# Janus Ga2SSe-Based van der Waals Heterojunctions as a Class of Promising Candidates for Photocatalytic Water Splitting: A DFT Investigation

Fan Yang, Marie-Christine Record, Pascal Boulet

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Fan Yang, Marie-Christine Record, Pascal Boulet. Janus Ga2SSe-Based van der Waals Heterojunctions as a Class of Promising Candidates for Photocatalytic Water Splitting: A DFT Investigation. Crystals, 2025, 15 (8), pp.728. 10.3390/cryst15080728. hal-05530195

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Article

# Janus Ga₂SSe-Based van der Waals Heterojunctions as a Class of Promising Candidates for Photocatalytic Water Splitting: A DFT Investigation

Fan Yang ID, Marie-Christine Record *ID and Pascal Boulet ID

IM2NP, Faculty of Sciences, CNRS, Aix-Marseille University, Avenue Escadrille Normandie-Niemen, 13013 Marseille, France; fan.yang.1@etu.univ-amu.fr (F.Y.); pascal.boulet@univ-amu.fr (P.B.)

* Correspondence: m-c.record@univ-amu.fr

## Abstract

Addressing global energy and environmental issues calls for the development of effective photocatalysts capable of enabling solar-driven water splitting, a key route toward sustainable hydrogen generation. In this work, we conducted a detailed density functional theory (DFT) study on three bilayer van der Waals (vdW) heterojunctions, Ga₂SSe/GaP, Ga₂SSe/PtSSe, and Ga₂SSe/SnSSe, each explored in four distinct stacking configurations, with Ga₂SSe serving as the base monolayer. We assessed their structural stability, electronic properties, and optical responses to determine their suitability for photocatalytic water splitting. The analysis showed that Ga₂SSe/GaP and Ga₂SSe/SnSSe exhibit type-II band alignment, while Ga₂SSe/PtSSe displays a type-I alignment. Electrostatic potential profiles and Bader charge calculations identified SeGa₂S/SSnSe and SeGa₂S/SeSnS as direct Z-scheme systems, offering efficient charge carrier separation and robust redox potential. For effective water splitting, the band edges must straddle the water redox potentials. Our results indicate that configurations A and B in Ga₂SSe/GaP, along with C and D in Ga₂SSe/SnSSe, fulfill this requirement. These four configurations also exhibit strong absorption in both the visible and ultraviolet spectral ranges. Notably, configurations C and D of Ga₂SSe/SnSSe achieve high solar-to-hydrogen (STH) efficiencies, reaching 38.44% and 21.75%, respectively. Overall, our findings suggest that these direct Z-scheme heterostructures are promising candidates for water splitting photocatalysis.

**Keywords:** photocatalysis; metal dichalcogenides; direct Z-scheme; first-principles calculations; solar-to-hydrogen efficiency

## 1. Introduction

The need for sustainable hydrogen production has become increasingly urgent due to rising concerns over environmental degradation and dwindling fossil fuel resources. Among the emerging strategies, photocatalytic water splitting using solar energy has proved particularly interesting for generating hydrogen in a clean, renewable way [1–3]. Hydrogen stands out as a desirable energy carrier because of its high energy content and carbon-free combustion. However, current industrial production methods, such as steam methane reforming, are still heavily reliant on fossil resources and generate large quantities of CO₂ emissions [4,5]. As an alternative, solar-driven photocatalysis offers a low-emission, scalable route for yielding hydrogen by utilizing sunlight to decompose water molecules into hydrogen and oxygen [6,7].

<figure><img src="image_3.png" /></figure>

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Academic Editor: Thomas

M. Klapötke

Received: 2 July 2025

Revised: 12 August 2025

Accepted: 13 August 2025

Published: 16 August 2025

**Citation:** Yang, F.; Record, M.-C.; Boulet, P. Janus Ga₂SSe-Based van der Waals Heterojunctions as a Class of Promising Candidates for Photocatalytic Water Splitting: A DFT Investigation. *Crystals* **2025**, *15*, 728. https://doi.org/10.3390/cryst15080728

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
Titanium dioxide (TiO₂) [8] was among the first materials identified for photocatalytic applications and remains extensively studied. However, its wide bandgap (~3.2 eV) limits its absorption to a small portion of the solar spectrum, the ultraviolet, thus restricting its efficiency under solar illumination [9]. To address this limitation, transition metal dichalcogenides (TMDs) [10], particularly MoS₂ [11] and WS₂ [12], have been explored due to their favorable optical properties, layered structures, and adjustable electronic characteristics [13–15]. Despite these advantages, TMD monolayers often suffer from fast charge recombination, suboptimal charge mobility, and narrow absorption bandwidths, which undermine their overall performance in water-splitting reactions.

