A First-principles Study on Gas Sensitivity of Pd or Pt Loaded (5, 5) Carbon Nanotube with a Di-vacancy Defect to CO and NO
Kai QIU , Lei LI , Hong-Wei FAN , He-Zhuan WEI , Sheng-Li AN , Gui-Xiao JIA
Carbon nanotubes (CNTs) are a potential gas-sensitive sensor material due to their excellent physical and chemical properties[1] and good adsorption and desorption ability for most gas molecules[2, 3]. Compared to traditional gas sensors, CNTs have advantages of small size, high sensitivity, low operating temperature, fast response speed and strong selectivity, which have aroused great attention of scientific researchers[4-6]. However, due to the weak binding energy and a small number of charge transfer between perfect CNTs and gas molecules such as CO, NO, H2O, CH4 and CO2, they cannot be directly used as gas sensors to detect these gas molecules[7, 8].
In order to expand the detection range of CNTs and improve the sensitivity, researchers have taken a series of measures such as vacancy defects and metal loaded modifications to solve this problem. To modify CNTs with metal is one of the most important methods among these measures. It was found that the adsorption of Pd atoms on single-walled CNTs (SWCNTs) could well detect CH2O gas[8] and the load of Pd significantly improved the sensitivity of SWCNTs to H2[9], SO2[10], CH3OH[10], CH4[10], CO and NO[11] gas molecules. In addition, the load of Pt also improved the sensitivity of CNTs to CO[11], NO[11], SO2[12], NH3 and NO2[13] gases. The load of Rh[14], Ti[15], Au[16] and Al[17] increased the sensitivity of SWCNTs to monoxides. To introduce atomic defects is another important approach to improve the gas sensitivity of SWCNTs. An atomic vacancy is an active site for gas adsorption and changes the sensitivity of CNTs to gases. Zanolli et al.[18] used an ab initio method to study the adsorption of NO2, NH3, H2O and CO2 gas molecules on SWCNT with atomic vacancies and found that the presence of the vacancy defect significantly enhanced the binding force between gas molecules and the tube. It was found that co-modified SWCNTs with metal atoms and vacancy defects were more sensitive to gas molecules. Zhou et al.[19] found that loaded Al enhanced the interaction between SWCNTs with the vacancy defect and H2CO. Loaded Li on SWCNTs for the SO2 adsorption had the same function[20].
CO and NO gases bring great harm to environment and human health. Thus, it is of great significance to design gas-sensitive sensors used to detect these gases. The interaction between perfect carbon nanotubes and CO or NO is weak. Atomic vacancies in carbon nanotubes, among which a di-vacancy defect is a common one, are inevitably introduced during the preparation and post-treatment. Therefore, in this work the adsorptions of CO and NO on M (Pd or Pt) loaded or M and di-vacancy co-decorated (5, 5) tube are studied, and binding energies, their geometrical and electronic structures are discussed to explore the influence of atomic vacancy defect in SWCNTs on the gas sensitivity to CO or NO. This will provide a certain theoretical basis for the design of CO and NO gas sensors.
All calculations were performed using a projector augmented-wave (PAW) approach based on the density functional theory (DFT)[21, 22] and a Vienna ab initio simulation package (VASP)[23, 24]. A generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was employed. A 1×1×2 Monkhorst-Pack (MP) k-points mesh was used and energy cutoff for the plane wave was set to 350 eV. The energy convergence was set as 1 × 10-4 eV. The maximum conver-gence was 0.02 eV/Å. For all models, vectors a and b respectively along the tube diameter direction were both 17 Å to ensure no interaction between tubes. The vector c (12 layers of carbon atoms) along the tube axis direction is 17.2 Å (Fig. 1).
In order to study adsorptions of the CO and NO gas molecules on the M loaded di-vacancy (5, 5) tube (M-V2-CNT)(CO-M-V2-CNT or NO-V2-CNT), the divacancy (5, 5) tube (V2-CNT) previously studied was selected[25], shown in Fig. 1b. Binding energies Eb(1) between M and the perfect or the di-vacancy (5, 5) tube were calculated as follows,
|
|
(1) |
where EMV is the energy of the metal-loaded perfect (M-CNT) or di-vacancy (5, 5) tube (M-V2-CNT) system, and EM and Etube are energies of a single metal atom and perfect or di-vacancy (5, 5) tube, respectively. Binding energies Eb(2) between gas molecules and the metal-loaded perfect or di-vacancy (5, 5) tube were estimated as follows:
|
|
(2) |
where EMV+gas is the energy for the adsorption system of CO and NO on the metal-loaded perfect or di-vacancy (5, 5) tube (CO-M-CNT and NO-M-CNT or CO-M-V2-CNT and NO-M-V2-CNT). Egas is the energy of CO or NO molecule.
