Construction of a novel two-dimensional AgBiO₃/BiOBr step-scheme heterojunction for enhanced broad-spectrum photocatalytic performance

Abstract

Efficient S-scheme heterojunction photocatalysts were prepared through in situ growth of AgBiO₃ on BiOBr. The self-assembled hierarchical structure of AgBiO₃/BiOBr was formed from flower-like AgBiO₃ and plate-like BiOBr. The optimized AgBiO₃/BiOBr heterojunction possessed excellent visible-light photocatalytic degradation efficiency (83%) for ciprofloxacin (CIP) after 120 min, with 1.46- and 4.15-times higher activity than pure AgBiO₃ and BiOBr, respectively. Furthermore, the removal ratio of multiple organic pollutants including tetracycline, Rhodamine B, Lanasol Red 5B and methyl orange was also investigated. Environmental interference experiments demonstrated that high pollutant concentrations, low photocatalyst dose and the addition of ions (SO₄²⁻, PO₄³⁻, HPO₄²⁻, H₂PO₄⁻) inhibited the photocatalytic activities. Subsequently, a simultaneous degradation experiment showed the competitive actions between CIP and RhB for radicals, decreasing the photocatalytic activity of CIP. Furthermore, trapping and electron spin resonance experiments showed that h⁺ and ˙O₂⁻ played a certain role in the degradation process and that ˙OH acted as assistant.

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1 Introduction

As society has progressed, domestic sewage and industrial wastewater have had an extremely significant impact on human health and the environment, which has attracted widespread attention.^1–5 Recently, photocatalytic removal of organic matter and reduction in the amount of heavy metals has been developed as reliable ways to mediate waste using solar irradiation.^6,7 Because visible light comprises ∼46% of the energy of the solar spectrum, it is highly desired to develop a visible-light responsive photocatalyst with a narrow bandgap.^8,9

Bi-based photocatalysts such as BiVO₄,^10,11 Bi₂WO₆,¹² Bi₂MoO₆,¹³ Bi₂S₃ (ref. 14) and Bi₂O₃ (ref. 15) have received widespread attention because they possess suitable structures. For example, bismuth bromide oxide (BiOBr) has a layered structure characterized by [Bi₂O₂]²⁺ slabs interleaved with double slabs of Br atoms.^16,17 This exceptional structure is helpful for the absorbing activity of visible light and charge-separation of the photogenerated carriers. However, BiOBr possesses common problems like most semiconductor materials including poor light absorptivity and high recombination efficiency.^18,19 To overcome these issues, new synthesis methods have been employed. For example, Zhang prepared ultra-thin BiOBr nano-roundels in water-in-oil emulsion microspheres by a solvothermal route, which results in materials with excellent photocatalytic activity, and the degradation rate of organics reached as high as 99.2%.²⁰ Liu fabricated a novel n–n type BiOBr/Bi₂WO₆ composite photocatalyst by a one-step solvothermal approach. Photocatalytic degradation experiments revealed that BiOBr–Bi₂WO₆ = 8 : 1 demonstrated excellent tetracycline (TC) degradation (96%) within 120 min.²¹ Zou fabricated a novel BiOBr/FeWO₄ composite photocatalyst via a solvothermal approach. The BiOBr/FeWO₄ (4 : 1) showed excellent photocatalytic degradation activity (90%) after 1 h toward doxycycline due to the inhibition of recombination of photo-induced electrons and holes and the improvement of the light response range.²²

The development of Bi-based MBiO₃ (M = Li, Na, Ba, Ca) materials is an exciting area of research in heterogeneous photocatalysis because of the structural variability and excellent physical and chemical properties of these materials.²³ Most M elements are alkaline metals, and the MBiO₃-based structures usually have simple preparation methods, mild operating conditions and high product purity. Moreover, insoluble compounds will not cause new problems to the environment. Silver bismuthate (AgBiO₃) has a narrow band-gap, effectively absorbing visible light.^24,25 Nevertheless, the narrower band-gap often leads to faster recombination of photo-induced electron/hole pairs. Thus, pure AgBiO₃ usually exhibits a relatively poor photocatalytic activity.

