Light-driven removal of rhodamine B over SrTiO₃ modified Bi₂WO₆ composites

Abstract

SrTiO₃/Bi₂WO₆ hybrids were hydrothermally prepared and structurally examined by means of various chemical and physical techniques. Formation of heterojunction is well confirmed via HRTEM, XPS and PL and contributes immensely to the enhanced catalytic function of a composite in an assay of Rhodamine B (Rh B) photodegradation due to accelerated migration and separation of charge carriers at it. Among as-prepared samples, Bi₂WO₆ loaded with 8 wt% SrTiO₃ proves most photocatalytically active in light of a top degradation rate (D) of ca. 98.4% and a peak apparent kinetic rate (k) of 0.0463 min⁻¹. Besides, h⁺ and ·OH turn out to be the major reactive species for Rh B degradation. Finally, based on theoretical and experimental results, a possible reaction mechanism was proposed.

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1. Introduction

Semiconductor-based photocatalysis demonstrates its predominance over thermal and electrocatalysis by its unique properties such as environmental compatibility and resource conservation on account of substitution of solar energy for non-renewable energy and absence of additional heat suppliers in most cases. Therefore, it has been extensively explored and has found versatile application in H₂ evolution,^1–6 dye degradation^7–10 and CO₂ immobilization.^11,12 Undoubtedly, engineering and fabrication of a highly active semiconductor photocatalyst as an efficient energy converter center at this process. Among well-exploited promoter options, SrTiO₃, a typical cubic perovskite, has been proposed for use as an excellent n-type semiconductor material on account of a collection of outstanding physical and chemical performances including but not limited to (a) excellent (photo)chemical and thermal durability, (b) intriguing structural, electronic and optical attributes such as powerful reducibility of electrons on its conduction band (E_CB = −0.8 eV (ref. 13)) and (c) facile synthesis with various existing methods from earth-abundant feedstocks, thus it witnesses huge significant progresses in artificial photosynthesis and environmental purification.¹⁴ Nevertheless, it falls short of response to visible light due to unduly wide bandgap (ca. 3.1 eV) and suppressed recombination of photogenerated exactions that are urgent for its application.¹⁵ To address these issues, numerous attempts on its structural and chemical modification have been made.^14,16

Structural modification is put to effect by introduction of porous some structures (e.g. mesoporous-assembled SrTiO₃ (ref. 17)) or designated shapes (e.g. SrTiO₃ spheres¹⁸ and nanocubes¹⁹) on micro/nano scale. Generally, these samples are seldom visible-light-driven despite enhanced photocatalytic function available under UV irradiation. As a most promising alternative, chemical modification consisting mainly of chemical doping^14,16 and construction of heterostructure²⁰ enables a red shift of optical absorption to visible light and prompt separation of photoinduced charges, favorable for elevated light-simulated catalytic performance. With regard to contribution to the aforementioned behaviors, constructing SrTiO₃-based hybrid materials takes the lead. Visible-light-responsive semiconductors involving g-C₃N₄,²⁰ Cu₂O,²¹ NiFe₂O₄ (ref. 22) and BiVO₄ (ref. 23) were reported to function as good building blocks to construct functional assemblies with SrTiO₃. Notably, amongst the semiconductors, bismuth tungsten (Bi₂WO₆), the simplest member among Aurivillius family oxides with general formula Bi₂A_n−1B_nO_3n+3 (A = Ca, Sr, Ba, Pb, Bi, Na and K; B = Ti, Nb, Ta, Mo, W and Fe), has triggered keen interest among scholars on photocatalysis in view of (1) its unique layer structure composed of corner-sharing WO₆ octahedral sheets and bismuth oxide sheets,²⁴ (2) excellent visible-light-induced semiconducting behavior with tunable optical harvesting and separation efficiency of electron–hole pairs via morphological and chemical modification and desirable chemical stability^25,26 and (3) effortless preparation through hydrothermal treatment from widely available raw materials.^26,27 Such characteristics provide ample options to construct a broad set of Bi₂WO₆-based (photo)catalytic systems for CO₂ utilization,^24,28 environmental pollutant consumption^29,30 and H₂ and/or O₂ evolution.³¹ For example, Ma et al.³² developed a novel visible-light-excited trinary Z-scheme composite g-C₃N₄/RGO/Bi₂WO₆ (RGO = reduced graphene oxide) via a hydrothermal route for dechlorination and degradation of 2,4,6-trichlorophenol (TCP). The composite (D = 98%) turned out to be much preponderant in catalytic property relative to its reference samples (D = 52%, 58% and 62% for Bi₂WO₆, g-C₃N₄ and Bi₂WO₆/g-C₃N₄, respectively) attributed to the efficient visible-light utilization efficiency and the construction of Z-scheme system using RGO as a charge transmission bridge between the g-C₃N₄ and the Bi₂WO₆. The author highlighted that accelerated separation and migration of photoinduced electrons and holes were accompanied by unaltered redox ability of these excitons accumulated in the conduction band (CB) of g-C₃N₄ and in the valence band (VB) of Bi₂WO₆ for Z-scheme mechanism. However, in the absence of RGO, a flow of charge carriers along the double-transfer path is prone to take place,^33,34 indicative of the key role RGO played. The double-transfer path equally allows encouraged splitting of excitons at the expense of their redox capacity, leading to raised light-driven catalytic performance. The two mechanisms are also suitable to other Bi₂WO₆-based hybrids.^35,36 Nevertheless, the double-transfer mechanism attracts much more attention than Z-scheme path. Therefore, we try to develop an unreported double-transfer mechanism by coupling SrTiO₃ with Bi₂WO₆ to offer enhanced catalytic performance in Rh B removal compared with the individuals in this paper.

