Metal-Embedded Graphene as Potential Counter Electrode for Dye

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Metal-Embedded Graphene as Potential Counter Electrode for Dye-Sensitized Solar Cell Qun Liu, Ze-Sheng Li, and Shi-Lu Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03464 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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Industrial & Engineering Chemistry Research

Metal-Embedded Graphene as Potential Counter Electrode for Dye-Sensitized Solar Cell Qun Liu, Ze-Sheng Li and Shi-Lu Chen* Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China.

*Corresponding author: Tel.: +86 10 68918670; fax: +86 10 68913154; E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: It is very important to explore a cheap but efficient catalyst as counter electrode in dye-sensitized solar cell (DSSC). In the present work, density functional theory (DFT) calculations were performed to investigate the reduction of triiodide ion catalyzed by metal atom embedded in graphene. It is shown that the binding energy of a single Pt atom embedded into the divacancy of graphene (Pt@DV) is about -11.0 eV, larger than the location of Pt at single vacancy (Pt@SV). In the Pt@DV with a Pt loading of 25wt%, the adsorption energy of iodine atom and the dissociation energy of iodine anion are respectively -1.54 and 0.60 eV, indicating a potential high catalytic activity of Pt-graphene. Further investigations of diverse transition-metalembedded graphenes ranging from Sc to Zn imply that the Co-embedded graphene (in divacancy) may be a good candidate to be utilized as counter electrode in DSSC.


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1. INTRODUCTION Dye-sensitized solar cell (DSSC) has attracted particular attention due to its high energy conversion efficiency, low cost, and environmental-friendliness. Quite recently, the energy conversion efficiency of DSSC has reached up to 13%,1 which is still much lower than the theoretical limit of 31%.2 In the past two decades of researches, great efforts have been made to optimize various chemical components of DSSC to achieve higher efficiency, including the design of high-efficiency sensitizer,3-12 semiconductor,13,14 redox mediator,15 and counter electrode.16-19 One of important aspects is to explore the chemically stable material of counter electrode with a high electro-catalytic activity for the reduction of triiodide. In the investigated counter electrode (CE) materials, platinum is the superior one due to its high conductivity, electro-catalytic activity, and stability.20-24 However, Pt is an expensive element with a low abundance, which restricts its large-scale application in DSSC. In order to solve this issue, several Pt-free alternative materials have been proposed, such as CoS,25 carbon materials,26 conductive organic polymers,27 as well as composite materials.28 Among these, carbon materials, which often show high electrical conductivity, low cost, good electro-catalytic activity, and chemical stability, have been extensively studied. Since 1996, various carbonaceous materials, such as carbon black,29 carbon nanotubes,30,31 porous carbon,32,33 carbon sphere,34 and active carbon,35 have been investigated theoretically or experimentally as catalytic materials of counter electrode. Graphene, a one-layer sheet of hexagonal carbon atoms, has unique electronic properties and has been considered as a new generation of electronic material.36,37 Especially, graphene is expected to be an alternative material for the counter electrode of DSSC.38 Grätzel et al. used graphene nanoplatelets as the DSSC cathode and pointed out that the graphene cathode exhibited


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good performance in DSSCs.39 However, a perfect surface of graphene sheet has a poor catalytic activity for the reduction of I3 to I−.40-46 Interestingly, metal-atom- or metal-cluster-embedded graphenes, which still possess huge surface-to-volume ratio, were proposed to be more efficient heterogeneous catalysts than graphene.43,47-50 Kaukonen et al. studied the structural and electronic properties of metal-embedded graphenes which were used as cathodes in fuel cells by density functional theory (DFT) calculations.51 Recently, a graphene incorporated with 27% Pt nanoparticles was used as CE and achieved an efficiency of 2.91%.52 The use of the doped graphene as counter electrode in DSSC was also supported by another investigation,53 where different amounts of Pt nanoparticle loadings (10– 60%) on graphene sheet were studied and a solar-to-electricity conversion efficiency of 8.79% was achieved. The size of metal particles on the substrate surface is one of the most important factors that are related to the performance of a catalyst.54-57 Recently, theoretical and experimental results demonstrated that downsizing the particles or clusters to single atoms could enlarge the active surface area of the catalyst, thus increasing the catalytic activity.58-60 However, although the platinum single atoms deposited on the surface of graphene nanosheets have been successfully prepared using the atomic layer deposition technique,61 controlling the sizes of the catalysts remains challenges in experiments.62,63 With this situation, theoretical investigations can help us further understand the catalytic performance of the Pt atom embedded in graphene. Recently, based on DFT calculations, Sun et al. reported that the binding energy of Pt atom on nitrogen-doped carbon nanotubes was approximately nine times larger than that of Pt atom on the undoped surface.64 This is due to the strong interaction of the Pt d-orbitals with the p-orbitals of N, thereby preventing the sintering of Pt particles. Moreover, higher catalytic performance was obtained when the Pt atom embedded in nitrogen-doped carbon nanotubes was used.65


