Efficient Photocatalytic Degradation of RhB by Constructing Sn₃O₄ Nanoflakes on Sulfur-Doped NaTaO₃ Nanocubes

Abstract

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 (NaTaO₃) showed an improved photocatalytic property for the degradation of Rhodamine B (RhB). Sn₃O₄ nanoflakes were constructed on the surface of NaTaO₃ 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 Sn₃O₄/NaTaO₃ 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.

1. 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 above-mentioned 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, ABO₃ 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 (NaTaO₃) is a prominent ABO₃ 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, NaTaO₃-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, NaTaO₃ 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 NaTaO₃ (dopants: Sr²⁺ [15,17], La³⁺ [20,21], Ba²⁺ [22], Ca²⁺ [23], N or S [12,23]) or semiconductors loaded on NaTaO₃, such In₂S₃ [10], WO₃ [11], NiO [24], AgCl/Ag₂O [25], g-C₃N₄ [18,26], RuO₂ [27], and CdS [28] have been typically studied. These works were developed to improve the photocatalytic activity of NaTaO₃-based materials. The assessment of composite photocatalysts included photocatalytic water splitting and photodegradation of organic pollutants. These techniques and other strategies make NaTaO₃ a better photocatalyst. Concerning the aforementioned, the potentiality of NaTaO₃-based photocatalysts is apparent.

To date, heterovalent tin oxide Sn₃O₄ formed of mixed valences of Sn²⁺ and Sn⁴⁺ 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 Sn₃O₄ tin structure has been reported to influence the physical properties of tin oxide. As proposed by theoretical calculations, Sn₃O₄ 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, Sn₃O₄ 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 Sn₃O₄.

To the best of our knowledge, there is no research on the photocatalytic performance of the Sn₃O₄/NaTaO₃ heterostructure. Therefore, in the current work, sulfur-doped NaTaO₃ cubes were synthesized to modify the band structure of NaTaO₃. Based on the appropriate valance band (EVB ~3.01 eV) and conduction band edge (ECB ~0.99 eV) of NaTaO₃ [12,28], which can match well with that of Sn₃O₄ (EVB ~1.07 eV and ECB ~1.55 eV vs. NHE, pH = 7) [30], the heterojunction of Sn₃O₄/NaTaO₃ was synthesized. An enhanced photo-induced 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 NaTaO₃-based materials in various applications. It is expected to offer an upfront approach for developing highly stable and effective heterostructures for organic pollutant degradation.

2. Materials and Methods

2.1. Materials

Tantalum oxide (Ta₂O₅), hydrochloric acid (HCl), sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), tin(II) chloride dehydrate (SnCl₂·2H₂O), ethanol (C₂H₅OH), sodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O), and Rhodamine B (RhB) were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without any further purification. Deionized water was used throughout the study.

2.2. Synthesis

NaTaO₃ and sulfur anion doped in NaTaO₃ were prepared via the reported effortless hydrothermal approach. In a typical synthesis process, 0.442 g of Ta₂O₅, 1.2 g of NaOH, and a certain amount of Na₂S₂O₃·5H₂O 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 NaTaO₃ powders and S-doped NaTaO₃ powders, respectively; the latter had a calculated S:Ta molar ratio of 5%, where the amount of Na₂S₂O₃•5H₂O was 0.0248 g.

The Sn₃O₄/NaTaO₃ and Sn₃O₄/NaTaO₃-S were prepared via a facile hydrothermal coprecipitation method. First, 5.0 mmol of SnCl₂•2H₂O and 12.5 mmol of Na₃C₆H₅O₇•2H₂O were dissolved in 12.5 mL of deionized water and stirred for 5 min to get a transparent solution, where a certain amount of NaTaO₃ or NaTaO₃-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. Sn₃O₄/NaTaO₃ and Sn₃O₄/NaTaO₃-S composites were prepared with the molar ratio of Sn:Ta 1:1. Pure Sn₃O₄ was synthesized without the addition of cubic NaTaO₃ in a similar process.

The samples acquired above are denoted as NaTaO₃, NTO-S, Sn/NTO, Sn/NTO-S, and Sn₃O₄, respectively.

2.3. 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 BaSO₄ 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.

2.4. 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.

3. 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 NaTaO₃ 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), ($\overline{1}$02), ($\overline{1}$12), ($\overline{1}$21), and (022) diffraction planes, respectively. All the diffraction peaks can be readily indexed as a pure perovskite NaTaO₃ with a monoclinic structure (JCPDS No. 74-2478) [10,12], which can facilitate the separation of photo-induced electron–hole pairs. As has been reported, NaTaO₃ may exist in three polymorphs, which are the orthorhombic, cubic, and monoclinic phases [10,12]. All the phases share TaO₆ octahedra frameworks at the corners, along with sodium ions residing in the dodecahedral interspaces, but the local structure distortion is highly different among various polymorphs [12]. The TaO₆ octahedra of the monoclinic phase were close to the ideal perovskite, favoring the separation of photo-induced electrons and holes and representing better photocatalytic activity [14]. After doping the S anion, it can also be seen that the diffraction peaks of NTO-S series samples still matched the monoclinic structure. As for bare Sn₃O₄, its XRD pattern showed poorer crystallinity and lower diffraction peak intensity. The principal diffraction peaks may correspond to the (101), ($\overline{1}$20), (111), ($\overline{2}$10), ($\overline{1}$21), and (210) crystal planes of standard triclinic-phase Sn₃O₄ (JCPDS No.16-0737) [29,30,31,34]. Additionally, the XRD patterns of Sn/NTO and Sn/NTO-S have no detected impurities, corresponding to the aforementioned NaTaO₃ and Sn₃O₄.

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.

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 NaTaO₃ with Sn²⁺ 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, and Sn/NTO-S (molar ratio Sn/Ta = 1/1) heterostructures, along with pure Sn₃O₄. The bare NaTaO₃ presented steep absorption only in the UV region located at 308 nm. Sulfur ion doping in NaTaO₃ showed an intense absorption with an absorption red-shift of 323 nm. The pure Sn₃O₄ could absorb the light irradiation as its optical absorption was determined 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 NaTaO₃. 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: $\mathsf{\alpha }hv=\mathrm{A}{(hv-E_{\mathrm{g}})}^2$, 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 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 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 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.

Furthermore, the degradation rate for RhB can be acquired from the slope (min⁻¹) of –Ln(C_t/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 NaTaO₃. 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]. The h⁺ of the NTO-S transmitted to the VB of the Sn₃O₄ [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 •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 NaTaO₃, 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 Ta⁴⁺-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.

4. 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 Sn/NTO-S composites exhibited cubic NaTaO₃, or that NTO-S assembled by countless interlaced fine Sn₃O₄ 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 Sn₃O₄ or NaTaO₃. The calculated decolorization rate constant is much greater (1.3550 min⁻¹) than single Sn₃O₄ (0.0555 min⁻¹) or NaTaO₃ (0.3366 min⁻¹). This could be due to the enhanced light absorbance by the larger specific surface area of the Sn₃O₄ nanoflakes and the formation of the heterojunction between NTO-S and Sn₃O₄, which can separate photo-induced carriers efficiently. It is also proposed that the formed •O₂⁻ and h⁺ favored the photocatalytic degradation of RhB. This work highlights the importance of heterostructured Sn₃O₄/NaTaO₃, which may provide a lead to develop high potential and NaTaO₃-based photocatalysts for the degradation of organic dyes.