Efficient molecular oxygen utilization of micelle-based BiOCl for enhanced in situ H₂O₂ production induced photocatalytic removal of antibiotics

Abstract

Photocatalytic self-Fenton technology is regarded as a promising strategy for the removal of pollutants in wastewater, and the in situ H₂O₂ production rate is one of the main factors affecting contaminant degradation performance. However, insufficient utilization of molecular oxygen results in poor activity in photocatalytic H₂O₂ production. Herein, we propose a single-layer BiOCl nanoflower induced by biosurfactant with surface micelles to increase the oxygen adsorption and reduction rate. The DFT results demonstrated that hydrophobic groups (–CH₂–) on the surface of micelles increased the O₂ adsorption sites and then expanded the O₂ adsorption capacity of BiOCl samples, which was also confirmed by the enhancement of the O 1s peak intensity in the XPS spectra. The charge concentrated on the micelle surface can accelerate the reduction of molecular oxygen to reactive oxygen species, thus enhancing H₂O₂ production to reach 108.6 μM within 60 min in pure water. Simultaneously, single-layer BiOCl increased the utilization rate of H₂O₂ by decreasing the decomposition of H₂O₂ itself, resulting in a 16.8-fold increase in sulfamethoxazole degradation efficiency. These results can inspire further developments in the photocatalytic degradation of antibiotics based on in situ H₂O₂ production.

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Environmental significance

The misuse and improper treatment of antibiotics lead to their discharge into the external environment, such as water and soil, which poses potential threats to ecology and human health. In this work, we construct a BiOCl nanosheet photocatalyst with a self-assembled micelle (SA-BiOCl) via a simple hydrothermal method. More importantly, SA-BiOCl improves in situ H₂O₂ production by increasing the amount of O₂ in solution. The outstanding antibiotic degradation property was attributed to abundant reactive oxygen species, due to the efficient utilization of the in situ-produced H₂O₂. This study provides a new idea for photocatalytic self-Fenton treatment of antibiotics in wastewater.

Introduction

Antibiotics are pharmaceuticals and personal care products (PPCPs) that are widely used in industry, agriculture, medicine, and other fields,^1–3 and they cause water contamination that can pose a fatal threat to human health and the natural environment.^4,5 More importantly, antibiotics with a large π electron conjugation system will form compounds possessing increased toxicity after decomposition, and many traditional methods cannot completely remove them. Among the advanced oxidation processes, there has been excellent performance of photocatalytic self-Fenton techniques with all types of antibiotics.^6–8

At present, many studies have reported increased H₂O₂ production in self-Fenton processes by increasing oxygen utilization. For instance, Wu et al.⁹ used organic semiconductor (DAnTMS) photocatalytic decomposition of water to simultaneously produce H₂ and H₂O₂, and then utilized self-produced oxygen to increase the yield of H₂O₂. Qian et al.¹⁰ regulated reactive oxygen species by constructing electron channels and oxygen adsorption sites, greatly enhancing the efficiency of photogenerated electron transfer to adsorbed oxygen. Furthermore, Chen et al.¹¹ obtained a tubular g-C₃N₄ sample to construct a solid–liquid–air interface, and the honeycomb-like structure enhanced O₂ diffusion, increasing the yield of H₂O₂ in the channel. Due to the hydroxyl radicals (˙OH) derived from H₂O₂, which possess strong oxidation capabilities and are important in the decomposition of pollutants, it is crucial to construct photocatalytic systems with high molecular oxygen utilization rates to produce H₂O₂ for the mineralization of antibiotic wastewater.^12–15

Currently, many photocatalytic materials have been used for the photocatalytic oxygen reduction reaction (ORR) to produce H₂O₂ and degrade antibiotics.^16–23 Among the investigated photocatalysts, BiOCl is an important semiconductor with the appropriate energy band structure (3.2–3.7 eV) and particular lamellar structure.^24–26 However, there is inadequate surface adsorption sites with pure BiOCl, and its relatively thick lamellae hinder electron transfer, resulting in poor photocatalytic H₂O₂ production activity.^27–30 Therefore, how to realize an increase in the oxygen absorption and enrichment capacity and electron transfer efficiency of BiOCl is a conundrum. Amphiphilic surfactants are self-assembled into micelles that can be used as a template for controlling the size and shape of crystals, and then efficiently promoting electron transfer.^31–33 For instance, Yang et al.³⁴ introduced the structural guiding agent polyvinylpyrrolidone to regulate the crystal plane growth of BiOX (X = Cl, Br, I), resulting in a higher electron density and enhanced photocatalytic degradation activity. Surfactants can also expand the spacing between the layered materials during the synthesis process, forming a single atomic layer and weakening the van der Waals interactions between the layers.³⁵ In particular, variable valence metals prevent the rapid decay of the photocurrent and ensure stable visible light-induced photocurrent, and accelerate electron cycling inside the photocatalyst.³⁶ Furthermore, the micelles formed after the biosurfactant is dispersed in water induce the growth of the material, and also bond to the surface of the materials by hydrogen bonds or covalent bonds, forming different surface charge densities and increasing the adsorption properties of non-polar molecules. Surface micelles can also increase the adsorption capacity of oxygen, increasing the ORR rate and enhancing the activation performance of oxygen molecules to promote in situ H₂O₂ production.^37–39 Therefore, biosurfactant-induced single-layer BiOCl with variable valence metals and surface micelles is an interesting strategy for enhancing molecular oxygen utilization.

