ARTICLE pubs.acs.org/JPCC
Theoretical Prediction of Hydrogen Storage on ZnO Sheet H. Si,† L. J. Peng,§ James R. Morris,‡,§ and B. C. Pan*,† †
Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China ‡ Materials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6115, United States § Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-2200, United States ABSTRACT: Using first-principles calculations, we investigate the adsorption behavior of hydrogen on the planar hexagonal ZnO sheet. Our calculations find that the planar ZnO monolayer preferably adsorbs hydrogen molecules, where a hydrogen molecule attaches to one oxygen atom with binding energy of ∼0.13 eV. This implies that the interaction between a hydrogen molecule and the ZnO sheet is stronger than that between a hydrogen molecule and graphene. We predict that each oxygen atom in a ZnO sheet can adsorb two hydrogen molecules on opposite sides of the sheet, and thus the gravimetric density for hydrogen storage on ZnO sheet is evaluated to be about 4.7 wt % at zero temperature. Furthermore, our calculations show that the gravimetric density for hydrogen storage on ZnO sheet reaches 1.52.1 wt % at 298 K and 5 MPa. This suggests that, despite their weight, ZnO sheets may have potential applications in hydrogen storage.
1. INTRODUCTION Hydrogen, unlike the fossil fuels, is an ideal renewable energy carrier. However, one of the major bottlenecks for applications of hydrogen as an energy carrier is the lack of appropriate storage materials, in which hydrogen should be stored with high gravimetric and volumetric densities, as well as being released easily under moderate conditions. So far, considerable efforts have been made to explore appropriate hydrogen storage materials. For examples, scientists found that some metals and metallic alloys might serve for storage of hydrogen, where the gravimetric densities of hydrogen storage reached high values (for example, 7.6 wt % for Mg).1,2 In these cases, however, the atomic hydrogen atoms were strongly bonded with some of the host atoms, so that high temperatures3 were required for the desorption of hydrogen from the materials. Therefore, the materials with strong chemical bonds between atomic hydrogen and host atoms are not efficient for hydrogen storage in practice. Alternatively, graphene,47 a two-dimensional (2D) planar hexagonal honeycomb monolayer with a large surface area, may serve for hydrogen storage. Unlike the above materials that bond with atomic hydrogen chemically, graphene may catch hydrogen molecules via the van der Waals interaction. Previous research revealed that the adsorption energy of hydrogen molecules on graphene was about 8090 meV/H2.811 Such small energy values imply that a hydrogen molecule may stay on the graphene at very low temperature only; if the system is heated slightly, the adsorbed hydrogen molecules on the graphene may escape. As a consequence, the hydrogen uptake of graphene is practical at very low temperature only. r 2011 American Chemical Society
More recently, first-principles calculations by Freeman et al.12 revealed that ultrathin films with surface orientation along (0001) or (0001) direction of wurtzite semiconductors preferred to adopt graphitic sheets. Similarly, a planar graphene-like configuration of ZnO was also predicted by other theoretical groups.13,14 Later on, these theoretical predictions were experimentally confirmed.15 Structurally, a perfect ZnO sheet features the planar honeycomb lattice configuration, with an alternative arrangement of zinc and oxygen atoms. Notice that the two kinds of atoms have different electronegativity, and thus a significant fraction of electrons are transferred from zinc atoms to oxygen atoms, unlike the purely carbon structure graphene. We speculate that the negatively charged oxygen atom may polarize the adsorbed hydrogen molecules and cause much stronger physisorption of hydrogen molecules on the ZnO sheet or even catalyze the adsorbed H2 into atomic H. If the former case is true, a hydrogen molecule adsorbing on the ZnO sheet is more stable than that on graphene, potentially improving the storage of hydrogen via ZnO sheets. To evaluate this idea, it is necessary to investigate the interaction of either a hydrogen molecule or a hydrogen atom with the ZnO sheet, followed by examining the capacity of hydrogen storage for ZnO sheet. In the present work, by performing calculations at the level of the density functional theory, we find that the ZnO sheet preferably adsorbs molecular hydrogen rather than atomic hydrogen. The adsorbed hydrogen molecule can be desorbed from the sheet with Received: November 30, 2010 Revised: March 22, 2011 Published: April 20, 2011 9053
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Table 1. Calculated Energies of the Relaxed Six Adsorption Sites for Two Individual Hydrogen Atoms on ZnO Sheeta initial configuration
final configuration
relative energy (eV)
A-B1 A-B2 A-B3 A-B4 A-B5 A-B6
molecule atomic molecule atomic atomic atomic
0 0.72 0 0.30 1.31 1.41
a
The energy of A-B1 is taken as a reference. Here, A and Bi (i = 16) refer to Figure 1.
