First-Principles Investigation of the Adsorption Behaviors of CH2O on BN, AlN, GaN, InN, BP, and P Monolayers

CH2O is a common toxic gas molecule that can cause asthma and dermatitis in humans. In this study the adsorption behaviors of the CH2O adsorbed on the boron nitride (BN), aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), boron phosphide (BP), and phosphorus (P) monolayers were investigated using the first-principles method, and potential materials that could be used for detecting CH2O were identified. The gas adsorption energies, charge transfers and electronic properties of the gas adsorption systems have been calculated to study the gas adsorption behaviors of CH2O on these single-layer materials. The electronic characteristics of these materials, except for the BP monolayer, were observed to change after CH2O adsorption. For CH2O on the BN, GaN, BP, and P surfaces, the gas adsorption behaviors were considered to follow a physical trend, whereas CH2O was chemically adsorbed on the AlN and InN monolayers. Given their large gas adsorption energies and high charge transfers, the AlN, GaN, and InN monolayers are potential materials for CH2O detection using the charge transfer mechanism.


Introduction
In the past years monolayer 2D materials have elicited increasing attention because of their superior thermal, mechanical, and optoelectronic properties [1,2]. Therefore, studies that focus on the applications of monolayer materials are attractive because these materials exhibit excellent performance in both gas sensors and optoelectronic devices [2,3]. With the development of the research, an increasing number of monolayer materials, such as boron nitride (BN) [4] and antimonene [5], have been predicted and synthesized. Within the monolayer materials, the family of nitrides has become a popular research material in optoelectronic and electronic applications because of their outstanding properties [4,6]. Two-dimensional (2D) BN, aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) materials exhibit a graphene-like honeycomb structure with band gaps of 4.47 [4], 2.9 [6], 2.16 [7], and 1.48 eV [8], respectively. Rao et al. successfully prepared graphene-like analogs of BN flakes, which can be applied in the preparation of composites [9]. It has been reported that an AlN monolayer can epitaxially grow on Ag (111) via plasma-assisted molecular beam deposition [10]. Graphene-like GaN monolayers exhibit high electronic mobility, indicating their excellent potential for application in nanoelectronics [11]. Moreover, first-principles calculations demonstrate that hexagonal InN monolayers exhibit excellent electronic properties [8], and InN nanowires can be experimentally prepared [12]. Furthermore, the boron phosphide (BP) monolayer is theoretically predicted to exhibit remarkable electronic properties [13], and a layered BP was prepared ∆ρ = ρ (sub+molecule) − ρ molecule − ρ sub (2) where ρ (sub+molecule) , ρ molecule , and ρ sub denote the total charge densities of the adsorption system, CH 2 O, and substrate, respectively. Furthermore, the charge transfer (Q) was calculated using the Hirshfeld method, and the adsorption distance (d) was also calculated. The negative value of Q indicates that the CH 2 O molecule collects the electrons from the substrate, whereas d denotes the nearest distance between the substrate and the CH 2 O molecule.

Results and Discussion
The structures of the BN, AlN, GaN, InN, BP, and P monolayers are 4 × 4 supercells, and the lattice parameters of the primary cells are 2.51, 3.13, 3.21, 3.63, 3.21, and 3.31 Å, respectively. The calculations of our structural constants were observed to be in good agreement with those of others [4,[6][7][8]13,34]. While determining the most stable structures of CH 2 O on the BN, AlN, GaN, InN, and BP surfaces, three representative initial sites, including the top of X (herein after referred to as B, Al, Ga, In, and B) atoms (T 1 ), top of N or P atoms (T 2 ), and top of hexagon centers (T 3 ) were considered, as plotted in Figure 1a. For the P monolayer three representative initial sites were calculated, as depicted in Figure 1b. After structural optimization, the most stable adsorption structures for CH 2 O on the surface of the BN, AlN, GaN, InN, BP, and P monolayers could be selected by choosing the lowest E (sub+molecule) of the three initial adsorption positions T 1 -T 3 , as depicted in Figure 2. In the BN, AlN, GaN, and InN, monolayers, CH 2 O tended to be adsorbed on the top of B, Al, Ga, and In, respectively. After the gas adsorption of CH 2 O, the structures of AlN, GaN, and InN exhibited various deformation levels, whereas no obvious deformation was observed in the BN, BP, and P structures. In the subsequent discussion, all the results were calculated using these stable structures.
Materials 2018, 11, x FOR PEER REVIEW 3 of 8 method, and the adsorption distance (d) was also calculated. The negative value of Q indicates that the CH2O molecule collects the electrons from the substrate, whereas d denotes the nearest distance between the substrate and the CH2O molecule.

