X-ray Spectroscopy of Gold–Thiolate Nanoclusters - The Journal of

Sep 8, 2014 - For the convenience of researchers with little or no previous experience in X-ray spectroscopy, some background information on the theor...
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X‑ray Spectroscopy of Gold−Thiolate Nanoclusters Peng Zhang* Department of Chemistry, Dalhousie University, Halifax, NS B3R4J2, Canada ABSTRACT: Exciting research on gold−thiolate nanoclusters (AuSR NCs) has recently emerged thanks to the breakthroughs in their chemical synthesis and structural determination. Herein is presented representative work regarding X-ray spectroscopy studies of AuSR NCs, including clusters such as Au144, Au38, Au36, Au25, Au24Pt, and Au19. It is demonstrated that synchrotron-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) techniques are unique and powerful tools for probing the structural and electronic properties of AuSR NCs. Emphasis is placed on X-ray studies from a site- and element-specific perspective conducted under variable experimental conditions (e.g., solvation, temperature, etc.). For the convenience of researchers with little or no previous experience in X-ray spectroscopy, some background information on the theory of XAS and XPS techniques is also provided.

1. INTRODUCTION Gold nanostructures are among the most widely studied materials in nanoscience and nanotechnology.1 Understanding the relationship between the atomic structure and properties of these nanomaterials is crucial for both fundamental studies and the development of viable applications.2 However, a significant challenge remaining in this area is the precise control of nanoparticle size and composition.1,2 Most of the samples of gold nanoparticles exist as a mixture of particles of a range of sizes (Figure 1a), making it difficult to establish a precise understanding of their size-dependent properties.

opportunities to more definitively understand the structure− property relationships of gold nanostructures. In spite of the successful synthesis of composition-precise AuSR NCs, experimental studies on the atomic level structure and properties remain challenging due to the difficulty in growing single crystals for X-ray crystallography experiments. In this context, X-ray spectroscopy techniques such as X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) are desirable alternative tools for the structural and property studies of AuSR NCs and have been previously found very useful for the studies of a variety of nanomaterials.6−9 In this contribution, some representative work on X-ray spectroscopy studies of AuSR NCs conducted by my research group will be presented. Note that a few other groups (e.g., Tsukuda,10,11 Negeshi,12 Kumar,13−15 and Scott16) have also reported XAS results on AuSR NCs, which are not covered in this article. Following a brief introduction to XAS and XPS techniques, results on Au144, Au38, Au36, Au25, Au24Pt, and Au19 nanoclusters will be discussed. The usefulness of XAS and XPS to probe the structural and bonding properties from a site- and element-specific perspective under variable experimental conditions will be illustrated. Finally, concluding remarks will be presented to summarize the established progress and provide a brief outlook on the future direction of this research area.

Figure 1. Schematic illustration of (a) polydisperse and (b) monodispered gold nanostructures, with the Au102 shown as an example having staple-like motifs on the surface.

2. FUNDAMENTALS OF X-RAY SPECTROSCOPY 2.1. X-ray Absorption Spectroscopy (XAS). For the convenience of researchers without much experience in XAS, a tutorial introduction on this technique is presented in this

Following a recent breakthrough in the synthesis and structural determination of thiolate-protected Au102 nanoclusters (Figure 1b), research on the gold−thiolate nanoclusters (AuSR NCs) has emerged as an exciting area with a significant impact on gold-based nanoscience and nanotechnology.3−5 Besides many other interesting properties, these quantum-sized NCs with precisely controlled composition offer unprecedented © XXXX American Chemical Society

Received: July 31, 2014 Revised: September 3, 2014

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Figure 2. (a) Illustration of typical XAS spectrum consisting of XANES and EXAFS (inset: k-space EXAFS); (b) the L3-edge XANES of some 5d transition metals demonstrating the white line intensity trend with electronic transition mechanism shown on the right; (c) EXAFS of Pd foil and the best fit (fitted bond distance information also shown) and the structural model; (d) an illustration of simulated EXAFS with increased coordination number. Data in (b) were adapted from ref 6 with permission.