Herein, SrTiO₃/Bi₂WO₆ heterojunctions were synthesized via a simple two-step hydrothermal process for the first time. Besides, they were characterized by means of XRD, TEM, SEM, XPS, DRS, BET and PL. Construction of heterostructure is well-certificated and holds the main part in elevated photocatalytic function of Rh B removal. Among as-prepared samples, Bi₂WO₆ decorated with 8 wt% SrTiO₃ loading is the optimal sample (D = 98.4% and k = 0.0463 min⁻¹), far outperforming those of Bi₂WO₆ (D = 86.0% and k = 0.0218 min⁻¹). Furthermore, h⁺ and ·OH are confirmed as the major reactive species for this process. Finally, a possible reaction mechanism was proposed.

2. Experimental

2.1. Chemicals

Bismuth nitrate (Bi(NO₃)₃·5H₂O), sodium tungstate (Na₂WO₄·2H₂O), sodium hydroxide (NaOH), titanium tetrachloride (TiCl₄), strontium hydroxide (Sr(OH)₂·8H₂O), anhydrous ethanol (CH₃CH₂OH) and hydrochloric acid (HCl) were purchased from Shanghai Chemical Reagent Company in analytical grade, and used as received without further purification. Deionized water was used throughout the whole experiment.

2.2. Synthesis

2.2.1. Synthesis of SrTiO₃. The synthetic procedure of SrTiO₃ was described elsewhere.³⁷ Briefly, to homogeneous mixture of hydrochloric solution (2.3 mL, 2 M) and titanium tetrachloride (1.1 mL) was dropwise added aqueous strontium hydroxide (0.27 M, 40 mL) under constant agitation. The resulting mixture was hydrothermally treated at 180 °C for 48 h. After cooling to the room temperature naturally, the obtained precipitate was centrifuged, washed with distilled water and ethanol 4 times, dried at 80 °C and then well-ground for further use.

2.2.2. Synthesis of SrTiO₃/Bi₂WO₆ heterojunctions. In situ growth of Bi₂WO₆ on the surface of SrTiO₃ via hydrothermal method was performed as follows. First, an appropriate amount of SrTiO₃ was suspended in aqueous Bi(NO₃)₃·5H₂O (0.25 M, 40 mL) under vigorous stirring. Subsequently, aqueous Na₂WO₄·2H₂O (0.25 M, 20 mL) was introduced dropwise into the resulting suspension above. After the agitation for 0.5 h, the obtained mixture underwent hydrothermal treatment at 180 °C for 12 h. Upon cooling to the ambient temperature, the crude product was collected via centrifugation, rinsed with distilled water and ethanol several times, dried at 80 °C for 4 h and thoroughly ground into powder. According to the steps above, by changing the amount of SrTiO₃, a series of composites with various mass ratio of SrTiO₃ to Bi₂WO₆ or SrTiO₃ loading (x = 1, 8, 10, and 15 wt%) were synthesized. Similarly, Bi₂WO₆ and SrTiO₃ were fabricated as reference samples.

