Tailoring GaN nanorods with MoS₂ on tungsten foil for enhanced photoelectrochemical performance

Abstract

Gallium nitride (GaN) nanostructures are highly promising for photoelectrochemical (PEC) water splitting due to their excellent electron mobility, chemical stability, and large surface area. However, the wide bandgap (~3.4 eV) of GaN limits its ability to absorb a broad spectrum of solar radiation, restricting its PEC performance. To address this limitation, MoS₂/GaN nanorods (NRs) heterostructures for enhanced PEC applications were fabricated on thin tungsten foil using a combination of atmospheric pressure chemical vapor deposition (CVD) and laser molecular beam epitaxy (LMBE). The Raman spectroscopy and X-ray diffraction revealed the hexagonal phase of GaN and MoS₂. X-ray photoelectron spectroscopy examined the electronic states of the GaN and MoS₂. PEC measurements revealed that the MoS₂-decorated GaN NRs exhibited a photocurrent density of approximately172 µA/cm², nearly 2.5-fold compared to bare GaN NRs (~70 µA/cm²). The increased photocurrent density is ascribed to the Type II band alignment between MoS₂ and GaN, which promotes effective charge separation, the decrease in charge transfer resistance, and the increase in active sites. The findings of this work underscore that the CVD and LMBE technique fabricated MoS₂/GaN heterostructures on W metal foil substrate can provide the vital strategy to raise the PEC efficiency toward solar water splitting.

Graphical abstract

Keywords

GaN nanorods / MoS₂ / X-ray photoelectron spectroscopy / PEC water splitting.

1 Introduction

The world’s energy consumption is rising every year, and total reliance on non-renewable energy sources causes high concentrations of CO₂ in the atmosphere, which likely enhances global warming. Researchers globally are investigating diverse renewable energy technologies as sustainable solutions to mitigate this issue [1,2]. Hydrogen (H₂) generation through photoelectrochemical (PEC) water splitting is being demonstrated as a reliable and alternative source to replace non-renewable energy sources. The production of clean and green hydrogen energy by using semiconductors without the emission of harmful CO₂ makes it an environment-friendly technique for sustainable development [3,4]. Numerous materials like transition metal oxides, sulphides, phosphides, carbides, and hydroxides have been investigated after the discovery of TiO₂ in PEC water splitting [5–11]. Among these, group III-V semiconductors (GaN, GaAs, InN, InP, etc.) have emerged as important materials for PEC water splitting as they have some unique characteristics like band gap tunability, longer carrier lifetimes, and better crystal quality, making them good choices for PEC [12,13]. GaN is a direct band gap (3.4 eV) semiconductor that belongs to Group III-V and has suitable conduction and valence band levels that straddle with water redox and oxidation level. However, their fast recombination of photogenerated charge carriers and larger bandgap limit their ability to achieve optimum PEC efficiency [14]. To increase the PEC efficiency of GaN, several techniques are employed, including catalyst attachment to reduce the reaction overpotential, nano-structuring for increased carrier movement, morphological engineering for better light absorption, and creating heterojunctions/structures to inhibit the charge recombination and facile the transportation of charge carriers [15,16].

Recent years have seen significant interest in transition metal dichalcogenides (TMDCs) materials because of their numerous applications in optoelectronics, CO₂ reduction, solar fuels, energy storage devices, gas sensors, and futuristic energy research areas [17,18]. TMDCs with the general formula MX₂ (where M = Mo, W, and X = S, Se) are highly interesting due to the planar layers of X−M−X stacked via Van der Waals interactions. MoS₂, a layered TMDC with a hexagonal arrangement, offers excellent optoelectronic and catalytic properties, making it a promising material for today’s energy research field. MoS₂ has demonstrated the ability to be a double-edged sword by improving catalytic activity for water splitting and extending visible light absorption [19]. With a tuneable band gap (E_g = 1.3–1.9 eV) categorized by the number of layers, MoS₂ demonstrates n-type semiconducting nature, suggesting the potential use in PEC hydrogen generation. Additionally, it was recently thought that MoS₂ is a substantial catalyst for both photocatalytic and electrocatalytic water splitting, because of its numerous exposed edges and active sites that come from the sulfur edges of the MoS₂ crystal layers [20,21]. In several studies, MoS₂-based heterostructures such as MoS₂/TiO₂, MoS₂/ZnO, MoS₂/WS₂, MoS₂/WO₃, MoS₂/CdS, MoS₂/CoTe, etc., have shown an enhancement in photocurrent density and PEC efficiency [20–31].

