Comparison Study of Structural Properties and CO Adsorption on the

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Comparison Study of Structural Properties and CO Adsorption on the Cu/Au(111) and Au/Cu(111) Thin Films Wenyuan Wang, Hexia Shi, Li Wang, Zhe Li, Hong Shi, Kai Wu, and Xiang Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04783 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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The Journal of Physical Chemistry

Comparison Study of Structural Properties and CO Adsorption on the Cu/Au(111) and Au/Cu(111) Thin Films Wenyuan Wang, † Hexia Shi, † Li Wang, † Zhe Li, † Hong Shi, † Kai Wu‡,* and Xiang Shao†, * †

Department of Chemical Physics, CAS Key Laboratory of Urban Pollutant Conversion, Synergetic

Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡

BJNL, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China * To whom correspondence should be addressed: [email protected]; [email protected]

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ABSTRACT: Supported metal films are often found displaying extraordinary properties compared to their bulk counterparts. In this work, we investigate the atomic and electronic structures of both the Au/Cu(111) and Cu/Au(111) thin films with low temperature scanning tunneling microscopy (STM) and synchrotron radiation photoelectron spectroscopy (SRPES). The high-resolution STM images revealed that the Au films evaporated on Cu(111) at room temperature (RT) grow with compressed lattices which gradually evolve and restore to that of the bulk gold since the fourth layer. Both STM and SRPES evidenced that there are considerable Cu atoms incorporated into each layer of the Au films, whose concentrations decrease stepwisely along with the film thickening. As a reversed system, the growth of Cu films on Au(111) starts with agglomerating at the subsurface and adopting a (1 × 1) lattice within submonolayer coverage. The lattice quickly shrinks to that of bulk Cu(111) since the second layer yet the electronic property restores slower until the third layer. In each Cu film there were also intermixed Au atoms coming from the substrate, and their concentrations also decrease along with the film thickening. On both Au/Cu(111) and Cu/Au(111) submonolayer films, CO adsorptions were investigated and were found significantly higher than bare Au(111) but still weaker than Cu(111) surface. The adsorbed CO molecules were apparently connected to the incorporated Cu atoms in the surface layers yet the enhanced CO bindings were closely related to the electronic properties of the films. These findings are believed to shed new light on the atomic details of the Cu-Au bimetallic systems, thus deepen the understanding of the specific catalytic activities of the Cu-Au alloys.

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INTRODUCTION The tunable activity of a transition metal is one of the long goals of heterogeneous catalysis research. For the metal nanoparticles which are widely applied in many circumstances, both volume/dimension and composition factors are found playing crucial roles in determining their catalytic activities. The former can be correlated with the modified electronic properties induced by quantum size effect, whereas the latter is frequently attributed to a combination of steric ensemble effect and electronic effect1-3. Despite the remaining challenge in characterizing the atomistic processes on nanoparticle surfaces, valuable understanding was already achieved on well-defined model systems such as single crystals and supported ultrathin films4-7. In particular, supported metal films have provided an interesting system combining both electronic attenuation and compositional manipulation8-10. The evaporated atomic layers may experience dramatically different coordinative environment, leading to rigid shifts of the electronic bands. In addition, intermixing of atoms in the surface vicinity would create diverse surface sites with variable reactivities in binding various adsorbates5-7, 11. Such bimetallic system actually constitutes a proper model for the real catalyst surfaces under realistic conditions. The alloying of Au with different metals has been attracting more and more attention since it can reduce the consumption of the precious Au resource and in the meantime harvest enhanced catalytic activities in variable reactions12-14. Cu is one of the best selections owing to its low price and high tendency of alloying with Au2, 13. As a matter of fact, Au-Cu alloyed nanoparticles have been demonstrated with superior activities in both reductive transformation of CO2 and a variety 3



