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Controlled Pore Sizes in Monolayer CN act as Ultrasensitive Probes for Detection of Gaseous Pollutants (HF, HCN and HS) 2

Kalishankar Bhattacharyya, Saied Md Pratik, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11963 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Controlled Pore Sizes in Monolayer C2N act as Ultrasensitive Probes for Detection of Gaseous Pollutants (HF, HCN and H2S) Kalishankar Bhattacharyya, Saied Md. Pratik and Ayan Datta* Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur – 700032, Kolkata, West Bengal, India

Abstract Porous two-dimensional monolayers have gained interest both in theoretical and experimental research because of their controlled pore size and evenly distributed holes within the basal plane. Herein, we have investigated the efficient trapping of three pollutants (HF, HCN, and H2S) on the recently synthesized porous two-dimensional nitrogeneted holey carbon (C2N-h2D) monolayer using periodic density functional theory (DFT) and ab-initio molecular dynamics (AIMD) simulations. The differential binding positions/orientations of HF, HCN, and H2S in the cavity of C2N depend on their adsorption energy and charge transfer efficiency. H2S serves as a charge donor while HF and HCN act as charge acceptors. Our AIMD simulations demonstrate that the C2N monolayer is thermally stable in room temperature. Adsorption occurs through physisorption and no chemical bond is formed between the molecule and the C2N surface irrespective of the site of interaction. Electrostatic interactions and hydrogen bonding facilitate strong non-covalent interactions between the polar pollutants and C2N. I-V characteristics of the C2N monolayer in presence of the adsorbed molecules show an increase in current for H2S at a bias of 1.0 V. However, for acceptor molecules, it shows a decrease (HF, bias=3.0 V and HCN, bias=2.5 V). Such unique sensitivity of C2N towards donor/acceptor molecules advocates its application for gas sensing applications. 1

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Introduction:

Worldwide air pollution through the contamination of emerging toxic gases is one of the major threat to the modern civilization. This is particularly acute in developing countries like India (Delhi) and China (Beijing). Over the last few decades, excess emission of toxic gases into the environment have proven to be detrimental to human health and rapid shrinkage in energy resources.1,2 Hence, designing new gas sensor materials have paramount application to probe the pollutant molecule in the modern era. In the last decade, number of porous materials such as metal and or/metal free organic frameworks (MOF, COF),3-5 zeolites,6,7 metal oxides8 have been extensively used for adsorbents,9 gas-sensor10 and for catalytic degradation of pollutant/toxic gases. High selectivity, superior sensitivity, fast response, low cost and minimal environmental impact are pre-requisites for promising gas sensor materials.11,12 One of the most important criteria of materials to be used as a gas sensor is its optimal surface – to – volume ratio.13 Higher specific surface area of materials, are potent candidates for sensing applications as they can interact well with the gaseous molecules. In past few years, solid state materials,14 specifically nano-materials (semiconductor nanowire, carbon nanotubes)15,16 have been widely used to detect the small concentration of pollutant gaseous molecules. However, in many cases their chemical reactivity with the gaseous molecules makes their recyclability difficult. Hence, designing new materials with the mentioned physicochemical properties are important guidelines for nanoscale materials to act as sensor. Successful isolation of graphene sheet, through micromechanical exfoliation from graphite, has generated new avenue for the application of monoatomically thick two-dimensional materials.17,18 Larger surface area of graphene having the specific honeycomb structure with sp2 2


