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Borophene as a Promising Material for Charge-Modulated Switchable CO2 Capture Xin Tan, Hassan A. Tahini, and Sean C. Smith ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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ACS Applied Materials & Interfaces

Borophene as a Promising Material for Charge-Modulated Switchable CO2 Capture

Xin Tan1,2, Hassan A. Tahini2, Sean C. Smith2,∗ 1

Hunan Key Laboratory of Micro-Nano Energy Materials and Devices, School of Physics and

Optoelectronics, Xiangtan University, Hunan 411105, P. R. China

2

Integrated Materials Design Centre (IMDC), School of Chemical Engineering, UNSW Australia,

Sydney, NSW 2052, Australia

KEYWORDS: borophene nanosheets, charge-modulated switchable, CO2 capture, high selectivity, high capacity, density functional theory

∗

Corresponding author: [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT Ideal carbon dioxide (CO2) capture materials for practical applications should bind CO2 molecules neither too weakly to limit good loading kinetics, nor too strongly to limit facile release. Although charge-modulated switchable CO2 capture has been proposed as a controllable, highly selective and reversible CO2 capture strategy, the development of a practical gas-adsorbent material remains a great challenge. In this work, by means of density functional theory (DFT) calculations, we have examined the possibility of conductive borophene nanosheets as promising sorbent materials for charge-modulated switchable CO2 capture. Our results reveal that the binding strength of CO2 molecules on negatively charged borophene can be significantly enhanced by injecting extra electrons into the adsorbent. At saturation CO2 capture coverage, the negatively charged borophene achieve CO2 capture capacities up to 6.73×1014 cm-2. Contrary to other CO2 capture methods, the CO2 capture/release processes on negatively charged borophene are reversible with fast kinetics, and can be easily controlled via switching on/off the charges carried by borophene nanosheets. Moreover, these negatively charged borophene nanosheets are highly selective for separating CO2 from mixtures with CH4, H2 and/or N2. This theoretical exploration will provide helpful guidance in searching for experimentally feasible, controllable, highly selective, and high-capacity CO2 capture materials with ideal thermodynamics and reversibility.

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INTRODUCTION Global warming and climate change concerns are primarily problems of too much greenhouse gases, for which carbon dioxide (CO2) is the main component, in the atmosphere.1-3 One crucial challenge of efficiently capturing and storing CO2 is the exploration of a practical gas-adsorbent material, which should bind CO2 molecules with just the right thermodynamic balance: neither too weakly to limit good loading kinetics, nor too strongly to limit the desorption rate.4-6 Recently, a novel approach of charge-modulated switchable CO2 capture has been proposed using density functional theory (DFT) computations as a controllable, high selective and reversible CO2 capture strategy for hexagonal boron nitride (h-BN) nanomaterials.7 In detail, the CO2 molecule undergoes weak physisorption (through van der Waals interaction) on neutral h-BN nanomaterials, such as nanosheets and nanotubes. A charge-induced chemisorption interaction can be invoked by injecting excess negative charge into the h-BN nanomaterials, which strengthens the binding of CO2 to the surface of the material. Moreover, the chemisorbed CO2 can desorb from h-BN nanosorbents when the excess electrons are removed. Both the adsorption and desorption processes are spontaneous when the excess electrons are injected or removed. The CO2 capture/release process can be simply controlled by switching the charges carried by h-BN nanomaterials on/off, indicating the possibility of designing gas-adsorbent materials for charge-modulated switchable CO2 capture. However, the large band gap of h-BN (ca. 5.8 eV8,9) makes it challenging to effectively inject the required electrons into it, which is prerequisite for charge-modulated switchable CO2

