Warped C80H30 nanographene as a chemical sensor for CO gas: DFT studies

Accepted Manuscript Warped C80 H30 nanographene as a chemical sensor for CO gas: DFT studies

Saeed Jameh-Bozorghi, Hamed Soleymanabadi

PII: DOI: Reference:

S0375-9601(16)31439-6 http://dx.doi.org/10.1016/j.physleta.2016.11.039 PLA 24211

To appear in:

Physics Letters A

Received date: Revised date: Accepted date:

21 October 2016 23 November 2016 29 November 2016

Please cite this article in press as: S. Jameh-Bozorghi, H. Soleymanabadi, Warped C80 H30 nanographene as a chemical sensor for CO gas: DFT studies, Phys. Lett. A (2017), http://dx.doi.org/10.1016/j.physleta.2016.11.039

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Highlights • • • •

Unlike graphene, warped nanographene (NG) may be chemical sensor for CO gas CO adsorption energy on NG is about -10.92 kJ/mol By increasing the CO concentration the conductivity of NG is increased. A short recovery time of about 83 ps is predicted for CO desorption form NG sensor.

Warped C80H30 nanographene as a chemical sensor for CO gas: DFT studies

Saeed Jameh-Bozorghi, Hamed Soleymanabadi* Department of Chemistry, Faculty of Science, Hamedan branch, Islamic Azad University, Hamedan, Iran

*Corresponding author; Email: [email protected]

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Abstract

In 2013, synthesis of a grossly warped nanographene (C80H30, NG) was reported in Nature Chemistry which opens a new avenue in carbonaceous nanomaterial research. Here we investigated the chemical reactivity and electronic sensitivity of this NG to carbon monoxide (CO) gas, using density functional theory calculations. It was found that the CO molecule prefers to be added to a C-C bond at the center of NG similar to the [2+2] addition. The calculated adsorption energy is about -10.92 kJ/mol which is accompanied by a natural bond orbitals (NBO) charge transfer of 0.21 e form the NG to the CO molecule. Unlike the graphene, the electronic properties of NG are significantly affected by the CO adsorption. After the CO adsorption, the electrical conductivity of the NG is considerably increased which can be converted to an electrical signal. Thus, it is concluded that the NG may be a potential compound for the CO chemical sensors which the pristine graphene is not appropriate. We demonstrated that by increasing the concentration of the CO molecules the electrical conductivity is more increased. Also, a short recovery time of about 83 ps is predicted for NG sensor.

Keywords: Sensor, Nanographene, DFT, CO gas

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1. Introduction By the discovery of carbon nanotubes in 1991 [1] and graphene in 2004 [2], an enormous interest in the carbonaceous nanomaterial was awakened [3-8]. In addition to graphene, many researches have been focused on the nanographene, a zero-dimensional discrete fragment of graphene which its angling bonds are saturated with hydrogen [9-11]. The properties of nanographenes are different from the graphene because of the size confinement. For example, their band gap is larger than that of the graphene which makes them more suitable for different electronic applications [10]. Very recently synthesis of a new nanographene (C80H30) has been reported by Kawasumi et al. in Nature Chemistry [12] which opens a new avenue in carbonaceous nanomaterial research. This nanographene includes non-hexagonal rings (Fig. 1) which grossly warps its structure. This structure serves as a model for defects in graphene which has distinct electronic and optical properties from other all-carbon families and may find potential applications in optoelectronic devices [12]. As a unique asymmetric structure, different properties and potential applications of this warped nanographene and its similar structures have been inspected by many groups [13-15]. Toxic gas detection is of great importance and much efforts have been focused to develop a high sensitive, portable, fast response, and low cost sensor [16-30]. Nanostructures have attracted extensive attention due to their sensitive electronic properties and high surface/volume ratio [3135]. Graphene and other carbonaceous nanomaterials have been considerably explored as gas sensors [32-36]. The usual gas mechanism of gas sensing is based on the charge transfer between the gas molecule and sensor which may change the electrical resistance of the sensor [36-38]. Besides the expensive experimental methods, many theoretical approaches have been employed to investigate the gas adoption and detection processes [39-40]. Carbon monoxide (CO) is a highly 3

