Crystal Design and Photoactivity of TiO₂ Nanorod Template Decorated with Nanostructured Bi₂S₃ Visible Light Sensitizer

Abstract

In this study, TiO₂-Bi₂S₃ composites with various morphologies were synthesized through hydrothermal vulcanization with sputtering deposited Bi₂O₃ sacrificial layer method on the TiO₂ nanorod templates. The morphologies of decorated Bi₂S₃ nanostructures on the TiO₂ nanorod templates are controlled by the duration of hydrothermal vulcanization treatment. The Bi₂S₃ crystals in lumpy filament, nanowire, and nanorod feature were decorated on the TiO₂ nanorod template after 1, 3, and 5 h hydrothermal vulcanization, respectively. Comparatively, TiO₂-Bi₂S₃ composites with Bi₂S₃ nanowires exhibit the best photocurrent density, the lowest interfacial resistance value and the highest photodegradation efficiency towards Rhodamine B solution. The possible Z-scheme photoinduced charge separation mechanism and suitable morphology of Bi₂S₃ nanowires might account for the high photoactivity of TiO₂ nanorod-Bi₂S₃ nanowire composites.

1. Introduction

Nanorod arrays of TiO₂ are beneficial to provide direct channels for electron transport, reducing the recombination probability of electrons in the transmission process. The rod morphology helps to improve the electron injection and collection efficiency of TiO₂ semiconductor [1,2,3,4,5]. However, the wide band gap nature of TiO₂ engenders poor response to visible light [6,7]. In order to improve the photoactivity of TiO₂, the strategy of semiconductor coupling is often used to enhance the solar energy conversion and utilization of TiO₂ [8]. Several cases of TiO₂ coupled with a visible light sensitizer has been shown a promising approach to improve solar energy utilization efficiency. TiO₂ microspheres coupled with CdS nanoparticles enhance the light harvesting ability and suppress the electron-hole recombination of TiO₂ and CdS [9]. The electrospinning formed TiO₂/CuO composite nanofibers enhance light absorbance and interparticle charge transfer and lower the band gap energy, thus promoting absorbance and utilization of photon energy from a broader light spectrum [10]. Precise control of Bi₂O₃ coverage layer phase composition realizes the high photoactivity of the one-dimensional TiO₂-Bi₂O₃ composites [11]. The decoration of ZnFe₂O₄ crystallites onto TiO₂ improves the photoactivity of TiO₂ and enhance the photodegradation performance towards Methylene orange [12]. The TiO₂/Bi₂S₃ core–shell nanowire arrays demonstrate improved photocurrent density than that of pristine TiO₂ because of the broadened light absorption ability and the increased charge carrier separation efficiency [13]. Similarly, Bi₂S₃ nanowires/TiO₂ nanorod arrays exhibit an excellent photoelectrochemical activity [14]. Rosette-rod TiO₂/Bi₂S₃ shows substantial improvement in photoresponse in compared with pristine TiO₂ photoanode [15]. Furthermore, hydrothermal method deposition of TiO₂/Bi₂S₃ composite film is presented to be promising for applications of photoanode [16]. These examples clearly present the feasibility of construction of TiO₂-Bi₂S₃ heterogeneous system to be used in photoactive devices with an improved efficiency.

Among various visible-light sensitizers, Bi₂S₃ has attracted much attention because it is environmentally friendly and non-toxic. Bi₂S₃ has a high optical absorption coefficient and a suitable band gap, which can absorb most of the visible light spectrum [17]. Bi₂S₃ crystals with a large area growth and different morphologies have been realized through various chemical routes [18]. The photoactivity of Bi₂S₃ crystals is highly dependent on shape, size, and crystalline quality. How to precisely control the microstructures of Bi₂S₃ in order to fabricate TiO₂-Bi₂S₃ composites with satisfactory photoactivity is an important issue. In this study, Bi₂S₃ crystals with various microstructures were decorated on TiO₂ nanorod arrays via vulcanization of Bi₂O₃ sacrificial layer. The vulcanization of metal oxide to obtain metal sulfide has been shown to be a promising and easy approach to synthesize the metal sulfides with controllable microstructures [17,19]. Using metal oxide sacrificial layer to form the metal sulfide through vulcanization is an easy approach to control the microstructure of the as-synthesized metal sulfide. This approach is suitable for microstructural control of the sulfide crystals and thus tuning the physical properties of the samples. However, studies on synthesis of TiO₂-Bi₂S₃ composites through vulcanization of Bi₂O₃ layer and their photoactivities are lacking. A detailed vulcanization process-dependent microstructure evolution and photoactive properties of TiO₂-Bi₂S₃ composites are proposed in this study to realize the design of TiO₂-Bi₂S₃ composites with high photoactive performance.

