INTRODUCTION
Arsine (AsH3) is a dangerous material with high concern about skin, lung, and bladder owing to its extreme volatility and toxicity [1-3]. AsH3 is slightly soluble in water, colorless, and with mild garlic‐like odor in the gas form [4, 5]. It can trigger serious health problems even if a low amount of arsine gas was ingested into the human body causing long‐term chronic diseases such as arsenicosis and acute fatal intoxication [6-8]. Removal of AsH3 from a gas phase is an important matter, especially in industry. Hence, finding an efficient adsorbent for removing arsine gas from the atmosphere is essential. In this regard, several compounds have been recognized for AsH3 gas detection and removal. Among various compounds, graphitic carbon nitride (g-C3N4) has the highest selectivity for AsH3 molecules amongst the other adsorbents due to its excellent characteristics such as suitable band gap energy (Eg=2.7 eV), low cost, and excellent physicochemical stability [9-11]. These novel features make g-C3N4 a promising candidate for different fields, such as H2 production from H2O splitting, gas storage, reduction of CO2, and toxic gas sensors [12-14]. Additionally, a vast number of attempts have been made in recent years by several strategies such as embedding, decorating, and doping different elements to improvise the AsH3 gas sensing and removing via g-C3N4-based compounds [15-18].  
The effect of B and P atoms codoping on optical and electronic characteristics of g-C3N4 were demonstrated by Moshfegh and co-workers [19] using ab-initio simulations. They found that the incorporation of both P and B into the structure of g-C3N4 decreases the Eg for pristine g-C3N4 from 3.1 eV to 1.9 eV. Vovusha et al. [20] reported the adsorption behavior of CO2 on the Cr‒, Co‒, Ni‒, Mn‒, Sc‒, Fe‒ Zn‒, and Cu‒doped g-C3N4 systems using VASP code. The results illustrated that these modified g-C3N4 compounds could be used for carbon dioxide gas storage. In another work, the adsorption manner of SO2 molecules over the Ir/P‒, Rh/P‒, and Co/P‒codoped g-C3N4 compounds were studied by DFT calculations [21]. The results indicated that the Ir/P‒codoped g-C3N4 with the highest adsorption energy (Eads) of -3.52 eV can be successfully utilized for the detecting and removing of sulfur dioxide from the atmosphere. In another report, Basharnavaz and co-workers [22] reported the adsorption manner of NO over the pristine g-C3N4, Fe‒, Os‒, and Ru‒embedded g-C3N4 using DFT computations. They found that among these transition metal (TM)‒modified g-C3N4 systems, the Os‒embedded with Eads of -3.14 eV has a promising candidate for detecting and removing of NO gas. Furthermore, the adsorption of CO gas on the g-C3N4, Pd‒, Pt‒, and Ni‒embedded g-C3N4 were reported using ab-initio computations [23]. The results revealed that the g-C3N4, Pd‒, and Ni‒embedded g-C3N4 are non-magnetic, while Pt‒embedded g-C3N4 system induces a magnetic moment of 1.35 µB. In addition, they found that Pt‒embedded g-C3N4 with the highest Eads of -2.77 eV is an excellent candidate for detecting and removing carbon monoxide gas from the environment. Furthermore, the adsorption behavior of NO2 over the g-C3N4 and Rh‒, Ir‒, and Co‒embedded g-C3N4 were investigated in order to explore the removing abilities of TM‒modified g-C3N4 as NO2 sensor [24]. The results displayed that the interaction between NO2 gas and Ir‒embedded system (with the highest Eads=-4.47 eV) is higher than those of the other TM‒modified g-C3N4 compounds. In another work, the adsorption behavior of several toxic gases such as H2S, NO, and SO2 on the pure and Mo‒decorated g-C3N4 systems were reported using ab-initio computations [25]. The results demonstrated that the interaction energy between these toxic gas and Mo‒decorated g-C3N4 is stronger than that of the pure system. Herein, a summary about the adsorption of arsine gas on the various adsorbents along with Eads are summarized in Table 1 [26-32]. From this Table, it can be found that the adsorption energy of AsH3 gas molecules on the Fe‒doped single-walled carbon nanotube with the highest adsorption energy of -2.48 eV is higher than those of the other adsorbents [29]. 
In this research, we investigated the adsorption manner of AsH3 gas on the P‒doped, Cr‒, Mo‒, W‒embedded, and also Cr/P‒, Mo/P‒, and W/P‒codoped g-C3N4 systems using DFT computations in order to introduce suitable systems for application in sensing and removing of AsH3 gas. To the best of the authors’ knowledge there is no report on these TM/P‒modified g-C3N4 systems as AsH3 sensor, though numerous literature reports have focused on the adsorption of AsH3 gas on several adsorbents. 

