changes of lead silicate glasses induced by leaching

Changes of lead silicate glasses induced by leaching

C. Schultz-M�unzenberg, W. Meisel *, P. G�utlich

Institut f�ur Anorganische Chemie und Analytische Chemie, Universit�at Mainz, Standinger Weg 9, 55099 Mainz, Germany

Received 7 June 1997; received in revised form 17 March 1998

Abstract

The structural di�erences in the surface region between freshly fractured and leached silicate glasses containing 16.7,

18.8 and 44.4 mol% PbO, respectively, were investigated by photoelectron spectroscopy (XPS). The optical properties

and the thicknesses of leached layers were determined by re¯ection measurements. The binding energies of the O1s sig-

nal components for untreated samples can be ascribed to non-bridging (NBO) and bridging (BO) oxygen and oxygen

associated with lead as network former (OPb). The binding energy of OPb was found to be 529.1 � 0.2 eV. For quan-

titative conclusions, relative XPS sensitivity factors were determined for oxygen, silicon and lead in these glasses. The

experimental NBO/BO/OPb ratio found is in good agreement with the NBO/BO/OPb ratio calculated by use of the `dis-

crete bond model' (DBM). Leaching causes a removal of about 90% of the glass modi®ers. The O1s signal of silanol

groups and other O±H bonds formed appears at a higher binding energy as compared with the BO signal. Whereas

the glasses with smaller Pb contents exhibit thin and well con®ned leached layers, on the glass with the larger Pb content

a structured thick layer is formed. Its thickness growth follows a��tp

-dependence expected for a di�usion controlled pro-

cess. Pb at network forming sites is more stable with respect to leaching than network modifying Pb. After the removal

of Pb from network modifying sites, in addition to the formation of silanol and other O±H bonds a rebinding between

Si tetrahedra is found resulting in an increased density of the leached layer. Ó 1998 Elsevier Science B.V. All rights

reserved.

1. Introduction

Glass surfaces are known [1] to be altered by thein¯uence of corrosive gases or ¯uids: thin surfacelayers develop with di�erent optical properties,chemical composition, structure and bonding rela-tions as compared with bulk glass. These processesare of special interest for lead containing glasseswhich are widely used for optical purposes [2].The change of optical properties and the leachingkinetics of surfaces as function of pH, tempera-

ture, or chemical composition of the leaching solu-tion were investigated in recent studies [1±5].Several authors have compared the removal ofthe divalent cation, Pb2�, and the enrichment ofthe hydrogen content with leaching processes of al-kali silicate glasses [2,3].

Studies of binding energies in surfaces can becarried out by di�erent methods [6]. Photoelectronspectroscopy (XPS) shows the oxidation state andchemical environment via the `chemical shift' ofthe element considered. In this way, a distinctionbetween bridging (BO) and non-bridging (NBO)oxygen [7] is known as well as the chemical shiftof the oxygen in silanol groups or other O±Hbonds from the O1s signal [8]. In some recent

Journal of Non-Crystalline Solids 238 (1998) 83±90

* Corresponding author. Tel.: +49-6131 393282; fax: +49-6131

392990; e-mail: [email protected].

0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 5 8 0 - 8

papers [9,10], XPS spectra of the systemxPbO(1 ) x)SiO2 resolve the di�erently bound oxy-gen and lead by means of the O1s and the Pb4fsignal. Two di�erent types of bonds for oxygen(BO and NBO) have been found in lead silicateglasses with a PbO content less than 50 mol%.For a PbO content >50 mol%, a third peak (OPb)is identi®ed for oxygen in a lead network. It isknown that in glasses with small amounts ofPbO, lead is a network modi®er (NWM) and de-creases the number of bonds between the [SiO4]4ÿ

tetrahedra [11]. At high PbO contents Pb±O±Pbbonds begin to form in the glass and Pb buildsup its own network with structural units of[PbO4]4ÿ tetrahedra/pyramids [12±14].

