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Article

First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures

by
Meiping Liu
1,
Yidan Lu
2,
Jun Song
2,
Benyuan Ma
2,
Kangwen Qiu
2,
Liuyang Bai
2,
Yinling Wang
2,
Yuanyuan Chen
3,* and
Yong Tang
2,3,*
1
School of Intelligent Manufacturing, Huanghuai University, Zhumadian 463000, China
2
Henan Key Laboratory of Smart Lighting, School of Energy Engineering, Huanghuai University, Zhumadian 463000, China
3
Polymer, Recycling, Industrial, Sustainability and Manufacturing (PRISM), Technological University of the Shannon: Midlands Midwest, N37 HD68 Athlone, Ireland
*
Authors to whom correspondence should be addressed.
Molecules 2024, 29(14), 3303; https://doi.org/10.3390/molecules29143303
Submission received: 18 June 2024 / Revised: 5 July 2024 / Accepted: 11 July 2024 / Published: 12 July 2024
(This article belongs to the Section Physical Chemistry)
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Abstract

Heterostructure catalysts are highly anticipated in the field of photocatalytic water splitting. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are proposed in this work, and the electronic structures were revealed with the first-principles method to explore their photocatalytic properties for water splitting. The results found that the thermodynamically stable AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are indirect semiconductors with reduced band gaps of 1.75 eV and 1.84 eV, respectively. These two heterostructures have been confirmed to have type-Ⅰ band alignments, with both VBM and CBM contributed to by the Sc2CF2 layer. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures exhibit the potential for photocatalytic water splitting as their VBM and CBM stride over the redox potential of water. Gibbs free energy changes in HER occurring on AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are as low as −0.31 eV and −0.59 eV, respectively. The Gibbs free energy change in HER on the AlN (GaN) layer is much lower than that on the Sc2CF2 surface, owing to the stronger adsorption of H on AlN (GaN). The AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures possess significant improvements in absorption range and intensity compared to monolayered AlN, GaN, and Sc2CF2. In addition, the band gaps, edge positions, and absorption properties of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures can be effectively tuned with strains. All the results indicate that AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are suitable catalysts for photocatalytic water splitting.

