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Hydrogen separation by porous phosphorene: A periodical DFT study
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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Hydrogen separation by porous phosphorene: A periodical DFT study Yayun Zhang a,c, Feng Hao c, Hang Xiao c, Chao Liu a,*, Xiaoyang Shi c, Xi Chen b,c,** a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, College of Power Engineering, Chongqing University, Chongqing 400030, China b ICAM, SV Laboratory, School of Aerospace, Xi'an Jiaotong University, Xi'an 710049, China c Columbia Nanomechanics Research Center, Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027, USA
article info
abstract
Article history:
We have examined theoretically the stability of porous phosphorene and its application in
Received 12 July 2016
hydrogen separation from gas mixture by employing first-principles calculations. The self-
Received in revised form
passivated pore of phosphorene was designed by removing six phosphorous atoms,
16 October 2016
reaching to the formation of covalent bonds among marginal atoms spontaneously. The
Accepted 18 October 2016
gas permeability and selectivity were obtained for the porous phosphorene membrane.
Available online xxx
The results indicated that the self-passivated defect in phosphorene is inert to the gas mixture containing N2, CO, CO2, H2O, and CH4 molecules, and the porous phosphorene
Keywords:
performed high selectivity for hydrogen over other gas molecules compared with previous
Hydrogen separation
graphene and silicene-based membranes. Our results unveiled the great potential of
Porous phosphorene
porous phosphorene as a promising membrane in hydrogen purification.
Gas mixture
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
First-principles calculation
Introduction Hydrogen, a renewable and ecologically clean resource, is regarded as an important chemical component, which is widely used in sorts of industrial processes such as fuel cell, chemical hydrogenation, and semiconductor processing. Usually, to generate hydrogen in industry, reforming of hydrocarbon fuels followed with water-gas shift reaction is adopted [1,2]. While the overall outcomes simultaneously contain impurity gases like CH4, CO and CO2, which should be
removed before getting access to the pure H2 utilization. Among major technologies for hydrogen purification, membrane separation is gaining increasing attention due to inherent advantages of low energy consumption and simple operation. Past decades have witnessed to the development of growing numbers of nanoporous materials including metal, zeolite, silica, carbon based and polymer membranes [3e13]. However, the reported hydrogen selectivities of these materials hitherto do not sufficiently meet practical applications. Therefore, exploring new membranes with high separation performance is in high demand.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected] (X. Chen). http://dx.doi.org/10.1016/j.ijhydene.2016.10.108 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
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Two-dimensional (2D) materials, since the graphene was discovered experimentally, have scraped a strong continuous wind in a wide range of applications [14]. Their novel irreplaceable structures and properties make them as super stars in areas of gas sensing [15], adsorption [16], and separation [17]. For example, graphene, a single-atom-thick planar membrane with the strongest structure, becomes attractive for its potential application as an excellent membrane [18,19], because it has a controllable pore size, stable structure, and efficient permeability that are necessary principles being a good membrane [4]. Later, the verification of the stability of porous graphene is conducted by the experiment with a focused electron beam of the transmission electron microscope [20]. A detailed molecular simulation using different pore sizes also provides a further prediction of porous graphene in hydrogen separation [21]. In addition, by introducing hydrogenated and nitrogen functionalized porous graphene after removing two adjacent rings, a high permeability for H2 relative to CH4 can be obtained [22]. Likewise, the porous carbon nanotube also shows the capacity of H2 separation from gas mixture [23,24], so as to other kinds of porous graphene with intrinsic subnanometer pores, such as polyphenylene [25], graphdiyne [26], and graphyne [27]. Whereas, the side effect of graphene slotting will cause the high chemical reactivity of edged carbon atoms with dangling bonds in a nanopore, especially when the porous graphene is exposed to the gases of CO, CO2, and NO [28e30]. Thus, it is important to protect these carbon atoms by decorating with hydrogen and nitrogen atoms [22]. Other 2D material family members, h-BN [31,32] and silicene [33,34], have also been proposed possessing the potential of hydrogen purification theoretically. In the latest years, a new member of 2D material family coming from a single layer of black phosphorus (BP), namely phosphorene, has received rapidly increasing attention since it was discovered in 2014 [35]. Unlike graphene, the atoms of phosphorene are arranged in a puckered honeycomb lattice, in which each phosphorus atom bonds with three adjacent phosphorus atoms [36]. The single layer phosphorene shows a vital advantage of having a finite band gap [37], high charge carrier mobility of around 1000 cm2/V [35], and mechanical properties [38e40]. Additionally, phosphorene also possesses other interesting and useful features, e.g. the anisotropic electrical conductance, high operating frequencies, and ambipolar behavior [41,42]. It is noteworthy that mono- and few-layer BP flakes are reported less stable than graphene, due to its tendency to oxidize [43,44]. However, some strategies have been provided to overcome this drawback and made it possible to use oxidized and defect phosphorene under the condition with oxygen molecules [45e48], which expanded the application of the phosphorene. Interestingly, recent studies also proposed strong gas sensitivity for phosphorene [49,50], which opened the window of dealing gas issues by using this new kind of 2D material. Therefore, taking these advantages into consideration, it can be proposed here that it is of possibility to use porous phosphorene to separate hydrogen from other gases. A very recent theoretical studies indicated that the porous phosphorene can maintain the stability and is even more thermodynamic stable than that of edge passivation with hydrogen or oxygen atom [51]. Different
from the porous graphene, the edge atoms of porous phosphorene can rebuild bonds with each other and then becomes the self-passivated pore. Thus, it is distinctly convenient to use phosphorene with divacancy defects as a hydrogen separation membrane without additional protection. In order to testify our prediction, first-principles calculations were employed in the present study and our calculation results showed that the self-passivated porous phosphorene indeed has the remarkable capacity of hydrogen separation. Specifically, the proposed porous phosphorene showed a high permeability and selectivity of H2 over N2, CO, CO2, H2O, and CH4. More importantly, the self-passivated pore could maintain its stability under the higher temperature and chemically inert to the considered gas molecules. The theoretical calculation presents the reasonability of our former thought and indicates the great potential of phosphorene-based membranes in hydrogen purification.
Computational methods In our studies, all calculations were performed by using the generalized gradient approximation (GGA), and the PerdeweBurkeeErnzerhof (PBE) functional [52] was employed for the exchange correlation energy of interacting electrons, which is implemented in the Quantum ESPRESSO package [53]. The Gaussian smearing method was applied to describe the total energy, with the width of smearing of 0.05. Electronion interactions were calculated based on ultrasoft pseudopotentials, and the kinetic energy cutoff is set to 30 Ry, which is sufficient for the plane wave expansion. The van der Waals (vdW) correction proposed by Grimme (D2) [54] was also included in order to give a good description of long-range vdW interactions [55,56]. The climbing image nudged elastic band (CI-NEB) method was employed for searching energy barrier of the penetration of gas molecule through the porous phosphorene. Five images were inserted between the initial and final states. A 6 5 supercell (19.8 A 22.8 A) of phosphorene (120 phosphorous atoms) was adopted to simulate the infinite planar sheet. A vacuum layer of 20 A thick was added along the direction perpendicular to the surface in order to minimize the artificial interactions between the sheet and its periodic images. The MonkhorstePack method with the centered k-point grid was used for integration in the Brillouin zone. The optimized lattice constants of phosphorene unit cell are 3.31 2.28 A, which are consistent with previous experimental and theoretical results [35,57].