While the use of mono- or bilayer MoS₂-decorated GaN on ITO/FTO-coated glass substrate and other rigid, non-flexible substrates has been extensively studied in PEC water splitting [29,32,33], utilizing flexible substrates for photoelectrode fabrication is highly advantageous for roll-to-roll production processes, offering considerable weight reduction relative to traditional glass or hard substrates. This approach supports the development of lightweight, portable, and easily deployable PEC systems. Thereby, the photoelectrodes with strong chemical and thermal stabilities, superior flexibility, and durability are highly sought for use in cutting-edge water-splitting applications [34–36]. The laser molecular beam epitaxy (LMBE) was used to grow GaN nanorods (NRs) and r-GO/GaN nanocolumns heterostructures on metal foils for PEC applications [35,36]. However, the PEC performance of GaN NRs grown on thin metal substrates coupled with MoS₂ remains unexplored. In this work, an atmospheric pressure chemical vapor deposition (AP-CVD) method was used to grow MoS₂ on LMBE-grown GaN NRs/W foil. The PEC experiments revealed an enhancement in photocurrent density (approximately 172 µA/cm²) for the MoS₂/GaN NRs heterostructure, compared to GaN NRs (~70 µA/cm²). This enhancement is related to the easy separation and transportation of photogenerated charge carriers, attributed to the facile Type II band alignment between MoS₂ and GaN NRs. This type of binary heterostructure, fabricated on the metal substrate, presents a suitable approach to increase the PEC performance by using scalable and large-area CVD coupled with LMBE for large-scale roll-to-roll PEC device fabrication.

2 Method

2.1 LMBE growth of GaN nanostructure on W metal substrate

The LMBE technique (base pressure of approximately 2×10⁻¹⁰ Torr) was used to grow GaN NRs on 99.99% pure, 0.127 mm thick W metal foil at 700 °C. Initially, organic solvents (acetone and isopropyl alcohol) were used to chemically clean the W foils, then dried with pure N₂ gas. The W metal foil was then thermally cleaned for 30 min at 850 °C in the main growth chamber. Using an R.F. nitrogen plasma cell with a forward power of 400 W and a nitrogen gas (semiconductor grade) flow of 1.1 SCCM, energy-dense nitrogen plasma species were directed to nitridate the thermally cleaned W foil for 20 min at 850 °C [36,37]. Using a KrF excimer laser (λ = 248 nm, τ = 25 ns) with an energy density of about 3 J/cm² and an R.F. nitrogen plasma with a nitrogen flow rate of 0.4 SCCM at an R.F. power of 250 W, the GaN NRs were deposited on nitridated W foil at 700 °C by ablation of a 99.9999% pure solid GaN target.

2.2 CVD growth of MoS₂ film on GaN/W foil

The MoS₂ film was grown using a 3-zone CVD furnace using the phase-down method. The CVD system was equipped with two small alumina boats that held 200 mg of sulfur and 25 mg of powdered molybdenum oxide. The GaN grown on W foil and MoO₃ powder were placed in Zone 2, whereas the sulfur powder was placed in Zone 3. The temperature of sulfur powder (Zone 3) was set at 200 °C, while the growth temperature of Zone 2 (GaN NRs/W) was set at 700 °C. The growth procedure was conducted for 1 h at a constant temperature of 700 °C in an Ar gas flow of 200 SCCM. The experimental steps of growth of MoS₂/GaN NRs heterostructure on W foil are summarized in Fig. 1(a).

2.3 Materials characterization and PEC measurements

A Renishaw Raman spectrometer with an excitation laser source of 785 nm (backscattering mode) was used to reveal the structural quality of the deposited samples. The crystallinity of the fabricated samples was assessed using X-ray diffraction (XRD) with having Cu Kα X-ray source (λ = 0.154 nm). The field emission scanning electron microscopy (FESEM) was used to investigate the surface morphology of fabricated samples. To study the electronic states, X-ray photoelectron spectroscopy (XPS) was employed, having a monochromatic Al Kα X-ray source with an energy of 1486.6 eV. A He-Cd laser source with an excitation wavelength of 325 nm was used to measure the photoluminescence. The PEC experiments were performed in a 0.5 mol/L Na₂SO₄ electrolyte solution (pH = 7) under an AM of 1.5 G simulating sunlight conditions. All the PEC experiments were performed under the same conditions for GaN NRs and MoS₂-coated GaN NRs on W foils. The detailed discussion of PEC measurements was described in Singh et al. [19].

