Nanoengineered Transparent, Free-Standing, Conductive

Oct 20, 2009 - Engineering, UniVersity of Connecticut, 191 Auditorium Road, Unit 3222, Storrs, Connecticut 06269-3222. ReceiVed: July 28, 2009; ReVise...
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J. Phys. Chem. C 2009, 113, 19525–19530

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Nanoengineered Transparent, Free-Standing, Conductive Nanofibrous Membranes Wenzhao Jia,†,§ Ying Wang,†,§ Joysurya Basu,† Timothy Strout,† C. Barry Carter,† Ali Gokirmak,‡ and Yu Lei*,† Department of Chemical, Materials and Biomolecular Engineering and Department of Electrical and Computer Engineering, UniVersity of Connecticut, 191 Auditorium Road, Unit 3222, Storrs, Connecticut 06269-3222 ReceiVed: July 28, 2009; ReVised Manuscript ReceiVed: September 28, 2009

Transparent conductive nanofibrous membranes have been successfully fabricated by sputter-coating a metal (Au, Pd, Pt, Ni, Ag, or Au/Pd alloy) onto a water-soluble-polymer nanofibrous template, followed by the dissolution of the template in a water bath. The size of the conductive nanofibers can be facilely controlled by adjusting the diameter of the polymer nanofibers as well as the sputter-coating time. The as-prepared samples were characterized by SEM, TEM, FTIR, and TGA, and the results reveal that the as-prepared conductive nanofibers consist of many metal nanoparticles held together after the dissolution of the polymer template, which is likely due to the coalescence of the metal nanoparticles as well as the bridging effect of the polymer chains between the adjoining metal nanoparticles. The transmittance of the film decreases but the conductivity of the film increases with the time of sputter-coating. The as-prepared transparent nanofibrous membrane also shows good mechanical and metallic properties, and its resistance displays humidity-dependent behavior most likely attributed to the swelling effect of the highly hydrophilic polymer chain that bridges the metal nanoparticles. This study provides a promising route to the facile synthesis of conductive nanofibers, which may have great potential in various applications. 1. Introduction Electrospun nanofibers have triggered considerable research activity in the past decade because of their great potential in numerous applications such as electronics, optics, sensors, tissue engineering, drug delivery, filtration, and so forth. The process of electrospinning is carried out under a high-voltage electrostatic field. As a polymer solution is fed through a syringe needle, it is charged by the high applied voltage. When the electric field overcomes the surface tension of the solution, a fine charged jet is ejected from the needle tip and attracted to an oppositely charged or grounded collector, which leads to the formation of ultrafine fibers after the solvent evaporates. The formed fibers are left on the collector in the form of a randomly oriented nonwoven mesh with an intrinsically high surface area to volume ratio and porosity.1-4 Electrospinning has been successfully used to generate continuous fibers with diameters in the range of a few micrometers to less than 100 nm.5-7 Recently, a great amount of attention has been paid to the preparation and design of different conducting/semiconducting nanofibers (metal, metal oxide, etc.) and nanocomposites by using electrospun polymer nanofibers as a template or substrate.8-11 Typically, there are two major processing methods used to synthesize these types of nanomaterials via electrospinning. The first method is to employ electrospun polymer nanofibers as templates for material coatings through different techniques such as chemical vapor deposition,12 atom vapor deposition,13 and sol-gel chemistry.14 The second method is to electrospin the mixture of polymer and the precursor of the desired material to form nanofibers.15-17 In both cases, the polymer template is removed by thermal degradation in the temperature range of * Corresponding author. E-mail: [email protected]. † Department of Chemical, Materials and Biomolecular Engineering. ‡ Department of Electrical and Computer Engineering. § Equal contributions.

