Electronic and structural properties of gas adsorbed graphene- silicene hybrid as a gas sensor
Hamad Rahman Jappor Department of Physics, College of Education for Pure Sciences, University of Babylon, Hilla, Iraq.
E-mail address:
[email protected] ABSTRACT The adsorption of NH3 , NO 2 , NO and SO 2 molecules on graphene-silicene (GS) hybrid is studied by density functional theory (DFT). We have found that a strong chemisorption adsorption of NH3 , NO2 and NO on GS hybrid with adsorption energies more than 1eV, due to the strong interaction, NH3 , NO 2 , and NO on GS hybrid could catalyse or activate, suggesting the possibility of GS hybrid as a metal-free catalyst, except the adsorption of NH3 and NO 2 on GS hybrid at site A. Additionally, we have found a weak physisorption of adsorbed SO 2 on GS hybrid with an adsorption energy less than 1eV (0.377 and 0.685 eV), demonstrating that GS hybrid could be a fine NH3, NO2 and SO 2 sensor at site A . Furthermore, an energy gap (Eg) of GS hybrid is opened depending on gas adsorption. It is shown that GS hybrid with NH3 and SO 2 gas molecules adsorption at site B have the ability to donating an electron. The results indicate that the smaller value of the energy of lowest unoccupied molecular orbital (ELUMO) are for adsorption SO2 on GS hybrid, these values show that a propensity of the molecule to accepter electrons. Keywords: Graphene-silicene hybrid; Electronic and structural properties; Gas sensor.
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1. INTRODUCTION Graphene is the first two-dimensional (2D) nanomaterials that was discovered in 20041 , from that time until now graphene has paying much attention due to its extraordinary mechanical, structural, optical, and electronic properties2-5 . After the discovery of graphene, several 2D nanomaterials
(monolayer materials)
were
synthesized experimentally6-8 and anticipated theoretically9-13 . The discovery of 2D demonstrates has new properties arise when a bulk material is thinned down to a single layer. Until now, a number of different 2D materials with interesting properties are well-known that comprise allotropes of several elements such as silicene, germanene, stanene, phosphorene, and arsenene. Silicene that was studied with a number of theoretical works14-16 is a two-dimensional crystal of silicon atoms on a honeycomb lattice, and similar to graphene, but the atomic structure of silicene is buckled, where the height of buckling about 0.44 Å between the unequal silicon atoms9 . Silicene are obtained either by the synthesis of one-dimensional isolated silicon nanoribbons (SiNRs) grown on substrates of Ag (100) or Ag (110)17 , or by mean of the chemica l exfoliation of CaSi2 18 . Silicene has attracted a great attention due to its interesting properties such as high-carrier mobility, quantum hall effect, superconductivity, and ferromagnetism. Chemical, electronic, or structural modifiability make graphene and silicene and other two-dimensional materials perfect materials for most nanotechno lo gy applications. It has been reported that graphene can be used to detect various gas molecules, such as CO, CO 2 , O2 , H2 S, NO, NO 2 , CH4 , H2 O and NH3 molecules, etc.1922 ,
due to rapid response time, the possibility of interaction between the adsorbed
molecules and surface of graphene surface and the chemical stability23 . On the other hand, silicene can be used as a highly sensitive molecule sensor for NO, NH3 , and NO2, CO2 , N2 , H2 24-27 . Taking into account similarities between and silicene and graphene,
2
we can obtain graphene-silicene
(GS) hybrid by mixing silicon and carbon.
Additionally, the combining carbon and silicon atoms in alternating arrangement create stable graphene-silicene monolayer in a planar hexagonal lattice with sp2 hybridizatio n, instead of sp3 hybridization that is usually preferred in Si28 , this hybridization result in an important difference in the properties of graphene and silicene. Recently, electronic and structural properties of silicene/graphene monolayer have been studied
29–31. In this
paper, we study the adsorption of NH3 , NO 2 , NO and SO 2 on the electronic and properties of GS hybrid using density functional theory because of their importance in great practical interest for industrial and environmental applications. Also, we believe that the sensitivity of graphene or silicene to molecules can be further improved by mixing silicon and carbon atoms in an alternating arrangement. Besides, most of the earlier work concentrated on common molecules on silicene, graphene, or doped graphene and expected comparatively low adsorption energies32,
33 .
