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Article

Three-Dimensional Bi-Enriched Bi2O3/Bi2MoO6 Z-Scheme Heterojunction: Augmented Photocatalytic Phenol Degradation

by
Congyu Cai
,
Shuwen Wang
,
Pingping Wan
,
Haoying Cai
,
Minhui Pan
and
Weiwei Wang
*
College of Life Science and Chemistry, Minnan Science and Technology College, Quanzhou 362332, China
*
Author to whom correspondence should be addressed.
Inorganics 2025, 13(7), 227; https://doi.org/10.3390/inorganics13070227
Submission received: 20 June 2025 / Revised: 3 July 2025 / Accepted: 4 July 2025 / Published: 6 July 2025
(This article belongs to the Special Issue Advances in Inorganic–Organic Composite Photocatalysts for Energy and Environmental Applications)
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Abstract

A three-dimensional Bi-enriched Bi2O3/Bi2MoO6 Z-scheme heterojunction photocatalyst was successfully synthesized via a facile one-step hydrothermal method for efficient phenol degradation under visible light. Structural and morphological characterizations (SEM, TEM, and XRD) confirmed the formation of a nanoflower-like architecture with a high specific surface area of 81.27 m2/g. Optical and electrochemical analyses revealed efficient charge separation and extended visible-light response. Under visible-light irradiation (λ > 420 nm), this heterojunction (Bi2O3:Bi2MoO6 = 3:7) demonstrated exceptional performance, degrading 97.06% of phenol (30 mg/L) within 60 min. XPS analysis confirmed the Z-scheme charge transfer mechanism: Photogenerated electrons in the conduction band of Bi2O3 (−0.59 eV) facilitated the generation of ·O2− radicals, while holes in the valence band of Bi2MoO6 (2.44 eV) predominantly produced ·OH radicals. This synergistic effect resulted in highly efficient mineralization and degradation of phenol.

Graphical Abstract

1. Introduction

Photocatalytic technology has attracted considerable attention as a sustainable approach for addressing environmental pollution and energy-related challenges, especially through the degradation and mineralization of persistent organic pollutants. However, its large-scale application is still hindered by the low efficiency of existing photocatalysts, mainly due to rapid charge recombination and limited light-harvesting ability. Therefore, developing high-performance photocatalysts with enhanced charge separation and extended visible-light absorption remains a critical objective for practical environmental applications [1,2]. Phenolic compounds in wastewater streams pose significant environmental challenges due to their persistent chemical nature and considerable ecotoxicity [3]. These characteristics render conventional treatment methods largely ineffective, necessitating the development of advanced remediation technologies. As such, the identification and optimization of efficient treatment approaches for phenolic contaminants has remained an active area of scientific investigation. Bismuth oxide–bismuth molybdate (Bi2MoO6) is a typical Bi(III)-based semiconductor with visible-light responsiveness. It exhibits excellent ionic conductivity, a narrow bandgap for visible-light activation, and notable environmental compatibility. Its distinctive layered crystalline architecture, coupled with remarkable photostability and cost-effective synthesis, has positioned it as an increasingly important candidate for environmental remediation applications, particularly in photocatalytic pollutant degradation [4]. However, the widespread application of Bi2MoO6-based photocatalysts faces substantial challenges due to inherent material limitations, including inefficient charge carrier separation, rapid electron–hole recombination, and inadequate visible-light absorption. To address these fundamental issues, researchers have focused on three principal modification approaches: surface modification with noble metals, strategic elemental doping, and the construction of heterojunctions [5,6,7]. The construction of heterojunctions serves as an effective strategy to enhance photocatalytic performance by simultaneously broadening the optical absorption spectrum and facilitating the separation of photogenerated charge carriers. This synergistic mechanism effectively suppresses electron–hole recombination, thereby significantly improving photocatalytic activity [8]. Notably, various bismuth-containing semiconductors share analogous layered architectures, facilitating the formation of heterojunctions between different bismuth-based photocatalysts [9]. However, heterojunction construction often suffers from interfacial lattice mismatch between constituent materials, which may impede efficient charge transfer across the interface. The bismuth-rich bismuth oxide–bismuth iodide (BiIO), a novel bismuth-based oxide with a layered crystal structure analogous to bismuth molybdate, addresses this limitation. Its elevated Bi and O content induces pronounced material anisotropy and significant orbital hybridization effects, which promote the formation of highly dispersed conduction and valence bands, thereby facilitating charge carrier transport and ultimately enhancing photocatalytic performance [10,11,12].
Above all these advantages, a three-dimensional novel Bi-enriched Bi2O3/Bi2MoO6 (short for BBO/BMO) Z-scheme [8,9] heterojunction photocatalyst was successfully synthesized via a facile one-step hydrothermal method. Through rational metal self-doping modification, the optimized BBO/BMO system exhibited exceptional interfacial compatibility, significantly improving structural stability while simultaneously enhancing charge separation and extending the optical absorption spectrum. Notably, the catalyst demonstrated efficient degradation of phenol in wastewater within 60 min, with its photocatalytic activity enhanced by two-fold compared to the unmodified material. The efficient photocatalytic process of BBO/BMO involves the following three steps. First, self-doped atoms optimize the internal electric field, enhancing the migration rate of photogenerated electrons and accelerating their transfer to the catalyst surface, where they trap O2 and reduce it to reactive ·O2− radicals. Second, the constructed Z-scheme heterojunction facilitates effective charge separation of photogenerated electron–hole pairs, promoting hole (h+) formation and improving catalytic efficiency. Third, the visible light response range is broadened, accelerating the formation of reactive oxygen species (ROS) such as ·O2− and ·OH, which effectively degrade and mineralize pollutants. All in all, this study provides new insights into designing self-doped photocatalysts, paving the way for their large-scale application in water remediation [13,14,15,16,17,18].

