Adsorption behavior of formaldehyde on ZnO (101¯0) surface: A first principles study

Accepted Manuscript ¯ Title: Adsorption behavior of formaldehyde on ZnO (1010) surface: A first principles study Authors: Wentao Jin, Guangde Chen, Xiangyang Duan, Yuan Yin, Honggang Ye, Dan Wang, Jinying Yu, Xuesong Mei, Yelong Wu PII: DOI: Reference:

S0169-4332(17)31781-6 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.125 APSUSC 36325

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Please cite this article as: Wentao Jin, Guangde Chen, Xiangyang Duan, Yuan Yin, Honggang Ye, Dan Wang, Jinying Yu, Xuesong Mei, Yelong Wu, Adsorption behavior ¯ surface: A first principles study, Applied Surface of formaldehyde on ZnO (1010) Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.125 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

̅𝟎) surface: a first principles Adsorption behavior of formaldehyde on ZnO (𝟏𝟎𝟏 study Wentao Jin1, Guangde Chen1, Xiangyang Duan1, Yuan Yin1, Honggang Ye1, Dan Wang1, Jinying Yu2, Xuesong Mei3, and Yelong Wu1,a) 1MOE

Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China

2School

3

of Physics, Northwest University, Xi’an, Shaanxi 710049, China

State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China.

a)

Author to whom correspondence should be addressed. Email address: [email protected]

Graphical Abstract

Highlights 

Electrostatic interactions derived from the polarization of formaldehyde molecule play an important role in determining the geometric structures. A chain-like adsorption model is proposed.

 1



Adsorbed formaldehyde molecule can overcome a very small energy barrier to break their C=O double bond, indicating the application potential of (101̅0) planes of ZnO in formaldehyde degradation.



The highest occupied orbital of adsorbed CH2O is lifted into the ZnO band gap as a deep donor level. The adsorbed CH2O will not have a big impact on the conductivity of the ntype ZnO samples.

ABSTRACT In a first principles study of the formaldehyde adsorption on ZnO surface, we found a novel chain adsorption structure on ZnO (101̅0) plane. This adsorption structure results from the electrostatic interactions between those adsorbed formaldehyde molecules and the unique arrangement of Zn-O surface dimers on (101̅0) plane. This adsorption mechanism has the potential to extend to other wurtzite materials’ (101̅0) plane and other similar cases. As the physical adsorption configurations are unstable, the chemical adsorption has to happen. The electronic properties show that the C=O double bond in CH2O turns into C-O single bond and the highest occupied molecule orbital (HOMO) of formaldehyde is lifted into ZnO band gap becoming the hole trapping center. These results may be meaningful for formaldehyde degradation and detection.

1. Introduction Formaldehyde is an important precursor to many industrial materials, such as various resins, which are widely used in wood processing and the textile industry. This leads to the indoor formaldehyde pollution [1-3]. As a volatile organic compound with irritation and mutagenicity [4], formaldehyde has negative effects 2

on respiratory and endocrine system of human body [5, 6]. Lots of efforts have been made to detect and decompose formaldehyde [7-11]. Although it’s not difficult to detect and identify formaldehyde in the laboratory, bulky instrumentation and complex analytical methods are not so good for widespread use. In order to obtain convenient real-time gas sensors, conductometric gas sensors or chemiresistors made of semiconductor materials have been widely investigated [12]. As an II-VI compound semiconductor with many excellent characteristics, ZnO and its nanostructures have been used to build sensors to detect formaldehyde [13-16]. Zhang et al. manufactured ZnO-nanostructure-based formaldehyde sensors, which showed high sensitivity and selectivity to formaldehyde in comparison with many other gases [17, 18]. And in view of the proper band gap size, ZnO also has a good performance in photocatalysis research, and it has been used to degrade formaldehyde [19-22]. So far, a lot of ZnO nanomaterials with different frameworks and morphologies have been prepared and used as sensing materials for formaldehyde and photocatalysts for formaldehyde degradation [15-17, 22-24]. For all of these applications, understanding the mechanism of formaldehyde adsorption on ZnO surfaces is a key issue. However, there is few related theoretical work reported. Jones and et al. have investigated CH2O on ZnO (0001) surface, because they believed that only ZnO (0001) surface was active when they were using Cu/ZnO binary catalyst to synthesize methanol [25]. Actually, the ZnO (101̅0) surface also plays an important role in most cases, especially for its nanostructures. It is a common knowledge that there are four conventional low-index surfaces for 3

