Electrospinning of ceramic nanofibers: Fabrication, assembly and applications

This paper provides a brief review of current research activities that focus on the synthesis and controlled assembly of inorganic nanofibers by electrospinning, their electrical, optical and magnetic properties, as well as their applications in various areas including sensors, catalysts, batteries, filters and separators. We begin with a brief introduction to electrospinning technology and a brief method to produce ceramic nanofibers from electrospinning. We then discuss approaches to the controlled assembly and patterning of electrospun ceramic nanofibers. We continue with a highlight of some recent applications enabled by electrospun ceramic nanofibers, with a focus on the physical properties of functional ceramic nanofibers as well as their applications in energy and environmental technologies. In the end, we conclude this review with some perspectives on the future directions and implications for this new class of functional nanomaterials. It is expected that this review paper can help the readers quickly become acquainted with the basic principles and particularly the experimental procedure for preparing and assembly of 1D ceramic nanofiber and its arrays.


Introduction
One-dimensional (1D) nanostructures such as nanowires, nanotubes, nanoribbons and nanofibers are continuing to be at the forefront of nanoscience and nanotechnology [1][2][3][4][5][6][7][8]. In particular, ceramic with 1D nano-architectures received increasing interest since it provides a good material system to investigate the dependence of electrical, optical, thermal and mechanical properties on dimensionality and size reduction [9][10][11][12][13][14]. 1D ceramic nanomaterials with their large surface-to-volume ratio have the potential for use in various applications where high porosity is desirable [11][12][13]. For example, inorganic nanowires are expected to play an important role as promising building blocks for nanoscale electronic [9,10,15], optoelectronic [15,16] and sensor device [12,17,18]. Most recently, ceramic nanowires have been explored as effective electrode materials for electrochemical energy storage devices [19,20]. In the past two decades, worldwide efforts in both the theory and the experimental investigation of growth, characterization and applications of 1D inorganic nanostructures have resulted in a mature, multidisciplinary field. For practical application of 1D ceramic nanomaterials, ⎯⎯⎯⎯⎯⎯⎯⎯ scalable materials production and convenient devices integration are extremely important. Technological advances over have resulted in the realization of several competing processes for fabricating nanometer-size ceramic materials. So far, many kinds of 1D ceramic nanomaterials have been successfully synthesized by a rich variety of methods, and the detailed research information on these 1D nanostructures can be readily seen in the pertinent literature [10,15,18,[21][22][23].
Among various chemical or physical synthetic approaches, electrospinning appears to be the most straightforward and versatile technique for generating 1D nanostructures [24][25][26]. Electrospinning was first patented in the US in 1902; however, the process was largely forgotten until the 1990s [27,28]. Electrospinning is currently the only technique that allows the fabrication of continuous fibers with diameters down to a few nanometers. Fibers with complex architectures, such as core-shell fibers or hollow fibers, can be produced by special electrospinning methods [24][25][26]. It is also possible to produce structures ranging from single fibers to ordered arrangements of fibers. Electrospinning is not only employed in university laboratories, but is also increasingly being applied in industry. The scope of applications, in fields as diverse as optoelectronics, sensor technology, catalysis, filtration, and medicine, is very broad [27,29,30]. However, electrospinning has been largely limited to the fabrication of nano bers of organic polymers materials, including synthetic and natural polymers, polymer alloys, and polymers loaded with chromophores, nanoparticles, or active agents, as it is relatively convenient to prepare a polymer solution or melt with appropriate rheological properties required for electrospinning [27,29,30]. Metals and ceramics are usually considered to be not directly spinnable [27,31] . Recent efforts by several research groups have made it possible to generate ceramic nano bers using electrospinning technique [24,[28][29][30][32][33][34][35]. This review will present a brief overview of recent progress in this research area, with a focus on the work carried out in our group [16,. After briefly introducing the basic principles of electrospinning, this paper will review the general experimental procedures for the preparation of electrospinning ceramic nano bers, and will highlight several unique features associated with the technique and illustrate the potential of electrospinning in future research related to ceramic processing. We will also discuss the controlled assembly and patterning of electrospun ceramic nanofibers. Finally, we will highlight some recent achievements on the study of physical properties of electrospun ceramic nano bers and their practical applications in the context of nanoelectronics, catalysis, environmental science, and energy technology [16,. By the end, we also provide some personal perspectives on the future development of this technique. It is expected that this review paper can help the readers quickly become acquainted with the basic principles and particularly the experimental procedure for preparing and assembly of 1D ceramic nanofiber and its arrays.

