Structural studies of nanocrystalline oxides

We report the results of structural studies of samples of nanocrystalline tin oxide, zirconia, magnesia, alumina and silica, prepared by sol–gel techniques (including the addition of silica to restrict grain growth) and high-energy ball milling. XRD, EXAFS/XANES and MAS-NMR analyses were used to characterise the materials. EXAFS showed that nanocrystals of ZrO₂ and SnO₂ prepared by sol–gel methods are highly crystalline, consistent with previous EXAFS studies of sol–gel prepared nanocrystalline oxides [A.V. Chadwick, M.J. Pooley, K.E. Rammutla, S.L.P. Savin, A. Rougier, J. Phys.: Condens. Matter, 15 (2003) 431.; A.V. Chadwick, G.E. Rush, in Nanocrystalline Metals and Oxides: Selected Properties and Applications, (Kluwer, New York), (2002), p.133.]. In contrast, the sol–gel prepared SiO₂ appeared amorphous, even after prolonged heating at 1200 °C. The EXAFS of the ball-milled samples showed clearly attenuated signals that cannot be attributed solely to particle size. Ball milling of quartz crystals (which have a simple Q₄) first broadens the NMR line and then generates Q₃ and then Q₂ lines indicating the generation of amorphous materials in the samples.     

Rare-earth doped (Lu0.85Y0.15)2SiO5 nanocrystalline powders obtained by polymer assisted sol–gel synthesis

In the present work we explored the possibility of obtaining nanocrystalline (Lu_0.85Y_0.15)₂SiO₅ (LYSO) powders using polymer assisted sol–gel method. The synthesis started from TEOS as alkoxide precursor while polyethylene glycol with average molecular weight of 4000 was used as fuel. The resulting powders were obtained by firing gels in two ways: in conventional furnace and in microwave oven, with further annealing. This modified sol–gel synthetic route enabled production of pure phase LYSO powders at much lower temperatures (1050 °C) compared to classical, -solid-state methods (1400 °C). Crystallization kinetics are examined using differential thermal analysis, and rather low values of crystallization activation energies (around 12 kJ/mol) were found, revealing good potential of this method for low-temperature production of LYSO powder.     

Effect of nano-sized SiO2 on the optical properties of YVO4:Eu3+ phosphors

In this paper, YVO₄: 1%Eu³⁺ phosphor was synthesized via solid state method at 1100 °C. Then TEOS was used as the source of silica, to coat the phosphors, using sol–gel approach. HRTEM analysis confirmed the formation of adhered and smooth coating layer with the thickness of 40–50 nm. From the experiments and characterizations, we found that although the amounts of added SiO₂ to the phosphors were not remarkable, but it resulted in enhancement of photoluminescence properties. Interestingly, under the excitation wavelength of 310 nm, the efficiency of the phosphors increased by about 20%. Also, a considerable effect of coating layer on decrease in surface oxygen vacancies was studied using ESR technique. Finally it was found that SiO₂ coating of YVO₄:Eu³⁺ phosphors, improves both chemical stability and thermal quenching, effectively.     

Preparation of proton conductive composites with CsHSO4/CsH2PO4 and phosphosilicate gel

New composite materials were prepared using cesium hydrogen sulfate (CsHSO₄) or cesium dihydrogen phosphate (CsH₂PO₄) and phosphosilicate gel (P₂O₅–SiO₂ gel). In X-ray diffraction patterns of these composites, diffraction peaks due to Cs₂H₅(SO₄)₂(PO₄) and CsH₅(PO₄)₂ were observed for CsHSO₄–(P₂O₅–SiO₂ gel) composites and CsH₂PO₄–(P₂O₅–SiO₂ gel) composites, respectively. These composites showed high conductivities in the order of 10^− 3 S cm^− 1 at 150 °C due to melting of Cs₂H₅(SO₄)₂(PO₄) or CsH₅(PO₄)₂ in the composites. In the cooling process, the CsHSO₄–(P₂O₅–SiO₂ gel) composites kept relatively high conductivity to 110 °C where solidification of Cs₂H₅(SO₄)₂(PO₄) occurs, whereas CsH₂PO₄–(P₂O₅–SiO₂ gel) composites showed relatively high conductivity continuously to ambient temperature.     

