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B-N@Graphene: Highly Sensitive and Selective Gas Sensor Indrani Choudhuri, Nandini Patra, Arup Mahata, Rajeev Ahuja, and Biswarup Pathak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07359 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 17, 2015

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The Journal of Physical Chemistry

B-N@Graphene: Highly Sensitive and Selective Gas Sensor Indrani Choudhuri,† Nandini Patra,§,‡ Arup Mahata,† Rajeev Ahuja,§,⊥ Biswarup Pathak,†,‡,* †

Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology (IIT) Indore, M.P., India

§

Condensed Matter Theory Group, Department of Physics and Astronomy, Box 516, Uppsala University, 751 20 Uppsala, Sweden

‡

Center for Material Science and Engineering, Indian Institute of Technology (IIT) Indore, M. P., India

⊥

Applied Materials Physics, Department of Material Science and Engineering, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden Email: [email protected]

1. Abstract: We have performed Density Functional Theory (DFT) calculations to study the gas (CO, CO2, NO and NO2) sensing mechanism of pure and doped (B@, N@ and B-N@) graphene surfaces. The calculated adsorption energies of the various toxic gases (CO, CO2, NO, and NO2) on the pure and doped graphene surfaces show, doping improves adsorption energy and selectivity. The electronic properties of the B-N@graphene surfaces change significantly compared to pure, B@ and N@graphene surfaces, while selective gas molecules are adsorbed. So, we report B-N co-doping on graphene can be highly sensitive and selective for semiconductor-based gas sensor.

Keywords: Graphene, Co-doping, Band gap, Gas sensor

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2. Introduction: Gas sensors are very much important to detect the presence of various toxic gases, which are very much harmful to organic life.1 Gaseous pollutants, which release in a large amount from the industries and automobiles, spread in our environments very rapidly and affect the human health, ozone layer and global climates. Semiconductor based gas sensors are highly demanding because of their exceptional stability and surface reactivity in the ambient conditions. The primary challenge of the semiconductor based gas sensor is to meet the demand for higher sensitivity and selectivity so that even the traces amount of toxic gases can be detected. The metal oxide based semiconductors are quite efficient2-5 but they are not environment friendly.6 So, the two dimensional metal free semiconducting material with excellent surface, optical and electrical properties could be the most promising gas sensing material. Graphene is the thinnest possible material,7-9 with the largest surface area (3000 m2 per gram), and got remarkable strength and stiffness, which can be stretched up to 20%, elastically. Theoretically monolayer of graphene and its electronic properties were proposed as early as in 1947.10-11 However, forty years later the material is synthesized and showed similar electronic properties as proposed theoretically.9,12 Thus it clearly shows the importance of theoretical studies in the prediction of new structures and their properties. There are number of researchers believe graphene can be an excellent sensor owing to its two dimensional structure and excellent electrical, optical and mechanical properties.13-15 Recently, Schedin et al.16 showed graphene is a unique and attractive sensing material to detect all the individual events when some gaseous molecules attaches or detaches from its surface. Even its local carrier concentration changes when gaseous molecules are adsorbed which leads to changes in resistance. Graphene is an exceptionally low noise material thus very sensible towards the charge fluctuations, so a suitable candidate for chemical 2 Environment ACS Paragon Plus

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detection.17-18 Graphene based devices are reported to be a promising candidate for DNA sequencing.19 as DNA can interact with graphene through π-π stacking which diminishes the ballistic conductance of graphene based materials.20-22 Despite of all, the zero band gap and absence of dangling bonds in graphene limits its devicebased applications. Various approaches have been taken to fabricate the high-performance graphene devices by engineering their band gaps for improving their semiconducting properties.23 Doping is an efficient approach, which tunes the electronic properties of graphene. Substitutional doping of graphene with different atoms (e.g. B, N, B-N) affects the sp2 hybridization of carbon atoms.24 The reason for choosing the N and B atoms to replace the C atoms is the atomic masses of these dopants are closer to carbon atom. The electron rich and deficient nature of N and B atoms helps to alter significantly the electronic properties of the host material. The researches of Lherbie et al.25 studied the charge mobility and conductivity of the system by doping graphene with different concentrations of B and N impurities. Cervantes-Sodi et al.26 theoretically predicted the band gap opening in graphene by elemental (B and N) doping. Since then, there are many experimental evidence of band gap opening in graphene by B/N doping.27-29 Panchakarla et al.27 experimentally synthesized B/N doped graphene and confirmed the Fermi energy shifting (p/n doping) as theoretically proposed. Fan et al.30 theoretically predicted band gap opening of graphene by doping small boron-nitride domains. Later Bepete et al. experimentally31 confirmed such predictions. So, lately, theoretical studies becoming an essential tool for predicting new materials and their properties. There are several theoretical and experimental studies on adsorption of gaseous molecules on pure and doped graphene surfaces.32-36 Ko et al.32 recently demonstrated that graphene can selectively sense NOx molecules at room temperature. The sensitivity was up to 9% in presence of 100 ppm NO2 gas. So, theoretical calculations are needed to show the changes in electronic properties of graphene in presences of gases. Zhang et al.33 showed how