A promising strategy to overcome these issues is the design of semiconductor heterojunctions [16]. By engineering the interface between different materials, heterostructures can improve charge separation and broaden light absorption. Successful photocatalytic activity requires proper alignment of the band edges: the valence band maximum (VBM) must lie above the water oxidation potential [17,18], while the conduction band minimum (CBM) should lie below the reduction potential [19,20]. Hence, the relative positioning of energy bands critically influences the overall efficiency of the photocatalytic process [21,22].

Two heterojunction architectures, type-II [23] and direct Z-scheme [24], are particularly effective at enhancing charge separation by spatially separating photogenerated electrons and holes across different materials [25]. Z-scheme heterostructures are especially attractive, as they maintain strong redox potentials by localizing the oxidation and reduction reactions on distinct layers, thereby improving both charge separation and chemical reactivity [26]. Recent theoretical investigations have validated the advantages of such configurations. For example, Yang et al. [27] reported that $Ga_2SSe/Bi_2O_3$ heterojunctions, featuring a Z-scheme-type mechanism for the transfer of charges, achieve efficient light harvesting and reduced charge recombination, resulting in solar-to-hydrogen (STH) conversion efficiencies surpassing 10%. Similarly, Kumar et al. [28] found that the $C_2N/WS_2$ heterojunction exhibits a favorable type-II alignment, with separated charge carriers enhancing photocatalytic performance under visible light. In contrast, type-I heterostructures are typically ineffective for complete water splitting because both electrons and holes tend to accumulate in a single material, hampering the redox reactions. For instance, Kim et al. [29] identified the limited activity of the $BiVO_4/Fe_2O_3$ system as a result of its type-I band configuration, which obstructs hole transfer and lowers efficiency.

Building on this context, we selected GaP, PtSSe, and SnSSe as complementary materials to $Ga_2SSe$, owing to their documented photocatalytic properties for visible-light-driven water splitting.

Sun et al. [30] confirmed sustained submicromolar hydrogen evolution from visible-light-irradiated suspensions of Pt-decorated GaP nanowires using methanol as a sacrificial electron donor. Meinhardová et al. [31] further fabricated GaP/TiO₂ Z-scheme heterostructures via wet impregnation, leveraging methanol’s dual role as both sacrificial agent and current multiplier to enhance hydrogen production through photocatalytic methanol reforming.

PtSSe has also attracted attention for its excellent photocatalytic potential. Peng et al. [32], using first-principles calculations, demonstrated that monolayer PtSSe possesses excellent photocatalytic water-splitting potential, featuring high visible-light absorption, appropriate band edge positions, and efficient carrier separation and transport. Furthermore, Janus PtSSe-based van der Waals heterostructures have shown outstanding visible-light harvesting capabilities and high solar-to-hydrogen (STH) conversion efficiencies [33]. Similarly, Janus T-SnSSe monolayers exhibit robust photocatalytic activity for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), with strong redox
capabilities under illumination [34]. Moreover, ZrSSe/SnSSe heterostructures demonstrate significant water-splitting potential, featuring thermodynamically favorable OER activation in acidic conditions (pH = 0) and efficient visible-light absorption [35].

Ga₂SSe itself has emerged as a highly promising photocatalyst. Li et al. [36] demonstrated that Janus Ga₂SSe nanotubes exhibit exceptionally high hole mobility up to 2.89 × 10⁴ cm²V⁻¹s⁻¹, significantly suppressed electron-hole recombination, exceptional oxidation capability, and strong visible-light absorption. These synergistic properties establish Ga₂SSe as an outstanding photocatalyst candidate. Zhang et al. [37] established that Type II van der Waals GaSe/CN and Ga₂SSe/CN heterostructures exhibit optimal band alignments, thereby satisfying the thermodynamic requirements for oxygen evolution reactions. Remarkably, these structures maintain thermodynamic feasibility across a broad pH range, while achieving remarkable solar-to-hydrogen conversion efficiencies of up to 15.11%. These properties establish their outstanding application prospects as photocatalysts for efficient water splitting. On the basis of the above studies, we employed density functional theory (DFT) calculations to investigate a series of van der Waals (vdW) heterojunctions [38] incorporating Ga₂SSe as the base material. Specifically, we explored three combinations, namely Ga₂SSe/GaP, Ga₂SSe/PtSSe, and Ga₂SSe/SnSSe, each modeled with four distinct stacking sequences. Our investigation encompassed structural stability, band structures, band alignment, interfacial charge redistribution, relative position of band edges with respect to redox potentials, optical absorption behavior, and solar-to-hydrogen (STH) efficiency [39]. The results highlight that stacking configurations C and D in the Ga₂SSe/SnSSe system exhibit a direct Z-scheme alignment with superior STH performance, making them promising photocatalyst candidates for efficient solar water splitting.