The object of this paper is to di�erentiate be-tween the surface structure and the bulk structureof leached glasses with special interest on a glasswith high PbO content.

2. Experimental

Lead silicate glass samples with 16.7, 18.8 and44.4 mol% PbO and di�erent alkali contents (glass-es F2, F5 and SF57, respectively) were investigated(Table 1). The samples were manufactured bySchott Glaswerke Mainz, Germany. The XPS mea-surements were carried out by use of a 150° spher-ical analyzer (VG Escalab MKII, MgKa radiation).Highly resolved spectra (O1s, Pb4f and Si2s) wererecorded under ultra high vacuum (UHV) condi-tions (2 ´ 10ÿ10 mbar) and a pass energy of 20eV. All instrumental settings were held identicalduring the course of these experiments. Surveyspectra with a lower energy resolution were usedto assign the signals to the respective elements.

Clean, untreated surfaces of glass rods (diame-ter 8 mm, length 18 mm) broken in a fracture stageunder UHV conditions, were measured immedi-

ately thereafter by XPS. This procedure avoidscontamination of the glass surface by residual gas-es and possible reactions of these gases with theglass components. The C1s signal was used to de-termine the degree of the contamination as well asto correct the energy scale for charging e�ects. Theposition of the signal of aliphatic carbon was as-sumed to have a binding energy of 285.0 eV.

Leaching was performed by immersion of pol-ished discs (diameter 18 mm, thickness 4 mm) insolutions of di�ering compositions with di�eringpH's under constant stirring at 20°C. The majorityof the experiments was performed with acid solu-tions of pH 3.7 prepared by adding H2SO4 to dis-tilled water. Further leaching experiments werecarried out in distilled water and in used commer-cial polishing slurry (pH 8.3). The thicknesses andrefractive indices of the layers were determined byre¯ection measurements on a spectralphotometer(Bruins Omega10) in the UV/VIS region at an an-gle of incidence of 10°.

3. Results

3.1. Re¯ection spectroscopy

The thicknesses and the refractive indices of thelayers were determined by re¯ection spectroscopy.A recursion formula [15] was used to calculate re-¯ection spectra. Hereby, the refractive indices andthicknesses of the layers were varied to get the bestagreement between experimental and calculatedspectra. For a single layer, the distance betweenthe maximum values in the re¯ectivity is in¯uencedmainly by the thickness of the layer. The ampli-tude of the maxima is given mainly by the di�er-ence in the refractive indices in the layer and inthe bulk. Additional layers lead to more compli-cated spectra. The shape of ni�k� in the region be-tween 300 and 750 nm was assumed to be the samefor the bulk and the layers, whereas the absolutedi�erence of ni between bulk and layer was variedin the ®tting procedure. By leaching of sampleSF57 in acid solution, a single surface layer wasobserved with refractive index, nD� 1.46(nD(bulk)� 1.77). This reduction is ascribed tothe reduced content of Pb in the leached layer.

Table 1

Composition of the glasses SF57, F2, and F5

SiO2 PbO Alkali oxides

mol% mol% mol%

SF57 54.0 44.4 1.6

F2 72.5 16.7 10.8

F5 70.5 18.8 10.7

84 C. Schultz-M�unzenberg et al. / Journal of Non-Crystalline Solids 238 (1998) 83±90

Fig. 1 shows a re¯ection spectrum for SF57 lea-ched in acid solution and the dependence of thelayer thickness on the leaching time. In goodagreement with recently published data [3], thethickness shows a

��tp

-growth, expected for a di�u-sion controlled process. After a leaching time of30 h and a ®nal thickness of 1350 nm, intrinsicstress destroys the surface layer. The applicabilityof the recursion formula from Ref. [15] shows thatthe thickness of the transition region between theleached layer and the bulk material is small(<20 nm) compared to the thickness of the layer.Leaching with polishing slurry (pH 8.3) leads toa more complicated series of layers with a gradualtransition between them. Samples F2 and F5 weremore stable in acid and basic media than SF57.After leaching F2 and F5 for 200 h in a solutionof pH 3.7, the thickness of the layers did notexceed 40 nm. The refractive indices werenD� 1.47 � 0.01, close to that of SF57. The im-proved corrosion behavior of lead silica glasseswith addition of alkali oxides is well known [2].

