Lewis AcidâBase Adducts for Improving the Selectivity and Sensitivity

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Lewis Acid-Base Adducts for Improving the Selectivity and Sensitivity of Graphene based Gas Sensors Indrani Choudhuri, Debopriya Sadhukhan, Priyanka Garg, Arup Mahata, and Biswarup Pathak ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00031 • Publication Date (Web): 25 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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Lewis Acid-Base Adducts for Improving the Selectivity and Sensitivity of Graphene based Gas Sensors Indrani Choudhuri,† Debopriya Sadhukhan,† Priyanka Garg,† Arup Mahata,† Biswarup Pathak, †,#,* †

Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology (IIT) Indore, Indore, M.P. 452020, India

#

Centre for Material Science and Engineering, Indian Institute of Technology (IIT) Indore, Indore, M. P. 452020, India Email: [email protected]

Abstract: The first principles calculations are performed to study the gas (NH3, NO2 NO and N2O) sensing properties of pure and doped (B@, Al@ and Ga@) graphene surfaces. Interactions between the gas molecules (NH3, NO2, NO and N2O) and the graphene surfaces are improved due to the doping on graphene. So, the dopants are carefully chosen to form the Lewis acidbase pairs between the dopants and gas molecules. Formation energy calculations and abinitio molecular dynamics simulations (AIMD) are carried out to evaluate their thermodynamic and thermal stabilities, respectively. The electronic properties of the Al@graphene change significantly when a selective gas molecule (NO2) is adsorbed. Thus, we report, Al@graphene can be a promising material for the highly selective and sensitive semiconductor based gas sensor.

Keywords: Graphene, Boron, Aluminium, Gallium, Doping, Gas sensor

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Over the past few decades, industrialization and modernization led to produce more and more toxic gases into the environment.1-2 Such gases have a negative effect on the environment. Hence, gas sensors are very much important to detect the presence of toxic gases.3-5 Micro and nanotechnology based gas sensors have the capability of detecting a minute quantity of gas selectively with a measurable response.6 Among all the semiconductor based gas sensors, graphene has drawn the tremendous attentions for its sensing precision toward a variety of gases.7-9 The graphene based sensor is categorized as a substantial chemiresistor because it changes its electrical resistivity10-12 with the change of its chemical environment.13-14 Graphene is the thinnest possible material,7-9 with a large surface area (3000 m2 per gram); thus a promising material for gas sensing. Interestingly, graphene and its electronic properties were theoretically proposed in 1947.15-16 However, it took another 40 years to synthesize and reproduce same properties as proposed theoretically.17-18 Thus, theoretical studies are very important for predicting new materials and their properties. It is believed that graphene can be an excellent sensor owing to its two dimensional structure, lowest signal-to-noise ratio and excellent electrical, optical and mechanical properties.1-2 Graphene is a highly sensitive material which detects all the individual events when a gas molecule attaches to or detaches from its surface.7 However, it is very difficult to prepare a perfect single layer graphene with zero band gap. Various approaches have been taken to improve the performance of graphene based devices and band gap engineering is one of the most promising approaches for changing its electronic properties.19-23 Doping is one the most efficient methods to improve the electronic properties of the material. Substitutional doping (such as B, N, B-N) in graphene affects the sp2 hybridization of carbon atoms which in turn changes the electronic properties of the system.24 Lherbier et al.25 showed that the charge mobility and conductivity of graphene changes with B-/N-impurities. Similarly, Cervantes-Sodi et al.26 predicted band gap opening in graphene by elemental B/N doping. Since then, there are many experimental reports on band gap opening in graphene by B/N-doping.27-29 B-, N-, and B-N-doped graphene 2 ACS Paragon Plus Environment

