Steering Surface Reaction at Specific Sites with Self-Assembly

Steering Surface Reaction at Specific Sites with Self-Assembly Strategy Xiong Zhou,† Fabian Bebensee,‡ Mingmei Yang,† Regine Bebensee,‡ Fang Cheng,† Yang He,† Qian Shen,† Jian Shang,† Zhirong Liu,† Flemming Besenbacher,‡ Trolle R. Linderoth,*,‡ and Kai Wu*,† †

BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Sino-Danish Center for Molecular Nanostructures on Surfaces and Interdisciplinary Nanoscience Center, Aarhus University, DK-8000 Aarhus C, Denmark

‡

S Supporting Information *

ABSTRACT: To discern the catalytic activity of different active sites, a self-assembly strategy is applied to confine the involved species that are “attached” to specific surface sites. The employed probe reaction system is the Ullmann coupling of 4-bromobiphenyl, C6H5C6H4Br, on an atomically flat Ag(111) surface, which is explored by combined scanning tunneling microscopy, synchrotron X-ray photoelectron spectroscopy, and density functional theory calculations. The catalytic cycle involves the detachment of the Br atom from the initial reactant to form an organometallic intermediate, C6H5C6H4AgC6H4C6H5, which subsequently self-assembles with its central Ag atom residing either on 2-fold bridge or 3-fold hollow sites at full coverage. The hollow site turns out to be catalytically more active than the bridge one, allowing us to achieve site-steered reaction control from the intermediate to the final coupling product, p-quaterphenyl, at 390 and 410 K, respectively. KEYWORDS: steering surface reaction, self-assembly strategy, active site, Ullmann coupling, scanning tunneling microscopy

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The substrate-induced confinement effect has already been proved effective to tune surface reactions.22−25 Besides, the confinement effect may also be created by the mobile reacting molecules themselves with the assembly strategy.26,27 Recently, the self-assembly strategy was successfully applied to steer the regioselectivity of the dehydrocyclization reaction and suppress defluorinated coupling of 4,4′-bis(2,6-difluoropyridin-4-yl)1,1′:4′,1″-terphenyl (BDFPTP) molecules.26 It is anticipated that the molecular assembly may also invoke the twodimensional assembly confinement effect to “pin down” the target molecules around active sites. Although the reaction molecules may still be able to slightly diffuse off from the active site, their reaction kinteics can be profoundly different, which provides us a possible method to steer the surface reaction sites and directly compare their activities. The Ullmann coupling reaction is of great significance in industry and nanotechnology and has therefore attracted numerous studies28−33 in the past century. It is proposed that an organometallic intermediate is involved in the catalytic reaction.34−41 In this study, the activities of hollow and bridge sites on the Ag(111) surface for the Ullmann coupling of aryl

recisely discerning catalytic sites and associating them with specific chemical reactions are two immensely difficult tasks, which are, however, prerequisites to a fundamental understanding of heterogeneous catalysis and the ultimate control of catalytic reactions. Many previous studies have shown that low-coordinated surface defect sites such as steps,1−3 kinks,4 corners and edges,5−7 elbows of reconstructed Au(111),8 or even vacancies9 and adatoms10 can function as active sites for a wide variety of catalytic reactions. On ordered surfaces, some special sites, i.e., the “C7” centers on Fe(111) and Fe(211) for ammonia synthesis,11,12 may exhibit special catalytic activity, as suggested by various indirect experimental measurements13−15 and theoretical calculations.16,17 Hollow, bridge, and top sites on atomically flat surfaces also show different adsorption or activation ability.18−21 Direct observation of the catalytic difference between these sites on atomically flat surfaces remains a great challenge. This challenge is further compounded by the fact that most previously studied catalytic reactions involve small molecules such as NH3, CO, and O2, which are extremely mobile and therefore cannot be directly detected by scanning tunneling microscopy (STM) under reaction conditions. In this work, we instead choose a target molecule that can be “fixed” at reactive sites through an assembly confinement effect so that submolecular resolution images can be obtained with STM before and after the reaction. © 2017 American Chemical Society

Received: July 12, 2017 Accepted: August 15, 2017 Published: August 15, 2017 9397

