- Email: [email protected]
Tunable photoluminescence in sol–gel derived silica xerogels
November 2000
Materials Letters 46 Ž2000. 86–92 www.elsevier.comrlocatermatlet
Tunable photoluminescence in sol–gel derived silica xerogels Jun Lin a,) , K. Baerner b a
Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 159 Renmin Street, Changchun 130022, People’s Republic of China b IV Physics Institute, UniÕersity of Gottingen, Bunsenstr. 11-13, D-37073, Gottingen, Germany ¨ ¨ Received 14 March 2000; accepted 28 April 2000
Abstract Silica xerogels prepared by sol–gel method show blue emission under UV excitation with a smaller Stokes shift. The luminescent properties have been investigated under various preparation conditions and compositions. The silica xerogels show similar luminescent properties when using C 2 H 5 OH and N, N-dimethylformamide ŽDMF. as solvents, which are very different from those when using dimethylsulfoxide ŽDMSO. as solvent, i.e., a red shift of excitation and emission has been observed in the latter case. The emission intensity of the silica xerogels also depends on the water content and pH of the starting reaction solution. The introduction of organic group Ž –CH 3 . in the silica xerogel modifies the network structure and further changes their luminescence properties. Heat treatment results in the decomposition of the organic Ž –SiCH 3 . groups, which eliminates the old luminescent centers and produces new luminescent centers in longer wavelength simultaneously. q 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55-m; 78.55Kz Keywords: Sol–gel; Silica xerogel; Photoluminescence
1. Introduction Sol–gel derived silica gel Žaerosol or xerogel. is of great importance based on its wide applications in optics, catalysts, sensors and solar energy collectors w1–4x. Organic dyes w5x and rare earth ions w6–8x have been incorporated into the microporous silica gel, which is generally considered to possess an optically inert framework. However, the sol–gel derived silica gel is not optically inert, but optically active, i.e., it shows luminescence under UV excita)
Corresponding author. E-mail address: [email protected] ŽJ. Lin..
tion w9,11x. Of particular interest and importance is that, a new class of stable and efficient white photoluminescent silicate materials has been obtained from an alkoxysilane and a carboxylic acid through the sol–gel route w12,13x. Another related report is the sol–gel derived undoped urea cross-linked organic– inorganic xerogels which display strong blue-light emission w14x. In short, the amount of work focused on the luminescence phenomena in the undoped silica gel itself is limited, and the luminescence mechanism is not very clear yet. Thus, in this paper we will report a more detailed study on the photoluminescence of sol–gel derived silica xerogels and organic–inorganic hybrid silica xerogels. First, vari-
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 1 4 7 - 6
J. Lin, K. Baernerr Materials Letters 46 (2000) 86–92
87
ous factors in the sol–gel process, such as solvents, pH value, water content are selected to investigate their influence on the luminescence properties of the resultant silica xerogel, and then organic group –CH 3 is introduced in the silica xerogel by using CH 3 SiŽOC 2 H 5 . 3 as one of the precursors; temperature effect on the luminescence properties of the hybrid silica xerogel has also been studied.
kV, 25 mA.. All measurements were performed at room temperature.
2. Experimental
The blue emission in silica xerogel has been reported by Garcia et al. w10x. Here we focus on the dependence of the photoluminescence properties of
All chemicals employed in the present work were purchased from Aldrich and used as received. Pure silica xerogels were prepared by hydrolysis and condensation of tetraethoxysilane SiŽOC 2 H 5 .4 ŽTEOS. under different conditions. In studying the solvent effects, 10 ml TEOS was mixed with 4 ml H 2 O and 5 ml solvent wethanol EtOH, or N, N-dimethylformamide ŽDMF., or dimethylsulfoxide ŽDMSO.x, under magnetic stirring, respectively. The pH values of these three solutions were kept at 2 by adding concentrated hydrochloric acid ŽHCl. by drops. Gelation took place in 10 days. The resulting silica xerogels are denoted as SXŽEtOH., SXŽDMF. and SXŽDMSO., respectively. Silica xerogels were also obtained under different pH values and water contents ŽH 2 OrTEOS mole ratio.. In order to decrease the experimental errors, all other parameters were kept constant to the greatest extent when adjusting the pH or changing the H 2 OrTEOS mole ratios. Pure silica xerogels were obtained by drying the wet silica gel at 1208C for 3 days. Organic–inorganic hybrid silica xerogels were prepared by the simultaneous hydrolysis and condensation of TEOS and methyltriethoxysilane CH 3 SiŽOC 2 H 5 . 3 ŽMTES . Žmolar ratio TEOSrMTESs 2:1. using DMF as solvent under acid condition ŽpH s 2.. The resultant hybrid xerogels, labeled as HSXŽTM., were heated at 1208C, 2008C, 4008C and 6008C for 3 days, respectively. Photoluminescence excitation and emission spectra were recorded with a Hitachi F-3010 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. FT-IR spectra were measured using an IFS-66V FT-IR spectrophotometer ŽBruker.. X-ray powder diffraction ŽXRD. were checked using a D-5000 X-ray diffractometer ŽSiemens. with CuK a Ž l s 0.15405 nm. radiation Ž40
3. Results 3.1. Dependence of the photoluminescence properties of the silica xerogel on the experimental conditions
Fig. 1. Excitation Ža. and emission Žb. spectra of SXŽEtOH. Ž1., SXŽDMF. Ž2. and SXŽDMSO. Ž3. after heat treatment at 1208C.
