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Ga-doped phagraphene as a superior media for sensing of carbon monoxide: A detailed theoretical investigation
Physica E 116 (2020) 113710
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Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Ga-doped phagraphene as a superior media for sensing of carbon monoxide: A detailed theoretical investigation Morteza Rouhani *, Sanaz Kord, Zohreh Mirjafary Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: DFT Sensor CO Phagraphene Carbon monoxide Computational
The adsorption of highly toxic carbon monoxide molecule (CO) on pristine, Al, Si, and Ga-doped phagraphene are studied using Density Functional Theory (DFT) calculations at M06-2X level of theory with 6-311Gþþ (d,p) basis set, to explore the potential abilities of phagraphene as an effective CO detection sensor. The full relaxed geometries of CO molecule, pristine and doped phagraphenes and related CO/pristine and doped-phagraphene complexes are specified and the variety parameters such as energy of Highest Occupied Molecular Orbital (EHOMO), energy of Lowest Unoccupied Molecular Orbital (ELUMO), gap energy (Eg), energy of adsorption (Ead), basis set superposition error (BSSE), gap energy alterations percent (%ΔEg), Global Electron Density Transfer (GEDT), energy of Fermi level (EF), work function (Φ), work function alterations percent (%ΔΦ), Density of States (DOS) patterns and Molecular Electrostatic Potential (MEP) profiles were calculated and analyzed. The results demonstrate that the CO molecule has a very poor physical adsorption on the pristine phagraphene. The calculated Ead value of CO/Si-doped phagraphene complex ( 4.2043 kcal/mol) is moderate, however, the calculated %Eg value (þ2.3498%) shows that Si-doped phagraphene has a low sensibility to CO molecule. The Ead calculations also indicate that the adsorption of CO molecule on Ga-doped phagraphene (Ead ¼ 6.9653 kcal/ mol) has the shorter recovery time compared with Al-doped phagraphene (Ead ¼ 8.1450 kcal/mol). Therefore, Ga-doped phagraphene can be considered as an promising gas sensing platform for detecting CO molecule.
1. Introduction Nowadays, the atmosphere pollution due to the fast industrial development and growing automobile traffic have turned to a critical problem. The fast, accurate and precise detection and monitoring of polluting hazardous gas molecules in very low concentrations is an essential demand in recent years. Among all the pollutant, carbon monoxide (CO) is a very dangerous gas in air [1,2]. Carbon monoxide is a transparent, odorless and tasteless gas which is called as an “invisible killer”. It is sorely poisonous gas and in a very low concentrations (9 ppm) can cause to some anti-health effects. Thus, the early detection of CO in the air is entirely essential [3]. Recently, different gas sensors have been used for various environ mental and security purposes [4,5]. The gas sensor with high sensibility and selectivity to target molecule as well as having low cost and simple assembly is of interest for industrial applications. The sensibility and selectivity are especially important parameters for a gas sensor. Because a typical gas sensor with high sensibility will has improved limit of detection and with high selectivity will respond to the target gas,
individually. Due to the mentioned reasons, attention to these two critical parameters in gas sensors have been increased, considerably [6–8]. Carbon allotropes are known to be promising materials as chemical gas sensors [9]. Among them, phagraphene is a novel carbon allotrope which its structure is consisted from 5-6-7 carbon rings [10]. Recently, Wang and coworkers on the basis of systematic evolutionary structure searching, have suggested this two-dimensional carbon structure. They have shown that because of its sp2 binding features and density of atomic packing, this structure has lower energy amount than most of the two-dimensional carbon allotropes [10]. Furthermore, this two-dimensional chemical structure is demonstrated to offer distorted Dirac cones. Phagraphene is a promising component in subsequent generation of nanoelectronic materials [11,12]. Very recently, Babazadeh and his coworkers studied the ability of the phagraphene as a novel nanocarrier for delivery of adrucil as an anticancer agent, based on DFT calculations [13]. They demonstrated that adrucil molecule has a very poor interaction with the pure phagraphene surface. Therefore they doped phagraphene with various impurity atoms
* Corresponding author. , E-mail addresses: [email protected], [email protected] (M. Rouhani). https://doi.org/10.1016/j.physe.2019.113710 Received 25 March 2019; Received in revised form 25 June 2019; Accepted 6 September 2019 Available online 9 September 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.
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involving B, Al and Si. They showed that the interaction of adrucil with doped phagraphene was increased as follows: Al-doped > Si-do ped > B-doped > pristine phagraphene. Thapa and coworkers found that the Li diffusion is preferred during intercalated and adsorbed state [14]. They demonstrated that the bulk-phagraphene has high specific capacity of 558 mAh/g, favorable negative formation energy, and stable positive open circuit voltage index for high Li intercalation. Sing and coworkers were performed detailed DFT calculations to appraise the mechanical characteristics of penta-graphene and phagraphene compared them with graphene, graphane, and pentaheptite [15]. They demonstrated that both penta-graphene and phagraphene have ultimate tensile strength and the strain smaller than that of graphene. Yuan et al., investigated realization of the magnetic phase transition and half-metallicity for phagraphene nanoribbons in virtue of functional groups (OH and CN) with various external electric fields and coverage fractions [15]. Due to the novelty of computational study on phagraphene as a sensor surface and in continuing our previous computational studies using Density Functional Theory (DFT) [16–21], the electronic sensi tivity of pristine and Al, Si and Ga doped phagraphene towards CO molecule was investigated via DFT approach. The results revealed the promising future of phagraphene in the development of CO gas sensors.
gadget for calculating the work function before and after the adsorption of a target gas molecule [30]. The surrounding gas molecule varies the work function (ΔΦ) which can be detected, accordingly. Even, some times it is possible to calculate the concentration of a target gas from ΔΦ value [31]. The gate voltage will be changed due to changing of work function of the sensor, which makes an electrical noise and caution the exposure with target gas [32]. The types of sensors which act on the basis of work function have some advantages compared with the other types such as fast, precise and accurate response and therefore they are used to detect and identify the various kinds of gas molecules in different industries [33–35]. According to the physical definition, the work function (Φ) is the least energy needed to release one electron to an infinite distance from the Fermi level of a molecule.
2. Computational details
EF ¼ EHOMO þ ðELUMO
Φ¼ Velðþ∞Þ
Vel(þ∞) in the equation (2) refers to the electrostatic potential energy of the electron with infinite interval from Fermi level which has been considered as zero. EF is the energy of the Fermi level. Thus, it is logical to rewrite the equation (2) as following: Φ¼
EPhagraphene
ECO þEBSSE
(3)
EF
The energy of Fermi level can be calculated as follows: EHOMO Þ=2
(4)
EHOMO and ELUMO in equation (4) mention to the energies of HOMO and LUMO levels, respectively. In a typical semiconductor, the change of Fermi level alters the work function and thus varies the field emission specifications because the densities of electron current from a hypo thetical level is affiliate exponentially to negative value of Φ according to the classical Richardson Dushman equation [36]: � � js ¼ AT2 exp W kT (5)
The optimization of chemical structures, molecular electrostatic potential (MEP), energy predictions, density of states (DOS) analyses and natural bond orbitals (NBO) were performed on a CO/phagraphene complexes M06-2X level of theory with 6-311Gþþ (d,p) basis set as performed in the GAMESS suite of programs [22]. GaussSum code was utilized to draw density of states (DOS) plots [23]. The attention to only HOMO and LUMO may not provide a pragmatic explanation of the frontier orbitals because, the neighboring orbitals may display quasi-degenerate energy levels in the boundary region. One of the most important application of the DOS plots is to show the molecular orbitals compositions and also their contributions in chemical bonding via the positive and negative charges gathered by αβDOS and TDOS diagrams. The αβDOS demonstrates the bonding and sum of positive and negative electrons with the nature of the interaction of the two orbitals, atoms or groups. It should be noted that positive amounts of the αβDOS show a bonding interaction, negative amounts show an anti-bonding interac tion and zero values demonstrate non-bonding interactions [24]. In our study, the resulted outputs from GaussSum program for pristine phag raphene, pristine phagraphene/CO, Si-doped phagraphene and Si-doped phagraphene/CO have related data about αDOS spectrum, βDOS spec trum, α-occupied orbitals, α-virtual orbitals, β-occupied orbitals and β-virtual orbitals. However, in the case of Al-doped phagraphene, Al-doped phagraphene/CO, Ga-doped phagraphene and Ga-doped phagraphene/CO only the main DOS spectrum, occupied orbitals and virtual orbitals have been provided. The M06 has 27% Hartree-Fock exchange part. It has been introduced for computation of the elec tronic properties [25–27]. The M06 functional gives comparable elec tronic properties with the general B3LYP but also involves weak dispersion interactions [28]. The CO molecule adsorption energy was calculated as follows: Ead ¼ ECO=Phagraphene
(2)
EF
In equation (5) js refers to the current density of the emission (mA/ mm2) and A is Richardson’s constant. Also, T is temperature (K), W is the work function of the cathode material (J or eV) and k is the Boltzmann constant (1.3806488 � 10 23 J K 1 or 8.6173324e 5 eV K 1) [37,38]. The A value itself can be obtained from equation (6): A ¼ 4 � πmek2 =h3 e1202 mA=mm2 K2
(6)
In the equation (6), m, e, and h refer to the mass of electron, elementary charge, and Plank’s constant, respectively. The molecular electrostatic potential (MEP) patterns were achieved by the Avogadro program, which can be defined as the potential that a unit of positive charge experiences at each place around the molecule. This potential feeling can be attributed to the distribution of the electron density in the typical molecule. One of the useful applications of MEP pattern is to figure out the intermolecular interactions. Furthermore, the electrostatic potential is utilized to realize the chemical reactivity of the molecule, so that parts of negative potential are attended to be locations of nucleophilic attack and protonation. Also, the locations of positive potential may be considered as the electrophilic sites [39]. Density of States (DOS) plots were obtained by the GaussSum code. Moreover, formula (7) was used to calculate the Global Electron Density Transfer (GEDT) [40]; X GEDT ¼ qA (7)
(1)
In equation (7), qA is the pure NBO charge of the atoms in complex systems and GEDT is the charge value which is transferred between the target molecule and adsorbent.
