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Chemical Physics Letters

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Research paper

Theoretical study of oxygen molecules adsorption on M3C12S12 (M=Co,Rh)—Class 2D metal – organic frameworks

Min Ruana,⁎, Qing Yangb,⁎, Menghao Wub, Baoshan Wangc, Junming Liud

a Institute of Materials Science and Engineering, Hubei Key Laboratory of Mine Environmental Pollution Control & Remediation, Hubei Polytechnic University, Huangshi,Chinab School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, Hubei 430074, Chinac College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Chinad Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, China

H I G H L I G H T S

• O2 molecules are chemically bonded with Co and Rh atoms of the 2D Co3C12S12 and Rh3C12S12 surface.

• Oxygen molecules adsorption narrowed the band gap from semiconductive to be metallic.

• The spin magnetic moment on each Co and Rh atom decrease from 1.0 to 0.2 μB and increase from 0.0 to 0.1 μB respectively.

• It is selective functionalization of the 2D MOFs as their specific domains are exposed to atmosphere.

A R T I C L E I N F O

Keywords:2D MDTFirst-principles calculationsChemical adsorptionSelective functionalizations

A B S T R A C T

The influence of O2 molecules adsorption on 2D CoDT and RhDT surface were investigated via ab initio cal-culations. It turns out that upon the chemically adsorption of O2 molecules, semiconductive CoDT and RhDTwith a band gap of respectively 0.218 and 0.101 eV both become metallic, while their magnetic moment on eachmetal atom will respectively decrease from 1.0 to 0.2 μB and increase from 0.0 to 0.1 μB. This may render aconvenient approach of “oxygen doping/spintronics” at ambient conditions, where selective functionalizationsof 2D MOFs can be realized as their specific domains are exposed to atmosphere.

1. Introduction

Metal-organic frameworks (MOFs) have become a focus study inmaterials chemistry [1] and promise a wide range of potential appli-cations including gas sorption [2], separation materials [3], and che-mical sensors [4], owing to their exceptional porosity that is con-structed by joining metal-containing units with organic linkers usingstrong bonds forming open crystalline frameworks [5,6]. However,porous MOFs have received far less attention in a number of desirabletechnologies such as photonic, electronic devices, thermoelectrics, andresistive sensing because they usually exhibit with very low electricalconductivity [7,8].

A recent breakthrough is the synthesis of 2D MOFs with a honey-comb lattice akin to graphene [9–11] and attractive physical/chemicalproperties due to the planar pi-conjugation with full charge delocali-zation in the 2D plane [12]. They have been predicted to be potentialuseful in electronic devices, such as chemiresistive sensors [13],

supercapacitors [14] and organic topological insulators [15]. Thegeometric structures and the electronic properties of 2D MOFs can betuned using different combinations of various ligand molecules andmetal centers [16]. For example, 2D NiDT (nickel bis(dithiolene),Ni3C12S12) nanosheet that has been successfully synthesized by Kambeet al [17,18] is a semiconductor. Band structure calculation

shows that native undoped NiDT has a topological insulator (TI)state within a band gap of Dirac band opened up by spin–orbit coupling(SOC) at around 0.5 eV above the Fermi level [15,19]. Its electricalconductivity can be high up to 160 S cm−1 at 300 K in controllableoxidation states [20], analogous to 2D graphene/graphene oxide.Campbell et al. [21] demonstrated that conductive Cu3(HITP)2 na-nosheet display a chemiresistive response towards ammonia, whileNi3(HITP)2 do not display an observable response. Sarkar et al. [22]found that unabsorbed Cobalt bis(dithioline) and its saturated bis-COadsorbed molecule provide remarkably distinct Ι-V responses, whichbecomes a signal for detection of CO gas. Liu et al. [23] calculated the

https://doi.org/10.1016/j.cplett.2019.07.009Received 29 March 2019; Received in revised form 26 June 2019; Accepted 3 July 2019

⁎ Corresponding authors.E-mail addresses: ruanmin@hbpu.edu.cn (M. Ruan), qyanghust@hust.edu.cn (Q. Yang).

Chemical Physics Letters 731 (2019) 136581

Available online 04 July 20190009-2614/ © 2019 Elsevier B.V. All rights reserved.

