We conducted theoretical investigation of the structural and electronic properties of Pt-functionalized graphene and NH-doped Pt-functionalized graphene, which are shown to be efficient materials for hydrogen storage. Nitrene radical dopant was an effective addition required for enhancing the Pt binding on the graphene sheet. We found that up to three H2 molecules could be adsorbed by Pt-functionalized graphene with an average binding energy in the range 3.049−1.731eV eV. The most crucial part of our work is measuring the effect of nitrene radical on Pt-functionalized graphene. Our calculations predicted that the addition of NH radicals on Pt-functionalized graphene enhance the binding of Pt on graphene, which helps also to avoid the desorption of Pt(H2)n (n=1-3) complexes from graphene sheet. Our results also predict Pt-functionalized NH-doped graphene is a potential hydrogen storage medium for on-board applications.
The rapidly growing needs of energy can only be fulfilled if the current sources of energy could be replaced with such sources, which are safe, pollution free, abundantly available, cheap, and recyclable. Hydrogen has great potential to be a carrier of energy for future and is a perfect alternative for fossil fuels. Especially for transportation applications, hydrogen is thought to be a clean carrier of energy in fuel cell vehicles. It has the highest energy density of 120 MJ kg-1, among all the known fuels. However, even after extensive research of many years, storage of hydrogen is still a hurdle towards the transition to a hydrogen economy. The main problem of using hydrogen as a fuel is its gaseous nature. Therefore, it is crucial and need of the hour to find a practical system to store hydrogen safely with high gravimetric and volumetric density at ambient conditions 1. A target of 5.5 wt. % has been set by the U.S. Department of Energy (DOE) for on-board storage of H2 to be met by 2017 2. During the past decade, tremendous research has been carried out on solid-state materials for several applications including hydrogen storage for fuel cell powered vehicles despite having problems with kinetics and reversibility 3, 4. Functionalized carbon nanostructures are considered promising in the context of hydrogen storage. Carbon nanostructures in their pristine form are chemically very inert for their applications in most of the technological fields like hydrogen storage. In order to increase their reactivity many techniques can be adopted. The common technique for functionalizing carbon nanostructures is doping of foreign atoms on the surface or in the interior of the nanostructures. Transition metals (TM) are one of the most studied dopants on carbon nanostructures in the recent past and they have been explored particularly in the pursuit of suitable hydrogen storage media 5, 6.
Extensive research shows that suitable hydrogen storage capacity and H2 adsorption binding energies can be achieved by doping these carbon-based nanomaterials with boron, or aluminum by decorating them with light alkali metals (Li and Na), alkaline earth metals (Ca and Sr), light transition metals (Sc through Ni) and other transition metals anchored on mono vacancies such as Palladium, Platinum, Titanium, Nickel, and Niobium 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17. Many researchers have reported the preparation of graphene-supported Pt-based catalysts and that all of them showed enhanced electrocatalytic performance for methanol oxidation 18, 19, 20. In order to enhance the sensing effect, graphene were usually functionalized by metal and metal oxides, such as Pd 21, 22, 23, 24, Pt 25, 26, 27, TiO2 27 and SnO2 26. Particularly, Pd or Pt displays high reactivity for hydrogen sensing, because they can disassociate H2 molecules even at room temperature 28, 29.
Experimental studies have shown that it is possible to use perfluorophenyl-azide to generate nitrene radicals and immobilized graphene on silicon wafers as reported in reference 30. Despite the interesting influence that nitrene radical had on graphene 31, theoretical investigation of Pt-functionalized NH-doped graphene for hydrogen uptake have not yet been addressed. It therefore is important to determine the effectiveness of Pt– functionalized graphene and Pt–functionalized NH-doped graphene for hydrogen storage. Motivated by the challenge to develop hydrogen storage materials for practical applications, the aim of this work is to provide a detailed description of the interactions of H2 with both Pt-functionalized and NH-doped Pt-functionalized graphene surfaces. To achieve this goal, we performed first-principles calculations based on density functional theory (DFT) for a systematic study of the geometric stability and hydrogen storage capacity of Pt-functionalized graphene and NH-doped Pt-functionalized graphene. First, Pt-functionalized graphene was investigated. Next, the hydrogen adsorption of Pt-functionalized was evaluated. Finally, we investigated the effect of NH radical on Pt-functionalized graphene.