## 2. Materials and Methods

All first-principles calculations involved the Vienna Ab initio Simulation Package (VASP) [40,41]. Projector augmented-wave (PAW) pseudopotentials [42] were used to model ion-valence electron interactions. For the exchange-correlation functional, the Perdew-Burke-Ernzerhof (PBE) formulation within the generalized gradient approximation (GGA) framework was employed [43,44]. To improve the reliability of the predicted electronic structure (particularly the bandgap, band edge alignment, and optical behavior) the hybrid Heyd-Scuseria-Ernzerhof functional (HSE06) was used in subsequent calculations [45].

The plane-wave basis set was truncated at a kinetic energy cutoff of 450 eV. Brillouin zone integration was carried out with a 4 × 4 × 1 Monkhorst-Pack grid, which was applied consistently for structural relaxation and electronic property evaluations. We set convergence thresholds of 10⁻⁵ eV for the total energy and 0.05 eV/Å for the maximum force acting on atoms. To accurately capture the weak interlayer forces present in van der Waals (vdW) heterostructures, the DFT-D3 dispersion correction method was included [46]. To suppress artificial interactions between periodic images along the out-of-plane axis, a 20 Å vacuum spacing was applied along z. Furthermore, Bader charge analysis was used to quantify charge transfer across the heterostructure interface [47]. This method partitions the electron density in real space using zero-flux surfaces and is well-suited to plane-wave DFT calculations, as it requires only the total charge density on a fine grid. In contrast, Mulliken population analysis relies on a projection onto localized orbitals, which is not available in VASP. Moreover, Mulliken charges are known to be strongly basis-dependent and may overestimate charge transfer, making Bader analysis a more reliable choice in this context.
## 3. Results

### 3.1. Structural Configuration and Stability of the Heterostructures

The Janus Ga₂SSe monolayer, which exhibits a hexagonal lattice structure, was selected as the foundational material for building bilayer heterostructures by pairing it with other Janus monolayers identified from the Materials Explorer database [48].

The relaxed lattice parameter of the Ga₂SSe monolayer was determined to be 3.71 Å. To ensure structural compatibility and minimal strain at the interface, a lattice mismatch threshold of less than 0.5% was adopted. Applying this constraint, three monolayers, namely GaP, PtSSe, and SnSSe, were selected as appropriate counterparts for heterojunction construction. Their optimized lattice constants (3.89Å for GaP, 3.64 Å for PtSSe, and 3.76 Å for SnSSe), along with their respective indirect bandgaps (2.65 eV, 2.06 eV, and 1.50 eV), are detailed in Figure S1 and Table S1 [32,35,49,50].

Given the intrinsic asymmetry of Janus structures, each bilayer heterostructure can adopt four possible stacking arrangements (labeled A through D), distinguished by the specific atomic species positioned opposite each other across the van der Waals (vdW) interface. These geometrical configurations are illustrated in Figure 1.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Top and side views of the monolayers Ga₂SSe, GaP, PtSSe, and SnSSe, along with the four possible stacking configurations (Patterns A–D) constructed for the Ga₂SSe/GaP, Ga₂SSe/PtSSe, and Ga₂SSe/SnSSe heterostructures. The rhombus outlines indicate the unit cell of each structure.</figcaption></figure>

To evaluate the energetic stability of these bilayer systems, the interfacial interaction energy (Eint) was computed for all stacking patterns, with values summarized in Table 1. The interaction energy is defined as follows [51]:

$$E_{\text{int}} = E_{\text{Ga}_2\text{SSe/monolayer}} - E_{\text{Ga}_2\text{SSe}} - E_{\text{monolayer}} \quad (1)$$
where $E_{Ga_2SSe/monolayer}$, $E_{Ga_2SSe}$, and $E_{monolayer}$ represent the total energies of the heterojunction, the $Ga_2SSe$ monolayer, and the paired monolayer, respectively. In both Table 1 and the rest of the article, the nomenclature adopted to unambiguously name the different heterostructure patterns corresponds to the top-down atomic sequence. The negative $E_{int}$ values across all configurations indicate that these heterostructures are energetically stable, primarily due to vdW interactions at the interface. This confirms their suitability for further electronic and optical property investigations.

