Hongta Yang

Preparation and physical properties of superhydrophobic papers.

In this study, we developed a facile method for preparing a superhydrophobic paper surface using a multi-layer deposition of polydiallyldimethylammonium chloride (polyDADMAC) and silica particles, followed by a fluorination surface treatment with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS, CF(3)(CF(2))(5)CH(2)CH(2)Si(OC(2)H(5))(3)). The superhydrophobic wood fiber products prepared in this study have contact angles of water greater than 150 degrees and sliding angles less than 5 degrees. Besides their high water repelling property, the superhydrophobic paper products kept a high tensile strength at high relative humidity condition. The superhydrophobic paper products also showed high resistance to bacterial contamination.

Large-Scale Colloidal Self-Assembly by Doctor Blade Coating

This article reports a simple, roll-to-roll compatible coating technology for producing 3D highly ordered colloidal crystal-polymer nanocomposites, colloidal crystals, and macroporous polymer membranes. A vertically beveled doctor blade is utilized to shear align silica microsphere-monomer suspensions to form large-area nanocomposites in a single step. The polymer matrix and the silica microspheres can be selectively removed to create colloidal crystals and self-standing macroporous polymer membranes. The thickness of the shear-aligned crystal is correlated with the viscosity of the colloidal suspension, and the coating speed and the correlations can be qualitatively explained by adapting the mechanisms developed for conventional doctor blade coating. We further demonstrate that the doctor blade coating speed can be significantly increased by using a dual-blade setup. The optical properties of the self-assembled structures are evaluated by normal-incidence reflection measurements, and the experimental results agree well with the theoretical predictions using Braggs law and a scalar wave approximation model. We have also demonstrated that the templated macroporous polymers with interconnected voids and uniform interconnecting nanopores can be directly used as filtration membranes to achieve size-exclusive separation of particles.

Vapor detection enabled by self-assembled colloidal photonic crystals

Here we report the sensitive and reversible detection of vapors by using self-assembled colloidal photonic crystals. The condensation of various vapors in the interstitials of silica colloidal photonic crystals leads to red-shift and amplitude reduction of optical stop bands. A linear relationship between wavelength shift and vapor partial pressure has been observed for a variety of vapors including ethanol, water, and toluene. Importantly, the sensitivity of colloidal photonic crystal-based vapor detectors can be improved by nearly two orders of magnitude by using a new full-peak analysis technique that takes advantage of the manifest amplitude reduction of optical stop bands during vapor condensation. Optical simulation based on a scalar-wave approximation model shows that the predicted optical responses during vapor condensation in colloidal photonic crystals agree well with experimental results. The condensation of vapors between submicrometer-scale microspheres, a topic that has received little examination, has also been investigated by both experiments and theoretical calculations. Predictions based on a modified Kelvin equation match with the experiments for a wide range of vapor partial pressures.

Scalable fabrication of superhydrophobic hierarchical colloidal arrays

Here we report a scalable bottom-up technology for assembling hierarchical colloidal arrays with superhydrophobic surface. Non-close-packed (NCP) colloidal multilayers, which facilitate the formation of more hydrophobic surface than close-packed arrays due to a higher fraction of entrapped air in between colloidal particles, are first fabricated by a simple spin-coating technology. Uniform silica nanoparticles are then assembled on the NCP microsphere arrays by a second spin-coating process. After surface functionalization of silica particles with fluorosilane, the resulting hierarchical colloidal arrays exhibit superhydrophobic surface with high apparent water contact angle (159°) and low contact angle hysteresis (4.7°). The experimental results on both the wettability and contact angle hysteresis can be qualitatively explained by adapting the Cassies model. This spin-coating-based colloidal self-assembly technology is compatible with standard microfabrication and enables large-scale production of superhydrophobic coatings that could find important technological applications ranging from self-cleaning diffractive optics to microfluidic devices.

Cicada-Wing-Inspired Self-Cleaning Antireflection Coatings on Polymer Substrates.

The cicada has transparent wings with remarkable self-cleaning properties and high transmittance over the whole visible spectral range, which is derived from periodic conical structures covering the wing surface. Here we report a scalable self-assembly technique for fabricating multifunctional optical coatings that mimic cicada-wing structures. Spin-coated two-dimensional non-close-packed colloidal crystals are utilized as etching masks to pattern subwavelength-structured cone arrays directly on polymer substrates. The resulting gratings exhibit broadband antireflection performance and superhydrophobic properties after surface modification. The dependence of the cone shape and size on the antireflective and self-cleaning properties has also been investigated in this study.

