Sb₂S₃/Sb₂O₃ Heterojunction for Improving Photoelectrochemical Properties of Sb₂S₃ Thin Films

Abstract

We prepared antimony metal films via electrodeposition, followed by the synthesis of Sb₂S₃ films through a chemical vapor phase reaction. Finally, an Sb₂O₃ film was deposited onto the Sb₂S₃ film using a chemical bath method, successfully constructing a heterojunction photocathode of Sb₂S₃/Sb₂O₃; the synthesized Sb₂S₃/Sb₂O₃ heterojunction is classified as a Type I heterostructure. The resulting Sb₂S₃/Sb₂O₃ heterojunction exhibited a photocurrent density of −0.056 mA cm⁻² at −0.15 V (vs. RHE), which is 1.40 times higher than that of Sb₂S₃ alone under simulated solar illumination. Additionally, the Sb₂S₃/Sb₂O₃ heterojunction demonstrated a lower carrier recombination rate and a faster charge transfer rate compared to Sb₂S₃, as evidenced by photoluminescence and electrochemical impedance spectroscopy tests. For these reasons, the Sb₂S₃/Sb₂O₃ heterojunction obtained a hydrogen precipitation rate of 0.163mL cm⁻² h⁻¹, which is twice the hydrogen precipitation rate of Sb₂S₃, under the condition of 60 min of light exposure. The significant enhancement in photoelectrochemical performance is attributed to the formation of the Sb₂S₃/Sb₂O₃ heterojunction, which improves both carrier separation and charge transfer efficiency. This heterojunction strategy holds promising potential for visible light-driven photoelectrochemical water splitting.

1. Introduction

Today, global population growth, rising energy consumption, and climate change pose significant threats to future energy security. Conventional fossil fuel combustion is a major source of greenhouse gas emissions, and over time, these resources are increasingly unable to meet humanity’s growing energy demands. As a result, the search for cost-effective, sustainable, renewable, and clean alternative energy sources has become a key area of research. This effort aims not only to meet daily energy needs but also to address the environmental challenges associated with traditional energy production [1,2,3,4]. Since Fujishima and Honda first demonstrated the photoelectrochemical decomposition of water for hydrogen production using TiO₂ photoanodes in 1972, photoelectrochemical (PEC) water splitting has made significant progress. Solar-driven PEC water splitting has become one of the most promising methods for hydrogen production. The construction of efficient and stable photoelectrodes is the critical factor in the PEC water decomposition system [5,6].

In recent years, antimony–sulfur compounds (such as Sb₂S₃, Sb₂Se₃, etc.) have gained significant attention from researchers due to their suitable bandgap and high light absorption properties. Among these, antimony sulfide (Sb₂S₃), which is highly abundant and environmentally friendly, stands out for its high light absorption coefficient. Its narrow bandgap, which aligns well with the solar spectrum, makes it an excellent candidate for photoelectrodes. As a result, Sb₂S₃ has attracted considerable interest in the study of photoelectrochemical water splitting [7]. Sb₂S₃ is a direct bandgap p-type semiconductor with excellent properties, including a narrow bandgap (~1.72 eV) which matches well with the solar spectrum, and band edges Ec = 0.22 V (vs. NHE) and Ev = 1.94 V (vs. NHE) [8] that align well with the solar spectrum. It also boasts a high light absorption coefficient (α ~ 10⁵ cm⁻¹), low cost, a low melting point (~550 °C), and outstanding stability in air and humidity [9,10,11]. These exceptional characteristics make Sb₂S₃ a highly attractive material for photovoltaic applications. Therefore, it is widely used in photodetectors [12,13,14], supercapacitors [15,16], solar cells [17,18,19], and photocatalysts [20,21]. However, the PEC performance of pristine Sb₂S₃ remains significantly lower than its theoretical potential [22]. The photogenerated carriers in Sb₂S₃ under light illumination exhibit limited diffusion lengths and short lifetimes. They are also less stable due to significant electron–hole recombination rates and insufficient charge mobility [23]. This is because the Fermi energy levels of the Sb₂S₃ photoelectrodes are pinned by deep energy level defects, leading to low separation and transport efficiency of the photogenerated carriers. To overcome these limitations, researchers have explored and investigated various modification techniques, including the construction of heterostructures, elemental doping, and the incorporation of co-catalysts.

