j.1551-2916.2010.03831.x
Facile Preparation of Hierarchically Porous TiO 2Monoliths
George Hasegawa,Kazuyoshi Kanamori,w Kazuki Nakanishi,and Teiichi Hanada
Department of Chemistry,Graduate School of Science,Kyoto University,Kyoto 606-8502,Japan
Monolithic titania (TiO 2)with multiscale porous structures have been successfully prepared via the sol–gel route accompanied by phase separation utilizing a chelating agent and mineral salt.The TiO 2gels obtained possess well-de?ned macropores derived from spinodal decomposition and mesopores as interstices of anatase TiO 2nanocrystallites.Most of the chelating agent were removed by hydrolysis and subsequent decarbonation through the gradual solvent exchange from ethanol to water.The TiO 2particles comprising TiO 2gel skeletons spontaneously converted from amorphous to anatase through the solvent exchange pro-cess in a mild condition at 601C.The present method of the fabrication of porous TiO 2monoliths is advantageous for wide-spread applications because the reaction occurs under an almost neutral condition and does not require a hydrothermal process,which was indispensable to strengthen the monolith in the method previously reported.
I.Introduction
P
OROUS
inorganic materials have been attracting signi?cant attention 1in widespread application ?elds such as electron-ics,2,3catalysis,4sensing,5biomedical science,6and separation science.7,8Hierarchically,porous metal oxides with different levels of pores are intensively studied in recent years.9–17The presence of micropores (o 2nm)contributes to a large surface area,which enhances contact between solid and ?uid phases,while the presence of mesopores (2–50nm)and macropores (450nm)increases mass transport through the material and increases the available surface area.Materials with hierarchi-cally multimodal porous structures consequently show im-proved properties compared with those with single-sized pores.As the applications in different ?elds invariably require simul-taneous controls of the material shape (powder,9,11–13,15?lm,10monolith,14,16,17etc.)and the pore structure,numerous porous inorganic materials have been synthesized by the sol–gel method utilizing structure-directing agents,such as latex spheres,(micro)emulsions,micelles of surfactant,etc.9–17
Of the many porous inorganic materials,titania (TiO 2)is of particular interest due to its unique characteristics and a lot of efforts have been made to obtain porous TiO 2.18–22Hierarchi-cally,porous TiO 2monoliths are especially important for chro-matographic applications because of their selective adsorption of organophosphates,such as nucleotides and phospholipids.23–28However,there are only a few reports describing the formation of porous TiO 2monoliths 29–33because titanium alkoxide exhib-its remarkably high reactivity,which generally leads to the het-erogeneous precipitation of TiO 2rather than the formation of
monolithic gels.Konishi et al .32,33prepared macroporous TiO 2monoliths from colloidal TiO 2and titanium alkoxide under a strongly acidic condition.However,the preparation under a milder condition is desired for widespread applications.
Recently,we developed a simple method for attaining the homogeneous gelation of monolithic TiO 2utilizing a chelating agent and mineral salt under a mild condition.34Although the chelating agents,such as b -ketone esters and b -diketones,are well known to decrease the reactivity of titanium alkoxide spe-cies by coordinating to the Ti atoms,35the addition of only a chelating agent does not allow us to acquire homogeneous TiO 2monoliths but friable opaque gels that are formed as aggregates of TiO 2precipitates.The key of our method is the addition of a conjugate base of strong acids,such as nitrate ions and halide ions,which further stabilize the chelated species and decrease the reactivity during,in particular,hydrolysis.This method is highly advantageous because the reaction occurs in a nearly neutral condition in one-pot and the gelation time is easily controlled by varying starting compositions.
In this work,we report the preparation of hierarchically po-rous TiO 2monoliths by utilizing the above-mentioned sol–gel method with a chelating agent and mineral salt.Well-de?ned interconnected macropores are formed as a result of concurrent phase separation and the sol–gel transition,while micro-and mesopores are formed as interstices in-between TiO 2particles (crystallites after calcination),which are embedded in the skel-etons.Ethyl acetoacetate,which was used as a chelating agent in this study,was removed by hydrolysis followed by decarbona-tion of the resultant acetoacetic acid.The rigid crack-free TiO 2monoliths obtained were crystallized into anatase and rutile by calcination,resulting in promising materials for a lot of appli-cations owing to their multimodal pore structure integrated in robust monoliths and the unique characteristics of TiO 2.
