Structure and Electronic Properties of Interface-Confined Oxide

Oct 16, 2017 - The controlled fabrication of nanostructures has often used a substrate template to mediate and control the growth kinetics. Electronic...
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The Structure and Electronic Properties of Interface-Confined Oxide Nanostructures Yun Liu, Yanxiao Ning, Liang Yu, Zhiwen Zhou, Qingfei Liu, Yi Zhang, Hao Chen, Jianping Xiao, Ping Liu, Fan Yang, and Xinhe Bao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06164 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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The Structure and Electronic Properties of Interface-Confined Oxide Nanostructures Yun Liu1†, Yanxiao Ning1†, Liang Yu1, Zhiwen Zhou1,2, Qingfei Liu1,2, Yi Zhang1,2, Hao Chen1,2, Jianping Xiao1, Ping Liu3, Fan Yang1*, Xinhe Bao1* Affiliations: 1

State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. 2

University of Chinese Academy of Sciences, Beijing 100049, China.

3

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States. *Corresponding authors. E-mail: [email protected] (F.Y.); [email protected] (X.B.) †These authors contributed equally to this work.

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Abstract The controlled fabrication of nanostructures has often used a substrate template to mediate and control the growth kinetics. Electronic substrate-mediated interactions have been demonstrated to guide the assembly of organic molecules or the nucleation of metal atoms, but usually at cryogenic temperatures, where the diffusion has been limited. Combining STM, STS and DFT studies, we report the strong electronic interaction between transition metals and oxides could indeed govern the growth of low-dimensional oxide nanostructures. As a demonstration, a series of FeO triangles, which are of the same structure and electronic properties, but with different sizes (side length > 3 nm), are synthesized on Pt(111). The strong interfacial interaction confines the growth of FeO nanostructures, leading to a discrete size distribution and a uniform step structure. Given the same interfacial configuration, as-grown FeO nanostructures expose not only identical edge/surface structure, but also exhibit the same electronic properties, as manifested by the local density of states and local work functions. We expect the interfacial confinement effect can be generally applied to control the growth of oxide nanostructures on transition metal surfaces. These oxide nanostructures of the same structure and electronic properties are excellent models for studies of nanoscale effects and applications. Keywords: FeO nanostructures, scanning tunneling microscopy, strong metal-oxide interaction, interfacial confinement, local work function, local density of states

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The physical and chemical properties of nanostructures depend on their size, shape and morphology. The controlled fabrication of ordered metal and semiconductor nanocrystals at surfaces, however, remains a difficult challenge. Often time, a corrugated surface template with periodic patterns is required to inhibit the diffusion of surface adatoms, such that the shape and size of nanocrystals could be controlled. For instance, a metal substrate with well-defined dislocation patterns,1 or more frequently used lately, the buckled two-dimensional material surfaces2 are good templates for the nucleation of nanostructures. On planar metal surfaces, electronic substrate-mediated interactions have also been reported to guide the assembly of organic molecules, or to mediate the nucleation of metal atoms at cryogenic temperatures.3 Yet, the dominance of electronic interaction in the growth of supported nanostructure could reach beyond low temperatures, where the nucleation is restricted by the limited diffusivity of surface atoms. We report here a strong electronic interaction in guiding the growth, namely the shape, structure and size distribution of supported nanostructures, and rendering the formation of well-defined nanocrystals with identical shape and structures, which are stable at room temperature or above. The strong interaction between late transition metals and reducible metal oxides has been known in catalysis, and found to alter the catalytic performance of metal catalysts.4,5 Since Georg-Maria Schwab proposed the inverse catalyst configuration,6 i.e. oxide nanostructures or thin films grown on metal surfaces, for the study of metal-oxide interaction, inverse catalysts have been widely used to investigate the interfacial chemistry at the molecular level,7–9 and demonstrated excellent catalytic performances in a number of reactions, such as water-gas shift,7,8 CO oxidation8,10 and CO2

conversion.11

For

instance,

FeO

nanostructures

exposing

coordinatively-unsaturated ferrous (CUF) sites confined on Pt, which are susceptible to oxidation to form FeO2 at 450 K,12–15 could be stabilized by CO/H2 on Pt and have shown excellent performance for the preferential oxidation of CO in excess H2 at room temperature.16–20 As the importance of the metal-oxide interface has been increasingly recognized in catalysis, the nature of interfacial interaction in tuning the structure and electronic properties of supported nanostructures remains to be explored

