Mass Spectrometry and Dynamics of Gold Adatoms Observed on the

Letter pubs.acs.org/NanoLett

Mass Spectrometry and Dynamics of Gold Adatoms Observed on the Surface of Size-Selected Au Nanoclusters Z. W. Wang and R. E. Palmer* Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, B15 2TT, United Kingdom S Supporting Information *

ABSTRACT: We report the imaging, mass spectrum, and dynamical behavior of adatoms and small clusters observed on the surface facets of size-selected, truncated octahedral gold clusters, AuN (N = 923 ± 23), via aberration-corrected scanning transmission electron microscopy. Our quantitative atom counting measurements show that most (∼70%) of the species on the surface are single Au adatoms. Such species are now proposed as key elements of the atomic structure of both monolayer-protected nanoclusters (nanoparticles) and self-assembled monolayers and may also play a role in gold nanocatalysis. The adatoms are found on both {100} and {111} facets with similar probabilities.

KEYWORDS: Size-selected clusters, gold, adatoms, surface dynamics, aberration-corrected, STEM

T

The size-selected Au clusters were produced in a radio frequency magnetron-sputtering, gas aggregation cluster beam source.25−28 A novel lateral time-of-flight mass filter was used to achieve size-selection with a nominal precision of M/ΔM = 20 (i.e., Au923 has 923 ± 23 atoms) prior to softlanding at 500 eV per cluster on copper TEM grids covered with amorphous carbon. The STEM investigation was performed in a JEOL instrument (JEM2100F) with a spherical-aberration corrector (CEOS GmbH). A probe convergence angle of 19 mrad and a low probe current of 5 pA were used in these experiments. The signal was collected by a HAADF detector with inner and outer collection angles of 62 and 164 mrad. The STEM images acquired at such collection angles possess good signal-to-noise ratios and the STEM intensities are Z-contrast dominated.29,30 A spatial resolution of 1 Å can readily be achieved under optimized conditions. Figure 1a shows a 3D intensity plot of the HAADF-STEM image of one Au923 cluster, where the array of peaks corresponds directly to the atomic columns in the truncated octahedral structure presented here. Decahedral and icosahedral morphologies are also observed, but in this report we focus on the truncated octahedral structure, because they present clearly both {100} and {111} facets (labeled in Figure 1c) “edge-on” to the beam. The intensity of each column in Figure 1a is proportional to the number of atoms it contains, due to the incoherent nature of the high-angle scattering.31−40 Figure 1b is an intensity profile along the line X−X′ in Figure 1a, where the fall in column intensity away from the cluster center reflects the truncation of the octahedral structure. Figure 1c shows a profile view of the same HAADF image as panel a; we

he atomic structure of self-assembled alkanethiol monolayers on the Au{111} surface has recently been illuminated by a series of experiments which propose that the alkanethiol species are bound to gold adatoms on top of the {111} plane.1−6 In parallel, the same type of moiety has been identified in monolayer-protected Au nanoparticles through Xray diffraction measurements,7−10 consistent with first principles calculations which also demonstrate the driving force of electronic shell closing in the gold core.11,12 The new architectures now proposed deviate significantly from the idea of a sharp boundary between a raft of alkanethiol molecules and the outermost Au{111} surface atomic layer or nanoparticle facet. These discoveries are of particular interest given the potential applications of the systems in areas such as catalysis, biological labeling, and sensing.13−17 In this Letter, we demonstrate direct atomic imaging and mass spectrometry of adatoms observed on top of the facets of bare Au923±23 clusters (923 is a magic number), as well as the dynamical behavior of the Au adatoms populations. Our quantitative investigation employs aberration-corrected scanning transmission electron microscopy (STEM) in the high-angle annular dark-field (HAADF) imaging mode. Such low-coordination Au adatoms may play a role in both molecular reaction catalysis14 and the catalytic growth of nanostructures,18 analogous to the lowcoordination corner and edge atoms in metal nanoparticles whose high activity has been experimentally demonstrated in both CO oxidation19 and electron-transfer reactions.20 DFT calculations also indicate increased activity for low-coordination atoms.21,22 In addition, the nanocluster surface dynamics reported in this study may relate to the activity fluctuation observed in the redox catalysis of individual Au nanoparticles.23 More generally, size-selected Au clusters are archetypes in developing our understanding of the atomic structures of nanosystems.24 © 2011 American Chemical Society

