Direct Z-scheme GaN/WSe₂ heterostructure for enhanced photocatalytic water splitting under visible spectrum

Abstract

van der Waals heterostructures are widely used in the field of photocatalysis due to the fact that their properties can be regulated via an external electric field, strain engineering, interface rotation, alloying, doping, etc. to promote the capacity of discrete photogenerated carriers. Herein, we fabricated an innovative heterostructure by piling monolayer GaN on isolated WSe₂. Subsequently, a first principles calculation based on density functional theory was performed to verify the two-dimensional GaN/WSe₂ heterostructure and explore its interface stability, electronic property, carrier mobility and photocatalytic performance. The results demonstrated that the GaN/WSe₂ heterostructure has a direct Z-type band arrangement and possesses a bandgap of 1.66 eV. The built-in electric field is caused by the transfer of positive charge between the WSe₂ layers to the GaN layer, directly leading to the segregation of photogenerated electron–hole pairs. The GaN/WSe₂ heterostructure has high carrier mobility, which is conducive to the transmission of photogenerated carriers. Furthermore, the Gibbs free energy changes to a negative value and declines continuously during the water splitting reaction into oxygen without supplementary overpotential in a neural environment, satisfying the thermodynamic demands of water splitting. These findings verify the enhanced photocatalytic water splitting under visible light and can be used as the theoretical basis for the practical application of GaN/WSe₂ heterostructures.

---

1. Introduction

Hydrogen energy is anticipated to be an alternative fuel to fossil fuels in the near future,^1–3 and in this case, photocatalytic water splitting is an attractive approach to produce hydrogen.⁴ The traditional photocatalytic material TiO₂ has a wide band gap (BG) of 3.20 eV, which is only activated by ultraviolet light with a wavelength of less than 385 nm and its hydrogen production efficiency is extremely low.⁵ Alternatively, two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as WSe₂, MoS₂, and MoSe₂ have significant application prospects in photocatalytic water splitting due to their excellent electronic properties, high carrier mobilities and visible-light response. In particular, it has been confirmed theoretically that monolayer WSe₂ has robust photoluminescence and high carrier mobility (705 cm² V⁻¹ s⁻¹), which are superior to that of MoS₂ and MoSe₂.^6–8 Nevertheless, the photocatalytic performance of monolayer WSe₂ is still limited owing to its high transmittance and poor photogenerated electron–hole separation efficiency. Hence, the use of a co-catalyst to reduce the electron–hole recombination is strongly recommended.

In recent years, some studies have proven that the construction of van der Waals heterostructures (vdWHs) is a valid approach to improve the photocatalytic efficiency of 2D materials.^9,10 vdWHs retain the electronic properties of individual layers and the interface effect of heterostructures endow them with some properties that are not present in their respective components, such as regulating the bandgap energy and improving the segregation efficiency of photogenic electron–hole pairs.^11,12 Traditional type II heterostructures have the advantages of response in an expanded spectrum range and promoted carrier separation,¹³ but their redox ability is poor. Thus, the design and fabrication of novel direct Z-scheme photocatalysts are attracting increasing interest to improve the redox ability and transfer performance of photo-generated charges.^14–16

GaN monolayers are very promising for application in high-performance opto-electronic devices due to their semiconducting character with a suitable bandgap of about 2.3 eV,^17,18 which is narrower than that of bulk GaN.¹⁹ Previous research disclosed that GaN shares an identical hexagonal configuration and related lattice constants with TMDCs, making them compatible.²⁰ Also, a GaN thin film was prepared via chemical vapor deposition from SL-WSe₂/c-sapphire to achieve GaN/WSe₂ heterostructures.²¹ R. Meng et al. revealed that the existence of band offsets and intrinsic electric fields leads to reinforced photocatalytic activity between WSe₂/GaN and WS₂/GaN.²² Shaoqian Yin et al. studied the effects of modifying the electric field and strain on the optical and electronic characteristics of GaN/WSe₂ heterostructures with various stacking configurations. However, a systematic study has not been performed to date on the photocatalytic performance of GaN/WSe₂ heterostructures.²³

In this study, the structural stability, electronic properties, carrier mobilities, and photocatalytic performance of GaN/WSe₂ heterostructures were explored via first-principles calculations. The calculations of the energy band gap presented that the GaN/WSe₂ heterostructure is a representative direct Z-scheme with a built-in electric field from GaN to WSe₂. Meantime, the carrier mobilities of the GaN/WSe₂ heterostructure, which influence the dissociation efficiency, was also amplified. In addition, the calculation of the Gibbs free energy of the GaN/WSe₂ system clarify the oxygen evolution reaction (OER) process. Consequently, it was inferred that GaN/WSe₂ heterostructures, which possess superior photocatalytic capacities under visible light, are favorable photocatalysts in the field water splitting.

