Photocatalysis, a promising green catalysis technology, was widely investigated to address environmental pollution and mitigate energy shortage [1-6]. The use of solar-driven photocatalysts could effectively promote organic pollutant degradation. Owing to the low cost, high activity, and optimal stability of TiO₂ catalysts, earnest efforts have been made to enhance photocatalytic performance based on these compounds[7-13]. However, the wide band gap (3.4 eV) of TiO₂ shows a narrow solar absorption range with rapid photogenerated charge carrier recombination[14, 15]. The use of co-catalysts, mostly noble metals (e.g., Pt, Au, and Pd), has been demonstrated as a successful strategy for improving electron migration. Furthermore, wider absorption semiconductors, such as WO₃[6, 16, 17], CuO [18, 19], CdS [5, 20, 21], SnS₂ [22], MoS₂[23-26], BiOX (X = Cl, Br, I) [27-30], BiVO₄ [31-34], g-C₃N₄ [35-42], and red/black phosphorous [43-45], have been employed to greatly enhance photocatalytic efficiency [46]. Up till now, other novel strategies have been investigated to reduce the recombination of photogenerated carriers and accelerate the transfer of electron-hole pairs [40, 46-52]. Constructing a built-in electric field could significantly accelerate photogenerated carrier separation by heterostructure [53-55]. Fabricating a tight-contact heterostructure was considered a key factor in forming an effectively built-in electric field [56]. Compared with the physical method, in-situ conversion can more easily facilitate the construction of an intimate interface and a built-in electric field for accelerating electron transfer with high photocatalytic performance. The formation of heterostructure photocatalysts through step-by-step physical composition could not successfully produce a built-in electric field [57]. Therefore, the method of in-situ conversion was applied to fabricate heterostructure photocatalysts with high photocatalytic performance [58, 59]. Such an in-situ conversion route allows the fabrication of uniform heterostructure photocatalysts with a tight contact and strong interaction. This effectively accelerates the transfer of photogenerated carriers [60]. Compared with a simple physical mechanical mixing synthesis method, the in-situ synthesis method for the as-prepared catalysis displays a more uniform structure. Moreover, this in-situ synthesis method provides a strategy for forming a tight-contact heterostructure between two compounds [61]. From the eco-friendly and chemical stability view, tin oxide (SnO₂) was employed as a promising photocatalyst [62]. Although SnO₂ served as a wide band gap photocatalyst, it exhibited optimal electron transfer performance, demonstrating the potential for use as an electron transport layer in solar cells [63, 64]. Furthermore, as a metal sulfide photocatalyst, SnS₂ with a narrow band gap structure exhibited a visible light response [65, 66]. However, SnS₂ suffered from high photogenerated carrier recombination. Thus, forming a heterostructure between SnO₂ and SnS₂ could not only enhance the visible light response range but also could improve the separation efficiency of photogenerated carriers. Moreover, SnS₂ could be obtained in-situ from SnO₂ through the ion-exchange method.

In the present work, SnO₂/SnS₂ heterostructure photocatalyst fabricated via an in-situ ion-exchange method was utilized to drive the photocatalytic water treatment. Compared with pristine SnO₂ and the mechanically mixed sample of SnO₂ and SnS₂, the as-prepared SnO₂/SnS₂ heterostructure sample exhibited a stronger interaction, demonstrating enhanced photo-degradation performance. The results of photocatalytic activity tests and photoelectrochemical measurements verified that this in-situ-induced SnO₂/SnS₂ heterostructure exhibited favorable photogenerated electron-hole pair separation efficiency. Moreover, the active species trapping experiments showed that optimal electron transfer performance could enhance the utilization of photogenerated electron to produce superoxide radicals to degrade organic pollutants. Furthermore, the calculated EIS Bode plots suggested that as-prepared heterostructure photocatalysts exhibited the longest electron lifetime.

