Structure and photoluminescence properties of sol gel TiO SiO films

Transcription

1 Thin Solid Films 413 (00) Structure and photoluminescence properties of sol gel TiO SiO films Chun Feng Song, Meng Kai Lu*, Ping Yang, Dong Xu, Duo Rong Yuan State Key Laboratory of Crystal Material, Shandong University, Jinan 50100, PR China Received 3 August 001; received in revised form 16 April 00; accepted 6 May 00 Abstract TiO SiO films (TiySis1:1) have been prepared by the sol gel method. The structure and photoluminescence (PL) properties of the films heat-treated at different temperatures have been investigated. The films appear composed of TiO nanocrystallites in anatase form embedded in SiO -rich matrix. Emission bands are observed in all the heat-treated films. This PL is attributed to various defects resulting from thermal treatment and crystallization of TiO and to energy levels in the mixed oxide matrix. 00 Elsevier Science B.V. All rights reserved. Keywords: Sol gel; Heat-treatment; Luminescence; Oxides 1. Introduction Titania silica (TiO SiO ) based films have aroused considerable interest for optics application, including antireflective coatings and optical planar waveguides, due to their high thermal stability, high chemical durability, low thermal expansion coefficient, and flexibly adjustable refractive index w1,x. In addition, TiO SiO mixed oxides can also be considered as attractive materials for catalytic applications due to their high catalytic activity and selectivity w3,4x. Sol gel technique offers an excellent method for the preparation of silica titania films. To obtain a good quality film, a condensation step is required. This involves a heat-treatment process. That can, in turn, influence the film structure. Many studies on the structure of sol gel derived TiO SiO films have been performed, including infrared (IR) spectroscopy, X-ray diffraction (XRD), nuclear magnetic resonance, transition electron microscopy, and atomic force microscopy w,5 9x. Photoluminescence (PL) techniques have been widely used to investigate the structure and properties of active sites on the surface of supported (or bulk) metal oxides, zeolites, etc. w10x. Such techniques are highly sensitive and non-destructive. To our knowledge, the PL proper- *Corresponding author. Fax: q address: mklu@icm.sdu.edu.cn (M.K. Lu). ties of sol gel derived TiO SiO films have never been reported. In the present paper, TiO SiO films were prepared by the sol gel method. The structure and PL properties of films heat-treated at different temperatures were investigated. In addition, the PL mechanism was discussed.. Experimental TiO SiO films, with a TiySi molar ratio of 1, were prepared using tetraethylorthosilicate (TEOS) and tetrabutyl titanate (TBT) as raw materials. TEOS was first hydrolyzed for 1 h with water under magnetic stirring at room temperature. Ethanol and acetylacetone were used as solvent system and hydrochloric acid as a catalyst. TBT was added to the partially hydrolyzed TEOS solution. The mixture was heated at 70 8C for 15 min and then cooled to room temperature. Water was then added drop by drop. The final volume ratio of TEOS:ethanol:acetylacetone:water was 1:0.5:1:3. The sol was left for 1 day at room temperature. Thoroughly cleaned silica glass substrates were used for sol gel spin-coating deposition. The sol gel solution was spun at 3500 rpm for 30 s. The films were then dried at 110 8C in air for 30 min. This process was repeated twice. The final 3-layers coatings were annealed in air at 550, 650, 750, 850 and 950 8C for 30 min, respectively. A /0/$ - see front matter 00 Elsevier Science B.V. All rights reserved. PII: S Ž

2 156 C.F. Song et al. / Thin Solid Films 413 (00) Fig. 1. TG curve of gel dried at room temperature. TiO SiO gel with an identical TiySi composition was obtained after solution aging at room temperature for 1 week. The same heat-treatments were performed on the gel. Powder samples were obtained by grinding the gels heat-treated at the different temperatures. Excitation and emission spectra were measured using an Edinburgh FLS90 combined fluorescence lifetime and steady state spectrometer. Fourier transform infrared (FTIR) absorption spectra were collected using a 5DX FTIR spectrometer. Only the powder samples were studied by FTIR because the silica substrate has a strong absorption at approximately 1066 cm due to Si O bonds, which hampered the film study. XRD patterns of films were taken using a Rigaku Dymax-rA X-ray diffractometer operate with a Cu Ka radiation (ls A) at 40 kv00 ma. A secondary graphite crystal monochromator was used. The DTA TG curves were recorded using a NETZSCH STA 409 EP simultaneous thermal analysis on a powder sample dried at room temperature. Fig.. DTA of gel dried at room temperature. easily as the Ti content andyor sintering temperature increasing. Gunji et al. has reported that, for sol gel derived TiO SiO (TiySis1:1) powders, crystallization of TiO occurs at approximately C. Fig. 3 shows the XRD patterns of films heat-treated at different temperatures. All the films annealed at 650 8C or more exhibit only a broad diffraction peak approximately 5.38, which