Enhanced Photocatalytic Activity of BiVO₄/Bi₂S₃/SnS₂ Heterojunction under Visible Light

Abstract

Heterojunction photocatalysts have attracted a significant amount of attention due to their advantages over a single photocatalyst and, particularly, their superior spatial charge separation. Herein, the BiVO₄/Bi₂S₃/SnS₂ heterojunction was synthesized via solvothermal synthesis with different ratios of BiVO₄ to SnS₂. The photodegradation rate of the 0.03 BiVO₄/SnS₂ sample for rhodamine B removal is 2.3 times or 2.9 times greater than that of a single SnS₂ or BiVO₄, respectively. The chemical bond between photocatalysts is confirmed by X-ray photoelectron spectroscopy (XPS), and the synchronized shift observed in binding energies strongly indicates the electron screening effect at the heterojunction. A Z-scheme model is proposed to explain charge transfer pathway in the system, in which the formation of Bi₂S₃ plays a crucial role in the enhanced photocatalytic performance of the heterojunction.

1. Introduction

In recent years, a tremendous effort has been made to develop efficient photocatalysts due to their potential applications in water splitting [1,2,3], CO₂ reduction [4,5], and pollutant removal [6,7,8]. A good photocatalyst needs a suitable bandgap to work in the visible light region, in addition to satisfying other important criteria, e.g., abundance, low cost, nontoxicity, low electron–hole recombination rate, and stability. Photocatalysts such as SnS₂ and BiVO₄ fit most of these conditions; in particular, they can absorb visible light because their bandgaps are 2.18–2.4 eV [1,9]. Although both materials have many advantages as a single photocatalyst, incorporating them with different photocatalysts to form a heterojunction has proved very effective to attain superior photocatalytic activity [1,10,11,12]. The interface can act as a bridge allowing electrons to migrate between photocatalysts, which, in turn, suppress the photoinduced electron–hole recombination and improve the performance of photocatalysts [11,13]. In addition, the heterojunction may also induce a Z-scheme system in which the redox ability is enhanced via coupling two or more narrow-bandgap semiconductors [1,14].

To date, no attempt has been made to fabricate the BiVO₄/SnS₂ heterojunction despite many publications on SnS₂ and BiVO₄ [7,15]. One of the reasons for this could be an occurrence of the reaction between Bi³⁺ and S²⁻ that leads to formation of Bi₂S₃ at elevated temperatures [6,16]. It would be challenging to obtain a BiVO₄/SnS₂ heterojunction without formation of Bi₂S₃. However, the composites containing Bi₂S₃, such as Bi₂S₃/ZnS [17], Bi₂S₃/MoS₂ [5,18], and TiO₂/Bi₂S₃ [3], have been shown to promote the photocatalytic activity. Furthermore, earlier researchers have demonstrated that both SnS₂ and BiVO₄ were successfully fabricated with Bi₂S₃ to produce Bi₂S₃/SnS₂ and Bi₂S₃/BiVO₄, respectively, exhibiting enhanced photocatalytic performance in dye degradation [15,19,20,21]. Additionally, ternary heterojunction based on Bi₂S₃ (e.g., Bi₂O₃/Bi₂S₃/MoS₂ [2], Bi₂S₃/SnS₂/Bi₂O₃ [6], and Bi₂O₃/Bi₂S₃/MoS₂ [16]) have also been proven to improve the photocatalytic activity. Therefore, viewed from a different perspective, Bi₂S₃ formation from the reaction between SnS₂ and BiVO₄ might enhance photocatalytic performance of the composite via formation of a ternary heterojunction.