In this work, we constructed BiOCl nanoflowers with self-assembled micelles (SA-BiOCl) on the surface, and they were induced by saponin powder via a simple hydrothermal method. As a soft template, self-assembled micelles increased the layer spacing between BiOCl during synthesis to form monolayer nanosheets. At the same time, hydrophobic and hydrophilic groups were grafted onto the BiOCl surface to form surface micelles, which provide more reactive sites and increase the adsorption capacity of oxygen. More importantly, the presence of surface micelles and the change of Bi valence during the ORR process improved the separation and mobility of photogenerated carriers, thereby increasing the activation of oxygen molecules for enhancing in situ H₂O₂ production. The outstanding degradation property was attributed to the abundant ˙OH, which was generated due to the efficient utilization of in situ-produced H₂O₂. Then, the photocatalytic antibiotic degradation was evaluated under solar illumination, and the enhanced adsorption of oxygen was verified by comparing the yield of H₂O₂ under He and air conditions. Furthermore, this method can also be expanded and applied to other BiOX (BiOBr, BiOI) systems, which proved its universality. Hence, such nano-photocatalysts modified with biosurfactants increase oxygen adsorption and the electron reduction rate to boost the utilization of molecular oxygen. This is an effective strategy for in situ H₂O₂ production and provides a novel idea in the field of photocatalytic self-Fenton environmental purification.

Experimental section

2.1 Materials

The reagents used in the experiment are listed in the ESI.†

2.2 Synthesis of BiOCl

The synthesis method of BiOCl is described in the ESI.†

2.3 Synthesis of SA-BiOCl

First, 2 mmol Bi(NO₃)₃·5H₂O (0.970 g), 2 mmol of NaCl (0.116 g), and 0.1 g saponin powder were dissolved in 70 ml deionized water and 10 ml ethanol in a 100 ml beaker, and then fully stirred for 30 min. Subsequently, the solution was injected into a 150 ml Teflon liner and heated for 18 hours at 160 °C. After the mixed solution was cooled at room temperature, the suspension was washed several times with deionized water and ethanol until the upper solution was clarified. The sample was dried in a vacuum oven at 70 °C, and was named SA-BiOCl. Similarly, the other reaction conditions remained unchanged, and 0.05 g, 0.2 g, and 0.5 g saponin powder was added. These samples were named SA_0.05-BiOCl, SA_0.2-BiOCl, and SA_0.5-BiOCl, respectively.

2.4 Photocatalytic H₂O₂ production

In a beaker, 50 mg of photocatalyst was added to 50 ml of deionized water, and oxygen adsorption equilibrium was achieved by stirring for 30 min in the dark. Then, 1.5 ml of the suspension was removed every 30 min under visible light irradiation, the filtered solution was centrifuged, and the absorbance was measured at 350 nm wavelength by iodometry.

2.5 Photocatalytic degradation of pollutants

In a glass bottle, 25 mg catalyst was added to 50 mL contaminant solution. Then, 1.5 ml solution was removed at fixed intervals and tested after filtration (the detailed test methods for the different contaminants are provided in section 4 of the ESI.†).

Results and discussion

The synthesis of monolayer nanostructures with different morphologies and sizes via the micelles of surfactants is highly attractive, as a variety of micellar structures play the role of the soft template. The SA-BiOCl was fabricated as illustrated in Fig. 1a. Saponin powder contains a variety of surface-active groups. When the charge on the surface of the surfactant is the same as that of the hydrophilic ion part of the solution, the hydrophobic tail of the surfactant is more easily adsorbed on the photocatalyst surface than the ionic part of the molecule due to the repulsion of charge. The surfactant is covered by plentiful oxygen in the hydrophobic endpoint, and is adhered to the surface of the catalyst with the micelle. In addition, a proper amount of saponin powder can inhibit the hydrolysis of Bi³⁺ and increase the free Bi³⁺ ions in the solution. Then, Bi³⁺ has the ability to loosen the structure with negatively charged micelles so that the micelles are more easily bound and remain on the surface of the BiOCl.⁴⁰

Fig. 1 (a) Schematic diagram of surface micelle formation principle of single-layer SA-BiOCl. Optical microscope images of (b) saponin, (c) saponin after hydrothermal reaction, (d) BiOCl, (e) SA-BiOCl before hydrothermal reaction, (f) SA-BiOCl after 8 hours of hydrothermal reaction, and (g) SA-BiOCl after 18 hours of hydrothermal reaction.

