2. Literature Review
2.3. Silica Surface 1. Surface Species
2.3.4. Surface Treatments 1. “Cleaning”
2.3.4.3. Treatments on Silica Glass
an attempt to determine structural changes. The apparent stress optical constant increases with increasing temperature of fiber formation. The thermal expansion behavior showed an anomaly, which was expected of the disilicate glasses. The density and diameter data were compared and it was deduced that thin fibers exhibit radial expansion and high drawing stresses, where thick fibers show radial shrinkage and small drawing stresses.121 Another study by Murach and Brukner on silica glass fibers showed very different results.122 Silica glass fibers exhibit axial shrinkage and radial expansion over the temperature range. It was also found that there was a large degree of anisotropy in the open structure of the three-dimensional network, which confirms the existence of asymmetric hollows in the fibers.
These hollows are oriented by the fiber drawing process and thus these fibers have much higher stresses than their bulk counterparts.122 More treatment analyses on silica glasses are described in the following section.
bonded surface silanol groups and the formation of slightly strained siloxane bonds. A relaxation of the structure of silica near these strained bonds reduces the stress. The second stage is between 600-1200°C that incorporates reconstruction of separate surface sites upon removal of free silanol groups. The properties of the remaining surface hydroxyl groups change, such as the appearance of stable sites that oppose rehydroxylation in the formation of new electron-acceptor and reactive centers.53 Kuzuu et al.
performed a study on the structural change in the near-surface region of a silica glass block with heat treatment.55 They also used infrared spectra to follow the changes and found that certain absorption bands were induced following a heat treatment, which they called annealing, at 1160°C for 120 hours. The appearance of certain absorption bands only occurred on certain surfaces of the glass block, either the side surfaces or the top surface.55 It was found that the OH reduction on the top surface was approximately 45 ppm with the heat treatment and that of the side surfaces was 60 ppm.55
Modifying the silica surface with stress in a wet or dry atmosphere results in different relaxation states in the surface.127 This was determined by Webb III et al. through molecular dynamics simulations of a glass surface creating an interface with a crystal surface. It was determined that some of the stresses created in this interface were retained after separation and consisted mainly of strained siloxane bonds. These strained bonds may react more readily with water. It was observed in their simulations that a surface in the presence of water before and after interface creation had an increase in the number of Si−O bond ruptures and siloxane bond formations.127 In addition, there were a higher occurrence of Si−O bonds reacting to form Si−OH sites.127
One very interesting investigation recently carried out by Nazabal et al. concentrated on the structural modification of silica surfaces induced by thermal poling.128 Thermal poling is accomplished by placing a glass in an electric field at an elevated temperature and inducing a permanent second-
order nonlinear susceptibility.129-131 The second harmonic (SH) signal generation is measured since the SH can be induced only in isotropic materials such as glass.132 The charge transport of mobile ions driven by the poling field from the anode to the cathode creates a negatively charged depletion layer at the anode surface.129,130 After or during poling, ions begin to deposit and create a large frozen-in electrostatic field in the anodic surface layer.128 Bridging Si−O−Si bonds are broken during poling and are only partly restored after thermal treatment. There is also a decrease of hydroxyls associated with an increase of non-bridging oxygen species.128
Silanol levels on synthetic quartz crystals were examined by Wood et al. as a function of surface treatment.75 Their surface treatments included;
water plasma, heating in a vacuum to 500°C, heating in a vacuum to
>1000°C, and an inert gas ion bombardment in vacuum.75 Secondary ion mass spectroscopy (SIMS) was utilized to follow the ion ratios (SiOH+:SiO+, SiOH+:Si+, and OH−:O−) as a function of temperature.75 The water plasma treatment resulted in a maximum of surface hydroxylation and the >1000°C heat treatment resulted in a minimum of surface hydroxylation.75 They found a linear relationship between ion ratio and temperature.75 Ion beam sputtering has also been used as a method for inducing changes in a silica surface. Flamm et al. studied the topographical effects of low-energy argon ion beam sputtering with an AFM.133 As ion beam sputtering is a common technique for depth profiling it is important to understand how this technique alters the surface of a silica glass. It was determined that the sputtering caused a ripple feature to appear on the surface and that the wavelength of this feature was dependent on the energy of the incident ion beam.133
Point defects on the silica surface were created by Radtsig through chemical modification and analyzed using ESR, IR and optical spectroscopy.134 Point defects are highly reactive and implies that either a silicon or oxygen atom on the surface has a different number of nearest
neighbors than chemically appropriate. Radtsig applied mechanical treatments such as cleaving under vacuum and also a thermochemical treatment consisting of the creation of Si−OCH3 from Si−OH followed by a high temperature pyrolysis.134 Silanone groups, (≡Si−O)2Si=O, were detected on the surface of mechanically activated silica and thermochemically activated silica. Various other sites were stabilized through these treatments such as paramagnetic sites and diamagnetic sites. Paramagnetic sites include; silyl radicals (tri-coordinated silicon atoms (≡Si−O−)3Si*)), oxy radicals (non-bridging oxygen atoms (≡Si−O−)3Si−O*)), and peroxide radicals (≡Si−O−)3Si−O−O*). Diamagnetic point defects include; silylene or bicoordinated silicon atoms (Si atom is linked to two lattice oxygen atoms) and silanone groups ((≡Si−O)2Si=O).134
Other chemical modifications on silica surfaces have included adsorption of acetaldehyde, methyl acetate, ethyl acetate, and methyl trifluoroacetate.135 Through the use of microcalorimetric analysis, IR spectroscopy and temperature-programmed-desorption studies it was determined that the chemistries adsorb to silica primarily through two hydrogen bonds per adsorbate. This involves the donation of electrons from the lone-pair orbital on the adsorbate’s carbonyl oxygen to hydrogen atoms in the surface silanol groups.135 This was detected through IR due to the shifts in the C=O and O−H bond stretching bands.135 Oleic acid, polymethyl methacrylate, and polyacrylic acid were coated on soda-lime-silicate and silica glasses and measured using IR spectroscopy by Lee and Condrate Sr..136 They posed three possible reactions of the organics with the glass surfaces. The first two being that the organic compounds with an alcohol (−OH) or a carboxylate (−COOX, where X is H or CH3) group bonds with the glass through hydrogen bonding or chelate bonding (forming several bonds to a metal ion). The third possibility is that a condensation or esterification reaction occurs between the functional groups of organics and the silanol groups on the glass surface.136 They found that metal-chelate species are
formed when organic compounds with carboxylate groups are placed on soda- lime silicate surfaces.136 The soda-lime silicates contained numerous metal ions with which to form bonds such as sodium, calcium, magnesium, aluminum and potassium. The bonding species was not identified for the silica coated glasses. Other chemical modifications of the silica surface include a wide range of organosilane, alkoxysilane or aminosilane treatments. Organosilanes create a bridge between the inorganic glass surface and any organic molecule. Treating a surface with a silane is a common method to enhance bonding and, therefore, the silanes are typically referred to as coupling agents.123,137