Received: September 1, 2011 Revised: November 19, 2011 Published: November 29, 2011 91

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Figure 1. continued isolated atom in (c) is marked by the arrow A, and the region with obscure features due to the movement of adsorbates during imaging is marked by the arrow D in (d). Panel (e) shows an atomic model of a truncated-octahedral Au923 cluster, based on the images above, to orientate the reader with respect to (c,d). The electron beam direction is along the Z-axis. The inset is the ⟨110⟩ view of this model. Note that the atomic model is not an exact 3D reconstruction from the experimental images (c) or (d) and does not include the surface adatoms.

clearly see atomic species on the edge-on {111} and {100} facets of the nanoparticle aligned with the beam. An atomic model of a truncated-octahedral Au923 cluster is shown in Figure 1e, which provides a good view of the surface facets of this structure.41 Unlike the occasional isolated atoms found freely diffusing on the substrate, such as the one marked A in Figure 1c, these surface species are generally found in highsymmetry sites with respect to the surface lattice structure of the edge-on facets of the nanocluster. Figure 1d is the next frame recorded after Figure 1c (6 s later) in a series of HAADFSTEM images and shows that the number and positions of the surface species also display significant changes between frames. While the aberration-corrected STEM images of the Au923 nanoclusters clearly reveal their atomic column structure as well as atomic-like species on their surface facets, one cannot immediately conclude that these surface species are individual atoms. Thus we performed a quantitative investigation using the atom counting method we have demonstrated previously42,43 in which the Au923 clusters themselves took the role of mass standards in the intensity analysis. The integrated intensity of each surface adsorbate was measured by integrating over a 2D polygon whose edges bisect adjacent adsorbates/ columns with a subtraction of background intensity obtained using a region around the adsorbate outside the polygon as a reference. The effect of electron channeling on the integrated intensities of the clusters was calibrated by statistical analysis of seven truncated-octahedral clusters and showed that for clusters measured “on-axis” (⟨110⟩ zone axis) the intensity was ∼17% (±6%) larger, on average, than for “off-axis” (see Supporting Information). Since this channeling does not occur for single atoms, a compensation of 17% was applied to the mass measurement of single atoms when on-axis clusters were used as mass balances. By this method, we determined the mass of all the surface species on facets B and C in Figure 1 panels c and d. The mass of each of the individual surface species labeled in Figure 1 panels c and d is shown in Figure 2a. The graph shows that most of the adatoms/adparticles lie near the dotted line corresponding to 1 atom, except for the adsorbates numbered 5 and 6, which consist of clusters of 2−3 atoms, and adsorbate number 4 where the intensity is reduced because the atom moves (see below) during imaging. We believe such atomic movement also leads to the obscure contrast of some adsorbates in the region marked by arrow D as well as the appearance of adatom number 9 in Figure 1d. The latter feature consists of two small bright dots, but the distance between them is only around half an atomic distance, consistent with the movement of a single adatom during the image acquisition period. Figure 2b is the mass spectrum of the adsorbed Au species, which results from the analysis of 15 truncatedoctahedral Au923±23 clusters. We conclude that most (69 ± 5%), but not all, of the adsorbates are single Au adatoms. The largest

Figure 1. (a) A 3D intensity plot of a truncated octahedral Au923 cluster obtained by HAADF-STEM imaging. (b) Intensity profile along the line X−X′ in (a) in which 3 pixels (0.028 nm/pixel) are binned. (c) A false color display of the same HAADF image as (a). Panel (d) is the next frame after (c) in a series of HAADF-STEM images obtained with a speed of 6 s/frame. In (c,d), the cluster is projected along a ⟨110⟩ axis with six facets clearly visible; the uppermost and lowermost are {100} and the others are {111}. The false color is used to display the adatoms clearly. The images are processed using a smoothing kernel (averaging 3 × 3 neighboring pixels). Individual adsorbates are labeled in (c) and (d). To aid in orientation, adsorbate number 6 is labeled in both (a) and (c). An 92

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Figure 2. (a) Mass analysis of the adsorbed species labeled in Figure 1c,d, using the Au923 clusters themselves as mass standards. The error bars include estimates of the resolution of the cluster beam mass spectrometer and statistical errors in calibrating the channelling effect as well as the errors in the signal/background level measurements. (b) Mass spectrum of the adsorbates observed on the facets of 15 truncated-octahedral Au923 clusters.