wurtzite ZnO: {0001}, {0001̅}, {101̅0} and {112̅0}. The polar surfaces {0001} and {0001̅} are often the exposed surfaces along the growth direction of the material [26-28]. The {101̅0} surface and the {112̅0} surface are nonpolar surfaces and comparatively simple and steady. Moreover, the {101̅0} surface is more stable, and it is often the most common exposed surface for ZnO nanostructures [29]. In the case of a typical ZnO nanowire with a standard hexagonal column shape, the two tips of the nanowire consist of polar surfaces ; all the other six side surfaces are usually {101̅0} surfaces, which play a crucial role in determining the physical and chemical properties of the nanowire. Thus, to get more insight into relevant reactions and further improve the device performance, investigations of the formaldehyde adsorbed on the ZnO (101̅0) surface are necessary. In this work, we have studied the adsorption of formaldehyde on ZnO (101̅0) surface by using first-principles calculations. The possible adsorption geometries, adsorption energies and their dependence on coverage, and the electronic structures are systematically investigated. 2. Computational methods All calculations were performed by using the density functional theory in the PBE [30] generalized gradient approximation. The Vienna ab-initio simulation package (VASP) was used in our work [31, 32]. Electron-ion interactions were treated with the projector augmented wave method [33, 34]. We set plane wave cut-off energy at 450 eV, and employed the Monkhorst-Pack sampling scheme [35] with a k4

point mesh density of 9 × 6 × 1 for the (1 × 1) surface cell. The lattice parameters of the slab model of ZnO (101̅0) surface were from a ZnO primitive cell optimized with the same accuracy, i.e., a = 3.282 Å, c = 5.294 Å. The slab model contained a 14 Å of vacuum space in the z direction and five ZnO bilayers. The bottom two bilayers were fixed in the bulk configuration during calculation, and the top three bilayers and formaldehyde molecules were allowed to relax freely until the force on each ion was less than 0.03 eV/Å. The dangling bonds of the bottom surface are saturated by pseudohydrogen atoms. It is a common problem that DFT calculations always underestimate the band gap of semiconductors. In this work we ignored the wrong band gap value and only cared about the relative energy level positions. 1

The adsorption energy 𝐸𝑎𝑑 per H2 CO is defined as 𝐸𝑎𝑑 = 𝑛 (𝐸𝑡𝑜𝑡𝑎𝑙 − 𝐸𝑠𝑙𝑎𝑏 − 𝑛𝐸𝑓𝑜𝑟𝑚𝑎𝑙𝑑𝑒ℎ𝑦𝑑𝑒 ), in which 𝐸𝑡𝑜𝑡𝑎𝑙 and 𝐸𝑠𝑙𝑎𝑏 denote energies of the total adsorbed model and the slab model, respectively; 𝐸𝑓𝑜𝑟𝑚𝑎𝑙𝑑𝑒ℎ𝑦𝑑𝑒 denotes the energy of one free formaldehyde molecule and 𝑛 is the number of formaldehyde molecules. Transition states were calculated with the climbing-image nudged elastic band method (CINEB) [36, 37]. 3. Results and discussions 3.1. Adsorption structures and adsorption energies Various possible adsorption structures were investigated, and we obtained the four most stable configurations as shown in Fig. 1. For all of the adsorption structures, we found that the O atom of CH2O was always bonded to a surface Zn 5

