This study successfully constructed an S-scheme cadmium tungstate/bismuth tungstate heterojunction photocatalyst driven by an interfacial built-in electric field, significantly improving its photocatalytic degradation performance. Owing to the intimate interfacial contact between CdWO4 nanorods and Bi2WO6 nanoflowers, the resulting S-scheme heterojunction effectively enhanced the separation and migration of photogenerated charge carriers, thereby promoting the generation of superoxide and hydroxyl radicals. The optimized CdWO4/Bi2WO6 heterojunction achieved 84% degradation of sulfadiazine (SDZ) within 180 minutes. Its apparent rate constant, k, was 1.62 and 10.2 times those of pristine Bi2WO6 and CdWO4, respectively, while the mineralization efficiency reached 54.39%. After four recycling tests, the degradation efficiency remained at 72.2%. Density functional theory calculations combined with liquid chromatography–mass spectrometry revealed that the main SDZ degradation pathways involved S–N bond cleavage, N–C bond cleavage, and a Smiles rearrangement. This work provides a new strategy for constructing interfacial built-in-electric-field-driven S-scheme heterojunctions for the photocatalytic oxidation of SDZ.
In February 2026, the Journal of Colloid and Interface Science published online the latest photocatalysis research by the teams of Professor Xuejun Zou and Professor Yuying Dong from Dalian Minzu University and Professor Jun Ke from North China Electric Power University. The work reports the design of a built-in-electric-field-driven S-scheme cadmium tungstate/bismuth tungstate heterojunction and investigates its photocatalytic performance. Huanbo Ni is the first author, while Xuejun Zou, Yuying Dong, and Jun Ke are the co-corresponding authors.
The extensive use of antibiotics has caused serious environmental contamination. Sulfadiazine (SDZ), which has a stable molecular structure and contains a sulfonamide group, can persist in the environment and pose major threats to ecosystems and human health. Existing mainstream technologies for treating SDZ-containing wastewater commonly suffer from low energy efficiency, difficulty in selectively disrupting its aromatic-ring–pyrimidine-ring structure, and low mineralization efficiency. The degradation process may also produce sulfonamide analogues that are even more difficult to decompose, resulting in secondary pollution.
Photocatalysis, an important branch of advanced oxidation processes, has become a major research focus because it is environmentally friendly, can utilize solar energy, and offers strong degradation potential. Constructing heterojunctions is an effective approach to improving photocatalytic performance. S-scheme heterojunctions can efficiently promote charge separation, although achieving directional migration of high-energy photogenerated carriers and establishing effective interfacial interactions remain challenging.
Tungstate materials are promising photocatalysts because of their distinctive optoelectronic properties. Cadmium tungstate, CdWO4, has a suitable bandgap, while its self-activated luminescence can alleviate electron–hole recombination. Bismuth tungstate, Bi2WO6, has a layered perovskite structure, abundant active sites, rapid charge transfer, and a high quantum yield. Combining the two into a binary tungstate heterostructure can integrate their advantages and address the low charge-separation efficiency and limited active sites of single-component tungstates.
In this work, substrate synthesis was precisely controlled to prepare a binary tungstate heterostructure composed of CdWO4 nanorods and Bi2WO6 nanoflowers. By exploiting their complementary structures and optoelectronic properties, spatially separated oxidation and reduction regions were established, photogenerated-carrier recombination was suppressed, light absorption was improved, and reactive-oxygen-species generation was regulated. The resulting system enabled efficient SDZ degradation in water and offers a new solution to limitations encountered in conventional advanced oxidation technologies.
The equipment used in this study was the Perfectlight PLS-CS300 xenon lamp light source. By optimizing the lamp-body structure and light-guiding configuration, the output optical power was substantially increased. Test results showed an improvement of more than 30% in optical power at different operating currents when all other conditions, including arrangement distance, were kept the same. This level of output enhancement can support broader, faster, and more efficient photocatalytic applications.
Figure 1. (a, b) XRD patterns of Bi2WO6, CdWO4, and CWBW-X (X = 1–4).
X-ray powder diffraction was used to characterize the crystalline phases of Bi2WO6, CdWO4, and the CWBW heterojunctions. Their diffraction peaks matched standard cards JCPDS 84-0038 and 73-1126, respectively, confirming the successful synthesis of the target tungstate phases. Characteristic CdWO4 reflections, including the (110) plane, appeared at 2θ = 23.25°, while Bi2WO6 showed characteristic reflections such as the (131) plane at 2θ = 28.38°.
