Scrupulous Probing of Bifunctional Catalytic Activity of Borophene

Jul 16, 2018 - charge transfer mechanism between the surface and the adsorbate. Ti- functionalized ... present energy crisis.1−3 Among the various r...
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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Scrupulous Probing of Bifunctional Catalytic Activity of Borophene Monolayer: Mapping Reaction Coordinate with Charge Transfer Amitava Banerjee,*,† Sudip Chakraborty,*,† Naresh K. Jena,*,† and Rajeev Ahuja*,†,‡ †

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Condensed Matter Theory Group, Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden ‡ Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden S Supporting Information *

ABSTRACT: We have envisaged the hydrogen evolution and oxygen evolution reactions (HER and OER) on two-dimensional (2D) noble metal free borophene monolayer based on first-principles electronic structure calculations. We have investigated the effect of Ti functionalization on borophene monolayer from the perspective of HER and OER activities enhancement. We have probed the activities based on the reaction coordinate, which is conceptually related to the adsorption free energies of the intermediates of HER and OER, as well as from the vibrational frequency analysis with the corresponding charge transfer mechanism between the surface and the adsorbate. Tifunctionalized borophene has emerged as a promising material for HER and OER mechanisms. We believe that our probing method, based on reaction coordinate coupled with vibrational analysis that has been validated by the charge transfer mechanism, would certainly become as a robust prediction route for HER and OER mechanisms in coming days. KEYWORDS: reaction coordinate, vibrational frequency, hydrogen evolution reaction, oxygen evolution reaction, borophene

T

have been emerged as the most promising candidate for such water splitting, especially for HER.7,11 But the bottleneck for such materials is the cost-effectiveness along with the geopolitical abundance, in order to achieve industrial production of hydrogen by employing them. The current state-of-the-art for designing an efficient catalyst is therefore based on the abundant and cheap elements of the periodic table to replace the precious metal based catalyst for HER and OER mechanism with lowering of overpotential.12−15 Since the successful exfoliation of monolayer graphene, the role of dimensionality has been strengthened from the perspective of fundamental materials properties16−18 and wide applications of two-dimensional (2D) materials in the field of nanoelectronics and energy.19−25 A recent member of this family of 2D materials is a one-atom-thick boron layer or borophene,26 which has gathered a considerable amount of attention in the materials community since its discovery. The directional transport properties have been investigated by Padilha et al.,27 whereas its applicability in the field of energy and sensing applications has been studied from the firstprinciples electronic structure calculations.28−34 The anisotropic optical properties of borophene have also been

he unsettled global energy demand along with the monotonic depreciation of fossil fuels paves the quest for renewable energy resources. Due to its highest energy density and environment friendly abundance, hydrogen has become the tangible most futuristic alternative to solve the present energy crisis.1−3 Among the various routes of hydrogen production, photocatalytic water splitting is one of the most propitious ways in the scientific community, since its inception by Fujishima and Honda in 1972.4 They envisaged TiO2 semiconductor as the first photocatalytic material, which could efficiently use solar energy to split water into hydrogen and oxygen. This water splitting mechanism, therefore, consists of a reduction and an oxidation half-reaction. The reduction half-reaction always takes place in the cathode to produce hydrogen, and hence this process is named as hydrogen evolution reaction (HER).5,6 On the other hand, the oxidation half-reaction takes place in the anode, where oxygen is generated, for which this process is named as oxygen evolution reaction (OER).7−9 The optimum catalysts for HER and OER require the lowering of overpotential associated with these reactions. At present, we get less than 5% of the world’s hydrogen10 through this water splitting process generating hydrogen and oxygen, which needs a substantial improvement in near future. Rational design of an efficient catalyst is therefore the prime focus to expedite hydrogen production. In the past couple of decades, noble metals such as platinum (Pt) © XXXX American Chemical Society

