Class 2D metal – organic frameworks

(1)

Contents lists available atScienceDirect

Chemical Physics Letters

journal homepage:www.elsevier.com/locate/cplett

Research paper

Theoretical study of oxygen molecules adsorption on M

₃

C

₁₂

S

₁₂

(M = Co, Rh) — Class 2D metal – organic frameworks

Min Ruan

^a,⁎

, Qing Yang

^b,⁎

, Menghao Wu

^b

, Baoshan Wang

^c

, Junming Liu

^d

aInstitute of Materials Science and Engineering, Hubei Key Laboratory of Mine Environmental Pollution Control & Remediation, Hubei Polytechnic University, Huangshi, China

bSchool of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

cCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan, China

dLaboratory of Solid State Microstructures and Department of Physics, Nanjing University, China

H I G H L I G H T S

•

^O²molecules are chemically bonded with Co and Rh atoms of the 2D Co₃C₁₂S₁₂and Rh₃C₁₂S₁₂surface.

•

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 0.2μBand increase from 0.0 to 0.1μBrespectively.

•

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 MDT

First-principles calculations Chemical adsorption Selective functionalizations

A B S T R A C T

The inﬂuence of O2molecules adsorption on 2D CoDT and RhDT surface were investigated via ab initio calculations. It turns out that upon the chemically adsorption of O₂molecules, semiconductive CoDT and RhDT with a band gap of respectively 0.218 and 0.101 eV both become metallic, while their magnetic moment on each metal atom will respectively decrease from 1.0 to 0.2μBand increase from 0.0 to 0.1μB. This may render a convenient approach of“oxygen doping/spintronics”at ambient conditions, where selective functionalizations of 2D MOFs can be realized as their speciﬁc domains are exposed to atmosphere.

1. Introduction

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

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

supercapacitors [14] and organic topological insulators [15]. The geometric structures and the electronic properties of 2D MOFs can be tuned using diﬀerent combinations of various ligand molecules and metal centers [16]. For example, 2D NiDT (nickel bis(dithiolene), Ni3C12S12) nanosheet that has been successfully synthesized by Kambe et 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 electrical conductivity can be high up to 160 S cm⁻¹at 300 K in controllable oxidation states [20], analogous to 2D graphene/graphene oxide.

Campbell et al. [21] demonstrated that conductive Cu3(HITP)2 nanosheet display a chemiresistive response towards ammonia, while Ni3(HITP)2do not display an observable response. Sarkar et al.[22]

found that unabsorbed Cobalt bis(dithioline) and its saturated bis-CO adsorbed molecule provide remarkably distinctΙ-V responses, which becomes a signal for detection of CO gas. Liu et al.[23]calculated the

https://doi.org/10.1016/j.cplett.2019.07.009

Received 29 March 2019; Received in revised form 26 June 2019; Accepted 3 July 2019

⁎Corresponding authors.

E-mail addresses:[email protected](M. Ruan),[email protected](Q. Yang).

Available online 04 July 2019

T

(2)

is much more convenient compared with conventional doping or covalent functionalizations. Selective functionalizations of 2D MOFs can be easily realized for interfacial devices as their speciﬁc domains are exposed to atmosphere. Here we propose that 2D MDT (M = Co, Rh) are such candidates. Our results reveal that their magnetic and electronic properties can be tuned by the adsorption of oxygen molecules, rendering a convenient approach of“oxygen doping/spintronics”

at ambient conditions.

2. Methods

Periodic, density-functional theoretical (DFT) calculations im- plemented in the Dmol³program[28]was applied for the slab calculation. To accurately account for the van der Waals (vdW) interactions, self-consistentﬁeld (SCF) energies of the systems were corrected for dispersion forces using the DFT-D3 method developed by Grimme et al.

