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Communication

Ru-Doped Single Walled Carbon Nanotubes as Sensors for SO2 and H2S Detection

by
Navaratnarajah Kuganathan
1,2,* and
Alexander Chroneos
1,2
1
Department of Materials, Imperial College London, London SW7 2AZ, UK
2
Faculty of Engineering, Environment and Computing, Coventry University, Priory Street, Coventry CV1 5FB, UK
*
Author to whom correspondence should be addressed.
Chemosensors 2021, 9(6), 120; https://doi.org/10.3390/chemosensors9060120
Submission received: 30 April 2021 / Revised: 21 May 2021 / Accepted: 23 May 2021 / Published: 24 May 2021
(This article belongs to the Section Materials for Chemical Sensing)
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Abstract

:
Carbon nanotubes are of great interest for their ability to functionalize with atoms for adsorbing toxic gases such as CO, NO, and NO2. Here, we use density functional theory in conjunction with dispersion correction to examine the encapsulation and adsorption efficacy of SO2 and H2S molecules by a (14,0) carbon nanotube and its substitutionally doped form with Ru. Exoergic encapsulation and adsorption energies are calculated for pristine nanotubes. The interaction of molecules with pristine nanotube is non-covalent as confirmed by the negligible charge transfer. The substitutional doping of Ru does not improve the encapsulation significantly. Nevertheless, there is an important enhancement in the adsorption of molecules by Ru-doped (14,0) nanotube. Such strong adsorption is confirmed by the strong chemical interaction between the nanotube and molecules. The promising feature of Ru-doped nanotubes can be tested experimentally for SO2 and H2S gas sensing.

1. Introduction

Single walled nanotubes (SWNTs) have attracted great interest due to their promising mechanical, chemical, and thermal properties and high surface area [1,2,3,4,5]. In recent years, there have been several experimental and theoretical studies showing the potential applications of SWNTs [6,7,8,9,10,11,12]. Applications include the use of SWNTs in energy storage and energy conversion devices [13,14], high-strength composites [15,16,17], nanoprobes and sensors [18,19,20], actuators [21,22], electronic devices [23,24], catalysis [25,26], and hydrogen storage media [27,28].
SWNTs are promising candidate materials to detect harmful gaseous molecules at low concentration [29,30,31]. Such detection is crucial to monitor environmental pollution. Many experimental and theoretical studies show that low concentration of small gaseous molecules such as NO2, NH3, NO, CO2, and CH4 can be trapped by SWNTs [32,33,34,35].
Modification of the surface of SWNTs via metal doping has been shown to be an efficient strategy to improve the adsorption of gaseous molecules [36,37,38,39]. A variety of transition metal doped SWNTs have been modelled theoretically to increase the efficacy of NH3 and NO2 molecules [36,37,38,39,40,41]. Though there are many studies focusing on the capture of nitrogen-containing pollutants by metal-doped nanotubes, a few studies have considered the adsorption of sulfur-containing pollutants. Sulfur-containing pollutants are also important to consider for removal as they can damage agriculture, aquatic life, and building structures. Numerous theoretical simulations have considered the interaction of SO2 and H2S with pristine SWNTs and concluded that the interaction between the nanotube and SO2 and H2S molecules are weak [42,43,44]. Metal doped SWNTs have also been considered theoretically to enhance the interaction of those molecules [45,46]. Zhang et al. [47] used density functional theory (DFT) simulation to study the adsorption of SO2 and H2S molecules on the Au-doped SWNT. Atom functionalized carbon nanotubes have been recently considered by Liao et al. [48] for H2S sensing and splitting.
In this study, we use spin-polarized mode of DFT simulations together with dispersion to study the encapsulation and adsorption of SO2 and H2S gases with pristine and Ru-doped SWNT. Simulations enabled the calculation of the encapsulation/adsorption energies, charge transfer, and the electronic nature of the resultant composites relative to that of pristine SWNT.

