Hydrogen Induced Clustering of Metal Atoms in Oxygenated Metal

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Hydrogen Induced Clustering of Metal Atoms in Oxygenated Metal Surfaces Yaguang Zhu, Dongxiang Wu, Qianqian Liu, Jerzy T. Sadowski, and Guangwen Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00750 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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The Journal of Physical Chemistry

Hydrogen Induced Clustering of Metal Atoms in Oxygenated Metal Surfaces Yaguang Zhu1, Dongxiang Wu1, Qianqian Liu1, Jerzy T. Sadowski2, Guangwen Zhou1* 1Department

of Mechanical Engineering & Materials Science and Engineering Program, State University of New York, Binghamton, NY 13902, USA

2Center

for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA

Abstract Nearly all metals form spontaneously an oxygenated surface upon exposure to air. Here we demonstrate that the reaction of such an oxygenated surface with hydrogen results in the clustering of metal atoms. Using scanning electron microcopy, X-ray photoelectron spectroscopy and in-situ lowenergy electron microscopy, we show that Cu atoms in the oxygenated Cu(110) surface self-assemble into Cu clusters upon the hydrogen induced loss of the chemisorbed oxygen. It is shown that the clustering of Cu atoms occurs preferentially along the upper side of step edges formed by neighboring terraces of the substrate and boundaries of heterophase domains on the same terrace, followed by the spreading across the entire surface as the reaction progresses toward completion. Using densityfunctional theory calculations, we show that the heterogeneous clustering of Cu atoms is induced by step-crossing barriers that hinder Cu atoms crossing descendent steps, thereby resulting in 3D aggregation of Cu atoms on the upper side of step edges. These results may find broader applicability to tailor the formation of metal clusters for elucidating the intrinsic properties and functionalities of nanoclusters.

*To whom correspondence should be addressed: [email protected]

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1. Introduction Nanosized clusters of metals differ significantly in their properties from the bulk phase and have received much attention in connection with promising applications across a wide range of fields including nanoscale devices1, quantum transport2, heterogeneous catalysis3, drug delivery4,5, etc. When working with these small metal clusters, it is essential to synthesize clusters so as to understand their fundamental nature and to allow for practical applications by controlling their functionalities. Various methods have been established for the synthesis of metal clusters that can be generally classified in physical vapor deposition6,7 and chemical reduction of metal salts8. A fundamental limitation for the physical vapor deposition approach is the lack of surfactants during the atomic aggregation and cluster growth, for which individual metal clusters coarsen or fuse with other clusters3,9. In contrast, the chemical approach involves the use of ligands that prevent the aggregation between clusters. However, ligands can also passivate the cluster surface that may lead to losses in the surface properties such as the catalytic reactivity resulting from the low coordination number atoms located at steps, corners and edges of the clusters10,11. Therefore, exploring the synthetic routes capable of producing nanosized metal clusters while simultaneously preventing the clusters from agglomeration and passivation are of great interest. Here we describe the formation of metal clusters by hydrogen-induced decomposition of an oxygenated metal surface into metal atoms that subsequently self-assemble into metal clusters. Using scanning tunneling microscopy (STM) and in-situ low-energy electron microscopy (LEEM) along with density-functional theory (DFT) modeling, we develop an atomistic understanding of the microscopic process leading to the clustering of metal atoms through the surface reaction. Because nearly all metals (except for Au) react spontaneously with oxygen to form an oxygenated surface, it is expected that the process is versatile and enables production of ultra-pure metal clusters owing to the spontaneous desorption of the gaseous reaction product (water molecules) from the surface.