Sb₂O₃ is an n-type semiconductor with a bandgap width of 3.0 eV, E_c = 0.32 V (vs. NHE), and E_v = 3.32 V (vs. NHE) [8,24]. Tigau [25] prepared Sb₂O₃ thin films using the evaporation method and found that the resulting polycrystalline thin films exhibited good crystalline quality, a face-centered cubic structure, and a lattice constant of a = 11.14 Å. The films also featured a selectively grown (222) crystal plane and demonstrated superior electronic conductivity at room temperature. Meanwhile, Sb₂O₃ also has strong UV absorption [26], optical refractive index [27], and electrochemical catalytic effect [28].

Research has been conducted on Sb₂S₃/Sb₂O₃ composite materials. Li et al. [29] modified TiO₂ photoanodes using Sb₂S₃/Sb₂O₃, resulting in the creation of Sb₂S₃/Sb₂O₃/TiO₂ composite photoanodes. Subsequent studies assessed the cathodic protection effectiveness of these composite photoanodes on 304 stainless steel, revealing improved performance relative to pure TiO₂. The Sb₂S₃/Sb₂O₃/TiO₂ composite photoelectrode demonstrated optimal performance to achieve an open circuit potential (OCP) of −0.76 V, which is significantly lower than the corrosion potential of 304 stainless steel. Additionally, Jiang et al. [30] successfully synthesized Sb₂O₃/Sb₂S₃ nanocomposite materials with n–n heterojunctions, utilizing them as photoanodes for photoelectrochemical water splitting, achieving a photocurrent density 40 times greater than that of bare Sb₂O₃ photoanodes. However, all previously reported Sb₂S₃/Sb₂O₃ composite materials have been employed exclusively as photoanodes. This study explores the potential of Sb₂S₃ as the photoactive layer at the bottom of photoelectrochemical cathodes, since its band gap is well positioned in relation to the energy distribution of the solar spectrum and it exhibits a high absorption coefficient in the visible light range. In contrast, Sb₂O₃ exhibits superior catalytic performance, strong corrosion resistance, and notable ultraviolet absorption properties, making it an ideal candidate for the catalytic layer at the top of the photoelectrochemical cathode. Through photoelectrochemical testing, we analyze and compare the photoelectrochemical properties of Sb₂S₃/Sb₂O₃ with those of Sb₂S₃, investigating photocurrent density, photogenerated charge carrier recombination rates, and charge transfer pathways.

2. Materials and Methods

2.1. Materials

Potassium antimony tartrate (K(SbO)C₄H₄O₆ 1/2H₂O, AR, 500 g, 99.0%, Tianjin Windship Chemical Reagent Technology Co., Tianjin, China), ammonium chloride (NH₄Cl, AR, 500 g, 99.5%, Tianjin Windship Chemical Reagent Technology Co., Tianjin, China), antimony trichloride (SbCl₃, AR, 500 g, 99.0%, Aladdin, Shanghai, China), triethanolamine (TEA, AR, 500 mL, 98.0%, Hengxing Reagent, Tianjin, China), sulfur powder (S, AR, 500 g, 98.0%, Tianjin Windship Chemical Reagent Technology Co., Tianjin, China), ethanol (C₂H₅OH, AR, 2500 mL, 99.7%, Chengdu Cologne Chemical Co., Chengdu, China), sulfuric acid (H₂SO₄, AR, 2500 mL, 95.0~98.0%, Chuandong Chemical, Chongqing, China), hydrochloric acid (HCl, AR, 2500 mL, 98.0%, Chuandong Chemical, Chengdu, China), sodium hydroxide (NaOH, AR, 500 g, 96.0%, Tianjin Windship Chemical Reagent Technology Co., Tianjin, China), and fluorine doped tin oxide (FTO, Gu Luo glass, Luoyang, China) glass substrates (14–16 Ω cm⁻¹, 25 mm × 15 mm, 2.2 mm) were acquired. All chemical products were of analytical grade and used as received without further purification.

2.2. Preparation of the Sb₂S₃/Sb₂O₃ Heterojunction

Preparation of Sb₂S₃ film—The synthesis of Sb₂S₃ was accomplished through the electrodeposition of antimony metal onto fluorinated tin oxide (FTO)-coated substrates, followed by annealing in a sulfur atmosphere. The electrolyte solution used was a mixed solution of potassium antimony tartrate (K₂(Sb₂(C₄H₂O₆)₂) at 5.5 mmol L⁻¹ and ammonium chloride (NH₄Cl) at 100 mmol L⁻¹. The pH of the mixed solution was adjusted to 1.3 using 0.1 mol L⁻¹ HCl, resulting in the final electrolyte solution, with a deposition potential of −0.8 V (vs. SCE) for a duration of 30 min. Subsequently, the deposited antimony metal was annealed in a sulfur-rich environment at 250 °C. The desired temperature was achieved by ramping at approximately 5 °C per min. In this process, the Sb metal was situated in zone 1 (250 °C), while elemental sulfur powder (1 g) was placed in zone 2 (200 °C). The vulcanization duration was 30 min, after which the assembly was allowed to cool to room temperature within the furnace [31,32].

