing (red line) incidence of the beam on the sample surface for (a) SAM, (b) NP(1), and (c) NP(2). The blue lines are obtained by subtracting the normal from the grazing signal, and evidence the observed angular (dichroic) effects. To evidence the polarization dependence of the main π* feature, the aromatic ring π* region is zoomed and reported in (d) SAM, (e) NP(1), and (f) NP(2).

methyl groups of the trimethylsilyl moiety (extremely intense in the NP(1) spectrum, as will be discussed in the following), a large broadening at about 288.7 eV that is thought to derive from the overlapping of a high energy π* plus σ* C−H, and a broad band around 293 eV assigned to C 1s → σ* C−C excitations of the methyl groups. The broad bands around 303 and 310 eV are respectively assigned to σ* CC benzene-like and σ* CC bonds. The NP(2) NEXAFS spectrum (bottom of Figure 3) is similar to that of SAM, with the same feature positions and very similar intensities; the NP(1) spectrum, on the other hand, appears different due to the very high intensity of the C 1s → σ* C−H methyl group feature (288.5 eV). The other resonances observed in the SAM and NP(2) spectra are still observable, but appear of small intensity in comparison with this intense peak. Accordingly with XPS findings, this behavior is ascribed to the bigger amount of physisorbed thiolate molecules observed in NP(1) with respect to NP(2), exposing their terminal −Si(CH3)3 moieties on the external surface of the NP. As will be discussed in section 3.3, the terminal trimethylsilyl groups appear closely packed, giving rise to dichroic effects in NP(1) AD-NEXAFS spectra. 3.3. Molecular Orientation and Self-Assembling Behavior: Angular Dependent NEXAFS. C K-edge NEXAFS spectra of samples SAM, NP(1), and NP(2) were acquired in angular dependent mode by varying the incidence angle of the linearly polarized (95%) synchrotron radiation on the sample surface, from grazing (20°) to normal (90°); the comparison between the dichroic effects observed in the SAM and NP spectra will allow us to probe the molecular organization behavior of SEB ligands grafted to AuNPs.

The NEXAFS resonance intensity depends on the orientation of the transition dipole moment (μ) of the probed molecules relative to the polarization vector (p) of the incoming radiation. If the directions of the electric field of the incoming radiation (E) and μ are parallel to each other, the intensity is at the maximum. On the other hand, the excitation does not occur when the vectors E and μ are orthogonal to each other.38 As shown in Figure 4a−c, the intensities of the first and second features observed in the SAM, NP(1), and NP(2) spectra show a small but clear variation with the beam incidence angle (linear dichroism), suggesting an high level of molecular organization. The average tilt angle of SAM (i.e., the angle between the molecular axis and the normal to the substrate) was obtained by the quantitative analysis of the angular dependence of the first π* resonance (attributed to C 1s−π* aromatic ring transition) intensity, following the procedure reported by Stöhr.38 For the SAM sample the average tilt angle was calculated by applying the simple formula I(θ , α) = 1 + (1/2)(3 cos2 θ − 1)(3 cos2 α − 1)

where α is the angle between the π* vector orbital and the normal to the surface plane (in this case the average tilt angle of molecular axis, φ, is simply 90° − α) and θ is the incidence angle of the X beam on the sample. A schematic drawing reporting the angles used to determine the molecular arrangement on the substrate is reported in Figure 5. The tilt angle value for SAM was calculated as φ = 30 ± 6°. This finding is in excellent agreement with the equilibrium geometry of SEB molecule bonded to a small cluster of Au atoms calculated by DFT. As shown in Figure 5, the geometry optimization results 8165



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Figure 5. Geometry-optimized SEB molecule bonded to an Au11 cluster of atoms. The angles used to determine the molecular arrangement on the substrate are indicated.

Figure 6. Calculated polarization dependent XAS spectrum for C12 carbon atom with the electric E field parallel (E∥) and perpendicular (E⊥) to the plane containing the carbon atoms of the CH3 groups of the Si(CH3)3 moieties.

showed a favored configuration with the SEB molecule in an upright position (φ = 32.4°), bonded to on-top sites, and the optimized S−Au bond length was estimated as 2.3 Å. Concerning to the SAM sample thickness, the evaluated average tilt angle leads to calculation of the expected value that can be compared to the one experimentally determined by the SR-XPS Au 4f signal attenuation and to the calculation result. Using a calculated value as dcalc equals the length of the molecule, and estimating a S−Au bond length value of 2.3 Å from DFT molecular structure optimization (Figure 5), the thiol SAM thickness was estimated by

hypothesize that SEB molecules tend to lie down on the NP(2) surface, giving rise to a thin shell of ligands, constituted by 40% of molecules covalently bonded to the gold core atoms plus 60% of physisorbed thiols, stabilizing the core−shell structure by intermolecular interactions. Like in NP(1), as the amount of molecules physisorbed on the NP surface increases (about 80% of SEB molecules are physisorbed), a more dense packing is obtained; a thicker shell of ligands is found by XPS analysis and only the −Si(CH3)3 groups exposed to the external AuNP surface appear oriented, giving rise to the nice dichroic effects observed in Figure4c,f (the two limit configurations are drafted in Figure 7).

