Sol–gel preparation and characterization of antibacterial and self-cleaning hybrid nanocomposite coatings

ZnO–TiO₂, SiO₂–TiO₂, and SiO₂–TiO₂–ZnO hybrid nanocomposite coatings were synthesized based on sol–gel precursors including tetramethoxysilane (TMOS), 3-glycidoxypropyl trimethoxysilane (GPTMS), tetra(n-butyl orthotitanate) (TBT), and zinc acetate dihydrate. The hybrid network was characterized by FTIR, FESEM, and EDAX techniques. Results indicated that inorganic particles’ size was of nanoorder (20–30 nm), with very uniform distribution and dispersion. Photocatalytic and self-cleaning activities of these coatings were further investigated by degradation of methylene blue in an aqueous solution (20 ppm) at visible light irradiation, indicating photocatalytic performance of the coatings containing ZnO and TiO₂ nanoparticles. The antibacterial effect of the coatings was investigated for inhibition and inactivation of cell growth, with the results showing the same antibacterial activity for ZnO–TiO₂ and SiO₂–TiO₂–ZnO coatings against Escherichia coli and Staphylococcus aureus; the activity was, however, higher than that of SiO₂–TiO₂ hybrid nanocomposite coatings.     

Grain size effect and microstructure influence on the energy storage properties of fine‐grained BaTiO3‐based ceramics

The dependence of energy storage properties on grain size was investigated in BaTiO₃‐based ferroelectric ceramics. Modified BaTiO₃ ceramics with different grain size were fabricated by two‐step sintering method from BaTiO₃ powders doped with Al₂O₃ and SiO₂ by aqueous chemical coating. For samples doped with ZnO sintering aid in addition to Al₂O₃‐SiO₂, the density and breakdown strength increased significantly. In general, samples with smaller grains have lower polarization but higher energy storage efficiency. Al₂O₃‐SiO₂‐ZnO‐doped samples with average grain size of 118±2 nm have an energy density of 0.83±0.04 J/cm³. Obvious segregation of doping elements in second phase and grain boundary was observed by TEM‐EDS. Impedance spectroscopy further explains the relationship between microstructure and properties. Compared to common energy storage ceramics, the grain size of this low‐cost ceramics sintered at relatively low temperature is small, and the pilot scale production has been well completed. All these features make the utilization in multilayer devices and industrial mass production possible. In addition, the obtained rules are helpful in further development of energy storage ceramics.     

Characterization of wollastonite coatings prepared by sol–gel on Ti substrate

Wollastonite coatings were prepared by sol–gel on Ti substrate and their microstructures have been studied. The phase compositions and the surface morphologies of these coatings were examined by X-ray diffraction and scanning electron microscopy. Thermal behavior of dried gel was examined by differential scanning calorimetry (DSC) and thermogravimetry (TG). There are many cracks among coatings and particles with size about 200–300 nm distributing inside cracks. DSC and TG results show that the glass transformation temperature of dried gel is about 850°C. After calcined at temperature 900°C, the phase of coatings consists of wollastonite, SiO₂, and CaSi₂O₅.     

Effect of silica doping on the densification and grain growth in zinc oxide

The ability of silica (SiO₂) in controlling the densification and grain growth behavior of nano crystalline zinc oxide (ZnO) has been systematically studied. It has been observed that SiO₂ acts as a sintering inhibitor in the ZnO–SiO₂ system up to 4 wt.% limiting value beyond which densification behavior of the system remains almost unchanged, especially above 1100 °C. The addition of SiO₂ to ZnO retards grain growth which in turn results a finer ultimate grain size as compared to the undoped ZnO. However, stabilization in grain size occurs at ≥4 wt.% SiO₂ addition. It has been observed that SiO₂ incorporation changes the grain growth mechanism up to 4 wt.% addition, beyond which no remarkable changes was noticed. The grain growth (n) shows distinctly different slopes as a function of sintering time for the SiO₂ doped ZnO systems than undoped ZnO. The different slopes tend to indicate that different diffusion mechanisms and probably the formation of a secondary phase (Zn–Si–O) at the grain boundary control the densification and grain growth. The thermal expansion coefficient of the system has been found to decrease substantially beyond 4 wt.% SiO₂ addition to ZnO.     

