Study on the adsorption selection of CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_4$$\end{document} on CuO (110) versus (111) surfaces: a density functional theory study

Finding the active sites of suitable metal oxides is a key prerequisite for detecting CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_4$$\end{document}. The purpose of the paper is to investigate the adsorption of CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_4$$\end{document} on intrinsic and oxygen-vacancies CuO (111) and (110) surfaces using density functional theory calculations. The results show that CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_4$$\end{document} has a strong adsorption energy of −0.370 to 0.391 eV at all site on the CuO (110) surface. The adsorption capacity of CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_4$$\end{document} on CuO (111) surface is weak, ranging from −0.156 to −0.325 eV. In the surface containing oxygen vacancies, the adsorption capacity of CuO surface to CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_4$$\end{document} is significantly stronger than that of intrinsic CuO surface. The results indicate that CuO (110) has strong adsorption and charge transfer capacity for CH4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_4$$\end{document}, which may provide experimental guidance.


Introduction
In recent years, the development of new gas sensors has received widespread attention. In particular, people are sensitive to toxic, flammable, explosive, and polluting gases present in homes, industrial sites, and the environment [1,2]. Methane is highly explosive and flammable, so developing a methane sensor with low cost, high sensitivity, good selectivity, and stable performance has become a hot spot [3][4][5][6]. In the study of CH 4 adsorption materials, it is usually microporous carbon [7], metal-organic Frameworks [8], layered InN [9], and Ni surface [10]. In recent years, there have been many studies on methane sensors, mainly including light interference methane sensors, metal oxide semiconductor gas sensors, infrared adsorption fiber optic methane sensors, and electrochemical methane sensors. Compared with other types of methane sensors, metal oxide semiconductor gas sensors are cheaper and people are more interested in their research [11][12][13][14][15]. Among transition metal oxide (TMO) compounds, copper oxide (CuO) is an important transition metal p-type semiconductor material and exhibits unique properties, with a narrow bandgap (1.3-1.6 eV) [16,17]. It has good thermal stability and high durability [18], so it is widely used in many fields such as gas sensors [19,20] and catalyst carriers [21,22].
At present, there have been some studies on CuO semiconductor adsorption of small-molecule gases. Maimaiti et al. [23] studied the two different reduction mechanisms of oxygen vacancies (V O ) formation and H 2 adsorption on the surface of CuO (111) through the first-principles. CuO reduction provides a mechanism for Cu. Warren et al. [24] found that multilayers of water can be seen at low temperatures, and the surface monolayer has strong chemisorption with CuO samples. Moreno et al. [25] found through the first-principles that the surface of CuO (110) has a better adsorption effect than intrinsic CuO (intrinsic refers to non-defected), and found the best adsorption position of CO 2 on the surface of CuO (110). Hao et al. [26] systematically studied the CuO (111) surface, the effect of surface structure on the adsorption and activation of H 2 was studied, including ideal V O and pre-coated O atoms surfaces. Sun et al. [27] found that CuO (111) also exhibited good adsorption and dissociation properties for O 2 , and the presence of V O could increase the chemical activity of O 2 dissociation and significantly improve the catalytic activity of CuO (111). But the interaction between CH 4 and CuO is unclear. This motivated us to study the adsorption between CuO surface and CH 4 .
This article starts with the working mechanism of the CuO semiconductor adsorption CH 4 gas sensor, the adsorption mechanism of CH 4 on (111) and (110) surfaces under intrinsic and V O was studied. By comparing the adsorption energy and charge transfer amount, we can conclude that the CuO (110) surface has a strong adsorption capacity for CH 4 , and it has the strongest adsorption capacity at the CuO (110) with V O . The active sites of CuO for CH 4 molecules were found and the adsorption mechanism was elucidated, which provided good guidance for the experiment of CH 4 detection or sensor design.

Model and computational method
For the two important crystal face (110) and (111) of CuO, we modeled three-layer slab 2 × 4 and 4 × 2 supercells containing 96 atoms, respectively. The periodicity of the boundary is suitable for the whole supercell, which can be replicated periodically throughout space. Figure 1 shows the side and top views of CuO (110) and (111) surfaces. Five different surface adsorption sites are considered for each surface, including central 1, central 2, O atom, Cu 1 atom, Cu 2 atom, named A, B, C, D, E, respectively. Directly above each site, there are three different configurations of CH 4 molecules, named up (U), parallel (P), and down (D). When the molecules are adsorbed to the surface site, we use X-Y to express it (X represents the adsorption site, Y represents the orientations of CH 4 ). In all geometric optimization calculations, The bottom layer of CuO with three-layer structure is fixed to maintain its volume to better study the surface adsorption of CH 4 . A vacuum layer perpendicular to the surface for 15 Å in the Z direction is sufficient to avoid periodic plate-to-plate interactions in the atomic layer.
All calculations in this paper using the density functional theory (DFT-D) [28] including dispersion force correction are performed using the CASTEP package [29] and the generalized gradient approximation (GGA) functional. The Perdew-Burke-Ernzerhof (PBE) [30] was selected for calculation. The cut-off energy is set to 420 eV. The geometry of the structure is considered convergent until the energy is reduced to 1.0 × 10 −5 Ha/ atomic and each atomic force is less than 0.05 eV/Å. The Monkhorst-Pack k-point is 2 × 2 × 1. The interaction strength between CuO surface and CH 4 molecules are expressed by the adsorption energy ( E ads ) via [31] where E CuO∕CH 4 , E CuO , and E CH 4 are the total energies of the CuO/CH 4 , CuO, and CH 4 systems, respectively.

