Synthesis and characterization of thermoluminescent glass-ceramics Li2O-Al2O3-SiO2:CeO2

Vitroceramic powders of Li₂O-Al₂O₃-SiO₂ systems (LAS), doped with 1% (LAS:1Ce) and 10% (LAS:10Ce) molar of cerianite (CeO₂) were synthesized by means of the gelification technique of metal formates of aluminum and lithium, in the presence of tetraethoxy silane and CeO₂. The gels obtained were dried (120 °C, 2.5 h), calcined (480 °C, 5 h) and sinterized (1250 °C, 30 min). The sinterized samples were characterized by X-ray difraction (XRD), scanning electron microscopy (SEM) and microchemical analysis (EDS). There is evidence for a mixture of two phases of 64% β-spodumene (Li₂O-Al₂O₃-4SiO₂) and 36% β-eucryptite (Li₂O-Al₂O₃-2SiO₂). The LAS:1Ce system was enriched in aluminum, the LAS:10Ce system showed areas of heterogeneous composition; some regions with a shortage of CeO₂, while others zones with cerium cumulus. From the microscopy images it was found that CeO₂ acts as a densificant agent in LAS system, favoring the sintering in the host. The chemical route and the sintering processes utilized allow the production of samples exhibiting an acceptable linear correlation between total thermoluminescent emission intensity and the irradiation dose when the CeO₂ concentration is low (less than 1%), opening the possibility of using this kind of glass-ceramic in dosimetry.     

Characterization and catalytic performance of pure and Li2O-doped CuO/CeO2 catalysts

Pure and Li₂O-doped CuO/CeO₂ catalysts calcined at 500 °C were prepared by impregnation method. The catalysts are characterized by DTA, TG-DTG, XRD, IR, TEM, nitrogen adsorption at −196 °C and the catalytic decomposition of hydrogen peroxide at 30 °C.The effects of molar ratio, heat treatment time and the doping on the structural, surface and catalytic properties of nanocrystalline Cu/Ce-mixed oxides system have been studied. It was found that the catalytic activity of ceria-supported copper oxide catalysts increased by increasing both the heat treatment time and dopant content. However, the pure Cu/Ce-mixed oxide solids containing 10 wt.% CuO exhibited the best performance. The characterization results indicated that the higher surface area, the formation of solid solution between copper and cerium oxides, and the high dispersion of copper species on the ceria were responsible for the high catalytic activity of the CuO/CeO₂ catalysts.     

Immobilization of glucose oxidase using CoFe2O4/SiO2 nanoparticles as carrier

Aminated-CoFe₂O₄/SiO₂ magnetic nanoparticles (NPs) were prepared from primary silica particles using modified StÖber method. Glucose oxidase (GOD) was immobilized on CoFe₂O₄/SiO₂ NPs via cross-linking with glutaraldehyde (GA). The optimal immobilization condition was achieved with 1% (v/v) GA, cross-linking time of 3 h, solution pH of 7.0 and 0.4 mg GOD (in 3.0 mg carrier). The immobilized GOD showed maximal catalytic activity at pH 6.5 and 40 °C. After immobilization, the GOD exhibited improved thermal, storage and operation stability. The immobilized GOD still maintained 80% of its initial activity after the incubation at 50 °C for 25 min, whereas free enzyme had only 20% of initial activity after the same incubation. After kept at 4 °C for 28 days, the immobilized and free enzyme retained 87% and 40% of initial activity, respectively. The immobilized GOD maintained approximately 57% of initial activity after reused 7 times. The K_M (Michaelis-Menten constant) values for immobilized GOD and free GOD were 14.6 mM and 27.1 mM, respectively.     

