Sb₂Se₃/CdSe thin film photocathode for efficient hydrogen production

Abstract

A highly efficient narrow-band-gap photoelectrode plays a vital role in achieving solar-to-hydrogen conversion. In this work, we report a novel Sb₂Se₃/CdSe/Pt photocathode, and the activity of this photocathode was studied. The photocathode was fabricated via electrodeposition followed by annealing and surface modification with a CdSe monolayer and Pt cocatalyst. The photocurrent density of the Sb₂Se₃/CdSe/Pt photocathode is ca. −6.9 mA cm⁻² at −0.2 V_RHE in near-neutral pH buffered solution, which is 2.3 times that of the Sb₂Se₃/Pt photoelectrode. The enhanced photoelectrochemical (PEC) performance is owing to the successful construction of the Sb₂Se₃/CdSe p–n heterojunction, which contributes to effective charge separation and transfer. Additionally, the effect of the Sb₂Se₃ layer thickness on the photocurrent was investigated, which suggests that an appropriate thickness of Sb₂Se₃ layer is necessary for sufficient light absorption and efficient hole transport. This work develops a novel Sb₂Se₃-based p–n heterojunction photocathode using a facile fabrication approach, which provides a practical pathway for developing cost-effective PEC devices with wide solar spectrum utilization.

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1. Introduction

Photoelectrochemical hydrogen production via water splitting is one of the most promising and challenging techniques for sustainable solar energy storage,^1,2 and utilizes semiconductor materials as photoelectrodes, enabling the direct conversion of solar energy into storable hydrogen. Sb₂Se₃, as a low-band-gap (1.2–1.0 eV) semiconductor, has been attracting tremendous interest due to its exceptional optoelectronic properties and material advantages. The narrow band gap and high absorption coefficient (∼10⁵ cm⁻¹ near the absorption edge) of Sb₂Se₃ enable broad and efficient solar harvesting.^3–5 Furthermore, Sb₂Se₃ has economic and environmental benefits, including earth abundance, low cost comparable to copper, and non-toxicity. Crucially, the Sb–Se phase diagram contains only one binary compound, namely orthogonal Sb₂Se₃,⁶ which makes it easier to control the phase and defects compared with multinary chalcogenides like Cu(In,Ga)(S,Se)₂ and Cu₂ZnSn(S,Se)₄.^7,8 In addition, the high electron mobility (16.9 cm² V⁻¹ S⁻¹) of Sb₂Se₃ (ref. 9) is conducive to charge transport in thick films, simultaneously minimizing recombination losses and maximizing light absorption.

The performance of the Sb₂Se₃ photocathode is significantly influenced by the fabrication method, and there are multiple approaches available for preparing Sb₂Se₃ photocathodes. The photocurrent density of the Sb₂Se₃ film prepared via solution-based approaches can reach −13 mA cm⁻² at 0 V_RHE, but this method depends on dissolving the Sb source and the Se source in toxic organic solvents, such as hydrazine hydrate, TGA and 2-mercaptoethanol, and the uniformity in the photocathode films is also an issue for scaling up the method.^10–12 Besides, the low melting point of Sb₂Se₃ (ca. 600 °C) and its high saturated vapor pressure (ca. 1200 Pa at 550 °C)⁶ enable Sb₂Se₃ thin-film deposition via thermal evaporation technology.^13,14 However, the quality of film by thermal deposition technology is easily affected by the distance between the evaporation source and the substrate, the temperature of the substrate, and the interface properties, which are difficult to control.^15,16 Among the various film fabrication techniques, the electrodeposition method is the most promising in terms of scalability and operability.¹⁷ By varying solution composition and electrodeposition parameters, the morphology and thickness can be tuned under ambient conditions, making this approach economical and practical. Despite these advantages, minimal efforts have been made to synthesize Sb₂Se₃ photocathode by the electrodeposition method and use them for photoelectrochemical applications.

To improve the PEC water splitting performance, typical n-type semiconductors (e.g., CdS,¹⁴ In₂S₃,¹⁸ TiO₂ (ref. 19)) are deposited onto p-type substrates as a buffer layer, thus establishing p–n junctions to enhance charge separation and suppress charge recombination. Among the n-type semiconductors, CdSe stands out due to its favorable band structure alignment and superior charge transport properties,²⁰ making it a promising candidate for PEC applications.^21,22 Consequently, engineering an Sb₂Se₃/CdSe heterojunction photocathode represents an effective strategy for achieving high-performance PEC water splitting.

