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Pd/TiO2 Nanofibrous Membranes and Their Application in Hydrogen Sensing Wenzhao Jia, Liang Su, Yu Ding, Ashley Schempf, Ying Wang, and Yu Lei* Department of Chemical, Materials and Biomolecular Engineering, UniVersity of Connecticut, 191 Auditorium Road, Storrs, Connecticut 06269 ReceiVed: June 4, 2009; ReVised Manuscript ReceiVed: August 3, 2009
Pd nanofibers were successfully fabricated using the electrospun TiO2 nanofibers as templates through a simple electroless plating method. The effect of electroless plating parameters on Pd deposition was investigated, and the optimum conditions were determined in terms of the best morphology and integrity of the deposited palladium. The as-prepared membranes were characterized by field emission scanning electron microscopy, energy-dispersive X-ray spectrometry, X-ray diffraction, and transmission electron microscopy. By controlling the electroless plating time, continuous Pd-layer coated or dense Pd nanoparticle decorated nanofibers have been obtained. The as-prepared Pd/TiO2 nanofibrous membranes show good mechanical and conductive properties, offering an excellent sensing platform. Their application for hydrogen detection is demonstrated, and the developed hydrogen sensor exhibits fast response, excellent recovery behavior, and good sensitivity. Two different hydrogen sensing mechanisms were realized in Pd/TiO2 nanofibrous membranes prepared with different deposition times: the resistance increase caused by the formation of palladium hydride and resistance decrease due to a lattice-expansion-induced “break junction”. This study provides a promising route to the facile synthesis of metal nanofibrous membranes through electroless plating, which may have great potential in various applications. 1. Introduction Nanostructures have been intensively investigated because of their unique properties and diverse applications. Among different nanostructures, continuous nanofibrous membranes have attracted more attention recently due to their inherent large specific surface area and highly porous structure, which facilitate the adsorption/desorption process and minimize the diffusion resistance of molecules to the surface of materials. The electrospinning process has been widely used to generate nanofibers with a tunable diameter,1 and the as-electrospun nanofibrous membranes have already been applied in drug delivery and tissue engineering,2,3 sensors,4,5 catalysis,6 ultrafiltration,7 and transistors.8 Diverse nanofibers with various components have been fabricated so far. For example, nanoparticle-decorated fibers were produced by electrospinning the composite polymer solution, followed by additional treatments, such as reduction and chemical reaction.9,10 Calcination in the air11,12 and carbonization in inert gas13 are also commonly used to remove the polymer and generate the desired inorganic nanofibers. Furthermore, it has also been reported that electrospun nanofibers can serve as templates/substrates for the growth of Pt nanoparticles/nanowires or the assembly of several precious metal nanoparticles, endowing the template materials with unique catalytic or antibacterial activity.6,14 On the other hand, due to the hot wave of “hydrogen economics” in recent years, Pd nanomaterials have been extensively studied in the hydrogen-related fields due to their remarkable permselectivity for hydrogen and excellent catalytic properties. For instance, palladium with different structures, such as nanoparticles, ultrathin films, and nanowires, has been widely applied in fuel cell, hydrogen separation/purification, membrane reactors, and hydrogen sensors.15-23 Besides these applications, * To whom correspondence should be addressed. E-mail: ylei@ engr.uconn.edu.
