An Efficient Two-Step Technique for Nitrogen-Doped Titanium Dioxide

Nov 17, 2006 - A series of nitrogen-doped TiO2 nanocatalysts have been synthesized successfully by a two-step hydrolysis-calcination method in which c...
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J. Phys. Chem. C 2007, 111, 1010-1014

An Efficient Two-Step Technique for Nitrogen-Doped Titanium Dioxide Synthesizing: Visible-Light-Induced Photodecomposition of Methylene Blue Junwei Wang, Wei Zhu, Yinqing Zhang, and Shuangxi Liu* Institute of New Catalytic Materials Science, College of Chemistry, Nankai UniVersity, Tianjin 300071, P R China ReceiVed: September 20, 2006; In Final Form: October 24, 2006

A series of nitrogen-doped TiO2 nanocatalysts have been synthesized successfully by a two-step hydrolysiscalcination method in which condensed HNO3 (∼16 mol/L) was first used. In contrast to the traditional hydrolysis method, the time consumed in the hydrolysis process was shortened greatly. The content of nitrogen decreases as the calcination temperature increases, resulting in the variation of bandgaps of the as-synthesized N-doped TiO2 from 1.55 to 2.95 eV. Decomposition of methylene blue under visible light irradiation (λ g 400 nm) has been carried out to evaluate the photocatalytic activity of the as-synthesized N-doped nanocatalysts. Compared to P25, greatly improved photocatalytic activity for water contaminant decomposition under visible light irradiation was obtained due to the doping of nitrogen into the titania system. The crystallinity of the photocatalysts was found to be less influential than the nitrogen content in determining the photocatalytic activity.

Introduction Nitrogen-doped TiO2 has roused great interest in both environment and energy fields. Currently, two main trends have been applied for doping TiO2, namely, metal-element doping and the nonmetal doping. In the first trend, various transitional metals, including Fe3+, Mo5+, Ru3+, Re4+, V4+, and Rh3+, have been reported to be capable of tuning the electronic structure of TiO2-based materials and improving their photocatalytic activity.1 Especially, Anpo et al. have developed a unique visible-light-responsive second-generation TiO2 by means of an advanced metal-ion-implantation method.2 However, these metal-doped TiO2 materials suffer from thermal instability and low quantum efficiency because of increased carrier trapping after doping, or require expensive facilities in the ion implantation case.3-5 In contrast, the nonmetal doping trend, first discovered by Sato in 1986 and rekindled by Asahi et al. in 2001, seems to be more successful.4,6,7 A few preparation methods for main group dopants such as N-, S-, C-, I-, P-, and B-doped TiO2 catalysts as well as some theoretical calculations have been reported.3,4,6,8,9 In particular, the presence of substitutional N atoms in the TiO2 matrix improves absorption in the visible region and leads to a corresponding photochemical activity.9 So far, several methods have been reported for preparing nitrogen-doped TiO2,3,4,7,10 and considerable success has been achieved in decreasing the band gap and increasing the photocatalytic activity. However, in most cases, preparation of nitrogen-doped TiO2 demands NH3 or N2 flow at high temperature or hydrolysis for quite a long time; therefore, room environment and simple operations cannot meet the need of these methods to prepare high-quality nitrogen-doped TiO2. In addition, the nitrogen source used in some of these methods is not in the same phase with the titanium source, making the doping process even tougher. We hereby propose a two-step hydrolysis-calcination technique for preparing nitrogen-doped TiO2. The hydrolysis * Corresponding author. E-mail: [email protected]. Phone: +8622-23509005. Fax: +86-22-23509005.

