[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.
Full-text · Article · Sep 2011 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The structure of paramagnetic centers (PMC) in carbon-doped silica is studied by the EPR and quantum chemistry methods. Three
types of radicals, ≡Si-·CH2, (≡Si-)2
·CH and (≡Si-)3C·, are identified in which the free valence is localized on the impurity carbon atom. Their structure is determined by using
samples enriched by isotopes 2D, 29Si, and 13C. The assignment of the obtained radiospectroscopic parameters of radicals is confirmed by quantum-chemical calculations
of model systems. Based on the obtained data, a conclusion is made that paramagnetic centers (the so-called EX-centers) discovered
earlier by the EPR method in silicon oxidation products have the structure (≡Si-)3C·, i.e. are not the intrinsic defects of the material but are related to impurity carbon atoms. Differences in the spectral
characteristics of the (≡Si-)3C· groups observed in experiments are caused by the amorphous structure of silica. The kinetic nonequivalence of the (≡Si-)3C· radicals in the reaction of the hydrogen atom detachment from the H2 molecule is established (the activation energies for different PMC fractions lie in the range from 10 to 17 kcal/mol). The
quantum-chemical calculations of model systems performed in the paper suggest that the differences observed in the reactivity
of radicals are related to their spatial structure. It is found that the high-temperature pyrolysis of the (≡Si-)3C-H and (≡Si-O-)3Si-H groups is accompanied by the quantitative regeneration of free radicals (≡Si-)3C· and (≡Si-O-)3Si·. The probable mechanism of carbon atom embedding from. (≡Si-O-)3Si-O-CH3 groups to silica accompanied by the formation of (≡Si-)4C groups is analyzed.
No preview · Article · Mar 2011 · Kinetics and Catalysis
[Show abstract][Hide abstract] ABSTRACT: Two variants of the surface-nonbridging oxygen hole center, (=Si-O)(3)Si-O(center dot) and (=Si-O)(2)(H-O)Si-O(center dot), stabilized in porous films of silica nano-particles were investigated by time resolved luminescence excited in the visible and UV spectral range by a tunable laser system. Both defects emit a photoluminescence around 2.0 eV with an excitation spectrum evidencing two maxima at 2.0 and 4.8 eV, this emission decreases by a factor similar to 2 on increasing the temperature from 8 up to 290 K. However, the different local structure influences the emission lineshape. the quantum yield and the decay lifetime. Such peculiarities are discussed on the basis of the symmetry properties of these defects.
[Show abstract][Hide abstract] ABSTRACT: By studying the site-selective luminescence spectra of oxidized silica nanoparticles we identify the electronic and the vibrational lines associated with the surface nonbridging oxygen, equivalent to Si-O(center dot). This defect emits a zero-phonon line inhomogeneously distributed around 2.0 eV with full width at half maximum of 0.04 eV, weakly coupled with the local Si-O(center dot) stretching mode whose frequency is measured to be 920 cm(-1). These findings are different from those of the well-characterized defect in the bulk silica thus evidencing structural peculiarities of the surface defect that is characterized by a nearly unperturbed nonbridging oxygen and of the nanoparticles that induce a narrower inhomogeneous broadening of the electronic transition.
[Show abstract][Hide abstract] ABSTRACT: We investigated the red luminescence in a porous film of silica nanoparticles, originating from surface nonbridging oxygen hole centers. The excitation spectrum was measured from 1.8 to 8.0 eV by a tunable laser system and a synchrotron radiation source; this spectrum evidences a peak at 2.0 eV, nearly overlapping with the emission, and an ultraviolet broadband with peaks at 4.8 and 6.0 eV. The emission is characterized by a spectrum with two subbands split by 0.07 eV, its decay occurs with lifetime longer than 30 mu s and undergoes a thermal quenching by a factor similar to 2 with increasing temperature from 10 to 290 K. The optical characteristics of surface and bulk centers are discussed on the basis of the reported experimental results and quantum chemical calculations.
[Show abstract][Hide abstract] ABSTRACT: We report a time-resolved luminescence study of non-bridging oxygen hole centre in two silica systems, bulk and surface, under excitation with a tunable laser. This comparison shows similarities in the emission range, around 1.9 eV, and in the excitation spectrum consisting of two bands at 2.0 eV and 4.8 eV. The main differences concern the emission lineshape and the decay: the bulk non-bridging oxygen hole centre is characterized by a single band that decays in similar to 10 mu s, whereas the surface defect shows a composite luminescence spectrum with two sub-bands decaying with a longer lifetime 30-40 mu s. These experimental outcomes evidence the effects of the host silica network on defect optical properties.
No preview · Article · Jan 2008 · Nuovo Cimento- Societa Italiana di Fisica Sezione B
[Show abstract][Hide abstract] ABSTRACT: This chapter discusses the physico-chemical processes that occur on the surface of very important material—silica. Indeed, from one hand, a set of practically important reactions occurs on the silica surface. From another hand, this surface could be considered as a model, and the experimental data obtained for such a model system can be compared with the theoretical results and used for planning new experiments. The chapter also describes the methods of generating point defects, quantum-chemical modeling of the properties (optical, IR, and ESR) of point defects on silica surface, inhomogeneity of physico-chemical properties of the point defects stabilized on silica surface, comparison of spectral properties of the surface and bulk defects in silica, mechanisms of point defect rearrangement, and design of the reactive intermediates with desired structure on silica surface. The structure, spectral, dia- and paramagnetic characteristics of point defects is also be considered.