toxic gas which human is dealing with it. Its detection is a challenging problem because of its weak interaction with the sensor devices [41]. It has been shown that pristine graphene and carbon nanotubes cannot detect the presence of CO gas [42, 43]. Sometimes, engineering the structure of nanomaterials by doing, functionalization, creating defects, etc helps to overcome this problem [35-54]. But the manipulation of the structure is an expensive task and finding a pristine sensor is of great importance. Here we investigate the adsorption behavior and electronic sensitivity of the newly synthesized warped C80H30 nanographene (NG) to CO gas by means of density functional theory (DFT) calculations. 2. Computational methods Energy calculations, geometry optimizations, molecular electrostatic potential (MEP) [55], natural bond orbitals (NBO) [56] and density of states (DOS) [57] analyses were performed on a NG and different CO/NG complexes using the B3LYP functional [58, 59] augmented with an empirical dispersion term [60] (B3LYP-D) with 6-31G (d) basis set. The B3LYP has been revealed to be a reliable and commonly employed density functional in the study of different nanomaterials [61-70]. The GAMESS suite of program was employed to perform all the calculations [71]. GaussSum program was used to get the DOS plots [72]. We defined the adsorption energy as: Ead = E[(CO)n/NG] – E(NG) - nE(CO) + EBSSE

(1)

where E[(CO)n/NG] corresponds to the energy of the complex in which n CO molecules are adsorbed on the NG, E(NG) is the energy of the isolated NG, E(CO) is the energy of a single CO molecule, and EBSSE is the energy of the basis set superposition error. The corrections for BSSE are calculated to be about 2.1-2.8 kJ/mol. We used the counterpoise method of Boys and Bernardi to calculate the EBSSE [73]. Regarding previous recommendations [74] B3LYP/ 6-31G (d) was employed to predicted the nucleus independent chemical shift (NICS) values based on the gauge 4

independent atomic orbital (GIAO) approach [75]. The NICS is a descriptor of local aromaticity introduced by Schleyer et al. [76]. It is computed as a negative value of the absolute shielding measured in the center of a definite ring, NICS(0). 3. Results and discussion 3.1. The NG specifications As it was shown in Fig .2, the studied NG contains 80 carbon atoms in a network of 26 rings, with 30 hydrogen atoms decorating the rim. This sheet of carbon is greatly distorted from planarity due to existence one pentagon in the center, and 5 heptagons embedded in the hexagonal lattices of carbon atoms. The C-C bonds of pentagon are about 1.393 to 1.413 Å, in agreement with the experimental X-ray result of 1.410 Å [14]. The b bonds (Fig. 1) which is shared between two hexagonal rings are in the range of 1.362-1373 Å. The reported experimental value for these bonds is about 1.356 Å [14]. Bonds c and d (Fig. 1) are about 1.452-1.473 and 1.422-1465 Å and the experimental values are 1.480 and 1.444 Å, respectively. The NICS analysis indicates that the pentagonal ring is anti-aromatic with the positive NICS (0) value of about +5.7 ppm and six hexagons around the pentagon are aromatic with the negative NICS (0) values of about -3.88 to 4.02 ppm. The aromaticity of ten rim hexagons is much more than the internal ones with NICS (0) values in the range of -8.63 to -9.13 ppm, because of the structural distortion of the internal hexagons. Also, heptagons have more anti-aromatic character than the pentagon with NICS (0) values in the range of +7.93 to +8.13 ppm. The HOMO and LUMO of NG lie at -5.11 and -2.09 eV which produce an HOMO-LUMO energy gap (Eg) of about 3.02 eV. This Eg is proportional to the Ȝmax of about 491 nm which has been observed in the experimental UV-Vis spectrum [14]. Based on the well-known equation E =

5

hc/Ȝ, the experimental Ȝmax of 491 nm corresponds to 2.53 eV (optical gap) which is somewhat smaller than our calculated Eg (3.02 eV). However, it is well-known that the theoretical Eg is smaller than the optical gap because of the electron-hole exciton creation upon the electron excitation

(in

the

experimental

UV-Vis

spectrum)

[77].