300-900 °C in order to extract the desired nanofibers. If necessary, additional treatments such as reduction, electroless deposition, or reaction with H2S may also be applied as a supplementary step.17,18 A wide range of materials with various morphologies have been manufactured through these methods (e.g., Pd19 or Ag2S20 nanoparticles embedded in polymer nanofibers, metallic (Al, Au, Ni and Cu) core-shell nanotubes,12,21 ZnS/Cu/PVA composite nanofibers,17 zinc chloride-activated porous carbon nanofibers,22 Cu nanowires,23 etc.). The synthesized nanomaterials, especially the conducting metallic nanofibers, have great potential in many fields such as sensing and electrocatalysis. Even though these synthesis routes are elegant, the high-temperature processing requirements and/or complicated reactions involved in the synthesis greatly limit their further application. Herein we report a simple and convenient method to produce transparent, free-standing, and conductive nanofibrous membranes via water-soluble-polymer nanofiber templates. Poly(Nisopropylacrylamide) (PNIPAm) and poly(acrylic acid) (PAA) were chosen to generate electrospun nanofiber templates because of their good water solubility. Rather than other methods that require high-temperature treatment for a relatively long time to decompose the polymer template, followed by hydrogen reduction of metal oxides to metals, our approach simply utilizes sputter-coating to deposit various metals on the top layers of templates, followed by dissolution of the water-soluble templates in a water bath, resulting in transparent, free-standing nanofibrous membranes with good mechanical and conductive properties. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG), and UV-vis transmittance spectra were used to characterize the as-prepared samples. The transmittance of the film decreases while the conductivity increases with the time of sputter-coating (or the film thickness). It was also demonstrated that the resistance of the as-prepared

10.1021/jp905023e CCC: $40.75  2009 American Chemical Society Published on Web 10/20/2009

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conductive nanofibrous membrane shows humidity-dependent behavior, which makes it a potential stimuli-responsive material. 2. Experimental Section 2.1. Preparation of Polymer Nanofiber Templates. Watersoluble poly(N-isopropylacrylamide) (PNIPAm, Aldrich) and poly(acrylic acid) (PAA, Aldrich, 35 wt % solution in water, MW∼250 000) were used to fabricate the nanofibrous templates. Briefly, PNIPAm was dissolved in ethanol to prepare 30 wt % solution and was electrospun using a 21 gauge needle at a flow rate of 0.8 mL/h at an applied potential of 20 kV with a collection distance of 10 cm, and 30% PAA was prepared and electrospun using a 19 gauge needle at a flow rate of 0.3 mL/h at an applied potential of 20 kV and a collection distance of 15 cm. Polymer nanofibers were collected on a grounded aluminum foil until a relatively thick membrane was formed. 2.2. Preparation of Transparent and Conductive Nanofibers. The sputter-coating targets (Au/Pd alloy, Pt, Au, Pd, Ni, and Ag) were purchased from EB Science and had a purity of 99%. A Au/Pd alloy (60 wt % Au and 40% Pd) was sputtercoated onto a PNIPAm nanofibrous template for a certain time with a constant current of 20 mA (Polaron Instrument Inc., SEM coating unit E5100). The Au/Pd-coated nanofibrous template was gently peeled off from the aluminum foil and then transferred to a water bath to dissolve the polymer template. The transparent free-standing Au/Pd nanofibrous membranes were carefully collected and placed on the appropriate substrates for characterization. As a further demonstration, the Au/Pd alloy, Pt, Au, Pd, Ni, and Ag have also been sputter-coated onto PAA nanofibrous membranes using a similar procedure. 2.3. Material Characterization. A JEOL 6335F fieldemission scanning electron microscope (FESEM) was used to examine the morphology and the size of the as-prepared conductive nanofibers. To investigate the morphology of the as-prepared conductive nanofibers further, the Au/Pd alloy nanofibers were broken down into small fragments by ultrasonication and dropped onto a gold substrate for SEM analysis. A Tecnai T12 transmission electron microscope (TEM) operated at 120 kV was used for chemical and morphological imaging. The STEM and EDS detectors were used in conjunction with chemical mapping of the samples. The sample for TEM was prepared by placing the as-prepared nanofibrous membrane onto carbon-film-coated copper grids. Fourier transform infrared spectroscopy (FTIR) measurements were made using a Thermo Nicolet Magna-IR 560 spectrometer equipped with a Spectra Tech IR-Plan microscope. Both Au/Pd and an electrospun PNIPAm nanofiber film were ground into a dry BaF2 disk prior to data collection, and the spectra were analyzed using Omnic software. Thermogravimetry (TG) was performed using a TA Instruments TGA Q500 under air flow of 60 mL min-1 at a heating rate of 10 °C min-1 from room temperature to 800 °C. The UV-vis transmittance spectra of the PNIPAm nanofibrous template and the as-prepared Au/Pd conducting nanofibrous films with different sputter-coating times were obtained using a Shimazu UV-1700 spectrometer. The conductivity was measured using a homemade four-point probe, and the thickness of the film was obtained using Veeco-Dektak 150 surface profiler. 2.4. Humidity-Dependent Resistance. The Au/Pd sputtercoated PNIPAm nanofibrous membrane was gently peeled off from the aluminum foil and transferred to a water bath to dissolve the polymer template. The free-standing Au/Pd nanofibrous membrane was collected and placed on a pair of interdigitated electrodes (IDE) to fabricate the resistor-type device for the study of humidity-dependent resistance. The

