BCl3 Adsorption on Pristine, S-Doped, and Cr-Doped Graphynes: A DFT Study

The adsorption of boron trichloride (BCl 3 ) was explored onto pristine, S-doped, and Cr-doped graphyne through density functional theory computations. The interaction of BCl 3 with pristine graphyne was weak and, thus, this sheet cannot be used as a sensor. Although S-doping strengthens the interaction, the S-doped sheet cannot still be used as a sensor. However, the reactivity and sensitivity of the sheet are significantly increased toward BCl 3 by replacing the C atom of graphyne with the transition metal Cr. The HOMO-LUMO gap of Cr-doped graphyne reduces from 2.18 to 1.38 eV following the adsorption of BCl 3 , which significantly increases the electrical conductivity. Thus, the great change in the conductivity can be converted into an electronic signal, indicating that Cr-doped graphyne may be a promising sensor for BCl 3 . Also, its work function is considerably decreased by the adsorption process, indicating that it can also work as a work function-type sensor for BCl 3 detection.


Introduction
Bromine trichloride (BCl3) is a dangerous toxic substance that can be used to refine copper, zinc, magnesium, and aluminum alloys to remove carbides, nitrides, and oxides from molten metal. It is used as a source of boron to increase the amount of British thermal unit for use in the propulsion of rockets and high-energy fuels [1]. Therefore, it is very important to identify this dangerous gas, which has rarely been addressed. The large surface area of nanostructures is highly susceptible to the adsorption of gas molecules, for example, the graphene nanoplate has been used as a sensor to adsorb many hazardous gases [2,3]. In addition, the electronic properties of this nanostructure are sensitive toward the presence of chemical vapors, which is the reason for the attraction of these materials to nanostructures [4][5][6]. Also, doping is a method that is used to enhance the sensitivity of electronic properties and the adsorption performance of nanostructures [7].
Graphyne nanosheet is one of the carbon allotropes proposed in 1987 by Baughman et al [8]. It is composed of a crystal lattice consisting of carbon with sp and sp 2 hybridization. Carbon, which participates in the formation of a hexagonal ring by sp 2 hybridization, and another carbon, by sp hybridization which binds these hexagonal rings together. The difference between this nanosheet and graphene is the many triple bonds it has in its network structure, which greatly changes its electronic and optical properties [9]. The advantages of this change are high chemical stability, large surface area and high electrical conductivity of graphyne [10][11][12]. A wide range of applications has been defined for graphyne, for example in electronics, chemical sensors, energy storage systems, and as electrodes in batteries [13][14][15][16]. The aim of the present work is to investigate the adsorption of BCl3 onto graphyne nanosheets to find an easy and fast way to identify this gas using density functional theory calculations.

Computational methods
An 86-carbon graphyne nanosheet was selected as an adsorbent for BCl3. Hydrogen atoms were used to saturate its ends to decrease the boundary effects. All calculations such as energy calculations, geometry optimization, and state density analysis on pristine, Cr-doped and S-doped graphyne were carried out through augmented B3LYP, i.e, B3LYP-D*. Here, Grimme's "D" term for evaluating dispersion forces that are weak. As a basis set, 6-31G (d) was used and all calculations were done by using GAMESS software [11]. We used GaussSum [17] software to draw the density of state (DOS) diagram. Based on previous studies, B3LYP is used for nanostructures and it exhibits high performance in the field [18][19][20].
After the adsorption of the BCl3 molecule onto pure graphyne as well as S-doped and Crdoped graphyne, its adsorption energy is calculated by the equation below: Here, E(sheet) is pure or doped graphyne sheet energy, E(BCl3) is the BCl3 molecule energy, E(BCl3/sheet) designates the energy of the sheet that adsorbed the BCl3 molecule. For the structures under study, the positive energy of the adsorption shows that this adsorption is exothermic. The correction of the basis set superposition error was making for interactions. The HOMO-LUMO energy gap (Eg) was computed according to the following equation: Where ELUMO and EHOMO designate the energy of the lowest and the highest unoccupied molecular orbitals respectively.