corresponds to the (101) lattice plane of anatase. No rutile phase can be detected even after heating at high temperature. Due to size effect, the XRD peaks broaden and their widths become larger as the particles become smaller w11x. The average crystallite size of TiO heat-treated at 850 and 950 8C have been estimated from the Debye Scherrer formula as approximately 3 and 3.6 nm, respectively. The average crystallite size below 850 8C is approximately 1 nm. It can be seen that the crystallite size of TiO increases largely when heat-treated above 750 8C. In addition, the 3. Results and discussion The DTA and TG curves of the powder sample are shown in Figs. 1 and, respectively. It can be seen from Fig. 1 that there is a strong weight loss between 50 8C and approximately 480 8C, after what no significant weight loss is observed. Fig. shows an endothermic peak approximately 190 8C and an exothermic peak approximately 640 8C. The weight loss below 00 8C and endothermic peak are due to the evaporation of water and organic compounds. Above 00 8C, the weight loss is attributed to the thermal decomposition of the remnant organic compounds w8x. The lack of significant weight loss above 480 8C indicates that most of organic compounds have been burnt off. The exothermic peak is attributed to the crystallization of TiO issued from a phase separation between SiO and. The phase separation is a general behavior in the sol gel derived TiO SiO systems, which takes place all the more Fig. 3. XRD patterns of films heat-treated at different temperatures.

3 C.F. Song et al. / Thin Solid Films 413 (00) Fig. 4. FTIR spectra of powder samples heat-treated at different temperatures. full width at half maximum decreases weakly between 650 and 750 8C, indicating a slight crystallite size increase. From these features, we can consider that a TiO SiO phase separation and TiO nucleation occurs in the temperature range between 550 and 750 8C, and a complementary crystallization and crystallite growth at higher temperature. Fig. 4 shows the FTIR absorption spectra of powder samples annealed at different temperatures. The strongest absorption peak approximately 1060 cm is assigned to the asymmetric vibration of Si O Si bonds. The absorption peak approximately cm is attributed to the stretching vibration of Si OH, Si OR (R: alkoxy groups) specious and Si O Ti bonds w8,9x. The band approximately 400 cm presumably corre- sponds to TO mode of crystalline anatase w1,13x. This band increases in intensity above 750 8C, which confirms XRD data showing that crystallite growth mainly takes places above 750 8C. The broad band approximately 3400 cm is attributed to the O H stretching vibration of different Si OH (silanol) species and to molecular water as well, and the band at 16 cm is attributed to adsorbed water. Both the bands decrease in intensity with increasing annealing temperature, showing that the hydroxy groups are removed gradually from the film. In our study, the SiOH specious may be present even at higher temperature e.g C. The weak absorption bands approximately 900 cm, as can be observed in all the samples even for those annealed at 950 8C, indicate the presence of hydrocarbon specious, which could come from remnant alkoxy groups (such as from SiOR specious). The weak absorption approximately 300 cm is due to the asymmetric stretching vibration of atmospheric CO. In addition, the band approximately cm can be clearly observed to shift towards lower wavenumber with increasing temperature, from approximately 1000 cm after annealing at 550 8C to 95 cm after annealing at 950 8C. These position variations could be related to a complex combination of SiOR, SiOH and SiO TiO component variations (including phase separation and TiO crystallization) together with structural changes of the silica-rich matrix during the heat-treatment process. These evidences show that a rise in the heat-treatment temperature leads to a decrease in the amount of hydroxy group and a structural reconstruction of TiO SiO network together with TiO crystallization. The excitation and emission spectra of films heattreated at different temperatures are shown in Fig. 5 and Fig. 6. All excitation spectra (lems440 nm) show a broad band centered approximately 380 nm and prominent peak approximately 350 nm (Fig. 5). At temperatures in the range C, the relative intensity of the band approximately 380 nm increase with increasing heating temperature and then decrease above 850 8C. When the 380 nm band appears weak, the 350 nm peak is more easily to be observed. It can be seen from Fig. 6 that all the emission spectra (l s380 nm) consist of ex a broad band centered approximately 440 nm, with three peaks at 484, 657 and 691 nm, and a serious of peaks between 550 and 600 nm. All of these emission bands become intense with increasing heating temperature below 850 8C, and then become weak above 850 8C for the bands approximately 440 and 484 nm while other peaks change slightly. The emission of the film heat- Fig. 5. Excitation spectra of films heat-treated at different temperatures (lems440 nm) (the slit width is 4.33 mm for a; 1.86 mm for b and 0.73 mm for c).