Jia et al. membrane covered the whole IDE area, which features 20 pairs of gold fingers with a finger width of 3 µm and a gap size of 3 µm on top of a SiO2 layer. The device was introduced into a homemade 5 cm3 sealed glass chamber with gas inlet/outlet ports. The device circuit was subjected to a fixed 0.1 V dc bias, and the current was continuously measured by a CHI-601C electrochemical analyzer (CH Instruments Inc., Austin, TX). The electrical resistance between the IDE electrodes was calculated by applying Ohm’s law (R ) V/I). Different humidity levels were obtained by flowing dry air through various saturated salt solutions at room temperature (20 ( 2 °C), and the actual relative humidity was calibrated with a commercial humidity sensor. The flow rate was regulated by a computer-controlled gas mixing system (S-4000, Environics Inc., Tolland, CT), and the total flow rate was set to 1.2 L/min for the experiments. All experiments were carried out at room temperature. In a typical experiment, the sample was first stabilized in dry air and then exposed to humid air for 2 min, followed by dry air for 5 min to recover the resistance, and then the cycle was repeated. The normalized resistance change is defined as ∆R/R0% ) [(R R0)/R0] × 100%, where R0 is the initial electrical resistance of the sensor in dry air and R is the measured real-time resistance upon exposure to humid air. As a comparison, the Pd/Au alloy was also directly sputtered onto interdigitated electrodes under the same sputter-coating conditions used to fabricate the control. 3. Results and Discussion The PNIPAm solution was successfully electrospun into a nanofibrous membrane. Sputter-coating of the Au/Pd alloy on the top layers of the polymer nanofiber template caused the color change from white to black. The Au/Pd-coated membrane was carefully peeled off from the aluminum foil and transferred to a water bath (Figure 1A). After the polymer template was dissolved in water, the free-standing, transparent nanofibrous membrane was transferred to a Petri dish. Figure 1B shows a visible-light image of the as-prepared transparent Pd/Au nanofibrous membrane. The membrane has excellent mechanical properties and can be easily fabricated to as large as 5 cm × 4 cm and could even be made larger. The size of the Au/Pd nanofibrous membrane is solely dependent on the dimensions of the template and the chamber size of the sputter coater. A typical SEM image of the Au/Pd nanofibrous membrane is presented in Figure 1C. One can see that the transparent Au/Pd membrane maintains a fibrous morphology similar to that of the template but the Au/Pd nanofibers are more like ribbons with a width of several hundred nanometers. Furthermore, the SEM image at high magnification reveals some fusing points formed after the Au/Pd coating (inset of Figure 1C). To investigate the integrity of the conductive nanofibrous membrane and its resistance, a current versus voltage study was performed for the Au/Pd nanofibrous membrane with a size of 0.6 cm × 0.7 cm (Figure 1D). The linear dependence of the current on the applied potential follows Ohm’s law, reflecting the metallic nature of the Au/Pd nanofibers. On the basis of the observed data, the resistance of the tested sample was 10.22 kΩ. The resistance of the Au/Pt nanofibrous membrane was dependent on fabrication conditions such as the diameter and density of polymer nanofibers in templates and the sputter-coating time. The resistance of the sample was very stable under ambient conditions, indicating the good stability of the as-prepared Au/ Pd nanofibers. The detailed morphology and structure of Au/ Pd nanofibers were further characterized and analyzed by TEM and SAED. Figure 2A,B shows typical TEM images of an uncoated PNIPAm nanofiber and a Au/Pd nanofiber after