To investigate the transformational mode of saponin powder, the morphology of saponin powder before and after the hydrothermal process was observed through an optical microscope. When pure saponin powder is uniformly distributed in solution, it forms micelles (Fig. 1b). Under the action of external energy, the micellar vesicles in saponin powder began to agglomerate and shrink, forming a dense stacked structure (Fig. 1c). Fig. 1d shows that pure BiOCl exists in the form of nanosheets. After adding saponin powder, the adsorption effect of micelles resulted in increased intermolecular forces of Bi, O, and Cl atoms, leading to the accumulation of nanosheets (Fig. 1e). As the reaction progressed, micelles attached to the surface of BiOCl and gradually aggregated (Fig. 1f), which is similar to the transformation of saponin powder during the synthesis of SA-BiOCl. Finally, stable flower-like nanospheres were formed by self-assembly after 18 h (Fig. 1g). Therefore, saponin powder not only provides surface-active sites for O₂, but also plays a crucial role in the formation of the SA-BiOCl nanoflower structure.

3.1 Morphology and structure of SA-BiOCl

Initially, SEM was used to reveal the microstructure evolution from BiOCl to SA-BiOCl. It was clear that the structure and morphology of SA-BiOCl were significantly different from that of BiOCl. Pure BiOCl exhibits a neat edge and smooth surface nanoplate with a diameter of approximately 1 μm (Fig. 2a). The self-assembled micelles formed by saponin powder could be controlled to adjust the size of the nanosheets, and stacked to assemble a nanoflower structure, with a nanoflower diameter of 300–500 nm (Fig. 2b). Through observing the enlarged TEM images (Fig. 2c and d), we found that the flower spherical structure of clusters was composed of abundant nanosheets, and the thickness of SA-BiOCl was estimated to be 30 nm. This result was also supported by the evolution of saponin powder in the optical microscope image.

Fig. 2 SEM images of (a) BiOCl, (c) SA-BiOCl; TEM images of (b) BiOCl, (d) SA-BiOCl; (e) FT-IR spectra of BiOCl, SA-BiOCl, and saponin; (f) the molecular structures of SA-BiOCl; (g) BET curves and (h) pore distribution of samples.

In the early growth stage, BiOCl crystals were grown along the micellar template into single-layer nanosheets, which was due to the inability of the template to limit the growth of unsealed nanosheets. At this stage, the edges of the unclosed nanosheets grow and extend along their edge planes. Subsequently, the dangling bonds on the surface of BiOCl tend to be saturated, so that the lamellar material is more densely stacked.^41,42 Additionally, the densely stacked sheets are bent, which increases the stability of the dangling bonds on the surface. This indicates that saponin powder in a specific concentration range can guide the growth of single-layer BiOCl nanoflowers.

The structure of the surface of SA-BiOCl was examined by Fourier transform infrared (FT-IR) spectroscopy. In the FT-IR spectrum (Fig. 2e), the absorption of SA-BiOCl at 3550–3416 cm⁻¹ is attributed to the –OH stretching frequency caused by intramolecular hydrogen bonding and the N–H stretching vibration peak.⁴³ Due to the deformation vibration of the N–H bond, SA-BiOCl generates corresponding absorption peaks at 1616 cm⁻¹ and 1056 cm⁻¹, which confirms that amino and hydroxyl groups are attached to the surface of SA-BiOCl. The C–H bond tensile vibration induced a peak at 2922 cm⁻¹.⁴⁴ It was confirmed that hydrophilic and hydrophobic groups covered the surface of SA-BiOCl and formed surface micelles (Fig. 2f). Additionally, the specific surface area of SA-BiOCl was 23.9 m² g⁻¹ (Table S1†), which is approximately 7 times higher than that of BiOCl (3.3 m² g⁻¹). The addition of surface-active sites was attributed to the increase in the specific surface area and surface micelles. The peak at 527 cm⁻¹ is a Bi–O stretching vibration peak, which confirms the existence of a Bi–O bond in the samples.⁴⁵Fig. 2g shows the N₂ adsorption/desorption isotherms of BiOCl and SA-BiOCl, which are type IV isotherms, and the obvious hysteresis loops prove the existence of mesoporous structures. Additionally, there were numerous micropores in the surface of SA-BiOCl, with a size of approximately 8 nm (Fig. 2h), and the unique structure and large specific surface area of SA-BiOCl may facilitate the adsorption and photocatalytic activity of the substrate.

