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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Growth, Structure and Catalytic Properties of ZnO Grown on CuO/Cu(111) Surfaces x

x

Mausumi Mahapatra, Jindong Kang, Pedro J Ramirez, Rebecca Hamlyn, Ning Rui, Zongyuan Liu, Ivan Orozco, Sanjaya D. Senanayake, and Jose A. Rodriguez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09243 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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The Journal of Physical Chemistry

Growth, Structure and Catalytic Properties of ZnOx Grown on CuOx/Cu(111) Surfaces

Mausumi Mahapatra,a Jindong Kang,b Pedro J. Ramírez,c Rebecca Hamlyn,b Ning Rui,a , Zongyuan Liu,a Ivan Orozco, b Sanjaya D. Senanayake b José A. Rodriguez a, b, * a

Chemistry Department, Brookhaven National Laboratory, Upton, NY, 11973 USA

b c

Department of Chemistry, SUNY at Stony Brook, Stony Brook, NY, 11794 USA

Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020-A, Venezuela

* Corresponding

author: Bldg. 555A, Brookhaven National Lab, P.O. Box 5000, Upton, NY 11973-5000, *[email protected]

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Abstract A catalyst with a ZnO/Cu configuration plays an important role in the synthesis of methanol from CO2 hydrogenation. In this study, scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) were used to investigate the growth mode of small coverages of ZnOx, θoxi < 0.3 monolayer, on a Cu(111) substrate. Our results show that, the mode of growth, size and shape of the ZnO nanoparticles are strongly dependent on the Zn deposition temperature. In a set of experiments, Zn was deposited on Cu(111) or CuOx/Cu(111) surfaces at 300 K with subsequent exposure to O2 at higher temperatures (400-550 K) which exhibited small particles of ZnO (< 20 nm in size) on the surface. The deposition of Zn onto CuOx/Cu(111) at elevated temperatures (450600 K) in an oxygen ambient produced large ZnO islands (300-650 nm in size) which were very rough and spread over several terraces of Cu(111). XPS/Auger spectra showed that all the preparation conditions stated above led to the formation ZnO/CuOx/Cu(111) surfaces where the oxidation state of zinc was uniform. Catalytic tests showed that all these surfaces were active for the hydrogenation of CO2 to methanol but only the systems prepared at 600 K displayed long term stability under reaction conditions.


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Introduction The increasing levels of carbon dioxide, a major greenhouse gas in the atmosphere is a major concern for the human society. As a result, today a large effort has been devoted to minimize the production of CO2 and also to capture and convert CO2 to value-added chemicals. The conversion of CO2 to methanol is an attractive approach in this regard. The most commonly used catalyst for the CO2 hydrogenation to methanol is Cu/ZnO/Al2O3. Alumina helps in the dispersion of the copper and zinc oxide active phase. Due to the complexity of powder catalysts dynamic changes in the active phase under reaction conditions are not fully understood. The configuration of the active phase of the catalyst (ZnCu bimetallic site vs ZnO-Cu interfacial site) and the role of ZnO are widely debated in literature. 1-9 Several studies propose that under reaction condition the partial reduction of ZnO occurs leading to the formation of alloys of different configuration, ZnCu or Zn/Cu, as the active phase 1-3 4-5 and other studies show that ZnO stabilizes Cu nanoparticles and induces a synergistic effect that tunes the catalyst in favor of selective products. 6-9 In one of the first studies aimed at understanding the Cu/ZnO synergy, it was argued that the active phase consisted of metal particles encapsulated by the oxide, a configuration which generated a schottky junction at the interface, responsible for the catalytic activity. 10 It has been suggested in a combined experimental and theoretical work that a copper step with close vicinity to ZnO on the copper surface creates an ensemble which is very active in methanol synthesis.1

A study based on transmission electron microscopy (TEM) and

X-ray photoelectron spectroscopy (XPS) showed the dynamics of the Cu/Zn nanoparticles under oxidizing and reducing conditions: upon oxidation the Cu-Zn metal cluster turns into two separate CuO and ZnO nanocrystals which can be reduced under hydrogen pressure to form ZnO decorated



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Cu clusters which are suggested to be the confined units of Cu/ZnO methanol synthesis catalyst. 11

The idea of a ZnO/Cu configuration as the active phase for a CO2  CH3OH transformation10,11 has been reinforced by the results of recent studies.6,12,13,14 Experiments performed using highresolution transmission electron microscopy (HRTEM) showed the presence of a ZnO overlayer on top of Cu particles during the CO2 hydrogenation reaction over a Cu/ZnO/Al2O3 catalyst; a result which demonstrates the relevant role played by a ZnO-Cu interface.6 This ZnO overlayer was metastable and had a graphite-like structure instead of the typical hexagonal wurtzite or cubic zincblende lattices seen for bulk zinc oxide.6 A specific CO stretching frequency has been found in infrared (IR) spectroscopy for this ZnO overlayer in contact with copper suggesting the existence of a strong interaction between the oxide and metal.

12

Furthermore, ZnO can be

promoted with metals such as Al, Ga and Mn to facilitate the migration of the oxide host to the copper particles and enhance catalytic activity.