COMPUTATIONAL METHODS
In this study, all of the relaxed computations were performed with the Quantum Espresso (QE) package based on DFT computations. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange‒correlation was utilized to explain the correlation and exchange effects. It should be noted that the used DFT exchange-correlation functional tends to underestimate adsorption energy, therefore the DFT computations coupled with a van der Waals (vdW)-inclusive corrections of Grimme (DFT-D) are carried out to improve the computations. A Monkhorst-Pack (MP) grid of 1×1×8 was carried out to perform Brillouin-zone (BZ) integrations. The kinetic energy cut-off equal to 90 Ry was chosen and a vacuum space of 18 Å was inserted along the z-direction of g-C3N4 surface to avoid the interaction between the periodic layers. 
The Eads of AsH3 gas over the P‒doped and TM/P‒modified g-C3N4 systems can be calculated by expression as given below [33]:

where , , and are the total energy of AsH3 adsorbed over the g-C3N4 systems, free AsH3 gas, and g-C3N4, respectively. A more negative value of Eads suggests that the adsorption behavior of AsH3 gas molecule over the g-C3N4 surface is energetically more favorable.
Additionally, one of an important parameter for the adsorption process of a toxic gas sensing device is the recovery time (), thus the  of AsH3 gas from P‒doped and Cr/P‒, Mo/P‒, and W/P‒modified g-C3N4 systems can be predicted from the following equation [34]:

where , , , and  indicates attempted frequency (=1012 s-1), Boltzmann’s constant ( 8.62×10-5 eV K-1), adsorption energy and temperature, respectively. The computed recovery time for the P‒doped, Cr‒, Mo‒, W‒embedded, Cr/P‒, Mo/P‒, and W/P‒codoped g-C3N4 systems are seen to be 3.90×10-4, 6.42×1028, 4.79×1032, 2.11×1040, 1.34×1029, 4.96×1033 and 3.24×1040 s, respectively. According to this equation, the more negative value for the adsorption energy leads to the extended recovery time due to prolonged desorption of gas molecules from the surface of adsorbent. Thus, it is inferred that the strong interaction energy between AsH3 gas and W/P‒codoped g-C3N4 system with the highest 
Eads=-3.105 eV revealed that the g-C3N4‒based materials, gradually recover to its a stable initial state [35].

RESULTS AND DISCUSSION
In this article, a systematic theoretical investigation of AsH3 gas adsorption by P‒doped, Cr‒, Mo‒, and W‒embedded, and also Cr/P‒, Mo/P‒, and W/P‒codoped g-C3N4 compounds were explored using DFT computations. The optimized structures for adsorption of AsH3 molecules over the P‒doped and TM/P‒codoped g-C3N4 systems are shown in Fig. 1. The results of the literature review displayed that the adsorption energy of AsH3 gas on different TM‒modified adsorbents from As atom is stronger than that of H atom [29, 32]. Comparing optimized structures of P‒doped and Cr/P‒, Mo/P‒, and W/P‒codoped g-C3N4 systems in Fig. 1, it is clearly seen that there is a remarkable change in relaxed structures of these g-C3N4 before and after the adsorption of AsH3 gas. On the other hand, with adsorption of AsH3 gas molecules on the g-C3N4 system and also codoping of g-C3N4 with TM and P elements, the primary flat structures of pure g-C3N4 automatically changed to buckle structure [36, 37].
To further explore the effect of AsH3 gas adsorbing on the electronic characteristics of g-C3N4 systems, the band structures plots for P‒doped, Cr‒, Mo‒, W‒embedded, and also Cr/P‒, Mo/P‒, and W/P‒codoped g-C3N4 systems are demonstrated in Fig. 2 and also the corresponding data are listed in Table 2. It is worth noting that the Fermi energy level was set to zero energy scale (red dotted line). The results of band structures revealed that the Eg for a pure g-C3N4 at the DFT calculation is smaller than the experimental value (Eg=2.70 eV), because the GGA functional underestimate the fundamental gap energy. It should be noted that the underestimation of Eg in the present investigation will not affect the ultimate conclusion because we aim to make the comparison for the adsorption performance and electrical characteristics of pure and TM/P‒modified g-C3N4 systems with and without AsH3 molecules. As shown in Fig. 2, with adsorption of AsH3 molecules on the g-C3N4 and also codoping of Cr, Mo, W, and P atoms, the Eg of system (Eg=2.7 eV) is considerably decreased. Furthermore, it was found that the EF of g-C3N4 system upshifts into the conduction band edge after the adsorption of AsH3 molecules and also codoping of P and TM atoms, indicating the improvement of the conductivity of these modified g-C3N4 systems. Additionally, these results indicated that the AsH3‒adsorbed and TM/P‒modified g-C3N4 systems have semi-metallic properties because valence band and conduction band energy levels have not crossed each other at near the Fermi energy. Thus, it is inferred that the electrical conductivity of g-C3N4 systems are considerably modulated by adsorption of AsH3 and also codoping with Cr, Mo, W, and P atoms (see Table 2).         