[29]The Kohn-Sham equation was solved in a self-consistent manner under the generalized gradient approximation (GGA) [30]. The functional of GGA functionals is Perdew-Burke-Eruzerhof PBE[31]with the all-electron double numerical (DND) basis sets[32]. The Monkhorst- Pack k-meshes are set to 7 × 7 × 1 in the Brillouin zone and the nearest distance between two adjacent layers is set to 18 Å. During the structural relaxation, all the atoms were relaxed. The convergence criteria applied for geometry optimization were enforced to 10⁻⁵au 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 Ni atoms of NiDT lattice structure with Co and Rh, respectively and then optimized with all atoms relaxed. To avoid unphysical interlayer interactions, the slabs were separated by a vacuum region of 18 Å. In this work, we calculated the adsorption energies according to the following equation,

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

in whichE (slab + adsorbate),E (slab), andE(adsorbate)were the calculated 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 be L = 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 Rh atoms, the optimized lattice structure with the lowest energy for 2D CoDT and RhDT was L = 14.76 Å and 15.16 Å, respectively. The optimized lattice structures of 2D NiDT, CoDT, RhDT were shown asFig. 1.

The dashed gray diamond denotes the unit cell in the calculations of the electronic properties. The structural, energetic, electronic and magnetic eﬀects of absorbed oxygen molecules on CoDT and RhDT 2D surface were investigated.

3.2 O2molecules adsorption at CoDT unit cell sheet

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

considered during the calculation. It can be seen fromFig. 2that all of the O2molecules were chemically adsorbed on CoDT surface with the CoeO 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 O2molecules adsorption on both sides of the surface symmetrically. The displacement of the Co atoms from the mean benzene plane of CoDT(O2) is 0.339 Å, which is bigger than others. When there were two and six O2 molecules adsorption on the surface,Δd is negative with the same spin states oxygen compare to the O2molecules with opposite spin states. The∠CoeOeO is also smaller with opposite spin states adsorption oxygen than that of the same spin states of about 120°. The adsorption energy per one O2

molecule for CoDT(O2), CoDT(2O2), CoDT(3O2) and CoDT(6O2) sheet system is 15.3, 9.5, 11.7 and 8.1 kCal/mol, respectively. For the opposite spin states of O2molecules of CoDT(2O2) and CoDT(6O2), the adsorption energy is 9.4 and 3.1 kCal/mol respectively, which is smaller than the same spin states of 9.5 and 8.1 kCal/mol. It was clear that oxygen molecules were likely to absorb with the same spin states. It concluded that the energetic predominant adsorption states of nO2

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

The adsorption of O2changed the magnetic configurations obvious with the magnetic moments listed inTable 1. The free-standing CoDT film are ferromagnetic with the magnetic moment of 2.9 μBper unit cell, consistent with Sarkar’s report[23], which means that 1.0μBper Co atom. The magnetic moment of the CoDT system is localized around the Co atoms, confirming that the ferromagnetism mainly arises from the one unpaired electron in the Co d orbital due to the dsp² hybridization of Co metal atoms. The magnetic configurations of Co atoms display various changes in terms of the number of adsorbed O2mole- cules. When one oxygen molecule adsorbed on the Co atom, the magnetic moment of the Co decreased to be 0.2μBwith others almost 1.0μB, and it was 0.3μBwhen there were 2O2molecules with up spin states adsorbed on the same Co atom with other 2Co atoms 1.0μB. If the two O2molecules with opposite spin states adsorbed on the same Co atom, the magnetic moment would decreased to be 0.0 μB. The magnetic moments of all 3Co atoms decreased to be 0.3μBwhen 3O2molecules absorbed on 3Co atoms respectively. The magnetic moments of Co atoms decreased to be 0.2, 0.3 and 0.2μBrespectively when 6O2mo- lecules absorbed on with the same spin states and to be 0.0 with three 3O2molecules up spin states and other 3O2molecules down spin states on the other side of the CoDT sheet. The interesting changes in the magnetic moments configurations encourage us to investigate the electronic properties of O2adsorbed CoDTfilms.

