Rational Design of Hyperbranched Nanowire Systems for Tunable Superomniphobic Surfaces Enabled by Atomic Layer Deposition

Tuning 3-D Nanomaterial Architectures Using Atomic Layer Deposition to Direct Solution Synthesis

ConspectusThe ability to synthesize nanoarchitected materials with tunable geometries provides a means to control their functional properties, with applications in biological, environmental, and energy fields. To this end, various bottom-up and top-down synthesis processes have been developed. However, many of these processes require prepatterning or etching steps, making them challenging to scale-up to complex, nonplanar substrates. Furthermore, the ability to integrate nanomaterials into hierarchical arrays with precise control of feature spacing and orientation remains a challenge.One approach to overcome these patterning challenges is the use of surface modification layers to guide the resulting geometry of nanomaterial architectures grown from the substrate. A powerful strategy to accomplish this is what we will refer to as "surface-directed assembly," where the resulting geometric parameters (feature size, shape, orientation) are predetermined by the initial surface layer. In particular, the use of Atomic Layer Deposition (ALD) to form a surface layer, followed by solution-based growth processes, has the ability to synthesize architected structures with tunable geometries on complex, nonplanar surfaces.Over the past decade, we have reported a series of studies where surface-directed assembly is used to synthesize ZnO nanowires (NWs) on top of a variety of substrates. In this case, a thin film of ZnO is deposited onto the substrate using ALD, which can guide the NW diameter, spacing, and angular orientation with respect to the substrate by controlling epitaxial relationships. Furthermore, we have shown that by depositing a submonolayer overcoat of a secondary material (e.g., amorphous TiO2), nucleation sites are partially blocked, which can further tune the spacing between nanowires while minimizing changes to their other geometric properties. This approach can be used to generate multilevel hierarchical structures, such as hyperbranched NW arrays with tunable control of each level of hierarchy using ALD. Finally, we have demonstrated that the tunable control of geometric parameters can be scaled-up to curved, nonplanar substrates. This highlights the power of ALD to conformally and uniformly deposit the seed layers on complex substrates with subnanometer precision.To complement these seeded hydrothermal approaches, we expanded this strategy to include conversion chemistry of the initial ALD seed layers. For example, by replacing ZnO with Al2O3 as the seed layer without changing the hydrothermal growth conditions, Al-Zn layered-double hydroxide nanosheets can be formed instead of nanowires. In another example of conversion chemistry, a solution anion-exchange process was used to incorporate sulfur into ALD metal oxide films. In both of these conversion processes, the properties of the initial ALD film enabled tuning of the resulting nanostructure geometry.In this Account, we describe the use of ALD to guide the growth of diverse nanomaterial systems, with tunable control over their geometry and composition. We further show how these approaches can be used to tune functional properties for a range of applications, including superomniphobic surfaces, antibiofouling coatings, and photocatalysis. We conclude with an outlook on how the combination of ALD and solution synthesis can enable future directions in scalable nanomanufacturing to overcome the limitations of traditional top-down and bottom-up approaches.

Bioinspired Zwitterionic Nanowires with Simultaneous Biofouling Reduction and Release

Marine biofouling is a complex and dynamic process that significantly increases the carbon emissions from the maritime industry by increasing drag losses. However, there are no existing non-toxic marine paints that can achieve both effective fouling reduction and efficient fouling release. Inspired by antifouling strategies in nature, herein, a superoleophobic zwitterionic nanowire coating with a nanostructured hydration layer is introduced, which exhibits simultaneous fouling reduction and release performance. The zwitterionic nanowires demonstrate >25% improvement in fouling reduction compared to state-of-the-art antifouling nanostructures, and four times higher fouling-release compared to conventional zwitterionic coatings. Fouling release is successfully achieved under a wall shear force that is four orders of magnitude lower than regular water jet cleaning. The mechanism of this simultaneous fouling reduction and release behavior is explored, and it is found that a combination of 1) a mechanical biocidal effect from the nanowire geometry, and 2) low interfacial adhesion resulting from the nanostructured hydration layer, are the major contributing factors. These findings provide insights into the design of nanostructured coatings with simultaneous fouling reduction and release. The newly established synthesis procedure for the zwitterionic nanowires opens new pathways for implementation as antifouling coatings in the maritime industry and biomedical devices.

