A Systematic Theoretical Study of Electronic Structures and Stability of

Moreover, it is found that the electronic properties near the Fermi level are mainly dominated by the transition-metal and carbon atoms, indicating th...
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A Systematic Theoretical Study of Electronic Structures and Stability of Transition-Metal Adsorbed Graphdiyne Clusters Xiaojun Li, and Deng-Hui Xing J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11572 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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A Systematic Theoretical Study of Electronic Structures and Stability of Transition-Metal Adsorbed Graphdiyne Clusters Xiaojun Li,a,b,* Denghui Xingb

aSchool

of Science, Xi’an University of Posts and Telecommunications, Xi’an 710121, Shaanxi, China

bDepartment

of Chemistry, Tsinghua University, Beijing 100084, China

*Corresponding Author E-mail: [email protected] (X. Li)

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Abstract: Graphdiyne (GDY), a new two dimensional carbon allotrope, has attracted much attention due to the unique structural features with sp- and sp2-hybridized carbon atoms. For the first time, we have systematically performed a theoretical investigation on the electronic structures and stabilities of the transition-metal adsorbed GDY (namely TM@GDY (TM = Sc-Zn)) clusters by means of density functional theory calculations. Accordingly, the TM@GDY (TM = Sc-Mn) clusters with partially-filled 3d metals have distortedly in-plane conjugated framework, whereas in the Zn@GDY there is a large distance between fully-filled Zn atom and GDY’s surface. The analysis of binding energy reveals that the TM@GDY (TM = Sc, Ti, V, Ni) clusters possess the larger structural stabilities than others, and transition-metal atoms can modulate the electronic structures. Moreover, the natural charge always transfers from metal to the GDY framework, and the transferred charges strongly depend on increasing atomic number, especially for the 4s orbital of metal. On basis of the energy decomposition, it is found that the net contributions from Mn to Ni are relatively small, leading to the slightly strong binding energies. These predicted results will provide a fundamental reference for screening the model of atomic catalysts and further explore the potential applications.

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1. Introduction All the time, carbon-based allotropes, e.g., fullerene, nanotube, graphite, etc, have been widely applied in many research areas and solved many practical problems as novel functional materials, e.g., energy,1-5 molecular switching,6-7 sensors,8-10 catalysis,11-14 electronics,15-17 and so on, while these carbon materials have played crucial role in the reformation of next-generation optoelectronic nanodevices.18-22 In fact, the tremendous interest recognized as candidate materials ascribes to their unique electronic structures and physicochemical properties,23-26 especially the two-dimensional (2D) materials with the highly conjugated architecture, such as graphene (sp2-hybridized), are of special interest owing to its unique structural properties, and can be considered as the promising candidates of novel optoelectronic devices.27-29 On the other hand, scientists have still devoted themselves to design and synthetize the novel 2D carbon allotrope materials. Up to 2010, the 2D graphdiyne (GDY) material, regarded as new carbon allotrope, was successfully synthesized by Li and co-workers,30 using a cross-coupling reaction with hexaethynyl benzene. Structurally, GDY is composed of self-assembly 18-member ring hexagon unit, which comes from the benzene ring and diacetylenic linkages (−C≡C−C≡C−), forming a largely delocalized π-conjugated architecture (Figure 1a). Compared with graphene, GDY possesses the fascinating physicochemical properties, including the large nonlinear optical susceptibility,31-32 high electronic conductivity and mobility,33-34 extreme thermal resistance,35 etc., that is because that GDY has the special structural features with sp- and sp2-hybridized carbon atoms, making it more attractive for future applications of novel nanostructured materials. By now, a great number of previous studies both experimentally and theoretically have explored the structures and electronic properties of GDY for potential applications in electrode materials,36-39 semiconductor devices,40 hydrogen storage,41-43 electrode materials, and catalysis.44-45 3