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capture. To overcome the this problem, two basic design strategies has been presented to develop the practical candidate materials: (1) design composite systems that will facilitate charge transfer to gas-adsorbent materials with poor electric conductivity, such as h-BN,10,11 and (2) explore intrinsic conductive gas-adsorbent materials, which have large amounts of active adsorption sites.12-14 Borophene, a new type of two-dimensional (2D) boron sheets, has been very recently grown successfully on single-crystal Ag (111) substrates.15-16 Different types of borophene, such as β12 sheet, χ3 sheet and so on, have been observed by scanning tunnelling microscopy15,16 and predicted by DFT computations17. Moreover, both experimental and theoretical results have shown that all these borophene are metallic materials with high electronic conductivity,16,17 which suggest that the charge states of borophene could be easily modified experimentally, and should satisfy the prerequisite for charge-modulated switchable CO2 capture. The extensive exploration of novel nanostructures as practical gas-adsorbent materials for charge-modulated switchable CO2 capture inspired us to ask an interesting question: can the intrinsic conductive borophene nanosheets be utilized as good sorbent materials for chargemodulated switchable CO2 capture? To address this question, we adopted a systematic investigation of the adsorption energies of CO2 molecules on neutral and negatively charged borophene, and kinetic process of CO2 adsorption/desorption on negatively charged borophene by means of comprehensive DFT computations. We found that the binding strength of CO2 molecules on negatively charged borophene can be significantly enhanced when extra

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electrons are injected into the adsorbent. At saturation CO2 capture coverage, the negatively charged borophene achieve CO2 capture capacities up to 13.5×1014 cm-2. Contrary to other CO2 capture methods, the CO2 capture/release processes on negatively charged borophene are reversible with fast kinetics, and can be easily controlled via switching on/off the charges carried by borophene nanosheets. Moreover, these negatively charged borophene nanosheets are highly selective for separating CO2 from mixtures with CH4, H2 and/or N2. This theoretical exploration will provide helpful guidance in searching for experimentally feasible, controllable, highly selective, and high-capacity CO2 capture materials with ideal thermodynamics and reversibility.

METHODS Our DFT calculations were carried out using the linear combination of atomic orbital and spin-unrestricted method implemented in Dmol3 package.18 The generalized gradient approximation (GGA) in Perdew-Burke-Ernzerhof (PBE) functional form19 together with an all-electron double numerical basis set with polarization function (DNP) were adopted, which have shown to be adequate for hexagonal B3620 and B2C graphene21. Considering the standard PBE functional is incapable of giving an accurate description of weak interactions, we adopted a DFT+D (D stands for dispersion) approach with the Grimme vdW correction in our computations.22 The real-space global cutoff radius was set to be 4.3 Å. DMol3 is a commercial (and academic) software package which uses DFT with a numerical radial

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function basis set to calculate the electronic properties of molecules, clusters, surfaces and crystalline solid materials from first principles. The real-space global radius cutoff is actually applied to the generation of the numerical basis sets. Here we chose β12 sheet of borophene16,17 as an example to investigate the CO2 capture/release processes on borophene nanosheets. A (2×4) supercell with periodic boundary conditions in the x-y plane (Figure 1(a)) was employed, and the vacuum space was set to larger than 20 Å in the z direction to avoid interactions between periodic images. Some of our calculations were repeated using a (3×6) supercell, and the differences in the calculated energies between the smaller and larger unit cells were found to be very small (< 0.05 eV). In geometry optimizations, all the atomic coordinates were fully relaxed up to the residual atomic forces smaller than 0.001 Ha/Å, and the total energy was converged to 10-5 Ha. The Brillouin zone integration was performed on a (5×5×1) Monkhorst-Pack k-point mesh.23 In order to simulate negatively charged borophene nanosheets, we introduced excess electrons into the supercell. For example, for 3.5 e- negatively charged borophene, we introduced excess 3.5 electrons into the (2×4) supercell.

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Figure 1. (a) Top (upper) and side (lower) views of the optimized structure of (2×4) β12 sheet of borophene. The unit cell of borophene is denoted by black dash lines, and B1, B2 and B3 denote different B atoms in borophene unit cell. (b) The calculated band structure and project density of states (PDOS) of (1×1) β12 sheet of borophene. The blue dashed line denotes the Fermi level.

To study the gas adsorption on borophene, we defined the adsorption energy of CO2, CH4, H2, and N2 molecules on borophene as follows /

(1)

where , , , and are the total energy of borophene with the adsorbed gas, isolated borophene nanosheet, isolated gas molecule, and number of gas molecules adsorbed on borophene. Based on this definition, a more negative adsorption energy denotes a stronger adsorption of the gas molecule to borophene. Here, we used the Mulliken method24 to determine the electron distribution and transfer mechanism.