1. Introduction

The efficient use of solar energy has been widely regarded as one of the most prominent approaches to addressing the issues of energy depletion and environmental pollution. Photocatalytic water splitting into H2 and O2 provides a more practical scheme for the efficient utilization of solar energy [1]. Numerous semiconductors have been developed to serve photocatalytic water splitting [2,3,4,5,6,7]. However, there are some basic but quite strict requirements for semiconductor photocatalysts [8,9]. Firstly, the valence band maximum (VBM) of a photocatalyst should be lower than the reduction potential of O2/H2O (E[O2/H2O]); meanwhile, its conduction band minimum (CBM) should exceed the reduction potential of H+/H2 (E[H+/H2]). In addition, photocatalysts also require superior light absorption in the visible region and low carrier recombination, which can provide enough photo-generated carriers for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Over the last few decades, some mono-component photocatalysts, such as TiO2 [10], ZnO [11], and BiVO4 [12], have been found with the capabilities for water splitting. Yet, these photocatalysts suffer from the disadvantages of poor visible light absorption and serious carrier recombination, limiting the application process of photocatalytic water splitting [13,14]. Therefore, the primary challenge that needs to be addressed for the widespread implementation of photocatalytic water splitting is to develop novel high-performance catalysts with an appropriate band gap, superior visible light absorption behavior, and a low carrier recombination rate [15]. Heterostructure photocatalysts [5,16], composed of multi-components, generally possess these aforementioned advantages and are among the catalysts most likely to be implemented in the future.
The emerging two-dimensional (2D) materials of atomic thickness have been discovered with notable potential for photocatalytic water splitting owing to their high cost-effectiveness, massive specific surface areas, and abundant active sites [17]. Transition metal carbides (MXene) are presently one of the most popular 2D materials, with prospective applications in solar energy conversion, such as photocatalysis and photovoltaics. Sc2CF2 is a semi-conductive MXene, and it has been predicted to have an electron mobility and thermal conductivity of up to 103 cm2*V*−1*s−1 and 472 W*m−1*K−1, respectively [18]. Even though its high carrier mobility and thermal conductivity are highly beneficial for photocatalytic water splitting, the VBM of Sc2CF2 exceeds E[O2/H2O], resulting in its inability to catalyze to the OER [19]. As other key members of 2D materials, AlN and GaN have recently been prepared by the MOCVD, PVT, MBE, and MEEG techniques [20,21,22,23]. These two nitrides are reported to exhibit unique optoelectronic and mechanical performance [24,25], and they have been considered as candidates for future optoelectronic devices [26,27,28,29,30,31,32]. For example, 2D AlN shows great promise in ultraviolet optoelectronic and laser diode applications [20,33], while 2D GaN has been investigated as a decent semiconductor for heterostructures and photocathodes [34,35]. Interestingly, the energy levels of VBM and CBM meet the requirements of photocatalytic water splitting [36]. However, they are all wide-band gap semiconductors with an ultraviolet absorption response only. Thus, their performances of photocatalytic water splitting are far below the requirement of real-world applications. Furthermore, several previous reports have proven the successes of heterostructures in optimizing the photocatalytic performance of MXene and nitrides [37,38,39]; nevertheless, the observations of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are still missing. Considering the favorable electronic nature of previously synthesized Sc2CF2 and nitrides (AlN, GaN), it therefore makes sense to investigate the novel properties and prospective photocatalytic performance of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures.
In this work, AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures were developed, and their electronic structures were investigated using the first-principles method to reveal the application potential for photocatalytic water splitting. Firstly, the stabilities of these two heterostructures were confirmed with binding energy (Eb) calculations and ab initio molecular dynamic (AIMD) simulations. Then, the results demonstrated that AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures possess type-Ⅰ band alignments. In addition, the charge density difference, band edge positions, free energy for HER, and absorption properties were also studied, indicating their promising abilities for photocatalytic water splitting. Finally, the effects of strain on the aforementioned performances were also revealed. The findings in this work extend the insights of novel photocatalysts and provide the theoretical foundation for experimental research to develop high-performance photocatalysts for water splitting.

