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Adsorption of CO, NO, and NH3 on ZnO monolayer decorated with noble metal (Ag, Au)
Applied Surface Science 508 (2020) 145202
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Adsorption of CO, NO, and NH3 on ZnO monolayer decorated with noble metal (Ag, Au) Yongfeng Qua, Jijun Dinga, Haiwei Fua, Jianhong Pengb, Haixia Chena, a b
T
⁎
College of Science, Xi’an Shiyou University, Xi’an, Shaanxi 710065, China College of Physics and Electronic Engineer, Qinghai Nationalities University, Xining, Qinghai 810007, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO monolayer Noble metal decoration Theoretical calculation Adsorption properties
The stable adsorption configuration, electronic structure and magnetic property of CO, NO, and NH3 adsorbed on pristine and noble metal (Ag, Au) atom decorated ZnO monolayer (ZnO-ML) are investigated using density functional theory (DFT). The results show that all three kind of gas molecules are physically absorbed on pristine ZnO-ML during the exothermic process. At the same time, the adsorption performance of the ZnO-ML is enhanced by doping noble metal (Ag, Au) atoms. Both Ag and Au atom doping can greatly enhance adsorption ability of the ZnO-ML to the CO and NO molecules, except for the NH3 molecules. For NO molecules, the adsorption energies of NO on the Ag, and Au atom decorated ZnO-ML are −0.92 eV and −1.30 eV, respectively, which are over 5 and 7 times larger than that of NO on the pristine ZnO-ML, respectively. Therefore, both the Ag, and Au atom decorated ZnO-ML are more sensitive for CO and NO molecules contrast to the ZnO-ML. These results provide insight into the adsorption properties of ZnO-ML, which could promote the further application of ZnO materials in the gas sensing field.
1. Introduction Graphene, a single atom thick honeycomb structure consisted of carbon atoms and arranged periodically, was successfully synthesized in 2004 [1]. It has been attracted extensive attention due to some excellent properties including stable geometric structure, excellent electronic properties, and large surface to volume ratio, etc. [2–4]. Graphene has promising application potential in optoelectronic devices [5], composites [6], and electronics [7]. However, both stable carbon atom sp2 hybridization structures and gapless feature make graphene unsuitable for electrical switch applications and gas sensor design [8]. Finding new interesting phenomena behind the planar structure and enlarging relevant applications is greatly inspiring our efforts to find other two-dimensional material with a moderate band gap. Among semiconductor oxides, zinc oxide (ZnO) has been extensively investigated owing to its excellent optical, electrical, and piezoelectric properties [9,10]. Bulk ZnO is an excellent semiconductor material with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, which can be used in solar cells, ultraviolet light-emitting diodes, and transparent conductive films [11,12], and so on. However, the researchers predict that the ZnO monolayer (ZnO-ML) could be more interesting properties than the bulk structures [13] by theoretical studies. Besides, ZnO-ML with ⁎
honeycomb structure, in which zinc and oxygen atoms are arranged in a single layer plane, has been successfully synthesized on a Pd(1 1 1) surface[14]. The two-dimensional ZnO-ML is a nonmagnetic semiconductor nanomaterial with a wide band gap of 3.25–4.0 eV, which is promising candidates in low-dimensional electronic devices [15]. For the last several years, a lot of research work has been devoted to the manipulation of the ZnO-ML surface to modify its electronic structure by different ways including the inclusion of intrinsic defects, doping of impurity atoms, and other adsorbates [16–19]. For example, Topsakal et al. found that the local magnetic moment of the ZnO-ML is caused by the Zn-vacancy defects [16]. Ren et al. reported that the magnetism of the ZnO monolayer is induced by doping 3d transition metals (Cr, Mn, Fe, Co, Ni, and Cu) [17]. Also, the transition metal (V, Cr, Mn, Fe, and Co) decorated ZnO monolayer are investigated using the first-principles calculation. Results indicated that V-decorated ZnO monolayer has the largest binding energy compared with other transition metals [18]. Zhang et al. found that the magnetism of non-metals (C, N and B) doped ZnO-ML can be regulated by adsorption of CO. At the same time, both C and N doped ZnO-ML can be transformed into nonmagnetic semiconductors, while B-doped ZnO-ML is turned into a ferromagnetic half metal [19]. More interestingly, it is well known that surface decoration is a direct and effective strategy to modify material properties at the atomic
Corresponding author. E-mail address: [email protected] (H. Chen).
https://doi.org/10.1016/j.apsusc.2019.145202 Received 4 November 2019; Received in revised form 10 December 2019; Accepted 26 December 2019 Available online 29 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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scale. Of course, the gas sensing properties of two-dimensional materials can also be improved by surface decoration [20,21]. For instance, Co-decorated graphene sheets exhibit high sensitivity toward NO, H2S, and HCN molecules [22]. Ag-Al co-doping can tune the band gap of ZnO more flexible [23]. However, the effect of gas molecules, especially for CO, NO, and NH3 molecules, adsorbed on noble metal atom (Ag, and Au) decorated ZnO-ML has hardly been investigated. CO, NO, and NH3 are all common gases in the atmosphere, in which CO and NO are colorless and odorless gases, while NH3 has a strong pungent smell [24,25]. CO, NO, and NH3 can seriously damage the oxygen delivery function of the human body. When the gas concentration is high, it will lead to hypoxia death. Besides, these gases will seriously pollute the environment. Therefore, it is necessary to effectively monitor and detect toxic gas molecules. Furthermore, investigation on the adsorption of CO, NO, and NH3 molecules helps to evaluate the gas sensing properties of noble metal atoms (Ag, Au) decorated ZnO-ML nanomaterials. In this work, based on density functional theory (DFT), the adsorption structures of CO, NO, and NH3 adsorbed on noble metal (Ag, Au) decorated ZnO-ML are investigated. On the one hand, the geometry and adsorption structure of CO, NO, and NH3 adsorbed onto pristine ZnO-ML (CO/ZnO-ML, NO/ZnO-ML, and NH3/ZnO-ML) are investigated to obtain the most stable adsorption structures. The band structure, electronic structure, charge transfer, magnetism, and adsorption energy base on the most stable configuration are calculated. The total density of states (TDOS) and the partial density of states (PDOS) of ZnO-ML before and after CO, NO, and NH3 adsorption are investigated to explore the interaction between gas molecules and ZnOML. On the other hand, the geometry and adsorption structure of CO, NO, and NH3 adsorbed onto noble metal (Ag, Au) decorated ZnO-ML (CO/Ag-ZnO-ML, NO/Ag-ZnO-ML, NH3/Ag-ZnO-ML, CO/Au-ZnO-ML, NO/Au-ZnO-ML, and NH3/Au-ZnO-ML) are investigated to acquire the most stable configuration. The band structure, adsorption energy, charge transfer, magnetism, and the DOS of CO, NO, and NH3 adsorbed on noble metal (Ag, Au) decorated ZnO-ML are calculated. Results show that the noble metal (Ag, Au) decorated ZnO-ML is more sensitive for CO and NO molecules detection than that of the undecorated ZnO-ML. However, the adsorption energy of NH3 adsorbed on both noble metal (Ag, Au) decorated ZnO-ML and pristine ZnO-ML are almost the same.
Fig. 1. The optimized geometry of ZnO-ML, and four possible adsorption sites including the top of Zn atoms (TZn), the top of O atoms (TO), above the center of the Zn-O bond (B), and the center of Zn-O hexagon (H).
