Synthesis of double Z-scheme CdS/Bi₂O₂CO₃/BiOCl heterojunction photocatalysts for degradation of rhodamine B under visible light

Abstract

In this paper, novel CdS/Bi₂O₂CO₃/BiOCl photocatalysts with a double Z-scheme heterojunction were successfully obtained by a simple one-step hydrothermal method. XRD, SEM, and TEM were used to verify the formation of the catalysts, and FTIR and XPS were used to evaluate their chemical composition and binding energy. To evaluate the performance of these photocatalysts, the degradation of RhB was performed under visible light and the effects of the catalyst dose and pH on the photocatalytic performance were examined. The results showed that the 10 mol% CdS/Bi₂O₂CO₃/BiOCl catalyst (0.1 g, pH = 5) showed the best activity, with a degradation rate of 97% in 90 min. The BET results showed that the composites have stronger adsorption properties. Electrochemical test results demonstrated that the Z-scheme heterojunction was responsible for promoting the carrier separation efficiency. The combination of strong visible light absorption and the effective separation of photogenerated electron–hole pairs remarkably enhanced the catalytic activity.

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1. Introduction

Protecting water resources and the removal of pollutants from water environments are increasingly important environmental issues.¹ However, traditional high-temperature and biological methods used to remove pollutants from water exhibit high energy consumption and easily introduce secondary pollution. Recently, photocatalysis has been widely investigated to replace traditional water treatment methods because it is clean, efficient, and low-cost.² Many photocatalysts have been widely investigated, including ZnO₂,³ g-C₃N₄,⁴ graphene,⁵ bismuth-based photocatalysts,^6–8 and so on. Among them, bismuth-based photocatalysts have been thoroughly studied owing to their simple preparation process and high responsiveness in the UV region.⁹ These bismuth-based catalysts include BiPO₄,¹⁰ Bi₂S₃,¹¹ BiOCl,¹² and Bi₂O₂CO₃.¹³ However, individual semiconductor materials exhibit drawbacks such as a poor response to visible light and rapid carrier recombination.^14–16 Therefore, the catalytic efficiency of these materials is not sufficient. To improve the conversion ability of catalysts, many methods have been proposed to modify Bi-based photocatalysts, such as noble metal deposition,¹⁷ non-metal doping,¹⁸ and preparing composites with other semiconductors to form heterojunction structures.^19,20

Heterojunction structures have attracted an increasing amount of attention because they can be used for the effective separation and migration of carriers. For instance, binary Z-scheme or type II heterojunction photocatalysts such as PbBiO₂Br/BiOBr,²¹ Ag₂O/Bi₂O₂CO₃,²² and BiVO₄/CeO₂ (ref. 23) can reduce the recombination of carriers, and they also exhibit a broader optical response range. Unfortunately, the electrostatic repulsion between electrons (or holes) prevents their effective transfer, so the redox ability of carriers is weakened in the conventional type II heterojunction mechanism. In contrast, in Z-scheme heterostructures, holes and electrons with poor redox ability will recombine. Meanwhile, the stronger oxidative holes and reductive electrons are still retained in the semiconductor materials of the heterostructure, resulting in high catalytic activity.²⁴

On the basis of these binary heterojunction systems, new ternary heterojunction systems have gradually been investigated. Compared with single heterojunctions, double heterojunctions have higher carrier separation efficiency, longer lifetimes, and larger visible response ranges. For example, the BiVO₄/CDs/CdS photocatalysts prepared by Kang et al. showed better performance for the production of hydrogen via water decomposition.²⁵ This was due to the effective separation of carriers and the extended lifetime of electrons and holes in the photocatalyst. Gao et al. successfully synthesized a g-C₃N₄/BiOI/Bi₂O₂CO₃ photocatalyst by an in situ conversion approach which showed good performance for the degradation of RhB.²⁶ Luo et al. also prepared a CdS/Bi₂MoO₆/BiOBr photocatalyst that effectively degraded up to 94.41% of tetracycline hydrochloride (TC).²⁷ While these ternary photocatalysts show good promise in terms of catalytic performance, their preparation requires multi-step synthetic methods, and convenient one-step strategies have only rarely been reported.

