Exchange between interstitial oxygen molecules and network oxygen atoms in amorphousSiO2studied byO18

Abstract

Amorphous SiO 2 (a-SiO 2) thermally annealed in an oxygen atmosphere incorporates oxygen molecules (O 2) in interstitial voids. When the thermal annealing is performed in 18 O 2 gas, interstitial 18 O 2 as well as interstitial 16 O 18 O and 16 O 2 are formed due to the oxygen exchange with the a-SiO 2 network. The a 1 g (v = 0) → X 3 − g (v = 1) infrared photoluminescence band of interstitial O 2 was utilized to quantitatively analyze the oxygen exchange, taking into account the influences of common network modifiers in synthetic a-SiO 2 (SiOH, SiF, and SiCl groups). The presence of network modifiers does not significantly change the average rate of 18 O transfer from interstitial O 2 to the a-SiO 2 network and its activation energy, suggesting that the network modifiers themselves do not serve as preferential oxygen exchange sites. When the concentration of SiOH groups is low, the oxygen exchange rate is distributed, indicating that only a small part of the network oxygen atoms participates in the oxygen exchange. However, the distribution of the oxygen exchange rate is distinctly narrow in the sample with high SiOH concentration. It is attributed to the redistribution of the network 18 O atoms and the modification of the a-SiO 2 network topology caused by reactions with mobile interstitial water molecules, which are transiently formed by dehydroxylation of paired SiOH groups. The activation energy for the average oxygen exchange rate is larger than that of the permeation of interstitial O 2 in a-SiO 2. Furthermore, the average exchange-free diffusion length of interstitial O 2 below 900 • C (1 μm) is far larger than the scale of the interstitial voids in a-SiO 2 (1 nm). These observations confirm that the oxygen exchange is not necessarily involved in the permeation of interstitial O 2 .

O2 Diffusion in Amorphous SiO2 Nanoparticles Probed by Outgassing

“…A prominent role of amorphous structure and density of the material has been put forward both by experimental and simulative works, suggesting that higher material density inhibits molecule diffusion. , Furthermore, it has been shown that diffusion of the O 2 can occur without interaction with the network oxygen or by contrast with a molecule–network exchange of atoms. , This latter process is negligible below 900 °C where the exchange-free diffusion length reaches values of ∼1 μm. As a final concern, it has been evidenced that network modifiers could have minor effects on the O 2 migration, and the more relevant species could be SiOH through a dehydroxylation process, causing an interstitial water driven exchange of oxygen with the SiO 2 network at high temperatures. , …”

“…As a final concern, it has been evidenced that network modifiers could have minor effects on the O 2 migration, and the more relevant species could be SiOH through a dehydroxylation process, causing an interstitial water driven exchange of oxygen with the SiO 2 network at high temperatures. 26,27 The precedent studies have been typically carried out at temperatures above 500 °C due to a limitation in the detectability of the O 2 in bulk systems caused by the low penetration. Studies at lower temperature and in nanodimensional silica are limited and deserve to be done in view of the above-mentioned applications at the nanoscale.…”

Diffusion and Reactions of Photoinduced Interstitial Oxygen Atoms in Amorphous SiO2 Impregnated with 18O-Labeled Interstitial Oxygen Molecules

“…Comparison between eq and the initial decay of f * shown in Figure yielded a k F value of ∼4 × 10 –24 cm 5 J –1 . The exchange rate per second ( k F F at F = 20 mJ cm –2 ) was ∼8 × 10 –26 cm 3 s –1 , which is of approximately equal magnitude as the rate of thermal oxygen exchange at 700 °C in this type of glass (∼1 × 10 –25 cm 3 s –1 ).…”

“…The 18 O fraction of interstitial O 2 , f *, decreased monotonically with F 2 laser fluence, F . The decrease in f * was due to the supply of 16 O from the Si–O–Si network , normalO 18 normalO 18 + ≡Si− normalO 16 −Si≡ ⇄ normalO 16 normalO 18 + ≡Si− normalO 18 −Si≡ normalO 16 normalO 18 + ≡Si− normalO 16 −Si≡ ⇄ normalO 16 normalO 16 + ≡Si− normalO 18 −Si≡ …”

“…This observation confirms the photoassisted oxygen exchange between interstitial O 2 and the Si–O–Si network. The reversible second-order exchange rate constant, k F , defined for the forward (eq ) and backward (eq ) reactions, , may be expressed by normald f * normald F = k italicF 2 ( N * − f * N T ) where N T and N * are the concentrations of the total and 18 O-labeled oxygen atoms, respectively, in the Si–O–Si network. Under the approximation that N * ≪ N T ( N */ N T ≲ 10 –4 ), eq can be solved as f * = f 0 * .25em exp ( − k F N T F / 2 ) d f * d F true| F = 0 = prefix− f 0 * k italicF N normalT 2 …”