First-Principles Study on Direct Z-Scheme SnC/SnS₂ Heterostructures for Photocatalytic Water Splitting

Abstract

Direct Z-scheme heterojunctions are known for their unique carrier mobility mechanism, which significantly improves photocatalytic water splitting efficiency. In this study, we use first-principles simulations to determine the stability, electrical, and photocatalytic properties of a SnC/SnS₂ heterojunction. Analyses of the projected energy band and state density demonstrate that the SnC/SnS₂ heterojunction exhibits an indirect band gap of 0.80 eV and a type-II band alignment. Analysis of its work function shows that the SnC/SnS₂ heterojunction has a built-in electric field pointing from the SnC monolayer to the SnS₂ monolayer. The band edge position and the differential charge density indicate that the SnC/SnS₂ heterostructure exhibits a Z-scheme photocatalytic mechanism. Furthermore, the SnC/SnS₂ heterojunction exhibits excellent visible-light absorption and high solar-to-hydrogen efficiency of 32.8%. It is found that the band gap and light absorption of the heterojunction can be effectively tuned by biaxial strain. These results demonstrate that the fabricated SnC/SnS₂ heterojunction has significant photocatalysis potential.

1. Introduction

The current global situation is facing two key challenges: energy scarcity [1,2] and environmental pollution [3]. The depletion of old fossil energy sources [4] and air pollution [5] resulting from their use have a considerable negative influence on our quality of life and future possibilities for sustainable development. The search for new clean energy sources is urgent. Hydrogen energy is the best energy source for future sustainable development [6,7], and the semiconductor photocatalytic hydrolysis technique is regarded as the most efficient method of hydrogen production [8]. Two-dimensional (2D) materials exhibit superior photocatalytic properties compared with those of semiconductor materials [9,10]. However, in some 2D materials, rapid photogenerated electron–hole recombination results in diminished photocatalytic activity [11,12]. In this regard, heterojunctions [13,14] have been proposed as an effective strategy to overcome the deficiency of electron–hole recombination and diminished photocatalytic activity.

By utilizing the energy band difference between two semiconductors, photogenerated electrons and holes at the interface of heterojunctions can be separated, thereby improving carrier separation efficiency [15,16]. Photocatalytic heterostructures include type-II and Z-scheme heterostructures. However, conventional type-II heterojunctions face challenges in effectively separating photogenerated electrons and holes at the heterojunction interface due to limitations in their band structure, which consequently reduces the efficiency of photocatalytic reactions. To overcome this difficulty, Yu et al. [17] introduced the concept of direct Z-type heterostructure photocatalysts. Z-type heterojunctions generate photogenerated electrons and holes in different semiconductor materials, thereby separating them at the interface and improving carrier separation efficiency. Numerous studies on Z-type heterojunctions have been reported, including GeC/HfS₂ [18], g-C3N4/MoS₂ [19] and GaN/BS [20].

The graphene-like structure SnC exhibits structural stability and an indirect bandgap [21,22]. It also exhibits excellent electron conductivity and superior light absorption [22]. In addition, SnC can efficiently convert light energy into chemical energy, making it a promising photocatalytic material [23,24]. SnC has high theoretical capacity and conductivity, making it suitable as an anode material for potassium- and sodium-ion batteries [25,26]. In addition, SnC has a suitable band edge position and can be used as a photocatalytic reduction reaction material [25,26]. Currently, several studies on SnC heterojunction photocatalysis have been reported. Because of the suitable band gap of SnC, a heterojunction composed of SnC and HfSSe exhibits high interlayer carrier recombination [27]. In addition, a high light absorption capacity [28] was reported in a heterojunction formed by SnC and BAs. A high energy conversion rate [29] was found in a heterojunction composed of SnC and PtSe2.

