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2006, 110, 16819-16822 Published on Web 08/09/2006
Multifunctional, Catalytic Nanowire Membranes and the Membrane-Based 3D Devices Wenjun Dong, Andrew Cogbill, Tierui Zhang, Samrat Ghosh, and Z. Ryan Tian* Chemistry and Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701 ReceiVed: June 16, 2006; In Final Form: July 8, 2006
Making large-scale, multifunctional, paper-like, free-standing membranes (FSM) and the FSM-based 3D macroscopic devices purely from long inorganic functional nanowires is challenging in many nanomaterials systems. Here we report synthesis of long nanowires of the catalytic titanate for direct fabrications of the FSM and the FSM-based 3D devices that are among the first to show unusual potentials in photocatalysis, write-erase-rewrite, microfiltration, and controlled release.
Introduction Immense effort is devoted to the study of one-dimensional (1D) nanomaterials including nanowires and nanotubes due largely to the unique physical and chemical properties associated with the 1D structural confinements in nanoscale.1 Lately, oxide nanowires (e.g., manganese oxide,2 vanadium oxide,3 etc.) and carbon nanotubes4,5 have been organized into paper-like 2D freestanding membranes (FSM) for new applications. However, large scale fabrication of robust, thermal-stable, and multifunctional macroscopic 3D devices directly from the 1D nanomaterials has remained a challenge. We herein show a new hydrothermal synthesis of TiO2-based long nanowire catalysts that have been directly cast into thermalstable, robust, and multifunctional FSM-based 2D paper and 3D devices (e.g., tube, bowl, cup) in nearly any macroscopic size and shape. Making long 1D nanostructures and then organizing them properly have been found to be critical in casting robust 2D FSMs directly out of the 1D nanomaterials.2-5 Thusassembled nanowires have shown unusual potentials in applications such as catalytically splitting water and cracking oil, making protection mask and armor, fabricating flame-retardant fabric, filtering bacteria, photoassisted rewriting, sensing,6,7 controlled drug releases, and regenerating tissues. Experimental Section 1. Synthesizing Long Nanowires and Casting FSM and the FSM-Based 3D Device. We have developed a synthetic route for the solution synthesis of TiO2-based long nanowires above 160 °C for 1-7 days. This is because in general a shorter reaction time at lower temperatures would result in short nanotubes and therefore brittle FSM.8 In a typical synthesis, 0.30 g of TiO2 powder (Degussa P25) was introduced into 40 mL of 10 M alkali solution in a 150 mL Teflon-lined autoclave container. After the hydrothermal reaction in an oven for 7 days above 160 °C, a white pulp-like product of the long nanowires was collected, washed with distilled water or dilute acid, then cast on the macroscopic templates and/or molds made of either ashless filter-paper (Whatman) or polyethylene film, and then dried at room tem* Corresponding author. E-mail:
[email protected].
10.1021/jp0637633 CCC: $33.50
perature (RT). This casting-drying process was repeated for several times at RT, and followed thereafter by a heating at 40-100 °C in an oven for 1-20 h. The 2D membrane paper can be formed from the drying of the pulp-like slurry of the long nanofibers on the plastic plate. In likewise fabricating the 3D device, the templates and molds of polyethylene can be easily detached by hand, while those made of the ashless filter paper can be readily removed by either an open flame or a heating in a furnace at ∼500 °C in air. 2. Materials Characterization. In this work, scanning electron microscopy (SEM), energy-dispersive X-ray analyses (EDX), transmission electron microscopy (TEM), and X-ray diffraction (XRD) studies have been used to characterize the catalytic titanate nanowires (NWs) formed in the new hydrothermal syntheses. The SEM and EDX work was done on a Philips ESEM XL30 microscope. The XRD data were collected on a Philips X’Pert X-ray diffractometer. Our TEM study was carried out on a JEOL X-100 microscope. 