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silica xerogels on the solvents, water content and pH value in the starting reaction solutions. The silica xerogels derived by using different solvents clearly show distinctive luminescence properties. Fig. 1 gives the excitation and emission spectra of SXŽEtOH.Ž1., SXŽDMF.Ž2. and SXŽDMSO.Ž3., respectively. For SXŽEtOH., its excitation spectrum consists of a main broad band peaking at 363 nm with a shoulder at 305 nm and two weak peaks at 236 and 243 nm, and the corresponding emission spectrum contains a broad band from 360 to 600 nm, with a maximum value at 439 nm. The Stokes shift amounts to 4770 cmy1 . The excitation and emission spectra of SXŽDMF. look similar to those of SXŽEtOH., but their maxima are located at 359 and 428 nm with a Stokes shift of 4500 cmy1 , respectively. Great difference has been observed for the case of SXŽDMSO., whose excitation and emission maxima shift to 402 and 464 nm Ža shoulder at 483 nm., respectively. The Stokes shift for the luminescence of SXŽDMSO. is 3320 cmy1 . The emission intensity of the peak at 430 nm of SXŽDMF. was studied as a function of H 2 OrTEOS mole ratio Ž R . and pH value of the starting reaction solution. The results are shown in Figs. 2 and 3, respectively. From Fig. 2 it can be seen that both lower or higher R values are not good for improving the blue emission intensity, and the maximum emission intensity is obtained when R amounts to 8. Also the emission intensity increases with increasing the pH value until pH s 8, and then decreases with further increase of the pH value, as indicated in Fig. 3.
Fig. 2. Emission intensity of SXŽDMF. as a function of H 2 OrTEOS mole ratio of the starting reaction solution.
Fig. 3. Emission intensity of SXŽDMF. as a function pH of the starting reaction solution.
3.2. Luminesence properties of hybrid silica xerogels Similar to the pure silica xerogels, the HSXŽTM. also show luminescence in the blue region under UV excitation. However, their luminescence behavior becomes more complicated, which has been investigated as a function of heat treatment temperatures between 1208C and 6008C. Fig. 4 shows the excitation and emission spectra of HSXŽTM. after heat treatment at 1208C. These spectra look like those in Fig. 1Ž1,2. for pure silica xerogels which are also treated at 1208C, except that the excitation bands at 242 and 300 nm become stronger relative to the main band at 360 nm and well resolved. The shape of the emission spectra obtained by exciting into 300 and 360 nm are the same, both of which have a peak at 438 nm with a long tail extended to 600 nm. This indicates that these two excitation bands originate
Fig. 4. Excitation Ža. and emission Žb. spectra of HSXŽTM. after heat treatment at 1208C.
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Fig. 7. Emission spectra of 4008C treated HSXŽTM. under different excitation. Fig. 5. Comparison of the emission spectra of HSXŽTM. under 360 nm excitation after heat treatment at different temperatures.
from the same emission center. However, the emission spectrum excited by short UV 242 nm clearly shows two well resolved peaks at 438 and 456 nm as well as a weak peak at a longer wavelength of 568 nm, suggesting that the 242 nm excitation band originates from another kind of emission center. This means there exist at least two distinctive emission centers in the 1208C treated HSXŽTM.. Fig. 5 shows a comparison of the emission spectra of HSXŽTM. samples treated from 1208C to 6008C Žexcited by 360 nm.. From this figure, it is clearly seen that with increasing heat treatment temperature the emission intensity decreases and the maximum peak value shifts to longer wavelength. The emission is completely quenched after 6008C treatment. A more detailed study on the luminescence properties of the 2008C and 4008C treated HSXŽTM.
Fig. 6. Emission spectra of 2008C treated HSXŽTM. under different excitation.
samples reveals that their emission centers have completely changed compared with those of the 1208C treated sample. Fig. 6 presents the emission spectra of 2008C treated HSXŽTM. sample excited by different wavelengths. Obviously, the photoluminescence spectra are strongly dependent on the excitation energy, i.e., the emission shifts to high energy with increasing excitation energy Žexcitation at 428, 390 and 361 nm yields emission in 503, 468 and 446 nm, respectively.. This indicates that new luminescent centers have formed after the 2008C treatment. The emission centers have changed again after 4008C treatment. For example, excitation into 361 and 404 nm yields broad band emission at 463 and 491 nm, respectively, both of which have a shoulder at 478 nm, as shown in Fig. 7. In summary, with increasing heat treatment temperature for the HSXŽTM., new emission centers with lower emission energy and weaker emission intensity have formed.
