English

• The widespread use of antibiotics in recent years has raised serious environmental and public health concerns [1]. To address this challenge, visible-light-driven photocatalysis has emerged as a potential solution for controlling antibiotic pollution owing to its environmentally benign nature, mild operating conditions, and potent radical-mediated oxidation capacity [2]. Consequently, the rational fabrication of high-performance photocatalysts is crucial for advancing photocatalytic technologies that effectively degrade antibiotics in wastewater systems. In this regard, as a typical bismuth-based photocatalyst, Bi₂WO₆ possesses a distinctive layered structure, excellent chemical stability, and outstanding visible-light response, exhibiting a great advantage for photocatalytic remediation applications [3,4]. Nevertheless, the severe recombination of photoproduced charges has restricted the photocatalytic efficiency of pure Bi₂WO₆ [5,6]. In recent years, heterojunction engineering has emerged as an effective approach to address this issue, especially given the growing focus on S-scheme heterojunction photocatalysts [7,8]. The S-scheme heterojunction is typically composed of two semiconductor materials with staggered energy bands. When two materials come into contact, the difference in their Fermi levels causes electron migration at the interface, resulting in band bending and the formation of an internal electric field (IEF). Notably, driven by the IEF, the low-value photogenerated electrons (e^-) and holes (h⁺) undergo recombination, while those with strong redox ability are preserved. Consequently, the S-scheme heterojunction achieves dual functionality: (ⅰ) Significantly promoting the spatial separation of photoproduced e^--h⁺ pairs while (ⅱ) maintaining the strong redox capabilities essential for photocatalytic processes [9,10].

  Carbon quantum dots (CQDs), an emerging class of zero-dimensional carbon nanomaterials, exhibit excellent dispersibility, stable chemical structure, tunable optical bandgap, outstanding surface features, and strong light absorption capacity [11,12]. These exceptional properties enable CQDs to offer unique benefits in the development of advanced photocatalytic materials. Recent investigations have revealed that S-scheme heterojunctions can be constructed by coupling CQDs with appropriate semiconductor materials. Recent advances have demonstrated several CQD-based S-scheme heterojunction systems, including N-CQDs/g-C₃N₄ [13,14], CQDs/Cu₂O [15], N-CQDs/TiO₂ [16], Yb-MnO₂/N-CQDs [17], CQDs/Bi₂MoO₆ [18] and CQDs/BiOBr [19], all of which have significantly enhanced photocatalytic activity. Despite the significant progress achieved, S-scheme photocatalysts that combine CQDs with semiconductors for antibiotic degradation are still in their nascent stages of development, with considerable potential yet to be harnessed for performance enhancement. Defect engineering, particularly through the introduction of OVs, is recognized as a potential tactic to increase the photocatalytic performance of semiconductor materials [20,21]. OV can not only expand light absorption but also modify the intrinsic bandgap structure of semiconductors, acting as trap centers for photoelectrons to facilitate the transfer/separation of photoproduced charges [22,23]. For example, Zhong et al. enhanced the photocatalytic activity of selective benzene oxidation to phenol by regulating the OV on the TiO₂ surface [24]. Notably, while OVs have been extensively studied in single-component semiconductor photocatalysts, the mechanistic understanding of their role in S-scheme heterojunctions as binary photocatalysts remains limited. These observations inspire us to design S-scheme heterojunctions by incorporating OV-modified Bi₂WO₆ onto CQDs, aiming to improve the photocatalytic removal efficiency of emerging antibiotic contaminants from contaminated water.

  Inspired by previous research, we fabricated a 3D nanoflower-shaped CQD/Bi₂WO₆-OV S-scheme heterojunction photocatalyst through a combination of hydrothermal synthesis and alkaline etching techniques (Fig. S1 in Supporting information). Systematic analyses reveal that the OV introduced in Bi₂WO₆ augments the Fermi level disparity between Bi₂WO₆ and the CQDs, facilitating the transfer of more e^- from Bi₂WO₆-OV to the CQDs. This process strengthens the IEF at the interface, thereby improving the separation/transfer of photoproduced charges. In situ XPS, EPR, and density functional theory (DFT) evidence of the S-scheme charge transfer process between Bi₂WO₆-OV and the CQDs. The optimized catalyst achieves 95.2% CIP degradation within 40 min under visible light (λ > 420 nm), outperforming most reported Bi₂WO₆-based photocatalysts.

  The microstructures of Bi₂WO₆, CQDs/Bi₂WO₆, and CQDs/Bi₂WO₆-OV were examined via scanning electron microscopy (SEM) analysis. The SEM micrographs in Fig. S2a (Supporting information) show that Bi₂WO₆ has a 3D nanoflower structure made from nanosheets approximately 22 nm thick. The images in Fig. S2b (Supporting information) indicate that, after adding the CQDs, the overall morphology of the CQD/Bi₂WO₆ composite remains a 3D nanoflower, with no significant change from that of pure Bi₂WO₆. However, the nanosheet surface appears rougher, likely due to the hydrothermal loading of CQDs onto the Bi₂WO₆ nanosheets. Owing to their small size, visualizing CQDs with SEM remains challenging. The SEM micrographs in Fig. S2c (Supporting information) show that the microstructure of the CQD/Bi₂WO₆ composite remained mostly the same after adding OV, implying that the NaOH treatment did not cause significant corrosion. Additionally, Fig. S2d (Supporting information) shows the SEM‒EDS elemental mapping results, confirming a uniform distribution of elements (Bi, W, O and C) in the CQD/Bi₂WO₆-OV nanocomposite.

  The structure of the as-synthesized CQDs was characterized via transmission electron microscopy (TEM) and fluorescence spectroscopy. As depicted in Fig. S3 (Supporting information), the CQDs display a near-spherical morphology without interparticle aggregation and emit intense blue fluorescence under a 365 nm UV lamp. These findings demonstrate that highly crystalline fluorescent CQDs were synthesized via a hydrothermal approach using lignin as the precursor. The TEM micrograph in Fig. 1a shows that pure Bi₂WO₆ is composed of nanosheets, which aligns with the SEM findings. The HR-TEM micrographs of Bi₂WO₆ (Figs. 1b and c) reveal distinct lattice fringe spacings of 0.272 nm, assigned to the (002)/(200) crystal planes of Bi₂WO₆ [25]. Fig. 1d displays TEM images of CQDs/Bi₂WO₆-OV at various magnifications. Compared with that of pure Bi₂WO₆, the surface roughness of the CQD/Bi₂WO₆-OV composite is greater, with many quantum dots (6 nm in diameter) distributed across its surface (Fig. 1e). The HR-TEM image of CQDs/Bi₂WO₆-OV (Fig. 1f) shows two sets of clear lattice fringes: A spacing of 0.272 nm matching the Bi₂WO₆ (002)/(200) planes and another fringe with a 0.212 nm spacing associated with the CQD (100) crystal plane [26]. These results provide compelling evidence for the robust interfacial adhesion between the CQDs and Bi₂WO₆.