Due to its suitable band gap, good photovoltaic performance, and unique optical properties, SnS₂ exhibits potential for use in a wide variety of fields [30,31,32]. In the field of optoelectronic devices, Huang et al. [33] employed chemical vapor deposition (CVD) to prepare SnS₂ nanosheets with excellent optoelectronic properties. In the field of battery materials, SnS₂ exhibits good conductivity and chemical stability and has been a popular electrode material for lithium batteries [34,35,36]. Chen et al. [37] synthesized SnS₂ thin films using CVD to optimize preparation conditions and achieve efficient photoelectrochemical water decomposition under visible light. In addition, a SnSe₂/SnS₂ heterojunction with an exceptionally high switching ratio and high responsivity in response to electric fields has been fabricated [38]. A SnO₂/SnS₂ heterojunction with remarkable activity and exceptional stability in electrocatalytic hydrogen evolution reactions (HERs) has also been reported [39]. A recent study indicated that heterojunction constructed from SnC and CdS exhibit excellent photocatalytic performance [40,41].

In this work, by using first-principles calculations, we constructed a SnC/SnS₂ heterojunction and investigated its electronic and photocatalytic properties. We found that the SnC/SnS₂ is a z-scheme heterojunction that effectively separates photogenerated electrons and holes. Furthermore, the SnC/SnS₂ heterojunctions exhibit excellent light absorption performance and high solar-to-hydrogen (STH) efficiency, making it a promising photocatalytic material.

2. Computation Details

All calculations were carried out using the Vienna Ab initio Simulation Package (VASP), which is based on density functional theory (DFT) [42]. To describe the exchange–correlation interaction, the Perdew–Burke–Ernzerhof (PBE) function was used, which is based on the generalized gradient approximation (GGA) [43]. The cut-off energy was set at 500 eV and a k-point grid of $9\times 9\times 1$ was sampled in the Brillouin zone. All atoms were relaxed sufficiently to reach the convergence criteria of $1\times {10}^{-5}$ eV for total energy and $0.01\mathrm{e}\mathrm{V}/\mathrm{\AA }$ for maximum force. Furthermore, because the PBE functional typically results in an underestimation of the band gap and optical absorption, we used a hybrid Heyd–Scuseria–Ernzerhof functional (HSE06) method to calculate the band structure [44]. In particular, the DFT-D3 method was applied to correct the interlayer van deer Waals (vdW) interaction [45]. The vacuum thickness along the z-axis was set to 20 $\AA$ to prevent interactions between adjacent layers. In addition, dipole corrections were used to obtain more accurate potential energy and forces [46].

The thermal stability of the SnC/SnS₂ heterojunction was verified via using ab initio molecular dynamics (AIMD) [47]. A 5 × 5 × 1 supercell was used to calculate the energy over 5000 fs at 300 K. In addition, Phonopy 2.12.0 software was used to analyze the phonon dispersion spectrum to investigate the dynamic stability of the SnC/SnS₂ heterojunction [48].

3. Results and Discussion

3.1. Geometric Structure and Stability

To construct the heterojunction, we calculated the structural and electronic properties of each component in the SnC/SnS₂ heterojunction. The top and side views of the optimized crystal structure of each material are shown in Figure 1. The optimized lattice constants of the SnC and SnS₂ monolayers are 3.59 and 3.70 Å, respectively, which are consistent with the values reported before [29,49]. The band structure calculated using the HSE06 method indicates that the SnC and SnS₂ monolayers possess indirect bandgaps of 1.77 eV and 2.46 eV, respectively, which agree well with those reported in previous reports [49,50]. The valence band maximum (VBM) and conduction band minimum (CBM) of SnC are −4.87 and −3.10 eV, respectively. In contrast, the VBM and CBM of SnS₂ are −7.17 and −4.71 eV, respectively.

The lattice constants are close; thus, the constructed heterostructure composed of SnC and SnS₂ only exhibits a lattice mismatch rate of 2.96%. By shifting and rotating the SnC layer, we constructed six configurations for the SnC/SnS₂ heterojunction: AA, AB, AC, BA, BB, and BC (Figure 2). The three configurations of BA, BB and BC were constructed by rotating the SnC layer by 180°. A and B on the left of the name represent the Sn and C atoms in the upper SnC layer of the heterojunction, respectively. A, B, and C on the right side of the name represent the alignment mode, where the upper atoms align with the lower S atom, the upper S atom, and the Sn atom in the lower SnS₂ layer, respectively.