Bi₂O₂CO₃ is an n-type semiconductor with alternating stacks of [Bi₂O₂]²⁺ sheet layers and CO₃²⁻ layers. However, the wide band gap (3.2–3.5 eV) of Bi₂O₂CO₃ has significantly limited its application.²⁸ In contrast, BiOCl has a suitable energy band position, providing it with a good light response in the UV region.²⁹ CdS is a material with some visible light photocatalytic activity, but its carrier recombination rate is too high. Due to the narrow band gap and low carrier lifetime of CdS, B.G. Mishra et al. coupled CdS with wide band gap semiconductors to fully exploit its role.³⁰ Therefore, it is proposed that combining Bi₂O₂CO₃, BiOCl, and CdS in a new semiconductor will take advantage of their respective advantages to provide an enhanced catalytic effect. However, to the best of our knowledge, a CdS/Bi₂O₂CO₃/BiOCl composite photocatalyst has not been reported yet.

Inspired by this speculation, we synthesized a series of CdS/Bi₂O₂CO₃/BiOCl ternary visible light photocatalysts using an easy one-step strategy. The structure, properties, and catalytic mechanism of the as-obtained samples were comprehensively evaluated using various characterization techniques. The ternary photocatalysts exhibited excellent RhB degradation performance and maintained high stability after four repeated experiments. In addition, a double Z-scheme heterojunction mechanism was confirmed according to radical trapping experiments, ESR tests, and the band location. This work offers insight into the preparation and application of highly efficient bismuth-based composite photocatalysts.

2. Experimental section

2.1 Synthesis of Bi₂O₂CO₃, BiOCl, CdS, Bi₂O₂CO₃/BiOCl, Bi₂O₂CO₃/CdS, and BiOCl/CdS

The synthesis process for obtaining Bi₂O₂CO₃ (BSC), BiOCl (BOC), CdS, Bi₂O₂CO₃/BiOCl (BSC/BOC), Bi₂O₂CO₃/CdS (BSC/CdS), and BiOCl/CdS (BOC/CdS) is described in the ESI.†

2.2 Synthesis of CdS/Bi₂O₂CO₃/BiOCl

First, bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, 2.021 g), urea (CH₄N₂O, 0.25 g), and potassium chloride (KCl, 0.315 g) were added to 25 mL of distilled water and continuously stirred for 1 h to obtain solution A. Cadmium chloride hemi(pentahydrate) (CdCl₂·2.5H₂O) (0.048 g, 0.096 g, 0.192 g) and thiourea (CH₄N₂S) (0.080 g, 0.160 g, 0.320 g) were mixed in 15 mL water and evenly stirred to obtain solution B. Then, solutions B and A were mixed and stirred for 1 h. Finally, the mixed solution was added to a PTFE-lined reactor and heated at 160 °C for 24 h. After cooling, each obtained sample was centrifuged several times, filtered, and dried at 60 °C for 24 h. The different ratios of CdCl₂·2.5H₂O and CH₄N₂S resulted in ternary composites with 5, 10, and 20 mol% CdS content. These products (5, 10, and 20 mol% CdS/Bi₂O₂CO₃/BiOCl) were denoted 5CBB, 10CBB, and 20CBB, respectively. Details of the ternary composite photocatalyst synthesis process are illustrated in Fig. 1.

Fig. 1 Fabrication of CdS/Bi₂O₂CO₃/BiOCl.

