water interactions

Journal of Non-Crystalline Solids 55 (1983) 445-449 North-Holland Publishing Company

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R A M A N S T U D Y OF G L A S S / W A T E R I N T E R A C T I O N S Gregory J. E X A R H O S and William E. C O N A W A Y * Battelle Pacific Northwest Laboratory, Richland, Washington 99352, USA

Received 22 November 1982

Extensive investigations designed to understand mechanisms responsible for glass dissolution in aqueous media rely on quantitative solution analyses for leached species [1] in addition to substrate analyses using surface sensitive probes (XPS, SEM, SIMS) [2]. Microstructural and surface atom distribution information afforded by these techniques require that the substrate be transferred to a high vacuum environment for the analysis. Such conditions may lead to alteration of the surface Structure and bonding associated with the w a t e r / s u b s t r a t e interaction. In situ vibrational R a m a n measurements are reported for binary silicate glass fibers in contact with deionized water. Surface induced bonding changes resulting from ion exchange reactions with the glass and subsequent hydration of the structure were followed by means of the observed R a m a n spectra. Glass of composition N a 2 0 . 3 S i O 2 was obtained by melting frit, prepared from a soluble a m m o n i u m silicate (Ludox AS40) and sodium carbonate, in a Pt crucible for one hour at 1500°C. Thin fibers (25/~m diameter) were drawn from the melt and the glass stoichiometry was verified through its Raman spectrum [3]. Sectioned fibers 1 cm in length were mounted in Teflon holders and inserted into Spectrosil grade fluorescence cells filled with deionized water. R a m a n spectra of fibers in and out of leachant were excited in both 90 ° forward and 180 ° backscattering configurations using 150 mW of 514.5 nm excitation from an Ar + laser. Scattered light was analyzed using a Spex 1403 double monochromator equipped with photon counting electronics. Measurements were obtained daily for samples leached at 25°C and 90°C. R a m a n spectra of vitreous S i O 2 and a powdered form of silicic acid having the composition 0.8(H20 ) - SiO 2 were also measured. Observed changes in the R a m a n spectrum of N a 2 0 . 3 S i O 2 during leaching can be explained by ion exchange reactions between sodium cations localized in ionic glass sites and hydrogen ions from the solution. Total exchange of sodium cations for these thin fibers occurs for leaching times approaching one week; hydrogen ions become localized at non-bridging oxygen sites in the * NORCUS assignment from Case Western Reserve University, US Department of Energy contract DE-AM06-76-RLO-2225 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland

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G.J. Exarhos, W.E. Conaway / Glass~water interactions

Na20.3SiO2

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Fig. 1. Raman spectra obtained from samples measured in air in a 180° backscattering geometry.

glass. Owing to the small mass of glass fiber and large solution volume, no significant change in solution p H was recorded during leaching. Figure 1 depicts 180 ° backscattered R a m a n spectra measured in air of pristine N a 2 0 . 3 S i O 2 glass (a) and the same fiber following one week of 25°C aqueous leaching (b). Spectrum (a) is representative of the trisilicate composition [3] and exhibits two strong features at 1100 and 540 cm-~ assigned to symmetric stretching vibrations involving silicon/non-bridging oxygen, and