The diffraction patterns of the CWBW-X composites were similar to that of pristine Bi2WO6. Enhanced peak intensities at 2θ = 28.49° and 32.91°, together with obvious peak shifts relative to CdWO4, resulted from lattice interactions between the two components and confirmed their coexistence and successful heterostructure formation. Changes in the full width at half maximum of the strongest diffraction peak indicated that increasing the concentrations of Bi3+ and WO42− accelerated Bi2WO6 nucleation. The increased number of nuclei intensified spatial constraints during crystal growth and inhibited further grain enlargement. In addition, lattice strain induced by component interactions impeded atomic migration and rearrangement, ultimately affecting crystallite size.
Figure 2. (a, b) SEM images of CdWO4; (c, d) SEM images of Bi2WO6; (e, f) SEM images of CWBW-3 at 10 µm and 3 µm; (g, h) TEM images of CWBW-3; and (i) elemental mapping of CWBW-3.
Scanning electron microscopy was used to characterize the morphologies of CdWO4, Bi2WO6, and the optimized CWBW-3 composite. CdWO4 exhibited a typical nanorod morphology consistent with its monoclinic structure and P2/c space group. Bi2WO6 formed nanoflowers capable of providing abundant catalytic active sites. In CWBW-3, CdWO4 nanorods grew on the surface of the Bi2WO6 nanoflowers. The resulting hybrid structure displayed mesoporous characteristics that could effectively promote molecular mass transfer during the reaction.
Transmission electron microscopy confirmed that the morphologies of the two tungstates were consistent with the SEM observations. In the high-resolution TEM images, the lattice spacing marked in dark blue corresponded to the CdWO4 (010) plane, while the light-cyan marking corresponded to the Bi2WO6 (112) plane, confirming successful construction of the CdWO4/Bi2WO6 composite. Elemental mapping of CWBW-3 further showed that Bi, Cd, O, and W were uniformly distributed throughout the sample.
Figure 3. N2 adsorption–desorption isotherms and corresponding pore-size distribution curves, shown in the insets, of pristine (a) CdWO4, (b) Bi2WO6, and (c–f) CWBW-X composites (X = 1–4).
BET analysis was performed on pristine CdWO4, Bi2WO6, and the CWBW composites to investigate the structure–activity relationship governing photocatalytic performance. The pristine materials adsorbed very little nitrogen at low relative pressures, indicating small intrinsic pore volumes and low porosity. Their abrupt adsorption increase at high pressure originated primarily from interparticle voids rather than intrinsic pores. These macroporous or disrupted mesoporous structures could hinder mass transfer and accelerate recombination of photogenerated carriers.
In contrast, the CWBW composites displayed enhanced adsorption at low pressure and type-IV isotherms with hysteresis loops, confirming a more regular mesoporous structure. Although all composites had lower BET surface areas, ranging from 4.76 to 6.45 m2/g, than the pristine phases, their pore structures were more ordered and optimized. Pore diameter varied with composition: CWBW-2 had the smallest mesopore diameter of 21.7 nm, whereas CWBW-4 had the largest at 48.7 nm.
This structural optimization improved the accessibility of active sites and mass-transfer efficiency, compensating for the reduction in surface area and enhancing photocatalytic activity. The study confirmed that heterojunction formation reduced the specific surface area while compositionally regulating pore structure. The intrinsic mesopores in the composites not only provided additional adsorption sites for pollutants but also suppressed electron–hole recombination through spatial confinement, reduced mass-transfer resistance, and improved light utilization. These effects worked synergistically with heterojunction-mediated charge transfer, highlighting the critical role of mesoporous architecture in optimizing photocatalytic activity.
Figure 4. (a) UV–Vis diffuse-reflectance spectra and Tauc plots; (b) PL spectra; (c) I–t curves; (d) EIS Nyquist plots; (e) Cdl plots; (f) LSV curves in the dark; (g) LSV curves under visible-light irradiation; (h) Mott–Schottky plots of CdWO4, Bi2WO6, and CWBW-3 catalysts; (i) schematic relative band alignment of the CdWO4/Bi2WO6 heterostructure; and (j) FT-IR transmittance spectra.
To systematically evaluate the photocatalytic potential of the CWBW system, the study investigated its optical properties, electrochemical behavior, and band structure. Fourier-transform infrared spectroscopy was also used to reveal how heterostructure formation regulated the photoelectric effect at the levels of bond characteristics and lattice microstructure.