DOI: 10.1021/acsnano.7b04900 ACS Nano 2017, 11, 9397−9404

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Figure 1. Sequential evolution of the Ullmann coupling of BBP molecules on Ag(111) via thermal activation. V = 0.2 V, I = 0.1 nA. (a) Catalytic reaction process. (b) Image of the intact BBP molecules adsorbed on Ag(111) at 120 K. (c) Assembly structure of Ag-COIs formed by RT annealing. The variation of the brightness of the central parts for the intermediates shows the coexistence of two distinct surface species, H-type (hollow sites, marked by black arrows) and B-type (bridge sites, marked by red arrows) intermediates. (d) Some H-type AgCOIs (black elliptical circles) converted into the final p-quaterphenyl (QP) product at 390 K. (e) All H-type Ag-COIs converted into the QP product at 400 K, and all left B-type Ag-COIs remained unreacted. (f) Most B-type Ag-COIs, except for those highlighted by red elliptical circles, converted into QP at 410 K. (g) All Ag-COIs converted into the product QP at 420 K. Black arrows indicate the cracks generated after the completion of the coupling reaction on Ag(111). Inset: STM image of an enlarged area. Black dash lines indicate the molecular axis orientations of the corresponding species.

halides, i.e., 4-bromobiphenyl (C6H5C6H4Br, denoted as BBP, Figure 1a), have been extensively explored. Combined STM and synchrotron X-ray photoelectron spectroscopy (XPS) experiements, together with density functional theory (DFT) calculations, show that the aforementioned catalytic sites on atomically flat Ag(111) can be unequivocally identified. The difference in the activities of these two sites can be further exploited to steer the thermally triggered surface reactions in a stepwise manner.

RESULTS AND DISCUSSION When deposited on Ag(111) at low temperatures (STM: 120 K; XPS: 110 K, see Experimental Methods for more details on the molecular deposition), the BBP molecules form various molecular clusters, as shown by the STM image in Figure 1b (all STM images hereafter adopt the same color scale bar in Figure 1).42,43 The clusters involve two distinct arrangements of the BBP molecules: head-to-head clovers and head-to-tail triangles, in which the molecules are held together by halogen and weak hydrogen bonds.44−46 Corresponding XPS measurements (Figure 2a) show peaks originating from C 1s and Br 3d. The XPS peak in the C 1s region can be fitted with a majority contribution at a binding energy (BE) of 284.5 eV, attributed to a benzyl ring carbon not directly bonded to Br, and a minority contribution at a higher BE of 285.5 eV, ascribed to C−Br. These two contributions have an 11:1 intensity ratio, in good agreement with the stoichiometry of BBP. The Br 3d BE features (3d5/2 = 71.7 eV and 3d3/2 = 70.6 eV) indicate no abstraction of Br from the molecules. The BBP molecules are thus molecularly adsorbed at the surface.

Figure 2. XPS data of the corresponding Br 3d (left panel) and C 1s (right panel) features of the BBP reactant molecularly adsorbed on a Ag(111) single crystal at 110 K at full coverage (a). The Ag-COI was formed at 323 K (b), and the final QP product was formed at 483 K (c).

When annealed at mild temperatures (STM: 300 K for extended time; XPS: 323 K for 10 min), a new assembly 9398