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Similarly, the gold atom interacts weakly with a non-defective graphene sheet, but is adsorbed exothermically on a defective graphene sheet.66 Okamoto et al. also reported that the stability of the metal clusters absorbed on the graphene with carbon vacancies was higher than that on the defect-free graphene.67 Therefore, it is supposed that the metal-atom-embedded graphene is stable enough to be utilized as catalyst and may be potential material for efficient counter electrode in DSSC. With this, it is worth to inspect if Pt-atom-embedded graphene (Pt-graphene) is also able to exhibit a high catalytic behavior as CE. In this work, we studied several different weight ratios of Pt-graphenes by means of the firstprinciple calculations and evaluated the adsorption energies along with the bonding situation of the Pt-graphene-iodine complexes. Furthermore, the adsorption energies of I atom on other transition metal-embedded graphenes (TM-graphene) ranging from Sc to Zn were compared. It was found that the complex of Co atom located at divacancy graphene is the most suitable candidate as CE of DSSC.

2. COMPUTATIONAL DETAILS All quantum chemical calculations were performed with Gaussian 09 package.68 The geometries were optimized using the density functional theory (DFT) B3LYP69 functional coupled with a 631G(d, p) basis set for C, H atoms, and a LANL2DZ basis set for metal atom in the acetonitrile (CH3CN) solution (the dielectric constant ε = 35.688). Bulk solvent effects of CH3CN were included by means of the conductor-like polarized continuum model (C-PCM).70 The binding energy (Eb) of metal atom embedded in graphene was calculated by: Eb = EMGN – (EM + EGN)

(1)

where EMGN represents the energy of the complex of metal-atom-embedded graphene, while EM and EGN are the energies of metal atom and separated graphene, respectively.


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I3 reduction reaction could be written as: I3 (sol) ↔ I2 (sol) + I- (sol)

(2)

I2 (sol) ↔ 2I*

(3)

I* + e- ↔ I- (sol)

(4)

where sol represents the acetonitrile solvent and * means the atom adsorbed on the electrode surface. The process of (2) was fast,71 so that the whole reduction reaction and overall electro catalytic activity are determined by the steps of (3) and (4). Hou et al.72 pointed out that the barriers for I2 dissociation and I* desorption are related to the adsorption energy of I, that is, the binding strength of I atom on the counter electrode plays a key role in determining the catalytic activity. Therefore, the adsorption energy of I atom on the metal-atom-embedded graphene was calculated in the present work. The adsorption energy (Eads) of iodine atom adsorbed on metalatom-embedded graphene was calculated by: Eads = EMGN+I - (EI + EMGN)

(5)

where EMGN+I is the total energy of the graphene system with iodine atom, while EI represent the energy of I atom. The dissociation energy (Edes) along the dissociation curve was calculated as follows: Edes = E(R) - EMGN+I

(6)

where E(R) is calculated by setting the I atom 10 Å far from the graphene system. There should be no interaction between the I and the graphene when their distance reaches 10 Å. Fukui function f(r)73,74 is simulated using the Multiwfn program.75 This function has been widely used in the prediction of reactive site, and can be calculated for three situations: Nucleophilic attack:

f  (r )   N 1 (r )   N (r )

(7)

Electrophilic attack:

f  (r )   N (r )   N 1 (r )

(8)


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f  (r )  f  (r )  N 1 (r )   N (r ) f (r )   2 2 0

Radical attickattack:

(9)

where f +(r) denote the ability of nucleophilic attack ability, while f -(r) and f 0(r) mean the abilities of electrophilic attack and radical attack, respectively. ρN(r) is the density function of neutral molecule, while ρN-1(r) and ρN+1(r) are cation and anion at the neutral geometry, respectively.