An exciton is

electrically

neutral quasiparticle composed from an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force [77]. The HOMO level of NG is more located on the peripheral rings of pentagon and the LUMO is mainly on the pentagon and c bonds (Fig. 2). The MEP plot indicates that the pentagonal ring and the rim carbon atoms have more negative electrostatic potential compared to the other ones. This trend is in consistent whit the MEP plot of corannulene molecule to which it has been experimentally indicated that the charge is accumulated at the apex site [78]. 3.2. CO adsorption on the NG To find the most stable CO/NG complex, we put the CO molecule from its O of C head on different sites of the NG including center of pentagon, heptagons and hexagons, bridge sites of CC bonds and C atoms. After the relax optimization process it is found that the CO molecule tends to be adsorbed on the b bond so that it C head is on a carbon atom of the pentagonal ring as shown in Fig. 3 (complex 1-CO). The b bonds are the shortest C-C bonds with the highest double character. Thus, the interaction is a semi [2+2] addition which makes a tetragonal ring between CO and C-C bonds in the complex 1-CO. The length of newly formed C-C bond is about 1.606 Å and the C-O bond is increased from 1.318 Å in free CO molecule to 1.382 Å in the complex (Fig. 3). The interacting b bond enlarged by about 0.189 Å. An NBO charge about 0.21 e is transferred from the NG to the CO molecule. The calculated adsorption energy is about -10.92 kJ/mol.

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As it was shown in Table 1, the electronic properties of NG are intensely affected by the adsorption of CO molecule. Especially, the LUMO level is shifted to lower energies and the Eg of the NG is decreased. Here we aim to inspect the electronic sensitivity of the NG to the CO gas. For this purpose we employed a prevalent and simple method which relates the Eg to the population of conduction electrons of a semiconductor as follows [79]: N = A T3/2 exp(-Eg/2kT)

(2)

where k is the Boltzmann's constant and A (electrons/m3K3/2) is a constant. This equation shows the electrical conductivity of the NG will exponentially change by a change in the Eg because of conduction electron population increase. This change can be converted to an electrical signal which help the detection of CO gas presence. This method has been repeatedly employed to investigate the sensitivity of different nanostructures to a definite chemical and sometimes its results have shown a good consistency with the experiment [79-82]. In the complex 1-CO, the LUMO level is significantly shifted from -2.09 eV (in the NG) to -2.43 eV (Table 1). This change can be explained by the fact that the CO molecule chiefly interacts with the LUMO level of the NG via its lone pairs. In Fig. 4, the LUMO profile of the complex 1-CO displays that the shape of the LUMO is meaningfully altered by shifting from the surface of NG on the CO molecule. This is in agreement with the large change in the energy of LUMO. Comparing the DOSs of the bare NG and complex 1-CO in a plot in Fig. 4 demonstrates that after the CO adsorption a new state is appeared at -2.43 eV through the Eg. As this state is the LUMO, and the CO molecule mainly contributes in its creation. However, the LUMO stabilization significantly reduces the Eg of the sheet by about 18.5%. The Eg is reduced from 3.02 eV in the bare NG to 2.43 eV in the complex 1-CO. It can be concluded that the NG becomes more

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semiconductor the presence of the CO molecule. Therefore, CO gas can be detected by the NG, producing an electrical signal. Using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, Wang et al.[42] have shown that the CO molecule prefers to be adsorbed on a carbon atom of graphene from its C head, releasing an energy of about -0.01 eV (~ -0.96 kJ/mol). They showed that the electronic properties of graphene is not sensitive to the CO gas. Wanno and Tabtimsai [43] predicted a stronger interaction between the CO and graphene by using B3LYP functional. They found that the adsorption energy is about -1.28 kcal/mol (~ -5.35 kJ/mol), and the CO molecule does not affect the electronic sensitivity. These findings indicate that the sensitivity and reactivity of NG are much more than those of the pristine graphene sheet. The recovery of a sensor is an essential issue and experimentally it is achieved by exposure to UV light or by heating the sensor to higher temperatures [83]. The recovery time of NG nanographene for CO gas can be calculated from the following equation of transition theory: IJ = ȣ-1 exp(- Ead/kT)

