Amorphous SiO2 Matrix Research Articles

UV resonance Raman spectroscopic insight into titanium species and structure-performance relationship in boron-free Ti-MWW zeolite

The titanium species and structure-performance relationship in boron-free Ti-MWW zeolite have been thoroughly studied by 244nm excited UV resonance Raman spectroscopy for the first time. It was surprisingly found out that acid treatment not only removed a part of the dominant non-framework TiOm (m=5, 6) species but also made most of them transformed into TiO4 species, which has never been reported before. Interestingly, acid treatment of as-synthesized and directly calcined Ti-MWW resulted in the formation of active framework TiO4 species and inactive TiO4 species in amorphous SiO2 matrix, respectively. Only the samples obtained from acid treatment of as-synthesized Ti-MWW and subsequent calcination showed catalytic activity in 1-hexene epoxidation. Importantly, despite higher Si/Ti ratio, framework Ti(OSi)4 species-dominant Ti-MWW showed better catalytic performance than the relatively hydrophilic framework Ti(OSi)3OH species-dominant one. These understandings would be beneficial for further study of Ti-MWW and development of highly efficient titanosilicate zeolites.

Atomistic simulation of the thermal conductivity in amorphous SiO2 matrix/Ge nanocrystal composites

We use nonequilibrium molecular dynamics computer simulations with the Tersoff potential aiming to provide a comprehensive picture of the thermal conductivity of amorphous SiO2 (a–SiO2) matrix with embedded Ge nanocrystals (nc–Ge). The modelling predicts the a–SiO2 matrix thermal conductivity in a temperature range of 50<T<500 K yielding a fair agreement with experiment at around room temperature. It is worth noticing that the predicted room-temperature thermal conductivity in a–SiO2 is in very good agreement with the experimental result, which is in marked contrast with the thermal conductivity calculated employing the widely used van Beest-Kramer-van Santen (BKS) potential. We show that the thermal conductivity of composite nc–Ge/a–SiO2 systems decreases steadily with increasing the volume fraction of Ge inclusions, indicative of enhanced interface scattering of phonons imposed by embedded Ge nanocrystals. We also observe that increasing the volume fractions above a certain threshold value results in a progressively increased thermal conductivity of the nanocomposite, which can be explained by increasing volume fraction of a better thermally conducting Ge. Finally, non-equilibrium molecular dynamics simulations with the Tersoff potential are promising for computing the thermal conductivity of nanocomposites based on amorphous SiO2 and can be readily scaled to more complex composite structures with embedded nanoparticles, which thus help design nanocomposites with desired thermal properties.

Influence of stress on the properties of Ge nanocrystals in an SiO2 matrix

In this work, self-assembled Ge quantum dot (QD) formation in a dielectric matrix is explored. Of particular interest were their structural and optical properties, in order to understand the stress build-up in such a process and its impact on the material properties during processing. To this end, thin films consisting of (Ge + SiO2)/SiO2 multilayers grown by RF magnetron sputtering were deposited at room temperature. Annealing of such films at 873 K in inert N2 atmosphere produced, at the position of the Ge-rich SiO2 layers, a high lateral density (about 1012 cm−2) of Ge QDs with a good crystallinity. SiO2 spacer layers separated the adjacent Ge-rich layers, where the Ge QDs were formed with a diameter of about the size of the (Ge + SiO2) as-deposited layer thickness, and created a good vertical repeatability, confirmed by the appearance of a Bragg sheet in two-dimensional small-angle X-ray scattering patterns. The structural analysis, by wide-angle X-ray diffraction, grazing-incidence small-angle X-ray scattering and transmission electron microscopy, has shown that the described processing of the films induced large compressive stress on the formed QDs. Optical analysis by time-resolved photoluminescence (PL) revealed that the high density of crystalline Ge QDs embedded in the amorphous SiO2 matrix produced a strong luminescence in the visible part of the spectrum at 2–2.5 eV photon energy. It is shown that the decay dynamics in this energy range are very fast, and therefore the transitions that create such PL are attributed to matrix defects present in the shell surrounding the Ge QD surface (interface region with the matrix). The measured PL peak, though wide at its half-width, when analysed in consecutive short spectral segments showed the same decay dynamics, suggesting the same mechanism of relaxation.

