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Ca and K decorated germanene as hydrogen storage: An ab initio study
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Ca and K decorated germanene as hydrogen storage: An ab initio study Kurt Irvin M. Rojas a,*, Al Rey C. Villagracia a,b,d, Joaquin Lorenzo Moreno a,b, Melanie David a,b, Nelson B. Arboleda Jr. a,b,c a
Physics Department, De La Salle University, Taft Ave., 1004 Manila, Philippines ANIMoS Research Unit, CENSER, Taft Ave., 1004 Manila, Philippines c ~ an, Laguna, Philippines DLSU-Science and Technology Complex, Bin d Department of Agricultural and Biosystems Engineering, The University of Arizona, Tucson, AZ 85721, USA b
article info
abstract
Article history:
The hydrogen storage capacity and performance of Ca and K decorated germanene were
Received 10 September 2017
studied using density functional theory calculation. The Ca and K adatoms were found to
Received in revised form
be sufficiently bonded to the germanene without clustering at the hollow site. Further
7 January 2018
investigation has shown an ionic bonding is apparent based on the charge density differ-
Accepted 13 January 2018
ence and Bader charge analysis. Upon adsorption of H2 on the decorated germanene, it was
Available online xxx
found that the Ca and K decorated systems could adsorb 8 and 9 H2 molecules, respectively.
Keywords:
435 meV), suggesting weak physisorption. The charge density profile revealed that the
Hydrogen storage
electron of H2 moved toward the adatom decoration without leaving the local region of H2.
Density functional theory
This suggests that a dipole-dipole interaction was apparent and consistent with the energy
Physisorption
range found. Finally, the gravimetric density obtained from the adsorption of H2 on the
The adsorption energies of H2 molecules were within the Van der Waals energy (400e
decorated germanene shows that this material is a potential for H2 storage media. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the impending depletion of fossil fuel reserves, significant effort has been expended in the development and implementation of technologies for sustainable alternative energy resources [1e4]. One of the potential alternatives is utilizing hydrogen as an energy source. Hydrogen is abundant in nature and when made into an energy carrier, it can be considered as a clean synthetic fuel. Hydrogen is being considered as an energy carrier due to its high gravimetric energy density, safety manageability, and environmentally benign nature of combustion [5e7]. Although the use of
hydrogen might seem ideal, there is still a lot of room for development in the production and storage aspects [8e11]. Between the two facets of hydrogen-based energy, the storage aspect poses the most significant impact in realizing the hydrogen industry. To compare, hydrocarbon and hydrogen can be stored at 1000 kg m3 and 71 kg m3 [7], respectively. Storage technologies such as pressurized gas tank and cryogenic liquid storage are already in development. However, the use of such technologies for mobile application poses safety and weight concerns [12e15]. An alternative storage can be constructed through material-based approach where the material stores the hydrogen by weak interactions. Chemisorption and
* Corresponding author. E-mail address: [email protected] (K.I.M. Rojas). https://doi.org/10.1016/j.ijhydene.2018.01.071 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Rojas KIM, et al., Ca and K decorated germanene as hydrogen storage: An ab initio study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.071
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physisorption are means that can be utilized for the attraction. However, chemisorption due its strong bonding poses a challenge on the extraction energy requirement of the stored hydrogen, which leaves physisorption to be of better approach [16]. The main advantage of physisorption mechanism is that it produces materials with lightweight property, and quick H2 recharge and extraction rate. On the other hand, physisorption method has the lowest hydrogen yield among the available storage methods [16]. In the material-based approach, investigations are typically done through density functional theory (DFT) simulations and experiments on bulk surfaces and nanostructures such as graphene, nanoribbon, nanoring, and fullerene. Graphene and single-walled carbon nanotubes, two nanostructures commonly known with the largest surface areas, yield hydrogen storage capacity below the standard set by the department of energy [16,17]. In response to this challenge, studies have been conducted in exploring possible functionalization of the materials to increase the hydrogen storage capacity [18e24]. It was shown that decoration and doping could indeed increase the hydrogen storage capacity of the material [25e30]. In decorated graphene studies, the metal decoration was observed to likely form a cluster due to lower binding energy as compared to its cohesive energy [31,32]. Upon further investigation, this cluster formation was found to decrease the hydrogen storage capacity by inhibiting
Fig. 1 e Geometry visualization of germanene from the top (a) and side (b) view. The binding sites for the Ca and K are shown in (c).
hydrogen approach and attraction. Thus, it was recommended that the decorations be in isolated states [33]. Since graphene cannot hold decorations in isolated states, silicene (a graphene allotrope made of Si) was preferred since it was found to keep the decorations bound in isolated states [31,34]. Furthermore, Na, Ca, Mg, and K decorated silicene was said to exceed the hydrogen storage capacity goal of 5.5 wt% [35]. Germanene, a Ge allotrope akin to silicene, has been recently synthesized on gold substrate [36]. Germanene has a similar structure with silicene but with a larger interatomic distance and buckling height [37]. Studies have been conducted to investigate the structural, electronic and hydrogen interaction aspects of germanene [37e41]. However, metal decorations and their capability in storing hydrogen are not yet extensively elucidated. Due to the high uptake potential of materials decorated with K and Ca [35,42,43], these two were chosen as the decoration for the germanene substrate. In this study, the various properties such as the structure, electronic nature, binding mechanism, and hydrogen storage capacity of the K and Ca decorated material were examined.
Methodology The study utilized spin-polarized density functional theory calculations through the Vienna Ab initio simulation package
Fig. 2 e The adsorption sites for the 1st H2 are labeled from A-F in (a). From the 2nd H2 onwards, there are more adsorption sites to be considered due to the removal of symmetry as an effect of the 1st H2. The adsorption sites for the 2nd H2 onwards is described by sites A-S of in (b). Two initial orientations were considered for approach of hydrogen, the parallel (left) and perpendicular (right) with respect to the Metal (M) decoration.
