Synergism Between Rutile and Anatase TiO2 Particles In

Applied Catalysis A: General 244 (2003) 383–391

Synergism between rutile and anatase TiO2 particles inphotocatalytic oxidation of naphthalene

Teruhisa Ohnoa, Kojiro Tokiedaa, Suguru Higashidab, Michio Matsumuraa,∗a Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

b Department of Industrial Chemistry, Osaka Prefectural College of Technology, 26-12 Saiwai, Neyagawa, Osaka 572-8572, Japan

Abstract

Photocatalytic oxidation of naphthalene was investigated in a mixed solution of acetonitrile and water using various kindsof titanium dioxide (TiO2) powders as the photocatalysts and molecular oxygen as the electron acceptor. The main productfrom naphthalene is 2-formylcinnamaldehyde. For this reaction, anatase small TiO2 particles, which are commonly used asphotocatalyst, are inactive, probably because band bending is necessary for the oxidation of naphthalene. If the particles arenot extremely small, pure rutile and pure anatase powders show fairly high activity, and those containing both anatase andrutile phases show the highest activity. When a pure anatase powder is partly (about 90%) converted to the rutile form by heattreatment, the activity is largely enhanced. The activity of pure rutile particles is also enhanced by physically mixing themwith a small amount of small-sized anatase particles, which are inactive for this reaction. These results can be explained bythe synergism between rutile and anatase particles. We consider that electrons are transferred from rutile particles to anataseparticles, i.e. naphthalene is mainly oxidized on rutile particles and oxygen is mainly reduced on anatase particles. Thiselectron transfer process is supported by electrochemical properties of TiO2 electrodes for reduction of oxygen.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Titanium dioxide; Anatase; Rutile; Photocatalyst; Naphthalene; 2-Formylcinnamaldehyde

1. Introduction

Titanium dioxide (TiO2)-mediated heterogeneousphotocatalysis has attracted great attention becauseof its potential applications to decomposition ofpollutants in water and air[1–5]. TiO2 and someother semiconductor photocatalysts have also beenextensively studied for the purpose of solar energyconversion[6–11]. Organic synthesis is another ac-tive research field of TiO2 photocatalysts[12–18]. Inmany applications, anatase TiO2 powders consistingof particles with a large surface area are used as the

∗ Corresponding author. Tel.:+81-6-6850-6695;fax: +81-6-6850-6699.E-mail address: [email protected] (M. Matsumura).

photocatalysts. These powders are advantageous toadsorb compounds included in the reaction systemat low concentrations. Typical compounds which ef-ficiently react on fine anatase powders are phenols[19] and olefins[20]. In contrast to these compounds,oxidation of water efficiently proceeds on large rutileTiO2 particles [4,5,9]. It has been known from thestudy of photoelectrochemistry of TiO2 electrodesthat a band bending is necessary for oxidation of wa-ter. Thus, it is considered that the TiO2 photocatalystparticles for this reaction must be large enough todevelop a space charge near the surface[6].

We have reported that naphthalene is efficientlyoxidized in a mixed solvent of water and acetoni-trile on UV-irradiated TiO2 powders using oxy-gen as the electron acceptor[17,18]. The main

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384 T. Ohno et al. / Applied Catalysis A: General 244 (2003) 383–391

product is 2-formylcinnamaldehyde; this was er-roneously described as 1,8-dihydroxynaphthalenein our previous papers[17,18]. Production of2-formylcinnamaldehyde was reported by Soana et al.[4] and Theurich et al.[5] as an intermediate ofphotocatalytic degradation of naphthalene in aqueoussolutions. In the course of our study, we found thata TiO2 powder containing both anatase and rutileparticles showed the highest activity[18]. This resulthinted that there is a synergistic effect between rutileand anatase particles in this reaction. In the presentstudy, in order to confirm this effect, we investigatedphotocatalytic activities of TiO2 powders containingrutile and anatase phases at different ratios.

