Morphological control and strong light scattering in macroporous TiO2 monoliths prepared via a colloid-derived sol–gel route

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  • Science and Technology of Advanced M

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    by multiple scattering and interference. Important exam-

    Pore formation is a very promising technique fortailoring the scattering strength as well as for obtainingstrongly scattering media [810]. Recently, we have

    separation induced by the hydrolysis and polycondensation

    TiO2 a fascinating material for photonic applications in thevisible regions. In spite of this advantage, few works havebeen performed on porous TiO2. One reason for this fact is

    ARTICLE IN PRESSthe difculty in preparing porous TiO2 monoliths; in thesolgel systems derived from titanium alkoxides, it isusually difcult to control the structural development in

    1468-6996/$ - see front matter r 2006 NIMS and Elsevier Ltd. All rights reserved.

    doi:10.1016/j.stam.2006.04.014

    Corresponding author. Tel.: +8175 383 2432; fax: +81 75 383 2420.E-mail address: fujita@dipole7.kuic.kyoto-u.ac.jp (K. Fujita).ples are coherent backscattering (CBS) [14] and Andersonlocalization [57]. In order to realize the photon localiza-tion in the random media, light must be elastically andextremely strongly scattered. The elastic scattering meansthat light absorption is negligibly small in the system. Thestrong scattering is obtained when the wavelength of lightis comparable to the size and spatial separation of therandomly distributed scatterers. In addition, the scatteringstrength increases with an increase in refractive-indexcontrast.

    of alkoxysilane and the subsequent freezing of the structureby the solgel transition [12]. The control over the size anddensity of macropores enabled us to tailor the scatteringstrength, although the scattering strength was weakbecause of the low refractive index of SiO2 skeleton(n1.46).Titanium dioxide (titania, TiO2) with the rutile-type

    structure is transparent for light in the wide range of thevisible spectrum. The absence of light absorption, alongwith its high refractive index (n2.7), makes rutile-typethe permanent morphology by the solgel transition. The macroporous morphology, i.e., the size and volume fraction of continuous

    macropores, can be tailored by adjusting the amount and/or molecular weight of PEO and the TiO2 concentration in the starting

    solution. During the heat treatment at temperatures above 1000 1C, the skeleton is sintered into fully dense body, and the crystallinestructure is transformed from anatase to rutile phases, while keeping the macroporous morphology. We show that the rutile-type TiO2-

    based macroporous monoliths are strongly scattering media for visible light and that the scattering strength can be controlled by the

    macroporous morphology.

    r 2006 NIMS and Elsevier Ltd. All rights reserved.

    Keywords: Titanium dioxide; Solgel method; Phase separation; Macroporous materials; Colloid; Multiple light scattering; Interference of light waves

    1. Introduction

    It is well established that the propagation of light indielectrically disordered media (random media) is affected

    fabricated macroporous monoliths in silica (SiO2)-basedsolgel systems, and investigated the light-scatteringproperties [11]. The macroporous morphology is formedvia the development of a transient structure of phasebackscattering. Well-dened macroporous bicontinuous structures are formed when the transient structure of phase separation is xed asMorphological control and strong lmonoliths prepared via a c

    Koji Fujitaa,, Junko Konishia, KaDepartment of Material Chemistry, Graduate School of Engineer

    bDepartment of Chemistry, Graduate School of Science, Kyoto Uni

    Received 3 February 2006; received in revis

    Available onlin

    Abstract

    Macroporous titania (TiO2) monoliths have been prepared via t

    the presence of poly(ethylene oxide) (PEO), and the light-scaaterials 7 (2006) 511518

    ht scattering in macroporous TiO2lloid-derived solgel route

    uki Nakanishib, Kazuyuki Hiraoa

    Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

    ity, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

    orm 14 April 2006; accepted 14 April 2006

    October 2006

    solgel route started from aqueous anatase-type titania colloid in

    ing properties have been investigated by means of coherent

    www.elsevier.com/locate/stam

  • ARTICLE IN PRESS

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    ofthe course of the hydrolysis and polycondensation becauseof the rapid polymerization reaction. Only recently, a rststudy on the scattering strength of TiO2 monoliths withmacroporous bicontinuous morphology was reported,where the strong scattering of visible light was demon-strated [13]. The successful fabrication of porous TiO2monoliths is achieved by the use of aqueous titania colloidinstead of highly reactive titanium alkoxide [14,15]. The pHincrease due to the hydrolysis of formamide allows us tocontrol the aggregation and gelation of titania colloid, andan addition of poly(ethylene oxide) (PEO) to the reactionmixture induces the phase separation in the system. Themacroporous morphology derived from the solgel processaccompanied by phase separation can be precisely con-trolled by the composition and reaction temperature of thestarting solution. Our previous study [13] showed that thepore size and porosity of macroporous TiO2 monoliths can

    Table 1

    The calculated starting compositions and notations of samples

    Sample Content (g)