2. Computational Methods

The spin-polarized DFT code VASP (Vienna Ab initio simulation package) was used to perform all calculations [49,50]. The generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE) was applied to model the exchange-correlation term [51]. The valence electronic configurations for C, S, O, and H were 2s2 2p2, 3s2 3p4, 2s2 2p4, and 1s1 respectively. A plane-wave basis set with a cut-off of 500 eV and the standard projected augmented wave (PAW) potentials [52] as implemented in the VASP code were used. A 2 × 2 × 1 Monk-horst Pack [53] k-point mesh was used to model pristine SWNT and molecules encapsulated or adsorbed-SWNT structures. For calculations on the molecules encapsulated or adsorbed-SWNT, periodic boundary conditions were applied to enforce a minimum lateral separation of 30 Å between structures in adjacent unit cells. In all cases the dimension of the cell was 30 Å × 30 Å ×17.28 Å. The number of atoms in the simulation box for all configurations are provided in the electronic Supplementary Information (see Table S1).
A conjugate gradient algorithm [54] was used to optimize the structures. The Hellman-Feynman theorem with Pulay corrections was used to obtain the forces on the atoms. Forces on the atoms in all optimized configurations were smaller than 0.04 eV/Å. The van-der Waals interaction was included in the form of semi-empirical pair-wise force field as implemented by Grimme et al. [55]. The Bader charge analysis [56,57] was carried out to calculate the charge transferred between the nanotube and the molecules. Initial magnetic moments for all atoms were set to one (e.g., MAGMOM = 227*1 for SO2@SWNT, which has 227 atoms in total).
The encapsulation energy of SO2 molecule was calculated by considering the difference in the total energy of the SO2@SWNT and the total energies calculated for an isolated SO2 molecule and an isolated SWNT.
Eenc = E(SO2@SWNT) − ESWNT − ESO2
where E(SO2@SWNT) is the total energy of SO2 encapsulated within a SWNT; ESWNT and ESO2 are the total energies of a SWNT and an isolated gas phase SO2 molecule. Similar equations were used to calculate the encapsulation energies of H2S and adsorption energies of SO2 and H2S.

3. Results

3.1. Encapsulation of SO2 and H2S within SWNT

The encapsulation of SO2 and H2S molecules was first considered within SWNT. In all cases we used a (14,0) semiconducting nanotube. Figure 1 shows the relaxed configurations and charge density plots showing the interactions between the nanotube and the molecules. Optimized configurations in different orientations are provided in the electronic Supplementary Information (see Figure S1). Both SO2 and H2S molecules occupy the center of the nanotube. The encapsulation energies calculated for SO2 and H2S are −0.27 eV and −0.20 eV, respectively (see Table 1). This indicates that both molecules are energetically stable inside the nanotube compared to their isolated gaseous forms. The interaction between the molecules and the nanotube is non-covalent. This is further confirmed by the charge density plots (see Figure 1c,d), a very small charge transfer from the nanotube to the molecules and almost zero magnetic moments as calculated for the pristine nanotube. The calculated density of states (DOS) plot shows that (14,0) nanotube is a semiconductor (band gap = 0.4 eV). DOS plots calculated for encapsulated and adsorbed complexes are not significantly affected (see Figure 1c–e).

3.2. Adsorption of SO2 and H2S on the Surface of SWNT

Next, we considered the adsorption of SO2 and H2S molecules on the surface of SWNT. The optimized configurations and charge densities showing the interaction of molecules with the SWNT are shown in Figure 2. Optimized configurations in different orientations are provided in the electronic Supplementary Information (see Figure S2). Adsorption is exoergic for both SO2 and H2S molecules meaning that the SWNT is capable of adsorbing these molecules on the surface (see Table 2). Both relaxed structures and charge density plots show that the interaction with the surface is weak. The adsorption energy calculated for SO2 molecule is slightly more negative than that calculated for H2S molecule. The weak adsorption is further confirmed by the negligible charge transfer and zero magnetic moments. The Fermi levels calculated of the complexes are not altered significantly and retain the semiconducting character of the SWNT (band gap = 0.4 eV).

3.3. Ru-Doped SWNT

Aiming to improve the encapsulation or adsorption efficacy, a single Ru atom was substitutionally doped on the surface of SWNT. The relaxed configuration together with bond distances and Bader charges are shown in Figure 3. The doped Ru atom is in a trigonal pyramid configuration with an outward displacement (see Figure 3a). The Ru-C bond distances are longer than the C-C bond distances (see Figure 3b). The bonding interaction between the Ru atom and the nanotube is further confirmed by the charge density plot (see Figure 3c). The Bader charge on the Ru atom is calculated to be +1.01, meaning that ~one electron has been donated by the Ru atom to the nearest neighbor C atoms (see Figure 3d). This is partly due to the larger electronegativity of C (2.55) than that of the Ru atom (2.20) [58]. The doping of the Ru atom significantly affects the DOS plot with some Ru states appearing near the Fermi level (see Figure 3e), leading to narrow-gap semiconductor (band gap = 0.1 eV). This is further confirmed by the atomic DOS plot of Ru (see Figure 3f).