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of oxidation reactions1-2, 15. Therefore, the Cu/Au(111) and Au/Cu(111) bimetallic films have attracted people’s attention and received considerable exploration. A recent work of Rodriguez and co-workers16 combined ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS) and density-functional-theory (DFT) calculations, and demonstrated that at room temperature (RT) and submonolayer coverage the evaporated Cu tends to aggregate at the subsurface region of the Au(111) substrate. Grillo et al.17 monitored the deposition of submonolayer Cu films on Au(111) with scanning tunneling microscopy (STM) and proposed a double-layer growth mode. Frieble and co-workers18 investigated the structure and redox activity of the Cu/Au(111) films under electrolyte conditions. On the other hand, the Au/Cu(111) films have been relatively less studied. Only a few works have been reported which mostly focused on the surface state changes of the Cu(111) substrate after the addition of Au adlayers19. As far as we know, the interface structures of both Au/Cu(111) and Cu/Au(111) bimetallic systems are not yet completely clear. Particularly, investigations of the local structural/electronic properties of these films and their influences on the adsorbed species are still in short supply. In this work, we have studied both Au/Cu(111) and Cu/Au(111) thin films with low temperature STM in combination with synchrotron radiation photoelectron spectroscopy (SRPES). The high resolution STM images clearly revealed the atomic structures of both films while the scanning tunneling spectroscopy (STS) and SRPES uncovered the evolution of their electronic properties against the film thickness change. In the case of Cu/Au(111) film, at submonolayer coverage and RT the Cu atoms are found aggregating at the subsurface region and adopting a pseudomorphic (1×1) structure of the Au(111) substrate. But on thicker Cu layers that 4


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are formed at higher coverage, the lattice quickly restores to that of Cu(111). In contrast, Au evaporated on Cu(111) forms true adlayer since the low coverage but adopts a compressed lattice. With the coverage increasing, the lattice in thicker Au films expands and restores back to the bulk value of Au since fourth layer. Therefore, at submonolayer coverage regime, both Cu/Au(111) and Au/Cu(111) islands are actually terminated with a gold layer but with obviously deviated structures relative to Au(111). CO adsorption experiments demonstrated that these films present enhanced CO binding compared to the Au(111) surface, which can be attributed to the incorporated Cu atoms as well as the up-shift of the d-levels in the surfaces of the bimetallic systems.

EXPERIMENTAL METHODS The experiments were conducted on a low-temperature STM (LT-STM, Createc Co.) which is housed in an ultrahigh vacuum (UHV) system with base pressure as low as 1 × 10-10 mbar. The Au(111) and Cu(111) surfaces were prepared by repeated cycles of Ar+ sputtering and annealing. Copper and gold depositions were performed on home-made evaporators which are constructed by winding either a high-purity copper or gold wire (Goodfellow, 99.99%) around a tungsten filament, respectively. During deposition the substrates were always held at room temperature (RT). The evaporation speed was controlled by varying the filament current while the film coverage was determined directly from STM measurements. CO (Air Product, 99.999%) was introduced by background feeding through a variable leak valve while keeping the partial pressure at ~5.0 × 10-8 mbar. Varied sample temperatures were controlled during CO adsorption 5



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before being finally transferred into the STM stage for imaging. All STM images were collected at liquid nitrogen (LN2) temperature with electrochemically etched tungsten tips. The STS measurements were performed with an internal lock-in amplifier set with frequency of 413 Hz and amplitude of 200 mV. The SRPES measurements of the Au/Cu(111) films were performed at the “Catalysis and Surface Science End-station” at the BL11U beamline in the National Synchrotron Radiation Laboratory (Hefei). The beamline offers soft irradiation with energies of 20 to 600 eV and a resolution (E/∆E) better than 104 at 29 eV. The photoelectrons are detected by a hemispherical analyzer (VG 4000) positioned around 5 cm away from the sample. Noticeably, the Au 4f signals were collected along the surface normal in order for increasing the signal intensity whereas the Cu 3p were collected at a grazing angle of 70º for better surface sensitivity. The SRPES data were analyzed with the XPSPEAK program. A Shirley background was applied for the Au 4f spectra while linear background for Cu 3p. For all peak fittings a mixed Lorentzian–Gaussian function was used.

RESULTS AND DISCUSSION

1.

Atomic Structure and Electronic Properties of the Au/Cu(111) Films.

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Figure 1. STM images of the sub-monolayer Au films grown on Cu(111): (a) 0.1 ML, (b) 0.7 ML. (c) Enlarged image with increased contrast showing the superstructure of the 1L-Au film. (d) Zoomed-in and (e) atomically resolved STM images, as well as (f) the proposed model for the specific triangular feature. The inset in (c) shows the FFT pattern of the superstructure. Black triangles mark the repeating triangular features. White circles in (e) highlight the incorporated Cu atoms in the Au lattice. Tunneling conditions: (a) I = 670 pA, U = − 0.25 V; (b) I = 120 pA, U = 1 V; (c, d) I = 130 pA, U = − 1 V；(e) I = 550 pA, U = - 12 mV.

