Enhanced Visible‑Light Photocatalysis and Self‑Cleaning via CuS‑Decorated TiO₂/PVDF Nanofibers

Abstract

CuS nanoflowers were successfully anchored onto anatase TiO₂/polyvinylidene fluoride (PVDF) nanofibers through a single low‑temperature hydrothermal step following electrospinning of a tetrabutyl orthotitanate (TBOT)/PVDF precursor. The Cu²⁺ and thiourea stoichiometry governed both the crystallinity and the morphology of the CuS phase. Incorporation of CuS reduced TiO₂’s band‑gap and markedly improved electron–hole separation, yielding a photocatalytic degradation rate of Rhodamine B (RhB) that was three times higher than that of pristine TiO₂/PVDF under visible‑light irradiation. The process preserved the flexibility of the fibers and conferred a self‑cleaning capability: dye droplets were photodegraded within 2 h, and hydrophobicity enabled dust removal by rolling water droplets. These findings demonstrate a flexible, recyclable, and self‑cleaning photocatalyst with strong practical potential for water remediation.

Introduction

Anthropogenic activities have accelerated environmental degradation across air, soil, and water systems. In particular, water contamination by toxic organics demands sustainable remediation strategies. Photocatalysis, pioneered by Fujishima and Honda, offers a green avenue for degrading pollutants under light. Titanium dioxide (TiO₂), though chemically robust, suffers from a wide band‑gap (~3.2 eV) and rapid carrier recombination, limiting visible‑light activity. Numerous approaches—doping, sensitization, and heterojunction engineering—have been explored to extend TiO₂’s absorption and improve charge separation. Coupling TiO₂ with narrow‑gap semiconductors such as CuS (band‑gap ~2.0 eV) is a promising route that avoids heavy‑metal contaminants.

Previous studies have shown that CuS/TiO₂ composites enhance photocatalytic performance, yet many reports involve powdered or brittle materials that are difficult to recover. Here, we present a flexible CuS/TiO₂/PVDF nanofiber system fabricated by one‑step hydrothermal treatment of electrospun TBOT/PVDF fibers. The low‑temperature process preserves fiber integrity, while the CuS nanoflower morphology maximizes interfacial contact and charge transfer. We systematically investigate how Cu²⁺ loading affects crystal growth, optical absorption, and photocatalytic efficiency, and we evaluate the material’s self‑cleaning behaviour under visible light.

Materials and Methods

Materials

PVDF (FR904), N,N‑dimethylformamide (DMF, AR 99.5 %), acetone (CP 99.0 %), TBOT (CP 98.0 %), copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, AR 99.0 %), thiourea (AR 99.0 %), Rhodamine B, methyl blue, and methyl orange were all purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purification.

Electrospinning of TBOT/PVDF Fibers

4.0 g PVDF was dissolved in 10 g acetone and 10 g DMF, stirred at 40 °C until transparent. 10 mL TBOT was added and stirred 1 h at 40 °C to obtain the precursor solution. Using a 5 mL syringe with a blunt needle, the solution was electrospun at 9 kV, 1.8 mL h⁻¹, and a tip‑collector distance of 11 cm. Fibers were collected on a rotating stainless‑steel drum (≈250 rpm) wrapped in aluminum foil. After drying at 60 °C for 10 h, the fibers were cut into 2.5 × 2.5 cm pieces for hydrothermal treatment.

Hydrothermal Fabrication of CuS/TiO₂/PVDF Fibers

A 1:2 molar ratio of Cu(NO₃)₂·3H₂O to thiourea was dissolved in 30 mL deionized water and stirred 30 min. The TBOT/PVDF pieces were placed in a 50 mL stainless‑steel autoclave with the solution, sealed, and heated at 150 °C for 24 h. During this step, TBOT hydrolyzed to TiO₂ while CuS nucleated and grew on the fiber surface. The resulting fibers were washed with ethanol and water, then dried at 60 °C for 10 h. Samples prepared with 0.1, 0.5, and 1 mmol Cu(NO₃)₂·3H₂O were designated Cu 0.1, Cu 0.5, and Cu 1, respectively.

Characterization

• PXRD (Rigaku SmartLab) 10–90° 2θ, Cu‑Kα (λ = 1.5418 Å).
• SEM (Phenom Pro) and TEM (JEOL JEM‑2100 Plus) for morphology.
• XPS (Thermo Escalab 250Xi) for elemental analysis.
• Diffuse reflectance UV‑Vis (Shimadzu UV‑2600) with BaSO₄ standard; Kubelka–Munk transformation used.
• Photoluminescence (Hitachi F‑2500) at 320 nm excitation.