T

adsorption of single gas molecule on two-dimensional MDT (M=Fe,Co, Ni, Pd, and Pt) films. The geometric structure and the electronicproperties of 2D MOFs can be tuned using different combinations ofvarious ligand molecules and metal centers. Experimental studies haveshown that Ni atoms in NiDT may be replaced by other transition metalatoms [24,25], and some of them were also predicted to exhibit TI statearound 0.5 eV away from Fermi level [26,27].

If we can obtain a 2D MOF with electronic/magnetic propertiessensitive to the adsorption of oxygen molecules, it can exhibit twodistinct behaviors when separated from/exposed to the air. Such tuningis much more convenient compared with conventional doping orcovalent functionalizations. Selective functionalizations of 2D MOFscan be easily realized for interfacial devices as their specific domainsare exposed to atmosphere. Here we propose that 2D MDT (M=Co,Rh) are such candidates. Our results reveal that their magnetic andelectronic properties can be tuned by the adsorption of oxygen mole-cules, rendering a convenient approach of “oxygen doping/spintronics”at ambient conditions.

2. Methods

Periodic, density-functional theoretical (DFT) calculations im-plemented in the Dmol3 program [28] was applied for the slab calcu-lation. To accurately account for the van der Waals (vdW) interactions,self-consistent field (SCF) energies of the systems were corrected fordispersion forces using the DFT-D3 method developed by Grimme et al.[29] The Kohn-Sham equation was solved in a self-consistent mannerunder the generalized gradient approximation (GGA) [30]. The func-tional of GGA functionals is Perdew-Burke-Eruzerhof PBE [31] with theall-electron double numerical (DND) basis sets [32]. The Monkhorst-Pack k-meshes are set to 7×7×1 in the Brillouin zone and the nearestdistance between two adjacent layers is set to 18 Å. During the struc-tural relaxation, all the atoms were relaxed. The convergence criteriaapplied for geometry optimization were enforced to 10−5 au for energy,0.002 au/Å for force, and 0.005 Å for maximum displacement.

CoDT and RhDT 2D lattice structures were built by replacing the Niatoms of NiDT lattice structure with Co and Rh, respectively and thenoptimized with all atoms relaxed. To avoid unphysical interlayer in-teractions, the slabs were separated by a vacuum region of 18 Å. In thiswork, we calculated the adsorption energies according to the followingequation,

Eads=E (slab)+E (adsorbate)− E (slab+ adsorbate)

in which E (slab+ adsorbate), E (slab), and E(adsorbate) were the cal-culated electronic energies of species adsorbed on the sheets, the free-standing sheets, and the gas-phase molecules, respectively [33].

3. Results and discussion

3.1 Optimized lattice structure of CoDT and RhDT

The optimized 2D NiDT lattice structure was found to beL=14.76 Å, which was in good agreement with the experimental value(14–15 Å) [17]. After the replacement of Ni atoms with Co and Rhatoms, the optimized lattice structure with the lowest energy for 2DCoDT and RhDT was L=14.76 Å and 15.16 Å, respectively. The opti-mized lattice structures of 2D NiDT, CoDT, RhDT were shown as Fig. 1.The dashed gray diamond denotes the unit cell in the calculations of theelectronic properties. The structural, energetic, electronic and magneticeffects of absorbed oxygen molecules on CoDT and RhDT 2D surfacewere investigated.

3.2 O2 molecules adsorption at CoDT unit cell sheet

The optimized top and side views structures of CoDT surface withdifferent number (n=1, 2, 3 and 6) of adsorption oxygen molecules

were shown in Fig. 2. The geometric parameters, band gaps and mag-netic moments were listed in Table 1. The distance between Co and theadsorbed O atom is dCo-O, ΔdO–O is the OeO bonds lengths of the ad-sorbed O2 and the free O2 molecule. It can be seen that the OeO bondsall have been stretched about 0.03–0.05 Å compared to the free O2

system of 1.225 Å. Δd is the displacement of the metal atoms from themean benzene plane due to the relaxation of the atoms. The angle ofCoeOeO is ∠CoeOeO, and Eads is the adsorption energy per oneoxygen molecule adsorbed on the surface, which is defined as Eads =(ECoDT+nEO2− ECoDT(O2))/n. The spin states of O2 molecules wereconsidered during the calculation. It can be seen from Fig. 2 that all ofthe O2 molecules were chemically adsorbed on CoDT surface with theCoeO bond length of about 2 Å. The CoeO distances of one or three O2

