Atomic ordered doping leads to enhanced sensitivity of phosgene gas detection in graphene nanoribbon: a quantum DFT approach

We explore a novel sensor for detection of phosgene gas by graphene derivatives such as pristine and doped graphene nanoribbons via first principles calculations. The interaction of phosgene molecule with various edge and center doped configurations of boron, phosphorus and boron-phosphorus co-doped armchair graphene nanoribbon (AGNR) and zigzag graphene nanoribbon (ZGNR) is investigated through density functional theory (DFT). P-doped systems showcase chemisorption, displaying enhanced sensitivity to phosgene detection as reflected by a more negative adsorption energy values, accompanied by a prominent charge transfer due to the doping. Regardless of nanoribbon geometry, the binding energies of P-doped systems exhibit notable uniformity within the range of −8.01 eV to −8.49 eV, however the adsorption energies in ZGNR are significantly lower than those observed in AGNR. Due to much higher(lower) electron-donating (accepting) capacity of phosphorous(boron) atoms in comparison to ‘C’ atom, substitutional doping with ‘P’ or ‘B’ atoms in AGNR has signifiant impact on the structural, electronic and adsorption properties of the nanoribbons. We observe that phosphorus doped configurations (edge/center) effectively interact with phosgene molecule with higher adsorption that corresponds to the chemisorption phenomenon. The strongest adsorption energy (−8.83 eV) is obtained for P doped configurations, followed by that for B+P co-doped AGNR (−4.23 eV). These results suggest significantly stronger adsorption of phosgene gas on P doped AGNR than on any other systems reported so far. Band structure analysis estimates that by phosphorus doping, changes in the band gap is significant and it also shows prominent changes in the band structures. Isosurface electronic charge density plots identify that the transfer of charge takes place from graphene system to phosgene molecule. Thus, significant variation in adsorption and electronic properties of P doped AGNR reveal that these geometries immensely promote the detection of phosgene gas, and may be considered as promising chemical sensor for phosgene removal.