Received: May 23, 2018 Accepted: July 16, 2018 Published: July 16, 2018 A

DOI: 10.1021/acsaem.8b00813 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

well, where the most important intermediate is H*. Therefore, we have performed the adsorption free energy analysis to form the reaction coordinate that maps all the intermediates in order to find the corresponding overpotential. As most of the catalysis reactions have taken place in room temperature, and as our DFT calculations are corresponding to the T = 0 K state, therefore the temperature effect is considered to evaluate ΔG through zero-point energy difference (ΔZPE) and entropic contribution (TΔS) for each of the individual adsorptions of the intermediates in HER and OER, as per the following relation:

investigated from the perspective of a promising photovoltaic material. It is worth mentioning here that hydrogenation of borophene leads to a material called borophane which can behave as a Dirac material. These kind of reports certainly motivate us to investigate the importance of tuning the associated electronic properties and band gaps of borophene under the influence of elemental functionalization and their corresponding effects on catalytic activity from the perception of HER and OER mechanisms. As mentioned earlier about the presence of titanium (Ti) in the first photocatalytic material to be found on Earth, we are encouraged to envisage the HER and OER activity of borophene monolayer with the effect of Ti functionalization. This has formed the foundation of this work in addition to the probing of catalytic activities based on a reaction coordinate approach coupled with the vibrational frequency analysis, which will be discussed in subsequent sections. The role of computational modeling of catalytic materials from the perspective of their HER and OER efficiencies would certainly provide the experimentalists a valuable lead to make visible progress in the state-of-the-art of heterogeneous catalysis. For instance, in concepts such as volcano plot,7 which correlates the exchange current densities for HER with hydrogen adsorption free energy, density functional theory (DFT) has been proven to be immensely useful. Ideally, for optimum HER catalyst, the hydrogen adsorption energy needs to be balanced between the values corresponding to the phsyisorption and chemisorption, a feature shown by Pt, and therefore, it lies at the apex of such a volcano plot. Such atomistic modeling entails better understanding of the OER and HER processes and makes reliable advancements to the expanding field of water splitting from screening, optimizing, and postulating efficient catalytic systems. As mentioned earlier, the HER takes place in the cathode of an electrochemical cell through proton reduction in the following manner: 2H+ + 2e− → H* + H+ + e− → H 2

ΔG = Eads + ΔZPE − T ΔS

For the ideal catalyst case, to approach thermoneutrality, the value of ΔG needs to be close to zero. Consideration of zeropoint energy is primarily based on the vibrational frequency analysis. In this work, we have thoroughly investigated the vibrational frequency in the case of a monohydrogenated, as well as Ti-functionalized borophene monolayer, along with the corresponding buckling height. We have shown that not only the reaction coordinate but also the vibrational frequency analysis are equally important for catalytic activity prediction. The primary descriptor adsorption energy reveals the fact of chemisorption or physisorption, which one can also conclude from the change in the vibrational frequency between the pristine and adsorbed borophene monolayer. This has also been verified as proof of concept with the help of Bader charge analysis and the charge distribution between the surface and adsorbate. We have shown how the introduction of Ti not only changes overpotential values in the reaction coordinate but also tunes the vibrational frequency and the buckling height, as the consequence of the charge transfer between borophene monolayer with hydrogen and Ti. We have performed first-principles DFT based spinpolarized electronic structure calculations (refer to the Computational Methodology section in the Supporting Information) in order to find the minimum energy configurations of monohydrogenated and Ti-functionalized borophene monolayer. Panels a and b of Figure 1 depict the 2D pristine buckled borophene sheet from the top and side perspectives, where T1, T2 and B1, B2 are the top and bottom boron atoms, respectively, with similar bond lengths T1−T2

(1)

whereas the OER in the anode undergoes a four electron pathway in order to generate oxygen as the byproduct. The OER mechanism is started with adsorption of water at the anode that gives rise to OH* along with an electron and hole pair. This OH* is then adsorbed further as O* with another electron−hole pair, which reacts with the water to create OOH* and then subsequently molecular oxygen. The whole OER process is comprised of four reaction steps, where an electron−hole pair is formed at each individual step, which justifies the name of four electron pathway process, as follows: H 2O + * → OH* + H+ + e−

(2)

OH* → O* + H+ + e−

(3)

O* + H 2O → OOH* + H+ + e−

(4)

OOH* → O2 ↑ +H+ + e−

(5)

(6)

where * denotes an active site on the surface where the adsorption is taking place. It is worth mentioning the similar influence of pH and the electrode potential (U) on the adsorption free energy (ΔG) of each of the intermediates (O*, OH*, and OOH*) in each step of OER, and therefore the theoretical overpotential is found to be independent of these two parameters. The same is true for the HER mechanism as