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sheets are experimentally synthesized and show similar electronic properties as predicted theoretically.27 There are several experimental and theoretical studies on the gas sensing properties of the pure and doped graphene sheets and some of these results are very promising for semiconductor based gas sensor.28,32-36 However, for their practical usages, the sensitivity of the semiconductor based gas sensor is very important as the concentrations of the gases could be in the ppm/ppb level.29-33 Yang et al.29 demonstrated that graphene based sensor on paper substrate shows prompt response in the presence of 200 ppm NO2 gas under 0.5% strain. Ko et al.30 showed that graphene can be very selective towards NOx molecules at room temperature. The sensitivity was up to 9% in the presence of 100 ppm NO2 gas. Zhang and co-workers31 theoretically studied the interactions between the graphene (defected and B/N doped) and the gas molecules to understand their gas sensing properties. They reported that the gas sensing properties of the graphene could be drastically improved by introducing dopants and defects. Dai et al.32 reported the gas sensing property of B/N/Al and S doped graphene sheets for small gas molecules. So, previous theoretical studies mainly focused on the adsorption properties of pure, defected and doped graphene sheets. Recently, Al and Ga doped graphene sheets are experimentally reported for nitrogen based gas sensors.33-34 Lv et al.33 demonstrated that Al and Ga doping enhance the adsorption and dissociation of N2O. Cho et al.34 reported that Pd and Al nanoparticles incorporated graphene enhance the sensing ability of NH3 and NO2, respectively. So, it is interesting to know that incorporation of such nanoparticles change the sensitivity and selectivity towards the gas molecules. In this work, we have theoretical investigated the B-, Al- and Ga-doped graphene to improve its gas sensing efficiency and selectivity towards N-based greenhouse gases (NH3, NO2, NO and N2O). It is very important for graphene to interact with the gas molecules so that its electronic properties will be changed. However, graphene does not have any dangling bonds. Thus, it does not interact strongly with the gas molecules. Here the gases and dopants (group 13 elements) are chosen in such


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a way that they form a Lewis acid-base pair and thus improve the sensitivity and selectivity towards the gas molecules. Computational Details: We have used the Vienna ab initio simulation package (VASP)35 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.36 The projected augmented wave (PAW) method37 is employed using an energy cut-off of 470 eV to describe the electronic wave function. Thus the generalized gradient approximation (GGA-PBE) within projector-augmented wave (PAW) methodology is adopted to investigate the electronic properties of such materials. Self-consistency was achieved with a convergence tolerance set at 10-4 eV and force at 10-2 eV/Å. We have included semi-empirical DFT-D3 type of dispersion correction38 as the longrange interaction present between the surface and the gaseous molecules. For all the calculations, 20 Å of vacuum was set to avoid interactions between the periodic images. The Brillion zone is integrated using Monkhorst-Pack generated sets of 11×11×1 k-points.39 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. Bader charge analysis is done40-42 to understand the charge transfer process. We have used Henkelman programme43 with near-grid algorithm refine-edge method for the Bader charge calculations. The total energy convergence criterion is set to 10-4 eV for the AIMD simulations. The temperature of the system during MD simulation is controlled by the nose thermostat model.44 The adsorption energy (EA) of the gases (NH3, NO2, NO, and N2O) on the pure and doped graphene surfaces is calculated using the following equation. EA = (Egraphene+ Egas)–Egas-graphene

(1)


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Here Egas-graphene represents the total energy of the gaseous molecules adsorbed on the pure graphene surfaces. Egraphene and Egas represent the single point energy of the graphene and gas molecule, respectively. Results and discussion: Pure graphene for gas sensor: A two-atom unit cell is considered for the hexagonal graphene structure. Hexagonal graphene structure is fully relaxed and the calculated carbon-carbon bond distance (1.42 Å) is very much in agreement with experimental C-C bond distance (1.42 Å) of graphene.45 A 4×4 supercell of 32 carbon atoms is constructed to study the interaction between the graphene and the gas molecules (Figure 1). The charge density of pure graphene (Figure S1, Supporting Information) shows the electrons are highly delocalized over the graphene surface. Bader atomic charge analysis shows that all the carbon atoms of graphene have the same number of electrons (4e). As can be seen from the total density of states (TDOS) of pure graphene (Figure S1, Supporting Information) that graphene does not have a bandgap and the orbital densities mainly come from the C 2p orbitals.45 The change in the electronic properties is one of the most important criteria for a semiconductor based gas sensor.46 When a gas molecule is adsorbed on the semiconductor’s surface, then the semiconductor interacts with the gas molecule which in turn tunes the band position. Such changes near the Fermi level control the flow of electrons from the valance band (VB) to the conduction band (CB). So, the main objective of this study is to find out the changes in the electronic properties due to the adsorption of gas molecules.47-48 The gas molecules are detected based on the conductance change in FET49-51 (Field-effect Transistors) semiconducting channels due to the adsorption of gas molecules. The adsorption of gas molecules on the surface of the semiconducting channel either changes its local surface potential or directly dopes the


