Hui Sun,
Gaoyang Liang,
Bingge Chen,
Jingqi Jia and
Hongxia Jing
*
School of Chemistry and Chemical Engineering, North University of China, Taiyuan 030051, P. R. China. E-mail: jhx820215@126.com
First published on 22nd April 2025
To address the limited visible-light absorption and rapid charge recombination of Bi2WO6 photocatalysts, this work constructs a Z-scheme ZnO/Fe3+-doped Bi2WO6 heterojunction via a hydrothermal-calcination method. The Fe3+ doping induces the formation of oxygen vacancies and optimizes the band structure, which cooperates with the interface reconstruction of ZnO to expand the light absorption to 480 nm. The hierarchical pore structure simultaneously enhances the mass transfer efficiency, and finally realizes the efficient degradation of tetracycline under visible light (the removal rate is 95.5% in 60 minutes, and the rate is 2.28 times higher than that of the pure phase) and the stable cycle performance is good. Mechanistic studies demonstrate that Z-scheme charge transfer driven by an interfacial built-in electric field ensures effective carrier separation, with photogenerated holes (h+) as key reactive species. The proposed “defect-heterojunction-interface trinity” strategy establishes a new design scheme for bismuth-based Z-scheme photocatalysts.
Photocatalytic oxidation has emerged as a promising strategy for TC remediation, leveraging solar-driven reactive oxygen species (ROS) generation to achieve contaminant degradation.10–12 Among various photocatalysts, bismuth tungstate (Bi2WO6) has garnered significant attention in visible-light-driven photocatalysis due to its unique layered structure and moderate bandgap (about 2.8–3.0 eV).13–16 However, pristine Bi2WO6 exhibits unsatisfactory photocatalytic TC degradation efficiency (more than 60% under optimal conditions), primarily due to: (1) limited visible-light absorption range (λ < 450 nm), which underutilizes 43% of solar spectrum energy; (2) rapid electron-hole recombination, resulting in low quantum efficiency; and (3) insufficient surface active sites for adsorbing TC macromolecules (dimensions: 1.2 × 0.6 nm).14,17–19 Although modification strategies such as metal ion doping (e.g., Sm3+ elevating degradation rate to 80% (ref. 20)) and heterojunction construction (e.g., g-C3N4 composites achieving 4-fold rate constant enhancement21) show partial success, single-component modifications fail to synergistically address light absorption, charge separation, and substrate adsorption. They face inherent trade-offs in performance and limited performance improvement.
Z-Scheme heterojunctions, inspired by natural photosynthesis, have emerged as a promising strategy to enhance photocatalytic performance. Unlike traditional Type-II heterojunctions, Z-scheme systems enable spatially separated redox reactions by retaining photogenerated electrons and holes with stronger redox potentials. This mechanism not only suppresses charge recombination but also enhances the generation of reactive oxygen species (ROS), such as hydroxyl radicals (·OH) and superoxide radicals (·O2−), which are critical for the degradation of tetracycline. Recent studies have demonstrated that Z-scheme heterojunctions, exhibit superior photocatalytic activity compared to conventional heterojunctions due to their unique charge transfer pathways.22–24
This study develops a high-efficiency Bi2WO6-based photocatalytic system through synergistic integration of Fe3+ doping and ZnO heterojunction engineering. The dual modification of Fe3+ and ZnO achieved triple enhancement: broadening spectral response via defect engineering, improving charge separation through a Z-scheme pathway, and optimizing TC adsorption-mass transport via hierarchical porosity. Mechanistic studies identify photogenerated holes (h+) as the dominant reactive species, advancing fundamental understanding of charge behavior. Furthermore, the “defect-heterojunction-interface trinity” design framework unifies structural optimization, reaction dynamics control, and system adaptability, establishing a novel paradigm for rationally developing bismuth-based Z-scheme photocatalysts toward complex antibiotic-polluted environments.
