Directional Growth of One-Dimensional CO2 ... - ACS Publications

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article 2

Directional Growth of One-Dimensional CO Chains on ZnO(10¯10) Hong Shi, Shiqi Ruan, Huihui Liu, Li Wang, Wenyuan Wang, Xinguo Ren, Bing Wang, Kai Wu, and Xiang Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08358 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Directional Growth of One-dimensional CO2 Chains on ZnO(10૚0) Hong Shi,1 Shiqi Ruan,1 Huihui Liu,1 Li Wang,1 Wenyuan Wang,1 Xinguo Ren,2,3,* Bing Wang,4 Kai Wu5,* 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 4

5

HFNL, University of Science and Technology of China, Hefei 230026, China

BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

Corresponding authors: [email protected], Tel: +86-551-63600765 [email protected], Tel: +86-551-63600830; [email protected], Tel: +86-10-62754005

Abstract: Atomic insights into the interaction of CO2 with the mixed-terminated ZnO(101ത0) surface were achieved in detail by low-temperature scanning tunneling microscopy (LT-STM) together with density functional theory (DFT) calculations. The binding site and adsorption geometry were directly imaged by LT-STM, revealing that the CO2 molecules are chemisorbed and turned into surface carbonate species. The strong interaction of CO2 with the ZnO(101ത0) surface in turn activates the surface, i.e. reconstructs the local surface such that facilitates further CO2 bindings, leading to the formation of one-dimensional assembly structure which grows along the [0001ത] direction. DFT simulations indicated that the superior agglomeration energies along 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

directions as well as the CO2-induced surface reconstruction are responsible for the directional growth of the surface carbonate chains.

2


Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction: Capture of gas phase CO2 molecules and their transformations into value-added chemicals are of fundamental importance to the human society, due to the societal and public concerns of global warming issues caused by the release of CO2 into the atmosphere.1 Metal oxide surfaces have been extensively investigated for this purpose, such as alkaline earth metal oxides,2, 3 TiO2,4 CeO2,5 RuO26 and ZnO,7 or their combinations.8, 9 Particularly, ZnO has been widely applied as active materials for catalyzing the production of methanol and small alkane/olefin from CO2-containing feedstock.10, 11 Despite the intensive studies, the mechanism of CO2 activation on ZnO-based catalysts is still elusive.12 In this work, we report a detailed low-temperature scanning tunneling microscopy (LT-STM) study on the interaction of CO2 with ZnO(101ത0), the most stable and abundant ZnO surface, in combination with accurate density functional theory (DFT) simulations. Our findings shed new light on the adsorption and activation mechanism of CO2 during its chemical fixing process. The interactions of CO2 with ZnO surfaces have been extensively studied with various surface science techniques as well as theoretical calculations.13-16 Some general understandings have been achieved, such as the different binding strengths on different ZnO faces and the formation of CO2- intermediates before its transition into other higher-grade chemical products.9,

13

However, the subtle differences in adsorption configurations of CO2 and their influence in the surface processes on ZnO are not fully understood. Therefore, more inputs from the high-spatial-resolution characterizations are highly desirable. Under such circumstances, we have conducted a LT-STM study of in-situ adsorption of CO2 molecules on the ZnO(101ത0) surface at liquid nitrogen (LN2) temperature. In the particular low-coverage regime, we observed that the adsorbed CO2 formed unexpectedly one-dimensional (1D) carbonate chains along the [0001ത] direction of the substrate. Such ordered species may have significant impacts on the CO2-involved surface reactions on ZnO.

