Sol2013gel templating of membranes to form thick, porous

Sol2013gel templating of membranes to form thick, porous

(Parte 1 de 2)

Sol–gel templating of membranes to form thick, porous titania, titania/zirconia and titania/silica films

Jan H. Schattka,{a Edeline H.-M. Wong,b Markus Antoniettia and Rachel A. Caruso*b

Received 31st October 2005, Accepted 2nd February 2006 First published as an Advance Article on the web 20th February 2006 DOI: 10.1039/b515421a

Cellulose acetate, cellulose nitrate, polyamide, polyethersulfone and polypropylene membranes have been used as templates in which sol–gel chemistry was conducted to fabricate porous metal oxide films. Dilution of the metal alkoxide solution allowed for variation in the total amount of inorganic deposited per membrane. Multiple coatings with dilute precursor gave control of the final wall thickness. The correlation between the morphology of the metal oxide and the various structures of the membrane templates indicates the concise coating of the organic material during the templating process. Substantial variation in structure and characteristic properties of the membranes (i.e., ionic/nonionic, hydrophilic/hydrophobic, functional groups) did not hamper the coating mechanism. Multiple coatings could also be applied with variation in the type of metal oxide precursor; this ‘sequential coating’ approach yielded complex structured materials of layered metal oxides, such as TiO2 and ZrO2. Coatings followed by casting (filling of the void space) gave a unique TiO2 coated bimodal pored (macroporous/mesoporous) silica.

Templating allows control over the greater and finer structure of a material. A sacrificial template, which acts as a support around which the final material is built before being removed, is commonly used for the preparation of materials with a specified outer structure as well as a predetermined inner arrangement. Numerous porous materials are fabricated using templating procedures to provide control over the inner porous structure.

The use of templates to form macroporous solids has recently gained attention from the research community. Templates such as colloidal crystals,1,2 polymer gels,3,4 virus fibers,5,6 egg-membrane,7 echinoid skeletal plates,8 and emulsions9,10 are some examples. The final structures obtained bear some resemblance to the initial template and generally exhibit properties that could not be achieved without the structure directing agent.

Membranes have varying morphology dependent on their composition and mode of fabrication. A number of membranes have been shown to act as templates using a variety of chemical techniques. For example, alumina membranes have been widely applied for producing a range of tubular materials.1 The use of cellulose acetate membranes as templates was shown for the formation of titanium dioxide and zirconium dioxide films using sol–gel procedures and metal alkoxide precursors.12 Cellulose materials have also been used as templates using aqueous metal salt solutions, to form high surface area, thermally stable metal oxide materials.13

Additionally cellulose acetate has been used for the infiltration of preformed nanoparticles to produce porous silicalite structures14 and porous metal oxide films.15 Bacterial cellulose membranes have recently been used as templates for the synthesis of titania networks.16

In this paper five organic membranes have been applied as templates: cellulose acetate, cellulose nitrate, polyamide, polyethersulfone and polypropylene. The membranes vary in their morphology, hydrophilic/hydrophobic and non-ionic/ ionic characteristics as well as their high or low non-specific adsorption. These sacrificial templates were used for the fabrication of porous metal oxide films by conducting sol–gel chemistry within the pores of the membrane. The final metal oxide structure has been examined as a function of the different membrane morphologies, the precursor concentration used during the templating process and the number of coatings applied to the membrane. The addition of a second metal oxide, such as zirconium dioxide, to TiO2 is known to alter the physicochemical properties of the titania.17 We have previously demonstrated the possibility of using mixed precursor solutions or mixed preformed nanoparticles in conjunction with the templating approach to form mixed oxides having different morphological properties and photocatalytic activities.18,19 Here, a new approach is described where individual coatings of different metal oxides can be applied by a sequential coating technique, wherein a number of layers (i.e., two) of the initial metal oxide are deposited before the second metal oxide is synthesized.

This is demonstrated for TiO2 and ZrO2. The crystal phase of the titania and zirconia in the materials obtained using this sequential coating of membranes is compared with the metal oxide structures formed when mixing the two metal oxide precursors before applying the sol–gel/templating approach to polymer gels.18 aMax Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany bSchool of Chemistry, The University of Melbourne, Melbourne 3010, Australia. E-mail: rcaruso@unimelb.edu.au; Fax: +61 3 9347 5180; Tel: +61 3 8344 7146 { Current address: Degussa AG, Rohm Specialty Acrylics, Rodenbacher Chaussee 4, Postcode 915-125d, D-63457 Hanau- Wolfgang, Germany.