2.3. Photocatalytic test

Photocatalytic activities of SrTiO₃/Bi₂WO₆ heterojunctions were evaluated by photocatalytic degradation of Rh B under simulated sunlight. A 500 W Xe lamp without any UV cutoff filter was used as a light source. A mixture of a composite (0.2 g) and aqueous Rh B (30 mg L⁻¹, 250 mL) was magnetically stirred for 60 min in dark to establish the adsorption–desorption equilibrium between them. Afterwards, the reaction was allowed for 90 min with light and stirring on. The temperature control of the whole system at ca. 25 °C was achieved by a flow of cooling water. At intervals of 15 min, 5 mL of suspension was collected and centrifuged to remove composite particles. The concentration of the remaining supernatant was determined by concentration-dependent absorbance on a UV spectrophotometer. Rh B degradation rate (D) and apparent rate constant (k) as indicators of photocatalytic performance of as-prepared samples can be calculated from the following equations:

D = (1 − C/C_e) × 100% (1)

kt = ln(C_e/C) (2)

where C_e (mg L⁻¹) and C (mg L⁻¹) respectively denote the equilibrium concentration in dark and instantaneous concentration after a period of time t (min) under irradiation.

2.4. Characterization

X-ray diffraction (XRD) patterns were collected on a Bruker D8-Discover instrument with Cu-Kα radiation (λ = 1.54178 nm). Morphology and size of samples were observed through transmission electron microscopy (TEM, FEI Tecnai G2-F30) and field-emission scanning electron microscopy (SEM, JSM-7800F). Chemical composition was determined via X-ray photoelectron spectroscopy (XPS) (VG Multiab-2000) using a PHI Quantum 2000 XPS system with a monochromatic Al-Ka source and a charge neutralizer. UV-vis diffuse reflection spectroscopy (DRS) was measured with a Shimadzu UV 3600 spectrometer. Photoluminescence (PL) spectra were taken at room temperature on a fluorospectrophotometer (Fluoromax-4) using a Xe lamp as an excitation source. The BET surface areas of the samples were obtained from a Micromeritics ASAP 2020M system at liquid nitrogen temperature (77 K).

3. Results and discussion

3.1. XRD analysis

XRD investigation is conducted to unequivocally illuminate the crystal structures of as-prepared samples. Typical XRD patterns of these samples are summarized in Fig. 1, along with the standard curves of orthorhombic-phase Bi₂WO₆ (JCPDS no. 73-1126) and cubic-phase SrTiO₃ (JCPDS no. 35-0734). With regard to bare Bi₂WO₆ (Fig. 1a), a series of narrow, sharp characteristic peaks located at 28.4, 32.8, 47.1, 55.9 and 58.6° are separately assigned to (113), (200), (220), (313) and (226) planes. Perfect accordance of these peaks with their counterparts in the standard Bi₂WO₆ curve indicates the sample with orthorhombic phase. For another thing, interplanar spacing of (113) and (200) planes are in several estimated as 0.316 and 0.273 nm. In the case of pristine SrTiO₃ (Fig. 1f), main diffraction peaks at 32.4, 39.9, 46.5, 57.7 and 67.6° correspond to the (110), (111), (200), (211) and (220) planes, respectively. It can be identified as a sample with cubic perovskite structure in comparison with those in standard pattern of cubic-phase SrTiO₃. Also, the (110) plane has a interplanar spacing of ca. 0.276 nm. In terms of SrTiO₃/Bi₂WO₆ hybrids (Fig. 1b–e), the major diffraction signals analogous to pure Bi₂WO₆ are evidently observed, suggesting that crystal structure of Bi₂WO₆ is insensitive to SrTiO₃ loading. Besides, characteristic peaks associated with SrTiO₃ are scarcely visible, possibly ascribed to low SrTiO₃ loading in the composites, which is similar to those described by Ge et al.³⁴ and Liu et al.¹⁵

Fig. 1 XRD patterns of (a) Bi₂WO₆, (f) bare SrTiO₃, and ((b)–(e)) SrTiO₃/Bi₂WO₆ heterojunctions with 1, 8, 10, 15 wt% SrTiO₃ loading, respectively.

