Photoelectrochemical and Photocatalytic Properties of SnS/TiO₂ Heterostructure Thin Films Prepared by Magnetron Sputtering Method

Abstract

Tin(II) sulfide(SnS)/titanium(IV) oxide (TiO₂) heterostructure thin films were prepared by radio-frequency magnetron sputtering to investigate the enhancement effect of the formed heterojunction on the photocatalytic performance. By adjusting the sputtering time to vary the thickness of the SnS layer, the crystallinity and light-absorption properties of the light-absorbing layer and the quality of the heterojunction interface were effectively controlled, thereby optimizing the fabrication process of the heterojunction. It was found that when the SnS layer thickness was 244 nm and the TiO₂ layer thickness was 225 nm, the heterostructure film exhibited optimal photoelectrochemical performance, generating the highest photocurrent of 3.03 µA/cm² under visible light, which was 13.8 times that of a pure TiO₂ film and 2.4 times that of a pure SnS film of the same thickness. Additionally, it demonstrated the highest degradation efficiency for methylene blue dye. The improved photoelectrochemical performance of the SnS/TiO₂ heterostructure film can be primarily attributed to the following: (1) the incorporation of narrow-bandgap SnS effectively broadens the light-absorption range, improving visible-light harvesting; (2) the staggered band alignment between SnS and TiO₂ forms a type-II heterojunction, significantly enhancing the charge carrier separation and transport efficiency. The present work demonstrated the feasibility of magnetron sputtering for constructing high-quality SnS/TiO₂ heterostructures, providing insights into the design and fabrication of photocatalytic heterojunctions.

1. Introduction

Photocatalytic technology has garnered significant attention among various environmental remediation approaches due to its notable advantages of high efficiency, environmental friendliness, and mild reaction conditions. This technique utilizes solar energy to excite electrons in the photocatalyst, generating electron–hole pairs that subsequently produce highly reactive oxygen species, including hydroxyl radicals (^•OH) and superoxide radicals (O₂^−•). These reactive species demonstrate remarkable efficacy in degrading organic pollutants into harmless H₂O and CO₂ [1,2,3].

A variety of binary or ternary semiconductor materials, including oxides [4], nitrides [5], chalcogenides [6], and perovskites [7], have been employed in photocatalytic applications. In addition to single-phase semiconductors, heterojunctions formed by combining two or more semiconductor materials have attracted extensive research attention in photocatalysis [8,9]. Currently, the most prevalent heterojunctions can be classified into three main types: type-II, Z-scheme, and S-scheme heterojunctions [10]. In type-II heterojunctions, the band structures of the two semiconductors exhibit a staggered alignment, where electrons migrate to the semiconductor with a lower conduction band potential, while holes transfer to the material with a higher valence band potential, thereby achieving effective charge separation [11].

As one of the earliest semiconductor materials employed in photocatalysis, TiO₂ possesses advantageous characteristics including non-toxicity, low cost, and excellent chemical stability [12]. With a bandgap of approximately 3.2 eV [13], TiO₂ can only absorb ultraviolet light with wavelengths shorter than 387 nm, which severely limits its solar energy utilization and practical photocatalytic efficiency [14]. Various modification strategies have been developed to enhance the photocatalytic performance of TiO₂. The visible-light response can be extended through noble metal deposition [15,16], element doping [17,18,19], and sensitization with narrow-bandgap semiconductors [20,21,22]. Designing one-dimensional nanostructures, such as nanowires or nanotube arrays, can shorten the carrier diffusion path [2]. In addition, introducing defect states such as oxygen vacancies can increase the carrier concentration and enhance electrical conductivity [23]. Coupling TiO₂ with narrow-bandgap semiconductors represents a well-established and effective strategy that simultaneously addresses two critical aspects. The narrow-bandgap component extends optical absorption into the visible-light region, while the formed heterojunction structure enhances charge separation and transport through built-in interfacial electric fields, collectively leading to significantly improved photocatalytic efficiency [24,25,26].