To overcome these drawbacks and elevate photocatalytic performance, heterostructure construction with other semiconductors aiming to improve charge transfer efficiency has been widely studied.^26,27 Shen prepared flower-like AgBiO₃ loaded on GO/NCDs, which exhibited significantly enhanced degradation activities towards organic pollutants.²⁸ Wu synthesized a novel binary AgBiO₃/g-C₃N₄ composite via in situ growth of AgBiO₃ on g-C₃N₄, which exhibited improved degradation performance towards organic pollutants of methyl orange (MO), tetracycline (TC), and phenol.²⁹ More recently, a new step-scheme heterojunction (S-scheme heterojunction) concept was proposed by Yu et al.³⁰ This structure induces the migration of the photo-induced electrons and holes and preserves their redox ability under the influence of an internal electric field due to the differences of the Fermi level of each semiconductor. Wang synthesized S-scheme Sb₂WO₆/BiOBr photocatalysts via a precipitation–deposition method, which exhibited enhanced photocatalytic performance compared with pure Sb₂WO₆ and BiOBr for removal of NO.³¹ The enhanced photocatalytic performance for the Sb₂WO₆/BiOBr composite was attributed to the S-scheme charge transfer path. It is necessary to combine appropriate semiconductors to improve light absorption capacities of materials, and also to improve the separation and transfer of photogenerated electrons and holes for diverse applications.^32,33

In this manuscript, a novel S-scheme AgBiO₃/BiOBr heterojunction was successfully obtained via a facile hydrothermal and in situ precipitation method. The morphology and optical properties of the materials were systematically investigated. The photocatalytic activities of AgBiO₃/BiOBr were tested towards antibiotic ciprofloxacin (CIP) pollutants as a model pollutant under visible-light irradiation. The influences of catalyst dose and initial concentration of CIP on the catalytic performance of AgBiO₃/BiOBr were discussed. Additionally, a photocatalytic mechanism was also proposed.

2 Experimental

2.1. Chemicals

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), potassium bromide (KBr), acetic acid (CH₃COOH), silver nitrate (AgNO₃), sodium bismuthate (NaBiO₃), sodium chloride (NaCl), sodium phosphate (Na₃PO₄·12H₂O), disodium hydrogen phosphate (Na₂HPO₄·12H₂O), potassium dihydrogen phosphate (KH₂PO₄), and sodium sulphate (Na₂SO₄) were purchased from Sinopharm Group Co., Ltd. Isopropanol (IPA), disodium ethylenediaminetetraacetic acid (EDTA-2Na), and 1,4-benzoquinone (BQ) were acquired from Aladdin Reagent Co., Ltd.

All chemicals used were analytical grade reagents and were used without further purification.

2.2. Synthesis of BiOBr nanosheets

The BiOBr nanosheets were obtained by a solvothermal method. Generally, 1.587 g of Bi(NO₃)₃·5H₂O were first added to a 200 mL flask containing a mixed solution of 35 mL of deionized (DI) water and 5 mL of acetic acid. Separately, 0.389 g of KBr was dissolved in 30 mL of DI water. Then the KBr solution was added dropwise to the first solution and stirred for 1 h. The reaction mixture was subsequently transferred to a 100 mL Teflon-lined autoclave and heated at 160 °C for 12 h. After heating, the samples were cooled to room temperature. The samples were then washed three times with DI water and absolute ethanol successively, and then dried at 60 °C for 12 h.

2.3. Synthesis of AgBiO₃/BiOBr composite

A certain amount of BiOBr powder was added into 50 mL of DI water and sonicated for 10 min. 0.5 mmol AgNO₃ was subsequently added. After 30 minutes of additional sonication, 0.5 mmol of NaBiO₃ was added to the above solution and stirred for 15 min at 25 °C. After that, the mixture was centrifuged at 5500 rpm. Then the final product was washed three times with DI water and ethanol and dried in a vacuum oven at 60 °C for 24 h. The mass ratio of AgBiO₃ and BiOBr were 1 : 1, 1 : 5, 1 : 10 and 1 : 15, which are referred to AB-1, AB-5, AB-10 and AB-15, throughout this manuscript. Pristine AgBiO₃ was prepared via similar procedures without adding BiOBr. The preparation process of AgBiO₃/BiOBr composite is described in Scheme 1.