To prepare the sample, 2.0 g of SnCl₂·2H₂O (Aladdin, ≥ 99.99%) and tert-butanol (Aladdin, ≥ 99.5%) were mixed together under rigorous magnetic stirring to form a homogenous solution. Meanwhile, 5.00-mL glycerin (Aladdin, ≥ 99.0%) and 2.25-mL cyclohexane (Aladdin, AR) were added to the above solution. The solution was kept under vigorous stirring with the formation of a transparent solution and then transferred into a 50-mL Teflon-lined stainless-steel autoclave. It was further heated at 200 ℃ for 18 h in an oven and naturally cooled to room temperature. The resulting solid powders were collected by centrifugation and washed with deionized water and alcohol several times. After drying in vacuum at 80 ℃ and roasting at 500℃ for 4 h, SnO₂ was obtained. To obtain the SnO₂/SnS₂ heterostructures, 0.9 g of as-obtained SnO₂ sample was dispersed in deionized water, and a 0.6-g sulfur source C₂H₅NS (Aladdin, ≥ 99.0%) was added into the mixture and stirred for 30 min. Then, the mixture was transferred into a 50-mL Teflon-lined stainless-steel autoclave and was heated at 160 ℃ for 30 h. The as-obtained powders were collected by centrifugation and washed with deionized water and alcohol several times, followed by drying at 80 ℃ under vacuum. For comparison, pure SnS₂ was also prepared by adding excess sulfur source C₂H₅NS [67].

X-ray diffraction (XRD) patterns were obtained in parallel mode (2θ from 20° to 80°) using a Rigaku Dmax-3C Advance X-ray diffractometer (Cu-K_α radiation, λ = 1.5406 Å). Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were performed on a Hitachi S-4800 and JEOL 2010F, respectively. The specific surface area (A_BET) of the as-prepared samples was tested using a nitrogen adsorption and desorption isotherm and calculated by applying the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models on the desorption branches (Micromeritics, TriStar II 3020). Diffuse reflectance spectra over the range 200–800 nm were collected through a UV-vis spectrophotometer (DRS, MC-2530) equipped with an integrating sphere assembly, using BaSO₄ as a reference. X-ray photoelectron spectroscopy (XPS) spectra were recorded by a PerkinElmer PHI 5000C ESCA system. The contaminant carbon (C 1s = 284.6 eV) was used as a reference for calibrating the binding energies.

The photocatalytic pollutant degradation tests of all as-prepared samples were evaluated by the degradation of Methylene blue (MB), Rhodamine B (RhB), and Methyl orange (MO) in aqueous solution. Each test was carried out at 25 ℃ in a homemade glass reactor, containing 50 mg of as-prepared catalyst in 100-mL pollutant solution with an initial concentration of 10 mg/L under magnetic stirring at a speed of 500 rpm. Before light irradiation, 1 h of dark adsorption was necessary to reach adsorption-desorption equilibrium. Then, a 300-W Xe lamp (AULIGHT, CEL-HXF300/CEL-HXUV300) with a 400-nm cutoff filter (λ> 400 nm) was initiated for photocatalytic degradation reaction. At each hour, the pollutant solution was sampled and centrifuged to remove the solid catalyst, followed by analysis on a UV spectrophotometer (UV 7504/PC) to determine the concentration of the residual organic molecules at their characteristic wavelength.

All the photoelectrochemical measurements were performed on a CHI660E electrochemical workstation with a standard three-electrode setup. A 300 W Xe lamp with a 400 nm cutoff filter (λ > 400 nm) was utilized as the light source. The doctor-blade method was used to prepare the photoelectrode for the photoelectrochemical measurements [68]. Briefly, 10 mg of as-prepared sample coated on FTO served as the work electrode with a 2 cm² working area, Pt foil (2 cm²) served as the counter electrode, and saturated calomel electrode (SCE) was used as the reference electrode. Meanwhile, 0.5 mol/L of Na₂SO₄ aqueous solution was used as the electrolyte for photoelectrochemical measurements. The photocurrent response curves were obtained with an on/off pulse cycle of 50 s at an applied potential of 0.5 V vs. SCE by chronoamperometry. Electrochemical impedance spectroscopy (EIS) analyses were collected at an open-circuit voltage over the frequency range of 10⁶–10^–1 Hz with an alternating current voltage of 5 mV. The Motto-Schottky plots were obtained at 1000 Hz to determine the flat-band potential of those prepared samples.

