Amplified and localized photoswitching of TiO2 by micro- and nanostructuring

Understanding the light induced hydrophilicity of metal-oxide thin films

Photocatalytic effects resulting in water splitting, reduction of carbon dioxide to fuels using solar energy, decomposition of organic compounds, and light-induced hydrophilicity observed on surfaces of various metal oxides (MOx), all rely on the same basic physical mechanisms, and have attracted considerable interest over the past decades. TiO2 and ZnO, two natively n-type doped wide bandgap semiconductors exhibit the effects mentioned above. In this study we propose a model for the photo-induced hydrophilicity in MOx films, and we test the model for TiO2/Si and ZnO/Si heterojunctions. Experimentally, we employ a wet exposure technique whereby the MOx surface is exposed to UV light while a water droplet is sitting on the surface, which allows for a continuous recording of contact angles during illumination. The proposed model and the experimental techniques allow a determination of minority carrier diffusion lengths by contact angle measurements and suggest design rules for materials exhibiting photocatalytic hydrophilicity. We expect that this methodology can be extended to improve our physical understanding of other photocatalytic surface effects.

Recovery of Al2O3 from hazardous Al waste as a reinforcement particle for high-performance Ni/Al2O3 corrosion resistance coating via ultrasonic-aided supercritical-CO2 electrodeposition

The unprocessed dumping of aluminium wastes in the landscape leads to generation of heat and toxic gases, which are detrimental to the ecosystem. Motivated by the waste-to-wealth notion, we demonstrated the recovery of aluminium oxide nanoparticles (Al₂O₃NPs) from domestic aluminium wastes via a sonochemical approach and synthesis of nickel/aluminium oxide (Ni/Al₂O₃) coating via ultrasonic-coupled supercritical carbon dioxide (US-SC-CO₂) electrodeposition method for higher corrosion resistance performance. The physical characterization and material confirmation of prepared films were examined by microscopic and various spectroscopic techniques. The electrochemical corrosion resistance studies were explored via potentiodynamic polarization and electrochemical impedance spectroscopy techniques. Based on the results, the US-SC-CO₂ strategy exposed an improved distribution of Al₂O₃ NPs assimilation in Ni matrix, higher corrosion resistance, and microhardness. The integration of ultrasonic irradiation into the SC-CO₂ process promises an enhanced coating quality. Thereby, the novel US-SC-CO₂ approach for Ni/Al₂O₃ synthesis is expected to achieve a sustainable green impact in real-world applications.

Directionally Guided Droplets on a Modular Bottom-Up Anisotropic Locally Ordered Nickel Nanocone Superhydrophobic Surface

The anisotropic surface prepared by the top-down etching technology shows unique advantages in terms of functional superhydrophobicity. However, it still has a shackle of the smallest etching size, which largely restricts the development of better superhydrophobicity. Therefore, it is still a huge challenge to realize the stepless size adjustment of an anisotropic surface in order to achieve better functionalization. In this work, a bottom-up approach inspired via the modular segmented preparation technology has been used to successfully build an anisotropic, locally ordered functionalized unique superhydrophobic structure, whose contact and rebound time of water droplets is extremely short. Furthermore, this structure with artfully arranged "tracks", which has a relatively large contact angle value, not only lasts more than 15 consecutive bounce cycles in the same direction, where the droplets after merging still bounce, but also exhibits a significant anisotropic sliding behavior, which is presented in different sliding angles, toward droplets rolling in different directions and has lower adhesion work and better self-cleaning and anti-fouling performance. Besides, some mechanisms such as the reduction-replacement-reduction cycle and repulsion-adhesion-switching have been proposed especially in modular preparation and anisotropic sliding behavior. More importantly, this sorted bottom-up structure has great potential for achieving higher efficiency of functionalized superhydrophobicity and other related applications.

Improving Photocatalytic Performance Using Nanopillars and Micropillars

A recent research emphasis has been placed on the development of highly crystallized nanostructures as a useful technology for many photocatalytic applications. With the unique construction of semiconductor transition metal oxide nanostructures in the form of nanopillars—artificially designed pillar-shaped structures grouped together in lattice-type arrays—the surface area for photocatalytic potential is increased and further enhanced through the introduction of dopants. This short review summarizes the work on improving the efficiency of photocatalyst nanopillars through increased surface area and doping within the applications of water splitting, removal of organic pollutants from the environment, photoswitching, soot oxidation, and photothermalization.

