Plasmonic nanoparticles, particularly gold (Au) and silver (Ag), have emerged as promising materials for enhancing solar-thermal membranes in wastewater desalination. Their unique ability to absorb and convert light into localized heat enables efficient vapor generation, making them ideal for addressing water scarcity challenges. The integration of these nanoparticles into membrane systems leverages their plasmonic properties to improve energy efficiency, vapor production rates, and salt rejection, while innovations in nanostructure design enhance durability in harsh brackish water environments.
The localized heating effect of plasmonic nanoparticles arises from surface plasmon resonance (SPR), where incident light excites collective oscillations of conduction electrons. For Au nanoparticles, SPR typically occurs in the visible to near-infrared range (520–900 nm), while Ag nanoparticles exhibit strong absorption in the 400–500 nm range. When embedded in a solar-thermal membrane, these nanoparticles concentrate heat at the nanoscale, minimizing thermal losses and enabling rapid water evaporation. Studies have demonstrated that plasmonic heating can achieve localized temperatures exceeding 100°C at the nanoparticle surface, even under moderate solar irradiation (1–5 suns). This hyperlocalized heating significantly enhances vapor generation rates, with reported values ranging from 1.2 to 3.6 kg m⁻² h⁻¹ under 1 sun illumination, depending on nanoparticle loading and membrane design.
Vapor generation efficiency is further influenced by light-trapping nanostructures that maximize solar absorption. Hierarchical structures, such as porous membranes with vertically aligned channels or plasmonic nanoparticle-decorated aerogels, improve light absorption by multiple scattering and trapping. For instance, membranes incorporating Au nanorods with tailored aspect ratios exhibit broadband absorption, capturing a wider solar spectrum compared to spherical nanoparticles. Similarly, Ag nanocubes with sharp corners enhance near-field effects, further concentrating heat. These innovations have led to solar-thermal conversion efficiencies exceeding 80% in optimized systems, a significant improvement over conventional solar stills.
Salt rejection in plasmonic solar-thermal membranes is achieved through a combination of thermal and structural mechanisms. The rapid vaporization at the nanoparticle surface leaves dissolved salts behind, while the membrane’s porous structure prevents salt accumulation from clogging the system. Some designs incorporate hydrophilic-hydrophobic heterostructures, where plasmonic nanoparticles are embedded in hydrophobic layers to facilitate vapor escape while repelling salt ions. Experimental results show salt rejection rates above 99% for brackish water with salinities up to 10,000 ppm. Additionally, the localized heating reduces the risk of membrane fouling, as high temperatures inhibit biofilm formation and precipitate salts in controlled zones.
Durability in brackish water remains a critical challenge, as prolonged exposure to high salinity and varying pH levels can degrade plasmonic nanoparticles. To address this, researchers have developed protective coatings, such as ultrathin silica or graphene layers, that shield nanoparticles from corrosion without compromising optical properties. For example, Ag nanoparticles encapsulated in a 2–5 nm silica shell retain over 90% of their plasmonic activity after 30 days in saline solutions. Similarly, Au nanoparticles functionalized with thiolate ligands exhibit enhanced stability in harsh environments. These advancements extend the operational lifespan of solar-thermal membranes, making them viable for long-term wastewater desalination.
Recent innovations also explore hybrid plasmonic systems, where Au or Ag nanoparticles are combined with photothermal polymers or carbon-based materials to enhance performance. For instance, plasmonic nanoparticles dispersed in a polyvinyl alcohol (PVA) matrix benefit from the polymer’s water transport properties while maintaining efficient heat generation. Other systems integrate plasmonic nanoparticles with reduced graphene oxide (rGO) to improve mechanical strength and thermal conductivity. Such hybrids demonstrate synergistic effects, achieving vapor generation rates of up to 4.0 kg m⁻² h⁻¹ under 1 sun irradiation.
The scalability of plasmonic solar-thermal membranes depends on cost-effective fabrication methods. Techniques like spin-coating, dip-coating, or in-situ reduction enable large-area deposition of nanoparticles on flexible substrates. Roll-to-roll manufacturing has been explored for producing meter-scale membranes with uniform nanoparticle distribution, critical for industrial applications. Economic analyses suggest that plasmonic-enhanced membranes could reduce the energy consumption of desalination by 30–50% compared to reverse osmosis, provided nanoparticle costs are optimized.
Future directions include optimizing nanoparticle arrangements for directional vapor flow and exploring alternative plasmonic materials like aluminum or copper, which offer lower material costs. Machine learning approaches are being employed to predict optimal nanostructure configurations for specific water compositions, further advancing the technology’s adaptability.
In summary, plasmonic nanoparticles in solar-thermal membranes represent a transformative approach to wastewater desalination. Their ability to generate localized heat, coupled with innovations in light-trapping nanostructures and durability, addresses key challenges in energy efficiency and salt rejection. As research progresses, these systems are poised to play a pivotal role in sustainable water purification technologies.