Thickness dependency of sol-gel derived ZnO thin films on gas sensing behaviors

ZnO thin films were fabricated by a sol-gel method using Zn(CH₃COO)₂·2H₂O as starting material in order to prepare an acetone gas sensor. A homogeneous and stable solution was prepared by dissolving the zinc acetate in a solution of ethanol and monoethanolamine. The sol-gel solution is coated on alumina substrates with various thicknesses by spin coating technique and heat treated to grow crystalline ZnO thin films. The effect of thickness on physical and electrical properties of as deposited ZnO thin films has been studied. The as deposited ZnO thin films were characterized by X-ray diffraction spectroscopy, field emission scanning electron microscopy and atomic force microscopy. The root mean square surface roughness factors increase with thickness of the films and found 3.9, 6.6, 9.0, and 11.28 nm for 80-, 220-, 450- and 620-nm-thin films respectively. The activation energies of the films are calculated from the resistance temperature characteristics. The sensitivities of the ZnO films towards the acetone gas were determined at an operating temperature of 200 °C. The sensitivity towards acetone vapor is strongly depending on surface morphology of the ZnO thin films.     

Novel synthesis of aligned Zinc oxide nanorods on a glass substrate by sonicated sol-gel immersion

For the first time, aligned ZnO nanorod structured thin films have been synthesized on a glass substrate, which had been coated with an Al-doped ZnO thin film, using the sonicated sol-gel immersion method. These nanorods were found to have an average diameter of 100 nm and an average length of 500 nm, with hexagonal wurtzite phase grew preferentially along the c-axis direction. A sharp ultra-violet (UV) emission centred at 383 nm corresponding to the free exciton recombination was observed in a room temperature photoluminescence (PL) spectrum. The prepared ZnO nanorod structured thin film is transparent in the visible region with an average transmittance of 78% in the 400-800 nm wavelength range and high absorbance properties in the UV region (< 400 nm). The results indicate that the prepared ZnO nanorods are suitable for ultra-violet photoconductive sensor applications.     

Optical and field emission characteristics of anodic aluminium oxide/ZnO hybrid nanostructure

Arrays of ZnO nanowires (NWs) were fabricated within the well-distributed pores of anodic aluminium oxide (AAO) template by a simple chemical method. The photoluminescence (PL) and field emission (FE) properties of the AAO/ZnO NWs hybrid structure were investigated in detail. The hybrid nanostructure exhibits interesting PL characteristics. ZnO NWs exhibit UV emission at 378 nm and two prominent blue-green emissions at about 462 and 508 nm. Intense blue emission from the AAO template itself was observed at around 430 nm. Herein, for the first time we report the FE characteristics of the ZnO/AAO hybrid structure to show the influence of the AAO template on the FE property of the hybrid structure. It is found that the turn-on electric field of the vertically grown and aligned ZnO NWs within the pores of AAO template is lower than the entangled unaligned ZnO NWs extracted from the template. Although the AAO template exhibits no FE current but it helps to achieve better FE property of the ZnO NWs through better alignment. The turn-on electric field of aligned NWs was found to be 3 V μm⁻¹ at a current of 0.1 μA. Results indicate that the AAO embedded ZnO NW hybrid structure may find useful applications in luminescent and field emission display devices.     

Hybrid nanostructures of titanium-decorated ZnO nanowires

Hybrid nanostructures of titanium (Ti)-decorated zinc oxide (ZnO) nanowire were synthesized. Various thick Ti films (6 nm, 10 nm, and 20 nm) were coated to form a titanium oxide (TiO) coating layer around ZnO nanowires. Transmission electron microscope analysis was performed to verify the crystallinity and phases of the TiO layers according to the Ti-coating thickness. Under UV illumination, a bare ZnO nanowire showed a conventional n-type conducting performances. With a Ti coating on a ZnO nanowire, it was converted to a p-type conductor due to the existence of electron-captured oxygen molecules. It discusses the fabrication of Ti-decorated ZnO nanowires including the working mechanisms with respect to UV light.     

Fabrication of TiO2 nanotube film by well-aligned ZnO nanorod array film and sol-gel process

High density TiO₂ nanotube film with hexagonal shape and narrow size distribution was fabricated by templating ZnO nanorod array film and sol-gel process. Well-aligned ZnO nanorod array films obtained by aqueous solution method were used as template to synthesize ZnO/TiO₂ core-shell structure through sol-gel process. Subsequently, TiO₂ nanotube array films survived by removing the ZnO nanorod cores using wet-chemical etching. Polycrystalline anatase TiO₂ nanotube films were ∼ 1.5 μm long and ∼ 100 nm in inter diameter with a wall thickness of ∼ 10 nm.     

