THE NATURE OF MULTILAYERED TiO 2-BASED PHOTOCATALYTIC FILMSPREPARED BY A SOL-GEL PROCESS

P. Sawunyama, A. Yasumori*, and K. OkadaDepartment of Inorganic Materials, Tokyo Institute of Technology, 2–12-1 O-okayama,

Meguro-ku, Tokyo 152, Japan

(Refereed)(Received August 18, 1997; Accepted September 2, 1997)

ABSTRACTTransparent anatase TiO2-based multilayered photocatalytic films on po-rous alumina and glass substrates were synthesized via a sol-gel process.All films had a sponge-like microstructure and a mean crystallite dimen-sion of ca. 8 nm. The simple anaerobic decomposition of acetic acidreaction was used as the standard test system for evaluating the photocat-alytic activity of the films. Doping with iron(III) impeded the photocata-lytic activity. © 1998 Elsevier Science Ltd

KEYWORDS: A. thin films, B. sol-gel chemistry, C. electron microscopy, D.microstructure, D. catalytic properties

INTRODUCTION

TiO2-based photocatalytic technology is making inroads in such diverse applications asenvironmental remediation, organic compound transformation [1–4], and self-cleaning ma-terials, e.g., TiO2-coated windows and bathroom tiles [5,6]. Except for the latter, TiO2 isnormally used in the form of particulate suspensions. In recent years, however, interest hasshifted toward nanometer-sized particles (crystallite dimension# 10 nm) in the form ofcolloids or film networks on supports [7].

An attractive property of nanometer-sized photocatalytic films or systems is the ability toalter the electrochemical potentials of the photogenerated charge carriers by simply modi-fying the size of the particles. This desirable feature, coupled with negligible band bending,

*To whom correspondence should be addressed.

Materials Research Bulletin, Vol. 33, No. 5, pp. 795–801, 1998Copyright © 1998 Elsevier Science LtdPrinted in the USA. All rights reserved

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means that the major deactivation event in photocatalysis, that of electron-hole recombina-tion, is kept at an absolute minimum. Moreover, film networks do not suffer from one of themajor drawbacks of particulate materials: that of recovery of the photocatalyst at the end ofthe reaction.

Nonetheless, the photocatalytic activity of TiO2 films is often governed by factors such asfilm preparation procedure, density or microstructure of films, crystalline phase, and thepresence of impurities or dopants, as well as the nature of the support. Although numerousreports [7–10] have been made on the preparation and photocatalytic activity of TiO2 filmson glass, research into TiO2 films on other supports, for example, porous ceramic materials,has been largely lacking. Porous supports offer, among other features, increased reactionsurfaces, the ability to support a multifunctional catalytic system comprising, for example, adispersed metal and metal oxide and possible sieve properties for selective or controlledsynthetic work. In this paper, we report on the nature and photocatalytic activities ofmultilayered TiO2-based films supported on porous alumina substrates. The photocatalyticactivities of the films were evaluated using the anaerobic decarboxylation of acetic acid, alsoknown as the photo-Kolbe reaction [11] as the test system:

CH3COOHO3TiO2hn

CO2 1 CH4 DG 5 52.3 kJ mol21 (1)

EXPERIMENTAL

Materials. TiO2 sols were prepared via a sol-gel process [12]. Briefly, this involved theacetic acid (0.033 mol dm23) catalyzed hydrolysis of acetylacetone (0.028 mol dm23)modified titanium(IV) tetraisopropoxide (0.028 mol dm23) in isopropanol (140 mL) at 80°Cunder an inert atmosphere. The resulting monodispersed sol was stable for several months.Similarly, iron(III)-doped TiO2 sols were prepared by adding the requisite amount of dopant,iron(III) acetylacetonate (concentration; 0.5, 1.0, 1.5, and 2.0% atomic weight of TiO2,respectively), to the precursor solutions. Films of uniform thickness were deposited on cleanglass slides (20 mm3 20 mm) and porous alumina membranes (diameter: 25 mm; Whatmanfilter discs, pore size 0.02 mm), respectively, by spin coating. A standard multilayered filmon glass was fabricated by repeating the coating cycle eight times, i.e., spin coating, dryingat room temperature, and drying at 300°C for 10 min. For films on alumina substrates, aporous SiO2 film (from ethanolic SiO2 sol) was deposited by spin coating. This was followedby the sputter coating of platinized photocatalysts (Pt) and/or the deposition of the desirednumber of layers of TiO2 by spin coating. The resulting multilayered films were calcined at400°C for 4 h. Figure 1 is an illustration of a typical platinized TiO2 multilayered film on analumina substrate.

All other materials (Wako Pure Chemicals) used were of the highest grade available.Doubly distilled deionized water was used throughout.

