Xue-Mei Li

What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces

Superhydrophobic surfaces have drawn a lot of interest both in academia and in industry because of the self-cleaning properties. This critical review focuses on the recent progress (within the last three years) in the preparation, theoretical modeling, and applications of superhydrophobic surfaces. The preparation approaches are reviewed according to categorized approaches such as bottom-up, top-down, and combination approaches. The advantages and limitations of each strategy are summarized and compared. Progress in theoretical modeling of surface design and wettability behavior focuses on the transition state of superhydrophobic surfaces and the role of the roughness factor. Finally, the problems/obstacles related to applicability of superhydrophobic surfaces in real life are addressed. This review should be of interest to students and scientists interested specifically in superhydrophobic surfaces but also to scientists and industries focused in material chemistry in general.

Reactive self-assembled monolayers on flat and nanoparticle surfaces, and their application in soft and scanning probe lithographic nanofabrication technologies

Self-assembly is an interesting process both for its biological relevance and because it provides a novel approach to complex structures having nanometer-scale dimensions. These structures are difficult or impossible to prepare by traditional methods. In this article, a general review on the use of self-assembled monolayers for chemical patterning is provided. In the first part, functional group transformation of SAMs on flat gold surfaces by chemical reactions is discussed in detail. The use of monolayer-protected gold nanoparticles as model systems for flat gold surfaces is covered with special focus on the transformation of functional groups on the outer SAM layer. Furthermore, techniques that have been used to pattern SAMs are discussed. Among these techniques, microcontact printing and scanning probe lithography are discussed in detail for both their advantages and limitations.

Nanosized Mn–Ru binary oxides as effective bifunctional cathode electrocatalysts for rechargeable Li–O2 batteries

Mn–Ru binary oxides have been synthesized via a hydrothermal approach and their performance as bifunctional catalysts for the air electrode is evaluated in non-aqueous Li–O2 secondary batteries. Characterization of the catalysts by X-ray diffractometry and transmission electron microscopy confirms that the as-prepared oxides contain γ-MnO2 and hydrous RuO2 with the morphology of fusiform nanorods and nanoparticles, respectively. Linear scanning voltammetric measurements reveal that the binary oxides exhibit remarkable electrocatalytic activities towards both the oxygen reduction and oxygen evolution reactions. Li–O2 batteries with Mn–Ru oxides as cathode catalysts show a higher discharge voltage plateau and a higher specific capacity of 6500 mA h gcarbon−1 than those with Ketjen black (KB) alone, and the subsequent charge voltage is ca. 500 mV lower than that with KB. Moreover, much enhanced cyclability of Li–O2 batteries is achieved with a capacity retention ratio of 73.2% after 5 cycles. The cyclability is maintained for 50 cycles without sharp decay under a limited discharge depth of 1100 mA h gcarbon−1, suggesting that such a bifunctional electrocatalyst is a promising candidate for the air electrode in Li–air batteries.

Formation of gold colloids using thioether derivatives as stabilizing ligands

Thioethers were used as adsorbates for preparing gold nanoparticles. Different thioether derivatives having from 1 to 4 thioether functionalities were synthesized. Colloids were prepared in a two-phase system, and characterized by 1H NMR and transmission electron microscopy (TEM). The stability of colloids protected by thioethers increases with the number of ligands per molecule. Monothioethers need longer chain lengths or costabilization by (oct)4NBr in order to give stable, redispersible gold colloids. Gold colloids stabilized by the bis(thioether) 5 could not be redispersed after precipitation. Colloids stabilized by the tris(thioether) 6 were only formed at elevated temperature (60°C) indicating the need of chain reorientation for attaining stable colloids. Tris(thioether) 7 gave stable colloids at room temperature, which could be redispersed even after precipitation. Tetrakis(thioether) 8 gave the smallest particle size and narrowest size distribution

Kinetics of (3-aminopropyl)triethoxylsilane (APTES) silanization of superparamagnetic iron oxide nanoparticles.

Silanization of magnetic ironoxide nanoparticles with (3-aminopropyl)triethoxylsilane (APTES) is reported. The kinetics of silanization toward saturation was investigated using different solvents including water, water/ethanol (1/1), and toluene/methanol (1/1) at different reaction temperature with different APTES loading. The nanoparticles were characterized by Fourier transform infrared spectroscopy, vibrating sample magnetometry, transmission electron microscopy, and thermal gravimetric analysis (TGA). Grafting density data based on TGA were used for the kinetic modeling. It is shown that initial silanization takes place very fast but the progress toward saturation is very slow, and the mechanism may involve adsorption, chemical sorption, and chemical diffusion processes. The highest equilibrium grafting density of 301 mg/g was yielded when using toluene/methanol mixture as the solvent at a reaction temperature of 70 °C.

