materials

Article Computational Study of Novel Semiconducting Sc2CT2 (T = F, Cl, Br) MXenes for Visible-Light Photocatalytic Water Splitting

Shaoying Guo 1,2,*, Hao Lin 1, Jiapeng Hu 1, Zhongliang Su 3 and Yinggan Zhang 4,*

1 Provincial Key Laboratory of Eco-Industrial Green Technology, College of Ecology and Resource Engineering, Wuyi University, Wuyishan 354300, ; [email protected] (H.L.); [email protected] (J.H.) 2 Fujian Provincial Key Laboratory of Pollution Control & Resource Reuse, , Fuzhou 350007, China 3 Xiamen Products Quality Supervision & Inspection Institute, Xiamen 361005, China; [email protected] 4 College of Materials, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, , Xiamen 361005, China * Correspondence: [email protected] (S.G.); [email protected] (Y.Z.)

Abstract: Seeking candidate photocatalysts for photocatalytic water splitting, via visible light, is of great interest and importance. In this study, we have comprehensively explored the crystal

structures, electronic properties, and optical absorbance of two-dimensional (2D) Sc2CT2 (T = F, Cl, Br) MXenes and their corresponding photocatalytic water splitting, under the visible-light region, by

first-principles calculations. Herein, we have proposed that 2D Sc2CT2 MXenes can be fabricated from their layered bulk compounds, alternatively to the traditional chemical etching method. Creatively,

we proposed Sc2CT2 (T = F, Br) as new materials; the band edge alignments of Sc2CF2 can be tuned to  meet the water redox potentials at pH = 8.0. It is highlighted that Sc2CF2 shows outstanding optical  spectra harvested under visible-light wavelength regions, and efficient separation of photo-induced

Citation: Guo, S.; Lin, H.; Hu, J.; Su, electrons and holes in different zones. These present results provide eloquent evidence and open a Z.; Zhang, Y. Computational Study of new door on the photocatalysis applications of such novel semiconducting MXenes.

Novel Semiconducting Sc2CT2 (T = F, Cl, Br) MXenes for Visible-Light Keywords: MXenes; photocatalyst; first-principles calculations Photocatalytic Water Splitting. Materials 2021, 14, 4739. https:// doi.org/10.3390/ma14164739 1. Introduction Academic Editor: Juan M. Coronado Hydrogen production, through photocatalytic water splitting via visible-light irra- diation, is a promising solution to solve the environmental issues by achieving low-cost, Received: 11 July 2021 nonpolluting, and environmentally friendly energy [1,2]. A suitable photocatalyst must Accepted: 20 August 2021 Published: 22 August 2021 fulfill at least the following three requirements: moderate band gaps, appropriate band alignments, and efficient separation of photo-generated electron–hole pairs. This is to

Publisher’s Note: MDPI stays neutral say, the band gap of the semiconductor photocatalyst must straddle the redox potential of − with regard to jurisdictional claims in water, as follows: the conduction band minimum (CBM) must be higher than 4.44 eV, + published maps and institutional affil- which is the redox potential of H /H2; and the valence band maximum (VBM) must be iations. lower than −5.67 eV, which is the redox potential of H2O/O2 [3–5]. Accordingly, 1.23 eV is the smallest band gap for a semiconductor photocatalyst. To exploit solar radiation more efficiently, a value of around 1.8 eV is an ideal gap for an efficient photocatalyst [2,6,7]. Moreover, the efficiency of a photocatalyst must always be hampered by the recombination of photo-induced electron–hole pairs. Therefore, the efficient carrier separation is another Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. vital point in semiconductor photocatalysts. However, finding an efficient semiconduc- This article is an open access article tor photocatalyst for hydrogen generation, to meet all the above requirements, is still an distributed under the terms and ongoing challenge [5]. conditions of the Creative Commons Recently, two-dimensional (2D) transition metal carbides/nitrides (MXene) were pro- Attribution (CC BY) license (https:// duced from their corresponding MAX phases, where M stands for early transition metal, A creativecommons.org/licenses/by/ stands for IIIA or IVA element, and X represents C or N [8–16]. MXenes have been exten- 4.0/). sively explored for miscellaneous applications as photocatalysts [17–19], electromagnetic

Materials 2021, 14, 4739. https://doi.org/10.3390/ma14164739 https://www.mdpi.com/journal/materials Materials 2021, 14, 4739 2 of 10

