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Preparation and characterization of Al and B co-doped cerium containing sol–gel derived silica glasses
January 2000
Materials Letters 42 Ž2000. 200–206 www.elsevier.comrlocatermatlet
Preparation and characterization of Al and B co-doped cerium containing sol–gel derived silica glasses A. Patra ) , G. De, D. Kundu, D. Ganguli Sol–Gel DiÕision, Central Glass and Ceramic Research Institute, JadaÕpur, Calcutta 700 032, India Received 12 April 1999; accepted 1 August 1999
Abstract Glasses containing Ce were prepared by the sol–gel method at 10008C. Two parallel series of experiments, one in oxygen and other in nitrogen atmosphere, were carried out for synthesis, where parallel series of glasses were prepared with addition of boron and aluminium. The densification behaviour of the gels has been studied by physical property measurements, as also UV–Visible–NIR and FTIR spectroscopy. The shifting of the absorption cut-off of the glasses towards shorter wavelengths with change in the atmosphere from oxygen to nitrogen and also with addition of boron or aluminium co-doping indicate that the concentration of Ce 4q ions is decreased. This is proposed to take place through electron trapping by Ce 4q Žleading to formation of Ce 3q . and hole trapping by non-bridging oxygens. q 2000 Elsevier Science B.V. All rights reserved. PACS: 82.70.Gg; 81.20.Pe; 76.30.Kg; 33.20.Lg; 33.20.Kf; 33.20.Ea Keywords: Sol–gel; Glasses; Cerium; UV–Visible–NIR spectroscopy
1. Introduction Glasses doped with different rare-earth ions are potential candidates for optical applications w1x. Cerium-doped glasses have many applications w1x; among those, one of the most important applications of these glasses is as cover glasses on solar cells in space satellites w2x. Such glasses should have good radiation stability, UV light cut-off and high visible transmission properties. These glasses Ž100-mm thick. should have 1% transmission ŽT. between 300 and 320 nm, 50% T at 345 nm and above 86% T in the visible range w2x. Cerium ions have two types of )
Corresponding author. E-mail address: [email protected] ŽA. Patra.
oxidation states, i.e., Ce 3q Žcerous. and Ce 4q Žceric. in oxide glasses. The proportion between the two oxidation states depends on both the basicity of the glass matrix and the preparation conditions w3x. The effect of co-dopants Že.g., Al and P. was found to influence the oxidation states and fluorescence properties w4x. The co-doping effect can be explained by changes in the coordination structures including coordination number, symmetry and basicity of oxygen ligand. Only a limited amount of work has been performed on the behaviour of cerium-doped silica glasses prepared by sol–gel processing w5–8x. In the present work, an investigation was made on sol– gel-derived Al and B co-doped cerium containing silica glasses for evaluation of the effect of the
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 9 9 . 0 0 1 8 4 - 6
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co-dopants and heating atmosphere ŽO 2 and N2 . on the oxidation states of cerium. The thermal densification process of the alkoxide-derived gel to glass was studied by UV–VIS–NIR and FTIR spectroscopies.
2. Experimental xCeO 2 –yM 2 O 3 – w100 y Ž x q y .xSiO 2 Ž x s 0.02– 1.0, y s 0.02–1.0 and M s Al and B. mol% sol compositions were prepared from tetraethyl orthosilicate ŽTEOS, purum grade, Fluka., ammonium ceric nitrate, ŽExcelar grade, Qualigens., aluminium nitrate ŽExtrapure, SD Chemicals., boric acid ŽAnalar grade, BDH, India. double-distilled, deionized water, HCl and NH 4 OH. Ammonium ceric nitrate and aluminium nitraterboric acid were dissolved separately in the required amounts of acidulated water. The solutions were added into TEOS under stirring. The molar ratio of H 2 O:TEOS:HCl was 14:1:0.01. The final pH of the sol was adjusted to 3.5. The sol Ž10–20 ml. was cast into Petri dishes for obtaining thin gels having thickness 0.4–0.6 mm after drying at 608C. The dried gels were heated up to 10008C under oxygen and nitrogen atmospheres with a soaking period of 0.5 h in each case. CS represents 0.02CeO 2 –99.98SiO 2 Žequivalent mol%.. CAS and CBS represent 0.02CeO 2 –0.02Al 2 O 3 –99.96SiO 2 and 0.02CeO 2 –0.02B 2 O 3 –99.96SiO 2 , respectively. The samples were cooled to room temperature and were characterized by UV–VIS–NIR spectroscopy ŽUV-3101 PC, Shimadzu. immediately. They were also characterised by FTIR spectroscopy ŽNicolet 5PC. and BET surface area analyser ŽAutosorb-1, Quanta Chrome.. All measurements were performed at room temperature.