2. Calculation methods and models

The theoretical analyses were manipulated entirely through the Vienna ab initio simulation package (VASP)²⁴ within the projector augmented plane-wave (PAW) pseudopotentials using density functional theory (DFT). The Perdew–Burke–Ernzerhof (PBE) algorithm was adopted to confirm the exchange-correlation functional.²⁵ Two-dispersion correction of DFT-TS²⁶ and DFT-D3 (ref. 27) was considered in the computation for clarifying the impacts of non-covalent forces. The valence electron schemes were as follows: 3d¹⁰4s²4p¹ for Ga, 2s²2p³ for N, 5d⁴6s² for W, and 4s²4p⁴ for Se. A Monkhorst Pack k-point grid of 7 × 7 × 1 and a cutoff energy of 480 eV were employed in the first Brillouin zone. The maximum force was set as 0.01 eV Å⁻¹ and the energy convergence threshold was 10⁻⁵ eV.

The space group of P6/m2 was chosen for isolated GaN and WSe₂. The lattice constants of the structure-optimized GaN (a = b = 3.200 Å, γ = 120°, see Table 1) and WSe₂ monolayer (a = b = 3.212 Å, γ = 120°, see Table 1) were identical to the anterior empirical and theoretical results.^28–30 The GaN/WSe₂ heterostructure was configured by the relaxed 2 × 2 lateral periodicity of monolayer GaN(001) and WSe₂(001), as shown in Fig. 1(c)–(e). To avoid interlayer interactions, a vacuum spacing of 15 Å was adopted perpendicularly for the GaN/WSe₂ heterostructures.

Table 1 Calculated lattice parameters (Å), lattice mismatch ratio (%), cohesive energies (meV Å⁻²), mismatch energies (meV Å⁻²), vdW binding energies (meV Å⁻²) and equilibrium interlayer distances (Å) of GaN/WSe₂ heterostructures using the dispersion-correction DFT-TS and DFT-D3 approaches after geometric relaxation

Model	Method	GaN	WSe2	GaN/WSe2	Rmis	Ecoh	Emis	Evdw	d
		a1 = b1	a2 = b2	a = b
V	DFT-D3	3.200	3.296	3.246	3.00	1.84	−0.78	2.61	3.09
VI	DFT-TS	3.212	3.338	3.261	3.92	4.14	−2.55	6.69	3.32

---

Fig. 1 Stereo matching patterns of six canonical parallel configurations of GaN/WSe₂ heterostructures. (a) model I, where the N atoms in interface are on the Se atoms straightly; (b) model II, where the Ga atoms are located directly above the middle of the W–Se bonds; (c) model III, where the N atoms in the interface are straightly on top the middle of the W–Se bonds; (d) model IV, where the Ga atoms are located directly above the W atoms; (e) model V, where Ga atoms are located directly on the Se atoms; and (f) model VI, where the N atoms are set on the W atoms.

3. Results and discussion

3.1 Interface stability

To inspect the impact of the interfacial interactions between the GaN and WSe₂ nanosheets on the structural stability of GaN/WSe₂ combinations, the compositional dependence of the total energy is discussed. Six representative parallel alignments of GaN/WSe₂ heterostructures are displayed in Fig. 1. To calculate the structural stability of the six stacked models quantitatively, the D3 and TS dispersion corrections were included to calculate the relative total energies of the six patterns (compared with the steadiest model) applying the PBE method, respectively, as shown in Fig. 2. The results show that the different calculation methods have analogous variation tendencies in the related total energies of the six configurations, demonstrating that the computational outcomes are dependable. Also, the figure shows that model V has the lowest relative energy among the models with the DFT-D3 algorithm, while model VI has the lowest relative energy compared with the others with DFT-TS algorithm. Hence, all sequent calculations were built on these two models.

Fig. 2 (a) Relative total energies of six models for GaN/WSe₂ heterostructures with DFT-TS and DFT-D3 approaches after geometrical optimization, where the illustration exhibits the corresponding side view of the heterostructure. (b) Dependence of the cohesive energies of model V and model VI with interlayer distance d.

To further study the structural stability of the GaN/WSe₂ heterostructure, the lattice mismatch ratio and mismatch energy between the two monolayers were calculated. The lattice mismatch ratio is described as R_mis=(a₂ − a₁)/a₁, where a₁ and a₂ represent the lattice constants of the GaN and WSe₂ monolayers, respectively. Table 1 lists the R_mis of the GaN/WSe₂ heterostructures calculated by the DFT-D3 and DFT-TS methods, which are 3.00% and 3.92%, respectively, showing a perfect match.³¹ Moreover, the lattice mismatch energies of the GaN/WSe₂ heterostructures were determined using the following equation:

ΔE_mis = [E_(GaN)a + E_(WSe2)a − E_(GaN) − E_(WSe2)]/S (1)

where E_(GaN)a and E_(WSe2)a are the overall energies of isolated GaN and WSe₂ under the equilibrium lattice parameters of the GaN/WSe₂ heterostructures, respectively. E_GaN and E_WSe2 indicate the whole energies of the single GaN and WSe₂ nanosheets before contact. S represents the interfacial area of the heterostructure. The mismatch energy results are exhibited in Table 1. ΔE_mis computed by DFT-D3 and DFT-TS approaches is 1.84 meV Å⁻² and 4.14 meV Å⁻², respectively, which is substantially below that of WS₂/WSe₂,³⁴ MoS₂/WSe₂,³³ and graphene/WSe₂.³² In the heterostructure, the lattice mismatch of the isolated GaN and WSe₂ caused by strain-driven interactions is almost negligible.