13

This is consistent with another article which

highlights the importance of a strong interface contact between ZnO and Cu as a necessary condition for good catalytic activity.7 It has been observed in operando X-ray electron diffraction and IR spectroscopy that the ZnO particles adopt a hexagonal prism like morphology under CO2 hydrogenation reaction conditions.9 Electronic interaction between the polar facets of the ZnO with the Cu enhances the catalytic activity for the CO2 hydrogenation reaction. 9 A recent article by Kattel et al. showed that an inverse catalyst prepared by deposition of ZnO nanoparticles on Cu(111) exhibits catalytic activity much higher than the traditional metal/oxide catalysts.14 In that work it was also demonstrated that a Zn-Cu(111) alloy transforms to a ZnO/Cu configuration under reaction conditions further enhancing the active role played by the ZnO-Cu interface in methanol synthesis.14 4 ACS Paragon Plus Environment

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The superior performance of

the ZnO/Cu(111) inverse catalyst

in methanol synthesis is

encouraging as a starting point in the design of technical catalysts.14-15. In principle, in a ZnO/Cu configuration one could see crystal structures or face terminations not seen for bulk ZnO.16-27 However, there is not detailed study of the growth mode of ZnO on Cu(111) at temperatures relevant for the hydrogenation of CO2 (500-600 K).16,24 Previous studies have examined the growth of ZnO on Cu(111) at 300 K or using NO2 as the oxidizing agent.16,18,24,27 Particles of ZnO grown under these conditions are not stable at elevated temperatures and disappear from the surface.13,16 On that basis, here we are focusing on an in depth structural characterization of the ZnO/Cu overlayer by using STM and XPS. The temperature dependent growth (300-600 K) and surface morphology of the ZnO overlayer on Cu(111) is explored for θoxi < 0.3 monolayer, the coverage where maximum activity for methanol synthesis was observed.12,13 In this work ZnO overlayers are prepared under similar experimental conditions as used for CO2 hydrogenation studies (500600 K) in an effort to correlate the surface structure with the proposed active phase of the catalyst. 14-15

For growth at elevated temperatures, we found very large islands of ZnO (300-650 nm in size)

not seen in previous studies.16,18,24,27 The results of the article are organized as follows. The first two sections focus on characterizing the surface structure of Zn/Cu(111) alloys prepared by vapor depositing Zn onto Cu(111) at 300 K and the oxidation of these surface alloys to ZnOx/CuOx/Cu(111) at elevated temperatures. In the next section STM and XPS experiments are carried out for Zn vapor deposited on to a previously oxidized copper, CuOx/Cu(111), at 300 K followed by oxidation at higher temperature. Next, we explore the surface structure when ZnOx/CuOx/Cu(111) was prepared by vapor depositing Zn onto CuOx/Cu(111) at 550 K in 5×10

-7

Torr oxygen background. Finally, we show the surface

evolution when the ZnOx/CuOx/Cu(111)

overlayer was prepared by depositing Zn onto a 5

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CuOx/Cu(111) at ~450 K in oxygen ambient and then heated to higher temperatures. For a set of these ZnOx/CuOx/Cu(111) systems, we performed studies of CO2 hydrogenation activity. Experimental Methods All of the STM experiments were conducted by using an Omicron STM chamber with a background pressure of 5 × 10 -10 Torr. A Cu(111) single crystal (Princeton Scientific Corp.) was cleaned by cycles of Ar ion sputtering and annealing (850 K, 10 min). The STM images were collected at 300 K by using a Pt-Ir tip. CuOx/Cu(111) thin films were prepared by exposing the clean Cu(111) surface to 5×10

-7

Torr oxygen at 550 K for 15 min. Zn was vapor deposited on

clean Cu(111) or a pre-oxidized CuOx/Cu(111) surface by using a metal evaporator. A commercial SPECS AP-XPS chamber equipped with a PHOIBOS 150 EP MCD-9 detector, located at the Chemistry Division of BNL, was used for XPS analysis. The surfaces containing Zn/Cu and ZnO/Cu interfaces were generated in a preparation chamber connected to the analysis chamber. The spectra for the Cu 2p, Zn 2p, Cu LMM, Zn LMM and O 1s regions were collected by using a Mg K-alpha (1253.6 eV) anode. Zn was vapor deposited on clean Cu(111) or a previously prepared CuOx/Cu(111) surface by using a metal evaporator. The coverage of Zn on Cu(111) was calibrated by using Auger electron spectroscopy (AES). Catalytic tests were performed using an instrument which combined a ultra-high vacuum (UHV) chamber for surface characterization (base pressure ~5×10-10 Torr) with a batch micro-reactor.12,13 This set-up allowed the transfer of the sample from the UHV chamber to the reactor (and vice versa) without exposure to air. In the UHV chamber surface characterization could be performed using XPS, AES, temperature programmed desorption (TPD), low-energy electron diffraction (LEED), and ion-scattering spectroscopy (ISS). A combination of XPS and AES was used to verify


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the oxidation state of a given ZnO-Cu(111) catalyst before and after reaction. In the experiments for the hydrogenation of CO2, the ZnO/CuOx/Cu(111) sample was moved into the batch reactor at ~ 300 K, then the two reactant gases, a mixture of 0.049 MPa (0.5 atm) of CO2 plus 0.441 MPa (4.5 atm) of H2, were introduced and the device was warmed to a reaction temperature of 550 K. The yields of methanol and CO were measured by a mass spectrometer and/or a gas chromatograph.12,13 The amount of molecules (methanol or CO) generated in each catalytic test was normalized by the sample active area and the total reaction time.

Results and Discussion A. Preparation of Zn/Cu(111) alloys A low coverage of Zn (0.1 ML) was vapor deposited onto Cu(111) at 300 K and the corresponding STM image is shown in Figure 1(a). Following Zn deposition, small patches of various shape and size distribution are observed near Cu(111) step edges. Figure 1(b) shows a zoomed in image and a line profile measurement performed across a patch. The line profile measurement suggest that the patches are about one atomic layer high (~2.5 Å) compare to bare Cu(111). Due to lack of atomic resolution in the STM images it is not possible to assign whether the patches are pure Zn or a ZnCu intermixed layer. It has been previously reported in a STM study that upon adsorption, the Zn atoms are initially localized at the Cu(111) step edges and then migrate inside the upper terrace to form a ZnCu surface alloy16 . It has been proposed that ZnCu surface alloy formation occurs by a “place exchange” mechanism where Zn atoms displace the surface Cu atoms. The formation process of the ZnCu alloy was further investigated theoretically and the results indicated that there is a relatively high activation barrier for the Zn insertion into the Cu substrate which



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favors the pseudomorphic growth of Zn islands over the Zn-Cu place exchange mechanism.

