Modulation of wall thickness of Al-doped ZnO nanotubes by nanolamination of atomic layer deposition for oxygen sensing

The high surface-to-volume ratio and feature dimensions of the gas sensors are the key factors for improving the gas response. In this study, a novel method to prepare an Al-doped ZnO (AZO) nanotube oxygen sensor with tunable wall thickness is reported via the ZnO–Al₂O₃ nanolamination of atomic layer deposition (ALD) using tris(8-hydroxyquinoline) gallium nanowire (GaQ₃NW) as a template. The ALD of Al₂O₃ significantly enhances wall uniformity and decreases the wall thickness of the AZO nanotubes. In addition, the incorporation of Al₂O₃ allows full coverage of AZO on GaQ₃NWs. With an increase in the Al₂O₃ fraction, the carrier concentration increases, but the depth of the depletion layer and gas response of the nanotube sensor are reduced. The gas response of the nanotubes is inversely proportional to wall thickness, suggesting that it is a function of the surface-to-volume ratio. When the wall thickness is decreased to 12 nm, the gas response of AZO nanotubes with 2% Al increases significantly to 7. This can be explained by the grain control model, because thin wall leads to the formation of fully charge-depleted nanotubes.     

Carbothermal Synthesis of the Nanostructures of Al2O3 and ZnO

Nanostructures of Al₂O₃ and ZnO have been synthesized by a carbothermal route involving the reaction of the metal or the metal oxide with carbon. In the case of Al₂O₃, nanowires and nanotubes are obtained starting with Al metal and active carbon or graphite. ZnO nanowires are obtained by the reaction of zinc oxalate or ZnO with active carbon or multiwalled carbon nanotubes. The Al₂O₃ and ZnO nanostructures obtained have been characterized by X-ray diffraction, electron microscopy and photoluminescence spectroscopy. These nanostructures are likely to be of use as catalyst supports and in other technological applications.     

Cure Characteristics,Mechanical and Thermal Properties of Al2O3 and ZnO Reinforced Silicone Rubber

The effects of alumina (Al₂O₃) and zinc oxide (ZnO) fillers on the curing characteristics, thermal and mechanical properties of silicone rubber were studied. Rheometer results indicate that the incorporation of ZnO fillers retards the curing process, whereas an enhancement in cure rate was observed for Al₂O₃. Higher maximum torque (M_H) and minimum torque (M_L) values was also observed for ZnO silicone rubber compounds compared to Al₂O₃. Thermogravimetric analysis (TGA) showed that ZnO silicone rubber compounds are thermally more stable than Al₂O₃; however, the coefficient of thermal expansion of the Al₂O₃ silicone rubber compounds are lower than that of ZnO. Comparison in mechanical strength between the two silicone rubber hybrids indicates that ZnO is a better reinforcement filler, as evidenced in the tensile strength, elongation at break, and modulus at 300% elongation.     

Carbon nanotubes improve the adhesion strength of a ceramic splat to the steel substrate

A scratch technique was used to measure the adhesion strength of plasma sprayed carbon nanotube (CNT) reinforced aluminum oxide (Al₂O₃) splat on the steel substrate. The effect of adding carbon nanotube on the adhesion strength of a single splat was studied by varying the CNT content as 0, 4 and 8 wt.% in the Al₂O₃ matrix. Higher lateral force was required by the nanoindenter tip to detach CNT reinforced Al₂O₃ splats as compared to Al₂O₃ splat. The adhesion strength increased significantly from 0.52 ± 0.05 MPa for Al₂O₃ splat to 4.21 ± 0.49 MPa for Al₂O₃-4 wt.% CNT splat and 7.36 ± 3.96 MPa for Al₂O₃-8 wt.% CNT splat. A high variation in the adhesion strength of Al₂O₃-8 wt.% CNT splat was due to varying degree of CNT dispersion in the matrix. A significant increase in the adhesion strength of Al₂O₃-CNT splat was attributed to its better mechanical interlocking with the substrate as a result of enhanced melting and spreading caused by the higher thermal conductivity of nanotubes. CNTs also form anchors between the splat and the substrate resulting in higher adhesion strength.     

