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. 2022 Dec 21;3(1):176–184. doi: 10.1021/jacsau.2c00580

Periodic Arrays of Metal Nanoclusters on Ultrathin Fe-Oxide Films Modulated by Metal-Oxide Interactions

Xuda Luo †,‡, Xiaoyuan Sun †,§, Zhiyu Yi †,‡, Le Lin †, Yanxiao Ning †,*, Qiang Fu †,∥,*, Xinhe Bao †,∥
PMCID: PMC9875227  PMID: 36711105

Abstract

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Rational design of highly stable and active metal catalysts requires a deep understanding of metal–support interactions at the atomic scale. Here, ultrathin films of FeO and FeO2–x grown on Pt(111) are used as templates for the construction of well-defined metal nanoclusters. Periodic arrays of Cu clusters in the form of monomers and trimers are preferentially located at FCC domains of FeO/Pt(111) surface, while the selective location of Cu clusters at FeO2 domains is observed on FeO2–x/Pt(111) surface. The preferential nucleation and formation of well-ordered Cu clusters are driven by different interactions of Cu with the Fe oxide domains in the sequence of FeO2-FCC > FeO-FCC > FeO-HCP > FeO-TOP, which is further validated by density functional theory calculations. It has been revealed that the p-band center as a reactivity descriptor of surface O atoms determines the interaction between metal adatoms and Fe oxides. The modulated metal-oxide interaction provides guidance for the rational design of supported single-atom and nanocluster catalysts.

Keywords: metal-oxide interaction, p-band center, Cu nanoclusters, Fe oxide films, scanning tunneling microscopy

Introduction

Single-atom and nanocluster catalysts have attracted great attention due to their high catalytic performance and atom utilization efficiency.1−3 The precise size control of single-atom and nanocluster catalysts and their stability during reactions remain a great challenge due to their high surface energy and strong tendency toward aggregation. Oxides are widely used as supports for metal atoms and nanoclusters in industrial catalysis and play a critical role to anchor these active nanostructures. Further, metal-oxide interactions can modulate the electronic structures and catalytic performance of supported metal catalysts,4−6 and the formed metal-oxide interfaces show a catalytic synergistic effect to present high catalytic activity and selectivity.7−9 Consequently, the interaction between metal atoms/nanoclusters and oxide supports has been extensively investigated in heterogeneous catalysis. It has been well recognized that the metal-oxide interaction can be effectively tailored by surface structures of oxide supports such as crystal face,10,11 surface defects,12,13 and surface functional groups.14 Nevertheless, the fundamental understanding of the metal-oxide interaction and interfacial bonding still deserves further exploration.

In real heterogeneous catalytic systems, the structures of metal on oxide supports are very complex. It is difficult to obtain a deeper understanding of the metal-oxide interaction at the atomic scale, which can be explored by model catalyst systems with fewer variables and controllable structures. Oxide single crystals such as TiO2(110)15,16 and Fe3O4(001)17,18 have been used as supports for metal nanostructures to explore metal-oxide interactions. However, the poor conductivity of most oxide bulks limits their application for surface science characterizations. Ultrathin oxide films supported on metal single crystals are also applied to support single atoms or nanoclusters,19−21 allowing atomic resolution of surface structures by surface analytic techniques. In addition, two-dimensional oxide films grown on metal surfaces often show moiré superstructures due to the lattice mismatch between oxide overlayers and metal surfaces, which can be used as templates for the construction of ordered metal atoms and nanoclusters,22,23 similar to previous findings of well-ordered metal nanoclusters supported on graphene24,25 and hexagonal boron nitride.26,27 On the basis of well-defined metal atoms and nanoclusters supported on ultrathin oxide films, the coordination environment and electronic structures of metal nanostructures can be definitely characterized, allowing us to understand the metal-oxide interaction at the atomic scale.

