Electrospinning Machine| Multi-Electric Field-Enhanced CuInS2-ModifiedTiO2(Anatase)/TiO2(Rutile)/PVDF Nanofiber Membrane for Multifunctional Piezo-Photocatalysis

"Energy shortage and water pollution are major global challenges that urgently require the development of clean energy and efficient pollution control technologies. Photocatalytic technology, with its green and sustainable characteristics, has become an ideal solution for simultaneously achieving energy conversion (e.g., hydrogen production) and pollutant degradation. However, traditional photocatalysts generally suffer from problems such as high carrier recombination rates and low solar light utilization efficiency, while multifunctional broad-spectrum catalysts remain relatively scarce.

In recent years, S-scheme heterojunctions have become a research hotspot as they promote charge separation through built-in electric fields while preserving strong redox capabilities. Further introduction of piezoelectric materials can utilize mechanical energy (water flow, ultrasound, etc.) to generate piezoelectric fields, which synergistically enhance carrier separation efficiency when combined with photogenerated electric fields.

However, existing piezoelectric photocatalytic materials (such as ZnO, MoS2, etc.) are mostly brittle powders that are easily lost and have poor cycling stability, limiting their practical applications. Therefore, designing new piezoelectric photocatalytic materials that combine high activity, flexibility, and easy recyclability is of great significance for promoting the synergistic management of environmental and energy issues."

Recently, Professor Xiangting Dong's research team at Changchun University of Science and Technology published their latest findings titled "Multi-Electric Field-Enhanced CuInS2-Modified TiO2(Anatase)/TiO2(Rutile)/PVDF Nanofiber Membrane for Multifunctional Piezo-Photocatalysis" in the internationally renowned journal Advanced Functional Materials. This study presents an exceptional strategy to address water pollution and energy crises through rational microstructure design, component regulation, and multi-electric field coupling, which enables efficient charge separation and rapid transport via multiple driving forces and charge transfer channels. Professor Dong serves as the corresponding author, with postdoctoral researcher Feng Sun as the first author.

The research designed and constructed a self-supporting, multi-electric field synergistic piezoelectric photocatalyst - a CuInS2 nanocube-modified TiO2(anatase)/TiO2(rutile)/PVDF [CuTi(AR)P] dual S-scheme heterojunction nanofiber membrane. By dispersing different TiO2 crystal phases within one-dimensional PVDF nanofibers, the system ensures sufficient photogenerated charge carrier production and fast migration. Meanwhile, the vertical growth of three-dimensional CuInS2 nanocubes on the one-dimensional fiber surfaces provides large specific surface area and broadens the light response range.

The gradient band alignment of the four functional components creates a multi-electric field synergistic effect through the built-in electric field (dual S-scheme heterojunction) and piezoelectric field (PVDF), significantly reducing charge recombination probability. Leveraging the synergistic effects of band engineering, nanostructure interfaces, and broad-spectrum design, the 0.50CuTi(AR)P catalyst demonstrated remarkable piezo-photocatalytic efficiencies under stirring-assisted simulated sunlight: 100.00% Cr(VI) reduction (60 min), 94.30% ciprofloxacin degradation (50 min), and H2/H2O2 production rates of 893.3 and 1174.7 μmol h-1 g-1, respectively, achieving multifunctional performance.

The self-supporting, multi-electric field synergistic CuInS2 nanocube-modified TiO2(anatase)/TiO2(rutile)/PVDF nanofiber membrane dual S-scheme heterojunction piezoelectric catalyst was fabricated through a combination of electrospinning and hydrothermal methods. In the XPS spectra, the binding energy shifts of Ti 2p, O 1s, Cu 2p, In 3d, and S 2p reflect charge transfer from CuInS2 to Ti(A)P and Ti(R)P in CuTi(AR)P, with the electron transfer being closely related to heterojunction formation.

Fig. 1: Synthesis schematic, XRD, and XPS.

The morphology of the samples was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ti(A)P, Ti(R)P, and Ti(AR)P exhibited nanofiber morphology with high aspect ratios, while CuInS2 displayed well-defined cubic structures. When 0.50 mmol CuInS2 was loaded onto Ti(A)P, Ti(R)P, and Ti(AR)P respectively, the CuInS2 nanoparticles were uniformly distributed on the surface of 0.50CuTi(AR)P. Furthermore, elemental mapping confirmed the homogeneous distribution of Ti, O, F, N, In, Cu, and S elements across the 0.50CuTi(AR)P surface, verifying its successful synthesis.