Structural and optical properties of doped ZnO/SiO2 nanocomposite

The synthesis of ZnO/SiO₂ nanocomposite doped with Co, Cu and Mn nanoparticles were performed using sol-gel method. Rietveld refinement analysis was done for the X-ray diffraction data and results showed that all samples are single wurtzite hexagonal phase and have an average crystallite size ~14 nm. The lattice parameters, bond length, crystallite size, microstrain, dislocation density and the system volume were studied in details. Transmission electron microscopy technique showed that nanoparticles characterized by an elliptical shape. Fourier transform infrared and scanning electron microscope techniques proved the presence of SiO₂ in the system. The energy gap values of both undoped and doped samples with Co, Mn and Cu were 2.84, 2.47, 2.71 and 2.8 eV, respectively. Photoluminescence spectra for undoped and doped ZnO/SiO₂ were investigated in details.     

Synergetic effect of NiO and SiO2 on the sintering and properties of 8 mol% yttria-stabilized zirconia electrolytes

Yttria-doped zirconia electrolytes (e.g., 8 mol% yttria-stabilized ZrO₂, 8YSZ) have been considered to be the most promising candidates for applications in solid oxide fuel cells (SOFC). Due to the ubiquitous presence of SiO₂ impurities and wide use of Ni-containing anodes, it is therefore of great technical importance to understand the synergetic effect of NiO and SiO₂ on densification, grain growth and ionic conductivities (especially the grain boundary (GB) conduction) of zirconia electrolytes. In this study, three groups of 8YSZ ceramics, with Si contents of ∼30, ∼500 and ∼3000 ppm, have been designed. 1 at% NiO was added into these materials by a wet chemical method. The addition of SiO₂ has a negative effect on the sintering and densification, while the introduction of 1 at% NiO reduced the sintering temperature and promotes grain growth of the zirconia ceramics. However, the presence of small amount of NiO prevented full densification of 8YSZ ceramics. NiO also led to a decrease by ∼33% in grain interior (GI) conductivity, with little effect on the GB conduction of high-purity 8YSZ (∼30 ppm SiO₂). However, the coexistence of NiO and SiO₂ is extremely detrimental to total conductivity by significantly reducing the GB conduction. Moreover, it is observed that, unlike the 8YSZ-doped SiO₂ with only, whose GB conduction increases greatly with increasing sintering temperature, the GB conduction of the NiO and SiO₂ codoped samples is less sensitive to sintering temperature.     

Preparation and characterization of nanocrystalline and mesoporous strontium titanate thin films at room temperature

The low temperature perovskite-type strontium titanate (SrTiO₃) thin films and powders with nanocrystalline and mesoporous structure were prepared by a straightforward particulate sol–gel route. The prepared sol had a narrow particle size distribution with hydrodynamic diameter of about 17 nm. X-ray diffraction (XRD) revealed that the synthesized powders had a perovskite-SrTiO₃ structure with preferable orientation growth along the (1 0 0) direction. TEM images showed that the average crystallite size of the powders annealed in the range 300–800°C was around 8 nm. FE-SEM analysis and AFM images revealed that the deposited thin films had mesoporous and nanocrystalline structure with the average grain size of 25 nm at 600°C. Based on Brunauer–Emmett–Taylor (BET) analysis, the synthesized powders showed mesoporous structure with BET surface area in the range 92–75 m²/g at 400–600°C. One of the smallest crystallite sizes and one of the highest surface areas reported in the literature were obtained, which can be used in many applications, such as photocatalysts.     

Incorporation of carbon nanotube and graphene in ZnO nanorods-based hydrogen gas sensor

In this study, ZnO nanorods (NRs) were grown using solgel/hydrothermal methods on SiO₂, carbon nanotube (CNT)/SiO₂, and graphene/SiO₂ substrates to form hydrogen gas sensing chips. Results indicate that ZnO NRs/CNT/SiO₂ structures exhibited better H₂ sensing performance than the other two types of ZnO NRs-based structures. Furthermore, multiple electrical and material characterizations show that ZnO NRs/CNT/SiO₂ structures had a stronger (002) crystalline phase, with nanorod fusion near the bottom, and more oxygen-related defects. Owing to their small size, simple fabrication, and low cost, the ZnO NRs/CNT/SiO₂based H₂ gas sensors are promising for future industrial H₂ sensing applications.     