interference shielding [20], battery anodes [21,22], heavy-metal removal [23], etc. Especially, for the photocatalytic community, monolayer Zr2CO2 and Hf2CO2 MXenes have been eval- uated as semiconductor photocatalysts. They not only exhibit suitable band gaps and band alignments, but can also separate photo-generated electrons and holes effectively [17–19]. Except for Zr2CO2 and Hf2CO2, the semiconducting feature was characterized in some other Sc-based MXenes, such as Sc2CO2 and Sc2C(OH)2, which possess the band gaps of 2.96 eV and 0.77 eV, respectively [24,25]. Experimentally, a layered Sc2CCl2 bulk crystal, with a unit cell that is isostructural to a 2D MXene, was successfully synthesized, which implied the feasibility for the fabrication of monolayer Sc2CCl2 [26]. Theoretically, the calculated optical bandgap of one-layer (1 L), 2 L and 3 L Sc2CCl2 is about 1.5 eV by the GW + RPA method [27]. Accordingly, the monolayer Sc2CCl2 is a promising optoelectronic material, due to the excellent light absorbance [27]. In addition, the analogue bulk crystal Y2CF2 has been directly synthesized through a high-temperature solid-state reaction a while ago [28,29]. The crystal has the ability to photo-reduce protons into H2 in the range of visible light [28]. Inspired by these studies, and considering the analogous of F, Cl, and Br, we wonder if Sc2CT2 MXenes (T = F, Cl, Br) are endowed with outstanding features as efficient semiconductor photocatalysts for water splitting. Thanks to the progress of the density functional theory, the computational method is now a very important approach for designing new functional material. In the present work, we have comprehensively explored the exfoliate energy, geometrical structure, kinetic and thermal stabilities, as well as electronic properties and optical absorbance of 2D Sc2CT2 (T = F, Cl, Br) MXenes, using first-principles calculations. Especially, we have extensively explored the possibility of Sc2CT2 (T = F, Cl, Br) MXenes to be used for visible-light photocatalytic water splitting. Our study can make contributions to the discovery and applications of Sc-based MXenes, as semiconductor photocatalysts for water-splitting hydrogen production.

2. Methods Our first-principles calculations were performed using the projector augmented- wave (PAW) [30] of generalized gradient approximation (GGA) with Perdew–Burke– Ernzerhof (PBE) [31] formalism, implemented in the Vienna ab initio simulation package (VASP) [32,33]. The energy cutoff of the plane-wave expansion was 550 eV. The thickness of the vacuum space was 25 Å in the z direction. The reciprocal space was represented by a Monkhorst-Pack scheme with 11 × 11 × 1 k-point grid meshes. The convergence criteria of energy for both electrons and ions were, respectively, 1 × 10−5 eV and 1 × 10−6 eV. Phonon dispersions were retrieved by the density functional perturbation theory (DFPT) as embedded in the PHONOPY code to study the kinetic stability [34,35], for which a 4 × 4 × 1 supercell with 4 × 4 × 1 k-point grid meshes was employed. The DFT-D3 van der Waals (vdW)-corrected methods [36] were performed for the improved descriptions of the layered bulk crystals. The calculation results were dealt by the ALKEMIE code [37]. To obtain accurate dielectric functions, we employed the time-dependent Hartree–Fock calculations (TDHF) [38] by the Heyd–Scuseria–Ernzerhof (HSE06) [39] calculations, which have taken the excitonic effects into account.

3. Results and Discussion

First, the bulk and monolayer structures of Sc2CT2 (T = F, Cl, Br) are displayed in Figure1. The bulk phase is crystallized in the trigonal space group of P3m1, in which the upper and lower sides of the Sc2C plane are halogenated by two T atomic layers. Moreover, the center C atoms are coordinated by six Sc atoms, leading to an octahedron of Sc6C. Table1 lists the calculated lattice parameters a (Å) and Sc–C bond lengths L (Å) of the Sc6C octahedron of the monolayer Sc2CT2. The lattice parameters a (Å) and c (Å) of the bulk Sc2CT2 are listed for comparison as well. According to our calculations, the lattice constant, via the structural optimization for bulk Sc2CCl2, is 3.445 Å, which is in line with the experimental result [26]. As the surface groups change from F to Br, the Materials 2021, 14, x FOR PEER REVIEW 3 of 10

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Sc2CT2 are listed for comparison as well. According to our calculations, the lattice constant, via the structural optimization for bulk Sc2CCl2, is 3.445 Å, which is in line with the exper- latticeimental parameters result [26]. increase As the slightly,surface whichgroups is chan attributedge from to theF to raising Br, the of lattice the halogen parameters element in- atomiccrease slightly, radius. Inwhich the Sc is6 attributedC octahedron to the of raising the monolayer of the halogen Sc2CT2 element, the changes atomic in radius. the Sc–C In bondthe Sc lengths6C octahedron show the of samethe monolayer trend as the Sc surface2CT2, the groups changes from in F the to Br.Sc–C It isbond worth lengths noting show that thethe calculatedsame trend lattice as the parameters surface groupsa of the from monolayer F to Br. ScIt 2isCT worth2 are almostnoting identicalthat the calculated with that oflattice their parameters bulk phases, a of indicating the monolayer that the Sc interlayer2CT2 are almost interaction identical between with thethat atomic of their layers bulk arephases, very indicating weak. Thus, that the the monolayer interlayer Sc interact2CT2 ision likely between to be isolatedthe atomic from layers the correspondingare very weak. bulkThus, phase. the monolayer Sc2CT2 is likely to be isolated from the corresponding bulk phase.

Figure 1. ((aa)) Crystal Crystal structure structure of of bulk bulk Sc Sc2CT2CT2 (T2 =(T F, = Cl, F, Br). Cl, Br).(b) Side (b) view Side viewand (c and) top ( cview) top of view single- of 2 2 single-layerlayer Sc CT Sc. 2CT2.