In equation (1) ECO/Phagraphene is the total energy of the adsorbed CO molecule on the phagraphene (or doped phagraphene) surface. The counter poise approach was used for calculating basis set superposition error (BSSE) for correcting all adsorption energies [29]. In this research, the sensitivity of work function (Φ) of the phag raphene and doped-phagraphene in the presence of CO molecule was investigated. The work function-based gas sensors use the Kelvin approach for action. These types of sensors use from a Kelvin oscillator
3. Results and discussion Initially, the CO molecule as well as pristine and doped phag raphenes were optimized separately at M06-2X/6-311Gþþ(d,p) level of theory. The performance of M06 suite of density functionals like M06-2X 2
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
for calculating the geometric structures and energies have been inves tigated by Dessent and coworkers [41]. They presented a comparative assessment between M06 suite and popular B3LYP density functional. They found that all of the M06 functions show well in prediction of global minimum conformer, conformer relative energies and week physical interactions. They demonstrated that the B3LYP is considerably less well for the canonical in the cases that dispersion interactions has considerable relation with conformer energies. As mentioned earlier, hybrid functionals are a class of approximations to the exchan ge–correlation energy functional in DFT that combine a share of exact exchange from Hartree–Fock theory with the remain of the exchan ge–correlation energy from ab initio or empirical sources. The precise exchange energy functional is represented in terms of the Kohn–Sham orbitals rather than the density, so is called an implicit density func tional. The hybridization with Hartree–Fock exchange supplies a easy scheme for amending the calculation of many molecular characteristics, such as bond lengths, vibration frequencies and atomization energies which cause to be explained poorly with simple ab initio functionals [25]. According to these explanations and this fact that there are weak interactions between CO molecule and pristine and heteroatom-doped phagraphene, we selected the M06-2X as a suitable and precise level of theory for this research. The optimized structure of CO molecule, its HOMO, LUMO and MEP profiles are shown in Fig. 1. The MEP pattern indicates that O head in CO has the highest electrostatic potential ag gregation and the C atom has considerable positive electrostatic po tential. In CO molecule, the bonding orbitals reside more on the oxygen atom, and the anti-bonding orbitals reside more on the carbon atom. The reactivity of CO typically arises from its HOMO when donating elec trons. When acting as an electron pair acceptor, its LUMO is significant. It can be seen that the HOMO of CO is a molecular orbital which puts significant electron density on the carbon atom. The LUMO of CO is the π* orbitals. The lobes of the LUMO are larger on the carbon atom than on the oxygen atom. Fig. 2 shows the optimized structure of phagraphene. It can be seen that the optimized phagraphene has 16 rings consisted from 47 carbon atoms with 17 hydrogen atoms located on the edges. The significant distortion of phagraphene from planar shape is due to the existence of pentagonal and heptagonal rings more in addition to hexagonal rings [13]. The MEP and DOS profiles for pristine phagraphene also are shown in Fig. 2. The high electrostatic potential distribution as well as considerable negative charges are seen on the carbon atoms and the hydrogen atoms in the rims of the flake possess slight positive charges. The DOS plot of pristine phagraphene shows that the HOMO-LUMO gap of energy is 3.5878 eV for this sheet. Due to this fact that the gas mol ecules approach in various directions to the adsorbent surface, the different adsorption geometries were prepared as input files for each case. For this purpose, the single CO molecule was located at three various directions (perpendicular oriented from C head, from O head and in parallel situation with respect to the surface of the phagraphene) at an interval of 2 Å above C or doped heteroatom atom in the surface. After all the structures were permitted to entirely relax, the interactions of the CO molecule with pristine and doped phagraphene sheet were explained regarding their adsorption energies. According to the
(a)
(b)
(c)
equation (1), the adsorption energy with most negative value demon strates the strongest adsorption of CO molecule on phagraphene surface. Therefore, most energetically desirable adsorption orientations were selected for further computations. 3.1. CO adsorption on the pristine phagraphene For studying the CO adsorption on the pristine phagraphene, the different adsorption orientations of CO on the pristine phagraphene were examined. It was demonstrated that the adsorption of CO molecule on pristine phagraphene is preferred in nearly parallel orientation (Fig. 3). Table 1 shows various calculated parameters such as EHOMO, ELUMO, Eg, Ead, %ΔEg, GEDT, EF, Φ and %ΔΦ for both pristine phag raphene and CO/phagraphene complex. It can be seen that the energies of HOMO, and LUMO levels of the pristine phagraphene are about 5.4128 and 1.8250 eV, respectively. Therefore, the Eg is about 3.5878 eV. The adsorption energy for the CO/pristine phagraphene complex was obtained 2.1962 kcal/mol with calculated BSSE value 0.021 kcal/mol. Due to approximately large distance between CO molecule and pristine phagraphene (CCO … CPhagraphene ¼ 3.29 Å and OCO … CPhagraphene ¼ 3.22 Å) and partial negative adsorption energy, it can be said that the adsorption of the CO molecule on the pristine phagraphene can be considered as a weak physical adsorption. The calculated GEDT analysis for CO/pristine phagraphene shows that there is a very feeble charge transfer 0.007 e from the phagraphene to the CO molecule. Also, the MEP pattern for the adsorption of CO molecule on the pristine phagraphene is shown in Fig. 3 which demonstrates a partial overlapping between the orbitals of CO molecule and pristine phagraphene. Moreover, the calculated DOS plot for CO/pristine phagraphene shows that the electronic properties of phagraphene are not altered during the adsorption of CO molecule on the phagraphene surface. HOMO and LUMO level of energies are slightly stabilized to 5.4264 and 1.8478 eV, respectively. Therefore, the Eg value is decreased about 0.257%, insignificantly. Also, the Φ is somewhat increased (by about 0.0182 eV). In the sensors which they work on the basis of alteration in work function, there is a considerable sensitivity to value of ΔΦ because the electron emission will exponentially grow by Φ decrease [42]. However, the ΔΦ value for CO/pristine phagraphene is increased nearly 0.5021% which is not perfect. Therefore, the CO molecule has a very weak physical adsorption on the pristine phag raphene which means that pristine phagraphene has a finite capacity for adsorption of CO molecule. To improve the sensitivity of phagraphene towards CO molecule, one carbon atom of the phagraphene was substituted with various doping atoms i.e. aluminum (Al), silicon (Si) and gallium (Ga). It should be mentioned that we located the CO molecule in three different positions on the various rings (pentagonal, hexagonal, and heptagonal) of phagraphene surface [13]. After the full relaxation, it was observed that the CO molecule tends to interact with the pentagonal ring of phagraphene. So, pentagonal, hexagonal, and heptagonal junction carbon was selected as a best doping position in phagraphene [13]. 3.2. CO adsorption on the Al-doped phagraphene The effects of substitution of Al atom on the geometric and electronic properties of the phagraphene, and also on the adsorption of CO mole cule on it were studied. Fig. 4 shows that the substitution of Al atom instead of one of carbon atoms, leads almost considerable geometric distortion due to aluminum’s relatively bigger atomic radius compared with carbon atom. In the fully relaxed Al-doped phagraphene, the Al atom is projected significantly out of the phagraphene surface in order to decrease forced strain. The calculated bond lengths for Al–C bond in Aldoped phagraphene are 1.88, 1.87 and 1.84 Å which are significantly much longer than the corresponding C–C bonds in the pristine phag raphene which are 1.43, 1.42 and 1.39 Å. The MEP profile for Al-doped phagraphene is represented in Fig. 4. The remarkable negative potential
(d)
Fig. 1. The chemical structure (a), HOMO (b), LUMO (c) and molecular elec trostatic potential (MEP) (d) plot of carbon monoxide (CO) molecule. 3
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
(d)
Fig. 2. The optimized structure from top view (a), side view (b), calculated molecular electrostatic potential (MEP) (c) and density of states (DOS) plot (d) of pristine phagraphene at M06-2X/6-311Gþþ(d,p) level of theory.