Fig. 8. XRD patterns of HSXŽTM. after heat treatment at different temperatures.
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charge transfer mechanism w10x and carbon impurity mechanism w12x, have been proposed to explain the luminescent phenomena in the sol–gel derived silica gels. In the preparation of SiO 2 gel by sol–gel processing, a glass network is formed by the reaction of hydrolysis and condensation of alkoxysilane precursors. The reactions remove the organic groups by replacing them with hydroxyl groups. Thermal treatments result in the formation of various defect centers, such as: Ž1. Fig. 9. FT-IR spectra of HSXŽTM. after heat treatment at different temperatures.
Ž2. XRD and FT-IR spectra were used to investigate the structure evolution of the HSXŽTM. samples during the heat treatment process, as shown in Figs. 8 and 9, respectively. Not much information can be obtained from the XRD, i.e. only a broad band at 2 u s 238 presents independence of the treatment temperatures, indicating that the xerogels remain amorphous in the whole heat-treatment process. Obvious changes are observed in the FT-IR spectra of HSXŽTM.. For the HSXŽTM. sample after heat treatment at 1208C, the FT-IR spectrum mainly consists of absorption peaks ranging from 400 to 1500 cmy1 , which are located at 440 cmy1 Ž d , Si`O`Si ., 798 cmy1 Ž ns , Si`O`Si., 836 cmy1 Ž d , SiCH 3 ., 940 cmy1 Ž n , Si`OH., 1057 cmy1 , 1155 cmy1 Ž nas , Si`O`Si. and 1279 cmy1 Ž n , SiCH 3 ., respectively w15x. With increasing heat treatment temperatures, the absorption peaks Ž440, 798, 1057 and 1155 cmy1 . of Si`O`Si bonds become stronger and shift to higher wavenumbers, and those of SiCH 3 Ž831, 1279 cmy1 . become weaker and disappear completely after the 6008C treatment. This is because of the pyrolysis of SiCH 3 groups at higher temperature in the HSXŽTM. sample.
4. Discussion The origin of the luminescence in sol–gel derived silica gels has not been well understood yet. Three possible mechanisms, i.e., defect mechanism w9x,
Ž3. where the arrow x denotes bond cleavage sites w9x. The localization of an electron in the dangling sp 3 silicon in SiO 3 units or the localization of an electron-hole in a 2 p orbital of the single bonded oxygen forms the defects. A strong electron–photon coupling causes the luminescence. This is the socalled defect mechanism. For the charge transfer mechanism, it is believed that the emission from silica gel is related to a charge transfer process between silicon and the oxygen atoms in the SiO 2 gel network w10x. The carbon impurity mechanism has been used to explain the luminescence phenomena that occurred in the system of silane coupling agents acting with the organic acid, where the emission is considered to be from the introduction of carbon impurities in the `O`Si`O` network by forming `O`C`O` or `Si`C` bonds w12x. It has been found that the structure of the sol–gel derived oxide networks are strongly affected by the concentration of the interacting species, their ratios, the reaction mediumŽsolvent., the pH, the catalyst and the temperature during condensation w16x. For example, reaction medium Žsolvent. will influence the final oxide contents contents, higher reactants concentration will result in a higher number of bridg-
J. Lin, K. Baernerr Materials Letters 46 (2000) 86–92
ing oxygens Ži.e., higher degree of polymerization., and pH will influence the structure of oxide network by influencing the reaction rate w16x. Thus, our experimental results in Section 3.1 can be generally understood. The luminescence properties Žpeak position and emission intensity. of the silica xerogels depend strongly on the solvents, water contents and pH values of the starting reaction solutions used. This is because the structure of the silica xerogel network depends on these factors. According to the defect mechanism, different network structure will produce different defects during the drying process, and further different luminescent centers and luminescence properties in the silica xerogels. It is well known that the Stokes shift, defined as the energetic difference between the maximum of the excitation spectrum and related maximum of the emission spectrum, is strongly connected with the electron–phonon coupling w14x. The Stokes shift for the luminesence of the silica xerogel is small Ž3000– 4800 cmy1 . and there is a strong overlap between the excitation and emission spectra. This suggests that there is a strong self-absorption for the luminescence of silica xerogel, i.e. energy transfer takes place from one emission center to another. At the present stage, it is difficult to explain our experimental results in more detail. Here we can say that the silica xerogels are potentially a kind of tunable phosphors whose luminescent properties can be adjusted by selecting different solvents as the reaction medium, water content and pH value of the starting reaction solution. The silica gel network has been modified by using CH 3 SiŽOC 2 H 5 . 3 as one of the reactants Žanother component is TEOS., and related reactions can be expressed as follows: CH 3 Si Ž OC 2 H 5 . 3 q H 2 O ™ CH 3 Si Ž OC 2 H 5 . 2 Ž OH . q C 2 H 5 OH
Ž 4.