To identify the most stable configuration, the binding energies of the six heterostructures were calculated. The following formula was used to calculate the binding energy E_b [51]:

$${\mathrm{E}}_{\mathrm{b}}={\mathrm{E}}_{\mathrm{SnC}/\mathrm{Sn}{\mathrm{S}}_2}-{\mathrm{E}}_{\mathrm{SnC}}-{\mathrm{E}}_{\mathrm{Sn}{\mathrm{S}}_2}$$

(1)

where ${\mathrm{E}}_{\mathrm{SnC}/\mathrm{Sn}{\mathrm{S}}_2}$, ${\mathrm{E}}_{\mathrm{SnC}}$, and ${\mathrm{E}}_{\mathrm{Sn}{\mathrm{S}}_2}$ denote the total energies of SnC/SnS₂ vdWHs, SnC, and SnS₂ layers, respectively.

Table 1. Optimized lattice constants, equilibrium interlayer distance d, and binding energy E_b for the stacking of SnC/SnS₂ heterostructure.

Stacking	AA	AB	AC	BA	BB	BC
$a=b\left(\mathrm{\AA }\right)$	3.612	3.619	3.615	3.621	3.612	3.616
$d\left(\mathrm{\AA }\right)$	3.632	3.273	3.064	3.157	3.720	2.961
$E_b\left(\mathrm{e}\mathrm{V}\right)$	−3.859	−3.921	−3.967	−3.958	−3.946	−3.983

shows that the binding energy of each configuration is negative, confirming their energy stability. The BC configuration exhibits the lowest binding energy (−3.983 eV). Table 1 also lists the optimized lattice constants and equilibrium interlayer distances for the six heterostructures. The interlayer distances for the AA, AB, AC, BA, BB, and BC configurations are 3.632, 3.273, 3.064, 3.157, 3.720, and 2.961 Å, respectively. This suggests that the SnC and SnS₂ monolayers are connected and bound together by interlayer vdW forces.

To further verify the stability of the BC configuration, we applied AIMD. Figure 3a shows that during a 5000-fs AIMD simulation at 300 K, no deformation was observed in the structural integrity of the SnC/SnS₂ heterojunction structure and that the energy remains within a certain range, demonstrating its stability. Meanwhile, we monitored the lattice constants and the average bond distance values before and after the simulation. It was found that the lattice constants remain almost unchanged during the simulation, while the average bond distance is within $\pm$5%. It further demonstrates the stability of SnC/SnS₂ heterojunction. Furthermore, we calculated the phonon spectrum of the SnC/SnS₂ heterojunction with BC configuration. The results presented in Figure 3b indicate the absence of any negative frequency within the Brillouin zone. This implies that the SnC/SnS₂ vdW heterostructure with BC configuration is stable [52]. Therefore, in the following discussion, we use the BC structure as the typical configuration for the SnC/SnS₂ heterojunction.

3.2. Electronic Properties

To investigate the electronic properties of the SnC/SnS₂ heterostructure, we calculate its band structure and density of states (DOS) using the HSE06 method. As shown in Figure 4a, the SnC/SnS₂ heterostructure exhibits an indirect bandgap of 0.80 eV. The CBM of the heterostructure is positioned at the M point, which is contributed by SnS₂. Its VBM is situated at the K point, which is contributed by SnC. This suggests that the heterostructure exhibits a type-II band alignment, which effectively inhibits the recombination of photogenerated electrons and holes, thereby enhancing photocatalytic efficiency.