II.Experimental Procedure
Titanium (IV)n -propoxide (Ti(OPr)4)and polyethylene oxide (PEO,M w 510000)were purchased from Sigma-Aldrich Co.(St.Louis,MO).Ethyl acetylacetonate (EtAcAc)and 1-propanol (PrOH)were purchased from Tokyo Chemical Industry Co.Ltd.(Tokyo,Japan).Ammonium nitrate (NH 4NO 3)was obtained from Kishida Chemical Co.Ltd.(Osaka,Japan).All reagents were used as received.Distilled water was used in all experiments.In the typical synthesis,given amounts of Ti(OPr)4,PrOH,and EtAcAc were mixed in a glass container.After the complete mixing,PEO was added to the resultant homogeneous yellow solution and the PEO was completely dissolved at 601C.The solution was then cooled to 401C,and 1M NH 4NO 3aqueous was slowly added under vigorous stirring.After stirring for 3min,the homogeneous solution obtained stood at 401C for 24h.The typical gelation time of the samples was from 30min to 1h.The starting compositions are listed in Table I.Except for the cases specially mentioned,the wet gels obtained sequentially underwent the solvent exchange process in an open container at 601C in the following solutions:ethanol (EtOH),EtOH/H 2O 59/1,EtOH/H 2O 58/2,EtOH/H 2O 57/3,EtOH/H 2O 51/1,and EtOH/H 2O 53/7each for more than 8h,and sub-sequently immersed in H 2O at 601C for 24h.The resultant wet gels were dried at 401C for 48h.The dried gels were calcined at
M.Menon—contributing editor
This work was supported by the Grant-in-Aid for Scienti?c Research (No.20750177for K.K.and 20350094for K.N.)from the Ministry of Education,Culture,Sports,Science,and Technology (MEXT),Japan.Also acknowledged is the Global COE Program ‘‘Inter-national Center for Integrated Research and Advanced Education in Materials Science’’(No.B-09)of the MEXT,Japan,administrated by the Japan Society for the Promotion of Science (JSPS).K.K.is also indebted to the ?nancial support by Research for Promoting Technological Seeds from Japan Science and Technology Agency (JST).
w
Author to whom correspondence should be addressed.e-mail:kanamori@kuchem.kyoto-u.ac.jp
Manuscript No.27404.Received January 18,2010;approved March 22,2010.
J ournal
J.Am.Ceram.Soc.,93[10]3110–3115(2010)
DOI:10.1111/j.1551-2916.2010.03831.x r 2010The American Ceramic Society
3110
various temperatures for2h with a rate of raising temperature at1001C/h.
The microstructures of the fractured surfaces of the samples were observed under scanning electron microscopy(SEM,JSM-6060S,JEOL Ltd.,Akishima,Japan)and?eld-emission SEM (FE-SEM,JSM-6700F,JEOL).A mercury porosimeter(Pore Master60-GT,Quantachrome Instruments,Boynton Beach, FL)was used to characterize the macropores of the samples, while a nitrogen adsorption–desorption apparatus(Belsorp mini II,Bel Japan Inc.,Toyonaka,Japan)was used to characterize the meso-and micropores of the samples.Before each nitrogen adsorption–desorption measurement,the sample was degassed at2001C under vacuum for more than6h.Infrared absorption spectra were recorded using an FT–IR spectrometer(FT-IR-8300,Shimadzu Corp.,Kyoto,Japan)using ground samples that were mixed with KBr to yield a1wt%sample.A total of 100scans were recorded with a resolution of4cmà1.The crystal structure was con?rmed by powder X-ray diffraction(XRD) (RINT Ultima III,Rigaku Corp.,Akishima,Japan)using a Cu K a radiation(l50.154nm)as an incident beam.Helium pycnometry(Accupyc1330,Micromeritics Instrument Corp., Norcross,GA)was used to determine the true density of the heat-treated samples.Bulk density of each sample was calcu-lated from the sample weight and volume.Porosity(%)of each sample was calculated as(1à[bulk density]/[true density])?100. Compressive mechanical testing was performed using a uniaxial electromechanical testing device(EZ Graph,Shimadzu Ltd., Japan)using cylindrical-shaped samples.Fracture stress was determined as the stress at which cracks appeared in the sample.
III.Results and Discussions Homogeneous TiO2gels are generally transparent and can be obtained from titanium alkoxide by the sol–gel method utilizing a chelating agent(EtAcAc)in the presence of strong acid anion as reported previously.34The strong acid anion is deduced to play the role of blocking agents which prevent Ti atoms from being exposed to nucleophilic reactions.Various mineral salts including strong acid anions such as Clà,Brà,Ià,and NO3àcan be used for this method.In this study,we chose NH4NO3be-cause it can be completely removed by pyrolysis and highly pure TiO2can be obtained.The amounts of EtAcAc and1M NH4NO3aqueous were?xed as the molar ratio of Ti(OPr)4/ EtAcAc/H2O/NH4NO351/1.1/3.0/0.056for convenience.
