Thin Solid Films 519 (2011) 8077–8084

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Structural characterization and photocatalytic activity of ultrathin TiO2 filmsfabricated by Langmuir–Blodgett technique with octadecylamine

Masashi Takahashi a,⁎, Koichi Kobayashi a, Kazuo Tajima b

a Department of Chemistry and Energy Engineering, Tokyo City University, 1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158-8557, Japanb Department of Chemistry, Kanagawa University, 3-27 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan

⁎ Corresponding author. Tel.: +81 357070104; fax: +E-mail address: mtakahas@tcu.ac.jp (M. Takahashi).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.06.077

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 November 2010Received in revised form 31 May 2011Accepted 23 June 2011Available online 30 June 2011

Keywords:OctadecylamineLangmuir–Blodgett filmPhotocatalytic activityPotassium titanium oxalateTitanium dioxide thin filmAtomic force microscopy

TiO2 thin filmswith nanometer-scale thicknesses were prepared by the hydrolysis of titanium potassium oxalateusing octadecylamine (ODA) Langmuir–Blodgett (LB) films as templates. After optimizing conditions inimmersion process, the amount of TiO2 generated in the ODA LB filmwas found to be precisely controlled by thenumber of deposited ODA layers. Morphological measurements showed that uniform TiO2 film with surfaceroughnessof less than1.3 nmcouldbeprepared from themonolayerLBfilms through subsequent heat-treatmentprocess, while generation of cracks became less noticeable on the 5-layer film after heat-treatment at lowerholding temperaturewith slowheating rate. In addition, photocatalytic activities of the TiO2filmswere examinedfrom the decomposition of cadmium stearate (CdSt) LB films and stearic acid (SA) cast films for different timeintervals of irradiation with UV light. Atomic force microscopymeasurements showed that an almost flat surfaceof the CdSt LB film changes to a moth-eaten appearance as a result of decomposition under UV light irradiation.Furthermore, the post-heat-treatment at higher temperatures resulted in decreased photocatalytic activity of theTiO2 film for the decomposition of SA cast film.

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1. Introduction

Among the inorganic films made up of metal oxides, TiO2 films havearoused interest owing to their potential use as anode material forphotoelectrical energy conversion [1], photocatalyst for decomposingwater or organic compounds [2–4], and coating material for improvingcertain surface properties [5]. To obtain thin TiO2 films, severalmethodsincluding sol-gel method, chemical vapor deposition and pyrosolmethod as well as conventional dip-, spray- or spin-coating and screenprinting of a colloidal solution have been employed so far. While well-defined structures of the TiO2 films are required to characterize theirfunctionalities quantitatively, these preparation methods are stillinsufficient in precise control of film thickness on the nanometer-scaleand uniformity over large substrate areas. Although, electrochemicaldeposition has recently emerged as a promising method for preparingTiO2 films with controlled structure [6,7], substrates available in thismethod are essentially restricted to conductive plates.

Alternatively, versatile Langmuir–Blodgett (LB) technique, usuallyadopted to fabricate highly orderedmolecularfilm assemblies of variouslong-chain amphiphiles, has been extended to the preparation of TiO2

films over the past fewdecades [8–16]. For example, ultrathin TiO2 filmswere synthesized from titanium alkoxides through a two-dimensionalsol-gel-process [8,9]. Multilayer deposition was demonstrated for

colloidal TiO2 nanoparticles or nanosheets to fabricate orderedorganic/inorganic structures [10]. Further, densely packed exfoliatedtitania nanosheet film could be prepared by the LB technique withoutany amphiphilic additives [11]. In our previous studies, we reported thepreparation of titania nanotube LB films by direct spreading ofhydrophobized particles onto the surface of aqueous subphase [17].The structure of the LB films obtained by subsequent deposition wasfound to consist of piles of rod-like particulate monolayers in whichnanotube bundles were arranged in a manner resembling floating logs.TiO2 particulate LB films also could be fabricated from Langmuirmonolayers on which TiO2 particles were adsorbed two-dimensionallyfrom a colloidal subphase [12,18]. In this case, the TiO2 particles in thecolloidal dispersion were negatively charged at near-neutral pH;therefore, for the adsorption of the particles by Coulombic interactions,we employed cationic amphiphiles of octadecylamine (ODA) as a filmmaterial throughout the experiments.