process, which occurs within the homogeneous phase, does not require any critical conditions other than room environment, thus overcoming the problems mentioned above. Additionally, compared to the traditional hydrolysis method reported by Burda et al.,11 which spends approximately 10 h on the hydrolysis process, our technique needs far less time for the hydrolysis (less than 1 h). A number of techniques, including XRD, UVvis diffuse reflectance spectra (UDRS), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) have been employed to explore the properties of these samples. In comparison with the commercial photocatalyst Degussa P25, a considerably enhanced activity of the visible-light-induced photodecomposition of methylene blue was found. Experimental Section The preparation process of nitrogen-doped TiO2 goes as follows: Titanium(IV) isopropoxide Ti(OCH(CH3)2)4 (3.0 g) (Aldrich 97%) was added dropwise into 15.0 mL of H2O under continuous stirring. A homogeneous sol or colloidal solution was obtained immediately after the contact of titanium isopropoxide with water. Concentrated nitric acid (4.0 mL) was then added slowly to the sol to achieve a translucent solution. After vigorous stirring for half an hour, the translucent solution became completely transparent. Aqueous ammonium hydroxide solution (AR, Kai Tong Chem. Inc.) with an ammonia content of 25-28% was added drop by drop into the transparent solution until pH ) 9. A precipitate or floccule could be observed upon addition of the ammonia solution, and the color of the precipitate grew deeper from white to dark yellow with increasing amount of added ammonia. The precipitate was filtered and washed with deionized water and dried in the air at room temperature. The sample obtained this way was the precursor of N-doped TiO2, which was amorphous powder (according to XRD results, not shown) composed mainly of titanium dioxide together with a certain amount of ammonia and nitrate absorbed on it. To eliminate the absorbed substance and acquire N-doped TiO2, this precursor was further calcined at various temperatures, viz. 250, 350, 450, 550, and 650 °C for 2 h. The obtained samples

10.1021/jp066156o CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2006

Efficient Two-Step Technique

Figure 1. X-ray diffraction patterns of N-doped TiO2 calcined at (a) 250, (b) 350, (c) 450, (d) 550, and (e) 650 °C. There is an increase in the size of the crystallite along with increasing temperature, but no phase change up to 650 °C is observed on N-doped TiO2.

were designated correspondingly as X250, X350, X450, X550, and X650 for conveniency. The colors of these N-series samples differ from tan (X250) to light yellow (X650) with increasing calcination temperature. The structure of the as-synthesized samples was determined by a powder XRD (Rigaku D/max-2500) diffractometer with Cu KR radiation (40 kV and 100 mA). Raman spectra were recorded with a Bruker RFS100/S Raman Spectrometer equipped with an air-cooled Nd:YAG laser emitting at a wavelength of 1064 nm, and a liquid-nitrogen-cooled germanium detector with an extended spectral band range of 3500 to 50 cm-1. Diffuse reflection spectra of the samples were obtained using a UVvisible diffuse reflectance spectrometer (JASCO Corp., V-570, Rev. 1.00). Fourier transform infrared (FTIR) spectra were recorded on a Bruker VECTOR 22 spectrometer. X-ray Photoelectron Spectra (XPS) were obtained using a Kratos Axis Ultra DLD spectrometer with a monochromated Al-Ka X-ray source (hν ) 1486.6 eV). The photocatalytic activities of the N-doped TiO2 samples and the commercially available TiO2 photocatalyst, Degussa P25, have been studied under visible light by photodecomposition of methylene blue (MB). The light source used in this work is a household desktop lamp with a 40-watt tungsten bulb, of which the wavelength range is usually considered as 400-2500 nm. For a typical photocatalytic experiment, 0.18 g of photocatalyst was added to 200 mL of aqueous solution containing ∼30 ppm methylene blue. The solution was stirred at a constant rate for 30 min without light illumination before a sample of this solution was taken to reduce the adsorption influence on the absorbance measurement. Another sample was taken after irradiation of the solution with continuous stirring for 20 h. The absorbance of these two solutions was measured using a SP-722 spectrometer at λ ) 650 nm (λmax for MB). Both samples were treated by centrifugation before the measurement. Results and Discussion Figure 1 shows the XRD patterns of the as synthesized N-doped TiO2. It could be concluded from Figure 1 that pure anatase phase was obtained for all samples except for the one calcined at 250 °C, which remains to be amorphous titania. Generally, phase transformation of pure TiO2 from anatase to rutile occurs at ∼500 °C. It is clear that the presence of nitrogen in titania has the effect of restraining this phase transformation. The anatase phase was considered to be more photoactive than the rutile phase;12,13 hence, doping of nitrogen may have the effect of restraining the photoactivity from decreasing when

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1011

Figure 2. UV-vis diffuse reflection spectra of N-doped TiO2 calcified at (a) 250, (b) 350, (c) 450, (d) 550, and (f) 650 °C and (e) P25.

