Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study

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Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study Vaishali Sharma a, Hardik L. Kagdada a, Jinlan Wang b,c, Prafulla K. Jha a,* a

Department of Physics, Faculty of Science, The M. S. University of Baroda, Vadodara, 390002, Gujarat, India School of Physics, Southeast University, Nanjing, 211189, China c Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha, Hunan, 410081, China b

highlights

graphical abstract


HER activity of graphene quantum dot “triangulene” is studied.
Triangulene provides better HER with hydrogen adsorption energy of 0.264 eV.
Site dependent influence of Pt over GQD is evaluated for hydrogen storage.
Hydrogen on Pt decorated GQD with bridge, hollow and top sites results in D-mode.

article info

abstract

Article history:

In the ever growing demand of future energy resources, hydrogen production reaction has

Received 17 March 2019

attracted much attention among the scientific community. In this work, we have investi-

Received in revised form

gated the hydrogen evolution reaction (HER) activity on an open-shell polyaromatic hy-

26 June 2019

drocarbon (PAH), graphene quantum dot “triangulene” using first principles based density

Accepted 4 September 2019

functional theory (DFT) by means of adsorption mechanism and electronic density of

Available online xxx

states calculations. The free energy calculated from the adsorption energy for graphene quantum dot (GQD) later guides us to foresee the best suitable catalyst among quantum

Keywords:

dots. Triangulene provides better HER with hydrogen placed at top site with the adsorption

Graphene quantum dots

energy as 0.264 eV. Further, we have studied platinum decorated triangulene for

Adsorption mechanism

hydrogen storage. Three different sites on triangulene were considered for platinum atom

Hydrogen evolution reaction

adsorption namely top site of carbon (C) atom, hollow site of the hexagon carbon ring near

Site dependence

triangulene's unpaired electron and bridge site over CeC bond. It is found that the platinum

Hydrogen storage

atom is more stable on the hollow site than top and bridge site. We have calculated the density of states (DOS), highest occupied molecular orbitals (HOMO), lowest unoccupied

* Corresponding author. E-mail address: [email protected] (P.K. Jha). https://doi.org/10.1016/j.ijhydene.2019.09.021 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Sharma V et al., Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.021

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molecular orbitals (LUMO) and HOMO-LUMO gap of hydrogen molecule adsorbed platinum decorated triangulene. Our results show that the hydrogen molecule (H2) dissociates instinctively on all three considered sites of platinum decorated triangulene resulting in Dmode. The fundamental understanding of adsorption mechanism along with analyses of electronic properties will be important for further spillover mechanism and synthesis of high-performance GQD for H2 storage applications. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction To solve the world's energy crisis caused by the excessive reliance on fossil fuels energy, enrooting clean, economical and renewable energy is a promising and much needed solution [1,2]. Hydrogen is extensively considered as costeffective, non-polluting, and sustainable source of energy. The exploration for hydrogen storage is receiving great attention in whole scientific society, despite a need for modification due to its weak intermolecular interaction making difficult to store hydrogen under ambient condition [3]. Hydrogen evolution reaction (HER) is one of the most significant electrochemical reactions from both theoretical and practical aspects [4]. In particular, the HER has attained excellent performance by using platinum (Pt) group metals as catalysts [5,6]. However, its further development is enormously limited due to the inadequacy and high cost of Pt metals. Significant attempts have been made in the last few decades to scrutinize novel materials that can replace or modify the noble metals for efficient hydrogen production [7e9]. Currently, dimensionality is progressing as one of the leading factors behind the fundamental properties of materials in addition to the atomic arrangements and compositions [10e12]. Recently discovered zero-dimensional graphene quantum dots (GQDs) are new and arising members of graphene family (comprising unique properties) [13e15] with sizes usually below 10 nm [16,17]. GQDs acquire size-shape dependent bandgap and unique photoluminescence (PL) properties due to quantum confinement and edge effects. Moreover, they have low-toxicity, chemical inertness, biocompatibility and no photo-bleaching properties, promoting their applications in many fields, including photoelectronics and catalysis [16,17]. Further to achieve the full potential of GQDs, GQDs-based nanocomposites have been studied [18,19]. Various nanocomposites based on nobel metals like aluminium, silver, platinum and palladium with GQDs were also studied [20e23]. He et al. [23] reported GQDs/Pt nanocomposites and found enhanced electrocatalytic activity in oxygen reduction reactions (OER), in comparison with commercial Pt/C catalysts. On the other hand, hydrogen storage remains to be the primary bottleneck for environment-friendly, renewable and cost-effective development of fuel cell systems [24,25]. Carbon-based materials have long been classified as capable candidates for hydrogen storage [26e28]. The hydrogen storage in carbon materials is extensively divided in two

