Adsorption of Gas Molecules on Graphene Doped with Mono and Dual Boron as Highly Sensitive Sensors and Catalysts

Document Type: Research Paper

Authors

1 Directorate of Education Babylon, Ministry of Education, Iraq

2 Department of Physics, College of Education for Pure Sciences, University of Babylon, Iraq

10.22052/JNS.2020.02.003

Abstract

First-principle calculations have been investigated to study the adsorption of the molecules (SO2, CO, NH3, CO2, NO2, and NO) on the surface of mono boron (B) B-doped and dual B-doped graphene sheets to explore their potential applications as sensors. Our findings indicate that the adsorption of (CO and NH3) on B-doped graphene and (CO and SO2) on dual B-doped graphene are weak physisorption with adsorption energy between (0.128 to 0.810) eV. However, the adsorption of (CO2, NO2, SO2, and NO) on B-doped graphene and (CO2, NH3, NO and NO2) on dual B-doped graphene are strong chemisorption. The strong interaction of (CO2, NO2, SO2, and NO) on B-doped graphene and (CO2, NH3, NO and NO2) on dual B-doped graphene demonstrating that B-doped graphene and dual B-doped graphene could catalyse or activate, suggesting the possibility of B-doped graphene and dual B-doped graphene as a catalyst. Moreover, the energy gap of B-doped graphene and dual B-doped graphene is opened upon adsorption of (CO, CO2, NH3, NO, NO2 and SO2) in various ways. Our calculations demonstrate the feasibility of B-doped graphene may be a good sensor for (CO and NH3) and dual B-doped graphene could be a good sensor for (CO and SO2).

Keywords


INTRODUCTION
The need for scale down sensors with fast response, high reliability, high sensitivity, high selectivity, low cost, and quick recovery has encouraged the scientists to try to find different gas sensing systems based on new nanomaterials. Graphene, the first discovered two-dimensional (2D) atomic crystal [1,2], Graphene represents a new material concept with only one atom thick, supplying scientists with another method to enter low-dimensional physics. By means of the research deepening, the manufacturing revolution carried by graphene will be immeasurable. Carbon is the versatile element on the crust of the earth and it is set up on the surface of the earth in dissimilar allotropes as diamonds, graphite, coke, and charcoal respectively. The newer allotropes of carbon were discovered such as graphene, carbon nanotubes (CNTs) and fullerenes [3–5]. Graphene is the youngest known allotrope of carbon, which is a two-dimensional and one-atom-thick material consisting of sp2 hybridized carbon atoms organized in the structure of a honeycomb. These allotropes of carbon are extensively used in research, that is, from biomedical to environmental applications owing to their unique physical and chemical properties [6]. The exceptional properties of carbon and nanomaterials, such as electronic, thermal, optical, mechanical and transport properties make them encouraging candidates for various potential applications [7–11]. From several experimental and theoretical studies, it is observed that the transport and electronic properties are exceedingly sensitive to change in the local chemical environment [12–14], has attracted unlimited interest thanks to its astonishing properties, for example, high carrier mobility, high surface-volume ratio, high chemical stability, low electronic temperature noise, fast response time, and high thermal stability.
Ergo, it offers promise in the development of ultrasensitive gas sensors with high selectivity, fast recovery, high packing density, and low power consumption [15,16]. The applicability of graphene in the field of gas sensing has been widely investigated both experimentally [17,18]. and theoretically [19–24]. At present, there are three types of modification ways for the maximum conventional graphene, including electrostatic field tuning, chemical modification, and atomic doping [25–28]. Nowadays, the most feasible methods to control the semiconductor performance of graphene is the atomic doping which regarded as one of which can open the band gap of graphene, and significantly improve its electronic and optical properties [29–32]. It has been demonstrated that the graphene-based gas sensors sensitivity can be significantly improved by introducing dopants or defects [33–38]. Despite the great development and advances made in this field, there are still some restrictions in these nanomaterials, including but not limited to exposure to the mixture of gases, weak anti-interference faculty, delayed gas response, and low selectivity …. etc. Since the surface of nanomaterial doesn’t only interact with one type of research gas, and most scientists only select the best from the adsorption outcomes to explain the selectivity of the surface of nanomaterial, while disregarding the interaction between other gas molecules and the surface of nanomaterial that has a very large influence on the adsorption results and they can’t judge the type of adsorbed gas [39]. Owing to the effect of other gases, when the multi-adsorption happens, the academics can’t differentiate which gas molecule has interacted with the surface of the material, so that the results can’t be judged exactly. Consequently, it is very necessary to discover a sensing material capable of detecting the type of adsorbed gas molecules [40]. Boron atom is the most common candidate of p-type dopant for carbon materials. With the development of synthesis techniques, B-doped graphene has been fabricated [41]. In this work, first-principles methods depending on the density functional theory (DFT) are used to examine the electronic properties of the graphene after the interaction with toxic gas molecules such as NH3, SO2, CO2, NO2, NO and CO. The sensitivity of the sensors is evaluated from the variations in their electronic transport properties. Our results reveal the promising future of graphene in the development of ultrahigh sensitive gas sensors [42].