CO2 reforming of methane over vanadia-promoted Rh/SiO2 catalysts

The reforming of methane with carbon dioxide over rhodium dispersed on silica, Rh/SiO₂, and vanadia-promoted silica, Rh/VO_x/SiO₂, was studied by kinetic test reactions under differential conditions in a temperature range from 723 to 773 K. Transmission infrared spectroscopy was applied to observe the interaction of CO₂ with the catalysts and the formation of surface intermediates during the CO₂–CH₄ reforming reaction. To analyze carbon deposition XP spectroscopy and TPO was carried out. It has been shown that the promotion of Rh/SiO₂ catalysts with vanadium oxide enhances the catalytic activity for CO₂ reforming of methane and decreases the deactivation by carbon deposition. This is attributed to the formation of a partial VO_x overlayer on the Rh surface, which reduces the size of accessible ensembles of Rh atoms required for coke formation and creates new sites at the Rh–VO_x interfacial region that are considered to be active sites for the activation/dissociation of carbon dioxide. This revised version was published online in June 2006 with corrections to the Cover Date.     

SiC Nanorods Grown on Electrospun Nanofibers Using Tb as Catalyst: Fabrication, Characterization, and Photoluminescence Properties

Well-crystallized β-SiC nanorods grown on electrospun nanofibers were synthesized by carbothermal reduction of Tb doped SiO₂ (SiO₂:Tb) nanofibers at 1,250 °C. The as-synthesized SiC nanorods were 100–300 nm in diameter and 2–3 μm in length. Scanning electron microscopy (SEM) results suggested that the growth of the SiC nanorods should be governed by vapor-liquid-solid (VLS) mechanism with Tb metal as catalyst. Tb(NO₃)₃ particles on the surface of the electrospun nanofibers were decomposed at 500 °C and later reduced to the formation of Tb nanoclusters at 1,200 °C, and finally the formation of a Si–C–Tb ally droplet will stimulate the VLS growth at 1,250 °C. Microstructure of the nanorod was further investigated by transmission electron microscopy (TEM). It was found that SiC <111> is the preferred initial growth direction. The liquid droplet was identified to be Si₈₆Tb₁₄, which acted as effective catalyst. Strong green emissions were observed from the SiC nanorod samples. Four characteristic photoluminescence (PL) peaks of Tb ions were also identified.     

Reactions of methylcyclopentane on Rh–Pt catalyst prepared by underpotential deposition of Rh on Pt/SiO2

Rh was deposited on to a well-characterized 3.1% Pt/SiO₂ (InCat-1) parent catalyst by underpotential deposition method to obtain a model Rh–Pt bimetallic catalyst. TEM and EDS was used to determine its mean particle size and bulk composition: the particles of ca. 3 nm contained ca. 60% Pt and 40% Rh. The Rh–Pt catalyst was tested in methylcyclopentane (MCP) reaction between 513 K and 603 K and 60–480 Torr H₂ pressure (with 10 Torr MCP). The parent Pt/SiO₂ as well as a 5% Rh/SiO₂ catalyst were also studied for comparison. Four subsequent treatments with O₂ and H₂ up to T = 673 K were applied on the bimetallic catalyst before the catalytic runs. The overall activity showed positive hydrogen order on all samples, bimetallic Rh–Pt resulting in the lowest TOF values. Ring opening and hydrogenolysis products, as well as unsaturated hydrocarbons were formed from MCP. The selectivity of ring opening products and fragments over Rh–Pt catalyst was between the values observed on Pt and Rh, while the selectivity towards benzene formation was highest on the bimetallic sample, especially at higher temperatures. “Selective” ring opening occurred on all samples, resulting mostly in 2 and 3-methylpentane and less hexane. Different pretreatments with H₂ and O₂ affected slightly the dispersion values and the catalytic behavior of Rh–Pt sample. The selectivities of the Rh–Pt catalyst being between the values observed for Pt/SiO₂ and Rh/SiO₂ indicates that the sample studied represented a real bimetallic catalyst, resembling both components and exhibiting at the same time, new properties in addition to those, characteristic of Pt or Rh. Dedicated to Konrad Hayek.     

Impacts of Post-metallisation Processes on the Electrical and Photovoltaic Properties of Si Quantum Dot Solar Cells

As an important step towards the realisation of silicon-based tandem solar cells using silicon quantum dots embedded in a silicon dioxide (SiO₂) matrix, single-junction silicon quantum dot (Si QD) solar cells on quartz substrates have been fabricated. The total thickness of the solar cell material is 420 nm. The cells contain 4 nm diameter Si quantum dots. The impacts of post-metallisation treatments such as phosphoric acid (H₃PO₄) etching, nitrogen (N₂) gas anneal and forming gas (Ar: H₂) anneal on the cells’ electrical and photovoltaic properties are investigated. The Si QD solar cells studied in this work have achieved an open circuit voltage of 410 mV after various processes. Parameters extracted from dark I–V, light I–V and circular transfer length measurement (CTLM) suggest limiting mechanism in the Si QD solar cell operation and possible approaches for further improvement.     