molecules adsorption on one side of the 2D CoDT surface is about 1.9 Å,which is shorter 0.2 Å than that of 2 or 6 O2 molecules adsorption onboth sides of the surface symmetrically. The displacement of the Coatoms from the mean benzene plane of CoDT(O2) is 0.339 Å, which isbigger than others. When there were two and six O2 molecules ad-sorption on the surface, Δd is negative with the same spin states oxygencompare to the O2 molecules with opposite spin states. The ∠CoeOeOis also smaller with opposite spin states adsorption oxygen than that ofthe same spin states of about 120°. The adsorption energy per one O2

molecule for CoDT(O2), CoDT(2O2), CoDT(3O2) and CoDT(6O2) sheetsystem is 15.3, 9.5, 11.7 and 8.1 kCal/mol, respectively. For the op-posite spin states of O2 molecules of CoDT(2O2) and CoDT(6O2), theadsorption energy is 9.4 and 3.1 kCal/mol respectively, which issmaller than the same spin states of 9.5 and 8.1 kCal/mol. It was clearthat oxygen molecules were likely to absorb with the same spin states. Itconcluded that the energetic predominant adsorption states of nO2

(n= 2, 6) molecules was that all oxygen molecules were the same spinstates, which would be focused on in the discussion.

The adsorption of O2 changed the magnetic configurations obviouswith the magnetic moments listed in Table 1. The free-standing CoDTfilm are ferromagnetic with the magnetic moment of 2.9 μB per unitcell, consistent with Sarkar’s report [23], which means that 1.0 μB perCo atom. The magnetic moment of the CoDT system is localized aroundthe Co atoms, confirming that the ferromagnetism mainly arises fromthe one unpaired electron in the Co d orbital due to the dsp2 hy-bridization of Co metal atoms. The magnetic configurations of Co atomsdisplay various changes in terms of the number of adsorbed O2 mole-cules. When one oxygen molecule adsorbed on the Co atom, the mag-netic moment of the Co decreased to be 0.2 μB with others almost 1.0 μB,and it was 0.3 μB when there were 2O2 molecules with up spin statesadsorbed on the same Co atom with other 2Co atoms 1.0 μB. If the twoO2 molecules with opposite spin states adsorbed on the same Co atom,the magnetic moment would decreased to be 0.0 μB. The magneticmoments of all 3Co atoms decreased to be 0.3 μB when 3O2 moleculesabsorbed on 3Co atoms respectively. The magnetic moments of Coatoms decreased to be 0.2, 0.3 and 0.2 μB respectively when 6O2 mo-lecules absorbed on with the same spin states and to be 0.0 with three3O2 molecules up spin states and other 3O2 molecules down spin stateson the other side of the CoDT sheet. The interesting changes in themagnetic moments configurations encourage us to investigate theelectronic properties of O2 adsorbed CoDT films.

The electronic band structures along the GKM direction of the of theCoDT unit cell sheet Brillouin Zone with different number of O2 mo-lecules adsorption are shown as in Fig. 3. The chemisorptions of O2 onthe CoDT surface lead to a transition from the semiconducting state tothe metallic state. The band gap of the free-standing CoDT was0.218 eV, which is the same as references [15,16]. And it decreases to0.109 eV when there was one O2 molecule adsorbed on the surface. Theband gap decreased to be 0.054 eV when there were two oxygen mo-lecules adsorption with up spin states. It became to be metallic whenthere were 3O2 and 6O2 adsorption. The orbitals of O2 molecule, CoDTsurface and CoDT(O2) are shown as Fig. 3(a). It was clear that the π*

orbital of O2 hybridized strongly with the dxz/dyz orbital of CoDT film,

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which induced a charge transfer from the O2 molecule to the na-nosheets, resluting in an upward shift of the Fermi level shown asFig. 3(b)–(f). It meant that the electronic characteristic of CoDT surfacecan be controlled by the number of chemically adsorbed O2 molecules.

3.3 O2 molecules adsorption at RhDT unit cell sheet

Geometric parameters, band gaps and magnetic moments of the O2-adsorbed nanosheets were listed in Table 2. There were two modes ofO2 molecule adsorption on RhDT unit cell sheet. One was that only oneO atom chemically adsorbed with Rh atom to form OeRh bond shownin Fig. 4(a), and the other was both of the two O atoms of O2 moleculeto form chemical bond with the same Rh atom shown in Fig. 4(b). Thesurface deformed dramatically of the second mode with the differenceof OeO bond with 0.099 Å compared with the free O2 molecule, and thedifferences of other modes were about 0.03 Å. The RheOeO angle was118.6° of the single RheO bond surface and it was 71.4° whose O2 wasalmost parrallel with the surface.