Ultrathin FeO film grown on Pt(111) has a moiré structure with different stacking configurations, including face-centered cubic (FCC), hexagonal close-packed (HCP), and TOP domains, where different Fe–O layer distances and surface dipoles have been reported.28−32 An FeO/Pt(111) surface has been used as a template for the construction of well-ordered Au atom arrays and most Au atoms are located at FCC domains.33,34 FeO/Pt(111) can be oxidized into FeO2–x/Pt(111),35−38 which also has a superstructure similar to FeO/Pt(111)36 and is another potential template for the growth of metal cluster arrays. Surface O atoms in FeO and FeO2–x films have nearly identical coordination structures and lattice parameters but present different activity for CO oxidation,36,38 which may show different interactions with metals. The well-defined FeO and FeO2–x films with similar O-terminated surfaces offer ideal models to understand how the surface O property influences metal-oxide interactions.

In the present work, atomically flat FeO and FeO2–x films on Pt(111) were used as templates for the growth of Cu atoms and clusters. Scanning tunneling microscopy (STM) investigations indicate that Cu atoms prefer to locate at FCC domains of the FeO film and FeO2 domains of the FeO2–x film, forming well-ordered Cu nanocluster arrays. Density functional theory (DFT) calculations show the adsorption strengths of Cu atoms on different domains of FeOx following the sequence of FeO2-FCC > FeO-FCC > FeO-HCP > FeO-TOP. The interaction is determined by the reactivity of surface O atoms in the Fe oxide films, which can be described by the p-band center.

Methods

Experimental Details

All experiments were performed in a two-chamber ultrahigh vacuum (UHV) system. The preparation chamber comprises X-ray photoelectron spectroscopy (XPS), evaporators, and cleaning facilities. XPS spectra were acquired using a dual-anode X-ray source (SPECS) with Mg Kα (1253.6 eV) and Al Kα (1486.6 eV) radiation. A CreaTec low-temperature STM (LT-STM) is equipped in the STM chamber. The temperature of STM measurement is held at 78 K. The base pressures of the STM and preparation chambers are 5 × 10–11 and 1 × 10–10 mbar, respectively.

A Pt(111) single crystal (MaTeck) was cleaned by cycles of Ar+ sputtering (1 keV, 10 μA) and annealing at 1050 K in UHV. When needed, the sample was annealed in 1 × 10–7 mbar O2 at 700 K to remove carbon species on the Pt(111) surface. The clean Pt(111) surface was checked by STM. The FeO film was grown on Pt(111) by deposition of Fe atoms from a Knudsen cell (CreaTec) in an O2 atmosphere with Pt(111) held at room temperature (RT) and annealing in an O2 atmosphere afterward. To get the FeO2–x/Pt(111) film, the FeO/Pt(111) surface was exposed to O3 at RT and then annealed in UHV. O3 dosing composed of 10% O3 and 90% O2 was generated by a high-purity ozone generator system.39,40 Cu was evaporated from another Knudsen cell (CreaTec), and Ce was evaporated from an e-beam evaporator (SPECS) with controllable evaporation flux onto FeO or FeO2–x surfaces, which were held at 100 K or RT. STM images were processed with SPIP software.

Computational Details

Spin-polarized DFT calculations were implemented using a plane-wave basis set in the Vienna Ab initio Simulation Package (VASP 5.4).41 The exchange–correlation energy was treated using the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA).42 The projected-augmented wave (PAW) pseudopotentials were utilized to describe the core electrons, and a cutoff energy of 400 eV was used for the plane-wave expansion.43 The following valence electron configurations were included in the self-consistent field calculations: Pt (5d9 and 6s1), Cu (3d10 and 4s1), Fe (3d6 and 4s2), and O (2s2 and 2p4). In addition, the van der Waals (vdW) dispersion forces were corrected by the vdW–DF (optPBE) functionals, which showed a highly accurate description of oxides.44,45 An on-site Hubbard term Ueff = U – J was added to address the open-shell d-electrons with 2.71 eV for Fe in FeO/Pt(111) and with 2.97 eV for Fe in FeO2–x/Pt(111).46 The water-based reference state for the calculations has been adopted to avoid an incorrect description of the gas phase O2 reference with standard DFT methods.44 The energies and residual forces were converged to 10–5 eV and 0.02 eV Å–1, respectively.