2. Experiment

2.1. Chemicals

Detailed materials were provided in S1 (Supporting Information).

2.2. Synthesis of Photocatalysts

2.2.1. Synthesis of Bi2MoO6 (BMO)

Bi(NO3)3·5H2O (1 mmol) and Na2MoO4·2H2O (0.5 mmol) were dissolved in 20 mL ethylene glycol. Subsequently, 40 mL of anhydrous ethanol was added to this mixture, followed by continuous stirring for 45 min to form Solution A. Solution A was then transferred into a 100 mL PTFE-lined stainless steel autoclave and subjected to hydrothermal treatment at 160 °C for 24 h. After natural cooling to room temperature, the resulting precipitate was collected by filtration and alternately washed three times with anhydrous ethanol and ultrapure water to remove residual impurities. Finally, the product was dried at 60 °C for 5 h to obtain the BMO sample.

2.2.2. Synthesis of Bi-Enriched Bi2O3 (BBO)

Bi(NO3)3·5H2O (1 mmol) was dissolved in 20 mL of ethylene glycol. Subsequently, 40 mL of anhydrous ethanol was added to this mixture, followed by continuous stirring for 45 min to form Solution B. Solution B was then transferred into a 100 mL PTFE-lined stainless steel autoclave and subjected to hydrothermal treatment at 160 °C for 24 h. After cooling naturally to room temperature, the resulting precipitate was collected by filtration and alternately washed three times with anhydrous ethanol and ultrapure water to remove residual impurities. The product was dried at 60 °C for 5 h to obtain the BBO sample.

2.2.3. Synthesis of Bi2O3/Bi2MoO6 (BBO/BMO) Heterojunction

The Bi2O3/Bi2MoO6 composites with controlled molar ratios (5:5, 4:6, 3:7, 2:8, and 1:9) were synthesized through precise stoichiometric adjustment of precursor Solutions A and B, shown in Figure 1. Each pair of Solutions A and B, adjusted to the desired molar ratios, was thoroughly mixed and stirred for 1 h, and then transferred into a 200 mL PTFE-lined stainless steel autoclave and subjected to hydrothermal reaction at 160 °C for 24 h. After natural cooling to room temperature, the resulting products were collected by filtration and washed three times with anhydrous ethanol and ultrapure water. Finally, the samples were dried at 60 °C for 5 h, yielding the BBO/BMO samples with the respective molar ratios.