In this study, the BiVO₄/Bi₂S₃/SnS₂ heterojunction was thus synthesized by combining SnS₂ and BiVO₄ via ultrasonic mixing and solvothermal synthesis. The formation of Bi₂S₃ during the synthesis would then constitute a ternary heterojunction with SnS₂ and BiVO₄. Various molar ratios of BiVO₄ to SnS₂ were prepared to investigate the properties of three-phase photocatalysts (BiVO₄, Bi₂S₃, and SnS₂₎ and to optimize the amount of BiVO₄ in the composite. Photocatalytic performance of the composite was compared with pure BiVO₄ and SnS₂, and the charge transfer pathway in the heterojunction system was examined. A Z-scheme model was then proposed to consistently explain the enhanced photocatalytic activity of BiVO₄/Bi₂S₃/SnS₂ heterojunctions.

2. Results and Discussions

2.1. Crystal Structure of the Samples

The crystalline phases of as-prepared samples with different ratios of BiVO₄ to SnS₂ were investigated through XRD (X-ray diffraction) patterns as shown in Figure 1. The characteristic peaks at 14.9°, 28.3°, 32.3°, 49.5°, and 52.4° correspond to (001), (100), (101), (110), and (111) planes of hexagonal SnS₂, respectively (ICDD PDF No. 00-023-0677). Other visible peaks can be observed at 18.7°, 19.0°, 28.8°, and 30.5°, assigned to (110), (011), ($\overline{1}$21), and (040) planes of monoclinic BiVO₄, respectively (ICDD PDF No. 00-014-0688). In addition to these obvious peaks, there are four small peaks at 22.3°, 23.6°, 24.9°, and 25.2°, which can be assigned to (202), (101), (130), and (310) planes of orthorhombic Bi₂S₃, respectively (ICDD PDF No. 01-089-8965). The XRD patterns of individual synthesized materials (BiVO₄, SnS₂, and Bi₂S₃) are shown in Figure S1. These characteristic diffraction peaks confirm three crystal phases of SnS₂, B₂S₃, and BiVO₄ in the composites. As the content of BiVO₄ during the synthesis is reduced, the diffraction intensities of BiVO₄ and Bi₂S₃ decrease and almost disappear for the 0.01 BiVO₄/SnS₂ sample due to the very low content of BiVO₄ and Bi₂S₃ in the composite. According to the result, the content of Bi₂S₃ depends on the initial concentration of BiVO₄ during the hydrothermal synthesis.

2.2. Morphology and Microstructure of the Samples

Figure 2a,b reveals agglomerates of SnS₂ that were composed of ultrafine nanoparticles with diameter of approximately 50 nm. On the other hand, BiVO₄ exhibits micron-sized polyhedral particles with a smooth surface, i.e., particular features of microcrystals with many facets, as observed in Figure 2c. The ultrafine nanoparticles of SnS₂ and microparticles of BiVO₄ explain a large difference (more than 100 times) in the specific surface area of SnS₂ and BiVO₄ in

Table 1. Comparison of molar/weight percent BiVO₄, specific surface area, and pseudo-first-order-kinetic degradation constant, for SnS₂, BiVO₄, and BiVO₄/SnS₂ composites with various BiVO₄-to-SnS₂ ratios.

Sample	Molar Percent of BiVO4 (mole %)	Weight Percent of BiVO4 (wt %)	BET Specific Surface Area (m2/g)	Degradation Rate Constant k (10−3 min−1)	R2
SnS2	0	0	75.7	0.60	0.9408
0.01 BiVO4/SnS2	0.99	1.74	83.3	1.40	0.9990
0.03 BiVO4/SnS2	2.91	5.05	74.7	1.60	0.9975
0.06 BiVO4/SnS2	5.66	9.60	68.5	1.25	0.9965
0.12 BiVO4/SnS2	10.71	17.53	56.3	0.68	0.9955
BiVO4	100	100	0.6	0.55	0.9625

. Furthermore, Figure 2d shows a micrograph image of the 0.03 BiVO₄/SnS₂ composite, in which the nanoparticles of SnS₂ are attached to, and cover, the particles of BiVO₄. Based on the EDS elemental mapping of 0.03 BiVO₄/SnS₂ in Figure 3, the elements of Sn, S, Bi, V, and O are uniformly dispersed in the composite, indicating good distribution of BiVO₄ and SnS₂ after the incorporation.

