Efﬁcient Photocatalytic Degradation of RhB by Constructing Sn 3 O 4 Nanoﬂakes on Sulfur-Doped NaTaO 3 Nanocubes

: Band structure engineering and heterojunction photocatalyst construction are efﬁcient approaches to improve the separation of photo-induced electrons and holes, along with enhancing light response ability. By sulfur doping, sodium tantalite (NaTaO 3 ) showed an improved photocatalytic property for the degradation of Rhodamine B (RhB). Sn 3 O 4 nanoﬂakes were constructed on the surface of NaTaO 3 nanocubes, forming a surface heterostructure via a simple hydrothermal process, initially. This heterostructure endows the photocatalyst with an enhanced charge separation rate, resulting in an improved photocatalytic degradation of RhB. Moreover, a possible mechanism over Sn 3 O 4 /NaTaO 3 and the photodegradation pathway of RhB were proposed as the combined effect of photo-induced electrons and holes. This facile process for band structure engineering and heterostructure construction provides the possibility for the practical application of high-efﬁciency photocatalysts. To prove the rationality of the proposed reaction pathway and investigate the capabilities of photo-induced e − /h+ pairs in the semiconductors, PL emission spectra of NaTaO 3 , NTO-S, Sn/NTO, Sn/NTO-S, and Sn 3 O 4 were carried out, respectively. PL emission arises from the recombination of free carriers, and a higher PL intensity reveals a higher recombination rate of photogenerated e − /h+ pairs. By contrast, a weaker PL intensity indicates a lower recombination rate of photo-excited e − /h+ pairs, and accordingly, much more photo-induced of states in TaO 6 After the S anion in NaTaO , the PL intensity of NTO-S was weaker than that of pure NaTaO 3 . indicated that the NTO-S could suppress the recombination of e − /h+ pairs. Noticeably, the intensities of the PL spectra of Sn/NTO and the Sn/NTO-S heterojunction photocatalysts strongly decreased with the Sn/NTO heterojunctions formed as compared to pure cubic NaTaO 3 ; this can be seen at the maximum PL intensity at around 469 nm. The lower PL intensity implied the efﬁcient inhibition of photogenerated pairs and therefore higher photocatalytic activity in the UV light region, which could effectively explain the variation of the photocatalytic activities of the different samples.


Introduction
In recent years, the extensive treatment of wastewater containing organic pollutants has led to a negative impact on the environment and serious consequences for human health. Although considerable efforts and strategies have been taken to alleviate the abovementioned problems, the photocatalytic oxidation of water as an advanced sewage disposal mechanism is of confocal interest and is considered an effective solution [1][2][3][4][5][6][7]. For this reason, there has been an upsurge in research on a variety of photocatalysts. Among these, ABO 3 perovskite-type oxides have been widely studied, due to their high performance in photocatalytic reactions, such as with overall water splitting and the photodegradation of organic pollutants, which provide a potential approach to solve our urgent energy and environmental problems [8,9]. Sodium tantalite (NaTaO 3 ) is a prominent ABO 3 perovskite-like oxide with a high performance in photocatalytic reactions, including abundance, photochemical stability, photodegradation of organic pollutants, water splitting, and low environmental impact, which attracts a lot of attention for its favorable layered structure and distinctive separation effectiveness in charge separation [10][11][12][13][14][15][16][17][18][19][20]. Hence, NaTaO 3 -based photocatalysts can intrinsically decrease the recombination rate of photo-induced carriers and promote the efficiency of photocatalytic systems. It has been well-documented that tantalates are superior to other metal oxides. Moreover, NaTaO 3 is a new type of wide bandgap semiconductor material, which has a bandgap of about 4.0 eV at room temperature [10]. To enhance the photocatalytic performance, metal or non-metal ions doped with NaTaO 3 (dopants: Sr 2+ [15,17], La 3+ [20,21], Ba 2+ [22], Ca 2+ [23], N or S [12,23]) or semiconductors loaded on NaTaO 3 , such In 2 S 3 [10], WO 3 [11], NiO [24], AgCl/Ag 2 O [25], g-C 3 N 4 [18,26], RuO 2 [27], and CdS [28] have been typically studied. These works were developed to improve the photocatalytic activity of NaTaO 3 -based materials. The assessment of composite photocatalysts included photocatalytic water splitting and photodegradation of organic pollutants.
