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Cite This: J. Phys. Chem. C 2018, 122, 1753−1760

Atomic-Scale Understanding of Gold Cluster Growth on Different Substrates and Adsorption-Induced Structural Change Qiang Li,† Deqiang Yin,*,‡,§ Junjie Li,*,§,∥ and Francis Leonard Deepak∥ †

School of Mechanical Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China College of Aerospace Engineering, Chongqing University, Chongqing 400044, China § Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ∥ International Iberian Nanotechnology Laboratory (INL), Avenida Mestre Jose Veiga, Braga 4715-330, Portugal Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 2, 2018 at 06:43:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: To fully understand the properties of functional nanocatalysts including nanoclusters and nanoparticles, it is necessary to know the geometric and electronic structures of the nanostructure. The catalytic properties of noble metal nanoclusters can often be improved by the formation of heterostructures on different support, but little is known about their atomic-scale structures and their interaction with the support materials. Here, we report the size- and support-dependent structures for Au nanoclusters by combining aberrationcorrected scanning transmission electron microscopy and density functional theory calculations. We demonstrate lattice-induced epitaxial coherence growth for Au nanoclusters on crystalline substrate of (110) MgO, different from the chainlike or random structures on amorphous carbon support. The time sequential atomic scale observations confirm that Au clusters tend to easily migrate on crystalline MgO support. DFT calculations based on the experiment results imply the CO adsorption on MgO supported Au clusters prefer to bind at apex sites, and the adsorption can induce the 3D structural change of the supported nanoclusters, resulting in the formation of linear and bridge CO species. The results should help to clarify the atomistic origin of shape-, number-, and support-dependent catalytic activity in the supported Au clusters catalysts. inline mass spectrometry.27 However, due to the limitation of spatial resolution, it is difficult to obtain atomic scale information for the catalysts by these methods. In principle, transmission electron microscopy (TEM) is one of the most powerful tools for the atomic scale characterization of nanomaterials.28 In particular, with the development of aberration-corrected electron microscopies, the Z-contrast (Z: atomic number) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) can provide two-dimensional (2D) information directly with single-atom sensitivity, which is suitable for the study of supported nanostructures at the atomic scale.29−32 Recently, Li et al. obtained 3D atomic-scale structures of large Au clusters (309 ± 6 atoms) by electron tomography in a Cs-corrected STEM.33 Browning et al. solved the structure of supported osmium clusters based on 2D STEM images.34 Tendeloo et al. reported the atomic scale dynamics of ultrasmall germanium clusters on carbon support under electron irradiation within the microscope.35 The results highlighted the influence of composition, size, and geometric structure of nanomaterials on their properties. However, due to the rapid atomic migration and electron damage to the ultrasmall structures, as well as the strong

1. INTRODUCTION Supported noble metal clusters or nanoparticles have attracted a great deal of attention lately due to their extraordinary catalytic activities in many reactions, such as in the oxidation of CO at low temperature,1−3 low-temperature water-gas shift reaction,4,5 hydrogen evolution reaction,6,7 and so on.8−10 The properties, however, strongly depend not only on the composition, size, and geometric structure of the metal nanoclusters or nanoparticles but also on their interactions with the different supports.11−14 The smaller the nanoparticle, the stronger is the dependence on the geometric structures and the strong interactions between metal nanostructures and support materials.12 To understand these properties, many theoretical works have been implemented to predict the atomic structures of the supported ultrasmall nanostructures.15−17 Synchrotron-radiation experiments have been used extensively in the characterization of the nanocatalysts shape, distance, and height.18−22 It is worthy to note that the dynamic observations can also be realized by using such methods. For example, Schwartzkopf et al. show real-time investigation of the growth and migration kinetics of metal nanostructures during magnetron sputter deposition with high spatial resolution by means of in situ microbeam grazing incidence small-angle X-ray scattering (μGISAXS),23−26 and Hejral et al. report a direct study of the catalysts under catalytic CO oxidation reaction conditions using in situ high-energy grazing incidence X-ray diffraction and © 2017 American Chemical Society

Received: December 6, 2017 Revised: December 27, 2017 Published: December 28, 2017 1753