2.3. Optical Absorption Properties

UV–Vis diffuse reflectance spectra (DRS) were used to study optical properties of as-prepared samples. The reflectance spectra were converted to absorption spectra via the Kubelka–Munk function as shown below, in Equation (1) [19,22]:

F(R) = α/S = (1 − R)²/2R,

(1)

where F(R), α, S, and R are the Kubelka–Munk function, absorption coefficient, scattering coefficient, and reflectance, respectively. As illustrated in Figure 4a, all composites exhibit absorption edges around the wavelength of 550 nm, indicating that they are photoactive in the visible light region. The curves of 0.06 BiVO₄/SnS₂ and 0.12 BiVO₄/SnS₂ are noticeably different from the remainder as their absorption regions extend beyond 550 nm. This could be attributed to an increase in the content of Bi₂S₃ in the composite.

The bandgaps (E_g) can be estimated using the following formula:

αhυ = B(hυ − E_g) ^n/2,

(2)

where hυ and B are incident photon energy and a constant associated with the material, respectively. The coefficient α can be obtained via the Kubelka–Munk function from Equation (1). The F(R) is commonly used to replace α in the Tauc plot ((αhυ)^2/n vs. hυ) [11,19,23]. In addition, SnS₂, Bi₂S₃, and BiVO₄ have been reported to be direct transition semiconductors, thus n = 1 is used in the equation [19,24]. Thus, the (F(R)hυ)² vs. hυ graph can be plotted as in Figure 4b, and the E_g values are evaluated by extrapolating the linear part of the curve to intercept the F(R) = 0 line. The E_g values of BiVO₄, SnS₂, and Bi₂S₃ are accordingly estimated to be 2.38, 2.18, and 1.46 eV, respectively, which are similar to those in previous reports [22,25]. In addition, the effective E_g values of 0.01 BiVO₄/SnS₂, 0.03 BiVO₄/SnS₂, 0.06 BiVO₄/SnS₂, and 0.12 BiVO₄/SnS₂ are estimated to be 2.17, 2.17, 2.11, and 2.04 eV, respectively. It appears that the apparent E_g value of the composite is decreased with an increase in BiVO₄. This bandgap modification may be ascribed to small composition variation of SnS₂, BiVO₄, and Bi₂S₃, especially near the interface. This phenomenon is also reported in previous publications [3,16,19].

2.4. X-Ray Photoelectron Spectroscopy (XPS)

The XPS survey spectrum of 0.03 BiVO₄/SnS₂ composite is shown in Figure 5a, demonstrating that it consists of Sn, S, Bi, V, and O elements in agreement with the EDS result. The chemical state of each element, investigated via the high-resolution XPS analyses, is shown in Figure 5b,c. As observed in Figure 5b, two separate spectra with peaks at 494.89 and 486.49 eV with an energy difference of 8.4 eV correspond to Sn 3d_3/2 and Sn 3d_5/2 of Sn⁴⁺, respectively, for a common SnS₂ [4,10,19]. On the other hand, for the 0.03 BiVO₄/SnS₂ composite, it is noted that both Sn 3d peaks shift to the higher binding energy side by about 0.30–0.35 eV, compared with those of pure SnS₂. The blueshift observed in the Sn 3d spectra is also found in, and synchronous with, the S 2p spectra.

As shown in Figure 5c, the S 2p_3/2 (161.34 eV) and S 2p_1/2 (162.52 eV) peaks, corresponding to typical Sn-S bonds of pure SnS₂ [4,9,10], also exhibit a blueshift in the binding energy by 0.1–0.3 eV for the 0.03 BiVO₄/SnS₂ composite. Furthermore, the synchronized binding energy shift is also observed, with an opposite direction, for the Bi 4f spectra. The spectral peaks for the 0.03 BiVO₄/SnS₂ composite at 158.75 and 164.34 eV, which belong to Bi 4f_7/2 and Bi 4f_5/2 of BiVO₄, respectively, exhibit the redshift, by 0.2–0.5 eV, compared with Bi 4f peaks (159.23 and 164.53 eV) of pure BiVO₄ [15]. Importantly, the synchronous phenomena are reproducible.