adsorbed on pristine or noble metal (Ag, Au) decorated ZnO-ML system, ZnO-ML or noble metal (Ag, Au) decorated ZnO-ML, and isolated gas molecules in the same supercell, respectively. A negative Ead means that the adsorption process is exothermic. 3. Results and discussion 3.1. The adsorption of CO, NO, and NH3 on pristine ZnO-ML 3.1.1. Adsorption configurations and geometry structures For each molecule, four adsorption sites are considered. They are named as the top of a zinc atom (TZn), the top of an oxygen atom (TO), above the center of the Zn-O bond (B), and the center of Zn-O hexagon (H) (see Fig. 1), respectively. Meanwhile, the different initial orientations of the gas molecules are also investigated to obtain a most stable adsorption configuration. For the diatomic (CO, NO), there are three possible initial orientations at each adsorption site. Take the CO molecule as an example, the molecular plane is parallel to or perpendicular (C atom points to the ZnO-ML or away from it) to the ZnO-ML surface plane. Furthermore, two initial orientations of the NH3 molecule are examined, one is the N atoms pointing away from the substrate, and the other is the N atoms pointing to the substrate. The calculation results are summarized in Table 1. From Table 1, the CO molecules prefer to adsorb on the H site. The configuration with the CO molecule adopts tilted adsorption, in which the C atoms are pointing to the Zn atoms on the substrate (Fig. 2(a)). The adsorption structure has the largest adsorption energy (−0.21 eV), the shorter distance between the CO and substrate (D = 2.59 Å), and a small charge transfer (0.07 e) from CO molecules to ZnO-ML, which indicates that the process is weak physical adsorption. In the case of NO
2. Computational details All calculations are performed by using the Dmol3 module, within the generalized gradient approximation (GGA) of Perdew-BurkeErnzerhof (PBE) [26,27]. The double numerical plus polarization (DNP) basis set is employed, and the DFT semi-core pseudopotential (DSPP) is selected. The Tkatchenko-Scheffler scheme is used in all calculations to correct van der Waals forces [28]. The convergence tolerance of energy of 1.0 × 10−5 Ha is set, and the maximum force and displacement are 0.002 Ha/Å and 0.005 Å, respectively. For geometry structure optimization, the global orbital cutoff (5.0 Å) is set, and the smearing energy (0.005 Ha) is used. The Brillouin zone is sampled using a 10 × 10 × 1 Monkhorst-Pack grid in geometry optimization. For the calculation of electronic structure, we use a 20 × 20 × 1 Monkhorst-Pack grid. To ensure the precision of the electronic structure calculation, energy tolerance is set as 1.0 × 10−6 Ha. The system consists of 4 × 4 ZnO monolayer supercell (32 atoms) with a distance of 15 Å to avoid the interlayer interaction between adjacent ZnO monolayer layers. Besides, the charge transfers are calculated based on the Mulliken charge analysis. For gas molecule adsorbed on both pristine ZnO-ML and the noble metal (Ag, Au) decorated ZnO-ML, the adsorption energy Ead is determined by
Table 1 Optimized geometric parameters of CO, NO, and NH3 adsorbed on different adsorption sites of ZnO-ML: adsorption energy (Ead), adsorption distance of nearest-neighbor atoms between the gas molecule and substrate (D), and charge transfer (Q) between gas molecule and substrate. Molecule
Sites
D (Å)
Q (e)
Ead (eV)
CO
B H TO TZn B (optimized to TZn) H TO TZn B H TO (optimized to TZn) TZn
2.59 2.59 2.57 2.60 2.76 2.77 3.04 2.76 2.20 3.17 2.20 2.20
0.07 0.07 0.08 0.08 0.02 0.01 0 0.02 0.16 0.06 0.16 0.16
−0.20 −0.21 −0.19 −0.20 −0.18 −0.17 −0.15 −0.18 −0.62 −0.28 −0.63 −0.63
NO
NH3
Ead = EZnO + gas − (EZnO + Egas) where EZnO + gas , EZnO and Egas are total energies of the gas molecule 2
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Fig. 2. The most stable adsorption configurations (top and side view) of (a) CO, (b) NO, and (c) NH3 adsorbed on ZnO-ML. The C, N, and H atoms are shown in black, blue, and white balls, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
adsorption, the NO molecules prefer to adsorb on TZn site with the largest adsorption energy (−0.18 eV). At the same time, the NO initially adsorbed at B site is moving spontaneously to TZn site after relaxation. Thus, the TZn site adsorption configuration is the most stable. The NO molecule adopts tilted adsorption, in which the N atoms are pointing to the Zn atoms on the substrate (Fig. 2(b)). For the most stable configuration, the system has a large adsorption distance (D = 2.76 Å) and a very small charge transfer (0.02 e) from NO molecules to substrate, which indicates NO molecules are a very weak dopant. It is found that the charge transfer of adsorption configuration at the TO site is zero, which indicates that the charge carrier concentration of the substrate is constant. For NH3 adsorption, the configuration with NH3 is vertical to the substrate plane in which that the N atoms point to the Zn atoms of the substrate (Fig. 2(c)). This configuration has larger adsorption energy (−0.63 eV) and a shorter adsorption distance between NH3 and ZnO-ML (D = 2.20 Å). The TZn site is the optimal adsorption site with a large charge transfer (0.16 e) from NH3 to ZnO-ML, which indicates that there is weak interaction between them. Also, the configuration of NH3 adsorbed at H site has smaller charge transfer and adsorption energy compared with other adsorption sites. These results indicate that ZnO-ML is a promising material for NH3 capture and separation.
Fig. 3. The band structures of (a) ZnO-ML, (b) CO, (c) NO, and (d) NH3 adsorbed on ZnO-ML based on the most stable configuration. The Fermi level is set to zero energy and indicated by the pink dashed line. The red and blue lines represent spin-up and spin-down bands, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.1.2. Electronic properties To understand the interaction between three gas molecules and ZnO-ML, electronic properties (band structures and DOS) of the adsorbed systems are investigated. The spin band structures of pristine ZnO-ML and adsorbed systems are calculated (Fig. 3). The spin-up and spin-down band structures of pristine ZnO, CO/ZnO, and NH3/ZnO are identical, indicating that CO and NH3 molecules adsorbed ZnO-ML systems are nonmagnetic. However, the spin-up and spin-down bands near the Fermi level of the NO molecules adsorbed on the ZnO-ML structure are asymmetric, which indicates that the adsorbed system is magnetic and the NO adsorption breaks the spin degeneracy (Fig. 3(c)). In contrast to the pristine ZnO-ML, both the conduction band and valence band of the adsorbed system become more discrete due to more overlap among different atoms in the outer electron orbit. For CO, and NH3 adsorbed ZnO-ML systems, the conduction band numbers are increased (Fig. 3(b) and (d)). Especially for NO molecules adsorbed on the ZnO-ML systems, there are two flat spin-polarized bands near the Fermi level in the spin-up bands, and one of them coincides with Fermi level. In the spin-down bands, there are two spin-polarized bands above the Fermi level, and they are both far from the Fermi level. The spin density of states (DOS) comprising TDOS and PDOS are calculated to obtain electron distribution and magnetic properties. Fig. 4 shows the TDOS and PDOS of (a) CO, (b) NO, and (c) NH3 adsorbed on pristine ZnO-ML based on the most stable adsorption configuration. The TDOS plots of CO/ZnO-ML and NH3/ZnO-ML present excellent symmetry between spin-up and spin-down plots owing to the
nonmagnetic system, but apparent asymmetry appears in TDOS of NO/ ZnO-ML near the Fermi level. To make a further insight into interactions between gas molecules and ZnO-ML, the TDOS before and after CO, NO, and NH3 adsorption are compared. The PDOS of the atom’s electron orbital is analyzed. The TDOS of conduction band moves towards the low energy region after adsorption of CO. Besides, there appear some new peaks, including the peak near −8.98 eV attributed to C-2s orbitals and the peak near 2.95 eV belonged to C-2p orbitals. It is observed that weak hybridizations appear between Zn and C near −5 eV and −3.3 eV (Zn-3d, C-2s, and C-2p), near 5.8 eV (Zn-4s, Zn-3p, and C-2p). These weak hybridization states between C and Zn atoms indicating weak physical interactions and weak physical adsorption, which is consistent with the analysis results of adsorption energy and charge transfer. For NO adsorbed on ZnO-ML, it is found that the TDOS undergoes a significant left shift, which mainly results from the newly occupied states just below the Fermi level. Also, obvious asymmetry appears near the Fermi level is mainly belong to N-2p orbitals, which result in the non-zero magnetic moment of the adsorption system. Several overlap states appear near −6.5 eV (Zn-3d, N-2s and N-2p) and 5.3 eV (N-2p, Zn-3p and Zn-4s). As NH3 adsorbed on ZnO-ML, the N-2p states and Zn-3d states are overlapped around −4.89 eV, −2.62 eV and 3
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Fig. 4. The TDOS and PDOS of (a) CO, (b) NO, and (c) NH3 adsorbed on ZnO-ML based on the most stable adsorption configuration. The Fermi level is set to zero energy and indicated by the black dashed line.
Table 2 Optimized geometric parameters of CO, NO, and NH3 adsorbed on noble metal (Ag, Au) decorated ZnO-ML: adsorption energy (Ead), adsorption distance of nearest-neighbor atoms between the gas molecule and decorated substrate (D), charge transfer (Q) between gas molecule and decorated substrate and total magnetic moment of the adsorbed system (M).
Fig. 5. The spin charge density distribution (top and side view) of NO adsorbed on ZnO-ML, the isosurface value for the spin charge is 0.02 e/Å3.