Table 1. The calculated lattice parameters a (Å) and c (Å) of bulk Sc2CT2, the lattice parameters a (Å) Table 1. The calculated lattice parameters a (Å) and c (Å) of bulk Sc2CT2, the lattice parameters a (Å) and Sc–C bond lengths L (Å) of monolayer Sc2CT2. and Sc–C bond lengths L (Å) of monolayer Sc2CT2. Sc2CF2 Sc2CCl2 Sc2CBr2 Sc2CF2 Sc2CCl2 Sc2CBr2 a 3.285 3.445 3.498 Bulk a 3.285 3.445 3.498 Bulk c 6.481 8.854 9.295 c 6.481 8.854 9.295 a a 3.252 3.2523.431 3.4313.507 3.507 MonolayerMonolayer L L 2.266 2.2662.330 2.3302.358 2.358

Herein, the 3D bulk crystals are composed of stacked MXene sheets, as displayed in Herein, the 3D bulk crystals are composed of stacked MXene sheets, as displayed inFigure Figure 1. 1Distinct. Distinct from from the thetraditional traditional chemic chemicalal etching etching method method to fabricate to fabricate MXene, MXene, which whichis environmentally is environmentally unfriendly, unfriendly, owing to owing the using to the of usinghydrofluoric of hydrofluoric acid (HF), acid we introduce (HF), we introducean alternative an alternative isolation approach, isolation approach, which has which been has successfully been successfully used for used graphene for graphene manu- facturing. Figure 2 displays the calculated exfoliation energy of Sc2CT2, and the exfoliation manufacturing. Figure2 displays the calculated exfoliation energy of Sc 2CT2, and the exfoliationenergy of graphite energy ofis graphitealso shown is also as a shownbenchm asark. a benchmark. As is observed, As is the observed, evaluated the exfoliation evaluated 2 exfoliationenergy of graphite energy ofis graphite~0.32 J/m is, ~0.32according J/m 2to, according our calculation, to our calculation,which is consistent which is with consis- the tentexperimental with the experimental measurement measurement and previous and theo previousretical study theoretical [40,41]. study Amazedly, [40,41]. Amazedly, the calcu- lated exfoliation energies for all the explored Sc2CT2 are much lower than that of graphite, the calculated exfoliation energies for all the explored Sc2CT2 are much lower than that of suggesting that the monolayer Sc2CT2 is very likely to be obtained from its bulk phases, graphite, suggesting that the monolayer Sc2CT2 is very likely to be obtained from its bulk phases,by similar by experimental similar experimental methods methods as graphene as graphene.. Especially, Especially, the exfoliation the exfoliation energyenergy of Sc2CF of2 2 2 Scis 2onlyCF2 ~0.23is only J/m ~0.23. Such J/m value. Such is nearly value istwo-thirds nearly two-thirds of that for of graphite, that for graphite,indicating indicating the rank- theing rankingpossibility possibility for experimental for experimental fabrication. fabrication. The stability acts a pivotal part in the practical application and experimental fabrica- tion of 2D Sc2CT2. Therefore, the phonon dispersion spectra calculations were employed, to assess the kinetic stabilities of 2D Sc2CT2. Moreover, ab initio molecular dynamics (AIMD) calculations were adopted, to evaluate the thermal stabilities. As the phonon curves illustrated in the upper panel of Figure3, there is not any appreciable imaginary acoustic dispersion branch of the phonon curve for all the Sc2CT2, confirming the kinetic stabilities of the three monolayer MXenes. More importantly, the highest phonon spectra frequencies of all the investigated Sc2CT2 are up to 20 THz. This value is comparable with that of the other monolayer material, implying the rough junction among the Sc, C, and T atoms [19]. The total energy changes and structure snapshots at 300 K, sustaining 9 ps

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of Sc2CT2, are displayed in the lower panel of Figure3. As is shown, the total energies are swinging very slightly and the atoms are well maintained around their equilibrium positionsFigure 2. The under calculated the temperature exfoliation energies field, indicating of 2D Sc2CT the2 and thermal graphite. stabilities The inset of theis the monolayer schematic Materials 2021, 14, x FOR PEER REVIEWof exfoliation process. 4 of 10 Sc2CT2. Hence, the kinetic stabilities and thermal stabilities of the Sc2CT2 monolayer have been approved. The stability acts a pivotal part in the practical application and experimental fabrica- tion of 2D Sc2CT2. Therefore, the phonon dispersion spectra calculations were employed, to assess the kinetic stabilities of 2D Sc2CT2. Moreover, ab initio molecular dynamics (AIMD) calculations were adopted, to evaluate the thermal stabilities. As the phonon curves illustrated in the upper panel of Figure 3, there is not any appreciable imaginary acoustic dispersion branch of the phonon curve for all the Sc2CT2, confirming the kinetic stabilities of the three monolayer MXenes. More importantly, the highest phonon spectra frequencies of all the investigated Sc2CT2 are up to 20 THz. This value is comparable with that of the other monolayer material, implying the rough junction among the Sc, C, and T atoms [19]. The total energy changes and structure snapshots at 300 K, sustaining 9 ps of Sc2CT2, are displayed in the lower panel of Figure 3. As is shown, the total energies are swinging very slightly and the atoms are well maintained around their equilibrium posi- tions under the temperature field, indicating the thermal stabilities of the monolayer Sc2CT2. Hence, the kinetic stabilities and thermal stabilities of the Sc2CT2 monolayer have FigureFigure 2.2.The The calculated calculated exfoliation exfoliation energies energies of of 2D 2D Sc ScCT2CTand2 and graphite. graphite. The The inset inset is theis the schematic schematic of been approved. 2 2 exfoliationof exfoliation process. process.