calcined. The crystallite size could be estimated using the Debye-Scherer equation

D ) 0.94λ/β cos θ where D is the crystal size, λ is the wavelength of X-ray radiation (0.1548 nm for Cu KR radiation), β is the full width at half-maximum, and θ is the diffraction angle.14 The calculated result is 18, 23, 24, and 30 nm for X350, X450, X550, and X650, respectively. We noticed that the size difference between X450 and X550 is rather small compared to that between X550 and X650 or X350 and X450. This difference in the size change reflects that the agglomeration of the TiO2 crystal is not a continuous process. It is interrupted by the removal of nitrogen from TiO2, which occurred mainly between 450 and 550 °C (concluded from Figure 2). The UV-visible diffuse reflection spectra of N-doped samples calcined at various temperatures together with P25 are shown in Figure 2. X250 appears to be quite different from others with its distinctly great absorption in the visible light region. Accordingly, X250 is dark brown while others are yellow. One reason for the broad absorbance may be the existence of the residual carbon in X250. However, the concentration of carbon must be very low, judging from the absence of the C-H bending vibration at around 2900 cm-1 in the FTIR spectra (Figure 3B). The absorption edge for N-doped TiO2 samples shifts greatly toward a larger wavelength (∼800 nm for X250) compared to that of P25 (∼ 420 nm). However, this capability decreases with increasing calcination temperature, especially when the temperature is above 550 °C. The band gap of the as-synthesized samples calculated by the absorption edge changes from 1.55 to 2.95 eV. Similar circumstances have been reported before by Sathish et al.15 This decrease may be caused by the loss of nitrogen in TiO2 during the heating process because it would become much easier for nitrogen to be replaced by oxygen in the air with increasing temperature. Fortunately, however, in most cases, we do not need to calcify titania to temperatures as high as 550 °C in respect that both the crystallization degree (Figure 1) and the photocatalytic activity (Table 1) of the N-doped TiO2 sample are already good enough after being calcified at 350 °C. In Figure 3A, the Fourier transform infrared (FTIR) spectra of HNO3 (concentrated) treated Degussa P25 before and after calcination at 350 °C together with that of N-doped TiO2 calcined at 350 °C and pure anatase TiO2 are shown. The peak at ∼1383 cm-1 in spectra c corresponds to the nitrate ions. After heating to at 350 °C, this peak disappears, indicating the complete removal of nitrate ions or at least the content of the remaining nitrate is not detectable.

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Wang et al.

TABLE 1: Comparison of the Properties of Nitrogen-Doped Titanium Dioxide Calcined at Various Temperatures Together with Degussa P25 photodecomposition of methylene blue (%) photocatalyst

calcination temperature (°C)

crystalline phase

250 350 450 550 650 250

75% anatase 25% rutile amorphous pure anatase pure anatase pure anatase pure anatase amorphous

Degussa P25 X250b X350 X450 X550 X650 X250d

crystallite size (nm)a

18 23 24 30

band gap (eV)c

first run

2.98

7.0

1.55 2.21 2.17 2.17 2.88 1.55

93.1 62.9 52.8 36.8 7.9 6.3

a Crystallite size was estimated using the Debye-Scherer equation. b The character “X” represents N-doped TiO2. edge. d Without light illumination.

Figure 3. FTIR spectra of (A) (a) N-doped TiO2 calcined at 350 °C, (b) HNO3 (concentrated) treated P25 calcined at 350 °C, (c) HNO3 (concentrated) treated P25 without calcination, and (d) pure anatase TiO2; (B) N-doped TiO2 calcined at (a) 250, (b) 350, (c) 450, (d) 550, and (e) 650 °C.