2. Results and Discussions

Firstly, the geometries and electronic structures of AlN, GaN, and Sc2CF2 monolayers were studied. The three optimized honeycomb geometries based on PBE functional are shown in Figure 1a, and the corresponding lattice constants were determined to be 3.22 Å, 3.29 Å, and 3.17 Å, respectively, matching up with earlier results [19,40,41,42]. The band structures of Sc2CF2, AlN, and GaN monolayers, calculated using the PBE and HSE06 functional, are exhibited in Figure 1b–d. It is clear from the band structures that all three monolayers are indirect band gap semiconductors. The VBM and CBM of the AlN (GaN) monolayer can observed at the K-point and G-point, respectively; however, the positions of VBM and CBM for the Sc2CF2 monolayer are generated at the M-point and K-point. The values of the band gap determined by the PBE functional are 2.92 eV, 2.19 eV, and 1.02 eV, whereas the in-turn results based on the HSE06 functional are 4.01 eV, 3.28 eV, and 2.09 eV. All the findings of the band gap are almost the same as the previous reports [36,38,43].
Since there are almost similar lattice constants between the AlN (GaN) layer and the Sc2CF2 layer, the AlN/Sc2CF2 (GaN/Sc2CF2) heterostructures are constructed by stacking and translating the AlN (GaN) layer on the Sc2CF2 layer. As presented in Figure 2, there are six generally observed stacking configurations (SCs) of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures that have been taken into account. Following sufficient structural relaxation based on the PBE functional, the lattice constants of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures given in Table 1 are similar to those of AlN and GaN monolayers, respectively, which may be attributed to the higher mechanical properties of AlN and GaN monolayers compared to that of the Sc2CF2 monolayer. The interlayer distances d for the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures within SC-I are the smallest, with the values of 2.71 Å and 2.82 Å, respectively. The minimum interlayer distance suggests the probable presence of significant interlayer coupling, which may profoundly influence the stabilities and electronic structures of the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. To confirm the most feasible SC, the values of Eb for AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures have been given as follows:
E b = E het −   E AlN   ( E GaN ) − E Sc 2 CF 2 S
where Ehet, EAlN (EGaN), and ESc2CF2 represent the energy of the heterostructure, AlN (GaN) monolayer, and Sc2CF2 monolayer, respectively, whereas S is the interface area. All the values in Table 1 for the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are negative, indicating their thermodynamic stabilities. As expected, SC-I was checked to be optimal, as the corresponding AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures had the lowest values of −22.17 meV*Å−2 and −30.82 meV*Å−2. The results of d and Eb suggest that the AlN and GaN can establish stable heterostructures with Sc2CF2 through van der Waals bonding [44]. Therefore, subsequent research concentrates mainly on the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures in SC-I.
Furthermore, the AIMD simulations were performed to ascertain the thermodynamic stabilities of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures in SC-I. Overall, the results of the total potential energy shown in Figure 3a demonstrate that the energy convergence of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures remains stable throughout the entire AIMD simulation process, with only minor fluctuations. At the same time, the final snapshots shown in Figure 3b,c still retain their complete structures without clear structural distortion, illustrating their thermodynamic stabilities. In addition, the phonon spectra of the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures in SC-I, shown in Figure S1, further confirm the dynamical stabilities.
Figure 4 shows the projected band structures and projected density of states (PDOSs) for AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. Both the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are indirect semiconductors with band gap values of 1.75 eV and 1.84 eV, respectively. As indicated in the projected band structures, the VBM and CBM of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are primarily populated by the Sc2CF2 layer, resulting in the formation of type-I band alignments in these heterostructures. For the two heterostructures, according to the PDOS, it is evident that Sc-3d orbitals make a substantial contribution to CBM and VBM, while the role of C-2p orbitals in the VBM is more significant than that in CBM, whereas the F-2p orbitals contribute an insignificant amount compared to both the VBM and CBM. However, the GaN layer provides a more substantial contribution to the VBM of the GaN/Sc2CF2 heterostructure compared to the AlN layer in the AlN/Sc2CF2 heterostructure. This difference may be due to the higher energy level of VBM in GaN compared to AlN. Furthermore, the band structures of the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures in all six SC are calculated by the PBE and HSE06 functional. The band structures in Figures S2–S5 indicate that all the heterostructures are indirect band gap semiconductors. As exhibited in Table 1, the results obtained from the PBE and HSE06 suggest that SC has no clear influence on the band gap.
The effect of interlayer coupling is particularly considerable on the electronic structure of heterostructures [45]. While the AlN (GaN) monolayer comes into contact with the Sc2CF2 monolayer, electrons spontaneously migrate from the layer with a smaller work function (W) to another layer until their Fermi levels approach equally. The values of W for AlN, GaN, and Sc2CF2 monolayers can be calculated with expression (2) to comprehend the electron migration mechanism:
W = E vac − E F