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and requires in-depth investigation. Particularly, the controlled fabrication of the metal-oxide

interface

with

well-defined

structures

and

their

atom-level

characterization have imposed a major challenge for the understanding of interfacial catalysis and chemistry.9,21,22 In this study, using FeO nanostructures supported on Pt(111) as an example, we explored the role of interfacial interaction in tuning the structural and electronic properties of supported two-dimensional (2D) nanostructures. FeO (wüstite) is a metastable iron oxide phase, but often desired for energy applications23,24 owing to its rich defect chemistry.12,13,25 Our previous work has shown FeO nanostructures could be stabilized on Pt(111) for room-temperature CO oxidation.16,19 Here, a series of different-sized FeO NSs were synthesized on Pt(111), with their geometric and electronic properties measured by scanning tunneling microscopy and spectroscopy (STM/STS). Our study demonstrates the dominance of metal-oxide interaction in determining the properties of supported nanostructures, which provides not only a route for the controlled fabrication of supported 2D structures, but also an understanding for tuning the geometric and electronic properties of oxide nanostructures. Results and discussions Structure of FeO nanostructures FeO nanostructures (NSs) were prepared on the Pt(111) surface, following the recipe described in the experimental section.25,26 Figure 1a shows FeO NSs are well dispersed on Pt(111) and exhibit the polar-bilayer structure of FeO(111). The shape of FeO NSs are typically triangles, truncated triangles or their coalesced form. The triangular FeO NSs are predominantly equilateral. Here, we define the sizes of these FeO NSs using equivalent diameters (d=2ඥܵ/ߨ), where S is the surface area of FeO NS. By analyzing the structure of these different-sized FeO NSs, we found the size distribution and structure of FeO NSs are indeed determined by the strong interaction between FeO and Pt, i.e. an interface-confined growth of FeO NSs.

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We analyze first the surface structure of FeO NSs. Figure 1c displays the structural model of FeO(111) on Pt(111). Due to the lattice mismatch between FeO and Pt(111), Fe atoms occupy successively fcc, hcp and top sites on Pt(111), rendering the formation of periodically arranged fcc-, hcp- and top- FeO domains. In STM, these three types of FeO domains could be distinguished with their different apparent heights, which were modulated by the locations of FeO on Pt(111). The identification of FeO domains in STM topography has also been analyzed previously, by combining STM images taken in the field-emission regime and DFT simulations.27,28 In the field-emission regime, STM images display huge contrast among different FeO domains, which is caused by the differences in local work functions (LWFs) and less influenced by the tip state. Using STM images taken in the field-emission regime as a reference.25,27–29 we could assign FeO domains in regular STM image (Figure 1d), obtained at near the Fermi level (EF). From Figure 1e, top-FeO domains with the lowest LWF appear as the brightest protrusions, while fcc-FeO domains with the highest LWF are depicted as the darkest depressions, which is consistent with previous studies.27–29 Correspondingly, hcp-FeO domains in STM image taken at near EF (Figure 1d) were found to exhibit the highest apparent height, whereas top-FeO domains display the lowest apparent height in the regular STM image. The periodicity of the moiré pattern, or the lattice vector of the moiré supercell, is ~ 25 Å, corresponding to the coincidence lattice between FeO and Pt(111) [8×d(Fe-Fe) ≈ 9×d(Pt-Pt)]. The average lattice spacing of FeO NSs, as measured from STM, is 3.10 Å, same as that of the FeO film on Pt(111) .28,30,31 With the above domain assignment, one could observe that FeO triangles with the side length below 4 nm (Figure 2a) consist of only fcc-FeO and hcp-FeO domains. As FeO NSs have a side length longer than 4 nm (Figure 2b, c), these NSs will have all three types of FeO domains, with top-FeO domains being surrounded by fcc-FeO and hcp-FeO domains. Interestingly, all corners and edges of FeO NSs display the apparent height of fcc-FeO domains, while top-FeO domains are always at the center, indicating the structure of FeO NSs is controlled by the different stabilities of these

