CO and H2 Activation over g-ZnO Layers and w-ZnO(0001) - ACS

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CO and H2 activation over g-ZnO layers and w-ZnO(0001) Hao Chen, Le Lin, Yifan Li, Rui Wang, Zhongmiao Gong, Yi Cui, Yangsheng Li, Yun Liu, Xinfei Zhao, Wugen Huang, Qiang Fu, Fan Yang, and Xinhe Bao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03687 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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CO and H2 activation over g-ZnO layers and w-ZnO(0001) Hao Chen,a, b† Le Lin,c† Yifan Li,d† Rui Wang,e, f Zhongmiao Gong,e Yi Cui,e YangshengLi,a, b Yun Liu,a Xinfei Zhao,a, b Wugen Huang,a, b Qiang Fu,a Fan Yang,a* Xinhe Baoa* a

State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience,

iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China b University c

of Chinese Academy of Sciences, Beijing 100049, China

School of Physical Science and Technology, ShanghaiTech University, Shanghai

201210, China d

Department of Chemical Physics, University of Science and Technology of China,

Hefei 230026, China e

Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and

Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou 215123, China f

Nano Science and Technology Institute, University of Science and Technology of

China, Suzhou 215123, P. R. China.

KEYWORDS: ZnO, CO oxidation, NAP-STM, XPS, oxygen vacancy, surface polarity

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ABSTRACT Graphene-like ZnO (g-ZnO) nanostructures (NSs) and thin films were prepared on Au(111) and their reactivities towards CO and H2 were compared with that of wurtzite ZnO (w-ZnO) (0001) single crystal. The interaction and reaction between CO/H2 and the different types of ZnO surfaces were studied using near-ambient-pressure scanning tunneling microscopy (NAP-STM), X-ray photoelectron spectroscopy (XPS) and Density functional theory (DFT) calculations. The reactivity of the w-ZnO(0001) surface towards CO and H2 was found to be more prominent than those on the surfaces of g-ZnO/Au(111). CO oxidation took place primarily at the edge sites of w-ZnO(0001) and the interface between g-ZnO NSs and Au(111), while g-ZnO thin films on Au(111) appeared inert at below 600 K. Similarly, the w-ZnO(0001) surface could dissociate H2 at 300 K, accompanied by a substantial surface reconstruction, while g-ZnO on Au(111) appeared inert for H2 activation at 300 K. DFT calculations showed that the reactivities of ZnO surfaces towards CO could be related to the formation energy of oxygen vacancy (EOvf), which could be related to the charge transfer to lattice oxygen atoms or surface polarity.


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INTRODUCTION Zinc oxide, ZnO, has received tremendous interest because of its wide applications in semiconductor1, optical2 and chemical industries3. ZnO-based catalysts have been used in a number of industrial applications, such as the water gas shift reaction4-6 and methanol synthesis7-8 and increasingly recognized for novel catalytic processes, such as the direct synthesis of light olefins from syngas9-10. For methanol synthesis, ZnO is considered as a major catalytic component in the Cu/ZnO/Al2O3 catalyst for the hydrogenation of carbon monoxide and carbon dioxide. But the role of ZnO has not been clear and has caught tremendous attention in current research3, 7, 11. In syngas conversion, Jiao et al.9 reported recently a direct and highly selective process by using nano-composite catalysts comprising of metal oxides and zeolites. In this nanocomposite catalyst, ZnO was employed as a main oxide component for the activation of CO and H2. Yet, the mechanism of CO and H2 activation remains to be explored. To address the catalytic role of ZnO, it is essential to study the interaction between ZnO and reactants, such as CO, CO2 and H2, on well-defined model catalysts. Currently, model ZnO catalysts could be prepared via the growth of ZnO layers on metal single crystals or through the usage of ZnO single crystals exposing different surface planes. The synthesis of crystalline ZnO layers have been reported on a number of planar metal surfaces, such as Pd(111)12, Pt(111)13-14, Au(111)15-17, Ag(111)18-19, Cu(111)18 and Brass(111)20. These crystalline ZnO thin films are stoichiometric, with the surface structure similar to the (0001) facet of wurtzite ZnO, but become flattened and thus expose a non-polar graphene-like ZnO (g-ZnO) surface. The wurtzite structure