The stability acts a pivotal part in the practical application and experimental fabrica- tion of 2D Sc2CT2. Therefore, the phonon dispersion spectra calculations were employed, to assess the kinetic stabilities of 2D Sc2CT2. Moreover, ab initio molecular dynamics (AIMD) calculations were adopted, to evaluate the thermal stabilities. As the phonon curves illustrated in the upper panel of Figure 3, there is not any appreciable imaginary acoustic dispersion branch of the phonon curve for all the Sc2CT2, confirming the kinetic stabilities of the three monolayer MXenes. More importantly, the highest phonon spectra frequencies of all the investigated Sc2CT2 are up to 20 THz. This value is comparable with that of the other monolayer material, implying the rough junction among the Sc, C, and T atoms [19]. The total energy changes and structure snapshots at 300 K, sustaining 9 ps of Sc2CT2, are displayed in the lower panel of Figure 3. As is shown, the total energies are swinging very slightly and the atoms are well maintained around their equilibrium posi- tions under the temperature field, indicating the thermal stabilities of the monolayer Sc2CT2. Hence, the kinetic stabilities and thermal stabilities of the Sc2CT2 monolayer have been approved.

Figure 3. Phonon dispersion curves curves (upper (upper panel), panel), total total energy energy chan changesges and and structure structure snapshots snapshots (lower (lower panel) panel) from from 0 0to to 9 9ps ps from from AIMD AIMD calculations calculations for for (a ()a Sc) Sc2CF2CF2, 2(b,()b Sc) Sc2CCl2CCl2, and2, and (c) (Scc)2 ScCBr2CBr2. 2.

To understand the electronic properties of monolayer Sc2CT2, the total and partial density of states (DOS) are represented in Figure4. Note that the DOS of Sc 2CF2, Sc2CCl2, and Sc2CBr2 exhibit very similar features, which is in relation to their alike atomic config- urations. For the explored Sc2CF2 and Sc2CCl2, the CBM is mainly contributed by Sc 3d states, while the VBM is entirely dominated by C 2p states and Sc 3d states. In addition, the C 2p orbits and Sc 3d orbits are forcefully hybridized [42]. The CBM of Sc2CBr2 is contributed by Sc 3d states as well, but the VBM of it is not only dominated by C 2p states and Sc 3d states, but also contributed by Br 4p states. Moreover, the calculated band gaps for Sc2CF2, Sc2CCl2, and Sc2CBr2 are 1.85 eV, 1.70 eV, and 1.54 eV, respectively, which are endowed with high harvest under the visible-light region. Note that the band gaps of all three 2D Sc2CT2 are larger than 1.23 eV, which meet the minimum value required

Figure 3. Phonon dispersion curves (upper panel), total energy changes and structure snapshots (lower panel) from 0 to 9 ps from AIMD calculations for (a) Sc2CF2, (b) Sc2CCl2, and (c) Sc2CBr2.

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To understand the electronic properties of monolayer Sc2CT2, the total and partial density of states (DOS) are represented in Figure 4. Note that the DOS of Sc2CF2, Sc2CCl2, and Sc2CBr2 exhibit very similar features, which is in relation to their alike atomic config- urations. For the explored Sc2CF2 and Sc2CCl2, the CBM is mainly contributed by Sc 3d states, while the VBM is entirely dominated by C 2p states and Sc 3d states. In addition, the C 2p orbits and Sc 3d orbits are forcefully hybridized [42]. The CBM of Sc2CBr2 is con- tributed by Sc 3d states as well, but the VBM of it is not only dominated by C 2p states and Sc 3d states, but also contributed by Br 4p states. Moreover, the calculated band gaps for Materials 2021, 14, 4739 5 of 10 Sc2CF2, Sc2CCl2, and Sc2CBr2 are 1.85 eV, 1.70 eV, and 1.54 eV, respectively, which are en- dowed with high harvest under the visible-light region. Note that the band gaps of all three 2D Sc2CT2 are larger than 1.23 eV, which meet the minimum value required for an efficient photocatalyst. Hence, it can be concluded that all the Sc2CT2 MXenes are promis- for an efficient photocatalyst. Hence, it can be concluded that all the Sc2CT2 MXenes are promisinging candidates candidates as semiconductor as semiconductor photocatalysts, photocatalysts, in the aspect in the aspectof bandgaps. of bandgaps.

Figure 4. The total and partialpartial densitydensity ofof states states of of ( a(a)) Sc Sc2CF2CF22,(, (bb)) Sc Sc22CClCCl22,, andand ((cc)) ScSc22CBrCBr22. The Fermi Fermienergy energy is marked is marked by dashed by dashed lines. lines.