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evidence to confirm the existence of the organometallic intermediate. The structural unit cell for the Ag-COIs (Figure S1b and c) contains two Ag-COI complexes and four Br atoms, in agreement with the stoichiometry of the deposited BBP molecules. The unit cell parameters are a = 1.85 ± 0.05 nm, b = 1.76 ± 0.05 nm, and θ = 85 ± 4° (Figure 6a), corresponding to a = |7c1 + 6c2| = 1.89 nm, b = |3c1 + 4c2| = 1.76 nm, and θ = 87.1° (Figure 6b), where c1 and c2 are unit vectors of the unit cell for the Ag(111) surface lattice. A close look at the STM images in Figure 1c reveals that the Ag-COIs systematically exhibit two different brightness levels of the central protrusion. Interestingly, these two species organize into arrays according to the brightness of the central protrusion such that we can distinguish B arrays (red arrows) and H arrays (black arrows) in Figure 1c where the B species has a larger apparent height. To precisely determine the adsorption sites with an atomic resolution for the Ag-COIs, the coverage of the reactant BBP is controlled to be about 0.8 monolayer so that both the surface intermediate island and its nearby Ag substrate could be simultaneously imaged and directly compared on the same terrace. We first spot an area where a densely packed AgCOI island and nearby sparse Ag-COIs coexist (Figure 4a). By controlling the scanning conditions, we are able to clearly observe either the differently oriented Ag-COIs in the molecular island (Figure 4b) and at the periphery of the island or the periodically arranged Ag atoms in the Ag(111) substrate (Figure 4c) in the same area. A simple superposition of both images (Figures 4b and c) gives Figure 4d. By extending the surface Ag lattice across the Ag-COI island (white grid in Figure 4d), it is immediately noticed that the central Ag atoms in the H species are positioned at 3-fold hollow sites (formed by the three underlying Ag atoms in triangular arrangement, as indicated by blue dots), and the B species, at 2-fold bridge sites (formed by two nearest-neighbor Ag atoms), respectively. Line profiles over these two types of species (Figure 5a) quantify the apparent height difference to be 0.05 ± 0.02 Å. A proposed model is given later on. This explanation for the apparent height difference finds further convincing evidence from STM manipulation experiments of the Ag-COI, as depicted in Figure 5b. The protrusion of the intermediate (the bottom one) in the image lights up immediately after its central Ag atom is repositioned from a hollow to a bridge site and the manipulated molecule orients against the unmanipulated one (the top one) at the angle 46°, exactly identical to that shown in Figure 1c. To examine the adsorption energy of the Ag-COI on the Ag(111) surface, DFT calculations are performed with the central Ag atom located on top, bridge, and hollow sites (Figure 5c). The top site possesses the highest adsorption energy (0.26 eV higher with respect to the hollow site). These results support the assignment of the two adsorption states to the hollow and bridge sites.31 The calculated heights of the central Ag atoms are 2.496 Å (bridge) and 2.443 Å (hollow), respectively. This is in agreement with the detected difference in the apparent height, 0.05 Å. Moreover, at the hollow site the adsorption angle between the Ag-COI and underlying Ag lattice is 4°, while at bridge site it is 10°, in agreement with the DFT calculations. The treatment of the sample at 390 K for 5 min caused some of the H-type intermediates to be transformed into oval-shaped molecules (∼1.8 nm in length) without a central protrusion (highlighted by black ellipses in Figure 1d, ∼40% H-type intermediates being transformed). We ascribe this change to

structure evolves (Figure 1c) where each entity is in the shape of a three-kernel-peanut with a length of ∼2.1 nm, about twice as long as the intact BBP molecule (∼1.0 nm), and a distinct bright protrusion appears at the center of the entity. The corresponding Br 3d XPS measurement shows a new dominating doublet peak at lower BE (3d5/2 = 69.0 eV and 3d3/2 = 68.0 eV), characteristic of AgBr,47 indicating that Br is detached from the BBP molecule and bound to the Ag substrate (Figure 2b). Under proper scanning conditions, the detached Br atoms could be clearly imaged between the peanut-shaped entities (see Figure S1b and c). In the C 1s XPS, the main peak significantly shifts toward lower BE, as compared with the situation prior to annealing, and displays a new shoulder at its low-BE side. Qualitatively, these changes in BE indicate that biphenyl moieties, C6H5C6H4−, generated by the Br detachment instead couple to Ag atoms, which are considerably less electronegative than Br, and hence can donate electrons to cause a downshift in BE. The increase of the work function for the sample may also cause the downshift of the BE for C 1s in this reaction step;48 however, the BE variation for C 1s is likely determined by the electronegativitiy of its connecting species such as Br or Ag in our experiments because the BE shifts upward during the formation of the p-quaterphenyl (QP) product. The peanut-shaped entities are therefore attributed to Ag-coordinated organometallic intermediates (denoted as AgCOI), C6H5C6H4AgC6H4C6H5. This assignment is supported by the C 1s XPS peak, which can be fitted with components at a BE of 284.0 eV (ascribed to C in the phenyl rings not attached to heteroatoms) and a small feature at 283.0 eV (ascribed to CAg). A similar organometallic intermediate has been previously proposed for the Ullmann coupling reaction of 4,4″-dibromo-pterphenyl, BrC6H5C6H4C6H5Br.35 To prove that the bright central protrusion is indeed a Ag atom, we targeted one intermediate by the STM tip and applied a sudden voltage pulse. As shown in Figure 3, this manipulation resulted in