3.880 g Bi(NO3)3·5H2O, 1.319 g Na2WO4·2H2O, and a stoichiometric amount of Fe(NO3)3·9H2O (corresponding to Fe3+:
Bi2WO6 molar ratios of 0.1
:
100, 0.26
:
100, 0.52
:
100, and 0.78
:
100) were dissolved in 40 mL deionized water. The subsequent steps (pH adjustment, ultrasonication, hydrothermal treatment, and calcination) were identical to the synthesis of pristine Bi2WO6. The synthesized products were named as 0.1Fe3+
-doped Bi2WO6, 0.26Fe3+
-doped Bi2WO6, 0.52Fe3+
-doped Bi2WO6, and 0.78Fe3+
-doped Bi2WO6.
η = [(A0 − At)/A0] × 100% | (1) |
Additionally, experiments were carried out to identify the primary reactive species involved in the photocatalytic process.
In the experiment, isopropyl alcohol (IPA) was introduced as a scavenger for hydroxyl radicals (·OH), and 1,4-benzoquinone (BQ) for superoxide radicals (·O2−), and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) for photogenerated holes (h+).
Fig. 2 comparatively analyzes the morphological evolution of pristine Bi2WO6 and ZnO/Fe3+-doped Bi2WO6 via SEM characterization. The unmodified Bi2WO6 exhibits a compact platelet morphology with an average lateral dimension of 0.5 μm (Fig. 2a–c). In stark contrast, the ZnO/Fe3+-doped Bi2WO6 heterostructure demonstrates significantly expanded nanosheets (1–2 μm) featuring a fluffy architecture composed of interconnected ultrathin layers (Fig. 2d–f). The resultant hierarchical porous structure achieves a substantially enhanced specific surface area, which facilitates reactant molecule adsorption and maximizes exposure of catalytically active sites.
Fig. 3 systematically characterizes the microstructure and elemental distribution of the ZnO/Fe3+-doped Bi2WO6 nanocomposites. The low-magnification TEM image (Fig. 3a) reveals, a translucent lamellar architecture with lateral dimensions of 1–2 μm consistent with the fluffy morphology observed by SEM. High-resolution TEM imaging (Fig. 3b–d) demonstrates that ultrathin ZnO nanosheets are uniformly anchored on the Bi2WO6 substrate, forming well-defined 2D heterointerfaces through edge-contact interactions. The TEM-EDS elemental mapping (Fig. 3e) deciphers the spatial distribution characteristics: Bi and W exhibit continuous distributions across the basal planes, Zn displays edge-enriched patterns at the nanosheet peripheries, and Fe is homogeneously dispersed within the Bi2WO6 lattice. This multiscale structural consistency—spanning crystallographic features, morphological evolution, and elemental localization—conclusively validates the precision-engineered architecture.
Fig. 4 reveals the atomic-scale interfacial characteristics of the ZnO/Fe3+-doped Bi2WO6 heterojunction through high-resolution TEM (HRTEM) and STEM analyses. The TEM/SEM images clearly show nanoscale ZnO particles uniformly deposited on the surface of Fe3+-doped Bi2WO6 nanosheets, forming defect-free and tightly bonded interfacial contacts (Fig. 4a and b). HRTEM analysis of the interfacial region (Fig. 4a) demonstrates a lattice spacing about 0.316 nm for Fe3+-Bi2WO6, corresponding to the (131) plane of the orthorhombic Bi2WO6. This spacing exhibits lattice expansion compared to the undoped sample (0.315 nm), attributed to Fe3+-induced lattice distortion. Adjacent to this, a spacing of 0.191 nm matches the (102) plane of wurtzite ZnO, indicating coherent lattice alignment between the two phases with no amorphous interfacial layers. Further interfacial analysis (Fig. 4b) reveals additional crystallographic features: 0.1621 nm and 0.1923 nm lattice spacings correspond to the ZnO (110) and Bi2WO6 (202) planes, respectively. These values show negligible deviation from standard reference data (JCPDS #39-0256 for Bi2WO6; JCPDS #36-1451 for ZnO), confirming the structural integrity of both phases. This provides direct evidence for the precisely constructed heterojunction, where ZnO nanoparticles establish strong electronic coupling with the Fe3+-doped Bi2WO6 substrate. Such interfacial characteristics are critical for enabling efficient charge transfer during photocatalytic processes.
