Theoretical Study of Adsorption Behavior of Dimethylamine and Ammonia on Al- and Ga-Doped BN Monolayer Surfaces Based on DFT

Endogenous volatile organic compounds (VOCs) can reflect human health status and be used for clinical diagnosis and health monitoring. Dimethylamine and ammonia are the signature VOC gases of nephropathy. In order to find a potential gas sensitivity material for the detection of both signature VOC gases of nephropathy, this paper investigated the adsorption properties of dimethylamine and ammonia on Al- and Ga-doped BN monolayers based on density functional theory. Through analyzing the adsorption energy, adsorption distance, charge transfer, density of states, and HOMO/LUMO, the results indicated that the adsorption effect of Al- and Ga-doped BN monolayers to dimethylamine and ammonia is probably good, and these nanomaterials have the potential to be applied for nephropathy monitoring and clinical diagnosis.


INTRODUCTION
As an important physiological process of the human body, respiration is one of the ways for the human body to exchange substances with the environment. 1−3 Relevant studies have shown that human exhaled gases contain thousands of volatile organic compounds (VOCs), 4 among which endogenous VOCs can reflect human health status and be used for clinical diagnosis and health monitoring. 5,6 For example, the concentration changes of ethane and other alkanes are related to lipid peroxidation. 7 NO and CO can diagnose the presence of lung cancer. 8 In addition, when the kidney is damaged or pathological changes occur, ammonium ions and urea cannot be metabolized and discharged from the kidney in time, resulting in an increase in the concentration of free ammonia, which will pass through the blood barrier in the form of ammonia and dimethylamine and exhale out of the human body. As a result, dimethylamine and ammonia are the signature VOC gases of nephropathy. 9 The method of human health monitoring and clinical diagnosis based on exhaled gases has the advantages of being noninvasive and safe and allowing for fast sampling. However, the existing detection equipment has some disadvantages, such as complex structure, huge volume, high price, and so on. The detection of exhaled gases via a gas sensor can effectively reduce the cost and shorten the detection time. Therefore, a pressing task and challenge is to develop effective, low-cost, and gas sensitive material, as shown in Figure 1.
Due to the strong responsivity and high sensitivity, 10−13 boron nitride (BN) is a potential material for gas sensors. For example, Sajjad et al. 14 explored the gas sensitivity of a boron nitride monolayer to CO 2 , and Lin 15 found that the h-BN monolayer has a fast response and excellent repeatability to ethanol and NH 3 . Deng 16 studied the adsorption of SO 2 on the (8, 0) boron nitride nanotube. In addition, atomic doping dramatically improves the gas sensitivity of intrinsic BN. 17,18 Moreover, as the metallic congener of B, Al is more active, and the gas sensitivity is enhanced after doping Al on BN. Wang et al. 19 modified the BN monolayer with Al for the ability to absorb CO 2 . Peyghan et al. 20 developed Al-doped single walled BN nanotubes and found Al doping can notably enhance the adsorption energy of CO. Heidari et al. 21 found an Al-doped h-BN monolayer is a potential material for adsorbing mercaptans.
Considering the above research, in this paper, the gas sensitivity of an Al-doped BN monolayer (Al−BN) to dimethylamine and ammonia was studied based on density functional theory (DFT). The work will guide the manufacture of a gas sensor and provide fundamental gas-sensitivity information of an Al-doped BN monolayer as a possible candidate for a resistance chemical sensor applied in nephropathy monitoring and clinical diagnosis.

COMPUTATION DETAILS
In this paper, all the quantum mechanics calculations were performed via the Dmol 3 module 22−24 based on DFT. The generalized gradient approximation (GGA) of the Perdew− Burke−Ernzerhof (PBE) functional was used to treat the electron exchange and correlation. 25,26 The dual numerical plus polarization (DNP) basis set was adopted. 27,28 The Brillouin zone was sampled as a 9 × 9 × 1 Monkhorst−Pack mesh k-point in geometric optimization. The DFT-D methods of Grimme were used to deal with the van der Waals interaction 29,30 Maximum force, energy tolerance accuracy, and maximum atom displacement were selected as 0.002 Ha/Å, 1.0 × 10 −5 Ha, and 5 × 10 −3 Å, respectively. 31 After a comparison, a 4 × 4 × 1 supercell with a 20 Å vacuum slab was built to avoid the interaction between two adjacent layers.