Selective CO oxidation in hydrogen-rich gas over CuO/CeO2 catalysts

A series of CuO/CeO₂ catalysts with different Cu-Ce compositions were synthesized by co-precipitation method and characterized by X-ray diffraction, H₂-TPR, CO-TPD, SEM and X-ray photoelectron spectroscopy (XPS) techniques. The effects of Cu-Ce composition and water vapor on the catalytic properties for the selective CO oxidation in the hydrogen-rich gas were investigated. The results indicated that CuO (10%)/CeO₂ catalyst remained the maximum CO conversion and selectivity at 140 and 160 °C, while the performance of CuO/CeO₂ catalysts deteriorated with the CuO molar ratio further increased. The interfacial CuO and CeO₂ interaction and synergistic effect enhanced the redox properties of CuO/CeO₂ catalyst and the highly dispersed copper species were proposed as the active sites for the selective CO oxidation. The blockage of catalytic active sites by absorbed water and the formation of CO-H₂O surface complexes reduced the activity of CuO (10%)/CeO₂ catalyst. The decreasing of surface lattice oxygen and absorbed oxygen species and the agglomeration of copper particles were the plausible interpretations for the deactivation of CuO (10%)/CeO₂ catalyst.     

Prepare high efficiency Mn-doped Zn2SiO4 green phosphors in air using nano-particles

In this study, nano-scale precursors of ZnO, SiO₂, and MnO₂ powders were used to prepare mixtures with the compositions of 2ZnO+SiO₂+X mol% MnO₂ (X=MnO₂/2ZnO, abbreviated as Zn₂SiO₄-X-MnO₂), where 2≤X≤5. The mixed Zn₂SiO₄-X-MnO₂ mixtures were calcined from 900 to 1300 °C in air in order to synthesize Zn₂SiO₄:Mn²⁺ green phosphors. The X-ray diffraction patterns of Zn₂SiO₄-X-MnO₂ particles indicated that ZnO was present in the 900 °C-calcined Zn₂SiO₄-X-MnO₂ phosphors, but not in particles calcined at temperatures of 1000 °C and higher. However, the unapparent secondary phase of ZnMnO₃ was found in the 1200 and 1300 °C-calcined Zn₂SiO₄-5-MnO₂ compositions. The luminescent characteristics of Zn₂SiO₄-X-Mn²⁺ phosphors were compared with that of a commercial product (Nichia Corp., Japan). The photoluminescence (PL) intensity of 1200 °C-calcined Zn₂SiO₄-4-MnO₂ phosphors was higher and the decay times of all synthesized Zn₂SiO₄-X-MnO₂ phosphors were longer than those of the commercial product.     

The surface properties and the activities in catalytic wet air oxidation over CeO2-TiO2 catalysts

No-noble metal CeO₂-TiO₂ catalysts prepared by sol-gel method were developed and examined for catalytic wet air oxidation (CWAO) of acetic acid. The structure of the catalysts was measured by BET, SEM, XRD, XPS and DTA-TG. We investigated the effect of the interactions of Ce and Ti on the structure of CeO₂-TiO₂ catalysts. The mechanisms of the relationships between the different content of Ti and the activity of CeO₂-TiO₂ catalysts were discussed. The results showed that the average crystal size of CeO₂ decreased and the surface areas increased; the low valence of Ce³⁺ increase, and the chemisorbed oxygen slightly decreased with the increase of Ti content on the surface of CeO₂-TiO₂ catalysts. The order of the activity in CWAO of acetic acid followed: Ce/Ti 1/1 > Ce/Ti 3/1 > Ce/Ti 1/3 > Ce/Ti 5/1 > CeO₂ > TiO₂ > no catalyst. In CWAO of acetic acid, the optimal atomic ratio of Ce and Ti was 1, and the highest COD removal was over 64% at 230 °C, 5 MPa and 180 min reaction time over Ce/Ti 1/1 catalyst. The excellent activity and stability of CeO₂-TiO₂ catalysts was observed in our study.     