Understanding particle size and distance driven competition of interparticle interactions and effective single-particle anisotropy

Magnetic response of single-domain nanoparticles (NPs) in concentrated systems is strongly affected by mutual interparticle interactions. However, particle proximity significantly influences single-particle effective anisotropy. To solve which of these two phenomena plays a dominant role in the magnetic response of real NP systems, systematic study on samples with well-defined parameters is required. In our work, we prepared a series of nanocomposites constituted of highly-crystalline and well-isolated CoFe2O4 NPs embedded in an amorphous SiO2 matrix using a single-molecule precursor method. This preparation method enabled us to reach a wide interval of particle size and concentration. We observed that the characteristic parameters of the single-domain state (coercivity, blocking temperature) and dipole–dipole interaction energy () scaled with each other and increased with increasing , where dXRD was the NP diameter and r was the interparticle distance. Our results are in excellent agreement with Monte-Carlo simulations of the particle growth. Moreover, we demonstrated that the contribution of acting as an additional energetic barrier to the superspin reversal or as an average static field did not sufficiently explain how the concentrated NP systems responded to an external magnetic field. Alternations in the blocking temperature and coercivity of our NP systems accounted for reformed relaxations of the NP superspins and modified effective anisotropy energy of the interacting NPs. Therefore, the concept of modified NP effective anisotropy explains the magnetic response of our concentrated NP systems better than the concept of the energy barrier influenced by interparticle interactions.

Optical and Electrochemical Properties of Co&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;/SiO&lt;sub&gt;2&lt;/sub&gt; Nanocomposite

Co3O4/SiO2 nanocomposite was obtained via citrate-gel method. 22 nm in size Co3O4 particles in an amorphous SiO2 matrix were obtained. Co3O4:SiO2 wt. ratio was verified using EDX and found to be close to the 80 % nominal ratio. The UV-Vis measurements showed broad absorption bands at around 440 and 790 nm. The optical band gap values were 1.95 and 1.35 eV respectively, being close to those observed for pure Co3O4 nanoparticles. The electrochemical measurements in 5 M KOH solution were performed using a three-electrode type system. Co3O4/SiO2 nanocomposite exhibited high specific capacitance reaching 758 F g-1 at 2.5 mV s-1. Very small solution and electrode-electrolyte interfacial charge transfer resistances were obtained when impedance spectra were analysed.

Structure and optical transmission spectra of ZnS–SiO2 nanocomposite films deposited at low temperatures

The results of studying the growth of a ZnS–SiO2 nanocomposite film by discrete thermal evaporation at lowered condensation temperatures using an ultrahigh-vacuum setup are presented. It is shown that ZnS–SiO2 nanocomposite films contain an amorphous SiO2 matrix and zinc-sulfide ZnS nanocrystals. The nanocrystallite shape and sizes depend on the condensation temperature.

Controllable synthesis, growth mechanism, structure and optical properties of ZnO–SiO2 nanocomposites

ZnO quantum dots (QDs) have been successfully encapsulated in the amorphous SiO2 matrix by a modified Stober method. The effects of the sequence of adding the ZnO QDs and tetraethyl orthosilicate (TEOS) and the hydrolysis time of TEOS on the structure, ordered and optical properties of ZnO–SiO2 nanocomposites were investigated in detail. When the TEOS was added to the solution of ZnO QDs, the QDs were disorderly dispersed within the amorphous SiO2 matrix. On the other hand, as adding the ZnO QDs to the already hydrolyzed TEOS solution, the self-assembly of ZnO–SiO2 nanocomposites was observed through a general route of zero-dimensional (0D) → 1D → 3D with the hydrolysis time of TEOS increased. The possible formation mechanisms were schematically illustrated. PL results showed that the PL peak intensity and the position of UV emission were sensitive to the hydrolysis time of TEOS, the luminescence intensity of the UV emission for the ZnO–SiO2 nanocomposites reached the highest value when t = 6 h. These ZnO–SiO2 nanocomposites would show a wider application for its particular structure, stability and enhanced emission, such as the conjugation of the nanocrystals to biological entities after functionalization.

Synthesis of Amorphous Fe-Doped SiO&lt;sub&gt;2&lt;/sub&gt; Anode Nanomaterial via Sol-Gel Method

Silicon-based anode is one of the most promising anode materials for next generation batteries due to its high theoretical capacity of about 4200 mAh/g. However, the hurdle of using such high capacity anode material is its large volumetric change during lithiation and delithiation causing capacity fading. In this study, in order to circumvent the large volumetric change, nanograined size SiO2 incorporated with Fe having compositions of FexSi1-xO2 (x=0.05, 0.10) have been synthesized using a modified sol-gel processing and fully characterized for its structure, morphology, and thermal properties. From the different characterization results, the samples synthesized at low processing temperatures (400 and 600 °C) suggest an amorphous structure with grain size of about 5 nm and Fe successfully incorporated into the amorphous nanograined SiO2 matrix.