To elucidate the adsorption interaction between the GaN and WSe₂ monolayers, the two above-mentioned methods were chosen to attain the interfacial cohesive energy, E_coh, at different interlayer distances, d. E_coh is expressed as follows:

E_coh = (E_GaN/WSe2 − E_GaN − E_WSe2)/S (2)

where E_GaN/WSe2, E_GaN, and E_WSe2 correspond to the whole energies of relaxed GaN/WSe₂ heterostructure, GaN monolayer and WSe₂ monolayer, separately. The E_coh of the GaN/WSe₂ composites at the most stable interlayer spacing is −0.78 meV Å⁻² and −2.55 meV Å⁻² for DFT-D3 and DFT-TS, respectively, as displayed in Table 1. It is noticeable that the negative E_coh values imply stable heterostructures.^35,36 Therefore, the cohesion between GaN and WSe₂ stabilizes the geometry. In addition, the E_coh trends calculated using the two methods are very similar to the changes in the interfacial space, proving the dependability of the calculated results.

Finally, the van der Waals energy was introduced to quantitatively describe the magnitude of the interlayer van der Waals force, which is defined as and its magnitude is determined by the lattice mismatch energy and the interface binding energy. The calculation formula is as follows:

E_vdW = |E_coh| + |ΔE_mis| (3)

The calculated results are 2.61 eV and 6.69 eV, which are within the normal vdW binding energy range,^37,38 indicating the existence of slight van der Waals forces in the GaN/WSe₂ heterostructures.

In the case of the GaN/WSe₂ heterostructure, the equilibrium interfacial space between GaN and WSe₂ is 3.09 Å with DET-D3 and 3.32 Å with DFT-TS, respectively, which is the canonical distance of vdW force.³⁹ Besides, given that R_mis under DET-D3 is smaller than that with DET-TS, the subsequent calculations are based on model V of the GaN/WSe₂ heterostructure with the DET-D3 method. The results of model VI of the GaN/WSe₂ heterostructure with the DET-TS method are demonstrated in the ESI.†

3.2 Electronic property

The band arrangements of the GaN monolayer, WSe₂ monolayer and GaN/WSe₂ composites were investigated using the PBE functional module. The Fermi level of the three systems was set as the level zero energy, and the high symmetry sites G (0,0,0), M (0,0.5,0) and K (−0.333,0.667,0) in the Brillouin zone were used as observation routes to study the band alignment of the system with the range limited from −4 eV to 5 eV. Fig. 3(a) shows that monolayer GaN is a semiconductor with an indirect BG of 2.13 eV, where the valence band maximum (VBM) is settled at the K point, while the conduction band minimum (CBM) is settled at the G point. This is theoretically in accordance with the established research.⁴⁰ The VBM and CBM of the single-layer WSe₂ were both set near the K point, which denotes that WSe₂ is a semiconductor with a direct BG of 1.65 eV. The calculated results of WSe₂ are similar to that reported by R. S. Meng,⁴¹ as demonstrated in Fig. 3(b).

Fig. 3 Energy band diagrams of (a) monolayer GaN, (b) WSe₂ nanosheet, and (c) model V of GaN/WSe₂ heterostructure. (d) Calculated TDOS of GaN/WSe₂ heterostructure and PDOS of GaN and WSe₂ employing the PBE algorithm. (e) PDOS of Ga and N atoms in GaN. (f) PDOS of W and Se atoms in WSe₂.

Fig. 3(c) presents the band diagram of the GaN/WSe₂ heterostructure. Both the VBM and CBM are located at the high symmetry point K, and thus it is a direct semiconductor with a BG of 1.47 eV. Compared with the indirect BG semiconductor, the direct bandgap semiconductor has a higher absorption coefficient for photo-generated electron–hole pairs and higher light utilization rate, and thus it is more suitable for photocatalysis.⁴² The forbidden bandwidth of the GaN/WSe₂ heterostructure is 1.47 eV, which is slightly lower than that of the GaN and WSe₂ monolayers, and its energy levels are denser. This is conducive to the migration of electrons, thereby improving the photocatalytic activity. Also, the BG of the GaN/WSe₂ heterostructure is larger than that required for photocatalytic water splitting, which is 1.23 eV. In addition, comparing the three images in Fig. 3(a–c), it can be found that the energy band structure of the GaN/WSe₂ heterostructure is similar to the energy band alignments of the two monolayers, and almost retains that of GaN and WSe₂ to a large extent. Therefore, it can be speculated that the binding force between the heterostructure layers is weak van der Waals force, and the interaction force when the two monolayers are combined is small, which also corresponds with the above-mentioned result.