COMPUTATIONAL METHODS AND MODELS
Computational Methods
All calculations are performed using DFT, the geometric structures were completely optimized using Gaussian 09 program package [43]. We chose the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [44,45], to describe the exchange and correlation energy in the structural optimizations and total energy calculations. The system is modelled including 42 carbon atoms of B-doped graphene and dual B-doped graphene was created. The corresponding dopant concentration is 2.4% and 4.8% of B-doped graphene and dual B-doped graphene respectively. The adsorption energy (Ead) of molecules on the B-doped graphene (Ead (gas+ B-doped graphene)) and dual B-doped graphene (Ead (gas+ dual B-doped graphene)) is defined as follows [46,47]:

Ead (gas+ B-doped graphene)= E(gas+ B-doped graphene) - (EB-doped graphene + Egas)                                               (1)

Ead (gas+ dual B-doped graphene)= E(gas+ dual B-doped graphene) - (Edual B-doped graphene + Egas)                              (2)

where E(gas+B-doped graphene) and E(gas+dual B-doped graphene) are the total energies of the relaxed molecule on the B-doped graphene and dual B-doped graphene respectively, EB-doped graphene and Edual B-doped graphene are the energies of isolated B-doped graphene and dual B-doped graphene and Egas is the energy of the isolated gas molecule. The diversity of relative energy of the highest occupied molecular orbital (EHOMO) and the energy of lowest unoccupied molecular orbital (ELUMO) of free B-doped graphene and dual B-doped graphene and adsorbed molecule on B-doped graphene and dual B-doped graphene gives the mechanism of interaction. The HOMO can be defined as an electron donor because of having an excess of electrons whereas the LUMO is lacking electrons and therefore it has a power of accepting electrons.
Computational Models The graphene model was cut from optimized graphite model. Graphene is a hexagonal monolayer atomic structure with a stable and symmetrical nature. The graphene molecular formula is C. Each one of carbon coordinates with other three atoms of carbon (arranged in a honeycomb polygon) and shapes a covalent molecule. The model of graphene as shown in Fig. 1 built using 3×3 supercell geometries. The lengths of C–C bonds are 1.42 Å [48], which is in agreement with the description of the literature [49].