Systems
Ead (eV)
D (Å)
Q (e)
M (μB)
Ag-ZnO-ML/CO Ag-ZnO-ML/NO Ag-ZnO-ML/NH3 Au-ZnO-ML/CO Au-ZnO-ML/NO Au-ZnO-ML/NH3
−0.61 −0.92 −0.60 −0.85 −1.30 −0.64
2.07 2.12 2.38 1.97 2.07 2.36
−0.05 −0.18 0.13 0.07 −0.07 0.24
1 0 1 1 0 1
Fig. 6. The most stable adsorption configurations (top and side view) of CO, NO, and NH3 adsorbed on Ag decorated ZnO-ML ((a)–(c)) and Au decorated ZnO-ML ((d)–(f)). The Ag and Au atoms are shown in cyan and yellow balls, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 4
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molecular orientations of the CO, NO, and NH3 have been discussed above. The most stable adsorption configurations are selected to discuss the band structure, DOS, charge transfer, and magnetism. For the most stable configurations, it is obvious that the noble metal (Ag, Au) atom gives rise to the slightly structural deformations of the ZnO-ML surface plane (Fig. 6). In the case of CO adsorption, the CO molecules prefer to adopt tilted adsorption configuration in which that C atom closes to the top of the adsorption sites (Fig. 6(a) and (d)). For NO adsorbed system, NO molecules are inclined to adsorb on the noble metal (Ag, Au) atom decorated substrate, in which the N atoms close to the adsorption sites (Fig. 6(b) and (e)). For NH3 adsorbed on noble metal (Ag, Au) atom decorated ZnO-ML, the NH3 molecules are vertical to the substrate plane, and the N atoms close to Ag and Au atom, respectively (Fig. 6(c) and (f)). The calculated total magnetic moment (M), adsorption distance (D), charge transfer (Q), and adsorption energy (Ead) of the most stable configurations are given in Table 2. The results show that noble metal (Ag, Au) decoration can prominently improve the adsorption performance of ZnO-ML for CO and NO molecules. The adsorption energy of CO on Ag decorated ZnO-ML is −0.61 eV, which is more than 2.9 times of that on pristine ZnO-ML. The adsorption energy of CO on Au decorated ZnO-ML is −0.85 eV, which is over 4 times larger than of that on pristine ZnO-ML. Besides, the adsorption energies of NO on Ag and Au decorated ZnO-ML are −0.92 eV and −1.30 eV, respectively, which are more than 5 and 7 times of that on pristine ZnO-ML, respectively. In contrast to ZnO-ML, the adsorption energy of the NH3 on Ag decorated ZnO-ML slightly lower than that of on ZnO-ML is −0.60 eV. However, the adsorption performance of Au decorated ZnOML for NH3 is barely enhanced. These results indicate that the noble metal (Ag, Au) atom decorated ZnO-ML may be used as good sensing materials for CO and NO. 3.2.2. Electronic properties The spin band structures are calculated for the noble metal (Ag, Au) decorated ZnO-ML and gas molecules adsorbed on noble metal (Ag, Au) decorated ZnO-ML based on most energy favorable structures (Fig. 7). It can be seen that the band structures of noble metal (Ag, Au) decorated ZnO-ML and CO and NH3 adsorbed noble metal (Ag, Au) decorated ZnO-ML show obvious asymmetry near Fermi level owing to the zeromagnetic moment of the system. However, a good symmetry appears in band structures of NO adsorbed noble metal (Ag, Au) decorated ZnOML. These results show that NO adsorbed noble metal (Ag, Au) decorated ZnO-ML are the non-magnetic semiconductors, but other systems are magnetic. It is observed that Ag decorated ZnO-ML with and without adsorption of CO and NH3 is a magnetic semiconductor (Fig. 7(a), (b) and (d)). The Fermi level intersects the spin-up band and lies in the band gap of the spin-down band, which indicates that NH3 adsorbed Ag decorated ZnO-ML exhibits a half-metallic property (Fig. 7(d)). However, it is found that a new impurity band appears in the band gap of the NO adsorbed Ag decorated system (Fig. 7(c)). The Au decorated ZnO-ML is a magnetic semiconductor (Fig. 7(e)). It is observed that a spin-up band intersects with the Fermi level, which indicates CO and NH3 adsorbed Au decorated ZnO-ML is half metal (Fig. 7(f) and (h)). Besides, it is found that a new impurity band also appears between conduction bands and valence bands of the NO adsorbed on the Au decorated system, and the conduction band becomes more discrete (Fig. 7(g)). Next, to further investigate the interaction between three gas molecules and noble metal (Ag, Au) atom decorated ZnO-ML, the TDOS and PDOS of noble metal (Ag, Au) atom decorated ZnO-ML based on the most stable adsorption configurations are depicted (Fig. 8). It can be seen that all TDOS experiences significant left shift after adsorption of gas molecules, which mainly results from the newly occupied states just below the Fermi level. In addition, the TDOS plots of NO/Ag-ZnO-ML and NO/Au-ZnO-ML present good symmetry between spin-up and spindown plots owing to the system is nonmagnetic. However, the obvious asymmetry appears in TDOS of CO/Ag-ZnO-ML, NH3/Ag-ZnO-ML, CO/
Fig. 7. The band structures of Ag decorated ZnO-ML (a), CO, NO, and NH3 adsorbed on Ag decorated ZnO-ML ((b)–(d)), Au decorated ZnO-ML (e), and CO, NO, and NH3 adsorbed on Au decorated ZnO-ML ((f)–(h)) based on the most stable adsorption configurations.
−0.60 eV, respectively. A new peak of TDOS appears near −7.62 eV attributed to N-2p orbitals. The strong overlap is hardly observed near the Fermi level, which indicates that the NH3 adsorption configuration is physisorption. For further insight into the magnetic moment of the adsorption system, the spin charge density distribution of NO/ZnO-ML is depicted (Fig. 5). For NO/ZnO-ML, the magnetism is mainly distributed on the NO molecules in which that the spin density of N atom is larger than that of O atom. Besides, there is almost no spin density located on the atoms of the substrate.
3.2. The adsorption of CO, NO, and NH3 on noble metal (Ag, Au) decorated ZnO-ML 3.2.1. Adsorption configurations and geometry structures In order to improve the adsorption performance of ZnO-ML to CO, NO, and NH3 molecules, the noble metal (Ag, Au) atoms adsorbed on ZnO-ML surface are investigated. The initially adsorption site and 5
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Fig. 8. The TDOS and PDOS of CO, NO, and NH3 adsorbed on Ag decorated ZnO-ML ((a)–(c)) and Au decorated ZnO-ML ((d)–(f)) based on the most stable configuration.
Table 3 The magnetic moment (μB) of CO and NH3 adsorbed on Ag-ZnO-ML and AuZnO-ML. Structure
Atom type
M(μB)
CO/Ag-ZnO-ML
C O in CO Ag O below N Ag O below C O in CO Au O below N Au O below
0.24 0.11 0.21 0.06 0.06 0.33 0.23 0.30 0.13 0.20 0.11 0.10 0.32 0.33
NH3/Ag-ZnO-ML
CO/Au-ZnO-ML
NH3/Ag-ZnO-ML
Ag
Ag
Au
Au
Au-ZnO-ML, and NH3/Au-ZnO-ML, especially near Fermi level. The PDOS analysis exhibits that two spin-polarized peaks appear near the Fermi level at −0.35 eV and 0.8 eV in spin-up states after CO adsorption, which are mainly attributed to C-2p, Ag-5p, and Ag-5s orbitals, respectively (Fig. 8(a)). In spin-down states, the two spin-polarized peaks also appear near the Fermi level and 0.96 eV above the Fermi level, which is also mainly attributed to C-2p, Ag-5p, and Ag-5s orbitals, respectively. It is observed that hybridizations appear between C and Ag near −0.35 eV (C-2p and Ag-5s), 0.8 eV and 3 eV (C-2p and Ag-5p), and thus the interaction between C and Ag atom is enhanced, which results in improving the adsorption performance of ZnO-ML for CO
Fig. 9. The spin charge density distribution (top and side view) of CO, and NH3 adsorbed on Ag-ZnO-ML ((a)–(b)) and Au-ZnO-ML ((c)–(d)), the isosurface value for the spin charge is 0.02 e/Å3. 6
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(−0.18 eV, 0.02 e), which means that ZnO-ML has promising prospects for NH3 sensing. The adsorption capacity of pristine ZnO-ML can be greatly enhanced by doping noble metal (Ag, Au) atoms. The Ag and Au atom doping can greatly enhance the adsorption ability of the ZnO-ML to the CO and NO molecules, except for the NH3 molecules. Especially for NO molecules, the adsorption energies of NO on the noble metal (Ag, Au) decorated ZnO-ML are −0.92 eV and −1.30 eV, which are 5 and 7 times more than that of on the pristine ZnO-ML, respectively. Therefore, both Ag and Au atom decorated ZnO-ML are more sensitive for CO and NO molecules contrast to the ZnO-ML. These results provide insight into the adsorption properties of ZnO-ML, which could promote the further application of ZnO materials in the gas sensing field.