atom, and another bond was formed between the atomic group CH2- and a surface O. Therefore, two possible adsorption sites can be determined: site A, the formaldehyde molecule is above a Zn-O dimer; site B, the formaldehyde molecule is above the gap between two adjacent Zn-O dimers. The adsorbed CH2O molecules have two different structures: pristine sp2 planar formaldehyde configurations (PA and PB), and distorted sp3 hybridized configurations (DA and DB). The subscripts A and B denote the site A and site B, respectively. The adsorption energies of the four configurations are given in Fig. 2. We considered the adsorption coverage from 1/9 monolayer (ML) to 1 ML (i.e., full coverage). The adsorption energies of PA and PB are higher than those of DA and DB because of the weak bonding. In the DA configuration, the Zn-O surface dimer is broken (see Fig. 1). The distance between the surface Zn and surface O increases from 1.87 Å to about 2.3 Å. And the surface atoms form two strong bonds with the adsorbed CH2O. As we can see from Table 1, the bond lengths 𝑑𝑂−𝑍𝑛 and 𝑑𝐶−𝑂 of DA configuration are the minimum ones compared with the other three configurations, indicating the strongest covalent bonding. These strong covalent bonds compensate the energy loss of the surface dimer breaking, making the adsorption energies of DA comparable with those of DB for an isolated CH2O (1/9ML, 1/6ML and 1/4ML in Fig. 2). At full coverage, A surface reconstruction happens, as shown in Fig. 3. All the surface Zn-O dimers are broken, and they bond with their neighbor ones, forming new surface dimers. As a result, the adsorbed CH2O molecules locate between these new formed 6

dimers, rather than the top of the Zn-O dimers. Therefore, at this point, the DA configuration at 1ML is similar to the DB configuration. In the DB configuration, in addition to forming two covalent bonds, the system also gains benefit from electrostatic interactions by rotating the adsorbed CH2O molecule. Figure 4 illustrates the electrostatic interactions in the D B configuration at different coverages. As shown in Fig. 4(a), the electrostatic interaction is the attraction between one H atom of CH2O and a surface O atom, which is similar to hydrogen bonding, but weaker than hydrogen bonding, i.e. quasi hydrogen bond. Because of the attraction, this H atom gets closer to the surface O through rotating the CH2O molecule. Moreover, the height of this H atom is lower than the other H atom (see Fig. 1), which is also the effect of the electrostatic attraction. This quasi hydrogen bond has great influence on the adsorption energy of the DB configuration, especially for 1/2 ML. At 1/2 ML, new quasi hydrogen bonds form between the adsorbed CH2O molecules along the [0001] direction as shown in Fig. 4(b). Meanwhile, the rotation of CH2O strengthens this interaction, which results in the significant decrease of the adsorption energy of DB at 1/2 ML in Fig. 2. This adsorption energy is the minimum value in all cases. At 1 ML, as shown in Fig. 4(c), because of the full coverage, the repulsion between the positively charged H atoms appears, accordingly, the adsorption energy increases. A rotation angle θ is introduced to describe the rotation of the adsorbed CH2O induced by the electrostatic interaction. It is defined as the acute angle between the C-O bond of CH2O and the [0001] direction, see Fig. 4(a) and Table 1. The θ of the DB 7

configuration is about 20° at coverage lower than 1ML, but it increases to 25° at 1ML, which can be attributed to the H-H repulsion. The DA configuration also has this rotation angle for the same reason. But the θ of the DA configuration is smaller than 15.11° at coverages lower than 1ML, because of the stronger covalent bond and the narrow space of site A. However, due to the surface reconstruction, the θ of the DA configuration significantly increases to 27.19° at 1ML. 3.2. Chain adsorption model Although the four configurations are different, they have one thing in common: the negatively charged O of CH2O is always bonded to surface Zn and the positively charged CH2- atomic group is always bonded to surface O. According to the laws that opposite charges attract and like charges repel, each CH2O attracts the adjacent ones along the [0001] direction and repels those along the [112̅0] direction, as shown in Fig. 4(c). The adsorption energies of all four configurations increase at 1ML (see Fig. 2) because of the repulsion in [112̅0] direction. For coverages lower than 1 ML, adsorbed CH2O molecules are more likely to avoid close arrangement along the [112̅0] direction. To confirm the repulsion in [112̅0] direction, we have tested the structures that the adsorbed CH2O molecules were closely arranged along this direction at lower coverages. As expected, the adsorption energies are quite high, and even higher than the corresponding energies at 1ML. The tendency to arrange along the [0001] direction is confirmed at the coverage of 1/2 ML in a (2 × 2) supercell with two CH2Os. We tested all possible distribution 8