Fig. 1a displays the schematic of the in-situ synthesis process for SnO₂/SnS₂ heterostructure photocatalysts. SnO₂ was synthesized by the hydrothermal method in tert-butanol solution. The as-formed SnO₂ powders were further transformed to SnO₂/SnS₂ heterostructures via the ion-exchange route by using C₂H₅NS as a sulfur source. To determine the crystal structures and phase of the as-prepared samples, XRD patterns for these samples were obtained. As shown in Fig. 1b, the XRD patterns of SnO₂ and SnS₂ are in good agreement with JPCDS No. 41-1445 and JPCDS No. 23-0677 for both standard SnO₂ and SnS₂ crystals. Furthermore, the as-prepared SnO₂/SnS₂ heterostructure sample displayed both SnO₂ and SnS₂ crystal phase. The as-prepared SnO₂/SnS₂ heterostructure sample shows the characteristic peaks of both SnO₂ and SnS₂ crystal phases. The as-formed samples were also investigated by scanning electron microscopy (SEM). As shown in Fig. 1c, the as-prepared composites comprised many typical hexagonal nanosheets with an average size of 1 μm. These uniform nanosheets were also covered with some nanoparticles, which could be ascribed to the remaining SnO₂ species. This is demonstrated in Fig. 1d as well, further indicating that pure SnO₂ was composed of many nanoparticles. To transfer the remaining SnO₂ to SnS₂, such composites were further treated by a sulfuration process with the formation of perfect hexagonal nanosheets, as shown in Fig. 1e. The results of the XRD patterns and SEM images of SnO₂ and SnS₂ shows that SnO₂/SnS₂ heterostructure could be obtained by the sulfuration of SnO₂ nanocrystals.

Fig. 2b shows the TEM images of an SnO₂/SnS₂ heterostructure sample, revealing SnO₂ nanoparticles loaded onto the surface of SnS₂ hexagonal nanosheets. The high-resolution transmission electron microscopy (HRTEM) image of such a heterostructure sample was marked with two characteristic lattice spacings of 0.26 and 0.31 nm, ascribed to SnO₂(101) and SnS₂(100), respectively. Such results also could confirm the existence of SnO₂ and SnS₂ in the heterostructures, further supporting the results of the XRD pattern. Moreover, the HRTEM image also verified the in-situ formation of the heterostructure through the hydrothermal method with ion exchange. The inset graph of Fig. 2a displays the SAED pattern of the as-prepared SnO₂/SnS₂ heterostructure sample. The formation of heterostructure also could be confirmed from the SAED pattern with the characteristic ring radius of SnO₂ and spots of SnS₂. The characteristic ring radius shows the (101) facet of SnO₂ with a crystal lattice spacing of 0.26 nm. Meanwhile, the spots in SAED image indicate the (100) facet of SnS₂ with a lattice fringe of 0.31 nm. The analysis of the SAED pattern is consistent with the results of the HRTEM image, confirming the formation of the SnO₂ and SnS₂ heterostructure. The EDX mapping images of the SnO₂/SnS₂ heterostructure were also collected. As shown in Figs. 2c,2d, and 2e, the hexagonal nanosheets structure exhibits an abundant sulfur element distribution, and the nanoparticles show an elemental oxygen distribution. Such results indicate the formation of a SnO₂/SnS₂ heterostructure during the hydrothermal treatment of SnO₂ through the ion-exchange route. The formation of heterostructure between SnO₂ and SnS₂ may be beneficial for the enhancement of the photocatalytic performance of the as-formed samples.

The absorption ability of catalysts is considered as one of the key indicators of catalytic performance. Fig. 3 shows the nitrogen adsorption-desorption isotherms of as-prepared SnO₂, SnO₂/SnS₂ heterostructure, and SnS₂. The as-prepared heterostructure samples exhibited excellent reversibility from the desorption-desorption plots. As shown in Table 1, both pure SnO₂ and SnS₂ exhibited low surface areas of 33.20 and 18.39 m²/g, respectively. Nevertheless, it was interesting that the partial transfer of SnO₂ to hexagonal SnS₂ nanosheets resulted in a significant increase in the surface area to 89.57 m²/g. Such a high surface area of the as-prepared SnO₂/SnS₂ heterostructure could be particularly important for increasing photocatalytic activity owing to its strong adsorption capability for trapping both light and reactants [69, 70]. SnO₂/SnS₂ and the SnO₂ & SnS₂ mix have similar surface areas of 89.57 and 92.40 m²/g, respectively. Thus, the enhanced photocatalytic degradation performance of SnO₂/SnS₂ is derived from the tight contact heterostructure rather than from the more active sites.