2.3. Characterizations

Detailed materials were provided in S2 (Supporting Information).
In situ FTIR spectra synthesis: The dynamic evolution process was monitored using FTIR spectra (Nicolet iS10, Thermo Fisher Scientific (Waltham, MA, USA)) at different time points. Prior to measurement, the samples were purged with N2 for 20 min in the sample chamber to eliminate the adsorbent contaminants on the surface and achieve stabilization. After collecting the background spectrum at t = 0 min, which was subsequently subtracted from all measurements, the gas source was switched to a CO2:N2 mixture (2:8 v/v) with simultaneous light illumination (λ > 420 nm, provided by a 300 W Xe lamp equipped with a cut-off filter). Samples were collected at time points of 1 min, 4 min, 8 min, 12 min, 16 min, and 20 min.

2.4. Photocatalytic Activity Evaluation

2.4.1. Degradation Experiments

Detailed materials were provided in S3 (Supporting Information).

2.4.2. Active Species Trapping Experiments

To elucidate the roles of reactive species in the photodegradation of phenol, IPA (20 mmol/L), BQ (10 mmol/L), and EDTA-2Na (10 mmol/L) were employed as scavengers for hydroxyl radicals (·OH), superoxide radicals (·O2−), and holes (h+), respectively. These concentrations were optimized based on preliminary quenching experiments and established protocols to ensure effective suppression of the target species while minimizing interference with the catalytic process.

3. Results and Discussion

3.1. Morphology and Structural Characterization

3.1.1. SEM and TEM

The morphological and microstructural characteristics of the catalysts were systematically investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 2a–c, BMO exhibits a well-defined three-dimensional (3D) hollow nanoflower-like architecture with an average diameter of approximately 1.41 μm. In contrast, the BBO samples primarily consist of two-dimensional (2D) nanosheets with an average thickness of about 10–20 nm, hierarchically assembled into spherical microparticles with 3.23 μm in diameter [19]. Remarkably, the BBO/BMO composite demonstrates a distinct hierarchical structure, where ultrathin BBO nanosheets are uniformly distributed on the surface of the hollow spherical BMO framework (Figure 2c,d). These composite nanosheets appear more detailed and regularly structured compared to their pure BBO counterparts, clearly indicating the successful formation of an integrated heterojunction structure. High-resolution TEM (HRTEM) analysis of the BBO/BMO samples reveals two distinct lattice fringes with measurable interplanar spacings. As shown in Figure 2e, the observed lattice spacing of 0.315 nm matches well with the (131) plane of BMO, while the 0.320 nm spacing corresponds to the (111) plane of BBO. Furthermore, Figure 2f demonstrates an additional lattice spacing of 0.328 nm, which can be indexed to the (012) plane of metallic Bi nanoparticles.

3.1.2. XRD

X-ray diffraction (XRD) analysis was performed to characterize the crystal structures of BMO, BBO, and BBO/BMO samples, as illustrated in Figure 3a. The XRD pattern of the as-synthesized BMO sample displayed well-defined diffraction peaks at 2θ angles of 28.3°, 32.5°, 46.7°, and 55.6°, with a high intensity and a narrow full-width at half-maximum, thereby demonstrating its excellent crystallinity. These characteristic peaks were successfully indexed to (131), (200), (202), and (133) crystallographic planes of Bi2MoO6 (PDF#21-0102), confirming the phase purity of the synthesized material. Additionally, the XRD pattern of the BBO/BMO sample revealed two significant features. Firstly, the characteristic peak at 2θ = 28.3° exhibited a slight leftward shift, and the additional diffraction peaks emerged at 27.9°, 46.4°, and 55.1°, respectively, corresponding to the (111), (220), and (311) planes of Bi2O3 (PDF#27-0052). Secondly, distinct peaks appeared at 27.2°, 37.9°, and 39.6°, corresponding to the (012), (104), and (110) planes of metallic Bi (PDF#44-1246). The agreement between XRD and TEM analyses confirms the presence of metallic Bi0, which originates from the reduction of Bi3+ by ethylene glycol during hydrothermal synthesis. Compared to pure BMO, the diffraction peaks showed noticeable sharpening with a reduced full-width at half-maximum (FWHM), clearly demonstrating the successful construction of the BBO/BMO heterojunction [17,19].

