First-Principles Study on the Mechanism of Hydrogen Decomposition

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First-Principles Study on the Mechanism of Hydrogen Decomposition and Spillover on Borophene Xianfei Chen, Jia Liu, Wentao Zhang, Bei-Bei Xiao, Peicong Zhang, and Longsan Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05019 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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The Journal of Physical Chemistry

First-Principles Study on the Mechanism of Hydrogen Decomposition and Spillover on Borophene Xianfei Chen1, 2*, Jia Liu1, Wentao Zhang1, Beibei Xiao3, Peicong Zhang1, Longsan Li2 1

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China

2

Postdoctoral Innovation Practice Base of Sichuan Konkasnow New Material Co.,Ltd, Yaan 625400, China

3

School of Energy and Power Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China

ABSTRACT

Borophane, a derivate of borophene, has been shown to eliminate the phonon imaginary frequency of borophene entirely with enhanced structural stability and be a 2D Dirac material with many appealing properties similar to its counterpart graphene. However, the mechanisms involved in borophene hydrogenation are still unclear, which are essential to borophane fabrication in experiment and benefit our understanding of borophene functionalization. In this

ACS Paragon Plus Environment

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work, we investigate H2 adsorption and dissociation with (without) an external field and the subsequent spillover of H atoms on borophene based on density functional theory (DFT) to shed light on the procedure of borophene hydrogenation. We find that the incorporation of positive electric fields could facilitate the borophane formation with shallower energy barriers for H2 decomposition and H atoms present ultra-high mobility on borophene under positive field. The origin of the field modulated energy barriers has been discussed. Our work provides an alternative method to hydrogenate borophene, which contributes to the application of borophane in ultrahigh speed electronic devices.


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1. INTRODUCTION Graphene, an allotrope of carbon arranged into two-dimensional (2D) honeycomb lattices with flat monolayer structure, was synthesized firstly through mechanical exfoliation of graphite in 2004.1 Afterwards, many excellent properties of graphene have been identified for promising applications, such as high chiral quantum hall effects (QHE), ultrahigh electron mobility, zero band gap and metallic properties.2,3 Boron is the element located before carbon in the periodic table, which shares similar capabilities with carbon in producing stable covalent bonds and thus holds great potential to form 2D materials. Accordingly, several boron nanosheets with 2D configurations have been predicted in theory, such as snub-type4 and α-, β-, γ- and δ-types boron sheets,5-9 struc-η (η = 1/4, 1/5, 1/6 etc.) boron sheets10 and so on. More interestingly, graphenelike two-dimensional ionic boron with double Dirac cones has been predicted, which is the first report about 2D ions boron to be stable at atmospheric pressure.11 These advancements greatly enrich our understanding of boron-based 2D materials that present complex chemical bonding characteristic. Nevertheless, 2D boron nanosheet, namely monolayer borophene, has never been realized in experiment until the pioneer work of Mannix et al. where they found that borophene could be synthetized on the (111) surface of Ag substrate,12 opening new possibility for discovering novel properties of 2D borophene and its derivatives. The structure of borophene regards quasi-planar B7 cluster as the basic repeating unit cell,12 which was composed mainly of B6 hexagonal rings and an additional B atom situated above the center of B6 hexagon. As a result, borophene is a monolayer boron nanosheet with regular fluctuations. The Ag substrate could promote the growth of borophene, but on the other hand, it might restrict the applications of borophene in some fields. It has been confirmed that the imaginary frequency parts of phonon band existed in the freestanding borophene may lead to the instability