is distributed on the Al atom location. It can be seen from Table 2 that HOMO level destabilized slightly from 5.4128 eV to 4.8326 eV and LUMO level stabilized significantly from 1.8250 eV to 3.0811 eV after the Al-doping process. Thus Eg was calculated about 1.7515 eV which is represented also by its DOS plot. Therefore, during doping of phagraphene flake by Al atom, the HOMO-LUMO gap was closed considerably to the extent of 1.8363 eV. In the next step, we have studied the CO molecule adsorption on the Al-doped phagraphene by placing the CO above the Al atom with various orientations involving C head, O head toward Al atom and with parallel axis relative to phagraphene. After fully relaxation, it was seen that the CO molecule can be adsorbed on the phagraphene surface with O head orientation toward Al atom with 2.160 Å OCO⋯AlAl-doped phag raphene (Fig. 5). Table 2 shows that the CO/Al-doped phagraphene complex revealed the Ead of 8.1450 kcal/mol, which was higher than the calculated Ead value for CO/pristine phagraphene complex ( 2.1962 kcal/mol). This observation can be attributed to the presence of Al atom which acts as a proper Lewis acid which simplifies the charge transfer from CO molecule to Al atom in Al-doped phagraphene. This transition has been confirmed by the calculated GEDT analysis which shows þ0.118 e charge transfer from CO molecule to Al atom in Al-
doped phagraphene. The adsorption of CO molecule on the Al-doped phagraphene destabilized both HOMO and LUMO levels of energies to the extent of 0.0397 eV and 0.3998 eV, respectively. Thus, the Eg is changed from 1.7515 eV to 2.116 eV and opened by almost 20.5595% which also is shown by DOS plot (Fig. 5). The change in HOMO and LUMO levels of energies after adsorption leads to slight change in EF and Φ values which causes 5.8788 in %ΔΦ amount. 3.3. CO adsorption on the Si-doped phagraphene Moreover, a C atom in phagraphene was replaced by Si atom and the effects of substitution on the electronic, structural, and CO adsorption characteristics of the phagraphene, were studied. As expected, by replacing the C atom by Si, structure of the sheet was remarkably dis torted (Fig. 6). In the fully relaxed Si-doped phagraphene, the Si atom impurity is located out of the surface till decrease stress due to its larger radius compared to the C atom. The calculated bond length is 1.71, 1.80 and 1.81 Å for the Si–C bonds in the Si-doped phagraphene which is considerably larger than the corresponding C–C bonds in the pristine phagraphene (Fig. 6). The calculated MEP profile for Si-doped phag raphene shows a broad distribution of electrostatic potential around the 4
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
Fig. 3. The optimized structure from top and side view (a), calculated molecular electrostatic potential (MEP) profile (b) and density of states (DOS) plot (c) of CO/ pristine phagraphene complex at M06-2X/6-311Gþþ(d,p) level of theory. Table 1 The calculated EHOMO (eV), ELUMO (eV), Eg (eV), Ead (kcal/mol), %ΔEg, GEDT (e ), EF (eV), Φ (eV) and %ΔΦ for pristine phagraphene and CO/phagraphene complex. Configuration Pristine phagraphene CO/phagraphene
EHOMO 5.4128 5.4264
ELUMO 1.8250 1.8478
Eg
Ead
%ΔEg
GEDT
3.5878 3.5786
–
–
–
2.1962
Si atom (Fig. 6). Similar to Al-doped phagraphene, it can be found from Table 3 that HOMO level of energy in phagraphene destabilized a little from 5.4128 eV to 5.3463 eV and LUMO level of energy stabilized remarkably from 1.8250 eV to 2.2865 eV after the Si-doping process. In the effect of Si atom substitution, Eg value was changed from 3.5878 eV to 3.0598 eV which is shown also by its DOS plot. In the following, we have studied adsorption of CO molecule on the Si-doped phagraphene by placing the CO molecule up the Si atom with various orientations involving from C head, O head, and with parallel axis towards phagraphene surface with 2 Å distance. After fully relaxa tion, it was seen that the CO molecule preferred to be adsorbed indi rectly almost from its C head toward Si atom in Si-doped phagraphene (Fig. 7). The CCO⋯SiSi-doped phagraphene distance was calculated as 3.271 Å. The interaction of CO molecule with Si atom in the Si-doped phagraphene from electrostatic potential viewpoint is presented by MEP calculation (Fig. 7). After physical adsorption, the HOMO level of energy stabilized from 5.3463 eV to 5.3638 eV and LUMO level of energy destabilized from 2.2865 eV to 2.2321 eV and therefore increased Eg from 3.0598 eV to 3.1317 eV namely 2.3498% (Table 3).
0.257
0.007
EF 3.6189 3.6371
Φ
%ΔΦ
3.6189 3.6371
– þ0.5021
According to the GEDT analysis, the charge transfer from CO molecule to Si-doped phagraphene (þ0.034 e ) is considerably lower than that Aldoped phagraphene (þ0.118 e ) because Al atom can act as a Lewis acid site which reinforces the charge transition from CO molecule. The CO/Si-doped phagraphene complex shows Ead of 4.2043 kcal/mol, which is lower than the Ead amount for CO/Al-doped phagraphene ( 8.1450 kcal/mol) which reveals strong evidence for the weaker physical adsorption of CO molecule on Si-doped phagraphene compared with Al-doped phagraphene. Furthermore, adsorption of CO molecule on Si-doped phagraphene surface leads to a little increase in EF from 3.8164 eV to 3.7979 eV and therefore causes negligible decrease in %ΔΦ amount ( 0.4871%). 3.4. CO adsorption on the Ga-doped phagraphene According to our expectation, by substituting a C atom by a Ga atom, the spatial structure of phagraphene is considerably distorted at the doped location (Fig. 8). The Ga–C bond lengths were calculated about 1.86, 190 and 1.91 Å which are much longer than the corresponding C–C 5
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(a)
(b)
(c)
(d)
Fig. 4. The optimized structure from top view (a), side view (b), calculated molecular electrostatic potential (MEP) (c) and density of states (DOS) plot (d) of Aldoped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory. Table 2 The calculated EHOMO (eV), ELUMO (eV), Eg (eV), Ead (kcal/mol), %ΔEg, GEDT (e ), EF (eV), Φ (eV) and %ΔΦ for Al-dopoed phagraphene and CO/Al-doped phag raphene complex. Configuration Al-doped phagraphene CO/Al-doped phagraphene
EHOMO 4.8326 4.7929
ELUMO 3.0811 2.6813
Eg
Ead
%ΔEg
GEDT
1.7515 2.1116
–
– þ20.5595
– þ0.118
8.1450
bonds in the pristine phagraphene (Fig. 8). Therefore, the Ga atom was significantly projected out of the sheet. The calculations showed that the HOMO and especially LUMO levels of energies in phagraphene were remarkably altered after the Ga-doping process. The HOMO and LUMO level of energies were changed from 5.4128 eV to 4.8887 eV and from 1.8250 eV to 3.1009 eV respectively, which resulted to gap closing and change Eg value from 3.5878 eV to 1.7878 eV (see DOS profile in Fig. 7) due to the stabilizing of LUMO level of phagraphene after Ga doping (Table 4). The calculated MEP pattern for Ga-doped phagraphene showed greater distribution of electrostatic potential in the Ga-doped site (Fig. 7). The adsorption of the CO molecule on the Ga-doped phagraphene was explored in the next step (Fig. 9). After fully optimization, it was seen that the CO molecule preferred to be physically adsorbed directly from its O head toward Ga atom in Ga-doped phagraphene (Fig. 9). The
EF 3.9568 3.7371
Φ
%ΔΦ
3.9568 3.7371
–
5.8788
OCO…GaGa-doped phagraphene distance was calculated as 2.181 Å. The MEP pattern shows the efficient interaction of CO molecule with Ga atom in the Ga-doped phagraphene (Fig. 9). The changes in the various elec tronic characteristics of the phagraphene after adsorption of CO mole cule were investigated. It can be seen that the HOMO and LUMO levels of energies are both destabilized from 4.8887 eV to 4.8114 eV and from 3.1009 to 2.7162 eV, respectively. Moreover, the Eg value in Ga-doped phagraphene is increased from 1.7878 eV to 2.0952 eV after adsorption of CO molecule on phagraphene which equals to þ17.1943% increase. Fermi level of energy is destabilized from 3.9947 eV to 3.7638 eV which offers a reduction of 6.1374% in %ΔΦ. The GEDT analysis shows a considerable charge transfer þ0.092 e from CO molecule to Ga-doped phagraphene. The Ead energy 6.9653 kcal/mol shows that adsorption of CO molecule on Ga-doped phagraphene is a type of considerable physical adsorption and it can be concluded that CO 6
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
Fig. 5. The optimized structure from top and side view (a), calculated molecular electrostatic potential (MEP) profile (b) and density of states (DOS) plot (c) of CO/ Al-doped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory.
molecule has acceptable interaction with Ga-doped phagraphene surface. Using DFT calculations, the adsorption of CO molecule on the pris tine, Al, Si and Ga doped phagraphene was explored in order to detect a proper work function type sensor. The obtained results show that the CO molecule interacts mainly via its O head with heteroatom-doped phag raphene. The amount of adsorption energies for CO molecule on pris tine, Al, Si and Ga doped phagraphene are 2.1962, 8.1450, 4.2043 and 6.9653 kcal/mol, respectively. Moreover, the %ΔΦ values were calculated þ0.5021, 5.8788, 0.4871 and 6.9653%, respectively. It can be seen that the CO molecule has a very weak physical adsorption on phagraphene surface. Also, the work function of pristine phagraphene is not properly affected by CO molecule and therefore, the pristine phag raphene is not a suitable work function type sensor for CO molecule. The substitution of heteroatom instead of carbon atom, leads to significant geometric distortion and projection out of heteroatom due to its rela tively bigger atomic radius compared with carbon atom. It can be a desirable phenomenon because the projected out heteroatom is more accessible with target molecule (in this study CO molecule) for adsorption and hence, the sensitivity of the adsorbent can be increased. However, this observation is not fixed and constant for all cases because alteration of Eg value before and after adsorption of the target molecule strongly depends on changes in HOMO and LUMO levels of energy.