Figure 1 shows STM topographies of the submonolayer Au film deposited on Cu(111) at RT. Here the uncovered Cu(111) area is termed as ex-Cu while the first layer Au is termed as 1L-Au. Thicker Au adlayers are defined in the same way. As can be seen in Figure 1a, at very low coverage the evaporated Au grows into irregular dendritic structures which all end at the downhill step edges of the Cu(111) substrate, indicating a diffusion controlled growth mode with 7



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nucleation at Cu steps20-22. With more Au deposited, the fractal branches merge into wider islands while keeping monolayer thickness until a coverage around 0.5 ML. After that and before the completion of the first layer, 2L-Au islands start to form at the middle of the 1L-Au patches, as can be seen in Figure 1b. The enlarged image in Figure 1c clearly reveals a superstructure consisting of triangular depressions (marked by the black triangles) whose side lengths are around 2 nm. The triangles are distributed in a hexagonal pattern with equivalent intervals around 4 nm, as dictated by the inserted Fast-Fourier-Transform (FFT) image. Figures 1d and 1e show the zoomed-in and atomically resolved images of a triangular feature, respectively, demonstrating that the gold lattice is actually continuous at the peripheries of the triangle but showing lower contrast. In addition, outside of the periphery regions there are also some low contrast sites (highlighted by the white circles) which can be assigned as the incorporated Cu atoms. With the atomic resolution, the lattice periodicity of the 1L-Au film can be precisely determined as 0.272 nm (+/- 0.005 nm), which is exactly the mean value of that of Au(111) (0.288 nm) and Cu(111) (0.255 nm). Previous studies of the Ag film on Cu(111)23-25 reported a (9 × 9) superstructure with similar triangular features. In that case the superlattice was deduced from overlapping the Ag(111) lattice (0.288 nm) with the Cu(111) substrate. The triangular structure was explained as the antiphase domain boundary after losing a row of Cu atoms inside the triangular region. Here we propose a similar (16 × 16) superstructure for the observed superstructure in 1L-Au/Cu(111) film by considering the lattice commensurability between the 1L-Au film and Cu(111) substrate: 0.272 nm × 15 = 0.255 nm × 16 = 4.08 nm. In addition, a tentative model for the triangular structure is 8


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shown in Figure 1f (see Figure S1 for the wide range images of the superstructure), which corresponds to a specific defect structure in the subsurface Cu layer. As illustrated by the model, 8 Cu atoms are removed from a Cu triangle consisting of 36 atoms and the left 28 Cu atoms form a smaller triangle and shift to the original center. In this way, if the interatomic distance of Cu keeps unchanged (0.255 nm), there are three equivalent anti-phase domain boundaries formed at the periphery of the new triangle. In this model, the length of each anti-phase domain boundary is about 8 × 0.255 nm = 2.04 nm, perfectly consistent with the measured side-length of the triangular features. We notice that the realistic film is not perfectly periodic but contains various deviations, as seen in Figure1c. This is probably due to the intermixing of Cu and Au in both the adlayer and the substrate surface which interferes with the ordering of local atomic structures.

Figure 2. STM images of 2.7 ML (a) and 4.5 ML (b) gold films deposited on Cu(111) at RT. (c−e) Atomic resolution STM images of 2L-Au, 3L-Au and 4L-Au films, respectively. (f) STM 9



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derived lattice constants of the Au adlayers as a function of film thickness. (g) Averaged dZ/dV spectra obtained on different Au adlayers. Tunneling conditions: (a) I = 810 pA, U = 1.17 V; (b) I = 900 pA, U = 1 V; (c) I = 870 pA, U = − 18 mV; (d) I = 800 pA, U = − 6 mV; (e) I = 800 pA, U = − 13 mV.