Self-assembled biomimetic superhydrophobic hierarchical arrays

Here, we report a simple and inexpensive bottom-up technology for fabricating superhydrophobic coatings with hierarchical micro-/nano-structures, which are inspired by the binary periodic structure found on the superhydrophobic compound eyes of some insects (e.g., mosquitoes and moths). Binary colloidal arrays consisting of exemplary large (4 and 30 μm) and small (300 nm) silica spheres are first assembled by a scalable Langmuir-Blodgett (LB) technology in a layer-by-layer manner. After surface modification with fluorosilanes, the self-assembled hierarchical particle arrays become superhydrophobic with an apparent water contact angle (CA) larger than 150°. The throughput of the resulting superhydrophobic coatings with hierarchical structures can be significantly improved by templating the binary periodic structures of the LB-assembled colloidal arrays into UV-curable fluoropolymers by a soft lithography approach. Superhydrophobic perfluoroether acrylate hierarchical arrays with large CAs and small CA hysteresis can be faithfully replicated onto various substrates. Both experiments and theoretical calculations based on the Cassies dewetting model demonstrate the importance of the hierarchical structure in achieving the final superhydrophobic surface states.

Macroporous photonic crystal-based vapor detectors created by doctor blade coating

We report the achievement of rapid and reversible vapor detection by using 3D macroporous photonic crystals created by a continuous and scalable bottom-up technology. Capillary condensation of a condensable vapor in the interconnected macropores with ∼74% porosity leads to the increase of the effective refractive index of the diffractive medium, resulting in the redshift of the optical stop bands. The wavelength shift is linearly proportional to the vapor partial pressure for a spectrum of vapors. Optical simulation and theoretical prediction based on Kelvin equation suggest that a liquid film is formed on the walls of the macropores during vapor condensation.

Enhanced removal of diclofenac from water using a zeolitic imidazole framework functionalized with cetyltrimethylammonium bromide (CTAB)

Diclofenac represents one of the most common pharmaceuticals and personal care products (PPCPs) in municipal wastewater. To enhance the removal of diclofenac from water, a cationic surfactant, cetyltrimethylammonium bromide (CTAB), was introduced into a zeolitic imidazole framework (ZIF)-67. CTAB is added originally to minimize the use of ligands during the synthesis of ZIF-67; however CTAB present in ZIF-67 also increases the surface charge and thus enhances the adsorption capacity up to 10 times compared to CTAB-free ZIF-67. Factors influencing the diclofenac adsorption are investigated, including CTAB loading, temperature and pH. A loading of 0.274 mol-CTAB/mol-Co2+ is suggested as an optimal loading to prepare a relatively efficient CTAB-ZIF-67, while over loading of CTAB could not fully incorporate CTAB into ZIF-67. In view of the diclofenac adsorption kinetics and isotherm, the adsorption of diclofenac to CTAB-ZIF-67 was considered to involve a strong affinity between diclofenac and CTAB-ZIF-67 owing to the electrostatic attraction between the carboxylic acid of diclofenac and the quaternary amine of CTAB. Considering that the removal of diclofenac from urine is of great interest, separation of diclofenac from urine using CTAB-ZIF-67 was evaluated and also employed to investigate the effect of co-existing ions. We also determined the desorption behavior of diclofenac from CTAB-ZIF-67 to provide insight for the recyclability of CTAB-ZIF-67 for diclofenac adsorption.

Generalized Fabrication of Monolayer Nonclose-Packed Colloidal Crystals with Tunable Lattice Spacing

Here, we report a simple colloidal transfer technology that enables scalable fabrication of monolayer nonclose-packed silica colloidal crystals on a large variety of substrates. Two-dimensional colloidal crystals with an unusual nonclose-packed structure are first assembled on silicon wafers by a spin-coating technique. A poly(vinyl alcohol) (PVA) film cast upon the spin-coated colloidal crystal is used to transfer the nonclose-packed particle arrays onto various substrates. The lattice spacing of the transferred monolayer colloidal crystal can easily be adjusted by thermally treating the PVA-silica spheres composite film for varied durations. We also have demonstrated the templating fabrication of periodic arrays of gold nanodots using a transferred monolayer nonclose-packed colloidal crystal as a structural template. The resultant plasmonic array exhibits high surface-enhanced Raman scattering enhancement factor (~3.8 × 10(7)) for adsorbed benzenethiol molecules.

Reusable macroporous photonic crystal-based ethanol vapor detectors by doctor blade coating.

This research reports the development of sensitive and reversible vapor detection by using three-dimensional macroporous photonic crystals. A scalable and roll-to-roll compatible doctor blade coating technology is utilized to fabricate flexible macroporous poly(ethoxylated trimethylolpropane triacrylate) (PETPTA) films with hexagonal close-packed pores which are interconnected. The pores are then coated with a layer of poly(2-hydroxyethyl methacrylate) (PHEMA) to create macroporous PHEMA/PETPTA films. The condensation of vapors in the PHEMA coated macroporous films leads to the increase of both the PHEMA swelling degree and the effective refractive index of the diffractive medium, resulting in the red-shift and amplitude reduction of the optical stop bands. The optical measurements reveal that the diffraction from the as-prepared macroporous photonic crystals sensitively monitors the vapor pressure of ethanol since the PHEMA layer displays a great volume dependence on ethanol due to a decreased Flory-Huggins mixing parameter. The dependence of the diffraction wavelength on vapor pressure and the reproducibility of vapor sensing have also been investigated in this study.