Materials Letters 63 (2009) 1699–1701

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Materials Letters

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Preparation and properties of Cu-grafted transparent TiO2-nanosheet thin films

Akira Nakajima ⁎, Yoshinori Akiyama, Sayaka Yanagida, Tomohisa Koike, Toshihiro Isobe,Yoshikazu Kameshima, Kiyoshi OkadaDepartment of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro, Tokyo 152-8552, Japan

⁎ Corresponding author. Tel.: +81 3 5734 2525; fax: +E-mail address: anakajim@ceram.titech.ac.jp (A. Nak

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.05.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 April 2009Accepted 7 May 2009Available online 14 May 2009

Keywords:Interfacial charge transferNanosheetPhotocatalystTiO2

Layer-by-layer

Transparent TiO2 nanosheet thin films were prepared using layer-by-layer method with poly(diallyldi-methylammonium chloride) as a counter polymer. The possibility of photocatalysis by the light in the longwavelength range was examined by grafting Cu on the film. Photocatalytic decomposition activity for gaseous2-propanol under UV illumination without light for electron excitation of TiO2 nanosheet was increased byCu grafting, suggesting that the interfacial charge transfer mechanism is effective also for nanosheet films.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The photoinduced reaction of titanium dioxide has been wellstudied from the discovery of water breakdown of TiO2 [1].Particularly, many researchers have investigated the strong oxidationpower of photogenerated radical species from the standpoint ofapplication for water and air purification [2–6]. Ultraviolet (UV)irradiation onto TiO2 generates electron and hole pairs, therebyrespectively reducing and oxidizing adsorbates on the surface andproducing radical species such as OH radicals and O2

−. These radicalscan decompose most organic compounds and bacteria.

Because of its band gap, TiO2 requires UV with wavelength of lessthan 380 nm for a photocatalytic reaction. To narrow its band gap,doping of metal ions or anions such as N and S is commonly used [7–10]. However, such doping engenders the localization of photogener-ated holes in a narrow band by themetal ions or anions, and decreasesthe strong oxidation power originating from the valence band (VB) ofTiO2.

Very recently, Irie et al. reported that grafting Cu(II) ions onto TiO2

provides an efficient visible-light sensitive photocatalyst by interfacialcharge transfer (IFCT) mechanism [11]. In this catalyst, charge transferfrom VB of TiO2 to Cu(II) ions on the surface is feasible by visible light(ca. 450 nm) because of the redox potential of Cu(II)/Cu(I) (0.3–0.5 Vvs. SHE). Since oxygen reduction by multiple electrons occurs at theCu ion site, the original strong oxidation power of TiO2 is retained.However, this mechanism is reported for rutile-type TiO2 powder, andhas not been applied to TiO2 nanosheet [12,13] thin films to date.

81 3 5734 3355.ajima).

ll rights reserved.

The coating of TiO2 nanosheets has high transparency, highdurability, and strong oxidation power. However, because of thequantum size effect, its band gap (3.8 eV, ca. 327 nm) [14] is largerthan that of normal TiO2 (3.2 eV, ca. 388 nm for anatase). The potentialof TiO2 nanosheet for industrial use will be increased if photocatalysisby the light in the long wavelength range is feasible using an IFCTmechanism. For this study, we prepared transparent TiO2 nanosheetfilms using layer-by-layer (LBL) method [15,16]. Their photocatalyticactivity was examined under UV illumination with and without thelight required for electron excitation of the TiO2 nanosheet.

2. Experimental

2.1. Sample preparation

In the present study, TiO2 nanosheet was prepared from a Csintermediate with following Sasaki et al.'s studies [11,12]. Cesiumcarbonate (CsCO3; Wako Pure Chemical Industries, Ltd., Japan) andrutile type TiO2 (Toho Titanium, Japan) were mixed to 1:5.3 (massratio). The mixture was heated at 800 °C for 40 h, then cooledgradually by 30 °C/h to room temperature. The obtained Cs0.68Ti1.83O4

was ground using a mortar and pestle. The powder (1.0 g) wasdispersed into 1 M HCl (100 mL) and stirred for 3 days. After stirring,the powder was separated from the liquid by filtration, then dried atroom temperature for 1 day. The resultant H0.68Ti1.83O4 powder (0.4 g)was dispersed into 0.02 M tetrabutylammonium (TBA; Sigma-AldrichCorp., USA) aqueous solution (100 mL) and stirred for 1 week. Aftercentrifugation at 6000 rpm, the supernatant solution was decantedand used as a TiO2 nanosheet suspension.

Because TiO2 nanosheets possess sufficient negative charge in thebasic solution, we used poly(diallyldimethylammonium chloride)

Fig. 2. UV–VIS absorbance spectra change of the (PDDA/TiO2)n self-assembly layerusing LBL method (Inset: Optical photograph of prepared films).