Results and discussion The stability of porous phosphorene In this part, the vacancy defect in phosphorene sheet is constructed and its stability under different ambient conditions are investigated. The porous phosphorene is designed by removing six phosphorous atoms (atoms within the black circle), which is shown in Fig. 1(a). Smaller defect is not studied due to the fact that gas molecules cannot pass the hole. And energy barriers of gas molecules going through the
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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Fig. 1 e Optimized geometries of pure phosphorene (a) and self-passivated porous phosphorene (b) with vacancy defect in 6 £ 5 supercell.
larger vacancy defect are very low according to previous calculation. Therefore, only this kind of defect is selected and proper to investigate its capacity of gas purification. As the monolayer phosphorene actually has two layers of phosphorous atoms, the vacancy defect is more complex than those of graphene and silicene. As a consequence, after geometry optimization of the vacancy defect, an abnormal decagon ring is generated in order to avoid the dangling bond at the edge of the hole, which is described in Fig. 1(b). The optimized pore has the size of length: 7.11 A and width: 5.53 A, in good agreement with previous theoretical results [51]. Although former study has presented the stability of this kind of vacancy defect by introducing no dangling bonds [51], here further investigations are completed via ab initio molecular dynamic simulation (ab-MD) to confirm its stability under real working conditions. The simulations are run under NVT for 5 ps at 300 K and 500 K separately for the phosphorene sheet as shown in Fig. 2(a) and (b) respectively. It can be seen that the stability of pore structure is maintained at both 300 K and 500 K as shown in Fig. 2(c) and the porous structure of phosphorene cannot be disrupted even at higher temperature. Furthermore, in order to testify whether the phosphorous atoms of pore will chemically react with gas molecules investigated here, another ab-MD simulation is conducted with porous phosphorene exposing to these gas molecules, which is shown in Fig. 2(d). After 5 ps running under 500 K, it still shows the intact output structure of porous phosphorene, implying chemically inert to the considered gases.
The adsorption of various gas molecules Before permeating through the pores of phosphorene, gas molecules will firstly be adsorbed to the vacancy area. In order
to evaluate the stability of molecules adsorption on porous phosphorene, the adsorption energy is defined as: Ea ¼ EGas=Phosphorene EGas EPhosphorene where EGas/Phosphorene, EGas, and EPhosphorene are the total energy of gas molecule adsorption on porous phosphorene, a single gas molecule, and porous phosphorene, respectively. The adsorption of different gas molecules, including H2, N2, CO, CO2, H2O, and CH4, are studied and results are summarized in Table 1. Here, we define the distance (D0) as the length between the center of gas molecules and the upper plane phosphorous atoms because the monolayer phosphorene has two-layer-liked structure. Table 1 shows that the calculated A (for H2O) to 2.67 equilibrium distance (D0) ranges from 1.11 A. Obviously, the adsorption energy of H2O on porous (for CH4) phosphorene with a binding energy value of 0.36 ev considering the vdW correction is higher than others and this is in accordance with its hydrophilic property [50]. On the contrary, the weakest interaction locates between hydrogen and porous phosphorene with only 0.04 ev. Therefore, the self-passivated porous phosphorene is chemically inert to these gas molecules, and the interaction between gas molecules and pore is mainly caused by the vdW force, which also agrees with results from our former ab-initio molecular simulations.
The processes of gas separation In general, an energy barrier exists when one gas molecule permeates through a porous material, due to the interaction between gas molecule and atoms of the material, and this interaction varies to different gas molecules and porous materials. A better porous material should have the character
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
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Fig. 2 e The outcome structures of porous phosphorene after 5 ps NVT ab-initio MD at 300 K (a) and 500 K (b) and corresponding temperature changes vs simulation time described in (c). (d) Presents the results in the gases atmosphere after 5 ps NVT ab-initio MD at 500 K. that the target gases can get through the pore easily while others cannot. In this part, permeation of studied gas molecules through the self-passivated pore of phosphorene are investigated in detail. Here, the target gas is the hydrogen and the processes of hydrogen permeating containing three steps: from adsorption site moving into the pore, reaching to the site called transition state where the interaction is the highest,
Table 1 e The DFT-D2 calculated equilibrium distance D0 between the center of gas molecules and the plane of (A) upper layer phosphorous atom with corresponding binding energy Ea (ev) for gas molecules adsorption on porous phosphorene, and the diffusion energy barrier Eb (ev) for molecules passing through the pore. DFT-D2 H2 N2 CO CO2 H2O CH4
D0
Ea
Eb
2.66 2.35 1.74 2.31 1.11 2.67
0.04 0.15 0.21 0.22 0.36 0.17
0.07 0.85 0.83 0.97 0.85 1.36
and leaving the hole for the adsorption position on the other site of the porous phosphorene. As for the hydrogen, the most stable state of H2 adsorption on porous phosphorene before passing the pore (SS-in) is that in which the H2 molecular is parallel to the plane of phosphorene sheet above the center of the vacancy, as shown in Fig. 2. A slight change happens to the location of adsorption after H2 getting through the pore and the whole system reaches its stable structure at SS-out. Five sites along the path from SS-in to SS-out are set to search the energy barrier (Eb) of the hydrogen permeation using the CINEB calculation. The diffusion energy barrier is defined by Equation (1): Eb ¼ ETS ESSin
(1)
where, ETS and ESS-in represent the total Gibbs free energy of the transition state (TS) of H2 when passing through the selfpassivated vacancy of phosphorene and the SS-in of H2 initial adsorption on porous phosphorene. The energy barriers of various gas molecules getting through the porous phosphorene are present in Fig. 3, and the corresponding diffusion energy barriers are listed in Table 1. Specifically, the diffusion
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
energy barrier for hydrogen molecule passing through the self-passivated vacancy defect of phosphorene is only 0.07 ev, which is smaller than that of porous graphene [22], twodimensional polyphenylene [25], and divacancy defect of silicene [34]. This negligible diffusion energy barrier of H2 suggests that the hydrogen molecule can permeate the porous phosphorene at the moderate temperature and pressure without external assistance. The optimized structure of the TS shows that the saddle point of the whole pathway energy locates at the site a little above the center of pore, which is caused by the abnormal shape of the self-passivated vacancy defect, as well as the situations of other gas molecules. This is also the reason to the non-smooth energy curve along the path of gas permeation in the Fig. 3. The calculated diffusion energy barriers are 0.85, 0.83, 0.97, 0.85, 1.36 ev for N2, CO, CO2, H2O, and CH4, respectively, which are much larger than that of H2 (0.07 ev). Therefore, porous phosphorene with selfpassivated defect can block these gas molecules and works as a promising hydrogen purification membrane. In order to obtain the hydrogen separation efficiency of porous phosphorene, the selectivity for H2 relative to other gas molecules (N2, CO, CO2, H2O, and CH4) permeating through the pores of phosphorene is estimated with the Arrhenius Equation (2) [22,25,27,34]: SH2 =Gas ¼
AH2 exp EH2 RT rH2 ¼ rGas AGas expf EGas =RTg
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and 1021 for H2 over N2, CO, CO2, H2O, and CH4, respectively. Compared with previous graphene-based [22], silica [3], and silicene [34] membranes, porous phosphorene exhibits the remarkable selectivity for H2 over H2O, CO2, N2, and CO, and the comparable capacity of purifying H2 over CH4. Therefore, it can be conducted that the porous phosphorene has a great potential in hydrogen separation based on the calculation results above and following analysis. First of all, the self-passivated phosphorene pore is chemically inert to the considered gas molecules as confirmed by the ab-initio molecular simulation. Because the phosphorous atoms bonding to the removed atoms from the phosphorene can form covalent PeP bonds spontaneously, which avoids leaving dangling bonds among the atoms around the vacancy defect. Similarly, the self-saturation of dangling bonds also exists in divacancy defect of silicone [34]. Whereas, this is hardly achieved in porous graphene proposed in literature [22], resulting in requesting the protection of edged carbon atoms with the assistance of hydrogen or other functional groups [28e30]. Although the porous graphene with divacancy defect can also form CeC covalent bonds, the pore size gives an unsatisfied result for hydrogen purification [59]. The self-passivated porous phosphorene in present work, on the contrary, has a reasonable pore size, obtaining a comparable capacity of separating hydrogen from the view of geometry. Therefore,
(2)
where r is the diffusion rate, A is the diffusion prefactor and here it is assumed that the diffusion prefactor of gas molecules studied is identical as A ¼ 1011 s1 [58], E is the diffusion energy barrier, and R is the gas constant. The temperaturedependent diffusion rate and selectivity profiles are investigated, which are shown in Fig. 4. It can been seen from Fig. 4(a) that the diffusion rates of H2 molecules are much higher than those of N2, CO, CO2, H2O, and CH4, consistent with the calculated diffusion energy barriers. As a result, the selectivity of hydrogen over other gases studied is extremely high in a wide temperature ranges. For example, as shown in Table 2, at room temperature (K ¼ 300 K), using porous phosphorene can achieve a high selectivity on the order of 1013, 1012, 1015, 1013,
Fig. 3 e Relative energy for H2, N2, CO, CO2, H2O and CH4 passing through the self-passivated pore of phosphorene as a function of adsorption height.
