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Article

Dual Defect-Engineered BiVO4 Nanosheets for Efficient Peroxymonosulfate Activation

by
Jiabao Wu
1,
Meiyu Xu
2,
Zhenzi Li
2,
Mingxia Li
1,* and
Wei Zhou
2,*
1
School of Chemistry and Materials Science, Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, China
2
Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China
*
Authors to whom correspondence should be addressed.
Nanomaterials 2025, 15(5), 373; https://doi.org/10.3390/nano15050373
Submission received: 7 February 2025 / Revised: 21 February 2025 / Accepted: 27 February 2025 / Published: 28 February 2025
(This article belongs to the Special Issue Surface Microstructure Regulation of Low-Dimensional Nanomaterials Photocatalysts for Energy and Environmental Applications)
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Abstract

Defects and heteroatom doping are two refined microstructural factors that significantly affect the performance of photocatalytic materials. Coupling defect and doping engineering is a powerful approach for designing efficient photocatalysts. In this research, we successfully construct dual defect-engineered BiVO4 nanosheets (BVO-N-OV) by introducing N doping and oxygen vacancies through ammonium oxalate-assisted thermal treatment of BiVO4 nanosheets. Due to the combined enhancement of band structure and surface properties from N doping and oxygen vacancies, the obtained BVO-N-OV nanosheets demonstrate improved visible light absorption, effective charge transfer efficiency, and increased active sites. As a result, the constructed BVO-N-OV/PMS system demonstrates significantly enhanced ciprofloxacin (CIP) removal performance under visible light illumination. The highest rate constant for CIP degradation over BVO-N-OV/PMS system is 7.9, 1.9, and 6.6 times greater than pristine BiVO4 (BVO), oxygen vacancy-enriched BiVO4 (BVO-OV), and N-doped BiVO4 (BVO-N), respectively. Even in a broad pH range (3.0–11.0) with various anions, the BVO-N-OV/PMS/Vis system still demonstrates stable and excellent CIP removal performance. This study seeks to provide valuable insights into the interaction between defect and doping engineering in photocatalytic activation of PMS, thereby proposing new strategies for designing effective photocatalyst/PMS systems for wastewater treatment.