High-speed deposition of tetragonal-ZrO2-dispersed SiO2 nanocomposite films by laser chemical vapor deposition

Tetragonal-ZrO2 (t-ZrO2)-dispersed amorphous SiO2 composite films were prepared by laser chemical vapor deposition at high deposition rates of 40–300μmh−1. Without SiO2 the films prepared at deposition temperatures of 869–1246K contained monoclinic ZrO2. ZrO2 was stabilized in its tetragonal phase in an amorphous SiO2 matrix at deposition temperatures of 867–1170K. The t-ZrO2 particle size decreased from 21 to 11nm with increasing deposition temperature. In the amorphous SiO2 matrix, t-ZrO2 nanoparticles formed a network structure at lower deposition temperatures, whereas the t-ZrO2 nanoparticles were homogeneously dispersed at higher deposition temperatures.

Hydrothermal synthesis and characterization of Co2.85Si0.15O4 solid solutions and its carbon composite as negative electrodes for Li-ion batteries

Co2.85Si0.15O4 solid solution was successfully synthesized using a facile hydrothermal method for the first time. The structural and morphological features of prepared powders were thoroughly investigated by different techniques. The Rietveld refinement assured the formation of spinel structured Co2.85Si0.15O4 without any impurity phases. The FT-IR and Raman spectrums revealed the presence of SiO2, and carbon with the Co2.85Si0.15O4 solid solution. The X-ray photoelectron spectroscopy inferred that the Co exists in +2 and +3 oxidation state and Si exists in multivalence state. Surface morphological analysis demonstrated that the formation of cube shape Co2.85Si0.15O4 microparticles are embedded in to amorphous SiO2 matrix, which was confirmed using SAED pattern. The inactive nature of amorphous SiO2 in Co2.85Si0.15O4 was confirmed by CV studies. Consequently, it was electrochemically active while making composite with carbon, since it reduces SiO2 into SiOx. The cycling stability of Co2.85Si0.15O4@C composite provided superior electrochemical performance than the pristine Co2.85Si0.15O4. The composite delivers the specific capacity of 444mAhg−1 at 75mAg−1 after 50 cycles with feeble capacity fading. The EIS spectra corroborates the composite exhibit the lower charge transfer resistance (Rct) and solid electrolyte interphase resistance (RSEI) compared to pristine Co2.85Si0.15O4. This further confirmed that carbon composite enhanced the inherent conductive nature and rate capability of pristine Co2.85Si0.15O4 electrode.

Strong quantum confinement effect and reduced Fröhlich exciton-phonon coupling in ZnO quantum dots embedded inside a SiO2 matrix.

ZnO quantum dots (QDs) embedded in an amorphous SiO2 matrix were examined in depth by using variable-temperature photoluminescence (PL) and optical reflectance spectroscopies. Compared with ZnO bulk crystals, ZnO quantum dots with an average size of 4 nm exhibit a strong quantum confinement effect, evidenced by a large blue shift in both PL and reflectance peaks of excitons. More interestingly, a remarkably reduced long-range Fröhlich interaction was revealed in ZnO QDs. These fascinating effects may make ZnO QDs a very appealing system in the fields of optoelectronics and others.

Pure dipolar-interacted CoFe2O4 nanoparticles and their magnetic properties

The mono-dispersed and uniform CoFe2O4 nanoparticles were synthesized by the thermal decomposition of Fe(acac)3 and Co(acac)2. Then the CoFe2O4 nanoparticles were diluted in amorphous SiO2 matrix with different CoFe2O4 nanoparticles’ concentrations. All samples show the positive or negative exchange bias behavior, indicating the presence of canted spin layer at the CoFe2O4 nanoparticles’ surface. The large effective anisotropy constant (3.38×106erg/cm3) was observed, which can be attributed to the induced surface anisotropy by the canted surface spins. The reduced magnetization (Mr/Ms) was dominated by the interparticle dipolar interaction while the coercivity (Hc) was determined by the synergistic effects of the surface anisotropy, interparticle dipolar interaction and interface effect. By suitably diluting CoFe2O4 in the SiO2 matrix, the high Hc (3056Oe) and the Mr/Ms (0.63) can be obtained, which is larger than most of those reported before. The present work is meaningful for revealing the underlying mechanism in nano-scaled magnetic system and improving the magnetic performance.