The adsorption energy of Cu on FeOx/Pt(111) is

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where ECu, EFeOx/Pt(111), and EFeOx/Pt(111)+Cu are the energies of Cu atoms in the gas phase, bare FeOx/Pt(111), and the structures after Cu atom adsorption on FeOx/Pt(111) surfaces, respectively.

The p-band center (εp) of surface O is defined as

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Model Constructions

We built up two basic models to corroborate the geometric structures of single-atom Cu located on FeOx/Pt(111). The FeO/Pt(111) interface was modeled using a moiré superstructure (√84 × √84)R10.9°-FeO/Pt(111) with a period of 2.5 nm, which amounts to placing an FeO-(√67 × √67) monolayer on a Pt(111)-(√84 × √84) substrate with a three-layer thickness.7,29,47,48 The FeO2–x/Pt(111) interface was constructed by shifting the O sublattice of FeO/Pt(111) from one Fe hollow site to another and adding extra 19 O atoms into the Fe–Pt interlayer in FCC domains of the new FeO/Pt(111) surface, and the intercalated O atoms are located at Pt-top sites. During the structure relaxation, the FeOx overlayer and the top two Pt layers were fully relaxed and the bottom Pt layer was constrained. The (1 × 1 × 1) k-point grids were exploited for all FeOx/Pt(111) structures.

Results and Discussion

Atomic Structures of FeO/Pt(111) and FeO2–x/Pt(111) Surfaces

The full monolayer (ML) FeO film on Pt(111) was prepared by depositing Fe atoms in an O2 atmosphere (1 × 10–7 mbar) onto the Pt(111) surface held at RT and then annealing in the O2 atmosphere (5 × 10–8 mbar) at 730 K (Figure S1a). The atomically resolved STM image of the ML FeO film shows that a hexagonal moiré pattern (Figure 1a) with a periodicity of 2.5 nm (lower panel in Figure 1c) is formed due to the lattice mismatch between Pt(111) (dPt-Pt = 0.27 nm) and FeO (dFe–Fe = 0.31 nm). According to the literature,49−51 the FeO/Pt(111) surface features an O–Fe–Pt (from top to bottom) layered structure, as shown in Figure 1d. From the top view, the moiré patterns can be divided into three regions including FCC domains (both Fe and O on Pt-hollow sites), HCP domains (Fe on Pt-hollow site and O on Pt-top site), and TOP domains (Fe on Pt-top site and O on Pt-hollow site).31 The attribution of different regions in STM images has been well established.29,31,47,48

Figure 1.

Figure 1

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Atomically resolved STM images of FeO/Pt(111) (a) (7 × 7 nm2; I = 0.5 nA, V = 0.1 V) and FeO2–x/Pt(111) (b) (7.8 × 7.8 nm2; I = 0.1 nA, V = 0.1 V); FCC, HCP and TOP domains are labeled in the images. (c) Line profiles of FeO (lower) and FeO2–x (upper) surfaces, corresponding to the red and blue lines in panels (a, b), respectively. Top and side views of the optimized structures of FeO/Pt(111) (d) and FeO2–x/Pt(111) (e) superstructures of (√84 × √84)R10.9° indicated by a yellow parallelogram. The white circles mark the three different domains. Pt: dark blue; Fe: purple; surface O: orange; and interface O: pink. Notably, the HCP and TOP domains of the FeO2–x/Pt(111) surface have the same O–Fe bilayer structures as those of the FeO/Pt(111) surface. (f) XPS O 1s spectra of the FeO/Pt(111) (red) and FeO2–x/Pt(111) (blue) surfaces; fitting peaks of FeO2–x in purple, pink, and green represent surface lattice oxygen, interface lattice oxygen, and surface hydroxyl, respectively.

