Jin Zhai

Two-dimensional graphene bridges enhanced photoinduced charge transport in dye-sensitized solar cells.

As a novel two-dimensional (2D) material, graphene shows great benefits in electric and material science. Compared to 1D nanomaterials, it may show more excellent properties. Here, we introduced graphene as 2D bridges into the nanocrystalline electrodes of dye-sensitized solar cells, which brought a faster electron transport and a lower recombination, together with a higher light scattering. On the basis of these advantages, the short-circuit current density was increased by 45% without sacrificing the open-circuit voltage, and the total conversion efficiency was 6.97%, which was increased by 39%, comparing with the nanocrystalline titanium dioxide photoanode, and it was also much better than the 1D nanomaterial composite electrode.

Directional water collection on wetted spider silk.

Many biological surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometre-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fibre structure that forms after wetting, with the ‘wet-rebuilt’ fibres characterized by periodic spindle-knots made of random nanofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liquid drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibres that mimic the structural features of silk and exhibit its directional water-collecting ability.

Hierarchically Ordered Macro−Mesoporous TiO2−Graphene Composite Films: Improved Mass Transfer, Reduced Charge Recombination, and Their Enhanced Photocatalytic Activities

Hierarchically ordered macro-mesoporous titania films have been produced through a confinement self-assembly method within the regular voids of a colloidal crystal with three-dimensional periodicity. Furthermore, graphene as an excellent electron-accepting and electron-transporting material has been incorporated into the hierarchically ordered macro-mesoporous titania frameworks by in situ reduction of graphene oxide added in the self-assembly system. Incorporation of interconnected macropores in mesoporous films improves the mass transport through the film, reduces the length of the mesopore channel, and increases the accessible surface area of the thin film, whereas the introduction of graphene effectively suppresses the charge recombination. Therefore, the significant enhancement of photocatalytic activity for degrading the methyl blue has been achieved. The apparent rate constants for macro-mesoporous titania films without and with graphene are up to 0.045 and 0.071 min(-1), respectively, almost 11 and 17 times higher than that for pure mesoporous titania films (0.0041 min(-1)).

Bioinspired super-antiwetting interfaces with special liquid-solid adhesion.

Super-antiwetting interfaces, such as superhydrophobic and superamphiphobic surfaces in air and superoleophobic interfaces in water, with special liquid-solid adhesion have recently attracted worldwide attention. Through tuning surface microstructures and compositions to achieve certain solid/liquid contact modes, we can effectively control the liquid-solid adhesion in a super-antiwetting state. In this Account, we review our recent progress in the design and fabrication of these bioinspired super-antiwetting interfaces with special liquid-solid adhesion. Low-adhesion superhydrophobic surfaces are biologically inspired, typically by the lotus leaf. Wettability investigated at micro- and nanoscale reveals that the low adhesion of the lotus surface originates from the composite contact mode, a microdroplet bridging several contacts, within the hierarchical structures. Recently high-adhesion superhydrophobic surfaces have also attracted research attention. These surfaces are inspired by the surfaces of gecko feet and rose petals. Accordingly, we propose two biomimetic approaches for the fabrication of high-adhesion superhydrophobic surfaces. First, to mimic a sticky geckos foot, we designed structures with nanoscale pores that could trap air isolated from the atmosphere. In this case, the negative pressure induced by the volume change of sealed air as the droplet is pulled away from surface can produce a normal adhesive force. Second, we constructed microstructures with size and topography similar to that of a rose petal. The resulting materials hold air gaps in their nanoscale folds, controlling the superhydrophobicity in a Wenzel state on the microscale. Furthermore, we can tune the liquid-solid adhesion on the same superhydrophobic surface by dynamically controlling the orientations of microstructures without altering the surface composition. The superhydrophobic wings of the butterfly (Morpho aega) show directional adhesion: a droplet easily rolls off the surface of wings along one direction but is pinned tightly against rolling in the opposite direction. Through coordinating the stimuli-responsive materials and appropriate surface-geometry structures, we developed materials with reversible transitions between a low-adhesive rolling state and a high-adhesive pinning state for water droplets on the superhydrophobic surfaces, which were controlled by temperature and magnetic and electric fields. In addition to the experiments done in air, we also demonstrated bioinspired superoleophobic water/solid interfaces with special adhesion to underwater oil droplets and platelets. In these experiments, the high content of water trapped in the micro- and nanostructures played a key role in reducing the adhesion of the oil droplets and platelets. These findings will offer innovative insights into the design of novel antibioadhesion materials.

Accurate Control of Multishelled ZnO Hollow Microspheres for Dye-Sensitized Solar Cells with High Efficiency

A series of multishelled ZnO hollow microspheres with controlled shell number and inter-shell spacing have been successfully prepared by a simple carbonaceous microsphere templating method, whose large surface area and complex multishelled hollow structure enable them load sufficient dyes and multi-reflect the light for enhancing light harvesting and realize a high conversion efficiency of up to 5.6% when used in dye-sensitized solar cells.