Fig. 4 e Diffusion rate (a) and selectivity for gas molecules (H2, N2, CO, CO2, H2O and CH4) passing through the selfpassivated defect of phosphorene as a function of temperature.
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
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Table 2 e The DFT-D2 calculated selectivity (S) of H2 relative to other gas molecules at room temperature (T ¼ 300 K), including H2/N2, H2/CO, H2/CO2, H2/H2O, and H2/CH4, and corresponding comparison results (diffusion energy barrier Eb (ev) and selectivity S for H2 relative to other gas molecule) of previously proposed porous membranes. Membranes Reference Eb (H2) S(H2/N2) S(H2/CO) S(H2/CO2) S(H2/H2O) S(H2/CH4) a b
Phosphorene
Graphene
Silica
Silicene
This work 0.07 1013 1012 1015 1013 1021
Ref. [15] 0.22a, 0.33b / / / / 1023, 108
Ref. [2] / 102 / 10 / 103
Ref. [26] 0.34 1011 1011 1011 102 1022
Hydrogenated porous graphene in Ref. [15]. Nitrogen functionalized porous graphene in Ref. [15].
the self-passivated porous phosphorene with larger size is not studied in this work, because it cannot theoretically perform a good result. In addition, as a metalloid element ranking later in the periodic table of elements, phosphorous atom has a larger vdW force radius compared with carbon and silicon, making it more difficult for gases like N2, CO, CO2, H2O, and CH4 with larger kinetic radius to passing through the porous phosphorene. It should also be noted that the theoretically calculated H2 selectivity of porous phosphorene here is an ideal result. In reality, the permeability and selectivity of traditional hydrogen separation membranes largely depend on many other factors in practical applications, including crystalline defect, membrane thickness and slight structure change. Besides, some early studies showed that the phosphorene is not stable in the water and in the condition with oxygen [43,60]. While the latest experiments has presented an efficient method to maintain its stability in these adverse conditions [44,46,48]. Additionally, the purpose of present work is located in hydrogen separation, and other gas like H2O is attended as impurity with negligible concentration. Therefore, the side influence from water reaches to its lowest position. So the character of porous phosphorene in gas separation should be noticed to all of us rather than remaining silence continually, and the phosphorene could be a new competitor for graphene [61].
Conclusion In present work, we have demonstrated that the phosphorene with self-passivated defects can be applied as a promising hydrogen separation membrane by conducting firstprinciples density function theory. Hydrogen molecule can permeate the porous phosphorene with surmountable diffusion energy barrier of only 0.07 eV. The porous phosphorene exhibits a high selectivity on the order of 1013, 1012, 1015, 1013, and 1021 for H2 over N2, CO, CO2, H2O, and CH4, at ambient temperature, respectively. Compared with graphene-based membranes, the self-passivated defect in phosphorene can be easily experimentally realized without needing protection from decorating other atoms. Furthermore, the porous phosphorene is chemically inert to most gas molecules according to the result from ab-initio molecular dynamics. Given these
advantages, our theoretical studies firstly unveil the great potential of porous phosphorene in hydrogen purification applications. Although the monolayer of phosphorene can be obtained through the experiment successfully [62], more efforts should also be paid to overcome the stability of phosphorene in the atmosphere with oxygen and water in the future to extend its application in a wider field.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51576019) and Chongqing university postgraduates' innovation project (CYB15016). One of the authors, Yayun Zhang, would like to acknowledge financial support from the Chinese Scholarship Council (CSC).
references
[1] Hufton J, Mayorga S, Sircar S. Sorption-enhanced reaction process for hydrogen production. AIChE J 1999;45:248e56. [2] Wang X, Wang N, Li M, Li S, Wang S, Ma X. Hydrogen production by glycerol steam reforming with in situ hydrogen separation: a thermodynamic investigation. Int J Hydrogen Energy 2010;35:10252e6. [3] De Vos RM, Verweij H. High-selectivity, high-flux silica membranes for gas separation. Science 1998;279:1710e1. [4] Pandey P, Chauhan R. Membranes for gas separation. Prog Polym Sci 2001;26:853e93. [5] Oyama S, Lee D, Hacarlioglu P, Saraf R. Theory of hydrogen permeability in nonporous silica membranes. J Membr Sci 2004;244:45e53. [6] Adhikari S, Fernando S. Hydrogen membrane separation techniques. Industrial Eng Chem Res 2006;45:875e81. [7] Ockwig NW, Nenoff TM. Membranes for hydrogen separation. Chem Rev 2007;107:4078e110. [8] David E, Kopac J. Development of palladium/ceramic membranes for hydrogen separation. Int J Hydrogen Energy 2011;36:4498e506. [9] Deng X, Luo D, Qin C, Qian X, Yang W. Hydrogen isotopes separation using frontal displacement chromatography with Pd-Al2O3 packed column. Int J Hydrogen Energy 2012;37:10774e8. [10] Doucet R, Gardner CL, Ternan M. Separation of hydrogen from hydrogen/ethylene mixtures using PEM fuel cell technology. Int J Hydrogen Energy 2009;34:998e1007.
