pubs.acs.org/Langmuir © 2009 American Chemical Society
Polymer Gel Templating of Free-Standing Inorganic Monoliths for Photocatalysis Xiaojuan Fan,†,‡ Honghan Fei,† David H. Demaree,† Daniel P. Brennan,† Jessica M. St. John,† and Scott R. J. Oliver†,* † Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, and ‡Department of Physics and Physical Science, Marshall University, Huntington, West Virginia 25755
Received September 10, 2008. Revised Manuscript Received February 14, 2009 We have developed a simple, low-cost process to fabricate free-standing porous metal oxide monoliths. Various swollen polymers and hydrogels possessing an open network structure are infiltrated with pure liquid metal alkoxide. Hydrolysis followed by chemical or thermal degradation of the polymer leads to bulk porous monoliths, TiO2 and SiO2 as initial examples. The titania solids were subsequently employed as photocatalysts under UV light and monitored for adsorption. The materials show efficient reusable photocatalytic ability as compared to pure-phase nanoparticle titanium oxide.
Introduction There is ever-increasing interest in the development of porous inorganic metal oxide membranes or particles. Such materials are used as molecular sieves,1 photocatalysts,2 gas filters,3 dyesensitized solar cells,4 hydrogen storage medium,5 and water purifiers6,7 due to their remarkable chemical stability and mechanical reliability.8-11 One well-known photocatalytic material is TiO2, a wide band gap semiconductor extensively studied for the degradation of environmental pollutants in wastewater and air upon UV light irradiation. One common form of titania for photocatalytic activity is nanoparticle powder distributed in solution. Nanoparticles provide high surface area and accelerate reactions that occur at the catalyst surface. The suspended powder form, however, makes recovery after the reaction difficult. In contrast to common suspended nanoparticle powders, porous structures can improve catalytic efficiency due to the internal high surface area and greater degree of interaction with the reactants. Interior pores and channels also serve as individual nanoscale reactors so that reactions take place in a confined space.12 TiO2 porous membranes have been prepared based on sol-gel methods with the assistance of polymer templates. Thermal removal of the polymer yields porous membranes *Corresponding author.
[email protected]. (1) Sun, T.; Ying, J. Y. Nature (London) 1997, 389, 704–706. (2) Hu, X. F.; Kuai, S. L.; Yu, Y.; Troung, V. V. J. Inorg. Mater. 2005, 20, 1463– 1466. (3) Alvin, M. A.; Lippert, T. E.; Lane, J. E. Am. Ceram. Soc. Bull. 1991, 70, 1491–1498. (4) Oregan, B. O.; Gratzel, M. Nature (London) 1991, 353, 737–740. (5) Seayad, A. M.; Antonelli, D. M. Adv. Mater. 2004, 16, 765–777. (6) Gracia, F.; Holgado, J. P.; Gonzalez-Elipe, A. R. Langmuir 2004, 20, 1688– 1697. (7) Yamashita, H.; Harada, M.; Tanii, A.; Honda, M.; Takeuchi, M.; Ichihashi, Y.; Anpo, M.; Iwamoto, N.; Itoh, N.; Hirao, T. Catal. Today 2000, 63, 63–69. (8) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127–1129. (9) Rowsell, J. L. C.; Millward, A. R.; Park, K. S.; Yaghi, O. M. J. Am. Chem. Soc. 2004, 126, 5666–5667. (10) Panella, B.; Hirscher, M. Adv. Mater. 2005, 17, 538-+. (11) Tran, D. T.; Zavalij, P. Y.; Oliver, S. R. J. J. Am. Chem. Soc. 2002, 124, 3966–3969. (12) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature (London) 2001, 412, 169–172.