Preparation of Sb₂S₃/Sb₂O₃ photoelectrode—Sb₂O₃ was prepared using a chemical bath deposition method. Specifically, 1.5 mL of triethanolamine was added to 60 mL of an SbCl₃ solution, which was adjusted to a concentration of 0.05 M. Under continuous stirring, saturated NaOH was added to adjust the pH to 7.80. Following this preparation, the previously synthesized Sb₂S₃ was vertically immersed in the resulting solution and maintained at a temperature of 60 °C for 15 min to facilitate the formation of the Sb₂S₃/Sb₂O₃ heterojunction [30,33].

2.3. Characterization

The crystallization phase of all the samples was investigated using an X-ray diffractometer (Shimadzu, XRD-6000, Cu Kα radiation, λ = 0.154056 nm, Kyoto, Japen) in the diffraction angle region of 10–80°. Scanning electron microscopy (SEM, ZEISS Sigma 300, Carl Zeiss AG, Oberkochen, Germany) and a corresponding Energy Dispersive Spectrometer (EDS) were used to perform a fine analysis of the microstructure, micromorphology, and composition of the sample. The transmittance spectra of the samples were obtained using an UV–Vis–NIR spectrophotometer (PE lambda 750, PerkinElmer, Inc., Waltham, MA, USA) in a wavelength range of 200–1000 nm at room temperature. The elemental composition and chemical valence of the samples were studied by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA). Steady-state photoluminescence (PL) spectra were examined by an Edinburgh FLS1000(Edinburgh Instruments Ltd., Edinburgh, UK) spectrofluorimeter. Sb₂S₃ and the Sb₂S₃/Sb₂O₃ heterojunction were excited with an excitation light of 320 nm, and the emission spectra were tested from 230 to 850 nm.

2.4. PEC Measurements

All PEC measurements were performed in a three-electrode system, in which the sample, Pt electrode, and saturated calomel electrode (SCE) were used as working, counter, and reference electrodes, respectively. A 0.5 M H₂SO₄ solution was employed as the electrolyte. Light illumination was performed from the front of the electrode. A 300 W xenon lamp with an AM1.5G filter was used to simulate the global standard sunlight spectrum (AM1.5G 100 mW·cm⁻², Xenon lamp and AM1.5G filter are from Beijing CEC Jinyuan, Beijing, China, the xenon lamp model is CEL-HXF300), and the scan rate of the linear sweep voltammetry was 2 mV/s. The test temperature was 25 °C, the pressure was atmospheric, and the diameter of the prepared photoelectrode was inserted into the solution to start the test. All work potentials were determined vs. RHE using the following Equation (1) [34]:

$$\mathrm{E}\left(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E}\right)=\mathrm{E}\left(\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{C}\mathrm{E}\right)+0.2483+0.0591\times \mathrm{p}\mathrm{H}$$

(1)

3. Results and Discussion

3.1. Characterization of the Phase Structure of Sb₂S₃ and the Sb₂S₃/Sb₂O₃ Heterojunction

The crystalline phase structures of monomeric Sb₂S₃ films and the Sb₂S₃/Sb₂O₃ heterojunction were analyzed and compared using X-ray diffraction (XRD) patterns. Figure 1 illustrates that the characteristic peaks of Sb₂S₃ thin films are located at 17.66°, 25.02°, 29.36°, and 32.48°, corresponding to the (120), (310), (211), and (212) crystallographic planes, respectively. These observations align with the orthorhombic phase of Sb₂S₃ (JCPDS 42-1393), wherein the (211) crystallographic plane is identified as the optimal growth orientation.

For the Sb₂S₃/Sb₂O₃ heterojunction samples, in addition to the peaks characteristic of Sb₂S₃, additional peaks at 13.80°, 27.76°, 32.16°, and 46.08° were detected. These peaks correspond to Sb₂O₃ (JCPDS 71-0365) and are associated with the (111), (222), (400), and (440) crystallographic planes, respectively, with the (222) plane being the preferred growth orientation. Furthermore, the peaks at 26.58°, 33.77°, 37.77°, 51.75°, 61.75°, and 65.74° are attributed to SnO₂ from the FTO glass substrate. Notably, no additional impurity diffraction peaks were observed in the XRD spectra, indicating that samples with well-defined crystallinity were successfully obtained. The (120) and (310) crystal faces indeed exhibit higher diffraction intensity compared to the (211) crystal face. However, when processing the data using Jade, it was found that the proportion of the diffraction peaks corresponding to the (120) and (310) crystal faces is not very significant, while the (211) crystal face has a larger proportion in the diffraction peak at 29.36°. Therefore, the (211) crystal face is considered to be the optimal growth plane.