  Figure 1

  

  Figure 1.  (a-c) TEM micrographs of pure Bi₂WO₆ and (d-f) CQDs/Bi₂WO₆-OV at different magnifications. (g) XRD patterns, (h) N₂ adsorption‐desorption isotherms and (i) EPR spectra of various samples.

  DownLoad: Full-Size Img PowerPoint

  The X-ray diffraction (XRD) patterns of all the samples (Fig. 1g) exhibit exclusively Bi₂WO₆ diffraction peaks, with no observable CQD peaks in the CQD/Bi₂WO₆ composite, which is likely due to the low CQD loading (<5 wt%). Compared with those of Bi₂WO₆, distinct D and G bands were observed in the Raman spectra of the CQDs/Bi₂WO₆ (Fig. S4 in Supporting information), further confirming the formation of CQDs. The N₂ adsorption‒desorption isotherms (Fig. 1h) reveal that all the catalysts exhibited a typical type Ⅳ shape, indicating the presence of narrow slit-like macropores and mesopores in the catalyst [27]. The specific surface areas of Bi₂WO₆, Bi₂WO₆-OV, CQDs/Bi₂WO₆, and CQDs/Bi₂WO₆-OV are 23.21, 24.36, 32.51, and 42.29 m²/g, respectively (Table S1 in Supporting information). The pore size distribution in Fig. S5 (Supporting information) indicates that CQD/Bi₂WO₆-OV has a narrower pore size distribution. These results demonstrate that incorporating CQDs and OVs onto Bi₂WO₆ 3D nanoflowers increases the specific surface area of the catalyst, which offers more reactive sites for pollutant adsorption and degradation.

  Fig. S6 (Supporting information) shows the high-resolution XPS spectra of O 1s and the relative amount of different oxygen species. The O 1s peaks of CQDs/Bi₂WO₆-OV and Bi₂WO₆-OV can split into three peaks at 530.2, 531.8, and 533.2 eV, corresponding to lattice oxygen, OVs, and adsorbed oxygen, respectively (Fig. S6a in Supporting information) [28]. The area of the three peaks and the corresponding oxygen species content were calculated through area integration. The semi-quantitative analysis results indicate that compared with Bi₂WO₆-OV, the relative content of OV in CQD/Bi₂WO₆-OV is slightly reduced (Fig. S6b in Supporting information), which may be due to the electronic defect states of OV on the surface of Bi₂WO₆-OV being offset by lone pair electrons of oxygen atoms on the surface of CQDs [29]. EPR spectroscopy was utilized as another powerful technique to confirm the existence of OV. As shown in Fig. 1i, the Bi₂WO₆-OV and CQD/Bi₂WO₆-OV nanoflowers exhibit similar EPR signals at g = 2.003, whereas the bare Bi₂WO₆ and CQD/Bi₂WO₆ samples present no signal, further indicating the presence of OV in Bi₂WO₆-OV and CQD/Bi₂WO₆-OV [30].

  The relationship between the loading level of the CQDs (controlled via the lignin precursor dosage) and the photocatalytic efficiency of Bi₂WO₆ was explored. As presented in Fig. S7 (Supporting information), pristine Bi₂WO₆ degraded only 60.3% of the CIP after 40 min of visible light illumination. Notably, the CQDs/Bi₂WO₆–2 prepared with the addition of 140 mg of lignin exhibited the optimal degradation efficiency (80.8%) for CIP. Among them, the first-order rate constant (k = 0.04266 min^-1) of CQDs/Bi₂WO₆–2 is 2.33 times greater than that of pristine Bi₂WO₆ (k = 0.01834 min^-1). These findings demonstrate that introducing a suitable amount of CQDs can improve the photocatalytic degradation performance of Bi₂WO₆ toward CIP.

  The removal efficiency of CIP was further assessed under visible-light irradiation across various catalysts. Fig. 2a shows that the concentration of the CIP solution remains stable under light irradiation without a photocatalyst, indicating the negligible self-photodegradation capability of CIP. Additionally, the CQDs exhibit a limited ability to degrade CIP under visible light. Under the same conditions, the CQD/Bi₂WO₆ composite exhibits a higher CIP removal efficiency than the original CQDs and Bi₂WO₆. After further introduction of OVs, the CQD/Bi₂WO₆-OV composite exhibits optimal photocatalytic activity, achieving 95.2% CIP degradation within 40 min of visible light irradiation. The corresponding rate constants were obtained by fitting the first-order reaction kinetics model (Table S2 in Supporting information). The fitting results are shown in Fig. S8 (Supporting information), which indicates that the photodegradation of CIP is consistent with the first-order reaction kinetics model. As presented in Fig. 2b, the first-order rate constant of CQDs/Bi₂WO₆-OV for CIP degradation is k = 0.06279 min^-1, which is 1.47 times greater than that of CQDs/Bi₂WO₆ (k = 0.04266 min^-1), 3.42 times greater than that of Bi₂WO₆ (k = 0.01834 min^-1), 2.18 times greater than that of Bi₂WO₆-OV (k = 0.02879 min^-1), and 30.78 times greater than that of CQDs (k = 0.00204 min^-1). Furthermore, the effects of different catalysts on the mineralization of CIP were evaluated. Fig. S9 (Supporting information) shows that the CQD/Bi₂WO₆-OV composite achieves 46.83% CIP mineralization within 60 min of visible light illumination, which is 1.63 times greater than that of Bi₂WO₆-OV (28.66%) and 1.44 times greater than that of CQDs/Bi₂WO₆ (32.51%). The above results indicate that constructing heterojunctions with CQDs and creating OVs can effectively promote the degradation and mineralization of CIP by CQDs/Bi₂WO₆-OV under visible light.

  Figure 2

  

  Figure 2.  (a) Photocatalytic removal and (b) first-order rate constants of CIP by various catalysts under visible light exposure. (c) Cycling tests of CQDs/Bi₂WO₆-OV for CIP degradation. (d) Impact of pH on CIP removal, (e) degradation of CIP in the presence of distinct inorganic ions and HA, (f) degradation of CIP in various water sources.