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
[11] Huang F, Meng C. Hydrophobic platinumpolytetrafluoroethylene catalyst for hydrogen isotope separation. Int J Hydrogen Energy 2010;35:6108e12. [12] Hwang K-R, Lee S-W, Lee D-W, Lee C-B, Ji S-M, Park J-S. Bifunctional hydrogen membrane for simultaneous chemical reaction and hydrogen separation. Int J Hydrogen Energy 2014;39:2614e20. [13] Zhang Q, Liu G, Feng X, Chu KH, Deng C. Hydrogen networks synthesis considering separation performance of purifiers. Int J Hydrogen Energy 2014;39:8357e73. [14] Novoselov K, Geim AK, Morozov S, Jiang D, Katsnelson M, Grigorieva I, et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005;438:197e200. [15] Yoon HJ, Yang JH, Zhou Z, Yang SS, Cheng MM-C. Carbon dioxide gas sensor using a graphene sheet. Sensors Actuators B Chem 2011;157:310e3. [16] Dai J, Yuan J, Giannozzi P. Gas adsorption on graphene doped with B, N, Al, and S: a theoretical study. Appl Phys Lett 2009;95:232105. [17] Bernardo P, Drioli E, Golemme G. Membrane gas separation: a review/state of the art. Industrial Eng Chem Res 2009;48:4638e63. [18] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183e91. [19] Geim AK. Graphene: status and prospects. Science 2009;324:1530e4. M. Electron beam nanosculpting of [20] Fischbein MD, Drndic suspended graphene sheets. Appl Phys Lett 2008;93:113107. [21] Du H, Li J, Zhang J, Su G, Li X, Zhao Y. Separation of hydrogen and nitrogen gases with porous graphene membrane. J Phys Chem C 2011;115:23261e6. [22] D-e Jiang, Cooper VR, Dai S. Porous graphene as the ultimate membrane for gas separation. Nano Lett 2009;9:4019e24. [23] Bucior BJ, Chen D-L, Liu J, Johnson JK. Porous carbon nanotube membranes for separation of H-2/CH4 and CO2/ CH4 mixtures. J Phys Chem C 2012;116:25904e10. [24] Gu C, Gao GH, Yu YX. Density functional study of the adsorption and separation of hydrogen in single-walled carbon nanotube. Int J Hydrogen Energy 2004;29:465e73. [25] Li Y, Zhou Z, Shen P, Chen Z. Two-dimensional polyphenylene: experimentally available porous graphene as a hydrogen purification membrane. Chem Commun 2010;46:3672e4. [26] Jiao Y, Du A, Hankel M, Zhu Z, Rudolph V, Smith SC. Graphdiyne: a versatile nanomaterial for electronics and hydrogen purification. Chem Commun 2011;47:11843e5. [27] Zhang H, He X, Zhao M, Zhang M, Zhao L, Feng X, et al. Tunable hydrogen separation in spesp2 hybridized carbon membranes: a first-principles prediction. J Phys Chem C 2012;116:16634e8. [28] D-e Jiang, Sumpter BG, Dai S. Unique chemical reactivity of a graphene nanoribbon's zigzag edge. J Chem Phys 2007;126:134701. [29] Huang B, Li Z, Liu Z, Zhou G, Hao S, Wu J, et al. Adsorption of gas molecules on graphene nanoribbons and its implication for nanoscale molecule sensor. J Phys Chem C 2008;112:13442e6. [30] Vanin M, Gath J, Thygesen KS, Jacobsen KW. First-principles calculations of graphene nanoribbons in gaseous environments: structural and electronic properties. Phys Rev B 2010;82:195411. € rter W, Muntwiler M, Tamai A, Greber T, [31] Corso M, Auwa Osterwalder J. Boron nitride nanomesh. Science 2004;303:217e20. [32] Lim SH, Luo J, Ji W, Lin J. Synthesis of boron nitride nanotubes and its hydrogen uptake. Catal Today 2007;120:346e50.
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[33] Vogt P, De Padova P, Quaresima C, Avila J, Frantzeskakis E, Asensio MC, et al. Silicene: compelling experimental evidence for graphenelike two-dimensional silicon. Phys Rev Lett 2012;108:155501. [34] Hu W, Wu X, Li Z, Yang J. Porous silicene as a hydrogen purification membrane. Phys Chem Chem Phys 2013;15:5753e7. nek D, et al. [35] Liu H, Neal AT, Zhu Z, Luo Z, Xu X, Toma Phosphorene: an unexplored 2D semiconductor with a high hole mobility. ACS Nano 2014;8:4033e41. [36] Li L, Yu Y, Ye GJ, Ge Q, Ou X, Wu H, et al. Black phosphorus field-effect transistors. Nat Nanotechnol 2014;9:372e7. nek D. Semiconducting layered blue [37] Zhu Z, Toma phosphorus: a computational study. Phys Rev Lett 2014;112:176802. [38] Peng X, Wei Q, Copple A. Strain-engineered direct-indirect band gap transition and its mechanism in two-dimensional phosphorene. Phys Rev B 2014;90:085402. [39] Wei Q, Peng X. Superior mechanical flexibility of phosphorene and few-layer black phosphorus. Appl Phys Lett 2014;104:251915. [40] Elahi M, Khaliji K, Tabatabaei SM, Pourfath M, Asgari R. Modulation of electronic and mechanical properties of phosphorene through strain. Phys Rev B 2015;91:115412. [41] Buscema M, Groenendijk DJ, Blanter SI, Steele GA, van der Zant HS, Castellanos-Gomez A. Fast and broadband photoresponse of few-layer black phosphorus field-effect transistors. Nano Lett 2014;14:3347e52. [42] Koenig SP, Doganov RA, Schmidt H, Neto AC, Oezyilmaz B. Electric field effect in ultrathin black phosphorus. Appl Phys Lett 2014;104:103106. [43] Ziletti A, Carvalho A, Campbell DK, Coker DF, Neto AC. Oxygen defects in phosphorene. Phys Rev Lett 2015;114:046801. [44] Carvalho A, Neto AHC. Phosphorene: overcoming the oxidation barrier. ACS Central Sci 2015;1:289e91. [45] Li P, Zhang D, Liu J, Chang H, Ye Sun, Yin N. Air-stable black phosphorus devices for ion sensing. ACS Appl Mater Interfaces 2015;7:24396e402. [46] Pei J, Gai X, Yang J, Wang X, Yu Z, Choi D-Y, et al. Producing air-stable monolayers of phosphorene and their defect engineering. Nat Commun 2016:7. [47] Edmonds M, Tadich A, Carvalho A, Ziletti A, O'Donnell K, Koenig S, et al. Creating a stable oxide at the surface of black phosphorus. ACS Appl Mater Interfaces 2015;7:14557e62. [48] Zhao Y, Wang H, Huang H, Xiao Q, Xu Y, Guo Z, et al. Surface coordination of black phosphorus for robust air and water stability. Angew Chem 2016;128:5087e91. [49] Kou L, Frauenheim T, Chen C. Phosphorene as a superior gas sensor: selective adsorption and distinct IeV response. J Phys Chem Lett 2014;5:2675e81. [50] Cui S, Pu H, Wells SA, Wen Z, Mao S, Chang J, et al. Ultrahigh sensitivity and layer-dependent sensing performance of phosphorene-based gas sensors. Nat Commun 2015:6. [51] Hashmi A, Farooq MU, Hong J. Long-range magnetic ordering and switching of magnetic state by electric field in porous phosphorene. J Phys Chem Lett 2016;7:647e52. [52] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865. [53] Giannozzi P, Baroni S, Bonini N, Calandra M, Car R, Cavazzoni C, et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J Phys Condens Matter 2009;21:395502. [54] Antony J, Grimme S. Structures and interaction energies of stacked grapheneenucleobase complexes. Phys Chem Chem Phys 2008;10:2722e9. n JG. Improved [55] Bucko T, Hafner Jr , Lebegue S, Angya description of the structure of molecular and layered
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
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[56] [57] [58]
[59]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
crystals: ab initio DFT calculations with van der Waals corrections. J Phys Chem A 2010;114:11814e24. Kagimura R, Chacham H. Knots in a graphene nanoribbon. Phys Rev B 2012;85:125415. Brown A, Rundqvist S. Refinement of the crystal structure of black phosphorus. Acta Crystallogr 1965;19:684e5. Blankenburg S, Bieri M, Fasel R, Mu¨llen K, Pignedoli CA, Passerone D. Porous graphene as an atmospheric nanofilter. Small 2010;6:2266e71. Leenaerts O, Partoens B, Peeters F. Graphene: a perfect nanoballoon. Appl Phys Lett 2008;93:193107.
[60] Wang G, Slough WJ, Pandey R, Karna SP. Degradation of phosphorene in air understanding at atomic level. 2D Materials 2016;3:025011. [61] Bagheri S, Mansouri N, Aghaie E. Phosphorene: a new competitor for graphene. Int J Hydrogen Energy 2016;41:4085e95. [62] Lu W, Nan H, Hong J, Chen Y, Zhu C, Liang Z, et al. Plasmaassisted fabrication of monolayer phosphorene and its Raman characterization. Nano Res 2014;7:853e9.
