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

Thin Solid Films 519 (2011) 8077–8084

Contents lists available at ScienceDirect

Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Structural characterization and photocatalytic activity of ultrathin TiO2 films fabricated by Langmuir–Blodgett technique with octadecylamine Masashi Takahashi a,⁎, Koichi Kobayashi a, Kazuo Tajima b a b

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

a r t i c l e

i n f o

Article history: Received 12 November 2010 Received in revised form 31 May 2011 Accepted 23 June 2011 Available online 30 June 2011 Keywords: Octadecylamine Langmuir–Blodgett film Photocatalytic activity Potassium titanium oxalate Titanium dioxide thin film Atomic force microscopy

a b s t r a c t TiO2 thin films with nanometer-scale thicknesses were prepared by the hydrolysis of titanium potassium oxalate using octadecylamine (ODA) Langmuir–Blodgett (LB) films as templates. After optimizing conditions in immersion process, the amount of TiO2 generated in the ODA LB film was found to be precisely controlled by the number of deposited ODA layers. Morphological measurements showed that uniform TiO2 film with surface roughness of less than 1.3 nm could be prepared from the monolayer LB films through subsequent heat-treatment process, while generation of cracks became less noticeable on the 5-layer film after heat-treatment at lower holding temperature with slow heating rate. In addition, photocatalytic activities of the TiO2 films were examined from the decomposition of cadmium stearate (CdSt) LB films and stearic acid (SA) cast films for different time intervals of irradiation with UV light. Atomic force microscopy measurements showed that an almost flat surface of 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 the TiO2 film for the decomposition of SA cast film. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Among the inorganic films made up of metal oxides, TiO2 films have aroused interest owing to their potential use as anode material for photoelectrical energy conversion [1], photocatalyst for decomposing water or organic compounds [2–4], and coating material for improving certain surface properties [5]. To obtain thin TiO2 films, several methods including sol-gel method, chemical vapor deposition and pyrosol method as well as conventional dip-, spray- or spin-coating and screen printing of a colloidal solution have been employed so far. While welldefined structures of the TiO2 films are required to characterize their functionalities quantitatively, these preparation methods are still insufficient in precise control of film thickness on the nanometer-scale and uniformity over large substrate areas. Although, electrochemical deposition has recently emerged as a promising method for preparing TiO2 films with controlled structure [6,7], substrates available in this method are essentially restricted to conductive plates. Alternatively, versatile Langmuir–Blodgett (LB) technique, usually adopted to fabricate highly ordered molecular film assemblies of various long-chain amphiphiles, has been extended to the preparation of TiO2 films over the past few decades [8–16]. For example, ultrathin TiO2 films were synthesized from titanium alkoxides through a two-dimensional sol-gel-process [8,9]. Multilayer deposition was demonstrated for

⁎ Corresponding author. Tel.: + 81 357070104; fax: + 81 357072163. E-mail address: [email protected] (M. Takahashi). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.06.077

colloidal TiO2 nanoparticles or nanosheets to fabricate ordered organic/inorganic structures [10]. Further, densely packed exfoliated titania nanosheet film could be prepared by the LB technique without any amphiphilic additives [11]. In our previous studies, we reported the preparation of titania nanotube LB films by direct spreading of hydrophobized particles onto the surface of aqueous subphase [17]. The structure of the LB films obtained by subsequent deposition was found to consist of piles of rod-like particulate monolayers in which nanotube bundles were arranged in a manner resembling floating logs. TiO2 particulate LB films also could be fabricated from Langmuir monolayers on which TiO2 particles were adsorbed two-dimensionally from a colloidal subphase [12,18]. In this case, the TiO2 particles in the colloidal 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 film material throughout the experiments. As a candidate process, Ganguly et al. demonstrated that the use of long-chain amine monolayers and an aqueous subphase containing potassium titanium oxalate (K2TiO(C2O4)2; PTO) results in formation of a complex at the air–water interface [19], yielding TiO2 films via LB deposition and post-heat-treatment [20]. Meanwhile, spectroscopic investigations showed that colloidal TiO2 clusters were simultaneously generated in the subphase by slow hydrolysis of PTO upon aging [13]. Thus, it is of concern that the TiO2 clusters might be adsorbed unevenly on the floating monolayer. To develop this process, we conducted the study toward using LB films based on our previous works dealing with adsorption of dye on