  DownLoad: Full-Size Img PowerPoint

  The stability of CQDs/Bi₂WO₆-OV was evaluated via recycling tests for the photocatalytic degradation of CIP. As evidenced in Fig. 2c, the CIP removal rate slightly decreased with each successive cycle, but remained stable at 84.0% after four cycles under visible light. The XRD patterns (Fig. S10 in Supporting information) before and after use show no change in the characteristic diffraction peaks, confirming the structural integrity of the composite. Moreover, the SEM images (Fig. S11 in Supporting information) reveal that the original morphology of CQDs/Bi₂WO₆-OV was well preserved after use. To further investigate the reasons for the performance decline of CQDs/Bi₂WO₆-OV after use. The Fourier transform infrared (FTIR) and XPS spectra of CQDs/Bi₂WO₆-OV were tested before and after use. The FTIR spectrum of the used sample (Fig. S12a in Supporting information) reveals no positional shift in peak positions but a reduction in intensity, indicating an alteration in its surface chemical state. This finding is supported by XPS analysis, which shows a concomitant attenuation in the intensities of the C 1s, O 1s, Bi 4f, and W 4f signals (Figs. S12b-f in Supporting information). These changes are likely attributable to the partial detachment of CQDs and the filling of surface OVs during the reaction.

  Furthermore, the photocatalytic performance of CQDs/Bi₂WO₆-OV was investigated under various pH values, coexisting anions, humic acid, and real water conditions. As depicted in Fig. 2d, the CQD/Bi₂WO₆-OV exhibits high photocatalytic efficiency toward CIP within a wide pH range of 6.8‒11.1. Notably, compared with acidic environments (pH 3.08‒5.1), alkaline environments (pH 9.1‒11.1) are more favorable for CIP degradation. This phenomenon may be due to the reduced redox potential of OH^-/^•OH at higher pH values [31], which facilitates more efficient OH^- oxidation by the CQD/Bi₂WO₆-OV photocatalytic system to generate additional ^•OH radicals. Fig. 2e shows the effects of the presence of both inorganic salts and humic acid (HA) in water on the photocatalytic degradation performance of CIP. Compared with ultrapure water, common anions (SO₄^2-, NO₃^-, CO₃^2-, Cl^-) and HA have negligible effects on CIP photodegradation, and the total removal rate remains above 88.0% within 40 min. However, they have an impact on the kinetics of the CIP photodegradation reaction, which may result from their competition with CIP for reaction sites on the CQD/Bi₂WO₆-OV surface, as well as their ability to function as scavengers of active species [32,33]. The impact of actual water bodies on the CIP degradation performance is shown in Fig. 2f. The removal efficiencies of CIP in tap water, lake water, and river water were 93.6%, 87.5% and 86.0%, respectively. Further kinetic analysis (Fig. S13 in Supporting information) indicates that compared to the other three types of water, ultrapure water has a high removal efficiency and the fastest degradation rate for CIP, due to the absence of active species inhibitors in ultrapure water [34]. These findings indicate that CQDs/Bi₂WO₆-OV have great potential for purifying antibiotic pollutants in actual water bodies.

  The light collection ability and band structure of the samples were evaluated via UV‒visible diffuse reflectance spectroscopy (UV‒vis DRS) and XPS valence band (XPS‒VB) analysis. As presented in Fig. 3a, the original Bi₂WO₆ exhibited excellent photoresponsivity in the range of 200–450 nm. The inherent absorption edge of Bi₂WO₆ redshifts after the introduction of the CQDs and OVs, indicating an improvement in its utilization efficiency of visible light. Moreover, the band gap energy (E_g) of the catalysts was calculated via the Tauc plot approach based on UV‒vis DRS spectra [35]. As given in Fig. S14 (Supporting information), the E_g of Bi₂WO₆-OV is 2.58 eV, which is lower than that of the original Bi₂WO₆ (2.65 eV). According to previous reports, OVs may generate defect levels below the conduction band (CB) of Bi₂WO₆, thereby shortening the bandgap and expanding the light absorption range [36]. In addition, Fig. 3b shows the UV‒vis DRS of the CQDs, which exhibit significant absorption throughout the visible light range of 200–800 nm, corresponding to a narrow bandgap of 1.88 eV (inset of Fig. 3b). The XPS-VB spectra of the samples were recorded to analyze the band structure. As given in Fig. 3c, the VBxps positions of Bi₂WO₆, Bi₂WO₆-OV, and the CQDs were 2.42, 2.22, and 2.48 eV, respectively. The VB position (vs. NHE) was calculated via the following equation: E_VB (vs. NHE) = Φ + VB_XPS - 4.44 eV, where Φ (4.2 eV) and 4.44 eV are the work functions of the XPS analyzer and the vacuum level, respectively [37]. The VB positions (vs. NHE) of Bi₂WO₆, Bi₂WO₆-OV, and the CQDs were calculated to be 2.18, 1.98, and 2.24 V, respectively. Thus, the CB potentials of Bi₂WO₆, Bi₂WO₆-OV, and CQDs were determined via the equation: E_g = E_VB - E_CB, which yielded values of −0.47, −0.60, and 0.36 V (vs. NHE), respectively. Based on these measurements, the band structures of Bi₂WO₆, Bi₂WO₆-OV, and the CQDs were determined and are summarized in Fig. S15 (Supporting information).

  Figure 3

  

  Figure 3.  UV‒vis DRS of (a) Bi₂WO₆-based catalyst and (b) CQDs. (c) XPS-VB spectra of Bi₂WO₆, Bi₂WO₆-OV and CQDs. (d) EIS spectra, (e) PL spectra and (f) TRPL measurements of different samples. In situ irradiated XPS for (g) W 4f, (h) Bi 4f and (i) C 1s of CQDs/Bi₂WO₆-OV in the dark or under visible light exposure.