RESULTS AND DISCUSSION
Electronic Structure of Doped Graphene
Graphene was doped with B and dual B atom, respectively, and the optimized models are shown in Fig. 2 (a) and (b). Doping graphene with other atoms or molecules is efficient ways to modify the structural and electronic properties of graphene. The doped atom is substituted with C atom. B-doped graphene and dual B-doped graphene retain the planar form of graphene after full relaxation and their corresponding model are given in Fig. 2 (a) and (b).
The free boron atom has an atomic configuration is 2s22p1. When graphene is doped, the boron atom faultlessly binds with the three neighbouring carbon atoms and undergoes a sp2 hybridization. Because of the dopant, the graphene a little deforms, the three B-C bonds for B-graphene are found to be 1.51 Å, while three B-C bonds for dual B-graphene is found to be 1.49 Å, which agrees with the theoretical results [50,51], which is increased than that of C-C bond length 1.42 Å in graphene, also note that the B1-B2 bond length for dual B-graphene is found to be 1.57 Å. All of those structural differences for B- and dual B-graphene could cause the variations of the electronic and structural properties [52]. However, the doping of boron atoms into graphene only results in a structural defect, it is not found unpaired electrons or distortion in the planar structure of the graphene sheet. Our results are in good agreement with the previous work that confirmed the planar structure of B-graphene [53].
Table 1 clarifies the effect of B-doped and dual B-doped on the electronic properties of graphene. The B-doped graphene has a large value of EHOMO, and small values of ELUMO and the Fermi energy (EF) compared with dual B-doped graphene. As is clear table the Fermi energy (EF) which is calculated from the EHOMO and ELUMO (EF = (EHOMO + ELUMO)/2) equal to -5.160 eV for B-doped graphene and -5.265 eV for dual B-doped graphene. The computed energy gap (Eg) for B-doped graphene is larger than dual B-doped graphene, however, the B-doped graphene and dual B-doped graphene are semiconductors with Eg equal to 1.307 eV and 0.172 eV, respectively.
It is clear that band gap is decreasing with increasing the concentrations of boron. The band gap decreasing upon doping is a well-known general phenomenon in semiconductors, not just in the present study. Shallow level donor impurities create energy levels in the band gap near the conduction band edge and shallow acceptor impurities create energy levels near the valence band edge. With the increase in the amount of boron concentration, the density of states of these dopants increases and forms a continuum of states just like in the bands and effectively the bandgap decreases. In other words, the system shows hole doping properties because the boron is a p-type dopant, consequently causing transport in Dirac point overhead the Fermi energy. At the hole doping, most states are dragged overhead Fermi energy level, this leads to decrease in the band gap. Thus it is possible to alter the Eg of graphene sheet by doping B atom in the graphene sheet, as a result, this can affect the electronic properties of graphene [54,55].
There are various main peaks in the valence and conduction band, Fig. 3 (a) and (b) illustrates the DOS, the highest number of degenerate states in the conduction and valence bands is 5 for B-doped graphene and dual B-doped graphene. It is evident that there are states available for the occupation at high DOS for a specific energy level and no states can be occupied at a zero- DOS for energy level. Moreover, doping with dual B atom leads to a slight increase in the DOS in the conduction and valence bands in comparison with those of B-doped graphene as shown in Fig. 3.