The surface morphologies of monolithic Sb₂S₃, Sb₂O₃, and the Sb₂S₃/Sb₂O₃ heterojunction were analyzed and compared using field emission scanning electron microscopy (SEM). As illustrated in Figure 2, the Sb₂S₃ films exhibited a bottom-up stacking of lamellar morphology, which facilitates greater absorption of visible light due to their increased exposed surface area. However, a significant number of pinholes were observed between the lamellae, which could allow direct contact of the electrolyte with the FTO glass substrate. This situation may compromise the photoelectrochemical performance and stability of Sb₂S₃.

In contrast, the Sb₂O₃ films comprised prismatic morphology with uniformly distributed smaller particles on the FTO glass substrate, indicating that the Sb₂O₃ films synthesized via chemical bath deposition possess good crystallization. The surface morphology of the Sb₂S₃/Sb₂O₃ heterojunction featured larger granular morphology attributed to Sb₂O₃. Notably, when Sb₂O₃ was grown on the Sb₂S₃ substrate, the morphology changed, and the growth of Sb₂O₃ effectively covered the existing pinholes in Sb₂S₃. This alteration addresses the issue of electrochemical performance degradation linked to those pinholes.

Additionally, the elemental distributions of monomeric Sb₂S₃, Sb₂O₃, and the Sb₂S₃/Sb₂O₃ heterojunction were investigated through energy dispersive X-ray spectroscopy (EDS) and mapping diagrams. As illustrated in Figure 3, the distribution of sulfur (S) and antimony (Sb) atoms within the selected range indicates a non-homogeneous distribution of S atoms in Sb₂S₃, which confirms the presence of multiple pinholes (Figure 3a). In contrast, the distribution of oxygen (O) and Sb atoms in the Sb₂O₃ film is notably uniform (Figure 3b). Furthermore, the mapping of O atoms in the Sb₂S₃/Sb₂O₃ heterojunction demonstrates that the observed large granular morphology corresponds to Sb₂O₃ (Figure 3c).

Figure 4 shows the EDS spectrum and compositional data for Sb₂S₃, Sb₂O₃, and the Sb₂S₃/Sb₂O₃ heterojunction. The EDS spectrum of Sb₂S₃ (Figure 4a) indicates that the atomic ratios of S atoms and Sb atoms are 61.94% and 38.06%, respectively, which is very close to the atomic ratio of Sb₂S₃. For Sb₂O₃, the corresponding atomic ratios of O atoms and S atoms are 54.44% and 45.56% (Figure 4b). In the Sb₂S₃/Sb₂O₃ heterojunction, the atomic ratios of O atoms, S atoms, and Sb atoms are 45.60%, 12.34%, and 42.06%, respectively (Figure 4c). It can be observed that the proportion of S atoms is relatively low, which may be attributed to the greater thickness of the Sb₂S₃/Sb₂O₃ heterojunction, resulting in an inability to detect all the S atoms.

Figure 5 shows the cross-sectional morphology of Sb₂S₃, Sb₂O₃, and the Sb₂S₃/Sb₂O₃ heterojunction. Based on the scaling of the scale bar, the thicknesses of the Sb₂S₃, Sb₂O₃, and the Sb₂S₃/Sb₂O₃ heterojunction were estimated to be approximately 2.85 μm, 2.00 μm, and 9.45 μm, respectively.

To further investigate the surface composition and chemical states of the Sb₂S₃, Sb₂O₃, and Sb₂S₃/Sb₂O₃ heterojunction samples, X-ray photoelectron spectroscopy (XPS) analyses were performed, as illustrated in Figure 6. The full-scan XPS spectra (Figure 6a) reveal the coexistence of antimony (Sb), sulfur (S), and oxygen (O) elements in both the Sb₂S₃ and Sb₂S₃/Sb₂O₃ heterojunction. As shown in the figure, other significant peak values include Sb 3p1, Sb 3p3, Sb 4d, and S 2s. Additionally, the peak of C 1s originates from CO₂ in the atmosphere. Notably, the O present in Sb₂S₃ primarily originates from atmospheric oxygen, while in Sb₂O₃, it coexists with both Sb and O elements. As shown in Figure 6b, the signal for the Sb-S bonds in the Sb₂S₃/Sb₂O₃ heterojunction is relatively weak. The XPS testing is related to the thickness of the heterojunction, and the thickness of 9.45 µm for the Sb₂S₃/Sb₂O₃ heterojunction can lead to inaccuracies in detecting deeper signals.