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Figure 1. (A) Au/Pd sputter-coated PNIPAm nanofibrous membrane. (B) Visible light image of the as-prepared Au/Pd nanofibrous membrane using PNIPAm nanofibers as a template. (C) Typical SEM picture of Au/Pd nanofibers. The inset shows the high-magnification SEM, indicating the fusing point formed. (D) I-V curve of the Au/Pd nanofibrous membrane after the dissolution of PNIPAm.

Figure 2. (A) Typical TEM image of an uncoated PNIPAm nanofiber. (B) Typical TEM image of a Au/Pd nanofiber after the dissolution of PNIPAm. (The inset corresponds to the dotted circled portion of the Au/Pd nanofiber.) (C) Schematic of the proposed coalescence of Au/Pd nanoparticles and the bridging effect of the polymer chains between adjacent Au/Pd nanoparticles. (D) TG curves of PNIPAm nanofibers and the as-prepared Au/Pd nanofibers after template removal. (E) FTIR spectra for PNIPAm nanofibers and the as-prepared Au/Pd nanofibers after template removal.

template removal. It can be seen that the surface of an uncoated PNIPAm nanofiber appears to be relatively smooth. However,

many Au/Pd nanoparticles that are ∼10 nm in size (Figure 2B inset) are clearly recognized on the surface of the sputter-coated

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Figure 3. UV-vis transmittance spectra of the as-prepared conductive nanofibrous films with different sputter-coating times. The inset presents the thickness vs the sputter-coating time.

nanofiber after template removal, which may be attributed to nucleation and island formation during the sputter-coating process. Some of the nanoparticles coalesce together. The SAED pattern indicates that the coated Au/Pd is amorphous (data not shown). It is also interesting that the metal nanoparticles did not fall apart during the dissolution of the polymer template, which may be attributed to the coalescence of the Au/Pd nanoparticles as well as the bridging effect of the polymer chains attached to adjacent Au/Pd nanoparticles (Figure 2C). To prove the presence of the polymer in the final product, thermogravimetric (TG) analysis was carried out on an uncoated PNIPAm nanofibrous template and Au/Pd nanofibers after template removal (Figure 2D). The TG of PNIPAm nanofibers shows that it is thermally stable up to 240 °C. Its initial weight loss of ∼10.4% is observed at up to 110 °C, which can be attributed to the evaporation of the physically adsorbed water molecules. The largest weight loss (87.5%) occurs in the temperature range from 240 to 418 °C, which can be assigned to the degradation of the polymer backbone. A total weight loss of 98.6% has been observed at up to 800 °C. On the contrary, the thermal behavior of the as-prepared Au/Pd nanofibers after template removal is different from that of PNIPAm. The initial weight loss of ∼7.74% at up to 135 °C can be ascribed to the loss of physically adsorbed water. The discontinuous weight losses of 46.2% from 135 to 412 °C and 26.4% from 412 to 720 °C are attributed to the degradation of polymer in Au/Pd nanofibers, and the discontinuity likely arises from the interaction between the polymer chains and Au/Pd nanoparticles. At up to 800 °C, the remaining weight of 19.4% can be assigned to Au/Pd because of its high thermal stability. Furthermore, the presence of PNIPAm in the Au/Pd nanofibers after template removal was also demonstrated by FTIR spectroscopy in Figure 2E. The optical properties of the as-prepared conductive nanofibers are further investigated with respect to the sputter-coating time and/ or the film thickness. As shown in the inset of Figure 3, one can see that the longer sputter-coating time results in a thicker conductive nanofibrous film after the removal of the template. Consequently, the transmittance of the film decreases with the increase in sputter-coating time (or film thickness) (Figure 3). In addition, the increase in the sputtering-coating time from 45 to 200 s also augments the conductivity from near zero to 103.51 S/cm. Besides PNIPAm, other water-soluble polymers can also serve as templates. As a further demonstration, electrospun PAA nanofibers were used as templates to fabricate various conductive nanofibrous membranes, including the Au/Pd alloy, Pt, Au,