Table 5 shows the Eg values for pristine, Al, Si and Ga doped phag raphene and also %ΔEg values after doping and adsorption. It can be seen that the Eg values are significantly changed during doping process of Al and Ga heteroatoms ( 104.8415% and 100.6824%, respectively) which demonstrates that HOMO and LUMO levels of energy have been altered considerably. However, Si-doped phagraphene shows the high est alteration in Eg value during CO adsorption process (3.1317%). So, it is necessary to consider to the other important factor means Ead. The CO molecule is adsorbed on the Ga-doped phagraphene from its O-head with adsorption energy about 6.9653 kcal/mol. It can be deduced that the adsorption of CO molecule on Ga-doped phagraphene is not rather much to prolong the CO release from surface and is not so little to prevent effective adsorption. The work function of Ga-doped phagraphene is significantly decreased 6.1374% by CO molecule adsorption because of the destabilization of both HOMO and LUMO level of energies. It can be concluded that the Ga-doped phagraphene may be a promising electronic sensor for CO molecule with a short re covery time. It can be a work function type sensor due to the consider able change of work function upon the adsorption process. However, in order to gain more evidences about the efficiency of Ga-doped phag raphene for detecting of CO molecule, the Thermodynamic character istics and transport aspects of CO molecule adsorption on pristine and doped phagraphene was studied. 7
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(a)
(b)
(c)
(d)
Fig. 6. The optimized structure from top view (a), side view (b), calculated molecular electrostatic potential (MEP) (c) and density of states (DOS) plot (d) of Sidoped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory. Table 3 The calculated EHOMO (eV), ELUMO (eV), Eg (eV), Ead (kcal/mol), %ΔEg, GEDT (e ), EF (eV), Φ (eV) and %ΔΦ for Si-dopoed phagraphene and CO/Si-doped phagraphene complex. Configuration Si-doped phagraphene CO/Si-doped phagraphene
EHOMO 5.3463 5.3638
ELUMO 2.2865 2.2321
Eg
Ead
%ΔEg
GEDT
3.0598 3.1317
–
– þ2.3498
– þ0.034
4.2043
EF 3.8164 3.7979
Φ
%ΔΦ
3.8164 3.7979
–
0.4871
3.5. Thermodynamic characteristics of CO adsorption onto pristine and doped phagraphene
3.6. Transport aspects of CO/pristine and Al, Si, Ga-doped phagraphene complexes
In the next step, the changes in Gibbs free energy (ΔG) and enthalpy (ΔH) were calculated at ambient pressure conditions (P ¼ 1 atm and T ¼ 298.14 K) in order to evaluate the thermodynamic possibility of adsorption of the CO molecule on the pristine and doped phagraphene. The results shows that the amounts of ΔG and ΔH (ΔG, ΔH) for CO molecule adsorption on the pristine, Al, Si, and Ga-doped phagraphene are 0.11 ( 0.61), 0.82 ( 1.68), 0.53 ( 1.09), and 0.98 ( 1.89), respectively. Based on calculated values, all the adsorption processes except CO adsorption on pristine phagraphene are considerably exothermic and therefore thermodynamically desirable.
To have a more precise view into the adsorption of CO molecule on the pristine and doped phagraphene, the molecular electrostatic po tential (MEP) profiles of CO/pristine and doped phagraphene are calculated which were shown for each case earlier. It can be seen that except of CO adsorption on pristine phagraphene, a considerable overlap exist between CO gas molecule and the doped-phagraphene surface, disclosing the incidence of a considerable physical adsorption which have been confirmed by calculated interaction intervals and adsorption energies. The overlapping between orbitals and consequence charge transfers leads to significant variations in the electronic properties of the phag raphene which is useful as sensor. The gap energy (Eg) can specify the electrical conductivity (σ) according to the following relation (relation 8
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(a)
(b)
(c)
Fig. 7. The optimized structure from top and side view (a), calculated molecular electrostatic potential (MEP) profile (b) and density of states (DOS) plot (c) of CO/ Si-doped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory.
1a) [43,44]:
σ α exp
Eg =2kB T
�
of CO molecule on Si-doped phagraphene and significantly decreases upon adsorption on Al-doped and Ga-doped phagraphene. Although Ead value of CO/Si-doped phagraphene ( 4.2043 kcal/mol) are almost high, the calculated %Eg value (þ2.3498%) emphasizing that Si-doped phagraphene has a low and unacceptable sensitivity to CO molecule. The other main factor for the assessment of the efficiency of a typical gas sensor is the recovery time of the target molecule from adsorbent surface. Recent studies demonstrated that the heating of graphene-based gas sensor in vacuum or under UV irradiation at 150 � C will cause to recover surface during 100–200 s [45]. The relation (2a) represents re covery time evaluation on the basis of adsorption energy: � τ α υ0 1 exp Ead =kB T (2a)
(1a)
In the relation (8), kB is the Boltzmann constant and T is the tem perature. It is obvious that, a small shift in the Eg value significantly changes the electrical conductivity. As mentioned earlier, the band gap of the phagraphene as an adsorbent is changed after adsorption of CO molecule. The energy band gap of pristine phagraphene (3.5878 eV) decreases slightly to 3.5786 eV after poor physical interaction with CO molecule ( 0.257%). This negligible change in the band gap of pristine phagraphene after interaction with CO molecule is due to the very weak physical interaction between them which confirms with a very small charge transfer ( 0.007 e ). However, in the case of CO molecule adsorption on the Al, Si and Ga-doped phagraphene, the energy gap values were changed from 1.7515 eV to 2.1116 eV (þ20.5595%), from 3.0598 eV to 3.1317 eV (þ2.3498%) and from 1.7878 eV to 2.0952 eV (þ17.1943%) in CO/Al-doped phagraphene, CO/Si-doped phagraphene and CO/Ga-doped phagraphene, respectively. These results are completely in agree with charge transfer values which are calculated with GEDT analysis (þ0.111, þ0.034 and þ 0.092 e , respectively). Therefore, the electrical conductivity slightly decreases after adsorption
In the relation (2), υ0 is attempt frequency, kB is the Boltzmann constant and T is the temperature. According to the relation (2), if the adsorption energy changes a little more negative, the recovery time increases, exponentially. Thus, the adsorption of CO molecule on Gadoped phagraphene (Ead ¼ 6.9653 kcal/mol) has the shortest recov ery time compared with Al-doped phagraphene (Ead ¼ 8.1450 kcal/ mol). Finally, it should be added that the modification of fullerene surface 9
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Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
(d)
Fig. 8. The optimized structure from top view (a), side view (b), calculated molecular electrostatic potential (MEP) (c) and density of states (DOS) plot (d) of Gadoped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory. Table 4 The calculated EHOMO (eV), ELUMO (eV), Eg (eV), Ead (kcal/mol), %ΔEg, GEDT (e ), EF (eV), Φ (eV) and %ΔΦ for Ga-dopoed phagraphene and CO/Ga-doped phag raphene complex. Configuration Ga-doped phagraphene CO/Ga-doped phagraphene
EHOMO 4.8887 4.8114
ELUMO 3.1009 2.7162
Eg
Ead
%ΔEg
GEDT
1.7878 2.0952
–
– þ17.1943
– þ0.092
6.9653
by doping impurity atoms is an effective approach to increase its sensitivity to gas molecules, experimentally [42,46]. The different strategies also have been employed for improving the sensitivity of graphene to different gaseous species. Graphene and graphene like surfaces have been investigated for the construction of gas sensors. Yuan and Shi [47] have discussed about the recent progresses in the synthesis of graphene materials for sensing purposes and the related techniques. All these findings demonstrate that the Ga-doped phagraphene can be a promising nanosensor for CO detection due to its considerable conduc tance variations, modest Ead, and acceptable recovery time. Therefore, we think that Ga-doped phagraphene can show a great potential for detection of gas molecules and its ability for detection of other toxic molecules in air such as NO, NO2, SO2 … should be studied in coming
EF 3.9948 3.7638
Φ
%ΔΦ
3.9948 3.7638
–
6.1374
investigations. So, we can be very hopeful that our proposed Ga-doped phagraphene could be experimentally synthesized and after optimiza tion of morphologies of sensing layers and configurations of sensing surfaces, it could be introduced as an effective CO gas sensor. 4. Conclusions Using DFT calculations, the adsorption process of CO molecule on the pristine and Al, Si and Ga-doped phagraphene was studied to discover a work function type sensor. The obtained data demonstrate that the CO molecule is adsorbed on the Ga-doped phagraphene from its O-head with releasing an energy about 6.9653 kcal/mol. The work function of Ga-doped phagraphene is considerably affected by CO molecule 10
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(a)
(b)
(c)
Fig. 9. The optimized structure from top and side view (a), calculated molecular electrostatic potential (MEP) profile (b) and density of states (DOS) plot (c) of CO/ Ga-doped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory. Table 5 The calculated Eg (eV) and %ΔEg for pristine, Al, Si and Ga-dopoed phagraphene. Values Eg %ΔEg (after doping)a %ΔEg (after adsorption)b a b
Adsorbent Pristine phagraphene
Al-doped phagraphene
Si-doped phagraphene
Ga-doped phagraphene
3.5878 – 3.5786
1.7515 104.8415 2.1116
3.0598 17.2560 3.1317
1.7878 100.6824 2.0952
The values are calculated compared with pristine phagraphene. The values are calculated compared with pure adsorbent.
( 6.1374) and therefore, Ga-doped phagraphene is an appropriate work function type sensor. It was deduced that the Ga-doped phagraphene may be a promising work function type sensor for CO molecule with a short recovery time.
Ethical statement/conflict of interest We have no conflicts of interest to declare. Acknowledgments The authors thank, Islamic Azad University, Science and Research 11
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Branch for the support and guidance.
