Si Bridges Research Articles

EFFECTS OF SURFACE OXYGEN ON THE ELECTRONIC PROPERTIES OF SILICON NANOCLUSTERS

We have studied the effects of surface oxygen and its bond structure on the electronic properties of silicon nanoclusters by means of density functional theory calculations. The results of the energy gap as a function of the nanocluster size in hydrogen-terminated and oxygen-adsorbed silicon clusters provide a well interpretation of several experiments. The nature of electronic and optical properties of silicon nanoclusters has been discussed and attributed to the oxygen in both the Si=O double bond structure and Si–O–Si bridge structure.

A new application of clay-supported vanadium oxide catalyst to selective hydroxylation of benzene to phenol

The highly selective direct hydroxylation of benzene with hydrogen peroxide to phenol was observed on a clay-supported vanadium oxide catalyst. Under mild reaction conditions at 313 K, high selectivity to phenol of 94% was realized on the clay-supported vanadium oxide catalyst at 14% conversion of benzene, comparable with the well-known TS-1 catalyst. On the other hand, vanadium on other supports and other metals on the clay support gave inferior catalytic performances. V–O–Al and V–O–Si bridges in the catalyst may be responsible for the reaction.

The structure of the SiO2∕Si(100) interface from a restraint-free search using computer simulations

The structure of the interface between SiO2 and Si(100) is investigated using the replica-exchange method driven by classical molecular dynamics simulations based on ab initio-derived interatomic potentials. Abrupt interfaces are shown to be unstable, whereas a substoichiometric oxide forms at the transition between the two materials that exhibits Si atoms in all three intermediate oxidation states, in agreement with experiment. A number of physical characteristics are found to be consistent with experimental data, including the distribution of Si atoms with different oxidation states, the increase in atom density and the stability of a pseudo-cristobalite pattern at the interface as well as the presence of Si–O–Si bridge bonds between the substrate and the suboxide.

Multipole-refined charge density study of diopside at ambient conditions

The experimental multipole electron density, ρ(r), of diopside was derived from high-resolution single-crystal diffraction at room temperature. Its topological analysis revealed predominantly ionic Si–O bonding, as found in electron density studies of other silicates. In particular, the non-bridging Si–O bonds are slightly less ionic in character than the bridging Si–O bonds. The Ca–O and Mg–O bonds are classified as pure closed-shell ionic interactions. An analysis of –∇2ρ(r) showed the presence of maxima around the oxygen atoms, associated to lone pairs domains that are involved in bonds with the surrounding ions. Calculation of atomic basins gave net charges of –1.56(12), 3.11(17), 1.79(13) and 1.88(18) e for O (averaged), Si, Ca and Mg atoms, respectively. O···O interactions between the O atoms at the vertices of the SiO4 tetrahedron were also detected from the topological analysis of ρ(r), and indicate a cooperative interaction among the lone pairs of neighbouring oxygen atoms. All these results were also confirmed by periodic restricted Hartree–Fock (RHF) calculations.

Influence of the growth temperature on the atomic distribution of TEOS deposited SiO2 films

Dispersion analysis was performed on low pressure chemically vapor deposited SiO2 films from tetraethylorthosilicate (TEOS) vapors at temperatures between 635 and 820°C. The transmission spectra were best reproduced using four Lorentzian oscillators within the range 900–1400cm−1 and two Lorentzian oscillators within the range 700–900cm−1. This is justified if one considers that two kinds of Si–O–Si bridges exist in films: those bearing characteristics similar to bulk fused silica (bulk-like bridges) and those located near the interfaces and grain boundaries (boundary-like bridges). The ratio of the populations of bulk to boundary-like bridges increases with deposition temperature from 1.2:1 at 635 to 2.1:1 at 820°C. Postdeposition annealing also causes an increase of bulk-like bridges at the of boundary-like ones. The distribution of the Si–O–Si angles in TEOS SiO2 films, related to the way atoms are arranged in them, is not a simple but it is a superposition of two Gaussians with different central values and full width at half maximum. The distance between the central angles of the two Gaussians was found to be between 7.7° for films deposited at 635°C and decreases to reach a value equal to 6.3° for those deposited at 820°C.