Pd nanomaterials have also been widely applied in many other areas, such as biosensors for the detection of glucose, uric acid, dopamine, and ascorbic acid.24-26 Generally, methods such as electroless plating, electrochemical plating, and chemical vapor deposition are commonly used in the deposition of Pd and other metals. Compared with chemical vapor deposition and electrochemical plating, electroless plating of metals can provide dense coating on both conducting and nonconducting surfaces, requires relatively simple equipment, and adapts a flexible procedure; therefore, it has been widely used to deposit either continuous metal films or metal nanoparticles on different substrate surfaces.21-23,27-30 Herein, we report the fabrication of Pd/TiO2 nanofibers by the electroless plating method for the first time with controllable size and surface morphology. An electrospun TiO2 nanofibrous membrane was used as the plating template. Electroless plating parameters were investigated for the effect on the morphology and integrity of the deposited Pd layer. Moreover, with different plating times, both continuous Pd-layer coated and dense Pd nanoparticle decorated nanofibers have been obtained. Field emission scanning electron microscopy (FESEM), energydispersive X-ray (EDX), X-ray diffraction (XRD), and transmission electron microscopy (TEM) were applied to characterize the as-prepared samples. We further demonstrated the hydrogen sensing ability of the Pd/TiO2 nanofibrous membranes. The asfabricated sensors exhibited good sensitivity, fast response, and excellent recovery upon hydrogen “on and off” within several seconds. This study provides a simple strategy to fabricate metal nanofibrous membranes, which is promising in various applications. 2. Experimental Section 2.1. Preparation of TiO2 Nanofiber Templates. TiO2 nanofibers were prepared by following a similar procedure described elsewhere.11 Briefly, a 1 mL solution containing 0.1 g of titanium tetraisopropoxide (Sigma-Aldrich), 0.2 mL of acetic acid, 0.03 g
10.1021/jp9052727 CCC: $40.75 2009 American Chemical Society Published on Web 08/24/2009
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TABLE 1: Compositions of the Sensitization and Activation Solutions and Electroless Plating Bath sensitization and activation solution SnCl2 (g/L) HCl (37 wt %, ml/L) PdCl2 (g/L) HCl (37 wt %, ml/L)
1 2.5 0.25 2.5
5 2.5 0.25 2.5
electroless plating solution PdCl2 (mM) Na2EDTA (M) NH4OH (M) NH2NH2 (mM) volume (mL)
15 0.11 3.17 11 2
28.2 0.188 4.5 15 2
of poly(vinylpyrrolidone) (PVP) (Mw ≈ 1.3 × 106, Sigma-Aldrich), and 0.7 mL of ethanol was prepared and electrospun using a 23 gauge needle with a flow rate of 0.3 mL/h at an applied potential of 7 kV and a collection distance of 5 cm. The collected composite nanofibers were kept in the air overnight, followed by calcination in a muffle furnace at 500 °C for 3 h to remove PVP and generate a TiO2 nanofibrous membrane. 2.2. Electroless Plating of Palladium. To coat the TiO2 nanofibers with palladium, the TiO2 nanofibrous membrane was first sensitized and activated with SnCl2 and PdCl2 acidic solutions sequentially and the procedure was repeated three times. A redox reaction occurs in which surface-bound Sn2+ is oxidized to Sn4+ and Pd2+ is reduced to form Pd nuclei. After the sensitization/activation, the membrane was transferred to a Pd electroless plating bath for further deposition. With the addition of hydrazine to the electroless plating solution, the Pd precursor was reduced and deposited on the surface of the activated TiO2 nanofibers. Finally, the Pd-coated TiO2 nanofibrous membrane was rinsed with deionized water and dried in ambient atmosphere. The compositions of the sensitization/ activation solutions and palladium electroless plating solution are listed in Table 1. The effects of the Sn-Pd sensitization/ activation time, the concentration of SnCl2, the plating concentration of the palladium precursor, and the electroless plating time on the morphology and integrity of Pd layer were investigated. 2.3. Material Characterization. A JEOL 6335F field emission scanning electron microscope (FESEM) and energydispersive X-ray spectrometer (EDX) were employed to examine the morphology and the elements of the as-prepared nanofibers. The average diameter of the nanofibers and their standard deviation were calculated using ∼50 randomly chosen nanofibers in the SEM images. The thickness of the Pd layer was estimated by the difference between the average radius of nanofibers before and after Pd electroless plating. An X-ray diffraction (XRD) pattern was collected using Cu KR radiation on a diffractometer. A Tecnai T12 transmission electron microscope (TEM) was used for morphological imaging. The sample for TEM was prepared by ultrasonicating the Pd/TiO2 nanofibrous membrane in ethanol for several minutes and then dropping it on carbon-film-coated copper grids. 2.4. Hydrogen Sensing. The resistor-type H2 sensors were fabricated by placing two electrodes on the Pd/TiO2 nanofibrous membrane with a gap of ∼1 mm. The device was placed into a homemade 5 cm3 sealed glass chamber with gas inlet/outlet ports and connected to the CHI-601C electrochemical analyzer (CH Instruments Inc., Austin, TX). The circuit was subjected to a fixed voltage (0.1 V dc) at room temperature, and the current was continuously recorded. In all experiments, different concentrations of H2 were obtained by diluting the 5% H2 with dry
Figure 1. Typical SEM images of electrospun PVP composite nanofibers (A) and TiO2 nanofibers after calcination (B).