Increasing the Au coverage resulted in thicker Au films on Cu(111), as shown by the STM images in Figures 2a and 2b, which presents a clear layer-by-layer growth mode. Figures 2c-e present the atomically resolved images obtained on the 2L-Au, 3L-Au, and 4L-Au islands, respectively. One can clearly identify the Au lattice with random disturbances of dim spots which are attributed to the incorporated Cu atoms (as highlighted by the white circles). This disturbance is rather strong and cause significant corrugations of the STM topographies, as can be clearly viewed by the profiles shown in Figures S2 and S3. Based on the atomic resolution images, the lattice periodicity of the 2L-Au, 3L-Au and 4L-Au films can be determined as 0.280 ± 0.005 nm, 0.285 ± 0.005 nm and 0.288 ± 0.005 nm, respectively. Compiling together the lattice constant of 1L-Au (0.272 nm) into the plot (Figure 2f), one can immediately realize the gradual expansion of the Au lattice along with the thickening of the film until the forth layers. For thicker films, the lattice constants maintain the value of bulk gold. One may expect to see distinct moiré patterns on each Au island (lower than 4 layers), which is however unobserved by STM. This is possibly due to the complexity of overlapping more than two different periodic lattice, as well as the reduced ordering in each layer due to the interference of the doped Cu atoms. Moreover, we did not see the herring-bone reconstruction of the Au (111) surface even on the 6L-Au islands. This can be 10


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attributed to the lattice strains induced by the compressed Au layers in surface vicinity, whose influence can be reserved until very thick films. The structural deviation from the bulk gold would necessarily lead to the deviation in electronic properties for the Au films, which can be detected by STS measurements. Figure 2g shows the typical constant-current dZ/dV spectra that were collected on differently thick Au films as well as the uncovered Cu(111) area. These spectra are characterized by a sequence of peaks originating from the Gundlach oscillations which are induced by field emission resonances (FER) of the electrons26. Usually a detailed fitting of the spectrum can give a precise measurement of the local work function (WF) of the surface27. Here we only take the first FER peak (f-FER) as a rough estimation of WF, which can already give reliable evaluations of different surfaces. The f-FER of ex-Cu can be determined at 4.6 eV which agrees very well with the WF of Cu(111) obtained by other methods28. It is worth noting that the pronounced peak appearing at 4.0 eV can be assigned to the L-edge of the (111) orientated Au film, despite of the gradually evolved interatomic distances.

29-30

Therefore, on 1L-Au, 2L-Au, and 3L-Au films, the f-FER was

sequentially measured as 5.1, 5.5, and 5.6 eV, respectively, showing a stepwise blue-shift along with the thickening of deposited Au. Such evolution of the WF actually reflects that the attachment of Au overlayers have caused gradual down-shift of the d-band of a Cu(111) surface, which is in line with previous theoretical31-32 and experimental studies1, 33-34. On the films thicker than 3 Layers, the f-FER stayed almost unchanged, as with the entire spectra, suggesting that the electronic properties of the corresponding Au films may already have reached a bulk-like case.

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Figure 3. SRPES measurements of (a) Au 4f and (b) Cu 3p transitions on bare Cu(111) and differently thick Au/Cu(111) films. (c) The intensity evolutions of the deconvoluted Au (Au-1 and Au-2) and Cu (Cu-1 and Cu-2) species as a function of film coverage. (d) Comparison of the attenuation of the experimentally measured (solid curve) with the theoretically predicted (dashed curve) Cu 3p transitions as a function of the thickness of the Au films.

Complementing the microscopic information obtained from STM, we also investigated the gold films with SRPES measurements to gain more surface-averaged chemical information. Figures 3a and 3b show the Au 4f and Cu 3p spectra recorded on differently thick gold films, respectively. Obviously, along with the thickening of the films, Au 4f signal gradually increases while Cu 3p deceases, as also demonstrated by the plots in Figures 3c and 3d. Detailed 12


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deconvolution analyses revealed that both Au 4f and Cu 3p may consist of two sets of peaks corresponding to two types of species. The Au species with relatively lower binding energies (B.E.) is defined as Au-1 while those with higher B.E. as Au-2. As can be seen in Figures 3a and 3c, Au-1 dominates in the 1ML-Au and only slightly decreases along with growth of Au films (see solid green curve in Figure 3c). Concomitantly, Au-2 increases gradually and starts to dominate the Au 4f signals since the 3ML- Au film (see dotted green curve in Figure 3c). On the 2ML-Au film, Au-1 and Au-2 show comparable intensities. In terms of B.E., the 4f (7/2) of Au-1 and Au-2 appears at 83.9 eV and 84.3 eV on the 1ML-Au film, respectively. Both gradually red-shift along with the film growth, and finally reach 83.7 eV and 84.0 eV on the 7 ML-Au film, respectively. Noticeably, the 4f (7/2) of a metallic bulk Au is usually measured as 84.0 eV35-36. We notice that usually the surface atoms have considerably lower coordination number than the bulk ones hence showing lightly lower binding energies37-39. Combining all the information, we tentatively attribute Au-1 as the Au atoms that always expose at the top surface whereas Au-2 the Au atoms residing beneath the surface and experiencing bulk-like coordination environment. Therefore, all the ultrathin Au films show a relatively higher B.E. compared to the bulk Au, possibly due to the charge accumulation induced by the work function difference against the Cu(111) substrate13, 40-41. And the thinner the film, the stronger the effect. It has to be pointed out that owing to the research history of growing Au films, the cleaned Cu(111) substrate was also detected with weak Au 4f signals peaking at ~ 84.2 eV. This can be assigned as the dissolved Au atoms in the Cu(111) surface region (see Figure S4) and will not be discussed in the current paper. 13



