Gas sensing performance of nanocrystalline ZnO prepared by a simple route

The nanocrystalline powders of pure and Al³⁺-doped ZnO with hexagonal structure were prepared by a simple hydrothermal decomposition route. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and the microstructure by transmission electron microscopy (TEM). All the compositions exhibited a single phase, suggesting a formation of solid solution between Al₂O₃ and ZnO. DC electrical properties of the prepared nanoparticles were studied by DC conductivity measurements. The indirect heating structure sensors based on pure and doped ZnO as sensitive materials were fabricated on an alumna tube with Au electrodes. Gas-sensing properties of the sensor elements were measured as a function of concentration of dopant, operating temperature and concentrations of the test gases. The pure ZnO exhibited high response to NH₃ gas at an operating temperature of 200 °C. Doping of ZnO with Al³⁺ increased its response towards NH₃ and the Al³⁺-doped ZnO (3.0 wt% Al₂O₃) showed the maximum response at 175 °C. The selectivity of the sensor elements for NH₃ against different reducing gases like LPG, H₂S and H₂ was studied. The results on response and recovery time were also discussed.     

Novel method for fabrication of NiO sensor for NO2 monitoring

Nickel oxide (NiO) sensor films were prepared on glass substrate by a sol–gel spin coating technique. These films were characterized for their structural and morphological properties by means of X-ray diffraction, field emission scanning microscopy and atomic force microscopy. The NiO films are oriented along (200) plane with the cubic crystal structure. These films were utilized in nitrogen dioxide gas (NO₂) sensor. The dependence of the NO₂ response on operating temperature, NO₂ concentration was investigated. The NiO film showed selectivity for NO₂ over Cl₂ compared to H₂S $ \left( {{\text{S}}_{{{\text{NO}}_{ 2} }} /{\text{S}}_{{{\text{Cl}}_{ 2} }} = 3 7. 5,{\text{ S}}_{{{\text{NO}}_{ 2} }} /{\text{S}}_{{{\text{H}}_{ 2} {\text{S}}}} = 3. 4} \right) $ . The maximum NO₂ response of 23.3 % with 85 % stability at gas concentration of 200 ppm at 200 °C was achieved. The response time of 20 s and recovery time of 498 s was also recorded with same operating parameters.     

A ZnO nanorod based layered ZnO/64° YX LiNbO3 SAW hydrogen gas sensor

A zinc oxide (ZnO) nanorod based surface acoustic wave (SAW) sensor has been developed and investigated towards hydrogen (H₂) gas. The ZnO nanorods were deposited onto a layered ZnO/64° YX LiNbO₃ substrate using a liquid solution method. Micro-characterization results revealed that the diameters of ZnO nanorods are around 100 and 40 nm on LiNbO₃ and Au (metallization for electrodes), respectively. The sensor was exposed to different concentrations of H₂ in synthetic air at operating temperatures between 200 °C and 300 °C. The study showed that the sensor responded with highest frequency shift at 265 °C. At this temperature, stable baseline and fast response and recovery were observed.     

Synthesis and characterization of CdO-doped nanocrystalline ZnO:TiO2-based H2S gas sensor

This paper reports the preparation and gas-sensing characteristic of ZnO:TiO₂-based hydrogen sulfide (H₂S) gas sensor with different mol% of CdO by polymerized complex method. The structural and gas-sensing properties of ZnO:TiO₂ materials have been characterized using X-ray diffraction and gas-sensing measurement. The electrical resistance response of the sensor based on the materials was investigated at different operating temperatures and different gas concentrations. The sensor with 10 mol% CdO-doped ZnO:TiO₂ shows excellent electrical resistance response toward H₂S gas. The cross sensitivity was also checked for reducing gases like CH₄, CO and H₂ gas. The selectivity and sensitivity of ZnO:TiO₂-based H₂S gas sensor were improved by the addition of 10 mol% of CdO at an operating temperature of 250 °C.     

