Hydroxyl groups in high-purity silica glass

Transcription

1 Journal of Non-Crystalline Solids 261 (2000) 186±194 Hydroxyl groups in high-purity silica glass V.G. Plotnichenko *, V.O. Sokolov, E.M. Dianov Fibre Optics Research Center, General Physics Institute, Russian Academy of Sciences, 38 Vavilov Street, Moscow, Russian Federation Received 01 June 1999; received in revised form 31 August 1999 Abstract IR absorption spectra of high-purity silica glass (SiO 2 ) of di erent types containing from 0.1 to 10 3 ppm of hydroxyl (OH) groups were studied and quantum-chemical calculations of di erent OH structures in the silica glass network were performed. On their basis a new approach to the problem of OH groups embedded in the glass network was proposed. It was shown that OH groups in high-purity SiO 2 occur mainly singly rather than as paired groups. Some of them are linked by hydrogen bonds to bridging oxygen atoms and not to each other, as is considered now. A new decomposition of the fundamental OH-stretching vibrational band into components was proposed. Ó 2000 Published by Elsevier Science B.V. All rights reserved. PACS: Jv; Bh; Ly; Nr 1. Introduction The hydroxyl (OH) group is one of the major impurities in silica glass, increasing its optical losses in the middle and near infrared. The vibrational absorption bands of OH groups in silanol centres (Si±OH) at the wavelengths of 2.72, 1.39, 0.9 lm etc. restrict the area of possible applications of glasses in various elds of science and technology, especially in bre and laser optics. Purity of glasses and the quality of the manufacturing process of glasses and glassy bres are usually veri ed by measuring the intensity of the OH absorption band. For di erent types of highpurity silica glass is the OH content ranges from less than 0.01 ppm to more than 1000 ppm. * Corresponding author. Tel.: ; fax: address: victor@fo.gpi.ac.ru (V.G. Plotnichenko). The mechanism of interaction of OH groups with the glass network is of considerable interest both from a scienti c point of view and in relation to the problem of optical bre environmental resistance. As shown in Refs. [1±5], the shapes of both the fundamental and overtone OH vibrational absorption bands are complicated owing to OH groups embedded in the glass network in di erent structures. The aim of this work was to understand the most probable structure for the OH groups embedded in the silica glass network and their manifestations in the IR absorption spectra. Fig. 1 shows the fundamental absorption band of the O±H stretching vibration with a maximum at about 3672 cm 1 (2.72 lm) and the commonly accepted scheme of OH groups arrangement in silica glass [1±5]. This band is believed to consist of four Gaussian components, m 1 ˆ cm 1, m 2 ˆ cm 1, m 3 ˆ cm 1 and m 4 ˆ cm 1. The m 1 component is assigned to `free' or `gas-like' OH groups, /00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S ( 9 9 )

2 V.G. Plotnichenko et al. / Journal of Non-Crystalline Solids 261 (2000) 186± Fig. 1. OH-stretching vibration band for silica glass (thick solid line) and its decomposition (thick dash line) into components (thin solid lines) used in Refs. [1±5]. the m 4 component is thought to be a vibration of the OH group hydrogen-bonded to another one in a linear con guration and the m 2 and m 3 components are ascribed to two neighbouring OH groups hydrogen-bonded to each other in cyclic con guration (Fig. 1). In the rst overtone region the frequencies corresponding to these components were found to be 7260, 7220 and 7100 cm 1 (no absorption corresponding to m 4 was observed) [3] or 7350, 7250, 7204 and 7070 cm 1 [4]. However, identi cation of the overtone components in Ref. [4] causes doubts: in our opinion, the 7350 cm 1 component should be attributed to a combined vibration of OH groups and the glass network, as was done in Ref. [3]. Similar results (with slightly di erent interpretation) were obtained in Ref. [5] for the rst three components of the fundamental vibration. However the 3510 cm 1 band occurred only after c irradiation and was consequently assigned to other structures with hydrogen-bonded OH groups. In addition, the following temperature dependence of the rst-overtone components was found in Ref. [4]: the 7350 cm 1 component practically did not change, the 7250 cm 1 one decreased (relative change K 1 ) and the 7204 cm 1 one increased ( K 1 ) with a temperature increase from room to 600 C. In contrast, in Ref. [5], the corresponding fundamental-band components were found to change as follows: the 3690 cm 1 component decreased ( K 1 ), the 3665 cm 1 one practically did not change and the 3620 cm 1 one decreased ( K 1 ) up to 600 C and further practically did not change with the temperature increasing up to 800 C. Most likely these and other contradictions are explained by an ambiguity or poor choice of decomposition