3.4. Encapsulation of SO2 and H2S within Ru-Doped SWNT

Next, the molecules were allowed to encapsulate with Ru-doped SWNT. The relaxed structures, charge density plots, and DOS plots are shown in Figure 4. Optimized configurations in different orientations are provided in the electronic Supplementary Information (see Figure S3). Table 3 reports the encapsulation energies, charge transferred between the nanotube and the molecules, and the net magnetic moments of the complexes. Encapsulation is exoergic for both SO2 and H2S molecules. There is a slight enhancement in the encapsulation energies. However, they are still non-covalent. This is consistent with the encapsulated structures and charge density plots (see Figure 3). The charge transfer is minimal in both cases though there is a slight increase in the charge transfer for the encapsulation of SO2. The SWNT encapsulated with SO2 exhibits a small magnetic moment of 0.30. The net magnetic moment of the SWNT encapsulated with H2S is zero. The total DOS plot shows that the SWNT encapsulated with SO2 is semi-metallic while the SWNT encapsulated with H2S is a narrow gap semiconductor (see Figure 4c,d).

3.5. Adsorption of SO2 and H2S on the Surface of Ru-Doped SWNT

Finally, we considered the adsorption of molecules on the surface of Ru-doped SWNT. Figure 5 shows the relaxed structure of the SO2 adsorbed on the surface of Ru-doped SWNT together with the charge density plot and the total DOS plot. Optimized configurations in different orientations are provided in the electronic Supplementary Information (see Figure S4). The SO2 molecule is chemically bonded via one of its oxygen atoms with the Ru atom, forming a Ru-O chemical bond (see Figure 5a,b). The Ru-O bond distance is calculated to be 2.171 Å. Adsorption is significantly enhanced upon doping. The calculated adsorption energy is −1.08 eV stronger by 0.85 eV than that calculated for the pure surface of the SWNT (see Table 4). The stronger adsorption is further confirmed by the charge density plot (see Figure 5c) and the significant charge transfer (0.52 e) from SWNT to the SO2 molecule. The total density plot shows that the resultant complex is metallic (refer Figure 5d). The magnetic moment of 0.67 implies that the resultant complex is magnetic.
In the case of H2S molecule, there is a strong chemical interaction between the Ru atom and the S atoms in H2S (see Figure 6). This is also confirmed by the bonding interaction between the Ru atom and the S atoms. The Ru-S bond length is calculated to be 2.463 Å slightly longer than the Ru-O bond observed in the interaction of SO2 with Ru-adsorbed SWNT. The calculated adsorption energy is −1.00 eV meaning that the doping enhanced the adsorption by 0.85 eV. The strong adsorption is evidenced by the charge density plot. The Bader charge analysis shows that a small amount of charge is transferred from the nanotube to the H2S molecule. The calculated DOS plot shows that the resultant complex retains its semiconducting character (band gap = 0.4 eV) though there is a dispersion in the valence and conduction bands by Ru states.

4. Conclusions

Carbon nanotubes provide a high inner and outer surface area to trap SO2 and H2S molecules. The efficacy of adsorption can be improved by modifying the surface of nanotube. Spin-polarized DFT simulations together with dispersion correction were employed to examine the encapsulation and adsorption efficacy of SO2 and H2S molecules by a pristine (14,0) SWNT and Ru-doped SWNT. Both SO2 and H2S were encapsulated and adsorbed exothermically but non-covalently by pristine SWNT, suggesting that molecules are more stable on the surface than their isolated gaseous forms. The doping of the Ru atom improved the encapsulation very slightly. However, strong adsorption is found for both molecules by the Ru-doped SWNT. Such strong adsorption is confirmed by the chemical interaction between the S or O atom on the guest molecule side and the Ru atom on the nanotube side. To conclude, SWNT and its doped form with the Ru atom are shown to encapsulate and adsorb both SO2 and H2S molecules. The promising feature of Ru-doped SWNT for the significant adsorption of gases should be verified experimentally.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/chemosensors9060120/s1, Figure S1: Optimized configurations of SWNT encapsulated by (a) SO2 molecule and (b) H2S molecule, Figure S2: Optimized configurations of SWNT adsorbed by (a) SO2 molecule and (b) H2S molecule, Figure S3: Optimized configurations of Ru-doped SWNT encapsulated by (a) SO2 molecule and (b) H2S molecule, Figure S4: Optimized configurations of Ru-doped SWNT adsorbed by (a) SO2 molecule and (b) H2S molecule, Table S1: Number atoms in a SO2 and H2S molecule encapsulated within SWNST or adsorbed on the SWNT.