These techniques and other strategies make NaTaO 3 a better photocatalyst. Concerning the aforementioned, the potentiality of NaTaO 3 -based photocatalysts is apparent.
To date, heterovalent tin oxide Sn 3 O 4 formed of mixed valences of Sn 2+ and Sn 4+ has garnered noticeable attention. One third of Sn atoms are located in Sn (II) tetrahedral coordination sites and two-thirds are located in Sn (IV) octahedral sites [29][30][31]. Additionally, the Sn 3 O 4 tin structure has been reported to influence the physical properties of tin oxide. As proposed by theoretical calculations, Sn 3 O 4 has been experimentally confirmed to possess absorption bands in a limited visible-light region, which may have promising applications as a potential photocatalyst in environmental remediation and energy conversion. With a direct bandgap energy of around 2.56 eV, Sn 3 O 4 has also been widely studied for its relatively negative conduction band edge and moderate charge transport features. The proper energy band makes it a better material for constructing heterojunction catalysts with other semiconductor materials [32][33][34][35][36][37]. Hence, it is very vital to broaden the photocatalytic active spectral range and improve the separation of carriers of Sn 3 O 4 .
To the best of our knowledge, there is no research on the photocatalytic performance of the Sn 3 O 4 /NaTaO 3 heterostructure. Therefore, in the current work, sulfur-doped NaTaO 3 cubes were synthesized to modify the band structure of NaTaO 3 . Based on the appropriate valance band (EVB~3.01 eV) and conduction band edge (ECB~0.99 eV) of NaTaO 3 [12,28], which can match well with that of Sn 3 O 4 (EVB~1.07 eV and ECB~1.55 eV vs. NHE, pH = 7) [30], the heterojunction of Sn 3 O 4 /NaTaO 3 was synthesized. An enhanced photoinduced charge separation was highly expected, which results in the enhanced photocatalytic degradation of RhB. This work provides an ordinary and low-cost method for the large-scale production of NaTaO 3 -based materials in various applications. It is expected to offer an upfront approach for developing highly stable and effective heterostructures for organic pollutant degradation.

Synthesis
NaTaO 3 and sulfur anion doped in NaTaO 3 were prepared via the reported effortless hydrothermal approach. In a typical synthesis process, 0.442 g of Ta 2 O 5 , 1.2 g of NaOH, and a certain amount of Na 2 S 2 O 3 ·5H 2 O were added into a Teflon-lined stainless steel autoclave (50 mL capacity) that was filled with deionized water to 75% of the total volume. The autoclave was sealed and put into a preheated oven to perform hydrothermal treatment at 180 • C for 12 h. After cooling to room temperature, the acquired precipitates were collected by centrifugation and thoroughly washed with deionized water several times, and then dried at 80 • C for 12 h before further characterization and photocatalytic reaction. The obtained products were NaTaO 3 powders and S-doped NaTaO 3 powders, respectively; the latter had a calculated S:Ta molar ratio of 5%, where the amount of Na 2 S 2 O 3 •5H 2 O was 0.0248 g.
The Sn 3 O 4 /NaTaO 3 and Sn 3 O 4 /NaTaO 3 -S were prepared via a facile hydrothermal coprecipitation method. First, 5.0 mmol of SnCl 2 •2H 2 O and 12.5 mmol of Na 3 C 6 H 5 O 7 •2H 2 O were dissolved in 12.5 mL of deionized water and stirred for 5 min to get a transparent solution, where a certain amount of NaTaO 3 or NaTaO 3 -S (0.175 g) was added. Then, 12.5 mL of 0.2 M NaOH aqueous solution was added to the above-mentioned solution, with vigorous stirring followed by ultrasonic treatment and the solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and maintained at 180 • C for 12 h. The acquired powder was washed with deionized water and ethanol several times and then finally dried at 60 • C for 12 h. Sn 3 O 4 /NaTaO 3 and Sn 3 O 4 /NaTaO 3 -S composites were prepared with the molar ratio of Sn:Ta 1:1. Pure Sn 3 O 4 was synthesized without the addition of cubic NaTaO 3 in a similar process.