2.3 Characterization

Crystal phase analysis of all products was performed with X-ray powder diffraction (D/MAX-2500/PC). The FTIR spectra of the samples were obtained with an infrared spectrometer (Nicolet IS10). The surface elements and chemical valence bonds of products were evaluated using X-ray photoelectron spectroscopy (XPS, ESCALAB XI+). The microstructure and morphology of the samples were observed by SEM (JSM-6700F) and TEM (JEM-2100 plus). UV-vis diffuse reflectance spectra were obtained with a UV-vis spectrophotometer (Cary 5000). An Aptar PGSTAT302N electrochemical workstation was used to determine the photoelectric properties of the photocatalysts. Time-resolved transient fluorescence lifetimes were measured with an FLSP920 instrument. Electron spin resonance (ESR) was performed with a Bruker A-300.

2.4 Photocatalytic performance tests

The photocatalytic behaviour of CdS/Bi₂O₂CO₃/BiOCl was evaluated by degrading RhB under visible light. A 300 W xenon lamp was used to simulate sunlight (λ > 420 nm). First, 0.1 g catalyst was added to 100 mL (20 mg L⁻¹) of RhB solution, and this mixture was stirred for 30 min to achieve adsorption–desorption equilibrium. Next, the lamp was turned on and 5 mL samples were removed for evaluation at 15 min intervals. Each sample was tested by measuring the maximum absorbance peak of the RhB solution at 554 nm. At the end of each photocatalytic experiment, all catalyst samples were recovered and washed to evaluate their reusability. Subsequent photocatalytic tests were performed using the same experimental parameters. Finally, the active species of the ternary composite photocatalysts were explored by ESR tests and radical capture experiments.

3. Results and discussion

3.1 Structure and morphology

The crystal phases of the as-obtained photocatalysts were analysed by XRD. The XRD patterns of BOC, BSC, and CdS along with their PDF cards are revealed in Fig. 2a. The hexagonal phase of CdS (PDF#41-1049), tetragonal phase of BOC (PDF#06-0249), and tetragonal phase of BSC (PDF#41-1488) were observed.^31–33 The XRD patterns of BSC/BOC and the ternary photocatalysts are provided in Fig. 2b. In the BSC/BOC binary composite diffraction pattern, the characteristic peaks of BOC were more prominent and obvious than those of BSC, indicating that BOC was the main phase. The characteristic peaks of BSC were observed at 12.9°, 30.3°, 42.3°, and 52.4° (marked by “♦” in Fig. 2b). In addition, the peak intensity of BOC was enhanced when BSC was complexed with BOC, suggesting that BSC and BOC interacted with each other instead of simply being mechanically adhered. Wang et al. believed that this was due to the formed heterojunction causing structural changes.²⁷

Fig. 2 (a) XRD patterns of CdS, BOC, and BSC with their respective PDF cards; (b) XRD patterns of BSC/BOC, 5CBB, 10CBB, and 20CBB.

BOC was still the dominant phase based on the diffraction peaks of the ternary composites. The BSC peaks are indicated with “♦” markers while those of CdS are marked with “•” symbols. With increasing CdS content, the CdS peak intensity was enhanced, while the intensity of the BOC peaks at 12.0°, 24.1°, and other locations decreased as the increasing amount of CdS covered the surface of BOC. Yin et al. suggested that lattice distortion also leads to a decline in crystallinity, implying that the phases interacted with each other to some extent.²⁸ The XRD patterns of Bi₂O₂CO₃/10 mol% CdS (BSC/10CdS) and BiOCl/10 mol% CdS (BOC/10CdS) are shown in Fig. S1.† Overall, these results indicate that each catalyst was correctly prepared, and the absence of extra peaks demonstrates their high purity.

