Synthesis,characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles

Herein, we report the simple synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles prepared by facile hydrothermal method. The synthesized nanoneedles were characterized through different characterization techniques to examine its crystallinity, phase structure, morphological, compositional, optical, vibrational and gas sensing properties. The detailed morphological studies revealed that the Ag-doped ZnO nanoneedles are assembled into non-homogeneously distributed flower-shaped structures which are grown in high density. Further characterizations confirmed that the synthesized nanoneedles are pure, possessing well-crystallinity and exhibiting good optical and vibrational properties. The synthesized Ag-doped ZnO nanoneedles were used as functional material to fabricate high sensitive acetone gas sensors. The effect of operating temperature and concentration of the acetone were analyzed for detailed sensing performance of synthesized nanoneedles. By detailed sensing experiments, the response and recovery times of 10 s and 21 s, respectively were calculated at acetone concentration of 100 ppm at an optimized operating temperature of 370 °C. A maximum sensitivity of 30.233 was recorded at 370 °C operating temperature for 200 ppm of acetone for the fabricated acetone sensor based on Ag-doped ZnO nanoneedles.     

Preparation,characterization of 1D ZnO nanorods and their gas sensing properties

The 1D ZnO nanorods (NR's) were grown with Zinc (Zn) ion precursor concentration variation on seed layer glass substrate by the low temperature hydrothermal method and utilized for nitrogen dioxide (NO₂) gas sensing application. Zn ion precursor concentration varied as 0.02, 0.03, 0.04, 0.05 and 0.06 M and thin films were characterized for structural, morphological, optical, electrical, surface defect study and gas sensing properties. All the film showed dominant orientation along the (002) direction, the intensity of the peak vary with the length of the nanorods. SEM cross images confirmed that nanorods had vertical alignment perpendicular to the plane of the substrate surface. The PL intensity of oxygen vacancy related defects for prepared samples was found to be linearly proportional to gas sensing phenomena. This result in good agreement with the theoretical postulation that, oxygen vacancies plays the important role for adsorption sites to NO₂ molecule. The gas sensing performance was studied as a function of operating temperature, Zn ion precursor concentration variation, and gas concentration. The maximum gas response is 113.32–100 ppm NO₂ gas at 150 °C for 0.05 M sample out of all prepared samples. Additionally, ZnO thin film sensor has potential to detect NO₂ as low as 5 ppm.     

Schottky barrier effect of ZnO modified methyl glycol thin films for detection of hydrogen sulfide gas

Highly nanocrystalline ZnO modified methyl glycol thin films have been deposited on a p-type silicon substrate via the sol–gel spin coating manner. The morphology of the as-deposited film was scrutinized using scanning electron microscopy. I–V characteristics of the as-prepared ZnO film under vacuum and in open air were monitored. The results showed that the ZnO films have a barrier height of 0.38 eV under vacuum and 0.62 eV in open air. The Schottky barrier height between ZnO grains was determined for different reducing gases. The ZnO film showed high sensitivity to H₂S gas compared with other reducing gases due to the reduction of barrier height between ZnO grains. The as-prepared ZnO film was annealed at four different temperatures. X-ray diffraction manifested that the wurtzite hexagonal structure of ZnO deviated from ideality at annealing temperature greater than 650 °C. The barrier height of ZnO film decreased due to the increase of annealing temperature up to 650 °C and then decreased. The results also confirmed that the change of barrier height strongly affected the sensitivity of ZnO film.     

ZnO thin film nanostructures for hydrogen gas sensing applications

A ZnO thin film-based gas sensor was fabricated using a SiO₂/Si substrate with a platinum comb-like integrated electrode and heating element. The structural characteristics, morphology, and surface roughness of the as-grown ZnO nanostructure were investigated. The film revealed the presence of a c-axis oriented (002) phase with a grain size of 20.8 nm. The sensor response was tested for hydrogen concentrations of 50, 70, 100, 200, 400, and 500 ppm at the optimum operating temperature of 350 °C. The sensitivities towards 50 and 200 ppm of hydrogen gas at 350 °C were approximately 78% and 98%, respectively. A linear response was observed for hydrogen concentrations within the range of 50 ppm–200 ppm. These results demonstrated the potential application of the ZnO nanostructure for the fabrication of cost-effective and high-performance gas sensors.     

