CB como template na síntese de nanofios de TiO2

CB como template na síntese de nanofios de TiO2

Synthesis of mesoporous titania networks consisting of anatase nanowires by templating of bacterial cellulose membranes

Dayong Zhang and Limin Qi*

Received (in Cambridge, UK) 9th February 2005, Accepted 5th April 2005 First published as an Advance Article on the web 15th April 2005 DOI: 10.1039/b501933h

Mesoporous titania networks consisting of interconnected anatase nanowires have been synthesized by using unique bacterial cellulose membranes as natural biotemplates; the titania networks exhibit enhanced photocatalytic activity compared with titania microfiber networks.

Recently,thecontrolled synthesisof porous,crystalline titaniawith tailored pore structure and size has attracted much attention because of its potential applications in various areas including photovoltaics, photocatalysis, separation, chemical sensing, and optical devices. In particular, bio-inspired fabricationby templating or nanocasting procedures1 has been demonstrated to be a versatile route to either macroporous (pore size .50 nm) or mesoporous(pore size 2–50 nm) titania. A variety of templates, such as colloidalcrystals,2 emulsions,3 reverse micelles,4 foams,5 and polymer gels,6 membranes7 and beads,8 have been employed for the synthesis of macroporous crystallineTiO2 while ordered mesoporousTiO2 films with anatase nanocrystalliteshave been fabricated by using nonionic amphiphilic block copolymer templates.9 On the other hand, considerable attention has been paid to the shape-controlled synthesis of anisotropic titania nanorods or nanowires due to the unique properties of onedimensional (1D) nanostructures;10 moreover, interlinked structures of titania nanofibers have been prepared and considered as potential functional biocompounds for bone-tissue engineering.1 It is expected thatthe combinationof a mesoporous structure and an interlinking nanowire network would endow the titania material with unique properties and multiple functions. Therefore, it is worthwhile to explore the synthesis of mesoporous titania networks constructedby interconnectedcrystalline titania nanowires.

For the preparationof porous inorganicmaterialswith tailored structures, a rich variety of biological structures with complex morphologies have been used as sophisticated templates. Typical examples of natural biological templates include bacterial threads,12 echinoid skeletal plates,13 eggshell membranes,14 insect wings,15 pollen grains,16 plant leaves,17 and wood.18 Notably, hierarchical, nanotubular titania structures with nano-precision replicationfrom natural cellulose matrices such as filter paper, cloth, and cotton were prepared by a novel surface sol-gel process.19 Furthermore, hierarchical, crystalline TiC networks were fabricated by carbothermal reduction of titania-coated cellulose paper.20 It is noted that although the chain-like biopolymer cellulose is predominantly isolated from plants, bacterial cellulose (BC) is produced by bacteria, especially the acetic acid bacteria Acetobacter.Bacterial cellulose, which has recentlybeen studied for useas artificialskinand blood vesselsand as a substrate for tissue engineering of cartilage, is identical to plant cellulosewith respect to molecular structurebut it has an ultrafine nanofiber network structure and unique properties including high crystallinity, high water holding capacity, high tensile strength, and mouldability during formation.21 In this work, BC membranes were used as biological templates, for the first time, for synthesizingmesoporousTiO2 networks consisting of interconnected anatase nanowires. The obtained mesoporous

TiO2 networks exhibited enhanced photocatalytic activity com- paredw itht he TiO2 networkstemplatedby eggshell membranes.14 BC pellicleswerefirst prepared by staticcultureof Suzhousweet wine koji containing multiple species of acetic acid bacteria in 10 wt%glucose solutionat roomtemperaturefor 30 days.The BC pellicles (y5 m in thickness) formed at the air/liquid interface werewashedwithwater,cutintopieces(about2 cm 6 2c m),a nd purifiedin 0.2M NaOH solutionat 100 uC for 3 h. Afterwashing with water thoroughly, the BC pellicles were dehydrated and transferred into isopropanol via a gradual solvent exchange process14 and the resultantBC membranes (y1m m int hickness) were stored in isopropanol prior to use. For the sol-gel titania coating, the BC membranes were dipped into a closed vessel containing a solution of tetra-n-butyltitanium,acetylacetone and isopropanol with a volumeratioof 1 : 0.4 : 19 for 24 h. Then, the membranes were filtered under reduced pressure to remove solution held in the membranes and held in air at room temperature for 48 h to complete the hydrolysis reaction. Finally, the obtained BC–titaniahybrid membranes were heated at 500 uC in air for 6 h, resultingin the formation of crystalline titaniathin films (y0.2 m in thickness).

The samples were characterized by scanning electron microscopy (SEM,F EI STRATAD B235,1 0k V),t ransmissione lectron microscopy (TEM, JEOL JEM 200CX, 160 kV), high-resolution TEM (HRTEM, FEI TECNAI F30, 300 kV), X-ray powder diffraction (XRD, Rigaku Dmax-2000, Cu Ka) and nitrogen adsorption–desorptionmeasurements(Micromeritics ASAP2010). The photocatalytic activity of the obtained porous titania films was characterized by measuring the titania-assisted photodegradation of the xanthene dye Rhodamine B,2 with the eggshell membrane-templatedtitania filmse mployeda sac ontrols ample for comparison.The titaniafilms were broken into smallerpieces (y100 m) and suspensions containing the titania photocatalysts (100 mg L21) and Rhodamine B (y1025 M) were placedin the dark for 30 min beforeilluminationto allowsufficientadsorption of Rhodamine B. The stirredsuspensions were illuminated with a 250 W high-pressure mercury lamp 25 cm high over the solution*liminqi@pku.edu.cn

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This journal is The Royal Society of Chemistry 2005 Chem. Commun., 2005, 2735–2737 | 2735 andthechangeofthe RhodamineB concentrationwithirradiation time was monitored by measuring the UV-vis absorption of the suspensions centrifuged to remove titaniaat differentperiods.