Fig. 5 systematically characterizes the surface chemical states and elemental coordination environments of the ZnO/Fe3+-doped Bi2WO6 composite and Bi2WO6. The full-spectrum analysis (Fig. 5a) clearly detects characteristic signal peaks corresponding to five elements: Bi, W, O, Zn, and Fe, which are fully consistent with the designed material composition. The high-resolution Bi 4f spectrum (Fig. 5b) exhibits two typical peaks at 158.9 eV and 164.23 eV, assigned to the Bi3+ 4f7/2 and 4f5/2 orbitals, respectively.29,30 The binding energy separation matches standard Bi2WO6, confirming the structural integrity of the Bi–O–W framework. It can be seen from the Bi 4f spectrum that the ZnO/Fe3+-doped Bi2WO6 moves to a higher binding energy direction than Bi2WO6, which confirms the driven directional electron transfer along the Bi2WO6 → Fe3+ → ZnO path. It also reveals the synergistic effect of heterojunction engineering and ion doping in optimizing the electronic structure. The O 1s spectrum (Fig. 5c) is deconvoluted into three chemical states: the dominant peak at 529.73 eV corresponds to lattice oxygen (OL) in Bi2WO6, the secondary peak at 530.40 eV arises from oxygen vacancy defects (OV), and the higher binding energy peak at 531.61 eV originates from surface-adsorbed oxygen species (OA).31–33 The relative intensity ratio (OV/OL = 0.15) indicates the presence of abundant active sites in the material and the concentration of oxygen vacancies is significantly higher than that of Bi2WO6. The Zn 2p spectrum (Fig. 5d) displays symmetric doublet peaks at 1021.86 eV (Zn 2p3/2) and 1044.91 eV (2p1/2), with a spin-orbit splitting energy of 23.05 eV, which is fully consistent with Zn2+ characteristics, confirming the existence of ZnO as an independent phase.29,34,35 In the W 4f spectrum (Fig. 5e), the characteristic peaks at 35.18 eV (4f7/2) and 37.36 eV (4f5/2) verify the stable presence of W6+.29,36 The absence of significant peak broadening demonstrates that Fe3+ doping does not disrupt the WO6 octahedral structure. Notably, the Fe 2p spectrum (Fig. 5f) exhibits characteristic peaks at 712.31 eV (2p3/2) and 724.68 eV (2p1/2), confirming the successful incorporation of Fe3+.33,37 These results mutually corroborate the successful construction of the ZnO/Fe3+-doped Bi2WO6 system.
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Fig. 5 XPS spectra of ZnO/Fe3+-doped Bi2WO6 and Bi2WO6 sample: (a) survey of the photocatalyst; (b) Bi 4f; (c) W 4f; (d) O 1s; (e) Zn 2p; (f) Fe 2p. |
Fig. 6 systematically elucidates the pore topology and mass transfer enhancement mechanisms of the ZnO/Fe3+-doped Bi2WO6 nanocomposite. The N2 adsorption–desorption isotherm (Fig. 6) exhibits Type IV characteristics with an H3-type hysteresis loop, consistent with slit-shaped pores in layered materials, aligning with TEM-observed stacked nanosheets. The BJH pore size distribution curve (Fig. 6) reveals a bimodal pore structure: a dominant peak at 23.02 nm (mesoporous region, 2–50 nm) and a secondary peak extending to 72.5 nm (macropore region, >50 nm). This hierarchical architecture originates from Fe3+ doping-induced lattice strain and ZnO heterointerface engineering.
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Fig. 6 Nitrogen adsorption–desorption isotherm, and pore size distribution of ZnO/Fe3+-doped Bi2WO6. |
The BET specific surface area (25.2 m2 g−1) shows a 26% enhancement compared to pristine Bi2WO6 (literature value about 20 m2 g−1 (ref. 38–40)), with a total pore volume of 0.1221 cm3 g−1, where mesopores account for half of the total pore volume. Microstrain induced by lattice distortion expands the interlayer spacing of Bi2WO6, while the spatial confinement effect of ZnO nanosheets suppresses dense stacking of layered plates. These synergistic interactions create an interconnected pore network. This unique structure endows the material with dual mass transfer advantages: (1) mesopores enhance tetracycline (TC) molecule enrichment via Kelvin capillary condensation, significantly increasing local concentration; (2) macropores serve as rapid mass transfer channels, substantially improving molecular diffusion rates.