High thermal annealing effect on structural and optical properties of ZnO–SiO2 nanocomposite

The effect of annealing temperature on photoluminescence (PL) of ZnO–SiO2 nanocomposite was investigated. The ZnO–SiO2 nanocomposite was annealed at different temperatures from 600°C to 1000°C with a step of 100°C. High Resolution Transmission Electron Microscope (HR-TEM) pictures showed ZnO nanoparticles of 5nm are capped with amorphous SiO2 matrix. Field Emission Scanning Electron Microscope (FE-SEM) pictures showed that samples exhibit spherical morphology up to 800°C and dumbbell morphology above 800°C. The absorption spectrum of ZnO–SiO2 nanocomposite suffers a blue-shift from 369nm to 365nm with increase of temperature from 800°C to 1000°C. The PL spectrum of ZnO–SiO2 nanocomposite exhibited an UV emission positioned at 396nm. The UV emission intensity increased as the temperature increased from 600°C to 700°C and then decreased for samples annealed at and above 800°C. The XRD results showed that formation of willemite phase starts at 800°C and pure willemite phase formed at 1000°C. The decrease of the intensity of 396nm emission peak at 900°C and 1000°C is due to the collapse of the ZnO hexagonal structure. This is due to the dominant diffusion of Zn into SiO2 at these temperatures. At 1000°C, an emission peak at 388nm is observed in addition to UV emission of ZnO at 396nm and is believed to be originated from the willemite.

Enhancement of adsorption efficiency of methylene blue on Co3O4/SiO2 nanocomposite

Single and well-crystalline Co3O4 phase imbedded in an amorphous SiO2 matrix has been obtained by novel aqueous solution method. The structural and morphological properties are investigated using X-ray diffraction, Fourier transform infrared spectrometer, and N2 adsorption–desorption techniques. The apparent crystallite size for Co3O4 was found to be about 13.5 nm, which elucidates the rule of poly ethylene glycol in preventing particle’s agglomeration; moreover, the pours structure of the composite enhances its adsorption ability. Co3O4/SiO2 has a high ability to absorb methylene blue from an aqueous solution. The removal percent of Methelene blue (MB) by Co3O4/SiO2 has reached 95.7%. The effect of various experimental parameters, such as initial dye concentration, contact time, and dose were investigated. Co3O4/SiO2 nanocomposite shows high adsorption capacity of 53.87 mg g−1, which is larger than the adsorption capacity of MB on other materials. Both of Langmuir and Freundlich models were used to analyze the equilibrium adsorption data. The pseudo-second-order model was found to be the most appropriate model to represent the present data. Co3O4/SiO2 nanocomposite material is proposed as a potential adsorbent for water treatment.

Structural, optical and electrical properties of sol–gel prepared mesoporous Co3O4/SiO2 nanocomposites

Abstract Structures, optical and electrical properties of Co3O4/SiO2 nanocomposites are reported. Well crystalline Co3O4 nanoparticles embedded in an amorphous SiO2 matrix is formed, and confirmed by XRD and FTIR measurements upon calcination of gel precursors up to 800 °C. The obtained nanocomposites have high surface area ∼126–312 m2 g−1, and the Co3O4 particle size was ∼7–15 nm. The optical properties of the Co3O4/SiO2 nanocomposites indicate the presence of two energy gaps; both of them are smaller than those reported for the Co3O4 bulk phase. The first is varied from 1.32 to 1.44 eV and the second one is varied from 1.76 to 1.87 eV depending on the particles size. DC conductivity was measured in the temperatures range 300–673 K. The activation energy for DC conduction varies with particle size. The conduction mechanism was suggested to be through small polarons and variable range hopping mechanisms, at high and low temperatures respectively.

Off-Stoichiometry Spectroscopic Investigations of Pure Amorphous Silica and N-Doped Silica Thin Films

Cathodoluminescence spectroscopy and X-ray photoelectron spectroscopy were concurrently used to investigate the local physicochemical nature of the amorphous lattice in pure SiO2 and N-doped SiO2 thin films prepared by radiofrequency magnetron sputtering (the latter samples deposited under a set of different conditions of N2 partial pressure). The main aim of this investigation was twofold: (i) to extend our knowledge of the physical and chemical structure of SiO2 films and (ii) to explore our capacity of manipulating, fine tuning, and measuring their stoichiometry characteristics. The presence of nitrogen atoms in the amorphous host structure was confirmed to significantly affect the formation of oxygen-deficient centers, nonbridging oxygen hole centers, and other kinds of defect complexes. The main challenge here was to relate the variations in type and concentration of these peculiar defects to the processing conditions and to the amount of nitrogen incorporated in the SiO2 amorphous matrix. The evolut...

