Regulation of Hierarchical Structure for Micro/Nanomaterials and Their Applications on Environmental and Energy Management

The rapid rise in global population has escalated energy demand and environmental strain. Micro-/nanostructured materials offer versatile solutions, addressing diverse challenges like air and water purification, catalysis, and energy conversion/storage. Through techniques like casting, spraying, and lithography, hierarchical structures are engineered onto these materials, enhancing their compatibility, design flexibility, and mechanical stability. However, some critical limitations including complex, expensive, as well as nonuniform fabrication processes, energy-intensive operation, and instability under extreme environments still hinder the commercialization of hierarchical structures and the expansion of their application scope. This dissertation aims to develop novel micro/nanomaterials with special hierarchical structure and wettability using facile, scalable, and energy-efficient techniques for air purification, air harvesting, and water descaling.

First, we fabricate an electrothermal air filter by facile electrospinning and scalable polymerization, featuring a hierarchical structure of microscale polyester (PET) fibers, nanoscale polyacrylonitrile (PAN) fibers, and thinner polypyrrole (PPy) nanoparticles self-assembled nanowires, which achieves high-efficiency filtration and active sterilization. Through fine-tuning PAN precursor concentration and electrospinning duration, the filter exhibits robust filtration efficiency of up to 97.90% and low wind resistance of 76 Pa. Hierarchically conductive network enables air filter to rapid heat to 130°C in 10 seconds with superior efficiency of 473.54 °C cm² W^-1, along with electrothermal sterilization reaching 99.48% efficiency within 10 minutes, thus ensuring an air filter assembled purifier powered by solar panel cleans air in 5 minute in a 55 L room. This strategy optimizes permeability and electroconductivity through tailored hierarchical structures, offering insights for next-gen conductive porous materials applicable in smart textures, desalination, oil-spill cleanup, electromagnetic shielding, thermal energy storage, and more.

 

 Second, we fabricate a LiCl@MOF-loaded honeycomb-patterned nanofibrous membrane and a dense CNT-loaded electrothermal membrane by electrospinning for atmospheric water adsorption and desorption, respectively. Combination of four scales of hierarchical structures of honeycomb-inspired nanofibrous networks significantly improved the moisture transport rate. Apart from adsorbing water molecules and loading porous MOF nanoparticles, overlapped nanofibers with capillary forces can adsorb deliquescent salts stably, reducing the water vapor pressure on fibers and improving the driving force for water diffusion. Benefiting from the progressive and hierarchical hygroscopic sites, the water adsorption ability of the membrane is up to 0.8 g g^-1 hr^-1 at 25 °C and 85% RH. Combining with the electrothermal fibrous heater, the water can be desorbed within 30 minutes under 120 °C, indicating an all-weather and all-day multicyclic water harvesting system is promising, whose water outcome can not only be used in domestic but also serve agricultural and industrial activities in the future.

Third, we use a one-step electroplating method to generate a cauliflower-like micro/nanostructure on metal, creating a superhydrophobic surface. The capillary force of hierarchical configuration captures lubricating oil forming a stable slippery lubricant-induced surface (SLIPS), providing prolonged anti-scaling performance at high temperatures. Surface free energy and roughness analysis of the surfaces reveal the anti-scaling mechanism. Real-world trials show SLIPS reduces scale mass by over 200% due to its low surface free energy (4.3 mJ m⁻²) and exceptional smoothness (Ra= 41 ± 8 nm). SLIPS also enhances heat transfer efficiency by increasing bubble departure frequency eightfold compared to uncoated surfaces. Coating a spiral tube with SLIPS improves flowability and reduces pressure drop. Additionally, SLIPS shows compatibility with mechanical vibration, aiding in descaling. This scalable coating method is anticipated to find applications in various industrial high-temperature processes prone to scale formation.

In summary, we have employed adaptive and cost-effective processing methods to fabricate novel micro/nanomaterials capable of addressing the current challenges in the environmental and energy sectors. By systematically studying the assembly processes of these micro/nanomaterials, we have optimized their hierarchical structures. The unique properties that arose from the special hierarchical structures effect such as multiple heterogeneous interfaces, confined space and large specific areas are discussed and some representative applications in air purification, atmospheric water harvesting, and water descaling are presented. We believe that our research will yield important insights to design hierarchical configurations in a more facile way for diverse broad applications.