3.2. Photoelectron spectroscopy

After a removal of the background by Shirley'smethod [16], all spectra were ®tted by use of Voigt

pro®les [17]. In addition to the Ka1;2lines, the high

energy satellite lines Ka3;4and Ka0 ;a00 have been ta-

ken into consideration. O1s spectra of both brokenand leached samples are shown in Fig. 2. F2 andF5 spectra are similar to each other and, therefore,only spectra of F5 are reproduced in Fig. 2. TheO1s signal of all broken glasses was found to con-sist of two superimposed peaks at 532.2 � 0.1 and530.3 � 0.1 eV. However, the spectrum of SF57had an additional peak at 529.1 � 0.2 eV. Thepeak with the binding energy of 532.2 eV is as-cribed to bridging oxygen covalently bound totwo silicon atoms [7]. The peak at 530.3 eV canbe associated with non-bridging oxygen connect-ing an Si ion with a network modi®er [10]. In thecase of SF57, there exists only lead as NWM,whereas in the case of F2 and F5 there exist threedi�erent elements as NWMs. This leads to abroader half width at the half maximum HWHM(2.5 eV) of the NBO signal for F2 and F5 as com-pared to SF57 (2.0 eV). The third maximum at529.1 eV with a smaller intensity (5.4%) in thespectrum of SF57 is of interest and will be ex-plained below. Changes in the leached layers com-pared with the virgin glass are: the center ofgravity of the O1s signals of leached SF57, F2and F5 shifted towards larger binding energies(cf. Fig. 2). After leaching in acid solution, in ad-dition to the BO and NBO binding energies at532.2 and 530.3 eV, respectively, a peak at533.3 � 0.1 eV appears. We ascribed it to O±Hbonds �H2O; Si±O±H; OHÿ; . . .� [18]. The rela-tive chemical shifts of these di�erent bonds areso small that they cannot be resolved from eachother in the XPS spectra. According to the factthat the information depth of XPS is much lessthan the thicknesses of the leached layers, similarspectra were found for all leached samples. Dueto the leaching of the Pb, the NBO signal decreasesto the limit of detection. For SF57, the peak at529.1 eV is not detected. The major contribution tothe O1s signal is now that resulting from the BOs.

The Pb4f and Si2s signals (Fig. 3) were decom-posed into their components in the same way asfor the O1s signal. Two facts require a simulta-neous ®tting of these signals: (i) The intensity ofthe background at a distinct energy is propor-tional to the area of the signal above this energy

Fig. 1. An example of an experimental (�) and a calculated ())

re¯ection spectrum of SF57 leached in acid solution (pH 3.7),

and the thickness growth of the leached layer (inset).

C. Schultz-M�unzenberg et al. / Journal of Non-Crystalline Solids 238 (1998) 83±90 85

(Shirley's method [16]). In this way, the back-ground of the Pb4f5=2 and Si2s is mainly in¯uencedby the Pb4f7=2. (ii) The Pb4f5=2-Ka1;2

signal is super-imposed on the Ka3;4

satellites of the Si2s.All Pb4f signals have the same multiplet split-

ting (4.9 eV � 0.2 eV). In the following, the bind-

ing energies are assumed to belong to the Pb4f7=2

signal. The Pb4f signal of broken samples, F2and F5, is well described by a single doublet (leadexists as NWM) at 139.6 � 0.1 eV. The Pb4fsignal of SF57 has two doublets appearing at139.6 � 0.1 eV and 138.0 � 0.3 eV, respectively.

Fig. 2. Decomposition of the O1s signal of leached (pH 3.7) and freshly fractured samples, and the residuals (inset) as percentage of the

signal height.