<table><caption>**Table 1.** Interfacial interaction energies ($E_{int}$) of the studied heterojunctions.</caption><thead><tr><th>Heterojunction</th><th>E<sub>int</sub> (meV)</th><th>Heterojunction</th><th>E<sub>int</sub> (meV)</th></tr></thead><tbody><tr><td>SGa<sub>2</sub>Se/PGa</td><td>-83.91</td><td>SeGa<sub>2</sub>S/GaP</td><td>-120.17</td></tr><tr><td>SeGa<sub>2</sub>S/PGa</td><td>-67.86</td><td>SGa<sub>2</sub>Se/GaP</td><td>-133.17</td></tr><tr><td>SGa<sub>2</sub>Se/SPtSe</td><td>-207.88</td><td>SeGa<sub>2</sub>S/SPtSe</td><td>-186.09</td></tr><tr><td>SGa<sub>2</sub>Se/SePtS</td><td>-192.33</td><td>SeGa<sub>2</sub>S/SePtS</td><td>-182.22</td></tr><tr><td>SGa<sub>2</sub>Se/SSnSe</td><td>-144.32</td><td>SeGa<sub>2</sub>S/SSnSe</td><td>-136.98</td></tr><tr><td>SGa<sub>2</sub>Se/SeSnS</td><td>-159.14</td><td>SeGa<sub>2</sub>S/SeSnS</td><td>-144.76</td></tr></tbody></table>

## 3.2. Electronic Properties

The projected band structures of the $Ga_2SSe/GaP$, $Ga_2SSe/PtSSe$, and $Ga_2SSe/SnSSe$ van der Waals heterostructures, computed using the HSE06 hybrid functional, are presented in Figure 2. Each heterostructure was analyzed in four distinct stacking arrangements. In the cases of $Ga_2SSe/GaP$ and $Ga_2SSe/SnSSe$, the valence band maximum (VBM) and conduction band minimum (CBM) are located on different constituent layers, indicating a staggered (type-II) band alignment. This configuration facilitates spatial separation of photoexcited charge carriers, which is favorable for photocatalytic performance.

Conversely, for $Ga_2SSe/PtSSe$, both the VBM and CBM are confined to the same monolayer, signifying a straddling (type-I) band alignment. Such an arrangement tends to localize electrons and holes within the same layer, promoting recombination and reducing photocatalytic efficiency. Based on this analysis, only the heterostructures exhibiting type-II alignment were selected for further investigation.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** *Cont.*</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Projected electronic band structures for the Janus Ga₂SSe/GaP, Ga₂SSe/PtSSe, and Ga₂SSe/SnSSe vdW heterojunctions, obtained using the HSE06 functional. Results are shown for the four stacking configurations: (a) Pattern A, (b) Pattern B, (c) Pattern C, and (d) Pattern D.</figcaption></figure>

To further explore the electronic behavior of the Ga₂SSe/GaP and Ga₂SSe/SnSSe van der Waals heterojunctions, we examined their electrostatic potential profiles along the out-of-plane (z) direction. As illustrated in Figure 3, each heterojunction exhibits an electrostatic potential difference (ΔΦ) across the interface. Based on the model introduced by Li et al. [52], the bandgap condition for polar materials can be modified as E_g > 1.23 ΔΦ eV, where ΔΦ is the potential offset across the junction (with ΔΦ > 0).

<figure><img src="image_7.png" /><figcaption>**Figure 3.** *Cont.*</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 3.** Electrostatic potential profiles across the interfaces of $Ga_2SSe/GaP$ and $Ga_2SSe/SnSSe$ van der Waals heterojunctions, shown for the four stacking configurations: (a) Pattern A, (b) Pattern B, (c) Pattern C, and (d) Pattern D. The blue dotted lines delineate the GaP (left column) and SnSSe slabs (right column), and the red dotted lines delineate the $Ga_2SSe$ slab (left and right columns) in the heterostructure.</figcaption></figure>

