Optimization of the Response Surface for the Construction of Hollow Solar Interfacial Evaporators Using Carrageenan/PVA Bimetric Hydrogels Coupled with AgBr@Ti₂O₃

Developing low-cost, efficient and durable solar-driven interfacial evaporators from sustainable feedstocks is important for practical solar desalination. Herein, carrageenan was extracted from Eucheuma using a response surface methodology-optimized alkaline treatment and further combined with poly(vinyl alcohol) to construct a double-network hydrogel. By integrating AgBr@Ti₂O₃ photothermal materials, a hollow carrageenan-based evaporator, denoted as HCE, was fabricated for solar-driven water evaporation. The response surface model reliably predicted carrageenan yield and gel strength under optimized conditions, including an NaOH concentration of 7%, an alkali-treatment temperature of 77℃ and an alkali-treatment time of 80 min. With the addition of 12% KCl, the carrageenan yield reached 73.4%, while the gel strength increased to 1279.6 g cm⁻². Fourier-transform infrared spectroscopy confirmed that the extracted product possessed the characteristic structural features of κ-carrageenan. AgBr was generated in situ on the Ti₂O₃ surface, forming an AgBr@Ti₂O₃ composite with an expanded solar absorption range. Benefiting from its interconnected porous network, rapid water wettability and hollow vapor-transport channel, the HCE achieved an evaporation rate of 5.15 kg m⁻² h⁻¹ under 1 sun irradiation. Differential scanning calorimetry and Raman spectroscopy revealed that the apparent evaporation enthalpy of water in the HCE decreased from 2349 J g⁻¹ for bulk water to 1695 J g⁻¹, suggesting that the double-network hydrogel regulated the hydrogen-bonding environment of water and reduced the energetic barrier for evaporation. Moreover, the HCE showed good cyclic stability, desalination capability and moderate antibacterial activity during saline-water evaporation. This work provides a sustainable and cost-effective strategy for constructing natural polysaccharide-based solar interfacial evaporators.