The density of states (DOS) usually reflects the distribution of electrons in each system. To further elucidate the electronic structure of the GaN/WSe₂ heterostructure, we also calculated the density of states of the GaN/WSe₂ heterostructure, GaN monolayer and WSe₂ monolayer. The density curve range was −2 eV to 3 eV for analysis, as illustrated in Fig. 3(e and f). The VBM of monolayer GaN consists mainly of Ga-4p and N-2p states, whereas the CBM consists of N-2p states, are displayed in Fig. 3(e). The VBM and CBM of monolayer WSe₂ consist primarily of W-5d and Se-4p states, as presented in Fig. 3(f). Apparently, the VBM is derived from GaN (orange shaded region in Fig. 3(e)), while the CBM of the GaN/WSe₂ heterostructure originates from WSe₂ (blue shaded region in Fig. 3(f)). The overlap may be caused by orbital hybridization, and the occurrence of orbital overlap will result in a reduction in the BG, which is more advantageous to improve the catalytic performance of the photocatalytic material.⁴³ It is well-known that the valence band offset (VBO) and the conduction band offset (CBO) are at the related sites of the VBM and the CBM of two sides of the interfacial space, respectively, which can considerably modify the charge transfer capacity of the heterostructures under illumination.⁴⁴ In addition, VBO, CBO, and BG can be defined as ΔE_V = E_{GaN VBM} − E_{WSe2 V}, ΔE_C = E_{GaN CBM} − E_{WSe2 CBM}, and E_g = EWSe₂ CBM − EGaN VBM, respectively. Under the equiponderant interfacial condition, the correlation of VBO, CBO, the BG values is VBO (0.12 eV) < CBO (0.65 eV) < BG (1.47 eV). Accordingly, it can be reliably inferred that the GaN/WSe₂ heterostructure shows a type-II or Z-scheme band structure, which is consistent with the conclusion of band alignment, leading to the spatial segregation of the photogenerated carrier pairs.⁴⁵

The charge transfer phenomenon at the heterostructure interface can be illuminated by calculating the interfacial work function of the GaN monolayer, WSe₂ monolayer and GaN/WSe₂ heterostructures.⁴⁶ The calculation formula is as follows:

W = E_vac − E_F (4)

where E_vac and E_F express the vacuum energy level and the Fermi energy level, respectively, and the vacuum energy level is taken as 4.5 eV. The GaN/WSe₂ heterostructure curve is the sum of the GaN monolayer and WSe₂ monolayer. The surface work functions of the GaN monolayer (Fig. 4), WSe₂ monolayer and GaN/WSe₂ heterostructure are 4.340 eV, 5.125 eV and 4.954 eV, respectively, and the work function of the heterostructure lies between that of isolated GaN and WSe₂. and the electrons usually flow from the low side of the work function to the high side. Therefore, when the WSe₂ and GaN monolayers are in contact, electrons aggregate at the interface of the WSe₂ side to form a negative region, and the holes accumulate at the GaN side to form a positive region, thereby forming a built-in electric field pointing from the GaN layer to the WSe₂ monolayer. Given that the Fermi levels of the two monolayers are different, the energy bands shift accordingly until the Fermi levels of the two monolayers reach equilibrium. The presence of a built-in electric field can improve the mobility of carriers, thereby enhancing the dissociated efficiency of photo-induced electron–hole pairs, reducing the recombination probability of carriers, and improving the photocatalytic properties of the heterostructure.

Fig. 4 Calculated surface work functions of: (a) GaN monolayer; (b) WSe₂ monolayer; and (c) GaN/WSe₂ heterostructure.

The position of the band edge of a semiconductor is very important for evaluating its redox ability and photocatalytic performance. It is apparent that with the emergence of heterostructures, the E_F of two pristine materials will achieve equilibrium. Consequently, the isolated GaN intimately contacts with WSe₂, which will provoke a negative shift of 0.614 V in E_F for GaN and induce a positive shift of 0.171 V for the WSe₂ nano-slab. Due to the migration of electrons from monolayer GaN to WSe₂ with abandoned holes in the GaN nanolayer, the edge potentials in the CB and VB of GaN are −0.73 V and 1.40 V at the GaN/WSe₂ heterostructure after equalizing, singly. Meanwhile, that of the WSe₂ slab is 0.02 V and 1.67 V, respectively. It can be deduced that the VB edge potential of WSe₂ is 0.27 V lower than that of monolayer GaN; meantime, the CB edge of GaN is 0.75 V larger than that of the WSe₂ slab. As shown in Fig. 5, the photogenerated carriers obey two different routes, as follows: (1) electron transformation: on the one hand, the photogenerated electrons transfer from the VB to the CB under illumination. On the other hand, the built-in electric field of the interface prevents electrons from migrating from the isolated GaN to WSe₂. Meanwhile, the occurrence of CBO impedes electrons implanting in the CB of the monolayer WSe₂, and the photo-induced holes in the VB of the single WSe₂ hinder their transitions to the VB of the GaN nanosheet. (2) Recombination of charges: in the CB of isolated WSe₂, the photogenerated electrons will rapidly recombine with the photogenerated holes in the VB of the isolated GaN, by virtue of the close range of charge conveyance between the WSe₂ and GaN layer. Accordingly, photogenerated carriers can excellently detach after the construction of the heterostructure. Finally, it was determined that the GaN/WSe₂ heterostructure is a representative direct Z-scheme semiconductor, which is consistent with the consequences of band alignment and DOS. The type of band arrangement can make electron–hole pairs separate effectively and diminish the recombination of carriers, increase the lifetime of minority carriers, and simultaneously retain the redox ability of the internal carriers, which is further enhanced compared to the traditional type-II heterostructure photocatalytic properties.