Photo-induced water–oil separation based on switchable superhydrophobicity–superhydrophilicity and underwater superoleophobicity of the aligned ZnO nanorod array-coated mesh films

Stimulus-responsive surface wettability, especially photoresponsive surface wettability, has been intensively studied. Meanwhile, multifunctional surfaces, especially for the treatment of oil contaminated water, have aroused worldwide attention. Recently, pH-responsive surfaces with controllable oil–water separation have also been reported. However, photoresponsive water–oil separation is still a challenge. Here we report photo-induced water–oil separation based on the switchable superhydrophobicity–superhydrophilicity and underwater superoleophobicity of aligned ZnO nanorod array-coated mesh films, which shows excellent controllability and high separation efficiency of different types of water–oil mixtures in an oil–water–solid three-phase system. The underwater superoleophobicity of the aligned ZnO nanorod array-coated stainless steel mesh film can effectively prevent the mesh film from being polluted by oils. This work is promising in photo-induced water–oil mixture treatments such as water-removal from a micro-reaction system and controllable filtration, and may also provide interesting insight into the design of novel functional devices based on controllable surface wettability.

Wetting and anti-wetting on aligned carbon nanotube films

This review covers recent advances in the wettability of aligned carbon nanotubes (ACNT). Carbon nanotubes (CNT) are inherently somewhat hydrophilic, with a water contact angle of less than 86°. When they are arranged in a textured manner on substrates having different surface topographies, different wettabilities are exhibited. These range from hydrophilic to hydrophobic, and even superhydrophobic, and with isotropic to anisotropic contact angle (CA) hysteresis. If chemical modification is involved, the wettability can be adjusted from superhydrophobic to superhydrophilic on a certain structured ACNTs. Here, we first examine the structural influence of isotropic roughness on this effect (including nano-structures and hierarchical structures), where isotropic wetting (including wetting and anti-wetting) is observed. Water can wet the nano-structured CNT alignment, resulting in self-assembly; on the other hand, superhydrophobicity is durable on hierarchical and chemically-modified nano-structured alignments. Secondly, the effects of anisotropic roughness on wetting behavior are discussed. Finally, we suggest the remaining challenges in the field, and several practical applications of ACNT possessing special wettability.

Bioinspired construction of Mg–Li alloys surfaces with stable superhydrophobicity and improved corrosion resistance

A facile method was utilized for the construction of the bioinspired superhydrophobic Mg–Li alloy surfaces with peonylike micronanoscale hierarchical structures. The resultant materials were characterized by scanning electron microscope, x-ray photoelectron spectroscopy, and water contact angle measurements. The obtained Mg–Li alloy surfaces presented the dramatically improved corrosion resistance and long-term stable superhydrophobic properties with a static water contact angle of about 160° and a small sliding angle of less than 5°, which may extend the practical application of Mg–Li alloys in industrial and high-technology fields.

High photostability and quantum yield of nanoporous TiO2 thin film electrodes co-sensitized with capped sulfides

Photoelectrochemical electrodes have been prepared by sequential deposition of quantum sized PbS, CdS and ZnS particles on TiO2 nanocrystalline films. Their photoelectrochemical properties have been studied in a two-electrode system which conforms more closely to practical conditions. The results show that the ternary sulfide PbS/CdS/ZnS co-sensitized TiO2 electrode generates incident photon-to-current conversion efficiency (IPCE) as high as nearly 100% under irradiation with 400 nm light; moreover its photostability is strongly improved, and this is the first report of this so far. The highest photoelectrical conversion efficiency is obtained for the TiO2/PbS/CdS/ZnS electrode, and is about twice as much as that of TiO2/PbS.

Underwater superoleophobic porous membrane based on hierarchical TiO2 nanotubes: multifunctional integration of oil–water separation, flow-through photocatalysis and self-cleaning

Functional porous membranes with special surface wettability have been applied widely for the treatment of water contamination. Herein, we demonstrated a novel underwater superoleophobic porous membrane with multifunctions such as oil–water separation, flow-through photocatalysis and self-cleaning. The porous membrane was fabricated by electrochemical formation of hierarchical TiO2 nanotubes on the surface of porous titanium, followed by calcination in air. Due to its superhydrophilicity and underwater superoleophobicity, the porous membrane achieved the separation of oily substances from water by allowing water to permeate through the membrane. The photocatalysis of hierarchical TiO2 nanotubes in the porous membrane was used to decompose toxic organic molecules during the permeation of polluted water through the membrane. In some cases, when the porous membrane was contaminated by organic molecules in the environment and lost their unique surface wettability, the UV-induced self-cleaning function of hierarchical TiO2 nanotubes recovered its original wettability. This multifunctional porous membrane demonstrated potential application against water contamination.