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The Cu 3p spectra in Figure 3b can also be deconvoluted into two sets of peaks. The Cu species with higher B.E. (termed as Cu-1) decrease gradually along with the Au film growth while that with lower B.E. (termed as Cu-2) appear since the 2ML-Au and take a very slow increase in parallel. Therefore, the Cu-1 species must be originated from pure Cu(111) surface and have a B.E. (for Cu 3p (3/2)) of 74.9 eV42-43. It gradually red-shifts to 74.7 eV for the 7ML-Au film, possibly resulting from the same electrostatic field constructed at the Au/Cu interface. In contrast, the Cu-2 species show much lower B.E. of 73.3 eV and are basically positioned at this value throughout all the Au films. Based on our STM data shown above, we tentatively attribute this Cu-2 species to the Cu atoms intermixed into the Au films. Their positive correlation with the Au coverage can thus be well explained considering more intermixed Cu atoms can be detected in thicker films. Moreover, their existence would seriously influence the attenuation of the Cu 3p signals against the changes of gold films. The dashed grey curve in Figure 3d shows the predicted Cu 3p attenuation (see the detailed calculation method in the Supporting Information) based on an ideal layer-by-layer model44. One can clearly see that the experimentally measured intensities decrease much slower than the prediction. Such deviation can be reasonably attributed to the intermixing of Cu atoms in the added Au layers. 2.

Atomic Structure And Electronic Properties Of The Cu/Au(111) Films.

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Figure 4. STM topographies of the submonolayer Cu/Au(111) films with the coverage of (a) 0.3 ML and (b) 0.7 ML. (c) Magnified view and (d) atomically resolved image of the 1L-Cu film. Tunneling conditions: (a) I = 130 pA, U = − 0.87 V; (b) I = 190 pA, U = − 0.75 V; (c) I = 160 pA, U = − 1.57 V; (d) I = 1200 pA, U = − 8 mV.

Cu/Au(111) can be regarded as a reversed system compared to Au/Cu(111), which also contains the interfaces between Cu and Au as well as the Cu-Au alloyed phases. As shown in Figures 4a and 4b, the deposited Cu atoms tend to form condensed “patches” instead of fractal-like structures along the step edges, which is apparently different from the growth behavior of Au on Cu(111). The Cu “patches” are usually enveloped by sharp boundaries running along the close-packed directions of the Au(111) substrate. The height of the Cu “patches” were measured around 0.19 nm, slightly lower than the mono-step height of the Cu (111) surface (0.208nm)

17, 45

. The single layer Cu (termed as 1L-Cu, similar terms are applied to thicker Cu 15



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films) islands present a curved reconstruction pattern with relatively low ordering, which is similar to the herringbone reconstruction of the uncovered Au(111) area (termed as ex-Au), but significantly different from the triangular reconstruction of Au/Cu(111). The turning point of the curved structure can serve as seed points for second layer Cu growth when the coverage is raised above half monolayer, as shown in Figure 4b. The magnified (Figure 4c) and the atomically resolved (Figure 4d) images revealed that this reconstructed 1L-Cu film contains many dim species with atomic size, as highlighted by the white circles. Based on the atomic resolution image, the periodicity of the hexagonal lattice can be determined as 0.288 ± 0.005 nm, which is exactly the same as Au(111) surface. Previously Rodriguez and co-workers16 concluded from their ISS, XPS and DFT calculation study that at room temperature the evaporated Cu tends to aggregate at the subsurface region of Au(111). Our own study of melamine adsorption on the submonolayer Cu/Au(111) film46 also demonstrated that the submonolayer Cu film is terminated with a gold layer. These results strongly support the proposition that the observed “1L-Cu islands” actually reside at the subsurface region and adopt a (1 × 1) structure against the Au(111) substrate, whose tops are overlaid with a gold layer, as illustrated by the model in Figure S5. With this model, the observed bright atoms in Figure 4d are actually the gold atoms residing on the subsurface Cu patches, while the encircled dim species may be assigned as the incorporated Cu atoms oversitting on either Cu atoms or intermixed Au atoms at the subsurface layer, as also presented in Figure S5. With these understandings, the curved reconstruction of the 1L-Cu film can be explained as tailored buckling of Au(111) surface in presence of many “Cu impurities”.