Pristine graphyne
Graphyne is one of the carbon allotropes in which single, resonance bonds (in its hexagonal ring) and triple bonds (-C≡C-) between its carbons can be seen, as shown Figure 1. As mentioned earlier, it has two types of carbon atoms in terms of hybridization, one is sp-hybridized (tagged as C1), which connects carbon hexagons, and the other is sp 2 -hybridized hexagons (tagged as C2).
After the optimization procedure was performed, the bond length between the carbons in the hexagonal ring was computed to be 1.42Å, which indicates the existence of a resonant bond between them. There is a bond length of 1.22 Å between the two carbon atoms of C2, which suggests a triple bond. Furthermore, the bond length C1-C2 bond length is 1.41Å. By comparing it with the bond between two carbons of ethane (~1.53 Å), it can be inferred that there is a resonance π bond between the two C1 and C2 carbons. Due to the presence of C2 atoms in graphyne, this configuration is more unstable in terms of energy than graphene [7]. To obtain the most stable BCl3 that is adsorbed onto graphyne configuration, we evaluated various interaction models. We showed at least three states by considering the BCl3 molecule parallel to the sheet and in two states perpendicular to the graphyne sheet, and compared them to find the most stable state ( Fig. 2). The data obtained from the optimization of these configurations are given in Table 1.
The results of Fig. 2 show that complex A is the most stable complex for this type of interaction. However, with Ead of 13.21 kcal/mol, it exhibits a van der Waals interaction which is weak. The interaction of B and C complexes with adsorption energies of 10.87 and 10.21 kcal/mol, respectively, are weaker than those of complex A. The most stable BCl3/graphyne (A) configuration was obtained, in which the BCl3 molecule interacted with the graphyne sheet horizontally with an equilibrium distance of 3.23 Å and an Ead of 13.21 kcal/mol, and these observations confirm that the interaction is weak. According to Table 1 and as shown in Fig. 1, the graphyne Eg is 2.57eV, which is reduced to 2.26, 2.42, and 2.51eV for configurations A, B, and C, respectively, which is not a significant change. Therefore, graphyne is still a semiconductor after the adsorption of BCl3 because the weak adsorption of BCl3 occurs without a large change in Eg.

S-doped graphyne
After replacing one of the C1 and C2 atoms in graphyne, the optimized structures are shown in indicating that the interaction is almost weak.

Cr-doped graphyne
Moreover, in the graphyne sheet, instead of C1 and C2 atoms, a chromium (Cr) atom is replaced and its influence is explored over the geometric structure and electronic characteristics of the graphyne sheet (Fig. 5). The Cr atom in the graphyne nanosheet disrupts its geometric structure because it is larger than carbon, and it was placed slightly higher than the surface of the sheet (Fig.   5). The Cr-C bonds are longer than the carbon-carbon bonds in the graphyne sheet, Cr-C1 and Cr-C2 bond lengths are 1.88 and 1.67Å, respectively. The Cr-doped graphyne data are provided in To investigate the interaction between the BCl3 molecule and Cr-doped graphyne, the BCl3 molecule was placed in different directions above the Cr atom (shown in Fig. 6). Two of the orientations were shown to be local minima, which we introduced as N and M.
In this equation, k designates the Boltzmann constant and σ is the electrical conductivity [21]. At a constant temperature, the lower the amount of Eg, the higher the electrical conductivity. So, when the adsorption procedure causes a decrease in Eg, the electrical conductance of Cr-doped graphyne increases significantly. To better evaluate the sensitivity of the configurations under study, the changes in the work function (Φ) were investigated before and following the adsorption process.
Φ of a semiconductor is the least amount of work needed for extracting an electron from the Fermi level. The re-examination of the gas-induced Φ by the suspended amplitude effect modifiers has been accepted for several years as the basis for the realization of a sensor operating system [22].
Theoretically, in vacuum, the released electron current densities are defined as follows: Where A designates the Richardson constant (A/m 2 ), T designates the temperature (K).Φ designates the work function as mentioned above. We computed the Φ values as follows: Where EF designates the energy of the Fermi level and Einf designates the electrostatic potential at infinity, which is presumed to be equal to 0. We subtracted Φ of the sheet from that of the complexes and obtained Φ changes (∆Φ). Φ for pristine graphyne was about 3.90 eV and changed very slightly after adsorbing the BCl3 molecule, which can be ignored. Also, changes in Φ of Sdoped graphyne are very small after BCl3 is adsorbed. But when BCl3 is adsorbed onto Cr-doped graphyne, Φ is significantly reduced from 3.81 to 3.39 eV. According to Eq. 4, there is an exponential relationship between the emitted current density and Φ. Therefore, it can be said that after the adsorption of BCl3, by decreasing the Φ, the current density of the emitted electron increases dramatically. Accordingly, we think that Cr-doping in graphyne is a promising way to increase the sensitivity of graphyne toward BCl3 which pristine graphyne did not.

Conclusions
The interaction of BCl3 was explored with pristine, S-doped, and Cr-doped graphyne by using Cr-doped graphene may be a Φ-type sensor for BCl3.           Figure 1 Optimized structure of graphyne and its density of state (DOS). Models for three stable adsorption states for a BCl3 molecule on the pristine graphyne, distances are in Å.  Models for two stable adsorption states for a BCl3 molecule on the different S-doped graphyne, distances are in Å.    Partial DOS plot of Cr-doped graphyne/ BCl3 nanostructure complex.

Figure 8
The HOMO pro le of Cr-doped graphyne/ BCl3 nanostructure complex.