    TiO2a PEO (Mv) Form-

    P300-T215 1.98 0.0300 (300,000) 1.16

    P325-T215 1.98 0.0325 (300,000) 1.16

    P350-T215 1.98 0.0350 (300,000) 1.16

    P375-T215 1.98 0.0375 (300,000) 1.16

    P300-T154 1.98 0.0300 (300,000) 1.16

    P325-T154 1.98 0.0325 (300,000) 1.16

    P350-T154 1.98 0.0350 (300,000) 1.16

    P1000-T110 1.98 0.1000 (100,000) 1.16

    aThe amount of titania in aqueous colloidal titania.bThe sum of the amount of nitric acid in aqueous colloidal titania andcThe sum of the amount of water in aqueous colloidal titania and that

    K. Fujita et al. / Science and Technology512be controlled by adjusting the TiO2 concentration and/orthe reaction temperature, and that the scattering strengthcan be altered by the morphological control. The amountand molecular weight of PEO also have signicantinuences on the macroporous morphology since theirparameters alter the timing of the onset of phase separationrelative to the solgel transition [14,15]. Here, we haveprepared rutile-type TiO2 monoliths with a variety ofmacroporous bicontinuous structures by adjusting thestarting compositions and examined their scatteringproperties. In particular, the relationship between theresultant macroporous morphology and the scatteringstrength is discussed.

    2. Experimental

    2.1. Sample preparation and characterization

    Aqueous dispersion of titania colloid was employed asthe titania source (STS-010, pH 1.7, Ishihara SangyoKaisha, Ltd., Japan). The colloidal particles had theanatase-type structure, and the primary particle size wasabout 7 nm. PEO [HO(-CH2-CH2-O-)nH] having viscosity-average molecular weights (Mv) of 100,000 and 300,000(Aldrich Chemical Co., Inc., Milwaukee, WI, USA) wereused as the polymer component to induce the phaseseparation. Formamide and 1M aqueous solution of nitricacid (HNO3) (Hayashi Pure Chemical Ind., Japan) wereutilized as the solvents to control the gelation andaggregation of titania colloid. The calculated startingcompositions and their notations are listed in Table 1.Gel samples were prepared from 21.5, 15.4, and 11.1wt%of TiO2 concentrations. The sample preparation is asfollows. First, appropriate amounts of 1M aqueous nitricacid and PEO were mixed in a glass tube. Then, formamideand aqueous colloidal titania were added under vigorousstirring in an ice bath. After being stirred for 10min, theresultant homogeneous solution was allowed to gel at 40 1C

    Fraction of TiO2 (wt%)

    de HNO3b H2O

    c

    0.177 5.84 21.5

    0.177 5.84 21.5

    0.177 5.84 21.5

    0.177 5.84 21.5

    0.322 8.96 15.4

    0.322 8.96 15.4

    0.322 8.96 15.4

    0.479 14.17 11.0

    added separately in the process of gel preparation.

    ed separately in the process of gel preparation.

    Advanced Materials 7 (2006) 511518in a closed condition. The wet gel was aged at the sametemperature for 24 h and was subjected to solvent exchangewith 2-methyl-2-propanol. The solvent exchange wasrepeated ve times. The wet gel thus obtained was freeze-dried using a vacuum device (VFD-21S, Shinku DeviceCo., Japan). Some of the dried gels were heat-treated attemperatures between 600 and 1200 1C for 1 h. A scanningelectron microscope (SEM, S-2600N, Hitachi Ltd., Japan)and a eld-emission scanning electron microscope (FE-SEM, JSM-6700F, JEOL Ltd., Japan) were used to observethe morphology of dried gels or heat-treated gels. The sizedistribution of micrometer-range pores was measured by amercury porosimetry (PORESISER-9320, MicromeriticsCo., USA), and the pore size distribution in mesoporeregime was determined by nitrogen adsorption-depositionmeasurements (ASAP-2010, Micrometrics Co., USA).

    2.2. Light-scattering measurements

    For the purpose of evaluating the scattering strength ofsamples, CBS experiments were performed so that the

  • transport mean free path of the light, corresponding to theaverage length after which the propagation direction of thelight was randomized by scattering, could be obtained. Acollimated beam of the 488-nm line from an Ar+ laser wasreected by a beam splitter and incident on the samplesurface with a small angle from the normal incidence. Thesample was rotated around an axis normal to the samplesurface in order to average out speckles. The scattered lightaround the backscattered direction was collected by a lenswith a focal point of 5 cm and detected using a charge-coupled device (CCD) placed at the focal point. Thetransport mean free path was calculated from thedependence of backscattered intensity on the scatteringangle [4].

