Abstract
Density functional theory (DFT) studies at B3LYP/6-31G (d) (Becke, 3-parameter, Lee-Yang-Parr) level were performed to evaluate adsorption interactions between ethylene oxide (EO) molecule, and pristine and transition metals (TM) (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) doped ZnO nanocluster (TM-doped Zn12O12). The adsorption energy (Ead), band gap energy (Eg), Mulliken charge transfer (QT) and molecular electrostatic potential (MEP) were calculated to examine the sensitivity of the Zn12O12 and its TM-doped forms toward EO detection. It was found that in contrast to the pristine Zn12O12, the electronic properties of TM-doped Zn12O12 were sharply sensitive to the presence of EO gas molecules. The results revealed that among the studied TM-doped Zn12O12, Cr- and V-doped Zn12O12 have great potential applicability as EO sensor, due to their highest Eg change (ΔEg) values, after the EO adsorption. Moreover, the density of state (DOS) calculations confirmed that strong electronic interaction between Cr- and V-doped Zn12O12 and EO molecules can makes them interesting empirical candidate for detection and adsorptive removal of EO gas molecules.
1 Introduction
Ethylene oxide (EO) is a colorless, flammable and highly reactive gas at the ambient temperature, having a slightly sweet odor (Bashir et al., 2016). EO (C2H4O) is the smallest cyclic ether which has been utilized in some industrial production processes such as solvents, antifreeze, pesticide, sterilization, textiles and pharmaceuticals (Rebsdat and Mayer, 2001). Exposure to toxic gas EO molecule may cause to dizziness, convulsions, cataracts, skin allergy, central neuropathy, and cancer (Steenland et al., 1991). Consequently, to protect human health and the environment purpose, detection of EO in the atmosphere using low cost, easy operation and sensitive gas sensors seems to be an important issue to research.
Due to their small size, low-power need, high detection sensitivity and low cost production, the recent sensitive solid state sensors are widespread devices for detection of hazardous and toxic gases in the environment (Capone et al., 2003). During the recent years, the low sensitivity of bulk materials in sensor fabrication processes has been overcome by developing exploring some new-generation of nanomaterials with the unique properties and many potential applications, ranging from electronics to chemical sensors (Afshari and Mohsennia, 2018; Beheshtian et al., 2012a; Mahdavian, 2012; Mohsennia et al., 2018; Rakhshi et al., 2018; Rezaei-Sameti and Yaghoobi, 2015; Yoon et al., 2011). Metal oxides, such as MgO, ZnO, BeO, WO3, SnO2, and TiO2, are commonly employed in chemical gas sensors, changing the electrical conductivity of these materials upon adsorption of gaseous molecules (Beheshtian et al., 2012b; Beheshtian et al., 2013a; Degler et al., 2016; Epifani et al., 2016; Kakemam and Peyghan, 2013; Kerkez and Boz, 2014; Kim et al., 2015; Peyghan and Noei, 2014; Peyghan and Yourdkhani 2014; Samadizadeh et al., 2015; Yang and Yao, 2000; Ziat et al., 2014;).
The ZnO is a semi-conductive metal oxide with a wide direct band gap which has high prospects in optoelectronics, especially because of its high excitation binding energy (Duan et al., 2006). ZnO nanostructured materials including nanowires, nanotubes, nanoclusters, and nanoparticles have been frequently probed as sensors for different gases including such as F2, NH3, CH4, O2, NO and CO (Lahmer, 2016; Peyghan and Noei, 2013; Peyghan et al., 2014; Peyghan et al., 2015). Because of low sensitivity of the substrate materials for sensing applications, different methods such as metal doping, non-metal doping and vacancy-defect have been developed to manipulate their geometry to enhance the electron transport and electronic properties (Alver et al., 2018; Aslanzadeh, 2016; Gong et al., 2006; Hadipour et al., 2015; Özgür et al., 2005; Sahay and Nath, 2008; Zhu et al., 2012). Among the various doping substances and strategies, the incorporation of few percent of transition metal (TM) ions into semiconducting oxide materials has shown the enhancement of both electronic and magnetic properties due to oxygen vacancies and created interstitial vacancies at host lattice positions (Ivanov et al., 2004; Kaushik et al., 2013; Marcel et al., 2005).