Fabrication and wetting behaviour of micro/nanostructured mushroom-shaped silver pillar surface

This manuscript presents a simple, one-step method for the fabrication of micro/nanostructured metal-based superhydrophobic surfaces via electroplating using stacked polycarbonate membranes with nanoscale and microscale pores as a template. The two-tiered mushroom-shaped silver pillar arrays include a top layer composed of nanopillars and a bottom layer composed of T-shaped micropillars. The presence of the re-entrant surface structures with a strong resistance pin the droplets to the cap's ridge and prevent water droplets from penetrating into the valleys of the rough surface, thus resulting in an increase in water contact angle (WCA). Compared with microstructured mushroom-shaped surfaces (WCA = 148°, sliding angle (SA) ∼ 26°) and nanostructured surfaces (WCA = 151.5°, SA ∼ 4.8°), the micro/nanostructured mushroom-shaped pillar arrays (WCA = 154.1°, SA ∼ 2°) exhibit remarkable superhydrophobic properties with high CA and low SA. This new micro/nanostructured surface will have a potential application in metal-based superhydrophobic materials.

Application of ZnO Nanorods Based Whispering Gallery Mode Resonator in Optical Immunosensors

In this research a whispering gallery mode (WGM) resonator based on vertically oriented ZnO nanorods, which were formed on silicon surface (silicon/ZnO-NRs), has been applied in the design of optical immunosensor that was dedicated for the determination of grapevine virus A-type (GVA) proteins. Vertically oriented ZnO-NRs were grown on silicon substrates by atmospheric pressure metal organic chemical vapor deposition (APMOCVD) and the silicon/ZnO-NRs structures formed were characterized by structural and optical methods. Optical characterization demonstrates that silicon/ZnO-NRs-based structures can act as 'whispering gallery mode' (WGM) resonator where quasi-whispering gallery modes (quasi-WGMs) are generated. These quasi-WGMs were experimentally observed in the visible and infrared ranges of the photoluminescence spectra. In order to design an immuno-sensing system the anti-GVA antibodies were immobilized on the surface of silicon/ZnO-NRs and in this way silicon/ZnO-NRs/anti-GVA structure was formed. The immobilization of anti-GVA antibodies and then the interaction of silicon/ZnO-NRs/anti-GVA structure with GVA proteins (GVA-antigens) resulted in an opposite shifts of the WGMs peaks in the visible range of the photoluminescence spectra observed as a defect-related photoluminescence emission of ZnO-NRs. Here designed silicon/ZnO-NRs/anti-GVA immuno-sensing structure demonstrates the sensitivity towards GVA-antigens in the concentration range of 1-200 ng/ml. Bioanalytical applicability of the silicon/ZnO-NRs-based structures in the WGMs registration mode is discussed.

A nanofabricated plasmonic core-shell-nanoparticle library

Three-layer core-shell-nanoparticle nanoarchitectures exhibit properties not achievable by single-element nanostructures alone and have great potential to enable rationally designed functionality. However, nanofabrication strategies for crafting core-shell-nanoparticle structure arrays on surfaces are widely lacking, despite the potential of basically unlimited material combinations. Here we present a nanofabrication approach that overcomes this limitation. Using it, we produce a library of nanoarchitectures composed of a metal core and an oxide/nitride shell that is decorated with few-nanometer-sized particles with widely different material combinations. This is enabled by resolving a long-standing challenge in this field, namely the ability to grow a shell layer around a nanofabricated core without prior removal of the lithographically patterned mask, and the possibility to subsequently grow smaller metal nanoparticles locally on the shell only in close proximity of the core. Focusing on the application of such nanoarchitectures in plasmonics, we show experimentally and by Finite-Difference Time-Domain (FDTD) simulations that these structures exhibit significant optical absorption enhancement in small metal nanoparticles grown on the few nanometer thin dielectric shell layer around a plasmonic core, and derive design rules to maximize the effect by the tailored combination of the core and shell materials. We predict that these structures will find application in plasmon-mediated catalysis and nanoplasmonic sensing and spectroscopy.

Programmable wettability on photocontrolled graphene film

Surface materials with specific wettability play important roles in a wide variety of areas from science to industry. We present a novel paraffin-infused porous graphene film (PIPGF) with programmable wettability. Because of graphene's photothermal property, the paraffin in the PIPGF was in transition between liquid and solid in response to near-infrared (NIR) light irradiation. Thus, we imparted the film with a dynamic and reversible transition between a slippery and a rough surface as the remotely tunable wettability. In addition, with the integration of NIR masks, the paraffin could melt at corresponding patterns on the PIPGF, which formed special flow pathways for the slipping droplets. Therefore, the PIPGF could provide programmable wettability pathways for the spatiotemporal droplet manipulation by flexibly changing the NIR masks. We demonstrated these programmable wettability pathways to not only simplify liquid handling in the microplates and droplet microarrays technology but also to provide distinctly microfluidic microreactors for different purposes, such as practical blood grouping diagnosis. These features indicated that the photocontrollable PIPGF would be amenable to a variety of applications, such as microfluidic systems, laboratory-on-a-chip settings, and droplet manipulations.