17

Their montecarlo simulation results showed that at low coverages (0.25 ML) the Zn adatoms coalesce to form compact islands. However, at and above room temperature, the place exchange between the Zn adatoms and Cu atoms in upper terrace sets in, which is more pronounced at higher temperatures. The ZnCu intermixing is less pronounced at higher Zn coverages, favoring the formation of larger Zn islands within the adlayer. The structure of Zn thin films (1 to 2.5 ML Zn coverage) on Cu(111) was recently investigated by the Freund group by using STM and XPS18. Their STM images of Zn/Cu(111) systems showed the formation of irregularly shaped patches on top of Cu(111) which they assigned to the Zn containing area with some possible

ZnCu

intermixing. The shape of the Zn containing patches observed by our STM images have striking similarity to that observed by Freund and coworkers and can be assigned to a Zn/Cu intermixed overlayer. Similar to the findings of the Freund group, we have also observed Cu step edge degradation after Zn exposure which is characteristics of ZnCu alloying at the step edges. B. Oxidation of Zn/Cu(111) alloy surfaces After the initial characterization, the Zn/Cu(111) surface (as shown in Figure 1a, b) was exposed to 5×10-7 Torr of molecular oxygen at 300 K for 20 minutes and the resulting STM image is shown in Figure 2a,b. After oxygen exposure, the surface underwent various morphological changes as shown in Figure 2a. The noticeable changes after oxygen exposure are: rough morphology of the Cu step edges, appreance of triangular pits (highlighted by a yellow triangle) and large size Zn/Cu islands (highlighted by a yellow circle) on Cu terraces. Figure 2b shows a zoomed-in image which clearly shows the triangular pits and the rough morphology of the islands. A line profile measurements performed across one island in Figure 2b suggests that the average height of the island is ~3.5 Å compare to 2.5 Å before the oxygen dose (Figure 1b line profile). This suggests 8 ACS Paragon Plus Environment

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that the Zn/Cu islands are oxidized after exposed to oxygen at 300 K. Matsumoto et al. reported the formation of rough step edges and triangular pits on the terraces following oxygen adsorption on Cu(111) at 300 K which they assigned to step and terrace copper oxide, respectively19. In a line of agreement with Matsumoto et al. we assign the triangular pits to terrace copper oxide. However, the step edge morphology in our experimental result can be a mixed Zn/Cu oxide . Figure 2c shows the STM image after the surface was heated slowly to 460 K in a 5×10-7 Torr of O2 ambient for 20 minutes and then cooled to 300 K. As a result of oxidation at 460 K, the copper surface got smoother with no visible pits or holes. A regular patteren developed on Cu (shown in the inset in Figure 2c) which can be assigned to the CuOx-44 structure as reported previously. 1920

In addition, the surface area of the Zn/Cu oxidized islands got smaller in size and the average

height increased to ~ 6 Å, suggesting the formation of islands with 2 to 3 layers of zinc oxide. Those oxide islands were not ordered and despite several attempts we wrere not able to gain any atomic scale infromation about their morphology . Simillar disorderd oxide formation has been reported by the Freund group following oxidation of Zn/Cu alloys.18 They have reported the formation of well orderd ZnO islands when Zn was vapor deposited on Cu(111) at 300 K in an oxygen ambienet followed by UHV annealing the surface to 550 K. Depositing Zn in an oxygen background minimizes the Zn/Cu intermixing which helps the formation of a well ordered ZnO overlayer on the Cu substrate.18 The oxidation process of Zn on Cu(111) was further studied by XPS/AES and the results are shown in Figure 3. Deposition of 0.1 ML Zn on Cu(111) shows the characteristic LMM auger peak for metallic Zn (Figure 3 a).21 Upon exposing the surface to oxygen at 300 K, the line shape changed and showed a peak at ~ 264.5 eV and a small shoulder at 261.5 eV. The peak at 264.5 eV corresponds to the oxidized Zn and the shoulder at 261.5 eV indicates the presence of a small 9 ACS Paragon Plus Environment


amount of metallic Zn.21 Upon heating the surface to 460 K and 550 K, the 264.5 eV peak got bigger and the shoulder at 261.5 eV peak got smaller indicating the gradual conversion of metallic Zn to Zn2+ at higher temperatures. 21 C. Oxidation of Zn/CuOx/Cu(111) We further explored the surface chemistry when Zn was vapor deposited onto CuOx/Cu(111) at 300 K followed by oxidation to higher temperatures. In other words, first Cu(111) was oxidized at 550 K to form a layer of the CuOx-44 structure on which Zn was vapor deposited at 300 K. The oxidation of 0.2 ML Zn on CuOx/Cu(111) was studied by XPS/AES and the results are shown in Figure 4a. Upon deposition of Zn onto a CuOx/Cu(111) substrate, the majority of the Zn LMM signal comes from metallic Zn, whereas a small portion of the Zn signal corresponds to oxidized Zn. 21 This is not surprising based on the fact that Zn has a higher oxofilicity than Cu. The Zn gets oxidized by taking surface oxygen atoms initially present in the copper oxide. Upon exposing the surface to oxygen at 300 K for 20 minutes, oxidized zinc contributes to the major Zn LMM peak while there is a small portion of Zn in a metallic state, evidenced by the small shoulder present at 261.5 eV.