The adsorption energy E ad represents the change of the gas molecules adsorption system, as shown in formula 1: 32 in which E gas/Al−BN represents the total system energy after gas molecule adsorption, and E gas and E Al−BN represent the energy of the gas molecule and Al−BN monolayer, respectively.
The charge transfer Q t-Hirshfeld between the Al−BN monolayer and gas molecules is estimated via Hirshfeld analysis. If Q t-Hirshfeld < 0, it denotes the electrons transfer from the Al−BN monolayer to gas molecules during the adsorption process. And the electrons transfer from gas molecules to the Al−BN monolayer when Q t-Hirshfeld > 0. 34

Optimization of the Al-Doped BN Monolayer, the
Ga-Doped BN Monolayer, Dimethylamine, and Ammonia. Due to the number of valence electrons of an Al atom being the same as that of a B atom but different from that of a N atom, there are no unpaired electrons after substituting B by Al; however, the electron deficiency after substituting N by Al will lead to the structural instability of a BN monolayer. In addition, the total formation energy of substituting B by Al (5.9951 eV) is much lower than that of substituting N by Al (14.6428 eV). Considering the above factors, we only simulated the situation that one B atom was substituted by Al. The optimized geometries of the BN and Al−BN monolayer are exhibited in Figure 2(a,b). It can be found that the length of the B−N bond is 1.445 Å, and six identical B−N bonds form a regular hexagon. Whereas, the geometric configuration of the Al−BN monolayer is irregular around the Al atom. The lengths of Al−N and two adjacent B−N bonds are 1.714, 1.426, and 1.494 Å, respectively. The bandgap of the Al−BN monolayer only decreases 0.215 eV compared with that of the BN monolayer, as shown in Figure 3, which implies the doping of the Al atom has little effect on the electronic properties of the BN monolayer.
Due to Ga and Al being in the same chemical group, we also only simulated the situation that one B atom was substituted by Ga for a Ga-doped BN monolayer, and the optimized geometry of the Ga−BN monolayer is exhibited in Figure 2  implying that the covalency of Ga−BN is greater than those of Bn and Al−BN monolayers.
In addition, there are 0.531 e electrons transferred from Al to adjacent N and 0.504 e electrons transferred from Ga to adjacent N, and they are quite more than the charge of 0.193 e transferring from B to adjacent N in an intrinsic BN monolayer according to Hirshfeld analysis.
The optimized geometry of dimethylamine, namely C 2 H 7 N, is shown in Figure 4 The optimized geometry of ammonia, namely NH 3 , is exhibited in Figure 4 The optimized adsorption configurations and parameters of dimethylamine on the Al−BN monolayer are shown in Figure  5 and Table 1. Compared with M2 and M3 modes, the adsorption energies and charge transfer are closed to each other, but the adsorption distance of 3.493 Å in M2 mode is larger than that of 2.681 Å in M3 mode. In addition, the Al doping region of the monolayer concaves downward after adsorbing dimethylamine. As far as dimethylamine adsorbing on the Al−BN monolayer by M1 mode, a very different phenomenon has emerged. The Al doping region of the monolayer bulges upward after adsorption. The adsorption energy of −2.033 eV in M1 mode is the biggest in the three modes. The charge transfer in M1 mode increases to 0.345 e, which is 1 order of magnitude greater than those in M2 and M3 modes. The reason could be that a Lewis acid−base interaction arises, where dimethylamine is the Lewis base and the Al atom is the Lewis acid. 35 And the interaction between dimethylamine and Al−BN in M1 mode is probably chemisorption. Considering the above factors, the adsorption effect of the Al−BN monolayer to dimethylamine is probably good, and the most likely adsorption mode is M1 mode.
In order to further estimate the interaction mechanism between dimethylamine and the Al−BN monolayer, the total density of states (TDOS) and partial density of states (PDOS) distributions are discussed and exhibited in Figure 6. As for the TDOS configuration of M1 mode, there are two novel peaks appearing near −19 and −12 eV, and they are contributed by the C 2 H 7 N according to the PDOS. The conduct band region is dominated by the 1s orbital of H atoms of C 2 H 7 N. In addition, an impurity state appears near the conduct band  bottom, which is contributed by the 3p orbital of Al, the 2p orbital of N, the 2p orbital of C, and the 1s orbital of H.