3 Results and discussion

The 45° tilt view FESEM image of GaN NRs grown on W foil using the LMBE process is displayed in Fig. S1(a) (see Electronic Supplementary Material), showing the formation of dense hexagonally-shaped GaN NRs on W foil. The diameter and length of GaN NRs were estimated statistically using high-resolution FESEM image (Fig. 1(b)) to be 50–70 nm and 150–170 nm, respectively. Figure S1(b) shows the FESEM image after the growth of MoS₂ on GaN NRs. It evidences the deposition of MoS₂, in which MoS₂ is densely interconnected with GaN NRs. The MoS₂/GaN NRs heterostructure’s high-resolution FESEM image is demonstrated in Fig. 1(c). The NRs’s diameter and length were obtained to be 90–110 nm and 200 and 220 nm, respectively. The enhanced diameter and slight increase in length of GaN NRs indicate the presence of MoS₂ on GaN NRs, resulting in changed surface features. To investigate the quality and vibrational modes of GaN NRs, Raman spectroscopy was utilized. Figure 1(d) shows the stacked Raman spectra of bare GaN NRs and the MoS₂/GaN NRs sample. The LMBE-grown GaN shows highly intense peaks corresponding to the E₂ (high) mode at a peak position of 567.2 cm⁻¹. The E₂ (high) peak for stress-free GaN is reported to be 567.6 cm⁻¹ [38]. The close proximity of the observed Raman peak to 567.6 cm⁻¹ suggests that GaN NRs are nearly stress-free [39]. When compared to the Raman data of GaN/W with MoS₂-coated GaN, the heterostructure displays a total of five vibrational modes related to MoS₂ and GaN compounds. Out of them, three peaks could be indexed for MoS₂. Two strong MoS₂ Raman peaks, representing the E¹_2g and A_1g vibrational modes, are located at 381.3 and 407.0 cm⁻¹, respectively [40]. These peaks were found to be separated by 25.7 cm⁻¹ and confirm the formation of MoS₂ with an estimated thickness of 50–70 nm [41]. Raman mode separation between E¹_2g and A_1g in the range of 25.4 to 25.9 cm⁻¹ is indicative of bulk MoS₂, with an estimated thickness of approximately 50–70 nm as reported by Huang et al. [42]. The combination of the 3D architecture of vertically aligned GaN nanorods and the polycrystalline nature of the W metal foil could influence the subsequent CVD growth of MoS₂, promoting the formation of relatively thicker bulk MoS₂ (approximately 50–70 nm) thin film. The Raman data of MoS₂ on bare W foil is shown in supplementary Fig. S2(a), which further shows the formation of bulk MoS₂. The Raman spectrum of MoS₂/GaN shows that the intensity of the GaN E₂ (high) mode is significantly reduced and slightly shifted toward a lower wavenumber of 565.1 cm⁻¹ compared to bare GaN NRs (567.2 cm⁻¹) [42]. The spectral shift indicates that GaN NRs possess a tensile stress after deposition of MoS₂ on GaN NRs. To study the crystallinity and crystal structure of GaN NRs and MoS₂/GaN NRs samples, the stacked XRD plot is depicted in Fig. S1(c). The lower part of the figure shows the strongest peak at a position of 34.5° that could be indexed for the GaN (002) plane, while a small peak at 63.6° position is related to the (103) lattice planes of GaN [43,44]. The observed XRD peaks of GaN are related to the hexagonal wurtzite crystal structure (JCPDS card No. 50-0792) [45,46]. The remaining three peaks at 2θ values of 40.3, 58.6 and 73.6° correspond to (110), (200), and (211) lattice planes of W foil substrate, respectively [47]. The upper section of Fig. S1(c), the MoS₂/GaN NRs depict several XRD peaks corresponding to the MoS₂ film. The diffracted peaks situated at 2θ values of 14.4, 29.2, 49.4, and 53.6° are related to 2H hexagonal phase of MoS₂ (002), (004), (105), and (106) planes, respectively (JCPDS card No. 37-1492) [48]. The remaining other peaks are assigned to lattice planes of GaN and W foil. The (002) plane of GaN is shifted toward a lower angle (from 34.5 to 33.0°) when compared with bare samples, which typically arises due to lattice mismatch or interfacial stress during heterostructure formation.