Besides the bandgap size, band edge alignmentsalignments are another predominantpredominant factor to facilitate the water reaction. We We have thereforetherefore calculated the band edge alignmentsalignments of Sc CT in Figure5, to assess the performance of these newly designed photocatalysts. Sc2CT22 in Figure 5, to assess the performance of these newly designed photocatalysts. Ac- According to the band alignments in Figure5, 2D Sc CCl and Sc CBr exhibit very similar cording to the band alignments in Figure 5, 2D Sc22CCl22 and Sc22CBr22 exhibit very similar band edge alignment characters. Interestingly, the VBM of Sc CCl and Sc CBr are located band edge alignment characters. Interestingly, the VBM of Sc22CCl22 and Sc22CBr22 are located at a more positive potential than the water redox potential of H O/O (1.23 eV vs. normal at a more positive potential than the water redox potential of H2 2O/O22 (1.23 eV vs. normal hydrogen electrode, NHE), implying the remarkable oxidation capabilities of the photo- generated holes. This This advantage can be appl appliedied in capturing the electrons from organicorganic pollutants, such as methylene blue [19]. However, it is a pity that the CBMs of Sc2CT2 pollutants, such as methylene blue [19]. However, it is a pity that the CBMs+ of Sc2CT2 are are also located at a more positive potential than the redox potential of H /H2 (0 V vs. also located at a more positive potential than the redox potential of H+/H2 (0 V vs. NHE), NHE), indicating that they are not appropriate for photocatalytic water splitting. While for indicating that they are not appropriate for photocatalytic water +splitting. While for Sc2CF2, the CBM is much more negative than the redox potential of H /H2 (0 V vs. NHE), Sc2CF2, the CBM is much more negative than the redox potential of H+/H2 (0 V vs. NHE), but the VBM position is not more positive than the redox potential of O2/H2O (1.23 eV vs. but the VBM position is not more positive than the redox potential of O2/H2O (1.23 eV vs. NHE), suggesting the unapplicable band alignments for water splitting. From the above Materials 2021, 14, x FOR PEER REVIEWNHE), suggesting the unapplicable band alignments for water splitting. From the 6above of 10 analysis, one can observe that the band positions of 2D Sc2CF2, Sc2CCl2 and Sc2CBr2 are analysis, one can observe that the band positions of 2D Sc2CF2, Sc2CCl2 and Sc2CBr2 are not not suitable as semiconductor photocatalysts under ambition conditions. suitable as semiconductor photocatalysts under ambition conditions.

FigureFigure 5. 5. BandBand alignments alignments with with respect respect to tothe the water water reduction reduction and and oxidation oxidation potential potential levels levels of of

ScSc22CTCT22 MXenes.MXenes.

From Figure 5, the band alignments of Sc2CF2 are not appropriate under ambient con- ditions. However, in the practical application of a photocatalysis, the water redox poten- tial is related to the value of pH in the solutions. Hence, the water oxidation potential of O2/H2O can be expressed by the following equation [43]:

OX E =− 5.67 eV + pH × 0.059 eV (1) O/HO22 According to Equation (1), one can shift the oxidation level of water upward in Figure 5. Thus, the band edge alignments of Sc2CF2 can be tuned to straddle the redox potential of water. As illustrated in Figure 6, in ambient conditions, Sc2CF2 is not an appropriate photocatalyst, due to its unsuitable band edge alignments. While in a pH = 8.0 solution, the oxidation level and the reduction potential of water will shift upwards by 0.472 V. We have plotted the band alignments of Sc2CF2 in Figure 6, from which one can observe that the VBM locates more positively than the hydrogen reduction level (0 V vs. NHE), and the CBM locates more negatively than the oxygen oxidation level (1.23 eV vs. NHE). The results show that Sc2CF2 has a favorable band position for water splitting in a pH = 8.0 solution. Moreover, the energy difference between VBM and the oxygen reduction poten- tial (oxidizing power) of Sc2CF2 is 0.25 eV, and the energy difference between CBM and the hydrogen reduction potential (reducing power) is 0.38 eV. Furthermore, we can mod- erate the water-oxidizing power and reducing power of Sc2CF2 by changing the pH to 9.0. Notably, the oxidizing power increases to 0.31 eV, while the reducing power decreases to 0.32 eV. Such values suggest that Sc2CF2 has excellent band edge positions as photocata- lysts, which is very helpful for protecting the chemical equilibriums during the water- splitting reaction process.

Figure 6. Band alignments with respect to water redox potentials of Sc2CF2 in ambient conditions, pH = 8 solution, and pH = 9 solution.

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Materials 2021, 14, 4739 Figure 5. Band alignments with respect to the water reduction and oxidation potential levels 6of of 10 Sc2CT2 MXenes.