Toward Advanced Superomniphobicity: Hierarchical Insights from Serif-T Nanostructures to Microscale Wrinkles

Developing a superomniphobic surface that exceeds the static and dynamic repellency observed in nature's springtails for various liquids presents a significant challenge in the realm of surface and interface science. However, progress in this field has been particularly limited when dealing with low-surface-tension liquids. This is because dynamic repellency values are typically at least 2 orders of magnitude lower than those observed with water droplets. Our study introduces an innovative hierarchical topography demonstrating exceptional dynamic repellency to low-surface-tension liquids. Inspired by the structural advantages found in springtails, we achieve a static contact angle of >160° and the complete rebound of droplet impact with a Weber number (We) of ∼104 using ethanol. These results surpass all existing benchmarks that have been reported thus far, including those of natural surfaces. The key insight from our research is the vital role of the microscale air pocket size, governed by wrinkle wavelength, in both static and dynamic repellency. Additionally, nanoscale air pockets within serif-T nanostructures prove to be essential for achieving omniphobicity. Our investigations into the wetting dynamics of ethanol droplets further reveal aspects such as the reduction in contact time and the occurrence of a fragmentation phenomenon beyond We ∼ 350, which has not been previously observed.

Reactive Multilayer Coating As Versatile Nanoarchitectonics for Customizing Various Bioinspired Liquid Wettabilities

In last few decades, multilayer coatings have achieved enormous attention owing to their unique ability to tune thickness, topography, and chemical composition for developing various functional materials. Such multilayer coatings were mostly and conventionally derived by following a simple layer-by-layer (LbL) deposition process through the strategic use of electrostatic interactions, hydrogen bonding, host-guest interactions, covalent bonding, etc. In the conventional design of multilayer coatings, the chemical composition and morphology of coatings are modulated during the process of multilayer constructions. In such an approach, the postmodulations of the porous multilayers with different and desired chemistries are challenging to achieve due to the lack of availability of readily and selectively reactive moieties. Recently, the design of readily and selectively reactive multilayer coatings (RMLCs) provided a facile basis for postmodulating the prepared coating with various desired chemistries. In fact, by taking advantage of the inherent ability of co-optimizing the topography and various chemistries in porous RMLCs, different durable bioinspired liquid wettabilities (i.e., superhydrophobicity, underwater superoleophobicity, underwater superoleophilicity, slippery property, etc.) were successfully derived. Such interfaces have enormous potential in various prospective applications. In this review, we intend to give an overview of the evolution of LbL multilayer coatings and their synthetic strategies and discuss the key advantages of porous RMLCs in terms of achieving and controlling wettability properties. Recent attempts toward various applications of such multilayer coatings that are strategically embedded with different desired liquid wettabilities will be emphasized.

Durability studies of underwater superoleophobic graphene oxide coated wire mesh

Due to the increased industrial oily wastewater, developing a successful oil/water separation mechanism is a ubiquitous challenge. As oil/water separation is an interfacial phenomenon, a straightforward way is to utilize the special wettability of novel materials towards oil and water. In this work, we intend to construct a durable membrane/mesh that can have a selective response towards oil and water based on the difference in surface tension. Graphene oxide (GO) is one such material that exhibits in-air hydrophilicity and underwater superoleophobicity. GO-coated wire meshes can act as membranes with excellent efficiency for oil/water separation, but they lack long-term durability for repeated use under different environments. We created GO*-coated wire meshes by dip coating multiple layers of GO with intermediate air plasma treatment. While the multiple steps of coating ensured complete coverage of the mesh with GO, plasma treatment improved the binding of the GO coating to the wire mesh. After coating five GO layers, the mesh is subjected to mild plasma treatment to improve the porosity. The GO*-coated mesh is extremely hydrophilic in air, and the underwater oil contact angles (CA) are ≥125° for different oils. To test the long-term durability, the GO*-coated mesh is continuously immersed underwater in acidic and basic media, and the underwater oil CA is measured at different immersion times. The initial durability results are very promising and show that the GO*-coated mesh retains a significant level of underwater oleophobicity even after 60 days of continuous immersion in water.

