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

    ami

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    add

    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 p