Si Ž OC 2 H 5 . 4 q H 2 O ™ Si Ž OC 2 H 5 . 3 Ž OH . q C 2 H 5 OH
Ž 5.
2CH 3 Si Ž OC 2 H 5 . 2 Ž OH .
™ H 3 C Ž H 5 C 2 O. 2 Si`O`Si Ž OC 2 H 5 . 2 CH 3 q H 2O
Ž 6.
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2Si Ž OC 2 H 5 . 3 Ž OH .
™ Ž H 5 C 2 O. 3 Si`O`Si Ž OC 2 H 5 . 3 q H 2 O
Ž 7.
CH 3 Si Ž OC 2 H 5 . 2 Ž OH . q Si Ž OC 2 H 5 . 3 Ž OH .
™ H 3 C Ž H 5 C 2 O. 2 Si`O`Si Ž OC 2 H 5 . 3 q H 2 O Ž 8. The hybrid silica xerogel network has been formed by the continuous occurrence of the above hydrolysis ŽEqs. Ž4. and Ž5.. and condensation ŽEqs. Ž6. – Ž8.. reactions. The final structure can be identified by the FT-IR spectra, where the absorption peaks belonging to Si`O`Si, Si`OH and Si`CH 3 are present ŽFig. 9.. Due to the introduction of –CH 3 group in the silica xerogel network, its luminescence properties have been modified. In Fig. 4, it can be found that the intensity ratio of the excitation band 242r360 nm Ž r s 0.77. has been greatly enhanced and the emission band shifts to a longer wavelength Ž l max s 438 nm., as compared with those in Fig. 2 for the pure silica gel Ž r s 0.43, l max s 430 nm.. According to the emission spectra in Fig. 4, it can be assumed that the 242 and the 360 nm bands are originated from two different emission centers. It seems that the introduction of –CH 3 group in the silica gel network is more favorable for the formation of emission center with excitation band at 242 nm. Carbon Ž –CH 3 . indeed plays some role in the luminescence of HSXŽTM., as the above mentioned carbon impurity mechanism indicated w12x. Heat treatment would change the structure of the HSXŽTM.; especially, the Si–CH 3 group would decompose to a large degree. This can be seen clearly from the FT-IR spectra ŽFig. 9.. As a result, the luminescence takes on a red shift and decreases in intensity ŽFig. 5. under 360 nm UV excitation. After heat treatment at 6008C, the luminescence disappeared while the Si–CH 3 group decomposes completely. We cannot say that the quenching of the luminescence is merely because of the decomposition of Si–CH 3 groups, but they must have some relationship. Furthermore, heat treatment Ž2008C and 4008C. also cause the formation of new luminescent centers with longer excitation and emission wavelengths, as shown in Figs. 6 and 7, which are completely different with those in the 1208C treated sample. This is also due to the structure change of
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the Si`O networks on heating. In summary, we can say that the luminescence properties of silica xerogel can also be adjusted by introducing organic components and heat treatment.
5. Conclusions From this study, it can be concluded that the sol–gel derived silica xerogel itself is a kind of interesting photoluminescent material whose luminescence properties can be adjusted by changing the reaction medium, water content, pH of the starting reaction solution, as well as the introduction of organic groups in the gel network and subsequent heat treatment. It is believed that there is still a lot of work to be done to clear up the luminescence phenomena in silica gels.
Acknowledgements This project was supported by the Bairenjihua of Chinese Academy of Sciences.