The surface chemistry of GaN NRs and MoS₂/GaN NRs samples was studied using the XPS technique. Figure S3(a) displays the results of the XPS survey scan of GaN/W foil and verifies the presence of Ga, N, O, and C elements at the surface of GaN NRs. The high-resolution core-level spectrum of Mo 3d is shown in Fig. 2(a). As presented, three peaks were identified in the high-resolution Mo 3d spectrum and were found to be at binding energies of 226.4, 229.2, and 232.4 eV. Two peaks that correspond to the bonding between Mo and S are found at 229.2 and 232.4 eV. The Mo 3d_5/2 and Mo 3d_3/2 spin-orbit coupled peaks, respectively, are dispersed to these peaks. The peak at 226.4 eV is attributed to S 2s, which confirms the Mo-S interaction and suggests the successful bonding with GaN NRs. The S 2p spectrum (Fig. 2(b)) shows two peaks at 162.1 and 163.3 eV, corresponding to the S 2p_3/2 and S 2p_1/2 electronic states, respectively. The XPS data of bare MoS₂ is depicted in supplementary Figs. S2(b) and S2(c), which further confirms the chemical composition of MoS₂ on W foil [48,49]. The comparison of core-level spectra of Ga 2p of GaN NRs and the MoS₂/GaN NRs spectrum is presented in Fig. 2(c). The GaN NRs display two predominant peaks at binding energies of 1117.9 and 1144.7 eV. These peaks originate from Ga−N bonding and are distributed to Ga 2p_3/2 and Ga 2p_1/2 spin-orbit coupled peaks, respectively [46]. The binding energy peaks were found to be at 1117.7 (Ga 2p_3/2) and 1144.6 eV (Ga 2p_1/2) for Ga 2p of MoS₂/GaN NRs sample. The peaks are slightly shifted toward the lower binding energy region after deposition of MoS₂, suggesting a change in the chemical environment of Ga atoms. Figure 2(d) shows the deconvoluted XPS spectrum of the N 1s photoelectron of both samples. The prominent peak centered at 398.5 eV position comes from Ga−N bonding. The additional peak at approximately 400 eV originates from surface-adsorbed nitrogen species or nitrogen interstitials, which could serve as trap states and potentially affect the electronic properties of GaN [35,45]. The remaining three peaks at binding energy positions of 394.6, 396.1, and 397.0 eV could be indexed for the Ga Auger signal, further supporting the successful growth of GaN with well-defined chemical states [46]. Similarly, in the MoS₂/GaN NRs heterostructure, the N 1s core-level spectrum shows multiple binding energy peaks. The prominent peak at 398.3 eV, associated with Ga-N bonding, is slightly shifted toward a lower binding energy compared to bare GaN. This shift indicates electronic interactions between MoS₂ and GaN. Figure S3(b) displays the valence band maxima (VBM) spectra of GaN NRs. The VBM position was determined by linearly extrapolating the valence band spectrum’s low binding energy edge. In relation to the surface Fermi level, the VBM value of GaN was determined to be 3.26 eV, indicating the n-type nature of GaN.

The PEC performance was evaluated for GaN and MoS₂/GaN photoelectrodes in 0.5 mol/L Na₂SO₄ electrolyte under 100 mW/cm² of simulated sunlight. The schematic representation of the three-electrode PEC system is displayed in Fig. 3(a). The open circuit potential (OCP) measurement was performed under chopped light conditions of the fabricated photoelectrodes (Fig. 3(b)). The GaN and MoS₂/GaN NRs photoelectrodes exhibited n-type conductivity, as evidenced by the observed negative change in OCP values. This shift indicates the generation of photogenerated charge carriers upon light exposure. The change in ΔV_OCP values for GaN and MoS₂/GaN was calculated to be 0.204 and 0.192 V vs. Ag/AgCl, respectively. Generally, the higher ΔV_OCP is related to higher surface potential, which suppresses the surface charge recombination [50,51]. The lower ΔV_OCP observed for MoS₂/GaN suggests that the bulk MoS_2, which generally exhibits weaker surface potential modulation, could introduce additional surface states or trap-assisted recombination centers that modify the surface potential [52]. The linear sweep voltammetry (LSV) profiles of the fabricated photoelectrodes in both dark and light environments are shown in Fig. 3(c). The materials’ ability to effectively use light for water splitting is demonstrated by the increase in current density in light conditions in comparison to the dark current. The MoS₂/GaN NRs heterostructure demonstrates a higher photocurrent under illumination than GaN, indicating that the introduction of the MoS₂ creates additional active sites to create a large number of electron-hole pairs, increasing the PEC performance. The onset potential (V_on) for water oxidation is determined from the corresponding LSV plot by identifying the potential at which the dark current density intersects with the tangent drawn at the point of maximum slope in the photocurrent density curve [53]. The onset potentials of −0.275 and −0.267 V vs. Ag/AgCl for GaN and MoS₂/GaN, respectively, indicate a more positive shift in V_on for the heterostructure. This anodic shift suggests that MoS₂/GaN exhibits slightly slower oxidation kinetics, likely due to the presence of defects or surface states at the photoelectrode surface [53]. In Fig. 3(d), the applied bias to photon current conversion efficiency (ABPE) was calculated according to previous studies [19,34,54]. The ABPE values were found to be 0.021 and 0.043% at 0.75 V vs. RHE for GaN and MoS₂/GaN, respectively. The higher ABPE value for MoS₂/GaN indicates that the heterostructure is more efficient in utilizing the applied voltage to generate photocurrent.