Introduction
Monitoring of poisonous and harmful gases such as nitrogen based gases (NO and NO 2 ), carbon based gases (CO and CO 2 ), sulphur based gases (SO, SO 2 and SO 3 ), hydrogen cyanide(HCN) and phosgene gas (COCl 2 ) has been a challenge for scientific society for the improvement of human health and the environment [1].In a recent interesting work, Fadardi et al demonstrated the adsorption of some common hazardous gases (CO, NO, NO 2 , and CO 2 ) on the stanene armchair nanoribbon [2].Phosgene is a toxic and colorless gas which has been used as a chemical weapon during the first world war.It is a volatile organic compound with a chemical formula of COCl 2 [3].It is observed from literature that it is indeed harmful and difficult to detect.It is released during combustion of carbon tetrachloride, trichloro-ethylene and other halogenated hydrocarbons present in common materials like plastic foam.Its exposure can cause severe effects on the human health such as lung damage, blurred vision, throat and eye irritation and even cause death when inhaled at low concentration [3,4].Therefore, phosgene content in work environments needs to be monitored using a sensitive, and precise technology in order to be controlled in the air.Till date, various nanomaterials from nanotubes to nanopowders have been proposed as sensors [5][6][7][8][9][10][11]. Since, the discovery of carbon based nanomaterials such as carbon nanotubes [12,13], fullerenes, and carbon nano cones, due to their better electric, physical [14] and mechanical properties, they are considered promising candidates for diverse applications including gas sensing [15,16].The adsorption and sensing of phosgene in sensor technology is still scarce.The simulation study of such a highly toxic gas help us in designing of good sensor devices.
Carbon based nanostructures, like graphene, carbon nanotubes [17], attracted more attention due to their outstanding properties, such as large surface area, high mechanical strength, and high stability [18][19][20][21].Felegari and Hamedani [22] investigated the adsorption of phosgene molecule on γ-graphyne, BN and Si doped graphynes using DFT calculations and also studied the sensitivity of graphyne towards the phosgene adsorption.Their calculated results show that pristine and BN doped graphynes have adsorption energies (E ads ) of −0.11 eV and −0.21 eV which shows physical nature of adsorption but Si doped graphyne have an E ads of −1.65 eV.Scdoped BN nanotubes were used by J Beheshtian et al [23] for the detection of phosgene molecule.A very low value of E ads of 0.06 eV indicates its poor sensitivity towards phosgene detection.Pankaj Srivastava et al investigated the sensing behavior of bare zigzag boron nitride nanoribbon (ZBNNR) for the toxic gas phosgene with use of DFT calculations.Their finding reveals that ZBNNR cannot be considered as good material for the sensing of phosgene due to very low value of E ads of 0.08 eV [24].Shakerzadeh et al [25] studied the phosgene adsorption onto pristine, Al and Ga doped boron nitride nanoclusters.According to their results, E ads for pristine, Al and Ga doped boron nitride nanoclusters ranges from −0.19 eV to −0.94 eV.This indicated the physisorption nature or low adsorption ability of boron nitride nanoclusters towards phosgene detection.Baei et al [26] determined the sensing ability of aluminium nitride nanocage towards COCl 2 molecule.Their results revealed that the electronic properties of aluminium nitride nanocage are not so much sensitive towards adsorption of phosgene molecule having E ads ranges from −0.13 eV to −0.59 eV.Furthermore, Zhang et al [27] used first-principles calculations to study sensing behavior of the phosgene molecule on undoped graphene and transition metal (TM) doped (TM = Zr, Mo, Ti, Mn, Fe, Co) graphene.They predicted that the adsorption behavior of phosgene molecule with pristine graphene is physisorption, with a very low adsorption energy of −0.55 eV.However, adsorption energies for transition metal doped graphene lies in range of −0.82 eV to −1.67 eV [28].While doping with transition metals promotes stronger chemical adsorption of phosgene on the doped graphene surface, thereby making TM doped graphene a potential candidate for phosgene sensing, unfortunately its experimental realization is challenging [28].Transition metal adatoms readily diffuse and demonstrate strong cluster formation tendency at room temperature [28], thereby posing technical difficulties.
Introduction of defects and dopants into carbon nanostructures are well-recognized approaches to tune their physical and chemical properties [29,30].Latest advanced control technology entails high-level precision in the design and synthesis of nanostructures at atomic scale, for example, through bottom-up synthesis of molecular precursors [31,32], which indeed opens up possibilities of substitutional doping of impurity atoms into the nanoribbons in a precise location.To increase the extent of gas adsorption and chemical sensing properties, materials such as boron, phosphorus and boron phosphorous co-doped AGNR have been introduced recently [33][34][35].Doping with boron and phosphorus atoms result in strong covalent type doping and the physiochemical properties of the resulting doped AGNRs are similar to that of carbon atoms.Therefore, we understand their utility in phosgene detection.The main goal of this work is to study the effect of ordered doping of heteroatoms (boron, phosphorus and boron phosphorus co-doping) on sensing behavior of graphene nanoribbons towards phosgene detection based on DFT calculations.This type of study has not been previously reported in literature to the best of our knowledge.Using first-principles study, we plan to explain adsorption behavior, charge transfer, band structures and density of states of present nanosystems.

Theoretical details
First-principles calculations based on DFT as implemented in ATK-VNL Toolkit [33] were performed for pristine, 'B', 'P' and 'B+P' doped graphene nanoribbons.The generalized gradient approximation (GGA) with in Perdew-Burke-Ernzerhof (PBE) parameterization [36] was used as exchange correlation functional, since it often proves to be sufficient for gas molecule adsorption, particularly chemisorption on graphene based nanoribbons [37,38].Norm-conserving pseudopotential [39] was considered for the present study and the density mesh cutoff was fixed at 75 Hartree.The Brillouin zone was sampled by 100×1×1 k-mesh, corresponding to the one-dimensional periodicity of the structure along the x axis.The selection of such fine k-mesh, as done in a previous work [18], ensures convergence of the band gap, particularly in case of the structures that are found to be metalic.The stress and force tolerance had been selected as 0.05 eV A −3 and 0.05 eV Å, respectively.A fixed value of 300 K for electron temperature was used for the simulation.For graphene nanoribbon, a vacuum of 20 Å was introduced into system to avoid interactions between adjacent layers along the z-axis.The Vienna ab-initio simulation package (VASP) software based on DFT was used to study the charge density difference plots of various configurations for studied nano systems.