Based on the above considerations, Sb₂Se₃-based thin-film photocathode for PEC water splitting was fabricated via a three-step process: electrodeposition in an alkaline selenosulfate electrolyte, followed by appropriate annealing, and rational surface engineering with a CdSe layer and Pt catalyst. A photocurrent density of 6.9 mA cm⁻² has been achieved for Sb₂Se₃/CdSe/Pt photocathode at −0.2 V_RHE in a near-neutral electrolyte. The influence of the Sb₂Se₃ layer thickness on the PEC performance is analyzed, which suggests that an appropriate thickness of Sb₂Se₃ layer is necessary for sufficient light absorption and efficient holes transport. The novel Sb₂Se₃-based p–n heterojunction photocathode with efficient PEC performance and scalable fabrication method demonstrates the great potential of developing cost-effective PEC devices with wide solar-spectrum utilization for H₂ evolution.

2 Experimental

2.1 Fabrication of Sb₂Se₃ film

The electrolyte consists of an ammonia buffer aqueous solution (0.1 M, pH = 9.0) containing sodium selenosulfate and antimony complex. Sodium selenosulfate solution was prepared via a modified literature method.²⁰ Briefly, Se powder(0.789 g), Na₂SO₃(3.15 g) and deionized water (50 mL) were added to a three-necked round bottom flask, purged with N₂ for 20 minutes, then refluxed at 70 °C for 10 hours to obtain Na₂SeSO₃ solution. Meanwhile, C₈H₄K₂O₁₂Sb₂·3H₂O (1.25 mM) was dissolved separately in the ammonium buffer solution. The final electrolyte was obtained by mixing both solutions at a volume ratio of 7 : 3, followed by Ar purging for 15 minutes. Electrochemical deposition was conducted using a CHI 760 D electrochemical workstation in a three-electrode configuration, with commercial FTO-coated glass, Pt foil, and saturated calomel electrode (SCE) as the working electrode, the counter electrode, and the reference electrode, respectively. Potentiostatic electrodeposition at −0.94 V_SCE is carried out,²¹ with charge density passed ranging from 0.125–1.0C cm⁻². Post-deposition annealing treatments were conducted at 200 °C–300 °C for 20 min using a rapid heating furnace.

2.2 Chemical bath deposition (CBD) of CdSe

In a separate beaker, KOH(0.505 g) was added into 100 mL deionized water, then nitrilotriacetic acid (0.57 g) was added into the above solution until all KOH is dissolved. Subsequently, Cd(NO₃)₂·4H₂O(0.617 g) was added into the above solution with magnetic stirring for 20 min. The Sb₂Se₃/FTO was placed into the above transparent solution and leaned against the wall of the beaker, followed by blowing N₂ for 20 minutes and keeping it in a 70 °C water bath for 10 minutes. Finally, 10 mL of Na₂SeSO₃ solution was quickly transferred to the above solution and then kept for 1 hour. The obtained samples were washed with DI water and dried in vacuum.

2.3 Photo-assisted electrodeposition of Pt

Pt nanoparticles were deposited by photo-assisted electrodeposition method in a solution containing 0.1 mM H₂PtCl₆·6H₂O (99.9% metals basis) and 0.5 M Na₂SO₄. Each deposition was performed at a fixed potential of 0 V_RHE under simulated sunlight (AM 1.5 G) from a 300 W Xe lamp solar simulator with magnetic stirring.

2.4 Photoelectrochemical measurements

The PEC measurements were performed with a conventional three-electrode configuration in phosphate buffered solution (pH = 6.5, containing 0.5 M Na₂SO₄, 0.25 M Na₂HPO₄/NaH₂PO₄) under AM 1.5 G sunlight illumination. The light intensity at the electrode is controlled to 100 mW cm⁻². The measured potential (V_Ag/AgCl) could convert to reversible hydrogen electrode potential (V_RHE) using the Nernst equation: V_RHE = V_Ag/AgCl + 0.059 pH + 0.197. The Mott–Schottky plots were acquired at a frequency of 2 KHz in the darkness. The electrochemical impedance spectroscopy (EIS) tests were performed at 0 V_RHE within a frequency of 1 Hz to 100 KHz.