waves from the X-ray absorbing atom to the neighboring atoms of different shells (i.e., the first shell corresponding to the nearest neighboring atoms, the second shell to the second nearest-neighboring atom, and so on). By fitting the EXAFS experimental spectrum (Figure 2c) with a desirable theoretical model, accurate information on the distance between the X-ray absorber and its neighboring atoms can be determined. EXAFS can also provide size information on low-dimensional materials like nanoclusters. As is illustrated in Figure 2(d), the EXAFS peak intensity is directly related to the number of neighboring atoms (i.e., coordination number, or CN) around the X-ray absorbing atom. In general, the lower CN associated with smaller-sized clusters corresponds to a lower EXAFS peak intensity. Again, quantitative information on the CN can be obtained by fitting the EXAFS with a theoretical model. 2.2. X-ray Photoelectron Spectroscopy (XPS). XPS is another useful X-ray spectroscopy technique providing complementary information as to what XAS does. The XPS technique is more widely used for materials characterization than the XAS, and the data analysis is more straightforward.19 XPS studies of gold nanostructures will typically include the core-level Au 4f and valence band region. Figure 3 shows the core-level Au 4f XPS and valence band XPS of size-varied AuSR nanoparticles reported by Sham et al.20 At the core level, useful information is normally found from the peak position and a line shape analysis. Figure 3(a) shows that when the size of AuSR nanoparticles decreases the Au 4f peaks sensitively shift to higher binding energy. Meanwhile, a close comparison of the spectral line shape in Figure 3(b) reveals a systematic line broadening as the particle size decreases. These findings can be reasonably interpreted by considering the nanosize effect and surface metal−ligand interaction in the nanoparticles. Overall, these examples illustrate the sensitivity and usefulness of peak position and line shape analysis for the structure and bonding studies of gold nanostructures.

section. More detailed information on this topic can be found elsewhere.8,9 Generally speaking, the XAS technique can be simply treated as a cousin of the widely used UV−vis absorption spectroscopy. The major difference between these two absorption spectroscopy techniques is that XAS involves core-level electronic transitions, whereas the UV−vis absorption only involves valence-orbital related transitions. A typical XAS spectrum consists of two spectral regions (Figure 2a), that is, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The corelevel electronic transition to unoccupied valence states from the X-ray absorption process enables one to probe the electronic properties of the absorbing atom in the XANES region. For instance, Figure 2(b) demonstrates that the intensity of the first spectral feature (historically called “white line”) in XANES following the absorption edge is sensitive to the valence delectron count of transition metal elements when the core-level 2p electrons (L2,3-edge) are excited. A higher population of the unoccupied valence d-state corresponds to a higher intensity of the white line peak. This is illustrated in Figure 2(b) where there is a gradual decrease of the white line intensity for consecutive 5d transition metals (Re → Au).17 Theoretical framework has also been established to semiquantitatively evaluate the d-electron count of transition metal elements.18 The most useful structural information comes from the EXAFS region, which can provide very accurate information on the local structure of the absorbing atom (e.g., Au). In the literature, EXAFS is often plotted in wave vector k-space by simply changing the x-axis unit from energy (eV) to photoelectron wavenumber k (k = ℏ−1(2me(E − E0))1/2). The physical meaning of EXAFS is most obvious when the spectrum is plotted in the pseudo radial function (R) space by applying a Fourier-transformation on the k-space EXAFS. As is demonstrated in Figure 2(c), the EXAFS peaks in R-space roughly correspond to the scattering path of photoelectron B

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behavior (narrower valence band, approaching the structure of molecular orbitals).