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B/N doped and defected graphene interacts with various gas molecules. They have concluded that the gas sensing property of graphene can be drastically improved by introducing dopants and defects. Dai et al.34 theoretically reported how elemental (B/N/Al and S) doping on graphene improves the gas sensing properties. Such calculations are helpful in predicting the changes in electrical properties of graphene, which in turn helpful for gas sensing. But, most of the previous research mainly focused on the perfect or mono doped graphene32-34 or metal decorated35 graphene for gas sensing properties. Those studies rather predict the selectivity on the basis of adsorption energies and incorporation of metals limits their applicability. On the other hand, it is experimentally proved that co-doped graphene has many excellent applications towards sensor36-38 photocatalysis,39 ORR,40-41 supercapacitor. 42 Recently, Niu et al.36 experimentally reported that nitrogen and silica co-doped graphene nano-sheet is a very good sensor for NO2. But, whether such co-doping could improve the sensitive and selectivity of the gas sensing properties is not well studied theoretically. In reality, the graphene sheets, which are prepared by the available fabrication methods, have many defects. Deliberately or accidentally graphene could be doped by non- carbon elements. So, our primary aim is to co-dope graphene by environmental friendly non-metal (like B or N) elements using the state of the art theoretical techniques. This is done to improve the selectivity and sensitivity of the gas sensor. As such doping not only improves the interaction between the graphene sheet and the gaseous molecule but also changes their electronic properties. 3. Computational Details: We have used the Vienna ab initio simulation package (VASP)43 to do all the calculations. The exchange-correlation interaction is treated in the level of the GGA using the Perdew-BurkeErnzerhof (GGA-PBE) exchange-correlation functional.44 The projected augmented wave (PAW) method45 is employed using an energy cut-off of 470 eV to describe the electronic wave


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function. Thus generalized gradient approximation (GGA-PBE) within projector-augmented wave (PAW) methodology is adopted to investigate the electronic properties of such materials. In all the calculations, self-consistency is achieved with a tolerance in the total energy is set to 10-4 eV. We have included semi-empirical DFT-D3 type of dispersion correction46 as the longrange interaction present between the surface and the gaseous molecules. The Brillion zone is integrated using Monkhorst-Pack generated sets of 11×11×1 k-points.47 For the density of states (DOS) calculations, the Monkhorst-Pack generated 45×45×1 sets of k-points are used with a Gaussian smearing of 0.001 eV. The adsorption energy (EA) of the toxic greenhouse gases (CO, CO2, NO and NO2) on the pure and doped graphene surfaces is calculated using the following equation. EA = (Egraphene+Egas)–EGas-graphene

(1)

Here EGas-graphene represents the energy of the optimized structure of the gaseous molecules adsorbed on the pure graphene surfaces. The Egraphene and Egas represent the single point energy of the pure graphene and gas molecules respectively. The single point energy is calculated using the optimized coordinates of the respective systems. The charge density difference (CDD)48 is plotted to understand the nature of bonding between the pure/doped graphene and the gas molecules. The CDD is calculated using the following equation.

ρCDD = ρtotal - ᅹi ρifragments

(2)

Here the ρtotal is the total charge density of the system and ρifragments is the charge density of the individual fragments by which the system is made of. Here the charge density of the fragments (ρifragments) is calculated by a pseudo structure in which fragment part retains the same structure as in the total system but other parts are deleted. Here, we have mainly two 5 Environment ACS Paragon Plus