Proton conductivity in sol–gel-derived P2O5–TiO2–SiO2 glasses

The sol–gel method was applied to prepare the P₂O₅–TiO₂–SiO₂ glasses with high proton conductivities and chemical stability. The glasses were prepared by the reaction of the Ti- and Si-alkoxides with PO(OCH₃)₃ or H₃PO₄, followed by heating at 600 °C. The obtained glasses were porous, the average pore diameter of which was <2 and 4 nm for glasses prepared using PO(OCH₃)₃ and H₃PO₄, respectively. The phosphorous ions, occurring as PO(OH)₃ in the TiO₂-free glass, were polymerized with one or two bridging-oxygen ions per PO₄ unit with the increasing TiO₂ content. Despite the P₂O₅–SiO₂ binary glasses exhibiting high conductivities of ∼10⁻² S/cm at room temperature, they also dissolved after immersing for 24 h in water. The chemical stability of these glasses increased significantly on the addition of TiO₂.     

Surface coating of LiMn2O4 spinel via in situ hydrolysis route: effect of the solution

Thin SiO₂ shell consisting of nano-SiO₂ particles can be deposited on to LiMn₂O₄ through the facile hydrolysis of tetraethoxysilane (TEOS) as a solution in either methanol or ethanol. The structure and surface morphologies of the modified and pristine materials were characterized by means of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The choice of solvent (methanol vs. ethanol) for TEOS hydrolysis has a profound effect on the physical and electrochemical properties of the resultant SiO₂-coated LiMn₂O₄. Coated LiMn₂O₄ demonstrated an improved cycling ability compared to the uncoated counterpart. Moreover, samples coated using a TEOS–methanol solution showed better cycling ability in extended potential windows and at an elevated temperature than that coated using ethanol.     

Effect of the synthesis route on the structural properties and shape of the indium oxide (In2O3) nano-particles

Nano-crystalline indium oxide (In₂O₃) particles have been synthesized by sol–gel and hydro-thermal techniques. A simple hydro-alcoholic solution consisting indium nitrate hydrate and citric acid (in sol–gel method) and 1, 4-butandiol (in hydro-thermal method) have been utilized. The structural properties of indium oxide nano-powders annealed at 450 °C (for both methods) have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and specific surface area (SSA) analysis. Structural analysis of the samples shows cubic phase in sol–gel and cubic-hexagonal phase mixture in hydro-thermally prepared particles. The nano-particles prepared by sol–gel method have nearly spherical shape, whereas hydro-thermally-made ones display wire- and needle-like shape in addition to the spherical shape. The obtained In₂O₃ nano-particles surface areas were 23.2 and 55.3 in sol–gel and hydro-thermal methods, respectively. The optical direct band gap of In₂O₃ nano-particles were determined to be ∼4.32 and ∼4.24 eV for sol–gel and hydro-thermal methods, respectively. These values exhibit ∼0.5 eV blue shift from that the bulk In₂O₃ (3.75 eV), which is related to the particle size reduction and approaching the quantum confinement limit of nano-particles.     

Study of the synthesis of nanocrystalline mixed tantalum–zirconium carbide

The synthesis conditions of refractory tantalum–zirconium carbide Ta_0.8Zr_0.2C on the basis of Ta₂O₅–ZrO₂–C ultrafine initial blend prepared via the sol–gel method are explored. The initial blend is prepared via hydrolysis in the presence of Ta(OC₅H₁₁)₅ and [Zr(O₂C₅H₇)_4–x (OC₅H₁₁) _x ] carbon source polymer solutions, gel drying, and carbonization at a temperature of 450°C. A series of the carbothermal synthesis experiments is implemented at various temperatures and exposure times. The synthesis conditions are shown to affect not only the phase composition of products but also their oxidation resistance related to the particle size.     

Production and characterization of spodumene dosimetric pellets by prepared by pechini and proteic sol–gel route