The shift of binding energy in the core-level of XPS spectra should be largely attributed to the change in electron concentration of semiconductors due to the interaction between SnS₂ and BiVO₄ [26]. As the electron concentration decreases, the binding energy of the semiconductor increases, and vice versa, due to the so-called electron screening effect [27]. When a heterojunction is formed through chemical interaction, the Fermi energy levels of both materials are adjusted, allowing the electron transfer between materials to achieve equilibrium [10,26,27]. In the case of the 0.03 BiVO₄/SnS₂ composite, it implies that SnS₂ has a higher Fermi energy level than that of BiVO₄, thus the electron migrates from SnS₂ to BiVO₄. This consequently reduces the electron concentration of SnS₂ and increases that of BiVO₄. As a result, the Sn 3d and S 2p spectra of the composite exhibit the blueshift, while Bi 4f spectra exhibit the redshift.

Furthermore, the broad peaks of the Bi 4f exhibited in the 0.03 BiVO₄/SnS₂ composite may be caused by the presence of Bi₂S₃. The Bi 4f_7/2 and Bi 4f_5/2 spectral peaks of a typical Bi₂S₃ are reported to be at 158.4 and 163.8 eV [28], respectively, which are slightly lower than those of BiVO₄. Moreover, the S 2p peaks at 161.45 and 162.81 eV in the 0.03 BiVO₄/SnS₂ composite could also correspond to S²⁻ of Bi₂S₃ [29]. The results from XPS spectra and XRD patterns (Section 2.1) strongly suggest the coexistence of SnS₂, Bi₂S₃, and BiVO₄ in the composite. Additionally, the shifts in binding energy indicate that the interaction among SnS₂, Bi₂S₃, and BiVO₄ in the 0.03 BiVO₄/SnS₂ composite is a chemical bonding rather than a physical contact only.

2.5. Photocatalytic Activity

Photocatalytic activity was evaluated via concentration reduction in RhB (rhodamine B) dye solution over time, as shown in Figure 6a, where C₀ is the initial RhB concentration at the time of light irradiation (t = 0), and C_t is the RhB concentration at any sampling time during irradiation. Blank RhB (a control test) in Figure 6a demonstrates that the RhB dye solution was stable under this test condition without any photocatalyst in the solution and that the photolysis of RhB dye was negligible over 240 min of visible light irradiation. Conversely, the concentration of RhB decreased over time for all other RhB solutions containing any photocatalyst, evidencing that photocatalysts used in this study respond to visible light (λ > 420 nm). Importantly, all composited powders exhibit higher photocatalytic degradation of RhB than that of bare SnS₂, Bi₂S₃, or BiVO₄ under visible light irradiation for over 240 min. The temporal evolution of RhB absorption spectra for each photocatalyst can be seen in Figure S2.

The degradation rates were compared using a photodegradation rate constant (k), assuming a pseudo-first-order reaction model, ln(C₀/C_t) = kt, as shown in Figure 6b. The kinetic model fits the data well, and the k value of the 0.03 BiVO₄/SnS₂ sample is about 2.3 times or 2.9 times greater than that of single SnS₂ or BiVO₄, respectively (Table 1). The photocatalytic activity of the composite is enhanced as the content of BiVO₄ is increased from 0.01 to 0.03, whereas further addition of BiVO₄ leads to reduction in photocatalytic activity of the composites (0.06 BiVO₄/SnS₂ and 0.12 BiVO₄/SnS₂). It is thus reasonable to consider that an optimal molar ratio exists at a relatively low concentration of BiVO₄. One possible reason could be the variation in the “effective” specific surface area. As shown in Table 1, the specific surface area of the composites mostly decreases with an addition of BiVO₄, ascribed to the significantly small value of bare BiVO₄ (0.6 m²/g). Furthermore, the interfacial reaction and agglomeration of powders may affect the effective active area for photocatalytic activity. As previously mentioned, the formation of Bi₂S₃ is intensive as the amount of BiVO₄ in the composite is increased. Although a small amount of Bi₂S₃ would be favorable for the composite, excess content may lead to deterioration of the photocatalytic performance because Bi₂S₃ has low photocatalytic activity of RhB degradation compared to SnS₂ and BiVO₄, possibly due to its rapid recombination rate of the photogenerated electron-hole [17]. Thus, excessive Bi₂S₃ could act as a recombination center for electron-hole pairs in the composite, which is similar to the findings of previous studies [13,19].

