Hong-Qing Liang

Polydopamine-Coated Porous Substrates as a Platform for Mineralized β-FeOOH Nanorods with Photocatalysis under Sunlight

Immobilization of photo-Fenton catalysts on porous materials is crucial to the efficiency and stability for water purification. Here we report polydopamine (PDA)-coated porous substrates as a platform for in situ mineralizing β-FeOOH nanorods with enhanced photocatalytic performance under sunlight. The PDA coating plays multiple roles as an adhesive interface, a medium inducing mineral generation, and an electron transfer layer. The mineralized β-FeOOH nanorods perfectly wrap various porous substrates and are stable on the substrates that have a PDA coating. The immobilized β-FeOOH nanorods have been shown to be efficient for degrading dyes in water via a photo-Fenton reaction. The degradation efficiency reaches approximately 100% in 60 min when the reaction was carried out with H2O2 under visible light, and it remains higher than 90% after five cycles. We demonstrate that the PDA coating promotes electron transfer to reduce the electron-hole recombination rate. As a result, the β-FeOOH nanorods wrapped on the PDA-coated substrates show enhanced photocatalytic performance under direct sunlight in the presence of H2O2. Moreover, this versatile platform using porous materials as the substrate is useful in fabricating β-FeOOH nanorods-based membrane reactor for wastewater treatment.

Highly Stable, Protein-Resistant Surfaces via the Layer-by-Layer Assembly of Poly(sulfobetaine methacrylate) and Tannic Acid

Zwitterionic materials have received great attention because of the non-fouling property. As a result of the electric neutrality of zwitterionic polymers, their layer-by-layer (LBL) assembly is generally conducted under specific conditions, such as very low pH values or ionic strength. The formed multilayers are unstable at high pH or in a high ionic strength environment. Therefore, the formation of highly stable multilayers of zwitterionic polymers via the LBL assembly process is still challenging. Here, we report the LBL assembly of poly(sulfobetaine methacrylate) (PSBMA) with a polyphenol, tannic acid (TA), for protein-resistant surfaces. The assembly process was monitored by a quartz crystal microbalance (QCM) and variable-angle spectroscopic ellipsometry (VASE), which confirms the formation of thin multilayer films. We found that the (TA/PSBMA)n multilayers are stable over a wide pH range of 4-10 and in saline, such as 1 M NaCl or urea solution. The surface morphology and chemical composition were characterized by specular reflectance Fourier transform infrared spectroscopy (FTIR/SR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Furthermore, (TA/PSBMA)n multilayers show high hydrophilicity, with a water contact angle lower than 15°. A QCM was used to record the dynamic protein adsorption process. Adsorption amounts of bovine serum albumin (BSA), lysozyme (Lys), and hemoglobin (Hgb) on (TA/PSBMA)20 multilayers decreased to 0.42, 52.9, and 37.9 ng/cm(2) from 328, 357, and 509 ng/cm(2) on a bare gold chip surface, respectively. In addition, the protein-resistance property depends upon the outmost layer. This work provides new insights into the LBL assembly of zwitterionic polymers.

Hierarchically porous carbon membranes derived from PAN and their selective adsorption of organic dyes

Porous carbon membranes were favorably fabricated through the pyrolysis of polyacrylonitrile (PAN) precursors, which were prepared with a template-free technique-thermally induced phase separation. These carbon membranes possess hierarchical pores, including cellular macropores across the whole membranes and much small pores in the matrix as well as on the pore walls. Nitrogen adsorption indicates micropores (1.47 and 1.84 nm) and mesopores (2.21 nm) exist inside the carbon membranes, resulting in their specific surface area as large as 1062 m2/g. The carbon membranes were used to adsorb organic dyes (methyl orange, Congo red, and rhodamine B) from aqueous solutions based on their advantages of hierarchical pore structures and large specific surface area. It is particularly noteworthy that the membranes present a selective adsorption towards methyl orange, whose molecular size (1.2 nm) is smaller than those of Congo red (2.3 nm) and rhodamine B (1.8 nm). This attractive result can be attributed to the steric structure matching between the molecular size and the pore size, rather than electrostatic attraction. Furthermore, the used carbon membranes can be easily regenerated by hydrochloric acid, and their recovery adsorption ratio maintains above 90% even in the third cycle. This work may provide a new route for carbon-based adsorbents with hierarchical pores via a template-free approach, which could be promisingly applied to selectively remove dye contaminants in aqueous effluents.

Underwater superoleophobic meshes fabricated by poly(sulfobetaine)/polydopamine co-deposition

Porous meshes with superhydrophilicity and underwater superoleophobicity have attracted much attention for oil/water separation. In this work, poly(sulfobetaine methacrylate) (PSBMA), was co-deposited with polydopamine (PDA) to cover steel meshes and to endow them with characteristics for oil/water separation. Compared with PDA-modified meshes, the oil contact angle increases to 158.6 ± 8.0° and the sliding angle decreases to 3.9° for the PSBMA/PDA-modified ones, indicating superoleophobic behaviour underwater and ultra-low adhesive properties towards oil. These results are mainly attributed to the superhydrophilicity of PSBMA and the highly rough surface morphology in micro-nanoscale introduced during the co-deposition process. The PSBMA/PDA-modified meshes show excellent performance in gravity-driven oil/water separation. They are stable towards organic solvent treatment, and retain the wettability in sea water for a long time. This one-step PSBMA/PDA co-deposition method provides a convenient and effective approach to modify porous materials with underwater superoleophobicity for oil/water separation.