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Recent studies have also shown that GDY is promising anode materials than graphene due to the high capacity and mobility,46 and can be considered as the potential optical materials owing to the largest first hyperpolarizabilities.47-48 In 2017, Prof. Li and co-works49 designed several new methods for the chemical synthesis of the 2D GDY materials, and further explored their mechanical and electronic properties, and shown that the GDY materials can be applied in many fields, e.g., optoelectronics, optics and energy. Very recently, a new series of alkali metal-adsorbed graphdiyne structures with the low vertical ionization potentials was theoretically designed as fascinating candidates of novel nonlinear optical materials.32 However, the understanding of unique electronic properties will be a major challenges to extend the potential applications. Density functional theory (DFT) calculations is a appropriate tool to explore the structures and properties, e.g., the vibrational characterizations of the 2D graphdiyne sheets have been explored by means of DFT computations, and the strongest IR vibration in electronic properties is found to be enhanced in intensity by 2190 times from GDY2 to GDY12.50 Most intriguingly, the doping with transition atoms (TMs) into GDY has attracted great attention, and it can effectively tailor the electronic properties of GDY. On the other hand, the electronic and magnetic properties of single 3d transition-metal atoms adsorbed graphdiyne sheets were also predicted by Sun et al.,51 and the adsorption of transition-metal atoms not only modulates the electronic structures of GDY, but also produce the excellent magnetic properties. In 2017, the high catalytic activity on the Ag38/GDY system were reported for the understanding of the process of CO oxidation,52 which is mainly dominated by the intrinsic activity of Ag38 cluster and the vital role of GDY. Thus, it is important to explore the doping effects of transition metals on GDY, because the TM atoms can influence and change the electronic properties of GDY, and then making it more promosing as the tunable nanostructured materials and the model of atomic catalysts. However, to our knowledge, there is still limited systematic understanding for how the different TM atoms with partially/fully filled 3d orbitals (e.g., Sc – Zn) influence the 4

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electronic structures of GDY, while it is also necessary to probe the charge mechanism and orbital interactions of the 3d TMs doped on the GDY’s surface. In this work, we have systematically investigated the structures, chemical stabilities, and electronic properties of single 3d TM-adsorbed graphdiyne clusters (namely TM@GDY (TM = Sc-Zn)) using DFT calculations with the inclusion of dispersion correction, the TM-adsorbed GDY cluster model is shown in Figure 1b. Based on the obtained structures, the binding energy, chemical stability, natural charges, and orbital interactions between TM atoms and GDY are analyzed, and the dependence of adsorbed TMs on electronic properties of GDY is indicated.

Figure 1. Schematic structures of (a) single-layer graphdiyne (GDY), in which the shaded areas represent the 18-member ring (18MR) hexagon unit, and (b) the transition metal-adsorbed graphdiyne (TM@GDY) clusters.

2. Computational Details All the electronic structure calculations of the TM@GDY clusters, namely TMC30H12 (TM = Sc-Zn), were carried out by means of DFT with generalized gradient approximation (GGA) PBE exchange-correlation functional53-54 in combination with the 6-31+G(d) basis set for C and H atoms and the Stuttgart/Dresden (SDD) pseudopotential for TM atoms, as implemented in the Gaussian 09 suite of program.55 This method has been successfully applied to study the metal-adsorbed 2D carbon materials.56-57 For all the transition metal-adsorbed clusters studied here, the different electronic spin states (e.g., singlet, triplet..., or doublet, quartet, etc.) were examined for initial 5

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structure in order to estimate the relative stabilities of the low-lying isomers. To accurately predict structural and electronic properties of these complexes, we utilized the dispersion-corrected DFT (DFT-D3) proposed by Grimme,58-60 which can overcome the conventional density functionals in properly describing the van der Waals forces. Vibrational frequency analyses were performed to ensure that these cluster complexes obtained from DFT are real local minima without imaginary frequency, and zero-point vibrational corrections were taken into consideration in relative energies. The energy decomposition was utilized to detect the detail components between the interactions of TM and GDY by using the ADF package.61 The crystal orbital Hamilton population (COHP) was utilized to analysis the covalent interactions between TM and the GDY carbon framework by software LOBSTER.62-65 The molecular structures were visualized using the VMD program.66 The differences of natural charges and electronic configurations on metal atoms of the TM@GDY (TM = Sc-Zn) clusters were evaluated based on natural population analysis method proposed by Weinhold and co-workers, which was performed using the NBO 6.0 program.67 3. Results and Discussion 3.1 Optimized Geometries The optimized stable structures of the TM@GDY (TM = Sc-Zn) clusters are presented in Figure 2. The calculated structural properties, relative energies, and natural charges on metal atoms of these clusters obtained using the PBE functional are listed in Table 1. Structurally, it is found that the pure GDY cluster has the D3h symmetry with a 1A1 electronic state. At PBE/6-31+G(d) level of theory, the average C-C bond lengths on benzene ring for the GDY cluster are predicted to be 1.413 Å, and the C-C and C≡C bond lengths on the diacetylenic linkages are calculated to be 1.390 and 1.238 Å, respectively, which are in good agreement with the earlier reports,68 being 1.43, 1.39 and