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different fragments, the pure graphene sheet and adsorbed gases. In CDD the positive and negative charge density is plotted by blue and yellow colors respectively. 4. Results and Discussion: 4.1. Pure Graphene for gas sensor: Two atoms unit cell is considered for the graphene hexagonal structure. The structure is fully relaxed and the carbon-carbon bond length (1.42 Å) of the optimized structure matches with the experimental value of 1.42 Å.49 Two atoms unit cell is extended to 4×4 supercell of 32 carbon atoms to study the gaseous molecules interacting with the graphene surface. The total charge density of the pure graphene is plotted and we find the charges are highly delocalized over the graphene. Bader charge analysis is done50 using the Henkelman programme51-52 with near-grid algorithm refine-edge method. It shows, all the carbon atoms of graphene have same charges (~ 4e), thus confirms the delocalization. The electronic structure of pure graphene is one of the most exciting finding of this century. The total density (TDOS) and partial density of states (PDOS) of graphene is plotted in figure S1 in supporting information. It shows graphene has 0 eV band gap and carbon 2p orbitals are appearing both sides of the Fermi energy (0 eV to -6.0 eV and 0 eV to 6.0 eV) agrees well with the previous experimental and theoretical studies.53 The change in the semiconductor’s electronic properties due to the gaseous molecular adsorption is the most important criteria for semiconductor based gas sensor.54 Band theory suggests, when gaseous molecules are adsorbed, the valence band (VB) electrons interact with the gaseous molecules thus tunes the band position. Such tuning controls the electron flow from VB to CB, thus the current. So, the main objective of this study is to find out the changes in the semiconductor’s electronic properties due to the adsorption of the gases.


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(i)

(j)

(k)

(l)

Figure 1: The optimized structures of (a) CO, (b) CO2, (c) NO and (d) NO2 molecules adsorbed on graphene surface (Brown, blue, and red coloured balls denote C, N and O respectively). Charge density plots (Isosurface value 0.2322 e.Å-3) of (e) CO, (f) CO2, (g) NO and (h) NO2 molecules adsorbed on graphene surface. The CDD (Charge density difference) plots of (i) CO, (j) CO2, (k) NO and (l) NO2 molecules adsorbed on graphene surface. (Isosurface value 0.0036 e.Å-3). Now, the question is whether toxic/greenhouse gases (CO, CO2, NO, NO2) could be adsorbed on the pure graphene surfaces or not? The CO/CO2 and NO/NO2 molecules are adsorbed on the 32 atoms unit cell of graphene through C-/O- and N-/O- centre respectively. The optimized structures of the adsorbed gaseous molecules (CO, CO2, NO, and NO2) are presented in figure 1(a-d). The CO/CO2 molecules adsorbed through C- centre is thermodynamically 0.02/0.02 eV [Figure 1(a-b)] more stable than through the O- centre. The distance between the graphene and the adsorbed CO molecule is 3.08 Å, thus interacts weakly. Similarly NO/NO2 molecules are more stable (by 0.01/0.01 eV) while adsorbed through N-centre [Figure 1(c-d)]. Here again the interaction between the gaseous molecules (NO and NO2) and graphene surface is very weak as the distance between the surface and the gaseous molecules are more than 2.98 Å. The calculated adsorption energies for CO, CO2, NO, and NO2 molecules are 0.12, 0.11, 0.10, 0.26 eV respectively. Our results show NO2 interacts better among all the four gases and this is fairly



in agreement with previous experimental study, where they show graphene could selectively sense NO2.32 The total charge density and CDD [Figure 1(e-l)] plots confirm the electrostatic nature of the interaction between the graphene and the gas molecules. Therefore, we can say, the molecules are physisorbed on the graphene surface. Bader charges are calculated for the gas molecules adsorbed on the graphene surface. We find that there are no changes in the total Bader charges on the C1 as well on the neighbouring carbon atoms (C2, C3 and C4); [Table S1, Supporting Information] when gaseous molecules are adsorbed on the graphene. In fact total number of valance electrons on the CO (10e), CO2 (16e), NO (11e) and NO2 (17e) molecules remains constant even after adsorption on the graphene surface. Thus, there are no charge transfer between the graphene and the adsorbed gas molecules. Our adsorption energy, charge density and Bader charge analysis study shows that the interaction between the pure graphene and toxic gases are very weak.

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Figure 2: The total and partial density of states (TDOS/PDOS) are plotted for (a) CO, (b) CO2, (c) NO and (d) NO2 molecules adsorbed on pure graphene surfaces. (EF is set to zero and indicated by the black dashed line). We have plotted the TDOS and PDOS of the various gases adsorbed on the graphene surface [Figure 2 (a-d)]. The total density of states of graphene does not change much due to the adsorption of CO and CO2 molecules [Figure 2 (a-b)] but gives a sharp peak at Fermi energy