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is the most stable phase of ZnO under ambient conditions. Wurtzite ZnO (w-ZnO) (0001) is also a typical polar oxide surface and exhibits reconstructed surface planes with triangular islands and holes to compensate for the extra surface charge21-22. The structural characters of the reconstructed w-ZnO(0001) surface have not been clear at the atomic level. Previous studies on ZnO layers supported on Pt(111)14 and Pd(111)12 have suggested that the coplanar structure of g-ZnO(0001) layers would turn into the wurtzite structure of w-ZnO(0001) when the number of ZnO layers has surpassed a critical thickness23-25. It is thus interesting to compare the reactivity of the g-ZnO(0001) layers with that of the w-ZnO(0001) surface, owing to their structural similarity. In this article, we studied the interaction and reaction between CO/H2 and the surfaces of g-ZnO(0001) on Au(111) and w-ZnO(0001), using near-ambient-pressure scanning tunneling microscopy (NAP-STM), X-ray photoelectron spectroscopy (XPS, including NAP-XPS) and density functional theory (DFT) calculations. Our study analyzed the structures of g-ZnO and w-ZnO, as well as their reactivity differences towards CO and H2. We show that surface polarity could be used as a descriptor for the reactivity of ZnO structures.

EXPERIMENTAL SECTION The experiments were carried out in three ultrahigh vacuum (UHV) systems. The first system is equipped with a NAP-STM26-27 (SPECS, base pressure < 3×10−10 mbar), mass spectroscopy (Hiden), and cleaning facilities. The second system is equipped with XPS (SPECS, Al Kα source, base pressure < 5×10−10 mbar), a high temperature and high pressure reactor with inner Au coating and cleaning facilities. The third system is


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equipped with NAP-XPS (SPECS, base pressure < 5 × 10-10 mbar) with a monochromatic Al Kα source. The exposure to reaction gas is done by backfilling the chamber. The Au (111) single crystal was cleaned by cycles of Ar ion sputtering (1 keV, 10 μA) and UHV annealing at ~800 K. The w-ZnO(0001) single crystal (MTI) was cleaned by cycles of Ar ion sputtering (1 keV, 10 μA) and UHV annealing at ~1000 K. ZnO layers were prepared on Au(111) by evaporating Zn atoms in the presence of ~1×10−7 mbar NO2 at various substrate temperatures. NO2 molecules were dosed through the stainless steel tube directed at the surface of Au(111). The surface coverage of ZnO was estimated from deposition time. STM images were acquired using Pt/Ir tips with the bias voltage (VS) applied to the sample. STM images were processed with SPIP software from Image Metrology, Denmark. XPS spectra were analyzed by CasaXPS software with a Shirley background subtraction and the 70/30 Gaussian–Lorentzian fits. Zn 2p3/2 and O 1s XP spectra was used for the calculation of the Zn:O ratio. From STM and XPS, the surface of pristine BL g-ZnO is almost defect-free and peaks of Zn 2p3/2 and O 1s are highly symmetric. So, we considered that pristine g-ZnO are stoichiometric. In ex-situ XPS studies, the exposure of g-ZnO to high pressure gas led to the formation of a shoulder O 1s peak at ~532.0 eV, which could be attributed to water adsorption from background during high pressure experiments16. Thus, peak deconvolution was used to separate the influence of shoulder O 1s peak and only the peak area of lattice O 1s was used to calculate the Zn:O ratio after CO exposure. The analysis of XPS results was displayed in Table S1 and Table S2. For the comparison of the Zn:O ratio, peak areas of Zn 2p3/2 and O 1s after


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CO exposure at elevated temperatures were all normalized to their peak areas after CO exposure at 300 K since there was no CO oxidation at 300 K for g-ZnO. DFT calculations were carried out based on the Vienna Ab-initio Simulation Packages (VASP 5.4)28-32. The exchange-correlation potential effect was treated by the Perdew-Burke-Ernzerhof (PBE)33 at a level of the Generalized Gradient Approximation (GGA)34. The projected-augmented wave (PAW)35 pseudopotentials were utilized to describe the core electrons, and a plane-wave cutoff kinetic energy was set to 400 eV. In addition, the van der Waals (vdW) dispersion forces were corrected using zero damping DFT-D3 method of Grimme36 to account for the long-range interactions in this system. An on-site Coulomb repulsion interaction, the effective Ueff = U – J, was set at 4.7 eV for Zn 3d orbitals37-41. Overall, spin-polarized effect was considered. For BL g-ZnO film on Au(111), the atomic structures included a (7×7) ZnO(0001) overlayer on a (8×8) Au(111) slab with three atomic layers (with the top layer relaxed)15. Here, ZnO layers were slightly expanded with lattice mismatch 2.6%. A gamma point was specified to sample the surface Brillouin zone. BL g-ZnO NS was constructed by cutting the BL g-ZnO film to expose the step. For comparison, the w-ZnO(0001) system21, 42 was also constructed, where a (4×4)-ZnO(0001) supercell was employed with six slabs and the bottom two layers were frozen and the top layer was partially resected. A (2×2×1) Gamma centered grid was used for the k-point sampling.