Figure 3. STM images (0.5 V, 0.1 nA) of two intact Ag-COIs (left) and one Ag-COI broken into three pieces (right) after a sudden voltage pulse (3 V, 1 s) on the central Ag atom. The detached Br atom near the top-left Ag-COI is clearly visible. The knockoff Ag atom (noted as K-Ag) is the same as the one picked from the Ag(111) substrate and dropped onto the surface again by the STM tip (noted as T-Ag). The two broken C6H5C6H4− groups stay near the K-Ag atom.

formation of three fragments on the surface (three items in the right part), one Ag atom and two identical biphenyls in close proximity. The knockoff Ag atom (top-right Ag atom, marked K-Ag, in Figure 3) appeared identical to the one dropped by gentle contact of a Ag tip with the Ag substrate (the central Ag atom, marked T-Ag). This provides direct experimental 9399


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Figure 4. Determination of the adsorption sites of the intermediates on Ag(111). (a) STM image of an area showing the coexistence of a densely packed Ag-COI island and sparse Ag-COIs nearby (0.2 V, 0.1 nA). (b) Close-up STM image of the white square area in (a) showing two types of differently oriented Ag-COIs (0.1 V, 0.4 nA). (c) STM image of the same area as in (b) clearly showing the Ag(111) lattice near the Ag-COI island (−0.2 V, 3 nA). (d) Superposition of the bottom Ag-COI island in (b) and the top Ag lattice in (c) separated by the light blue dashed line. The white grid superposed indicates the substrate lattice deduced from the Ag atom image in the upper part. Two distinct adsorption sites can now be clearly distinguished, i.e., B-type Ag-COI located at the bridge sites (with a bright central protrusion) and H-type Ag-COI located at the hollow sites (with a dim central protrusion). Black solid and dashed lines indicate the underlying Ag lattice and molecule axis orientations, respectively. The hollow and bridge sites are highlighted by blue dots representing the Ag atoms.

Figure 5. (a) Line profiles of the Ag-COIs marked along the red and black lines in Figure 4b for the bridge and hollow sites, respectively. (b) STM tip manipulation of a Ag-COI from the hollow site to the bridge site, demonstrating the site-dependence of the central Ag protrusion (0.5 V, 0.1 nA). The superimposed white grid represents the underlying Ag(111) surface lattice according to real experimental measurements. The hollow and bridge sites are highlighted by blue dots representing the Ag atoms. (c) Adsorption energy of the Ag-COIs at different sites on the Ag(111) surface as a function of the angle between the long molecular axis of the intermediate and the underlying Ag lattice along either of the three equivalent surface directions. (d) Optimized perspective and top-view models of the Ag-COIs adsorbed at the hollow and bridge sites on Ag(111).

C6H5C6H4C6H4C6H5 (abbreviated as QP). The on-surface generated QP species form an angle of 50°, an increase of about

elimination of the central Ag atom in the Ag-COI on the hollow sites to form the final p-quaterphenyl product, 9400


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Figure 6. Evolution of the surface species on Ag(111) and their correspondingly proposed models during the reaction process. (a) STM image of two types of differently oriented Ag-COIs (0.5 V, 0.1 nA). Unit cell (yellow parallelogram) parameters: a = 1.85 ± 0.05 nm, b = 1.76 ± 0.05 nm, and θ = 85 ± 4°. (b) Corresponding unit cell model for the structure in (a) with parameters a = |7c1 + 6c2| = 1.89 nm, b = |3c1 + 4c2| = 1.76 nm, and θ = 87.1°. Ag atoms forming the hollow and bridge sties to anchor the central Ag atoms in the Ag-COIs are marked in yellow to enhance visibility. (c) All H-type Ag-COIs converted into QP, while all B-type Ag-COIs remain intact on Ag(111). Unit cell parameters: a = 1.83 ± 0.05 nm, b = 1.74 ± 0.05 nm, and θ = 83 ± 4°. (d) Proposed model for the unit cell in (c). (e) All Ag-COIs converted into the final product, QP. Since the QP molecule is shorter than Ag-COI, the unit cell parameters of the QP structure changed into a = 2.00 ± 0.05 nm, b = 1.55 ± 0.05 nm, and θ = 90 ± 2°. (f) Proposed model for (e) with unit cell parameters a = |7c1| = 2.02 nm, b = |3c1 + 6c2| = 1.50 nm, and θ = 90°, in good agreement with the observed ones shown in (e). Black solid and dashed lines indicate the underlying Ag lattice and molecule axis orientations, respectively.