3. Photocatalysis. Prior to the catalysis work, 10 mg of the FSM was soaked in 10 mL of 1 mol/L Mg(NO3)2 solution for 12 h at RT, then dried at RT, and then heated in air above 100 °C for 3 h. A UV lamp Entella (model B100 AP/R) was about 5 mm above the FSM. In the photocatalytic study, the solution concentrations were measured on a UV-visible spectrometer (HP 8453). Results and Discussion XRD patterns of the nanofibers confirm that the 1D nanowire samples resemble the titanate in lattice structure.9,10 Our XRD data have suggested that thus-formed nanowires should be the titanate phase [2θ ) 9.8° (001), 11.2° (200), 24.4° (110), and 29.7° (003), (JCPDS card No.: 47-0561)]. The titanate structure’s basic building unit is a TiO6-octahedron.11-14 The edgeshared [TiO6] octahedra would form a negatively charged layered structure. The countercations (e.g., Na+) sit in between the adjacent layers, thus resulting in variable interlayer distances depending on the size and the hydration degree of the cation, which would explain the flexibility of the long nanofibers.15,16 The FSM (Figure 1a) cast from a slow drying of the nanowire pulp can easily survive from multiple bends and folds (see the inset in Figure 1a), revealing the robust nature of the paper© 2006 American Chemical Society
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Figure 1. Robust 2D paper and 3D devices of nanowire. (a) An asformed FSM and a folded FSM paper (inset). (b) A FSM tube sitting in an FSM bowl that is next to an empty FSM cup.
like FSM formed typically by long and flexible fibers. A systematic study on varying the fabrication parameters has suggested that the FSM paper’s flexibility could be controlled by optimizing (a) the ratio of water to the nanowires in the pulp and (b) the time for drying the nanowire pulp. The preparation of such robust FSM has enabled us to directly cast the long nanowires, under the help of the 3D templates or molds, into macroscopic 3D devices such as tube, bowl, and cup (see Figure 1b). Such nanowire membrane devices, each weighing about 0.2-0.3 g and with a nearly 500 µm wall thickness, can be freely handled by hands and trimmed with scissors. To our knowledge, this is among the first attempts to cast at RT a pure inorganic nanofiber-based 3D ceramic device that can be cut by scissors. The successful casting of the FSM or 3D membrane devices would depend on the morphology and spatial organization of the nanofibers. The long nanofibers can self-organize into the robust FSM and 3D membrane devices while nanoparticles or short nanofibers cannot. Further, the controlled assemblies of the nanofibers can determine the robustness of the 3D membrane device. A multideck lateral structure can be clearly seen from the cross-section of the 3D device wall membrane (Figure 2a). The number of the decks is in line with the number of times the nanowire pulp has been introduced onto the templates for drying, which is consistent with the results from Figure 1a that the pulp drying is accompanied by the nanowire self-organization. The nanowire diameter ranges from 50 to 60 nm (see the inset in Figure 2a), which is in a good agreement with literature results that the hydrothermal synthesis at this temperature would generate nanowires9,10 rather than multiwalled nanotubes.8 Similar temperature effects on the structural transition from nanosheets to nanowires in the ZnO17 and MoO318 systems have been observed. The nanowires are typically longer than 0.1 mm (Figure 2b). The long nanowires have entangled into 3D voids (typically 1-10 µm at the opening, see the inset in Figure 2b) that could allow the membrane to expand and the nanowire to glide during
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Figure 2. SEM and TEM characterizations of the nanowire wall. (a) An SEM photograph from the cross-section of a FSM showing the multidecker texture, and a TEM picture (inset) demonstrating the average diameter of single nanowire about 50-60 nm. (b) A lowresolution FESEM photograph displaying many intertwined nanowires typically longer than 0.1 mm, and an inset showing the high-resolution FESEM photograph depicting the macropores of scaffolding nanowires.