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hybridized carbon atom has been shown to have an outstanding applications such as electronics,19 photonics,20 mechanical,21 chemical,22 energy storage,23 electrocatalysis24 and DNA sensing.25,26 In addition, theoretically measured the large surface area of graphene (~3000 m2 g-1) and it’s derivative like as graphene oxide,27 reduced graphene oxide,28 could be used as unequivocally efficient materials for gas sensing, adsorption, and separation of the small gaseous molecule. Inspired by the excellent application shown by graphene, similar other 2D materials such as silicene,29-31 hexagonal boron nitride (h-BN),32 germanene,33 phosporene,34 transition metal dichalcogenides (TMDs)35 has been explored in the application for next-generation optoelectronics devices.36 Unfortunately, inspite of numerous application of graphene due to its unusual structural and electronic properties, it is a zero band gap semiconductor.37 Thus, the gapless nature of graphene might be an obstacle to the application of logic gate and high-speed switching devices. To open up the band gap as well increasing the higher sensitivity in graphene, various possibilities have been explored. Several theoretical and experimental studies have been shown that the reactivity of graphene can be further enhanced by substitutional doping and defect engineering.38 Zhang and co-workers have reported that defective graphene could enhance adsorption energy and exhibit substantial charge transfer between adsorbate compared to the pristine graphene counterpart.39 Recently, we have shown that C, and Ge doped silicene can show excellent electronic and optical properties.40 Doping with n-type dopants ( e.g. O, N, S etc)39 and p-type ( B, Al etc)41 can reinforce the electronic properties which in turn changes the charge mobilities and semiconductor properties of graphene. It is important to note that structural defect in pristine graphene can also enhance the binding energy of molecules even at nano-molar concentration.39 However, synthesis of precisely controlled porous graphene is still challenging, and which makes it difficult to generate uniform graphene of uniform porosity.42,43 Indeed, a

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crucial role is played by the trade-off of the steric factor of adsorbate and the pore diameter of graphene. In addition, the carbon atoms located in the immediate vicinity of the pore should be chemically passivated in order to reduce the high reactivity of the resulting dangling bonds.44 To overcome this chemical hindrance, some graphene-like analogues such as graphyne,45 and graphdiyne46 with the uniformly distributed hole, were used for He, H, CO, CH4, N2 gas separation. Recently, Baek and co-workers have been successfully synthesized a new twodimensional (2D) nitrogen atom (N) containing hexagonal carbon composite, C2N-h2D crystal, by the bottom-up wet-chemical reaction.47 The advantage of single layer C2N is that it has a finite direct band gap of ~1.70 eV and field effect transistor device based on C2N exhibits an on/off ratio of 107 which is much larger than graphene. Therefore, C2N promises an exciting materials for application in electronic, optoelectronic, and catalysis.48,49 In addition, large surface area and the periodically distributed hole in C2N makes it an ideal 2D materials for applications in gas separation and purification.50 In this article, we have employed the first principle calculation to elucidate C2N as a potential candidate as an adsorbent for chemically hazardous pollutant gaseous molecules, namely, HF, HCN, and H2S. Based on the density functional theory (DFT), we have systemically explored the binding energy, adsorption distance, charge density difference, density of states (DOS) of these pollutants (HF, HCN, and H2S) upon adsorption on single layer C2N. In addition, we have performed ab-initio molecular dynamics simulation on the composite system to address the adsorption behaviour of gas molecules in a more realistic manner. We have demonstrated that the H –atoms of the pollutants specifically interact with the electronegative N-atoms of C2N instead of symmetric pore mouth. AIMD simulations show that the adsorption of these molecules on single layer C2N is due to physisorption arising entirely from weak van der Waals interactions 4


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(vdW) rather than chemisorption. Transport properties show that C2N can selectively sense electron-donor (H2S) and electron-acceptor (HF, HCN) with precise I-V characteristics.