RESULTS AND DISCUSSION Surface structures of g-ZnO(0001) and w-ZnO(0001)


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The growth and structures of ZnO NSs and thin films on Au(111) were displayed in Figure 1. ZnO NSs were prepared by evaporating Zn atoms onto Au(111) in the NO2 atmosphere. Consistent with literature reports, ZnO layers on Au(111) exhibit a flattened g-ZnO surface15, 43. When ZnO was deposited at ~450 K in NO2 and flashed at ~450 K in UHV, crystalline g-ZnO NSs of both monolayer (ML) and bilayer (BL) thickness could be observed on the surface of Au(111) (Figure 1a). Owing to the lattice mismatch between g-ZnO and Au(111), a moiré pattern with the lattice spacing of ~23 Å (Figure 1b) could be observed on g-ZnO. Since the lattice spacing of Zn or O atoms is ~3.3 Å (Figure 1c) in g-ZnO NSs, a coincidence lattice was suggested to form between the (7 × 7)-ZnO supercell and the (8 × 8)-Au(111) substrate (Figure S1a), assuming the ZnO lattice is well aligned with the major index of Au(111). The apparent height of ML ZnO NSs is ~1.8 Å and the apparent height of BL ZnO NSs is ~3.3 Å. When ZnO was deposited at 300 K and annealed at 600 K in NO2, one would observe dominantly BL g-ZnO NSs with roundish shapes on the Au(111) surface (Figure 1d). The atomic lattice of BL g-ZnO NSs was displayed in Figure 1e, which showed the same lattice constant of g-ZnO as that of ML g-ZnO NSs (Figure 1c). Note that, the growth of g-ZnO at elevated temperatures could cause embedded BL and ML g-ZnO NSs (Figure S1b-e), which were formed by the oxidation of AuZn surface alloy and could be differentiated by their corresponding apparent heights (Figure S1f). STM also showed the preferred growth of ZnO NSs on the fcc domains of Au(111), which caused the reconstruction of surface herringbones to surround ZnO NSs (Figure S1b). As the coverage of ZnO increases on Au(111), ZnO thin films covering the entire surface of


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Au(111) could be prepared. Figure 1f shows the g-ZnO thin film of ~3.7 ML thickness on Au(111).

Figure 1. The structures of g-ZnO NSs and layers on Au(111). (a-c) STM images of g-ZnO NSs prepared by depositing Zn atoms in 1×10-7 mbar NO2 with the substrate temperature of Au(111) held at ~450 K. (a) Large-scale STM image showing both ML and BL g-ZnO NSs on the Au terrace. (b-c) STM images showing (b) moiré structure (with super cell marked by the rhombus) and (c) atomic lattice of ML ZnO NS. Inset shows the structural model of ML g-ZnO NS (color representations: yellow-Au; red-Zn; grey-O). (d-f) STM images of g-ZnO NSs prepared by depositing Zn atoms in 1×10-7 mbar NO2 with the substrate temperature of Au(111) held at 300 K. (d) Large-scale STM image showing only BL g-ZnO NSs on the Au terrace. Inset shows the structural model of BL g-ZnO NS (color representations: yellow-Au; red-Zn; grey-O). (e) Highresolution STM image of BL g-ZnO NS. (f) STM image of g-ZnO thin film with layer thickness of ~3.7 ML on Au(111). Scanning parameters: (c) Vs= +0.30 V, It= 0.23 nA；(e) Vs= +1.4 V, It= 0.35 nA.

w-ZnO crystals along the (0001) direction typically expose the polar surfaces of Zn-terminated ZnO(0001) or O-terminated ZnO(000-1), where the accumulation of dipole moments causes the divergence of surface energy. As a polarity compensation mechanism, the surface of ZnO(0001) undergoes a reconstruction to exhibit triangular surface terraces and straight step edges (Figure 2a). The structural model used to


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simulate the w-ZnO(0001) surface was displayed in Figure 2b. The large density of triangular pits have been suggested as Zn vacancies, which maintained the charge neutrality of the ZnO(0001) surface21. In comparison, ZnO(0001) layers grown on metal substrates eliminates its surface polarity by the reconstruction to a planar graphene-like structure. While the transition from g-ZnO to w-ZnO has been suggested for g-ZnO layers on Pt(111) when the layer thickness of g-ZnO has reached 5 ML8, such structural transition has not been observed for g-ZnO films on Au(111). We tried to grow ZnO thin films with layer thickness over 5 ML, but found ZnO thin films exhibited the flat g-ZnO surface at ~4.5 ML thickness (Figure 2c) and would turn amorphous when the layer thickness reached ~8 ML (Figure 2d). Thus, we compare the reactivity of g-ZnO thin films on Au(111) with that of bulk w-ZnO(0001) surface directly in the following sections.