Figure 3. Density of adatoms on (a) the {100} and (b) the {111} facets of Au923 clusters (28 facets each). The inset to (a) is a schematic which defines the number of adatoms (Na), the number of atoms in the last complete layer of the facet (Nf) and their total, N.

24.1 ± 1.4 in Figure 1d for the {100} facet B, while the number of adspecies (here atoms) falls from 3 to 1 (as is also evident to the eye), so in this example 2 adatoms on the facet B seem to move to another facet. The migration of adatoms may also be responsible for the significant morphological changes of the clusters sometimes observed between frames, for example, the growth of the atomic column at the cluster edge (see Supporting Information). What is the cause of the apparent migration of adatoms between different facets of the Au923 clusters? The effect could in principle be activated directly by the electron beam incident at 200 kV in the STEM (i.e., by momentum transfer) or indirectly (by beam-induced heating) or by purely thermal effects (the sample is at room temperature).44−46 To address this question, we studied the stability of the adatoms as a function of electron beam current. For each dose rate investigated, we summed the differences in the total number of selvedge atoms (N), those in the outermost layer plus adatoms, at the same {100} facet of the same nanoparticle between successive frames over a series of 10 images (0.5 s/ frame). The result of this measurement is shown in Figure 4, which displays a clear increase in the mobility of the adatoms as the electron dose rate increases. We note, however, that the extrapolation of the data in Figure 4 to zero current indicates that purely thermal energy can drive the same dynamical process at room temperature, albeit at a reduced rate.

cluster of adatoms for which there is evidence is the tetramer, but this is rare. The surface adatoms and small clusters discussed are observed on both the {100} and {111} facets of the Au923 clusters, as Figure 1c,d show. To investigate whether one facet is favored over another, we performed a statistical analysis of the (time-averaged) adatom density (Na/N) on both types of facet. Here, N is defined as the sum of the number of adatoms on a facet (Na) and the number of atoms in the outermost surface layer of the facet (Nf), as illustrated in the inset to Figure 3a. One might expect that atomic adsorption would be more energetically favorable on the more open {100} facet than the {111} facet, as indicated for example by molecular dynamics simulations on FCC gold clusters (Wulff polyhedra), which show that adatoms on {111} facets can diffuse to {100} facets and remain trapped there, while the reverse process hardly occurs even at T = 500 K.44 However, this expectation is not reflected in the experimental results displayed in Figure 3a,b; they do not show the preferred appearance of adatoms on the {100} facets. The average densities are 0.11 ± 0.01 and 0.13 ± 0.01 for {100} and {111}, respectively. Note that the data is based on the quantitative intensities (i.e., atom counting) and not simply the number of distinctly visible species. A further point of interest is that a change in the total number of atoms (N) in/on each facet between consecutive STEM images in the measured sequences is often observed. For example, N changes by about 2 from 26.4 ± 1.6 in Figure 1c to 93

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ASSOCIATED CONTENT

S Supporting Information *

The calibration of the channeling effect and the diffuseness of the atomic columns observed between frames. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS We thank Dr. Feng Yin and Ahmed Abdela for deposition of the gold clusters. We acknowledge financial support from the EPSRC and TSB. The STEM instrument employed in this research was obtained through the Birmingham Science City project “Creating and Characterising Next Generation Advanced Materials”, supported by Advantage West Midlands (AWM) and part-funded by the European Regional Development Fund (ERDF).

Figure 4. Dynamics of adatoms, as represented by the sum of the differences in the number of atoms N at the same {100} facet between successive images of the same Au923 cluster as a function of electron beam dose rate, obtained over 10 frames (0.5 s per frame) for each dose rate.