Computational Details: Herein, first principle calculations were performed by VASP package.51 The inclusion of core – valence interactions were accounted by projector augmented wave (PAW) and the PerdewBurke-Ernzerhof (PBE) exchange-correlation functional was used within the

generalized

gradient approximation (GGA).52 Ionic core was described by the ultrasoft pseudopotentials. Van der Waals corrections were incorporated by Grimme’s DFT-D2 method.53 As shown in Figure 1, a 2×2×1 supercell containing 72 atoms were modeled from the optimized the unit cell of the C2N monolayer (12 C and 6 N atoms). The vacuum width along the z-direction of 15 Å is used to avoid the interaction with periodically neighboring images. Brillouin zone integation was described by the 5×5×1 Monkhorst-Pack k-point for structural relaxation and 13×13×1 k –point was used for density of states calculations (DOS). For structural optimization, a cut off energy of 500 eV were used and the energy and forces on each atom are converged till a cut-off of 10-4 eV and 10-3 eV/Å, respectively. We have performed Bader charge calculations using Henkelmans program54 to analyses and quantity the charge transfer occurring between the adsorbateadsorbent pair. To explore the most stable configuration of the C2N monolayer, we have calculated the binding energy (Eb) using the following formula: Eb = EC2N-gas - (EC2N+Egas)

(1)

where Eb is the binding energy of the gaseous molecules on C2N. EC2N-gas, Egas and EC2N represent the total energy of optimized adsorbate-adsorbent, isolated gaseous molecules and C2N 5



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monolayer respectively.We have also computed the minimum energy pathway (MEP) for the passage of the molecules through the holes in C2N and calculated the activation barrier based on the nudged elastic band method (NEB).55 The electronic transport properties were performed using the nonequilibrium Green’s function (NEGF) method within the Keldysh formalism as implemented in the TRANSSIESTA module of the SIESTA package.56,57 The electric current across the scattering region was calculated using the Landauer-Buttiker formula58 as 𝐼 V! = 𝐺!

!! 𝑇 !!

𝐸, V! dE

(2)

where G0 is the unit of the quantum conductance and T(E,Vb) is the electronic transmission probability at energy E under applied bias voltage (Vb) across the electrodes. For verifying the room temperature stability, molecular dynamics simulations were carried out using QUICKSTEP program as implanted in CP2K package.59 CP2K module uses a Gaussian plane-wave (GPW) approach where Kohn-Sham orbitals are generated in an atom-centered Gaussian basis set and auxiliary plane wave basis set are used for describing electronic charge density. In GPW calculations, we have used the norm-conserving Goedecker, Teter, and Hutter pseudopotentials to describe the core electrons and nuclei and double –ζ valence polarized basis set (DZVP-MOLOPT-SR-DTH) are used to treat the valence electron.60,61 Additionally, we also used Grimme’s DFT-D2 method to incorporate the non-covalent interaction.62 All simulations were carried out at 300K in the NVT ensemble where temperature was maintained using a NoséHoover thermostat.63,64 Velocity-Verlet algorithm65 were used to integrate the equation of motion taking 0.5 fs as the time step. All trajectory simulation were run up to 10 ps and last 5 ps were taken for analyses of the trajectory of gas molecules over the C2N monolayer.

6


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Results and Discussion: Figure 1 represents the optimized lattice structure in 2×2×1 supercell of C2N monolayer computed using density functional theory implemented in Vienna Ab initio Simulation Package (VASP). As shown in Figure 1, there are three type of unique bonds such as two kinds of C-C bonds and one C-N bond. The calculated lattice constants (8.33 Å) and C-C (1.42 Å, 1.47 Å) and C-N (1.34 Å) bond length are in good agreement with experimental and previous theoretical investigations.47-49 The pore diameter of the C2N is about 5.5 Å which is higher the van der Waals diameter of HF, HCN, and H2S. Due to the presence of six highly electronegative Natoms in the pore, C2N behaves an electron rich material with periodically distributed holes.66 Therefore, porous C2N acts a potential candidate for trapping and sensing the small pollutant gas molecules.