4 158 C.F. Song et al. / Thin Solid Films 413 (00) Fig. 6. Emission spectra of films heat-treated at different temperatures (lexs380 nm) (the slit width is 4.33 mm for a; 1.86 mm for b and 0.73 mm for c). treated at 750 8C appears to be most intense, while it is the weakest for the film heat-treated at 550 8C. PL due to defects in sol gel derived silica gels has been widely investigated w14 0x. As it will be explained below, these defects are induced by thermal treatment up to limit temperature, above which they gradually disappear with increasing temperature. Accordingly, the related PL should depend on heattreatment temperature. Oxygen related defects, such as ^Si, ^Si Si^, could result in a blue emission band w14 16x. Non-bridged oxygen hole centers ^Si O (NBOHC) is associated with PL bands at approximately 689 and 65 nm w19x. The introduction of carbon impurities in the O Si O network, in the form of O C O or Si C bonds, can also cause blue emission w14,0x. In addition, nanostructured titania can exhibit broad emission band centered at nm at room temperature w1,x. The main broad emission band observed at 440 nm in our spectra, which strongly depended on the annealing temperature, could be attributed to the oxygen related defects in the TiO SiO film. Emission peaks at 657 and 691 nm is ascribed to NBOHC. During the preparation of TiO SiO samples by the sol gel method, a mixed glass network is usually formed by hydrolysis (1) and condensation () of alkoxides precursors. Ti ions and Si ions are thus induced in tetrahedral sites of the mixed glass network. The reactions are described by the equations: ^M ORqH O ^M OHqHOR (1) ^M OH ^M O M^qH O () where M is Ti or Si. At low temperature, the reaction proceeds incompletely, and a large amount of ^M OR and ^M OH groups remain in the network. The subsequent thermal treatment can result in the formation of various defect centers, as expressed by the equations: ^M O xr R Ox M^ ^M O M^ (3) q ^M xo R R O xm^ ^M M^ (4) ^M O xrrx O M^ ^M O O M^ (5) ^M O xh H Ox M^ ^M O M^qH O ()600 8C) (6) Where the arrow x denotes the breaking bond w18,19,3x. According to our thermal analysis, the Eqs. (3) (5) most probably occur in the temperature C thermal range. It has been reported that these defects (^Si, ^Si Si^, ^Si O etc.) diminish and eventually disappear by heat-treatment above 700 8C w19x. In their study, Garcia et al. did not observe any PL above 600 8C w15x. However, in the TiO SiO system, these defects appear at higher temperature due to a TiO SiO phase separation followed by TiO crystal- lization, which can be expressed by the reaction shown in Eq. (7) So, the PL still can be observed above 600 8C. The TiO crystallites are in nanometer size. Because most of ions of the nanoparticles are located on the surface and non-saturated in coordination, electrons or holes may be excited easily and escape from the ions. As nanocrystallites are well dispersed in the matrix, more electrons or holes are excited and radiative recombination increases. Thus, the PL intensity can be enhanced greatly. The defects and nanocrystallites increase significantly in the thermal range between 550 and 750 8C because TiO nucleation occurs at this temperature range as discussed above. So, the PL intensity increases significantly when heat-treated at 750 8C. Heat-treatment at much higher temperature promotes the TiO crystallite growth and then the non-saturated ions decrease as the crystallites size becomes larger. In addition, the defects begin to