adsorption approaches: physisorption of hydrogen molecules, depending on weak van der Waals interactions; and chemisorption of hydrogen atoms, generated by the dissociation of hydrogen molecule. The latter approach is the reversible weak physisorption with pure carbon materials. However, the high adsorption capacity as a consequence of hydrogen molecule adsorption is unlikely at ambient temperature which is necessary for practical applications [24]. Substantial improvement in hydrogen storage capacity can be attained by doping materials with small amounts of metals that can act catalytically [28e31]. Recently, Yodsin et al. [32] theoretically investigated hydrogen adsorption and diffusion on platinumdecorated carbon nanocone (PteCNC). The presence of hydrogen atom on Pt catalyst effectively induces hydrogen diffusion process because of carbon-carbon surface, the PteH promotes the hydrogen shift from CeH bonds to its neighbouring C atom with energy barrier less than 0.5 eV under ambient conditions [32]. Cutting the graphene sheet leads to the several nanofragments of graphene, in which one can see various structures of polycyclic aromatic hydrocarbons (PAHs) like anthracene, pyrene, coronene, triangulene, phenalenyl and ultimately benzene. This results in the similar properties in PAHs and zero-dimensional nanographenes [33,34]. Among all PAHs, some consists closed-shell electronic configuration in their ground state. However, due to the topology of p electron arrangements [34], there are some PAHs [35,36] which acquire high spin, open-shell radical nature in their ground state and
polynuclear are also termed as open shell non-Kekule benzenoid. Thus, these systems can be considered more commonly as open-shell graphene fragments [35]. PAHs have been proposed as a possible catalyst for H2 formation [37,38]. Rasmussen et al. studied the role of pyrene in hydrogen evolution in interstellar medium and concluded that the binding of hydrogen to pyrene is exothermic and site dependent [37]. In present work, we study the role of graphene quantum dots (GQD) for HER and hydrogen storage. Quantum mechanical calculations based on density functional theory (DFT) were performed to study the adsorption mechanism and electronic properties of the aforementioned. We use open shell GQD, triangulene (C22H12) as a model system for HER. Furthermore, for hydrogen storage, we investigated the site dependent adsorption mechanism of hydrogen molecule (H2) over platinum decorated triangulene. The bridge (B), hollow (H) and top (T) sites were considered for the adsorption of H2 molecule to understand triangulene's capacity for hydrogen storage usage.

Please cite this article as: Sharma V et al., Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.021