Fig. 7 provides an in-depth analysis of the chemical bonding and defect evolution mechanisms at the ZnO/Fe3+-doped Bi2WO6 heterointerface through Fourier-transform infrared spectroscopy (FT-IR). The calcined sample exhibits a prominent absorption peak at 1300 cm−1 (Fig. 7), attributed to oxygen vacancies (VO) 3.2 associated with metal-oxygen lattice vibrations. The peak intensity increases significantly after calcination, combined with XPS quantification, confirming that high-temperature treatment induces oxygen deintercalation from the Bi2WO6 lattice, generating high-concentration VO defects.41,42 Gaussian peak deconvolution of the 1450 cm−1 feature resolves two components: a broadened peak at 1428 cm−1 corresponding to the δ(Zn–O) bending vibration in the ZnO lattice, and a shoulder peak at 1465 cm−1 originating from ν(Zn–O–W) heterointerface vibrations.43,44 This unique vibrational mode provides direct spectroscopic evidence of chemical bonding between ZnO and Bi2WO6. Spectral evolution analysis reveals critical structural changes: the disappearance of the adsorbed water hydroxyl (–OH) peak (1380 cm−1) indicates enhanced crystallinity, while the concurrent enhancement of VO defect peaks and ν(Zn–O–W) vibrational peaks confirms that the heterointerface achieves atomic-level bonding via solid-phase reaction.45
According to Tauc's eqn (2), the bandgaps of ZnO and Bi2WO6 are estimated based on their UV-vis DRS spectra (Fig. 8b). Here, α denotes the light absorption coefficient, h represents Planck's constant, ν denotes the light frequency, A is a proportionality constant, and n are the band energy and Eg signifies the bandgap energy. ZnO and Bi2WO6 are indirect semiconductors, so n is 1/2.
αhν =A(hν −Eg)n/2 | (2) |
The composite material exhibits a bandgap of 2.68 eV, which is 0.30 eV narrower than pristine Bi2WO6 (2.98 eV) and significantly lower than the Fe3+-doped system (2.89 eV). These results confirm the synergistic interplay between heterojunction engineering and ion doping in optimizing electronic structures.
Fig. 9 elucidates the charge separation enhancement mechanism of ZnO/Fe3+-doped Bi2WO6 through steady-state photoluminescence (PL) spectroscopy and carrier dynamics analysis. Under 470 nm laser excitation (matching the band-edge transition energy of Bi2WO6), the PL emission intensity of the composite is significantly reduced compared to pristine Bi2WO6, indicating effective suppression of non-radiative carrier recombination pathways. Combined Mott–Schottky measurements and HRTEM interfacial analysis reveal that a flat-band potential shift (ΔEfb = 0.57 eV) at the heterojunction interface induces a built-in electric field. This field drives directional electron migration along the Bi2WO6 → Fe3+ → ZnO transfer pathway. The synergy between defect-state trapping and charge transfer mechanisms significantly enhances photocatalytic degradation under visible light.
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Fig. 9 Fluorescence spectra of ZnO/Fe3+-doped Bi2WO6, pure Bi2WO6 and ZnO with different molar ratios. |
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Fig. 10 Impedance spectra (a), photocurrent response curves (b) and MS curves (c) of ZnO/Fe3+-doped Bi2WO6, Bi2WO6, ZnO and 0.26Fe3+-doped Bi2WO6. |
As shown in Fig. 10(c), the curves of ZnO/Fe3+-doped Bi2WO6, Bi2WO6, ZnO and 0.26Fe3+-doped Bi2WO6 all show positive slope characteristics in the test potential range (−2.0 to 0.2 V vs. SCE), which strictly conforms to the classical response behavior of n-type semiconductors. The flat band potentials of ZnO/Fe3+-doped Bi2WO6 (−1.45 eV), Bi2WO6 (−0.74 eV), ZnO (−1.42 eV) and 0.26Fe3+-doped Bi2WO6 (−0.85 eV) were determined by linear extrapolation method. It must be taken into account that their flat band potential (Efb) is −1.45 V (−1.25 V vs. NHE), −0.74 V (−0.54 V vs. NHE), −1.42 V (−1.22 V vs. NHE), −0.85 V (−0.65 V vs. NHE) relative to the Ag/AgCl distribution.46 The numerical differences reflect band engineering modulation.