3. RESULTS AND DISCUSSION 3.1 Structural and Electronic Properties of Pt-graphenes. The weight ratio of Pt to the entire molecule is defined as N% = MPt/(MC+MH+MPt), where M means the mass. Several different loadings of Pt-graphene sheets were built, which can be divided into two series, i.e., the Pt is located at single vacancy (SV) and divacancy (DV), respectively (Figure 1). In these structural models, the dangling bonds are terminated by H atoms to render surface atoms. The Pt atom was initially placed in the middle of defective graphene. It is found that, in the four Pt@SV sheets (Figure 1a) with different Pt loadings, all Pt-C bond distances are close to 1.98 Å, and the dihedral angles of ∠C1-C2-C3-Pt composed of Pt atom and the graphene plane is about 56°. In the four Pt@DV sheets (Figure 1b), the Pt atom is located in the middle of the divacancy, and the Pt-C bond distances are about 2.0 Å, slightly longer than those in Pt@SV (~1.98 Å). It is further found that the dihedral angle of ∠C1-C2-C3Pt decreases with the weight ratio of Pt decreases, which means that the Pt atom gradually approaches the graphene plane. In all other cases, the binding energies of Pt@DV (Eb ~ -11.38 eV) are larger than those of Pt@SV with the same Pt loadings (Eb ~ -8.53 eV), which indicates much stronger interactions between Pt and the neighboring C atoms in Pt@DV. The binding energies of Pt@DV are also much larger than those of the complexes with the Pt atom absorbed


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on the perfect graphene (Table S1), which shows a similar trend to the previous studies.65,66

Figure 1. Optimized structures of Pt-graphene sheets with Pt located at single vacancy (Pt@SV, a) and divacancy (Pt@DV, b). The color coding: blue (platinum), white (hydrogen), and gray (carbon).

Table 1 Binding energy (Eb, eV), the distance between Pt atom and neighboring C atom (b, Å) and the dihedral angles of Pt atom and the graphene plane (dihedral, °) Pt@SV

Pt@DV

Complexes

Eb

b

dihedral

Complexes

Eb

b

dihedral

42wt%

-8.95

1.98

55.6

42wt%

-11.84

2.00

14.9

28wt%

-8.32

1.98

56.3

28wt%

-11.33

2.00

13.8

25wt%

-8.31

1.99

56.7

25wt%

-10.96

2.01

11.2

19wt%

-7.49

1.97

56.5

19wt%

-5.60

1.99

8.1


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Figure 2. (a) Isosurface of f -(r) (the most positive value is green) and (b) Electrostatic potential map of the Pt-graphene sheet (the most negative value is in red while the most positive value is in blue). The 25wt% Pt@DV was chosen as the example.

The isolated Pt atom has a partially filled d-shell and half-filled 6s orbital. When a carbon atom in graphene is replaced by a Pt atom, the charge is transferred from the Pt atom to the neighboring carbon atoms. The natural bond orbital (NBO)76 analysis shows that in the Pt@SV complex the charges at the Pt atom and each ligated C atom are +0.56e and -0.07e, respectively. In the Pt@DV complex, the corresponding charges are +0.35e and -0.10e, respectively. The population analysis indicates that the electron density on the Pt atom can transfer to the graphene sheet which acts as an electron withdrawing support. Fukui function plot of the Pt-graphene confirms that the Pt atom exhibits a strong electrophilicity (with 25wt% Pt@DV as the example, Figure 2a). It is clear that the most positive part of f -(r) function is mainly localized at the Pt center, indicating that the Pt atom is the favorite reactive site for electrophilic attack. The electrostatic potential map of the defective graphene surface (with 25wt% Pt@DV as the example, Figure 2b) also demonstrates that the Pt center most likely supplies electron for the reduction of I atom. All these results imply that the Pt atom might be the active site for the catalysis.


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3.2 Adsorption of I Atom on Pt-graphenes. The adsorption of I atom on the Pt atoms of Pt@SV and Pt@DV were calculated and the obtained adsorption energies (Eads) are given in Table 2. In principle, the negative value (exothermic reaction) is in favor of the adsorption of I atom, but the absolute value should not be too large as strong binding may hinder the following reactions. From table 2, it can be seen that the adsorption energies of I atom adsorbed on Pt@SV decrease with the increase of carbon quantity. This can be explained by the Frontier Orbital Picture (FOP) (Figure 3). In the Pt@SV complexes with a higher Pt weight, the HOMO orbital is more localized at the Pt atom, indicating a higher adsorption ability of Pt. When the mass ratio of Pt weight reaches 19wt%, the adsorption energy is still too large. The Pt@SV complexes thus seem to be improper as the counter electrode of DSSC. Table 2. Adsorption energies (Eads, eV) of I atom on Pt-graphene Complexes

Pt@SV

Pt@DV

42wt%

-2.55

-1.06

28wt%

-2.45

-1.10

25wt%

-2.35

-1.54

19wt%

-2.29

-

Different from the case of Pt@SV, the adsorption energies in Pt@DV increase with the increase of carbon quantity, except for the Pt@DV of 19wt%. We failed in optimizing the structure for iodine atom adsorbed on the 19wt% Pt@DV complex. This may be attributed to the localization of HOMO at the graphene instead of the Pt atom (see Figure 3). In addition, it was reported that the Pt-C bonds in the Pt@SV complex consists of three σ bonds and one π bond, while the Pt@DV complex shows four local σ bonds.43 As a result, it is easier for I atom to get electron from the Pt@SV complex than from the Pt@DV complex. This probably increases the adsorption probability of I atom on Pt@SV, thus resulting in larger adsorption energies.


