Journal of Non-Crystalline Solids 270 (2000) 163±171
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Cation site occupation by Ag /Na ion-exchange in R2O±Al2O3±SiO2 glasses Tetsuji Yano *, Tomonori Nagano, Jaeho Lee, Shuichi Shibata, Masayuki Yamane Department of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan Received 26 July 1999; received in revised form 29 September 1999
Abstract Ag /Na ion-exchanged R2 O±Al2 O3 ±SiO2 glasses with uniform concentration pro®le of Ag and Na were prepared by heat treatment in molten silver salt followed by holding at the same temperature in an ambient atmosphere. Their glass transition temperature (Tg ) and thermal expansion coecient (TEC) were measured and structures were investigated using 29 Si-MAS NMR, 27 Al-MAS NMR, IR and Raman spectroscopies. Both Tg and TEC decreased with increase of the exchange ratio, but Tg was still above the ion-exchange temperature of 400°C even for the fully exchanged sample. The 29 Si- and 27 Al-MAS NMR spectra were mostly unchanged and no sign of the structural alteration of the glass network was observed. On the other hand, the vibrational spectra showed remarkable peak shifts depending on the exchange ratio. From these structural results, it was found that when the exchange ratio was low, the introduced Ag ions were stabilized at the non-bridging oxygen (NBO) site, and then Na ions in Alé4 site were exchanged by Ag ions after full replacement of NBO sites, where é represents the bridging oxygen. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The ion-exchange technique has been applied to develop special glasses with high functionality such as chemically tempered glasses [1], gradient-index glass rods [2] and planar waveguides [3] by replacing alkali ions with other monovalent cations. One of the advantages of the ion-exchange technique is the introduction of a large amount of special ions such as Cu and Ag with a certain chemical state [4], while, in an ordinary melt-
* Corresponding author. Tel.: +81-3 5734 2523; fax: +81-3 5734 2845. E-mail address:
[email protected] (T. Yano).
quenched glass, coexistence of Cu2 or Ag0 colloid formation is inevitable. On the other hand, it was recently found that some structural alterations are induced by Ag / Na or Cu /Na ion-exchanges in silicate glasses [4±7] and they are re¯ected in IR, 29 Si-MAS NMR, 27 Al-MAS NMR and O1s X-ray photoelectron spectra. These structural alterations eventually aect various glass properties such as glass transition temperature, thermal expansion coecient, electrical conductivity and crystallization temperature. In Ag /Na ion-exchanges of 34R2 O66SiO2 (mol%) glass, for example, both glass transition temperature and thermal expansion coecient decreased as the exchange proceeded [5]. The degree of these structural alterations and, hence, the changes in glass properties were
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considered to be dependent on the composition of initial glasses as well as the type and the concentration of cations to be introduced by the ion-exchange technique. For example, Araujo reported that color of Ag /Na ion-exchanged glasses in the system R2 O±Al2 O3 ±SiO2 becomes appreciable due to colloid formation as the amount of non-bridging oxygen increases [8]. In the glasses of the R2 O± Al2 O3 ±SiO2 system, monovalent cations occupy two chemical sites: the site near a non-bridging oxygen (NBO) and the one near a bridging oxygen which surrounds an Al3 ion in four fold coordination (Alé4 ) [9,10]. Araujo also suggested that the introduced Ag ions are subjected to the different in¯uences of local environment depending on the occupied site. One of the important properties of Ag and Cu ions is their high mobility in a glass, which is attributed to their unique electronic con®gurations [11]. The mobility of an ion also depends on the occupied site [12±14], thus the investigation of the preferential site for introduced Ag ions is important to develop new functional glasses. This paper reports the detailed structural investigation by NMR, IR and Raman spectroscopies of the chemical sites occupied by ionexchanged Ag ion in the glasses of 20R2 O± 10Al2 O3 ±70SiO2 and 25R2 O±25Al2 O3 ±50SiO2 (mol%, R Na, Ag or Na + Ag) systems.