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1.24 Å, repsectively. These results further demonstrate that the GDY cluster model constructed by us is appropriate. Y

Y X

X

Z

Z

Y

Y X

X

Z

Z

Figure 2. Optimized stable geometries of transition metal-adsorbed graphdiyne unit TM@GDY (TM = Sc-Zn) clusters: (a) for TM = Sc-Mn, (b) for TM = Fe-Ni, (c) for TM = Cu, and (d) for TM = Zn. The gray, white, and colour balls represent the C, H, and TM atoms, respectively.

To locate the stable structures of the TM-adsorbed GDY clusters, we tried to consider several possible adsorption sites of TMs on the GDY surface as initial structures, e.g., the hollow in 18MR, metal bridge, and above sites of surface. According to our calculations, we see that the most stable structures of the TM@GDY (TM = Sc-Mn) clusters are slightly distorted when the TM atoms are adsorbed on the GDY surface (Figure 2a), of which the Sc@GDY structure with low spin electronic state (doublet) is energetically preferred, and is more stable in energy than its identical one with high spin state (quartet) by about 1.06 eV. Similar cases are for Ti@GDY and V@GDY. Nevertheless, it is observed that the Cr@GDY and Mn@GDY clusters adopt the high spin electronic states as their ground-state structures (Table 1), e.g., triplet and quartet, respectively. Moreover, the average TM-C bond lengths (RTM-C) between metals and the C atoms in different spin states are also predicted for TM@GDY. As can be seen from Table 1, the RTM-C bond lengths of TM@GDY clusters 7

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dramatically increase along with an increase of spin states, e.g., 2.137-2.214 Å for Ti-C bonds, and the calculated RTM-C bond lengths for Cr@GDY are 2.074 Å, which are slightly smaller than those (2.287 Å) reported previously.51 Noteworthily, as for the Fe, Co, and Ni dopants, the TM-adsorbed GDY structures are rather regular with C2v symmetry (Figure 2b), in which the Fe@GDY structure in its triplet spin state is obviously lower in energy than its singlet and quintet spin states by about 0.73 and 0.84 eV, respectively. The Co@GDY cluster adopts an open-shell doublet electronic ground state, whereas the closed-shell singlet electronic state is found for Ni@GDY. Interestingly, the calculated Ni-C bond lengths are predicted to be 1.943 Å in Ni@GDY, which are larger than the previous report (1.885 Å),51 and the predicted bond lengths are close to the expected covalent bond lengths (~ 1.95 Å), indicating the strong chemisorption between Ni and GDY. As for the Cu@GDY cluster, it displays the C2 symmetry with 2A state (Figure 2c), which is more stable than its quartet electronic state by about 1.73 eV. More remarkably, for the Zn@GDY cluster, the adsorbed Zn atom is located above the surface of GDY, forming the six equivalent Zn-C ‘bonds’ (Figure 2d). Meanwhile, the average Zn-C ‘bond’ lengths are predicted to be much larger (3.661 Å) due to the weak interaction between the Zn atom and GDY. However, for its high spin configurations, the Zn atom deviates from central site of GDY to the acetylenic linkages, resulting in the shorter Zn-C bonds, e.g, 2.204 and 2.531 Å. Table 1. Relative energies (∆E, in eV) and average TM-C bond lengths (RTM-C, in Å) in different spin states, natural charges (Charges) and electronic configurations (NECs) on metal atoms of TM@GDY (TM=Sc-Zn) at the PBE/6-31+G(d)(C,H)/SDD(TM) level of theory. Natural Population on Metal Atom Cluster