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with pore size ranging from 1.5 nm to tens of nanometers.12 Such membranes are sintered at 400-500 °C in order to obtain crystalline TiO2, often shrinking to a condensed and/or cracked structure. A variety of other synthetic strategies have been reported to prepare bulk porous structures with enhanced surface area compared to condensed nanoparticles. For example, bulk porous TiO2 solids with controlled pore volume and size were prepared by a solvothermal hot-press technique.13 A robust, porous titania with inherently large surface area would be ideal for photocatalytic applications. Polymer swelling is a well-known phenomenon. Polymer gels or membranes with network channels have long been used as supports for organic, biomolecular, and macromolecular separations.14-17 A common polymer gel with fine network structure is agarose, commonly used as separation medium in the electrophoresis of DNA fragments. The open network structure is an ideal template for the formation of inorganic metal oxide porous structures. Recently, a simple and very cost-effective method was developed using agarose gel as template.18,19 Scanning electron microscopy (SEM) reveals the highly uniform agarose microstructure with pore size of approximately 100 nm.18 A number of porous metal oxides derived from agarose gels were reported such as TiO2, SiO2, ZrO2, SnO2, and Nb2O5.19 The general procedure is to construct agarose gel sections and place into nonaqueous solvent followed by metal alkoxide precursor. Metal alkoxide infiltrates the channels within the swollen agarose gel. Thermal treatment removes the polymer while crystallizing the metal oxide. It is presumed that the metal oxide porous structure forms during the heating and only part of polymer remains in the monolith. Pore size and volume (13) Bosc, F.; Lacroix-Desmazes, P.; Ayral, A. J. Colloid Interface Sci. 2006, 304, 545–548. (14) Xu, H. Y.; Liu, X. L.; Li, M.; Chen, Z.; Cui, D. L.; Jiang, M. H.; Meng, X. P.; Yu, L. L.; Wang, C. J. Mater. Lett. 2005, 59, 1962–1966. (15) Guyot, A. React. Polym. 1989, 10, 113–129. (16) Blank, Z.; Reimschuessel, A. C. J. Mater. Sci. 1974, 9, 1815–1822. (17) Cole, K. D.; Gaigalas, A.; Akerman, B. Electrophoresis 2006, 27, 4396– 4407. (18) Brody Biotechniques 2005, 38, 60–60. (19) Huang, F. Z.; Zhou, M. F.; Cheng, Y. B.; Caruso, R. A. Chem. Mater. 2006, 18, 5835–5839.
Published on Web 3/31/2009
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Fan et al. Scheme 1. Overall steps for the preparation of inorganic monoliths from two different swollen polymer gels.
can be controlled by the agarose content of the hydrogel and gelling temperature. Direct thermal removal of agarose, however, may significantly shrink pore size and often cracks or fractures the product. An alternative method to degrade the polymer prior to thermal treatment may avoid these problems, retaining the shape and size of the monolith after heating. Another potentially useful swollen polymer template not studied previously is poly(acrylamide) (PAM), which preserves its threedimensional topology during drying with controllable pore and wall size.15 The common test for photocatalytic activity of TiO2 powder is to monitor the degradation rate of methylene blue (MB) under UV irradiation.20 The powder is suspended in an aqueous solution of MB and centrifuging is necessary to dilute the solution and allow quantitative determination by UV-vis spectroscopy. In contrast, porous, rigid solids are superior for photocatalysis and wastewater purification, since the solid remains at the bottom of the MB reaction solution, eliminating the need to centrifuge or filter. In addition, the larger surface area increases photochemical turnover as well as physical adsorption. In this paper, we report the fabrication of inorganic porous monoliths using swollen agarose as template, where the polymer is first degraded chemically prior to calcination. Titania was chosen for its potential photocatalytic activity and studied by the reaction of MB under (or in the absence of ) UV light.
Experimental Section Materials. All materials including poly(acrylamide) (PAM, Acros), tetraethylorthosilicate (Acros), agarose powder (LE for biochemistry, Acros), TBE buffer (Tris/Borate/EDTA, Fisher Scientific), absolute ethanol (100%, Aldrich), titanium n-butoxide (Alfa Aesar), titanium isopropoxide (Alfa Aesar), and dimethylsulfoxide (DMSO, Fisher Scientific) were used as purchased. Synthesis. Two types of agarose gels were prepared by adding either 0.3 or 0.4 g of agarose powder to 20 mL TBE buffer or deionized water and stirring at room temperature to give 1.5 wt % or 2.0 wt % aqueous solutions, respectively. The solution was then heated and gently stirred until the agarose beads dissolved at around 60 to 70 °C. The mixture was then cooled slowly to about 50 °C with stirring to remove bubbles, then poured into a 100 15 mm2 plastic Petri dish and allowed to cool to room temperature. The final semitransparent solid gel was cut into ca. 1 1 0.5 cm3 squares using a razor blade. The agarose square pieces were placed in a closed vial with 5 mL of absolute ethanol for 2 to 3 days at room temperature. During this time, the water in the gel solvent exchanged with the ethanol. The exchanged gel pieces were placed in a second closed vial containing 5 mL of neat Ti alkoxide precursor for 2 to 3 days. The gel pieces were then taken out of the precursor liquid and placed in a third closed vial with 5 mL of DMSO for 3 to 21 days. The final composite products were heated within 2 h of (20) Zhou, J. F.; Zhou, M. F.; Caruso, R. A. Langmuir 2006, 22, 3332–3336.