2. Experiments

The synthesis of TiO₂ nanorod template on the F-doped SnO₂ (FTO) substrate was realized through a hydrothermal method at 170 °C. Other detailed parameters and preparation procedures have been described elsewhere [4]. The Bi₂O₃ thin films were sputter deposited on the TiO₂ nanorod template using a radio-frequency magnetron sputtering system. A metallic Bi disc was used as the target (99.9 wt%, 2 inches in diameter); the substrate temperature was fixed at 410 °C. The working atmosphere is mixed Ar/O₂ with a ratio of 1/1, the working pressure is 20 mtorr and sputtering power is 30 W. The sputtering duration is 50 min. The Bi₂O₃ thin-film coated TiO₂ nanorod templates are further immersed in a 20 mL reaction solution containing 0.1M thiourea and sealed in a Teflon-lined autoclave for a hydrothermal reaction at 160 °C for 1, 3, and 5 h to obtain TiO₂-Bi₂S₃ composites (named as BT-1, BT-3, and BT-5, respectively.).

Crystallographic structures of as-synthesized samples were investigated by X-ray diffraction (XRD) analysis using Cu Kα radiation with a two-theta scan range of 20–60° and scan rate of four degrees per min. The morphologies of the as-synthesized samples were investigated using a field emission scanning electron microscopy (FE-SEM). The morphology, high resolution images, crystallographic structure, and composition of BT-1 and BT-3 composite samples were investigated by high-resolution transmission electron microscopy (HR-TEM). X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental binding energies of the as-synthesized samples. The diffuse reflectance spectra of the as-synthesized samples were recorded by using UV-vis spectrophotometer (Jasco V750) at the wavelength range of 200–800 nm. Photoelectrochemical (PEC) and electrochemical impedance spectroscopy (EIS) properties of various photoanodes were investigated using the potentiostat (SP150, BioLogic, Orlando, FL, USA). During measurements, an Ag/AgCl electrode were used as the reference electrode herein. The 0.5 M Na₂SO₄ solution was used as electrolyte and the light irradiation source for measurements is excited from 100 W Xe arc lamp. The 10 mL RhB solution (5 × 10⁻⁵ M) was used as target dye solution for photodegradation experiments. The RhB solution containing various photocatalysts with different irradiation durations (0, 15, 30, 45, and 60 min) was carried out to understand the photocatalytic activity of various as-synthesized samples.

3. Results and Discussion

The morphology of initial pristine TiO₂ rod array template was clearly observed from the SEM images in Figure 1a. As seen, the TiO₂ rods are uniformly grown on the entire substrate; moreover, TiO₂ rods have a regular cross-section and the side-wall surfaces are smooth. Figure 1b displays the morphology of BT-1. A large number of lumpy filament-like crystals are covered on the top region of the TiO₂ rod template. Figure 1c shows that abundant nanowires with around 0.8 μm length were grafted on the TiO₂ rod template for the BT-3 sample. Such a morphology is similar to that of Bi₂S₃ nanowire-decorated TiO₂ rod heterostructures synthesized via a facile two-step hydrothermal growth process reported by Liu et al. [14]. Compared to Figure 1c, after extending the hydrothermal duration to 5 h, the diameter of one dimensional Bi₂S₃ thickened, and the length increases from 0.8 μm to 1.2 μm as exhibited in Figure 1d. Some aggregations of one-dimensional Bi₂O₃ crystals appear on the TiO₂ rod template. It has been shown that Bi₂O₃ oxide layer is easily etched and transfer to Bi₂S₃ phase during a vulcanization process with sulfur ions in the reaction solution [20].