3.1.3. N2 Adsorption–Desorption Isotherms

As is shown in Figure 3b, N2 adsorption–desorption isotherm analysis revealed that BMO, BBO, and BBO/BMO samples exhibited type IV isotherms with distinct hysteresis loops at relative pressures (P/P0 > 0.4), indicative of mesoporous nature. Specifically, the BMO sample displayed an H1-type hysteresis loop in the P/P0 range from 0.8 to 1.0, which is characteristic of capillary condensation occurring in the uniform cylindrical pores of its nanoflower-like spherical structure. The relatively steep adsorption branch suggests a narrow pore size distribution, predominantly cylindrical in shape. The Brunauer–Emmett–Teller (BET) specific surface area of BMO was determined to be 36.45 m2/g, with primary pore diameters ranging from 10 to 30 nm. The BBO sample exhibited a distinct H3-type hysteresis loop within the relative pressure (P/P0) range from 0.45 to 1.0, indicative of its slit-shaped pores formed by the stacking of plate-like particles. This unique pore structure is beneficial for the formation and transfer of photogenerated charge carriers. However, its specific surface area is smaller than that of BMO, measured at 27.51 m2/g, primarily attributed to its larger pore size distribution. Compared to pristine BMO and BBO, the BBO/BMO sample successfully integrates the advantageous features of both components, further confirming the effectiveness of their combination. The composite consists of nanoflower-like spherical BMO acting as a substrate, with finer and thinner BBO nanosheets uniformly dispersed between the petal-like structures. Despite having a similar pore size distribution, the composite significantly enhanced the specific surface area of 81.27 m2/g, which is 2.2 times that of BMO and 2.9 times that of BBO (Table 1). This structural optimization not only facilitates electron generation on the BMO surface but also promotes efficient charge transfer across the BBO layers, thereby providing a solid foundation for improved photocatalytic performance [2,3,7].

3.2. Surface Chemical States and Electron Distribution

The interfacial electronic structure of the catalyst was analyzed via XPS, as shown in Figure 4a. The survey spectrum confirms the co-existence of Bi, Mo, and O elements in the BBO/BMO composite. Notably, the Bi/Mo atomic ratio (9.0) is significantly higher than that of pristine BMO (6.2), providing compelling evidence for the formation of a heterojunction structure (Table S2). As shown in Figure 4b, the Bi 4f spectrum exhibits two characteristic spin–orbit doublets corresponding to Bi 4f7/2 and Bi 4f5/2, with subpeaks at 159.3 eV assigned to Bi0 and subpeaks at 164.6 eV assigned to Bi3+. Notably, a reverse valence transition of Bi occurs, where the Bi 4f7/2 peak in BBO/BMO (164.6 eV) shifts to a lower binding energy, indicating an increase in electron density around Bi atoms, making them more likely to exhibit reduction through electron acquisition. Moreover, the Bi0/Bi3+ ratio in BBO (1.4) is the highest compared to that in BMO (1.3) and BBO/BMO (1.3), which is calculated from the XPS peak areas, confirming that the BBO component in the heterojunction exhibits the highest degree of electron enrichment (Table S2). The Mo 3d spectrum (Figure 4c) reveals a positive binding energy shift of +0.4 eV for Mo6+ in BBO/BMO compared to BMO, indicating electron transfer toward Bi2O3 via Bi–O–Mo bridging bonds. This observation strongly supports the proposed Z-scheme mechanism [20,21].
As illustrated in Figure 4d, the characteristic peaks of O1s were found to be two sub-peaks for all three catalysts: a main peak at ~530.2 eV corresponding to lattice oxygen (OL), and a secondary peak at ~531.7 eV assigned to surface-adsorbed oxygen (OA). The OL component in the BBO/BMO catalyst accounts for 78.05%, an intermediate value between BBO (68.22%) and BMO (80.82%). These results confirm successful composite formation through Bi-O-Mo interfacial bonding [4,7,11]. Concurrently, the binding energy of the OA peak in BBO/BMO exhibits a positive shift of 0.3 eV, from 531.7 eV to 532.0 eV, which is not only beneficial for enhancing the catalytic performance and increasing surface reactive oxygen species, but also conducive to the formation of ·O2− and ·OH radicals. Furthermore, the enhancement of free radical reactions consequently accelerates pollutant degradation. EPR analysis (Figure 4e) confirms that the Bi-O-Mo interface facilitates oxygen vacancy generation and enhances radical activity, as evidenced by the substantially stronger symmetric signal at about g = 2.003 attributed to electrons trapped on oxygen vacancies, observed in the BBO/BMO composite compared to the pure BBO and BMO samples [18].