Photocatalytic performance of Ru doped anatase mounted on silica for reduction of carbon dioxide

Ru doped anatase supported on silica was prepared by solid-state dispersion method and examined for the photocatalytic reduction of carbon dioxide in aqueous medium at ambient conditions. To assist in interpreting the photocatalytic behaviour of Ru-TiO2/SiO2, reference systems consisting of Ru doped TiO2 and TiO2 supported on SiO2 were also analyzed and the conditions were optimized. Ru/TiO2 photocatalysts with metal loadings of 0.1, 0.3, 0.5 and 1.0wt% were prepared by impregnation method and a series of TiO2/SiO2 catalysts with low TiO2 (1, 3, 5 and 10wt%) contents were prepared by solid-state dispersion method. The photocatalysts were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), UV–vis DRS, FT-IR and Raman spectroscopy. The results showed that TiO2 particles with Ru ions have higher photocatalytic activity than undoped TiO2 and the optimum Ru loading was found to be 0.5wt%. Nevertheless, the yield increased notably when TiO2 was supported on SiO2. This strong enhancement suggests that in 10wt% TiO2/SiO2 the efficiency of charge separation is strongly influenced through the presence of TiOSi bridging bonds. On the contrary, Ru-TiO2/SiO2 has no significant improvement in activity over TiO2/SiO2 except that it shows nearly quadruple times higher activity for the formation of methanol than Ru/TiO2. The difference in the photocatalytic activity is related to its physico-chemical properties.

An investigation of the acid properties of UL-ZSM-5 by FTIR of adsorbed 2,6-ditertbutylpyridine and aromatic transalkylation test reaction

The surface acidity of semicrystalline zeolitic mesoporous UL-ZSM-5 aluminosilicate materials of different degree of crystallinity (UL-zeolites were introduced recently in our laboratory [D. Trong On, S. Kaliaguine, Nanoporous Materials—Science and Engineering, Imperial College Press, 2005; D. Trong On, S. Kaliaguine, Angew. Chem. Int. Ed. 40 (2001) 3248; A. Ungureanu, D. Trong On, E. Dumitriu, S. Kaliaguine, Appl. Catal. A: Gen. 254 (2003) 203]) has been investigated by means of FTIR spectroscopy of adsorbed 2,6-ditertbutylpyridine (DTBPy) and the test reaction of transalkylation of toluene with 1,3,5-trimethylbenzene. FTIR experiments and catalytic tests allowed also characterizing the surface acidity of the starting amorphous mesostructured aluminosilicate precursor and the external acid sites of crystalline ZSM-5 zeolite. The results indicated that most of the Brönsted acidic OHs accessible to DTBPy molecules are terminal on the mesopore surfaces of both UL-ZSM-5 materials and mesostructured aluminosilicate and also on the external surface of ZSM-5. These sites, which are responsible for the O H stretching band near 3745 cm −1, are of medium strength. Their density correlated well with the external surface area. On the other hand, bridging Si OH Al sites generating O H stretching band at 3612 cm −1 and conferring the medium-strong Brönsted acidity of UL-ZSM-5 and ZSM-5 are located predominantly in the micropores of the zeolitic ZSM-5 phase and thus not accessible to bulky DTBPy. Just like the external surface of ZSM-5, the mesoporous surface of UL-ZSM-5 carries a small fraction of bridging Brönsted acid sites which are likely located in the pore mouth region of their ZSM-5 zeolitic phase. The catalytic activity of the distinct aluminosilicate materials under study as well as the selectivity towards the different reactions involved in the transalkylation process has been found to strongly depend on the nature, density, strength and accessibility of the characteristic Brönsted acid sites.

Ab initio calculation of the structural and electronic properties of the SiC (100) Surfaces

In this work, we have calculated ab initio the structural and electronic properties of both the C- and Si-terminated SiC (100) surfaces in the c(2×2) and (2×1) reconstruction patterns, respectively. Based on our results, we found that the Si-terminated surfaces is dominated by weak bonded Si-dimers, which is stabilized only at Si-rich conditions, leading to a (3×2) or more complex reconstruction patterns, as verified experimentally. Also, our results show that the C-terminated surfaces is characterized by strong triply-bonded C-dimers, in a c(2×2) reconstruction pattern, which consists of C2 pairs over Si bridge sites, which is consistent with the experimental results.