The crystal phase and composition of the BiOCl and SA-BiOCl samples were analyzed by X-ray diffraction (XRD) patterns (Fig. 3a). Three sharp characteristic diffraction peaks emerged at 2θ of 25.9°, 32.5°, and 33.4°, corresponding to the (101), (110), and (102) crystal facets of the BiOCl standard structure (JCPDS no. 06-0249), respectively, and confirming that the prepared samples belong to the tetragonal phase.^46,47 In the process of BiOCl synthesis, with the addition of saponin powder, the diffraction peak of SA-BiOCl decreases, which was possibly due to the widening of the layer spacing to form single-layer BiOCl. The diffraction peak of SA-BiOCl was weaker and broader than the relative strength of the standard pattern, indicating that saponin powder mainly induces the dominant growth of (110) crystal facets.³⁴ This was also proved by the HRTEM image analysis in Fig. 3b. Pure BiOCl exhibits uniform lattice fringes with a plane spacing of 0.344 and 0.267 nm, which are reflected as the (101) and (102) facets, respectively. As illustrated in Fig. 3c, there are different lattice stripes for SA-BiOCl corresponding to the (101) and (110) facets, respectively, which correspond to the XRD results. Additionally, EDS mapping (Fig. 3d) confirmed that there was a uniform distribution to the elemental composition of SA-BiOCl, and the Bi/Cl atomic ratio approached 1.

Fig. 3 (a) XRD patterns of BiOCl and SA-BiOCl samples. HRTEM images of (b) BiOCl and (c) SA-BiOCl. (d) EDS mapping of SA-BiOCl (Bi, O, Cl).

3.2 The surface micelle increases molecular oxygen adsorption

During the SA-BiOCl synthesis process, the stability of self-assembled micelles changed according to various concentrations of saponin powder. Fig. 4a shows that the micelle state was the most stable in the solution with 0.1 g of saponin powder. There are differences in the non-polar forces within the self-assembled micelles with different levels of saponin, resulting in different surface charge densities. The surface polarity changes when there are large quantities of groups on the surface of SA-BiOCl, manifesting an increase in electron density around SA-BiOCl. The density of states (DOS) was analyzed by density functional theory (DFT) calculations. As shown in Fig. 4b and c, the valence band (VB) and conduction band (CB) are composed of a hybrid of the O 2p, Bi 6p, and Cl 3p orbitals. The electronic structures and DOS results of BiOCl and SA-BiOCl reveal that the main difference between the two is the Cl 3p orbital near the Fermi energy level in SA-BiOCl. The introduction of saponin significantly activates the interaction of other atoms with the Cl atom,⁴⁸ indicating that the introduction of surface micelles increased the amount of electrons around the Fermi energy and facilitated electron transfer.

Fig. 4 (a) Zeta potential of different concentrations of saponin powder. Calculated TDOS of (b) BiOCl and (c) SA-BiOCl; XPS of (d) O 1s. (e) Schematic diagram showing the state of O₂ in the surface micelles of SA-BiOCl.

Moreover, the interaction between atoms can be seen in the Raman spectra (Fig. S1†). BiOCl is a tetragonal phase structure with Raman symmetric vibrational modes of A_1g, B_1g, and E_g. The strongest band existing at 145 cm⁻¹ was assigned to the stretching mode of the internal Bi–Cl bond, while the peak of the other A_1g was generated by the stretching mode of the external Bi–Cl bond at 61 cm⁻¹. The peak at 201 cm⁻¹ belongs to the internal Bi–O stretch of E_g, and the weakest peak was assigned to the O atom belonging to E_g/B_1g mode. The survey spectrum (Fig. S2†) showed that the composition of BiOCl and SA-BiOCl included Bi, Cl, and O. The Bi 4f spectrum of SA-BiOCl was broadened compared with BiOCl (Fig. S3†), and it can be fitted by two peaks, which are located at approximately 164 and 159 eV. The SA-BiOCl binding energies shifted by 0.2 eV, which may be related to the variation in the surface energy caused by external functional groups and surface micellar structure.³⁴ In addition, a similar change occurred in Cl 2p (Fig. S4†). Moreover, the peaks of the O 1 s at approximately 530 and 531 eV are related to the lattice oxygen and the oxygen absorbed at the surface, respectively (Fig. 4d).⁴⁹ This was due to the encapsulation of oxygen and attachment to the surface of BiOCl during micellar aggregation (Fig. 4e).

The peak intensity of surface oxygen increases with introduction of the saponin powder, implying that the amount of oxygen absorbed at the surface increased. Also, the O element in the XPS elemental analysis has been increased (Table 1). The new peak appearing at 533.5 eV represents –OH on SA-BiOCl, and this is consistent with our theoretical model.⁵⁰ Noticeably, the change in the surface energy may cause SA-BiOCl to increase the adsorption of O₂, which indicates increased O₂ utilization efficiency in the photocatalytic H₂O₂ production⁴² that could be proved by the measurement of contact angles and surface free energy (Fig. S5†). The surface free energy of the as-synthesized BiOCl and SA-BiOCl was 93.06 mN m⁻¹ and 92.70 mN m⁻¹, respectively, which indicates that the surface micelles of SA-BiOCl can promote the adsorption of organic molecules.⁵¹

