An STM and DFT Study

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Identifying Different Adsorption States of Methanol on ZnO(10 #10): An STM and DFT Study Shiqi Ruan, Zhe Li, Hong Shi, Wenyuan Wang, Xinguo Ren, and Xiang Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00576 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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The Journal of Physical Chemistry

Two different adsorption configurations of methanol on a ZnO(10-10) surface were identified with high resolution STM 214x180mm (168 x 168 DPI)

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Identifying Different Adsorption States of Methanol on ZnO(1010): An STM and DFT Study Shiqi Ruan,1,ǂ Zhe Li,1,ǂ Hong Shi,1 Wenyuan Wang,1 Xinguo Ren,2,3 and Xiang Shao1,2,* 1 Department

of Chemical Physics, CAS Key Laboratory of Urban Pollutant Conversion, University of Science and Technology of China, Hefei 230026, China

2

Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China 3

Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei 230026, China

ǂ These

authors contribute equally to this work

To whom correspondence should be addressed: [email protected]

1


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ABSTRACT: The adsorption and organization state of methanol on ZnO surface is of importance for understanding the mechanism of the related (photo) catalytic reactions. In this work, by using high resolution scanning tunneling microscopy (STM) in combination with density functional theory (DFT), we have unambiguously identified both the physi- and chemisorbed methanol species on the nonpolar ZnO(1010) surface whose distribution obviously depends on the temperature. The physisorption of methanol dominates at liquid nitrogen temperature but can transform into chemisorption upon either thermal annealing or electron injection. Moreover, the chemisorbed methanol mostly retains an undissociated state and tends to form a special onedimensional chain structure along the [0001] direction mediated by the intermolecular hydrogen bonding interactions. These findings are believed to provide fundamental information for a deepened understanding of methanol chemistry over the ZnO surface.

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1. INTRODUCTION Being one of the most important industrial chemicals and one of the most promising fuels, the methanol has steered a large number of research interests from both fundamental and practical aspects.1-4 Particularly, the atomic understanding of the interactions of methanol with various catalytic surfaces is of great importance for revealing the reaction mechanisms under heterogeneous conditions. The achieved knowledge is harvested not only from the recent applications of high-resolution characterization techniques including scanning tunneling microscopy and spectroscopy (STM)

5-7

but also from the fast developments of the theoretical

simulations8-9. Francis et al.10 studied the oxidation of methanol on the oxygen-covered Cu(110) surface. Yates and co-workers11 observed the clustering of physisorbed methanol on the Cu(111) surface at low temperatures. Guo et al.12 and Feng et al.13 independently studied the photocatalytic decomposition of methanol over the rutile TiO2(110) surface, and obtained new understanding picturing slightly different from what Henderson et al.14 have addressed with traditional techniques such as temperature programmed desorption (TPD) and infrared reflected adsorption spectroscopy (IRAS). ZnO is the important component of the industrially applied Cu/ZnO/Al2O3 catalyst for methanol synthesis from syngas15-20. It has also been regarded as the promising transparent electrode for methanol fuel cell 21-23. Therefore, a number of studies have been conducted aiming at unveiling the interactions/reactions of methanol with various ZnO surfaces24-29. Generally, methanol was found interacting with the ZnO surface rather strongly by forming chemical bonds30, 3


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but resulting in whether molecular or dissociated species upon adsorption is still under debate31-33. Au et al. 34 and Jones et al.30 conducted independent researches on ZnO single crystals. Through the characterizations of a number of spectroscopies including ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and variable-energy photoelectron spectroscopy (PES), they suggested that methanol dissociates into methoxy and hydroxyl species upon adsorption at room temperature. These propositions were supported by the recent STM works conducted at room temperature by Iwasawa and co-workers 35. Without the assistance of theory, they observed two types of ordered adsorption structures of methanol on the ZnO(1010) surface and tentatively assigned them to different dissociative configurations. However, controversial results were claimed by Kiss et al.36 based on their studies with infrared spectroscopy in combination with HAS (helium atom scattering) spectroscopy. They demonstrated that the chemisorbed methanol organized into a (2×1) superstructure with only half of the molecules dissociated into methoxyl and hydroxyls, similar to the case of water on the same surface 37. Notably, the previous studies mostly considered the collective behaviors of methanol on the ZnO surface which were deduced from surface averaging spectroscopies. The single molecular information is still vacant and heavily desired. Herein, we have conducted a low temperature STM work in combination with density functional theory (DFT) calculations of methanol adsorption on the ZnO(1010) surface. We particularly focused on the low coverage of methanol adsorbed either at liquid nitrogen (LN2) temperature or room temperature (RT). Our in situ dosing technique facilitated the direct recognition of both weakly and strongly bound methanol species, which are 4



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attributed to physisorbed and chemisorbed methanol molecules, respectively. Furthermore, upon either electron stimulation or thermal annealing the physisorbed methanol can be transformed readily into the chemisorbed state, the latter further organizing into a specific chain-like structure along the [0001] direction of the surface. To the best of our knowledge, these findings are novel and may shed new lights on the reaction mechanism of methanol on ZnO surfaces.

