Journal of Alloys and Compounds 474 (2009) 326–329
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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 . Most studies on MT thin ﬁlms were focused on synthesis, emphasizing on control parameters such as pH [3,4], moisture , water content , 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 . 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 . 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 . 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 . 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
Y. Shen et al. / Journal of Alloys and Compounds 474 (2009) 326–329
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 . In contrast, lines at 148, 402, 521, and 631 cm−1 , which characterize the anatase phase TiO2 , 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.
Y. Shen et al. / Journal of Alloys and Compounds 474 (2009) 326–329
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 ),
where h is the incident photon energy and C is a constant . 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 . 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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