Please cite this article in press as: Rojas KIM, et al., Ca and K decorated germanene as hydrogen storage: An ab initio study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.071
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Table 1 e Calculated germanene configuration comparison. a ( A) Bond Buckling length ( A) height ( A) Trivedi et al. (2014) [37] Ye et al. (2014) [40] Li et al. (2014) [41] Calculated Average Standard deviation % Coefficient of variation
4.034 4.063 3.95 4.06 4.03 0.05 1.31
2.443 2.439 2.387 2.44 2.43 0.03 1.14
0.737 0.677 0.65 0.69 0.69 0.04 5.28
(VASP) to perform structural optimization, single point energy and electronic property calculations [44] for a non-empirical approach. The Perdew-Burke-Ernzerhof variant of generalized gradient approximation (GGA-PBE) was used for the exchange correlation potential [45,46] for an. Projectoraugmented-wave (PAW) potentials were used with 450 eV of cutoff energy. The Brillouin zone was sampled using a 5 5 1 Gamma centered grid for the structural calculations and 11 11 1 Gamma centered grid for the electronic property calculations. Energy convergence criterion was set to 104 eV and atomic relaxations were set to continue until forces on each atom were less than 0.02 eV A1. The Grimme method (DFT-D2) was used for the Van der Waals correction [47]. Dipole corrections were considered in the calculation.
Model of the system The system is a super cell composed of a 4 4 unit cell of germanene as shown in Fig. 1a. The super cell was projected infinitely along the x, y, and z axes and a vacuum of 15 A (Fig. 1b) was set to preserve the single layered model, resulting to an infinite single layer germanene sheet. This model could simulate a pristine germanene at the middle of a relatively large germanene sheet.
Decoration of germanene Each decoration was placed above the germanene surface on various distinct sites, namely, Hollow (H), sublattice A (valley, V), sublattice B (Top, T), and in-between sublattice A & B (Bridge, B) as shown in Fig. 1c. The system was allowed to move in the x, y, z directions. The amount of
Fig. 3 e The visualization of the final and most stable configuration of a Caedecorated and K-decorated germanene.
adsorption or sticking is described by the “binding” energy, EB defined as EB ¼ EGe þ EM EM=Ge
(1)
where EB , EGe , EM , and EM=Ge are the binding energy, pristine germanene energy, isolated metal decoration energy, and energy for metal decorated germanene, respectively.
H2 adsorption approach After determining the most stable decorated germanene configuration, the hydrogen molecule was placed on top of various distinct adsorption sites near the decoration (Fig. 2a and b). The systems were relaxed to its final configuration. The vertical and horizontal orientations of H2 were considered relative to the surface normal (Fig. 2c). This served as a key step in determining the true stable from the metastable
Table 2 e Adatom adsorption on germanene. This table shows: the binding energy (Eb) of the adatom on the considered sites, the cohesive energy (Ec), Eb/Ec ratio, the binding energy on graphene (Eb graphene) and silicene (Eb silicene), the adatom height h measured from the furthest Ge of sublattice B, the nearest distance between the adatom and Ge (DM-Ge), and the difference between the maximum and minimum z coordinates of Ge atoms in sublattice A (DzA) and B (DzA). Atom Site Eb (eV) Ec (eV) Eb/Ec (eV) Eb graphene (eV) Eb silicene (eV) h ( A) DM-Ge ( A) DzA (A) DzB (A) Ca
K
a b
Hol Top Val Hol Top Val
2.22 2.12 2.18 1.83 1.57 1.72
1.84a
1.21
0.63b
2.19b
0.93a
1.96
0.80b
1.90b
1.86 0.59 1.64 2.58 2.39 2.69
2.89, 3.26 3.06 3.36 3.42, 3.89 3.15 3.45
0.29 0.79 1.11 0.16 0.30 0.16
0.34 2.53 0.30 0.12 0.78 0.08
Kittle (2010) [50]. Lin & Ni (2012) [31].
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Eadspro ¼ EH2 þ EM=Ge=ðn1ÞH2 EM=Ge=nH2
(3)
Charge density difference and bader charge analysis The charge density difference (CDD) and Bader charge analysis (BCA) determined the change in electron population in a certain region in a visual and numerical value, respectively. The CDD (BCA) is the difference between the charge density (charge) of the combined and isolated systems. CDD and BCA are defined in Eqs. (4) and (5), respectively. These were used to determine the nature of the adsorption mechanism apparent in the system. Dr ¼ rAB rA rB
(4)
DQ ¼ QAB QA QB
(5)
where Dr and DQ are the changes in charge density and charge, respectively. Also rx and Qx denote the charge density and charge of system x. In the study, the systems of focus are the Germanene-adatom (AB), Germanene (A), and Adatom (B).
Results and discussion figures Structural properties of pristine germanene The lattice constant a, bond length and buckling height were 4.06 A, 2.44 A, and 0.69 A, respectively. The calculated results on the structural properties of germanene were found to be comparable with the results from similar studies as shown in Table 1.