DOI: 10.1021/acs.jpcc.7b12037 J. Phys. Chem. C 2018, 122, 1753−1760

Article

The Journal of Physical Chemistry C

Figure 1. Typical HAADF STEM images of Au clusters on (110) MgO substrate and intensity profiles: (a−l) the atomic scale structures of Au2−10 and Au12 observed in the sample; (m−t) the structural evolution from 2D to 3D growth under the drive of minimalizing the surface energy; (f1, o1, s1) the corresponding intensity profiles in Figures 1f, 1o, and 11.

sample or Cu grids (with amorphous carbon layer) at room temperature using molecular beam epitaxy (MBE) in a highvacuum chamber with a base pressure of ∼8.0 × 10−7 Pa. Prior to the growth, MgO (110) TEM specimen was annealed at 973 K in air for 30 min to remove the amorphous layers. High-purity Au (99.95%) was thermally evaporated from the effusion cell at 1475 K for ∼1 s in order to achieve a low density of Au clusters and nanoparticles. The chamber background pressure was 2.0 × 10−5 Pa during deposition, and thermal evaporation rate of Au was less than 0.01 nm/s. The deposited samples were used directly for the STEM and energy-dispersive X-ray spectroscopy (EDS) characterizations. 2.2. TEM Observations. HAADF-STEM images were obtained using a double-aberration-corrected Titan Cubed G2 60-300 analytical microscope operated at 200 kV (with spatial resolution of ∼0.7 nm), which was equipped with a monochromator and an X-FEG Schottky high brightness source, offering an unprecedented opportunity to probe structures with sub-angstrom resolution. HAADF-STEM images were recorded on a charge-coupled device (CCD) camera (2k × 2k, Gatan UltraSan TM 1000), and a detector with an inner semiangle of over 60.0 mrad and a probe convergence angle of ∼22 mrad were adopted. 2.3. DFT Calculations. Calculations were implemented using Vienna ab initio simulation package (VASP) within the framework of DFT.36 The projector-augmented wave (PAW) method was applied for the electron−ion interactions.

interaction between the nanomaterials and supports, the clear 2D image of clusters and the geometric structure are still not fully understood and clearly established. Here, combining the state-of-the-art aberration-corrected STEM and density functional theory (DFT) calculations, we provide size- and support-dependent structural information at atomic scale for supported metal nanostructures on different substrates. To obtain an enhanced Z-contrast image (Z: atomic number), we adopted Au (Z = 79) as a model metal and magnesium oxide (MgO) (ZMg = 12) and amorphous carbon (Z = 6) as support materials. Our results confirm that the Au clusters tend to form lattice-induced coherent structures on the crystalline support and chainlike or random structures on the amorphous support. Interestingly, the Au clusters tend to easily migrate on the surface of MgO (110) but are stabilized by the amorphous carbon. Further DFT calculations based on the experimental results imply that the CO adsorption of MgO supported Au clusters is preferentially bound at apex sites. It can also induce the 3D structural change of the supported nanoclusters and result in bridge or linear connection between CO and Au atoms.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Sample Preparation and Characterization. To avoid an unwanted solvent effect, the specimens for STEM imaging were prepared by directly evaporating Au onto MgO (110) TEM 1754


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Figure 2. Typical HAADF STEM images of Au clusters on amorphous carbon support and intensity profiles: (a−r) the atomic scale structures of Au1−10 observed in the sample; (s−x) the structural evolution from 2D to 3D growth under the drive of minimalizing the surface energy. Panels s1, u1, and x1 show the corresponding intensity profiles in Figures 2s, 2u, and 2x.