where Evac and EF refer to the energies of the vacuum level and the Fermi level, respectively. As shown in Figure S6, the corresponding values of W for the AlN, GaN, and Sc2CF2 monolayers are 5.23 eV, 5.18 eV, and 4.95 eV. These findings indicate the electron transfer from the Sc2CF2 monolayer to the AlN and GaN layers. Thus, the electron and hole concentration regions emerge near the AlN (GaN) and Sc2CF2 layers, respectively. In Figure S7, because of the electrostatic induction [46,47,48], electrons in the AlN (GaN) layer are repelled by the electrons on the surface of Sc2CF2, causing upward band bending of the AlN (GaN) layer, while the band of Sc2CF2 bends downward. When the heterostructures are exposed to light irradiation, electrons excited on the CBM of the AlN (GaN) layer can spontaneously flow to the CBM of the Sc2CF2 layer; then, these electrons can be used for the HER. Owing to the potential barriers [37], holes are retained in the VBM of AlN (GaN) and Sc2CF2 layers for OER. Bader charge calculations revealed that 0.095 |e| electrons flow from the Sc2CF2 monolayer to the AlN monolayer, while the number of electrons transferring to the GaN monolayer was 0.041|e| [49]. As illustrated in Figure 5a,b, AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures were found with the W of 5.14 eV and 5.07 eV, respectively. Moreover, the potential drops introduced by the electron migration were observed in AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, with values of 9.35 eV and 7.24 eV, respectively. The potential drop establishes a built-in electric field orientating from the AlN (GaN) layer and Sc2CF2 layer. The potential drops in AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are much greater than those of the past reported InSe/MoS2 (2.55 eV) [50], GaN/GeS (less than 0.5 eV) [51], and CrSSe/MoS2 (2.30 eV) [52] heterostructures. The large potential drops result in the generation of strong built-in electric fields in AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, and the built-in electric fields are beneficial for carrier separation in photocatalysis and photovoltaic processes [53]. In practical applications, defect and doping engineering can be implemented in the interfaces of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures to enhance the strength of the built-in electric field, which helps promote the separation of charge carriers. In addition, the charge density difference ρ can be used to track the charge transfer and obtain information on the interactions between different components in heterostructures [54]. The ρ for AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are calculated by expression (3):
∆ ρ = ρ het   −   ρ AlN ( ρ GaN )     −   ρ Sc 2 CF 2
where ρhet, ρAlN (ρGaN) and ρSc2CF2 express the charge densities of the AlN/Sc2CF2 (GaN/Sc2CF2) heterostructure and AlN (GaN) and Sc2CF2 monolayers, respectively. From the results plotted in Figure 5c,d, the electron migration from the Sc2CF2 layer to the AlN (GaN) layer is evidently noticeable, and the transfer behavior is more pronounced in the AlN/Sc2CF2 heterostructure, conforming to the previous statements.
To reveal the potential of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures for photocatalytic water splitting, the energy levels of VBM and CBM for two heterostructures and three freestanding monolayers were evaluated with the method proposed by Toroker [55]. As can be seen from Figure 6a, the positions of VBM and CBM for the AlN and GaN monolayers match the demands of photocatalytic water splitting, and their wide band gaps allow them to work within the extensive pH range. For the Sc2CF2 monolayer, its CBM and VBM are beyond E[H+/H2] and E[O2/H2O], respectively, indicating that it has the catalytic ability for HER but is incapable of OER. The CBM and VBM of the AlN/Sc2CF2 heterostructure are located at −4.29 eV and −6.04 eV, respectively, whereas the GaN/Sc2CF2 heterostructure possesses the values of −3.92 eV and −5.76 eV. Moreover, the GaN/Sc2CF2 heterostructure has been identified with a wide work pH range from 0 to 7. Figure S8 shows the band alignments of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, and it is evident that the AlN, GaN, and Sc2CF2 monolayers in heterostructures are assessable for photocatalytic water splitting. The Gibbs free energy change ΔG of HER is valuable to estimate the photocatalytic property from a thermodynamic perspective [56], and the ΔG for HER occurring on both AlN, GaN, and Sc2CF2 surfaces in the heterostructure is calculated to assess the feasibility of HER, driven by the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. Nine possible sites for H-atom adsorbing on the heterostructure, as shown in Figure S9, were considered in this work. The calculation details and stable adsorption structures (Figure S10) are provided in the Supplementary Materials. The structure that had the lowest energy was chosen to be the most stable adsorption site. The H-atom was adsorbed on the F atom of the Sc2CF2 layer, similar to the surfaces of AlN and GaN, it was located above the N atom. The results in Figure 6b demonstrate that the ΔG for HER on the Sc2CF2 layer in AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures was 1.75 eV and 1.17 eV. As these present values of ΔG are smaller than the 2.63 eV for N-Ni3S2/NF [57], which is available for driving HER experimentally, HER on the Sc2CF2 surface in the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures should likewise be experimentally practicable. However, the HER occurring on the AlN and GaN surfaces in AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures is more favorable, for which the values of ΔG are −0.31 eV and −0.59 eV, respectively. The ΔG values of HER on AlN and GaN surfaces are close to those of cobalt phosphide catalysts reported with good performances of photocatalytic water splitting [58] and smaller than that of the freestanding AlN monolayer [59]. The smaller ΔG for HER on the AlN and GaN surfaces in heterostructures may be attributed to their large overpotentials, as shown in Table S2 [60]. As is known from Figure S11, the p-band center was calculated to further comprehend better HER performance on AlN and GaN surfaces. The values of the N-2p band center for H adsorption on AlN and GaN surfaces are −1.77 and −3.02 eV, while the center values of the F-2p band were predicted to be −6.98 and −6.53 eV for adsorption on Sc2CF2 in AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. A lower band center means the stronger adsorption strength of H on the N atom compared to that on the F atom [61]. Therefore, the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures with appropriate ΔG exhibit attractive application potential as photocatalysts for water splitting to produce clean hydrogen energy.