Scheme 1 Schematic of the in situ fabrication of the AgBiO₃/BiOBr heterojunction.

2.4. Characterization

The morphology of AgBiO₃/BiOBr was investigated by high-resolution transmission electron microscopy (HRTEM, JEM-2100, Japan) and scanning electron microscopy (SEM, SU8010, Japan) equipped with energy dispersive spectroscopy (EDS). Zeta potential were measured by a Malvern Zetasizer instrument (Zetasizer Nano Zs, Britain). The X-ray diffraction (XRD) patterns of AgBiO₃/BiOBr heterojunctions were collected with a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 0.15406 nm) at 2θ ranging between 10° and 80°. The specific surface areas of the AgBiO₃/BiOBr heterojunctions were evaluated using N₂ adsorption isotherm on a Gemini-2380 (Quantachrome) system. The UV-vis diffuse reflection spectra were measured using a Lambda 950 UV-vis spectrophotometer. A Pick Quant FLuo Time 300 spectrometer was used to collect the steady-state photoluminescence (PL) spectra of the samples. The structures of the AgBiO₃/BiOBr samples were also characterized with X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The electron spin resonance (ESR) spectra were measured on a Bruker ER300-SRC instrument.

2.5. Evaluation of visible-light-driven photocatalytic performance

The photocatalytic activities of the AgBiO₃/BiOBr samples were evaluated by degradation of CIP in an aqueous solution under visible-light irradiation (350 W Xe lamp with a UV-cutoff filter, λ > 420 nm). The photocatalysts (40 mg) were dispersed in CIP solution (40 mL at a concentration of 20 mg L⁻¹). The suspensions were stirred magnetically for 30 min in the dark to confirm adsorption–desorption equilibrium prior to irradiation. As the reaction proceeded, an aliquot (3 mL) was gathered at various times and centrifuged to remove the photocatalyst for UV-vis measurements (UV-2450) at the maximum absorption wavelength of 273 nm for CIP.

3 Results and discussion

3.1. Physicochemical properties

The XRD patterns of the samples are depicted in Fig. 2. For bare BiOBr, all of the diffraction peaks can be well indexed to the tetragonal phase of JCPDS card no. 09-0393.³⁴ The diffraction peaks at 19.0°, 21.3°, 28.7°, 31.7°, 35.9°, 46.7° and 57.5° are assigned to the (101), (012), (104), (110), (113), (116), and (215) crystal planes, respectively. These assignments are consistent with the standard diffraction patterns of AgBiO₃ (JCPDS 89-9072).²⁹ For the AgBiO₃/BiOBr heterojunction, the presence of AgBiO₃ does not affect the BiOBr diffraction peak position (Fig. S1†), indicating that AgBiO₃ nanoflowers grew on BiOBr nanosheets without any chemical reaction. The peak intensities of the BiOBr (001) and (002) planes decreased as the mass ratio of AgBiO₃ to BiOBr increased, indicating that the AgBiO₃ affected the orientation of BiOBr growth.