We performed calculations based on Density Functional Theory (DFT) using DMol3 codes as implemented on Material Studio 7.0 with the atomic orbitals developed by B.Delley as the basis sets 32, 33. The exchange correlation interaction between electrons is described using (local density approximation) (LDA) with the Perdew Wang correlation 34 throughout this paper. It is well known that, the LDA method predicts notably higher binding energy compared to (generalized gradient approximation) (GGA) method 35, 36. Some researchers have predicted that relativistic effects should be considered for DFT calculations for a transition metal 37, 38. So we chose the all-electron with relativistic correction core treatment method and the numerical double numerical plus polarization basis set (DNP), which has shown accurate results 39, 40. The energy minimization was done with the convergence tolerance energy of 1.0 10-6 Ha. All positions of the atoms were relaxed such that the force on each atom was less than 0.002 Ha/Å. The Brillouin zone was sampled using 5 x 5 x 1 Monkhorst-Pack mesh of special k-points 41. We chose 4.5 Å as the real space global orbital cutoff radius. To improve the convergence, thermal smearing was set to 0.005 Ha. To model the graphene sheets, a supercell containing 50 atoms with periodic boundary conditions along the x and y-axes was used. The optimized lattice parameters of the supercell were a = b = 12.30Å and c = 30 Å (a, and b are the distances in the plane of graphene, and c is the distance between two adjacent graphene sheet selected to avoid any interaction between periodic images).
The binding energy of the Platinum atom on graphene was calculated as:
 | (1) |
where, Egra+Pt is the total energy of the graphene sheet with Platinum (Pt) decoration, Egrap is the total energy of the pristine graphene sheet, EPt is the total energy of a free Pt atom.
The binding energy of the Platinum atom on graphene doped with nitrene (NH) was calculated as:
 | (2) |
where, Egrap+NH+Pt is the total energy of the graphene sheet with Platinum (Pt) decoration, and NH doping Egrap+NH is the total energy of the graphene doped with Nh, EPt is the total energy of a free Pt atom.
The adsorption energy of the Hydrogens atom on graphene decorated with Pt was calculated as:
 | (3) |
where, Egrap+Pt+H2 is the total energy of the graphene sheet with Pt)decoration and H2 adsorbed, Egrap+Pt is the total energy of the graphene decorated with Pt, EH2 is the total energy of a free H2 molecule.
The adsorption energy of the Hydrogens atom on graphene decorated with Pt and NH was calculated as:
 | (4) |
where, Egrap+NH+Pt+H2 is the total energy of the graphene sheet with Pt and NH decoration and H2 adsorbed, Egrap+NH+Pt is the total energy of the graphene decorated with Pt and, EH2 is the total energy of a free H2 molecule.
First, we have studied two configurations, an atomic configuration (At) and a molecular configuration (Mol) to see the most stable. The binding energies are respectively 1.048eV and 3.560eV for At and Mol, as shown in Table 1. This is in agreement with the results found in the literature 42. Therefore, we notice that the Molecular configuration is the most stable configuration. However, for the two configuration, we are witnessing a dissociation of the H2 molecule when it is bound to platinum. The inter-atomic distance between Pt-H are respectively 1.633 Å, 1.505 Å for At and Mol configuration. The interatomic distance H-H are respectively 3.265Å and 2.038Å respectively for At and Mol. Thus, we see that there isn't the formation of Kubas, which is in agreement with the literature.
Thus, in the rest of our study, we will only focus on the Mol configuration since it is more stable.
3.2. Hydrogen Adsorption on Molecular ConfigurationThen we studied the number of hydrogen atoms that Pt could adsorb in the molecular configuration.
We note that the addition of H2 causes a decrease of the adsorption energy. We have 3.560 eV, 2.414 eV, 2.237 eV and 1.753 eV respectively for H2, 2H2 and 4H2. We note that Pt can make covalent bonds with hydrogen up to 4H2, beyond that there is a phenomenon of physiosorption. Similarly, as the Table 2 shows, there is no formation of kuba complex because the lowest distances between H are greater than 0.768 except for the case of 5H2 for the fifth H2. We also see that the adsorption energy decreases as the number of H2 increases.
Since the aim of this study is to evaluate the effect of the presence of Pt atom on graphene upon H2 adsorption, the binding of Pt atom at three high symmetry points on graphene was studied to determine the stable location of Pt atom on graphene. The symmetry points are the bridge (B) site above the center of a carbon-carbon bond, the top (T) site directly above a carbon atom and the hollow (H) site above of a hexagon ring.