2

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while NO/NO2 molecules [Figure 2 (c-d)] are adsorbed. These sharp peaks are [Figure 2 (cd)] coming from the 2p orbitals of the N and O atoms of the NO and NO2. But, electronic properties did not change much due to the gas adsorption. Thus, we believe pure graphene cannot be a promising material for semiconductor based gas sensor. The B-, N- and B-N doped graphene is studied to improve the interaction with the gaseous molecules. Moreover, B-doped graphene (B@graphene) is a p-type semiconductor so hole is the major carrier. Now, when oxidizing gases (NO2, CO2) come in contact with the semiconductor it increases the number of hole in the VB. As a result, the conductivity of the semiconductor increases. But is case of reducing gases (NO, CO) the effect is opposite. Similarly, N-doped graphene (N@graphene) is an n-type semiconductor. So, when it comes in contact with the reducing gases the number of electron increases in valance band. So the electrical conductivity increases. It also shows opposite effect for oxidizing gases. Therefore such doping is very important for improving the sensitivity and selectivity of the gas sensors.

4.1. B@-, N@-, B-N@graphene for gas sensor: 4.1.1 Formation energy: One boron/nitrogen atom is doped in the 32 atomic unit cell of graphene, which corresponds to 3.12 % of boron/nitrogen doping. The doping concentration is varied from 3.12% to 50.00% by doping 3 (9.38%), 5 (15.60%), 7 (21.80%) and 16 (50.00%) boron/nitrogen atoms in the 32 atomic unitcell of graphene. The doping is done such a way that the B/N-B/N distances are the longest to avoid severe structural distortion. Similarly, the B-N co-doping is done by doping 2, 6, 10, 14 and 16 pairs of B-N atoms, which correspond to 6.25%, 18.75%, 31.25%, 43.75% and 50.00% doping. The formation energy (Ef) of the B-, N- doped and B-N-doped graphene system is calculated using the following equations. Ef = Edoped-graphene – Egraphene-x(EB/N-EC)


(3)


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Ef = EB-N@graphene – Egraphene]-x(EB + EN )-2EC

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(4)

Here, Edoped-graphene and Egraphene are the energy of the optimized structure of doped-graphene and pure graphene sheet respectively and ‘x’ is the number of dopant atoms. EB/N and EC is the atomic energy of doped (B/N) and carbon atom respectively. EB and EC are calculated from the α-rhombohedral boron and graphene structure respectively. EN is calculated from the total energy of the N2 molecule. We have varied the doping concentration from 3.12 to 50.00% for all the cases as B-, N-and B-N doped graphene is experimentally achieved up to 13.00%, 15.60% and 27.00%respectively.55-56 So, we assume the B-, N- and B-N doped graphene can be synthesized up to 50.00% doping concentration. The formation energy value for various doping concentrations is given in supporting information (Table S2). Our graphical representation of doping concentration vs. formation energy [Figure 3] clearly shows that B-N co-doping is energetically more favorable than N-doping than B-doping. Such trend also observed experimentally57 and this could be due to their (C and N) similar sizes (C = 0.73 Å and N= 0.71 Å) than the B atom (0.84 Å).

Figure 3. Graphical representation of the change in doping formation energy of B, N and B-N co-doped graphene with increasing dopant concentration. 4.1.2 B@Graphene:


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As boron has one less valance electron than carbon therefore such type of doping will lead into a p-type of semiconductor. We have optimized the 3.12% B@graphene and the structure is slightly distorted as the B–C bond length is increased to 1.48 Å, thus deviated from the C–C bond length of 1.42 Å. The charge density plot shows that there is an electron deficient area created around the B centre and the nature of the B-C bond is not purely covalent. Bader charge analysis also shows that B is positively charged (+1.97) than the neighbouring C (-0.57) atoms. The TDOS and PDOS of 3.12% B@graphene is plotted to understand the electronic structure of B-doped graphene. We find some orbital density at the Fermi energy region and such orbital density coming from the dopants interacting with the neighbouring carbon atoms to form degenerate states at the Fermi energy. Such doping shifts the Fermi energy (by 0.76 eV) below the Dirac point, thus makes B-doped graphene a p-type semiconductor as theoretically predicted by Cervantes-Sodi and co-workers.26 Though such B-doping does not open up the band gap at the Fermi energy but creates a gap (0.20 eV) above the Fermi energy, [Figure S2, Supporting Information] behaves like a degenerate semiconductor.58-59 Thus, B-doped graphene’s electronic properties can be controlled by tuning the position of the Dirac point using the gate voltage.26, 59 The boron doping concentrations are increased to open up the band gap at the Fermi energy. Therefore, we have doped 3, 5, 7 and 16 boron atoms in the 32-atom sheet of carbons, corresponds to 9.38%, 15.60%, 21.80% and 50.00% of boron doping. But the increase of B doping concentration leads to a systematic opening up of band gap (0.20 eV to 2.56 eV) above the Fermi energy. After 50.00% of boron doping, 2.56 eV of band gap is produced above the Fermi energy. We assume as the doping concentration increases (hole concentration increases), the amount of degenerate states appearing at the Fermi energy level increases thus the effective band gap is shifted towards the more positive side [Figure S3-S6, and Table S3, Supporting Information].