Figure 6b displays the high-resolution XPS spectra for Sb in Sb₂S₃, Sb₂O₃, and the Sb₂S₃/Sb₂O₃ heterojunction. The binding energy peaks for Sb 3d_5/2 and Sb 3d_3/2 in Sb₂S₃ were observed at 527.45 eV and 536.82 eV, respectively, indicative of the Sb-S bond. Additionally, a smaller binding energy peak for Sb 3d_5/2 at 530.21 eV corresponds to Sb-O bonding, a result of the inadvertent oxidation of Sb₂S₃ due to exposure to air. For Sb₂O₃, the binding energy peaks for Sb 3d_5/2 and Sb 3d_3/2 were found at 529.59 eV and 538.87 eV, respectively, consistent with Sb-O bonding. The binding energy peaks for the Sb 3d in the Sb₂S₃/Sb₂O₃ heterojunction can be deconvoluted into four peaks located at 528.67 eV, 529.45 eV, 538.07 eV, and 538.80 eV, corresponding to the Sb-S and Sb-O bonds at the Sb 3d_5/2 and Sb 3d_3/2 positions, respectively. It is noteworthy that the intensities of the binding energy peaks associated with the Sb-S bonds were relatively low, likely due to the fact that Sb₂S₃ resides in the lower layer and the probing depth of the XPS analysis was not sufficient to reveal significant contributions from that layer [35,36]. XPS further demonstrated the successful synthesis of the Sb₂S₃/Sb₂O₃ heterojunction.

3.2. Analysis of Optical Properties of Sb₂S₃ and the Sb₂S₃/Sb₂O₃ Heterojunction

The UV–visible absorption spectra of the Sb₂S₃, Sb₂O₃, and Sb₂S₃/Sb₂O₃ samples were measured using an ultraviolet–visible spectrometer (UV-Vis) over the wavelength range of 190–1000 nm. As shown in Figure 7a, the Sb₂S₃ thin films demonstrate strong visible light absorption in the range of 300 to 700 nm. In contrast, the Sb₂O₃ thin films exhibit significant ultraviolet absorption but are less effective in utilizing visible light. Notably, the visible light absorption capability of the Sb₂S₃/Sb₂O₃ heterojunction is considerably reduced. This observation aligns with findings by Yao et al. [26], which indicated that a TiO₂/Sb₂O₃ heterojunction possesses a weaker light-absorbing capacity compared to monomeric TiO₂. This reduction in absorption may be attributed to the higher optical refractive index of Sb₂O₃, which results in a considerable portion of visible light being reflected, with only a small fraction absorbed by Sb₂S₃. Furthermore, analysis of the SEM images reveals that the decreased visible light absorption is likely due to the Sb₂O₃ particle morphology covering the surface of the Sb₂S₃ film, thereby obstructing light penetration and absorption.

The optical band gaps (E_g) of Sb₂S₃ and Sb₂O₃ were accurately calculated using Tauc plots as shown in Equation (2) [37]:

$${\left(\mathsf{\alpha }\mathrm{h}\mathrm{v}\right)}^{\mathrm{n}}=\mathrm{A}\left(\mathrm{h}\mathrm{v}-{\mathrm{E}}_{\mathrm{g}}\right)$$

(2)

where α is the optical absorption coefficient (cm⁻¹), hν is the energy of the incident photon (eV), E_g is the Tauc’s bandgap (eV) of the sample, and the value of n depends on the energy band nature of the sample. In the Tauc plot, a linear region is typically observed, and the intersection of this linear section with the horizontal axis represents the optical band gap. As illustrated in Figure 7b, the optical band gaps (E_g) of Sb₂S₃ and Sb₂O₃ are approximately 1.60 eV and 3.21 eV, respectively. Additionally, the valence band maxima (VBM) of Sb₂S₃ and Sb₂O₃ were determined using high-resolution X-ray photoelectron spectroscopy (XPS). Figure 7c shows that the VBM of Sb₂S₃ is −1.44 eV, while that of Sb₂O₃ is 0.72 eV.

