Facile Construction of 2D/2D ZnIn2S4-Based Bifunctional Photocatalysts for H2 Production and Simultaneous Degradation of Rhodamine B and Tetracycline

A two-dimensional/two-dimensional (2D/2D) TiO2/ZnIn2S4 photocatalyst was reasonably proposed and constructed by a two-step oil bath-hydrothermal method. TiO2 nanosheets uniformly grown on the surface of ZnIn2S4 nanosheets and a synergetic effect between the TiO2 and ZnIn2S4 could highly contribute to improving the specific surface area and hydrophilicity of ZnIn2S4 as well as accelerating the separation and transfer of photon-generated e−-h+ pairs, and thus enhancing the visible-light photocatalytic degradation and H2 evolution performance of ZnIn2S4. Rhodamine B (RhB) and tetracycline (TC) were simultaneously selected as the target pollutants for degradation in the work. The optimum photocatalytic RhB and TC degradation properties of TiO2/ZnIn2S4-10 wt% were almost 3.11- and 8.61-fold higher than that of pure ZnIn2S4, separately, while the highest photocatalytic hydrogen evolution rate was also observed in the presence of TiO2/ZnIn2S4-10wt% and 4.28-fold higher than that of ZnIn2S4. Moreover, the possible photocatalytic mechanisms for enhanced visible-light photocatalytic degradation and H2 evolution were investigated and proposed in detail. Our research results open an easy pathway for developing efficient bifunctional photocatalysts.


Introduction
Since the concept of sustainable development was proposed, the production of clean energy and the treatment of wastewater with persistent organic pollutants have attracted increasing attention from researchers [1][2][3][4]. Compared to conventional treatment methods, photocatalysis technology by semiconductors has some advantages of clean, easy operation and high efficiency, which is considered to be promising in the territory of alleviating energy shortages and environmental crises [5,6]. Numerous scholars have been endeavoring to probe newfashioned semiconductors photocatalysts with superior activity and good stability to achieve effective hydrogen production and pollutant degradation in the past few decades [7,8]. Among the semiconductors photocatalysts, ternary metal chalcogenide semiconductors, such as CuCo 2 S 4 , ZnIn 2 S 4 , and CaIn 2 S 4 , have obtained exceeding attention in the domain of photocatalysis research owing to the advantages of small band gaps, outstanding photoconversion capacity, and good stability [9][10][11].

Synthesis of ZnIn2S4 Nanosheets
ZnIn2S4 was prepared via an oil-bath process according to the former literature [12]. 272 mg of ZnCl2, 442 mg of InCl3, and 300 mg of TAA were dissolved into 50 mL of deionized water (pH = 2.5), and heated at 80 °C for 2 h. After cooling, ZnIn2S4 could be obtained by separating, washing, and drying under a vacuum at 60 °C overnight. Finally, 500 mg of ZnIn2S4 was dispersed into 100 mL of methanol with continuous ultrasound treatment.

Synthesis of TiO2/ZnIn2S4 Nanosheets
The specific reaction process was illustrated in Figure 1. Firstly, 10 mL of HF was slowly dropped into a 100 mL of Teflon-lined autoclave reactor containing 25 mL of tetrabutyl titanate and heated at 200 °C for 40 h. After cooling, the precipitates were thoroughly separated by centrifugation and then dried under vacuum at 60 °C for overnight. Subsequently, the precipitates (3.1 mg, 15.5 mg, 18.6 mg, and 31 mg) and ZnIn2S4 dispersion liquid (19.6 mL, 18 mL, 17.6 mL, and 16 mL) were added in 0.1 M NaOH solution under stirring for 24 h, separately, then washed with deionized water till the pH = 7 and dried at 100 °C in a vacuum drying chamber. Finally, TiO2/ZnIn2S4 composites with different weight percent of TiO2 (2 wt%, 10 wt%, 12 wt% and 20 wt%) were obtained via the procedure and marked as TiO2/ZnIn2S4-2 wt%, TiO2/ZnIn2S4-10 wt%, TiO2/ZnIn2S4-12 wt% and TiO2/ZnIn2S4-20 wt%, respectively. Further, the identical method was also utilized to synthesize blank TiO2 in the absence of ZnIn2S4.

Density Functional Theory (DFT) Calculation
To calculate the band gaps and work functions of TiO 2 and ZnIn 2 S 4 , the model of TiO 2 (1 × 2 × 1 supercell) and ZnIn 2 S 4 (1 × 1 × 2 supercell) were first built and given in Figure S1. Subsequently, the calculations were performed by utilizing the Vienna ab initio Simulation Package (VASP), which implements the DFT with a generalized gradient approximation (GGA) and super-soft pseudopotential method. Calculations were carried out by utilizing the Predew-Burke-Ernzerhof (PBE) scheme. The electron wave functions were described by the projector augmented wave (PAW) method with a cutoff energy of 300 eV and a K-point of 2 × 3 × 2.