In this study, first-principles density functional theory (DFT) calculations have been performed on the electronic structure of TM-doped Zn12O12 (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) to evaluate the effect of metal dopants on the geometric structure and electronic structure of Zn12O12 to explore behavior of pristine and TM-doped Zn12O12 in the presence of EO molecules. Here, some theoretical parameters such as adsorption energy (Ead), band gap energy (Eg), global hardness (η), Mulliken charge transfer (QT), molecular electrostatic potential (MEP) and density of electron state (DOS) were analyzed and compared. On the basis of the calculated results of EO/TM-doped Zn12O12 adsorbed complexes, we theoretically predicted that Cr- and V-doped Zn12O12 are suitable candidates for design and fabrication of sensing devices for detection of EO gas molecules, even in low concentration.
2 Computational studies
All the calculations were performed using Gaussian 09 program package (Frisch et al., 2013) at the level of DFT with B3LYP/6-31G (d) (Becke, 3-parameter, Lee-Yang-Parr) (Becke, 1988; Lee et al., 1988). For Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu atoms, the standard LANL2DZ (Los Alamos National Laboratory (LANL) including double-zeta (DZ)) basis set was used. Convergence of calculations was checked by comparison of the maximum energy and maximum displacement with the corresponding threshold values (Ha and (1 × 10―6 Ha and 6 × 10―4 Å, respectively). The DOS for all the studied structures was plotted by the Gauss Sum 2.1.4 program (O’boyle et al., 2008). The Ead is defined as follows:
where E(EO/nanocluster), Enanocluster and EEO are respectively correspond to energy of the adsorbed complexes, Zn12O12 nanocluster and EO molecule. The obtained negative values of Ead indicated exothermic nature of the adsorption process for all the studied adsorbed complex systems. The calculated Ead values have been corrected by the basis set superposition error (BSSE) using the counter-poise (CP) correction (Boys and Bernardi, 1970). The charge transfer between EO molecule and the adsorbents (i.e., pristine and TM-doped Zn12O12) was calculated by Mulliken charge analysis from the difference of charge concentration on EO before and after the adsorption.
The band-gap energy (Eg = ELUMO − EHOMO = − (A− I)) and the global hardness (η = (I−A)/2)), where LUMO, HOMO, A and I refer respectively to the lowest unoccupied molecular orbital, highest occupied molecular orbital, electron affinity of molecule and ionization potential (Chattaraj et al., 2006; Koopmans, 1934; Phillips, 1961). The sensitivity of the pristine and TM-doped Zn12O12 for detection of EO was evaluated by calculation of % in following form:
where Eg1 and Eg2 are Eg values before and after adsorption of EO molecules.
3 Results and discussion
3.1 EO adsorption on pristine Zn12O12
As shown in Figure 1, the structural geometry of pristine Zn12O12 has been made of eight hexagons (B66) and six tetragons (B46) with tetrahedral rotational symmetry with inversion symmetry. The calculated average bond lengths of the B66 (D1) and B46 (D2) were about 1.85 and 1.93 Å, respectively (Table 1). The angles in tetragons (A1, A2) and hexagons (A3, A4) were 91.97, 86.92, 126.41 and 112.37 degree, respectively. Recently, the optimized structure of Zn12O12 using DFT/B3LYP with 6-31G (d) for oxygen and LANL2DZ for zinc showed that the distances D1 and D2 were 1.91 and 1.99 Å, respectively (de Oliveira et al., 2015). The calculated A1, A2, A3 and A4 angles in this work were obtained to be 90.8, 88.9, 123.6 and 116.2 degree, respectively. From the comparison, we concluded that the results are in good agreement with the calculated data reported in the literature (de Oliveira et al., 2015).
