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Hydrogen storage in N- and B-doped graphene decorated by small platinum clusters: A computational study
Accepted Manuscript Full Length Article Hydrogen Storage in N- and B-Doped Graphene Decorated by Small Platinum Clusters: A Computational Study I-Nan Chen, Shiuan-Yau Wu, Hsin-Tsung Chen PII: DOI: Reference:
S0169-4332(18)30450-1 https://doi.org/10.1016/j.apsusc.2018.02.106 APSUSC 38564
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
8 December 2017 2 February 2018 9 February 2018
Please cite this article as: I-N. Chen, S-Y. Wu, H-T. Chen, Hydrogen Storage in N- and B-Doped Graphene Decorated by Small Platinum Clusters: A Computational Study, Applied Surface Science (2018), doi: https://doi.org/10.1016/ j.apsusc.2018.02.106
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Hydrogen Storage in N- and B-Doped Graphene Decorated by Small Platinum Clusters: A Computational Study
I-Nan Chen, Shiuan-Yau Wu and Hsin-Tsung Chen*
Department of Chemistry, Chung Yuan Christian University, Chung Li District, Taoyuan City 32023, Taiwan
* Corresponding author. E-mail: [email protected] (H.-T.C.); Tel: +886-3-265-3324 1
Abstract: In this work, we perform density functional theory (DFT) calculations to investigate the hydrogen adsorption on Pt4 cluster supported on pristine, B-, and N-doped graphene sheets. It is found that the doping B or N atom in the graphene could enhance the interaction between the Pt4 cluster and the supporting substrate. The first H2 molecule is found to be dissociative chemisorption on the three substrates. Further, dissociative and molecular adsorption of multiple H2 molecules are co-adsorbed on the three substrates. In addition, the interaction between Pt4(H2)x and the substrate is illustrated for the stability of Pt4(H2)x on the substrate. AIMD simulation is also performed to verify the stability and hydrogen storage. Accordingly, the B-graphene is predicted to be the most potential materials for hydrogen storage among these three materials.
2
Introduction Over the past decade, hydrogen has been considered as environment friendly source of energy due to its lightweight, high energy density.1-4 Regarding the safety, stimulating the search for better hydrogen storage and transport materials becomes very important.5-8 Metal hydrides and hydrogen rich compounds9-11 have been used for hydrogen storage medium. However, those media have disadvantages of slow kinetic and poor reversibility.5 Solid-state materials is the alternative for solving the safety problem.5 One of the most promising materials is carbon-based material including fullerene, carbon nanotube, graphene, and 3-D carbon nanomaterials which has been studied as either catalyst supports or catalysts in many applications.12-23 In this study, we considered heteroatom-doped graphene as a medium to improve its hydrogen storage capacity. However, to fulfill the requirements for hydrogen storage capacity in these carbon-based materials at ambient pressure and room temperature with the desired binding energy of 0.1 ~ 0.8 eV,6, 24 further enhancement of hydrogen storage properties of carbon-based materials is needed. Metals decorated on the carbon-based material have been investigated and the interaction between hydrogen molecules and the material can be improved via electrostatic forces.25-28 Metal-doped graphene is demonstrated to be more preferable in comparison with Metal-doped fullerene and carbon nanotube because both sides of graphene could be readily used to enhance the hydrogen storage.29-33 Previous studies have showed that Pt clusters and carbon-supported Pt clusters can be applied to hydrogen storage.34-38 However, metals tend to aggregate to form bigger size of clusters rather than being finely dispersed on the substrate because the metal–metal interaction is much stronger than the metal–host materials which decreases their efficiency of hydrogen storage capacity. Introducing heteroatoms (N and B) in graphene is the approach to increase the interaction between metal and the support to 3
avoid forming larger clusters.16, 39-42 In view of this aspect, theoretical work may play a leading role and provide useful information to solve this problem. To the best of our knowledge, no comparative study on heteroatom-doped (N and B) graphene decorated by Pt cluster for hydrogen storage has been explored. In the current work, we systematically perform a computational study of hydrogen adsorption on Pt4 cluster supported on pristine, B-, and N-doped graphene.