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channel, resulting in change of the FET conductance. Experiments52-53 and theoretical54-55 studies concluded that the charge transfer from the adsorbed gas molecules to the semiconducting channel is the dominant mechanism for the current responses. The different current responses to different gases supported the charge transfer mechanism. These current responses due to the adsorption of gas molecules were realized experimentally by measuring the Ids–Vg curve before and after adsorption of gases. Here Ids refers to drain to source current and Vg is the gate to source voltage.1

Figure 1: Optimized structures of (a) NH3, (b) NO2, (c) NO and (d) N2O adsorbed graphene. Here brown, blue, red and beige colour balls denote C, N, O, and H, respectively. Electrostatic potential (ESP) plots (Isosurface value: 0.03 e.Å-3) of (e) NH3, (f) NO2, (g) NO and (h) N2O adsorbed graphene surfaces. The blue and red colours denote less and more electron dense area in the electrostatic potential surface. TDOS and PDOS of (i) NH3, (j) NO2, (k) NO and (l) N2O adsorbed graphene surfaces. The Fermi level is set to zero and indicated by the black dashed line. The adsorption behaviour of the gas molecules is investigated on a 32-atom supercell of graphene. All possible orientations of the gas molecules are considered for the adsorption and the most stable geometries of the graphene+gas molecules are shown in Figure 1(a-c). We


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have mainly considered the situation where the gas molecules are adsorbed on the same side of graphene. We have mainly considered the situation where the gas molecules are adsorbed on one side of the graphene. In general, graphene is deposited on a substrate material (mainly Si/SiO2) for such kind of applications.1 So, it is unlikely that both side of the graphene is available for gas molecule adsorption. Interestingly, NH3 is adsorbed via the H-atom where as NO2, NO and N2O prefer to adsorb via the N-centre. However, interactions between the gas molecules (NH3, NO2, NO and N2O) and the graphene surfaces are very weak in nature. In all the cases, the distance between the graphene and the gas molecule is more than 3 Å. The calculated adsorption energies for NH3, NO2, NO and N2O molecules are 0.14, 0.38, 0.10 and 0.19 eV, respectively, which are very much in agreement with previous studies.31 The distances between pure graphene and gas molecules are more than 3 Å, indicating a physical adsorption, though the graphene-NO2 interaction is stronger compared to graphene-NO, graphene-N2O and graphene-NH3.30 The electrostatic potential is plotted on the total electron density surface by VESTA56 [Figure 1(d-f)] to show a more qualitative and illustrative analysis of the charge distribution. It shows that NH3 is adsorbed on graphene surface via positively charged H atoms of NH3 due to the electrostatic interaction. Other gas molecules (NO2, NO, N2O) are adsorbed via negatively charged N atoms. It is clearly show that there are no sign of orbital overlaps between the graphene and the gas molecules. Mainly electrostatic interactions are present between the graphene and the gas molecules. Bader charge analysis of the graphene+gas systems also complements our ESP results. It is found that Hs in NH3 have +0.38 charge on them which help NH3 molecule to adsorb on graphene via H atoms. On the other hand, N in NO2, NO and N2O gas molecules have +δ charges on them. Bader charge analysis also shows that the graphene [Table S1, Supporting Information] and the gas molecules retain their total number of valence electrons [NH3 (8e), 7 ACS Paragon Plus Environment

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NO2 (17e), NO (11e) and N2O (16e)]. This suggests, there is no net electron transfer between the graphene and the gas molecules. Adsorption energy, electrostatic potential plots and Bader charge analysis indicate that the interaction between the graphene and the gas molecules are of very weak in nature. We have plotted the TDOS and PDOS (partial density of states) of graphene+gas [Figure 1(gi)] systems to find out the changes in electronic properties due to the adsorption of gas molecules. The density of states of graphene does not change much due to the adsorption of NH3/N2O [Figure 1(i) and 1(l)] but it gives a very sharp pick at the Fermi when NO/NO2 molecules [Figure 1(j-k)] are adsorbed. This is mainly due to the weak interaction between the C 2p orbitals of graphene and N 2p orbitals of NO/NO2 molecules. But the linear energy dispersion at the Fermi level is similar for all the three cases. Therefore, the overall change in electronic properties due to the adsorption of gas molecules is not good enough for the sensitivity and selectivity. The B-, Al-, and Ga-doped graphene systems are studied to improve the interactions between the graphene and gas molecules.57-60 Moreover, B-, Al-, and Ga@graphene systems are ptype semiconductors, so holes are the major carrier here. Now, when N-based gases (NH3, NO2, NO and N2O) come in contact of the doped-graphene, then the lone pairs of the gas molecule interact with the surface which in turn changes the electronic properties of the material.61