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Metal-based catalysts are widely used in catalytic oxidation processes such as the water-gas-shift reactions and methanol synthesis/oxidation. Upon the exposure to an oxygen-containing atmosphere during the oxidation process, the metal surface undergoes a series of changes starting with the oxygen chemisorption induced restructuring to oxide nucleation and growth12–14. Meanwhile, hydrogen can be produced at the surface by disproportion of hydroxyl species during the catalytic reaction. Compared with the extensive studies on the interaction of oxygen with metallic surfaces, the understanding on the reaction between hydrogen and the oxygenated metal surfaces is still quite limited. Oxygen chemisorption induced surface restructuring is well established for many metal surfaces15–17. In contrast, it is much less understood regarding how the oxygenated metal surface evolves upon reacting with hydrogen. Copper is a transition metal that functions as an important industrial catalyst because of its ability to change its oxidation state and to adsorb other substances onto its surface and activate them in the catalytic process. Cu can be easily oxidized even at room temperature and oxygen atoms that chemisorb on the Cu surface can act as active sites in catalytic reactions including water-gas reaction18, methanol synthesis and methanol oxidation19. Consequently, the adsorption process of oxygen on Cu surfaces has been studied as a prototype in dealing with the metal oxide catalysis. Cu(110), the most open surface of the low-index FCC faces, has been studied extensively for understanding oxygen chemisorption induced surface restructuring phenomena20–22. Upon the oxygen exposure, the Cu(110) surface develops two sequential reconstructions, which are the added-row (21)-O with a saturated oxygen coverage of 0.5 monolayer (ML) and the c(62)-O with the saturated oxygen coverage of 2/3 ML12,14,17,23–25. The former is characterized by the formation of Cu-O-Cu rows along the [001] direction in every other [110]-(1 × 1) spacing of the (110) substrate while the latter contains two adjacent [001]oriented Cu-O-Cu rows in every three [110]-(1 × 1) spacing of the substrate14,26–29. Using STM and LEEM, we show the clustering of Cu atoms in the Cu(110)-c(62)-O upon the loss of the surface 3



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oxygen via its reaction with H2 gas. Since the surface reaction does not involve chemical surfactants and the reaction product of H2O molecules desorb from the surface, this de-oxygenation process results in self-assembled Cu clusters that are surfactant-free and may work as an ideal system for understanding the intrinsic properties and functionalities of materials at the nanoscale.

2. Experimental and Computational Section Our experiments involved a two-step process. At first, an atomically clean Cu(110) surface was exposed to O2 gas to develop a well-ordered c(62)-O surface. Subsequently the c(62)-O surface was exposed to the flow of H2 gas at elevated temperature. The Cu(110) single crystal (purchased from Princeton Scientific Corp., purity: 99.9999%) is a top-hat shaped disc (1 mm in thickness and 8 mm in diameter), cut to within 0.1 of the (110) crystallographic orientation and polished to a mirror finish. The Cu(110) surface was cleaned by repeated cycles of Ar+ bombardment (510−5 Torr of Ar+, 1 μA cm−2, 1 kV) at room temperature followed by annealing at 600C for 10 min until no impurities could be detected by several surface sensitive methods including Auger electron spectroscopy (AES), X-ray photoelectron microscopy (XPS), STM and LEEM imaging, and low-energy electron diffraction (LEED). The freshly cleaned Cu(110) surface was exposed to pO2= 110-5 Torr and 350C via introducing oxygen gas (purity = 99.9999%) to the vacuum chamber through a variable-pressure leak valve. Oxygen dosing under this condition results in well-ordered c(62) reconstruction, as shown previously14 and also confirmed by our own STM imaging and LEED. The Cu(110)-c(62)-O surface was then exposed to the flow of H2 gas at the pressures varying from 110-8 to 110-5 Torr at 150C. An ultrahigh vacuum (UHV) variable-temperature scanning tunneling microscope system (Omicron VT-STM XA) was employed to image the surface morphology and atomic structure of the Cu(110) surface after the O2 and H2 exposures. STM tips were made from an electrochemically etched polycrystalline tungsten 4


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wire and flashed using e-beam heating (1kV, 2mA) several times to evaporate adsorbates and native oxide before STM imaging. To ensure the pristine nature of the tungsten tip, the O2 and H2 dosing was performed in a sample preparation chamber (with a base pressure of ~310-10 Torr), followed by sample transfer to the STM chamber (with the base pressure of ~110-11 Torr) for imaging of the surface. All STM images were acquired at room temperature in the constant-current mode with the bias voltage applied to the sample. The H2 dosing was conducted in an added-on manner by exposing the sample to H2 for ~ 30 min each time and then transferring the sample to the STM chamber. XPS experiments were performed in a SPECS XPS system with a base pressure 210-10 Torr. The O1s core levels were excited by Al Kα radiation. Similar as the STM experiments, the Cu(110) crystal was first cleaned with cycles of sputtering and annealing until no O and C spectra could be detected by XPS. Oxygen was then directly introduced to the XPS system through a leak valve to form an oxygenated surface, followed by subsequent exposure to hydrogen gas. XPS measurements were performed interruptedly for different amounts of the gas dosing. To complement the interrupted STM and XPS measurements, uninterrupted, real-time LEEM imaging was employed to monitor the surface evolution during the H2 exposure. All LEEM images were obtained in bright-field, at an electron energy of 13.5 eV, and in real time at the high temperature during the flow of H2 gas in an UHV system with a base pressure below 110-10 Torr. Low-energy electron diffraction (LEED) patterns were obtained at an electron energy of 40 eV. Density functional theory calculations were performed using the Vienna ab-initio simulation package (VASP) with the PW91 generalized gradient approximation and projector augmented wave potentials using a cutoff energy of 380 eV. The Brillouin-zone integration was performed using (2121) K-point meshes based on Monkhorst–Pack grids and with broadening of the Fermi surface according to the Methfessel-Paxton smearing technique with a smearing parameter of 0.2 eV. We calculated the lattice constant of Cu to be 3.64 Å, which is in good agreement with previous calculations26,30–32. As 5