Strain is a common effect at the interface of the heterostructure that considerably impacts its electronic structure [62,63]. The band structures and absorption coefficients of strained AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are calculated using HSE06 functional to explore the effect of biaxial strain on their electronic structures and absorption properties. The strain ε is defined as follows:
ε = a   − a 0 a 0
where a and a0 stand for the lattice constants of strained and free heterostructures. Six strains of −6%, −4%, −2%, 2%, 4%, and 6% were applied to the heterostructures. The band structures and PDOS of strained AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are exhibited in Figures S12–S15.
It is evident that all heterostructures maintain an indirect band gap feature with type-Ⅰ band alignment since the VBM and CBM of strained heterostructures are contributed to by AlN and GaN, respectively. As shown in Figure 7a,b, the effect of strains on the band gaps of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures is clear, with lattice compression and tension reducing and increasing the bandgap, respectively. With strains varying from −6% to 6%, the values of band gap for the strained AlN/Sc2CF2 heterostructures increase from 1.02 eV to 2.2 eV, whereas those of the strained GaN/Sc2CF2 heterostructures change from 1.13 eV to 2.17 eV. The energy levels of VBM and CBM for the strained AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are displayed in Figure 7c,d. The tendency of band gaps for strained heterostructures can be understood from the opposite effects of the lattice strain on the energy levels of VBM and CBM. What the opposite effects mean is that the lattice tension causes the VBM energy level to decrease, and it also leads to an increase in the CBM energy level. Hence, the energy differences between VBM and CBM expand, leading to an increase in band gaps. According to the band edge positions, the −2%-strained AlN/Sc2CF2 heterostructure and the −2%-strained GaN/Sc2CF2 heterostructure still maintain their capabilities for photocatalytic water splitting. As the lattice constants of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures expand, their overpotentials increase, which potentially leads to improved photocatalytic activities.
Light absorption performance is an important indicator for photocatalysts, which determines the upper limit of photogenerated carriers used for subsequent HER and OER. The absorption coefficients of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures and their freestanding monolayer components can be calculated using expression (5):
α ω = 2 ω ε 1 2 ω + ε 2 2 ω − ε 1 ( ω )
where ω is the photon frequency, and ε1( ω ) and ε2( ω ) stand for the real and imaginary parts of the dielectric function. As proven by Figure 8, the absorption behaviors of AlN and GaN monolayers are only observed in the ultraviolet region with low intensities due to their wide band gaps. The absorption activity of the Sc2CF2 monolayer is clear, with strong absorption intensity in the visible region. As for AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, their absorption range expands due to the reduced band gaps. Since the Sc2CF2 layers occupy VBM and CBM in the heterostructures, the absorption behaviors of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are comparable to that of the Sc2CF2 monolayer. However, the absorption intensities of heterostructures are enhanced, which might be ascribed to interlayer coupling [64,65]. It can be inferred that the superior absorption activities of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures will produce more carriers for HER and OER. Hence, these two heterostructures possess better performances in photocatalytic water splitting than those of AlN, GaN, and Sc2CF2 monolayers.
For strained AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, the absorption coefficients shown in Figure 9 demonstrate the modulation of strains on their optical absorption performances. As previously stated, compressive strains decrease the band gaps of the heterostructures, resulting in the expanded ranges of absorption for the compressed AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. The absorption ranges of tensile-strained AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures become narrower as their band gaps increase. However, there is another noticeable fact in that the absorption intensities of tensile-strained AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, particularly in the UV range, are superior to those of compressed systems. The enhanced absorption intensity can be attributed to the changes in the charge density difference in the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures under different strains, as shown in Figure S16. It is evident that the changes in lattice have a significant impact on interlayer coupling. Specifically, more charge transfer from the Sc2CF2 layer to the AlN (GaN) layer in the lattice expanded heterostructures, and the transfer behaviors weaken in the compressed AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. The changes in charge transfer may directly influence the strength of the built-in electric field and potential drop, which are responsible for facilitating the migration of photogenerated carriers, thereby enhancing the absorption intensity [45,66,67]. These findings have validated the enhanced absorption capabilities of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, as well as the strained ones, indicating their considerable potential for applications in photocatalytic water splitting.