deposition, as shown in Figure S2. After deposition, the sizes of the deposited Au nanoparticles on both substrates were checked by low-magnification STEM images and confirmed that the average size of the nanoparticles is near 2−3 nm based on statistical analysis in Figures S3 and S4. From the Z-contrast HAADF-STEM images, the brighter contrast for heavier atoms in each atomic column can be seen, which means that the brighter atoms, clusters, and nanoparticles in both samples represent Au, consistent with the EDS results shown in Figures S5 and S6. Figures 1a−t show atomic-resolution HAADF STEM images of the MgO (110) supported Au nanoclusters with different sizes. We can see that the Au nanoclusters show an epitaxial coherent growth on the crystalline substrate, which is similar to the lattice induced epitaxial growth mechanism of thin films.39−42 According to the atomic model in Figure S1a, the Au atom aligned on the MgO (110) plane can sit atop either Mg or O columns. From previous DFT calculations it has been revealed that the Au atoms sitting atop O columns is more stable than atop Mg column in the Au nanocrystal/MgO (110) heterointerface because of a lower binding energy.43 Interestingly, all the Au atoms sit atop Mg column in our experimental results. The difference can be attributed to the existing Mg vacancies on the surface of (110) MgO support produced by irradiation damage during the fabrication of TEM samples.44−47 What is more, under the induction effect of the lattice, most of the ultrasmall nanoclusters tend to form a symmetric atomic

The generalized gradient approximation (GGA) of Perdew et al. (PW91) was employed to describe the exchange-correlation functional.37,38 The single particle Kohn−Sham wave function was expanded using the plane wave with a cutoff energy of 400 eV. The irreducible Brillouin zone was sampled with the regular Monkhorst−Pack grid of 6 × 6 × 6 k points. Since the exact structure of amorphous carbon is not known, the carbon substrate was not taken into account in our calculations.35 The initial 3D atomic models were established based on the experimental 2D images. The MgO (110) supported Au clusters or free Au clusters were put in a sufficiently large vacuum space in order to isolate them from each other, and the configurations were fully optimized. All atoms in the model were fully relaxed until the magnitude of force on each atom is less than 0.005 eV/Å.

3. RESULTS AND DISCUSSION Since intensity of an atomic column in HAADF images is directly proportional to atomic number Z (∼Z1.7) as well as with the number of atoms, we adopted Au (Z = 79) as the model metal and magnesium oxide (ZMg = 12) (MgO) and amorphous carbon (Z = 6) as support materials in order to obtain an enhanced Z-contrast image. An MgO (110) single crystal TEM sample was selected for the experiment because the Mg and O columns can be separately viewed along [110] in high-resolution STEM image, as shown in Figure S1. The amorphous carbon structure was also checked by high-resolution TEM and diffraction before 1755


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mechanism, different from the lattice induced mechanism in Figure 1. The corresponding intensity profiles in Figures 1s, 1u, and 1x reveal that the height of Au clusters or nanoparticles are varied from 1 to 2 to 10 layers along with the increase of size. To shed further light on the migration of Au nanocluster on crystalline and amorphous substrates, we implement direct dynamic observations for the nanoclusters on both substrates under electron irradiation. The stability of Au nanocluster on substrates is important to understand the change of properties in catalytic process. Figures 3a−f show the dynamic process of the Au nanoparticles on MgO (110) support. Figures 3a−d show the migration of an Au cluster (marked as the red arrow) on the support. Figures 3e,f show a rapid coalescence with a big nanoparticle located on its lower right direction. In contrast, Au nanoclusters tend to be stable on amorphous carbon support (marked by the red arrow in Figures 3g−l), the reason being that the carbon films can stabilize gold clusters with a binding energy of ∼0.3 eV per Au atom.51 The rapid migration of the ultrasmall Au clusters on the surface of crystalline support is harmful to their catalytic properties. Recently, Dollinger et al. report that size-selected Au clusters can be created on porous TiO2 and act as a high efficient catalysts for the bromination of 1,4-dimethoxybenzene in solution.52 The results identify that the active sites of supported Au nanoclusters can be created and stabilized in porous TiO2 structure using gas phase methods. In the present study, our results show another possible route to stabilize the ultrasmall metal clusters using amorphous supports. What is more, although the Au cluster finds it difficult to migrate on the carbon support, the ultrasmall cluster with about 30 atoms show a structural evolution from a near-square structure to an elongated structure under electron irradiation. To obtain 3D structures of the Au clusters based on the 2D STEM images, we took the MgO supported Au4, Au7, and Au8 and carbon supported Au4 and Au8 as examples and conducted a series of DFT calculations. The initial atomic models were constructed based on atomic x and y coordinates in HAADFSTEM images and assumed possible z values. After full relaxation, the models keep close to initial configuration, demonstrating that they indeed corresponding to local minima and that the measured clusters are stable. If there is a considerable discrepancy between the experimental images and relaxed structure, we adjusted the initial models and conducted further optimizations. Figure 4 shows the relaxed models of MgO (110) supported Au4, Au7, and Au8 and free Au4 and Au8 (since the exact structure of amorphous carbon is not known, the carbon support was not considered for the calculations35). It is worthy to note that for unclear images or complex systems the statistical parameter estimation theory is usually applied to estimate the structural parameters, and then the ab initio calculations are implemented to evaluate the stability of the structure.31,35 However, the Cs-corrected microscopy with a high spatial resolution of ∼0.7 nm allows us to clearly determine the atomic columns in the ultrasmall Au clusters. From the projections along different orientation, we can see that the Au clusters on MgO tend to show a 3D geometry rather than an absolutely in-plane structure like calculated free Au clusters.15 Considering the lattice mismatch of 1.6% between Au nanocrystal and (110) MgO, the Au−Au bond length in the Au nanocluster should be different from the value of ∼2.88 Å in the bulk.53 Therefore, we statistically obtain the change of Au−Au bond length in MgO supported Au4 from 3.040 to 3.072 Å, Au7 from 2.918 to 3.050 Å,