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Computational methodology All the geometrical optimization, adsorption mechanism and electronic properties calculations were performed with the Gaussian 09 program [39]. We have performed all calculations using spin-unrestricted density functional theory by means of Becke three parameters hybrid functional with Lee-YangPerdew correlation functionals (B3LYP) because of the openshell character of triangulene [40,41]. The hybrid functionals are the mixture of Hartree-Fock (HF) exchange along with density functional theory (DFT) exchange-correlation functionals. B3LYP functional uses LYP expression for non-local correlation and VWN function III for local correlation. The LANL2DZ with effective core potential (ECP) basis set was used for hydrogen molecule on platinum decorated triangulene which replaces 1s through 2p electron of the heavy atoms with a potential field for considerable computational savings. LANL2DZ defines D95V on first row [42], Los Alamos ECP plus DZ on NaeLa, Hf-Bi [43,44]. Visualizations of molecular orbitals are rendered with GaussView (version 5) using the results of DFT calculations [45]. The plots of density of states (DOS) are generated using Multiwfn software [46]. Frequency calculations were performed at the same level as the geometry optimization to confirm no imaginary frequencies. The platinum atom is modelled as being adsorbed at one of three different positions: (i) bridge site, that is above the middle point of carbon-carbon bond, (ii) hollow site, that is above centre of the aromatic ring of carbon, and (iii) top site, as name suggests on top of the carbon atom of triangulene. Throughout the manuscript, all three sites were named as triþPt (B) for bridge site, triþPt (H) for hollow site and triþPt (T) for top site. Moreover, to attain the accuracy in the protocol we further re-optimized the structures for single point calculation at same level of theory in all considered systems. The adsorption energy (Ead) between the systems was calculated by; Ead ¼ Etriþpt  Etri þ Ept




Ead ¼ EtriþptþH2  Etriþpt þ EH2

(1)


(2)

Where Etriþpt is the optimized energy of system in which platinum is adsorbed on triangulene (on three different sites), Etri is the optimized energy of pristine triangulene and EH2 is the optimized energy of hydrogen molecule. With this definition, a negative Ead value shows a stable adsorption complex on the triangulene.

Results and discussion Hydrogen evolution reaction (HER) To understand the core of our calculations, we first analysed the structural properties of triangulene before and after the adsorption of hydrogen. An open-shell PAH triangulene molecule (C22H12) modelled as a GQD exists in triplet groundstate and has been recently synthesized [47] and studied theoretically [35]. In our previous work, we have investigated

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the geometrical and electronic properties of triangulene in both singlet and triplet states and concluded that the triangulene is most stable in its triplet state and therefore called triplet-ground-state polybenzoid [35]. Accordingly, in present work, we have only analysed electronic properties of triangulene together with hydrogen incorporation in its triplet states. Fig. 1(a) presents top and side views of initial structure of triangulene with hydrogen. The hydrogen atom is placed at a distance of 2.50  A above carbon atom. Fig. 1(b) shows top and side view of the optimized structure of hydrogen adsorbed triangulene. No imaginary frequencies were found during the ground-state calculation, indicating that the optimized structure represents a true minimum. After optimization, the hydrogen atom is repelled when placed on pristine triangulene (Fig. 1(b)). The distance becomes 3.38  A from the 2.50  A. This may be due to the nature of GQD to adsorb molecules at the edges. In order to understand the adsorption mechanism of hydrogen on triangulene for HER, we have calculated the adsorption energy (Ead) with the formula given in equation (1) and presented them in Table 1. For hydrogen on pristine triangulene, the adsorption energy is 0.264 eV. This low adsorption energy is good for HER mechanism. Further, we have evaluated the adsorption of hydrogen over the edges of GQD and found the Ead of 2.910 eV. The initial and optimized structures of hydrogen over the edge of GQD are presented in Supplementary Material. This high Ead may not be of significance for HER when hydrogen is placed at the edge of GQD. Therefore, we have analysed the electronic properties of GQD at top site only. Fig. 2 shows the electronic density of states (DOS) of pristine and hydrogen adsorbed triangulene. The intrinsic band gap of triangulene changes a little by the inclusion of hydrogen atom. The energy bandgap (Eg ) of triangulene is tabulated in Table 1. The energy bandgaps (Eg ) are calculated through the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO). The expression for energy bandgap is given by following equation: Eg ¼ ELUMO  EHOMO

(3)