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and nitrogen atmospheres were 23.8 and 53.9 m2rg, respectively. These results indicated that the CS and CBS gels were more densified under oxygen atmosphere compared to nitrogen atmosphere under the same heating schedule. However, CAS gels showed reverse results. All densified samples still contained open porosity even after heating at 10008C. 3.2. UV–VIS spectra 3.2.1. CE-doped SiO2 gel deriÕed glass The absorption spectra ŽFigs. 1 and 2. of CS gels heated at 10008C under oxygen and nitrogen atmospheres were made with 2.1 " 0.05-mm thick samples. The cut-off in the case of CS glasses heated under oxygen atmosphere was observed at 394.4 nm, which shifted to 376.4 nm for samples heated under nitrogen atmosphere. Further, the colour of the CS glasses changed with the atmosphere from oxygen Žyellow. to nitrogen Ždeep violet.. As a consequence, the samples treated in nitrogen showed a broad absorption near 515 nm ŽFig. 3.. To obtain the UV absorption bands of the above, thin gels samples Žthickness 0.50 " 0.05 mm. were used. A strong absorption band at 256 nm was observed in the case
3. Results 3.1. Surface area The surface areas of CS gels heated at 10008C under oxygen and nitrogen atmospheres were 43.6 and 72.0 m2rg, respectively; those of CAS gels heated at 10008C under oxygen and nitrogen atmospheres were 286.0 and 188 m2rg, respectively. The same of CBS gels heated at 10008C under oxygen
Fig. 1. UV–VIS spectra of CS Ž1.; CBS Ž2., and CAS Ž3. glasses heated at 10008C under oxygen atmosphere.
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Fig. 2. UV–VIS spectra of CS Ž1.; CBS Ž2. and CAS Ž3. glasses heated at 10008C under nitrogen atmosphere.
of CS glasses heated under oxygen atmosphere at 10008C ŽFig. 4., whereas an absorption band at 250 nm was observed in the case of samples heated under nitrogen. The half-width of this band was 4622
Fig. 4. Optical absorption spectrum of CS glass Žthicknesss 0.50 "0.05 mm. heated at 10008C under oxygen atmosphere.
and 5066 cmy1 under oxygen and nitrogen atmosphere heated samples, respectively.
Fig. 3. Optical absorption spectrum of CS glass heated at 10008C under nitrogen atmosphere.
3.2.2. CE– (Al and B)-co-doped SiO2 gel deriÕed glass The absorption spectra of CAS and CBS glasses Žthicknesss 2.1 " 0.05 mm. after densification at 10008C under oxygen and nitrogen atmospheres are shown in Figs. 1 and 2, respectively. The absorption cut-off of CAS glasses shifted from 302.8 to 285.2 nm when the glasses were prepared under oxygen and nitrogen atmospheres, respectively. The cut-off of CBS glasses densified under oxygen was observed at 302 nm, which shifted to 297.2 nm for samples heated under nitrogen atmosphere. The absorption cut-off of both the co-doped samples shifted to lower wavelength when compared with the same of CS glasses. A further shifting of absorption cut-off values towards lower wavelengths was also observed when the samples were annealed in nitrogen atmosphere instead of oxygen. Unlike the CS glasses, the CAS and CBS glasses were colourless after heating
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in all atmospheres. A strong absorption band peaked at 256 nm was observed in CAS samples Žthickness 0.5 " 0.05 mm. heated under different atmospheres. The half band widths are 4668 and 4874 cmy1 under oxygen and nitrogen atmosphere heated CAS samples. A strong absorption band peaked at 248 nm was observed in CBS glasses Žthickness 0.5 " 0.05 mm. heated under both oxygen and nitrogen atmospheres. The half band widths for this band are 5046 and 5382 cmy1 for oxygen and nitrogen atmospheres, respectively. 3.3. Infrared spectra 3.3.1. NIR spectra (900–2500 nm) NIR spectral studies of Ce-containing silica gels and Al and B co-doped samples Ždensified under oxygen and nitrogen atmospheres at 10008C. showed typical bands at 1368 and 2200 nm. The band at 1368 nm has been assigned to the first overtone of the fundamental O`H stretching of ‘‘free silanol group w8,9x. The band at 2200 nm was assigned to a combination of the fundamental O`H stretching of ‘‘free SiOH groups with the fundamental symmetric stretching mode of the silica network w8,9x. Figs. 5–7 show the NIR spectra of CS, CAS and CSB gels
Fig. 5. NIR spectra of CS glasses heated at 10008C under oxygen Ždashed line. and nitrogen Žsolid line. atmospheres.
Fig. 6. NIR spectra of CAS glasses heated at 10008C under oxygen Ždashed line. and nitrogen Žsolid line. atmospheres.
heated at 10008C under oxygen and nitrogen atmospheres, respectively. The intensity of the band at 1368 nm decreased with increase in the temperature of densification and a small shift of this band from
Fig. 7. NIR spectra of CBS glasses heated at 10008C under oxygen Ždashed line. and nitrogen Žsolid line. atmospheres.