2. COMPUTATIONAL METHODS The calculations have been performed within Gaussian 09 package34 and density functional theory formalism using Perdew–Burke–Ernzerhof (PBE) functional35 at 6311G basis set. The PBE exchange–correlation functional is a type of generalized gradient approximation (GGA) functionals, that is able presentation a fundame nta l understanding of the structural and electronic properties of monolayer material (2D materials) such as graphene, silicene, germanene, etc. Using the above computatio na l methods, we investigate the adsorption influence of gas molecules (NO, NO 2 , NH3 , and SO2 ) on the structural and electronic properties of GS hybrid that resulting from mixing both silicene and graphene. The geometry optimization of the GS hybrid at which has the minimum energy. The convergence criteria in Gaussian software involving
3
gradients maximum force, maximum displacement, root-mean-square (RMS) force, RMS
displacement
are
reported
as
0.000450
Ha/Bohr,
0.001800
Bohr,
0.000300Ha/radian, and 0.001200 radians, respectively. The values EHOMO and ELUMO are represent the highest occupied molecular orbital energy and the lowest unoccupied molecular orbital energy, respectively, are deduced by full population analysis. The structure of pristine GS hybrid are shown in Figure 1, the structure of GS hybrid has been constructed with 42 atoms including 21carbon and 21 silicon atoms, carbon and silicon atoms are positioned successively in the hexagonal structure of the planar structure of GS hybrid.
Fig. 1. Top and side views of the optimized structure of pristine GS hybrid. There are two different adsorption sites: A which stand for the position above the silicon atom and B above the carbon atom.
3. RESULTS AND DISCUSSION 3.1. Electronic Structure of Pristine GS Hybrid We used two-dimensional system consist of graphene-silicene (GS) hybrid. The average optimised bond length for carbon-silicon is 1.82Å. This value is in agreement 4
with other calculations for SiC nanoribbons36 , silicene/graphene nanoribbons37,38 , and silicene/graphene hybrid28,39 . In the first of all, we calculated the electronic properties of pristine GS hybrid. The LUMO and HOMO acts as an electron acceptor and electron donor, repectively. One can be seen from the results that there are a slight difference between the ELUMO (5.06eV) and the EHOMO (-5.15eV) of GS hybrid. It is worth mentioning that the Fermi energy (EF) which is calculated from the HOMO and LUMO (EF= (HOMO + LUMO)/2) equal to -5.105eV. The energy gap (Eg) is one of the essential properties of material, according to results, GS hybrid is a semiconductor with energy gap in the range of 0.1 eV. Figure 2 illustrated the density of states (DOS) of GS hybrid. There are various main peaks in the valence and conduction band, and the highest number of degenerate state are 6, which indicate existing some states for occupation at high density of states for a particular energy level. Also, there are no states in the energy level at a zero DOS. 0
10
6
DOS (electrons/eV)
5
4
3
2
1
0 -20
-15
-10
-5
0
Energy (eV)
Fig. 2. Density of states (DOS) of pristine GS hybrid.
3.2. Adsorption of gas molecules on GS hybrid To find the favorable configuration of adsorption, a comprehensive study on the gas molecules adsorption (NO, NO2 , NH3 , and SO2 ) on GS hybrid is positioned at two 5
various occupation sites as following: The A-site direct above silicon atom, B-site direct above carbon atom as in Fig.1. The adsorption energy (Ead) is defined as Ead= Egas/ GS hybrid - (EGS hybrid-Egas)
(1)
where Egas/ GS hybrid is the energy of GS hybrid adsorbed with gas, EGS hybrid is the energy of pristine GS hybrid, and Egas the energies of the isolated gas molecule. At Ea smaller than zero, the adsorption energy of the total system is a smaller than the sum energy of GS hybrid and isolated gas molecules, therefore the reaction is exothermic, in this reaction, more energy releases due to the greater adsorption energy. However, if the energy needed when adsorption energy larger than zero, it is comparatively difficult for the reaction to remain. The NH3 molecules are adsorbed to GS hybrid, by means of carbon and silicon atom as shown in Fig.3(a) and Fig.3(b). The obtained bond lengths for Si-N and C–N are 2.19 and 1.44Å, respectively, which are in good agreement with the other result for the bond length of silicon-nitrogen26 and carbon- nitrogen19 . The results of adsorption of NH3 on GS hybrid show that the calculated EHOMO, ELUMO and EF for GS hybrid at site A are less than at site B, also, the Ea for GS hybrid at site A are less than at site B as shown in Table 1. The adsorption energy indicates that the binding strength between NH3 and GS hybrid is moderate and Ea equal to -0.523eV for at site A. Thus, GS hybrid can be used to detect NH3 at site A because the desorptionadsorption equilibrium of NH3 on GS hybrid at this site is without difficulty built. Nevertheless, NH3 on GS hybrid at site B could motivate or catalyze this adsorbate because of the strong interaction, proposing the ability of GS hybrid as a metal- free catalyst, a similar result is found for silicene and graphene with NH3 molecule26 . The energy gap for hybrid at sites A (0.09eV) and B (0.003eV), are smaller than pristine; lead to decrease the energy gap.