PAPER w.rsc.org/materials | Journal of Materials Chemistry

1414 | J. Mater. Chem., 2006, 16, 1414–1420 This journal is The Royal Society of Chemistry 2006

Layered metal oxides have been fabricated by screen printing particulate material or spin-coating and dip-coating onto flat substrates for applications in photovoltaics20 and for photoinduced charge separation.21 The layer-by-layer deposition approach has also achieved TiO2/ZrO2 films on flat substrates with either hetero-layered oxides22 or an oxide with gradient composition.23 The sol–gel templating technique applied here affords a macroporous film composed of a TiO2 and ZrO2 three dimensional network, with the titania and zirconia nanoparticles side-by-side throughout the structure.

That is, the zirconia has coated one side only of the titania layer with the presence of the template preventing complete surrounding of the titania with zirconia in the final material. When a mixture of silica precursor and porogen is applied to the titania coated template the silica fills the pore space between the titania walls, the end result being a material with a bimodal pore structure (macro and meso-pores) in the silica with the macropores coated in crystalline titania.

Experimental

Materials

The templates used were ultrafiltration membranes: cellulose acetate (CA), cellulose nitrate (CN), polyamide (PA), polyethersulfone (PES) and polypropylene (P) membranes with particle retentions24 of 450 nm or 220 nm. Table 1 lists suppliers and the abbreviations used to refer to the membranes during the remainder of the text. Isopropanol (9.7%) and the sol–gel precursors, titanium(IV) isopropoxide (TIP, 9.99% or 97%), zirconium(IV) propoxide (ZrP, 70% (m/m) in 1-propanol), and tetramethylorthosilicate (TMOS), plus the surfactant polyoxyethylene (10) cetyl ether (C16E10) and HCl (37%) used for the formation of porous silica were obtained from Aldrich. The water used throughout the synthesis was treated in a threestage Millipore Milli-Q Plus 185 purification system.

Templating

The metal alkoxides (TIP and ZrP) were diluted with their corresponding alcohols to yield solutions of set weight percent. For example, monitoring the changes in the total inorganic incorporated during templating required alkoxide solutions varying in weight percent (5 through to 9.9 wt%), while multiple coatings were applied from alkoxide solutions of 17 wt% TIP and 10 wt% ZrP. The membranes were soaked in the alkoxide solutions for 5 minutes before they were transferred into a water–alcohol mixture (1 : 1 by volume) and afterwards into water for a period of 5 minutes each, to hydrolyze the precursors. The hybrid materials obtained were dried at 60 uC between glass slides in order to keep the membranes flat. For the multiple coating samples this procedure was repeated up to 5 times with either the same precursor solution or with a variation in the metal alkoxide. The addition of silica during the templating procedure followed a previously published method.25 Briefly, the TMOS, C16E10 and 0.01 M aqueous HCl were mixed (2 : 1 : 1 by weight) to obtain a homogeneous solution before adding the membrane template for 6 min.

To remove the template the coated membranes were heated between the glass slides in an oven under various programs (depending on the membrane being studied). For CA/P/PA the samples were calcined at 550 uC for 4 h (heating rate of 5 uC min21). CN membranes were heated initially to 150 uC (heating rate of 5 uC min21), then slower heating from 150 uC to 200 uC (heating rate of 1 uC min21), followed by 200 uCt o 550 uC at a rate of 5 uC min21, and finally the sample was heated at 550 uC for 4 h. This avoided vigorous combustion at the ignition point of CN (180 uC), which was crucial to maintain the morphological characteristics of the template in the final inorganic structure. The PES membranes were heated to 700 uC for 4 h (heating rate of 4.5 uC min21). For these samples the glass slides were removed before heating to prevent the deformation and melting of the glass. A steady air flow was used during the calcination process.

Characterization

The morphology of the samples was examined by scanning and transmission electron microscopy (SEM and TEM). For SEM analysis pieces of the membranes and inorganic films were mounted on a carbon coated stub so that freshly broken surfaces (lying perpendicular to the flow direction through the membrane) were exposed. (Note: in some cases the membranes were frozen in liquid nitrogen before being snapped to expose a fresh surface.) The samples were sputter coated before observation with a Zeiss DSM 940 or Philips XL30 FEG Field Emission SEM. Cross-sectional images of the structure were obtained by examining thin (50–100 nm) slices of the material on a Zeiss EM 912 Omega or Philips CM120 BioTwin TEM. To prepare the samples they were first embedded in poly(methyl methacrylate) (PMMA) or LR White Resin and then cut into ultra thin sections using a Leica ultracut UCT ultramicrotome.

The crystal phase of the final inorganic oxides was determined using wide angle X-ray scattering (WAXS), employing an Enraf-Nonius PDS-120 instrument. To assess the mass ratio of inorganic material to CA or PA template for the variation in weight percent of precursor, thermogravimetric analysis (TGA) was carried out under oxygen at a ramp of 20 uC min21 to 800 uC using a Netzsch TG 209/DSC 200.