21

Upon heating the surface to higher temperatures, the intensity of the shoulder

decreased suggesting the gradual conversion of Zn to Zn2+ at 550 K which is similar to what was observed in Figure 3. Figure 4b shows an STM image when 0.2 ML Zn was vapor deposited onto CuOx/Cu(111) at 300 K and then heated to 550 K in 5×10-7 Torr of O2 ambient for 20 minutes. It shows the formation of ZnO aggregates of various sizes on the surface. It can be observed that the majority of the smaller ZnO aggregates look rounded with the large clusters exhibiting a triangular shape. The height of the clusters varied and fell in the range between 0.9 to 1.6 nm. The 0001 facets in the wurzite ZnO crystal have a step height of 5.2 Å for two layers of ZnO (0001). On this basis it can 10 ACS Paragon Plus Environment

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be said that the observed ZnO aggregates roughly contain 3 to 6 ZnO layers. This suggest that the interaction between the ZnO layers is stronger than the interaction between the ZnOand CuOx/Cu(111). . D. Zn deposition on CuOx/Cu(111): 550-600 K in an oxygen ambient It has been previously reported that the stability of the ZnOx overlayer prepared by the oxidation of a Zn/Cu alloy is different to that prepared by the deposition of zinc on copper at 600 K in oxygen ambient.

15

The ZnOx formed by the former method was less stable at the elevated

temperatures (500-600 K) typically used for the CO2 hydrogenation reaction. STM experiments were carried out to characterize the surface structure and growth morphology of ZnO/CuOx/Cu(111) prepared at 550-600 K. The Cu(111) sample was heated to 550 or 600 K and exposed to 5×10 -7 Torr of O2 for ten minutes to ensure oxidation of the Cu surface to the CuOx44 structure. The sample temperature was maintained at the elevated temperature and Zn was dosed in an ambient of 5×10

-7

Torr of O2. Zn LMM Auger showed the formation of ZnO on a

CuOx/Cu(111) substrate. The dominant surface features observed under this preparation condition are large triangular islands of ZnO on top of CuOx/Cu(111) as shown in Figure 5. The size of the islands is considerably larger and the nucleation density is very low under this preparation condition as compare to the previous method, (Figure 4) when Zn was deposited at 300 K followed by heating to 550 K in oxygen. Figure 5a,b show large scan area STM images for ZnO coverages 0.1 and 0.2 ML, respectively. With an increase in coverage from 0.1 to 0.2 ML, both the island density and the average size of the islands increased. Line profile measurements performed on various islands indicate that the average side length of the triangular islands at 0.1 ML oxide coverage is 300-400 nm and in the broad range of 300-650 nm for 0.2 ML coverage. The large size of the islands and the low nucleation density indicates that, at high temperatures, a critical size 11 ACS Paragon Plus Environment


nuclei formation is necessary to stabilize the Zn on the surface rather than diffusing into the Cu bulk. The edges of the ZnOx islands coincide with the Cu(111) lattice direction. High-resolution images revealed that the bare surfaces surrounding the triangular islands are a CuOx-44 structure (shown in the inset in Figure 5a) as expected. It has been observed that the majority of the triangular islands grow near step edges although there are some instances where they grow on flat terraces. The shape of the triangles varies; it can be a perfect isosceles triangle, or a triangle with truncated edges as shown in Figure 5b. However, all the triangular islands have some common features: they have depressions in the center, rugged morphology, contain several layers, and very porous in nature. Figure 5c and d show the zoomed in 3D view of the area highlighted in Figure 5a and b, respectively, clearly showing rough morphology and depressions inside the islands. Figure 6 shows examples of the most commonly observed island structures at 0.2 ML ZnO coverage. It has been observed that a majority of the triangular islands grow across multiple step edges (as shown in Figure 6a) in the so-called carpet mode.22 23 The step edges which occupy the islands are heavily kinked (compare to the smooth step edges observed on Cu(111)), which suggest that the step edges change to accommodate the growing ZnO islands. The growth mechanism of the triangular islands across the step edges can be a result of the seeding and growth process as explained in the case carpet growth mode of graphene on Ru. 22 We need further investigation but it can be speculated that when a growing ZnO island meets a descending step edge, some of the surface Cu atoms underneath the ZnO island has been removed at some localized spot. At these spots the ZnO island attaches to the lower terrace and provides the seed for the growth of the island in the lower terrace. There are also some instances (as shown in Figure 6b), where a triangular island grows over a terrace without crossing the step edges. This type of island growth is often accompanied by the 12 ACS Paragon Plus Environment

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presence of a large number of Cu steps bunched together unlike monoatomic steps spaced apart on clean Cu. It is unlikely to observe this kind of step edge morphology on clean Cu(111), and is proposed to be induced by the growth of ZnO islands. One explanation is that, Cu atoms from the nearby empty terraces are transported to the terrace which occupy the growing ZnO island, so that the island can grow in the same terrace level without traversing the steps. This mass transfer results in the expansion of the ZnO covered Cu terraces and the shrinking of nearby empty Cu terraces forming multi-steps. This kind of island and step multi-step growth morphology is reported by the growth of graphene on Ru. 22 Line profile II performed across 6b shows that the average height of the particles across the island is ~6 Å. The zoomed-in image of the area highlighted in Figure 6b is shown in Figure 6c where the four different steps inside the ZnO island (step height ~2.5 Å) are clearly visible. As mentioned before, it is often observed that when an island grows over a stepped surface, the Cu step and the terraces change to underlay the growing ZnO island. It is likely that there are four copper steps created locally to underlay the ZnO overlayer. There are also some cases where the ZnO islands are observed on flat terraces as shown in Figure 6d, which have similar rough morphology as observed in Figure 6a and b. There are rare instances where multiple triangles coalesce and some non-triangular islands are observed as shown in the Figures S1 and S2 respectively. Interestingly the non-triangular islands do not contain big depressions in the middle as observed in the case for triangular islands. This maybe a consequence of variations in size and a different type of growth mode with respect to the CuOx/Cu(111) substrate. The line profile measurements performed across various islands (an example is shown in the line profile I performed on Fig 6a) show that the initial step height between the copper oxide and the first layer of the ZnO island is ~ 3 Å and the step height between the first layer and the protrusion 13 ACS Paragon Plus Environment