We can clearly see from the figure (Fig (2)) that the HOMO is slightly shifted with the adsorption of hydrogen, while LUMO is unaffected (not shown in the figure but table clearly shows). This can be attributed to the low adsorption energy (0.264 eV) arising from the weak interaction between hydrogen and triangulene. The negligible charge transfer (3.75104 e) between hydrogen and triangulene further confirms this. The HER is considered as one of the significant electrochemical reactions that comprise proton reduction at the electrode to produce hydrogen first and then H2 gas as an aftermath. The capability of a given surface to catalyse the HER is determined by exchange current density, which can be related to the free energy of adsorbed hydrogen (H2) when the reaction is at equilibrium. The free energy of H in the adsorbed state can be defined as the following equation: DGH* ¼ Ead þ DEZPE  TDSH

(4)

Here, Ead is the hydrogen adsorption energy, DEZPE presents

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Fig. 1 e (a) Initial and (b) optimized structure of hydrogen adsorbed over triangulene.

Table 1 e Calculated HOMO, LUMO, bandgap (Eg) and adsorption energies (Ead) of pristine and hydrogen adsorbed triangulene. System triangulene trianguleneþh

HOMO (eV)

LUMO (eV)

Eg (eV)

Ead (eV)

6.397 6.389

2.664 2.661

3.730 3.720

e 0.264

the zero-point energy of hydrogen in both adsorbed and gaseous phases with the variation in values from 0.00 to 0.04 eV [48e50]. The last term shows the entropy difference of hydrogen in both adsorbed and gaseous phases with the

experimental value of 0.40 eV. While, H* represents hydrogen adsorption. Exploring the facts that the vibrational entropy is small in the adsorbed state which further shows that the adsorption entropy of hydrogen is DSH z  1=2 DSOH , where D SOH represents the entropy of hydrogen in gaseous phase. As a result, the free energy related to the adsorption can be given by equation: DGH* ¼ DEad þ 0:24 eV, where DGH* should be zero to acquire good catalyst for HER. This shows that adsorption energy plays important role in HER and should be in the vicinity of 0.24 eV. Form the Table 1, we can clearly see that hydrogen adsorption on pristine triangulene results in the adsorption energy of 0.264 eV. Therefore, HER of pristine triangulene gives the best results and is a promising candidate. Our pristine triangulene shows better HER activity than hydrogen monomer on pyrene and coronene PAH as their binding energy lies in the range of 0.6e1.6 eV and 0.6e1.4 eV respectively [37]. Moreover, our calculated value of Ead with pristine triangulene suggests better HER performance as compared to the platinum and palladium surfaces [51].

Hydrogen storage

Fig. 2 e Density of states (DOS) of triangulene and hydrogen adsorbed triangulene. The vertical dotted line shows the HOMO energy.

In order to study the role of GQD for hydrogen storage, the interaction of platinum with triangulene (C22H12) was studied first. To study the effect of the catalyst on loading and unloading of hydrogen on GQD, Pt atom was used with three different sites (bridge, hollow and top). Calculations with a single Pt atom over a triangulene were preformed first. Fig. 3(aec) presents top and side views of initial structures of triangulene with platinum atom. The platinum atom is placed at a distance of 2.50  A above all three sites of triangulene, triþPt (B), triþPt (H) and triþPt (T). No imaginary frequencies were found during the ground-state calculation, indicating that the optimized structure represents a true potential

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Fig. 3 e Initial structure of platinum adsorbed over triangulene including three different sites (a) bridge (b) hollow and (c) top for platinum.