In addition to the photocatalytic activity of the material, stability and reusability are also important to determine its value in practical applications. Thus, the reusability test was also conducted by collecting the used sample (0.03 BiVO₄/SnS₂) after irradiation via centrifuge and putting it back into the fresh RhB solution. The same procedure for the photocatalytic activity measurement was repeated in this reusability test. Figure 6c shows the degradation efficiencies of 0.03 BiVO₄/SnS₂ composite over four cycles, illustrating stability and reusability of the 0.03 BiVO₄/SnS₂ composite. A slight loss (around 6%) of the degradation efficiency in the 4th run could be due to a loss of photocatalyst during the recovery process. Furthermore, the stability of the composite was also confirmed by XRD pattern before and after irradiation (Figure S3), in which the crystal structure of 0.03 BiVO₄/SnS₂ was not significantly altered after RhB degradation process.

To determine main reactive species responsible for RhB photodegradation by 0.03 BiVO₄/SnS₂ during irradiation, scavenger tests were conducted using various types of scavengers. In this study, methanol (10 mM) [6], ascorbic acid (AA, 10 mM) [30], and isopropyl alcohol (IPA, 10 mM) [16] were used as the scavengers for superoxide radicals (•O₂⁻), holes (h⁺), and hydroxyl radicals (•OH), respectively. As shown in Figure 6d, an addition of methanol into the system has only a slight effect on RhB degradation, indicating that •O₂⁻ radicals play a minor role in the degradation process. Conversely, AA (h⁺ scavenger) and IPA (•OH scavenger) greatly suppress photocatalytic RhB degradation. The result strongly suggests that h⁺ and •OH radicals are the main reactive species for the RhB photodegradation process with the 0.03 BiVO₄/SnS₂ photocatalyst under visible light (λ > 420 nm) irradiation.

2.6. Possible Mechanism for Photocatalytic Activity Enhancement of BiVO₄/Bi₂S₃/SnS₂ Heterojunction

A plausible photocatalytic mechanism of the BiVO₄/Bi₂S₃/SnS₂ heterojunction could be explained by understanding the band energy structures of the photocatalysts as illustrated in Figure 7. The empirical Equations (3) and (4) are often used to estimate the band edge of the semiconductor such as conduction band edge potential (E_CB) and valence band edge potential (E_VB) [15,19]:

E_VB = χ − E^e + 0.5E_g

(3)

E_CB = E_VB − E_g,

(4)

where, χ is absolute electronegativity of a semiconductor, determined by the geometric mean of absolute electronegativity values of constituent atoms. In this study (Section 2.3), E_g values of SnS₂, Bi₂S₃, and BiVO₄ are 2.18, 1.46, and 2.38 eV, respectively. The absolute electronegativity values of Bi, V, O, Sn, and S are 4.69, 3.6, 7.54, 4.3, and 6.22 eV, respectively, taken from Pearson’s experimental data [31]. The value E^e is the energy of the free electron on the hydrogen scale, which is 4.5 eV. According to this calculation, E_CB and E_VB of SnS₂ are estimated to be −0.09 and 2.09 eV, respectively, whereas BiVO₄ exhibits E_CB of 0.47 eV and E_VB of 2.85 eV, and Bi₂S₃ possesses E_CB of 0.05 eV and E_VB of 1.51 eV. The E_CB and E_VB values of these photocatalysts in the present study are similar to those of previous reports [11,16,19].