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adopted in the previous work, the surface was modeled with a periodically repeated five-layer slab. The bottom two layers of Cu atoms were fixed at the lattice position and all the other atoms were allowed to fully relax during optimization until all the force components acting the atoms were less than 0.015 eV/Å. A vacuum gap of 12 Å was used to separate successive slabs. The climbing image nudged elastic bands (CI-NEB) method was employed to calculate the reaction barriers, where five intermediate images between the initial and final states were used. The atomic structures are visualized using the VESTA package.

3. Results and Discussion The freshly cleaned Cu(110) surface was first exposed to 1x10-5 Torr of oxygen at 350C for 30 min to form a well-defined c(62)-O reconstructed layer. Fig. 1(a) illustrates a representative STM image, showing the formation of a continuous and uniform adsorbate phase on the Cu(110) surface consisting of atomically flat terraces separated by straight and parallel steps. The apparent height of the steps measured by the STM imaging is ~ 1.3 Å, which is consistent with the height of monoatomic steps on the pristine Cu(110). High-resolution STM images as shown in Fig. 1(b) confirm a c(62)-O reconstructed surface, where the unit cell delineated by the dashed black box matches well with the DFT-optimized c(62) structure in the inset. Hydrogen gas was then introduced to the chamber to react with the Cu(110)-c(62)-O adsorbate phase at 150C. Fig. 1(c) illustrates an STM image of the surface morphology after 18 Langmuir (where 1 L=10-6 Torr·s) of H2 exposure, which shows that the initially straight steps become roughened along with the formation of clusters, mainly on the upper side of the step edges. As marked with dashed lines, domains with parallel atomic chains are visible on the terraces adjacent to the step edges. The atomic chains in these domains are parallel to the [001]-oriented Cu-O-Cu chains in the existing (62) regions. Fig. 1(d) is a zoom-in STM image of the region with atomic chains shown in Fig. 1(c). The spacing 6


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between adjacent chains is measured to be ~ 5 Å. These structure features are consistent with the (21) structuring that is characterized by the [001]-oriented Cu-O-Cu chains in every other [110]-(1 × 1) spacing of the (110) substrate. Therefore, the H2 exposure results in the c(62)(21) phase transformation. The atomic surface densities of O and Cu in the c(62)-O structure are 0.07/Å2 and 0.09/Å2, respectively, while the (21) structure has a smaller atomic surface density (i.e., 0.07/Å2) of Cu and O14,33,34. Correspondingly, the H2 induced c(62)(21) transformation involves the loss of chemisorbed oxygen and ejection of Cu atoms from the parent c(62). In this process, the chemisorbed oxygen is lost via its reaction with adsorbed hydrogen to form H2O molecules that easily desorb from the surface29. As shown in Fig. 1(c), Cu atoms ejected from the c(62)(21) transformation aggregate into clusters preferentially on the upper side of the step edges.

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Fig. 1: (a) Topographic STM image of the Cu(110)-c(62)-O surface formed by exposing clean Cu(110) at 350C to pO2=110-5 Torr for 40 min. (b) Zoomed-in STM image showing the atomic structure of the c(62)-O reconstruction formed from the oxygen exposure, the unit cell is delineated by the black rectangle. Inset is a DFT optimized c(62)-O structure. (c) STM image showing the surface morphology after 18 L of H2 exposure at 150C, (21) domains are outlined by dashed lines. (d) Zoom-in STM image showing the atomic chains of the (21) reconstruction in the regions adjacent to step edges, as 8


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marked by the dashed lines in (c). The unit cell is delineated by the black rectangle. Inset is a DFT optimized (21) structure. Blue and grey balls represent Cu atoms, and red balls represent O atoms in the insets of (b, d). The tunneling conditions for the STM imaging: IT = 0.1 nA, VB = -1 V for (a, c) and IT = 1 nA, VB = -0.3 V for (b, d).