As a candidate process, Ganguly et al. demonstrated that the use oflong-chain amine monolayers and an aqueous subphase containingpotassium titanium oxalate (K2TiO(C2O4)2; PTO) results in formation ofa complex at the air–water interface [19], yielding TiO2 films via LBdeposition and post-heat-treatment [20]. Meanwhile, spectroscopicinvestigations showed that colloidal TiO2 clusters were simultaneouslygenerated in the subphase by slow hydrolysis of PTO upon aging [13].Thus, it is of concern that the TiO2 clusters might be adsorbed unevenlyon the floating monolayer.

To develop this process, we conducted the study toward using LBfilms based on our previous works dealing with adsorption of dye on

Fig. 1. Schematic illustration of the TiO2 film preparation process.

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the ODA LB films [21,22]. Specifically, the sequence of the preparationsteps was modified to make the process applicable to an ODA LB filmas a template for nucleation and crystal growth of TiO2 from PTO(Fig. 1). The present procedure is considered to have the advantagesof reducing PTO usage, as well as preventing excess build up of TiO2

clusters in the multilayered film. In this study, we first examinedinfluences of preparation conditions on the structure of the TiO2 films,so as to prepare ultrathin TiO2 films with uniform and controlledthickness. The photocatalytic properties of the TiO2 films were nextdemonstrated by decomposition of cadmium stearate (CdSt) LB filmsand stearic acid (SA) cast films. From atomic force microscopy(AFM) measurements, we also observed surface morphologicalchange of the CdSt LB film during photodegradation on the TiO2 film.

2. Experimental details

ODA (≥99% pure, Aldrich Chemical Company, Inc.) and PTO (≥90%pure, Wako Pure Chemical Industries, Ltd.) were used as the filmmaterial and Ti source, respectively. These materials were of thehighest grade andwere used without further purification. Preparationof the LB films was carried out using a Langmuir trough (HBMAP-typeLB film balance, Kyowa Interface Science). Subphases were madeusing distilled deionized water. An ODAmonolayer was spread from a1.0×10−3 mol dm−3 chloroform solution on the aqueous subphaseat 20 °C; afterward, left for 10 min to allow the spreading solvent toevaporate. To avoid protonation of amino group in the ODA molecule,the pH of the subphase was adjusted to be in the range of 10.1–10.5 byadding sodium hydroxide, thereby forming a typical condensed ODAmonolayer [22]. The monolayer was stable for withstanding highsurface pressures up to 60 mNm−1. Limiting area of the condensedODA monolayer was 0.22 nm2 molecule−1, indicating a close-packedarrangement of the hydrocarbon chains. Deposition of the ODAmonolayer on solid substrate was performed at a surface pressure of45 mN m−1 using the conventional vertical dipping method withdipping and withdrawal speed of 7 mmmin−1. We used calciumfluoride plate, quartz plate, glass plate, and silicon wafer as the solid

Fig. 2. Time course of UV–vis spectrum of 5-layer ODA LB film on a quartz plate duringimmersion in 1×10−3 mol dm−3 PTO solution at 20 °C. Inset indicates the changes inabsorbance at 240 nm with immersion time for the 3- and 5-layer ODA LB films.

substrates for different experiments. To prevent re-spreading of thedeposited layer, a drying time of at least 10 min was used betweendips of the substrate.

TiO2 thin layers were generated by immersing as-deposited ODALB films (1–9 layers) in an aqueous PTO solution at 20 °C for givenperiod of times. In this process, which is analogous to a reaction withammonia, the PTO is believed to react with ODA to produce TiO2 inthe LB layers. Subsequently, the ODA–TiO2 LB films were sintered inair for 60 min at different holding temperatures (300, 400, 500, and600 °C). According to a previous report on the thermal analysis ofODA-modified single-walled carbon nanotubes, a reduction inweight due to the reacted ODA was observed in simultaneousthermogravimetry–differential thermal analysis around 300 °C [23].Thus, while the boiling point of ODA is 347 °C at ambient pressure,the heating temperatures applied in the present experiments aresufficient to eliminate ODA from the films.