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of borophene structure with the long-wavelength transversal vibrations,13,14 and finally contribute to the observable distorting of the 2D structure.12 In light of that, utilization of borophene need integrate with the Ag substrate to guarantee its structural stabilization, which would greatly limit its application. Previous works confirmed that the electronic properties and structural stability of several 2D materials such as graphene could be improved through chemical functionalization,15 where hydrogenation is the most significant method.16-19 Indeed, it has been demonstrated recently that boron hydride sheets, a hydrogenated counterpart of 2D boron sheets, could improve their dynamical stability.20 Hydrogenating borophene into borophane could not only eliminate the phone imaginary frequency entirely, but also introduce several exciting properties different from its counterpart.13,20,21 For example, the in-plane Young’s module of pristine borophene reaches 398 GPa·nm along the zigzag direction while only 170 GPa·nm along the armchair direction,12 indicating strong anisotropic. However, it is attenuated over hydrogenation, where the Young’s modulus turn to be 172 and 111 GPa·nm along these two directions respectively,22 much smaller than those of borophene due to the reducing of B-B bond strength. More interestingly, hydrogenation brings about Dirac band dispersion into borophane,13,20 enabling ultrahigh fermion mobility,23 chiral anomaly,24 suppression of back scattering24 and so forth similar to graphene. Owing to the Dirac band structure from the unpaired pz orbital of C atoms,25 the Fermi velocity of graphene could reach up to 8.2×105 m/s.26,27 However, the Fermi velocity of borophane could reach up to 3.5×106 m/s predicated under HSE06 level,13 which is almost four times higher than that of graphene. Moreover, borophane has recently been demonstrated to be a promising anode material for Na ion battery, inferring its great potential in electrochemical applications.28


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However, the mechanism involved in the hydrogenation of borophene into borophane has not been specified up to now, which is essential for its experimental synthesis in future. In such context, decomposition of H2 molecule and the hydrogen spillover followed on the surface of borophene, all of which refer to the feasibility for borophane fabrication, have been investigated meticulously in this work. In order to reduce the energy barrier of borophene hydrogenation, an external perpendicular electric field is applied and its effects on H2 adsorption and dissociation are also discussed. 2. CALCULATION METHOD All calculations were performed in DMol3 module based on spin-polarized density functional theory (DFT) using generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) as the exchange-correlation function.29 The basis set was specified as double numerical plus polarization (DNP), while all electron including some relativistic effect is used as the core treatment. We chose DFT-D2 method within the Grimme scheme to describe the Van der Waals interactions existed between H2 and borophene.30,31 The geometry structures were fully relaxed until the force, displacement and energy tolerance are less than 0.002 Ha/ Å (1 Ha = 27.211 eV), 0.005Å and 1×10-5 Ha, respectively. Linear synchronous transition/quadratic synchronous transition (LST/QST) and nudged elastic band (NEB) tools were utilized to review the dissociation and diffusion pathways of H2 and H atoms on borophene. The k-point was set to be 9×9×1 for Brillouin zone integration. The H-H bond length (lH-H) in each H2 was set to be 0.740 Å before geometry optimization in accordance with its experimental value.32 The vacuum layer was set to be larger than 24 Å to diminish the interactions between interlayers. Adsorption energy (Ead) of a single H2 physically adsorbed on borophene is defined as, Ead = Eborophene+H2 – (Eborophene + EH2)

(1)


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where Eborophene+H2 is the total energy of borophene with a single H2 adsorbed on it, while the Eborophene and EH2 represent the energy of pristine borophene and that of a single H2, respectively. Besides, the binding energy (Eb) of isolated H atom chemically adsorbed on borophene is denoted as, Eb = [Eborophene+2H – (Eborophene + EH2)] / 2