2.5 Materials characterizations

X-ray diffraction (XRD) was conducted to analyze the crystallographic structure of the samples using an X-ray diffractometer with Cu Kα radiation. Scanning electron microscope (JEOL JSM 7800 FE) equipped with energy dispersive X-ray spectroscope (EDX) was employed to observe the morphology and elemental distribution of the photoelectrode. Optical absorption properties of the films were measured with a spectrophotometer (Cary 5000, Agilent). The optical absorption (A) of the films was calculated using the formula: A = 100−T−R, where T denotes transmittance and R corresponds to reflectance.

3. Results and discussion

3.1 Fabrication and surface modification of Sb₂Se₃ thin films

As shown in Fig. 1a, the Sb₂Se₃ film was grown onto a FTO coated glass substrate by facile electrodeposition, which was then annealed to obtain a high-quality polycrystalline thin film. Then, the surface of the annealed-Sb₂Se₃ thin film was modified with n-type CdSe via CBD to form a p–n junction. The deposited Sb₂Se₃ (Fig. 1c), 250 °C annealed Sb₂Se₃ (Fig. 1d) and CdSe-modified 250 °C annealed Sb₂Se₃ (Fig. 1e) exhibits macroscopic homogeneity. XRD patterns were obtained to analyze the phase composition and structure. As shown in Fig. S1, the deposited Sb₂Se₃ film exhibits no obvious diffraction peaks apart from those originating from the FTO substrate, implying the amorphous structure for the deposited Sb₂Se₃ films. In contrast, all annealed Sb₂Se₃ films display sharp diffraction peaks matching the orthorhombic Sb₂Se₃ (PDF No. 15-0861),²² demonstrating that thermal annealing is essential for inducing the structural transformation from disordered amorphous state to well-ordered crystalline phase. Annealed Sb₂Se₃ film is comprised of densely packed spherical particles with an average size of ca. 100 nm (Fig. 1h). After modification with CdSe, a uniform CdSe overlayer is formed on the surface (Fig. 1i). XRD patterns for Sb₂Se₃/CdSe are presented in Fig. S2. It is observed that the new peaks occurred at 25.3°, 42.2°, and 49.7°, corresponding to the (111), (220), and (311) planes of cubic CdSe.²³ Fig. S3 shows that CdSe particles with an average particle size of 70 nm are uniformly distributed on FTO. The EDS elemental mappings further verify the homogeneous distribution of Sb, Se and Cd elements across the film (Fig. S4). The cross-sectional SEM image reveals that the deposited Sb₂Se₃ layer with a thickness of ca. 140 nm covers the FTO substrate (Fig. 1j). For the CdSe-modified Sb₂Se₃ film (Fig. 1k), a CdSe overlayer with a thickness of ca. 20 nm coats the underlying Sb₂Se₃ film.

Fig. 1 (a) Schematic diagram of fabrication process for Sb₂Se₃/CdSe. (b–e) Optical images of the FTO substrate, deposited Sb₂Se₃, 250 °C annealed Sb₂Se₃, and Sb₂Se₃/CdSe. (f–i) Top-view SEM images of the FTO substrate, deposited Sb₂Se₃, 250 °C annealed Sb₂Se₃, and Sb₂Se₃/CdSe. (j and k) Cross-sectional SEM images of the Sb₂Se₃ and Sb₂Se₃/CdSe.