Photocatalytic activity of multielement doped TiO2 in the degradation of congo red

TiO₂ although considered a promising photocatalyst for the degradation of aqueous pollutants, it suffers from poor absorption in the visible region and hence requires ultraviolet (UV) light for activation. To make TiO₂ a visible active photocatalyst, multielement (C, N, B, and F) doping has been done. The synthesised CNBF/TiO₂ catalysts were calcined at different temperatures and characterized by XRD, BET surface area, UV DRS, XPS, HRSEM-EDAX, and TEM techniques. These catalysts found to show less band gap values when compared to bare TiO₂. These catalysts were tested for their catalytic activity towards the degradation of a textile dye - congo red (CR) under different reaction conditions. It was found that the photocatalytic activity was dependent on both doping of multielement and the calcination temperature of CNBF/TiO₂. The co-doped catalysts which were calcined at 400 °C and 600 °C (100% intensity in anatase phase) were found to be the best catalysts (100% decolourisation of CR in 21/2 h and 2 h respectively). TOC analysis carried out for the samples at the reaction time of 5 h showed very high percentage (83%) degradation of CR over CNBF/TiO₂ catalysts calcined at 600 °C when compared to the other catalysts calcined at different temperatures. CNBF/TiO₂ (1000 °C) showed very less photocatalytic activity due to the formation of rutile phase.     

Dielectric properties of barium titanate ceramics modified by SiO2 and by BaO-SiO2

The influence of SiO₂ on the dielectric properties of barium titanate ceramics was investigated. SiO₂ had been doped solely and together with BaO into barium titanate before calcination. X-ray diffraction showed that all the ceramics were of a pure perovskite phase after sintering at 1275 °C for 2 h. For SiO₂-doping, there was about 2.5 °C increase in Curie temperature per molar percentage of doping and the leakage current was obviously increased, especially at low voltages for relatively high doping levels. While for the co-doping of SiO₂ and BaO, there was little change in Curie temperature. The point defects formed through the dopings were proposed responsible for the effects. It was suggested that SiO₂ is important to barium titanate ceramics not only for sintering but also for modifying their properties.     

Preparation and properties of electroless Ni-P-SiO2 composite coatings

Dispersible SiO₂ nanoparticles were co-deposited with electroless Ni-P coating onto AISI-1045 steel substrates in the absence of any surfactants in plating bath. The resulting Ni-P/nano-SiO₂ composite coatings were heat-treated for 1 h at 200 °C, 400 °C, and 600 °C, respectively. The hardness and wear resistance of the heat-treated composite coatings were measured. Moreover, the structural changes of the composite coatings before and after heat treatment were investigated by means of X-ray diffraction (XRD), while their elemental composition and morphology were analyzed using an energy dispersive spectrometer (EDS) and a scanning electron microscope (SEM). Results show that co-deposited SiO₂ particles contributed to increase the hardness and wear resistance of electroless Ni-P coating, and the composite coating heat-treated at about 400 °C had the maximum hardness and wear resistance.     

SiO2 film electret with high surface potential stability

The plasma surface treatment and ion implantation were utilized to improve the stability of charge storage in the SiO₂ film electret. It was found that the SiO₂ films treated by argon plasma with the arcing at 700 V for 15 min, or implanted by 150 keV (kilo electron volt) Ar⁺ with a dose of 2 × 10¹¹ cm⁻², after being negatively charged, showed a remnant negative potential as large as 90% of the primary value after being stored in a glass container with desiccant for 10 days. It was also found that after being negatively charged at room temperature and aged at 200-350 °C for several times, the SiO₂ films implanted by 150 keV Ar⁺ had a relatively high remnant potential and it did not decay significantly even after being heated at the aging temperature of 200-350 °C in room atmosphere for 60 min.     