  DownLoad: Full-Size Img PowerPoint

  The charge migration/separation kinetics of the photocatalysts were investigated via photoluminescence (PL), electrochemical impedance spectroscopy (EIS), and time-resolved photoluminescence (TRPL). As given in Fig. 3d, compared with the Bi₂WO₆, Bi₂WO₆-OV, and CQD/Bi₂WO₆ heterojunctions, the CQD/Bi₂WO₆-OV heterojunction has a smaller resistance radius arc under the same conditions, indicating its excellent interface charge transfer ability [38]. The separation kinetics of the photoinduced charges were analyzed through PL and TRPL measurements. As depicted in Fig. 3e, the PL intensities of Bi₂WO₆-OV and CQDs/Bi₂WO₆ are significantly lower than that of pure Bi₂WO₆ after the introduction of CQDs and OV. At the same time, CQDs/Bi₂WO₆-OV presented the lowest PL signal, indicating that constructing heterojunctions with CQDs and introducing OVs can promote the effective separation of photogenerated charges [39]. In addition, TRPL analysis (Fig. 3f and Table S3 in Supporting information) reveals that, compared with Bi₂WO₆ (τ_avg: 0.97 ns), Bi₂WO₆-OV (τ_avg: 1.02 ns), and CQDs/Bi₂WO₆ (τ_avg: 1.22 ns), the CQDs/Bi₂WO₆-OV had a longer carrier lifetime (τ_avg: 1.62 ns), demonstrating the significant enhancement of space charge separation in CQDs/Bi₂WO₆-OV [40]. Therefore, the photogenerated charges in the CQD/Bi₂WO₆-OV composites can be maximally utilized for photocatalytic reactions.

  The charge transfer direction between the CQDs and Bi₂WO₆-OV was determined via in situ irradiation XPS. Before analysis, all the XPS spectra were energy-calibrated with the adventitious carbon C 1s peak set at 284.8 eV as an internal reference. As given in Fig. 3g, the two peaks of the W 4f orbital in CQDs/Bi₂WO₆-OV are ascribed to the binding energies of W 4f_5/2 and W 4f_7/2, respectively [41]. Under light irradiation, these peaks shift toward lower binding energies, confirming the accumulation of e^- in Bi₂WO₆-OV. Similar trends were recorded in the in situ XPS spectra of Bi 4f (Fig. 3h). The C 1s XPS spectrum (Fig. 3i) reveals three characteristic peaks at 284.8, 285.9, and 288.2 eV, corresponding to C-C/C═C, C-O, and C═O bonds, respectively, which are consistent with the typical chemical structure of CQDs. Notably, these peaks shift toward higher binding energies under light irradiation [42]. The in situ XPS results strongly confirmed that the photogenerated e^- transferred from the CQDs to Bi₂WO₆-OV.

  Additionally, ultraviolet photoelectron spectroscopy (UPS) was employed to determine the work functions (Φ_s) of Bi₂WO₆, Bi₂WO₆-OV, and the CQDs [43]. The cutoff edges (E_cutoff) of Bi₂WO₆, Bi₂WO₆-OV and the CQDs are located at 16.11, 16.72 and 15.51 eV, respectively (Fig. S16 in Supporting information). The Φ values of Bi₂WO₆, Bi₂WO₆-OV and CQDs can be calculated to be 5.11, 4.50 and 5.71 eV, respectively, via the following equation: Φ = hν - E_cutoff, where hν denotes the energy of the incident photon (21.22 eV) [44,45]. Furthermore, the Fermi levels (E_f) of Bi₂WO₆, Bi₂WO₆-OV and the CQDs are calculated to be −5.11, −4.5 and −5.71 eV, respectively, by applying the equation Φ = E_v - E_f, where E_v represents the potential of the vacuum (0 eV). The different Φ values between two semiconductors are the key factors for charge transfer at the semiconductor interface. The greater the difference in Φ between the two semiconductors is, the stronger the IEF at the interface, and the higher the efficiency of interface charge transfer [46,47]. Obviously, after introducing OVs into Bi₂WO₆, the Φ of Bi₂WO₆-OV decreases, leading to an increase in E_f. The Φ difference between the CQDs and Bi₂WO₆ is 0.6 eV, whereas that between the CQDs and Bi₂WO₆-OV increases to 1.21 eV. Therefore, the introduction of OVs modulated the diameter of Bi₂WO₆, increased the diameter difference between the CQDs and Bi₂WO₆, provided a stronger driving force for interface charge transfer, and enhanced the performance of the CQD/Bi₂WO₆-OV S-scheme heterojunction in degrading CIP. As presented in Fig. S17 (Supporting information), the E_f of Bi₂WO₆-OV is greater than that of the CQDs. In close contact, e^- spontaneously transfers from Bi₂WO₆-OV to the CQDs until E_f at the interface reaches equilibrium. Simultaneously, the energy band of Bi₂WO₆-OV that loses e^- bends upward, whereas the energy band of the CQDs that gain e^- bends downward. The e^- depletion layer and e^- accumulation layer are formed in Bi₂WO₆-OV and CQDs, respectively, resulting in IEF and band bending from Bi₂WO₆-OV to the CQD direction in the space charge layer. During photoexcitation, the photoproduced e^- in the CB of the CQDs spontaneously migrates toward Bi₂WO₆-OV and recombines with h⁺ in the VB of Bi₂WO₆-OV. The electron-rich Bi₂WO₆-OV serves as the reduction reaction site, whereas the hole-rich CQDs serve as the oxidation reaction site. The constructed S-scheme heterojunction between the CQDs and Bi₂WO₆-OV synergistically promotes the spatial separation of photoexcited e^--h⁺ pairs while maintaining their high redox potentials.

  To elucidate the charge transfer mechanism, the 3D charge density differences of CQDs/Bi₂WO₆-OV and CQDs/Bi₂WO₆ were computed via DFT. Figs. 4a, b, d and e denote the charge density differences of CQDs/Bi₂WO₆ and CQDs/Bi₂WO₆-OV, respectively. Blue and yellow refer to the charge depletion and accumulation regions, respectively. e^- are readily transferred from the Bi₂WO₆ and Bi₂WO₆-OV sides to the CQD side through the heterojunction interface, leading to a positive charge on the Bi₂WO₆ and Bi₂WO₆-OV sides near the interface and a negative charge on the CQD side. The redistribution of charges results in the generation of IEF from Bi₂WO₆ and Bi₂WO₆-OV to the CQDs near the interface, which aligns with the results of the work function and in situ XPS. Notably, the CQD/Bi₂WO₆-OV system results in a stronger charge density difference than the CQD/Bi₂WO₆ system does, resulting in an enhanced IEF effect at the CQD/Bi₂WO₆-OV interface due to the modification of OV, effectively increasing the spatial separation/transfer of photoproduced charges. Bader charge analysis revealed that the number of free electrons transferred from Bi₂WO₆ and Bi₂WO₆-OV to the CQDs was 0.74 e and 1.06 e, respectively (the fundamental charge), which further supports the above conclusion. As depicted in Figs. 4c and f, the planar average charge density difference of CQDs/Bi₂WO₆-OV along the z-axis is positive for the CQDs and negative for Bi₂WO₆-OV, further confirming e^- migration from Bi₂WO₆-OV to the CQDs under nonilluminated conditions. Moreover, the planar average charge density difference along the z-axis of CQDs/Bi₂WO₆-OV is significantly greater than that of CQDs/Bi₂WO₆, which is in good agreement with the charge density difference analysis. These findings provide clear evidence for both an S-scheme charge transfer pathway and a strengthened IEF in the CQD/Bi₂WO₆-OV composite.