1. Introduction

Antibiotics and other persistent, refractory organic pollutants are present in various water environments, posing a serious threat to ecosystems and human health [1,2]. Therefore, it is urgent to explore a green and efficient strategy for the removal of pollutants from water. The peroxymonosulfate-based advanced oxidation process (PMS-AOPs) can generate various highly oxidative reactive oxygen species (ROS) to break down organic contaminants, thus making it a promising technology for organic wastewater treatment [3,4,5]. However, the O-O bond in PMS has a bond length of 1.453 Å with a bond energy of 377 kJ/mol, suggesting the activation of PMS is challenging [4,6]. Among the various PMS activation strategies, photocatalysis has attracted significant interest because of its benefits such as energy conservation, sustainability, and environmentally friendly, making it a hot topic in current research [7]. However, most of the developed photocatalysts face issues like low solar light absorption efficiency, severe charge recombination, and insufficient reactive active sites, leading to inefficient PMS activation processes and a significant gap in many practical requirements [8]. Therefore, it is of great significance to conduct in-depth research on the influence of structure on the performance of photocatalytic materials and to develop efficient and stable photocatalysts for PMS activation.
The catalytic efficiency of photocatalytic materials is closely related to numerous structural factors. Defects and heterogeneous atom doping are two key microscopic structural factors that affect the performance of photocatalytic materials. Through precise calculations and simulations, density functional theory (DFT) modeling can reasonably design operations such as nitrogen doping and defect formation, providing important theoretical support for experiments, and has been validated in numerous studies in recent years [9,10]. On one hand, the presence of defects is a common phenomenon in material synthesis of photocatalytic nanomaterials. Defects can effectively regulate the performance of photocatalysts. For example, oxygen vacancy defects, which are commonly found in metal oxides, can optimize conductivity and band structure, thereby significantly enhancing photon absorption, exciton generation, and carrier separation processes. More importantly, oxygen vacancy defects act as highly active reaction sites, promoting the adsorption and activation of substrate molecules [11,12,13]. Especially for PMS activation reactions, the highly symmetric ion distribution on both sides of the oxygen vacancy is beneficial for the lateral adsorption of the two oxygen atoms in the O-O bond of the PMS molecule onto the catalyst. In addition, the abundant electrons confined by the oxygen vacancy can accelerate the interfacial electron transfer between the PMS molecules and the catalyst, thus promoting PMS activation [14,15,16]. Therefore, further research on the impact of defects on photocatalytic PMS activation will be beneficial for constructing efficient and stable photocatalytic PMS activation systems. Moreover, impurity energy levels will be generated within the semiconductor’s band gap due to the introduction of heteroatoms, thereby narrowing the band gap and broadening light absorption. Also, heteroatom doping leads to an uneven distribution of charges, thereby creating an internal electric field that facilitates the migration and separation of photo-generated charge carriers [17,18]. For PMS activation reactions, heteroatom doping will create numerous asymmetric sites, thus reducing the energy barrier for PMS adsorption and activation on the photocatalyst surface and accelerating the breaking of the O-O bond in PMS and the production of reactive oxygen species (ROS) [19,20]. Both defect engineering and doping engineering can regulate photocatalytic PMS activation reactions. Actually, during the semiconductor doping process, the introduction of dopants is often accompanied by the generation of defects [21,22,23]. In other words, defects and heteroatoms may interact with each other during the photocatalytic PMS activation process. Studies have shown that coupling heterogeneous atoms and vacancy defects is an effective strategy to improve photocatalytic performance. For example, Zhang et al. [24] developed a Nitrogen-doped BiOCl atomic layers photocatalyst, homogeneously doped N and co-engineered OV reconstructed the electronic structure of BiOCl, thus kinetically facilitating CO2-to-CO reduction. However, whether coupling defects and heteroatoms can promote the PMS activation for pollutants degradation remains to be studied. Systematically studying the combined enhancement of defect and doping engineering on photocatalytic PMS activation performance will promote the understanding of the essence of photocatalysis and the development of efficient photocatalytic materials.
Two-dimensional (2D) layered nanomaterials have been extensively investigated in photocatalysis due to the large specific surface area and the short bulk carrier migration distance [25,26,27,28,29,30,31,32,33,34]. More importantly, compared to the corresponding bulk materials, the surface atoms of 2D ultrathin nanosheets have lower escape energy, making them more easily replaced by heteroatoms or escape from the lattice [35]. Therefore, 2D ultrathin nanosheets can serve as an ideal structural model for defect engineering and doping engineering.
In this study, layered BiVO4 is employed as a model material to validate this concept. By performing ammonium oxalate-assisted post-heat treatment on BiVO4 nanosheets, nitrogen doping and oxygen vacancy dual defects were created. The structure, photoelectrochemical properties, and photocatalytic performance of the resulting catalysts were characterized and evaluated in detail. Experimental results show that dual defects synergistically optimize the band structure, enhance light absorption, and promote the migration and separation of photo-generated carriers. In addition, the introduction of oxygen vacancies and nitrogen doping optimized the surface properties, thereby providing abundant sites for the adsorption and activation of PMS molecules. Consequently, compared to the pristine BiVO4 (BVO), oxygen-deficient BiVO4 (BVO-OV), and N-doped BiVO4 (BVO-N), the dual defect-engineered BiVO4 nanosheets (BVO-N-OV) exhibited significantly enhanced PMS activation performance under visible light irradiation. This work is anticipated to offer valuable insights into the coupling mechanism of defect engineering and doping engineering in photocatalytic PMS activation, and to propose new strategies to enhance the photocatalytic efficiency of materials through microstructure regulation.

2. Experimental Section

2.1. Materials

All materials used in this work are commercially available and used without any treatment. The purchased chemicals and the corresponding providers are listed as follows: Bismuth nitrate pentahydrate (Bi(NO3)3⋅5H2O 99%, Aladdin, Shanghai, China), Ammonium metavanadate (NH4VO3 99%, Sinopharm, Beijing, China), Sodium dodecylbenzenesulfonate (C18H29NaO3S 95%, Aladdin, Shanghai, China), Oxalic acid (C2H2O4 99%, Aladdin, Shanghai, China), Ammonium oxalate monohydrate (C2H8N2O4⋅H2O AR, Sinopharm, Beijing, China), Sodium hydroxide (NaOH 99%, Sinopharm, Beijing, China), Nitric Acid (HNO3 AR, Kermel, Tianjin, China), Potassium hydrogen monnopersulfate (KHSO5⋅0.5KHSO4⋅0.5K2SO4 99%, Aladdin, Shanghai, China), Ciprofloxacin (C17H18FN3O3 98%, Macklin, Shanghai, China), Ethanol (C2H5OH 99%, Sinopharm, Beijing, China).