Mechanism of Nanovoid Formation at the ZnO–Glass Interface in Planar Multilayered Structures of AlOx–ZnO–Glass

Functional porous materials require easy fabrication methods with controllability of a wide range of pore size and its density for practical applications including optical devices. The Kirkendall effect based on unbalanced material diffusion provides such a possibility in conjunction with material configurations of multilayers. This study reports a formation of nanoscale pores within ZnO films in planar multilayered structures of Al₂O₃–ZnO‐aluminosilicate glass and demonstrates the mechanism of forming relatively large nanopores in ZnO near the ZnO–glass interface via stress‐promoted Kirkendall diffusion. Experimental characterizations supported by atomic simulation reveal that an enhanced in‐plane tensile stress in the ZnO films with increasing the thickness of the neighboring Al₂O₃ films can promote the diffusivity of the Zn atoms and the pore growth in the ZnO films. The pore size and location in the intermediate ZnO layer of the Al₂O₃–ZnO–glass is alterable by simply selecting the thickness of the Al₂O₃ layer. Promoted diffusion of the Zn atoms enables to fabricate porous planar ZnO films with pore sizes up to a few hundred nm with an enhanced light scattering ability. These findings offer a promising route to produce porous planar films through in‐depth understanding of diffusivity enhancement in glass–metal oxide couples.     

Texture development in Fe-doped alumina ceramics via templated grain growth and their application to carbon nanotube growth

Fe-doped alumina (Fe-Al₂O₃) materials with a controlled microstructure could be designed for some special uses such as a substrate for carbon nanotube growth. In this study, Fe-doped Al₂O₃ ceramics with varying degrees of texture were prepared via Templated Grain Growth method and utilized for carbon nanotube synthesis by Catalytic Chemical Vapor Deposition in order to investigate how α-Al₂O₃ crystal orientation affects carbon nanotube growth in polycrystalline ceramics. The degree of texture increased with the Fe content in the presence of liquid phase. Three kinds of carbon filaments (few-wall carbon nanotubes bundles, individual multi-wall nanotubes and carbon nanofibres) were observed over Fe-doped Al₂O₃ ceramics with varying degrees of texture depending on the surface roughness, crystallographic orientation and the size of the catalyst nanoparticles. While well-textured substrates with a rough surface led to a small amount of randomly oriented carbon nanotube bundles, perpendicularly oriented individual multi-wall nanotubes were obtained over relatively smooth single crystal α-Al₂O₃ platelet surfaces (basal planes) which remained in the matrix without growing.     

Enhanced red photoluminescence in a nanocomposite thin film composed of Bi3.6Eu0.4Ti3O12 and Au-decorated ZnO nanorods

This paper firstly reports on a series of nanocomposite thin films composed of a ferroelectric Bi_3.6Eu_0.4Ti₃O₁₂ (BET) and Au-decorated ZnO nanorods (Au-ZnO), which are prepared by a ultraviolet-induced hybrid chemical solution method. The effects of Au nanoparticles (NPs) and ZnO nanorods on the significantly red photoluminescence enhancement of Eu³⁺ ions are investigated. The results indicate that the larger near band edge (NBE) emission of ZnO by the SPR effect of Au NPs can make an energy transfer from ZnO to Eu³⁺ ions. Simultaneously, the depression of the deep-level emission of ZnO can improve the monochromaticity in the visible range. Furthermore, the dielectric and ferroelectric properties of the thin films are also enhanced. The findings suggest that the nanocomposite thin films of a ferroelectric BET and Au-ZnO could be used as a multifunctional material in the ferroelectric optoelectronic devices with bright light emitting.     