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Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Ga-doped phagraphene as a superior media for sensing of carbon monoxide: A detailed theoretical investigation Morteza Rouhani *, Sanaz Kord, Zohreh Mirjafary Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: DFT Sensor CO Phagraphene Carbon monoxide Computational
The adsorption of highly toxic carbon monoxide molecule (CO) on pristine, Al, Si, and Ga-doped phagraphene are studied using Density Functional Theory (DFT) calculations at M06-2X level of theory with 6-311Gþþ (d,p) basis set, to explore the potential abilities of phagraphene as an effective CO detection sensor. The full relaxed geometries of CO molecule, pristine and doped phagraphenes and related CO/pristine and doped-phagraphene complexes are specified and the variety parameters such as energy of Highest Occupied Molecular Orbital (EHOMO), energy of Lowest Unoccupied Molecular Orbital (ELUMO), gap energy (Eg), energy of adsorption (Ead), basis set superposition error (BSSE), gap energy alterations percent (%ΔEg), Global Electron Density Transfer (GEDT), energy of Fermi level (EF), work function (Φ), work function alterations percent (%ΔΦ), Density of States (DOS) patterns and Molecular Electrostatic Potential (MEP) profiles were calculated and analyzed. The results demonstrate that the CO molecule has a very poor physical adsorption on the pristine phagraphene. The calculated Ead value of CO/Si-doped phagraphene complex ( 4.2043 kcal/mol) is moderate, however, the calculated %Eg value (þ2.3498%) shows that Si-doped phagraphene has a low sensibility to CO molecule. The Ead calculations also indicate that the adsorption of CO molecule on Ga-doped phagraphene (Ead ¼ 6.9653 kcal/ mol) has the shorter recovery time compared with Al-doped phagraphene (Ead ¼ 8.1450 kcal/mol). Therefore, Ga-doped phagraphene can be considered as an promising gas sensing platform for detecting CO molecule.
1. Introduction Nowadays, the atmosphere pollution due to the fast industrial development and growing automobile traffic have turned to a critical problem. The fast, accurate and precise detection and monitoring of polluting hazardous gas molecules in very low concentrations is an essential demand in recent years. Among all the pollutant, carbon monoxide (CO) is a very dangerous gas in air [1,2]. Carbon monoxide is a transparent, odorless and tasteless gas which is called as an “invisible killer”. It is sorely poisonous gas and in a very low concentrations (9 ppm) can cause to some anti-health effects. Thus, the early detection of CO in the air is entirely essential [3]. Recently, different gas sensors have been used for various environ mental and security purposes [4,5]. The gas sensor with high sensibility and selectivity to target molecule as well as having low cost and simple assembly is of interest for industrial applications. The sensibility and selectivity are especially important parameters for a gas sensor. Because a typical gas sensor with high sensibility will has improved limit of detection and with high selectivity will respond to the target gas,
individually. Due to the mentioned reasons, attention to these two critical parameters in gas sensors have been increased, considerably [6–8]. Carbon allotropes are known to be promising materials as chemical gas sensors [9]. Among them, phagraphene is a novel carbon allotrope which its structure is consisted from 5-6-7 carbon rings [10]. Recently, Wang and coworkers on the basis of systematic evolutionary structure searching, have suggested this two-dimensional carbon structure. They have shown that because of its sp2 binding features and density of atomic packing, this structure has lower energy amount than most of the two-dimensional carbon allotropes [10]. Furthermore, this two-dimensional chemical structure is demonstrated to offer distorted Dirac cones. Phagraphene is a promising component in subsequent generation of nanoelectronic materials [11,12]. Very recently, Babazadeh and his coworkers studied the ability of the phagraphene as a novel nanocarrier for delivery of adrucil as an anticancer agent, based on DFT calculations [13]. They demonstrated that adrucil molecule has a very poor interaction with the pure phagraphene surface. Therefore they doped phagraphene with various impurity atoms
* Corresponding author. , E-mail addresses: [email protected], [email protected] (M. Rouhani). https://doi.org/10.1016/j.physe.2019.113710 Received 25 March 2019; Received in revised form 25 June 2019; Accepted 6 September 2019 Available online 9 September 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.
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involving B, Al and Si. They showed that the interaction of adrucil with doped phagraphene was increased as follows: Al-doped > Si-do ped > B-doped > pristine phagraphene. Thapa and coworkers found that the Li diffusion is preferred during intercalated and adsorbed state [14]. They demonstrated that the bulk-phagraphene has high specific capacity of 558 mAh/g, favorable negative formation energy, and stable positive open circuit voltage index for high Li intercalation. Sing and coworkers were performed detailed DFT calculations to appraise the mechanical characteristics of penta-graphene and phagraphene compared them with graphene, graphane, and pentaheptite [15]. They demonstrated that both penta-graphene and phagraphene have ultimate tensile strength and the strain smaller than that of graphene. Yuan et al., investigated realization of the magnetic phase transition and half-metallicity for phagraphene nanoribbons in virtue of functional groups (OH and CN) with various external electric fields and coverage fractions [15]. Due to the novelty of computational study on phagraphene as a sensor surface and in continuing our previous computational studies using Density Functional Theory (DFT) [16–21], the electronic sensi tivity of pristine and Al, Si and Ga doped phagraphene towards CO molecule was investigated via DFT approach. The results revealed the promising future of phagraphene in the development of CO gas sensors.
gadget for calculating the work function before and after the adsorption of a target gas molecule [30]. The surrounding gas molecule varies the work function (ΔΦ) which can be detected, accordingly. Even, some times it is possible to calculate the concentration of a target gas from ΔΦ value [31]. The gate voltage will be changed due to changing of work function of the sensor, which makes an electrical noise and caution the exposure with target gas [32]. The types of sensors which act on the basis of work function have some advantages compared with the other types such as fast, precise and accurate response and therefore they are used to detect and identify the various kinds of gas molecules in different industries [33–35]. According to the physical definition, the work function (Φ) is the least energy needed to release one electron to an infinite distance from the Fermi level of a molecule.
2. Computational details
EF ¼ EHOMO þ ðELUMO
Φ¼ Velðþ∞Þ
Vel(þ∞) in the equation (2) refers to the electrostatic potential energy of the electron with infinite interval from Fermi level which has been considered as zero. EF is the energy of the Fermi level. Thus, it is logical to rewrite the equation (2) as following: Φ¼
EPhagraphene
ECO þEBSSE
(3)
EF
The energy of Fermi level can be calculated as follows: EHOMO Þ=2
(4)
EHOMO and ELUMO in equation (4) mention to the energies of HOMO and LUMO levels, respectively. In a typical semiconductor, the change of Fermi level alters the work function and thus varies the field emission specifications because the densities of electron current from a hypo thetical level is affiliate exponentially to negative value of Φ according to the classical Richardson Dushman equation [36]: � � js ¼ AT2 exp W kT (5)
The optimization of chemical structures, molecular electrostatic potential (MEP), energy predictions, density of states (DOS) analyses and natural bond orbitals (NBO) were performed on a CO/phagraphene complexes M06-2X level of theory with 6-311Gþþ (d,p) basis set as performed in the GAMESS suite of programs [22]. GaussSum code was utilized to draw density of states (DOS) plots [23]. The attention to only HOMO and LUMO may not provide a pragmatic explanation of the frontier orbitals because, the neighboring orbitals may display quasi-degenerate energy levels in the boundary region. One of the most important application of the DOS plots is to show the molecular orbitals compositions and also their contributions in chemical bonding via the positive and negative charges gathered by αβDOS and TDOS diagrams. The αβDOS demonstrates the bonding and sum of positive and negative electrons with the nature of the interaction of the two orbitals, atoms or groups. It should be noted that positive amounts of the αβDOS show a bonding interaction, negative amounts show an anti-bonding interac tion and zero values demonstrate non-bonding interactions [24]. In our study, the resulted outputs from GaussSum program for pristine phag raphene, pristine phagraphene/CO, Si-doped phagraphene and Si-doped phagraphene/CO have related data about αDOS spectrum, βDOS spec trum, α-occupied orbitals, α-virtual orbitals, β-occupied orbitals and β-virtual orbitals. However, in the case of Al-doped phagraphene, Al-doped phagraphene/CO, Ga-doped phagraphene and Ga-doped phagraphene/CO only the main DOS spectrum, occupied orbitals and virtual orbitals have been provided. The M06 has 27% Hartree-Fock exchange part. It has been introduced for computation of the elec tronic properties [25–27]. The M06 functional gives comparable elec tronic properties with the general B3LYP but also involves weak dispersion interactions [28]. The CO molecule adsorption energy was calculated as follows: Ead ¼ ECO=Phagraphene
(2)
EF
In equation (5) js refers to the current density of the emission (mA/ mm2) and A is Richardson’s constant. Also, T is temperature (K), W is the work function of the cathode material (J or eV) and k is the Boltzmann constant (1.3806488 � 10 23 J K 1 or 8.6173324e 5 eV K 1) [37,38]. The A value itself can be obtained from equation (6): A ¼ 4 � πmek2 =h3 e1202 mA=mm2 K2
(6)
In the equation (6), m, e, and h refer to the mass of electron, elementary charge, and Plank’s constant, respectively. The molecular electrostatic potential (MEP) patterns were achieved by the Avogadro program, which can be defined as the potential that a unit of positive charge experiences at each place around the molecule. This potential feeling can be attributed to the distribution of the electron density in the typical molecule. One of the useful applications of MEP pattern is to figure out the intermolecular interactions. Furthermore, the electrostatic potential is utilized to realize the chemical reactivity of the molecule, so that parts of negative potential are attended to be locations of nucleophilic attack and protonation. Also, the locations of positive potential may be considered as the electrophilic sites [39]. Density of States (DOS) plots were obtained by the GaussSum code. Moreover, formula (7) was used to calculate the Global Electron Density Transfer (GEDT) [40]; X GEDT ¼ qA (7)
(1)
In equation (7), qA is the pure NBO charge of the atoms in complex systems and GEDT is the charge value which is transferred between the target molecule and adsorbent.
