Sanghyuk Wooh

Efficient Light Harvesting with Micropatterned 3D Pyramidal Photoanodes in Dye‐Sensitized Solar Cells

3D TiO2 photoanodes in dye-sensitized solar cells (DSCs) are fabricated by the soft lithographic technique for efficient light trapping. An extended strategy to the construction of randomized pyramid structure is developed by the conventional wet-etching of a silicon wafer for low-cost fabrication. Moreover, the futher enhancement of light absorption resulting in photocurrent increase is achieved by combining the 3D photoanode with a conventional scattering layer.

Synthesis of Mesoporous Supraparticles on Superamphiphobic Surfaces

A method for mesoporous supraparticle synthesis on superamphiphobic surfaces is designed. Therefore, supraparticles assembled with nanoparticles are synthesized by the evaporation of nanoparticle dispersion drops on the superamphiphobic surface. For synthesis, no further purification is required and no organic solvents are wasted. Moreover, by changing the conditions such as drop size and concentration, supraparticles of different sizes, compositions, and architectures are fabricated.

Surface Modification of TiO2 Photoanodes with Fluorinated Self-Assembled Monolayers for Highly Efficient Dye-Sensitized Solar Cells.

Dye aggregation and electron recombination in TiO2 photoanodes are the two major phenomena lowering the energy conversion efficiency of dye-sensitized solar cells (DSCs). Herein, we introduce a novel surface modification strategy of TiO2 photoanodes by the fluorinated self-assembled monolayer (F-SAM) formation with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS), blocking the vacant sites of the TiO2 surface after dye adsorption. The F-SAM helps to efficiently lower the surface tension, resulting in efficient repelling ions, e.g., I3(-), in the electrolyte to decrease the electron recombination rate, and the role of F-SAM is characterized in detail by impedance spectroscopy using a diffusion-recombination model. In addition, the dye aggregates on the TiO2 surface are relaxed by the F-SAM with large conformational perturbation (i.e., helix structure) seemingly because of steric hindrance developed during the SAM formation. Such multifunctional effects suppress the electron recombination as well as the intermolecular interactions of dye aggregates without the loss of adsorbed dyes, enhancing both the photocurrent density (11.9 → 13.5 mA cm(-2)) and open-circuit voltage (0.67 → 0.72 V). Moreover, the combined surface modification with the F-SAM and the classical coadsorbent further improves the photovoltaic performance in DSCs.

Stable Hydrophobic Metal-Oxide Photocatalysts via Grafting Polydimethylsiloxane Brush

Polydimethylsiloxane (PDMS) can be grafted to metal-oxide photocatalysts such as titanium oxide by simple UV irradiation in solution or melt. The PDMS graft metal oxides are still photocatalytically active. They are hydrophobic, liquid repellent, self-cleaning, prevent biofouling and are long-term stable even in UV light.

Silicone Brushes: Omniphobic Surfaces with Low Sliding Angles

Losing contact: Omniphobic surfaces can be readily produced by acid-catalyzed graft polycondensation of dimethyldimethoxysilane (PDMS). Droplets show a very small contact angle hysteresis as well as a low sliding angle of only a few degrees. The nm-thick PDMS layer is neither easily washed away nor depleted. This method offers a novel approach towards the preparation of super-liquid-repelling surfaces.

A Photocatalytically Active Lubricant-Impregnated Surface

Lubricant impregnated surfaces (LISs) exhibit sliding angles below 5°. A LIS is presented that possesses photocatalytic activity as well as improved liquid repellency. In a single-step reaction, the surface of photocatalytic mesoporous TiO2 substrate is modified by grafting polydimethylsiloxane (PDMS) brush and the residual non-bound PDMS serves as lubricant. Since the lubricant and the hydrophobic layer are chemically identical, the grafting PDMS layer is stably swollen by the lubricant PDMS, which inhibits direct contact of liquid drops to the solid substrate. Liquid drops such as water, methanol, and even low-surface-tension fluorocarbons, slide on the surface with tilt angles below 1°. The surface exhibits long-term stable photocatalytic activity while retaining its liquid repellency. This photocatalytic activity allows photocatalytic chemistry, for example, decomposition of organics, on LIS to be carried out.