Fermi level positions of SnS₂, Bi₂S₃, and BiVO₄ with respect to their VBs (valence bands) can be determined from the potential difference between VBM (VB maximum) and Fermi level from the valence band XPS spectra in Figure S4 [12]. The difference between the Fermi level and VBMs of SnS₂, Bi₂S₃, and BiVO₄ is found to be 1.3, 0.3, and 2 eV, respectively. The result indicates that SnS₂ and BiVO₄ are n-type semiconductors, whereas Bi₂S₃ is a p-type semiconductor, which is in agreement with prior studies [2,6,16]. Our XPS results suggest that the photocatalysts are chemically bonded at the interface, and their Fermi levels are aligned after contact; as a result, their band energy structures are accordingly adjusted, creating a heterojunction as depicted in Figure 7 [12,16].

To understand the charge-transfer pathway in the BiVO₄/Bi₂S₃/SnS₂ heterojunction system, we explore and examine two possible models for the charge transfer process as proposed in Figure 8. All of the photocatalysts in both models are capable of generating electron-hole pairs upon visible light illumination due to their narrow bandgaps. In model A, the photogenerated electrons (e⁻) would migrate from the CB (conduction band) of Bi₂S₃ to CBs of SnS₂ and BiVO₄. Thus, •O₂⁻ radicals would not be produced by the heterojunction since the CBs of both SnS₂ and BiVO₄ are incapable of reducing dissolved O₂ to produce •O₂⁻ because their CBs are less negative than −0.33 eV (O₂/ •O₂⁻) [16]. On the other hand, the generated holes (h⁺) would move from the VBs of both SnS₂ and BiVO₄ to the VB of Bi₂S₃. In this scenario, the heterojunction also could not oxidize H₂O to form •OH because VB of Bi₂S₃ is less positive than 2.72 eV (for •OH/H₂O) [8,16]. It has been reported that •O₂⁻, •OH, and h⁺ play major roles in the RhB photodegradation process [16,32,33]. Although model A might improve charge separation of photoinduced electrons and holes, it would be less efficient as the composite would be unable to produce •O₂⁻ and •OH to degrade the RhB dye. Moreover, the model is inconsistent with our trapping experiment (our scavenger test in Section 2.5) in which •O₂⁻ and •OH were observed in the RhB degradation process.

A more plausible scenario might be that of model B in Figure 8, involving a Z-scheme process. Based on this model, we can explain the presence of •O₂⁻, •OH, and h⁺ for the RhB degradation process. In the Z-scheme process, the photoinduced e⁻ from the CBs of both SnS₂ and BiVO₄ could migrate to and recombine with h⁺ in the VB of Bi₂S₃. Consequently, e⁻ from the CB of Bi₂S₃ would be able to react and reduce the dissolved O₂ in H₂O to produce •O₂⁻ because the CB of Bi₂S₃ is more negative than −0.33 eV. According to our scavenger test, •O₂⁻ played only a minor role in the RhB degradation process by the 0.03 BiVO₄/SnS₂ heterojunction, which fits well with the current model due to the low optimal content of Bi₂S₃ in the composite. Meanwhile, since h⁺ in the VB of Bi₂S₃ quickly recombine with e⁻ of BiVO₄ and SnS₂, it would leave h⁺ in the VB of BiVO₄ to oxidize H₂O and generate •OH (2.72 vs. NHE); similarly, h⁺ are also left behind in the VB of SnS₂. As the VB of SnS₂ is not negative enough to produce •OH, h⁺ would decompose RhB directly due to its strong oxidation ability [16,32]. Thus, the Z-scheme model is consistent with the experimental result, and may be used to explain the charge-transfer pathway of the BiVO₄/Bi₂S₃/SnS₂ heterojunction system.

