Low Temperature Heterolytic Adsorption of H2 on ZnO

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Low Temperature Heterolytic Adsorption of H on ZnO(101 #0) Surface Hong Shi, Hao Yuan, Zhe Li, Wenyuan Wang, Zhenyu Li, and Xiang Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01447 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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Low Temperature Heterolytic Adsorption of H2 on ZnO(𝟏𝟎𝟏𝟎) Surface Hong Shi†, Hao Yuan‡ , Zhe Li†, Wenyuan Wang†, Zhenyu Li‡,‖,* and Xiang Shao†,‖,* † Department

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

‖

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

Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China * To whom correspondence should be addressed: [email protected]; [email protected]


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Abstract: The interaction of molecular hydrogen with a ZnO(1010) surface at cryogenic conditions was investigated by in-situ scanning tunneling microscopy (STM) experiments combined with density functional theory (DFT) calculations. The H2 molecules were found splitting at extremely low temperature around 20 K and growing into a novel one-dimensional chain structure along the [0001] direction, which extends as long as several hundred nanometers and possesses substantial thermal stability up to around 220 K. Both high resolution STM images and DFT calculations reveal that the initial H2 splitting occurs through a physisorbed precursor state followed by forming heterolytic bonds to the Zn and O ions belonging to two neighbored Zn-O pairs of the ZnO(1010) surface. These findings have shed new light on the active roles of ZnO in various hydrogen related techniques as well as hydrogenation reactions.



1. Introduction The interaction of hydrogen molecules with metal oxides has received increasing interests in the past decades.1-4 This is mostly owing to the importance of hydrogen as fuels and as feedstock for chemical industries, as well as to the popular involvements of oxides in these processes. Being a technically important metal oxide, ZnO has been widely applied in electronics and photoelectronics, sensors and catalysis.5,6 Its interaction with hydrogen can have important consequences on each aspect of the related applications. Previous studies of exposing atomic hydrogen to ZnO demonstrated the formation of strongly bound surface H species.7,8 But the synergistically H doping is usually unavoidable which can significantly alter the property of the ZnO material.9,10 In contrast, molecular hydrogen was theoretically considered interacting weakly with the stoichiometric ZnO surfaces,11 even though ZnO-based catalysts have been practically utilized in industrial transformation of syngas into methanol and other hydrocarbons.12,13 Very recently, Mun et al.14 has demonstrated with near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) and temperature programmed desorption spectroscopy (TPD) that under high pressure conditions H2 can dissociatively adsorb on the subsurface oxygen anions of a ZnO(0001) surface at above room temperatures. Nevertheless, atomic level experiments of H2 adsorption on ZnO have not been reported yet, which is however crucial for unveiling the mechanism of hydrogen activation and hydrogenation reactions.15,16 In this work, we have investigated the adsorption of H2 on the nonpolar ZnO(1010) surface with low temperature scanning tunneling microscopy (STM) in combination with density functional theory (DFT) calculations. Our in-situ dosing experiments directly evidenced that H2 exposure cannot lead to any adsorption at above liquid nitrogen temperature, but surprisingly, at cryogenic conditions below 55 K, the dosed H2 can split into atomic hydrogen and form chain-like structures which can be stabilized until close to room temperature (RT).