is ~3 to 5 Å. This suggests that there is one atomic layer of ZnO on top of CuOx/Cu(111) and 3D ZnO structures of 2 to 3 layers high aggregate on top of the first layer. In other words, the large ZnO triangular islands is made by the formation of one atomic layer ZnO sheet, on top of which small 3D ZnO clusters aggregate. One possibility for this kind of growth morphology is that, the lattice mismatch between the ZnO first layer and the Cu substrate creates a high surface free energy in the interfacial layer which does not favor the growth of subsequent ZnO layers which leads to the formation of clusters on top of the first layer. Due to the rough morphology, it was not possible to identify the structure of the ZnO overlayer: whether it has a planar (nonpolar) or wurzite (polar) structure. However, it has been often observed in our STM images that the large triangular islands are associated with central depressions. It has been reported in the literature that the polar Znterminated ZnO crystal shows a triangular reconstruction on the surface.25,26 The resulting triangular islands have a rugged morphology and contain central depressions. The rugged morphology and the formation of holes in the triangular islands is explained as a mechanism for the stabilization of the polar surfaces. 24-26 The surface dipole of the Zn-terminated ZnO island is cancelled by forming Zn deficient pits and step edges which are terminated by oxygen. Thus, our experimental images show evidence for the formation of a polar ZnO phase on copper. According to the literature, the growth of a well-defined ZnO thin film on Cu(111) is a challenging step.

18, 27

Freund et al recently reported the formation of ordered ZnO thin films when Zn was

vapor deposited onto Cu(111) under an oxygen ambient followed by step wise heating the surface to 550 K.18 However, the ZnO overlayer prepared by that method did not cover the entire Cu(111) surface with the formation of 3D islands instead of layer-by-layer growth. In a recent article Zhao et al show that the use of an atomic oxygen source is advantageous for the growth of ZnO thin films on Au(111) and Cu(111) surfaces.27 Their experimental results demonstrate the formation of 14 ACS Paragon Plus Environment

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a well ordered ZnO film on Cu(111) by using NO2 as the oxidizing agent which was not achieved when oxygen was used.27 E. Zn deposition on CuOx/Cu(111): 450 K in an oxygen ambient In order to explore the effect of temperature on the growth morphology of ZnOx on CuOx/Cu(111), experiments were carried out by dosing Zn onto CuOx/Cu(111) at 450 K in an O2 background of 5×10-7 Torr. The resulting STM images of ZnO/CuOx/Cu(111), θoxide = 0.15 monolayer collected at 300 K are summarized in Figure 7. At 450 K, the formation of triangular ZnO islands near the step edges was observed similar to the 550 K growth (Figure 5 and 6). However, at 450 K the average size of the islands is much smaller (200-300 nm in size) compare to the 550 K growth. Figure 7a shows a ZnO island growing over multiple Cu steps in a carpet mode growth and Figure 7b shows a zoomed-in image which clearly shows the morphology of the island. At 450 K, the ZnO islands consists of a large number of triangular pits, the first, second, and third incomplete layers of ZnO leveled as A, B, C and D respectively in Figure 7b. Line profile measurement performed across A, B, C,D

is shown in the right. The triangular pits (A) contain CuOx 44

structure, the height of the first and the second ZnO layers with respect to the pits (cu-44) are ~3.5 Å and 6 Å respectively. The island morphology at 450 K is slightly different to that observed at 550 K: At 450 K, the ZnO particles inside the island are more spread out and there are large number of pits throughout the island whereas at 550 K, the particles are sintered and the island often contains a central depression. Interestingly, the pits observed at 450 K and the central depressions observed at 550 K expose bare CuOx/Cu thus providing a direct interface between ZnO-Cu in each ZnO unit. In the next step, we did a stepwise oxidation of the surface to higher temperatures and the results are shown in Figure 8. It is noted that in order to heat, the sample has to be transferred to the 15 ACS Paragon Plus Environment


manipulator in the preparation chamber and then transfer back to the STM stage for scanning. Therefore, it was not possible to target one triangular island and observed the structure evolution in the successive oxidation steps. The images reported in Figure 8 are the most commonly observed island types at various oxidation temperatures. Figure 8a shows one triangular ZnO island growth over multiple step edges observed at 450 K which has a very porous structure as explained before. The surface was then slowly heated to 550 K in the presence of 5×10-7 Torr O2 background for 20 minutes and then cooled to 300 K for imaging. As shown in Figure 8b, after oxidation to 550 K the islands maintain the triangular frame as observed at 450 K. However, after heating to 550 K, the depressions or holes inside the island are mostly centered and the island morphology closely resembles to that observed at 550 K growth. Increasing the oxidation temperature to 600 and 700 K, (Figure 8c and d) leads to disruption of the triangular structures and creates more open structures indicating some Zn loss form the surface. Line profile measurement performed across the islands (shown in the right) suggests that the average height of the particles inside the island increase with increase in oxidation temperature (6 Å at 450 K to 2 nm at 700 K), indicating sintering at higher temperature. This is further studied by XPS and is shown in Figure 9. XPS clearly show the reduction in Zn 2p signal when the ZnO/CuOx/Cu(111) prepared at 450 K was annealed to higher temperatures. The corresponding Zn LMM Auger spectra showed a decrease in the amount of ZnO that was present on the CuOx/Cu(111) substrate when the annealing temperature increased from 450 to 700 K. F. Catalytic implications of the structural heterogeneity for the ZnO islands As mentioned in the Introduction, a HR-TEEM study revealed that the active phase of ZnO-Cu based catalysts under CO2 hydrogenation reaction conditions consists of Cu particles encapsulated by a ZnO overlayer with a graphite-like structure.6 Our experimental results show that different 16 ACS Paragon Plus Environment