Photocatalytic Hydrogen Generation
The photocatalytic hydrogen experiments were conducted in an automatic online gas analysis system (Labsolar-6A, Perfectlight, Beijing, China). A Xenon lamp of 300 W (PLS-SXE 300C, λ > 420 nm) was employed to supply the visible-light source. During the process, 20 mg of as-fabricated photocatalysts were added into a 60 mL of mixed solution (50 mL deionized water and 10 mL of TEOA) without adding H 2 PtCl 6 , and then the reaction container was installed into the photocatalytic reaction instrument and the distance between the light source and the solution was about 16 cm. Ahead of starting the reaction, the entire installation was vacuumed to remove the air until the system pressure was beneath 1.0 Kpa. Then, turned on the light source and operated the program, automatic sampling every 60 min. In the whole reaction process, the temperature of circulating cooling water was always controlled at about 5 • C. Finally, the generated blended gas was transferred to gas chromatography (GC9790) equipped with a TCD detector (LabSolar-IIIAG, Perfectlight, Beijing, China) to further detect and calculate the production of hydrogen. To investigate the reusability of the binary heterostructure, recycled hydrogen production was carried out four times using TiO 2 /ZnIn 2 S 4 -10 wt% as the photocatalyst. The apparent quantum efficiencies (AQE) for H 2 evolution of λ = 400, 420, and 500 nm were determined in a 75 mL Pyrex glass reactor. The apparent quantum efficiency (AQE) could be determined using the following equation: AQE = 2 × the number of H 2 evolved molecules the number of incident photons × 100%

Photodegradation Activity Evaluation
The visible-light photocatalytic degradation activity was evaluated by the degradation of fresh TC (10 mg/L) and RhB (30 mg/L) solution. The light source was provided by 500 W Xenon light (PLS-SXE 500C) equipped with a UV cutoff filter (λ > 420 nm). Firstly, 10 mg of photocatalyst was dispersed into the 50 mL of TC solution and 50 mL of RhB solution, respectively. Then, the above-mentioned solution was transferred to the photocatalytic reaction apparatus (XPA-7) and kept stirring in the dark for 60 min to obtain an adsorptiondesorption equilibrium. After turning on the Xenon light, the photocatalytic degradation reaction was starting. At a given interval, a 4 mL aliquot of mixture was taken out utilizing a syringe with a needle and then filtrated using a 0.22 µm Millipore filter to obtain the residual solution, the concentration of which at the maximum absorption wavelength (355 nm for TC and 554 nm for RhB) was monitored using a PerkinElmer UV-vis spectrophotometer (Lambda 35). Moreover, the recycled photodegradation experiments were carried out four times using TiO 2 /ZnIn 2 S 4 -10 wt% as the photocatalyst at the same condition. Once the degradation experiment was over, the remaining sample in the beaker was immediately recycled by separation, washing, and drying for the next cyclic degradation experiment. The degradation efficiency (De %) was calculated by the equation: In the formula, C 0 and C t denote the concentrations at the initial time and after each stage of degradation, separately. As for the trapping experiments, equal amounts (1.0 mM) of scavengers were added to the TC solution to capture active radicals.

Results and Discussions
The text continues here. The XRD patterns were used to recognize the phase composition and the structure of samples, and the results are exhibited in Figure 2. The diffraction  [33]. The diffraction peaks of ZnIn 2 S 4 and TiO 2 can be seen simultaneously in the XRD patterns of TiO 2 /ZnIn 2 S 4 composites, the characteristic peak intensities of TiO 2 generally increased, while the diffraction peaks of ZnIn 2 S 4 gradually decrease with the increased content of TiO 2 . Further, no other peaks of the third (impurity) phase are detected in all XRD patterns, implying the successful construction of the TiO 2 /ZnIn 2 S 4 composites.
In the formula, C0 and Ct denote the concentrations at the initial time and after each stage of degradation, separately. As for the trapping experiments, equal amounts (1.0 mM) of scavengers were added to the TC solution to capture active radicals.