The chemical environment and oxidation state of the elements present on the surface of the as-synthesized samples were investigated using X-ray photoelectron spectroscopy. The XPS spectra of AB-5 heterojunction together with pure AgBiO₃ and BiOBr are shown in Fig. 1. As depicted in Fig. 1b, the survey spectrum of AB-5 was composed of Bi, Br, O and Ag. The binding energies at 159.06 eV and 164.36 eV for Bi 4f_7/2 and Bi 4f_5/2 indicate the existence of Bi³⁺ in AB-5 (Fig. 1c).³⁵ In the Br 3d spectrum, two characteristic peaks at 67.52 eV and 68.92 eV are assigned to Br 3d_5/2 and Br 3d_3/2 of Br⁻, respectively (Fig. 1d). From Fig. 1e, it can be seen that the peak of O 1s located at 529.88 eV, which corresponds to lattice oxygen.³⁶ The peaks at 531.50 eV and 534.03 eV are associated with adsorbed oxygen and surface O–H or H₂O, respectively.³⁷ In the Ag 3d spectrum of AB-5 shown in Fig. 1f, the peaks at 367.12 eV and 373.14 eV are ascribed to Ag 3d_5/2 and Ag 3d_3/2, respectively. These peaks demonstrate the existence of Ag⁺ in the composite.²⁵ The binding energies of Bi 4f, Br 3d, O1s, and Ag 3d for the AB-5 heterojunction all exhibited a negative shift compared with those of pure BiOBr and AgBiO₃. These simultaneous shifts are likely due to strong electronic interactions between BiOBr and AgBiO₃.

Fig. 1 (a) XRD patterns of as-synthesized samples; XPS spectra of BiOBr, AgBiO₃, and AB-5 composite: (b) survey XPS spectra (c) Bi 4f, (d) Br 3d, (e) O 1s, and (f) Ag 3d.

Fig. 2a reveals that pure AgBiO₃ possesses as flower-like structure composed of nanosheets with diameters around 600 nm. On the other hand, BiOBr exhibits a plate-like morphology (Fig. 2b). Both AgBiO₃ and BiOBr have analogous plate-like structures and interactions due to van der Waals forces, which facilitates and stabilizes AgBiO₃ over the BiOBr surface and results in the formation of a tight AgBiO₃/BiOBr junction. After the deposition of AgBiO₃, the flower-like semiconductor was inserted into disordered plate-like BiOBr, whereas the outer surfaces of BiOBr became rough (Fig. 2c and d).

Fig. 2 The SEM image of (a) AgBiO₃, (b) BiOBr and (c and d) AB-5 heterojunction.

TEM imaging (Fig. 3a) verifies that the flower-like AgBiO₃ was successfully loaded on the surface of the plate-like BiOBr. The corresponding magnified HRTEM image (Fig. 3b) exhibits distinct lattice fringes of 0.276 nm and 0.310 nm in accordance with the (110) plane of BiOBr and (104) plane of AgBiO₃, respectively. The EDS elemental mapping indicates that Br, Bi, O and Ag are uniformly distributed at the surface of the AB-5 heterojunction (Fig. 4), further confirming that the as-synthesized composite contains AgBiO₃ and BiOBr.

Fig. 3 The TEM image (a) and HRTEM image (b) of AB-5 heterojunction.

Fig. 4 EDS elemental mapping of AB-5 heterojunction.

The data in Fig. 5 demonstrates that the as-synthesized samples show a classical IV isotherm, suggesting that the material contains mesoporous. The measured S_BET values of BiOBr, AgBiO₃ and AB-5 heterojunction were 2.53, 20.99 and 5.78 m² g⁻¹, respectively. The surface area of AB-5 heterojunction was almost two times higher than that of BiOBr, suggesting that the introduction of AgBiO₃ improves the specific surface area of the materials. For the AB-5 heterojunction, the pore size distribution curve exhibits a sharp peak at 1.2–3.0 nm and a very broad peak at 10–30 nm, together which are indicative of a mesoporous structure. The higher specific surface area and the porous structure of AB-5 heterojunction effectively promote the kinetics of photocatalytic reaction due to the increase number of active sites.³⁸

Fig. 5 N₂ adsorption–desorption isotherms and the corresponding pore volume.

The as-synthesized photocatalysts were studied by UV-Vis diffuse reflectance spectroscopy to investigate their optical properties (Fig. 6a). The absorption edges of BiOBr are located at 442 nm, while AgBiO₃ possesses intense absorption between 200–800 nm. Compared to BiOBr, the absorption band edge of the AgBiO₃/BiOBr heterojunction shows a distinct red shift, indicating that AgBiO₃ improves visible-light absorption capacity. This result might contribute to the enhanced production of photogenerated electron–hole pairs. The band-gap of the samples was calculated by the Tauc equation as follows eqn (1).

