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.     

WO3薄膜气敏光学传感特性研究

WO3薄膜是良好的光学气敏传感器材料.采用溶胶凝胶法制备了WO3掺杂薄膜,对样品在不同浓度氢气气氛中的气敏光学性质、敏感度及响应时间进行了测试、分析和计算,并结合光传输理论给出了气敏薄膜的光学变化机制,理论分析与实验结论吻合.     

Chemical and optical properties of dye-doped sol-gel films

Sol-gel silica coatings doped with an acid indicator were prepared and their behaviour vs pH in aqueous media were studied. Absorption spectroscopy was used to characterize the reversible optical response of doped films both in liquid and gaseous media. Several features such as: a) strong influence of acid concentration on films colour change, b) reversibility without fatigue all over the pH range, and c) spontaneous regain of initial basic absorption when coatings were sensitized under acid vapour; pointed out that dye molecules were incorporated in the surface of the silica films pores. On the other hand, coatings resistance vs alkaline attack showed that damage quickly proceeded if films surface were inhomogeneous, due to direct corrosion of the glass substrate.     

基于相移检测的有机改性溶胶-凝胶光学氧敏感膜研究

以四乙氧基硅烷(TEOS)和二甲基二乙氧基硅烷(DDS)为共聚前驱体,Ru(phen)3Cl2为荧光指示剂,采用溶胶凝胶技术,通过改变DDS/TEOS比值,制备了基于荧光猝灭原理的有机改性氧敏感膜.探讨了成膜条件,研究了DDS的掺杂量对敏感膜结构和性能的影响.结果表明,掺入适量的有机硅醇盐DDS,敏感膜均匀无龟裂,有较好的机械性和柔韧性.将敏感膜组装到基于相移检测的光纤氧传感器并对气态氧进行检测,检测下限达2×10-6,响应时间＜30 s;对溶解氧进行检测,检测下限达0.5 mg/L,响应时间＜50 s.与纯TEOS敏感膜相比,有机改性敏感膜具有更好的重复性和稳定性,更有利于溶解氧的测量.     

Effect of annealing in a various oxygen atmosphere on structural,optical, electrical and gas sensing properties of MoxOy thin films

Molybdenum oxide thin films were thermally evaporated on a glass substrate and monitored by an annealing process in a variable oxygen atmosphere. The effects of post annealing condition on the microstructural, morphological, optical and electrical properties were investigated using X-ray diffraction, Raman spectroscopy, atomic force microscope, spectroscopic ellipsometry and impedance spectroscopy. As-deposited amorphous films crystallized into tetragonal metastable phase of Mo₅O₁₄ on annealing at 500 °C in vacuum and air. This structure transformed to stable orthorhombic of MoO₃ with annealing in oxygen environment. The optical parameters such as the refractive index, extinction coefficient, optical band gap energy and the Urbach energy were calculated from Cauchy formalism. Ellipsometric measurements reveal that the samples present optical gap located between 3.24 and 3.90 eV when the atmosphere becomes rich on oxygen. The variation of the conductivity in terms of the temperature shows an electrical behavior with oxygen environment. Finally, it has been found that MoO₃ thin films had high sensitivity to ethanol, which made them as a good candidate for the ethanol sensor.     

Effects of oxygen annealing on gas sensing properties of carbon nanotube thin films

Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition have been investigated as resistive gas sensors towards NO₂ oxidizing gas. Effects of air oxidative treatment dramatically influence the nanotubes’ electrical resistance as determined by volt-amperometric measurements. In particular the electrical measurements show that electrical behavior of the CNT films can be converted from semiconducting to metallic through thermal treatments in oxygen. The electrical response was then measured exposing the films to sub-ppm NO₂ concentrations (100 ppb in air) at 165 °C. Upon exposure to NO₂, the electrical resistance of CNTs was found to decrease. The obtained results demonstrate that nanotubes could find use as a sensitive chemical gas sensor for (a) the fast response accompanied by a high sensitivity to sub-ppm NO₂ exposure, and (b) the precise recover of the base resistance value in absence of NO₂ at a fixed operating temperature, likewise indicating that intrinsic properties measured on as prepared nanotubes may be severely changed by extrinsic oxidative treatment effects.     

Structural, optical and gas sensing properties of spin coated nanocrystalline zinc oxide thin films

Synthesis of nanocrystalline zinc oxide thin films by sol gel spin coating technique and its application as ammonia gas sensor is presented in this paper. The synthesized sample is pure zinc oxide with hexagonal wurtzite structure. The lattice parameters are: a = 3.2568 Å and c = 5.210 Å. Average crystallite size is of the order of 58 nm. SEM studies show that growth of the film takes place with folded structure, increasing the open surface area of the film. Optical study revealed that band gap of ZnO is 3.25 eV with direct band to band transitions. Gas sensing characteristics showed that ZnO film is sensitive as well as fast responding to ammonia gas at 573 K. A high sensitivity for ammonia gas indicates that the ZnO films are selective for this gas. The rise time and recovery time are 25 and 80 s, respectively. The mechanism of gas sensing is explained adequately.     