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Figure 3. The HOMO orbital spatial distribution of the complexes of Pt@SV (a) and Pt@DV (b).

Figure 4. Hamiltonian kinetic energy density K(r) in the Pt-graphene-I complex with the 25wt% Pt@DV as the example. The red lines are positive values of K(r) and the blue lines are negative values of K(r). The values of ρ and K (r) at the bcp on the Pt-I bond paths are shown.

To further scrutinize the properties of the Pt-graphene-I complex, with the 25wt% Pt@DV as the example, the Hamiltonian kinetic energy density K(r)77 was plotted in Figure 4 together with the values of 𝜌 at the Pt-I bond critical point (bcp).78 The K(r) is defined as K(r) = G(r) −1/4∇2𝜌, where G(r) is the local kinetic energy density. In the plot of K(r), the covalent and non-covalent


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regions of the molecular space are directly related with the positive and negative values of their functions, respectively. From Figure 4, it can be seen that, at the Pt-I bond critical point, 𝜌 is small and the K(r) is positive. This suggests that the Pt-I bond is a weak covalent bond.

Figure 5. Electron density difference map between the Pt-graphene-I complex and the two isolated parts. The red indicates the increase of electron density while the blue indicates the decrease. The 25wt% Pt@DV was chosen as the example.

The charge transfer from the Pt-graphene surface to I atom was analyzed by the NBO calculation. With the 25wt% Pt@DV as the example, the I atom shows a negative charge of 0.27e, which means that electron have transferred from the Pt-graphene surface to the I atom. This shows that the Pt-graphene features as an electron donor, and the I atom is an acceptor upon the adsorption on the Pt-graphene surface. Furthermore, the electron density difference of this system (shown in Figure 5) was plotted by calculating the electron density difference between the adsorbed system and the two isolated parts (i.e., the I atom and Pt-graphene). It is shown that the charge depletes around the I and Pt atoms, and accumulates around the Pt-I bond.


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3.3 Dissociation of Iodide Ion from Pt-graphenes. The dissociation of iodine ion from the Pt@DV and Pt@SV were investigated by scanning the Pt-I distance of the Pt-graphene-I anions (Figure 6). We failed in optimizing the structure with I atom adsorbed on the 19wt% Pt@DV complex. Therefore, there is no line of Pt@DV/19% for the dissociation of I- from the 19wt% Pt@DV graphene sheet. It can be seen that the dissociation energy in the 25wt% Pt@SV system (1.67 eV) is much larger than those in the three Pt@DV systems (about 0.60 eV). The dissociation energy in the 25wt% Pt@DV is the smallest, implying a good catalytic activity for the reduction of I−/I3−. This is the reason why we focused on the complex properties of I adsorbed on the 25wt% Pt@DV in the earlier sections. 1.8

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt@DV/42% Pt@DV/28% Pt@DV/25% Pt@SV/25%

1.2

0.6

0.0 3

4

5

6

7

8

9

bond length (Å)

Figure 6. Potential energy profiles for the dissociation of I- from diverse Pt-graphenes. 3.4 Candidate Materials. In order to explore efficient and cheaper catalyst as the counter electrode in DSSC, we studied diverse transition metal-embedded graphenes (TM-graphenes) ranging from Sc to Zn. The TM-graphenes at all possible spin multiplicity state have been optimized (Table S2). The ground-state multiplicity states were used to calculate the properties for the metal-embedded graphenes. Using the model of 25wt% Pt-graphene, we examined the Eb


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of TM-graphenes and Eads of I atom on TM-graphene. Similar to the case of Pt-graphene, both single vacancy (TM@SV) and divacancy (TM@DV) were considered (the Cartesian coordinates are given in the Supporting Information). The Eb of transition metals embedded into graphene surface were calculated (Figure 7). TM@SV TM@DV

-5

-10

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-15

-20

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Pt

transition metal

Figure 7. The binding energy of M atom embedded in graphene.

