Modifying the Atomic and Electronic Structures of Gold Nanocrystals

Article pubs.acs.org/JPCC

Modifying the Atomic and Electronic Structures of Gold Nanocrystals via Changing the Chain Length of n‑Alkanethiol Ligands Yong Jiang,† Peidong Yin,† Yuanyuan Li, Zhihu Sun, Qinghua Liu, Tao Yao,* Hao Cheng, Fengchun Hu, Zhi Xie, Bo He, Guoqiang Pan, and Shiqiang Wei* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, People’s Republic of China ABSTRACT: The interfacial atomic and electronic structures of ligand-protected nanomaterials are vital factors but are inadequately known. Here, we demonstrate that the adsorption geometry, as well as the electronic structures of Au−S interface, can be tailored via varying the hydrocarbon “tail” lengths of n-alkanethiol ligands. Fully nalkanethiols (n = 3, 8, and 12) capped Au nanocrystals of 3.0 nm were characterized in solution by X-ray absorption fine structure at the Au L3-edge. With increasing alkyl length, it is found that the headgroup S atom occupies the nanocrystals surface sites with gradually higher coordinations, along with the progressively shortened Au−S bond length. As a result, the strongest Au−S interactions coming from the longest n-alkanethiols capping lead to the most significant d charge transfer from the surface Au layer to the S atoms.

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INTRODUCTION Gold nanocrystals have attracted considerable interest for decades because of their promising applications in a variety of areas such as nanoelectronics, catalysis, and biomedicine.1−7 In most practical cases, the Au nanocrystals are coated with an organic surfactant layer to impart the desired stability and functionality.8 The coverage of alkanethiols that incorporate sulfhydryl (-SH) as a linking group is the most common and successful strategy to functionalize Au nanocrystals with specific physical and chemical properties.9,10 For example, the modification of surface states by precise control of thiols coatings can markedly improve the catalytic selectivity of epoxybutane formation from 11 to 94% at equivalent reaction conditions.11 To synthesize water-soluble Au nanoparticles for biological applications, the “ligand-exchange” method involving the displacement of thiols for other ligands weakly bound to gold is used.12,13 Given the ubiquitous use of the archetypal Aualkanethiols systems, the detailed information of the interfacial interaction between the thiols and the nanocrystals is thus essential if their special functionality is to be fully exploited. An alkanethiol molecule is commonly composed of two parts (Figure 1a): the sulfur headgroup that covalently binds with a nanocrystal and the hydrocarbon chain of variable length ((C− C)n chain).14,15 Usually, by virtue of changing the chain backbone, the surface chemistry properties of nanocrystals can be varied from hydrophobic to hydrophilic.15,16 It has also been reported that the ordering of the n-alkanethiols on Au surfaces depends markedly on the chain length, showing a good ordering of the standing-up configuration when n is increased.17−19 To date, however, few studies have been carried out to understand the mechanism regarding why the tailed alkyl © 2012 American Chemical Society

Figure 1. (a) Schematic drawing shows the 3-nm gold nanocrystals capped by the n-alkanethiols of variable hydrocarbon chain lengths (different number n of carbon atom). (b) Au L3-edge XANES data for n-alkanethiols (n = 3, 8, and 12) capped Au nanocrystals (noted as Au−Cn), along with the Au foil data for reference. The insert shows the zoom-in white lines. Au L3-edge k2-weighted extended X-ray absorption fine structure oscillations [k2χ(k)] (c) and their Fourier transforms (d) for the samples and the foil.

Received: September 27, 2012 Revised: October 30, 2012 Published: November 1, 2012 24999

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and X-ray absorption near-edge structure (XANES) spectra were measured in transmission mode using ionization chambers to measure the radiation intensity. XANES measurements were conducted in transmission mode in the energy range from 30 eV below to 70 eV above the Au L3-edge. After the reaction, the solution samples were then transferred to a 10-mm-thick Teflon cell for the XAFS measurements. Because of the weak absorptions from the air and solution around this energy, high signal-to-noise XAFS data can be obtained using such cell thickness. The initial precursor solution and Au foil were also measured for comparison.