Author Contributions

Computation, N.K.; writing, N.K.; analysis and editing, N.K., writing—original draft preparation, N.K.; writing—review and editing, A.C. Both authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Imperial College London is acknowledged for providing computing facilities.

Conflicts of Interest

The author declares no conflict of interest.

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Chemosensors 09 00120 g001 550
Figure 1. (a) Relaxed structure of a single SO2 molecule encapsulated within SWNT, (b) charge density plot showing the interaction of SO2 molecule with nanotube, (c) DOS plot of a pristine SWNT, (d) DOS plot of SO2@SWNT, (e) DOS plot of H2S@SWNT, (f) relaxed structure of a H2S molecule inside the nanotube, and (g) its charge density plot. Vertical dashed lines correspond to the Fermi level.
Figure 1. (a) Relaxed structure of a single SO2 molecule encapsulated within SWNT, (b) charge density plot showing the interaction of SO2 molecule with nanotube, (c) DOS plot of a pristine SWNT, (d) DOS plot of SO2@SWNT, (e) DOS plot of H2S@SWNT, (f) relaxed structure of a H2S molecule inside the nanotube, and (g) its charge density plot. Vertical dashed lines correspond to the Fermi level.
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Figure 2. (a) Relaxed structure of a single SO2 molecule adsorbed on the surface of SWNT, (b) charge density plot showing the interaction of SO2 molecule with the nanotube, (c) DOS plot of SO2-adsorbed nanotube, (d) relaxed structure of H2S molecule adsorbed on the surface of the nanotube, (e) its charge density plot, and (f) DOS plot of H2S-adsorbed nanotube. Vertical dashed lines correspond to the Fermi level.
Figure 2. (a) Relaxed structure of a single SO2 molecule adsorbed on the surface of SWNT, (b) charge density plot showing the interaction of SO2 molecule with the nanotube, (c) DOS plot of SO2-adsorbed nanotube, (d) relaxed structure of H2S molecule adsorbed on the surface of the nanotube, (e) its charge density plot, and (f) DOS plot of H2S-adsorbed nanotube. Vertical dashed lines correspond to the Fermi level.
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Figure 3. (a) Relaxed configuration of the Ru-doped SWNT, (b) calculated bond distances measured in Angström around the defect, (c) charge density plot showing the interaction between the Ru atom and the nanotube, (d) Bader charge on the Ru atom and the nearest neighbor C atoms, and (e) total DOS plot and (f) the atomic DOS plot calculated for the Ru atom. Vertical dashed lines correspond to the Fermi level.
Figure 3. (a) Relaxed configuration of the Ru-doped SWNT, (b) calculated bond distances measured in Angström around the defect, (c) charge density plot showing the interaction between the Ru atom and the nanotube, (d) Bader charge on the Ru atom and the nearest neighbor C atoms, and (e) total DOS plot and (f) the atomic DOS plot calculated for the Ru atom. Vertical dashed lines correspond to the Fermi level.
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Figure 4. (a) Relaxed structure of a single SO2 molecule encapsulated within the Ru-doped SWNT, (b) charge density plot showing the interaction of SO2 molecule with the nanotube, and (c) the total DOS plot of the encapsulated configuration. Similar plots are shown for H2S encapsulated SWNT (d–f). Vertical dashed lines correspond to the Fermi level.
Figure 4. (a) Relaxed structure of a single SO2 molecule encapsulated within the Ru-doped SWNT, (b) charge density plot showing the interaction of SO2 molecule with the nanotube, and (c) the total DOS plot of the encapsulated configuration. Similar plots are shown for H2S encapsulated SWNT (d–f). Vertical dashed lines correspond to the Fermi level.
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Figure 5. (a) Relaxed structure of a single SO2 molecule adsorbed on the surface of Ru-doped SWNT, (b) side view of the relaxed configuration showing the Ru-C and Ru-O bond distances measured in Angström, (c) charge density plot showing the interaction of SO2 molecule with the nanotube, and (d) the total DOS plot of the adsorbed configuration. Vertical dashed lines correspond to the Fermi level.