3.2. SEM, TEM and hydrothermal synthesis mechanism analysis

3.2.1. SEM and TEM analysis. A meticulous comparison on surface morphology and particle size of as-prepared samples are thoroughly made by means of SEM and TEM graphs presented in Fig. 2. Fig. 2a exhibits homogeneous and shaggy microspheres (ca. 6 μm in size) composed of Bi₂WO₆ sheets, which can be seen more clearly in the inset. In Fig. 2b, bare SrTiO₃ displays quasi-spherical particles with a smooth surface and a narrow size distribution from 50 to 310 nm. Fig. 2c describes surface-rough and subsphaeroidal SrTiO₃/Bi₂WO₆ particles in the size range of ca. 2–4 μm. The magnified graph reveals SrTiO₃ pellets randomly “suspended” in the ocean of Bi₂WO₆ sheets (insets in Fig. 2c). HRTEM of 8 wt% SrTiO₃/Bi₂WO₆ in Fig. 2d displays a clear fringe with lattice spacing of 0.315, 0.271 and 0.275 nm respectively indexed as (113) and (200) planes of Bi₂WO₆ and (110) plane of SrTiO₃, which is in good agreement with XRD results. Therefore, structural preservation of the two individuals occurs before and after their combination. More importantly, the image provides a convincing evidence for construction of the intimate contact on nanoscale level between SrTiO₃ and Bi₂WO₆, opening a door to elevated photocatalytic function by perfecting transmission paths of excitons.

Fig. 2 SEM of Bi₂WO₆ (a) and SrTiO₃ (b) and SEM (c) and HRTEM (d) of 8 wt%-SrTiO₃/Bi₂WO₆.

TEM-EDX mapping serves as a powerful tool in analysis of elemental component and distribution of samples. To obtain the knowledge above, mapping of 8 wt%-SrTiO₃/Bi₂WO₆ sample (Fig. 3) is provided for detailed discussion, coupled with the selected TEM area for EDX mapping analysis (Fig. 3a). Remarkably, Sr and Ti dominates the quasi-spherical area with an even distribution (Fig. 3e and f), leaving the other behind for Bi and W (Fig. 3c and d). Also, O fills the whole area. Due to definition of SrTiO₃ and Bi₂WO₆ as quasi-spherical and sheet-like powder by SEM observations, it can be concluded that Sr and Ti mainly exist in SrTiO₃ and Bi and W in Bi₂WO₆. In other words, the two plain components only closely contacts with each other at their interfaces with little elemental exchange.

Fig. 3 (a) TEM image and ((b)–(f)) the corresponding EDX mapping of O, Bi, W, Ti and Sr, respectively, for 8 wt% SrTiO₃/Bi₂WO₆.

3.2.2. Synthesis mechanisms of samples. It is well-known that synthesis mechanisms of samples enable profound insights of their preparations. Detailed description of related mechanisms are as follows.
3.2.2.1 The formation of SrTiO₃ spheres. For SrTiO₃ spheres, a possible mechanism of their formation begins with gradual hydrolysis of TiCl₄ in HCl to TiO₂ with elevated alkalinity due to drop-by-drop addition of Sr(OH)₂. Afterwards, the obtained TiO₂ undergoes dissolution–precipitation route described elsewhere.^18,38 Firstly, TiO₂ is converted to a dissoluble and highly reactive intermediate Ti(OH)₆²⁻ upon further addition of Sr(OH)₂. Subsequently, the active species immediately combines with Sr²⁺ to form SrTiO₃ nuclei. The hydrothermal condition in this work favors self-assembly of these nuclei to SrTiO₃ pellets with a minimum surface energy. The related chemical steps can be represented with the following formulas:

H⁺ + OH⁻ → H₂O (3)

Ti⁴⁺ + 2H₂O → TiO₂ + 4H⁺ (4)

TiO₂ + 2OH⁻ + 2H₂O → Ti(OH)₆²⁻ (5)

Ti(OH)₆²⁻ + Sr²⁺ → SrTiO₃ + H₂O (6)

