INTRODUCTION

Ethylene oxide (EO) is the smallest cyclic ether, commonly used in all industries such as oilfield, detergent, cosmetic, polyurethane-based foams, and pharmaceuticals [1]. This gas is colorless, flammable, and highly reactive at the environment and temperature room, having a slight sweet odor [2]. So far, the dangerous health effects of occupational EO exposure, especially corneal burns, eyes damage, skin irritation and allergy, lack of sensory neuropathy, and bloody damage, have been extensively studied [3]. Exposure to EO also can result in dizziness, nausea, headache, convulsions, and blisters and might supply upward thrust to vomiting and coughing. Additionally, both animal and human studies have demonstrated that EO gas acts as a carcinogen factor that can cause leukemia and other sorts of cancers. Consequently, to keep the health of humans, the surrounding of humans and the environment according to green chemistry and avoid toxic gas, the detection of EO gas inside the environment and the atmosphere using cost-effective, proper action and sensitive gas sensors seem to be an essential issue and title to investigation and appear novel sensors [4, 5].

Due to their small size, down power and strength need, excessive finding sensitivity, and cost-effective production, the recent sensitive solid-state sensors are widespread devices for detecting toxic and hazardous gases in the environment [6]. In recent years, many reports on detecting toxic and reactive gases by solid-state gas sensors have been published in [7–9]. In the last decade, exploring a few new generations of nanomaterials has opened new perceptions into the attractive properties of their many capability applications, starting from electronics to chemical sensors [10, 12]. Among them, graphene having a two-dimensional network of carbon atoms with the sp2 hybrid has possessed significant attention in many research fields. Due to its many exciting properties, such as large specific surface region and specific electrical characteristics (more mobility with surprisingly less electrical noise), a wide range of experimental and theoretical studies on the properties of pristine and functionalized graphene has been reported in scientific journals [12–15]. According to the experimental and theoretical investigations and reports, it is shown that the sensitivity of gas sensors based on graphene has been significantly improved by the advent of defects and dopants on pristine graphene (PG) [16]. Recently, we have investigated the effect of external electric fields (EFs) on the adsorption of EO on pristine and Al-doped coronene [17–19]. It was shown that the applied EFs could enormously improve the potential application of coronene-based sensors to detect EO in trace amounts [20]. Graphene doped with B, N, and Si dopants has shown strong binding with most typical gases such as carbon dioxide, nitrogen dioxide, ammonia, and dinitrogen [21–31].

The purpose of this research is to obtain fundamental insights into the effect of adsorbed EO molecules on the electrical properties of PG, silicon- (SiG), aluminum- (AlG), and boron-doped graphenes (BG), and how these new characteristics are having the application potential for the fabrication of gas sensing devices with higher sensitivity. In this study, some possible orientations of the EO toward the PG and graphene doped systems were taken into considered. Here, several theoretical parameters such as the adsorption energies (Ead), chemical potential (μ), electrophilicity index (ω), band-gap energy (Eg), maximum amount of electronic charge (ΔNmax), density of electron state (DOS), partial density of state (PDOS), Mulliken charge transfer, molecular electrostatic potential (MEP) maps, global hardness (η), and softness (S), were analyzed and compared. Finally, it was concluded that the electronic properties of SiG and AlG, contrary to PG and BG, strongly modified in the presence of EO.

COMPUTATIONAL METHODS AND DETAILS

Selected graphene includes 1000 and 22 atoms (26 hydrogen and 96 carbon atoms) with zigzag sides, and the sides saturated with hydrogen have been shown in Fig. 1a. All C–C bond lengths and C–C–C angles were calculated after relaxations (Table 1) that are in compromise settlement with formerly suggested values [32, 33]. All calculations were accomplished with the Gaussian 09 program package [34]. Density functional theory (DFT)/X3LYP (extended hybrid functional combined with Lee–Yang–Parr) technical method with the 6-31G(d) basis set were used for optimizing geometry and energy of single-point calculation of PG and doped graphene structures; also all analysis was in the gas phase. Also, the X3LYP functional is an extended functional, which significantly betters the precision for van der Waals interaction calculations with respect to general used B3LYP functional [20, 35].

Fig. 1.
figure 1

Optimized geometries of PG (a), SiG (b), AlG (c), and BG (d).

