Resolution of bridging oxygen signals from O 1s spectra of silicate glasses using XPS: Implications for O and Si speciation

We used an advanced charge compensation system on an X-ray Photoelectron Spectrometer to yield linewidths from O 1s, Si 2p and Pb 4f spectra of 1.22, 1.35, and 1.10 eV, respectively. These linewidths (eV) are the narrowest obtained for silicate glasses, on any X-ray Photoelectron Spectrometer, to date. The exceptional resolution reveals two O 1s peaks in the PbO–SiO2 glasses studied. One clearly resolved, high binding energy O 1s peak represents the bridging oxygen signal and the second, lower energy peak represents both non-bridging oxygen and metal-bridging oxygen contributions. These data allow quantification of bridging oxygen contents without detailed deconvolution because both the peak width and intensity are determined solely by the spectral data. The intensity of the bridging oxygen signal decreases systematically with decreased SiO2 content; however, the measured bridging oxygen abundance is greater than predicted if all Pb atoms in the glass are assumed to be associated with two non-bridging oxygen atoms (i.e., O–Pb–O units). There remains, for example, a significant quantity of bridging oxygen in the glass at the orthosilicate composition (Mol. frac.: 0.67 PbO, 0.33 SiO2). We demonstrate that bridging oxygen, non-bridging oxygen and metal-bridging oxygen exist at this composition and at all glass compositions studied, including the 0.50 PbO, 0.50 SiO2 glass. Equilibrium thermodynamic (speciation) calculations indicate that at least three silicate species dominate the glass: a network species (SiO2), a (SiO44-) monomeric species and a trimeric ring-like species (Si3O92-). With these species, the bridging oxygen contents are accurately modeled in PbO–SiO2 glasses over the compositional range 0.3 PbO, 0.70 SiO2 to 0.67 PbO, 0.33 SiO2, and there is a remarkable agreement between the modeled bridging oxygen and the measured bridging oxygen contents with this study and previous studies. However, we do not intend to imply that the SiO2, (SiO44-) and (Si3O92-) are the only species present in the glass structure. In addition, this study shows that the Si 2p spectrum consists of one peak, fitted with one doublet, which shifts systematically to higher binding energy with increased SiO2 content. We propose that this shift results from a more intense signal from the networked (more siliceous) species that are located at higher binding energy.