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1368 nm Ž2008C. to 1364 nm Ž10008C. was observed in all cases, indicating the silanol group environment changed when the material achieved increased density w9x. A band at 2210 nm with a shoulder at 2240 nm was obtained in the 10008C heat-treated samples. The two Ž1368, 2200 nm. significant absorption bands were found to be unique stretching vibration of adjacent silanol groups and their overtones and combinations in alkoxide-derived silica gels. None of these peaks have been eliminated by heating; instead, they have only decreased in intensity with increasing temperatures. Clearly, the gels were not completely dehydrated. However, the intensities of these bands were lower under oxygen atmosphere heat-treated glasses compared to those glasses under nitrogen atmosphere, indicating the lower OH content. Another band observed near 1900 nm was connected with hydroxyl group, associated with physical pore water w9,10x. The band at 1900 nm ŽFigs. 5 and 7. disappeared for samples prepared under oxygen atmosphere. These results indicated that CS and CBS gels were more densified under oxygen atmosphere than under nitrogen atmosphere, other conditions remaining the same; this corresponds with the surface areas values. On the other hand, the CAS gels
Fig. 8. IR spectra of CAS glasses heated at 10008C under oxygen Žsolid line. and nitrogen Ždashed line. atmospheres.
Fig. 9. IR spectra of CBS glasses heated at 10008C under oxygen Žsolid line. and nitrogen Ždashed line. atmospheres.
showed similar behaviour in different atmospheres because of high surface areas in both cases. The NIR spectra revealed decrease in hydroxyl groups in the gel with increasing densification temperature, i.e., with decreasing surface areas. 3.3.2. IR spectra (2000–400 cm y 1) Detailed studies have been made on the evolution of IR spectra of these gels with increasing temperature of heat-treatment under different atmospheres up to gel–glass conversion. The results are summarized here. All 608C dried gels Žwith and without Al and B. showed a peak at around 1384 cmy1 which was assigned to NOy 3 ions. The absence of this band for all 2008C-heated gels indicated the removal of NOy 3 from the samples. A peak at 965 cmy1 due to the proton-bonded terminal non-bridging Si`Oy groups decreased in intensity with increasing treatment temperature up to 9508C. The absorption band at 1080 cmy1 for all 608C gels was assigned to Si`O`Si asymmetric stretching. This band shifted to a longer wavenumber, 1090 cmy1 for 9008C heated samples, suggesting a strengthening of the Si`O`Si bonding w5x. On further heat-treatment of all these gels at 10008C under different atmospheres, the typical band
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due to Si`OH Ž965 cmy1 . disappeared and the band for Si`O`Si stretching shifted to 1105 cmy1 , indicating complete densification of SiO 2 . 1.0 mol% of ŽAl 2 O 3 and B 2 O 3 . co-doped 1.0 mol% CeO 2 containing silica gels were used for IR study. In the case of CAS samples, an absorption band at 972 cmy1 due to Si`Oy Žnon-bridging oxygen. was observed after densified at 10008C under two different atmospheres ŽFig. 8.. This result suggested that aluminium acts as network modifier into silica network at 10008C. For B co-doped samples, two new bands at 1400 and 915 cmy1 obtained ŽFig. 9. after heattreatment at 10008C were due to B`O stretching and Si`O`B stretching; the latter confirmed that boron was incorporated into the silica network at 10008C w11x.
4. Discussion It is well-known that the absorption behaviours of Ce 4q- and Ce 3q-doped silica glasses were different, e.g., Ce 4q ions absorbed at 260 nm and Ce 3q ion at 320 nm w1x. The former absorption was attributed to charge transfer and the latter, to 4f–5d transition. Because of the high oscillator strength of the charge transfer band, the absorption of Ce 3q around 320 nm was obscured by the former band ŽFig. 4.. From our previous studies w6,7x, it has been shown that the intensity of the luminescence of glass activated by Ce in nitrogen atmosphere was much higher compared to oxygen atmosphere-heated samples. This also confirmed the generation of higher concentration of Ce 3q ions in nitrogen atmosphere-heated samples. The main emission peak centered around 450 nm due to Ce 3q on steady-state excitation, while additional peaks in the vicinity of the main peak were also observed w6,7x. The existence of several peaks can be attributed to non-equivalence of the Ce ions, i.e., coordination number changing with the surrounding ligands from one site to another. It was thus inferred that the cerium ion resided in different sites for different atmospheres of heating. The yellow coloration of the CS glasses Žin oxygen atmosphere. is known to be caused by the tail of the charge transfer band reaching into the visible range w3,12x. Another cause of this yellow colour may be explained to be due to the increase of coordination
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number of Ce 4q ion during heat-treatment in oxygen as pointed out by Blinkova et al. w12x. 2 Ce 3q On q O 2
™ 2 Ce
4q
Onq1
Ž 1.