In equation (1) ECO/Phagraphene is the total energy of the adsorbed CO molecule on the phagraphene (or doped phagraphene) surface. The counter poise approach was used for calculating basis set superposition error (BSSE) for correcting all adsorption energies [29]. In this research, the sensitivity of work function (Φ) of the phag raphene and doped-phagraphene in the presence of CO molecule was investigated. The work function-based gas sensors use the Kelvin approach for action. These types of sensors use from a Kelvin oscillator
3. Results and discussion Initially, the CO molecule as well as pristine and doped phag raphenes were optimized separately at M06-2X/6-311Gþþ(d,p) level of theory. The performance of M06 suite of density functionals like M06-2X 2
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for calculating the geometric structures and energies have been inves tigated by Dessent and coworkers [41]. They presented a comparative assessment between M06 suite and popular B3LYP density functional. They found that all of the M06 functions show well in prediction of global minimum conformer, conformer relative energies and week physical interactions. They demonstrated that the B3LYP is considerably less well for the canonical in the cases that dispersion interactions has considerable relation with conformer energies. As mentioned earlier, hybrid functionals are a class of approximations to the exchan ge–correlation energy functional in DFT that combine a share of exact exchange from Hartree–Fock theory with the remain of the exchan ge–correlation energy from ab initio or empirical sources. The precise exchange energy functional is represented in terms of the Kohn–Sham orbitals rather than the density, so is called an implicit density func tional. The hybridization with Hartree–Fock exchange supplies a easy scheme for amending the calculation of many molecular characteristics, such as bond lengths, vibration frequencies and atomization energies which cause to be explained poorly with simple ab initio functionals [25]. According to these explanations and this fact that there are weak interactions between CO molecule and pristine and heteroatom-doped phagraphene, we selected the M06-2X as a suitable and precise level of theory for this research. The optimized structure of CO molecule, its HOMO, LUMO and MEP profiles are shown in Fig. 1. The MEP pattern indicates that O head in CO has the highest electrostatic potential ag gregation and the C atom has considerable positive electrostatic po tential. In CO molecule, the bonding orbitals reside more on the oxygen atom, and the anti-bonding orbitals reside more on the carbon atom. The reactivity of CO typically arises from its HOMO when donating elec trons. When acting as an electron pair acceptor, its LUMO is significant. It can be seen that the HOMO of CO is a molecular orbital which puts significant electron density on the carbon atom. The LUMO of CO is the π* orbitals. The lobes of the LUMO are larger on the carbon atom than on the oxygen atom. Fig. 2 shows the optimized structure of phagraphene. It can be seen that the optimized phagraphene has 16 rings consisted from 47 carbon atoms with 17 hydrogen atoms located on the edges. The significant distortion of phagraphene from planar shape is due to the existence of pentagonal and heptagonal rings more in addition to hexagonal rings [13]. The MEP and DOS profiles for pristine phagraphene also are shown in Fig. 2. The high electrostatic potential distribution as well as considerable negative charges are seen on the carbon atoms and the hydrogen atoms in the rims of the flake possess slight positive charges. The DOS plot of pristine phagraphene shows that the HOMO-LUMO gap of energy is 3.5878 eV for this sheet. Due to this fact that the gas mol ecules approach in various directions to the adsorbent surface, the different adsorption geometries were prepared as input files for each case. For this purpose, the single CO molecule was located at three various directions (perpendicular oriented from C head, from O head and in parallel situation with respect to the surface of the phagraphene) at an interval of 2 Å above C or doped heteroatom atom in the surface. After all the structures were permitted to entirely relax, the interactions of the CO molecule with pristine and doped phagraphene sheet were explained regarding their adsorption energies. According to the
(a)
(b)
(c)
equation (1), the adsorption energy with most negative value demon strates the strongest adsorption of CO molecule on phagraphene surface. Therefore, most energetically desirable adsorption orientations were selected for further computations. 3.1. CO adsorption on the pristine phagraphene For studying the CO adsorption on the pristine phagraphene, the different adsorption orientations of CO on the pristine phagraphene were examined. It was demonstrated that the adsorption of CO molecule on pristine phagraphene is preferred in nearly parallel orientation (Fig. 3). Table 1 shows various calculated parameters such as EHOMO, ELUMO, Eg, Ead, %ΔEg, GEDT, EF, Φ and %ΔΦ for both pristine phag raphene and CO/phagraphene complex. It can be seen that the energies of HOMO, and LUMO levels of the pristine phagraphene are about 5.4128 and 1.8250 eV, respectively. Therefore, the Eg is about 3.5878 eV. The adsorption energy for the CO/pristine phagraphene complex was obtained 2.1962 kcal/mol with calculated BSSE value 0.021 kcal/mol. Due to approximately large distance between CO molecule and pristine phagraphene (CCO … CPhagraphene ¼ 3.29 Å and OCO … CPhagraphene ¼ 3.22 Å) and partial negative adsorption energy, it can be said that the adsorption of the CO molecule on the pristine phagraphene can be considered as a weak physical adsorption. The calculated GEDT analysis for CO/pristine phagraphene shows that there is a very feeble charge transfer 0.007 e from the phagraphene to the CO molecule. Also, the MEP pattern for the adsorption of CO molecule on the pristine phagraphene is shown in Fig. 3 which demonstrates a partial overlapping between the orbitals of CO molecule and pristine phagraphene. Moreover, the calculated DOS plot for CO/pristine phagraphene shows that the electronic properties of phagraphene are not altered during the adsorption of CO molecule on the phagraphene surface. HOMO and LUMO level of energies are slightly stabilized to 5.4264 and 1.8478 eV, respectively. Therefore, the Eg value is decreased about 0.257%, insignificantly. Also, the Φ is somewhat increased (by about 0.0182 eV). In the sensors which they work on the basis of alteration in work function, there is a considerable sensitivity to value of ΔΦ because the electron emission will exponentially grow by Φ decrease [42]. However, the ΔΦ value for CO/pristine phagraphene is increased nearly 0.5021% which is not perfect. Therefore, the CO molecule has a very weak physical adsorption on the pristine phag raphene which means that pristine phagraphene has a finite capacity for adsorption of CO molecule. To improve the sensitivity of phagraphene towards CO molecule, one carbon atom of the phagraphene was substituted with various doping atoms i.e. aluminum (Al), silicon (Si) and gallium (Ga). It should be mentioned that we located the CO molecule in three different positions on the various rings (pentagonal, hexagonal, and heptagonal) of phagraphene surface [13]. After the full relaxation, it was observed that the CO molecule tends to interact with the pentagonal ring of phagraphene. So, pentagonal, hexagonal, and heptagonal junction carbon was selected as a best doping position in phagraphene [13]. 3.2. CO adsorption on the Al-doped phagraphene The effects of substitution of Al atom on the geometric and electronic properties of the phagraphene, and also on the adsorption of CO mole cule on it were studied. Fig. 4 shows that the substitution of Al atom instead of one of carbon atoms, leads almost considerable geometric distortion due to aluminum’s relatively bigger atomic radius compared with carbon atom. In the fully relaxed Al-doped phagraphene, the Al atom is projected significantly out of the phagraphene surface in order to decrease forced strain. The calculated bond lengths for Al–C bond in Aldoped phagraphene are 1.88, 1.87 and 1.84 Å which are significantly much longer than the corresponding C–C bonds in the pristine phag raphene which are 1.43, 1.42 and 1.39 Å. The MEP profile for Al-doped phagraphene is represented in Fig. 4. The remarkable negative potential
(d)
Fig. 1. The chemical structure (a), HOMO (b), LUMO (c) and molecular elec trostatic potential (MEP) (d) plot of carbon monoxide (CO) molecule. 3
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(a)
(b)
(c)
(d)
Fig. 2. The optimized structure from top view (a), side view (b), calculated molecular electrostatic potential (MEP) (c) and density of states (DOS) plot (d) of pristine phagraphene at M06-2X/6-311Gþþ(d,p) level of theory.