αhν = A(hν − E_g)^n/2 (1)

where α, h, ν, A, and E_g are the optical absorption coefficient, Planck constant, photon frequency, a constant and band gap energy, respectively. In addition, the electronic transitions adhere to the n value: direct-allowed or indirect-allowed transitions for n = 1 or n = 4, respectively. Because BiOBr³⁹ and AgBiO₃ (ref. 24) are both indirect-allowed semiconductors, E_g can be computed from the plot of (αhν)^1/2 as a function of hν.³⁹ As shown in Fig. 6b–d, BiOBr, AgBiO₃ and AB-5 heterojunction bandgaps were computed to be 2.74 eV, 0.80 eV and 2.53 eV, respectively.

Fig. 6 (a) UV-vis spectra of the as-prepared samples; (b–d) the band gaps of AgBiO₃, BiOBr and AB-5 heterojunction.

Fig. 7 displays the valence band (VB) position of the samples as evaluated tested by VB-XPS spectra. The VB positions of BiOBr and AgBiO₃ were computed to be 2.11 eV and 0.12 eV, respectively. Subsequently, the conduction band (CB) position was computed by the energy band position formula as follows (2):

E_CB = E_VB − E_g (2)

Fig. 7 (a and b) Valence-band XPS spectra of AgBiO₃ and BiOBr.

The CB positions of BiOBr and AgBiO₃ were computed to be −0.63 eV and −0.68 eV, respectively. The energy band potentials of BiOBr and AgBiO₃ are depicted in Table 1.

Table 1 Energy band engineering analysis of AgBiO₃ and BiOBr

Samples	ECB for NHE^a/eV	EVB for NHE^a/eV	Eg/eV
a NHE: normal hydrogen electrode.
AgBiO3	−0.68	0.12	0.80
BiOBr	−0.63	2.11	2.74

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3.2. Photocatalytic performance

The photocatalytic activities of the as-prepared materials toward CIP in aqueous solutions were evaluated under visible-light irradiation. As shown in Fig. 8a, adsorption experiments were carried out in the dark for 30 min. When irradiated with visible light, the CIP exhibited negligible degradation over time in the absence of the photocatalyst. With the addition of BiOBr and AgBiO₃, the degradation efficiencies of CIP reached 20% and 57% after 120 min under visible-light irradiation, respectively. After introducing AgBiO₃, the AgBiO₃/BiOBr heterojunction (AB-1, AB-5, AB-10, and AB-15) all possessed better photocatalytic activity than that of pure BiOBr with respective efficiencies of 80%, 83%, 65%, and 50%. In particular, AB-5 exhibited the highest degradation efficiency. This result demonstrated that a moderate amount of AgBiO₃ could promotes composite degradation efficiency due to formation of heterojunctions at the interface between AgBiO₃ and BiOBr, enhancing visible light absorption and separation efficiency of photogenerated carriers. The mechanical mixture of 5 wt% AgBiO₃ and BiOBr were also investigated as a control, and it exhibited lower efficiencies than the AgBiO₃/BiOBr heterojunction. This lower efficiency is attributed to poor interfacial contact between AgBiO₃ and BiOBr during mechanical mixing.

Fig. 8 (a) Degradation efficiencies, (b) the kinetic fit of CIP using the as-prepared photocatalysts under visible-light irradiation; Degradation efficiencies of the CIP solution by (c) various AB-5 dosage ([CIP] = 20 mg L⁻¹), (d) various concentration of CIP ([AB-5] = 40 mg), (e) different pH and (f) under different ion (SO₄²⁻, PO₄³⁻, HPO₄²⁻ and H₂PO₄⁻); (g) photodegradation of TC, RhB, LR5B, MO and CIP; (h) degradation efficiency of different solvents in the presence of AB-5; (i) time-dependent absorption spectra of CIP and RhB degradation.

