First-principles study of methanol adsorption on heteroatom-doped phosphorene

Dongdong Liu,Yongling Shi,Li To,Dfeng Yn,Ru Chen,*,Shungyin Wng,c,d,*

a State Key Laboratory of Chem/Bio-Sensing and Chemometrics,Provincial Hunan Key Laboratory for Graphene Materials and Devices,College of Chemistry and Chemical Engineering,Hunan University,Changsha 410082,China

b ICQD/Hefei National Laboratory for Physical Sciences at Microscale,and Key Laboratory of Strongly-Coupled Quantum Matter Physics,Chinese Academy of Sciences and Department of Physics,University of Science and Technology of China,Hefei230026,China

c Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province,Shenzhen University,Shenzhen 518060,China

d Shenzhen Research Institute of Hunan University,Shenzhen 518057,China

Key words:Methanol adsorption Gas sensing Heteroatomdoping Phosphorene Density functional theory calculation

ABSTRACT First-principles calculations based on van der Waals(vd W)corrected density functional theory(DFT)are fi rstly employed to investigate the adsorption of methanol(CH 3OH)gas molecule on pristine and X-doped phosphorene(X=B,C,N and O).The CH3OH gas molecule is placed on the top of different phosphorene surfaces,the w hole adsorption systems are fully optimized by using Vienna ab initio simulation package(VASP).The calculation results demonstrate that both pristine and heteroatomdoped phosphorene are sensitive to CH3OH gas molecule with a moderate adsorption energy and an excellent charge transfer.Among all the investigated adsorption con fi gurations,CH3OH gas molecule is physically absorbed on pristine phosphorene and heteroatom-doped phosphorene.The N and O doping improve the adsorption of phosphorene with CH3OH gas molecule,while B and Cdoping are almost not bene fi cial compared to the pristine phosphorene.The results suggest that N-doped and O-doped phosphorene are ideal candidates used for CH3OH gas sensing.

Methanol has been used as a clean energy resource to tackle global energy shortage and environmental pollution problems[1–4].However,the methanol gas is highly poisonous,and inhaling the high concentrations of methanol gas or absorpting of methanol through the skin is often extremely harmful to human beings[5].For these reasons,it is especially necessary to develop a reliable and selective methanol gas sensor.In the past few years,there has been performed extensive work on methanol adsorption and decomposition,or methanol gas sensing based on different materials with various surface,such as Zn O(1010)surface[1],PtRu Pt(111)[2],Ru(0001)surfaces[3],MoS2surface[4],Ni(Pd)-decorated grapheme[6],graphene supported Pt13nanoclusters[7],TbOx(111)surfaces[8],Pt3Ni(111)surface[9]and Al-doped Zn O thin fi lms[10].

Most recently,phosphorene(single layer black phosphorus)has been successfully fabricated by mechanical cleavage from black phosphorus[11].Similar to graphene and MoS2,phosphorene is a single layered tw o-dimensional(2D)nanomaterial[12].Theoretical calculation revealed that phosphorene is an intrinsic p-type semiconductor material with the direct band gap of about 0.73 eV[13,14].Besides,phosphorene possesses a large freecarrier mobility(up to 1000 cm2V?1s?1),good mechanical fl exibility,linear dichroism,optical responses and high on/off ratios(up to 104mobility)[15–19].And more significantly,phosphorene has an anisotropic electric conductance and is sensitive to environmental gas molecules[16–19],thus has many promising applications in nanoelectronic devices.Therefore,many experiments and theoretical calculations have been performed to study the adsorption and dissociation of small gas molecules over phosphorene surface.Zhang and Jing reported the fi rst-principle study of the adsorption of tetracyano-pquinodimethane(TCNQ)and tetrathiafulvalene(TTF)molecule on phosphorene[12,20].Cai researched the physisorbed adsorption of CO,H2,H2O,NH3,NO,NO2,and O2on phosphorene[21].Kulish surveyed the metal adatoms on single-layer phosphorene based on density functional theory[22].Yu,Arabieha,Lalitha and Kuanga further investigated the gases adsorption of heteroatomdoped phosphorene[16–18,23].In 2015,Carmen researched methanol gas sensing based on layered black phosphorus using electrochemical impedance spectroscopy as the detection method[5].The results indicated that the sensing for methanol detection based on black phosphorus is high-ef fi ciency and cost-effective.However,there are no studies about the adsorption properties of methanol gas molecule over phosphorene surface by means of the density functional theory calculation.