Figure 1. Top view of the fully optimized 2×2×1 supercell of C2N monolayer. Cyan and blue spheres represent carbon and nitrogen atoms, respectively. The four unique adsorption sites for the adhesion of HF, HCN, and H2S are also demonstrated. To investigate the thermal stability of porous C2N monolayer, we have employed the BOMD studies at 300K from the DFT optimized structure. In our 10 ps MD simulation, C2N monolayer attains equilibrium at ~ 2.0 ps and does not exhibit significant deformation, as confirmed by the 7



convergence of RMSD plot (See Suppor. Info. File). In fact, it is also observed that C2N monolayer maintains planarity along with a small variation in pore diameter (5.47±.2 Å) throughout the simulations. In order to understand the favorable binding position of the molecules on C2N, we have considered all the four unique possible adsorption sites namely, the center of the pore (A), center of the triangular N atom (B), top of the benzene ring (C) and top of the pyrazine ring (D) of the C2N as shown in Figure 1. It has been found that HF gas molecule adsorbs preferentially on B site with the distance of 2.51 Å amongst the other possible configuration available in C2N. Based on the highest binding energy of the C2N-HF composite, the most stable conformations is shown in Figure 2, where the HF molecule is located almost perpendicularly with the C2N sheet and H atom of HF molecule points towards the B center rather than the center of cavity of C2N monolayer. As compared with the isolated HF molecule, the bond length of H-F bond gets elongated from 0.88 Å to 0.96 Å within the B site. From the density of states ( DOS ) of HF-C2N composite, one can see that the contribution of HF electronic states arises around ~2 eV in Valence Band Maximum (VBM) and 5.5 eV in Conduction Band Minimum (CBM) which are far away from the Fermi level, hence the band gap of HF-C2N monolayer remains almost unaltered (1.69 eV). To gain a microscopic understanding of mechanism of HF adsorption on C2N, we have performed BOMD simulation of the HF-C2N complex at 300K. We have considered the DFT optimized geometries as an initial configuration for the simulation where H atom of HF molecule point towards the B center. As shown in Figure 2, HF maintains its mobile nature across the porous center of C2N without exhibiting any bond formation with C2N monolayer throughout the whole simulation. It is also important to note that HF does not get desorbed or detected from the C2N pore at the room temperature. Sampling on the various 8


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configuration of HF on the C2N surface reveal that HF mostly resides perpendicular to C2N wherein H atom oriented towards to the N atom of the pore. C2N-HF equilibrates within ~ 3ps which can be attributed to the strong non-covalent interaction between polar HF adsorbent and the surface. During the production simulation run, we have computed the distance of centre of mass of HF molecule with the C2N sheet. This remains in the range of ~2.6 to 3.05 Å from the pore cavity.67 Clearly, such long-range distance between the HF and C2N confirms the physisorption mediated by the non-covalent interaction.

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Figure 2. The most stable conformation of HF binding on C2N monolayer. (a) top (b) side view of the HF adsorbed on C2N. (c) DOS and (d) charge density difference of the HF-C2N complex (isosurface value =2×10-3 e/Å). Snapshots of various configuration of HF molecule at C2N monolayer. (e) Top (f) side view during 0, 5 and 10 ps BOMD simulation at constant

10


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temperature (300K) and pressure (1 atm.). Cyan, blue, grey and white spheres represent carbon, nitrogen, fluorine and hydrogen atom respectively. In the case of HCN adsorption on C2N, we examined the above mentioned binding positions including with H atom points towards the C2N surface. Binding energy confirms the strongest binding interaction arises at A site. In this lowest energy conformation, it is observed that the HCN is nearly perpendicular to the C2N layer where H atom of HCN located toward the porous site with the distance of 2.63 Å (See Figure 3). Bond length of C≡N in HCN changes from 1.15 Å to 1.16 Å due to adsorption confirming again weak binding (physisorption) of HCN on C2N, which is in agreement to the previous theoretical results of polar toxic gas adsorption on graphene, germenene and silicene.68 As shown in Figure 3, the density of states (DOS) demonstrate that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of HCN are positioned far away from the Fermi level which again indicates the negligible reactivity of HCN towards the C2N. As anticipated, adsorption of HCN does not affect the band gap (1.62 eV) as well electronic properties of C2N. To gauge the room temperature stability of HCN over C2N, we also performed BOMD simulation of HCN on C2N. Figure 3 depicts the snapshots of HCN over the C2N sheet at the various time intervals.