Principle of electrospinning
Electrospinning is based on inducing static electrical charges on the molecules of a solution at such a density that the self-repulsion of the charges causes the liquid to stretch into a ber in an electric eld [27]. Provided there is no breakage in the stretched solution, a single strand of continuous ber is formed upon solvent evaporation. When a high voltage is applied to the solution, the ohmic current distributes the charges throughout the molecules [27]. As the solution is ejected from the spinneret tip, the ohmic current transits to a predominantly convective current. The charges are transported from the electrospinning tip to the target through the deposition of the ber. The current stops oscillating when the deposition becomes stable [29]. This can be used to monitor the spinning process. The electrical charges used for electrospinning can be positive, negative or both (alternating current). Although most reported electrospinning experiments were carried out using a positive potential, it has been shown that a negative potential produces nano bers with a narrower diameter distribution [59,60]. This was explained by the fact that electrons can be dispersed more rapidly and uniformly than the much heavier protons.
In a typical electrospinning setup, a high-voltage source is connected to a metallic needle, which is attached to a solution reservoir (Fig. 1). The needle has a relatively small ori ce that concentrates the electric charge density on a small pendant drop of solution. Although a metallic spinneret such as a needle is convenient for the application of charge to the solution, the process also works if a high voltage is applied to the solution using a dedicated electrode with a nonconducting spinneret. A porous cylinder has also been used for electrospinning, and it is possible to induce charges on a free solution droplet without direct contact to form a ber. The theoretical modeling of a viscous leaky dielectric solution subjected to a critical voltage showed that it becomes unstable in an electric eld when the surface tension can no longer maintain its static equilibrium [27,31,59]. At this voltage, protrusions form on the solution surface and jets of solution are ejected. Any perturbation or nonuniformity on the solution surface will concentrate charges in regions with higher curvature. If the curvature is suf ciently large for the potential difference between such a region and the collector to reach a critical value, the solution erupts from the surface and accelerates towards the collector. This has given rise to numerous designs in which drums, spikes, ridges and disks have been used to dispense the solution for electrospinning. The diameter and morphology of electrospun nanofibers can be controlled by adjusting the following parameters during electrospinning [27,31,