Figure 1. (Top view) Six possible initial adsorption sites of two isolated hydrogen atoms on the ZnO sheet. A and B represent the adsorption sites of the first and second H atom, respectively.
an adsorption energy of 0.110.13 eV. More importantly, the simulated gravimetric density for hydrogen storage on ZnO sheet reaches 1.52.1 wt % at 298 K and 5 MPa, which is significantly better than most current materials. Given the significant weight increase of ZnO compared to carbon, this improved gravimetric uptake is particularly of interest.
2. COMPUTATIONAL DETAILS In our calculations, density functional theory (DFT) implanted in the SIESTA package1618 is employed for structural optimization and total energy calculations. The electron exchangecorrelation potential is treated within the local density approximation (LDA). The norm-conserving pseudopotentials generated via the TroullierMartins scheme19 are used to describe the interaction of valence electrons, which are expressed in fully separable form developed by Kleiman and Bylander.20,21 Double-ζ basis sets plus polarization orbitals (DZP) are utilized for Zn and O atoms, and double-ζ basis sets (DZ) are used for H atoms. The local density approximation (LDA) with the exchangecorrelation potential in the form of PerdewZunger (PZ)22 is adopted. A supercell consisting of 5 5 unit cells of perfect ZnO sheet is taken into account to simulate an infinite ZnO sheet, in which a vacuum region of 20 Å perpendicular to the plan of the sheet is employed. Such a large vacuum region can avoid the interaction between images caused from the periodic boundary condition. The Brillouin zone is sampled with a 6 6 1 MonkhorstPack grid. The conjugate gradient (CG) algorithm is adopted to fully relax the structures until the residual force acting on each atom is no more than 0.02 eV/Å. The optimal length of ZnO bond for the perfect sheet in our calculations is 1.90 Å, which is slightly shorter than the corresponding one in the ZnO bulk.23 3. ADSORPTION OF HYDROGEN ON A ZNO SHEET We first study the adsorption behavior of the isolated hydrogen atoms on the planar ZnO sheet. Initially two isolated hydrogen atoms are respectively placed above a Zn atom and an O atom which are the first-nearest (A and B1 in Figure 1) or the second-nearest (A and B3 in Figure 1) neighbors to each other. After optimization, the two separate hydrogen atoms may spontaneously form one hydrogen molecule, and the formed hydrogen molecule attaches to the oxygen atom, as shown in
Figure 2. (Top view) Eight possible initial adsorption sites of a single hydrogen molecule on ZnO sheet.