Figure 2. Surface structures of w-ZnO(0001) and ZnO thin films on Au(111). (a) STM image of wZnO(0001) showing the reconstructed surface with triangular islands and dark pits. (b) Structural model of the w-ZnO(0001) surface (color representations: grey-Zn; red/pink-O). (c) STM image of


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the ZnO film with layer thickness of ~4.5 ML on Au(111). (d) STM image of the ZnO film with layer thickness of ~8 ML on Au(111).

The interaction and reaction between CO and g-ZnO As we studied the interaction and reaction between CO and g-ZnO on Au(111), we examined mainly BL g-ZnO NSs and thin films. ML g-ZnO NSs are not stable on Au(111) and would reconstruct to BL g-ZnO NSs at elevated temperatures. When BL g-ZnO NSs were exposed to 0.39 mbar CO at 300 K, no adsorbates were observed on BL g-ZnO NSs on both terrace and edge sites (Figure 3a-b). Similarly, g-ZnO thin films of ~3.7 ML thickness on Au(111) remained inert for CO adsorption at 300 K when they were exposed to 0.36 mbar CO (Figure 3c-d) or higher (a few mbar CO). Thus, both BL g-ZnO NSs and thin films on Au(111) appeared inactive for CO adsorption or reaction at 300 K.

Figure 3. In-situ STM images of BL g-ZnO NS and g-ZnO thin film on Au(111) before and after the exposure to high pressure CO at 300 K. (a-b) BL g-ZnO NS before (a) and after (b) the exposure of 0.39 mbar CO. (c-d) g-ZnO thin film with layer thickness of ~3.7 ML on Au(111) before (c) and


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after (d) the exposure of 0.36 mbar CO. On both surfaces, no changes were observed on the moiré structures after CO exposure.

Figure 4. The reactivity of BL g-ZnO NSs on Au(111). (a) Large-scale STM image of BL g-ZnO NSs after the exposure to 0.27 mbar CO at ~420 K. Moiré pattern disappeared from the g-ZnO surface. (b) STM image of BL g-ZnO NS after the exposure to 0.27 mbar CO at ~480 K. The squared area was magnified in (c) and white arrows marked the corrugated edges formed by the reaction with CO. (d) Atomic resolution STM image showing the missing of neighboring oxygen atoms on BL g-ZnO NSs after the reaction with CO at ~480 K. White grid lines were imposed to illustrate the oxygen lattice. The corresponding structural model was displayed in (e). The dashed circles mark the missing atoms in (d). STM images in (a-d) were taken at 300 K after the evacuation of CO. (f) NAP-XPS C1s spectra for BL g-ZnO NSs on Au(111) in 0.4 mbar CO at elevated temperatures. The carbon peaks at ~284 eV were attributed to contamination from high pressure background. Gaseous CO gave the peaks at above 291 eV. (g) XPS peak intensity ratio of Zn2p3/2 over O1s (IZn/IO) for BL g-ZnO NSs on Au(111) as a function of annealing temperature in 0.27 mbar CO (red line) and in UHV (blue line). (h) Zn LMM Auger peaks of BL g-ZnO NSs on Au(111) as a function of annealing temperatures in 0.27 mbar CO. Pristine BL g-ZnO NSs before reaction (black) and Zn metal films (purple) deposited on Au(111) were also displayed for reference. Scanning parameters: (b) Vs= +1.8 V, It= 0.13 nA; (e) Vs= +0.028 V, It= 0.86 nA.


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We examined further the interaction and reaction between CO and BL g-ZnO at elevated temperatures. Figure 4a showed that the moiré pattern disappeared from the surface of BL g-ZnO NSs after they were exposed to 0.27 mbar CO at ~420 K. In contrast to the smooth edges of BL g-ZnO NSs before CO exposure, the edges of gZnO NSs appeared irregular after CO exposure, indicating the occurrence of CO oxidation at edge sites to form CO2 and leave step edges. After the exposure to 0.27 mbar CO at ~480 K, the step edges of g-ZnO NSs became highly corrugated (Figure 4b-4c), which suggested that Zn atoms at edge sites were destabilized in the ZnO lattice, accompanying the consumption of edge O atoms by CO, and would subsequently leave the ZnO lattice or desorb from the surface. Consistently, Pan et al. reported that the reactivity of CO oxidation on ZnO/Pt(111) was linear to the density of interfacial sites between g-ZnO NSs and Pt(111)41. Note that, there are dark holes formed on the surface of BL g-ZnO NSs after CO oxidation at ~480 K. Figure 4d displayed atomic resolution STM image of these dark features, which could be assigned as O oxygen vacancy clusters on g-ZnO NSs caused by the reaction between surface O atoms and CO. The corresponding structure is illustrated in Figure 4e. The observation of oxygen vacancy clusters corroborated that Zn atoms could leave the original lattice position upon the loss of oxygen atoms from neighboring sites. When BL g-ZnO NSs were cooled to 300 K after the reaction with CO, their surfaces could also appear with a (√3×√3)R30° adsorbate structure (Figure S2). Since NAP-XPS showed no carbonate peak in the C 1s spectra (Figure 4f) and ex-