Characterization. UV-vis spectra of films supported on glass were recorded with a doublebeam spectrophotometer (Jasco UVIDEC-610). Film thickness was estimated by profilom-etry (Surfcom-Tokyo Seimitsu). The texture of films on porous alumina was observed byfield emission scanning electron microscope [(FE-SEM), Jeol JSM-890S]. X-ray diffraction(XRD) patterns were recorded using monochromated Cu Ka radiation with a RigakuGeigerflex diffractometer.

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Photocatalytic Activity Evaluation. A 300 W Xe arc lamp, operated at maximum power,was used as the light source. A thermostated Pyrex vessel (volume: 1000 mL) with aprovision for circulating the head space gas was used as the photochemical reaction vessel.In a typical experiment, the photocatalyst, supported on a sample holder (area: 1 cm2), wasplaced in the vessel; the coated surface faced the irradiation source, and 700 mL of a 1429ppm (ca., 0.14%) acetic acid solution was added. The solution was purged with He for ca.,30 min before irradiation. The gas phase was circulated continuously, and gas phase sampleswere taken at appropriate intervals throughout and analyzed for carbon dioxide by gaschromatography (Shimadzu GC-6AM with a gaskuropak 54 column, length, 2.0 m; o.d., 5mm; i.d., 3 mm; and a thermal conductivity detector interfaced to an integrator, ShimadzuChromatopac CR6A). Helium was used as the carrier gas.

RESULTS

XRD. XRD patterns of TiO2-based films supported on glass and porous alumina substrateswere recorded in the range 2u 5 70–10°. All XRD patterns were identical for the samenumber of layers regardless of the nature of the substrate. The XRD data revealed that themultilayered films were composed of highly oriented anatase TiO2 crystallites, i.e., orientedalong the (101) crystallographic plane. Peak intensity of the (101) plane increased with anincreasing number of coating applications. No crystallographic planes of iron oxides wereidentified for Fe31-doped films. This could be due to the low concentrations of the Fe31

dopant and also to the possible incorporation of Fe31 within the anatase lattice. The meancrystallite dimension of the films perpendicular to the (101) plane was estimated using theScherrer equation [13]. The mean crystallite dimension was ca. 8 nm.

UV-vis and Film Texture. UV-vis absorption spectra of films on glass are shown in Figure2. All films were transparent and exhibited interference patterns. An increase in the number

FIG. 1An illustration of a typical platinized multilayered photocatalyst.

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of layers and doping with Fe31 resulted in a red-shift of absorption edges of the films.However, Fe31 absorption peaks were not evident. It is possible that doping of TiO2 withFe31 resulted in the incorporation of Fe31 into the TiO2 lattice as suggested above.

It was possible to estimate film thickness using the UV-vis transmission spectra data byinvoking the following relationship [14]:

d 5 ml1l2/(2[n(l1l2 2 n(l2)l1]) (2)

whered is film thickness (in nm),m is the number of oscillations between the two minimaor maxima occurring at wavelengthsl1 andl2 (in nm), andn(l1) andn(l2) are the refractiveindices of the film atl1 andl2, respectively. The refractive index of anatase in the visibleregion is ca. 2.49–2.55 [15]. Calculated film depths of the multilayered films are presentedin Table 1.

Analyses of film surfaces by profilometry revealed that the surfaces of all films were veryjagged. Due to the low resolution of this technique, however, detailed analyses of the surfaceswere not performed. Nevertheless, film thickness of films deposited on glass substrates wasestimated using this technique, and the results are shown in Table 1. From these results, it wasestimated that each coating application deposited a TiO2 layer approximately 30-nm thick.This result was confirmed by FE-SEM (vide infra).

FE-SEM images of a typical platinized multilayered film (i.e., 20 TiO2 coatings, Pt film,and SiO2 film) are shown in Figure 3. Figure 3A reveals that the surface morphology of thefilms consists of a granular and rough microtexture, more or less sponge-like. Figure 3B

FIG. 2UV-vis absorption spectra of TiO2 films on glass. The number of layers are (from left toright) 4, 6, 8, 12, 16, and 20.

TABLE 1Film Thickness As Determined By Profilometry

and UV-Visible Spectra Data

Number of layers

4 8 12 16 20

Thicknessd/nm (UV-vis) 158 302 416 464 596Thicknessd/nm (profilometry) 150 250 - 400 600

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shows the cross section of the fractured multilayered film. The thickness was determined tobe ca. 600 nm. Thus, each TiO2 coating application deposited a film ca. 30-nm thick. Thisresult is in close agreement with those obtained by profilometry and from UV-vis spectra(vide supra). A higher magnification of the edge profile in Figure 3B is depicted in Figure 3C.The micropores are clearly visible. A backscattered electron image of the fractured crosssection showing the Pt film (white) beneath the TiO2 layers is presented in Figure 3D. Thethickness of the Pt film was estimated to be ca. 20 nm. Thus the TiO2 film network wascomposed of agglomerates of smaller crystallites with channels ranging in size from a fewnanometers to perhaps a few tens of nanometers, giving rise to a fairly porous photocatalystfilm.