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2. Experimental and Calculation Methods The experiments were carried out on a commercial LT-STM system from Createc Co. (Germany), which is housed in an ultrahigh vacuum (UHV) chamber with base pressure of 1.0×10-10 mbar. The ZnO(1010) single crystal (Princeton Scientific Corp.) was cleaned by cycles of Ar+ sputtering with subsequent annealing to ~ 1000 K. To avoid the contamination from residual gases, usually the as-prepared ZnO sample was transferred into the STM stage as fast as possible after the last annealing treatment. Hydrogen gas (Air product Co., 99.999%) was purified by a LN2-cooled trap before introduced into the chamber via leak valves. For in-situ H2 adsorption, the STM tip was retracted by about 5 mm away from the surface and then approached again to find the same area after dosing H2. The H2 gas was introduced through a tubular doser directed to a shutter-protected hole on the cold shield of the STM. The H2 pressure was read from the ion gauge before and after exposing, and was usually controlled below 5.0×10-8 mbar. In order to avoid thermal cracking of H2, during exposing the ion gauges were all switched off just before opening the shutter and switched on again after the shutter was closed. While liquid helium was always filled for cooling, a 2450-sourcemeter was used to heat the STM stage to a scheduled temperature between 10 K and 77 K. For ex-situ exposing experiments the cleaned ZnO(1010) crystal was cooled to around 20 K on a 4-axis manipulator. After H2 exposure, the sample was then transferred into the STM stage which was kept at 77 K within ~15 seconds to avoid warming up of the sample as little as possible. Post-annealing treatments of the H2-covered samples were all performed on the manipulator which was pre-cooled to certain temperatures. All ex-situ exposed and post-annealed samples were scanned at 77 K. All STM images were acquired in constant-current mode using an electrochemically etched gold tip. First-principles calculations were performed based on DFT as implemented in the Vienna ab initio simulation package (VASP) 17,18 within the project-augmented wave (PAW) framework19,20. The generalized gradient



approximation in form of the Perdew-Burke-Ernzerhof functional (GGA-PBE) was used to describe the exchange-correlation effect 21. A DFT-D2 correction was applied to accurately describe the van der Waals interaction22. A plane-wave cutoff of 500 eV was used for all calculations. The convergence criteria of energy and force were set to 10-5 eV and 0.01 eV/ Å, respectively. A ZnO(1010)-(3×3) surface with four double layers was built and the last two double layers were fixed. Lattice parameters used were in accordance with our previous work23 (a=3.317 Å, c=5.314 Å). Adsorption energy is defined as follows: Eads = Etot – Esuf – EH2, where Eads, Etot and EH2 represent energies of the whole system, the surface, and the H2 molecule, respectively. We also carried out climbing-image nudged elastic band (CI-NEB)24 calculations with 5 images to find the H2 dissociation energy barrier.

3. Results and Discussion ZnO(1010) is the most stable facet of ZnO crystal which can be prepared as atomically flat under ultrahigh vacuum (UHV) conditions.25 It also dominates the exposed surfaces of various ZnO nanomaterials.26 As shown in Figure 1a, the clean ZnO(1010) surface is stoichiometric and free of any observable point defects at the top surface. The regularly distributed bright chains running along the [1210] direction are usually assigned to the surface Zn ion rows imaged at positive sample bias.23 Inset in Figure 1a shows the atomically resolved image wherein each Zn ion presents as a round protrusion by which the rectangle unit cell can be well defined. After in-situ dosing 0.8 L (Langmuir, 1L=10-6 torr ▪ s) of H2 at ~20 K, we immediately observed a number of straight and dotted chains running along the [0001] direction, as shown in Figure 1b. Extended exposure led to the growth of the chains both in length and concentration, as shown in another sequence of in-situ exposure experiment presented in Figure S1. The long chains usually get terminated at either a step edge or a subsurface defect.23 At larger exposure such as 15 L, compact


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domains with (2×1) periodicity relative to the substrate lattice were formed, as shown by Figures 1c and 1d. Noticeably, similar chainlike structures have been proposed for both water and CO2 molecules on the same surface.23, 27

But their formations require higher temperature when the molecules can overcome the diffusion barrier on the

surface. Therefore, here at such low temperature, only H2 and its adsorption on ZnO(1010) surface can explain the observation. Actually, in Figure 1e we propose the observed linear structure as an H-chain structure along the [0001] direction which is formed by the heterolytic adsorption of H2 on both surface Zn2+ and O2- sites. The simulated STM image displays perfect consistency with the experimental topography obtained at positive sample bias, as shown in Figure 1f, wherein the Zn-H species are imaged as round protrusions while the O-H species present as dark.