The component with the largest amplitude (139.6eV) results from lead as NWM (92.1%). The sec-ond part of the signal will be explained below.The Pb4f signals of leached glasses are attenuatedcompared to the Si2p signal (Fig. 3). In the spec-

tra of F2 and F5, however, only the Pb4f line at139.6 � 0.1 eV appears, the same as in the bro-ken samples. For all three glasses, a smallamount of NWM Pb remains after leaching.For SF57, obviously another signal appears at

Fig. 3. XPS Pb4f and Si2s signals of leached (pH 3.7) and freshly fractured samples, and the residuals (inset) as percentage of the signal

height.


137.2 � 0.2 eV in addition to the NWM signal ofPb at 139.6 eV.

Relative sensitivity factors of Si2s, O1s andPb4f for the broken samples were derived fromthe areas of the respective XPS signals as relatedto the known contents of lead, oxygen and siliconin the glasses. In this way, a determination of theelement concentration in the layers of the leachedglasses became possible. The intensity of the XPSsignal depends on di�erent factors (e�ective crosssection, glass matrix, attenuation length, instru-mental parameters; . . . [11,19]) and, therefore, onlyrelative factors were used and all the measure-ments were performed with identical experimentalsettings. The relation between XPS intensities, Ii,and element concentrations, ci, is described by

Ii

Ij� fi � ci

fj � cj� f i

j �ci

cj:

Under the given experimental conditions, therelative sensitivity factors, f i

Si, related to Si aref Pb

Si � 16:3� 1:5 and f OSi � 2:9� 0:3. The elemen-

tal compositions in the surface region of samplesleached at pH 3.7 for 1 h are shown in Table 3.By further experiments the composition was found

to be nearly independent of the kind of the leach-ing solution and of the duration of leaching. Theleached samples had a smaller O/Si ratio than thebroken samples, and a reduction of the Pb contentby about 90% was observed.

4. Discussion

The experimental results have shown that a dis-tinction between BO and NBO is possible for thesamples broken in UHV. The two sublines in theXPS O1s (BO, NBO) as well as the single line inthe Pb4f7=2 signals indicate that all Pb ions are in-corporated on NWM sites for F2 and F5. The ap-pearance of a third subline in the O1s signal (OPb)and the second subline in the Pb4f7=2 signal ofSF57 indicate that in this sample some Pb occupiesa small number of network former (NWF) sites inaddition to the NWM sites.

Assuming that all Pb is on modifying sites, it ispossible to calculate a theoretical BO/NBO ratioby use of the DBM [8] and of the glass composi-tion. Depending on the content of NWMs, thenumber of BO bound to the quaternary siliconspecies changes from i� 0 to 4. Table 2 showsthe calculated BO and NBO portions as comparedto the measured subline intensities of the XPS O1ssignal. There is a good agreement, within about1%, for F2 and F5. The calculation of the BO/NBO ratio for SF57 requires the knowledge ofthe relative amount of NWF Pb. The BO/NBO ra-tio increases for a large portion of NWF Pb anddecreases for a large portion of NWM Pb. The in-tensity of the second line in the Pb4f7=2 signal(8.2%), we assume, is the portion of the Pb onNWF sites. From this additional informationand the sample composition we recalculate the

Table 2

A comparison of experimentally found and calculated NBO/BO/BOPb ratios of the XPS O1s signal

NBO BO BOPb

exp. calc. exp. calc. exp. calc.

% % % % % %

SF57 38.5 � 0.2 38.5 56.1 � 0.3 56.1 5.4 � 0.2 (5.4)

F2 64.6 � 0.4 65.5 35.4 � 0.3 34.5 ) )F5 67.4 � 0.4 68.5 32.6 � 0.3 31.5 ) )

Table 3

O and Pb atomic concentrations derived from the spectra by

use of relative sensitivity factors, related to Si (cSi� 1)

cO cPb

SF57 broken 2.85 � 0.11 0.82 � 0.10

leached 2.40 � 0.20 0.1 � 0.01

F2 broken 2.42 � 0.15 0.26 � 0.04

leached 2.10 � 0.14 0.04 � 0.03

F5 broken 2.38 � 0.17 0.25 � 0.05

leached 2.30 � 0.15 0.04 � 0.03


distribution of BO, NBO and OPb. In fact, there isa very good agreement, within the experimental er-rors, for SF57, if the third line in the O1s signal isascribed to NWF Pb in structural units of[PbO4]4ÿ tetrahedra/pyramids.