G.J. Exarhos, W.E. Conaway / Glass/water interactions

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silicon/bridging oxygen motions, respectively. Observed features for the leached glass spectrum may be interpreted in terms of measured spectra of vitreous silica and silicic acid. V'brational features exhibited by vitreous SiO 2 at 450, 800, 1065, and 1200 cm-~ can quantitatively be understood in terms of a continuous random network model [4] incorporating fourfold coordinated silicon and twofold coordinated oxygen. The remaining two sharp lines observed have previously been ascribed to localized "defect" modes in the glass represented by broken bonds or an alternate atom coordination number [5]. Galeener [6] has recently suggested that the 606 cm-~ feature could be explained by the existence of planar threefold rings in the structure. Assignment of the 495 cm-~ band is still under discussion. The Raman spectrum of a stable silicic acid approaching the metacomposition is depicted in fig. l(d). The relatively low non-bridging oxygen stretching frequency, 971 cm ~, is indicative of a significant number of non-bridging oxygen atoms in the structure [7]. Particle size effects lead to increased scattering at higher frequencies and therefore a rising baseline results. Incorporation of H + into glass fibers of composition N a 2 0 . 3 S i O 2 is manifested by dramatic changes in the Raman spectrum. The leached fiber spectrum (b) appears to be a linear combination of the vitreous SiO 2 (c) and silicic acid (d) spectra. However, features assigned to "defect modes" in vitreous SiO 2 are not apparent in the leached fiber spectrum. Either such localized features were not present in the starting material or bonding defects (broken bonds, unsatisfied coordination) are susceptible to attack by aqueous species. These ideas are consistent with results presented by Pantano and Kerey on alkali aluminosilicate glasses where glass durability decreases with increasing alkali (more non-bridging oxygens) in the structure [8]. Experiments on fibers leached at 90°C for two days gave identical results. Freshly drawn fibers react rapidly with water. This effect is manifested in the OH stretching region of the Raman spectrum by the appearance of four broad bands at 3250, 3425, 3600, and 3650 cm-1. Such features intensify and broaden during leaching into a diffuse band centered at around 3450 cm-1 which is comparable to the broad band exhibited by silicic acid. Attempts to measure OH Raman active vibrations of fibers in HzO are precluded by strong solvent Raman scattering in this region. In situ acquired Raman spectra during leaching of N a 2 0 - 3SiO 2 glass fibers are depicted in fig. 2. Vibrational bands decrease in intensity as a function of time and broaden to the low frequency side. This observation suggests formation of an increasing number of non-bridging oxygen sites caused by aqueous attack of the silicon-bridging oxygen network. The feature at 1640 cm -1 is intrinsic to water. As sodium cations are replaced in the structure, the features assigned to silicic acid and vitreous silica begin to appear. The very weak Raman spectrum (d) is comparable to the Raman spectrum of the fiber removed from solution (fig. lb). In this study, Raman spectroscopy has been successfully used as an in situ

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G.J. Exarhos, W.E. Conawa), / Glass/water interactions

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Fig. 2. In situ Raman spectra of Na20.3SiO2 fiber in water measured in a 90 ° forward scattering geometry.

structural probe to follow glass leaching in aqueous solutions. Leaching glass fibers in water does lead to formation of a surface graded refractive index which tends to minimize the amount of Raman scattered radiation thereby degrading the signal. Though not discussed here, removing the fiber from solution minimizes this loss of scattering power and reduces background noise without significantly altering bonding interactions within the leached layer.

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This work was supported by the US Department of Energy, Office of Basic Energy Sciences under contract DE-AC06-76RLO 1830. W.E. Conaway also acknowledges partial support from the Department of Energy under NORCUS contract DE-AM06-76-RLO-2225. We wish to thank Mr. G.D. Maupin for preparing the glass fibers.

References [1] [2] [3] [4] [5] [6] [7] [8]

J.C. Tait and C.D. Jensen, J. Non-Crystalline Solids 49 (1982) 363. C.Q. Buckwalter, L.R. Pederson and G.L. McVay, J. Non-Crystalline Solids 49 (1982) 397. S.A. Brauer and W.B. White, J. Non-Crystalline Solids 23 (1977) 261. P.N. Sen and M.F. Thorpe, Phys. Rev. B15 (1977) 4030. R.H. Stolen, J.T. Krouse and D.R. Kurkjian, Disc Faraday Soc. 50 (1970) 103. F.L. Galeener, J. Non-Crystalline Solids 49 (1982) 53. T. Furulsawa, K.E. Fox and W.B. White, J. Chem. Phys. 75 (1981) 3226. C.G. Pantano and T.A. Kerey, Paper 115-G-82 presented at 84th Annual American Ceramic Soc. Meeting, Cincinnati, Ohio, May 2-5, 1982.