2. Experimental 2.1. Glass preparation A 150 g batch of glass 20Na2 O±10Al2 O3 ± 70SiO2 (mol%) was prepared by melting reagent grade Na2 CO3 , Al2 O3 and high-purity silica sand. A glass of the composition 25Na2 O±25Al2 O3 ± 50SiO2 (mol%) was also prepared using reagent grade Na2 CO3 , ®ne Al2 O3 and SiO2 powders. The glass compositions were selected to choose the typical site ratio: in the former glass, a half of Na ion occupies NBO sites and the remaining half occupies Alé4 sites, while in the latter one, all Na ions occupy Alé4 sites.
Glasses, which were once melted in an electric furnace for 2 h, were crushed into cullets about 2 mm in size and re-melted to enhance homogenization. The melting temperature was 1600°C for the 20Na2 O±10Al2 O3 ±70SiO2 glass and 1650°C for the 25Na2 O±25Al2 O3 ±50SiO2 glass. After pouring onto a graphite plate, the glasses were annealed for 1 h at the temperatures 10°C above their glass transition temperatures. Then the stress-free glasses were cooled to room temperature at a rate of 1°C/min. 2.2. Ag /Na ion-exchange Ag /Na ion-exchange was carried out on the sample glasses of about 10 15 0:3±0:4 mm by soaking them in a fused salt bath of 48Ag2 SO4 ± 52(AgCl)2 (mol%). The ion-exchange temperature was chosen as 400°C to hinder possible crystallization due to the reduction of Tg [5±7]. Soaking time for the glass of the composition 20Na2 O±10Al2 O3 ±70SiO2 (mol%) (Glass A) was changed from 15 min to 13 h to prepare the samples with various Na and Ag contents. After soaking, the sample glass platelet was subjected to the heat treatment in an air at 400°C so that the total treatment time was 72 h to equalize the concentration of Ag ion throughout the sample. It should be noted that during this treatment the introduced Ag ions are allowed to settle in the stable sites. For the glass of 25Na2 O±25Al2 O3 ± 50SiO2 (mol%) (Glass B), soaking±time was only 13 h to obtain a fully exchanged sample. The concentration of Ag ion introduced by the ionexchange was determined by energy dispersive X-ray spectroscopy, EDX, on the scanning electron microscope. 2.3. NMR spectroscopy measurement 29 Si- and 27 Al-MAS NMR was carried out on the original glass and the ion-exchanged glass samples in the series of Glass A. Measurements were made on the pulverized samples contained in ZrO2 sample tube using high resolution solid state FT-NMR spectrometer JNM-GSX270 (JEOL) with multi-nuclear MAS probe. The experimental conditions for 29 Si-MAS NMR were:
T. Yano et al. / Journal of Non-Crystalline Solids 270 (2000) 163±171
resonance frequency, 53.43 MHz; delay time, 30 s; spinning rate, 3.9±4.0 kHz; accumulation times, 240±1000. Chemical shift was referenced to polydimethylsilane which has a shift of )33.8 ppm from tetramethylsilane (TMS). In 27 Al-MAS NMR, the conditions were: resonance frequency, 70.26 MHz; delay time, 10 s; spinning rate, 3.9± 4.0 kHz; accumulation times, 200. Al2 (SO4 )3 was utilized as the standard sample which has a shift of 0 ppm from Al(H2 O)3 6 . 2.4. Vibrational spectroscopy measurement FT Raman scattering spectroscopy measurement was made on all the original and the ionexchanged glasses. Surfaces of the platelet samples were optically polished. The measurement was carried out using a Raman spectrometer under the following conditions: wavelength of the excitation: CW-Nd:YAG laser (k 1064 nm); laser power: 1.5 W; resolution: 4 cmÿ1 ; accumulation times: 24,000±32,000; detection angle: back-scattering. The obtained spectra were corrected with respect to the base-line and reduced by B ose±Einstein correction given by Eq. (1): Icorr
m
m0 ÿ m
4
1 ÿ exp
hm ÿ kT
165
orange. X-ray diraction analysis was performed on these samples to check the state of silver in the glass. The results showed only a broad halo pattern and no signal due to silver metal was detected. Taking into account the employed scanning condition, the amount of Ag metal in glass was estimated to be less than 1 mol% and almost all the introduced Ag ions stayed as monovalent cation in glass. Fig. 1 plots the changes of the glass transition temperature, Tg , and thermal expansion coecient, TEC, of the samples of Glass A against the ion-exchange ratio x Ag=
Na Ag. Both Tg and TEC decreased with increasing exchange ratio x. Tg of the exchanged samples of Glass A was always above the ion-exchange temperature of 400°C. An in¯ection was observed in Tg in the vicinity of x 0:5, over which its slope became smaller. TEC of Glass B also decreased from 116 3 10ÿ7 Kÿ1
in x 0 to 102 3 10ÿ7 Kÿ1
in x 1:0. Tg of the sample with x 0 was measured as 758 5 C, but that of the sample with x 1:0 was not clearly observed.