The intensity of the steady-state photoluminescence (PL) spectra is commonly employed to characterize the complexation of photogenerated electrons and holes; a higher intensity indicates a greater degree of carrier complexation. Figure 8 presents the PL spectra of Sb₂S₃ and the Sb₂S₃/Sb₂O₃ heterojunction, excited by a 320 nm laser. As shown in the figure, the PL spectrum of Sb₂S₃ exhibits characteristic peaks at 494 nm, 550 nm, and 623 nm. In contrast, the PL spectrum of Sb₂O₃ displays a characteristic peak at 550 nm. Similarly, the Sb₂S₃/Sb₂O₃ heterojunction shows a prominent peak only at 550 nm, which may originate from the fluorescence characteristic peaks of both Sb₂S₃ and Sb₂O₃ at this wavelength. Notably, the characteristic peaks of Sb₂S₃ at 494 nm and 623 nm are nearly undetectable in the Sb₂S₃/Sb₂O₃ heterojunction, indicating that their intensity is significantly lower than that observed for Sb₂S₃ alone. Thus, the Sb₂S₃/Sb₂O₃ heterojunction effectively suppresses carrier complexation [38].

3.3. Schematic Diagram of the Energy Band of the Sb₂S₃/Sb₂O₃ Heterojunction

To ascertain the energy band alignment of the Sb₂S₃/Sb₂O₃ heterojunction and to construct an energy band diagram, high-resolution X-ray photoelectron spectroscopy (XPS) was utilized to measure the valence band maximum (VBM) positions for both Sb₂S₃ and Sb₂O₃. The VBM values for Sb₂S₃ and Sb₂O₃ were found to be −1.44 eV and 0.72 eV, respectively. Additionally, UV–visible spectroscopy inferred the optical band gaps (Eg) of Sb₂S₃ and Sb₂O₃ to be approximately 1.60 eV and 3.21 eV, respectively. Furthermore, the XPS results revealed the core energy levels of both Sb₂S₃ and Sb₂O₃. According to the Kraut method [39], the valence band offset (ΔEv) and the conduction band offset (ΔEc) of the Sb₂S₃/Sb₂O₃ heterojunction can be calculated using Equations (3) and (4) [35,40,41]:

$$\begin{gathered} ∆{\mathrm{E}}_{\mathrm{v}}=\left({\mathrm{E}}_{\mathrm{S}\mathrm{b} {3\mathrm{d}}_{5/2}}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{S}}_3}-{\mathrm{E}}_{\mathrm{V}\mathrm{B}\mathrm{M}}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{S}}_3}\right)+\left({{\mathrm{E}}_1}_{\mathrm{S}\mathrm{b} {3\mathrm{d}}_{5/2}}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{S}}_3/{\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_3}-{{\mathrm{E}}_2}_{\mathrm{S}\mathrm{b} {3\mathrm{d}}_{5/2}}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{S}}_3/{\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_3}\right) \\ -\left({\mathrm{E}}_{\mathrm{S}\mathrm{b} {3\mathrm{d}}_{5/2}}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_3}-{\mathrm{E}}_{\mathrm{V}\mathrm{B}\mathrm{M}}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_3}\right) \end{gathered}$$

(3)

$$∆{\mathrm{E}}_{\mathrm{c}}={\mathrm{E}}_{\mathrm{g}}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{S}}_3}+∆{\mathrm{E}}_{\mathrm{v}}-{\mathrm{E}}_{\mathrm{g}}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_3}$$

(4)

In this context, ${\mathrm{E}}_{\mathrm{S}\mathrm{b} 3\mathrm{d}5/2}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{S}}_3}$ and ${\mathrm{E}}_{\mathrm{S}\mathrm{b} 3\mathrm{d}5/2}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_3}$ represent the core energy levels of Sb₂S₃ and Sb₂O₃, which are 527.45 eV and 529.59 eV, respectively; ${{\mathrm{E}}_1}_{\mathrm{S}\mathrm{b} 3\mathrm{d}5/2}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{S}}_3/{\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_3}$ is the core energy level of the wide bandgap Sb₂O₃ in the Sb₂S₃/Sb₂O₃ heterojunction, at 529.45 eV; and ${{\mathrm{E}}_2}_{\mathrm{S}\mathrm{b} 3\mathrm{d}5/2}^{{\mathrm{S}\mathrm{b}}_2{\mathrm{S}}_3/{\mathrm{S}\mathrm{b}}_2{\mathrm{O}}_3}$ is the core energy level of the narrow bandgap Sb₂S₃, at 528.67 eV. The calculated values for the valence band offset (ΔE_v) and the conduction band offset (ΔE_c) are 0.80 eV and −0.81 eV, respectively. Based on these band offsets, a schematic energy band diagram of the Sb₂S₃/Sb₂O₃ heterojunction is illustrated in Figure 9.

