Hydroxyl-Group Identification Using O K-Edge XAFS in Porous Glass Fabricated by Hydrothermal Reaction and Low-Temperature Foaming

Abstract

Porous glass was prepared by the hydrothermal reaction of sodium borosilicate glass, and oxygen-ion characterization was used to identify the hydroxyl groups in its surface area. A substantial amount of “water” was introduced into the ionic structure as either OH− groups or H2O molecules through the hydrothermal reaction. When the hydrothermally treated glass was reheated at normal pressures, a porous structure was formed due to the low-temperature foaming resulting from the evaporation of H2O molecules and softening of the glass. Although it was expected that the OH− groups would remain in the porous glass, their distribution required clarification. Oxygen K-edge X-ray absorption fine structure (XAFS) spectroscopy enables the bonding states of oxygen ions in the surface area and interior to be characterized using the electron yield (EY) and fluorescence yield (FY) mode, respectively. The presence of OH− groups was detected in the O K-edge XAFS spectrum of the porous glass prepared by hydrothermal reaction with a corresponding pre-edge peak energy of 533.1 eV. In addition, comparison of the XAFS spectra obtained in the EY and FY modes revealed that the OH− groups were mainly distributed in the surface area (depths of several tens of nanometers).

Visualization of elemental distributions and local analysis of element-specific chemical states of an Arachnoidiscus sp. frustule using soft X-ray spectromicroscopy

“…As we mentioned above, the O K-XAS spectra shown in Fig 5b were quite similar to those of SiO 2 (glass) reported in a previous paper [ 41 , 42 ]. As in the case of the Si K-XAS spectra, the chemical states were found to be similar in each ROI.…”

“…Then, we sorted the normalized intensities by energy to generate the K-XAS spectrum, as shown in Fig 4b . The resultant O K-XAS spectrum shown in Fig 4b is definitely very similar to that of SiO 2 (glass) reported in a previous paper [ 41 , 42 ].…”

“…To generate the K-XAS spectrum, we set the ROIs, where ROI stands for the region of interest, at arbitrary locations inside the X-PEEM image, as shown in Fig 4a, and normalized the ROI intensity recorded in each image using the area of the ROI. Then, we sorted the normalized intensities by energy to generate the K-XAS spectrum, as shown in Fig 4b. The resultant O K-XAS spectrum shown in Fig 4b is definitely very similar to that of SiO 2 (glass) reported in a previous paper [41,42].…”

“…g ., Si-OH, whereas the main structure at the steep rise is associated with bridging oxygen, e . g ., Si-O-Si [ 42 ].…”

“…Furthermore, we recognize the weak structure, i.e., the doublet peak, is observed at 531 and 532 eV in each curve. These small peaks are thought to be associated with nonbridging oxygen ions at 531 eV and with hydroxyl groups, e.g., Si-OH, whereas the main structure at the steep rise is associated with bridging oxygen, e.g., Si-O-Si [42].…”