up to 500 K leads to the QP molecules beginning to desorb from the surface. The experimental observations described above strongly suggest that the conversion from Ag-COI to QP is site-steered with a reaction rate that depends on the adsorption site for the central Ag atom in the Ag-COI. If the molecules became rather mobile at the reaction temperature of 390 K such that the mixed structure in Figure 1e is formed by reassembly upon cooling, one would expect the Ag-COIs to occupy the most preferred adsorption sites, the hollow ones, according to the calculations. In our experiment, however, the Ag-COIs mainly sit at the bridge sites (red arrows). The main reason lies in that the molecule mobility is largely suppressed by the surface spatial “confinement” of the molecules in the intermediate assembly, and the in-plane rotation of the species is restricted along a specific direction.27 To verify that the molecules are indeed “attached” around the assumed reaction sites, STM images of the Ag-COIs are recorded at room temperature, i.e., the reaction temperature for the formation of the Ag-COIs (Figure S3). Due to thermally enhanced drift and molecular vibrations at the surface, molecules in the image becomes fuzzy, compared with those in Figure 1c imaged at low temperatures. However, the intermediate packing patterns remain the same at both room and low temperatures. In order to achieve submolecular resolution, our STM imaging is acquired at low temperatures after the sample is swiftly cooled from high temperatures where the Ullmann coupling reaction takes place. If the Ag-COIs were not turned into the coupling products, QP, on specific sites, then a randomly mixed structure rather than the alternately packed

4°, against the intact B-type intermediate in neighboring rows. Further annealing to 400 K for 5 min leads to virtually all Htype Ag-COIs on the hollow sites transforming into the final QP products (Figure 1e), resulting in alternating rows, as indicated by black and red arrows (∼2% B-type intermediates transformed). The coexistence of the Ag-COIs on bridge sites and the generated QP products on the hollow sites at the atomically flat Ag(111) surface has never been experimentally observed before. A temperature elevation up to 410 K for 5 min causes the majority of the B-type Ag-COIs on the bridge sites to transform into QP molecules, as shown in Figure 1f, where some unreacted Ag-COIs are marked by the red ellipses (∼70% Btype intermediates transformed). The produced QP molecules form an angle of 60° in neighboring rows. Final heating to 420 K for 5 min results in complete conversion of all Ag-COIs into the QP products. In the corresponding XPS measurements (Figure 2c), the C 1s peak remarkably narrows down and loses its shoulder at low BE, indicating a uniform chemical state for the C atoms. The peak position (BE 284.1 eV) is in between the features for BBP and Ag-COI in which the phenyl groups are connected to electronegative (Br) and electropositive (Ag) species, respectively. As a control experiment, commercially available QP molecules are deposited onto Ag(111) for comparison (Figure S2d), resulting in assembly structures with the same STM appearance as for the on-surface-generated QP molecules, except for the cracks in the monolayer of the latter (which is discussed later on). We therefore conclude that the Ullmann coupling reaction has completed and attribute the oval features observed in Figure 1d−g to the final QP molecules. Annealing 9401


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CONCLUSIONS In summary, the Ullmann coupling reaction of the BBP molecules on Ag(111) undergoes a stepwise catalytic procedure as the reaction temperature is elevated. Specifically, the molecularly adsorbed BBP reactant detaches the Br atom to form the Ag-COI, whose central Ag atom resides either on the bridge or hollow site at full coverage, as steered by the selfassembly strategy. The hollow site is catalytically more active than the bridge one, enabling us to achieve a site-steered reaction from the Ag-COI to the final coupling product QP at 390 and 410 K, respectively. This study demonstrates that the self-assembly strategy can be well employed to confine reaction species around the active sites even on an atomically flat surface, allowing to steer and observe surface reactions on specific sites at the submolecular level and advancing our understanding of surface chemistry and catalysis.