Doped (B, Al, Ga) graphene for gas sensor: Formation energy: One B/Al/Ga atom is doped in the 32 atomic unit cell of graphene, which corresponds to 3.12% of elemental doping. The concentration is increased from 3.12 to 6.25% by doping 2 atoms in the 32 atomic unit cell of graphene. 6.25% doping is done in such a way that the B-B/Al-Al/Gahas the maximum distances to avoid any structural distortion. 8 ACS Paragon Plus Environment

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Our graphical representation [Text 1, Figure S2, Supporting Information] of formation energy vs. doping concentration clearly shows that B-doping is favourable over Al- and Ga-doping. This could be due to their (C and B) similar sizes (C = 0.73 Å and B= 0.84 Å) compared to Al (1.21 Å) and Ga (1.22 Å). The calculated binding energies for B, Al and Ga atoms are 13.88, 6.78, and 5.54 eV, respectively. This can be explained from the size of the atoms (atomic radius: Ga>Al>B). Our calculated cohesive energies are 5.92, 3.53 and 2.96 eV for the B, Al and Ga bulk structures, respectively.62 Thus, these dopants will not form clusters, as dopant’s binding energy is higher than their cohesive energy and the distance between the doped atoms are more than 8.60 Å (for 6.25% doping).

Thermal Stability: We have checked the thermal stability of the 3.12% and 6.25% B-, Al-, and Ga@graphene sheets by performing the ab initio molecular dynamics (AIMD) simulations at room temperature (300 K).63-64 The AIMD simulations are performed on the 4×4 supercell structures to check their thermal stabilities at room temperature. All the simulations are performed for 5 ps (5000 fs) with a time step of 1 fs. We did not find any structural reconstructions [Figure S3-S5, Table S2, Supporting Information] for any of the doped systems throughout the simulation period. The structural parameters (Table S2, Supporting Information) are tabulated before and after the simulation and we find they are very much comparable. In all the cases, we find minimal energy fluctuations [Figure S3-S5, Supporting Information] throughout the simulation period, which suggests such doped systems, are very much stable at the room temperature (300 K).

Boron-doped graphene (B@graphene): As B has one less valence electron than C therefore B-doping in graphene makes itself a p-type semiconductor. The B–C bond length (1.48 Å) in 3.12% B@graphene [Table S3, Supporting


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Information] is higher than the C–C bond length (1.42 Å) of graphene. Thus B-doping in graphene distorts the geometry of graphene. Moreover, an electron deficient area appears [Figure S6, Supporting Information] in the charge density plot of B@graphene. On the other hand, Bader charge analysis shows that B is positively charged (+1.96) where as its neighbouring C (-0.57) atoms are negatively charged. Thus, charge transfer occurs from B to C atoms in B@graphene. The TDOS and PDOS of 3.12% B@graphene is plotted to understand its orbital properties. We find C 2p orbitals are appearing at the Fermi, which could be due to the interaction between the B and neighbouring carbon atoms to form states at the Fermi. The Fermi level (by 0.76 eV) is shifted below the Dirac point due to such doping. Therefore B-doping in graphene creates a gap (0.11 eV) above the Fermi level and such type of semiconductors are called degenerate semiconductors.65-67 Thus B-doped graphene electronic properties can be controlled by tuning the position of the Dirac point using gate voltage.67 The B doping concentration is increased to 6.25% to achieve a band gap opening at the Fermi. We find there is a band gap of 0.32 eV above the Fermi level for the higher B-doping concentration. The amount of degenerate states appearing at the Fermi increases as the B-doping concentration increases (hole concentration increases). As a result, the effective band gap is shifted towards the more positive side. The interaction of the gaseous molecules on the doped-graphene surface is studied for low (3.12%) and high doping (6.25%) concentrations to maximize the effect on the electronic structure. We have maintained an equal ratio for the dopant and gas molecule for our study. Gas molecules prefer to be adsorbed at the B-site of the B@graphene (3.12%) via N-centre of gas molecules [Figure 2(a-c)]. This could be due to the Lewis acid-base based interactions between the dopants and the gas molecules. We find, NH3 is preferred to be adsorbed via Ncentre in B@graphene to form a stable Lewis acid-base pair. The B-N distances are 1.74, 1.66,