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type of zinc oxide structures can be formed on Cu(111) depending on the methodology followed for the preparation. The sample temperature used for the preparation has a strong effect on the size and stability of the ZnO particles. In our studies, we did not see graphite-like structures as seen for the powder catalyst under reaction conditions.6 However, in catalytic tests, we found that our ZnO/CuOx/Cu(111) systems were much more active for CO2 hydrogenation than Cu(111). Figure 10 compares the catalytic activity of systems where ~ 30% of the copper substrate was covered with ZnO. The ZnO overlayer was prepared at 400 or 600 K to induce the formation of overlayers of ZnO with different sizes (10-20 nm versus 300-500 nm) and morphologies as described above. For clean Cu(111), the measured catalytic activity was ~ 3 x 1012 methanol molecules cm-2 s-1. The addition of ZnO to the copper substrate enhanced the catalytic activity by two orders of magnitude. For a sample preparation at 400 K, one has a smaller particle size (see above) but the system has a limited stability at 600 K. Initially the catalytic activity of this type of sample is high, but it drops with time as a consequence of the disappearance of ZnO from the surface of the catalyst, as seen in XPS/Auger measurements (bottom of Figure 11). On the other hand, the ZnO/CuOx/Cu(111) sample prepared at 600 K has a bigger ZnO particle size (see above), less catalytic activity, but higher stability in Figures 10-11. For both samples, the measured line shape for the Zn LMM Auger spectra (Figure 11) points to the presence of ZnO on the copper surface during reaction 21. In other study we found that the ZnO/CuOx/Cu(111) systems generated by a direct oxidation of ZnCu alloys are able to catalyze well the oxidation of CO and the water-gas shift reaction at relatively low temperatures.28 The present work shows the best approach for dealing with the issue of catalyst stability at the high temperatures necessary for the conversion of CO2 to methanol when the formation of an active phase with a ZnO/Cu configuration has been detected for copper-zinc oxide powder catalysts.6,10,11 To get high stability, the ZnO overlayer must be 17 ACS Paragon Plus Environment


growth at 550-600 K. The growth approaches previously tested at lower temperatures16,18,24,27 did not generate the very large islands of ZnO necessary for obtaining a zinc oxide/copper catalyst stable at elevated temperatures. Conclusion In this study, STM and XPS were used to investigate the growth mode of small coverages of ZnO, θoxi < 0.3 monolayer, on a Cu(111) substrate. The oxidation of ZnCu alloys produced small clusters of a ZnO on top of a CuOx/Cu(111) substrate. On the other hand, the ZnO/CuOx/Cu(111) overlayer prepared by vapor deposition of Zn at 500-600 K in an oxygen ambient produced very large triangular islands of ZnO. The triangular island structures were very rough and contained big depressions which encapsulate the Cu. The porous nature of the island provides high surface area and deep holes can provide a direct ZnO-Cu interface, thus giving a configuration which can be a very active catalyst for methanol production. The ZnO islands prepared at 450 K contain large number of pits which expose bare Cu, thus providing a large number of ZnO/Cu interfaces inside each unit. Our experimental results show that from a stability point of view it is best to grow the ZnO overlayer at elevated temperatures (500-600K). Particle sintering and a significant loss of Zn from the surface were observed when the ZnO overlayer was grown at 400-450 K and then annealed to higher temperatures in 5×10

-7

Torr oxygen ambient or under the reaction conditions for CO2

hydrogenation to methanol.

Supporting Information Available 18 ACS Paragon Plus Environment

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STM images of rarely observed non triangular ZnO islands on top of CuOx/Cu(111). This material is available free of charge via the Internet at http://pubs.acs.org

Notes The authors declare no competing financial interest

Acknowledgements The research carried out at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Science and Office of Basic Energy Sciences under contract No. DE-SC0012704. SDS is supported by a U.S. DOE Early Career Award. PR is grateful for financial support by BID and EN-SCN.

References

1. Behrens, M.; Studt, F.; Kasatkin, I.; Kühl, S.; Hävecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; Kniep, B.-L. The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts. Science 2012, 1219831. 2. Kuld, S.; Thorhauge, M.; Falsig, H.; Elkjær, C. F.; Helveg, S.; Chorkendorff, I.; Sehested, J. Quantifying the Promotion of Cu Catalysts by ZnO for Methanol Synthesis. Science 2016, 352, 969-974. 3. Kuld, S.; Conradsen, C.; Moses, P. G.; Chorkendorff, I.; Sehested, J. Quantification of Zinc Atoms in a Surface Alloy on Copper in an Industrial‐Type Methanol Synthesis Catalyst. Angew. Chem. 2014, 126, 6051-6055. 4. Nakamura, J.; Choi, Y.; Fujitani, T. On the Issue of the Active Site and the Role of ZnO in Cu/ZnO Methanol Synthesis Catalysts. Top. Catal. 2003, 22, 277-285. 5. Grunwaldt, J.-D.; Molenbroek, A.; Topsøe, N.-Y.; Topsøe, H.; Clausen, B. In Situ Investigations of Structural Changes in Cu/ZnO Catalysts. J. Catal. 2000, 194, 452-460. 6. Lunkenbein, T.; Schumann, J.; Behrens, M.; Schlögl, R.; Willinger, M. G. Formation of a ZnO Overlayer in Industrial Cu/ZnO/Al2O3 Catalysts Induced by Strong Metal–Support Interactions. Angew. Chem. 2015, 127, 4627-4631. 7. Kondrat, S. A.; Smith, P. J.; Wells, P. P.; Chater, P. A.; Carter, J. H.; Morgan, D. J.; Fiordaliso, E. M.; Wagner, J. B.; Davies, T. E.; Lu, L. Stable Amorphous Georgeite as a Precursor to a High-Activity Catalyst. Nature 2016, 531, 83. 19 ACS Paragon Plus Environment