The binding energy of the first mode was 14.2 kCal/mol, and it wasonly 0.1 kCal/mol, which meant that the second mode was unstable. Itwas 10.8 kCal/mol per one O2 molecule when two O2 molecules ad-sorbed on RhDT surface with up states, and it was 8.6 kCal/mol when

the two O2 molecules with opposite spin states. The binding energy of3O2 adsorption on RhDT surface was 12.8 kCal/mol. The binding en-ergy was 10.6 kCal/mol when 6O2 with up spin states and was 8.6kCal/mol with opposite states. From the binding energy, it was clearthat oxygen molecules were more likely absorbed on the RhDT sheetwith the first adsorption mode with up spin states.

The magnetic moment of the free-standing RhDT sheet was only 0.0μB per Rh atom, which was much smaller than that of CoDT of 1.0 μB.The magnetic moment was unchanged when one O2 molecule adsorbedon. The magnetic moments of Rh atom increased to be 0.1 μB with twoO2 molecules adsorbed on with other two Rh atoms were 0.0 μB. Whenthere were 3O2 or 6O2 adsorbed on Rh atoms, the magnetic momentsincreased to be 0.1 μB for all of the Rh atoms. It was clear that themagnetic characteristic of the RhDT surface can also be controlled bythe number of chemically adsorbed O2 molecules.

The electronic band gap was 0.101 eV for the free-standing RhDTsurface, which was about half of the free-standing CoDT surface. Whenthere was one O2 molecule adsorbed on the sheet, the band gap de-creased dramatically to be 0.001 eV, and it was metallic when therewere more than two O2 molecules adsorbed on the surface. The orbitalof O2 molecule, RhDT surface and RhDT(O2) are shown as Fig. 5(a). Italso due to the orbital hybridization between O2 and the Rh d orbitals

Fig. 1. Optimized 2D lattice structure of (a) NiDT, (b) CoDT and (c) RhDT. Carbon, Sulfur, Nickel, Cobalt and Rhodium atoms are in grey, yellow, blue, purple andorange, respectively. The dashed gray diamond denotes the unit cell in the calculations. (For interpretation of the references to colour in this figure legend, the readeris referred to the web version of this article.)

Fig. 2. Top and side views of the optimized structures with different number of O2 molecules adsorption on CoDT unit cell surface. (a) 1O2, (b) 2O2, (c) 3O2, (d) 6O2.

Table 1Geometric parameters, band gaps and magnetic spin moments of the O2-adsorbed nanosheets. DCo–O is the distance between the metal atoms and the adsorbed Oatom, ΔdO–O is the difference between the OeO bonds length for the adsorbed and free O2 molecule, Δd is the displacement of the metal atoms from the meanbenzene plane, and the angle of CoeOeO, and the adsorption energy Eads are given. Eads is the adsorption energy per O2 molecule adsorbed on the surface, which isdefined as Eads = (ECoDT+ nEO2 − ECoDT(O2))/n (n= 1, 2, 3, 6).

Substrates dCo–O (Å) ΔdO–O (Å) Δd (Å) ∠CoeOeO (°) Eads (kCal/mol) Magnetic spin moment per Co atom (μB) Band gap (eV)

CoDT 1.0, 1.0, 1.0 0.218CoDT(O2) 1.909 0.038 0.339 119.0 15.3 0.2, 1.1, 1.0 0.109CoDT(2O2) 2.096 0.027 0.071 120.1 9.5 0.3, 1.0, 1.0 0.054CoDT(2O2)updown 1.963 0.054 −0.003 110.5 9.4 0.0, 1.1, 1.1 0.136CoDT(3O2) 1.919 0.037 0.098 118.9 11.7 0.3, 0.3, 0.3 MetallicCoDT(6O2) 2.075 0.029 0.032 119.7 8.1 0.2, 0.3, 0.2 MetallicCoDT(6O2) updown 2.056 0.033 −0.123 113.8 3.1 0.0, 0.0, 0.0 Metallic

