Engineered superhydrophilicity and superhydrophobicity of graphene-nanoplatelet coatings via thermal treatment

Only-sp2 nanocarbon superhydrophobic materials - Synthesis and mechanisms of high-performance

Superhydrophobic systems have fascinated the human kind since the earliest observations of the repellence of water droplets by biological systems. Currently, superhydrophobic materials (SHMs), often inspired by nature and engineered as thin coatings, become an important class of complex systems with numerous industrial implementations. The most important applications of SHMs cover waterproof, self-cleaning, anti-/deicing, anti-fogging, and catalytic systems/units, e.g., in textiles, civil and military engineering, automotive and space industry, and water-from-oil separating systems. In a few above areas, SHMs proved also to be tailorable as smart, i.e., reversibly stimuli-responsive and/or recyclable solutions. In all of those emerging fields, carbon - as the 'sixth element' - represents one of the most prospective components, also in the 'only‑carbon'-based systems. The versatility of carbon (nano)materials, supported by their surface and morphology/topology tunability at from the nano- to macroscale, is vital in the manufacturing of high-performance SHMs. Here, we review only-sp2-hybridized nanocarbon SHMs, i.e., materials exhibiting water contact angle (WCA) >150°, from molecular design to synthesis and evaluation of their application-oriented properties, including WCA. The nanocarbons - pristine/as-made, (non-)covalently functionalized and in a form of carbon‑carbon composites - are analyzed according to their dimensionality: 0D fullerenes, 1D carbon nanotubes (CNTs), 2D graphene, and 3D carbon nanofibers (CNFs). Importantly, this review intends to provide premises toward novel sp2-nanocarbon SHMs, indicating nanowettability and Hansen Solubility Parameters the key ones.

Advances in the research of carbon-, silicon-, and polymer-based superhydrophobic nanomaterials: Synthesis and potential application

With the rapid development of science and technology, superhydrophobic nanomaterials have become one of the hot topics from various subjects. Due to their distinct properties, such as superhydrophobicity, anti-icing and corrosion resistance, superhydrophobic nanomaterials are widely used in industry, agriculture, defense, medicine and other fields. Hence, the development of superhydrophobic materials with superior performance, economical, practical features, and environment-friendly properties are extremely important for industrial development and environmental protection. Aimed to provide a scientific and theoretical basis for the subsequent study on the preparation of composite superhydrophobic nanomaterials, this paper reviewed the latest progress in the research of superhydrophobic surface wettability and the theory of superhydrophobicity, summarized and analyzed the latest development of carbon-based, silicon-based and polymer-based superhydrophobic nanomaterials in terms of their synthesis, modification, properties and structure sizes (diameters), discussed the problems and unique application prospects of carbon-based, silicon-based and polymer-based superhydrophobic nanomaterials.

Remarkable Thermal Performance Enhancement of Micro Heat Pipes with Graphene-Nanoplatelet Nano-Wicks

The ultrafast water permeation property of graphene nanoplatelets (GNPs) synergically enhances the evaporation and water circulation processes in a micro heat pipe (MHP). An MHP is a promising phase-change heat-transfer device capable of transferring large amounts of heat energy efficiently. The hydrophobic, atomically smooth carbon walls of GNPs nanostructures provide a network of nanocapillaries that allows water molecules to intercalate frictionlessly among the graphene layers. Together with the attraction force of the oxygenated functional groups, a series of hydrophobic and hydrophilic surfaces are formed that significantly improve the water circulation rate. The intercalation of water molecules encourages the formation of water-thin film for film-wise evaporation. The effect of nano-wick thickness on the thermal performance of the MHP is investigated. A thinner GNP nano-wick is more favorable to film-wise evaporation while a thicker nano-wick promotes a higher water circulation rate from the condenser to the evaporator, leading to the existence of an optimal thickness. By benchmarking with the uncoated MHP, the thermal conductance of an MHP with a 46.9-µm GNP nano-wick manifests a maximum enhancement of 128%. This study provides insights on the feasible implementation of GNP nano-wicks into a highly efficient micro-scale electronics cooling device for environmental sustainability.

Wettability Controlled Surface for Energy Conversion

To achieve clean and high-efficiency utilization of renewable energy, functional surfaces with controllable and patternable wettability are becoming a fast-growing research focus. In this work, a laser scribing strategy to fabricate patterned graphene surfaces that are capable of energy conversion in different forms is demonstrated. Using the laser raster-scanning and vector-scanning modes, two distinct surface structures are constructed on polybenzoxazine substrate, yielding a superhydrophilic (LSHL) surface and superhydrophobic (LSHB) surface, respectively. Of particular note is that the unique hierarchical structure of LSHB surface has endowed it with quite a robust superwetting behaviors. Further profiting from the flexibility of the processing method, wettability patterns with spatially resolved LSHL and LSHB regions are designed, achieving the conversion of surface energy to liquid kinetic energy. This also offers a tractable approach to fabricate wettability-engineered devices that enable the directional, pumpless transport of water by capillary pressure gradient and the selective surface cooling via jet impingement. In addition, the LSHB surface demonstrates the high conversion of electric-to-thermal energy (222 °C cm² W^-1 ) and light-to-thermal energy (88%). Overall, the material system and processing method present a promising step forward to developing easy-fabricated graphene surfaces with spatially controlled wettability for efficient energy utilization and conversion.

Influence of substrate on ultrafast water transport property of multilayer graphene coatings

The ultrafast water transport in graphene nanoplatelets (GNPs) coating is attributed to the low friction passages formed by pristine graphene and the hydrophilic functional groups which provide a strong interaction force to the water molecules. Here, we examine the influence of the supporting substrate on the ultrafast water transport property of multilayer graphene coatings experimentally and by computational modelling. Thermally cured GNPs manifesting ultrafast water permeation are coated on different substrate materials, namely aluminium, copper, iron and glass. The physical and chemical structures of the GNPs coatings which are affected by the substrate materials are characterized using various spectroscopy techniques. Experimentally, the water permeation and absorption tests evidence the significant influence of the substrate on the rapid water permeation property of GNPs-coating. The water transport rates of the GNPs coatings correspond to the wettability and the free surface energy of their substrates where the most hydrophilic substrate induces the highest water transport rate. In addition, we conduct molecular dynamics (MD) simulations to investigate the transport rate of water molecules through multilayer GNPs adjacent to different substrate materials. The MD simulations results agree well with the experimental results inferring the strong influence of the substrate materials on the fast water transport of GNPs. Therefore, selection of substrate has to be taken into consideration when the GNPs-coating is placed into applications.