Usually Q�i�-units are used to describe the[SiO4]4ÿ tetrahedra [20], where i means the numberof BO connecting the respective tetrahedron toNWFs. The O/NWF ratio allows a calculation ofthe distribution of the Q�i�-units (O/NWF� (4 ) i) + i/2 and O/NWF� 2±4). For SF57(44.4 mol% PbO), a relatively large fraction ofQ�2� units and a small fraction of Q�3� units arepresent, whereas in F2 and F5, because of thesmaller content of NWM, mainly Q�3� units andonly a small fraction of Q�4� units exist.

Structural and compositional changes in theleached layers as detected by XPS are as follows:In good agreement with recently published data[1], the thickness had a

��tp

-growth, expected for adi�usion controlled process. For all three samples,only a small amount of Pb remained after leach-ing. This remnant can be understood as an equilib-rium between the sample and the concentration ofthe Pb ions in the solution. Accordingly, the NBOline in the XPS O1s signal is not detected. The ma-jor contribution to the O1s signal is now that re-sulting from the BOs. The existence of two linesin the Pb4f7=2 signal and the disappearance of theO1s peak at 530.3 eV shows that network forminglead in the leached layers has a di�erent chemicalenvironment compared to that in the bulk struc-ture. The increased intensity of the Pb±BO signalas compared to the Pb±NBO signal (see Fig. 3,SF57 spectra) indicates also that the dissolutionof the network forming Pb by leaching is less prob-able than that of network modifying Pb.

The ratios O/Si and Pb/Si were calculated byweighing the intensity of the Pb4f, O1s and Si2ssignal with the relative sensitivity factors deter-mined from the fractured samples (cf. Table 3).Although some water from the solution shouldbe incorporated into the network, the leached sam-ples show a smaller O/Si ratio than the brokensample. In SF57, the Q�i� distribution increasesfrom an average value i� 2.32 (i.e. Q�2� and Q�3�

units predominate) for the bulk material toi � 3.2 (i.e. Q�3� and Q�4� units predominate) for

the leached layers. These data imply that the leach-ing process not only replaces Pb on NWM sites bysilanol groups but also changes some of the NWF±NBO sites to NWF±BO sites. Furthermore, thisstructural recon®guration is also demonstratedby the composition of the XPS O1s signal: in theleached layer both the OOH and the NBO contrib-ute less to the intensity of the whole O1s signalthan the NBO to the whole signal intensity ofthe bulk.

5. Conclusions

We demonstrated a distinction between the dif-ferently bound oxygen species by XPS. For thetwo glasses of small PbO content, lead exists inthe form of NWM. In the sample SF57, exhibitinga high Pb content, lead exists also in the form ofNWF. The NBO/BO/OPb ratios of the O1s peaksof freshly fractured surfaces agree well with the ra-tios calculated from the known glass compositions.Leaching in acid media produces a single layerwith less Pb and with a reduced refractive indexwhose thickness grows as

��tp

. An equilibrium be-tween Pb in the layer and Pb in solution is ob-served. NWF sites for Pb are more stable withrespect to leaching than NWM sites. The reducedO/NWF ratios in the layers show that there is alarger fraction of bonds between NWF (i.e. Si±O±Si) compared to the bulk. Leaching producesa layer with a Pb content smaller than in the bulkand a more dense Si network.

Acknowledgements

One of the authors (C.S.-M.) gratefully ac-knowledges the support by the Schott GlaswerkeFonds and helpful discussions with D. Sprenger.

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changes of lead silicate glasses induced by leaching

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