We note, as Table 3 shown, that the bridge configuration has the largest Eb with 2.611 eV followed by Hollow with 2.461 eV then Top configuration with 2.351 eV. We also see that the Pt-C distance is greater in the Bridge configuration with a value of 2.037 Å, followed by the Hollow configuration with 2.031 Å and then the Top configuration with 1.967 Å as shown in the table. These results are in agreement with the literature 43, 44.
To better understand the principle of charge transfer, we performed the Mulliken charge analysis. The charge of the Pt for the configurations T, B and H being respectively 0.053e, 0.045e, 0.060. The charges carried by the carbons directly linked to the Pt are for the top -0.244e, for the bridge -0.221e and -0.221e and -0.162e for the Hollow. Thus, we see that for the three configurations the Pt played the role of donor while the graphene played the role of acceptor. The study of the DOS (Cf Figures 1a, b) shows a shrinkage of the band at the Fermi level, which is due as shown by the Mulliken charge analysis to the electron transfer from the Pt atom to the graphene sheet.
We study here the hydrogen adsorption process on the Pt-decorated graphene for the three configurations (Top, Bridge and Hollow). For the different configurations (Top, Bridge and Hollow) we notice that unlike the Pt alone on the graphene, the H2 adsorption is stronger for the Top configuration followed by the Hollow configuration and then the bridge configuration. However, even if there is no covalent bond, we see an electron transfer as shown by the electron density on Figure. 2. The Ead are respectively 3.049eV, 2.939eV and 2.791eV for the Top, Hollow and Bridge as given in table. The distance C-Pt being respectively 2,384, 2,360 and 2,354 for Top, Bridge and Hollow. We notice that the C-Pt distances are higher than for the Pt alone, which is quite logical.
Similarly, by comparing the distances Pt-H with the distance of the PtH2 alone we notice an increase in the Pt-H distance with respectively 1.538 Å, 1.536 Å and 1.536 Å for the T, B and H. This is quite logical because of the addition of a Pt interaction with the graphene resulting to a reduction of the interaction with the H2 then an increase of the Pt-H distance. Likewise, for the PtH2, we observe a dissociation of the H2 as shown by the distances H-H (see table).
In the following, we have studied for the Top configuration the number of H2 that the Pt-decorated graphene can adsorb.
In this part, we reported on the adsorption of H2 molecules on Top Pt-functionalized 4x4 graphene. The computational results presented below show that Pt-functionalized flat graphene sheets may bind up to three H2 molecules per Pt atom to reach saturation. We observed that when three H2 molecules are bonded, one additional H2 molecule breaks the connection between Pt atom and graphene. We observed that the stability of the Pt(H2)n on graphene was decreasing when the number of hydrogen molecules adsorbed on it was increasing. Table 5 summarizes the binding energy of H2 on Pt-functionalized graphene as the number of H2 molecules increased along with, the average equilibrium parameters. For simplicity we denote the average distance between Pt and H atoms (d_Pt-H), between H-H atoms (d_H-H) and the relative distance of Pt atoms to the nearest carbon atom (d_C-Pt).
We then analyzed the successive addition of H2 molecules to Pt-functionalized graphene. Figure 3 represents the optimized structure for different numbers of H2 molecules to illustrate the saturation adsorption capacity of H2 on Pt-functionalized graphene.
The first H2 molecule is adsorbed with a binding energy of -3.049eV on Pt-G. We also observed that the average distance between Pt atom and hydrogen atoms (d_Pt-H) was 1.536Å, and the bond length (d_H-H) was equal to 1.514Å. The first three H2 molecules were strongly adsorbed on the Pt atom as shown in Figure 3.
The Ead energies varied between 3.049— 1.731eV, the average d_H-H was between 0.943—1.514Å, the average d_Pt-H ranged between 1.536—1.864Å and the d_Pt-C was between 2.377—2.523Å as shown in Table 5. The d_Pt-H were between 1.536— 1.864Å as shown in Table 3, which were in good agreement with distances reported in the literature 42, 44. We found that Pt-graphene achieved saturation with three adsorbed molecules, as after adding one more H2 molecule a weak physisorption was observed with d_Pt-C of 3.769Å for the for H2 molecule. Our results showed that metal functionalized graphene play an important role crucial factor in the H2 storage capacity when compared to H2 adsorption on pristine graphene as reported in the early literature 45, 46, 47.