Figure 3. A sequence of atomic scale HAADF STEM images for small Au nanoclusters on different support: (a−f) the rapid migration and coalescence of Au clusters on MgO (110) substrate; (g−l) amorphous carbon-stabilized Au clusters.

arrangement. Figures 1a−l show the 2D structures of Au2−10 and Au12 observed in our sample, which are energetically stable based on the previous theoretical prediction. In principle, for each free AuN (N = 3−13), the DFT calculations usually provide several energy-stabilized geometric structures.15 However, some of the predicted energy-stabilized geometric structures for free Au clusters are missing in the MgO (110) supported Au clusters but are seen in the following amorphous carbon supported Au clusters. Figures 1m−t show the structural evolution from 2D to 3D growth, driven by minimalizing the surface energy and total energy.48−50 It is worthy to note that the images are taken at very thin regions (corresponding thick region image is also shown in Figure S7), and the intensity profiles in Figures 1f,o,s reveal that the height of Au clusters varied from one to a few layers with the increase of size. To uncover the influence of lattice-induced effect on the geometric structures of Au nanoclusters, an amorphous carbon thin film was used as another support to deposit Au nanoparticles. Figure 2 shows the amorphous carbon supported Au nanoclusters. We can see that some of the chainlike structures are seen without the MgO supported Au nanoclusters, such as Au3, Au4, Au6, Au7, and Au8 in Figures 2b,d,j,k,p. In addition, most of the nanoclusters show irregular morphologies. Even for the nanoparticles with the size of ∼2 nm they show a disordered cluster structure (as shown in Figures 2v,w and Figure S8). In Figure 2x, the Au nanocluster with a size of ∼3 nm shows a partially ordered structure rather than a crystal structure, which can be attributed to the size which is far from the critical crystalline size in classical heterogeneous nucleation and growth 1756


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Figure 4. Relaxed 3D geometries viewed from different directions. The atomistic models are constructed based on the experimental images shown in Figures 1i (a), 1g (b), 1h (c), 1c (d), 2f (e), and 2o (f), and a full structural relaxation is conducted.

and Au8 from 2.613 to 2.760 Å, as shown in Table S1. The results show an atom number (N) dependent Au−Au bond length in the Au nanoclusters on the support of MgO (110) substrate. The Au−Au bond length is changed from near Mg−Mg bond length of ∼3.05 Å to the length in bulk Au accompanied by the increase of the numbers of atoms, which is in accordance with the change of morphology of Au nanoclusters from 2D to 3D transition in Figure 1. The Au/MgO system is an important catalytic system, especially for the oxidation of CO at low temperatures.43,54 Because of the difficulty to get real atomic models for the supported systems, many calculations of CO adsorption have been focused on the free Au clusters or nanocrystals.55 To uncover the influence of CO adsorption on the structure of MgO supported Au clusters, we implement the DFT calculation of CO adsorption based on the obtained atomic models of Au7/MgO and Au4/MgO in Figure 5. Figures 5a−g show the singe CO adsorption of MgO supported Au7 from atom 1 to 7 labeled in Figure 4b1. Figures 5h,i show the single CO adsorption of MgO supported Au7 from atom 1 to 2 marked in Figure 4c1. In both clusters, we can see that single CO adsorption can induce the structural fluctuation of the supported Au nanoclusters from one 3D structure to another 3D structure. Moreover, the different adsorption positions result in different geometric structures as well as different connections between CO and Au atoms, such as the CO adsorption in the position 3 in Figure 4b; the carbon can