1700 A. Nakajima et al. / Materials Letters 63 (2009) 1699–1701

(PDDA, typical Mw 100,000–200,000; Sigma-Aldrich Corp., USA) as acounter positively charged polymer for layer-by-layer coating usingCoulombic attraction. A PDDA (10 g dm−3) solution was prepared bydissolving a certain mass of PDDA in a TBA solution of pH 9.

Pure quartz (SiO2) glass plates (30 mm×60 mm×1 mm; TosohCorp., Tokyo, Japan) were used as substrates. After normal cleaningprocedures [16], substrates were immersed into the PDDA solution for20 min. They were then rinsed with water and dried at 60 °C in anoven under an air atmosphere. Then the sample was immersed in aTiO2 nanosheet suspension for 20 min and rinsed with water. Thesamples were dried at 60 °C under an air atmosphere. Subsequently,(PDDA/TiO2)5 thin films were obtained by repeating these proceduresfive times. Then, at the top of the TiO2 nanosheet coating, Cu ion wasgrafted by immersing 0.1 mM CuCl2 (Wako) solution for 20 min.

After deposition, strong UV light was illuminated onto the samplesusing a Hg–Xe lamp for 20 h to decompose the interlayer PDDA [16].The illumination intensity at the sample surface was 66 mW/cm2 atλ=365 nm. Finally, (TiO2)5 with Cu grafting (hereinafter denoted asCu/(TiO2)5) film was obtained. To compare the effect of Cu grafting,pure (TiO2)5 film (without Cu grafting) was also prepared.

2.2. Evaluation

The crystalline phase was evaluated using X-ray diffraction (XRD;XRD-6100, Shimadzu Corp., Japan). The respective chemical composi-tions of powder samples were evaluated using X-ray fluorescenceanalysis (XRF; RIX-2000; Rigaku Corp., Japan). Using a UV–VIS–NIRscanning spectrophotometer (V-630; Jasco Inc., Japan), UV-VISabsorption spectra were evaluated. The prepared films' morphologywas evaluated using noncontact-mode of an atomic force microscope(AFM, JSPM 5200; JEOL, Japan). A Pt-Ti coated Si cantilever (forceconstant: 0.95 N/m) was used for this measurement. The surfacechemical composition was measured using X-ray photoelectronspectroscopy (XPS, 5500MC; Perkin-Elmer PHI Co., U.S.A.) with a MgKα X-ray line (46.950 eV). The takeoff angle was 45°. The bindingenergy scales were referenced to 284.5 eV, as determined by locationsof peaks on the C1s spectra of hydrocarbon (CHx) for correcting thedeviation.

Photocatalytic activity was evaluated according to the decomposi-tion of gaseous 2-propanol (Wako). Detailed experimental conditionswere described in a previous report [16]. After dark storage for threehours, UV illumination was conducted using a UV illuminator (LA-410UV-1; Hayashi Watch Works Co. Japan) equipped with a Hg–Xelamp (MX4010). This light source contains several peaks in 280–450 nm and the strongest peak at 365 nm. The colored glass filter, UV-33 (absorbed UVb300 nm, Asahi Glass Co. Ltd. Japan) was used tolimit the wavelength range of light illumination (Fig. 1). In all-light

Fig. 1. Wavelength spectra of the light source with and without the color filter.

and UV-limited illumination cases, the intensity for 365 nm UV lightwas changed: from 30 mW/cm2 (all-light) to 24 mW/cm2 (UV-limited), and from 20 mW/cm2 (all-light) to 17 mW/cm2 (UV-limited).

3. Results and discussion

3.1. Characterization of Cu/(TiO2)5 thin films

Analyses using XRD and XRF revealed that the obtainedCs0.68Ti1.83O4 powder was almost single phase and that its Cs/Ti ratiowas the same as the designed value (0.36). Themajority of Cs ion in thepowder was dissolved during subsequent stirring in HCl solution, andthe final CsO concentration in the powder was decreased to 0.3mass%.Fig. S-1 (in the Appendix A) shows the XRD pattern of the coating ofTiO2 nanosheet suspension before and after drying on the glass. Whenthe coating is wet, the XRD pattern implies an amorphous-like (poorcrystallization) state. However, a strong peak appears after drying,suggesting that the nanosheet was stacked during drying. The Tyndalleffect was also observed as a result of irradiation of the TiO2 nanosheetsuspension by the laser beam (Fig. S-2 in the Appendix A).

Fig. 2 depicts the UV–VIS absorbance spectra change of the(PDDA/TiO2)n self-assembly layer during LBL method. The UVabsorbance was increased continuously by both PDDA and TiO2

deposition with almost identical absorbance each time, suggestingthat homogeneous coating was attained at each step of immersionduring film processing. Subsequently, AFM analysis revealed that theaverage surface roughness values (Ra) of the films were, respectively,around 1.3 nm and 1.7 nm for Cu/(TiO2)5 film and (TiO2)5 film andsurface Cu concentration ([Cu/(Ti+O+Cu)]) was revealed to be 0.5mass% by XPS. The chemical state of Cuwas unclear by XPS because ofits low concentration. Fig. 2 presents photographs of prepared films.Both films show high transparency to visible wavelength light.