heating or under mechanical stress, thus making the nanowire’s 2D and 3D assemblies to be robust and thermally stable. After the 3 h calcination period at 700 °C in air, the wall of the 3D objects still retains the characteristic multidecker structure (see Supporting Information, S1) of the entangled nanowires (see Supporting Information, S2). After the calcination at 800 °C, however, the wall membrane became brittle. This is because the original nanowire morphology has changed to one that is typically 20 µm long and 100-400 nm wide. Our XRD data suggest that the nanowire should be the TiO2-B phase (a ) 12.1787, b ) 3.7412, c ) 6.5249 Å; β ) 107.0548°) after the calcination at 700 °C (see Supporting Information, S3), and then a mixture of TiO2-B and anatase after the clacination at 800 °C. Both XRD patterns agree well with the results reported in the literature.19,20 Being different from other paperlike materials,2-5 this macroporous nanowire paper could be very useful in high-temperature catalysis. An immediate application of the white membrane of the organized long catalytic nanowires is to use the write-eraserewrite function under the help of the UV irradiation. TiO2 is commonly utilized as an inexpensive and nontoxic photocatalyst. After being excited by UV light, the TiO2 can catalyze dye degradation.22 On our FSM, four characters (“UARK”) of water-based ink (1.0 × 10-2 mol/L crystal violet) were exposed to the UV light in air. After 15 min, all four characters disappeared (Figure 3, a,b).21 This writing-erasing cycle has been repeated for four times on this FSM (21.4 mg), each showing the same result (Figure 3b). Four such times of the UV irradiation, on the other hand, caused little change to the same characters that were written on the regular printing paper (49.0 mg) in the first cycle. In addition, such inorganic nanowire paper can be potentially useful in many harsh environments below 700 °C. Yearly, 9.5 million hectares are deforested globally, and 35% of commercial wood is used for paper production.23 Therefore,
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Figure 3. Photoassisted write-erase on the nanowire wall membrane and regular printing paper. (a) 4th writing on a nanowire paper and the 1st writing on the printing paper using the ink of crystal violet. (b) Results from the 4th erasing by the UV irradiation.
use of such rewritable, erasable, and heat-resistant inorganic nanowire paper might help save the disappearing forests. The catalytic activity of TiO2-B has been demonstrated to be better than that of other TiO2 phases.24 Due to the macroporous nature, such robust nanowire-based membrane catalysts should have unique potentials for photocatalytic decompositions of organic pollutants25 such as nerve agent simulants (NAS)26 [e.g., (C2H5O)2P(O)(H2CSC6H5) from Aldrich]. Being different from the literature results,27,28 our data from a 15-min UV irradiation at RT on a piece of the nanowire-based wall membrane have shown that the NAS concentration (50 mL, ∼4.5×10-7 mol/L, originally) was reduced by 67.8% (Figure 4a). The spectroscopic measurements of the NAS concentrations were done on an HP 8453 UV-visible spectrometer. Without the catalyst, the NAS concentration decrease after the same UV irradiation was lower than the detection limit. Another blank test, using this wall membrane without the UV irradiation, caused the NAS concentration to decrease by about 1.0%, implying that nearly 66.8% of the NAS concentration drop (67.8% - 1.0%) should mainly attribute to the photocatalytic decomposition rather than the surface adsorption on the catalyst. In our TEM/SEM/XRD studies, no MgO nanoparticles could be seen on the nanofiber catalyst. The EDX study, however, shows that about 0.85 wt % of the Mg element exists in this catalyst. Both results have implied that the Mg species would likely be in a form of highly dispersed cluster(s), which encourages us to do the HRTEM work to identify the shape/ size and then the role of the Mg-containing particles in this photocatalysis in our future work. Parallel tests using the P25 and anatase TiO2 powder (325 mesh, Alfa Aesar) of the same weight resulted in the reduction of the NAS concentration by 35.0% and 8.0% (Figure 4a), respectively. The wall membrane is obviously far superior to the P25 and anatase powder in the NAS photodecomposition, suggesting that the nanowire membrane could be an exciting new photocatalyst. During the UV irradiation, the solution temperature increase was negligible. In heterogeneous catalysis, utilizations of such nanowire FSM catalysts could minimize (i)
J. Phys. Chem. B, Vol. 110, No. 34, 2006 16821
Figure 4. (a) Decreases of NSA concentration by the FSM wall with and without UV irradiation, and by the P25 and anatase TiO2 under the UV irradiation. (b) Controlled drug release within 4 days using the wall membrane.