Nitric acid oxidation of Si (NAOS) method for low temperature fabrication of SiO2/Si and SiO2/SiC structures

We have developed low temperature formation methods of SiO₂/Si and SiO₂/SiC structures by use of nitric acid, i.e., nitric acid oxidation of Si (or SiC) (NAOS) methods. By use of the azeotropic NAOS method (i.e., immersion in 68 wt% HNO₃ aqueous solutions at 120 °C), an ultrathin (i.e., 1.3-1.4 nm) SiO₂ layer with a low leakage current density can be formed on Si. The leakage current density can be further decreased by post-metallization anneal (PMA) at 200 °C in hydrogen atmosphere, and consequently the leakage current density at the gate bias voltage of 1 V becomes 1/4-1/20 of that of an ultrathin (i.e., 1.5 nm) thermal oxide layer usually formed at temperatures between 800 and 900 °C. The low leakage current density is attributable to (i) low interface state density, (ii) low SiO₂ gap-state density, and (iii) high band discontinuity energy at the SiO₂/Si interface arising from the high atomic density of the NAOS SiO₂ layer.For the formation of a relatively thick (i.e., ≥10 nm) SiO₂ layer, we have developed the two-step NAOS method in which the initial and subsequent oxidation is performed by immersion in ∼40 wt% HNO₃ and azeotropic HNO₃ aqueous solutions, respectively. In this case, the SiO₂ formation rate does not depend on the Si surface orientation. Using the two-step NAOS method, a uniform thickness SiO₂ layer can be formed even on the rough surface of poly-crystalline Si thin films. The atomic density of the two-step NAOS SiO₂ layer is slightly higher than that for thermal oxide. When PMA at 250 °C in hydrogen is performed on the two-step NAOS SiO₂ layer, the current-voltage and capacitance-voltage characteristics become as good as those for thermal oxide formed at 900 °C.A relatively thick (i.e., ≥10 nm) SiO₂ layer can also be formed on SiC at 120 °C by use of the two-step NAOS method. With no treatment before the NAOS method, the leakage current density is very high, but by heat treatment at 400 °C in pure hydrogen, the leakage current density is decreased by approximately seven orders of magnitude. The hydrogen treatment greatly smoothens the SiC surface, and the subsequent NAOS method results in the formation of an atomically smooth SiO₂/SiC interface and a uniform thickness SiO₂.     

A novel method for immobilization of Co tetraphenylporphyrins on P(4VP-co-St)/SiO2: Efficient catalysts for aerobic oxidation of ethylbenzenes

In this paper, poly(4vinylpyridine-co-styrene) (P(4VP-co-St)) was grafted on silica gel particles in the manner of “grafting from”, and the grafting particle P(4VP-co-St)/SiO₂ was gained. The grafting particle P(4VP-co-St)/SiO₂ is a novel kind of supports for immobilizing metalloporphyrin catalysts. Then, the immobilization of cobalt tetraphenylporphyrin (CoTPP) on the supports P(4VP-co-St)/SiO₂ was carried out via the axial coordination reaction between CoTPP and the pyridine groups of the grafted P(4VP-co-St), resulting in the heterogenised catalysts CoTPP-P(4VP-co-St)/SiO₂. The synthesized catalysts were characterized by FTIR and the axial coordination process between CoTPP and the grafted P(4VP-co-St) was confirmed by UV-vis. The effects of various factors on the immobilization reaction of CoTPP were studied in detail. Finally, the catalytic performance of CoTPP-P(4VP-co-St)/SiO₂ in the catalytic oxidation process of ethyl benzene was investigated. The experimental results show that the axial coordination reaction is a very easy and novel method for favorably immobilizing CoTPP onto the P(4VP-co-St)/SiO₂ surfaces. During the immobilization process of CoTPP on P(4VP-co-St)/SiO₂, the most bonding amount of CoTPP (0.19 g/g) is obtained under the lower temperature (5 °C) and the higher concentration of CoTPP(6.0 mg/ml) lasting 4 h. Moreover, the supported catalyst CoTPP-P(4VP-co-St)/SiO₂ can effectively activate the dioxygen, and obviously catalyze the transform of ethylbenzene into acetophenone. So it exhibits the fine catalytic activity.     