Multiplicity

∆E

RTM-C

Charge s

Sc@GDY

doublet

0.00

2.317

1.37

[core]4s0.123d1.474p0.014d0.01

quartet

1.06

2.320

singlet

0.00

2.137

1.03

[core]4s0.173d2.774p0.014d0.02

triplet

0.19

2.207

Ti@GDY

NECs

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V@GDY Cr@GDY

Mn@GDY

Fe@GDY

Co@GDY Ni@GDY

Cu@GDY Zn@GDY

quintet

1.21

2.214

doublet

0.00

2.080

quartet

0.14

2.156

singlet

0.90

2.023

triplet

0.00

2.074

quintet

0.04

2.186

doublet

0.86

2.083

quartet

0.00

2.102

sextet

0.77

2.132

singlet

0.73

1.856

triplet

0.00

1.940

quintet

0.84

2.096

doublet

0.00

1.917

quartet

1.15

1.976

singlet

0.00

1.943

triplet

1.29

1.965

quintet

2.94

1.968

doublet

0.00

2.045

quartet

1.73

2.047

singlet

0.00

3.661

triplet

1.68

2.204

quintet

3.17

2.531

0.74

[core]4s0.263d3.954p0.015s0.014d0.02

0.70

[core]4s0.253d4.994p0.015s0.014d0.03

0.90

[core]4s0.263d5.784p0.024d0.03

0.73

[core]4s0.283d6.954p0.014d0.01

0.62

[core]4s0.313d8.044p0.015s0.014d0.01

0.60

[core]4s0.293d9.084p0.014d0.01

0.89

[core]4s0.363d9.714p0.02

0.00

[core]4s2.003d10.00

3.2 Relative Stability In order to explore the relative stability of the TM-adsorbed GDY clusters, we computed the binding energy (Eb), which can be defined using the reaction TM@GDY  GDY + TM, as following: Eb = E(GDY) + E(TM)  E(TM@GDY)

(1)

where E(GDY), E(TM) and E(TM@GDY) represent the total energies including the zero-point vibrational correction for graphdiyne, isolated TM atom, and the TM-adsorbed graphdiyne clusters, respectively. According to the definition, the complexes with more positive Eb values are expected to possess the strong binding between TM and graphdiyne cluster, indicating more stable TM-adsorbed graphdiyne complexes.

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Figure 3. The binding energy (Eb, in eV) of the TM@GDY (TM = Sc-Zn) clusters obtained using the PBE functional.

The calculated binding energies of the TM-adsorbed graphdiyne clusters are plotted in Figure 3. One can see that the large binding energies are found for TM@GDY (TM = Sc, Ti, Cr), being 3.81, 4.20, and 3.55 eV, respectively, indicating these TM-adsorbed complexes are energetically favorable. Of which the largest Eb value of 4.20 eV is predicted for the adsorption of Ti atom on the surface. Moreover, it is recently found that the Sc and Ti atoms provide the most stable binding on the GDY surface reported by Lin,44 with 6.07 and 6.43 eV at the PBE level of theory, respectively. For the adsorbed Cr-Ni atoms, their binding energies with graphdiyne cluster increase smoothly along with increasing order of atomic number, from 1.89 to 2.45 eV, which are closely related with the partially filled 3d orbitals of transiton-metal atoms. Previous studies predicted that the binding energies of Feand Ni-adsorbed GDY sheets are 1.91 and 2.73 eV, respectively,51 which are consistent with our calculations. However, the binding energies decrease quickly for the adsorbed Cu and Zn atoms. Especially, the Zn@GDY cluster is predicted to have the low binding energy (-0.40 eV), which ascribes to the fully filled orbitals of Zn atom and hardly transfers electron to GDY, implying that the Zn-adsorbed GDY cluster is thermodynamically instable because of the negative Eb value. To further examine the electron density redistribution (∆ρ) of TM-adsorbed graphdiyne clusters, 10

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we performed a charge density differences (CDD) to describe the interaction between the TM atoms and its neighboring carbon atoms, defined as: ∆ρ = ρ(TM@GDY) - ρ(GDY) - ρ(TM)

(2)

and the CDD plots are shown in Figure 4. We can see that as for the Sc@GDY and Ti@GDY clusters, the charge is mainly accumulated on the Sc-C and Ti-C bonds, indicating their strong interactions. By contrast, the charge density significantly reduces between the Cr atom and neighboring carbon atoms, and there are some accumulations of charge density on the two Ni-C bonds. This behavior demonstrates stronger binding of the Ni-C bonds than the Cr-C bonds. All of these results are consistent with the relative stability of the TM@GDY clusters, reflected by the binding energies above.