chain lengths influence the Au−S interface structure as well as the electronic properties. Several factors may account for this hindrance: one is the lack of the comparable nanocrystals that have the identical size or shape while capped by different nalkanethiols; another is the limitation of the characterization technique that is sensitive to the local adsorption structures and can be performed in solution. Various experimental techniques including infrared-visible sum-frequency, X-ray standing wave and X-ray crystallography have been applied to investigate the interface structures.20,21 For instance, by using coherent X-ray scattering, Watari et al. reported a large differential stress induced by thiols adsorption on a Au nanocrystal.20 On the theoretical side, the ab initio calculations have predicted a few geometrically reasonable positions on Au(111) and Au(100) surface for S (ontop, bridge, hollow) but are mostly limited in the short alkanethiols.22,23 The above obtained results, however, provide relatively scarce information on the Au−S interfacial local structure and electronic properties of nancrystals of several nanometers. To solve the above issues, in this work, we have prepared a series of similar-sized Au nanocrystals capped by n-alkanethiols of different hydrocarbon lengths (model shown in Figure 1a). The X-ray absorption fine structure (XAFS) technique as a sensitive local-structural probe is used to determine the atomic and electronic structures of the Au nanocrystals.24,25 It is shown that the evolutions of Au−S interface structures and density states are strongly dependent on the chain lengths of nalkanethiols. The theoretical calculations further confirm that the charge transfer occurs significantly in the case of long nalkanethiols capping.

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RESULTS AND DISCUSSION The effects of the alkyl chain length on the atomic structure and electronic properties of Au−S interfaces are revealed by using XANES and EXAFS techniques at the Au L3-edge. Figure 1b shows the normalized Au L3-edge XANES spectra for the nalkanethiol-capped Au nanocrystals (Au−Cn) in the colloidal solution together with the Au foil as the reference. A substantial variation can be found that the intensity of main absorption peaks A corresponding to white-line raises as the hydrocarbon chain length (n) increases, and all are more intense than that of the bulk (the insert in Figure 1b). This indicates that the electronic properties of surface Au atoms in the Au−S interfaces are truly modified via changing the alkyl chain length (n), which will be inferred in detail later. With regard to the atomic structures for Au−S interface, we plot the EXAFS spectra in parts c and d of Figure 1. The oscillation k2χ(k) functions for the Au nanocrystals capped by different nalkanethiols show a remarkable difference in the low k region (k = 4−6 Å−1), which can be ascribed to the distinct interactions between gold atom and lighter backscattering sulfur atom (Figure 1c). This difference can be further manifested in Fourier-transform (FT) in Figure 1d. The FT curves of the samples demonstrate an apparent Au−S peak located at 1.87 Å26 together with relatively weaker Au−Au bonding peaks in the range of 2.0−3.5 Å. Of note, the intensity of Au−S peak gradually decreases in the order of Au−C12 > Au−C8 > Au− C3, along with the high−R shifting of its position from 1.90 Å to 1.95 Å and 1.98 Å. These intensity and position variation laws of the Au−S peak imply that the head S atom in long nalkanethiol coordinated stronger with surface Au atoms than that in the short one. The XAFS results thus reveal that the head S atom in n-alkanethiols with different hydrocarbon “tail” length truly modulate the Au−S interface states. To examine if the sizes or morphologies of the Au nanocrystals have been changed when the different nalkanethiol are capped, we performed TEM and UV−vis measurements shown in Figure 2. Parts a−c of Figure 2 show the TEM images of Au nanocrystals capped by n-alkanethiols (n = 3, 8, and 12, respectively). It can be seen that nearly all of the particles possess the same mean diameter of ∼3 nm with a narrow size distribution, as well as the same spherical shape. The UV−vis results of the samples in Figure 2d also confirm this point, showing only a similar absorption bands peaked at 507 nm that is originated from the characteristic plasmon resonance absorbance for spherical Au nanocrystals.27 Therefore, these Au nanocrystals capped by n-alkanethiols, bearing the uniform size and shape, provide suitable models for us to unravel the influence of chain length on the Au−S interface states. The evidence of the similar size and shape for the nalkanethiol-capped Au nanocrystals (n = 3, 8, and 12) by TEM

