Two-Dimensional Lamellar Stacked Bi₂O₃/CeO₂ Type-II Heterojunctions Promote Carrier Separation to Enhance Ciprofloxacin Oxidation

Abstract

The development of efficient and stable photocatalysts is critical for addressing water pollution challenges caused by persistent organic contaminants. However, single-component photocatalysts often suffer from rapid photogenerated carrier recombination and limited visible-light absorption. In this study, a two-dimensional lamellar stacked Bi₂O₃/CeO₂ type-II heterojunction photocatalyst (BC) was successfully synthesized in situ by a topological transformation strategy induced by high-temperature oxidation of monolithic Bi. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses confirmed the uniform distribution of Bi₂O₃ nanosheets on CeO₂ surfaces, forming an intimate interfacial contact that enhances charge separation and transfer efficiency. Photoluminescence (PL) spectroscopy, UV–visible diffuse reflectance spectroscopy (DRS), and electrochemical characterization revealed extended visible-light absorption (up to 550 nm) and accelerated electron migration in the heterojunction. Under simulated sunlight, the optimized BOC (3:1) composite exhibited a ciprofloxacin (CIP) degradation rate constant 2.30 and 5.63 times higher than pure Bi₂O₃ and CeO₂, respectively. Theoretical calculations validated the type-II band alignment with conduction and valence band offsets of 0.07 eV and 0.17 eV, which facilitated efficient spatial separation of photogenerated carriers. This work provides a rational strategy for designing heterojunction photocatalysts and advancing their application in water purification.

1. Introduction

In the context of continuous societal development, water pollution has emerged as a pressing environmental issue on a global scale, particularly concerning toxic, highly stable, and bioaccumulative complex organic compounds [1,2,3]. To tackle this challenge, there is an urgent need to develop efficient and environmentally friendly nanotechnologies for water remediation. Among various approaches, the oxidation process of reactive species in semiconductor photocatalytic materials stands out as an effective method for degrading pollutants due to its environmental friendliness and no secondary pollution [4,5]. However, photocatalysts often face challenges in practical applications, such as the easy recombination of photogenerated carriers, low quantum efficiency, and poor stability [6,7,8].

Among various catalysts, CeO₂ as a rare earth oxide has gained widespread application in multiple fields due to its non-toxicity, abundant O vacancies, and unique 4f electron structure [9]. Notably, the distinctive electronic structure of CeO₂ enables the reversible conversion between the oxidized state Ce⁴⁺ and the reduced state Ce³⁺ [10,11]. This characteristic endows CeO₂ with excellent redox properties and oxygen storage capacity, facilitating the enhancement of photocatalytic reactions. However, CeO₂ faces inherent limitations as a single-component photocatalyst, such as limited absorption in the visible-light range beyond 400 nm and the rapid recombination of photogenerated electron–hole pairs [12,13,14], which significantly limits its potential in photocatalytic applications. To overcome these issues, various strategies have been employed to enhance the photocatalytic efficiency of CeO₂, including morphology control, metal and non-metal doping, and heterostructure design. For instance, Li et al. prepared nanosheet CeO₂ with different grain sizes using a wet-chemical method, and PL spectra analysis revealed that smaller grain sizes (8.2 nm) exhibited stronger photocatalytic activity due to higher concentrations of O vacancies, which favored enhanced light absorption and charge transfer [15]. Lai et al. fabricated Fe-doped porous CeO₂ using a combustion method, and the introduction of Fe³⁺ reduced the bandgap energy of CeO₂ [16]. Moreover, heterojunction designs have been widely adopted due to their ability to effectively separate photogenerated charge carriers [17,18]. For example, Chen et al. constructed a three-dimensional flower-like Ag₆Si₂O₇/CeO₂ p-n heterojunction using a precipitation method, which effectively enhanced the material’s visible-light harvesting capability [19]. Similarly, Wen et al. reported the efficient degradation and mineralization of EFA molecules under visible light by an Ag₂O/CeO₂ p-n heterojunction [20]. These studies demonstrate that constructing heterojunctions based on CeO₂ is an effective strategy to enhance photocatalytic performance. Bi₂O₃, with a narrow bandgap of 2.8 eV, has the potential to generate superoxide radicals and hydroxyl radicals in photocatalytic processes, making it an excellent semiconductor photocatalyst [21,22,23]. The presence of the lone pair of electrons in Bi 6s² can induce internal polarization within the material, effectively suppressing the recombination of photogenerated electron–hole pairs and promoting their separation and transfer [24,25]. However, Bi₂O₃ still faces the challenge of rapid carrier recombination [26,27]. According to previous studies, the highly stable CeO₂, with its unique f and d electron orbital structure, can be coupled with Bi₂O₃ to achieve efficient interfacial charge transfer [28]. Moreover, given that the band alignment of Bi₂O₃ and CeO₂ is essentially staggered [22], the construction of a type-II heterojunction-based photocatalyst is considered an ideal strategy for improving its photocatalytic performance.