2.2. Synthesis

Preparation of BiVO4: The flake-like BiVO4 was synthesized using a hydrothermal method. 0.57 mmol of sodium dodecylbenzenesulfonate (SDBS) and 2.0 mmol of Bi(NO3)3·5H2O were dissolved in 10 mL of 4M HNO3 aqueous solution and stirred for 1 h. 2.0 mmol of NH4VO3 were dispersed in 10 mL of 2 M NaOH aqueous solution, Then, the above solution was added to the Bi(NO3)3·5H2O solution under stirring for 0.5 h. Next, 2 M NaOH were dropwisely added into the above solution to adjust the pH of the mixture to 6–7 under stirring for 0.5 h. Then, the mixed solution was transferred to a 100 mL stainless steel autoclave (PTFE-lined) and hydrothermal treated at 200 °C for 2 h. After cooling to room temperature, the products were washed with deionized water and ethanol and dried at 80 °C for 8 h.
Preparation of BiVO4-OV: Completely mixed 100 mg of the original BiVO4 nanosheets with 150 mg of oxalic acid. The above mixture was placed in an Ar atmosphere and heated it to 280 °C at a rate of 2 °C/min, maintained for 2 h. Subsequently, the samples were washed with anhydrous ethanol to eliminate any remaining impurities. Lastly, the samples were vacuum-dried at 60 °C for 8 h until completely dry.
Preparation of BiVO4-N: 100 mg of the original BiVO4 nanosheets were heated in an NH3 atmosphere at a rate of 2 °C/min to 280 °C, and maintained for 2 h.
Preparation of BiVO4-N-OV: Completely mixed 100 mg of the original BiVO4 nanosheets with 170 mg of ammonium oxalate monohydrate. The mixture was placed in a muffle furnace and heated to 280 °C at a rate of 2 °C/min, then maintained at 280 °C for 1.5 h, 2 h, and 2.5 h, respectively. Subsequently, the samples were washed with anhydrous ethanol to eliminate any remaining impurities. Lastly, the samples were vacuum-dried at 60 °C for 8 h until completely dry.