energy surface (PES) minimum. For the platinum atom adsorbed on bridge site of triangulene i.e. between the CeC bonds, the platinum atom prefers to be with 9C carbon atom (numbering can be seen from the figure). The bond length between carbon and platinum atoms becomes 2.14  A which is similar to the Pt adatom on graphene [52]. Similar nature has been observed when platinum atom is placed on top site of triangulene with the similar bond length of 2.14  A. In both cases platinum atom prefers bridge site. For platinum adsorption on hollow site, platinum atom is adsorbed on 7C carbon atom with the bond length of 2.05  A. It is evident from Table 2 and Fig. 4(aec) that the platinum atom is chemically adsorbed on all three considered sites indicating that the open d-shell of platinum and two unpaired electrons of triangulene play important role in developing the interaction between them. Among all, triþPt (H) acquires highest adsorption energy with the value of 1.994 eV. However, other two sites triþPt (B) and triþPt (T) give similar adsorption energy with the value of 1.210 eV. This similar adsorption energy of triþPt (B) and triþPt(T) may be attributed to the fact that Pt prefers to be on bridge site which is in good agreement with the Pt adsorption on pristine graphene [52]. The adsorption energy of Pt over triangulene is slightly high in accordance with the energy of Pt over coronene [53]. Further, an important parameter of hydrogen storage system is the surface adsorption and dissociation of hydrogen with the storage matrix. Fig. 5(aec) shows the initial structure of platinum adsorbed triangulene with H2 molecule over all three considered sites. To investigate the interactions between H2 molecule and Pt adsorbed triangulene, we have placed H2 at the distance of 2.00  A. The optimized structures of platinum decorated triangulene H2 over it are presented in Fig. 6(aec). After the adsorption, the HeH bond length of triþPtþH2 (B), triþPtþH2 (H) and triþPtþH2 (T) is 1.90, 1.80 and 1.90  A respectively,

 We which is longer than its equilibrium distance of 0.74 A. found that the single platinum atom over all three considered sites is capable of adsorbing H2 molecule while completely dissociating it. Our study is in agreement with the other reported studies of H2 over platinum decorated carbon-based materials [28e31,53]. Analogous to the above adsorption of platinum on triangulene, triþPtþH2 (B) and triþPtþH2 (T) show same value of adsorption energies. However, the structural property is slightly different as the Pt with H2 is adsorbed on different sites, almost opposite in triþPtþH2 (B) and triþPtþH2 (T) cases. Moreover, this did not change the electronic properties and adsorption energies of triþPtþh (B) and triþPtþh (T). Basically, H2 adsorption over metal decorated carbonbased materials are categorized into two distinctive types; first is the Kubas adsorption (K-mode), which is nondissociative H2 adsorption [54] and secondly the dihydride adsorption in which H2 is dissociated (D-mode) [55]. Our study reveals the H2 dissociation with Pt decorated GQD. For d10 transition metals such as Ni, Pt, and Pd, several studies report the stability of H2 adsorption on carbon-based materials both in K- and D-modes, which further depends on position and configuration [55e58]. However, in our study, the H2 adsorption over Pt decorated triangulene with three different sites (bridge, hollow and top) results in D-mode. The adsorption energies are calculated through equation (2) and presented in Table 2. The calculated adsorption energies are 2.029 eV, 1.192 eV and 2.029 eV for triþPtþH2 (B), triþPtþH2 (H) and triþPtþH2 (T) respectively. The bridge and top site show same and highest adsorption energy due to their similar structural properties analogues with the adsorption of Pt over triangulene on both aforementioned sites. As a result, GQD is a good material for metal support in the H2 storage application. Fig. 7(aeb) shows the DOS of platinum decorated triangulene and H2 on platinum

Table 2 e Calculated HOMO, LUMO, bandgap (Eg) and adsorption energies (Ead) of platinum and hydrogen molecule adsorbed triangulene. System HOMO (eV) LUMO (eV) Eg (eV) Ead (eV)

triþPt (B)

triþPt (H)

triþPt (T)

triþPtþH2 (B)

triþPtþH2 (H)

triþPtþH2 (T)

5.614 2.844 2.320 1.210

5.418 3.352 2.060 1.994

5.612 2.844 2.310 1.210

5.770 3.140 2.630 2.029

5.690 3.160 2.530 1.192

5.770 3.140 2.630 2.029

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Fig. 4 e Optimized structure of platinum adsorbed triangulene including three different sites (a) bridge (b) hollow and (c) top for platinum.