Fig. 5 Electronic energy band alignments for isolated GaN and WSe₂ before contact, after contact, and under illumination, displaying the segregation and transformation mechanisms of photoinduced charges.

The two important potentials for water splitting are the O₂/H₂O electrode potential of 1.23 V, which can generate oxygen, and the standard hydrogen electrode H⁺/H₂O potential of 0 V. Given that the GaN/WSe₂ heterostructure is a Z-type heterostructure, the potential at the CB position of the heterostructure is −1.06 eV and the VB position is 1.47 eV. Comparing the water splitting potential, it can be known that the energy at the CBM in the GaN/WSe₂ heterostructure is more negative than that of the standard hydrogen electrode (H⁺/H₂O), and the VBM is more positive than that of the O₂/H₂O electrode. The BG of the GaN/WSe₂ heterostructure is larger than the potential required for water splitting, and thus it can be deduced that the GaN/WSe₂ heterostructure can be applied for either the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER), making it a good photocatalytic heterostructure.

The formation of a heterostructure will induce interactions between the interfaces, leading to the transfer and redistribution of charges at the interface. This can be studied by calculating the differential charge density of the heterostructure. The plane differential density is defined as Δρ, which can be calculated using the following formula:

Δρ = ρ_GaN/_WSe2 − ρ_GaN−ρ_WSe2 (5)

where ρ_GaN/WSe2 represents the charge density of the entire GaN/WSe₂ heterostructure system, and ρ_GaN and ρ_WSe2represent the charge density of the GaN monolayer and WSe₂ monolayer, respectively. The plane charge differential density curve and three-dimensional charge differential density map in the Z direction of the GaN/WSe₂ heterostructure were obtained, as shown in Fig. 5.

For the plane charge differential density curve in Fig. 5, the blue area represents the depletion of charges and the yellow area represents the charge accumulation. Electrons accumulate in the WSe₂ slab and are consumed in the GaN slab, elucidating that electrons migrate from the GaN to WSe₂ nanosheet. To quantitatively articulate the charge density, the Bader charge analysis under equilibrium was executed. The negative charges of 0.016 e per atom shift from the GaN nanosheet to the WSe₂ after they contact each other. It is generally acknowledged that the variation in the populations is consistent with the conclusions of the charge density difference. Simultaneously, in Fig. 5, it indicates that for the benefit of implementing stability, segments of electrons in GaN relocate to WSe₂ in the interface, which causes a positive charge region in the surface of GaN and negative charge in that of WSe₂. It can be confirmed that the GaN/WSe₂ heterostructures can take advantage of the built-in electric field pointing from GaN to WSe₂ to efficiently separate the photogenerated electrons and holes (Fig. 6).

Fig. 6 Plane-averaged and 3D charge density difference of GaN/WSe₂ heterostructures calculated using the PBE method. The inset shows a side view of the charge density difference of the GaN/WSe₂ heterostructure with the isosurface value of 0.0005 e Bohr⁻³.

3.3 Carrier mobility

Generally, the carrier mobility originates from the deformation potential, which is a pivotal factor to estimate photocatalytic properties and should be calculated systematically.⁴⁷ The methods for the calculation of the carrier mobility are elaborated in the ESI.† By utilizing compressive and tensile strains, the in-plane C_2D and E₁ⁱ of the isolated GaN, WSe₂ and GaN/WSe₂ were assessed by fitting the data into parabolic and linear curves, which are exhibited in Fig. 7, S5 and S6 in the ESI,† respectively. Also, Table 2 displays the acquired m*, C_2D, E₁ⁱ and μ_2D.

Fig. 7 (a) Top vision of orthogonal primitive cell of GaN/WSe₂ heterostructure. (b) Variations in relative total energy versus strains of GaN/WSe₂. Tendencies of CBM and VBM versus strains for GaN/WSe₂, which set E_vac as zero-point in the directions of (c) zigzag and (d) armchair. The linear curves offer the deformation potentials of GaN/WSe₂ heterostructures.