Electrospinning of ceramic nanofibers
Electrospinning technique has been recognized as a fabrication method for polymer and carbon nanofibers [59]. Since 2002, electrospinning has been further explored as a high efficiency method for the generation of 1D ceramic nano bers [26,29,32]. In the past decade, various ceramic nano bers are fabricated by the combination of two conventional techniques: electrospinning and sol-gel [32,34]. Generally, ceramic nano bers are made by the electrospinning of ceramic precursors in the presence of polymer followed by calcination at higher temperatures. In order to generate well-controlled and high-quality ceramic nano bers by electrospinning, one typically process has to use the following procedure: (1) Preparation of an electrospinning solution containing a polymer and sol-gel precursor to the ceramic material, (2) Electrospinning of the solution under appropriate conditions, generate precursor nanofibers containing inorganic precursor and polymer assistant materials.
(3) Calcination of the precursor nano bers at high temperature to remove polymers and obtain the ceramic phase.
Doping or incorporation of foreign materials into a solid ceramic matrix material is often required for many applications such as catalysis and chemical sensors [12]. One remarkable feature that distinguishes the electrospinning approach from other techniques for preparing ceramic nano bers is that it is very simple and convenient to incorporate or encapsulate other materials into the nano bers and nanotubes. In an process, functional materials (molecules or nanoparticles) can be easily doped or incorporated into nano bers by adding these materials or their precursors to the spinning solutions. For example, Lin et al. fabricated aluminum-doped zinc oxide (AZO) nano bers with precisely controlled Al dopant concentration by adding aluminum nitrate in electrospinning solutions (Fig. 3) [16,39]. The diameter of AZO nano bers decreased and became uniform when the concentration of Al increased from 0 to 3.0 at.%. The dependence of the morphology and diameter of the electrospun nano bers on the processing parameters, including PVA concentration, applied voltage, and the Al dopant, was investigated in their research. It was found that the viscosity and conductivity were of most importance to the morphology of the e-spinning bers, given constant electric eld strength and ow rate [16,39]. Another example for this controlled doping is Ag-ZnO nanofibers by electrospinning [47,48]. Ag nanoparticles with controlled concentration have been successfully loaded in ZnO nanofibers by Lin et al. (Fig. 4).
Electrospinning technique can also be used for fabricating nanofibers composed by non-oxide ceramics including carbide, boride, nitride, silicide and sulphide [38,50,[79][80][81][82][83][84][85][86][87]. Wu et al. synthesized nitride nanofibers via electrospinning for the first time [50]. Group III N alloys, particularly GaN, have a huge potential for revolutionary semiconductor device con gurations. Extremely long GaN nano bers on a large scale can be synthesized based on electrospinning. The strategy to obtain GaN nano bers can be divided into three steps: (1) The electrospinning of precursor composite nano bers that comprise polymer and gallium nitrate; (2) Calcination of the composite nano bers in air to thoroughly remove organic components and to form gallium oxide nano bers; (3) In situ conversion of the as-prepared oxide nano bers into nitride nano bers in an ammonia atmosphere at high temperature [50]. SEM image shows that the continuous structure of the initial Ga 2 O 3 nano ber was maintained after ammonization. The length of the synthesized GaN nano bers could reach the centimeter scale, and the average ber diameter is only 40 nm, which indicates an ultrahigh aspect ratio of the GaN nano bers (Fig. 5a). TEM images (Fig. 5b) also reveal the brillar structure of the ammoniated samples. A selected-area electron-diffraction (SAED) pattern shows that the nano bers are polycrystalline, and a  high resolution TEM image indicates that the crystalline size of the nano ber is less than 10 nm (insets of Fig. 5b). This approach will be applicable to the synthesis of other nitride nano bers such as InN, AlN, and InGaN. It is expected that this technique will make the large-scale production and assembly of nitride functional nanomaterials with practical applications possible [50]. Lin et al. demonstrated that using the electrospinning technique coupled with a thermal treatment approach, ZnS nano bers can be prepared by sulfurizing the electrospun ZnO nano bers (as a template) at 500 in an H2S atmosphere (Fig. 6) [38]. The ZnS nanofibers were found to be polycrystalline, with an average grain size of 20 nm (Fig. 6d). The morphology of ZnS nano bers was found to be rough and uneven, which was considered to be correlated with the processing conditions. Their study demonstrated that electrospinning could be a general approach of fabricating 1D-nanostructured sul de with rough surface and high surface area for potential applications in nanoscale devices like high performance sensors [38].

Controlled assembly of ceramic nanofibers
Well-aligned and highly ordered structures are often required for device applications, especially in the fabrication of electric and photonic devices like field effect transistors, sensors and diodes. Many approaches have been developed to control the alignment of electrospun nano bers. Xia group developed a gap technique, as shown in Fig. 7a, by which one can collect the electrospun nano bers as a uniaxially aligned arrays over a large area [24,27,29,30,[32][33][34]88]. The setup is essentially the same as the conventional one except for the introduction of an insulating (e.g. made of air, quartz, or polystyrene) gap in the middle of the collector, with a width that can be varied from hundreds of micrometers to several centimeters. The insulating gap makes the electric eld lines in the vicinity of the collector split into two fractions pointing toward opposite edges of the gap (see the electrostatic force analysis in Fig. 7b). The as-spun, charged nano bers are expected to experience electrostatic forces from two sources: the strong external eld (F1) between the spinneret and the collector, and the repulsion between adjacent, charged bers (F2). As a result, the bers are stretched perpendicular to the edges of the gap [33]. Since the bers can keep highly charged after deposition, the strong repulsion between the deposited and incoming nano bers can facilitate a parallel alignment between them as this alignment has the lowest energy. Therefore, the alignment can be signi cantly improved as the deposition is increased. By using this modified electrospinning technique, well-aligned ceramic nanofiber arrays (Fig. 7) or single nanofiber deposited in controlled directions can be fabricated.
Our group developed a newly modified electrospinning technique for generating highly oriented ceramic nano bers with a length of several centimeters. Very different from the conventional electrospinning process, the bers are collected between the electrodes [41]. The experimental con guration for our modi ed electrospinning setup is illustrated in Fig. 8a. Unlike the conventional setup, a metallic triangular tip without any solution supply system was used as the electrospinning source. The tip in a H 2 S atmosphere for 2 h (c), and a dark-eld transmission electron microscopy image there of (d). An energy-dispersive X-ray spectroscopy result and electron-diffraction pattern recorded from the ZnS nano bers are inserted, respectively [38]. Reprinted with permission from Lin D D, Wu  was prepared by cutting down a 1-mm-thick aluminum sheet into a 1-5 cm rectangle with a triangular tip. The apex angle of the triangular was set as 60°. The tip was used as the electrospinning source; it helped to establish a Tylor cone while electrospinning. A grounded coin was placed close to the aluminum sheet as the counter electrode of the system. A bundle of aligned electrospun bers was then formed between the spinning tip and the collector (Fig. 1c). The bers were then transferred on a silicon wafer. The key to success is the reduction of interelectrode distance and the use of an ultra strong electric eld, which is about six times higher than conventional conditions [41].