Enhanced acetone detection using Au doped ZnO thin film sensor

Zinc oxide (ZnO) thin films are prepared using sol–gel method for acetone vapor sensing. Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) was taken as starting material and a stable and homogeneous solution was prepared in ethanol by deliquescing the zinc acetate and distinct amount of monoethanolamine as a stabilizing agent. The prepared solution was then coated on silicon substrates by spin coating method and then annealed at 650 °C for preparing ZnO thin films. The thickness of the film was maintained at 410 nm. The structural, morphological and optical studies were done for the synthesized ZnO thin films. The operating temperature and sensor response is considered to be an important parameter for the gas sensing behavior of any material. Therefore, the present study examined the effect of sensing behavior of 3% v/v gold (Au) doped ZnO thin films as a sensor. The response characteristics of 410 nm ZnO thin film for temperature ranging from 180 to 360 °C were determined for the acetone vapors. The reported study provides a significant development towards acetone sensors, where a very high sensitivity with rapid response and recovery times are reported with lowered optimal operating temperature as compared to bare ZnO nano-chains like structured thin films. In comparison to the bare ZnO thin films giving a response of 63 at an operating temperature of 320 °C, a much better response of 132.3 was observed for the Au doped ZnO thin films at an optimised operating temperature of 280 °C for a concentration of 500 ppm of acetone vapors.     

Thermolysis of Urea Complexes of Uranyl Nitrate

Quantitative parameters of thermolysis of uranyl nitrate urea complexes, [UO₂(NO₃)₂{(NH₂)₂·CO}₂], [UO₂(H₂O){(NH₂)₂CO}₄](NO₃)₂, and [UO₂(H₂O){(NH₂)₂CO}₅](NO₃)₂ at 175, 200, and 225°C were measured. Thermolysis of [UO₂(NO₃)₂{(NH₂)₂CO}₂] at 200°C affords the biuret complex of uranyl nitrate in a 90% yield. The urea ligands in the hydrated complexes completely transform into biuret at 175°C. Thermolysis of [UO₂(H₂O){(NH₂)₂CO}₅](NO₃)₂ yields the biuret-cyanurate complexes of uranyl nitrate. The features of thermolysis of the uranyl nitrate complexes originate from the chemical transformations of urea at elevated temperatures.     

Enhanced H2 sensing properties of a-plane ZnO prepared on c-cut sapphire substrate by sputtering

Zinc oxide (ZnO) thin films have been prepared on c-plane sapphire substrate by magnetron sputtering technique. The influence of deposition time on the structural, optical and photoluminescence properties of the films have been investigated. XRD patterns reveal the growth of preferentially oriented (101) non-polar a-plane ZnO film with hexagonal wurtzite structure. The PL peak shifts towards lower wavelength for deposition time up to 20 min, which is in consistent with the results obtained from UV absorption studies. The blue shift in the PL peak confirms the possibility for quantum confinement effect. The band gap energy of the film increases from 3.33 to 3.38 eV, indicating enhanced quantum confinement effects. FESEM micrographs showed that the films have a smooth and dense morphology with uniform grain growth. Hydrogen sensing measurements indicated that a-plane ZnO film on c-sapphire showed higher response than c-plane ZnO film reported earlier. The sensor response of 44 nm thick ZnO film exhibit highest response of 145 towards 500 ppm H₂ gas at the operating temperature of 200 °C.     

Influence of effective surface area on gas sensing properties of WO3 sputtered thin films

WO₃ thin films having different effective surface areas were deposited under various discharge gas pressures at room temperature by using reactive magnetron sputtering. The microstructure of WO₃ thin films was investigated by X-ray diffraction, scanning electron microscopy, and by the measurement of physical adsorption isotherms. The effective surface area and pore volume of WO₃ thin films increase with increasing discharge gas pressure from 0.4 to 12 Pa. Gas sensors based on WO₃ thin films show reversible response to NO₂ gas and H₂ gas at an operating temperature of 50-300 °C. The peak sensitivity is found at 200 °C for NO₂ gas and the peak sensitivity appears at 300 °C for H₂ gas. For both kinds of detected gases, the sensor sensitivity increases linearly with an increase of effective surface area of WO₃ thin films. The results demonstrate the importance of achieving high effective surface area on improving the gas sensing performance.     