The SEM image shownin Fig. 1a gives an overview of the BC membranetemplate, whichsuggeststhattheBC membranehasan ultrafine network structurecomprisinginterwovennanofiberswith diameters less than 100 nm, which is much smaller than the diameters of typical plant cellulose bundles (y10 m). An enlarged image presented in Fig. 1b shows that the nanofibers generallyadopt a ribbon-like morphology with widths 40–100 nm and thickness y10 nm. It is noted that the high-magnification imageis not veryclear due to the poor conductivityof the organic substance; however, the image quality can be largely improved after the BC membranes are coated with titania. As shown in Fig. 1c, a typicalSEM imageof theBC–titaniahybrid membranes clearly exhibits interconnected ribbon-likenanofibers that faithfully replicate the morphology of the original BC template. Thermogravimetric analysis (TGA) of the BC–titania hybrid membranessuggested that the BC template started pyrolyzing at y280 uC and was completely pyrolyzed by 500 uC, leaving9 wt% inorganicsolid. Aftercalcination at 500 uC to remove thetemplate, porous titania networks comprising interwoven nanowires typically 20–30 nm in diameter were obtained (Fig. 1d). It was indicated that considerable shrinkage occurred during template removal and the original ribbon-like morphology was not preserved probably due to the very thin thickness of the ribbons.

Fig.2a showsa typicalTEM image of the interconnected titania nanowires and the related electron diffraction pattern (ED) exhibits sharp rings corresponding to the anatase crystals, indicatingthe polycrystalline structure of the titania nanowires. A representative HRTEMimageof thetitaniananowiresis shown in Fig. 2b, which revealed the presence of many crystallites showingclearanataselattice fringes, confirming thatthe nanowires consisted of anatase nanocrystals. The XRD pattern of the obtainedtitanianetworksdemonstrated that anatase was actually the only crystal phasepresentin the product(Fig. 3a).An average crystallitesize of about 14 nm was estimated according to line width analysis of the (101) reflection based on the Scherrer formula.Nitrogensorptionisothermsof thetitanianetworksalong with the Barret–Joyner–Halenda(BJH) pore size distributionplot are presented in Fig. 3b, which suggest that the networks are basically mesoporous with pore sizes predominantly less than 40 nm and a pore size distribution centered around 9 nm. The BET specific surface area and pore volume were measured to be 61m2 g21 and0.20 cm3 g21, respectively. This resultsuggested that the mesopores mainly originated from the interstices between the interconnected titaniananowires,in good agreementwiththeSEM observations.

Fig. 4 illustratesthe photodegradationof RhodamineB in the presence of the BC membrane-templatedtitanianetworksas well as itsphotodegradationin the presenceof themacroporoustitania networks obtained by sol-gel coating of eggshell membranes followed by calcination at 500 uC, as reported previously.14 It suggests that the current mesoporous titania networks exhibit a considerably enhanced photocatalytic activitycomparedwith the macroporous titania networks obtained througha similarsol-gel nanocasting procedure. It is noted that the macroporous titania networks showedboth crystallinity and surface areas comparable to the present mesoporous titania networks but much larger diametersof the interwovenfibers (typicallyy1 m). Therefore, the higher photocatalytic activity of the mesoporous titania networks consisting of anatase nanowires may be attributed to the larger accessible surface areas.

Fig. 1 SEM images of BC membranes (a, b), BC–titania hybrid membranes (c), and titania networks templated by BC membranes (d). Arrows indicate the twisted structureof ribbon-like nanofibers.

Fig. 2 TEM (a) and HRTEM (b) images of titania nanowires constituting networks templated by BC membranes. Inset shows the corresponding ED pattern.

2736 | Chem. Commun., 2005, 2735–2737 This journal is The Royal Society of Chemistry 2005

In summary, mesoporous titania networks consisting of interconnected anatase nanowires have been synthesized by using unique bacterial cellulose membranes as natural biotemplates. The novel titania nanowirenetworks may findpotential applications in areas including photocatalysis, photovoltaics, and bone-tissue engineering. The templating strategy is generally extendable to the synthesis of mesoporous nanowire networks of other metal oxide systems.

Dayong Zhang and Limin Qi* State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry, Peking University, Beijing 100871, P. R. China. E-mail: liminqi@pku.edu.cn

Notes and references

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D. G. Shchukin and R. A. Caruso, Adv.Funct. Mater., 2003,13, 789. 8( a) U. Meyer,A. Larsson,H.-P.HentzeandR.A.Caruso,Adv.Mater., 2002, 14,1 768; (b) D. C. Shchukin and R. A. Caruso, Chem. Mater., 2004, 16, 2287; (c)H . Zhang, G.C .H ardy, Y.Z .K himyak, M. J. Rosseinsky and A. I. Cooper, Chem. Mater.,2 004, 16, 4245. 9( a)E .L .C repaldi, G. Soler-Illia,D .G rosso, F. Cagnol,F .R ibot and

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Fig. 3 XRD pattern (a) and nitrogenadsorption–desorption isotherms (b)of titanianetworkstemplatedby BC membranes. Insetshowsthe poresize distributiondeterminedfrom the desorptionbranch.

Fig. 4 Photodegradationof RhodamineB monitoredas the normalized concentration change versus irradiation time in the presence of titania networks templatedby BC membranes(a) and eggshellmembranes (b).

This journal is The Royal Society of Chemistry 2005 Chem. Commun., 2005, 2735–2737 | 2737