Organosilicon Dendritic Networks in Porous Ceramics for Water Purification

Triethoxysilyl-functionalized poly(propylene imine) (DAB32) dendrimer and poly(ethylene imine) (PEI5) hyperbranched polymer were conveniently prepared in chloroform and were subsequently allowed to impregnate porous ceramic filters. Following hydrolysis of triethoxysilyl moieties to Si−OH, polycondensation occurred affording networks containing Si−O−Si bridges with simultaneous formation of Si−O−M bridges resulting from the interaction of Si−OH with M−OH of the ceramic surface. In this manner, covalent binding of the organosilicon dendritic polymers is achieved at the ceramic surface. These porous ceramic filters, impregnated with organosilicon dendritic polymers, were employed for water purification. The concentration of polycyclic aromatic compounds in water was reduced to the level of a few ppb by continuous filtration of contaminated water through these filters. The filters loaded with pollutants were effectively regenerated by treatment with acetonitrile.

Microfabricated strained substrates for Ge epitaxial growth

The manipulation of strain in micromachined silicon structures presents an opportunity in the control of surface processes in epitaxial growth. With appropriate fabrication techniques, the magnitude, crystallographic direction, and symmetry of the strain at a Si surface can be precisely controlled with this strategy. Synchrotron x-ray microdiffraction techniques allow simultaneous independent measurements of the strain and bending in these structures and serve to calibrate the fabrication process. Bending is the dominant source of strain in a microfabricated Si bridge loaded at its ends by silicon nitride thin films that we have used as a strained substrate in studies of Ge epitaxial growth. The total strain difference between the top and bottom of the bent bridge exceeds 10−3 in present structures and can potentially be increased in optimized devices. These micromachined substrates complement other methods for producing strained silicon and silicon–germanium structures for improved electrical device performance and for fundamental studies of epitaxial growth.

Incorporation of Aluminium and Iron into the Zeolite MCM‐58

The hydrothermal synthesis of zeolite MCM-58 is investigated with N-benzylquinuclidinium bromide as a structure-directing agent in order to isomorphously substitute aluminium or iron for silicon. Al-MCM-58 was synthesised in a wide range of nSi/nAl ratios (from 19 to 56), and Fe-MCM-58 was successfully prepared in a narrower range of nSi/nFe ratios (from 18 to 36). The obtained products were characterised by XRD, SEM, 27Al MAS NMR, 29Si MAS NMR, FTIR, and ESR spectroscopy. Two different calcination procedures, viz. in a stream of nitrogen and air or in a stream of ammonia, were used in order to modify the acid sites in the zeolite. FTIR spectroscopy before and after the adsorption of [D3]acetonitrile and pyridine was employed to determine the concentration and type of the acid sites. The acid forms of Al(Fe)-MCM-58 are characterised by the vibrations of bridging Si–OH–Al(Fe) groups at 3628 cm–1 and 3564 cm–1 (Al-MCM-58) or 3646 cm–1 and 3520 cm–1 (Fe-MCM-58). The acid sites of both zeolites Al-MCM-58 and Fe-MCM-58 are accessible for [D3]acetonitrile and pyridine, and all materials contain substantial numbers of Lewis sites (Al-MCM-58: 50 % of the total acid sites; Fe-MCM-58: 90 %) over the whole range of nSi/nAl (nSi/nFe) ratios studied. Only a small increase in the concentration of Bronsted acid sites was achieved after calcination of the as-synthesised samples in a flow of ammonia. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Structure of multicomponent SiO 2–Al 2O 3–Fe 2O 3–CaO–MgO glasses for the preparation of fibrous insulating materials

Glasses used for the fabrication of fibrous insulating materials are made from natural raw materials, mainly basalts. Basalt alloys, however, show relatively high crystallization capability, which is disadvantageous for throwing and durability of the fibres prepared. In the work new raw materials, such as melaphyres and diabase from the Silesia region of Poland were applied for the preparation of aluminosilicate alloys. In order to study their crystallization ability the glasses were heated in an electric furnace at various temperatures for various time periods. Depending on the raw material used as well as the temperature of heat-treatment amorphous or crystalline materials were obtained. Crystalline phases were identified based on X-ray diffraction studies. It was found that magnetite/titanomagnetite crystallized in the first step. Then pyroxenes phases of diopsides or augite type appeared in the systems. Spectroscopic investigations in the mid IR region were carried out for all the glasses. This made it possible to determine the influence of thermal treatment on the structural changes of glasses (changes in the spectra shapes). Locations of the bands due to Al–O–Si and Si–O–Si bridges vibrations suggested that in most cases the augite-type phases were present in the systems (aluminium coordination number equal to 4 and 6). Appearance of aluminium in coordination 4 and 6 was confirmed by NMR investigations (two clear bands in the spectra).