The electronic band structures along theGKMdirection of the of the CoDT unit cell sheet Brillouin Zone with diﬀerent number of O2mo- lecules adsorption are shown as inFig. 3. The chemisorptions of O2on the CoDT surface lead to a transition from the semiconducting state to the metallic state. The band gap of the free-standing CoDT was 0.218 eV, which is the same as references[15,16]. And it decreases to 0.109 eV when there was one O2molecule adsorbed on the surface. The band gap decreased to be 0.054 eV when there were two oxygen molecules adsorption with up spin states. It became to be metallic when there were 3O2and 6O2adsorption. The orbitals of O2molecule, CoDT surface and CoDT(O2) are shown asFig. 3(a). It was clear that theπ^* orbital of O2hybridized strongly with thedxz/dyzorbital of CoDTﬁlm,

(3)

which induced a charge transfer from the O2 molecule to the nanosheets, resluting in an upward shift of the Fermi level shown as Fig. 3(b)–(f). It meant that the electronic characteristic of CoDT surface can be controlled by the number of chemically adsorbed O2molecules.

3.3 O2molecules adsorption at RhDT unit cell sheet

Geometric parameters, band gaps and magnetic moments of the O2- adsorbed nanosheets were listed inTable 2. There were two modes of O2molecule adsorption on RhDT unit cell sheet. One was that only one O atom chemically adsorbed with Rh atom to form OeRh bond shown inFig. 4(a), and the other was both of the two O atoms of O2molecule to form chemical bond with the same Rh atom shown inFig. 4(b). The surface deformed dramatically of the second mode with the diﬀerence of OeO bond with 0.099 Å compared with the free O2molecule, and the diﬀerences of other modes were about 0.03 Å. The RheOeO angle was 118.6° of the single RheO bond surface and it was 71.4° whose O2was almost parrallel with the surface.

The binding energy of theﬁrst mode was 14.2 kCal/mol, and it was only 0.1 kCal/mol, which meant that the second mode was unstable. It was 10.8 kCal/mol per one O2molecule when two O2molecules adsorbed on RhDT surface with up states, and it was 8.6 kCal/mol when

the two O2molecules with opposite spin states. The binding energy of 3O2adsorption on RhDT surface was 12.8 kCal/mol. The binding energy was 10.6 kCal/mol when 6O2with up spin states and was 8.6 kCal/mol with opposite states. From the binding energy, it was clear that oxygen molecules were more likely absorbed on the RhDT sheet with theﬁrst adsorption mode with up spin states.

The magnetic moment of the free-standing RhDT sheet was only 0.0 μBper Rh atom, which was much smaller than that of CoDT of 1.0μB. The magnetic moment was unchanged when one O2molecule adsorbed on. The magnetic moments of Rh atom increased to be 0.1μBwith two O2molecules adsorbed on with other two Rh atoms were 0.0μB. When there were 3O2or 6O2adsorbed on Rh atoms, the magnetic moments increased to be 0.1μBfor all of the Rh atoms. It was clear that the magnetic characteristic of the RhDT surface can also be controlled by the number of chemically adsorbed O2molecules.

The electronic band gap was 0.101 eV for the free-standing RhDT surface, which was about half of the free-standing CoDT surface. When there was one O2molecule adsorbed on the sheet, the band gap decreased dramatically to be 0.001 eV, and it was metallic when there were more than two O2molecules adsorbed on the surface. The orbital of O2molecule, RhDT surface and RhDT(O2) are shown asFig. 5(a). It also due to the orbital hybridization between O2and 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 and orange, respectively. The dashed gray diamond denotes the unit cell in the calculations. (For interpretation of the references to colour in thisﬁgure legend, the reader is referred to the web version of this article.)

Fig. 2.Top and side views of the optimized structures with diﬀerent number of O₂molecules adsorption on CoDT unit cell surface. (a) 1O₂, (b) 2O₂, (c) 3O₂, (d) 6O₂.

Table 1

Geometric parameters, band gaps and magnetic spin moments of the O2-adsorbed nanosheets. D_Co–Ois the distance between the metal atoms and the adsorbed O atom,Δd_O–Ois the diﬀerence between the OeO bonds length for the adsorbed and free O₂molecule,Δd is the displacement of the metal atoms from the mean benzene plane, and the angle of CoeOeO, and the adsorption energy Eadsare given. Eads is the adsorption energy per O2molecule adsorbed on the surface, which is deﬁned as Eads= (ECoDT+ nEO2−ECoDT(O2))/n (n = 1, 2, 3, 6).