3. Results and Discussion

Scheme 1 depicts the preparation method of the dual defect-engineered BiVO4 nanosheet photocatalyst. Initially, BiVO4 nanosheets were prepared through a hydrothermal process. Subsequently, the dual defect-engineered BiVO4 nanosheets were obtained by annealing the BiVO4 nanosheets with ammonium oxalate assistance. For comparison, N-doped BiVO4 nanosheets were prepared by annealing the BiVO4 nanosheets in an NH3 atmosphere. Meanwhile, oxygen vacancy-rich BiVO4 nanosheets were synthesized by oxalate-assisted annealing of BiVO4 nanosheets in an inert atmosphere. The X-ray diffraction (XRD) patterns of the synthesized BVO-N-OV, BVO, BVO-N, and BVO-OV samples well matched with monoclinic BiVO4 (JCPDS: 14-0688) (Figure 1a) [36]. In the XRD patterns of the obtained materials, no additional diffraction peaks other than those of BiVO4 were observed, indicating the high purity of the synthesized samples. In addition, as the amount of N doping increases, the XRD diffraction peak corresponding to the (121) plane at 28.9° gradually shifted (Figure 1b), indicating that N atoms were successfully integrated into the BiVO4 lattice or substituted O atoms [37]. Scanning electron microscope (SEM) images revealed that the pristine BiVO4 displayed a thin, smooth-surfaced nanosheet morphology (Figure 1c). After introducing N doping and oxygen vacancy defects, the morphology and dimension of the BiVO4 nanosheets remained largely unchanged (Figure 1d-f). In addition, transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) images (Figure 1g,h) further validated the nanosheet morphology of the dual defect-engineered BiVO4. The lattice fringe spacing of 0.255 nm aligns with the (002) plane of monoclinic BiVO4. The lattice spacing value of the (002) crystal plane of the dual defect-engineered BiVO4 nanosheets was similar to that of the pristine BiVO4, likely due to the similar radii of N atoms (0.75 Å) and O atoms (0.73 Å). In addition, as shown in Figure 1i–l and Figure S1, EDX element mapping confirmed N incorporation and indicated that N, O, Bi, and V elements were uniformly spread across both BVO-N and BVO-N-OV.
X-ray photoelectron spectroscopy (XPS) was further employed to examine the impact of N doping and oxygen vacancy defects on the chemical composition and surface chemical states of the BiVO4 nanosheets. The O 1s XPS spectra of the samples (Figure 2a) displayed two peaks around 529.7 and 531.9 eV for the BVO sample, attributed to lattice oxygen atoms (OL) and surface-adsorbed oxygen species (OA), respectively. In contrast, the O 1s XPS spectra of the BVO-OV, BVO-N, and BVO-N-OV samples showed a new peak at approximately 530.7 eV, which could be attributed to defect-state oxygen. The result proved the presence of oxygen defects in the samples [38,39]. Moreover, the O 1s XPS peak at 530.7 eV for the BVO-N-OV sample exhibited a larger peak area compared to the BVO-OV and BVO-N samples, indicating that the BVO-N-OV sample had a higher concentration of oxygen defects. This also suggested a strong relationship between N doping and O defects. Notably, after the introduction of oxygen defects, the area of the peak corresponding to OA at approximately 531.9 eV significantly increased. This indicates that the creation of oxygen defect sites facilitates the adsorption of oxygen-containing species such as O2, promoting the generation of highly oxidative ROS. The N 1s spectra of the BVO-N and BVO-N-OV samples (Figure 2b) presented two peaks at binding energies of approximately 398.4 and 399.9 eV, corresponding to lattice N atoms and chemically adsorbed N atoms, respectively [40]. This result further confirmed that N atoms were effectively integrated into the BiVO4 nanosheet lattice through ammonium oxalate-assisted thermal treatment and NH3 atmosphere heat treatment. The Bi 4f XPS spectra of all samples (Figure 2c) showed two peaks corresponding to the Bi 4f7/2 and Bi 4f5/2 signals of Bi3+ in BiVO4. The Bi 4f peaks for BVO located at approximately 159.1 and 164.3 eV. By contrast, the Bi 4f XPS peaks for the BVO-OV, BVO-N, and BVO-N-OV samples shifted slightly to higher binding energies. This shift could be due to lattice distortion resulting from N doping and oxygen defects [21,41]. Similarly, the V 2p XPS spectra of all samples (Figure 2d) showed two peaks at approximately 516.8 and 524.1 eV, which were ascribed to the V 2p1/2 and V 2p3/2 signals of BiVO4, respectively. Furthermore, the V 2p XPS peaks for the BVO-OV, BVO-N, and BVO-N-OV samples also shifted to higher binding energies than the BVO sample. The slight increase in the binding energies of the Bi 4f and V 2p peaks for the BVO-OV, BVO-N, and BVO-N-OV samples can be attributed to a decrease in electron cloud density, which is caused by the effects of N doping and oxygen defects [41]. The presence and generation of defects in the materials were further confirmed through room-temperature Electron Paramagnetic Resonance (EPR) analysis. As shown in Figure 2e, all the samples exhibited an EPR signal at g = 2.003, confirming oxygen vacancy defects in the samples [42]. The EPR signals of the BVO-OV, BVO-N, and BVO-N-OV samples were significantly stronger than that of the BVO sample, indicating that the ammonium oxalate-assisted thermal treatment and NH3 atmosphere heat treatment processes could construct oxygen vacancy defects in the BiVO4 nanosheets, thereby increasing the concentration of oxygen vacancy defects. Moreover, the BET specific surface area of the samples was tested using N2 adsorption-desorption isotherms. As shown in Figure 2f, the BET specific surface areas of the BVO, BVO-OV, BVO-N, and BVO-N-OV samples were 1.15, 4.55, 2.54, and 6.84 m2/g, respectively. This suggested that the introduction of N doping and oxygen vacancies could significantly enhance the specific surface area. The dual defect-engineered BVO-N-OV sample exhibited the largest specific surface area among them. More surface active sites can be exposed, thus promoting the adsorption and activation of PMS molecules on the BiVO4 nanosheets.