form a bridge connection between two Au atoms rather than linear connection in other atomic position of 1−2 and 4−7 in Figure 4b1 (Figure S9). Figures 5j,k show the CO saturated adsorption on MgO supported Au7 and Au4 in Figures 4a,c. The relaxed model show similar 3D to 3D structural fluctuations with the single CO adsorption. To further uncover the CO preferential sites, we have carried out statistical analysis of the changes in total energy and Au−C bonding length at different sites. The results show that the Au−C bond length can vary from 1.88 to 2.22 Å. The relative total energy curves for CO molecule adsorbed to an apex site (green curve) and to an edge site (red curve) of the Au7 in Figure 4b1 are shown in Figure 6. The significant energetic and bonding differences between the two types of adsorption sites imply that CO molecule prefer to bind to the apex sites rather than the edge sites of the supported small gold clusters.

4. CONCLUSIONS We present an atomic-scale observation and show a quantitative interpretation about the supported Au clusters. The results confirm that the lattice can induce an epitaxial coherence growth of metal clusters on crystal substrates and a chainlike or random growth mechanism on amorphous support. Compared to the rapid migration of Au cluster on crystalline MgO support, the amorphous carbon substrate can stabilize the Au clusters. On the basis of the clear 2D STEM images, we demonstrate that 1757


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Figure 5. CO adsorption-induced structural changes on MgO (110) supported Au clusters viewed from different directions: (a−g) single CO adsorption of MgO supported Au7 from atom 1 to 7 labeled in Figure 4b1; (h, i) the single CO adsorption of MgO supported Au7 from atom 1 to 2 marked in Figure 4c1; (j, k) the CO saturated adsorption on MgO supported Au7 and Au4 in Figures 4a1 and 4b1.

quantitative interpretation can uncover the 3D structures of the hetero-nanostructures. Further DFT calculations imply that the CO adsorption can induce the change of geometry of the supported Au nanoclusters and result in different

connections between CO and Au atoms. Such a combined atomic-scale study provides insights into 3D structures of the ultrasmall Au nanoclusters on crystalline and amorphous substrates and helps to clarify the atomistic mechanism of 1758


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Figure 6. Relative total energy curves for adsorption of a CO molecule to an apex (atoms 1, 3, 5, and 7 in Figure 4b1) and to an edge site (atoms 2, 4, and 6 in Figure 4b1) of the Au7 cluster in Figure 4b1.

shape-, number-, and support-dependent catalytic activity in the case of supported Au cluster catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12037. Atomic model and HAADF-STEM image of pure (110) MgO, HAADF-STEM image, high-resolution TEM image and corresponding fast Fourier transformation (FFT) of amorphous carbon support, low-magnification and atomic scale HAADF STEM images after deposition of Au clusters onto the MgO and carbon support, the size distribution of deposited Au clusters on MgO and carbon supports, EDS mapping and spectrum of Au clusters, HAADF and corresponding annular bright-field (ABF) STEM images of Au clusters on MgO (110) and amorphous carbon support, atomic models of CO adsorbed Au clusters, the atom number (N) dependent Au−Au bond length (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.Y.). *E-mail: [email protected] (J.L.). ORCID

Junjie Li: 0000-0001-7098-8366 Francis Leonard Deepak: 0000-0002-3833-1775 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.J.L. appreciates the Chinese Scholarship Council for financial support. JJL and FLD acknowledge the financial support from the N2020: Nanotechnology Based Functional Solutions (NORTE-01-0145-FEDER-000019), supported by Norte Portugal Regional Operational Program (NORTE2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). This work was supported in part by the National Natural Science Foundation of China (NSFC) (grant no. 11302141).

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REFERENCES

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