8078

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

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

the ODA LB films [21,22]. Specifically, the sequence of the preparation steps was modified to make the process applicable to an ODA LB film as a template for nucleation and crystal growth of TiO2 from PTO (Fig. 1). The present procedure is considered to have the advantages of reducing PTO usage, as well as preventing excess build up of TiO2 clusters in the multilayered film. In this study, we first examined influences of preparation conditions on the structure of the TiO2 films, so as to prepare ultrathin TiO2 films with uniform and controlled thickness. The photocatalytic properties of the TiO2 films were next demonstrated by decomposition of cadmium stearate (CdSt) LB films and stearic acid (SA) cast films. From atomic force microscopy (AFM) measurements, we also observed surface morphological change of the CdSt LB film during photodegradation on the TiO2 film.

ODA (≥99% pure, Aldrich Chemical Company, Inc.) and PTO (≥90% pure, Wako Pure Chemical Industries, Ltd.) were used as the film material and Ti source, respectively. These materials were of the highest grade and were used without further purification. Preparation of the LB films was carried out using a Langmuir trough (HBM AP-type LB film balance, Kyowa Interface Science). Subphases were made using distilled deionized water. An ODA monolayer was spread from a 1.0 × 10 − 3 mol dm − 3 chloroform solution on the aqueous subphase at 20 °C; afterward, left for 10 min to allow the spreading solvent to evaporate. 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 by adding sodium hydroxide, thereby forming a typical condensed ODA monolayer [22]. The monolayer was stable for withstanding high surface pressures up to 60 mN m − 1. Limiting area of the condensed ODA monolayer was 0.22 nm 2 molecule − 1, indicating a close-packed arrangement of the hydrocarbon chains. Deposition of the ODA monolayer on solid substrate was performed at a surface pressure of 45 mN m − 1 using the conventional vertical dipping method with dipping and withdrawal speed of 7 mm min − 1. We used calcium fluoride plate, quartz plate, glass plate, and silicon wafer as the solid

substrates for different experiments. To prevent re-spreading of the deposited layer, a drying time of at least 10 min was used between dips of the substrate. TiO2 thin layers were generated by immersing as-deposited ODA LB films (1–9 layers) in an aqueous PTO solution at 20 °C for given period of times. In this process, which is analogous to a reaction with ammonia, the PTO is believed to react with ODA to produce TiO2 in the LB layers. Subsequently, the ODA–TiO2 LB films were sintered in air for 60 min at different holding temperatures (300, 400, 500, and 600 °C). According to a previous report on the thermal analysis of ODA-modified single-walled carbon nanotubes, a reduction in weight due to the reacted ODA was observed in simultaneous thermogravimetry–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 are sufficient to eliminate ODA from the films. Amounts of TiO2 and organic film materials on the solid substrate were estimated by ultraviolet–visible (UV–vis) spectroscopy (Shimadzu UV-3100PC) and Fourier transform infrared (FTIR) spectroscopy (JASCO FT/IR-8900), respectively. Surface information for the films such as the elemental composition and the valence states at different preparation stages was acquired using X-ray photoelectron spectroscopy (XPS) capability of an SSX-100 spectrometer (Surface Science Instruments) at a takeoff angle of 35° with a monochromatic Al Kα X-ray source (hν = 1486.7 eV) operating at 10 kV, 13 mA and a charge neutralizer. The charge-up shift was corrected with respect to the C1s peak at 284.6 eV for energy calibration. Surface morphologies of the LB films were studied by AFM (Digital Instruments Nanoscope IIIa). All AFM images were obtained in the tapping-mode operation under atmospheric conditions. Cantilevers used were commercially available etched silicon probe (Veeco, TESP-type) with a length of 125 μm and resonant frequencies of 303–383 kHz. In addition, it should be mentioned that we also attempted structural characterization of the TiO2 films using X-ray diffractometers (Rigaku RINT2500, 2000 and UltimaIII) with a CuKα (λ = 0.154 nm) radiation source (40 kV, 40–300 mA). However, no diffraction peak appeared in θ/2θ scan mode with a small angle of incidence, probably because the film thickness was extremely small.