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 19

Experimental and Theoretical Methods: STM 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 (LEED) optics and quadrupole mass spectrometer (QMS, pfeiffer). The ZnO(101ത0) single crystals were purchased from Princeton Scientific Corp. and have dimensions of 10×3×0.5 ݉݉ଷ . The samples were cleaned by repeated Ar+ sputtering at 2 keV followed by annealing to 900 K in O2 atmosphere (1.0 × 10-6 mbar) for tens of cycles, until the sharp (1×1) LEED pattern can be observed. To ensure the sample cleanness, usually the last annealing was performed without oxygen and the sample was transferred into the STM stage as fast as possible. Subsequent STM characterizations confirmed this operation can well protect the surface from contaminations. All the STM measurements were conducted at LN2 temperature. The images were acquired in constant-current mode using an electrochemically etched gold tip. CO2 (99.995%, Linde AG) was dosed onto the sample at the STM stage through a tubular doser which approaches close to the shutter-protected hole on the LN2-cooled shield. During in-situ dosing operations, the tip was retracted away from the surface as far as about 5 mm, which completely ruled out the shadowing effect of the tip. The exposing pressures were monitored from the ion gauge readings and controlled below 1×10-8 mbar. The exposure amounts were calculated from the gauge reading and the exposing time. All calculations were performed with the generalized-gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE)17 as implemented in the all electron FHI-aims code package.18,19 As shown previously, 13,20 the PBE functional is well suited for describing binding energies of adsorbate on ZnO surfaces. 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). For CO2 molecule in free space, the optimized C-O bond length is 1.170 Å which is 0.008 Å larger than the experimental value of 1.162 Å. The ZnO(101ത0) surface was model by a (4×4) slab consisting of four ZnO layers with additional 35 Å vacuum to avoid 4


Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the interaction between the repeated slabs. Geometry relaxation was carried out by adding CO2 onto ZnO surface with the last two ZnO layers fixed. To evaluate the energy preference of various CO2 aggregates, free CO2 molecules were added on top of different surface O atoms as starting configurations, thus constructing different aggregations. The adsorption energy is defined as ‫ܧ‬௔ௗ௦ = ‫ܧ‬஼ைమ /௓௡ை − n ∙ ‫ܧ‬஼ைమ − ‫ܧ‬௓௡ை , wherein ‫ܧ‬஼ைమ , ‫ܧ‬௓௡ை and ‫ܧ‬஼ைమ /௓௡ை denote energies of free CO2 molecule, bare ZnO surface and the whole adsorption system, respectively, and n denotes the number of the adsorbed CO2 molecules. We also carried out a climbing-image nudged elastic band (CI-NEB) calculation to find the minimum energy path for CO2 migration from [12ത10] direction to [0001]. The threshold of residual forces for NEB convergence was set as 0.2 eV/Å and the threshold for the climbing-image method was set as 0.05 eV/Å to get an accurate energy barrier. This calculation was conducted in a 4×4 supercell with two CO2 adsorbed on the ZnO(101ത0). The initial and final structures for these two calculations are the same. But CO2 migration directions are opposite.

Result and Discussion: ZnO(101ത0) is the only face of ZnO crystal that can be atomically flat and un-reconstructed after routine UHV sputtering-annealing treatments.21,22 As shown in Fig. 1a, the as-prepared surface displays large terraces defined by sharp steps running along and directions. Near-atomic resolution can be routinely achieved, showing the bright parallel lines running along the directions which are normally assigned as the Zn2+ rows on the surface.22 Occasionally, the Zn2+ ions can be resolved as bright spots at even closer tip-surface distances, thus the rectangular surface lattice can be clearly imaged as shown by the inset in Fig. 1a. On this sample, no obvious lattice defects were observed at the surface, which differs from what were reported in the literatures that Zn-O dimer vacancies frequently appear on the UHV-cleaned ZnO surface.23 This is possibly due to the relatively high annealing temperature that we have employed. Actually, we frequently observed dim and bright dot-like species that 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 19

show uniform nanometer size and disperse evenly on the surface, as marked by the circles in Fig. 1a. The zoom-in images in Figs. 1b and 1c clearly demonstrate that these species do not cause any lattice discontinuity, suggesting that they do not position on the top surface, but possibly locate under the surface, i.e. in subsurface region. Therefore, we tentatively assign them as two kinds of different subsurface defects (More detailed investigations of their nature and properties will be given in a forthcoming paper).