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situ XPS showed a water peak in the O 1s spectra (Figure S3), the adsorbate pattern in Figure S2 could be attributed to background water adsorption. Consistently, DFT calculations (Figure S4) showed a rather weak adsorption energy of CO2 (-0.30 eV) on the surface of BL g-ZnO/Au(111). The reaction between CO and g-ZnO NSs on Au(111) was also measured by exsitu XPS sequentially. Before CO exposure, XPS measurements showed that UHV annealing at up to 600 K did not cause obvious changes in the peak intensity ratio of Zn 2p3/2 over O 1s (IZn/IO), which is consistent with the intact moiré pattern of g-ZnO NSs in STM at up to 600 K (Figure S5). In 0.27 mbar CO, IZn/IO was found to increase with the reaction temperature (Figure 4g), because of the continuous reaction between lattice O and CO to form CO2 and leaving the surface. Figure 4h showed that upon CO reduction, surface coverage of Zn would also decrease with the increasing reaction temperature, which is consistent with the etching of BL g-ZnO NSs in STM at elevated temperatures. The reduction of ZnO and the subsequent diffusion/desorption of Zn atoms became much accelerated at ~523 K and above, causing a sharp increase of IZn/IO subsequently (Figure 4g). Meanwhile, Zn LMM Auger spectra also showed that the intensity of Zn2+ decreased with the increasing reaction temperature and the main peak shifted to higher kinetic energies at above 523 K, indicating the reduction of ZnO24 and consistent with XPS measurements (Figure S3). In contrast, g-ZnO thin films of ~3.7 ML thickness on Au(111) appeared inert to CO under similar reaction conditions as described above. NAP-STM shows the moiré lattice of the g-ZnO film could still be clearly resolved (Figure 5a) and no obvious


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change in the surface morphology was detected during in-situ scanning under 0.27 mbar CO at ~460 K (Figure S6). After the evacuation of CO, no oxygen vacancies or adsorbates were resolved on the film surface (Figure 5b). Consistently, IZn/IO from XPS measurements stayed nearly constant (Figure S7). Zn LMM spectra also remained the same during CO exposure at elevated temperatures, which confirmed the structural integrity of g-ZnO thin films on Au(111). Similarly, BL g-ZnO film on Au(111) was also not reactive to CO under the similar reaction conditions (Figure S7). Thus, g-ZnO NSs on Au(111) are much more active towards CO than the closed g-ZnO thin films on Au(111).

Figure 5. The reactivity of g-ZnO thin films with layer thickness of ~3.7 ML on Au(111). (a) NAPSTM image of g-ZnO thin film in 0.27 mbar CO at ~460 K. (b) STM image of g-ZnO thin film after the evacuation of CO and being cooled to 300 K. The moiré structure of g-ZnO thin films was preserved throughout CO treatments. (c) Zn LMM Auger peaks of g-ZnO thin film on Au(111) as a function of annealing temperature in 0.27 mbar CO. Pristine g-ZnO thin film before reaction (black) and Zn metal film (purple) deposited on Au(111) were also displayed for reference.

The interaction and reaction between CO and w-ZnO(0001) To maintain charge neutrality at the surface, Zn-terminated w-ZnO(0001) undergoes surface reconstruction to expose triangular islands with straight step edges at the surface. These triangular islands could retain their structural integrity at elevated