Photocatalytic Activity. Irradiation of an aqueous acetic acid solution under anaerobicconditions in the presence of a TiO2-based photocatalytic film resulted in the evolution ofCO2 and CH4 as the main gaseous products. The activities of the different TiO2-basedphotocatalysts on porous alumina substrates were evaluated by monitoring the evolution ofCO2 quantitatively. In a typical experiment, the desired photocatalyst (8 layers of TiO2) wasimmersed in a 0.14% (0.02 mol dm23) aqueous acetic acid solution, and irradiation wascarried out as outlined in the “Experimental” section. The temperature of the reaction

FIG. 3FE-SEM images of a multilayered photocatalyst on porous alumina substrate after heattreatment at 400°C for 4 h. (A) surface morphology, (B) fractured surface, (C) a highermagnification of the image shown in B, and (D) a backscattered electron image of panel Ashowing the Pt layer (white).

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solution was maintained at 40°C, and irradiation was carried out for 5 h. Table 2 summarizesthe results of this investigation.

DISCUSSION

Films prepared in this work were transparent and had a porous microstructure; however, theexact dimensions of the film pores were uncertain. The rough surface texture and the porousmicrostructure of the film networks were formed from the agglomeration of elementary unitsduring calcination. The colloidal particles of the TiO2 sol had an estimated average particlesize of ca. 5 nm. Thus, the porosity of the films was largely controlled by the elementaryparticle sizes, although sintering during calcination could have resulted in the production oflarger particles. Although a highly dense film is desirable for efficient charge diffusion, anopen network is crucial for a good photocatalyst.

All photocatalytic films prepared in this work were of the anatase phase and were orientedalong the (101) plane. In contrast, Weinberger and Garber [9] have reported the synthesis ofporous anatase films oriented along the (220) crystallographic plane by a reactive magnetronsputtering technique. It is not clear how preferred crystallite orientation affects photocatalyticactivity, and thus, this subject warrants further investigation.

The illumination of a photocatalyst with light energy, hn $ Ebg, results in the generationof electrons and holes. The photogenerated charge carriers rapidly diffuse to the surface ofthe photocatalyst, and, in the case of platinized photocatalysts, the driving force for chargecarrier separation is believed to be the potential gradient between the Pt electrode and thesolution. Therefore, the separation of reductive sites from oxidative sites was achievedthrough a judicious arrangement of the multilayered photocatalytic film. The separation ofreaction centers with a concomitant minimization of charge carrier recombination would thusimprove the efficiency of the overall photocatalytic process.

In this study, the platinized-undoped photocatalyst was the most active, and the platinized-doped films were the least active. For Fe31-doped films, although the nature of the Fe31

TABLE 2Amount of CO2 Evolved in the Photodecomposition

of Acetic Acid Mediated By a TiO2-Based Photocatalytic Filmon Porous Alumina

PhotocatalystRate

(mmol cm22 h21)Amount of CO2 evolved in

5 h (mmol cm22)

TiO2 3.0 15.0Fe/TiO2 1.2 6.2Fe/TiO2 (1.0%) 2.7 13.7Fe/TiO2 (2.0%) 2.2 11.2Pt/TiO2 4.0 20.0Pt/Fe/TiO2 (0.5%) 1.5 7.7Pt/Fe/TiO2 1.8 9.1Pt/Fe/TiO2 (2.0%) 1.6 7.8

Experimental conditions: 0.14% acetic acid solution, initial pH5 ca.3.19,surface area of photocatalyst film5 1.0 3 1024 m2, temperature5 40°C.

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centers in anatase is unknown, work by Gratzel and Howe [16] suggests that the Fe31 ionsare substitutionally located in Ti41 lattice sites. Of course, the substitutional procedure isexpected to influence the placement of the Fe31 ions within the TiO2 lattice. It is possible thatthe low reactivity of Fe31-doped films may have been due to the fact that Fe31 ions acted ascharge carrier recombination sites.

Nevertheless, the photocatalytic materials reported in this work hold promise for suchapplications as environmental remediation, self-cleaning materials, and controlled synthesis.Further work into other TiO2-mediated photocatalytic reactions is necessary before some ofthe properties of the TiO2-based multilayered photocatalysts reported in this study can beconclusively demonstrated.

CONCLUSIONS

Porous films synthesized in this work had several unique features: (1) high surface area tovolume ratios, (2) high optical transparency, (3) a novel way of separating the reductive sitesfrom the oxidative sites, and (4) possible molecular sieve properties. The separation ofreductive sites from oxidative sites was achieved through a prudent arrangement of TiO2

layers, Pt electrode, and the porous silica–alumina base. Doping with Fe31 ions wasdetrimental to the overall rate of acetic acid decomposition.

ACKNOWLEDGMENTS

P.S. thanks UNESCO/MONBUSHO for the award of a research fellowship.

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