In addition, the standard redox potentials for the $H^+/H_2$ and $O_2/H_2O$ half-reactions in aqueous solutions are given by the following expressions [53,54]:

$$E_{H^+/H_2} = -4.44 + 0.059 \times pH \quad (2)$$

$$E_{O_2/H_2O} = -5.67 + 0.059 \times pH \quad (3)$$

For photocatalytic water splitting to occur efficiently, the positions of the conduction and valence band edges of the heterostructures must appropriately straddle these redox levels. At pH = 0, the reduction and oxidation potentials correspond to -5.67 eV and -4.44 eV, respectively. To evaluate the alignment of the heterojunction band edges with respect to these potentials, we used the following relations [55]:

$$E_{VBM} = -I = -\chi - 0.5E_g \quad (4)$$

$$E_{CBM} = -A = E_{VBM} + E_g \quad (5)$$

Here, I is the ionization potential, A is the electron affinity, χ is the electronegativity, and $E_g$ is the bandgap. These equations enable us to determine the conduction band minimum (CBM) and valence band maximum (VBM) positions of the heterostructures and assess whether the electronic structure of the materials is suitable for water-splitting reactions. The input quantities used for the band alignment were obtained from the fully relaxed heterostructure including van der Waals interactions.
The calculated VBM and CBM positions for the Ga₂SSe/GaP and Ga₂SSe/SnSSe heterojunctions are reported in Table S2. Bader charge analysis further reveals subtle but consistent interlayer charge transfers in the SeGa₂S/SSnSe and SeGa₂S/SeSnS systems, distinguishing them from conventional type-II alignments. Specifically, in SeGa₂S/SSnSe, a net transfer of approximately 0.0004 |e| occurs from the SSnSe layer to the SeGa₂S layer, while in SeGa₂S/SeSnS, about 0.0038 |e| is transferred from SeSnS to SeGa₂S. Although small in magnitude, these values are obtained from fully converged calculations using a dense k-point grid and high energy cutoffs, with total charge conservation accurate within 2 × 10⁻⁶ |e| . These charge redistribution patterns, combined with the band alignment, support a direct Z-scheme mechanism in both heterostructures.

Unlike type-II heterojunctions, where photogenerated electrons and holes are sepa-
rated between the layers, Z-scheme heterojunctions involve a unique interlayer recombina-
tion pathway, wherein electrons from one layer recombine with holes from the other in a
Z-shaped transfer scheme. This approach preserves high-energy charge carriers, namely
holes with strong oxidative power in one layer and electrons with strong reductive capa-
bility in the other, thus maintaining the overall redox potential and significantly boosting
photocatalytic efficiency.

Depending on the heterojunction type, the band edge positions of both type-II and direct Z-scheme heterojunctions need to be adjusted using the ΔΦ value, as illustrated in Figure 4. Since the standard redox potentials of water are referenced to the highest vacuum energy level set as zero, and because the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) take place on different monolayers in type-II and direct Z-scheme heterojunctions, the standard redox potentials for the H⁺/H₂ and O₂/H₂O redox couples must be corrected according to the following expressions:

$$
E_{H^+/H_2} = -4.44 - \Delta\Phi + 0.059 \times pH \quad (6)
$$

$$
E_{O_2/H_2O} = -5.67 - \Delta\Phi + 0.059 \times pH \quad (7)
$$

Figure 5 presents the adjusted band edge positions for the Ga₂SSe/GaP and Ga₂SSe/SnSSe van der Waals heterojunctions. The arrows labeled hv indicate the absorption of photons leading to electron excitation from the valence to the conduction band. In type-II heterojunctions, photogenerated electrons in the donor layer (with higher CBM energy) transfer to the CB of the acceptor layer, while holes migrate in the opposite direction, enabling spatial separation of charge carriers and facilitating the HER and OER at different sites. In contrast, in direct Z-scheme heterostructures, electrons in the CB of the acceptor recombine with holes in the VB of the donor layer at the interface (shown with curved arrows in panels c and d). This recombination preserves the most reducing electrons and the most oxidizing holes in the respective layers, optimizing photocatalytic activity for water splitting. Therefore, the conduction and valence band edges of the SGa₂Se/PGa, SeGa₂S/PGa, SeGa₂S/SSnSe, and SeGa₂S/SeSnS heterojunctions straddle the redox potentials for both HER and OER, fulfilling the thermodynamic criteria required for water splitting within a range of acidic and alkaline environments. These four heterojunctions can serve as promising photocatalytic materials suitable for water splitting across a variety of pH conditions.
<figure><img src="image_9.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_12.png" /></figure>

**Figure 4.** Band alignment of the studied heterostructures relative to water redox potentials, corrected by vacuum level offset ($\Delta\Phi$): (a–c) Type-II heterojunctions—(a) “1” = GaP/SGa₂Se, GaP/SeGa₂S, “2” = SGa₂Se/SSnSe; (b) PGa/SGa₂Se, PGa/SeGa₂S; (c) “1” = SGa₂Se/SeSnS. (c,d) Direct Z-scheme heterostructures—(c) “2” = SeGa₂S/SSnSe; (d) SeGa₂S/SeSnS. Dashed lines show $H^+/H_2$ and $O_2/H_2O$ redox levels. Red, dashed arrows indicate reaction sites and types.