Fig. 8b depicted the reaction kinetics, which were modeled by a pseudo-first-order kinetic model: ln(C₀/C_t) = kt. The rate constants of AgBiO₃, BiOBr, the AB-1, AB-5, AB-10, and AB-15 heterojunctions and the mechanical mixture were 0.00550, 0.00204, 0.01344, 0.01497, 0.00798, 0.00555 and 0.00529 min⁻¹, respectively. The rate constants of AB-5 were 2.72 and 7.34 times higher than those of AgBiO₃ and BiOBr, respectively, suggesting that a suitable quantity of AgBiO₃ improves the photocatalytic performance of AgBiO₃/BiOBr.

The effects of different reaction conditions were also investigated. In Fig. 8c, the concentration of CIP solution was 20 mg L⁻¹ (pH = 7). When the dosage of AB-5 heterojunction increases from 20 mg to 40 mg, the photocatalytic degradation rate of CIP increases from 40% to 83% after 120 min under visible-light irradiation. The improved photocatalytic activity originates from the greater quantity of active species.⁴⁰ However, when the photocatalytic dosage further increases to 50 mg, the degradation rate of CIP is not significantly improved. Therefore, considering the cost of photocatalyst, 40 mg was considered to be optimal for subsequent investigations. As shown in Fig. 8d, the photocatalytic degradation efficiency of CIP was reduced, increasing the initial concentration of CIP. The phenomenon might be because the higher concentration of CIP consumes the generated radicals faster.

Fig. 8e shows that the AB-5 heterojunction exhibits excellent photocatalytic activity at pH 7. At neutral pH, AB-5 and CIP are electroneutral, which allows the CIP molecule to interact with the AB-5. As a result, AB-5 can readily adsorb CIP and show increased photodegradation activity. When the pH is decreased below 6.09, CIP exists its cationic state,⁴¹ which induces electrostatic repulsions between the positively charged surface of AB-5 (Fig. 9) and protonated CIP molecules, which serves to attenuate the adsorption capacity of AB-5.⁴² Therefore, at pH values above 7, the photocatalytic activity of AB-5 was significantly decreased. This phenomenon likely occurs because AB-5 undergoes chemical changes under alkaline conditions.

Fig. 9 Zeta potential of AB-5 heterojunction.

It is widely known that the real wastewaters often contain various inorganic salts, which could affect the efficacy of the pollutant treatment. Therefore, the effect of several common anions in wastewater (SO₄²⁻, PO₄³⁻, HPO₄²⁻ and H₂PO₄⁻) at a concentration of 1 mmol L⁻¹ on CIP degradation in the AB-5 system was studied. As shown in Fig. 8f, the addition of SO₄²⁻ slightly affects the degradation efficiency of CIP (72%). This decrease in degradation efficiency might be because SO₄²⁻ consumes ˙OH to produce ˙SO₄⁻,^43,44 thus lowering the degradation rate of CIP. Furthermore, the CIP degradation ratio was significantly reduced in the presence of PO₄³⁻, HPO₄²⁻ and H₂PO₄⁻. Compared to HPO₄²⁻ (21%) and H₂PO₄⁻ (27%), the degradation efficiency of AB-5 towards CIP became almost completely inhibited by PO₄³⁻, which might be due to the consumption of photogenerated holes by PO₄³⁻. With the reaction of photo-induced holes, PO₄³⁻ is transformed into HPO₄²⁻. Similarly, HPO₄²⁻ is changed into H₂PO₄⁻ after reacting with the photo-induced holes. However, changing PO₄³⁻ into HPO₄²⁻ is more kinetically facile than transforming HPO₄²⁻ into H₂PO₄⁻. Consequently, the inhibition effect of PO₄³⁻ is stronger than those of HPO₄²⁻ and H₂PO₄⁻.⁴⁵