components of the experimentally measured absorption bands. In particular, taking into account the note made above on interpretation of the 7350 cm 1 band eliminates such contradictions, at least partially. In all the papers cited above most of the OH groups (or even all of them) in silica glass were assumed to interact with each other in pairs. Again, usually no distinction is made between the vibrational frequency of single (`free' or `gas-like') OH groups, m 0, and the frequency of OH groups hydrogen-bonded in linear con guration, m 1 (Fig. 1). As shown in what follows, this is inconsistent with quantum-chemical calculations. Such a point of view has evidently arisen from studying the interaction of water molecules with silica glass, with the H 2 O Si±O±Si! Si±OH HO±Si reaction being of the rst importance. However, for the high-purity silica glasses used in bre optics, it seems much more natural to suggest that the OH groups occur in glass mainly in the form of single silanol groups, Si±OH, not interacting with each other, but bound in di erent ways with the glass network. We assume that OH groups can form hydrogen bonds not only with each other but also with the oxygen atoms of normal bridging linkage, Si±O±Si, similar to OH groups linked by hydrogen bonding to the oxygen matrix in YAG [6]. Besides, if a hydrogen bond is formed between two OH groups, the vibrational frequencies m 0 and m 1 should di er from one another. Therefore, we have

3 188 V.G. Plotnichenko et al. / Journal of Non-Crystalline Solids 261 (2000) 186±194 analyzed the shape of the absorption band assigned to the fundamental OH stretching vibration and its decomposition into components for di erent types of silica glass, with the OH concentration ranging from a fraction of ppm to more than 10 3 ppm (from 10 4 to 10 1 wt%) and have performed quantum-chemical calculations of possible ways that both single and paired OH groups can form in the silica glass network. 2. Experimental The absorption band of the fundamental OHstretching vibration for di erent types (I, II, III and IV according to classi cation given in Refs. [7,8]) of high-purity silica glass made by various technologies (Russian trademarks KU, KV, KUVI, KI and KS-4V) is shown in Fig. 2. The absorption spectra were measured on a vacuum infrared Fourier spectrometer with the noise level not worse than 0.1% (in transmission) and resolution of 1 cm 1. Thickness of the glass specimens under investigation varied from 0.2 to 250 mm. The absorption spectrum of the KS-4V silica glass specimen with OH contents less than 10 ppb (according to our measurements) was used as the base line to take into account the multiphonon absorption edge of the glass network. The shape of the OH fundamental absorption band turned out to be the same for all the glasses investigated within the accuracy of the measurements. Only in certain silica glasses (for example, of the KV type) manufactured from natural quartz and thus containing monovalent and divalent metal impurities with concentration of hundreds of ppm, the width of the band is somewhat broader (approximately by 2±3 cm 1, the total width being 110 cm 1 ). This allows one to conclude that embedding of OH groups in the silica glass network depends neither on the glass manufacturing method nor on the OH concentration, at least in the range from tens of ppb to more than 1000 ppm. In our opinion, this is possible only if OH groups occur in the glass bulk mainly as single centres but not as paired OH groups hydrogenbonded with each other. However it should be emphasized that this conclusion is restricted to the bulk only and not to the surface. The analysis of di erent methods for decomposition of the OH fundamental absorption band into components described by Voigt functions was performed using the v 2 criterion, a di erential moments method and a least-squares method. We found that only four components can be determined uniquely from the experimental spectra to ensure both required accuracy of decomposition relevant to accuracy of the measured spectra and unambiguity of the decomposition. The frequencies of components were found to be cm 1 (relative absorption was taken to be 1), cm 1 (0.5), cm 1 (2.5) and cm 1 (0.5) (see Fig. 3). Frequencies of the rst three components are in good agreement with the results of Refs. [1±5] while for the fourth component we obtained considerably higher frequency, 3565 cm 1 instead of 3510 cm Calculation method and cluster model Fig. 2. Absorption band of the fundamental stretching vibration of OH groups in high-purity silica glass of di erent types. All calculations were performed with the help of the MOPAC 93 package [9] using PM3 semiempirical quantum-chemical method known to describe hydrogen bonds adequately. Several reference calculations of characteristic con gurations were also performed by the AM1 method.