Amounts of TiO2 and organic film materials on the solid substratewere estimated byultraviolet–visible (UV–vis) spectroscopy (ShimadzuUV-3100PC) and Fourier transform infrared (FTIR) spectroscopy (JASCOFT/IR-8900), respectively. Surface information for the films such as theelemental composition and the valence states at different preparationstages was acquired using X-ray photoelectron spectroscopy(XPS) capability of an SSX-100 spectrometer (Surface Science In-struments) at a takeoff angle of 35° with a monochromatic Al Kα X-raysource (hν=1486.7 eV) operating at 10 kV, 13 mA and a chargeneutralizer. The charge-up shift was corrected with respect to the C1speak at 284.6 eV for energy calibration. Surface morphologies of the LBfilmswere studied byAFM(Digital InstrumentsNanoscope IIIa). All AFMimages were obtained in the tapping-mode operation under atmo-spheric conditions. Cantileversusedwere commercially available etchedsilicon probe (Veeco, TESP-type) with a length of 125 μm and resonantfrequencies of 303–383 kHz. In addition, it should bementioned thatwealso attempted structural characterization of the TiO2 films using X-raydiffractometers (Rigaku RINT2500, 2000 and UltimaIII) with a CuKα

(λ=0.154 nm) radiation source (40 kV, 40–300mA). However, nodiffraction peak appeared in θ/2θ scan mode with a small angle ofincidence, probably because the film thickness was extremely small.

Fig. 3. Plot of absorbance at 240 nm against PTO concentration for 5-layer ODA LB film.The ODA LB films were immersed in the PTO solutions at 20 °C for 60 min.

Fig. 4. Relationship between absorbance at 240 nm and number of ODA layers.Absorbance corresponds to generated amount of TiO2 during immersion in the1×10−3 mol dm−3 PTO solution at 20 °C.

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Photocatalytic activity of the TiO2 thin films was evaluated fromthe photodecomposition of both SA cast film and CdSt LB filmdeposited over the TiO2 films. The light source was a 500 W xenonshort-arc lamp (Usio SX-UI500XQ optical moduleX) equipped with awater-based IR-cut filter, and a UV transmitting-visible absorbingfilter (HOYA, U330). FTIR spectra were recorded over different timeintervals under irradiation with UV light to estimate the amount ofdecomposition from the change in the absorbance of the CH2

antisymmetric stretching vibration band at 2918 cm−1.

3. Results and discussion

3.1. Fabrication of ODA LB films and formation of TiO2 films

As a template for the formation of TiO2 film, ODA LB films wereprepared from the Langmuir monolayer on the subphase at a pHabove 10.1 (corresponding to pKa of long-chain amines). Applying asurface pressure of 45 mNm−1, multilayer deposition of the ODAmonolayer on the solid substrate was carried out with a transfer ratioclose to unity. The details of the experiments have been describedelsewhere [22].

Subsequently, the as-deposited ODA LB films were immersed in anaqueous PTO solution. To ensure the formation of a TiO2 film, UV–visspectra of the ODA LB films were measured at different stages in thepreparation procedure. After immersion in the aqueous PTO solution,a distinctive absorption bandwas observed below 350 nm (see Fig. 2).This result indicates formation of TiO2 because UV light absorption isassigned to the excitation of electrons from the valence band to theconduction band of TiO2. In contrast, whenwe employed an SA LB filminstead, no absorption band appeared in the UV–vis spectrum. Thus, itis clear that the amino groups in the ODA LB film catalyze hydrolysis ofthe titanium oxalate ions, so that nucleation and crystal growth areinduced to form a TiO2 thin film. Accordingly, the ODA LB film is

Fig. 5. AFM images at various preparation stages using monolayer ODA LB film: (a) as-depos

considered to play a major role as a template during the immersionprocess.

Since penetration of titanium oxalate ions into the inner layer ofthe ODA LB film may become a rate-determining step for thegeneration of TiO2, we first examined the influence of the immersionconditions on the structure of the TiO2 films. Fig. 2 shows the timecourse of UV–vis spectrum for the 5-layer ODA LB films duringimmersion in the 1×10−3 mol dm−3 PTO aqueous solution at 20 °C.Change in the band intensity in the UV region indicates that TiO2

gradually generates in the LB film with immersion time, and finallythe amount of TiO2 reaches a constant value. As illustrated in the insetof Fig. 2, we plotted the change in the absorbance at 240 nm againstimmersion time for the 3- and 5-layer ODA LB films. The results showthat the immersion time of ca. 20 min is required to achieveequilibrium for the 3-layer ODA LB film, while it is more than30 min for the 5-layer ODA LB film. In addition, when we employedthe ODA LB films with a larger number of built-up layers, a longerimmersion time was needed (not shown in the inset figure). On theother hand, a higher concentration of PTO could shorten theimmersion time required to attain a constant value of the generatedTiO2. However, the amounts of TiO2 were almost identical for ODA LBfilms with the same number of layers regardless of the concentrationof PTO.