Preparation of titanium dioxide/activated carbon composites using supercritical carbon dioxide

The penetration of titanium tetraisopropoxide (TTIP) dissolved in supercritical CO₂ into the nano-spaces of an activated carbon was studied for the preparation of a TiO₂-coated activated carbon. The conversion of TTIP to TiO₂ through thermal decomposition was confirmed by evolved gas analysis during heat treatment under a N₂ flow. Acetone was detected in the evolved gas, which suggested that some isopropoxide groups in TTIP reacted with the carbonyl groups on the activated carbon surface. This chemical reaction with carbon is expected to be advantageous for favorable attachment to the carbon surface. The crystallite size of anatase in the TiO₂/carbon composites was 4.1 nm, as estimated from the X-ray diffraction pattern, which almost corresponded to the graphene crystallite size; L_a (3.3-3.4 nm), as estimated from both the Raman spectrum and X-ray diffraction pattern. As the size of the crystallite prepared by bulk condensation of TTIP was more than 15 nm, these results confirmed that the anatase crystals were present in the carbon pores. Also, it was suggested that the crystal growth of TiO₂ was influenced by the carbon nano-spaces.     

Room Temperature Photocatalytic Oxidation of Carbon Monoxide Over Pd/TiO2–SiO2 Catalysts

The photocatalytic oxidation of carbon monoxide over TiO₂–SiO₂ and Pd/TiO₂–SiO₂ catalysts was studied. The catalyst samples were synthesized by using sol–gel technique coupled with hydrothermal treatment and all samples were hydrothermally treated before calcination in air. The catalyst samples were characterized by XRD, BET and DRIFTS techniques. The photocatalytic activity of the samples was determined by using circulated batch photoreactor coupled with in line gas transmission FTIR cell charged with 2,000 ppm carbon monoxide in air initially over 0.5 g of catalyst sample under 33 W (254 nm) irradiation power. XRD and BET results confirmed the presence of anatase phase and the decrease on the crystallite size of TiO₂ with SiO₂ addition which yield higher surface area and better dispersion of TiO₂ over mesoporous SiO₂. DRIFTS results indicated the presence of surface hydroxyls coordinated to Ti⁴⁺ and Si–O–Ti sites. All samples containing 10–90 % TiO₂ over SiO₂ exhibited significant photo oxidation activity at room temperature. The photocatalytic oxidation rate of carbon monoxide is favored by SiO₂ addition due to high surface area, better dispersion of TiO₂ particles and higher surface defects. The addition of PdO improves the photocatalytic activity significantly and the synergy between the TiO₂ and PdO phases.     

Synthesis and Characterization of Anti-fungus,Anti-corrosion and Self-cleaning Hybrid Nanocomposite Coatings Based on Sol–Gel Process

Anti-corrosion, anti-fungus, and self-cleaning properties of coatings containing ZnO–TiO₂, SiO₂–TiO₂ and SiO₂/TiO₂/ZnO nanoparticles synthesized based on sol–gel precursors using tetra methoxysilane, 3-glycidoxypropyl trimethoxysilane, tetra (n-butyl orthotitanate) and zinc acetate dihydrate were investigated by FESEM, EDAX and TEM analyses. Results indicated uniform dispersion of inorganic nanoparticles in the range of 20–40 nm in size. Anti-corrosion property of the hybrid coating was characterized by EIS measurements and parametrically analyzed in an equivalent circuit when the coating was exposed to salt solution. Results showed that, ZnO and TiO₂ nanoparticles enhance anti-corrosion property of the hybrid coatings. Anti-fungus and anti-bacterial properties of the coatings were determined by diameter of inhibition zone and inhabitation of bacterial growth, respectively. The coating containing ZnO and TiO₂ nanoparticles showed anti-fungus and anti-bacterial properties which were related to their photocatalytic properties. Degradation of methylene blue in aqueous solution was determined by UV–Visible tests which indicated self-cleaning property of the coatings containing ZnO and TiO₂ nanoparticles.     

Improved Energy Storage Properties of Fine‐Crystalline BaTiO3 Ceramics by Coating Powders with Al2O3 and SiO2

The multilayer structure of capacitor demands for fine grain size of dielectric ceramics in devices, because the thinner layer which needs ceramics with fine grain size is helpful in enlarging the capacitance. In this paper, the aqueous chemical coating method was utilized to modify the BaTiO₃ particles. The fine‐crystalline BaTiO₃ ceramics with an average grain size below 200 nm without abnormal grain growth by co‐coating Al₂O₃ and SiO₂ has been prepared. The phase composition, microstructures of coated particles and ceramics, and dielectric properties were investigated. For samples containing 3 wt% of Al₂O₃ and 1 wt% of SiO₂, the energy storage density is 0.725 J/cm³ and the efficiency of the ceramic samples can keep above 80%. The breakdown strength was improved to about 190 kV/cm.     