Optimization of mixed Langmuir-Blodgett films of a water insoluble porphyrin in a calixarene matrix for optical gas sensing

The gas sensing capabilities of Langmuir-Blodgett (LB) mixed films of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) and p-tert-butylcalix[8]arene (C8A) have been studied in this work. Although EHO is known to be very sensitive to NO₂ gas, this study demonstrates that the C8A matrix improves the sensing properties of the porphyrin molecules in the solid state. After the exposure to NO₂ and the subsequent recovery, the UV-vis spectrum of a C8A:EHO film shows no aggregation of the porphyrin. In atomic force microscopy (AFM) images, C8A:EHO films appear with sharper surfaces than those made of pure EHO, allowing a better accessibility of the gas molecules to the active binding sites. Multilayer LB films of the C8A:EHO system ranging from 2 to 40 layers have been prepared to study their response to NO₂ by UV-vis spectroscopy, and their kinetics reveal an important thickness dependence. Through the analysis of AFM images, it has been found that the surface roughness increases until the sample reaches 20 layers and then remains almost constant, which is related to the response time. The optimum film thickness has been found to be 20 layers, for which both the speed of response and the surface roughness are maximum.     

Nitric oxide-releasing fluorescence-based oxygen sensing polymeric films

The in vitro analytical performance of fluorescence-based oxygen sensing polymeric films prepared with silicone rubbers that spontaneously release nitric oxide (NO) is examined. The use of NO-release polymers for fabricating functional optical sensors is proposed as a potential solution to fingering biocompatibility and concomitant performance problems encountered with prototype intravascular optical oxygen sensors. Plasticized silicone rubber films formulated with two distinct types of diazeniumdiolate NO donors release NO for more than 24 h. The optical oxygen sensing films prepared by doping these NO release polymeric materials with oxygen indicators (pyrene/perylene donor/acceptor pair) display different analytical responses, as compared to controls without NO release capability. Nonlinear Stern-Volmer behavior is observed for single-layer NO release oxygen sensors owing to heterogeneous environments for the pyrene/perylene pair and a concomitant quenching of the fluorescence by excess amine sites in such films. In contrast, a dual-layer configuration using an underlying NO-release silicone rubber layer covered with a second polymeric layer containing the fluorescent indicators is shown to yield identical sensitivity and linearity toward oxygen as conventional non-NO-releasing oxygen sensing films, while still providing surface NO fluxes necessary to yield more thromboresistive devices.     

Thin polyaniline and polyaniline/carbon nanocomposite films for gas sensing

We have studied the gas sensing properties of five polyaniline-based materials—thick and thin PANI films, nanocomposite PANI/MWNT and PANI/SWNT films, and PANI nanogranules embedded in a polyvinylpyrrolidone matrix. The films (except for the latter) were deposited within the induction period of the polymerization process on gold interdigitated micro electrodes. Their sensitivity to NH₃, H₂, ethanol, methanol, and acetone was measured. The thin PANI film (~ 100 nm thick) prepared by a lift-off process had the sensitivity to ammonia below 0.5 ppm, which was higher than that of nanocomposite films. Two materials—thick PANI film and nanocomposite PANI/MWNT film—exhibited a shallow minimum in the temperature dependence of resistance (at 313 K and 319 K), which is a feature exploitable in practical applications, since the gas sensors should be insensitive to small temperature fluctuations at these temperatures.     

Tailoring molecular architectures with cobalt tetrasulfonated phthalocyanine: immobilization in layer-by-layer films and sensing applications

In the field of organic thin films, manipulation at the nanoscale can be obtained by immobilization of different materials on platforms designed to enhance a specific property via the layer-by-layer technique. In this paper we describe the fabrication of nanostructured films containing cobalt tetrasulfonated phthalocyanine (CoTsPc) obtained through the layer-by-layer architecture and assembled with linear poly(allylamine hydrochloride) (PAH) and poly(amidoamine) dendrimer (PAMAM) polyelectrolytes. Film growth was monitored by UV-vis spectroscopy following the Q band of CoTsPc and revealed a linear growth for both systems. Fourier transform infrared (FTIR) spectroscopy showed that the driving force keeping the structure of the films was achieved upon interactions of CoTsPc sulfonic groups with protonated amine groups present in the positive polyelectrolyte. A comprehensive SPR investigation on film growth reproduced the deposition process dynamically and provided an estimation of the thicknesses of the layers. Both FTIR and SPR techniques suggested a preferential orientation of the Pc ring parallel to the substrate. The electrical conductivity of the PAH films deposited on interdigitated electrodes was found to be very sensitive to water vapor. These results point to the development of a phthalocyanine-based humidity sensor obtained from a simple thin film deposition technique, whose ability to tailor molecular organization was crucial to achieve high sensitivity.     