Integrating Structural Colors with Additive Manufacturing Using Atomic Layer Deposition

We demonstrate tunable structural color patterns that span the visible spectrum using atomic layer deposition (ALD). Asymmetric metal-dielectric-metal structures were sequentially deposited with nickel, zinc oxide, and a thin copper layer to form an optical cavity. The color response was precisely adjusted by tuning the zinc oxide (ZnO) thickness using ALD, which was consistent with model predictions. Owing to the conformal nature of ALD, this allows for uniform and tunable coloration of non-planar three-dimensional (3D) objects, as exemplified by adding color to 3D-printed parts produced by metal additive manufacturing. Proper choice of inorganic layered structures and materials allows the structural color to be stable at elevated temperatures, in contrast to traditional paints. To print multiple colors on a single sample, polymer inhibitors were patterned in a desired geometry using electrohydrodynamic jet (e-jet) printing, followed by area-selective ALD in the unpassivated regions. The ability to achieve 3D color printing, both at the micro- and macroscales, provides a new pathway to tune the optical and aesthetic properties during additive manufacturing.

Geometric Optimization of Bismuth Vanadate Core-Shell Nanowire Photoanodes using Atomic Layer Deposition

In this study, systematic geometric tuning of core-shell nanowire (NW) architectures is used to decouple the contributions from light absorption, charge separation, and charge transfer kinetics in photoelectrochemical water oxidation. Core-shell-shell NW arrays were fabricated using a combination of hydrothermal synthesis of ZnO and atomic layer deposition (ALD) of SnO2 and BiVO4. The length and spacing of the NW scaffold, as well as the BiVO4 film thickness, were systematically tuned to optimize the photoelectrochemical performance. A photocurrent of 4.4 mA/cm2 was measured at 1.23 V vs RHE for sulfite oxidation and 4.0 mA/cm2 at 1.80 V vs RHE for water oxidation without a cocatalyst, which are the highest values reported to date for an ALD-deposited photoanode. Electromagnetic simulations demonstrate that spatial heterogeneity in light absorption along the core-shell NW length has a critical role in determining internal quantum efficiency. The mechanistic understandings in this study highlight the benefits of systematically optimizing electrode geometry at the nanoscale when designing photoelectrodes.

A Nanowire Nest Structure Comprising Copper Silicide and Silicon Nanowires for Lithium-Ion Battery Anodes with High Areal Loading

High loading (>1.6 mg cm^-2 ) of Si nanowires (NWs) is achieved by seeding the growth from a dense array of Cu₁₅ Si₄ NWs using tin seeds. A one-pot synthetic approach involves the direct growth of CuSi NWs on Cu foil that acts as a textured surface for Sn adhesion and Si NW nucleation. The high achievable Si NW loading is enabled by the high surface area of CuSi NWs and bolstered by secondary growth of Si NWs as branches from both Si and CuSi NW stems, forming a dense Si active layer, interconnected with an electrically conducting CuSi array (denoted Si/CuSi). When employed as Li-ion battery anodes, the Si/CuSi nest structure demonstrates impressive rate performance, reaching 4.1 mAh cm^-2 at C/20, 3.1 mAh cm^-2 at C/5, and 0.8 mAh cm^-2 at 6C. Also, Si/CuSi shows remarkable long-term stability, delivering a stable areal capacity of 2.2 mAh cm^-2 after 300 cycles. Overall, complete anode fabrication is achieved within a single reaction by employing an inexpensive Sn powder approach.

Design and applications of surfaces that control the accretion of matter

Surfaces that provide control over liquid, solid, or vapor accretion provide an evolutionary advantage to numerous plants, insects, and animals. Synthetic surfaces inspired by these natural surfaces can have a substantial impact on diverse commercial applications. Engineered liquid and solid repellent surfaces are often designed to impart control over a single state of matter, phase, or fouling length scale. However, surfaces used in diverse real-world applications need to effectively control the accrual of matter across multiple phases and fouling length scales. We discuss the surface design strategies aimed at controlling the accretion of different states of matter, particularly those that work across multiple length scales and different foulants. We also highlight notable applications, as well as challenges associated with these designer surfaces' scale-up and commercialization.