From Figure 5, the band alignments of Sc2CF2 are not appropriate under ambient con- From Figure5, the band alignments of Sc CF are not appropriate under ambient ditions. However, in the practical application of2 a photocatalysis,2 the water redox poten- conditions. However, in the practical application of a photocatalysis, the water redox tial is related to the value of pH in the solutions. Hence, the water oxidation potential of potential is related to the value of pH in the solutions. Hence, the water oxidation potential O2/H2O can be expressed by the following equation [43]: of O2/H2O can be expressed by the following equation [43]: OX EOX =− 5.67 eV + pH × 0.059 eV O/HOE22 = −5.67 eV + pH × 0.059 eV (1)(1) O2/H2O According to to Equation Equation (1), (1), one one can can shift shift the theoxidation oxidation level level of water of water upward upward in Figure in 5. Thus, the band edge alignments of Sc2CF2 can be tuned to straddle the redox potential Figure5. Thus, the band edge alignments of Sc 2CF2 can be tuned to straddle the redox of water. As illustrated in Figure 6, in ambient conditions, Sc2CF2 is not an appropriate potential of water. As illustrated in Figure6, in ambient conditions, Sc 2CF2 is not an appropriatephotocatalyst, photocatalyst, due to its unsuitable due to its unsuitableband edge bandalignments. edge alignments. While in a While pH = in8.0 a pHsolution, = 8.0 solution,the oxidation the oxidationlevel and the level reduction and the potential reduction of potential water will of shift water upwards will shift by upwards 0.472 V. We by 0.472have plotted V. We have the band plotted alignments the band of alignments Sc2CF2 in ofFigure Sc2CF 6,2 fromin Figure which6, fromone can which observe one canthat observethe VBM that locates the VBMmore locatespositively more than positively the hydrogen than the reduction hydrogen level reduction (0 V vs. level NHE), (0 V and vs. NHE),the CBM and locates the CBM more locates negatively more than negatively the oxygen than theoxidation oxygen level oxidation (1.23 eV level vs. (1.23NHE). eV The vs. NHE).results Theshow results that Sc show2CF2 that has Sca 2favorableCF2 has a band favorable position band for position water forsplitting water in splitting a pH = in 8.0 a pHsolution. = 8.0 solution.Moreover, Moreover, the energy the difference energy difference between between VBM and VBM the and oxygen the oxygen reduction reduction poten- potentialtial (oxidizing (oxidizing power) power) of Sc2 ofCF Sc2 is2CF 0.252 is eV, 0.25 and eV, the and energy the energy difference difference between between CBM CBM and andthe hydrogen the hydrogen reduction reduction potential potential (reducing (reducing power) power) is 0.38 is eV. 0.38 Furthermore, eV. Furthermore, we can we mod- can moderateerate the water-oxidizing the water-oxidizing power power and reducing and reducing power of power Sc2CF of2 by Sc changing2CF2 by changingthe pH to the9.0. pHNotably, to 9.0. the Notably, oxidizing the power oxidizing increases power to increases 0.31 eV, while to 0.31 the eV, reducing while the power reducing decreases power to decreases0.32 eV. Such to 0.32 values eV. Suchsuggest values that suggest Sc2CF2 thathas excellent Sc2CF2 has band excellent edge positions band edge as positions photocata- as photocatalysts,lysts, which is whichvery helpful is very for helpful protecting for protecting the chemical the chemical equilibriums equilibriums during duringthe water- the water-splittingsplitting reaction reaction process. process.

Figure 6. Band alignments with respect to water redox potentials of of Sc Sc22CFCF2 2inin ambient ambient conditions, conditions, pHpH == 8 solution, and pH = 9 solution.

Having confirmed the band gap and band edge alignments of Sc2CF2, we now turn to its optical spectra in the visible-light wavelength range, which is another vital criteria for photocatalysts to drive the water-splitting reaction efficiently. Figure7 displays the optical absorbance of Sc2CF2, obtained from TDHF-HSE06 and DFT-HSE06, and the light spectrum of silicon is plotted as a benchmark [44]. As is observed, the calculated optical absorbance of 2D Sc2CF2 are absolutely higher than that of silicon, in most visible-light regions. Moreover, the optical absorption, obtained by the TDHF-HSE06 method, is significantly larger than that evaluated by the standard DFT-HSE06 approach, which implies a strong excitonic effect in Sc2CF2, in the visible-light region. For the absorption coefficients that were evaluated by the TDHF-HSE06 method, 2D Sc2CF2 exhibits broad optical absorptions, with three optical spectra peaks at around ~400 nm, ~450 nm, and ~550 nm. It is worth noting that the optical 5 −1 coefficients of Sc2CF2 can reach up to ~2 × 10 cm at around ~550 nm, suggesting an extraordinary light harvesting ability. The outstanding optical performances imply that the Sc2CF2 monolayer is a very hopeful candidate as a semiconductor photocatalyst for hydrogen generation. Materials 2021, 14, x FOR PEER REVIEW 7 of 10

Having confirmed the band gap and band edge alignments of Sc2CF2, we now turn to its optical spectra in the visible-light wavelength range, which is another vital criteria for photocatalysts to drive the water-splitting reaction efficiently. Figure 7 displays the optical absorbance of Sc2CF2, obtained from TDHF-HSE06 and DFT-HSE06, and the light spectrum of silicon is plotted as a benchmark [44]. As is observed, the calculated optical absorbance of 2D Sc2CF2 are absolutely higher than that of silicon, in most visible-light regions. Moreover, the optical absorption, obtained by the TDHF-HSE06 method, is sig- nificantly larger than that evaluated by the standard DFT-HSE06 approach, which implies a strong excitonic effect in Sc2CF2, in the visible-light region. For the absorption coefficients that were evaluated by the TDHF-HSE06 method, 2D Sc2CF2 exhibits broad optical ab- sorptions, with three optical spectra peaks at around ~400 nm, ~450 nm, and ~550 nm. It is worth noting that the optical coefficients of Sc2CF2 can reach up to ~2 × 105 cm−1 at around Materials 2021, 14, 4739 ~550 nm, suggesting an extraordinary light harvesting ability. The outstanding optical7 of 10 performances imply that the Sc2CF2 monolayer is a very hopeful candidate as a semicon- ductor photocatalyst for hydrogen generation.