(2)

where Eborophene+2H is the total energy of borophene with two H atoms bonded on it, and the EH is the energy of an isolated H atom. 3. RESULTS AND DISCUSSION Similar to graphane,33 the entire processes for borophene hydrogenation could be divided into three steps. Firstly, a single H2 molecule adsorbs on borophene surface physically. Then, the adsorbed H2 dissociates into two H atoms that are chemically adsorbed on borophene surface with covalent bonds formed between H and B atoms. Finally, the H atoms diffuse on borophene from one B atom to the nearest one (i.e. spillover) until all B atoms are bonded with H. These processes should perform on both sides of borophene in light of the difficulty for H atom to penetrate through the borophene layer. 3.1. ADSORPTION OF H2 The adsorption of H2 molecules determines the initial states (IS) of the dissociation reactions, which also inflects the chemical activity of reactants and subsequent reaction path. Here, various adsorption sites have been considered to check for the stable configuration. Up to 19 adsorption configurations with H2 being parallel or perpendicular to borophene surface have been considered, whose Ead values are less than −0.1 eV. Here, three typical energy favorable adsorption sites have been identified as indicated in Figure 1, where H2 could adsorb above the center of B6 hexagon (denoted as CI), above the diamond formed by four B atoms (CII) and the


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B-B bond on crest (CIII), respectively. A borophene supercell of 3×5×1 is used to simulate the H2 adsorption, where the distance between two H2 molecules are 8.66 and 8.07 Å along the zigzag and armchair directions, large enough to avoid interactions therein. The light blue area in Figure 1 represent the unit cell whose lattice parameters are 2.89 (au) and 1.61 Å (bu) respectively, being in good agreement with the experimental value while bu is a bit shorter than that of borophene on Ag substrate (1.67 Å) in previous report.12 Also the bulking height (d) between the upper boron atoms plane and the lower one is 0.90 Å, which is larger than d ≈ 0.80 Å of borophene on Ag substrate.12 Such discrepancy could be attributed to the strong interaction between borophene and Ag substrate, which depress the corrugation of borophene. The calculated Ead values and corresponding configuration details are tabulated in Table 1. Generally, H2 adsorption in CII and CIII are metastable with Ead of −0.052 and −0.035 eV, respectively. The corresponding adsorption heights (h) are 2.68 and 3.20 Å, in line with the understanding that stronger interactions between H2 and substrate lead to shorter distance therein. The most energetic adsorption site lies in the CI with Ead = −0.053 eV and h = 2.68 Å. H2 adsorption on borophene have much lower Ead values compared with previous reported ones on silicene (−0.166 eV)34 and graphene (−0.153 eV)33. Ead of H2 on borophene could be reflected by the transferred charge values. The charge transfer from borophene to H2 (∆q) of these configurations are calculated to be −0.020, −0.020 and −0.014 e−1 as exhibited in Table 1, which are almost 1/3 of those in silicene (−0.058 e−1).34 Considering the fact that these Ead values are considerably small and comparable with the average kinetic energy of H2 at room temperature (kBT = 0.026 eV), it is plausible that H2 could obtain enough energy to adjust their adsorption configurations freely and might even still stay in gaseous state at room temperature. It should be noted that the incorporation of Van der Walls correction is necessary to describe the H2 adsorption, which could be clarified by the comparison of the adsorption


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energies (Ead) with (without) the –D term. As shown in Table S1, Ead without the D-term are positive, implying an endothermic process, while they turn out to be negative under DFT-D2 level. Results obtained from DFT-D3 per Grimme35,36 give the same conclusion. As a result, although the most favorable adsorption site for H2 located on borophene is CI, the decomposition mechanisms of H2 over CII and CIII have also been considered in the following part. Previous works have demonstrated that interactions between gas molecules and 2D materials as well as the electronic properties of several low-dimension materials could be modified by the application of external electric field (F).37,38 Hence an external F has thus been introduced to modulate the adsorption of H2 on borophene as well as the Ead. The dependence of Ead on F is shown in Figure 2, where the direction of a positive F indicated by the blue arrows is denoted as that pointing from borophene to H2 molecule. Indeed, the positive F would enhance the adsorption of H2 on borophene greatly as evidenced by the increased Ead. When F increase from 0 to 1.54 V/Å, the Ead values for CI change from −0.053 to −0.128 eV. Similar results could also be found for those of CII, where a nearly generated cure with that of CI could be found on Figure 2. F could also enhance the adsorption of H2 on borophene over CIII although their Ead are still much smaller than those on the other two configurations. 3.2. DISSOCIATION OF H2 Then, we turn to investigate its dissociation process to shed light on the borophene hydrogenation mechanism. After decomposition, H atoms could reside on meta, para and ortho sites of B6 hexagon as shown in Figure 3a. To estimate the stability of H atoms that are chemically bonded with borophene, we calculate the Eb according to the formula 2. H atoms adsorbed on meta sites show the most favorable bond strength with a Eb of −0.67 eV, larger than those on para (−0.59 eV) and ortho (−0.40 eV). In contrast, the H-B bond lengths (lH-B) are all