Photocatalytic degradation of CIP was used as a model reaction to explore the impact of N doping and oxygen vacancy defects on the photocatalytic activation of PMS by BiVO4 nanosheets. As shown in Figure 3a, all the samples demonstrated minimal photocatalytic CIP degradation performance without PMS. After adding a certain amount of PMS, the photocatalytic performance of all the samples improved significantly. This is because photocatalytic activation of PMS generates more highly oxidative ROS. It was evident that the photocatalytic performance of the dual defect-engineered BVO-N-OV sample was far superior to that of the BVO-OV, BVO-N, and BVO samples. First-order kinetic fitting was conducted for the PMS activation and CIP degradation process under visible light, as presented in Figure 3b. It was evident that the BVO-N-OV sample displayed the best photocatalytic PMS activation performance, with an apparent rate constant for CIP degradation as high as 0.08846 min−1 (Table S1), which was 7.9, 1.9, and 6.6 times higher than those of BVO, BVO-OV, and BVO-N, respectively. The above results indicated that N doping and oxygen vacancy defects exhibited a significant synergistic effect in promoting photocatalytic PMS activation. The coupling of doping engineering and defect engineering showed great potential for the design of efficient photocatalyst systems. In addition, the impact of the thermal treatment process on the photocatalytic PMS activation efficiency of the BVO-N-OV sample was further investigated. Figure 3c illustrated that the BVO-N-OV sample exhibited the highest photocatalytic activity when the ammonium oxalate-assisted thermal treatment time was 2 h. This might be ascribed to the optimal N doping levels and oxygen vacancy defects. Figure 3d showed that by increasing the PMS amount from 0 mg to 60 mg, under visible light irradiation within 30 min, the CIP removal rate increased from 4.0% to 91.0%. This could be explained by the fact that adding more PMS allowed for more efficient utilization of active sites, generating more SO4•−, •OH, and other ROS, thus enhancing the degradation of CIP. However, when the amount of PMS was further increased to 80 mg, the CIP degradation was slightly suppressed. This could be attributed to the catalyst’s limited active sites and the radical quenching effect from the excess PMS [43,44]. To evaluate the potential of the obtained dual defect-engineered BiVO4 nanosheets for treating real antibiotic wastewater, tests were conducted under various pH conditions, as shown in Figure 3e. At pH = 7.0, the BVO-N-OV sample showed the highest CIP degradation performance. When the pH was adjusted from 7.0 to 3.0 or 11.0, the CIP removal rate decreased from 91.3% to 83.8% and 87.9%, respectively. This could be due to strong acidic conditions potentially damaging the active sites on the catalyst surface, while H+ ions could also quench SO4•− and •OH, thereby hindering CIP degradation. On the other hand, strong alkaline conditions significantly increased the negative charge on the catalyst surface, hindering PMS adsorption and thus limiting CIP degradation [45,46,47]. Although both acidic and alkaline conditions hindered PMS activation, the CIP removal rate still reached over 83.8% within 30 min. This indicates that the prepared BVO-N-OV photocatalyst can efficiently activate PMS over a wide pH range, making it more promising for practical applications than the traditional Fenton reaction. In real antibiotic wastewater, various inorganic ions are typically present. To further investigate the adaptability of the BVO-N-OV photocatalyst to practical application environments, the CIP removal performance was tested with different anions. As shown in Figure 3f, the addition of CO32−, Cl−, NO3−, SO42−, HCO3−, and other anions had little impact on the photocatalytic removal efficiency of CIP. This further validated the excellent adaptability of the obtained catalyst to complex real-world environments. Furthermore, the stability and reusability of the BVO-N-OV photocatalyst were assessed through cycling tests, XRD and SEM characterization. After five consecutive cycles, the CIP removal rate only showed a slight decrease and still reached 90% (Figure 3g). Meanwhile, after five cycles of reaction, the XRD pattern of the BVO-N-OV photocatalyst showed no significant changes (Figure S2a), this suggested that its crystal phase structure remained stable throughout the reaction. In addition, the SEM images (Figure S2b) showed that after five cycles of reaction, the microscopic morphology of the BVO-N-OV photocatalyst remained sheet-like, with no significant changes compared to its original state. This further verified the catalyst’s strong structural stability after repeated use. All of the above results confirmed that the constructed dual defect-engineered BVO-N-OV photocatalyst exhibited excellent stability. Satisfyingly, the photocatalytic degradation performance of BVO-N-OV for CIP was superior to most of the reported Catalyst/PMS/Vis systems (Figure 3h), further confirming the key role of the synergistic effect of N doping and oxygen vacancy defects in promoting PMS activation and highlighting the significance of our work.
To explain the excellent performance of the BVO-N-OV photocatalyst, a series of electrochemical characterizations were conducted. As shown in the UV-Vis diffuse reflectance spectra (DRS) of the prepared samples (Figure 4a), the absorption edge of the BVO sample was approximately 500 nm. After introducing N doping or oxygen vacancies, its visible light absorption in the 500–800 nm range was significantly enhanced. Unexpectedly, the light absorption of the BVO-N-OV sample in the 500–800 nm range was weaker than that of BVO-OV, possibly because N atoms filled some of the oxygen vacancies. Furthermore, with the extension of the ammonium oxalate-assisted thermal treatment time, the visible light absorption capacity of the resulting dual defect-engineered BiVO4 nanosheets gradually increased (Figure 4b), indicating that the catalyst’s light absorption performance could be regulated and tuned by adjusting the preparation conditions. The significantly enhanced visible light absorption capacity could be one of the reasons behind the BVO-N-OV photocatalyst’s superior performance. The carrier separation and transfer behaviors of the obtained catalysts were examined through transient photocurrent response, electrochemical impedance spectroscopy (EIS), photoluminescence (PL) spectra, and time-resolved photoluminescence (TRPL) spectroscopy. Figure 4c presented the transient photocurrent responses of all the samples. Relative to the BVO sample, the photocurrent density of the BVO-OV, BVO-N, and BVO-N-OV samples was significantly enhanced, and the BVO-N-OV photocatalyst exhibited the highest photocurrent density. This result indicated that the introduction of N doping or