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

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.

2. Experimental details

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

Fig. 4. Relationship between absorbance at 240 nm and number of ODA layers. Absorbance corresponds to generated amount of TiO2 during immersion in the 1 × 10− 3 mol dm− 3 PTO solution at 20 °C.

Photocatalytic activity of the TiO2 thin films was evaluated from the photodecomposition of both SA cast film and CdSt LB film deposited over the TiO2 films. The light source was a 500 W xenon short-arc lamp (Usio SX-UI500XQ optical moduleX) equipped with a water-based IR-cut filter, and a UV transmitting-visible absorbing filter (HOYA, U330). FTIR spectra were recorded over different time intervals under irradiation with UV light to estimate the amount of decomposition 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 were prepared from the Langmuir monolayer on the subphase at a pH above 10.1 (corresponding to pKa of long-chain amines). Applying a surface pressure of 45 mN m − 1, multilayer deposition of the ODA monolayer on the solid substrate was carried out with a transfer ratio close to unity. The details of the experiments have been described elsewhere [22]. Subsequently, the as-deposited ODA LB films were immersed in an aqueous PTO solution. To ensure the formation of a TiO2 film, UV–vis spectra of the ODA LB films were measured at different stages in the preparation procedure. After immersion in the aqueous PTO solution, a distinctive absorption band was observed below 350 nm (see Fig. 2). This result indicates formation of TiO2 because UV light absorption is assigned to the excitation of electrons from the valence band to the conduction band of TiO2. In contrast, when we employed an SA LB film instead, no absorption band appeared in the UV–vis spectrum. Thus, it is clear that the amino groups in the ODA LB film catalyze hydrolysis of the titanium oxalate ions, so that nucleation and crystal growth are induced to form a TiO2 thin film. Accordingly, the ODA LB film is

8079

considered to play a major role as a template during the immersion process. Since penetration of titanium oxalate ions into the inner layer of the ODA LB film may become a rate-determining step for the generation of TiO2, we first examined the influence of the immersion conditions on the structure of the TiO2 films. Fig. 2 shows the time course of UV–vis spectrum for the 5-layer ODA LB films during immersion 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 finally the amount of TiO2 reaches a constant value. As illustrated in the inset of Fig. 2, we plotted the change in the absorbance at 240 nm against immersion time for the 3- and 5-layer ODA LB films. The results show that the immersion time of ca. 20 min is required to achieve equilibrium for the 3-layer ODA LB film, while it is more than 30 min for the 5-layer ODA LB film. In addition, when we employed the ODA LB films with a larger number of built-up layers, a longer immersion time was needed (not shown in the inset figure). On the other hand, a higher concentration of PTO could shorten the immersion time required to attain a constant value of the generated TiO2. However, the amounts of TiO2 were almost identical for ODA LB films with the same number of layers regardless of the concentration of PTO. Fig. 3 shows an adsorption isotherm of PTO for the 5-layer ODA LB films with an immersion time of 60 min. The amount of TiO2 generated increased with increasing PTO concentration and reached saturation around 1 × 10 − 4 mol dm − 3. This indicates that titanium oxalate ions are chemically adsorbed to the ODA LB film to generate TiO2. Taking these results into account, we employed the most appropriate conditions for completing the generation of TiO2, i.e., 60 min immersion in 1.0 × 10 − 3 mol dm − 3 PTO solution for the 1–5 layer-LB films, in the subsequent experiments. Next, we checked the generated amount of TiO2 for the LB films with different numbers of layers. As shown in Fig. 4, the ODA LB films processed with the PTO solution for sufficient immersion time offered an almost proportional relationship between absorbance at 240 nm and the number of ODA layers. Therefore, it is clear that the adsorption and subsequent hydrolysis of PTO stoichiometrically proceed with the build-up of ODA. This means that the amount of TiO2 (=thickness of TiO2 film) could be controlled by the number of LB layers. Such precise and facile control of the film thickness is considered to be the most important advantage of the present method. 3.2. Structural characterizations of the films at various preparation stages 3.2.1. Surface morphological analysis by AFM Surface morphologies of the monolayer and 5-layer LB films were observed using AFM measurements. Figs. 5 and 6 display change in AFM images for the LB films on a silicon wafer at different preparation 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

Fig. 5. AFM images at various preparation stages using monolayer ODA LB film: (a) as-deposited LB film, (b) after immersion in PTO solution, and (c) after heat-treatment at 500 °C.