Figure 4b shows the DOS of the SnC/SnS₂ vdW heterostructure. Based on this figure, the VBM of the heterojunction mostly originates from SnC, whereas its CBM originates from SnS₂. The partial DOS indicates that the CBM of the heterostructure is primarily derived from the Sn-p orbital. In contrast, its VBM is primarily derived from the S-p and C-p orbitals. These findings are consistent with the analysis of the band structures, indicating that the contact between SnC and SnS₂ exhibits a type-II band alignment. However, polar solutions such as water may have an effect on the stability and band gap of the heterojunction. Therefore, these factors should be taken into account in practical application [53].

To evaluate the interlayer charge transfer, we further explore the work function. The work functions of the SnC and SnS₂ monolayers and heterojunction were calculated as follows:

$$\mathsf{\varphi }=E_{vac}-E_F$$

(2)

where $\mathsf{\varphi }$, $E_{vac}$ and $E_F$ represent the work function, vacuum level and Fermi level, respectively. The work functions of the SnC monolayer and SnS₂ monolayer are 4.729 eV [54] and 6.586 eV [55], respectively. The disparity in work function between SnC and SnS₂ monolayers leads to the redistribution of charges. When the two monolayers come into contact, electrons move from the SnC monolayer to the SnS₂ monolayer and the holes are moved from the SnS₂ monolayer to the SnC monolayer until their Fermi levels are aligned. In addition, a built-in electric field is generated, and its direction changes from the SnC layer to the SnS₂ layer. The built-in electric field causes the bending of the band edges in SnC and similar bending for SnS₂ has been reported before in the literature [56]. In addition, as shown in Figure 5a, after the formation of the heterostructure, an electrostatic potential difference of 0.426 eV exists between the SnC layer and SnS₂ layers. Electrostatic potential differences affect the photocatalytic performance of photocatalytic materials [18].

To further evaluate the charge transfer, we also calculated the average charge density difference along the Z-axis and the differential charge density. The average charge density difference along the Z-axis is defined as follows:

$$\Delta \mathsf{\rho }\left(Z\right)=\int {\rho }_{SnC/Sn{S}_2}dxdy-\int {\rho }_{SnC}dxdy-\int {\rho }_{Sn{S}_2}dxdy$$

(3)

where $\int {\rho }_{SnC/Sn{S}_2}dxdy$, $\int {\rho }_{SnC}dxdy$ and $\int {\rho }_{Sn{S}_2}dxdy$ denote the charge densities of the SnC/SnS₂ heterostructure, SnC, and SnS₂ along the Z-axis, respectively. Figure 5b shows that charge density redistribution is primarily observed at the interface between SnC and SnS₂. Here, the positive charges are concentrated at SnC, whereas the negative charges accumulate at SnS₂. This creates a built-in electric field from the SnC to SnS₂ layer.

A suitable band edge position is of paramount importance in the construction of a photocatalytic heterostructure. The band arrangement of the SnC and SnS₂ monolayer in a heterojunction is shown in Figure 5c. According to Equations (4) and (5), at $pH$ = 0, the SnC layer only crosses the reduction potential of water (−4.44 eV) and the SnS₂ only crosses the oxidation potential of water (−5.67 eV). The energy bands exhibit a staggered arrangement.

$$E^{H^{+}/H_2}=-4.44\mathrm{e}\mathrm{V}+pH\times 0.059\mathrm{e}\mathrm{V}$$

(4)

$$E^{O_2/H_{2}O}=-5.67\mathrm{e}\mathrm{V}+pH\times 0.059\mathrm{e}\mathrm{V}$$

(5)

As presented in Figure 5d, when sunlight shines on the SnC/SnS₂ heterojunction, electrons in the valence band (VB) of each monolayer are stimulated to move to the conduction band (CB), resulting in an equal number of holes in the VB, thereby forming photogenerated electrons and holes. Furthermore, the built-in electric field can accelerate the recombination of photogenerated electrons at the CBM of SnS₂ and holes at the VBM of SnC. However, it can also hinder holes from the VBM of SnS₂ to the VBM of SnC. This analysis shows that the constructed SnC/SnS₂ heterojunction exhibits the character of a direct Z-scheme heterojunction photocatalyst.