Multifunctional Textiles Based on Three-Dimensional Hierarchically Structured TiO2 Nanowires

The development of three-dimensional (3D) micro-/nanostructures with multiscale hierarchy offers new potential for the improvement of the pristine textile properties. In this work, a polyester fabric coated with 3D hierarchically structured rutile TiO₂ nanowires (THNWP) was fabricated by a facile hydrothermal strategy. The THNWP samples exhibit markedly improved photocatalytic activities and antibacterial properties owing to their 3D hierarchical architecture constructed by one-dimensional nanowire structures, good crystallinity, excellent light-harvesting capability, and fast electron-transfer rate. Furthermore, the unique 3D hierarchical nanostructures also combine with the monofilament to produce ternary-scale hierarchy, which endows the fabric surface with outstanding superamphiphobicity after further facile fluorination treatment. The supportive air-pockets trapped within the unique ternary-scale architectures are proved to be the crucial factor in the achievement of high liquid repellency, and the highest performing superamphiphobic surface is capable of repelling liquids down to a minimal surface tension of 23.4 mN m^-1. We envision that our findings may possess great potential in the bottom-up design of high-performance textiles.

Designing Re-Entrant Geometry: Construction of a Superamphiphobic Surface with Large-Sized Particles

Re-entrant geometries can effectively trap air pockets beneath coating surfaces, prevent the penetration of low surface tension organic liquids, and achieve superamphiphobic performance. However, the creation of re-entrant geometries through particle-based spray coating remains a challenge. In the past decade, various studies have focused on the preparation of superamphiphobic coatings using ultrafine nanoparticles (10-15 nm) using conventional spray-coating methods. In this work, we aim to fabricate a spray-coated superamphiphobic surface using large particles with a hierarchical structure. The study systematically investigated the wetting behaviors of liquids with different topographies obtained using large particles (i.e., smooth, micro, nano, and micro/nanostructures) by different coating methods. The findings suggested that compared with the typical colloid template method, the surface obtained using the spray-coating method showed much greater roughness, which greatly enhanced the oleophobicity of the coating. Furthermore, only hierarchically monodisperse hollow SiO₂ spheres (MDH-SiO₂) showed excellent superamphiphobicity, which was independent of the hollow sphere size. While maintaining the coating roughness, by applying solid C@SiO₂ as a reference sample, the important role of the hollow structure of MDH-SiO₂ at the solid-liquid-air interface was confirmed. Nanosphere-surrounded hollow structures were shown to serve as a re-entrant type structure, preventing the imbibition of the liquid, finally leading to a stable Cassie state. This design strategy may provide useful guidelines for the fabrication of large particle-based spray-coated superamphiphobic surfaces.

Anti-biofouling NH3 gas sensor based on reentrant thorny ZnO/graphene hybrid nanowalls

Since toxic gas leakage may cause ecological environmental problems and even life-threatening damage, effective monitoring of toxic gas is of great importance and subject to increasing demand. However, complicated environmental factors, as well as various coexisting interferences can easily affect the sensitivity and selectivity of gas sensors, hindering their performance. Recent reports have successfully demonstrated the development of hierarchical nanostructures with desirable self-cleaning properties, yet gas sensors that can resist contamination have rarely been realized. Here, we developed a reentrant thorny ZnO/graphene hybrid nanowall structure that simultaneously repels liquid contamination and possesses NH3 gas sensing properties. The unique reentrant and hierarchical structure, featuring an interconnected vertical graphene nanowall framework with numerous ZnO nanospikes branched on the top nanowall, is highly repellent to liquids, even biofluids with low surface tension. The hierarchical structure consisting of gas sensing graphene and ZnO can be successfully applied as an NH3 gas sensor at room temperature, exhibiting not only excellent sensitivity, selectivity, and repeatability, but also outstanding stability even after bacterial contamination. This study provides a versatile method for fabricating reentrant and hierarchical structures with excellent liquid repellency, and offers a promising method for designing reliable gas sensors with anti-biofouling properties.