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2.20 and 1.97 Å for NH3, NO2, NO and N2O gas molecules, respectively. The calculated adsorption energies are 0.59, 1.24, 0.57 and 0.49 eV for NH3, NO2, NO and N2O gas molecules, respectively. Therefore, NO2 strongly interacts with the B@graphene. As a result, the B atom is pulled upward by 0.33 Å from the graphene surface. This could be due to the strong interactions between the positively charged N (of NO2) atom and π-electrons of graphene. As the distance between B@graphene and gas molecules are usually less than 2 Å, indicating a chemical adsorption. Zhang et al31 theoretically demonstrated that NO2 is preferred to adsorbed via Ncentre over O-centre. This is due to the stronger B-N bond (1.67 Å) formation which leads to higher adsorption energy (1.37 eV). We believe, this is one of the main driving forces for the N atoms to be closest atom in all the doped systems. The electrostatic potential plots [Figure 2(e-h)] and Bader charge analysis of B@graphene+gas systems show that the charge on B atom does not change much due to the adsorption of gas molecules (NH3, NO2, NO and N2O). ESP plots show that the gases are adsorbed at the electron deficient B site via N centre of the gas molecule. Even after adsorption, NO retains its total valence electron (10.99e), where as the total number of valence electrons are 7.86e, 17.55e and 15.89e for NH3, NO2 and N2O respectively [Figure S7, Table S3, Supporting Information]. This further confirms the charge transfer is significant while NO2 is adsorbed. Thereby, the electrostatic plots perfectly complement the Bader charge analysis. TDOS and PDOS are plotted for 3.12% B@graphene+gas systems in Figure 2(i-l). In case of the 3.12% B@graphene+NH3 system, there are some states (Figure 2i) appearing at the Fermi. Our PDOS analysis confirms that these states are mainly coming from H 1s and N/C 2p orbitals.


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Figure 2: Optimized structures of (a) NH3, (b) NO2, (c) NO and (d) N2O molecules adsorbed on 3.12% B@graphene (Brown, pink, blue, red and beige colour balls denote C, B, N, O and H respectively). Electrostatic potential (ESP) plots of (e) NH3, (f) NO2, (g) NO and (h) N2O molecules adsorbed 3.12% B@graphene (Isosurface value: 0.03 e.Å-3). The blue and red colours denote less and more electron dense area in the electrostatic potential surface. TDOS and PDOS of (i) NH3, (j) NO2, (k) NO and (l) N2O molecules adsorbed 3.12% B@graphene surfaces. The Fermi level is set to zero and indicated by the black dashed line. Thus orbital density appears at the Fermi region is mainly due to the presence NH3 non-bonding orbitals [Figure 2(i)]. On the other hand, in case of NO2 adsorption, it creates a significant gap just above the Fermi level. However, some low intensity states are still present at the Fermi energy [Figure 2(j)]. This is interesting as in all other cases (B@graphene, B@graphene+NH3, B@graphene+NO] the gap is appearing far above the Fermi energy where as in case of NO2 adsorption, the gap is appearing close to the Fermi. Similarly, NO and N2O adsorption creates some sharp states just above the Fermi energy, which are mainly contributed by the C, N, and O 2p orbitals. It means due to the adsorption of these gases there are certain changes in the electronic properties of B@graphene but not significant amount to improve its sensitive toward a particular gas. So, B@graphene may not be very sensitive material for the chemical gas sensor.


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Similarly, we have studied the gas adsorption properties of the 6.25% B@graphene system. The B-N distances are 1.80, 1.72, 2.18 and 2.14 Å for NH3, NO2, NO and N2O molecules respectively. Their adsorption energies (per gas molecule) are 0.53, 1.34, 0.70, and 0.32 eV respectively. So, adsorption behaviour does not change with concentrations. TDOS and PDOS of the 6.12% B@graphene+gas systems [Figure S7] show similar electronic properties as observed for the 3.12% B@graphene+gas systems. Similarly, AIMD simulations [Figure S7-S10 and Table S6, Supporting Information] for the 6.12% B@graphene+gas systems show the structures are stable at room temperature.