Solar cooling with water–ammonia absorption chillers and concentrating solar collector – Operational experience

Concentrating solar collectors provide high efficiency at high driving temperatures favourable for thermally driven chillers. Therefore, they offer applications for hot climates and industrial process integration, especially in combination with NH₃–H₂O chillers that provide refrigeration temperatures below 0 °C. The presented solar cooling installation comprises a linear concentrating Fresnel collector that provides the driving heat for two NH₃–H₂O absorption chillers at temperatures up to 200 °C. Chilled water temperatures are produced in the range between −12 °C and 0 °C. Collector capacities reached up to 70 kW at peak times and the total cooling capacity of both chillers showed peak values up to 25 kW. For good operating conditions, the thermal system EER was 0.8 and an electrical system EER of 12 was easily achieved. The system showed a sound operating behaviour. The performance of different operation and control strategies was analysed, evaluated and enhanced within the two year operation phase.     

Ethanol gas sensing properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-doped ZnO thick film resistors

The characterization and ethanol gas sensing properties of pure and doped ZnO thick films were investigated. Thick films of pure zinc oxide were prepared by the screen printing technique. Pure zinc oxide was almost insensitive to ethanol. Thick films of Al₂O₃ (1 wt%) doped ZnO were observed to be highly sensitive to ethanol vapours at 300°C. Aluminium oxide grains dispersed around ZnO grains would result into the barrier height among the grains. Upon exposure of ethanol vapours, the barrier height would decrease greatly leading to drastic increase in conductance. It is reported that the surface misfits, calcination temperature and operating temperature can affect the microstructure and gas sensing performance of the sensor. The efforts are, therefore, made to create surface misfits by doping Al₂O₃ into zinc oxide and to study the sensing performance. The quick response and fast recovery are the main features of this sensor. The effects of microstructure and additive concentration on the gas response, selectivity, response time and recovery time of the sensor in the presence of ethanol vapours were studied and discussed.     

Effect of growth temperature on structural, electrical and optical properties of dual ion beam sputtered ZnO thin films

ZnO epitaxial thin films were grown on p-type Si(100) substrates by dual ion beam sputtering deposition system. The crystalline quality, surface morphology, optical and electrical properties of as-deposited ZnO thin films at different growth temperatures were studied. Substrate temperature was varied from 100 to 600 °C at constant oxygen percentage O₂/(O₂ + Ar) % of 66.67 % in a mixed gas of Ar and O₂ with constant chamber pressure of 2.75 × 10^?4 mBar. X-Ray diffraction analyses revealed that all the films had (002) preferred orientation. The minimum value of stress was reported to be ?0.32 × 10¹⁰ dyne/cm² from ZnO film grown at 200 °C. Photoluminescence measurements demonstrated sharp near-band-edge emission (NBE) was observed at ~375 nm along with deep level emission (DLE) in the visible spectral range at room temperature. The DLE Peak was found to have decrement as ZnO growth temperature was increased from 200 to 600 °C. The minimum FWHM of the NBE peak of 16.76 nm was achieved at 600 °C growth temperature. X-Ray photoelectron spectroscopy study revealed presence of oxygen interstitials and vacancies point defects in ZnO film grown at 400 °C. The ZnO thin film was found to be highly resistive when grown at 100 °C. The ZnO films were found to be n-type conducting with decreasing resistivity on increasing substrate temperature from 200 to 500 °C and again increased for film grown at 600 °C. Based on these studies a correlation between native point defects, optical and electrical properties has been established.     

Nanoporous TiO2 thin film based conductometric H2 sensor

Nanoporous titanium dioxide (TiO₂) based conductometric sensors have been fabricated and their sensitivity to hydrogen (H₂) gas has been investigated. A filtered cathodic vacuum arc (FCVA) system was used to deposit ultra-smooth Ti thin films on a transducer having patterned inter-digital gold electrodes (IDTs). Nanoporous TiO₂ films were obtained by anodization of the titanium (Ti) thin films using a neutral 0.5% (wt) NH₄F in ethylene glycol solution at 5 V for 1 h. After anodization, the films were annealed at 600 °C for 8 h to convert the remaining Ti into TiO₂. The scanning electron microscopy (SEM) images revealed that the average diameters of the nanopores are in the range of 20 to 25 nm. The sensor was exposed to different concentrations of H₂ in synthetic air at operating temperatures between 100 °C and 300 °C. The sensor responded with a highest sensitivity of 1.24 to 1% of H₂ gas at 225 °C.     