The FTIR spectra of the prepared photocatalysts are displayed in Fig. 3a. The band at approximately 3440 cm⁻¹ in all the spectra was assigned to the O–H stretching vibration of atmospheric water.^34,35 The band at 1633 cm⁻¹ was caused by the H–O–H bending vibration of adsorbed water.³⁶ For pure BSC, absorption bands were observed at 544 cm⁻¹ (Bi–O stretching vibration), 845 cm⁻¹ (non-planar bending vibration of the CO₃²⁻ groups), and 1391 cm⁻¹ (asymmetric vibration of the CO₃²⁻ groups).³⁷ The BOC spectrum showed a peak at 525 cm⁻¹, which was ascribed to Bi–O vibrations.³⁸ The CdS spectrum has peaks at 1036 and 1403 cm⁻¹ caused by the vibration of the Cd–S bonds.²⁵Fig. 3b shows the corresponding enlarged view of the 500–1500 nm region in Fig. 3a. The Bi–O stretching vibration peak of 10CBB was observed in the range of 525 and 544 cm⁻¹. This peak was probably caused by the merging of the infrared bands of BSC and BOC.³⁰ The peak at 845 cm⁻¹ was significantly weakened and the other two BSC peaks (at 1036 and 1391 cm⁻¹) were slightly shifted. These changes indicate that chemical reactions occurred between the three components of the composite material. This was confirmed by further characterization, as discussed below.

Fig. 3 (a) FTIR spectra of BSC, BOC, CdS, and 10CBB and (b) enlarged view of the 500–1500 nm region of the FTIR spectra.

The microscopic morphology of the prepared samples was examined by SEM. BSC consisted of plates with lengths of about 200–400 nm (Fig. 4a). BOC consisted of large pieces with lengths of about 1–3 μm (Fig. 4b). However, CdS exhibited an irregular granular block structure (Fig. 4c). The binary BSC/BOC photocatalysts (Fig. 4d) showed the presence of small BSC plates and large BOC plates. Stacking BSC and BOC together increased the contact area, providing more opportunities for the transfer and recombination of electrons. The micromorphology of 10CBB is presented in Fig. 4e. CdS granules were uniformly distributed on the surfaces of BSC and BOC, benefiting from the high degree of dispersion provided by the hydrothermal method. These CdS particles can serve as electron transfer sites, promoting carrier separation and enhancing photocatalytic activity. The elemental mapping from the SEM image (Fig. S2†) and EDS spectra (Fig. S3†) further revealed the uniform spatial distribution of the Bi, C, Cl, O, S, and Cd elements in this sample.

Fig. 4 SEM images of (a) BSC, (b) BOC, (c) CdS, (d) BSC/BOC, and (e) 10CBB.

To further explore the micromorphology and microstructure of the heterojunction photocatalyst, 10CBB was analysed by TEM and HRTEM. As presented in Fig. 5a and b, both BSC and BOC had irregular plate structures and nanosized CdS particles were attached to these nanosheets. This was in good agreement with the SEM images. Fig. 5c shows an HRTEM image of the rectangular area marked in Fig. 5b. The observed crystal plane spacings of 1.720, 3.586, and 3.720 nm corresponded to the (202) plane of BOC, the (100) face of CdS, and the (011) face of BSC, respectively. Therefore, HRTEM analysis reveals that the CdS nanoparticles were chemically bonded with the two Bi-based crystalline phases, leading to the formation of a heterostructure. In addition, the heterojunction structure of 10CBB is also supported by the elemental mapping from the TEM image (Fig. S4†). The closer contact between the three phases creates favourable conditions for electron transfer.

Fig. 5 (a) and (b) TEM and (c) HRTEM images of 10CBB.

The surface elemental valence states and binding energy of 10CBB were studied using XPS. The presence of Bi, C, Cl, O, S, and Cd was demonstrated using a survey spectrum (Fig. 6a). The S 2p binding energy is close to that of Bi 4f. Therefore, the spectrum shown in Fig. 6b had four fitted peaks. The peaks at 159.3 and 164.6 eV corresponded to the Bi 4f_7/2 and Bi 4f_5/2 signals of Bi³⁺, respectively. The peaks at 160.2 and 165.8 eV were ascribed to S 2p_3/2 and S 2p_1/2.^39–41 The Cl 2p spectrum (Fig. 6c) was deconvoluted into two peaks at 198.0 and 199.7 eV, which were ascribed to the Cl 2p_3/2 and Cl 2p_1/2 signals of BiOCl, respectively.⁴² As shown in Fig. 6d, the three C 1s peaks at 284.5, 285.8, and 289.0 eV were associated with adventitious carbon, C–O groups, and O–C=O groups, respectively.^43,44 The Cd 3d peaks at 405.2 and 411.9 eV were ascribed to the Cd 3d_5/2 and Cd 3d_3/2 signals of CdS (Fig. 6e).⁴⁵ In the O 1s spectrum (Fig. 6f), the peaks at 529.9 and 531.1 eV were ascribed to the Bi–O bonds and the O atoms of carbonate groups, respectively.⁴⁶ All the characteristic peaks were clearly distinguished, demonstrating the successful preparation of the ternary heterojunction photocatalyst.