Table 1 Membrane abbreviations, suppliers, hydrophilicity, specific surface area (SA, as determined by BET theory) and thickness (from SEM) Membrane Abbreviation Supplier Hydrophilic SA/m2 g21 Thickness/m

Cellulose acetate, 450 nm CA45 Schleicher & Schull, Sartorius Yes 7 129 Cellulose nitrate, 450 nm CN45 Schleicher & Schull, Osmonics Yes 13 131 Polyamide, 450 nm PA45 Schleicher & Schull, Supelco Yes 12 164 Polyether sulfone, 220 nm PES22 Osmonics Yes 12 122 Polyether sulfone, 450 nm PES45 Osmonics Yes 8 110 Polypropylene, 220 nm P2 Osmonics No 30 163

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For the multiple coating experiments TGA was carried out under air using a variety of programs, reflecting those used as the calcination programs with additional heating to 800 uC, using a Mettler Toledo TGA/SDTA851e with attached autosampler and gas flow controller. Nitrogen sorption measurements (on a Micromeritics Tristar 3000 instrument) were conducted to determine the specific surface area of the materials, using the method of Brunauer, Emmet and Teller (BET).

Results and discussion

In an earlier communication,12 we reported that films of porous titanium dioxide or zirconium dioxide could be obtained by a sol–gel coating process of cellulose acetate membranes. The membranes were soaked in a neat solution of the alkoxide precursor and subsequently transferred into an aqueous alcohol solution, where hydrolysis and condensation of the precursor occurred. The metal oxide formed was deposited as an amorphous layer on the structured template. Calcining of the samples removed the organic template and lead to crystallization of the metal oxide giving a threedimensional, porous structure. On close inspection of these inorganic films the structure was not homogeneous over the complete cross section of the membrane, see Fig. 1. The outer parts of the films (Fig. 1b and d) had thicker inorganic walls which were a result of casting, and a significant deposit of excess TiO2 outside of the structured material was observed. The center of the structure (Fig. 1c) however was composed of much finer walls, as a result of coating of the initial membrane structure.

To prevent the build up of excess material and these casting effects, the method in which the precursor solution has been added to the membrane template was modified by diluting the alkoxide precursor with its corresponding alcohol before templating. TGA showed that the amount of inorganic material deposited in the PA membrane increased linearly with the concentration of the precursor (Fig. 2a). However, at too low weight percent of alkoxide in alcohol the structure dictated by the membrane was not maintained. For example, coating a polyamide membrane with an isopropanol solution

Fig. 1 Casting and excess material obtained when using neat precursor solutions during the templating of CA45 membranes. SEM images of a) a cross section of film; b), c), and d) higher magnification of the upper, middle and lower segments of the film.

Fig. 2 a) Weight of inorganic material per weight of polyamide membrane when templating with different precursor concentration solutions and b) when coating the same membrane several times with 17 wt% titanium(IV) isopropoxide in isopropanol.

1416 | J. Mater. Chem., 2006, 16, 1414–1420 This journal is The Royal Society of Chemistry 2006 of 17 wt% titanium(IV) isopropoxide resulted in a hybrid material with a metal oxide content of about 20% of the weight of the membrane; upon removal of the organic material the structure collapsed (Fig. 3a). At this dilution, the amount of inorganic material remaining is not sufficient to hold the network structure after removing the supporting membrane.

However, the hybrid material (membrane/amorphous TiO2 coating) can be templated again, which adds more inorganic material to the walls of the structure. After the second coating cycle, the metal oxide network was strong enough to maintain its morphology after calcination. Further coatings increase the amount of inorganic material per template (Fig. 2b), thereby reinforcing the structure and thickening the walls.

Fig. 3 shows TEM images of ultramicrotomed slices of the structures obtained by coating a PA membrane once, three times and five times with an isopropanol solution of 17 wt% titanium(IV) isopropoxide; the morphology of the template was preserved after the second coating cycle (not shown) and the walls become thicker with each additional coating step, which is consistent with the TGA data showing extra metal oxide was being introduced with every coating (Fig. 2b).

The amount of unstructured excess material, which is deposited on the outside of the membrane, was very low using the multiple coating method. Also complete filling of the pores in the outer parts of the membrane, i.e. ‘‘casting’’, was not observed for these samples prepared using the dilute alkoxide precursors. The SEM images in Fig. 4 illustrate the close resemblance of the morphology of the obtained inorganic material after three coating cycles using the polyamide membrane. The template shows large, almost spherical pores, separated by polyamide walls. As both sides of these walls are coated during the templating process, the inorganic network is exclusively composed of two very thin layers in close proximity to each other—the spacings between these layers were previously occupied by the template prior to its removal by calcination—and larger pores retained from the porosity of the template. While only the large pores can be seen in the SEM image (Fig. 4b), both these and the smaller pores in the structures are observed using microtomed samples and TEM (Fig. 4c).