Results and Discussions
The text continues here. The XRD patterns were used to recognize the phase composition and the structure of samples, and the results are exhibited in Figure 2. The diffraction peaks observed at 21.6°, 27.7°, 47.2°, 52.7°, and 55.6° can be well attributed to the (006), (102), (110), (116), and (022) crystal planes of hexagonal ZnIn2S4 (JCPDS No. 72-0773) [32]. For pure TiO2, a set of diffraction peaks located at 25.3° (101), 37.8° (004), 48.1° (200), 54.1° (105), 55.1° (211) and 62.8° (204) are consistent with the anatase TiO2 (JCPDS No. 21-1272) [33]. The diffraction peaks of ZnIn2S4 and TiO2 can be seen simultaneously in the XRD patterns of TiO2/ZnIn2S4 composites, the characteristic peak intensities of TiO2 generally increased, while the diffraction peaks of ZnIn2S4 gradually decrease with the increased content of TiO2. Further, no other peaks of the third (impurity) phase are detected in all XRD patterns, implying the successful construction of the TiO2/ZnIn2S4 composites. SEM and TEM were utilized to observe the morphology of the catalysts. As displayed in Figure 3a, pristine TiO2 showed a 2D nanosheet structure with different sizes. As for pure ZnIn2S4, a nanoflower-like structure assembled by the large number of nanosheets can be seen in Figure 3b. After coupling ZnIn2S4 with TiO2, SEM (Figure 3c) and TEM ( Figure 3d) images indicate TiO2 nanosheets grow on the surface of ZnIn2S4 nanosheets, forming an intimate 2D/2D contact interface between ZnIn2S4 and TiO2. The elemental distribution in the TiO2/ZnIn2S4 composite was analyzed using SEM-energy-dispersive X-ray spectroscopy ( Figure S3), and the result confirmed the homogeneous coexistence of Ti, O, Zn, In, and S elements. Moreover, high-resolution TEM analysis was conducted to investigate the microstructure information of the TiO2/ZnIn2S4-10 wt% composite, and the result is depicted in Figure 3e, the lattice fringes with d spacings of 0.352 and 0.322 nm can be seen, which can be assigned to TiO2 (101) SEM and TEM were utilized to observe the morphology of the catalysts. As displayed in Figure 3a, pristine TiO 2 showed a 2D nanosheet structure with different sizes. As for pure ZnIn 2 S 4 , a nanoflower-like structure assembled by the large number of nanosheets can be seen in Figure 3b. After coupling ZnIn 2 S 4 with TiO 2 , SEM (Figure 3c) and TEM ( Figure 3d) images indicate TiO 2 nanosheets grow on the surface of ZnIn 2 S 4 nanosheets, forming an intimate 2D/2D contact interface between ZnIn 2 S 4 and TiO 2 . The elemental distribution in the TiO 2 /ZnIn 2 S 4 composite was analyzed using SEM-energy-dispersive X-ray spectroscopy ( Figure S3), and the result confirmed the homogeneous coexistence of Ti, O, Zn, In, and S elements. Moreover, high-resolution TEM analysis was conducted to investigate the microstructure information of the TiO 2 /ZnIn 2 S 4 -10 wt% composite, and the result is depicted in Figure 3e, the lattice fringes with d spacings of 0.352 and 0.322 nm can be seen, which can be assigned to TiO 2 (101) and ZnIn 2 S 4 (102) facets, separately [34,35]. The above results further indicate the successful formation of the TiO 2 /ZnIn 2 S 4 hybrid.
XPS was explored to be aware of the surface element composition and chemical state of the TiO 2 /ZnIn 2 S 4 -10 wt% composite. As shown in Figure 4a, the XPS survey spectrum of TiO 2 /ZnIn 2 S 4 reveals the coexistence of Ti, O, S, Zn, and In elements, which is in keeping with the EDS test results. Figure 4b presents the XPS spectrum of O 1s, two characteristic peaks located at 530.6 and 531.92 eV can be attributed to the Ti-O bond and the -OH group, respectively [36]. The high-resolution XPS spectra of Ti 2p showed two characteristic peaks located at 458.34 and 463.89 eV (Figure 4c), assigning to Ti 2p 3/2 and Ti 2p 1/2 , separately [37]. In the high-resolution S 2p spectrum (Figure 4d), the binding energies of 161.06 and 162.31 eV can be assigned to the S 2p 3/2 and S 2p 1/2 , respectively, suggesting the occurrence of S 2− [38]. In Figure 4e, the peaks centered at 444.30 and 452.40 eV are ascribed to In 3d 5/2 and In 3d 3/2 , assigning to In 3+ binding state [39]. As for Zn 2p (Figure 4f), the peaks centered at 1021.34 and 1044.36 eV ascribed to 2p 3/2 and 2p 1/2 , respectively, which proves the existence of Zn 2+ [40]. and ZnIn2S4 (102) facets, separately [34,35]. The above results further indicate the successful formation of the TiO2/ZnIn2S4 hybrid. XPS was explored to be aware of the surface element composition and chemical state of the TiO2/ZnIn2S4-10 wt% composite. As shown in Figure 4a, the XPS survey spectrum of TiO2/ZnIn2S4 reveals the coexistence of Ti, O, S, Zn, and In elements, which is in keeping with the EDS test results. Figure 4b presents the XPS spectrum of O 1s, two characteristic peaks located at 530.6 and 531.92 eV can be attributed to the Ti-O bond and the -OH group, respectively [36]. The high-resolution XPS spectra of Ti 2p showed two characteristic peaks located at 458.34 and 463.89 eV (Figure 4c), assigning to Ti 2p3/2 and Ti 2p1/2, separately [37]. In the high-resolution S 2p spectrum (Figure 4d), the binding energies of 161.06 and 162.31 eV can be assigned to the S 2p3/2 and S 2p1/2, respectively, suggesting the occurrence of S 2− [38]. In Figure 4e, the peaks centered at 444.30 and 452.40 eV are ascribed to In 3d5/2 and In 3d3/2, assigning to In 3+ binding state [39]. As for Zn 2p (Figure 4f), the peaks centered at 1021.34 and 1044.36 eV ascribed to 2p3/2 and 2p1/2, respectively, which proves the existence of Zn 2+ [40].   XPS was explored to be aware of the surface element composition and chemical state of the TiO2/ZnIn2S4-10 wt% composite. As shown in Figure 4a, the XPS survey spectrum of TiO2/ZnIn2S4 reveals the coexistence of Ti, O, S, Zn, and In elements, which is in keeping with the EDS test results. Figure 4b presents the XPS spectrum of O 1s, two characteristic peaks located at 530.6 and 531.92 eV can be attributed to the Ti-O bond and the -OH group, respectively [36]. The high-resolution XPS spectra of Ti 2p showed two characteristic peaks located at 458.34 and 463.89 eV (Figure 4c), assigning to Ti 2p3/2 and Ti 2p1/2, separately [37]. In the high-resolution S 2p spectrum (Figure 4d), the binding energies of 161.06 and 162.31 eV can be assigned to the S 2p3/2 and S 2p1/2, respectively, suggesting the occurrence of S 2− [38]. In Figure 4e, the peaks centered at 444.30 and 452.40 eV are ascribed to In 3d5/2 and In 3d3/2, assigning to In 3+ binding state [39]. As for Zn 2p (Figure 4f), the peaks centered at 1021.34 and 1044.36 eV ascribed to 2p3/2 and 2p1/2, respectively, which proves the existence of Zn 2+ [40].  The specific surface area and water contact angles were measured to investigate the adsorption performance of photocatalysts, and the results are displayed in Figure 5. The nitrogen adsorption-desorption isotherms of TiO 2 , ZnIn 2 S 4 , and TiO 2 /ZnIn 2 S 4 -10 wt% showed type IV isotherms with the hysteresis loop of mesoporous structures (Figure 5a), the specific surface area of TiO 2 /ZnIn 2 S 4 -10 wt% was larger than that of TiO 2 and ZnIn 2 S 4 . It can be seen that the average pore sizes are between 2 and 50 nm (Figure 5b), which further confirmed the formation of a mesoporous structure. Meanwhile, water contact angles of TiO 2 , TiO 2 /ZnIn 2 S 4 , and ZnIn 2 S 4 were also measured to analyze the hydrophilicity and hydrophobicity of prepared materials. It can be observed from Figure 5c-e that contact angles of ZnIn 2 S 4 , TiO 2 /ZnIn 2 S 4 -10 wt% and TiO 2 were, respectively, 72.9 • , 15.6 • , and 8.9 • , manifesting the hydrophilicity of ZnIn 2 S 4 can be improved by coupling with TiO 2 . These results illustrated that interface contact exists between pollutants and the photocatalysts owing to the enhanced specific surface area and hydrophilicity, and thus it is expected to obtain excellent photocatalytic performance.
angles of TiO2, TiO2/ZnIn2S4, and ZnIn2S4 were also measured to analyze the hydrophilicity and hydrophobicity of prepared materials. It can be observed from Figure 5c-e that contact angles of ZnIn2S4, TiO2/ZnIn2S4-10 wt% and TiO2 were, respectively, 72.9°, 15.6°, and 8.9°, manifesting the hydrophilicity of ZnIn2S4 can be improved by coupling with TiO2. These results illustrated that interface contact exists between pollutants and the photocatalysts owing to the enhanced specific surface area and hydrophilicity, and thus it is expected to obtain excellent photocatalytic performance. The photoabsorptive behavior of TiO2, ZnIn2S4, and TiO2/ZnIn2S4-10 wt% was detected via UV-vis DRS spectra as shown in Figure 6a, the absorption wavelength of pristine ZnIn2S4 with steep edge was at approximate 560 nm in the visible-light areas, which presented favorable absorption capacity both in the visible and UV light, while the absorption edge of absolute TiO2 was located in about 406 nm. The photoabsorption ability of TiO2/ZnIn2S4-10 wt% exhibits a very close absorption profile with ZnIn2S4 with slightly diminished absorption and blue-shifted absorption edge, suggesting the introduction of TiO2 has a slight influence on the light absorption property of ZnIn2S4. As depicted in Figure 6b,c, the band gaps of TiO2 and ZnIn2S4 were calculated as 3.24 and 2.48 eV based on the equation: (αhν) 1/n = A(hν−Eg) [41], which were roughly matched with the results of DFT calculation ( Figure S3). Valence band XPS (VB-XPS) of TiO2 and ZnIn2S4 were also conducted to further understand the band structure of TiO2 and ZnIn2S4. As demonstrated in Figure 6d, the EVB-XPS values of pure TiO2 and ZnIn2S4 were 2.95 and 1.49 eV, respectively. Therefore, the VB potentials of the normal hydrogen electrode (EVB-NHE, pH = 7) of TiO2 and ZnIn2S4 were determined to be 2.71 and 1.25 eV based on the EVB vs. NHE = φ + EVB-XPS − 4.44, where φ is the work function (4.2 eV) of the XPS analyzer [42], while the ECB vs. NHE values of TiO2 and ZnIn2S4 could be computed as −0.53 and −1.23 eV using the The photoabsorptive behavior of TiO 2 , ZnIn 2 S 4 , and TiO 2 /ZnIn 2 S 4 -10 wt% was detected via UV-vis DRS spectra as shown in Figure 6a, the absorption wavelength of pristine ZnIn 2 S 4 with steep edge was at approximate 560 nm in the visible-light areas, which presented favorable absorption capacity both in the visible and UV light, while the absorption edge of absolute TiO 2 was located in about 406 nm. The photoabsorption ability of TiO 2 /ZnIn 2 S 4 -10 wt% exhibits a very close absorption profile with ZnIn 2 S 4 with slightly diminished absorption and blue-shifted absorption edge, suggesting the introduction of TiO 2 has a slight influence on the light absorption property of ZnIn 2 S 4 . As depicted in Figure 6b,c, the band gaps of TiO 2 and ZnIn 2 S 4 were calculated as 3.24 and 2.48 eV based on the equation: (αhν) 1/n = A(hν−E g ) [41], which were roughly matched with the results of DFT calculation ( Figure S3). Valence band XPS (VB-XPS) of TiO 2 and ZnIn 2 S 4 were also conducted to further understand the band structure of TiO 2 and ZnIn 2 S 4 . As demonstrated in Figure 6d, the E VB-XPS values of pure TiO 2 and ZnIn 2 S 4 were 2.95 and 1.49 eV, respectively. Therefore, the VB potentials of the normal hydrogen electrode (E VB-NHE , pH = 7) of TiO 2 and ZnIn 2 S 4 were determined to be 2.71 and 1.25 eV based on the E VB vs. NHE = ϕ + E VB-XPS − 4.44, where ϕ is the work function (4.2 eV) of the XPS analyzer [42], while the E CB vs. NHE values of TiO 2 and ZnIn 2 S 4 could be computed as −0.53 and −1.23 eV using the equation: E CB = E VB − E g [43]. Therefore, the overall band structure positions of TiO 2 and ZnIn 2 S 4 can be obtained and shown in Figure S4.
To uncover the positive influence of constructing heterojunction on the catalytic performances of ZnIn 2 S 4 , the separation and migration behaviors of photogenerated charges were deeply investigated. First, the PL spectra of ZnIn 2 S 4 and TiO 2 /ZnIn 2 S 4 -10 wt% were measured to monitor the recombination process of photoinduced charge carriers. Generally, the dramatically reduced PL intensity is regarded as a signal of effective charge separation [44]. As shown in Figure 7a, TiO 2 /ZnIn 2 S 4 -10 wt% exhibited a lower PL intensity than ZnIn 2 S 4 , suggesting a higher separation efficiency of photogenerated carriers in TiO 2 /ZnIn 2 S 4 -10 wt% composite. The time-resolved photoluminescence (TRPL) spectra were also acquired to investigate the detailed information about the decay behavior of photogenerated carriers, and the results are shown in Figure 7b. The average fluorescence lifetime (τ avg ) of TiO 2 /ZnIn 2 S 4 -10 wt% (581 ps) was longer than pristine ZnIn 2 S 4 (543 ps), implying that the coupling ZnIn 2 S 4 with TiO 2 can helpfully prevent the recombination of photoinduced carriers and obtain a longer fluorescence lifetime. To further clarify the en-hanced photogenerated charge transfer and separation efficiency, the photoelectrochemical performance was characterized and analyzed by transient photocurrent responses and EIS tests. As demonstrated in Figure 7c, the TiO 2 /ZnIn 2 S 4 -10 wt% showed a higher photocurrent density than ZnIn 2 S 4 , and the average photocurrent density of TiO 2 /ZnIn 2 S 4 -10 wt% was raised to be 2.11 mA·cm −2 , approximately 1.5-fold larger than that of pristine ZnIn 2 S 4 (1.39 mA·cm −2 ), implying that the construction of TiO 2 /ZnIn 2 S 4 composite can promote the photoexcited charge carrier transfer. Furthermore, the EIS plot of TiO 2 /ZnIn 2 S 4 -10 wt% composite exhibited a smaller semicircle than pristine ZnIn 2 S 4 , as observed in Figure 7d, manifesting a lesser electric resistance and more efficient charge transfer process existing in the TiO 2 /ZnIn 2 S 4 -10 wt% composite. These optical and photoelectrochemical properties demonstrated that the formation of TiO 2 /ZnIn 2 S 4 heterojunction was capable of elevating the separation and transfer efficiency of photogenerated carriers, thus obtaining the enhanced photocatalytic performance.
The visible-light photocatalytic H 2 generation activities of TiO 2 , ZnIn 2 S 4 , and TiO 2 / ZnIn 2 S 4 composites were evaluated in the presence of TEOA sacrificial reagent. As shown in Figure 8a, pure ZnIn 2 S 4 possessed a low photocatalytic performance due to the high recombination rate of photoexcited charge carriers and photocorrosion, while pristine TiO 2 had almost no catalytic activity, which could be attributed to its wide bandgap [45]. Notably, the photoactivity of ZnIn 2 S 4 was gradually improved along with the introduction of TiO 2 . Among all composites, the TiO 2 /ZnIn 2 S 4 -10 wt% composite showed the optimal H 2 rate of 650 µmol/h/g, which was 4.28-fold higher than that of pristine ZnIn 2 S 4 ( Figure 8b). The recycling tests were also conducted in the same reaction condition to investigate the durability performance of TiO 2 /ZnIn 2 S 4 -10 wt% photocatalyst. As is demonstrated in Figure 8c, the H 2 production amount throughout four successive cycles barely changed. Moreover, the crystal structure and morphology of TiO 2 /ZnIn 2 S 4 -10 wt% showed no noticeable changes by comparing the XRD pattern ( Figure S5a) or SEM image ( Figure S5b) of a used sample with the fresh sample. These test results manifested that the TiO 2 /ZnIn 2 S 4 -10 wt% composite possesses good photocatalytic stability. To further clarify the driving force in the photocatalytic process, the AQEs of TiO 2 /ZnIn 2 S 4 -10 wt% photocatalyst at 400, 420, and 500 nm were calculated as 1.3, 1.1, and 0.1%, respectively (Figure 8d), which exhibits a similar trend with the adsorption spectrum, indicating that the H 2 production reaction is a photocatalytic driven process.  [43]. Therefore, the overall band structure positions of TiO2 and ZnIn2S4 can be obtained and shown in Figure S4. To uncover the positive influence of constructing heterojunction on the catalytic performances of ZnIn2S4, the separation and migration behaviors of photogenerated charges were deeply investigated. First, the PL spectra of ZnIn2S4 and TiO2/ZnIn2S4-10 wt% were measured to monitor the recombination process of photoinduced charge car- The visible-light photocatalytic H2 generation activities of TiO2, ZnIn2S4, and TiO2/ZnIn2S4 composites were evaluated in the presence of TEOA sacrificial reagent. As shown in Figure 8a, pure ZnIn2S4 possessed a low photocatalytic performance due to the high recombination rate of photoexcited charge carriers and photocorrosion, while pristine TiO2 had almost no catalytic activity, which could be attributed to its wide bandgap [45]. Notably, the photoactivity of ZnIn2S4 was gradually improved along with the introduction of TiO2. Among all composites, the TiO2/ZnIn2S4-10 wt% composite showed the optimal H2 rate of 650 µmol/h/g, which was 4.28-fold higher than that of pristine ZnIn2S4 (Figure 8b). The recycling tests were also conducted in the same reaction condition to investigate the durability performance of TiO2/ZnIn2S4-10 wt% photocatalyst. As is demonstrated in Figure 8c, the H2 production amount throughout four successive cycles barely changed. Moreover, the crystal structure and morphology of TiO2/ZnIn2S4-10 wt% showed no noticeable changes by comparing the XRD pattern ( Figure S5a) or SEM image ( Figure S5b) of a used sample with the fresh sample. These test results manifested that the TiO2/ZnIn2S4-10 wt% composite possesses good photocatalytic stability. To further clarify the driving force in the photocatalytic process, the AQEs of TiO2/ZnIn2S4-10 wt% photocatalyst at 400, 420, and 500 nm were calculated as 1.3, 1.1, and 0.1%, respectively (Figure 8d), which exhibits a similar trend with the adsorption spectrum, indicating that the H2 production reaction is a photocatalytic driven process. To confirm the performance multiformity of the as-prepared samples, the photocatalytic degradation capacities of all samples were also investigated by using colorless TC and colored RhB as the simulated organic pollutants. The TC and RhB photodegradation curves of TiO2, ZnIn2S4, and TiO2/ZnIn2S4 composites were illustrated in Figure 9a and S6a, respectively. Almost no changes in the concentration of TC and RhB were noticed in the absence of a catalyst, suggesting that the self-degradation process could be ignored. The pure TiO2 displayed weak degradation activities within 60 min, while ZnIn2S4 showed high degradation activities than TiO2 due to a wider visible-light re- To confirm the performance multiformity