Durable Light-Driven Three-Dimensional Smart Switchable Superwetting Nanotextile as a Green Scaled-Up Oil-Water Separation Technology

Stimuli-responsive polymer architectures are attracting a lot of interest, but it still remains a great challenge to develop effective industrial-scale strategies. A single-stage and cost-effective approach was applied to fabricate a three-dimensional (3D) smart responsive surface with fast and reversibly switchable wetting between superhydrophobicity and superhydrophilicity/underwater superoleophobicity properties induced by photo and heat stimuli. Commercially available PVDF and P25TiO₂ as starting materials fabricated with a scaled-up electrospinning approach were applied to prepare 3D smart switchable PVDF-P25TiO₂ nanotextile superwetted by both UV and solar light that is simply recovered by heat at a reasonable time. The superhydrophilic/underwater superoleophobic photo-induced nanotextile will act in "water-removing" mode in which water quickly passes through and the oil is blocked on the surface. An acceptable recycling, reusing, and superior antifouling and self-cleaning performance arising from a TiO₂ photocatalytic effect makes it highly desired in a green scaled-up industry oily wastewater treatment technology. With these advantages, a large-scale industrial production process can be simply simulated by applying a conducting mesh-like collector substrate.

Drop Impact on Two-Tier Monostable Superrepellent Surfaces

Superrepellency is a favorable nonwetting scenario featuring a dramatic reduction of the solid/liquid contact area. The robustness of superhydrophobicity plays a central role in self-cleaning and anti-icing. Drop impacts happen ubiquitously in natural environments and often cause a notable extension of the solid/liquid contact area. This is associated with an enhanced affinity between water and the microtextures and therefore leads to irreversible breakdowns in the superhydrophobicity. This problem remains a major challenge and limits the practical applications of superrepellent materials. In order to find a solution, in this paper, a repeated Cassie-Wenzel-Cassie wetting state transition is studied at the microscale when a drop impacts a two-tier superhydrophobic surface. In this case, the surface is completely dry without any liquid residue after the drop rebounds. The present results exhibit a striking contrast to the conventional perspective. The influence of geometrical parameters of the textured surface on the spreading and retracting behaviors of the impact drops is quantified, as well as the time-dependence scaling laws. From a practical point of view, it is demonstrated that the self-cleaning and dropwise condensation may significantly benefit from this repeated wetting transition. Dirt particles or small droplets in deep textures are able to be taken away so that the functionality and the robustness of the superhydrophobicity may be significantly strengthened. The results reported in this study facilitate the design of functional superrepellent materials.

Trade-off in membrane distillation with monolithic omniphobic membranes

Omniphobic membranes are attractive for membrane distillation (MD) because of their superior wetting resistance. However, a design framework for MD membrane remains incomplete, due to the complexity of omniphobic membrane fabrication and the lack of fundamental relationship between wetting resistance and water vapor permeability. Here we present a particle-free approach that enables rapid fabrication of monolithic omniphobic membranes for MD desalination. Our monolithic omniphobic membranes display excellent wetting resistance and water purification performance in MD desalination of hypersaline feedwater containing surfactants. We identify that a trade-off exists between wetting resistance and water vapor permeability of our monolithic MD membranes. Utilizing membranes with tunable wetting resistance and permeability, we elucidate the underlying mechanism of such trade-off. We envision that our fabrication method as well as the mechanistic insight into the wetting resistance-vapor permeability trade-off will pave the way for smart design of MD membranes in diverse water purification applications.

Under-liquid dual superlyophobic nanofibrous polymer membranes achieved by coating thin-film composites: a design principle

Surfaces with under-liquid dual superlyophobicity have garnered tremendous interest because of their promising applications, but their unexplored underlying nature restricts the designed construction of such surfaces. Herein, we coated the thin-film composites with different terminal groups over the electrospun polyacrylonitrile nanofibrous membranes, which afforded the membranes excellent stability in organic solvents, as well as modulated under-liquid wetting behaviors. Among them, the representative under-liquid dual superlyophobic 4-cyan-Ph-terminated membrane could realize highly efficient separation of all types of oil/water mixtures and even emulsions. Moreover, we found that the under-liquid wetting behaviors could be classified in terms of the intrinsic water contact angle (θ w). By comparing the total interfacial energy, we proved that the under-liquid dual lyophobic surfaces were thermodynamically metastable. On this basis, we could predict the θ w of rough surfaces with the under-liquid dual lyophobicity in a given oil-water-solid system (e.g., 47.3-89.1° in cyclohexane-water-solid system, R = 2). This work provides a design principle for the fabrication of under-liquid dual superlyophobic surfaces, which will open potential applications in diverse fields in terms of such smart surfaces.