The density of states configurations of M2 and M3 modes are shown in Figure 6(b,c). It can be found that there are two novel peaks appearing at −12.5 and −11 eV in the valence band region. The conduct band region is dominated by the 3p orbital of Al. Moreover, a novel peak appears in the Fermi level and is totally contributed by dimethylamine according to the PDOS, and there is no hybridization between the Al atom and dimethylamine. In addition, there is also a little bit of orbital hybridization among the orbital of Al and those orbitals of atoms of dimethylamine near the conduct band bottom and the valence band maximum. The coupling among the orbitals of Al, H, C and N atoms in the impurity state near the conduct band bottom provides another theoretical support that a strong interaction occurred between Al−BN and dimethylamine by M1 mode with a strong charge transfer process, which probably leads the change of conductivity of such monolayer after dimethylamine. 36 The distributions of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) for the Al−BN monolayer and dimethylamine adsorbing on Al−BN monolayer are exhibited in Figures 7 and 8, respectively. The energy gap E g is shown in formula 2: Where, E LUMO and E HOMO represent the values of the LUMO and HOMO, respectively.
It can be observed that the distributions of the LUMO and HOMO become very uneven after dimethylamine adsorbs on the Al−BN monolayer. The energy gap decreases to 0.180 eV in M1 mode, an indication that the chemical activity of Al−BN will be significantly increased. 20 The LUMO is mainly distributed on the dimethylamine molecule, and the HOMO is mainly distributed on the Al−BN monolayer. However, the LUMO is mainly distributed on the Al−BN monolayer, and the HOMO is mainly distributed on the dimethylamine molecule in M2 mode. In addition, compared with those of Al−BN, the LUMO decreases to −0.065 eV and the HOMO increases to −0.190 eV in M2 mode. As a result, the energy     adsorption configurations of N1 are shown in Figure 9(a,b). The adsorption distance and adsorption energy are 2.043 Å and −2.125 eV, respectively. The charge transfer of 0.354 e is quite large, which indicates that a Lewis acid−base interaction probably happens during adsorption. In addition, the Al doping region of the monolayer bulges upward after adsorption of ammonia, and the three N−H bonds change to 1.025, 1.025, and 1.026 Å, respectively.
The optimized adsorption configurations of N2 are exhibited in Figure 9(c,d). It is intriguing that the closest distance of 2.041 Å between the adsorbate and adsorbent is from the N atom of ammonia to the Al atom of the Al−BN monolayer, shorter than the value we set initially and closer to that of N1 mode. However, H atoms gradually move away from the monolayer during the adsorption process, and the final optimized distance between H atom of ammonia to Al atom of Al−BN increases to 2.601 Å. In addition, the values of energy and change transfer also approximately equal to those of N1 mode. In a word, the optimized adsorption configuration of N2 mode is almost the same as those of N1 mode. From those, we can conclude that there could be a strong chemisorption interaction between ammonia and the Al−BN monolayer, and the most likely adsorption mode is N1 mode.
Due to the optimized adsorption configuration of N2 mode being almost the same as that of N1 mode, we only discuss the DOS configuration of N1 mode, and it is exhibited in Figure  10. One novel peak appears in the valence band region, mainly being contributed by the 1s orbital of H atom. The conduct band region is dominated by the 1s orbital of H atoms of NH 3 , and the conduct band minimum is smaller after ammonia adsorption, which leads to the pseudogap decrease and further results in the increase of the conductivity of Al−BN.
The distributions of LUMO and HOMO for ammonia adsorbing on the Al−BN monolayer are exhibited in Figure 11. The values of LUMO and HOMO are −0.052 and −0.211 eV, respectively. Compared with those of the Al−BN monolayer, the LUMO decreases to −0.052 eV, but the HOMO increases to −0.211 eV after ammonia adsorbs on the Al−BN monolayer. As a consequence, the energy gap decreases to 0.159 eV. In addition, the LUMO is mainly distributed on the ammonia molecule, whereas HOMO is mainly distributed on the Al−BN monolayer.

Adsorption Properties of Dimethylamine and Ammonia on the Ga−BN Monolayer.
Considering the number of valence electrons of a Ga atom is the same as that of an Al atom and both the most likely adsorption modes for dimethylamine and ammonia on Al-doped BN monolayer are the N atom of dimethylamine and ammonia approaching the Al−BN monolayer, we only consider the N atom of dimethylamine and ammonia approaching the Ga−BN monolayer as well.