Electrical transfer properties and FET devices
Our group reported the assembly and electrical properties of ultralong copper oxide (CuO) nano bers prepared by electrospinning method [35]. The prepared CuO nano bers were polycrystalline, with diameter of 60 nm and length over 100 μm. By using a designed ber collector, the electrospun CuO nano ber was deposited bridging two paralleled electrodes. Electrical measurement was conducted between the two electrodes. The conductivity of individual CuO nano ber was measured to be 3×10 3 S/cm. Field effect transistor behavior in single CuO nano ber was also observed, showing that CuO nano ber was intrinsic p-type semiconductor [35]. Another research in our group performed electrical measurement on single ZnO nanofibers. FET behavior in single ZnO nano ber was also observed, showing that the ZnO nano ber was an intrinsic n-type semiconductor [43].
Highly oriented GaN nano bers were fabricated by electrospinning. This technique indicated the possibility of precisely locating a patterned GaN nano ber with designed orientation, which is favorable for some practical applications such as electrical devices and lasers [50]. As an example, we assembled a single electrospun GaN nano ber into a eld-effect transistor device, which is schematically shown in the inset of Fig. 9d. The current-voltage (I sd -V sd ) characteristics of a GaN nano ber FET device at different gate voltages (from +60V to −60V with a 30V step) are shown in Fig. 9d. The current versus source-drain voltage and gate voltage were measured, the nano ber conductance increases for Vg greater than zero and decreases for Vg less than zero, which indicates that these nano bers are intrinsic n-type. This n-type behavior in nominally undoped GaN is believed to be attributed to the nitrogen vacancies and/or oxygen impurities [50].
Our results indicated that semiconductive ceramic nano bers can be directly assembled into FETs using the simple and versatile electrospinning method. The nano ber transistors should be useful in building low-cost logic and switching circuits, as well as highly sensitive chemical and biological sensors with reduced device dimensions.
It is of great interest to study oxide nanofibers with doping of selection of elements for enhancing and controlling their mechanical, electrical, and optical performance. Our group studied the electrical properties of single aluminum-doped zinc oxide nanofibers [16]. Typical current-voltage (I-V) curve through a single AZO nanowire with different Al concentrations is shown in Fig. 10, and the corresponding conductivity was calculated (Fig. 10). Compared to the pure ZnO nanowire with a conductivity of 2.55×10 -5 S cm -1 , the Al-doped ZnO nanowires show a steep increase up to 9.73× 10 -3 S cm -1 (Al = 5 at %), indicating a great enhancement in conductivity. Here, aluminum acts as a cationic dopant in the ZnO lattice, that is, the trivalent Al 3+ ion will occupy the divalent Zn 2+ site allowing electrons to move to the conduction band easily. This gives a net increase in the concentration of electrons, thereby enhancing the electrical conductivity [16]. Lin et al. studied electrical transfer properties of indium doped tin oxide (ITO) nanofibers [37]. Devices for I-V measurement and field-effect transistors (FETs) were assembled using ITO nanowires with top contact configurations. The effect of Sn doping on the electrical conductivity was significant in that it enhanced the conductance by over 10 7 times, up to ~1 S cm 1 for ITO nanowires with an Sn content of 17.5 at.%. The nanowire FETs were operated in the depletion mode with an electron mobility of up to 0.45 cm 2 V 1 s 1 and an on/off ratio of 10 3 .