3. PROPERTIES OF GOLD−THIOLATE NANOCLUSTERS (AUSR NCS) 3.1. Earlier Studies on Nonmonodisperse Samples. Before the compositionally precise AuSR NCs were synthetically attainable, X-ray spectroscopy such as XAS had already been employed to study nonmonodispersed AuSR nanostructures of various sizes and compositions. Representative XAS results in this area can be found in the publications by Ugarte et al.,21 Sham et al.,20 Fernandez et al.,22 Frenkel et al.,23 and so on. However, the low monodispersity nature of these samples made it impossible to achieve a precise understanding of the relationship between the particle size and properties. In addition, without using composition-precise samples, XAS structural information is limited to simple two-shell (Au−S and Au−Au) fitting results, and more detailed structural information from a site-specific perspective cannot be obtained.24,25 3.2. Au144(SR)60: The First XAS Study of CompositionPrecise AuSR NCs. The first XAS study of truly monodispersed AuSR NCs was conducted on Au144 by MacDonald et al.26 Prior to this XAS study, a density functional theory (DFT) study conducted by Lopez-Acevedo et al. has predicted a structural model on Au144 (Figure 4a).27 The structural information from EXAFS fitting analysis (Figure 4b) was found to be consistent with the DFT model. On the basis of Lopez-Acevedo’s model, a site-specific simulation (Figure 4c) further demonstrated the origin of experimental EXAFS features presented in Figure 4(b). For instance, the most intense EXAFS feature around 2 Å can be seen to originate from the Au−S bond (Figure 4c), and the intense EXAFS feature on the right of the Au−S scattering peak is due to the Au−Au interaction between the surface and core Au atoms. Using the simulation, it can also be deduced that the feature at around 4 Å in Figure 4b comes from the high-shell Au−Au interactions from the core Au atoms. Moreover, electronic and bonding properties of Au144 were comparatively studied with

Figure 3. (a) XPS Au 4f peak position analysis, (b) XPS Au 4f line shape analysis after aligning all the peaks to that of the bulk, and (c) XPS analysis on valence band shape and apparent spin−orbit (SO) splitting of AuSR nanoparticles of varied size and bulk Au. Adapted with permission from ref 20. Copyright 2003 American Physical Society.

The valence band XPS is also of particular use for the evaluation of electronic behavior of nanomaterials. Again, the AuSR nanoparticle samples are used as examples. As the particle size decreases, it is evident in Figure 3(c) that the valence band becomes narrower, which is coupled with a decrease of the apparent spin−orbit split. These observations indicate that as the particle size becomes smaller the electronic behavior of the nanoparticles is in transition from more bulklike behavior (broad valence band) to more molecule-like

Figure 4. (a) Au144 structural model predicted by DFT; (b) experimental EXAFS and best fit of Au144; (c) site-specific EXAFS simulations using the DFT structural model; (d) Au L3-edge XANES of Au144, Au38, and Au25; (e) S K-edge XANES of Au144, Au38, and Au25. Adapted with permission from ref 26. Copyright 2010 American Chemical Society. C

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MacDonald et al. reported the EXAFS of Au38 (model shown in Figure 6a) collected in both solid and toluene solution

Au38 and Au25 in an element-specific manner using both Au L3(Figure 4d) and S K-edge (Figure 4e) XANES. These results demonstrate the interesting size-dependent bonding properties of the AuSR NCs from both the metal (gold) and ligand (sulfur) perspective. The precise composition of the Au144 sample offered a valuable opportunity to precisely understand the size-dependent electronic properties of the AuSR NCs. Figure 5(a) and (b)

Figure 6. (a) Structural model of Au38 and (b) EXAFS of Au38 in solid and solution phases. Adapted with permission from ref 28. Copyright 2011 American Chemical Society.

(Figure 6b).28 As expected, the Au−Au bonding environment changed noticeably upon solvation, which is reflected in the change of the EXAFS feature around 3 Å in Figure 6(b). EXAFS fitting analysis shows that upon solvation the average Au−Au bonds in the Au core and the AuSR ligand shell expand by ca. 0.03 and 0.06 Å, respectively. In addition, the Au−S distance also increases by ca. 0.02 Å. Coincidently, the electronic behavior of the AuSR NCs was changed upon solvation, which was revealed by Au L3-edge XANES experiment and verified by ab initio calculations. These results can be accounted for based on the donation/back-donation Au−S bonding mechanism. The findings reported in this work imply that the solvation effect could be of potential use for tuning the electronic properties of AuSR NCs via controlling the solvation process and demonstrates the usefulness of XAS in detecting the structural change of AuSR NCs caused by the solvation effect. 3.4. Au25(SR)18: Solvent and Temperature Effects. The findings from solution-phase XAS study on Au38 stimulated further work to monitor the structural change of AuSR NCs in different conditions. MacDonald et al. conducted XAS studies on Au25 under varied conditions by changing the experimental temperature and the solvent type.25 Au25 was selected for a detailed study in this regard due to its structural simplicity (Figure 7a) and its extremely large surface area. In order to more completely reveal the structural change of Au25 from a site-specific perspective, a multishell EXAFS fitting procedure was used that included one Au−S path and three Au−Au paths (i.e., center−surface, surface−surface, and surface−staple Au− Au shells). The simulated spectrum in Figure 7(b) confirms that such a multishell procedure is feasible for the Au25 system in that distinct EXAFS peaks are well separated and correspond with these four unique scattering paths. A decrease in bond length is observed within the Au13 icosahedral core (i.e., Au−Au (1) and Au−Au (2) path) upon temperature decrease. This is accompanied by a noticeable increase in the average Au−Au(3) (surface−staple) bond length. The metal−metal bond contraction with temperature decrease is common for metallic nanoparticles. Thus, contraction of the Au13 core implies the metallic behavior of the core in response to the temperature change. The abnormal thermal expansion behavior of the Au− Au(3) path was proposed to be related to the molecule-like behavior of the SR−Au−SR−Au−SR ligand shell. The crystallographic study indicated that the aromatic phenyl rings from the phenylethanethiol ligand group together to accommodate pi−pi interactions in the surrounding ligand network of the NCs. Therefore, the hardness of these aromatic