Results and discussion
A supercell of 3×3×1 was modelled for AGNR to study the adsorption behavior of phosgene gas on its surface.The AGNR consists of 42 carbon (C) and 12 hydrogen (H) atoms.The bond lengths of 'C-C' and 'C-H' were taken as 1.42 Å and 1.09 Å, respectively [40] and the total length of AGNR was set to be 11.36 Å. Different orderly doped AGNRs structural models passivated with hydrogen atoms were designed for the study.The number of dimer carbon atom chains across the width direction defines the width of AGNR as 'W'.The predicted values of adsorption energies listed in table 1 with W = 9 are in better agreement with earlier published work [27], therefore we considered W = 9 in the present work.AGNR was assumed to be orderly doped with a row of 'B' atoms, 'P' atoms and co-doped 'B+P' atoms along the length direction (x axis) of the AGNR at the edge and the center position.Various sensing systems are formed by ordered doping of boron (B), phosphorus (P) and boron phosphorus (B+P) atoms into graphene nanoribbon with 6 'C' atoms substituted by 6 'B', 6 'P' and 3'B' +3'P' atoms.All the structural models of AGNR after addition of B, P and B+P atoms along with pristine form are depicted in figure 1.
For sensing of phosgene by AGNR systems, a phosgene molecule is placed initially at a distance <3 Å from graphene nanoribbon and the full system is allowed to relax to minimize the forces and energies.Phosgene (COCl 2 ) is a planar molecule with C=O (d 3 ) distance as 1.18 Å, the C−Cl (d 1 and d 2 ) distances as 1.74 Å and the Cl−C−Cl bond angle as 111.8° [4] (see figure 1(h)).H, O, C, B, P and Cl atoms are shown by white, red, brown, green, orange and purple color spheres in figure 1.