XPS spectra were used to further verify the formation of the SnO₂/SnS₂ heterostructure. Fig. 4a shows the XPS survey spectra of the SnO₂, SnS₂, and SnO₂/SnS₂ heterostructures; the results show the absence of elemental sulfur after in-situ ion exchange with SnO₂ used as a precursor. As shown in Fig. 4b, the binding energy of Sn 3d_5/2 in the SnO₂/SnS₂ heterostructure (486.43 eV) was between those of SnS₂ (486.31 eV) and SnO₂ (486.67 eV), corresponding to the formation of the heterojunction between SnO₂ and SnS₂. In addition, the S 2p orbital and the O 1s orbital were roughly at 161.72 and 530.71 eV, respectively, which also conformed to the standard value of SnO₂ and SnS₂ (Figs. 4c and 4d), and the combination of S^2– and O^2– caused the binding of Sn⁴⁺. Therefore, a strong interaction and intimate interface could be observed between SnO₂ and SnS₂. As such, it could be confirmed that the SnO₂/SnS₂ heterojunction was formed through the in-situ ion-exchange method. The solar light absorption capability was important for the photocatalyst, which could affect the sunlight utilization. Fig. 4e displays the results from the UV-vis diffuse reflection spectrometer of as-prepared SnO₂, SnS₂, and SnO₂/SnS₂ heterostructures. As suggested by Fig. 4e, all the samples possessed a strong ultraviolet light absorption, whereas in the visible region, the SnO₂/SnS₂ heterostructure showed a stronger absorption compared with the pure SnO₂ nanoparticles, because the narrow band gap of SnS₂ leads to strong visible-light absorption. This result indicates that the response of the heterostructure sample to sunlight was successfully extended from the ultraviolet range to the visible light range, improving its utilization of sunlight. Based on the diffuse reflection coefficient of the Kubelka-Munk equation (Eq. 1) [71], the band gaps (E_g) of SnO₂, SnS₂, and SnO₂/SnS₂ were calculated to be approximately 3.4, 2.2, and 2.2 eV, respectively (Fig. 4f).

(Eq.1)

where hv is the discrete photon energy, B is a constant of the material, and alpha (α) could be calculated by using the diffuse reflection coefficient of the Kubelka-Munk equation.

The photocatalytic degradation tests of as-prepared samples were performed by MB and MO degradation under visible light irradiation (300-W Xe lamp with a 400-nm cutoff filter). Fig. 5a exhibits the results of the photocatalytic degradation tests of MB, which shows that the as-prepared SnO₂/SnS₂ sample displayed the highest MB removal efficiency (1 - C/C₀) of 97.2% within 180 min under visible light irradiation, whereas the MB removal efficiencies for SnO₂, SnS₂, and the SnO₂ & SnS₂ mix were about 58.5%, 78.7%, and 64.1%, respectively, under the same conditions. The zeta potentials of the as-obtained samples were obtained to illustrate the surface charge of the as-prepared photocatalysts (Fig. S1). All the SnS₂-based samples exhibit negative potentials, which may be due to the effective adsorption of the cationic dye MB during the photo-degradation process. Similarly, Fig. 5b shows the MO photo-degradation efficiency, which indicates that the as-prepared SnO₂/SnS₂ sample also possessed the highest degradation efficiency of 97.9%, whereas the MO removal efficiencies for SnO₂, SnS₂, and the SnO₂ & SnS₂ mix were approximately 13.2%, 36.7%, and 40.2%, respectively. The degradation activities of RhB have been included in the revised manuscript. The results show that SnO₂/SnS₂ has the best photocatalytic degradation property of RhB, which is consistent with the results for MB and MO (Fig. S5). After a one-hour test, the RhB (initial concentration is 5 x 10^-6 mg/L) removal efficiency reaches 94.17% for SnO₂/SnS₂, whereas the removal efficiencies for SnO₂, SnS₂, and the SnO₂ & SnS₂ mix samples were approximately 54.2%, 79.6%, and 70.0%, respectively, under visible-light irradiation (λ > 400 nm). It could be concluded that the formation of heterostructure between SnO₂ and SnS₂ could effectively improve the performance of photocatalytic degradation.