Figure 3B shows the FTIR spectra of N-doped TiO2 calcined at various temperatures. No transmission peaks of nitrate are observed in any of these samples. In accordance with the XRD patterns shown in Figure 1, X250 shows a spectrum different from the others. All of these N-doped samples show an absorption peak at 1630 cm-1, although the intensities of these peaks differ from each other. Generally, the FTIR peak at ∼1630 cm-1 is attributed to the O-H bending vibration of adsorbed water molecules. In view of the decreasing intensity of this peak as the calcination temperature increases, it can be concluded that less void sites are left for N-doped samples calcined at high temperature. This loss of void sites indicates the occurrence of agglomeration due to the increasing calcination temperature. This conclusion is consistent with the XRD and Raman shift result shown in Figure 1 and Figure 5, respectively. The peak at ∼1402 cm-1 is observed clearly for X250 only. This peak can be attributed to the bending vibration mode of the N-H band, which may be formed by the doping nitrogen

second run

third run

92.2 62.3 51.9 35.8

92.0 62.3 52.2 35.5

c

Calculated from the absorption

with the absorbed H2O. The intensity of this N-H absorption peak decreases sharply with increasing calcination temperature and almost disappears when calcined above 350 °C. This again can be explained by the decrease of void sites due to the formation of the anatase crystal phase at 350 °C (Figure 1). According to Xiaobo Chen et al., the typical infrared absorption of N-O bands occurs at 1618 cm-1(antisymmetric stretch, very strong), 1318 cm-1 (symmetric stretch, weak), and 750 cm-1 (bend, strong).3 No peaks in these regions are observed, thus ruling out the possibility of significant formation or physisorption of NO2 species. Figure 4 shows the XPS spectra of pure anatase TiO2 and N-doped TiO2 calcined at 350 °C. There have been many reports concerning the XPS spectroscopy study of N-doped TiO2;3,4,8,11,16,17 in short, there seems to be no consensus on the assignment of the N 1s XPS results or even the position of the peaks. These disagreements may be caused by the different preparation procedures adopted by different groups. We would like to add our findings to enrich the variety of XPS results for N-doped TiO2. Three areas have been examined in this work: the N1s region around 400 eV (Figure 4C), the Ti2p region around 460 eV (Figure 4A), and the O1s (Figure 4B) region around 530 eV. The Ti 2p3/2 and Ti 2p1/2 core level appears at 458.6, 464.4 eV for pure TiO2 and 458.4, 464.2 eV for N-doped TiO2 calcined at 350 °C. It is to be noted that the XPS result in our finding for both Ti 2p3/2 and 2p1/2 in pure anatase TiO2 is different from that observed by Burda et al.3 They have reported a much higher binding energy of Ti 2p in Degussa P25. However, the XPS results of Ti 2p in N-doped TiO2 are not that different. Burda et al. attribute this to the difference in structure between P25, which is nonporous, and the porous TiO2 nanocolloid prepared through a sol-gel process. Although, a lower binding energy of Ti 2p in N-doped TiO2 has been found compared to that in pure anatase TiO2, which indicates that the electronic interaction of Ti with anions in the N-doped material is different from that in pure TiO2. This difference can be explained by the change of electron density around the Ti atom due to the formation of the N-Ti bond with the introduction of nitrogen into TiO2. It is known that the electronegativity (the ability of a bonded atom to pull electrons toward itself) of nitrogen is lower than that of oxygen. When the oxygen in TiO2 is substituted by nitrogen, the electron density around the Ti atom will increase. As a result, the binding energy of Ti decreases. Unlike titanium, the oxygen 1s core level peak shows almost the same binding energy (529.7 eV) in both N-doped TiO2 and pure anatase TiO2 (Figure 3c), indicating the similar nature of oxygen in doped and undoped TiO2. This result is consistent

Efficient Two-Step Technique

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Figure 5. Raman spectra of N-doped TiO2 calcined at (a) 250, (c) 350, (d) 450, (e) 550, and (f) 650 °C and (b) pure TiO2; (A) full length spectra; (B) expanded spectra for the Eg mode at 143.0 cm-1.

Figure 4. X-ray photoelectron spectra (XPS) for N-doped TiO2 and pure TiO2 (A) Ti 2p, (B) O 1s, and (C) N 1s. (a) represents N-doped TiO2 calcined at 350 °C, and (b) represents pure TiO2.