References w1x K.I. Jensen, J. Non-Cryst. Solids 145 Ž1992. 237. w2x O.S. Wolfbeis, R. Reisfeld, I. Oehme, Struct. Bonding 85 Ž1996. 51. w3x C. Houng-Van, B. Pommier, R. Harivololona, P. Pichat, J. Non-Cryst. Solids 145 Ž1992. 250. w4x L.L. Hench, J.L. Nogues, in: L.C. Klein ŽEd.., Sol–Gel Optics Processing and Application, Kluwer, 1994, p. 59. w5x R. Reisfeld, C.K. Jorgensen, Struct. Bonding 77 Ž1992. 207. w6x M.A. Zaitoun, T. Kim, C.T. Lin, J. Phys. Chem. B 102 Ž1998. 1122. w7x L.D. Carlos, R.A. Sa Ferreira, Phys. Rev. B 14 Ž1999. 10042. w8x E. Coroncillo, B. Viana, P. Escribano, C. Sanchez, J. Mater. Chem. 8 Ž1998. 507. w9x B.E. Yodas, J. Mater. Res. 5 Ž1990. 1157. w10x J. Garcia, M.A. Mondragon, C. Tellez, S.A. Campero, V.M. Castano, Mater. Chem. Phys. 41 Ž1995. 15. w11x M.R. Ayers, A.J. Hunt, J. Non-Cryst. Solids 217 Ž1997. 229. w12x W.H. Green, K.P. Le, J. Grey, T.T. Au, M.J. Sailor, Science 276 Ž1997. 1826. w13x V. Bekiari, P. Lianos, Langmuir 14 Ž1998. 3459. w14x R.A. Sa Ferreira, L.D. Carlos, V. de Zea Bermudez, Thin Solid Films 343r344 Ž1999. 476. w15x L.G. Britcher, D.C. Kehoe, J.G. Matisons, A.G. Swincer, Macromolecules 28 Ž1995. 3110. w16x B.E. Yodas, J. Polym. Sci., Part A: Polym. Chem. 24 Ž1986. 3475.
Materials Letters 46 Ž2000. 86–92 www.elsevier.comrlocatermatlet
Tunable photoluminescence in sol–gel derived silica xerogels Jun Lin a,) , K. Baerner b a
Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 159 Renmin Street, Changchun 130022, People’s Republic of China b IV Physics Institute, UniÕersity of Gottingen, Bunsenstr. 11-13, D-37073, Gottingen, Germany ¨ ¨ Received 14 March 2000; accepted 28 April 2000
Abstract Silica xerogels prepared by sol–gel method show blue emission under UV excitation with a smaller Stokes shift. The luminescent properties have been investigated under various preparation conditions and compositions. The silica xerogels show similar luminescent properties when using C 2 H 5 OH and N, N-dimethylformamide ŽDMF. as solvents, which are very different from those when using dimethylsulfoxide ŽDMSO. as solvent, i.e., a red shift of excitation and emission has been observed in the latter case. The emission intensity of the silica xerogels also depends on the water content and pH of the starting reaction solution. The introduction of organic group Ž –CH 3 . in the silica xerogel modifies the network structure and further changes their luminescence properties. Heat treatment results in the decomposition of the organic Ž –SiCH 3 . groups, which eliminates the old luminescent centers and produces new luminescent centers in longer wavelength simultaneously. q 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55-m; 78.55Kz Keywords: Sol–gel; Silica xerogel; Photoluminescence
1. Introduction Sol–gel derived silica gel Žaerosol or xerogel. is of great importance based on its wide applications in optics, catalysts, sensors and solar energy collectors w1–4x. Organic dyes w5x and rare earth ions w6–8x have been incorporated into the microporous silica gel, which is generally considered to possess an optically inert framework. However, the sol–gel derived silica gel is not optically inert, but optically active, i.e., it shows luminescence under UV excita)
Corresponding author. E-mail address: [email protected] ŽJ. Lin..
tion w9,11x. Of particular interest and importance is that, a new class of stable and efficient white photoluminescent silicate materials has been obtained from an alkoxysilane and a carboxylic acid through the sol–gel route w12,13x. Another related report is the sol–gel derived undoped urea cross-linked organic– inorganic xerogels which display strong blue-light emission w14x. In short, the amount of work focused on the luminescence phenomena in the undoped silica gel itself is limited, and the luminescence mechanism is not very clear yet. Thus, in this paper we will report a more detailed study on the photoluminescence of sol–gel derived silica xerogels and organic–inorganic hybrid silica xerogels. First, vari-
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 1 4 7 - 6
J. Lin, K. Baernerr Materials Letters 46 (2000) 86–92
87
ous factors in the sol–gel process, such as solvents, pH value, water content are selected to investigate their influence on the luminescence properties of the resultant silica xerogel, and then organic group –CH 3 is introduced in the silica xerogel by using CH 3 SiŽOC 2 H 5 . 3 as one of the precursors; temperature effect on the luminescence properties of the hybrid silica xerogel has also been studied.
kV, 25 mA.. All measurements were performed at room temperature.