molecules. The NO adsorption is different from the case of CO, Ag-ZnOML is a nonmagnetic semiconductor after NO adsorption (Fig. 8(b)). Several hybridizations appear between N-2p and Ag-4d near −7.80 eV, N-2p and Ag-5s near Fermi-level. Besides, the electronic properties of Ag-ZnO-ML can be effectively regulated by adsorption of NO, which results from the electronic state contribution of NO to TDOS of the adsorption systems near the Femi level. For NH3 adsorption, Ag-ZnOML is still a magnetic semiconductor after NH3 adsorption (Fig. 8(c)). However, the electronic states of NH3 have no contribution to the TDOS of the adsorption systems near 0 eV, which indicates that weak interaction between NH3 and Ag-ZnO-ML. There appear weak hybridizations between N and Ag near 3 eV (N-2s and Ag-5p) and −5.6 eV (N-2p and Ag-4d). The Au-ZnO-ML is a magnetic semiconductor after CO adsorption (Fig. 8(d)). A spin-up peak crosses the Fermi level, and a spindown peak appears at 0.4 eV near the Fermi level, which is mainly attributed to C-2p and Au-6 s states. The overlaps near Fermi-level refer to C-2s, C-2p, and Au-6s, near 1.38 eV and 3.82 eV refer to C-2p and Au6p, near −8.3 eV and −7 eV refer to C-2p, Au-6s, and Au-6p. These results suggest that the interaction between CO and Au-ZnO-ML is enhanced. In the case of NO adsorbed system, a new peak crosses the Fermi level and the other new peak at 0.84 eV near the Fermi level, which is mainly attributed to N-2p and Au-6s (Fig. 8 (e)). Besides, the electronic properties of Au-ZnO-ML can be effectively regulated by adsorption of NO, which results from the electronic state's contribution of NO to TDOS of the adsorption systems near the Femi level. Thus AuZnO-ML is promising for electronic applications, such as a gas sensor. The adsorption system of NH3 on Au decorated ZnO-ML is still a magnetic semiconductor (Fig. 8(f)). It is observed that a spin-up state crosses the Fermi level, which is mainly attributed to Au-6s and N-2p orbitals. There are several weak hybridizations appear between N-2p and Au-5d (near −2.5 eV and −5.4 eV), N-2p and Au-6s near Fermilevel, N-2s and Au-6p near 3.62 eV. These results indicate a weak interaction between NH3 and Au-ZnO-ML, indicating that Au-ZnO-ML barely enhances the adsorption ability for NH3. It is consistent with the analysis result of adsorption energy. For the magnetism distribution of CO, and NH3 on noble metal (Ag, Au) decorated ZnO-ML, the spin charge density distribution of CO/AgZnO-ML, NH3/Ag-ZnO-ML, CO/Au-ZnO-ML, and NH3/Au-ZnO-ML is depicted (Fig. 9). Also, the magnetic moment (μB) of CO and NH3 adsorbed on Ag-ZnO-ML and Au-ZnO-ML are calculated and summarized in Table 3. For CO/Ag-ZnO-ML, a larger area of spin density locates on the Ag atom and CO molecules, in which that the C atom has a larger portion of spin density than O atom, which is consistent with the calculation result of magnetic moment (Fig. 9(a)). However, for NH3/AgZnO-ML, most of the spin density is mainly resides on Ag atoms and O atoms below the Ag atoms. It is consistent with the magnetic moment of Ag (0.33 μB) and O below Ag (0.23 μB), while a small part of the spin density locates on N atoms in adsorbed NH3 (Fig. 9(b)). For CO/Au-ZnO-ML, the C atoms above Au have more spin density than those above Ag. The O atoms below Au have also more spin density than those below Ag (Fig. 9(c)). Besides, for NH3/Au-ZnO-ML, O atoms below Au have a larger area of spin density than those below Ag, while Ag atoms have more spin density than Au atoms (Fig. 9(d)). These results indicate that a strong magnetic interaction appears between CO molecules and noble metal (Ag, or Au) atoms, while a smaller magnetic interaction exists between NH3 molecules and Ag, or Au atoms.
CRediT authorship contribution statement Yongfeng Qu: Data curation, Investigation, Writing - original draft, Methodology, Software. Jijun Ding: Writing - review & editing. Haiwei Fu: Conceptualization, Investigation, Project administration. Jianhong Peng: Software, Validation. Haixia Chen: Supervision, Conceptualization, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Natural Science Foundations of China (Grant No. 11804273; 11447116), Science and Technology Plan Program in Shaanxi Province of China (Grant No. 2019GY-170; 2019GY-176; 2016JQ5037), Special Program for Scientific Research of Shaanxi Educational Committee (Grant No. 16JK1601), Graduate Student Innovative and Practical Ability Training Program of Xi’an Shiyou University (Grant No. YCS19211028). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, H.L. Stormer, Ultrahigh electron mobility in suspended graphene, Solid State Commun. 146 (2008) 351–355. [3] Q.Y. He, S.X. Wu, Z.Y. Yin, H. Zhang, Graphene-based electronic sensors, Chem. Sci. 3 (2012) 1764–1772. [4] J.J. Ding, H.X. Chen, L. Ma, H.W. Fu, X.J. Wang, Field emission of graphene oxide decorated ZnO nanorods grown on Fe alloy substrates, J. Alloy. Compd. 729 (2017) 538–544. [5] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Graphene photonics and optoelectronics, Nat. Photo. 4 (2010) 611–622. [6] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Graphene-based composite materials, Nature 442 (2006) 282–286. [7] H.R. Jiang, W. Shyy, M. Liu, L. Wei, M.C. Wu, T.S. Zhao, Boron phosphide monolayer as a potential anode material for alkali metal-based batteries, J. Mater. Chem. A 5 (2017) 672–679. [8] O. Leenaerts, B. Partoens, F.M. Peeters, Adsorption of H2O, NH3, CO, NO2, and NO on graphene: a first-principles study, Phys. Rev. B 77 (2008) 125416. [9] C.V.V.M. Gopi, M. Venkata-Haritha, L. Young-Seok, K. Hee-Je, ZnO nanorods decorated with metal sulfides as stable and efficient counter-electrode materials for high-efficiency quantum dot-sensitized solar cells, J. Mater. Chem. A 4 (2016) 8161–8171. [10] F.X. Liang, Y. Gao, C. Xie, X.W. Tong, Z.J. Li, L.B. Luo, Recent advances in the fabrication of graphene-ZnO heterojunctions for optoelectronic device applications, J. Mater. Chem. C 6 (2018) 3815–3833. [11] S. Dutta, S. Chattopadhyay, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Role of defects in tailoring structural, electrical and optical properties of ZnO, Prog. Mater Sci. 54 (2009) 89–136. [12] H.X. Chen, J.J. Ding, X.M. Wang, X.J. Wang, G.X. Chen, L. Ma, Enhanced mechanism investigation on violet-blue emission of ZnO films by incorporating Al and Zn to form ZnO-Al-Zn films, Opt. Mater. 62 (2016) 505–511. [13] F. Claeyssens, C.L. Freeman, N.L. Allan, Y. Sun, M.N.R. Ashfolda, J.H. Harding,
4. Conclusions In summary, the adsorption of CO, NO, and NH3 on the pristine, Agand Au-ZnO-ML is investigated by using DFT calculation. The CO, NO, and NH3 molecules can be physically absorbed near the TZn site of pristine ZnO-ML with the exothermic process. However, the adsorption energy (−0.63 eV) and charge transfer (0.16 e) of NH3 adsorbed on pristine ZnO-ML is larger than that of CO (−0.21 eV, 0.07 e) and NO 7
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Surf. Sci. 456 (2018) 351–359. [22] Y.N. Tang, D.W. Ma, W.G. Chen, X.Q. Dai, Improving the adsorption behavior and reaction activity of Co-anchored graphene surface toward CO and O2 molecules, Sens. Actuat. B 211 (2015) 227–234. [23] H.X. Chen, Y.F. Qu, L. Sun, J.H. Peng, J.J. Ding, Band structures and optical properties of Ag and Al co-doped ZnO by experimental and theoretic calculation, Physica E 114 (2019) 113602. [24] H.D. Kuhns, C. Mazzoleni, H. Moosmuller, R.E. Keislar, P.W. Barber, J.G. Watson, Nikolic, Remote sensing of PM, NO, CO and HC emission factors for on-road gasoline and diesel engine vehicles in Las Vegas, NV, Sci. Total Environ. 322 (2004) 123–137. [25] Y. Zeng, S.G. Wang, Comment on “Identification of major sources of atmospheric NH3 in an urban environment in northern China during wintertime”, Environ. Sci. Technol. 52 (2018) 362–363. [26] Q.G. Jiang, Z.M. Ao, D.W. Chu, Q. Jiang, Reversible transition of graphene from hydrophobic to hydrophilic in the presence of an electric field, J. Phys. Chem. C 116 (2012) 19321–19326. [27] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [28] A.A. Tamijani, A. Salam, M.P. De Lara-Castells, Adsorption of noble-gas atoms on the TiO2(110) surface: an ab initio-assisted study with van der Waals-corrected DFT, J. Phys. Chem. C 120 (2016) 18126–18139.