3.3. Photoelectrochemical and Photoluminescence Properties

The UV-vis diffuse reflectance spectra (DRS) of the synthesized BMO, BBO, and BBO/BMO samples are displayed in Figure 5a,b. All three samples show broad absorption bands within the 350–800 nm wavelength range. Particularly, the BBO/BMO composite exhibits a slight red shift in absorption edge relative to pristine BMO, suggesting that the constructed heterojunction significantly enhances the photoresponse range through effective band structure modulation. As is shown in Figure 5b, the band gap energies (Eg) of BMO, BBO, and BBO/BMO were calculated using the Kubelka–Munk transformation, yielding values of 2.81 eV, 2.32 eV, and 2.79 eV, respectively [22].
Furthermore, Mott–Schottky (M-S) analysis was performed to determine the flat-band potentials of the catalysts, as illustrated in Figure 5c. The positive slopes of the M-S plots for both BMO and BBO confirm their n-type semiconductor characteristics [23]. The flat-band potentials were determined to be ECB BMO= −0.47 (−0.37 eV vs. NHE) and ECB BBO= −0.69 (−0.59 eV vs. NHE). This upward shift in conduction band position can be attributed to Bi self-doping, which directly influences the photocatalytic performance of both materials. Based on the equation ECB =EVB − Eg [24,25], the valence band (EVB) positions of BMO and BBO were calculated to be approximately 2.44 eV (2.54 eV vs. NHE) and 1.73 eV (1.83 eV vs. NHE), respectively, where EVB represents the valence band potential, ECB the conduction band potential, and Eg the band gap energy. Accordingly, the energy band structures of BMO and BBO were constructed and presented in Figure 5e [26]. The composite catalyst clearly follows a Z-scheme heterojunction mechanism [8,9,27,28,29,30]. Under light irradiation, both components generate photogenerated electrons and holes, with the holes preferentially accumulating in the valence band (VB) of BBO. Since the redox potential of O2/·O2− is higher than that of electrons in the conduction band (CB) of BBO, the accumulated electrons at the CB are readily captured by O2 under visible light irradiation, subsequently reducing to form ·O2− reactive radicals. Meanwhile, the photoexcited electrons from the VB of BMO transfer to its CB and subsequently migrate to the VB of BBO, achieving effective charge carrier separation [31]. The accumulated holes in the VB of BMO react with H2O/OH− to generate highly reactive ·OH radicals. These reactive oxygen species ultimately contribute to the catalytic degradation process, which is consistent with the reactive oxygen species detection results shown in EPR. Additionally, electrochemical impedance spectroscopy (EIS) analysis, shown in Figure 5d, reveals that BBO/BMO exhibits a smaller semicircular radius compared to individual BMO and BBO, indicating enhanced charge separation efficiency and reduced charge transfer resistance in the composite. This facilitates the separation of photogenerated electron–hole pairs and consequently improves photocatalytic activity. Furthermore, photocurrent response measurements in Figure 5f demonstrate that BBO/BMO generates significantly stronger photocurrent signals than either BMO or BBO under visible light irradiation, confirming its superior charge separation efficiency, which is consistent with the EIS results.
The photoluminescence (PL) spectra of BBO, BMO, and BBO/BMO composite were further investigated (Figure 5g). It is found that the BBO/BMO composite exhibits significantly quenched PL intensity compared to its individual counterparts, demonstrating enhanced separation of electron–hole pairs and improved charge carrier mobility upon heterojunction formation [32]. This synergistic effect not only improves the photocatalytic efficiency but also facilitates superior charge transport kinetics, ultimately endowing the BBO/BMO heterostructure with remarkably enhanced catalytic activity.