Stability analysis and Binding energy calculations
Firstly, we studied the stability of all nano systems considered in present work.The stability of various configurations is evaluated by calculation of binding energies (E b ).Furthermore, binding energies (E b ) of dopant atoms are calculated by using equation (1) [41].
Here, t represents the total number of atoms, E T represents the total energy of structure, and E C , E H , E B , and E P represents the isolated energy of C, H, B, and P atoms, respectively.Further, p, q, r, and s represent the total number of C, H, P and B atoms, in one modeled unit cell, respectively Figure 1 shows the front view of atomic structures formed after doping with 6B, 6 P and 3B+3 P atoms into the graphene domain and corresponding configurations are formed: B edge AGNR, B center AGNR, P edge AGNR, P center AGNR, BP edge AGNR and BP center AGNR.The schematics for all studied nanosystems before and after relaxations are given in figure 1.
As shown in figure 1(a), after relaxation, negligible changes are seen in optimized geometries of pristine AGNR.In case of 'B' doped geometries, changes are minimal in optimized geometries as compared to other doped geometries as shown in figures 1(b)-(c).As in figure 1(d), in case of P edge AGNR, after optimization the bond between 'P' and 'H' is broken due to weak vander Waals forces of attraction and in P center AGNR (figure 1(e)), the repulsions between phosphorus atoms take place due to large atomic size of phosphorus atoms as compared to carbon atoms.On the other hand, in 'B+P' co-doped geometries (figures 1(f), (g)), boron and phosphorus atoms are little bit protruded from their plane as compared to phosphorus doped variants.The binding energies of various configurations follow the order: P edge AGNR (−8.11 eV) > P center AGNR (8.01 eV)> BP edge AGNR (−7.19 eV) > BP center AGNR (−7.17 eV) > B edge AGNR (−7.15 eV) > B center AGNR (−7.14 eV).
As summarized in table 1, the binding energies of 'P' doped geometries among the considered ones are the highest among all variants as phosphorus atoms are strongly interacting with graphene nanoribbons and distortion in these systems is minimal.However, the width of the nanoribbon, as well as the location of the dopant atoms in the lattice, may also play a role.In order to investigate this effect, we have investigated the adsorption energetics in two additional geometries with different nanoribbon width (n) in the P-doped case: P edge AGNR with a ribbon width of 5 graphene hexagons (n = 5), and P center AGNR with a ribbon width of 4   1, we can observe that the binding energies are all similar for the P-doped systems (between −8.01 eV and −8.49eV) regardless of the nanoribbon geometry, with a slight negative increase with an increasing width of the nanoribbon in both the armchair and zigzag cases.Furthermore, the adsorption energies in the ZGNR case are significantly lower than in the AGNR case, varying between −4.3 eV and −6.4 eV.Correspondingly, we can conclude that the adsorption process is more favorable in the AGNR case, and the adsorption mechanism is not affected significantly by the width of the nanoribbon.As such, in the following, we keep the focus of this manuscript to the AGNR geometry with a width of n = 3 hexagons.Gas sensing mechanism works in two ways: Physisorption and Chemisorption.Physisorption corresponds to weak van der Waals forces of attraction between gas molecule and substrate [42].The physisorption of molecules onto the nanoribbon substrate has been explored in the work of Fadardi et al [2], where they studied the adsorption of various common hazardous gases, including CO, NO, NO 2 , and CO 2 , on the stanene armchair nanoribbon.This process was found to have a less negative value of adsorption energy.Less charge gets transfer between phosgene molecule and graphene system in physisorption.Chemisorption represents strong chemical bonding between the gas molecule and the substrate and also has more negative value of adsorption energy.The whole adsorption phenomenon can be better understood in terms of range of energy of adsorption.Physisorption falls in the energy range of the order of 10 − 2 eV and for chemical type of bonding (sharing of electrons between gas and target material), adsorption energies are always greater than 1 eV or more [43].Larger values of E ads greater than 1 eV particularly indicate chemical bond formation.On the other hand, more charge will be transferred between graphene and phosgene molecule.The adsorption energies (E ads ) define the extent of interaction between the phosgene molecule and pristine or doped graphene nanoribbons, calculated by the following equation [44].
where, E AGNR = E doped/ undoped AGNR represents the total energy of doped/ undoped AGNR, E AGNR+GAS = E doped/ undoped AGNR + phosgene gives the same type of energy after adsorption of phosgene, and E GAS = E phosgene represents the energy of phosgene molecule, respectively.After adsorption of COCl 2 , negligible changes in optimized geometries of Pr edge AGNR and Pr center AGNR were observed.In order to consider the interaction of COCl 2 with 'B' doped graphene geometries, the phosgene molecule with its 'Cl' atom was placed parallel to 'B' doped configurations (figures S1 and S2).
After adsorption of phosgene, phosgene was relaxed on the 'B' atom with its 'Cl' atom head at a distance of 2.87 Å /2.94 Å (figures S1(f), S2(h)) for B edge AGNR /B center AGNR.After adsorption, bulges or protruding type changes were seen in their optimized geometries (see figures (S1(f), S2(h)) with adsorption energy values as −4.43 eV (B edge AGNR) and −1.76 eV (B center AGNR).The size of boron atom is very small or comparable to carbon atom, thus boron atoms were minorily pulled out from graphene nanoribbon surface.The changes produced in the optimized geometries and larger adsorption energies of B doped geometries indicate the chemisorption nature of adsorption as compared to pristine surface.Here, it is important to note that van der Waals interactions can play a pivotal role in influencing binding energies and site preferences, especially in scenarios where the physisorption of molecules onto the adsorbent takes place.However as we observe that phosphorus doped configurations (edge/center) effectively interact with phosgene molecule showing high adsorption energy that corresponds to the chemisorption phenomenon.The bond lengths and charge transfer, as will be discussed later, also indicate chemisorption.Therefore, for our systems, integrating van der Waals (vdW) calculations is anticipated to result in only marginal alterations, while the trends are expected to remain same/similar.Thus, for phosgene sensing, we use GGA exchange correlation, which proves sufficient to capture the trends and is previously adopted in many previous works for gas molecule adsorption on graphene nanoribbons [37,38,42].The changes in bond lengths and bond angle of phosgene molecule after interaction with B doped edge position were d 1 (C-Cl) = 1.73 Å, d 2 (C-Cl) =1.78 Å, d 3 (C=O) = 1.19 Å, Cl-C-Cl = 109°a nd at B doped center position, these were d1 (C-Cl) = 1.74 Å, d2 (C-Cl) =1.79 Å, d 3 (C=O) =1.19 Å, Cl-C-Cl = 110°.Similar observation of slight increase of bond lengths in adsorbed gas molecule has been previously reported by other researchers [42], indicating possible 'expansion of gas molecule' post adsorption.Thus, geometrical changes in phosgene molecule are significant after its interaction with 'B' doped geometries as compared to pristine surface.
In P edge AGNR and P center AGNR, initially, 'Cl' atom was facing towards 'P' atom at the edge and the center position (figures 2(a), (c)).After interaction with COCl 2 , 'P' atoms were pulled out from nanoribbon and 'O' atom of COCl 2 came closer to 'P' atom with a binding distance of 2.60 Å (figure (2(b)).On the other hand, when the phosgene molecule with its 'Cl' end interacts with the P center AGNR, the 'O' atom moved closer to 'P' atom with a binding distance of 2.22 Å (figure (2(d)).In P doped geometries, we observed that after adsorption of phosgene, all phosphorus atoms were found on the above of graphene plane instead of embedding into graphene nanoribbon which may be due to larger atomic radius of 'P' atom than that of 'C' atom.This significant difference in atomic radius of 'C' and 'P' atoms produced strain in graphene nanoribbon and phosphorus atoms were lifted from their positions figures (2(b), (d)).The adsorption of phosgene resulted into more bulges or protruding type structures as compared to other geometries.As shown in table 1, E ads between phosgene and P edge AGNR /P center AGNR were found to be −8.83eV/ −7.46 eV, respectively.Therefore, 'P' doping strengthened the adsorption of phosgene molecule significantly and resulting into better sensitivity of the graphene nanoribbon.The elevations that appeared in case of phosphorus doped structures were caused by the difference between atomic radius of 'C' atom and 'P' atom.The calculated value of bond lengths and bond angle of phosgene molecule after adsorption with P edge AGNR were d Regarding the geometry of the adsorption, similar results are observed in case of the increased nanoribbon width (figures 2(e)-(h)), with further increased adsorption energies.The high adsorption energy and small binding distance in case of P-doped AGNR indicates that there is strong interaction occurring between phosgene and P doped configurations owning to chemisorption which leads to chemical bond formation.In case of the ZGNR geometry, we can observe a significant buckling at the location of doping, caused by the increased strain introduced by the P-P bond, causing a significant buckling.As in the armchair case, the O atom of the phosgene molecule tends to beclosest to the P atoms, with a similar adsorption distance.However, the adsorption energy is reduced, suggesting a decrease in the strength of the chemical nature of the bonding.
P center AGNR (n = 4); The equal amount of acceptor (boron) and donor (phosphorus) impurities introduced into the graphene nanoribbon yielded the formation of BP edge /center AGNR.We observed that 'Cl' atom of phosgene molecule got adsorbed on BP edge AGNR and BP center AGNR with adsorption energies are −4.23 eV and −3.65 eV, respectively (table 1).The interaction of phosgene with BP doped configurations also results into the strongest type of chemical adsorption.After full structural relaxation, 'Cl' atom stabilized itself at a distance of 3.81 Å/ 3.47 Å (figures S2(j), S2(l)) for BP edge /center AGNR.Less elevated structures were obtained after interaction in BP doped configurations as compared to P doped geometries.The bond lengths and bond angle of phosgene molecule after adsorption with BP edge AGNR were found to be d It worths noting that our results of adsorption energies for B/P/B+P doped AGNR lie in range of −1.76 eV to −8.83 eV which are higher than that previously reported results for phosgene detection in systems such as Nb-doped arsenene (E ads ranges from −0.21 eV to −2.90 eV) [45] and Ti, Ni and Cu decorated borospherene (E ads ranges from −0.06 eV to −1.31 eV) [46], suggesting enhanced stability of the gas-solid adsorption complex, when B/P/B+P doped AGNR are used for sensing.
On the basis of higher adsorption energies, the edge doped configurations were predicted to be more energetically favorable.However, our results revealed that ordered doping is a better way to improve the adsorption of phosgene molecule and it fairly enhanced the adsorption energies to greater extent.The calculated results of adsorption energy established that 'P' doping in graphene domain is one of the prominent way for the detection of phosgene molecule.Thus, this new idea of ordered doping of acceptor and donor impurities (boron, phosphorus and boron-phosphorus) into graphene domain demonstrates the potential applicability of these materials towards phosgene detection.The P-case is most suitable for phosgene adsorption.Therefore, we will focus mainly on P-case thoroughly and discussed all the electronic properties for this case only.However, for completeness, the other cases have also been summarized in supplementary information (SI).