Figure 1. STM images of (a) the as-prepared clean ZnO(101ത0) surface (30 nm × 30 nm) and (d) after exposing to 0.5 L of CO2 at LN2 temperature. (b) and (c) are zoom-in images of the bright (red circle) and dim (dashed black circle) features with near-atomic resolution. (6 nm × 4 nm). Inset in (a) is the atomically resolved image showing the Zn lattice. The black box and blue rectangles highlight the singly dispersed and linearly organized CO2 molecules respectively. Inset in (d) is the amplified image of a single CO2 species. All images were acquired with U=2.0 V, I=500 pA.

After the ZnO surface was cleaned, CO2 gas was directly introduced onto the sample which was retained at the STM stage, with the STM tip was retracted far away from the surface. This operation was conducted with the purpose of preventing the ZnO sample from contamination by residual gas during exposure of CO2 outside the cold shield of STM. As shown in Fig. 1d, many dark lines appeared after about 0.5 L (Langmuir, 1×10-6 Torr﹒sec) CO2 dosage, indicating the CO2 molecules have assembled into various linear structures along the directions of ZnO at LN2 temperature. Here the CO2 molecules were imaged as bar-like pits, possibly due to the specific scanning condition as well as the termination of the tip.24 A closer look at the singly 6


Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dispersed CO2 molecules (marked by the black box and amplified in the inset in Fig. 1d) immediately reveals that they are located right between the bright lines, i.e. the Zn2+ rows on the surface. The blue rectangle encircles a typical linear CO2 ensemble running across more than ten Zn2+ rows, reaching around 5 nm in length. More detailed analyses of such ensembles will be given below. It is worth noting that in the whole image and many other larger images as well, we rarely identified (2×1) superstructure domains that have been proposed in previous reports.13-16 Our in-situ dosing strategy is crucial to build the correlation of the molecular adsorption and reactivity with the specific structural features of the surface. It is particularly important for studying the CO2 adsorption on ZnO, which is quite active in adsorbing residual water molecules in the UHV chamber, leading to undistinguishable structures comparable to CO2.25 Figs. 2a and 2b show the exactly same ZnO surface area before and after in-situ dosage of 0.05 L CO2, wherein the adsorbed CO2 molecules can be immediately identified, displaying as bar-like dark features randomly distributed on the surface. It is noticed that there exists no special correlation between the CO2 adsorption sites and the surface defects, as exemplified by the steps or the aforementioned subsurface defects shown in Figs. 1b and 1c. Zoom-in images of an individual CO2 molecule are given in Figs. 2c-2e, illustrating their topography dependence on the imaging bias voltage. By reference to the Zn2+ ion lattice (the superimposed yellow grids over the images), one can clearly identify that the adsorbed CO2 is centered in the middle of two Zn2+ ion rows and displays an unsymmetrical topography along the direction. This indicates that the CO2 molecule is positioned on top of surface O2- ions with its molecular plane parallel to the direction. According to the height profiles given in Fig. 2f, it appears that the neighboring Zn2+ and O2- ions on adjacent rows display significantly different contrast, in a comparison with those without nearby CO2. It is suggestive of the adsorbate-induced lattice relaxation accompanied by spatially charge redistribution.

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 19

Figure 2. STM characterization of the same area of ZnO(101ത0) (a) before and (b) after in-situ dosing CO2. Scan size: 30 nm x 30 nm, U=2.4 V, I=100 pA. (c)-(e): Zoom-in images of a single CO2 obtained with sample bias of 3.7 V, 2.8 V and 2.4 V respectively. The yellow grids represent the Zn lattice. Notice here the lattice grids around the CO2 molecules are intentionally uncovered for better viewing. Image sizes: 2.5 nm x 2.5 nm. (h) Corresponding profiles along the different arrows in (c-e). The dashed curve shows the surface corrugation in absence of CO2 adsorption. The grey rectangle marks the region of adsorbed CO2. (g) and (h) show the side and top views of the DFT simulated configuration of an isolated CO2 on ZnO(101ത0). In the ball models the oxygen and carbon atoms of CO2 are colored in blue and brown, respectively, while the oxygen and zinc atoms of the ZnO substrate are colored in red and grey, respectively.