of the as-prepared samples, the photocatalytic degradation capacities of all samples were also investigated by using colorless TC and colored RhB as the simulated organic pollutants. The TC and RhB photodegradation curves of TiO 2 , ZnIn 2 S 4 , and TiO 2 /ZnIn 2 S 4 composites were illustrated in Figures 9a and S6a, respectively. Almost no changes in the concentration of TC and RhB were noticed in the absence of a catalyst, suggesting that the self-degradation process could be ignored. The pure TiO 2 displayed weak degradation activities within 60 min, while ZnIn 2 S 4 showed high degradation activities than TiO 2 due to a wider visible-light response range. With respect to the TiO 2 /ZnIn 2 S 4 composites, all composites showed better photocatalytic performance than TiO 2 and ZnIn 2 S 4 . Among them, TiO 2 /ZnIn 2 S 4 -10 wt% possessed the optimum performance, and almost 95% of TC and 93% of RhB could be degraded. The photocatalytic activity was enhanced when the mass content of TiO 2 increased from 2% to 10%, then decreased as TiO 2 content further increased to 12% or even more, which may be attributed to excessive TiO 2 shielding the light absorption. To obtain the reaction rate constant "k", the photodegradation curves were further kinetically fitted by using the pseudo-first-order equation: −ln (C/C 0 ) = kt, the results were displayed in Figures 9b and S6b, separately. The k value of TiO 2 /ZnIn 2 S 4 -10 wt% composite was highest compared with other samples and was up to 0.04115 min −1 for TC and 0.04168 min −1 for RhB, which was almost 111 and 190 fold that of pure TiO 2 , and 26 and 6.65 fold that of individual ZnIn 2 S 4 . Meanwhile, the mineralization capacities of all kinds of photocatalysts were investigated by TOC measurement. As demonstrated in Figures 9c and S6c, the TOC removal efficiencies of TiO 2 /ZnIn 2 S 4 composites distinctly overtopped TiO 2 and ZnIn 2 S 4 under the irradiation of visible light. Among them, the TiO 2 /ZnIn 2 S 4 -10 wt% composite showed the highest TOC removal efficiency (83.5% for TC and 85.6 for RhB), which formed the correspondence with its doughty photocatalytic degradation abilities, and confirmed that the TiO 2 /ZnIn 2 S 4 had high mineralization capacities. To determine the reusability of TiO 2 /ZnIn 2 S 4 -10 wt% in the photocatalytic process, the photocatalytic cycle experiments were performed to investigate the reusable performance. As shown in Figures 9d and S6d, the photodegradation efficiency scarcely had changed after undergoing four consecutive cycles. In addition, the XRD pattern ( Figure S7a,c) and SEM image ( Figure S7b,d) of TiO 2 /ZnIn 2 S 4 -10 wt% illuminated the crystal structure and morphology of TiO 2 /ZnIn 2 S 4 -10 wt% before and after photodegradation cycling remained unchanged. The results demonstrated the splendid degradation stability of TiO 2 /ZnIn 2 S 4 -10 wt% during the photocatalytic process.  The work functions (Φ) were calculated to investigate the route of charge transfer at the contact interface of ZnIn2S4 and TiO2, and the results are given in Figure 10a. It was observed that the Φ of ZnIn2S4 is lower than that of TiO2, and thus the photoinduced electrons could transfer from ZnIn2S4 to TiO2 when ZnIn2S4 and TiO2 came in contact to construct a heterojunction. Subsequently, EDTA-2Na, t-BuOH, and BQ were selected in The work functions (Φ) were calculated to investigate the route of charge transfer at the contact interface of ZnIn 2 S 4 and TiO 2 , and the results are given in Figure 10a. It was observed that the Φ of ZnIn 2 S 4 is lower than that of TiO 2 , and thus the photoinduced electrons could transfer from ZnIn 2 S 4 to TiO 2 when ZnIn 2 S 4 and TiO 2 came in contact to construct a heterojunction. Subsequently, EDTA-2Na, t-BuOH, and BQ were selected in sequence as the scavengers of h + , • OH, and • O 2 − to further identify the roles of active species during the photodegradation process. As recorded in Figure 10b, varying degrees of photocatalytic activity suppression were observed after sacrificial agents were added with an order BQ > EDTA-2Na > t-BuOH, indicating • O 2 − and h + are main and secondary active substances, separately, while • OH has minimal impact on the photocatalytic reactions. Figure 9. Photocatalytic degradation rate (a), the pseudo-first-order kinetics fitted curves (b), and TOC removal rate (c) of TC over all samples; maintenance of catalytic performance of TiO2/ZnIn2S4-10 wt% (d).