Temperature enhanced gas sensing properties of diamond films

Nanocrystalline diamond (NCD) film was used as a functional part of gas sensor. The gas sensing properties of H-terminated nanocrystalline diamond films were examined to oxidizing gases (i.e., COCl₂ and humid air). Pronounced increase in the surface conductivity (3 orders of magnitude) was found after sensor exposure to phosgene gas and was explained by the surface transfer doping effect. We also present a possible way how to achieve sensor selectivity, i.e. how to distinguish between phosgene and humid air (the mostly present background gas in a common environment).     

Sol-gel cobalt oxide-silica nanocomposite thin films for gas sensing applications

Various metal oxide-silica nanocomposite films have been recently proposed as gas-sensitive materials. This paper presents results on cobalt oxide-SiO₂ mesoporous nanocomposite thin films templated by a cationic surfactant. The films were deposited on glass substrate by dip-coating process, using [Co(CH₃COO)₂]·4H₂O and tetraethoxysilane (TEOS) as starting materials. The effect of withdrawal speed, number of layers and thermal treatment on the crystalline structure, morphology, Co-doping states, optical, electrical and gas sensing properties of the thin films has been investigated using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, optical transmittance and room temperature photoreduction-oxidation data.     

Preparation of sol-gel films containing thiazole-based chromophore and their nonlinear optical performances

A push-pull thiazolylazo chromophore and alkoxysilane-terminated chromophore have been synthesized. Their structures were verified by elemental analysis, FTIR, UV-Visible spectra and ¹H NMR. Followed by a hydrolysis and copolymerization process of the alkoxysilane with poly(3-(trimethoxysilyl)propyl methacrylate) (PTMSPM), transparent hybrid films were obtained by spin-coating. From TGA thermogram the initial decomposition temperature of the hybrid film was determined to be 231 °C. The molecular hyperpolarizability of thiazolylazo chromophore was evaluated by solvatochromic method and the nonlinear optical coefficient value of the hybrid film was also calculated to be 56.8 pm/V by second harmonic generation (SHG) measurements. The poled film exhibits fairly high stability of optical nonlinearity in depoling experiment, implying its suitability for device applications.     

Beta zeolite supported sol-gel TiO2 materials for gas phase photocatalytic applications

Beta zeolite supported sol-gel TiO(2) photocatalytic materials were prepared according to a sol-gel route in which high specific surface area Beta zeolite powder was incorporated into the titanium isopropoxide sol during the course of the sol-gel process. This led to an intimate contact between the zeolite surface and the TiO(2) precursors, and resulted in the anchorage of large amounts of dispersed TiO(2) nanoparticles and in the stabilization of TiO(2) in its anatase form, even for high TiO(2) wt. contents and high calcination temperatures. Taking the UV-A photocatalytic oxidation of methanol as gas phase target reaction, high methanol conversions were obtained on the Beta zeolite supported TiO(2) photocatalysts when compared to bulk sol-gel TiO(2), despite lower amounts of TiO(2) within the photoactive materials. The methanol conversion was optimum for about 40 wt.% TiO(2) loading and calcination temperatures of 500-600°C.     

Tailoring the adhesion of optical films on polymethyl-methacrylate by plasma-induced surface stabilization

Adhesion of plasma-deposited optical and protective coatings, such as amorphous hydrogenated silicon nitride, SiN_1.3, on polymethyl-methacrylate (PMMA) substrates has been found to be limited by a cohesive failure inside the PMMA bulk. Using direct exposure to a low pressure plasma in helium or to vacuum ultraviolet (VUV) radiation generated from plasma, the adhesion of SiN_1.3 at high humidity and elevated temperature has been substantially increased. Using a multitechnique analytical approach, the enhanced adhesion was attributed to the initial etching of the weak boundary layer followed by formation of a crosslinked, graded, mechanically stabilized layer in the interfacial region (interphase), which possesses a physical thickness of 50 to 100 nm and a microhardness of about 2 GPa.     

Effect of Al dopants on the structural,optical and gas sensing properties of spray-deposited ZnO thin films

Undoped and Al-doped ZnO thin films were deposited on glass substrates by the spray pyrolysis method. The structural, morphological and optical properties of these films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–Vis spectroscopy, photoluminescence (PL) and photoconductivity (PC) measurements, respectively. XRD analyses confirm that the films are polycrystalline zinc oxide with the hexagonal wurtzite structure, and the crystallite size has been found to be in the range 20–40 nm. SEM and AFM analyses reveal that the films have continuous surface without visible holes or faulty zones, and the surface roughness decreases on Al doping. The Al-doped films have been found to be highly transparent (>85%) and show normal dispersion behavior in the wavelength range 450–700 nm. The doped films show only ultraviolet emission and are found to be highly photosensitive. Among all the films examined, at 300 °C the 1.0 at% Al-doped film shows the selective high response (98.2%) to 100 ppm acetone concentration over to methanol, ethanol, propan-2-ol, formaldehyde and hydrogen.