3.1. Structural Stability
In this study, we chose the hexagonal MoSe
2 and HfS
2 monolayers, which are able to fit each other well. After the structural optimization, the lattice constants of MoSe
2 and HfS
2 single layers were 3.30 Å and 3.61 Å, respectively, which agree with the previous values [
54]. Due to the small lattice mismatch of these single layers, we designed a heterojunction composed of 2 × 2 MoSe
2 and HfS
2 supercells. In order to probe into the steadiest stacked pattern, we studied various configurations. The steadiest configuration of the MoSe
2/HfS
2 heterojunction, whose equilibrium structure is presented in
Figure 1, will be studied in the subsequent parts.
In addition, the interlayer spacing of the MoSe
2/HfS
2 heterobilayer was 3.05 Å, as displayed in
Figure 1, which refers to the vertical distance from the nearest S atoms of the HfS
2 layer to the Se atoms of the MoSe
2 layer. This vertical separation between two layers is a typical vdW equilibrium distance, which is in accord with other 2D vdW heterostructures [
11,
44]. Thus, it was necessary to add the vdW corrections into our computation. The binding energy (E
b) of the MoSe
2/HfS
2 heterojunction can be defined as this equation: E
b = E
MoSe2/HfS2 − E
MoSe2 − E
HfS2, where E
MoSe2/HfS2, E
MoSe2, and E
HfS2 stand for the total energy of this heterostructure, isolated MoSe
2, and HfS
2 monolayers, respectively. After calculations, the bound energy of the steadiest configuration of the MoSe
2/HfS
2 heterojunction is –0.27 eV, which means that this composite is stable.
3.2. Electronic Properties
The band structures of related materials were calculated by employing the hybrid functional. As presented in
Table 1, the bandgaps of MoSe
2 and HfS
2 monolayers were 2.02 and 1.99 eV, respectively, which are consistent with the previous calculations [
54] and mean that the two layers can both utilize the visible light irradiation. Moreover, the valence band maximum (VBM) and conduction band minimum (CBM) of the HfS
2 monolayer were both lower than those of MoSe
2 single layer, which indicates that MoSe
2/HfS
2 heterobilayer can form a type II heterostructure. As we all know, all of the direct Z-scheme systems have a typical type II band alignment structure [
26]. Therefore, the MoSe
2/HfS
2 heterojunction can form a direct Z-scheme photocatalyst.
Meanwhile, the VBM of the HfS
2 single layer was able to stride over the standard oxidation potential for O
2/H
2O, whereas the CBM was 0.5 eV lower than the standard reduction potential for H
+/H
2. So, the HfS
2 monolayer can only be used for OER. At the same time, the VBM of the MoSe
2 monolayer was 0.04 eV higher than the standard oxidation potential, while the CBM could straddle the standard reduction potential, indicating that the MoSe
2 single layer can only be applied for HER. Thus, by combining the MoSe
2 and HfS
2 single layers, the MoSe
2/HfS
2 nanocomposite can be used for photocatalytic overall water splitting. As depicted in
Figure 2, the MoSe
2/HfS
2 heterostructure is a semiconductor with a direct bandgap of 0.53 eV, which is less than those of the individual MoSe
2 (2.02 eV) and HfS
2 (1.99 eV) single layers, indicating that this heterojunction can make the best of visible light and even enlarge the applied range of light to infrared light. As shown in the picture of the projected band structure, the VBM of this heterostructure is mainly composed of the MoSe
2 layer, while the CBM is primarily made up of the HfS
2 layer, which is in support of the separation of the photoinduced carriers.
In order to systematically study the photocatalytic ability of the MoSe
2/HfS
2 heterostructure, we employed a density of states (DOS) analysis. TDOS and PDOS represent the total and partial DOS, respectively. As displayed in
Figure 3, the VBM of this heterojunction chiefly consisted of the Mo 4d and Se 4p states, which were derived from the MoSe
2 layer. However, the CBM was primarily made up of the Hf 5d and S 3p orbitals, which were rooted in the HfS
2 slab. Thus, the VBM and CBM of this heterojunction were separated into different layers, which is consistent with the previous analysis about the band structure.
The effective separation of the photo-generated electron–hole pairs is an essential factor for improving the photocatalytic activity. Thus, we applied the charge density difference and Bader charge analysis to investigate the carrier migrating processes. The charge density differences of this heterostructure are defined as this: △ρ = ρ
MoSe2/HfS2 − ρ
MoSe2 − ρ
HfS2, where ρ
MoSe2/HfS2, ρ
MoSe2, and ρ
HfS2 indicate the charge density of the MoSe
2/HfS
2 nanocomposite, freestanding MoSe
2, and HfS
2 nanosheets, respectively. The small bandgap (0.53 eV) in this heterostructure was favorable to enhancing the interlayer carrier transfer. As depicted in
Figure 4, there is obvious charge accumulation and depletion near the interface. The MoSe
2 layer was apt to lose charge, while the HfS
2 slab tended to gain charge. Hence, the charge transfer between two layers brought about an internal electric field pointing from the MoSe
2 layer to the HfS
2 slab, which is in accordance with the previous report that the electric field generally points from the hydrogen evolution catalyst to the oxygen evolution catalyst [
25]. In order to evaluate the quantity of charge transfer, we employed the Bader charge analysis. The S and Se atoms were liable to acquire electrons, while the Hf and Mo atoms were apt to lose electrons. As a whole, the migrated electrons from the MoSe
2 layer to the HfS
2 slab were 0.052 e, which is in keeping with the charge density differences analysis.
After the light irradiation, the electrons on the VB of MoSe
2 and HfS
2 slabs were excited to the CB of these layers. The internal electric field of the MoSe
2/HfS
2 nanocomposite was in favor of the electron migration from the CB of HfS
2 to the VB of MoSe
2. Meanwhile, the electric field hindered the electron transfer from the CB of MoSe
2 to the CB of HfS
2 and the hole transfer from the VB of HfS
2 to the VB of MoSe
2. Moreover, because the CBM of HfS
2 and VBM of MoSe
2 were close to each other (0.69 eV)—as displayed in
Table 1—the photogenerated electrons in the CBM of HfS
2 layer could easily recombine with the holes in the VBM of the MoSe
2 layer. Finally, the photoinduced electrons in the CBM of MoSe
2 slab and the holes in the VBM of HfS
2 slab could both be preserved, which enabled the MoSe
2/HfS
2 nanocomposite to possess high redox ability. The schematic illustration of the Z-scheme photocatalytic mechanism for the MoSe
2/HfS
2 nanocomposite is illustrated in
Figure 5. Therefore, in comparison with the MoSe
2 and HfS
2 monolayers, the MoSe
2/HfS
2 heterostructure can improve the separation efficiency of the photogenerated carriers and enhance the redox ability.
In addition, stress is inevitable in industrial production and may also originate from the mismatch of the lattices between different materials. Thus, we investigated the band edge positions of the MoSe
2/HfS
2 nanocomposite as a function of in-plain strains. By applying the strains in the range from −6% to 6%, the calculated band edge positions of the MoSe
2/HfS
2 heterostructure remained almost unchanged, as displayed in
Figure 6. Thus, the band structures of MoSe
2/HfS
2 heterostructure are stable, which is different from the typical type II heterojunctions [
11,
15]. Therefore, the stable electronic property is in favor of future industrial applications.