8080

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

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.

underwent a characteristic change during the course of the immersion process. For instance, the formation of irregularly shaped patches was typically seen in the image. Such a change in the layer structure could be expected by lateral expansion of the monolayer. The generation of TiO2 at around the polar-head groups in the monolayer yields an area expansion and thus results in failure to retain the planar surface, pushing parts of the collapsed film out from the monolayer. After subsequent heat-treatment at 500 °C for 60 min in air, organic molecules were eliminated from the LB film, and then a thin film (monolayer TiO2 film) was left on the solid substrate. Cross-sectional analysis revealed that the monolayer TiO2 film has a uniform and almost even surface with a roughness of less than 1.3 nm. For the 5-layer ODA LB film in Fig. 6, the as-deposited LB film also had a uniform and flat surface, except for defects in the form of holes in

the layer structure, and it turned into an uneven surface after immersion in the PTO solution. Comparing AFM images at this stage, the roughness of the 5-layer LB film is much larger than that of the monolayer LB film due to area expansion of respective layers in the LB film. Similar to the change in Fig. 5c, heat-treatment at 300 °C reduced the surface roughness; consequently, a film with almost flat surface was formed on the substrate (5-layer TiO2 film). However, as seen in the image 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 is construed as a result of the shrinkage of TiO2 film during heating; thus, the crack generation is supposed to be promoted by heat-treatment at higher temperature. In fact, cracks became more noticeable as the heating temperature was increased to 600 °C (Fig. 7). In addition, crosssectional analysis allowed us to estimate the film thickness from the depth of the cracks to be 3–4 nm for the 5-layer TiO2 film.

Fig. 7. AFM images of 5-layer TiO2 films on a silicon wafer heat-treated at various temperatures with a 60 min holding time: (a) 300, (b) 400, (c) 500, and (d) 600 °C. The temperature was 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 °C, and (b) 500 °C for 60 min with fast heating (left) and slow heating (right).

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

Fig. 9. Survey-scan XPS spectra of 5-layer ODA-TiO2 LB film (a) before and (b) after heat-treatment at 500 °C for 60 min.

Since a rapid raise of temperature usually causes an increase in surface stress of the film, a slower heating rate would be effective for reducing the cracks of the film. To examine the influence of heating rate, the 5-layer ODA-TiO2 LB films were heat-treated at 300 and 500 °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 the electrical furnace, cracks hardly appeared in both the films heattreated at a rate of 1 °C min − 1. Accordingly, slow heating rate was confirmed 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 this study were transparent and bonded tightly to the substrates.

3.2.2. XPS analysis XPS analysis was studied to characterize change in surface composition of the films during the heat-treatment. Fig. 9 shows the XPS survey spectra of the 5-layer ODA-TiO2 LB films on the glass plate before and after heat-treatment at 500 °C for 60 min with fast heating (8 °C min − 1). For the film before heat-treatment, photoelectron peaks of the expected elements such as C 1s, N 1s, Ti 2p and O 1s could be observed in the spectrum (a), indicating presence of the TiO2 in the ODA matrix. In contrast, peaks ascribed to potassium element from PTO was absent in the spectrum. This allows us to infer that the amount of K + incorporated in the LB film is inherently small. Simultaneously, K + may be localized at the polar head group region of 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 at different temperatures; (b) 300 °C, (c) 400 °C, (d) 500 °C, and (e) 600 °C.