The transformation of the FeO film into the FeO2–x film has been achieved by atomic oxygen exposure at RT.37 Here, we find that the FeO/Pt(111) surface can be oxidized into FeO2–x/Pt(111) through the exposure of 30 L (Langmuir, 1 L = 1 × 10–6 Torr·s) O3 at RT and subsequent annealing in UHV at 500 K to desorb weakly bound oxygen species (Figure S1b). Figure 1b displays an atomically resolved STM image of the FeO2–x surface, which exhibits closely packed patterns with the same lattice spacing and moiré periodicity of the FeO/Pt(111) surface, but presents a larger surface corrugation (0.09 nm, upper panel in Figure 1c). According to the domain assignment in the STM image of a surface containing both unoxidized and oxidized regions (Figure S2), the brighter domains in the FeO2–x film are identified to be located at FCC domains of the FeO film, which originates from intercalated oxygen at the Fe–Pt interface and formation of O–Fe–O trilayer structures, while HCP and TOP domains of the the FeO2–x/Pt(111) surface remain as the O–Fe bilayer structures.37,38,48 According to STM results, the atomic model of the FeO2–x/Pt(111) surface was constructed and optimized (Figure 1e).37 Notably, each Fe atom in FeO2-FCC is sixfold-coordinated, which is higher than threefold-coordinated Fe in FeO-FCC, while the surface O atoms in both systems are all threefold-coordinated.

Following the oxidation of FeO into FeO2–x, the XPS O 1s peak shifts to a lower binding energy (BE) position (Figure 1f).35,52 Meanwhile, a larger full width at half-maximum can be seen in the spectrum, which means multiple components are present in it. It has been figured out that the surface oxygen has a lower BE compared with the interface oxygen in the O–Fe–O trilayer structures.35 The deconvoluted components (blue spectrum in Figure 1f) show that the BEs of the surface oxygen and interface oxygen are 528.8 and 529.4 eV, respectively, and the small peak at 530.2 eV is attributed to hydroxylation of surface oxygen.35

Deposition of Cu on FeO/Pt(111) and FeO2–x/Pt(111) at 100 K

After depositing 0.05 ML Cu on the FeO/Pt(111) surface at 100 K, highly ordered and dispersed clusters (0.16 count/nm2) are formed on the surface (Figure 2a), while a few aggregated nanostructures also exist. Fast Fourier transformation (FFT) analysis of the STM image shows a nearly sixfold symmetric diffraction pattern with a periodicity of 2.6 nm, close to that of the moiré pattern of the FeO film. Stripes rather than dots in the FFT pattern indicate that the FeO film consists of domains with different rotation angles with respect to the Pt(111) surface, the same as that in Figure S1a. Cu nanostructures are in the form of either monomers with a height of 0.19 nm or trimers with a height of 0.17 nm (Figure 2b). The spacing between neighboring atoms of the trimer clusters is around 0.44 nm (Figure 2e). The spatial distribution of the monomers and trimers in the three domains has been statistically analyzed, and the data are shown in Figure 2f, which suggest that Cu atoms or clusters mainly anchor at FCC domains but scarcely at HCP and TOP domains. Thus, the FCC domains have the strongest interaction with Cu atoms, in line with Au atoms on FeO.33,34

Figure 2.

Figure 2

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STM images of 0.05 ML Cu deposited at 100 K on FeO/Pt(111) (a) (100 × 100 nm2; I = 0.01 nA, V = 1 V) and (b) (10 × 10 nm2; I = 0.1 nA, V = 0.3 V), and FeO2–x/Pt(111) (c) (100 × 100 nm2; I = 0.01 nA, V = 1 V) and (d) (10 × 10 nm2; I = 0.1 nA, V = 0.5 V). The insets in panels (a, c) are their corresponding FFT images. FCC, HCP, and TOP domains are marked by squares, triangles, and circles, respectively. (e) Line profiles of Cu atoms and distances between Cu atoms correspond to the red line in panel (b) and blue and green lines in panel (d), respectively. (f) Statistical histograms of the number of Cu atoms in different domains in (b, d).