2. Experimental

2.1. Materials and instruments

Various kinds of titanium dioxide (TiO2) powdershaving anatase and rutile crystal structures were ob-tained from Catalysis Society of Japan (TIO-3, TIO-5),Ishihara Sangyo (ST-01, ST-21, and ST-41), TohoTitanium (TT-2, NS-51), and Japan Aerosil (P-25).The content of anatase and the surface area of thesepowders were as follows: ST-01: 100%, 192.5 m2/g;ST-21: 100%, 56.1 m2/g; ST-41: 100%, 8.2 m2/g;NS-51: 1.5%, 6.5 m2/g; TT-2: 99.9%, 24.0 m2/g;TIO-3: 0%, 48.1 m2/g; TIO-5: 9%, 2.5 m2/g; P-25:83.5%, 49.2 m2/g. Naphthalene was obtained fromWako Pure Chemical Industry, and was used af-ter three-time recrystallization from ethanol. Otherchemicals were obtained from commercial sources asguaranteed reagents and used without further purifi-cation. The contents of rutile and anatase phases inTiO2 powders were determined by X-ray diffractionspectroscopy (XRD, Philips, X’Pert-MRD). The mor-phology of TiO2 powders was examined by using ascanning electron microscope (SEM, Hitachi S-5000).The surface area of the powders was determined us-ing a surface area analyzer (Micromeritics, FlowSorbII 2300).

2.2. Photocatalytic reaction and analysis of theproducts

Photocatalytic reactions were typically carried outin Pyrex glass tubes filled with a mixture of acetonitrile

(3.6 g), water (0.3 g), naphthalene (0.1 g), and TiO2powder (0.1 g). TiO2 particles were suspended in thesolution by sonication for 1 min before the reaction.During the reaction, the solution was magneticallystirred and bubbled with O2 at a rate of 2.0 ml/min,and externally photoirradiated. A 500 W super pres-sure mercury lamp (Wacom, BMO-500DY) was usedas a light source, and a UV34-filter (Kenko Co.) wasemployed to remove deep UV light (λ < 340 nm)and prevent direct photoexcitation of naphthalene. Thelight intensity was controlled using fine stainless-steelmeshes as neutral density filters.

After photoirradiation for certain time periods, thesolution was analyzed with high-performance liquidchromatographs (HPLC, GL Science UV620 and Hi-tachi D-7500), which were equipped with an ODScolumn. Using a mixture of water and methanol (1:4)as the eluent, we found that at least three compoundsare produced from naphthalene. In order to identifythe products, we tried to isolate them from the reac-tion solution as follows: first, the solution was com-pletely evaporated under a reduced pressure. Second,the residue was dissolved in hexane, and the solutionwas fed to a silica gel column (Wakogel C-300). Whenhexane was used as the eluent, naphthalene was easilyremoved from the column, whereas the products wereadsorbed in the column. Third, after removal of naph-thalene from the column using hexane as the eluent,the eluent was changed to a mixed solvent of hexane,chloroform and ethanol, and the contents of chloro-form and ethanol were gradually increased. Finally,the fractionalized solutions were completely evapo-rated under reduced pressure. The collected substancein each fraction was analyzed by 400 MHz1H and100 MHz13C NMR using a JEOL JNM-AL400 NMRspectrometer. The molecular weight was determinedwith a GC–MS spectrometer (JEOL JMS-DX303HF)equipped with a DB-1 column.

2.3. Preparation of the TiO2 powders containingrutile and anatase phases

TiO2 powders containing both anatase and rutilephases were prepared by two methods. One method isa partial conversion of the anatase phase into the rutilephase by applying heat treatment. More precisely, weused a pure anatase TiO2 powder (TT-2) as the start-ing powder, and it was heated in aerobic atmosphere

T. Ohno et al. / Applied Catalysis A: General 244 (2003) 383–391 385

for 1 h at different temperatures. The other method isphysical mixing of a rutile powder with an anatasepowder at different ratios. A rutile powder (NS-51)was used as a standard material of the rutile particles,from which an anatase component (1.5%) originallyincluded in this powder was removed by a treatmentwith 10% HF solution for 24 h[21]. After this treat-ment, the powder was thoroughly washed with wateruntil fluorine atoms were completely removed fromthe surface. For mixing this pure rutile powder with ananatase (ST-01) powder, the powders were added towater at certain ratios, and the mixture was sonicatedfor 30 min.