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Full Length Article
Adsorption of CO, NO, and NH3 on ZnO monolayer decorated with noble metal (Ag, Au) Yongfeng Qua, Jijun Dinga, Haiwei Fua, Jianhong Pengb, Haixia Chena, a b
T
⁎
College of Science, Xi’an Shiyou University, Xi’an, Shaanxi 710065, China College of Physics and Electronic Engineer, Qinghai Nationalities University, Xining, Qinghai 810007, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO monolayer Noble metal decoration Theoretical calculation Adsorption properties
The stable adsorption configuration, electronic structure and magnetic property of CO, NO, and NH3 adsorbed on pristine and noble metal (Ag, Au) atom decorated ZnO monolayer (ZnO-ML) are investigated using density functional theory (DFT). The results show that all three kind of gas molecules are physically absorbed on pristine ZnO-ML during the exothermic process. At the same time, the adsorption performance of the ZnO-ML is enhanced by doping noble metal (Ag, Au) atoms. Both Ag and Au atom doping can greatly enhance adsorption ability of the ZnO-ML to the CO and NO molecules, except for the NH3 molecules. For NO molecules, the adsorption energies of NO on the Ag, and Au atom decorated ZnO-ML are −0.92 eV and −1.30 eV, respectively, which are over 5 and 7 times larger than that of NO on the pristine ZnO-ML, respectively. Therefore, both the Ag, and Au atom decorated ZnO-ML are more sensitive for CO and NO molecules contrast to the ZnO-ML. These results provide insight into the adsorption properties of ZnO-ML, which could promote the further application of ZnO materials in the gas sensing field.
1. Introduction Graphene, a single atom thick honeycomb structure consisted of carbon atoms and arranged periodically, was successfully synthesized in 2004 [1]. It has been attracted extensive attention due to some excellent properties including stable geometric structure, excellent electronic properties, and large surface to volume ratio, etc. [2–4]. Graphene has promising application potential in optoelectronic devices [5], composites [6], and electronics [7]. However, both stable carbon atom sp2 hybridization structures and gapless feature make graphene unsuitable for electrical switch applications and gas sensor design [8]. Finding new interesting phenomena behind the planar structure and enlarging relevant applications is greatly inspiring our efforts to find other two-dimensional material with a moderate band gap. Among semiconductor oxides, zinc oxide (ZnO) has been extensively investigated owing to its excellent optical, electrical, and piezoelectric properties [9,10]. Bulk ZnO is an excellent semiconductor material with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, which can be used in solar cells, ultraviolet light-emitting diodes, and transparent conductive films [11,12], and so on. However, the researchers predict that the ZnO monolayer (ZnO-ML) could be more interesting properties than the bulk structures [13] by theoretical studies. Besides, ZnO-ML with ⁎
honeycomb structure, in which zinc and oxygen atoms are arranged in a single layer plane, has been successfully synthesized on a Pd(1 1 1) surface[14]. The two-dimensional ZnO-ML is a nonmagnetic semiconductor nanomaterial with a wide band gap of 3.25–4.0 eV, which is promising candidates in low-dimensional electronic devices [15]. For the last several years, a lot of research work has been devoted to the manipulation of the ZnO-ML surface to modify its electronic structure by different ways including the inclusion of intrinsic defects, doping of impurity atoms, and other adsorbates [16–19]. For example, Topsakal et al. found that the local magnetic moment of the ZnO-ML is caused by the Zn-vacancy defects [16]. Ren et al. reported that the magnetism of the ZnO monolayer is induced by doping 3d transition metals (Cr, Mn, Fe, Co, Ni, and Cu) [17]. Also, the transition metal (V, Cr, Mn, Fe, and Co) decorated ZnO monolayer are investigated using the first-principles calculation. Results indicated that V-decorated ZnO monolayer has the largest binding energy compared with other transition metals [18]. Zhang et al. found that the magnetism of non-metals (C, N and B) doped ZnO-ML can be regulated by adsorption of CO. At the same time, both C and N doped ZnO-ML can be transformed into nonmagnetic semiconductors, while B-doped ZnO-ML is turned into a ferromagnetic half metal [19]. More interestingly, it is well known that surface decoration is a direct and effective strategy to modify material properties at the atomic
Corresponding author. E-mail address: [email protected] (H. Chen).
https://doi.org/10.1016/j.apsusc.2019.145202 Received 4 November 2019; Received in revised form 10 December 2019; Accepted 26 December 2019 Available online 29 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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scale. Of course, the gas sensing properties of two-dimensional materials can also be improved by surface decoration [20,21]. For instance, Co-decorated graphene sheets exhibit high sensitivity toward NO, H2S, and HCN molecules [22]. Ag-Al co-doping can tune the band gap of ZnO more flexible [23]. However, the effect of gas molecules, especially for CO, NO, and NH3 molecules, adsorbed on noble metal atom (Ag, and Au) decorated ZnO-ML has hardly been investigated. CO, NO, and NH3 are all common gases in the atmosphere, in which CO and NO are colorless and odorless gases, while NH3 has a strong pungent smell [24,25]. CO, NO, and NH3 can seriously damage the oxygen delivery function of the human body. When the gas concentration is high, it will lead to hypoxia death. Besides, these gases will seriously pollute the environment. Therefore, it is necessary to effectively monitor and detect toxic gas molecules. Furthermore, investigation on the adsorption of CO, NO, and NH3 molecules helps to evaluate the gas sensing properties of noble metal atoms (Ag, Au) decorated ZnO-ML nanomaterials. In this work, based on density functional theory (DFT), the adsorption structures of CO, NO, and NH3 adsorbed on noble metal (Ag, Au) decorated ZnO-ML are investigated. On the one hand, the geometry and adsorption structure of CO, NO, and NH3 adsorbed onto pristine ZnO-ML (CO/ZnO-ML, NO/ZnO-ML, and NH3/ZnO-ML) are investigated to obtain the most stable adsorption structures. The band structure, electronic structure, charge transfer, magnetism, and adsorption energy base on the most stable configuration are calculated. The total density of states (TDOS) and the partial density of states (PDOS) of ZnO-ML before and after CO, NO, and NH3 adsorption are investigated to explore the interaction between gas molecules and ZnOML. On the other hand, the geometry and adsorption structure of CO, NO, and NH3 adsorbed onto noble metal (Ag, Au) decorated ZnO-ML (CO/Ag-ZnO-ML, NO/Ag-ZnO-ML, NH3/Ag-ZnO-ML, CO/Au-ZnO-ML, NO/Au-ZnO-ML, and NH3/Au-ZnO-ML) are investigated to acquire the most stable configuration. The band structure, adsorption energy, charge transfer, magnetism, and the DOS of CO, NO, and NH3 adsorbed on noble metal (Ag, Au) decorated ZnO-ML are calculated. Results show that the noble metal (Ag, Au) decorated ZnO-ML is more sensitive for CO and NO molecules detection than that of the undecorated ZnO-ML. However, the adsorption energy of NH3 adsorbed on both noble metal (Ag, Au) decorated ZnO-ML and pristine ZnO-ML are almost the same.