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removal to 450 or 500 °C at a rate of 10 °C/min and soaked at that temperature for 2 h. The overall steps are summarized in Scheme 1. PAM gel samples were synthesized in various cross-linking ratios according to literature methods.21 The hydrogel samples were then submersed in ethanol to exchange the water for ethanol. The ethanol-soaked gel samples were next placed in a container of tetraethylorthosilicate (TEOS) and sealed for 1 week to allow the TEOS to infiltrate the porous polymer network. Finally, the samples were removed from the TEOS bath and allowed to react with atmospheric moisture for at least 3 days, yielding a PAMSiO2 composite material. These samples were not treated with DMSO but instead heated in a tube furnace to 600 °C at a rate of 10 °C/min in air and soaked for 2 h. Characterization and Photocatalysis. The solid metal oxide structures were characterized by powder X-ray diffraction (PXRD, Miniflex Plus, Rigaku Americas), SEM (WB-6, International Scientific Instruments), and TEM (CM-12, Philips). TEM samples were prepared by stirring the monoliths in ethanol and then cast onto a carbon-coated copper grid. Photocatalysis experiments were conducted by monitoring absorption spectra of aqueous MB solution after UV irradiation. A single porous TiO2 monolith (typical weight 0.17 g) was placed into 3 mL of 0.026 mM MB aqueous solution in a quartz cuvette (1.25 1.25 4.5 cm3). A 6 W compact UV lamp (UVL-56, UVP Inc., intensity 1.35 mW/cm2 at 365 nm) was used as the light source, with the sample holder placed on its side to maximize exposure. A UV-vis spectrometer (model 8452A, Hewlett-Packard) was used to determine the concentration of MB solution versus UV exposure time by alternating the sample holder between spectrometer and UV lamp. Control experiments without UV lamp exposure were also conducted to determine the degree of adsorption. Adsorption experiments for simple TiO2 powder prepared directly by adding water to Ti isopropoxide were also conducted for comparison.
Results and Discussion The TiO2 products templated by agarose gels are free-standing white monolithic solids (Figure 1), retaining their shape after calcination. Figure 1 shows SEM images of a typical porous TiO2 monolith from 2 wt % agarose gel and Ti butoxide as precursor. The cross-sectional view after fracturing the monolith shows an outer shell with thickness of ∼7 μm (arrows, Figure 1a). All attempts to remove the shell led to the breakage of the monolith into very small fragments. The inner region displays a homogeneous porous structure of submicrometer size (Figure 1b). Ground samples were also examined by TEM, illustrating an average particle size of ∼25 nm (Figure 2). The condensed, continuous shell is likely due to the immediate hydrolysis of Ti butoxide (eq 1) by air moisture upon removal from the Ti precursor solution. Indeed, a coating could be observed on the outer walls of the gel prior to immersion in DMSO. The inner (21) Hsiung, T. L.; Wang, H. P.; Lin, H. P. J. Phys. Chem. Solids 2008, 69, 383– 385.
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Figure 1. A typical TiO2 monolith from 2 wt % agarose: (a) cross section shows the outer surface shell (thickness ∼7 μm, arrows) and inner porous morphology; (b) higher-magnification image reveals the homogeneity of the interior.