Figure 5. (a) Relaxed structure of a single SO2 molecule adsorbed on the surface of Ru-doped SWNT, (b) side view of the relaxed configuration showing the Ru-C and Ru-O bond distances measured in Angström, (c) charge density plot showing the interaction of SO2 molecule with the nanotube, and (d) the total DOS plot of the adsorbed configuration. Vertical dashed lines correspond to the Fermi level.
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Figure 6. (a) Relaxed structure of a single H2S molecule adsorbed on the surface of Ru-doped SWNT, (b) side view of the relaxed configuration showing the Ru-C and Ru-S bond distances measured in Angström, (c) charge density plot showing the interaction of H2S molecule with the nanotube, and (d) the total DOS plot of the adsorbed configuration. Vertical dashed lines correspond to the Fermi level.
Figure 6. (a) Relaxed structure of a single H2S molecule adsorbed on the surface of Ru-doped SWNT, (b) side view of the relaxed configuration showing the Ru-C and Ru-S bond distances measured in Angström, (c) charge density plot showing the interaction of H2S molecule with the nanotube, and (d) the total DOS plot of the adsorbed configuration. Vertical dashed lines correspond to the Fermi level.
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Table
Table 1. Calculated encapsulation energies with respect to molecules, the amount of charge transferred between the nanotube and the molecule, and net magnetic moments of the pristine nanotube and the encapsulated complexes.
Table 1. Calculated encapsulation energies with respect to molecules, the amount of charge transferred between the nanotube and the molecule, and net magnetic moments of the pristine nanotube and the encapsulated complexes.
SystemEncapsulation Energy (eV)Charge TransferMagnetic Moment
SWNT------0.00
SO2@SWNT−0.270.110.10
H2S@SWNT−0.200.020.00
Table
Table 2. Calculated adsorption energies with respect to molecules, the amount of charge transferred between the nanotube and the molecule, and net magnetic moments of pristine nanotube and the adsorbed complexes.
Table 2. Calculated adsorption energies with respect to molecules, the amount of charge transferred between the nanotube and the molecule, and net magnetic moments of pristine nanotube and the adsorbed complexes.
SystemAdsorption Energy (eV)Charge TransferMagnetic Moment
SWNT------0.00
SO2_SWNT−0.230.090.00
H2S_SWNT−0.150.010.00
Table
Table 3. Calculated encapsulated energies with respect to molecules, the amount of charge transferred between the nanotube and the molecule and net magnetic moments of pristine nanotube and the adsorbed complexes.
Table 3. Calculated encapsulated energies with respect to molecules, the amount of charge transferred between the nanotube and the molecule and net magnetic moments of pristine nanotube and the adsorbed complexes.
SystemEncapsulation Energy (eV)Charge TransferMagnetic Moment
SWNT------0.00
SO2@Ru.SWNT−0.290.110.30
H2[email protected]−0.230.020.00
Table
Table 4. Calculated adsorption energies with respect to molecules, the amount of charge transferred between the nanotube and the molecule, and net magnetic moments of pristine nanotube and the adsorbed complexes.
Table 4. Calculated adsorption energies with respect to molecules, the amount of charge transferred between the nanotube and the molecule, and net magnetic moments of pristine nanotube and the adsorbed complexes.
SystemAdsorption Energy (eV)Charge TransferMagnetic Moment
SWNT------0.00
SO2_Ru.SWNT−1.080.520.67
H2S_Ru.SWNT−1.000.070.00
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Kuganathan, N.; Chroneos, A. Ru-Doped Single Walled Carbon Nanotubes as Sensors for SO2 and H2S Detection. Chemosensors 2021, 9, 120. https://doi.org/10.3390/chemosensors9060120

AMA Style

Kuganathan N, Chroneos A. Ru-Doped Single Walled Carbon Nanotubes as Sensors for SO2 and H2S Detection. Chemosensors. 2021; 9(6):120. https://doi.org/10.3390/chemosensors9060120

Chicago/Turabian Style

Kuganathan, Navaratnarajah, and Alexander Chroneos. 2021. "Ru-Doped Single Walled Carbon Nanotubes as Sensors for SO2 and H2S Detection" Chemosensors 9, no. 6: 120. https://doi.org/10.3390/chemosensors9060120

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