This study innovatively proposed a two-dimensional heterointerface construction strategy based on a topological transformation mechanism. The Bi₂O₃/CeO₂ (BOC) heterojunction photocatalyst with a 2D layered stacking structure was successfully constructed by using 2D layered CeO₂ nanosheets as the structural template and combining them with high-temperature calcination to induce the in situ oxidation of monolithic Bi to Bi₂O₃ on its surface. Experimental results demonstrated that the BOC (3:1) composite exhibited optimal photocatalytic degradation efficiency for fluoroquinolone antibiotics (CIP) and methylene blue (MB) under both solar- and visible-light irradiation. The performance enhancement was attributed to the unique advantages of the in situ growth strategy: the formation of an intimate heterointerface between Bi₂O₃ and CeO₂ established an efficient electron transport channel, significantly improving carrier migration rates. These findings confirmed that the in situ growth strategy provided a novel pathway for interface engineering in heterojunction photocatalysts.

2. Materials and Methods

2.1. Preparation of Photocatalysts

Bi₂O₃ nanoparticles: First, 3 mmol of Bi (NO₃)₃·5H₂O (MACKLIN, Shanghai, China) was dissolved in 20 mL of ethylene glycol and stirred continuously for 30 min at room temperature. The resulting solution was then transferred to an autoclave, where the reaction was carried out under hydrothermal conditions at 180 °C for 12 h. After the reaction, the product was filtered to yield a grey–black powder Bi. This powder was then placed in a muffle furnace, heated to 550 °C at a rate of 5 °C/min, and maintained at this temperature for 2 h. The final product obtained was a light yellow Bi₂O₃ powder.

CeO₂ nanosheets: An amount of 2.78 g of Ce (NO₃)₃·6H₂O (MACKLIN, Shanghai, China) was dissolved in 100 mL of deionized water to form a clear solution. To this solution, 1.5 g of NaHCO₃ was added, and the mixture was stirred for 30 min. The solution was then aged at 30 °C in a water bath for 24 h. After aging, the mixture was filtered and dried to obtain a white precipitate. This precipitate was placed in a muffle furnace and calcined at 500 °C for 4 h to produce yellow CeO₂ nanosheets.

Bi₂O₃/CeO₂ composite materials: The Bi and CeO₂ nanosheets were weighed in a specific mass ratio and ground together in a mortar to form a homogeneous mixture. This mixture was then placed in a muffle furnace, heated at a rate of 5 °C/min to 550 °C, and held at this temperature for 2 h. After the heat treatment, the composite material was removed and labeled as BOC.

Physical mixing Bi₂O₃/CeO₂ (p-BOC): Bi₂O₃ (non-metallic Bi) and CeO₂ were physically ground and mixed, maintaining a 3:1 mass ratio.