SnS has a relatively narrow optical bandgap with remarkable tunability. Its bandgap can be categorized into an indirect bandgap (1.0–1.5 eV) and a direct bandgap (1.3–2.3 eV), with specific values influenced by synthesis methods, lattice strain, and sulfur stoichiometry [27,28]. Coupled with its high optical absorption coefficient of 10⁴ to 10⁵ cm⁻¹ under visible light [29,30], SnS emerges as an ideal material for light-absorbing layers. Previous studies have demonstrated that SnS/TiO₂ heterostructures can effectively enhance photoelectric conversion efficiency [31,32]. Zhang et al. developed SnS/TiO₂ nanotube array (NTA) photoanodes with significantly enhanced photocatalytic degradation performance toward 2,4,6-trichlorophenol under visible-light irradiation. The construction of a type-II heterojunction by integrating SnS with TiO₂ suppressed the charge carrier recombination and improved separation efficiency. The nanostructure shortened charge diffusion pathways while increasing the specific surface area and exposing more active sites. After five consecutive degradation cycles, the SnS/TiO₂ NTAs maintain a removal efficiency of over 90% for the target pollutant, demonstrating excellent cycling stability and structural durability [33]. Successive ionic layer reaction (SILAR) remains the predominant fabrication technique, employing sequential immersion in cation and anion precursor solutions for thin film deposition [34,35]. Other established methods comprise hydrothermal synthesis [36], chemical bath deposition [37], and electrodeposition [33]. Beyond pollutant degradation, the SnS/TiO₂ heterojunction demonstrates versatile applications in solar cells [38], photocatalytic hydrogen production [39], and sensors [40]. Al Ahmed developed a solar cell using Zn₃P₂ as the hole transport layer, SnS as the light-absorbing layer, and TiO₂ as the electron transport layer, achieving 30.45% power conversion efficiency through optimized band alignment [38].

Currently, chemical methods predominantly serve as the primary approach for fabricating SnS/TiO₂ composites. Nevertheless, these solution-based processes are prone to generating impurity phases, either through excessive S²⁻ in the precursors or the oxidation of Sn²⁺ during reactions [34,41]. Such impurities not only compromise the light-absorption characteristics of SnS films due to bandgap misalignment but also introduce deep-level defects that significantly impair charge carrier mobility [42]. In contrast, physical deposition methods conducted under vacuum conditions minimize the introduction of impurities and oxides, facilitating the formation of high-quality SnS/TiO₂ heterojunction interfaces.

This study employed magnetron sputtering, a widely used physical vapor deposition method, to fabricate SnS/TiO₂ heterostructure thin films. Compared to conventional solution-based approaches, this method enabled the precise control over film thickness and composition. The resulting SnS films maintained phase purity, and their integration with TiO₂ effectively extended the light-absorption range. Furthermore, the heterostructure films exhibited significantly improved charge carrier transport efficiency, contributing to enhanced photocatalytic performance relative to the individual SnS and TiO₂ components. These results confirmed magnetron sputtering as a controllable fabrication method that enables the tuning of crystallinity and interface properties, demonstrating its potential for constructing well-defined SnS/TiO₂ heterostructures for photocatalytic applications.

2. Results and Discussion

2.1. Structural Characterization

When the sputtering power was fixed at 80 W, the deposition rate typically remained stable. Consequently, the film thickness of SnS generally increased linearly with a prolonged sputtering time. The thicknesses of SnS and TiO₂ layers in different samples are shown in

Table 1. Thicknesses of each layer in the SnS/TiO₂ thin films prepared with different sputtering time of SnS.

Sputtering Time of SnS (min)	Thicknesses of SnS (nm, ±4)	Thicknesses of TiO2 (nm, ±4)
8	102	220
12	244	225
16	359	217
20	462	231

.

The thickness of the TiO₂ layer was maintained at approximately 220 nm, while the thickness of SnS layer exhibited a linear dependence on the sputtering time, as shown in Figure 1a. The linear regression analysis yielded a coefficient of determination (R²) of 0.9918, confirming an excellent linear relationship between the thickness of SnS film and deposition time. Specifically, as the sputtering duration increased from 8 to 20 min, the corresponding thickness increased from 102 to 462 nm.

The XRD patterns of the SnS/TiO₂ heterostructure, pure SnS, and pure TiO₂ thin films are shown in Figure 1b. All SnS/TiO₂ films exhibited diffraction peaks corresponding to both SnS and TiO₂ phases. As the sputtering time of the SnS layer increased, the diffraction peaks of SnS became more intense while those of TiO₂ showed a gradual decrease in intensity. The SnS layers displayed an orthorhombic structure with a polycrystalline nature that matched well with the standard diffraction pattern (JCPDS No. 39-0354), with characteristic peaks observed at approximately 30.52°, 31.60°, and 42.60°, corresponding to the (101), (111), and (210) planes of SnS, respectively. With an increasing sputtering time, the (111) and (210) diffraction peaks shifted slightly toward lower angles, and no diffraction peaks corresponding to impurity phases such as SnS₂ or Sn₂S₃ were detected. Additionally, three diffraction peaks of TiO₂ were observed and identified as the anatase phase (JCPDS No. 21-1272), including the (101) plane peak at 25.29°, the (112) plane peak at 38.51°, and the (200) plane peak at 47.97°. The relatively weak intensity of these TiO₂ diffraction peaks is likely due to the fact that the TiO₂ layer was located beneath the SnS layer and had a relatively small thickness.

