1D Multi-Structured Micro/nanomaterials with Special Wettability

Yong Zhao*

* Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, P. R. China

zhaoyong@buaa.edu.cn

ABSTRACT Electrospinning is a straigthforward and versatile way to

fabricate mulilevel structured ultrafine fibers down to micro-/nanometer scale. In this paper, recent achievements of electrospun fibers with multilevel surface and inner structuers made by our group are presented. These electrospun multilevel structured materials provide wide applications in catalysis, filtration and adsroption, et al.

Keywords: electrospinning, micro-/nano fibers, multilevel structure, application

1 INTRODUCTION Nature creatures have been endowed a tremendous

number of excellent biological materials with fascinating structures, which exhibit numerous optimized functions. With deeper and deeper understanding of materials in nature, it has been known that most special properties of substances come from their chemical elemental composition as well as multilevel micro-/nanostructures. In recent years, considerable researches have concentrated on hierarchically complex structured materials especially on micro-/nanometer scale to seek benefits of these unique structure related properties.1-6, 7-9

Among micro-/nanomaterials’ family, one-dimensional (1D) materials like wires, tubes and fibers, are one of the most important and representative categories that have found broad applications in electronics, optics, sensors, catalysis, solar cells and many other cutting-edge domains.10-12 The simplest 1D nanostructure is a cylinder object with nanoscale diameter. Most of solid nanowires or nanofibers approximate to such cylinder shape because it is usually the lowest energy status for a cylinder object. A good number of elegant approaches have been developed in order to acquire all kinds of 1D micro-/nanomaterials from shorter wires to longer fibers. Besides scale reduction, another notable achievement on the development of microscopic 1D material is that scientists found multilevel structures also bring many fascinating variation on properties and functions. The multilevel structures are classified into two categories including surface structures and inner structures. Surface multilevel structures mainly refer to the down sized secondary structures on the primary cylinder 1D materials such as pine-tree like branch structure, porous structure, necklace structure and some other deformed structures. Inner multilevel structures, which are variants of tubular structure, mainly include

peapod structure, wire-in-tube structure, mutliwalled tube structure and multichannel structure. These multilevel structured 1D nanomaterials are attractive because they afford larger specific area or additional heterogeneous interfaces, which play important roles in enhancing nanosize effect. Therefore, scientists paid considerable attentions to prepare these interesting 1D nanomaterials.

In diverse approaches, chemical methods and physical deposition methods incline to produce shorter rods or wires, but they are often incapable of making longer fibers. In recent years, a versatile and promising electrospinning method is booming for generating micro-/nanofabric materials with well controllable compositions, structures properties and functionalities. Electrospinning is a simple technique that can stretches viscous liquids (often polymer solutions) into ultrafine fibers by using electrostatic force. So far as we know, this is the only universal method that could be applied to produce nanofibers from very broad materials from polymers to hybrids to inorganics. Moreover, the prominent advantage of electrospinning approach lies in that it has huge potential to tune the configuration of spinning setup as well as abundant post treatment methods for electrospun products. It thereby leaves vast room for researchers to tailor the structures and properties of electrospun fibers. During last decade, we focused our attentions on electrospinning multilevel structured materials. Here, we will exhibit some representative progress on electrospun multilevel structured functional micro-/nanofibers made by our group in recent years.

1. MULTILEVEL STRUCTURED MICRO/NANOFIBERS FABRICATED

BY ELECTROSPINNING 2.1 Surface multilevel structures

Electrospinning is a composite word composed by two words, i.e., electrostatic and spinning. Obviously, it is a technique that uses electrostatic force to spinning fibers. It is a very powerful method for creating diverse kinds of microstructures by controlling the concentration of electrospinning solution, applied voltage, electrospun distanse between the jet and the collector. For example, polystryrene could be esily electrospun from uniform smooth ultrafine fibers to the porous microsphere/nanofiber composite fibers with its unique structure, which can be realized by changing the electrospun conditions (Figure. 1(a), (b)).13 Addtionally, ultrathin polystryrene fibers with porous structures on their surfaces of which have been

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successfully fabricated (Figure. 1(c), (d)).14 Such nanoscaled pores caused by the solvent volatiling and distrubited on fibers’ surfaces, which can effective enlarge the specific surface areas than smooth ones. Knotted fibers can also be simply obtained by coaxial electrospinning (Figure. 1(e), (f)).15 Polystryrene acts as suporting fiber on which balsam-pear-like poly(methyl methacrylate) knots are distributed for Rayleigh instability effect.

Figure. 1 (a) SEM image of network structure of porous microsphere/nanofiber composite film. (b) surface nanostructure of a single porous microsphere. (c) SEM image of porous polystyrene fibers (d) Blue dotted square: amplification of fibrous surface. (e) SEM image of knotted fibers. PS fibers wrapped with PMMA knots can be observed. (f) orange dotted square: amplification of the fiber and knot. 2.2 Inner multilevel structures

Thanks to these two great progresses in the evolution of electrospinning: coaxial electrospinning16-19 and side-by-side electrospinning.16, 20 Not only multilevel structure surfaces can be fabricated, nanofibers with controllable inner multilevel structures can also be gained. Compared with other methods, it is attractive for a number of core-shell or an anisotropic material since it is a simple and efficient way. Notable examples include coaxial, microfluid manifold, triple-layer co-axial.

Based on the routine co-axial electrospinning, we improved multifluidic co-axial electrospinning that could fabricate core/shell ultrathin fibers with a special nanowire-in-microtube structure.21 As showing in Figure 2, TiO2

nanowire was embedded in TiO2 microtube. Three coaxial capillaries were assembled as the spinneret, and chemically inert middle fluid was introduced to work as the spacer between out and inner fluids. Under the protection of a proper middle fluid, various outer and inner fluid pairs even a couple of completely miscribe fluids can be used to form a three-layered core/shell structure, and by removing of the middle fluid, wire-in-tube structure can be prepared.

Figure.2 (a, b) Cross-section SEM image and TEM image of TiO2 fibers, which shows the interesting wire-in-tube structure. Such electrospinning system takes advantages of fabrication three-layered core/shell structure. However, it also shows limitation on producing multicomponent materials, especially on miscible components. So we improved on a novel multifludic electrospinning technology in order to generate multichannel hollow fibers by one-step. As shown in Figure 3a, the experimental setup is characterized by the hierarchical compound nozzle which is assembled by embedding two or more metallic capillaries separately into a blunt metal needle. Certain viscous fluid was fed to the blunt metal needle as the shell fluid, and other fluids that were immiscible with shell fluid were delivered into the compound hierarchical nozzle as inner fluid. By taking advantages of the multifluidic compound-jet electrospinning technique, hierarchical multichannel microtubes can be facile and straightforward fabricated. It was obvious that the middle wall of the tubes in the vertical direction is straight and divided the tube into two, three, four (Figure 3b, 3d, 3e).22 Such multichannel tubes were successfully used in storage and release thermal energy in a stepwise manner.23 Meanwhile, under controlled voltage and electrospun solution concentration during electrospray process, as-prepared particle sphere were of several inner chambers corresponding to the designed inner hierarchical compound nozzles (Figure 3c, 3f).24

As mentioned above, co-axial electrospinning can encapsulate functional materials into fibers to form hollow structures, multilevel structure, and many other special morphologies and shapes. However, since emulsion is metastable system that could demulsification when it is storage for long time or during electrospinning process, co-axial electrospinning shows limitations when used in emulsion system. Accordingly, we developed a versatile microemulsion electrospinning method to fabricate ultraporous inorganic oxide nanofibers with hierarchical

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Figure. 3 (a) Schematicillustration of the multifluidicelectrospinning setup. The immiscible inner and outer fluids are issued out separately from individual capillaries. After electrospinning, a fibrous film is collected on the counter electrode. SEM images of multichannel tubes with variable diameter and channel number. (b, d, e) Corresponding to tube with two, three and four channel number. Scale bars are 100 nm. (c, e) Two and four chamber spheres could also be fabricated by this method. Scale bars are 500 nm. inner structures (Figure 4).25 The transparent microemulsion is far more stable than obscure emulsion system. Therefore, it could be facilely electrospun to nanofibers by using such as metal alkoxide and paraffin oil system. After calcination, i.e., by selective removal of the oil phase, the ultrathin hybrid nanofibers decomposed to multilevel porous inorganic nanofibers.

Figure. 4 (a) Cross-sectional SEM images of porous TiO2nanofibers. (b) TEM image of nanofibers after calcination showing a discontinuous bamboo structure. Inset: a magnification of a single segment showing the existence of mesopores on the wall.

3 APPLICATIONS Electrospun multilevel structured materials endow large

number of predominant applications. They are competitive in many areas than bulk or the same sized materials without hierarchical structures. The biggest advantages of

electrospun materials are their very high surface to volume ratio, controllable porosity as well as easily accessibility for mass production and application. Herein, we present a few representative applications including in the areas of catalysis, sensor, filtration and adsorption.

3.1 Catalysis

The most straightforward and obvious exhibition of the advantageous of electrospinning is its application in catalysis. The huge specific surface and heterogeneous interfaces result in the celerity and high-efficiency which are the most important key factors for catalytic field. Here, we take TiO2 as an example because it has been considered as one kind of most effective photocatalysts for its stability, low-cost, nontoxicity and strong oxidizing power of the photo-generated holes with which most of organic compounds can be oxidized to nontoxic carbondioxide under ultraviolet irradiation.

Figure. 5 (a) SEM images of TiO2 fibers with different number of channels. (b) Plots of the decrease in acetaldehyde concentration and increase in CO2 product concentration vs. irradiation time during the photocatalytic degradation of acetaldehyde by TiO2 fibers with different channel number. (c) Dependence of ln(C0/Ct) on irradiation time in the initial 2 h photocatalytic reaction. It indicates clearly that increasing the channel number of TiO2 fibers systematically enhances the degradation rate of gaseous acetaldehyde molecules.

We prepared the solid fibers and normal hollow fibers (i.e. one channel fibers) by traditional electrospinning and coaxial electrospinning.26 The two channel fibers and three channel fibers were fabricated by a multifluidic compound-jet electrospinning method (Figure 5a). The photocatalytic activity of TiO2 fibers with different channel number was evaluated by the degradation of gaseous acetaldehyde. As showing Figure 5b, it indicates clearly that increasing the channel number of TiO2 fibers systematically enhances the degradation rate of gaseous acetaldehyde molecules. TiO2 fibers with three channels show the highest photocatalytic activity and reaction rate, which degrades 82% of initial acetaldehyde in a typical photocatalytic reaction of 3 h and

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initial rate constant (k), is 0.83 h-1 (Figure 5c). Such multichannel fibers structures not only presented high photocatalytic effecience, but also collected and reused easily. 3.2 Energy storage

Functional nanostructured architectures assembled from nanowires, nanobelts, nanotubes et al, offer tremendous impact in energy storge, such as solar cells and batteries. Recently, we prepared multicomponent phase change microfibers, which can storage and release thermal energy in a stepwise manner by a facile one-step multifluidic compound-jet electrospinning with temperature control.23 Due to special multichannel tubular microstructure in which controllably encapsulate different phase change materials into the channels independently, multiresponsive effect can be realized. It may open a new way in smart fiber materials for architectural energy strorage materials.

Figure. 6 Two-component phase change microfibers that were derived from two kinds of inner fluids, hexadecane and eicosane for five cycles. The insert shows side-view SEM image of the fibers where the inner phase change materials have been removed. The scale bar is 500 nm. 3.3 Filtration and adsorption

Filtration and adsorption is necessary and important in many areas. For example, in engineering field and environmental protection areas, materials which exhibit the capacity of filtration or adsorption oil from oil/water system own much favour. Meanwhile, many attentions have been focused on the fabrication of highly efficient filtrated and adsorpted materials.27, 28 Fibrous materials used for filter provide advantages of high filtration efficiency and low air resistance. Because of the very high surface area to volume ration and resulting high surface cohension as well as the possibility of recycle usage of electrospun membrane, electrospinning is the promise candidate in the application of filtration and adsorption. Some surperhydrophobic and oleophylic polymers, like polystyrene, are chosen for the fabrication of membrane with filtration and adsorption functions. Recently, we reported the fabrication of ultrathin polystyrene fibers with nanoscaled pores on fibrous surfaces (Figure 7).14 During the electrospinning process,

controlling the compositions of electropun solvent, porous structure could be easily obtain. Taking advantage of the second-order porous structure on polystyrene fibers, compared to smooth fibers without pores, the as-prepared membranes exhibited excellent oil adsorption capacity and oil selective capacity in oil/water media. The weight oil adsorption is over one hundred times higher than the membrane itself. Meanwhile, the unidirectional water penetration electrospun fibrous film was successfully obtained by our group.29 By controlling the films conponents, water can spontaneous penetrate from one side but block in another side. It may spark the application in filtration and seperation fields.

Figure.7(a) SEM image of porous polystyrene fibrous mat (inset: amplification of PS fiber). (b) Magnification of porous fiber surface. (c-e) Oil spill remove from water of polystyrene sample (oil dyed with Oil Red, motor oil).

4 CONCLUTION Electrospining, affording fabrication of thin fibers from

micrometers down to few nanometers with well controllable strucutures and chemical compositions, endows us to combine wide range of materials with different morphological structures that benefit for superior properties than bulk materials or the same sized materials. These materials are amazed not only for their various morphologies, but also for their multi-functions as well as a wide range of prospective and real applications, including in target and controllable drug delivery and release, tissue engineering, sensor, et al. We believe that continuous researches, explorations and developments will have to take place for electrospinning because it permanently demanded not only in academia but also in industry. Such improvements will strongly push the field forward on the fabrication of multilevel structures and inaugurate multi-functionalizations. We will devote ourselves on this field and hope that an exciting and flourishing futures as well as tremendous potential application will be brought out in this field.

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