Theoretical Calculation of the Gas-Sensing Properties of Pt-Decorated Carbon Nanotubes

Abstract

The gas-sensing properties of Pt-decorated carbon nanotubes (CNTs), which provide a foundation for the fabrication of sensors, have been evaluated. In this study, we calculated the gas adsorption of Pt-decorated (8,0) single-wall CNTs (Pt-SWCNTs) with SO₂, H₂S, and CO using GGA/PW91 method based on density functional theory. The adsorption energies and the changes in geometric and electronic structures after absorption were comprehensively analyzed to estimate the responses of Pt-SWCNTs. Results indicated that Pt-SWCNTs can respond to the three gases. The electrical characteristics of Pt-SWCNTs show different changes after adsorption. Pt-SWCNTs donate electrons and increase the number of hole carriers after adsorbing SO₂, thereby enhancing its conductivity. When H₂S is adsorbed on CNTs, electrons are transferred from H₂S to Pt-SWCNTs, converting Pt-SWCNTs from p-type to n-type sensors with improved conductivity. However, Pt-SWCNTs obtain electrons and show decreased conductivity when reacted with CO gas.

1. Introduction

Carbon nanotubes (CNTs) have structures with abundant pores, large surface-to-volume ratios, and strong adsorption and desorption capabilities for gases. Gas molecules that adsorb on the surface of CNTs change the shape of CNTs and trigger redistribution of electrons, leading to a macroscopic change in resistance [1]. Kong et al. [2] used chemical vapor deposition (CVD) to fabricate single-wall CNTs (SWCNTs) on SiO₂/Si substrates to detect NO₂ (2 ppm to 200 ppm) and NH₃ (0.1% to 1%) diluted in air or Ar. Results showed that the conductivity of SWCNTs decreases threefold after adsorbing NH₃, whereas the conductivity increases threefold after adsorbing NO₂. Unlike traditional gas sensors, CNT gas sensors exhibit faster response, higher sensitivity, smaller size, and lower working temperatures [3,4]. These advantages make CNTs suitable for application in industries, the medical field, and environmental protection. Recently, the CNT gas sensor has received extensive research attention and achieved certain interesting results. It is significant to study the gas sensitive properties of CNTs, which is the foundation of sensor design. In this paper, the gas sensitive property of CNTs is studied mainly through theoretical calculation analysis. Intrinsic CNTs can only detect several strong oxidizing and reducing gases, while other gases are only weakly adsorbed with low sensitivity because of the structure and chemical properties of intrinsic CNTs [5–7]. To overcome this limitation, some researchers have proposed various physical and chemical modifications, such as introduction of new active sites on the surface of CNTs. The authors in [8] showed that polar groups (COOH, NH₂, NO₂ and H₂PO₃) are promising candidates for enhancing CO₂ and CH₄ adsorption capacity by strengthening adsorption and activating exposed edges and terraces to introduce additional binding sites. Peng et al. [9] found B-doped CNT gas sensors have a good sensitivity to CO and H₂O. Besides, transition metals are rich in d-electrons and empty orbitals, wherein small gas molecules can bond strongly to the metal when adsorbed on the surface [10,11]. Studies [12] indicated that compared with intrinsic CNTs, CNTs with metal depositions have better sensitivity. For instance, Pt- and Au-functionalized CNTs are more sensitive by an order of magnitude for NO₂ and NH₃ detection than intrinsic CNTs. The response characteristics of sensors largely depend on the number of active sites, which can strengthen the response of sensors [11]. Pt can adsorb small molecules [13–16], and Pt has a good catalytic activity. CNTs have excellent physical and chemical properties and unique structures that can be used as a supporting material that influences the activity of Pt catalysts [17]. Conversely, the catalytic activity of Pt can improve the gas sensing properties of CNTs. Therefore the present study introduces Pt as a new active site. The adsorptive processes and properties of Pt-SWCNTs for SO₂, H₂S, and CO are calculated. These gases are highly toxic to the human body, making this research significant by providing theoretical support and a good foundation for the fabrication of suitable CNT-based sensors.