is distributed on the Al atom location. It can be seen from Table 2 that HOMO level destabilized slightly from 5.4128 eV to 4.8326 eV and LUMO level stabilized significantly from 1.8250 eV to 3.0811 eV after the Al-doping process. Thus Eg was calculated about 1.7515 eV which is represented also by its DOS plot. Therefore, during doping of phagraphene flake by Al atom, the HOMO-LUMO gap was closed considerably to the extent of 1.8363 eV. In the next step, we have studied the CO molecule adsorption on the Al-doped phagraphene by placing the CO above the Al atom with various orientations involving C head, O head toward Al atom and with parallel axis relative to phagraphene. After fully relaxation, it was seen that the CO molecule can be adsorbed on the phagraphene surface with O head orientation toward Al atom with 2.160 Å OCO⋯AlAl-doped phag raphene (Fig. 5). Table 2 shows that the CO/Al-doped phagraphene complex revealed the Ead of 8.1450 kcal/mol, which was higher than the calculated Ead value for CO/pristine phagraphene complex ( 2.1962 kcal/mol). This observation can be attributed to the presence of Al atom which acts as a proper Lewis acid which simplifies the charge transfer from CO molecule to Al atom in Al-doped phagraphene. This transition has been confirmed by the calculated GEDT analysis which shows þ0.118 e charge transfer from CO molecule to Al atom in Al-
doped phagraphene. The adsorption of CO molecule on the Al-doped phagraphene destabilized both HOMO and LUMO levels of energies to the extent of 0.0397 eV and 0.3998 eV, respectively. Thus, the Eg is changed from 1.7515 eV to 2.116 eV and opened by almost 20.5595% which also is shown by DOS plot (Fig. 5). The change in HOMO and LUMO levels of energies after adsorption leads to slight change in EF and Φ values which causes 5.8788 in %ΔΦ amount. 3.3. CO adsorption on the Si-doped phagraphene Moreover, a C atom in phagraphene was replaced by Si atom and the effects of substitution on the electronic, structural, and CO adsorption characteristics of the phagraphene, were studied. As expected, by replacing the C atom by Si, structure of the sheet was remarkably dis torted (Fig. 6). In the fully relaxed Si-doped phagraphene, the Si atom impurity is located out of the surface till decrease stress due to its larger radius compared to the C atom. The calculated bond length is 1.71, 1.80 and 1.81 Å for the Si–C bonds in the Si-doped phagraphene which is considerably larger than the corresponding C–C bonds in the pristine phagraphene (Fig. 6). The calculated MEP profile for Si-doped phag raphene shows a broad distribution of electrostatic potential around the 4
M. Rouhani et al.
Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
Fig. 3. The optimized structure from top and side view (a), calculated molecular electrostatic potential (MEP) profile (b) and density of states (DOS) plot (c) of CO/ pristine phagraphene complex at M06-2X/6-311Gþþ(d,p) level of theory. Table 1 The calculated EHOMO (eV), ELUMO (eV), Eg (eV), Ead (kcal/mol), %ΔEg, GEDT (e ), EF (eV), Φ (eV) and %ΔΦ for pristine phagraphene and CO/phagraphene complex. Configuration Pristine phagraphene CO/phagraphene
EHOMO 5.4128 5.4264
ELUMO 1.8250 1.8478
Eg
Ead
%ΔEg
GEDT
3.5878 3.5786
–
–
–
2.1962
Si atom (Fig. 6). Similar to Al-doped phagraphene, it can be found from Table 3 that HOMO level of energy in phagraphene destabilized a little from 5.4128 eV to 5.3463 eV and LUMO level of energy stabilized remarkably from 1.8250 eV to 2.2865 eV after the Si-doping process. In the effect of Si atom substitution, Eg value was changed from 3.5878 eV to 3.0598 eV which is shown also by its DOS plot. In the following, we have studied adsorption of CO molecule on the Si-doped phagraphene by placing the CO molecule up the Si atom with various orientations involving from C head, O head, and with parallel axis towards phagraphene surface with 2 Å distance. After fully relaxa tion, it was seen that the CO molecule preferred to be adsorbed indi rectly almost from its C head toward Si atom in Si-doped phagraphene (Fig. 7). The CCO⋯SiSi-doped phagraphene distance was calculated as 3.271 Å. The interaction of CO molecule with Si atom in the Si-doped phagraphene from electrostatic potential viewpoint is presented by MEP calculation (Fig. 7). After physical adsorption, the HOMO level of energy stabilized from 5.3463 eV to 5.3638 eV and LUMO level of energy destabilized from 2.2865 eV to 2.2321 eV and therefore increased Eg from 3.0598 eV to 3.1317 eV namely 2.3498% (Table 3).
0.257
0.007
EF 3.6189 3.6371
Φ
%ΔΦ
3.6189 3.6371
– þ0.5021
According to the GEDT analysis, the charge transfer from CO molecule to Si-doped phagraphene (þ0.034 e ) is considerably lower than that Aldoped phagraphene (þ0.118 e ) because Al atom can act as a Lewis acid site which reinforces the charge transition from CO molecule. The CO/Si-doped phagraphene complex shows Ead of 4.2043 kcal/mol, which is lower than the Ead amount for CO/Al-doped phagraphene ( 8.1450 kcal/mol) which reveals strong evidence for the weaker physical adsorption of CO molecule on Si-doped phagraphene compared with Al-doped phagraphene. Furthermore, adsorption of CO molecule on Si-doped phagraphene surface leads to a little increase in EF from 3.8164 eV to 3.7979 eV and therefore causes negligible decrease in %ΔΦ amount ( 0.4871%). 3.4. CO adsorption on the Ga-doped phagraphene According to our expectation, by substituting a C atom by a Ga atom, the spatial structure of phagraphene is considerably distorted at the doped location (Fig. 8). The Ga–C bond lengths were calculated about 1.86, 190 and 1.91 Å which are much longer than the corresponding C–C 5
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(a)
(b)
(c)
(d)
Fig. 4. The optimized structure from top view (a), side view (b), calculated molecular electrostatic potential (MEP) (c) and density of states (DOS) plot (d) of Aldoped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory. Table 2 The calculated EHOMO (eV), ELUMO (eV), Eg (eV), Ead (kcal/mol), %ΔEg, GEDT (e ), EF (eV), Φ (eV) and %ΔΦ for Al-dopoed phagraphene and CO/Al-doped phag raphene complex. Configuration Al-doped phagraphene CO/Al-doped phagraphene
EHOMO 4.8326 4.7929
ELUMO 3.0811 2.6813
Eg
Ead
%ΔEg
GEDT
1.7515 2.1116
–
– þ20.5595
– þ0.118
8.1450
bonds in the pristine phagraphene (Fig. 8). Therefore, the Ga atom was significantly projected out of the sheet. The calculations showed that the HOMO and especially LUMO levels of energies in phagraphene were remarkably altered after the Ga-doping process. The HOMO and LUMO level of energies were changed from 5.4128 eV to 4.8887 eV and from 1.8250 eV to 3.1009 eV respectively, which resulted to gap closing and change Eg value from 3.5878 eV to 1.7878 eV (see DOS profile in Fig. 7) due to the stabilizing of LUMO level of phagraphene after Ga doping (Table 4). The calculated MEP pattern for Ga-doped phagraphene showed greater distribution of electrostatic potential in the Ga-doped site (Fig. 7). The adsorption of the CO molecule on the Ga-doped phagraphene was explored in the next step (Fig. 9). After fully optimization, it was seen that the CO molecule preferred to be physically adsorbed directly from its O head toward Ga atom in Ga-doped phagraphene (Fig. 9). The
EF 3.9568 3.7371
Φ
%ΔΦ
3.9568 3.7371
–
5.8788
OCO…GaGa-doped phagraphene distance was calculated as 2.181 Å. The MEP pattern shows the efficient interaction of CO molecule with Ga atom in the Ga-doped phagraphene (Fig. 9). The changes in the various elec tronic characteristics of the phagraphene after adsorption of CO mole cule were investigated. It can be seen that the HOMO and LUMO levels of energies are both destabilized from 4.8887 eV to 4.8114 eV and from 3.1009 to 2.7162 eV, respectively. Moreover, the Eg value in Ga-doped phagraphene is increased from 1.7878 eV to 2.0952 eV after adsorption of CO molecule on phagraphene which equals to þ17.1943% increase. Fermi level of energy is destabilized from 3.9947 eV to 3.7638 eV which offers a reduction of 6.1374% in %ΔΦ. The GEDT analysis shows a considerable charge transfer þ0.092 e from CO molecule to Ga-doped phagraphene. The Ead energy 6.9653 kcal/mol shows that adsorption of CO molecule on Ga-doped phagraphene is a type of considerable physical adsorption and it can be concluded that CO 6
M. Rouhani et al.
Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
Fig. 5. The optimized structure from top and side view (a), calculated molecular electrostatic potential (MEP) profile (b) and density of states (DOS) plot (c) of CO/ Al-doped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory.
molecule has acceptable interaction with Ga-doped phagraphene surface. Using DFT calculations, the adsorption of CO molecule on the pris tine, Al, Si and Ga doped phagraphene was explored in order to detect a proper work function type sensor. The obtained results show that the CO molecule interacts mainly via its O head with heteroatom-doped phag raphene. The amount of adsorption energies for CO molecule on pris tine, Al, Si and Ga doped phagraphene are 2.1962, 8.1450, 4.2043 and 6.9653 kcal/mol, respectively. Moreover, the %ΔΦ values were calculated þ0.5021, 5.8788, 0.4871 and 6.9653%, respectively. It can be seen that the CO molecule has a very weak physical adsorption on phagraphene surface. Also, the work function of pristine phagraphene is not properly affected by CO molecule and therefore, the pristine phag raphene is not a suitable work function type sensor for CO molecule. The substitution of heteroatom instead of carbon atom, leads to significant geometric distortion and projection out of heteroatom due to its rela tively bigger atomic radius compared with carbon atom. It can be a desirable phenomenon because the projected out heteroatom is more accessible with target molecule (in this study CO molecule) for adsorption and hence, the sensitivity of the adsorbent can be increased. However, this observation is not fixed and constant for all cases because alteration of Eg value before and after adsorption of the target molecule strongly depends on changes in HOMO and LUMO levels of energy.