Intrinsic defects and structural phase of ZnS nanocrystalline thin films: effects of substrate temperature

Fabrications of ZnS nanocrystalline thin films at different substrate temperatures (T_S) of 200, 300 and 400 °C by means of pulsed laser deposition are presented. Thin film deposited at T_S of 200 °C is in cubic zinc-blende (ZB) structure while those deposited at T_S of 300 and 400 °C are in hexagonal wurtzite (W) phase. The grain size, surface roughness and bandgap of the films increases with increasing T_S. The zinc vacancies and interstitials in the films increases while sulfur vacancies decreases with increasing T_S. The variation of zinc and sulfur vacancies in ZnS films with T_S is responsible for structural phase transition from ZB to W which causes the change in energy bandgap.     

Synthesis and adsorption property of zinc rust of zinc hydroxynitrate

In order to clarify the formation condition of zinc rusts such as layered zinc hydroxynitrate (Zn₅(OH)₈(NO₃)₂·2H₂O: ZHN), ZnO particles were aged with aqueous Zn(NO₃)₂·6H₂O solution at 6–140 °C for 48 h. Further, adsorption of H₂O and CO₂ on ZHN was examined for simulating study of atmospheric corrosion of galvanized steel. The ZHN was formed at 6 °C and the ZnO completely disappeared, meaning the hydrolysis of ZnO particles in aqueous Zn(NO₃)₂·6H₂O solution to recrystallize as ZHN. Increasing the aging temperature improved the crystallinity of layered structure of ZHN, showing a maximum at 85 °C. The formed ZHN was hexagonal plate-like particles. The particle size was dependent of the crystallinity of layered structure of ZHN. The specific surface area of ZHN was decreased on elevating the aging temperature, showing a minimum at 85 °C. The adsorption of H₂O and CO₂ was enhanced on increasing the crystallinity of layered structure of ZHN, meaning that these molecules are adsorbed not only on particle surface but also in interlayer of ZHN. These facts infer that the preferred orientation of plate-like ZHN particles leads to the formation of compact rust layer on galvanized steel and to the enhancement of corrosion resistance.     

Preparation,characterisation and activity evaluation of CaCO3/ZnO photocatalyst

The photocatalyst CaCO₃/ZnO with high activity was prepared by coprecipitation method using (NH₄)₂CO₃, Zn(NO₃)₂ and Ca(NO₃)₂ as raw materials. The photocatalyst was characterised by X-ray powder diffraction, terephthalic acid photoluminescence probing technique (TA-PL), UV–vis diffuse reflectance spectroscopy and the fluorescence emission spectra. The photocatalytic activity of the photocatalyst was evaluated by photocatalytic oxidation of methyl orange and rhodamine B. The results showed that the photocatalytic activity of the photocatalyst was much higher than that of pure ZnO. The best mole ratio of Ca/Zn in the sample was 1?:?2, and its intensity of TA-PL was the strongest. The effect of heat treatment condition on the photocatalytic activity of the photocatalyst was investigated. The best preparation condition was about 650°C for 7?h. Compared with pure ZnO, the photoabsorption wavelength range of the CaCO₃/ZnO extends towards visible light and improves the utilisation of the total spectrum. The possible mechanisms of influence of CaCO₃ on the photocatalytic activity of CaCO₃/ZnO were also discussed.     

Influence of pyrolytic temperature on the properties of ZnO films optimized for H2 sensing application

In this work, we report the influence of pyrolytic temperature on the properties of ZnO films deposited by a novel spray pyrolysis deposition route. XRD results revealed an improvement in crystal quality of the films with increase in growth temperature. The optical measurements of the films show a maximum transmittance of ~85 % and the band gap of ~3.5 eV. Photoluminescence spectra revealed that the UV emission peaks at 385 nm is improved with increase in growth temperature upto 300 °C, which corresponds to the increase of optical quality and decrease of Zn interstitial defect in the films. Gold ohmic contacts were evaporated on the optimized ZnO film prepared at the substrate temperature of 300 °C, and response of the film to different concentrations of hydrogen (150–500 ppm) at room temperature was investigated. The ZnO sensor showed significant sensitivity to hydrogen for concentration as low as 150 ppm at room temperature, and the sensor response was observed to increase with increase in hydrogen concentration. The increased sensitivity of the film was attributed to the large roughness of the film revealed from AFM analysis. The results ensure the application of our novel sensor, to detect H₂ at low concentration and at room temperature.     