3.4. Assessment of Catalytic Activity

Figure 6 presents the photocatalytic degradation performance of phenol in simulated wastewater under visible light irradiation using three different photocatalysts. Compared to the photodegradation efficiencies of BBO and BMO, the composite photocatalyst demonstrates significantly enhanced degradation efficiency, achieving nearly complete phenol removal within 60 min, as shown in Figure 6a. As illustrated in Figure 6b, the BBO/BMO composite with a mass ratio of 3:7 exhibited optimal photocatalytic performance, achieving 99% degradation efficiency. These results demonstrate that BMO modification with BBO effectively enhances photocatalytic stability and degradation performance [33]. Further experiments were conducted to evaluate the effects of catalyst dosage and initial phenol concentration. For the optimal 3:7 BBO/BMO composite, the optimal catalyst dosage was determined to be 0.1 g (Figure 6c), and the initial phenol concentration ranging from 10 to 30 mg/L, with 30 mg/L selected as the standard concentration for subsequent experiments (Figure 6d) (the highest degradation efficiency was observed at 60 min). To identify the dominant reactive species in the photocatalytic process, radical trapping experiments were conducted under the controlled conditions: 0.1 g of the catalyst, 30 mg/L of phenol, and a reaction time of 60 min. Specifically, three scavengers were employed to identify the dominant reactive species: p-benzoquinone (BQ, 10 mM) for superoxide radicals (·O2−), EDTA-2Na (10 mM) for holes (h+), and isopropanol (IPA, 20 mM) for hydroxyl radicals (·OH). As depicted in Figure 6e, the addition of each scavenger during the degradation process resulted in varying degrees of suppression of the degradation efficiency [34,35]. Compared to the control system (without scavengers), the degradation efficiency of BBO/BMO decreased dramatically from 97% to 36% upon the addition of BQ (·O2− scavenger), indicating that ·O2− played a predominant role in the photocatalytic degradation process. When EDTA-2Na (h+ scavenger) and IPA (·OH scavenger) were introduced, the degradation efficiency was, respectively, reduced to 44% and 53.74% [36,37,38], suggesting the participation of both holes and hydroxyl radicals in the reaction mechanism. Overall, these results conclusively demonstrate that ·O2−, h+, and ·OH serve as the primary active species responsible for phenol degradation.
Meanwhile, the comparative FTIR spectra of the fresh and spent catalysts revealed that the characteristic vibrational bands associated with the catalyst’s core functional groups remained largely unchanged. Specifically, the peaks at 559 cm−1, 725 cm−1, and 840 cm−1 (assigned to Mo-Bi-O stretching modes, Mo-O stretching vibrations, and Mo-Bi-Mo bridging structures) demonstrated excellent consistency in both position and relative intensity before and after the catalytic reaction. This preservation of key structural fingerprints strongly indicates that the primary chemical bonds and framework of the catalyst material remained stable under the applied reaction conditions. Therefore, these FTIR results provide direct spectroscopic evidence supporting the robust chemical stability of the catalyst throughout the reaction process (Figure 6f).

3.5. Exploration of the Catalytic Mechanism

As is shown in Figure 7, the in situ FTIR spectra of BMO and BBO/BMO both display a broad peak between 3700 cm−1 and 3200 cm−1, corresponding to surface hydroxyl (-OH) stretching vibrations of surface hydroxyl groups (-OH). This observation confirms the presence of surface hydroxyl groups that serve as active sites for the generation of ·OH radicals during photocatalytic processes. The abundant surface hydroxyl groups significantly enhance the oxidative degradation capability of the catalysts. Notably, quantitative analysis reveals that the peak area of BBO/BMO is 1.08 times (at 4 min) and 1.13 times (at 8 min) larger than that of BMO, indicating its superior oxidative performance. Compared to BMO, BBO/BMO showed stronger absorption intensity and faster enhancement rate in this region, especially during 4–8 min, indicating that the introduction of BBO significantly enhanced the hydroxyl group density and reactivity. The C-H vibrational peaks were observed between 2980 cm−1 and 2950 cm−1, and the C-H region represents the presence of adsorbed organic pollutant molecules [39]. BBO/BMO showed more pronounced and sharper C-H absorption peaks in this region, indicative of its superior capacity to adsorb organic molecules. BBO/BMO shows more obvious and sharper C-H absorption peaks in this region, indicating its stronger adsorption capacity for organic molecules, which are conducive to the enrichment of reactants on the catalytic surface. Notably, compared to pristine BMO (Figure 7a), the BBO/BMO composite (Figure 7b) exhibits a 15.9% intensity enhancement at 1048 cm−1 (Table S4), accompanied by a 4 cm−1 blue shift from 1044 cm−1 to 1048 cm−1. These results clearly demonstrate that the incorporation of BBO significantly enhances the oxygen vacancy signals in the BBO/BMO composite, which is favorable for improving catalytic activity and well consistent with the EPR measurements [40,41].
In addition, a weak absorption peak at 1640 cm−1, assigned to C=O stretching vibration, potentially indicates the phenol oxidation to quinone. Another weak absorption peak observed at 1710 cm−1 is attributed to the characteristic pattern of cis-butenedioic acid, suggesting that further oxidation of quinone may lead to ring opening of the phenolic structure and facilitate subsequent oxidative degradation. Two distinct absorption peaks at 1323 cm−1 (weak) and 1093 cm−1 (strong) represent acid mineralization, where the intense peak at 1093 cm−1 implies progressive acidification over time [42]. In summary, the abundant oxygen vacancies facilitate the generation of strong hydroxyl radicals, which promote the complete degradation of phenol through a series of oxidation processes, including conversion to quinone intermediates, ring-opening reactions, and subsequent mineralization to carboxylic acids, ultimately leading to the formation of carbon dioxide and water [43].