Computational Methods The spin-polarized first-principles calculations with the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)43 functional were performed by using the Vienna ab initio simulation package (VASP).44,
45
The
Brillouin zone integration using the Monkhorst-Pack scheme with k-point mesh of 3 × 3 × 1 was selected.46 A energy cutoff of 400 eV was chosen. The convergence criterion of total energy is set to be 10-4 eV. As shown in Figure 1, the supercell containing 32 carbon atoms with periodic boundary conditions was modeled with a x-y plane of 9.84 × 8.52 Å2 for the infinite graphene sheet. The z direction with a 20 Å vacuum space was set to avoid the interactions between periodic boxes. The heteroatom-doped graphene was modeled by introducing N or B dopant to substitute C atom in the supercell as shown in Figure 1. The adsorption energy is predicted by using the equation of Eads = E[nH2-substrate] – E[(n-1)H2-substrate] – E[H2], where E[H2-substrate], E[(n-1)H2-substrate], and E[H2] are the total energies of nH2 molecule adsorbed on the substrate, (n-1)H2 molecules adsorbed on the substrate, and H2 gas-phase molecule, respectively. The average adsorption energy is computed by using the equation of Eavg = {E[nH2-substrate] – E[substrate] – nE[H2]}/n, where E[nH2-substrate], E[substrate], and E[H2] are the total energies of nH2 molecules adsorbed on the substrate, the bare substrate, and H2 gas-phase molecule, respectively. 4
The van der Waals dispersion (vdW) interaction on the hydrogen adsorption energy were considered by the DFT-D method.47 The vdW interaction can be negligible due to the relative energy less than 0.05 eV. Therefore, the vdW interaction was not included in the adsorption energy. In addition, we performed ab initio molecular dynamics (AIMD) simulations in a NVT canonical ensemble using the Nosé-Hoover thermostat 48 with 1 fs per time-step until time reached 1000 fs to study the stability of the complexes.
Results and discussion Figure 1 displays the optimization structures of pristine, B-, and N-doped graphene. The predicted C–C bond distance in the pristine graphene is 1.42 Å which is well consistent with published value of 1.42 ~ 1.44 Å.41, 49, 50 The calculated lengths of C–B and C–N bonds are 1.48 and 1.41 Å, respectively, which also agree with the theoretical results.41, 51 Then, we study the interaction between the mentioned substrate and Pt4 cluster. Possible adsorption configurations of the Pt4 cluster are initially calculated by placing the Pt4 cluster at several different sites on the pristine graphene as displayed in Figure 1. Three types of adsorption configuration for Pt4/graphene are found. The optimized adsorption structures and adsorption energies are illustrated in Supporting Information (see Figure S1). The energetically most favorable configuration of the Pt4/graphene is shown in Figure 2 in which the Pt4 cluster located at the hollow site of hexagon via two Pt atoms bound at the C–C bridge site. The predicted adsorption energy and the formed Pt–C bond are -1.52 eV and 2.23 Å, respectively. For B-, and N-doped graphene, the most stable geometry is depicted in Figure 2 by substituting a C atom to B or N atom of the most stable Pt4/graphene configuration. The adsorption energies of Pt4 cluster on the B-, and N-doped graphene is predicted to be -2.36 and -1.75 eV as 5
shown in Table 1. Compared with the pristine graphene, the interaction between the Pt4 cluster and the B-doped graphene is strongly enhanced by 0.84 eV while it is only 0.25 eV for N-doped graphene. As shown in Figure 2c, the Bader charge of Pt4 on the pristine, B-, and N-doped graphene is calculated to be +0.14, +0.17, and +0.05 eV, respectively, in which the charge transfer occurs from the Pt4 cluster to the substrate. For understanding the hydrogen storage capacity, we turn on to study the adsorption property of H2 molecules on the mentioned Pt4/substrate. The optimal geometries of nH2@Pt4/graphene, nH2@Pt4/B-graphene, and nH2 @Pt4/N-graphene are depicted in Figures 3 ~ 5. The related parameters including Eads, Eavg, and the distances of Pt–substrate, Pt–H, and H–H are listed in Tables 2 ~ 4. The first H2 molecule dissociates in H2@Pt4/graphene after optimization with the adsorption energy of -1.30 eV and the dissociation H–H length of 1.90 Å indicating that H2 molecule is dissociative chemisorption on the Pt4/graphene which is also reported by Psofogiannakis et al.52 Similar phenomenon is found in the system of Pt 4/B-graphene and Pt4/N-graphene. The calculated adsorption energies and the dissociation H–H lengths of H2 @Pt4/B-graphene and H2 @Pt4/N-graphene are -1.04 and -1.54 eV and 1.76 and 1.89 Å, respectively. The calculated adsorption energy increases in the order: H2@Pt4/B-graphene (-1.04 eV) < H2@Pt4/graphene (-1.30) < H2@Pt4/N-graphene (-1.54 eV). Regarding the property of reversible kinetic, the Pt4/B-graphene materials exhibits the promising candidate material for hydrogen storage among these three substrates. Additional H2 molecules are further attached to the Pt4/substrate until the substrate achieves saturation. The adsorption energy of second H2 molecule is calculated to be -0.84, -0.91, and -0.82 eV for the Pt 4/graphene, Pt4/B-graphene, and Pt4/N-graphene, respectively. It is found that one of hydrogen molecules becomes molecular adsorption with the H–H bond lengths of 1.02 and 1.01 Å for the Pt4/graphene and Pt4/B-graphene while both H2 molecules dissociate with the H–H 6