Figure 3a,b. However, when the two individual hydrogen atoms are respectively placed on two different kinds of atoms that are beyond second-nearest neighbors or on two oxygen atoms, these two hydrogen atoms do not spontaneously move together. Instead, they stay in their initial sites. Table 1 lists the calculated relative energies of the six typical cases. Apparently, one hydrogen molecule adsorbing on the sheet is more stable than two isolated hydrogen atoms on the sheet. To go further, we examine the adsorption behavior of individual hydrogen molecules on different sites of the planar ZnO sheet. For the ZnO sheet, there are four typical sites: atop a Zn atom, atop an O site, the bridge site between ZnO bond, and the hollow site in each hexagonal ring. When the HH bond of a single hydrogen molecule parallel or perpendicular to the plane of the ZnO sheet is considered, there are eight possible initial configurations for a single hydrogen molecule adsorbing on the ZnO sheet, as displayed in Figure 2. Namely, the HH bond is (I) perpendicular to the plane on top of an O atom, (II) perpendicular to the plane on top of a Zn atom, (III) perpendicular to the plane above the center of ZnO bond, (IV) parallel to the ZnO bond, (V) perpendicular to the ZnO bond, (VI) perpendicular to the plane above the center of a hexagon, (VII) parallel to a diagonal line of a hexagon, or (VIII) rotated by 30° with respect to case VII. It is found that most initial configurations do not remain after full relaxations. Instead, the orientation of each adsorbed hydrogen molecule is adjusted so that one hydrogen atom in the hydrogen molecule attaches to an oxygen atom only. Here, the hydrogen molecule bonds with the oxygen atom almost collinearly (Figure 4), with a vertical distance of around 2.10 Å between the H2 and the oxygen atom. To estimate the bonding strength between a H2 molecule and the O atom, we compute the binding energy. 9054
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Figure 3. Three optimal structures for a single hydrogen molecule adsorbing on the ZnO sheet. The left panels are the top views, and the right panels are the side views.
It is noted that the ZnO sheet interacting with hydrogen molecules through the van der Waals interaction cannot be precisely described by LDA or the generalized gradient approximation (GGA) functional.24 However, in some cases, LDA can be used as a reasonable approximation for the van der Waals interaction systems through fortuitous cancelations of errors between exchange and correlation energy.25,26 For example, the interlayer binding energy of graphite system is underestimated by 50% at the level of LDA, whereas GGA calculations fail to predict the binding ground state for graphite.27 According to this finding, it is expected that LDA calculations are able to predict the physical properties of hydrogen molecule adsorbing on ZnO sheet qualitatively. On the other hand, the localized atomic orbital basis sets used for the H2sheet system are larger than the corresponding ones for both the isolated sheet and the hydrogen molecule, which may cause an error in the binding energy.28 So in our calculations, the binding energy containing the basis-set superposition error (BSSE) correction29 is considered as below: Eb ¼ Etotal ðsheet þ H2 Þ ½Etotal ðsheetghost þ H2 Þ þ Etotal ðsheet þ H2ghost Þ
ð1Þ
The “ghost” atoms represent the additional basis sets centered at the atomic position of the ZnO sheet or hydrogen molecules without any atomic potential. As shown in Figure 4, for the most stable cases, each binding energy shows much stronger interaction between the hydrogen molecule and the ZnO sheet than that between the hydrogen molecule and the graphene sheet. We stress that the evaluated binding energies above are just in the range of 0.10.2 eV/H2, satisfying the desired values for the hydrogen storage.30,31 Mulliken population analysis can help us to understand the nature of the hydrogen molecule adsorbing on the ZnO sheet. As mentioned above, an oxygen atom is more electronegative than a zinc atom in the ZnO sheet, so the oxygen atom catches electrons from the zinc atom easily. Our calculations indicate that one oxygen atom in the ZnO sheet can gain 0.83 electron on average. Such an electronrich site (O atom) polarizes the adsorbed hydrogen molecule so that
the adsorbed hydrogen molecule has a dipole moment of about 0.05 e 3 Å. Thus, the electrostatic interaction between the polarized hydrogen molecule and the oxygen atom makes the hydrogen molecule collinear with the oxygen atom. Next, we focus on the adsorption behavior of more hydrogen molecules on the sheet. In this case, one hydrogen molecule is first attached to its favored site, as shown in Figure 3a, and then the second hydrogen molecule is placed on different sites of the sheet. The final results tell us that the two hydrogen molecules cannot be adsorbed on one oxygen atom at the same side of the sheet. This is because the repulsion between two hydrogen molecules locating at one oxygen atom is stronger than the attraction between the oxygen atom and the hydrogen molecule. Instead, two hydrogen