2.2. Characterization of Photocatalysts

In this experiment, X-ray diffraction (XRD) was used to determine the lattice structure and chemical composition of the materials. A copper target was used during testing, with a scanning range set as 5°~90° and a scanning speed of 10°/min. The surface morphology and structure of the samples were observed using scanning electron microscopy (SEM), while energy-dispersive X-ray spectroscopy (EDX) was employed to obtain the distribution of Bi, O, and Ce elements on the surface of the BOC composite material. The morphology and microstructure of the samples were observed using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM; JED-2300, Tokyo, Japan). To measure the distribution and chemical states of surface elements in the samples, X-ray photoelectron spectroscopy (XPS) analysis was performed: (1) Energy Calibration: All spectra were initially referenced to the C 1s peak (284.8 eV) of adventitious carbon on the sample surface. (2) Charge Compensation: A uniform energy offset of +0.32 eV was applied using Avantage (V-6.9.0) software to correct for surface charging effects. This adjustment was consistently implemented across all spectra (Bi 4f, Ce 3d, and O 1s) within the same sample batch. Ultraviolet–visible diffuse reflectance spectroscopy (UV-vis DRS, PE lambda 750, PerkinElmer, USA) was utilized to measure the light absorption range of each sample and calculate its bandgap. Photoelectrochemical tests were performed using a CHI760E (Shanghai Chenhua, Shanghai, China) electrochemical workstation with a three-electrode system (Ag/AgCl reference electrode, platinum wire counter electrode, and glassy carbon working electrode) in a quartz electrolytic cell with 0.05 M Na₂SO₄ electrolyte. The sample preparation procedure consisted of ultrasonically mixing 5 mg of powder with 1 mL of 5 wt% Nafion solution for 30 min, and 50 μL was taken and coated on a 1 × 1 cm² FTO conductive glass and dried at 60 °C for 2 h. The sample was prepared in the same manner as the sample. The test light source was a 300 W xenon lamp with an initial potential of 0 eV for photocurrent response testing and 1.5 eV for electrochemical impedance testing. The photoluminescence (PL) spectrum was used to evaluate the photoluminescent properties of the photocatalytic materials, with an excitation wavelength of approximately 320 nm. Electron spin resonance (ESR) technology was employed to detect the radicals playing a major role in the photocatalytic degradation process, providing further understanding of the photocatalytic activity mechanism.

2.3. Photocatalytic Activity Testing

The photocatalytic performance of the catalyst in degrading CIP and MB under a xenon lamp (CEL-HXUV300, Beijing China Education Au-light, Beijing, China) with a power of 300 W was tested. Initially, 20 mg of the catalyst was uniformly dispersed in a beaker containing 50 mL of pollution solution. Under dark conditions, the mixture was stirred for 40 min to achieve adsorption–desorption equilibrium. Subsequently, the xenon lamp light source was turned on. Every 20 min, 3 mL of the solution was withdrawn using a 5 mL syringe and filtered through a 0.45 µm microporous filter, repeating this process a total of 6 times. The concentration of CIP in the samples was measured using a UV–visible spectrophotometer (UV-3600, Shimadzu, Tokyo, Japan) at a detection wavelength of 270 nm. The photocatalytic degradation rate of ciprofloxacin can be calculated using the following formula, in accordance with the requirements of an academic paper:

k = −ln (C_t/C₀)/t

(1)

where C₀ is the initial concentration of ciprofloxacin, and C_t is the concentration of ciprofloxacin at different time points.

3. Results and Discussion

Characterizations

The microstructures and elemental distributions of Bi₂O₃, CeO₂, and BOC composites with different ratios were systematically investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). As shown in Figure 1a, Bi₂O₃ exhibited irregular block-like or granular morphology with micron-sized dimensions, while the CeO₂ sample in Figure 1b displayed a smooth sheet-like structure. When elemental Bi was in situ oxidized through high-temperature calcination on the surface of CeO₂ nanosheets to form Bi₂O₃, the surface roughness of BOC composites significantly increased. The Bi₂O₃ particles fragmented and adhered to the CeO₂ surface (Figure 1c and Figure S1), forming numerous irregular two-dimensional layered sheet stacking structures. This structural evolution likely originated from the topotactic oxidation process of elemental Bi at elevated temperatures: Bi was oxidized to Bi₂O₃ on the surface of CeO₂ at high temperatures. These observations suggested the potential formation of heterostructures between Bi₂O₃ and CeO₂ [29]. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images, presented in Figure 1d–g, further revealed the crystal structure of the BOC composite. In the HRTEM images, two distinct sets of lattice fringes were clearly observed, indicating the highly crystalline nature of the BOC composite. The lattice spacing of 0.302 nm corresponded to the (222) plane of Bi₂O₃, while the lattice spacing of 0.293 nm matched the (311) plane of CeO₂. Additionally, the interface between Bi₂O₃ and CeO₂ was clearly visible in the images, further confirming the successful construction of the BOC heterojunction. Figure 1h–j further demonstrated the distribution of Ce, Bi, and O elements in the BOC (3:1) sample, verifying the compositional consistency of the composite material.