Optical properties and photo sensors
Electrospun inorganic nanofibers have interesting optical properties due to its shrinked dimensions and nanoscale grain sizes. For example, our group studied the photoluminescence (PL) spectroscopy of electrospun GaN nanofibers [50]. Spectra were collected at room temperature, exited by a monochromatic source with excitation energy of 3.82 eV (325 nm). The PL spectrum (Fig. 11a) from GaN nano bers showed a broad peak at 363 nm (3.42 eV), and a small, narrow peak at 333 nm (3.72 eV). It is known that the size effect widens the bandgap of nanowires and that a blue-shift may occur to the PL spectrum; however, surface effects narrow the bandgap of nanowires. According to our experimental results, the primary bandgap of the nano bers (3.42 eV) is a little higher than that reported for bulk GaN (Wurtzite: 3.4 eV), which indicates a combination of the size effect and surface effect. As ammonia exists during the fabrication of GaN nanowires, surface passivation of GaN nanowires may exist, and accordingly modi es the bandgap, as reported for other nitride nanowires. The small peak at 333 nm demonstrated a blue-shift in the emission photon energy of the nanoparticles compared with the bandgap of bulk GaN, which shows that the emission peak is a result of the quantum Considering the wide direct band gap, nanometer-scale dimensions, and also taking advantage of the high aspect ratio and rough surfaces, electrospun ceramic nano bers are suitable for ultraviolet photodetection applications [50]. Figure 11b shows the photoconductance response of the GaN nano ber FET device fabricated by Wu et al. The measured Isd of the nano ber device showed a rapid increase from about 0.03 to 25 nA upon exposure. The electrospun GaN nano bers show much higher sensitivity to UV detection than that of single crystalline GaN nanowires. From their measurement data, the conductance of an electrospun GaN nano ber increases by 830 times when UV was on. In contrast, a single crystal GaN nanowire grown by the chemical vapour deposition (CVD) method showed a smaller response (~78) to UV light [50]. A similar enhancement of photoresponse within electrospun ceramic nanofibers has also been demonstrated in Ag/ZnO and AZO material systems. For example, photoactive material consisting of heterogeneous Ag-ZnO nanowires and nanofibers was prepared by electrospinning by Lin et al. Tunable UV photodetectors fabricated using Ag-ZnO NWs have shown high sensitivity up to over four orders of magnitude with relatively fast and stable response speed (Fig. 12) [48]. In conclusion, electrospun nanofibers is suitable for assembling next generation high-performance photo-sensor devices due to its unique polycrystalline nature and rough surfaces, which lead to a high surface area-to-volume ratio, and subsequently result in more photo generated carriers compared with a smooth single crystalline nanowires.

Memory device
Electrospun ceramic nanofibers have been assembled to functional nano-scale smart electronic devices including but not limited to FETs and sensors. Lin et al. developed single ZnO nanofiber based memories. A nanoscale memory device constructed from an individual AZO nanowire was observed to switch between two states differing in conductivity by about one order of magnitude, because of field-induced charge trapping and de-trapping (Fig. 13) [16]. These results demonstrate that multifunctional AZO nanostructures prepared by electrospinning have great potential applications in next-generation semiconductor devices, such as diodes and photodetectors, as well as transparent sensors and memories.