Figure 4. The charge density difference (CDD) of the TM@GDY (TM=Sc, Ti, Cr, Ni) clusters: (a) Sc@GDY, (b) Ti@GDY, (c) Cr@GDY, and (d) Ni@GDY. Yellow and blue indicate the depletion and accumulation of charge density, respectively. The isosurface value of charge redistribution is 0.01 e/bohr3.

3.3 Electronic Structures To systematically investigate the electronic features of the TM-adsorbed graphdiyne clusters, we performed the total (DOS) and partial (PDOS) density of states for the TM@GDY (TM=Sc, Ti, Cr, Ni) clusters, as shown in Figure 5, which involve the fragment contributions (metal atoms and C) 11

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of these complexes to the total density of states. One can see that the metal atoms can largely contribute to the occupied and unoccupied molecular orbitals of TDOS, and influence the electronic structures of the GDY cluster. Moreover, it is found that the electronic properties near the Fermi level are mainly dominated by the transition-metal and carbon atoms, indicating that the GDY and TM atoms have the strong interactions, i.e. covalent bonds, which should be very responsible for the structural stabilization of these adsorbed complexes. On the other hand, it is revealed from calculations that the free GDY cluster has the wide HOMO-LUMO energy gap (3.113 eV), but the TM@GDY complexes possess the reduced energy gap when the TM atoms are adsorbed on the GDY surface. Interestingly, the small HOMO-LUMO gaps are closely related with the electron redistributions of HOMO and LUMO orbitals, and corresponds to the low-kinetic stability, and then leads to the formation of the activated complexes for any potential reaction,47 which will be helpful to build the active centers for the potential application as single atomic catalysts.

Figure 5. Total (TDOS) and partial (PDOS) densities of states for the TM@GDY (TM=Sc, Ti, Cr, Ni) clusters: (a) Sc@GDY, (b) Ti@GDY, (c) Cr@GDY, and (d) Ni@GDY. The Fermi level is shifted to zero.

3.4 Charge Transfer and Populations 12

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To explore the charge distribution and valence electronic configurations on metal atoms, we performed a natural population analysis for the TM-adsorbed graphdiyne clusters, using NBO 6.0 program.67 The calculated natural charges on metal atoms are listed in Table 1, and depicted in Figure 6a. Obviously, the natural charges on metals strongly depend on the atom types, and the Sc and Ti atoms in TM@GDY clusters possess the large charges, being 1.37 and 1.03 electrons, respectively, implying that the two metal atoms can transfer more electrons to GDY to form strong intramolecular interactions between metal and GDY. It is interesting that the large charge transfer can modulate the electronic and magnetic properties of carbon-based materials.51 Based on the analysis of electronic configurations, on the other hand, the transferred charges on the Sc and Ti atoms come mainly from the contributions of 4s orbitals (1.88 and 1.83 electrons) while their 3d orbitals accept 0.47 and 0.77 electrons from GDY, respectively, when compared with isolated Sc and Ti atoms (3d14s2, 3d24s2, respectively), as shown in Figure 6b. Noteworthily, the Cr and Cu atoms present the same valence electrons on partially filled 4s orbitals, so they transfer few electrons (0.75 and 0.64) from this orbital to GDY, and the occupations of their 3d orbitals are slightly reduced by 0.01 and 0.29 electrons, respectively. As for the Zn@GDY cluster, there are fully filled 3d and 4s orbitals on the Zn atom, which are same to its isolated Zn atom (3d104s2), indicating the Zn atom doesn’t donate and attract any electron, and these orbitals are not involved in chemical bonding with the big Zn-C distances (3.661 Å) as discussed above. On the other hand, we can easily deduce from Figure 6a and 3, the transferred charges on metal atoms and the binding energies have the similar change tendency, and both of them can consistently reflect the relative stability of the TM-adsorbed graphdiyne clusters.