3. Computational Methods

All computational investigations in the present work were carried out using the Vienna ab initio simulation package (VASP) [68], which is based on the Density Functional Theory (DFT). The exchange–correlation functional was treated by the generalized gradient approximation within the Perdew–Burke–Ernzerhof (GGA-PBE) scheme [69]. Projector-augmented wave pseudopotentials (PAW) were utilized [70,71] with a cutoff energy of 500 eV. The lattice constant and atom position were fully relaxed based on the PBE functional until the energy and force fell to less than 10−5 eV and 0.01 eV*Å−1, respectively. The calculations of band structure and absorption coefficient were conducted with the Heyd–Scueria–Ernzerhof hybrid functional (HSE06) [72]. The van der Waals force, described with the DFT-D3 method [73], was considered in all calculations. The vacuum space of all 2D systems was set to 30 Å in the z-direction to shield neighboring interactions. The Gamma-centered scheme [74] k-points with grids of 15 × 15 × 1 and 21 × 21 × 1 were implemented to sample the Brillouin zone for structure optimization and property calculations. The NVT-ensembled AIMD simulations [75], using the algorithm of the Nosé–Hoover thermostat, were performed on the 4 × 4 × 1 supercell to confirm the thermal stabilities of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. The simulation temperature of 300 K was set, and the total simulation duration was 6 ps with a 1 fs step time. The VASPKIT package [76] and VESTA code [77] were employed for pre- and post-visualization.

4. Conclusions

AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are proposed in this work, and their electronic structures were investigated using the first-principles method to explore their photocatalytic properties. These thermodynamic stable heterostructures show type-I band alignments and their corresponding band gaps were found to be 1.84 eV and 1.75 eV. Furthermore, the clear potential drops of 9.35 eV and 7.24 eV were found to be present in AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, which can generate built-in electric fields to promote carrier separation. The band edge positions of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are suitable for photocatalytic water splitting. The Gibbs free energy change in HER that occurred on the AlN and GaN surfaces in two heterostructures was as low as −0.31 eV and −0.59 eV, respectively. The lower Gibbs free energy changes may be attributed to the stronger adsorption behaviors on AlN and GaN compared to that on Sc2CF2 in heterostructures. As for absorption performance, both the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures possess significant improvements in absorption range and intensity compared to the monolayered AlN, GaN, and Sc2CF2. Furthermore, strains can effectively tune the band gaps, edge positions, and absorption properties of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. The capabilities of photocatalytic water splitting for AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are kept over a wide strain range. All the findings above suggest that the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are promising catalyst candidates for photocatalytic water splitting. Some properties of carriers, such as their mobility, lifetime, and diffusion length, which play important roles in photocatalytic reactions, are still unknown and can be studied further in the future.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules29143303/s1, Table S1. The valence band offset (VBO) and conduct band offset (CBO) of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures; Table S2. The overpotential values for HER of different surfaces in AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures; Figure S1. The phonon spectrum of (a) AlN/Sc2CF2 and (b) GaN/Sc2CF2 heterostructures; Figure S2. The band structures of AlN/Sc2CF2 heterostructures by using the PBE functional; Figure S3. The band structures of AlN/Sc2CF2 heterostructures by using the HSE06 functional; Figure S4. The band structures of GaN/Sc2CF2 heterostructures by using the PBE functional; Figure S5. The band structures of GaN/Sc2CF2 heterostructures by using the HSE06 functional; Figure S6. The potentials for AlN, GaN, and Sc2CF2 monolayers; Figure S7. The carrier transfer mechanical of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. The MN represents AlN and GaN; Figure S8. The band alignments of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures; Figure S9. All the possible adsorption sites of H-atom on the (a) AlN layer and (b) Sc2CF2 layer in AlN/Sc2CF2 heterostructure. The same adsorption sites also have been considered in GaN/Sc2CF2 heterostructure; Figure S10. The stable adsorption of H atom on the AlN (GaN) and Sc2CF2 surface in AlN/Sc2CF2 (GaN/Sc2CF2) heterostructure; Figure S11. The PDOS distribution of H adsorbed on the surfaces of AlN (GaN) and Sc2CF2 in AlN/Sc2CF2 (GaN/Sc2CF2) heterostructure, respectively; Figure S12. The band structures of strained AlN/Sc2CF2 heterostructures by using the HSE06 functional; Figure S13. The PDOS of strained AlN/Sc2CF2 heterostructures by using the HSE06 functional; Figure S14. The band structures of strained GaN/Sc2CF2 heterostructures by using the HSE06 functional; Figure S15. The PDOS of strained GaN/Sc2CF2 heterostructures by using the HSE06 functional; Figure S16. The charge density difference of the strained AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures. References [78,79,80,81,82,83] are cited in Supplementary Materials.