The crystallite size (D) and microstrain (ε) of both SnS and TiO₂ were calculated using Scherrer’s formula (Equation (1)) and the strain equation (Equation (2)), respectively [43,44]:

$$\mathrm{D}={\displaystyle \frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta }}}$$

(1)

$$\mathsf{\epsilon }={\displaystyle \frac{\mathsf{\beta }}{4\mathrm{t}\mathrm{a}\mathrm{n}\mathsf{\theta }}}$$

(2)

where K is the Scherrer constant, λ denotes the X-ray wavelength (0.15406 nm), β represents the full width at half maximum (FWHM) of the diffraction peak, and θ is the Bragg diffraction angle.

The lattice parameters of both thin films were calculated using Bragg’s law (Equation (3)) and the interplanar spacing formula (Equation (4)) [45], where λ represents the incident X-ray wavelength, d_hkl denotes the interplanar spacing, θ is the diffraction angle, (h, k, l) are the Miller indices, and (a, b, c) stand for the lattice constants. SnS crystallizes in the orthorhombic system, while anatase TiO₂ adopts a tetragonal structure (with a = b) [46,47]. The calculated crystallite sizes and lattice parameters are summarized in

Table 2. Calculation of the structural parameters of the SnS/TiO₂ thin films prepared with different sputtering times of SnS.

Sputtering Time of SnS (min)	SnS					TiO2
	D ± 0.8 (nm)	ε ± 0.09 (×10−3)	a (Å)	b (Å)	c (Å)	D ± 0.8 (nm)	a (Å)	c (Å)
8	18.5	6.87	4.3199	10.8950	3.9765	27.0	3.7900	9.4453
12	22.6	5.44	4.3210	10.9415	3.9757	27.3	3.7928	9.4292
16	23.8	5.10	4.3219	11.1500	3.9775	27.2	3.7848	9.5549
20	24.6	4.95	4.3277	11.1054	3.9777	27.9	3.7900	9.4725
pure SnS	27.5	4.29	4.3279	11.1735	3.9771	/	/	/
pure TiO2	/	/	/	/	/	27.5	3.7899	9.4736

.

$$\mathsf{\lambda }=2{\mathrm{d}}_{\mathrm{h}\mathrm{k}\mathrm{l}}\mathrm{s}\mathrm{i}\mathrm{n}\mathsf{\theta }$$

(3)

$${\displaystyle \frac{1}{{{\mathrm{d}}_{\mathrm{h}\mathrm{k}\mathrm{l}}}^2}}={({\displaystyle \frac{\mathrm{h}}{\mathrm{a}}})}^2+{({\displaystyle \frac{\mathrm{k}}{\mathrm{b}}})}^2+{({\displaystyle \frac{\mathrm{l}}{\mathrm{c}}})}^2$$

(4)

The TiO₂ thin films had a grain size of about 27 nm, with lattice parameters close to standard values (a = b = 3.7852 Å, c = 9.5139 Å). For the SnS films, when the sputtering time increased from 8 to 20 min, the grain size grew from 18.5 nm to 24.6 nm, yet remained smaller than that of pure SnS films deposited directly on glass substrates. The variation in microstrain across different crystallographic planes is illustrated in Figure 2a, showing a consistent decreasing trend in microstrain for all observed planes with an increasing sputtering time. The most significant changes in both grain size and microstrain occurred when the sputtering time increased from 8 to 12 min, with subsequent prolongation showing diminished variation amplitudes. As the film thickness increased, the grains developed more completely through the growth and merging of smaller crystallites, resulting in larger grain sizes and improved crystallinity [48]. The standard lattice parameters for orthorhombic SnS are a = 4.3291 Å, b = 11.1923 Å, and c = 3.9838 Å. The deviations of lattice constants for different samples from standard values are presented in Figure 2b, as determined by the following Equation (5):

$${\mathsf{\delta }}_{\mathrm{a}}={\displaystyle \frac{\mathrm{a}-{\mathrm{a}}_0}{{\mathrm{a}}_0}};{\mathsf{\delta }}_{\mathrm{b}}={\displaystyle \frac{\mathrm{b}-{\mathrm{b}}_0}{{\mathrm{b}}_0}};{\mathsf{\delta }}_{\mathrm{c}}={\displaystyle \frac{\mathrm{c}-{\mathrm{c}}_0}{{\mathrm{c}}_0}}$$

(5)

Considerable deviations occurred at the shorter sputtering time of 8–12 min, while films deposited for 16–20 min showed better agreement with standard values. This behavior may be attributed to the insufficiently smooth surface of the TiO₂ films, which imposed a localized strain on the overlying SnS films and hindered proper grain growth, causing lattice distortion. Additionally, the poor lattice matching between the SnS films and substrate contributed to the deviation of lattice parameters [49].