oxygen vacancy defects could effectively improve the photo-generated carrier separation, and the BVO-N-OV photocatalyst demonstrated the highest carrier separation efficiency [48]. According to the EIS curves of the samples (Figure 4d), it was observed that the Nyquist arc radius of the BVO-OV, BVO-N, and BVO-N-OV samples were much smaller than the BVO sample, and the BVO-N-OV sample exhibited the smallest Nyquist arc radius. This suggested that the interface charge transfer resistance of the BVO-OV, BVO-N, and BVO-N-OV samples was smaller than that of the BVO sample, and the introduction of dual defects was more favorable for photo-generated carrier transfer than N doping or oxygen vacancy defects alone [49]. In addition, the PL intensity of the BVO-N-OV sample was much lower than the BVO, BVO-OV, and BVO-N samples (Figure 4e), further showing that dual defects effectively restrain the recombination of photo-generated carriers [50]. Furthermore, the photo-generated carrier lifetime was examined by TRPL spectroscopy, and the results are presented in Figure 4f and Table S2. The average carrier lifetimes (τave) of the BVO, BVO-OV, BVO-N, and BVO-N-OV samples were 8.56, 5.84, 8.02, and 2.18 ns, respectively. The sharp decay of τave indicated that the introduction of N doping and oxygen vacancy defects improved the carrier transfer efficiency of the catalyst [51,52]. All the aforementioned characterization results suggested that N doping and oxygen vacancy defects synergistically enhanced photo-generated carrier separation and migration, resulting in the excellent photocatalytic performance of the BVO-N-OV catalyst.
The band structure determines the electronic and optical properties of a material, and is a key factor influencing its photocatalytic activity and selectivity. Thus, the band gaps (Eg) of all the samples were calculated from the Tauc plots based on the Kubelka-Munk function (Figure 4g) [53]. The calculated Eg values of the BVO, BVO-OV, BVO-N, and BVO-N-OV samples were 2.42, 2.31, 2.35, and 2.32 eV, respectively. This result showed that the introduction of N doping and oxygen vacancy defects narrowed the band gap, thereby extending the material’s light absorption range. The energy level structure of the obtained photocatalysts was further determined using the VB-XPS spectrum. As shown in Figure 4h, the valence band (VB) potentials of the BVO, BVO-OV, BVO-N, and BVO-N-OV samples were 2.48, 1.98, 2.23, and 1.62 eV, respectively. Accordingly, the conduction band (CB) potentials of the BVO, BVO-OV, BVO-N, and BVO-N-OV samples were calculated using the equation ECB = EVB − Eg, yielding values of −0.06, −0.33, −0.12, and −0.70 eV, respectively. Therefore, the energy band structures of all the samples were established and shown in Figure 4i. It was clear that after the introduction of N doping or oxygen vacancy defects, the band gap narrowed, and the conduction band potential also became more negative. Among them, the BVO-N-OV sample exhibited the most negative CB potential, which might be responsible for its excellent photocatalytic performance. These results indicated that doping and defect engineering could synergistically optimize the materials’ energy band structure, thereby enhancing the photocatalytic activity.
To investigate the carrier migration behavior of the BVO-N-OV photocatalyst and the PMS activation pathway and reveal the CIP degradation mechanism, active species trapping tests were carried out to identify the primary reactive species in the CIP degradation process. As shown in Figure 5a, when ethanol (EtOH) and tert-butyl alcohol (TBA) were added as SO4•− and •OH scavengers into the BVO-N-OV/PMS/Vis system [8,44], CIP removal rate dropped from 91% to 50% and 70%, respectively. This suggested that SO4•− and •OH were crucial in the CIP degradation. When β-carotene and p-benzoquinone (p-BQ) were introduced as scavengers for 1O2 and •O2−, the CIP removal rate dropped from 91% to 80% and 56%, respectively. This highlighted the crucial role of 1O2 and •O2− in the CIP removal process, with •O2− having a more significant impact than 1O2. When oxalic acid (AO) was added to the system as a hole (h+) scavenger, the removal rate of CIP sharply decreased from 91% to 60%, indicating that h+ might be crucial to the CIP degradation process. To further validate the results of the active species trapping experiments, EPR was used to study free radical generation during the reaction in the BVO-N-OV/PMS system. As seen in Figure 5b–d, although the BVO-N-OV/PMS system was able to generate DMPO-•O2−, DMPO-•OH, DMPO-SO4•− and TEMP-1O2 signals under dark conditions [8,44,54,55,56], the signal intensities were very weak. This indicated that the BVO-N-OV/PMS system generated only a limited number of free radicals in the dark. As expected, after 5 min of visible light irradiation, the signal intensities of all free radicals in the BVO-N-OV/PMS system were significantly enhanced, indicating the key role of the photocatalytic effect in the free radical generation process. The significantly enhanced free radical generation level could explain the excellent CIP removal performance of the BVO-N-OV/PMS/Vis system. Based on the experimental findings and analysis above, a potential reaction mechanism is suggested (Scheme 2). Visible light irradiation led to the generation of photoelectrons and holes in BVO-N-OV. The photo-induced electrons were excited to the CB, while the photo-induced holes remained in the valence band VB. The introduction of N doping and oxygen vacancy defects created an impurity energy level within the band gap, enhancing light absorption and facilitating the temporary storage of the excited electrons, suppressing their recombination with the photo-induced holes. The electrons in the CB of BiVO4 nanosheets reacted with PMS and dissolved O2 to generate SO4•−, •OH, •O2−, and 1O2. The more negative CB potential of BVO-N-OV aided PMS activation, leading to the generation of more ROS. The generated ROS oxidized and decomposed CIP into small molecular products such as H2O and CO2. The VB potential of BiVO4 nanosheets (+1.62 eV) is too negative to oxidize OH− into •OH. Even so, the photo-induced h+ has a strong oxidizing ability and could directly oxidize CIP into small molecular products such as H2O and CO2. In a word, the construction of dual defects synergistically promoted the photocatalytic activation of PMS and the multipath generation of ROS, achieving efficient CIP removal.