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RESULTS AND DISCUSSION Geometric and Electronic Properties of β12 Sheet of Borophene. The optimized structure of β12 sheet of borophene in a (2×4) supercell was optimized without any symmetry constraint by DFT computations. As shown in Figure 1(a), the borophene nanosheet is a strictly planar rectangular monolayer with lattice constants a = 5.08 Å and b = 2.92 Å. Analyzing the electronic band structure and project density of states (PDOS) of β12 sheet of borophene (Figure 1(b)) revealed that this planar nanosheet is metallic, and the states at the Fermi level originate mainly from B-2p. All these results are consistent with previous calculations.16 Since metallic materials are referred as very good electrical conductor, we expect that such an intrinsically metallic feature of the borophene leads to good electrical conductivity and high electron mobility, which could facilitate the requisite excess electrons be effectively injected into borophene nanosheets for charge-modulated switchable CO2 capture. Adsorption of a Single CO2 on Neutral and 3.5 e- Negatively Charged Borophene Nanosheets. We next shift our attention to the adsorption of a single CO2 on neutral and negatively charged borophene. In the unit cell of β12 sheet of borophene, there are three different B atoms denoted as B1, B2 and B3 in Figure 1(a). Here, we considered all the possible adsorption sites: directly on top of a B atom, above the center of a honeycomb-like hole, and bridge site above the B-B bond. Figure 2 shows the lowest-energy configurations of a CO2 molecule absorbed on B3 site of the neutral and 3.5 e- negatively charged borophene. Please note that the

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lowest-energy configurations of a CO2 molecule absorbed on B1 and B2 sites of the neutral and 3.5 e- negatively charged borophene are shown in Figure S1and Figure S2, respectively. On neutral borophene nanosheets (Figure 2(a), Figure S1(a) and Figure S2(a)), the CO2 molecular has a linear shape, which is parallel to borophene and locates on top of one B atom. The distances between the C atom of CO2 and closest B atom are about 3.309-3.507 Å, and the geometry of the linear CO2 molecule is similar to a free CO2 molecule. Mulliken population analysis shows that only a negligible amount of electrons (about 0.001-0.009 e-) transfer from CO2 molecule to borophene. In this case, the CO2 molecule is weak physisorption on neutral borophene with small adsorption energy of -0.15 to -0.19 eV.

Figure 2. Top (upper) and side (lower) views of the lowest-energy configurations of a CO2 absorbed on B3 site of the (a) neutral and (b) 3.5 e- negatively charged borophene. The light magenta, red and grey balls denote the B, O and C atoms, respectively. The adsorption energies of a CO2 on neutral and 3.5 e- negatively

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charged borophene are listed in the figures.

When extra 3.5 e- are injected into borophene supercell (Figure 2(b), Figure S1(b) and Figure S2(b)), the CO2 molecule is strongly adsorbed at one B atom, especially B2 and B3 sites, and changes from physisorption into chemisorption on 3.5 e- negatively charged borophene. The distances between the C atom of CO2 and B atom of brophene are shortened to 1.737-1.780 Å. Moreover, the O-C-O angle is bent from 178.9-179.7º to 125.2-126.2º, the two double C=O bonds are elongated from 1.176 to 1.269-1.273 Å, and the charge transfer from borophene to CO2 molecule increase to 0.83-0.86 e-. For the 3.5 e- negatively charged case, the adsorption energies of CO2 molecule at B1, B2 and B3 sites are remarkably enhanced to -0.69, -1.23 and -1.35 eV, respectively. Please note that the binding strength of CO2 adsorbed at B2 and B3 sites are much stronger than CO2 molecule on other high-performance adsorbents with the adsorption energies ranging from -0.4 to -0.8 eV,6 which indicate the negatively charged borophene is an ideal gas-adsorbent material for CO2 capture. We also simulated the adsorption of CO2 molecule on positively charged borophene, and found that the CO2 molecule is weak physisorption on positively charged borophene with small adsorption energy less than -0.20 eV, which indicate that the binding strength of CO2 on positively charged borophene are not enhanced in comparison with the neutral case. Please note that 3.5 e- negatively charged borophene is an example for relatively large negatively charged borophene, where CO2 molecule is chemisorbed on borophene with strong binding strength.