The cerium ion resided in higher coordination numbers in oxygen atmosphere and in low coordination numbers in nitrogen atmosphere heat-treated samples and the proportion of ceric ions increased in the Ce-containing samples prepared in O 2 atmosphere. The shifting of the absorption cut-off for CS glasses towards shorter wavelengths with a change in the atmosphere from oxygen to nitrogen indicated that the concentration of ceric ions was decreased w13x. As already mentioned, a violet colour Žband at ; 515 nm. has been recorded for CS glasses prepared at 10008C under nitrogen atmosphere ŽFig. 3.. This may be due to trapping of holes on non-bridging oxygen atoms in the high temperature range; the trap may be deep enough so that absorption corresponding to promotion of the holes into the valence band can be observed in the visible range w14x. In sol–gel processing, the silica gel backbone is the O`Si`O network, which is susceptible to various defects w15,16x. Recently, the blue emission arising from Ce ŽIII. and that obtained from the silica gel which was connected with defects in the silica matrix have been distinguished by time-resolved spectroscopy w7x. The short decay time and high halfwidth of emission band are typical for the defect-related luminescence bands in the silica matrix, which are probably due to oxygen vacancies w7x. The Oy states Žor positive holes., also called oxygen-associated hole centers w17x, were one of the possible defects in silica gel matrix. They represented defect electrons in the O 2y matrix and a possible form in which it occurred is the self-trapped holes, Si`Oy`Si w17x. Therefore, it is possible that the ejected electron from the oxygen-associated hole centers react with Ce 4q encapsulated in sol–gel glasses to produce Ce 3q-hole complex. This reduction process, Ce ŽIV. q e Ce ŽIII. was possibly observed in the present case when the Ce-doped samples were heated under N2 atmosphere. For the first time, an absorption band at 515 nm is observed for Ce-doped silica xerogel. This was considered to originate from the ey–hq pairs. The shift of the
™
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absorption cut-off of CAS and CBS glasses to shorter wavelengths as compared to CS glasses indicated that the ratio of Ce 4qrCe 3q decreased in aluminium and boron co-doped samples. The increase in band width on heating under nitrogen atmosphere indicated an increase in the concentration of Ce ŽIII.. Lin et al. w13,18x showed that aluminium ions prefer mainly six wAlO6 x and four wAlO4 x coordination in Ce`Al co-doped glasses prepared either in oxygen or nitrogen atmosphere. The formation of four coordinated species, i.e., wAlO4 xy was favoured when concentrations of Ce and Al are very high. Therefore, in the present system where the concentrations were very low, Al should exist in six coordinated form wAlO6 x 3. Alternatively, as wAlO6 x 3y has more charge density than wAlO4 xy unit, the formation of wAlO6 x 3y is advantageous to balance the excess charge of cerium ions. Recently, Qiu et al. w19x showed the photoreduction of Sm3q to Sm2q, where the holes were trapped by non-bridging oxygen ions as well as by tetrahedral coordinated boron and aluminium atoms, while electrons were trapped by Sm3q ion. Considering the above mechanism, we may propose that in the case of CAS and CBS glasses, some of the electrons were trapped by the Ce 4q, which led to the formation of Ce 3q and holes were trapped by non-bridging oxygen ions and as a result, the absorption cut-off shifted towards shorter wavelengths.
5. Conclusions Small amounts of cerium in sol–gel-derived silica glasses showed UV cut-off property. The absorption cut-off wavelength shifted towards shorter wavelengths when Ce containing silica glasses were pre-
pared at 10008C under nitrogen atmosphere instead of oxygen atmosphere. The shifting of the absorption cut-off of CAS and CBS glasses towards shorter wavelengths as compared to CS glasses indicated that the ratio of Ce 4qrCe 3q decreased in aluminium and boron co-doped samples.
References w1x R. Reisfeld, C.K. Jorgensen, in: R. Reisfeld, C.K. Jorgensen ŽEds.., Laser and Excited States of Rare-Earths, SpringerVerlag, 1977. w2x K.M. Flyles, Glass Technology 32 Ž1991. 40. w3x G. Zhenan, J. Non-Cryst. Solids 52 Ž1982. 337. w4x Y. Ishii, K. Arai, H. Namikawa, M. Tanaka, A. Negishi, T. Handa, J. Am. Ceram. Soc. 70 Ž1987. 72. w5x A. Patra, D. Kundu, D. Ganguli, J. Sol-Gel Sci. Technol. 9 Ž1997. 65. w6x R. Reisfeld, H. Minti, A. Patra, D. Ganguli, M. Graft, Spectrochimica Acta, Part A 54 Ž1998. 2143. w7x R. Reisfeld, A. Patra, G. Panczer, M. Gaft, Optical Material, Žin press.. w8x A. Patra, D. Ganguli, Phys. Chem of Glasses Žaccepted.. w9x C.C. Perry, X. Li, J. Chem. Soc., Faraday Trans. 87 Ž1991. 761. w10x L.L. Hench, J.K. West, Chem. Rev. 90 Ž1990. 33. w11x M.A. Villegas, J.M.F Navarro, J. Mater. Sci. 23 Ž1988. 2464. w12x G.B. Blinkova, Sh.A. Vakhidov, A.Kh. Islamov, I. Nuritdinov, Kh.A. Khaidarova, Glass Phys. Chem. 20 Ž1994. 283. w13x S.-L. Lin, C.-S. Hwang, J. Ceram. Soc., Jpn. 103 Ž1995. 1209. w14x R.J. Araujo, N.F. Borrelli, SPIE, 1590 Submolecular Glass Chemistry and Physics Ž1991. 138. w15x J. Zink, B. Dunn, J. Ceram. Soc., Jpn. 99 Ž1991. 878. w16x A. Patra, D. Ganguli, J. Non-Cryst. Solids 144 Ž1992. 111. w17x A. Mohammed, T. Zaitoun, T. Kim, C.T. Lin, J. Phys. Chem. B 102 Ž1998. 1123. w18x S.-L. Lin, C.-S. Hwang, J.-F. Lee, J. Mater. Res. 11 Ž1996. 2641. w19x J. Qui, K. Miura, T. Suzuki, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 74 Ž1999. .