8. Behrens, M.; Furche, A.; Kasatkin, I.; Trunschke, A.; Busser, W.; Muhler, M.; Kniep, B.; Fischer, R.; Schlögl, R. The Potential of Microstructural Optimization in Metal/Oxide Catalysts: Higher Intrinsic Activity of Copper by Partial Embedding of Copper Nanoparticles. ChemCatChem 2010, 2, 816-818. 9. Martin, O.; Mondelli, C.; Cervellino, A.; Ferri, D.; Curulla‐Ferré, D.; Pérez‐Ramírez, J. Operando Synchrotron X‐Ray Powder Diffraction and Modulated‐Excitation Infrared Spectroscopy Elucidate the CO2 Promotion on a Commercial Methanol Synthesis Catalyst. Angew. Chem. Int. Ed. 2016, 128, 11197-11202. 10. Frost, J. Junction Effect Interactions in Methanol Synthesis Catalysts. Nature 1988, 334, 577. 11. Holse, C.; Elkjær, C. F.; Nierhoff, A.; Sehested, J.; Chorkendorff, I.; Helveg, S.; Nielsen, J. H. Dynamic Behavior of CuZn Nanoparticles under Oxidizing and Reducing Conditions. J. Phys. Chem. C 2015, 119, 2804-2812. 12. Schumann, J.; Kröhnert, J.; Frei, E.; Schlögl, R.; Trunschke, A. Ir-Spectroscopic Study on the Interface of Cu-Based Methanol Synthesis Catalysts: Evidence for the Formation of a ZnO Overlayer. Top. Catal. 2017, 60, 1735-1743. 13. Schumann, J.; Eichelbaum, M.; Lunkenbein, T.; Thomas, N.; Alvarez Galvan, M. C.; Schlögl, R.; Behrens, M. Promoting Strong Metal Support Interaction: Doping ZnO for Enhanced Activity of Cu/ZnO: M (M= Al, Ga, Mg) Catalysts. ACS catal. 2015, 5, 3260-3270. 14. Kattel, S.; Ramírez, P. J.; Chen, J. G.; Rodriguez, J. A.; Liu, P. Active Sites for CO2 Hydrogenation to Methanol on Cu/ZnO Catalysts. Science 2017, 355, 1296-1299. 15. Palomino, R. M.; Ramírez, P. J.; Liu, Z.; Hamlyn, R.; Waluyo, I.; Mahapatra, M.; Orozco, I.; Hunt, A.; Simonovis, J. P.; Senanayake, S. D. et al. Hydrogenation of CO2 on ZnO/Cu (100) and ZnO/Cu (111) Catalysts: Role of Copper Structure and Metal–Oxide Interface in Methanol Synthesis. J. Phys. Chem. B 2018, 122, 794-800. 16. Sano, M.; Adaniya, T.; Fujitani, T.; Nakamura, J. Formation Process of a Cu− Zn Surface Alloy on Cu (111) Investigated by Scanning Tunneling Microscopy. J. Phys. Chem. B 2002, 106, 7627-7633. 17. Funk, S.; Bozzolo, G.; Garcés, J.; Burghaus, U. Atomistic Modeling of Zn Deposition on the Low Index Faces of Copper. Surf. Sci. 2005, 600, 195. 18. Liu, B.-H.; Groot, I. M.; Pan, Q.; Shaikhutdinov, S.; Freund, H.-J. Ultrathin Zn and ZnO Films on Cu (111) as Model Catalysts. Appl. Catal., A 2017, 548, 16-23. 19. Matsumoto, T.; Bennett, R.; Stone, P.; Yamada, T.; Domen, K.; Bowker, M. Scanning Tunneling Microscopy Studies of Oxygen Adsorption on Cu (1 1 1). Surf. Sci. 2001, 471, 225245. 20. An, W.; Xu, F.; Stacchiola, D.; Liu, P. Potassium‐Induced Effect on the Structure and Chemical Activity of the CuxO/Cu (1 1 1)(X≤ 2) Surface: A Combined Scanning Tunneling Microscopy and Density Functional Theory Study. ChemCatChem 2015, 7, 3865-3872. 21. Schön, G. Auger and Direct Electron Spectra in X-Ray Photoelectron Studies of Zinc, Zinc Oxide, Gallium and Gallium Oxide. J. Electron Spectrosc. Relat. Phenom. 1973, 2, 75-86. 22. Gunther, S.; Danhardt, S.; Wang, B.; Bocquet, M.-L.; Schmitt, S.; Wintterlin, J. Single Terrace Growth of Graphene on a Metal Surface. Nano lett. 2011, 11, 1895-1900. 23. Sutter, P. W.; Flege, J.-I.; Sutter, E. A. Epitaxial Graphene on Ruthenium. Nat. Mater. 2008, 7, 406.


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24. Thang, H. V.; Tosoni, S.; Pacchioni, G. Evidence of Charge Transfer to Atomic and Molecular Adsorbates on ZnO/X (111)(X= Cu, Ag, Au) Ultrathin Films. Relevance for Cu/ZnO Catalysts. ACS Catal. 2018, 8, 4110-4119. 25. Mora-Fonz, D.; Lazauskas, T.; Farrow, M. R.; Catlow, C. R. A.; Woodley, S. M.; Sokol, A. A. Why Are Polar Surfaces of ZnO Stable? Chem. Mater. 2017, 29, 5306-5320. 26. Dulub, O.; Diebold, U.; Kresse, G. Novel Stabilization Mechanism on Polar Surfaces: ZnO (0001)-Zn. Phys. Rev. Lett. 2003, 90, 016102. 27. Xinfei, Z.; Hao, C.; Hao, W.; Rui, W.; Yi, C.; Qiang, F.; Fan, Y.; Xinhe, B. Growth of Ordered ZnO Structures on Au (111) and Cu (111). Acta Phys.-Chim. Sin. 2018, 34, 1373-1380. 28. Mahapatra , M.; Gutierrez , R.A.; Kang , J.; Rui , N.; Hamlyn , R.; Liu , Z.; Orozco , I.; Ramirez , P.J.; Senanayake, S.D.; Rodriguez, J.A , The Behavior of Inverse Oxide/Metal Catalysts: CO Oxidation and Water-gas Shift Reactions over ZnO/Cu(111) Surfaces, Surf. Sci. 2018, doi: https://doi.org/10.1016/j.susc.2018.09.008