4. Conclusions

This study successfully constructed a Z-scheme BBO/BMO heterojunction photocatalyst via a facile one-step hydrothermal synthesis method. The catalyst exhibits photocatalytic activity across a broad visible-light spectrum and was characterized and tested for degrading organic pollutants in simulated water. The systematic characterization and organic pollutant degradation experiments have led to the following key findings: First of all, the BBO/BMO heterostructure achieves remarkable enhancement in photocatalytic phenol degradation efficiency through the synergistic modification of surface hydroxyl groups, heterojunction optimization, and oxygen vacancy introduction. These integrated modifications collectively promote efficient separation of photogenerated charge carriers while substantially increasing the production of reactive oxygen species, thereby significantly boosting the overall photocatalytic performance. Secondly, the positive slopes observed in the Mott–Schottky (M-S) plots confirm the n-type semiconductor characteristics of both BBO and BMO. The successful combination of a Z-scheme heterojunction, through their composite formation, significantly enhances the separation efficiency of photogenerated charge carriers, thereby effectively improving the overall photocatalytic performance. Thirdly, radical scavenging experiments conclusively identified superoxide radicals (·O2−), holes (h+), and hydroxyl radicals (·OH) as the predominant reactive species responsible for the highly efficient photocatalytic degradation of phenol, achieving 90% removal efficiency within 60 min under optimal conditions.
Last but not least, this study provides new insights into the rational design of high-efficiency photocatalysts, offering promising strategies for environmental remediation applications and expanding the potential scope of photocatalytic technologies.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics13070227/s1, Figure S1: XRD patterns of BMO, BBO and different BBO/BMO ratios; Table S1: Comparison of the kinetics of PN degradation in recently reported photo-Fenton processes; Table S2: The chemical state of Bi, Mo and O in the catalysts; Table S3: Relative Signal Amplitude Comparison; Table S4: In-situ FTIR Spectral Shifts (Δcm−1) and Reactivity Metrics: BMO vs. BBO/BMO. References [44,45,46,47,48,49,50,51,52,53,54,55,56,57,58] are cited in the supplementary materials.

Author Contributions

Conceptualization, Writing—original draft, Software, C.C.; Writing—review and editing, C.C., M.P. and W.W.; Writing—review, W.W.; performed experiments, S.W., P.W. and H.C. All authors have read and agreed to the published version of the manuscript.