3. Materials and Methods

3.1. Materials

Thioacetamide (C₂H₅NS, 98%), acetic acid (CH₃COOH, 99.7%), ethylene glycol ((CH₂OH)₂, 99.5%), L-ascorbic acid (C₆H₈O₆, 99.6%), and thiourea (H₂NCSNH₂, 98%) were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). The tin(IV) chloride pentahydrate (SnCl₄·5H₂O, 98%), ammonium vanadate (V) (NH₄VO₃, 99.0%), bismuth (III) nitrate pentahydrate (Bi(NO₃)₃·5H₂O 99.5%), methanol (CH₃OH, 99.8%), and isopropyl alcohol ((CH₃)₂CHOH, 99.7%) were supplied by Nacalai Tesque (Kyoto, Japan). The rhodamine B (C₂₈H₃₁ClN₂O₃, > 95%) was purchased from Tokyo Chemical Industry (Tokyo, Japan). All chemicals were analytical grade and used without further purification. Deionized water (DI water) was obtained from the Direct-Q water purification system (Millipore).

3.2. Preparation of SnS₂

Preparation of SnS₂ was conducted via a typical synthesis method. The powder of SnCl₄·5H₂O (5 mmol) was dissolved in 40 mL of 5% (v/v%) acetic acids under magnetic stirring. Then, 10 mmol amount of thioacetamide was added to the solution. After 30 min of vigorous stirring to achieve a homogeneous solution, the solution was transferred to a stainless-steel autoclave attached with a Teflon liner to fill 80% of its maximum capacity (50 mL). It was then put in a preheat electric oven at 150 °C for 12 h under autogenous pressure. After letting the autoclave cool to room temperature, SnS₂ precipitate was collected via centrifuge at 4000 rpm for 5 min. The SnS₂ was then washed several times with DI water and ethanol and dried at 90 °C overnight.

3.3. Preparation of BiVO₄

Typical hydrothermal synthesis was used to prepare BiVO₄. The first solution was prepared by dissolving 2.43 g of Bi(NO₃)₃·5H₂O in 20 mL of 2 M HNO₃ acid, and the second solution was made by dissolving an equimolar amount of NH₄VO₃ in 2 M NaOH solution. Then, the second solution was poured drop by drop into the first solution. The clear solution turned yellow as BiVO₄ precipitate was formed. The mixture solution was continually stirred for another 10 min before adding 1 mL of acetic acid into the solution. After 1 h of stirring, the solution was transferred to the stainless-steel autoclave with a Teflon liner. The autoclave was heated at 180 °C for 24 h. Finally, BiVO₄ precipitate was collected, washed, and dried, following the same procedure described in Section 3.2.

3.4. Preparation of BiVO₄/Bi₂S₃/SnS₂ Composites

The composite was prepared using an ultrasonic mixing and a solvothermal synthesis. A specific amount of BiVO₄ (0.01, 0.03, 0.06, and 0.12 mmol) was mixed with 1 mmol of SnS₂ in 40 mL of ethylene glycol. The mixture was thoroughly mixed via ultrasonication at 45 kHz for 1 h before transferring to the autoclave. It was then sealed and heated at 150 °C for 8 h. The final product of the composite was obtained through washing and drying, following the same procedure as stated in Section 3.2 and Section 3.3. The composites with different ratios of BiVO₄ to SnS₂ were denoted, respectively, as 0.01 BiVO₄/SnS₂, 0.03 BiVO₄/SnS₂, 0.06 BiVO₄/SnS₂, and 0.12 BiVO₄/SnS₂. For reference, a bare Bi₂S₃ was synthesized using solvothermal synthesis (detailed in the Supplementary Information).