Table 1 The XPS elemental analysis of the as-prepared samples

Materials	Elements
	Bi (atomic%)	O (atomic%)	Cl (atomic%)
BiOCl	28.39	42.69	28.92
SA-BiOCl	26.47	46.10	27.44

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To verify the adsorption capacity of the micelles for O₂, different samples were dispersed in water, and the content of bubbles was observed under the optical microscope. As shown in Fig. S6a,† the optical microscope image reveals that a large number of micellar vesicles were formed in a solution containing only saponin powder to wrap O₂. The bubbles in the solution of SA-BiOCl (Fig. S6b†) were significantly higher than those in pure BiOCl (Fig. S6c†) and BiOCl with saponin powder added (Fig. S6d†), which implies that there is increased adsorption enrichment ability of the surface micelle of SA-BiOCl for O₂. To verify the encapsulation and adsorption of micelles, methylene blue (MB) adsorption experiments were carried out. Fig. S7† shows the adsorption of MB at fixed intervals by BiOCl, SA-BiOCl, and BiOCl with saponin powder added in the dark. SA-BiOCl had the most optimal adsorption effect on MB, and there was also increased adsorption of MB by BiOCl material with saponin powder, proving that the hydrophilic group in the micelle plays a promoting role in the adsorption of organic pollutants. Hence, the surface micelle can increase the diffusion of reactants and products between the active sites of SA-BiOCl, thereby promoting photocatalytic activity.

3.3 Optical properties and photoelectrochemical measurements of SA-BiOCl

To further clarify the impact of the surface micelle on the photocatalytic activity of BiOCl, the optical and photoelectrochemical properties were measured. A series of BiOCl samples was evaluated for their light-harvesting ability by UV-vis diffuse reflectance spectroscopy (DRS). Upon increasing the amount of saponin powder, the color of the synthesized samples gradually deepened. Fig. 5a shows that there was a slight redshift in the absorption edge of SA-BiOCl compared with that of BiOCl, which increased the absorption effect of visible light. In addition, there was stronger visible light adsorption for the flower-like material compared with the sheet-like material due to its relatively large specific surface area. This improves the separation efficiency of carriers, thus narrowing the bandgap and increasing the photocatalytic activity.⁵² However, as the micelles on the BiOCl surface increase, a large number of micelles will stack on the material surface, hindering electron transfer between channels.

Fig. 5 (a) DRS of samples. (b) The absorption versus energy curve of BiOCl and SA-BiOCl. (c) Band structure of the samples. (d) PL spectrum of as-prepared samples. (e) Photocurrent of a series of samples. (f) The Nyquist plots of EIS for BiOCl and SA-BiOCl.

Based on these results, Tauc plots (ahν)² = A (hν − E_g) were used to calculate the bandgaps of all BiOCl and SA-BiOCl samples. Fig. 5b shows that the band gaps of BiOCl and SA-BiOCl were 3.51 and 3.43 eV, respectively. Compared with BiOCl, the bandgap of SA-BiOCl was narrower, which indicates that an adjustable bandgap can be realized by surface energy regulation using the biosurfactant modification method. Moreover, there was a substantial increase in the absorption of visible light when the biosurfactant was added to 0.1 g, which can be related to the surface-active groups and the decrease in surface energy.

To further understand the band structure, the position of the CB was determined through the measurement of Mott–Schottky (MS) curves under different frequencies (Fig. S8†). The CBs of BiOCl and SA-BiOCl were −0.59 and −1.22 eV, respectively. After the introduction of saponin powder, the CB of BiOCl was sharply reduced, and the reduction performance of surface O₂ was greatly improved. Moreover, the MS curves showed that the slope of the potential curve was positive, with typical n-type semiconductor characteristics. The CB position of SA-BiOCl was approximately −1.22 eV, which indicates that the CB electrons can achieve the reduction of O₂ (Fig. 5c). In contrast, pure BiOCl can scarcely reduce O₂ to produce ˙O₂⁻.

Furthermore, the photocatalytic activity of semiconductors is mainly related to the migration efficiency of photogenerated electrons. We also measured the photoluminescence (PL), photocurrent response, and electrochemical impedance spectroscopy (EIS) to determine the photoelectrochemical properties. In the PL spectra (Fig. 5d), the main strong emission peak was for pure BiOCl and SA-BiOCl at approximately 622 nm (λ_excitation = 250 nm). The intensity for SA-BiOCl was significantly decreased in comparison to that of BiOCl, which indicates that SA-BiOCl has a lower e⁻–h⁺ pair recombination rate.⁵³ Therefore, it can be inferred that the appearance of surface micelles and the change in the surface energy of SA-BiOCl inhibit the recombination of photogenerated e⁻ and h⁺. Obviously, the photocurrent density produced by SA-BiOCl was 3.6 times higher than that produced by BiOCl under visible light irradiation (Fig. 5e), indicating that the electron and hole separation efficiency was higher for SA-BiOCl. At the same time, at the moment of turning on and off the lamp, the photocurrent produced a strong change, indicating that the sample was sensitive to visible light. The Nyquist plot radius of EIS for SA-BiOCl shows a smaller semicircular diameter, indicating that the interfacial charge transfer efficiency of SA-BiOCl was higher than that of BiOCl (Fig. 5f), which is consistent with the photocurrent density. It mainly benefited from the change in surface energy of SA-BiOCl, which accelerated the electron transfer from the interlamination to the surface.