Optical characterization showed that the absorption edge of the CWBW composites was approximately 450 nm, representing a pronounced redshift relative to pristine Bi2WO6 and CdWO4. Their bandgaps narrowed to 2.76–2.82 eV, substantially below those of Bi2WO6 at 3.06 eV and CdWO4 at 3.19 eV. This promoted efficient visible-light harvesting and the generation of larger numbers of photogenerated carriers. Photoluminescence measurements showed much weaker emission intensities for the composites than for the pristine phases, confirming that heterojunction formation effectively inhibited carrier recombination.
Photoelectrochemical testing demonstrated that CWBW-3 produced the strongest and most stable photocurrent response, reflecting excellent charge-separation, transfer efficiency, and photoelectrochemical stability. Its smaller EIS Nyquist semicircle indicated lower charge-transfer resistance and faster interfacial charge transport. Cyclic-voltammetry-derived double-layer capacitances for the CWBW samples ranged from 0.112 to 0.131 mF cm−2, all higher than those of pristine Bi2WO6 and CdWO4. This indicated a larger electrochemically active surface area and a higher density of catalytic active sites. Linear sweep voltammetry further confirmed that the composites generated markedly higher photocurrent under visible light, with CWBW-3 showing the best overall photoelectrochemical performance.
Mott–Schottky plots gave flat-band potentials of −0.66 V and −0.61 V versus SCE for CdWO4 and Bi2WO6, respectively. Their positive slopes confirmed that both were n-type semiconductors. Calculations combining these values with the measured bandgaps produced valence-band-edge positions of 2.45 eV for CdWO4 and 2.21 eV for Bi2WO6. These results clarified the relative band structures and confirmed the thermodynamic feasibility of CdWO4/Bi2WO6 heterojunction formation.
FT-IR peaks at 3415, 1620, and 2358 cm−1 were attributed to physically adsorbed water vapor and carbon dioxide and did not affect the intrinsic photoelectrochemical properties. Peaks at 1225 and 1078 cm−1 corresponded to stretching vibrations of W–O–W bridging bonds in the tungstates. This stable bridging-oxygen framework provided efficient pathways for photogenerated-carrier transport and reduced recombination during migration. Bi–O bond peaks at 756–698 cm−1 confirmed the successful incorporation of the bismuth-containing component. The Bi–O bonds helped regulate the bandgap, broaden the visible-light response, and improve carrier generation, separation, and transport. These FT-IR results provided direct structural evidence for the enhanced photoelectric effect and were consistent with the photoelectrochemical measurements.
Figure 5. (a) SDZ photodegradation efficiency under visible-light irradiation; (b) kinetic curves used to quantify the SDZ photodegradation rate; (c) TOC-removal kinetics of CWBW-3; (d) cycling stability of the optimal catalyst during SDZ photodegradation over five cycles; (e) calculated differential charge density and equipotential surfaces; (f) spatial electron gain/loss distribution at the CdWO4/Bi2WO6 heterojunction; (g–i) in situ XPS spectra of (g) Cd 3d, (h) Bi 4f, and (i) W 4f for CWBW-3 under dark and illuminated conditions; and (j–l) EPR spectra detecting photogenerated reactive species, including h+, ·O2−, and ·OH.
A xenon lamp with a light intensity of 3.3 mW/cm2 was used to simulate solar irradiation and evaluate SDZ degradation by the CWBW system. Density functional theory calculations, in situ XPS, and electron paramagnetic resonance were combined to investigate the catalytic mechanism.
After 30 minutes of dark adsorption, pristine CdWO4 and Bi2WO6 adsorbed only 2% and 7% of SDZ, respectively, whereas the CWBW-X composites achieved adsorption rates of 6%–8%. Their richer mesoporous structures therefore provided improved adsorption and created favorable conditions for subsequent photocatalytic degradation. Under illumination, SDZ degradation efficiencies were 68% for pristine Bi2WO6, 17% for pristine CdWO4, and 70%–84% for the CWBW-X composites. CWBW-3 delivered the best result at 84%, with a pseudo-first-order rate constant of 0.01081, substantially higher than those of the pristine phases and the other composites.
CWBW-3 achieved a total organic carbon removal efficiency of 54.39%, confirming that SDZ could be efficiently mineralized into nontoxic small molecules such as CO2 and H2O. After four degradation cycles, its efficiency declined. Following centrifugal recovery, filtration, and calcination-based regeneration, the degradation efficiency recovered to 72.2%, demonstrating good recyclability. The incomplete recovery was likely associated with the loss of active components during catalyst collection.