air through a computer-controlled gas mixing system (S-4000, Environics Inc., Tolland, CT) and the total flow rate was set to be 1.7 L/min. In a typical experiment, the sensor was first exposed to dry air to obtain the baseline, then switched to a certain concentration of H2 for 200 s, followed by dry air for 200 s to recover the sensor response, which is defined as one cycle. For each H2 concentration, three cycles were repeated. The electric resistance of the sensor is calculated by applying Ohm’s law (R ) V/I). 3. Results and Discussion Typical SEM images of the as-electrospun nanofibers before and after calcination are presented in Figure 1A,B. One can see that the electrospun PVP composite nanofibers had an average diameter of 177.4 nm, whereas the TiO2 nanofibers obtained after calcination shrunk to 117.2 nm. In addition, both nanofibers are relatively uniform and have small size variation. It is well-known that TiO2 has very good chemical and thermal stability. Therefore, it could serve as a good template in electroless plating of Pd. After sensitization and activation, the TiO2 nanofibrous membrane changed from white to brown, indicating the formation of Pd nuclei on the surface of TiO2 nanofibers. The activated TiO2 nanofibrous membrane was then carefully transferred to Pd electroless plating solution. After the addition of the reducing agent, hydrazine, the membrane turned gray in a few minutes at room temperature due to the palladium deposition through the following chemical reaction. 2Pd(NH3)2+ + N2H4 + 4OH- f 2Pd0 + 2NH3 + N2 + 4H2O
(1)
Systematic experiments were conducted to investigate the effect of process parameters (e.g., the Sn-Pd sensitization/ activation time, the SnCl2 concentration, the concentration of palladium precursor, and the electroless plating time) on the morphology and integrity of the Pd layer. The morphologies of nanofibers after Pd electroless plating under various conditions are shown in Figure 2, and the corresponding experimental conditions and analysis results are listed in Table 2. It is wellknown that Pd grows from nuclei during the electroless plating process, so two concentrations of sensitization solution (1 g/L SnCl2 vs 5 g/L SnCl2) were first evaluated. As shown in Figure 2A,B and Table 2, A and B, with higher concentration of SnCl2 (5 g/L), the deposited Pd layer was much thicker even though the plating time was shorter; meanwhile, the diameter deviation of the nanofibers was much bigger than that of the Pd/TiO2 nanofibers using a lower concentration of SnCl2 (1 g/L). In addition, it is observed that microspherical Pd aggregates were formed on the surface of the nanofibers when 5 g/L SnCl2 was used in the sensitization step. Therefore, the application of 1
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Figure 2. Typical SEM images of Pd/TiO2 nanofibers synthesized with different electroless plating conditions. The scale bar is 1 µm.
g/L of SnCl2 in the sensitization step seems to favor the preparation of relatively uniform and smooth Pd/TiO2 nanofibers and it was used for subsequent experiments. To study the effect of sensitization/activation time, various sensitization/activation times (5, 10, and 15 min) were examined. It is observed in Figure 2A,C,F and Table 2, A, C, and F, that the larger diameter of Pd/TiO2 nanofibers accompanies the longer sensitization/ activation time, whereas the size deviation decreases with the increase of activation time, which suggests that, with longer sensitization/activation time, more Sn2+ could bind to the TiO2 nanofiber surface and more Pd nuclei are formed, resulting in a faster electroless plating rate and more uniform growth of Pd. Furthermore, by carefully comparing the morphology of Pd/ TiO2 nanofibers under different sensitization/activation times (Figure 2A,C,F), it is revealed that some Pd aggregates were present on the samples using a 5 or 15 min sensitization/ activation, whereas such aggregates were not observed for the sample with 10 min of sensitization/activation, indicating that 10 min of sensitization/activation gives the best morphology among the tested conditions. In addition, by comparing the samples prepared under different electroless plating times (1 vs 2 h, Figure 2E,F and Table 2, E and F), one can see that, with the increase of deposition time from 1 to 2 h, the thickness of the Pd layer was increasing almost 2-fold from 109.8 to 198.9 nm, indicating that the Pd was gradually deposited on the surface during the 2 h period without depleting the Pd precursor. Furthermore, higher concentrations of Pd precursor (28.2 vs 15 mM) led to a thicker Pd layer (217.5 vs 176.4 nm) with a bigger