3.4 Performance and mechanism of photocatalytic H₂O₂ production

To reveal the effect of the surface micelle on the H₂O₂ performance in the constructed SA-BiOCl, the structure of SA-BiOCl was further clarified by DFT calculation. The charge density differences (Fig. 6a and b) reflect that the charge transfer of H₂O and O₂ reacts with external groups on the surface of SA-BiOCl. The yellow region between H₂O and O₂ for SA-BiOCl mainly appears in the middle of the external functional groups and H₂O and O₂, indicating the high transfer efficiency of electrons on the surface micelles of SA-BiOCl. More importantly, compared with H₂O, when O₂ is in contact with surface micelles, the charge is more concentrated near the surface micelles.

Fig. 6 The charge density differences in the interface between adsorbed (a) H₂O and (b) O₂ with SA-BiOCl. (c) Calculated reaction energies of each step of H₂O₂ production on the samples. (d) Photocatalytic H₂O₂ production under different atmospheres within 90 min. (e) The formation rate constant (k_f) of BiOCl and SA-BiOCl. (f) Photocatalytic production of H₂O₂ over SA-BiOCl with scavengers. (g) EPR spectra of ˙OH for samples under dark and light conditions.

It can be intuitively seen that the non-polar forces inside the micelles contribute to the transfer of photoproduct charge between SA-BiOCl and O₂. In fact, the surface micelles of SA-BiOCl form additional active sites with open channel structures that can effectively promote the full interaction between electrons and O₂ in CBs. The above results reveal that SA-BiOCl samples with surface micelles are beneficial for enhancing the O₂ utilization efficiency. The entire process of H₂O₂ production was further studied by DFT calculations, where O₂ was initially adsorbed to groups on the surface and reacted with unpaired electrons. Fig. 6c shows that SA-BiOCl requires lower energy in the determination step of ˙O₂⁻ and is more likely to generate H₂O₂ over BiOCl.

The photocatalytic H₂O₂ production by BiOCl and SA-BiOCl was monitored in pure water (no organic electron donor) under visible light irradiation. Fig. 6d shows that SA-BiOCl displayed efficient properties in photocatalytic H₂O₂ production under visible light, and the H₂O₂ concentration reached 121.5 μM within 90 min, which was 7.99 times that of pure BiOCl (15.2 μM). In addition, the H₂O₂ production of SA-BiOCl was compared with other works in Table 2. The O₂ equilibrium condition is widely regarded as a key parameter, due to the photocatalytic generation of H₂O₂ being the effect of the O₂ reduction reaction.⁵⁴ To illustrate the role of molecular oxygen in H₂O₂ production, experiments under an air atmosphere and He atmosphere equilibrium conditions were also carried out. In an atmosphere of He, the yields of H₂O₂ over BiOCl and SA-BiOCl were 10.6 μM and 45.3 μM within 90 min, respectively. Compared with the H₂O₂ production in the air, there was still a certain amount of H₂O₂ production in the He atmosphere. This phenomenon is mainly caused by the slow release of O₂ encased in micellar vesicles. From Fig. 6e, the formation rate constant (K_f) of SA-BiOCl reached 1.81 μM min⁻¹. Subsequently, the photocatalytic decomposition of H₂O₂ was evaluated (Fig. S9†). The decomposition rate (K_d) of SA-BiOCl for H₂O₂ was 1.64 μM min⁻¹, indicating that SA-BiOCl possesses higher in situ H₂O₂ utilization efficiency. The photocatalyst turns gray after the ORR, which may be due to Bi⁰ precipitates during the process. Inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis was further performed to explore the leaching of elemental Bi during the photocatalytic reaction (Table 3). The amount of leached Bi in the BiOCl sample was 0.05%, and the leached amount of SA-BiOCl was 1.34%. This phenomenon is due to Bi³⁺ being reduced to Bi⁰ during the reaction process, and the construction of the Bi⁰/BiOCl heterostructure, which accelerates the internal charge migration and conversion efficiency of BiOCl. In addition, Bi⁰ is continuously oxidized by oxygen to Bi³⁺ in the ORR process, so that elemental Bi reaches a stable cyclic utilization and conversion equilibrium. This process not only accelerates the utilization efficiency of molecular oxygen, but it also ensures the stability of the material structure.