(a) Optimized structure of Zn12O12, D and A means distance and angle, respectively; (b) Optimized structure of EO/Zn12O12; (c) DOS, HOMO and LUMO profiles of pristine Zn12O12; (d) DOS, HOMO and LUMO profiles of EO/Zn12O12; (e) Calculated electrostatic potential on the molecular surface of Zn12O12. Color ranges, in a.u.: blue, more positive than 0.0054; green, between 0.0054 and 0; yellow, between 0 and -0.0206; red, more negative than -0.0206.
The bond lengths of the B66 (D1) and B46 (D2) and the angles A1, A2, A3 and A4 in the surface of pristine and TM-doped Zn12O12.
| System | D1(Å) | D2(Å) | A1(deg) | A2(deg) | A3(deg) | A4(deg) |
|---|---|---|---|---|---|---|
| Zn12O12 | 1.85 | 1.93 | 91.97 | 86.92 | 126.41 | 112.37 |
| Sc-doped Zn12O12 | 1.94 | 1.98 | 91.91 | 89.07 | 123.69 | 121.27 |
| Ti-doped Zn12O12 | 1.84 | 1.86 | 90.59 | 88.38 | 129.32 | 104.57 |
| V-doped Zn12O12 | 1.78 | 1.85 | 93.14 | 87.07 | 127.62 | 106.08 |
| Cr-doped Zn12O12 | 1.76 | 1.87 | 90.05 | 88.77 | 129.69 | 103.99 |
| Mn-doped Zn12O12 | 1.81 | 1.89 | 90.20 | 89.61 | 128.55 | 103.82 |
| Fe-doped Zn12O12 | 1.81 | 1.87 | 90.50 | 89.88 | 127.15 | 118.11 |
| Co-doped Zn12O12 | 1.82 | 1.86 | 90.71 | 88.64 | 127.75 | 107.21 |
| Ni-doped Zn12O12 | 1.86 | 1.89 | 90.61 | 89.44 | 127.39 | 106.80 |
| Cu-doped Zn12O12 | 1.85 | 1.89 | 90.78 | 88.49 | 126.87 | 107.18 |
For better understanding of the effect of EO adsorption on the electronic property of Zn12O12, DOS plots of the EO/Zn12O12 adsorbed complex were calculated and analyzed. The energy values of HOMO and LUMO were obtained to be -6.99 and -2.86 eV, resulting in Eg = 4.13 eV. In accordance with the DOS plots as shown in Figure 1, the HOMO and LUMO respectively are more localized on the O and Zn atoms. Therefore, it is clear that the Zn atoms are suitable sites for adsorption of EO molecules. On the other hand, in the most stable structure of EO/Zn12O12 adsorbed complex, O atom of EO molecule was attracted by Zn atom of Zn12O12. As shown by the MEP of Zn12O12 in Figure 1, it can be seen that the negative regions above the O atoms (red colors) are stronger than the positive ones of the Zn atoms (blue colors).
The optimized geometry structure of EO/Zn12O12 adsorbed complex is shown in Figure 1. The newly formed O–Zn bond and calculated adsorption energy were about 2.81 Å and -16.04 kcal/mol, respectively. The interaction between EO and Zn12O12 surface showed a charge transfer of -0.14 e from Zn12O12 to EO molecule. As show in Table 2, after the adsorption of EO molecule on Zn12O12, its electronic properties including HOMO, LUMO, and Eg were not significantly changed (4.13 to 4.15 eV). Therefore, it can be concluded that pristine Zn12O12 is not notably sensitive to the adsorption and detection of EO molecules.