Figure 1. (a) Bird’s eye view and (b) side view of borophene; (c) bird’s eye view and (d) side view of monohydrogenated borophene; (e) variation of buckling height in monohydrogenated borophene in comparison with pristine borophene. Green and light pink represent boron and hydrogen, respectively. B

DOI: 10.1021/acsaem.8b00813 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials and B1−B2 and with the corresponding buckling height of 0.877 Å. In the process of hydrogenation of the sheet starting with a single hydrogen, as depicted in Figure 1c,d, it has been found that the H adatom is attached on top of the boron atom having the B−H bond length of 1.204 Å. The corresponding T1−T2 and B1−B2 bond lengths are also changed as a consequence, which is higher to lower from the adsorbed H on B atom, as compared to that particular B atom in the pristine sheet. This is due to the change in the electronic environment with the charge redistribution in the vicinity of the B atom, where the hydrogen has been adsorbed. The uniform charge distribution between the B atoms in the pristine monolayer has been changed drastically with the hydrogen adsorption, which enables the charge transfer from boron to form a B−H bond. We have represented the corresponding buckling height variation in Figure 1e, where the white rings indicate the buckling height scale in angstroms and the numbers “1−12” denote the nth B atoms that are present in the area of the considered sheet. It shows that with hydrogenation the buckling height varies from 0.311 to 1.053 Å. We have also determined the vibrational frequency of the adsorbed hydrogen, which has attained a value of 2443.15 cm−1 corresponding to the B−H bond length of 1.204 Å. In the subsequent stage, we have functionalized the borophene sheet with Ti. We have considered several different configurations to dope Ti (Supporting Information Figure S1) on various sites such as on hollow, top, and bridge positions of the monolayer in order to find the minimum energy configuration of Ti-functionalized borophene. The bird’s eye and side views of the relaxed lowest energy Ti-functionalized borophene structure are shown in Figure 2a,b. Panels c and e of Figure 2 are depicting the minimum energy structure of

adsorbed hydrogen on the Ti-functionalized monolayer from top and side perspectives, respectively. There is a strong coordination of Ti adatom with the pristine sheet that has been reflected in terms of various shorter and longer B1−B2, B1−T1, B2−T2, and T1−T2 bond lengths and buckling height variation, due to the substantial charge distribution in the Ti-functionalized borophene sheet. The buckling height is ramping strongly with Ti functionalization as compared to the pristine monolayer of borophene, as represented clearly in Figure 2d. With the hydrogenation, this buckling height is ramping more as shown by the distorted blue circle in Figure 2d. This ramping is primarily happening due to the nonuniform charge distribution around the Ti-functionalized site on the borophene sheet. The vibrational frequency analysis determines the frequency corresponding to the B−H bond length of 1.213 Å that has been changed to 2364.66 cm−1 after the pristine sheet is functionalized with Ti. This change is also the consequence of the charge redistribution between the surface and the adsorbate under the influence of functionalization. These observations are also validated with the adsorption energy (Ea), which is basically the required energy to dissociate the Ti adatom from the pristine borophene sheet and move it further away from the sheet, that has been determined as follows: Ea = (E B(Ti) − E B − E Ti)

(7)

where EB(Ti) is the total energy of the Ti adsorbed borophene sheet, EB is the total energy of the borophene sheet, and ETi is the total energy of an isolated Ti atom. The adsorption energy corresponding to the Ti functionalization is −5.88 eV, which turns out to be a strong binding between Ti and borophene monolayer. This also confirms that, in the case of more than a single Ti adatom that could be adsorbed on borophene, they tend to make a monolayer in place of clustering due to the strong binding between Ti and B atoms of the pristine sheet. This monolayer formation is more favorable as compared to the cluster formation, which is evident from the fact that the adsorption energy of a single Ti on borophene is 1.03 eV more than the cohesive energy of titanium itself, which is 4.85 eV/ atom.35 In this regard, it is also worth mentioning that we have tried adsorbing hydrogen to envisage HER on a Ti-functionalized sheet as well, but the real strong binding with the sheet is not favorable for optimum HER mechanism, which requires a trade-off balance between chemisorption and the physisorption limit. The monolayer growth in the case of functionalization is always more favorable than the clustering of the adatom. This has also been confirmed from the charge redistribution of the Ti-functionalized monolayer as compared to the pristine sheet. We now analyze the adsorption free energy diagram at equilibrium potential and pH = 0, which shows that the associated proton transfer for the catalytic reaction on borophene monolayer is exothermic. The adsorption free energy of the pristine borophene sheet is 0.41 eV, which has substantially decreased to 50% of the initial value with the inclusion of Ti functionalization. The associated adsorption free energy for HER of 0.20 eV is much more thermoneutral while getting close to the ideal Pt catalyst.7 Although our pristine 2D borophene has shown promise as an HER catalyst as compared to another similar system,31−34 however the corresponding adsorption free energy ΔG for HER, as shown in Figure 3a, resided away from 0.0 eV, which has been brought back to the close proximity of ideal HER with the help

