First-Principles Investigation of the Adsorption Behaviors of CH2O on BN, Aln, Gan, Inn, BP, and P Monolayers

Delft University of Technology

First-principles investigation of the adsorption behaviors of CH 2 O on BN, AlN, GaN, InN, BP, and P monolayers

Feng, Chuang; Qin, Hongbo; Yang, Daoguo; Zhang, Guoqi DOI 10.3390/ma12040676 Publication date 2019 Document Version Final published version Published in Materials

Citation (APA) Feng, C., Qin, H., Yang, D., & Zhang, G. (2019). First-principles investigation of the adsorption behaviors of CH O on BN, AlN, GaN, InN, BP, and P monolayers. Materials, 12(4), 1-8. [676]. https://doi.org/10.3390/ma120406762 Important note To cite this publication, please use the final published version (if applicable). Please check the document version above.

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

Chuang Feng 1, Hongbo Qin 1,* , Daoguo Yang 1 and Guoqi Zhang 1,2

1 School of Mechanical and Electronic Engineering, Guilin University of Electronic Technology, Guilin 541004, China; [email protected] (C.F.); [email protected] (D.Y.); [email protected] (G.Z.) 2 EEMCS Faculty, Delft University of Technology, 2628 Delft, The Netherlands * Correspondence: [email protected]; Tel.: +86-773-2290108

Received: 29 January 2019; Accepted: 18 February 2019; Published: 25 February 2019

Abstract: 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.

Keywords: monolayer materials; CH2O; ﬁrst-principles calculation; gas sensor

1. 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

Materials 2019, 12, 676; doi:10.3390/ma12040676 www.mdpi.com/journal/materials Materials 2019, 12, 676 2 of 8 by Li et al. [14]. In addition, the blue phosphorus (P) monolayer was synthesized by Zhang et al. [15], and the monolayer P exhibited a remarkable potential for applications in nanodevices [16]. Ga, In, and P are critical raw materials for the EU while they are not critical for China [17], and the application of 2D materials can significantly reduce the amount of material necessary for the realization of a sensor. Owing to their atomic thicknesses, large surface areas, and excellent electrical properties, monolayer 2D materials can become the new-generation sensitive materials of gas sensors [18]. In a previous study, the electronic characteristics of the monolayer materials were altered using some special adsorbed gas molecules [3]. Based on the investigation conducted by Zhou et al., P was ultrasensitive to NO, NO2, and NH3 gas molecules, indicating that the P monolayer is a potential material for the detection of these molecules [19]. Furthermore, antimonene was sensitive to NO, NH3, and SO2 gas molecules. Its electronic characteristics are altered after these gas molecules are adsorbed [3]. The investigation conducted by Li et al. indicated that monolayer MoS2 was suitable for detecting the O2, NO, and NO2 molecules [20]. Therefore, the application of monolayer materials in gas sensors is currently considered to be a popular research field. Furthermore, various theoretical works and experiments have proved that BN, AlN, GaN, InN, BP, and P are potential sensitive materials that can be applied in gas sensors [19,21–25]. For example, ultrathin BN nanosheets can be used for detecting ethanol at high operating temperatures, and Lin et al. reported that the gas sensors showed a rapid response and recovery [21]. According to the investigation conducted by Wang et al., the AlN monolayer was suitable for detecting H2, CO, CO2, NO, and O2 via the charge transfer mechanism [22]. CH2O is a common toxic gas molecule that may lead to asthma and dermatitis in humans. Thus, detecting its presence and removing it from the environment is considered to be important. To the best of our knowledge, various traditional materials, such as ZnO [26], and NiO [27], have been considered for detecting CH2O; however, these materials have complex structures and work at high temperatures. Therefore, identifying new sensitive materials for detecting CH2O is useful. Several studies have exhibited that 2D materials may denote excellent gas detection performance [28,29]. In addition, investigations of the interactions between the CH2O molecule and the monolayer substrates BN, AlN, GaN, InN, BP, and P have been rare to date. In this study, the interactions between CH2O and the six monolayer substrates (i.e., BN, AlN, GaN, InN, BP, and P) were investigated using the first-principles calculation. This work can help researchers to study and predict the application of the six monolayer substrates in detecting CH2O.