Table 2 Carrier effective masses and (m₀), elastic modulus C^zig_2D and C^ar_2Dm (Nm⁻¹), deformation potentials E^zig₁ and E^arm₁ (eV), and carrier mobilities μ^zig_2D and μ^arm_2D (cm² V⁻¹ s⁻¹) of isolated GaN, WSe₂ and GaN/WSe₂ under the zigzag and armchair traces at 300 K

Structure	Type			C^zig2D	C^arm2D	E^zig1	E^arm1	μ2Dzig	μ2D^arm
GaN	Electron	1.01	1.83	256.16	279.78	−4.30	−2.46	213.93	396.91
								171 (ref. 48)	304 (ref. 49)
	Hole	1.26	1.19	256.16	279.78	−0.98	−1.26	3705.20	2574.64
								3185 (ref. 48)	2396(ref. 49)
WSe2	Electron	1.99	2.50	203.14	177.15	2.24	−3.26	193.79	63.84
								142 (ref. 50)
	Hole	1.79	2.52	203.14	177.15	1.49	−2.40	511.96	122.25
								583 (ref. 51)
GaN/WSe2	Electron	0.20	0.25	318.04	331.43	−5.99	−5.38	4149.37	4328.33
	Hole	0.54	0.61	318.04	331.43	−3.22	−2.91	2101.71	2395.94

---

The monolayer GaN and WSe₂ and GaN/WSe₂ heterostructure exhibit different C^zig_2D and C^arm_2D in the zigzag and armchair directions, which determines that their physical stress responses are totally anisotropic. In addition, the carrier mobilities of the WSe₂ monolayer is lower than that of nanosheet GaN because of its smaller elastic modulus and higher deformation potential. These conclusions are in accordance with previous theoretical and experimental results.^48–51 Alternatively, the limitations of the deferent rates of photogenerated carriers are mainly owing to the low electron mobilities of the GaN slab and low carrier mobilities of the WSe₂ slab, essentially increasing the recombination of photogenerated electron–hole pairs. Monolayer GaN and WSe₂ detrimentally obstruct the photocatalytic activities, and thus is significant to fabricate GaN/WSe₂ heterostructures. Consequently, the electron and hole mobilities of the GaN/WSe₂ heterostructure are 4149.37 cm² V⁻¹ s⁻¹ and 2101.71 cm² V⁻¹ s⁻¹ along the zigzag direction, and 4328.33 cm² V⁻¹ s⁻¹ and 2395.94 cm² V⁻¹ s⁻¹ along the armchair direction, respectively, indicating that the electrons in the GaN/WSe₂ heterostructure, which have a tendency of spreading and moving along both the zigzag and armchair directions, are superior to holes. Additionally, water oxidation reactions take place in the WSe₂ nanosheet through photogenerated holes, the water reduction reactions emerge in the GaN monolayer, as can be seen in Fig. 5. In comparison to the single layers, the electron mobilities of the GaN/WSe₂ heterostructure in the zigzag pathway achieve a remarkable enhancement, which are 19 times and 21 times that of the isolated GaN and WSe₂, respectively. Meanwhile, 11 times and 67 times versus monolayer GaN and WSe₂ are achieved along the armchair pathway. Correspondingly, the hole mobilities of the GaN/WSe₂ heterostructure in the zigzag direction and armchair direction are improved by 4 times and 19 times compared with that of nano-slab WSe₂, respectively. In short, GaN/WSe₂ heterostructures with noticeable carrier mobilities exhibit tremendous potential for application in the photocatalytic field.

3.4 Photocatalytic performance

To additionally validate if water splitting reactions will initiate spontaneously, the thermodynamic practicability of applying the GaN/WSe₂ heterostructures as photocatalysts was explored. The complete water splitting mechanism of the GaN/WSe₂ heterostructure under light irradiation was divided into the HER and oxygen OER. Herein, the Gibbs free energy (ΔG) of the water splitting reaction were obtained from the report by Nørskov et al.⁵² The method for the calculations of ΔG is elaborated in the ESI.† Acid (pH = 0) and neutral (pH = 7) conditions were both considered. Universally, the HER tends to occur under a suitable potential than the OER, and thus investigating the thermodynamic driving force for the OER is sufficient.

When the OER reaction happens, the structures of the most stable OH*, O*, and OOH* intermediates adsorbed at the WSe₂ interface during the standard four-electron transport reaction paths and the relative variation in ΔG are elucidated in Fig. 8. The computed ΔG of OH*, O*, and OOH* in the dark (U_h = 0 V) when pH = 0 is 1.74 eV, 2.81 eV and 4.87 eV, respectively. Therefore, the emergence of the OOH* intermediate is the rate-determining process, and thus the OER process can proceed spontaneously when providing a 1.47 eV external potential afforded by photogenerated holes. It is apparent that the computed ΔG is enhanced in the initial and third steps, but decreases in the second and fourth reaction when pH = 0 and U_h = 1.23 V.

