[Show abstract][Hide abstract] ABSTRACT: We demonstrate that a porous film of silica nanoparticles emits a bright visible luminescence associated
with defects stabilized by oxygen chemisorption at oxygendeficient center sites. Time-resolved spectra excited by a tunable laser allow us to distinguish the luminescence at 1.99 eV, characteristic of the nonbridging oxygen hole center (NBOHC) (Si�O)3Si�O•, and a fast and a slow emission: the first
(lifetime τ ≈ 25 ns) is peaked at 2.27 eV with an excitation spectrum centered at 5.5 eV; the second (τ ≈ 7.5 μs) is peaked at 2.41 eV and is excited around 3.2 and 5.2 eV. Reaction in an air atmosphere leads to the disappearance of the NBOHC luminescence and of the fast band, whereas the slow one remains stable. On the basis of the comparison with previous experimental and computational works, we discuss the role of the silanone Si=O and of the dioxasilyrane Si(O2) as the emitting defects.
The Journal of Physical Chemistry C 01/2011; 115:19476. · 4.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report a comparative study on the optical activity of surface and interior Ge–oxygen deficient centers in pressed porous and sol–gel Ge-doped silica, respectively. The experimental approach is based on the temperature dependence of the two photoluminescence bands at 4.2 (singlet–singlet emission, S1→S0) and 3.1eV (triplet–singlet emission, T1→S0), excited within the absorption band at about 5eV. Our data show that the phonon assisted intersystem crossing process, linking the two excited electronic states, is more effective for surface than for interior centers in the temperature range 5–300K. For both centers, a distribution of the activation energies of the process is found. Based on the results of quantum chemical calculations of the electronic structure of (HO)2Ge: molecule it is suggested that the electronic de-excitation pathway involves two excited triplet states (S1→T2→T1→S0) and shows a structural dependence on the O–Ge–O angle.
Journal of Non-Crystalline Solids 01/2005; 351(21-23):1805-1809. · 1.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The methods of optical, ESR, and IR spectroscopy were used to obtain data on the structure and mechanism for the formation of the products in the reaction of dioxasilirane groups (Si–O)2Si 2 (DOSG) stabilized on the silica surface. Depending on the regime of the reaction (temperature and methane pressure), the process is accompanied by the formation of various products: methoxy (–O–CH3) and ethoxy (–O–C2H5) groups. The process mechanism is elucidated: this is a free-radical reaction in which paramagnetic sites are generated in the reaction between DOSG and methane molecules. The formation of final products is due to the reactions >Si(O)(OCH3) + CH4 >Si(OH)(OCH3) + CH3 and >Si(O–CH2)(OH) + CH3 >Si(OH)(OC2H5). The ratio of the rate constants of methyl radical addition to (Si–O)2Si: and (Si–O)2Si 2 at room temperature was determined experimentally (4.6 1.0).
Kinetics and Catalysis 02/2004; 45(2):273-278. · 0.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The methods of optical and IR spectroscopy and quantum chemistry were used to obtain data on the direction and kinetics of the reaction of a silanone (=SiO)2Si=O with a CH4 molecule and a methyl radical. Two mechanisms of methylation of silanone groups, molecular and free-radical, are studied. Both processes are accompanied by the formation of (=SiO)2Si(OH)(CH3) groups. The rate constant of the molecular process is determined and its activation energy is estimated (17 kcal/mol). A methyl radical adds to the silicon atom in a silanone group to form the oxy radical (=Si–O)2Si(O·)(CH3). This radical carries a free-radical process of silanone group methylation. The main channel for the pyrolysis of (=Si–O)2Si(OH)(CH3) groups is their decomposition with the abstraction of a methane molecule. The activation energy of this process is ~70 kcal/mol. Quantum chemical methods were employed to obtain data on possible intermediates in the processes studied and these results are used to interpret spectral and kinetic data.
Kinetics and Catalysis 01/2004; 45(2):265-272. · 0.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Experimental and quantum chemical study of reactive silica surface methylation is carried out. The main product of the reaction is the (Si–O)2Si(H)(CH3) groups, which are formed via a radical-chain process with the participation of methane molecules and paramagnetic and diamagnetic defects on the oxide surface. Spectral (optical and IR) characteristics of the groups participating in the process (Si–O)2Si–CH3, (Si–O)2Si(H)(CH3), and (Si–O)2Si(CH3)(CH3) are determined. Information on the kinetics of separate steps of the process is obtained including rate constants and the activation energies of steps.
Kinetics and Catalysis 12/2003; 45(1):14-23. · 0.54 Impact Factor