Influence of the annealing temperature on the IR properties of SiO 2 films grown from SiH 4 + O 2

Dispersion analysis was performed on low pressure chemically vapor deposited (LPCVD) SiO 2 films grown from SiH 4 + O 2 at 425 °C. The transmission spectra were analyzed using four Lorentz oscillators within the range 900–1400 cm −1. It was found that the distribution of the Si O Si angles is a superposition of two Gaussians; one corresponding to bridges located in the bulk of the film and one corresponding to bridges located close to the boundaries of the film namely the interfaces of the films and the grain boundaries. The ratio between the bulk like Si O Si bridges over the boundary bridges was found equal to 0.61:1 indicating that films grown from SiH 4 + O 2 contain a higher number of boundary Si O Si bridges relative to those located in the bulk of the film. After annealing for 30 min at temperatures in the range from 550 to 950 °C, films were found to have a lower thickness. The calculated ratio of the two distributions after annealing have shown a clear reduction in the concentration of the boundary bridges as the temperature of annealing increases, in advance of the bridges located in the bulk of the film. For the film annealed at 950 °C for 30 min the ratio was found equal to 4.0:1 which is the same to that of thermally grown films at the same temperature.

Variable anchoring of boron in zeolite beta

The transformation of boron species in H–B-beta and H,Na–B-beta zeolites has been studied by 11B and 1H MAS NMR as well as 1H{ 11B} Rotational Echo Double Resonance in combination with infrared spectroscopy. Tetrahedral and trigonal boron are found in the hydrated zeolites. Only trigonal boron is observed in the pure proton form for the dehydrated zeolite. The degree of boron anchoring in the framework (number of B–O–Si bridges) increases with higher dehydration temperature. The 11B isotropic chemical shift of trigonal boron correlates with the number of B–O–Si connectivities, and can be utilized to identify B(OSi) x (OH) 3− x species with different degrees of framework anchoring, x: 10.5 ± 0.7 ppm ( x = 3), 15.4 ± 0.5 ppm ( x = 1), and 18.5 ± 0.6 ppm ( x = 0). The transformation of boron anchoring and coordination numbers is reversible, depending on the conditions (temperature, humidity). Five 1H NMR and four IR lines are resolved for H–B-beta and assigned to three different classes of OH groups: class A are defect silanol groups (1.7 ppm/3745 cm −1), class B are Si–OH⋯B [3] sites and weakly hydrogen bonded SiOH and BOH groups (2.0, 2.3 ppm/3735 cm −1), and class C groups are due to hydrogen bonded SiOH, BOH and possibly water (3.0, 4.3 ppm/∼3680, 3530 cm −1).

Characterization of the location and interfacial states of gallium in gallium/MCM-41 composites

A broad based approach, using nitrogen adsorption, X-ray diffraction, DSC, solid-state NMR and X-ray photoelectron spectroscopy, has been used to characterize a series of Ga/MCM-41 composite materials. Rather than filling the mesochannels of the MCM-41 it is found, through this combination of techniques, that most of the Ga is present in the spaces between the particles of MCM-41. High-angle XRD and 71Ga NMR indicate that at room temperature most of the Ga is in the liquid metal state. 29Si and 1H MAS NMR, and XPS reveal that at the Ga/MCM-41 interface the Ga reacts with silanol groups to form new Ga + states like Si O Ga, or bridging Si(OH)Ga hydroxyls.

Simulation of FTIR reflection spectra in non normal incidence

Reflection spectra in non normal incidence of SiO2 films consist of an extra band attributed to the Longitudinal Optical (LO) mode of vibration of the Si–O–Si bridge. In order to simulate spectra taken in non normal incidence we developed analytical expressions describing reflection in non normal incidence of an absorbent film placed on a non absorbent medium. Similar expressions found in the literature are not precise leading to unreliable results for the case of SiO2 films. The optical model is based on the matrix method considering the existence of four Lorentz oscillators same as for the case of the normal incidence. We found that the LO band is anticipated by the optical model without the need of adding extra oscillators for the description of this band. It was also found that the refractive index of the film is depending from the angle of incidence. Increase of the angle of incidence causes a small decrease of the real part of the refractive index in the range 1100–1300 cm−1 while the imaginary part of the refractive index slightly increases in the same range of wavenumbers.