Table 5 shows the Eg values for pristine, Al, Si and Ga doped phag raphene and also %ΔEg values after doping and adsorption. It can be seen that the Eg values are significantly changed during doping process of Al and Ga heteroatoms ( 104.8415% and 100.6824%, respectively) which demonstrates that HOMO and LUMO levels of energy have been altered considerably. However, Si-doped phagraphene shows the high est alteration in Eg value during CO adsorption process (3.1317%). So, it is necessary to consider to the other important factor means Ead. The CO molecule is adsorbed on the Ga-doped phagraphene from its O-head with adsorption energy about 6.9653 kcal/mol. It can be deduced that the adsorption of CO molecule on Ga-doped phagraphene is not rather much to prolong the CO release from surface and is not so little to prevent effective adsorption. The work function of Ga-doped phagraphene is significantly decreased 6.1374% by CO molecule adsorption because of the destabilization of both HOMO and LUMO level of energies. It can be concluded that the Ga-doped phagraphene may be a promising electronic sensor for CO molecule with a short re covery time. It can be a work function type sensor due to the consider able change of work function upon the adsorption process. However, in order to gain more evidences about the efficiency of Ga-doped phag raphene for detecting of CO molecule, the Thermodynamic character istics and transport aspects of CO molecule adsorption on pristine and doped phagraphene was studied. 7
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(a)
(b)
(c)
(d)
Fig. 6. The optimized structure from top view (a), side view (b), calculated molecular electrostatic potential (MEP) (c) and density of states (DOS) plot (d) of Sidoped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory. Table 3 The calculated EHOMO (eV), ELUMO (eV), Eg (eV), Ead (kcal/mol), %ΔEg, GEDT (e ), EF (eV), Φ (eV) and %ΔΦ for Si-dopoed phagraphene and CO/Si-doped phagraphene complex. Configuration Si-doped phagraphene CO/Si-doped phagraphene
EHOMO 5.3463 5.3638
ELUMO 2.2865 2.2321
Eg
Ead
%ΔEg
GEDT
3.0598 3.1317
–
– þ2.3498
– þ0.034
4.2043
EF 3.8164 3.7979
Φ
%ΔΦ
3.8164 3.7979
–
0.4871
3.5. Thermodynamic characteristics of CO adsorption onto pristine and doped phagraphene
3.6. Transport aspects of CO/pristine and Al, Si, Ga-doped phagraphene complexes
In the next step, the changes in Gibbs free energy (ΔG) and enthalpy (ΔH) were calculated at ambient pressure conditions (P ¼ 1 atm and T ¼ 298.14 K) in order to evaluate the thermodynamic possibility of adsorption of the CO molecule on the pristine and doped phagraphene. The results shows that the amounts of ΔG and ΔH (ΔG, ΔH) for CO molecule adsorption on the pristine, Al, Si, and Ga-doped phagraphene are 0.11 ( 0.61), 0.82 ( 1.68), 0.53 ( 1.09), and 0.98 ( 1.89), respectively. Based on calculated values, all the adsorption processes except CO adsorption on pristine phagraphene are considerably exothermic and therefore thermodynamically desirable.
To have a more precise view into the adsorption of CO molecule on the pristine and doped phagraphene, the molecular electrostatic po tential (MEP) profiles of CO/pristine and doped phagraphene are calculated which were shown for each case earlier. It can be seen that except of CO adsorption on pristine phagraphene, a considerable overlap exist between CO gas molecule and the doped-phagraphene surface, disclosing the incidence of a considerable physical adsorption which have been confirmed by calculated interaction intervals and adsorption energies. The overlapping between orbitals and consequence charge transfers leads to significant variations in the electronic properties of the phag raphene which is useful as sensor. The gap energy (Eg) can specify the electrical conductivity (σ) according to the following relation (relation 8
M. Rouhani et al.
Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
Fig. 7. The optimized structure from top and side view (a), calculated molecular electrostatic potential (MEP) profile (b) and density of states (DOS) plot (c) of CO/ Si-doped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory.
1a) [43,44]:
σ α exp
Eg =2kB T
�
of CO molecule on Si-doped phagraphene and significantly decreases upon adsorption on Al-doped and Ga-doped phagraphene. Although Ead value of CO/Si-doped phagraphene ( 4.2043 kcal/mol) are almost high, the calculated %Eg value (þ2.3498%) emphasizing that Si-doped phagraphene has a low and unacceptable sensitivity to CO molecule. The other main factor for the assessment of the efficiency of a typical gas sensor is the recovery time of the target molecule from adsorbent surface. Recent studies demonstrated that the heating of graphene-based gas sensor in vacuum or under UV irradiation at 150 � C will cause to recover surface during 100–200 s [45]. The relation (2a) represents re covery time evaluation on the basis of adsorption energy: � τ α υ0 1 exp Ead =kB T (2a)
(1a)
In the relation (8), kB is the Boltzmann constant and T is the tem perature. It is obvious that, a small shift in the Eg value significantly changes the electrical conductivity. As mentioned earlier, the band gap of the phagraphene as an adsorbent is changed after adsorption of CO molecule. The energy band gap of pristine phagraphene (3.5878 eV) decreases slightly to 3.5786 eV after poor physical interaction with CO molecule ( 0.257%). This negligible change in the band gap of pristine phagraphene after interaction with CO molecule is due to the very weak physical interaction between them which confirms with a very small charge transfer ( 0.007 e ). However, in the case of CO molecule adsorption on the Al, Si and Ga-doped phagraphene, the energy gap values were changed from 1.7515 eV to 2.1116 eV (þ20.5595%), from 3.0598 eV to 3.1317 eV (þ2.3498%) and from 1.7878 eV to 2.0952 eV (þ17.1943%) in CO/Al-doped phagraphene, CO/Si-doped phagraphene and CO/Ga-doped phagraphene, respectively. These results are completely in agree with charge transfer values which are calculated with GEDT analysis (þ0.111, þ0.034 and þ 0.092 e , respectively). Therefore, the electrical conductivity slightly decreases after adsorption
In the relation (2), υ0 is attempt frequency, kB is the Boltzmann constant and T is the temperature. According to the relation (2), if the adsorption energy changes a little more negative, the recovery time increases, exponentially. Thus, the adsorption of CO molecule on Gadoped phagraphene (Ead ¼ 6.9653 kcal/mol) has the shortest recov ery time compared with Al-doped phagraphene (Ead ¼ 8.1450 kcal/ mol). Finally, it should be added that the modification of fullerene surface 9
M. Rouhani et al.
Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
(d)
Fig. 8. The optimized structure from top view (a), side view (b), calculated molecular electrostatic potential (MEP) (c) and density of states (DOS) plot (d) of Gadoped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory. Table 4 The calculated EHOMO (eV), ELUMO (eV), Eg (eV), Ead (kcal/mol), %ΔEg, GEDT (e ), EF (eV), Φ (eV) and %ΔΦ for Ga-dopoed phagraphene and CO/Ga-doped phag raphene complex. Configuration Ga-doped phagraphene CO/Ga-doped phagraphene
EHOMO 4.8887 4.8114
ELUMO 3.1009 2.7162
Eg
Ead
%ΔEg
GEDT
1.7878 2.0952
–
– þ17.1943
– þ0.092
6.9653
by doping impurity atoms is an effective approach to increase its sensitivity to gas molecules, experimentally [42,46]. The different strategies also have been employed for improving the sensitivity of graphene to different gaseous species. Graphene and graphene like surfaces have been investigated for the construction of gas sensors. Yuan and Shi [47] have discussed about the recent progresses in the synthesis of graphene materials for sensing purposes and the related techniques. All these findings demonstrate that the Ga-doped phagraphene can be a promising nanosensor for CO detection due to its considerable conduc tance variations, modest Ead, and acceptable recovery time. Therefore, we think that Ga-doped phagraphene can show a great potential for detection of gas molecules and its ability for detection of other toxic molecules in air such as NO, NO2, SO2 … should be studied in coming
EF 3.9948 3.7638
Φ
%ΔΦ
3.9948 3.7638
–
6.1374
investigations. So, we can be very hopeful that our proposed Ga-doped phagraphene could be experimentally synthesized and after optimiza tion of morphologies of sensing layers and configurations of sensing surfaces, it could be introduced as an effective CO gas sensor. 4. Conclusions Using DFT calculations, the adsorption process of CO molecule on the pristine and Al, Si and Ga-doped phagraphene was studied to discover a work function type sensor. The obtained data demonstrate that the CO molecule is adsorbed on the Ga-doped phagraphene from its O-head with releasing an energy about 6.9653 kcal/mol. The work function of Ga-doped phagraphene is considerably affected by CO molecule 10
M. Rouhani et al.
Physica E: Low-dimensional Systems and Nanostructures 116 (2020) 113710
(a)
(b)
(c)
Fig. 9. The optimized structure from top and side view (a), calculated molecular electrostatic potential (MEP) profile (b) and density of states (DOS) plot (c) of CO/ Ga-doped phagraphene at M06-2X/6-311Gþþ(d,p) level of theory. Table 5 The calculated Eg (eV) and %ΔEg for pristine, Al, Si and Ga-dopoed phagraphene. Values Eg %ΔEg (after doping)a %ΔEg (after adsorption)b a b
Adsorbent Pristine phagraphene
Al-doped phagraphene
Si-doped phagraphene
Ga-doped phagraphene
3.5878 – 3.5786
1.7515 104.8415 2.1116
3.0598 17.2560 3.1317
1.7878 100.6824 2.0952
The values are calculated compared with pristine phagraphene. The values are calculated compared with pure adsorbent.
( 6.1374) and therefore, Ga-doped phagraphene is an appropriate work function type sensor. It was deduced that the Ga-doped phagraphene may be a promising work function type sensor for CO molecule with a short recovery time.
Ethical statement/conflict of interest We have no conflicts of interest to declare. Acknowledgments The authors thank, Islamic Azad University, Science and Research 11
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Branch for the support and guidance.
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