Figure 2 shows the possible formation processes of Bi₂S₃ nanostructures on the TiO₂-Bi₂O₃ composite rod template via different vulcanization processes in this study. Notably, in aqueous solution the solubility of Bi₂S₃ is much lower than that of Bi₂O₃. When the TiO₂-Bi₂O₃ composite rods are placed in a reaction solution containing a large amount of S²⁻ ions, there is a high possibility of S²⁻ ions transfer toward to the Bi₂O₃ surface as exhibited in (a). Furthermore, due to solubility disparity, the formation of Bi₂S₃ nuclei on Bi₂O₃ will occur during the vulcanization process because of the ion exchange of S²⁻ from the sulfur precursor solution and O²⁻ from the Bi₂O₃ surface. The generation of Bi₂S₃ crystals will continually proceed with a result of Bi³⁺ reacting with S²⁻ as exhibited in schematic (b). Further extending reaction duration, the Bi₂S₃ nuclei will slowly stack up to form loose filament structure at the given vulcanization condition of schematic (c). When an increase of the vulcanization reaction duration to 3 h, the loose and lumpy Bi₂S₃ clusters will transfer into abundant, distinguishable, and separable nanowire crystals covered on the TiO₂ template as displayed in schematic (d). When the reaction duration reaches 5 h, the one-dimensional Bi₂S₃ crystals continue to aggregate into a thicker rod structure, as shown in schematic (e).

Figure 3a–c demonstrate the XRD patterns of vulcanization-treated BT-1, BT-3, and BT-5 samples. In addition to the Bragg reflections originated from the FTO substrate, the characteristic diffraction peaks centered at approximately 26.61°, 36.18°, and 54.76° can be indexed to (110), (101), and (211) planes of rutile TiO₂ (JCPDS No. 00-021-1276), revealing high crystallinity of the TiO₂ template. Notably, after hydrothermal vulcanization, several visible Bragg reflections originated from orthorhombic Bi₂S₃ phase could be clearly observed (JCPDS No. 017-0320) in the BT-1, BT-3, and BT-5 samples. Figure 3a–c present that addition of thiourea in hydrothermal reaction solution can vulcanize the Bi₂O₃ sacrificial oxide layer to transfer into the corresponding sulfide phase in BT-1, BT-3, and BT-5. The prolonged vulcanization process (from 1 to 5 h) improves Bragg reflection intensity of Bi₂S₃ phase of the samples, revealing an improved crystallinity of the samples. This might be associated with crystal morphology transformation of the loose and lumpy filament-like Bi₂S₃ crystals of BT-1 to one-dimensional Bi₂S₃ rods of BT-5 with prolonged vulcanization process. No Bragg reflections associated with Bi₂O₃ and other impurity phases were detected in the XRD patterns, revealing the vulcanization processes herein successfully synthesized the TiO₂-Bi₂S₃ composite structure, and the as-synthesized composite structures are crystalline.

Figure 4a shows the low-magnification TEM image of the BT-1. It can be observed that Bi₂S₃ flakes decorated on the TiO₂ rod. Figure 4b is the HRTEM image taken from the red square in Figure 3a. The distinct lattice fringes with a distance of approximately 0.28 nm are from the interplane spacing of orthorhombic Bi₂S₃ (2 2 1) and the lattice fringes with a spacing of 0.33 nm are from the interplane spacing of tetragonal TiO₂ (1 1 0). Figure 4c showed the selected area electron diffraction (SAED) pattern of several BT-1. Several clear diffraction spots arranged in centric rings are associated with the orthorhombic Bi₂S₃ (2 1 1), (2 2 1), and (4 3 1) planes and the rutile TiO₂ (1 1 0) plane, which agrees with the result observed by XRD analysis. This demonstrates the well-constructed TiO₂-Bi₂S₃ heterogeneous structure of BT-1, and a good crystallinity of the composites. The EDS spectrum in Figure 4d shows Ti, O, Bi, and S are the main elements in this composite structure. Furthermore, the particular element line scan intensity distributions displayed in Figure 4e reveals the construction of Bi₂S₃ flakes on the TiO₂ rod for the BT-1.