Figure 2. (a) Bird’s eye view and (b) side view of Ti-functionalized borophene; (c) bird’s eye view and (e) side view of monohydrogenated Ti-functionalized borophene; (d) variation of buckling height Ti-decorated borophene in comparison with pristine borophene. Green, light pink, and sky blue represent boron, hydrogen, and Ti atoms, respectively. C

DOI: 10.1021/acsaem.8b00813 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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as compared to that of the gas phase. The hydration effect in the case of Ti-functionalized borophene monolayer is really exciting to see, where the binding energy of −0.349 eV leads to the favorable thermoneutral reaction. Figure 3b depicts the reaction coordinate associated with the OER mechanism as explained earlier through eqs 2−6. We have obtained the adsorption free energies after adsorbing O*, OH*, and OOH* in different structural configurations on pristine and Ti-functionalized borophene sheets in order to find the relaxed structures with the lowest energy values. We have determined ΔG1 and ΔG2 for both the cases of pristine and Ti-functionalized sheets as follows: ΔG1B = 1.165 eV,

ΔG1B + Ti = 1.904 eV,

ΔG2 B = −3.945 eV,

ΔG2 B + Ti = − 0.357 eV

(9)

The possible rate-determining step in the pristine case is O* oxidation to OOH*, whereas in the case of Ti-functionalized sheet, the corresponding rate-determining step is OH* oxidation to O*. Moreover, under the influence of Ti functionalization, the subsequent reaction barrier, O* to OOH*, has become really negligible, which indicates the easy formation of OOH*. Finally, it has been observed that the Ti functionalization diminishes the difference between ΔG1 and ΔG2, which can be related to the over potential lowering of OER, and therefore higher catalytic efficiency. In order to have a profound understanding about the HER mechanism on pristine and Ti-functionalized borophene from the charge transfer mechanism, we have performed Bader charge analysis based on the charge density difference between hydrogenated and dehydrogenated sheets. The average net charge per atom is calculated by taking the difference between valence charge and Bader charge per atom. In the case of pristine borophene, the average net charge per B atom is 0.024 and per H is −0.529, while the net charge of B that is attached with H is 0.762, which indicates that the adsorbed H gains charge from the sheet. In the case of Ti-functionalized borophene monolayer, Ti loses charge, while the average net charge of Ti atom is 0.303. This enables further rearrangement of the charge distribution scenario of the pristine sheet near the vicinity of Ti functionalization, which is reflected in the crumbling of pristine sheet near the site of Ti functionalization. This has also been observed in the corresponding buckling height. After the adsorption of a single hydrogen atom, the net charge per Ti atom has become 0.297, which is losing charge, while the average net charge per B atom is 0.004, so overall, the borophene sheet is losing a very negligible amount of charge. The net charge of B, which is attached with H is 0.523 and average net charge of H is −0.369. Therefore, it is observed, in the Ti-functionalized borophene case, that H is gaining less charge, which is also depicted in the charge density difference plot (Figure 4b of upper panel). This is also supporting our finding based on reaction coordinate analysis, that Tifunctionalized borophene is a better candidate for good HER catalyst as compared to the pristine case. The less amount of charge taken by hydrogen atom from the monolayer in the case of Ti-functionalized borophene monolayer has also been reflected in the less strong B−H bond with Ti. These observations are also positively connected to the previous finding, longer B−H bond length and lower wavenumber under the influence of Ti functionalization as shown in Figures 1d and 2e. The lower panel of Figure 4 is showing the projected density of states of borophene monolayer in (a)