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domains. Previous theoretical studies on the FeO film on Pt(111) have shown that fcc-FeO domains are thermodynamically most stable, while top-FeO domains are the least stable.29 Thus, with all edges located in the fcc-FeO domains could help stabilize the edges of FeO NSs and lower the total energy of FeO NSs. We then look into the edge structure of FeO NSs. The edges of as prepared FeO NSs typically expose two-coordinated Fe or O atoms, which were also termed as coordinatively-unsaturated Fe (CUF) or O (CUO) sites.25,28,29 For a hexagonal FeO island, CUF sites would alternate with CUO sites as the edge termination (Figure 1b). The (truncated) triangular shape of FeO NSs in Figure 1a indeed implies the preference of one edge structure, which is the CUF edge as revealed by high resolution STM images (Figure 2). FeO triangles, all edges are uniformly CUF-terminated, which is independent to the size of FeO NSs. The CUF edge appears curvy in STM with periodic indentations, which are caused by an electronic effect originated from the different locations of Fe atoms on Pt(111). Element-specific STM images have shown that there are no missing Fe or O atoms along the edge.25 However, the coincidence lattice [8×d(Fe-Fe) ≈ 9×d(Pt-Pt)] dictates that CUF atoms would also shift gradually and periodically, from three-fold hollow sites to top sites of Pt. DFT simulation shows that CUF atoms at top sites would contract very slightly towards the fcc sites of Pt(111), which is not obvious from the optimized structural model(Figure 2e). Rather, a drastic reduction in the electronic states near EF for top-Fe atoms cause their much-lowered brightness in STM (Figure 2f). The periodic indentation in the edge is thus caused by CUF atoms at near top sites repeated every super lattice. As the coincidence lattice determines the moiré pattern, it affects also the edge structure of FeO NSs.

Size distribution of triangular FeO NSs The size distribution of triangular FeO NSs is also modulated by the coincidence lattice between FeO and Pt(111). We examined 267 FeO triangles with atomic resolution and plotted their size distribution in Figure 3. Here, the number of CUF atoms along each edge of the equilateral FeO triangle is termed as n, the

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stoichiometry of FexOy triangles could be calculated as

x = n(n + 1) / 2

and

y = n ( n − 1) / 2 . Figure 4 displays a series of FeO triangles, with their side lengths

ranging from n = 10 to n = 14 (n = 8, 9, 15, 16 were not observed). These FeO triangles display the same atomic structure and consist of only two types of domains, with the hcp-FeO domain at the center and fcc-FeO domains at the edge. The structural models (Figure 4) also show that the vertex of FeO triangles are always approximately at the fcc sites of Pt. For Fe66O55 (n = 11), the vertex could locate at exactly the fcc sites with minimal lattice contraction (Figure 2). To understand the stabilities of edge and vertex Fe atoms at different locations of Pt(111), we used an Fe6O3 cluster, whose Fe atoms are all at edges and the vertex, to compare the total energies of FeO at the fcc, hcp or top sites of Pt(111). DFT calculations show that Fe6O3 at fcc sites of Pt gives a total energy 0.52 eV lower than that of Fe6O3 at hcp sites of Pt, whereas Fe6O3 on top sites of Pt is thermodynamically unstable. Therefore, FeO NSs always have fcc-FeO domains at the edge and the vertex, since these positions are less stable and need be located on the most stable sites of Pt(111). Particularly, the stabilization of vertex Fe atoms is key to maintain the triangular shape of FeO NSs. Figure 5a shows that the formation energies of Fe6O3 is so sensitive to its locations on Pt(111) that a 0.12 nm shift away from the perfect fcc sites could raise the formation energy by 0.47 eV/Fe. That means to locate the vertex of FeO NSs at fcc sites could bring major energy gains, while continuous increase of edge Fe atoms would unavoidably put the vertex Fe atoms onto the unstable top sites (Figure 5c). As such, FeO triangles with n = 8, 9, 15, 16 are not observed in our study. On the other hand, FeO triangles with vertex Fe atoms at the fcc sites of Pt could all be observed with Fe66O55 (n = 11) exhibiting the largest population in the histogram, since its vertex Fe atoms could locate at exactly the fcc sites with minimal lattice distortion (Figure 4). The histogram of FeO triangles shows three maxima peaks at n = 11, 19 and 27(Figure 3), which is an arithmetic progression with a common difference of 8. That means, the size difference between neighboring maxima peaks (8 × d(Fe-Fe))

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corresponds to the size of the coincidence lattice between FeO and Pt(111). The histogram of FeO triangles suggests that the most stable (and thus populated) FeO triangles always have their vertex Fe atoms at the fcc sites of Pt(111) and the population of FeO triangles depends on the stability of their vertex Fe atoms. Other FeO triangles (e.g., n = 15, 16 and 22~26) would lose their corners to prevent the vertex Fe atoms at the top sites of Pt(111), which are highly unstable. Thus, a discrete distribution of the side lengths of FeO NSs could be observed, which was modulated by the coincidence lattice between FeO and Pt(111). The preferred locations of edge and corner Fe atoms at the fcc sites of Pt(111) determine the discrete size distribution of FeO. At its heart, the strong electronic interaction between FeO and Pt(111) dictates the edge energy of FeO NSs, which subsequently determines the preferred locations, size and domain composition for FeO NSs, and thus a templated growth on Pt(111).