Jia et al. size deviation (Figure 2C,D and Table 2, C and D), suggesting that a lower Pd precursor quantity would be favorable in preparing a thinner Pd layer without sacrificing the uniformity and surface smoothness. Under all examined conditions, the optimum electroless plating condition in terms of the morphology and the integrity of the Pd layer was determined to be 10 min of sensitization/activation with 1 g/L SnCl2 in the sensitization solution and 0.25 g/L PdCl2 in the activation solution, 15 mM PdCl2 in the electroless plating bath, and a 2 h deposition time at room temperature. The Pd/TiO2 nanofibrous membrane obtained under such conditions was used in the following characterization and H2 sensing. The successful coating of Pd on TiO2 nanofibers was further confirmed by EDX analysis. As shown in Figure 3A, only the Pd element was detected, which can be attributed to the fact that the surface of TiO2 was fully covered by a palladium layer. Figure 3B shows the XRD pattern of the as-prepared sample, which could be assigned to the Pd face-centered cubic (fcc) crystalline structure based on JCPDS card no. 46-1043. The TiO2 nanofibrous membrane is very fragile. However, the Pd deposition on TiO2 nanofibers enhances the mechanical property of the as-prepared Pd/TiO2 nanofibrous membrane. The Pd/TiO2 nanofibrous membrane possesses an ultrathin layer of Pd, is free-standing and highly porous, and can be easily handled with tweezers, offering a number of advantages for various applications ranging from sensing to fuel cells and catalytic reactions. Because of the renowned hydrogen adsorption property of palladium, the as-prepared Pd/TiO2 membrane was then used to fabricate a resistor-type hydrogen sensor as an applicable demonstration. The Pd/TiO2 nanofibrous membrane was highly conductive, and the hydrogen sensing response has been monitored for hydrogen concentration in the range of 0.3 to 2%. Figure 4A shows a typical time-dependent resistance change with different concentrations of H2 (0.3, 0.5, 1, 1.5, and 2%) in the air. Exposure of the sensor to hydrogen shows a prompt increase of the sensor resistance and reaches a steady state in a few seconds. When Pd is exposed to H2, hydrogen molecules dissociate into hydrogen atoms that react with Pd to form palladium hydride (PdHx). The resistivity of PdHx is slightly higher than that of pure Pd, resulting in resistance increase. When the hydrogen in the carrying air is removed, the sensor resistance immediately returns to its initial value due to the dissociation of hydrogen from PdHx, indicating fast recovery and good reversibility of the sensor response. For each H2 concentration, three on-off cycles were performed and the responses were stable and reproducible. Sensitivity and response time are two important parameters to evaluate the performance of hydrogen sensing devices.16,22 Sensitivity was defined as the percentage of resistance change, (∆R/R0)% ) [(Rm - R0)/ R0] × 100%, where R0 is the initial electrical resistance of the sensor in air and Rm is the maximum resistance upon exposure to H2. Figure 4B shows the calibration curve of sensitivity versus H2 concentration, which exhibits the good linearity in the examined H2 concentration range. It is clearly shown that higher concentration of H2 results in higher response and the sensitivity of 5.43 per 1% H2 was achieved. The linear relationship between resistance change and H2 concentration has been reported in the literature and can be explained as follows. Based on Langmuir isotherm theory and as a good approximation, the resistance increase upon hydrogen exposure is proportional to the surface coverage of hydrogen, which is further proportional to the hydrogen partial pressure (or concentration) when the hydrogen partial pressure is low.22 Response time is defined here as the time required for the sensor to reach 90% of the
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TABLE 2: Electroless Plating Conditions and Corresponding Results
A B C D E F
SnCl2 concentration (g/L)
sensitization and activation time (min)
PdCl2 plating concentration (mM)
plating time
average diameter (nm)
standard deviation (nm)
thickness of Pd layer (nm)
1 5 1 1 1 1
5 5 10 10 15 15
15 15 15 28.2 15 15
2h 45 min 2h 2h 1h 2h
419.3 542.3 469.9 552.1 336.7 515
78.5 149.7 70.2 116.4 53.6 58.1
151.1 212.5 176.4 217.5 109.8 198.9
maximum resistance change after the sensor is exposed to hydrogen. For all hydrogen concentrations tested, the response of the Pd/TiO2 nanofibrous membrane to hydrogen concentration
Figure 3. EDX analysis (A) and XRD patterns (B) for the Pd/TiO2 nanofibrous membrane with 2 h of electroless plating.