molecules respectively adsorb on the top sites of two oxygen atoms, with average binding energy of about 0.12 eV/H2. If more hydrogen molecules are added on the same side of the ZnO sheet, all hydrogen molecules respectively adsorb on the top sites of oxygen atoms until oxygen atoms in the sheet are covered fully by the hydrogen molecules. All of the above calculations are relevant to the case of hydrogen molecules adsorbing on oxygen atoms at the same side of the ZnO sheet. We then assume that the hydrogen molecules may adsorb on both sides of the sheet. Interestingly, we find that each oxygen atom can adsorb two hydrogen molecules on the both sides of the sheet, as shown in Figure 5. As a result, the gravimetric density for hydrogen storage in an isolated ZnO sheet is about 4.7 wt %. For the practical use of hydrogen storage, we not only pay attention to the adsorption behavior of hydrogen but also look at the desorption behavior of hydrogen from the materials. Because of this, we turn to how the adsorbed hydrogen molecule releases from a ZnO sheet. For this purpose, the releasing procedure of an individual H2 molecule either from the bare ZnO sheet (named as case A) or from the ZnO sheet that is fully covered by hydrogen molecules (named as case B) is carefully examined. In our calculations, the distance (D) between an oxygen atom and its nearest H atom in the hydrogen molecule is gradually enlongated, followed by structural relaxation with fixed D at each step. Figure 6 shows the relative energies as a function of the distance D for both extreme cases. Clearly, the energy in each case becomes higher as the distance D increases; when the separation is beyond 7 Å, the relative energies in cases A and B, respectively, reach 0.137 and 0.11 eV. We stress that the relative energy varies with distance D monotonically. This strongly implies that a hydrogen molecule near the ZnO sheet may spontaneously adsorb on an oxygen atom, without overcoming any energy barrier; on the other hand, the adsorbed hydrogen molecule may be released by overcoming an energy barrier of about 0.137 or 0.11 eV for case A or B. Here, the small difference of the energy barriers between cases A and B is reasonable: in comparison to case A, the sheet in case B is fully covered by hydrogen molecules. In the latter case, the polarized hydrogen molecules in the ZnO sheet repel each other, which assists the release of an adsorbed hydrogen molecule from the ZnO sheet. So, the energy barrier in case B is smaller than that in case A.
4. SIMULATIONS AT FINITE TEMPERATURES AND PRESSURES All calculations and analysis above are done at zero temperature. In fact, hydrogen uptake at room temperature and moderate pressure is of more practical interest. Because of this, we then estimate the hydrogen uptake on a single ZnO sheet at finite 9055
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Figure 4. Optimized configurations for adsorption of a single hydrogen molecule at the eight sites on ZnO sheet. 9056
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Figure 5. Full coverage of hydrogen molecules on both sides of a ZnO sheet: (a) top view; (b) side view.
Figure 6. Desorption energy curve of one hydrogen molecule desorbed from ZnO sheet for (a) only one hydrogen molecule adsorbed on the ZnO sheet and (b) the ZnO sheet being covered fully by hydrogen molecules.
temperature and pressure through employing an efficient thermodynamics method that was introduced in our previous work.32 This method has been proven to be accurate and much faster than quantum mechanical approaches33 and grand canonical Monte Carlo (GCMC)34 methods. On the basis of this efficient thermodynamics method,32 we introduce the internal pressure Pint(r), the pressure of the adsorbed phase at position r, which differs from the external gas pressure Pext(r) by Pint ðrÞ ¼ Pext exp½ Eads ðrÞ=KB T
ð2Þ
where Eads(r) is the local adsorption potential, KB is the Boltzmann constant, and T is the temperature. An empirical hydrogen equation of state (EOS)35 is then used to predict the local adsorbed gas density F(x, y, z) based on the temperature T and the calculated internal pressure Pint(x, y, z). Note that the local gas density varies with position since the local adsorption energy is a R function of position. The total gas uptake [w = V F(x, y, z) dV] will be calculated by integrating the local gas density over the volume of the entire system. Ideally, the adsorption energy Eads(r) would be mapped out at all locations above the ZnO sheet, to provide a full H2 density profile near the sheet. This would require a very large number of calculations, which is challenging for DFT calculations. To provide a simpler estimate, we assume that the adsorption energies of different configurations in Figure 4 vary with distance D from the ZnO sheet in a manner similar to Figure 6a scaled by their respective binding energies. We generate curves for different positions along the lattice by rescaling the energy curves of Figure 6a by a factor of E/Emin; here E is the binding energy displayed in Figure 4 and Emin = 0.137 eV/molecule is the strongest binding energy.