Aluminium-doped graphene (Al@graphene): The results of B-doping influence us to dope with a heavier group 13 element to improve the gas sensing properties. In the 3.12% Al@graphene, the Al-C bond distances (1.74 Å) are higher than the neighbouring C-C bond (1.39 Å) distances. Thus such doping distorts the geometry of the pure graphene. Electron density and Bader charge analysis is done to understand the bonding nature of the Al-C bond. Here, Al is positively charged (+2.23) than the surrounding C atoms (-0.63). It indicates that significant amount of charge transfer occurs from the Al to C atoms of Al@graphene. PDOS confirms [Figure S11-14, Table S4 and S7, Supporting Information] that the impurity states appearing at the Fermi level are mainly come from the C 2p orbitals. Hence electron deficient atom breaks the delocalization of π-electron density and thus creates some states at the Fermi level. Such degenerate semiconducting behaviour is also seen in the 6.25% Al@graphene system.


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Figure 3: Optimized structures of (a) NH3, (b) NO2, (c) NO and (d) N2O adsorbed on 3.12% Al@graphene. (Brown, Turquoise, blue, red and beige coloured balls denote C, Al, N, O and H respectively). Electrostatic potential (ESP) plots of (e) NH3, (f) NO2, (g) NO and (h) N2O adsorbed on 3.12% Al@graphene. (Isosurface value: 0.03 e.Å-3). The blue and red colours denote less and more electron dense area in the electrostatic potential surface. TDOS and PDOS of (i) NH3, (j) NO2, (k) NO and (l) N2O molecules adsorbed on 3.12% Al@graphene surface. The Fermi level is set to zero and indicated by the black dashed line. The Al-N distances in 3.12% Al@graphene+gas systems are 2.01, 1.94, 1.97 and 1.99 Å for NH3, NO2, NO and N2O molecules, respectively [Figure 3(a-d)]. Such doping distorts the planarity of the sheet and Al goes out of the plane by 0.86, 0.98, 0.95 and 0.91 Å due to the adsorption of NH3, NO2, NO and N2O gas molecules, respectively. The calculated adsorption energies are 1.52, 2.54, 1.57 and 1.69 eV for NH3, NO2, NO and N2O respectively. Therefore, Al@graphene interacts strongly with the gas molecules and Al@graphene-NO2 has higher binding energy than other gas molecules. So, we can say that the gas molecules are chemisorbed on 3.12% Al@graphene. Electrostatic potential plots [Figure 3(e-h)] show that the gas molecules are adsorbed at the highly positively charged dopant Al via Lewis acidbase interaction. Bader charge analysis also gives quantitative assessment about the changes


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in charge on Al atom due to the gas adsorption. Interestingly, the total number of electrons on NH3 (7.81e), NO2 (16.68e), NO (10.89e) and N2O (15.88e) decreases significantly after the adsorption of the gas molecules. The net charge transfer is maximum when a NO2 gas molecule is adsorbed. The TDOS and PDOS analysis of Al@graphene+gas systems show [Figure 3(i-l)] similar trend as observed for B@graphene+gas systems. Here also the C and N 2p orbitals present at the Fermi energy for Al@graphene+NH3, Al@graphene+NO and Al@graphene+N2O systems. However, to our surprise, there is a band gap opening at the Fermi energy for Al@graphene+NO2 system. Al@graphene interacts strongly with the NO2 molecule which could be due to the Lewis acid-base based interactions. We find the net charge transfer is maximum for Al@graphene+NO2 system as the NO2 molecule acts as an electron donor and Al@graphene as an acceptor. As a result, NO2 reduces the hole carrier of Al@graphene and stabilizes the orbital density.34 Hence, Al@graphene+NO2 system opens up a band gap of 0.25 eV. So, we predict, Al@graphene could be a very sensitive and selective gas sensor for NO2. Our results are very much in agreement with the previous experimental report34 where they demonstrated Al nano-particle loaded graphene for NO2 sensing. Similarly, the gas adsorption properties of the 6.25% Al@graphene is studied whether concentration plays an important role toward such sensitivity and selectivity. The calculated adsorption energies are 1.41, 2.61, 1.51 and 1.57 eV for NH3, NO2, NO and N2O gas molecules respectively. TDOS and PDOS studies [Figure S13, Supporting Information] show similar electronic properties as observed for 3.12% Al@graphene+gas molecules. We have found that 3.12% Al@graphene shows a great selectivity and sensitivity towards NO2 molecule. Furthermore, we have considered gas molecules adsorbed on both side of Al@graphene as this is the most sensitive system for NO2. So, we have considered two configurations for this. 15 ACS Paragon Plus Environment