How drops start sliding over solid surfaces

It has been known for more than 200 years that the maximum static friction force between two solid surfaces is usually greater than the kinetic friction force—the force that is required to maintain the relative motion of the surfaces once the static force has been overcome. But the forces that impede the lateral motion of a drop of liquid on a solid surface are not as well characterized, and there is a lack of understanding about liquid–solid friction in general. Here, we report that the lateral adhesion force between a liquid drop and a solid can also be divided into a static and a kinetic regime. This striking analogy with solid–solid friction is a generic phenomenon that holds for liquids of different polarities and surface tensions on smooth, rough and structured surfaces. A liquid droplet is shown to slide across a solid surface subject to friction forces analogous with those between two solids. The phenomenon is generic, and closes a gap in our understanding of liquid–solid friction.

Spontaneous jumping, bouncing and trampolining of hydrogel drops on a heated plate

The contact between liquid drops and hot solid surfaces is of practical importance for industrial processes, such as thermal spraying and spray cooling. The contact and bouncing of solid spheres is also an important event encountered in ball milling, powder processing, and everyday activities, such as ball sports. Using high speed video microscopy, we demonstrate that hydrogel drops, initially at rest on a surface, spontaneously jump upon rapid heating and continue to bounce with increasing amplitudes. Jumping is governed by the surface wettability, surface temperature, hydrogel elasticity, and adhesion. A combination of low-adhesion impact behavior and fast water vapor formation supports continuous bouncing and trampolining. Our results illustrate how the interplay between solid and liquid characteristics of hydrogels results in intriguing dynamics, as reflected by spontaneous jumping, bouncing, trampolining, and extremely short contact times.Drops of liquid on a hot surface can exhibit fascinating behaviour such as the Leidenfrost effect in which drops hover on a vapour layer. Here Pham et al. show that when hydrogel drops are placed on a rapidly heated plate they bounce to increasing heights even if they were initially at rest.

Cylindrical chains of water drops condensing on microstructured lubricant-infused surfaces

We studied the condensation of water drops on a micro-structured lubricant-infused surfaces. Hierarchical micro-prism surfaces were fabricated by soft imprinting with wet TiO2 nanoparticle paste. After hydrophobization, the patterned surfaces were infused with silicone oil as a lubricant. When cooling at high humidity (over 80%), water drops nucleate and start growing on the surface. Once they have reached a certain size, the drops at neighboring channels of the micro-prisms attract each other and spontaneously form cylindrical chains. These chains of drops align perpendicular to the prism array. The morphology and the length-to-width ratio of the chains of drops depend on the thickness of the lubricant layer. This new concept of water drop alignment on lubricant-infused surfaces offers a new route for pattern formation with condensed drops.

Isolated Mesoporous Microstructures Prepared by Stress Localization-Induced Crack Manipulation

Cracks observed in brittle materials are mostly regarded as defects or failures. However, they can be a valuable tool when implemented in a controlled way. Here, we introduce a strategy to control the crack propagation of mesoporous micropatterns (prisms and pyramids), which leads to the isolation of well-defined microstructures. Mesoporous micropatterns were fabricated by the soft imprinting technique with wet TiO2 nanoparticle (NP) pastes, followed by sintering to remove organic components. Since the volume of the paste significantly shrinks during the sintering step, stress is localized at the edge of micropatterns, in good agreement with finite element method simulations, creating well-defined cracks and their propagation. It was demonstrated that the degree of stress localization is determined by the thickness of residual layers, NP size, and heating rate. After controlled crack propagation and delamination of microparticles from the substrates, mesoporous microwires and microparticles were successfully produced and functionalized from the isolated mesoporous prisms and pyramids. The method proposed in this study for controlled crack manipulation and delamination opens a door for straightforward and economical fabrication of well-defined mesoporous microparticles.