<figure><img src="image_13.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

**Figure 5.** *Cont.*
<figure><img src="image_17.png" /><figcaption>Figure 5. Corrected band edge positions of the Ga₂SSe/GaP and Ga₂SSe/SnSSe vdW heterojunctions for (a) Pattern A, (b) Pattern B, (c) Pattern C, (d) Pattern D. The valence and conduction bands are represented by rectangles. Vertical arrows indicate electronic transitions from the valence band to the conduction band. Inclined arrows labeled hv represent photon absorption leading to electron excitation. Other inclined arrows indicate charge transfer between the heterostructure components. Red dashed arrows indicate reaction sites and types.</figcaption></figure>

### 3.3. Optical and Photocatalytic Properties

Light absorption capability is widely recognized as a critical factor for efficient water-splitting photocatalysis. Figure 6a,b present the absorption coefficients of patterns A and B for the Ga₂SSe/GaP vdW heterojunctions, as well as patterns C and D for the Ga₂SSe/SnSSe vdW heterojunctions, alongside those of their respective monolayers. The absorption coefficient (α) was computed using the dielectric function formula [56]:

$$ \alpha(\omega) = \frac{\omega\sqrt{2}}{c} \left[ \sqrt{\varepsilon_1^2(\omega) + \varepsilon_2^2(\omega)} - \varepsilon_1(\omega) \right]^{1/2} \quad (8) $$

where the real (ε₁) and imaginary (ε₂) parts of the dielectric function are connected through the Kramers–Kronig relations, with ε₂ derived from ε₁. Compared to the isolated monolayers, particularly GaP and SnSSe, the heterostructures exhibit similar absorption profiles in the visible range, with a marked enhancement in the ultraviolet region. Despite the overall reduction in band gaps, a consistent red-shift of the absorption onset is not observed. This can be explained by analyzing the orbital contributions at the band edges. In the Ga₂SSe/PGa heterostructure, the VBM is mainly composed of phosphorus orbitals, whereas the CBM does not involve p contributions. In contrast, in Ga₂SSe alone, the VBM is dominated by sulfur and selenium orbitals, which are also absent in the CBM. This element-specific segregation of states across the band edges restricts efficient optical transitions near the gap and prevents a noticeable shift of the absorption edge. Moreover, the projected density of states (Figure 2) does not reveal the presence of mid-gap states or significant electronic reconstruction. In direct Z-scheme heterostructures, partial hybridization near the valence band indicates limited interlayer orbital mixing. These subtle interfacial effects
impact the joint density of states and optical transition probabilities, particularly in the UV region, where enhanced absorption is observed. The changes in the absorption spectra are therefore governed not by the emergence of new states, but by the nature and spatial distribution of orbital contributions at the band edges, along with interface-driven modifications to the optical selection rules. This enhancement suggests that patterns A and B of Ga₂SSe/GaP, along with patterns C and D of Ga₂SSe/SnSSe, are strong candidates for photocatalytic water splitting applications.

<figure><img src="image_18.png" /><figcaption>**Figure 6.** Optical absorption spectra of the Ga₂SSe/GaP vdW heterojunctions (a) and the Ga₂SSe/SnSSe vdW heterojunctions (b), together with those of their corresponding monolayers, calculated using the HSE06 functional. The vertical green dotted lines delineate the boundaries of the visible wavelength region.</figcaption></figure>