    3. Results

    3.1. Characterization of macroporous and mesoporous

    structures

    Fig. 1 shows the micrometer-range morphology for driedTiO2 gels prepared from 21.5wt% TiO2 concentration andvaried PEO contents. Figs. 1(a)(d) represents SEM imagesof P300-T215, P325-T215, P350-T215, and P375-T215

    We have also inspected the variation in gel morphologywith heat treatment. A typical result is shown for P325-T154 sample in Fig. 2. Figs. 2(a) and (b) correspond to FE-SEM images depicting the micrometer-range morphologybefore and after heat treatment at 1000 1C for 1 h,respectively. One can see that the heat-treated sampleretains the macroporous bicontinuous morphology,although overall shrinkage is observed. Figs. 2(c) and (d)represent the higher-magnication FE-SEM photographstaken for the fracture surfaces of TiO2 skeletons inFigs. 2(a) and (b), respectively. The dried gel is composedof the aggregate of nanoparticles, while the application ofheat treatment at 1000 1C brings about the growth of particlesize accompanied by the sintering of TiO2 skeleton. We alsoconrmed that the macroporous bicontinuous structure ismaintained even after the heat treatment at 1200 1C.Fig. 3 shows the pore size distribution in the micrometer

    range determined by the mercury intrusion method forsamples heat-treated at 1200 1C for 1 h. All the samplespossess sharp pore size distributions, which are character-istic of spinodal decomposition. For samples preparedfrom a xed TiO2 concentration (21.5 or 15.4wt% TiO2)and varied PEO contents, the minor change in PEOcontent affects strongly the macroporous bicontinuous

    ARTICLE IN PRESSK. Fujita et al. / Science and Technology of Advanced Materials 7 (2006) 511518 513samples, respectively. All the gel samples exhibit typicalmacroporous bicontinuous structures, and the sizes ofskeleton and macropores increase with increasing the PEOcontent in the starting mixture. A similar result wasobserved for gels prepared from 15.5wt% TiO2 concentra-tion and varied PEO content.Fig. 1. SEM photographs of dried gels prepared with 21.5wt% TiO2 concen

    T215, and (d) P375-T215.morphology; the increase in PEO content leads to theincreases in both the size and volume of macropores. Thepore volume depends more signicantly on the TiO2concentration in starting compositions. It is clear that thesample prepared from the lower TiO2 concentration tendsto yield the larger pore volume.tration and varied PEO content: (a) P300-T215, (b) P325-T215, (c) P350-

  • ARTICLE IN PRESSofK. Fujita et al. / Science and Technology514As shown in Figs. 2(c) and (d), the nanometer-rangestructure of TiO2 skeleton is considerably affected by theheat treatment. For the purpose of exploring the effect of

    0

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    1 100.2Pore diameter (m)

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    P1000-T110P300-T154P325-T154P350-T154P300-T215P325-T215P350-T215P375-T215

    Fig. 3. Pore size distributions measured by the mercury intrusion method

    for samples heat-treated at 1200 1C for 1 h.

    Fig. 2. FE-SEM photographs of dried TiO2 gel (P325-T154) and that heat-trea

    of the dried and heat-treated gels, respectively, and (c) and (d) correspond to hig

    in (a) and (b), respectively.ted at 1000 1C for 1 h: (a) and (b) depict the micrometer-range morphology

    Advanced Materials 7 (2006) 511518heat treatment on the nanometer-range structure in moredetail, we carried out nitrogen adsorption measurements sothat the pore size distribution in the mesopore regime couldbe determined. A representative result is shown for P300-T215 sample in Fig. 4. The pore size distributions wereobtained from the adsorption branch of the isotherm bythe BarretJoyer-Harenda (BJH) method. The mesoporevolume decreases gradually at elevated temperatures, andnally vanishes at 1000 1C. In other words, the mesoporescollapse almost completely during the heat treatment at1000 1C, indicating that TiO2 skeleton is sintered into thefully dense body. This result is consistent with themorphology change as observed in Figs. 2(c) and (d). Aswe reported earlier [13], heating at temperatures above900 1C also causes the complete transformation of thecrystalline structure of TiO2 skeleton from anatase intorutile phases. Thus, rutile-type TiO2 monoliths with fullysintered skeleton and well-dened continuous macroporescan be fabricated via the solgel process incorporatingphase separation and the subsequent heat treatment.

    3.2. CBS

    Light-scattering properties have been characterized byCBS measurements. CBS is observed as an increase in thereected intensity from a medium at the exact back-scattering direction as a consequence of the interference ofwaves propagating along time-reversed optical paths [14].

    her-magnication images for the fracture surfaces of skeletons as observed

  • ARTICLE IN PRESSy of0.8

    1

    -1 )

    dried gel600 C700 C

    K. Fujita et al. / Science and TechnologThe CBS yields a cone in the plot of the backscatteredintensity versus the scattering angle, and the full-width ofhalf-maximum of the cone is inversely proportional to thetransport mean free path l [4]. Here, we chose three sampleswith almost the same pore size but different porositiesbased on the data of Fig. 3 and compare their scattering

    5 10 50 1000

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    Pore diameter (nm)

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    1000 C

    Fig. 4. Pore size distributions evaluated...

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