We next focused on the effect of nitrene radical-doped single-sided Pt-functionalized graphene for hydrogen storage. A number of theoretical papers focus on NH-doped graphene and its properties. Pablo et al. used nitrene radical to tune the optical properties of graphene 31. Kutana et al report that when NH groups are attached to graphene, the C-C bonds are broken inducing a strong local deformation on the graphitic structure 48. Motivated by the transformation effects of these groups on carbon-based materials, we found it crucial to know their reactivity in the presence of Pt-functionalized graphene for hydrogen storage. We carried out the calculations in two parts: First, we doped one NH radical on Pt functionalized graphene on a single-sided in order to determine the most favourable site for the NH radical. Second, we investigated the effect of the nitrene radical on the successive H2 adsorption. We placed NH radical on three different site as show in Figure 4 (A, B and C) in order to check the most favorable site. Tableau 3 show the binding energy (Eb), the distance between Pt atom and the nearest carbon atom (d_C-Pt ).
In Table 6 we notice that with the addition of NH Eb increases for the three configurations (Top, Bridge and Hollow). With a bigger Eb for the C Top. We also note that as for platinum only the most stable configuration in terms of Eb is the Top configuration. Similarly, as shown in table and in Figure 4, we observe for certain configurations a covalent bond between the Pt-N which tends to avoid the dissociation of the Pt as we will see in part 9.
We also note that doping with NH increases for all configurations the distance C-Pt as shown in the table. Therefore, the largest Eb is due to the interaction of Pt with N as we can see in the Figure 4.
Thus, we can notice that the doping with the nitrene causes an increase of the Eb then a greater stability of the Pt on the graphene.
To better understand the binding mechanisms, we studied the charge transfer phenomena by the Mulliken technique. The atom of N, which is directly related to the graphene (C1-Top configuration), gains on average -0.397e while the H, which is directly related to it, has a charge of 0.276e. We see that the N behaves like an electron acceptor. The Pt has a load of + 0.148e while the two carbons directly linked to it have -0.043e and -0.190e, respectively. Therefore, we see that as for Pt alone with graphene, Pt also plays here the role of electron donor. The strong attraction between NH-doped graphene and Pt-functionalized graphene can be explained by the force of attraction generated by Pt and NH radicals, which prevented Pt atom from migrating on the graphene sheet.
As for Pt-Graphene, before studying the storage capacity we studied the most favorable site for Hydrogen adsorption.
We note that for Top and Bridge configuration we are seeing an increase in Ead except for A Top and Bridge as shown in the Table 7. However, for Hollow configuration, doping with NH decreases Ead. We see, as shown in Figure 5, that the covalent bond between Pt and N tends to increase Ead. Similarly, we note that the distance Pt-H increases in general for the three configurations as shown in table. We also note that the distance C-Pt tends to be reduced. The shortening of the d_C-Pt distance implied an increasing of the average binding energy of H2 molecules on NH-Pt-G which varied between 3.389eV—2.543eV. All these results show that NH created a fundamental defect that enhanced the binding of Pt on graphene, and that is crucial for the reversible adsorption and desorption processes.
We notice that the most suitable configuration for H2 storage is the B-Top configuration.
As these results show, the adsorption energy of H2 on NH-doped Pt-functionalized graphene was increased (Ead= -3.389eV) compared to the case of Pt-functionalized graphene. This result shows the strong influence of the NH-doping on the adsorption of H2. This is in a good agreement on what is reported in the literature 14, 25. Also Seenithurai.S et al. have reported this energy decrease when using Li-decorated double vacancy graphene 49, which means that the presence of the nitrene radical enhanced the binding of Pt atoms on graphene and prevented their migration. This result is promising for achieving reversible adsorption /desorption of hydrogen in this system.
We then studied the successive addition of H2 molecules on B-Top-Pt-functionalized graphene. Figure 6 shows, respectively, the optimized structures for nH2 addition on NH-doped Pt-functionalized graphene (B Top NH-Pt-G).
We calculated the sequential hydrogen adsorption energies and these along with the various equilibrium parameters: adsorption energy (Ead), the distance between Pt atom and the nearest C atoms (d_C-Pt) are summarized in Table 8.
We then analyzed the successive addition of H2 molecules and found that the interaction strength was achieved after adsorbing three H2 molecules because the four H2 molecule was quite far from a Pt atom at 2.949Å. As Table 5 also shows the adsorption energy of H2 molecules decreased as we increased their concentration on the NH-doped Pt-functionalized graphene. We observed that the distances of the three H2 molecules to the Pt atom were respectively, 1.542Å, 1.534Å, 1.552Å. If we added one additional H2 molecule, after relaxation the result shows that this molecule was located at a distance of 3.726Å, and the analysis of the electron density (Cf Figure 6) show that there is no interaction between this H2 and the NH-doped Pt-functionalized graphene.