Polymer Membranes with Vertically Oriented Pores Constructed by 2D Freezing at Ambient Temperature.

Polymer membranes with well-controlled and vertically oriented pores are of great importance in the applications for water treatment and tissue engineering. On the basis of two-dimensional solvent freezing, we report environmentally friendly facile fabrication of such membranes from a broad spectrum of polymer resources including poly(vinylidene fluoride), poly(l-lactic acid), polyacrylonitrile, polystyrene, polysulfone and polypropylene. Dimethyl sulfone, diphenyl sulfone, and arachidic acid are selected as green solvents crystallized in the polymer matrices under two-dimensional temperature gradients induced by water at ambient temperature. Parallel Monte Carlo simulations of the lattice polymers demonstrate that the directional process is feasible for each polymer holding suitable interaction with a corresponding solvent. As a typical example of this approach, poly(vinylidene fluoride) membranes exhibit excellent tensile strength, high optical transparence, and outstanding separation performance for the mixtures of yeasts and lactobacilli.

Polymer fibers with hierarchically porous structure: combination of high temperature electrospinning and thermally induced phase separation

Isotactic polypropylene (iPP) fibers with hierarchically porous structure were successfully prepared by electrospinning at 200 °C combined with thermally induced phase separation (TIPS). Dioctyl phthalate (DOP) and dibutyl phthalate (DBP) are typical diluents for iPP in TIPS and are used as solvents for electrospinning. An ionic liquid was added to increase the solution conductivity, facilitate the electrospinning process, and maintain a stable cone-jet electrospinning mode. Theoretical calculation demonstrates that the jet cools rapidly, and phase separation takes place in the jet during its travelling path, as the system traverses across the phase diagram from the single phase region to the metastable region. For the iPP/DOP system, the surface morphology of fibers changes from aligned microvoids bridged by fibrils to a wrinkled structure with the addition of ionic liquid, as the ionic liquid inhibits iPP crystallization. The pore morphology can also be modulated by varying co-diluent composition. Open pores appear on the fiber surface and the cross-section varies from closed cellular pores to a bi-continuous structure with the increase of DBP content in the co-diluent, which clearly demonstrates the phase separation mechanism changes from solid–liquid to liquid–liquid phase separation. The as-spun porous fibers show more than a 100-fold increase in specific surface area compared with the non-porous ones. The main advantages of this method are the pore formation process has a precise mechanism and the pore morphology is well correlated with the phase diagram. Furthermore, it is readily extended to other polymers with TIPS. Highly porous poly(vinylidene fluoride) (PVDF) fibers can be easily prepared from PVDF/DBP solution with a 157-fold increase in specific surface area.

Poly(vinylidene fluoride) separators with dual-asymmetric structure for high-performance lithium ion batteries

Dual-asymmetric poly(vinylidene fluoride) (PVDF) separators have been fabricated by thermally induced phase separation with dimethyl sulfone (DMSO2) and glycerol as mixed diluents. The separators have a porous bulk with large interconnected pores (~1.0 μm) and two surfaces with small pores (~30 nm). This dual-asymmetric porous structure endows the separators with higher electrolyte uptake amount and rapider uptake rate, as well as better electrolyte retention ability than the commercialized Celgard 2400. The separators even maintain their dimensional stability up to 160 °C, at which temperature the surface pores close up, leading to a dramatic decrease of air permeability. The electrolyte filled separators also show high ion conductivity (1.72 mS∙cm―1) at room temperature. Lithium iron phosphate (LiFePO4)/lithium (Li) cells using these separators display superior discharge capacity and better rate performance as compared with those from the commercialized ones. The results provide new insight into the design and development of separators for high-performance lithium ion batteries with enhanced safety.

Polysulfone membranes via thermally induced phase separation

Polysulfone (PSF) membranes have gained great attention in the fields of ultrafiltration, microfiltration, and thin film composite membranes for nanofiltration or reverse osmosis. For the first time, it is proposed to fabricate PSF membranes via thermally induced phase separation (TIPS) process using diphenyl sulfone (DPSO2) and polyethylene glycol (PEG) as mixed diluent. DPSO2 is chosen as a crystallizable diluent, while PEG is considered in terms of molecular weight (Mw) and dosage. We systematically investigate the interactions between PSF, DPSO2 and PEG based on the simulation calculations and solubility parameter theory. It is inferred that DPSO2 has an excellent compatibility with PSF, and the addition of PEG results in the ternary system thermodynamically less stable and then facilitates its liquid-liquid (L-L) phase separation. SEM images indicate that cellular-like pores are obvious throughout the membrane when the PEG content in the mixed diluent is 25 wt%−35 wt%. We can facilely manipulate the pore size, water flux and mechanical properties of PSF membranes with the dosage of PEG-200, the Mw of PEG or the cooling rate. The successful application of TIPS can provide a new approach for structure manipulation and performance enhancement of PSF membranes.