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Figure 3. The most stable conformation of HCN binding on C2N materials. (a) Top (b) side view of the HCN adsorbed on C2N. (c) DOS and (d) charge density difference of the HF-C2N complex (isosurface value =2×10-3 e/Å). (e) Top and (f) side views of snapshots representing the various position of HCN on C2N monolayer at 0,5 and 10 ps BOMD simulation at constant temperature (300K) and pressure (1 atm.). Cyan, blue and white spheres represent carbon, nitrogen and hydrogen atom respectively. 12


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During the production run of the BOMD simulation in HCN-C2N, HCN remains within ~2.753.15 Å of the C2N and it is driven towards the N center of the cavity, which can be confirmed from higher probability of finding a N atom of C2N towards the H of HCN composed to that of a C-atom as observed from radial distribution functions (RDF) plots (See Figure S2). Simultaneously, our simulation also show that no chemical bond is formed during the adsorption of HCN. For H2S, we also investigated the binding position of H2S starting from various initial geometries similar to the above two molecules considering both S atom and H atom pointing towards the adsorption sites simultaneously. The most stable conformation as presented in Figure 6, is that for the H atom of H2S pointing towards the A-site while S atom flipped away from the C2N surface. Adsorption onto C2N sheet, the H-S-H angle and H-S bond length changes to 90.64° and 1.35 Å respectively compared to 109.47° and 1.31 Å of isolated H2S molecule which is similar to the adsorption of H2S on group-IV 2D materials.69 The large change in the H-S-H angle (ΔӨ = 19°) changes the electronic structure of the H2S-C2N complex substantially. Hence, unlike HF and HCN, H2S is capable of tuning the band-gap of C2N on adsorption. From the DOS of the H2S-C2N complex, we can observe that the HOMO of the H2S close to the Fermi level. Because of the contribution of H2S molecule in the DOS of the system, the band gap is lowered considerably to the 0.86 eV to the pristine C2N (1.66 eV).

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Figure 4. (a) Top and (b) side view of the H2S adsorption on C2N sheet. (c) DOS and (d) charge density difference plot of the H2S-C2N complex. (isosurface value =2×10-3 e/Å). (e) Top and (f) side views of snapshots representing the various position of H2S molecule on C2N monolayer at 0, 5, and 10 ps BOMD simulation at constant temperature (300K) and pressure (1 atm.). Cyan, blue, yellow and white spheres represent carbon, nitrogen, and sulphur and hydrogen atom respectively.

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To elucidate the binding mechanism of H2S on C2N, we have also performed BOMD simulation on the aggregate. The snapshots of the C2N-H2S complex at different simulation timescale are shown in Figure 4. After equilibration of the system, we observed back and forth movement of the H-atoms in H2S in the A site which may be attributed to the repulsion from the π-rich surface of C2N. However, the RDF plots between H atom of H2S and N of A-site maximizes at 2.8 Å (See Figure S2), which further confirms that inspite of the positional fluctuation, H2S prefers to remain within the immediate vicinity of the N-atoms. Again no evidence for the formation of any chemical bond to the surface is found. To quantify the physiochemical origin of pollutant gas adsorption on C2N, we have calculated the binding energy of the most stable conformation, charge transfer between adsorbateadsorbent, energy barriers for hopping across the pore, and band-gap of the average adsorbateadsorbant distance (See Table 1).