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1.21 Å without obvious fluctuation. However, H atoms adsorption leads to the upward of corresponding B atoms as exhibited in Figure 3d, which contribute to the partial structure deformation of borophene. Consequently, lB-B between the H-bonded B and its nearest one on the crest increase from 1.61 Å (pristine borophene) to 1.73, 1.72 and 1.71 Å, respectively. Similar deformations have also been found on silicene and graphene upon hydrogenation.33,34 To this end, four possible H2 dissociation pathways could be identified in light of the IS and FS considered above which are schematically illustrated in Figure 3a. The calculated energy barriers (Ebar) are 0.64, 1.01, 0.60 and 1.76 eV, respectively and the corresponding reaction energies (Er) are −1.29, −1.14, and −1.29 and −0.77 eV. Hence, path III is the most energy favorable path for H2 dissociation with maximum Er while minimal Ebar. The configurations of IS, TS and FS of path III along with the charge distribution estimated by Hirshfeld population analysis are depicted in Figure 3b-d respectively. It is found that the H2 decomposition process could be described as two sub-steps. Firstly, the physically adsorbed H2 molecule approach the borophene (h declines from 2.68 to 1.41 Å) with corresponding B atoms protruded from the quasi-plane layer of borophene and then chemically adsorption of H2 forms with elongated H-H bond length (lH-H increases from 0.75 to 0.85 Å from IS to TS). However, activating H2 molecule with elongated lH-H configuration consumes much energy, which contributes to the major part of Ebar. Subsequently, the adsorbed H2 depart from each other with the formation of strong covalent bond between H and B atoms (i.e. from TS to FS). As shown in Figure 3c, charge analysis demonstrates that both H atoms and the B atoms underneath in TS are positively charged, indicating the existence of strong coulomb repulsion that facilitates subsequent separation of H2. Therefore, the activation of H2 and its subsequent chemically adsorption becomes the ratelimiting step determining the decomposition course, where energy import is required. In contrast,


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path IV is the most impossible pathway with minimum Er and maximal Ebar, which can be rationalized by the shortest lH-H and concomitant strong repulsion interaction in TS. Electric field is considered here as a catalyst to investigate the possibility of reducing the Ebar.39 The calculated Ebar as a function of applied F are illustrated in Figure 4, where nearly linear relationships are found. Generally, path I and III present the minimum energy paths with nearly generated energy profile, suggesting that the configuration of weak adsorbed H2 has little effect on the Ebar. Besides, a positive F could result in the reduction of Ebar while they increase monotonously under negative F. Taking path III as an example, the Ebar decrease from 0.60 to 0.47 eV with F rise up to 1.54 V/Å. Meanwhile, the values of Er increase from −1.29 to −1.42 eV instead, attributing to the enhanced bonding strength between H atoms and borophene in FS under F. Simultaneously, Eb of H atoms adsorbed on borophene increase from −0.67 to −0.78 eV under the same F. However, a negative F increases from 0 to −1.54 V/Å, the attendant Ebar increase from 0.60 to 0.74 eV, while Er decrease from −1.29 to −1.22 eV. The different modulation of Er under F originate in the different response of negatively charged H atom and positively charged B underneath under F as shown in Figure 3d. It should be noted that the Ebar is insensitive to F, and thus the strength of F we used is a little bit large. However, we note that F larger than 1 V/Å was used in field effect devices experimentally. 40 As a consequence, in many previous simulation works, F range from 1 V/Å to have been applied to silicene,34 MoS2,41 and graphene42 systems. In order to get further insight into the linear dependent behavior of Ebar under F, the reactions along path III under F = ±1.54 V/Å are plotted in Figure 5b and c. It is important to note that the atomic structures of TS state for H2 dissociation have little change under different F. Thus, the diversification of Ebar under different F mainly results from the modulation of bonding strength