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In order to understand the underlying mechanism of CO2 capture on negatively charged borophene, the electron density distributions of the frontier orbitals (i.e., the lowest unoccupied molecular orbital (LUMO), LUMO+1, and LUMO+2) of neutral borophene, and the differences of electron density distributions between the neutral and 2 e- negatively charged borophene are considered in Figure 3(a)-(d). Figure 3(a)(c) clearly shows that the frontier orbitals (LUMO, LUMO+1, and LUMO+2) of neutral borophene are predominantly distributed on the B2 and B3 atoms, indicating that when an extra electron is introduced to neutral borophene the electron will fill the p-like orbitals of B2 and B3 atoms. This is confirmed by the comparison of the differences in electron density distributions of neutral and 2 e- negatively charged borophene (Figure 3(d)). Since CO2 is a Lewis acid and it prefers to accept electrons during reaction, the B atoms (B2 and B3 atoms) of negatively charged borophene can donate electrons to CO2 molecule, forming a new bond between B atoms of borophene and the C atom of CO2 (Figure 3(f)). This is significantly different from the case that CO2 on neutral borophene (Figure 3(e)), and could explain why the CO2 molecule has a strong interaction with B2 and B3 atoms of negatively charged borophene.

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Figure 3. (a) LUMO, (b) LUMO+1, and (c) LUMO+2 of a neutral borophene nanosheet. These orbitals are drawn with an isosurface value of 0.03 e/Å3. The colors of the orbitals show the wave function (yellow, positive; blue, negative). (d) The differences of electron density distributions between the neutral and 2 e- negatively charged borophene. Yellow refer to electron-rich area, and the isosurface value is 5×10-7 e/Å3. The total charge density distribution of a single CO2 molecule adsorbed at B3 site on (e) neutral and (f) 3.5 e- negatively charged borophene. The isosurface value is 0.6 e/Å3. Please note that there is an overlap of the electron densities of the C atom of CO2 molecule and B3 atom of borophene in (f), which indicates the formation

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of a new bond.

To study the kinetic process of CO2 adsorption/desorption on 3.5 e- negatively charged borophene, we investigated the energy change of a CO2 adsorbed on borophene after 3.5 extra electrons are injected or extracted. For the adsorption process (Figure 4(a)), we initiated the calculation with the most stable configuration of a CO2 physisorption on neutral borophene. Then 3.5 extra electrons were introduced into borophene supercell, and we examined the energy changes as the system relaxes to the 3.5 e- negatively charged optimized geometry. For the desorption process (Figure 4(b)), we initiated the calcualtion with the most stable configuration of a CO2 chemisorption on 3.5 e- negatively charged borophene. Then 3.5 electrons were extracted from borophene supercell, and the system was allowed to relax to form a physisorbed CO2 molecule. As 3.5 extra electrons were injected into neutral borophene, the interactions between the CO2 and the 3.5 e- negatively charged borophene were significantly enhanced compared to the neutral borophene case. Therefore, the physisorbed CO2 molecule spontaneously relaxes to chemisorption configuration, and the adsorption process is exothermic by 1.13 eV without any energy barrier. For the desorption process, when 3.5 electrons are removed from the 3.5 e- negatively charged borophene, the CO2 molecule spontaneously desorbs from borophene and returns to the weakly bound configuration. Similar to the adsorption process, the desorption process is also exothermic by 1.99 eV without any energy barrier. In a word, the CO2 capture/release processes on negatively charged borophene

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are reversible with fast kinetics, and can be easily controlled via switching on/off the charges carried by borophene nanosheets.

Figure 4. The energy change of (a) the relaxation (capture) of a physisorbed CO2 on borophene after 3.5 extra electrons are injected, and (b) the reverse relaxation (release) process of a chemisorbed CO2 from borophene after 3.5 extra electrons are removed from the supercell.