The optimized adsorption configurations of dimethylamine and ammonia are exhibited in Figure 12, and the adsorption parameters are shown in Table 2. It is intriguing that both the adsorption energies of −1.975 eV for dimethylamine and   as a Lewis base and the Ga atom acts as a Lewis acid in the adsorption process. However, all the adsorption distances are closed to 2.00 Å, and the Ga doping region of monolayer bulges upward after dimethylamine and ammonia adsorption. The adsorption parameters also indicate that there are strong interactions occurring between the Ga−BN monolayer and dimethylamine or ammonia, respectively.
The TDOS and PDOS distributions are exhibited in Figure  13 to estimate the interaction mechanism between dimethylamine, ammonia, and the Al−BN monolayer, respectively. As for dimethylamine, there is one novel peak appearing in the valence band region, which is contributed by C 2 H 7 N via the analysis of PDOS. The strong adsorption interaction between C 2 H 7 N and the Ga−BN monolayer can be reflected by the orbital hybridization near the conduction band minimum (CBM), where the 2p orbital of N atom, the 2p orbital of the C atom, and the 1s orbital of the H atom of the C 2 H 7 N molecule are coupled with the 4s orbital of the Ga atom of the Ga−BN monolayer. As for ammonia, it can be found that one novel peak appears in the conduction band region due to the 1s orbital of H atom of NH 3 and it is hybridized with the 4s orbital of the Ga atom. The DOS in the Fermi level does not change after NH 3 adsorption. The DOS distributions are also the strong interaction between the two kinds of the signature VOC gases of nephropathy and Ga−BN monolayer.
The distributions of the LUMO and HOMO for Ga−BN monolayer, dimethylamine, and ammonia adsorbing on the Ga−BN monolayer are exhibited in Figure 14. It could be found that the values and distributions of the LUMO and HOMO change significantly after dimethylamine and ammonia adsorption. The LUMO is more localized on the dimethylamine and ammonia. Due to the energy gaps decreasing much from 0.195 to 0.180 and 0.156 eV, respectively, the chemical activity of such systems will be significantly increased. In addition, the values of the LUMO and HOMO are −0.030 and −0.210 eV, respectively, for dimethylamine adsorbing on Ga−BN. Compared with dimethylamine adsorbing on the Al−BN monolayer, both of the values of the LUMO and HOMO increase 0.001 eV. As a consequence, the energy gap is also 0.180 eV. As for ammonia, the values of LUMO and HOMO are −0.053 and −0.209 eV, respectively. Comparing with ammonia adsorbing on the Al− BN monolayer, the energy gap decreases a little to 0.156 eV.   Currently, the quantum mechanics calculation study on the surface adsorption of dimethylamine has not been reported yet; however, there are many reports about surface adsorption of ammonia. And the adsorption parameters of ammonia on several kinds of adsorbents are shown in Table 3. The adsorption energies and electronic properties of ammonia adsorbing on the Al-and Ga-BN are significantly higher compared to those reported 37−41 for it adsorbing on pristine graphene, and it is also comparable to the reported adsorption energy of ammonia on Ni-doped graphyne. 38

CONCLUSIONS
In order to find a potential gas sensitivity material for the detection of nephropathy's signature VOC gases, namely dimethylamine and ammonia, the adsorption properties of dimethylamine and ammonia on Al-and Ga-doped BN monolayers are comprehensively investigated based on DFT. The conclusions are summarized: (1) The process of dimethylamine adsorbing on a Al−BN monolayer is exothermic, and the most likely adsorption mode is the M1 mode (the N atom of dimethylamine approaching the Al−BN monolayer). And there could be chemisorption between dimethylamine and the Al− BN monolayer because of the large adsorption energy, the large amount of charge transfer, and the short adsorption distance. (2) The most likely adsorption mode for ammonia on an Aldoped BN monolayer is N1 mode (the N atom of ammonia approaching the Al−BN monolayer). There is also a strong chemical interaction between ammonia and the Al−BN monolayer. (3) The adsorption parameters of dimethylamine and ammonia on the Ga-doped BN monolayer are close to those on the Al-doped BN monolayer. And the adsorption effect of the Ga−BN monolayer to dimethylamine and ammonia is probably good as well.