(3)

where T is temperature, k is the Boltzmann’s constant ( ~ 8.318 * 10-3 kJ/mol.K), and ȣ the attempt frequency. The attempt frequency of 1012 s-1 has been previously employed to the recovery of carbon nanotubes from NO2 molecules at 298 K [84]. With this attempt frequency a recovery time 83 ps can be predicted for NG, indicating that the NG sensor benefits from a short recovery time. As a comparison, a recovery time of about 9 ms has been observed for NO2 desorption from the surface of N-doped carbon nanotubes [85]. 3.3. Effect of concentration of CO gas

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The effect of concentration of the CO molecules (or gas pressure) is investigated on the adsorption properties and electronic sensitivity of the NG sheet. To this aim, the adsorption of 2 to 5 CO molecules on the b bonds are explored. For the second CO molecule, it is located far from the first adsorbed CO in order to reduce the steric effect between CO molecules. Fig. 5 indicates the complex in which 2 CO molecules are adsorbed on the NG (complex 2-CO). Calculated adsorption energy per CO molecule for the complex 2-CO is about -10.88 kJ/mol which is approximately equal to that of the complex 1-CO. Overall, the adsorption energy per CO molecule is equal for the all adsorption states (Table 1). The NBO charge transfer per CO molecule (0.22 e) is also similar to that in the complex 1-CO. In the 2-CO complex, the LUMO level is considerably shifted from -2.09 eV (in the NG) to -2.47 eV (Table 1). The change of Eg is about 20.6% which is slightly higher than the case 1-CO complex. For adsorption of three CO molecules, it is inevitable that at least two of them be near each other. Thus, our calculations indicate that the most stable structure is that in which one of the CO molecules is attached from its O head to the carbon atom of pentagon as shown in Fig. 5. This is due to the favorable interaction of O head of the CO with the C head of the nearby CO molecule. The adsorption energy per CO molecule for configuration 3-CO is about -10.91 kJ/mol and the change of the Eg is slightly larger than that in the 1-CO and 2-CO complexes. Also, the most stable complexes 4-CO and 5-CO are shown in Fig. 5. Table 1 indicates that compared to the adsorption energy change which is negligible by increasing the number of CO molecules, the electronic properties are more affected. Overall, the LUMO level is more affected by increasing the CO molecules and the Eg is more narrowed. The LUMO profile of 5-CO complex in Fig. 6 reveals that similar to the case of 1-CO complex the LUMO is shifted on the CO molecules. This finding indicates that the electronic 9

properties of the NG molecule are affected by the concentration or pressure of the CO gas. It can be concluded that existing non-hexagonal rings in the structure of the graphene sheet increases the sensitivity of the sheet toward the CO gas. As a comparison, Jiang et al. [86] have performed the DFT calculations to explore the interactions between CO molecule and the pristine and defected graphene sheets. They revealed that existing the defect in the structure of the graphene layers significantly increases the electronic sensitivity and reactivity of the graphene toward CO gas. Their results are in consistence with our findings. 3.4. UV-Vis spectrum The time dependent DFT calculations are performed to obtain the UV-Vis spectrums for the bare NG and its complexes with different numbers of CO. the UV-Vis plots are shown in Figs. 1S and 2S (supplementary materials) and the results are collected in Table 1. The calculated Ȝmax of bare NG, 1-CO, 2-CO, 3-CO, 4-CO, and 5-CO is appeared at 482.97, 583.99, 608.96, 635.18, 660.38, and 682.12 nm, respectively. The corresponding Eopt values are about 2.57, 2.12, 2.04, 1.95, 1.88, and 1.82 eV, respectively (Table 1) which are slightly smaller than the corresponding Eg values. The calculated Ȝmax for pristine NG is in good agreement with the experimental value of 491 nm [14]. The results indicates that by the adsorption of first CO molecule the Ȝmax of NG shows a large redshift which considerably reduces the Eopt of the NG. This is similar to the reduction of the Eg and can produce an optical signal which will help to the detection of CO molecule. By increasing the number of the adsorbed CO molecules, the Ȝmax shows a larger redshift and, thus the Eopt is more decreased which is in consistence with the Eg reduction. Compounds with different Ȝmax values have different colors and this matter can be used in reorganization of the concentration of the adsorbed CO molecules. It can be concluded that both Eg and Eopt of the NG are significantly 10