Fig. 8 (a) Four-electron route of the OER with the rapid production of OH*, O* and OOH* intermediates on the WSe₂ side of the heterostructures. Computational ΔG for the four-electron route of the OER pathway at (b) pH = 0 and (c) pH = 7.

Interestingly, when the external potential realizes 1.67 V with pH = 0, the various trends of ΔG are the same as the conditions at pH = 0 and U_h = 1.23 V. The results indicate that the OER step of water splitting will not spontaneously originate at pH = 0. Until the external potential reaches 1.88 V at pH = 7, the ΔG of the ultimate processes (ΔG_A, ΔG_B, ΔG_C, and ΔG_D) shifts to negative values, as exhibited in Fig. 8(c) (presented by the green line), which ascertains that the OER process will spontaneously occur at pH = 7 with light illumination. In conclusion, the GaN/WSe₂ heterostructures can initiate water decomposition without a thermodynamic driving force under irradiation in a neutral environment.

4. Conclusions

In summary, we comprehensively investigated the electronic, optical and photocatalytic properties of the GaN/WSe₂ heterostructure. Model V manifested a beneficial Z-scheme band arrangement with a built-in electric field pointing from GaN to WSe₂, while model VI displayed a detrimental type I band alignment. In the case of model V, the electron mobilities of the GaN/WSe₂ heterostructure are significantly enhanced compared to that of monolayer GaN and WSe₂, and the hole mobilities enlarge substantially compared to that of isolated WSe₂. In addition, the CB edge of GaN and VB edge of the WSe₂ monolayer can yield robust reduction and oxidation reaction, respectively. Then, we explored the photocatalytic OER on the heterostructure and found that the reaction spontaneously proceeded in a neutral environment. These findings provide a theoretical basis for the practical application of 2D GaN/WSe₂ heterostructures and provide insight for subsequent research on van der Waals heterostructures.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by East China University of Science and Technology (200237). This work was financed by Science and Technology Commission of Shanghai Municipality, China [Grant No. 17DZ1201405].

