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

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.


Introduction
As society has progressed, domestic sewage and industrial wastewater have had an extremely signicant impact on human health and the environment, which has attracted widespread attention. [1][2][3][4][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 BiVO 4 , 10,11 Bi 2 WO 6 , 12 Bi 2 MoO 6 , 13 Bi 2 S 3 (ref. 14) and Bi 2 O 3 (ref. 15) have received widespread attention because they possess suitable structures. For example, bismuth bromide oxide (BiOBr) has a layered structure characterized by [Bi 2 O 2 ] 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/Bi 2 WO 6 composite photocatalyst by a one-step solvothermal approach. Photocatalytic degradation experiments revealed that BiOBr-Bi 2 WO 6 ¼ 8 : 1 demonstrated excellent tetracycline (TC) degradation (96%) within 120 min. 21 Zou fabricated a novel BiOBr/FeWO 4 composite photocatalyst via a solvothermal approach. The BiOBr/FeWO 4 (4 : 1) showed excellent photocatalytic degradation activity (90%) aer 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 MBiO 3 (M ¼ 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 24,25 Nevertheless, the narrower band-gap oen leads to faster recombination of photo-induced electron/ hole pairs. Thus, pure AgBiO 3 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 ower-like AgBiO 3 loaded on GO/NCDs, which exhibited signicantly enhanced degradation activities towards organic pollutants. 28 Wu synthesized a novel binary AgBiO 3 /g-C 3 N 4 composite via in situ growth of AgBiO 3 on g-C 3 N 4 , 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 inuence of an internal electric eld due to the differences of the Fermi level of each semiconductor. Wang synthesized S-scheme Sb 2 WO 6 /BiOBr photocatalysts via a precipitation-deposition method, which exhibited enhanced photocatalytic performance compared with pure Sb 2 WO 6 and BiOBr for removal of NO. 31 The enhanced photocatalytic performance for the Sb 2 WO 6 /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 AgBiO 3 /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 AgBiO 3 /BiOBr were tested towards antibiotic ciprooxacin (CIP) pollutants as a model pollutant under visible-light irradiation. The inuences of catalyst dose and initial concentration of CIP on the catalytic performance of AgBiO 3 /BiOBr were discussed. Additionally, a photocatalytic mechanism was also proposed.
All chemicals used were analytical grade reagents and were used without further purication.

Synthesis of BiOBr nanosheets
The BiOBr nanosheets were obtained by a solvothermal method. Generally, 1.587 g of Bi(NO 3 ) 3 $5H 2 O were rst added to a 200 mL ask 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 rst solution and stirred for 1 h. The reaction mixture was subsequently transferred to a 100 mL Teon-lined autoclave and heated at 160 C for 12 h. Aer 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.

Synthesis of AgBiO 3 /BiOBr composite
A certain amount of BiOBr powder was added into 50 mL of DI water and sonicated for 10 min. 0.5 mmol AgNO 3 was subsequently added. Aer 30 minutes of additional sonication, 0.5 mmol of NaBiO 3 was added to the above solution and stirred for 15 min at 25 C. Aer that, the mixture was centrifuged at 5500 rpm. Then the nal 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 AgBiO 3 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 AgBiO 3 was prepared via similar procedures without adding BiOBr. The preparation process of AgBiO 3 /BiOBr composite is described in Scheme 1.

Characterization
The morphology of AgBiO 3 /BiOBr was investigated by highresolution 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 AgBiO 3 /BiOBr heterojunctions were collected with a Bruker D8 Advance diffractometer using Cu Ka radiation (l ¼ 0.15406 nm) at 2q ranging between 10 and 80 . The specic surface areas of the AgBiO 3 /BiOBr heterojunctions were evaluated using N 2 adsorption isotherm on a Gemini-2380 (Quantachrome) system. The UV-vis diffuse reection 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 AgBiO 3 /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.

Evaluation of visible-light-driven photocatalytic performance
The photocatalytic activities of the AgBiO 3 /BiOBr samples were evaluated by degradation of CIP in an aqueous solution under visible-light irradiation (350 W Xe lamp with a UV-cutoff lter, l > 420 nm). The photocatalysts (40 mg) were dispersed in CIP solution (40 mL at a concentration of 20 mg L À1 ). The suspensions were stirred magnetically for 30 min in the dark to conrm 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.
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 AgBiO 3 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 Bi 3+ 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 À , 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 H 2 O, respectively. 37 In the Ag 3d spectrum of  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 shi compared with those of pure BiOBr and AgBiO 3 . These simultaneous shis are likely due to strong electronic interactions between BiOBr and AgBiO 3 . Fig. 2a reveals that pure AgBiO 3 possesses as ower-like structure composed of nanosheets with diameters around 600 nm. On the other hand, BiOBr exhibits a plate-like morphology (Fig. 2b). Both AgBiO 3 and BiOBr have analogous plate-like structures and interactions due to van der Waals forces, which facilitates and stabilizes AgBiO 3 over the BiOBr surface and results in the formation of a tight AgBiO 3 /BiOBr junction. Aer the deposition of AgBiO 3 , the ower-like semiconductor was inserted into disordered plate-like BiOBr, whereas the outer surfaces of BiOBr became rough ( Fig. 2c and d).