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Recalling the 1L-Au/Cu(111) film addressed above, one may notice that always a single Au layer tends to localize at the surfaces of both Au/Cu and Cu/Au ultrathin films. Such a configuration can be simply understood by considering that Au(111) has substantially lower surface energies (SE) than Cu(111) (1.50 vs 1.83 J/m2) 47-49. Therefore, the bimetal system tends to expose the Au layer to gain lower energy. Such a rule also fits with the fact of growing Cu on Ag(111) (SE~1.25 J/m2) and Pt(111) (SE~2.48 J/m2), which forms Ag-top/Cu-sub23-25 and Cu-top/Pt-sub structures50-52, respectively.

Figure 5. STM images of (a) 2.2 ML and (b) 4.5 ML Cu film deposited on Au(111) at RT. (c) Atomically resolved image obtained on 2L-Cu. (d) Plot of the STM derived lattice constants of the Cu films as a function of film thickness. (e) Averaged dZ/dV spectra of different Cu films on 17



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Au(111). The spectrum achieved on Au(111) is also presented for reference. Tunneling conditions: (a) I = 400 pA, U = −1 V; (b) I = 120 pA, U = 1.13 V; (c) I = 300 pA, U = −20 mV.

Evaporating more Cu onto the Au substrate lead to the growth of multilayer films, which follows an obvious layer-by-layer mode, as displayed by the STM images in Figures 5a and 5b. For the Cu patches of two up to six layers, a quasi-hexagonal superstructure can be observed, whose periodicity was measured around 4 nm. The atomic resolution of the 2L-Cu film (Figure 5c) reveals a lattice constant of 0.255 nm, which is exactly the same as that of Cu(111). The same lattice constant is well preserved in thicker films. Moreover, profile measurements (not shown here) across the steps of 2L-Cu and thicker films revealed a constant step height of 0.22 nm, which is consistent with the single step height on Cu(111). With all these evidences, we propose that since the 2L-Cu, the Cu films basically restore to a Cu(111) termination. The observed moiré patterns can be understood as a result of overlapping the Cu lattice (0.255 nm) with the Au(111) lattice (0.288 nm) of the substrate. Similar to the case of Au films on Cu(111), the low ordering of the moiré pattern can be attributed to the interruptions induced by the intermixed Au atoms into the Cu lattice, which show different contrast (highlighted by white circles) in the atomically resolved image as shown in Figure 5c. Analogous to the Au films on Cu(111), the thickness-dependent atomic structures of the Cu films on Au(111) imply a gradually evolved electronic property on different Cu adlayers. In Figure 5e, we present typical dZ/dV spectra that have been collected on different Cu/Au(111) films. The spectrum of Au(111) is also presented showing the f-FER at ~ 5.5 V, dictating a 18


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surface WF of 5.5 eV which is comparable to that measured by other techniques. It is noted that the strong peak at ~ 3.5 eV is originated from the upper edge of the Au L-gap instead of any vacuum states related with Gundlach oscillation29-30. For the dZ/dV spectra of the Cu films, one can clearly see the down-going trend of the f-FERs along with the increase of film thickness, which is obviously contrary to the evolution on the Au films as presented in Figure 2. Specifically, the f-FERs read 5.2 V and 5.0 V for 1L-Cu and 2L-Cu films, respectively, and maintained at ~ 4.7 V for 3L-Cu and thicker films. It needs to be mentioned that even on the thick Cu films (up to 6 layers) there can be Au atoms intermixed into the top Cu layer. But their concentrations are low and would not exert significant influence on the electronic properties of the Cu films, as dictated by the adsorption of melamine molecules reported in our previous study.36