Various adsorption sites of M on the perfect (5, 5) tube are considered, including the hole (H) site, the top (T) site of the C atom and two bridging sites of the C−C bond (the C−C bond is inclined (B1)or vertical (B2) on the tube axis), see Fig. 1a. The calculated results show that Pd or Pt is easily adsorbed at the B1 site. The C−Pd(Pt) bond lengths are 2.115 (2.066 Å) and 2.134 Å (2.063 Å), respectively, and the corresponding Eb(1) is 1.60 eV (2.35 eV), indicating a chemical bond formed between C and Pd or Pt.
In order to understand the effect of a di-vacancy defect on the geometric structure of the M loaded (5, 5) tube, we consider possible load sites of M on the di-vacancy (5, 5) tube (Fig. 1b). The di-vacancy defect in the (5, 5) tube is symmetrical. There are two different C-ring hole sites: 5-ring (H1) and 8-ring (H2) ones, and eight different C−C bond bridging sites: C(6)−C(9), C(1)−C(2), ···, C(6)−C(7) and C(7)−C(8) ones. The calculated results show that the C(6)−C(7) bond at the defect area of the di-vacancy (5, 5) tube easily chemically adsorbs one M atom, and M and the C(6)−C(7) bond form a three-membered ring structure. Compared to M-CNT, the C-Pd(Pt) bonds in M-V2-CNT are shortened to 2.092 (2.038 Å) and 2.124 Å (2.059 Å), respectively. The corresponding Eb(1) is increased to 1.88 eV (2.89 eV), indicating that the covalence between loaded M and C in the (5, 5) tube is enhanced, that is, a divacancy defect enhances the loading of M on the (5, 5) tube.
Li et al.[11] studied adsorptions of CO and NO gases on the Pd or Pt loaded perfect (8, 0) tube. For these adsorption structures, the C−O bond in CO or N−O in NO was perpendicular to the tube axis and C or N was adsorbed at the top position of the Pd or Pt atom. Therefore, this work only considers these adsorption models of CO and NO on the M-CNT. Calculation results show that CO or NO is chemically adsorbed on the top M atoms, forming M−C/N bonds (M and C in CO or N in NO form M−C/N bonds) and the Pd (Pt)−C/N bond lengths (DM−C/N) are 1.896 (1.845 Å) /1.878 Å (1.813 Å), shown in Table 1. The Pd or Pt atom is an active position to adsorb CO and NO, unlike their physical adsorption on the perfect (5, 5) tube. After CO or NO adsorption on the M-CNT, the C(tube)−M (the distance DC(tube)−M between M and its closest two C atoms in the (5, 5) tube) bond length is significantly changed. In the CO adsorption structure the C(tube)-Pd (Pt) bond lengths are elongated to 2.155 (2.172 Å) and 2.407 Å (2.310 Å), respectively (Table 1). Thus, the CO or NO adsorptions weaken the loading of Pd and Pt on the (5, 5) tube, which is consistent with other research results[11].
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| DC(tube)–M (Å) | DM–C/N (Å) | DC/N–O (Å) | Eb (eV) | Q (e) | ||
| CO-Pd-CNT | 2.155 | 2.407 | 1.896 | 1.159 | 1.87 | 0.017 |
| CO-Pd-V2-CNT | 2.140 | 2.308 | 1.902 | 1.160 | 1.79 | 0.016 |
| CO-Pt-CNT | 2.172 | 2.310 | 1.845 | 1.165 | 2.75 | 0.038 |
| CO-Pt-V2-CNT | 2.138 | 2.247 | 1.856 | 1.165 | 2.02 | 0.037 |
| NO-Pd-CNT | 2.196 | 2.214 | 1.878 | 1.188 | 1.62 | 0.057 |
| NO-Pd-V2-CNT | 2.156 | 2.248 | 1.839 | 1.185 | 1.79 | 0.058 |
| NO-Pt-CNT | 2.167 | 2.170 | 1.813 | 1.189 | 2.44 | 0.081 |
| NO-Pt-V2-CNT | 2.071 | 2.298 | 1.798 | 1.186 | 2.08 | 0.091 |
The Eb(2) of CO and NO adsorptions on the Pt-CNT are 2.75 and 2.44 eV, respectively, larger than the Eb(2) (1.87 and 1.62eV) of CO and NO adsorptions on Pd-CNT. The Pd-CNT easily desorbs CO and NO molecules. In addition, it can be seen from Table 1 that the M-CNT transfers a small number of charges to CO or NO, which is different from the Pd or Pt loaded (8, 0) tube[11].