Depositing 0.05 ML Cu on FeO2–x at 100 K produces a higher density (0.38 count/nm2) of Cu atoms and clusters and almost no aggregated islands on the FeO2–x film (Figure 2c) in contrast with Cu deposition on the FeO film. The FFT image shows a sixfold symmetric diffraction pattern, which stems from the well-ordered Cu cluster arrays with a periodicity of 2.6 nm, close to that of the clean FeO2–x surface (Figure S1b). The high-resolution STM image indicates that most of the clusters sit on FeO2-FCC domains (Figure 2d) with a height of around 0.15 nm (Figure 2e). Only a few isolated Cu atoms are located on HCP domains but none of the Cu atoms are on TOP domains (Figure 2f). Each cluster sitting in the FeO2-FCC domain consists of one or a few atoms, which are distributed randomly within the domain. The distances between neighboring Cu atoms are 0.55 and 0.63 nm, respectively (Figure 2e), which are larger than those of the Cu clusters on FeO. The thermal stability of the Cu nanostructures on FeO and FeO2–x films has been investigated by annealing the as-prepared samples to RT. As shown in Figure S3, Cu atoms and clusters on FeO aggregate to form large clusters and nanoparticles with an irregular distribution, while the well-ordered Cu cluster arrays are still observed on the FeO2–x surface, indicating higher thermal stability of Cu clusters on the FeO2–x film.

Deposition of Cu on FeO/Pt(111) and FeO2–x/Pt(111) at RT

Deposition of 0.05 ML Cu on the FeO/Pt(111) surface at RT induces a much less density of Cu clusters (0.02 count/nm2) and aggregation of Cu atoms in the form of linear nanostructures (Figure 3a), which are selectively grown at the domain boundaries (Figure S4) and composed of nanoparticles with a length of several nanometers and a height of around 0.26 nm (Figure 3d). It is inferred that the interaction between the Cu and FeO film is so weak that the deposited Cu atoms can easily diffuse and aggregate on the FeO surface at RT. The XPS Fe 2p signal has almost no change after deposition of 0.7 ML Cu at RT (Figure S5a). The X-ray-induced Cu LVV Auger peak indicates that the deposited Cu atoms on FeO at RT are still metallic (Figure S5b). Therefore, the linear nanostructures observed in Figure 3a originate from the aggregated Cu atoms, which preferentially nucleate at the FeO domain boundaries and further diffuse through the FeO film.

Figure 3.

Figure 3

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STM images of (a) 0.05 ML Cu deposited at RT on FeO/Pt(111) (100 × 100 nm2; I = 0.01 nA, V = 1 V), (b) 0.05 ML Cu deposited at RT on FeO2–x/Pt(111) (100 × 100 nm2; I = 0.01 nA, V = 1 V), and (c) red square in panel (b) (10 × 10 nm2; I = 0.1 nA, V = 0.3 V). The inset in panel (b) is its corresponding FFT image. (d) Line profiles of Cu nanostructures correspond to the red line in panel (a) and blue and green lines in panel (c), respectively. XPS Cu 2p spectra (e) and Cu LVV Auger spectra (f) of 0.5 ML Cu deposited on FeO/Pt(111) at 100 K and on FeO2–x/Pt(111) at RT.

Depositing Cu on the FeO2–x/Pt(111) surface at RT can still produce ordered nanostructures (Figure 3b). Most clusters contain more than one atom (Figure 3c) and have two different apparent heights of 0.16 and 0.20 nm (Figure 3d). Compared with the Cu nanostructures deposited on FeO2–x at 100 K, the lower cluster density (0.15 count/nm2) and larger aggregates of Cu atoms have been observed on the surface with Cu deposited at RT. However, the dispersion of Cu on FeO2–x is much higher than that on FeO in the case of RT deposition, suggesting a stronger interaction between Cu and FeO2–x than FeO. Figure 3c shows that most Cu clusters are still located at FeO2-FCC domains and a few Cu clusters are observed on HCP domains, similar to the phenomenon of deposited Cu on FeO2–x at 100 K.

