Global freshwater scarcity necessitates innovative water treatment solutions. Traditional methods like ion exchange, distillation, reverse osmosis, and nanofiltration, while effective, suffer from high energy consumption and costs. Solar-driven interfacial water evaporation emerges as a promising sustainable alternative, harnessing solar energy to minimize environmental impact and link water and energy resources. However, current technologies face challenges, including organic matter contamination of the condensate and salt accumulation on the evaporator surface. Integrating photocatalysis with photothermal processes offers a potential solution to degrade organic pollutants during evaporation. This study explores the synergistic effect of light and heat generated during interfacial evaporation on photocatalytic activity, using a molybdenum disulfide (MoS₂) membrane.

Existing research has explored various photothermal materials, including carbon-based materials, plasmonic metals, semiconductors, and polymers, for solar water evaporation. Semiconductors, particularly metal sulfides with narrow band gaps (like MoS₂), are attractive due to their efficient sunlight-to-heat conversion, chemical durability, and abundance. MoS₂ is known for its narrow band gap (1.89 eV) and broad absorption range, making it suitable for both photothermal evaporation and photocatalytic water purification. However, studies integrating these processes to leverage the synergistic effects of light and heat remain limited. Previous research has examined individual components of this system (solar evaporation and photocatalysis) but a combined efficient approach had not yet been achieved. This research addresses this gap by examining the synergistic combination of photothermal and photocatalytic processes for optimal water purification.

The researchers synthesized MoS₂ nanosheets on carbon fiber cloth (CF) using a solvothermal approach, creating a MoS₂/CF membrane. The synthesis involved a mixed solvent of water and DMF, using Na₂MoO₄·2H₂O as the Mo source and L-cysteine as the S source. Characterization techniques included scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-Vis-NIR spectroscopy to analyze the membrane's morphology, structure, and optical properties. The hydrophilicity of the MoS₂/CF membrane was assessed by measuring the contact angle of water. Photothermal conversion performance was evaluated by monitoring surface temperatures under simulated solar irradiation. The light absorption capabilities of the membrane were calculated using the Beer-Lambert law. For photocatalytic activity evaluation, Rhodamine B (RhB) was used as a model organic pollutant. The degradation efficiency of RhB was determined under simulated sunlight, and the degradation rate constant was calculated. Water evaporation performance was measured by monitoring the weight change of the system under 1 sun illumination. The evaporation rate and solar evaporation efficiency were calculated. A numerical simulation was conducted to study the temperature distribution and heat transfer within the system. The impact of light intensity on evaporation and degradation rates was examined, along with the effect of RhB concentration. Density functional theory (DFT) calculations were used to predict the reactivity of RhB and identify the most active sites for free radical attack. Finally, the desalination capability of the membrane was evaluated using high-salinity water. The chemical composition of the condensed water was analyzed using UV spectrophotometry to confirm the purification efficiency.

The MoS₂/CF membrane exhibited a high water evaporation rate of 2.07 kg m⁻² h⁻¹ under 1 sun irradiation and an 82% degradation efficiency of RhB within 60 minutes. The evaporation rate for saturated brine reached 1.369 kg m⁻² h⁻¹. The high performance was attributed to the efficient photothermal conversion of MoS₂, resulting in high interfacial energy localization. The interconnected porous structure of the MoS₂ nanosheets facilitated vapor escape and water transport. The membrane's superior light absorption (91.51% in the 250-2500 nm range) contributed to its high photothermal efficiency. The MoS₂/CF exhibited enhanced photocatalytic activity due to the synergistic effect of photothermal and photocatalytic processes. The elevated temperature during evaporation increased the rate constant of interfacial electron transfer, accelerating free radical generation and RhB degradation. DFT calculations confirmed that N43 and N44 atoms in RhB are the most active sites for free radical attack. The membrane showed excellent desalination performance, producing freshwater from high-salinity water at a rate of 1.56 kg m⁻² h⁻¹. The collected condensate showed no RhB absorption peak, confirming the effective purification of the water. The results from the numerical simulation were in good agreement with the experimental data, validating the accuracy of the simulation.

The results demonstrate the successful integration of solar-driven water evaporation and photocatalytic degradation using a multifunctional MoS₂/CF membrane. The synergistic effect of photothermal and photocatalytic processes significantly enhanced the overall performance of the system, exceeding that of individual processes. The high evaporation rate and efficient pollutant removal demonstrate the potential of this technology for sustainable water purification and desalination. The findings highlight the importance of considering the combined effects of heat and light in designing efficient solar water purification systems. The use of MoS₂, a readily available and environmentally friendly material, further enhances the sustainability of the approach. The performance of this membrane is superior to many previously published results. This system shows potential for providing clean water in regions facing water scarcity or water contamination issues.

This study successfully designed and fabricated a multifunctional MoS₂/CF membrane for integrated solar-driven water evaporation and water purification. The system exhibits high efficiency in both water evaporation and organic pollutant degradation, demonstrating a sustainable solution for addressing global water challenges. Future research could focus on optimizing the membrane structure and exploring other photocatalytic materials for further performance enhancement. Scalability and cost-effectiveness analysis would also be important steps toward practical implementation.

While the study demonstrates high performance under simulated solar irradiation, further investigation is needed to assess the long-term stability and durability of the MoS₂/CF membrane under real-world conditions. The study used RhB as a model pollutant; further research should investigate the membrane's effectiveness in degrading other types of organic pollutants and mixtures thereof. The scalability and cost-effectiveness of the proposed technology also require further investigation to evaluate its potential for large-scale applications.