the downstream separation and weight loss of catalysts, (ii) the use of catalytic supports and binders, and (iii) the cost due to the ease of recycling the catalyst via the calcination. Aqueous suspensions of polystyrene latex microspheres (Alfa Aesar, 0.75, 1, 2, and 2.5 µm in diameter, respectively), each in the concentration of 0.0025% (wt), have been used for investigating the permeability of the nanowire membrane.2 Each nanowire cup’s microfiltration was completed within 5 min. No 2-µm microspheres were detected in 1 mL of the filtered sample. Those of 0.75 µm and 1 µm, however, penetrated the wall membrane in the parallel tests, suggesting a size-exclusion function of the 3D devices in filtrations of micrometer-sized particles. Potentially, the membrane can be used for filtrations of bacteria spores in civil defense and environmental cleaning. The unique integration of the permeation and the photocatalysis of the membrane cup have been further demonstrated. In this work, one such cup pretreated in the abovementioned Mg(II) solution was filled with the NAS solution and then irradiated by a UV lamp (Entela, model B100 AP/R) from one side of the cup. After 15 min of the UV irradiation, 3.0 mL of the permeated-catalyzed solution was collected, with (32.0 ( 1.0) % of the NAS instantly decomposed. If a circular UV lamp could be applied around the cup, the concentration reduction could be comparable to that in the Figure 4a (FSM/UV). This result demonstrates an exciting application potential of the 3D devices in the industrial continuous flow-filtration-catalysis at different temperatures, where the reactant-catalyst contact time is limited. Macroporous 3D devices, walled by the scaffolding nanowires, are useful in controlled drug release.29 In our study, a section of the wall membrane (74.0 mg) was presoaked in a 100 mL solution of 0.001 mol/L crystal violet for 12 h at RT, and then placed in 10 mL of fresh water at RT. The controlled release was monitored on an HP 8453 UV-visible spectrometer. After every 24 h of the release, the FSM was transferred into another 10 mL of fresh water. As shown in Figure 4b, the controlled release reached a maximum at 24 h, and was effective for at least 4 days. Furthermore, it can be expected that the 3D scaffolds of the nanowires, after being coated with growth
16822 J. Phys. Chem. B, Vol. 110, No. 34, 2006 hormone, could be very useful in directing the growth of stem cells for potential applications in regenerative medicine.30,31 Acknowledgment. The authors thank X. Peng, B. Durham, Z.-L. Wang, S. Suib, J. Liu and H. Fan for helpful discussions. Help from X. Peng’s lab in the UV-vis work and that from Y. Chen and S. Goeke in the TEM imaging, and the support from the Arkansas Bioscience Institute are highly appreciated. Supporting Information Available: The SEM image (S1), the TEM image (S2), and the XRD pattern (S3). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (2) Yuan, J.; Laubernds, K.; Villegas, J.; Gomez, S.; Suib, S. L. AdV. Mater. 2004, 16, 1729. (3) Gu, G.; Schmid, M.; Chiu, P.-W.; Minett, A.; Fraysse, J.; Kim, G.-T.; Roth, S.; Kozlov, M.; Mun˜oz, E.; Baughman, R. H. Nature Mater. 2003, 2, 316. (4) Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Terrones, M.; Dresselhaus, M. S. Nature 2005, 433, 476. (5) Zhang, M.; Fang S.; Zakhidov A. A.; Lee S. B.; Aliev A. E.; Williams C. D.; Atkins K. R.; Baughman R. H. Science 2005, 309, 1215. (6) Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes C. A.; Ong, K. G. Nanotechnology 2006, 17, 398. (7) Varghese, O. K.; Yang, X.; Kendig, J.; Paulose, M.; Zeng, K.; Palmer, C.; Ong, K. G.; Grimes, C. A. Sens. Lett. 2006, 4, 120. (8) Tian, Z. R.; Voigt, J, A.; Liu, J.; Mckenzie, B.; Xu, H. J. Am. Chem. Soc. 2003, 125, 12384. (9) Armstrong, A. R.; Armstrong, G.; Canales, J.; Bruce, P. G. Angew. Chem., Int. Ed. 2004, 43, 2286.
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