Figure 4. Typical SEM pictures of various conductive nanofibrous membranes using PAA nanofibers as templates: (A) Au/Pd alloy, (B) Pt, (C) Au, (D) Pd, (E) Ni, and (F) Ag. The insets present highmagnification SEM images.

Pd, Ni, and Ag. As shown in Figure 4, all of the conductive nanofibers after template removal have a continuous, smooth surface and good uniformity. The diameters of the various conductive nanofibers are slightly larger than those of the PAA template because of the coating of metals. The slight difference in the observed morphology for different conductive nanofibers is possibly due to the properties of the metals, which may affect the sputtering rate as well as the interaction with the polymer template. To investigate the detailed morphology, Au/Pd nanofibers using PAA nanofibers as a template were selected and ultrasonicated to generate small fragments. As shown in Figure 5A, the nanobelts and semicylindrical nanoshells can be clearly observed. In addition, the intersections of the fibers indicate the formation of fusing points. The observed structures can be explained as follows. Before ultrasonication, the semicylindrical shells are held by the “knot” structures and maintain nanofibrous structures. However, after ultrasonication, the small free fragments generated are fully extended and open their semicylindrical shells to form nanobelt structures, whereas some of the fragments near the fusing points still display semicylindrical shell structures because of the constrained force. The thickness of the conductive nanofibers can be conveniently controlled by the sputtering time. TEM images of an uncoated PAA nanofiber and a Au/Pd nanofiber after the dissolution of the PAA template also demonstrated that metal nanoparticles were held together on nanofibers likely because of the coalescence of Au/Pd nanoparticles as well as the bridging effect of the polymer chains between adjacent nanoparticles (Figure 5B,C), which was also demonstrated by the metal mapping shown in Figure 6. In this Figure, the formation of islands can clearly be observed. EDS mapping shows that both of the metals are homogeneously distributed. There might be a little segregation at the edges. In the EDS maps, some black regions can be observed. This indicates that those regions are free of any metal and also the

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Figure 5. (A) Representative SEM image of Au/Pd nanofiber fragments after the dissolution of PAA, followed by ultrasonication. (B) Typical TEM image of uncoated PAA nanofibers. (C) Typical TEM image of Au/Pd nanofibers after the dissolution of PAA.

Figure 6. One Au/Pd nanofiber and its corresponding metal mappings.

Figure 7. (A) Comparison of the humidity-dependent resistance change of a sputter-coated Au/Pd 2-D film and the as-synthesized Au/Pd nanofibrous membrane upon periodic exposure to 60% RH at an applied dc bias of 0.1 V and room temperature. (B) Typical real-time humidity-dependent resistance change of the as-prepared Au/Pd nanofibers upon periodic exposure to various humidities at an applied dc bias of 0.1 V and room temperature. The sample was first exposed to humid air for 2 min, followed by dry air for 5 min to recover the resistance, and then the cycle was repeated.

presence of polymer. These observations further substantiate the postulate that metal/alloy islands form and some of them coalesce during sputtering; thus, the coalescence of the islands and the remaining polymer chains from the template impart mechanical stability to the nanofibers. As demonstrated above, different conductive nanofibrous membranes can be facilely fabricated using polymer templates. The dimensions of the conductive nanofibers can also be easily controlled by changing the diameter of the templated nanofibers and the sputter-coating time. That is, by controlling the diameter of the polymer nanofibers, we can easily fine tune the size of the conductive nanofibers, and by controlling the sputtering time, the metal thickness can be adjusted. In addition, conductive nanofibers with fusing points improve the strength of the nanofibrous membrane, providing the flexibility for various applications. Not only can a single layer of conductive nanofibrous membranes be fabricated but also layer-by-layer conductive nanofibers can be potentially prepared by sequentially sputtercoating different metals, followed by the removal of the polymer