Figure 4. (A) Typical response of the device to periodic exposure of different hydrogen concentrations at an applied dc bias of 0.1 V and room temperature. The device was prepared using the Pd/TiO2 nanofibrous membrane with 2 h of electroless plating. (B) Corresponding calibration curve plotted with sensitivity (∆R/R0 × 100%) vs H2 concentration.
change was obtained within several seconds and then the equilibrium was reached. The fast and stable response was possibly benefited from the thin Pd layer (176.4 nm) and the nanofibrous structure of the membrane that provides less diffusion and transportation resistance and high specific surface area for H2 dissociation. It has been well-understood that, upon exposure to high concentration of H2, the formation of PdHx will cause the increase of the size/volume of Pd. It has also been reported that, at a H2 concentration above 1%, a dense Pd 2-D film based H2 sensor may malfunction due to considerable stress in the film or stress mismatch at the film/substrate interface caused by the palladium lattice or volume expansion.17 However, such a phenomenon was not observed in our experiments. The developed Pd/TiO2 nanofibrous membrane can detect a high concentration of H2 without any failure observed. The higher surface area of the nanofibers and highly porous structure of the nanofibrous membrane may help to minimize such stress caused by the volume expansion of PdHx and thus remarkably enhance the stability of the Pd/TiO2 nanofibrous membrane upon the absorption of high concentration of hydrogen. As the TiO2 nanofibers are fully covered by the Pd layer, the sensing mechanism is mainly attributed to resistance increase of the continuous Pd nanofibrous membrane due to the formation of palladium hydride. Various Pd-based hydrogen sensors have been reported. On the basis of the sensing mechanism, these Pd-based hydrogen sensors can be classified into two groups: the resistance increase upon exposure to H2 due to the formation of PdHx and the resistance decrease of the discontinuous Pd layer due to the “break junctions” of the PdHx-induced lattice expansion and the appearance of new conductive pathways.31,32 As mentioned above, Pd grows from nuclei during the electroless plating process. Under sufficient electroless plating time (e.g., 2 h), the nanofibrous template is fully covered by the Pd layer and our hydrogen sensor discussed above falls into the first category. However, with shorter electroless plating time, the assembly of Pd nanoparticles instead of a uniform Pd layer on TiO2 nanofiber could be expected, which may provide a different H2 sensing mechanism. To verify this hypothesis, electroless plating was designed with shorter deposition time. Simply, the TiO2 nanofibrous template was taken out from the plating solution at different times (10, 15, and 20 min), and the deposited Pd was assessed with SEM. Figure 5A shows the TiO2 nanofibers with 10 min of Pd deposition, and clearly reveals that the deposited Pd consists of a lot of Pd nanoparticles sitting on TiO2 nanofibers next to each other. TEM was further used to characterize the sample and verify the particle configuration of Pd (Figure 5B). It is necessary to point out that ultrasonication was applied to prepare the TEM sample and some of the Pd nanoparticles detached from the nanofibers so that the Pd nanoparticles were less dense than those shown in the SEM image. The TEM image also supports that the Pd is present mostly like individual nanoparticles or aggregates attached to the surface of nanofibers. When the deposition time was
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Figure 5. (A, B) A typical SEM image and TEM image of the sample prepared with 10 min of electroless plating, respectively. (C, D) Typical SEM images of the sample prepared with 15 and 20 min of electroless plating, respectively.