Reactively sputtered MoO3 films for ammonia sensing

We report the gas-sensing properties of ion-beam sputter deposited MoO₃ thin-films. The change in the DC conductivity was measured in dry N₂ with 10% O₂ in the presence of up to 490 ppm of NH₃, NO, NO₂, C₃H₆, CO and H₂. At ∼440 °C the film was found to be very sensitive to NH₃, with 490 ppm increasing the conductivity by approximately a factor of 70. This was approximately 17 times greater than the response to the other gases. The NH₃ response was strongly affected by the accompanying levels of O₂, NO₂ and H₂O. For example, changing the accompanying O₂ levels from 1% to 20% decreased the NH₃ response by approximately a factor of 20. Similarly, the presence of 100 ppm NO₂ (in 10% O₂) decreased the NH₃ response by approximately a factor of three, and 1% water vapor decreased it by more than a factor of two. The NH₃ response, however, was relatively unaffected by 100 ppm of accompanying NO, C₃H₆, CO or H₂. XPS measurements show that the increased conductivity in the presence of NH₃ was also accompanied by a partial reduction of the surface MoO₃. We observed an increase in the resistance of the films after extended time at elevated temperatures.     

EDTA-assisted phase conversion synthesis of (Gd0.95RE0.05)PO4 nanowires (RE = Eu,Tb) and investigation of photoluminescence

Abstract

Hexagonal (Gd_0.95RE_0.05)PO₄·nH₂O nanowires ~300 nm in length and ~10 nm in diameter have been converted from (Gd_0.95RE_0.05)₂(OH)₅NO₃·nH₂O nanosheets (RE = Eu, Tb) in the presence of monoammonium phosphate (NH₄H₂PO₄) and ethylene diamine tetraacetic acid (EDTA). They were characterized by X-ray diffraction, thermogravimetry, electron microscopy, and Fourier transform infrared and photoluminescence spectroscopies. It is shown that EDTA played an essential role in the morphology development of the nanowires. The hydrothermal products obtained up to 180 °C are of a pure hexagonal phase, while monoclinic phosphate evolved as an impurity at 200 °C. The nanowires undergo hexagonal→monoclinic phase transformation upon calcination at ≥600 °C to yield a pure monoclinic phase at ~900 °C. The effects of calcination on morphology, excitation/emission, and fluorescence decay kinetics were investigated in detail with (Gd_0.95Eu_0.05)PO₄ as example. The abnormally strong ⁵D₀→⁷F₄ electric dipole Eu³⁺ emission in the hexagonal phosphates was ascribed to site distortion. The process of energy migration was also discussed for the optically active Gd³⁺ and Eu³⁺/Tb³⁺ ions.     

Effect of ZnO doping on morphology and electrochemical properties of sub-micron RuO2 sensing electrode of DO sensor

Thick-film 20 mol% ZnO-doped RuO₂ sensing electrodes (SEs) were fabricated by screen-printing technique on the platinised alumina substrate of the planar electrochemical dissolved oxygen (DO) sensor. The effect of ZnO doping on morphology, electrochemical properties and sensing characteristics of the sensor was investigated. It was found that ZnO doping has not only improved the SE structure, but has also enhanced selectivity of the DO sensor. Selectivity testing exhibited that the presence of Cl⁻, Li⁺, SO₄²⁻, NO³⁻, Ca²⁺, PO₄³⁻, Mg²⁺, Na⁺ and K⁺ with a concentration range of 10⁻⁷ to 10⁻¹ mol/L in the solution had practically no effect on the sensor's emf. The reason in enhancement of the sensor characteristics could be related to the establishment of the better structured SE as more advanced crystallization is achieved for the doped RuO₂-SE.