Preparation and photoelectric properties of ordered mesoporous titania thin films

Preparation and photoelectric properties of ordered mesoporous titania thin films

Journal of Alloys and Compounds 474 (2009) 326–329 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 474 (2009) 326–329

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Preparation and photoelectric properties of ordered mesoporous titania thin films Yue Shen a,b , Junchao Tao a , Feng Gu b , Lu Huang b , Jian Bao a , Jiancheng Zhang b , Ning Dai a,∗ a b

National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, 200083, China School of Material Science and Engineering, Shanghai University, Shanghai, 200072, China

a r t i c l e

i n f o

Article history: Received 23 April 2008 Received in revised form 18 June 2008 Accepted 19 June 2008 Available online 2 September 2008 Keywords: Mesoporous Titania Photoelectric properties

a b s t r a c t Ordered mesoporous titania (MT) thin films have been grown on Si and indium tin oxides (ITO) substrates by evaporation-induced self-assembly (EISA) technique. The films have honeycomb-like structures and are consisted of anatase nanocrystallites, as evidenced from Raman spectra and high resolution transmission electron microscopy. The band-gap energies of the mesoporous titania thin films are larger than that of bulk TiO2 and are tunable through controlling film processing. Refractive index and extinction coefficient of the mesoporous titania thin films were determined using spectroscopic ellipsometry and were found to depend on the film thickness. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Mesoporous titania (MT) has attracted a great deal of attention due to its strong oxidizing and reducing ability under UV light irradiation, rendering the material very suitable for applications in solar cell electrodes, photocatalysis, gas-sensor, electrochromic display devices, and etc.[1,2]. In order to increase the utility of mesoporous titania materials, especially in electronic and photonic applications, it is essential to synthesize mesoporous titania thin films with the framework of nanocrystalline anatase [3]. Most studies on MT thin films were focused on synthesis, emphasizing on control parameters such as pH [3,4], moisture [5], water content [6], and the resulting nanostructures. One of the major problems in preparing MT thin films is that conventional thermal treatment often leads to collapse of the mesoporous TiO2 network, which significantly limits their application. Evaporation-induced self-assembly (EISA) is an advanced synthetic approach for mesoporous titania that allows for tuning the inorganic condensation rate with the formation of an organized liquid crystal template [7]. It utilizes very dilute initial solutions from which a liquid crystalline mesophase is gradually formed upon evaporation. The slow co-assembly of an inorganic network around this liquid crystalline phase permits the formation of well-defined mesostructured material.

∗ Corresponding author. Tel.: +86 21 65161674; fax: +86 21 65830734. E-mail addresses: [email protected] (Y. Shen), [email protected] (N. Dai). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.090

Fabrication of mesoporous TiO2 thin films with enhanced nanocrystallinity is required by several technologically demanding applications, such as photocatalysis and thin film solar cell, where the semiconducting and the photovoltaic behaviors of TiO2 thin films are largely dependent on the crystallinity. It is thus necessary to carry out a delicate study on the dependence of the photoelectric property on nanostructures of MT thin films. Some works have been done in this field. Cernigoj et al. have studied photo-catalytic activity of TiO2 thin films produced by surfactant-assisted sol–gel technique and shown that the material with anatase crystal structures have greater photo-catalytic activity than that with rutile forms [8]. Wang et al. have prepared Pt-embedded mesoporous TiO2 thin film of the cubic anatase structure and demonstrated the photo-driven killing ability of Micrococcus lylae cells on the film [9]. Frindell et al. reported the sensitized luminescence properties of mesoporous titania thin films doped with trivalent europium. They observed a bright narrow bandwidth emission from the europium activator ions through energy transfer from the semiconducting titania nanoparticle array [10]. In this paper, we report the study on electrical and optical properties of pure mesoporous titania thin films prepared by the EISA method. We find that the band-gap energies, refractive index and extinction coefficient of the mesoporous titania thin films are tunable through controlling film processing. The properties of pure mesoporous titania thin films play important roles in increasing photoelectric exchange efficiencies. 2. Experimental Mesoporous titania thin films were prepared via the following procedure. Hydrochloric acid solution and acetylacetone (AcAc) were added to ethanol/P123