Herein,we fi rstly presented the study of the adsorption properties of methanol gas molecule on pristine and X-doped phosphorene(X=B,C,Nand O)based on fi rst-principlecalculations.In theadsorption models,the CH3OHgasmoleculewasplaced on the top of different phosphorenes,O??H bond of CH3OH was parallel to phosphorene surfaces,and the w hole adsorption systemswere fully optimized by using Vienna ab initio simulation package(VASP).Our resultsshow that theadsorption of CH3OHgasmoleculeon N-doped and O-doped phosphorene is more efficient compared to the pristine phosphorene.However,B-doped and C-doped phosphorene are almost not bene fi cial.Our present study providesa valuable reference for methanol gas adsorption and sensor based on phosphorene-based material.

Our calculations are performed based on the density functional theory(DFT)and electron-ion interactions are investigated with the use of the projector augmented w ave(PAW)method,which implemented in the VASP[13,14].The exchange corrective interaction is described by the generalized gradient approximation(GGA)with the parametrization of Perde-Burke-Ernzerhof(PBE)and we perform non spin-polarized calculations for all of the structural optimizations[24].The van der Waals density function(vd W-DF)of Becke88 optimization(optB88)is employed to improve the description of the PBEfunction[13,14].A planewave basis set with an energy cutoff of 450 eV is employed for the planew ave expansion of the w ave function.The atomic positions are fully relaxed until satisfying an energy convergence of 1?10?4eV and force convergence of 0.05 eV/?.

We adapt a 3?3 phosphorene supercell along the x and y directions of the p hosp horene layer.The lattice constant of the primitive unit cell is a=3.32?,b=4.32?,and 36 atom s are contained.In order to safely avoid the interaction between the periodically repeated structures,a vacuum thickness of 20? between the layers is used.Thus,the methanol-phosphorene adsorp tion systems are composed of a 10.04??13.29??22.16supercell.The fi rst Brillouin zone is sampled using a Monkhorst-Pack(MP)grid corresp onding to 5?1?5 k-point m esh for both geom etry optim ization and electronic properties calculations.In order to estimate the charge distribution and corresp ond ing charge transfer(D Q)between the CH3OH gas molecule and phosp horene,Bader charge analysis method is adopt[17,20,22,25].

In order to characterize the stability of the CH3OH gas molecule adsorption on phosphorene,the adsorption energy is determined as:

w here EphosphorenetCH3OH,EphosphoreneandECH3OHstand for the total energy of the molecular adsorption on phosphorene,the isolated phosphorene layer and the single CH3OH molecule,respectively.According to this de fi nition,a more negative Eaindicates a more favorable interaction.

To better understand the nature of the adsorption mechanism,the isosurface of the charge density difference(CDD)D r of the methanol-phosphorene adsorption systems is de fined as:

w here rphosphorenetCH3OH,rphosphoreneand [13_TD DIFF]IFF]rCH3OHstand for the charge distribution of the molecular adsorption on phosphorene,the isolated phosphorene layer and the single CH3OH molecule respectively.According to this de fi nition,a positive D Q indicates the transfer of electrons from CH3OH molecule to phosphorene.

The optimized geometry of single layer phosphorene from both the top and side view s is show n in Fig.1.The relaxed lattice constant is a=3.35?and b=4.43?along the armchair and zigzag directions.The calculated P??Pbondslength in and out of the plane are 2.24?and 2.26?,and the corresponding bond angles are 96.5?and 102.3?,respectively.These are in good agreement with the previous theoretical studies[13,14].

The electronic properties of the single layer phosphorene are investigated through the density of states(DOS)and the band structure,which are show n in Fig.S1(Supporting information).The energy gap of the single layer phosphorene is about 0.73 eV,and the valance band minima(VBM)and conduction band maxima(CBM)are both located at the high symmetry point in the reciprocal space.These reveal that phosphorene is a direct bandgap semiconductor,which is in agreement with previous researches[13,14,20].

The pristine phosphorene has sp3hybridization,w here Pbonds with three adjacent P with a lone pair of electrons,resulting in a puckered structure[14–16].Fig.2 shows the schematic illustration of top and side view s of the optimized X-doped phosphorene structures(X=B,C,N and O),and Table 1 show s the calculated results of lattice constants,bond lengths and distances between tw o atoms in pristine and X-doped phosphorene,respectively.Here,we de fine P atom on a,b and c sites as Pa,Pband Pcrespectively.In pristine phosphorene,dX-Paand dX-Pb/cis 2.26?and 2.24?.The X-Padistances in B,C,N and O-doped phosphorene systems are found to be 1.87,1.81,1.80 and 3.19?,and X-Pb/cdistances are 1.94,1.81,1.79 and 1.72?respectively,which agree with the previous calculations[18,26,27].

Fig.1.The top and side view s of the optimized geometry of single layer phosphorene and three adsorption sites:the hollow site(H),the bridge site(B),and the top site(T).

Table 1 The calculated results of lattice constants,bond lengths and distances between tw o atoms in pristine and X-doped phosphorene,respectively.