5 C.F. Song et al. / Thin Solid Films 413 (00) diminish. Therefore, the PL intensity decreases with increasing temperature above 750 8C. In the sol gel derived TiO SiO system, Ti ions are well distributed in the TiO SiO network before phase separation. After higher temperature treatment such as after phase separation, it is likely that part of titanium ions remain in the silica-rich matrix. The radius of Si ions is 0.41 A while it is 0.68 A for Ti. The larger tetrahedron of Ti could cause little distortions in the TiO SiO network or in the silica-rich matrix. In this case, some energy levels under 3 ev may be generated in the distorted TiO SiO system (The band gap for bulk TiO is 3.03 ev, and 10.5 ev for SiO with non-bridged oxygen). We speculate that the recombination of the electrons and holes trapped by these energy levels may result in the emission peaks at 489 nm and in the nm region. When nucleation and crystallization of TiO occurs, more electrons and holes excited from TiO nanocrystallites could be trapped by these levels. Thus, the PL intensity of these emission peaks increases greatly when heat-treated to higher temperature. 4. Conclusion TiO SiO films prepared by the sol gel method and heat-treated at temperatures up to 950 8C appear composed of TiO nanocrystallites in anatase form embedded in a silica-rich matrix. A TiO SiO phase separation and TiO nucleation occurs in the range C, and a complementary crystallization and crystallite growth above 750 8C. Thermal treatment and TiO crystallization can result in the formation of various defects such as ^M, ^M O. In addition, distortions in the distorted TiO SiO oxides lead to the creation of energy levels below 3 ev. These defects cause visible PL of the films. The TiO nanocrystallites can enhance the PL intensity. The defects and crystallite size change with thermal treatment, which make PL change with increasing heat-treatment temperature. Acknowledgments The author thanks the Natural Science Foundation of China for support (the number is ). References w1x X. Hao, X. Fan, Z. Wang, M. Wang, Mater. Lett. 51 (001) 45. wx R. Almeida, J. Sol Gel Sci. Tech. 13 (1998) 51. w3x J.R. Sohn, H.J. Jang, J. Catalysis 13 (1991) 563. w4x C. Odenbrand, S.L.T. Andersson, L.A.H. Andersson, J.G.M. Brandin, G. Busca, J. Catalysis 15 (1990) 541. w5x T. Gunji, T. Kasahara, Y. Abe, J. Sol Gel Sci. Tech. 13 (1998) 975. w6x T. Kogure, T. Umezawa, J. Am. Ceram. Soc. 8 (1999) 348. w7x W. Que, Y. Zhou, L. Lam, Y.C. Chan, Z. Sun, S.D. Cheng, H.P. Li, C.H. Kam, J. Mater. Sci. Lett. 19 (000) 147. w8x W. Que, Y. Zhou, Y.L. Lam, Y.C. Chan, S.D. Cheng, Z. Sun, C.H. Kam, J. Sol Gel Sci. Tech. 18 (000) 77. w9x O. Martins, R. Almeida, J. Sol Gel Sci. Tech. 19 (000) 651. w10x T. Toyoda, T. Hayakawa, K. Abe, T. Shigeari, Q. Shen, J. Lumin (000) 137. w11x P. Yang, M.K. Lu, D. Xu, D. Yuan, G. Jun, Chem. Phys. Lett. 336 (001) 76. w1x R.J. Gonzalez, R. Zallen, Phys. Rev. B55 (1997) w13x I.M.M. Salvado, J. Non-Cryst. Solids 147 & 148 (199) 56. w14x M.A. Garcia, E. Borsella, S.E. Paje, J. Llopis, M.A. Villegas, R. Polloni, J. Lumin. 93 (001) 53. w15x M.A. Garcia, S.E. Paje, M.A. Villegas, J. Llopis, Mater. Lett. 43 (000) 3. w16x S. Okuzaki, K. Okude, T. Ohishi, J. Non-Cryst. Solids 65 (000) 61. w17x N. Chiodini, F. Meinardi, F. Morazzoni, A. Paleari, R. Scotti, Appl. Phys. Lett. 76 (000) 309. w18x J. Lin, K. Baerner, Mater. Lett. 46 (000) 86. w19x I.I. Hinic, G.M. Stanisic, Z.V. Popovic, J. Sol Gel Sci. Tech. 14 (1999) 81. w0x Y.C. Zhu, C.X. Ding, J. Solid State Chem. 145 (1999) 711. w1x Y. Lei, L.D. Zhang, J. Mater. Res. 16 (001) wx W.H. Green, K.P. Le, J. Grey, T.T. Au, M.J. Sailor, Science 76 (1997) 186. w3x H.J. Bi, W.P. Cai, H.Z. Zhang, B.D. Yao, J. Mater. Res. 15 (000) 364.