4 V.G. Plotnichenko et al. / Journal of Non-Crystalline Solids 261 (2000) 186± and angles characteristic for silica glass and OH groups. Then all coordinates of all atoms in each cluster were optimized by PM3 method. We modelled the formation of the hydrogen bond between the oxygen atoms of the OH group and the oxygen atom of Si±O±Si linkage or of the second OH group by xing the distance R O±O between these oxygen atoms and optimizing coordinates of all other atoms in each cluster. This procedure was iterated for several values of R O±O distance from 10.0 to 1.0 A for the clusters consisting of two parts and from the largest possible distance for the given ring to 1.0 A for all rings. According to our estimations the accuracy of vibrational frequencies and transition amplitudes was not worse than 10% in our approach. Fig. 3. Experimental absorption band of the stretching vibration of OH groups (thick solid line) and its decomposition (thick dash line) into components (thin solid lines), according to the present work. To calculate properties of the single silanol group (Si±OH) we used a cluster containing the HO±SiO 3 tetrahedron bound with three neighbouring SiO 4 tetrahedra. A pair of such clusters was used to calculate the hydrogen-bonded pair of Si±OH groups. To calculate the Si±OH group interacting with the Si±O±Si linkage we used the same cluster together with another one containing two SiO 4 tetrahedra with common bridging oxygen atom. The outer oxygen atoms of the SiO 4 tetrahedra in all the clusters were bound with SiH 4 groups to eliminate surface e ects. In crystalline polymorphs of silicon dioxide the Si±O±Si linkages are known to form mainly six-membered ring structures. The same rings and as well the rings with both smaller and greater number of SiO 4 tetrahedra (presumably from three to eight) are thought to occur in silica glass [10,11]. Generally speaking, both interaction of the silanol group with the Si±O±Si linkages and mutual interaction of two silanol groups may depend on size of the rings where they reside. Therefore we considered all possible cases of such interactions in the rings containing from 3 to 8 SiO 4 tetrahedra. Initially, the bond lengths and valence angles in the clusters were taken equal to average lengths 4. Discussion Fig. 4 shows calculated dependence of the cluster total energy on the R O±O distance between the oxygen atoms of the OH group and the Si±O±Si linkage and on the R O±O distance between oxygen atoms of two OH groups. In nite R O±O distance corresponds to the zero point of the energy. Fig. 4. Formation of hydrogen bond between OH group and Si±O±Si linkage, (a) and between two OH groups, (b).