Figure 5a shows the low-magnification TEM image of the BT-3 structure scratched from the sample. The sidewall of the composite showed an undulated feature because of the decoration of Bi₂S₃ crystals. However, a broken top region of the structure was observed in Figure 5a. Because the preparation of TEM sample with a scratched off method will destroy the integrity of the BT-3 composite, the distinct two-layered structure of the BT-3 composite is not observed herein, as revealed in the aforementioned SEM observation. Figure 5b displays the HRTEM image taken from the red square region in Figure 5a. The distinct lattice fringes with a distance of approximately 0.28 nm are associated with the interplanar spacing of Bi₂S₃ (2 2 1). Figure 5c demonstrates the EDS spectrum taken from the nanostructure in Figure 5a. In addition to Cu and C originated from the TEM grid, the Ti, O, Bi, and S are the main constituent elements in this composite structure, which proves that TiO₂ and Bi₂S₃ phases coexist in the composite structure. Figure 5d presents a low-magnification image of the scratched nanowire structure from BT-3 sample. The nanowire has the diameter of 23 nm, and the surface is smooth. The nanowire shows a little bent state. Figure 5e shows the HRTEM image taken from the red square in Figure 5d. The distinct and ordered lattice fringes shows the single crystalline quality of the Bi₂S₃ nanowire. Similarity, a distinguishable Bi₂S₃ (2 2 1) lattice image in the one-dimensional Bi₂S₃ crystal in an orthorhombic structure has been shown in WO₃/Bi₂S₃ composite synthesized via chemical bath deposition [21]. Figure 5f shows the EDS spectrum taken from the single Bi₂S₃ nanowire, where the EDS spectrum indicates high purity of Bi₂S₃ composition of the nanowire.

Figure 6 shows the characteristic XPS spectral lines of various samples. The main constituent elements of Bi, S, Ti, and O are detected in BT-1, BT-3, and BT-5, supporting the existence of Bi₂S₃ and TiO₂ in the composite structure. Moreover, the relatively weak Ti signals from the spectra in comparison with that of the Bi signals, revealing the capping layer of Bi₂S₃ phase on the TiO₂ template for the test samples, and this spectral feature has widely been observed in the composite structure, having obvious layering characteristics [9]. In addition to the C signal that originated from the sample contamination on exposure to ambient air, no impurity was detected in the as-synthesized samples. In order to further investigate the elemental binding states of the Bi₂S₃ capping layer, the XPS narrow scan spectra of Bi 4f for BT-1, BT-3, and BT-5 are displayed in Figure 7a–c, respectively. Two sharp and distinct peaks centered at approximately 157.3 eV and 162.6 eV, which are assigned to Bi 4f_7/2 and Bi 4f_5/2 of Bi₂S₃, respectively [22]. The tiny peak located between the two distinct characteristic peaks of Bi 4f is originated from S 2p. The binding energies of Bi 4f core-level peaks corresponded to the characteristic binding state of Bi³⁺ in the Bi₂S₃, revealing the well formation of the Bi₂S₃ phase through vulcanizing the Bi₂O₃ layer. No metallic Bi or Bi₂O₃ appeared after vulcanization.

Notably, the narrow scan spectra of S 2p region of BT-1, BT-3, and BT-5 in Figure 8 reveal the characteristic peaks centered at approximately 162.6 eV and 160.1 eV, which are assigned to S 2p_1/2 and S 2p_3/2, respectively. The S 2p binding energies herein associated with the aforementioned Bi 4f binding states evidenced the formation of Bi-S bonds in the capping layer synthesized via the given vulcanization processes in this study [23].