3.2.2.2 The formation of Bi₂WO₆ spheres. According to synthetic process of Bi₂WO₆ in Section 2.2, total amount of Na₂WO₄ corresponds just to stoichiometric proportion that completely precipitates Bi³⁺. Therefore, dissoluble tungstenic acid (H₂WO₄) and tungstic oxide (WO₃) can hardly result. In other words, Bi₂WO₆ is the only precipitate.^39,40 During formation of Bi₂WO₆ microspheres, Bi³⁺ species are at first reversibly hydrolyzed to BiONO₃. Next, WO₄²⁻ ions combine with Bi³⁺, leading to Bi₂WO₆ nucleation. What's more, hydrothermal route (180 °C, 12 h) enables growth of Bi₂WO₆ cores to sheets and their subsequent integration into microspheres. These steps can be summarized with the following equations:

Bi³⁺ + H₂O + NO₃⁻ ⇌ BiONO₃ + 2H⁺ (7)

2Bi³⁺ + WO₄²⁻ + 2H₂O → Bi₂WO₆ + 4H⁺ (8)

3.2.2.3 In situ decoration of SrTiO₃ pellets with Bi₂WO₆ microspheres. To begin with, SrTiO₃ pellets are constructed following eqn (3)–(6) and serve as a base for their excellent chemical contact with Bi₂WO₆, leading to elevated separation efficiency of light-induced excitons and thus enhanced photocatalytic performance. Next, entry of Bi³⁺ precursor into SrTiO₃ suspension results in adsorption on the surface of SrTiO₃ pellets (SrTiO₃·Bi³⁺) and thus slightly higher binding energy of Bi³⁺ (Fig. 4d). Moreover, upon addition of WO₄²⁻ species, they instantly bind with chemisorbed and unbounded Bi³⁺ to form Bi₂WO₆ amorphous cores. Finally, these cores are hydrothermally driven to form sheet-like Bi₂WO₆ and further self-assemble to Bi₂WO₆ microspheres that wrap SrTiO₃ pellets (Fig. 2c). The chemical process except eqn (3)–(6) present below:

SrTiO₃·Bi³⁺ + WO₄²⁻ + 2H₂O → SrTiO₃·Bi₂WO₆ + 4H⁺ (9)

Fig. 4 (a) The survey spectrum, ((f)–(h)) O 1s levels of Bi₂WO₆, SrTiO₃ and 8 wt%-SrTiO₃/Bi₂WO₆ and high-resolution XPS spectra of (b) Ti 2p, (c) Sr 3d, (d) Bi 4f, (e) W 4f.

3.3. XPS analysis

XPS measurements are implemented to identify the surface composition and chemical state of as-prepared samples. The binding energies obtained in the XPS analysis are calibrated using C 1s peak at 284.6 eV.⁴¹ XPS spectra of 8 wt%-SrTiO₃/Bi₂WO₆, SrTiO₃ and Bi₂WO₆ are summarized in Fig. 4. Fig. 4a exhibits a wide scan XPS spectrum for these samples. Sr, Ti and O form SrTiO₃ and Bi, W and O constitute Bi₂WO₆. Interestingly, unlike four other elements, Ti is “absent” for 8 wt%-SrTiO₃/Bi₂WO₆ due to excessive signal overlap of Ti 2p with that of Bi 4d (marked with dashed rectangle). Similar incident occurs to O 1s for SrTiO₃ in 8 wt%-SrTiO₃/Bi₂WO₆. In Fig. 4b, the characteristic doublet at 464.0 and 458.3 eV related to Ti 2p_1/2 and Ti 2p_3/2 indicates the presence of Ti⁴⁺ in SrTiO₃.¹³ However, Ti 2p signal at 458.1 eV appears very faint in view of signal overlap as before (see Fig. 4a). Sr²⁺ in the composite is well-determined by a pair of peaks at 134.0 and 132.3 eV (Fig. 4c) assigned to Sr 3d orbit.¹⁵ The binding energies of 159.1 and 164.4 eV (Fig. 4d) are ascribed to the Bi 4f_7/2 and Bi 4f_5/2, evidencing that Bi in the composite exists in the 3+ oxidation state. The binding energies of 37.4 eV and 35.4 eV (Fig. 4e) separately assigned to W 4f_5/2 and W 4f_7/2 support W⁶⁺ in the hybrid.⁴² The O 1s spectra seem more complex than those of other elements. To make them understood, they are all well deconvoluted and exhibited in Fig. 4f and g. Evidently, XPS spectrum of O 1s for Bi₂WO₆ (Fig. 4f) presents a triplet at 532.7, 530.9 eV and 530.0 eV, corresponding to the absorbed oxygen and the lattice oxygen W–O and Bi–O in the framework of Bi₂WO₆.^33,43,44 For SrTiO₃ (Fig. 4g), those peaks at 532.3, 530.2 and 529.2 eV are associated with H, Ti and Sr bonded oxygen, respectively.^45,46 In the case of the composite (Fig. 4h), O 1s spectrum of SrTiO₃ can hardly be observed in light of its relatively low loading (8 wt%) and peak overlap with Bi₂WO₆ (Fig. 4a). Therefore, only the latter O 1s spectrum can be found. Notably, relative to those of plain Bi₂WO₆ and SrTiO₃, Bi 4f, W 4f, Sr 3d and O 1s spectra of the composite (Fig. 4c–h) sample undergo slight chemical shifts, providing a solid evidence for construction of the heterostructure between plain Bi₂WO₆ and SrTiO₃ as TEM does.