Spodumene is an aluminosilicate that has proven suitable for high-dose TL dosimetry of beta or gamma rays. Due to the presence of lithium in its chemical composition (LiAlSi₂O₆ – β-LAS), it has potential as neutron dosimeter as well. This silicate may be obtained naturally or synthetically. The synthetic LAS has been produced by solid state reaction and conventional sol–gel, whose difficulty arises from the need to employ high temperatures and high cost reagents, respectively. Alternative routes like Pechini and proteic sol–gel methods are promising, because they can reduce production costs and the possibility of environmental pollution. This work aimed at producing spodumene with the proteic sol–gel method using edible unflavored gelatin as a precursor and also with the Pechini method. The products were characterized physically and morphologically, and their applicability as TL dosimeter was investigated, comparing the sensitivity of samples produced by different methods. Two sets of samples were produced using different sources of silicon, tetraethyl orthosilicate (TEOS, Si(C₂H₅O)₄) and silica (SiO₂). The materials produced were characterized by X-ray diffraction and by thermal analysis in order to evaluate their structural properties, as well as possible temperature-dependent changes in physical or chemical properties. The syntherized pellets produced with these crystals were irradiated with a ⁹⁰Sr–⁹⁰Y source and their TL glow curves were evaluated. The production of β-LAS was successful by both methods, either using silica or TEOS as a silicon source. The crystals were obtained using much lower temperatures than by methods described in literature. We observed that the method of powder production was critical to develop a radiation detector: the best TL material was the powder produced using silica and the Pechini Method.     

Spectroscopic properties of Co2+: ZnAl2O4 nanocrystals in sol–gel derived glass–ceramics

Transparent glass–ceramics containing zinc–aluminum spinel (ZnAl₂O₄) nanocrystals doped with tetrahedrally coordinated Co²⁺ ions were obtained by the sol–gel method for the first time. The gels of composition SiO₂–Al₂O₃–ZnO–CoO were prepared at room temperature and heat-treated at temperature ranging 800–950 °C. When the gel samples were heated up to 900 °C, ZnAl₂O₄ nanocrystals were precipitated. Co²⁺ ions were located in tetrahedral sites in ZnAl₂O₄ nanocrystals. X-ray diffraction analysis shows that the crystallite sizes of ZnAl₂O₄ crystal become large with the heat-treatment temperature and time, and the crystallite diameter is in the range of 10–15 nm. The dependence of the absorption and emission spectra of the samples on heat-treatment temperature were presented. The difference in the luminescence between Co²⁺ doped glass–ceramic and Co²⁺ doped bulk crystal was analysed. The crystal field parameter D_q of 423 cm⁻¹ and the Racah parameters B of 773 cm⁻¹ and C of 3478.5 cm⁻¹ were calculated for tetrahedral Co²⁺ ions.     

Controllable synthesis of spherical silicon and its performance as an anode for lithium-ion batteries

Spherical silicon is controllably synthesized by the hydrolysis of tetraethylorthosilicate (TEOS) with the addition of different contents of ammonia to form SiO₂, then reduced by magnesium powder in argon atmosphere at 900 °C for 3 h. The experimental results show that the electrochemical performance of the as-prepared silicon anode is much affected by the morphology of silicon, and the spherical silicon with a particle size of 250–300 nm shows a reversible capacity of 1,345.8 mAh g^?1 with the capacity retention of 83.2 % after 20 cycles. The relationship between the electrochemical performance of the spherical silicon and the diameters of silicon sphere makes it possible to control the performance of the silicon anode by adjusting the hydrolysis conditions of TEOS.     

Light absorption in Ge nanoclusters embedded in SiO2: comparison between magnetron sputtering and sol–gel synthesis

SiGeO films have been produced by a sol–gel derived approach and by magnetron sputtering deposition. Post-thermal annealing of SiGeO films in forming gas or nitrogen atmosphere between 600 and 900 °C ensured the phase separation of the SiGeO films and synthesis and growth of Ge nanoclusters (NCs) embedded in SiO₂. Rutherford backscattering spectrometry analysis evidenced a similar Ge concentration (~12 %), but a different Ge out-diffusion after annealing between the two types of techniques with the formation of a pure SiO₂ surface layer (~30 nm thick) in sol–gel samples. The thermal evolution of Ge NCs has been followed by transmission electron microscopy and Raman analysis. In both samples, Ge NCs form with similar size increase (from ~3 up to ~7 nm) and with a concomitant amorphous to crystalline transition in the 600–800 °C temperature range. Despite a similar Ge concentration, a significant lower NCs density is observed in sol–gel samples attributed to an incomplete precipitation of Ge, which probably remains still dispersed in the matrix. The optical absorption of Ge NCs has been measured by spectrophotometry analyses. Ge NCs produced by the sol–gel method evidence an optical band gap of around 2 eV, larger than that of NCs produced by sputtering (~1.5 eV). These data are presented and discussed also considering the promising implications of a low-cost sol–gel based technique towards the fabrication of light harvesting devices based on Ge nanostructures.     