Figure Captions Figure 1 STM images of 0.1 ML Zn/Cu(111) collected at 300 K: a) Large scale (500 nm2 ), b)

Zoomed in image (125 nm2), the line profile measurement performed across a Zn/Cu patch is shown in the right. Images were collected at 0.1 nA, +1.3 V tunneling conditions Figure 2 Oxidation of 0.1 ML Zn/Cu(111): a) Large scale(500 nm2) STM image collected after the Zn/Cu(111) surface was oxidized at 300 K, b) zoomed in image (250 nm2) after the oxidation at 300 K, a triangular pit and an oxidized Zn/Cu island are highlighted by yellow triangle and circle respectively, the line profile measurement performed across an oxidized Zn/Cu island is shown below, c) 500 nm2 STM image after oxidation at 460 K, the inset shows the CuOx-44 substrate . All images were collected at 0.1 nA, +1.3 V tunneling conditions Figure 3 Zn LMM AES profile of 0.1 ML Zn deposited onto Cu(111) at 300 K plus subsequent oxidation at higher temperatures: a) Zn/Cu(111) at 300 K, b) oxidation at 300 K, c) oxidation at 460 K, d) oxidation at 550 K



Figure 4 0.2 ML Zn deposited onto CuOx/Cu(111) at 300 K followed by oxidation at higher temperatures: a) Zn LMM Auger profile where the different oxidation temperatures are indicated in the figure , b) Large scan area STM image (800 nm2) showing the formation of ZnO clusters of various size distribution on CuOx/Cu(111), oxidation temperature 550 K, image were collected at 0.1 nA, +0.8 V tunneling conditions Figure 5 STM images of the ZnO/CuOx/Cu(111) overlayer prepared when Zn was vapor deposited onto a CuOx/Cu(111) at 550 K in 5× 10 -7 Torr oxygen ambient. Top Panels: a) large scale (1500 nm2) image showing the triangular ZnO island on top of CuOx/Cu(111) , θZnO = 0.1 ML, the inset shows the CuOx/Cu(111) surface; b) large scale (1500 nm2) image showing three triangular ZnOisland on top of CuOx/Cu(111), θZnO = 0.2 ML. Bottom panels: 3D images of the structures shown in the top panels. All images were collected at 0.1 nA, +1.3 V tunneling conditions Figure 6 STM images of the most commonly observed ZnO islands at ~550 K, θoxi = 0.2 ML: a) 800 nm2 size STM image showing the growth of one ZnO island over multiple Cu step edges, b) 600 nm2 size STM image showing the growth of one ZnO island on a single terrace and Cu step faceting induced by ZnO, c) zoomed in image (220 nm2) of the area highlighted by a square in b which clearly shows the four different steps in the ZnO island, d) 1000 nm2 image showing the growth of one ZnO island on a flat terrace. The line profile measurements performed on a and b are shown below the respective figures. All images were collected at 0.1 nA, +1.3 V tunneling conditions Figure 7 ZnO/CuOx/Cu(111) systemprepared when Zn was vapor deposited onto a CuOx/Cu(111) surface at 450 K in 5×10 -7 Torr oxygen ambient, θoxi = 0.15 ML a) 250 nm2 size image showing the growth of one ZnO island over multiple step edges, b) 3D view of the island shown in part “a”, c) Zoomed in image showing a clear view of the different layers inside a ZnOisland. Images were collected at 0.1 nA, +1.3 V tunneling conditions Figure 8 STM images of ZnO/CuOx/Cu(111) prepared by vapor depositing Zn onto CuOx/Cu(111) at 450 K in 5×10 -7 Torr oxygen ambient and then annealed to high temperatures in oxygen background a) as prepared surface at 450 K, b) annealed to 550 K, c) annealed to 600 K and d) annealed to 700 K . The line profile measurements are shown in the right. All images were 300 nm2 size, tunneling conditions used are 0.1nA, +1.2 V Figure 9 Zn 2P XP spectra collected by vapor depositing Zn onto CuOx/Cu(111) at 450 K in 5×10 -7 Torr oxygen ambient and then heated to high temperatures in oxygen background. A clear reduction in Zn 2P XP signal intensity was observed as a result of high temperature heating. Figure 10 Reaction rate for the production of methanol on ZnO/CuOx/Cu(111) surfaces prepared by depositing Zn under an O2 atmosphere (5 x 10-7 Torr) on CuOx/Cu(111) at 400 and 600 K. In each case, ~ 30% of the copper substrate was covered by ZnO. T= 550 K, PH2= 4.5 atm, PCO2= 0.5 atm. Figure 11 Zn LMM Auger spectra collected after performing the synthesis of methanol on ZnO/CuOx/Cu(111) surfaces prepared by depositing Zn under an O2 atmosphere (5 x 10-7 Torr) on CuOx/Cu(111) at 400 and 600 K. In each case, ~ 30% of the copper substrate was covered by ZnO. 22 ACS Paragon Plus Environment

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T= 550 K, PH2= 4.5 atm, PCO2= 0.5 atm. The Zn Auger spectra were collected after removing the reaction mixture and flashing the sample to 500 K. No signal was seen in the C 1s XPS region.

TOC GRAPHIC



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Fig. 1


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Fig. 2



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Fig. 3


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Fig. 4



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Fig. 5


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Fig. 6



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Fig. 7


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Fig. 8



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Fig. 9


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Fig. 10



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Fig. 11


Cu

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