Density of state analysis (DOS)
To have a better understanding of the adsorption phenomenon and to figure the effect of doping of B and P atoms on the electronic properties of graphene system, density of states(DOS) has been calculated.From the DOS analysis in figure 3, we observed that before adsorption of COCl 2 , broad peaks are observed which is denoted by orange lines and after adsorption, peaks are shown by brown lines.In figure , fermi level (E F ) is observed at 0 eV.

DOS of pristine AGNR
The DOS of Pr edge AGNR indicates that after the interaction of phosgene with Pr edge AGNR (figure 3(a)), peaks are observed around −2.4 eV, −2.9 eV, −3.8 eV in the valence band 'VB' (left to fermi level) and 1.8 eV in the conduction band 'CB' (right to fermi level) having corresponding DOS values are 90 eV −1 , 52 eV −1 , 51 eV −1 and 120 eV −1 .While in case of Pr center AGNR (figure 3(b)), however, peaks are seen at around −2.3 eV, −2.9 eV, −3.8 eV in VB and 2.8 eV in CB having corresponding DOS values are 85eV 1 , 80 eV −1 , 70 eV −1 and 25 eV −1 .After adsorption of COCl 2 , a little change was observed in DOS peaks around fermi level (E F ) for pristine AGNR at both edge and center position.From DOS analysis, we found that only Cl-3p states participate in DOS with C-2p states.The electronic properties of the pristine system are not greatly affected by adsorption (figures 3(a), (b)).