2.4. Electrochemical study using TiO2 ceramicelectrodes

Rutile-form TiO2 ceramics were prepared by press-ing a TiO2 powder (TIO-5) into disks (1 cm in diame-ter), followed by heating at 1300◦C in air for 1 h. TheTiO2 ceramics thus obtained were heated in a streamof hydrogen at 650◦C for 4 h in order to make themelectrically conductive. TiO2 electrodes were fabri-cated by connecting a copper wire to the back sur-face of the conductive TiO2 ceramics via Ga–In alloy.The electrochemical measurements were carried outin aqueous solutions of 0.1 M (M: mol/dm3) H2SO4under potentiostatic conditions using a potentiostat(Nikkokeisoku, NPOT-2501), an Ag/AgCl referenceelectrode, and a platinum plate counter electrode. Inorder to clarify the catalytic effect of anatase particlesfor reduction of oxygen, fine anatase particles (ST-21)were loaded on the rutile ceramic TiO2 electrodes byrubbing them against the electrode surface.

3. Results and discussions

3.1. Photocatalytic reaction of naphthalene on TiO2photocatalysts

The main product generated from naphthalene as aresult of the photocatalytic reaction was confirmed tobe 2-formylcinnamaldehyde by 400 MHz1H NMR,100 MHz 13C NMR and GC–MS spectra. Detaileddata of NMR and GS–MS spectra for the identifi-cation of 2-formylcinnamaldehyde are as follows:(E)-2-formylcinnamaldehyde:1H NMR (400 MHz,

CDCl3, d) 10.23 (1H, s), 9.81 (1H, d,J = 7.8 Hz),8.57 (1H, d,J = 18.1 Hz), 7.89 (1H, m), 7.66 (3H,m), 6.68 (1H, dd,J = 18.1 and 7.8 Hz);13C NMR(100 MHz, CDCl3, d) 193.6, 192.6, 149.2, 135.3,134.5, 133.9, 133.5, 132.2, 130.7, 127.9; (CI–MS),m/e 161 (M+ + H), Calcd. MW for C10H8O2 160.17.(Z)-2-Formylcinnamaldehyde:1H NMR (400 MHz,CDCl3, d) 10.1 (1H, s), 9.72 (1H, d,J = 8.0 Hz),8.16 (1H, d,J = 11.7 Hz), 7.93 (1H, m), 7.70 (3H,m), 6.35 (1H, dd,J = 11.7 Hz, 8.0 Hz); (CI–MS),m/e161 (M+ + H), Calcd. MW for C10H8O2 160.17. AnNMR spectrum of 2-formylcinnamaldehyde isolatedfrom the reaction solution is shown inFig. 1. Thespectrum indicates that bothE andZ isomers are in-cluded. The component of theE isomer was increasedby leaving the reaction solution overnight. This resultsuggests that theZ isomer is the original product thatis spontaneously transformed into the thermodynami-cally more stableE isomer. By recrystallization fromhexane, we obtained (E)-2-formylcinnamaldehyde asneedle crystals. Isolation of the pureZ isomer wasunsuccessful. Besides 2-formylcinnamaldehyde, thereare at least two other products which give HPLCpeaks at different retention times. These compoundswere not isolated either. These compounds may behydroxy-2-formylcinnamaldehyde, and hydroxy-1,4-naphthoquinone, which were reported by Soana et al.[4] and Theurich et al.[5] as the intermediates ofthe photocatalytic reaction of naphthalene in aqueoussolutions. From the comparison of the amounts of2-formylcinnamaldehyde produced and naphthaleneconsumed, the yield of 2-formylcinnamaldehyde wasestimated to be about 85%.