3.5. Characterization

X-ray diffraction (XRD) measurement was conducted using a Rigaku RINT2100 at 40 kV and 30 mA with Cu Kα radiation (λ = 1.5418 Å), where the diffractogram was obtained via scanning the sample in a 2θ angle range from 10° to 80°. Microstructures and morphologies of the samples were investigated using an FE-SEM (Hitachi SU6600 Scanning Electron Microscope) equipped with Bruker EDX operated at 15 and 20 kV for SEM and EDX, respectively, where all samples were coated with Au via sputtering prior to the analyses. UV–Vis diffuse reflectance spectra (DRS) were obtained using a Lambda 750S UV/Vis/NIR Spectrophotometer with BaSO₄ as reference. X-ray photoelectron spectroscopy (XPS, JPS-9030 X-ray photoelectron spectrometer) was conducted with Mg Kα radiation using C 1s = 284.8 eV as reference. The Brunauer–Emmett–Teller (BET) specific surface area was measured and evaluated using nitrogen gas adsorption with a FlowSorb III 2305 Micromeritics Instrument (Shimadzu, Japan).

3.6. Photocatalytic Activity Measurement

Evaluation of photocatalytic activity was conducted via the photodegradation of RhB dye under visible light irradiation using a 500 W Xe lamp (Ushio, UXL-500D-O) equipped with a cutoff filter (λ > 420 nm). The intensity of light from the Xe lamp was 100 mW/cm² calibrated using a Spectroradiometer (S-2440 model II). After passing through the cutoff and water filters, the sample solution received around 40 mW/cm² of light intensity. In each measurement process, 30 mg of the photocatalyst was put into 80 mL of RhB solution (5 mg/L, 0.01 mmol/L) in a beaker with 100 mL capacity. The solution was magnetically stirred and maintained at 25 °C during irradiation. About 3 mL of sample solution was taken every 60 min, and the photocatalyst powder was filtered out by syringe filter (0.22 µm, PTFE). The absorbance of each RhB solution was measured using a UV–Vis spectrophotometer (Lambda 750S UV/Vis/NIR). Then, the concentration of RhB was determined from the absorbance intensity at λ_max = 554 nm. Before irradiation, the establishment of adsorption–desorption equilibrium between photocatalyst and RhB solution was achieved to ensure an accurate result of photocatalytic activity.

Since SnS₂ exhibits exceptionally strong adsorptivity for RhB dye [23], an initial dye concentration (C₀) for each photocatalytic test would be different, depending on the ratio of BiVO₄/SnS₂ composites, after achieving an adsorption–desorption equilibrium. To ensure that all samples had relatively similar initial concentrations at the start of illumination, all samples were subjected to the adsorption–desorption process twice. Each sample was added to a RhB solution, which was then agitated by sonication for 10 min and magnetically stirred for 60 min in the dark. Then, the sample was collected via centrifuge and put into a fresh RhB solution, where the adsorption–desorption procedure was performed again prior to the photocatalytic test. This devised method allowed an initial RhB concentration (C₀) to be similar for each photocatalytic test, using various composite ratios.

4. Conclusions

In summary, three-phase photocatalysts consisting of BiVO₄, Bi₂S₃, and SnS₂ were prepared by chemical reaction between BiVO₄ and SnS₂ via solvothermal synthesis. The composite photocatalyst produced via a molar ratio of 1:0.03 (SnS₂:BiVO₄) demonstrates the highest photocatalytic performance for RhB degradation among our prepared samples. The composite is proved to be stable after several cycles under visible light irradiation. The main reactive species for the photocatalytic degradation of RhB for 0.03 BiVO₄/SnS₂ are h⁺ and •OH, whereas •O₂⁻ also plays a minor role in the degradation process. The enhanced photocatalytic activity is attributed to the formation of Bi₂S₃, allowing a suitable condition for the electron pathway (Z-scheme) in the BiVO₄/Bi₂S₃/SnS₂ heterojunction. We believe that our discovery of the beneficial formation of Bi₂S₃ could encourage more research that focuses on materials prone to reaction with each other at elevated temperature and pressure. The finding may provide a different approach for preparation of ternary heterojunctions by taking advantage of the chemical reaction between combined photocatalysts.