5 190 V.G. Plotnichenko et al. / Journal of Non-Crystalline Solids 261 (2000) 186±194 Fig. 5. Si±OH group hydrogen-bonded with Si±O±Si linkage. Fig. 6. Si±OH group hydrogen-bonded with Si±O±Si linkage in the six-membered ring. For interaction of the OH group with the Si±O±Si linkage the total energy has two minima at R O±O 0:35 nm and R O±O 0:27 nm (Fig. 4(a)). The rst minimum is caused not so much by hydrogen bonding (though certain contribution is still present), but mainly by Coulomb and dipole electrostatic interaction of the hydrogen atom of the OH group with the bridging oxygen atoms in the opposite part of the cluster. Calculation shows that such weakly-bound states occur also for Si± OH groups in ve- and six-membered rings but they are absent in three-, four-, seven- and eightmembered rings. The second minimum corresponds to formation of the hydrogen bond between the oxygen atom of the OH group and the bridging oxygen atom. Height of the energy barrier preventing the hydrogen bond from destruction does not exceed 0.1 ev. According to the classi cation given in Ref. [12] such hydrogen bond is the `weak' one. Fig. 5 shows the equilibrium con guration of the OH group hydrogenbonded with the Si±O±Si linkage at R O±O 0:27 nm and Fig. 6 presents the OH group hydrogen-bonded with the Si±O±Si linkage in the six-membered ring. Calculation shows that similar hydrogen-bonded state occurs in ve-, seven- and eight-membered rings. Notice that the hydrogen bond in the rings is always formed with the bridging oxygen atom most distant from the OH group (in other words, one lying in the opposite part of the ring) owing to the transversal ring bending which is energetically more favourable than any deformation accompanied by a change of Si±O±Si linkage length. In smaller rings (three- and four-membered ones) the hydrogen bond between the oxygen atom of the OH group and bridging oxygen atom is not formed. This could be explained by high rigidity of small rings relative to both the transversel bending and the deformations accompanied by a change of the Si±O±Si linkages lengths. With two OH groups approaching each other, the minima of the cluster total energy clusters arise at R O±O 0:27 nm and at R O±O 0:22 nm. They correspond to the formation of the hydrogen bond between oxygen atoms of the OH groups at rst in linear and then in cyclic con gurations (Fig. 4(b)). the height of the energy barrier preventing the hydrogen bond from destruction is 0.35 ev for the linear con guration and less than 0.1 ev for the cyclic one. This allows one to classify the hydrogen bond in both con gurations as `moderate' [12]. Notice that for the OH group interacting both with the Si±O±Si linkage and the second OH group, the ground-energy bound states occur at approximately the same distance, R O±O 0:27 nm. Fig. 7 shows calculated equilibrium con gurations of hydrogen-bonded states of two OH groups, the linear (a) and cyclic (b). The linear

6 V.G. Plotnichenko et al. / Journal of Non-Crystalline Solids 261 (2000) 186± Fig. 8. Hydrogen-bonded pair of Si±OH groups in the sixmembered ring (linear con guration). Cyclic con guration is unstable for OH groups incorporated in the same ring. Fig. 7. Hydrogen-bonded pair of Si±OH groups: (a) linear con guration, (b) cyclic con guration. con guration of two hydrogen-bonded Si±OH groups in the six-membered ring is shown in Fig. 8. As in the case of interaction of OH groups with Si±O±Si linkages, a hydrogen-bonded state occurs also in the rings containing from ve to eight members and is absent in three- and fourmembered rings. The hydrogen bond in the rings is formed only in the linear con guration and only between Si±OH groups located in the opposite (most distant from each other) silicon sites of the ring. The cyclic con guration in all the rings appears to be unstable, since it can be formed only due to considerable deformation of the rings which does not occur under usual conditions. So we consider several possible types of embedding of the OH groups in the silica glass network. Fig. 9 shows decomposition of the experimental absorption band of the OH stretching vibration in KU glass into corresponding seven components with the calculated frequencies. Notice however that the experimental accuracy does not allow us to determine all the parameters of all the component bands and so this decomposition should be considered as an illustration only. The probability of the dipole vibrational excitation (IR absorption) of the OH groups for all above-described types of