X-ray diffraction (XRD) patterns were employed as a critical tool for determining the chemical composition and crystal structure of the samples. As shown in Figure 2a and Figure S2, the composite material exhibits a strong diffraction peak near 26.90° according to the standard reference card PDF#04-001-7782, which is attributed to the monoclinic crystal system structure of Bi₂O₃, corresponding to the (120) crystal plane. It is noteworthy that the original precursor material cited in PDF#04-007-9968 observed the characteristic peak of the (012) crystal plane belonging to the monoclinic state Bi at 27.2°. However, after high-temperature calcination, no peaks belonging to the monoclinic state Bi appeared in the composite. Meanwhile, the new diffraction peak of Bi₂O₃ (120) crystal face is located at 26.90°, which is only 0.3° away from the characteristic peak of monoclinic bismuth, indicating that the monoclinic bismuth has undergone lattice remodeling and in situ oxidation at high temperatures. In addition, weaker diffraction peaks were observed at 25.75°, 33.25° and 46.31°, corresponding to (002), (111), (200), and (221) crystal faces, respectively. Also, several distinct characteristic peaks of CeO₂ were found in the XRD spectra at 28.57°, 33.11°, 47.53°, and 56.40°, respectively [30]. These peaks belong to the (111), (200), (220), and (311) crystal planes of the CeO₂ cubic crystal system, respectively, as determined from the standard reference card PDF#04-002-2968, and no other stray peaks were found. No additional diffraction peaks were observed in the composites as in previous similar composite systems [31,32], further confirming the successful fusion of the two components, both Bi₂O₃ and CeO₂. These findings were consistent with the SEM and TEM analyses, demonstrating the effective formation of a heterojunction between Bi₂O₃ and CeO₂ in the composite.

The XPS full-spectrum analysis (Figure S2, Supporting Information) revealed strong peaks for Bi 4f, O 1s, and Ce 3d in the BOC material, confirming the presence of Bi, O, and Ce elements. In the high-resolution XPS spectrum of Bi 4f (Figure 2b), characteristic peaks of Bi 4f 7/2 and Bi 4f 5/2 were observed at 158.6 eV and 163.9 eV [33,34], respectively, in pure Bi₂O₃, corresponding to Bi³⁺. In composites, these peaks are shifted to higher binding energies (blue shift) to 158.9 eV and 164.2 eV, respectively. This shift was attributed to the interaction and combination of Bi₂O₃ and CeO₂ at high temperatures, where electrons from Bi₂O₃ were transferred to CeO₂, resulting in a reduction in the electron cloud density around the Bi atoms [35,36]. For the O 1s spectrum (Figure 2c), the peaks at 529.19 eV, 530.79 eV, and 532.01 eV in the BOC (3:1) composite were assigned to Ce-O, Bi-O, and hydroxyl oxygen (-OH) formed by lattice oxygen, respectively. As shown in Figure 2d, the deconvolution of the Ce 3d fine spectrum revealed eight peaks, which were divided into two spin–orbit split doublets, Ce 3d 5/2 and Ce 3d 3/2, labeled as A1–A4 and B1–B4, respectively. Among these, A1, A3, A4, B1, B3, and B4 were attributed to the photoexcitation of Ce (IV), while A2 and B2 were assigned to the photoionization of Ce (III) [37,38]. The presence of these peaks indicated that mixed valence states of Ce³⁺ and Ce4+ coexisted in both CeO₂ and the BOC composite materials. The presence of Ce³⁺ ions induced lattice distortion in CeO₂ and generated oxygen vacancies, thereby enhancing catalytic activity [39]. Notably, after the introduction of Bi₂O₃, the binding energy of Ce 3d in the composite materials exhibited a slight decrease, suggesting an increase in the electron cloud density around the Ce atoms. In contrast to the behavior observed in the Bi 4f results, the XPS analysis further indicated strong electronic coupling between Bi₂O₃ and CeO₂. This interaction effectively suppressed the recombination of charge carriers during the photocatalytic process, thereby improving the overall photocatalytic efficiency [40].