Funding

Provincial Young and Middle-aged Teachers Education and Research Project of Fujian (Science and Technology) (JAT241231); Provincial First-class Undergraduate Major Construction Project for “Environmental Science” in 2021 (SJZY-2022-02); Provincial First-class Undergraduate Program of Virtual Simulation Experimental Teaching in 2020 (SJKC-2020-04); Provincial Education and Teaching Research of Fujian in 2024 (FBJY20240020).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 13 00227 g001
Figure 1. Schematic representation for fabrication of catalyst.
Figure 1. Schematic representation for fabrication of catalyst.
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Figure 2. SEM images of BMO (a,a1), BBO (b,b1), BBO/BMO (c,c1), HRTEM of BBO/BMO (d–f).
Figure 2. SEM images of BMO (a,a1), BBO (b,b1), BBO/BMO (c,c1), HRTEM of BBO/BMO (d–f).
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Figure 3. XRD patterns of the synthesized BMO, BBO, and BBO/BMO (a), N2 adsorption–desorption isotherms and PSD curves (inset) of BMO, BBO, and BBO/BMO (b).
Figure 3. XRD patterns of the synthesized BMO, BBO, and BBO/BMO (a), N2 adsorption–desorption isotherms and PSD curves (inset) of BMO, BBO, and BBO/BMO (b).
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Figure 4. XPS survey spectra of BMO, BBO, and BBO/BMO (a), Bi 4f (b), Mo 3d (c), O 1s (d), ESR spectra of BMO, BBO, and BBO/BMO (e).
Figure 4. XPS survey spectra of BMO, BBO, and BBO/BMO (a), Bi 4f (b), Mo 3d (c), O 1s (d), ESR spectra of BMO, BBO, and BBO/BMO (e).
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Figure 5. UV-vis DRS (a), (αhυ)2 versus hυ plots (b), Mott–Schottky curves of sample BMO, BBO, and BBO/BMO (c), electrochemical impedance spectra (d), energy band structure diagrams of the samples (e), band structure diagrams of BMO and BBO (f), PL spectra (g).
Figure 5. UV-vis DRS (a), (αhυ)2 versus hυ plots (b), Mott–Schottky curves of sample BMO, BBO, and BBO/BMO (c), electrochemical impedance spectra (d), energy band structure diagrams of the samples (e), band structure diagrams of BMO and BBO (f), PL spectra (g).
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Figure 6. Removal of phenol in different photocatalysts (a), mass ratio (b), catalyst dosage (c), initial phenol concentration (d), inhibitory effect (e), FT-IR spectra (f), and (C0 = 30 mg/L, V = 0.1 L, m = 0.1 g, T = 298 K, and t = 0–120 min).
Figure 6. Removal of phenol in different photocatalysts (a), mass ratio (b), catalyst dosage (c), initial phenol concentration (d), inhibitory effect (e), FT-IR spectra (f), and (C0 = 30 mg/L, V = 0.1 L, m = 0.1 g, T = 298 K, and t = 0–120 min).
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Figure 7. In situ FTIR spectra characterizing sample BMO (a) and BBO/BMO (b).
Figure 7. In situ FTIR spectra characterizing sample BMO (a) and BBO/BMO (b).
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Table 1. BET surface area and pore size of BMO, BBO, and BBO/BMO.
Table 1. BET surface area and pore size of BMO, BBO, and BBO/BMO.
SamplesSBET
m2/g		VP
cm3/g		dP
nm
BMO36.450.12613.84
BBO27.510.11216.35
BBO/BMO81.270.21110.36
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Cai, C.; Wang, S.; Wan, P.; Cai, H.; Pan, M.; Wang, W. Three-Dimensional Bi-Enriched Bi2O3/Bi2MoO6 Z-Scheme Heterojunction: Augmented Photocatalytic Phenol Degradation. Inorganics 2025, 13, 227. https://doi.org/10.3390/inorganics13070227

AMA Style

Cai C, Wang S, Wan P, Cai H, Pan M, Wang W. Three-Dimensional Bi-Enriched Bi2O3/Bi2MoO6 Z-Scheme Heterojunction: Augmented Photocatalytic Phenol Degradation. Inorganics. 2025; 13(7):227. https://doi.org/10.3390/inorganics13070227

Chicago/Turabian Style

Cai, Congyu, Shuwen Wang, Pingping Wan, Haoying Cai, Minhui Pan, and Weiwei Wang. 2025. "Three-Dimensional Bi-Enriched Bi2O3/Bi2MoO6 Z-Scheme Heterojunction: Augmented Photocatalytic Phenol Degradation" Inorganics 13, no. 7: 227. https://doi.org/10.3390/inorganics13070227

APA Style

Cai, C., Wang, S., Wan, P., Cai, H., Pan, M., & Wang, W. (2025). Three-Dimensional Bi-Enriched Bi2O3/Bi2MoO6 Z-Scheme Heterojunction: Augmented Photocatalytic Phenol Degradation. Inorganics, 13(7), 227. https://doi.org/10.3390/inorganics13070227

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