3.2 Effect of calcination temperature on PEC properties of Sb₂Se₃-Based electrodes

The PEC performance of Sb₂Se₃-based electrodes was investigated under simulated sunlight (AM 1.5 G). As shown in Fig. 2a, the bare p-type Sb₂Se₃ electrodes annealed at 200–300 °C displays low cathodic photocurrents of only tens of µA cm⁻². The poor PEC activity of unmodified p-type Sb₂Se₃ photoelectrodes implies the necessity of further enhancement through rational interface engineering. After the surface decoration of Pt nanoparticles (Fig. 2b), the photocurrent densities are significantly enhanced. Particularly, the Sb₂Se₃/Pt photocathode annealed at 250 °C achieves the highest photocurrent density (J_ph) of ca. −3.0 mA cm⁻² at −0.2 V_RHE. The charge transfer rate during the photoelectrochemical reaction was explored by EIS measurements. As shown in Fig. 2c, the Nyquist curves of Sb₂Se₃/Pt show an approximate semicircle, which are fitted using an R (CR) circuit (see inset of the Fig. 2c). R_s, R_ct, CPE₁ correspond to solution resistance, charge transfer resistance and capacitance. It is observed that 250 °C annealed Sb₂Se₃/Pt has the lower R_ct (1904 Ω cm²) under illumination conditions than the R_ct of 200 °C and 300 °C annealed sample (2714 and 2506 Ω cm²), indicating the effective charge transfer and separation at the electrode–electrolyte interface on the Sb₂Se₃/Pt electrode annealed at 250 °C.

Fig. 2 LSV plots of (a) bare Sb₂Se₃ and (b) Sb₂Se₃/Pt photocathodes. The Sb₂Se₃ samples were calcined at 200, 250 and 300 °C. (c) Nyquist plots of Sb₂Se₃/Pt electrodes conducted at 0 V_RHE under illuminated conditions and (d) Mott–Schottery plots of Sb₂Se₃ under darkness condition.

Mott–Schottky measurements were carried out to determine the carrier density (N_d). N_d can be calculated from the M–S slope using the following equation:

where e₀, ε₀ and ε are the charge of an electron, vacuum permittivity and the relative dielectric constant, respectively. is the slope of the Mott–Schottky plot.^24,25 Assuming equivalent ε for all the samples, a larger Mott–Schottky slope corresponds to a lower carrier density. Relevant parameters are summarized in Table S1. As shown in Fig. 2d, the slope of the M–S plots for the sample annealed at 250 °C reaches the minimum, leading to the higher density of trapped electrons, which can be explained by the XRD and SEM results. The lower annealed temperature (200 °C) induces a lower crystallinity (Fig. S5), thus reducing the electron mobility. Conversely, the excessively high annealing temperature can result in the volatilization of Se, thus generating numerous Se vacancies. These vacancies serve as recombination centers that can trap the photogenerated carriers, ultimately deteriorating the performance of PEC device, which is consistent with observations in previously reported Ar-annealed Sb₂Se₃ thin-film solar cells.¹³ As shown in Fig. S5c and f, Sb₂Se₃ film annealed at 300 °C exhibits obvious pores on the surface, indicating that vaporization occurs at the temperature, which results in the decomposition of performance. Based on the transmittance spectrum of 250 °C annealed Sb₂Se₃ (Fig. S6), the direct band gap is determined to be 1.33 eV.

Table 1 compares the performance of Sb₂Se₃ thin films prepared by different fabrication methods including electrodeposition, thermal evaporation and spin coating. Sb₂Se₃ film prepared by electrodeposition achieved a photocurrent of 30 µA cm⁻², which is comparable with that obtained by other methods. Therefore, Sb₂Se₃ thin films produced by electrodeposition along with a moderate annealing offers a competitive balance of PEC performance, operability, and cost-effectiveness. However, further enhancement on the photocurrent is required to catch up the state-of-the-art photocathodes. To address this limitation, CdSe layer is introduced to form a beneficial p–n heterojunction with Sb₂Se₃, as discussed in the following section.

Table 1 The comparison of PEC performance of this work with other Sb₂Se₃ electrodes

Photoelectrodes	Preparation method	Test condition	Photocurrent at 0 VRHE/µA cm^−2	Reference
Sb2Se3	Electrodeposition	Nea-neutral electrolyte	30	This work
Sb2Se3	Thermal evaporation	Nea-neutral electrolyte	∼2	22
Sb2Se3	Spin coating	0.5 M H2SO4 electrolyte	∼90	26

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3.3 Effect of CdSe on PEC properties of Sb₂Se₃-Based electrodes