The light absorption properties of Bi₂O₃, CeO₂, and BOC composites with varying ratios were investigated using ultraviolet–visible diffuse reflectance spectroscopy (UV-Vis DRS). As illustrated in Figure 2e, pure Bi₂O₃ and pure CeO₂ exhibited strong light absorption primarily in the ultraviolet region. In contrast, the BOC (3:1) heterojunction composite demonstrated a reduced absorption intensity in the ultraviolet region, while its absorption edge was redshifted to 550 nm, accompanied by significantly enhanced absorption in the visible-light region. Based on the SEM analysis results, the particle size of Bi₂O₃ in the BOC composite was reduced, and partial adherence to the surface of CeO₂ nanosheets was observed, resulting in tighter interfacial contact. This facilitated the formation of the heterojunction [41], which promoted the red shift of the absorption edge through an interfacial charge transfer mechanism, thereby extending the material’s light absorption range to a broader wavelength region [22]. The bandgap energies of Bi₂O₃ and CeO₂ were determined using the Tauc plot method [31,42,43] (Figure S3a), yielding values of 2.84 eV and 2.94 eV, respectively. These results indicated that both materials could be excited under the visible-light range. Furthermore, from the VB-XPS spectra (Figure S3b), the valence band potentials of Bi₂O₃ and CeO₂ were measured to be 1.46 eV and 2.43 eV, respectively. Using the formula E_CB = E_VB − E_g, their conduction band values were calculated to be −1.38 eV and −0.47 eV, respectively.

To further investigate the generation and transport efficiency of photogenerated charge carriers in the samples, photoelectric current response tests were conducted. A higher photocurrent density typically indicated a greater separation efficiency of photogenerated electron–hole pairs. When the light source was turned on, all samples generated photocurrent (as shown in Figure 3a), demonstrating that these materials possessed good photoresponsive properties. Notably, the photocurrent density of the BOC (3:1) composite was significantly higher than that of the individual components and other composite ratios, maintaining a high photocurrent response throughout the test. This observation suggested that the interfacial contact area between the two phases in the heterostructure played a crucial role in the effective migration of charge carriers [22]. Nyquist plots (Figure 3b) were employed to characterize the charge transfer impedance of the materials. The results revealed that the radius of the impedance arc for the BOC composite was significantly smaller than that of pure CeO₂, indicating a substantial reduction in charge transfer impedance, which implied a higher charge transfer efficiency for the BOC composite. Furthermore, the BOC (3:1) composite exhibited a smaller impedance arc radius, attributed to the moderate Bi₂O₃ content and the advantages of its lamellar structure. Photoluminescence (PL) spectroscopy was used to study the recombination behavior of electron–hole pairs, where a lower peak intensity indicated higher charge capture efficiency and slower recombination rates [44]. As shown in Figure 3c, the PL intensity of the BOC (3:1) composites was significantly reduced compared to that of pure CeO₂ and Bi₂O₃, suggesting the existence of effective charge transfer between CeO₂ and Bi₂O₃. This improvement in the efficiency of electron–hole pair separation further supported the formation of a heterogeneous structure at the interface between the two metal oxides through chemical bonding [45]. Additionally, the high specific surface area and sheet-like morphology of CeO₂ facilitated better integration with Bi₂O₃, further enhancing the transport efficiency of charge carriers [46,47].

