Effect of Precursors on Lithium Infiltrated Silica Gels Studied by Xps
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EFFECT OF PRECURSORS ON LITHIUM INFILTRATED SILICA GELS STUDIED BY XPS S.F. HO AND L. C. KLEIN Rutgers-The State University of New Jersey, Ceramics Dept., Piscataway, NJ 08855-0909.
P.O. Box 909,
ABSTRACT Lithium silicate gels of composition 15 mole % lithia - 85 mole % silica were prepared by soaking highly porous silica gels in aqueous LiNO3 or methanolic LiOH solutions. The fracture surfaces of these samples were studied by X-ray photoelectron spectroscopy (XPS). High resolution XPS core level spectra were collected such that both chemical shift and atomic concentration could be determined. While the average lithium content was slightly higher in LiNO3 gels, only some of the lithium was associated with the silica network. In contrast, most of the lithium in LiOH gels was associated with the network. INTRODUCTION Lithium silicate gels have been prepared in two ways [1]. First, they have been prepared by hydrolyzing tetraethyl orthosilicate (TEOS) with aqueous solutions of lithium salts [2,3]. These are referred to as polymerized gels. Second, they have been prepared by soaking silica gels formed from colloidal sols in lithium salt solutions [4]. These are referred to as infiltrated gels. While the same bulk composition has been achieved with both methods, there are differences between polymerized and infiltrated gels because of the scale on which the two components are mixed [5]. The polymerized gels typically have porosity below 5 mm while the infiltrated gels have pore sizes about 250 nm [6]. The incorporation of lithium in both types of gels has been studied with MASNMR [5,7] and X-ray diffraction analysis [7]. The chemical uniformity has been determined through sectioning and atomic absorption spectroscopy [4,8). Only in the case of the polymerized gels has XPS been used to follow the incorporation of lithium salts and the evolution of residues from the process [9]. In the previous study [9), polymerized gels were prepared with lithium nitrate using two concentrations of methanol [10]. Both thin films and fracture surfaces were analyzed. The most interesting result of this study was the interaction of methanol and nitrates, where the nitrates were able to oxidize the organics. A similar study was carried out here on infiltrated gels prepared with both a ueous LiNO0 and methanolic LiOH. The reason for using XPS is that while Li NMR can detect the environment [5), XPS can provide semiquantitative analysis of the gel chemistry. Earlier studies using XPS in alkali silicate glasses [11-13) and, in one case, a sodium silicate gel [14) serve as good references. EXPERIMENTAL PROCEDURES Silica gels were prepared by mixing colloidal silica (Ludox-HS40,
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
534
O0s
0
C
c
SI2s
1000
808
GOO 400 Binding Energy (eV)
200
Figure 1 - XPS wide scan of colloidal silica gel before infiltration
Table I
Binding Energies (BE) and Atomic Concentrations for LiNO 3 and LiOH Infiltrated Gels LiNO Atomic Concantration
LiOH Atomic Concentration
19.8
23.5
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