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The interaction of the gaseous molecules on the B-doped-graphene is studied for low (3.12%) and high doping (50.00%) concentrations to get the minimum and maximum effect on the electronic structure of the doped graphene systems. The concentrations of the gas molecules are varied along with their doping concentrations. For the 3.12% doped-system, the dopant and gaseous molecular ratio is 1:1 whereas 16:4 for the 50.00% doping concentration. The dopant vs. gaseous molecular ratio (16:4) is increased to minimize the interaction between the gaseous molecules.

Figure 4: The optimized structures of (a) CO, (b) CO2, (c) NO and (d) NO2 molecules adsorbed on 3.12% B@graphene surface (Brown. green, blue, and red coloured balls denote C, B, N and O respectively). Charge density plot of (e) CO, (f) CO2, (g) NO and (h) NO2 molecules adsorbed on 3.12% B@graphene surface (Isosurface value 0.2322 e.Å-3). The CDD plots of (i) CO, (j) CO2, (k) NO and (l) NO2 molecules adsorbed 3.12% B@graphene surface. (Isosurface value 0.0036 e.Å-3). Gaseous molecules are most stable while adsorbed at the B-site of the B-doped graphene (3.12%) [Figure 4(a-d)]. They are adsorbed via C-/O- and N-/O- centre for CO/CO2, NO/NO2 molecules respectively. Our relative energetic study shows CO and CO2 molecules are preferred to be adsorbing via C-centre by 0.60 and 0.09 eV respectively. The distance between the B and C, of CO and CO2 gas molecules are 1.59 and 3.14 Å respectively. The adsorption energies calculated for the most stable CO and CO2 structures are 0.66 and 0.12 eV respectively.


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Therefore, CO interacts with the B-doped graphene strongly whereas CO2 interacts weakly. This is because CO2 is a linear molecule thus it prefers to be adsorbed in a parallel fashion. As a result, the electrostatic interaction between the negatively charged O (of CO2) and the graphene

π-electrons might be strong enough to repel the molecule. Similarly, NO and NO2 molecules are preferred to be adsorbing via N-centre than the O-centre [Figure 4(c-d)]. The relative energetics study show structures adsorbed via N-centre are stable by 0.38 eV and 0.48 eV respectively. The B-N distances are 2.22 Å and 1.65 Å for the NO and NO2 gases respectively. The calculated adsorption energies for the most stable NO and NO2 structures are 0.58 and 1.23 eV respectively. The charges on the N of NO2 are higher than on the NO thus the interaction between the B and the bent NO2 molecule is stronger than the NO. So, the adsorption energy and charge density analysis shows that CO/CO2 molecules are physisorbed whereas NO/NO2 molecules are chemisorbed on the 3.12% B@graphene surface. Previous theoretical studies demonstrated similar trend in gas adsorptions on B doped graphene surfaces.33-34 Zhang et al.33 reported that NO and NO2 bind more strongly than CO. Our calculated gaseous molecular adsorption energy trends are very much similar with previous theoretical reports on B@graphene surfaces.33-34 The calculated adsorption energy of the gases on the 50.00% B@graphene system shows similar trend as observed in 3.12% B@graphene. It ranges from 0.17 eV to 0.73 eV per gas molecules. We have also plotted the charge density and CDD for the 50.00% B-doped+gas systems to understand the extent of interactions. The adsorption energy and charge density analysis shows similar trend in bonding as observed in 3.12% B@graphene surface.