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Firstly, we have tried to adsorb two NO2 gas molecules from both side (in opposite fashion) of 3.12% Al@graphene. We find that the structure is not a minima in the potential energy surface. One of the NO2 molecules is going far from the surface. So, we believe 3.12% Al@graphene is capable to adsorb one NO2 molecule at a time. Secondly, we tried to adsorb two NO2 molecules from both side (in opposite fashion) of the 6.25% Al@graphene . We find that this configuration is energetically less stable (by 1.03 eV) than the case where both the NO2 molecules are adsorbed on same side of the graphene. As the concentrations are higher so their orbital densities are presented at the Fermi level. As a result, 6.25% Al@graphene+NO2 system does not open up the band gap at the Fermi. Thus, we believe concentration plays an important role for their sensitivity.

Gallium-doped graphene (Ga@graphene): We have doped Ga in place of B/Al to improve the gas sensing property of graphene. Ga doping in graphene further distorts the planarity of the graphene sheet. The Bader charge analysis of 3.12% Ga@graphene shows that Ga is positively charged (+1.02) than its surrounding C atoms (-0.30). TDOS analysis confirms the presence of Ga-C mixed orbital at the Fermi energy and thus makes it a degenerate semiconductor. Higher (6.25%) Ga-doping concentration leads to the more shifting of the Dirac point, and thus leads to a broad orbital density around the Fermi energy. Due to the high Ga-doping concentration, a band gap of 0.35 eV generated above the Fermi level. For the 3.12% Ga@graphene+gas systems, the Ga-N distances are 2.09, 2.01, 2.06 and 2.03 Å for NH3, NO2, NO and N2O molecules with adsorption energies of 1.15, 2.07, 1.20 eV, 1.03 eV respectively [Figure 4 (a-d)]. Due to the gaseous molecular adsorption, Ga comes out of the plane by 0.88, 1.00, 0.99 and 1.06 Å for NH3, NO2, NO and N2O molecules, respectively. Thus, the gaseous molecular adsorption energies are lower on Ga@graphene 16 ACS Paragon Plus Environment

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system compared to Al@graphene system but higher than B@graphene system. The electrostatic potential plots show the adsorption of N-based gases on the top of the positively charged Ga atom. Bader charge analysis shows there is a significant amount of charge transfer occurs from Ga to the surrounding C-atoms [Figure S16-S18, Table S5, Supporting Information]. Interestingly, NH3 (7.80e) donates electrons to the Ga@graphene where as NO2 (17.52e), NO (11.24e) and N2O (16.14e) withdraw electrons from the Ga@graphene sheet. Interestingly, our TDOS studies show, band gap opening [Figure 4 (i-l)] at the Fermi energy for Ga@graphene+NO2 and Ga@graphene+NO systems. However, there are some states appear at the Fermi energy for Ga@graphene+NH3 and Ga@graphene+N2O system. The bandgap is higher (0.36 eV) for Ga@graphene+NO2 system than the Ga@graphene+NO (0.23 eV) system. So, such band gap opening certainly shows sensitivity toward NO and NO2 gas molecules. But for Ga@graphene, the selectivity is not good as it opens up the band gap for both the systems. Thus Ga@graphene can’t be a selective gas sensor.

Figure 4: Optimized structures of (a) NH3, (b) NO2, (c) NO and (d) N2O adsorbed on 3.12% Ga@graphene. (Brown, orange, blue, red and beige coloured balls denote C, Ga, N, O and H respectively). Electrostatic potential (ESP) plots of the (e) NH3, (f) NO2, (g) NO and (h) N2O 3.12% Ga@graphene (Isosurface value: 0.03 e.Å-3). The blue and red colours denote less and


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more electron dense area in the electrostatic potential surface. TDOS and PDOS of (i) NH3, (j) NO2, (k) NO (l) N2O molecules adsorbed on 3.12% Ga@graphene surface. The Fermi level is set to zero and indicated by the black dashed line.