To further explore the reason for the enhancement of photocatalytic performance, various probe experiments were performed. Combined with the UV-vis DRS analysis, Fig. 5c shows the activity tests for MB photo-degradation under UV light irradiation (λ = 365 nm, 100 mW). Before the photocatalytic reaction, the as-prepared samples were stirred in 10 x 10^‒6 mg/L of MO solution under dark for 3 h, ensuring adsorption-desorption equilibrium. The results indicated that the as-prepared SnO₂/SnS₂ sample still exhibited the highest MB removal efficiency (44.1%) with 300 min. However, SnO₂, SnS₂ and the SnO₂ & SnS₂ mix possessed low MB removal efficiencies (4.6%, 28.0% and 9.5%, respectively). Such results verified that the formation of SnO₂/SnS₂ heterostructure could greatly improve photocatalytic degradation performance. To further understand the mechanism of photo-degradation and photogenerated carrier transfer process, the active species trapping experiments were performed for exploring active species during the process of photo-degradation. As shown in Fig. 5d, for the as-prepared SnO₂/SnS₂, the degradation efficiency of MB was clearly inhibited when 0.1 mmol AgNO₃ as electron (e^–) scavenger was added into the reaction solution. The photo-degradation efficiency decreased only a little after adding 0.2 vol% methanol (ME) as a hole (h⁺) scavenger and 0.2 vol% isopropyl alcohol (IPA) as the hydroxyl radical (HO^•) scavenger. These results suggest that the photogenerated electrons generated superoxide radicals (O₂^•–) as the major active species during the photocatalytic process. As is known, superoxide radicals are generated from dissolved oxygen. Bubbling N₂ could remove the dissolved oxygen, inhibiting the generation of superoxide radicals. Hence, the control experiment by bubbling N₂ was conducted to verify the role of superoxide radicals. Fig. S4 shows the control experiment of bubbling N₂ to remove dissolved oxygen from the MB solution, revealing the dramatic decrease in the photo-degradation efficiency of MB. Such results indicate that the absence of dissolved oxygen suppresses the generation of superoxide radicals, which decreases the photo-degradation efficiency. Therefore, it could be indirectly verified that superoxide radicals are the active species during the photo-degradation process in which SnO₂/SnS₂ serves as the photocatalyst. The active species trapping experiments for SnO₂ (Fig. 5e) indicates that the MB degradation efficiency suffered a slight decrease upon the introduction of AgNO₃ (0.1 mmol) as an electron scavenger or IPA (0.2 vol%) as a hydroxyl radical scavenger. However, the hole scavenging experiment demonstrated that the photo-activity for degrading MB could be maintained in the presence of 0.2 vol% ME (as hole scavenger). Thus, it was reasonable that the active species of SnO₂ were photogenerated electrons and hydroxyl radical. As shown in Fig. 5f, the results of the trapping experiments showed that introducing the three scavengers could decrease the photo-degradation efficiency of MB for SnS₂. Therefore, hydroxyl radical, photogenerated holes, and electrons may play the same role for degrading MB under visible-light irradiation in the case of SnS₂. Based on these above results, the built-in electric field in SnO₂/SnS₂ heterostructure can be considered to have changed the photogenerated carrier transfer pathway, allowing more photogenerated electrons to react with O₂ with the formation of superoxide radicals. Fig. 5g displayed the lifetime measurements of the SnO₂/SnS₂ heterostructure for the photo-degradation of MB. The results showed that the as-prepared heterostructure sample could be recycled 7 times in treating the MB solution, suggesting excellent stability. Both the morphologies and crystallinity of in-situ SnO₂/SnS₂ could be well maintained, even after being utilized 7 times (Figs. S2 and S3), indicating its good stability during the photo-degradation process.

To further verify the effect of heterostructure, photoelectrochemical measurements of such samples were tested. Photocurrent response measurements were used to demonstrate the photogenerated carrier separation and migration. As shown in Fig. 6a, a clear comparison could be observed in the photocurrent density curves for the as-prepared samples under visible-light irradiation (λ > 400 nm). The SnO₂/SnS₂ heterostructure displayed the highest photocurrent intensity of 21.5 μA/cm², which was about 307 times higher than that of SnO₂ (0.07 μA/cm²), 28 times higher than that of SnS₂ (0.76 μA/cm²), and 21 times higher than that of the SnO₂ & SnS₂ mix (1.03 μA/cm^-2). Such results indicate that the highest photocurrent of SnO₂/SnS₂ could be attributed to the existence of the heterostructure, which is beneficial for the photogenerated electron-hole transfer and separation process. Moreover, to further confirm the transfer and separation of photogenerated carriers, electrochemical impedance spectra (EIS) were analyzed for these as-prepared samples. The EIS Nyquist plots of SnO₂, SnS₂, and SnO₂/SnS₂ are displayed in Fig. 6b. Among those samples, the EIS spectra of the as-prepared SnO₂/SnS₂ sample had the smallest capacitance arc, which indicated the fastest separation and transfer of photogenerated electron-hole pairs. Such results confirmed that the formation of heterostructure in SnO₂/SnS₂ could enhance the photocatalytic performance. These were in good agreement with the results of the photo-degradation tests. The carrier lifetime (τ_e) of those as-prepared samples could be calculated based on the EIS Bode plots (Figs. 6c–6f), demonstrating the carrier transfer process according to the equation [72].