In order to unravel the interaction details of CO2 on the ZnO surface, we have conducted DFT calculations based on the FHI-aims code.18,19 Figs. 2g and 2h show the side and top views of an optimized CO2 molecule on the ZnO(101ത0) surface. Two surface Zn-O dimers (labeled as O1-Zn1 and O2-Zn2) are involved in the CO2 binding. The carbon atom of the CO2 is positioned almost right on top of a substrate O ion (labeled as O2), while its two oxygen atoms strongly bind to adjacent Zn ions (labeled as Zn1 and Zn2), both results indicate that a tridentate configuration is formed. The CO2 adopts a bent configuration, where the O-C-O bond angle becomes about 130o, similar to that for a surface carbonate species.13,

14

This configuration

implies that a partial charge flow from the ZnO substrate to the chemisorbed CO2 molecule may 8


Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


occur, which is further supported by the Mulliken charge analysis (see Fig. S2 for the detailed charge distribution around the surface-bound CO2). Furthermore, the whole molecule sits vertically on the surface, with its molecular plane parallel to the (12ത10) plane of ZnO. These calculations are in excellent agreement with our STM observations and the proposed adsorption configuration of CO2 on the same surface but at higher coverages.13-16 We notice that the CO2-bonded surface atoms, including Zn1, O2 and Zn2 are significantly affected, thus deviated from original positions. In particular, Zn1 and Zn2 are experiencing drastically different bonding configurations, well explaining their topographic variations under the STM imaging, as shown in Figs. 2c-2e. The configuration variation also spreads to the neighboring oxygen rows along the direction, resulting in slight topographic changes at these positions as well. More detailed information about the bond lengths and angles of the optimized CO2 configuration on the ZnO(101ത0) surface is given Fig. S1. The calculated DFT adsorption energy within the Perdew-Burke-Ernzerhof (PBE)

17

generalized gradient approximation of an individual CO2 amounts to -0.67 eV, in consistence with the chemisorption picture. Correspondingly, the surface-bound CO2 molecules are rather stable under the repeated STM tip interactions. No displacement was observed even under relatively rigorous scanning conditions such as high bias and/or high current. Upon more dosage of CO2 molecules onto the surface, chain-like CO2 aggregates gradually develop, as clearly shown by the sequential images in Fig. 3 that were acquired exactly at the same place after each dosing step.26 More interestingly, if one focused on the species marked by the arrows in the images, one immediately recognize the 1D chain growth of CO2 is not isotropic but instead adapts preferentially to the [0001ത] direction of the surface. The complete growth process can be better viewed in the video presented in the supporting information.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 19

Figure 3. Sequential STM images obtained at the same area after each batch dosing of CO2 for (a): 0.007 L (b): 0.017 L, (c): 0.037 L, and (d): 0.087 L. The arrows point to the growth direction of the CO2 chains. The bright curves at the right bottom corners are the steps serving as the markers. Image size: 20 nm × 20 nm. All images were acquired at U = 2.6 V and I = 320 pA.

Both the 1D assembling behavior and the directional growth of CO2 chains are quite surprising, as in the literatures the (2×1) superstructure was the only proposed ordered phase of CO2 at low coverages.13,15 Why would the CO2 molecules aggregate along the [0001ത] direction rather than stack against the Zn-O dimer rows of the surface? To answer this question, we have calculated the formation energies of different CO2 aggregates based on a (4×4×4) super cell of the ZnO surface. As shown in Fig. 4, four distinct geometries of the CO2 dimers were calculated. The calculated adsorption energies per molecule indicates that the largest energy gain (-0.75 eV versus -0.67 eV) occurs when two CO2 molecules adsorb next to each other along the direction. And the energy increases with the chain length (see Table S1). The stabilization of the chain structure possibly results from the attractive dipole-dipole interactions between the bent CO2 molecules since their planes are aligned along the direction. In contrast, in a stack fashion the dipole-dipole interactions of the neighboring CO2 may be repulsive. Therefore, when 10


Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


two CO2 molecules adsorb on the same Zn-O row along the direction, either closely stacked into a (1×1) or separated by one lattice away to form a (2×1) pattern, the adsorption energies would remarkably decrease by 0.3 eV and 0.1 eV, respectively, with respect to the single molecule case. The energies would decrease along as more molecules are attached. Fig. 4d shows a specific adsorption geometry with two CO2 molecules adsorbed at adjacent rows but in the next-nearest neighbor (NNN) positions against each other. Its adsorption energy slightly increases compared to the singly dispersed molecules. Therefore, the close linear connection of CO2 molecules along the direction is superior to the sparsely distributed adsorption, manifesting the preferential formation of long carbonate chains even at very low coverage. This is consistent with our STM observations. Along the direction, however, the CO2 molecules would prefer staying away from each other. In fact, our calculations demonstrate that the previously reported (2×1) superstructure can hardly form when the coverage is below 0.3 ML. The higher the coverage, the denser the linear structures, until recognizable (2×1) superstructures forms. Actually, in our in-situ dosing experiments, (2×1) islands were only observed at higher coverages and were frequently accompanied by the physisorbed CO2 molecules (Figs. S3 and S4), rendering their similar formation probabilities.

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 19

Figure 4. Averaged adsorption energies per molecule of various CO2 aggregations on the surface. (a): (2×1) alignment along . (b): (1×1) chain along . (c): (1×1) alignment along . (d): two CO2 adsorbed on NNN positions at the adjacent rows.

The directional growth of the CO2 chains is obviously kinetically controlled, since the final adsorption structures of CO2 are undistinguishable no matter the new molecules attach from the head or the tail of the already-existing chain. As shown in Fig. 5a where the local atomistic bonding of the surface ions around a chemisorbed CO2 molecule is presented, one can find that the Zn1 and Zn2 cations drastically differ in their coordination configurations. Both Zn cations are bonded to four O2- anions. But Zn1 is much closer to the center of a tetrahedron whereas Zn2 is much closer to the face of the tetrahedron. Therefore, one may anticipate that it is much easier for Zn2 to form bonds with the incoming inactivated CO2 molecule which has normally a linear O-C-O configuration. As a matter of fact, this anticipation is strongly supported by our CI-NEB calculations. 27, 28 Fig. 5b shows the CI-NEB results performed using the “aims-Chain” tool which is modified from the Atomic Simulation Environment (ASE) package.29 We started with two CO2 molecules adsorbed in a (2×1) pattern. Then Molecule 2 was fixed but Molecule 1 was allowed to move until it reached either of the neighboring sites of Molecule 2 along the directions and form a dimer. The black curve displays the energy change for the dimer formation 12


Page 13 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


along the [0001] direction, while the red curve for the [0001ത] direction. Both reaction paths climb three barriers, the second of which can be taken as the rate-determining (RD) one since it contains the highest-energy intermediate. Therefore, the overall reaction barrier can be determined from the energy difference between the initial state and the highest transition state. As shown in Fig. 5b, the activation energy along [0001ത] is slightly lower (~0.02 eV) than along [0001] direction, indicating that the former is the reaction preferred direction, particularly at low temperatures. It is noted that the calculation in Fig. 5b only shows the possible reaction path corresponding to one preset adsorption geometry of two CO2 molecules. By integrating all kinds of adsorption possibilities as encountered in real exposing process, the preference of chain growth along the [0001ത] direction can be significant, which finally turns out to be a prominent phenomenon in our in-situ STM observations conducted at LN2 temperature.