Fig. 1. A 1H NMR spectrum of 2-formylcinnamaldehyde isolatedfrom the reaction solution. Solvent: CDCl3.


In the present study, we paid attention to 2-formyl-cinnamaldehyde and measured its amounts fromthe analysis of its HPLC peak areas of the reactionsolutions. The rate of the production of 2-formyl-cinnamaldehyde depended on the TiO2 powders usedas the photocatalysts, but 2-formylcinnamaldehydewas always the main product of the photocatalyticreaction under our experimental conditions using anykind of TiO2 powders. Among the TiO2 powderswe investigated, a powder (P-25), which containsboth anatase and rutile particles[22], showed thehighest photocatalytic activity for the production of2-formylcinnamaldehyde. Similarly, powders con-taining both rutile and anatase phases showed highactivity, even if the component of the anatase phaseis a few percent. Pure anatase particles showed fairlyhigh activity, if their sizes are not extremely small.Pure rutile particles with considerable size also showfairly high activity, though the value is lower than

Fig. 2. Photocatalytic activity of TiO2 powder (T-22) heat-treated at various temperatures for the production of 2-formylcinnamaldehyde.For comparison, the activity of the P-25 powder is shown on the right end. The reaction was carried out for 1 h in a mixed solution ofCH3CN (3.6 g) and water (0.3 g) containing TiO2 (0.1 g) and naphthalene (0.78 mmol). The solution was bubbled with O2 and externallyirradiated (λ > 340 nm). The content of rutile phase and the particle size of TiO2 powders are shown beneath the graph.

that of pure anatase particles with nearly the samesize. However, very small anatase powders (ST-01and ST-21), which are commonly used as the photo-catalysts, showed very low activity in this reaction.

3.2. Photocatalytic production of2-formylcinnamaldehyde using heat-treated TiO2powders

In order to elucidate the reason why the powdersconsisting both rutile and anatase phases show highphotocatalytic activity, we systematically synthesizedTiO2 powders containing these two phases at differ-ent ratios. Our first approach to obtaining a series ofTiO2 samples with different rutile and anatase con-tents is a partial conversion of the crystal phase of apure anatase powder (TT-2) into rutile phase by heattreatment at different temperatures.Fig. 2 shows thephotocatalytic activity of these TiO2 samples for the


production of 2-formylcinnamaldehyde from naphtha-lene and the content of rutile phase in them. The high-est activity inFig. 2 is observed for the sample treatedat 800◦C, which contains 90% of the rutile phase. Itsphotocatalytic activity was nearly the same as that ofP-25, although the contents of the rutile phase are dif-ferent for the two materials. This result suggests thatsome synergism between anatase and rutile TiO2 par-ticles exists in this reaction, and that their optimal ratiodepends on the sample preparation conditions. How-ever, in the case of these samples prepared by heattreatment, the change in the crystallinity and the sizeof TiO2 particles may also affect the photocatalyticactivity, and it is difficult to estimate the separate con-tribution from each of these effects.

3.3. Photocatalytic reaction on TiO2 powdersprepared by physically mixing rutile particles withsmall anatase particles

In order to further prove the synergism between ru-tile and anatase TiO2, we physically mixed some ru-tile powder with some anatase powder. In this study,we used the NS-51 (rutile) powder, which shows fairlyhigh activity, and the ST-01 (anatase) powder, whichshows very poor activity for this reaction. The NS-51powder consists of large (about 400 nm) rutile par-ticles, and the ST-01 powder consists of very fine(about 7.0 nm) anatase particles. The anatase com-ponent (1.5%), which was originally included in theNS-51 powder, was removed by immersing the NS-51powder in 10% HF (seeSection 2). The size of ru-tile particles included in the NS-51 powder was notchanged by the HF treatment. The pure rutile pow-der thus obtained and the anatase powder (ST-01)were added at various ratios, keeping the total amountat 100 mg, to a mixed solution of acetonitrile (3.6 g)and water (0.3 g), and this mixture was sonicated for30 min. After this mixing procedure, naphthalene wasadded to the suspension and the photocatalytic reac-tion was carried out.