one (Figure 1e) of the Ag-COI and QP species should have been experimentally observed by STM because a sudden temperature quench of the sample should freeze most surface species in position. This also indicates that the QP products are in situ generated by the Ag-COIs sitting on the hollow sites. In addtion, large-scale STM images of the QP herringbonelike structure due to on-surface reaction depict irregular cracks, as marked by the black arrows in Figure 1g (large-scale images of Figure 1c and e are given in Figure S2). These cracks are not present in the control experiment using the commercial QP molecules (Figure S2). The detailed structural information provided in Figure 6a, c, and e show that the superstructure unit cells for the Ag-COI and QP structures are commensurate (see the corresponding models in Figure 6b, d, and f). The unit cell for the final QP structure is slightly smaller than that for the Ag-COI. The changes of the angle between adjacent molecules support these models. The cracks in the final QP monolayer are thus formed by release of the yield stress, which is caused by the shrinkage of the QP molecules after completion of the catalytic reaction on Ag(111). If the product molecules were free to diffuse on the surface, these structural defects would have been annealed out. The observed cracks therefore provide further evidence that the QP product is indeed generated on site. Finally, due to the different lattice parameters of the Cu(111) substrate, although the intermediate packing mode on Cu(111) (Figure 7) looks similar to that on Ag(111) (Figure 1c),

EXPERIMENTAL METHODS STM Experiments. The STM images and STS data were acquired on a Unisoku ultra-high-vacuum (UHV) low-temperature STM. Single-crystal substrates were cleaned by cycles of Ar+ sputtering and then annealed at about 500 °C. The flatness and cleanness of the substrates were verified by STM imaging. Most STM experiments were performed by using a W tip, which was prepared by electrochemical corrosion in 2 mol/L NaOH solution, except for the experiment in Figure 3b, where a Ag tip was employed and fabricated by electrochemical corrosion in mixed solution with a volume ratio of 25−28% aqueous ammonia/pure ethyl alcohol/3% hydrogen peroxide solution = 1:1:1. BBP was bought from Alfa Aesar (≥98% purity) and dosed onto the substrate from a self-made Ta boat at room temperature without further purification. In order to avoid undesired dosing, the Ta boat was kept in an individually pumped compartment separated by a gate valve from the main chamber. The coverage was controlled by the dosing time. XPS Measurements. The XPS experiments were performed at the bending magnet beamline SX-700 at the ASTRID storage ring. The UHV chamber with a base pressure of about 2 × 10−10 mbar was equipped with a ion-sputtering gun and sample heating facilities to clean the Ag(111) substrate. Clean samples were obtained after repeated cycles of sputtering with Ar+ ions at a kinetic energy of 1.5 keV for 20 min (sample current ≈ 8 μA) and annealing to 823 K for 10 min. The cleaning cycles were repeated until no carbon and bromine contaminations could be detected in XPS. BBP was bought from ACROS (≥99% purity) and dosed onto the Ag(111) substrate from a quartz crucible at room temperature, which was quite similar to the procedure used in the STM chamber. All spectra were collected at normal emission with the X-rays impinging onto the sample at an angle of 45° with respect to the plane of the sample. Each Br 3d spectrum was obtained by adding up 10 sweeps of the electron energy range with a step size of 0.1 eV and an acquisition time of 1 s per step. In order to obtain a high signal-to-noise ratio to capture the slight differences in the C 1s spectra, 120 sweeps were added up per spectrum and the step size was reduced to 0.05 eV/step. The XPS data for molecular adsorption was collected at 110 K, while other XPS data were collected at 300 K, which is either equal to or lower than the annealing temperature so that the molecular chemical states would be retained. Curve fitting was carried out by using the software package OriginPro 8. All curves were fitted with Voigt profiles, since the peak shapes are typically determined by a Lorentzian contribution due to the limited lifetime of the core hole state and a Gaussian broadening, mostly due to the line shape of the exciting X-rays from an X-ray monochromator and electron scattering and detection in the spectrometer.49 Gaussian contributions may also be related to thermal excitation processes. Chemical, structural, and electronic (by dopants) inhomogeneities in the surroundings of the emitting atoms often contribute to the Gaussian broadening.