2. EXPERIMENTAL AND CALCULATION METHODS All experiments were performed on a Createc LT-STM housed in a UHV chamber with base pressure of 5.0 × 10-11 mbar. The sample preparation chamber has a base pressure of 1.0 × 10-10 mbar and is equipped with low energy electron diffraction optics (LEED, Specs) and quadrupole mass spectrometer (QMS, Pfeiffer). The ZnO(1010) single crystals (Princeton Sci.) were cleaned by repeated Ar+ sputtering at 2 keV followed by annealing to 900 K in O2 atmosphere (1.0 × 10-6 mbar) for cycles until a sharp (1×1) LEED pattern can be observed. The sample cleanness was ensured by flashing to annealing temperature followed by fast transfer into the STM chamber. All the STM measurements were conducted at LN2 temperature. The images were acquired in constantcurrent mode using an electrochemically etched gold tip. Dehydrated methanol (99.99%, Aldrich) was purified by freeze-pump-thaw cycles before introduced into the UHV chamber via a high precision variable leak valve (VAT). For in situ dosing experiments, the STM tip was retracted for about 5 mm away from the surface and the methanol beam was guided by a capillary that approaches close to the shutter-protected hole on the LN25


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cooled shield. For ex-situ adsorption, methanol was also directed by the tubular doser which opens about 20 mm away from the sample surface. The dosages were monitored by the pressure rise and the exposing time. All calculations were performed with the generalized-gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE)38 as implemented in the all electron FHI-aims code package3940.

For bulk wurtzite ZnO we obtained optimized lattice parameters of 3.288 Å and 5.273 Å,

respectively, which compare well with the experimental values (3.250 Å and 5.207 Å)

41.

The

ZnO(1010) surface was modeled by a (4×4) slab consisting of four ZnO layers with additional 35 Å vacuum to avoid the interaction between the repeated slabs. Geometry relaxation was carried out by adding methanol onto ZnO surface with the last two ZnO layers fixed. To evaluate the energy preference of various methanol aggregates, free methanol molecules were added on top of different surface O atoms as starting configurations, thus constructing different aggregations. The adsorption energy is defined as 𝐸𝑎𝑑𝑠 = 𝐸𝐶𝐻3𝑂𝐻/𝑍𝑛𝑂 ―n ∙ 𝐸𝐶𝐻3𝑂𝐻 ― 𝐸𝑍𝑛𝑂 wherein

𝐸𝐶𝐻3𝑂𝐻, 𝐸𝑍𝑛𝑂 and

𝐸𝐶𝐻3𝑂𝐻/𝑍𝑛𝑂 denote energies of free methanol molecule, bare ZnO surface and the whole adsorption system, respectively, and n denotes the number of the adsorbed methanol molecules. We also carried out a climbing-image nudged elastic band (CI-NEB)42-43 calculation with the help of the external code ASE44 to find the minimum energy path for methanol migration along both [1210] and [0001] directions. The calculations were conducted based on a (4×4) supercell of ZnO(1010) with one methanol molecule adsorbed. The threshold of residual forces for NEB convergence was set as 0.2 eV/Å and the threshold for the climbing-image method as 0.05 eV/Å in order to get an accurate energy barrier. Notice for both directional diffusions the initial and final 6



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configurations of the methanol are of the same energy. Demonstrative atomic models were all built with the VESTA software.45

3. RESULTS AND DISCUSSION 3.1 Adsorption of Methanol at Liquid Nitrogen Temperature. The flat and clean ZnO(101 0) surface can be routinely prepared and the detailed STM characterizations can be referred to previous work of our lab.41, 46 We then subjected the as-prepared surface to methanol adsorption experiments at different temperatures. We first present the adsorption result at LN2 temperature (77 K). As shown in Figure 1, by comparing exactly the same position before (Figure 1a) and after in-situ exposure to methanol vapor for 0.07 L (Figure 1b) and 0.20 L (Figure 1c) (L denotes Langmuir, 1 L=10-6torr · sec), one can clearly identify two types of adsorbates distributed randomly on the surface. The dominant one (around 90%) has a dumbbell-like shape and is aligned on the bright parallel lines which are assigned as the Zn2+ rows on the surface.