Fig. 3 shows an adsorption isotherm of PTO for the 5-layer ODA LBfilms with an immersion time of 60 min. The amount of TiO2

generated increased with increasing PTO concentration and reachedsaturation around 1×10−4 mol dm−3. This indicates that titaniumoxalate ions are chemically adsorbed to the ODA LB film to generateTiO2. Taking these results into account, we employed the mostappropriate conditions for completing the generation of TiO2, i.e.,60 min immersion in 1.0×10−3 mol dm−3 PTO solution for the 1–5layer-LB films, in the subsequent experiments.

Next, we checked the generated amount of TiO2 for the LB filmswith different numbers of layers. As shown in Fig. 4, the ODA LB filmsprocessed with the PTO solution for sufficient immersion time offeredan almost proportional relationship between absorbance at 240 nmand the number of ODA layers. Therefore, it is clear that theadsorption and subsequent hydrolysis of PTO stoichiometricallyproceed with the build-up of ODA. This means that the amount ofTiO2 (=thickness of TiO2 film) could be controlled by the number ofLB layers. Such precise and facile control of the film thickness isconsidered to be the most important advantage of the presentmethod.

3.2. Structural characterizations of the films at various preparationstages

3.2.1. Surface morphological analysis by AFMSurface morphologies of the monolayer and 5-layer LB films were

observed using AFM measurements. Figs. 5 and 6 display change inAFM images for the LB films on a silicon wafer at differentpreparation stages. In the images for the monolayer ODA LB film(Fig. 5), the as-deposited film revealed an almost uniform layer,except for projections of dust. However, the monolayer LB film

ited LB film, (b) after immersion in PTO solution, and (c) after heat-treatment at 500 °C.

Fig. 6. AFM images at various preparation stages using 5-layer ODA LB film: (a) as-deposited LB film, (b) after immersion in PTO solution, and (c) after heat-treatment at 300 °C.

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underwent a characteristic change during the course of theimmersion process. For instance, the formation of irregularly shapedpatches was typically seen in the image. Such a change in the layerstructure could be expected by lateral expansion of the monolayer.The generation of TiO2 at around the polar-head groups in themonolayer yields an area expansion and thus results in failure toretain the planar surface, pushing parts of the collapsed film outfrom the monolayer. After subsequent heat-treatment at 500 °C for60 min in air, organic molecules were eliminated from the LB film,and then a thin film (monolayer TiO2 film) was left on the solidsubstrate. Cross-sectional analysis revealed that the monolayer TiO2

film has a uniform and almost even surface with a roughness of lessthan 1.3 nm.

For the 5-layer ODA LB film in Fig. 6, the as-deposited LB film alsohad a uniform and flat surface, except for defects in the form of holes in

Fig. 7. AFM images of 5-layer TiO2 films on a silicon wafer heat-treated at various temperaturewas elevated to the target point in 60 min.

Fig. 8. AFM images of 5-layer TiO2 films on a silicon wafer heat-treated at (a) 300

the layer structure, and it turned into an uneven surface afterimmersion in the PTO solution. Comparing AFM images at this stage,the roughness of the 5-layer LB film is much larger than that of themonolayer LB film due to area expansion of respective layers in the LBfilm. Similar to the change in Fig. 5c, heat-treatment at 300 °C reducedthe surface roughness; consequently, a filmwith almostflat surfacewasformed on the substrate (5-layer TiO2 film). However, as seen in theimage in Fig. 6c, slight cracks were found to appear on the 5-layer TiO2

film after heat-treatment at a heating rate of 5 °C min−1. This isconstrued as a result of the shrinkage of TiO2 film during heating; thus,the crack generation is supposed to be promoted by heat-treatment athigher temperature. In fact, cracks became more noticeable as theheating temperaturewas increased to 600 °C (Fig. 7). In addition, cross-sectional analysis allowed us to estimate the film thickness from thedepth of the cracks to be 3–4 nm for the 5-layer TiO2 film.

s with a 60 min holding time: (a) 300, (b) 400, (c) 500, and (d) 600 °C. The temperature

°C, and (b) 500 °C for 60 min with fast heating (left) and slow heating (right).