6
Fig.3. The side view for the adsorbate (a) NH3 , (c) NO 2 , (e) NO, (g) SO 2 on GS hybrid at site A, (b) NH3 , (d) NO 2 , (f) NO, (h) SO 2 on GS hybrid at site B.
7
On can see from the calculated EF for adsorbed GS hybrid that this value is smaller than this of pristine. The table shows the calculated HOMO and LUMO energies for pristine is smaller than adsorbed NH3 on GS hybrid, this indicates that HOMO and LUMO energies of adsorbed NH3 on GS hybrid are less stable when compared with pristine. The density of states of GS hybrid as a function of energy level in Fig.4(a) and Fig.4(b), illustrate that the adsorption of NH3 on GS hybrid for at site B is conformable with the DOS of pristine GS hybrid, whereas DOS of at site A are larger than the DOS of pristine GS hybrid.
Table 1. Structural and electronic properties of adsorption of NH3 , NO 2 , NO, and SO2 on GS hybrid in the present work. NH3 adsorption
NO2 adsorption
NO adsorption
SO 2 adsorption
at site A
at site B
at site A
at site B
at site A
at site B
at site A
at site B
Ea
-0.523
2.719
-0.143
2.357
1.379
2.497
0.377
0.685
Eg
0.09
0.003
0.035
0.074
0.015
0.146
0.357
0.135
EHOMO
-5.055
-4.906
-5.093
-5.113
-4.98
-5.01
-5.249
-5.243
ELUMO
-4.964
-4.904
-5.057
-5.038
-4.96
-4.86
-5.160
-5.112
EF
-5.009
-4.905
- 5.076
-5.075
- 4.97
- 4.93
- 5.204
- 5.177
Property (eV)
The adsorption of gas molecule NO 2 on GS hybrid are depicted in Fig.3(c)and Fig.3(d), the distance between C-N is 1.44Å, while Si-N is 2.19Å. Moreover, the angle of Si-NO and C-N-O are 120o and 118◦, it can be noticed from the results (Table 1) that the Ea, Eg, and EF of GS hybrid at site A are smaller than of GS hybrid at site B, this reveals that adsorption of NO 2 on GS hybrid at site A is exothermic adsorptions, additiona lly,
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the energy gaps for GS hybrid are smaller than those of pristine. However, the computed EF for GS hybrid at sites A and B, are smaller than pristine. We found that GS hybrid at site B is chemisorption with large adsorption energies, but then again GS hybrid at site A are physisorption with small adsorption energies. We found that GS hybrid at site B is strongly reactive to NO 2 , in other words, GS hybrid at site B strongly bound to NO 2 is not appropriate for gas sensor applications due to their slow desorption from GS hybrid. The calculated HOMO and LUMO energies for GS hybrid at sites A and B are larger than those pristine. The calculated EHOMO for GS hybrid at site B are smaller than at site A and ELUMO for GS hybrid at site A are smaller than at site B. DOS of adsorbed GS hybrid is illustrated in Fig.4(c) and Fig.4(d), the Figures show that the DOS for all adsorbed GS hybrid are comparable to the DOS of pristine GS hybrid. Fig.3(e) and Fig.3(f), show the adsorption of NO on GS hybrid at sites A and B, respectively. The bond length of optimized atomic of bond length Si-N is 2.17Å and angle of Si-N-O is 121o while C-N is 1.46Å and angle of C-N-O is 119o , as indicated, this value is similar to of adsorption of NO 2 . The Ea and Eg for GS hybrid at site A are smaller than GS hybrid at site B (Table 1). The obtained results of the adsorption energy for GS hybrid at site A is 1.379eV; this value agrees with the other calculations for gas adsorption on silicene26 . The Eg for GS hybrid at site B is larger than pristine, on the other hand, Eg for GS hybrid at site A is smaller than pristine. It can be seen that the adsorption energy for all GS hybrid sheet in the range of (1.379-2.497) eV are a strong chemisorption, compared with their chemisorption on graphene19 , therefore, the GS hybrid is not suitable as the sensor of NO. But then, due to the strong interaction, GS hybrid could activate NO molecule, subsequently, the ability of GS hybrid as a metalfree catalyst. In the table, we summarize the HOMO and LUMO energy of the
9
adsorption of NO on GS hybrid. The results show that EHOMO and ELUMO of pristine GS hybrid are larger than all adsorbed GS hybrid. The density of states of GS hybrid is illustrated in Fig.4, the figure shows that the DOS for all adsorbed GS hybrid are comparable with the DOS of pristine GS hybrid. Fig.4(g) and Fig.4(h) show the adsorption of SO 2 on GS hybrid at sites A and B, respectively, The Si-S and C-S bond lengths are 2.39Å and 3.15Å, respectively, these values are agreement with those in Refs26, 40 . The C-S-O angle is 122o and Si-S-O angle is 112o . In this section, we presented the adsorption of SO 2 on GS hybrid in Table 1. The energy gap for all adsorbed is larger than pristine. The table shows that the HOMO and LUMO energy of the adsorption of SO 2 on GS hybrid are smaller than pristine. The computed EF for pristine is smaller than all adsorbed GS hybrid. The Table 4, shows that the EF and Eg of GS hybrid at site B are smaller than at site A. On the other hand, the adsorption energy of GS hybrid at site B are larger than GS hybrid at site A. From our results that are displayed in Table 1, one can see that GS hybrid at site A has the weakest adsorption strength of 0.377eV while GS hybrid at site B has adsorption energy of 0.685. The SO 2 -based gases generally have smaller adsorption energies than those for other gas adsorption, indicating that GS hybrid is more sensitive to Sulphurbased toxic gases, consequently, this configuration is able to detect SO 2 gas with high sensitively efficiently. The density of states of armchair GS hybrid in Fig.4(g) and Fig.4(h) show that the adsorption of gas molecules SO 2 on GS hybrid leads to an increase in the highest number of density of states in the energy bands for all GS hybrid, in comparison with pristine GS hybrid. It is clear from Table 1 that the adsorption energies for all configuration at site A are lower than adsorption energy at site B.