Selective steam reforming of methanol over silica-supported copper catalyst prepared by sol–gel method

Silica-supported copper prepared by a sol–gel method can selectively catalyze methanol steam reforming to hydrogen and carbon dioxide at 250 °C. The catalytic activity increases with the copper content up to 40 wt.%. The selectivity to carbon monoxide with the catalysts containing 20–40 wt.% of copper is significantly lower than that with a commercial Cu/ZnO/Al₂O₃ catalyst. Copper particles are highly dispersed in the catalyst whose Cu content is 20 wt.% or less. After the reaction at 250 °C the particles are present as Cu₂O with the mean crystallite size less than 4 nm. In the catalyst with the Cu content of 30–50 wt.%, the fine Cu₂O particles coexist with large metallic Cu particles whose mean crystallite size is 30–40 nm after the reaction. The large metallic particles are supposed to contribute to the reaction as well as the fine Cu₂O particles although the surface area is estimated to be significantly smaller than that of the latter.     

TiO2/ZnO double-layer broadband antireflective and down-shifting coatings for solar applications

Making full use of sunlight in solar cells requires reducing the reflection of light and minimizing spectral mismatch. Here, a TiO₂/ZnO double-layer coating with both wider band antireflection and down-shifting performance was prepared. TiO₂ sols and ZnO nanoparticles were synthesized via the sol-gel method and then successively coated on the surface of the Si substrate by dip-coating. Computational simulations were used to obtain the optimal refractive index and thickness of the coatings. In the experiments, the thicknesses of the TiO₂ and ZnO coatings were adjusted by changing the lifting speed, and the refractive index of the TiO₂ and ZnO coatings were adjusted by adding the porosity inducing agent and varying the concentration of the solution. The TiO₂/ZnO coating reduces the reflectivity of the silicon substrate by 24.97% in the 400–1100 nm band, and the ZnO nanoparticles can convert light at approximately 345 nm–527 nm, reducing the spectral mismatch of the solar cell. The photocurrent of solar cells coated with TiO₂/ZnO coatings was markedly improved, with an increase of 29% in the average photocurrent at 300–800 nm. Herein, TiO₂/ZnO coatings have the potential to benefit the development of multifunctional coatings that are important for improving the efficiency of solar cells.     

A Novel Nanocomposite Matrix Based on Silylated Chitosan and Multiwall Carban Nanotubes for the Immobilization of Urease

A nanocomposite matrix made up of silylated chitosan and multiwall carbon nanotubes (CHIT–SiO₂–MWCNTs) was fabricated to investigate the immobilization of urease (Urs). Urs enzyme was covalently immobilized with the CHIT–SiO₂–MWCNTs matrix using glutaraldehyde as a linker. The resulting Urs/CHIT–SiO₂–MWCNTs biomatrix was characterized with Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and cyclic voltammetry (CV) taking CHIT–SiO₂–MWNTs as a reference. The influence of various parameters on Urs enzyme activity within the matrix was investigated including pH, temperature, and time. The Michaelis–Menten constant and apparent activities for the Urs enzyme were calculated to be 0.51 mM and 89.02 mg/cm², respectively; indicating CHIT–SiO₂–MWCNTs nanocomposite matrix has a high affinity to immobilize Urs enzyme.     

Photoluminescence of supported vanadia catalysts: linear correlation between the vanadyl emission wavelength and the isoelectric point of the oxide support

Photoluminescence spectra of vanadium oxide supported on SiO₂, TiO₂, Al₂O₃, K‐doped Al₂O₃, ZnO and MgO were recorded. All the vanadium‐containing solids exhibit room‐temperature luminescence in the 550–700 nm region upon excitation from 250 to 400 nm. Significant spectral shifts of the emission maximum wavelength (λ_phos) depending on the acid/basic nature of the support have been observed. In this way, the higher the isoelectric point (or zero point charge, pzc) of the oxide support, the higher is the λ_phos of the supported vanadia catalysts. The linear relationship between these two properties clearly proves that the characteristics of the oxide support has a direct influence on the phosphorescence, and hence on the electronic states, of the vanadyl group. Kinetic analysis of the luminescence decay of supported catalysts has also been determined. This revised version was published online in August 2006 with corrections to the Cover Date.