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Table 1.   Bond lengths and angles around C1, C2, C3, and X (X = C, Si, Al, B) in PG, SiG, AlG, and BG
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The Ead of EO molecule on the PG, SiG, AlG, and BG was computed by the following equation:

$${{E}_{{{\text{ad}}}}} = {{E}_{{{\text{(EO}} + {\text{adsorbent)}}}}} - {\text{(}}{{E}_{{{\text{adsorbent}}}}} + {{E}_{{{\text{EO}}}}}{\text{)}} + {{{{\delta }}}_{{{\text{BSSE}}}}}{\text{,}}$$
(1)

where \({{E}_{{{\text{(EO}} + {\text{adsorbent)}}}}}\), Eadsorbent, EEO are the total energy of the adsorbed complex systems, the energy of isolated adsorbents (i.e., PG, SiG, AlG, and BG), and the energy of the free EO molecules, respectively, basis set superposition error (BSSE) was calculated to improve Ead, casting off basis functions overlap effects. The transfer of charge between the EO molecules and the adsorbent surfaces was computed through Mulliken population analysis from the difference of concentration of charge on EO earlier than and after adsorption.

Finally, from the optimized structures, ΔNmax, η, S, μ, and ω of the investigated systems were calculated. In an N-electron system, having general energy of E and external potential of \(\nu (r)\), μ is given by [36]:

$${{\mu }} = {{\left( {\frac{{\partial E}}{{\partial N}}} \right)}_{{\nu (r)}}}{\text{.}}$$
(2)

Similarly, η is represented in terms of the second derivative of energy for the potential external \(\nu (r)\) and is obtained by the following equation [37]:

$${{\eta }} = {{\left( {\frac{{{{\partial }^{{\text{2}}}}E}}{{\partial {{N}^{{\text{2}}}}}}} \right)}_{{\nu (r)}}},$$
(3)

η and μ can be approximated as the following formulation [9, 36]:

$${{\mu }} = \left( {\frac{{ - (I + A)}}{2}} \right){\text{,}}$$
(4)
$${{\eta }} = \left( {\frac{{I - A}}{2}} \right){\text{,}}$$
(5)

where A and I are electron affinity of the molecule and the ionization potential, respectively, the frontier molecular orbitals (HOMO to LUMO) near is suggested by Koopmans’s theorem, for closed-shell systems is very proper in describing chemical reactivity and stability of molecules based on the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Demonstrate the ionization potential of the molecule and the amount of electron affinity with the HOMO and LUMO orbital energies, respectively. With using the Koopmans’ theorem, A and I values can be correlated with the frontier orbitals by the following relation [38–41]:

$$I = {{-}}{{E}_{{{\text{HOMO}}}}}{\text{,}}$$
(6)
$$A = {{-}}{{E}_{{{\text{LUMO}}}}}{\text{.}}$$
(7)

The Eg, ω, S, and ΔNmax can be obtained from equations [9, 42].

$${{E}_{{\text{g}}}} = {{E}_{{{\text{LUMO}}}}} - {{E}_{{{\text{HOMO}}}}}{\text{,}}$$
(8)
$$\omega = \frac{{{{\mu }^{2}}}}{{2\eta }}{\text{,}}$$
(9)
$$S = \frac{1}{{2\eta }}{\text{,}}$$
(10)
$$\Delta {{N}_{{{\text{max}}}}} = - \frac{\mu }{\eta }{\text{.}}$$
(11)

Therefore, at the same time as the amount of ω displays the tendency of the system to obtain additional electronic charge from the environment, the amount of ΔNmax characterizes the charge valence of the molecule [43].

RESULTS AND DISCUSSION

The sensitivity of PG, SiG, AlG, and BG surfaces for adsorption of EO molecules were investigated by the DFT calculations, confirming that the chemical behavior of graphene can be significantly affected by doping of metal [44]. The optimized structures of the doped graphenes were obtained, as shown in Fig. 1. In contrast, for doping with B, when a carbon atom in the flat planes of graphene is substituted with Si or Al atoms, a profound evolution has been shown in the SiG and AlG planar structure, which may be due to the bigger size of Al and Si compared with the C atom. The graphene structure deformation, which can be seen near the doping positions of the Al and Si atoms, is painted in Figs. 1b and 1c. The computed angles and bond lengths of the adsorption systems have been shown in Table 1 [45].

Structure Optimization and Geometry

To put and assess the optimum adsorption configurations, the EO molecule was firstly situated at different positions at the top surface of the graphene in different points. After complete relaxation, the optimal configurations, achieved from the other preliminary states have been inspected to perceive the superlative energetically stable state. The most stable configurations of EO, after absorption on the PG, SiG, AlG, and BG are summarized in Fig. 2.

Fig. 2.
figure 2

Optimized geometries of EO/PG (a), EO/SiG (b), EO/AlG (c), and EO/BG (d).