Magnetic properties
Magnetic properties of ceramic nanofiber systems have been widely studied [55,61,[89][90][91][92][93]. For example, Fe-doped TiO 2 -SnO 2 hybrid nano ber has been prepared via electrospinning method. The magnetic property versus Fe doping content was characterized 55 . The magnetic properties have also been characterized for the Fe-TiO 2 /SnO 2 nano bers before and after annealing under vacuum atmosphere, and nano bers show obvious ferromagnetic property at room temperature as they are annealed (Fig. 14a) [55]. Wu el al. showed that the simple solution-based electrospinning synthetic method allows convenient assembly and precise doping of Mn or other elements into the GaN nanofiber matrix and the Mn-doped GaN  (Fig. 14b) [50]. Their result shows a new way to the facile synthesis of diluted magnetic semiconductor (DMS) nano bers, which is of signi cant importance for constructing spintronic nanodevices such as ultrasensitive magnetic eld sensors and spin FETs.

Phase transition
Nano-structured ceramics are also suggested to have different thermal stabilities compared with their bulks due to the shrinked dimensions and size effects. Our group studied the phase stability of ferroelectric and piezoelectric ceramic nanofibers. BaTiO 3 nano bers with diameters ranging from 92 to 182 nm have been prepared by electrospinning [45]. XRD and SEM revealed that when the annealing temperature is higher than 600 , completely crystallized BaTiO 3 nano bers could be obtained and the grain size almost kept a constant value (~13.7 nm) when the annealing time lasted longer than 8 h at 600 . Besides, the diameters of bers decreased when the annealing temperature increased from 500 to 600 , but at elevated temperature higher than 600 , the bers became thicker and also exhibited a rougher surface feature. Tetragonal to cubic phase transition in BTO nanofibers were monitored using Raman spectroscopy (Fig. 15). They found that the electrospun BaTiO 3 nano bers have much higher Tc (~220 ) than bulk BaTiO 3 ceramics (~130 ) [45]. This effect can be understood by considering the shrinked sizes in BaTiO 3 nanofibers.
The reduced grain size in electrospun BaTiO 3 nano bers caused larger grain-boundary areas, and further relieved internal stresses in materials due to grain-boundary sliding. Smaller internal stresses, relaxed by grain boundaries, could decrease the free energy of the ferroelectric phase, and thus increase the Curie temperature. Our research indicates that electrospinning ceramic nanofibers may have different phase stability of ceramics than in bulks, which provides an exciting opportunity for controlling the electrical, optical and ferroelectric performance of ceramics.

Energy storage
Electrospinning ceramic nanofiber, which has extremely high surface area according to its small size and provides fast charge-transfer channels along its 1D nanostructure, has been considered as an ideal material system for energy storage applications . For example, Mai et al. demonstrated high performance lithium ion battery electrode based on electrospun vanadium oxide nanofibers [133]. They prepared ultralong hierarchical vanadium oxide nanowires with diameter of 100-200 nm and length up to several millimeters using the low-cost starting materials by electrospinning combined with annealing. The hierarchical nanowires were constructed from attached vanadium oxide nanorods of diameter around 50 nm and length of 100 nm. The initial and 50th discharge capacities of the ultralong hierarchical vanadium oxide nanowire cathodes are up to 390 and 201 mAh/g when the lithium ion battery cycled between 1.75 and 4.0 V (Fig. 16). When the battery was cycled between 2.0 and 4.0 V, the initial and 50th discharge capacities of the nanowire cathodes are 275 and 187 mAh/g. Compared with self-aggregated short nanorods synthesized by hydrothermal method, the ultralong hierarchical vanadium oxide nanowires exhibit much higher capacity. This is due to the fact that self-aggregation of the unique nanorod-innanowire structures has been greatly reduced because of the attachment of nanorods in the ultralong nanowires, which can keep the effective contact areas of active materials, conductive additives, and electrolyte large and fully realize the advantage of nanomaterial-based cathodes. This demonstrates that ultralong hierarchical vanadium oxide nanowire is one of the most favorable nanostructures as cathodes for improving cycling performance of lithium ion batteries [133].