3.3. Photocatalytic Performance

The light absorption coefficient is an important index of heterostructure photocatalytic performance. Therefore, we computed the light absorption coefficients of the SnC and SnS₂ monolayers and SnC/SnS₂ heterostructure. For a more precise calculation of light absorption, all calculations were performed using the HSE06 function. The light absorption coefficient can be calculated as follows [57]:

$$\mathsf{\alpha }\left(\mathsf{\omega }\right)={\displaystyle \frac{\sqrt{2\mathsf{\omega }}}{\mathrm{c}}}{\left\{{\left[{\mathsf{\epsilon }}_1^2\left(\mathsf{\omega }\right)+{\mathsf{\epsilon }}_2^2\left(\mathsf{\omega }\right)\right]}^{\frac{1}{2}}-{\mathsf{\epsilon }}_1\left(\mathsf{\omega }\right)\right\}}^{{\displaystyle \frac{1}{2}}}$$

(6)

where $\mathrm{c}$ and $\mathsf{\omega }$ denote the light speed and angular frequency in vacuum, respectively, and ${\mathsf{\epsilon }}_1$ and ${\mathsf{\epsilon }}_2$ denote the real and imaginary parts of the dielectric constant, respectively.

Figure 6 shows that the two monolayers exhibit low visible-light absorption. However, the SnC/SnS₂ heterostructure exhibits a substantial increase in its absorption coefficient within the visible-light spectrum. This phenomenon can be attributed to the decrease in the band gap following the combination of the SnC and SnS₂ monolayers.

In addition, we calculated the STH efficiency to further assess the photocatalytic capacity of the SnC/SnS₂ heterostructure. The recombination of interlayer charges in direct Z-scheme heterostructures results in the consumption of electrons and holes. The STH efficiency ${\eta }_{STH}$ can be expressed as follows [58]:

$${\eta }_{STH}=0.5\times {\eta }_{abs}\times {\eta }_{cu}$$

(7)

where ${\eta }_{abs}$ denotes a physical quantity that represents the ability of materials to absorb light and ${\eta }_{cu}$ denotes the proportion of electrons and holes involved in the photoelectric conversion process, also known as the carrier utilization efficiency. ${\eta }_{abs}$ can be calculated as follows:

$${\eta }_{abs}={\displaystyle \frac{{\int }_{E_g}^{\infty }P\left(\hslash \omega \right)d\left(\hslash \omega \right)}{{\int }_0^{\infty }P\left(\hslash \omega \right)d\left(\hslash \omega \right)}}$$

(8)

where $P\left(\hslash \omega \right)$ denotes the photon flux density of solar radiation under standard test conditions (AM1.5G) at a photon energy of $\hslash \omega$; $E_g$ denotes the band gap of the SnC/SnS₂ heterostructure. ηcu can be expressed as follows:

$${\eta }_{cu}={\displaystyle \frac{\Delta G{\int }_E^{\infty }\frac{P\left(\hslash \omega \right)}{\hslash \omega }d\left(\hslash \omega \right)}{{\int }_{E_g}^{\infty }P\left(\hslash \omega \right)d\left(\hslash \omega \right)}}$$

(9)

where $\Delta G$ denotes the minimum electrochemical potential difference required for a water decomposition reaction in a standard state and $E$ represents the minimum photon energy required fo the water splitting reaction to occur, which is calculated as follows:

$$E=\left\{\begin{array}{c}{E}_g,\left(\chi \left(H_2\right)>0.2,\chi \left(O_2\right)>0.6\right)\\ E_g+0.2-\chi \left(H_2\right),\left(\chi \left(H_2\right)<0.2,\chi \left(O_2\right)>0.6\right)\\ E_g+0.6-\chi \left(O_2\right),\left(\chi \left(H_2\right)>0.2,\chi \left(O_2\right)<0.6\right)\\ E_g+0.8-\chi \left(H_2\right)-\chi \left(O_2\right),\left(\chi \left(H_2\right)<0.2,\chi \left(O_2\right)<0.6\right)\end{array}\right.$$

(10)

where $\chi \left(H_2\right)$ and $\chi \left(O_2\right)$ denote the overpotentials of the HER and oxygen evolution reaction (OER), respectively.