References

1. W. Ager Joel and A. Lapkin Alexei, Science, 2018, 360, 707–708.
2. F. Gutiérrez-Martín, A. B. Calcerrada, A. de Lucas-Consuegra and F. Dorado, Renewable Energy, 2020, 147, 639–649.
3. C. J. Quarton, O. Tlili, L. Welder, C. Mansilla, H. Blanco, H. Heinrichs, J. Leaver, N. J. Samsatli, P. Lucchese, M. Robinius and S. Samsatli, Sustainable Energy Fuels, 2020, 4, 80–95.
4. Y. Wu, C. Cao, C. Qiao, Y. Wu, L. Yang and W. Younas, J. Mater. Chem. C, 2019, 7, 10613–10622.
5. H. Zhuang, W. Chen, W. Xu and X. Liu, Int. J. Energy Res., 2020, 44, 3224–3230.
6. W. Zhao, Z. Ghorannevis, L. Chu, M. Toh, C. Kloc, P. H. Tan and G. Eda, ACS Nano, 2012, 7, 791–797.
7. G. Hao, R. Zhang and W. Zhang, et al., Acta Phys. Sin., 2022, 71, 01–04.
8. P. Tonndorf, R. Schmidt, P. B Ttger, X. Zhang, J. B. Rner, A. Liebig, M. Albrecht, C. Kloc, O. Gordan and D. Zahn, Opt. Express, 2013, 21, 4908–4916.
9. X. Zhang, Z. Meng, D. Rao, Y. Wang, Q. Shi, Y. Liu, H. Wu, K. Deng, H. Liu and R. Lu, Energy Environ. Sci., 2016, 9, 841.
10. H. Wang, X. Li and J. Yang, ChemPhysChem, 2016, 17, 2100.
11. W. Hu, J. Yang and B. N. L. E. Lawrence, J. Mater. Chem. C, 2017, 5, 12289–12297.
12. W. Xia, L. Dai, P. Yu, X. Tong, W. Song, G. Zhang and Z. Wang, Nanoscale, 2017, 9, 4324–4365.
13. J. LOW, J. YU and M. JARONIEC, et al., Adv. Mater., 2017, 29, 1601694.
14. J. Yu, et al., Phys. Chem. Chem. Phys., 2013, 15, 16883–16890.
15. A. Meng, B. Zhu, B. Zhong, L. Zhang and B. Cheng, Appl. Surf. Sci., 2017, 422, 518–527.
16. F. Liu, R. Shi, Z. Wang, Y. Weng, C. M. Che and Y. Chen, Angew. Chem., Int. Ed., 2019, 58, 11791–11795.
17. H. Ahin, S. Cahangirov, M. Topsakal, E. Bekaroglu and S. Ciraci, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 80, 155453.
18. A. R. Fareza, F. A. Nugroho, F. F. Abdi and V. Fauzia, J. Mater. Chem. A, 2022, 10, 24280.
19. W. M. Hoi, B. Oliver and R. J. Kaplar, et al., J. Mater. Res., 2021, 23, 36.
20. P. Gupta, et al., Sci. Rep., 2016, 6, 23708.
21. M. Tangi, P. Mishra, C. Tseng, T. K. Ng, M. N. Hedhili, D. H. Anjum, M. S. Alias, N. Wei, L. Li and B. S. Ooi, ACS Appl. Mater. Interfaces, 2017, 9, 9110–9117.
22. R. Meng, et al., Sci. China Mater., 2016, 59, 1027–1036.
23. S. Yin, Q. Luo, W. Dong, G. Guo, X. Sun, Yi Li, Y. Tang, Z. Feng and X. Dai, Phys. E: Low-Dimens. Syst. Nanostructures, 2022, 142, 115258.
24. G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169–11186.
25. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868.
26. A. Tkatchenko and M. Scheffler, Phys. Rev. Lett., 2009, 102, 073005.
27. S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104.
28. Y. Jiang, et al., Comput. Mater. Sci., 2017, 138, 419–425.
29. R. Zhang, G. Hao and X. Ye, et al., J. Appl. Phys., 2021, 129, 174302.
30. Yi Ding, et al., Phys. B, 2011, 406, 2254–2260.
31. J. He, M. Zhang and Q. Xiao, J. Nanjing Univ., 2006, 1, 63–102.
32. K. Si, J. Ma, C. Lu, Y. Zhou, C. He, D. Yang, X. Wang and X. Xu, Appl. Surf. Sci., 2020, 507, 145082.
33. R. Zhang, G. Hao, X. Ye, S. Gao and H. Li, Phys. Chem. Chem. Phys., 2020, 22, 23699–23706.
34. K. Si, J. Ma, C. Lu, Y. Zhou, C. He, D. Yang, X. Wang, X. X. Xu, C. Wang, W. Xiong, Y. Liu, D. Yang, X. Zhang and J. Xu, Nanotechnology, 2021, 33, 085705.
35. C. He, F. S. Han, J. H. Zhang and W. X. Zhang, J. Mater. Chem. C, 2020, 8, 6923–6930.
36. R. Zhang, G. Hao, H. Li, X. Ye, S. Gao, X. Yuan and C. Liu, J. Phys. Chem. Solids, 2020, 143, 109466.
37. D. K. Gorai and T. K. Kundu, Appl. Surf. Sci., 2022, 590, 153104.
38. R. Zhang, G. Hao and X. Ye, et al., Phys. Chem. Chem. Phys., 2020, 22, 23699–23706.
39. W. Zhang, G. Hao and R. Zhang, et al., J. Phys. Chem. Solids, 2021, 157, 110189.
40. X. T. Zhu, Y. Xu, Y. Cao, Y. Q. Zhao, W. Sheng, G. Z. Nie and Z. Ao, Appl. Surf. Sci., 2021, 542, 148505.
41. R. S. Meng, Sci. China Mater., 2016, 59(12), 1027–1036.
42. P. Shan, SIn₂Te/TeIn₂Se: a type-II heterojunction as a water-splitting photocatalyst with high solar energy harvesting, Xiangtan Univ., 2021.
43. F. Zhou, H. Xiang, J. Liu and B. Liu, Mater. Eng., 2018, 46, 9–19.
44. Z. Chen, X. Ma, J. Hu, F. Wan, P. Xu, G. Wang, M. Wang, S. Deng and C. Huang, New J. Chem., 2021, 45, 16520–16528.
45. R. Zhang, G. Q. Hao, H. B. Li, X. J. Ye, S. Gao, X. Yuan and C. Liu, J. Phys. Chem. Solids, 2020, 143, 109466–109471.
46. G. Hao, R. Zhou and W. Lei, et al., Phys. Chem. Solids, 2022, 165, 110684.
47. F. Zhuang, H. Guoqiang, R. Zhou, W. Lei, Y. Deng, X. Ye, H. Li and R. Zhang, Mater. Today Commun., 2022, 33, 104295.
48. W. J. Yin, X. L. Zeng, B. Wen, Q. X. Ge, Y. Xu, G. Teobaldi and L. M. Liu, Front. Phys., 2021, 16, 11467.
49. K. Ren, Y. Liu, J. Yu and W. C. Tang, Chem. Phys., 2020, 528, 110539.
50. W. Liu, J. Kang, D. Sarkar, Y. Khatami, D. Jena and K. Banerjee, Nano Lett., 2013, 13, 1983–1990.
51. R. Zhang, F. Zhuang and R. Zhou, et al., J. Phys. Chem. Solids, 2022, 171, 111014.
52. J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard and H. Jónsson, J. Phys. Chem. B, 2004, 108, 17886–17892.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra00928a

‡ Co-first authors: these authors contributed equally.

---