2. Computation Model and Methods

We built (8,0) SWCNTs and gas molecule models using Materials Studio (Accelrys, San Diego, CA, USA), a molecular dynamics simulation software. The geometries and properties of the system were derived using the quantum mechanics program DMol³ code (Accelrys). We adopted GGA to treat the electronic exchange and correlation effects, as described by PW91 [18]. Pt is a heavy metal with an atomic number of 78; therefore, DFT semi-core pseudopotentials [19] were used to manage the interactions between the nucleus and the valence electron. To ensure accuracy, the energy threshold and self-consistent field convergence criteria were set to 2.72 × 10⁻⁴ and 2.72 × 10⁻⁵ eV, respectively. The space orbital cutoff radius was set to 0.40 nm, whereas the Brillouin zone k-point sampling was performed in a 1 × 1 × 2 [20,21] Monkhorst–Pack mesh. A 2.50 nm × 2.50 nm × 0.85 nm periodic boundary was adopted to avoid the interaction between adjacent cells.

Two SWCNT unit cells were selected as intrinsic CNTs to build Pt-SWCNTs. According to references [11,22], Pt can easily adsorb on the vacancy defects of SWCNTs, and its adsorption energy is 6.400 eV, which is greater than that of Pt adsorbed on perfect crystal surface (2.750 eV). Accordingly, the present research selected this Pt-SWCNT model with geometry optimization structure, as shown in Figure 1.

3. Results and Discussion

The radius of a Pt atom is 0.183 nm, which is greater than that of a C atom (0.070 nm). Thus, Pt is highlighted on the CNT surface. The bond lengths between the Pt atom and three adjacent C atoms changed from 0.142 nm to 0.199, 0.199, and 0.189 nm for Pt-C1, Pt-C2, and Pt-C3, respectively. These results are consistent with the reference [22].

The quantum chemical energies of Pt-SWCNTs and gas molecules, as well as the optimized structure of the adsorption systems (E_Pt-SWCNTs, E_gas, and E_{gas-Pt-SWCNTs}) were calculated. The adsorption energy (E_b) between a gas molecule and CNTs can be calculated by the following formula:

$$E_b=E_{\mathit{\text{gas}}-Pt-\mathit{\text{SWCNTs}}}-E_{Pt-\mathit{\text{SWCNTs}}}-E_{\mathit{\text{gas}}}$$

(1)

At E_b < 0, the energy of the absorption system is less than the total energy of gas molecules and Pt-SWCNTs. Therefore, the reaction is exothermic and spontaneous. Greater adsorption energy releases more energy during the reaction process. However, when E_b > 0 it is relatively difficult for the reaction to continue because of the energy required.

In actual practice, the gas-sensitive response of the sensor is evaluated by the changes in electrical characteristics (e.g., resistance) of sensors. Therefore, we also calculated and analyzed the electronic structure of Pt-SWCNTs, gas molecules, and adsorption system. E_HOMO and E_LUMO represent the highest occupied molecular orbital (HOMO) energy and the lowest unoccupied orbital (LUMO) energy, respectively. Eg is the difference of E_LUMO and E_HOMO, and Q is the net charge of the system. The parameters are defined as follows:

$$E_{L-H}=|E_{{\mathit{\text{LUMO}}}^{(Pt-\mathit{\text{SWCNTs}})}}-E_{{\mathit{\text{HOMO}}}^{(\mathit{\text{gas}})}}|$$

(2)

$$E_{H-L}=|E_{{\mathit{\text{LUMO}}}^{(\mathit{\text{gas}})}}-E_{{\mathit{\text{HOMO}}}^{(Pt-\mathit{\text{SWCNTs}})}}|$$