increased from 10 to 20 min, the deposited Pd was relatively uniform and a continuous Pd layer was observed on the top layer of the template (Figure 5D). However, not surprisingly, Pd nanoparticle decorated nanofibers in deeper layers were also observed. In addition, based on the conductivity measurement, it is found that the Pd/TiO2 nanofibrous membrane became conductive after 10 min of Pd plating, and the resistance decreased with the increase of electroless plating time. As a compromise, the Pd/TiO2 nanofibrous membrane with 15 min of Pd deposition was then prepared (Figure 5C) and used to fabricate another device for H2 sensing. The response and calibration curve are presented in Figure 6. One can see that both the decrease and the increase of resistance are observed upon the exposure to low and high concentration of hydrogen, respectively. Exposure to a lower concentration of H2 (e0.5%) caused a rapid resistance decrease, as expected, which appears to involve closing gaps between Pd nanoparticles. The closed nanogaps reverted to open when purging with the carrying air, and the response showed good reversibility. Surprisingly, the response to 0.3% H2 is higher than that to 0.5% H2. This result indicates that the sensor response of this device is controlled by the trade-off between the break-junction-induced resistance decrease and PdHx-formation-induced resistance increase. When 1% and higher percentages of H2 were supplied, palladium adsorbed more H2 and the PdHx-formation-induced resistance increase was dominating in the response. Especially for 1% H2 sensing, the resistance initially increased sharply and then decreased gradually after a while and finally reached the steady state, which implies that the palladium hydride was formed quickly and increased the resistance of the existing electron conducting pathways first. With further increase of hydrogen exposure time, the expansion of the Pd nanoparticles may close some gaps and form some new conductive pathways, resulting in resistance decrease accordingly. Finally, the whole process reaches steady state and the overall resistance is stabilized. The domination regions of the PdHx-induced resistance increase and the PdHx-induced break junction can be clearly observed in the calibration curve (Figure 6B), where the H2 concentration is
Jia et al.
Figure 6. (A) Typical response of the device to periodic exposure of different hydrogen concentrations at an applied dc bias of 0.1 V and room temperature. The device was prepared using the Pd/TiO2 nanofibrous membrane with 15 min of electroless plating. (B) Corresponding calibration curve plotted with sensitivity (∆R/R0 × 100%) vs H2 concentration.
low, the sensitivity is a negative value, and the effect of the break junction is significant, whereas at high H2 concentration, the resistance increases because the PdHx-formation-induced resistance increase is becoming the dominating factor. By combining different sensor mechanisms into one device and adjusting the electrolesss plating time, different sensitivity and detection ranges can be facilely achieved. Thus, it provides an excellent sensor platform with superior variability. The result demonstrates that the sensor performance can be fine-tuned to provide unique information, thus satisfying various applications. 4. Conclusion In summary, a Pd-coated TiO2 nanofibrous membrane has been facilely synthesized by electroless plating of Pd on a TiO2 nanofibrous template. Controllable Pd loading can be obtained by simply adjusting palladium electroless depositing factors, such as sensitizer concentration, sensitization/activation time, the amount of Pd precursor, and the plating time. Specifically, a nanofibrous membrane with a continuous Pd layer (like a coresheath structure) or aggregated Pd nanoparticles was achieved with different deposition times. The prepared Pd/TiO2 nanofibrous membrane showed good mechanical and electrical properties and provides an excellent material for hydrogen sensing. The huge surface and porous structure of the nanofibrous membrane enable fast response and recovery for hydrogen detection. Upon the exposure to hydrogen, the nanofibrous membrane with a continuous Pd layer exhibited resistanceincreasing behavior and its normalized resistance change (sensitivity) shows good linearity with H2 concentrations in the detection range. With a shorter deposition time, a lot of Pd nanoparticles instead of a continuous Pd layer are present on the nanofiber surface, which showed unique response to different hydrogen concentrations, possibly caused by the competition of two processes: the H2-induced increase of resistance due to