Substrates d_Co–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 0.218

CoDT(O2) 1.909 0.038 0.339 119.0 15.3 0.2, 1.1, 1.0 0.109

CoDT(2O2) 2.096 0.027 0.071 120.1 9.5 0.3, 1.0, 1.0 0.054

CoDT(2O2)updown 1.963 0.054 −0.003 110.5 9.4 0.0, 1.1, 1.1 0.136

CoDT(3O2) 1.919 0.037 0.098 118.9 11.7 0.3, 0.3, 0.3 Metallic

CoDT(6O2) 2.075 0.029 0.032 119.7 8.1 0.2, 0.3, 0.2 Metallic

CoDT(6O2) updown 2.056 0.033 −0.123 113.8 3.1 0.0, 0.0, 0.0 Metallic

(4)

Fig. 3.(a) Orbitals of O2, CoDT and CoDT(O2). The band gap structures along theGKMdirection of the of CoDT unit cell surface Brillouin Zone with diﬀerent number of O2molecules 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 and red 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 this ﬁgure legend, the reader is referred to the web version of this article.)

Table 2

Geometric parameters, band gaps and magnetic spin moments of the O₂-absorbed nanosheets. D_Rh–Ois the distance between the metal atoms and the adsorbed O atom, dO–Ois the OeO bond length of the adsorbed O2molecule,Δd is the displacement of the metal atoms from the mean benzene plane, and the angle of RheOeO, and the adsorption energy Eadsare given. Eads is the adsorption energy per O2molecule adsorbed on the surface, which is deﬁned as Eads= (ERhDT+ nEO2− E_RhDT(O2))/n (n = 1, 2, 3, 6).

Substrates d_Rh–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 0.101

RhDT(O2) 2.068 0.032 0.266 118.6 14.2 0.0, 0.0, 0.0 0.001

RhDT(=O2) 2.097 0.099 0.637 71.4 0.1 0.0, 0.0, 0.0 0.163

RhDT(2O2) 2.180 0.028 0.050 119.1 10.8 0.1, 0.0, 0.0 Metallic

RhDT(2O2) updown 2.174 0.034 0.034 119.0 8.6 0.0, 0.0, 0.0 Metallic

RhDT(3O2) 2.096 0.032 0.094 118.8 12.8 0.1, 0.1, 0.1 Metallic

RhDT(6O2) 2.190 0.036 0.035 119.1 10.6 0.1, 0.1, 0.1 Metallic

RhDT(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 O2molecules with diﬀerent spin states adsorbed on RhDT unit cell surface. (a) 1O2, (b) two O atoms of O2double bond with Rh atom, (c) 2O2with up spin states, (d) 3O2, (e) 6O2with up states.

(5)

which can generates more electronic bands around Fermi level which facilitate electron transport leading to a dramatic conductivity en- hancement in the O2-adsorbedﬁlms. The band gap structures along the GKMdirection of the of RhDT unit cell surface Brillouin Zone with diﬀerent O2molecules adsorbed on were shown as inFig. 5(b)–(f).

4. Conclusions

The inﬂuences of O2 molecules adsorption on the electric and magnetic properties of CoDT and RhDT nanosheets were investigated.

The results showed that O atom chemically bonded with Co/Rh atom with the bond length of about 2 Å. The binding energy per O2molecule was the biggest of 15.2 and 14.2 kCal/mol with only one O2molecule adsorbed on CoDT and RhDTﬁlms respectively. The smallest binding energy per O2molecule was for 6O2molecules adsorption, and it was 8.1 and 10.6 kCal/mol for CoDT and RhDT ﬁlm respectively. It was ferromagnetic of the free-standing CoDT sheet with 1.0μBper Co atom.