DFT calculations showed that, in the ground state, electrons transferred from Bi2WO6 to CdWO4. Pronounced interfacial charge-density changes indicated strong electronic interaction, and the charge-density profile along the z direction quantitatively confirmed this interfacial electron transfer. In situ XPS showed that, under illumination, the Cd 3d and W 4f peaks of CWBW-3 shifted toward higher binding energies, while the Bi 4f peaks shifted in the opposite direction. This confirmed directional electron transfer through W–O bonds from CdWO4 to Bi2WO6, consistent with the DFT calculations and strongly supporting the proposed S-scheme charge-transfer mechanism.
EPR analysis detected abundant stable surface oxygen vacancies at g = 2.002. These vacancies can activate O2, trap photogenerated electrons, and promote reactive-oxygen-species formation. Under illumination, the h+, ·O2−, and ·OH signals of CWBW-3 were much stronger than those of the pristine phases and increased further with irradiation time. This demonstrated that more high-energy holes accumulated to generate ·OH, while ·O2− production also increased markedly. The results revealed built-in-electric-field-driven S-scheme carrier migration and identified the major reactive species involved in SDZ degradation.
Figure 6. (a) Concept and formation of S-scheme heterojunction photocatalysts and (b) schematic photocatalytic mechanism of the CWBW heterojunction.
Based on the DFT calculations, XPS and ESR results, and band-structure analysis, the study proposed an S-scheme heterojunction mechanism for the CWBW catalyst. The Fermi level, conduction band, and valence band of Bi2WO6 are all higher than those of CdWO4. When the absorbed photon energy equals or exceeds the bandgap, valence-band electrons are excited into the conduction band.
After the two semiconductors contact each other in the dark, electrons transfer from Bi2WO6 to CdWO4, forming an interfacial built-in electric field and inducing band bending. Under illumination, less-reactive photogenerated electrons and holes recombine along the bent band edges. The highly negative conduction-band potential of Bi2WO6, −0.85 eV versus NHE, allows its accumulated photogenerated electrons to reduce dissolved oxygen to ·O2−. Meanwhile, photogenerated holes in the CdWO4 valence band at 2.45 eV versus NHE can oxidize OH− to ·OH. Both reactive species efficiently oxidize and degrade SDZ molecules. The S-scheme heterojunction therefore promotes efficient charge separation and transfer while extending carrier lifetimes.
Figure 7. (a) Molecular structure and optimized geometry; (b) HOMO and LUMO orbital distributions; (c) electrostatic-potential map; (d) Fukui-function isosurfaces, f−, f+, and f0; (e) Mulliken charge distribution and Fukui-index values of SDZ; and (f) proposed SDZ transformation pathways in the CdWO4/Bi2WO6 S-scheme heterostructure system.
To further clarify SDZ degradation pathways in the CWBW S-scheme heterojunction system, the study combined frontier-molecular-orbital analysis, electrostatic-potential mapping, and Fukui-function analysis. DFT calculations provided the Mulliken charge distribution and average local softness of SDZ, while LC–MS was used to systematically identify degradation mechanisms and transformation pathways.
SDZ consists of an aminobenzene unit and a pyrimidine ring connected through a sulfonamide group. Its HOMO is mainly distributed over the aminobenzene moiety, whereas its LUMO is concentrated on the pyrimidine ring. The HOMO–LUMO energy gap reaches 4.80 eV, making SDZ difficult to degrade under natural light without a catalyst. In the CWBW system, the oxidative pathway involves h+ attack on the HOMO-rich aminobenzene region, while the reductive pathway involves electron injection into the LUMO-rich pyrimidine ring.
The negative electrostatic-potential regions of SDZ are concentrated on the aminobenzene ring and the nitrogen atom of the sulfonamide group, making them susceptible to h+ attack. Positive-potential regions occur on the pyrimidine ring and nearby carbon atoms, making them susceptible to electron attack. These results are consistent with the orbital analysis.
Fukui-function calculations showed that N17, C1, and C5 are prone to electrophilic reactions, mainly involving h+; C13, C15, and N12 on the pyrimidine ring are susceptible to nucleophilic reactions involving electrons and ·O2−; and N17, C13, and C15 are major radical-reaction sites involving ·OH. Quantitative average-local-softness analysis further confirmed that ·OH preferentially attacks N17, C13, and C15, identifying these positions as key initial sites for SDZ degradation.