Figure 7. CalculatedCalculated optical optical absorption absorption coefficients coefficients of ofSc2 ScCF22CF compared2 compared with with the experimental the experimental spectrumspectrum ofof intrinsicintrinsic silicon.silicon. TheThe colorfulcolorful backgroundbackground isis thethe referencereference solarsolar spectralspectral irradianceirradiance [45]. [45]. The efficient separation of photo-induced electrons and holes can guarantee high efficiencyThe efficient of a semiconductor separation of photocatalyst.photo-induced Figureelectrons8a showsand holes the can electron guarantee localization high ef- functionsficiency of (ELFs) a semiconductor of 2D Sc2CF photocatalyst.2, to investigate Figu there spatial 8a shows charge the distributions. electron localization As presented func- intions Figure (ELFs)8a, of the 2D electrons Sc2CF2, to are investigate transferred the from spatial Sc atomscharge to distributions. C atoms and As F presented atoms, and in completelyFigure 8a, the gathered electrons around are transferred C atoms and from F atoms, Sc atoms leading to toC highlyatoms and localized F atoms, surroundings. and com- Accordingly,pletely gathered the around positively C atoms charged and region F atoms, around leading the to Sc highly atoms localized will attract surroundings. the photo- generatedAccordingly, electrons, the positively while the charged negatively region charged around region the around Sc atoms the Cwill and attract F atoms the will photo- trap thegenerated photo-activated electrons, holes. while Hence, the negatively the photo-induced charged region electron–hole around pairs the C tend and to F aggregatedatoms will intrap different the photo-activated areas, resulting holes. in efficient Hence, carrierthe photo-induced separation in electron–hole the Sc2CF2 monolayer. pairs tend to As ag- is known,gregated most in different photo-generated areas, resulting electrons in efficient are gathered carrier at theseparation CBM, and in the photo-generated Sc2CF2 mono- holeslayer. areAs is gathered known, atmost the ph VBMoto-generated [4,46]. We electrons have then are computed gathered at the the band CBM, decomposed and photo- chargegenerated density holes ofare the gathered Sc2CF2 atmaterial the VBM in [4,4 Figure6]. We8. have From then the bandcomputed decomposed the band chargedecom- density,posed charge we can density observe of thatthe theSc2CF CBMs2 material are entirely in Figure contributed 8. Fromby the the bandd electrons decomposed from thecharge Sc atoms, density, while we can the VBMsobserve are that completely the CBMs dominated are entirely by contributed the Sc atoms by and the pd electronselectrons fromfrom the Sc C atoms, while which the is inVBMs line are with completely the partial dominated DOS that by is displayedthe Sc atoms in and Figure p elec-4a. Astrons is observed,from the C the atoms, CBMs which are mainlyis in line distributed with the partial at the DOS Sc–F that zones, is displayed and the VBMin Figure are located4a. As is at observed, the C zones, the thus CBMs the are photo-induced mainly distributed electrons at andthe Sc–F holes zones, will transfer and the to VBM the Sc–F are zoneslocated and at the C zones, C zones, respectively. thus the photo-induced Therefore, the electrons photo-excited and holes electron–hole will transfer pairs to the can Sc– be Materials 2021, 14, x FOR PEER REVIEW 8 of 10 separatedF zones and efficiently C zones, at respectively. different zones Therefore, in the monolayerthe photo-excited Sc2CF2 ,electron–hole which is a benefit pairs forcan the be photocatalysisseparated efficiently water-splitting at different reaction zones onin the monolayer Sc 2CFCF22, whichphotocatalyst. is a benefit for the photocatalysis water-splitting reaction on the monolayer Sc2CF2 photocatalyst.

Figure 8. (a) The The ELF ELF contour contour plot perpendicular toto thethe (100)(100) directiondirection forfor 2D2D ScSc22CF2.. The The band band decomposed decomposed charge charge density of the (b)) CBMCBM andand ((cc)) VBMVBM forfor 2D2D ScSc22CF2..