2. Materials and Methods

All the calculations related to the interactions between CH2O and the monolayer substrates (i.e., BN, AlN, GaN, InN, BP, and P) were performed based on density functional theory (DFT) [30], using the Dmol3 package [31]. The generalized gradient approximation was performed using the Perdew–Burke–Ernzerhof (PBE) method [32]. For geometry optimizations, the convergence thresholds for displacement, energy gradient, and energy were considered to be 5 × 10−3 Å, 2 × 10−3 Ha/Å, −5 and 1 × 10 Ha, respectively. While investigating the CH2O adsorption, the van der Waals interaction was considered to be critical. Hence, the method of Grimme [33] was employed while performing all the calculations. Furthermore, the Monkhorst–Pack mesh of 12 × 12 × 1 was selected for calculating the electronic structures. The gas adsorption system comprised a 4 × 4 supercell and one CH2O molecule inside, in which a vacuum of 25 Å was constructed. To evaluate the gas adsorption strength between these monolayer materials and the gas molecules, the gas adsorption energy was deﬁned as

Ea = E(sub+molecule) − Emolecule − Esub (1) where E(sub+molecule), Emolecule, and Esub denote the total energies of the adsorption system, CH2O, and substrate, respectively. Further, the charge density difference was calculated as Materials 2019, 12, 676 3 of 8

∆ρ = ρ − ρ − ρ (2) Materials 2018, 11, x FOR PEER REVIEW (sub+molecule) molecule sub 3 of 8

where ρ(sub+molecule), ρmolecule, and ρsub denote the total charge densities of the adsorption system, method,Materials 2018and, 11the, x FORadsorption PEER REVIEW distance (d) was also calculated. The negative value of Q indicates that3 of 8 CH2O, and substrate, respectively. Furthermore, the charge transfer (Q) was calculated using the theHirshfeld CH2O molecule method, collects and the the adsorption electrons distancefrom the (substrate,d) was also whereas calculated. d denotes The the negative nearest value distance of Q betweenmethod, the and substrate the adsorption and the distance CH2O molecule. (d) was also calculated. The negative value of Q indicates that indicates that the CH2O molecule collects the electrons from the substrate, whereas d denotes the the CH2O molecule collects the electrons from the substrate, whereas d denotes the nearest distance nearest distance between the substrate and the CH2O molecule. 3.between Results theand substrate Discussion and the CH2O molecule. 3. ResultsThe structures and Discussion of the BN, AlN, GaN, InN, BP, and P monolayers are 4 × 4 supercells, and the 3. Results and Discussion latticeThe parameters structures of of the the primary BN, AlN, cells GaN, are InN, 2.51, BP, 3.13, and P3.21, monolayers 3.63, 3.21, are and 4 × 43.31 supercells, Å, respectively. and the lattice The calculationsparametersThe structures of theour primarystructural of the cellsBN, cons areAlN,tants 2.51, GaN, were 3.13, InN, observed 3.21, BP, 3.63, and to 3.21, beP monolayersin and good 3.31 agreement Å, respectively.are 4 × with4 supercells, Thethose calculations of andothers the [4,6–8,13,34].oflattice our structuralparameters While constants ofdetermining the primary were the observed cells most are stable to 2.51, be instructures 3.13, good 3.21, agreement of 3.63, CH 23.21,O withon andthe those BN,3.31 ofAlN, Å, others respectively. GaN, [4, 6InN,–8,13 and ,The34]. calculations of our structural constants were observed to be in good agreement with those of others BPWhile surfaces, determining three representative the most stable initial structures sites, includ of CHing2O the on top the of BN, X (herein AlN, GaN, after InN, referred and BPto as surfaces, B, Al, Ga,three[4,6–8,13,34]. In, representativeand B) Whileatoms (Tdetermining initial1), top sites, of N includingthe or Pmost atoms stable the (T top2), structures and of X top (herein of of hexagon CH after2O referredon centers the BN, to(T asAlN,3) were B, Al,GaN, considered, Ga, InN, In, andand BP surfaces, three representative initial sites, including the top of X (herein after referred to as B, Al, asB) plotted atoms (Tin1 ),Figure top of 1a. N orFor P atomsthe P monolayer (T2), and top three of hexagon representative centers initial (T3) were sites considered, were calculated, as plotted as depictedinGa, Figure In, and in1a. Figure B) For atoms the 1b. P(TAfter monolayer1), top structural of N threeor Poptimizati atoms representative (Ton,2), and the topmost initial of stable hexagon sites wereadsorption centers calculated, (Tstructures3) were as depicted considered, for CH2 inO as plotted in Figure 1a. For the P monolayer three representative initial sites were calculated, as onFigure the surface1b. After of structural the BN, AlN, optimization, GaN, InN, the BP, most and stable P monolayers adsorption could structures be selected for CH 2byO onchoosing the surface the depicted in Figure 1b. After structural optimization, the most stable adsorption structures for CH2O lowestof the BN,E(sub+molecule) AlN, GaN, of the InN, three BP, andinitial P monolayers adsorption couldpositions be selected T1–T3, as by depicted choosing thein Figure lowest 2.E (sub+molecule)In the BN, on the surface of the BN, AlN, GaN, InN, BP, and P monolayers could be selected by choosing the AlN,of the GaN, three and initial InN, adsorption monolayers, positions CH2O Ttended1–T3, as to depicted be adsorbed in Figure on the2. Intop the of BN, B, Al, AlN, Ga, GaN, and andIn, lowest E(sub+molecule) of the three initial adsorption positions T1–T3, as depicted in Figure 2. In the BN, respectively.InN, monolayers, After CHthe2 Ogas tended adsorption to be adsorbed of CH2O, on the the structures top of B, Al, of Ga, AlN, and GaN, In, respectively. and InN exhibited After the AlN, GaN, and InN, monolayers, CH2O tended to be adsorbed on the top of B, Al, Ga, and In, variousgas adsorption deformation of CH levels,2O, the whereas structures no ofobvious AlN, GaN, deformation and InN was exhibited observed various in the deformation BN, BP, and levels, P structures.whereasrespectively. no Inobvious theAfter subsequent the deformation gas adsorptiondiscussion, was observed allof theCH 2resuO, in thelts the werestructures BN, calculated BP, and of PAlN, structures.using GaN, these and Instable the InN subsequentstructures. exhibited discussion,various deformation all the results levels, were whereas calculated no usingobvious these deformation stable structures. was observed in the BN, BP, and P structures. In the subsequent discussion, all the results were calculated using these stable structures.