The morphology of the SnS/TiO₂ thin films are shown in Figure 3. Figure 3a,b presents the surface and cross-sectional views of the heterostructure film, respectively, where the SnS layer was deposited for 12 min. The SEM images clearly revealed the heterostructure. Figure 3c–f display the AFM images of various samples, showing that the film surface exhibited enhanced planarity as the SnS sputtering time increased from 8 to 20 min. As shown in

Table 3. Root mean square (RMS) of the SnS/TiO₂ thin films prepared with different sputtering times of SnS.

Sputtering Time of SnS (min)	RMS (nm)
8	1.93
12	1.74
16	1.63
20	1.60

, the root mean square (RMS) roughness ranged from 1.60 to 1.93 nm.

The elemental composition of SnS/TiO₂ films prepared with different sputtering times of SnS showed no significant variations. Therefore, representative EDS and XPS spectra from the sample with the 8 min sputtering time of SnS are presented in Figure 4. The EDS spectrum revealed the elemental distribution, with atomic percentages for different samples summarized in

Table 4. Atomic percentages of the SnS/TiO₂ thin films prepared with different sputtering times of SnS.

Sputtering Time of SnS (min)	Ti (At%)	O (At%)	Sn (At%)	S (At%)
8	21.08	45.85	18.24	14.83
12	15.24	32.18	29.01	23.57
16	7.06	14.97	43.18	34.79
20	4.63	9.69	47.36	38.32

. As the SnS layer thickness increased, the atomic percentages of Sn and S progressively rose, while those of Ti and O correspondingly decreased. In the heterostructure films, the Ti:O ratio was approximately 1:2.12, close to the theoretical value of 1:2. The Sn:S ratio was about 1.23:1, indicating a Sn-rich and S-deficient composition in the SnS films. This deviation primarily arises from the much higher saturated vapor pressure of S compared to Sn, causing the preferential re-evaporation of S atoms from the film surface during deposition.

Due to the shallow probing depth of XPS, the characterization of heterostructure films was conducted in two stages: first, the pure TiO₂ film was analyzed to obtain high-resolution spectra of Ti and O elements, shown in Figure 4e,f; subsequently, after the SnS layer deposition, another measurement was performed to determine the elemental composition and chemical states of the SnS film. Figure 4b shows the XPS survey spectrum of the upper SnS layer, while Figure 4c,d presents the high-resolution spectra of Sn and S elements, respectively. In the Sn 3d spectrum, the peaks at 485.5 eV and 494.0 eV corresponded to the Sn 3d_5/2 and Sn 3d_3/2 energy levels, respectively, with a binding energy difference of 8.5 eV, confirming the presence of Sn²⁺ [50]. The S 2p spectrum exhibited two peaks at 160.3 eV and 161.5 eV, attributed to the spin–orbit splitting of S²⁻ [51]. Combined with XRD results, it was demonstrated that the phase-pure nature of the magnetron-sputtered SnS layer effectively avoided detrimental effects from impurity phases on the light-absorption properties. Figure 4e,f displays the high-resolution spectra of Ti and O elements from the pure TiO₂ film. The Ti 2p spectrum showed characteristic peaks at 458.4 eV of Ti 2p_3/2 and 464.1 eV of Ti 2p_1/2, with a spin–orbit splitting of 5.7 eV, consistent with Ti⁴⁺ [52]. The O 1s spectrum revealed two components: the peak at 529.8 eV originated from Ti-O bonds, while the peak at 531.7 eV was assigned to surface -OH groups [53].

Figure 5a presents the UV-Vis-NIR diffuse reflectance spectra of SnS/TiO₂ heterostructure thin films prepared with different sputtering time of SnS. The Tauc equation (Equation (6)) was employed to calculate the bandgaps of both pure SnS and TiO₂ films [54,55]:

$${\left(\mathsf{\alpha }\mathrm{h}\mathsf{\nu }\right)}^{{\displaystyle \frac{1}{\mathrm{n}}}}=\mathrm{A}(\mathrm{h}\mathsf{\nu }-{\mathrm{E}}_{\mathrm{g}})$$

(6)