bond length of 1.76 Å on the Pt4/N-graphene. The adsorption energy is predicted in the range of -1.67 to -0.05 eV from the third until the eighth molecule for the Pt4/graphene. Notably, dissociative and molecular adsorption of multiple H2 molecules are co-adsorbed on the Pt4/graphene. For the seventh H2 molecule, it exhibits smaller adsorption energy of -0.05 eV which is below the range of physisorption. As seen in Figure 3g, the seventh and eighth H2 molecule is far away from the Pt4/graphene with the distances of 2.26 and 2.10 Å and the adsorption energies of -0.05 and -0.05 eV. The predicted H–H bond lengths are 0.76 and 0.78 Å which is almost the same as H2 gas-phase (0.75 Å). Therefore, only six H2 molecules can adsorb on the Pt 4/graphene. Similar behavior of hydrogen adsorption is also found on the Pt4/B-graphene and Pt4/N-graphene. The calculated adsorption energies are in the range of -1.34 to -0.01 and -1.71 to -0.05 eV for the cases of Pt 4/B-graphene and Pt4/N-graphene. As seen in Figure 4h, the eighth H2 molecule is far away from the Pt4/B-graphene with the distance of 3.48 Å and the adsorption energy of -0.01 eV resulting in seven H2 molecules binding to Pt4/B-graphene. For the Pt4/N-graphene, six H2 molecules are found to adsorb on the substrate. The Bader charge analysis is performed to explain the interaction between H2 and the substrate. As shown in Figure S2 of Supporting Information, the Bader charge of the Pt4 cluster becomes more positive after the adsorption of a hydrogen molecule. This clearly indicates that the Pt4 cluster on the substrate polarizes the H2 molecule and the charge transfer takes place from the Pt4 cluster to H2 molecule resulting in the dissociation of H─H bond. As seen in Tables 2 ~ 4, the average adsorption energy decreases when increasing the number of adsorbed H2 molecule. The calculated average adsorption energies are in the range of -1.30 ~ -0.81, -1.04 ~ -0.71, and
-1.54 ~ -0.76, for the Pt 4/graphene,
Pt4/B-graphene, and Pt4/N-graphene. Although the average adsorption energy (-0.71 ~ -0.76 eV/H2) of 8 H2 molecules on the substrate is close to the optimal energy range 7
(-0.10 ~ -0.80 eV) for effective cyclic adsorption/desorption process, this quantity cannot be used for situation where various adsorption (dissociative chemisorption, molecular chemisorption, and physisorption) are co-existed due to the overestimation of the adsorption energy for the physiosorbed species.
To verify the stability of Pt4(H2)n on the substrate, we calculate the adsorption energy of Pt4(H2)n on the substrate by the question of Eads = E[Pt4(H2)n-substrate] – E[substrate] – E[Pt4(H2)n]. The results are summarized in Table S1 (see Supporting Information). It is found that without losing the contact between Pt4(H2)n and the substrate after the optimization, there are 4, 7, and 4 H2 molecules adsorbed on Pt4 for Pt4/graphene, Pt4/B-graphene, and Pt4/N-graphene, respectively. The calculation results show B- and N-doped can enhance the interaction between Pt 4(H2)n and the supporting substrate. AIMD simulation is also performed to verify the stability and hydrogen storage in the above materials. We consider three different temperatures (200, 300, and 400 K) for AIMD simulation according to the recommended temperature of 233~393 K by the U.S. DOE for H2 delivery.53 The results are shown in Supporting Information (Figures S3 ~S5). The Pt4(H2)6 on the pristine and N-doped graphene is found to be structurally unstable even at 200 K but the Pt4(H2)4 can stable below the temperature of 300 K. For the case of B-doped graphene, the Pt4(H2)7 is found to be structurally stable even at 400 K. The AIMD simulation results support the DFT results of adsorption energy of Pt 4(H2)n on the substrate. According to the stability of Pt4(H2)n on the substrate and the hydrogen storage capacity, the B-graphene is predicted to be the most potential materials for hydrogen storage among these three materials.
Conclusion 8
The H2 adsorption and storage on pristine, B-, and N-doped graphene sheets decorated with Pt4 cluster have investigated by employing first-principles calculations with the GGA-PBE level of theory. Compared to pristine graphene, the interaction between the Pt4 cluster and the supporting substrate is enhanced by 0.84 and 0.23 eV for the B- and N-doped graphene. According to our calculations, the first H2 molecule is likely to be dissociative chemisorption on the three materials while co-existed dissociative and molecular adsorption for multiple H2 molecules are found. The Bader charge analysis is carried out and clearly indicates that the Pt4 cluster on the substrate polarizes the H2 molecule with the charge transfer from the Pt4 cluster to H2 molecule resulting in the dissociation of H─H bond. According to the stability of Pt4(H2)x on the substrate and the hydrogen storage behavior studied from DFT and AIMD simulations, the B-graphene is predicted to be the most potential materials for hydrogen storage. Our calculation results may be useful for developing the carbon-based system and for the application of hydrogen storage.
Acknowledgment The authors would like to thank the Chung Yuan Christian University (CYCU), Ministry of Science and Technology (MOST) and National Center for Theoretical Sciences (NCTS), Taiwan, for supporting this study, under Grant Numbers MOST 106-2113- M-033-003,
105-2113-M-033-008,
104-2113-M-033-010
and
103-2632-M-033-001-MY3 and the use of facilities at the National Center for High-Performance Computing, Taiwan.