Water reclamation from shale gas drilling flow-back fluid using a novel forward osmosis–vacuum membrane distillation hybrid system

This study examined the performance of a novel hybrid system of forward osmosis (FO) combined with vacuum membrane distillation (VMD) for reclaiming water from shale gas drilling flow-back fluid (SGDF). In the hybrid FO-VMD system, water permeated through the FO membrane into a draw solution reservoir, and the VMD process was used for draw solute recovery and clean water production. Using a SGDF sample obtained from a drilling site in China, the hybrid system could achieve almost 90% water recovery. Quality of the reclaimed water was comparable to that of bottled water. In the hybrid FO-VMD system, FO functions as a pre-treatment step to remove most contaminants and constituents that may foul or scale the membrane distillation (MD) membrane, whereas MD produces high quality water. It is envisioned that the FO-VMD system can recover high quality water not only from SGDF but also other wastewaters with high salinity and complex compositions.

Formation of superhydrophobic polymerized n-octadecylsiloxane nanosheets.

Superhydrophobic nanosheet assemblies of polymerized n-octadecylsiloxane (PODS) were obtained on glass using n-octadecyltricholorsilane (OTS) as a starting material and acetone as the extracting agent. The coating layers were characterized by infrared spectroscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, and contact angle measurements. It was shown that the PODS nanosheets consist of stacks of bilayered polysilsequioxane of OTS in a head-to-head arrangement. The relative humidity of the environment was found to control the self-assembly and eventually the morphology of the final coating. A coating-formation mechanism was proposed to describe the formation of OTS nanosheets on the basis of quick OTS hydrolysis, fast self-assembly, and slow polycondensation (i.e., after OTS hydrolysis, nanosheets can be formed only when the self-assembly process proceeds at a faster rate than polycondensation, which implies critical controlling factors in the formation of well-ordered OTS self-assembled monolayers (SAMs)).

Composite hollow fiber nanofiltration membranes for recovery of glyphosate from saline wastewater

A high performance versatile composite hollow fiber nanofiltration (NF) membrane is reported for the separation of glyphosate from saline waste streams. Preparation of SPEEK based on an amorphous poly (ether ether ketone, PEEK) was investigated. The membrane was prepared by coating sulfonated polyether ether ketone (SPEEK) onto a polyethersulfone (PES) ultrafiltration (UF) hollow fiber membrane. The composite membrane was characterized by water permeability, scanning electron microscopy, and rejection toward sodium sulfate (Na₂SO₄), sodium chloride (NaCl), and calcium chloride (CaCl₂). About 90% rejection toward sulfate anions and only 10% rejection for calcium cations were obtained. A water permeability around 10-13 LMHBar and 90% rejection for polyethylene glycol (PEG) with a molecular weight of 4000-6000 Da were observed. In the separation of glyphosate from saline wastewater, the membrane rejected less than 20% of NaCl and higher than 90% of glyphosate at an operating pressure of 5 bars and pH = 11.0. An economic analysis indicated that the cost for recovery of glyphosate was comparably low to the value gained by an increase in the productivity. The results may lead to a new promising low energy solution for the environmental problem faced by the herbicide industry.

Conversion of a Metastable Superhydrophobic Surface to an Ultraphobic Surface

Superhydrophobic surfaces in Wenzel and metastable wetting state were prepared and the conversion of such surfaces to ultraphobic surfaces was reported by the application of a fine-scale roughness. Silicon nitride substrates with hexagonally arranged pillars were prepared by micromachining. The two-scale roughness was achieved by coating these substrates with 60 nm silica nanoparticles. The surface was made hydrophobic by silanization with octadecytrichlorosilane (OTS). Wettability studies of the silicon nitride flat surface, silicon nitride pillars, and the surfaces with two-scale roughness showed that a two-scale roughness can effectively improve the hydrophobicity of surfaces with a higher apparent contact angle and reduced contact angle hysteresis when the original rough surface was in a metastable or Wenzel state. This study shows the pathway of converting a metastable hydrophobic surface to an ultraphobic surface by the introduction of a fine-scale roughness, which adds to the literature a new aspect of fine-scale roughness effect.

Cellulose triacetate forward osmosis membranes: preparation and characterization

Abstract Cellulose triacetate (CTA) forward osmosis membranes were prepared by immersion precipitation. Following a standard recipe for the preparation of reverse osmosis membranes, the concentrations of polymer, additives (lactic acid, methanol), and solvent composition in the casting solution were investigated with respect to membrane permeation and salt rejection. The membrane preparation parameters including the evaporation time and the coagulation bath temperature were investigated as well. A CTA membrane with a quasi double-skinned morphology was obtained. The CTA FO membrane showed a water flux higher than 10 kg/m2 h using 2 M glucose water solution as the draw solution and 0. 5 M NaCl as feed. A NaCl rejection of 95% was obtained.