α represents the absorption coefficient, A is a constant, and the value of n depends on the electronic transition characteristics of the semiconductor. The direct bandgap of SnS was calculated with n = 1/2, while TiO₂, as an indirect bandgap semiconductor, was assigned n = 2 [56,57]. In indirect transitions, the energy contribution from electron–phonon interactions is much smaller than the photon energy. Near the band edges, the influence of phonon energy on the absorption coefficient is relatively weak. Therefore, the phonon energy is approximated as a perturbation to the bandgap and directly incorporated into the effective bandgap value [54,58]. The Tauc plot of the pure SnS and TiO₂ film is shown in Figure 5b,c, with the corresponding absorption spectra provided as insets. The pure SnS film, deposited with a sputtering time of 20 min and a thickness of 456 nm, exhibited a bandgap of 1.27 ± 0.03 eV, while the pure TiO₂ film with a thickness of 231 nm had a bandgap of 3.26 ± 0.03 eV. The SnS thin film with a narrow bandgap exhibited superior visible-light absorption compared to TiO₂. When composited with TiO₂ to form the heterostructure, the resulting films demonstrated a pronounced red shift in absorption edge and significantly enhanced the visible-light-absorption capacity. As the sputtering time of SnS layers increased from 8 to 12 min, the films showed substantial improvement in light absorption. However, the further extension of the deposition time yielded diminishing returns, with the 20 min sample displaying a slightly lower absorption than the 16 min counterpart. This thickness-dependent optical behavior can be attributed to several interrelated factors. Thinner films suffer from incomplete photon absorption due to a limited optical path length, while increased thickness promotes more efficient light harvesting through extended propagation paths and improved crystallinity [59]. This improvement correlates with grain growth and structural perfection, which collectively induce bandgap narrowing and broaden the spectral response. Beyond optimal thickness, the absorption efficiency approaches saturation, with potential performance degradation caused by intensified light scattering and reflection losses [60]. Notably, the most significant enhancements in both grain size and light absorption occur during the initial 8 to 12 min of thickness increase, highlighting the critical influence of SnS crystallinity in determining the optical performance of heterostructure system.

2.2. Photoelectrochemical Characterization

Transient photocurrent tests were performed under visible-light irradiation using SnS/TiO₂ heterostructure thin films prepared with different sputtering times of SnS as photoanodes, with the results shown in Figure 6a. When the deposition time of SnS layer increased from 8 to 12 min, the photocurrent density of the heterostructure films exhibited a significant enhancement, rising from 1.98 µA/cm² to 3.03 µA/cm². However, the further prolongation of the sputtering time and corresponding increase in the SnS layer thickness led to a noticeable decline in photocurrent generation under visible light, with the 20 min sample showing a photocurrent density comparable to that of the 8 min sample. This phenomenon can be attributed to the improved crystalline quality and structural ordering of SnS layers with increasing thickness, which initially enhances their light-absorption capability. Nevertheless, when the film thickness exceeds an optimal range, the absorption approaches saturation while charge carrier scattering at grain boundaries and interfaces becomes more pronounced, ultimately reducing charge transport efficiency and photocurrent density [60,61]. Pure SnS and TiO₂ films of equivalent thickness to the optimized 12 min heterostructure served as controls to demonstrate the photoelectrochemical performance improvement. The current–time curves of these three configurations are presented in Figure 6b. The pure TiO₂ film demonstrated a minimal photocurrent density due to its poor visible-light absorption. While the pure SnS film generated a photocurrent density of 1.28 µA/cm², the optimized heterostructure achieved 3.03 µA/cm², representing a 13.8-fold enhancement over the pure TiO₂ film and a 2.4-fold improvement compared to SnS.

To analyze the differences in photoelectrochemical performance among the various structures, electrochemical impedance spectroscopy (EIS) measurements were conducted to elucidate the charge transport and interfacial reaction mechanisms that may occur in different film architectures. Figure 7 displays the Nyquist plots of the samples, where the impedance (Z) is defined as a standard complex function comprising a real component (Z′) and an imaginary component (Z″) [62]:

$$Z\left(\omega \right)=Z^{\prime }\left(\omega \right)+i{Z}^{″}\left(\omega \right)$$

(7)

All EIS spectra exhibited semicircular arcs, indicating that the electrochemical processes at the semiconductor/electrolyte interface were primarily controlled by charge transfer [62]. The spectra were fitted using the equivalent circuit shown in the figure, where R_s represents the solution resistance, CPE denotes the constant phase element, and R_ct is the charge transfer resistance at the electrode surface. The fitted parameters are listed in

Table 5. Fitting parameters of the electrochemical impedance spectra.