XPS measurements were performed to figure out the valence states of Cu overlayers grown on FeO and FeO2–x. To increase the XPS signal intensity, the deposited Cu coverage is increased to 0.5 ML (Figure 3e). No shakeup satellites around 942.0 eV characteristic for Cu2+ are observed in all Cu 2p spectra, which excludes the presence of Cu2+ species.53 Further, Cu 2p BE of the Cu/FeO2–x surface is 0.3 eV lower than that of the Cu/FeO surface. Since the Cu 2p BE difference between Cu0 and Cu+ is about 0.1 eV, the 0.3 eV BE difference might derive from different interfacial charge transfers and size effects.54 Cu LVV Auger peaks are used to distinguish the valence states, confirming the coexistence of Cu0 (918.0 eV) and Cu+ (915.8 eV) species on the Cu/FeO and Cu/FeO2–x surfaces (Figure 3f).53 Deposited Cu atoms on FeO and FeO2–x films are both partially oxidized, which derives from the charge transfer at the Cu–FeOx interfaces. The proportion of Cu+ on Cu/FeO2–x is much higher, which indicates that more electrons of Cu are transferred to FeO2–x, and the Cu–FeO2–x interaction is stronger than Cu–FeO.

Theoretical Analysis of Cu on FeO/Pt(111) and FeO2–x/Pt(111)

DFT calculations were carried out to figure out the geometric structures of single-atom Cu located on FeO/Pt(111) and FeO2–x/Pt(111) surfaces and to unravel the nature of metal-oxide interactions. We first investigate the stability of Cu atoms at O-fcc, O-hcp, and O-top sites in the three domains of FeO/Pt(111) and FCC domains of FeO2–x/Pt(111) (Figure S6). The optimized configurations and the adsorption energies (Eads, see the Methods Section) for Cu adsorbed on a total of 12 sites are summarized in Table 1. In the FeO-FCC domain, the most stable Cu location is found at the O-fcc site (Figure 4a) with an adsorption energy of −2.19 eV. In the FeO-HCP domain, only O-fcc and O-top sites are obtained because the Cu atom initially placed at the O-hcp site is spontaneously relaxed to the most favorable O-fcc site (Eads = −2.10 eV). In the FeO-TOP domain, it is found that Cu located at the O-hcp site can evolve into an O-bridge site where the Cu adatom is coordinated by two oxygen atoms, whose stability (Eads = −1.94 eV) exceeds those at O-fcc and O-top sites. In the FeO2-FCC domain, Cu located at the O-fcc site is favored with the adsorption energy of −3.25 eV compared with O-hcp and O-bridge sites. Note that the Cu atom initially put at the O-top site is relaxed to the O-bridge site. Therefore, the thermodynamic stability of Cu adsorption sites on the FeO/Pt(111) and FeO2–x/Pt(111) surfaces follows an order of FeO2-FCC > FeO-FCC > FeO-HCP > FeO-TOP, as shown in Figure 4b. This calculated trend well explains the STM observation that Cu preferentially sits on FCC domains of both FeO/Pt(111) and FeO2–x/Pt(111) surfaces, and Cu interacts more strongly with the FeO2–x/Pt(111) surface than with the FeO/Pt(111) surface.

Table 1. Calculated Cu Adatom Adsorption Sites and Adsorption Energies (Eads, eV).

domain FeO-FCC FeO-HCP FeO-TOP FeO2-FCC
initial configuration O-fcc O-hcp O-top O-fcc O-hcp O-top O-fcc O-hcp O-top O-fcc O-hcp O-top
optimized configuration O-fcc O-hcp O-top O-fcc O-fcc O-top O-fcc O-bridge O-top O-fcc O-hcp O-bridge
adsorption energy –2.19 –2.05 –1.82 –1.99 –2.10 –1.73 –1.57 –1.94 –1.34 –3.25 –3.09 –3.20
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Figure 4.