3.4. DRS analysis

A remarkable insight into optical property and band gaps of as-prepared samples is available via DRS as illustrated in Fig. 5. In Fig. 5a, bare Bi₂WO₆ sample is characterized by a strong band-edge absorption in the visible light region with an absorption edge at 430 nm. As for pure SrTiO₃, it is endowed with forceful near-visible band-edge absorption and an absorption edge at 396 nm. Noticeably, with respect to the hybrids, typical semiconducting optical harvesting curves stand out. In particular, the x-dependent successively blue shift of these curves in comparison with that of Bi₂WO₆ indicates a tunable optical property as a third knock-down witness for construction of the heterostructure between the pure components.^1,9

Fig. 5 The optical properties (a) and bandgap energies (b) of as-prepared samples.

The bandgap (E_g) of a semiconductor is calculated according to the following equation (Fig. 5b):^47–49

αhν = A(hν − E_g)^n/2 (10)

where α, ν, E_g and A represent the absorption coefficient, the light frequency, the band gap and a constant, respectively. E_gs of SrTiO₃ and Bi₂WO₆ are determined to be 3.05 (n = 4 (ref. 50)) and 2.88 eV (n = 2 (ref. 51)), respectively, which are consistent with the previous reports.^22,23,52 Besides, 8 wt%-SrTiO₃/Bi₂WO₆ corresponds to a hybrid semiconductor with E_g = 2.93 eV (n = 2).

3.5. PL analysis

It is well-established that PL intensity functions as an indicator of separation efficiency of light-excited charges in light of origin of PL emission from charge recombination. As a result, PL analyses of SrTiO₃, Bi₂WO₆ and 8 wt%-SrTiO₃/Bi₂WO₆ are conducted and illustrated in Fig. 6. Obviously, all samples are endowed with a peak emission wavelength of ca. 465 nm possibly due to their approximate band gaps (e.g. E_g = 2.88, 3.05 and 2.93 eV respectively for Bi₂WO₆, SrTiO₃ and 8 wt%-SrTiO₃/Bi₂WO₆). In Fig. 6a, typical quenched PL intensity of 8 wt%-SrTiO₃/Bi₂WO₆ indicates enhanced separation efficiency of charge carriers via efficient construction of the heterostructure between Bi₂WO₆ and SrTiO₃. Besides, x-dependent PL intensity observed in Fig. 6b suggests separation efficiency as a function of x. More specifically, enhanced separation efficiency of excitons (decreased PL intensity of samples) coincides with increased x (x ≤ 8 wt%) or diminished x (x > 8 wt%). To conclude, the optimal separation efficiency is achieved at x = 8 wt% and correspond to the peak photocatalytic performance (Fig. 8). The trend is possibly associated with insufficient formation of the heterojunction by excessively low SrTiO₃ loading or creation of new recombination sites by its overloading.

Fig. 6 PL patterns of SrTiO₃, Bi₂WO₆ and 8 wt%-SrTiO₃/Bi₂WO₆.