Pd/TiO2 Nanofibrous Membranes the formation of less conductive PdHx and the break-junctioninduced decrease of resistance due to the formation of new conducting pathways. This study provides a promising route for the facile and cost-effective synthesis of metallic nanofibers, and the as-synthesized metallic nanofibers show great promise in the applications of sensory devices, catalysis, and other fields. Acknowledgment. We greatly appreciate the funding from NSF and DHS. Points of view in this document are those of the author(s) and do not necessarily represent the official position of the funding agencies. L.S. and Y.W. also are thankful for the partial support from the UConn Center for Environmental Science and Engineering. References and Notes (1) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670. (2) Sill, T. J.; von Recum, H. A. Biomaterials 2008, 29, 1989. (3) Barnes, C. P.; Sell, S. A.; Boland, E. D.; Simpson, D. G.; Bowlin, G. L. AdV. Drug DeliVery ReV. 2007, 59, 1413. (4) Ding, B.; Wang, M. R.; Yu, J. Y.; Sun, G. Sensors 2009, 9, 1609. (5) Yoon, J.; Jung, Y. S.; Kim, J. M. AdV. Funct. Mater. 2009, 19, 209. (6) Formo, E.; Lee, E.; Campbell, D.; Xia, Y. N. Nano Lett. 2008, 8, 668. (7) Yoon, K.; Hsiao, B. S.; Chu, B. J. Mater. Chem. 2008, 18, 5326. (8) Wu, H.; Lin, D.; Zhang, R.; Pan, W. J. Am. Ceram. Soc. 2008, 91, 656. (9) Behler, K. D.; Stravato, A.; Mochalin, V.; Korneva, G.; Yushin, G.; Gogotsi, Y. ACS Nano 2009, 3, 363. (10) Jin, M.; Zhang, X. T.; Nishimoto, S.; Liu, Z. Y.; Tryk, D. A.; Murakami, T.; Fujishima, A. Nanotechnology 2007, 18, 075605. (11) Li, D.; Xia, Y. N. Nano Lett. 2003, 3, 555. (12) Wu, H.; Sun, Y.; Lin, D. D.; Zhong, R.; Zhang, C.; Pan, W. AdV. Mater. 2009, 21, 227.
J. Phys. Chem. C, Vol. 113, No. 37, 2009 16407 (13) Lai, C.; Guo, Q. H.; Wu, X. F.; Reneker, D. H.; Hou, H. Nanotechnology 2008, 19, 195303. (14) Dong, H.; Wang, D.; Sun, G.; Hinestroza, J. P. Chem. Mater. 2008, 20, 6627. (15) Kumar, M. K.; Ramaprabhu, S. Int. J. Hydrogen Energy 2007, 32, 2518. (16) Sun, Y. G.; Wang, H. H. AdV. Mater. 2007, 19, 2818. (17) Ding, D. Y.; Chen, Z. AdV. Mater. 2007, 19, 1996. (18) Jeon, K. J.; Lee, J. M.; Lee, E.; Lee, W. Nanotechnology 2009, 20, 135502. (19) Hu, X. J.; Huang, Y.; Shu, S. L.; Fan, Y. Q.; Xu, N. P. J. Power Sources 2008, 181, 135. (20) Nair, B. K. R.; Harold, M. P. J. Membr. Sci. 2007, 290, 182. (21) Nair, B. K. R.; Choi, J.; Harold, M. P. J. Membr. Sci. 2007, 288, 67. (22) Yu, S. F.; Welp, U.; Hua, L. Z.; Rydh, A.; Kwok, W. K.; Wang, H. H. Chem. Mater. 2005, 17, 3445. (23) Rahimi, F.; Zad, A. I. J. Phys. D: Appl. Phys. 2007, 40, 7201. (24) Lu, J.; Do, I.; Drzal, L. T.; Worden, R. M.; Lee, I. ACS Nano 2008, 2, 1825. (25) Claussen, J. C.; Franklin, A. D.; ul Haque, A.; Porterfield, D. M.; Fisher, T. S. ACS Nano 2009, 3, 37. (26) Huang, J. S.; Liu, Y.; Hou, H. Q.; You, T. Y. Biosens. Bioelectron. 2008, 24, 632. (27) Yeung, K. L.; Christiansen, S. C.; Varma, A. J. Membr. Sci. 1999, 159, 107. (28) Byeon, J. H.; Hwang, J. Surf. Coat. Technol. 2008, 203, 357. (29) Tao, D.; Wei, Q. F.; Cai, Y. B.; Xu, Q. X.; Sun, L. Y. J. Coat. Technol. Res. 2008, 5, 399. (30) Rohan, J. F.; Casey, D. P.; Ahern, B. M.; Rhen, F. M. F.; Roy, S.; Fleming, D.; Lawrence, S. E. Electrochem. Commun. 2008, 10, 1419. (31) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (32) Khanuja, M.; Kala, S.; Mehta, B. R.; Kruis, F. E. Nanotechnology 2009, 20, 015502.
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