(Eq.2)

The calculated τ_e values (Table 2) of the as-prepared SnO₂, SnO₂/SnS₂, and SnS₂ samples were 13.1, 41.6, and 19.3 μs, respectively. The carrier lifetime of the SnO₂/SnS₂ heterostructure was 41.6 μs, much longer than the values for pure SnO₂ and SnS₂, suggesting that the formation of the heterostructure between SnO₂ and SnS₂ could greatly prolong the lifetime of the photo-generated electrons. In addition, such a prolonged carrier lifetime of SnO₂/SnS₂ indicated a drastically different carrier transfer pathway from both pure SnO₂ and SnS₂. As known, electron-holes pairs were easily recombined on the surface or in the bulk of SnO₂ and SnS₂. Constructing the SnO₂/SnS₂ heterostructure allows the built-in electric field in the heterostructure. Owing to the rapid electron transportation ability of SnO₂, the photogenerated carrier on the conductor band of SnS₂ could be transferred to the conductor band of SnO₂, forming the special photogenerated carrier transfer pathway of the SnO₂/SnS₂ heterostructure. Therefore, a built-in electric field was constructed to form the photogenerated carrier transportation process in the SnO₂/SnS₂ heterostructure, leading to a high carrier separation efficiency. For better comparison, the electron lifetime of the mechanically mixed sample (SnO₂ & SnS₂ mix) was also determined to be approximately 5.03 μs, about 1/8 that of the SnO₂/SnS₂ heterostructure. This further demonstrated the importance of the built-in electric field. As a result, the as-prepared heterostructure samples displayed strong photocatalytic activity for degrading organic pollutants.

A possible photocatalytic degradation mechanism could be constructed for the SnO₂/SnS₂ heterostructure photocatalyst. The optimal photo-degradation performance could be ascribed to the formation of the heterostructure, which not only extended the sunlight absorption range but also accelerated the transfer and separation of photogenerated electron-hole pairs. The Mott-Schottky plots (Figs. 7a–c) of the as-prepared samples were obtained from electrochemical measurements analyzing the conduct band (CB) position. The CB positions for SnO₂, SnS₂, and SnO₂/SnS₂ were −0.105, −0.245, and −0.206 V (vs. NHE), respectively. Combined with the UV-vis DRS analysis, the energy band structures could be estimated for all samples (Fig. 4f). Based on the energy band structures and active species trapping experiments, a possible photocatalytic mechanism for the SnO₂/SnS₂ heterostructure could be described as shown in Fig. 7d. Under visible-light irradiation, electrons could be excited to the conduct band (CB), leaving holes in the valance band of SnS₂. The photogenerated electrons could be rapidly transferred to the CB of SnO₂, owing to the fast electron transport ability of SnO₂. Therefore, with the assistant of the SnO₂/SnS₂ heterostructure, a built-in electric field was constructed to accelerate the photogenerated carrier transfer and reduce the recombination of photogenerated electron-hole pairs. Moreover, the photogenerated electrons could be used to produce superoxide radical (O₂^•−), as a strong oxidizing agent for degrading organic pollutants.

SnO₂/SnS₂ heterostructure photocatalyst was prepared via the in-situ conversion of SnO₂ through ion exchange. The as-formed heterostructure effectively boosted the photocatalytic performance for treating organic pollutants. The results of photo-degradation tests and photoelectrochemical measurements showed that a built-in electric field in the SnO₂/SnS₂ heterostructure could effectively improve the separation of photogenerated electron-hole pairs and prolong carrier lifetime, producing an enhanced photocatalytic efficiency.