Fig. 2(a) shows a large-scale STM image of the typical morphology of the surface after 18,000 L of H2 exposure at 150C. The surface morphology changed significantly, namely with the dramatic shrinkage in the (62) coverage along with further aggregation of Cu clusters on the upper side of step edges and some Cu clusters sparsely distributed over the terraces. As measured from the STM images shown in Fig. 2, the c(62) domains appear as protrusions that are ~0.65 Å higher than the surrounding (21) regions on the same terrace, consistent with previous work14,33,35. The c(62) coverage (appearing as bright domains in Fig. 2(a)) drops to ~ 40%. Fig. 2(b) is a zoomed-in STM image crossing the stepped region, which shows that both the upper and lower terraces have been transformed into the (21) reconstruction under the large amount of H2 exposure. The c(62)(21) transformation also results in c(62)/(21) heterophase boundaries. Fig. 2(c) is a zoomed-in STM image of such a boundary formed by the c(62) and (21) domains on the same terrace. As can be seen in Fig. 2(c), Cu atoms aggregate preferentially on the (21)/c(62) boundary. The formation of (21) domains at different places of the surface also results in boundaries formed by the growth of (21) domains. Fig. 2(d) is a zoomed-in STM image of such a boundary of two (21) domains on the same terrace, which shows that Cu atoms have the tendency to aggregate into clusters along the boundary. In addition, Cu atoms can also form clusters on the (21) terrace, as seen in Fig. 2(d). Fig. 2(e) illustrates the surface geometry profiles of several Cu clusters as marked in Fig. 2(b-d), which show that the clusters have the height of ~ 5 Å and width of ~ 1.8 nm. It is worth mentioning that the measured surface height of the clusters may not be very accurate because STM is sensitive to both the 9



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geometric and electronic structures of the surface and it is experimentally challenging to separate these on an atomic scale. By assuming a cone shape of the cluster, the number (N) of Cu atoms in the cluster can be estimated as N=4πr2ℎ/3𝑎3, where r and h are respectively the radius (at the cluster base) and surface height of the cluster from the STM measurements, and a the lattice parameter (3.61 Å) of Cu. In this way, it can be estimated that the relatively large clusters shown in Figs. 2(b-d) consist of about 50 Cu atoms per cluster.

Fig. 2: (a) Topographic STM image after the Cu(110)-c(62)-O surface after 18,000 L of H2 exposure at 150C. (b, c, d) Zoom-in STM images of the areas marked by dotted black boxes b, c, d in (a), respectively. (e) Surface profiles along the black, red and blue lines marked in (b, c, d), respectively. The tunneling conditions for all the STM imaging are IT = 0.5 nA, and VB = -2 V. 10


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Fig. 3: (a) Topographic STM image of the Cu(110)-c(62)-O surface after 216,000 L of H2 exposure at 150C. (b) Surface profile along the blue line marked in (a) revealing the height difference between the (21) and c(62) regions. (c) Zoom-in STM view of the area marked by the dotted black box in (a). The tunneling conditions for the STM imaging are IT = 0.2 nA, VB = -2 V.

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Fig. 3(a) shows an STM image of the surface morphology after 216,000 L of H2 exposure at 150C. Cu clusters aggregate mainly on the (21) terrace while the c(62) structure remains relatively intact. Fig. 3(b) displays an STM surface height profile taken along the blue line in Fig. 3(a). The (21) region is ~ 0.72 Å above the c(62) area, indicating that the two areas are separated by a monoatomic step with the (62) area on the lower side of the step edge. As shown in Fig. 3(a), the upper terrace has been transformed into the (21) reconstruction while the lower terrace adjacent to the step edge still maintains the c(62) structure, further confirming that the upper terrace of the step edge is more favorable for hydrogen adsorption that induces the c(62)(21) transformation and the extra Cu atoms released from the surface phase transformation accumulate on the upper terrace of the step edge. The chain-like continuity of the Cu clusters on the (21) terrace suggests that Cu atoms aggregate along the boundaries between (21) domains, consistent with the surface feature shown in Fig. 2(d). Fig. 3(c) is a zoom-in STM view of the area marked by the dotted black rectangle in Fig. 3(A), which shows a small (21) depression with its rim consisted partly of the step edge and partly of the c(62)/(21) boundary on the lower terrace. Cu clusters aggregate along the rim of the (21) depression and the clusters along the upper side of the step edge have larger sizes than the clusters along the rim on the lower terrace.

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Fig. 4: (a) Topographic STM image of the Cu(110)-c(62)-O surface after 360000 L of H2 exposure at 150C. (b) Size distribution histogram of Cu clusters in (a). The tunneling conditions for the STM imaging are IT = 0.1 nA, VB = -1.5 V.