  Figure 4

  

  Figure 4.  Top view (a, d) and side view (b, e) of the charge density difference, plane average charge density difference Δρ (z) (c, f) of CQDs/Bi₂WO₆ and CQDs/Bi₂WO₆-OV. (g) Transient photocurrent density, (h) SPV and (i) IEF intensity of CQDs/Bi₂WO₆ and CQDs/Bi₂WO₆-OV.

  DownLoad: Full-Size Img PowerPoint

  Typically, the intensity of the IEF serves as a key kinetic factor governing charge separation, making its quantitative evaluation an essential step. According to the theory proposed by Kanata et al., the IEF of a given material shows a monotonic positive correlation with both its surface charge density and surface photovoltage [48]. Notably, the CQD/Bi₂WO₆-OV sample achieves a high surface charge density of 0.2554 µC/cm², as determined by transient photocurrent measurements (Fig. 4g). Steady-state surface photovoltage (SPV) measurements revealed a significant surface potential of 227.1 µV for CQDs/Bi₂WO₆-OV (Fig. 4h). These results further demonstrate that the CQD/Bi₂WO₆-OV heterostructure has an enhanced photogenerated charge separation efficiency compared with its CQD/Bi₂WO₆ counterpart [49]. According to the surface charge density and surface photovoltage results (Table S4 in Supporting information), the IEF intensity of CQDs/Bi₂WO₆-OV is 1.84 times greater than that of CQDs/Bi₂WO₆ (Fig. 4i). As a result of the enhanced IEF and well-designed S-scheme heterojunction in CQDs/Bi₂WO₆-OV, highly efficient charge migration and separation can be achieved.

  The reactive species generated by CQDs/Bi₂WO₆-OV during the photocatalytic process were identified via in situ EPR. As presented in Figs. 5a and b, the CQD/Bi₂WO₆-OV does not exhibit DMPO-^•OH or DMPO-^•O₂^- characteristic signals under dark conditions. A distinct four-line DMPO-^•OH signal with a 1:2:2:1 intensity ratio and six intense lines of DMPO-^•O₂^- were observed under visible light, confirming the generation of ^•OH and ^•O₂^- radicals. As the irradiation time increased, the increasing peak intensities of the DMPO-^•OH and DMPO-^•O₂^- signals increased, indicating a corresponding increase in the concentration of these two free radicals. These findings demonstrate that the constructed CQD/Bi₂WO₆-OV S-scheme heterojunction facilitates visible-light-driven generation of ^•OH and ^•O₂^- radicals, which are primarily responsible for CIP degradation via oxidative cleavage. Reactive species quenching experiments were performed to assess the contributions of ^•OH, ^•O₂^-, and h⁺ to the photocatalytic removal of CIP by CQDs/Bi₂WO₆-OV. As given in Fig. 5c, the addition of L-ascorbic acid (AA) significantly inhibited CIP degradation, revealing that ^•O₂⁻ was the primary reactive species for the photocatalytic removal of CIP. Moreover, the addition of Na₂C₂O₄ (h⁺ scavenger) and isopropanol (IPA, ^•OH scavenger) also inhibited CIP degradation to some extent, confirming the secondary roles of h⁺ and ^•OH in the photocatalytic process. The corresponding rate constants were obtained by fitting the first-order reaction kinetics model (Fig. S18 in Supporting information). Furthermore, compared with the quencher-free system, the addition of AA, Na₂C₂O₄, and IPA reduced the reaction rate constants for CIP removal to 0.00553, 0.02414, and 0.03799 min^-1, respectively (Fig. 5d). This further indicates that ^•O₂⁻ is the main reactive species for CIP degradation, followed by ^•OH and h⁺.

  Figure 5

  

  Figure 5.  EPR peaks of (a) DMPO-^•O₂^- and (b) DMPO-^•OH. (c) Trapping experiments for the removal of CIP over CQDs/Bi₂WO₆-OV. (d) Effect of different scavengers on the reaction rate constants. (e) Potential mechanism of CQDs/Bi₂WO₆-OV for CIP photodegradation.

  DownLoad: Full-Size Img PowerPoint

  Based on experimental analysis and theoretical calculations, the photocatalytic mechanism of the CQD/Bi₂WO₆-OV heterojunction for CIP degradation was further investigated. As revealed in Fig. 5e, the e^- in the CQDs and Bi₂WO₆-OV transitions from their respective VB to CB under irradiation with light. Under IEF driving, e^- in the CB of the CQDs recombines with h⁺ in the VB of Bi₂WO₆-OV, whereas highly reactive h⁺ and e^- are retained in the CQD/Bi₂WO₆-OV heterojunction. The VB potential of the CQDs is more positive than that of OH^-/^•OH (1.99 V vs. NHE) [50], and the CB potential of Bi₂WO₆-OV is more negative than that of O₂/^•O₂^- (−0.33 V vs. NHE) [51]. Under these conditions, the h⁺ in the VB of the CQDs can oxidize OH- to generate ^•OH, and the e^- in the CB of Bi₂WO₆-OV can reduce O₂ to form ^•O₂^-. Consequently, an S-scheme heterojunction is successfully constructed between the CQDs and Bi₂WO₆-OV, which simultaneously facilitates (ⅰ) efficient separation and directional migration of photoinduced charges and (ⅱ) abundant production of ^•O₂^- and ^•OH radicals.