region likely arises from the gradual polymerization of the titania precursor in the undried, hygroscopic DMSO. This pretreatment disrupts the agarose cross-linkages,22 with the majority of the disconnected polymer remaining in the monolith prior to calcination (Figures S6 and S7, Supporting Information). The final product after the air thermolysis step shows only trace organic residue and water. TiðOC4 H9 Þ4 þ 2H2 OfTiO2 þ 4C4 H9 OH
ð1Þ
A related morphology was found for a lower 1.5 wt % agarose gel and Ti isopropoxide precursor (Figure 3). The product again exhibits a condensed outer shell (Figure 3a) and an interior porous morphology (Figure 3b). The lower weight ratio of the initial polymer gel likely leads to a less continuous agarose network and in turn a more condensed TiO2 network. Compared with the above structure from higher weight percent agarose gel, the morphology is less homogeneous and the pore features increase from the submicrometer scale to the micrometer scale. We obtained phase-pure anatase for all samples after calcination at 450 to 500 °C (Figure 4). Simply increasing calcination temperature to 700 °C resulted in pure rutile, also confirmed by PXRD. The formation of anatase TiO2 rather than rutile TiO2 is advantageous, since the former has far greater efficient photocatalytic activity, as will be discussed. The Scherrer equation estimates the crystal size of the TiO2 monoliths to be ∼23.6 nm and the rutile phase from higher temperature to be ∼50.4 nm. The mass changes throughout the monolith preparation procedure were monitored (Table 1). The mass reduced from 0.54 to 0.35 g after ethanol exchange, indicating that there is less solvent remaining in the polymer matrix. During the titanium butoxide soaking, a ∼10% volume shrinkage occurred along with a decrease in mass from 0.35 to 0.19 g. The butoxide groups are likely exchanged by the ethanol trapped in the agarose slab, and the resultant precursor infiltrates the gel. Following immersion in DMSO for 5 days, there is a slight decrease in the total weight from 0.19 to 0.15 g. Finally, annealing at 500 °C led to removal of agarose and the anatase TiO2 monolith. The materials produced using PAM gels were also free-standing monoliths, such as porous SiO2 grown in 4 wt % cross-linked PAM (Figure 5a). The corresponding magnified image for this sample is displayed in Figure 5b. Figure 6 presents the monolith morphology for higher cross-linking ratios of 6 and 8 wt % PAM samples. These SiO2 solids show less-ordered morphology than the titania samples from agarose. In all cases, an open structure (22) Kenawy, E. R. React. Funct. Polym. 1998, 36, 31–39.
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lacking long-range order is observed with relatively little change for the different cross-linking ratios. In addition, attempts to use transition metal alkoxides based on Ti and Zr yielded only very fine fibers following calcination, likely due to the more rapid hydrolysis known for these metals. Unlike the commercially available agarose gels that can dehydrate by exchanging water for ethanol over several days, we believe the starting PAM gel structure is not as well-defined. Attempts to improve morphology by fine-tuning the weight percent of the PAM gels and swelling time were not successful. Our TiO2 and SiO2 solids are formed employing swollen agarose and PAM templates. The main difference between our method and that of Caruso and co-worker previously published titania structure is the disruption of the agarose by DMSO prior to calcination. Our monoliths possess a very different morphology based on our DMSO treatment as well as longer duration of metal precursor soaking. Attemps to form silica in the agarose system were not successful. Consequently, neither polymer gel can template both silica and titania. We have, however, expanded the general method to a new polymer (PAM) while also overcoming the difficulty of templating silica structures. We therefore chose the agarose-grown TiO2 monoliths for further investigation of both photocatalytic activity and surface adsorption. Since our materials are free-standing, particle suspension is not required and UV-vis spectra therefore can be recorded by simply switching the sample holder between lamp and spectrometer. It was found that both catalysis and adsorption occur, inducing a dramatic reduction in the concentration of MB in solution. Figure 7 shows the change in MB absorbance vs UV irradiation time in the presence of the TiO2 monolith without stirring. The initial absorption (top curve) shows the concentration before UV light exposure. The sharp absorption peaks at 655 nm, 664 nm, and 672 nm disappeared by 80 min of exposure to UV light. After 160 min, ca. 77% of the MB was decomposed (gray curve, Figure 7). The concentration of MB was fitted to the following first order exponential decay eq 2: R ¼ A expð -T=t1 Þ þ R0
ð2Þ
where R is remaining concentration, T is exposure time, t1 is decay coefficient, and R0 and A are constants. From the calculation, A = 0.91, t1 = 87.4 min, and R0 = 0.11 (inset, Figure 7). The photocatalytic reaction is entirely reproducible using the same amount of sample (see Supporting Information). The efficiency is similar to that of typical TiO2 powder catalysts, but the latter required stirring and a more intense UV lamp.23 For example, commercial P25 TiO2 powder used as photocatalyst decomposes MB by ∼70% under intense UV lamp (40 W) exposure for 150 min with stirring.23 The bottom curve in Figure 7 is the spectrum after 18 h with neither UV irradiation nor stirring, reaching 87% decrease in MB concentration. The higher percentage is attributed in part to surface adsorption rather than only photocatalysis. A detailed study of the adsorption ability of the porous TiO2 solids (again grown from 1.5 wt % agarose) was therefore conducted by not exposing to UV light. 0.161 g of the TiO2 solid was placed in a test tube containing the MB solution and stirred in the absence of light. The solids became blue, whereas the solids did not change color for the above experiments under UV (in both cases, the solution became colorless by sight). These results imply that MB molecules are also adsorbed on the titania, reducing the MB concentration (further details are discussed in the Supporting Information). (23) Garcia, R. B.; Vidal, R. R. L.; Rinaudo, M. Poimeros 2000, 10, 155–161.