Figure 9a is a schematic energy band diagram of the Sb₂S₃/Sb₂O₃ heterojunction under dark conditions. Based on the valence band and conduction band offset energies, the Sb₂S₃/Sb₂O₃ heterojunction is classified as a Type I heterojunction. In a Type I heterojunction, photo-excited holes and electrons tend to transfer to the semiconductor with a narrower bandgap [40,42]. However, due to the changes in surface potential induced by illumination in semiconductors, the Surface Photovoltage (SPV) is defined as the change in surface potential caused by light exposure, which is used to characterize the separation of photo-generated charge carriers. The presence of a surface space charge region (SCR) resulting from the potential difference at the semiconductor surface typically leads to n-type semiconductors exhibiting a positive SPV, where the band edges of the SCR bend upward. Conversely, p-type semiconductors exhibit a negative SPV, with the band edges of the SCR bending downward [43]. Therefore, the band edges of Sb₂S₃ bend downward, while those of Sb₂O₃ bend upward. The bending of the valence band at the interface facilitates the transfer of photo-generated holes from Sb₂O₃ to the high valence band of Sb₂S₃. The potential barrier caused by the bending of the conduction band prevents the photo-generated electrons from Sb₂O₃ from moving to the conduction band side of Sb₂S₃ (Figure 9b). In this process, the photo-generated electrons from Sb₂O₃ can be retained in the conduction band with a higher reduction potential, participating in photocatalytic hydrogen evolution reactions, while the photo-generated holes transfer to the Sb₂S₃ side and move to the counter electrode under the influence of an external voltage to participate in oxygen evolution reactions. This special Type I heterostructure and band bending achieve the separation of photo-generated electron–hole pairs, enhancing the separation efficiency of photo-generated charge carriers.

3.4. Photoelectrochemical Characterization of Sb₂S₃ and the Sb₂S₃/Sb₂O₃ Heterojunction

The transient photocurrent response (TPR) and electrochemical impedance (EIS) of a photocatalyst determine the intensity and migration efficiency of the material’s photogenerated carriers. The photogenerated electron–hole separation, migration efficiency, and stability of Sb₂S₃ and the Sb₂S₃/Sb₂O₃ heterojunction were tested using an electrochemical workstation coupled with a 300 W xenon lamp.

Figure 10 presents the linear scanning voltage (J-V) plots for both single Sb₂S₃ and the Sb₂S₃/Sb₂O₃ heterojunction, with applied bias voltages ranging from 0.45 V to −0.15 V (vs. RHE). The measurements were conducted with a 5 s on-time and off-time, at a sweep rate of 2 mV s⁻¹. It was observed that both single Sb₂S₃ and the Sb₂S₃/Sb₂O₃ heterojunction generate photocurrents upon photoelectric excitation. Notably, the photocurrent response of the Sb₂S₃/Sb₂O₃ heterojunction is significantly greater than that of Sb₂S₃, indicating that the heterojunction is more effective in generating photogenerated carriers upon photoexcitation. The photocurrent density of the Sb₂S₃/Sb₂O₃ heterojunction consistently exceeds that of Sb₂S₃ across the range of applied bias voltages. For instance, at −0.15 V (vs. RHE), the photocurrent density reaches −0.056 mA cm⁻², which is 1.40 times greater than that of Sb₂S₃, which has a photocurrent density of −0.040 mA cm⁻². Furthermore, the photocurrent densities of the Sb₂S₃/Sb₂O₃ heterojunction are 3.28, 2.6, 1.88, 1.51, and 1.37 times higher than those of Sb₂S₃ at the applied bias voltages of 0.35 V, 0.25 V, 0.15 V, 0.05 V, and −0.05 V (vs. RHE), respectively.

Figure 11a illustrates the time–current (I-t) profiles for the monomer Sb₂S₃ and the Sb₂S₃/Sb₂O₃ heterojunction. These profiles were obtained to assess the stability of the photocathodes during an electrochemical process under continuous illumination at an applied bias voltage of −0.15 V (vs. RHE) for 2 h. The current density at the light state of Sb₂S₃ remains stable throughout the illumination period, showing little to no change, and stabilizes at −0.015 mA cm⁻². Conversely, the current density at the light state of the Sb₂S₃/Sb₂O₃ heterojunction initially experiences a decline but stabilizes after approximately 20 min of illumination, maintaining a consistent value of −0.075 mA cm⁻²; this stabilized current is five times greater than that of Sb₂S₃.