The Journal of Physical Chemistry 8

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Figure 5: The total and partial density of states (TDOS/PDOS) are plotted for (a) CO, (b) CO2, (c) NO and (d) NO2 molecules adsorbed on 3.12% B@graphene surfaces. (EF is set to zero and indicated by the black dashed line). The TDOS and PDOS are plotted [Figure 5 (a-d)] for the 3.12% B@graphene+gas molecules. In case of 3.12% B@graphene+CO2 system, neither band gap opening nor any CO2 orbitals present at the Fermi energy. It concludes that in presence of CO2 on B@graphene does not make any difference in the electronic structure of B@graphene. On the other hand, adsorption of CO, NO creates some impurity states at the Fermi energy level [Figure 5 (a,c)]. PDOS analysis confirms the impurity states appearing at the EF mainly coming from the gaseous C (CO) and N (NO) 2p orbitals. On the other hand, NO2 gas adsorption opens up the band gap [0.50 eV, (Figure 5d)] at the Fermi energy. Hence, we can say B-doped graphene can be a selective and sensitive gas sensor for the NO2 molecule. Similarly, we have studied the electronic structure of the 50.00% B@graphene+gas systems. There is no band gap opening at the Fermi energy due to the gaseous (CO, CO2, NO and NO2) molecular adsorption. For the higher doping concentration, the degenerate orbitals of B appear at the Fermi energy, thus not good for sensor. 4.1.3 N@Graphene: Similarly, we thought if we dope graphene with a more electro-negative atom such as nitrogen (N) then the interaction between the nitrogen doped graphene and the gaseous molecules can be improved. On the other hand, nitrogen has one more valance electron than the carbon atom


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therefore such doping could lead to an n-type semiconductor. So, if we dope one N atom in the 32 atoms unitcell of graphene, which corresponds to 3.12% of nitrogen doping. We have relaxed the N@graphene and the optimized structure retains the similar geometry as pure graphene. This is because nitrogen and carbon are of same size thus the N-C bond distances (1.41 Å) are close to C-C bond distances (1.42 Å) of graphene. Charge density analysis shows more electron density around the N atom of N@graphene. Our Bader charge calculation also shows that N is more (-1.22) negatively charged than the surrounding C atoms (0.30). Thus we assume the extra electron is localized mainly on the nitrogen atom. The TDOS and PDOS are plotted to understand the electronic structure of the N-doped graphene. From the TDOS analysis, we find band gap opens up by 0.20 eV below the Fermi energy. Here Fermi energy position shifted 0.76 eV above the Dirac point thus N@graphene is an n-type semiconductor. Some orbital density appearing just below the Fermi energy (-0.15 eV) and PDOS analysis confirms that this density is mainly coming from the N 2p Thus N-doped graphene’s electronic properties can be controlled by tuning the position of the Dirac point using gate voltage.26 We assumed, if we increase the N doping concentration then we would open up the graphene’s band gap significantly. Hence, we have doped 3, 5, 7 to 16 nitrogen atoms in the 32 atomic unit cell of graphene, corresponds to 9.38%, 15.60%, 21.80% to 50.00% of N doping. As the N doping concentration increases, the effective band gap of N@graphene system increases from 0.15 eV to 2.82 eV [Figure S7-S9, Supporting Information] below the Fermi energy. Here also some impurity states appearing at the Fermi energy. This can also be explained as degenerate states coming from the N and C 2p orbitals. As N has more electron than C so the degenerate states are coupled with the valence band (VB) side. These types of systems called the degenerate semiconductor.58


The Journal of Physical Chemistry (i)

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(j)

(k)

(l)

Figure 6: The optimized structures of (a) CO, (b) CO2, (c) NO and (d) NO2 molecules adsorbed on 3.12% N@graphene surface (Brown, blue and red coloured balls denote C, N and O respectively). Charge density plot of (e) CO, (f) CO2, (g) NO and (h) NO2 molecules adsorbed on 3.12% N@graphene surface (Isosurface value 0.2322 e.Å-3). The CDD plots of (i) CO, (j) CO2, (k) NO and (l) NO2 molecules adsorbed on 3.12% N@graphene surface. (Isosurface value 0.0036 e.Å-3). The gaseous molecular adsorptions are studied at the N-centre of 3.12% N@graphene to see whether the interaction between the N and gaseous molecules could be improved or not. CO adsorbed through C-center is thermodynamically more stable by 0.01 eV than through the Ocenter [Figure 6 (a-d)]. The B-C bond distance between the B and adsorbed CO gas molecule is 3.21 Å, thus weakly interacting. Similarly CO2 is 0.01 eV more stable when adsorbed via Ccenter than through the O-center and the B-C distance is 3.08 Å. Thus both the gases (CO and CO2) interact weakly with the N-doped graphene. The adsorption energies calculated to be 0.11 and 0.12 eV for the most stable CO and CO2 structures respectively. Similarly, NO and NO2 gases are preferred to be adsorbed via N-center by 0.02 eV and 0.16 eV respectively. Their adsorption energies are calculated to be 0.18 and 0.26 eV respectively. The distance between the N and adsorbed NO and NO2 gases are 3.05 Å and 3.02 Å respectively. Therefore the interaction between the N-doped graphene and the NO/NO2 gas molecules are comparably stronger. Our