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Figure 2. TEM images of a ground TiO2 monolith prepared from 1.5 wt % agarose: (a) a single particle; (b) larger particles were also observed.
Figure 3. (a) Outer shell surface a TiO2 monolith from 1.5 wt % agarose gel and Ti isopropoxide. (b) Cross-sectional view showing a similar interior morphology.
Figure 5. (a) SEM image of a calcined silica monolith from 4 wt % PAM (scale bar: 0.75 mm); (b) magnified SEM image (scale bar: 3.0 μm).
Figure 6. SEM images of the final calcined product from (a) 6 wt % PAM (scale bar: 3.0 μm); (b) 8 wt % PAM (scale bar: 2.5 μm). Figure 4. PXRD patterns for all TiO2 monoliths after annealing at 500 °C for 2 to 4 h indicate phase-pure anatase. Table 1. Mass for a Typical Sample (1 1 0.5 cm3) Prepared in 1.5 wt % Agarose agarose slab
mass (g)
before ethanol exchange after ethanol exchange after titanium precursor soaking after DMSO soaking after annealing
0.54 0.35 0.19 0.15 0.07
For comparison, the adsorption ability of TiO2 powder was also tested. The anatase titania nanoparticles were prepared by simply allowing Ti isopropoxide to hydrolyze in air. Annealing the powders at 950 °C formed pure rutile-phase nanoparticles. 0.161 g of either anatase or rutile TiO2 powder were placed in MB solution overnight, again in the dark without UV irradiation. Both UV-vis absorption spectra are provided (Supporting 5838
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Figure 7. UV-vis absorption spectra of the MB solution in the presence of a TiO2 monolith catalyst and UV light irradiation (365 nm, 6 W, intensity 1.35 mW/cm2; inset: relative intensity of the 655 nm maximum vs time). Langmuir 2009, 25(10), 5835–5839
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Information). The concentration of MB only decreased 30% after 24 h of soaking with anatase TiO2 powder and 15% under the same conditions with rutile powder. These values are far less than both reaction with and adsorption by the porous TiO2 monoliths. These data indicate that the powder is likely only adsorbing on the surface of the powder nanoparticles. It was further observed that the TiO2 monoliths change from white-gray to deep blue after soaking in MB solution overnight. The colorless TiO2 powder becomes only pale white-blue, confirming that far greater adsorption occurs on the monoliths than on the powder. In addition, the blue stained monoliths can be decolorized by exposure to UV light for several hours, allowing reuse of the materials for further photocatalysis and dye removal.
Conclusions A simple method has been developed for preparing porous metal oxide catalysts, using polymer gels swollen in neat metal alkoxide, followed by chemical and thermal decomposition. Unlike the commercially available P25 TiO2 nanoparticles, our porous free-standing monoliths can easily be recovered from solution for reuse. The open morphologies also possess
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sufficient uniformity to allow reproducible use as photocatalysts. The photocatalytic ability of the porous TiO2 solids is similar to that of nanoparticle powder but can easily be recovered and reused. The materials possess a condensed outer shell and an interior structure with varying pore size, depending on the polymer gel and inorganic precursor. Further tuning of the properties should be possible by varying polymer gel, inorganic precursor, and growth conditions. Large area monoliths are possible since the size depends only on that of the original agarose gel, which can be cast into any desired size or shape. Acknowledgment. The authors wish to acknowledge Prof. Jin Zhang and Abe Wolcott at UC Santa Cruz for assistance with TEM and Prof. Yat Li at UC Santa Cruz for help with SEM measurements. Supporting Information Available: Additional UV-vis data as well as infrared spectra and thermogravimetic analyses. This material is available free of charge via the Internet at http://pubs.acs.org.
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