Fig. 1. The optimized geometry of ZnO-ML, and four possible adsorption sites including the top of Zn atoms (TZn), the top of O atoms (TO), above the center of the Zn-O bond (B), and the center of Zn-O hexagon (H).
adsorbed on pristine or noble metal (Ag, Au) decorated ZnO-ML system, ZnO-ML or noble metal (Ag, Au) decorated ZnO-ML, and isolated gas molecules in the same supercell, respectively. A negative Ead means that the adsorption process is exothermic. 3. Results and discussion 3.1. The adsorption of CO, NO, and NH3 on pristine ZnO-ML 3.1.1. Adsorption configurations and geometry structures For each molecule, four adsorption sites are considered. They are named as the top of a zinc atom (TZn), the top of an oxygen atom (TO), above the center of the Zn-O bond (B), and the center of Zn-O hexagon (H) (see Fig. 1), respectively. Meanwhile, the different initial orientations of the gas molecules are also investigated to obtain a most stable adsorption configuration. For the diatomic (CO, NO), there are three possible initial orientations at each adsorption site. Take the CO molecule as an example, the molecular plane is parallel to or perpendicular (C atom points to the ZnO-ML or away from it) to the ZnO-ML surface plane. Furthermore, two initial orientations of the NH3 molecule are examined, one is the N atoms pointing away from the substrate, and the other is the N atoms pointing to the substrate. The calculation results are summarized in Table 1. From Table 1, the CO molecules prefer to adsorb on the H site. The configuration with the CO molecule adopts tilted adsorption, in which the C atoms are pointing to the Zn atoms on the substrate (Fig. 2(a)). The adsorption structure has the largest adsorption energy (−0.21 eV), the shorter distance between the CO and substrate (D = 2.59 Å), and a small charge transfer (0.07 e) from CO molecules to ZnO-ML, which indicates that the process is weak physical adsorption. In the case of NO
2. Computational details All calculations are performed by using the Dmol3 module, within the generalized gradient approximation (GGA) of Perdew-BurkeErnzerhof (PBE) [26,27]. The double numerical plus polarization (DNP) basis set is employed, and the DFT semi-core pseudopotential (DSPP) is selected. The Tkatchenko-Scheffler scheme is used in all calculations to correct van der Waals forces [28]. The convergence tolerance of energy of 1.0 × 10−5 Ha is set, and the maximum force and displacement are 0.002 Ha/Å and 0.005 Å, respectively. For geometry structure optimization, the global orbital cutoff (5.0 Å) is set, and the smearing energy (0.005 Ha) is used. The Brillouin zone is sampled using a 10 × 10 × 1 Monkhorst-Pack grid in geometry optimization. For the calculation of electronic structure, we use a 20 × 20 × 1 Monkhorst-Pack grid. To ensure the precision of the electronic structure calculation, energy tolerance is set as 1.0 × 10−6 Ha. The system consists of 4 × 4 ZnO monolayer supercell (32 atoms) with a distance of 15 Å to avoid the interlayer interaction between adjacent ZnO monolayer layers. Besides, the charge transfers are calculated based on the Mulliken charge analysis. For gas molecule adsorbed on both pristine ZnO-ML and the noble metal (Ag, Au) decorated ZnO-ML, the adsorption energy Ead is determined by
Table 1 Optimized geometric parameters of CO, NO, and NH3 adsorbed on different adsorption sites of ZnO-ML: adsorption energy (Ead), adsorption distance of nearest-neighbor atoms between the gas molecule and substrate (D), and charge transfer (Q) between gas molecule and substrate. Molecule
Sites
D (Å)
Q (e)
Ead (eV)
CO
B H TO TZn B (optimized to TZn) H TO TZn B H TO (optimized to TZn) TZn
2.59 2.59 2.57 2.60 2.76 2.77 3.04 2.76 2.20 3.17 2.20 2.20
0.07 0.07 0.08 0.08 0.02 0.01 0 0.02 0.16 0.06 0.16 0.16
−0.20 −0.21 −0.19 −0.20 −0.18 −0.17 −0.15 −0.18 −0.62 −0.28 −0.63 −0.63
NO
NH3
Ead = EZnO + gas − (EZnO + Egas) where EZnO + gas , EZnO and Egas are total energies of the gas molecule 2
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Fig. 2. The most stable adsorption configurations (top and side view) of (a) CO, (b) NO, and (c) NH3 adsorbed on ZnO-ML. The C, N, and H atoms are shown in black, blue, and white balls, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
adsorption, the NO molecules prefer to adsorb on TZn site with the largest adsorption energy (−0.18 eV). At the same time, the NO initially adsorbed at B site is moving spontaneously to TZn site after relaxation. Thus, the TZn site adsorption configuration is the most stable. The NO molecule adopts tilted adsorption, in which the N atoms are pointing to the Zn atoms on the substrate (Fig. 2(b)). For the most stable configuration, the system has a large adsorption distance (D = 2.76 Å) and a very small charge transfer (0.02 e) from NO molecules to substrate, which indicates NO molecules are a very weak dopant. It is found that the charge transfer of adsorption configuration at the TO site is zero, which indicates that the charge carrier concentration of the substrate is constant. For NH3 adsorption, the configuration with NH3 is vertical to the substrate plane in which that the N atoms point to the Zn atoms of the substrate (Fig. 2(c)). This configuration has larger adsorption energy (−0.63 eV) and a shorter adsorption distance between NH3 and ZnO-ML (D = 2.20 Å). The TZn site is the optimal adsorption site with a large charge transfer (0.16 e) from NH3 to ZnO-ML, which indicates that there is weak interaction between them. Also, the configuration of NH3 adsorbed at H site has smaller charge transfer and adsorption energy compared with other adsorption sites. These results indicate that ZnO-ML is a promising material for NH3 capture and separation.
Fig. 3. The band structures of (a) ZnO-ML, (b) CO, (c) NO, and (d) NH3 adsorbed on ZnO-ML based on the most stable configuration. The Fermi level is set to zero energy and indicated by the pink dashed line. The red and blue lines represent spin-up and spin-down bands, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.1.2. Electronic properties To understand the interaction between three gas molecules and ZnO-ML, electronic properties (band structures and DOS) of the adsorbed systems are investigated. The spin band structures of pristine ZnO-ML and adsorbed systems are calculated (Fig. 3). The spin-up and spin-down band structures of pristine ZnO, CO/ZnO, and NH3/ZnO are identical, indicating that CO and NH3 molecules adsorbed ZnO-ML systems are nonmagnetic. However, the spin-up and spin-down bands near the Fermi level of the NO molecules adsorbed on the ZnO-ML structure are asymmetric, which indicates that the adsorbed system is magnetic and the NO adsorption breaks the spin degeneracy (Fig. 3(c)). In contrast to the pristine ZnO-ML, both the conduction band and valence band of the adsorbed system become more discrete due to more overlap among different atoms in the outer electron orbit. For CO, and NH3 adsorbed ZnO-ML systems, the conduction band numbers are increased (Fig. 3(b) and (d)). Especially for NO molecules adsorbed on the ZnO-ML systems, there are two flat spin-polarized bands near the Fermi level in the spin-up bands, and one of them coincides with Fermi level. In the spin-down bands, there are two spin-polarized bands above the Fermi level, and they are both far from the Fermi level. The spin density of states (DOS) comprising TDOS and PDOS are calculated to obtain electron distribution and magnetic properties. Fig. 4 shows the TDOS and PDOS of (a) CO, (b) NO, and (c) NH3 adsorbed on pristine ZnO-ML based on the most stable adsorption configuration. The TDOS plots of CO/ZnO-ML and NH3/ZnO-ML present excellent symmetry between spin-up and spin-down plots owing to the
nonmagnetic system, but apparent asymmetry appears in TDOS of NO/ ZnO-ML near the Fermi level. To make a further insight into interactions between gas molecules and ZnO-ML, the TDOS before and after CO, NO, and NH3 adsorption are compared. The PDOS of the atom’s electron orbital is analyzed. The TDOS of conduction band moves towards the low energy region after adsorption of CO. Besides, there appear some new peaks, including the peak near −8.98 eV attributed to C-2s orbitals and the peak near 2.95 eV belonged to C-2p orbitals. It is observed that weak hybridizations appear between Zn and C near −5 eV and −3.3 eV (Zn-3d, C-2s, and C-2p), near 5.8 eV (Zn-4s, Zn-3p, and C-2p). These weak hybridization states between C and Zn atoms indicating weak physical interactions and weak physical adsorption, which is consistent with the analysis results of adsorption energy and charge transfer. For NO adsorbed on ZnO-ML, it is found that the TDOS undergoes a significant left shift, which mainly results from the newly occupied states just below the Fermi level. Also, obvious asymmetry appears near the Fermi level is mainly belong to N-2p orbitals, which result in the non-zero magnetic moment of the adsorption system. Several overlap states appear near −6.5 eV (Zn-3d, N-2s and N-2p) and 5.3 eV (N-2p, Zn-3p and Zn-4s). As NH3 adsorbed on ZnO-ML, the N-2p states and Zn-3d states are overlapped around −4.89 eV, −2.62 eV and 3
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Fig. 4. The TDOS and PDOS of (a) CO, (b) NO, and (c) NH3 adsorbed on ZnO-ML based on the most stable adsorption configuration. The Fermi level is set to zero energy and indicated by the black dashed line.