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Figure 6. (a) Natural charges and (b) Charge transfer of 3d and 4s orbitals on metal atoms of the TM@GDY (TM = Sc-Zn) clusters.

3.5 Orbital Interaction In order to study the orbital interactions between single metal atom and GDY surface, the energy decomposition and negative crystal orbital Hamiltonian population (COHP) for these clusters are performed, as shown Table 2 and Figure 7. As can be seen from Table 2, the net contributions, EPauli + Eelectrostatic, are actually the effect that reduces the contributions of orbital interactions in real binding energies. The energy decomposition results show that the net contributions from Mn to Ni are relatively small compared to the orbital interactions, resulting in the strong binding energies. The integrated pCOHP results show that from Cr to Mn the bonding effect between metal and carbon framework gets stronger (Table 3), indicating a larger binding energy of Mn compared to Cr. From Mn to Co, the integrated pCOHP gets larger which is consistent to the tendency of Eb of metal atoms. Compared to other metals, it is a large anti-bonding region in pCOHP of Cu1/GDY which explains 14

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the relative weak binding interaction between Cu atom and GDY. Table 2. Energy Decomposition of the M1/GDY (M=Cr, Mn, Fe, Co, Ni, Cu) cluster. System

Pauli Repulsion/eV

Electrostatic Interactions/eV

Net Contributions/eV

Orbital Interactions/eV

Sc Ti V Cr Mn Fe Co Ni Cu Zn

13.67 21.28 23.26 20.93 16.06 16.47 15.89 12.45 16.93 0.28

-6.90 -13.16 -14.25 -12.49 -11.03 -11.42 -11.90 -9.89 -9.29 -0.20

6.78 8.12 9.01 8.44 5.03 5.05 3.98 2.56 7.64 0.09

-17.93 -20.14 -21.34 -21.33 -18.04 -14.22 -12.16 -8.97 -9.08 -0.08

Figure 7. Projected COHP (pCOHP) of M1/GDY (M=Cr, Mn, Fe, Co, Ni, Cu). Negative pCOHP (-pCOHP) indicating the bonding effect below the Fermi level. All Fermi levels are set to zero in each figure. Table 3. The integrated pCOHP between M1 and GDY. M1/GDY

Integrated (-pCOHP)

Cr Mn Fe Co Ni Cu

0.9073 0.9682 1.1243 1.2783 1.3228 0.4725

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4. Conclusions In summary, we have systematically studied the structures, stabilities, and electronic properties of the TM@GDY (TM = Sc-Zn) clusters using the density functional theory calculations. It is found that the most stable structures of TM@GDY (TM = Sc-Mn) with partially-filled 3d metal are slightly distorted in-plane, whereas other metal-adsorbed GDY clusters are symmetrical (e.g., C2v, Cs), with the exception of Zn@GDY, in which the fully-filled Zn atom is located above the GDY surface with the large bond distances, resulting in the weak interaction between Zn atom and GDY. The analysis of binding energy shows that the TM@GDY (TM = Sc, Ti, V, Ni) clusters possess the higher structural stabilities than others, and these relative stabilities can be reflected by the charge density differences, whereas the Zn@GDY cluster is thermodynamically instable because of the negative Eb value. Meanwhile, we find that the charge always transfers from metal to the GDY framework, and the transferred charges strongly depend on increasing atomic number, especially for the 4s orbital of metal, resulting in tunable electronic properties of GDY. Additionally, using energy decomposition, it is seen that the net contributions from Mn to Ni are relatively small, leading to the strong binding energies, while the integrated pCOHP results show that from Cr to Mn the bonding effect between metal and carbon framework gets stronger, indicating a larger binding energy of Mn compared to Cr. Herein, it is hopeful that the fundamental research will provide the important reference for exploring the potential applications in efficiently single atomic catalysts.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21603173), and the Natural Science Foundation of Shaanxi Province (No. 2016JQ5110). The authors gratefully 16

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