The calculated Ead (kcal/mol), HOMO, LUMO, Ef and Eg (eV) for the studied systems. The average total Mulliken charge transfer from the adsorbents to EO molecule (QT). The nearest atom-to-atom distance between the O atom of EO, X (X = Zn, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) of pristine and TM-doped Zn12O12 (Å) and their Eg change (ΔEg) values after the EO adsorption.
| System | Ead | dX–O (Å) | QT(e) | EHOMO | ELUMO | Eg | Ef | η | ΔEg% |
|---|---|---|---|---|---|---|---|---|---|
| Zn12O12 | – | – | – | -6.99 | -2.86 | 4.13 | -4.93 | 2.07 | – |
| EO/Zn12O12 | -16.04 | 2.81 | -0.14 | -6.67 | -2.52 | 4.15 | -4.60 | 2.08 | 0.48 |
| Sc-doped Zn12O12 | – | – | – | -3.65 | -2.52 | 1.13 | -3.09 | 0.57 | – |
| EO/Sc-doped Zn12O12 | -19.92 | 2.23 | -0.15 | -3.68 | -2.51 | 1.17 | -3.10 | 0.59 | 3.53 |
| Ti-doped Zn12O12 | – | – | – | -4.05 | -2.83 | 1.22 | -3.44 | 0.61 | – |
| EO/Ti-doped Zn12O12 | -20.55 | 2.22 | -0.16 | -4.00 | -2.74 | 1.26 | -3.37 | 0.63 | 3.28 |
| V-doped Zn12O12 | – | – | – | -5.20 | -2.88 | 2.32 | -4.04 | 1.16 | – |
| EO/V-doped Zn12O12 | -36.65 | 2.00 | -0.18 | -5.62 | -2.64 | 2.98 | -4.13 | 1.49 | 28.45 |
| Cr-doped Zn12O12 | – | – | – | -3.99 | -2.79 | 1.20 | -3.39 | 0.60 | – |
| EO/Cr-doped Zn12O12 | -60.04 | 1.97 | -0.19 | -3.83 | -2.12 | 1.71 | -2.98 | 0.86 | 42.50 |
| Mn-doped Zn12O12 | – | – | – | -5.03 | -2.67 | 2.36 | -3.85 | 1.18 | – |
| EO/Mn-doped Zn12O12 | -31.91 | 2.01 | -0.15 | -4.99 | -2.51 | 2.48 | -3.75 | 1.24 | 5.08 |
| Fe-doped Zn12O12 | – | – | – | -4.92 | -2.53 | 2.39 | -3.73 | 1.20 | – |
| EO/Fe-doped Zn12O12 | -35.98 | 2.00 | -0.16 | -5.18 | -2.38 | 2.80 | -3.78 | 1.40 | 17.15 |
| Co-doped Zn12O12 | – | – | – | -5.71 | -2.34 | 3.37 | -4.03 | 1.69 | – |
| EO/Co-doped Zn12O12 | -31.11 | 2.02 | -0.15 | -5.82 | -2.27 | 3.55 | -4.05 | 1.78 | 5.34 |
| Ni-doped Zn12O12 | – | – | – | -5.75 | -2.71 | 3.04 | -4.23 | 1.52 | – |
| EO/Ni-doped Zn12O12 | -33.31 | 2.01 | -0.17 | -5.54 | -2.33 | 3.21 | -3.94 | 1.61 | 5.59 |
| Cu-doped Zn12O12 | – | – | – | -6.48 | -2.78 | 3.70 | -4.63 | 1.85 | – |
| EO/Cu-doped Zn12O12 | -29.21 | 2.05 | -0.16 | -6.46 | -2.64 | 3.82 | -4.55 | 1.91 | 3.24 |
3.2 EO adsorption on the TM-doped Zn12O12
To tailor the sensitivity and sensing range of doped Zn12O12 toward EO molecule, a Zn atom was replaced by different TM including Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The optimized structures are shown in Figure 2. The geometrical parameters (i.e., bond lengths and angles) of the TM-doped Zn12O12 showed in the Table 1.