A comparison of the UV-vis absorption spectra of TiO₂ template and various TiO₂-Bi₂S₃ samples are displayed in Figure 9a. The absorption of the TiO₂ template is mainly in the UV light range. The decoration of Bi₂S₃ extends the absorption range of the TiO₂ template to the visible light range because of the narrow energy gap of Bi₂S₃. This is consistent with the feature of optical absorption spectra for Bi₂S₃ nanorod/TiO₂ nanoplate composites [24]. Moreover, in the previous TiO₂/Bi₂S₃ core–shell nanowire arrays synthesized via a successive ionic layer adsorption and reaction method, the absorption of the composite extends to cover the visible light range, and the decoration of Bi₂S₃ phase increases the light absorption ability of the pristine TiO₂ nanowires [8]. Notably, the band gap energies of the pristine TiO₂ and Bi₂S₃ are evaluated in Figure 9b,c, respectively, from Tauc plots based on Kubelka–Munk function [25]. The TiO₂ has a band gap energy of approximately 3.02 eV, and the band gap energy of the reference Bi₂S₃ derived from the sputtering deposited Bi₂O₃ layer and then vulcanized using 0.1 M thiourea is approximately 1.3 eV. Based on the evaluated band gap energy, the construction of the TiO₂/Bi₂S₃ heterostructure herein improves the light harvesting ability.

Figure 10a shows the transient photocurrent performance of various photoelectrodes under repeated on/off irradiation cycles at 0.5 V (vs. Ag/AgCl). It is known that the transient photocurrent performance is highly associated with the photoresponse, charge carrier transport speed, charge carrier separation efficiency, and charge carrier recombination rate of samples. When the light is on, the photocurrent densities of all samples swiftly rise to a stable value of 0.12, 0.81, 0.93, and 0.47 mA/cm² for TiO₂, BT-1, BT-3, and BT-5, respectively. When the light is off, the photocurrent densities of the samples drop to their initial dark current density value instantaneously. Such a fast rise and fall process of the photocurrents indicates that carrier transport and separation in the prepared photoelectrodes proceed quickly [11]. Furthermore, the photoresponse of the BT-3 increased approximately three times higher than that of the TiO₂ template. The BT-1 and BT-5 also exhibited enhanced photoresponses in comparison with the TiO₂ template. Among various composite structure, the BT-3 has the highest photocurrent response, indicating it has the lowest photogenerated electron/hole recombination rate. By contrast, the BT-5 exhibits a lower photocurrent response among various composites. This might be attributed to the following reason. Substantially excessive Bi₂S₃ deposition leads to an increase in the number of recombination centers, and as a result, a decrease in photocurrent density has been observed due to the loss of photogenerated electrons [15]. The results herein prove that decoration of Bi₂S₃ crystals onto the TiO₂ template can effectively improve the photoinduced charge separation efficiency of the TiO₂ template, resulting in more efficient charge migration and higher photocurrent density of the TiO₂-Bi₂S₃ composite structure. A similar improved transient photocurrent performance because of decoration of visible light sensitizer onto the wide band gap semiconductor with a suitable band alignment has been demonstrated in TiO₂-Bi₂O₃ and ZnO-Sn₂S₃ heterostructures [11,25]. Figure 10b presents the Nyquist plots of various photoelectrodes. The semicircular radius of the Nyquist plots can reflect the charge transfer resistance of the photoelectrode materials. As show in Figure 10b, the arc radius of the TiO₂-Bi₂S₃ composites herein are smaller than that of the TiO₂ template, revealing that formation of TiO₂/Bi₂S₃ heterojunction is beneficial for interfacial charge transfer. Comparatively, BT-3 has the smallest radius of the Nyquist curve among various samples. The possible equivalent circuits shown in Figure 10c are used to fit the EIS Nyquist results, where Rs, R1, R2, and CPE represent the solution resistance, semiconductor depletion layer resistance, charge transfer resistance, and chemical capacitance, respectively [26]. Herein, the R2 is evaluated from the fitting results of the Nyquist plots at the low frequency region. The R2 values of the pristine TiO₂, BT-1, BT-3, and BT-5 are 3500, 583.3, 463.4, and 925 Ω, respectively. The BT-3 exhibited the smallest R2 value. As the vulcanization duration was further prolonged, a larger size of Bi₂S₃ crystallites deposition occurred, which might prolong the electron diffusion path in the BT-5 composite, resulting a larger R2 value among various composites. Notably, charge transfer resistances of the BT-1, BT-3, and BT-5 are much lower than that of TiO₂. This indicates that decoration of Bi₂S₃ onto the TiO₂ could effectively reduce the photoinduced charge transfer resistance of the TiO₂ template and enhance the separation efficiency of charge carriers. Both the transient photocurrent and Nyquist plot results herein support an effective separation of photogenerated electron–hole pairs and faster interfacial charge transfer occurred on the BT-3 interface, which might lead to the enhanced photocatalytic performance.