As shown in Figure 5c, $\chi \left(H_2\right)$ is equal to $4.44-2.83=1.61\mathrm{e}\mathrm{V}$, and $\chi \left(O_2\right)$ is equal to $7.06-5.67=1.39\mathrm{e}\mathrm{V}$. Using Equations (5)–(8), the STH efficiency of the SnC/SnS₂ heterostructure is 32.8%. Typically, STH efficiency above 10% is proposed for commercial use. The STH efficiency of the SnC/SnS₂ heterostructure is significantly higher than that reported in the literature, e.g., GaTe/AsP (14.1%) [59], PtTe₂/Sb₂S₃ (28.8%) [60] and β-SnSe/HfS₂ (14.29%) [61]. This implies that the SnC/SnS₂ heterojunction has broad prospects in the industry for clean energy.

To further evaluate the photocatalytic capacity of the SnC/SnS₂ heterojunction, we calculated the Gibbs free energy redox reaction at pH = 0. The Gibbs free energy of redox reactions can be calculated as follows [62,63]:

$$∆\mathrm{G}=∆\mathrm{E}+∆E_{ZPE}-T∆S+∆G_U$$

(11)

where ΔE, $∆E_{ZPE}$ and $T∆S$ denote the differences in the total energy, zero-point energy, and entropy between products and reactants of the reactions, respectively. The temperature (T) in the calculation is set to 298.15 K. And $∆G_U=-eU$ represents the potential with respect to the standard water reduction potential of the standard hydrogen electrode (SHE).

The Gibbs free energy change curves of HER and OER for the SnC/SnS₂ heterojunction are shown in Figure 7. The HER process comprises the following two steps:

$$\ast +H^{+}+e^{-}\to H\ast$$

(12)

$$H\ast +H^{+}+e^{-}\to H_2+\ast$$

(13)

where ∗ represents the adsorption site of heterostructure and $H\ast$ denotes the heterojunction after the adsorption of H atoms. We find that the energy of $H^{\ast }$ is the lowest at the top position of the C atom. Therefore, we chose this structure to calculate the free energy. As shown in Figure 7a, in the absence of an external potential, the free energy of the heterojunction HER reaction is −0.72 eV. The OER process can be concluded as follows:

$$\ast +H_{2}O\to OH\ast +H^{+}+e^{-}$$

(14)

$$OH\ast \to O\ast +H^{+}+e^{-}$$

(15)

$$\mathrm{O}\ast +H_{2}O\to OOH\ast +H^{+}+e^{-}$$

(16)

$$OOH\ast \to \ast +O_2+H^{+}+e^{-}$$

(17)

where ∗ denotes the active site of heterostructure and $OH\ast$, $O\ast$ and $OOH\ast$ denote the configuration of the heterojunction after adsorption of OH, O, and OOH, respectively. We found that the lowest energy points of $OH\ast$, $\mathrm{O}\ast$, $OOH\ast$ are the top position of the S atom, the top position of the Sn atom, and the top position of the S atom, respectively. And we used the corresponding structure with the lowest energy to calculate the free energy. For OER, the photogenerated hole potential (U_h) represents the energy difference from SHE to VBM of SnS₂ [63]. When the external potential is 0 V, the overpotential of heterojunction is 2.5 V, and the overall OER curve shows an upward trend. When the external potential is 1.23 V, the heterojunction overpotential decreases; however, the OER still cannot proceed spontaneously. When U = U_h = 2.62V, the overall OER curve shows a downward trend, indicating that the OER is exothermic and is spontaneous under light.