embedding of OH groups into silica glass network is shown in Fig. 10. The calculated frequency of OH stretching vibration of free (interacting neither with each other nor with Si±O±Si linkages) OH groups is 3690 cm 1 (x 0 in Fig. 9). In the weakly-bound state of the OH group and the Si±O±Si linkage (R O±O 0:35 nm) the frequency of the OH stretching vibration is 3670 cm 1 (x 1 ) and the IR absorption of this band is 4 times as high as that of the free OH group. In the hydrogen-bonded state of the OH group and the Si±O±Si linkage (R O±O 0:27 nm) the frequency of the OH vibrations varies from 3625 cm 1 to 3615 cm 1 in the clusters without rings and in di erent rings. In view of the calculation accuracy, it is reasonable to take the frequency of the stretching vibration for all such OH groups to be 3620 cm 1 (x 2 ). IR absorption of the OH group hydrogen-bonded with the Si±O±Si linkage is 25 times as high as that of the free Si±OH group. In the hydrogen-bonded pair of OH groups in linear con guration, two vibrations occur (x 3 and x 4 in Fig. 9) with calculated frequencies varying

7 192 V.G. Plotnichenko et al. / Journal of Non-Crystalline Solids 261 (2000) 186±194 Fig. 10. Probability of vibrational excitation of various types of OH groups in silica glass: (a) solitary (`free' or `gas-like') OH group, (b) OH group weakly bound with bridging oxygen atoms, (c) OH group hydrogen-bonded with bridging oxygen atom, (d) hydrogen-bonded pair of OH groups (linear con guration), (e) hydrogen-bonded pair of OH groups (cyclic con- guration). See text for details. Fig. 9. Experimental absorption band of the stretching vibration of OH groups in silica glass (thick solid line) and its decomposition (thick dash line) into components with calculated frequencies (thin solid lines). from 3655 cm 1 to 3670 cm 1 and from 3580 cm 1 to 3600 cm 1, respectively, for clusters without rings and for di erent rings (Figs. 7 and 8). In view of the calculation accuracy, we accept the average frequencies, 3660 cm 1 (x 3 ) and 3590 cm 1 (x 4 ). IR absorption corresponding to x 3 and x 4 is found to be 4 times higher and 30 times higher, respectively, than absorption of the free OH group. It should be emphasized that the vibrational frequencies of both OH groups hydrogen-bonded in linear con guration changes signi cantly in comparison with the free OH groups and not only frequency of one of these OH groups, as has been assumed practically in all previous works [1±5]. It is caused by the wellknown e ect of redistribution of electronic density in the hydrogen bond formation [12,13]. In the cyclic con guration two equivalent hydrogen bonds are formed with the co-phase (x cm 1 ) and anti-phase (x cm 1 ) vibrations. Intensity of corresponding IR absorption turns out to be approximately 20 and 10 times higher than absorption of the free OH group, respectively. Turning back to our decomposition shown in Fig. 3, we conclude that the component of the OH absorption band at 3690 cm 1 is caused by vibrations of single free OH groups (x 0 ). Both the OH groups weakly-bound with bridging oxygen atoms and the pairs of hydrogen-bonded (in linear con- guration) OH groups contribute to the component at 3660 cm 1 (x 1 and x 3 ). Both the single OH groups hydrogen-bonded with the Si±O±Si linkages (x 2 ) and the pairs of OH groups hydrogen-bonded with each other in linear con guration (x 4 ) contribute to the wide band near 3630 cm 1. Nominally, only the pairs of hydrogen-bonded (in cyclic con guration) OH groups can contribute to the component at 3565 cm 1 (x 5 and x 6 ). However, accuracy of the decomposition in this

8 V.G. Plotnichenko et al. / Journal of Non-Crystalline Solids 261 (2000) 186± part of the absorption band is considerably worse and therefore the contribution of the pairs of OH groups hydrogen-bonded with each other in linear con guration (x 4 ) must not be ruled out as well. The results of the absorption band decomposition for KU glass and the calculation data allow us to make the following conclusions. The net concentration of OH groups weakly-bound with bridging oxygen atoms and that of the OH groups hydrogen-bonded with each other in linear con- guration is about 10% with the concentration of free OH groups taken as 100%. The net concentration of the single OH groups hydrogen-bonded with the Si±O±Si linkages and of the OH groups hydrogen-bonded with each other in linear con guration is about 10% as well. Finally, the concentration of OH groups hydrogen-bonded with each other in cyclic con guration is not more than 2%. Hence the concentrations of OH groups weakly-bound with bridging oxygen atoms and of OH groups hydrogen-bonded with the Si±O±Si linkages are of the same order of magnitude. Unfortunately, the experimental data available do not allow us to estimate separately the concentration of the OH groups interacting with Si±O±Si linkages and hydrogen-bonded with each other in linear or cyclic con gurations. 