Corrosion resistance of BIS 2062-grade steel coated with nano-metal-oxide mixtures of iron, cerium, and titanium in the marine environment

BIS 2062-grade carbon steel is extensively used for fishing boat construction. The steel is highly susceptible to corrosion on the hull and welding joints under marine environment. Here, we demonstrate the application of a novel multifunctional nano-metal-oxide mixture comprised of iron, titanium, and cerium as a marine coating to prevent corrosion. The electrochemical performance of nano-metal-oxide mixture coatings, applied over boat-building steel, was evaluated at 3.5% NaCl medium. The nano-mixture surface coatings showed an efficient corrosion resistance with increased polarization resistance of 6043 Ω cm2 and low corrosion current density of 3.53 × 10−6 A cm−2. The electrochemical impedance spectral data exhibited improvement in the polarization resistance of outermost surface and internal layers. The coating responded faster recovery to normal state when subjected to an induced stress over the coating. The nano-material in the coating behaves as a semiconductor; this enhanced electronic activity over the surface of the steel.

Friction and Wetting Transitions of Magnetic Droplets on Micropillared Superhydrophobic Surfaces

Reliable characterization of wetting properties is essential for the development and optimization of superhydrophobic surfaces. Here, the dynamics of superhydrophobicity is studied including droplet friction and wetting transitions by using droplet oscillations on micropillared surfaces. Analyzing droplet oscillations by high-speed camera makes it possible to obtain energy dissipation parameters such as contact angle hysteresis force and viscous damping coefficients, which indicate pinning and viscous losses, respectively. It is shown that the dissipative forces increase with increasing solid fraction and magnetic force. For 10 µm diameter pillars, the solid fraction range within which droplet oscillations are possible is between 0.97% and 2.18%. Beyond the upper limit, the oscillations become heavily damped due to high friction force. Below the lower limit, the droplet is no longer supported by the pillar tops and undergoes a Cassie-Wenzel transition. This transition is found to occur at lower pressure for a moving droplet than for a static droplet. The findings can help to optimize micropillared surfaces for low-friction droplet transport.

Robust hybrid elastomer/metal-oxide superhydrophobic surfaces

We introduce a new type of hybrid material: a nanostructured elastomer covered by a hard photoactive metal-oxide thin film resembling the exoskeleton of insects. It has extreme water repellency and fast self-recovery after damage. A new fabrication method for replicating high aspect ratio, hierarchical re-entrant aluminum structures into polydimethylsiloxane (PDMS) is presented. The method is based on a protective titania layer deposited by atomic layer deposition (ALD) on the aluminum template. The ALD titania transfers to the elastomeric scaffold via sacrificial release etching. The sacrificial release method allows for high aspect ratio, even 100 μm deep and successful release of overhanging structures, unlike conventional peeling. The ALD titania conformally covers the 3D multihierarchical structures of the template and protects the polymer during the release etch. Afterwards it prevents the high aspect ratio nanostructures from elasticity based collapse. The resulting nanostructured hybrid PDMS/titania replicas display robust superhydrophobicity without any further fluoro-coating or modification. Their mechanical and thermal robustness results from a thick nanostructured elastomeric layer which is conformally covered by ceramic titania instead of a monolayer hydrophobic coating. We have demonstrated the durability of these replicas against mechanical abrasion, knife scratches, rubbing, bending, peel tape test, high temperature annealing, UV exposure, water jet impingement and long term underwater storage. Though the material loses its superhydrophobicity in oxygen plasma exposure, a fast recovery from superhydrophilic to superhydrophobic can be achieved after 20 min UV irradiation. UV-assisted recovery is correlated with the high photoactivity of ALD titania film. This novel hybrid material will be applicable to the large area superhydrophobic surfaces in practical outdoor applications.

Recent Advances in TiO2 -Based Nanostructured Surfaces with Controllable Wettability and Adhesion

Bioinspired surfaces with special wettability and adhesion have attracted great interest in both fundamental research and industry applications. Various kinds of special wetting surfaces have been constructed by adjusting the topographical structure and chemical composition. Here, recent progress of the artificial superhydrophobic surfaces with high contrast in solid/liquid adhesion has been reviewed, with a focus on the bioinspired construction and applications of one-dimensional (1D) TiO2-based surfaces. In addition, the significant applications related to artificial super-wetting/antiwetting TiO2-based structure surfaces with controllable adhesion are summarized, e.g., self-cleaning, friction reduction, anti-fogging/icing, microfluidic manipulation, fog/water collection, oil/water separation, anti-bioadhesion, and micro-templates for patterning. Finally, the current challenges and future prospects of this renascent and rapidly developing field, especially with regard to 1D TiO2-based surfaces with special wettability and adhesion, are proposed and discussed.

Surface functionalized H2Ti3O7nanowires engineered for visible-light photoswitching, electrochemical water splitting, and photocatalysis

The mechanism of the visible-light driven photoelectrochemical properties of surface engineered H₂Ti₃O₇nanowires has been demonstrated.