3. Comparison of the CO adsorption on single layer Au/Cu(111) film and Cu/Au(111) film.

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Figure 6. STM images showing the adsorption results of dosing 30 L of CO to (a) 1L-Au and (b) 1L-Cu surfaces which were held at 100 K. (c) The profiles of the different CO species highlighted in the insets of (a) and (b). (d) Statistical results of the adsorption amount of CO on different sample surfaces. (e) The statistical results of the adsorption amount of CO as a function of sample temperature on 1L-Au (black curve) and 1L-Cu (red curve) surfaces, respectively. Each data point was averaged from six STM images. (f) Compiled dZ/dV spectra showing the work function differences between the four surfaces. Tunneling conditions: (a) I = 130 pA, U = 1 V; (b) I = 110 pA, U = − 3 V.

Among the films discussed above, the ultrathin ones present the most characteristic properties owing to the specific interface structures. In addition, the ultrathin metal adlayers can often find counterparts in bimetallic nanoparticles wherein the added metal is enriched at the surface region. In the current study, the monolayer Cu/Au(111) and Au/Cu(111) films both hold a similar 20


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Au-top/Cu-sub bilayer surface yet residing on different substrates, i.e. Au(111) for the former and Cu(111) for the latter. For both Au/Cu(111) and Au/Cu/Au(111) surfaces, the atomically resolved STM images have revealed quite different lattice constants, and the STS measurements have demonstrated rather different electronic properties. These factors should lead to drastically different chemical properties on these two structurally similar surfaces. Figures 6a and 6b show the STM results of exposing 30 L (Langmuir, 1 L = 10-6 torr ▪ sec) CO to both ultrathin films while keeping the samples at 100 K. Clearly, there were markedly number of CO molecules adsorbed on the films at this temperature, which is in sharp contrast to the result of CO exposure to Au(111)53-54. The protrusive topography of CO may be caused by the special modification of the tip apex with a CO molecule.55 Once the CO is removed from the tip, the depressive topography immediately recovers, as can be found in Figure S6 in the supplementary materials. Some brighter protrusions with oval shapes (highlighted by the ellipses) can be assigned to two CO molecules residing in nearest neighboring surface sites56, as evidenced by the profiles shown in Figure 6c. In Figures S7 we show the CO exposure results of the exposed Cu(111) and Au(111), respectively. On the ex-Cu(111) region, an ordered (√3 × √3-R30o) CO phase was observed55, 57, whereas on the ex-Au(111) only ignorable number of adsorbates can be found. This is in line with the literatures that CO interacts extremely weakly with Au(111) and desorbs before 60 K53, whereas it forms variety of adsorption phases on Cu(111) dependent on the coverage and desorbs completely at around 200 K58-60. In Figure 6a one may also notice that many of the CO molecules reside around the dark peripheries of the triangular features. In contrast, on the 1L-Cu/Au(111) film, CO adsorb with smaller concentration and reside mostly 21



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around the ridge-like reconstruction on the surface. As discussed above, the triangle peripheries on Au/Cu submonoalyer film is related to the anti-phase domain boundaries at the Cu(111) substrate while the ridge region on reconstructed Cu/Au submonolayer film is rich of incorporated Cu atoms. Therefore, CO adsorption on both ultrathin films may be related to the Cu-induced surface features. In Figure 6d we compare the concentrations of the adsorbed CO molecules on the different surfaces, which is found following the order of Cu(111) >> 1L-Au/Cu > 1L-Cu/Au >> Au(111), dictating an sequentially weakened CO adsorbability with more and more Au added. To gain a more detailed comparison of the two ultrathin films, we further conducted CO exposures of the same amount (30 L) but at varied sample temperatures. The results can give qualitative evaluations of the surface interaction strengths with CO molecules. The STM-derived CO concentration on the specific 1L-Au and 1L-Cu films are plotted in Figure 6e as a function of temperature. The corresponding STM images are presented in Figures S8 and S9. It can be seen that upon increasing the exposure temperature, the amount of adsorbed CO quickly reduced. For the 1L-Cu/Au(111) film, hardly no CO molecules were adsorbed when the sample temperature reached ~200 K, dictating a upper limit for CO adsorption at the experimental conditions. Whereas for the 1L-Au/Cu(111) film, this limit significantly increased to around 250 K, indicating a stronger interaction with CO compared to the 1L-Cu/Au(111) film. The different activities to CO of the distinct surfaces should originate from their different electronic properties. According to the work of Nørskov and co-workers61-62, the d-band centroid (ϵd) of a metal usually plays a decisive role in the adsorption energy of molecules. Generally, the 22