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Hydrogen separation by porous phosphorene: A periodical DFT study Yayun Zhang a,c, Feng Hao c, Hang Xiao c, Chao Liu a,*, Xiaoyang Shi c, Xi Chen b,c,** a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, College of Power Engineering, Chongqing University, Chongqing 400030, China b ICAM, SV Laboratory, School of Aerospace, Xi'an Jiaotong University, Xi'an 710049, China c Columbia Nanomechanics Research Center, Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027, USA
article info
abstract
Article history:
We have examined theoretically the stability of porous phosphorene and its application in
Received 12 July 2016
hydrogen separation from gas mixture by employing first-principles calculations. The self-
Received in revised form
passivated pore of phosphorene was designed by removing six phosphorous atoms,
16 October 2016
reaching to the formation of covalent bonds among marginal atoms spontaneously. The
Accepted 18 October 2016
gas permeability and selectivity were obtained for the porous phosphorene membrane.
Available online xxx
The results indicated that the self-passivated defect in phosphorene is inert to the gas mixture containing N2, CO, CO2, H2O, and CH4 molecules, and the porous phosphorene
Keywords:
performed high selectivity for hydrogen over other gas molecules compared with previous
Hydrogen separation
graphene and silicene-based membranes. Our results unveiled the great potential of
Porous phosphorene
porous phosphorene as a promising membrane in hydrogen purification.
Gas mixture
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
First-principles calculation
Introduction Hydrogen, a renewable and ecologically clean resource, is regarded as an important chemical component, which is widely used in sorts of industrial processes such as fuel cell, chemical hydrogenation, and semiconductor processing. Usually, to generate hydrogen in industry, reforming of hydrocarbon fuels followed with water-gas shift reaction is adopted [1,2]. While the overall outcomes simultaneously contain impurity gases like CH4, CO and CO2, which should be
removed before getting access to the pure H2 utilization. Among major technologies for hydrogen purification, membrane separation is gaining increasing attention due to inherent advantages of low energy consumption and simple operation. Past decades have witnessed to the development of growing numbers of nanoporous materials including metal, zeolite, silica, carbon based and polymer membranes [3e13]. However, the reported hydrogen selectivities of these materials hitherto do not sufficiently meet practical applications. Therefore, exploring new membranes with high separation performance is in high demand.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected] (X. Chen). http://dx.doi.org/10.1016/j.ijhydene.2016.10.108 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
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Two-dimensional (2D) materials, since the graphene was discovered experimentally, have scraped a strong continuous wind in a wide range of applications [14]. Their novel irreplaceable structures and properties make them as super stars in areas of gas sensing [15], adsorption [16], and separation [17]. For example, graphene, a single-atom-thick planar membrane with the strongest structure, becomes attractive for its potential application as an excellent membrane [18,19], because it has a controllable pore size, stable structure, and efficient permeability that are necessary principles being a good membrane [4]. Later, the verification of the stability of porous graphene is conducted by the experiment with a focused electron beam of the transmission electron microscope [20]. A detailed molecular simulation using different pore sizes also provides a further prediction of porous graphene in hydrogen separation [21]. In addition, by introducing hydrogenated and nitrogen functionalized porous graphene after removing two adjacent rings, a high permeability for H2 relative to CH4 can be obtained [22]. Likewise, the porous carbon nanotube also shows the capacity of H2 separation from gas mixture [23,24], so as to other kinds of porous graphene with intrinsic subnanometer pores, such as polyphenylene [25], graphdiyne [26], and graphyne [27]. Whereas, the side effect of graphene slotting will cause the high chemical reactivity of edged carbon atoms with dangling bonds in a nanopore, especially when the porous graphene is exposed to the gases of CO, CO2, and NO [28e30]. Thus, it is important to protect these carbon atoms by decorating with hydrogen and nitrogen atoms [22]. Other 2D material family members, h-BN [31,32] and silicene [33,34], have also been proposed possessing the potential of hydrogen purification theoretically. In the latest years, a new member of 2D material family coming from a single layer of black phosphorus (BP), namely phosphorene, has received rapidly increasing attention since it was discovered in 2014 [35]. Unlike graphene, the atoms of phosphorene are arranged in a puckered honeycomb lattice, in which each phosphorus atom bonds with three adjacent phosphorus atoms [36]. The single layer phosphorene shows a vital advantage of having a finite band gap [37], high charge carrier mobility of around 1000 cm2/V [35], and mechanical properties [38e40]. Additionally, phosphorene also possesses other interesting and useful features, e.g. the anisotropic electrical conductance, high operating frequencies, and ambipolar behavior [41,42]. It is noteworthy that mono- and few-layer BP flakes are reported less stable than graphene, due to its tendency to oxidize [43,44]. However, some strategies have been provided to overcome this drawback and made it possible to use oxidized and defect phosphorene under the condition with oxygen molecules [45e48], which expanded the application of the phosphorene. Interestingly, recent studies also proposed strong gas sensitivity for phosphorene [49,50], which opened the window of dealing gas issues by using this new kind of 2D material. Therefore, taking these advantages into consideration, it can be proposed here that it is of possibility to use porous phosphorene to separate hydrogen from other gases. A very recent theoretical studies indicated that the porous phosphorene can maintain the stability and is even more thermodynamic stable than that of edge passivation with hydrogen or oxygen atom [51]. Different
from the porous graphene, the edge atoms of porous phosphorene can rebuild bonds with each other and then becomes the self-passivated pore. Thus, it is distinctly convenient to use phosphorene with divacancy defects as a hydrogen separation membrane without additional protection. In order to testify our prediction, first-principles calculations were employed in the present study and our calculation results showed that the self-passivated porous phosphorene indeed has the remarkable capacity of hydrogen separation. Specifically, the proposed porous phosphorene showed a high permeability and selectivity of H2 over N2, CO, CO2, H2O, and CH4. More importantly, the self-passivated pore could maintain its stability under the higher temperature and chemically inert to the considered gas molecules. The theoretical calculation presents the reasonability of our former thought and indicates the great potential of phosphorene-based membranes in hydrogen purification.
Computational methods In our studies, all calculations were performed by using the generalized gradient approximation (GGA), and the PerdeweBurkeeErnzerhof (PBE) functional [52] was employed for the exchange correlation energy of interacting electrons, which is implemented in the Quantum ESPRESSO package [53]. The Gaussian smearing method was applied to describe the total energy, with the width of smearing of 0.05. Electronion interactions were calculated based on ultrasoft pseudopotentials, and the kinetic energy cutoff is set to 30 Ry, which is sufficient for the plane wave expansion. The van der Waals (vdW) correction proposed by Grimme (D2) [54] was also included in order to give a good description of long-range vdW interactions [55,56]. The climbing image nudged elastic band (CI-NEB) method was employed for searching energy barrier of the penetration of gas molecule through the porous phosphorene. Five images were inserted between the initial and final states. A 6 5 supercell (19.8 A 22.8 A) of phosphorene (120 phosphorous atoms) was adopted to simulate the infinite planar sheet. A vacuum layer of 20 A thick was added along the direction perpendicular to the surface in order to minimize the artificial interactions between the sheet and its periodic images. The MonkhorstePack method with the centered k-point grid was used for integration in the Brillouin zone. The optimized lattice constants of phosphorene unit cell are 3.31 2.28 A, which are consistent with previous experimental and theoretical results [35,57].