Fig. 6 XPS survey spectrum (a) of 10CBB: (b) Bi 4f and S 2p, (c) Cl 2p, (d) C 1s, (e) Cd 3d, and (f) O 1s.

In summary, the XRD, FTIR, HRTEM, EDS, and XPS results confirmed the presence of CdS, Bi₂O₂CO₃, and BiOCl in the heterojunction.

3.2 Optical properties

The UV-vis diffuse reflectance spectra of the catalysts are shown in Fig. 7a. Both BSC and BOC had absorption edges at about 370 nm and a strong response in the UV region. However, the visible light responsiveness of BSC/BOC was not improved. In contrast, the ternary photocatalyst samples showed significantly enhanced absorption intensity in the visible light region. This was potentially due to the superior visible light absorption properties of CdS and the formation of the heterostructure improving the electron excitation efficiency of the ternary material.⁴¹ The band gaps of the pure components were obtained through Tauc's equation:⁴⁷

αhv = A(hv − Eg)^n/2 (1)

where A, α, v, h, Eg, and n denote the scaling factor, absorbance, incident light frequency, Planck constant, forbidden band width, and optical leap type, respectively. As shown in Fig. 7b, the band gaps of CdS, BSC, and BOC were 2.20, 3.30, and 3.36 eV, respectively. The VB and CB potentials of the semiconductor were determined by eqn (2) and (3):⁴⁸

E_VB = X − Ee + 0.5Eg (2)

E_CB = E_VB − Eg (3)

where Ee is the free energy under the hydrogen standard, E_CB is the conduction band potential, X is the electronegativity of the material, and E_VB is the valence band potential. The X values of CdS, BSC, and BOC were 5.05, 6.54, and 6.33 eV, respectively.^31,47–49 The E_VB values of CdS, BSC, and BOC were calculated to be 1.65, 3.69, and 3.51 eV, and their E_CB values were calculated to be −0.55, 0.39, and 0.15 eV, respectively.

Fig. 7 (a) UV-vis diffuse reflectance spectra of the as-obtained catalysts and (b) estimated bandgaps of CdS, BOC, and BSC.

3.3 Photocatalytic activity detection

The RhB degradation activity of the prepared photocatalysts is shown in Fig. 8a. Using blank RhB as a reference, it was observed that BSC and BOC had almost no degradation effect on RhB. This was because they could absorb only UV light. It should be noted that the RhB degradation efficiency of CdS was only about 33%, even though CdS had a good visible light response. This was most likely because the rapid recombination of carriers in CdS reduced its photocatalytic efficiency. Furthermore, as reported by Fan et al., small CdS particles can easily agglomerate, which reduces their specific surface area and active sites.⁵⁰ The RhB degradation rate of the BSC/BOC composite reached 56%, which was probably due to the formation of a single heterojunction in this composite material.⁵¹ The ternary photocatalysts showed a markedly improved degradation performance, and 10CBB had the best catalytic effect, with an RhB degradation efficiency of 97% in 90 min. This was because the excellent visible light response and double heterostructure of 10CBB allowed for the more efficient transfer of electrons. A low loading of CdS is not enough to provide more active sites to improve the photocatalytic activity, while a too high loading leads to inactivation of the active sites. Therefore, the photocatalytic performance of 5CBB and 20CBB is poorer than that of 10CBB. For comparison, the performance of the binary BSC/10CdS and BOC/10CdS systems is also provided in Fig. S5.† These binary photocatalysts showed a much worse performance than the ternary 10CBB catalyst. In addition, to demonstrate that the dye is degraded by photocatalytic degradation rather than adsorption driven dye removal, we subjected 10CBB to a longer (120 min) dark reaction, and the result is shown in Fig. S6.† The curve almost equilibrates after 30 min, indicating that adsorption and desorption reach equilibrium, and will not drive dye removal.