In this study, a comprehensive evaluation of the photocatalytic performance of Bi₂O₃, CeO₂, and their composite BOC (3:1) against ciprofloxacin (CIP) and methylene blue (MB) pollutants was conducted under simulated sunlight and visible-light conditions. As illustrated in Figure 3d, the concentration of CIP decreased gradually over time. Compared to CeO₂, the degradation efficiency of the BOC composites initially increased and then decreased with the rising content of Bi₂O₃. The BOC (3:1) composite exhibited superior photocatalytic activity compared to other compositions, individual components, and the physically mixed sample (p-BOC). As shown in Figure 3e, analysis using the pseudo-first-order kinetic model revealed that the degradation rate constant of BOC (3:1) was 0.02002 min⁻¹ (Table S1), which was 2.30 times higher than that of Bi₂O₃ and 5.63 times higher than that of CeO₂. This indicated that integrating CeO₂ with Bi₂O₃ significantly enhanced the photocatalytic activity of CeO₂. As shown in Figure 3f, cycling experiments demonstrated that the degradation rate of CIP by BOC (3:1) remained consistently above 80% during the first four cycles, indicating good stability in the photocatalytic degradation process. XRD and DRS spectra showed no significant changes before and after catalyst use (Figure S4), confirming the practical feasibility of BOC (3:1). Combined with the earlier DRS analyses, the composite exhibited a strong photoresponse in the visible-light range. Combined with the earlier DRS analyses, the composite exhibited a strong photoresponse in the visible-light range. Consequently, degradation experiments were performed under visible light for 30 mg/L methylene blue (MB). As depicted in Figure S5, the BOC (3:1) composite material achieved 80% degradation of MB within 120 min, with a degradation kinetic constant of 0.01309 min⁻¹ (Table S2). These findings further confirmed that the BOC (3:1) heterojunction material exhibited enhanced utilization of visible-light energy.

To elucidate the photocatalytic degradation mechanism of CIP, the primary active species generated during the photocatalytic process of the BOC (3:1) composite were identified. Specific scavengers were employed to target different active species: TEMPO for superoxide radicals (•O₂⁻), IPA for hydroxyl radicals (•OH), and AO for holes (h⁺). The scavenger experiment results revealed that •O₂⁻ and h⁺ played significant roles in the degradation process (Figure 3g). Furthermore, electron spin resonance (ESR) spectra of the BOC (3:1) materials indicated that the ESR signals of DMPO–•O₂⁻ were weak in the dark (Figure 3h,i). However, under UV–visible irradiation, distinct signal peaks corresponding to •O₂⁻ were observed. Additionally, a reduction in the peaks of the TEMPO-h⁺ spectra (Figure S6) indicated the presence of h⁺. Based on the above characterization and experimental results, it was speculated that a type-II charge transfer mechanism was formed in the BOC (3:1) heterojunction.

The mechanism for the enhanced photocatalytic performance under UV–visible-light irradiation is illustrated in Figure 4 [48,49]. A type-II heterojunction was formed between Bi₂O₃ and CeO₂ at high temperatures. PL spectroscopic analysis shows that the fluorescence intensity of BOC (3:1) is significantly reduced compared with that of pure Bi₂O₃ and CeO₂, suggesting that the oxygen vacancies are mainly distributed on the surface of the material rather than in the bulk phase, and thus are more inclined to promote carrier separation rather than triggering the composite of the bulk phase [50,51,52]. Driven by the built-in electric field in the BOC (3:1) type-II heterojunction, the photogenerated electrons migrate from the conduction band of Bi₂O₃ (CB, −1. 38 eV) to the conduction band of CeO₂ (CB, −0.47 eV) and are captured by O₂ adsorbed by the oxygen vacancies on the surface to generate a strongly oxidized •O₂⁻ (E(O₂/•O₂⁻) = −0.33 eV). At the same time, the hole is enriched in the valence band (VB, 1.46 eV) of Bi₂O₃, whose potential is not sufficient to directly oxidize H₂O to produce -OH (E(H₂O/-OH) = 2.38 eV), and the decrease in fluorescence intensity confirms that the surface oxygen vacancies selectively trap the hole, greatly delaying the electron–hole complex. This synergistic mechanism can effectively separate the electron–hole pair and thus promote the photocatalytic performance of the BOC (3:1) complexes.