Surprisingly, our TDOS studies for 6.25% Ga@graphene+gas systems show similar electronic properties for all the cases. There are some states appearing at the Fermi level for all the Ga@graphene+gas systems. Thus for higher concentrations, band gap does not open at the Fermi. Hence such system is neither selective nor sensitive toward any nitrogen based gases. Thermal Stability of Gas adsorbed doped graphene: We have also performed the molecular dynamics simulation for all four gases (NH3, NO2, NO and N2O) adsorbed on B/Al/Ga@graphene at room temperature. Firstly, MD simulations are performed for the 3.12% B@graphene+gas systems to check their thermal stability at room temperature. Interestingly, NH3 desorbs from the B@graphene surface at the early stage of the simulation. On the other hand, NO2, NO and N2O remain intact to the surface. B atom retains the same position in the B@graphene+NO2, B@graphene+NO, and B@graphene+N2O systems. Thus, it shows that B@graphene is a stable system in presence of the gas molecules and the B@graphene surface interacts strongly with the NO2, NO and N2O gas molecules.


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Figure 5: MD simulations of (a) NH3 (b) NO2 (c) NO and (d) N2O adsorbed 3.12% Al@graphene.

MD simulations (Figure 5) are performed to check whether Al@graphene+gas systems are stable at room temperature or not? The Al-N and Al-C distances remain constant throughout the simulation period. We find the total energy fluctuation is minimal for all the cases. As Al@graphene is the best system. The electronic structure analyses are performed after AIMD simulations for 3.12% Al@graphen+gas systems to understand the sensitivity of the system over a period time. For this, we have plotted DOS/PDOS of Al@graphen+gas systems after a 5ps AIMD simulation. We find that the electronic properties (Figure S15) remain same after the MD simulation. In fact, Al@graphene+NO2 shows a 0.25 eV band gap even after the MD. Thus, it remains sensitive and selective over a period of time. AIMD simulations are performed at the room temperature for the Ga@graphene+gas systems [Figure S19-S20, Table S6 Supporting Information] to check their thermal stabilities. In this case we have found shiver energy fluctuation in every case. After MD, the Ga is going out of the graphene plane in the presence of NH3, NO2, NO and N2O gas by 0.94, 1.39 Å, 1.38 Å and 1.31 Å respectively. Hence Ga-C bond is also elongated after MD. It concludes that Ga@graphene loses its stability at room temperature with time in the presence of these three gases. The same is true with 6.25% Ga concentration. It means that Ga@graphene is not favourable for the chemical gas sensor material.

Conclusion: In this study, we have performed DFT calculations to investigate the gas (NH3, NO, NO2 and N2O) sensing mechanism of pure and doped (B@, Al@, and Ga@) graphene surfaces. The 19 ACS Paragon Plus Environment

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interactions between the graphene surface and gas molecules are improved by B-/Al-/Gadoping. Group 13 elemental doping was done to improve the interactions by forming a Lewis acid-base pair. Adsorption energies, charge densities and Bader charge analysis are studied to understand their nature of the interactions. MD simulations are performed to confirm that all the systems are thermally stable at room temperature. Electronic structure calculations are performed to understand their selectivity and sensitivity towards a particular gas. Interestingly, 3.12% Al@graphene changes its electronic properties while a NO2 gas molecule is adsorbed to its surface. Thus, such doped system shows sensitivity and selectivity toward a particular gas (NO2) molecule. However, 6.25% Al@graphene+NO2 system does not change its electronic structure at the Fermi energy. Thus, concentration plays an important role towards their sensitivity and selectivity. Similarly, 3.12% Ga@graphene changes its electronic properties in the presence of NO and NO2 molecules. However, such system is not good selective for a particular gas. Thus, we believe, Al@graphene could be a very sensitive and selective semiconductor based gas sensor.

Acknowledgments: 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)] and DSTSERB. I. C., P. G. and A. M. thank MHRD for the research fellowship.

Associated Content: * Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website. File name: Supporting Information


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Optimized structures (graphene, graphene+gases, B@graphene, 6.25%B@graphene+gases, Al@graphene, 6.25%Al@graphene+gases, and Ga@graphene and 6.25% Ga@graphene) and their respective MD simulations, TDOS/PDOS, charge densities, electrostatic potential (ESP) plots and Bader charges are given in the Supporting Information file (PDF).

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Table of Content (TOC):

Al@graphene for semiconductor based selective gas sensor.


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