Figure 7. Large-area STM image of randomly appearing QP (marked by light blue elliptical circles) converted from the C6H5C6H4CuC6H4C6H5 intermediates (peanut-shaped features) on Cu(111) at 450 K (V = 0.2 V, I = 0.1 nA). This is completely different from the cases shown in Figure 1d and e on Ag(111), where the QP products are initially generated from the Ag-COIs sitting on the bridge sites.

random occurrence of the QP formation at the surface is routinely observed. This means that the steered surface reactions at active sites on Ag(111) are controlled in synergy by the sizes of the BBP, Ag-COIs, and QP species involved, the underlyging substrate lattice, and the self-assembly strategy as well. 9402


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ACS Nano DFT Calculations. DFT calculations were carried out using the Vienna ab initio simulation package (VASP)50 with projector augmented wave (PAW) potentials.51 A kinetic energy cutoff of 400 eV was found to cover the total energy of our systems. The exchange− correlation functional was treated with the generalized gradient approximation in the Perdew−Burke−Ernzerhof form.52 van der Waals forces between the biphenyl groups and the surface were not included due to limited computational resources. In the computations, a three-layered slab model with a 3 × 9 supercell was employed to describe the Ag(111) surface. During the relaxation, the two bottom layers of Ag(111) were fixed at their bulk values. Adjacent slabs were separated by about 10 Å to avoid the interaction between them. A Monkhorst−Pack k-point mesh of 1 × 5 × 1 was used for k-point sampling of the Brillouin zone. STM images were simulated within the Tersoff−Hamann approximation.53

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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/acsnano.7b04900. STM imaging of surface species, large-area STM imaging of surface species, and room-temperature imaging of AgCOIs (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (K. Wu). *E-mail: [email protected] (T. R. Linderoth). ORCID

Trolle R. Linderoth: 0000-0001-9008-7581 Kai Wu: 0000-0002-5016-0251 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was jointly supported by NSFC (91527303, 21333001) and MOST (2017YFA0204702), China. We acknowledge support from the Danish Council for Independent Research Natural Sciences and the Danish National Research Foundation. F.B. thanks the Alexander von Humboldt Foundation for a Feodor Lynen Fellowship. We thank Zheshen Li for technical support in the synchrotron XPS measurements at the SX-700 beamline at ASTRID. REFERENCES (1) Zambelli, T.; Wintterlin, J.; Trost, J.; Ertl, G. Identification of the “Active Sites” of a Surface-catalyzed Reaction. Science 1996, 273, 1688−1690. (2) Dahl, S.; Logadottir, A.; Egeberg, R. C.; Larsen, J. H.; Chorkendorff, I.; Törnqvist, E.; Nørskov, J. K. Role of Steps in N2 Activation on Ru(0001). Phys. Rev. Lett. 1999, 83, 1814. (3) Vang, R. T.; Honkala, K.; Dahl, S.; Vestergaard, E. K.; Schnadt, J.; Laegsgaard, E.; Clausen, B. S.; Norskov, J. K.; Besenbacher, F. Controlling the Catalytic Bond-breaking Selectivity of Ni Surfaces by Step Blocking. Nat. Mater. 2005, 4, 160−162. (4) Saywell, A.; Schwarz, J.; Hecht, S.; Grill, L. Polymerization on Stepped Surfaces: Alignment of Polymers and Identification of Catalytic Sites. Angew. Chem., Int. Ed. 2012, 51, 5096−5100. (5) Fu, Q.; Li, W.-X.; Yao, Y.; Liu, H.; Su, H.-Y.; Ma, D.; Gu, X.-K.; Chen, L.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. Interface-Confined Ferrous Centers for Catalytic Oxidation. Science 2010, 328, 1141− 1144. (6) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.; Somorjai, G. A. Platinum Nanoparticle Shape Effects on Benzene Hydrogenation Selectivity. Nano Lett. 2007, 7, 3097. 9403


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Steering Surface Reaction at Specific Sites with Self-Assembly

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