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temperatures (up to 530 K in our study) as they were at room temperature (Figure S8). When exposed to CO, NAP-STM image (Figure 6a) showed no obvious change at the step or pit sites at 300 K, although these step and pit sites expose coordinatively unsaturated sites. Under 0.33 mbar CO, no obvious changes were found in the surface morphology of w-ZnO(0001) at ~340 K (Figure 6b), where triangular islands on wZnO remained with straight step edges. However, the reaction between CO and wZnO(0001) could be observed at higher temperatures. It was found that, when the surface temperature was raised to ~360 K, the surface of w-ZnO(0001) changed drastically with the straight steps of the ZnO terrace etched into the sawtooth shape and the size of pits in the ZnO terrace increased (Figure 6c). In-situ STM images (Figure S9) showed further that the etching process occurs from both the step edge and the inside rim of the pits. The loss of edge O atoms by reacting with CO caused unbalance of surface charges. To maintain the charge neutrality, Zn atoms also diffused away from the step edges, such that the surface of ZnO(0001) appeared etched. When the surface temperature was further raised to ~530 K, the steps of wZnO(0001) were etched severely and no triangle steps were obvious from the surface due to the enhanced CO2 desorption at higher temperature (Figure 6d-e). Apparently, the etching speed is faster at the step edge than from inside the pits. When CO reacts with lattice O on w-ZnO(0001), O atoms at the step edges or the inside rim of pits are both undercoordinated and favorable sites for CO oxidation. However, the structural model (Figure S10) shows that the triangular islands expose 2-coordinated O atoms at corner sites of step edges, which are expected to be more reactive than 3-coordinated


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O atoms at step sites. Meanwhile, the inside rim of pits does not expose 2-coordinated O atoms. Since the removal of corner O atoms could trigger sequential removal of Zn atoms and the generation of new corner O atoms, step edges of w-ZnO(0001) could exhibit higher activity in CO oxidation than the inner rim of pits on w-ZnO(0001).

Figure 6. The reactivity of the w-ZnO(0001) surface towards CO. (a-e) NAP-STM images of the w-ZnO(0001) surface in 0.33 mbar CO at elevated temperatures. (a) at 300 K. (b) at ~340 K. No obvious changes in the surface morphology were observed. (c) at ~360 K. The steps appeared etched and pits appeared enlarged due to the reaction between lattice O and CO. (d-e) in 0.33 mbar CO at ~530 K. Small triangular islands disappeared and the curvature of steps increased. White arrows indicated the etched steps. (f) NAP-XPS C1s spectra for w-ZnO(0001) in 0.4 mbar CO at elevated temperatures. The carbonate peak at ~289 eV, marked by dashed rectangle, appeared at between 300-500 K. Gaseous CO gave the peaks at above 291 eV. The carbon peaks at ~284 eV were attributed to contamination from high pressure background. (g) STM image after the evacuation of 0.33 mbar CO and cooled to 300 K. The terrace appeared highly disordered with adsorbates.

NAP-XPS on w-ZnO(0001) showed the presence of a small carbonate peak in the C 1s spectra (Figure 6f) at between 300-500 K, indicating a strong adsorption of CO2 on w-ZnO(111). Our DFT calculations (Figure S4) also suggested a strong adsorption


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energy of CO2 at -1.79 eV on the terrace sites of w-ZnO(0001). The formation of carbonate could block active sites on w-ZnO(0001) for CO oxidation. But, our STM study has shown that the continuous etching of step edges could be observed at above 360 K on w-ZnO(0001). Thus, the formation of carbonates was likely caused by CO2 adsorption on surface terraces. At high temperatures, the presence of surface carbonates is an equilibrium between the adsorption and desorption of CO2. The lifetime of surface carbonates might be too short to be caught in NAP-STM measurements. Although NAP-STM did not resolve surface carbonates at high temperatures, the appearance of surface carbonates on w-ZnO(0001) could be observed at 300 K after the evacuation of CO (Figure 6g). In comparison, w-ZnO(0001) is more active than g-ZnO NSs or thin film on Au(111) in the reaction with CO. The reaction between H2 and ZnO at 300 K The interaction and reaction between H2 and g-ZnO/w-ZnO were also compared under NAP conditions and demonstrated a similar reactivity trend as described above. Figure 7a displays STM images of BL g-ZnO NSs in 1.69 mbar H2 at 300 K, which showed that the surface of g-ZnO NSs remains intact, with the surface moiré pattern well-preserved. In contrast, the exposure of w-ZnO(0001) to 1.33 mbar H2 at 300 K leads to significant changes on the surface of w-ZnO(0001), where high density of triangular clusters emerged and caused the roughening of the w-ZnO(0001) surface (Figure 7b). The hydrogenation of the w-ZnO(0001) surface at 300 K could be confirmed from XPS measurements (Figure 7c), which detected the appearance of a shoulder OH peak at 531.4 eV in the O1s spectra. The 0.2 eV shift of lattice O peak


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towards higher binding energy could be attributed to band bending induced by H2 adsorption44. Upon the dissociation of H2 on w-ZnO(0001), the extra charge from hydrogen adsorption caused unbalance in surface dipole moments and O atoms are likely to be lifted to form OH species, as such the charge neutrality could be preserved at the surface. These OH groups are responsible for surface reconstruction and the formation of triangular clusters on w-ZnO(0001). The planar Zn-terminated wZnO(0001) surface was only recovered after cycles of Ar ion sputtering and UHV annealing at ~1000 K (Figure 7d). Our study thus shows w-ZnO(0001) is more reactive towards the dissociative adsorption of H2 than g-ZnO.