The experimental observation that the {100} and {111} facets of the clusters are approximately equally populated by adatoms, even when there is evidence for migration between facets, is an enigmatic but intriguing result in this work. Updated theoretical calculation shows a significantly enhanced binding energy for Au adatoms on the more open (unreconstructed) {100} versus {111} facets. 44,47 One intriguing possibility is that the {100} facets are unstable with respect to a local “hex” reconstruction into surface rafts akin to the {111} surface. The extended Au {100} surface, like Pt {100}, exhibits this reconstruction.48 The method of imaging reported here makes the experimental observation of such an effect elusive, since we accumulate intensity right through the cluster. A diffuseness of the atomic columns at the surface of the {100} facet viewed side-on may be expected and indeed is sometimes observed, but the possible effect needs a thoroughgoing crystallographic analysis beyond the scope of the present work. Other possible explanations of the similar populations of {100} and {111} facets include local surface premelting and diffusion barriers/prefactors that do not respect the static binding energies. In summary, we have reported atomic imaging and mass analysis of gold adatoms and small clusters observed on the surface of size-selected gold (Au923±23) nanoclusters. We have exploited quantitative atom counting in the aberrationcorrected scanning transmission electron microscope, using the clusters themselves as mass standards, to show that most (∼70%), but not all, of the adsorbed species are individual Au atoms. They are observed to locate on both {100} and {111} facets with similar probabilities, while sequential imaging shows that they seem to migrate dynamically between different facets. While this observed dynamical process is found to be enhanced by the electron beam current, it persists in the extrapolation to zero dose, that is, in the purely thermal regime. The adatoms observed, and their dynamics, may play a role in a diversity of processes, such as molecular catalysis, catalytic growth of nanowires and the formation of alkanethiol-passivated nanoparticle structures. The same quantitative approach in combination with variable sample temperature may lead to further insights into the dynamics of the adatoms reported here.

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REFERENCES

(1) Maksymovych, P.; Sorescu, D. C.; Yates, J. T. Phys. Rev. Lett. 2006, 97, 146103. (2) Grönbeck, H.; Odelius, M. Phys. Rev. B 2010, 82, 085416. (3) Mazzarello, R.; Cossaro, A.; Verdini, A.; Rousseau, R.; Casalis, L.; Danisman, M. F.; Floreano, L.; Scandolo, S.; Morgante, A.; Scoles, G. Phys. Rev. Lett. 2007, 98, 016102. (4) Cossaro, A.; Mazzarello, R.; Rousseau, R.; Casalis, L.; Verdini, A.; Kohlmeyer, A.; Floreano, L.; Scandolo, S.; Morgante, A.; Klein, M. L.; Scoles, G. Science 2008, 321, 943. (5) Chaudhuri, A.; Lerotholi, T. J.; Jackson, D. C.; Woodruff, D. P.; Dhanak, V. Phys. Rev. Lett. 2009, 102, 126101. (6) Voznyy, O.; Dubowski, J. J.; Yates, J. T.; Maksymovych, P. J. Am. Chem. Soc. 2009, 131, 12989. (7) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430. (8) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754. (9) Qian, H.; Eckenhoff, W. T.; Zhu, Y.; Pintauer, T.; Jin, R. J. Am. Chem. Soc. 2010, 132, 8280. (10) Lopez-Acevedo, O.; Tsunoyama, H.; Tsukuda, T.; Häkkinen, H.; Aikens, C. M. J. Am. Chem. Soc. 2010, 132, 8210. (11) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157. (12) Pei, Y.; Gao, Y.; Zeng, X. C. J. Am. Chem. Soc. 2008, 130, 7830. (13) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (14) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (15) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (16) Murray, R. W. Chem. Rev. 2008, 108, 2688. (17) Kaden, W. E.; Kunkel, W. A.; Kane, M. D.; Roberts, F. S.; Anderson, S. L. J. Am. Chem. Soc. 2010, 132, 13097. (18) Allen, J. E.; Hemesath, E. R.; Perea, D. E.; Lensch-Falk, J. L.; Li, Z. Y.; Yin, F.; Gass, M. H.; Wang, P.; Beloch, A. L.; Palmer, R. E.; Lauhon, L. Nat. Nanotechnol. 2008, 3, 168. (19) Overbury, S. H.; Schwartz, V.; Mullim, D. R.; Yan, W. F.; Dai, S. J. Catal. 2006, 241, 56. (20) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (21) Hvolbaek, B.; Janssens, T. V. W.; Clausen, B. S.; Falsig, H.; Christensen, C. H.; Norskov, J. K. Nanotoday 2007, 2, 14. (22) Mpourmpakis, G.; Andriotis, A. N.; Vlachos, D. G. Nano Lett. 2010, 10, 1041. (23) Xu, W.; Kong, J. S.; Yeh, Y.-T. E.; Chen, P. Nat. Mater. 2008, 7, 992. (24) Li, J.; Li, X.; Zhai, H. J.; Wang, L. S. Science 2003, 299, 864. 94