Figure 3. Free energy diagram for (a) hydrogen evolution reaction and (b) oxygen evolution reaction at equilibrium on pristine and Tifunctionalized borophene monolayer.7,31

of Ti functionalization. Here, one observation is noteworthy, that the more negative adsorption free energy ΔG for the pristine sheet as compared to the Ti-functionalized sheet indicates the tendency of a stronger B−H bond, which decreases the H−H recombination and therefore hydrogen evolution probability. One reason why this borophene monolayer could be better than the other similar system31 is because of the structural coordination difference of the B atom. The present case has only six-coordinated boron, whereas the previously reported structure is a combination of five- and sixcoordinated boron rings, which can lead to the asymmetric charge distribution unlike in the present case of all sixmembered boron rings. It is also possible to obtain the exchange current density associated with the HER mechanism as per the following empirical relation: 1 i0 = −ek 0 , for ΔG H* < 0 1 + exp(ΔG H*/kT ) (8) where, k0 is the rate constant. This relation is mostly sensitive toward the thermochemical environment, and therefore the evaluated current density can be underestimated under the experimental conditions such as oxide formation, defects, and solvation effect. The obtained exchange current density is inversely proportional to the Tafel slope. As shown in the inset of Figure 3a, under the influence of Ti functionalization, the exchange current density is getting higher and consequently will depict a lower Tafel slope, which basically indicates better HER activity as compared to the pristine case. We have also investigated the hydration effect on HER in our considered system, where the binding energy is the primary descriptor. We have observed a strong binding for pristine borophene in the presence of an implicit water environment, with the corresponding binding energy of −0.748 eV, which is higher D

DOI: 10.1021/acsaem.8b00813 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

In this work, we have introduced a robust theoretical approach to probe the catalytic activity based on the reaction coordinate aided with vibrational frequency analysis and validated through charge transfer mechanism. Based on this probing foundation, we have thoroughly envisaged the HER and OER activity on a Ti-functionalized borophene monolayer. We have found that the introduction of Ti would certainly enhance the catalytic activity of a pristine borophene monolayer from the perspectives of both HER and OER. We have also found that the vibrational frequency analysis of the surface adsorbate interaction is strongly connected to the reaction coordinate, from where we determine the overpotential value. We have found Ti introduction has lowered the overpotential with an amount of 0.20 eV, which was double in the pristine case. In the case of OER, Ti functionalization drastically reduces the overpotential of the reaction pathway between O* and OOH*. This eventually reduced the overall overpotential of OER reaction in the case of a Ti-introduced borophene monolayer as compared to the pristine one. Buckling height has also a significant role in mechanical properties, especially with the directional strain. Mostly sp3 hybridization is the reason for the buckling. Strong and weak hybridization effects are reflected respectively by decreasing and increasing the interlayer B1−T1 bond length. Therefore, it can be speculated that more bucking means increasing the B1− T1 bond length and decreasing the B1−B2 or T1−T2 (σ) bond. In the Ti adsorbed case this alternation bond length helps to maximize the Ti coordination. It is noteworthy to mention that this change in bond length is correlated with the charge distribution, as the alternation of charge density also triggers the bond strength B−H bond, which is one key factor for HER. The overall observation is that Ti addition increases the average buckling height and changes the charge density distribution of the pristine sheet in such a way that reduces the overpotential for HER. We believe that our computational probing approach based on reaction coordinate and vibrational frequency, along with the charge density distribution, would be a future prospect to predict new catalytic materials. The experimental investigations of our theoretical prediction would certainly be worth carrying forward in the scientific community.

Figure 4. Upper panel: Charge density difference of (a) B with 1H (b) B(Ti) with 1H. Isosurface, 0.004 a.u. Insets: Planner views of charge density differences. Lower panel: Projected density of states of (a) pristine, (b) hydrogenated, (c)Ti-functionalized, and (d) hydrogenated and Ti-functionalized borophene monolayer.