Characterizations
The crystalline structures of the products were characterized by X-ray diffraction (XRD) on a Bruker D8 Advance powder X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). The morphologies of the as-obtained catalysts were recorded by field emission scanning electron microscopy (FE-SEM, HITACHIS-4800). The microstructures of the samples were recorded using high-resolution transmission electron microscopy (HRTEM, JOEL JEM 2100). UV-vis diffuse reflectance spectra (DRS) of the catalysts were acquired on a UV-vis spectrophotometer (UV-2550, Shimadzu) with an integrating sphere attachment within the wavelength range from 200 nm to 800 nm along with BaSO 4 as the reflectance standard. The photoluminescence (PL) emission spectra were carried out with an FLS920 fluorescence spectrometer at room temperature under the excitation wavelength of 300 nm.

Photocatalytic Degradation of RhB
The photocatalytic activities of as-obtained products determined by the degradation of RhB were conducted in a photoreaction apparatus, an XPA-II photochemical reactor (XPA-II, Nanjing Xujiang Machine-electronic Plant, China). In typical processing, a 30 mg catalyst was added to 30 mL of RhB solution (20 mg/L). The mixer was stirred for 30 min in the dark to achieve the adsorption-desorption equilibrium between the dye and the surface of the catalyst, then the reaction system was performed via the irradiation of a 300 W mercury lamp and by continuously stirring under favorable ambient conditions. At certain intervals of time, 4 mL of the mixed solution was collected and taken out for centrifugation to remove the catalyst and analyzed using a UV-vis spectrophotometer (UV-6100, Metash). The residual samples were collected for repeated photocatalytic reactions.

Results and Discussion
To know the phase composition of hetero photocatalyst, crystallinity, and purity, XRD was performed and is presented in Figure 1. The XRD pattern of NaTaO 3 shows that the peaks at 22.9 • , 32.5 • , 40.1 • , 46.6 • , 52.5 • , 57.8 • , 57.9 • , and 67.9 • are attributed to the (100), (101), (111), (200), (102), (112), (121), and (022) diffraction planes, respectively. All the diffraction peaks can be readily indexed as a pure perovskite NaTaO 3 with a monoclinic structure (JCPDS No. 74-2478) [10,12], which can facilitate the separation of photo-induced electron-hole pairs. As has been reported, NaTaO 3 may exist in three polymorphs, which are the orthorhombic, cubic, and monoclinic phases [10,12]. All the phases share TaO 6 octahedra frameworks at the corners, along with sodium ions residing in the dodecahedral interspaces, but the local structure distortion is highly different among various polymorphs [12]. The TaO 6 octahedra of the monoclinic phase were close to the ideal perovskite, favoring the separation of photo-induced electrons and holes and representing better photocatalytic activity [14]. After doping the S anion, it can also be seen that the diffraction peaks of NTO-S series samples still matched the monoclinic structure. As for bare Sn 3 O 4 , its XRD pattern showed poorer crystallinity and lower diffraction peak intensity. The principal diffraction peaks may correspond to the (101), (120),  The morphologies and microstructural details of the as-prepared NaTaO3, NTO-S, Sn/NTO, and Sn/NTO-S were discussed based on the SEM and TEM images. Figure 2 contains representative SEM images of the as-obtained samples. As shown (inset of Figure  2a,b), NaTaO3 and NTO-S presented an approximate cubic structure with an average size between 300 nm and 500 nm, while the surfaces of some of them were a little coarse. Figure  2c,d shows the synthesized Sn3O4 clusters with low and high magnification, respectively, which are composed of thin flakes with a size range between 100 nm and 400 nm and a thickness of approximately 5-10 nm. Then the SEM images of Sn/NTO ( Figure 2e) and Sn/NTO-S ( Figure 2f) are displayed. Their surfaces were rough, which consisted of Sn3O4 nanoflakes and might provide still more active faces and a larger specific area beneficial to heterogeneous nucleation and assembly growth of Sn3O4 nanoflakes. The morphologies and microstructural details of the as-prepared NaTaO 3 , NTO-S, Sn/NTO, and Sn/NTO-S were discussed based on the SEM and TEM images. Figure 2 contains representative SEM images of the as-obtained samples. As shown (inset of Figure 2a,b), NaTaO 3 and NTO-S presented an