To assess the effectiveness of the photocatalytic water-splitting process, the solar-to-hydrogen (STH) conversion efficiency is a key metric, measuring how well the photocatalyst converts sunlight into hydrogen production under zero applied bias. The STH efficiency ($\eta_{STH}$) consists of two components: the light absorption efficiency ($\eta_{abs}$) and the carrier utilization efficiency ($\eta_{cu}$), which can be expressed as follows [57,58]:

$$ \eta_{abs} = \frac{\int_{E_g}^{\infty} P(\hbar\omega)d(\hbar\omega)}{\int_{0}^{\infty} P(\hbar\omega)d(\hbar\omega)} \qquad (9) $$

$$ \eta_{cu} = \frac{\Delta G \int_{E}^{\infty} \frac{P(\hbar\omega)}{\hbar\omega} d(\hbar\omega)}{\int_{E_g}^{\infty} P(\hbar\omega) d(\hbar\omega)} \qquad (10) $$

where $P(\hbar\omega)$ represents the AM1.5G solar irradiance at photon energy $\hbar\omega$, and $E_g$ denotes the bandgap energy. $\Delta G$ corresponds to the minimum potential difference needed for water splitting (1.23 eV), while $E$ stands for the photon energy involved in the photocatalytic process, defined as follows [57,58]:

$$ E = \begin{cases} E_g, & \text{if } \chi(H_2) \ge 0.2 \text{ and } \chi(O_2) \ge 0.6 \\ E_g + 0.2 - \chi(H_2), & \text{if } \chi(H_2) < 0.2 \text{ and } \chi(O_2) \ge 0.6 \\ E_g + 0.6 - \chi(O_2), & \text{if } \chi(H_2) \ge 0.2 \text{ and } \chi(O_2) < 0.6 \\ E_g + 0.8 - \chi(H_2) - \chi(O_2), & \text{if } \chi(H_2) < 0.2 \text{ and } \chi(O_2) < 0.6 \end{cases} \qquad (11) $$

where $\chi(H_2)$ and $\chi(O_2)$ represent the overpotentials for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. The solar-to-hydrogen efficiency ($\eta_{STH}$) is then calculated using the following formula [57,58]:

$$ \eta_{STH} = \eta_{abs} \times \eta_{cu} \qquad (12) $$
Moreover, since the intrinsic electric field significantly influences the electron-hole separation, the solar-to-hydrogen efficiency ($η_{STH}$) should be corrected accordingly, as follows [57,58]:

$$η'_{STH} = η_{STH} \times \frac{\int_{0}^{\infty} P(\hbar\omega)d(\hbar\omega)}{\int_{0}^{\infty} P(\hbar\omega)d(\hbar\omega) + \Delta\Phi \int_{E_g}^{\infty} \frac{P(\hbar\omega)}{\hbar\omega} d(\hbar\omega)} \quad (13)$$

Here, $\Delta\Phi$ represents the electrostatic potential difference. As reported in Table 2, the corrected solar-to-hydrogen efficiencies ($η'_{STH}$) for patterns A and B in the Ga₂SSe/GaP heterojunctions are 5.99% and 8.48%, respectively. By contrast, patterns C and D in the Ga₂SSe/SnSSe heterostructures shows significantly higher values of 38.44% and 21.75%, respectively.

<table><caption>**Table 2.** Overpotentials (in eV) for the hydrogen evolution reaction ($\chi(H₂)$) and oxygen evolution reaction ($\chi(O₂)$) at pH = 7, along with the efficiencies for light absorption ($η_{abs}$), carrier utilization ($η_{cu}$), solar-to-hydrogen conversion ($η_{STH}$), and the corrected solar-to-hydrogen efficiency ($η'_{STH}$).</caption><thead><tr><th>Heterojunction</th><th>χ (H₂)</th><th>χ (O₂)</th><th>η<sub>abs</sub> (%)</th><th>η<sub>cu</sub> (%)</th><th>η<sub>STH</sub> (%)</th><th>η'_{STH} (%)</th></tr></thead><tbody><tr><td>SGa₂Se/PGa</td><td>1.14</td><td>1.12</td><td>13.85</td><td>45.99</td><td>6.37</td><td>5.99</td></tr><tr><td>SeGa₂S/PGa</td><td>0.75</td><td>0.82</td><td>18.35</td><td>48.56</td><td>8.91</td><td>8.48</td></tr><tr><td>SeGa₂S/SSnSe</td><td>0.63</td><td>1.36</td><td>56.36</td><td>68.21</td><td>38.44</td><td>38.44</td></tr><tr><td>SeGa₂S/SeSnS</td><td>0.13</td><td>1.03</td><td>44.61</td><td>55.28</td><td>24.66</td><td>21.75</td></tr></tbody></table>

These enhanced efficiencies in the Z-scheme arrangements not only outperform the type-II counterparts but also surpass the conventional 10% benchmark commonly cited for practical solar-to-hydrogen applications. For comparison, widely studied reference materials typically report significantly lower STH efficiencies. TiO₂ (anatase), for instance, generally achieves only around 0.1 to 1.0% [59], mainly due to its wide band gap and UV-limited absorption. MoS₂ in monolayer or few-layer form performs slightly better, with reported values in the range of approximately 1.0 to 2.0%, particularly when combined with heterostructures or co-catalysts [60]. Perovskites-based photocatalysts such as MoS₂-NiFe-MAPbBr₃ may reach efficiencies between 10 and 15% [61], though their operational instability under realistic conditions remains a critical limitation. These values are based on representative experimental and theoretical studies available in the literature, although reported STH values often vary depending on specific experimental setups or device architectures.