Enhancing the thermoelectric properties of super-lattice Al2O3/ZnO atomic film via interface confinement

Aluminum oxide (Al₂O₃)/zinc oxide (ZnO) thin films deposited via atomic layer deposition (ALD) are demonstrated to enhance their thermoelectric properties by manipulating them with a nano-thick Al₂O₃ interface. The overall superlattice structure is tuned by varying the ZnO ALD sequence and the Al₂O₃ ALD sequence while maintaining the same composition. An aluminum-doped zinc oxide (AZO) thin film is deposited at 250 °C, and the Al₂O₃ thickness in the superlattice is gradually increased from 0.13 nm to 1.23 nm. The total film composition is fixed at 2% AZO. We observe that an efficient superlattice structure is made with a specific Al₂O₃ thickness. The thermal conductivity is significantly decreased from 0.57 W/mK to 0.26 W/mK as the thickness of the Al₂O₃ layer is increased. Additionally, the absolute Seebeck coefficient is increased from 14 μV/K to 65 μV/K. This may be caused by the interface confinement effect and interface scattering between the ZnO layer and the Al₂O₃ layer. The figure of merit ZT value is 0.14 for the most efficient structure.     

Synergetic effect of combined use of Cu–ZnO–Al2O3 and Pt–Al2O3 for the steam reforming of methanol

Commercial Cu–ZnO–Al₂O₃ catalysts are used widely for steam reforming of methanol. However, the reforming reactions should be modified to avoid fuel cell catalyst poisoning originated from carbon monoxide. The modification was implemented by mixing the Cu–ZnO–Al₂O₃ catalyst with Pt–Al₂O₃ catalyst. The Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture created a synergetic effect because the methanol decomposition and the water–gas shift reactions occurred simultaneously over nearby Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalysts in the mixture. A methanol conversion of 96.4% was obtained and carbon monoxide was not detected from the reforming reaction when the Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture was used.     

Efficient production of hydrogen and multi-walled carbon nanotubes from ethanol over Fe/Al2O3 catalysts

Fe/Al₂O₃ catalysts with different Fe loadings (10-90 mol%) were prepared by hydrothermal method. Ethanol decomposition was studied over these Fe/Al₂O₃ catalysts at temperatures between 500 and 800 °C to produce hydrogen and multi-walled carbon nanotubes (MWCNTs) at the same time. The results showed that the catalytic activity and stability of Fe/Al₂O₃ depended strongly on the Fe loading and reaction temperature. The Fe(30 mol%)/Al₂O₃ and Fe(40 mol%)/Al₂O₃ were both the effective catalyst for ethanol decomposition into hydrogen and MWCNTs at 600 °C. Several reaction pathways were proposed to explain ethanol decomposition to produce hydrogen and carbon (including nanotube) at the same time.     

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.     

One-dimensional ZnO nanostructure growth prepared by thermal evaporation on different substrates: Ultraviolet emission as a function of size and dimensionality

Large-scale uniform one-dimensional ZnO nanostructures were fabricated through thermal evaporation via the vapor solid mechanism on different substrates. The effects of Si (100), Si (111), SiO₂ and sapphire substrates with constant oxygen treatment on the morphology and diameter of ZnO nanostructures were investigated. It is found that the type of substrate has a great effect on the shape and diameter of the synthesized nanowires, nanorods, and nanotubes. It is noticed that the size and dimensionality were the most influential parameters on both structural and optical properties of the grown ZnO nanostructures. X-ray diffraction analysis confirms the stability of the wurtzite crystal structure for all grown ZnO nanostructures and the preferred orientation is substrate dependent. The crystallinity as well as the defects within the crystal lattice of the grown ZnO nanostructures was studied through Raman spectroscopy. The photoluminescence spectra of ZnO nanostructures grown on Si (100), Si (111), SiO₂ and sapphire substrates showed two peaks at a near-band-edge (NBE) emission in the ultraviolet region and a broad deep-level emission (DLE) around the green emission.     

Crystallisation in SiO2 –Al 2 O 3 –ZnO– TiO 2 glass: process mechanism

Abstract

Devitrification of a frit based on the oxide system SiO₂–Al₂O₃–ZnO, using bulk glass as well as powdered glass samples with different particle sizes was studied. The following major crystalline phases were identified: gahnite (ZnAl₂O₄ ), α-quartz, cristobalite, and a solid solution exhibiting the β-quartz structure, which contained SiO₂ , Al₂O₃ , and ZnO and whose composition varied with temperature. Depending on the size of the glass (frit) samples, gahnite was found to devitrify directly from the initial glassy phase or from the solid solution with a β-quartz structure, which appeared to act as a precursor.     