Iobs ;
1
where Icorr and Iobs are the reduced and observed Raman intensity at Raman shift m from the excitation wavenumber m0 , respectively. The corrected Raman spectra were de-convoluted into several Gaussian-bands using Levenberg±Marquard least square ®tting method [15]. FTIR measurement was also made using a KBr pellet. The resolution was 4 cmÿ1 and the number of data accumulation was 40.
3. Results 3.1. Glass properties Most of the ion-exchanged glass samples obtained were colorless, but some of Glass A with high amount of Ag ion were tinged with yellow or
Fig. 1. Plots of the glass transition temperature Tg (s) and the thermal expansion coecient (h) of the samples of Glass A against the ion-exchange ratio x Ag=
Na Ag. The solid lines are drawn as guides for eye.
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3.2.
29
Si- and
27
Al-MAS NMR spectra
29
Si-MAS NMR spectra for the ion-exchanged samples of Glass A series are shown in Fig. 2. All the spectra consist of one broad band at around )93 ppm. This broad resonance band is in the range of the chemical shift of Q3 (0Al), Q4 (1Al) and Q4 (2Al) [11] as shown in the upper part of the ®gure. The peak position shifted to higher
frequency side and the full width at a half maximum (FWHM) increased with the progress of the ion-exchange, but they were quite small compared with the experimental resolution. In 27 Al-MAS NMR spectra (which were not shown here), only one peak was observed and their spectral shape did not change with the ion-exchange ratio. The peak position was about )80 ppm from Al(H2 O)3 6 , which is assigned to four-fold coordinated Al3 ion. 3.3. IR spectra IR spectra of the ion-exchanged samples of Glass A series are shown in Fig. 3. A large broad band was observed around 1100 cmÿ1 in all the samples, which is attributed to an overlap of the stretching vibrations of Si±é (bridging) and Si±Oÿ (non-bridging) bonds. Their band-shape did not change remarkably with the ion-exchange.
Fig. 2. 29 Si-MAS NMR spectra of the samples with various x of the system Glass A. Typical chemical shift ranges of Qm (nAl) are shown by bars on the top of the ®gure.
Fig. 3. Infrared absorption spectra of the samples of Glass A having various x values by KBr pellet method.
T. Yano et al. / Journal of Non-Crystalline Solids 270 (2000) 163±171
167
Fig. 4. Infrared absorption spectra of the samples of Glass B having x 0 and 1.0 by KBr pellet method.
Appreciable changes were found in the region of 600±800 cmÿ1 . With the progress of the ion-exchange, the broad band clearly split into two peaks, whose positions are represented by the thin lines. The band of the initial glass
x 0 consisted from the bands at 757 cmÿ1 and 720 cmÿ1 , which are assigned to the stretching vibrations of Si±O±Si and Si±O±Al, respectively. Si±O±Si vibration band slightly shifted toward higher frequency side about 4 cmÿ1 from x 0 to x 1, while the peak position of the vibration of Si±O±Al shifted largely toward lower wavenumber side at x > 0.5. Fig. 4 shows IR spectra of Glass B. A large broad band around 1100 cmÿ1 is assigned to the vibration of Si±é (bridging) and did not change before and after the ion-exchange. The 700 cmÿ1 band, which is assigned to the stretching vibration of Si±O±Al because Glass B has the composition of Al/Si 1, shifted toward the lower wavenumber side from 692 cmÿ1 to 667 cmÿ1 between before and after the ion-exchange. 3.4. Raman spectra Raman spectra for the ion-exchanged glasses are shown in Figs. 5 and 6 for the samples of Glass A and Glass B, respectively. The Raman band in the region of 900±1200 cmÿ1 is assigned to the asymmetric stretching vibration of SiO4 tetrahe-
Fig. 5. Raman spectra of the samples of Glass A having various x values. All the spectra were reduced using B ose±Einstein correction.