Charge Density Effects on Single CO2 Adsorption on Negatively Charged Borophene Nanosheets. For charge-modulated switchable CO2 capture method, the charge density on adsorbents determines the binding strength of CO2 molecule on gasadsorbent materials. To investigate the charge density effects on CO2 capture on negatively charged borophene, we studied CO2 capture on negatively charged borophene with different charge densities. Here, the charge densities of borophene was defined as follows

where , and are the charge densities of borophene, the total charge and the

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surface area in a (2×4) supercell, respectively. The surface area of borophene in (2×4) supercell can be calculated as 8 , where and b are the lattice constants of (2×4) supercell. Figure 5 shows the adsorption energies of a CO2 adsorbed at B3 site on negatively charged borophene and the charge transfer from borophene to CO2 molecule as functions of charge densities. The adsorption energy of CO2 on borophene is small (+0.18 ~ -0.35 eV) for small negative charge density case (< 21.0×1013 e-/cm2), and charge transfer from borophene to CO2 molecule is negligible. However, when the negative charge density is larger than 21.0×1013 e-/cm2, the binding strength of CO2 and the charge transfer from borophene to CO2 enhanced dramatically with the increase of the charge density, which suggest that the CO2 can only adsorb on negatively charged borophene with large charge density. Considering the required adsorption energy of CO2 on high-performance gas-adsorbent material is -0.4 to -0.8 eV,6 we defined the required charging density for CO2 capture on negatively charged borophene is more than 25.2×1013 e-/cm2. Please note that the CO2 adsorption energy depends insensitively on charge density for small negative charge density region, but becomes strongly dependent on charge density for large negative charge density region. This is because CO2 molecule is physisorbed on borophene with small negative charge density, where the charge transfer from borophene to CO2 is relatively small (red line in Figure 5). In small negative charge density region, the CO2 adsorption energy is small and depends slightly on charge density on borophene. Moreover, as the negative charge density on

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borophene increase, more electrons transfer to CO2 molecule, leading CO2 molecule negative charged. Therefore, the repulsive interaction between negatively charged borophene and negatively charged CO2 molecule increases as the negative charge on borophene increases, this is the reason why the increasing negative charge will have a little more positive absorption energy in small negative charge density region. On the contrary, CO2 is chemisorbed on borophene with large negative charge density (Figure 2(b)), where a new bond between B atoms of borophene and the C atom of CO2 is formed (Figure 3(f)). In this case, the charge transfer from borophene to CO2 is large and is dramatically increased as the negative charge density on borophene increases (red line in Figure 5). The increased charge transfer results in much stronger binding of the chemisorbed CO2 molecule on borophene. This is the reason why the CO2 adsorption energy is strongly dependent on charge density for large negative charge density region.

Figure 5. The adsorption energies of a CO2 adsorbed at B3 site on negatively charged

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borophene and the charge transfer from borophene to CO2 molecule as functions of charge densities. The gray area denotes the adsorption region.

CO2 Capture Capacity on Negatively Charged Borophene Nanosheets. The CO2 capture capacity is another important ceria for high-performance gas-adsorbent materials. To estimate CO2 capture capacity, we explored the maximum number and the average adsorption energies of captured CO2 molecules on negatively charged borophene as functions of negative charge densities, as shown in Figure 6(a). Here, we used (2×2) supercell, and the maximum number of captured CO2 for different negatively charged borophene is determined by gradually increase of the number of adsorbed CO2 on negatively charged borophene until no more CO2 molecule can be chemisorbed. We calculated the average adsorption energy of captured CO2 as the total adsorption energy divided by the maximum number of captured CO2. For small charge density (≤ 21.0×1013 e-/cm2), no CO2 can be captured by negatively charged borophene. As we increase the negative charge density from 25.2×1013 to 63.9×1013 e/cm2, one, two and four CO2 molecules are chemisorbed on the negatively charged borophene with the average adsorption energy of chemisorbed CO2 molecules ranging from -0.79 to -2.82 eV (Figure 6(a)). Further increase number of CO2 molecules in the supercell leads to some CO2 molecules moving far away from the adsorbent during the geometry optimization even if we further increase the charge density of borophene. Thus, we define the saturation CO2 capture coverage of negatively charged borophene as four chemisorbed CO2 molecules chemisorbed in each (2×2)

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supercell (i.e. CO2 capture capacity 6.73×1014 cm-2), which shows in Figure 6(b) and (c).

Figure 6. (a) The maximum number and the average adsorption energies of captured CO2 molecules on negatively charged borophene as functions of negative charge densities. (b) Top and (c) side views of the lowest-energy configuration of four chemisorbed CO2 molecules on (2×2) negatively charged borophene with charge density 57.2×1013 e-/cm2.