Table 1. Binding energy (Eb), charge transfer (ΔQ) between molecule and the C2N monolayer, distance (dH-C2N) of the H atom of the gas molecules to the C2N adsorption sites, band Gap (Eg), and energy barrier for gas molecules passing through the C2N monolayer at DFT based calculations. Average adsorbate-adsorbent distance (dmol-C2N), and molecular bond length distance (dH-X) from BOMD simulation. Molecule

Eb

ΔQ (e)

(kcal/mol)

dH-C2N

Eg

(Å)

(eV)

Nature

E‡ (eV)

dmol-C2N

dH-X (Å)

(Å)

HF

-10.0

-0.027

2.51

1.69

acceptor

1.78

2.95

0.96

HCN

-14.5

-0.021

2.63

1.66

acceptor

2.80

2.75

1.11

15



H2S

-13.1

0.028

2.43

0.86

donor

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1.33

2.76

1.36

The average distance of the center-of-masses for all gaseous molecules with C2N lies in the range 2.76-2.95 Å (See Figure S2) which solely arises due to the strong electrostatic interaction between electropositive H atom of the pollutants and the π surface of C2N. In Table 1, we have observed that the binding energy of all gases are in the range of -10.0 kcal/mol to -14.5 kcal/mol. The moderate to weak binding energy of the adsorbate-C2N complex as the binding mechanism strongly supports physisorption instead of chemisorption. For the electron-accepting molecules, HF and HCN, the binding energy are -10.0 kcal/mol (-14.5 kcal/mol) with the adsorbate-C2N distance 2.51 Å (2.63 Å). In the case of H2S, the lowest energy configuration (Eb = -13.1 kcal/mol) leads to 1.35 Å for the H-S bond length and 90.64° for the H-S-H bond angle after adsorption. Such distinct deformation of H2S after adsorption demonstrates that H2S perturbs the electron in C2N compared to the other two gases molecules. We have plotted the fluctuation for the bond length variation of H-F, H-CN and H-SH of HF, HCN and H2S molecules, for the last 5 ps of our simulations. Figure 5 shows that the bond-length oscillate near their isolated gas-phase values with marginal standard deviation values (dH-F = 0.88 Å, dH-CN = 1.07 Å, dH-SH = 1.31 Å). The average bond length of H-F, H-CN, and H-SH are 0.94 Å, 1.08 Å, and 1.34 Å and their variation from their isolated values arises from vibrational as well as thermal motion. This indeed confirms that HF/HCN/H2S do not undergo bond-cleavage on adsorption to C2N and processes like H-atom spillage and poisoning of the surface due to saturation of the reactive site are highly unlikely. This is a boost for reversible surface detection application for our C2N monolayer.

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Figure 5. The bond length (in Å) variation of (a) H-F (b) H-CN (c) H1-SH2 and (d) H2-SH1 of HF, HCN and H2S molecules on adsorption of C2N monolayer during BOMD simulation. The red line denotes the time-averaged bond-lengths.(e) Bond angle ( in degree) variation of H-S-H on the C2N monolayer during BOMD simulations. Furthermore, we have also demonstrated the hydrogen bond formation during the gaseous molecules adsorption on the C2N. As shown in Table 1, the shortest distance between H and N atom of HF and C2N monolayer are smaller than 2.75 Å, van der Waals cutoff distance of H and N atom, which is often attributed the presence of hydrogen bond of the type X-H…N. Also, the angle of the NC2N…H-F is 172°, which closes to linearity is also considered as a favorable condition for hydrogen bond.70 Similarly, the angle of NC2N…H-C and and NC2N…H-S of HCN and H2S are in 131.8° and 157.5° which are well within the angle distortion patterns for N…H-C and N…H-S.71 As shown in Figure 5(e), the bond angle variation of H…S…H on the C2N 17