DOS of P edge / P center AGNR
The phosphorus 'P' atom significantly influences the electronic properties of AGNR and results in the modification of total DOS for both configurations such as P edge AGNR and P center AGNR.The DOS analysis of both systems (figures 3(c), (d)) reflects very sharp and more intense peaks around E F after the adsorption of phosgene.These sharp and intense DOS peaks for both cases are caused by hybridization between P-3p states and Cl-3p states of the phosgene molecule.In case of P edge AGNR (figure 3(c)), DOS peaks are observed nearer to −3.9 eV, −3.8 eV, −3.7 eV, −3 eV, −2.9 eV, −2 eV, −1.9 eV in VB and 1.5 eV, 3.2 eV, 3.4 eV in CB having corresponding DOS values are 65 eV −1 , 100 eV −1 , 90 eV −1 , 187 eV −1 , 250 eV −1 , 250 eV −1 , 230 eV −1 and 190 eV −1 , 100 eV −1 , 85 eV −1 .On the other hand, P center AGNR (figure 3(d)), shows DOS peaks nearer to −3.8 eV, −2.9 eV, −1.5 eV in VB and 1.1 eV, 1.9 eV, 2.6 eV, 3.8 eV in CB with DOS values are 115 eV −1 , 140 eV −1 , 140 eV −1 and 60 eV −1 , 90 eV −1 , 110 eV −1 , 115 eV −1 , respectively.The DOS peaks confirm that these systems can be considered as good adsorbing materials for phosgene detection.The significant difference between the DOS of pristine and 'P' doped AGNR is due to the higher electronegativity of the 'P' atom than that of 'C' atom.

Band structure analysis
To get more insights into the adsorption of phosgene molecule onto doped graphene nanoribbons, the band structures were analyzed.After the adsorption of gas molecule, the delocalization of orbitals and charge transfer take place, which result into changes in the electronic structure of materials, making them suitable for gas sensing applications.The electrical conductivity (σ) of different built-in materials can be easily determined by the band gap equation (3) [47] which is as follows   S1.Thus, large band gap changes are observed for P doped configurations that results into creation of new energy states in valence and conduction bands of 'P' doped AGNR.Such prominent increase of band gap due to phosgene molecule adsorption was observed also on Carbon nanotubes and Boron Nitride nanotubes [48], indicating possibility of modulation of the electrical properties of the phosgene-adsorbent system.This large change in band gap is due to more negative value of E ads that lies in chemisorption region.Thus, we can conclude that 'P' doped systems may be considered as promising materials in phosgene detection due to large band gap change, providing electric signals and having large negative value of adsorption energies among all other configurations.

Charge transfer analysis
In order to examine the phosgene adsorption capability of pristine and different doped configurations, the charge density difference images were analyzed.The charge transfer between the phosgene molecule and different variants were analyzed by Mulliken atomic charge analysis [49].With the use of Mulliken population, the charge transfer was calculated by using the following equation: where DQ, Q a and Q b indicate charge transfer between phosgene gas and different variants, Mulliken charges of the gas after and before the adsorption, respectively.The values of the charge transfer are listed in the table S1.
When the phosgene molecule interacts with pristine and doped (B/P/B+P) AGNR, a significant charge gets transferred from the graphene nanoribbons to the phosgene.A parallel observation of significant charge transfer from the nanoribbon to phosgene occurred when the phosphorene nanosheet was subjected to phosgene gas molecules [50].This reinforces our finding that the COCl2 gas molecule functions as a potent acceptor due to its oxidizing nature with respect to P doped graphene nanoribbons, resulting in electron transitions from the P   Thus, the computational data of charge transfer is consistent with adsorption energy study.
To further understand the interaction mechanism between phosgene and 'P' doped AGNR, the charge density difference images are shown in figure 6.These images illustrated that an overlap between phosgene and P doped geometries indicating a strong covalent interaction between phosgene and P doped AGNR (edge/center).Figures 6(a), (b)) shows a large amount of charge transfer from P doped AGNR to phosgene having values are 0.07e and 0.08e for P edge AGNR and P center AGNR, respectively.These results of charge transfer are supported by data obtained from adsorption energy analysis.