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between activated H2 and borophene in TS states. To confirm our speculation, we calculate the Eb of H2 and the concomitant atomic charges in TS states as shown in Table S2. Note that the Eb values are positive, indicating that there is an endothermic process from IS to TS. Also, an inverse proportion relation exists between Eb and Ebar, where a stronger Eb (lower Eb) leads to smaller Ebar. A lower Eb suggests that the configuration of TS state are more stable and behave lower energy, which bridges the energy gap between TS and IS, resulting in the reduction of Ebar. Furthermore, the origin of such F-dependent Eb could be understood by analyzing the density of states (DOS) combined with molecule orbital theory.43 In TS, H–B bonds form through linear combination of the H-1s orbital and B-2p orbital, turning out to be a pair of bonding (σ) and antibonding orbital (σ*). The former is fully occupied while partially filled situation appears in σ* orbital. According to the molecular orbital theory, more electrons in B would lead to higher σ* occupancy and weaker Eb. In light of that, we calculate the PDOS of H and corresponding B atoms in TS under different F as shown in Figure 6. Interactions between H and B atoms depend on the orbital hybridization of lowest bands, determined by the overlapping of electronic orbitals. It could be found that the electronic hybridization between H atoms and corresponding B atoms is extremely strong at 0 ~ −8 eV without F (Figure 6a). Moreover, the orbital hybridizations show little changes upon F as shown in Figure 6b-c, supporting the observation that F has negligible effect on the configuration of TS as mentioned above. However, the number of electrons in B-2p orbital is 2.00 e-1 which presents a declining tendency upon positive F (1.91 e-1 under 1.51V/ Å) while opposite trend exists upon negative F (2.11 e-1). Accordingly, positively F is expected to decline the Eb, which facilitate the dissociation of H2 on borophene with reduced Ebar. In contrast, negative F presents an inhibition effect on borophene hydrogenation. 3.3. SPILLOVER OF H ATOM


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The spillover procedure of H atom is the final step for borophene hydrogenation. Several emblematical configurations of IS regarding H atom(s) on the same side and opposite of borophene are considered and depicted in Figure 7. It is found that strong interactions existed between H atoms and borophene result in the deformation of borophene layer as exhibited in Figure 7, being consistent with previous report.44 For clarity, diffusion of isolated H atom on borophene is considered firstly to simulate the distant migration where the H atom diffuses at somewhere far from others. With one H atom adsorbed on borophene (denoted as SI), there are three feasible migration pathways as indicated in Figure 7a, namely to the neighboring location in the B6 hexagon (path I), to the opposite location (path II) and to the second neighboring location (path III). Note that the hydrogenation ratio here is 1 H atom per 24 B atoms. The calculated Eb (Eb = −0.61 eV, lH-B = 1.21 Å) is less than that of fully hydrogenated borophane,44 but lH-B is a little larger,21 inferring that the stability could enhanced with the rising of the hydrogenation ratio. The corresponding results are tabulated in Table 2. Er are calculated to be zero with no energy difference between IS and FS along these pathways, since all the B atoms possess identical potential energies. The Ebar are 0.48, 0.58 and 0.58eV for pathways I–III, respectively. LST/QST and NEB calculations confirm that path II is not an elementary reaction that could perform through path I and III, with the latter being the rate limiting step. To this end, the minimum diffusion barrier involves H migration along the ridge of borophene with an Ebar of 0.48eV as depicted in Figure 8a. The corresponding diffusion time τ could be calculated by equation, τ = dH2/2D