Supporting Information Tables S1 and Figures S1−S5. This material is available free of charge.
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Table 1. The calculated adsorption energy (Ea), average Pt-Pt bond length ( distance between Pt and substrate (DPt-sub), average Pt-C bond length (
Pt-C)
adsorbed on the substrate. substrate
Ea (eV)
graphene
–1.52
2.59
2.11
2.23
B-graphene
–2.36
2.59
2.10
2.25
N-graphene
–1.75
2.63
1.99
2.35
Pt-Pt
(Å)
13
DPt-sub (Å)
Pt-C
(Å)
Pt-Pt),
of Pt4
Table 2. Calculated some important parameters for nH molecules on Pt4/graphene. Pt4/graphene
Distance
Eads
Eavg
Number of H2
Pt-G (Å)
Pt-H (Å)
H-H (Å)
(eV)
(eV)
1 2 3 4
2.28 2.20 2.35 3.34
1.56 1.60 1.55 1.73
1.90 1.65 1.84 1.92
-1.30 -0.84 -1.67 -0.93
-1.30 -1.07 -1.27 -1.19
5 6 7 8
3.41 3.40 3.73 4.16
1.64 1.76 2.62 2.10
1.03 0.87 0.76 0.78
-1.15 -0.47 -0.05 -0.05
-1.18 -1.06 -0.92 -0.81
14
Table 3. Calculated some important parameters for nH molecules on Pt4/B-graphene. Pt4/B-graphene
Distance
Eads
Eavg
Number of H2
Pt-BG(Å)
Pt-H(Å)
H-H(Å)
(eV)
(eV)
1 2 3 4
2.29 2.33 2.47 2.39
1.56 1.65 1.57 1.76
1.76 1.01 1.62 0.88
-1.04 -0.91 -1.34 -0.81
-1.04 -0.97 -1.09 -1.02
5 6 7 8
2.66 2.75 2.79 2.83
1.73 1.67 1.69 3.48
2.44 3.11 1.74 0.75
-1.07 -0.38 -0.13 -0.01
-1.03 -0.93 -0.81 -0.71
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Table 4. Calculated some important parameters for nH molecules on Pt4/N-graphene. Pt4/N-graphene
Distance
Eads
Eavg
Number of H2
Pt-NG(Å)
Pt-H(Å)
H-H(Å)
(eV)
(eV)
1 2 3 4
2.19 2.17 2.26 3.35
1.56 1.57 1.56 1.73
1.89 1.76 1.81 0.90
-1.54 -0.82 -1.71 -0.72
-1.54 -1.18 -1.36 -1.20
5 6 7 8
3.43 3.41 3.73 4.16
1.72 1.75 2.62 2.55
0.92 0.88 0.76 0.76
-0.83 -0.41 -0.06 -0.05
-1.12 -1.01 -0.87 -0.76
16
Figure Captions Figure 1. Optimized structures of (a) graphene (b) Boron-doped graphene (c) Nitrogen-doped graphene and possible adsorbed site (Hollow, Bridge, Top) for Pt 4 on the substrate.
Figure 2. (a) and (b) are top and side views for optimized structures of Pt 4 on pristine graphene, B-doped graphene, and N-doped graphene from left to right. (c) The calculated Bader charge for Pt 4 on pristine graphene, B-doped graphene, and N-doped graphene, respectively.
Figure 3. Optimized structures of nH2 @Pt4/graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
Figure 4. Optimized structures of nH2@Pt4/B-graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
Figure 5. Optimized structures of nH2 @Pt4/N-graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
17
Figure 1. Optimized structures of (a) Graphene (b) Boron-doped graphene (c) Nitrogen-doped graphene and possible adsorbed site (Hollow, Bridge, Top) for Pt 4 on the substrate.
18
Figure 2. (a) and (b) are top and side views for optimized structures of Pt 4 on pristine graphene, B-doped graphene, and N-doped graphene from left to right. (c) The calculated Bader charge for Pt 4 on pristine graphene, B-doped graphene, and N-doped graphene, respectively.
19
Figure 3. Optimized structures of nH2 @Pt4/graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
20
Figure 4. Optimized structures of nH2@Pt4/B-graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
21
Figure 5. Optimized structures of nH2@Pt4/B-graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
22
Graphical Abstract:
We have performed density functional theory (DFT) calculations to investigate the hydrogen adsorption on Pt 4 cluster supported on pristine, B-, and N-doped graphene sheets. Calculation results show that the doping B or N atom in the graphene could enhance the interaction between the Pt 4 cluster and the supporting substrate. The B-doped graphene is predicted to be the most potential materials for hydrogen storage among these three materials.
23
Highlights
Hydrogen adsorption on Pt 4 cluster supported on pristine, B-, and N-doped graphene sheets have been studied by using theoretical calculations. Doping B or N atom in the graphene could enhance the interaction between the Pt4 cluster and the supporting substrate. Dissociative and molecular adsorption of multiple H2 molecules are co-adsorbed on the three substrates. The B-graphene is predicted to be the most potential materials for hydrogen storage among these three materials.