Sputtering Time of SnS (min)	Rct (kΩ)	Rs (Ω)	CPEdl (µF)	n	k0 (×10−9 m/s)	J0 (×10−8 A/cm2)
8	99.85	10.55	47.70	0.74	13.11	12.65
12	79.96	15.65	14.94	0.82	16.37	15.80
16	123.32	10.52	48.49	0.73	10.61	10.24
20	168.33	14.02	31.22	0.74	7.78	7.50
SnS	523.53	25.13	26.71	0.84	2.50	2.41
TiO2	289.52	13.47	16.54	0.89	4.52	4.36

. When the sputtering time of SnS increased from 8 to 12 min, the R_ct of the SnS/TiO₂ films decreased from 99.85 kΩ to 79.96 kΩ. However, with further increases in sputtering time, the R_ct gradually rose to 168.33 kΩ, demonstrating that the interfacial charge transfer efficiency initially improved but subsequently deteriorated with an increasing SnS layer thickness. To further investigate the kinetics of the relevant electrochemical processes, the electron transfer rate constant (k⁰) at the film/electrolyte interface was calculated using Equation (8) [62]:

$${\mathrm{k}}^0={\displaystyle \frac{\mathrm{R}\mathrm{T}}{{\mathrm{n}}^2{\mathrm{F}}^2\mathrm{A}{\mathrm{R}}_{\mathrm{c}\mathrm{t}}\mathrm{C}}}$$

(8)

where R, T, and n represent the universal gas constant, reaction temperature, and number of electrons involved in the S²⁻/S redox reaction, respectively, F denotes the Faraday constant (~96,485 C/mol), A is the electrode surface area, and C is the concentration of the S²⁻/S redox couple in solution.

The exchange current density (J₀) was additionally calculated using Equation (9) [63], with the results listed in Table 5:

$${\mathrm{J}}_0={\displaystyle \frac{\mathrm{R}\mathrm{T}}{\mathrm{n}\mathrm{F}{\mathrm{R}}_{\mathrm{c}\mathrm{t}}}}$$

(9)

When the SnS layer was deposited for 12 min, the heterostructure film achieved maximum values for both k⁰ of 1.637 × 10⁻⁸ m/s and J₀ of 1.580 × 10⁻⁷ A/cm², demonstrating that an optimal SnS thickness can significantly enhance the charge transfer efficiency. However, when the sputtering time exceeded 12 min, both k⁰ and J₀ decreased, likely because the increased thickness of the light-absorption layer began to adversely affect charge carrier transport pathways, thereby elevating recombination probability [64].

The pure SnS and TiO₂ films of equivalent thickness exhibited R_ct values of 523.53 kΩ and 289.52 kΩ, respectively. In contrast, the heterostructure demonstrated significantly reduced R_ct values, accompanied by substantially enhanced k⁰ and J₀ values. To further investigate the charge transfer facilitation mechanism in the heterojunction, a Mott–Schottky (M-S) analysis was performed on both SnS and TiO₂ films, with the resulting curves presented in Figure 8. Equation (10) is as follows [65]:

$${\displaystyle \frac{1}{{\mathrm{C}}_{\mathrm{S}\mathrm{C}}^2}}={\displaystyle \frac{2}{{\mathrm{e}\mathsf{\epsilon }}_{\mathrm{r}}{\mathsf{\epsilon }}_0{\mathrm{N}}_{\mathrm{D}}}}(\mathrm{V}-{\mathrm{V}}_{\mathrm{f}\mathrm{b}}-{\displaystyle \frac{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}{\mathrm{e}}})$$

(10)

where C_sc represents the space charge layer capacitance, ε_r is the relative dielectric constant of the semiconductor, ε₀ denotes the vacuum permittivity, e is the elementary charge, N_D stands for the donor concentration, V is the applied potential, V_fb indicates the flat-band potential, k_B is the Boltzmann constant, and T is the temperature. The flat-band potentials (V_fb vs. RHE) of the TiO₂ and SnS films were determined to be −0.31 V and −0.67 V, respectively, by extrapolating the linear portion of the M-S curves to the intercept with the x-axis. For n-type semiconductors, the conduction band potential (E_CB vs. RHE) can be approximated as the V_fb (vs. RHE) [66]. Combined with the bandgap values obtained from UV-Vis-NIR spectroscopy, the valence band potentials (E_VB vs. RHE) were calculated using Equation (11) [67]:

$${\mathrm{E}}_{\mathrm{V}\mathrm{B}}={\mathrm{E}}_{\mathrm{C}\mathrm{B}}+{\mathrm{E}}_{\mathrm{g}}$$

(11)

The calculated (E_VB vs. RHE) of the TiO₂ and SnS thin films were 2.95 V and 0.60 V, respectively. Based on these results, the energy band diagrams of TiO₂ and SnS were constructed, as shown in Figure 9. Under visible-light irradiation, electrons in SnS were excited from the valence band to the conduction band. The formation of a type-II heterojunction between TiO₂ and SnS facilitated the directional charge carrier migration, where photogenerated electrons were transferred from the conduction band of SnS to TiO₂, while holes migrated toward and accumulated in the valence band of SnS [10]. This spatial separation of electrons and holes not only enhanced the charge separation efficiency but also intensified the upward band bending at the SnS/electrolyte interface [35,68]. As a result, hole injection into the electrolyte was promoted, accelerating interfacial oxidation reactions. Consequently, the charge transfer resistance (R_ct) at the semiconductor/electrolyte interface was significantly reduced, indicating markedly improved charge transfer kinetics.