Growth of hierarchical gold clusters for use in superomniphobic electrodes

We developed a method to fabricate a superomniphobic gold electrode by synthesizing hierarchical gold clusters on a gold substrate and treating the surface with low surface energy materials. The reduction of gold ions was repeated several times, causing the gold microparticles to grow in random directions and form hierarchical gold clusters. Treatment of the gold structures with perfluorothiol resulted in a superhydrophobic surface that also exhibited superoleophobicity for oils and liquids with surface tensions as low as 25.6 mN. The resulting electrode was not contaminated by hydrophilic and hydrophobic liquids, and by analyzing the current-voltage characteristics of the electrode with a PEDOT:PSS solution droplet, the electrode was found to be waterproof.

In Situ Generation of Zinc Oxide Nanobushes on Microneedles as Antibacterial Coating

This paper introduces a facile and scalable method to generate a layer of antibacterial coating on microneedles. The antibacterial coating (i.e., zinc oxide nanobushes) is generated on the surface of gold-coated polystyrene microneedles using the hydrothermal growth method. The antimicrobial property is examined using the agar diffusion test with both gram-positive and gram-negative bacteria.

Springtail-inspired superomniphobic surface with extreme pressure resistance

Both high static repellency and pressure resistance are critical to achieving a high-performance omniphobic surface. The cuticles of springtails have both of these features, which result from their hierarchical structure composed of primary doubly reentrant nanostructures on secondary microgrooves. Despite intensive efforts, none of the previous studies that were inspired by the springtail were able to simultaneously achieve both high static repellency and pressure resistance because of a general trade-off between these characteristics. We demonstrate for the first time a springtail-inspired superomniphobic surface displaying both features by fabricating a hierarchical system consisting of serif-T-shaped nanostructures on microscale wrinkles, overcoming previous limitations. Our biomimetic strategy yielded a surface showing high repellency to diverse liquids, from water to ethanol, with a contact angle above 150°. Simultaneously, the surface was able to endure extreme pressure resulting from the impacts of drops of water and of ethylene glycol with We >> 200, and of ethanol with We ~ 53, which is the highest pressure resistance ever reported. Overall, the omniphobicity of our springtail-inspired fabricated system was found to be superior to that of the natural springtail cuticle itself.

Cassie-Levitated Droplets for Distortion-Free Low-Energy Solid-Liquid Interactions

Despite the rapid advent of superomniphobic materials, there is a lack of methodologies to accurately investigate the ultralow-energy interactions taking place on these interfaces. For instance, universally employed models such as the pendant droplet often fail to provide representative information on the wetting properties of superomniphobic surfaces. The delicate balance between the forces acting at the droplet-surface and droplet-needle interfaces can easily result in heavily distorted droplet profiles. Here, we introduce a Cassie-levitating droplet model which overcomes the limitations of the pendant droplet model, allowing a distortion-free assessment of the interactions between super(amphi)omniphobic materials and low surface tension liquids. Comparative analysis in wetting of low surface tension fluids such as hexadecane (∼27.47 mN/m) on superamphiphobic surfaces via the Cassie-levitating and pendant droplet models reveals up to 70° (800%) deviations in the estimated contact angle hysteresis. A theoretical framework is developed to assess experimentally observed profile distortions against ideal gravity-induced sagging of droplet shapes during dynamic droplet expansion and contraction cycles. Notably, pendant droplets resulted in up to 50% distortion while the Cassie-levitating ones achieved less than just 10%. We believe that the Cassie-levitating droplet model bears ample potential for the characterization of the rapidly emerging family of superomniphobic materials, setting the basis for their future engineering in numerous emerging applications.