Results and Discussion
The structures of the BN, AlN, GaN, InN, BP, and P monolayers are 4 × 4 supercells, and the lattice parameters of the primary cells are 2.51, 3.13, 3.21, 3.63, 3.21, and 3.31 Å, respectively. The calculations of our structural constants were observed to be in good agreement with those of others [4,[6][7][8]13,34]. While determining the most stable structures of CH2O on the BN, AlN, GaN, InN, and BP surfaces, three representative initial sites, including the top of X (herein after referred to as B, Al, Ga, In, and B) atoms (T1), top of N or P atoms (T2), and top of hexagon centers (T3) were considered, as plotted in Figure 1a. For the P monolayer three representative initial sites were calculated, as depicted in Figure 1b. After structural optimization, the most stable adsorption structures for CH2O on the surface of the BN, AlN, GaN, InN, BP, and P monolayers could be selected by choosing the lowest E(sub+molecule) of the three initial adsorption positions T1-T3, as depicted in Figure 2. In the BN, AlN, GaN, and InN, monolayers, CH2O tended to be adsorbed on the top of B, Al, Ga, and In, respectively. After the gas adsorption of CH2O, the structures of AlN, GaN, and InN exhibited various deformation levels, whereas no obvious deformation was observed in the BN, BP, and P structures. In the subsequent discussion, all the results were calculated using these stable structures.  The calculated values of Ea, Q, and d for the most energetically stable adsorption structures are presented in Table 1. For CH2O on the surface of the BN, GaN, BP, and P monolayers, the values of Ea were −0.283, −0.456, −0.249, and −0.188 eV, respectively. Furthermore, the values of Ea for CH2O on method, and the adsorption distance (d) was also calculated. The negative value of Q indicates that the CH2O molecule collects the electrons from the substrate, whereas d denotes the nearest distance between the substrate and the CH2O molecule.