Materials Letters 42 Ž2000. 200–206 www.elsevier.comrlocatermatlet
Preparation and characterization of Al and B co-doped cerium containing sol–gel derived silica glasses A. Patra ) , G. De, D. Kundu, D. Ganguli Sol–Gel DiÕision, Central Glass and Ceramic Research Institute, JadaÕpur, Calcutta 700 032, India Received 12 April 1999; accepted 1 August 1999
Abstract Glasses containing Ce were prepared by the sol–gel method at 10008C. Two parallel series of experiments, one in oxygen and other in nitrogen atmosphere, were carried out for synthesis, where parallel series of glasses were prepared with addition of boron and aluminium. The densification behaviour of the gels has been studied by physical property measurements, as also UV–Visible–NIR and FTIR spectroscopy. The shifting of the absorption cut-off of the glasses towards shorter wavelengths with change in the atmosphere from oxygen to nitrogen and also with addition of boron or aluminium co-doping indicate that the concentration of Ce 4q ions is decreased. This is proposed to take place through electron trapping by Ce 4q Žleading to formation of Ce 3q . and hole trapping by non-bridging oxygens. q 2000 Elsevier Science B.V. All rights reserved. PACS: 82.70.Gg; 81.20.Pe; 76.30.Kg; 33.20.Lg; 33.20.Kf; 33.20.Ea Keywords: Sol–gel; Glasses; Cerium; UV–Visible–NIR spectroscopy
1. Introduction Glasses doped with different rare-earth ions are potential candidates for optical applications w1x. Cerium-doped glasses have many applications w1x; among those, one of the most important applications of these glasses is as cover glasses on solar cells in space satellites w2x. Such glasses should have good radiation stability, UV light cut-off and high visible transmission properties. These glasses Ž100-mm thick. should have 1% transmission ŽT. between 300 and 320 nm, 50% T at 345 nm and above 86% T in the visible range w2x. Cerium ions have two types of )
Corresponding author. E-mail address: [email protected] ŽA. Patra.
oxidation states, i.e., Ce 3q Žcerous. and Ce 4q Žceric. in oxide glasses. The proportion between the two oxidation states depends on both the basicity of the glass matrix and the preparation conditions w3x. The effect of co-dopants Že.g., Al and P. was found to influence the oxidation states and fluorescence properties w4x. The co-doping effect can be explained by changes in the coordination structures including coordination number, symmetry and basicity of oxygen ligand. Only a limited amount of work has been performed on the behaviour of cerium-doped silica glasses prepared by sol–gel processing w5–8x. In the present work, an investigation was made on sol– gel-derived Al and B co-doped cerium containing silica glasses for evaluation of the effect of the
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 9 9 . 0 0 1 8 4 - 6
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co-dopants and heating atmosphere ŽO 2 and N2 . on the oxidation states of cerium. The thermal densification process of the alkoxide-derived gel to glass was studied by UV–VIS–NIR and FTIR spectroscopies.
2. Experimental xCeO 2 –yM 2 O 3 – w100 y Ž x q y .xSiO 2 Ž x s 0.02– 1.0, y s 0.02–1.0 and M s Al and B. mol% sol compositions were prepared from tetraethyl orthosilicate ŽTEOS, purum grade, Fluka., ammonium ceric nitrate, ŽExcelar grade, Qualigens., aluminium nitrate ŽExtrapure, SD Chemicals., boric acid ŽAnalar grade, BDH, India. double-distilled, deionized water, HCl and NH 4 OH. Ammonium ceric nitrate and aluminium nitraterboric acid were dissolved separately in the required amounts of acidulated water. The solutions were added into TEOS under stirring. The molar ratio of H 2 O:TEOS:HCl was 14:1:0.01. The final pH of the sol was adjusted to 3.5. The sol Ž10–20 ml. was cast into Petri dishes for obtaining thin gels having thickness 0.4–0.6 mm after drying at 608C. The dried gels were heated up to 10008C under oxygen and nitrogen atmospheres with a soaking period of 0.5 h in each case. CS represents 0.02CeO 2 –99.98SiO 2 Žequivalent mol%.. CAS and CBS represent 0.02CeO 2 –0.02Al 2 O 3 –99.96SiO 2 and 0.02CeO 2 –0.02B 2 O 3 –99.96SiO 2 , respectively. The samples were cooled to room temperature and were characterized by UV–VIS–NIR spectroscopy ŽUV-3101 PC, Shimadzu. immediately. They were also characterised by FTIR spectroscopy ŽNicolet 5PC. and BET surface area analyser ŽAutosorb-1, Quanta Chrome.. All measurements were performed at room temperature.