Fig. 9. Survey-scan XPS spectra of 5-layer ODA-TiO2 LB film (a) before and (b) afterheat-treatment at 500 °C for 60 min. Fig. 11. Plots of (αhν)1/2 vs. photon energy for TiO2 films: (a) before heat-treatment,

and after heat-treatment at different temperatures; (b) 300 °C, (c) 400 °C, (d) 500 °C,and (e) 600 °C.

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Since a rapid raise of temperature usually causes an increase insurface stress of the film, a slower heating rate would be effective forreducing the cracks of the film. To examine the influence of heatingrate, the 5-layer ODA-TiO2 LB films were heat-treated at 300 and500 °C with fast heating (5–8 °C min− 1) and slow heating(1 °C min−1), followed by AFM measurements. As shown in Fig. 8,although slower heating substantially prolonged residence time in theelectrical furnace, cracks hardly appeared in both the films heat-treated at a rate of 1 °C min−1. Accordingly, slow heating rate wasconfirmed to be favorable for suppressing the rise in surface stress,offering the films with smooth and homogeneous surface. In addition,regardless of the presence of cracks, all the TiO2 films prepared in thisstudy were transparent and bonded tightly to the substrates.

3.2.2. XPS analysisXPS analysis was studied to characterize change in surface

composition of the films during the heat-treatment. Fig. 9 shows theXPS survey spectra of the 5-layer ODA-TiO2 LB films on the glass platebefore and after heat-treatment at 500 °C for 60 min with fast heating(8 °C min−1). For the film before heat-treatment, photoelectronpeaks of the expected elements such as C 1s, N 1s, Ti 2p and O 1scould be observed in the spectrum (a), indicating presence of the TiO2

in the ODA matrix. In contrast, peaks ascribed to potassium elementfrom PTO was absent in the spectrum. This allows us to infer that theamount of K+ incorporated in the LB film is inherently small.Simultaneously, K+ may be localized at the polar head group regionof the ODA molecules in inner layers of the LB film at a depth of a few

Fig. 10. UV–vis spectra of (a) ODA-TiO2 LB films, and the films after heat-treatment atdifferent temperatures; (b) 300 °C, (c) 400 °C, (d) 500 °C, and (e) 600 °C.

nanometers from the surface, which also causes less escapingphotoelectrons from the film.

It should be noted that, based on the surface compositions obtainedfrom Ti 2p and N 1s peaks, the atomic ratio of Ti/N within a fewnanometers from the surface was estimated to be 0.91. Taking intoaccount dissociation constants of oxalic acid (pKa1=1.27, pKa2=4.27at 25 °C) and pH value of 3.7 for the 1.0×10−3 mol dm−3 aqueous PTOsolution, we calculated the Ti/N ratio of 0.83, which is almost inagreement with the experimental value. This result gives furthersupport to the above-mentioned consideration that the reaction of PTOand ODA proceeds stoichiometrically to yield the TiO2 film under theappropriate conditions.

In the spectrum (b), peaks for K 2p, Si 2s and Si 2p are located at294, 152 and 101 eV, respectively. Appearance of the K 2p peak maybe caused by segregation of potassium compounds from inner layer tothe surface of the film during heat-treatment. Also, the presence ofcracks on the TiO2 film can be confirmed from the Si 2s and Si 2p peaksbecause these peaks originated from the glass substrate. On the otherhand, disappearance of N 1s peak in the spectrum (b) indicates thatthe ODA template is completely removed from the heat-treated film.Similarly, most of the other organic compounds are considered to bedecomposed by the heat-treatment, though a relatively weak C1speak due to residual carbon and/or adventitious hydrocarbon isobserved.

In the corresponding high-resolution spectra of Ti 2p (not shown),the peaks of spirited spin-orbit doublet components (2p1/2 and 2p3/2)appeared approximately at 464.1 and 458.3 eV regardless of the heat-treatment. The two peaks were very symmetrical, and the energysplitting of 5.8 eV between them was close to that expected fromstandard binding energy tables. Thus, these results indicate that the Tiat the surface exists in the 4+ oxidation state in a tetragonalstructure [24], which is hardly changed during the heat-treatment atleast up to 500 °C.