From Figure 7, it can be seen that the Eb of TM@DV (-18 eV ~ -23 eV) are larger than those of TM@SV (-4 eV ~ -12 eV). Among all the candidates, the most stable one is Ti@DV complex. To determine the most suitable catalyst, the adsorption energies of I atom on TM-graphene were computed (Table 3). It is found that the adsorption energies of I adsorbed on TM@SV are too large (~ -12.4 eV), indicating TM@SV may be improper as the counter electrode of DSSC. In contrast, the adsorption energies of I adsorbed on TM@DV are much smaller (~ -1.9 eV). In particular, the Co@DV complex possesses the suitable adsorption energy, which implies that it may have comparable catalytic ability to the Pt@DV complex. To further evaluate the catalytic activities of Pt@DV and Co@DV complexes, we performed the calculations on the reaction


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pathway of iodine reduction, involving the cleavage of I2 into two I atoms and the removal of the adsorbed I as an I- anion. The results show that, in both Pt@DV and Co@DV complexes, when the I2 molecule was adsorbed it was immediately dissociated into two I atoms with one I atom located on the top of metal atom (Figure 8), which is consistent with our previous work.16 Table 3. Adsorption energy (Eads, eV) of I atom adsorption on TM-graphene Metal

M@SV

M@DV

Sc

-13.09

-1.85

Ti

-13.26

-2.48

V

-12.89

-1.76

Cr

-14.46

-3.21

Mn

-12.46

-1.78

Fe

-2.26

-2.29

Co

-13.61

-0.50

Ni

-13.70

-1.83

Cu

-14.42

-1.34

Zn

-13.66

-1.87

Figure 8. The geometric structures of I2 molecules adsorbed on the Pt@DV (a) and Co@DV (b) complexes.


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Pt@DV/25% Co@DV Ti@DV

2.4

1.8

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.2

0.6

0.0 3

4

5

6

7

bond length (Å)

Figure 9. Potential energy profiles for the dissociation of I- from Pt@DV (black), Co@DV (red), and Ti@DV (blue) complexes.

The potential energy profiles for the dissociation of iodine anion from the Pt@DV and Co@DV surfaces are given in Figure 9. The case of the most stable Ti@DV was also calculated (Figure 9). It is shown that the dissociation energies are 0.60 and 0.59 eV in the Pt@DV and Co@DV systems, respectively. Compared to the case of Ti@SV (2.29 eV), the dissociation energy of Co@DV decreases about 74%. When the iodine is completely isolated in the Co@DV system, the Mulliken charge of iodine is close to -1e, indicating that I gets one electron from the Co@DV and desorbs as an anion. These results imply that the Co@DV also possesses a high catalytic activity for the iodine reduction. Thus the Co@DV complex is proposed as a potential CE material of DSSC.

4. CONCLUSION On the basis of the first-principle calculations, we investigate the possibility of Pt-graphene as a potential high-efficiency CE in DSSC. Four different loadings of Pt-containing graphene sheets


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(19wt%, 25wt%, 28wt%, and 42wt%), involving single vacancy and divacancy graphenes, were constructed and analyzed in terms of structural and electronic properties. It is found that graphene is able to induce the positive charge of the embedded Pt atom and thus makes it ready to react with the I atom. The binding energy of a single Pt atom onto divacancy is about -11.0 eV, which not only ensures the high stability of the embedding, but also vigorously excludes the diffusion and aggregation of embedded Pt atoms. Among all models, the Pt@DV with around 25wt% loading of Pt is the most appropriate material as the CE of DSSC. In 25wt% Pt@DV, the calculated adsorption energy of iodine atom (-1.54 eV) and the dissociation energy of iodine anion (0.60 eV) imply the potential high catalytic activity of Pt-graphene. With the further analysis of diverse transition metal-embedded graphenes ranging from Sc to Zn, it is proposed that the Co@DV, i.e., Co atom embedded into divacancy in graphene, may be a good candidate to be utilized as counter electrode in DSSC. It is expected that these results will facilitate the development of low-cost and high-efficiency catalysts based on graphene.

ACKNOWLEDGMENTS This work is financially supported by the Major State Basic Research Development Programs of China (2011CBA00701), the National Natural Science Foundation of China (21373027, 21473010), the 111 Project (B07012), Beijing Nova Program (Z151100000315055), and Beijing Key Laboratory for Chemical Power Source and Green Catalysis (2013CX02031).


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Supporting Information Properties of the complexes with Pt atom absorbed on the perfect graphene, the energies of TMgraphenes with different multiplicities, and Cartesian coordinates of the optimized structures.


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