4. Conclusions In conclusion, we have comprehensively explored thethe photocatalyticphotocatalytic waterwater splittingsplitting of Sc22CT2 (T = F, Cl,Cl, Br)Br) MXenes, MXenes, as as semiconductor semiconductor photocatalysts photocatalysts for for hydrogen hydrogen generation, genera- tion,based based on first-principles on first-principles calculations. calculations. Firstly, Fi therstly, exfoliation the exfoliation energy energy of bulk-layered of bulk-layered Sc2CT2 Schas2CT been2 has calculated, been calculated, which iswhich much is smaller much smaller than that than of that graphite, of graphite, implying implying that the that mono- the layer Sc CT can be isolated by mechanical cleavage from its bulk phases. Additionally, monolayer2 Sc2 2CT2 can be isolated by mechanical cleavage from its bulk phases. Addition- ally, as demonstrated by phonon calculations and ab initio molecular dynamicssimula- tions, the Sc2CT2 monolayer shows kinetic and thermal stabilities. Moreover, the calcu- lated band gaps for Sc2CF2, Sc2CCl2, and Sc2CBr2 are 1.85 eV, 1.70 eV, and 1.54 eV, respec- tively. However, in ambient conditions, Sc2CT2 is not a suitable photocatalyst, due to its inappropriate band edge position. It is highlighted that the band edge alignments of Sc2CF2 can be tuned to straddle the redox potential of water, by shifting the oxidation level of water upwards, which can be realized by introducing a pH = 8.0 solution. Furthermore, by changing the pH to 9.0, the oxidizing power and reducing power of Sc2CF2 can be mod- erated to a harmonious level, which is very helpful to keep the chemical balance during the redox reaction. Meanwhile, 2D Sc2CF2 exhibits a high optical absorption, up to 2 × 105 cm−1, in visible-light wavelengths. Notably, the photo-excited electron–hole pairs of Sc2CF2 will separate efficiently. Our exciting results pave the way for designing more MXenes as semiconductor photocatalysts for hydrogen generation.

Author Contributions: Conceptualization, S.G. and Y.Z.; methodology, S.G. and Y.Z.; validation, S.G. and Y.Z.; formal analysis, S.G. and Y.Z.; investigation, H.L., J.H. and Z.S.; writing—original draft preparation, S.G.; writing—review and editing, Y.Z.; visualization, H.L., J.H. and Z.S.; super- vision, Y.Z.; funding acquisition, S.G. All authors have read and agreed to the published version of the manuscript. Funding: This work is financially supported by Natural Science Foundation of Fujian Province (2020J01411), Young and middle-aged Scientific Research Project of Education Department of Fujian Province (JAT190773), Scientific Research Fund project of Wuyi University (XD201703) and Fujian Provincial Key Laboratory of Pollution Control & Resource Reuse Open Project Fund. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request. The data presented in this study are avail- able on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Simon, T.; Bouchonville, N.; Berr, M.J.; Vaneski, A.; Adrović, A.; Volbers, D.; Wyrwich, R.; Döblinger, M.; Susha, A.S.; Rogach, A.L.; et al. Redox shuttle mechanism enhances photocatalytic H2 generation on Ni-decorated CdS nanorods. Nat. Mater. 2014, 13, 1013–1018. 2. Chen, X.; Liu, L.; Peter, Y.Y.; Mao, S.S. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 2011, 331, 746–750. 3. Sa, B.; Li, Y.-L.; Qi, J.; Ahuja, R.; Sun, Z. Strain engineering for phosphorene: The potential application as a photocatalyst. J. Phys. Chem. C 2014, 118, 26560–26568.

Materials 2021, 14, 4739 8 of 10

as demonstrated by phonon calculations and ab initio molecular dynamicssimulations, the Sc2CT2 monolayer shows kinetic and thermal stabilities. Moreover, the calculated band gaps for Sc2CF2, Sc2CCl2, and Sc2CBr2 are 1.85 eV, 1.70 eV, and 1.54 eV, respectively. However, in ambient conditions, Sc2CT2 is not a suitable photocatalyst, due to its inap- propriate band edge position. It is highlighted that the band edge alignments of Sc2CF2 can be tuned to straddle the redox potential of water, by shifting the oxidation level of water upwards, which can be realized by introducing a pH = 8.0 solution. Furthermore, by changing the pH to 9.0, the oxidizing power and reducing power of Sc2CF2 can be moderated to a harmonious level, which is very helpful to keep the chemical balance during the redox reaction. Meanwhile, 2D Sc2CF2 exhibits a high optical absorption, up to 2 × 105 cm−1, in visible-light wavelengths. Notably, the photo-excited electron–hole pairs of Sc2CF2 will separate efficiently. Our exciting results pave the way for designing more MXenes as semiconductor photocatalysts for hydrogen generation.

Author Contributions: Conceptualization, S.G. and Y.Z.; methodology, S.G. and Y.Z.; validation, S.G. and Y.Z.; formal analysis, S.G. and Y.Z.; investigation, H.L., J.H. and Z.S.; writing—original draft preparation, S.G.; writing—review and editing, Y.Z.; visualization, H.L., J.H. and Z.S.; supervision, Y.Z.; funding acquisition, S.G. All authors have read and agreed to the published version of the manuscript. Funding: This work is financially supported by Natural Science Foundation of Fujian Province (2020J01411), Young and middle-aged Scientific Research Project of Education Department of Fujian Province (JAT190773), Scientific Research Fund project of Wuyi University (XD201703) and Fujian Provincial Key Laboratory of Pollution Control & Resource Reuse Open Project Fund. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request. The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

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