To explore the effect of modified CdSe layer on the PEC performance, the LSV measurements were examined. As shown in Fig. 3a, the Sb₂Se₃/CdSe/Pt photocathode generates superior PEC performance, achieving a high J_ph of −6.9 mA cm⁻² at −0.2 V_RHE and a large onset potential (V_on) of 0.004 V_RHE (defined as the voltage at which the J_ph reaching −0.5 mA cm⁻²). In a sharp comparison, the bare Sb₂Se₃ electrode showed a very weak cathodic current (tens of µA cm⁻²) (Fig. 1a) due to severe charge recombination and sluggish HER kinetics. Although depositing Pt as a cocatalyst, the J_ph only reached −3.0 mA cm⁻² at −0.2 V_RHE for Sb₂Se₃/Pt photocathode, with a less positive V_on of −0.1 V_RHE. Fig. S7 displays statistical distribution of J_ph and V_on for Sb₂Se₃/Pt and Sb₂Se₃/CdSe/Pt photoelectrodes, proving satisfactory reproducibility. To further elucidate the underlying mechanism of charge transfer and separation, transient photocurrent measurements were carried out. As shown in Fig. 3b, both two J–T curves show an observable ‘spike-like’ transient, i.e., a rapid initial increase in photocurrent upon illumination, followed by a decay to a steady-state value, meaning much electrons being trapped. According to the proportion of the steady-state photocurrent (J_ss) and the instantaneous photocurrent (J_ss), the charge transfer efficiency (η_tran) values of Sb₂Se₃/Pt and Sb₂Se₃/CdSe/Pt devices were determined to be 12.3% and 44.1%, respectively. Based on the light-harvesting efficiency (Fig. S8) derived from wavelength-dependent absorbance (Fig. 3c) and AM 1.5 G solar spectrum (Fig. S9), the calculated η_sep value for Sb₂Se₃/CdSe/Pt device (6.2%) is higher than Sb₂Se₃/Pt (4.1%).^27,28 Fig. S9 illustrates that the theoretical photocurrent density (J_abs) values for the two photocathodes are very close, being 18.8 mA cm⁻². Notably, the experimentally obtained J_ph of 0.5 mA cm⁻² is much lower than its corresponding J_abs value of 18.8 mA cm⁻², which is owing to the limited carrier dynamics.

Fig. 3 (a) LSV curves of bare Sb₂Se₃, Sb₂Se₃/Pt and Sb₂Se₃/CdSe/Pt photoelectrodes. (b) Transient photocurrent response and (c) absorption spectra for the as-obtained samples. (d) Nyquist plots, and (e) the corresponding Bode plots of Sb₂Se₃/Pt and Sb₂Se₃/CdSe/Pt obtained at 0 V_RHE under illumination. (f) Mott–Schottery plots of different photocathodes without illumination.

To elucidate the interface charge transfer process, EIS measurements were performed on the Sb₂Se₃/CdSe photoelectrode. As shown in Fig. 3d, the Nyquist plot of Sb₂Se₃/CdSe/Pt photoelectrode shows an approximate semicircle and a straight line. Considering the additional interface of CdSe, R (CR)(CR)W circuit are used to simulate the Nyquist plot of Sb₂Se₃/CdSe/Pt. The fitting values of all the electrochemical parameters are summarized in Table 2. A remarkably decreased R_ct value of 224.0 Ω is observed for Sb₂Se₃/CdSe/Pt electrode as against 1904 Ω for Sb₂Se₃/Pt. Thus, CdSe coating could facilitate the charge transfer of Sb₂Se₃ layer, thereby improving the photocurrent response. Generally, a lower characteristic peak frequency in Bode phase plots indicates a longer electron lifetime.^29,30 As shown in the Fig. 3e, Sb₂Se₃/CdSe/Pt photocathode exhibits a lower characteristic peak frequency (1.8 Hz) than the Sb₂Se₃/Pt photocathode (21.5 Hz), indicating the longer electron lifetime and, therefore, lower carrier recombination rate.

Table 2 Electrochemical parameters of photoelectrodes obtained from EIS study

Samples/parameters	Rs/Ω	Rct/Ω	CPE1/µF cm^−2	R1/Ω	CPE2/µF cm^−2
Sb2Se3/Pt	70.24	1904	55.78	—	—
Sb2Se3/CdSe/Pt	59.26	224.0	20.61	10.84	11.83