High-resolution TEM (HRTEM) analysis revealed lattice spacings of 0.351 nm, 0.325 nm, and 0.321 nm, corresponding to the (101) plane of TiO2 (anatase), (110) plane of TiO2 (rutile), and (112) plane of CuInS2, respectively, further confirming the successful construction of 0.50CuTi(AR)P.

Piezoresponse force microscopy (PFM) was employed to analyze the piezoelectric properties of PVDF, 0.50CuTi(AR)P, and 1.00CuTi(AR)P. The amplitude images showed distinct domain walls, while phase images indicated randomly oriented electrical domains. Under applied voltages (±6V, ±15V, ±20V), all three samples exhibited characteristic "phase hysteresis" loops and "butterfly-shaped" amplitude loops, with nearly 180° phase reversal, confirming the existence of polarization switching. The introduction of heterostructures significantly enhanced the piezoelectric response of PVDF, establishing a foundation for highly efficient piezo-photocatalysis.

Fig. 2: SEM/TEM morphology, elemental mapping, and PFM.

Pollutant Treatment Performance:
The Cr(VI) reduction experiments demonstrated that 0.50CuTi(AR)P achieved 100% reduction efficiency within 60 min, significantly outperforming single-component materials [CuInS₂ (24.40%), Ti(A)P (40.10%), and Ti(R)P (33.80%)] and binary composites [CuTi(A)P (57.60%), CuTi(R)P (53.70%), and Ti(AR)P (63.60%)]. The heterojunction structure optimized charge carrier separation, though excessive CuInS₂ loading degraded performance. With an optimal heterostructure ratio, 0.50CuTi(AR)P exhibited the highest piezo-photocatalytic activity.

Further, 0.50CuTi(AR)P degraded 94.3% of ciprofloxacin (CIP) in 50 min, with efficiencies 3.04-, 2.23-, and 2.62-fold higher than CuInS₂, Ti(A)P, and Ti(R)P, respectively, consistent with the Cr(VI) reduction trend. Under combined light-piezoelectric stimulation, the removal efficiencies for Cr(VI) and CIP reached 100% (1.85× pure photocatalysis, 3.73× pure piezocatalysis) and 94.3% (1.81× pure photocatalysis, 4.25× pure piezocatalysis), confirming remarkable synergy between photocatalytic and piezoelectric effects.

Renewable Energy Production:
Under 360 min of light-piezoelectric co-action, 0.50CuTi(AR)P generated 402.0 μmol of H₂ at a rate of 893.3 μmol h⁻¹ g⁻¹, 1.76× higher than pure photocatalysis (507.1 μmol h⁻¹ g⁻¹), while maintaining >870.1 μmol h⁻¹ g⁻¹ after 4 cycles. Its H₂O₂ production rate reached 1174.7 μmol h⁻¹ g⁻¹, far exceeding pure photocatalysis (665.4 μmol h⁻¹ g⁻¹) and piezocatalysis (58.7 μmol h⁻¹ g⁻¹), with excellent cycling stability.

Through heterostructure design and multi-field synergy, 0.50CuTi(AR)P simultaneously addressed environmental remediation and energy production, demonstrating broad application potential.

Fig. 3: Cr(VI)/CIP removal, pH/mechanical effects, H₂/H₂O₂ rates.

he 0.50CuTi(AR)P catalyst degraded CIP through three primary pathways, with intermediate products ultimately mineralized into small molecules such as H2O and CO2 via oxidative processes. Toxicity evaluation (T.E.S.T.) demonstrated that 0.50CuTi(AR)P effectively degraded CIP while significantly reducing its toxicity.

Rice cultivation experiments provided further validation: plants grown in CIP solutions treated with the catalyst exhibited growth conditions comparable to those in tap water, and markedly superior to plants in untreated CIP solutions. These results confirm the catalyst's effectiveness in eliminating CIP's biotoxicity and mitigating its adverse effects on crops.

Fig. 4: CIP degradation pathways, toxicity assessment, rice growth.

DFT calculations revealed that the work functions of TiO₂ (anatase), TiO₂ (rutile), and CuInS₂ were 6.5 eV, 6.3 eV, and 4.9 eV, respectively. Under illumination, electrons migrated from CuInS₂ (higher Fermi level, EF) to Ti(A)P and Ti(R)P (lower EF), establishing a built-in electric field [CuInS₂→Ti(A)P/Ti(R)P] that induced band bending. This S-scheme heterojunction mechanism effectively separated charge carriers while preserving strong redox capabilities, thereby facilitating CIP degradation, Cr(VI) reduction, and H₂/H₂O₂ generation.

Fig. 5: Proposed mechanism.

Paper link: https://doi.org/10.1002/adfm.202505795