Fig. 4(a) depicts an STM image showing the surface morphology after 360,000 L of H2 exposure at 150C. The entire surface is now covered with a high density of Cu cluster, indicating that the surface has lost its reconstructions with this large amount of hydrogen exposure. Upon the complete loss of the chemisorbed oxygen from the surface, Cu atoms in the parent reconstructed layer aggregate into clusters across the surface. Fig. 4(b) illustrates the size distribution histogram of the clusters in Fig. 4(a), which shows an average size of 1.6 nm, slightly smaller than the Cu clusters formed from the reaction shown in Fig. 2. This difference in the cluster sizes can be attributed to the difference in hydrogen pressure. In Fig. 2, the oxygenated Cu(110) surface was exposed to the H2 gas flow of ~ 110-8 Torr whereas in Fig. 4 the surface was exposed to a higher H2 pressure (~110-5 Torr) in order to speed up the reaction. The higher H2 gas pressure results in a higher nucleation density of Cu clusters and thus a

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smaller zone to capture mobile Cu atoms for cluster growth, thereby yielding a smaller average cluster size.

Fig. 5: Photoemission spectra of the O 1s region for the oxygenated Cu(110) surface formed from the oxygen dosing at pO2=1x10-5 Torr and 350C for 30 min, followed by subsequent H2 exposures at 150C.

To further confirm the Cu clustering, XPS measurements were performed to identify the chemical nature of the surface upon the similar oxygen and hydrogen exposures as the STM experiments. Similar as the STM experiments, a freshly cleaned Cu(110) surface was exposed to pO2=1x10-5 Torr at 350C for 30 min to form the oxygenated surface (e.g., the c(62)-O reconstructed 14


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surface). As shown in Fig. 5, the oxygen exposure results in appreciable intensity in the O1s core level (black line) centered at the binding energy (BE) = 530.3 eV, which corresponds to chemisorbed oxygen32,36,37. Upon subsequent H2 exposures at 150C, the intensity of the O1s peak drops gradually, indicative of the loss of chemisorbed O due to its reaction with adsorbed H to form H2O molecules that desorb from the surface. Particularly, the O 1s peak disappears after 360000 L of H2 exposure, indicating that the surface becomes oxygen-free. This also confirms that the clusters observed from the STM imaging, as shown in Fig. 4a, are Cu clusters instead of any other oxygen-containing adsorbate species. The STM imaging and XPS measurements shown above demonstrate that the hydrogeninduced loss of chemisorbed oxygen results in the clustering of Cu atoms released from the parent oxygen chemisorbed layer. Two main features can be noted from the STM observations. First, surface steps are effective trapping centers for hydrogen adsorption for reaction with chemisorbed oxygen in the upper terrace, thereby inducing the c(62)(21) transformation there, as shown in Fig. 1(c). The preferential adsorption of hydrogen along the step edge is further confirmed using DFT modeling. Based on the STM results as shown in Fig. 1(a), we construct a stepped surface consisting of c(62) reconstructed terraces separated by a [110]-oriented monoatomic step steps, and then examine the adsorption of H2 molecules and atomic H at different surface sites around the step edge. Numbering of the various adsorption sites considered is marked in Fig. 6(a, b). Our DFT calculations indicate that the upper side of the step edge (i.e., site 1) is more favorable than any other sides with the adsorption energies of -1.24 eV for molecular hydrogen and -1.06 eV for atomic hydrogen. Therefore, hydrogen adsorbs preferentially along the upper side of the step edge and reacts with adjacent O atoms to form H2O molecules that desorb from the surface and destabilize Cu and O atoms within the c(62) region, thereby resulting in the c(62)(21) transformation along the