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pristine or B case, (b) hydrogenated or B−H case, (c) Tifunctionalized or B(Ti) case, and (d) hydrogenated and Tifunctionalized or B(Ti)−H case. The spin channel density is the same for all cases containing hydrogen, whereas a strong overlap of d−p orbital and therefore d−p hybridization has been observed in the case of Ti-functionalized borophene. In the respective panels , the overlaps of the p orbital of B at 1.203 eV, with the B−H case at 0.778 eV along with the B(Ti) case at 1.534 eV and B(Ti)−H at 1.278 eV, have been marked with a green strip in order to show the overlap and how they shifted in different cases distinctively. Moreover, we also note here that the shift between B and B−H cases is quite significant (0.425 eV), whereas the difference between corresponding B(Ti) and B(Ti)−H amounts to only 0.256 eV. This can be interpreted keeping in perspective the stronger binding of H with pristine borophene vis-á-vis relatively moderate binding (of H) with Ti−borophene. Stronger binding in the former case leads to a larger ΔG (negative), whereas Ti functionalization tunes the chemical interaction of H with the substrate in a favorable manner with a visible impact of pushing ΔG closer to zero.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00813. Computational methodology, minimum energy configuration of Ti adsorbed on the borophene surface, and the intermediates for OER (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(A.B.) E-mail: [email protected]; amitava.banerjee@ physics.uu.se. *(S.C.) E-mail: [email protected]; sudip.chakraborty@ physics.uu.se. *(N.K.J.) E-mail: [email protected], naresh.jena@ physics.uu.se. *(R.A.) E-mail: [email protected]. Fax: +46 184713524. Tel.: +46 728772897. ORCID

Amitava Banerjee: 0000-0002-3548-133X E

DOI: 10.1021/acsaem.8b00813 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (19) Kubota, Y.; Watanabe, K.; Tsuda, O.; Taniguchi, T. Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure. Science 2007, 317, 932. (20) Aufray, B.; Kara, A.; Vizzini, S.; Oughaddou, H.; Léandri, C.; Ealet, B.; Le Lay, G. Graphene-like Silicon Nanoribbons on Ag(110): A Possible Formation of Silicene. Appl. Phys. Lett. 2010, 96, 183102. (21) Vogt, P.; De Padova, P.; Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M. C.; Resta, A.; Ealet, B.; Le Lay, G. Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon. Phys. Rev. Lett. 2012, 108, 155501. (22) Bianco, E.; Butler, S.; Jiang, S.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. Stability and Exfoliation of Germanane: A Germanium Graphane Analogue. ACS Nano 2013, 7, 4414−4421. (23) Zhu, F.-F.; Chen, W.-J.; Xu, Y.; Gao, C.-L.; Guan, D.-D.; Liu, C.- H.; Qian, D.; Zhang, S.-C.; Jia, J.-F. Epitaxial Growth of TwoDimensional Stanene. Nat. Mater. 2015, 14, 1020−1025. (24) Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tomańek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033−4041. (25) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308−5309. (26) Mannix, A. J.; Zhou, X.-F.; Kiraly, B.; Wood, J. D.; Alducin, D.; Myers, B. D.; Liu, X.; Fisher, B. L.; Santiago, U.; Guest, J. R.; Yacaman, M. J.; Ponce, A.; Oganov, A. R.; Hersam, M. C.; Guisinger, N. P. Synthesis of Borophenes: Anisotropic, Two-Dimensional Boron Polymorphs. Science 2015, 350, 1513. (27) Padilha, J. E.; Miwa, R. H.; Fazzio, A. Directional Dependence of the Electronic and Transport Properties of 2D Borophene and Borophane. Phys. Chem. Chem. Phys. 2016, 18, 25491−25496. (28) Zhang, Y.; Wu, Z.-F.; Gao, P.-F.; Zhang, S.-L.; Wen, Y.-H. Could Borophene Be Used as a Promising Anode Material for HighPerformance Lithium Ion Battery? ACS Appl. Mater. Interfaces 2016, 8, 22175−22181. (29) Shukla, V.; Wärnå, J.; Jena, N. K.; Grigoriev, A.; Ahuja, R. Toward the Realization of 2D Borophene Based Gas Sensor. J. Phys. Chem. C 2017, 121, 26869−26876. (30) Jena, N. K.; Araujo, R. B.; Shukla, V.; Ahuja, R. Borophane as a Benchmate of Graphene: A Potential 2D Material for Anode of Li and Na-Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9, 16148. (31) Mir, S. H.; Chakraborty, S.; Jha, P. K.; Warna, J.; Soni, H.; Jha, P.; Ahuja, R. Two-dimensional Boron: Lightest Catalyst for Hydrogen and Oxygen Evolution Reaction. Appl. Phys. Lett. 2016, 109, 053903. (32) Rupp, C. J.; Chakraborty, S.; Anversa, J.; Baierle, R. J.; Ahuja, R. Rationalizing the Hydrogen and Oxygen Evolution Reaction Activity of Two-Dimensional Hydrogenated Silicene and Germanene. ACS Appl. Mater. Interfaces 2016, 8, 1536−1544. (33) Mir, S. H.; Chakraborty, S.; Warna, J.; Narayan, S.; Jha, P. C.; Jha, P. K.; Ahuja, R. A Comparative Study of Hydrogen Evolution Reaction on pseudo-monolayer WS2 and PtS2: insights based on the density functional theory. Catal. Sci. Technol. 2017, 7, 687. (34) Almeida, R.; Banerjee, A.; Chakraborty, S.; Almeida, J.; Ahuja, R. Theoretical Evidences Behind Bifunctional Catalytic Activity in Pristine and Functionalized Al2C monolayer. ChemPhysChem 2018, 19, 148. (35) Kittel, C. Introduction to Solid State Physics, 8th ed.; John Wiley & Sons: Hoboken, NJ, USA, 2005.