Metal Decoration
Fig. 4 e The CDD image and atom labels are shown in (a) and (b). It shows the regions with electron migration as implied by the electron rich and poor region. For a clearer view, a 2D cut viewed from the side is presented in (c). In (c), the mechanism is more visually apparent to be ionic. (c) presents the data for low concentration (blue) and high concentration (red) gradient. (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.) configuration as investigated in our previous study [48]. To determine the true stable configuration, the average adsorption energy at the final configurations was compared to see the largest adsorption energy. The average adsorption energy Eadsave is defined as Eadsave ¼ nEH2 þ EM=Ge EM=Ge=nH2
n
(2)
where n is the number of H2 currently in the system and EM=Ge=nH2 is the energy of nH2 on decorated germanene. The addition of H2 was allowed if the adsorption energy of the single H2 being adsorbed was attractive and was within or greater than the Van der Waals energy range (40e435 meV) [49]. This is labeled in this paper as “progressive” adsorption energy, Eadspro , and expressed as
The measured values and image of the results for the relaxation of the system are presented in Table 2 and Fig. 3, respectively. The Ca and K were observed to have the largest binding energies at the hollow site of the germanene surface. The adatoms also bind more strongly on germanene than on graphene but behave very similar with silicene. The structure difference between a flat (graphene) and buckled (silicene/ germanene) honeycomb structure greatly affect the binding of decorations. Furthermore, the binding energy of the adatom on germanene was found to be greater than the cohesive energy [50] which signifies that the K and Ca is stable at that decoration concentration and is unlikely to form bulk cluster. To describe the binding mechanism of Ca and K on germanene, the CDD (Fig. 4a and b) and 2D plot (Fig. 4c) were plotted. As shown, there is an electron rich region near the primary Ge, while an electron poor region at the adatom (Ca & K). The charge transfer results (Table 3) confirmed that Ca and K lost 1.2e and 0.70e, respectively. This significant difference in electron density suggests a purely ionic bonding between the adatom and germanene. In comparing sublattices A and B primary Ge atoms (Figs. 1b, 4a and 4b), there is a higher concentration of electron on sublattice B primary Ge atom, which suggests a stronger bonding. This was also proven by the charge transfer results where the sublattice B charge gain is larger than that of sublattice A. A movement of electrons of
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Table 3 e Charge transfer of the adatom (on hollow) and the surrounding Ge thru BCA. Division
Adatom Primary Ge
Secondary Ge
DQ (e)
Atom
Adatom 1 Ge 12 Ge 13 Ge 14 Ge 19 Ge 20 Ge 21 Ge 11 Ge 6 Ge 15 Ge 22 Ge 27 Ge 18
GeCa
GeK
1.21 þ0.33 þ0.09 þ0.33 þ0.09 þ0.33 þ0.06 0.02 þ0.01 0.03 þ0.03 0.03 þ0.01
0.70 þ0.18 þ0.03 þ0.18 þ0.03 þ0.18 þ0.03 0.01 þ0.04 0.01 þ0.04 0.01 þ0.04
the secondary Ge was also apparent in the CDD image (Fig. 4a and b), which was due to the dipole-dipole interaction induced by the electron-rich or electron-poor region at the primary Ge atom and adatom, respectively. The total density of states of the bare germanene is presented in Fig. 5a. The up- and down-spin states, being symmetric, are degenerate for a given energy level, which suggests a non-magnetic character of the material. There was no gap present. However, the DOS is zero at the Fermi energy which suggests a zero-gap semi-metal material. The total DOS of K-decorated germanene (Fig. 5b) shows degenerate spin-up and spin down states for a given energy level that suggests a non-magnetic character. The available states at the Fermi energy level suggest that the material is metallic. The Dirac point from the earlier total DOS of bare germanene was shifted below fermi energy level has slightly opened up a gap of 34 meV upon decoration. The total DOS of Ca-decorated germanene (Fig. 5c) shows slightly non-degenerate (asymmetric) spin-up and spin-down states for a given energy level that suggests a slightly magnetic character. There were available states at the Fermi energy level, which suggests a metallic property of the material. The Dirac cone was similarly shifted below Fermi energy with a gap opening of 57 meV.
H2 adsorption on decorated germanene Upon multiple calculations for the various initial setups of H2, the configurations with the highest adsorption energy were determined and plotted upon addition of H2 (Fig. 6). For both decorations, decreasing trends in adsorption energy were observed as the number of H2 increased. This suggests that oversaturation of H2 is possible and evident when more H2 is added leading to an adsorption energy falling below minimum energy limit and a repulsive behavior. For these H2, the progressive adsorption energy falls within the Van der Waals energy range of interaction. The final configurations of H2 are presented in Fig. 7 as H2 is increased. The figures show that a K and Ca edecorated 4 4 germanene system can adsorb 9 and 8 H2, respectively. In
Fig. 5 e The density of states of (a) pristine, (b) K-decorated, and (c) Ca-decorated germanene.
Table 4, the adsorption profile of the final system with maximum H2 was presented. Here it was found that the progressive adsorption energy was near the borderline minimum of the Van der Waals energy range (400 meV). To describe the interaction in the perspective of electron migration, the CDD calculation was done on K and Ca edecorated germanene systems with one H2 each. The CDD results (Fig. 8) for both K and Ca systems have similar mode of interaction. The electrons of the H2 migrated in
Please cite this article in press as: Rojas KIM, et al., Ca and K decorated germanene as hydrogen storage: An ab initio study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.071
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Table 4 e H2 on K e decorated and Ca e decorated germanene adsorption profile. # H2 H2 h ( A) DH2-K DH-H Eads-prog Eads-ave ( A) ( A) (eV) (eV) Ge Adatom
Fig. 6 e Average adsorption energy.
the direction of the adatoms K and Ca. However, the electrons still remained within the local region of H2. This suggests that although the electron is attracted by the adatom, there is no apparent charge transfer, thus the nature of binding is a dipole-dipole attraction. This interpretation is supported by the adsorption energy within the Van der Waals range.
9 H2 on K-decorated germanene 2.70 0.43 2.87 1st H2 2.94 0.66 2.85 2nd H2 3rd H2 3.06 0.79 2.81 2.87 0.59 2.84 4th H2 3.27 1.00 2.84 5th H2 4.98 2.70 2.87 6th H2 5.33 3.06 4.26 7th H2 6.00 3.72 4.91 8th H2 6.39 4.11 6.59 9th H2 8 H2 on Ca-decorated germanene 2.65 1.06 2.46 1st H2 2.69 1.10 2.48 2nd H2 2.49 0.90 2.54 3rd H2 2.61 1.01 2.57 4th H2 4.21 2.61 2.62 5th H2 4.95 3.36 4.28 6th H2 5.16 3.57 4.56 7th H2 4.88 3.29 4.32 8th H2
0.7567 0.7564 0.7571 0.7563 0.7549 0.7523 0.7514 0.7508 0.7512 0.7706 0.7683 0.7667 0.7636 0.7558 0.7513 0.7511 0.7510
0.06
0.0931
0.04
0.1511
Fig. 8 e CDD of H2 on (a) K, and (b) Ca e decorated germanene.
Fig. 7 e Shows the final configuration for every addition of H2 on the Ca-decorated germanene (a ~ h) and K-decorated germanene (i ~ q).
With regards to the total DOS, the adsorption of H2 on Cadecorated germanene opened an energy gap of 11 meV (Fig. 9a) at the Fermi energy level. This is another interesting property that transpired in the adsorption of H2 on the Cadecorated germanene. The H2 transforms the Ca-decorated germanene from metallic nature to a semiconducting nature. Also, the formerly asymmetric form of the DOS is now symmetric and thus rendering the whole material a nonmagnetic. For the case of the H2 on K-decorated germanene (Fig. 9b), the non-magnetic and metallic nature does not change even with the adsorption of H2. In this study, only the coverage of 1 adatom per 32 Ge was considered and calculated. A conjecture was made such that the number of H2 does not decrease as the adatom coverage increases so that the storage capacity can be hypothetically determined with the 1:32 H2 storage as the basis. The H2 storage capacity in terms of gravimetric density is presented in Table 5. The results show that the obtained gravimetric density has surpassed the requirement of 5.5 wt% H2.
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Fig. 9 e The total DOS of H2 on Ca-decorated germanene (a) shows that it has a semiconductor and non-magnetic properties. On the other hand, the total DOS of H2 on K-decorated germanene (b) shows that it has metallic and non-magnetic properties.
Table 5 e Gravimetric density. Coverage
Single Isolated One-sided coverage Two-sided coverage
Adatom & H2 set: Ge ratio
1:32 16:32 (filled hollow) 32:32 (filled hollow)
Gravimetric density wt% Ca
K
0.68 8.01 12.52
0.76 8.96 13.97
Summary and conclusion Based on the results, K and Ca decoration were observed to prefer the hollow site with binding energies greater than their cohesive energies. This suggests that isolated decoration are stable on germanene as opposed of their stability in graphene. The binding of K and Ca transformed the germanene into a metallic material with non-magnetic and slightly magnetic nature, respectively. The bonding mechanism of the decoration with the substrate is of an ionic nature as described by the CDD and BCA. The adsorption of H2 on the decorated substance produced the following results: a. H2 on Ca-decorated germanene 1. Sufficiently large adsorption energy a. Electron of H2 moves towards Ca but remains within H2 2. Shift from metallic to semiconductor and from magnetic to non-magnetic 3. Achieved 8 H2 (surpasses Ca decorated silicene) b. H2 on K-decorated germanene a. Sufficiently large adsorption energy b. Electron of H2 moves towards K but remains within H2 c. Retains electronic and magnetic property d. Achieved 9 H2 (surpasses Ca decorated silicene) Finally, with the calculation of the gravimetric density, the one-sided and two-sided full coverage is at least 8 wt% and 12.5 wt % and as such can pass the hydrogen storage capacity requirement.
In further studies, other decorations can be used. Other variants of germanene or silicene similar to silicon carbide can also be explored. Also, it is equally important to explore the properties of the germanene with the substrate it was formed on.
Acknowledgment The calculations in this study were performed using the facilities of the High-Performance Computing Laboratory (HPCL) of the Center of Natural Sciences and Environmental Research (CENSER), De La Salle University (DLSU) and the Advance Nanomaterials Investigation & Molecular Simulations (ANIMoS) research unit, CENSER, DLSU. We would also like to thank the support of the Physics Department of DLSU through the Computational Materials Design Research Group (CMDRG).
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[29] Enriquez JIG, Moreno JLV, David MY, Arboleda NB, Lin OH, Villagracia ARC. DFT investigation on the electronic and water adsorption properties of pristine and N-Doped TiO2 nanotubes for photocatalytic water splitting applications. J Electron Mater 2017;46(6):3592e602. [30] Hong-Zhe P, Yong-Long W, Kai-Hua H, Ming-Zhen W, Yu O, Li C. First-principles study of hydrogen adsorption on titanium-decorated single-layer and bilayer graphenes. Chin Phys B 2013;22(6):67101. [31] Lin X, Ni J. Much stronger binding of metal adatoms to silicene than to graphene: a first-principles study. Phys Rev B 2012;86:1e9. [32] Nakada K, Ishii A. In: Gong Jian, editor. DFT calculation for adatom adsorption on graphene, graphene simulation. InTech; 2011. https://doi.org/10.5772/20477. [33] Sun Q, Wang Q, Jena P, Kawazoe Y. Clustering of Ti on a C 60 surface and its effect on hydrogen storage. J Am Chem Soc 2005;127:14582e3. [34] Sahin H, Peeters FM. Adsorption of alkali, alkaline earth and transition metal atoms on silicene. Phys Rev B 2013;87:1e9. [35] Wang Y, Zheng R, Gao H, Zhang J, Xu B, Sun Q, et al. Metal adatoms-decorated silicene as hydrogen storage media. Int J Hydrogen Energy 2014;39(26):14027e32. vila ME, Xian L, Cahangirov S, Rubio A, Le Lay G. [36] Da Germanene: a novel two-dimensional Germanium allotrope akin to Graphene and Silicene. N J Phys 2014;16:95002. [37] Trivedi S, Srivastava A, Kurchania R. Silicene and germanene: a first principle study of electronic structure and effect of hydrogenation-passivation. J Comput Theor Nanosci 2014;11:781e8. [38] Pang Q, Zhang Y, Zhang JM, Xu KW. Functionalization of lowdimensional honeycomb germanium with 3d transitionmetal atoms. Comput Mater Sci 2011;50(5):1717e24. € gl U. Stability of germanene under [39] Kaloni TP, Schwingenschlo tensile strain. Chem Phys Lett 2013;583:137e40. [40] Ye M, Quhe R, Zheng J, Ni Z, Wang Y, Yuan Y, et al. Tunable band gap in germanene by surface adsorption. Phys E 2014;59:60e5. [41] Kresse G, Furthmu¨ller J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a planewave basis set. Comput Mater Sci 1996;6:15e50. [42] Li SS, Zhang CW, Ji WX, Li F, Wang PJ, Hu SJ, et al. Tunable electronic and magnetic properties in germanene by alkali, alkaline-earth, group III and 3d transition metal atom adsorption. Phys Chem Chem Phys 2014;16(1):15968. [43] Khan S, Khan MS. Computational study of hydrogen adsorption on potassium-decorated boron nitride nanotubes. Nano 2011;1(2):103e10. [44] Beheshti E, Nojeh A, Servati P. A first-principles study of calcium-decorated, boron-doped graphene for high capacity hydrogen storage. Carbon N Y 2011;49(5):1561e7. [45] Kresse G, Furthmu¨ller J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev B 1996;54(16):11169e86. [46] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77(18):3865e8. [47] Grimme S. Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J Comput Chem 2006;27(15):1787e99. [48] Rojas KI, Villagracia AR, Arboleda NB. H2 adsorption on K decorated germanene surface: an ab-initio investigation. Mater Res Express 2016;3(11):115015. [49] Lee L. The chemistry and Physics of solid adhesion. In: Lee Lieng-Huang, editor. Fundamentals of Adhesion. New York: Springer; 1991. p. 1e86. [50] Kittel C. Introduction to solid state physics. 8th ed. New York: Wiley; 2010. p. 49e85.
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Ca and K decorated germanene as hydrogen storage: An ab initio study Kurt Irvin M. Rojas a,*, Al Rey C. Villagracia a,b,d, Joaquin Lorenzo Moreno a,b, Melanie David a,b, Nelson B. Arboleda Jr. a,b,c a
Physics Department, De La Salle University, Taft Ave., 1004 Manila, Philippines ANIMoS Research Unit, CENSER, Taft Ave., 1004 Manila, Philippines c ~ an, Laguna, Philippines DLSU-Science and Technology Complex, Bin d Department of Agricultural and Biosystems Engineering, The University of Arizona, Tucson, AZ 85721, USA b
article info
abstract
Article history:
The hydrogen storage capacity and performance of Ca and K decorated germanene were
Received 10 September 2017
studied using density functional theory calculation. The Ca and K adatoms were found to
Received in revised form
be sufficiently bonded to the germanene without clustering at the hollow site. Further
7 January 2018
investigation has shown an ionic bonding is apparent based on the charge density differ-
Accepted 13 January 2018
ence and Bader charge analysis. Upon adsorption of H2 on the decorated germanene, it was
Available online xxx
found that the Ca and K decorated systems could adsorb 8 and 9 H2 molecules, respectively.
Keywords:
435 meV), suggesting weak physisorption. The charge density profile revealed that the
Hydrogen storage
electron of H2 moved toward the adatom decoration without leaving the local region of H2.
Density functional theory
This suggests that a dipole-dipole interaction was apparent and consistent with the energy
Physisorption
range found. Finally, the gravimetric density obtained from the adsorption of H2 on the
The adsorption energies of H2 molecules were within the Van der Waals energy (400e
decorated germanene shows that this material is a potential for H2 storage media. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the impending depletion of fossil fuel reserves, significant effort has been expended in the development and implementation of technologies for sustainable alternative energy resources [1e4]. One of the potential alternatives is utilizing hydrogen as an energy source. Hydrogen is abundant in nature and when made into an energy carrier, it can be considered as a clean synthetic fuel. Hydrogen is being considered as an energy carrier due to its high gravimetric energy density, safety manageability, and environmentally benign nature of combustion [5e7]. Although the use of
hydrogen might seem ideal, there is still a lot of room for development in the production and storage aspects [8e11]. Between the two facets of hydrogen-based energy, the storage aspect poses the most significant impact in realizing the hydrogen industry. To compare, hydrocarbon and hydrogen can be stored at 1000 kg m3 and 71 kg m3 [7], respectively. Storage technologies such as pressurized gas tank and cryogenic liquid storage are already in development. However, the use of such technologies for mobile application poses safety and weight concerns [12e15]. An alternative storage can be constructed through material-based approach where the material stores the hydrogen by weak interactions. Chemisorption and
* Corresponding author. E-mail address: [email protected] (K.I.M. Rojas). https://doi.org/10.1016/j.ijhydene.2018.01.071 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Rojas KIM, et al., Ca and K decorated germanene as hydrogen storage: An ab initio study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.071
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physisorption are means that can be utilized for the attraction. However, chemisorption due its strong bonding poses a challenge on the extraction energy requirement of the stored hydrogen, which leaves physisorption to be of better approach [16]. The main advantage of physisorption mechanism is that it produces materials with lightweight property, and quick H2 recharge and extraction rate. On the other hand, physisorption method has the lowest hydrogen yield among the available storage methods [16]. In the material-based approach, investigations are typically done through density functional theory (DFT) simulations and experiments on bulk surfaces and nanostructures such as graphene, nanoribbon, nanoring, and fullerene. Graphene and single-walled carbon nanotubes, two nanostructures commonly known with the largest surface areas, yield hydrogen storage capacity below the standard set by the department of energy [16,17]. In response to this challenge, studies have been conducted in exploring possible functionalization of the materials to increase the hydrogen storage capacity [18e24]. It was shown that decoration and doping could indeed increase the hydrogen storage capacity of the material [25e30]. In decorated graphene studies, the metal decoration was observed to likely form a cluster due to lower binding energy as compared to its cohesive energy [31,32]. Upon further investigation, this cluster formation was found to decrease the hydrogen storage capacity by inhibiting
Fig. 1 e Geometry visualization of germanene from the top (a) and side (b) view. The binding sites for the Ca and K are shown in (c).