Figure 1. Three representative initial adsorption sites for (a) boron nitride (BN), (aluminum nitride) AlN, (gallium nitride) GaN, (indium nitride) InN, and boron phosphide (BP) and for (b) phosphorus Figure 1. Three representative initial adsorption sites for (a) boron nitride (BN), (aluminum nitride) AlN, (P).Figure 1. Three representative initial adsorption sites for (a) boron nitride (BN), (aluminum nitride) (gallium nitride) GaN, (indium nitride) InN, and boron phosphide (BP) and for (b) phosphorus (P). AlN, (gallium nitride) GaN, (indium nitride) InN, and boron phosphide (BP) and for (b) phosphorus (P).

FigureFigure 2. 2. MostMost stable stable adsorption adsorption structures structures of of CH CH2O2O on on the the surface surface of of the the (a ()a )BN, BN, (b (b) )AlN, AlN, (c ()c )GaN, GaN,

(d(d) )InN, InN, (e (e) )BP, BP, and and ( (ff)) P P monolayers. monolayers. Figure 2. Most stable adsorption structures of CH2O on the surface of the (a) BN, (b) AlN, (c) GaN, The(d) InN,calculated (e) BP, andvalues (f) Pof monolayers. 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 wereThe −0.283, calculated −0.456, values −0.249, of and Ea, Q−0.188, and eV,d for respectively. the most energetically Furthermore, stab thele valuesadsorption of Ea structuresfor CH2O onare thepresented surface ofin pristineTable 1. grapheneFor CH2O was on the−0.162 surface eV [29], of the indicating BN, GaN, that BP CH, and2O wasP monolayers, easier to adsorb the values on the of aforementionedEa were −0.283, −materials0.456, −0.249, than andpristine −0.188 graphene. eV, respectively. The calculated Furthermore, values ofthe d valuesfor BN, of GaN, Ea for BP, CH and2O onP 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

Materials 2019, 12, 676 4 of 8

The calculated values of Ea, Q, and d for the most energetically stable adsorption structures are presented in Table1. For CH 2O 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 were 2.990, 2.361, 3.328, and 3.230 Å, respectively, which were considerably larger than the bond length between the atom in CH2O and the substrate (i.e., lH–N = 1.03 Å, lO–Ga = 1.87 Å, lO–B = 1.48 Å, and lC–P = 1.38 Å, respectively) [35]. Thus, the adsorption of CH2O on these materials was considered to exhibit the trends of physical adsorption. The Ea values for CH2O 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 (lC–N = 1.46 Å), indicating that chemical adsorption may be observed in these two cases.

Table 1. Ea, Q, and d of the gas adsorption systems of CH2O adsorbed on the BN, AlN, GaN, InN, BP, and P monolayers.

Substrate Site Ea (eV) Q (e) d (Å)

BN T1 −0.283 −0.019 2.990 (H–N) AlN T1 −1.044 −0.206 1.566 (C–N) GaN T1 −0.456 −0.107 2.361 (O–Ga) InN T1 −1.046 −0.319 1.555 (C–N) BP T3 −0.249 −0.065 3.328 (O–B) PT1 −0.188 −0.067 3.230 (C–P)

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 CH2O 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 CH2O molecule gained electrons from these substrates. The Q values of the CH2O adsorbed on pristine graphene [29] and MoS2 [28] were −0.008 and −0.010 e, respectively, indicating that the charge transfers between CH2O and these materials were more noticeable than that to pristine MoS2 or graphene. To further investigate the charge transfers between CH2O 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 Figure3. For CH 2O on the surface of the AlN, GaN, and InN monolayers yellow regions were observed to be localized around the substrates, indicating that CH2O obtains electrons from the substrates. However, for CH2O 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 CH2O. To perform an in-depth investigation of the adsorption of CH2O on the BN, AlN, GaN, InN, BP, and P monolayers, the density of states (DOSs) for the CH2O substrate systems were calculated, as depicted in Figure4. Notably, for all the adsorption systems, the contribution of the electronic levels of CH2O was observed between −2.5 and 2.5 eV, which is around the Fermi level (Ef). Note that the DOSs near Ef may exhibit a remarkable effect on the electronic characteristics of materials [3,22]. Thus, the existence of CH2O may have different degrees of inﬂuence on the electronic properties of these materials. To perform an in-depth investigation into the effects of CH2O on the electronic characteristics of the substrate, the band structures were also calculated, as shown in Figure5. The band structure is an important parameter for determining the electrical properties of materials [24]. The band Materials 2019, 12, 676 5 of 8

gaps of CH2O 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: Eg–BN = 4.67 eV; Materials 2018, 11, x FOR PEER REVIEW 5 of 8 Eg–AlN = 3.43 eV; Eg–GaN = 2.45 eV; Eg–InN = 0.83 eV; Eg–BP = 0.94 eV; and Eg–P = 1.97 eV (refer to Figure S1 of “Supplementary Materials”). This result implies that the adsorption of CH2O had a noticeable Materials 2018, 11, x FOR PEER REVIEW 5 of 8 effectof the on large the electricalband gap, characteristics BN is an insulator of these and monolayer not suitable materials, for application except for in BP. gas Because sensors. of For the CH large2O on the surface of the P monolayer, the Ea and Q values were quite small even though CH2O band gap, BN is an insulator and not suitable for application in gas sensors. For CH2O on the surface ofinfluenced the large the band electronic gap, BN characteristics is an insulator of and the not mo suitablenolayer. forThis application result indicates in gas thatsensors. P was For not CH the2O of the P monolayer, the Ea and Q values were quite small even though CH O inﬂuenced the electronic on the surface of the P monolayer, the Ea and Q values were quite 2small even though CH2O most suitable material for detecting CH2O in our study. The AlN, GaN, and InN monolayers may characteristicsinfluenced the ofelectronic the monolayer. characteristics This result of the indicates monolayer. that PThis was result not the indicates most suitable that P materialwas not the for detectinghave excellent CH O potential in our study. for the The detection AlN, GaN, of CH and2O. InN monolayers may have excellent potential for the most suitable2 material for detecting CH2O in our study. The AlN, GaN, and InN monolayers may detection of CH O. have excellent potential2 for the detection of CH2O.