Photocatalytic activity of the mixed powder isshown by the solid line inFig. 3 as a function of thecontent of the NS-51 (rutile) powder. An importantpoint is that the mixed powders show higher activitythan pure anatase or pure rutile powder. This resultstrongly suggests the existence of a synergistic ef-fect between anatase and rutile powders. Another

important point is that the photocatalytic activity isdrastically increased by adding a small amount of theanatase ST-01 powder to the rutile NS-51 powder,whereas the increase in the activity is small when asmall amount of the rutile powder is added to theanatase powder, i.e. the slope at the left-hand side ofthe peak is smaller than that at the right-hand side. Inaddition, the activity of the NS-51 powder before re-moving the anatase component, which contains only1.5% anatase, shows fairly high activity, as shown byan open square. These results suggest that the pres-ence of a small amount of anatase phase is importantfor attaining high photocatalytic activity.

For comparison, we mixed the ST-01 powder witha type of anatase powder (ST-41) that consists of par-ticles of about 200 nm. In this case, the photocatalyticactivity decreased with the increase of the content ofthe ST-01 powder, as shown by the broken line inFig. 3. We were unable to test the combination of largerutile powder and very small rutile powder, becausevery small rutile powder is not available.

Fig. 4shows the SEM pictures of the anatase ST-01(a) and rutile NS-51 (b) powders, and the mixed (c

Fig. 3. Photocatalytic activity of TiO2 powders prepared by mix-ing ST-01 powder with NS-51 powder (solid line) or ST-41powder (broken line) at various ratios for the production of2-formylcinnamaldehyde. NS-51 powder was used after removingthe anatase component (1.5%) originally included in the powder.The open square represents the activity of NS-51 powder with-out removing the anatase component. Reaction conditions are thesame as shown inFig. 2.


Fig. 4. SEM images of (a) ST-01 powder, (b) NS-51powder, and their mixtures at ratios of (c) 9:1 and (d) 1:9, respectively.

and d) powders. The SEM picture of the mixed pow-der with 90% of rutile (d), which showed nearly thehighest photocatalytic activity, reveals that the fineanatase particles are effectively loaded on each rutileparticle. In the case of the powder with 10% of ru-tile (c), a rutile particle is loaded with many anataseparticles. In this powder, besides such anatase-loadedrutile particles, many anatase agglomerates are seenby SEM observation (not shown inFig. 4). Since theST-01 powder shows very low activity in this reaction,these agglomerates are considered to be ineffective.The high activity of the mixed powders with a rutilecontent of about 70–90% is, therefore, attributed tothe effective loading of fine anatase particles on rutileparticles. It should be noted that the activity of thispowder is higher than those of the P-25 powder andthe heat-treated TT-2 powder (seeFig. 2). The optimalcontent of the anatase particles in the photocatalyst isconsidered to be dependent on the mixing conditions

and, probably, on the particle size of the original pow-ders.

The absorption spectra of 300 nm thick rutile andanatase films are shown inFig. 5. The anatase film wasdeposited on a quartz substrate by the radio-frequencyspattering method, and the rutile film was obtainedby heating the film at 900◦C in air. The differencein the onset of the absorption of the films is due totheir optical band gaps[23]. The poor photoabsorptionof anatase TiO2 at around 365 nm, which is the maincomponent of emitted photons from the light source,suggests the possibility that this optical property is thereason for the low photocatalytic activity of the pow-ders with high anatase contents, as shown inFig. 3.However, this possibility is refuted by the fact thata sufficient amount of TiO2 particles (100 mg) wasadded to the reaction solution (3.5 ml). Under such ex-perimental conditions, the effective optical path lengthof TiO2 particles suspended in the reaction solution


Fig. 5. Absorption spectra of 300 nm thick rutile and anataseTiO2 films deposited on quartz substrates. Oscillations seen atwavelengths longer than 400 nm are due to interference of lightby the thin TiO2 films.

is about 50�m, and the irradiated UV light is effec-tively absorbed by the particles, even when they areall anatase. As a consequence, the low activity of pureanatase powder should not be ascribed to its opticalproperties but to its low photocatalytic activity for theoxidation of naphthalene. This conclusion is also sup-ported by the fact that the ST-01 powder shows highactivity for oxidation of 2-propanol, as discussed later.