Figure 11 shows the Mott–Schottky (M–S) plots of TiO₂, Bi₂S₃, and TiO₂-Bi₂S₃ composites performed at 1 kHz. All the samples were measured with a positive slope in the M–S plots, revealing an n-type nature of the composed semiconductors. The flat band potential (E_fb) of various samples can be calculated from the x intercept of the linear region in the M–S plots according to the M–S equation [27]. Furthermore, the normal hydrogen electrode (NHE) potential can be converted from the Ag/AgCl reference electrode as NHE = V(Ag/AgCl)—0.197 V. The E_fb of TiO₂ and Bi₂S₃ referenced samples are −0.21 and −0.45 V vs. NHE, respectively. Comparatively, the flat band potential of the TiO₂-Bi₂S₃ composites has a negative shift compared to the TiO₂ template as exhibited in Figure 11c–e. Moreover, the tangent slope in the linear region of the M–S plots for the TiO₂-Bi₂S₃ composites is smaller than the TiO₂ template, demonstrating the decoration of Bi₂S₃ crystallites onto the TiO₂ template increases carrier concentration and reduce the charge recombination rate of the composites. A more negative shift of the flat band potential with respective to TiO₂ template is observed for the BT-3 among various TiO₂-Bi₂S₃ composites; moreover, the smaller tangent slop in the linear region of the M–S plot was observed for the BT-3. These reveal a superior electronic property of the BT-3 than that of other TiO₂-Bi₂S₃ composites. It has been shown that the crystallite size and number of visible-light sensitizers affect the photoinduced charge separation efficiency of the visible-light sensitizer-decorated TiO₂ [28]. Moreover, the crystal quality of the semiconductor has been proved to affect its photoactivity [29]. The BT-3 because of suitable Bi₂S₃ crystallite quality and size shows the superior photogenerated carrier density and effective interface transfer ability among various composites in this study.