Combining the theoretical results with LC–MS data, four principal degradation routes were proposed, with three core pathways. Pathway I involves cleavage of the sulfonamide S–N bond, followed by oxidation, hydroxylation, reduction, and eventual mineralization of intermediates into small molecules. Pathway II proceeds through a Smiles rearrangement that generates an isomeric intermediate before further mineralization. Pathway III involves N–C bond cleavage, splitting the molecule into sulfonamide–benzene and pyrimidine fragments that subsequently undergo hydroxylation and radical attack to form sulfur-containing and amino-containing small molecules. Pathway IV begins with ·OH attack at N17 to form a specific intermediate. Overall, SDZ degradation is initiated mainly by ·OH attack on the S–N bond, N–C bond, and –NH2 group.
This work successfully synthesized an S-scheme heterojunction photocatalyst composed of rod-shaped CdWO4 and flower-like Bi2WO6 through a hydrothermal process followed by calcination. Compared with pristine Bi2WO6 and CdWO4, the heterojunction exhibited substantially improved photocatalytic performance. Under xenon-lamp irradiation, the optimized CWBW-3 system achieved 84% SDZ degradation within 180 minutes and a mineralization efficiency of 54.39%. After four reuse cycles, the degradation efficiency remained at 72.2%, demonstrating good recyclability and structural stability.
Experimental characterization and DFT calculations showed that the S-scheme heterojunction structure was the key factor behind the performance enhancement. It not only efficiently separated photogenerated electrons and holes and promoted the formation of superoxide and hydroxyl radicals, but also lowered the energy barriers for breaking the stable aromatic and pyrimidine rings in SDZ. FMO analysis, ESP mapping, Fukui-function calculations, and LC–MS results revealed that the main degradation routes involved S–N bond cleavage, N–C bond cleavage, and a Smiles rearrangement. This study provides experimental and theoretical support for the relationship between S-scheme charge transfer and antibiotic degradation and offers a new paradigm for constructing efficient photocatalytic systems to treat antibiotic-contaminated water.
Huanbo Ni is a master’s student in the 2024 cohort at Dalian Minzu University. His research focuses on the design and preparation of environmental photocatalytic materials and their application in aquatic-pollution remediation. He has published four academic papers.
Xuejun Zou is an associate professor at Dalian Minzu University. His research interests include environmentally functional materials, environmental catalysis, field-assisted catalysis using external fields such as light, electricity, magnetism, and electromagnetic waves, and pollution-control chemistry. He has led six provincial- or ministerial-level projects and published more than 40 academic papers, including over 20 SCI-indexed papers and two ESI highly cited papers. He holds 10 granted Chinese invention patents and has received two Liaoning Provincial Natural Science Academic Achievement Awards.
Yuying Dong is a professor at Dalian Minzu University. Her research covers the analysis and ecological effects of trace pollutants, structure–property–activity relationships of pollutants, multimedia modeling of pollutant–environment interactions, photocatalyst design, and pollution-control applications. She has led three projects funded by the National Natural Science Foundation of China and one subproject under China’s 863 Program. She has published more than 100 papers, including over 50 SCI-indexed papers and two ESI highly cited papers, authored four academic books, and received more than 10 granted Chinese invention patents.
Professor Dong serves as a reviewer for journals including Environmental Science, Environmental Chemistry, Asian Journal of Ecotoxicology, and Applied Catalysis B. She is also a peer-review expert for the National Natural Science Foundation of China and a member of the Professional Committee on Persistent Organic Pollutants of the Chinese Society for Environmental Sciences. In education, she has led several projects under the Liaoning Provincial Education Science Plan, edited the 13th Five-Year Plan textbook Environmental Science published by Science Press, published more than 30 education-reform papers, and received multiple teaching awards, including first prize in the Liaoning Provincial Micro-Lecture Competition, first prize for a provincial high-quality open course, and provincial teaching-achievement awards.
Huanbo Ni, Jingyan Tang, Xuejun Zou, et al. “Designing Interfacial Built-In Electric Field-Driven S-Scheme Cadmium Tungstate/Bismuth Tungstate Heterojunction for Boosting Photocatalytic Performance.” Journal of Colloid and Interface Science, 2026, 711, 140068.
https://doi.org/10.1016/j.jcis.2026.140068
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