24
S0169-4332(18)30450-1 https://doi.org/10.1016/j.apsusc.2018.02.106 APSUSC 38564
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
8 December 2017 2 February 2018 9 February 2018
Please cite this article as: I-N. Chen, S-Y. Wu, H-T. Chen, Hydrogen Storage in N- and B-Doped Graphene Decorated by Small Platinum Clusters: A Computational Study, Applied Surface Science (2018), doi: https://doi.org/10.1016/ j.apsusc.2018.02.106
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Hydrogen Storage in N- and B-Doped Graphene Decorated by Small Platinum Clusters: A Computational Study
I-Nan Chen, Shiuan-Yau Wu and Hsin-Tsung Chen*
Department of Chemistry, Chung Yuan Christian University, Chung Li District, Taoyuan City 32023, Taiwan
* Corresponding author. E-mail: [email protected] (H.-T.C.); Tel: +886-3-265-3324 1
Abstract: In this work, we perform density functional theory (DFT) calculations to investigate the hydrogen adsorption on Pt4 cluster supported on pristine, B-, and N-doped graphene sheets. It is found that the doping B or N atom in the graphene could enhance the interaction between the Pt4 cluster and the supporting substrate. The first H2 molecule is found to be dissociative chemisorption on the three substrates. Further, dissociative and molecular adsorption of multiple H2 molecules are co-adsorbed on the three substrates. In addition, the interaction between Pt4(H2)x and the substrate is illustrated for the stability of Pt4(H2)x on the substrate. AIMD simulation is also performed to verify the stability and hydrogen storage. Accordingly, the B-graphene is predicted to be the most potential materials for hydrogen storage among these three materials.
2
Introduction Over the past decade, hydrogen has been considered as environment friendly source of energy due to its lightweight, high energy density.1-4 Regarding the safety, stimulating the search for better hydrogen storage and transport materials becomes very important.5-8 Metal hydrides and hydrogen rich compounds9-11 have been used for hydrogen storage medium. However, those media have disadvantages of slow kinetic and poor reversibility.5 Solid-state materials is the alternative for solving the safety problem.5 One of the most promising materials is carbon-based material including fullerene, carbon nanotube, graphene, and 3-D carbon nanomaterials which has been studied as either catalyst supports or catalysts in many applications.12-23 In this study, we considered heteroatom-doped graphene as a medium to improve its hydrogen storage capacity. However, to fulfill the requirements for hydrogen storage capacity in these carbon-based materials at ambient pressure and room temperature with the desired binding energy of 0.1 ~ 0.8 eV,6, 24 further enhancement of hydrogen storage properties of carbon-based materials is needed. Metals decorated on the carbon-based material have been investigated and the interaction between hydrogen molecules and the material can be improved via electrostatic forces.25-28 Metal-doped graphene is demonstrated to be more preferable in comparison with Metal-doped fullerene and carbon nanotube because both sides of graphene could be readily used to enhance the hydrogen storage.29-33 Previous studies have showed that Pt clusters and carbon-supported Pt clusters can be applied to hydrogen storage.34-38 However, metals tend to aggregate to form bigger size of clusters rather than being finely dispersed on the substrate because the metal–metal interaction is much stronger than the metal–host materials which decreases their efficiency of hydrogen storage capacity. Introducing heteroatoms (N and B) in graphene is the approach to increase the interaction between metal and the support to 3
avoid forming larger clusters.16, 39-42 In view of this aspect, theoretical work may play a leading role and provide useful information to solve this problem. To the best of our knowledge, no comparative study on heteroatom-doped (N and B) graphene decorated by Pt cluster for hydrogen storage has been explored. In the current work, we systematically perform a computational study of hydrogen adsorption on Pt4 cluster supported on pristine, B-, and N-doped graphene.
Computational Methods The spin-polarized first-principles calculations with the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)43 functional were performed by using the Vienna ab initio simulation package (VASP).44,
45
The
Brillouin zone integration using the Monkhorst-Pack scheme with k-point mesh of 3 × 3 × 1 was selected.46 A energy cutoff of 400 eV was chosen. The convergence criterion of total energy is set to be 10-4 eV. As shown in Figure 1, the supercell containing 32 carbon atoms with periodic boundary conditions was modeled with a x-y plane of 9.84 × 8.52 Å2 for the infinite graphene sheet. The z direction with a 20 Å vacuum space was set to avoid the interactions between periodic boxes. The heteroatom-doped graphene was modeled by introducing N or B dopant to substitute C atom in the supercell as shown in Figure 1. The adsorption energy is predicted by using the equation of Eads = E[nH2-substrate] – E[(n-1)H2-substrate] – E[H2], where E[H2-substrate], E[(n-1)H2-substrate], and E[H2] are the total energies of nH2 molecule adsorbed on the substrate, (n-1)H2 molecules adsorbed on the substrate, and H2 gas-phase molecule, respectively. The average adsorption energy is computed by using the equation of Eavg = {E[nH2-substrate] – E[substrate] – nE[H2]}/n, where E[nH2-substrate], E[substrate], and E[H2] are the total energies of nH2 molecules adsorbed on the substrate, the bare substrate, and H2 gas-phase molecule, respectively. 4
The van der Waals dispersion (vdW) interaction on the hydrogen adsorption energy were considered by the DFT-D method.47 The vdW interaction can be negligible due to the relative energy less than 0.05 eV. Therefore, the vdW interaction was not included in the adsorption energy. In addition, we performed ab initio molecular dynamics (AIMD) simulations in a NVT canonical ensemble using the Nosé-Hoover thermostat 48 with 1 fs per time-step until time reached 1000 fs to study the stability of the complexes.