Fig. 8 (a) Degradation curves of RhB (pH = 5, 1 g L⁻¹ 10CBB), (b) corresponding kinetic curves, (c) UV-vis spectra of RhB at 400–700 nm for 10CBB at different light reaction times and (d) cyclic photocatalytic experiments of 10CBB.

The kinetic constant composite first-order dynamic equations of the photocatalysts are presented in Fig. 8b. The k values of BOC, BSC, CdS, BSC/BOC, 5CBB, 10CBB, and 20CBB were 0.0010, 0.0013, 0.0033, 0.0070, 0.0227, 0.0282, and 0.0091 min⁻¹, respectively. The catalytic activity of 10CBB can be attributed to its higher electron excitation efficiency, better separation efficiency, and stronger light absorption intensity. The UV-vis spectra of RhB on 10CBB at different light reaction times in the range of 400–700 nm are displayed in Fig. 8c. The peak at 554 nm significantly declined in intensity with time, indicating that RhB was successfully degraded. Degradation experiments were performed with the recovered 10CBB sample to explore its stability (Fig. 8d). The degradation rate of 10CBB was still approximately 83% after four repeated tests. The slight decline was caused by sample loss during the recovery process.⁵² XRD was used to evaluate 10CBB after the reaction, as shown in Fig. S7.† The diffraction pattern of 10CBB did not significantly change after four repeated tests, indicating the stability of this sample during the photocatalytic reaction. The RhB degradation performance of 10CBB was compared with those of recently reported photocatalysts. As shown in Table S1,† it can be found that 10CBB has lower energy consumption and higher degradation efficiency.

3.4 Effects of different factors on photocatalytic performance

3.4.1 Effects of reaction pH. The effect of different pH values (pH = 3, 5, 7, 9, 11) on the photodegradation of RhB by 10CBB was explored with the other conditions being the same, and the results are shown in Fig. 9a. At pH = 5, 7 and 9, 10CBB had high photocatalytic activity with degradation efficiencies of 97%, 92% and 94%, respectively. At pH = 3, the degradation rate was 83%, which may be due to the partial dissolution of the 10CBB composite photocatalyst under strongly acidic conditions, resulting in mass loss.⁵³ Under strongly alkaline conditions, RhB was barely degraded, which may be the result of a high concentration of OH⁻ and charge interactions, hindering the contact between the photocatalyst and dye.⁵⁴

Fig. 9 (a) 10CBB degradation curves of RhB at different pH values; (b) 10CBB with different dosage degradation curves of RhB.

3.4.2 Effects of catalyst dose. The amount of catalyst also affects the result of photodegradation. As shown in Fig. 9b, when the catalyst content increased from 0.025 g L⁻¹ to 1 g L⁻¹, the photocatalytic activity also increased. This was due to there being more active sites. However, when the catalyst content increased to 1.5 g L⁻¹, the degradation efficiency decreased, because the excessive photocatalyst blocked the absorption of visible light, so the carrier separation efficiency decreased.⁵⁵ Therefore, the selection of an appropriate amount of photocatalyst is more favourable to accelerating the photodegradation process.

