Orientation Control of Block Copolymer Thin Films Placed on Ordered Nanoparticle Monolayers

We investigate orientation and lateral ordering of poly­(styrene-<i>block</i>-methyl methacrylate) (PS-<i>b</i>-PMMA) diblock copolymer (diBCP) thin films placed on ordered nanoparticle (NP) monolayers. The densely packed NP monolayers were prepared on silicon substrates with the Langmuir–Blodgett (LB) deposition technique. The perpendicular domain orientation of BCP thin films is obtained on the ordered NP monolayers due to the nanoscale regular roughness which exerts the elastic deformation on the BCP nanodomains and suppresses the substrate-induced parallel orientation. The effect of BCP film thickness as well as the NP size on the orientation of BCP nanodomains was systematically investigated. We also demonstrate the defect-tolerant ordering of the perpendicular orientation of BCP thin films on the NP-vacant sites

Shaping the Assembly of Superparamagnetic Nanoparticles

Superparamagnetism exists only in nanocrystals, and to endow micro/macro-materials with superparamagnetism, superparamagnetic nanoparticles have to be assembled into complex materials. Most techniques currently used to produce such assemblies are inefficient in terms of time and material. Herein, we used evaporation-guided assembly to produce superparamagnetic supraparticles by drying ferrofluid droplets on a superamphiphobic substrate in the presence of an external magnetic field. By tuning the concentration of ferrofluid droplets and controlling the magnetic field, barrel-like, cone-like, and two-tower-like supraparticles were obtained. These assembled supraparticles preserved the superparamagnetism of the original nanoparticles. Moreover, other colloids can easily be integrated into the ferrofluid suspension to produce, by co-assembly, anisotropic binary supraparticles with additional functions. Additionally, the magnetic and anisotropic nature of the resulting supraparticles was harnessed to prepare magnetically actuable microswimmers

Controlling the Structure of Supraballs by pH-Responsive Particle Assembly

Supraballs of various sizes and compositions can be fabricated via drying of drops of aqueous colloidal dispersions on super-liquid-repellent surfaces with no chemical waste and energy consumption. A “supraball” is a particle composed of colloids. Many properties, such as mechanical strength and porosity, are determined by the ordering of a colloidal assembly. To tune such properties, a colloidal assembly needs to be controlled when supraballs are formed during drying. Here, we introduce a method to control a colloidal assembly of supraballs by adjusting the dispersity of the colloids. Supraballs are fabricated on superamphiphobic surfaces from colloidal aqueous dispersions of polystyrene microparticles carrying pH-responsive poly­[2-(diethylamino)­ethyl methacrylate]. Drying of dispersion drops at pH 3 on superamphiphobic surfaces leads to the formation of spherical supraballs with densely packed colloids. The pH 10 supraballs are more oblate and consist of more disordered colloids than the pH 3 supraballs, caused by particle aggregates with random sizes and shapes in the pH 10 dispersion. Thus, the shape, crystallinity, porosity, and mechanical properties could be controlled by pH, which allows broader uses of supraballs

Controlling the Structure of Supraballs by pH-Responsive Particle Assembly

Supraballs of various sizes and compositions can be fabricated via drying of drops of aqueous colloidal dispersions on super-liquid-repellent surfaces with no chemical waste and energy consumption. A “supraball” is a particle composed of colloids. Many properties, such as mechanical strength and porosity, are determined by the ordering of a colloidal assembly. To tune such properties, a colloidal assembly needs to be controlled when supraballs are formed during drying. Here, we introduce a method to control a colloidal assembly of supraballs by adjusting the dispersity of the colloids. Supraballs are fabricated on superamphiphobic surfaces from colloidal aqueous dispersions of polystyrene microparticles carrying pH-responsive poly­[2-(diethylamino)­ethyl methacrylate]. Drying of dispersion drops at pH 3 on superamphiphobic surfaces leads to the formation of spherical supraballs with densely packed colloids. The pH 10 supraballs are more oblate and consist of more disordered colloids than the pH 3 supraballs, caused by particle aggregates with random sizes and shapes in the pH 10 dispersion. Thus, the shape, crystallinity, porosity, and mechanical properties could be controlled by pH, which allows broader uses of supraballs

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

Effective Passivation of Nanostructured TiO₂ Interfaces with PEG-Based Oligomeric Coadsorbents To Improve the Performance of Dye-Sensitized Solar Cells

A novel poly­(ethylene glycol) (PEG) based oligomeric coadsorbent was employed to passivate TiO₂ photoanodes resulting in the large increase in both open-circuit voltage (<i>V</i>_oc) and short-circuit current density (<i>J</i>_sc) primarily because of the reduced electron recombination by the effective coverage of vacant sites as well as the negative band-edge shift of TiO₂. The effective suppression of electron recombination was evidenced by electrochemical impedance spectroscopy (EIS) and by stepped light-induced transient measurements of photocurrent and voltage (SLIM-PCV). The work function measurements also showed that the existence of coadsorbents on TiO₂ interfaces is capable of shifting the band-edge of TiO₂ photoanodes upwardly resulting in the increase in photovoltage. In addition, the coadsorbent was proven to be effective even in the presence of common additives such as LiI, 4-<i>tert</i>-butylpyridine, and guanidinium thiocyanate. The effect of Li⁺ cation trapping by ethylene oxide units of the coadsorbent was particularly notable to significantly increase <i>V</i>_oc at a small expense of <i>J</i>_sc. Consequently, the introduction of novel PEG-based oligomeric coadsorbents for TiO₂ photoanodes is quite effective in the improvement of photovoltaic performance because of the simultaneous increase in both <i>V</i>_oc and <i>J</i>_sc

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 1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-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