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EXPERIMENT SECTION Materials. Tetrachloroauric acid (Aldrich, 99.9+%), propanethiol (Aldrich, C3H8S, n = 3), octanethiol (Aldrich, C8H18S, n = 8), dodecanethiol (Aldrich, C12H26S, n = 12), ethanol, acetone, triphenylphosphine (Aldrich, PPh3), and tertbutylamine-borane (Aldrich, TBAB) were used as received without further purification. ClAuPPh3, in the form of white powder, was synthesized by reacting HAuCl4·nH2O with PPh3. Preparation of n-Alkanethiol-Capped Au Nanocrystals. At first, 0.050 g of AuClPPh3 was dissolved in 40 mL of ethanol solvent at room temperature under vigorous stirring. Then, 0.088 g of TBAB used as the reductant was added into the solution. After 2 h, 0.4 mmol n-alkanethiol (C3H8S, C8H18S or C12H26S) was added into the above mixture for the preparation of Au nanoparticle capped with thiols of different alkyl lengths. Here, the n-alkanethiols are in excess, and the mole ratio of alkanethiol to Au is near 4:1, so as to make Au nanoparticle surface fully capped by alkanethiols. The mixture was kept stirring for several days to complete the reaction. Characterizations. Transmission electron microscopy (TEM) images were obtained with a JEOL 2010 system. The samples for TEM were prepared by dropping the reaction solution onto Cu TEM grids directly and drying in air. The UV−vis spectra were recorded on a UV-2501PC/2550 spectrophotometer in the wavelength range of 200−800 nm and corrected by ethanol as background absorption. As reference, the initial precursor solution was also measured. Xray absorption fine structure (XAFS) measurements were performed at U7C XAFS station in NSRL (National Synchrotron Radiation Laboratory, P.R. China) and 4W1B beamline of Beijing Synchrotron Radiation Facility. Au L3-edge (11919 eV) extended X-ray absorption fine structure (EXAFS) 25000

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From Table 1, we can easily find that the Au−Au coordination numbers and bond lengths of samples in the interior are quite similar to those of Au foil. This is consistent with the UV−vis and TEM results that the body nanocrystals suffer little influence upon the covering by different nalkanethiols. It is known that the 3-nm Au nanocrystals have a high percentage of surface atoms that are exposed (ca. 40% used in fitting), which would contribute substantial signals to XAFS probing. The obtained surface Au−Au bond lengths show a high extent of contraction relative to that of Au foil, in agreement with the reported coherent X-ray diffraction measurement, which shows near-surface stress induced by thiol adsorption in a single Au nanocrystal.20 Moreover, the coordination numbers and bond lengths for the surface Au−Au display a slight reduction with the increase of n, which imply that the surface Au structures have higher disorder for the longer n-alkanethiols coverage, as can be confirmed by the increased surface disorders (σ2) with n. By consideration of the invariable shape, size, as well as the internal structures, the evolutions of surface Au−Au bond lengths and coordination numbers are the results from the distinct Au−S interactions by the n-alkanethiols with different lengths. With regard to the Au−S interfacial interactions, it can also be found that the Au−S bond has n-dependent length of 2.41, 2.37, and 2.32 (±0.03) Å and coordination numbers of 0.9, 1.8, and 3.0 (±0.3) from n = 3, 8, and 12. Given the comparable coverage for the three n-alkanethiols on nanocrystal surface, we consider that the distinct Au−S intensities in FT curves can be ascribed to the different Au−S bonding modes. Inferred from the coordination number, we can deduce that the average Au− S coordination numbers of 0.9, 1.8, and 3.0 are almost identical with the expected value of the ontop, bridge, and hollow sites, respectively, which have schematically shown in parts a−c of Figure 3. For example, we speculate that, for the dodecanethiol capping, the Au−S interface shell is composed of the selfassembled network of Au−S bonds with a typical tetrahedral configuration formed by 3 Au surface atoms and a head S atom (Figure 3c). This hollow adsorption configuration with high symmetry is expected to possess a stronger Au−S interaction in relation to that for the ontop and bridge sites, which can be reflected by its shortest Au−S bond length of 2.32 Å. Our results that the S atom favors the ontop adsorption site for the shortest n-alkanethiol (Au−C3) is in keeping with the previous photoelectron diffraction studies of short methylthiolate (CH3S) on Au(111) surface.28 They identified atop sites