8081

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.

nanometers from the surface, which also causes less escaping photoelectrons from the film. It should be noted that, based on the surface compositions obtained from Ti 2p and N 1s peaks, the atomic ratio of Ti/N within a few nanometers from the surface was estimated to be 0.91. Taking into account dissociation constants of oxalic acid (pKa1 = 1.27, pKa2 = 4.27 at 25 °C) and pH value of 3.7 for the 1.0 × 10− 3 mol dm− 3 aqueous PTO solution, we calculated the Ti/N ratio of 0.83, which is almost in agreement with the experimental value. This result gives further support to the above-mentioned consideration that the reaction of PTO and ODA proceeds stoichiometrically to yield the TiO2 film under the appropriate conditions. In the spectrum (b), peaks for K 2p, Si 2s and Si 2p are located at 294, 152 and 101 eV, respectively. Appearance of the K 2p peak may be caused by segregation of potassium compounds from inner layer to the surface of the film during heat-treatment. Also, the presence of cracks on the TiO2 film can be confirmed from the Si 2s and Si 2p peaks because these peaks originated from the glass substrate. On the other hand, disappearance of N 1s peak in the spectrum (b) indicates that the ODA template is completely removed from the heat-treated film. Similarly, most of the other organic compounds are considered to be decomposed by the heat-treatment, though a relatively weak C1s peak due to residual carbon and/or adventitious hydrocarbon is observed. 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 heattreatment. The two peaks were very symmetrical, and the energy splitting of 5.8 eV between them was close to that expected from standard binding energy tables. Thus, these results indicate that the Ti at the surface exists in the 4+ oxidation state in a tetragonal structure [24], which is hardly changed during the heat-treatment at least up to 500 °C.

3.2.3. UV–vis spectra Fig. 10 shows UV–vis spectra of the 5-layer TiO2 films on a quartz plate with different heat-treatments. These curves exhibited high transparency of the films in the visible light region, but a slightly-higher baseline could be noticed when the film was calcined at 300 °C. This may be attributed to reflectance from particles generated by heattreatment typically at lower temperature. With increasing holding temperature, the heat-treated TiO2 film showed a weaker absorption in the UV–vis region and the absorption edge wavelength gradually shifted toward the shorter wavelength. To estimate the indirect band gap energy for these films, (αhν)1/2 was plotted against photon energy (hν), where α represents absorption coefficient [25,26]. The band gaps

8082

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

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 of absorbance 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.

of TiO2 films could be derived from the abscissa intercepts of the linear portion of the curves. As shown in Fig. 11, the estimated band gap energy of 3.5 eV for the ODA-TiO2 LB film (curve a) changed to 3.3 eV after heat-treatment at 300 °C (curve b). This can be ascribed to the growth of TiO2 crystallites during heat-treatment. On the other hand, the band gap energies of the heat-treated TiO2 films increased from 3.3 to 3.6 eV with increasing holding temperature. As is well known, heattreatment at higher temperatures generally promotes increase in nanoparticle size, resulting in decrease of band gap energy due to quantum size effects [27]. In contrast, the blue-shift of absorption edge appearing in the present results is opposite to this tendency. This may be explained by particle internal stress which leads to the change of energy gap structure. Similar to the literature, effect of the particle internal 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 using UV light of 35 mW cm − 2 intensity under ambient conditions. As a solid substrate, we used UV-transparent quartz plate in the following experiments. As an organic decomposing material, we first employed CdSt LB films because they offer a uniform coating with controlled thickness. Fig. 12(a) shows a series of FTIR spectra for the photodecomposition of CdSt LB film on the 5-layer TiO2 film heattreated at 500 °C. Bands peaking at 2852 and 2918 cm − 1 are assigned to CH2 symmetric and asymmetric stretching vibrations, respectively. The intensities of these peaks were gradually decreased over irradiation 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 CdSt loading on the TiO2 film. Thus, it is clear that the CdSt LB film on the TiO2 film was decomposed faster than that on the bare quartz plate,