Because as-synthesized wet gels contain EtAcAc which coor-dinates to Ti atoms,and TiO2networks are consequently weak, the dried gels break into pieces after calcination.Removing EtAcAc and extended aging in water are therefore needed.In order to remove EtAcAc from TiO2networks,as-synthesized wet gels were immersed in EtOH/H2O and EtAcAc was hydro-lyzed to acetoacetic acid.The generated acetoacetic acid was immediately decarbonized with the forming acetone and CO2as shown in Fig.1.36The EtOH/H2O ratio has to be gradually re-duced so that the hydrolysis of EtAcAc is allowed to take place slowly.The gels are cracked when directly immersed in H2O.In addition,this process was conducted in an open container so that the pressure does not increase with generating gaseous CO2. Through this process,the yellow gels turn to white and crack-free TiO2monoliths are obtained after drying.As for the sample T2.5-350,the linear shrinkage percentages during the solvent exchange and drying are about33%and30%,respectively;the dried TiO2gel was about47%of the size of as-synthesized wet TiO2gel.
The addition of PEO,which adsorbs on the condensates through the hydrogen bonding,induces phase separation and TiO2monoliths with cocontinuous macropores can be obtained from the particular compositions as shown in the TiO2–solvent–PEO pseudoternary system of the composition–morphology re-lationship(Fig.2).The mass of TiO2was calculated from the amount of Ti(OPr)4assuming all Ti(OPr)4are ideally converted into TiO2.The mass of the solvent phase,which contains PrOH (including PrOH released from Ti(OPr)4),EtAcAc,and residual H2O,was calculated from the mass of the starting sol,TiO2,and PEO.Closed circle,double circle,crossed circle,and open circle denote nanoporous structure,isolated pores,cocontinuous structure,and particle aggregates,respectively.Figure2indi-cates that the desired cocontinuous structure was formed from a TiO2concentration at o11mass%.
Figures3(a)–(e)shows SEM images of dried TiO2gels pre-pared by varying only the amount of PEO.As the amount of PEO increases,the structures of the dried gels change from nanoporous,isolated pores,cocontinuous,and to particle ag-gregates because phase separation tendency increases and the more coarsened structures are frozen by gelation.37The mor-phology change from isolated pores to cocontinuous occurred due to an increase of the solvent mixture-based?uid phase. Monolithic gels with the morphologies of nanoporous and iso-lated pores could not be obtained without cracks because the gas generated during the removal of EtAcAc increases the pressure in the gels and breaks them into pieces when the pressure ex-ceeds the fracture strength of the gels.Gels with the morphology of particle aggregates were brittle.Crack-free gels can be ob-tained in the case of the morphology of a cocontinuous structure as shown in Fig.3(f).The continuous macropores help to prevent the pressure inside the gels from increasing to keep the monolithic shape of the gel.
The more detailed macropore characteristics of the TiO2 gels with cocontinuous structure are investigated by mercury porosimetry as shown in Fig.4.It indicates that the macropores
Table I.Starting Compositions and Resultant Morphology of the Dried Samples
Sample Ti(OPr)4(mL)PrOH(mL)EtAcAc(mL)1M NH4NO3aqueous(mL)PEO(g)Morphology
T2.5-250 5.0 2.5 2.5 1.00.250Nanoporous
T2.5-300 5.0 2.5 2.5 1.00.300Isolated pores
T2.5-325 5.0 2.5 2.5 1.00.325Cocontinuous
T2.5-350 5.0 2.5 2.5 1.00.350Cocontinuous
T2.5-375 5.0 2.5 2.5 1.00.375Cocontinuous
T2.5-400 5.0 2.5 2.5 1.00.400Cocontinuous
T2.5-425 5.0 2.5 2.5 1.00.425Cocontinuous
T2.5-450 5.0 2.5 2.5 1.00.450Particle aggregates T2.5-475 5.0 2.5 2.5 1.00.475Particle
aggregates
Fig.1.Ethyl acetoacetate converts to acetoacetic acid by hydrolysis.
The generated acetoacetic acid immediately decomposes into acetone
and carbon dioxide.