2. Experimental
The blue emission in silica xerogel has been reported by Garcia et al. w10x. Here we focus on the dependence of the photoluminescence properties of
All chemicals employed in the present work were purchased from Aldrich and used as received. Pure silica xerogels were prepared by hydrolysis and condensation of tetraethoxysilane SiŽOC 2 H 5 .4 ŽTEOS. under different conditions. In studying the solvent effects, 10 ml TEOS was mixed with 4 ml H 2 O and 5 ml solvent wethanol EtOH, or N, N-dimethylformamide ŽDMF., or dimethylsulfoxide ŽDMSO.x, under magnetic stirring, respectively. The pH values of these three solutions were kept at 2 by adding concentrated hydrochloric acid ŽHCl. by drops. Gelation took place in 10 days. The resulting silica xerogels are denoted as SXŽEtOH., SXŽDMF. and SXŽDMSO., respectively. Silica xerogels were also obtained under different pH values and water contents ŽH 2 OrTEOS mole ratio.. In order to decrease the experimental errors, all other parameters were kept constant to the greatest extent when adjusting the pH or changing the H 2 OrTEOS mole ratios. Pure silica xerogels were obtained by drying the wet silica gel at 1208C for 3 days. Organic–inorganic hybrid silica xerogels were prepared by the simultaneous hydrolysis and condensation of TEOS and methyltriethoxysilane CH 3 SiŽOC 2 H 5 . 3 ŽMTES . Žmolar ratio TEOSrMTESs 2:1. using DMF as solvent under acid condition ŽpH s 2.. The resultant hybrid xerogels, labeled as HSXŽTM., were heated at 1208C, 2008C, 4008C and 6008C for 3 days, respectively. Photoluminescence excitation and emission spectra were recorded with a Hitachi F-3010 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. FT-IR spectra were measured using an IFS-66V FT-IR spectrophotometer ŽBruker.. X-ray powder diffraction ŽXRD. were checked using a D-5000 X-ray diffractometer ŽSiemens. with CuK a Ž l s 0.15405 nm. radiation Ž40
3. Results 3.1. Dependence of the photoluminescence properties of the silica xerogel on the experimental conditions
Fig. 1. Excitation Ža. and emission Žb. spectra of SXŽEtOH. Ž1., SXŽDMF. Ž2. and SXŽDMSO. Ž3. after heat treatment at 1208C.
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silica xerogels on the solvents, water content and pH value in the starting reaction solutions. The silica xerogels derived by using different solvents clearly show distinctive luminescence properties. Fig. 1 gives the excitation and emission spectra of SXŽEtOH.Ž1., SXŽDMF.Ž2. and SXŽDMSO.Ž3., respectively. For SXŽEtOH., its excitation spectrum consists of a main broad band peaking at 363 nm with a shoulder at 305 nm and two weak peaks at 236 and 243 nm, and the corresponding emission spectrum contains a broad band from 360 to 600 nm, with a maximum value at 439 nm. The Stokes shift amounts to 4770 cmy1 . The excitation and emission spectra of SXŽDMF. look similar to those of SXŽEtOH., but their maxima are located at 359 and 428 nm with a Stokes shift of 4500 cmy1 , respectively. Great difference has been observed for the case of SXŽDMSO., whose excitation and emission maxima shift to 402 and 464 nm Ža shoulder at 483 nm., respectively. The Stokes shift for the luminescence of SXŽDMSO. is 3320 cmy1 . The emission intensity of the peak at 430 nm of SXŽDMF. was studied as a function of H 2 OrTEOS mole ratio Ž R . and pH value of the starting reaction solution. The results are shown in Figs. 2 and 3, respectively. From Fig. 2 it can be seen that both lower or higher R values are not good for improving the blue emission intensity, and the maximum emission intensity is obtained when R amounts to 8. Also the emission intensity increases with increasing the pH value until pH s 8, and then decreases with further increase of the pH value, as indicated in Fig. 3.
Fig. 2. Emission intensity of SXŽDMF. as a function of H 2 OrTEOS mole ratio of the starting reaction solution.
Fig. 3. Emission intensity of SXŽDMF. as a function pH of the starting reaction solution.
3.2. Luminesence properties of hybrid silica xerogels Similar to the pure silica xerogels, the HSXŽTM. also show luminescence in the blue region under UV excitation. However, their luminescence behavior becomes more complicated, which has been investigated as a function of heat treatment temperatures between 1208C and 6008C. Fig. 4 shows the excitation and emission spectra of HSXŽTM. after heat treatment at 1208C. These spectra look like those in Fig. 1Ž1,2. for pure silica xerogels which are also treated at 1208C, except that the excitation bands at 242 and 300 nm become stronger relative to the main band at 360 nm and well resolved. The shape of the emission spectra obtained by exciting into 300 and 360 nm are the same, both of which have a peak at 438 nm with a long tail extended to 600 nm. This indicates that these two excitation bands originate
Fig. 4. Excitation Ža. and emission Žb. spectra of HSXŽTM. after heat treatment at 1208C.