According to semiconductor electrochemistry theory, the relationship between Efb and conduction band minimum (ECB) for n-type semiconductors is: ECB = Efb − 0.1 eV (pH = 7).46,47 Therefore, the ECB of 0.26Fe3+-doped Bi2WO6 is −0.75 eV, and that of ZnO is −1.32 eV. The Efb of 0.26Fe3+-doped Bi2WO6 is negatively shifted by 0.11 eV compared with that of pure Bi2WO6, indicating that the substitution of Fe3+ for W6+ sites induces the formation of oxygen vacancies. The Efb of ZnO/Fe3+-doped Bi2WO6 is further negatively shifted by 0.60 eV due to the dual effects of the built-in electric field and the space charge layer reconstruction.
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Fig. 11 Degradation curves (a–c) and kinetic curves (d) of different materials under simulated sunlight irradiation. |
This performance breakthrough originates from Fe3+ doping-induced oxygen vacancy concentration enhancement (XPS quantification: OV/OL = 0.15). Upon constructing the ZnO heterojunction (mass ratio 1.71:
1), the degradation efficiency surges to 95.5% (Fig. 11b), with a reaction rate constant (k) of 0.05343 min−1 (Fig. 11d), which is 2.28-fold higher than that of Fe3+-doped Bi2WO6 (0.02344 min−1) and 2.7-fold greater than pristine Bi2WO6 (0.01983 min−1), establishing it as one of the highest reported values among similar materials.
The performance improvement is attributed to the triple synergy mechanism: (1) heterojunction interface engineering: HRTEM and SAED analysis show that ZnO nanosheets (thickness 8 nm) form a coherent interface with Bi2WO6 through edge contact, and the Mott–Schottky test shows that the interface generates a certain intensity of built-in electric field (ΔEfb = 0.57 eV); (2) synergistic effect of defect states: Fe3+ replaced W site to cause lattice distortion; (3) multi-stage mass transfer channels: BET analysis showed that the hierarchical pore structure (mesoporous 23 nm, macroporous 72.5 nm) significantly increased the diffusion rate of TC molecules, and finally achieved a breakthrough improvement in photocatalytic performance.
Fig. 12a evaluates the cycling stability of the ZnO/Fe3+-doped Bi2WO6 heterojunction. Over five consecutive cycles, the TC degradation efficiency remains at 95.5%, 92.65%, 90.09%, 89.96%, and 87.89%, with a cumulative decay of only 7.61 percentage points, demonstrating exceptional structural durability. This stability arises from the lattice reinforcement, and pore preservation mechanisms, highlighting its potential for industrial applications in complex aqueous environments.
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Fig. 12 The cycle experimental diagram (a) and active species capture experimental diagram (b) of ZnO/Fe3+-Bi2WO6 catalyst. |
Fig. 12b reveals the dominant active species via radical trapping experiments. Under the addition of scavengers for ·O2− (BQ), ·OH (IPA), and h+ (EDTA-2Na), the tetracycline degradation rates decreased to 65.4%, 69%, and 10.3%, respectively, confirming photogenerated holes (h+) as the primary drivers of degradation.
EVB = Eg + ECB | (3) |
The valence band (VB) are calculated as 1.88 eV and 2.14 eV.48 The built-in field drives the photogenerated electrons to migrate from the CB of Fe3+-doped Bi2WO6 to the VB of ZnO, which effectively separates the carriers and reduces the recombination of photogenerated carriers. The photogenerated e− with stronger reduction ability on the CB of ZnO and the strong oxidation ability h+ of VB of Bi2WO6 are retained, forming a unique Z-type charge transfer pathway (Fig. 13).
In contrast, a conventional Type-II heterojunction—where electrons from the CB of Fe3+-doped Bi2WO6 would migrate to the VB of ZnO—is limited by the insufficient oxidation capacity of ZnO's VB holes (1.88 eV vs. the ·OH/·OH− oxidation potential of 1.99 eV). This contradicts scavenger experiments and fails to achieve efficient degradation. The Z-scheme system, however, enables effective separation of photogenerated electrons and holes, resulting in a tetracycline degradation rate constant of 0.5343 min−1 and highly efficient degradation.
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