Figure 4

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(a) Optimized configurations of the most stable Cu adsorption sites in the three domains of FeO/Pt(111) and the FCC domain of FeO2–x/Pt(111). Pt: dark blue; Fe: purple; surface O: orange; interface O: pink; and Cu: white. (b) Adsorption energies of Cu location on the three domains of FeO/Pt(111) and the FCC domain of FeO2–x/Pt(111). (c) Projected density of states (PDOS) for p-orbitals of surface O in the three domains of FeO/Pt(111) and the FCC domain of FeO2–x/Pt(111). Inserted values denote the positions of the p-band center (εp).

The reactivity of surface O is mainly dependent on its electronic structure, which can be reflected by the density of states (DOS) population and quantitatively described by the p-band center.55−57Figure 4c shows the p-orbitals projected density of states (PDOS) of surface O atoms in different regions of the FeOx/Pt(111). Surface O in the FeO2 domain exhibits a richer DOS population around the Fermi level compared to that of FeO, which qualitatively indicates a high reactivity of surface O in the FeO2 domain. Through quantifying the p-band center (εp) of surface O (see Methods Section for the formula), we find that surface O in FeO2-FCC possesses a higher εp (−2.37 eV) than that in FeO/Pt(111) indicating the highest interaction with Cu adsorption. Furthermore, εp values of surface O are −3.12 eV for FeO-FCC, −3.22 eV for FeO-HCP, and −3.53 eV for FeO-TOP, respectively. It is well consistent with the interaction order of surface O in different regions of FeOx/Pt(111) with the Cu adatom, i.e., FeO2-FCC > FeO-FCC > FeO-HCP > FeO-TOP. The increase of surface O reactivity in the FCC domain of the FeO film stems from the enhancement of the interfacial Fe–Pt bond strength, i.e., the shorter spatial separation between the FeO overlayer and Pt substrate and the stronger charge transfer.48 As for the FeO2 domain, the Fe–Pt bonding at the FeO/Pt interface is replaced by much stronger Fe–O binding, inducing a higher reactivity of the surface O. Therefore, we conclude that the electronic effect plays a dominant role in Cu interaction with surface O where the surface O atom with the more positive p-band center can enhance adsorption of Cu.

Discussion

Depositing Cu on the FeO/Pt(111) surface at 100 K and RT produces highly dispersed cluster arrays and aggregated linear nanostructures, respectively. At 100 K, Cu atoms have limited diffusion ability to avoid further agglomeration and reside at the lowest energy domains (FCC domains). However, at RT, Cu atoms can nucleate and aggregate at the defect sites such as domain boundaries, which indicates a weak interaction between Cu atoms and the FeO surface. As a result, the regular distribution of Cu atoms and clusters on the FeO film at 100 K is from both preferential bonding at FCC domains and limited diffusion ability at this temperature. Deposition of Cu on the FeO2–x/Pt(111) surface at both 100 K and RT produces Cu clusters with a regular spatial distribution. Most Cu atoms are observed to selectively locate on FeO2-FCC domains. A higher dispersion degree of Cu atoms on the FeO2–x film indicates the limited migration of Cu atoms over FeO2 domains and the stronger interaction between Cu atoms and FeO2, which has been validated by XPS and DFT results.

The FeO/Pt(111) and FeO2–x/Pt(111) surfaces have the similar coordination structure of surface O atoms but show significantly different interactions with Cu adatoms. They both can be used as model catalysts to understand the influence of the reactivity of surface O atoms on the metal-oxide interaction. DFT calculation results show that Cu atoms on the FeO and FeO2 surfaces all prefer to locate at the O-hollow site and thus have the same coordination environment. Therefore, the interaction between Cu atoms and FeOx mainly depends on the property of surface O atoms, and the p-band center has been used as a descriptor to directly reflect their electronic properties. As reported in previous studies, the p-band center of surface O could be used as a descriptor for the reactivity of surface O for C–H activation.56,57 Here, we find that surface O whose p-band center is closer to Fermi energy shows a higher bonding tendency with metals. As shown in Figure S7, a good and quantitative scaling relation exists between the εp and Cu–FeOx interaction. The p-band center of surface O can be used as a new descriptor for metal-oxide interactions, which is helpful for the rational design of single-atom and cluster catalysts supported on oxides.