Local work functions and local density of states of supported FeO NSs We then proceed to examine local electronic properties of FeO NSs on Pt(111). Although STM topography has demonstrated the geometric structure of FeO NSs is insensitive to size, local electronic measurements could provide more information regarding the interaction between FeO and Pt, as well as the geometric environment, since small structural variations could lead to drastic changes in local electronic properties. Meanwhile, LWFs and local density of states (LDOS) are usually influenced by interfacial charge transfer, which are of center importance in understanding the chemical properties of supported NSs.32–35 To obtain LWFs, scanning tunneling spectroscopy (STS) was taken in closed loop under field-emission resonance (FER) conditions. As the bias voltage exceeds the work function of the sample, excited electrons enter the vacuum gap between STM tip and the sample, and are influenced by the surface potential of the substrate. The series of bound image states could be measured by FER-STS and manifested as a series of standing wave peaks in the spectra.32,36 The energy positions of low-lying resonance peaks of FER-STS are linear to surface potentials and could be compared to give the

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differences in LWFs.37–39 Figure 6a shows FER-STS spectra taken on the different domains of an FeO NS. LWFs increased in the order of top-FeO (4.2 eV) < hcp-FeO (4.5 eV) < fcc-FeO (5.1eV), which is consistent with DFT calculations.27,40,41 Note that, for a finite-sized tip, the lack of lateral resolution in FER causes the measurement of LWFs being influenced by neighboring FeO domains with lower work function and stronger FER intensity. The low-lying resonance peaks could have mixed signals from neighboring regions with strong intensity.34 Specifically, FER-STS on the hcp-FeO domain would display a 4.2 eV shoulder peak from neighboring top-FeO domains, because the low-lying resonance peak for top-FeO domains (4.2 eV) has the strongest intensity. Meanwhile, FER-STS on fcc-FeO domains would show two shoulder peaks from the neighboring top-FeO and hcp-FeO domains. Similar FER-STS spectra have also been observed in the measurements on the 1 ML FeO film on Pt(111).34 To illustrate the evolution of LWFs across the FeO surface, a series of dI/dV spectra were recorded along the line in Figure 6b and the color-coded dI/dV spectra (Figure 6c) show clearly that the evolution of FER peaks matches with the switch of FeO domains. At the center of the FeO NS, top-FeO domain gave the strongest FER peak at 4.2 eV, which shifts gradually to 4.5 eV on hcp-FeO domain with decreasing intensity. The energy position of the lowest FER peak on Pt(111) is 6.0 eV, which is 0.1 eV higher than the actual work function of Pt(111) and the difference could be attributed to the tip-induced Stark shift.36,39 Thus, FER-STS spectra give not only LWFs, but also the positions of FeO on Pt(111). Interestingly, the vertex and edges of the FeO NS, located at the fcc-FeO domain, show only a broad and weak FER peak at ~4.2 eV (Figure 6c and Figure 7a, b), which is much lower than the 5.1 eV peak regularly observed for fcc-FeO inside the FeO NSs (Figure 6a). The lowered LWF for fcc-FeO domains at the edge could be attributed to the excess electron density of CUF atoms that reduces the dipole of FeO.21,37,42 LWF on different-sized FeO triangles and other FeO NSs were further compared by FER-STS (Figure 7). LWFs were measured at the top-FeO, hcp-FeO and fcc-FeO domains of these FeO NSs, as well as their vertex and edge sites. We found that LWF