Figure 1. STM images of the exact ZnO(1010) surface area before (a) and after (b) dosing 0.8 L H2 molecule at 22 K. Image size: 36 × 36 nm. Tunneling condition: 3.9 V, 100 pA. Inset in (a) shows the atomic resolution of the clean ZnO(1010) surface obtained at LN2 temperature. The red rectangle highlights the unit cell of the surface. Image size: 4.2 nm ×4.2 nm. Tunneling condition: 2.5 V, 2 nA. (c, d) Large size (30 × 30 nm) and magnified (10 × 10 nm) STM images of the same sample as (a) and (b) but exposed to 15 L of H2 at 22 K. The blue box highlights the unit cell of



the (2 × 1) superstructure. Tunneling conditions: (c) 3.9 V, 100 pA; (d) 3.0 V, 50 pA. (e) Optimized ball model of an infinite H-chain along [0001] direction on ZnO(1010). H, Zn, and O atoms are colored in white, blue (surface) and grey (subsurface), red (surface) and pink (subsurface), respectively. (f) Overlapping of experimental with simulated STM images of the H-chain showing perfect consistency. The blue and red lines mark the rows of surface Zn and O ions, respectively.

The proposed dissociative adsorption of H2 is surprising at the first glance considering that the H-H breaking occurs at cryogenic temperature as low as 20 K. We also performed a series of in-situ dosing experiments at raised temperatures until 77 K, which clearly turned out a stepwisely reduced adsorption of H2 with the temperature rise, as shown in Figure S2 of the supporting information. Experimentally, when the sample was warmed up to about 55 K, no H-chains could be observed even with a much larger exposure up to 15 L. Such a low temperature threshold, as well as the negative correlation between the H2 adsorption probability with the sample temperature strongly suggest a precursor-mediated adsorption mechanism which has been frequently evidenced on metal surfaces.28 Its validity for certain oxide surfaces such as H2/ZnO can also be possible despite of scarce investigations on the adsorption dynamics having been reported. More interestingly, when annealing the sample with pre-adsorbed H2 we found that the H-chains can be stabilized to well above the adsorption temperature limit (55 K). As shown in Figure 2a, we prepared an “ex-situ” sample by dosing about 150 L of H2 onto the ZnO(1010) surface which was held on the manipulator and cooled to around 20 K, and the scanning at liquid nitrogen temperature (77 K) still observed almost 0.2 ML of H-chain on the surface. Subsequent annealing to ~200 K saw negligible reduction of the H-chains, as shown in Figure 2b. Even after annealing to RT, there were still some H-chains reserved on the surface (Figure 2c). Figure 2d shows the plot of coverage versus annealing temperature. One can find the fast desorption actually occurs


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at around 220 K, which is by far higher than the usual desorption of other weak adsorbates such as CO.29,30 This desorption behavior also unambiguously excludes any possibility that some contaminations from residual gas of the UHV chamber such as H2O and CO2 have formed the observed chain structures, because the desorption temperatures of the latter two are well above RT.31,32

Figure 2. The STM images of a H2/ZnO(1010) sample after annealing to different temperatures. (a) 77 K (b) 200 K and (c) 300 K. The sample was prepared by exposing 150 L of H2 to a clean ZnO(1010) kept outside of STM stage and ~ 20 K. (d) The statistical results of the surface coverage of H2 versus annealing temperature. Image size: 100 nm × 100 nm. Tunneling condition: 3.9 V, 100 pA. All images were obtained at 77 K.