The LSV measurements under chopped illumination, as presented in Fig. 4(a), demonstrate an enhancement in photocurrent density for the MoS₂/GaN heterostructure compared to GaN. This improvement indicates the stability and effectiveness of MoS₂ decoration in boosting the PEC performance of GaN NRs, which is analogous to transient (I‒t) photocurrent measurements. Figure 4(b) depicts the I‒t data of GaN NRs for 30 s ON−OFF cycles at 0.2, 0.4, and 0.6 V vs. Ag/AgCl. A photocurrent density of approximately 70 µA/cm² is observed for GaN NRs at 0.6 V vs. Ag/AgCl. This is comparable to the previously documented photocurrent density of approximately 60 µA/cm² (0.8 V vs. Ag/AgCl) for GaN nanocolumns on Nb foil [35]. Figure 4(c) displays the I‒t data of MoS₂ decorated GaN NRs at 0.2, 0.4, and 0.6 V vs. Ag/AgCl. The MoS₂/GaN NRs heterostructure recorded the improved photocurrent density of approximately 172 µA/cm² at 0.6 V vs. Ag/AgCl. After the deposition of r-GO onto GaN nanocolumns, the photocurrent density was enhanced approximately 1.9-fold compared to GaN nanocolumns [35]. However, the deposition of MoS₂ onto GaN NRs showed a more pronounced improvement in the photocurrent density of approximately 2.5 and 3.4-fold compared to bare GaN and MoS₂ (~50 µA/cm²) (Fig. S4(a)), respectively. The improved photocurrent density of the MoS₂/GaN NRs heterostructure is attributed to the absorption of the wide spectrum of light, and improved charge separation and migration due to the suitable band alignment between MoS₂ and GaN. The photocurrent density of various MoS₂-based heterostructures is presented in Table 1, which suggests the effectiveness of MoS₂ in heterostructures to improve the PEC efficiency.

The electrode-electrolyte properties of photoelectrodes were accessed with the help of EIS measurements. The Nyquist plot shows that the radius of the arc and is directly related to the interfacial charge transfer resistance (R_ct). The lower arc of radius results in minimal interfacial resistance at the interface. From the plot, it is evident that the MoS₂/GaN NRs heterostructure has a lower arc of radius, which ensures reduced interfacial resistance for electron transport. Furthermore, the EIS data were fitted using the equivalent circuit (Randles circuit) shown in the inset of Fig. 4(d). The charge transfer resistance is represented by the R_ct, and the electrolyte resistance and double layer capacitance are represented by R_s and C_dl, respectively, in the Randles circuit. From the fitted data, the R_ct values were obtained to be 934.2 and 735.6 Ω for GaN and MoS₂/GaN, respectively. The lower R_ct value represents the improved conductivity and catalytic activity of the heterostructure interface [34,43,49]. Additionally, EIS of MoS₂/GaN was conducted in 0.5 mol/L H₂SO₄ to evaluate stability and variations in charge transfer resistance. The MoS₂/GaN heterostructure shows the increased charge transfer resistance (850.6 Ω) in 0.5 mol/L H₂SO_4, suggesting that the interface is more resistive in alkaline conditions, possibly due to surface modifications or changes in interfacial charge dynamics. However, the presence of a stable impedance response across different electrolytes demonstrates its electrochemical stability despite variations in electrolytes.