(3)

$$\mathrm{\Delta }{Q}_{\mathit{\text{SWCNTs}}}=Q_{{\mathit{\text{SWCNTs}}}^{(\mathit{\text{gas}}-Pt-\mathit{\text{SWCNTs}})}}-Q_{{\mathit{\text{SWCNTs}}}^{(Pt-\mathit{\text{SWCNTs}})}}$$

(4)

$$\mathrm{\Delta }{Q}_{Pt}=Q_{P{t}^{(\mathit{\text{gas}}-Pt-\mathit{\text{SWCNTs}})}}-Q_{P{t}^{(Pt-\mathit{\text{SWCNTs}})}}$$

(5)

3.1. SO₂

SO₂ is colorless, corrosive, and has a strong pungent odor. Moreover, when SO₂ is dissolved in bodies of water, sulfurous acid rain is generated, which is harmful to the environment. SO₂ can also form sulfuric acid when dissolved in water, which can irritate the mucous membrane of the eyes and nose.

The full geometric optimization of the Pt-SWCNTs and SO₂ adsorption model is shown in Figure 2. An oxygen atom O1 points to Pt, with Pt-O1 and Pt-S distances of 0.212 and 0.245 nm, respectively. The reaction adsorption energy is –1.225 eV (

Table 1. Adsorption energy and structural parameters of Pt-SWCNTs adsorption.

	SO2	H2S	CO
dPt-C1/nm	0.205	0.200	0.205
dPt-C2/nm	0.201	0.202	0.202
dPt-C3/nm	0.196	0.192	0.194
Eb/eV	–1.225	–0.977	–1.386

), which denotes an exothermic and spontaneous reaction. By contrast, the reaction adsorption energy of intrinsic SWCNTs is –0.830 eV, so Pt-doping enhances the interaction between SO₂ and SWCNTs. Pt is not only a sensing element of Pt-SWCNTs, but also an active site. Strong interaction with gas molecules adsorbed on the surface results in deformation of Pt-SWCNTs and elongation of the Pt-C bond.

The frontier orbital energy difference of SO₂ and Pt-SWCNTs is E_H-L ≪ E_L-H. A Pt-SWCNT electron only needs to overcome a 0.158 eV energy barrier to transfer to SO₂, whereas a SO₂ electron needs to overcome a 3.818 eV energy barrier to transfer to Pt-SWCNTs. Therefore, Pt-SWCNTs provide electrons to SO₂ in the adsorption process. A portion of electrons fill the anti-bonding orbital of S-O1, changing the bond length from 0.143 nm to 0.165 nm. O2 is far from the CNT surface, so the interaction is small, allowing only a small change in the bond length of S-O2 (0.150 nm).

According to the respective Mulliken charge populations, SWCNTs of Pt-SWCNTs have 0.147 positive charge and Pt has 0.147 negative charge before adsorption. After the adsorption process, SWCNTs have 0.509 positive charge, whereas Pt has 0.116 negative charge. SO₂ obtains 0.393 electrons during the adsorption reaction with Pt-SWCNTs, which is 4.6 times than intrinsic SWCNTs (

Table 2. Adsorption energy and charge transform of intrinsic SWCNTs adsorption.

	Eb/eV	Qgas/e
SO2-SWCNTs	–0.830	–0.067
H2S-SWCNTs	–0.591	0.013
CO-SWCNTs	–0.157	0.006

). Charge variation (ΔQ_SWCNTs, ΔQ_Pt) of SWCNTs and Pt are 0.362 and 0.031, respectively (

Table 3. Electrical structure parameters of the adsorption structures.

System	QSWCNTs/e	QPt/e	Qgas/e	ΔQSWCNTs/e	ΔQPt/e
Pt-SWCNTs	0.147	–0.147
SO2-Pt-SWCNTs	0.509	–0.116	–0.393	0.362	0.031
H2S-Pt-SWCNTs	–0.019	–0.266	0.285	–0.166	–0.119
CO-Pt-SWCNTs	0.166	–0.347	0.181	0.019	–0.200

). Therefore, SO₂ obtains electrons mainly from SWCNTs, whereas the Pt exhibits a small charge change.