4. Conclusions

In conclusion, we successfully constructed dual defect-engineered BiVO4 nanosheets by introducing N-doping and oxygen vacancies through ammonium oxalate-assisted thermal treatment. The prepared BVO-N-OV photocatalyst efficiently activated PMS under visible light irradiation, achieving a 91.0% removal rate of CIP within 30 min. The apparent rate constants for CIP removal in the BVO-N-OV/PMS/Vis system were 7.9, 1.9, and 6.6 times those of BVO, BVO-OV, and BVO-N, respectively. More importantly, even under a wide pH range (3.0–11.0) and with different anions, the BVO-N-OV/PMS/Vis system still demonstrated stable and excellent CIP removal performance. The results of active species trapping experiments and EPR tests confirmed that the key reactive species involved in the CIP degradation process include SO4•−, •OH, •O2−, 1O2 and h+. The excellent photocatalytic performance of the BVO-N-OV/PMS/Vis system could be attributed to the following factors: (1) BiVO4 has a relatively wide visible light absorption range. Its band structure enables it to effectively absorb visible light and generate photoexcited carriers (electrons and holes). (2) N doping and oxygen vacancy defects synergistically optimized the energy band structure, enhancing visible light absorption and promoting the separation of photo-generated carriers; (3) The introduction of oxygen vacancies and N doping optimized the surface properties, supplying numerous active sites that accelerated the adsorption and activation of PMS molecules. This research is anticipated to offer valuable insights into the coupling mechanism of defect engineering and doping engineering in photocatalytic PMS activation and new ideas for developing and fabricating efficient photocatalysts/PMS systems for wastewater treatment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano15050373/s1, Figure S1: EDX elemental mapping profiles of BVO-N; Figure S2: (a) XRD patterns and of BVO-N-OV before and after photocatalytic reaction; (b) SEM image of BVO-N-OV after photocatalytic reaction; Table S1: The rate constants k for activating PMS to degrade CIP by BVO, BVO-N, BVO-OV and BVO-N-OV; Table S2: The transient fluorescence lifetimes of BVO, BVO-N, BVO-OV and BVO-N-OV.

Author Contributions

Conceptualization, M.L. and W.Z.; methodology, W.Z.; software, M.X.; validation, M.X. and Z.L.; formal analysis, Z.L.; investigation, J.W. and M.X.; resources, Z.L.; data curation, J.W.; writing—original draft preparation, J.W.; writing—review and editing, M.L. and W.Z.; visualization, M.L.; supervision, Z.L.; project administration, W.Z.; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (52172206, 22275103), the Shandong Province Natural Science Foundation (ZR2022QD062, ZR2024MB155), the Basic Research Project of Science, Education and Production Integration Pilot Project (2023PY015), and the Talent Research Project of Qilu University of Technology (Shandong Academy of Sciences) (2024RCKY018, 2024RCKY036).

Data Availability Statement

No additional data are available.