Synthesis and photocatalytic properties of BiOCl nanowire arrays

Bismuth oxychloride (BiOCl) nanowire arrays have been successfully prepared employing the Anodic Aluminum Oxide (AAO) template assisted sol-gel method. Nanowires of 100 nm diameter and length 2-6 μm, assembled in the porous of AAO templates, were formed. XRD and HRTEM results show that the nanowires are pure BiOCl polycrystal phase without Bi₂O₃ or BiCl₃. The photocatalytic activity of BiOCl nanowire arrays was investigated by the degradation of Rhodamine B dye solution under UV irradiation.     

Hybrid p-type ZnO film and n-type ZnO nanorod p-n homo-junction for efficient photovoltaic applications

Simple hybrid p-n homo-junctions using p-type ZnO thin films and n-type nanorods grown on fluorine tin oxide (FTO) substrates for photovoltaic applications are described. The ZnO nanorods (1.5 μm) were synthesized via an aqueous solution method with zinc nitrate hexahydrate and hexamethylenetetramine on ZnO seed layers. The 10-nm-thick ZnO seed layers showed n-type conductivity on FTO substrates and were deposited with a sputtering-based method. After synthesizing ZnO nanorods, aluminum-nitride co-doped p-type ZnO films (200 nm) were efficiently grown using pre-activated nitrogen (N) plasma sources with an inductively-coupled dual-target co-sputtering system. The structural and electrical properties of hybrid p-n homo-junctions were investigated by scanning electron microscopy, transmittance spectrophotometry, and I-V measurements.     

Aqueous synthesis of interconnected ZnO nanowires using spray pyrolysis deposited seed layers

Interconnected ZnO nanowires were grown in a two-stage process, using spray pyrolysis deposited ZnO seed layers as a nucleation platform for subsequent hydrothermal growth. We present a comparison between the effect of these spray pyrolysis deposited seed layers and well-ordered sputter deposited seed layers, along with their respective ZnO nano-morphologies that were obtained via hydrothermal growth. It will be shown that the growth of interconnected ZnO nanowires was influenced by the physical and crystallographic orientations of the underlying seed crystallites. Sputtered seed layers resulted in fairly vertical nanorods which were approximately 80 nm in width, while seed layers deposited by spray pyrolysis resulted in arrays of interconnected ZnO nanowires measuring approximately 15 nm in width.     

ZnO thin films prepared by a single step sol-gel process

ZnO thin films were prepared on fused silica from a single spin-coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C₂H₃O₂)₂], monoethanolamine [H₂NC₂H₄OH ] and isopropanol. Crystallization annealing was performed over the range 500 to 650 °C. X-ray analysis showed that thin films were preferentially orientated along the [002] c-axis direction of the crystal. The films had a transparency of greater than 85% in the visible region for sol-gels with a zinc content of up to 0.7 M and exhibited absorption edges at ∼ 378 nm. The optical band-gap energy was evaluated to be 3.298-3.306 eV. Photoluminescence showed a strong emission centered at ca. 380 nm along with a broad yellow-orange emission centered at ca. 610 nm. Single step sol-gel thin film deposition in the film thickness range from 80 nm to 350 nm was demonstrated. The effect of sol-gel zinc concentration, film thickness and crystallization temperature on film microstructure, morphology and optical transparency is detailed. A process window for single spin coating deposition of c-axis oriented ZnO discussed.     

Seeded growth of ZnO nanorods from NaOH solutions

Perpendicularly aligned arrays of corrugated ZnO nanorods were grown onto gold patterned LiTaO₃ substrates, coated with a sputtered ZnO seed layer. During the growth process, these substrates were held submerged in an aqueous solution comprising a 1:40 mol ratio mix of zinc nitrate hexahydrate to sodium hydroxide. The substrates were placed in a custom apparatus residing in an autoclavable storage bottle. Scanning electron micrographs, which were taken at different deposition intervals, suggest that the growth mechanism of ZnO nanorods initiates with the etching of the ZnO sputtered seed layer into hexagonal bases (> 500 nm across), from where multiple protrusions (40 nm-100 nm in width) grow atop these hexagonal bases. Such nanoprotrusions later coalesce into larger nanorods. Uniformly distributed high density corrugated nanorods, with proximal spacing between adjacent nanorods of approximately 20 nm-50 nm, were observed over the entire surface.     