FeO/Pt(111) films can be used as templates for the construction of well-defined Cu cluster arrays only at 100 K, while deposition of Cu atoms on FeO2–x/Pt(111) at 100 K and RT can form regular metal arrays. In addition, we find that Ce atoms are randomly distributed on the FeO film but form ordered cluster arrays on the FeO2–x films at 100 K and RT (Figure S8). Due to the low reactivity of surface O of FeO, most of the metals have weak interactions with the FeO film and present low thermal stability, which results in the formation of aggregated nanostructures. Furthermore, the difference in the interaction between metals and different domains of the FeO film is small. Therefore, only Cu and Au cluster arrays with regular distribution form on the FeO film at low temperatures. In contrast, the FeO2 domain with high surface O reactivity can have strong interactions with more metal atoms such as Ce resulting in highly dispersed metal clusters with relatively high thermal stability. In addition, the surface reactivity of the FeO2 domain and surrounding FeO domain has a sufficiently large difference such that many metals have the tendency to locate at the FeO2 domains forming periodic arrays. Therefore, the FeO2–x film might be a more universal template for the construction of well-ordered metal cluster arrays in contrast with the FeO film.

Conclusions

FeO/Pt(111) and FeO2–x/Pt(111) surfaces with moiré patterns are used as templates for the construction of well-ordered Cu cluster arrays at 100 K. Cu atoms deposited on FeO at RT aggregate significantly forming linear nanostructures, while the regular distribution of clusters can be observed after depositing Cu on FeO2–x at RT. DFT calculation results show that Cu atoms prefer to locate at O-fcc sites of FCC domains of FeO and FeO2–x. The adsorption strength of a single Cu atom on FeO/Pt(111) and FeO2–x/Pt(111) follows an order of FeO2-FCC > FeO-FCC > FeO-HCP > FeO-TOP, which can be correlated with the p-band center of the surface O. Therefore, the stronger interaction between Cu atoms and FeO2 results from the higher surface O reactivity as described by the p-band center.

Acknowledgments

This work was financially supported by the National Key R&D Program of China (2021YFA1502800), National Natural Science Foundation of China (Nos. 21825203, 22288201, and 91945302), Photon Science Center for Carbon Neutrality, and Liaoning Revitalization Talents Program (XLYC1902117).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.2c00580.

  • Extended results, including STM images, XPS data, and DFT models (PDF)

Author Contributions

X.L., Y.N., and Q.F. conceived and designed the experiments; X.L. and Z.Y. carried out the preparation of experimental samples and STM and XPS characterizations; X.S. and L.L. carried out DFT calculations; and X.L. and X.S. drafted the original manuscript; All authors, including X.L., X.S., Z.Y., L.L., Y.N., Q.F., and X.B., contributed to the analysis, interpretation, discussion of results, and the revisions of the manuscript. CRediT: Xuda Luo data curation, formal analysis, investigation, methodology, writing-original draft, writing-review & editing; Xiaoyuan Sun data curation, formal analysis, investigation, writing-original draft; Zhiyu Yi data curation, formal analysis, investigation, writing-review & editing; Le Lin formal analysis, methodology, writing-original draft, writing-review & editing; Yanxiao Ning conceptualization, formal analysis, investigation, supervision, writing-original draft, writing-review & editing; Qiang Fu conceptualization, funding acquisition, methodology, project administration, supervision, writing-review & editing; Xinhe Bao conceptualization, funding acquisition, project administration, writing-review & editing.

The authors declare no competing financial interest.

Supplementary Material

au2c00580_si_001.pdf (992.6KB, pdf)

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Supplementary Materials

au2c00580_si_001.pdf (992.6KB, pdf)

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