TiO2 nanowire-templated hierarchical nanowire network as water-repelling coating

Extraordinary water-repelling properties of superhydrophobic surfaces make them novel candidates for a great variety of potential applications. A general approach to achieve superhydrophobicity requires low-energy coating on the surface and roughness on nano- and micrometre scale. However, typical construction of superhydrophobic surfaces with micro-nano structure through top-down fabrication is restricted by sophisticated fabrication techniques and limited choices of substrate materials. Micro-nanoscale topographies templated by conventional microparticles through surface coating may produce large variations in roughness and uncontrollable defects, resulting in poorly controlled surface morphology and wettability. In this work, micro-nanoscale hierarchical nanowire network was fabricated to construct self-cleaning coating using one-dimensional TiO2 nanowires as microscale templates. Hierarchical structure with homogeneous morphology was achieved by branching ZnO nanowires on the TiO2 nanowire backbones through hydrothermal reaction. The hierarchical nanowire network displayed homogeneous micro/nano-topography, in contrast to hierarchical structure templated by traditional microparticles. This hierarchical nanowire network film exhibited high repellency to both water and cell culture medium after functionalization with fluorinated organic molecules. The hierarchical structure templated by TiO2 nanowire coating significantly increased the surface superhydrophobicity compared to vertical ZnO nanowires with nanotopography alone. Our results demonstrated a promising strategy of using nanowires as microscale templates for the rational design of hierarchical coatings with desired superhydrophobicity that can also be applied to various substrate materials.

Hydrogen assisted synthesis of branched nickel nanostructures: a combined theoretical and experimental study

The selective adsorption of small molecules over specific facets plays an important role in morphology controlled synthesis of metal nanocrystals. In the present work, hydrogen is found to be a good capping agent for direct synthesis of branched nickel nanocrystals, i.e., secondary branching (Ni-SB) nanoparticles and multipods (Ni-MP). Using ab initio thermodynamics and the Wulff construction principle, it has been found that: (i) in the presence of hydrogen (P_H2 = 6 bar), the facet structure stability follows the order of Ni(100) > Ni(111) > Ni(110), resulting in competitive over-growth along the 〈111〉 and 〈110〉 directions; (ii) with increasing hydrogen pressure, the Ni deposition rate over the crystal surface increases as a result of more Ni²⁺ reduction; the competition between deposition and surface diffusion, therefore, becomes the vital factor for the nanocrystal growth process; (iii) the diffusion energy barrier of a surface Ni atom on Ni(111) is lower than that on Ni(110), especially on hydrogen covered surfaces, indicating that the kinetic over-growth only along the 〈111〉 direction producing Ni-MP will be dominant under P_H2 = 14 bar; (iv) the ab initio based Wulff construction principle predicts the shapes and morphologies at different hydrogen pressures which is further confirmed with HRTEM results. Finally, compared with nickel nanoparticles (Ni-NP) synthesized in the absence of hydrogen, the hydrogen assisted branched Ni nanomaterials, especially the Ni-MP, show higher catalytic activities for hydrogenation reactions of acetophenone and nitrobenzene.

Bio-inspired interlocking random 3-D structures for tactile and thermal sensing

Hierarchical nanostructures are tailored and used routinely in nature to accomplish tasks with high performance. Their formation in nature is accomplished without the use of any patterning process. Inspired by the performance of such structures, we have combined 2-D nanosheets with 1-D nanorods for functioning as electronic skin. These structures made in high density without any patterning process can be easily assembled over large areas. They can sense pressures as low as 0.4 Pa, with a response time in milliseconds. Further, these structures can also detect temperature changes with a non-linear response in the 298-400 K range, which is similar to skins perception of thermal stimuli. We illustrate this effect by showing that the device can differentiate between two 10 µl water droplets which are at room temperature and 323 K respectively.

Slippery surface based on lubricant infused hierarchical silicon nanowire film

Slippery surface based on lubricant infused hierarchical Si nanowire films was developed, which provided low contact angle with liquid droplet, while possessing liquid repellent property upon slight tilting.