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(25) Goldby, I. M.; von Issendorff, B.; Kuipers, L.; Palmer, R. E. Rev. Sci. Instrum. 1997, 68, 3327. (26) von Issendorff, B.; Palmer, R. E. Rev. Sci. Instrum. 1999, 70, 4497. (27) Pratontep, S.; Preece, P.; Xirouchaki, C.; Palmer, R. E.; SanzNavarro, C. F.; Kenny, S. D.; Smith, R. Phys. Rev. Lett. 2003, 90, 055503. (28) Pratontep, S.; Carroll, S. J.; Xirouchaki, C.; Streun, C. M.; Palmer, R. E. Rev. Sci. Instrum. 2005, 76, 045103. (29) Wang, Z. W.; Li, Z. Y.; Park, S. J.; Abdela, A.; Tang, D.; Palmer, R. E. Phys. Rev. B 2011, 84, 073408. (30) Wang, Z. W.; Palmer, R. E. J. Phys.: Conf. Ser., in press. (31) Crewe, A. V.; Wall, J.; Langmore, J. Science 1970, 168, 1338. (32) Howie, A. J. Microscopy 1979, 117, 11. (33) Nellist, P. D.; Pennycook, S. J. Ultramicroscopy 1999, 78, 111. (34) Singhal, A.; Yang, J. C.; Gibson, J. M. Ultramicroscopy 1997, 67, 191. (35) Li, Z. Y.; Young, N. P.; Di Vece, M.; Palomba, S.; Palmer, R. E.; Bleloch, A.; Curley, B. C.; Johnston, R. L.; Jiang, J.; Yuan, J. Nature 2008, 451, 46. (36) Muller, D. A. Nat. Mater. 2009, 8, 263. (37) Lebeau, J. M.; Findlay, S. D.; Allen, L. J.; Stemmer, S. Nano Lett. 2010, 10, 4405. (38) Krivanek, O. L.; Chisholm, M. F.; Nicolosi, V.; Pennycook, T. J.; Corbin, G. J.; Dellby, N.; Murfitt, M. F.; Own, C. S.; Szilagyi, Z. S.; Oxley, M. P.; Pantelides, S. T.; Pennycook, S. J. Nature 2010, 464, 571. (39) Van Aert, S.; Batenburg, K. J.; RoSSell, M. D.; Erni, R.; Van Tendeloo, G. Nature 2011, 470, 374. (40) Hartel, P.; Rose, H.; Dinges, C. Ultramicroscopy 1996, 63, 93. (41) Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33. (42) Young, N. P.; Li, Z. Y.; Chen, Y.; Palomba, S.; Di Vece, M.; Palmer, R. E. Phys. Rev. Lett. 2008, 101, 246103. (43) Wang, Z. W.; Toikkanen, O.; Yin, F.; Li, Z. Y.; Quinn, B. M.; Palmer, R. E. J. Am. Chem. Soc. 2010, 132, 2854. (44) Baletto, F.; Mottet, C.; Ferrando, R. Surf. Sci. 2000, 446, 31. (45) Marks, L. D. Rep. Prog. Phys. 1994, 57, 603. (46) Agrawal, P. M.; Rice, B. M.; Thompson, D. L. Surf. Sci. 2002, 515, 21. (47) Ferrando, R. Private communication. (48) Havu, P.; Blum, V.; Havu, V.; Rinke, P.; Scheffler, M. Phys. Rev. B 2010, 82, 161418.

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