The rate-controlling step of the TiO2 photocatalyst is well known toshift from mass transport (ln(C(t)/C0)=−kt) to surface reaction (C(t)=C0−kt) with decreasing light intensity [17,18]. Where t is theillumination time, C0 and C(t) are concentrations of the decomposi-tion substances in the vessel at the initial state and time t, and k is areaction constant. Preliminary experiments using the (TiO2)5 filmrevealed that the rate-controlling step for the decomposition ofgaseous 2-propanol is changed from mass transport to surfacereaction using the colored glass filter (Fig. S-3 in the Appendix A).

Fig. 3 portrays a comparison of photocatalytic activity between Cu/(TiO2)5 and (TiO2)5 films with and without the UV light required forelectron excitation of the TiO2 nanosheet. Differences between these

Fig. 3. Comparison of photocatalytic activity between Cu/(TiO2)5 and (TiO2)5 films (a)with and (b) without the UV light required for electron excitation of the TiO2 nanosheet.

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two films in initial adsorption capability during dark storage were notgreat. For full-spectrum light illumination, (TiO2)5 film providedbetter photocatalytic activity than Cu/(TiO2)5 film (reaction constantsfor Cu/(TiO2)5 and (TiO2)5 films were, respectively, 8.4×10−2 and32.7×10−2 h−1), which is attributable to the decrease of oxidationsites on the film surface by Cu grafting and the efficiency difference ofthe charge transfer.

In marked contrast, the opposite result was shown for UVillumination when the light for electron excitation of TiO2 nanosheetwas cut by the glass filter. Because UV is slightly filtered in the vessel,the (TiO2)5 film possesses slight photocatalytic activity. In this case,despite a lower oxidation site, the Cu/(TiO2)5 film provides betterphotocatalytic activity than the (TiO2)5 film (reaction constants forCu/(TiO2)5 and (TiO2)5 films were 1.5×10−2 and 1.0×10−2 h−1).These results suggest that photocatalysis by the light in the longwavelength range is feasible for TiO2 nanosheet films by Cu grafting. Itis inferred that the IFCT mechanism is also effective not only for rutilepowders but also for these materials. Detailed quantitative analyses ofthe relation between Cu grafting amount and photocatalytic activityare points to be addressed in future work.

4. Conclusions

In this study, transparent TiO2-nanosheet thin films were preparedusing LBL method, the possibility of photocatalysis by the light in thelong wavelength range was tested by grafting Cu onto the film.Photocatalytic decomposition activity for gaseous 2-propanol underUV illuminationwithout light for electron excitation of TiO2 nanosheetwas increased by Cu grafting. This result suggests that IFCT is alsoeffective for nanosheet films.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.matlet.2009.05.016.

References

[1] Honda K, Fujishima A. Nature 1972;238:37–8.[2] Kawai T, Sakata T. Nature 1980;286:474–6.[3] Ollis DE, Al-Ekabi H, editors. Photocatalytic Purification and Treatment of Water

and Air. Amsterdam: Elsevier; 1993. p. 747.[4] Rosenberg I, Brock JR, Heller A. J Phys Chem 1992;96:3423–8.[5] Takeda N, Torimoto T, Sampath S, Kuwabara S, Yoneyama H. J Phys Chem 1995;99:

9986–91.[6] Hoffmann MR, Martin ST, Choi W, Bahenemann DW. Chem Rev 1995;95:69–96.[7] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Science 2001;293:269–71.[8] Irie H, Watanabe T, Hashimoto K. J Phys Chem B 2003;107:5483–6.[9] Ohno T, Mitsui T, Matsumura M. Chem Lett 2003;32:364–5.[10] Mrowetz M, Balcerski W, Hoffmann MR. J Phys Chem B 2004;108:10617–7273.[11] Irie H, Miura S, Kamiya K, Hashimoto K. Chem Phys Lett 2008;457:202–5.[12] Sasaki T, Watanabe M. J Am Chem Soc 1998;120:4682–9.[13] Sasaki T, Watanabe M. J Phys Chem B 1997;101:10159–61.[14] Sakai N, Ebina Y, Takada K, Sasaki T. J Am Chem Soc 2004;126:5851–8.[15] Sasaki T, Ebina Y, Tanaka T, Harada M, Watanabe M. Chem Mater 2001;13:4661–7.[16] Yanagida S, Nakajima A, Sasaki T, Kameshima Y, Okada K. Chem Mater 2008;20:

3757–64.[17] Ohko Y, Fujishima A, Hashimoto K. J Phys Chem B 1998;102:1724–9.[18] Ohko Y, Hashimoto K, Fujishima A. J Phys Chem A 1997;101:8057–62.