47

The minority

species (around 10%) is also residing on the Zn2+ rows but has a circular shape. The exclusive formation of these two types of species was confirmed by large number of scannings which were performed in different surface regions and with wide bias range. For convenience, we shall denote the dumbbell-like one as physisorbed (termed as Phys-Me) and the circular-shaped one as chemisorbed (termed as Chem-Me) methanol molecules. The reasons will be presented below. Figure 1d shows the height profiles of these adsorbed species along the [1210] direction. The dips correspond to the dark rim of the adsorbates. Therefore, the length of the Phys-Me species is 7


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measured as 0.84 nm while the Chem-Me as 0.52 nm, respectively. Both values are much larger than the size of the surface unit cell along this direction. In addition, the interval between the two lobes of the Phys-Me species is around 0.26 nm, which is smaller than the unit cell size of ZnO along the [1210] direction.

Figure 1. STM image sequence of (a) bare ZnO (1010) surface and after in-situ dosing (b) 0.07 L and (c) 0.20 L of methanol at 77 K. The black arrows in (a) indicate the [0001] and [1210] directions of the ZnO substrate. These orientations hold for all the images presented in this work. The solid red and dashed blue boxes in (b) mark the dumbbell-like physisorbed and circular-shaped chemisorbed methanol species, respectively. (d) Height profiles of the two different species along the red and blue lines in (c). Imaging conditions: 4.3 V and 0.14 nA. All STM images have the same size of 11.6×11.6 nm2. To understand the nature of the methanol-induced species, we have conducted DFT calculations on the adsorption configurations of methanol on the ZnO(1010) surface. We 8



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introduced methanol molecules with various orientations, angles and heights above the ZnO(1010) slab, and finally obtained two stable adsorption structures as shown in Figure 2. The one shown in Figures 2a (side view) and 2c (top view) is denoted as a Chem-Me since it has an adsorption energy of -1.25 eV. In this case, the methanol molecule binds to the surface via its oxygen atom and forms a strong chemical bond to the surface Zn cation whose length is only slightly longer than that of the Zn-O bond in the ZnO bulk (2.07 Å versus 2.02 Å). In addition, the OH group of the methanol forms a weak hydrogen bond to the surface O anion. Figures 2b and 2d show the side and top views of the Phys-Me on the ZnO(1010) surface. In this configuration, the methanol molecule binds to the surface only through a weak hydrogen bond connecting its OH group with the surface O anion, and results in a much smaller adsorption energy of -0.58 eV. Therefore, the Chem-Me actually forms a bidentate binding to the surface with its methyl group somehow fixed to one side of the vertical (1210) plane that is indicated by the dashed line in Figure 2c. This turns out that under the STM imaging, the chemisorbed methanol is always imaged as one stable and round protrusion. Similar topography was also reported for chemisorbed methanol on TiO2(110) surface.12,13 In contrast, the physisorbed methanol has a monodentate binding to the surface and its methyl group can readily rotate along this single bond. Under the moderate interaction with the STM tip, the physisorbed methanol can be imaged as a two-lobe feature due to the equal appearance of its methyl group at both sides of the vertical (1210) plane as indicated by the dashed line in Figure 2d. Such duplicated imaging of a single molecule was also reported on other weakly bound adsorbate systems such as CO2 on TiO2(110) and CO on the CuO film grown on Cu(110) surface.48,49 When the tip-surface interaction is increased, the physisorbed methanol would probably be moved by the 9


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tip, as will be discussed later. Figures 2e and 2f show the high-resolution STM images of chemisorbed and physisorbed methanol, respectively, which were obtained by a CO-terminated tip. The white grids represent the rectangular lattice of the surface Zn ions which is reproduced from the atomically resolved image taken at the same area. One can clearly see that the centers of the protrusions (marked by the red dots) in both of the Chem-Me and the Phys-Me species shift aside the top sites of the anchoring Zn positions (marked by black triangles). The STM images show very good agreement with the theoretical models. It needs to be mentioned that aside the above two configurations we also calculated a dissociative adsorption state of methanol on the ZnO(1010) surface, and resulted in rather small adsorption energy (around -0.8 eV).36,50 Considering that its formation requires breaking the O-H bond hence becomes much more difficult in comparison with the above two adsorption states, we take this species disfavored under our experimental condition hence unobserved at all.