Fig.3.The top and side views of the most stable methanol adsorption con fi gurations on(a)pristine,(b)B-doped,(c)C-doped,(d)N-doped and(e)O-doped phosphorene.

We then discuss the adsorption of CH3OH gas molecule on single layer phosphorene and heteroatom-doped phosphorene.Three adsorption sites are considered as Fig.1 show s:the hollow site(H),the bridge site(B),and the top site(T)[22,28].In the adsorption models,the CH3OH gas molecule is placed on the top of different phosphorenes,O??H bond of CH3OH is parallel to phosphorene surfaces,and the w hole systems are fully optimized by using VASP.Fig.3 show s the most stable methanol-phosphorene adsorption con fi gurations.For abetter understanding of the adsorption of CH3OH gas molecule on these different phosphorene surfaces,the adsorption sites,adsorption distances(d)between CH3OH gas molecule and the phosphorene surface and their corresponding adsorption energies(Ea)(Eq.(1))are calculated and listed in Table 2.The d refers to the distance between oxygen atom of CH3OHmolecule and the surface of phosphorene or heteroatomdoped phosphorene.It can be observed that the CH3OH molecule’s orientations are almost unchanged and the adsorption distances vary from 2.1? to 2.83?.For the pristine phosphorene,the adsorption energy is?0.24 eV and the adsorption distance is 2.74?,which indicates the adsorption behavior is allowed in thermodynamics and the gas molecule is physically absorbed on the phosphorene surface.This is agree with the previous experimental results[6,12,16,21].Moreover,the adsorption energies between CH3OH with B-doped,C-doped,N-doped and O-doped phosphorene are?0.26,?0.25,?0.29 and?0.23 eV,and thecorresponding distances are 2.78,2.83,2.51 and 2.10?,respectively.The adsorption energies and corresponding distances between CH3OH with B-doped and C-doped samples are similar to the pristine phosphorene.However,N-doped phosphorene possess the significantly shorter adsorption distance and largest adsorption energy.O-doped phosphorene has the shortest adsorption distance although the adsorption energy is close to the pristine phosphorene.These indicate N-doped and O-doped phosphorene are more effective for the adsorption of CH3OHgasmolecule due to the shorter adsorption distance and larger adsorption energy,while B-doped and C-doped phosphorene are almost not bene fi cial compared to the pristine phosphorene.

Table 2 The adsorption sites,adsorption distances and adsorption energies for methanol adsorption on the pristine phosphorene and heteroatom-doped phosphorene after relaxation.

In order to estimate the D r(Eq.(2))and corresponding charge transfer(D Q)between CH3OH gas molecule and phosphorene,Bader charge analysis is implemented as show n in Fig.4.

For the O-doped phosphorene,the amount of electrons transferred from CH3OH gas molecule to O-doped phosphorene is the largest(up to 0.04 e),which leads to the shortest adsorption distance(the adsorption energy is not the largest).Also,the N-doped phosphorene has larger charge transfer(near 0.03 e)with corresponding shorter adsorption distance and larger adsorption energy w hen compared to the pristine phosphorene.

However,the amount of the transferred electrons for B-doped and C-doped phosphorene are obviously smaller(about 0.02 e),which results in the longer distances and similar adsorption energies compared to the pristine phosphorene.Furthermore,it could be observed from the charge density difference that the charges are remarkably redistributed after adsorbing CH3OH gas molecule and the charge is transfered from CH3OH to N-doped or O-doped phosphorene.This gives a further explanation that N-doped and O-doped phosphorene are more effective for the adsorption of CH3OH gas molecule.

Fig.4.Charge density difference(CDD)for the adsorption of CH3OH gas molecule on(a)pristine,(b)B-doped,(c)C-doped,(d)N-doped and(e)O-doped phosphorene.The isosurface value is 0.001 e/?3.The yellow regions show the charge accumulation,w hereas the blue regions represent the charge depletion.

In conclusion,we have fi rstly performed a theoretical research on the adsorption properties between methanol gas molecule and pristine or heteroatom-doped phosphorene by using VASP.The calculation results show that N-doped and O-doped phosphorene have larger charge transfer values with corresponding shorter adsorption distances and larger adsorption energies for the adsorption of methanol compared to the pristine phosphorene,while B-doped and C-doped phosphorene are not desired.Our results suggest that N-doped and O-doped phosphorene are ideal candidates w hen used for CH3OH gas sensing.

Acknow ledgments

This work was supported by the National Natural Science Foundation of China(Nos.21701043,21573066,51402100),the Provincial Natural Science Foundation of Hunan (Nos.2016JJ1006,2016TP1009),the Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province and Shenzhen Science and Technology Program(No.JCYJ20170306141659388).

Appendix A.Supp lementary data

Supplementary data associated with this article can be found,in the online version,at https://doi.org/10.1016/j.cclet.2018.01.041.