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Fig. 3. (a) Orbitals of O2, CoDT and CoDT(O2). The band gap structures along the GKM direction of the of CoDT unit cell surface Brillouin Zone with different numberof O2 molecules with up spin states adsorbed on. (b) free-standing sheet; (c) 1O2; (d) 2O2; (e) 3O2; (f) 6O2. Fermi level is marked by a thin green line. The black andred line corresponds to the band gap structures of alfa (spin up) and beta (spin down) electrons, respectively. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Table 2Geometric parameters, band gaps and magnetic spin moments of the O2-absorbed nanosheets. DRh–O is the distance between the metal atoms and the adsorbed Oatom, dO–O is the OeO bond length of the adsorbed O2 molecule, Δd is the displacement of the metal atoms from the mean benzene plane, and the angle of RheOeO,and the adsorption energy Eads are given. Eads is the adsorption energy per O2 molecule adsorbed on the surface, which is defined as Eads = (ERhDT+ nEO2 −ERhDT(O2))/n (n= 1, 2, 3, 6).

Substrates dRh–O (Å) ΔdO–O (Å) Δd (Å) ∠RheOeO (°) Eads (kCal/mol) Magnetic spin moment (μB) Band gap (eV)

RhDT 0.0, 0.0, 0.0 0.101RhDT(O2) 2.068 0.032 0.266 118.6 14.2 0.0, 0.0, 0.0 0.001RhDT(=O2) 2.097 0.099 0.637 71.4 0.1 0.0, 0.0, 0.0 0.163RhDT(2O2) 2.180 0.028 0.050 119.1 10.8 0.1, 0.0, 0.0 MetallicRhDT(2O2) updown 2.174 0.034 0.034 119.0 8.6 0.0, 0.0, 0.0 MetallicRhDT(3O2) 2.096 0.032 0.094 118.8 12.8 0.1, 0.1, 0.1 MetallicRhDT(6O2) 2.190 0.036 0.035 119.1 10.6 0.1, 0.1, 0.1 MetallicRhDT(6O2) updown 2.167 0.034 0.008 119.1 8.6 0.0, 0.0, 0.0 Metallic

(a) (b) (c) (d) (e)

Top

Side

Fig. 4. Optimized structures of O2 molecules with different spin states adsorbed on RhDT unit cell surface. (a) 1O2, (b) two O atoms of O2 double bond with Rh atom,(c) 2O2 with up spin states, (d) 3O2, (e) 6O2 with up states.

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which can generates more electronic bands around Fermi level whichfacilitate electron transport leading to a dramatic conductivity en-hancement in the O2-adsorbed films. The band gap structures along theGKM direction of the of RhDT unit cell surface Brillouin Zone withdifferent O2 molecules adsorbed on were shown as in Fig. 5(b)–(f).

4. Conclusions

The influences of O2 molecules adsorption on the electric andmagnetic properties of CoDT and RhDT nanosheets were investigated.The results showed that O atom chemically bonded with Co/Rh atomwith the bond length of about 2 Å. The binding energy per O2 moleculewas the biggest of 15.2 and 14.2 kCal/mol with only one O2 moleculeadsorbed on CoDT and RhDT films respectively. The smallest bindingenergy per O2 molecule was for 6O2 molecules adsorption, and it was8.1 and 10.6 kCal/mol for CoDT and RhDT film respectively. It wasferromagnetic of the free-standing CoDT sheet with 1.0 μB per Co atom.The magnetic moment of the Co atom decreased to be 0.2/0.3 μB whenthe Co atom bonded with O atom of the O2 molecules adsorbed on. Thefree-standing RhDT is a non-magnetic system, but the adsorption of O2

molecules can induce spin polarizations for the RhDT(O2) system withmagnetic moments of 0.1 μB per Rh atom. The band gap for the free-standing CoDT and RhDT surface was 0.218 and 0.101 eV, respectively.When there was one O2 adsorbed on, the band gap decreased to be0.109 and 0.001 eV for CoDT and RhDT sheet respectively. The bandgap was narrowed to be 0.054 eV for CoDT and metallic for RhDT with2O2 adsorbed on. CoDT and RhDT were all metallic with 3O2 or 6O2

absorption. Our results reveal that their electronic and magneticproperties can be tuned by the adsorption of oxygen molecules, ren-dering a convenient approach of “oxygen doping/spintronics” at am-bient conditions.

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Acknowledgments

This work was financially supported by the National Natural ScienceFoundation of China (No. 51801058).

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