Results and discussion Figure 1 displays the optimization structures of pristine, B-, and N-doped graphene. The predicted C–C bond distance in the pristine graphene is 1.42 Å which is well consistent with published value of 1.42 ~ 1.44 Å.41, 49, 50 The calculated lengths of C–B and C–N bonds are 1.48 and 1.41 Å, respectively, which also agree with the theoretical results.41, 51 Then, we study the interaction between the mentioned substrate and Pt4 cluster. Possible adsorption configurations of the Pt4 cluster are initially calculated by placing the Pt4 cluster at several different sites on the pristine graphene as displayed in Figure 1. Three types of adsorption configuration for Pt4/graphene are found. The optimized adsorption structures and adsorption energies are illustrated in Supporting Information (see Figure S1). The energetically most favorable configuration of the Pt4/graphene is shown in Figure 2 in which the Pt4 cluster located at the hollow site of hexagon via two Pt atoms bound at the C–C bridge site. The predicted adsorption energy and the formed Pt–C bond are -1.52 eV and 2.23 Å, respectively. For B-, and N-doped graphene, the most stable geometry is depicted in Figure 2 by substituting a C atom to B or N atom of the most stable Pt4/graphene configuration. The adsorption energies of Pt4 cluster on the B-, and N-doped graphene is predicted to be -2.36 and -1.75 eV as 5
shown in Table 1. Compared with the pristine graphene, the interaction between the Pt4 cluster and the B-doped graphene is strongly enhanced by 0.84 eV while it is only 0.25 eV for N-doped graphene. As shown in Figure 2c, the Bader charge of Pt4 on the pristine, B-, and N-doped graphene is calculated to be +0.14, +0.17, and +0.05 eV, respectively, in which the charge transfer occurs from the Pt4 cluster to the substrate. For understanding the hydrogen storage capacity, we turn on to study the adsorption property of H2 molecules on the mentioned Pt4/substrate. The optimal geometries of nH2@Pt4/graphene, nH2@Pt4/B-graphene, and nH2 @Pt4/N-graphene are depicted in Figures 3 ~ 5. The related parameters including Eads, Eavg, and the distances of Pt–substrate, Pt–H, and H–H are listed in Tables 2 ~ 4. The first H2 molecule dissociates in H2@Pt4/graphene after optimization with the adsorption energy of -1.30 eV and the dissociation H–H length of 1.90 Å indicating that H2 molecule is dissociative chemisorption on the Pt4/graphene which is also reported by Psofogiannakis et al.52 Similar phenomenon is found in the system of Pt 4/B-graphene and Pt4/N-graphene. The calculated adsorption energies and the dissociation H–H lengths of H2 @Pt4/B-graphene and H2 @Pt4/N-graphene are -1.04 and -1.54 eV and 1.76 and 1.89 Å, respectively. The calculated adsorption energy increases in the order: H2@Pt4/B-graphene (-1.04 eV) < H2@Pt4/graphene (-1.30) < H2@Pt4/N-graphene (-1.54 eV). Regarding the property of reversible kinetic, the Pt4/B-graphene materials exhibits the promising candidate material for hydrogen storage among these three substrates. Additional H2 molecules are further attached to the Pt4/substrate until the substrate achieves saturation. The adsorption energy of second H2 molecule is calculated to be -0.84, -0.91, and -0.82 eV for the Pt 4/graphene, Pt4/B-graphene, and Pt4/N-graphene, respectively. It is found that one of hydrogen molecules becomes molecular adsorption with the H–H bond lengths of 1.02 and 1.01 Å for the Pt4/graphene and Pt4/B-graphene while both H2 molecules dissociate with the H–H 6
bond length of 1.76 Å on the Pt4/N-graphene. The adsorption energy is predicted in the range of -1.67 to -0.05 eV from the third until the eighth molecule for the Pt4/graphene. Notably, dissociative and molecular adsorption of multiple H2 molecules are co-adsorbed on the Pt4/graphene. For the seventh H2 molecule, it exhibits smaller adsorption energy of -0.05 eV which is below the range of physisorption. As seen in Figure 3g, the seventh and eighth H2 molecule is far away from the Pt4/graphene with the distances of 2.26 and 2.10 Å and the adsorption energies of -0.05 and -0.05 eV. The predicted H–H bond lengths are 0.76 and 0.78 Å which is almost the same as H2 gas-phase (0.75 Å). Therefore, only six H2 molecules can adsorb on the Pt 4/graphene. Similar behavior of hydrogen adsorption is also found on the Pt4/B-graphene and Pt4/N-graphene. The calculated adsorption energies are in the range of -1.34 to -0.01 and -1.71 to -0.05 eV for the cases of Pt 4/B-graphene and Pt4/N-graphene. As seen in Figure 4h, the eighth H2 molecule is far away