d = dcalc cos φ + 2.3

where φ is the tilt angle estimated by NEXAFS measurements = 30°; dcalc = 8.1 Å. A thickness of 9.3 Å was obtained. As for the dichroic effects observable in AuNP angular dependent NEXAFS spectra (Figure 4b,c,e,f), a qualitative observation of the intensity variations suggests that molecular organization still occurs to some extent. It is necessary to distinguish between the two AuNPs taken as examples of thick (NP(1)) and thin (NP(2)) ligand shells. For NP(1) (Figure 4e), no strong angular dependent effects are observed for the feature associated with benzene ring orientation (285.5 + 286.0 eV); on the other hand, the feature at about 288.5 eV relative to CH3 groups of −Si(CH3)3 moieties (Figure 4b) shows a strong dichroic behavior, similar to those observed in thick layers of organometallic polymers based on square-planar [PtCl 2 (PBu 3 ) 2 ] complexes 49 as well as on the transPtCl2(PBu3)2 sample itself (Figure S11 in the Supporting Information). This is also supported by theoretical calculations; the major fraction of the dichroic nature of the experimental spectra could be approximately simulated by considering the polarization dependent absorption of the C12 carbon atom as shown in Figure 6. The NP(2) angular dependent NEXAFS spectrum (Figure 4c) is similar to that of SAM, but the dichroic effects are opposite. Focusing on the first two π* features (285.5 and 286.0 eV), which are indicative for aromatic ring orientation (Figure 4d,f), it is noteworthy that in the NP(2) spectra they present an intensity minimum at normal incidence that increases at grazing geometry. This behavior, on “flat” surfaces, would be indicative of molecules lying nearly flat with the plane of the aromatic ring almost parallel to the surface. In the case of molecules bonded to “bended” surfaces, as in capped NPs, a quantitative evaluation of the average tilt angle is of difficult interpretation, but combining the information acquired by SRXPS and angular dependent NEXAFS we can reasonably

Figure 7. Schematic representation of SEB molecule arrangement on the surface of (a) NP(2) (thin molecular shell) and (b) NP(1) (thick shell), evidencing the closely packed CH3 groups on NP(1) surface.

4. CONCLUSIONS With the aim of comparing different interaction modes of stabilizing ligands, depending on the geometry of the anchoring gold surface (flat or spherical), a set of well-defined functionalized AuNPs, as well as a self-assembling monolayer of the same ligand anchored on “flat” gold surface, have been prepared and characterized by SR-induced techniques. The chemical bond between the capping molecule and gold nanoparticles and substrate, as well as the thiol/Au interface and thickness of the molecular overlayer, have been investigated by means of SR-XPS; AuNPs-SEB molecular structure and SEB molecule organization in the overlayer have been studied by means of angular dependent NEXAFS at the C K-edge. With the aim of attaining a deeper level of interpretation of the experimental data, density functional theory (DFT) calculations have been carried out for the SEB molecule, both free and bonded to a small cluster of gold atoms. The combined XPS 8166