3.2.3. UV–vis spectraFig. 10 shows UV–vis spectra of the 5-layer TiO2 films on a quartz

plate with different heat-treatments. These curves exhibited hightransparency of the films in the visible light region, but a slightly-higherbaseline could be noticed when the film was calcined at 300 °C. Thismay be attributed to reflectance from particles generated by heat-treatment typically at lower temperature. With increasing holdingtemperature, the heat-treated TiO2 film showed a weaker absorption inthe UV–vis region and the absorption edge wavelength graduallyshifted toward the shorter wavelength. To estimate the indirect bandgap energy for these films, (αhν)1/2 was plotted against photon energy(hν), where α represents absorption coefficient [25,26]. The band gaps

Fig. 12. Photocatalytic activity of the 5-layer TiO2 film for decomposition of CdSt LB film; (a) FTIR spectra of CdSt LB films irradiated with UV light for 0–240 min, and (b) plots ofabsorbance at 2918 cm−1 (CH2 antisymmetric stretch) as a function of irradiation time: (□) 5-layer TiO2 film, and (●) bare quartz plate. The CdSt LB film was deposited on the TiO2

film heat-treated at 500 °C.

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of TiO2 films could be derived from the abscissa intercepts of the linearportion of the curves. As shown in Fig. 11, the estimated band gapenergy of 3.5 eV for the ODA-TiO2 LB film (curve a) changed to 3.3 eVafter heat-treatment at 300 °C (curve b). This can be ascribed to thegrowth of TiO2 crystallites during heat-treatment. On the other hand,the band gap energies of the heat-treated TiO2 films increased from 3.3to 3.6 eV with increasing holding temperature. As is well known, heat-treatment at higher temperatures generally promotes increase innanoparticle size, resulting in decrease of band gap energy due toquantum size effects [27]. In contrast, the blue-shift of absorption edgeappearing in the present results is opposite to this tendency. This maybe explained by particle internal stress which leads to the change ofenergy gap structure. Similar to the literature, effect of the particleinternal stress is supposed to exceed the quantum size effect [28].

3.3. Photocatalytic activities for decomposition of organic thin-films

The photocatalytic performance was tested for the TiO2 films usingUV light of 35 mW cm−2 intensity under ambient conditions. As asolid substrate, we used UV-transparent quartz plate in the followingexperiments. As an organic decomposing material, we first employedCdSt LB films because they offer a uniform coating with controlledthickness. Fig. 12(a) shows a series of FTIR spectra for thephotodecomposition of CdSt LB film on the 5-layer TiO2 film heat-treated at 500 °C. Bands peaking at 2852 and 2918 cm−1 are assignedto CH2 symmetric and asymmetric stretching vibrations, respectively.The intensities of these peaks were gradually decreased overirradiation time. We plotted peak absorbance at 2918 cm−1

vs. irradiation time together with a control sample in Fig. 12(b).Since these bands originate from hydrocarbon chains of CdSt,decrease in the absorbance directly translates to a decrease in CdStloading on the TiO2 film. Thus, it is clear that the CdSt LB film on theTiO2 film was decomposed faster than that on the bare quartz plate,

Fig. 13. AFM images of CdSt LB film on the 5-layer T

indicating a photocatalytic activity of the TiO2 film. Similar to theprevious report for TiO2 films prepared by a dip-coating technique[29], the reaction proceeded according to pseudo-first-order reactionkinetics, i.e., [CdSt]t=[CdSt]0 exp(−kt), where [CdSt]t and [CdSt]0 arethe concentrations of CdSt at time t=0 and t, respectively, and k is thepseudo-first-order constant. The k value for the TiO2 film wascalculated to be 7.0×10−3 min−1 from the data in Fig. 12(b), whichis almost one order of magnitude less than that reported for SAdecomposition under 0.5 mW cm−2 UV light irradiation [29].

Change in surface morphologies during UV light irradiation wasobserved by AFM measurements. Fig. 13 shows AFM images of theCdSt LB film at different photodecomposition stages on the 5-layerTiO2 film heat-treated at 500 °C. These images clearly display the CdStLB film, initially having an almost flat and homogeneous surface,gradually eroding due to exposure to UV radiation, changing to amoth-eaten appearance. This indicates that the film underwentinhomogeneous photodecomposition at the submicrometer levelprobably because photocatalytic reaction centers were distributedover the TiO2 surface. In the image at 30 min irradiation, weparticularly found that most of the holes are arranged in lines,reflecting distribution pattern of the active sites in the catalyst film. Inaddition, when the measurement of tapping-mode AFMwas repeatedin the same scanning area, enlargement of pore size was observed forthe UV irradiated LB films. This can be interpreted as the result ofinternal corrosion of the CdSt LB film, leading to embrittlement of therigid film structure. Namely, taking into account the fact that thecontact between CdSt LB film and TiO2 film is due to van der Waalsinteraction and the photodecomposition occurs at the TiO2 filmsurface, we inferred that void spaces are generated in the solid-stateLB film by the UV irradiation. These void spaces usually inhibit theCdSt LB film from sufficient contact with the TiO2 film, therebydecreasing the decomposition rate even in the early stage ofirradiation as indicated by the small k value.

iO2 film at different UV light irradiation times.