To evaluate the range of synthetic materials that can be degraded, several molecules including CIP, TC, LR5B, RhB and MO were investigated. As shown in Fig. 8g, the removal ratios of CIP, TC, LR5B, RhB and MO were 83%, 87%, 98%, 77% and 95%, respectively. The differences in degradation rations are due to each contaminant's distinct molecular structure and adsorption characteristics. Interestingly, we found that the AB-5 had excellent removal efficiencies toward LR5B, MO and TC, depending on adsorption rather than degradation. Moreover, when CIP and RhB coexisted, the mutual effect was investigated (Fig. 8i). The CIP degradation ratio decreased from 83% to 48% in presence of RhB. The reason for this decrease in removal efficiency might be due to competitive interactions for active species between RhB and CIP.⁴⁶

To further understand the photocatalytic applicability of AB-5 in an actual environment, CIP photodegradation in different solvents with AB-5 was investigated. In these experiments, DI water was used as a control. Fig. 8h exhibits the effect of different solvents on the photocatalytic process. When CIP was dissolved in DI water, lake water, tap water and deep well water, the photocatalytic degradation efficiencies were 83%, 28%, 24% and 17%, respectively. The photocatalytic degradation of CIP in environmental waters was significantly inhibited. This phenomenon may be related to inorganic ions and organic pollutants, which could consume the free radicals produced by the AB-5 heterojunction.

3.3. Photocatalytic mechanism

To observe the migration and separation efficiency of the photo-induced charge, PL spectroscopy was performed. Generally, a weaker PL intensity reflects a lower recombination probability. As shown in Fig. 10, With the addition of AgBiO₃, the PL intensity of AgBiO₃/BiOBr heterojunction (AB-5) decreased compared to pure BiOBr, suggesting that the recombination of charge carriers was inhibited. AB-5 exhibits the effective charge separation efficiency, which is instrumental in improving photocatalytic activity. The electrochemical experiment is conducted to investigate the separation and transfer property of photo-induced electron–hole pairs. As shown in Fig.S3,† AB-5 exhibited the highest photocurrent density via switch On/Off under visible light irradiation, indicating that a lower recombination and more rapid charge transfer could be achieved via heterostructure.

Fig. 10 PL spectra of the as-prepared samples.

Trapping experiments were performed to further interrogate the photocatalytic mechanism of CIP degradation. Benzoquinone (BQ), isopropanol (IPA), and disodium ethylenediaminetetraacetate (EDTA-2Na) were used as scavengers for superoxide radicals (˙O₂⁻), hydroxyl radicals (˙OH), and photo-induced holes (h⁺), respectively.⁴⁷ Fig. 11a shows the degradation efficiencies of CIP in the presence of various scavengers. Compared to the sample without radical scavenger, the degradation efficiencies of AB-5 toward CIP decreased to 23%, 62%, and 8% upon addition of BQ, IPA, and EDTA-2Na, respectively. These results indicate that the photo-induced holes and superoxide radicals played a critical role in the degradation process, and hydroxyl radicals act as assistants. Additionally, the generation of reactive oxygen species (ROS) using the AB-5 composite was detected using DMPO and TEMPO spin-trapping ESR techniques, which elucidated the degradation mechanism when using the combined pollutants. As shown in Fig. 11b, the strong signal in the dark implied the presence of TEMPO-h⁺, which gradually weakened under visible-light irradiation, suggesting that photo-induced holes are the main active species.^48,49 In Fig. 11c and d, the typical signals of DMPO-˙O₂⁻ and DMPO-˙OH were tested with the AB-5 composite under visible-light irradiation,⁵⁰ suggesting that ˙O₂⁻ and ˙OH radicals emerged during the degradation process.

Fig. 11 (a) Trapping experiments of active species during photodegradation of CIP in the presence of AB-5 under visible-light irradiation; (b) TEMPO-h⁺, (c) DMPO-˙O₂⁻and (d) DMPO-˙OH adducts on AB-5 under visible-light irradiation.