Adsorption of Gas Molecules on Doped Graphene
The B- and dual B-graphene are studied to improve the interaction with the gaseous molecules. The boron has one less valence electron than carbon, therefore such type of doping will lead into a p-type semiconductor so hole is the main carrier, which indicates that the boron acts as an acceptor, and the gas molecules acts as a donor, this causes a large transfer of charge, and therefore a strong interaction between the electron-deficient B atom and the electron-donating atom of gas molecules, the reason that the gas molecules are adsorbed above the boron sites and not interacting with graphene carbons.
We study the adsorption of CO, CO2, NO, NO2, NH3 and SO2 gases on B- and dual B-graphene near the dopant site. After adsorption of these gases, we try to examine its effect on structural, and electronic properties of B- and dual B-graphene. The optimized adsorption structure of CO adsorbed on B- and dual B-graphene is shown in Fig. 4(a) and (b), respectively. Meanwhile, we find that bond lengths of B-C, B1-C and B2-C are 2.28 Å, 2.04 Å and 2.35 Å, respectively, which is basically decreased with the increase of electrons in the elements, while the angles of B-C-O, B1-C-O and B2-C-O are 128o, 92o and 92o, respectively.
It can be noticed from Table 2, the Eg for adsorbed B-graphene is smaller than those of B-graphene, this point out that the Eg decreases with the adsorbed B-graphene, whereas the Eg for adsorbed dual B-graphene is larger than those of dual B-graphene, this indicates that Eg increases with the adsorbed dual B-graphene. The computed EF for adsorption of CO on B-graphene is larger than the calculated value for B-graphene, whereas EF for adsorption of CO on dual B-graphene is smaller than the calculated value for dual B-graphene. It has been found that the EHOMO for adsorbed dual B-graphene is larger than the dual B-graphene and ELUMO for adsorbed dual B-graphene is smaller than dual B-graphene, conversely, EHOMO for adsorbed B-graphene is smaller than the B-graphene, while the ELUMO for adsorbed B-graphene is larger than the B-graphene. The results show that the high value for EHOMO is -5.813 eV, this value shows a propensity of the molecule to bestow electrons, whereas the lowest value of ELUMO is -4.506 eV, this indicates a propensity of the molecule to accept electrons. The Ead for adsorbed B-graphene and dual B-graphene are 0.81 eV and -0.503 eV corresponding to weak physisorption [56,57]. The binding strength of CO with B- and dual B-graphene are intermediate with the values of Ead. Thus, B- and dual B-graphene can be successfully used to detect CO since the adsorption-desorption equilibrium of CO on B- and dual B-graphene are easily built.
Fig. 4(c) and (d) present that the most stable form of adsorbed CO2 on B- and dual B-graphene, respectively. It has been found that bond lengths of B-C, B1-C and B2-C are 1.88Å, 2.23Å and 1.77Å, respectively, which basically decrease with the increase of electrons in the elements, meanwhile, the angles between the bonds B-C, B1-C and B2-C with the planes are 92o, 87o and 52o of B- and dual B-graphene, this is similar to the adsorption of CO on B- and dual B-graphene. Table 2 clarifies the effect of CO2 adsorption on the electronic properties of B- and dual B-graphene. The results show that the Eg reduces with the adsorbed CO2 on B- and dual B-graphene in comparison with B- and dual B-graphene. The adsorption of CO2 on B- and dual B-graphene is larger than 1eV. Therefore, is a strong chemisorption, one can see from Table 2 that EHOMO for adsorption of CO2 on B-graphene is smaller than B-graphene, while EHOMO for adsorbed dual B-graphene is larger than those of dual B-graphene. in return ELUMO for adsorption of CO2 on B- and dual B-graphene are larger than those of B- and dual B-graphene. The deduced results illustrate this larger value EHOMO of adsorbed B-graphene is -5.813 eV, this value appears a propensity of the molecule to donate electrons and smaller value ELUMO for B-graphene is -4.506 eV and display a propensity of the molecule to accepter electrons.
The NH3 molecules are adsorbed to B- and dual B-graphene, via B atoms, as shown in Fig. 4(e) and (f). The bond lengths of B-N, B1-N and B2-N are 1.75 Å, 1.69 Å and 2.73 Å, respectively, basically decreases with the increase of electrons in the elements. Those values are consistent with the other for the length of bond B-N [53], while the angles between the bonds B-N, B1-N and B2-N with the planes of B- and dual B-graphene are 103o, 109o and 36o, respectively. The overall results of adsorbed NH3 on B- and dual B-graphene are summarized in Table 2. The adsorption energies indicate that the tying strength of NH3 with B-graphene is (-0.430 eV). Thus, B-graphene can be used to detect NH3. Nevertheless, NH3 on dual B-graphene could catalyse or activate this adsorbate due to the strong interaction, suggesting the possibility of dual B-graphene as catalyst. It is obvious from Tables 1 and 2 the adsorbed dual B-graphene lead to increase the Eg in comparison with dual B-graphene. The table shows the calculated EHOMO and ELUMO of adsorption of NH3 on B- and dual B-graphene are smaller than those of B- and dual B-graphene.