Nanostructured Cerium-doped ZnO thin film – A breath sensor

Breath analysis has emerged as one of the accurate diagnostic techniques, which can be used to correlate disease conditions with the abnormal concentrations of certain biomarkers present in the exhaled breath. In this background, thin film based chemi-resistive Cerium-doped ZnO sensors were developed for detecting acetone and ethanolamine, which are the notable biomarkers of diabetes mellitus and carcinoma respectively. Ce-doped ZnO thin films were deposited on glass substrates using spray pyrolysis technique. X-ray diffraction patterns confirmed the formation of Ce-doped ZnO thin films with polycrystalline hexagonal wurtzite structure. Scanning electron micrographs and optical absorption spectra of Ce-doped ZnO thin films revealed the decrease in average grain size and increase in band gap respectively with an increase in Ce-dopant concentration. ZnO films with 0.004 and 0.008 M of Ce-dopant concentrations showed a better room temperature sensing response towards acetone and ethanolamine respectively. Highly selective nature of the developed sensing elements towards acetone and ethanolamine with swift response and recovery times can be considered as a non-invasive and cost effective method for the detection of diseases like diabetes mellitus and carcinoma.     

Novel ozone gas sensor based on ZnO nanostructures grown by the microwave-assisted hydrothermal route

Good quality ZnO nanostructures were obtained by the microwave-assisted hydrothermal synthesis, at low reaction temperatures, using zinc acetate as the starting precursor. X-ray diffraction confirmed the crystallinity of the ZnO nanostructures, which resulted free of impurities. Field emission gun scanning electron microscopy analysis revealed that the ZnO nanostructures crystallized at 120 °C were more homogeneous and had a constant diameter along the entire wire length, exhibiting an ideal defect density that favors the gas sensing response. A new ozone gas sensor based on these nanostructures was evaluated at low exposure times (15 s) by recording the change in the film resistance. The ZnO nanostructures showed good sensitivity even at low ozone concentration (100 ppb), and fast response and short recovery time at 200 °C, demonstrating great potential for a variety of applications. Two main effects were observed: the first one is intrinsic to that of the sample, while the second is a consequence of the surface and interface complex cluster defects, which produce extrinsic defects.     

Studies on NO2 gas sensing properties of sprayed Co1−xMnxFe2O4 (0≤x≤0.5) spinel ferrite thin films

The Co_1−xMn_xFe₂O₄ (0≤x≤0.5) spinel ferrite thin films were deposited on quartz substrates by chemical spray pyrolysis technique. The effect of Mn substitution on to the structural, electrical, dielectric and NO₂ gas sensing properties of cobalt ferrite thin films was studied. The X-ray diffraction analysis reveals that deposited films exhibit spinel cubic crystal structure. The lattice constant increases with the increase in Mn²⁺ content. The decrease in resistivity with increase in temperature suggests that the films have a semiconducting nature. The room temperature dielectric properties such as dielectric constant (ε′), loss tangent (tanδ), dielectric loss (ε′′) and AC conductivity have been studied in the frequency range 20 Hz–1 MHz. The film shows the highest sensor response at moderately low (150 °C) operating temperature. The effect of operating temperature, gas concentration, film selectivity and substitution of Mn on to gas response is carefully studied. The manganese substituted cobalt ferrite films are extremely selective towards NO₂ with a 20 times gas response compared with other gases. The gas response achieved nearly 92% of its initial value after 150 days, indicating good stability of the films.     