Effects of Atmosphere on Viscosity of Selected Bituminous and Low-Rank Coal Ash Slags

The viscosities of nine coal ash slags were measured in air as a function of temperature. Selected slags were also tested in reducing atmosphere and in air with 10% water vapor. In air, the viscosities of the slags can be related to the mole ratio of basic to acidic oxides. An improved fit is obtained from the ratio of oxide ions contributing to the bases to the oxide contributing to the acids. The change in viscosity observed in using hydrogen instead of air appears to be related to the amount of iron originally in the slag. When water vapor is present in air, the viscosity of a high-silica glassy slag decreased. The viscosities of three other slags, with lower silica content, increased slightly, with the increase in potassium content. In the former case, water likely enters the slag and disrupts Si−O−Si bridges. In the latter case, water may form metal−hydroxide complexes with the alkali cations such as potassium. A second effect of water vapor is to reduce the temperature at which the slag begins to crystallize (the temperature of critical viscosity). These effects of water on coal ash slag viscosity are similar to those on magmas, even if with lower magnitude.

Dissociative adsorption of water in CaNaA zeolites studied by TG, DRIFTS and 1H and 27Al MAS NMR spectroscopy

Three CaNaA zeolites, in which, respectively, 49%, 65% and 83% of the Na + cations were exchanged with Ca 2+ ions, along with a NaA sample were studied by thermogravimetry, temperature-programmed DRIFT spectroscopy and MAS NMR. Thermogravimetric desorption profiles of the Ca 2+ exchanged zeolites show a low-temperature peak between 423 and 508 K as well as two high-temperatures peaks at 853 and 973 K, which are missing in case of the NaA sample. Three main bands at 3603–3611, 3495–3510 and 3552–3565 cm −1, of which the first two are due to bridging Si–OH–Al groups located in 8- and 6-rings, respectively, whereas the third corresponds to Ca(OH) + groups dominate the DRIFT spectra of zeolite samples exchanged with Ca 2+ ions. The intensity of the band at 3603–3611 cm −1 decreases significantly until it vanishes almost completely at 823 K. A simultaneous decreases of the intensity of the band of Ca(OH) + groups is indicative of a low-temperature dehydroxylation process between these two species. The intensity of the other Si–OH–Al group bands at 3495–3510 cm −1 shows almost no change up to 823 K, thus suggesting that these groups participate in a high-temperature dehydroxylation process. A weak band in the region 3670–3680 cm −1 is attributed to Al–OH groups of extra-framework aluminum species formed during dehydration of the CaNaA zeolites. Peaks at 4.3 and 5.2 ppm in the 1H MAS NMR spectra are assigned to Si–OH–Al groups in 8- and 6-rings, respectively. 27Al MAS NMR spectra show two main peaks at 57.3 and 78.7 ppm, which are due to tetrahedrally coordinated aluminum and calcium alumosilicate groups. A weak signal between 12 and 13 ppm ascribed to octahedrally coordinated aluminum disappears at higher temperatures.

Structural transitions in silica glass: thermo-mechanical anomalies and polyamorphism

Using molecular dynamics computer simulations, we have studied the nature of structural transitions in crystalline and amorphous silica. Atomic interactions were modeled using a charge-transfer three-body potential that was developed to describe the charge redistribution upon breaking and forming of bonds, especially in systems that exhibit mixed covalent-ionic bonding. Two types of transitions, namely reversible and irreversible, occur in silica glass. The reversible transition is based on the same mechanism as in the α- to β-cristobalite phase transformations in crystalline silica, i.e., the abrupt rotation of Si–O–Si bridges around the Si–Si axis. Unlike in the crystal, in the glass, these rotations occur sporadically and remain localized, spreading over a range of temperature or pressure. Thus the transition is gradual. We show that this mechanism is responsible for the anomalous thermo-mechanical behaviors of silica glass, e.g., stiffening upon heating and softening upon compression. The irreversible transition is based on bond exchanges. In particular, smaller rings in the structure dissolve in favor of larger ones as the density of the structure increases. This mechanism causes permanent densification of silica glass and constitutes the basis for polyamorphism in this system.

Film strain and compositional changes in thermally nitrided silicon dioxide thin films

The nature of the Si–O bonds, composition, and film strain changes in thermally nitrided silicon dioxides in NH 3 at 1000 °C and 1 atm is investigated with Fourier transform infrared spectroscopy and spectroscopic ellipsometry. Shifts of the transverse optical and longitudinal optical phonons of O in Si–O–Si bridging bonds are the results of both compositional and strain within the silicon dioxide matrix. Red shifts during the initial nitridation period are followed by blue shifts at longer nitridation times. Red shifts can be explained by nitrogen incorporation and strain accumulation, while bulk film strain relaxation in nitrided oxides is most likely responsible for the blue shifts.