To understand the mechanism of H2 adsorption and the formation of chain structure on the ZnO(1010) surface, we have performed DFT calculations. Two possible heterolytic adsorption configurations of an isolated H2 molecule on ZnO(1010) was firstly considered in Figures 3a and 3b, while Figures 3c and 3d present the corresponding charge density dispersions. In Figure 3a, the two split H atoms bind to the neighbored Zn and O ions belonging to two different Zn-O pairs ((Zn-O)a and (Zn-O)b). While in Figure 3b, the splitting of H2 takes place right above a single



Zn-O pair and form concerted bonds of Oa-H and Zna-H. In both cases the Zn-H has a positive charge density while the O-H is negative, as shown in Figures 3c and 3d. In addition, the Zn-H is geometrically more protrusive than the O-H species, hence being imaged as bright under STM. Energetically, the calculated adsorption energy (Ea) of configuration I (2H:Zna-Ob) is around -0.30 eV while that of configuration II (2H:Zna-Oa) is around -0.51 eV. Both are essentially exothermic hence thermodynamically permissible. Considering the narrow and low temperature window for H2 adsorption, we propose that the H2 splitting occurs through a physisorbed precursor state followed by a strongly bound chemisorption state.28 As shown by the inset in Figure 3e, the physisorbed H2 orients in the (1210) plane and lies between the Zn-O pairs with an adsorption energy of -0.19 eV. Setting it as the initial state, we performed climb-image nudge energy barrier (CI-NEB) calculations for the two heterolytic adsorption configurations. The calculated plot in Figure 3e reveals a formation barrier of 0.19 eV for I but 0.55 eV for II, respectively. Clearly, configuration I is more kinetically preferred while II is more thermodynamically preferred. Which configuration would take over may depend on the experimental conditions.


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Figure 3. Two optimized configurations of the heterolytically adsorbed H2 molecule on (a) two neighbored Zn-O pairs and (b) one single Zn-O pair, respectively. (c) and (d) are the corresponding top-view models of (a) and (b), respectively. The charge density around the hydrogen atoms are presented in yellow (positive) and cyan (negative) colors. (e) The dissociation barriers for a physically adsorbed H2 to form the two heterolytic adsorption configurations of (a) and (b), respectively. The side view model of the physisorbed H2 is shown in the inset.

In our experiments the singly dispersed H2 was rarely observed under our conditions. Alternatively, we always observed the H-chains on the ZnO(1010) surface, even at sufficiently low exposures, which strongly suggests an extremely small barrier for the diffusion of H2 on the surface (calculated to be ~ 0.08 eV), as well as for the H-chain growth. In Figures 4a and 4b we show the calculated configurations of the shortest H-chain (H2 dimer) along the [0001] direction, formed through two different pathways starting respectively from configuration I (Figure 3a) and



II (Figure 3b) of a singly adsorbed H2. The growth barrier is shown in Figure 4c, wherein the slight difference of the anisotropic growth along [0001] and [0001] directions were also considered. One can immediately recognize the prominently low barrier (~0.1 eV) for chain growth from configuration I (pathway I), which is even lower than the case of single H2 adsorption shown in Figure 3. Notably, with the attachment of a second H2, the thermodynamic advantage of configuration II (pathway II) is significantly reduced. Further growth of the H-chain through the two pathways would more or less follow the same energetic picture as shown in Figure 4c. And the energetic difference of the two end-states finally becomes completely negligible for an extended H-chain. In this respect, the pathway I earns an overwhelming advantage of low barrier thus can be largely preferable mechanism for the observed long H-chains under STM. In addition, we found the structural detail of configuration I can better explain our STM observations of the termination of H-chains at the step edges. As shown in Figures 4d and 4e, along the [0001] direction, the H-chains always end with an unoccupied Zn row to the Zn-terminated step edge along [1210], in the opposite direction the H-chains can extend to the sharp edge of the O-terminated [1210] step. Such a phenomenon can be perfectly explained by the model in Figure 4a since in the [0001] direction the Zn ion of the terminated Zn-O pair is always unoccupied. In contrast, the model of Figure 4b shows that the H-chain terminates with a Zn-H species in the [0001] direction, which would be imaged as a protrusion instead of the bare ZnO surface. This evidence further strengthens our proposal that the observed H-chain is originated from H2 splitting over two neighbored Zn-O pairs and the chain growth should follow pathway I. In this scenario we can easily understand why no isolated dissociated H2 was observed with STM even though they show sufficiently high diffusion barrier on the surface (lowest for the Zn-H along the [1210] direction, see Figure S3). This is because pathway I provides a recombination reaction channel with much lower barrier (~0.3 eV, see Figure 3), and the yielded physisorbed H2 can readily diffuse on the surface until captured by another dissociated H2.