patterns, and found that the chain structure along [0001] direction is the most energetically favorable structure. These results are plotted in Fig.2 at 1/2 ML. For the coverages of 1/9 ML, 1/6 ML and 1/4 ML of the four configuration in Fig. 2, we conventionally use (3 × 3), (3 × 2) and (2 × 2) surface supercells to isolate adsorbates. And here we tested this chain model at lower coverages of 1/4 ML, 1/6 ML and 1/9 ML by using (9 × 1), (6 × 1), and (4 × 1) surface cells, respectively ( i.e., kept the supercell along [0001] direction unchanged and enlarged the supercell along [112̅0] direction to decrease the coverage ). It is found that the adsorption energy of the chain model nearly does not change as the coverage varies. The results of the DB chain configuration are plotted in Fig.2, whereas the results of PA, PB and DA are omitted for brevity, because the energies at 1/2 ML in Fig.2 can be good representatives for the chain adsorption energies. The adsorption energies for these chain modes of PA, PB and DA configurations are just a little lower than those with isolated CH2O (1/9 ML, 1/6 ML and 1/4 ML), but the chain model of DB configuration has extremely low adsorption energy. This can be attributed to the additional forming of quasi hydrogen bonds mentioned above in DB configuration. From this chain adsorption model, we may draw a picture for the adsorption mechanism of formaldehyde on ZnO (101̅0) surface. The electrostatic repulsion from the adsorbed CH2O molecules would push the approaching free CH2O away from locating at its adjacent sites in the [112̅0] direction (see Fig. 5(a)). On the other hand, the electrostatic attraction may guide the free CH2O to the adjacent site in the [0001] direction (see Fig. 5(b)). Thus, the chain-like adsorption is very likely to occur 9

when formaldehyde is adsorbed on ZnO (101̅0) surface. This mechanism may also give some inspirations for the adsorption of molecules containing aldehyde group on similar surfaces of materials. 3.3. The instability of PA and PB In the issues of formaldehyde adsorption on materials, there are usually two kinds of adsorption configurations: one with intact sp2 planar formaldehydes perpendicular to the material’s surface [25, 38-44] and another with distorted formaldehydes in sp3 hybridization [42, 44]. In this work, PA and PB belong to the former case, and DA and DB belong to the latter case. Given the same adsorption site of PA and DA (or PB and DB), it’s natural to wonder how the PA (or PB) configuration can transform to the DA (or DB) configurations. Using the climbing-image nudged elastic band method (CNEB), we have calculated these transforming processes and obtained the transition states with the minimum energies. The results are given in Fig. 6. Two coverages are taken into consideration: 1/4 ML representing the isolated adsorption, and 1/2 ML representing the chain adsorption. The energy barriers are about 0.1 eV for PA to DA, and about 0.01 eV for PB to DB, which are very small. These structure transformations can happen easily, i.e., the PA and PB configurations are instable in realistic cases. The fourfold-coordinated C atom with sp3 hybridization in DA and DB configurations implies the breaking of C=O bond in pristine formaldehyde, which is confirmed by the investigation of molecular orbitals in Sec. 3.4. Therefore, if the DA 10