10
0
0
10
6
6
(a)
(b)
5
DOS (electrons/eV)
5
DOS (electrons/eV)
10
4
3
2
4
3
2
1
1
0 -20
-15
-10
-5
0 -20
0
-15
-10
6
Alpha DOS Spectrum Beta DOS Spectrum
4
3
2
-5
0
Alpha DOS Spectrum Beta DOS Spectrum
(d)
5
DOS (electrons/eV)
DOS (electrons/eV)
5
4
3
2
1
1
0 -20
-15
-10
-5
0 -20
0
-15
Energy (eV) 6
(e)
Alpha DOS Spectrum Beta DOS Spectrum
Alpha DOS Spectrum Beta DOS Spectrum
(f)
5
DOS (electrons/eV)
5
4
3
2
1
4
3
2
1
0 -20
-15
-10
-5
0 -20
0
-15
Energy (eV) 10
-5
0
(g)
6
0
10
(h)
5
DOS (electrons/eV)
5
4
3
2
1
0 -20
-10
Energy (eV)
0 6
-10
Energy (eV)
6
DOS (electrons/eV)
0
6
(c)
DOS (electrons/eV)
-5
Energy (eV)
Energy (eV)
4
3
2
1
-15
-10
-5
0 -20
0
Energy (eV)
(b)
-15
-10
-5
0
Energy (eV)
Fig. 4. Density of states for the adsorbate (a) NH3 , (c) NO 2 , (e) NO, (g) SO 2 on GS hybrid at site A, (b) NH3 , (d) NO 2 , (f) NO, (h) SO 2 on GS hybrid at site B. 11
4. CONCLUSIONS In summary, based on the DFT, the adsorptions of NH3 , NO 2 , NO and SO 2 gases on GS hybrid at two different occupation sites, A and B sites have been investigated. The energy gap, adsorption energy, Fermi energy, density of states, ELUMO and EHOMO are calculated. The results suggest that the adsorption of SO 2 on GS hybrid is weak physisorption, and the adsorption of NH3 , NO and NO 2 on GS hybrid at B site is a strong chemisorption, due to gases slow desorption from GS hybrid at this site, the GS hybrid is not suitable as the sensor of NH3 , NO, and NO 2 at this site. Nevertheless, GS hybrid could catalyze or activate these adsorbate, proposing the ability of GS hybrid as a metal-free catalyst. On the other hand, the results suggest GS hybrid could be a good NH3 , NO 2 sensor at A site, in the same way, GS hybrid can be used to detect SO2 at A and B sites since the adsorption–desorption equilibrium of SO2 on GS hybrid at these sites is easily built; indicating that GS hybrid is more sensitive to the sulphurbased toxic gases. There are many main peaks in all DOS; a high density of state at a particular energy level denotes that there are numerous states existing for occupation. The present results reveal that the density functional theory is a good and powerful in a calculation of the effect of gas adsorption on the electronic structure of GS hybrid. References and Notes 1. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, and A.A. Firsov, Science 306, 666 (2004). 2. J. Dai, J. Yuan and P. Giannozzi, Appl. Phys. Lett. 95, 232105 (2009). 3. D. Chen, L. Tang, and J. Li, Chem. Soc. Rev. 39, 3157 (2010). 4. B. Wanno and C. Tabtimsai, Superlattice Microstruct. 67, 110 (2014). 5. A. S. Rad, Appl. Surf. Sci. 357, 1217 (2015).
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