hydrogen approach and attraction. Thus, it was recommended that the decorations be in isolated states [33]. Since graphene cannot hold decorations in isolated states, silicene (a graphene allotrope made of Si) was preferred since it was found to keep the decorations bound in isolated states [31,34]. Furthermore, Na, Ca, Mg, and K decorated silicene was said to exceed the hydrogen storage capacity goal of 5.5 wt% [35]. Germanene, a Ge allotrope akin to silicene, has been recently synthesized on gold substrate [36]. Germanene has a similar structure with silicene but with a larger interatomic distance and buckling height [37]. Studies have been conducted to investigate the structural, electronic and hydrogen interaction aspects of germanene [37e41]. However, metal decorations and their capability in storing hydrogen are not yet extensively elucidated. Due to the high uptake potential of materials decorated with K and Ca [35,42,43], these two were chosen as the decoration for the germanene substrate. In this study, the various properties such as the structure, electronic nature, binding mechanism, and hydrogen storage capacity of the K and Ca decorated material were examined.
Methodology The study utilized spin-polarized density functional theory calculations through the Vienna Ab initio simulation package
Fig. 2 e The adsorption sites for the 1st H2 are labeled from A-F in (a). From the 2nd H2 onwards, there are more adsorption sites to be considered due to the removal of symmetry as an effect of the 1st H2. The adsorption sites for the 2nd H2 onwards is described by sites A-S of in (b). Two initial orientations were considered for approach of hydrogen, the parallel (left) and perpendicular (right) with respect to the Metal (M) decoration.
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Table 1 e Calculated germanene configuration comparison. a ( A) Bond Buckling length ( A) height ( A) Trivedi et al. (2014) [37] Ye et al. (2014) [40] Li et al. (2014) [41] Calculated Average Standard deviation % Coefficient of variation
4.034 4.063 3.95 4.06 4.03 0.05 1.31
2.443 2.439 2.387 2.44 2.43 0.03 1.14
0.737 0.677 0.65 0.69 0.69 0.04 5.28
(VASP) to perform structural optimization, single point energy and electronic property calculations [44] for a non-empirical approach. The Perdew-Burke-Ernzerhof variant of generalized gradient approximation (GGA-PBE) was used for the exchange correlation potential [45,46] for an. Projectoraugmented-wave (PAW) potentials were used with 450 eV of cutoff energy. The Brillouin zone was sampled using a 5 5 1 Gamma centered grid for the structural calculations and 11 11 1 Gamma centered grid for the electronic property calculations. Energy convergence criterion was set to 104 eV and atomic relaxations were set to continue until forces on each atom were less than 0.02 eV A1. The Grimme method (DFT-D2) was used for the Van der Waals correction [47]. Dipole corrections were considered in the calculation.
Model of the system The system is a super cell composed of a 4 4 unit cell of germanene as shown in Fig. 1a. The super cell was projected infinitely along the x, y, and z axes and a vacuum of 15 A (Fig. 1b) was set to preserve the single layered model, resulting to an infinite single layer germanene sheet. This model could simulate a pristine germanene at the middle of a relatively large germanene sheet.
Decoration of germanene Each decoration was placed above the germanene surface on various distinct sites, namely, Hollow (H), sublattice A (valley, V), sublattice B (Top, T), and in-between sublattice A & B (Bridge, B) as shown in Fig. 1c. The system was allowed to move in the x, y, z directions. The amount of
Fig. 3 e The visualization of the final and most stable configuration of a Caedecorated and K-decorated germanene.
adsorption or sticking is described by the “binding” energy, EB defined as EB ¼ EGe þ EM EM=Ge
(1)
where EB , EGe , EM , and EM=Ge are the binding energy, pristine germanene energy, isolated metal decoration energy, and energy for metal decorated germanene, respectively.
H2 adsorption approach After determining the most stable decorated germanene configuration, the hydrogen molecule was placed on top of various distinct adsorption sites near the decoration (Fig. 2a and b). The systems were relaxed to its final configuration. The vertical and horizontal orientations of H2 were considered relative to the surface normal (Fig. 2c). This served as a key step in determining the true stable from the metastable
Table 2 e Adatom adsorption on germanene. This table shows: the binding energy (Eb) of the adatom on the considered sites, the cohesive energy (Ec), Eb/Ec ratio, the binding energy on graphene (Eb graphene) and silicene (Eb silicene), the adatom height h measured from the furthest Ge of sublattice B, the nearest distance between the adatom and Ge (DM-Ge), and the difference between the maximum and minimum z coordinates of Ge atoms in sublattice A (DzA) and B (DzA). Atom Site Eb (eV) Ec (eV) Eb/Ec (eV) Eb graphene (eV) Eb silicene (eV) h ( A) DM-Ge ( A) DzA (A) DzB (A) Ca
K
a b
Hol Top Val Hol Top Val
2.22 2.12 2.18 1.83 1.57 1.72
1.84a
1.21
0.63b
2.19b
0.93a
1.96
0.80b
1.90b
1.86 0.59 1.64 2.58 2.39 2.69
2.89, 3.26 3.06 3.36 3.42, 3.89 3.15 3.45
0.29 0.79 1.11 0.16 0.30 0.16
0.34 2.53 0.30 0.12 0.78 0.08
Kittle (2010) [50]. Lin & Ni (2012) [31].