template in water. Such a simple technique has enormous advantages and can be conveniently extended to synthesize multilayered conductive nanofibrous membranes cost-effectively. For example, inexpensive metals can be sputtered first and serve as the supporting layer, and then noble metals can be sputtered as an ultrathin top layer on it with controllable thickness that is determined by the sputtering time, thus reducing the overall cost and maintaining the same performance. Additionally, such a multilayered conductive nanofibrous membrane is free-standing and very porous and can be facilely attached to any materials, offering a number of advantages for various applications ranging from fuel cells to electronic devices to catalytic reactions to sensing. Furthermore, because the metal nanoparticles are bridged by the very hydrophilic polymer chains, one can expect that the as-prepared conductive nanofibers would have humidity-dependent resistance. To demonstrate this, the developed conductive, very porous Au/Pd nanofibrous membrane was exposed to dry air and humid air periodically and the normalized resistance change was recorded. Figure 7A represents the typical

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normalized resistance change of the Au/Pd nanofibers and the sputter-coated Au/Pd 2-D film (control as a comparison) upon periodic exposure to 60% RH. Only a small increase in the normalized resistance change upon exposure to humid air was observed for the control (Au/Pd 2-D film), whereas a dramatic resistance change was observed when Au/Pd nanofibers were used. The enhanced humidity-dependent resistance can be attributed to several factors. First, the conductive nanofibers have a very porous structure and possess a high specific surface area, allowing water molecules to access the material’s surface with minimal diffusional resistance. Second, the PNIPAm bridging the adjacent Au/Pd nanoparticles is very hydrophilic and tends to absorb moisture, which causes the swelling of the polymer chains. The swelling of the polymer can result in an increase in the distance between metal nanoparticles on the nanofibers, thus increasing the resistance of the nanofibers. Figure 7B shows the normalized resistance change of Au/Pd nanofibers upon periodic exposure to various humidities. The corresponding humidity-dependent resistance change is presented in Figure 1S. One can see that the resistance of Au/Pd nanofibrous membrane responses to the humidity changes quickly and exhibits humidity-dependent behavior. By purging with dry air, the resistance can almost return to the original value. The fast change and good recovery of resistance can be attributed to the very porous structure, the ultrathin nanofibrous film, and the high hydrophilicity of the polymer chains bridging the Au/Pd nanoparticles. These features make the as-prepared conductive nanofibrous membrane a potential stimulus-responsive material. 4. Conclusions In summary, we have demonstrated a new strategy for the facile synthesis of various conductive nanofibrous membranes. Transparent, free-standing, ultrathin conductive nanofibrous membranes were successfully prepared by sputter-coating Ni, Pt, Au, Pd, Ag, or Au/Pd alloy onto electrospun water-soluble polymer nanofibers, followed by template removal in a water bath. The as-synthesized conductive nanofibers exhibit continuous, uniform morphology and some fusing points. The size of the conductive nanofibers can be controlled by simply adjusting the diameter of the polymer nanofibers, and the thickness of the sputter-coated metal can be controlled by the sputtering time. The transmittance of the film decreases whereas the conductivity increases with the sputter-coating time (or the film thickness). In addition, the as-prepared transparent, free-standing nanofibrous membrane shows good mechanical and conductive properties, and its resistance displays humidity-dependent behavior most likely attributed to the swelling effect of the very hydrophilic polymer chains bridging the metal nanoparticles. This study provides a promising route for the facile and costeffective synthesis of transparent, conductive nanofibers, and