3.6. BET and DFT analysis

BET specific surface areas (S_BET), N₂ adsorption–desorption curves (the red line) and DFT pore-size distributions (the black line) are provided in Fig. 7. To begin with, S_BET ascends up to 35.7 m² g⁻¹ (x ≤ 8 wt%) and then descends (x > 8 wt%) for the composites. The trend agrees well with those of the amount of Rh B (A_d) adsorbed in the dark on the surface of composites, D and k (Fig. 8). In summary, 8 wt%-SrTiO₃/Bi₂WO₆ with a peak S_BET provides the optimal photocatalysis, indicating that S_BET is beneficial for enhanced photocatalytic performance. Next, according to IUPAC recommendation, Fig. 7 displays typical II isothermal curve. Finally, DFT pore-size distributions confirm samples as mesoporous materials. The samples other than 15 wt%-SrTiO₃/Bi₂WO₆ have a dominate pore size of ca. 3 nm and 4.6 nm. The amount variation tendency of both pores coincides with those of S_BET and A_d for the composites.

Fig. 7 BET specific surface areas (S_BET), N₂ adsorption–desorption curves (the red line) and DFT pore-size distributions (the black line) of (a) Bi₂WO₆, (b) 1 wt%-SrTiO₃/Bi₂WO₆, (c) 8 wt%-SrTiO₃/Bi₂WO₆, (d) 10 wt%-SrTiO₃/Bi₂WO₆ (e) 15 wt%-SrTiO₃/Bi₂WO₆ and (f) SrTiO₃.

Fig. 8 x-Dependent D, k and A_d over SrTiO₃/Bi₂WO₆ heterojunctions.

Besides, adsorption capacity of samples (the amount of adsorbed Rh B in the dark on the surface of the as-prepared samples) is calculated according to the formula A_d = (1 − C_e/C₀) × 100%, where A_d, C₀ and C_e stand for adsorption capacity of the as-prepared samples, the concentration of original Rh B solution (30 mg L⁻¹) and Rh B equilibrium concentration in dark. Thus, A_d values of samples are obtained and summarized in Fig. 8. Markedly, with regard to the composites, A_d is on the rise to 8.91% (x ≤ 8 wt%) and then decrease to 5.53% (x > 8 wt%), which varies well with S_BET (Fig. 7).

3.7. Evaluation of photocatalytic performance

Rh B works as a model contaminant to test photocatalytic activity of as-prepared samples. Fig. 8 displays the effect of x on D, k and A_d. Evidently, the augmented D and k are coincident with increased x up to 8 wt%. Inversely, exorbitant x (8–15 wt%) results in reduced D and k. To conclude, 8 wt%-SrTiO₃/Bi₂WO₆ (D = 98.4% and k = 0.0463 min⁻¹) is much more photocatalytically active than Bi₂WO₆ (x = 0, D = 86.0%, k = 0.0218 min⁻¹), which derives mainly from perfective separation route of electrons and holes through the interfaces between SrTiO₃ and Bi₂WO₆ as disclosed by highly quenched PL intensity (Fig. 6a), which is in line with the literature results.^7,52

Moreover, the sample offers catalytic superiority over any other composite. One of the reasons is possibly associated with best separation efficiency of excitons as PL analysis indicates (Section 3.5). For another, the suitable S_BET (Fig. 7) and optical absorption (Fig. 5a) favor Rh B adsorption and activation.

Controlled experiments are crucial for knowledge of the photocatalytic process. As a result, these experiments were conducted (Table 1). Remarkably, 8 wt%-SrTiO₃/Bi₂WO₆ far surpasses plain SrTiO₃ and Degussa P25 (Table 1, Title 2 and 3) with reference to light-driven catalytic behavior in virtue of overwhelming advantage of optical response and excitonic splitting. Blank tests (Table 1, Title 4–6) evidence that Rh B photodegradation is certainly a light-stimulated catalytic process rather than a thermal-induced catalytic or self-degradation one.

Table 1 Controlled experiments

Title	Samples	D^a/%
a Under the condition described in Experimental section.b In dark during the whole process.
1	8 wt%-SrTiO3/Bi2WO6	98.4
2	SrTiO3	38.1
3	Degussa P25	17.5
4	None	0
5	8 wt%-SrTiO3/Bi2WO6	16.3^b
6	None	0^b

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3.8. Photostability exploration

Photostability is a considerably critical attribute for an excellent semiconductor material. Therefore, related tests were performed (Fig. 9). Obviously, 8 wt%-SrTiO₃/Bi₂WO₆ maintains chemically (Fig. 9a) and structurally (Fig. 9b) durable after used 4 times. In a word, it is beyond doubt that it deserves an outstanding promoter for the excellent stability toward the sunlight.