Energy band structures of M-CNT, CO-M-CNT and CO-M-CNT systems are calculated to explore electronic structure changes before and after CO and NO adsorptions on the M-CNT, see Fig. 2. The perfect (5, 5) tube is a conductor. The Pd (or Pt) adsorption slightly opens the band gap of the perfect (5, 5) tube and it was 0.04 eV (or 0.08 eV). The adsorptions of CO and NO on M-CNT also do so and the band gaps are 0.02~0.12 eV (Fig. 2a and 2b). Therefore, M-CNT can not be used to well detect CO and NO molecules due to the small band gap changes before and after CO and NO adsorptions.
In all, M-CNT can chemically adsorb CO and NO molecules, and the loading of Pd on perfect (5, 5) tube improves their adsorptions. However, M-CNT can not effectively detect CO and NO molecules due to the unapparent band gap changes before and after their adsorptions.
Similar CO-M-V2-CNT and NO-M-V2-CNT models to CO-M-CNT and NO-M-CNT models are selected, namely the C−O bond in CO or N−O in NO is perpendicular to the tube axis and C in CO or N in NO is loaded on the M, shown in Fig. 1(c). The geometrical structures of CO-M-V2-CNT and NO-M-V2-CNT are similar to CO-M-CNT and NO-M-CNT. CO and NO are chemically adsorbed at the top of the M atom to form M−C/N bonds, and the Pd(Pt)−C/N bond lengths are 1.902 (1.839 Å) and 1.856 Å (1.798 Å), respectively, see Table 1. The C(tube)-Pd(Pt) bond lengths change significantly after the CO and NO adsorptions on M-V2-CNT. After CO adsorption, C(tube)-Pd(Pt) bonds are elongated to 2.140 (2.138 Å) and 2.308 Å (2.247 Å), respectively. After NO adsorption, their bond lengths are 2.156 (2.071 Å) and 2.248 Å (2.298 Å), respectively, see Table 1. This indicates that the adsorbed CO and NO weaken the interaction between the di-vacancy (5, 5) tube and the loaded M. Eb(2) of CO and NO adsorptions on the Pd-V2-CNT are both 1.79 eV, close to those (1.87 and 1.62 eV) of their adsorptions on Pd-CNT. Eb(2) for Pt-V2-CNT is 2.02 and 2.08 eV, respectively, smaller than those (2.75 and 2.44 eV) of their adsorption on Pt-CNT. The presence of di-vacancy defect had no significant effect on the charge transfer amount between CO or NO and M-CNT, however, it weakens the binding force between Pt-CNT and CO or NO, conducive to the desorption of CO or NO.
The energy band structures of V2-CNT, M-V2-CNT, CO-M-V2-CNT and NO-M-V2-CNT systems are calculated to clarify electronic structure changes before and after CO and NO adsorptions on the tube (see Fig. 2). The band gap of the perfect (5, 5) tube is opened to 0.26 eV when generating a di-vacancy defect. The band gap of V2-CNT remains after the loading of M, similar to the perfect (5, 5) tube after loading M. The band gap changes of M-V2-CNT after CO and NO adsorptions are different. The NO adsorption makes the valence band of M-V2-CNT pass through the Fermi level, while the CO adsorption nearly does not change the band gap of M-V2-CNT. Thus, M-V2-CNT has gas sensitive selectivity to NO and can be used to detect NO gas rather than detect CO gas. This may be due to the fact that NO with (1σ)2(2σ)2(1π)4(3σ)2(2π)1 valence electron configuration contains a higher singlet-occupied 2p orbital, which would lead to easy reaction with M-V2-CNT.