Figure 3. Charge density differences for the CH2O adsorbed on the surface of the (a) BN, (b) AlN, (c) 3 FigureGaN, (d 3.) ChargeInN, (e) density BP, and differences (f) P monolayers for the CHwith2O an adsorbed isosurface on thevalue surface of 0.003 of thee/Å (a.) The BN, blue (b) AlN, and (Figureyellowc) GaN, 3.contours (Charged) InN, denote density (e) BP, the and differences increase (f) P monolayers andfor the decrease CH with2O ofadsorbed an electrons, isosurface on respectively. the value surface of 0.003of the e/Å (a) 3BN,. The (b) blue AlN, and (c) 3 yellowGaN, (d contours) InN, (e denote) BP, and the increase(f) P monolayers and decrease with of an electrons, isosurface respectively. value of 0.003 e/Å . The blue and yellow contours denote the increase and decrease of electrons, respectively.

Figure 4. Density of states (DOSs) of CH2O on the surface of the (a) BN, (b) AlN, (c) GaN, (d) InN, Figure 4. Density of states (DOSs) of CH2O on the surface of the (a) BN, (b) AlN, (c) GaN, (d) InN, (e) (e) BP, and (f) P monolayers. BP, and (f) P monolayers. Figure 4. Density of states (DOSs) of CH2O on the surface of the (a) BN, (b) AlN, (c) GaN, (d) InN, (e) BP, and (f) P monolayers.

Materials 2018, 11, x FOR PEER REVIEW 6 of 8

Materials 2019, 12, 676 6 of 8 Materials 2018, 11, x FOR PEER REVIEW 6 of 8

Figure 5. Band structures of CH O on the surface of the (a) BN, (b) AlN, (c) GaN, (d) InN, (e) BP, and (f) Figure 5. Band structures of CH22O on the surface of the (a) BN, (b) AlN, (c) GaN, (d) InN, (e) BP, and Figure 5. Band structures of CH2O on the surface of the (a) BN, (b) AlN, (c) GaN, (d) InN, (e) BP, and P monolayers. (f) P monolayers.(f) P monolayers. To obtain detailed conﬁrmation about the type of gas adsorption behavior, the total electronic To obtainTo obtain detailed detailed confirmation confirmation about about the the type type of of gasgas adsorption behavior, behavior, the the total total electronic electronic densities were also calculated, as depicted in Figure6. The slices of electronic densities can help densitiesdensities were werealso alsocalculated, calculated, as depictedas depicted in in Figure Figure 6. 6. TheThe slices ofof electronic electronic densities densities can can help help to to to determine the occurrence of a new chemical bonding. For instance, when CH2O is adsorbed on determinedetermine the occurrence the occurrence of aof new a new chemical chemical bonding. bonding. ForFor instance, when when CH CH2O2 Ois adsorbedis adsorbed on theon the the BNBN monolayer, monolayer, no no obviousobvious charge charge dist distributionribution was was observed observed between between the CH the2O CHand2 BNO and atoms, BN and atoms, BN monolayer, no obvious charge distribution was observed between the CH2O and BN atoms, and and thethe adsorption adsorption distance distance waswas 2.992.99 Å, implying that that the the type type of of gas gas adsorption adsorption behavior behavior was was physical physical the adsorption distance was 2.99 Å, implying that the type of gas adsorption behavior was physical adsorption.adsorption. Similarly, Similarly, the the gas gas adsorption adsorption behavior behavior types of of CH CH2O2 Oadsorbed adsorbed on the on GaN, the GaN, BP, and BP, P and 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 P monolayers were also observed to follow the trend of physical adsorption. In the case of CH2O 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 on the AlN and InN surfaces, the charge distribution between the CH2O and the substrates were the AlN and InN surfaces, the charge distribution between the CH2O and the substrates were apparent,apparent, considering considering the smallthe small adsorption adsorption distances, distances, considerableconsiderable adsorption adsorption energies, energies, and and large large apparent,charge considering transfers, revealing the small the adsorption formation of distances, new covalent considerable bonds. As previoadsorptionusly mentioned, energies, theand gas large charge transfers, revealing the formation of new covalent bonds. As previously mentioned, the gas chargeadsorption transfers, of revealing CH2O was the observed formation to noticeably of new co influencevalent bonds. the electrical As previo propertiesusly mentioned,of the AlN, InNthe gas adsorption of CH2O was observed to noticeably inﬂuence the electrical properties of the AlN, InN adsorptionand GaN of CH monolayers.2O was observed Given that to the noticeably gas adsorption influence types th ofe CHelectrical2O on the properties AlN and ofInN the surfaces AlN, InN and GaN monolayers. Given that the gas adsorption types of CH2O on the AlN and InN surfaces and GaNfollowed monolayers. the trend ofGiven chemical that adsorption, the gas adsorption they exhibit types an excellent of CH potential2O on the for AlN catalyzing and InN CH 2surfacesO or followedas disposable the trend gas of chemicalsensors for adsorption, CH2O detection. they For exhibit CH2O an on excellent the GaN potentialsurface, a chemical for catalyzing bond was CH 2O followed the trend of chemical adsorption, they exhibit an excellent potential for catalyzing CH2O or or as disposableobserved between gas sensors the gas formolecule CH2O and detection. the substrate. For CHCH2OO was on therefore the GaN easily surface, desorbed a chemical from the bond as disposable gas sensors for CH2O detection. For CH2O on the GaN surface, a chemical bond was was observedGaN monolayer between after the adsorption. gas molecule Moreover, and the compared substrate. with CH In,2O the was resources therefore of easilyGa in China, desorbed USA, from observed between the gas molecule and the substrate. CH2O was therefore easily desorbed from the the GaNand monolayer the EU are considerable after adsorption. [17], indicating Moreover, that compared GaN is suitable with for In, use the in resources gas sensors of Gafor inCH China,2O GaN monolayer after adsorption. Moreover, compared with In, the resources of Ga in China, USA, USA, anddetection. the EU are considerable [17], indicating that GaN is suitable for use in gas sensors for and the EU are considerable [17], indicating that GaN is suitable for use in gas sensors for CH2O CH O detection. detection.2