with that reported by Sathish et al. However, we do not observe the broadening effect mentioned in their work. For the nitrogen 1s case, previous studies have obtained the typical binding energy of nitrogen in TiN,17 NH3,18 and NaNO33 at 397.2, 398.8, and 408 eV, respectively. From this point of view, it is clear that the element adjacent to nitrogen directly influences its binding energy. The stronger the electronegativity of the adjacent element, the higher the binding energy of nitrogen. In this work, the content of nitrogen element in the N-doped TiO2 calcined at 350 °C is estimated to be ∼2% (molar ratio) from the XPS result. The nitrogen 1s core level from N-doped TiO2 shows a main peak at 399.6 eV, which lays higher than that of TiN and lower than that of NO3-. We attribute this to the binding energy of the N atom in the environment of O-Ti-N. In addition, two small peaks are observed at 397.0 and 403.0 eV, respectively. The lower can be attributed to the

formation of N-Ti-N, and the higher can be attributed to the formation of Ti-N-O. The weak intensity for both small peaks indicates the trace amount of nitrogen in these two forms. Figure 5 shows the Raman spectra of pure and N-doped TiO2 calcined at various temperatures. According to literature,19-25 the phase of TiO2 can be identified by the Raman peaks over 140-700 cm-1 range. Typically, for anatase TiO2, there are six Raman active fundamental modes at 142(Eg), 197(Eg), 397(B1g), 518(A1g + B1g), and 640 cm-1 (Eg), respectively. We can conclude from Figure 5A that all of the N-doped TiO2 samples except the one calcined at 250 °C (X250) show spectra corresponding to the typical anatase TiO2. The Raman spectra of pure anatase TiO2 are given for comparison. The peaks become sharper and stronger as the calcination temperature increases. One of the reasons for this sharpening trend is the increasing crystal size.26 Thus, Figure 5 can lead to the same conclusion with Figure 1 that all of the N-doped TiO2 except X250 are anatase phase and that the crystal size of these samples increases with increasing calcination temperature. Additionally, the Raman peaks shift slightly for the N-doped samples. For instance, the Eg mode at 142.0 cm-1 for pure anatase shifts to 144 cm-1 for X350, X450, and X550. For X650, it is interesting to note that this Eg mode shifts back to 142 cm-1. Obviously, this backward shift cannot be attributed to the crystal size effect because the size of the N-doped TiO2 increases monotonically with increasing calcination temperature. In combination with the UV-vis diffuse reflection spectra of the N-doped samples (Figure 2), we attribute this shift in the peak position to the doping of nitrogen. The doping process changes the composition of the TiO2 crystal and thus the vibration mode. The more nitrogen contained in the system, the larger the shift for the peaks. In Figure 2, X650 shows almost the same spectrum as

1014 J. Phys. Chem. C, Vol. 111, No. 2, 2007 that of pure TiO2, indicating that the nitrogen doped in the system nearly disappears after calcination at 650 °C. This decrease causes the Raman peaks to shift backward. The photocatalytic activities of the N-doped TiO2 nanocatalysts and Degussa P25 under visible light have been studied by the photodecomposition of the water contaminant methylene blue. The results of these photoreactions together with a comparison in properties of various photocatalysts have been made in Table 1. Compared to the commercial photocatalyst Degussa P25, which is almost nonactive in the visible region, the nitrogen-doped TiO2 samples exhibit much better photoactivity. The stability of the catalysts has been tested by using the photocatalysts repeatedly three times. No visible change of the photoactivity has been observed throughout these three runs. One may raise the doubt of whether it is the photocatalyst that plays the key role in decomposing methylene blue because methylene blue can absorb visible light itself. This doubt, however, is not necessary considering the case using P25 or X650 as photocatalyst. If the decomposition of methylene blue is due to the light absorbance itself, then the efficiency of the decomposition using a different photocatalyst may not vary so much as is shown in Table 1. To ascertain the role of light radiation in the decomposition process, we have tested the effect of X250 without light radiation and the result is not unexpected. Without light, the decomposition process could hardly proceed. However, the photoactivity of the N-doped TiO2 samples decreases as the calcination temperature increases. This decrease in photoactivity can be explained by the loss of nitrogen due to the heating process, which can be concluded from the UV-vis diffuse reflection spectra of these samples (Figure 2). It is noteworthy that X250, which is amorphous N-doped TiO2, shows the highest photocatalytic activity, not the well crystallized samples. From this point of view, it can be concluded that nitrogen content is more important than crystallinity in determining visible light photoactivity. Conclusions Nitrogen-doped TiO2 was synthesized within a homogeneous phase through a simple but effective method. A combination of several characterization techniques leads to the conclusion of the formation of the O-Ti-N environment in anatase TiO2. The photocatalytic activity of the nitrogen-doped TiO2 was found to be greatly superior to that of the commercial P25 under visible light irradiation. The nitrogen content, rather than the crystallinity, plays a more important role in determining the photocatalytic activity of the photocatalyst. We believe that this preparation technique and the as synthesized photocatalysts, with further investigation and improvement, will be of great value to water pollution prevention as well as to the efficient utilization of solar energy.