Improved hydrophilicity of zinc oxide-incorporated layer-by-layer polyelectrolyte film fabricated by dip coating method

ZnO nanoparticles suspended in poly(acrylic acid) (PAA) were deposited onto layer-by-layer (LBL) polyelectrolyte (PET) films fabricated from poly(allylamine hydrochloride) (PAH) and PAA by dip coating method. Effect of etching time and concentration of ZnO suspension on hydrophilicity of the LBL-PET films before and after UV irradiation was examined using water contact angle measurement. 2.0 M PAH/PAA solutions with a dipping speed of 3.0 cm/min provided stable LBL-PET films with thickness sufficient for HCl etching. Glass substrates with the etched LBL-PET film dipped into 0.2 wt.% ZnO suspension exhibited the contact angle of 10° after irradiated by UV for 60 min.     

Structural and optoelectronic properties of transparent conductive c-axis-oriented ZnO based multilayer thin films with Ru interlayer

ZnO and Ru multilayer thin films are deposited using the sputtering deposition technique at room temperature. The effects of the Ru interlayer thickness and annealing temperature on the properties of multilayer thin films have been studied. An X-ray diffraction study reveals that ZnO layers are highly c-axis-oriented. The use of an Ru interlayer improves the crystalline quality of the subsequently deposited ZnO layers. Moreover, the crystalline quality of the entire structure is further enhanced through thermal annealing in a vacuum. Atomic force microscopy images show that the surface roughness of the multilayer thin films increases with a Ru interlayer thickness greater than 6 nm. The roughness of the film surface increases in correlation with annealing temperatures. This accounts for the decreased optical transmittance of the multilayer thin films annealed at temperatures higher than 450 °C. The electrical resistivity of multilayer thin films decreases with an increase in the metallic interlayer thickness. Thermal annealing at 450 °C causes low resistivity in multilayer thin films. The lowest resistivity reached ～5.4 × 10^?4 Ω cm for multilayer films with a 10-nm-thick Ru interlayer annealed at 450 °C.     

Polymeric semiconducting carbon from fullerene by pulsed laser ablation

The polymeric semiconducting carbon films are grown on silicon and quartz substrates by excimer (XeCl) pulsed laser deposition (PLD) technique using fullerene C₆₀ precursor. The substrate temperature is varied up to 300 °C. The structure and optical properties of the films strongly depend on the substrate temperature. The grain size is increased and uniform polymeric film with improved morphology at higher temperature is observed. The Tauc gap is about 1.35 eV for the film deposited at 100°C and with temperature the gap is decreased upto 1.1 eV for the film deposited at 250 °C and increased to about 1.4 eV for the film deposited at 300 °C. The optical absorption properties are improved with substrate temperature. Raman spectra show the presence of both G peak and D peak and are peaked at about 1590 cm^− 1 and 1360 cm^− 1, respectively for the film deposited at 100 °C. The G peak position remains almost unchanged while D peak has changed only a little with temperature might be due to its better crystalline structure compared to the typical amorphous carbon films and might show interesting in device such as, optoelectronic applications.     

Influence of annealing on the structural,optical and photoluminescence properties of ZnO thin films for enhanced H2 sensing application

Nanostructured zinc oxide (ZnO) thin film sensors were prepared by spray pyrolysis, and their structural, optical, photoluminescence and morphological properties were investigated by X-ray diffractometer, UV–vis spectrometer, photoluminescence spectrometer, and scanning electron microscope (SEM), respectively. The post-annealing of ZnO film in air at 400 °C was found to be effective for the distribution of grains and their sizes, which favors the c-axis orientation of the film. This enhancement is accompanied by an increase in the optical band gap from 3.4 eV to 3.53 eV, which confirms the uniformity of ZnO film prepared by using a specially designed spray nozzle. SEM micrograph after heat treatment revealed uniform distribution of particles with well grown grains of ZnO. Hydrogen sensing measurement indicated the annealed ZnO film to show much higher response than the as deposited film. To understand the enhancement of the sensing performance of the annealed ZnO film, the gas sensing mechanism of the film was proposed and discussed. The magnitudes of the sensor response as well as its dependence on annealing differ significantly depending on the crystallite size of the film.     