No preview · Article · Dec 2007 · Thin Films and Nanostructures
[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.
No preview · Article · Jul 2005 · Journal of Non-Crystalline Solids
[Show abstract][Hide abstract] ABSTRACT: We report a study of the emission decay from the singlet excited state of two fold coordinated Si and Ge centers stabilized on the surface of silica and Ge-doped silica. The PL lifetimes are of the order of nano-seconds and increase on decreasing the temperature. The results suggest that, for the surface centers, the phonon assisted intersystem-crossing process linking the excited states affects the decay rates, is effective down to low temperatures and is distributed because of the inhomogeneity of the defects.
Full-text · Article · Jan 2005 · physica status solidi (c)
[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.
No preview · Article · Mar 2004 · Kinetics and Catalysis
[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).
No preview · Article · Feb 2004 · Kinetics and Catalysis
[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.
No preview · Article · Dec 2003 · Kinetics and Catalysis
[Show abstract][Hide abstract] ABSTRACT: The review summarizes the results of experimental and quantum-chemical analysis of the structure, spectral parameters (optical, radio, and IR spectroscopic), and reactivity of the main types of paramagnetic and diamagnetic point defects that are stabilized on the activated silica surface or can be obtained by its directed chemical modification.
[Show abstract][Hide abstract] ABSTRACT: The paper presents the results of studies of spectra and reactivity of intrinsic defects of silica including two-coordinated silicon atoms ((≡Si–O–)2Si:, silylene centers, SCs). Such centers, stabilized on the surface of activated silica, are shown to differ in optical (absorption and luminescent) spectra and effects in various chemical processes. These effects are quantitatively analyzed. Possible differences in the SC geometry, resulting in the observed effects, are discussed using the quantum chemical calculations of molecular models.
No preview · Article · Oct 1998 · Journal of Non-Crystalline Solids
[Show abstract][Hide abstract] ABSTRACT: The photochemical transformations of the substituted allyl radicals Si(CH2)mCHCHCH2 (m = 0, 1, or 3) and SiO(CH2)mCHCHCH2 (m = 0−3) grafted onto a silica surface were studied. The cyclization of the allyl-type radicals resulting in the formation of the corresponding β-substituted cyclopropyl radicals Si(CH2)m-c-Pr• or SiO(CH2)m-C-Pr• (c-Pr = cyclopropyl) was found to be the primary photochemical process occurring upon photolysis at λ ≥ 370 nm. Further transformations of the resulting cyclopropyl radicals depend crucially on the m value. In the case of m = 0 or 1, the thermal isomerization of the cyclopropyl-type radicals yielding the parent allyl-type radicals is the main reaction pathway at room temperature. For m = 2 or 3 the reaction of intramolecular hydrogen atom transfer becomes the predominating process. This reaction results in the corresponding alkyl-type radicals SiCH(CH2)m-1-c-Pr• or SiOĊH(CH2)m-1-c-Pr formation. It was shown that the presence of electron-withdrawing substituents in β-position resulted in the decrease in the thermal stability of the cyclopropyl-type radicals. The experimental data obtained were compared with the results of quantum-chemical calculations.
No preview · Article · Jun 1998 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: A method for preparing >Si(R1(R2
.) structures (R1=CH3, CD3, or CH2−CH3, R2
. or CD2−CD2
.) grafted to a silica surface is developed. The reactions of intramolecular transfer of H (D) atoms between the R1 and R2
. fragments were studied by ESR. The directions and kinetic parameters of these reactions were established.
No preview · Article · Aug 1997 · Russian Chemical Bulletin
[Show abstract][Hide abstract] ABSTRACT: Kinetic trends, pathways, and parameters of the primary and secondary UV photochemical reactions of ''bulk'' and ''surface'' oxygen-deficient centers (ODC) in silica glasses with molecular hydrogen have been studied by UV absorption, luminescence, and ESR spectroscopies and the chemisorption method. The contribution of the singlet and triplet excited states of ODC to reaction with hydrogen at different temperatures and hydrogen pressures was established for ''surface'' ODC. The activation barrier to the H-2 reaction with ''surface'' ODC in the singlet excited state was determined as E(a) < 1 kcal/mol. The formation of paramagnetic H(I) centers has been observed, The kinetic data observed for ''surface'' ODS photoreactions have been applied to the analysis of similar reactions of hydrogen with photoexcited ODC when the diffusion limitations of reaction rates are taken into account. The H(I) centers escaped detection in UV-irradiated ''bulk'' ODC; this is attributed to the diffusion limitations and subsequent photodecomposition of the H(I) centers.
No preview · Article · Apr 1995 · The Journal of Physical Chemistry