3.4. Strain Engineering

According to previous research reports, strain engineering can effectively regulate the properties of 2D materials [64,65]. The above discussion demonstrates that the SnC/SnS₂ heterojunction exhibits excellent photocatalytic properties and is a candidate 2D vdW heterojunction material with significant application potential. Therefore, we investigated the effect of strain on the electronic and optical properties of the SnC/SnS₂ heterojunction.

In particular, the effect of biaxial strain on the SnC/SnS₂ heterojunction was investigated by applying a plane biaxial strain between −6% and 6%. Here, negative values represent compressive strains, while positive values denote tensile strains, respectively. The degree of strain was calculated as follows:

$$\mathsf{\epsilon }={\displaystyle \frac{a-a_0}{a_0}}\times 100\%$$

(18)

where $a_0$ and a denote the lattice constants of the initial and the post-strain heterojunction, respectively. In addition, we calculated the strain energy $E_s$ of the SnC/SnS₂ heterostructure to ensure that the strain degree was within the elastic range, as follows:

$$E_s=E_{\epsilon }-E_0$$

(19)

where $E_{\epsilon }$ and $E_0$ denote the energies of the SnC/SnS₂ heterostructure with and without strain, respectively.

As shown in Figure 8a, with an increase in tensile (compression) strain, the total energy of the SnC/SnS₂ heterojunction gradually increases, and a perfect quadratic function relationship is formed between them, indicating that SnC/SnS₂ heterojunction is a flexible material and exhibits reversibility when the external strain is within its elastic limit. As the degree of strain changes, the band gap of the SnC/SnS₂ heterojunction changes. At −2% strain, the heterojunction band gap reaches its maximum (0.81 eV). At 6% strain, the heterojunction band gap reaches the minimum (0.36 eV).

In addition, by applying biaxial strain on the SnC/SnS₂ heterojunctions, we investigated the optical properties of the studied heterojunctions. As shown in Figure 8b, $\alpha \left(\omega \right)$ varies with strain. Under compressive strain, the light absorption of the heterojunction exhibits no obvious change. Under tensile strain, the heterojunction absorption curve changes. Under 2% tensile strain, a high absorption peak at 500 mm~700 mm appears in the heterojunction. Under 4% tensile strain, a high absorption peak at 600–850 mm can be found in the heterojunction. Under 6% tensile strain, the heterojunction exhibits multiple high absorption peaks at 400–1100 mm. These results demonstrate that strain is an effective way to tune the optical properties of SnC/SnS₂ heterojunctions.

4. Conclusions

In this study, first-principles calculations were applied to explore the structure, electronic and photocatalytic properties of a SnC/SnS₂ heterojunction. The BC configuration exhibited the lowest binding energy; thus, its stability was verified. Our results demonstrate that the calculated band gap of the SnC/SnS₂ heterostructure exhibits a type-II band alignment. The predicted energy gap of our proposed SnC/SnS₂ heterostructure is 0.80 eV. In addition, the SnC/SnS₂ vdW heterostructure is identified as a typical Z-scheme heterostructure based on analyses of the work function, band alignment, and mean charge density difference. In particular, the STH efficiency we predicted in the SnC/SnS₂ heterojunction reaches 32.8%. Both the electronic structure as well as the optical properties of the heterojunction can be effectively modulated by biaxial strain. These results imply that the fabricated SnC/SnS₂ heterojunction is a promising material with excellent photocatalytic performance, which can be realized by applying biaxial strain in experiments. Thus, our calculations have a guiding significance to the experiment. In our previous work, we firstly investigated the physical properties of a GeSe/SnSe heterostructure in theory and found that it exhibits high carrier mobility, strong light absorption, and superior PCE properties [66]. Subsequently, we successfully prepared a GeSe/SnSe heterojunction and found that it exhibits excellent photoelectric properties and can be used as a near-infrared photodetector [67]. From a viewpoint of outlooking, the calculation expectation in this study can be used as a guide for subsequent experiments.