3.5 N₂ adsorption–desorption isotherms

The larger the specific surface area of the material, the stronger the adsorption properties and the more active sites can be provided. The N₂ adsorption–desorption isotherms of BSC, BOC, BSC/BOC, and 10CBB are displayed in Fig. 10a, and these samples exhibit type IV isotherms.²⁷ The specific surface areas (Fig. 10b) of BOC, BSC, BSC/BOC, and 10CBB were 0.544, 1.325, 1.838 and 9.331 m² g⁻¹, respectively. The results indicated that after the three substances were mixed together, the specific surface area increased significantly. In other words, the adsorption capacity for RhB was greatly enhanced.

Fig. 10 (a) N₂ adsorption–desorption isotherms of BOC, BSC, BSC/BOC, and 10CBB; (b) BET surface area of the corresponding materials.

3.6 Electrochemical characterization

The transient photocurrent responses of the as-prepared catalysts are shown in Fig. 11a. The photocurrent density of 10CBB was significantly enhanced compared to those of the other catalysts. This may be related to the heterostructure of 10CBB, which sharply improved the separation of carriers. The EIS Nyquist plots of these materials are presented in Fig. 11b. It is well-known that a smaller arc radius implies lower charge migration resistance. 10CBB had the smallest radius, suggesting that faster electron migration occurred on the 10CBB surface.

Fig. 11 (a) Transient photocurrent response and (b) EIS Nyquist plots of BOC, BSC, CdS and 10CBB, (c) PL spectra of all catalysts, (d) time-resolved transient fluorescence spectra of BSC, BOC, BSC/BOC and 10CBB.

The recombination of free electrons and holes leads to fluorescence intensity, and a high fluorescence intensity indicates a high carrier recombination rate. As indicated in Fig. 11c, the ternary photocatalyst had a lower fluorescence intensity, indicating a higher carrier separation ability. This was due to the influence of the heterostructure and the visible light absorption properties of this catalyst. The carrier separation kinetics were further investigated by fluorescence lifetime measurements, as shown in Fig. 11d. The curves were fitted by the following equation:³⁰

I(t) = I₀e^−t/τ (4)

where I(t) refers to the luminous intensity at time t, τ is the decay lifetime, and I₀ is the original intensity. The fluorescence lifetimes of BSC, BOC, BSC/BOC, and 10CBB were 0.37, 0.79, 0.88, and 0.91 ns, respectively. These results demonstrated that 10CBB had superior carrier separation and migration efficiency compared with the other catalysts.

3.7 Photocatalytic mechanism

In order to investigate the process of the photodegradation of RhB, ESR tests were performed using DMPO technology. As displayed in Fig. 12a, no characteristic DMPO–·O₂⁻ or DMPO–·OH signals were obtained in the dark. In contrast, both signals were easily observed after 5 min and 10 min of visible light irradiation, and the signal intensity was enhanced with the extension of illumination time. This indicated that both superoxide radicals (·O₂⁻) and hydroxyl radicals (·OH) coexisted during the photocatalytic degradation process. In addition, we detected the presence of ·OH in the 10CBB photocatalyst using terephthalic acid (TA). These reactive oxygen species are captured by TA to produce highly fluorescent 2-hydroxyterephthalic acid. With the generation of ·OH, the fluorescence intensity will gradually increase.⁵⁶ The result (Fig. 12b) showed that the holes in 10CBB have sufficient potential to produce ·OH. Similarly, nitroblue tetrazolium (NBT) was used as a molecular probe to investigate the presence of ·O₂⁻. The reaction of NBT with ·O₂⁻ resulted in the formation of methoxy derivatives, which led to the decrease of UV absorption strength.⁵⁷ The UV-vis absorbance spectra of NBT aqueous solution containing the 10CBB photocatalyst are shown in Fig. 12c, which strongly indicated the high yield of ·O₂⁻ in the 10CBB photocatalyst.

Fig. 12 (a) ESR spectra of the DMPO–·O₂⁻ and DMPO–·OH adducts in 10CBB aqueous suspension under dark and illuminated conditions (5 min and 10 min); (b) PL spectra of TA aqueous suspension containing the 10CBB photocatalyst; (c) UV-vis absorbance spectra of NBT aqueous solution containing the 10CBB photocatalyst; (d) experiments on the capture of active species during RhB photodegradation.