Results and discussion The stability of porous phosphorene In this part, the vacancy defect in phosphorene sheet is constructed and its stability under different ambient conditions are investigated. The porous phosphorene is designed by removing six phosphorous atoms (atoms within the black circle), which is shown in Fig. 1(a). Smaller defect is not studied due to the fact that gas molecules cannot pass the hole. And energy barriers of gas molecules going through the
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
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Fig. 1 e Optimized geometries of pure phosphorene (a) and self-passivated porous phosphorene (b) with vacancy defect in 6 £ 5 supercell.
larger vacancy defect are very low according to previous calculation. Therefore, only this kind of defect is selected and proper to investigate its capacity of gas purification. As the monolayer phosphorene actually has two layers of phosphorous atoms, the vacancy defect is more complex than those of graphene and silicene. As a consequence, after geometry optimization of the vacancy defect, an abnormal decagon ring is generated in order to avoid the dangling bond at the edge of the hole, which is described in Fig. 1(b). The optimized pore has the size of length: 7.11 A and width: 5.53 A, in good agreement with previous theoretical results [51]. Although former study has presented the stability of this kind of vacancy defect by introducing no dangling bonds [51], here further investigations are completed via ab initio molecular dynamic simulation (ab-MD) to confirm its stability under real working conditions. The simulations are run under NVT for 5 ps at 300 K and 500 K separately for the phosphorene sheet as shown in Fig. 2(a) and (b) respectively. It can be seen that the stability of pore structure is maintained at both 300 K and 500 K as shown in Fig. 2(c) and the porous structure of phosphorene cannot be disrupted even at higher temperature. Furthermore, in order to testify whether the phosphorous atoms of pore will chemically react with gas molecules investigated here, another ab-MD simulation is conducted with porous phosphorene exposing to these gas molecules, which is shown in Fig. 2(d). After 5 ps running under 500 K, it still shows the intact output structure of porous phosphorene, implying chemically inert to the considered gases.
The adsorption of various gas molecules Before permeating through the pores of phosphorene, gas molecules will firstly be adsorbed to the vacancy area. In order
to evaluate the stability of molecules adsorption on porous phosphorene, the adsorption energy is defined as: Ea ¼ EGas=Phosphorene EGas EPhosphorene where EGas/Phosphorene, EGas, and EPhosphorene are the total energy of gas molecule adsorption on porous phosphorene, a single gas molecule, and porous phosphorene, respectively. The adsorption of different gas molecules, including H2, N2, CO, CO2, H2O, and CH4, are studied and results are summarized in Table 1. Here, we define the distance (D0) as the length between the center of gas molecules and the upper plane phosphorous atoms because the monolayer phosphorene has two-layer-liked structure. Table 1 shows that the calculated A (for H2O) to 2.67 equilibrium distance (D0) ranges from 1.11 A. Obviously, the adsorption energy of H2O on porous (for CH4) phosphorene with a binding energy value of 0.36 ev considering the vdW correction is higher than others and this is in accordance with its hydrophilic property [50]. On the contrary, the weakest interaction locates between hydrogen and porous phosphorene with only 0.04 ev. Therefore, the self-passivated porous phosphorene is chemically inert to these gas molecules, and the interaction between gas molecules and pore is mainly caused by the vdW force, which also agrees with results from our former ab-initio molecular simulations.
The processes of gas separation In general, an energy barrier exists when one gas molecule permeates through a porous material, due to the interaction between gas molecule and atoms of the material, and this interaction varies to different gas molecules and porous materials. A better porous material should have the character
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
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Fig. 2 e The outcome structures of porous phosphorene after 5 ps NVT ab-initio MD at 300 K (a) and 500 K (b) and corresponding temperature changes vs simulation time described in (c). (d) Presents the results in the gases atmosphere after 5 ps NVT ab-initio MD at 500 K. that the target gases can get through the pore easily while others cannot. In this part, permeation of studied gas molecules through the self-passivated pore of phosphorene are investigated in detail. Here, the target gas is the hydrogen and the processes of hydrogen permeating containing three steps: from adsorption site moving into the pore, reaching to the site called transition state where the interaction is the highest,
Table 1 e The DFT-D2 calculated equilibrium distance D0 between the center of gas molecules and the plane of (A) upper layer phosphorous atom with corresponding binding energy Ea (ev) for gas molecules adsorption on porous phosphorene, and the diffusion energy barrier Eb (ev) for molecules passing through the pore. DFT-D2 H2 N2 CO CO2 H2O CH4
D0
Ea
Eb
2.66 2.35 1.74 2.31 1.11 2.67
0.04 0.15 0.21 0.22 0.36 0.17
0.07 0.85 0.83 0.97 0.85 1.36
and leaving the hole for the adsorption position on the other site of the porous phosphorene. As for the hydrogen, the most stable state of H2 adsorption on porous phosphorene before passing the pore (SS-in) is that in which the H2 molecular is parallel to the plane of phosphorene sheet above the center of the vacancy, as shown in Fig. 2. A slight change happens to the location of adsorption after H2 getting through the pore and the whole system reaches its stable structure at SS-out. Five sites along the path from SS-in to SS-out are set to search the energy barrier (Eb) of the hydrogen permeation using the CINEB calculation. The diffusion energy barrier is defined by Equation (1): Eb ¼ ETS ESSin
(1)
where, ETS and ESS-in represent the total Gibbs free energy of the transition state (TS) of H2 when passing through the selfpassivated vacancy of phosphorene and the SS-in of H2 initial adsorption on porous phosphorene. The energy barriers of various gas molecules getting through the porous phosphorene are present in Fig. 3, and the corresponding diffusion energy barriers are listed in Table 1. Specifically, the diffusion
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energy barrier for hydrogen molecule passing through the self-passivated vacancy defect of phosphorene is only 0.07 ev, which is smaller than that of porous graphene [22], twodimensional polyphenylene [25], and divacancy defect of silicene [34]. This negligible diffusion energy barrier of H2 suggests that the hydrogen molecule can permeate the porous phosphorene at the moderate temperature and pressure without external assistance. The optimized structure of the TS shows that the saddle point of the whole pathway energy locates at the site a little above the center of pore, which is caused by the abnormal shape of the self-passivated vacancy defect, as well as the situations of other gas molecules. This is also the reason to the non-smooth energy curve along the path of gas permeation in the Fig. 3. The calculated diffusion energy barriers are 0.85, 0.83, 0.97, 0.85, 1.36 ev for N2, CO, CO2, H2O, and CH4, respectively, which are much larger than that of H2 (0.07 ev). Therefore, porous phosphorene with selfpassivated defect can block these gas molecules and works as a promising hydrogen purification membrane. In order to obtain the hydrogen separation efficiency of porous phosphorene, the selectivity for H2 relative to other gas molecules (N2, CO, CO2, H2O, and CH4) permeating through the pores of phosphorene is estimated with the Arrhenius Equation (2) [22,25,27,34]: SH2 =Gas ¼
AH2 exp EH2 RT rH2 ¼ rGas AGas expf EGas =RTg
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and 1021 for H2 over N2, CO, CO2, H2O, and CH4, respectively. Compared with previous graphene-based [22], silica [3], and silicene [34] membranes, porous phosphorene exhibits the remarkable selectivity for H2 over H2O, CO2, N2, and CO, and the comparable capacity of purifying H2 over CH4. Therefore, it can be conducted that the porous phosphorene has a great potential in hydrogen separation based on the calculation results above and following analysis. First of all, the self-passivated phosphorene pore is chemically inert to the considered gas molecules as confirmed by the ab-initio molecular simulation. Because the phosphorous atoms bonding to the removed atoms from the phosphorene can form covalent PeP bonds spontaneously, which avoids leaving dangling bonds among the atoms around the vacancy defect. Similarly, the self-saturation of dangling bonds also exists in divacancy defect of silicone [34]. Whereas, this is hardly achieved in porous graphene proposed in literature [22], resulting in requesting the protection of edged carbon atoms with the assistance of hydrogen or other functional groups [28e30]. Although the porous graphene with divacancy defect can also form CeC covalent bonds, the pore size gives an unsatisfied result for hydrogen purification [59]. The self-passivated porous phosphorene in present work, on the contrary, has a reasonable pore size, obtaining a comparable capacity of separating hydrogen from the view of geometry. Therefore,
(2)
where r is the diffusion rate, A is the diffusion prefactor and here it is assumed that the diffusion prefactor of gas molecules studied is identical as A ¼ 1011 s1 [58], E is the diffusion energy barrier, and R is the gas constant. The temperaturedependent diffusion rate and selectivity profiles are investigated, which are shown in Fig. 4. It can been seen from Fig. 4(a) that the diffusion rates of H2 molecules are much higher than those of N2, CO, CO2, H2O, and CH4, consistent with the calculated diffusion energy barriers. As a result, the selectivity of hydrogen over other gases studied is extremely high in a wide temperature ranges. For example, as shown in Table 2, at room temperature (K ¼ 300 K), using porous phosphorene can achieve a high selectivity on the order of 1013, 1012, 1015, 1013,
Fig. 3 e Relative energy for H2, N2, CO, CO2, H2O and CH4 passing through the self-passivated pore of phosphorene as a function of adsorption height.