(3)

where dH is the diffusion distance along this pathway, and D represents the diffusion coefficient determined by D = PdH2ν · exp(−Ebar / kBT). P is the probability for the H atom diffusing to its


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nearest one, and the vibration frequency ν of borane molecule (B2H6) is about 1013 Hz.32 kB denotes the Boltzmann constant that is 8.617×10-5 eV/K, while T is set to be 298.15 K at room temperature. In path I, with Ebar = 0.48 eV and P = 1/2, D is calculated to be 1.14×10−11 cm2/s and τ is 1.30×10-5 s. In view of the fact that F could reduce Ebar for H2 dissociations, it is reasonable to forecast that the Ebar of H spillover could also be adjusted by external F. The results are plotted in Figure 8b, where Ebar is increased from 0.48 to 0.66 eV with the rising of F from 0 to −1.54 V/Å. Nevertheless, Ebar is decreasing when the direction of F is reversed, and finally turn out to be 0.30 eV at F = 1.54 V/Å. Also, the τ is calculated to be 1.18×10−8 s with D = 1.26×10−8 cm2/s, which is almost 103 times faster than that without F. The effects of Ag substrate on the diffusion of isolated H atom along path I-III have also been considered as depicted in Figure 8b. The calculated Ebar along these pathways are 0.62, 0.62 and 0.60 eV, respectively, being a little larger than those without Ag substrate due to the enhanced stability of borophene on substrate. On the other hand, the differences of Ebar between path I-III are smaller than those without the Ag substrate, indicating reduced anisotropic of H atom migration. Similarly, with F increased from 0.51 to 1.54 V/Å, the Ebar is reduced from 0.55 to 0.41 eV as shown in Figure 8b. The effect of H atoms nearby on the migration of H should be estimated upon increased hydrogenation ratio. In case of unilateral hydrogenation as exhibited in Figure 7b (i.e. SII), there are three possible reaction pathways, denoted as path IV-VI. The Ebar (Er) along path V and VI are 0.85 and 0.25 eV, respectively. Path IV could not take place due to higher priority of path VI than path IV. In addition, path VI shows the biggest Er and lowest Ebar, indicating that H atoms prefer to reside on the same ridge but with separated distribution to reduce the coulomb repulsion. The calculated τ is 1.68×10−9 s with D = 8.79×10−8 cm2/s, which is 1/7 of the τ along


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path I under F = 1.54 V/Å. Also, the spillover of H atom after diffusing along path VI (denoted as SIII) is considered with three pathways VII-IX as depicted in Figure 7c. Ebar of these pathways are 2.62, 1.21 and 0.88 eV, respectively, indicating that H atoms would prefer to diffuse on the same ridge of borophene. For the case of bilateral hydrogenation (i.e. SIV), four feasible migration pathways for H atom exist as depicted in Figure 7d denoted as path X-XIII. Ebar along these four pathways are 1.00, 1.45, 1.45 and 0.81 eV respectively, much larger than that of single H atom migration on borophene. These enhanced Ebar are beneficial to maintain the structure stability of borophane structural unit, similar to our and other results found on graphene and silicene.33,45,46 Further works should be performed to give a deep understanding about the interface stability between borophene/borophane. 4. CONCLUSION In summary, the adsorption and dissociation of H2 molecule on borophene with (without) external electric field combined with subsequent spillover of H atoms on borophene surface are investigated based on DFT calculations. It is found that the H2 adsorb hardly on borophene and might even remain gaseous state at room temperature with high decomposition Ebar of more than 0.60 eV. The positive electric field could not only increase the Ead of H2 on borophene, but also facilitate its dissociation followed with reduced Ebar of 0.47 eV under F = 1.54 V/Å. In contrast, negative F would increase the Ebar instead in light of the weakened bonding strength between activated H2 and borophene in TS configurations under different F. In addition, an isolated H atom present favorable mobility on borophene with a low Ebar of 0.48 eV that leads to a predicted τ of 1.30×10-5 s, which could be further decreased to be 0.30 eV with a reduced τ of 1.18×10-8 s under F = 1.54 V/Å. The adsorption of the opposite H atom increases the Ebar (0.81 eV) and thus hinders the migration of H atoms with enhanced stability. Our work provides a deep insight into