CH4, H2, and N2 Adsorption on Borophene Nanosheets. Currently, three types of gas mixtures that are of the most interest are targeted for capture technologies, namely, postcombustion (predominantly CO2/N2 separation), natural gas sweetening

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(CO2/CH4), and precombustion (CO2/H2) capture.25 In order to demonstrate the high selectivity of negatively charged borophene nanosheets for CO2 capture, the adsorption energies of CH4, H2, and N2 on neutral and negatively charged borophene are calculated and compared with those of CO2, which are listed in Figure 7. Clearly, for CH4, H2 and N2, the adsorption of these gases on neutral and negatively charged borophene is physical rather than chemical with small adsorption energy (less than 0.37 eV). Please note that when we inject 3.5 electrons into borophene, the N2 is repelled far away from borophene nanosheet, indicating 3.5 e- negatively charged borophene can’t capture N2 gas. Since the Dmol3 code cannot treat the charged system with gas molecule far away from the adsorbent, we cannot calculate the adsorption energies of N2 on 3.5 e- negatively charged borophene. On the contrary, although CO2 is weakly physisorbed adsorbed at neutral and 2 e- negatively charged borophene, the CO2 is chemisorbed on 3.5 e- negatively charged borophene with large adsorption energy of -1.35 eV. These results suggest that the negatively charged borophene has very high selectivity for capturing CO2 from CH4, H2 and/or N2 mixtures.

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Figure 7. The adsorption energies of CO2, CH4, H2, and N2 on neutral, 2 e- and 3.5 enegatively charged borophene.

CONCLUSIONS In summary, our DFT calculations have demonstrated that conductive borophene nanosheets are highly promising candidate for charge-modulated switchable CO2 capture. By modification of the charge state of borophene, the binding strength of CO2 molecules on negatively charged borophene can be significantly enhanced. Compared with other CO2 capture methods, the CO2 capture/release processes on negatively charged borophene are reversible with fast kinetics, and can be easily controlled via switching on/off the charges carried by borophene nanosheets. Moreover, these negatively charged borophene nanosheets are highly selective for separating CO2 from mixtures with CH4, H2 and/or N2. Here we note that several

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experimental methods, such as electrochemical methods, electrospray, electron beam, supercapacitor or by gate voltage control,26-28 have been proposed as efficient ways to modify the charge state of carbon-based and boron nitride nanomaterials. In addition, we also note that the minimum charge density for CO2 capture on negatively charged borophene is 25.2×1013 e-/cm2, which is comparable to other charge-modulated CO2 capture materials, for example, nitrogen-doped graphene nanosheets (~21.5×1013 e/cm2),14 h-BN nanosheets (~15.0×1013 e-/cm2),7 and graphitic carbon nitride (g-C4N3) nanosheets (~18.0×1013 e-/cm2).13 If we use a supercapacitor to charge borophene, the charge density 25.2×1013 e-/cm2 corresponds to about 40.4 micro-coulombs/cm2, which could be achieved with 3V charging at an average capacitance of ~13.5 microfarads. Therefore, we believe that negatively charged borophene is an experimentally feasible material for charge-modulated switchable CO2 capture. This theoretical exploration will provide helpful guidance in searching for experimentally feasible, controllable, highly selective, and high-capacity CO2 capture materials with ideal thermodynamics and reversibility, and at the same time driving further theoretical and experimental efforts in this direction.

AUTHOR INFORMATION Supporting Information The lowest-energy configurations of a CO2 absorbed on B1 site of the neutral and 3.5 e- negatively charged borophene. The lowest-energy configurations of a CO2 absorbed on B2 site of the neutral and 3.5 e- negatively charged borophene.

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Corresponding Author *Sean C. Smith. Email: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the Grants from National Natural Science Foundation of China (No. 51502255), and was undertaken with the assistance of resources provided by the National Computing Infrastructure (NCI) facility at the Australian National University; allocated through both the National Computational Merit Allocation Scheme supported by the Australian Government and the Australian Research Council grant LE120100181 (“Enhanced merit-based access and support at the new NCI petascale supercomputing facility, 2012-2015).

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Table of Contents (TOC)

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Borophene as a Promising Material for Charge ... - ACS Publications

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