dra, which appears at dierent wavenumbers depending on the number and the type of oxygen constituting the tetrahedron: non-bridging oxygen, bridging oxygen bonding to another SiO4 tetrahedron and bridging oxygen bonding to an Al3 ion, etc [16±18]. The band in the region of 400±700 cmÿ1 is assigned to the bending or stretching vibration of Si±O±Si bonds constituting middlerange network structures like 3-membered ring [18±20]. Glass A (Fig. 5) shows that the bands in the region of 900±1200 cmÿ1 shifted largely about 100 cmÿ1 with the progress of Ag /Na ion-exchange.
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Fig. 6. Raman spectra of the samples of Glass B having x 0 and 1. The spectra were reduced using B ose±Einstein correction. Typical Raman shift of Q4 (nAl) for n 1±4, are presented by bars at the top of the ®gure from the reference [18].
A remarkable change in its shape occurred between x 0:26 and 0.66, above which the band remained unchanged by further exchange. In contrast, Glass B had an asymmetric band at around 1000 cmÿ1 , and the ion-exchange shifted the peak position slightly toward the lower frequency side. Consequently, the tail at higher frequency side became a small shoulder. The former can be attributed to the asymmetric stretching vibration of Q4 (2Al) and the latter to that of Q4 (1Al).
4. Discussion 4.1. Alteration of network structure Both 29 Si-MAS NMR and 27 Al-MAS NMR spectra remained mostly unchanged by Ag /Na ion-exchange. It suggests that the alteration of the network structure did not occur in the glasses of
the present study, although the local environment of Ag ion may be dierent depending on the glass composition. This is quite dierent from the previous results reported for Ag /Na ion-exchange in the R2 O±SiO2 system [5,6] or Cu /Na ion-exchange in the R2 O±Al2 O3 ±SiO2 system [4] where a remarkable change was observed in the spectra. In these cases, the progress of the ion-exchange treatment decreased Tg monotonously and, above a certain exchange ratio, Tg turned out to be below the ion-exchange temperature. On this subject, it is quite reasonable for the present case to consider that Tg which was always higher than the ion-exchange temperature did not alter the glass network structure. However, the simultaneous decrease of Tg and TEC were also observed in this system, and this is also an appropriate case to investigate the correlation between the changes in these properties and the site-occupation by Ag ions in the glasses. Concerning the in¯ection point of Tg at around x 0.5 in Glass A, the co-existence of Al2 O3 in the glass should be paid much attention to, because it oers an alternative site for Na or Ag ion by the formation of four-fold coordination in the glass with R/Al > 1. 4.2. Eects of Ag ions on the vibrational spectra As shown in Figs. 3 and 4, only the Si±O±Al bond was aected largely by Ag /Na ion-exchange in IR spectra. In the case of Glass A, a shift toward the lower wavenumber side was observed only for x > 0.5. According to the IR absorption measurements of R2 O±Al2 O3 ±SiO2 glasses with R=Al 1
R Li, Na and K) by Roy [17], the band due to Si±O±Al stretching vibration shifted toward the lower wavenumber side in order Li ® Na ® K, and it was suggested that the increase of masses or ionic radii of alkali ions nearby Alé4 site was one of the main factors to cause the peak shift. The observed shift of Si±O±Al bond in Glass A also suggests that the proportion of Ag ion occupying Alé4 site begins to increase at x > 0.5. On the other hand, Raman spectra of Glass A showed an opposite trend to IR shift. That is, a remarkable change of the spectral band shape was already observed in the samples with low x values
T. Yano et al. / Journal of Non-Crystalline Solids 270 (2000) 163±171