Structure and dynamics of guest molecules confined in a mesoporous silica matrix: Complementary NMR and PDF characterisation

We present an experimental approach to study the structure and dynamics of molecular functional complexes in porous host matrices. Combining the results of Solid-State NMR and pair distribution function analysis based on total X-ray scattering data the structural arrangement and dynamical behaviour of Na2[Fe(CN)5NO]·2H2O (SNP) embedded in amorphous SiO2 matrix is investigated. We show that the SNP complexes are embedded as isolated complexes within the SiO2 pores and propose a structural model for the cation and anion arrangement. Additionally the NMR results demonstrate the rotational dynamics of the SNP complexes.

Sol-gel as a method to tailor the magnetic properties of Co1+yAl2-yO4

The magnetic properties of mesoscopic materials are modified by size and surface effects. We present a sol-gel method used to tailor these effects, and illustrate it on Co1+yAl2-yO4 spinel. Nanocomposites made of spinel oxide Co1+yAl2-yO4 particles dispersed in an amorphous SiO2 matrix were synthesized. Samples with various mass fractions -x of Co1+yAl2-yO4 in composite, ranging from predominantly SiO2 (x = 10 wt%) to predominantly spinel (x = 95 wt%), and with various Co concentrations in spinel y were studied. The spinel grain sizes were below 100 nm with a large size distribution, for samples with predominant spinel phase. Those samples showed Curie-Weiss paramagnetic behavior with antiferromagnetically interacting Co ions (? ? -100 K). The grain sizes of spinel stays confined in 100 nm range even in the spinel samples diluted with as low as 5 wt% concentration of amorphous SiO2. For the samples with predominant SiO2 the crystalline nanoparticles are well separated and of size of around 100 nm, but with presence of much smaller spinel nanoparticles of about 10 nm. The magnetic properties of the samples with predominant silica phase showed complex behavior, spin-glass magnetic freezing at the lowest temperatures and lower absolute value of ? and consequently lower exchange constant.

Structure and electrical transport in films of Ge nanoparticles embedded in SiO2 matrix

The films containing Ge nanoparticles embedded in SiO2 matrix were prepared by RF magnetron sputtering and subsequently by thermal annealing. Their structure was investigated by conventional transmission electron microscopy and high resolution transmission electron microscopy together with energy-dispersive X-ray spectroscopy. The electrical behavior of films was studied by measuring current–temperature and current–voltage characteristics. The structure investigation reveals two kinds of features: a low density of big Ge nanoparticles with sizes from 20 to 50 nm and a network of small amorphous Ge nanoregions/nanoparticles (5 nm size or less) with high density, both being embedded in amorphous SiO2 matrix. The electrical transport was shown to take place through the network of amorphous Ge nanoregions. At low temperature, the T−1/4 dependence of the current was evidenced, while at high temperature, the T−1 Arrhenius dependence was found. At both low and high temperatures, the conductivity is nearly constant. The behavior at low temperature was explained by the hopping mechanism on localized states located in a band near the Fermi energy, while at high temperature by the charge excitation to the extended states.

Optical and photocatalytic properties of TiO2/Ag–SiO2 nanocomposite thin films

The TiO2/Agx–(SiO2)1−x nanocomposite thin films with a bilayer structure were prepared via a chemical solution approach combined with a sol–gel spin-coating processing. The TiO2/Ag0.15–(SiO2)0.85 thin film is about 200nm in thickness including a top anatase–TiO2 layer approximately 60nm, while the approximately spherical Ag nanoparticles with 10–30nm in diameter were embedded uniformly in the amorphous SiO2 matrix of the sublayer. The UV–Vis absorption peaks appeared in the wavelength of 420nm due to the surface plasmon resonance (SPR) of Ag particles. The SPR peaks manifest a red-shift with increasing Ag content from 6 to 15at.%, whereas blue-shift from 15 to 20at.%. The photocatalytic activity enhanced by increasing Ag content up to 15at.% but reduced by further increasing Ag content. It is found that the optimal Ag loading for achieving the highest photocatalytic activity is 15at.%.