  DFT calculations based on the Fukui function were adopted to investigate the removal pathways of CIP. The optimized structure of the CIP is presented in Fig. 6c. Typically, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) identify the sites prone to electrophilic and nucleophilic reactions in photocatalytic processes [52]. As displayed in Fig. 6a, the HOMO orbitals of CIP are predominantly situated at C10, C2, C4, C5, and O13 atoms, whereas the LUMO orbitals are mainly found at C15, C8, C6, C3, C5, and N11 atoms. Moreover, the electrostatic potential (ESP) mapping of CIP reveals distinct charge localization, with strongly negative potentials concentrated at the O13 and O17 sites, whereas the N7, N22, and C14 regions exhibit significant positive potentials (Fig. 6b). Although these results indicate probable reactive sites, HOMO‒LUMO and ESP analyses alone are insufficient for the quantitative evaluation of reactive sites. Therefore, the Fukui function of CIP was used to quantify its local reactivity. As depicted in Fig. 6d, the C8, O17, N11, and O13 atoms have high f⁰ values and are susceptible to ^•OH attack; the C2, O13, N22, F12, and C4 atoms have higher f^- values and are vulnerable to h⁺ and ¹O₂ assaults; and the C4, C10, C9, O13, and O17 positions have high f⁺ values and are prone to preferential attacks from ^•O₂^-.

  Figure 6

  

  Figure 6.  (a) HOMO and LUMO, (b) ESP mapping, (c) optimized structure and (d) Fukui index of CIP.

  DownLoad: Full-Size Img PowerPoint

  LC‒MS technology was utilized to identify potential degradation intermediates of CIP, as illustrated in Fig. S19 (Supporting information). Based on the above DFT calculations and LC‒MS findings, three plausible pathways for CIP degradation were proposed. As revealed in Fig. 7a, the removal process of CIP essentially involves cleavage of the piperazine ring, F-substitution, depropylation, defluorination, and decarboxylation, among other steps. In pathway Ⅰ, CIP (m/z 332) undergoes initial attack by ^•O₂^- and ^•OH, resulting in cleavage and opening of the piperazine ring to generate P1 (m/z 334) and P2 (m/z 262). P2 is subsequently attacked by h⁺ and ^•O₂^-, triggering defluorination and depropylation to yield P3 (m/z 243) and P4 (m/z 205) [53]. In pathway Ⅱ, CIP (m/z 332) undergoes decarboxylation via ^•O₂^- attack to generate P6 (m/z 286). Then, P6 undergoes tetrahydropyridine ring cleavage under combined ^•O₂^- and ^•OH attacks, yielding P7 (m/z 262), followed by C‒N bond cleavage of P7 to form P8 (m/z 179) [54]. In pathway Ⅲ, CIP (m/z 332) undergoes h⁺-mediated piperazine ring oxidation and cleavage to produce P10 (m/z 361). P10 is subsequently attacked by h⁺ and ^•O₂^-, followed by hydrolysis to yield P11 (m/z 334) [55]. Then, P11 is converted to P12 (m/z 261) via amination and fluorine substitution reactions. Thereafter, P12 undergoes deamination and ring opening, affording P13 (m/z 197) and P14 (m/z 166). Finally, these intermediates are further oxidized to nontoxic small molecules via the combined action of ^•O₂^-, ^•OH, and h⁺, with some mineralized into CO₂ and H₂O.

  Figure 7

  

  Figure 7.  (a) Decomposition pathway of CIP by CQDs/Bi₂WO₆-OV. (b) Acute toxicity, (c) mutagenicity and (d) developmental toxicity of CIP and its intermediates.

  DownLoad: Full-Size Img PowerPoint

  The toxicity of CIP and its intermediates was assessed via the Toxicity Estimation Software Tool (T.E.S.T.) based on QSAR modeling. Fig. 7b shows the evolution of acute toxicity for CIP and its degradation intermediates during photodegradation, with nearly all the transformation products exhibiting elevated LC₅₀ values relative to those of CIP. Among them, three degradation products (P9, P13, and P14) presented significantly higher LC₅₀ values (>100 mg/L) than did CIP, demonstrating an order-of-magnitude decrease in acute toxicity. As presented in Fig. 7c, a similar phenomenon regarding mutagenicity is observed. Except for product P6, which is more mutagenic than CIP, the mutagenicity of the other products is lower than that of CIP. Notably, products P1, P13, P11, P14, P12, P3, P4, P5, P8, and P13 show no detectable mutagenicity. Fig. 7d further demonstrates that all the intermediates exhibit reduced developmental toxicity compared with that of CIP, with products P14 and P9 showing no detectable toxicity. The results demonstrate that the developed CQDs/Bi₂WO₆-OV can effectively reduce the toxicity of CIP and its byproducts under visible light illumination.

  Furthermore, the ecological toxicity of the solution following CIP degradation by CQDs/Bi₂WO₆-OV was assessed using mung bean sprout cultivation experiments. In these experiments, mung beans were soaked in 30 mL of tap water, a raw CIP solution (20 mg/L), and a treated CIP solution. After 7 days of cultivation at a constant temperature of 25 ℃, the growth results (Fig. S20 in Supporting information) reveal that the CIP solution significantly inhibited mung bean growth. In contrast, the mung beans in the treated CIP solution germinated successfully and grew well, indicating that the CQDs/Bi₂WO₆-OV can effectively reduce the ecological toxicity of the CIP solution with the assistance of visible light. Notably, the length of the bean sprouts in the treatment group is slightly shorter than that in the tap water control. This slight difference indicates that trace amounts of toxic intermediates still exist in the CIP solution after 40 min of photodegradation, implying that a longer irradiation time is necessary to achieve complete detoxification.

  In summary, we synthesized a novel 3D nanoflower-structured CQD/Bi₂WO₆-OV S-scheme heterojunction, which demonstrates favorable visible-light photocatalytic activity with a 95.2% CIP removal rate, outperforming most reported similar catalysts. Kinetic analysis revealed that the first-order rate constant k of CQDs/Bi₂WO₆-OV for CIP degradation was 3.4- and 30.8-fold greater than that of Bi₂WO₆ and CQDs, respectively. DFT calculations and UPS analysis indicate that the introduction of a moderate OV in the CQD/Bi₂WO₆-OV S-scheme heterojunction increases the Fermi level difference between Bi₂WO₆ and the CQDs. These findings indicate that more electrons are transferred from Bi₂WO₆-OV to the CQDs, resulting in a strengthened IEF at their interface, which enhances the separation of the photoproduced charge carriers. In situ XPS and EPR further confirmed the establishment of an S-scheme charge migration pathway between the CQDs and Bi₂WO₆-OV, promoting the generation of ^•OH and ^•O₂^- radicals and the photodegradation of CIP. Moreover, three potential degradation pathways of CIP were elucidated via LC‒MS and the Fukui index. The T.E.S.T. analysis predicts low toxicity of the intermediates, confirming that the CQD/Bi₂WO₆-OV S-scheme heterojunction effectively mitigates the ecological risk of CIP by minimizing the number of heterocyclic groups. This work provides insights into the design of advanced S-scheme heterojunctions with robust IEF to achieve superior charge separation and environmental remediation.