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upper side of the step edge with ejection of extra Cu atoms that aggregate into Cu clusters there, as confirmed with the STM imaging shown in Figs. 1-4. Our STM imaging also shows that the clustering of ejected Cu atoms starts from the upper side of the step edges and then spreads across the entire surface as the reaction progresses toward completion. The observed nucleation of Cu clusters adjacent to step edges clearly points to the effect of step-crossing barriers for mass transport and in modulating the surface concentration of Cu atoms for the clustering of Cu atoms. The step edges for the heterogeneous nucleation of Cu clusters include steps formed by neighboring terraces of the substrate (Figs. 1(c) and 2(b)) or c(62)/(21) boundaries due to the difference in the surface heights of (21) and c(62) domains on the same terrace (Fig. 3(c)). The clustering process requires surface diffusion of Cu atoms across the terrace and step edges. The locations for the clustering of Cu atoms depend on the interplay between the terrace diffusion and step edge crossing. There are additional kinetic barriers that hinder the crossing of Cu atoms via a step to a lower terrace, e.g., Ehrlich−Schwoebel (ES) barrier38,39. The ES barrier presents an extra energy barrier for an adatom crossing over a step edge from one terrace to another. We perform DFT calculations to evaluate the magnitudes of the surface diffusion barrier and step-crossing barrier in our system. As shown in the insets of Fig. 6(c), the two sites marked as purple are stable sites for Cu atoms on two adjacent (21) terraces. The barrier for a Cu atom crossing the step edge from the upper terrace to the lower terrace is ~1.13 eV whereas the barrier for diffusion of Cu atoms along the [001] channel of the (21) terrace is only ~0.75 eV. Therefore, the Cu atom encounters a significantly larger barrier for the step crossing to the lower terrace. Similarly, our DFT calculations show that the surface diffusion of a Cu atom (marked as grey in Fig. 6(d)) on the c(62) to the adjacent (21) domain on the same terrace encounters a barrier of ~ 1.24 eV by crossing the c(62)/(21) boundary, which is significantly higher than the in-channel diffusion of Cu atoms on either the (21) or c(62) terraces26,30,40. Because the step-crossing barrier hinders Cu adatoms crossing descendent steps, Cu adatoms bounce back from 16


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the step edge. This, therefore, results in a higher concentration of Cu atoms on the upper side of step edges, thereby giving rise to the nucleation of Cu clusters in these regions as shown in Figs. 1-3.

Fig. 6: (a) The side view and (b) top view of the atomic configuration of the stepped Cu(110)-c(62)-O surface for DFT calculations of molecular and atomic hydrogen adsorption at the different oxygen sites numbered from 1 to 8 including terrace sites, upper and lower side of a step edge and along the microfacet of the step edge. The dotted black line represents the atomic step, and the solid line corresponds to the [110] direction. The angel between those two lines are about 18 degrees. (c) DFT computed minimum energy path for the diffusion of a Cu atom by crossing a descendent step formed by two (21) terraces. (d) DFT computed minimum energy path for a Cu atom crossing a boundary formed by the (21) and c(62) domains on the same terrace. Red balls: O atoms; blue balls: Cu atoms in bulk; grey balls: Cu atoms in the (21) domain; gold balls: Cu atoms in the c(62) domain; purple balls: stable sites for a Cu adatom before and after its jump over the step edge. 17



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The atomic-scale STM imaging and DFT modeling of the H2-induced clustering of Cu atoms are further confirmed from real-time LEEM observations. Fig. 7 shows in-situ LEEM images extracted from supporting LEEM video 1. Initially, the Cu(110) surface was fully covered by the (62) reconstruction (Fig. 7(a)) formed by the oxygen exposure at pO2~1.410-6 Torr and T=600C. The (62) reconstruction was also confirmed by the LEED pattern as shown in Fig. 7(e). The (62) reconstructed surface was then exposed to pH2=110-8 Torr of H2 gas flow at T=550C (this temperature was chosen so that the reaction rate was not either too slow or too fast, conducive to the in-situ LEEM observations). Upon the H2 exposure, areas with bright contrast became visible and grew into a stripe morphology (Fig. 7(b)), indicating the c(62)(21) transformation, where the dark-contrast regions corresponded to the c(62) phase while the bright-contrast regions were transformed to the (21) phase. The co-existing of (21) and c(62) regions was confirmed by the LEED patterns shown in Fig. 7(f). With continued H2 exposure, the clusters of nanoparticles gradually became visible and populated across the entire surface, as shown in Figs. 7(c, d). Consistent with the STM imaging and DFT modeling illustrated in Figs. 1-4 and 6, the in-situ LEEM imaging showed that atomic steps are the favorable sites for initiating the c(62)(21) transformation as well as the clustering of Cu atoms ejected from the transformation. This can be seen in Figs. 7(c, d), where the slightly darker lines represent step edges between surface terraces and Cu clusters are relatively aligned along the step edges. Figs. 8(a-c) illustrate time-sequence zoom-in LEEM images showing more detail of the clustering of Cu atoms induced by the H2 exposure. As indicated in Fig. 8(a), atomic steps are still visible near the completion of the c(62)(21) transformation, where the small stripe-like, darkcontrast areas are remaining c(62) regions. Meanwhile, Cu clusters are visible along the step edges. It can also be noted in Figs. 8(a-c) that the contrast for surface steps became increasingly weakened with the continued hydrogen exposure. This can be attributed to the enhanced mobility of surface steps 18


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upon the further loss of chemisorbed oxygen, for which surface steps cannot be resolved clearly by the LEEM imaging. The loss in the image contrast from the atomic steps also suggests that the Cu surface became progressively pristine, for which the step edges became more susceptible to detachment and attachment of step atoms. This is also consistent with previous in-situ STM imaging, which showed that atomic steps on clean Cu(110) undergo fast thermal fluctuation even at room temperature while chemisorbed oxygen dramatically improves the stability of surface steps27.