and completed at x < 0.66. Raman bands in the wavenumber region of 900±1200 cmÿ1 are assigned to the stretching vibration of SiO4 tetrahedral unit and known to give various band shapes and peak positions depending on the coordination state in silicates materials [21]. Although Raman bands of Glass A consisted of several overlapped bands of dierent SiO4 units, those of NBO-free Glass B are quite informative to deconvolute and understand the spectral changes of Glass A. In NBO-free Glass B, Q4 (2Al) units are dominant spices based on LoewensteinÕs rule [22] and a tail observed at the higher wavenumber side is a sign of the coexistence of Q4 (1Al) (see at the top of Fig. 6 [18]). Since the replacement of Na ions nearby Alé4 by Ag ions did not induce a spectral change in the Glass B so largely, the eect of the exchange of Alé4 site on Raman band can be approximately neglected. In other words, the changes of the spectra of Glass A can be attributed mainly to the band shifts of Qm units with NBO. From the above consideration, the de-convolution procedures of the Raman band of Glass A were carried out, where the following restrictions were employed; (1) ®ve kinds of units, Q4 (0Al), Q4 (1Al), Q4 (2Al), Q3 (0Al,R) and Q3 (1Al,R), are dominant, and (2) only Q3 (0Al,R) and Q3 (1Al,R) are sensitive to the ion-exchange to show the shifts of peak position and the changes of relative intensity. The reason of the restriction (1) is as follows: From the composition of Glass A given as R=Al=Si 4=2=7, a half of R is used for the charge compensation of Alé4 tetrahedral units, and the rest half of R is for the formation of NBO (Q3 ). Therefore, the glass network would be formed by 2Q3 + 2Alé4 + 5Q4 . Under the LoewensteinÕs rule [22], this fraction means that the amount of Alé4 units is more than that necessary to form Si±O±Al bond with Q3 and less than that with all Q3 and Q4 units. Thus, at least, 5 kinds of units of Q4 (0Al), Q4 (1Al), Q4 (2Al), Q3 (0Al,R) and Q3 (1Al,R) should be considered, where Q4 (2Al) was introduced from the point of view of the structural ¯uctuation identical to the glassy materials. The initial values of the peak position and FWHM for the de-convolution procedures were taken from the literature [18,23,24]. Unfortunately, the values for Q3 (1Al,R) is not available in the
169
literature. Thus the peak position at about 100 cmÿ1 lower than that of Q3 (0Al,R) was taken as Q3 (1Al,R) because one additional Si±O±Al bond (n ® n + 1) cause a shift about 75±150 cmÿ1 toward lower wavenumber side on the vibration of Q4 (nAl) [18].
Fig. 7. The results of the de-convolution procedures made for the Raman spectra of Glass A Band assignment; 1: Q4 (1Al), 2: Q4 (2Al), 3: Q3 (0Al.Na), 4: Q3 (1Al.Na), 5: Q3 (0Al.Ag) and 6: Q3 (1Al.Ag).
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Fig. 7 shows the band de-convolution of Glass A by the least square calculation with Gaussiantype band elements. The Raman band of the initial glass could be de-convoluted by 4 band elements, Q4 (1Al) (No. 1 in the ®gure), Q4 (2Al) (No. 2), Q3 (0Al,Na) (No. 3) and Q3 (1Al,Na) (No. 4). With increase of the ion-exchange ratio, a couple of bands, Q3 (0Al,Ag) (No. 5) and Q3 (1Al,Ag) (No. 6) appeared at the lower wavenumber side. The spectrum of the fully exchanged glass sample could be deconvoluted by the bands No. 1, 2, 5 and 6. As shown in this ®gure, the observed changes in the spectral shape were well reproduced by using 6 band elements, and the main factor to cause the changes of the spectral shape was the shift of the vibration due to Q3 units (No. 3, 4, 5 and 6) by the replacement of nearby Na ion with Ag . That is, the peaks of Q3 (0Al,R) shifted from 1120 to 1040 cmÿ1 and Q3 (1Al,R) shifted from 970 to 930 cmÿ1 by the replacement of Na ® Ag, and the bands 3 and 4 already disappeared in the spectrum of x 0:66. Takada et al. [5] and Yamane et al. [6] measured IR re¯ectance spectra of Ag /Na ionexchanged R2 O±2SiO2 glass (Q3 unit is dominant) and reported that Si±é stretching vibration shifted toward the lower wavenumber side while that of Si±Oÿ nb stretching vibration toward the higher wavenumber side. Taking these observations into account, it is quite reasonable to consider that Ag occupying NBO site aected SiO4 tetrahedra directly and caused the shift of Raman band of Q3 toward the lower wavenumber side. The result by Matson et al. [20] also supported this trend of Raman band, where 1100 cmÿ1 band shifted toward the lower wavenumber side in order of Cs ® Li (with the increase of covalency between R±Oÿ nb bond). 4.3. Preferential site-occupation of Ag ions After the ion-exchange in molten salt, the postheat treatment at the same temperature gave enough time to form a uniform concentration pro®le of Ag /Na ions in the samples. At the same time, this procedure also allowed the introduced Ag ions to ®nd and occupy the energetically stable sites in the glasses. As shown in the spectroscopic results, the structural rearrangement of the net-