Fig. 5 e Initial structure of hydrogen on platinum adsorbed triangulene including three different sites (a) bridge (b) hollow and (c) top for platinum.

decorated triangulene. In the case of triþPt (H) the formation of the bottom of conduction band is slightly higher than the contribution of orbitals from triþPt (B) and triþPt (T). Slight increase in the height of the curve is observed in the proximity of the HOMO level (dash lines). The triþPt (B) and triþPt (T) show the exact overlapped peaks of DOS due to their same structural properties leading in the similar electronic properties. Moreover, in case of H2 adsorption, triþPtþH2 (H) dominate the conduction and valence bands, while triþPtþH2 (B) and triþPtþH2 (T) show similar peaks. The dotted line in the Fig. 7(aeb) shows the HOMO levels, that are slightly shifted in case of triþPtþH2 (B) and triþPtþH2 (T). To better understand the electronic properties of interacting systems, we also show

the frontier molecular orbitals of hydrogen adsorbed systems and presented them in Fig. 8(aec). The HOMO orbitals of the triþPtþH2 (B), triþPtþH2 (H) and triþPtþH2 (T) are preferentially localized at the hydrogen atom, whereas the LUMO is delocalized and shared by both the Pt adsorbate and the triangulene. The distribution of the HOMO and LUMO for triþPtþH2 (B) is same as in the case of the triþPtþH2 (T). Due to the formation of equivalent PteH (1.55e1.56  A) bonds after interaction between Pt adsorbed triangulene with hydrogen molecule, the more homogeneous electron density distribution occurs and thus LUMO are delocalized over the systems. Therefore, it is noteworthy that our results show hydrogen storage with platinum decorated triangulene. Additionally, our results will help in

Please cite this article as: Sharma V et al., Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.021

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Fig. 6 e Optimized structure of hydrogen molecule on platinum adsorbed triangulene including three different sites (a) bridge (b) hollow and (c) top for platinum.

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Fig. 7 e Density of states (DOS) of (a) Pt on triangulene (b) hydrogen adsorption on triþPt with three different sites (bridge, hollow and top).

Fig. 8 e HOMO and LUMO plot of (a) triþPtþH2 (B) (b) triþPtþ H2 (H) and (c) triþPtþ H2 (T). Please cite this article as: Sharma V et al., Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.021

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understanding the next step for hydrogen storage that is H2 spillover mechanism in graphene quantum dots which is imperceptible in experiments.

[5]

Conclusions

[6]

The zero-dimensional graphene quantum dot, triangulene is considered to study their catalytic activity on HER through adsorption mechanism and electronic density of states calculations based on density functional theory. The band gap of triangulene calculated by HOMO-LUMO shows little change by the inclusion of hydrogen atom. The electronic properties calculations and adsorption energy for HER activity of triangulene show the optimal adsorption energy of hydrogen over top for HER activity. Present work helps us in concluding that triangulene can be a good electrocatalyst. Furthermore, we have analysed site dependent platinum decorated triangulene for hydrogen storage. The platinum adsorption on hollow site was found to be the most stable configuration and determined by considering other two bridge and top sites for adsorption. It is noteworthy that after the adsorption of platinum on top site, it prefers bridge site leading to the same structural, electronic properties and hydrogen storage. The complete study with site dependency shows that platinum decorated triangulene can be selected as an efficient hydrogen storage element. Moreover, our obtained results are not only the bottom line for a better understanding of the H2 spill over phenomena in graphene quantum dots but they are also advantageous for further design and synthesis of highly effective transition metal decorated GQDs for H2 storage.

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Acknowledgement Authors acknowledge the financial assistance from the Department of Science & Technology under the IndoePoland program of cooperation on science and technology through project DST/INT/POL/P-33/2016.

Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.021. [16]

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Please cite this article as: Sharma V et al., Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.021