Fig. 9 (a) Reuse of 8 wt%-SrTiO₃/Bi₂WO₆ and (b) XRD patterns of fresh and 4^th cyclical 8 wt%-SrTiO₃/Bi₂WO₆.

3.9. Reaction mechanism

It is well-constructed that identification of primary reactive species in the degradation process enables proposal of reaction mechanism. Thereby, the reactive species trapping experiments were conducted. Ethylene diamine tetra acetic acid (EDTA), isopropanol (IPA) and benzoquinone (BQ) featured as scavengers for hole (h⁺), hydroxyl radical (OH·) and superoxide radical (·O₂⁻), respectively.^53–55 Fig. 10 definitely elucidates the considerably diminished D of 8 wt%-SrTiO₃/Bi₂WO₆ in the presence of EDTA or IPA, circumstantiating h⁺ and OH· as the main species. Conversely, ·O₂⁻ plays minor role in Rh B removal.

Fig. 10 Various D values of 8 wt%-SrTiO₃/Bi₂WO₆ after introduction of several scavengers.

On the other hand, the matched band structures are required for two semiconductors intended for highly efficient assembly of a hybrid. The valence band (VB) and conduction band (CB) potentials can be calculated from the two equations below:^56–59

E_VB = X − E^e + 0.5E_g (11)

E_CB = E_VB − E_g (12)

where X represents the absolute electronegativity of semiconductor (X = 6.36 for Bi₂WO₆ (ref. 60) and 5.19 for SrTiO₃ (ref. 37)), which means the atomic electron affinity and the first ionization energy; E^e is the energy of free electrons on the hydrogen scale (4.5 eV). Accordingly, E_CB and E_VB are respectively calculated to be 0.42 and 3.30 eV for Bi₂WO₆ and −0.83 and 2.22 eV for SrTiO₃.

On basis of the aforementioned discussion, a rational route for migration, isolation and capture of excitons since their birth is put forward as illustrated in Fig. 11. Under simulated sunlight, both Bi₂WO₆ and SrTiO₃ are excited to give birth to elections and holes. Subsequently, they are forced respectively to accumulate in the CB of Bi₂WO₆ and VB of SrTiO₃ due to the potential differences between their CBs (−0.83 and 0.42 eV) and VBs (2.22 and 3.30 eV), leaving their recombination suppressed and their lifetime lengthened as PL results indicate (Fig. 6). Thus, more excitons are allowed to run the photocatalytic reaction, which is mainly responsible for enhanced catalytic function. Next, elections are captured by chemisorbed O₂ to convert to OH· by way of ·O₂⁻.^61,62 However, holes hardly combine with OH⁻ to form OH· as a result of less positive potential in VB of SrTiO₃ (2.22 eV) compared with that of ·OH/OH⁻ (2.38 eV). Therefore, Rh B is transformed to its degradation products mostly via its oxidation by OH· and h⁺.

Fig. 11 Reaction mechanism of Rh B removal over 8 wt%-SrTiO₃/Bi₂WO₆ under simulated sunlight.

4. Conclusion

SrTiO₃/Bi₂WO₆ composites were prepared via a technically viable two-step hydrothermal method and submitted to structural exploration by means of XRD, TEM, SEM, XPS, DRS, BET and PL. The heterostructure between SrTiO₃ and Bi₂WO₆ is well-constructed and plays the main part in raised photocatalytic behavior in Rh B removal. 8 wt%-SrTiO₃/Bi₂WO₆ is selected as the optimal catalyst (D = 98.4% and k = 0.0463 min⁻¹) among as-prepared samples by perfecting the SrTiO₃ loading and optical and electric properties. h⁺ and OH· as the main species are beyond dispute. Based on the discussion above, a possible reaction was proposed.

Acknowledgements

This work was financially supported by the Fundamental Research Funds for the Central Universities of China (No. 3207045403 and 3207045409), National Natural Science Foundation of China (No. 21576050), Jiangsu Provincial Natural Science Foundation of China (BK20150604) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† These authors contributed equally to this work.

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