Based on the above experiment results, the possible estimation of the energy band was conducted. In Fig. 12, under visible-light irradiation electron–hole pairs were induced on the surface of AgBiO₃ and BiOBr. According to band structure analysis, the AgBiO₃/BiOBr contains a typical II-type heterojunction. Nevertheless, if the composite obeys the II-type heterojunction charge transfer mechanism, the photogenerated holes would accumulate in the VB of AgBiO₃ and would not have a sufficiently high oxidation potential (only 0.12 eV for VB of AgBiO₃) to generate ˙OH (˙OH/H₂O, +1.99 eV).⁵¹ Based on the energy band structure and free radical experiment results discussed above, the S-scheme photocatalytic mechanism is more suitable,^30,52,53 which is formed attributed to the coupling of chemical bonds, the large contact area and the difference of Fermi level between the AgBiO₃ and BiOBr molecules. The novel transfer pathways not only promoted the separation of photogenerated carriers and restrained the recombination of electrons and holes, but they also maintained stronger redox ability. The electrons and holes originate from CB of BiOBr and VB of AgBiO₃, respectively, which recombine with each other due to the internal electric field and band edge bending interaction. They retain photo-induced charge with higher redox potential. The stacked holes on the VB of BiOBr could react with H₂O to form ˙OH because VB potential was higher than the standard redox potential of ˙OH/H₂O (+1.99 eV vs. NHE). Therefore, the holes and ˙OH directly degrade organic pollutants adsorbed on the AB-5 composite surface. The CB potential edge of AgBiO₃ (−0.68 eV vs. NHE) efficiently produces ˙O₂⁻ (E (O₂/˙O₂⁻) = −0.33 eV vs. NHE). Thus, the proposed S-scheme mechanism route of photoinduced carrier charge separation is thermodynamically favorable in the composite.

Fig. 12 Proposed photocatalytic mechanism scheme of S-scheme AgBiO₃/BiOBr.

4 Conclusions

In this work, S-scheme AgBiO₃/BiOBr heterojunctions were designed through a facile hydrothermal and in situ precipitation method. XRD, XPS, SEM, TEM and BET characterization methods were systematically utilized to evaluate the morphology of the materials. The optimized AgBiO₃/BiOBr heterojunction possesses excellent performance for CIP degradation. The highest photodegradation efficiency of CIP was 83%, which was 1.46- and 4.15-times greater activity than those of pure AgBiO₃ and BiOBr, respectively. For other contaminants, the removal efficiencies were 98% (LR5B), 77% (RhB), 87% (TC), and 95% (MO). Environmental interfering experiments demonstrated that low catalyst dose, high CIP concentrations and the addition of ions (SO₄²⁻, PO₄³⁻, HPO₄²⁻ and H₂PO₄⁻) decreased the photocatalytic efficiency. The coupled S-scheme AgBiO₃/BiOBr heterojunction achieved enhanced visible-light response, improved the charge separation and suppressed the recombination of electron–hole, and preserved redox ability. Furthermore, an S-scheme mechanism was proposed from trapping experiments and ESR result, indicating that photo-induced holes and superoxide radicals both play a critical role in the degradation process and that hydroxyl radicals acted as assistants.

Author contributions

Jianhong Lu: Conceptualization, Methodology, Software, Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Junhong Bie: Software, Resources, Investigation. Formal analysis. Shuai Fu: Conceptualization, Software, Writing - review & editing, Resources, Visualization, Funding acquisition. Jiaqi Wu: Formal analysis, Data curation, Writing - review & editing. Qiang Huang: Methodology, Software. Pengli He: Investigation, Formal analysis. Zhe Yang: Investigation, Data curation, Funding acquisition. Xiuji Zhang: Investigation, Funding acquisition. Huijie Zhu: Project administration, Supervision. Peiyuan Deng: Conceptualization, Visualization, Funding acquisition.

Data availability statement

All data, models, and code generated or used during the study appear in the submitted article.

Conflicts of interest

There are no conflicts of interest.

Acknowledgements

The support of this work by the National Natural Science Foundation of China (Grant No. 51709141), the Key Science and Technology Research Projects of Henan Province (Grant No. 222102320376) and the Key Scientific Research Project in Universities of Henan Province (Grant No. 22B130001).

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Footnote

† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2ra03157d

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