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solutions followed by tetrabutyl titanate (TBOT) under vigorous stirring. After 0.5 h, distilled water was added to the mixed solution and aged with vigorous stirring at room temperature for 6 h. The molar ratio of P123/TBOT/ethanol/H2 O/AcAc/HCl is 0.025:1:28.5:30:0.5:0.005. Thin films were prepared by spin coating (2000 rpm, 20 s) the fresh solution onto Si or ITO (indium tin oxides) substrates. The assynthesized films were aged at 40 ◦ C for 2 days and then annealed at 150 ◦ C for 24 h. The thin films were subsequently calcined at a rate of 1 ◦ C min−1 to 300 ◦ C for 4 h (MT-1). Four layers (MT-4) and 10 layers (MT-10) thin film samples were obtained by repeating above process. Measured by spectroscopic ellipsometry measurement, the thickness of MT-1, MT-4, and MT-10 films is 199, 797, and 1733 nm, respectively. Nanocrystallites of the anatase phase in the mesoporous thin films were characterized using Horiba Jobin Yvon HR800 Raman spectrometer and the excited wavelength was 514 nm from an Ar ion laser. Transmission electron microscopic (TEM) images of MT thin films obtained using Japan JSM-2010F microscopy operating at an acceleration voltage of 200 kV. A Perkin-Elemer Lambda 2S spectrophotometer was used to measure the optical transmission spectra of the MT thin films on ITO substrates. Spectroscopic ellipsometric data were tested using a Jobin Yvon UVISEL/460-VIS-AGAS ellipsometer.

3. Results and discussion Raman spectra of MT-1, MT-4 and MT-10 films deposited on ITO substrates are shown in Fig. 1. It can be seen that no clear crystalline phase was formed in the MT-1. Special band of the Eg(1) mode of the TiO2 film at 148 cm−1 was tested in MT-4 [11]. In contrast, lines at 148, 402, 521, and 631 cm−1 , which characterize the anatase phase TiO2 [11], were observed on MT-10. There appeared a decrease in the width of the lowest frequency Eg(1) mode from MT-1 to MT4 then to MT-10. It has been testified that the narrowing of the Eg(1) mode is due to the increasing of TiO2 crystallite size, which

Fig. 1. Raman spectra of MT thin films.

can be accounted for by a combined mechanism involving phonon confinement and nonstoichiometry effects [12,13]. The increases of crystallite size from MT-1 to MT-4 and then to MT-10 is confirmed by the TEM studies presented in Fig. 2. Fig. 2 presents transmission electron microscopic (TEM) images of MT-1, MT-4 and MT-10 thin films. The samples seem to have honeycomb-like structures. The average pore size is about 7–8 nm, estimated from the TEM image in Fig. 2a. HR-TEM images in Fig. 2b, c, and d show that the framework of MT-1 calcined at 300 ◦ C is

Fig. 2. TEM images of (a) MT-1 (50,000×), (b) MT-1 (200,000×), (c) MT-4 (200,000×) and (d) MT-10 (200,000×). Inset: SAED patterns of MT thin films.

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Fig. 3. Energy gap (Eg ) of MT thin films. Inset indicates the optical transmission spectrum of MT-1.

mainly consisted of amorphous structures, while the nanostructures of MT-4 and MT-10 calcined at the same temperature are consisted of nanocrystallites. Apparently, a prolong calcining at low temperature helps to form good crystal structures and to protect the well-organized networks of MT thin films. From selected-area electron diffraction (SAED) patterns shown in the insets of Fig. 2c and d, the interplanar distance (d-value) of (1 0 1) and (0 0 4) planes of the MT is calculated to be about 3.35 and 2.3 Å, respectively. The optical transmission spectrum of MT-1 thin film grown on ITO was measured and shown in the inset of Fig. 3. The film is transparent for wavelength longer than 350 nm. The energy gap (Eg ) is determined by assuming a direct transition between the valence and the conduction bands and was estimated from the decreased portion on the transmission spectra. In terms of the Beer–Lambert law, transmittance T = exp(−˛L), where ˛ is the absorption coefficient and L is the thickness of the film. The absorption coefficient ˛ as a function of photon energy can be expressed by n