work structural units did not occur during the series of the ion-exchange and post-heat treatment, and hence the number of the sites for the cations of Na and Ag are considered not to change. The vibrational spectroscopy data revealed that Ag ions occupied preferentially NBO sites up to x 0.5 rather than Alé4 sites. In the region (0 < x < 0.5), Tg also decreased more steeply than in the region of x > 0.5 (see Fig. 1). This indicates that the process of the occupation of NBO sites by Ag ions can not be understood only by the simple replacement of Na ions, but also by a certain interaction between Ag ion and the local glass network which forms NBO site. Generally, Ag2 O components are not easy to incorporate into silicate glass system by the melt-quench method, and silicate glasses containing a large concentration of Ag ions can be made only by the ion-exchange procedure. The reason still has not been clearly understood, but as suggested by Araujo [8], the coexistence of NBOs in glass or melt would aect the stability of the valence of Ag ions in the glass melts. As mentioned in Section 3.1, most of the silver successfully remained as ions in the glass samples because the ion-exchange might be done at a temperature low enough not to bring about changes of the valence of silver. However, as previously reported O1s XPS data on Ag /Na ionexchanged silicate glass system [7], NBOs were heavily aected by the introduced Ag ions to cause a large chemical shift toward higher binding energy side. Houde-Walter et al. [25] measured Ag K-edge EXAFS on Ag /Na ion-exchanged silicate and aluminosilicate glasses, and showed that Ag ion in NBO site has a coordination number (CN) 2.1 with a shorter Ag±O distance (rAg±O 2.08), while ones in Alé4 sites have a larger CN about 2.5 and a longer rAg±O 2.23. In addition, the results of Na K-edge EXAFS by McKeown et al. [26] revealed that Na ions take large CNs of 6.4±7.6 (rAg±O 2.60±2.61) at NBO site and 5.1±6.4 at Alé4 site (rAg±O 2.61±2.62) in silicate and aluminosilicate glasses, respectively. Taking these results into consideration, by the replacement of Na by Ag , a larger decrease of CN would be caused at NBO sites than at Alé4 site. This means that a large relaxation of local-structure would be induced at NBO site when Ag ion
T. Yano et al. / Journal of Non-Crystalline Solids 270 (2000) 163±171
is incorporated, causing the decrease of the jumping frequency between neighboring sites similar to the mixed alkali eect. This phenomenon by Ag ion would lead to trap or stabilize itself at NBO site. The characteristic feature of Ag ion qualitatively represented by a large PaulingÕs electronegativity of about 1.9 [27] should be one of the factors to explain it, that is, more covalent bonds with surrounding oxygens might be formed than the case of Na ions. In order to depict these phenomena including the decrease of Tg and TEC more precisely, characteristics of Ag and Na ions in the glass should be investigated from other aspects, like their dynamic motions and relaxation against an external electric ®eld.
5. Conclusions Ag /Na ion-exchanged R2 O±Al2 O3 ±SiO2 glasses were prepared using two kinds of original glasses: 20Na2 O±10Al2 O3 ±70SiO2 (mol%) containing two types of alkali sites of non-bridging oxygen and Alé4 ; and 25Na2 O±25Al2 O3 ±50SiO2 (mol%) containing only Alé4 sites. The glass transition temperature and thermal expansion coecient decreased with the introduction of Ag ions. No structural alteration of the glass network was found in 29 Si- and 27 Al-MAS NMR spectra. IR and Raman spectra showed unique shifts of bands relating to non-bridging oxygen and Alé4 units. The introduced Ag ions are stabilized preferentially at non-bridging oxygen sites, and then begin to occupy Alé4 sites.
Acknowledgements The authors thank Professor Takashi Inoue and Dr Hiromi Saito of Tokyo Institute of Technology for performing the measurement of FT-Raman spectra.
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