Fig. 14. Photodecomposition curves of SA cast film as a function of UV-light irradiationtime; 5-layer TiO2 films without heat-treatment (○) and with heat-treatment at 300 °C(△), 500 °C (□), and 600 °C (w). Result for bare quartz plate (●) was also indicated.

Fig. 15. Photocatalytic activities of TiO2 films heat-treated at 300 °C for decompositionof SA cast film. The TiO2 films were prepared from ODA LB films with different numbersof layers by: (△) monolayer, (□) 5 layers, and (w) 9 layers. Result for bare quartz plate(●) was also indicated.

8083M. Takahashi et al. / Thin Solid Films 519 (2011) 8077–8084

Subsequently, influence of preparation conditions of the TiO2 filmson their photocatalytic activities was examined in the samemanner asused in Fig. 12. As an organic decomposing material, we employed SAcast films instead of the CdSt LB films in the following experiments. Inaddition to the heat-treated TiO2 films, we specially prepared the TiO2

film without heat-treatment. To eliminate organic compounds fromthe ODA-TiO2 LB film, the LB film was processed by ultraviolet/ozonetreatment with a UV-ozone cleaner (Nippon Laser & ElectronicsLab., NL-UV253).

Fig. 14 shows changes in absorbance at 2918 cm−1 for the 5-layerTiO2 films with different heat-treatments. The photodecompositionrates of SA for all TiO2 films were significantly larger than that for abare quartz plate. As to the TiO2 film heat-treated at 500 °C, a value of3.2×10−2 min−1 was estimated for k in the initial stage of irradiationuntil 30 min. Comparison of the k values for CdSt LB film and SA castfilm shows that the decomposition of SA cast film proceeded 4.5 timesfaster than that of CdSt LB film under the same conditions. Thisdifference in decomposition rate is interpreted to be due to softness ofthe SA film by which the SA film can keep contact with the surface ofphotocatalyst.

In general, calcination process has been applied to enhancephotocatalytic performance of TiO2 catalysts. This is because improve-ment of crystallinity by heat-treatment reduces trap sites associatedwithoxygenvacancy in the crystal lattice, being favorable for preventingthe electron–hole recombination. Nevertheless, our current resultsindicated that the photocatalytic activity of TiO2 film was decreasedwith increasing heat-treatment temperature. Although the reason ofthis tendency remains uncertain at present, one possible explanation isthat utilization efficiency of UV light below 330 nm was lowered withincreasingheating temperature due to theblue-shift of absorption in theUV region. Meanwhile, the rise in temperature caused decrease in theactive surface area of TiO2 as well as generation of cracks, which maydecrease photocatalytic activities of the TiO2 films.

Similar to the result for the 5-layer TiO2 films, we found a decreasein photocatalytic performance for the monolayer TiO2 films whenapplying higher holding temperatures (data not shown). Compared tothe heat-treatment at the same temperature in Fig. 14, the monolayerfilm was less active than the 5-layer film. To elucidate this trend,photodecomposition curves for the 1–9 layer films with heat-treatment at 300 °C are presented in Fig. 15. The figure indicatesthat photocatalytic activity of the TiO2 film is enhanced withincreasing number of layers. This can be explained by increase inthe light-absorption efficiency below 330 nm which promotesgeneration of electron–hole pairs. Furthermore, the thicker TiO2

films are supposed to have an advantage in charge separation by spacecharge layer at the film surface, while thickness of the TiO2 film, e.g.,

ca. 8 nm for 9-layer film, is small enough to allow the photoinducedelectron–hole pairs to persist while they diffuse as far as the TiO2

surface.