The NO molecules are adsorbed to B- and dual B-graphene, via B atoms, as shown in Fig. 4(g) and (h). The bond lengths of B-N, B1-N and B2-N are 1.78 Å, 1.97 Å and 2.45 Å, respectively. The angles of B-N-O, B1-N-O and B2-N-O are 96o, 116o and 113o, respectively. The results indicate that Ead for dual B-graphene is larger than B-graphene, in the same way the Eg for adsorbed B-graphene is smaller than B-graphene, while the Eg for adsorbed dual B-graphene is larger than dual B-graphene, It can be seen that the Ead for B- and dual B-graphene are a strong chemisorption, therefore, the B- and dual B-graphene are not suitable as a sensors of NO rather than B- and dual B-graphene can activate the adsorbate because of the strong interaction, proposing the possibility of B- and dual B-graphene as catalysts. Our results show that EHOMO of adsorption of NO on B-graphene is smaller than B-graphene, while EHOMO of adsorption of NO on dual B-graphene is larger than dual B-graphene, conversely, the ELUMO of adsorption of NO on B- and dual B-graphene are larger than B- and dual B-graphene.
The NO2 molecules are adsorbed to B- and dual B-graphene, via B atoms, as shown in Fig. 4(i) and (j). The optimized bond lengths of B-N, B1-N and B2-N are 1.66 Å, 2.06 Å and 2.65 Å. Those values are consistent with the other upshot for the length of bond B-N [24,58] at the same time that the angles between the bonds B-N, B1-N and B2-N with the planes of B- and dual B-graphene are 104o, 91o and 51o, respectively. Table 2 lists the results of adsorption of NO2 on B- and dual B-graphene, it can be noticed that the Eg for adsorbed B-graphene smaller than those of B-graphene, conversely, the Eg for adsorbed dual B-graphene is greater than dual B-graphene. We found the adsorption of NO2 on B- and dual B-graphene are a chemisorption for Ead greater than 1eV, therefore, the B- and dual B-graphene are not suitable as a sensors of NO2, rather than B- and dual B-graphene could activate this adsorbate, suggesting the possibility of B- and dual B-graphene as catalysts. The calculated EHOMO of adsorption of NO2 on B-graphene is smaller than B-graphene, while EHOMO of adsorption of NO2 on dual B-graphene is larger than dual B-graphene, conversely, the ELUMO of adsorption of NO2 on B-graphene is larger than B-graphene, while ELUMO of adsorption of NO2 on dual B-graphene is smaller than dual B-graphene.
The SO2 molecules are adsorbed to B- and dual B-graphene, via B atoms, as shown in Fig. 4(k) and (l). The bond lengths B-S, B1-S and B2-S are 1.80Å, 2.30Å and 2.60Å, respectively. While the angles between the bonds B-S, B1-S and B2-S with the planes of B- and dual B-graphene are 86o, 81o and 61o, respectively. If we discuss the adsorption of SO2 on B- and dual B-graphene, we can deduce that Eg for adsorbed B-graphene is larger than adsorbed dual B-graphene, the Ead for adsorbed dual B-graphene is smaller than adsorbed B-graphene, and EF for adsorbed B-graphene is smaller than adsorbed dual B-graphene. Also, HOMO and LUMO energies for the adsorbed dual B-graphene is larger than dual B-graphene, conversely, EHOMO of adsorption of SO2 on B-graphene is smaller than B-graphene, while ELUMO of adsorption of SO2 on B-graphene is larger than B-graphene. One can see from the overall results that are displayed in Table 2, that the adsorbed SO2 on dual B-graphene has a weakest adsorption of 0.128 eV with physisorption, indicating that dual B-graphene is sensitive to the gas SO2. While the adsorbed SO2 on B-graphene has the largest Ead of 7.146eV that studied in the case of SO2 adsorption with chemisorption. In other words, the B-graphene is not suitable as the sensor of SO2. Instead of this, B-graphene can activate that adsorbate because of the strong interaction, proposing the probability of B-graphene as catalyst.
The density of states (DOS) of CO adsorption on B- and dual B-graphene illustrated in Fig. 5(a) and (b), respectively, the Figure shows that DOS of B- and dual B-graphene with the adsorption of gas molecules CO are different from the B- and dual B-graphene, the highest of peaks become less, i.e., the conduction and valence bands are less with the highest number of DOS. The showing DOS to the adsorption CO2 on B- and dual B-graphene lead to decrease at DOS in the valence and conduction valence bands in comparison with DOS for B- and dual B-graphene, as depicted in Fig. 5(c) and (d), respectively. DOS to the adsorption NH3 on B-graphene leads to decrease DOS in the valence and conduction bands in comparison with DOS for B-graphene, whereas DOS for adsorption of NH3 on dual B-graphene as function of energy levels shows that the adsorption of NH3 on dual B-graphene is conformable with DOS of dual B-graphene, as depicted in Fig. 5(e) and (f), respectively. DOS for adsorption of NO on B- and dual B-graphene lead to a decrease in DOS in the conduction and valence bands in comparison with DOS of B- and dual B-graphene, as depicted in Fig. 5(g) and (h), respectively. On the other hand, DOS for adsorption of NO2 molecules on B- and dual B-graphene revealed that DOS decrease in comparison with DOS of B- and dual B-graphene, see Fig. 5(i) and (j), respectively.
Finally, DOS for adsorption of SO2 on B- and dual B-graphene in Fig. 5(k) and (l) illustrate that SO2 adsorbed gas molecules adsorbed cause a decrease in the maximum number of DOS in the valence and conduction bands, in comparison with B- and dual B-graphene.