  CRediT authorship contribution statement

  Haitao Ren: Writing – original draft, Funding acquisition, Formal analysis, Data curation. Zongcheng Miao: Resources, Methodology. Xiangbo Feng: Investigation. Abdelkader Labidi: Writing – original draft. Yuzhen Zhao: Visualization, Conceptualization. Chuanyi Wang: Writing – original draft, Supervision, Project administration.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This study was supported by the SAFEA of China (“Belt and Road” Innovative Exchange Foreign Expert Project, DL2023041004L), the Natural Science Basic Research Program of Shaanxi (No. 2025JC-YBQN-149) and the Xijing University Special Fund for High-Level Talents (No. XJ24B20).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2026.112557.

  
  
• 1. [1]

     Y. Lu, X. Zhou, Y. Zheng, et al., Water Res. 275 (2025) 123179. doi: 10.1016/j.watres.2025.123179

  2. [2]

     W.W. Chen, F.F. Huang, Q.Y. Qu, et al., Rare Met. 44 (2025) 3970–3980. doi: 10.1007/s12598-024-03174-x

  3. [3]

     W. Mao, X. Shen, L. Zhang, et al., Front. Environ. Sci. Eng. 18 (2024) 86. doi: 10.1007/s11783-024-1846-x

  4. [4]

     B. Li, X. Liu, H. Zhu, et al., Small 20 (2024) 2406074. doi: 10.1002/smll.202406074

  5. [5]

     S. Li, M. Cai, Y. Liu, et al., Chin. J. Catal. 43 (2022) 2652–2664. doi: 10.1016/S1872-2067(22)64106-8

  6. [6]

     X. Ning, A. Hao, X. Qiu, et al., Adv. Funct. Mater. 35 (2025) 2413217. doi: 10.1002/adfm.202413217

  7. [7]

     H. Ren, Z. Miao, Y. Zhao, et al., J. Alloy Compd. 1028 (2025) 180646. doi: 10.1016/j.jallcom.2025.180646

  8. [8]

     X. Li, Y. Wan, F. Deng, et al., Chin. Chem. Lett. 36 (2025) 111418. doi: 10.1016/j.cclet.2025.111418

  9. [9]

     L. Zhang, J. Zhang, J. Yu, et al., Nat. Rev. Chem. 9 (2025) 328–342. doi: 10.1038/s41570-025-00698-3

  10. [10]

      L. Sun, W. Yan, W. Wang, et al., Angew. Chem. Int. Ed. 137 (2025) e202511080. doi: 10.1002/ange.202511080

  11. [11]

      J. Zhao, C. Li, X. Du, et al., Small 18 (2022) 2200744. doi: 10.1002/smll.202200744

  12. [12]

      P. Wang, C. Chen, H. Ren, et al., Chin. Chem. Lett. 36 (2025) 110725. doi: 10.1016/j.cclet.2024.110725

  13. [13]

      X. Li, Q. Luo, L. Han, et al., J. Mater. Sci. Technol. 114 (2022) 222–232. doi: 10.1016/j.jmst.2021.10.030

  14. [14]

      Y. Xu, W. Hou, K. Huang, et al., Adv. Sci. 11 (2024) 2403607. doi: 10.1002/advs.202403607

  15. [15]

      J. Wang, S. Li, P. Ma, et al., Colloids Surf. A 690 (2024) 133779. doi: 10.1016/j.colsurfa.2024.133779

  16. [16]

      M. Karaca, Z. Eroğlu, O. Ağığlı, et al., ACS Omega 8 (2023) 26597–26609. doi: 10.1021/acsomega.3c03532

  17. [17]

      Z. Chen, D. Yang, Z. Liang, et al., Chem. Eng. J. 474 (2023) 145978. doi: 10.1016/j.cej.2023.145978

  18. [18]

      J. Liu, L. Ji, Q. He, et al., Sep. Purif. Technol. 363 (2025) 132196. doi: 10.1016/j.seppur.2025.132196

  19. [19]

      Y. Liu, X. Zhang, X. Li, et al., J. Water Process. Eng. 57 (2024) 104588. doi: 10.1016/j.jwpe.2023.104588

  20. [20]

      H. Li, J. Li, Z. Ai, et al., Angew. Chem. Int. Ed. 57 (2018) 122. doi: 10.1002/anie.201705628

  21. [21]

      H. Jiang, L. Wang, X. Yu, et al., Chem. Eng. J. 466 (2023) 143129. doi: 10.1016/j.cej.2023.143129

  22. [22]

      C. Yang, J. Zhang, G. Yu, et al., Adv. Energy Mater. 15 (2025) 2403713. doi: 10.1002/aenm.202403713

  23. [23]

      S. Gao, R. Wu, M. Sun, et al., Appl. Catal. B: Environ. 324 (2023) 122260. doi: 10.1016/j.apcatb.2022.122260

  24. [24]

      S. Zhong, D. Yu, Y. Ma, et al., Angew. Chem. Int. Ed. 25 (2025) e202502823. doi: 10.1002/anie.202502823

  25. [25]

      L. Liu, J. Liu, K. Sun, et al., Chem. Eng. J. 411 (2021) 128629. doi: 10.1016/j.cej.2021.128629

  26. [26]

      Y. Quan, B. Wang, G. Liu, et al., Chem. Eng. Sci. 232 (2021) 116338. doi: 10.1016/j.ces.2020.116338

  27. [27]

      Q. Wang, R. Zhao, H. Li, et al., Sep. Purif. Technol. 359 (2025) 130546. doi: 10.1016/j.seppur.2024.130546

  28. [28]

      X. Li, Q. Liu, F. Deng, et al., Appl. Catal. B: Environ. 314 (2022) 121502. doi: 10.1016/j.apcatb.2022.121502

  29. [29]

      X. Li, B. Kang, F. Dong, et al., Nano Energy 81 (2021) 105671. doi: 10.1016/j.nanoen.2020.105671

  30. [30]

      F. Xu, Y. He, J. Zhang, et al., Angew. Chem. Int. Ed. 64 (2025) e202414672. doi: 10.1002/anie.202414672

  31. [31]

      S. Zhou, L. Jiang, H. Wang, et al., Adv. Funct. Mater. 33 (2023) 2307702. doi: 10.1002/adfm.202307702

  32. [32]