Synthesis,characterization and enhanced acetone sensing performance of Pd loaded Sm doped SnO2 nanoparticles

In the present communication, we have presented a high performance acetone sensor based on Pd loaded Sm doped SnO₂ nanomaterial. The (0.5, 1, 2 and 3) wt% Pd loaded 6 mol% Sm doped SnO₂ nanoparticles were prepared using a co-precipitation method. The characterization of samples was done by using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FEG-SEM), Energy Dispersive Analysis by X-rays (EDAX), High Resolution-Transmission Electron Microscopy (HR-TEM) and Selective Area Electron Diffraction (SAED) techniques. The gas response studies such as sensitivity, selectivity and stability towards liquid petroleum gas, ammonia, ethanol and acetone were measured at 100 ppm concentrations. The results show that optimum Pd loading (2 wt%) results in smaller crystallite size (~3.1 nm), lower operating temperature (200 °C), higher gas response (94%),better selectivity, faster response (~3 s) and quicker recovery (~12 s) towards acetone.     

Structure and dielectric properties of barium titanate thin films for capacitor applications

BaTiO₃ is a typical ferroelectric material with high relative permittivity and has been used for various applications, such as multilayer ceramic capacitors (MLCCs). With the tendency of miniaturization of MLCCs, the thin films of BaTiO₃ have been required. In this work, BaTiO₃ thin films have been deposited on Pt-coated Si substrates by RF magnetron sputtering under different deposition conditions. The films deposited at the substrate temperature from 550 °C–750 °C show a pure tetragonal perovskite structure. The films deposited at 550 °C–625  °C exhibit (111) preferential orientation, and change to (110) preferential orientation when deposited above 650 °C. The film morphologies vary with working pressure and substrate temperature. The film deposited at 625 °C and 4.5 Pa has the relative permittivity of 630 and the loss tangent of 2% at 10 kHz.     

Self-assembled films of multi-wall carbon nanotubes used in gas sensors to increase the sensitivity limit for oxygen detection

We have developed a self-assembly method to obtain multi-wall carbon nanotube films applied in resistive gas sensor devices. This self-assembly method produces a homogeneous multi-wall carbon nanotube film with good adhesion to the substrate and an oxygen sensor device with a sensitivity 3.4 times higher than films made by regular casting deposition. A fast recovery time of 0.9 s was observed at 160 °C operation temperature.     

Heteroepitaxial ZnO/sapphire (0 0 0 1) structure prepared by sol–gel process

ZnO thin films were grown on sapphire (0 0 0 1) substrates by sol–gel process and their structural and optical properties were characterized in detail. High-quality texture was obtained by using precursor solution of zinc acetate and ethanolamine in 2-methoxyethanol, pyrolyzed at 300 °C, then heated at 500 °C, and finally annealed at 750 °C. Highly c-axis oriented ZnO films were confirmed by X-ray θ–2θ scan. A relatively high transmittance in the visible spectra range and clear absorption edge of the film were observed. Epitaxial relationship between ZnO and sapphire and photoluminescence of the film were examined by using a X-ray pole-figure analysis and He–Cd laser. Near-band-edge emission with a deep-level emission was observed.     