Figure 5. (a) XPS Au 4f spectra of Au144 and bulk Au; (b) XPS valence band of Au144 and bulk Au; (c) site-specific simulation of Au 5d DOS of Au144 and bulk Au. Adapted with permission from ref 26. Copyright 2010 American Chemical Society.

shows the experimental XPS core-level and valence band results of Au144 together with that of bulk Au. It appears that for Au144 only a very small Au 4f binding energy shift (0.2 eV) and valence band narrowing (5.1 eV for Au144 versus 5.3 eV for the bulk) were detected in comparison with the bulk. These results suggest that even for a 1.7 nm Au144 the NCs still are very much metal-like from the XPS core-level and valence band perspective. Furthermore, the precise composition of Au144 allowed for a site-specific simulation of the Au valence (Figure 5d) d-DOS using the DFT model (Figure 5c). The Au d-DOS profiles of the three presentative Au sites (i.e., the staple Au, the surface Au, and the core Au) are distinctly unique to their location in the nanocluster. The core Au shows the broadest d-DOS, indicating its metal-like electronic character. The staple Au, however, shows a very sharp d-DOS, characteristic of molecule-like Au species. As expected, the d-DOS width of surface Au lies in between those of the core and staple sites, suggesting an electronic behavior in between the metal-like core Au and the molecule-like staple Au. The averaged d-DOS simulation also verified the experimental results in Figure 5(b); that is, Au144 has a valence band that is only slightly narrower than that of the bulk Au. 3.3. Au38(SR)24: The Solution-Phase Local Structure. A unique feature of the XAS experiment is the capability to measure solution-phase samples. For such small particles as AuSR NCs, a large percentage of atoms are located on the surface. These surface atoms directly bond to the protecting thiolate ligands, which in turn interact with solvent molecules. As a result, the solvation process of AuSR NCs could result in a change of the NC structure, which can be sensitively probed by XAS. D

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Figure 7. (a) Structural model of Au25; (b) simulated EXAFS of Au25; and (c) bond distance fitting results from EXAFS of Au25 measured under different conditions. Adapted with permission from ref 25. Copyright 2011 American Chemical Society.

Figure 8. (a) Schematic illustration of structural models of Au19 and Au25; (b) XPS valence band data of Au19 and Au25; (c) EXAFS and multishell best fit; (d) a comparison of bond distance from EXAFS fitting results of Au19 (black striped) and Au25 (red). Adapted with permission from ref 29. Copyright 2012 American Chemical Society.

ligand tail interactions prevents the staple systems from contracting with the relatively soft Au13 core. The multishell EXAFS fitting results (Figure 7c) are compared when the temperature (10 vs 300 K) and sample state (solid vs polar solution vs nonpolar solution) are changed.