      X. Yang, H. Sun, G. Li, et al., Appl. Catal. B: Environ. 294 (2021) 120252. doi: 10.1016/j.apcatb.2021.120252

  33. [33]

      R. Chen, L. Li, Y. Gong, et al., J. Mater. Sci. Technol. 202 (2024) 67–81. doi: 10.1016/j.jmst.2024.02.080

  34. [34]

      J. Guo, H. Sun, X. Yuan, et al., Water Res. 219 (2022) 118558. doi: 10.1016/j.watres.2022.118558

  35. [35]

      S.D. Wang, L.Y. Huang, L.J. Xue, et al., Appl. Catal. B: Environ. 358 (2024) 124366. doi: 10.1016/j.apcatb.2024.124366

  36. [36]

      A. Mariappan, P. Mannu, T. Thiruppathiraja, et al., Chem. Eng. J. 476 (2023) 146720. doi: 10.1016/j.cej.2023.146720

  37. [37]

      Y. Meng, J. Li, H. Liu, et al., Small Methods 9 (2025) 2401510. doi: 10.1002/smtd.202401510

  38. [38]

      D. Zu, Y. Ying, Q. Wei, et al., Angew. Chem. Int. Ed. 63 (2024) e202405756. doi: 10.1002/anie.202405756

  39. [39]

      J. Wang, S. He, M. Zhang, et al., Adv. Energy Mater. 4 (2025) 2406048.

  40. [40]

      S. Li, M. Cai, Y. Liu, et al., Adv. Powder Mater. 2 (2023) 100073. doi: 10.1016/j.apmate.2022.100073

  41. [41]

      Y. Feng, D. Chen, Y. Zhong, et al., ACS Appl. Mater. Interfaces 15 (2023) 9221–9230. doi: 10.1021/acsami.2c19703

  42. [42]

      A. Wang, W. Hong, L. Li, et al., Energy Storage Mater. 44 (2022) 145–155. doi: 10.1016/j.ensm.2021.10.019

  43. [43]

      S. Li, C. You, F. Yang, et al., Chin. J. Catal. 68 (2025) 259–271. doi: 10.1016/S1872-2067(24)60181-6

  44. [44]

      L. Yuan, C. Tang, P. Du, et al., Angew. Chem. Int. Ed. 36 (2024) e202412340. doi: 10.1002/anie.202412340

  45. [45]

      X. Xia, Y. Jia, W. Wang, et al., J. Mater. Sci. Technol. 236 (2025) 301–309. doi: 10.1016/j.jmst.2024.12.093

  46. [46]

      J. Zhu, Q. Bi, Y. Tao, et al., Adv. Funct. Mater. 33 (2023) 2213131. doi: 10.1002/adfm.202213131

  47. [47]

      X. Wang, X. Wang, J. Huang, et al., Nat. Commun. 12 (2021) 4112. doi: 10.1038/s41467-021-24511-z

  48. [48]

      D. Liu, X. Yang, P. Chen, et al., Adv. Mater. 35 (2023) 2300655. doi: 10.1002/adma.202300655

  49. [49]

      L. Sun, X. Yu, L. Tang, et al., Chin. J. Catal. 52 (2023) 164–175. doi: 10.1016/S1872-2067(23)64507-3

  50. [50]

      Y. Zhong, S. Ma, D. Chen, et al., Water Res. 258 (2024) 121774. doi: 10.1016/j.watres.2024.121774

  51. [51]

      Y. Ai, J. Hu, X. Xiong, et al., Appl. Catal. B: Environ. 353 (2024) 124098. doi: 10.1016/j.apcatb.2024.124098

  52. [52]

      S.B. Prasanna, A.A. Jagtap, S. Ramanathan, et al., Chem. Eng. J. 505 (2025) 159579. doi: 10.1016/j.cej.2025.159579

  53. [53]

      V.D. Dang, J.A. Jr, T. Annadurai, et al., Chem. Eng. J. 422 (2021) 130103. doi: 10.1016/j.cej.2021.130103

  54. [54]

      C. Li, B. Cheng, J. Shen, et al., Sep. Purif. Technol. 360 (2025) 131116. doi: 10.1016/j.seppur.2024.131116

  55. [55]

      D. Liu, L. Jiang, D. Chen, et al., Chem. Eng. J. 482 (2024) 149165. doi: 10.1016/j.cej.2024.149165

• 

  Figure 1  (a-c) TEM micrographs of pure Bi₂WO₆ and (d-f) CQDs/Bi₂WO₆-OV at different magnifications. (g) XRD patterns, (h) N₂ adsorption‐desorption isotherms and (i) EPR spectra of various samples.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) Photocatalytic removal and (b) first-order rate constants of CIP by various catalysts under visible light exposure. (c) Cycling tests of CQDs/Bi₂WO₆-OV for CIP degradation. (d) Impact of pH on CIP removal, (e) degradation of CIP in the presence of distinct inorganic ions and HA, (f) degradation of CIP in various water sources.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  UV‒vis DRS of (a) Bi₂WO₆-based catalyst and (b) CQDs. (c) XPS-VB spectra of Bi₂WO₆, Bi₂WO₆-OV and CQDs. (d) EIS spectra, (e) PL spectra and (f) TRPL measurements of different samples. In situ irradiated XPS for (g) W 4f, (h) Bi 4f and (i) C 1s of CQDs/Bi₂WO₆-OV in the dark or under visible light exposure.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Top view (a, d) and side view (b, e) of the charge density difference, plane average charge density difference Δρ (z) (c, f) of CQDs/Bi₂WO₆ and CQDs/Bi₂WO₆-OV. (g) Transient photocurrent density, (h) SPV and (i) IEF intensity of CQDs/Bi₂WO₆ and CQDs/Bi₂WO₆-OV.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  EPR peaks of (a) DMPO-^•O₂^- and (b) DMPO-^•OH. (c) Trapping experiments for the removal of CIP over CQDs/Bi₂WO₆-OV. (d) Effect of different scavengers on the reaction rate constants. (e) Potential mechanism of CQDs/Bi₂WO₆-OV for CIP photodegradation.

  下载: 全尺寸图片 幻灯片
  

  Figure 6  (a) HOMO and LUMO, (b) ESP mapping, (c) optimized structure and (d) Fukui index of CIP.

  下载: 全尺寸图片 幻灯片
  

  Figure 7  (a) Decomposition pathway of CIP by CQDs/Bi₂WO₆-OV. (b) Acute toxicity, (c) mutagenicity and (d) developmental toxicity of CIP and its intermediates.

  下载: 全尺寸图片 幻灯片
•