approximate cubic structure with an average size between 300 nm and 500 nm, while the surfaces of some of them were a little coarse. Figure 2c As shown in Figure 3a,b, NaTaO 3 always shows the cubic morphology before and after constructing with Sn 3 O 4 . Additionally, the average size of those composites changes little, approximately 300 nm. Evidently, the cubic NaTaO 3 was essentially surrounded by sheet-like Sn 3 O 4 and a portion of small slices that were well dispersed on the outside surface of the cubic NaTaO 3 to compose a representative Sn/NTO heterojunction. Hence, Figure 3c illustrates a schematic diagram of the growth process of Sn/NTO in two steps. The first step is heterogeneous nucleation of Sn 3 O 4 seeds on the outside surface of the cubic NaTaO 3 , driven by the adsorption connecting -OH groups on the surface of the cubic NaTaO 3 with Sn 2+ in the precursor [31]. The second step is Sn 3 O 4 nanoflakes successfully growing an assembly on the outside surface of cubic NaTaO 3 . The heterojunction between Sn 3 O 4 and NaTaO 3 would facilitate photoresponse and interfacial electron transfer and therefore enhance the photocatalytic activity. As shown in Figure 3a,b, NaTaO3 always shows the cubic morphology before and after constructing with Sn3O4. Additionally, the average size of those composites changes little, approximately 300 nm. Evidently, the cubic NaTaO3 was essentially surrounded by sheet-like Sn3O4 and a portion of small slices that were well dispersed on the outside surface of the cubic NaTaO3 to compose a representative Sn/NTO heterojunction. Hence, Figure 3c illustrates a schematic diagram of the growth process of Sn/NTO in two steps. The first step is heterogeneous nucleation of Sn3O4 seeds on the outside surface of the cubic NaTaO3, driven by the adsorption connecting -OH groups on the surface of the cubic Na-TaO3 with Sn 2+ in the precursor [31]. The second step is Sn3O4 nanoflakes successfully growing an assembly on the outside surface of cubic NaTaO3. The heterojunction between Sn3O4 and NaTaO3 would facilitate photoresponse and interfacial electron transfer and therefore enhance the photocatalytic activity. The light properties of a material are very sensitive to and seriously influenced by their intrinsic microstructures and, therefore, any changes in electronic structure. Figure  4a determines the UV-vis diffuse reflectance spectra (DRS) of NaTaO3, NTO-S, Sn/NTO,  As shown in Figure 3a,b, NaTaO3 always shows the cubic morphology before and after constructing with Sn3O4. Additionally, the average size of those composites changes little, approximately 300 nm. Evidently, the cubic NaTaO3 was essentially surrounded by sheet-like Sn3O4 and a portion of small slices that were well dispersed on the outside surface of the cubic NaTaO3 to compose a representative Sn/NTO heterojunction. Hence, Figure 3c illustrates a schematic diagram of the growth process of Sn/NTO in two steps. The first step is heterogeneous nucleation of Sn3O4 seeds on the outside surface of the cubic NaTaO3, driven by the adsorption connecting -OH groups on the surface of the cubic Na-TaO3 with Sn 2+ in the precursor [31]. The second step is Sn3O4 nanoflakes successfully growing an assembly on the outside surface of cubic NaTaO3. The heterojunction between Sn3O4 and NaTaO3 would facilitate photoresponse and interfacial electron transfer and therefore enhance the photocatalytic activity. The light properties of a material are very sensitive to and seriously influenced by their intrinsic microstructures and, therefore, any changes in electronic structure. Figure  4a determines the UV-vis diffuse reflectance spectra (DRS) of NaTaO3, NTO-S, Sn/NTO, The light properties of a material are very sensitive to and seriously influenced by their intrinsic microstructures and, therefore, any changes in electronic structure. Figure 4a determines the UV-vis diffuse reflectance spectra (DRS) of NaTaO 3 , NTO-S, Sn/NTO, and Sn/NTO-S (molar ratio Sn/Ta = 1/1) heterostructures, along with pure Sn 3 O 4 . The bare NaTaO 3 presented steep absorption only in the UV region located at 308 nm. Sulfur ion doping in NaTaO 3 showed an intense absorption with an absorption red-shift of 323 nm. The pure Sn 3 O 4 could absorb the light irradiation as its optical absorption was determined to be around 473 nm. After coupling NaTaO 3 or NTO-S, it was found that Sn/NTO and Sn/NTO-S heterostructures exhibited a broader absorption peak in contrast to pure NaTaO 3 . The Sn/NTO composites presented a significant shift toward 484.7 nm. However, the Sn/NTO-S heterostructure showed a continuous red-shift to 523.5 nm. According to hv = 1240/λ and the Kubelka-Munk (KM) method, with the equation: αhv = A (hv − E g ) 2 , in which α, hv, E g, and A are the absorption coefficient, the photon energy, indirect bandgap, and a constant, respectively [10,11,31], the bandgap energies of NaTaO 3 , NTO-S, Sn 3 O 4 , Sn/NTO, and Sn/NTO-S shown in Figure 4b were calculated to be 4.03 eV, 3.84 eV, 2.62 eV, Crystals 2021, 11, 59 6 of 11 2.56 eV, and 2.37 eV, respectively. Hence, the bandgaps of the composites were greatly reduced. Interestingly, the bandgap values of Sn/NTO and Sn/NTO-S composites were not between pure NaTaO 3 and Sn 3 O 4 ; the optical transitions of the composites extend much wider. Consequently, it can be inferred that the bandgap value of the composite narrowed via sulfur doping; then, the light absorption was further enhanced by coupling Sn 3 O 4 . The enhanced light response could be attributed to the formation of a heterojunction between them [10,11,18,[26][27][28][29]31].
to be around 473 nm. After coupling NaTaO3 or NTO-S, it was found that Sn/NTO and Sn/NTO-S heterostructures exhibited a broader absorption peak in contrast to pure Na-TaO3. The Sn/NTO composites presented a significant shift toward 484.7 nm. However, the Sn/NTO-S heterostructure showed a continuous red-shift to 523.5 nm. According to hv = 1240/λ and the Kubelka-Munk (KM) method, with the equation: αℎ = A (ℎ − ) , in which α, hv, Eg, and A are the absorption coefficient, the photon energy, indirect bandgap, and a constant, respectively [10,11,31], the bandgap energies of NaTaO3, NTO-S, Sn3O4, Sn/NTO, and Sn/NTO-S shown in Figure 4b were calculated to be 4.03 eV, 3.84 eV, 2.62 eV, 2.56 eV, and 2.37 eV, respectively. Hence, the bandgaps of the composites were greatly reduced. Interestingly, the bandgap values of Sn/NTO and Sn/NTO-S composites were not between pure NaTaO3 and Sn3O4; the optical transitions of the composites extend much wider. Consequently, it can be inferred that the bandgap value of the composite narrowed via sulfur doping; then, the light absorption was further enhanced by coupling Sn3O4. The enhanced light response could be attributed to the formation of a heterojunction between them [10,11,18,[26][27][28][29]31]. To investigate the photocatalytic degradation capabilities of the as-obtained catalysts, we carried out a degradation properties assessment using different samples on RhB in a photocatalytic device that had been specially designed. The degradation of RhB and the corresponding kinetic plots are displayed in Figure 5a,b, respectively. Figure 5a shows the different relative concentration variations of the RhB over 140 min. C0 is the initial concentration of the RhB, and Ct is the concentration of the RhB after a reaction for t. The concentration of the RhB was almost constant, which implies that the RhB had attained the adsorption equilibrium in a short time and prior reaction. Therefore, it was confirmed that the decrease of RhB was a result of degradation as time went by. Cubic NaTaO3 presented a steady degradation rate under UV light irradiation. After 120 min, the concentration of the RhB decreased by 63%. The photodegradation of the RhB in the presence of pure Sn3O4 performed relatively slowest, as only 18% of the RhB was degraded in the 120 min illumination period. The reduced degradation rate possibly contributed to the inhibited electron transportation efficiency and narrow spectral response in bare Sn3O4, which led to the fast recombination of photo-induced electron-hole pairs and poor activity. It is well known that NaTaO3 exhibits low photocatalytic activity, even under UV illumination, due to its large bandgap. However, after having been doped in sulfur ion or coupled with Sn3O4, the photocatalytic degradation ability of NTO-S or Sn/NTO was very much enhanced in contrast to that of the pure NaTaO3 or the Sn3O4 catalyst. Interestingly, both displayed similar photocatalytic degradation rates of RhB under UV light irradiation. The Sn/NTO-S showed the best performance. The RhB was degraded almost entirely in 120 min, which also demonstrated that heterojunction structure plays a decisive role in the transportation and separation of photo-induced carriers. To investigate the photocatalytic degradation capabilities of the as-obtained catalysts, we carried out a degradation properties assessment using different samples on RhB in a photocatalytic device that had been specially designed. The degradation of RhB and the corresponding kinetic plots are displayed in Figure 5a,b, respectively. Figure 5a shows the different relative concentration variations of the RhB over 140 min. C 0 is the initial concentration of the RhB, and C t is the concentration of the RhB after a reaction for t. The concentration of the RhB was almost constant, which implies that the RhB had attained the adsorption equilibrium in a short time and prior reaction. Therefore, it was confirmed that the decrease of RhB was a result of degradation as time went by. Cubic NaTaO 3 presented a steady degradation rate under UV light irradiation. After 120 min, the concentration of the RhB decreased by 63%. The photodegradation of the RhB in the presence of pure Sn 3 O 4 performed relatively slowest, as only 18% of the RhB was degraded in the 120 min illumination period. The reduced degradation rate possibly contributed to the inhibited electron transportation efficiency and narrow spectral response in bare Sn 3 O 4 , which led to the fast recombination of photo-induced electron-hole pairs and poor activity. It is well known that NaTaO 3 exhibits low photocatalytic activity, even under UV illumination, due to its large bandgap. However, after having been doped in sulfur ion or coupled with Sn 3 O 4 , the photocatalytic degradation ability of NTO-S or Sn/NTO was very much enhanced in contrast to that of the pure NaTaO 3 or the Sn 3 O 4 catalyst. Interestingly, both displayed similar photocatalytic degradation rates of RhB under UV light irradiation. The Sn/NTO-S showed the best performance. The RhB was degraded almost entirely in 120 min, which also demonstrated that heterojunction structure plays a decisive role in the transportation and separation of photo-induced carriers. Furthermore, the degradation rate for RhB can be acquired from the slope (min −1 ) of -Ln(C t /C 0 )~t plots (Figure 5b); the calculated results of the decolorization rate constant κ (min −1 ) for NaTaO 3 , NTO-S, Sn/NTO, Sn/NTO-S, and Sn 3 O 4 were 0.3366, 0.5790, 0.6060, 1.3350, and 0.0555 min −1 , respectively. Evidently, the Sn/NTO-S catalyst exhibited the highest κ values (1.3550 min −1 ), which were about 4.0 times those of NaTaO 3 .
Based on the experimental results mentioned above, a schematic model of a proposed reaction pathway of degraded products is illustrated in Scheme 1. The band edge positionings of NaTaO 3 and Sn 3 O 4 were carefully made based on experimental results and earlier studies. Via DRS, the bandgap energy of NaTaO 3 (4.03 eV) was wider than that of Sn 3 O 4 (2.62 eV) [38]. The CB of NaTaO 3 was estimated to be −0.99 eV, as documented [10,39]. The narrowing of bandgap energy as a result of doping S anion is a very key factor. The photodegradation efficiency of NTO-S was found to be greater than that of pure NaTaO 3 . The reason may be that the substitution of resident O atoms within the NaTaO 3 lattice by sulfur atoms induced the local states just above the valence level as the new hybridized orbitals were formed [12,40]. Aside from the bandgap width, the photocatalytic degradation activity of a sample highly depended on the recombination of electron (e − )-hole (h + ) pairs [12,[41][42][43][44]. Under photo-irradiation, the photo-induced e−/h+ pairs on the CB/VB of each semiconductor were created. As reported previously, the principal active species for the photocatalytic degradation of RhB included hydroxyl radicals (•OH) and superoxide radicals (•O 2 − ), which were composed of the creation of h + and e − [34], and some researchers have explored intermediates and key reaction species [45][46][47]. S-doped catalysts displayed a less intense emission than pure NaTaO 3 samples, indicating a reduced e − /h + recombination and an enhanced RhB photodegradation performance [12]. Furthermore, the degradation rate for RhB can be acquired from the slope (min −1 ) of -Ln(Ct/C0) ~ t plots (Figure 5b); the calculated results of the decolorization rate constant κ (min −1 ) for NaTaO3, NTO-S, Sn/NTO, Sn/NTO-S, and Sn3O4 were 0.3366, 0.5790, 0.6060, 1.3350, and 0.0555 min −1 , respectively. Evidently, the Sn/NTO-S catalyst exhibited the highest κ values (1.3550 min −1 ), which were about 4.0 times those of NaTaO3.
Based on the experimental results mentioned above, a schematic model of a proposed reaction pathway of degraded products is illustrated in Scheme 1. The band edge positionings of NaTaO3 and Sn3O4 were carefully made based on experimental results and earlier studies. Via DRS, the bandgap energy of NaTaO3 (4.03 eV) was wider than that of Sn3O4 (2.62 eV) [38]. The CB of NaTaO3 was estimated to be −0.99 eV, as documented [10,39]. The narrowing of bandgap energy as a result of doping S anion is a very key factor. The photodegradation efficiency of NTO-S was found to be greater than that of pure Na-TaO3. The reason may be that the substitution of resident O atoms within the NaTaO3 lattice by sulfur atoms induced the local states just above the valence level as the new hybridized orbitals were formed [12,40]. Aside from the bandgap width, the photocatalytic degradation activity of a sample highly depended on the recombination of electron (e − )-hole (h + ) pairs [12,[41][42][43][44]. Under photo-irradiation, the photo-induced e−/h+ pairs on the CB/VB of each semiconductor were created. As reported previously, the principal active species for the photocatalytic degradation of RhB included hydroxyl radicals (•OH) and superoxide radicals (•O2 − ), which were composed of the creation of h + and e − [34], and some researchers have explored intermediates and key reaction species [45][46][47]. S-doped catalysts displayed a less intense emission than pure NaTaO3 samples, indicating a reduced e − /h + recombination and an enhanced RhB photodegradation performance [12]. This plausible mechanism comprised excitation of the CB of the Sn3O4; then, the e − on the CB of the NaTaO3 reacted with the available O2 to produce the superoxide radicals •O2 − . Then, the •O2 − degraded RhB, or the RhB intermediate, into degraded products [45].   [31], and holes contributed more to the high photocatalytic performance, while •OH showed less importance in its ability to degrade RhB [34]. In this case, the RhB reacted with h + and formed the intermediate RhB that was finally converted to degraded products. Comprehensively, the photocatalytic degradation mechanisms could be depicted as the interfacial charge transfer of the composite-enhanced RhB photosensitization process under irradiation, which could effectively separate and transfer photogenerated carriers. Therefore, the resultant •O 2 − and h + favored the photodegradation procedure, and the coupled semiconducting materials formed more dynamic catalytic centers that improved the efficiency of RhB dye removal. This suggested that high-quality interfaces in the heterostructure composites played a vital role in enhancing the catalytic performance. To prove the rationality of the proposed reaction pathway and investigate the capabilities of photo-induced e−/h+ pairs in the semiconductors, PL emission spectra of NaTaO 3 , NTO-S, Sn/NTO, Sn/NTO-S, and Sn 3 O 4 were carried out, respectively. PL emission arises from the recombination of free carriers, and a higher PL intensity reveals a higher recombination rate of photogenerated e−/h+ pairs. By contrast, a weaker PL intensity indicates a lower recombination rate of photo-excited e−/h+ pairs, and accordingly, much more photo-induced carriers can participate in the photocatalytic reaction [10,34]. As shown in Figure 6, the spectrum of NaTaO 3 nanocubes showed three peaks at around 395 nm, 451 nm, and 469 nm. The emission at 451 nm and 469 nm was due to band-to-band mixing of Ta 4+ -O − states in the octahedral TaO 6 motifs of NaTaO 3 [48]. After doping the S anion in NaTaO 3 , the PL intensity of NTO-S was weaker than that of pure NaTaO 3 . It indicated that the NTO-S could suppress the recombination of e−/h+ pairs. Noticeably, the intensities of the PL spectra of Sn/NTO and the Sn/NTO-S heterojunction photocatalysts strongly decreased with the Sn/NTO heterojunctions formed as compared to pure cubic NaTaO 3 ; this can be seen at the maximum PL intensity at around 469 nm. The lower PL intensity implied the efficient inhibition of photogenerated pairs and therefore higher photocatalytic activity in the UV light region, which could effectively explain the variation of the photocatalytic activities of the different samples.
RhB that was finally converted to degraded products. Comprehensively, the photocatalytic degradation mechanisms could be depicted as the interfacial charge transfer of the composite-enhanced RhB photosensitization process under irradiation, which could effectively separate and transfer photogenerated carriers. Therefore, the resultant •O2 − and h + favored the photodegradation procedure, and the coupled semiconducting materials formed more dynamic catalytic centers that improved the efficiency of RhB dye removal. This suggested that high-quality interfaces in the heterostructure composites played a vital role in enhancing the catalytic performance.
To prove the rationality of the proposed reaction pathway and investigate the capabilities of photo-induced e−/h+ pairs in the semiconductors, PL emission spectra of Na-TaO3, NTO-S, Sn/NTO, Sn/NTO-S, and Sn3O4 were carried out, respectively. PL emission arises from the recombination of free carriers, and a higher PL intensity reveals a higher recombination rate of photogenerated e−/h+ pairs. By contrast, a weaker PL intensity indicates a lower recombination rate of photo-excited e−/h+ pairs, and accordingly, much more photo-induced carriers can participate in the photocatalytic reaction [10,34]. As shown in Figure 6, the spectrum of NaTaO3 nanocubes showed three peaks at around 395 nm, 451 nm, and 469 nm. The emission at 451 nm and 469 nm was due to band-to-band mixing of Ta 4+ -O − states in the octahedral TaO6 motifs of NaTaO3 [48]. After doping the S anion in NaTaO3, the PL intensity of NTO-S was weaker than that of pure NaTaO3. It indicated that the NTO-S could suppress the recombination of e−/h+ pairs. Noticeably, the intensities of the PL spectra of Sn/NTO and the Sn/NTO-S heterojunction photocatalysts strongly decreased with the Sn/NTO heterojunctions formed as compared to pure cubic NaTaO3; this can be seen at the maximum PL intensity at around 469 nm. The lower PL intensity implied the efficient inhibition of photogenerated pairs and therefore higher photocatalytic activity in the UV light region, which could effectively explain the variation of the photocatalytic activities of the different samples.

Conclusions
On the whole, we have successfully synthesized a novel Sn3O4/NaTaO3 heterojunction with doping sulfur anion via a simple hydrothermal method. The bandgap value of NaTaO3 and the composite narrowed via sulfur doping, which induced an increase in light absorption. With coupling Sn3O4, it was found that the as-obtained Sn/NTO and

Conclusions
On the whole, we have successfully synthesized a novel Sn 3 O 4 /NaTaO 3 heterojunction with doping sulfur anion via a simple hydrothermal method. The bandgap value of NaTaO 3 and the composite narrowed via sulfur doping, which induced an increase in light absorption. With coupling Sn 3 O 4 , it was found that the as-obtained Sn/NTO and Sn/NTO-S composites exhibited cubic NaTaO 3 , or that NTO-S assembled by countless interlaced fine Sn 3 O 4 nanoflakes were successfully synthesized for the first time. The fabrication of enough interfaces with high qualities played a critical role in light absorption and enhancing photocatalytic performance. Assessed by the degradation of the RhB solution, the Sn/NTO-S composite presents a superior photoreactivity toward RhB degradation under UV light illumination in contrast to individual Sn 3 O 4 or NaTaO 3 . The calculated decolorization rate constant is much greater (1.3550 min −1 ) than single Sn 3 O 4 (0.0555 min −1 ) or NaTaO 3 (0.3366 min −1 ). This could be due to the enhanced light absorbance by the larger specific surface area of the Sn 3 O 4 nanoflakes and the formation of the heterojunction between NTO-S and Sn 3 O 4 , which can separate photo-induced carriers efficiently. It is also proposed that the formed •O 2 − and h + favored the photocatalytic degradation of RhB. This