Wang et al. Acknowledgment. This work was financially supported by the National Science Foundation of China (Grant Nos. 29973016 and 20233030) and the Ministry of Education of P R China. Note Added after ASAP Publication. Part C of Figure 4 was missing when this paper was published ASAP on November 17, 2006. The corrected version was published ASAP on November 30, 2006. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Anpo, M.; Dohshi, S.; Kitano, M.; Hu, Y.; Takeuchi, M.; Matsuoka, M. Annu. ReV. Mater. Res. 2005, 35, 1. (3) Chen, X.; Lou, Y.; Anna, C.; Samia, S.; Burda, C.; Gole, J. L. AdV. Funct. Mater. 2005, 15, 41. (4) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (5) Yamashita, H.; Honda, M.; Harada, M.; Ichihashi, Y.; Anpo, M.; Hirao, T.; Itoh, N.; Iwamoto, N. J. Phys. Chem. B 1998, 102, 10707. (6) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Appl. Phys. Lett. 2002, 82, 454. (7) Sato, S. Chem. Phys. Lett. 1986, 123, 126. (8) Park, J. H.; Kim, S.; Allen, J. B. Nano Lett. 2006, 6, 24. (9) Valentin, C. D.; Pacchioni, Gi.; Selloni, A. Phys. ReV. B 2004, 70, 085116. (10) Jang, J. S.; Kim, H. G.; Ji, S. M.; Bae, S. W.; Jung, J. H.; Shon, B. H.; Lee, J. S. J. Solid State Chem. 2006, 179, 1067. (11) Burda, C.; Lou, Y.; Chen, X.; Anna, C.; Samia, S.; John, S.; Gole, J. L. Nano Lett. 2003, 3, 1049. (12) Bersani, D.; Antonioli, G.; Lottici, P. P.; Lopez, T. J. Non-Cryst. Solids 1998, 232-234, 175. (13) Karla, R. R.; Enrique, A. R.; Daniel, R. J. Electrochem. Soc. 2006, 153, 1296. (14) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed.; Wiley: New York, 1974. (15) Sathish, M.; Viswanathan, B.; Viswanath, R. P.; Chinnakonda, S. G. Chem. Mater. 2005, 17, 6349. (16) Sakthivel, S.; Kisch, H. Chem. Phys. Chem. 2003, 4, 487. (17) Saha, N. C.; Tompkins, H. G. J. Appl. Phys. 1992, 722, 3072. (18) NIST/EPA Gas-Phase Infrared Database, http://webbook.nist.gov/ chemistry/; http://srdata.nist.gov/xps/. (19) Takahashi, T.; Nakabayashi, H.; Tanabe, J.; Yamada, N.; Mizuno, W. J. Vac. Sci. Technol., A 2003, 21, 1419. (20) Ohsaka, T.; Izumi, F.; Fujiki, Y. J. Raman Spectrosc. 1978, 7, 321. (21) Berger, H.; Tang, H.; Levy, F. J. Cryst. Growth 1993, 130, 108. (22) Tang, H.; Prasad, K.; Sanjines, R.; Schmid, P. E.; Levy, F. J. Appl. Phys. 1994, 75, 2042. (23) Robert, T. D.; Laude, L. D.; Geskin, V. M.; Lazzaroni, R.; Gouttebaron, R. Thin Solid Films 2003, 440, 268. (24) Bersani, D.; Antonioli, G.; Lottici, P. P.; Lopez, T. J. Non-Cryst. Solids 1998, 232-234, 175. (25) Ivandaa, M.; Music, S.; Gotic, M.; Turkovic, A.; Tonejc, A. M.; Gamulin, O. J. Mol. Struct. 1999, 480, 641. (26) Bersani, D.; Lottici, P. P.; and Ding, Xing-zhao. Appl. Phys. Lett. 1998, 72, 73.