Hollow replication was also obtained from all the other membranes studied with variation in morphological properties (Fig. 4d–f, and Fig. 5). The denser structure of the cellulose acetate membrane resulted in an interconnected network of hollow, nearly tubular, structures (Fig. 4d). Consequently, TEM images of ultramicrotomed sections show non-connected distorted oval patterns (Fig. 4f), as would be expected from a slice of the hollow, tubular material shown in the SEM images.

In Fig. 5 the SEM images of the titania films obtained using the PES, CN and P membranes with smaller particle retention size can be compared with the original membrane structure. Notably, the finer polymer fiber of the P membrane gives a finer TiO2 structure. The cellulose nitrate is quite similar in morphology to the cellulose acetate membrane.

These images show that the structural differences between the templates are transferred to the final titania morphology, as a coating is being applied in all cases. It is noteworthy that the method can be successfully utilized for the varying chemical nature of the templates. This makes the method potentially applicable to a lot of other materials and therefore to a great variety of available morphologies.

Specific surface areas have been calculated by BET analysis after gas sorption on a selection of the membranes coated three times with TiO2, see Table 2. Trends of surface area and membrane material cannot be elaborated from the data, as the variations in the calcination programs have significant

Fig. 3 TEM images of ultramicrotomed slices from the TiO2 networks obtained by coating a polyamide membrane once (a), three times (b), and five times (c) with dilute (17 wt% in isopropanol) titanium(IV) isopropoxide. The scale bar is the same for each image.

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influence on the crystallization process, the final crystal size and hence surface area. In all cases the surface area of the final titanium dioxide material was higher than that of the template membrane.

It is not only possible to vary the template and hence morphology, but also the metal oxide can be varied; for example zirconia structures are produced using zirconium propoxide, and many other metal alkoxide precursors can be used to produce a range of metal oxides. Additionally, using the ‘sequential coating’ approach it is possible to get complex structured hollow replicas with a consecutive, directional layered set-up of two (or more) metal oxides. Instead of coating several times with the same metal oxide, different precursor solutions are used for each coating cycle. This results in a layering of the metal oxides where the thickness and cohesiveness of the layers can also be controlled. It could be envisaged that such a technique would be applied for the synthesis of a charge separation device.

Fig. 4 SEM images of the polyamide membrane (a); the titanium dioxide structure obtained by coating this membrane three times with titanium(IV) isopropoxide (17 wt%) (b); and a TEM image of the ultramicrotomed slice of the polyamide templated inorganic structure (c). SEM images of (d) the cellulose acetate membrane and (e) the titanium dioxide structure obtained by coating this template three times with titanium(IV) isopropoxide (17 wt%), and (f) a TEM image of the ultramicrotomed slice of the cellulose acetate templated inorganic structure.

Fig. 5 SEM images of the polyethersulfone, cellulose nitrate and polypropylene membranes and the final titanium dioxide structures obtained from applying three coatings (17 wt% titanium(IV) isopropoxide) to these templates: a) PES22, b) CN22, c) P2, d) PES22–TiO2, e) CN22–TiO2, and f) P2–TiO2. The scale bar is the same for each image.

1418 | J. Mater. Chem., 2006, 16, 1414–1420 This journal is The Royal Society of Chemistry 2006

The example of TiO2/ZrO2 structures is shown here, where two layers of the titanium dioxide precursor were applied followed by two coatings of the zirconium dioxide precursor. No apparent difference in morphology between the structures obtained when layering different metal oxides or a single oxide is found using SEM. However, TEM reveals some contrast between the two different oxides (Fig. 6a). Additionally, darkfield TEM (using the (101) reflection of anatase at one single angle of diffraction) shows intense bright crystals which are iso-oriented on the ‘inner side’ of the network, that is, the side that was closest to the polymer before its removal (Fig. 6b). WAXS experiments supported the electron diffraction and gave scattering peaks corresponding to anatase titania and tetragonal zirconia (Fig. 7).

This organized, sequentially layered structure is substantially different to the material obtained when using a single mixed precursor solution during templating. The layering process builds separate amorphous metal oxide layers on the template, which on heating allow crystallization of the individual metal oxides with little or no influence of the second metal oxide. However, using a mixed titanium isopropoxide

Table 2 Film thickness and specific surface area (SA) of the three layered titanium dioxide films

(Parte 1 de 2)

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