from the Pt4/B-graphene with the distance of 3.48 Å and the adsorption energy of -0.01 eV resulting in seven H2 molecules binding to Pt4/B-graphene. For the Pt4/N-graphene, six H2 molecules are found to adsorb on the substrate. The Bader charge analysis is performed to explain the interaction between H2 and the substrate. As shown in Figure S2 of Supporting Information, the Bader charge of the Pt4 cluster becomes more positive after the adsorption of a hydrogen molecule. This clearly indicates that the Pt4 cluster on the substrate polarizes the H2 molecule and the charge transfer takes place from the Pt4 cluster to H2 molecule resulting in the dissociation of H─H bond. As seen in Tables 2 ~ 4, the average adsorption energy decreases when increasing the number of adsorbed H2 molecule. The calculated average adsorption energies are in the range of -1.30 ~ -0.81, -1.04 ~ -0.71, and
-1.54 ~ -0.76, for the Pt 4/graphene,
Pt4/B-graphene, and Pt4/N-graphene. Although the average adsorption energy (-0.71 ~ -0.76 eV/H2) of 8 H2 molecules on the substrate is close to the optimal energy range 7
(-0.10 ~ -0.80 eV) for effective cyclic adsorption/desorption process, this quantity cannot be used for situation where various adsorption (dissociative chemisorption, molecular chemisorption, and physisorption) are co-existed due to the overestimation of the adsorption energy for the physiosorbed species.
To verify the stability of Pt4(H2)n on the substrate, we calculate the adsorption energy of Pt4(H2)n on the substrate by the question of Eads = E[Pt4(H2)n-substrate] – E[substrate] – E[Pt4(H2)n]. The results are summarized in Table S1 (see Supporting Information). It is found that without losing the contact between Pt4(H2)n and the substrate after the optimization, there are 4, 7, and 4 H2 molecules adsorbed on Pt4 for Pt4/graphene, Pt4/B-graphene, and Pt4/N-graphene, respectively. The calculation results show B- and N-doped can enhance the interaction between Pt 4(H2)n and the supporting substrate. AIMD simulation is also performed to verify the stability and hydrogen storage in the above materials. We consider three different temperatures (200, 300, and 400 K) for AIMD simulation according to the recommended temperature of 233~393 K by the U.S. DOE for H2 delivery.53 The results are shown in Supporting Information (Figures S3 ~S5). The Pt4(H2)6 on the pristine and N-doped graphene is found to be structurally unstable even at 200 K but the Pt4(H2)4 can stable below the temperature of 300 K. For the case of B-doped graphene, the Pt4(H2)7 is found to be structurally stable even at 400 K. The AIMD simulation results support the DFT results of adsorption energy of Pt 4(H2)n on the substrate. According to the stability of Pt4(H2)n on the substrate and the hydrogen storage capacity, the B-graphene is predicted to be the most potential materials for hydrogen storage among these three materials.
Conclusion 8
The H2 adsorption and storage on pristine, B-, and N-doped graphene sheets decorated with Pt4 cluster have investigated by employing first-principles calculations with the GGA-PBE level of theory. Compared to pristine graphene, the interaction between the Pt4 cluster and the supporting substrate is enhanced by 0.84 and 0.23 eV for the B- and N-doped graphene. According to our calculations, the first H2 molecule is likely to be dissociative chemisorption on the three materials while co-existed dissociative and molecular adsorption for multiple H2 molecules are found. The Bader charge analysis is carried out and clearly indicates that the Pt4 cluster on the substrate polarizes the H2 molecule with the charge transfer from the Pt4 cluster to H2 molecule resulting in the dissociation of H─H bond. According to the stability of Pt4(H2)x on the substrate and the hydrogen storage behavior studied from DFT and AIMD simulations, the B-graphene is predicted to be the most potential materials for hydrogen storage. Our calculation results may be useful for developing the carbon-based system and for the application of hydrogen storage.
Acknowledgment The authors would like to thank the Chung Yuan Christian University (CYCU), Ministry of Science and Technology (MOST) and National Center for Theoretical Sciences (NCTS), Taiwan, for supporting this study, under Grant Numbers MOST 106-2113- M-033-003,
105-2113-M-033-008,
104-2113-M-033-010
and
103-2632-M-033-001-MY3 and the use of facilities at the National Center for High-Performance Computing, Taiwan.
Supporting Information Tables S1 and Figures S1−S5. This material is available free of charge.