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Also, to achieve the best desirable adsorption configurations on the various graphene doped systems, equilibrium distances between graphene and EO molecule (i.e., the center to the center nearest distance of atoms between EO gas and pristine and doped graphenes), Eg, Ead, molecular electronic parameters and the transfer of charge (Mulliken charge) were computed and noted in Table 2.

Table 2.   Ead, δBSSE, EHOMO, ELUMO, Eg, μ, η, S, ω, ΔNmax for PG, EO/PG, SiG, EO/SiG, AlG, EO/AlG, BG, and EO/BG. Charge transfer from the PG, SiG, AlG, and BG to the EO. The nearest atom-to-atom distance between the O and X (X = C, Si, Al, B) in EO and PG, SiG, AlG, BG
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Adsorption of EO on PG, SiG, AlG, and BG

The measured amounts of Ead of EO on PG, SiG, AlG, and BG have been indicated in Table 2. Under the achieved amounts, as expressed in Table 2, the negative or positive values of the Ead of the systems are related to an exothermic or endothermic adsorption mechanism, respectively. The calculated and measured Ead for EO on PG, SiG, AlG, and BG were obtained as 0.54, –6.07, –24.55, and 0.50 kcal mol–1, respectively. Under the achieved results presented in Table 2, the Ead is negative for SiG and EO/AlG, which demonstrate an exothermic adsorption process.

The least atom to atom distance between the Al and O in EO/AlG system was obtained to be 1.9651 Å and is much shorter than that of the other three systems. The distance between the Si and O in EO/SIG system was obtained to be 1.9981 Å. The results showed that there is a covalent bonding between the O group of EO and Al and Si atoms in the AlG and SiG systems. The atom-to-atom distance between the C and O atoms in EO/PG, B and O atoms in EO/BG systems were calculated to be 3.5042 and 3.5458 Å, respectively. The acquired outcomes of the distance between atoms displayed that the EO molecule undergoes weak physical adsorption (Van der Waals force and interaction) on the PG and BG systems (Table 2 and Fig. 2), justifying the higher Ead.

When nucleophilic or electrophilic molecular types are adsorbed on the absorbent surface, a charge transfer between the electron-donor-acceptor complexes is expected. For EO adsorption on SiG and AlG surfaces, the computed charge transfer by Mulliken population analysis was –0.23 e and –0.22 e, respectively, stating the charge transfer from SiG and AlG to the EO molecule. For the EO/PG complex system, a meager charge (0.01 e) turned into transferred from the EO molecule to the PG surface, and for EO/BG complex system, however, there is no charge transfer. According to the obtained Ead and charge transfer results, the weak adsorption of EO molecules on PG and BG surfaces with low Ead indicates an insufficient charge transfer.

MEP Maps

For additional study, MEP maps of PG, AlG, and EO/AlG were computed, and outcomes are proven in Fig. 3. The bright area in MEP maps mentions several electrons, which is an active space usable for chemical reactions. The graphical results displayed that the positive charge of the EO molecule works as an electron-donor (blue color in Fig. 3) and PG, AlG works as an electron-acceptor (red color in Fig. 3).

Fig. 3.
figure 3

MEP plots of PG (a), AlG (b), and EO adsorption on AlG (c). Color ranges, in a.u.: blue, more positive than 0.0015; green, between 0.0015 and 0; yellow, between 0 and –0.0255; red, more negative than –0.0255. (For interpretation of the references to color in the artwork, the reader is referred to the web version of the article.)

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Molecular Orbital Analysis (MO)

The energy discord between the nucleophile agent of the EO molecule (HOMO) and the electrophile agent of the pristine and doped graphenes (LUMO) was taken, and the HOMO to LUMO interactions was computed and considered.

The measured LUMO and HOMO energies of the different models are plotted in Table 2 and Fig. 4. It can be computed that doping operations are forcefully dependent on all the molecular orbital energy interval between the LUMO and HOMO molecular orbitals. According to the LUMO and HOMO molecular orbitals of PG, AlG, and EO/AlG as displayed in Fig. 4, the total density of LUMO and HOMO orbitals are localized at the end layers of PG, while in AlG and EO/AlG, they were considered near the Al atom. It was also shown that the HOMO energy level of the doped graphenes was more than PG, whereas their LUMO energy level was less than PG. Furthermore, it was shown that the LUMO and HOMO energies of the adsorbents reduce after EO adsorption.

Fig. 4.
figure 4

The HOMO and LUMO profiles of PG ((a) is HOMO and (b) is LUMO); AlG ((c) is HOMO and (d) is LUMO) and EO adsorption on AlG ((e) is HOMO and (f) is LUMO).