The work functions (Φ) were calculated to investigate the route of charge transfer at the contact interface of ZnIn2S4 and TiO2, and the results are given in Figure 10a. It was observed that the Φ of ZnIn2S4 is lower than that of TiO2, and thus the photoinduced electrons could transfer from ZnIn2S4 to TiO2 when ZnIn2S4 and TiO2 came in contact to construct a heterojunction. Subsequently, EDTA-2Na, t-BuOH, and BQ were selected in sequence as the scavengers of h + , • OH, and • O2 − to further identify the roles of active species during the photodegradation process. As recorded in Figure 10b, varying degrees of photocatalytic activity suppression were observed after sacrificial agents were added with an order BQ > EDTA-2Na > t-BuOH, indicating • O2 − and h + are main and secondary active substances, separately, while • OH has minimal impact on the photocatalytic reactions. The possible mechanisms for boosting photocatalytic pollutant degradation and H 2 production performances of TiO 2 /ZnIn 2 S 4 composites were proposed and illustrated in Figure 11 based on the aforementioned discussion. When TiO 2 nanosheets grew on the surface of ZnIn 2 S 4 nanosheets, a closed contact interface was formed between ZnIn 2 S 4 and TiO 2 . Under visible-light irradiation, TiO 2 could not absorb visible light due to its large band gap energy, while the electrons on the VB of ZnIn 2 S 4 could be easily excited to its CB and generate electron-hole pairs because of the small band gap energy. According to the DFT calculated results of work functions, the electrons would transfer from the CB of ZnIn 2 S 4 to that of TiO 2 , while h + left on the VB of ZnIn 2 S 4 , which leads to the spatial separation of electrons and holes with higher redox powers. For H 2 production, the photogenerated e − could easily reduce the surface-adsorbed protons to H 2 with h + being consumed by TEOA. Different from the H 2 production process, electrons on the CB of TiO 2 would firstly react with O 2 to obtain • O 2 − in the photodegradation process. Subsequently, the h + and • O 2 − participated in the photodegradation reaction due to their strong oxidation ability. As a result, the improved separation efficiency of photoinduced charge carriers would provide more carriers to participate in the photocatalytic reaction process to acquire enhanced photocatalytic performance. The possible mechanisms for boosting photocatalytic pollutant degradation and H2 production performances of TiO2/ZnIn2S4 composites were proposed and illustrated in Figure 11 based on the aforementioned discussion. When TiO2 nanosheets grew on the surface of ZnIn2S4 nanosheets, a closed contact interface was formed between ZnIn2S4 and TiO2. Under visible-light irradiation, TiO2 could not absorb visible light due to its large band gap energy, while the electrons on the VB of ZnIn2S4 could be easily excited to its CB and generate electron-hole pairs because of the small band gap energy. According to the DFT calculated results of work functions, the electrons would transfer from the CB of ZnIn2S4 to that of TiO2, while h + left on the VB of ZnIn2S4, which leads to the spatial separation of electrons and holes with higher redox powers. For H2 production, the photogenerated e − could easily reduce the surface-adsorbed protons to H2 with h + being consumed by TEOA. Different from the H2 production process, electrons on the CB of TiO2 would firstly react with O2 to obtain • O2 − in the photodegradation process. Subsequently, the h + and • O2 − participated in the photodegradation reaction due to their strong oxidation ability. As a result, the improved separation efficiency of photoinduced charge carriers would provide more carriers to participate in the photocatalytic reaction process to acquire enhanced photocatalytic performance.

Conclusions
In summary, a 2D/2D heterojunction consisting of ZnIn2S4 nanosheets and TiO2 nanosheets was fabricated by a facile two-step synthesis method for photocatalytic H2 evolution and pollutant degradation. The small TiO2 nanosheets deposited on the surface

Conclusions
In summary, a 2D/2D heterojunction consisting of ZnIn 2 S 4 nanosheets and TiO 2 nanosheets was fabricated by a facile two-step synthesis method for photocatalytic H 2 evolution and pollutant degradation. The small TiO 2 nanosheets deposited on the surface of large ZnIn 2 S 4 nanosheets, resulting in the formation of the close 2D/2D heterointerface contact, which contributes to providing sufficient and short paths for the separation and transfer of photoinduced charge. All TiO 2 /ZnIn 2 S 4 heterojunction photocatalysts possess higher photocatalytic activities than pure ZnIn 2 S 4 and TiO 2 . Among them, the TiO 2 /ZnIn 2 S 4 -10 wt% photocatalyst exhibits optimal H 2 evolution rate (650 µmol/h/g) and pollution degradation efficiencies (95% for TC and 93% for RhB) with excellent photocatalytic stability during four consecutive test cycles. The enhanced photocatalytic properties are believed to have originated from the accelerated charges separation and transfer as well as enhanced specific surface area and hydrophilicity. This work provides a practical strategy for preparing ZnIn 2 S 4 -based heterojunctions to act as highly efficient bifunctional photocatalysts for energy and environmental application.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano13162315/s1, Figure S1: structure models; Figure S2: The elemental mapping images; Figure S3: Calculated band structures; Figure S4: The energy band structure; Figure S5: XRD patterns and SEM image in the recycled photocatalytic H 2 development; Figure S6: Photocatalytic degradation rate, pseudo-first-order kinetics fitted curves, TOC removal rate and maintenance of catalytic performance; Figure S7: XRD patterns and SEM images in the recycling photocatalytic degradation.
Author Contributions: Y.C.: investigation, methodology, data curation, and writing-original draft preparation. L.Z.: data curation, validation, and formal analysis. Y.S. and J.L.: software, investigation. J.X.: resources, supervision, and writing-review and editing. L.Q. and X.X.: investigation and methodology. D.M.: validation and funding acquisition. P.L.: conceptualization, methodology, funding acquisition, and writing-review and editing. S.D.: project administration, supervision, and funding acquisition. All authors have read and agreed to the published version of the manuscript.
Funding: This study was funded by the National Natural Science Foundation of China (grant number 41763020) and the Natural Science Foundation of Jiangxi Province (grant numbers 20212BAB204020, 20212BAB204018, and 20202BABL214040).

Data Availability Statement:
The authors confirm that the data supporting the findings of this study are available within the article. Derived data supporting the findings of this study are available on request.

Conflicts of Interest:
The authors declare no conflict of interest.