Low temperature growth and optical properties of ZnO nanowires using an aqueous solution method

ZnO nanowires were grown on indium tin oxide (ITO) coated glass substrates at a low temperature of 90 degrees C using an aqueous solution method. The ZnO seeds were coated on the ITO thin films by using a spin coater. ZnO nanowires were formed in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2 x 6H2O) and hexamethylenetetramine (C6H12N4). The pH value and concentration of the solution play an important role in the growth and morphologies of ZnO nanowires. The size of ZnO naonowires increased as the concentration of the solution increased. It was formed with a top surface of hexagonal and tapered shape at low and high pH values respectively. Additionally, the single crystalline structure and optical property of the ZnO nanowires were investigated using high-resolution transmission electron microscopy and photoluminescence spectroscopy.     

Fabrication of bismuth oxide-tin oxide nanowires by direct thermal oxidation of Bi-Sn eutectic nanowires

Bismuth oxide-tin oxide (BiO_x-SnO_x) heterostructure nanowires with a diameter of 70 nm were fabricated by directly annealing Bi-Sn eutectic nanowires synthesized by the vacuum hydraulic pressure injection process. After removal of AAO (Anodic Aluminum Oxide) template with an etching solution, a spontaneous oxide was formed on nanowires to enclose the Bi-Sn eutectic alloys. While these nanowires went through the annealing process with the proper heating rate of 50 °C/min, the well-annealed oxide nanowires remain solid, straight and segmental. The results of cathodoluminescence (CL) spectrum and photoresponse proved that the products consisted of bismuth oxide and tin oxide. This fabrication methodology provides a simple way to produce one-dimensional oxide nanomaterials.     

Enhanced light to electricity conversion efficiency of CdS–ZnO composite nanorod based electrochemical solar cell

Cadmium sulfide–zinc oxide composite nanorods having at least 100 nm diameters were synthesized by a two-step chemical deposition technique. Polycrystalline nanorods of ZnO were grown on indium tin oxide coated quartz substrate by aqueous chemical growth technique. Cadmium sulfide was deposited on the surface of the ZnO nanorod thin film by chemical bath deposition. The X-ray diffraction results revealed the co-existence of polycrystalline CdS and ZnO, both having hexagonal structures. Neither any phase mixing nor any surface diffusion induced alloying was observed. Micro-Raman study detected a pair of optical phonons at 301 cm⁻¹ and 438 cm⁻¹ corresponding to hexagonal CdS and ZnO, respectively. An enhanced light to electricity conversion efficiency of 2.52% was recorded from CdS–ZnO photoanode based electrochemical solar cell under 0.5 sun illumination condition (50 mW cm⁻²). We observed a significant enhancement of short circuit current of the electrochemical solar cells due to addition of ionic salt solution to the electrolyte.     

Physical and sensing properties of ZnO:F:Al thin films deposited by sol-gel

Fluorine and aluminum-doped zinc oxide thin films, ZnO:F:Al, were prepared on soda-lime glass substrates by the sol-gel method and repeated dip-coating. The effect of the solution ageing and film thickness on the physical characteristics of the films was studied. Two ageing times, namely, two and seven days, and three different thicknesses, in the order of 220, 330, and 520 nm, were the main variables used in this work. As-deposited ZnO:F:Al films showed a high electrical resistivity, however after a vacuum thermal treatment, it was registered a significant decrease. Structural, optical, and morphological characterizations were carried out in vacuum-annealed films. The X-ray diffraction (XRD) patterns revealed that both as-deposited and vacuum-annealed ZnO:F:Al thin films were polycrystalline with a hexagonal wurtzite-type structure with a well-defined (002) diffraction peak, irrespective of the ageing time of the starting solution. The (002) peak shows a proportional increase with the thickness magnitude. An average crystallite size of about 20 nm was estimated using the well-known Scherrer's formula. From the surface morphological study it was observed that the grain size is almost independent of the ageing time of the starting solution, and the film thickness. Films presented an average optical transmittance in the visible range (400-700 nm) in the order of 90%, as well as a band gap of 3.3 eV. The gas-sensing properties of ZnO:F:Al thin films in an atmosphere containing different concentrations of carbon monoxide, and at different operation temperatures were probed. The highest sensitivity registered was of the order of 93%.     

On the structural, morphological, optical and electrical properties of sol-gel deposited ZnO:In films

Indium-doped zinc oxide thin films deposition was performed by the sol-gel technique using homogeneous and stable solutions of zinc acetate 2-hydrate and indium chloride in ethanol. Films were spin coated onto glass substrates. After drying and after a heat treatment at 450 °C, highly transparent (80%-90%) films were obtained. The effect on the structural, morphological, optical and electrical thin films properties of the dopant concentration was investigated. The temperature dependencies of the electrical conductivity under vacuum and in open atmosphere were analysed and discussed.     