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Fig. 7. Emission spectra of 4008C treated HSXŽTM. under different excitation. Fig. 5. Comparison of the emission spectra of HSXŽTM. under 360 nm excitation after heat treatment at different temperatures.
from the same emission center. However, the emission spectrum excited by short UV 242 nm clearly shows two well resolved peaks at 438 and 456 nm as well as a weak peak at a longer wavelength of 568 nm, suggesting that the 242 nm excitation band originates from another kind of emission center. This means there exist at least two distinctive emission centers in the 1208C treated HSXŽTM.. Fig. 5 shows a comparison of the emission spectra of HSXŽTM. samples treated from 1208C to 6008C Žexcited by 360 nm.. From this figure, it is clearly seen that with increasing heat treatment temperature the emission intensity decreases and the maximum peak value shifts to longer wavelength. The emission is completely quenched after 6008C treatment. A more detailed study on the luminescence properties of the 2008C and 4008C treated HSXŽTM.
Fig. 6. Emission spectra of 2008C treated HSXŽTM. under different excitation.
samples reveals that their emission centers have completely changed compared with those of the 1208C treated sample. Fig. 6 presents the emission spectra of 2008C treated HSXŽTM. sample excited by different wavelengths. Obviously, the photoluminescence spectra are strongly dependent on the excitation energy, i.e., the emission shifts to high energy with increasing excitation energy Žexcitation at 428, 390 and 361 nm yields emission in 503, 468 and 446 nm, respectively.. This indicates that new luminescent centers have formed after the 2008C treatment. The emission centers have changed again after 4008C treatment. For example, excitation into 361 and 404 nm yields broad band emission at 463 and 491 nm, respectively, both of which have a shoulder at 478 nm, as shown in Fig. 7. In summary, with increasing heat treatment temperature for the HSXŽTM., new emission centers with lower emission energy and weaker emission intensity have formed.
Fig. 8. XRD patterns of HSXŽTM. after heat treatment at different temperatures.
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charge transfer mechanism w10x and carbon impurity mechanism w12x, have been proposed to explain the luminescent phenomena in the sol–gel derived silica gels. In the preparation of SiO 2 gel by sol–gel processing, a glass network is formed by the reaction of hydrolysis and condensation of alkoxysilane precursors. The reactions remove the organic groups by replacing them with hydroxyl groups. Thermal treatments result in the formation of various defect centers, such as: Ž1. Fig. 9. FT-IR spectra of HSXŽTM. after heat treatment at different temperatures.
Ž2. XRD and FT-IR spectra were used to investigate the structure evolution of the HSXŽTM. samples during the heat treatment process, as shown in Figs. 8 and 9, respectively. Not much information can be obtained from the XRD, i.e. only a broad band at 2 u s 238 presents independence of the treatment temperatures, indicating that the xerogels remain amorphous in the whole heat-treatment process. Obvious changes are observed in the FT-IR spectra of HSXŽTM.. For the HSXŽTM. sample after heat treatment at 1208C, the FT-IR spectrum mainly consists of absorption peaks ranging from 400 to 1500 cmy1 , which are located at 440 cmy1 Ž d , Si`O`Si ., 798 cmy1 Ž ns , Si`O`Si., 836 cmy1 Ž d , SiCH 3 ., 940 cmy1 Ž n , Si`OH., 1057 cmy1 , 1155 cmy1 Ž nas , Si`O`Si. and 1279 cmy1 Ž n , SiCH 3 ., respectively w15x. With increasing heat treatment temperatures, the absorption peaks Ž440, 798, 1057 and 1155 cmy1 . of Si`O`Si bonds become stronger and shift to higher wavenumbers, and those of SiCH 3 Ž831, 1279 cmy1 . become weaker and disappear completely after the 6008C treatment. This is because of the pyrolysis of SiCH 3 groups at higher temperature in the HSXŽTM. sample.
4. Discussion The origin of the luminescence in sol–gel derived silica gels has not been well understood yet. Three possible mechanisms, i.e., defect mechanism w9x,
Ž3. where the arrow x denotes bond cleavage sites w9x. The localization of an electron in the dangling sp 3 silicon in SiO 3 units or the localization of an electron-hole in a 2 p orbital of the single bonded oxygen forms the defects. A strong electron–photon coupling causes the luminescence. This is the socalled defect mechanism. For the charge transfer mechanism, it is believed that the emission from silica gel is related to a charge transfer process between silicon and the oxygen atoms in the SiO 2 gel network w10x. The carbon impurity mechanism has been used to explain the luminescence phenomena that occurred in the system of silane coupling agents acting with the organic acid, where the emission is considered to be from the introduction of carbon impurities in the `O`Si`O` network by forming `O`C`O` or `Si`C` bonds w12x. It has been found that the structure of the sol–gel derived oxide networks are strongly affected by the concentration of the interacting species, their ratios, the reaction mediumŽsolvent., the pH, the catalyst and the temperature during condensation w16x. For example, reaction medium Žsolvent. will influence the final oxide contents contents, higher reactants concentration will result in a higher number of bridg-
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ing oxygens Ži.e., higher degree of polymerization., and pH will influence the structure of oxide network by influencing the reaction rate w16x. Thus, our experimental results in Section 3.1 can be generally understood. The luminescence properties Žpeak position and emission intensity. of the silica xerogels depend strongly on the solvents, water contents and pH values of the starting reaction solutions used. This is because the structure of the silica xerogel network depends on these factors. According to the defect mechanism, different network structure will produce different defects during the drying process, and further different luminescent centers and luminescence properties in the silica xerogels. It is well known that the Stokes shift, defined as the energetic difference between the maximum of the excitation spectrum and related maximum of the emission spectrum, is strongly connected with the electron–phonon coupling w14x. The Stokes shift for the luminesence of the silica xerogel is small Ž3000– 4800 cmy1 . and there is a strong overlap between the excitation and emission spectra. This suggests that there is a strong self-absorption for the luminescence of silica xerogel, i.e. energy transfer takes place from one emission center to another. At the present stage, it is difficult to explain our experimental results in more detail. Here we can say that the silica xerogels are potentially a kind of tunable phosphors whose luminescent properties can be adjusted by selecting different solvents as the reaction medium, water content and pH value of the starting reaction solution. The silica gel network has been modified by using CH 3 SiŽOC 2 H 5 . 3 as one of the reactants Žanother component is TEOS., and related reactions can be expressed as follows: CH 3 Si Ž OC 2 H 5 . 3 q H 2 O ™ CH 3 Si Ž OC 2 H 5 . 2 Ž OH . q C 2 H 5 OH
Ž 4.