Figure 12a shows the photodegradation level of various photocatalysts towards RhB solution. The percentage of photodegradation was calculated using the C/Co = It/Io. The Co and C are the initial and residual concentration of the RhB solution at t = 0 and at irradiation duration t, respectively, and can be evaluated from the intensity variation of absorbance spectra of the RhB solution with and without photocatalytic reaction [30]. Figure 12a demonstrates that the BT-1, BT-3, BT-5, and TiO₂ degrades 60.3%, 65.6%, 50.8%, and 36.1% of RhB solution, respectively, after 30 min irradiation. Furthermore, the degradation level of RhB solution reached 77.8%, 87.2%, 71.2%, and 57.1% after 60 min irradiation for the BT-1, BT-3, BT-5, and TiO₂, respectively. Notably, the dark balanced absorptions of various samples are also conducted to understand the initial catalysts’ surface dye absorption efficiency. The C/Co of the BT-1, BT-3, BT-5, and TiO₂ after 30 min dark balanced adsorption is 7.1%, 7.6%, 5.8%, and 4.2%, respectively. Comparatively, the BT-1 and BT-3 exhibit a slightly larger surface dye adsorption ability than that of BT-5 and TiO₂. Figure 12b demonstrates the discoloration of RhB solution with BT-3 under different irradiation durations. Apparent discoloration appeared in the RhB solution containing BT-3 with an increased irradiation duration. To quantitatively compare the photocatalytic activity of the as-prepared photocatalyst samples, the photodegradation data were fitted to the pseudo-first-order kinetics equation: ln(Co/C) = kt, where k is the apparent first-order rate constant [4]. Figure 12c presents the plot of ln(Co/C) versus t for various samples. The higher k value is observed for the TiO₂-Bi₂S₃ composites than that of pristine TiO₂. Moreover, the BT-3 demonstrates the highest k value of 0.03274 min⁻¹ among various samples, confirming the higher photocatalytic activity of BT-3 composite. The suitable morphology and decoration content of Bi₂S₃ crystallites on the TiO₂ template might account for the observed result. This has also been supported by visible-light CuO sensitizer decorated ZnO composite photocatalysts that crystal morphology and content of the CuO substantially affect the photocatalytic activity of CuO-ZnO nanocomposites [31]. Active groups of the photocatalytic degradation reaction were explored by the addition of free radical capture agents, as exhibited in Figure 12d. Herein, benzoquinone (BQ, ·O₂⁻ radical scavenger), tertiary butyl alcohol (t-BuOH, ·OH radical scavenger), and EDTA-2Na (EDTA-2Na, h⁺ radical scavenger) are used to explore the active groups of the photocatalytic degradation reaction. Notably, the photocatalytic efficiency of RhB solution with BT-3 was significantly inhibited after adding BQ, indicating that ·O₂⁻ is the main active substance for RhB degradation. Furthermore, the results herein reveal that a single scavenger could not completely prevent the dye degradation, and the h⁺ and ·OH also contribute some degrees of photodegradation towards RhB dyes.

In the M-S analysis, the flat band potentials of the TiO₂ and Bi₂S₃ are −0.21 and −0.45 eV, respectively. Furthermore, the E_CB bottoms of the TiO₂ and Bi₂S₃ can be evaluated to be −0.31 and −0.55 eV, respectively [32]. The valence band (VB) positions of the TiO₂ and Bi₂S₃ are evaluated to be 2.71 and 0.75 eV, respectively. According to the band structures of TiO₂ and Bi₂S₃, there are two possible migration mechanisms of photoinduced charge carriers in the TiO₂/Bi₂S₃ heterojunction as shown in Figure 13. However, the potential of O₂/·O₂⁻ is −0.33 eV, which is more negative than CB of TiO₂, and CB electrons are not easy to reduce O₂. At the same time, the photoinduced holes on Bi₂S₃ VB cannot oxidize H₂O to produce ·OH radicals, because the VB edge potential of Bi₂S₃ is more negative than the potential of H₂O/OH (2.27 V), as shown in Figure 13a. Based on the aforementioned discussions, type II photodegradation mechanism is not suitable for the as-synthesized TiO₂-Bi₂S₃ composites in this study. By contrast, a direct Z-scheme mechanism over the TiO₂/Bi₂S₃ heterostructure can be proposed in Figure 13b. Upon irradiation, TiO₂/Bi₂S₃ absorbs light greater than its band gap, electron-hole pairs are generated (Equations (1) and (2)), and RhB will also be excited to RhB* by light irradiation at the same time (Equation (3)). The RhB* injects electrons into the CB of Bi₂S₃ (Equation (4)). A similar phenomenon of electron injection from RhB* to the TiO₂ CB has been proposed in the literature [33]. The electrons in the TiO₂ CB position could migrate to the Bi₂S₃ VB, resulting in the effective separation of photoinduced charge carriers (Equation (5)). The RhB⁺ reacts with adsorbed dye to form intermediate products by the photosensitization process (Equation (6)). Parts of the holes in the TiO₂ VB will react with the intermediate product to degrade RhB dyes (Equation (7)). Moreover, the residual photoinduced holes in the TiO₂ VB will react with the H₂O molecules to generate the ·OH radicals (Equation (8)), and the electrons are gathered on Bi₂S₃ CB to produce ·O₂⁻ radicals (Equation (9)) [34]. Therefore, the RhB dyes are effectively photodegraded with BT-3 under irradiation (Equation (10)). A similar Z-scheme mechanism is shown in other TiO₂-based composite systems decorated with visible-light sensitizers [35,36,37].