October2010Facile Preparation of Hierarchically Porous TiO2Monoliths3111
in each gel are sharply distributed and the pore size and total pore volume increases with an increasing amount of PEO.The increase of pore size is due to the enhancement of phase sepa-ration,while the increase of pore volume is attributed both to the decrease of shrinkage during drying owing to the coarser cocontinuous structure,and to the increasing fraction of the ?uid phase containing PEO.This result also indicates that the macropore size of the gels can be easily controlled from 0.17to 5.4m m by adjusting the amount of PEO.
Figures 5and 6display XRD patterns and FT–IR spectra of the dried TiO 2gel with/without the solvent exchange (the pro-cess of removing EtAcAc)and the calcined TiO 2monoliths.Figure 5indicates that the dried TiO 2gel without the solvent exchange is amorphous because there exists no obvious diffrac-tion peaks,whereas that with the solvent exchange exhibits XRD patterns of the anatase crystal structure.It can be de-duced that the former includes EtAcAc-coordinating Ti atoms,which inhibits crystallization.By contrast,the latter contains much less EtAcAc,which promotes crystallization.The diffrac-tion patterns also indicate that the rutile phase was developed after calcination above 7001C.In Fig.6,the infrared absorption bands around 1620and 1550cm à1(marked with open circles)are attributed to the vibrations of acetylacetate groups coordi-nated to Ti.The broad bands below 900cm à1are characteristic
of the Ti–O–Ti networks.38The sharp absorption band at 1380cm à1is derived from the remaining NH 4NO 3in the spec-trum of the dried TiO 2without the solvent exchange.According to the spectra,the amount of chelating agent in TiO 2gels effec-tively decreases through the solvent exchange process.However,the absorption peak attributed to the chelating agent still re-mains in the sample heat treated at 4001C and the heat-treat-ment temperature that can remove the chelating agent from the sample is 5001–7001C.The chelating agent strongly coordi-nates in the primary TiO 2particles with a considerably small pore size and with few open pores,which makes the complete removal dif?cult.
The SEM images of the TiO 2monoliths calcined at 6001and 7001C for 2h are shown in Figs.7(a)and (b),respectively.Both samples retain their cocontinuous macroporous structure.How-ever,the fractured surface of the skeleton looks different be-tween two images;the rougher surface is observed in the sample calcined at 7001C than that at 6001C because of the phase trans-formation and crystal growth from anatase (6001C)to rutile (7001C).The more detailed images observed under FE-SEM of the calcined samples are shown in Fig.8.The particles in the skeleton become larger as the calcination temperature increases up to 6001C due to the growth of the anatase crystallites
as
Fig.2.Pseudoternary representation of the composition–morphology relationship in the titania–solvent–polyethylene oxide
system.
Fig.3.(a–e)Scanning electron micrographs of the dried titania (TiO 2)gels prepared with varied polyethylene oxide content,(a)T2.5-250,(b)T2.5-300,(c)T2.5-350,(d)T2.5-400,and (e)T2.5-475.(f)Appearance of the monolithic TiO 2
gel.
Fig.4.Pore size distributions of the titania gels calcined at 3001C pre-pared with varied polyethylene oxide content.
3112Journal of the American Ceramic Society—Hasegawa et al.Vol.93,No.10
shown in Figs.8(a)–(e).Moreover,signi?cant crystal growth can be con?rmed between 6001and 7001C.This change corresponds to the crystal phase transition from anatase to rutile as shown in Fig.5.
Figures 9(a)and (b)show the nitrogen adsorption–desorption isotherms of the TiO 2monoliths calcined at varied temepratures and the corresponding pore size distributions calculated by the BJH method using adsorption branch,respectively.In addition,the obtained parameters of the calcined TiO 2samples obtained by the nitrogen adsorption measurement are shown in Table II.According to the IUPAC classi?cation,the isotherms of the samples calcined below 6001C can be classi?ed into type-IV,while the isotherm of the sample calcined at 7001C can be regarded as those with collapsed meso-and micropores.The mesopores stem from the interstices of the anatase crystals and this result corresponds to the FE-SEM images in Fig.8.As described above,the growth of the anatase crystallites with an increasing calcination temperature leads to an enlargement of the mean pore diameter.Besides,the larger shrinkage reduces the pore volume when the heat treatment at the higher temper-ature is conducted.The TiO 2monolith consisting of rutile crys-tallites possesses almost no micropores and mesopores,which agrees with the FE-SEM observation.