Further, the trapping experiment of ·O₂⁻, ·OH, and h⁺ was also performed using benzoquinone (BQ), isopropyl alcohol (IPA), and disodium ethylenediaminetetraacetate (EDTA-2Na), respectively. As shown in Fig. 12d, the degradation efficiency was only 19% after the addition of EDTA-2Na, implying that h⁺ was the primary radical responsible for RhB degradation. Degradation rates of 41% and 50% were respectively achieved after adding BQ and IPA. These indicated that ·O₂⁻ and ·OH were also active species involved in the process of photodegradation.

Based on the above experiment and energy band structure, a reasonable mechanism of photodegradation was proposed. Prior to this, the conduction and valence band positions of BSC, BOC and CdS before contact were calculated according to eqn (1), (2) and (3) (Fig. 13a). After being in contact with each other to form a heterojunction, the energy levels of CdS, BOC, and BSC were moved in order to maintain the equilibrium of the Fermi energy level, and then the composite photocatalyst absorbed enough visible light energy, making the electrons excited from the VB to the CB.

Fig. 13 (a) Band structure before contact, (b) type II heterojunction mechanism, and (c) double Z-scheme heterojunction mechanism.

First, if it's a type II heterojunction system (Fig. 13b), electrons will transfer from the CB of CdS to the CBs of BOC and BSC. The CB potentials of BSC and BOC are more positive than that of O₂/·O₂⁻ (−0.33 eV vs. NHE), so the electrons are not able to reduce O₂. Similarly, h⁺ will eventually transfer to the VB of CdS. But the VB of CdS is more negative than that of H₂O/·OH (2.38 eV vs. NHE), so h⁺ cannot oxidize H₂O to produce ·OH.^58,59 Therefore, the traditional type II heterojunction mechanism is inconsistent with the ESR and radical capture experimental results reported in this work.

As a consequence, a more reasonable double Z-scheme heterostructure degradation process was also evaluated. As shown in Fig. 13c, the electrons on the CBs of BSC and BOC transfer to the VB of CdS. After the migration of electrons is completed, most of the electrons are located in the CB of CdS (<−0.33 eV), where they can reduce O₂ to produce ·O₂⁻. The holes occupy the VB of BSC and BOC (>2.38 eV), where they can oxidize H₂O to generate ·OH.^60,61 Thus, ·OH, ·O₂⁻ and h⁺ further degrade RhB to H₂O, CO₂, and other small molecules. The redox capacity of the double Z-scheme heterojunction mechanism is stronger than that of type II, which can effectively promote carrier separation.

4. Conclusions

In this study, we synthesized innovative ternary CdS/Bi₂O₂CO₃/BiOCl photocatalysts with a double Z-scheme heterojunction through a one-pot method. Compared with the single components, the ternary composite photocatalysts had a better adsorption performance, higher carrier separation efficiency, stronger visible light absorption and longer fluorescence lifetime. The photocatalytic activity test results showed that the CBB photocatalysts (0.1 g, pH = 5) have a superior degradation effect on RhB (20 mg mL⁻¹) under visible light. The high catalytic activity was still maintained after four cycles of testing. The reasons for the improved activity of the ternary photocatalysts include the narrowing of the forbidden band width and the light response range widening to the visible light area. And a double Z-scheme heterojunction is also formed, which increases the carrier separation efficiency and improves the electron migration path. We believe that this article will provide new ideas for the structural design of Bi-based ternary photocatalysts.

Author contributions

Yueyi Li: formal analysis, investigation, methodology, data curation, and writing – original draft. Yuehui Liu: formal analysis, investigation, methodology, and writing – original draft. Xuguang Liu: project administration and writing – review & editing. Xia Li: supervision, project administration, and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of Shandong Province (Approval No.: ZR2022ME054).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3re00407d

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