Low photocatalytic activity of anatase fine particlesis also observed for oxidation of water[6]. Photo-electrochemistry of rutile and anatase TiO2 electrodesshows that band bending in TiO2 is necessary to pho-tooxidize water on them[6]. This means that a spacecharge layer must be developed in TiO2 near the sur-face for the production of oxygen, which requires fourholes. It is known that TiO2 particles often containTi3+ species[21,24], which allow the development ofthe space charge layer. If the concentration of the Ti3+species is 5× 1016 cm−3, band bending of 0.2 V willbe generated with the development of 220 and 140 nmthick space charge layers for rutile (dielectric constantε = 114) and anatase (dielectric constantε = 48)particles, respectively. Hence, TiO2 powders consist-ing of particles smaller than 10 nm are unlikely to de-velop sufficient band bending, leading to low activityfor oxidation of water. Although the photooxidation ofnaphthalene on TiO2 electrodes has not been studied,the low activity of small anatase particles suggests thatband bending is necessary for this reaction as well.On the other hand, for the photocatalytic reactions ofalcohols, it is known that small anatase particles showhigh activities. The photoelectrochemistry reveals that

Fig. 6. Photocatalytic activity of TiO2 powders prepared by mix-ing ST-01 powder with NS-51 powder at various ratios for theproduction of acetone from 2-propanol. The reaction was carriedout for 0.5 h in an aqueous solution of 0.5 M 2-propanol. Theother conditions are the same as shown inFig. 2.

these compounds are easily oxidized on TiO2 elec-trodes without having large band bending[25].

Fig. 6 shows the photocatalytic oxidation of2-propanol on the mixed NS-51 and ST-01 powders.In this case, the pure rutile NS-51 powder shows lowactivity as in the case of oxidation of naphthalene.However, in contrast to the oxidation of naphthalene,the pure anatase ST-01 and some samples with highcontents of anatase phase show high activity, probablybecause 2-propanol is easily oxidized on small anataseparticles. The very high photocatalytic activity of theST-01 powder and the sample with the ST-01 con-tent of 90% is probably due to the current doublingmechanism, which is known for the photooxidationof alcohols on TiO2 electrodes[25,26]. An importantpoint is that the pure rutile powder is inefficient forthe reaction of 2-propanol as well as for the reactionof naphthalene. From this result, it is expected thatthe low activity of the pure rutile particles is relatedto the reduction process, i.e. the electron transfer tooxygen, which is common to the two reactions.

3.4. Electrochemical oxygen reduction at TiO2electrodes

The results thus far obtained suggest that the smallanatase particles loaded on rutile particles mediate theelectron transfer from the rutile particles to oxygen,


Fig. 7. Current–voltage characteristics of a rutile TiO2 ceramicelectrode before (solid line) and after (broken line) loading fineanatase particles (ST-21) on it. The solution is 0.1 M H2SO4 andwas bubbled with oxygen.

and enhance the total photocatalytic reaction. In or-der to obtain evidence for the above electron relayprocess, the electrochemical properties of rutile TiO2ceramic electrodes were examined. The solid line inFig. 7 shows the current–voltage (I–V) characteris-tics of the TiO2 ceramic electrode measured in 0.1 Msulfuric acid that was bubbled with oxygen. The ca-thodic current observed at potentials more negativeof −0.4 V versus Ag/AgCl is ascribed to reduction ofoxygen, because the current disappears if the bubblinggas is changed from oxygen to argon. When smallamounts of the anatase ST-21 particles are loaded onthe TiO2 ceramic electrodes, this current is largelyenhanced, as shown by the broken line inFig. 7.The current density differs to some extent among theelectrodes prepared under the same conditions. Thisis probably because the amount of the anatase TiO2particles loaded on the TiO2 electrodes differs. How-ever, the current density for the electrodes loaded withsmall anatase particles was always higher than that ofthe non-loaded electrode by about three to five times.This result strongly suggests that the electron transferfrom rutile TiO2 particles to oxygen is enhanced bydepositing small anatase TiO2 particles. The enhance-ment is considered to be due to the catalytic activityof the anatase surface being higher than that of therutile surface for the reduction of oxygen.