sensitive to the adsorption of CO molecules which makes the NG a promising candidate for CO detection. 4. Conclusion The adsorption of one to five CO molecules on the NG is investigated by means of DFT calculations. We found that the CO is adsorbed on the b bonds at the center of NG, releasing an energy of about 10.92 kJ/mol which is higher than the CO adsorption on the pristine graphene. The CO adsorption significantly stabilized the LUMO level of NG, thereby reducing the Eg which increases the electrical conductivity. It shows that the NG may be used as a chemical senor for CO molecules which cannot be detected by pristine graphene. Also, the electrical conductivity of NG is sensitive to the concentration of the CO gas. The results also indicates that by the adsorption of first CO molecule the Ȝmax of NG shows a large redshift which considerably reduces the Eopt of the NG. Finally, a short recovery time of about 83 ps is predicted for CO desorption from the surface of NG.

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Figure captions

Fig. 1. (a) A schematic view of the warped nanographe which is reprinted by permission from Macmillan Publishers Ltd: [Nature Chemistry] ref. 14, copyright (2013), (b) top view of the optimized nanogaphene, and (c) its side view. Fig. 2. (a) HOMO level, (b) LUMO level, and (c) molecular electrostatic potential of warped C80H30 nanographene. Color ranges in a.u.: blue, more positive than 0.010; green, between 0.010 and 0; yellow, between 0 and -0.010; red, more negative than -0.010. Fig. 3. The complex of 1-CO (CO/NG) in which a CO molecule is adsorbed at the center of the warped nanographene. Distances are in Å. Fig. 4. The DOS plots of the bare NG and the complex 1-CO. The LUMO level is that of 1-CO complex. Fig. 5. The optimized structures of 2-5 CO molecules are adsorbed on the warped nanographene. Fig. 6. The LUMO profile of 5-CO complex.

22

Table 1. Calculated adsorption energy per CO molecule (Ead, kJ/mol), HOMO, LUMO energies, HOMO-LUMO energy gap (Eg) in eV for NG and its complexes with CO molecules. ¨Eg indicates the change of Eg of NG after the adsorption of CO molecules. Q is the amount of the NBO charge which is transferred from NG to a CO molecule. The Ȝmax is in nanometer and optical gap (Eopt) in eV. Structure

Ead

Ȝmax

Eopt

EHOMO

ELUMO

Eg

%¨Eg

Q(e)

NG

-

482.97

2.57

-5.11

-2.09

3.02

-

0.21

1-CO

-10.92

583.99

2.12

-4.89

-2.43

2.46

-18.5

0.22

2-CO

-10.88

608.96

2.04

-4.87

-2.47

2.40

-20.5

0.22

3-CO

-10.91

635.18

1.95

-4.80

-2.43

2.37

-21.5

0.23

4-CO

-10.96

660.38

1.88

-4.80

-2.50

2.30

-24.2

0.23

5-CO

-10.98

682.12

1.82

-4.81

-2.61

2.20

-29.1

0.23

23

Fig. 1.

24

Fig. 2.

25

Fig. 3.

26

Fig. 4.

27

Fig. 5.

28

Fig. 6.

29