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the mechanism involved in fabrication of borophane that possess abundant properties with promising applications. ASSOCIATED CONTENT Supporting Information. Details of Ead of H2 on borophene under different D-terms, calculated Eb and charges of TS for H2 dissociation along path III under electric fields. AUTHOR INFORMATION Corresponding Author * Email: [email protected], Note The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge the supports from general programs of Sichuan Provincial Department of Education (No. 16ZB0100), program of science and technology Department of Sichuan Province (No. 2015GZ0050, 2015GZ0054, 2016GZ0283), Youth Foundation of Chengdu University of Technology (No. 2017QJ03).


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Table 1. Ead, h and Δ q of single H2 molecule physically adsorbed on borophene in configurations CI, CII and CIII. CI

CII

CIII

Ead (eV) −0.053 −0.052 −0.035 h (Å)

2.68

2.68

3.20

∆q (e−1)

−0.020 −0.020 −0.014

Table 2. Er and Ebar of H atom migration along path I–XIII for structures SI, SII, SIII and SIV. SI

SII

SIII

SIV

Path

I

II

III

IVb

V

VI

VII

VIII

IX

X

XI

XII

XIII

Er

0.00

0.00

0.00

−

0.01

−1.12

1.13

0.80

0.00

0.00

0.00

0.00

−0.13

Ebar

0.48

0.58a

0.58a

−

0.85

0.25

2.62

1.21

0.88

1.00

1.45a

1.45a

0.81

a

Not an elementary reaction;

b

Migration could not take place.


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Figure 1. 3×5×1 supercell of borophene adopted in this investigation. Configurations of H2 adsorbed in CI (a), CII (b) and CIII(c). h indicates the adsorption height of H2 molecule and d represents the bulking height of borophene defined as the distance between the upper borophene plane and the lower one. The pink and white balls represent the B and H atoms respectively. Figure 2. Dependence of Ead on F for H2 adsorbed in CI, CII and CIII. The blue arrows indicate the positive direction of F. Figure 3. Schematic illustration of H2 dissociation pathways on borophene (a). Top and side views of IS (b), TS (c) and FS (d) configurations along path III. The blue numbers around the atoms represent the atomic charges calculated by Hirshfeld population analysis. Figure 4. Ebar of H2 dissociations along path I-IV under external F. The dashed lines represent the fitting lines of Ebar values. Figure 5. The energy profiles of H2 dissociation under F = 0, 1.54 and -1.54 V/Å along path III. The energy of gas state is marked as zero, and the structures of IS, TS and FS are depicted as illustrations. Figure 6. PDOS for B and H atoms in TS structure along Path III under F = 0, 1.54 and -1.54 V/Å, where the Fermi level is indicated by the black dashed line. The yellow shadow denotes the occupied states of B-2p orbital up to Fermi level. Figure 7. Atomic structure of H atom(s) adsorbed on borophene SI (a), SII (b) SIII (c) and SIV (d). The blue arrows indicate the possible diffusion pathways of the H atom. Figure 8. The diffusion pathway of a single H atom on borophene along path I without substrate (a). The energy of IS is marked as zero, and the structures of IS, TS and FS are depicted. Ebar of


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H atom spillover along path I without substrate and along path III on Ag substrate under F (b). The blue balls in the illustration represent the Ag atoms, while the green arrows represent the spillover pathways.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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Figure 6.


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Figure 7.


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Figure 8.


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TOC Graphic


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First-Principles Study on the Mechanism of Hydrogen Decomposition

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