Table 2 A detailed comparison of photocatalytic H₂O₂ production by different photocatalysts

Photocatalysts	Conditions			Xe lamp	H2O2 production	Ref.
	Catalyst	Medium	Time
SNGQD/TiO2	25 mg	6 vol% 2-propanol	1.5 h	500 W	82.8 μM	55
rGO/TiO2/CoPi	50 mg	O2 atmosphere	1 h	300 W	58 μM	56
Bi2WO6	50 mg	0.43 mM phenol	1 h	300 W	8 μM	57
Cv-g-C3N4	100 mg	100 ml H2O	1 h	300 W	92 μM	58
CdS-GO	50 mg	30 ml H2O	12 h	300 W	128 μM	59
g-C3N4/PDI-BN-rGO	50 mg	30 ml H2O	24 h	2 kW	1233 μM	60
SA-BiOCl	50 mg	50 ml H2O	1 h	300 W	108.6 μM	This work

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Table 3 The corresponding ICP-OES results for the prepared samples

Samples	Before reaction (Bi)	After reaction (Bi)
BiOCl	0.04%	0.05%
SA-BiOCl	0.05%	1.34%

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To further explore the active species that influence H₂O₂ production, the reactive species scavengers were added over BiOCl and SA-BiOCl to clarify the production path. AgNO₃, EDTA-2Na, p-benzoquinone (BQ), and tert-butanol (TBA) were added as the quenchers of e⁻, h⁺, ˙O₂⁻, and ˙OH, respectively. As shown in Fig. 6f, EDTA-2Na trapped h⁺ in solution, reduced the consumption of electrons, and promoted the electron reduction reaction of oxygen. The H₂O₂ yield significantly decreased after the addition of AgNO₃, and there was little effect on the H₂O₂ production from the consumption of ˙OH by TBA, which proved that H₂O₂ was produced by the two-electron reduction process rather than by the transformation of ˙OH. The trapping agent used was 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to conduct the electron paramagnetic resonance (EPR) test and verify the active species in H₂O₂ production. Fig. S10† shows that the strongest ˙O₂⁻ signal was from SA-BiOCl under visible light, which proves that ˙O₂⁻ exists in large quantities as an intermediate product in the process of H₂O₂ generation, and provides evidence for the one-electron reduction process. Additionally, ˙O₂⁻ is an intermediate in the process of producing H₂O₂ and has a major role in the degradation of antibiotics. Combined with the above experiments, we found that ˙OH is not the main factor that produces H₂O₂. Thus, the signal of DMPO – ˙OH indicated that H₂O₂ decomposed into ˙OH to degrade pollutants (Fig. 6g).

3.5 Photocatalytic pollutant degradation performance

The most optimal photocatalytic performance was obtained from SA-BiOCl (SA_0.1-BiOCl) with 0.1 g saponin powder, and this was attributed to the surface micellar increased H₂O₂ production and utilization efficiency. To this end, we explored the effect of increasing H₂O₂ production upon the degradation of organic pollutants. As a comparison, we found that the degradation rates of MB by BiOCl and SA-BiOCl were 40.0% and 89.0% within 60 min in pure MB solution, respectively (Fig. S11†). In contrast, the degradation efficiencies of BiOCl and SA-BiOCl reached 65.1% and 100.0%, respectively, when H₂O₂ was added during the degradation process.

To assess the photocatalytic degradation performance of photocatalysts, diclofenac sodium (DCF) was used as a model in this work. As shown in Fig. 7a, the addition of saponin powder significantly impacted the photocatalytic performance of SA_x-BiOCl series samples. The degradation rate of BiOCl toward DCF was 60.5% in 120 min under visible light irradiation. In contrast, the photodegradation efficiency of SA-BiOCl was increased to 100%, and exhibited 1.65 times higher photocatalytic activity for DCF than that of pure BiOCl. This was due to an appropriate amount of saponin powder that formed a layered micelle in solution and adsorbed BiOCl on the surface during the hydrothermal process, controlling its growth to smaller diameters. Obviously, the nanoflower structure is the result of anisotropic growth following the Ostwald maturation mechanism. When the BiOCl solution contains a high concentration of saponin powder, spherical micelles will be established to wrap BiOCl, which restricts Bi³⁺ from entering the interlayer for self-assembly. As a result, excess surfactant acts as a growth inhibitor, limiting crystal growth in all directions and ultimately leading to a decrease in the crystallinity of BiOCl. At the same time, the coating of abundant micelles will enhance the agglomeration effect of BiOCl, increase the particle size, and reduce the surface-active sites of materials, eventually leading to a decrease in the photocatalytic activity.⁶¹

Fig. 7 (a) Photocatalytic degradation of DCF within 120 min. (b) The first-order reaction kinetic constants of DCF degradation, (c) quenching of DCF by BiOCl and SA-BiOCl under visible light, and (d) photocatalytic degradation of other antibiotics.