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Eadspro ¼ EH2 þ EM=Ge=ðn1ÞH2 EM=Ge=nH2
(3)
Charge density difference and bader charge analysis The charge density difference (CDD) and Bader charge analysis (BCA) determined the change in electron population in a certain region in a visual and numerical value, respectively. The CDD (BCA) is the difference between the charge density (charge) of the combined and isolated systems. CDD and BCA are defined in Eqs. (4) and (5), respectively. These were used to determine the nature of the adsorption mechanism apparent in the system. Dr ¼ rAB rA rB
(4)
DQ ¼ QAB QA QB
(5)
where Dr and DQ are the changes in charge density and charge, respectively. Also rx and Qx denote the charge density and charge of system x. In the study, the systems of focus are the Germanene-adatom (AB), Germanene (A), and Adatom (B).
Results and discussion figures Structural properties of pristine germanene The lattice constant a, bond length and buckling height were 4.06 A, 2.44 A, and 0.69 A, respectively. The calculated results on the structural properties of germanene were found to be comparable with the results from similar studies as shown in Table 1.
Metal Decoration
Fig. 4 e The CDD image and atom labels are shown in (a) and (b). It shows the regions with electron migration as implied by the electron rich and poor region. For a clearer view, a 2D cut viewed from the side is presented in (c). In (c), the mechanism is more visually apparent to be ionic. (c) presents the data for low concentration (blue) and high concentration (red) gradient. (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.) configuration as investigated in our previous study [48]. To determine the true stable configuration, the average adsorption energy at the final configurations was compared to see the largest adsorption energy. The average adsorption energy Eadsave is defined as Eadsave ¼ nEH2 þ EM=Ge EM=Ge=nH2
n
(2)
where n is the number of H2 currently in the system and EM=Ge=nH2 is the energy of nH2 on decorated germanene. The addition of H2 was allowed if the adsorption energy of the single H2 being adsorbed was attractive and was within or greater than the Van der Waals energy range (40e435 meV) [49]. This is labeled in this paper as “progressive” adsorption energy, Eadspro , and expressed as
The measured values and image of the results for the relaxation of the system are presented in Table 2 and Fig. 3, respectively. The Ca and K were observed to have the largest binding energies at the hollow site of the germanene surface. The adatoms also bind more strongly on germanene than on graphene but behave very similar with silicene. The structure difference between a flat (graphene) and buckled (silicene/ germanene) honeycomb structure greatly affect the binding of decorations. Furthermore, the binding energy of the adatom on germanene was found to be greater than the cohesive energy [50] which signifies that the K and Ca is stable at that decoration concentration and is unlikely to form bulk cluster. To describe the binding mechanism of Ca and K on germanene, the CDD (Fig. 4a and b) and 2D plot (Fig. 4c) were plotted. As shown, there is an electron rich region near the primary Ge, while an electron poor region at the adatom (Ca & K). The charge transfer results (Table 3) confirmed that Ca and K lost 1.2e and 0.70e, respectively. This significant difference in electron density suggests a purely ionic bonding between the adatom and germanene. In comparing sublattices A and B primary Ge atoms (Figs. 1b, 4a and 4b), there is a higher concentration of electron on sublattice B primary Ge atom, which suggests a stronger bonding. This was also proven by the charge transfer results where the sublattice B charge gain is larger than that of sublattice A. A movement of electrons of
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Table 3 e Charge transfer of the adatom (on hollow) and the surrounding Ge thru BCA. Division
Adatom Primary Ge
Secondary Ge
DQ (e)
Atom
Adatom 1 Ge 12 Ge 13 Ge 14 Ge 19 Ge 20 Ge 21 Ge 11 Ge 6 Ge 15 Ge 22 Ge 27 Ge 18
GeCa
GeK
1.21 þ0.33 þ0.09 þ0.33 þ0.09 þ0.33 þ0.06 0.02 þ0.01 0.03 þ0.03 0.03 þ0.01
0.70 þ0.18 þ0.03 þ0.18 þ0.03 þ0.18 þ0.03 0.01 þ0.04 0.01 þ0.04 0.01 þ0.04
the secondary Ge was also apparent in the CDD image (Fig. 4a and b), which was due to the dipole-dipole interaction induced by the electron-rich or electron-poor region at the primary Ge atom and adatom, respectively. The total density of states of the bare germanene is presented in Fig. 5a. The up- and down-spin states, being symmetric, are degenerate for a given energy level, which suggests a non-magnetic character of the material. There was no gap present. However, the DOS is zero at the Fermi energy which suggests a zero-gap semi-metal material. The total DOS of K-decorated germanene (Fig. 5b) shows degenerate spin-up and spin down states for a given energy level that suggests a non-magnetic character. The available states at the Fermi energy level suggest that the material is metallic. The Dirac point from the earlier total DOS of bare germanene was shifted below fermi energy level has slightly opened up a gap of 34 meV upon decoration. The total DOS of Ca-decorated germanene (Fig. 5c) shows slightly non-degenerate (asymmetric) spin-up and spin-down states for a given energy level that suggests a slightly magnetic character. There were available states at the Fermi energy level, which suggests a metallic property of the material. The Dirac cone was similarly shifted below Fermi energy with a gap opening of 57 meV.
H2 adsorption on decorated germanene Upon multiple calculations for the various initial setups of H2, the configurations with the highest adsorption energy were determined and plotted upon addition of H2 (Fig. 6). For both decorations, decreasing trends in adsorption energy were observed as the number of H2 increased. This suggests that oversaturation of H2 is possible and evident when more H2 is added leading to an adsorption energy falling below minimum energy limit and a repulsive behavior. For these H2, the progressive adsorption energy falls within the Van der Waals energy range of interaction. The final configurations of H2 are presented in Fig. 7 as H2 is increased. The figures show that a K and Ca edecorated 4 4 germanene system can adsorb 9 and 8 H2, respectively. In
Fig. 5 e The density of states of (a) pristine, (b) K-decorated, and (c) Ca-decorated germanene.