$${\mathrm{TiO}}_2+\mathrm{h}\mathsf{\upsilon }\to {\mathrm{TiO}}_2{ \mathrm{e}}^{-}\left(\mathrm{CB}\right)+{\mathrm{TiO}}_2{ \mathrm{h}}^{+}\left(\mathrm{VB}\right)$$

(1)

$${\mathrm{Bi}}_2{\mathrm{S}}_3+\mathrm{h}\mathsf{\upsilon }\to {\mathrm{Bi}}_2{\mathrm{S}}_3{ \mathrm{e}}^{-}\left(\mathrm{CB}\right)+{\mathrm{Bi}}_2{\mathrm{S}}_3{ \mathrm{h}}^{+}\left(\mathrm{VB}\right)$$

(2)

$$\mathrm{RhB}+\mathrm{h}\mathsf{\upsilon }\to { \mathrm{RhB}}^{*}\left(\mathrm{LUMO}\right)+{ \mathrm{RhB}}^{+}\left(\mathrm{HOMO}\right)$$

(3)

$${\mathrm{RhB}}^{*}\left(\mathrm{LUMO}\right)+{\mathrm{Bi}}_2{\mathrm{S}}_3{ \mathrm{e}}^{-}\left(\mathrm{CB}\right)\to {\mathrm{Bi}}_2{\mathrm{S}}_3{ \mathrm{total} \mathrm{e}}^{-}\left(\mathrm{CB}\right)$$

(4)

$${\mathrm{TiO}}_2{ \mathrm{e}}^{-}\left(\mathrm{CB}\right)+{\mathrm{Bi}}_2{\mathrm{S}}_3{ \mathrm{h}}^{+}\left(\mathrm{VB}\right)\to \mathrm{recombination}$$

(5)

$${\mathrm{RhB}}^{+}\left(\mathrm{HOMO}\right)+{\mathrm{RhB}}_{\left(\mathrm{abs}\right)}\to \mathrm{Intermediate}$$

(6)

$$\mathrm{Intermediate}+{\mathrm{TiO}}_2{ \mathrm{h}}^{+}\left(\mathrm{VB}\right)\to \mathrm{degradation} \mathrm{products}$$

(7)

$${\mathrm{TiO}}_2{ \mathrm{h}}^{+}\left(\mathrm{VB}\right)+{\mathrm{H}}_2\mathrm{O}\to ·\mathrm{OH}$$

(8)

$${\mathrm{Bi}}_2{\mathrm{S}}_3{ \mathrm{e}}^{-}\left(\mathrm{CB}\right)+{\mathrm{O}}_2\to ·{\mathrm{O}}_2{}^{-}$$

(9)

$$\mathrm{RhB}+·{\mathrm{O}}_2{}^{-}\to \mathrm{degradation} \mathrm{products}$$

(10)

4. Conclusions

TiO₂ nanorod array coated with Bi₂O₃ layer was used to vulcanize to form TiO₂-Bi₂S₃ composites with various morphologies. The hydrothermal vulcanization duration profoundly affects the microstructure, optical properties, and photoactivity of TiO₂-Bi₂S₃ composites. The decorated Bi₂S₃ crystals changed the morphology from filament, nanowire to nanorod with an increased vulcanization duration from 1, 3, and 5 h, respectively. The decoration of Bi₂S₃ crystals enhanced the light absorption capacity of the TiO₂ nanorod template. The improved photodegradation performance of the composites can be reasonably attributed to the construction of the direct Z-scheme heterojunction between the TiO₂ and Bi₂S₃. This study demonstrates that design of TiO₂-Bi₂S₃ composites with suitable morphology of decorated Bi₂S₃ crystals can tune the photoactivity of the composites, and the findings in this study may be of great value for the development of oxide-sulfide composites for ideal photosensitive device applications.