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and nitrogen atmospheres were 23.8 and 53.9 m2rg, respectively. These results indicated that the CS and CBS gels were more densified under oxygen atmosphere compared to nitrogen atmosphere under the same heating schedule. However, CAS gels showed reverse results. All densified samples still contained open porosity even after heating at 10008C. 3.2. UV–VIS spectra 3.2.1. CE-doped SiO2 gel deriÕed glass The absorption spectra ŽFigs. 1 and 2. of CS gels heated at 10008C under oxygen and nitrogen atmospheres were made with 2.1 " 0.05-mm thick samples. The cut-off in the case of CS glasses heated under oxygen atmosphere was observed at 394.4 nm, which shifted to 376.4 nm for samples heated under nitrogen atmosphere. Further, the colour of the CS glasses changed with the atmosphere from oxygen Žyellow. to nitrogen Ždeep violet.. As a consequence, the samples treated in nitrogen showed a broad absorption near 515 nm ŽFig. 3.. To obtain the UV absorption bands of the above, thin gels samples Žthickness 0.50 " 0.05 mm. were used. A strong absorption band at 256 nm was observed in the case
3. Results 3.1. Surface area The surface areas of CS gels heated at 10008C under oxygen and nitrogen atmospheres were 43.6 and 72.0 m2rg, respectively; those of CAS gels heated at 10008C under oxygen and nitrogen atmospheres were 286.0 and 188 m2rg, respectively. The same of CBS gels heated at 10008C under oxygen
Fig. 1. UV–VIS spectra of CS Ž1.; CBS Ž2., and CAS Ž3. glasses heated at 10008C under oxygen atmosphere.
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Fig. 2. UV–VIS spectra of CS Ž1.; CBS Ž2. and CAS Ž3. glasses heated at 10008C under nitrogen atmosphere.
of CS glasses heated under oxygen atmosphere at 10008C ŽFig. 4., whereas an absorption band at 250 nm was observed in the case of samples heated under nitrogen. The half-width of this band was 4622
Fig. 4. Optical absorption spectrum of CS glass Žthicknesss 0.50 "0.05 mm. heated at 10008C under oxygen atmosphere.
and 5066 cmy1 under oxygen and nitrogen atmosphere heated samples, respectively.
Fig. 3. Optical absorption spectrum of CS glass heated at 10008C under nitrogen atmosphere.
3.2.2. CE– (Al and B)-co-doped SiO2 gel deriÕed glass The absorption spectra of CAS and CBS glasses Žthicknesss 2.1 " 0.05 mm. after densification at 10008C under oxygen and nitrogen atmospheres are shown in Figs. 1 and 2, respectively. The absorption cut-off of CAS glasses shifted from 302.8 to 285.2 nm when the glasses were prepared under oxygen and nitrogen atmospheres, respectively. The cut-off of CBS glasses densified under oxygen was observed at 302 nm, which shifted to 297.2 nm for samples heated under nitrogen atmosphere. The absorption cut-off of both the co-doped samples shifted to lower wavelength when compared with the same of CS glasses. A further shifting of absorption cut-off values towards lower wavelengths was also observed when the samples were annealed in nitrogen atmosphere instead of oxygen. Unlike the CS glasses, the CAS and CBS glasses were colourless after heating
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in all atmospheres. A strong absorption band peaked at 256 nm was observed in CAS samples Žthickness 0.5 " 0.05 mm. heated under different atmospheres. The half band widths are 4668 and 4874 cmy1 under oxygen and nitrogen atmosphere heated CAS samples. A strong absorption band peaked at 248 nm was observed in CBS glasses Žthickness 0.5 " 0.05 mm. heated under both oxygen and nitrogen atmospheres. The half band widths for this band are 5046 and 5382 cmy1 for oxygen and nitrogen atmospheres, respectively. 3.3. Infrared spectra 3.3.1. NIR spectra (900–2500 nm) NIR spectral studies of Ce-containing silica gels and Al and B co-doped samples Ždensified under oxygen and nitrogen atmospheres at 10008C. showed typical bands at 1368 and 2200 nm. The band at 1368 nm has been assigned to the first overtone of the fundamental O`H stretching of ‘‘free silanol group w8,9x. The band at 2200 nm was assigned to a combination of the fundamental O`H stretching of ‘‘free SiOH groups with the fundamental symmetric stretching mode of the silica network w8,9x. Figs. 5–7 show the NIR spectra of CS, CAS and CSB gels
Fig. 5. NIR spectra of CS glasses heated at 10008C under oxygen Ždashed line. and nitrogen Žsolid line. atmospheres.
Fig. 6. NIR spectra of CAS glasses heated at 10008C under oxygen Ždashed line. and nitrogen Žsolid line. atmospheres.
heated at 10008C under oxygen and nitrogen atmospheres, respectively. The intensity of the band at 1368 nm decreased with increase in the temperature of densification and a small shift of this band from
Fig. 7. NIR spectra of CBS glasses heated at 10008C under oxygen Ždashed line. and nitrogen Žsolid line. atmospheres.
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1368 nm Ž2008C. to 1364 nm Ž10008C. was observed in all cases, indicating the silanol group environment changed when the material achieved increased density w9x. A band at 2210 nm with a shoulder at 2240 nm was obtained in the 10008C heat-treated samples. The two Ž1368, 2200 nm. significant absorption bands were found to be unique stretching vibration of adjacent silanol groups and their overtones and combinations in alkoxide-derived silica gels. None of these peaks have been eliminated by heating; instead, they have only decreased in intensity with increasing temperatures. Clearly, the gels were not completely dehydrated. However, the intensities of these bands were lower under oxygen atmosphere heat-treated glasses compared to those glasses under nitrogen atmosphere, indicating the lower OH content. Another band observed near 1900 nm was connected with hydroxyl group, associated with physical pore water w9,10x. The band at 1900 nm ŽFigs. 5 and 7. disappeared for samples prepared under oxygen atmosphere. These results indicated that CS and CBS gels were more densified under oxygen atmosphere than under nitrogen atmosphere, other conditions remaining the same; this corresponds with the surface areas values. On the other hand, the CAS gels
Fig. 8. IR spectra of CAS glasses heated at 10008C under oxygen Žsolid line. and nitrogen Ždashed line. atmospheres.