Author Contributions

Conceptualization, M.L., Y.C. and Y.T.; methodology, M.L. and Y.L.; software, Y.T., B.M.; validation, J.S., K.Q., L.B. and Y.W.; formal analysis, M.L. and Y.L.; investigation, M.L. and Y.T.; resources, M.L. and Y.W.; data curation, M.L., Y.L., J.S., B.M. and Y.T.; writing—original draft preparation, M.L. and Y.L.; writing—review and editing, Y.C. and Y.T.; visualization, M.L.; supervision, Y.C. and Y.T.; project administration, K.Q. and L.B.; funding acquisition, M.L., J.S. and Y.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the China Central Guidance on Local Science Technology Development Fund of Henan Province (No. Z20221343028), the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (IRTSTHN) (No. 24IRTSTHN020), the Natural Science Foundation of Henan Province (No. 232300420335 and No. 242300420256), the Scientific and Technological Breakthroughs in Henan Province (No. 232102230016, No. 242102210164 and No. 242102230157), the Key Scientific Research Project of Colleges and Universities in Henan Province (No. 23A480009), and the Youth project of National Scientific Research Project Cultivation Fund of Huanghuai University (No. XKPY-2022023).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 29 03303 g001
Figure 1. (a) The optimized AlN, GaN, and Sc2CF2 monolayers. The band structures for (b) AlN, (c) GaN, and (d) Sc2CF2 monolayers. The red-solid and blue-dotted lines in the band structures are the results using the PBE and HSE06 functional, respectively.
Figure 1. (a) The optimized AlN, GaN, and Sc2CF2 monolayers. The band structures for (b) AlN, (c) GaN, and (d) Sc2CF2 monolayers. The red-solid and blue-dotted lines in the band structures are the results using the PBE and HSE06 functional, respectively.
Molecules 29 03303 g001
Molecules 29 03303 g002
Figure 2. (a−f) The bottom and side views of AlN/Sc2CF2 (GaN/Sc2CF2) heterostructures. SC-I to SC-VI correspond to the six stacking configurations.
Figure 2. (a−f) The bottom and side views of AlN/Sc2CF2 (GaN/Sc2CF2) heterostructures. SC-I to SC-VI correspond to the six stacking configurations.
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Figure 3. (a) The total energies of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures in the AIMD simulation at a temperature of 300K, as well as their (b) initial and (c) final snapshots.
Figure 3. (a) The total energies of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures in the AIMD simulation at a temperature of 300K, as well as their (b) initial and (c) final snapshots.
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Figure 4. The projected band structure and PDOS of (a) AlN/Sc2CF2 and (b) GaN/Sc2CF2 heterostructures using the HSE06 functional. In the band structures, the green circles represent the contributions of the Sc2CF2 monolayer, while the cyan and magenta circles show those of the AlN and GaN layers, respectively.
Figure 4. The projected band structure and PDOS of (a) AlN/Sc2CF2 and (b) GaN/Sc2CF2 heterostructures using the HSE06 functional. In the band structures, the green circles represent the contributions of the Sc2CF2 monolayer, while the cyan and magenta circles show those of the AlN and GaN layers, respectively.
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Figure 5. The potential and charge density difference in (a,c) AlN/Sc2CF2 and (b,d) GaN/Sc2CF2 heterostructures. The isosurface of the insert was set to 3 × 10−4 e*Å−3.
Figure 5. The potential and charge density difference in (a,c) AlN/Sc2CF2 and (b,d) GaN/Sc2CF2 heterostructures. The isosurface of the insert was set to 3 × 10−4 e*Å−3.