Fig. 4 e Diffusion rate (a) and selectivity for gas molecules (H2, N2, CO, CO2, H2O and CH4) passing through the selfpassivated defect of phosphorene as a function of temperature.
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Table 2 e The DFT-D2 calculated selectivity (S) of H2 relative to other gas molecules at room temperature (T ¼ 300 K), including H2/N2, H2/CO, H2/CO2, H2/H2O, and H2/CH4, and corresponding comparison results (diffusion energy barrier Eb (ev) and selectivity S for H2 relative to other gas molecule) of previously proposed porous membranes. Membranes Reference Eb (H2) S(H2/N2) S(H2/CO) S(H2/CO2) S(H2/H2O) S(H2/CH4) a b
Phosphorene
Graphene
Silica
Silicene
This work 0.07 1013 1012 1015 1013 1021
Ref. [15] 0.22a, 0.33b / / / / 1023, 108
Ref. [2] / 102 / 10 / 103
Ref. [26] 0.34 1011 1011 1011 102 1022
Hydrogenated porous graphene in Ref. [15]. Nitrogen functionalized porous graphene in Ref. [15].
the self-passivated porous phosphorene with larger size is not studied in this work, because it cannot theoretically perform a good result. In addition, as a metalloid element ranking later in the periodic table of elements, phosphorous atom has a larger vdW force radius compared with carbon and silicon, making it more difficult for gases like N2, CO, CO2, H2O, and CH4 with larger kinetic radius to passing through the porous phosphorene. It should also be noted that the theoretically calculated H2 selectivity of porous phosphorene here is an ideal result. In reality, the permeability and selectivity of traditional hydrogen separation membranes largely depend on many other factors in practical applications, including crystalline defect, membrane thickness and slight structure change. Besides, some early studies showed that the phosphorene is not stable in the water and in the condition with oxygen [43,60]. While the latest experiments has presented an efficient method to maintain its stability in these adverse conditions [44,46,48]. Additionally, the purpose of present work is located in hydrogen separation, and other gas like H2O is attended as impurity with negligible concentration. Therefore, the side influence from water reaches to its lowest position. So the character of porous phosphorene in gas separation should be noticed to all of us rather than remaining silence continually, and the phosphorene could be a new competitor for graphene [61].
Conclusion In present work, we have demonstrated that the phosphorene with self-passivated defects can be applied as a promising hydrogen separation membrane by conducting firstprinciples density function theory. Hydrogen molecule can permeate the porous phosphorene with surmountable diffusion energy barrier of only 0.07 eV. The porous phosphorene exhibits a high selectivity on the order of 1013, 1012, 1015, 1013, and 1021 for H2 over N2, CO, CO2, H2O, and CH4, at ambient temperature, respectively. Compared with graphene-based membranes, the self-passivated defect in phosphorene can be easily experimentally realized without needing protection from decorating other atoms. Furthermore, the porous phosphorene is chemically inert to most gas molecules according to the result from ab-initio molecular dynamics. Given these
advantages, our theoretical studies firstly unveil the great potential of porous phosphorene in hydrogen purification applications. Although the monolayer of phosphorene can be obtained through the experiment successfully [62], more efforts should also be paid to overcome the stability of phosphorene in the atmosphere with oxygen and water in the future to extend its application in a wider field.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51576019) and Chongqing university postgraduates' innovation project (CYB15016). One of the authors, Yayun Zhang, would like to acknowledge financial support from the Chinese Scholarship Council (CSC).
references
[1] Hufton J, Mayorga S, Sircar S. Sorption-enhanced reaction process for hydrogen production. AIChE J 1999;45:248e56. [2] Wang X, Wang N, Li M, Li S, Wang S, Ma X. Hydrogen production by glycerol steam reforming with in situ hydrogen separation: a thermodynamic investigation. Int J Hydrogen Energy 2010;35:10252e6. [3] De Vos RM, Verweij H. High-selectivity, high-flux silica membranes for gas separation. Science 1998;279:1710e1. [4] Pandey P, Chauhan R. Membranes for gas separation. Prog Polym Sci 2001;26:853e93. [5] Oyama S, Lee D, Hacarlioglu P, Saraf R. Theory of hydrogen permeability in nonporous silica membranes. J Membr Sci 2004;244:45e53. [6] Adhikari S, Fernando S. Hydrogen membrane separation techniques. Industrial Eng Chem Res 2006;45:875e81. [7] Ockwig NW, Nenoff TM. Membranes for hydrogen separation. Chem Rev 2007;107:4078e110. [8] David E, Kopac J. Development of palladium/ceramic membranes for hydrogen separation. Int J Hydrogen Energy 2011;36:4498e506. [9] Deng X, Luo D, Qin C, Qian X, Yang W. Hydrogen isotopes separation using frontal displacement chromatography with Pd-Al2O3 packed column. Int J Hydrogen Energy 2012;37:10774e8. [10] Doucet R, Gardner CL, Ternan M. Separation of hydrogen from hydrogen/ethylene mixtures using PEM fuel cell technology. Int J Hydrogen Energy 2009;34:998e1007.