In summary, to pursue the search for material for storage of hydrogen for mobile applications, we used first-principles calculations based on density functional theory (DFT) to study the interaction of H2 with free Pt, Pt-decorated graphene complex and NH- Pt-decorated graphene. We predicted that the molecular configuration of PtH2 is more stable than the atomic configuration. We predicted also, that free Pt atoms could bind four H2 molecules in the binding energy interval (1.753− 3.560 eV) in the molecular configuration. Furthermore, our study of the interaction between Pt atoms and graphene demonstrated that the interaction between the Pt and the graphene is more stable when the Pt is in a bridge site with a binding energy equal to 2.611eV. However, for hydrogen storage, the interaction between Pt and H2 is more stable when the Pt is in a Top site with an adsorption energy of 3.049eV. We note also that, when the Pt is adsorbed to the graphene their adsorption energy with H2 decrease. Moreover, we show that our complex Pt-decorated graphene on the Top site can absorb three H2 in the energy window of 3.049−1.731eV in the atomic form. We also predicted that the successive addition of H2 molecules on Pt-decorated graphene induced the desorption of nH2−Pt complex from the graphene sheet when the number of H2 molecules was higher than three H2. The most crucial part of our work is measuring the effect of nitrene radical on the H2 adsorption of Pt-functionalized graphene. We demonstrated that the doping of Pt-decorated graphene by nitrene lead to a more stable interaction between Pt and the graphene because it has increase their binding energy. We also demonstrated that the most suitable site is at the top of graphene with a binding energy of 3.074eV. Likewise, for hydrogen storage, the interaction between Pt and H2 is more stable when the Pt is in a Top site with an adsorption energy of 3.389eV. We also note that, the addition of NH increase the adsorption of H2 on the Pt. Finally, we predict that the NH doped Pt-decorated graphene cannot adsorb like Pt-decorated graphene up to three H2 molecule, with adsorption energy between 1.534-3.389eV. Our results also predict Pt-functionalized NH-doped graphene is a good configuration for potential hydrogen storage.
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Published with license by Science and Education Publishing, Copyright © 2019 El Hadji Oumar Gueye, Abdoulaye Ndiaye Dione, Allé Dioum, Baye Modou Ndiaye, Papa Douta Tall and Aboubaker Chédikh Beye
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 distance between H (d_H-H) and distance between Pt and H (d_Pt_H)){kind=link}
 of H2 molecules on Pt atom along with their equilibrium distances between H-H atom (d_H-H) and Pt and H atoms (d_Pt-H)){kind=link}
, d_C-Pt of Pt atom to pristine graphene at three adsorption sites (T, B, and H)){kind=link}
){kind=link}
){kind=link}
 of one H2 molecules in graphene functionalized with Pt atoms (Top, Bridge, Hollow) along with their equilibrium distances between H-H atom (d_H-H), Pt and C atom (d_C-Pt) and Pt and H atoms (d_Pt_H)){kind=link}
, top (T) and hollow (H) sites with one H2 molecule. Here blue balls represents Pt, with balls represents H and grey balls carbons atoms of graphene){kind=link}
 of H2 molecules in graphene functionalized with Pt atoms (Top) along with their equilibrium distances between H-H atom (d_H-H), Pt and C atom (d_C-Pt) and Pt and H atoms (d_Pt_H)){kind=link}
 with one to four H2 molecules){kind=link}
, d_C-Pt of Pt-decorated graphene at three adsorption sites (T, B, and H) doped with NH in three different positions (A, B and C)){kind=link}
, top (T) and hollow (H) and NH-doped on different position (A, B and C)){kind=link}
 of one H2 molecules in graphene functionalized with Pt atoms (Top, Bridge, Hollow) and doped with NH in three different position (A, B and C) along with their equilibrium distances between H-H atom (d_H-H), Pt and C atom (d_C-Pt) and Pt and H atoms (d_Pt-H)){kind=link}
, top (T) and hollow (H) and NH-doped on different position (A, B and C) with one H2 molecule){kind=link}
 of H2 molecules in graphene functionalized with Pt atoms (Top) and doped with NH (B position) along with their equilibrium distances between H-H atom (d_H-H), Pt and C atom (d_C-Pt) and Pt and H atoms (d_Pt-H)){kind=link}
 and NH-doped (A position) with two to four H2 molecules){kind=link}