Acknowledgments

We gratefully acknowledge the support of the funding and the Development Plan of Youth Innovation Team in Colleges and Universities of Shandong Province.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Scheme 1. Schematic diagram of the synthesis of BiVO4, BiVO4-OV, BiVO4-N and BiVO4-N-OV.
Scheme 1. Schematic diagram of the synthesis of BiVO4, BiVO4-OV, BiVO4-N and BiVO4-N-OV.
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Figure 1. XRD patterns of the (a) BVO, BVO-N, BVO-OV and BVO-N-OV; (b) BVO and BVO-N-OV-1.5, BVO-N-OV, BVO-N-OV-2.5; SEM images of (c) BVO; (d) BVO-N; (e) BVO-OV; (f) BVO-N-OV; (g) TEM, (h) HRTEM and (i–l) EDX elemental mapping profiles of BVO-N-OV.
Figure 1. XRD patterns of the (a) BVO, BVO-N, BVO-OV and BVO-N-OV; (b) BVO and BVO-N-OV-1.5, BVO-N-OV, BVO-N-OV-2.5; SEM images of (c) BVO; (d) BVO-N; (e) BVO-OV; (f) BVO-N-OV; (g) TEM, (h) HRTEM and (i–l) EDX elemental mapping profiles of BVO-N-OV.
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Figure 2. XPS spectra of BVO, BVO-OV, BVO-N, BVO-N-OV (a) O 1s; (b) N 1s; (c) Bi 4f; (d) V 2p; (e) EPR spectra; (f) N2 adsorption-desorption isotherms of BVO, BVO-N, BVO-OV and BVO-N-OV.
Figure 2. XPS spectra of BVO, BVO-OV, BVO-N, BVO-N-OV (a) O 1s; (b) N 1s; (c) Bi 4f; (d) V 2p; (e) EPR spectra; (f) N2 adsorption-desorption isotherms of BVO, BVO-N, BVO-OV and BVO-N-OV.
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Figure 3. (a,c) CIP degradation experiments and (b) kinetic profile image in different systems. Reaction conditions: catalyst dosage: 10 mg, PMS dosage:60 mg, [CIP] = 20 ppm, [pH]0 = 6.5. The removal rate of CIP in different conditions, (d) PMS dosage; (e) solution pH; (f) the presence of inorganic anions. (g) Removal rate of CIP by 5 cycles of BVO-N-OV catalyst; (h) Comparison of the degradation rate of CIP under visible light for BVO-N-OV and other materials.
Figure 3. (a,c) CIP degradation experiments and (b) kinetic profile image in different systems. Reaction conditions: catalyst dosage: 10 mg, PMS dosage:60 mg, [CIP] = 20 ppm, [pH]0 = 6.5. The removal rate of CIP in different conditions, (d) PMS dosage; (e) solution pH; (f) the presence of inorganic anions. (g) Removal rate of CIP by 5 cycles of BVO-N-OV catalyst; (h) Comparison of the degradation rate of CIP under visible light for BVO-N-OV and other materials.
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Figure 4. UV–vis absorption spectra of (a) BVO, BVO-N, BVO-OV and BVO-N-OV; (b) BVO and BVO-N-OV-1.5, BVO-N-OV, BVO-N-OV-2.5; (c) Transient photocurrent response; (d) EIS; (e) PL spectra; (f) TRPL spectra; (g) band gap energy; (h) VB-XPS spectra; (i) band structures of BVO, BVO-N, BVO-OV and BVO-N-OV.
Figure 4. UV–vis absorption spectra of (a) BVO, BVO-N, BVO-OV and BVO-N-OV; (b) BVO and BVO-N-OV-1.5, BVO-N-OV, BVO-N-OV-2.5; (c) Transient photocurrent response; (d) EIS; (e) PL spectra; (f) TRPL spectra; (g) band gap energy; (h) VB-XPS spectra; (i) band structures of BVO, BVO-N, BVO-OV and BVO-N-OV.
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Figure 5. (a) Effect of scavengers on CIP degradation in the BVO-N-OV/PMS/Vis system; EPR spectra of (b) DMPO-•O2−, (c) DMPO-•OH/SO4•−, and (d) TEMP-1O2 adducts obtained for BVO-N-OV and PMS under darkness and irradiation.
Figure 5. (a) Effect of scavengers on CIP degradation in the BVO-N-OV/PMS/Vis system; EPR spectra of (b) DMPO-•O2−, (c) DMPO-•OH/SO4•−, and (d) TEMP-1O2 adducts obtained for BVO-N-OV and PMS under darkness and irradiation.
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Scheme 2. The proposed mechanism for the photocatalytic activation of PMS through BVO-N-OV process.
Scheme 2. The proposed mechanism for the photocatalytic activation of PMS through BVO-N-OV process.
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Wu, J.; Xu, M.; Li, Z.; Li, M.; Zhou, W. Dual Defect-Engineered BiVO4 Nanosheets for Efficient Peroxymonosulfate Activation. Nanomaterials 2025, 15, 373. https://doi.org/10.3390/nano15050373

AMA Style

Wu J, Xu M, Li Z, Li M, Zhou W. Dual Defect-Engineered BiVO4 Nanosheets for Efficient Peroxymonosulfate Activation. Nanomaterials. 2025; 15(5):373. https://doi.org/10.3390/nano15050373

Chicago/Turabian Style

Wu, Jiabao, Meiyu Xu, Zhenzi Li, Mingxia Li, and Wei Zhou. 2025. "Dual Defect-Engineered BiVO4 Nanosheets for Efficient Peroxymonosulfate Activation" Nanomaterials 15, no. 5: 373. https://doi.org/10.3390/nano15050373

APA Style

Wu, J., Xu, M., Li, Z., Li, M., & Zhou, W. (2025). Dual Defect-Engineered BiVO4 Nanosheets for Efficient Peroxymonosulfate Activation. Nanomaterials, 15(5), 373. https://doi.org/10.3390/nano15050373

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