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Figure 4. (a and b) Formation of H2 dimers from singly adsorbed H2 with configuration I and II, respectively. The red and yellow arrows indicate that the second H2 molecule attached to the right side of the first H2, while the dotted blue and green arrows indicate a reverse direction growth of H2 dimer. (c) CI-NEB calculations of the energy barriers along the four pathways shown in (a) and (b). Again, the initial configuration of the second H2 is physical adsorption in undissociated form. (d) STM image of the H-chains terminated at differently orientated step edges of a ZnO island. Size: 26 nm×18 nm. Tunneling condition: 3.0 V, 100 pA. (e) Magnified image focusing at the (0001) step showing an unoccupied Zn row at the termination of the H-chain. Size: 5.5 nm × 7.5 nm. Tunneling condition: 3.0 V, 100 pA.

In addition to unveiling the chain growth mechanism, the calculations of the H2 dimers clearly demonstrate a substantial energy gain upon forming chain-like structure along [0001] direction. As presented in Table 1, the averaged Ea can finally reach around -0.69 eV for each H2 in an infinite chain. In sharp contrast, stacking compactly in the [1210] direction would significantly reduce the adsorption energy of the heterolytically adsorbed H2, which



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thus becomes unfavorable and was not observed at all in our experiments. Moreover, the increasing trend of Ea of the [0001] chain strongly suggests that the chain growth is a self-catalyzed process, explaining the formation of long H-chains of up to several hundred nanometers spreading across the whole terrace. At the meantime, the detachment of H2 from a long H-chain becomes more difficult, which perfectly explains the significant thermal stability of the chain structure. In Table 1, we also notice the (2×1) superstructure has comparable Ea as the infinite [0001] chain. Therefore, their appearances were frequently observed at high coverage, as shown by the inset in Figure 1b. In contrast, the full coverage of H2 with (1×1) structure has a substantially lower adsorption energy thus never realized in our experiments.

Table 1. Adsorption energy and formation barrier for different H2 splitting configurations on ZnO(1010) surface Barrier

Ea*

(eV)

(eV)

2H : Zna-Ob

0.19

-0.30

2H : Zna-Oa

0.55

-0.51

2nd H2 splitting along Ⅰ’

0.08

-0.48

2nd H2 splitting along Ⅰ’’

0.11

-0.48

2nd H2 splitting along IⅠ’

0.54

-0.58

2nd H2 splitting along IⅠ’’

0.47

-0.58

Configuration

Infinite chain along [0001]

-0.69

Infinite chain along [1210]

-0.29

2H: (2×1)

-0.67

2H: (1×1)

-0.53

* Ea is averaged over all the adsorbed H2 molecules.

4. Conclusion In summary, with a combination of low-temperature STM experiments and DFT calculations, we demonstrate that H2 molecules can split on a ZnO(1010) surface at cryogenic temperatures and construct one-dimensional chain structure with substantial stability. The splitting of H2 is proposed through a weakly bound precursor state before


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forming concerted bonds with two neighbored Zn-O pairs. While the afterwards chain growth is revealed a self-catalyzed process meeting with both the extended chain length and the significant thermal stability observed in the experiments. Our results provide the direct evidence for the important roles of ZnO in activating hydrogen molecules. These findings could significantly deepen our understanding of the working mechanisms of ZnO under various circumstances with H2 in presence. Moreover, this work also establishes a novel strategy for preparing surface hydrogen species in a mild manner which can be valuable for extensive investigations of the H-related issues.

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

Notes The authors declare no competing financial interest.

Supplemental Material: Additional STM images showing temperature-dependent adsorption of H2.

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Low Temperature Heterolytic Adsorption of H2 on ZnO

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