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Moreover, Mott–Schottky analysis was conducted on Sb₂Se₃/Pt and Sb₂Se₃/CdSe/Pt electrodes to explore the interface energetics. The negative slope of the Mott–Schottky plots for both samples confirms the p-type semiconductor character of Sb₂Se₃ (Fig. 3f). Besides, carrier concentration increases significantly for Sb₂Se₃/CdSe/Pt electrode, compared to Sb₂Se₃/Pt (Table S2), implying the enhanced charge transport due to the formation of Sb₂Se₃/CdSe heterojunction.³¹ The flat band potential (E_fb) can be determined by the x-intercept of the linear region of the Mott–Schottky plots. The Sb₂Se₃/CdSe/Pt photoelectrode exhibits an E_fb value of 0.75 V_RHE, which is positively shifted by 150 mV relative to that of the Sb₂Se₃/Pt photocathode (0.6 V_RHE). This positive shift in E_fb agrees with the anodic shift in onset potentials (Fig. 3a), demonstrating enhanced driving force for interfacial charge separation and transfer.³²

The stability was evaluated by a long-time current–time test. The photocurrent of Sb₂Se₃/Pt and Sb₂Se₃/CdSe/Pt photocathodes decreased gradually from −0.11 to 0.06 mA cm⁻² and −0.56 to −0.41 mA cm⁻² respectively with a rapid decay in the first few minutes (Fig. S10). The photocurrent retained 73.2% of the initial value for Sb₂Se₃/CdSe/Pt photocathode after one hour of testing, showing a better photostability. XPS measurements were performed to probe the chemical species and chemical bonding state. By comparing the Sb 3d spectra (Fig. S11a) of the electrodes before and after testing, it can be seen that the peak area corresponding to O 1s increases remarkably, which is likely related to the precipitation of Sb₂O₃ on the film.³³ The Se 3d signal of the sample (Fig. S11b) before stability test can be fitted into two pairs of typical peaks, indicating the presence of Se²⁻and Se⁰ species.³⁴ After testing, the Se⁰ species disappeared and the peak area ratio of Se²⁻ reduced, showing that the loss of Se element occurs during testing. The peaks at 405.4 eV and 412.2 eV represent the Cd 3d_5/2 and Cd 3d_3/2 of Cd²⁺ (Fig. S11c), respectively, which are positively shifted after the stability test. These results indicate that the Se species is not stable, which results in the decay of photocurrent.

To investigate the effect of Sb₂Se₃/CdSe heterojunction on charge separation, the band structure of the Sb₂Se₃ and CdSe is carefully proposed. Before contact, the Fermi level of Sb₂Se₃ is near the valence band (VB) maximum. Based on the E_fb (Fig. 2d) and bandgap energy, the Fermi level, VB and conduction band (CB) are determined to be 5.08, 5.28 and 3.94 eV, respectively. According to the previous reports, CdSe behaves as a typical n-type semiconductor with the Fermi level, CB and VB positioned at 4.47, 4.35 and 6.07 eV,³⁵ respectively (Fig. 4a). After forming p–n heterojunction, the Fermi levels of Sb₂Se₃ and CdSe equilibrate (Fig. 4b). Under illumination, photoexcited electrons in Sb₂Se₃ migrate to the CB of CdSe, while holes from CdSe are promptly captured by the VB of Sb₂Se₃ (Fig. 4c). The accumulated electrons at CdSe are then channeled to Pt cocatalyst for H₂ production, simultaneously suppressing the recombination of carriers and enhancing interfacial charge transfer, ultimately boosting PEC activity.

Fig. 4 Schematic energy level alignment (a) before equilibrium, (b) at equilibrium and (c) quasi–static equilibrium of Sb₂Se₃ and CdSe films.

3.4 Effect of electrodeposited charge densities on PEC properties of Sb₂Se₃-Based electrodes

To investigate the influence of the thickness of light absorption layer on the performance, we fabricated the Sb₂Se₃/CdSe photocathode started with the deposition of Sb₂Se₃ layers by changing electrodeposited charge densities (0.125–1.0C cm⁻²). The cross-sectional microstructure of 0.125, 0.25, and 0.5C cm⁻² Sb₂Se₃/CdSe samples were observed by SEM (Fig. S12). It can be seen that with larger electrodeposited charge density, the Sb₂Se₃ layer becomes thicker, varying from 102 nm to 140 nm. The influence of charge density during electrodeposition on the PEC performance is illustrated in Fig. 5a. The largest photocurrent density of −6.9 mA cm⁻² at −0.2 V_RHE was achieved for the electrode that was electrodeposited for 0.25C cm⁻². The decreased photocurrent density was observed for the thicker light-absorption layer-based device, with a photocurrent density of −2.8 mA cm⁻² at −0.2 V_RHE.