Figure 7. H2 activation over BL g-ZnO NSs on Au(111) and w-ZnO(0001). (a) STM image of BL g-ZnO NSs on Au(111) after the exposure to 1.69 mbar H2 (image taken after the evacuation of H2). No obvious changes were observed on the moiré structures of g-ZnO or the herringbone structures of Au(111). (b) STM image of w-ZnO(0001) after the exposure to 1.33 mbar H2 at 300 K (image taken after the evacuation of H2). Significant morphology changes took place on w-ZnO(0001) with the appearance of high-density triangular clusters (marked by white circles). The side length of triangular clusters is ~3 nm. (c) XPS O1s spectra of w-ZnO(0001) before (red) and after (orange) the exposure to 100 mbar H2. (d) STM image on the recovery of the w-ZnO(0001) surface after


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ACS Catalysis

cycles of Ar ion sputtering and UHV annealing at ~1000 K.

Comparison of the reactivities between CO and g-ZnO/w-ZnO From above, we have compared the reactivities of g-ZnO on Au(111) and wZnO(0001). The surface of w-ZnO(0001) was found to exhibit a higher reactivity than that of g-ZnO NSs or thin films on Au(111). Among the different forms of g-ZnO, BL g-ZnO NSs are more reactive towards CO than BL g-ZnO thin films. The reactivity comparison was made based on the reaction between CO and lattice oxygen atoms on the various ZnO structures. Yet, such comparison could still provide an indication for the reactivity of ZnO structures in general, since CO oxidation on oxide surfaces has usually been suggested to occur via the Mars-van Krevelen mechanism45, i.e. CO molecules would react directly with lattice oxygen atoms. DFT calculations were performed to understand the reaction between CO and BL g-ZnO NS/film on Au(111) or w-ZnO(0001). Figure 8 displays the potential energy diagram of CO oxidation on BL g-ZnO NS and w-ZnO(0001). Consistent with experimental studies, CO molecules were found to adsorb weakly on these surfaces. The exothermic process of CO oxidation could take place via CO reaction with lattice O and the formation of CO2* is more exothermic on w-ZnO(0001) than on BL g-ZnO NS. Here, we assume that the formation energy of O vacancy (EOvf) could be used to describe the reactivity difference of these oxide surfaces. Our calculations (Table 1) show that EOvf is -2.29 eV for w-ZnO(0001), -1.48 eV for BL g-ZnO NS, and -0.83 eV for BL g-ZnO film (Figure S11). The sequence of EOvf on these surfaces agrees well with the reactivity trend for CO oxidation as measured in experimental studies: w-


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ZnO(0001) > BL g-ZnO NS > BL g-ZnO film.

Figure 8. DFT-calculated potential energy diagrams for CO oxidation over supported BL g-ZnO NS and w-ZnO(0001). The optimized structures of BL g-ZnO NS and w-ZnO(0001) are shown in panels. (Color representations: yellow- Au; grey- Zn; red- O; pink- O; black- C). The dashed circles mark the formation of O vacancies. * represents surface adsorption sites and Ov indicates surfaces with oxygen vacancy.

Through the analysis of projected density of states (pDOS) and charge density diffferences, the trend in EOvf could be understood and the reactivities of lattice oxygen atoms in different ZnO structures could be derived, in consistent with experimental studies. Figure 9a showed that O atoms on w-ZnO(0001) has richer 2p-states around the Fermi level than O atoms in BL g-ZnO NS. That means, O in BL g-ZnO NS has a stronger bonding than O on w-ZnO, as also reflected in the analysis of charge density differences (Figure 9b-9c). Bader charge analysis also suggested a charge transfer from ZnO to Au(111) at 0.02 |e| per ZnO, implying the reactivity of g-ZnO NS is affected by the interaction between Au(111) and ZnO. In fact, the enhanced catalytic activity exhibited at the metal-oxide interface has often been attributed to 1) the presence of dual catalytic sites and 2) the electronic properties of supported oxide NSs tuned by


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ACS Catalysis

interfacial interaction. Taking the ZnO/Cu(111) system as an example, Mahapatra et al.46 have suggested that the ZnO/Cu interface is highly active for catalyzing CO oxidation and the water-gas shift reaction. Palomino et al.47 showed that the ZnO/Cu interface is key to the enhanced binding and conversion of CO2 in methanol synthesis. The reactivity at the g-ZnO/Au interface is expected to be much lower than that of the g-ZnO/Cu interface, owing to the inertness of Au and the weaker interaction between ZnO and Au. DFT calculations39 have shown the interfacial electron transfer is more significant from Cu(111) to ZnO than the electron transfer from ZnO to Au(111).