Figure 5. (a) Bonding configurations of the neighbored Zn cations around an adsorbed CO2 molecule. (b) Comparison of the energy barriers for CO2 growth along [0001] (black) and [0001ത] (red) directions.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 19

Conclusions: In conclusion, we have investigated CO2 adsorption on the ZnO(101ത 0) surface by low temperature STM in combination with DFT calculations. Our in-situ characterization demonstrates that at low coverages the CO2 molecules tend to form 1D chain-like structures instead of aggregating into compact islands. The chain growth also clearly prefers the [0001ത] direction of the ZnO, induced by the activated Zn ions that are located unsymmetrically aside of the adsorbed CO2 molecule. The linear assembling structure together with the directional adsorption dynamics provides new insights of CO2 interactions with the ZnO surfaces, paving a route for better understanding of the related surface chemistry of ZnO, for instance, methanol synthesis and Fischer-Tropsch synthesis.

Supporting information: Additional STM images and calculation results, and a video of the 1D directional growth of CO2 on the surface.

Acknowledgement: We are grateful for the financial support of NSFC (91545128, 21333001, 11374276, 11574283) and MOST (2014CB932700). X. S. thanks the financial support of the Fundamental Research Funds for the Central Universities and the Thousand Talent Program for Young Outstanding Scientists of the Chinese government. The “Strategic Priority Research Program” of the Chinese Academy of Sciences (grant XDB01020100) is also acknowledged.

References: (1) Karl, T. R. Modern Global Climate Change. Science 2003, 302, 1719-1723. (2) Sun, Z.; Wang, J.; Du, W.; Lu, G.; Li, P.; Song, X.; Yu, J. Density Functional Theory Study on the Thermodynamics and Mechanism of Carbon Dioxide Capture by CaO and CaO Regeneration. RSC Adv. 2016, 6, 39460-39468. 14


Page 15 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Jensen, M. B.; Pettersson, L. G. M.; Swang, O.; Olsbye, U. CO2 Sorption on MgO and CaO Surfaces: A Comparative Quantum Chemical Cluster Study. J. Phys. Chem. B 2005, 109, 16774-16781. (4) Tan, S.; Zhao, Y.; Zhao, J.; Wang, Z.; Ma, C.; Zhao, A.; Wang, B.; Luo, Y.; Yang, J.; Hou, J. CO2 Dissociation Activated Through Electron Attachment on the Reduced Rutile TiO2(110)-1×1 Surface. Phys. Rev. B 2011, 84, 155418. (5) Albrecht, P. M.; Jiang, D.; Mullins, D. R. CO2 Adsorption As a Flat-Lying, Tridentate Carbonate on CeO2(100). J. Phys. Chem. C 2014, 118, 9042-9050. (6) Wang, Y.; Lafosse, A.; Jacobi, K. Adsorption and Reaction of CO2 on the RuO2(110) Surface. J. Phys. Chem. B 2002, 106, 5476-5482. (7) Noei, H.; Wöll, Ch.; Muhler, M.; Wang, Y. Activation of Carbon Dioxide on ZnO Nanoparticles Studied by Vibrational Spectroscopy. J. Phys. Chem. C 2011, 115, 908-914. (8) Burghaus, U. Surface Science Perspective of Carbon Dioxide Chemistry—Adsorption Kinetics and Dynamics of CO2 on Selected Model Surfaces. Catal. Today 2009, 148, 212-220. (9) Burghaus, U. Surface Chemistry of CO2 – Adsorption of Carbon Dioxide on Clean Surfaces at Ultrahigh Vacuum. Prog. Surf. Sci. 2014, 89, 161-217. (10) Spencer, M. S. The Role of Zinc Oxide in Cu/ZnO Catalysts for Methanol Synthesis and the Water–Gas Shift Reaction. Top. Catal. 1999, 8, 259-266. (11) Jiao, F.; Li, J.; Pan, X.; Xiao, J.; Li, H.; Ma, H.; Wei, M.; Pan, Y.; Zhou, Z.; Li, M. et al. Selective Conversion of Syngas to Light Olefins. Science 2016, 351, 1065-1068. (12) Behrens, M.; Studt, F.; Kasatkin, I.; Kuhl, S.; Havecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; Kniep, B. L. et al. The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts. Science 2012, 336, 893-897. (13) Wang, Y.; Kováčik, R.; Meyer, B.; Kotsis, K.; Stodt, D.; Staemmler, V.; Qiu, H.; Traeger, F.; Langenberg, D.; Muhler, M. et al. CO2 Activation by ZnO through the Formation of an Unusual Tridentate Surface Carbonate. Angew. Chem. Int. Ed. 2007, 46, 5624-5627. (14) Buchholz, M.; Weidler, P. G.; Bebensee, F.; Nefedov, A.; Wöll, Ch. Carbon Dioxide 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