Photo catalyst
Environmental problems such as air and water pollution have provided the impetus for sustained fundamental and applied research in the area of environmental remediation [139][140][141][142][143][144]. With the steady and fast growing field of nanoscience and nanotechnology, nanostructured ceramics like zinc oxide and titanium oxide has become a promising photocatalyst for its high catalytic activity, low cost, and environmental friendliness [50,51,55,57,  [145][146][147][148][149][150][151][152]. Major limitation of achieving high photocatalytic efficiency in the ceramic nanostructure systems is the quick recombination of charge carriers. Electrospinning ceramic nanofibers have extremely high surface area due to its small sizes, and further provide channels for quick charge transfer due to its 1D nanostructures. Therefore, electrospun ceramics have huge potential for photo catalyst applications [47,57,[145][146][147][148][149][150][151][152][153][154][155]. Our group fabricated photocatalytically active Ag-ZnO composite nanofibers by electrospinning (Fig. 17) [47]. The resultant heterostructure could promote the charge separation of the photogener ated electrons (e-) and holes (h+), allowing both of the e-and h+ participating in the overall photocatalytic reaction. The optimal photocatalytic activity of Ag-ZnO nanofibers exceeded that of pure ZnO nanofibers by a factor of more than 25 when the Ag concentration was kept at 7.5 atom %. Therefore, exploring the catalytic activity of such composite structures may pave the way for designing useful nanoscale building blocks for photocatalytic and photovoltaic applications [47]. Zhang et al. synthesized hybrid Fe-TiO 2 /SnO 2 nano be with combination of high photocatalytic activity and room temperature ferromagnetic properties. It is believed that this hybrid nano ber may serve as a candidate of new generation of visible light-excitable photocatalyst, which is also possibly easy for recycling and its potential applications are foreseeable in some elds such as water purifying and pollution treatment [55].

Filters and separators
Taking advantages of the highly porous fibrous nanostructures, ceramic nanofiber membrane can be used for filter and separator applications [128,[156][157][158][159]. separation layer on a porous substrate using large titanate nanofibers and smaller boehmite nanofibers [157]. Traditional ceramic separation membranes, which are fabricated by applying colloidal suspensions of metal hydroxides to porous supports, tend to suffer from pinholes and cracks that seriously affect their quality. Other intrinsic problems for these membranes include dramatic losses of flux when the pore sizes are reduced to enhance selectivity and dead-end pores that make no contribution to filtration. Electrospun ceramic nanofiber mats can successfully overcome these challenges. The ceramic nanofibers are able to divide large voids into smaller ones without forming dead-end pores and with the minimum reduction of the total void volume. The separation layer of nanofibers has a porosity of over 70% of its volume, whereas the separation layer in conventional ceramic membranes has a porosity below 36% and inevitably includes deadend pores that make no contribution to the flux. This radical change in membrane texture greatly enhances membrane performance. The resulting membranes were able to filter out 95.3% of 60-nm particles from a 0.01 wt % latex while maintaining a relatively high flux of between 800 and 1000 L/(m 2 ·h), under a low driving pressure (20 kPa). Such flow rates are orders of magnitude greater than those of conventional membranes with equal selectivity. Moreover, the flux was stable at approximately 800 L/(m 2 ·h) with a selectivity of more than 95%, even after six repeated runs of filtration and calcination. Use of different supports, either porous glass or porous alumina, had no substantial effect on the performance of the membranes; thus, it is possible to construct the membranes from a variety of supports without compromising functionality. The Darcy equation satisfactorily describes the correlation between the filtration flux and the structural parameters of the new membranes. The assembly of nanofiber meshes to combine high flux with excellent selectivity is an exciting new direction in membrane fabrication.

Conclusions and future perspective
Electrospun nano bers are extremely long. Such high-aspect-ratio nano bers made of ceramics provide an exciting materials system to investigate some fundamentally important problems, especially electrical, mechanical, magnetic and optical phenomena at the nanometer scale in ceramic science. Most ceramic nano bers prepared by electrospinning are polycrystalline, which is quite different from CDV-grown single crystalline inorganic nanowires. The grain or domain size can be comparable with the diameter of the bers, and may further provide unique functions like modified phase stability and high sensitivity to environmental. The electrospun nanofibers can be easily assembled for device integrations, which is important for future smart electronic devices in nanoscale. Another obvious advantage for electrospinning ceramic nanofibers is that the fabrication process is highly efficient and scalable. The low cost electrospun nanofibers hold potential for applications in photocatalyst and lithium ion battery electrodes. As both electrospinning and sol-gel processing techniques are being constantly improved, it is expected that progress in understanding the properties and applications of ceramic nano bers will be accelerated in the coming years.
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