To further verify the above photocatalytic enhancement mechanism, a detailed calculation of the band alignment of the BOC (3:1) composite was conducted. As illustrated in Figure S7, the energy differences between the core levels and the valence bands of CeO₂ and Bi₂O₃ were determined to be 880.35 eV and 157.22 eV, respectively. In the BOC (3:1) heterojunction, the energy difference between the core levels of Bi 4f and Ce 3d was measured to be 723.3 eV. Table S3 summarizes the core levels, valence band edge values, and bandgap energies of CeO₂, Bi₂O₃, and the BOC (3:1) sample [20,46].

Additionally, the conduction band offset (ΔEc) and valence band offset (ΔEv) of the BOC (3:1) heterojunction were calculated to be −0.07 eV and −0.17 eV, respectively. These values indicated that the conduction band (CB) and valence band (VB) of CeO₂ were positioned lower than those of Bi₂O₃. Based on these calculated results, the band structure diagram of the BOC (3:1) composite, as illustrated in Figure 5, was constructed. This diagram was consistent with the formation mechanism of a type-II heterojunction. Typically, ΔEc and ΔEv were determined using the following equations [53]:

$$\begin{array}{c}\mathsf{\Delta }{\mathrm{E}}_{\mathrm{V}}={({\mathrm{E}}_{\mathrm{C}\mathrm{e},3\mathrm{d}}-{\mathrm{E}}_{\mathrm{V},\mathrm{C}\mathrm{e}{\mathrm{O}}_2})}^{\mathrm{C}\mathrm{e}{\mathrm{O}}_2}-{({\mathrm{E}}_{\mathrm{B}\mathrm{i},4\mathrm{f}}-{\mathrm{E}}_{\mathrm{V},{\mathrm{B}\mathrm{i}}_2{\mathrm{O}}_3})}^{{\mathrm{B}\mathrm{i}}_2{\mathrm{O}}_3}-\mathsf{\Delta }{\mathrm{E}}_{\mathrm{C}\mathrm{L}}\\ \mathsf{\Delta }{\mathrm{E}}_{\mathrm{C}\mathrm{L}}={({\mathrm{E}}_{\mathrm{C}\mathrm{e},3\mathrm{d}}-{\mathrm{E}}_{\mathrm{B}\mathrm{i},4\mathrm{f}})}^{\mathrm{h}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}}\\ {\mathsf{\Delta }\mathrm{E}}_{\mathrm{C}}={\mathrm{E}}_{\mathrm{g}}^{\mathrm{C}\mathrm{e}{\mathrm{O}}_2}-{\mathrm{E}}_{\mathrm{g}}^{{{\mathrm{B}\mathrm{i}}_2\mathrm{O}}_3}+\mathsf{\Delta }{\mathrm{E}}_{\mathrm{V}}\end{array}$$

(2)

4. Conclusions

In conclusion, a BOC (3:1) heterojunction photocatalyst was successfully synthesized through a simple high-temperature in situ synthesis method. TEM and DRS characterizations indicated that a robust interfacial interaction was established between CeO₂ and Bi₂O₃. The reduction in Bi₂O₃ particle size resulted in a notable redshift in the absorption edge of the BOC (3:1) heterojunction, extending its light absorption range beyond 550 nm into the visible region. Furthermore, XPS analysis verified the successful formation of a type-II heterojunction, characterized by a conduction band offset of 0.07 eV and a valence band offset of 0.17 eV. Photoelectrochemical and EPR measurements further revealed that this heterostructure not only significantly enhanced visible-light absorption but also markedly improved the separation efficiency of photogenerated electron–hole pairs, thereby contributing to exceptional photocatalytic activity in the degradation of CIP and MB. This study provided a simple and efficient approach for constructing type-II heterojunction photocatalysts, offering novel strategies and insights for the degradation of complex pollutants in aqueous systems.