Figure 2. TEM images for the propanethiol- (n = 3) (a), octanethiol(n = 8) (b), and dodecanethiol- (n = 12) (c) capped Au nanocrystals. Scale bar = 20 nm. (d) UV−vis spectra of n-alkanethiol-capped Au nanocrystals and the precursor AuClPPh3.

and UV−vis confirm that the evolutions of XAFS results come from the distinct features of adatoms Au−S interface. Quantitative information can be obtained by the least-squares curve-fitting of the FT peaks in the R-range of 1.5−3.2 Å, through separating the contributions of Au atoms at the surface and in the core of a nanocrystal. The fitted analysis was carried out with the ARTEMIS programs of IFEFFIT. Effective scattering amplitudes and phase shifts for the Au−S and Au− Au pairs were calculated with the ab initio code FEFF8.0. The value of p for the Au NCs was determined by assuming the cuboctahedron shape of the Au particle. In terms of p, the Au− Au coordination numbers in the core and at the surface could be expressed as N0 × p and NAu−Ausurface × (1−p), respectively. Here N0 = 12 is the Au−Au coordination number in a facecentered cubic structured Au core, and NAu−Ausurface denotes the surface Au−Au coordination number was allowed to run free. In our case, for 3-nm Au nanocrystals, the value of p is about 0.6. After fitting the FT curves, we get all the structural parameters, which are listed in Table 1.

Table 1. Structural Parameters of Au Nanocrystals Extracted from Least-Squares Curve-Fitting of the EXAFS FT Peaks by Separating the Contributions of Au Atoms at the Surface and in the Core of the Particlesa sample Au foil Au−C3

Au−C8

Au−C12

bond Au−Au Au−S Au−Au Au−Au Au−S Au−Au Au−Au Au−S Au−Au Au−Au

(surface) (core) (surface) (core) (surface) (core)

pb

0.60

0.60

0.60

R (Å) 2.88 2.41 2.82 2.88 2.37 2.82 2.88 2.32 2.81 2.88

± ± ± ± ± ± ± ± ± ±

0.01 0.03 0.03 0.02 0.03 0.03 0.02 0.04 0.03 0.02

N 12 0.9 7.8 12 1.8 7.4 12 3.0 7.1 12

± 0.4 ± 0.3 ± 0.4 ± 0.3 ± 0.4

σ2 (10−3 Å) 8.0 6.5 12.3 11.4 6.0 14.6 13.2 3.6 15.2 13.0

± ± ± ± ± ± ± ± ± ±

0.3 1.0 1.1 1.1 1.0 1.1 1.1 1.0 1.1 1.1

ΔE 3.5 4.1 3.1 3.0 3.0 3.8 4.0 3.4 3.5 4.0

± ± ± ± ± ± ± ± ± ±

1.1 0.5 1.1 1.0 0.5 1.2 1.3 0.4 1.1 1.2

The Au−Au coordination numbers in the core of the nanocrystals were fixed during the fitting. bp is the fraction of Au atoms in the core of the nanocrystals. a

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systemic investigation on the alkanethiol-capped Au nanoparticles by Whitesides and Nuzzo and co-workers who had shown that monolayers formed from long-chain thiols are thermally more stable than films formed from short-chain thiols.32 Also, current−voltage measurements on molecular selfassembled monolayer junctions by Li et al. have shown that the charge injection barrier decreases with the increase of the molecular length, which is consistent with our findings that the charger transfer is facilitated in the case of long alkanethiol capping.33 To address this issue and confirm our experimental evidence, we conducted systematic density functional theory (DFT) calculations based on the atomic structural parameters by XAFS, using the Vienna ab initio simulation package (VASP).34 The obtained densities of states (DOS) for the series of configurations with different occupational sites of S atoms on the Au surface are shown in Figure 4. Here, we mainly focus on

Figure 3. Schematic representations of three different local adsorption structure models, including the structural parameters: (a) ontop site adsorption for Au−C3, (b) bridge site adsorption for Au−C8, and (c) hollow site adsorption for Au−C12. The bottom figures show the plan view of the Au (111) surface with different adsorption sites.