Fig. 7: Time sequence of LEEM images (supporting in-situ LEEM video 1, field-of-view: 10 m) visualizing the hydrogen induced c(62)(21) transformation and clustering of Cu atoms on the Cu(110). (a) The c(62) reconstructed surface formed by oxygen exposure at pO2~1.410-6 Torr and T~ 600C. (b-d) Exposure of the Cu(110)-c(62)-O surface to pH2 =110-8 Torr at T~ 550C results in the c(62)(21) phase transformation along with the clustering of Cu atoms ejected from the surface 19



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phase transformation. (e) LEED pattern from the c(62) reconstructed surface in (a). (f) Composite LEED patterns confirming the co-existence of (21) and c(62) domains at the surface in (b).

Fig. 8: (a-c) Time-lapse zoom-in LEEM images (supporting in-situ LEEM video 2, image size: 0.5  0.6 m) showing the formation and growth of Cu clusters at pH2 ~110-8 Torr and T=550C. (d) Evolution of Density of Cu clusters as a function of hydrogen exposure time. (e) Normalized average clusters area, D/D0, vs. hydrogen exposure time, where D represents the clusters area size measured from the LEEM image at time t and D0 the clusters area size at time t0.

The kinetics of the clustering of Cu atoms was also measured from the in-situ LEEM observations. Fig. 8(d) shows the evolution of the density of Cu clusters area with respect to the hydrogen exposure 20


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time, which resembles a sigmoidal trend. By combining the ex-situ STM imaging and in-situ LEEM imaging shown above, three reaction stages can be approximately distinguished from Fig. 8(d). In stage I, the clustering of Cu atoms takes place predominantly along step edges because hydrogen adsorption is initiated from the step edge regions. Because the density of surface steps does not change over time and the surface steps are still quite stable owing to the presence of a large amount of chemisorbed oxygen on the surface, the number density of Cu clusters increases quite slowly in this reaction stage. Further hydrogen exposure leads to the random nucleation of (21) domains along both the step edges and on the terraces, which results numerous c(62)/(21) and (21)/(21) boundaries. As shown in Figs. 2 and 3, these boundaries serve as the sinks of Cu atoms. Therefore, Cu clusters increases dramatically as a result of the formation of a large number of domain boundaries, which corresponds to reaction stage II shown in Fig. 8(d). Upon the completion of the c(62)(21) transformation, c(62)/(21) boundaries disappear and only (21)/(21) boundaries are available for nucleating Cu clusters. Thus, the increase in the Cu clusters slows down again, which corresponds to reaction stage III in Fig. 8(d). Therefore, the sigmoidal kinetics of the clustering of Cu atoms is controlled by the temporal evolution of the surface sites available for the nucleation of Cu clusters. The in-situ LEEM observations also provide insight into the growth kinetics of Cu clusters by measuring the evolution of the projected area of Cu clusters. As shown in Fig. 8(e), the average clusters area size (projected area) was observed to show a linear growth behavior in the first ~ 1000 s of hydrogen exposure and then reach a saturated size irrespective of continued hydrogen exposure. By comparing with the evolution in the number of Cu clusters shown in Fig. 8(d), the further hydrogen exposure only resulted in the formation of new clusters area but did not give rise to further growth of the existing clusters. This is consistent with the STM imaging in Figs. 2-4, which shows that the Cu clusters in the same images have a similar average size for the different extents of the hydrogen induced deoxygenation at constant temperature. 21



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The relatively uniform cluster size may suggest that there exists an active zone of capture of Cu atoms around each cluster. Cu atoms inside this capture zone may migrate to the cluster, leading to growth of the cluster. Therefore, the surface diffusion of Cu atoms should play an important role in the cluster growth. In an analogy to the oxide island growth controlled by the surface diffusion of oxygen atoms during the initial-stage oxidation of Cu13,40–44, the surface diffusion of Cu atoms to the perimeter of a Cu cluster creates a growth of rate 𝑑𝑁(𝑡) 𝑑𝑡