Results and Discussion
The structures of the BN, AlN, GaN, InN, BP, and P monolayers are 4 × 4 supercells, and the lattice parameters of the primary cells are 2.51, 3.13, 3.21, 3.63, 3.21, and 3.31 Å, respectively. The calculations of our structural constants were observed to be in good agreement with those of others [4,[6][7][8]13,34]. While determining the most stable structures of CH2O on the BN, AlN, GaN, InN, and BP surfaces, three representative initial sites, including the top of X (herein after referred to as B, Al, Ga, In, and B) atoms (T1), top of N or P atoms (T2), and top of hexagon centers (T3) were considered, as plotted in Figure 1a. For the P monolayer three representative initial sites were calculated, as depicted in Figure 1b. After structural optimization, the most stable adsorption structures for CH2O on the surface of the BN, AlN, GaN, InN, BP, and P monolayers could be selected by choosing the lowest E(sub+molecule) of the three initial adsorption positions T1-T3, as depicted in Figure 2. In the BN, AlN, GaN, and InN, monolayers, CH2O tended to be adsorbed on the top of B, Al, Ga, and In, respectively. After the gas adsorption of CH2O, the structures of AlN, GaN, and InN exhibited various deformation levels, whereas no obvious deformation was observed in the BN, BP, and P structures. In the subsequent discussion, all the results were calculated using these stable structures.  The calculated values of Ea, Q, and d for the most energetically stable adsorption structures are presented in Table 1. For CH2O on the surface of the BN, GaN, BP, and P monolayers, the values of Ea were −0.283, −0.456, −0.249, and −0.188 eV, respectively. Furthermore, the values of Ea for CH2O on the surface of pristine graphene was −0.162 eV [29], indicating that CH2O was easier to adsorb on the aforementioned materials than pristine graphene. The calculated values of d for BN, GaN, BP, and P The calculated values of E a , Q, and d for the most energetically stable adsorption structures are presented in Table 1. For CH 2 O on the surface of the BN, GaN, BP, and P monolayers, the values of E a were −0.283, −0.456, −0.249, and −0.188 eV, respectively. Furthermore, the values of E a for CH 2 O on the surface of pristine graphene was −0.162 eV [29], indicating that CH 2 O was easier to adsorb on the aforementioned materials than pristine graphene. The calculated values of d for BN, GaN, BP, and P were 2.990, 2.361, 3.328, and 3.230 Å, respectively, which were considerably larger than the bond length between the atom in CH 2 O and the substrate (i.e., l H-N = 1.03 Å, l O-Ga = 1.87 Å, l O-B = 1.48 Å, and l C-P = 1.38 Å, respectively) [35]. Thus, the adsorption of CH 2 O on these materials was considered to exhibit the trends of physical adsorption. The E a values for CH 2 O on the AlN and InN surfaces were −1.044 and −1.046 eV, respectively, which were considerably larger than those of other materials. Meanwhile, the adsorption distances were 1.566 and 1.555 Å, respectively, which were in the bonding range (l C-N = 1.46 Å), indicating that chemical adsorption may be observed in these two cases. Previous studies related to the (indium selenide) InSe and BP monolayers have demonstrated that the resistivity of the substrate can be altered by the adsorbed gas molecules using the charge transfer mechanism, implying that the value of Q plays an important role in the gas adsorption behavior [25,36]. The calculated values of Q for the CH 2 O adsorbed on the BN, AlN, GaN, InN, BP, and P monolayers were −0.019, −0.206, −0.107, −0.319, −0.065, and −0.067 e, respectively, revealing that the CH 2 O molecule gained electrons from these substrates. The Q values of the CH 2 O adsorbed on pristine graphene [29] and MoS 2 [28] were −0.008 and −0.010 e, respectively, indicating that the charge transfers between CH 2 O and these materials were more noticeable than that to pristine MoS 2 or graphene. To further investigate the charge transfers between CH 2 O and the substrates, the charge density differences were calculated, where the blue region represents an increase in the number of electrons, whereas the yellow region indicates electron reduction, as depicted in Figure 3. For CH 2 O on the surface of the AlN, GaN, and InN monolayers yellow regions were observed to be localized around the substrates, indicating that CH 2 O obtains electrons from the substrates. However, for CH 2 O on the surface of the BN, BP, and P monolayers, the blue or yellow regions were observed to be small, and the absolute values of Q were smaller than 0.07 e. These results indicate that the charge transfer in these cases was less evident when compared with that observed in case of the aforementioned materials (i.e., AlN, GaN, and InN). The charge transfers were reported to alter the number of charge carriers and resistance of the substrate [20]. Thus, the resistance of AlN, GaN, and InN may be noticeably altered after the adsorption of CH 2 O.
To perform an in-depth investigation of the adsorption of CH 2 O on the BN, AlN, GaN, InN, BP, and P monolayers, the density of states (DOSs) for the CH 2 O substrate systems were calculated, as depicted in Figure 4. Notably, for all the adsorption systems, the contribution of the electronic levels of CH 2 O was observed between −2.5 and 2.5 eV, which is around the Fermi level (E f ). Note that the DOSs near E f may exhibit a remarkable effect on the electronic characteristics of materials [3,22]. Thus, the existence of CH 2 O may have different degrees of influence on the electronic properties of these materials. To perform an in-depth investigation into the effects of CH 2 O on the electronic characteristics of the substrate, the band structures were also calculated, as shown in Figure 5. The band structure is an important parameter for determining the electrical properties of materials [24]. The band gaps of CH 2 O adsorbed on the BN, AlN, GaN, InN, BP, and P monolayers were 3.36, 3.15, 1.78, 1.02, 0.94, and 1.72 eV, respectively. The band gaps of pure substrates were as follows: E g-BN = 4.67 eV; E g-AlN = 3.43 eV; E g-GaN = 2.45 eV; E g-InN = 0.83 eV; E g-BP = 0.94 eV; and E g-P = 1.97 eV (refer to Figure  S1 of "Supplementary Materials"). This result implies that the adsorption of CH 2 O had a noticeable effect on the electrical characteristics of these monolayer materials, except for BP. Because of the large band gap, BN is an insulator and not suitable for application in gas sensors. For CH 2 O on the surface of the P monolayer, the E a and Q values were quite small even though CH 2 O influenced the electronic characteristics of the monolayer. This result indicates that P was not the most suitable material for detecting CH 2 O in our study. The AlN, GaN, and InN monolayers may have excellent potential for the detection of CH 2 O.         