Jia et al. the as-synthesized conductive nanofibers show great promise in applications of sensory devices, catalysis, and other fields. Acknowledgment. We greatly appreciate funding from a UConn large faculty research grant and the NSF. We also thank Professor Bru¨ckner of the UConn Chemistry Department for helpful discussions. Supporting Information Available: Correlation between the normalized resistance change and relative humidity for the assynthesized Au/Pd nanofibers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Theron, A.; Zussman, E.; Yarin, A. L. Nanotechnology 2001, 12, 384–390. (2) Casper, C. L.; Stephens, J. S.; Tassi, N. G.; Chase, D. B.; Rabolt, J. F. Macromolecules 2004, 37, 573–578. (3) Reznik, S. N.; Yarin, A. L.; Theron, A.; Zussman, E. J. Fluid Mech. 2004, 516, 349–377. (4) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87, 4531–4547. (5) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670– 5703. (6) Sigmund, W.; Yuh, J.; Park, H.; Maneeratana, V.; Pyrgiotakis, G.; Daga, A.; Taylor, J.; Nino, J. C. J. Am. Ceram. Soc. 2006, 89, 395–407. (7) Subbiah, T.; Bhat, G. S.; Tock, R. W.; Pararneswaran, S.; Ramkumar, S. S. J. Appl. Polym. Sci. 2005, 96, 557–569. (8) Ras, R. H. A.; Ruotsalainen, T.; Laurikainen, K.; Linder, M. B.; Ikkala, O. Chem. Commun. 2007, 13, 1366–1368. (9) Han, G. Y.; Guo, B.; Zhang, L. W.; Yang, B. S. AdV. Mater. 2006, 18, 1709–1712. (10) Kriha, O.; Becker, M.; Lehmann, M.; Kriha, D.; Krieglstein, J.; Yosef, M.; Schlecht, S.; Wehrspohn, R. B.; Wendorff, J. H.; Greiner, A. AdV. Mater. 2007, 19, 2483–2485. (11) Schechner, P.; Kroll, E.; Bubis, E.; Chervinsky, S.; Zussman, E. J. Electrochem. Soc. 2007, 154, B942–B948. (12) Bognitzki, M.; Hou, H. Q.; Ishaque, M.; Frese, T.; Hellwig, M.; Schwarte, C.; Schaper, A.; Wendorff, J. H.; Greiner, A. AdV. Mater. 2000, 12, 637–640. (13) Peng, Q.; Sun, X. Y.; Spagnola, J. C.; Hyde, G. K.; Spontak, R. J.; Parsons, G. N. Nano Lett. 2007, 7, 719–722. (14) Caruso, R. A.; Schattka, J. H.; Greiner, A. AdV. Mater. 2001, 13, 1577–1579. (15) Li, D.; Xia, Y. N. Nano Lett. 2003, 3, 555–560. (16) Panda, P. K.; Ramakrishna, S. J. Mater. Sci. 2007, 42, 2189–2193. (17) Wang, H. Y.; Lu, X. F.; Zhao, Y. Y.; Wang, C. Mater. Lett. 2006, 60, 2480–2484. (18) Bognitzki, M.; Becker, M.; Graeser, M.; Massa, W.; Wendorff, J. H.; Schaper, A.; Weber, D.; Beyer, A.; Golzhauser, A.; Greiner, A. AdV. Mater. 2006, 18, 2384–2386. (19) Demir, M. M.; Gulgun, M. A.; Menceloglu, Y. Z.; Erman, B.; Abramchuk, S. S.; Makhaeva, E. E.; Khokhlov, A. R.; Matveeva, V. G.; Sulman, M. G. Macromolecules 2004, 37, 1787–1792. (20) Lu, X. F.; Li, L. L.; Zhang, W. J.; Wang, C. Nanotechnology 2005, 16, 2233–2237. (21) Ochanda, F.; Jones, W. E. Langmuir 2005, 21, 10791–10796. (22) Kim, C.; Ngoc, B. T. N.; Yang, K. S.; Kojima, M.; Kim, Y. A.; Kim, Y. J.; Endo, M.; Yang, S. C. AdV. Mater. 2007, 19, 2341–2346. (23) Wang, Z. L.; Kong, X. Y.; Wen, X. G.; Yang, S. H. J. Phys. Chem. B 2003, 107, 8275–8280.

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