The contribution of each different configuration to the total adsorption is evaluated as follows. We consider there are four distinct types of adsorption sites in a ZnO unit cell. One is above an O atom (Figure 4, configuration I) with binding energy of 0.131 eV. The second site is above a Zn atom (Figure 4, configurations II and V) with an average binding energy of 0.076 eV. The third one is above the middle of a six-ring hollow, which is represented by configurations VI, VII, and VIII (Figure 4) with an average energy of 0.104 eV. The fourth site is above the ZnO bond as shown in configurations III and IV of Figure 4 with binding energy of 0.137 eV. The contribution of each type of adsorption site is estimated by its proportion in a unit cell. For example, there are one O atom, one Zn atom, one six-ring, and three ZnO bonds. Thus, the contribution is 1/6, 1/6, 1/6, and 1/2 for O atom, Zn atom, sixmembered ring, and ZnO bond, respectively. The total H2 uptake is integrated up to distances of 10 Å. For both sides of the sheet, the total uptake is calculated to be in the range 2.02.6 wt % at 298 K and 5 MPa, depending on the details of the assumed scaling behavior. Since the total uptake will continue to increase with the integration distance D, the excess uptake is a better parameter to evaluate the adsorption effects. The excess uptake is calculated by subtracting from the total uptake the mass of gas that would have occupied the same volume without adsorbent adsorbate interaction. For both sides of the ZnO sheet, the excess uptake is 1.52.1 wt % at 298 K and 5 MPa. When it is considered that most current materials, including cyclopentadienyl chromium hydrazide gels36 and ZnO nanowires,37 have less than 1 wt % hydrogen uptake under the same conditions, this suggests that the ZnO sheet is promising to meet the U.S. Department of Energy target for hydrogen storage. Our previous work on expanded graphite model showed our method slightly but systematically underestimated the adsorption compared to more detailed methods, indicating a possibly higher adsorption capacity on ZnO than indicated here. It is noted that, similar to our considered ZnO system, an h-BN system is also polarized, which was considered as a candidate for the hydrogen storage.3840 But the bonding character between the atoms in h-BN is different from that in ZnO sheet. In an h-BN system, each B atom bonds to its neighboring N atoms in the covalent character; and in a ZnO system, each O atom connects with its neighboring Zn atoms via a mixed bonding of covalent and ionic characters. This difference may cause the different bonding energies of hydrogen adsorbed in them. For example, the binding energy of hydrogen molecules in ZnO sheets is larger than that in h-BN sheets, and thus the uptake capability of hydrogen storage of the h-BN system is weaker than that of the ZnO sheet.
5. SUMMARY In summary, by using local density approximation, we investigate the adsorption and desorption behavior of hydrogen on the planar hexagonal honeycomb ZnO sheet. We find that the planar ZnO monolayer preferably adsorbs hydrogen molecules rather than dissociates hydrogen molecules. The adsorbed hydrogen molecules bind to the ZnO sheet, with binding energy of ∼0.13 eV/H 2 , which meets the desired energy range of 0.10.2 eV/H2 for hydrogen uptake. Furthermore, our thermodynamics calculations predict that the H2 uptake in the ZnO sheet reaches about 1.52.1 wt % at 298 K and 5 MPa. These features indicate that the monolayer ZnO sheet has potential for hydrogen storage. 9057
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by National Basic Research Program of China (2009CB939901). The HP-LHPC of USTC (University of Science and Technology of China) is acknowledged for computational support. Research was also sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division (L.J.P. and J.R.M.).
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