Photocatalytic degradation tests of methylene blue were conducted on the 12 min sputtered SnS/TiO₂ heterostructure film along with pure SnS and TiO₂ films of equivalent thickness. For first-order reactions, the photocatalytic rate constant can be calculated using the following equation [69,70]:

$$\ln \left({\displaystyle \frac{{\mathrm{C}}_0}{\mathrm{C}}}\right)=\mathrm{k}\mathrm{t}$$

(12)

The photocatalytic degradation kinetics were analyzed using the first-order reaction model, where C₀ represents the initial solution concentration, C is the concentration after irradiation time t, and k denotes the reaction rate constant. Figure 10a plots the −ln(C/C₀) values versus time for all samples, with the fitted R² values and k rate constants summarized in

Table 6. Calculation of the photocatalytic rate constants of different samples.

Samples	k	R2
SnS/TiO2	0.0170	0.9967
SnS	0.0121	0.9932
TiO2	0.0104	0.9910

. The R² values exceeding 0.99 confirmed that the photocatalytic degradation followed first-order kinetics for all samples. The pure SnS and TiO₂ films exhibited relatively low-rate constants, attributable to the poor visible-light absorption of TiO₂ and the rapid charge recombination of SnS, despite its strong visible-light harvesting capability. In contrast, the SnS/TiO₂ heterostructure film achieved the highest rate constant of 0.0170, demonstrating superior catalytic performance with 87% methylene blue degradation within 120 min under visible light. The reproducibility testing of SnS/TiO₂ demonstrated consistent performance across three successive cycles, as illustrated in Figure 10b. The rate constants for the second and third cycles were determined to be 0.0157 and 0.0151, respectively, while the third cycle achieved 84% degradation efficiency for methylene blue. The degradation mechanism likely involved the generation of reactive oxygen species such as hydroxyl radicals (^•OH) and superoxide radicals (O₂^−•) through redox reactions, as reported in similar photocatalytic systems [71,72,73]. These active species were responsible for the oxidative breakdown of the dye molecules.

The SnS/TiO₂ heterostructure demonstrated superior photoelectrochemical performance compared to its individual counterparts due to synergistic interactions across multiple stages [74]. Under visible-light irradiation, the narrow bandgap of SnS enabled efficient light absorption [27]. The type-II band alignment played a crucial role by facilitating charge separation, where photogenerated electrons in SnS transferred to the TiO₂ conduction band due to its more negative potential, while holes migrated to the SnS valence band, thereby effectively suppressing recombination and improving charge transport efficiency [69]. Additionally, the heterojunction interface likely contained unique active sites such as defect states that further enhanced redox reaction kinetics [75]. These combined effects achieved photocatalytic performance comparable to other TiO₂-based catalysts reported in other works such as N-doped TiO₂ [76], CdS/TiO₂ [20], and Cu₂O/TiO₂ [25] systems.

3. Materials and Methods

3.1. Preparation of SnS/TiO₂ Heterostructure Thin Films

SnS/TiO₂ heterostructure thin films were prepared using a vacuum deposition equipment (SY-500, Beijing Shengdeyu Vacuum Technology Co., Ltd., Beijing, China). Glass slides were used as substrates and sequentially cleaned by ultrasonication in acetone, absolute ethanol, and deionized water for 10 min each. TiO₂ thin films were deposited by radio-frequency magnetron sputtering using a TiO₂ target (99.99%, ZhongNuo Advanced Material Technology Co., Ltd., Beijing, China) with a sputtering power of 300 W for 30 min. During deposition, the Ar flow rate was maintained constant at 70 sccm with a working pressure of 0.55 Pa. The as-deposited TiO₂ films were then annealed in a tube furnace at 500 °C for 2 h in air. In our previous work, the preparation process of SnS thin films was optimized, including parameters such as sputtering power, annealing time, and annealing temperature. Subsequently, SnS thin films were deposited on the TiO₂ films by radio-frequency magnetron sputtering using a SnS target (99.99%, ZhongNuo Advanced Material Technology Co., Ltd., Beijing, China) with a constant sputtering power of 80 W. The sputtering time varied between 8, 12, 16, and 20 min for different samples. A final annealing process was performed in the tube furnace at 300 °C for 1 h under Ar atmosphere to complete the preparation of SnS/TiO₂ heterostructure thin films.