monlayer near ~90.4±5° firmly supports the approximately linear angle formed between the N of C2N with H of H2S molecule. In order to understand the electrostatic interactions between HF, HCN, and H2S with the C2N, charge density plot at adsorption site are also analyzed. As shown in Figure 2, the charge densities are redistributed on the cavity of the C2N layer where an amount of partial charge 0.027 ‫׀‬e‫ ׀‬are transferred from the C2N to HF, indicating that electrostatic interaction plays a major role during physisorption of HF in C2N. Similarly for the HCN-C2N interaction, only 0.021 ‫׀‬e‫ ׀‬is accumulated from C2N to HCN. Figure 3 shows that charge density is redistributed around HCN, however it is smaller compared to charge density in HF. Interestingly, 0.028 ‫׀‬e‫ ׀‬partial charges are transferred from the H2S to C2N, which unambiguously shows that the H2S behaves as a donor towards the C2N, which is similar to the adsorption behavior of H2S on the graphene, silicene and other analogue.68 From charge density plot in Figure 4, it is revealed that the charges get localized on adsorption site of H2S-C2N composite which clearly provides the physisorption binding of H2S by means of strong electrostatic interaction. We believe that such electrostatic interactions between small gaseous molecules with C2N within the non-covalent distance additionally stabilize the NC2N…H-X interactions. In addition, we have also explored the possibility of these gaseous molecules to diffuse through the C2N layer. Recently some two dimensional materials, and covalent organic frameworks (COFs) have been exhibited as promising materials for pollutant gas separation, and water desalination.72 We searched the minimum energy pathways (MEP) to calculate the activation barrier for the passage of the molecules from one side to another side of C2N based on the nudged elastic band (NEB) method. The activation barrier (E‡) of HF, HCN, and H2S are reported in Table1. As shown in Table 1, the E‡ of all gaseous molecules are much higher 18


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compared to the reported in the literature indicating that these are less prone to penetrate the C2N layer.72 It is also important to note that the charge density overlap near the vicinity of gas molecule adsorption site in C2N are stronger and hence it will be difficult for a gas molecule to overcome the repulsion of the large electron density surface on C2N. In correlation with charge transfer mechanism, the higher activation barrier, for HF and HCN, arises due to their electron accepting that result in a higher repulsion from C2N surface. In contrast for H2S, the low activation barrier arises due to acting as electron donating to C2N surface. However, in both cases, the high activation barrier clearly provides the reluctance of bond formation between any types of gas molecules (chemisorption) with the N atom of C2N surface.

Figure 6. (a) Schematic view of a two-probe model of C2N monolayer where semi-infinite left and right electrode regions (light-red shaded area) are in contact with central scattering region. (b) Current (I) Bias Voltage (V) characteristics of the C2N layer with adsorbed gaseous molecule at their respective binding sites. To quantitatively probe the gas sensing properties of C2N for electronic device fabrication, we have calculated electronic transport properties of C2N with and without adsorption using the NEGF method. We have used the two-probe system to examine the I-V characteristic where two19



probe system constitutes three regions, namely semi-infinite left and right electrode and central i.e. scattering region. For left, right and scattering region we have taken a 2×2 supercell structure. Figure 6 represents I-V curve for the C2N layer before and after HF, HCN, and H2S adsorption. The I-V characteristics of the pristine C2N sheet depict that up to a 1.0 V bias, no current is detected through the device. However, with increasing the bias voltage, only a very small current passes through the C2N sheet.

Figure 7. DOS of the adsorbed gaseous molecules into the C2N monolayer at each bias voltage. (a), (b), and (c) represents the HF-C2N; (d), (e), and (f) represents the HCN-C2N; (g), (h), and (i) represents the H2S-C2N composite. Left electrode density of states and right electrode density of states are in black and red lines respectively. It has been observed that the current exhibits non-linear relationship with the bias voltage and maximum current (0.002 µA) arises in the HF-C2N complex under an applied bias voltage of 2.5 20