Crystallization kinetics and growth mechanism of Pb(Zr0.52·Ti0.48)O3 nanopowders

In this study, Pb(Zr_0.52·Ti_0.48)O₃ nanopowders were synthesized via sol–gel process. Particle morphology, crystalline phases and thermal behavior were characterized by scanning electron microscopy, X-ray diffraction and simultaneous thermal analyzer, respectively. The X-ray diffraction pattern showed perovskite phase clearly. The non-isothermal activation energy for the perovskite crystallization in Pb(Zr_0.52·Ti_0.48)O₃ gel powders was 224.91 kJ mol^?1. Both growth morphology parameter (n) and crystallization mechanism index (m) are close to 3.0, indicating that the bulk nucleation is dominant in the perovskite PZT formation. To determine dielectric properties, the calcined Pb(Zr_0.52·Ti_0.48)O₃ nanopowders were pressed using uniaxial press. It was found that the Pb(Zr_0.52·Ti_0.48)O₃ disks, by sintering at 1,200 °C for 2 and 10 h, and at 1 kHz frequency, had 966 and 1,490 of the dielectric constant, respectively.     

Sol–gel synthesis and characterization of Li<Subscript>2</Subscript>CO<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite solid electrolytes

Composite solid electrolytes in the system (1???x)Li₂CO₃–xAl₂O₃, with x?=?0.0–0.5 (mole), were synthesized by a sol–gel method. The synthesis carried out at low temperature resulted in voluminous and fluffy products. The obtained materials were characterized by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy/energy-dispersive X-ray, Fourier transform infrared spectroscopy and AC impedance spectroscopy. Structural analysis of the samples showed an amorphous feature of Li₂CO₃ and traces of α-LiAlO₂, γ-LiAlO₂ and LiAl₅O₈. The prepared composite samples possess high ionic conductivities at 130–180 °C on account of the presence of lithium aluminates as well as the formation of a high concentration of an amorphous phase of Li₂CO₃ via this sol–gel preparative technique.     

Luminescence properties of green emission of SiO2/Zn2SiO4:Mn nanocomposite prepared by sol–gel method

Green light emitting Mn²⁺ doped Zn₂SiO₄ particles embedded in SiO₂ host matrix were synthesized by a sol–gel method. After the incorporation of ZnO:Mn nanoparticles in a silica monolith using sol–gel method with supercritical drying of ethyl alcohol in two steps, it was heat treated in air at 1200 °C for 2 h in order to obtain the SiO₂/α-Zn₂SiO₄:Mn nanocomposites. The microstructure of phosphor crystals was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). XRD results indicate that the pure phase α-Zn₂SiO₄ with rhombohedral structure was obtained after thermal treatment at 1200 °C. The SiO₂/α-Zn₂SiO₄:Mn nanocomposites with a Mn doping concentration of 1.5 at% exhibit two broadband emissions in the visible range: a strong green emission at around 525 nm and a second one in the range between 560 and 608 nm. This nanocomposite with a Mn doping concentration of 0.05 shows the highest relative emission intensity. Upon 255 nm excitation, the luminescence decay time of the green emission of Zn₂SiO₄:Mn around 525 nm is 11 ms. The luminescence spectra at 525 nm (⁴T₁–⁶A₁) and lifetime of the excited state of Mn²⁺ ions-doped Zn₂SiO₄ nanocrystals are investigated.     

Surface properties of palladium catalysts supported on ternary ZrO2–Al2O3–WOx oxides prepared by the sol–gel method: Study of the chemical state of the support

The surface properties of Pd and Pd–Pt catalysts supported on binary ZrO₂–WOx and ternary ZrO₂–Al₂O₃–WOx oxides prepared by the sol–gel method were studied. Special attention was paid to the study of the texture of the catalysts as well as the chemical state of tungstated zirconia and tungstated zirconia promoted with alumina in the palladium catalysts. The catalysts were tested in the isomerization of n-hexane and were characterized by N₂ physisorption, XRD, TPR, Raman spectroscopy, XPS and FT-IR of adsorbed pyridine. The catalysts had bimodal pore size distributions with mesopores in the range 55–70 Å and macropores of 1000 Å in diameter. The catalysts had a surface WOx coverage (4.4–6.0 W nm^?2) lower than that of the theoretical monolayer (7.0 W nm^?2). A lower acidity of the ternary ZrO₂–Al₂O₃–WOx oxide as compared to the binary ZrO₂–WOx oxide was found. Higher activity in the isomerisation of n-hexane was obtained in the Pd–Pt catalysts supported on ternary ZrAlW oxides prepared by sol–gel that is correlated with the coexistence on the surface of W⁴⁺ (WO₂) or W⁰ and W⁶⁺ (Al₂(WO₄)₃) species, ZrO₂ in the tetragonal phase and a high amount of ZrOx suboxides species in a low oxidation state (Zr³⁺ and Zr²⁺).     