To obtain detailed confirmation about the type of gas adsorption behavior, the total electronic densities were also calculated, as depicted in Figure 6. The slices of electronic densities can help to determine the occurrence of a new chemical bonding. For instance, when CH2O is adsorbed on the BN monolayer, no obvious charge distribution was observed between the CH2O and BN atoms, and the adsorption distance was 2.99 Å, implying that the type of gas adsorption behavior was physical adsorption. Similarly, the gas adsorption behavior types of CH2O adsorbed on the GaN, BP, and P monolayers were also observed to follow the trend of physical adsorption. In the case of CH2O on the AlN and InN surfaces, the charge distribution between the CH2O and the substrates were apparent, considering the small adsorption distances, considerable adsorption energies, and large charge transfers, revealing the formation of new covalent bonds. As previously mentioned, the gas adsorption of CH2O was observed to noticeably influence the electrical properties of the AlN, InN and GaN monolayers. Given that the gas adsorption types of CH2O on the AlN and InN surfaces followed the trend of chemical adsorption, they exhibit an excellent potential for catalyzing CH2O or as disposable gas sensors for CH2O detection. For CH2O on the GaN surface, a chemical bond was observed between the gas molecule and the substrate. CH2O was therefore easily desorbed from the GaN monolayer after adsorption. Moreover, compared with In, the resources of Ga in China, USA, and the EU are considerable [17], indicating that GaN is suitable for use in gas sensors for CH2O detection.  To obtain detailed confirmation about the type of gas adsorption behavior, the total electronic densities were also calculated, as depicted in Figure 6. The slices of electronic densities can help to determine the occurrence of a new chemical bonding. For instance, when CH 2 O is adsorbed on the BN monolayer, no obvious charge distribution was observed between the CH 2 O and BN atoms, and the adsorption distance was 2.99 Å, implying that the type of gas adsorption behavior was physical adsorption. Similarly, the gas adsorption behavior types of CH 2 O adsorbed on the GaN, BP, and P monolayers were also observed to follow the trend of physical adsorption. In the case of CH 2 O on the AlN and InN surfaces, the charge distribution between the CH 2 O and the substrates were apparent, considering the small adsorption distances, considerable adsorption energies, and large charge transfers, revealing the formation of new covalent bonds. As previously mentioned, the gas adsorption of CH 2 O was observed to noticeably influence the electrical properties of the AlN, InN and GaN monolayers. Given that the gas adsorption types of CH 2 O on the AlN and InN surfaces followed the trend of chemical adsorption, they exhibit an excellent potential for catalyzing CH 2 O or as disposable gas sensors for CH 2 O detection. For CH 2 O on the GaN surface, a chemical bond was observed between the gas molecule and the substrate. CH 2 O was therefore easily desorbed from the GaN monolayer after adsorption. Moreover, compared with In, the resources of Ga in China, USA, and the EU are considerable [17], indicating that GaN is suitable for use in gas sensors for CH 2 O detection. To obtain detailed confirmation about the type of gas adsorption behavior, the total electronic densities were also calculated, as depicted in Figure 6. The slices of electronic densities can help to determine the occurrence of a new chemical bonding. For instance, when CH2O is adsorbed on the BN monolayer, no obvious charge distribution was observed between the CH2O and BN atoms, and the adsorption distance was 2.99 Å, implying that the type of gas adsorption behavior was physical adsorption. Similarly, the gas adsorption behavior types of CH2O adsorbed on the GaN, BP, and P monolayers were also observed to follow the trend of physical adsorption. In the case of CH2O on the AlN and InN surfaces, the charge distribution between the CH2O and the substrates were apparent, considering the small adsorption distances, considerable adsorption energies, and large charge transfers, revealing the formation of new covalent bonds. As previously mentioned, the gas adsorption of CH2O was observed to noticeably influence the electrical properties of the AlN, InN and GaN monolayers. Given that the gas adsorption types of CH2O on the AlN and InN surfaces followed the trend of chemical adsorption, they exhibit an excellent potential for catalyzing CH2O or as disposable gas sensors for CH2O detection. For CH2O on the GaN surface, a chemical bond was observed between the gas molecule and the substrate. CH2O was therefore easily desorbed from the GaN monolayer after adsorption. Moreover, compared with In, the resources of Ga in China, USA, and the EU are considerable [17], indicating that GaN is suitable for use in gas sensors for CH2O detection.

Conclusions
In summary, the structure, charge transfers, and electronic characteristics of CH 2 O adsorbed on the BN, AlN, GaN, InN, BP, and P monolayers were investigated using first-principles calculations.
For the adsorption of CH 2 O on the BN, GaN, BP, and P surfaces, the gas adsorption behavior followed the trends of physical adsorption. By assessing the band structures and DOSs of the gas adsorption systems, it was found that the electronic characteristics of these materials was evidently altered by the adsorption of the CH 2 O molecule, except for the BP monolayer. The GaN monolayer was considered to be the most suitable material for detecting CH 2 O in our study because of its appreciable charge transfer and moderate adsorption energy. The adsorption of CH 2 O on the surface of the AlN and InN monolayers was observed to follow the trends of chemical adsorption, with large charge transfers and considerable adsorption energies, revealing that AlN and InN have excellent potential for catalyzing CH 2 O or in disposable gas sensors for CH 2 O detection.

Conflicts of Interest:
The authors declare no conflict of interest.