A comparison of the EXAFS results of the solid-state (300 K) and solution-phase samples (Figure 7c) shows a decrease in bond length of the Au13 core (i.e., the Au−Au(1) and Au− Au(2) path) accompanied by an increase in the Au−Au(3) bond length when the sample is dissolved in toluene (Figure 7c). This observation could be attributed to the weakened pi− E

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Figure 9. (A)−(D) Structural models and CN values of Au25 and Au24Pt with varied dopant location; (E) Pt L3-edge EXAFS of Au24Pt and the best fit; (F) Au L3-edge EXAFS of Au25 and Au24Pt. Adapted with permission from ref 31. Copyright 2012 American Chemical Society.

aimed to illustrate the sensitivity of structural and electronic properties from a small change in the AuSR NC composition, particularly the core size. Indeed, the two NCs do show different electronic properties; for instance, in the valence electron behavior shown in Figure 8b. Au19 exhibits a narrower and more positively shifted d-band than Au25, due to the expected smaller size of the Au19 core, further indicating that the electronic structure of Au25 is more metallic in nature. To correlate the electronic properties with the atomic structure, a multishell EXAFS fit was conducted. The results show considerable Au−Au bond expansion for the surface− surface and surface−staple Au paths, whereas the Au−S and center−surface Au−Au bond distance remain nearly unchanged. These findings were interpreted using Jiang’s DFT model that predicted a defective Au11 core within the Au19 NCs. The defective core structure of Au19 results in a lattice expansion and more molecule-like d-band behavior relative to the Au25 reference. Thus, these findings point out the sensitivity of structure and electronic behavior of the AuSR NCs upon change of the core composition, which can be successfully detected by XAS. 3.6. Au24Pt(SR)18: The Doping Effect. Doping the AuSR NCs with a metal heteroatom could offer a good opportunity to tailor their structure and properties (e.g., catalytic). However, without single-crystal X-ray crystallographic data, it is challenging to determine the location of the dopant atom and the doping effect on the atomic structure of the NCs. The element-specific nature makes X-ray spectroscopy well suited to overcome the above-mentioned challenges by selectively

pi stacking in the Au25 ligand shell due to the solvation in aromatic toluene solvent. The reduced hardness of the ligand shell caused by solvation in toluene could both release the surface tension of the Au13 core and weaken the pi−pi interaction of the ligand shell, resulting in a contraction of the core and a relaxation of the ligand shell. As for the solvation effect in polar solvent like acetonitrile, a further contraction of the Au13 core was observed, whereas the Au−Au(3) path remains essentially unchanged relative to that of the solid sample. The further contraction of the Au13 core was proposed to be caused by the charge donation from the solvent to the Au13, whereas the unchanged surface−staple Au−Au interaction can be accounted for by considering the weak interaction between the nonpolar aromatic ligand and the polar solvent. Overall, temperature- and solvation-dependent EXAFS results on Au25 illustrated the intriguing bonding properties of such NCs on the metal-like behavior of the Au13 core and the molecule-like behavior of the gold−thiolate ligand shell. 3.5. Au19(SR)13: The Sensitive Core Composition Effect. The Au25 system not only was used for an in-depth temperature-/solvation-dependent EXAFS study illustrated in the precedent section but also serves as an excellent reference for comparative structural studies with other NCs whose structure and properties are relatively unknown. Chevrier et al. reported such a study by closely comparing the XAS data of Au25 with Au19.29 The total structure of Au19 has not been reported, but Jiang et al. had predicted a promising structural model (Figure 8a) using DFT simulations and a “staple-fitness” methodology.30 Due to the similar size and composition of these two NCs (Au19 vs Au25), the XAS study by Chevrier et al. F

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Figure 10. (a) Structural models to illustrate each component in the Au36 nanocluster; (b) room-temperature (295 K) EXAFS of Au36 and best fit; (c) low-temperature (90 K) EXAFS of Au36 and best fit; (d) S K-edge XANES of Au36 and Au38; (e) simulated S K-edge XANES of Au36 and Au38. Adapted with permission from ref 28. Copyright 2013 American Chemical Society.

Au) bonding effect on the electronic behavior of the NCs. Overall, the element (Pt and Au)-specific XAS and XPS studies demonstrated that doping Au25 with a mono-Pt atom results in considerable changes to its metal−metal bond distance and the electronic behavior, implying the effectiveness of this method to tailor the structure and properties of the AuSR NCs. 3.7. Au36(SR)24: The fcc Core Geometry Effect. Typically, the face-centered cubic (fcc) structure is always observed in larger nanocrystalline gold, whereas non-fcc geometry (e.g., icosahedron-like core) is commonly found in ultrasmall (