Design and construction of CuS functionalized superhydrophobic cellulose paper with effective oil/water separation, and photothermally boosted antimicrobial and anti-/deicing performances

Technical Summary: Design and Construction of CuS Functionalized Superhydrophobic Cellulose Paper

Problem Statement
The paper addresses the escalating environmental threats posed by frequent oil spills and uncontrolled oily wastewater discharges, which endanger ecosystems and human health. While conventional treatment methods (e.g., skimming, bioremediation) exist, they often suffer from operational complexity, high energy demands, and limited scalability. Although superhydrophobic surfaces (SHSs) offer a promising alternative for oil/water separation, their practical application is frequently hindered by insufficient mechanical stability, high fabrication costs, and the tendency of functional nanoparticles (such as photothermal agents) to peel off substrates. Furthermore, existing materials often lack integrated functionalities such as photothermal-driven antimicrobial activity and anti-/deicing capabilities, which are crucial for operation in harsh, low-temperature environments.

Methodology
To address these challenges, the authors designed and fabricated a multifunctional superhydrophobic cellulose paper (CP) functionalized with copper sulfide (CuS). The fabrication process involved a multi-step modification strategy:

1. Substrate Preparation: Commercial cellulose paper (CP) served as the eco-friendly, renewable base.
2. PDA Coating: To solve the issue of weak interfacial contact between CuS and cellulose, a polydopamine (PDA) coating was applied via spontaneous oxidative polymerization of dopamine. This acted as a robust adhesive layer inspired by mussel proteins.
3. CuS Deposition: CuS nanoparticles were synthesized via a hydrothermal method and subsequently in-situ coated onto the PDA-modified paper. The PDA layer facilitated strong adhesion between the CuS particles and the cellulose fibers.
4. Hydrophobic Modification: The composite was immersed in an octadecylamine (ODA) solution. The long-chain hydrocarbons of ODA lowered the surface energy, completing the transition to a superhydrophobic state.

The resulting material, designated CP/PDA/CuS/ODA, was characterized using SEM, XRD, FTIR, and contact angle measurements. Its performance was evaluated through oil/water separation tests, durability assessments (mechanical, chemical, thermal, and UV), antifouling/self-cleaning tests, and photothermal evaluations (including antimicrobial and anti-/deicing capabilities).

Key Contributions and Results

• Superhydrophobicity and Superoleophilicity: The modified paper exhibited a water contact angle (WCA) of 156.4° and a virtually zero oil contact angle (OCA). This wettability was attributed to the synergistic effect of the hierarchical micro/nano-roughness provided by PDA/CuS clusters and the low surface energy of the ODA coating.
• Oil/Water Separation: The material demonstrated effective continuous gravity-driven separation of various oil/water mixtures (including cyclohexane, petroleum ether, n-hexane, dichloromethane, chloroform, and carbon tetrachloride).
  • Efficiency: Separation efficiencies exceeded 99.35% for all tested mixtures.
  • Flux: High permeation fluxes were achieved (ranging from ~8443 to ~10034 L m⁻² h⁻¹) without external pressure.
  • Absorption Capacity: The paper showed high oil absorption capacities (7.68–21.92 g/g) for various organic solvents and oils.
  • Reusability: The separation performance remained stable over 10–20 cycles.
• Durability: The material exhibited exceptional robustness, maintaining superhydrophobicity (WCA > 150°) after:
  • 100 tape-peeling cycles.
  • 100 folding cycles.
  • 10 sandpaper abrasion cycles.
  • Exposure to acidic (pH 1) and alkaline (pH 12) solutions, saltwater (up to 5 wt% NaCl), UV irradiation (12 h), and thermal treatment (up to 100°C).
  • Long-term storage (5 weeks) under ambient conditions.
• Antifouling and Self-Cleaning: The surface resisted contamination from various liquid pollutants (grape juice, cola, dye solutions) and solid contaminants (chalk powder). Water droplets rolled off the surface, carrying away dirt, demonstrating effective self-cleaning.
• Photothermal Performance: Leveraging the photothermal conversion capability of CuS and PDA:
  • Under 1.0 kW m⁻² solar irradiation, the surface temperature reached approximately 74.3–78.6°C within 60 seconds.
  • The material showed excellent photothermal stability over 10 light on-off cycles.
• Photothermal Antimicrobial Activity: Under light irradiation, the material efficiently inactivated both Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria, whereas the material in the dark showed no significant bactericidal effect.
• Anti-icing and Deicing:
  • Anti-icing: Solar irradiation extended the freezing time of water droplets by approximately 124.7% compared to non-irradiated conditions.
  • Deicing: The time required to melt pre-formed ice was reduced by approximately 80.8% under solar irradiation compared to dark conditions.

Significance
The paper claims that the fabricated CuS-functionalized superhydrophobic cellulose paper offers a versatile and superior platform for environmental remediation. By integrating efficient oil/water separation with photothermally boosted antimicrobial and anti-/deicing functionalities, the material addresses critical limitations of current superhydrophobic materials, specifically regarding durability and performance in low-temperature or contaminated environments. The authors posit that this design provides a novel pathway for realizing efficient oil absorption and water purification even under harsh conditions, making it highly promising for realistic industrial applications in oil spill remediation and water treatment. The use of abundant, renewable cellulose and low-cost, non-toxic components further supports its potential for scalable implementation.