9
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Table 1. The calculated adsorption energy (Ea), average Pt-Pt bond length ( distance between Pt and substrate (DPt-sub), average Pt-C bond length (
Pt-C)
adsorbed on the substrate. substrate
Ea (eV)
graphene
–1.52
2.59
2.11
2.23
B-graphene
–2.36
2.59
2.10
2.25
N-graphene
–1.75
2.63
1.99
2.35
Pt-Pt
(Å)
13
DPt-sub (Å)
Pt-C
(Å)
Pt-Pt),
of Pt4
Table 2. Calculated some important parameters for nH molecules on Pt4/graphene. Pt4/graphene
Distance
Eads
Eavg
Number of H2
Pt-G (Å)
Pt-H (Å)
H-H (Å)
(eV)
(eV)
1 2 3 4
2.28 2.20 2.35 3.34
1.56 1.60 1.55 1.73
1.90 1.65 1.84 1.92
-1.30 -0.84 -1.67 -0.93
-1.30 -1.07 -1.27 -1.19
5 6 7 8
3.41 3.40 3.73 4.16
1.64 1.76 2.62 2.10
1.03 0.87 0.76 0.78
-1.15 -0.47 -0.05 -0.05
-1.18 -1.06 -0.92 -0.81
14
Table 3. Calculated some important parameters for nH molecules on Pt4/B-graphene. Pt4/B-graphene
Distance
Eads
Eavg
Number of H2
Pt-BG(Å)
Pt-H(Å)
H-H(Å)
(eV)
(eV)
1 2 3 4
2.29 2.33 2.47 2.39
1.56 1.65 1.57 1.76
1.76 1.01 1.62 0.88
-1.04 -0.91 -1.34 -0.81
-1.04 -0.97 -1.09 -1.02
5 6 7 8
2.66 2.75 2.79 2.83
1.73 1.67 1.69 3.48
2.44 3.11 1.74 0.75
-1.07 -0.38 -0.13 -0.01
-1.03 -0.93 -0.81 -0.71
15
Table 4. Calculated some important parameters for nH molecules on Pt4/N-graphene. Pt4/N-graphene
Distance
Eads
Eavg
Number of H2
Pt-NG(Å)
Pt-H(Å)
H-H(Å)
(eV)
(eV)
1 2 3 4
2.19 2.17 2.26 3.35
1.56 1.57 1.56 1.73
1.89 1.76 1.81 0.90
-1.54 -0.82 -1.71 -0.72
-1.54 -1.18 -1.36 -1.20
5 6 7 8
3.43 3.41 3.73 4.16
1.72 1.75 2.62 2.55
0.92 0.88 0.76 0.76
-0.83 -0.41 -0.06 -0.05
-1.12 -1.01 -0.87 -0.76
16
Figure Captions Figure 1. Optimized structures of (a) graphene (b) Boron-doped graphene (c) Nitrogen-doped graphene and possible adsorbed site (Hollow, Bridge, Top) for Pt 4 on the substrate.
Figure 2. (a) and (b) are top and side views for optimized structures of Pt 4 on pristine graphene, B-doped graphene, and N-doped graphene from left to right. (c) The calculated Bader charge for Pt 4 on pristine graphene, B-doped graphene, and N-doped graphene, respectively.
Figure 3. Optimized structures of nH2 @Pt4/graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
Figure 4. Optimized structures of nH2@Pt4/B-graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
Figure 5. Optimized structures of nH2 @Pt4/N-graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
17
Figure 1. Optimized structures of (a) Graphene (b) Boron-doped graphene (c) Nitrogen-doped graphene and possible adsorbed site (Hollow, Bridge, Top) for Pt 4 on the substrate.
18
Figure 2. (a) and (b) are top and side views for optimized structures of Pt 4 on pristine graphene, B-doped graphene, and N-doped graphene from left to right. (c) The calculated Bader charge for Pt 4 on pristine graphene, B-doped graphene, and N-doped graphene, respectively.
19
Figure 3. Optimized structures of nH2 @Pt4/graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
20
Figure 4. Optimized structures of nH2@Pt4/B-graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
21
Figure 5. Optimized structures of nH2@Pt4/B-graphene (a) H2, (b) 2H2, (c) 3H2, (d) 4H2, (e) 5H2, (f) 6H2, (g) 7H2, and (h) 8H2.
22
Graphical Abstract:
We have performed density functional theory (DFT) calculations to investigate the hydrogen adsorption on Pt 4 cluster supported on pristine, B-, and N-doped graphene sheets. Calculation results show that the doping B or N atom in the graphene could enhance the interaction between the Pt 4 cluster and the supporting substrate. The B-doped graphene is predicted to be the most potential materials for hydrogen storage among these three materials.
23
Highlights
Hydrogen adsorption on Pt 4 cluster supported on pristine, B-, and N-doped graphene sheets have been studied by using theoretical calculations. Doping B or N atom in the graphene could enhance the interaction between the Pt4 cluster and the supporting substrate. Dissociative and molecular adsorption of multiple H2 molecules are co-adsorbed on the three substrates. The B-graphene is predicted to be the most potential materials for hydrogen storage among these three materials.
24