Sudip Chakraborty: 0000-0002-6765-2084 Naresh K. Jena: 0000-0002-8242-8005 Rajeev Ahuja: 0000-0003-1231-9994 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Carl Tryggers Stiftelse for Vetenskaplig Forskning (CTS), the Swedish Research Council (VR), STANDUP, and Erasmus Mundus (EMINTE) for the doctoral fellowship. SNIC and HPC2N are also acknowledged for providing computing time.

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REFERENCES

(1) Takanabe, K. Photocatalytic Water Splitting: Quantitative Approaches Toward Photocatalyst by Design. ACS Catal. 2017, 7, 8006−8022. (2) McDowall, W.; Eames, M. F. Forecasts, Scenarios, Visions, Backcasts and Roadmaps to the Hydrogen Economy: A Review of the Hydrogen Futures Literature. Energy Policy 2006, 34, 1236−1250. (3) Hook, M.; Tang, X. Depletion of Fossil Fuels and Anthropogenic Climate ChangeA review. Energy Policy 2013, 52, 797−809. (4) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (5) Roger, I.; Shipman, M. A.; Symes, M. D. Earth-abundant Catalysts for Electrochemical and Photoelectrochemical Water Split ting. Nature Rev. Chem. 2017, 1, 0003. (6) Trasatti, S. Work function, Electronegativity, and Electrochemical Behaviour of Metals: III. Electrolytic Hydrogen Evolution in Acid Solutions. J. Electroanal. Chem. Interfacial Electrochem. 1972, 39, 163−184. (7) Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23−J26. (8) Montoya, J. H.; Seitz, L. C.; Chakthranont, P.; Vojvodic, A.; Jaramillo, T. F.; Nørskov, J. K. Materials for Solar Fuels and Chemicals. Nat. Mater. 2017, 16, 70−81. (9) Yan, Y.; Xia, B.; Xu, Z.; Wang, X. Recent Development of Molybdenum Sulfides as Advanced Electrocatalysts for Hydrogen Evolution Reaction. ACS Catal. 2014, 4, 1693. (10) Gong, J.; Luque, R. Catalysis for Production of Renewable Energy. Chem. Soc. Rev. 2014, 43, 7466. (11) Sheng, W.; Myint, M.; Chen, J. G.; Yan, Y. Correlating the Hydrogen Evolution Reaction Activity in Alkaline Electrolytes with the Hydrogen Binding Energy on Monometallic Surfaces. Energy Environ. Sci. 2013, 6, 1509−1512. (12) Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16, 1097. (13) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I. B.; Nørskov, J. K. Computational High-throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nat. Mater. 2006, 5, 909. (14) Tan, S. M.; Sofer, Z.; Luxa, J.; Pumera, M. Aromatic- Exfoliated Transition Metal Dichalcogenides: Implications for Inherent Electrochemistry and Hydrogen Evolution. ACS Catal. 2016, 6, 4594−4607. (15) Abbas, M. A.; Bang, J. H. Rising Again: Opportunities and Challenges for Platinum-Free Electrocatalysts. Chem. Mater. 2015, 27, 7218−7235. (16) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (17) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Twodimensional Gas of Massless Dirac fermions Graphene. Nature 2005, 438, 197−200. (18) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric F

DOI: 10.1021/acsaem.8b00813 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX