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Sol?gel templating of membranes to form thick, porous, Notas de estudo de Engenharia de Produção

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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 emul-

sions9,10 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 mem-

branes 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.11 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 concentra-

tion 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, Röhm Specialty Acrylics, Rodenbacher Chaussee 4, Postcode 915-125d, D-63457 Hanau- Wolfgang, Germany.

PAPER www.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), poly-

ethersulfone (PES) and polypropylene (PP) 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 (99.7%) and the

sol–gel precursors, titanium(IV) isopropoxide (TIP, 99.999% or

97%), zirconium(IV) propoxide (ZrP, 70% (m/m) in 1-propa-

nol), 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 three-

stage 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 99.99 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/PP/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 uC to 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 deter-

mined 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/mm

Cellulose acetate, 450 nm CA45 Schleicher & Schüll, Sartorius Yes 7 129 Cellulose nitrate, 450 nm CN45 Schleicher & Schüll, Osmonics Yes 13 131 Polyamide, 450 nm PA45 Schleicher & Schüll, 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 PP22 Osmonics No 30 163

This journal is  The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 1414–1420 | 1415

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 auto-

sampler and gas flow controller. Nitrogen sorption measure-

ments (on a Micromeritics Tristar 3000 instrument) were

conducted to determine the specific surface area of the mate-

rials, 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 three-

dimensional, 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 PP membranes with smaller particle reten-

tion size can be compared with the original membrane struc-

ture. Notably, the finer polymer fiber of the PP 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.

This journal is  The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 1414–1420 | 1417

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) PP22, d) PES22–TiO2, e) CN22–TiO2,

and f) PP22–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, dark-

field 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 sub-

stantially 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

Membrane Thickness/mm SA/m2 g21

CA45 85 28 CN45 57 39 PA45 117 27 PES22 89 20 PES45 53 13 PP22 72 49

Fig. 6 The sequential coating approach was used to obtain complex materials: a) TEM and b) darkfield TEM images of the 2 layers of titanium

dioxide followed by 2 layers of zirconium dioxide; c) SEM image of the TiO2/SiO2 material (2 layers TiO2 followed by cast with silica in the presence

of C16E10); d) TEM image of an ultramicrotome of the TiO2/SiO2. Insert shows the mesoporous character of the silica as observed by TEM.

Fig. 7 X-Ray diffraction curves for the zirconia/titania materials

obtained using either a) the mixed precursor templating approach or b)

sequential coating of the template, and for reference c) the positioning

of the anatase titania (open) and tetragonal zirconia (grey) peaks.

This journal is  The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 1414–1420 | 1419

and zirconium propoxide solution with a similar Ti : Zr ratio

during templating gives an amorphous material under the

same calcination conditions (Fig. 7).18 Mixing the metal oxide

precursors prevented crystallization during the heating step

due to the more homogeneous distribution of Ti and Zr

throughout the structure.

The versatility of the sequential coating technique can be

demonstrated in the following example where we initially

employed the possibility to coat, and then cast the remaining

pore structure within the template for the formation of a TiO2/

SiO2 material. To make this structure potentially useful, we

applied a second molecular porogen in the casting process to

induce mesoporosity within the silica. In this case two layers of

TiO2 have been sequentially coated on the template followed

by soaking this hybrid material (membrane/amorphous TiO2)

in a silica precursor containing the additional porogen (the

nonionic surfactant C16E10). On removal of the membrane

a mesoporous silica structure (pore size y2.5 nm, surface area of the TiO2/SiO2 material is 593 m

2 g21) is obtained,

replicating both the overall geometry as well as the micrometer

sized pore architecture of the membrane, however with a layer

of crystalline titanium dioxide coating the macropores. An

SEM image and the TEM image of an ultramicrotomed

specimen of the TiO2/SiO2 material are shown in Fig. 6c and d,

respectively. At higher magnification, see insert, the meso-

porosity of the silica is apparent. It can be observed that the

silica remains crack-free, with the shrinking stress due to

condensation presumably taken up by the macropores.

Conclusions

The method of templating membranes via a sol–gel procedure

to generate inorganic crystalline materials with defined pore

size and architecture has been improved, using the principle of

‘multiple coating’. Diluting the precursor throughout the

coating procedure, but repeating the process until the desired

thickness of the layer is reached, substantially reduced excess

surface material and removed the casting effect observed at

the outer edges of the membrane. This technique allows a

very precise and homogeneous templating of membranes of

different materials such as polyamide, polypropylene, poly-

ethersulfone, cellulose acetate, and cellulose nitrate. A varia-

tion of the ratio of the amount of inorganic to organic material

obtained during templating can easily be achieved, which

allows control of structural density and the mechanical

stability of the resulting metal oxide film.

Further, this approach can be used to coat the template

with consecutive layers of different metal oxides (sequential

coating) or a layering of one metal oxide followed by casting of

the final volume with mesoporous silica; thus allowing the

preparation of complex structured materials.

Acknowledgements

Rona Pitschke is thanked for ultramicrotoming the samples

and assistance with TEM. Yitzhak Mastai is acknowledged for

conducting the darkfield TEM, and Meifang Zhou, Andreas

Zillessen and Kirsten Schwekendiek for technical support. The

Max Planck Society, Australian Research Council, Particulate

Fluids Processing Centre and The University of Melbourne’s

MRGS are appreciated for their financial support. RAC

acknowledges the Australian Research Council for an

Australian Research Fellowship.

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