= 2𝜋𝑟𝐽𝑠 ,

(1)

where N(t) is the number of Cu atoms in a cluster at time t, r is the radius of the circular profile of a cluster, 𝐽𝑠 is the actual flux of Cu atoms incorporated into the cluster. For 2D lateral growth of a diskshaped cluster, then by solving the above differential equation, the increase in the cluster area, A(t), with respect to time follows 𝐴(𝑡)~𝑡2. For 3D growth of a cluster, the cross-sectional area of the cluster is 𝐴(𝑡) = 𝜋Ω𝐽𝑠(𝑡 ― 𝑡0),

(2)

where Ω is the volume occupied by one Cu atom in the cluster. Therefore, the increase of the projected cluster area A has t2 dependence on reaction time for 2D growth and t dependence for 3D growth. As shown in Fig. 8(e), the kinetic data on the evolution of the projected area of the clusters show a linear dependence on reaction time before reaching a saturated cluster size. This agrees well with the 3D growth limited by surface diffusion of Cu atoms (i.e., Eq. (2)). The 3D growth kinetics also corroborates well with the 3D topology measured from STM imaging as shown in Figs. 2-4. Metallic clusters represent a new class of physical and chemical objects that differ from their bulk counterparts and are used in diverse fields including catalysis, optics, and plasmonics. The atomic structure and size of the clusters usually play a dominant role in determining the properties, but the surface of the clusters synthesized by wet chemical methods is often masked with a layer of chemical capping agents or solvents that may significantly change the chemical reactivity. For instance, the 22


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pristine Cu(111) surface does not show reactivity toward H2O adsorption and dehydrogenation45 whereas the oxygenated Cu(111) surface is active toward the dissociation of H2O molecules into OH species by the reaction of chemisorbed oxygen with H2O37. Similarly, the formation of Cu clusters on the pristine Cu(111) was also found active in promoting the dissociation of water molecules46. Therefore, gaining the knowledge of the intrinsic behavior of pristine clusters is a prerequisite for designing nanoclusters with well-defined chemical activity. As shown in the present work, the reaction of hydrogen with the oxygenated Cu(110) surface results in the formation of Cu clusters that are surfactant-free and may serve as an ideal system for elucidating the intrinsic properties and functionalities of Cu clusters. Most of the metals spontaneously develop an oxygenated surface when in contact with an oxygen-containing atmosphere. We expect broader applicability of our results in manipulating the self-assembly of other metal clusters by the reaction of oxygenated metal surfaces with hydrogen gas.

4. Conclusions Using a combination of STM and in-situ LEEM imaging, we have demonstrated that the reaction of the oxygenated Cu(110)-c(62)-O surface with hydrogen gas results in the clustering of Cu atoms upon the loss of the chemisorbed oxygen. It is shown that the clustering of Cu atoms happens initially along the upper side of the step edges formed by neighboring terraces of the substrate and boundaries of c(62) and (21) domains on the same terrace, which subsequently spreads across the entire surface as the reaction progresses. Based on the in-situ LEEM measurements on the size evolution of the clusters, it is shown that the growth of the 3D clusters is controlled by surface diffusion of Cu atoms. Using density-functional theory calculations, we show that the heterogeneous clustering of Cu atoms is induced by step-crossing barriers that hinder Cu atoms crossing descendent steps, thereby promoting the aggregation of Cu atoms in the regions adjacent to the upper side of step edges. 23



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Supporting Information: The Supporting Information is available: In-situ LEEM video 1: In-situ LEEM video showing the hydrogen induced c(62)(21) transformation and clustering of Cu atoms by the exposure of the Cu(110)-c(62)-O surface to pH2 =110-8 Torr at T~ 550C In-situ LEEM video 2: In-situ LEEM video showing the growth of Cu clusters by the exposure of the Cu(110)-c(62)-O surface to pH2 =110-8 Torr at T~ 550C

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0001135. The authors thank N. P. Guisinger for help with the STM experiments. This research used resources of the Center for Functional Nanomaterials and the Scientific Data and Computing Center, a component of the Computational Science Initiative, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Use of the Center for Nanoscale Materials, an Office of Science user facility at Argonne National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. This work used the computational resources from the Extreme Science and Engineering Discovery Environment (XSEDE) through allocation TG-DMR110009, which is supported by National Science Foundation grant number OCI-1053575.

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Hydrogen Induced Clustering of Metal Atoms in Oxygenated Metal

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