Fig. 9. IR spectra of CBS glasses heated at 10008C under oxygen Žsolid line. and nitrogen Ždashed line. atmospheres.
showed similar behaviour in different atmospheres because of high surface areas in both cases. The NIR spectra revealed decrease in hydroxyl groups in the gel with increasing densification temperature, i.e., with decreasing surface areas. 3.3.2. IR spectra (2000–400 cm y 1) Detailed studies have been made on the evolution of IR spectra of these gels with increasing temperature of heat-treatment under different atmospheres up to gel–glass conversion. The results are summarized here. All 608C dried gels Žwith and without Al and B. showed a peak at around 1384 cmy1 which was assigned to NOy 3 ions. The absence of this band for all 2008C-heated gels indicated the removal of NOy 3 from the samples. A peak at 965 cmy1 due to the proton-bonded terminal non-bridging Si`Oy groups decreased in intensity with increasing treatment temperature up to 9508C. The absorption band at 1080 cmy1 for all 608C gels was assigned to Si`O`Si asymmetric stretching. This band shifted to a longer wavenumber, 1090 cmy1 for 9008C heated samples, suggesting a strengthening of the Si`O`Si bonding w5x. On further heat-treatment of all these gels at 10008C under different atmospheres, the typical band
A. Patra et al.r Materials Letters 42 (2000) 200–206
due to Si`OH Ž965 cmy1 . disappeared and the band for Si`O`Si stretching shifted to 1105 cmy1 , indicating complete densification of SiO 2 . 1.0 mol% of ŽAl 2 O 3 and B 2 O 3 . co-doped 1.0 mol% CeO 2 containing silica gels were used for IR study. In the case of CAS samples, an absorption band at 972 cmy1 due to Si`Oy Žnon-bridging oxygen. was observed after densified at 10008C under two different atmospheres ŽFig. 8.. This result suggested that aluminium acts as network modifier into silica network at 10008C. For B co-doped samples, two new bands at 1400 and 915 cmy1 obtained ŽFig. 9. after heattreatment at 10008C were due to B`O stretching and Si`O`B stretching; the latter confirmed that boron was incorporated into the silica network at 10008C w11x.
4. Discussion It is well-known that the absorption behaviours of Ce 4q- and Ce 3q-doped silica glasses were different, e.g., Ce 4q ions absorbed at 260 nm and Ce 3q ion at 320 nm w1x. The former absorption was attributed to charge transfer and the latter, to 4f–5d transition. Because of the high oscillator strength of the charge transfer band, the absorption of Ce 3q around 320 nm was obscured by the former band ŽFig. 4.. From our previous studies w6,7x, it has been shown that the intensity of the luminescence of glass activated by Ce in nitrogen atmosphere was much higher compared to oxygen atmosphere-heated samples. This also confirmed the generation of higher concentration of Ce 3q ions in nitrogen atmosphere-heated samples. The main emission peak centered around 450 nm due to Ce 3q on steady-state excitation, while additional peaks in the vicinity of the main peak were also observed w6,7x. The existence of several peaks can be attributed to non-equivalence of the Ce ions, i.e., coordination number changing with the surrounding ligands from one site to another. It was thus inferred that the cerium ion resided in different sites for different atmospheres of heating. The yellow coloration of the CS glasses Žin oxygen atmosphere. is known to be caused by the tail of the charge transfer band reaching into the visible range w3,12x. Another cause of this yellow colour may be explained to be due to the increase of coordination
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number of Ce 4q ion during heat-treatment in oxygen as pointed out by Blinkova et al. w12x. 2 Ce 3q On q O 2
™ 2 Ce
4q
Onq1
Ž 1.