Optimized structures of TM-doped Zn12O12 and their adsorbed complexes with EO molecule. Distances are in Å.
After the doping process, the HOMO level was significantly shifted to higher energies, and consequently, the Eg was considerably narrowed. As shown in Table 2, in the case of Cr- and V-doped Zn12O12, the HOMO was increased from -6.99 to -3.99 and -5.20 eV, narrowing the Eg by about 2.93 and 1.81 eV, respectively. Similar trend was found for the other TM-doped Zn12O12. As shown, upon doping of the TM, Eg values of Zn12O12 were decreased from 4.13 eV in pristine Zn12O12 to 3.70-1.13 eV in the doped forms. The molecules having small Eg value and excitation energy which are known as soft molecules are more polarizable. Therefore, the soft molecules show electron density changes more easily than hard molecules with high Eg value, leading to higher chemical reactivity. According to the calculated η, the order of systems reactivity of the studied systems was as follows:
Sc-doped Zn12O12 > Cr-doped Zn12O12 > Ti-doped Zn12O12 > V-doped Zn12O12 > Mn-doped Zn12O12 > Fe-doped Zn12O12 > Ni-doped Zn12O12 > Co-doped Zn12O12 > Cu-doped Zn12O12 > Zn12O12.
The optimized structures of EO molecule adsorbed on the surface of TM-doped Zn12O12 from its O atom are shown in Figure 2. In order to consider the influence of doping on the electronic properties of Zn12O12, the DOS plots of Cr-doped Zn12O12, V-doped Zn12O12, EO/Cr-doped Zn12O12 and EO/V-doped Zn12O12 were calculated and the results of Cr- and V-doped Zn12O12 and respective adsorbed complex were presented in Figure 3. As shown in this figure, in the EO/Cr-doped Zn12O12 and EO/V-doped Zn12O12 adsorbed complexes, the HOMO levels of Cr- and V-doped Zn12O12 were shifted closer to the EO molecule. Furthermore, without the doping, from the DOS plots as shown in Figure 1, it was clearly visible that the DOS of Zn12O12 near the Fermi levels (Ef) showed no distinct changes after adsorption of EO molecules, while for the EO/Cr-doped Zn12O12 and EO/V-doped Zn12O12, the DOSs was shifted to more negative energies near the Fermi level, indicating a relative strength molecular interaction. In summary, as shown in Table 2, the doping with Sc, Ti and Cu atoms did not show a significant change on Eg value of the corresponding doped Zn12O12. However, the doping with Mn, Fe, Co, and Ni atoms revealed slightly improvement on the sensitivity of the respective doped Zn12O12 towards EO molecules.
(a) DOS, HOMO and LUMO profiles of V-doped Zn12O12 and EO/V-doped Zn12O12; (b) DOS, HOMO and LUMO profiles of Cr-doped Zn12O12 and EO/Cr-doped Zn12O12.
The gas sensor performance is generally influenced by changes in electrical conductivity (Δσ) of adsorbent materials after exposing gas molecules. The electrical conductivity (σ) at a given temperature (T) can be related to the Eg values according to: σ = A T3/2 exp (― Eg/2kT), where k is the Boltzmann’s constant and A (electrons/m3K3/2) is a constant (Li, 2006). According to the above relation, after the adsorption of EO molecule, the observed Eg change induces a change in the σ values, which then generates a recordable electric signal (Beheshtian et al., 2011; Beheshtian et al., 2013b; Soltani et al., 2013).