3.2. Characterization and Measurements

The crystal structure and phase composition of the samples were characterized by X-ray diffraction (Bruke D8 with Cu-Kα radiation, Billerica, MA, USA). Scanning electron microscopy (SEM) (ZEISS Gemini 300, Jena, Germany) and atomic force microscopy (AFM) (Bruker Dimension ICON, Billerica, MA, USA) were employed to analyze the surface and cross-sectional morphology. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha, Waltham, MA, USA) was used to determine the elemental composition and chemical states of the thin films. The optical absorption properties were evaluated using ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometry (Shimadzu UV-3600, Kyoto, Japan).

The photoelectrochemical performance of the samples was tested using a three-electrode system connected to an electrochemical workstation (CHI660E) at room temperature. The three-electrode system consisted of a working electrode (the thin-film samples deposited on FTO glass with an effective area of 1 cm²), a reference electrode (saturated calomel electrode, SCE), and a counter electrode (Pt disk electrode). A xenon lamp served as the light source, equipped with an optical filter to control the visible-light wavelength range between 420 and 780 nm, delivering an irradiance of 62 mW/cm² at the sample surface. The measurements were conducted in a 0.5 mol/L Na₂SO₄ aqueous electrolyte solution (pH = 7). All measured potentials were converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation (Equation (13)) [77], where E_RHE represents the potential relative to the reversible hydrogen electrode (RHE), E_SCE is the potential measured against the saturated calomel electrode (SCE), and $E_{SCE}^{\theta }$ denotes the standard electrode potential of SCE:

$${\mathrm{E}}_{\mathrm{R}\mathrm{H}\mathrm{E}}={\mathrm{E}}_{\mathrm{S}\mathrm{C}\mathrm{E}}+0.0591\mathrm{p}\mathrm{H}+{\mathrm{E}}_{\mathrm{S}\mathrm{C}\mathrm{E}}^{\mathsf{\theta }}({\mathrm{E}}_{\mathrm{S}\mathrm{C}\mathrm{E}}^{\mathsf{\theta }}=0.2415\mathrm{v}\mathrm{s}.\mathrm{N}\mathrm{H}\mathrm{E}\mathrm{a}\mathrm{t}25°\mathrm{C})$$

(13)

For transient photocurrent tests, an open-circuit potential was applied to the working electrode while recording current–time profiles under alternating light-on and light-off conditions. Each state (illuminated/non-illuminated) was maintained for 50 s, with a total test duration of 400 s. Electrochemical impedance spectroscopy (EIS) was performed under visible-light irradiation across a frequency range of 0.1 Hz to 100,000 Hz. A Mott–Schottky analysis was conducted in dark conditions at a fixed frequency of 1000 Hz with an AC amplitude of 10 mV.

The photocatalytic degradation performance of the thin films was evaluated using a methylene blue (MB) dye solution as the model pollutant. A film sample with a surface area of 1 × 1 cm² was immersed in 50 mL of MB aqueous solution (4 mg/L), consistent with our previous work [78]. Prior to illumination, the reactor was placed on a magnetic stirrer and kept in dark conditions for 30 min to establish adsorption–desorption equilibrium on the catalyst surface. During the photocatalytic test, the sample was irradiated with visible light from a xenon lamp positioned at a distance of 30 cm. Aliquots of 4 mL were collected at 20 min intervals over a total irradiation period of 120 min. The concentration of MB in each collected sample was immediately determined using an ultraviolet-visible spectrophotometer (UV-2300, Shanghai Techcomp Instruments Ltd., Shanghai, China).

4. Conclusions

SnS/TiO₂ heterostructure thin films were fabricated by RF magnetron sputtering with a fixed TiO₂ thickness of 225 nm and varying SnS deposition times. The characterization confirmed phase-pure SnS layers, with optimal optoelectronic performance at a 244 nm SnS thickness. The increased SnS thickness enhanced crystallinity, grain growth, and light absorption, but excessive thickness led to absorption saturation and reduced charge transport due to carrier scattering. The 12 min SnS sputtered heterostructure thin film exhibited a maximum photocurrent density of 3.03 µA/cm² under visible light, which was 13.8 times that of pure TiO₂ thin film and 2.4 times that of pure SnS thin film with an equivalent thickness. This enhancement arose from the following: (1) the narrow-bandgap SnS effectively extended the light absorption; (2) the type-II band alignment promoted charge separation. Photocatalytic tests showed 87% methylene blue degradation in 120 min, far outperforming pure SnS and TiO₂ films. These results demonstrated the feasibility of using a purely physical method for constructing SnS/TiO₂ heterojunctions with tunable structural parameters and controlled interface quality. Additionally, further investigations are required to examine the influence of pH values and initial pollutant concentrations on the degradation rate, as well as to explore the degradation mechanism through radical trapping experiments and byproduct analysis.