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Figure 2. Side (a, b) and top (c, d) view models of chemisorbed (a and c) and physisorbed (b and d) methanol molecules on the ZnO(1010) surface. The substrate Zn and O ions are colored in gray and red, respectively. The C, O, and H atoms in the methanol molecule are colored in green, blue and orange, respectively. The dashed lines in (c) and (d) indicate the (1210) plane perpendicular to the (1010) surface. (e) and (f) show the high resolution STM images (size of 2.2×2.2 nm2) of chemisorbed methanol and physisorbed methanol, respectively. The white grids represent the Zn lattice. Imaging conditions: 2.9 V and 0.80 nA. 3.2 Adsorption of Methanol at Room Temperature. As a general knowledge, the selective formation of different adsorption state may depend on the temperature. Carrying this idea, we also conducted methanol adsorption experiments with an ex-situ strategy (out of the STM shield) at room temperature. Before doing that, we carefully examined the possible influence of the residual gasses on the clean ZnO surface at RT. Thanks to the low base pressure of the chamber, no contaminated adsorption was observed. As shown in Figures 3a-3c, along with the methanol dosage increasing from 0.1 L to 0.9 L, more and more methanol molecules appeared on the surface and organized into many linear structures. In the magnified image shown in Figure 3d, one can clearly recognize that the linear structure is composed of oval protrusions corresponding to the chemisorbed methanol molecule. Meantime, sparsely distributed physisorbed methanol can still be observed (marked by the black circles) yet with only 20% of the total number. This is in sharp contrast to the case of 77 K where almost 80% of the methanol takes a physisorption state.

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Figure 3. STM images (size of 50×50 nm2) of dosing (a) 0.10 L, (b) 0.32 L and (c) 0.90 L of methanol at 300 K. (d) Zoom-in image (size of 20×20 nm2) of the area marked by the black box in (c). Black circles in (d) highlight the randomly distributed physisorbed methanol molecules. All STM images were taken with U=3.8 V and I=0.10 nA. The linear arrangement of methanol along the [0001] direction is somehow similar to the chain structure formed by CO2 on the same surface41, which is possibly due to the templating effect of the rectangular lattice of ZnO(1010). To better understand the formation mechanism of this ordered structure, we simulated and compared the methanol dimers with four different spatial arrangements. Figures 4a-4d show the optimized configurations of two chemisorbed methanol molecules in adjacent positions together with the adsorption energy averaged molecularly. Recalling in the single methanol case the adsorption energy is -1.25 eV, one can immediately recognize that the dimer formed by two adjacent methanol molecules along the [0001] direction is the most favorable (Figure 4a), while that along [1210] direction (Figure 4b) is most unfavorable. Other configurations (Figures 4c and 4d) show almost undistinguishable adsorption energy as the 12



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isolated methanol. Since we only considered the methanol dimer at the current stage, the energy preferences of the ordering structures can be magnified when the structure grows, as already found in the CO2 adsorption on the same surface.41 Therefore, when the total coverage is low, we mainly observed very short aggregations of methanol on the surface. But when the coverage increases to a certain value, considerable long methanol chains start to form, as clearly shown in Figures 3a and 3c, respectively.

Figure 4. Top view models and molecularly averaged adsorption energies for four different methanol dimers adsorbed on the ZnO(1010) surface. (a) (1x1) chain along [0001]. (b) (1×1) alignment along [1210]. (c) (2×1) alignment along [1210]. (d) Two methanol chemisorbed on next nearest-neighbor positions of the adjacent rows. 13


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3.3 One-Dimensional Diffusion of Physisorbed Methanol. Owing to the strong binding to the surface, the Chem-Me species (either isolated or assembled in linear structure) were found very stable under STM scanning with bias ranging from 2.5 V to as high as 4.7 V. In contrast, the PhysMe species were found diffusing along the [1210] direction even with usual scanning parameters, i.e. moderate bias around 3.0 V and low tunneling current less than 100 pA, reflecting a weak binding to the surface. Figures 5a-c show a sequence of images on a sample prepared at LN2 temperature, presenting clearly the displacement of the Phys-Me species (see the black arrow) during scanning. Using the Chem-Me as a marker, the displacement of the Phys-Me can be measured as 3.3 Å which is in consistency with the lattice constant of ZnO in the [1210] direction. These observations are perfectly in line with our CI-NEB calculations of the surface diffusion of methanol species on ZnO. Figure 5d shows the potential energy profile of both Phys-Me and ChemMe species along the [1210] direction, while Figures 5e and 5f show the selected snapshots of the adsorption configurations of both species along the reaction pathways, respectively. One can clearly see that the migration barrier is as low as 0.17 eV for the Phys-Me while significantly high of 0.62 eV for the Chem-Me. It is therefore unobservable for the Chem-Me species to displace at LN2 temperature.