Luminescence properties of Ge implanted SiO2:Ge and GeO2:Ge films

We have investigated cathodeluminescence (CL) of Ge implanted SiO₂:Ge and GeO₂:Ge films. The GeO₂ films were grown by oxidation of Ge substrate at 550 °C for 3 h in O₂ gas flow. The GeO₂ films on Ge substrate and SiO₂ films on Si substrate were implanted with Ge-negative ions. The implanted Ge atom concentrations in the films were ranging from 0.1 to 6.0 at%. To produce Ge nanoparticles the SiO₂:Ge films were thermally annealed at various temperatures of 600-900 °C for 1 h in N₂ gas flow. An XPS analysis has shown that the implanted Ge atoms were partly oxidized. CL was observed at wavelengths around 400 nm from the GeO₂ films before and after Ge⁻-implantation as well as from SiO₂:Ge films. After Ge⁻-implantation of about 0.5 at% the CL intensity has increased by about four times. However, the CL intensity from the GeO₂:Ge films was several orders of magnitude smaller than the intensity from the 800 °C-annealed SiO₂:Ge films with 0.5 at% of Ge atomic concentration. These results suggested that the luminescence was generated due to oxidation of Ge nanoparticles in the SiO₂:Ge films.     

Synthesis and characterization of pure and ZrO2-doped nanocrystalline CuO-NiO system

The physicochemical, surface and catalytic properties of pure and doped 0.25CuO-NiO solids prepared by sol-gel method were investigated. The dopant concentration was 2, 4 and 6 mol% ZrO₂. The solids investigated were calcined at 400 and 600 °C. The techniques employed were XRD, EDX, TEM, surface excess oxygen, nitrogen adsorption at −196 °C and catalytic oxidation of CO by O₂ using both static and flow methods. The results revealed that the investigated system dissolved 4 mol% ZrO₂ by heating at 400 °C. This process was accompanied by a significant increase in the S_BET and V_p with subsequent decrease in the (r) values of the doped adsorbent. ZrO₂-doping of the system investigated followed by calcination at 400 and 600 °C led to a considerable increase in its catalytic activity in CO oxidation by O₂ using static and flow methods. The doping process was not accompanied by any change in the activation energy of the catalyzed reaction.     

Effect of Li2O and CoO-doping of CuO/Fe2O3 system on its surface and catalytic properties

Physicochemical, surface and catalytic properties of pure and doped CuO/Fe₂O₃ system were investigated using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), nitrogen adsorption at −196 °C and CO-oxidation by O₂ at 80-220 °C using a static method. The dopants were Li₂O (2.5 mol%) and CoO (2.5 and 5 mol%). The results revealed that the increase in precalcination temperature from 400 to 600 °C and Li₂O-doping of CuO/Fe₂O₃ system enhanced CuFe₂O₄ formation. However, heating both pure and doped solids at 600 °C did not lead to complete conversion of reacting oxides into CuFe₂O₄. The promotion effect of Li₂O dopant was attributed to dissolution of some of dopant ions in the lattices of CuO and Fe₂O₃ with subsequent increase in the mobility of reacting cations. CoO-doping led also to the formation of mixed ferrite Co_xCu_1−xFe₂O₄. The doping process of the system investigated decreased to a large extent the crystallite size of unreacted portion of Fe₂O₃ in mixed solids calcined at 600 °C. This process led to a significant increase in the S_BET of the treated solids. Doping CuO/Fe₂O₃ system with either Li₂O or CoO, followed by calcination at 400 and 600 °C decreased its catalytic activity in CO-oxidation by O₂. However, the activation energy of the catalyzed reaction was not much affected by doping.     