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(4) Kishore, P. S.; Viswanathan, B.; Varadarajan, T. K. Synthesis and Characterization of Metal Nanoparticle Embedded Conducting Polymer−Polyoxometalate Composites. Nanoscale Res. Lett. 2008, 3, 14−20. (5) Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications. Chem. Rev. 2011, 111, 3669−3712. (6) Jain, P. K.; Huang, W.; El-Sayed, M. A. On the Universal Scaling Behavior of the Distance Decay of Plasmon Coupling in Metal Nanoparticle Pairs: A Plasmon Ruler Equation. Nano Lett. 2007, 7 (7), 2080−2088. (7) Jägeler-Hoheisel, T.; Cordeiro, J.; Lecarme, O.; Cuche, A.; Girard, C.; Dujardin, E.; Peyrade, D.; Arbouet, A. Plasmonic Shaping in Gold Nanoparticle Three-Dimensional Assemblies. J. Phys. Chem. C 2013, 117 (44), 23126−23132. (8) Ranjan, M.; Fackso, S.; Fritzsche, M.; Mukherjee, S. Plasmon Resonance Tuning in Ag Nanoparticles Arrays Grown on Ripple Patterned Templates. Microelectron. Eng. 2013, 102, 44−47. (9) Das, I.; Ansari, S. A. Nanomaterials in Science and Technology. J. Sci. Ind. Res. 2009, 68, 657−667. (10) Vitaliano, R.; Fratoddi, I.; Venditti, I.; Roviello, G.; Battocchio, C.; Polzonetti, G.; Russo, M. V. Self-Assembled Monolayers Based on Pd-Containing Organometallic Thiols: Preparation and Structural Characterization. J. Phys. Chem. A 2009, 113 (52), 14730−14740. (11) Fratoddi, I.; Venditti, I.; Battocchio, C.; Polzonetti, G.; Bondino, F.; Malvestuto, M.; Piscopiello, E.; Tapfer, L.; Russo, M. V. Gold Nanoparticle Dyads Stabilized with Binuclear Pt(II) Dithiol Bridges. J. Phys. Chem. C 2011, 115, 15198−15204. (12) Vitale, F.; Vitaliano, R.; Battocchio, C.; Fratoddi, I.; Giannini, C.; Piscopiello, E.; Guagliardi, A.; Cervellino, A.; Polzonetti, G.; Russo, M. V.; et al. Synthesis and Microstructural Investigations of Organometallic Pd(II) Thiol-Gold Nanoparticles Hybrids. Nanoscale Res. Lett. 2008, 3, 461−467. (13) Vitale, F.; Vitaliano, R.; Battocchio, C.; Fratoddi, I.; Piscopiello, E.; Tapfer, L.; Russo, M. V. Synthesis and Characterization of Gold Nanoparticles Stabilized by Palladium(II) Phosphine Thiol. J. Organomet. Chem. 2008, 693, 1043−1048. (14) Techane, S. D.; Gamble, L. J.; Castner, D. J. Multi-technique Characterization of Self assembled Carboxylic Acid Terminated Alkanethiol Monolayers on Nanoparticle and Flat Gold Surfaces. J. Phys. Chem. C 2011, 115, 9432−9441. (15) Battocchio, C.; Fratoddi, I.; Russo, M. V.; Polzonetti, G. NEXAFS Study of a Pt-Containing Rod-Like Organometallic Polymer (Pt-DEBP): Molecular Orientation onto HOPG, Au/Si(1 1 1), Cr/ Si(1 1 1) and Si(1 1 1) Surfaces. Chem. Phys. Lett. 2004, 400, 290− 295. (16) Kang, J. H.; Kim, Y. C.; Cho, K.; Park, C. E. Effects of Particle Size on the Molecular Orientation and Birefringence of Magnetic Nanoparticles/Polyimide Composites. J. Appl. Polym. Sci. 2006, 99, 3433−3440. (17) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. SelfAssembled Monolayers and Multilayers of Conjugated Thiols, a,oDithiols, and Thioacetyl-Containing Adsorbates. Understanding Attachments between Potential Molecular Wires and Gold Surfaces. J. Am. Chem. Soc. 1995, 117, 9529−9534. (18) Sonogashira, K. Development of Pd−Cu Catalyzed CrossCoupling of Terminal Acetylenes with sp2-Carbon Halides. J. Organomet. Chem. 2002, 653, 46−49. (19) Arcadi, A.; Cacchi, S.; Rasario, M. D.; Fabrizi, G.; Marinelli, F. Palladium-Catalyzed Reaction of o-Ethynylphenols, o((Trimethylsilyl)ethynyl)phenyl Acetates, and o-Alkynylphenols with Unsaturated Triflates or Halides: A Route to 2-Substituted-, 2,3Disubstituted-, and 2-Substituted-3-acylbenzo[b]furans. J. Org. Chem. 1996, 61, 9280−9288. (20) Fratoddi, I.; Gohlke, C.; Cametti, C.; Diociaiuti, M.; Russo, M. V. Self-Assembly of Nanostructured Polymetallaynes. Polymer 2008, 49, 3211−3216.

and NEXAFS study provided detailed information on the NP structure: the experimental results, supported by the DFT calculations, are consistent with hybrid systems in which the metallic Au core is surrounded by a shell of SEB molecules, whose thickness can be successfully evaluated by XPS analysis. Angular dependent NEXAFS data suggest that SEB molecules are organized on a “lying down” configuration on the AuNP surface in systems with a thin molecular shell. Interestingly, when the ligand shell becomes thicker the molecules attain a closely packed arrangement, exposing the methyl terminal groups on the external NP surface. The comparison between angular dependent NEXAFS spectra collected onto SAM and AuNPs-SEB demonstrated how this spectroscopic technique, extensively used up to now to investigate molecular selfassembling on macroscopic substrates, is equally able to individuate molecular organization in nanostructured systems.

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ASSOCIATED CONTENT

S Supporting Information *

Details about instruments, materials, synthesis schemes, Figures S1−S12, Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.in the Supporting Information.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 0039 06 57333388. Fax: 0039 06 57333390. Present Address ⊥

S.M.: Department of Physics, North Carolina State University, Raleigh, NC 27695, USA. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Ateneo Sapienza 2011/C26A11PKS2 and 2013/C26A13HRZ4 projects for financial support. This work has been partially supported by the Dipartimento di Chimica, Sapienza Università di Roma, through the Supporting Reseach Initiative 2013. S.M. is indebted to Prof. L. Pasquali (University of Modena) for precious and fruitful discussions.

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ABBREVIATIONS SR-XPS, synchrotron radiation induced X-ray photoelectron spectroscopy; NEXAFS, near edge X-ray absorption fine structure spectroscopy; AD-NEXAFS, angular dependent NEXAFS; SAM, self-assembling monolayer; AuNPs, gold nanoparticles

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REFERENCES

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