Figure 9. (a) DFT-calculated projected density of states corresponding to O 2p orbitals of supported BL g-ZnO NS (red) and w-ZnO(0001) (blue). (b-c) Charge density differences of edge O sites at (b) BL g-ZnO NS on Au(111) and (c) w-ZnO(0001). Blue circles indicate the decrease in local electron densities and yellow circles indicate the increase.

Further, surface polarity of w-ZnO and BL g-ZnO NS could be used as an


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indicator to explain the discrepancy in reactivity. The strength of polarity48 can be 1

estimated with: D𝑧 = 𝑁∑𝑞𝑖𝑧𝑖, where qi is the Bader charge value, zi is z coordinate of the atom and N is the number of Zn-O pairs. The average surface rumpling of BL gZnO NS is 0.07 Å, which is much smaller than that of w-ZnO(0001), 0.31 Å. Combined with the charge of atoms from surface layer, w-ZnO(0001) was found to exhibit higher surface polarity (1.47 D) than that of BL g-ZnO NS (0.77 D). The surface of BL g-ZnO NS could become more stable through depolarization with respect to w-ZnO. Inversely, higher polarity of w-ZnO could induce more flexibility of lattice O and thereby higher activity towards CO molecules (Table 1).

Table 1. Summary of DFT calculation results of BL g-ZnO on Au(111) and w-ZnO(0001).

a. Sites for the formation of O vacancy; b. Formation energies of O vacancy (EOvf) is defined as: 𝐸𝑂𝑣𝑓 = 𝐸𝑎𝑓𝑡𝑒𝑟 + 𝐸𝐶𝑂2 ― (𝐸𝑏𝑒𝑓𝑜𝑟𝑒 + 𝐸𝐶𝑂), where 𝐸𝑏𝑒𝑓𝑜𝑟𝑒 is the total energy of pristine structure without O vacancy, 𝐸𝑎𝑓𝑡𝑒𝑟 is the total energy of the reacted structure with one O vacancy, 𝐸𝐶𝑂and 𝐸𝐶𝑂2 are the energies of CO and CO2 molecule at their optimized gas phase geometry, respectively. c. Bader charge analysis49; d. Average rumpling of surface ZnO layer; e. Coordination number of active O site; f. Calculated surface or boundary polarity (also called dipole48).

SUMMARY

The reactivities of g-ZnO(0001) NSs and thin films on Au(111) and the wZnO(0001) surface towards CO and H2 were investigated using NAP-STM, XPS and


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ACS Catalysis

DFT calculations. On Au(111), crystalline g-ZnO NSs and thin films of different layer thickness could be prepared with their surface structures resolved at atomic resolution. At elevated temperatures, CO oxidation took place primarily at the edge sites of w-ZnO and the interface between g-ZnO NSs and Au(111). The desorption of CO2 left oxygen and zinc vacancies at surfaces. In contrast, g-ZnO thin films with layer thickness in between 2-4 ML on Au(111) are inert for CO oxidation under similar reaction conditions. From the onset reaction temperature, the reactivity trend could be reached as w-ZnO(0001)> BL g-ZnO NSs> BL g-ZnO thin films. Similarly, the w-ZnO(0001) surface exhibited a much higher reactivity towards the dissociative adsorption of H2 at 300 K, where OH groups induced the reconstruction and roughening of w-ZnO. In contrast, g-ZnO on Au(111) appeared inert in 1.69 mbar H2 at 300 K. The differences in reactivity among BL g-ZnO NSs or thin films on Au(111) and w-ZnO(0001) could be rationalized from the differences in EOvf, and further attributed to the charge transfer on oxygen atoms and surface polarity.

ASSOCIATED CONTENT Supporting Information This material is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Author *Email: [email protected]; *Email: [email protected]


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Author Contributions †These authors contributed equally to this work. NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by Ministry of Science and Technology of China (2017YFB0602205, 2016YFA0202803), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020200), Natural Science Foundation of China (21473191, 91545204).

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CO and H2 Activation over g-ZnO Layers and w-ZnO(0001) - ACS

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