Table 2 Optimized geometric parameters of CO, NO, and NH3 adsorbed on noble metal (Ag, Au) decorated ZnO-ML: adsorption energy (Ead), adsorption distance of nearest-neighbor atoms between the gas molecule and decorated substrate (D), charge transfer (Q) between gas molecule and decorated substrate and total magnetic moment of the adsorbed system (M).
Fig. 5. The spin charge density distribution (top and side view) of NO adsorbed on ZnO-ML, the isosurface value for the spin charge is 0.02 e/Å3.
Systems
Ead (eV)
D (Å)
Q (e)
M (μB)
Ag-ZnO-ML/CO Ag-ZnO-ML/NO Ag-ZnO-ML/NH3 Au-ZnO-ML/CO Au-ZnO-ML/NO Au-ZnO-ML/NH3
−0.61 −0.92 −0.60 −0.85 −1.30 −0.64
2.07 2.12 2.38 1.97 2.07 2.36
−0.05 −0.18 0.13 0.07 −0.07 0.24
1 0 1 1 0 1
Fig. 6. The most stable adsorption configurations (top and side view) of CO, NO, and NH3 adsorbed on Ag decorated ZnO-ML ((a)–(c)) and Au decorated ZnO-ML ((d)–(f)). The Ag and Au atoms are shown in cyan and yellow balls, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 4
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molecular orientations of the CO, NO, and NH3 have been discussed above. The most stable adsorption configurations are selected to discuss the band structure, DOS, charge transfer, and magnetism. For the most stable configurations, it is obvious that the noble metal (Ag, Au) atom gives rise to the slightly structural deformations of the ZnO-ML surface plane (Fig. 6). In the case of CO adsorption, the CO molecules prefer to adopt tilted adsorption configuration in which that C atom closes to the top of the adsorption sites (Fig. 6(a) and (d)). For NO adsorbed system, NO molecules are inclined to adsorb on the noble metal (Ag, Au) atom decorated substrate, in which the N atoms close to the adsorption sites (Fig. 6(b) and (e)). For NH3 adsorbed on noble metal (Ag, Au) atom decorated ZnO-ML, the NH3 molecules are vertical to the substrate plane, and the N atoms close to Ag and Au atom, respectively (Fig. 6(c) and (f)). The calculated total magnetic moment (M), adsorption distance (D), charge transfer (Q), and adsorption energy (Ead) of the most stable configurations are given in Table 2. The results show that noble metal (Ag, Au) decoration can prominently improve the adsorption performance of ZnO-ML for CO and NO molecules. The adsorption energy of CO on Ag decorated ZnO-ML is −0.61 eV, which is more than 2.9 times of that on pristine ZnO-ML. The adsorption energy of CO on Au decorated ZnO-ML is −0.85 eV, which is over 4 times larger than of that on pristine ZnO-ML. Besides, the adsorption energies of NO on Ag and Au decorated ZnO-ML are −0.92 eV and −1.30 eV, respectively, which are more than 5 and 7 times of that on pristine ZnO-ML, respectively. In contrast to ZnO-ML, the adsorption energy of the NH3 on Ag decorated ZnO-ML slightly lower than that of on ZnO-ML is −0.60 eV. However, the adsorption performance of Au decorated ZnOML for NH3 is barely enhanced. These results indicate that the noble metal (Ag, Au) atom decorated ZnO-ML may be used as good sensing materials for CO and NO. 3.2.2. Electronic properties The spin band structures are calculated for the noble metal (Ag, Au) decorated ZnO-ML and gas molecules adsorbed on noble metal (Ag, Au) decorated ZnO-ML based on most energy favorable structures (Fig. 7). It can be seen that the band structures of noble metal (Ag, Au) decorated ZnO-ML and CO and NH3 adsorbed noble metal (Ag, Au) decorated ZnO-ML show obvious asymmetry near Fermi level owing to the zeromagnetic moment of the system. However, a good symmetry appears in band structures of NO adsorbed noble metal (Ag, Au) decorated ZnOML. These results show that NO adsorbed noble metal (Ag, Au) decorated ZnO-ML are the non-magnetic semiconductors, but other systems are magnetic. It is observed that Ag decorated ZnO-ML with and without adsorption of CO and NH3 is a magnetic semiconductor (Fig. 7(a), (b) and (d)). The Fermi level intersects the spin-up band and lies in the band gap of the spin-down band, which indicates that NH3 adsorbed Ag decorated ZnO-ML exhibits a half-metallic property (Fig. 7(d)). However, it is found that a new impurity band appears in the band gap of the NO adsorbed Ag decorated system (Fig. 7(c)). The Au decorated ZnO-ML is a magnetic semiconductor (Fig. 7(e)). It is observed that a spin-up band intersects with the Fermi level, which indicates CO and NH3 adsorbed Au decorated ZnO-ML is half metal (Fig. 7(f) and (h)). Besides, it is found that a new impurity band also appears between conduction bands and valence bands of the NO adsorbed on the Au decorated system, and the conduction band becomes more discrete (Fig. 7(g)). Next, to further investigate the interaction between three gas molecules and noble metal (Ag, Au) atom decorated ZnO-ML, the TDOS and PDOS of noble metal (Ag, Au) atom decorated ZnO-ML based on the most stable adsorption configurations are depicted (Fig. 8). It can be seen that all TDOS experiences significant left shift after adsorption of gas molecules, which mainly results from the newly occupied states just below the Fermi level. In addition, the TDOS plots of NO/Ag-ZnO-ML and NO/Au-ZnO-ML present good symmetry between spin-up and spindown plots owing to the system is nonmagnetic. However, the obvious asymmetry appears in TDOS of CO/Ag-ZnO-ML, NH3/Ag-ZnO-ML, CO/
Fig. 7. The band structures of Ag decorated ZnO-ML (a), CO, NO, and NH3 adsorbed on Ag decorated ZnO-ML ((b)–(d)), Au decorated ZnO-ML (e), and CO, NO, and NH3 adsorbed on Au decorated ZnO-ML ((f)–(h)) based on the most stable adsorption configurations.
−0.60 eV, respectively. A new peak of TDOS appears near −7.62 eV attributed to N-2p orbitals. The strong overlap is hardly observed near the Fermi level, which indicates that the NH3 adsorption configuration is physisorption. For further insight into the magnetic moment of the adsorption system, the spin charge density distribution of NO/ZnO-ML is depicted (Fig. 5). For NO/ZnO-ML, the magnetism is mainly distributed on the NO molecules in which that the spin density of N atom is larger than that of O atom. Besides, there is almost no spin density located on the atoms of the substrate.
3.2. The adsorption of CO, NO, and NH3 on noble metal (Ag, Au) decorated ZnO-ML 3.2.1. Adsorption configurations and geometry structures In order to improve the adsorption performance of ZnO-ML to CO, NO, and NH3 molecules, the noble metal (Ag, Au) atoms adsorbed on ZnO-ML surface are investigated. The initially adsorption site and 5
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Fig. 8. The TDOS and PDOS of CO, NO, and NH3 adsorbed on Ag decorated ZnO-ML ((a)–(c)) and Au decorated ZnO-ML ((d)–(f)) based on the most stable configuration.
Table 3 The magnetic moment (μB) of CO and NH3 adsorbed on Ag-ZnO-ML and AuZnO-ML. Structure
Atom type
M(μB)
CO/Ag-ZnO-ML
C O in CO Ag O below N Ag O below C O in CO Au O below N Au O below
0.24 0.11 0.21 0.06 0.06 0.33 0.23 0.30 0.13 0.20 0.11 0.10 0.32 0.33
NH3/Ag-ZnO-ML
CO/Au-ZnO-ML
NH3/Ag-ZnO-ML
Ag
Ag
Au
Au
Au-ZnO-ML, and NH3/Au-ZnO-ML, especially near Fermi level. The PDOS analysis exhibits that two spin-polarized peaks appear near the Fermi level at −0.35 eV and 0.8 eV in spin-up states after CO adsorption, which are mainly attributed to C-2p, Ag-5p, and Ag-5s orbitals, respectively (Fig. 8(a)). In spin-down states, the two spin-polarized peaks also appear near the Fermi level and 0.96 eV above the Fermi level, which is also mainly attributed to C-2p, Ag-5p, and Ag-5s orbitals, respectively. It is observed that hybridizations appear between C and Ag near −0.35 eV (C-2p and Ag-5s), 0.8 eV and 3 eV (C-2p and Ag-5p), and thus the interaction between C and Ag atom is enhanced, which results in improving the adsorption performance of ZnO-ML for CO
Fig. 9. The spin charge density distribution (top and side view) of CO, and NH3 adsorbed on Ag-ZnO-ML ((a)–(b)) and Au-ZnO-ML ((c)–(d)), the isosurface value for the spin charge is 0.02 e/Å3. 6
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(−0.18 eV, 0.02 e), which means that ZnO-ML has promising prospects for NH3 sensing. The adsorption capacity of pristine ZnO-ML can be greatly enhanced by doping noble metal (Ag, Au) atoms. The Ag and Au atom doping can greatly enhance the adsorption ability of the ZnO-ML to the CO and NO molecules, except for the NH3 molecules. Especially for NO molecules, the adsorption energies of NO on the noble metal (Ag, Au) decorated ZnO-ML are −0.92 eV and −1.30 eV, which are 5 and 7 times more than that of on the pristine ZnO-ML, respectively. Therefore, both Ag and Au atom decorated ZnO-ML are more sensitive for CO and NO molecules contrast to the ZnO-ML. These results provide insight into the adsorption properties of ZnO-ML, which could promote the further application of ZnO materials in the gas sensing field.