Table III lists the relationship between the porosity and the compressive fracture stress of the TiO 2monoliths calcined at different temperatures.The skeletal densities of the TiO 2sam-ples calcined at 6001and 7001C are similar to the true density of anatase (3.893g/cm 3)and rutile (4.250g/cm 3),respectively.39This result agrees with the XRD patterns as shown in Fig.5.The true density of the TiO 2sample calcined at 3001C is lower than that of anatase because it still includes a chelating agent as shown in Fig.6.As the calcination temperature increases,the
bulk density largely increases as a result of the decrease of porosity.As for the mechanical strength,the compressive frac-ture stress sharply increases with the increasing calcination tem-perature.When the TiO 2monolith was calcined at 6001C,the chelating agent remaining in the TiO 2skeletons was pyrolyzed leading to the further formation of Ti–O–Ti.In addition,the denser porous structure also leads to the stronger skeleton.The compressive fracture stress consequently increases to a large ex-tent.On the other hand,the main factor of the increase in the strength from the sample calcined at 6001C to that calcined at 7001C is the crystal phase transition.As shown in Fig.8,during the constituent of the cocontinuous skeleton changes from ana-tase to rutile,signi?cant crystal growth occurs.This is accom-panied by the decrease of porosity with enhanced sintering.The cohesive strength between each crystallite in the rutile TiO 2monolith is stronger than that in the anatase TiO 2monolith because the contact areas between the neighboring crystallite become much larger.
IV.Conclusions
Hierarchically porous TiO 2monoliths have been successfully prepared via the sol–gel route utilizing the chelating method ac-companied by phase separation.The addition of mineral salt,NH 4NO 3together with a chelating agent,allows the gradual hydrolysis and polycondensation of Ti(OPr)4leading to homo-geneous TiO 2gel monoliths.The addition of an appropriate amount of PEO induces spinodal decomposition and well-de-?ned cocontinuous macroporous structure forms.Most of the chelating agent in the gels obtained can be removed by hydro-lysis followed by decarbonation during the stepwise
solvent
Fig.5.X-ray diffraction patterns of the as-dried gels with/without the solvent exchange and the titania samples calcined at different tempera-tures.The diffraction peaks attributed to anatase are indicated by closed circles,whereas those attributed to rutile are by open
squares.
Fig.6.Infrared absorption spectra of the as-dried gels with/without the solvent exchange and the titania samples calcined at different tempera-tures.The absorption bands assigned to the chelating agent are indicated by open
circles.
Fig.7.Scanning electron micrographs of the titania monoliths (T2.5-350)calcined at 6001C (a)and 7001C (b).
October 2010Facile Preparation of Hierarchically Porous TiO 2Monoliths 3113
Fig.8.Enlarged micrographs of the titania monoliths(T2.5-350)calcined at different temperatures observed under?eld-emission scanning electron microscopy;(a)as-dried,(b)3001C,(c)4001C,(d)5001C,(e)6001C,and(f)7001
C.
Fig.9.(a)Nitrogen adsorption–desorption isotherms and(b)the corresponding pore size distributions of the titania monoliths(T2.5-350)calcined at different temperatures.
Table II.Pore Characteristics of the Titania Monoliths
(T2.5-350)Calcined at Different Temperatures
Heat-treatment
temperature(1C)S BET(m2/g1)V p(cm/g1)w D p(nm)w 2002170.238 2.1 3001480.211 2.8 4001310.194 3.2 500830.150 4.1 600200.046 5.4 7000.60.002(6.1) Total pore volume V p and mean pore diameter D p obtained by the BJH method using adsorption branch.
Table III.Relationship Between Porosity and the Fracture Stress of the Titania Monoliths(T2.5-350)Calcined
at Different Temperatures
Heat-treatment
temperature(1C)
True density
(g/cm3)
Bulk density
(g/cm3)
Porosity
(%)
Fracture stress
(MPa) 300 3.63 1.28657.8?102 600 3.88 2.2342 2.0?103 700 4.19 2.5739 4.3?103
3114Journal of the American Ceramic Society—Hasegawa et al.Vol.93,No.10
exchange from EtOH to H2O.The constituents of the cocon-tinuous gel skeletons are converted from amorphous to anatase through the crystallization in warm water at the same time.The resultant macroporous TiO2monoliths also have well-de?ned mesopores attributed to the interstices of anatase nanocrystals. Calcination at7001C induces the transformation from anatase to rutile with losing micro-and mesopores,while maintaining the macroporous morphology.In addition,the calcined porous TiO2monoliths show a remarkably high strength(B4.3?103 MPa of compressive strength)and are highly promising for many applications,especially to separation media.
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