Partial density of states (PDOS) of Pd, Pt, N, O, C(6) and C(7) atoms in the NO-M-V2-CNT system is presented to explore the NO adsorption mechanism on M-V2-CNT, see Fig. 3. Obvious orbital overlaps occur between loaded Pd4d or Pt5d and N2p and O2p in NO near the Fermi energy level, which indicates that NO and M-V2-CNT have a strong interaction, consistent with the binding energy results. Pd4d, Pt5d, N2p, O2p, C(6)2p and C(7)2p all pass through the Fermi level after the NO adsorption and give rise to the band gap change of M-V2-CNT. It can be seen from Fig. 3 that the peaks of Pd4d or Pt5d, N2p and O2p at the Fermi energy level are significantly higher than those of C(6)2p and C(7)2p. Therefore, the loaded M and adsorbed NO mainly contribute to the band gap variation of NO-M-V2-CNT system.
In conclusion, CO and NO molecules are chemically adsorbed on the M-V2-CNT. The presence of a di-vacancy defect significantly reduces the interaction between CO or NO gas and the M-CNT, beneficial to the CO and NO desorption. The NO adsorption converts M-V2-CNT from a semiconductor to a conductor, creating an obviously electrical signal to detect the NO gas.
The presence of a di-vacancy defect enhances the Pd or Pt loading on the (5, 5) tube, and CO and NO gas molecules can be chemically adsorbed on the Pd or Pt loaded perfect (5, 5) tube or Pd or Pt loaded di-vacancy (5, 5) tube. The Pd or Pt loaded di-vacancy (5, 5) tube has a gas-sensitive selectivity to CO and NO. The adsorption of NO makes it convert from a semiconductor to a conductor, while the adsorption of CO does not work. In addition, the presence of a di-vacancy defect significantly weakens the interaction between the CO or NO gas molecule and the Pt loaded di-vacancy (5, 5) tube, urging the CO and NO gas desorption. In brief, the Pd or Pt loaded di-vacancy (5, 5) tube can be a potential gas sensor material to detect the NO gas. The work would provide a theoretical basis for the design of the NO gas sensor.
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Figure 1 Geometrical structures of a perfect (5, 5) tube (a), the (5, 5) tube with one di-vacancy (b) and the CO or NO adsorption on the M and di-vacancy co-decorated (5, 5) tube (c). Green, pink, red and yellow spheres are M, C(CO)/N(NO), O and C at the defect area, respectively
Figure 2 Energy band structures of CO/NO-Pd-CNT (a), CO/NO-Pt-CNT (b), CO/NO-Pd-V2-CNT (c) and CO/NO-Pt-V2-CNT (d)
Figure 3 PDOS of Pd, Pt, N, O, C(6) and C(7) in the NO-M-V2-CNT system. Fermi energy level is set to 0
Table 1. Geometrical Structure Parameters (D), Binding Energies (Eb(2)) and Charge Transfer Amount (Q) for Adsorption Systems of CO and NO on M-CNT and M-V2-CNT
| DC(tube)–M (Å) | DM–C/N (Å) | DC/N–O (Å) | Eb (eV) | Q (e) | ||
| CO-Pd-CNT | 2.155 | 2.407 | 1.896 | 1.159 | 1.87 | 0.017 |
| CO-Pd-V2-CNT | 2.140 | 2.308 | 1.902 | 1.160 | 1.79 | 0.016 |
| CO-Pt-CNT | 2.172 | 2.310 | 1.845 | 1.165 | 2.75 | 0.038 |
| CO-Pt-V2-CNT | 2.138 | 2.247 | 1.856 | 1.165 | 2.02 | 0.037 |
| NO-Pd-CNT | 2.196 | 2.214 | 1.878 | 1.188 | 1.62 | 0.057 |
| NO-Pd-V2-CNT | 2.156 | 2.248 | 1.839 | 1.185 | 1.79 | 0.058 |
| NO-Pt-CNT | 2.167 | 2.170 | 1.813 | 1.189 | 2.44 | 0.081 |
| NO-Pt-V2-CNT | 2.071 | 2.298 | 1.798 | 1.186 | 2.08 | 0.091 |
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