adsorption with a Au−S distance of 2.42 Å that is also close to our fitted value of 2.41 Å for Au−C3 sample. It has been reported that, for long-chain n-alkanethiols, the alkyl chains exist predominantly in an ordered, standing-up conformation, whereas the disordered, lying-down states are observed for the short n-alkanethiols.17,29 Hence, the self-assemblies of nalkanethiols with distinct steric effects would then lead to the respective adsorption sites with energy optimization. A longer alkanethiol could introduce more steric effect and thereby the headgroup S atoms will adopt more energetically stable bonding modes, such as hollow site configuration. Then, the system will tend to be more ordered and stable through the van der Waals interactions between thiols. Moreover, our findings could also explain the observations that the Au−S peak is more intense for the n-alkanethiols with higher n in EXAFS measurements.30 Then, we will turn to the modulation of electronic structures of Au−S interface, information of which can be again obtained from XANES results shown in Figure 1b. In the XANES region, three prominent peaks: peak A associated with a 2p3=2 to 5d5=2;3=2 dipole transition provides a probe of the unoccupied densities of d states at the Fermi level and peaks B and C originating from multiple-scattering contributions, can be observed for all spectra.7,31 As compared with Au foil, the nalkanethiol-capped Au nanoparticles (NPs) exhibit a much more intense white-line peak, while all the other peaks are shifted in energy and reduced in intensity. The latter phenomenon can be ascribed to the weakened multiplescattering effect for the surface Au atoms upon the nalkanethiols capping, which can be confirmed by the increased surface disorder as mentioned above. It should be noted that the magnitude of white-line peak is enhanced as the increase of n. A more intense white-line indicates an increase in d-hole population (d charge depletion in the nanocrystal) above the Fermi level, thus suggesting a larger amount of charge transfer from Au to S atoms. The phenomenon that the charge transfer is more significant in the case of higher-coordination sites is similar to the previous reports; for instance, the calculated 0.17 e− for atop site and 0.24 e− for bridge site.31 The alkyl is a good electron donor, facilitating the charge transfer in case of long alkanethiols capping. Therefore, for the case of dodecanethiol (n = 12) capping, the tetrahedral configuration formed has a stronger Au−S interactions than that of ontop and bridge configurations, resulting in a highest white-line peak in the XANES spectrum. This picture can also be confirmed by a

Figure 4. Electronic DOS as calculated within DFT for three different adsorption geometries of n-alkanethiol-capped Au nanocrystals.

the electronic structures (Au 5d state shown as the red line) of Au adatoms that that are bound to S atoms, since the nalkanethiols has little effect on the Au core that remain the same characters to Au bulk. From Figure 4, a remarkable difference can be observed around Fermi level. For the geometry of the hollow site, the Au 5d and S 3p states display the obviously increased state density just at the Fermi energy relative to that for the bridge and ontop sites. The eminently increased DOS reflects strong s-p-d hybridization, leading to the charge transfer (including d charge) from the Au NP to the thiol in agreement with the above XANES considerations that the geometry of the hollow site has the strongest Au−S interaction. Because of the comparable electronegativities of S and Au, the formation of covalent bonding is favored, which would facilitate the s-d hybridization in Au. Hence, for the strong Au−S bond in the geometry of the hollow site, the population of the d charge is increased due to the strong s-d hybridization, resulting in the enhancement of the d charge transfer.

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CONCLUSION In summary, this work has provided the long-needed quantitative characterization of the interfacial atomic and electronic structures of gold and n-alkanethiols systems, by using XAFS spectroscopy combined with DFT calculations. We 25002

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demonstrate that, by varying hydrocarbon “tail” lengths from short to long n-alkanethiols, the adsorption of the head S atoms on Au NPs surface can be changed correspondingly from low coordination ontop to high coordination at the bridge and hollow sites. The strongest Au−S interactions with significant charge transfer from Au NPs to alkanethiols exist in the configuration of the hollow site for the case of long dodecanethiol adsorption. We consider that the two factors of steric effect and electron donor together contribute to the strongest Au−S interactions in the case of long n-alkanethiols capping. On the basis of the results presented here, one can plan to extend the study to the functionalization of Au nanocrystal surfaces where the role of the chain length must be assessed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.Y.); [email protected] (S.W.). Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11135008, 11079004, 11205158, U1232132, 11175184, and 11079032), and by the Fundamental Research Funds for the Central Universities. The authors would like to thank NSRL and BSRF for the beam time.

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