The transfer of a large number of electrons during adsorption causes the redistribution of system charges. The density of states (DOS) near the Fermi level appears to be impure, for example there is a peak in –0.5eV. And the DOS between HOMO and LUMO changes. Figure 3 shows that these impure states are caused by SO₂ adsorption. The p orbitals of S and O atoms have a large overlap with the d orbitals of Pt atom, and it demonstrates that SO₂ can strongly hybridize with Pt [23]. This has significant effect on the frontier orbital of the adsorption system, which changes the HOMO and LUMO orbital formation, causing the change in the DOS. Figure 4a shows that the p orbitals of C1 and C3 form σ bond with S. In Figure 4b, the d orbitals of Pt and the p orbitals of S are hybridized. The frontier orbital energy gap E_g of the system is 0.285 eV after adsorbing SO₂, which is reduced by 0.047 eV compared with that in the non-adsorbed SO₂. This is beneficial for the transfer of electrons between HOMO and LUMO, thereby enhancing conductivity.

SO₂ adsorption on the surface of Pt-SWCNTs has large adsorption energy and can form a stable structure. The p-type Pt-SWCNTs [11] donate electrons and increase the number of hole carriers, reducing the frontier orbital energy, diminishing energy gap E_g, and enhancing conductivity. Pt-SWCNTs are highly responsive to SO₂, the doped Pt effectively improved the adsorption sensitivity of SWCNTs to SO₂.

3.2. H₂S

H₂S which is the simplest hydride of sulfur, is a colorless toxic gas that smells like rotten eggs and is strongly corrosive. It is also harmful to human health. The S in H₂S is at the lowest valence state, so it is strongly reducible.

The adsorption reaction of Pt-SWCNTs and H₂S is also exothermic, with E_b of –0.977 eV, more than intrinsic SWCNTs (–0.591eV in Table 2). The frontier orbital energy differences are E_H-L = 4.438 eV and E_L-H = 1.519 eV, therefore H₂S provides electrons to Pt-SWCNTs in this reaction. Mulliken charge analysis (Table 3) shows that H₂S donates 0.285 electrons almost 22 times more than intrinsic SWCNTs. Pt and SWCNTs have 0.019 and 0.266 electrons, respectively, after H₂S is adsorbed on Pt-SWCNTs (Figure 5). A large number of electron transformations convert Pt-SWCNTs from p-type to n-type. E_g of the adsorption system is 0.283 eV, which is reduced by 0.049 eV compared that with non-adsorbed H₂S, thus enhancing conductivity. H₂S-Pt-SWCNT frontier orbitals concentrate on Pt-SWCNTs, and H₂S is not involved in the composition of HOMO and LUMO orbitals. Figure 6 shows that the DOS of H₂S is not distributed between the HOMO and LUMO, and the DOS near the Fermi level is basically the same as that of Pt-SWCNTs, which is consistent with the results of frontier orbitals (Figure 7). The p orbitals of S have a large overlap with the d orbitals of Pt, and the strong interaction of them enhances the adsorption between H₂S and the nanotube surface.

From the comparison results of adsorption energy and transfer charge, it can be seen that doped Pt obviously improves the adsorption ability of the instrinsic SWCNTs to H₂S. H₂S adsorbs on the surface of Pt-SWCNTs and donates substantial electrons to Pt-SWCNTs, which converts CNTs from p-type to n-type. The frontier orbital energy is increased, while the conductivity is enhanced because of the decrease in the frontier orbital energy gap.

3.3. CO

CO is a colorless, non-irritating gas. However when it enters the human body, CO combines with blood hemoglobin, which prevents the union of hemoglobin and oxygen, leading to body tissue hypoxia and even suffocation. The C atom in CO has +2 valence electrons and that can be further oxidized to +4. Accordingly, CO is a reducing gas that provides electrons in reactions.