Adsorption on a ZnO(101ത0) Substrate Studied by Infrared Reflection Absorption Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 1672-1678. (15) Tang, Q.; Luo, Q. Adsorption of CO2 at ZnO: A Surface Structure Effect from DFT+U Calculations. J. Phys. Chem. C 2013, 117, 22954-22966. (16) Kotsis, K.; Stodt, D.; Staemmler, V.; Kováčik, R.; Meyer, B.; Traeger, F.; Langenberg, D.; Strunskus, Th.; Kunat, M.; Wöll, Ch. CO2 Adlayers on the Mixed Terminated ZnO(101ത0) Surface Studied by He Atom Scattering, Photoelectron Spectroscopy and Ab Initio Electronic Structure Calculations. Z. Phys. Chem. 2008,222, 891–915. (17) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (18) Blum, V.; Gehrke, R.; Hanke, F.; Havu, P.; Havu, V.; Ren, X.; Reuter, K.; Scheffler, M. Ab Initio Molecular Simulations with Numeric Atom-centered Orbitals. Comput. Phys. Commun. 2009, 180, 2175-2196. (19) Havu, V.; Blum, V.; Havu, P.; Scheffler, M. Efficient Integration for All-electron Electronic Structure Calculation Using Numeric Basis Functions. J. Comput. Phys. 2009, 228, 8367-8379. (20) Calzolari, A.; Catellani, A. Water Adsorption on Nonpolar ZnO(101̅0) Surface: A Microscopic Understanding. J. Phys. Chem. C. 2009, 113, 2896-2902. (21) Diebold, U.; Koplitz, L. V.; Dulub, O. Atomic-scale Properties of Low-index ZnO Surfaces. Appl. Surf. Sci. 2004, 237, 336-342. (22) Dulub, O.; Boatner, L. A.; Diebold, U. STM Study of the Geometric and Electronic Structure of ZnO(0001)-Zn, (0001ത)-O, (101ത 0), and (112ത 0) Surfaces. Surf. Sci. 2002, 519, 201-217. (23) Kováčik, R.; Meyer, B.; Marx, D. F Centers versus Dimer Vacancies on ZnO Surfaces: Characterization by STM and STS Calculations. Angew. Chem. Int. Ed. 2007, 46, 4894-4897. (24) The tip condition can be tuned by capturing or releasing an adatom or molecule from the tip apex. In our case, mostly possibly the tip apex was modified by a CO molecule that has been adsorbed from the residual gas inside the chamber. 16


Page 17 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Meyer, B.; Marx, D.; Dulub, O.; Diebold, U.; Kunat, M.; Langenberg, D.; Wöll, Ch. Partial Dissociation of Water Leads to Stable Superstructures on the Surface of Zinc Oxide. Angew. Chem. Int. Ed. 2004, 43, 6641-6645. (26) The STM images have been flattened and specially lightened, in order for clearer viewing of the CO2 molecules and the substrate lattice at the meantime. With this treatment, the step edges at the right bottom corner of the images display as bright curves. (27) Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978-9985. (28) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901-9903. (29) Bahn, S. R.; Jacobsen, K. W. An Object-oriented Scripting Interface to A Legacy Electronic Structure Code. Comput. Sci. Eng. 2002, 4, 56-66.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 19

18


Page 19 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO2 form molecular chains on ZnO(10-10) 50x45mm (300 x 300 DPI)


Directional Growth of One-Dimensional CO2 ... - ACS Publications

Recommend Documents