Furthermore, the photodegradation of DCF was fitted to the equation: ln(C₀/C) = kt,²⁸ and the kinetic rate of DCF degradation on SA-BiOCl (0.0961 min⁻¹) was higher than that of BiOCl (0.00713 min⁻¹), SA_0.05-BiOCl (0.05076 min⁻¹), SA_0.2-BiOCl (0.06722 min⁻¹), and SA_0.5-BiOCl (0.00709 min⁻¹) (Fig. 7b). Apart from DCF, BiOCl and SA-BiOCl were chosen to remove sulfanilamide, sulfamethoxazole (SMX), and acyclovir under visible light irradiation. As shown in Fig. 7c, the degradation rates of SA-BiOCl for sulfanilamide, SMX, and acyclovir were 51.1%, 48.4%, and 65.2% in 120 min, which were 12.8, 16.2, and 3.85 times higher than that of pure BiOCl, respectively. This result is interpreted as the surface free energy reduction caused by the surface micelles attached to SA-BiOCl, whereby organic pollutants are easily adsorbed onto the surface of SA-BiOCl. The increase in H₂O₂ production and its utilization rate will subsequently promote degradation efficiency. In addition, Fig. S12† shows the degradation results of DCF by SA-BiOCl in four cycles, which illustrates that the removal efficiency is nearly stable.

To explore the degradation mechanism of BiOCl samples, a quenching experiment was conducted. Regarding DCF as the target pollutant, EDTA-2Na, BQ, and TBA were added as quenchers of h⁺, ˙OH, and ˙O₂⁻, respectively. As shown in Fig. 7d, in the presence of BQ, TBA, and EDTA-2Na, the degradation efficiency of SA-BiOCl on DCF significantly decreased, indicating that the three active species all played a role in DCF degradation. Additionally, the adsorbed molecular oxygen combined with electrons to form ˙O₂⁻ and H₂O₂, which were simultaneously used for antibiotic degradation. Then, the H₂O₂ is decomposed into ˙OH, which plays a major role in degradation.

Degradation of antibiotics is difficult because of the stable aromatic ring structure. However, direct oxidization by h⁺ occurs through redox reactions with pollutants. In the process of the electron reduction of O₂, h⁺ plays a role in ring-opening and synergistically degrades pollutants. Therefore, h⁺ direct oxidation and increased reactive oxygen species synergistically promote the degradation of antibiotics over SA-BiOCl. The antibiotic degradation pathways are shown below:

Photocatalyst + hv → h⁺ + e⁻ (1)

h⁺ + antibiotics → low-molecular weight organic matter (2)

O₂ + 2H⁺ + 2e⁻ ⇌ H₂O₂ (3)

H₂O₂ + H⁺ + e⁻ → ˙OH + H₂O (4)

˙OH + antibiotics → CO₂ + H₂O (5)

It is possible to draw the conclusion that SA-BiOCl has achieved greater photocatalytic performance based on the aforementioned experimental findings. It was difficult to achieve electron–hole separation with pure BiOCl under visible light irradiation because of the large band gap. SA-BiOCl induced by biosurfactants effectively expanded its absorption range of sunlight and reduced the electron–hole recombination. Because of the presence of a mass of micellar vesicles on the SA-BiOCl surface, the photogenerated electrons migrated to the surface of the photocatalyst, and corresponding h⁺ was created. Then, the accumulated holes in the CB directly degraded the antibiotics through redox reactions. The electronics reduced the surface O₂ to produce H₂O₂, which was then decomposed into ˙OH and used for the oxidation of antibiotics (Fig. 8).

Fig. 8 Proposed mechanism for the photocatalytic degradation of pollutants by SA-BiOCl.

Conclusions

In this work, BiOCl with abundant self-assembled surface micelles was constructed to enhance molecular oxygen activation and H₂O₂ production, thus increasing the efficiency of antibiotic removal. The presence of surface micelles reduced the surface free energy of SA-BiOCl, which could be beneficial for adsorbing O₂ from the air. The surface micelles not only accelerated the charge transfer efficiency on the surface of SA-BiOCl, but they also reduced the Gibbs free energy of the 2e⁻ ORR process, thus promoting 2e⁻ ORR to increase the H₂O₂ production. Subsequently, H₂O₂ was converted into ˙OH under the action of SA-BiOCl, which degraded antibiotics in coordination with ˙O₂⁻. Under solar irradiation, the degradation efficiency of pollutants was 12.8 (sulfanilamide), 16.8 (SMX), and 3.85 (acyclovir) times higher than that of pure BiOCl. In brief, this work not only provides novel inspiration for the preparation of photocatalysts with a micellar structure, but it also provides a new method for the removal of antibiotics from wastewater.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partly supported by the National Natural Science Foundation of China (22276011, 21607034, 51978032, 52270086), Beijing Natural Science Foundation (8192011), Science and Technology General Project of Beijing Municipal Education Commission (KM202010016006), the Pyramid Talent Training Project of Beijing University of Civil Engineering and Architecture (JDYC20200313, JDLJ20200301), the Youth Beijing Scholars program, No. 024, Joint Project of the Beijing Municipal Education Commission and Municipal Nature Science Foundation (21JH0024).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2en00933a

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