Mechanically mixed ZnO‐Al2O3 catalysts in the synthesis of propylene carbonate via alcoholysis of urea

Alcoholysis of urea with 1,2‐propylene glycol (PG) is a potential industrial process for the synthesis of propylene carbonate (PC) with many advantages. However, the preparation of industrial catalysts such as Zn‐Al oxide by the co‐precipitation method will inevitably produce a large amount of salty water and building desalting equipment will increase the investment cost. The present work attempts to use a mechanically mixed catalyst consisting of commercial ZnO and Al₂O₃ to solve these problems. This type of catalyst was capable of catalyzing the reaction more than 5 times. The PC yield was gradually increased with each use. The best PC yield of 96.7% is comparable to the co‐precipitated Zn‐Al oxide catalysts and was achieved during the third use. In the reaction, ZnO and Al₂O₃ were dissolved into the reactants to form complexes, which homogenously catalyzed the reaction. It was found that the dissolved Zn and Al complex significantly influenced the PC yield and re‐precipitated in the late stage of the reaction. Furthermore, the partial dissolution of Al assisted the dissolution and precipitation of Zn, which improved the PC yield. After several dissolution‐precipitation cycles, ZnO‐Al₂O₃ was homogeneously mixed at the atomic scale. Interestingly, there was an adequate linear relationship between the amount of dissolved Zn and Al in each reaction, the linear correlation coefficient improved after each reaction, and the slope of the line (the ratio of dissolved Zn/Al) was 7.26 in the third use.     

Catalytic oxidation of benzene,toluene and p-xylene over colloidal gold supported on zinc oxide catalyst

Au was loaded (1.5 wt.%) on the supports (ZnO, Al₂O₃ and MgO) by a colloidal deposition method. For a range of low temperatures (50–300 °C), the catalytic activity of Au/ZnO was much greater than that of Au/Al₂O₃ and Au/MgO. In particular, for the Au/ZnO, the benzene conversion exceeded 80% at 150 °C. The results of catalyst characterization suggested that the high catalytic activity of the Au/ZnO might be attributed to the effects of strong metal–oxide interaction which is possibly originated from the small lattice parameter difference between Au {111} and ZnO {101} lattice planes.     

Electrical and optical properties of Al-doped ZnO and ZnAl2O4 films prepared by atomic layer deposition

ZnO/Al₂O₃ multilayers were prepared by alternating atomic layer deposition (ALD) at 150°C using diethylzinc, trimethylaluminum, and water. The growth process, crystallinity, and electrical and optical properties of the multilayers were studied with a variety of the cycle ratios of ZnO and Al₂O₃ sublayers. Transparent conductive Al-doped ZnO films were prepared with the minimum resistivity of 2.4 × 10⁻³ Ω·cm at a low Al doping concentration of 2.26%. Photoluminescence spectroscopy in conjunction with X-ray diffraction analysis revealed that the thickness of ZnO sublayers plays an important role on the priority for selective crystallization of ZnAl₂O₄ and ZnO phases during high-temperature annealing ZnO/Al₂O₃ multilayers. It was found that pure ZnAl₂O₄ film was synthesized by annealing the specific composite film containing alternative monocycle of ZnO and Al₂O₃ sublayers, which could only be deposited precisely by utilizing ALD technology.     

Annealing effects on the optical and morphological properties of ZnO nanorods on AZO substrate by using aqueous solution method at low temperature

Vertically aligned ZnO nanorods (NRs) on aluminum-doped zinc oxide (AZO) substrates were fabricated by a single-step aqueous solution method at low temperature. In order to optimize optical quality, the effects of annealing on optical and structural properties were investigated by scanning electron microscopy, X-ray diffraction, photoluminescence (PL), and Raman spectroscopy. We found that the annealing temperature strongly affects both the near-band-edge (NBE) and visible (defect-related) emissions. The best characteristics have been obtained by employing annealing at 400°C in air for 2 h, bringing about a sharp and intense NBE emission. The defect-related recombinations were also suppressed effectively. However, the enhancement decreases with higher annealing temperature and prolonged annealing. PL study indicates that the NBE emission is dominated by radiative recombination associated with hydrogen donors. Thus, the enhancement of NBE is due to the activation of radiative recombinations associated with hydrogen donors. On the other hand, the reduction of visible emission is mainly attributed to the annihilation of OH groups. Our results provide insight to comprehend annealing effects and an effective way to improve optical properties of low-temperature-grown ZnO NRs for future facile device applications.     