Conclusion
In present work, the adsorption and electronic response of pristine as well as B/P/B+P doped armchair (AGNR) and zigzag (ZGNR) graphene nanoribbons towards phosgene molecule have been analyzed by DFT approach.The computational results reveal that pristine AGNR interacts with phosgene molecule via vander Waals forces of attraction with a very less negative value of E ads .However P-doped systems demonstrated chemisorption and stronger sensitivity towards phosgene detection (more negative value of E ads ) and more charge gets transfer for this doping.We found that regardless of nanoribbon geometry, prominent binding energies for P-doped systems exhibit uniformity (within the range of −8.01 eV to −8.49 eV), displaying a slight negative correlation with nanoribbon width increase in both armchair and zigzag configurations; notably, adsorption energies in ZGNR are considerably lower than those in AGNR.Band structure study predicts that P doping into graphene nanoribbons results into a substantial improvement in phosgene detection which is confirmed by the emergence of extra energy levels in the valence and conduction band of graphene domain and large band gap change.The DOS analysis predicts that P-doping in graphene nanoribbon shows sharp and more intense peaks which is due to difference in electronegativities of 'C' and 'P' atoms.The calculated results of more negative E ads and large band gap variations for P doped geometries show an increased sensitivity towards phosgene detection.These findings provide a useful insight to develop the novel chemical sensors for phosgene detection by using Pdoping in graphene systems.In a nutshell, the ordered doping predicted to be a good strategy for improving the sensitivity of graphene towards phosgene molecule and leads to a significant increase of the adsorption energies.
graphene hexagons.These are shown on figures 2(e)-(h).Furthermore, four structures with similar doped graphene nanoribbons in zigzag geometry (ZGNR) are investigated to determine the effect of the crystallographic orientation of the nanoribbon.However, in these cases, only a local doping with two neighboring P atoms is used due to the close distance of neighboring lattice sites in the periodic direction, which causes an increased strain in the P-P bonds in the relaxed structure.The corresponding structures: P2 center ZGNR (n = 3 and n = 4) (figures 2(i)-(l)), and P2 edge (n = 3 and n = 4) (figures 2(m)-(p)).As shown in table

3. 2 .
Geometric parameters and Adsorption energy of all doped systems For detection of phosgene, pristine AGNR and 6 possible geometries with a ribbon width of 3 hexagons were considered to find out the most favorable geometry for adsorption.These adsorption geometries are shown on figure S1 and S2, and 'C' atom in pristine, 'B' atom in B edge AGNR, B center AGNR, BP edge AGNR and BP center AGNR were considered as active adsorption site due to accumulation of local positive charge around it
(a)-(b) and at center position in figures S1(c)-(d), after adsorption, the orientation of phosgene molecule was parallel to graphene nanoribbon and no significant changes were observed in optimized geometry of Pr edge AGNR and Pr center AGNR.However, after adsorption of phosgene molecule with pristine system at both edge and center position, 'Cl' atom moved away from pristine graphene system.After interaction, 'Cl' atom of phosgene molecule in case of Pr edge AGNR, stabilized itself at a distance of 3.56 Å (figure S1(b)) and in case of Pr center AGNR, it get relaxed at a distance of 3.47 Å (figureS1(d)).The bond lengths and bond angle of phosgene molecule after interaction with Pr-AGNR at edge position were found to be d1 (C-Cl) = 1.75 Å, d 2 (C-Cl) =1.74 Å, d 3 (C=O) =1.19 Å, Cl-C-Cl = 110°and at center position, these were d 1 (C-Cl) = 1.74 Å, d 2 (C-Cl) =1.74 Å, d 3 (C=O) = 1.19 Å, Cl-C-Cl = 110°,respectively.These minimal changes in bond lengths and bond angle of phosgene molecule suggests that the molecule remains intact even after its interaction with pristine surface.The adsorption of phosgene onto Pr-AGNR at edge and center position resulted into weak interaction with an adsorption energy (E ads ) of −0.65 eV.This indicates weak adsorption (van der Waals forces of attraction) and reveals physical nature of interaction.

Figure 3 .
Figure 3.The DOS plots of (a) Pr edge AGNR; (b) Pr center AGNR; (c)P edge AGNR; (d) P center AGNR before and after adsorption of phosgene.

Figure 5 .
Figure 5. Bar diagram of (a) adsorption energies and (b) change of band gap (eV) before and after phosgene adsorption for all variants.
atoms in the phosphorene nanosheet to the COCl2 gas.Here, phosgene molecule acts as a electron acceptor as shown in figure6, where, it is surrounded by yellow region (charge accumulation region) which shows the gain of charge in the adsorption process.On the other hand, 'C' atoms in pristine AGNR and doped atoms (B and P) in doped AGNR are surrounded by cyan region (charge depletion region) which indicates the loss of charge.It is observed that the phosgene molecule carries only a very small amount of charge of 0.01e (as depicted in table S1 and figures S6(a), (b)) when it interacts with pristine AGNR at both edge and center doped configuration.There is no overlap exists between phosgene and pristine AGNR (figures S6(a), (b)) which indicates weak type of interaction.Interaction of phosgene with 'B' atoms in B edge AGNR (figure S6(c)) and B center AGNR (figure S6(d)) transfers a charge of 0.03e and 0.02e towards phosgene molecule which is mainly obtained from the 'B' atom.When phosgene molecule interacts with the BP doped geometries, significant charge transfer takes place between the phosgene and BP doped geometries having values 003e and 0.08e for BP edge AGNR and BP center AGNR (figure S6(e),(f)).A small overlap exists between phosgene and B doped geometries (figure S6(c),(d)).