Microstructural evolution of sol-gel derived ZnO thin films

Zinc oxide thin films, with thicknesses between ∼ 20 and 450 nm, were prepared by spin-coating a sol-gel precursor solution (zinc acetate dihydrate and monoethanolamine in an isopropanol solvent) onto glass substrates, followed by heat treatment at temperatures through 773 K. At 298 and 373 K, the films exhibited the structure of a lamellar ZnO precursor, Layered Basic Zinc Acetate (LBZA). At higher temperatures, LBZA released intercalated water and acetate groups and dehydroxylated to form zinc oxide nanograins with wurtzite structure, which were preferentially oriented in the c-axis direction. Both the degree of the films' c-axis orientation and the topography of their surfaces varied with heat treatment and precursor concentration. For films calcined at 773 K, a minimum of micron-scale surface wrinkles coincided with a maximum in c-axis preference at intermediate concentrations, suggesting that release of mechanical stress during densification of thicker films may have disrupted the ordering process that occurs during heat treatment.     

Facile synthesis of single crystalline vanadium pentoxide nanowires and their photocatalytic behavior

High quality single crystalline vanadium pentoxide (V₂O₅) nanowires were grown on sapphire and ITO coated glass substrates using spin coating followed by annealing process. The nanowires formed by this method are found to be approximately 5 μm long with an average diameter of 100 nm. The thickness of the spin coated vanadium precursor film played a vital role to form uniform seed layers which are essential for the growth of high quality V₂O₅ nanowires. The growth mechanism was investigated with respect to temperature and thickness of the precursor film. The synthesized nanowires have been proven to be a potential photocatalyst for the degradation of toluidine blue O dye under ultraviolet irradiation.     

Investigations on structural, optical and electrical properties of p-type ZnO nanorods using hydrothermal method

Undoped and doped ZnO nanorods were grown from an aqueous solution at low temperature (90 °C) on sapphire (100) substrates coated with ZnO thin film annealed in air at 550 °C for 1 h. X-ray diffraction results show that these nanorods have wurtzite type structure, and they are oriented in the c-axis direction. The optical properties are examined by room temperature micro photoluminescence and Raman scattering analysis which confirm that the nanorods exhibit good optical and electrical properties. A strong enhancement of multiple-phonon Raman scattering process with longitudinal optical phonon overtone up to fifth order was observed. It is found that the thin film coating of ZnO plays an important role in the c-axis oriented growth of undoped and doped ZnO nanorods due to good lattice match between the thin film and nanorods.     

Broad band optical characterization of sol-gel TiO2 thin film microstructure evolution with temperature

Titanium dioxide (TiO₂) thin films have been produced by spin coating a titanium isopropoxide sol on silicon wafer substrates. The structural evolution of the thin films in terms of decomposition, crystallization and densification has been monitored as a function of annealing temperature from 100 to 700 °C using optical characterization and other techniques. The effect of annealing temperature on the refractive index and extinction coefficient of these TiO₂ thin films was studied in the range of 0.62 to 4.96 eV photon energy (250-2000 nm wavelength) using spectroscopic ellipsometry. Thermal gravimetric analysis and atomic force microscopy support the ellipsometry data and provide information about structural transformations in the titania thin films with respect to different annealing temperatures. These data help construct a coherent picture of the decomposition of the sol-gel precursors and the creation of dense layers of TiO₂. It was observed that the refractive index increased from 2.02 to 2.45 at 2.48 eV (500 nm) in sol-gel spin coated titania films for annealing temperatures from 100 °C to 700 °C.     

The properties of transparent semiconductor Zn1 − xTixO thin films prepared by the sol-gel method

Transparent semiconductor thin films of Zn_1 − xTi_xO (0 ≦ x ≦ 0.12) were deposited on alkali-free glass substrates by the sol-gel method. The effects of Ti addition on the crystallization, microstructure, optical properties and resistivity of ZnO thin films were investigated. The as-coated films were preheated at 300 °C, and then annealed at 500 °C in air ambiance. X-ray diffraction results showed all polycrystalline Zn_1 − xTi_xO thin films with preferred orientation along the (002) plane. Ti incorporated within the ZnO thin films not only decreased surface roughness but also increased optical transmittance and electrical resistivity. In the present study, the Zn_0.88Ti_0.12O film exhibited the best properties, namely an average transmittance of 91.0% (an increase of ~ 12% over the pure ZnO film) and an RMS roughness value of 1.04 nm.