Figure 5(a) shows the Mott-Schottky (M-S) graph of GaN and MoS₂/GaN NRs samples. The positive slope of both photoelectrodes indicates the n-type semiconducting nature [19,55]. By extrapolating the intercept on the X-axis of the M-S plot, the flat band potential (V_FB) for the GaN NRs and MoS₂/GaN NRs heterostructures was found to be −0.65 and −0.77 V vs. Ag/AgCl, respectively. The cathodic shift in the V_FB position indicates the better reduction capability of the MoS₂/GaN heterostructure [27]. The donor density (N_D) was estimated by extracting the slope from a linear fitted M-S plot. The N_D values were obtained to be 4.46×10¹⁷ and 6.28×10¹⁹ cm⁻³ for bare GaN and MoS₂/GaN, respectively, indicating the improved charge carrier concentration at the interface, hence resulting in improved PEC performance [19,56–58]. Figure 5(b) presents the photoluminescence (PL) spectra of both samples with a strong peak around approximately 361 nm (~3.4 eV), which corresponds to the near band-edge emission of GaN with a defect level peak in the 470–500 nm region. The reduction in PL intensity upon the introduction of MoS₂ suggests PL quenching, which is typically associated with the decrement in recombination events in MoS₂/GaN, indicating that MoS₂ facilitates electron transfer [56]. In Fig. 5(c), the bi-exponentially fitted time-resolved photoluminescence (TRPL) analysis was further employed to investigate the electron lifetime and recombination dynamics. The extracted lifetime components were found to be τ₁ = 0.42 ns, τ₂ = 8.98 ns for GaN and τ₁ = 0.34 ns, τ₂ = 5.53 ns for MoS₂/GaN. Herein, τ₁ corresponds to the nonradiative recombination lifetime, while τ₂ represents the radiative recombination lifetime and plays a crucial role in understanding the charge carrier transport behavior in semiconductor heterostructures. The observed reduction in the τ₂ value for the MoS₂/GaN heterostructure compared to GaN suggests an accelerated charge carrier extraction behavior, which is analogous to previous studies [19,59]. The stability of fabricated GaN and MoS₂/GaN photoelectrodes was investigated in 0.5 mol/L Na₂SO₄ electrolyte up to 30 min at 0.6 V vs. Ag/AgCl (Fig. 5(d)). Both samples, as well as MoS₂/W foil (Fig. S4 (b)) demonstrate excellent stability over the test duration. The sustained photocurrent response indicates that the photoelectrodes retain a significant level of activity, highlighting their potential for long-term PEC applications.

To understand the charge separation and extraction mechanism of the MoS₂/GaN heterostructure, the energy band alignment is presented in Fig. 6. The band gap of the bulk MoS₂ is approximately 1.3 eV, while the VBM is situated at roughly 1.1 eV in relation to the Fermi level [21,60]. The GaN is a wide band gap material with a band gap of 3.4 eV with VBM value of GaN/W is 3.26 eV (Fig. S3(b)) as per reported in Refs. [61–63]. The conduction band (CB) position was approximated using the formula E_{\mathrm{C}\mathrm{B}}=E_{\mathrm{V}\mathrm{B}}-E_{\mathrm{g}}, while taking these variables into account. The CB position was estimated to be −0.2 and −0.14 eV for MoS₂ and GaN, respectively. As illustrated in Fig. 6, a Type II heterostructure is formed between MoS₂ and GaN based on CB and VB positions. Due to the more negative shift in CB position, electrons will easily jump from the CB of MoS₂ to the CB of GaN, and later on transfer to the counter electrode through the external bias, where they help in the reduction process. Conversely, the more positive shift in the VB position of GaN will encourage the photogenerated holes to the flow toward the VB of MoS₂, contributing to the oxidation process at the photoanode surface. As a result, charge separation is made easier in Type II heterostructure, which eventually leads to better PEC performance.

4 Conclusions

In this study, the MoS₂/GaN NRs heterostructure was successfully fabricated by growth of MoS₂ using CVD on LMBE grown GaN NRs on W metal foil, which was confirmed by Raman and XPS spectroscopy. The MoS₂/GaN NRs heterostructure exhibited enhanced photocurrent density of approximately 172 µA/cm² compared to the GaN NRs (~70 µA/cm²). This increment in photocurrent density is attributed to the increased number of active sites provided by MoS₂, improved donor density, and minimized charge recombination due to the Type II band alignment of the MoS₂/GaN heterostructure. These results highlight the potential of the MoS₂/GaN NRs heterostructure on W foil for enhancing the PEC water splitting performance. Additionally, the feasibility of growing such binary nanostructures on metal foils suggests a promising pathway for developing scalable and roll-to-roll PEC devices by further improving PEC efficiency.

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