The magnetic moment of the Co atom decreased to be 0.2/0.3μBwhen the Co atom bonded with O atom of the O2molecules adsorbed on. The free-standing RhDT is a non-magnetic system, but the adsorption of O2

molecules can induce spin polarizations for the RhDT(O2) system with magnetic moments of 0.1μBper 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 O2adsorbed on, the band gap decreased to be 0.109 and 0.001 eV for CoDT and RhDT sheet respectively. The band gap was narrowed to be 0.054 eV for CoDT and metallic for RhDT with 2O2adsorbed on. CoDT and RhDT were all metallic with 3O2or 6O2

absorption. Our results reveal that their electronic and magnetic properties can be tuned by the adsorption of oxygen molecules, rendering a convenient approach of“oxygen doping/spintronics” at ambient conditions.

Declaration of Competing Interest

The authors declared that there is no conﬂict of interest.

Acknowledgments

This work wasﬁnancially supported by the National Natural Science Foundation of China (No. 51801058).

References

[1] A. Schoedel, M. Li, D. Li, M. O'Keeﬀe, O.M. Yaghi, Chem. Rev. 116 (2016) 12466.

[2] J.A. Mason, M. Veenstra, J.R. Long, Chem. Sci. 5 (2014) 32.

[3] J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev. 112 (2011) 869.

[4] L.E. Kreno, K. Leong, O.K. Farha, M. Allendorf, R.P. Van Duyne, J.T. Hupp, Chem.

Rev 112 (2012) 1105.

[5] H. Furukawa, K.E. Cordova, M. O'Keeﬀe, O.M. Yaghi, Science 341 (2013) 1230444.

[6] O.M. Yaghi, M. O'Keeﬀe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Nature 423 (2003) 705.

[7] L. Sun, M.G. Campbell, M. Dinca, Angew. Chem. 55 (2016) 3566.

[8] V. Stavila, A.A. Talin, M.D. Allendorf, Chem. Soc. Rev. 43 (2014) 5994.

[9] K.S. Novoselov, V.I. Fal, L.C. FalKo, P.R. Gellert, M.G. Schwab, K. Kim, Nature 490 (2012) 192.

[10] X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H. Zhang, Small 7 (2011) 1876.

[11] S. Chen, J. Dai, X.C. Zeng, Phys. Chem. Chem. Phys. 17 (2015) 5954.

[12] J. Cui, Z. Xu, Chem. Commun. 50 (2014) 3986.

[13] M. Campbell, M. Dincă, Sensors 17 (2017) 1108.

[14] D. Sheberla, J.C. Bachman, J.S. Elias, C.-J. Sun, Y. Shao-Horn, M. Dincă, Nat. Mater.

16 (2016) 220.

[15] Z.F. Wang, N. Su, F. Liu, Nano Lett. 13 (2013) 2842.

[16] H. Liu, X. Li, C. Shi, D. Wang, L. Chen, Y. He, J. Zhao, PCCP 20 (2018) 16939.

[17] T. Kambe, R. Sakamoto, K. Hoshiko, K. Takada, M. Miyachi, J.H. Ryu, S. Sasaki, J. Kim, K. Nakazato, M. Takata, H. Nishihara, J. Am. Chem. Soc. 135 (2013) 2462.

[18] T. Kambe, R. Sakamoto, T. Kusamoto, T. Pal, N. Fukui, K. Hoshiko, T. Shimojima, Z. Wang, T. Hirahara, K. Ishizaka, S. Hasegawa, F. Liu, H. Nishihara, J. Am. Chem.

Soc. 136 (2014) 14357.

[19] M. Wu, Z. Wang, J. Liu, W. Li, H. Fu, L. Sun, X. Liu, M. Pan, H. Weng, M. Dincă, L. Fu, J. Li, 2D Materials 4 (2016) 015015.

[20] J.J. Adjizian, P. Briddon, B. Humbert, J.L. Duvail, P. Wagner, C. Adda, C. Ewels, Fig. 5.(a) Orbitals of O2, RhDT and RhDT(O2). The band gap structures along theGKMdirection of the of RhDT unit cell surface Brillouin Zone with diﬀerent 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 and red 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.)

(6)