The cerium ion resided in higher coordination numbers in oxygen atmosphere and in low coordination numbers in nitrogen atmosphere heat-treated samples and the proportion of ceric ions increased in the Ce-containing samples prepared in O 2 atmosphere. The shifting of the absorption cut-off for CS glasses towards shorter wavelengths with a change in the atmosphere from oxygen to nitrogen indicated that the concentration of ceric ions was decreased w13x. As already mentioned, a violet colour Žband at ; 515 nm. has been recorded for CS glasses prepared at 10008C under nitrogen atmosphere ŽFig. 3.. This may be due to trapping of holes on non-bridging oxygen atoms in the high temperature range; the trap may be deep enough so that absorption corresponding to promotion of the holes into the valence band can be observed in the visible range w14x. In sol–gel processing, the silica gel backbone is the O`Si`O network, which is susceptible to various defects w15,16x. Recently, the blue emission arising from Ce ŽIII. and that obtained from the silica gel which was connected with defects in the silica matrix have been distinguished by time-resolved spectroscopy w7x. The short decay time and high halfwidth of emission band are typical for the defect-related luminescence bands in the silica matrix, which are probably due to oxygen vacancies w7x. The Oy states Žor positive holes., also called oxygen-associated hole centers w17x, were one of the possible defects in silica gel matrix. They represented defect electrons in the O 2y matrix and a possible form in which it occurred is the self-trapped holes, Si`Oy`Si w17x. Therefore, it is possible that the ejected electron from the oxygen-associated hole centers react with Ce 4q encapsulated in sol–gel glasses to produce Ce 3q-hole complex. This reduction process, Ce ŽIV. q e Ce ŽIII. was possibly observed in the present case when the Ce-doped samples were heated under N2 atmosphere. For the first time, an absorption band at 515 nm is observed for Ce-doped silica xerogel. This was considered to originate from the ey–hq pairs. The shift of the
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absorption cut-off of CAS and CBS glasses to shorter wavelengths as compared to CS glasses indicated that the ratio of Ce 4qrCe 3q decreased in aluminium and boron co-doped samples. The increase in band width on heating under nitrogen atmosphere indicated an increase in the concentration of Ce ŽIII.. Lin et al. w13,18x showed that aluminium ions prefer mainly six wAlO6 x and four wAlO4 x coordination in Ce`Al co-doped glasses prepared either in oxygen or nitrogen atmosphere. The formation of four coordinated species, i.e., wAlO4 xy was favoured when concentrations of Ce and Al are very high. Therefore, in the present system where the concentrations were very low, Al should exist in six coordinated form wAlO6 x 3. Alternatively, as wAlO6 x 3y has more charge density than wAlO4 xy unit, the formation of wAlO6 x 3y is advantageous to balance the excess charge of cerium ions. Recently, Qiu et al. w19x showed the photoreduction of Sm3q to Sm2q, where the holes were trapped by non-bridging oxygen ions as well as by tetrahedral coordinated boron and aluminium atoms, while electrons were trapped by Sm3q ion. Considering the above mechanism, we may propose that in the case of CAS and CBS glasses, some of the electrons were trapped by the Ce 4q, which led to the formation of Ce 3q and holes were trapped by non-bridging oxygen ions and as a result, the absorption cut-off shifted towards shorter wavelengths.
5. Conclusions Small amounts of cerium in sol–gel-derived silica glasses showed UV cut-off property. The absorption cut-off wavelength shifted towards shorter wavelengths when Ce containing silica glasses were pre-
pared at 10008C under nitrogen atmosphere instead of oxygen atmosphere. The shifting of the absorption cut-off of CAS and CBS glasses towards shorter wavelengths as compared to CS glasses indicated that the ratio of Ce 4qrCe 3q decreased in aluminium and boron co-doped samples.
References w1x R. Reisfeld, C.K. Jorgensen, in: R. Reisfeld, C.K. Jorgensen ŽEds.., Laser and Excited States of Rare-Earths, SpringerVerlag, 1977. w2x K.M. Flyles, Glass Technology 32 Ž1991. 40. w3x G. Zhenan, J. Non-Cryst. Solids 52 Ž1982. 337. w4x Y. Ishii, K. Arai, H. Namikawa, M. Tanaka, A. Negishi, T. Handa, J. Am. Ceram. Soc. 70 Ž1987. 72. w5x A. Patra, D. Kundu, D. Ganguli, J. Sol-Gel Sci. Technol. 9 Ž1997. 65. w6x R. Reisfeld, H. Minti, A. Patra, D. Ganguli, M. Graft, Spectrochimica Acta, Part A 54 Ž1998. 2143. w7x R. Reisfeld, A. Patra, G. Panczer, M. Gaft, Optical Material, Žin press.. w8x A. Patra, D. Ganguli, Phys. Chem of Glasses Žaccepted.. w9x C.C. Perry, X. Li, J. Chem. Soc., Faraday Trans. 87 Ž1991. 761. w10x L.L. Hench, J.K. West, Chem. Rev. 90 Ž1990. 33. w11x M.A. Villegas, J.M.F Navarro, J. Mater. Sci. 23 Ž1988. 2464. w12x G.B. Blinkova, Sh.A. Vakhidov, A.Kh. Islamov, I. Nuritdinov, Kh.A. Khaidarova, Glass Phys. Chem. 20 Ž1994. 283. w13x S.-L. Lin, C.-S. Hwang, J. Ceram. Soc., Jpn. 103 Ž1995. 1209. w14x R.J. Araujo, N.F. Borrelli, SPIE, 1590 Submolecular Glass Chemistry and Physics Ž1991. 138. w15x J. Zink, B. Dunn, J. Ceram. Soc., Jpn. 99 Ž1991. 878. w16x A. Patra, D. Ganguli, J. Non-Cryst. Solids 144 Ž1992. 111. w17x A. Mohammed, T. Zaitoun, T. Kim, C.T. Lin, J. Phys. Chem. B 102 Ž1998. 1123. w18x S.-L. Lin, C.-S. Hwang, J.-F. Lee, J. Mater. Res. 11 Ž1996. 2641. w19x J. Qui, K. Miura, T. Suzuki, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 74 Ž1999. .