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Figure 6. (a) The energy levels of VBM and CBM for the free monolayers and heterostructures. The black lines and dashed lines represent the redox potentials for water splitting at pH = 0 and 7, respectively. (b) Gibbs free energy diagram for HER on the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, respectively.
Figure 6. (a) The energy levels of VBM and CBM for the free monolayers and heterostructures. The black lines and dashed lines represent the redox potentials for water splitting at pH = 0 and 7, respectively. (b) Gibbs free energy diagram for HER on the AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, respectively.
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Figure 7. The band gaps and band edge positions of strained (a,c) AlN/Sc2CF2 and (b,d) GaN/Sc2CF2 heterostructures.
Figure 7. The band gaps and band edge positions of strained (a,c) AlN/Sc2CF2 and (b,d) GaN/Sc2CF2 heterostructures.
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Figure 8. Absorption coefficients of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, as well as those of AlN, GaN, and Sc2CF2 monolayers.
Figure 8. Absorption coefficients of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures, as well as those of AlN, GaN, and Sc2CF2 monolayers.
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Figure 9. Absorption coefficients of the strained (a) AlN/Sc2CF2 and (b) GaN/Sc2CF2 heterostructures compared with the two freestanding heterostructures.
Figure 9. Absorption coefficients of the strained (a) AlN/Sc2CF2 and (b) GaN/Sc2CF2 heterostructures compared with the two freestanding heterostructures.
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Table 1. Results of lattice constant a (Å), interlayer distance d (Å), binding energy Eb (meV*Å−2), and band gap Eg (eV) for AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures.
Table 1. Results of lattice constant a (Å), interlayer distance d (Å), binding energy Eb (meV*Å−2), and band gap Eg (eV) for AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures.
HeterostructureItemSC-ISC-IISC-IIISC-IVSC-VSC-VI
AlN/Sc2CF2a3.213.203.203.203.203.20
d2.713.382.933.372.843.03
Eb−22.17−8.74−13.62−9.69−18.25−10.54
EgPBE0.790.850.860.860.850.85
HSE061.751.701.711.701.701.70
GaN/Sc2CF2a3.253.223.243.223.223.21
d2.823.372.983.312.853.18
Eb−30.82−15.21−24.24−15.87−27.19−21.27
EgPBE0.900.950.960.950.930.95
HSE061.841.811.821.811.821.81
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Liu, M.; Lu, Y.; Song, J.; Ma, B.; Qiu, K.; Bai, L.; Wang, Y.; Chen, Y.; Tang, Y. First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures. Molecules 2024, 29, 3303. https://doi.org/10.3390/molecules29143303

AMA Style

Liu M, Lu Y, Song J, Ma B, Qiu K, Bai L, Wang Y, Chen Y, Tang Y. First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures. Molecules. 2024; 29(14):3303. https://doi.org/10.3390/molecules29143303

Chicago/Turabian Style

Liu, Meiping, Yidan Lu, Jun Song, Benyuan Ma, Kangwen Qiu, Liuyang Bai, Yinling Wang, Yuanyuan Chen, and Yong Tang. 2024. "First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures" Molecules 29, no. 14: 3303. https://doi.org/10.3390/molecules29143303

APA Style

Liu, M., Lu, Y., Song, J., Ma, B., Qiu, K., Bai, L., Wang, Y., Chen, Y., & Tang, Y. (2024). First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures. Molecules, 29(14), 3303. https://doi.org/10.3390/molecules29143303

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