Hierarchical heterostructure of porous NiO nanosheets on flower-like ZnO assembled by hexagonal nanorods for high-performance gas sensor

A novel hierarchical heterostructure consisting of porous NiO nanosheets and flower-like ZnO assembled by hexagonal nanorods was successfully fabricated by a simple two-step hydrothermal approach. Flower-like ZnO was obtained by the first step hydrothermal method. Through the second step hydrothermal method, porous NiO nanosheets grew on the surface of flower-like ZnO to realize integration of ZnO and NiO, so the p-n heterostructure between ZnO and NiO formed. The samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX). Gas sensing test results showed that the sensor based on NiO/ZnO composite exhibited superior sensing properties to acetone. The sensor response to 100 ppm acetone was about 205.14 at the optimum working temperature of 240 °C, and the response and recovery times were about 7 and 20 s, respectively. The enhanced response might be attributed to heterojunction and larger specific surface area provided by attached porous NiO nanosheets. The rapid response and recovery characteristics and improved selectivity attributed to the porous structure and good catalytic actions of NiO nanosheets.     

Influence of organics on ZnO coatings prepared by chemical solution deposition

Highly c-axis-oriented ZnO thin films were prepared on soda–lime–silica glass substrates by chemical solution deposition process using a zinc-naphthenate precursor as a starting material. In order to investigate the effect of pyrolysis temperature on properties of the films such as crystallinity, surface morphology and transmittance in visible spectra region, the precursor films were pyrolyzed at 300 °C or 500 °C, followed by final heat treatment at 600 °C. Highly transparent films in visible spectra region were obtained. The relationship between residual organics and properties of the annealed films was discussed.     

Microstructural evolution of YSZ electrolyte aerosol-deposited on porous NiO-YSZ

Dense, crack-free, ～7.5 μm thick, 8 mol% yttria stabilized zirconia (YSZ) film was aerosol deposited on porous NiO-YSZ anode substrates at room temperature without additional high-temperature sintering. The films’ microstructures and gas permeability were observed after annealing at various temperatures. The dense, gas-tight film that was observed up to 1000 °C became porous at higher temperatures probably due to structural instability related to oxygen non-stoichiometry. A cell using such film as electrolyte showed an open cell voltage of 1.10 V and a maximum power density of 0.51 W/cm² at 750 °C.     

Improved photoluminescence emission and gas sensor properties of ZnO thin films

In this article, we report a comparative study of the influence of pressure-assisted (1.72 MPa) versus ambient pressure thermal annealing on both ZnO thin films treated at 330 °C for 32 h. The effects of pressure on the structural, morphological, optical, and gas sensor properties of these thin films were investigated. The results show that partial preferential orientation of the wurtzite-structure ZnO thin films in the [002] or [101] planes is induced based on the thermal annealing conditions used (i.e., pressure assisted or ambient pressure). UV–vis absorption measurements revealed a negligible variation in the optical -band gap values for the both ZnO thin films. Consequently, it is deduced that the ZnO thin films exhibit different distortions of the tetrahedral [ZnO₄] clusters, corresponding to different concentrations of deep and shallow level defects in both samples. This difference induced a variation of the interface/bulk-surface, which might be responsible for the enhanced optical and gas sensor properties of the pressure-assisted thermally annealed film. Additionally, pressure-assisted thermal annealing of the ZnO films improved the H₂ sensitivity by a factor of two.     

Microstructure-related piezoelectric properties of a ZnO film grown on a Si substrate

We investigated the effect of grain size on the piezoelectric properties of ZnO using films of different grain sizes and a fixed thickness of 800 nm deposited on a Si substrate by pulsed laser ablation in the temperature range of 300–700 °C. All of the deposited films have a crystal structure with a c-axis orientation. The grain size of the grown films, characterized by transmission electron microscopy (TEM), increases with the deposition temperature. In contrast, their piezoelectric efficiency (PE, d₃₃), characterized by piezoelectric force microscopy (PFM), was found to initially increase with the deposition temperature up to 500 °C, after which it decreased with further increases in temperature. The maximum PE value is observed for the film deposited at 500 °C with a grain size of approximately 60 nm. The peculiar PE behavior observed was theoretically explained by a competition between the contribution of the c-axis orientation favoring a larger d₃₃ value due to the enhanced static asymmetry and the strong grain size effect that influences the piezoelectric polarization as a result of domain motion.