Enhancement of luminescence properties and the role of ZnO in Tb3+ ions doped zinc aluminum phosphate glasses

Terbium ion doped zinc aluminum phosphate (ZAP) glasses with composition (90−x)((90−y)P₂O₅–10Al₂O₃–yZnO)–xTb₂O₃ (x=0.5–9 in mol% and y=30, 35, 40 in mol%) have been prepared by melt quenching method, and the effects of the Tb₂O₃ and ZnO content on the luminescence properties have been studied by photoluminescence spectroscopies. It was found that the green emission peaked at 544 nm is significantly enhanced under higher Tb₂O₃ content, meanwhile the sensitization effect of ZnO content is confirmed from the enhanced main emission. The quenching effect attributed to the resonant energy transfer through the cross-relaxation mechanism is observed when Tb₂O₃ concentration is beyond 2.5 mol% due to the fact that more Tb³⁺ ions enhance the 4f→5d and 4f→4f electronic transitions through the dipole–dipole (d–d) interaction. Also, ZnO plays a role of the disperser to prevent non-radiative de-excitation process. A characteristic luminescence image of the (100−x)(60P₂O₅–10Al₂O₃–30ZnO)·xTb₂O₃ series glasses under UV excitation at 366 nm is presented for the first time, and the transition of luminescence suggests that the Tb³⁺-doped ZAP glasses are suitable for green and dual-color blue/green LED applications by modulation of Tb and ZnO composition.     

Synthesis of glass-ceramic glazes in the ZnO–Al2O3–SiO2–ZrO2 system

Glazes in the ZnO–Al₂O₃–SiO₂–ZrO₂ system with crystallization ability of gahnite (ZnO·Al₂O₃) and β-quartz solid solution (βqss) were synthesized. The compositions were designed based on calcium and magnesium oxide replacement (from a CaO–MgO–Al₂O₃–SiO₂ glass-ceramic glaze system) with zinc oxide and simultaneous increasing aluminum oxide. By this replacement, diopside eliminated and co-precipitation of gahnite and zirconium silicate observed. However, a little addition of Li₂O changes the crystallization path by precipitation of βq_ss and willemite (2ZnO·SiO₂) at low temperatures (800–900 °C) which dissolved into glaze by development of firing temperature. The experiments showed that while the micro-hardness of gahnite-based glass-ceramic glazes is almost equal with the diopside based one, it is more than the traditional floor tile glazes.     

Study on thermally conductive ESBR vulcanizates

The thermal conductivities of emulsion polymerized styrene-butadiene rubber (ESBR) vulcanizates filled with alumina (Al₂O₃), zinc oxide (ZnO), carbon nanotubes (CNTs), silicon carbide (SiC), are measured by steady-state method. The effects of types and loadings of the fillers and their mixture on thermal conductivities of the ESBR vulcanizates are investigated. The results show that the thermal conductivity of ESBR vulcanizates filled with alumina or zinc oxide, increases nearly linearly with increasing loading when the filler loading exceeded 20 phr; the ESBR vulcanizates filled with CNTs have the highest thermal conductivity at a given filler loading in comparison with other composite vulcanizates. At a given loading of 100 phr, the ESBR vulcanizate filled with two different particle sizes SiC of 1–3 and 5–11 μm at the mass ratio of 1:1 has the highest thermal conductivity and relatively good mechanical properties. The experimental results are analyzed using Geometric mean model and Agari’s equation to explain the effect of filler types and particle sizes on the formation of thermal conductive networks. The thermal conductivity of the ESBR vulcanizates filled with Al₂O₃ or ZnO or CNTs could be well predicted by optimized parameters using Agari’s equation for a polymer composite filled with mixtures of particles.