Table 4, the adsorption profile of the final system with maximum H2 was presented. Here it was found that the progressive adsorption energy was near the borderline minimum of the Van der Waals energy range (400 meV). To describe the interaction in the perspective of electron migration, the CDD calculation was done on K and Ca edecorated germanene systems with one H2 each. The CDD results (Fig. 8) for both K and Ca systems have similar mode of interaction. The electrons of the H2 migrated in
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Table 4 e H2 on K e decorated and Ca e decorated germanene adsorption profile. # H2 H2 h ( A) DH2-K DH-H Eads-prog Eads-ave ( A) ( A) (eV) (eV) Ge Adatom
Fig. 6 e Average adsorption energy.
the direction of the adatoms K and Ca. However, the electrons still remained within the local region of H2. This suggests that although the electron is attracted by the adatom, there is no apparent charge transfer, thus the nature of binding is a dipole-dipole attraction. This interpretation is supported by the adsorption energy within the Van der Waals range.
9 H2 on K-decorated germanene 2.70 0.43 2.87 1st H2 2.94 0.66 2.85 2nd H2 3rd H2 3.06 0.79 2.81 2.87 0.59 2.84 4th H2 3.27 1.00 2.84 5th H2 4.98 2.70 2.87 6th H2 5.33 3.06 4.26 7th H2 6.00 3.72 4.91 8th H2 6.39 4.11 6.59 9th H2 8 H2 on Ca-decorated germanene 2.65 1.06 2.46 1st H2 2.69 1.10 2.48 2nd H2 2.49 0.90 2.54 3rd H2 2.61 1.01 2.57 4th H2 4.21 2.61 2.62 5th H2 4.95 3.36 4.28 6th H2 5.16 3.57 4.56 7th H2 4.88 3.29 4.32 8th H2
0.7567 0.7564 0.7571 0.7563 0.7549 0.7523 0.7514 0.7508 0.7512 0.7706 0.7683 0.7667 0.7636 0.7558 0.7513 0.7511 0.7510
0.06
0.0931
0.04
0.1511
Fig. 8 e CDD of H2 on (a) K, and (b) Ca e decorated germanene.
Fig. 7 e Shows the final configuration for every addition of H2 on the Ca-decorated germanene (a ~ h) and K-decorated germanene (i ~ q).
With regards to the total DOS, the adsorption of H2 on Cadecorated germanene opened an energy gap of 11 meV (Fig. 9a) at the Fermi energy level. This is another interesting property that transpired in the adsorption of H2 on the Cadecorated germanene. The H2 transforms the Ca-decorated germanene from metallic nature to a semiconducting nature. Also, the formerly asymmetric form of the DOS is now symmetric and thus rendering the whole material a nonmagnetic. For the case of the H2 on K-decorated germanene (Fig. 9b), the non-magnetic and metallic nature does not change even with the adsorption of H2. In this study, only the coverage of 1 adatom per 32 Ge was considered and calculated. A conjecture was made such that the number of H2 does not decrease as the adatom coverage increases so that the storage capacity can be hypothetically determined with the 1:32 H2 storage as the basis. The H2 storage capacity in terms of gravimetric density is presented in Table 5. The results show that the obtained gravimetric density has surpassed the requirement of 5.5 wt% H2.
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Fig. 9 e The total DOS of H2 on Ca-decorated germanene (a) shows that it has a semiconductor and non-magnetic properties. On the other hand, the total DOS of H2 on K-decorated germanene (b) shows that it has metallic and non-magnetic properties.
Table 5 e Gravimetric density. Coverage
Single Isolated One-sided coverage Two-sided coverage
Adatom & H2 set: Ge ratio
1:32 16:32 (filled hollow) 32:32 (filled hollow)
Gravimetric density wt% Ca
K
0.68 8.01 12.52
0.76 8.96 13.97
Summary and conclusion Based on the results, K and Ca decoration were observed to prefer the hollow site with binding energies greater than their cohesive energies. This suggests that isolated decoration are stable on germanene as opposed of their stability in graphene. The binding of K and Ca transformed the germanene into a metallic material with non-magnetic and slightly magnetic nature, respectively. The bonding mechanism of the decoration with the substrate is of an ionic nature as described by the CDD and BCA. The adsorption of H2 on the decorated substance produced the following results: a. H2 on Ca-decorated germanene 1. Sufficiently large adsorption energy a. Electron of H2 moves towards Ca but remains within H2 2. Shift from metallic to semiconductor and from magnetic to non-magnetic 3. Achieved 8 H2 (surpasses Ca decorated silicene) b. H2 on K-decorated germanene a. Sufficiently large adsorption energy b. Electron of H2 moves towards K but remains within H2 c. Retains electronic and magnetic property d. Achieved 9 H2 (surpasses Ca decorated silicene) Finally, with the calculation of the gravimetric density, the one-sided and two-sided full coverage is at least 8 wt% and 12.5 wt % and as such can pass the hydrogen storage capacity requirement.
In further studies, other decorations can be used. Other variants of germanene or silicene similar to silicon carbide can also be explored. Also, it is equally important to explore the properties of the germanene with the substrate it was formed on.
Acknowledgment The calculations in this study were performed using the facilities of the High-Performance Computing Laboratory (HPCL) of the Center of Natural Sciences and Environmental Research (CENSER), De La Salle University (DLSU) and the Advance Nanomaterials Investigation & Molecular Simulations (ANIMoS) research unit, CENSER, DLSU. We would also like to thank the support of the Physics Department of DLSU through the Computational Materials Design Research Group (CMDRG).
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Please cite this article in press as: Rojas KIM, et al., Ca and K decorated germanene as hydrogen storage: An ab initio study, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.071