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V. It is important to note that the electronic properties of the C2N do not alter substantially during the physisorption of HF and HCN and hence, charge transfer induced adsorption mechanism might influence I-V characteristic.73 For electron-accepting HF and HCN, similar I-V curve appears compared to the pristine C2N. On HF adsorption, the current increases slightly when 1.5 V bias applied. In case of HCN, almost similar trend of increment of current with bias voltage is observed. In contrast, when H2S is adsorbed on the C2N, the current is dramatically increased at a small bias window. This can be correlated to that number of electronic states near Fermi region arising due to the increment bias voltage. It has been observed that the current decrease upon further increment of voltage and the reduction of current can directly be associated with the resistivity of the materials which may vary during the adsorption/desorption. In order to explore the non-linear behavior of the I-V characteristics, we have plotted the DOS of the left and right electrode of HF-C2N, HCN-C2N and H2S-C2N at different bias voltage (See Fig. 7). On application of bias voltage, the energy level of left and right electrode begin to shift simultaneously towards the right and left direction. Consequently, current starts to flow when more and more energy states of left and right electrodes come closer to each other, resulting in an overlap region. At zero bias voltage, there is no overlap between the energy states, therefore no current passes through the central scattering region (Fig.11 (a), (d) and (e)). As the bias voltage slowly increases, the possibilities of electrons occupying the states near the equilibrium Fermi level is enhanced. Under the bias voltage at 1.5 eV, energy states of the two electrodes in HFC2N leads to overlap between them near the Fermi level, and consequently current starts to flow through the scattering region (Fig. 11 (b)). Similarly, in HCN-C2N, the matching of the energy states of left and right electrodes start to overlap and current increases under bias voltage 1.5 eV, although amount of current is reduced substantially compared to HF-C2N ( Figure 11 (e)).

21



However, with the small increment of bias voltage (1.0 eV) in H2S-C2N, the current increases quickly which is attributed to the well matched energy level near Fermi level. After maximizing the current at 2.5 eV (1.0 eV) in HF-C2N (H2S-C2N), the decay in current occurs due to the mismatch of energy states of the two electrodes at 3.0 eV (1.5 eV) (Figure 11 (c),(i)). Nevertheless, the number of energy states of the two electrodes increases in HCN-C2N under bias voltage, resulting in an increment in its current. Thus, the transport properties calculation confirm the C2N material can behave as gas sensor towards the toxic gases either by increasing or decreasing current flow which can be directly calibrated to the experimentally measurable resistance capacity of the C2N and the mass of the pollutant.

Conclusion: In summary, we have demonstrated an efficient trapping of three important pollutants (HF, HCN, and H2S) molecules on C2N monolayer. We have systemically explored the most favorable binding site of each gas molecule based on the four different adsorption positions. Adsorption energy, charge density, and Bader charge analysis were investigated to understand the strength and nature of adsorption of the pollutant on these surfaces. Among the three pollutants, HCN is adsorbed most strongly with an adsorption energy (-14.5 kcal/mol) compared to the HF (-10.0 kcal/mol) and H2S (-13.1 kcal/mol), exhibiting the stronger affinity of HCN towards the C2N. Overall, the weak adsorption energy of these system arises mostly due to an electrostatic interaction between these polar molecules and C2N. Our AIMD simulations have shown that all the systems are thermally stable at room temperature and do not form any chemical bond between adsorbate and adsorbent. Interestingly, the strong N…H-X hydrogen bond formed

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between the N-atom of the pore and pollutant stabilize these complexes. The high electron density in the cavity of C2N provides a prohibitively large diffusion barrier of gases passing through them. Transport properties calculation on C2N during adsorption of these molecules show that the I-V characteristics of each of these molecules are unique and the rise and fall in the molecular conductance is directly an outcome of the inherent electron donating or accepting nature of these molecules. Hence, molecular electronics devices based on C2N monolayer can be an excellent detector device for reversible and ultrasensitive detector of such pollutants.

ASSOCIATED CONTENT Supporting Information RMSD plot of C2N monolayer, adsorption energy of various adsorption sites, radial distribution function of various pollutant gaseous molecules. AUTHOR INFORMATION Corresponding Author Corresponding Author: [email protected]. Phone: +91-33-24734971. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT KB gratefully acknowledge IACS for financial assistance in the form of doctoral fellowships. SMP thanks IACS for awarding Research Associate Fellowships. AD thanks DST, BRNS and INSA for partial funding. We thank CRAY supercomputer and IBM P7 cluster for computational facilities.

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