AB-5 shown in
TEM imaging (Fig. 3a) veries that the ower-like AgBiO 3 was successfully loaded on the surface of the plate-like BiOBr. The corresponding magnied HRTEM image (Fig. 3b) (Fig. 4), further conrming that the assynthesized composite contains AgBiO 3 and BiOBr.
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, AgBiO 3 and AB-5 heterojunction were 2.53, 20.99 and 5.78 m 2 g À1 , respectively. The surface area of AB-5 heterojunction was almost two times higher than that of BiOBr, suggesting that the introduction of AgBiO 3 improves the specic 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 specic 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 The as-synthesized photocatalysts were studied by UV-Vis diffuse reectance spectroscopy to investigate their optical properties (Fig. 6a). The absorption edges of BiOBr are located at 442 nm, while AgBiO 3 possesses intense absorption between 200-800 nm. Compared to BiOBr, the absorption band edge of the AgBiO 3 /BiOBr heterojunction shows a distinct red shi, indicating that AgBiO 3 improves visible-light absorption capacity. This result might contribute to the enhanced production of photogenerated electron-hole pairs. The bandgap of the samples was calculated by the Tauc equation as follows eqn (1).
where a, h, n, 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 ¼ 1 or n ¼ 4, respectively. Because BiOBr 39 and AgBiO 3 (ref. 24) are both indirect-allowed semiconductors, E g can be computed from the plot of (ahn) 1/2 as a function of hn. 39 As shown in Fig. 6b-d, BiOBr, AgBiO 3 and AB-5 heterojunction bandgaps were computed to be 2.74 eV, 0.80 eV and 2.53 eV, respectively. Fig. 7 displays the valence band (VB) position of the samples as evaluated tested by VB-XPS spectra. The VB positions of BiOBr and AgBiO 3 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): The CB positions of BiOBr and AgBiO 3 were computed to be À0.63 eV and À0.68 eV, respectively. The energy band potentials of BiOBr and AgBiO 3 are depicted in Table 1.

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 AgBiO 3 , the degradation efficiencies of CIP reached 20% and 57% aer 120 min under visible-light irradiation, respectively. Aer introducing AgBiO 3 , the AgBiO 3 /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 AgBiO 3 could promotes composite degradation efficiency due to formation of heterojunctions at the interface between AgBiO 3 and BiOBr, enhancing visible light absorption and separation efficiency of photogenerated carriers. The mechanical mixture of 5 wt% AgBiO 3 and BiOBr were also investigated as a control, and it exhibited lower efficiencies than the AgBiO 3 /BiOBr heterojunction. This lower efficiency is attributed to poor interfacial contact between AgBiO 3 and BiOBr during mechanical mixing. Fig. 8b depicted the reaction kinetics, which were modeled by a pseudo-rst-order kinetic model: ln(C 0 /C t ) ¼ kt. The rate constants of AgBiO 3 , 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 À1 , respectively. The rate constants of AB-5 were 2.72 and 7.34 times higher than those of AgBiO 3 and BiOBr, respectively, suggesting that a suitable quantity of AgBiO 3 improves the photocatalytic performance of AgBiO 3 /BiOBr.
The effects of different reaction conditions were also investigated. In Fig. 8c, the concentration of CIP solution was Fig. 7 (a and b) Valence-band XPS spectra of AgBiO 3 and BiOBr. 20 mg L À1 (pH ¼ 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% aer 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 signicantly 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 signicantly decreased. This phenomenon likely occurs because AB-5 undergoes chemical changes under alkaline conditions.
It is widely known that the real wastewaters oen contain various inorganic salts, which could affect the efficacy of the  , HPO 4 2À and H 2 PO 4 À ) at a concentration of 1 mmol L À1 on CIP degradation in the AB-5 system was studied. As shown in Fig. 8f,  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 signicantly inhibited. This phenomenon may be related to inorganic ions and organic pollutants, which could consume the free radicals produced by the AB-5 heterojunction.

Photocatalytic mechanism
To observe the migration and separation efficiency of the photoinduced charge, PL spectroscopy was performed. Generally, a weaker PL intensity reects a lower recombination probability. As shown in Fig. 10, With the addition of AgBiO 3 , the PL intensity of AgBiO 3 /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.
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 (cO 2 À ), hydroxyl radicals (cOH), 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-cO 2 À and DMPO-cOH were tested with the AB-5 composite under visible-light irradiation, 50 suggesting that cO 2 À and cOH radicals emerged during the degradation process. 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 AgBiO 3 and BiOBr. According to band structure analysis, the AgBiO 3 /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 AgBiO 3 and would not have a sufficiently high oxidation potential (only 0.12 eV for VB of AgBiO 3 ) to generate cOH (cOH/H 2 O, +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 AgBiO 3 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 AgBiO 3 , respectively, which recombine with each other due to the internal electric eld 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 H 2 O to form cOH because VB potential was higher than the standard redox potential of cOH/H 2 O (+1.99 eV vs. NHE). Therefore, the holes and cOH directly degrade organic pollutants adsorbed on the AB-5 composite surface. The CB potential edge of AgBiO 3 (À0.68 eV vs. NHE) efficiently produces cO 2 À (E (O 2 /cO 2 À ) ¼ À0.33 eV vs. NHE). Thus, the proposed S-scheme mechanism route of photoinduced carrier charge separation is thermodynamically favorable in the composite.

Conclusions
In this work, S-scheme AgBiO 3 /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 AgBiO 3 /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 AgBiO 3 and BiOBr, respectively. For other contaminants, the removal efficiencies were 98% (LR5B), 77% (RhB), 87% (TC), and 95% (MO decreased the photocatalytic efficiency. The coupled S-scheme AgBiO 3 /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 photoinduced holes and superoxide radicals both play a critical role in the degradation process and that hydroxyl radicals acted as assistants.