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calculated adsorption energy trends are in agreement on previous studies on N-doped graphene surfaces reported by Zheng and co-workers.33 When the gaseous molecules adsorbed on the N@Graphene surface, there is not much change in the Bader charges on the N and the surrounding C atoms. Thus proves that there are only weak interactions between the gases and N@graphene. Only in case of NO2 adsorption, the charge on the N atom changes (6.18) due to the stronger interaction with the NO2 molecule. Thus NO2 has far higher adsorption energy in comparison to the other three gases. The charge density and CDD [Figure 6(e-l)] is plotted to understand whether any orbital mixing present between the Nand gaseous molecules. The CDD analysis shows there is not enough orbital mixing between the N-doped graphene and the gas molecules. The calculated adsorption energy of the gases on the 50.00% N@graphene system ranges from 0.16 eV to 0.52 eV per gas molecule. The charge density and CDD plot confirms that there is only electrostatic attraction between the N atom and the adsorbed gases. 18


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Figure 7: The total and partial density of states (TDOS/PDOS) are plotted for (a) CO, (b) CO2, (c) NO and (d) NO2 molecules adsorbed on 3.12% N@graphene surfaces. (EF is set to zero and indicated by the black dashed line). The electronic structure (TDOS and PDOS) [Figure 7 (a-d)] of the N@graphene+gas molecules is studied for 3.12 and 50.00% N@graphene systems [Figure S10-S11 and Table S4; Supporting


4


Information] to understand effect on the electronic structure of the N@graphene due to the toxic gases adsorbed. The electronic structure of the 3.12% and 50.00%N@graphene+gas system does not open up any band gap at the Fermi energy. It means adsorption of the toxic gases did not alter the electrical current flow through the 3.12% N@graphene. Thus, N@graphene (3.12%) cannot be a promising semiconductor based gas sensor. 4.1.4 B-N@Graphene: We have considered B-N co-doping to check whether these type of doping could open up the band gap at the Fermi energy and such doping might improve the selectivity and sensitivity of the gas sensor. Therefore, we have doped one boron and nitrogen atom into the six membered ring of the 32 atomic unit cell of graphene [Figure 8a] in such a way that the interaction between the B and N is minimal. 30

(a)

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TDOS PDOS C (2p) PDOS B (2p) PDOS N (2p)

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Figure 8: The optimized structures of (a) 6.25% B-N@graphene (Front view) and (b) Charge density plots of 6.25% B-N@graphene (Isosurface value 0.2322 e.Å-3). (c) The TDOS and PDOS of 6.25% B-N@graphene. (EF is set to zero and indicated by the black dashed line). The charge density study [Figure 8b] shows that there is less electron density around the B atom but more on the N atom as found in their respective B@graphene and N@graphene systems. Bader charge analysis shows similar trend as B and N bears +1.97 and -1.19 charges respectively [Table S5, Supporting Information]. Interestingly, in the mono-doped case


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(B@graphene and N@graphene), the 2p orbitals of the dopants are mainly present at the Fermi energy thus we do not get clean band gap at the Fermi energy. Even, if we increase the concentration of B and N doping, the effective band gap shifts further and further from the Fermi energy. But if we co-dope (6.25% B-N doping) then the band gap opens up (0.24 eV) at the Fermi energy [Figure 8c]. This is because the total number of electrons in the system remains constant thus gives a clean band gap at the Fermi energy. We have also increased the BN concentration in the B-N@graphene from 6.25% to 18.75%, 31.25%, 43.75% and 50.00% and the calculated band gaps are 0.24, 0.90, 0.30, 0.19 and 0.11 eV respectively [Figure S12-S13, Table S6, Supporting Information]. As the B-N co-doping pattern is different for the higher concentrations (31.25% to 50.00%) thus the band gap opening are different. First, we have considered 6.25% B-N@graphene for our further study. The gaseous molecules (CO, CO2, NO, and NO2) are adsorbed at the B-site of the 6.25% B-N@graphene as B-interacts more strongly with the gas molecules in comparison to N.