The Eg of Cr- and V-doped Zn12O12 was significantly changed after the adsorption of EO (i.e., ΔEg = 42.50 and 28.45%, respectively). Based on the highest ΔEg% values, it can be concluded that Cr- and V-doped Zn12O12 have promising capability to be applied for fabrication of a favorable gas sensor for detection of EO molecules. In the practical performance of gas sensors, the recovery time (τ) is another important parameter for the practical applications, which should be as small as possible for device reusability. The τ can be expressed using the conventional transition state theory as:
The minimum atom-to-atom distance between the O atom of EO molecule with the Cr and V atoms of Cr- and V-doped Zn12O12 in the EO/Cr-doped Zn12O12 and EO/V-doped Zn12O12 adsorbed complexes was obtained to be 1.97 and 2.00 Å, respectively and was shorter than of the other adsorbed systems (Table 2). After adsorption of electrophilic or nucleophilic molecular species on an absorbent surface, a charge transfer is expected to occur from the electron donor to electron acceptor (e.g., from the pristine and doped Zn12O12 to EO). Therefore, the Mulliken charge analysis of the TM-doped Zn12O12 has been performed to evaluate their electron donor-ability. The calculated QT values for Cr-, V-, Ni-, Fe-, Cu-, Ti-, Co-, Mn-, Sc-doped Zn12O12 systems obtained to be -0.40, -0.38, -0.20, -0.18, -0.09, -0.09, -0.08, -0.04 and -0.06 e. This revealed that among the doped atoms, Cr and V atoms in the Cr- and V-doped Zn12O12 show a higher charges transfer tendency to their adjacent oxygen atoms, indicating a higher electron donor-ability. For the adsorbed complexes, as shown in Table 2, the highest QT values were, as expected, -0.19 and -0.18 e, respectively for the EO/Cr-doped Zn12O12 and EO/V-doped Zn12O12 systems, indicating stronger interaction between the doped Cr and V atoms and oxygen atoms of EO molecules due to the more charge transfers. For the other TM-doped Zn12O12, the calculated charge transfer was obtained to be in range of -0.14 to -0.17 e.
The Ead of EO molecule on the TM-doped Zn12O12 was in the following order: EO/Cr-doped Zn12O12 (-60.04) > EO/V-doped Zn12O12 (-36.65) > EO/Fe-doped Zn12O12 (-35.98) > EO/Ni-doped Zn12O12 (-33.31) > EO/Mn-doped Zn12O12 (-31.91) > EO/Co-doped Zn12O12 (-31.11) > EO/Cu-doped Zn12O12 (-29.21) > EO/Sc-dopedZn12O12 (-19.92) > EO/Ti-dopedZn12O12 (-16.04) kcal/mol. According to the obtained results, the Ead values were negative for all systems which indicate an exothermic adsorption process. As shown in Table 2, the Ead value in EO/Cr-doped Zn12O12 and EO/V-doped Zn12O12 is -60.04 and -36.65 kcal/mol, respectively, indicating a strong interaction with EO molecule.
Ultimately, according to the consistent computational results of Mulliken charge, Ead, atom-to-atom distance, DOS plot, ΔEg% and HOMO–LUMO orbital analysis, Cr- and V-doped Zn12O12 can be considered as potential material for adsorption and detection of EO molecules.
4 Conclusion
In the present work, we have investigated the electronic sensitivity and reactivity of pristine and TM-doped Zn12O12 to EO molecule using the DFT method. The calculated DOS of the adsorbed complexes indicated the strong interaction between Cr- and V-doped Zn12O12 and EO molecules. According to the results, the highest QT values were obtained to be -0.19 and -0.18 e, respectively form Cr-and V-doped Zn12O12 to EO molecule. Among the TM-doped Zn12O12, Cr-doped Zn12O12 with the calculated ΔEg% of 42.50% and Ead of -60.04 kcal/mol and V-doped Zn12O12 with the obtained ΔEg% of 28.45% and Ead of -36.65 kcal/mol showed superior sensitivity toward EO molecules. The computational results of two important key-characters of gas sensor devices (i.e., ΔEg and the resultant Δσ, as well as τ) indicated that the TM-doped Zn12O12 especially Cr- and V-doped Zn12O12 can be used to develop the new generation of sensitive, accurate and portable gas sensor devices.
Acknowledgments
The authors appreciate the partial financial support of this study by university of Kashan, Iran.
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