Photocatalytic Activity

Photodegradation of 5 mg L⁻¹ RhB was monitored under a 9‑W white LED (λ > 400 nm). A 60 mL RhB solution in a 100 mL quartz tube was equilibrated with 5 cm² of catalyst for 30 min in the dark. Light irradiation was applied at 4 cm distance; aliquots (3 mL) were sampled at regular intervals, centrifuged to remove particles, and measured at 554 nm. The concentration ratio C/C₀ represented the degradation efficiency. Recyclability was tested by successive runs after washing and drying.

Self‑Cleaning Assessment

• Static contact angles of H₂O, RhB, MO, and MB droplets measured with a Theta Attension instrument.
• Dye‑droplet photodegradation: 10 mg L⁻¹ droplets on the fiber surface, irradiated under LED, photographed periodically.
• Dust removal: fine dust was sprinkled on the fiber, a water droplet was dropped, and the surface was tilted to observe dust rolling off.

Results and Discussion

Structural and Morphological Characterization

XRD confirmed anatase TiO₂ (101, 004, 200, 211) and β‑PVDF peaks. CuS appeared as (102) and (006) reflections in Cu 0.1, evolving to (101), (103) in Cu 0.5 and Cu 1, indicating progressive crystallinity with increased Cu loading (Fig. 1). SEM images revealed that TiO₂/PVDF fibers had a rough surface; CuS deposition rendered the surface increasingly porous, culminating in hexagonal lamellar nanoflowers at 1 mmol Cu (Fig. 2). TEM/HRTEM (Fig. 3) showed clear lattice fringes (0.35 nm for TiO₂ (101), 0.31 nm for CuS (102)), confirming intimate heterointerface.

Compositional Analysis

XPS spectra displayed Ti 2p, O 1s, Cu 2p, and S 2p signals. Ti 2p₃/₂ at 459 eV, Cu 2p₃/₂ at 932 eV (Cu²⁺), and S 2p₃/₂ at 162 eV (S²⁻) verified the presence of TiO₂ and CuS. The S/Cu ratio (~1.27) reflected excess thiourea. The O/Ti ratio (>2:1) was attributable to PVDF’s fluorinated backbone.

Optical Properties

Diffuse reflectance spectra (Fig. 5) showed that CuS incorporation shifted absorption into the visible range; the absorption edge moved from 400 nm (TiO₂/PVDF) to 530 nm (Cu 1). Band‑gap calculations (Eg = 1240/λ_g) yielded a reduction from 3.2 eV to 2.0 eV with higher Cu loading (Table 2). Photoluminescence intensity decreased markedly in CuS‑decorated samples, indicating suppressed electron–hole recombination and enhanced charge separation.

Photocatalytic Performance

Under visible light, TiO₂/PVDF degraded 52.9 % of RhB in 50 min, whereas Cu 0.1, Cu 0.5, and Cu 1 achieved 85.7 %, 99.2 %, and 100 % degradation, respectively (Fig. 7a). Kinetic analysis (−ln C/C₀ = kt) gave rate constants of 9.8 × 10⁻³, 1.6 × 10⁻², 1.8 × 10⁻², and 2.9 × 10⁻² min⁻¹ for TiO₂/PVDF, Cu 0.1, Cu 0.5, and Cu 1, illustrating a three‑fold enhancement for Cu 1. Recyclability tests (Fig. 7b) confirmed negligible activity loss over five cycles.

Mechanistic Insight

Scavenger experiments indicated that in TiO₂/PVDF, superoxide radicals dominate, while in CuS/TiO₂/PVDF, photogenerated holes are the primary oxidants. A proposed heterojunction model (Fig. 8) shows CuS absorbing visible light, transferring electrons to TiO₂’s CB, and holes to CuS’s VB, thus driving efficient O₂⁻· and OH· formation for RhB oxidation.

Self‑Cleaning Efficacy

Static contact angles revealed hydrophobicity (H₂O ~110°, RhB ~95°) for all samples, slightly reduced upon CuS addition. Dye‑droplet photodegradation images (Fig. 10) demonstrated complete fading within 120 min. Dust‑removal tests (Fig. 11) showed that a single rolling water droplet eliminated surface dust, highlighting the practical self‑cleaning advantage.

Conclusions

We have demonstrated a facile, low‑temperature, one‑step synthesis of CuS‑decorated TiO₂/PVDF nanofibers that retain mechanical flexibility and exhibit superior visible‑light photocatalysis—three times the degradation rate of RhB relative to pristine TiO₂/PVDF. The CuS/TiO₂ heterojunction narrows the band‑gap, suppresses carrier recombination, and boosts charge separation. Moreover, the fibers exhibit durable self‑cleaning behavior, enabling dye degradation and dust removal under ambient light. These attributes make CuS/TiO₂/PVDF a promising, reusable, and maintenance‑free material for sustainable water purification and environmental remediation.