Figure 9: The optimized structures of (a) CO, (b) CO2 (c) NO (d) NO2 molecules adsorbed on 6.25% B-N@graphene surface. Charge density plots of (e) CO, (f) CO2, (g) NO, and (h) NO2 molecules absorbed on 6.25% B-N@graphene (Isosurface value 0.2322 e.Å-3). The CDD of (i) CO, (j) CO2, (k) NO, (l) NO2. (Isosurface value 0.0036 e.Å-3). The adsorption energies calculated for all these gaseous molecules found to be stronger (0.25 eV to 1.35 eV) with respect to their mono-doped systems. Among these gases, NO2 (1.36 eV), NO



(0.85 eV) and CO (0.77 eV) bind very strongly whereas CO2 (0.25 eV) weakly on the BN@graphene surface [Figure 9(a-d)]. We have plotted the charge density and CDD of the gases adsorbed on the B-N@graphene surface [Figure 9(e-l)] to see the interaction between the BN@graphene and gas molecules. The charge density and CDD also shows that NO2, NO and CO have considerable orbital mixing with the B of the B-N@graphene. Bader charge analysis shows that B gains 0.12e charge (+1.85) when CO is adsorbed and loses 0.11e (+2.08) when NO2 is adsorbed. As we know CO is a reducing gas so it transfers electrons to the B, whereas NO2 is an oxidizing gas thus oxidizes the B. Such considerable amount of electron transfer is only possible when they are bonded, thus proves that CO and NO2 gases are chemisorbed on B-N@graphene surface. Similarly, we have studied the adsorption property of these gas molecules on 50.00% BN@graphene as well The calculated adsorption energy of the gases on the 50.00% BN@graphene system ranges from 0.24 eV to 1.23 eV per gas molecules. The charge density and CDD is plotted for the 50.00% B-N@graphene system and it confirms that the gas molecules are chemisorbed on the 50.00% B-N@graphene surface. 4


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(a)

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Figure 10: The total and partial density of states (TDOS/PDOS) are plotted for (a) CO, (b) CO2, (c) NO and (d) NO2 molecules adsorbed on 6.25% B-N@graphene surface. (EF is set to zero and indicated by the black dashed line]. TDOS and PDOS of 6.25% B-N@graphene+gas molecules are plotted in [Figure 10 (a-d)]. The electronic structure of 6.25% B-N@graphene+CO/CO2 molecules [Figure 10 (a-b)] shows that


4

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there is not much change in the electronic structure when CO and CO2 molecules are adsorbed. As the band gap changes from 0.24 to 0.43 for CO and 0.24 to 0.35 for CO2. In case of CO the N 2p (loan pair) orbital appears just below the Fermi energy and the vacant O 2p orbital just above the Fermi energy. On the other hand there are impurity states appearing at the Fermi energy when NO/NO2 gases are adsorbed on the B-N@graphene surface [Figure (c-d)]. We have found same type interaction of NO with the B-N@graphene surface as we have discussed in B@graphene. So, there is no band gap at the Fermi energy. So, we find 6.25% B-N@graphene is highly sensitive and selective for N-based (NO and NO2) gas molecules. Similarly, the 50.00% B-N@graphene shows similar trend. So, the B-N@graphene could be a highly sensitive and selective semiconducting based gas sensor. 5. Conclusion: Here, we have performed Density Functional Theory (DFT) calculations to study the gas (CO, CO2, NO and NO2) sensing mechanism of pure and doped (B@, N@ and B-N@)graphene surfaces. The adsorption energy, charge density and CDD of the pure graphene+gas molecules are studied to understand their bonding and selectivity. The p- and n-type of doping certainly improves the interaction between the surface and the gaseous molecules but such doping does not change the band gap properties at the Fermi energy. Therefore semiconductor-based gas sensing technique is not promising for such type of doping. But the B-N co-doping changes the graphene’s electronic properties at the Fermi energy. Once the gaseous molecules are adsorbed on the B-N@graphene surface, the electronic properties change maximum for NO and NO2 but not for CO and CO2. Therefore B-N@graphene could be a promising semiconductor based gas sensor.

6. Acknowledgments:



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We thank IIT Indore for the lab and computing facilities. This work is supported by Council of Scientific and Industrial Research [CSIR, Grant number: 01(2723)/13/EMR(II)], New Delhi. A. M. thanks CSIR for the Senior Research Fellowship. R. A. thanks Swedish Research Council (VR), Swedish Institute, and Swedish Energy Agency for financial support and SNIC for computing time.

7. Associated Content: * Supporting Information Optimized structures (graphene, graphene+gases, B@graphene, 50%B@graphene+gases, N@graphene, 50.00%N@graphene+gases, and 50.00% B-N@graphene) and their respective TDOS/PDOS, charge densities and Bader charges are given in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

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B-N@graphene for semiconductor based selective gas sensor.


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