The total induced magnetic moment (Mtot) and geometrical parameters of AsH3 gas adsorbed over the P‒doped and Cr/P‒, Mo/P‒, and W/P‒modified g-C3N4 systems such as As–H bond length (dAs-H), TM‒Nedge bond length, and distance between As atoms of arsine molecules and these TM elements are listed in Table 2. As seen, the magnetic moment for W/P‒codoped (Mtot=0.39 µB) is higher than those of the Cr/P‒ and Mo/P‒modified g-C3N4 systems, because the weak interaction between Nedge and W atoms causes induced magnetic features in this system. Furthermore, it can be realized that with adsorption of AsH3 molecules on the W/P‒codoped g-C3N4, the Mtot reduces from 0.39 to 0.00 µB. This phenomenon attributed to strong overlapping between orbitals of W and As atoms. In addition, the bond lengths of As–H are 1.525 Å (P‒doped), 1.535 Å (Cr/P-codoped), 1.539 Å (Mo/P-codoped), and 1.541 Å (W/P-codoped) having a considerable variation in comparison with the bond length of free AsH3 gas (1.520 Å). From Table 2, it can be observed that the bond lengths of TM‒Nedge significantly change after adsorption of AsH3 molecules over the TM/P‒modified g-C3N4 compounds. It should be noted that the elongation of dAs-H in AsH3‒adsorbed W/P‒codoped g-C3N4 is higher than those of the other TM/P‒codoped systems. This phenomenon is mostly ascribed to the large electron transfer from the W/P‒codoped g-C3N4 to AsH3 gas. The maximum value of Eads for AsH3 adsorbed on the W/P‒codoped g-C3N4 with the smaller distance between W and AsH3 molecules indicating strong chemisorption for AsH3 on this system (see Table 2).
The Lowdin charges analysis before and after adsorption of AsH3 molecules on the P‒doped, TM‒embedded, and TM/P‒codoped g-C3N4 compounds are displayed in Table 2. The negative value of the electron transfer shows that the electron is transferred from TM‒modified g-C3N4 to AsH3 molecules. On the other hand, the AsH3 acts as an electronic charge acceptor and these TM‒modified g-C3N4 compounds behave as electronic charge donor [27, 28]. In addition, it was found that the charge transfer between orbitals of g-C3N4 and AsH3 gas in W/P‒codoped system is more than those of the other g-C3N4 systems. The results of Lowdin charge displayed that the interaction energy between W/P‒codoped and AsH3 molecules is higher than those of the other reported systems.
In order to further explain the electronic characteristics of the g-C3N4 systems, the partial density of states (PDOS) for the Nedge orbitals of the P‒doped, Cr‒, Mo‒, W‒embedded, Cr/P‒, Mo/P‒, and W/P‒codoped g-C3N4 compounds before and after adsorption of AsH3 molecules are demonstrated in Fig. 3. From this figure, we can see that with codoping of Cr, Mo, W, and P elements and also adsorption of AsH3 molecules on the g-C3N4, electron density close to the Fermi energy state for Nedge elements in these g-C3N4 systems remarkably increase, which is caused owing to the strong overlapping between orbitals of the different elements in the g-C3N4 systems and AsH3 gas. This shows that there is a considerable charge transfer between AsH3 gas and modified g-C3N4. In addition, it can be inferred that the sharp peak close to EF for the Nedge element in AsH3‒adsorbed W/P‒codoped g-C3N4 is more than those of the Cr/P‒, Mo/P‒ codoped compounds, which reveals a strong interaction energy between W element and AsH3 gas. Based on these results, it can be stated that the strong orbital hybridization between Cr, Mo, and especially W atoms and AsH3 gas enhances the gas adsorption ability of the g-C3N4 compounds to this toxic gas.
CONCLUSIONS
In this research, the adsorption manner of AsH3 gas over the P‒doped, Cr‒, Mo‒, W‒embedded, Cr/P‒, Mo/P‒, and W/P‒codoped g-C3N4 compounds were investigated by first‒principles study. The results of adsorption energy displayed that the sensitivity of g-C3N4 system for the adsorption of AsH3 gas can be considerably improved by introducing an appropriate transition metal (TM) dopant. Therefore, Cr/P–, Mo/P‒, and W/P‒codoped g-C3N4 are more appropriate for detection and adsorption of AsH3 gas than that of the pristine g-C3N4. The results of electronic band structures revealed that with adsorption of AsH3 molecules over the g-C3N4 systems and also codoping of TM and P atoms, the electrical conductivity of g-C3N4 remarkably reduces due to the induced new impurity energy levels close to Fermi energy level. Additionally, the results of relaxed structures indicated that with adsorption of AsH3 over the g-C3N4 systems and also modifying with these TM atoms, the initial structure of g-C3N4 system automatically chances from planar to wrinkles structure. Furthermore, the results of electron transfer indicated that the electron density accumulation region is located on the orbitals of AsH3 gas molecules, resulting from the electron transfer from TM/P‒codoped g-C3N4 systems to AsH3 gas. Based on these results, it can be state that the W/P–codoped g-C3N4 with the highest adsorption energy of -3.105 eV is more suitable than those of the Cr/P‒ and Mo/P‒codoped g-C3N4 systems for detecting and removing of AsH3 from the atmosphere. On the other hand, the W/P–codoped g-C3N4 cannn be a good candidate for toxic gas sensors, providing an avenue to facilitate the design of highly active g-C3N4-based gas sensors.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.