and DB are the only possible configurations, formaldehyde will be destroyed when it is adsorbed on ZnO (101̅0) surface. Moreover, from the Fig. 6, it seems that this distortion is unrecoverable (the energy barrier is more than 0.5 eV). Together with some possible consequent reactions, ZnO with exposed (101̅0) surface might have good performance as catalyst in formaldehyde degradation. 3.4. Electronic structures We have calculated the band structures of the systems, projected density of states (PDOS) of the adsorbed CH2O, and band decomposed charge densities. Figure. 7 shows the related results. For a free CH2O, seven molecular orbitals composed by valence states of C, O and H are taken into account and depicted in Fig. 7(a). From the perspective of molecular orbital theory, in a free CH2O, 3a1 and 4a1 are respectively the bonding ( σ ) and antibonding ( σ* ) orbitals from the 2s-orbitals of C and O; 1b2 and 2b2 are respectively the bonding ( π ) and antibonding ( π* ) orbitals from the C and O 2p-orbitals in the CH2O plane; and the C and O 2p-orbitals perpendicular to the CH2O plane generate the bonding ( π ) 1b1 orbital and the antibonding ( π* ) 2b1 orbital; 5a1 is the bonding orbital with σ symmetry deriving from the C and O 2p-orbitals along the C-O bond (the corresponding antibonding orbital 6a1 does not display here, as it is too high in energy); The six relatively lower orbitals are occupied by twelve valence electrons and the antibonding orbital 2b 1 is empty.

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In the PA and PB configurations (Fig. 7(b) and Fig. 7(c)), both of the electronic structures and the geometrical structures of CH2O are almost unchanged. From the DOS curves in Fig. 7(b) and Fig. 7(c), we can see that all the orbitals of the adsorbed formaldehyde nearly remain the same as that of a free one. The only change is that the 2b2 peak is getting broader. This is due to the coupling between the 2b2 orbital of the formaldehyde molecule and the dangling bond of the surface Zn. Moreover, the unoccupied 2b1 orbital is above the conduction band minimum, so there is no energy level existing in the band gap for PA and PB configurations. For the DA and DB configurations, the CH2O bonds to a surface Zn and a surface O through its O and C, respectively. The geometrical structure of CH2O changes from planar sp2 configuration to tetrahedral sp3 configuration, indicating that the double bond C=O turns into a single C-O bond. The 1b1 orbital disappears and turns into two independent p-like orbitals, which form the two covalent bonds. Therefore, we can see that the corresponding 2b1 antibonding orbital also disappears from the 𝐶𝐻 𝑂

conduction band (see Fig. 7(d) and Fig. 7(e)). The bond length 𝑑𝐶−𝑂2 increases from 1.21 Å in the free CH2O to 1.33 Å in the DB configuration, and to 1.37 Å in the DA configuration (see Table 1). The increasing of C-O bond length and the geometrical distortion raise all the energy levels of CH2O up. The raised 3a1 orbital splits into two levels because of interacting with O 2s band of ZnO. The 4a1 antibonding orbital does not change. The 1b2 and 5a1 orbitals are dispersed in the lower half of the ZnO valence band. There are two DOS peaks near the valence band maximum (VBM). The lower one is the p-orbital of formaldehyde O atom which bonds with a surface Zn. 12

This p-orbital is from the broken 1b1 orbital. The inset of Fig. 7(d) displays the charge density iso-surface of this peak for the DA configuration. The other DOS peak near the VBM is from the raised 2b2 orbital, see the inset of Fig. 7(e) which displays the charge density iso-surface of this peak. In the DA and DB configuration, the 2b2 orbital becomes the highest occupied state; it locates in the middle of the band gap for the DA configuration and gets close to the VBM in the DB configuration. From the aspect of electric properties of the material, we would conclude that, for native n-type ZnO samples, the adsorbed formaldehyde molecules on (101̅0) surface may not have much impact on the conductivity, because they only introduce fully occupied states close to the VBM or in the band gap. However, it will have great effect on the conductivity in p-type ZnO samples, as these fully occupied defect states would attract holes, i.e., the adsorption sites will become the hole trapping centers. Therefore, for the application of ZnO in conductometric formaldehyde sensors, using p-type sample would be an effective way to improve the performance. But for the n-type samples, the high index planes may work better than (101̅0) plane, as the samples Zhang et al. used [17, 18]. 4. Conclusion In this work, the geometry and electronic structures of the adsorption of formaldehyde on ZnO (101̅0) surface are investigated by first-principles calculations. It is found that the electrostatic interactions derived from the 13