Optical characterization of SiO2/Zn2SiO4:V nanocomposite obtained after the incorporation of ZnO:V nanoparticles in silica host matrix

Vanadium-doped Zn₂SiO₄ particles embedded in silica host matrix were prepared by a simple solid-phase reaction under natural atmosphere at 1200 °C after the incorporation of ZnO:V nanoparticles in silica monolith using sol-gel method with supercritical drying of ethyl alcohol in two steps. The obtained sample, exhibits a strong PL band in the visible range at 540 nm and two thin emission lines in the UV range at 394 and 396 nm under intensive power excitation. Photoluminescence excitation (PLE) measurements show different origins of the emission bands. It is suggested that radiative defects attributed to vanadium in the interfaces between Zn₂SiO₄ particles and SiO₂ host matrix resulting from heat treatment and zinc oxide excitonic emissions, were responsible for theses luminescence bands.     

Study of the synthesis and Cl2 gas-sensing properties of nanometer HNO3-doped SnO2

We focused on the effects of the inorganic acid HNO₃ on the gas-sensing properties of nanometer SnO₂ and prepared the powders via a dissolution-pyrolysis method. Furthermore, the powders were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectra (EDS). Several aspects were surveyed, including the calcining temperature, concentration of nitric acid and the working temperature. The results showed that the gas response of 3 wt% HNO₃-doped SnO₂ powders (calcined at 500 °C) to 10 ppm Cl₂ reached 316.5, at the working temperature 175 °C. Compared with pure SnO₂, appropriate HNO₃ could increase the gas sensitivity to Cl₂ gas more significantly.     

Characteristics of sandwich-structured Al2O3/HfO2/Al2O3 gate dielectric films on ultra-thin silicon-on-insulator substrates

Sandwich-structure Al₂O₃/HfO₂/Al₂O₃ gate dielectric films were grown on ultra-thin silicon-on-insulator (SOI) substrates by vacuum electron beam evaporation (EB-PVD) method. AFM and TEM observations showed that the films remained amorphous even after post-annealing treatment at 950 °C with smooth surface and clean silicon interface. EDX- and XPS-analysis results revealed no silicate or silicide at the silicon interface. The equivalent oxide thickness was 3 nm and the dielectric constant was around 7.2, as determined by electrical measurements. A fixed charge density of 3 × 10¹⁰ cm⁻² and a leakage current of 5 × 10⁻⁷A/cm² at 2 V gate bias were achieved for Au/gate stack /Si/SiO₂/Si/Au MIS capacitors. Post-annealing treatment was found to effectively reduce trap density, but increase in annealing temperature did not made any significant difference in the electrical performance.     

Sol-gel preparation and characterization of CoFe2O4-SiO2 nanocomposites

Magnetic nanocomposites formed by cobalt ferrite particles dispersed in a silica matrix were prepared by a sol-gel process. The effects of the thermal treatment temperature and the salt concentration on the structural and magnetic properties of the composites were investigated. By controlling these parameters, CoFe₂O₄/SiO₂ nanocomposites with different crystallite size and magnetic properties were obtained. By increasing the annealing temperature and salt concentration, composites with a progressive increase in the coercive field and of the density of magnetization were produced. In particular, a nanocomposite, with a Fe/Si molar concentration of 21%, obtained by drying the gel at 150 °C and further annealing at 800 °C, has a coercivity of 2000 Oe, which is more than twice higher than the coercivity of bulk cobalt ferrite.     

Magneto-optical study of slanted Co nanowires embedded in CeO2/SrTiO3(0 0 1)

Sequential pulsed laser deposition of CoO and CeO₂ at 650 °C under vacuum leads to the formation of a slanted Co nanowires assembly embedded in CeO₂/SrTiO₃(0 0 1) epilayers. High temperature magneto-optical Faraday measurements were performed, which revealed a Faraday ellipticity of 1.3° at a wavelength of 450 nm for 300 nm thick samples and which allowed to access the magnetic properties. From the analysis of the coercivity dependence on temperature, it is shown that the magnetic anisotropy of the slanted Co nanowires is dominated by shape anisotropy and that their magnetization reversal is localized.