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Figure 5. (a)-(c) Sequential STM images (4.7×2.3 nm2) showing the diffusion of one Phys-Me species along the [1210] direction. These images were acquired at 77 K. The white and dashed black lines mark the initial and final positions, respectively. Imaging condition: 3.5 V and 0.13 nA. (d) Potential energy profiles for Phys-Me and Chem-Me diffusion along the [1210] direction. (e) and (f) show the selected snapshots of the Phys-Me and Chem-Me species diffusing in the [1210] direction. The initial positions are marked by ① and ③, while the end positions are marked by ② and ④, respectively. It is noted that under STM imaging the Phys-Me migration is one-dimensional along the [12 10] direction, which means in other directions the migration would experience higher energy barrier. To verify this idea, we also calculated the migration of the Phys-Me species along the [0001] direction. As a result, the CI-NEB calculation always fell into the valley state corresponding exactly to the Chem-Me configuration, which means any diffusing attempt along the [0001] direction would switch the methanol molecule from physisorption into chemisorption state. The calculated 15


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barrier for the transformation reaction is 0.21 eV which is slightly bigger than the diffusion barrier along the [1210] direction. This sufficiently low barrier allows the transformation of Phys-Me into Chem-Me state upon thermal excitation. Therefore, it well explains our observations that ChemMe dominates the surface at RT. The reverse reaction barrier, converting Chem-Me back into PhysMe, is calculated as 0.89 eV which is sufficiently higher than the diffusion barrier of the Chem-Me along the [1210] direction. This explains why the Chem-Me was so stable on the surface that we scarcely observed its displacement under STM scanning. 3.4 Tip Induced Conversion Of Phys-Me Into Chem-Me State. The CI-NEB calculations unambiguously explained the population of the Phys-Me and Chem-Me species at low and room temperatures. It also suggests that the Phys-Me can be feasibly converted into the Chem-Me state under STM tip manipulations since the barrier is only 0.04 eV higher than that for migration along the [1210] direction. In Figures 6a-6c we show a sequence of STM images obtained at different biases. There were originally four Phys-Me and one Chem-Me species in the imaging area. After increasing the bias from 3.1 V to 4.6 V, one Phys-Me species first displaced along the Zn row and eventually transformed into a Chem-Me species, as highlighted by the black ovals. In the meantime, the other three Phys-Me molecules were all displaced from the original positions, reflecting the strong manipulation effect of the tip. Similar experiments were repeated at different positions for varying biases ranging from 3.2 V to 4.7 V, and the conversion events within the scanning area of the same size (20×20 nm2) were averaged from eight different positions for each bias. As plotted in Figure 6d, when the bias is greater than the threshold of 3.2 V, the conversion reaction starts to occur and speed up monotonically with the increase of the bias. Of course, the diffusion frequency 16



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of the Phys-Me also increases synchronically, as demonstrated by Figures 6a-6c. We assume in this process the Phys-Me molecules are excited by the hot electrons injected from the STM tip, which then relax into a lower-level state upon releasing the electron to the substrate. Such process has also been proposed for other adsorbate reactions on oxide surfaces.51

Figure 6. (a)-(c) Sequential STM images (size of 12×5 nm2) of a same area obtained with different biases: (a) 3.1 V, (b) 4.6 V and (c) 4.6 V. (d) Histogram plot of the conversion events of Phys-Me into Chem-Me as a function of sample bias.

CONCLUSION In conclusion, we have investigated the adsorption behavior of methanol on the ZnO(1010) surface with high resolution STM and DFT calculations. Two adsorption states including the physisorbed and chemisorbed methanol were recognized whose population on the surface strongly depends on the adsorption temperature. The physisorbed methanol can be readily displaced along the [1210] direction and can be transformed into chemisorbed state upon tip interactions. In the case of chemisorbed methanol, the molecules distribute randomly on the surface at low temperature but show preference to organize into linear structures orientating along the [0001] direction at room 17


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temperature. These results clearly unveil the interaction of methanol with the ZnO surface, which may shed new lights on the catalytic mechanism of methanol reaction based on a ZnO catalyst.

Acknowledgement: We are grateful for the financial support of NSFC (Grant No. 21872130, 91545128, 21333001) and the National Key Research and Development Program of China (Grant No. 2017YFA0205003).

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An STM and DFT Study

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