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higher ϵd, the stronger interaction with adsorbates. Experimentally, the core levels and the WF of a surface were found shifting synchronically with ϵd upon changing the surface orientation and components63-64. Therefore, we can use the work function as a descriptor to evaluate the relative shift of ϵd on different surfaces. Accordingly, the lower WF, the higher position of ϵd relative to the Fermi level, and the stronger the binding of the adsorbed molecules. As shown by the compiled plot in Figure 6f, pure Cu(111) and Au(111) have the lowest (4.6 eV) and highest (5.5 eV) WF, respectively, while 1L-Au/Cu(111) and 1L-Cu/Au(111) film have the intermediate values with the former being slightly lower than the latter (5.1 eV vs 5.2 eV). Such order of WFs reflects the reverse order of ϵd for the four surfaces. Hence, the CO adsorption activity follows the sequence as Cu(111) > 1L-Au/Cu > 1L-Cu/Au > Au(111). Apart from the overall trend in binding CO that is averaged across the surface, one may notice that the STM-derived adsorption temperature limit of both 1L-Au/Cu(111) and 1L-Cu/Au(111) ultrathin films are actually slightly higher than the TPD-derived desorption temperature on Cu(111)58-60. This can be attributed to the specifically strong binding of CO binding at the Cu atoms which are intermixed in the Au top layer, as deduced from the STM images. As demonstrated by the work of Rodriguez et al.16, the Cu atoms mixed in the Au-surf layer of the 1L-Cu/Au(111) film experience an up-shift of their d-states. In the case of the 1L-Au/Cu(111) film, a compressed lattice may cause a stronger coordination interaction hence result in even more prominent up-shift of the d-states. According to the Blyholder model65-68 of CO adsorption on metals, the up-shifted d-state would not only favor the hybridization of the 3dz2 of Cu with the 5σ orbital of CO, but also benefit the back donation from the filled d-states 23



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into the empty 2π orbital of CO. Consequently, the dispersed Cu atoms in both 1L-Cu/Au(111) and 1L-Au/Cu(111) films present in turn increased interactions to CO relative to those in pure Cu(111) surface. Therefore, the observed temperature limit of CO adsorption on these films are significantly higher than that on a Cu(111) surface.

CONCLUSIONS The atomic structures and CO adsorption properties of Au/Cu(111) and Cu/Au(111) films have been studied with STM and SRPES. STM shows that at RT the deposited Au thin films form a compressed lattice on the Cu(111) surface, which gradually evolves to the bulk value until the fourth layer. Such lattice evolution also finds correspondences in the evolution of electronic properties as deduced from the STS measurements. For the Cu films grown on Au(111), however, our results demonstrate that the deposited Cu atoms show strong intermixing with the Au substrate even at room temperature. The sub-monolayer Cu deposited on the Au(111) segregates at the subsurface with an Au skin layer on top. While the true Cu adlayers start to appear on the two-layer thick films whose lattice constant switches sharply to that of Cu(111). The interactions between CO with these ultrathin films are found significantly enhanced in comparison with on pure Au(111) surface. The Au/Cu(111) film is slightly more active than Cu/Au(111) film. The apparent binding sites of the CO molecules are determined as closely related to the surface Cu atoms. However, the STS measurements reveal that the specific electronic structures of the ultrathin films may be the real accounts behind the enhanced chemical properties. These findings 24


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are believed to provide a deepened understanding of the surface chemistry of Au-Cu bimetallic systems.

Acknowledgement We are grateful for the financial support of NSFC (91545128, 21333001) and MOST (2017YFA0205003, 2011CB808702). X. S. thanks the financial support of the Fundamental Research Funds for the Central Universities and the Thousand Talent Program for Young Outstanding Scientists of the Chinese government. Supporting information: Additional STM images, tentative model for 1L-Au and 1L-Cu films, and SRPES spectrum of Au-doped Cu(111) surface.

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TOC Graphic:

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Atomic structures and CO adsorption properties of Cu/Au(111) and Au/Cu(111) thin films were investigated with low-temperature STM and photoelectron spectroscopy. 97x75mm (300 x 300 DPI)


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Comparison Study of Structural Properties and CO Adsorption on the

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