indicating a photocatalytic activity of the TiO2 film. Similar to the previous report for TiO2 films prepared by a dip-coating technique [29], the reaction proceeded according to pseudo-first-order reaction kinetics, i.e., [CdSt]t = [CdSt]0 exp(−kt), where [CdSt]t and [CdSt]0 are the concentrations of CdSt at time t = 0 and t, respectively, and k is the pseudo-first-order constant. The k value for the TiO2 film was calculated to be 7.0 × 10 − 3 min − 1 from the data in Fig. 12(b), which is almost one order of magnitude less than that reported for SA decomposition under 0.5 mW cm − 2 UV light irradiation [29]. Change in surface morphologies during UV light irradiation was observed by AFM measurements. Fig. 13 shows AFM images of the CdSt LB film at different photodecomposition stages on the 5-layer TiO2 film heat-treated at 500 °C. These images clearly display the CdSt LB film, initially having an almost flat and homogeneous surface, gradually eroding due to exposure to UV radiation, changing to a moth-eaten appearance. This indicates that the film underwent inhomogeneous photodecomposition at the submicrometer level probably because photocatalytic reaction centers were distributed over the TiO2 surface. In the image at 30 min irradiation, we particularly found that most of the holes are arranged in lines, reflecting distribution pattern of the active sites in the catalyst film. In addition, when the measurement of tapping-mode AFM was repeated in the same scanning area, enlargement of pore size was observed for the UV irradiated LB films. This can be interpreted as the result of internal corrosion of the CdSt LB film, leading to embrittlement of the rigid film structure. Namely, taking into account the fact that the contact between CdSt LB film and TiO2 film is due to van der Waals interaction and the photodecomposition occurs at the TiO2 film surface, we inferred that void spaces are generated in the solid-state LB film by the UV irradiation. These void spaces usually inhibit the CdSt LB film from sufficient contact with the TiO2 film, thereby decreasing the decomposition rate even in the early stage of irradiation as indicated by the small k value.

Fig. 13. AFM images of CdSt LB film on the 5-layer TiO2 film at different UV light irradiation times.

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

8083

Fig. 14. Photodecomposition curves of SA cast film as a function of UV-light irradiation time; 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 decomposition of SA cast film. The TiO2 films were prepared from ODA LB films with different numbers of layers by: (△) monolayer, (□) 5 layers, and (w) 9 layers. Result for bare quartz plate (●) was also indicated.

Subsequently, influence of preparation conditions of the TiO2 films on their photocatalytic activities was examined in the same manner as used in Fig. 12. As an organic decomposing material, we employed SA cast films instead of the CdSt LB films in the following experiments. In addition to the heat-treated TiO2 films, we specially prepared the TiO2 film without heat-treatment. To eliminate organic compounds from the ODA-TiO2 LB film, the LB film was processed by ultraviolet/ozone treatment with a UV-ozone cleaner (Nippon Laser & Electronics Lab., NL-UV253). Fig. 14 shows changes in absorbance at 2918 cm − 1 for the 5-layer TiO2 films with different heat-treatments. The photodecomposition rates of SA for all TiO2 films were significantly larger than that for a bare quartz plate. As to the TiO2 film heat-treated at 500 °C, a value of 3.2 × 10 − 2 min − 1 was estimated for k in the initial stage of irradiation until 30 min. Comparison of the k values for CdSt LB film and SA cast film shows that the decomposition of SA cast film proceeded 4.5 times faster than that of CdSt LB film under the same conditions. This difference in decomposition rate is interpreted to be due to softness of the SA film by which the SA film can keep contact with the surface of photocatalyst. In general, calcination process has been applied to enhance photocatalytic performance of TiO2 catalysts. This is because improvement of crystallinity by heat-treatment reduces trap sites associated with oxygen vacancy in the crystal lattice, being favorable for preventing the electron–hole recombination. Nevertheless, our current results indicated that the photocatalytic activity of TiO2 film was decreased with increasing heat-treatment temperature. Although the reason of this tendency remains uncertain at present, one possible explanation is that utilization efficiency of UV light below 330 nm was lowered with increasing heating temperature due to the blue-shift of absorption in the UV region. Meanwhile, the rise in temperature caused decrease in the active surface area of TiO2 as well as generation of cracks, which may decrease photocatalytic activities of the TiO2 films. Similar to the result for the 5-layer TiO2 films, we found a decrease in photocatalytic performance for the monolayer TiO2 films when applying higher holding temperatures (data not shown). Compared to the heat-treatment at the same temperature in Fig. 14, the monolayer film was less active than the 5-layer film. To elucidate this trend, photodecomposition curves for the 1–9 layer films with heattreatment at 300 °C are presented in Fig. 15. The figure indicates that photocatalytic activity of the TiO2 film is enhanced with increasing number of layers. This can be explained by increase in the light-absorption efficiency below 330 nm which promotes generation of electron–hole pairs. Furthermore, the thicker TiO2 films are supposed to have an advantage in charge separation by space charge 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 photoinduced electron–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 using the ODA LB films as a template. Under optimized conditions, the generated amount of TiO2 in the LB films was proportional to the number of deposited ODA layers, which indicated that the thickness of the TiO2 films can be precisely controlled by adjusting the amount of ODA. From the topographic height profile of the AFM images, the film thickness was estimated to be 3–4 nm for the 5-layer TiO2 film. Also, thicker TiO2 films tended to crack during heat-treatment, while lower holding temperatures with slow heating rate reduced the appearance of cracks. Upon irradiating with UV light, photocatalytic decompositions of both CdSt LB film and SA cast film were observed on the surface of the TiO2 films. Among the TiO2 films with different heat-treatments, the film heat-treated at lower holding temperature showed higher photocatalytic activity for the decomposition of SA. In addition, an enhancement in the photocatalytic activity was obtained as the number of TiO2 layers increased. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