CONCLUSION
First-principles calculations have been performed to investigate the effect of adsorbed CO, CO2, NH3, NO, NO2 and SO2 molecules on the electronic properties of boron-doped graphene and dual boron-doped graphene surfaces. It can be deduced from our outcomes that the CO and NH3 adsorbed gas molecules on B-doped graphene is accomplished by physisorption interaction with moderate adsorption energy. Simultaneously, the of CO2, NO, NO2 and SO2 adsorption molecules on B-doped graphene are strong chemisorptions with higher adsorption energy. Furthermore, the B-doped graphene is strongly reactive with CO2, NO, NO2 and SO2 adsorbed gas molecules, because these gases slow desorption from B-doped graphene. Consequently, B-doped graphene is not practicable as a gas sensor for these molecules. On the other hand, the B-doped graphene can be used as a catalyst. Besides, our conjecture that B-doped graphene is adequate to sensitively monitor CO and NH3 sensors with moderate adsorption energy. However, the adsorption of gas molecules CO2, NH3, NO and NO2 on dual B-doped graphene undergo in a strong chemisorption interaction, so it presumably unsuitable for usage as a gas sensor for these gases. In the same time, the adsorptions of CO and SO2 on dual B-doped graphene are a weak physisorption with Ead ranging from 0.128 eV to -0.503 eV. Hence, it can be used as a good sensor for CO and SO2. It has been demonstrated that most of the adsorption energies are positive corresponds to the endothermic reaction, except that the adsorption of NH3 on B-doped graphene and the adsorption of CO, CO2, NH3, NO on dual B-doped graphene are negative corresponds to the exothermic reaction. Our results reveal that the electronic properties of graphene can be stirringly modulated by boron doping and molecules adsorption, which could be used to design chemical sensors and graphene, could be used to build sensors for the detection of particular molecules.

Conflict of Interest
The authors declare that there is no conflict of interests regarding the publication of this manuscript.

 

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