Figure 6. Slices of charge densities for the CH2O adsorbed on the (a) BN, (b) AlN, (c) GaN, (d) InN, (e) BP, and (f) P monolayers. The charge density ranges from 0 to 2 e/A3.

Figure 6. Slices of charge densities for the CH2O adsorbed on the (a) BN, (b) AlN, (c) GaN, (d) InN, 4. Conclusions 3 (Figuree) BP, and 6. Slices (f) P monolayers.of charge densities The charge for the density CH2O ranges adsorbed from on 0 tothe 2 ( e/Aa) BN,. (b) AlN, (c) GaN, (d) InN, 3 (e ) BP, and (f) P monolayers. The charge density ranges from 0 to 2 e/A . 4. Conclusions

4. ConclusionsIn summary, the structure, charge transfers, and electronic characteristics of CH2O adsorbed on the BN, AlN, GaN, InN, BP, and P monolayers were investigated using ﬁrst-principles calculations.

Materials 2019, 12, 676 7 of 8

For the adsorption of CH2O 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 CH2O molecule, except for the BP monolayer. The GaN monolayer was considered to be the most suitable material for detecting CH2O in our study because of its appreciable charge transfer and moderate adsorption energy. The adsorption of CH2O 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 CH2O or in disposable gas sensors for CH2O detection.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/12/4/676/s1, Figure S1: Band structures of pristine (a) BN, (b) AlN, (c) GaN, (d) InN, (e) BP, and (f) P. monolayers. Author Contributions: H.Q. and C.F. conceived this research; C.F. and H.Q. carried out the calculations; C.F. and H.Q. designed and wrote the article; D.Y. and G.Z. reviewed the article. All authors read and approved the final manuscript. Funding: Our investigation was sponsored by Guangxi Natural Science Foundation under grant Nos. 2016GXNSFBA380114 and 2018GXNSFAA281222, Guangxi Thousand Talents Training Plan of Middle-Aged and Young Backbone Teacher (Hongbo Qin), and the Innovation Project of GUET Graduate Education under grant No. 2018YJCX04. Conflicts of Interest: The authors declare no conflict of interest.

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