Fig. 5 (a) Current–potential curves of photocathodes deposited with different charge densities. (b) Absorption spectra and (c) Nyquist plots for the as-obtained samples. (d) Schematics of the photo absorption and carrier transport in the PEC devices based on Sb₂Se₃ films with different thicknesses.

Fig. 5b shows the absorption spectra of samples with different thickness. According to the transmittance and reflectance spectra of Sb₂Se₃ layer with different thickness (Fig. S13), the thick Sb₂Se₃ layer can absorb more photons than the thin counterpart (Fig. 5a). Hence, the reduced photocurrent for thin Sb₂Se₃ layer is likely related to the reduced carrier production. Blue and green photons with a wavelength of 375–600 nm are mostly absorbed close to the Sb₂Se₃/CdSe interface.^12,21 Photoexcited holes in this region need to travel a long distance to reach the back contact. Given the inherently low mobility and diffusion length of hole relative to that of electron for Sb₂Se₃, the thick Sb₂Se₃ layer photoelectrode likely suffers from sluggish hole diffusion kinetics, thereby diminishing overall carrier collection efficiency.

Moreover, EIS was performed to clarify the origin of the decreased photocurrent density for the thick Sb₂Se₃ layer-based cell. As indicated by the Nyquist plot (Fig. 5c) and the fitting results (Table S3), the thinnest and worst-performing electrode (102 nm thick sample) shows the largest R_ct (6.6 k Ω), while the corresponding R_ct for the 126 nm thick (0.25C cm⁻²) sample is the smallest (0.2 k Ω, respectively), which explains for the higher photocurrent density of the film prepared by passing 0.25C cm⁻² charge density in comparison to the other electrode. As the thickness increases further, a larger resistance was observed (0.7 k Ω). Experimental results reveal that merely increasing the thickness of absorber layer for better light absorption was ineffective for enhancing the photocurrent of the PEC device. As illustrated in Fig. 5d, the charge-transport process and optical absorption in the PEC device strongly depends on the absorber thickness. Too thick absorber layer could lead to the poor charge transport and too thin absorber layer could results in insufficient light absorption. An appropriate thickness of the absorber layer allows sufficient absorption as well as efficient charge transport without recombination.

4. Conclusions

In summary, a novel Sb₂Se₃/CdSe p–n heterojunction was successfully fabricated by a facile electrodeposition followed by annealing and surface modification. Factors affecting the photoelectrochemical activity such as the annealing temperature and electrodeposited charge density have been investigated and optimized. The Sb₂Se₃/CdSe/Pt photocathode with Sb₂Se₃ electrodeposited for 0.25C cm⁻² and annealed at 250 °C exhibits the optimal performance. The fabricated PEC device achieves a photocurrent density of ca. −6.9 mA cm⁻² at −0.2 V_RHE in near-neutral solution, which is 2.3 times higher than the Sb₂Se₃/Pt. The enhanced photoelectrochemical performance is owing to the successful construction of Sb₂Se₃/CdSe p–n heterojunction, which contributes to effective charge separation and transfer. This study demonstrates the potential of Sb₂Se₃-based low-band-gap photocathode for scalable solar hydrogen production, although a further improvement in the photocurrent density and stability through additional interface engineering is required.

Author contributions

Liting Wei: funding acquisition, data curation, formal analysis, investigation, writing – original draft, writing – review & editing. Yinhu Yu: formal analysis, investigation, writing – review & editing. Tong Chang: writing – review & editing Jiao Jiao: writing – review & editing. Yangfang Wu: writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI): XRD patterns, SEM images of photoelectrodes, Tauc plots, light-harvesting efficiency curves, integrated photocurrent density, stability tests, and fitting results of Nyquist plots. See DOI: https://doi.org/10.1039/d6ra00129g.

Acknowledgements

This work is supported by the Doctoral Research Initiation Project of Yuncheng University, China (No. YXBQ-202516) and Basic Research Program of Yuncheng City, China (No. YCKJYD-202538).

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