Figure 6 .
Figure 6.3D isosurface of charge density difference with the isosurface value of 0.0003 e Å −3 for (a) P edge AGNR, (b) P center AGNR after adsorption of phosgene (COCl 2 ).

Table 1 .
Adsorption energies of all studied configurations (in eV), binding distance, bond lengths and bond angles after adsorption of COCl 2 , binding energies(eV).
Variants Eads (adsorption energy) (in eV)Binding distance (in Å)Bond lengths after adsorption (in Å)Bond angle after adsorption(Cl-C-Cl) Binding Energy (eV) d1 (C-Cl) d 2 (C-Cl) d3 (C=O) and 'P' atom in P edge AGNR and P center AGNR was taken as active site for adsorption.All calculated parameters such as adsorption energies, binding distance, changes in bond lengths and bond angles after adsorption are listed in table 1 for these systems as well as the P-doped systems in different nanoribbon geometries.The binding distance is the closest distance of 'Cl' atom of phosgene molecule with doped (B, P) atom of AGNR.As shown in figureS1, phosgene molecule was placed parallel to Pr edge AGNR with chlorine ('Cl') atom of phosgene molecule facing towards graphene nanoribbons.Initially, phosgene molecule (COCl 2 ) with 'Cl' atom of phosgene interacts with 'C' atom at edge in figures S1 ).After the phosgene molecule adsorption, band gap change (ΔE g ) is very small for Pr edge AGNR, Pr center AGNR (0.09 eV, 0.07 eV).Thus in case of pristine, after adsorption of phosgene at both edge and center position, change in band gap is not significant.This indicates that undoped AGNR cannot be considered as active material for detection of phosgene molecule.3.4.1.B edge and B center AGNRThe doping of boron ('B') in pristine AGNR at edge and center position indicates its semi-metallic behavior.B edge AGNR configuration acquires a band gap (E g ) of 0.08 eV (direct band gap) and after adsorption, it opens the band gap to 0.72 eV (direct band gap) (figureS5(a)-(b)).In case of B center AGNR, the boron atom act as a p-type dopant shifting the fermi level in AGNR nearer to valence band than conduction band.The band gap of B center AGNR changes slightly i.e. from 0.02 eV (indirect band gap) to 0.25 eV (indirect band gap) (figureS5(c)-(d)).Since, the band gap change (ΔE g ) are 0.64 eV and 0.23 eV for B edge AGNR and B center AGNR.A small change in energy gap is observed for B doped geometries.3.4.2.BP edge and BP center AGNRWhen equal number of 'B' and 'P'atoms are added into the graphene system in a ordered way, for system BP edge AGNR, it acquires energy gap of 1.32 eV with a direct band gap and after adsorption, this changes to 0.96 eV (direct band gap material) (figure S5(e)-(f)).i.e. fine tunability of energy gap appears after detection of phosgene molecule.The BP center AGNR system attains a band gap of 1.42 eV (direct band gap semiconductor) and after interaction with COCl 2 , the band gap changes to 1.30 eV (direct band gap semiconductor) (figureS5(g)-(h).The band gap change (ΔE g ) are 0.36 eV and 0.12 eV for BP edge AGNR and BP center AGNR which is very small.This small change in the band gap for BP doped geometries indicates that these materials cannot be considered as good material for detection of phosgene molecule although these variants have more value of adsorption energies.3.4.3.P edge and P center AGNRThe doping of 'P' atoms in pristine AGNR at edge and center position indicates metallic and semi metallic behavior before adsorption of phosgene molecule.In case of P edge AGNR, the band gap behavior changes from metallic to semiconducting type (1.06 eV) i.e. direct band gap semiconductor (figures 4(a)-(b)).On the other hand, for P center AGNR, the band gap changes from 0.88 eV (semi-metallic) to 1.71 eV (direct band gap semiconductor) (figures 4(c)-(d)).After phosgene adsorption, opening of band gap takes place in case of P doped geometries.The band gap change (ΔE g ) is 1.06 eV and 0.83 eV for P edge AGNR and P center AGNR as shown in table