Results and discussion
To verify the consistency of the calculated and experimental values, we first calculated the lattice parameters of CH 4 . The

Density of states and charge transfer
For the most stable configurations B-D and C-P, the calculated charge transfer (Q) values are −0.17 and −0.20 e, respectively, as shown in Table 1, indicating that CH 4 adsorbs on the CuO surface as a charge acceptor and adsorbs charge from the surface. Next, to get a deeper understanding of the sensitivity mechanism, we computed the partial density of states (PDOS) for the B-D and C-P configurations (Fig. 2). It can be seen from Fig. 2a that the total density of states (TDOS) of CH 4 adsorbed by CuO (110) is mainly contributed by Cu-3d and O-2p orbitals, and there is little change before and after adsorption. After adsorption, the peak of CH 4 shifted to the left, which was contributed by the H-1s and C-2p orbitals of CH 4 at −3.4 eV. In addition, by observing the PDOS of the Cu atom that we selected next to CH 4 , it can be seen that the 3d orbital of the Cu atom before and after adsorption has a slight change.
These results indicate that the adsorption system of CH 4 by CuO (110) are stable. Similar to Fig. 2a, b shows the DOS of the C-P configurations and shows that the Cu-3d orbitals contribute almost to the energy band. There are no new electron peaks of CH 4 were observed on these two surfaces, but the electron peaks of CH 4 in the C-P ----− 0.03 --configuration were farther away from the Fermi level than those in the B-D configuration. On the one hand, it shows that no new bonds are generated after the system is adsorbed, on the other hand, the B-D configuration is more favorable. Besides, by comparing PDOS of CH 4 molecules before and after adsorption, it is found that there are two discrete peaks of DOS before adsorbing CH 4 molecules. When the CH 4 molecules are adsorbed, the peak shifts to the left about 4.5 eV, indicating that CH 4 molecules becomes stable after adsorption. However, due to the introduction of V O , the PDOS changes before and after the Cu atoms next to CH 4 are more obvious, and the 3d orbitals of the Cu atoms after adsorption move about 1 eV to the right, which may be the reason for the higher adsorption energy. For a more intuitive understanding of the interaction between CH 4 molecules and CuO surfaces, we calculated the Charge density difference, as shown in Fig. 3. Blue areas indicate the gain of electrons, and green areas indicate the loss of electrons. The apparent charge accumulation between the CH 4 molecules and the surface indicates the interaction between the CH 4 molecules and the surface. The Mulliken charge distribution of the CH 4 molecules before and after adsorption is listed in Table 2.

The effect of V O
By studying the adsorption mechanism of B-D and C-P configurations, we find that surfaces containing V O affect the adsorption. CuO(111) surfaces with V O enhance the adsorption capacity of CH 4 , while CuO(110) surfaces with V O have a negative effect on CH 4 . This is consistent with the results of Sun et al. [27] in studying the adsorption of CH 4 molecules on CuO(111) surface by V O . Some studies have also shown that the presence of V O will increase the surface conductivity and enhance the adsorption of CH 4 [37,38]. It is precise because of the existence of V O that unsaturated bonds are generated, which leads to a strong adsorption capacity of CuO(111) on CH 4 .

Conclusions
In conclusion, the adsorption behavior of CH 4 on CuO (110) and CuO (111) with intrinsic and V O was studied by DFT calculation. It was found that the adsorption capacity of CuO (110) was greater than that of CuO (111) when CH 4 was adsorbed on the CuO surface. On the intrinsic CuO(110) surface, the center of the six-membered ring is the active adsorption site, and on the CuO(111) with V O , the vacancy is the active adsorption site. During the adsorption process, DOS showed orbital hybridization between CH 4 molecules and surfaces, and the change of CH 4 molecules after adsorption is also obvious. And charge density difference can clearly show the electron transfer between CuO surface and CH 4 molecules. Therefore, this not only provides a reference for the active site of CH 4 adsorption on CuO surface, but also provides a reference for CuO to be a new sensor for detecting CH 4 .
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Fig. 3
Charge density difference of two configurations (isovalue = 0.002 e ∕Å 3 ), a B-D configuration, and b C-P configuration. Blue and green isosurfaces represent gains and losses of electrons, respectively