(˛h) = C(h − Eg ),

(1)

where h is the incident photon energy and C is a constant [14]. In this formula, n = 2 for direct transition and n = 1/2 for indirect transition. The (˛h)1/2 versus h relationship is shown in Fig. 3 for the MT thin films. The linear behavior of the curve at energy above 3.5 eV supports the assumption of indirect transition. The values of Eg , estimated by extrapolating the linear portion of the curve to (˛h)1/2 = 0, are 3.58, 3.49 and 3.42 eV for MT-1, MT-4, and MT-10, respectively. Those numbers are all larger than 3.2 eV for bulk anatase TiO2 . The change of Eg is, from the results of Figs. 1 and 2, most likely due to increasing nano-sizes of crystallinity of the anatase phase from MT-1 to MT-10. The ability to precisely control the sizes of the nano thin films indicates that Eg of the MT thin films is tunable. This property is extremely useful for many optoelectronic device applications including thin film solar cells. The refractive index of mesoporous TiO2 thin films is one of the fundamental properties to be considered [15]. Spectroscopic ellipsometry measurement was performed and a new amorphous formula was used to describe the dielectric function of MT-1 thin films, assuming a two layer model (TiO2 /Si) for the MT-1 thin films grown on Si substrates. The incident angle was 70◦ . Fig. 4a shows the refractive index n and extinction coefficient k of MT-1 thin films, derived from fitting the experimental spectroscopic ellipsometric data. A good fit is found between the model calculation and experimental data in the entire wavelength range. The thickness of MT-1 is about 199 nm. As shown in Fig. 4a, both the refractive index and

Fig. 4. (a) Refractive index n and extinction coefficient k of MT-1 and (b) refractive index n of MT thin films.

the extinction coefficient of the film decrease with increasing wavelength. In addition, the extinction coefficient is very small at long wavelength region where the films are transparent. Fig. 4b shows the refractive index n of MT-1, MT-4, and MT-10 thin films described by the new amorphous formula. According to the results presented in Fig. 4b, the refractive indexes of MT-1, MT-4 and MT-10 thin films are 1.72, 1.67 and 1.63, respectively, in contrast to 2.2 for bulk TiO2 in visible wavelength range. Decrease in refractive index with the film thickness is expected to be caused by increasing porosity due to prolong heating. 4. Conclusion In conclusion, ordered mesoporous titania thin films with nanocrystallites of anatase phase were successfully prepared by EISA at 300 ◦ C. Long duration of calcineing at low temperature appears to be helpful to stabilize the structures of well-organized networks of MT thin films and obtaining well-crystal structures at the same time. The energy gap (Eg ) of the MT thin films can be tuned by controlling the annealing process. Increasing the layer thickness of the mesoporous titania thin films appears to decreasing refractive indices. Acknowledgements This work is supported by Shanghai City Committee of Science and Technology (0752nm016, 07JC14058); Innovation Program of Shanghai Municipal Education Commission (08YZ08); K.C. Wong Education Foundation, Hong Kong; Shanghai Postdoctoral Scientific Program; and National Laboratory Foundation for Infrared Physics. One of the authors (ND) would like to thank the support of the “Out-

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standing Young Scholar” program by National Science Foundation in China (Grant No. 60225004) and Shanghai City Committee of Science and Technology in China (06XD14020). References [1] S. Ito, T. Takeuchi, S. Yanagida, Chem. Mater. 15 (2003) 2824. [2] S. Kambe, S. Nakade, T. Kitamura, J. Phys. Chem. B 106 (2002) 2967. [3] H.S. Yun, K.C. Miyazawa, H.S. Zhou, I. Honma, M. Kuwabara, Adv. Mater. 13 (10) (2001) 1377. [4] X.S. Li, G.E. Fryxell, J.C. Birnbaum, C.M. Wang, Langmuir 20 (2004) 9095. [5] K.S. Jang, M.G. Song, S.H. Cho, J.D. Kim, Chem. Commun. (2004) 1514. [6] E.L. Crepaldi, G.J. Soler-Illia, D. Grosso, F. Cagnol, F. Ribot, C.J. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770.

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