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
[11] Huang F, Meng C. Hydrophobic platinumpolytetrafluoroethylene catalyst for hydrogen isotope separation. Int J Hydrogen Energy 2010;35:6108e12. [12] Hwang K-R, Lee S-W, Lee D-W, Lee C-B, Ji S-M, Park J-S. Bifunctional hydrogen membrane for simultaneous chemical reaction and hydrogen separation. Int J Hydrogen Energy 2014;39:2614e20. [13] Zhang Q, Liu G, Feng X, Chu KH, Deng C. Hydrogen networks synthesis considering separation performance of purifiers. Int J Hydrogen Energy 2014;39:8357e73. [14] Novoselov K, Geim AK, Morozov S, Jiang D, Katsnelson M, Grigorieva I, et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005;438:197e200. [15] Yoon HJ, Yang JH, Zhou Z, Yang SS, Cheng MM-C. Carbon dioxide gas sensor using a graphene sheet. Sensors Actuators B Chem 2011;157:310e3. [16] Dai J, Yuan J, Giannozzi P. Gas adsorption on graphene doped with B, N, Al, and S: a theoretical study. Appl Phys Lett 2009;95:232105. [17] Bernardo P, Drioli E, Golemme G. Membrane gas separation: a review/state of the art. Industrial Eng Chem Res 2009;48:4638e63. [18] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183e91. [19] Geim AK. Graphene: status and prospects. Science 2009;324:1530e4. M. Electron beam nanosculpting of [20] Fischbein MD, Drndic suspended graphene sheets. Appl Phys Lett 2008;93:113107. [21] Du H, Li J, Zhang J, Su G, Li X, Zhao Y. Separation of hydrogen and nitrogen gases with porous graphene membrane. J Phys Chem C 2011;115:23261e6. [22] D-e Jiang, Cooper VR, Dai S. Porous graphene as the ultimate membrane for gas separation. Nano Lett 2009;9:4019e24. [23] Bucior BJ, Chen D-L, Liu J, Johnson JK. Porous carbon nanotube membranes for separation of H-2/CH4 and CO2/ CH4 mixtures. J Phys Chem C 2012;116:25904e10. [24] Gu C, Gao GH, Yu YX. Density functional study of the adsorption and separation of hydrogen in single-walled carbon nanotube. Int J Hydrogen Energy 2004;29:465e73. [25] Li Y, Zhou Z, Shen P, Chen Z. Two-dimensional polyphenylene: experimentally available porous graphene as a hydrogen purification membrane. Chem Commun 2010;46:3672e4. [26] Jiao Y, Du A, Hankel M, Zhu Z, Rudolph V, Smith SC. Graphdiyne: a versatile nanomaterial for electronics and hydrogen purification. Chem Commun 2011;47:11843e5. [27] Zhang H, He X, Zhao M, Zhang M, Zhao L, Feng X, et al. Tunable hydrogen separation in spesp2 hybridized carbon membranes: a first-principles prediction. J Phys Chem C 2012;116:16634e8. [28] D-e Jiang, Sumpter BG, Dai S. Unique chemical reactivity of a graphene nanoribbon's zigzag edge. J Chem Phys 2007;126:134701. [29] Huang B, Li Z, Liu Z, Zhou G, Hao S, Wu J, et al. Adsorption of gas molecules on graphene nanoribbons and its implication for nanoscale molecule sensor. J Phys Chem C 2008;112:13442e6. [30] Vanin M, Gath J, Thygesen KS, Jacobsen KW. First-principles calculations of graphene nanoribbons in gaseous environments: structural and electronic properties. Phys Rev B 2010;82:195411. € rter W, Muntwiler M, Tamai A, Greber T, [31] Corso M, Auwa Osterwalder J. Boron nitride nanomesh. Science 2004;303:217e20. [32] Lim SH, Luo J, Ji W, Lin J. Synthesis of boron nitride nanotubes and its hydrogen uptake. Catal Today 2007;120:346e50.
7
[33] Vogt P, De Padova P, Quaresima C, Avila J, Frantzeskakis E, Asensio MC, et al. Silicene: compelling experimental evidence for graphenelike two-dimensional silicon. Phys Rev Lett 2012;108:155501. [34] Hu W, Wu X, Li Z, Yang J. Porous silicene as a hydrogen purification membrane. Phys Chem Chem Phys 2013;15:5753e7. nek D, et al. [35] Liu H, Neal AT, Zhu Z, Luo Z, Xu X, Toma Phosphorene: an unexplored 2D semiconductor with a high hole mobility. ACS Nano 2014;8:4033e41. [36] Li L, Yu Y, Ye GJ, Ge Q, Ou X, Wu H, et al. Black phosphorus field-effect transistors. Nat Nanotechnol 2014;9:372e7. nek D. Semiconducting layered blue [37] Zhu Z, Toma phosphorus: a computational study. Phys Rev Lett 2014;112:176802. [38] Peng X, Wei Q, Copple A. Strain-engineered direct-indirect band gap transition and its mechanism in two-dimensional phosphorene. Phys Rev B 2014;90:085402. [39] Wei Q, Peng X. Superior mechanical flexibility of phosphorene and few-layer black phosphorus. Appl Phys Lett 2014;104:251915. [40] Elahi M, Khaliji K, Tabatabaei SM, Pourfath M, Asgari R. Modulation of electronic and mechanical properties of phosphorene through strain. Phys Rev B 2015;91:115412. [41] Buscema M, Groenendijk DJ, Blanter SI, Steele GA, van der Zant HS, Castellanos-Gomez A. Fast and broadband photoresponse of few-layer black phosphorus field-effect transistors. Nano Lett 2014;14:3347e52. [42] Koenig SP, Doganov RA, Schmidt H, Neto AC, Oezyilmaz B. Electric field effect in ultrathin black phosphorus. Appl Phys Lett 2014;104:103106. [43] Ziletti A, Carvalho A, Campbell DK, Coker DF, Neto AC. Oxygen defects in phosphorene. Phys Rev Lett 2015;114:046801. [44] Carvalho A, Neto AHC. Phosphorene: overcoming the oxidation barrier. ACS Central Sci 2015;1:289e91. [45] Li P, Zhang D, Liu J, Chang H, Ye Sun, Yin N. Air-stable black phosphorus devices for ion sensing. ACS Appl Mater Interfaces 2015;7:24396e402. [46] Pei J, Gai X, Yang J, Wang X, Yu Z, Choi D-Y, et al. Producing air-stable monolayers of phosphorene and their defect engineering. Nat Commun 2016:7. [47] Edmonds M, Tadich A, Carvalho A, Ziletti A, O'Donnell K, Koenig S, et al. Creating a stable oxide at the surface of black phosphorus. ACS Appl Mater Interfaces 2015;7:14557e62. [48] Zhao Y, Wang H, Huang H, Xiao Q, Xu Y, Guo Z, et al. Surface coordination of black phosphorus for robust air and water stability. Angew Chem 2016;128:5087e91. [49] Kou L, Frauenheim T, Chen C. Phosphorene as a superior gas sensor: selective adsorption and distinct IeV response. J Phys Chem Lett 2014;5:2675e81. [50] Cui S, Pu H, Wells SA, Wen Z, Mao S, Chang J, et al. Ultrahigh sensitivity and layer-dependent sensing performance of phosphorene-based gas sensors. Nat Commun 2015:6. [51] Hashmi A, Farooq MU, Hong J. Long-range magnetic ordering and switching of magnetic state by electric field in porous phosphorene. J Phys Chem Lett 2016;7:647e52. [52] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865. [53] Giannozzi P, Baroni S, Bonini N, Calandra M, Car R, Cavazzoni C, et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J Phys Condens Matter 2009;21:395502. [54] Antony J, Grimme S. Structures and interaction energies of stacked grapheneenucleobase complexes. Phys Chem Chem Phys 2008;10:2722e9. n JG. Improved [55] Bucko T, Hafner Jr , Lebegue S, Angya description of the structure of molecular and layered
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108
8
[56] [57] [58]
[59]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
crystals: ab initio DFT calculations with van der Waals corrections. J Phys Chem A 2010;114:11814e24. Kagimura R, Chacham H. Knots in a graphene nanoribbon. Phys Rev B 2012;85:125415. Brown A, Rundqvist S. Refinement of the crystal structure of black phosphorus. Acta Crystallogr 1965;19:684e5. Blankenburg S, Bieri M, Fasel R, Mu¨llen K, Pignedoli CA, Passerone D. Porous graphene as an atmospheric nanofilter. Small 2010;6:2266e71. Leenaerts O, Partoens B, Peeters F. Graphene: a perfect nanoballoon. Appl Phys Lett 2008;93:193107.
[60] Wang G, Slough WJ, Pandey R, Karna SP. Degradation of phosphorene in air understanding at atomic level. 2D Materials 2016;3:025011. [61] Bagheri S, Mansouri N, Aghaie E. Phosphorene: a new competitor for graphene. Int J Hydrogen Energy 2016;41:4085e95. [62] Lu W, Nan H, Hong J, Chen Y, Zhu C, Liang Z, et al. Plasmaassisted fabrication of monolayer phosphorene and its Raman characterization. Nano Res 2014;7:853e9.
Please cite this article in press as: Zhang Y, et al., Hydrogen separation by porous phosphorene: A periodical DFT study, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.10.108