The optimized adsorption structure of CO adsorbed on Pt-SWCNTs is shown in Figure 8, which is consistent with reference [24], C atoms point to Pt, whereas O atoms point away from the CNT surface. The adsorption reaction is exothermic. High adsorption energy results in a tight bond between gases and CNTs, as well as an interaction distance of 0.198 nm.

The same as the previous two gases, the adsorption energy and the transfer charge between Pt-SWCNTs and CO are increased obviously, enhancing the adsorption. Figure 9 shows that the p orbitals of C in CO have an overlap with the d orbitals of Pt, especially near the Fermi level. The DOS near the Fermi level is changed, and the peak at –7 eV is split, which is related to CO adsorption. The contributions of CO to HOMO and LUMO are mainly on the p orbitals of C and O atoms (Figure 10), which change the system configuration of frontier orbitals, changing the DOS of the system. During adsorption, CO provides 0.181 electrons (Table 3), p-type CNTs obtain electrons, and the number of hole carriers decreases. Frontier orbital energy and energy gap E_g increase, decreasing conductivity.

4. Discussion

Pt is a heavy metal with an atomic number of 78. Its outer core 5d orbitals have nine electrons and an unpaired d electron, so it easily absorbs electrons to reach a steady state. Pt doped into SWCNTs obtains electrons from C. In Pt-SWCNTs, Pt has 0.147 electrons, C1, C2, and C3 adjacent to Pt have 0.028, 0.029, and 0.097 electrons, respectively, which form an electron accumulation zone around the Pt atom. Given that Pt easily obtains electrons, when SO₂ reacts with Pt-SWCNTs, SWCNTs donate most of the electrons, and a small charge change in Pt (ΔQ_Pt) occurs. On the contrary, when the target gases are H₂S and CO, Pt exhibits a large charge change.

5. Conclusions

In this study, the adsorptions of three gases on the surface of Pt-SWCNTs were calculated based on DFT. The gas-sensing properties of Pt-SWCNTs were assessed according to the changes in adsorption energy, geometric structure, and electronic structure during adsorption. The main conclusions are as follows:

• The doped Pt effectively improves the adsorption sensitivity of intrinsic SWCNTs to the three kind of gases.

• The adsorption energy of the reaction between Pt-SWCNTs and SO₂ is large, and numerous electrons are transferred from CNTs to the target gases. The frontier orbital energies (E_HOMO and E_LUMO) and E_g are decreased, and the electrical conductivity of Pt-SWCNTs is enhanced. Pt-SWCNTs have high sensitivity to SO₂.

• H₂S is reduced when reacted with Pt-SWCNTs. H₂S provides a large number of electrons, converting CNTs from p-type to n-type. The frontier orbital energies are increased, whereas E_g is decreased, thereby enhancing conductivity.

• When CO is adsorbed on Pt-SWCNTs, CO provides electrons to p-type CNTs, decreasing the number of hole carriers. The frontier orbital energies and E_g are increased, decreasing conductivity.

Results of the theoretical calculation show that Pt-SWCNTs can respond to the three gases. The electrical characteristics of Pt-SWCNTs show different degrees of changes after adsorption of the test gases. As SO₂ is adsorbed on Pt-SWCNTs, the CNTs lose electrons, the number of hole carriers is increased, and conductivity is enhanced. As H₂S is adsorbed on the surface of CNTs, Pt-SWCNTs receive a large number of electrons and transform from p-type into n-type. The conductivity of Pt-SWCNTs is also enhanced. Comparing their adsorption energies and charge transformations, the sensitivity to SO₂ is higher than the sensitivity of Pt-SWCNTs to H₂S. Moreover, CO is an electron-donor gas, which reduces hole carriers and weakens conductivity. Therefore, Pt-SWCNTs can be used to fabricate gas sensors in detecting SO₂, H₂S, and CO gases.