Si Ž OC 2 H 5 . 4 q H 2 O ™ Si Ž OC 2 H 5 . 3 Ž OH . q C 2 H 5 OH
Ž 5.
2CH 3 Si Ž OC 2 H 5 . 2 Ž OH .
™ H 3 C Ž H 5 C 2 O. 2 Si`O`Si Ž OC 2 H 5 . 2 CH 3 q H 2O
Ž 6.
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2Si Ž OC 2 H 5 . 3 Ž OH .
™ Ž H 5 C 2 O. 3 Si`O`Si Ž OC 2 H 5 . 3 q H 2 O
Ž 7.
CH 3 Si Ž OC 2 H 5 . 2 Ž OH . q Si Ž OC 2 H 5 . 3 Ž OH .
™ H 3 C Ž H 5 C 2 O. 2 Si`O`Si Ž OC 2 H 5 . 3 q H 2 O Ž 8. The hybrid silica xerogel network has been formed by the continuous occurrence of the above hydrolysis ŽEqs. Ž4. and Ž5.. and condensation ŽEqs. Ž6. – Ž8.. reactions. The final structure can be identified by the FT-IR spectra, where the absorption peaks belonging to Si`O`Si, Si`OH and Si`CH 3 are present ŽFig. 9.. Due to the introduction of –CH 3 group in the silica xerogel network, its luminescence properties have been modified. In Fig. 4, it can be found that the intensity ratio of the excitation band 242r360 nm Ž r s 0.77. has been greatly enhanced and the emission band shifts to a longer wavelength Ž l max s 438 nm., as compared with those in Fig. 2 for the pure silica gel Ž r s 0.43, l max s 430 nm.. According to the emission spectra in Fig. 4, it can be assumed that the 242 and the 360 nm bands are originated from two different emission centers. It seems that the introduction of –CH 3 group in the silica gel network is more favorable for the formation of emission center with excitation band at 242 nm. Carbon Ž –CH 3 . indeed plays some role in the luminescence of HSXŽTM., as the above mentioned carbon impurity mechanism indicated w12x. Heat treatment would change the structure of the HSXŽTM.; especially, the Si–CH 3 group would decompose to a large degree. This can be seen clearly from the FT-IR spectra ŽFig. 9.. As a result, the luminescence takes on a red shift and decreases in intensity ŽFig. 5. under 360 nm UV excitation. After heat treatment at 6008C, the luminescence disappeared while the Si–CH 3 group decomposes completely. We cannot say that the quenching of the luminescence is merely because of the decomposition of Si–CH 3 groups, but they must have some relationship. Furthermore, heat treatment Ž2008C and 4008C. also cause the formation of new luminescent centers with longer excitation and emission wavelengths, as shown in Figs. 6 and 7, which are completely different with those in the 1208C treated sample. This is also due to the structure change of
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the Si`O networks on heating. In summary, we can say that the luminescence properties of silica xerogel can also be adjusted by introducing organic components and heat treatment.
5. Conclusions From this study, it can be concluded that the sol–gel derived silica xerogel itself is a kind of interesting photoluminescent material whose luminescence properties can be adjusted by changing the reaction medium, water content, pH of the starting reaction solution, as well as the introduction of organic groups in the gel network and subsequent heat treatment. It is believed that there is still a lot of work to be done to clear up the luminescence phenomena in silica gels.
Acknowledgements This project was supported by the Bairenjihua of Chinese Academy of Sciences.
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