5. Conclusion We studied the shape of the fundamental vibrational absorption band of OH groups and performed quantum chemical calculations of the structure and vibrational properties of such OH groups in silica glass. The main results and conclusions are as follows: The shape of the absorption band corresponding to the fundamental stretching vibration of OH groups (3672 cm 1 ) in di erent types of high-purity silica glass manufactured with di erent technologies does not depend or depends slightly on OH group concentration in wide range from 10 1 to 10 3 ppm. This demonstrates that OH group are present in the glass mainly as single groups not interacting with each other. In silica glass `moderate' hydrogen bonds between oxygen atoms of single OH groups and oxygen atoms of bridging Si±O±Si linkages with the energy about 0.06 ev and interatomic distance R O±O 0:27 nm and `weak' hydrogen bonds with the energy less than 0.05 ev and interatomic distance R O±O 0:35 nm can be formed. Between oxygen atoms of two OH groups in silica glass `moderate' hydrogen bonds either in linear con guration with energy about 0.35 ev and interatomic distance R O±O 0:27 nm or in cyclic con guration with an energy about 0.15 ev and interatomic distance R O±O 0:22 nm can be formed. The fundamental absorption band of the OHstretching vibration experimentally observed at 3672 cm 1 can be decomposed into the following components: cm 1 ± free single Si±OH silanol groups interacting neither with each other nor with Si±O±Si linkages cm 1 ± single OH groups slightly bound with bridging oxygen atoms and pairs of OH groups hydrogen-bonded with each other in linear con guration (vibration of one of the OH groups, namely one taking no part in the hydrogen bond) cm 1 ± single OH groups hydrogenbonded with Si±O±Si linkages and pairs of OH groups hydrogen-bonded with each other in linear con guration (vibration of one of the OH groups, namely one participating in the hydrogen bond) cm 1 ± pairs of OH groups hydrogen-bonded with each other in cyclic con guration and pairs of OH groups hydrogenbonded with each other in linear con guration (vibration of one of the OH groups, namely one participating in the hydrogen bond). To determine relative concentrations of all types of OH groups considered in the present paper, more precise decomposition of the experimental absorption band into more than four components is necessary. This is possible only if the accuracy of the absorption spectra measurement is considerably increased (by two orders of magnitude at least). Unfortunately the spectrophotometric equipment available now does not allow this.

9 194 V.G. Plotnichenko et al. / Journal of Non-Crystalline Solids 261 (2000) 186±194 Another way to verify experimentally the assumptions and results of the present paper is to try to change signi cantly the concentration ratio of OH groups of di erent types in glass by various types of in uence on glass, such as irradiation, heat or pressure treatment, resulting in changes of shapes of the OH absorption bands. References [1] G.E. Walrafen, J. Chem. Phys. 62 (1974) 297. [2] G.E. Walrafen, S.R. Samanta, J. Chem. Phys. 69 (1978) 493. [3] J. Stone, G.E. Walrafen, J. Chem. Phys. 76 (1982) [4] Y. Yokomachi, R. Tohmon, K. Nagasawa, Y. Ohki, J. Non-Cryst. Solids 95&96 (1987) 663. [5] S.Yu. Gerasimov, Fizi. Khim. Stekla, Glass Phys. Chem. 16 (1990) 759 (in Russian). [6] K.K. Deb, Spectroscopy Letters 15 (1982) 243. [7] R. Bruckner, J. Non-Cryst. Solids 5 (1971) 123,177. [8] O.V. Mazurin, M.V. Streltsina, T.P. Shvaiko-Shvaikovskaja, Properties of Glasses and Glassforming Melts, Reference Book, Part I, Nauka, Leningrad, USSR, 1993, p. 16 (in Russian). [9] J.J.P. Stewart, MOPAC Manual, Fujitsu Ltd., Tokyo, Japan, [10] F.L. Galeener, J. Non-Cryst. Solids 49 (1982) 53. [11] J.K. West, L.L. Hench, J. Non-Cryst. Solids 180 (1994) 11. [12] I.G. Kaplan, Introduction to Theory of Intermolecular Interactions, Nauka, Moscow, USSR, 1982 (in Russian). [13] K.Ya. Burstein, P.P. Shorygin, Quantum-chemical Calculations in Organic Chemistry and Molecular Spectroscopy, Nauka, Moscow, USSR, 1989 (in Russian).