4. Conclusions

Ultrathin TiO2 films were prepared by the hydrolysis of PTO usingthe ODA LB films as a template. Under optimized conditions, thegenerated amount of TiO2 in the LB films was proportional to thenumber of deposited ODA layers, which indicated that the thickness ofthe TiO2 films can be precisely controlled by adjusting the amount ofODA. From the topographic height profile of the AFM images, the filmthickness was estimated to be 3–4 nm for the 5-layer TiO2 film. Also,thicker TiO2 films tended to crack during heat-treatment, while lowerholding temperatureswith slow heating rate reduced the appearance ofcracks. Upon irradiatingwithUV light, photocatalytic decompositions ofboth CdSt LB film and SA cast film were observed on the surface of theTiO2 films. Among the TiO2 films with different heat-treatments, thefilm heat-treated at lower holding temperature showed higherphotocatalytic activity for the decomposition of SA. In addition, anenhancement in the photocatalytic activity was obtained as the numberof TiO2 layers increased.

References

[1] B. O'Regan, M. Gräzel, Nature 353 (1991) 737.[2] T. Kim, B. Sohn, Appl. Surf. Sci. 201 (2002) 109.[3] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1.[4] D. Chowdhury, A. Paul, A. Chattopadhyay, Langmuir 21 (2005) 4123.[5] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M.

Shimohigoshi, T. Watanabe, Nature 388 (1997) 431.[6] Y. Matsumoto, Y. Ishikawa, M. Nishida, S. Ii, J. Phys. Chem. B 104 (2000) 4204.[7] S. Karuppuchamy, N. Suzuki, S. Ito, T. Endo, Curr. Appl. Phys. 9 (2009) 243.[8] M. Oswald, V. Hessel, R. Riedel, Thin Solid Films 339 (1999) 284.[9] I. Moriguchi, H. Maeda, Y. Teraoka, S. Kagawa, J. Am. Chem. Soc. 117 (1995) 1139.

[10] T. Yamaki, K. Asai, Langmuir 17 (2001) 2564.[11] M. Muramatsu, K. Akatsuka, Y. Ebina, K. Wang, T. Sasaki, T. Ishida, K. Miyake, M.

Haga, Langmuir 21 (2005) 6590.[12] K. Muramatsu, M. Takahashi, K. Tajima, K. Kobayashi, J. Colloid Interface Sci. 242

(2001) 127.[13] C. Malitesta, A. Tepore, L. Valli, A. Genga, T. Siciliano, Thin Solid Films 422 (2002)

112.[14] Y. Tian, J.H. Fendler, Chem. Mater. 8 (1996) 969.[15] K.S. Mayya, V. Patil, P.M. Kumar, M. Sastry, Thin Solid Films 312 (1998) 300.[16] A. Serra, A. Genga, D. Manno, G. Micocci, T. Siciliano, A. Tepore, R. Tafuro, L. Valli,

Langmuir 19 (2003) 3486.[17] M. Takahashi, Y. Okada, K. Kobayashi, Chem. Lett. 37 (2008) 276.[18] M. Takahashi, H. Natori, K. Tajima, K. Kobayashi, Thin Solid Films 489 (2005) 205.[19] P. Ganguly, D.V. Paranjape, M. Sastry, Langmuir 9 (1993) 577.[20] D.V. Paranjape, M. Sastry, P. Ganguly, Appl. Phys. Lett. 63 (1993) 18.[21] M. Takahashi, K. Kobayashi, K. Takaoka, T. Takada, K. Tajima, Langmuir 16 (2000)

6613.

8084 M. Takahashi et al. / Thin Solid Films 519 (2011) 8077–8084

[22] M. Takahashi, K. Kobayashi, K. Takaoka, K. Tajima, J. Colloid Interface Sci. 203(1998) 311.

[23] G. Gabriel, G. Sauthier, J. Fraxedas, M. Moreno-Mañas, M.T. Martínez, C.Miravitlles, J. Casabó, Carbon 44 (2006) 1891.

[24] K. Bange, C.R. Ottermann, O. Anderson, U. Jeschkowski, M. Laube, R. Feile, ThinSolid Films 197 (1991) 279.

[25] P. Sudhagar, R. Sathyamoorthy, S. Chandramohan, Appl. Surf. Sci. 254 (2008) 1919.[26] S. Janitabar-Darzi, A.R. Mahjoub, A. Nilchi, Phys. E 42 (2009) 176.[27] J. Yu, H. Yu, B. Cheng, X. Zhao, J. Yu, W. Ho, J. Phys. Chem. B 107 (2003) 13871.[28] N. Xiao, Z. Li, J. Liu, Y. Gao, Thin Solid Films 519 (2010) 541.[29] P. Sawunyama, L. Jiang, A. Fujishima, K. Hashimoto, J. Phys. Chem. B 101 (1997)

11000.