3.5. Mechanism of electron transfer and totalreaction in the mixed powders

The conduction band of anatase TiO2 has been con-sidered to locate at a higher energy position than that

of rutile TiO2 by about 0.2 eV[27]. This relationshipof the energy levels is disadvantageous to the elec-tron transfer from rutile particles to anatase particles,which is assumed to be the reason for the synergis-tic effect between them. If holes are very effectivelyconsumed by naphthalene on the rutile particles, elec-trons accumulate in them, leading to an upward shiftof the conduction band[28]. However, this modelcontradicts with the opinion that the band bending isimportant for the oxidation of naphthalene. Hence,we have to consider that electrons are transferredfrom rutile particles to anatase particles via thermalactivation. This electron transfer is supported by thefact that electron transfer via energy barriers higherthan 0.2 eV is a common practice in semiconduc-tor and organic electronic devices[29,30]. Althoughdetails of the mechanism remain to be clarified, allthe results obtained from the photocatalytic activitiesof the mixed TiO2 particles and the electrochemicalreduction on the TiO2 electrodes demonstrate thatthe electrons are effectively transferred from rutileparticle (or electrode) to anatase particles.

As discussed above, rutile particles show strongerphotoabsorption than anatase particles (seeFig. 5).Hence, for example, if a mixed powder with 70% ofrutile particles and 30% of small anatase particles isphotoirradiated, most of the light is absorbed by the ru-tile particles. The electrons are considered to be trans-ferred to anatase particles loaded on them and reduceoxygen. On the other hand, holes probably react withnaphthalene on the rutile particles, because their largeparticle size is useful to drive the oxidation reaction.Bickley et al. [31] attributed high photocatalytic ac-tivity of the P-25 powder to the presence of rutile andanatase phases in it. They assumed that each effec-tive particle in the powder consists of an anatase coreand a thin rutile cover layer, and that the holes pho-togenerated in the anatase core are effectively trans-ferred to the rutile layer. We consider that they areright when they insist that the interaction between ru-tile and anatase phases is responsible to the high pho-tocatalytic activity of the P-25 powder. However, wehave revealed that the P-25 powder consists of rel-atively large rutile particles and very small anataseparticles, which separately form their agglomeratesbefore mixing[22]. We therefore consider that, underexperimental conditions of photocatalytic reactions,the agglomerates are destroyed and the rutile particles


are loaded with small anatase particles as the NS-51powder physically mixed with the ST-01 powder, asshown inFig. 4.

4. Conclusion

We found synergism between anatase and rutileparticles for the photocatalytic oxidation of naphtha-lene. By simply mixing these particles, we attainedhigher activity than those of the original powders.Furthermore, the activity is higher than those ofmany kinds of TiO2 powders obtained from differentsources. These results are explained by assuming thatrutile particles are advantageous to the oxidation ofnaphthalene. Thus, small anatase particles are consid-ered to be advantageous to the reduction of oxygen,probably because of the surface catalytic activity. Thecombination of large rutile particles and small anataseparticles is practically useful, because rutile particlesare generally produced at high temperatures and havelarge particle sizes, and anatase particles are producedat low temperatures and have small particle sizes.Although what we demonstrated as the synergismbetween rutile and anatase particles is only for theoxidation of naphthalene, we consider that a similareffect exists in photocatalytic reactions when oxygenis used as the electron acceptor.

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