polarization of formaldehyde molecule plays an important role in determining the geometrical structure and the system energies. We conclude that the electrostatic interactions will make the adsorbed CH2O molecules adopt a chain-like pattern on the ZnO (101̅0) surface. Based on this chain adsorption model, an adsorption mechanism of formaldehyde on ZnO (101̅0) surface is proposed. By studying the transition states between the four stable adsorption configurations, the two physical adsorption configurations with pristine sp2 formaldehyde molecules are proved to be unstable. They can overcome the small energy barriers and turn into another two chemically bonding configurations, in which the adsorbed CH2O rehybridizes from sp2 to sp3 and forms two strong covalent bonds with the surface Zn and O. From the perspective of molecule orbitals, the 1b1 bonding orbital of formaldehyde is broken in the latter two sp3 hybridized configurations. Accordingly, the C=O double bond turns into C-O single bond. By combining some possible subsequent reactions, ZnO with exposed (101̅0) surface may act as a catalyst in formaldehyde degradation. A fully occupied state originated from the formaldehyde HOMO orbital was lifted into the band gap and close to the VBM. As a hole trapping center, the adsorbed formaldehyde on ZnO (101̅0) will not have a big impact on the conductivity of the common n-type ZnO samples. Acknowledgements The authors gratefully acknowledge the financial support of the National Nature Science Foundation of China (Nos. 21373156, 11404253 and 11404260), National

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Key R&D Program of China (2016YFB1102500) and Natural Science Foundation of Education Bureau of Shannxi Province, China (No. 2015JQ1013).

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Figr-3

24

Figr-4

25

Figr-5

26

Figr-6

27

Figr-7

28

TABLE 1. Structure properties of the four adsorption configurations PA, PB, DA and DB. 𝑑𝑂−𝑍𝑛 , 𝑑𝐻−𝑂 and 𝑑𝐶−𝑂 , denote the bond lengths of O-Zn, H-O and C-O between the 𝐶𝐻 𝑂

adsorbed CH2O and the ZnO surface dimer respectively. 𝑑𝐶−𝑂2 denotes the bond length of C-O in CH2O. θ is defined as the acute angle between the C-O bond of CH2O and the [0001] direction. 1/9ML 𝑃𝐴

𝐷𝐵

1/2ML

1ML

2.190

2.195

2.199

2.255

2.345

𝑑𝐻−𝑂 (Å)

2.246

2.250

2.248

2.198

2.107

𝑑𝐶−𝑂2 (Å)

1.231

1.230

1.230

1.230

1.228

𝑑𝑂−𝑍𝑛 (Å)

2.150

2.150

2.156

2.177

2.245

𝑑𝐻−𝑂 (Å)

1.997

1.953

1.957

1.927

1.883

𝑑𝐶−𝑂2 (Å)

1.232

1.233

1.232

1.234

1.232

𝑑𝑂−𝑍𝑛 (Å)

1.899

1.898

1.898

1.888

1.891

𝑑𝐶−𝑂 (Å)

1.495

1.498

1.499

1.504

1.490

𝑑𝐶−𝑂2 (Å)

𝐶𝐻 𝑂

1.369

1.369

1.369

1.369

1.364

θ (deg)

15.11

12.26

12.51

10.86

27.19

𝑑𝑂−𝑍𝑛 (Å)

1.942

1.939

1.945

1.932

1.948

𝑑𝐶−𝑂 (Å)

1.574

1.570

1.578

1.550

1.541

𝑑𝐶−𝑂2 (Å)

1.330

1.331

1.330

1.346

1.348

θ (deg)

21.355

18.60

20.37

21.64

25.34

𝐶𝐻 𝑂

𝐷𝐴

1/4ML

𝑑𝑂−𝑍𝑛 (Å)

𝐶𝐻 𝑂

𝑃𝐵

1/6ML

𝐶𝐻 𝑂

29