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is not sensitive to the size or shape of FeO NSs, but dependent only on the geometric environment of FeO, i.e. the coordination number and the location of Fe atoms on Pt(111). In other words, LWF of FeO is dominated by the interfacial structure between FeO and Pt(111), rather than the size of FeO NSs. LDOS measurements were also carried out on a series of FeO NSs (Figure 8). Most STS spectra on the surface terraces of FeO NSs are almost featureless near EF except for a peak at ~ 0.5 eV above EF on the top-FeO domain. Previous studies have assigned this empty state to the contributions from dz2 orbitals of Fe,27,43 which is enhanced for top-Fe atoms due to the strong orbital hybridization between Pt and top-Fe in the z direction.43 Along the edges of FeO NSs, STS spectra are also mostly featureless, except at where the edge appears curvy (Figure 8a). As described in the structure of FeO NSs (Figure 2e), CUF atoms at the curvy part of step edge are near top sites of Pt(111), which thus display a similar STS spectra. The spatial distribution of the LDOS peak (Fe-3dz2) on different-sized FeO triangles are illustrated in Figure 8. It is clear that only the curvy part of the edge sites and top-FeO domains show a resonance peak at around + 0.5 eV. Note that, an empty state at ~ 1.5 eV above EF was also observed at the edge and vertex (Figure 8b), as well as the fcc-FeO domains of FeO NSs. Merte et al.28 reported that fcc-FeO domains show dominant features at > 1.2 eV above EF in the simulated partial DOS of O-2pz orbitals. Thus, the empty states at ~ 1.5 eV may originate from O atoms at the vertex and edge sites of FeO NSs. More importantly, the energy positions of LDOS peaks are approximately the same for different-sized FeO NSs (Figure 8b). Furthermore, both FeO triangles and FeO hexagons display the same LDOS peaks (Fe-3dz2) as long as the coordination and location of Fe atoms on Pt(111) are the same in these NSs (Figure 8 and Figure 9). Similar to the LWF measurements, the size or shape of FeO NSs (d > 3 nm) have no influence on the spatial distributions of electronic states near EF. Rather, the interfacial configurations of Fe-O-Pt determine the electronic properties of FeO NSs confined on Pt(111). Interface confined FeO NSs on Pt(111) We have now demonstrated that the growth of FeO NSs is dictated by the strong

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interaction between FeO and Pt(111), which causes the surface and edge structure, as well local electronic properties such as LWF and LDOS, are the same for FeO NSs of different sizes (d > 3 nm) and shapes. Our previous DFT calculations have shown that the interfacial binding energy between the FeO film and Pt(111) is approximately 1.40 eV per FeO. The interfacial binding would vary with the positions of Fe and O atoms on Pt(111), which also restrict the growth of FeO NSs to have all CUF edges located at the fcc-FeO domains. As such, a discrete size distribution of FeO triangles was observed and the same interfacial binding configurations at the FeO edges also led to their same electronic properties for chemical reaction. In comparison, we cite another set of well-defined NSs that has been well-studied, i.e. MoS2 triangles on Au(111).44–48 Like FeO NSs, MoS2 NSs also tend to grow the triangular shape on Au(111). However, within the same size regime as FeO NSs, MoS2 triangles show clearly the size-dependent edge structure. When the number of Mo atoms at each edge (n(Mo)) is smaller than 6, MoS2 triangles expose S-terminated edges. When n(Mo) > 6, MoS2 triangles would expose Mo-terminated edges. Unlike the discrete distribution of FeO triangles, the size distribution of MoS2 triangles is continuous in the size range of n(Mo) = 2~8.45,46 DFT calculations have shown that the interfacial binding energy between the MoS2 film and Au(111) is only 0.03~0.25 eV per MoS2.44,47 With a relatively weak interfacial interaction, the intrinsic structural and electronic properties of MoS2 were preserved and exhibit a strong size-dependent behavior.47,48 In contrast, the strong interfacial interaction confines the growth of FeO on Pt(111) and renders the size-independent properties of FeO NSs, which are mainly sensitive to the interfacial bonding configurations of Pt-Fe-O. One could thus utilize the strong interfacial interaction to fabricate nano-systems of identical properties. Conclusion In summary, we demonstrated a confined growth of supported FeO NSs dictated by the strong electronic interaction between FeO and Pt(111). The as-grown FeO NSs with sizes larger than 3 nm exhibit not only the same structure, but also dominantly a

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CUF-terminated step structure (CUF-step). Interfacial confinement helps the stabilization of CUF-step, which in turn tunes the length of steps to match with the coincidence lattice between FeO and Pt(111), such that most CUF atoms could locate at the most stable fcc sites of Pt(111). As a result, a discrete distribution of the step lengths and thus the NS sizes are observed for supported FeO NSs. We studied further the series of FeO triangles, which expose exclusively the CUF-step. From LDOS and LWF measurements, we found that local electronic structures of FeO NSs are not dependent to their size or shape, but rather determined by (and very sensitive to) their locations on Pt(111). Therefore, the geometric structure and electronic properties of FeO NSs are indeed governed by interfacial confinement, rendering FeO NSs with uniform CUF steps, which are particularly suitable for model catalytic studies. This study thus provided a strategy to utilize the strong electronic interaction between metal and oxides nanostructures to synthesize supported nanostructures with controlled properties. Since strong metal-oxide interaction has often been noted in catalysis, we expect such interface-confined growth could be general for the growth supported oxide NSs on transition metal surfaces.

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Experiment and Computational Methods The experiments were carried out in a combined ultrahigh vacuum (UHV) system equipped with low-temperature scanning tunneling microscope (LT-STM, Createc), XPS, UPS and the cleaning facilities. Base pressures of STM and preparation chamber are