B. O'Regan, M. Gräzel, Nature 353 (1991) 737. T. Kim, B. Sohn, Appl. Surf. Sci. 201 (2002) 109. A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1. D. Chowdhury, A. Paul, A. Chattopadhyay, Langmuir 21 (2005) 4123. R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. Y. Matsumoto, Y. Ishikawa, M. Nishida, S. Ii, J. Phys. Chem. B 104 (2000) 4204. S. Karuppuchamy, N. Suzuki, S. Ito, T. Endo, Curr. Appl. Phys. 9 (2009) 243. M. Oswald, V. Hessel, R. Riedel, Thin Solid Films 339 (1999) 284. I. Moriguchi, H. Maeda, Y. Teraoka, S. Kagawa, J. Am. Chem. Soc. 117 (1995) 1139. T. Yamaki, K. Asai, Langmuir 17 (2001) 2564. M. Muramatsu, K. Akatsuka, Y. Ebina, K. Wang, T. Sasaki, T. Ishida, K. Miyake, M. Haga, Langmuir 21 (2005) 6590. K. Muramatsu, M. Takahashi, K. Tajima, K. Kobayashi, J. Colloid Interface Sci. 242 (2001) 127. C. Malitesta, A. Tepore, L. Valli, A. Genga, T. Siciliano, Thin Solid Films 422 (2002) 112. Y. Tian, J.H. Fendler, Chem. Mater. 8 (1996) 969. K.S. Mayya, V. Patil, P.M. Kumar, M. Sastry, Thin Solid Films 312 (1998) 300. A. Serra, A. Genga, D. Manno, G. Micocci, T. Siciliano, A. Tepore, R. Tafuro, L. Valli, Langmuir 19 (2003) 3486. M. Takahashi, Y. Okada, K. Kobayashi, Chem. Lett. 37 (2008) 276. M. Takahashi, H. Natori, K. Tajima, K. Kobayashi, Thin Solid Films 489 (2005) 205. P. Ganguly, D.V. Paranjape, M. Sastry, Langmuir 9 (1993) 577. D.V. Paranjape, M. Sastry, P. Ganguly, Appl. Phys. Lett. 63 (1993) 18. 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, Thin Solid Films 197 (1991) 279.

[25] [26] [27] [28] [29]

P. Sudhagar, R. Sathyamoorthy, S. Chandramohan, Appl. Surf. Sci. 254 (2008) 1919. S. Janitabar-Darzi, A.R. Mahjoub, A. Nilchi, Phys. E 42 (2009) 176. J. Yu, H. Yu, B. Cheng, X. Zhao, J. Yu, W. Ho, J. Phys. Chem. B 107 (2003) 13871. N. Xiao, Z. Li, J. Liu, Y. Gao, Thin Solid Films 519 (2010) 541. P. Sawunyama, L. Jiang, A. Fujishima, K. Hashimoto, J. Phys. Chem. B 101 (1997) 11000.