Figure 11b presents the electrochemical impedance spectroscopy (EIS) patterns for Sb₂S₃, Sb₂O₃, and the Sb₂S₃/Sb₂O₃ heterojunction, which were evaluated under continuous illumination and an applied bias voltage of −0.15 V (vs. RHE). In this context, the diameter of the semicircle represents the charge transfer impedance (R₂) at the electrode–electrolyte interface, while R₁ corresponds to the solution resistance, and CPE₁ represents the transfer capacitance. A smaller semicircle diameter in the electrochemical impedance spectroscopy (EIS) pattern indicates lower impedance and reduced charge transfer resistance at the electrode–electrolyte interface. The results reveal that the Sb₂S₃/Sb₂O₃ heterojunction has the smallest semicircle diameter (as shown in Figure 11c), suggesting the lowest charge transfer resistance and the highest charge migration efficiency. This finding further reinforces the idea that the formation of the Sb₂S₃/Sb₂O₃ heterojunction enhances the migration efficiency of photogenerated carriers.

The photocatalytic hydrogen evolution performance of the electrodes was characterized using a photocatalytic evaluation system. With an applied voltage of −0.15 V, the samples were illuminated for 60 min, and the hydrogen produced was automatically detected using a gas chromatograph. The temperature of the entire experimental system was maintained at 25 °C. First, hydrogen calibration was conducted, in which 1 mL of hydrogen was injected into the photocatalytic system, resulting in a chromatographic peak area of 40.8321 for the 1 mL of hydrogen, with a peak retention time of 2.093 min. Based on this data, the hydrogen evolution rates for both Sb₂S₃ and the Sb₂S₃/Sb₂O₃ heterojunction were calculated. It is clear from Figure 12 and

Table 1. Hydrogen precipitation volume and rate of hydrogen precipitation from Sb₂S₃ and the Sb₂S₃/Sb₂O₃ heterojunction after 60 min of illumination.

Sample	Time of Appearance of Hydrogen Peak	Chromatographic Peak Area	Hydrogen Volume/mL	Hydrogen Precipitation rate/mL cm−2 h−1
1 mL standard hydrogen	2.039	40.8321	1.000	\
Sb2S3	2.098	4.9891	0.122	0.081
Sb2S3/Sb2O3	2.111	9.9438	0.244	0.163

that the hydrogen precipitation rate of Sb₂S₃ is 0.081 mL cm⁻² h⁻¹, whereas that of the Sb₂S₃/Sb₂O₃ heterojunction is about twice that of Sb₂S₃. This difference is undoubtedly attributed to the significant improvement in photoelectrocatalytic performance of the Sb₂S₃/Sb₂O₃ heterojunction.

4. Conclusions

In summary, the Sb₂S₃/Sb₂O₃ heterojunction was successfully synthesized through the electrodeposition of a metallic Sb film, followed by vulcanization and chemical bath deposition. Characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS), confirmed the formation of the Sb₂S₃/Sb₂O₃ heterojunction, characterized by larger particles in the upper layer and a lamellar morphology in the lower layer. The calculation results obtained from the Kraut method indicate that the synthesized Sb₂S₃/Sb₂O₃ heterojunction is categorized as a Type I heterostructure. The unique characteristics of this Type I heterostructure and band bending facilitate the separation of photogenerated charge carriers. Under simulated sunlight, the Sb₂S₃/Sb₂O₃ heterojunction exhibited a photocurrent density of −0.056 mA cm⁻² at −0.15 V, which is 1.40 times greater than that of monomeric Sb₂S₃. In the hydrogen precipitation experiment with a light time of 60 min, the hydrogen precipitation rate of the Sb₂S₃/Sb₂O₃ heterojunction is 0.163 mL cm⁻² h⁻¹, which is about twice that of Sb₂S₃. The mechanisms underlying the enhanced photocurrent were investigated using electrochemical impedance spectroscopy, steady-state fluorescence spectroscopy, and photoelectrochemical (PEC) measurements. The improved photoelectrochemical performance of the Sb₂S₃/Sb₂O₃ heterojunction can be attributed to a reduction in interfacial transport impedance and a significant increase in the efficiency of photogenerated carrier separation. The formation of a heterojunction between Sb₂S₃ and Sb₂O₃ effectively enhances the photoelectrochemical performance of the photoanode, addressing issues related to the separation efficiency of photogenerated carriers and the recombination rate in Sb₂S₃ films to some extent. Meanwhile, loading co-catalysts is an effective approach to improving the photoelectrochemical performance of photoanodes. In future studies, loading co-catalysts on the Sb₂S₃/Sb₂O₃ heterojunction is expected to further enhance its photoelectrochemical performance.