In terms of overpotentials, values for traditional photocatalysts are not always consistently reported, but it is generally acknowledged that TiO₂ and MoS₂ systems often require relatively high HER overpotentials, above 0.2–0.5 V [62–64], particularly in the absence of co-catalysts. The Z-scheme configurations examined in this study exhibit HER overpotentials of the same order of magnitude, in the range of 0.2–1.4 V, indicating similar energy efficiency and charge separation. For OER, similar overpotentials of about 0.2 V have been reported for optimized MoS₂/Ni₃S₂ heterostructures [63].

In this context, our predicted efficiencies, especially for the Ga₂SSe/SnSSe Z-scheme, are not only competitive with but also significantly higher than those of most conventional photocatalysts, clearly demonstrating the promise of such 2D heterostructures for next-generation solar hydrogen production. Furthermore, other theoretically investigated 2D heterojunction systems have also shown notable STH values, such as CdS/SPtSe with 37.50% [65], SiSe/SnSSe with 14.61% [66], ZrSSe/Ga₂SSe with 17.64% [67], and GaN/InS with 26.33% [68]. These comparative results further support the relevance and strong potential of the systems explored in our study.
**4. Conclusions**

This work underscores the promising capabilities of Ga₂SSe-based van der Waals heterojunctions for efficient solar-driven photocatalytic water splitting. Using comprehensive DFT simulations, we show that the Ga₂SSe/SnSSe (patterns C and D) and Ga₂SSe/GaP (patterns A and B) heterostructures feature ideal type-II band alignments and strong redox properties, meeting the essential criteria for water splitting. Importantly, patterns C and D exhibit a direct Z-scheme charge transfer mechanism, facilitating effective separation of photogenerated electron-hole pairs while maintaining powerful reduction and oxidation potentials. Each of these four heterojunctions satisfies the water-splitting conditions and demonstrates strong light absorption in both the visible and ultraviolet spectra. Particularly, the Ga₂SSe/SnSSe heterostructures in patterns C and D achieve exceptional solar-to-hydrogen conversion efficiencies, surpassing the commercial benchmark of 10% with values of 38.44% and 21.75%, respectively, outperforming many previously reported photocatalysts. These findings establish these Z-scheme heterojunctions as highly promising candidates for future photocatalytic water splitting applications.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst15080728/s1, Figure S1: Band structures of the Ga₂SSe monolayer (a), GaP monolayer (b), PtSSe monolayer (c), and SnSSe monolayer (d) calculated using the HSE06 functional. The Fermi energy is set to 0 eV; Table S1: Lattice parameter (a in Å) and band gap (Eg in eV) of the four monolayers, calculated using the HSE06 functional, along with a comparison with available literature data; Table S2: Electrostatic potential difference (ΔΦ), valence band maximum (VBM), and conduction band minimum (CBM) of the Ga₂SSe/GaP and Ga₂SSe/SnSSe van der Waals heterojunctions.

**Author Contributions:** Conceptualization, P.B. and M.-C.R.; methodology, F.Y., P.B. and M.-C.R.; software, F.Y.; validation, F.Y., P.B. and M.-C.R.; formal analysis, F.Y.; investigation, F.Y.; resources, P.B. and M.-C.R.; data curation, F.Y.; writing—original draft preparation, F.Y.; writing—review and editing, P.B. and M.-C.R.; visualization, F.Y.; supervision, P.B. and M.-C.R.; project administration, P.B. and M.-C.R.; funding acquisition, P.B. and M.-C.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Data will be made available through Zenodo depository upon request to the corresponding author.

**Acknowledgments:** The authors are thankful to the China Scholarship Council for financing the PhD thesis of F. Yang. This work was granted access to the HPC resources A0150806881 made by the "Grand Equipement National de Calcul Intensif (GENCI)". The "Centre de Calcul Intensif d'Aix-Marseille" is acknowledged for granting access to its high-performance computing resources.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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