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Quantum Molecular Analysis (QM)

The QM of reactivity in the frame of the DFT for EO adsorption on PG and doped graphenes were illustrated in Table 2. There exists an inverse communication between η and S. An enhancement in the hardness results in a reduction in softness which induces the enhancement of the consistency and decline of reactivity.

According to the obtained results of η, when EO was adsorbed on pristine and doped graphenes, the η amount of all the reviewed systems reduced appreciably, increasing the system reactivity content with other works in the study and literature [46]. The calculated μ were in the range of –3.31 to –3.56 eV. It was evident that EO adsorptions on PG and doped graphenes have led to a change in field emission currents. However, as shown in Table 2, the μ value of the pristine graphene changed slightly upon the adsorption of the EO gas molecule. The obtained ΔNmax was in the range 3.96 to 22.19 eV. The positive values of ΔNmax confirm that in all the models, the EO molecule acts as an electron donor. The calculated ΔNmax values are in qualified compromise with results gained from the HOMO to LUMO investigation and MEP maps.

DOS and PDOS Analysis

The obtained DOS and PDOS of the EO molecule, adsorbed on PG and doped graphene systems, are provided in Figs. 5 and 6, respectively. As proven in Fig. 5, the shapes of DOS graphs are nearly analogous to each other. Considering the critical role of Eg on the electronic and conductivity confidants of the studied systems, the Eg was calculated via HOMO, and LUMO energies and the data are shown in Table 2. The total charge density analysis illustrated that Eg in EO adsorption on PG and doped graphenes did not significantly change. The PDOSs of the B, Al, Si, C, and O atoms in EO/BG, EO/AlG, EO/SiG, and EO/PG complexes systems (s, p orbitals of O, B, Al, Si, and C) across the Fermi level were drawn in Fig. 6. The PDOS of the O, B, and C atoms in EO/BG and EO/PG complexes systems (s and p orbitals of O, B, and C) across the Fermi level have been proven in Figs. 6a and 6d. Based on the received outcomes, only p orbitals of B and C atoms have been placed across the Fermi level and have no overlap with the p orbital of the O atom. The PDOS of the O, Al, and Si atoms in EO/AlG and EO/SiG complexes systems (s and p orbitals of O, Al, and Si) across the Fermi level has been proven in Figs. 6b and 6c. Regarding received outcomes, the p orbital of Al and Si atoms suggests strong overlap with the p orbital of O across the Fermi level. This demonstrates that EO can be strongly hybridized with Al and Si atoms.

Fig. 5.
figure 5

(a–h) the DOS projected onto PG, EO/PG, SiG, EO/SiG, AlG, EO/AlG, BG, and EO/BG, respectively.

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Fig. 6.
figure 6

(a–d) the PDOS projected onto EO/PG, EO/SiG, EO/AlG, and EO/BG, respectively.

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Conclusively, based on the fixed results of Mulliken charge, Ead, atom to atom distance, the MEP maps, molecular electronic parameters, DOS, PDOS, and HOMO and LUMO orbital analysis, AlG and SiG can be explained as potential materials for the structure of sensitive solid-state gas sensors for EO tracing.

CONCLUSIONS

In the present research, the adsorption properties of EO gas molecules on PG, SiG, AlG, and BG were inspected using the DFT computational method. Due to its small Ead, large bonding distance and small net charge transfer between the PG and EO molecules, it was shown that the adsorption of EO molecules on PG was very poor. To enhance the adsorption capacity, sensitivity and conductivity of graphene, Si, Al, and B atoms were doped into graphene by substituting C atoms. The Ead value of the EO/SiG and EO/AlG systems (chemisorption) was significantly higher than that of the EO/BG and EO/PG systems (physisorption), which is consistent with the calculated results of the DOSs, PDOSs, MEP, QM, and MO analyses. Considering the excessive binding energies and brief binding distances of the adsorbed EO molecules on the active surfaces of SiG and AlG, it can be derived that EO molecules were bonded on SiG and AlG surfaces; also, the type of this bond was chemical. Furthermore, it was shown that the interaction of EO molecules has no significant agents on the electronic properties of PG. In contrast the action and reaction between EO molecules and the doped graphenes, especially SiG and AlG, revealed apparent modifications in the HOMO to LUMO gap of energy, indicating their electrical conductivity change. Therefore, based on the obtained results, AlG and SiG with a high surface-to-volume ratio (S/V) maybe both be considered as sensitive elements in the fabrication of electrochemical gas detection devices. However, we recommend AlG for developing sensitive solid-state gas sensors, especially for monitoring EO gas molecules.