molecules. The NO adsorption is different from the case of CO, Ag-ZnOML is a nonmagnetic semiconductor after NO adsorption (Fig. 8(b)). Several hybridizations appear between N-2p and Ag-4d near −7.80 eV, N-2p and Ag-5s near Fermi-level. Besides, the electronic properties of Ag-ZnO-ML can be effectively regulated by adsorption of NO, which results from the electronic state contribution of NO to TDOS of the adsorption systems near the Femi level. For NH3 adsorption, Ag-ZnOML is still a magnetic semiconductor after NH3 adsorption (Fig. 8(c)). However, the electronic states of NH3 have no contribution to the TDOS of the adsorption systems near 0 eV, which indicates that weak interaction between NH3 and Ag-ZnO-ML. There appear weak hybridizations between N and Ag near 3 eV (N-2s and Ag-5p) and −5.6 eV (N-2p and Ag-4d). The Au-ZnO-ML is a magnetic semiconductor after CO adsorption (Fig. 8(d)). A spin-up peak crosses the Fermi level, and a spindown peak appears at 0.4 eV near the Fermi level, which is mainly attributed to C-2p and Au-6 s states. The overlaps near Fermi-level refer to C-2s, C-2p, and Au-6s, near 1.38 eV and 3.82 eV refer to C-2p and Au6p, near −8.3 eV and −7 eV refer to C-2p, Au-6s, and Au-6p. These results suggest that the interaction between CO and Au-ZnO-ML is enhanced. In the case of NO adsorbed system, a new peak crosses the Fermi level and the other new peak at 0.84 eV near the Fermi level, which is mainly attributed to N-2p and Au-6s (Fig. 8 (e)). Besides, the electronic properties of Au-ZnO-ML can be effectively regulated by adsorption of NO, which results from the electronic state's contribution of NO to TDOS of the adsorption systems near the Femi level. Thus AuZnO-ML is promising for electronic applications, such as a gas sensor. The adsorption system of NH3 on Au decorated ZnO-ML is still a magnetic semiconductor (Fig. 8(f)). It is observed that a spin-up state crosses the Fermi level, which is mainly attributed to Au-6s and N-2p orbitals. There are several weak hybridizations appear between N-2p and Au-5d (near −2.5 eV and −5.4 eV), N-2p and Au-6s near Fermilevel, N-2s and Au-6p near 3.62 eV. These results indicate a weak interaction between NH3 and Au-ZnO-ML, indicating that Au-ZnO-ML barely enhances the adsorption ability for NH3. It is consistent with the analysis result of adsorption energy. For the magnetism distribution of CO, and NH3 on noble metal (Ag, Au) decorated ZnO-ML, the spin charge density distribution of CO/AgZnO-ML, NH3/Ag-ZnO-ML, CO/Au-ZnO-ML, and NH3/Au-ZnO-ML is depicted (Fig. 9). Also, the magnetic moment (μB) of CO and NH3 adsorbed on Ag-ZnO-ML and Au-ZnO-ML are calculated and summarized in Table 3. For CO/Ag-ZnO-ML, a larger area of spin density locates on the Ag atom and CO molecules, in which that the C atom has a larger portion of spin density than O atom, which is consistent with the calculation result of magnetic moment (Fig. 9(a)). However, for NH3/AgZnO-ML, most of the spin density is mainly resides on Ag atoms and O atoms below the Ag atoms. It is consistent with the magnetic moment of Ag (0.33 μB) and O below Ag (0.23 μB), while a small part of the spin density locates on N atoms in adsorbed NH3 (Fig. 9(b)). For CO/Au-ZnO-ML, the C atoms above Au have more spin density than those above Ag. The O atoms below Au have also more spin density than those below Ag (Fig. 9(c)). Besides, for NH3/Au-ZnO-ML, O atoms below Au have a larger area of spin density than those below Ag, while Ag atoms have more spin density than Au atoms (Fig. 9(d)). These results indicate that a strong magnetic interaction appears between CO molecules and noble metal (Ag, or Au) atoms, while a smaller magnetic interaction exists between NH3 molecules and Ag, or Au atoms.
CRediT authorship contribution statement Yongfeng Qu: Data curation, Investigation, Writing - original draft, Methodology, Software. Jijun Ding: Writing - review & editing. Haiwei Fu: Conceptualization, Investigation, Project administration. Jianhong Peng: Software, Validation. Haixia Chen: Supervision, Conceptualization, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the National Natural Science Foundations of China (Grant No. 11804273; 11447116), Science and Technology Plan Program in Shaanxi Province of China (Grant No. 2019GY-170; 2019GY-176; 2016JQ5037), Special Program for Scientific Research of Shaanxi Educational Committee (Grant No. 16JK1601), Graduate Student Innovative and Practical Ability Training Program of Xi’an Shiyou University (Grant No. YCS19211028). References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, H.L. Stormer, Ultrahigh electron mobility in suspended graphene, Solid State Commun. 146 (2008) 351–355. [3] Q.Y. He, S.X. Wu, Z.Y. Yin, H. Zhang, Graphene-based electronic sensors, Chem. Sci. 3 (2012) 1764–1772. [4] J.J. Ding, H.X. Chen, L. Ma, H.W. Fu, X.J. Wang, Field emission of graphene oxide decorated ZnO nanorods grown on Fe alloy substrates, J. Alloy. Compd. 729 (2017) 538–544. [5] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Graphene photonics and optoelectronics, Nat. Photo. 4 (2010) 611–622. [6] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Graphene-based composite materials, Nature 442 (2006) 282–286. [7] H.R. Jiang, W. Shyy, M. Liu, L. Wei, M.C. Wu, T.S. Zhao, Boron phosphide monolayer as a potential anode material for alkali metal-based batteries, J. Mater. Chem. A 5 (2017) 672–679. [8] O. Leenaerts, B. Partoens, F.M. Peeters, Adsorption of H2O, NH3, CO, NO2, and NO on graphene: a first-principles study, Phys. Rev. B 77 (2008) 125416. [9] C.V.V.M. Gopi, M. Venkata-Haritha, L. Young-Seok, K. Hee-Je, ZnO nanorods decorated with metal sulfides as stable and efficient counter-electrode materials for high-efficiency quantum dot-sensitized solar cells, J. Mater. Chem. A 4 (2016) 8161–8171. [10] F.X. Liang, Y. Gao, C. Xie, X.W. Tong, Z.J. Li, L.B. Luo, Recent advances in the fabrication of graphene-ZnO heterojunctions for optoelectronic device applications, J. Mater. Chem. C 6 (2018) 3815–3833. [11] S. Dutta, S. Chattopadhyay, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Role of defects in tailoring structural, electrical and optical properties of ZnO, Prog. Mater Sci. 54 (2009) 89–136. [12] H.X. Chen, J.J. Ding, X.M. Wang, X.J. Wang, G.X. Chen, L. Ma, Enhanced mechanism investigation on violet-blue emission of ZnO films by incorporating Al and Zn to form ZnO-Al-Zn films, Opt. Mater. 62 (2016) 505–511. [13] F. Claeyssens, C.L. Freeman, N.L. Allan, Y. Sun, M.N.R. Ashfolda, J.H. Harding,
4. Conclusions In summary, the adsorption of CO, NO, and NH3 on the pristine, Agand Au-ZnO-ML is investigated by using DFT calculation. The CO, NO, and NH3 molecules can be physically absorbed near the TZn site of pristine ZnO-ML with the exothermic process. However, the adsorption energy (−0.63 eV) and charge transfer (0.16 e) of NH3 adsorbed on pristine ZnO-ML is larger than that of CO (−0.21 eV, 0.07 e) and NO 7
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