Effects of heat treatment and TiO2 content on the optical properties of Eu3+ doped TiO2–SiO2 thin films

The photoluminescence properties of Eu³⁺-doped TiO₂–SiO₂ thin films were investigated. The films were deposited on silicon substrates by the sol–gel process using the dip-coating method. The molar ratio of TiO₂ content was varied from 25% to 100%, while Europium concentration was fixed to 1%. The obtained films were calcinated at various temperatures ranging from 400 °C to 1300 °C, which allowed determining the optimal conditions for the Eu³⁺ luminescence. Meanwhile, the structure of TiO₂–SiO₂ powders, prepared in the same conditions as the films, was also studied by Raman spectroscopy. It revealed the role of Europium and SiO₂ on the stabilization of the anatase phase and the importance of the silica matrix in the control of titania particle size.     

Electrocaloric effect in antiferroelectric PbZrO3 thin films

Antiferroelectric PbZrO₃ thin films have been deposited on Pt(111)/Ti/SiO₂/Si substrate by polymer modified sol–gel route. Temperature dependent P –E hysteresis loops have been measured at 51 MV/m within a temperature range of 40 °C to 330 °C. The maximum electrocaloric effect ～0.224 × 10^–6 K mV^–1 has been observed near the dielectric phase transition temperature (235 °C) of the thin films. The electrocaloric effect and its strong temperature dependence have been attributed to nearly first‐order phase transition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

TEOS-based low-pressure chemical vapor deposition for gate oxides in 4H–SiC MOSFETs using nitric oxide post-deposition annealing

The use of SiO₂/4H–SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) can be problematic due to high interface state density (D_it) and low field-effect mobility (μ_fe). Here, we present a tetra-ethyl-ortho-silicate (TEOS)-based low-pressure chemical vapor deposition (LPCVD) method for fabricating the gate oxide of 4H–SiC MOSFETs using nitric oxide post-deposition annealing. SiO₂/4H–SiC MOS capacitors and MOSFETs were fabricated using conventional wet and TEOS oxides. The measured effective oxide charge density (Q_eff) and D_it of the TEOS-based LPCVD SiO₂/4H–SiC MOS capacitor with nitridation were 4.27 × 10¹¹ cm⁻² and 2.99 × 10¹¹ cm⁻²eV⁻¹, respectively. We propose that the oxide breakdown field and barrier height were dependent on the effective Q_eff. The measured μ_fe values of the SiO₂/4H–SiC MOSFETs with wet and TEOS oxides after nitridation were, respectively, 11.0 and 17.8 cm²/V due to the stable nitrided interface between SiO₂ and 4H–SiC. The proposed gate stack is suitable for 4H–SiC power MOSFETs.     

Dielectric and piezoelectric properties of the Ba0.92Ca0.08Ti0.95Zr0.05O3 thin films grown on different substrate

Lead free Ba_0.92Ca_0.08Ti_0.95Zr_0.05O₃ (BCZT) thin films were deposited on Pt/Ti/SiO₂/Si and LaNiO₃(LNO)/Pt/Ti/SiO₂/Si substrates by a sol–gel processing technique, respectively. The effects of substrate on structure, dielectric and piezoelectric properties were investigated in detail. The BCZT thin films deposited on LNO/Pt/Ti/SiO₂/Si substrates exhibit (100) orientation, larger grain size and higher dielectric tunability (64%). The BCZT thin films deposited on Pt/Ti/SiO₂/Si exhibit (110) orientation, higher Curie temperature (75 °C), better piezoelectric property (d₃₃ of 50 pm/V) and lower dielectric loss (0.02). The differences in dielectric and piezoelectric properties in the two kinds of oriented BCZT films should be attributed to the difference of structure, in-plane stress and polarization rotation in orientation engineered BCZT films.