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Paracetamol-assisted zinc oxide (ZnO) highly porous micro-flakes were synthesized by a rapid two-step synthesis: precipitation (with sodium hydroxide) and calcination (at 250°C for 1 h). Addition of paracetamol during the synthesis not only inhibited the growth of ZnO grains but also originated self-assembly of the micro-flakes resulting in highly porous flower like structures. Increase in paracetamol concentration also increased porosity on ZnO microstructures due to the self-assembly of thinner flakes without any structural changes. X-ray diffraction (XRD) shows the preferential orientation of powders in the (101) direction of hexagonal structure. Raman spectra is dominated by E2 (high) optical mode due to vibration of oxygen atoms. Samples were tested for gas detection at 50, 100, 200, 400, 800 and 1000 (parts per million) PPM concentration of carbon dioxide (CO2). ZnO porous microstructures were obtained with a high concentration of paracetamol, enhancing the carbon dioxide sensing response from 20% to 90% with a response time of 60 s. These simple, low-cost and highly porous self-assembled ZnO structures with enhanced CO2 detection will be of interest for several researchers in the chemical sensor fabrication field.

Undoped and nickel-doped zinc oxide thin films were deposited on sodalime glass substrates by utilizing dip coating and ultrasonic spray pyrolysis deposition techniques. In both cases zinc acetate and nickel acetylacetonate were used as zinc precursor and nickel dopant source, respectively. XRD analysis confirms the ZnO wurtzite structure with (002) as the preferential orientation.SEM studies show the formation of two types of morphologies, primarily a porous spherical grains with a grain size distribution from 40 to 150 nm and another, rose-like structures with size distribution from 30 to 200 nm, based on different deposition techniques utilized. The elemental depth profiles across the films were investigated by the secondary-ion mass spectrometry (SIMS). Different gas sensing responses of all ZnO films were obtained for both propane and carbon monoxide gases, at different gas concentrations and operating temperatures. The highest sensing response (~6) for undoped ZnO films was obtained for films deposited by ultrasonic spray pyrolysis (USP). Nevertheless, the highest sensing response (~4 &times, 104) for doped ZnO films was obtained for films deposited by dip coating method. The behavior of sensing responses is explained in detail based on the morphological properties and the amount of Ni impurities incorporated into the crystal lattice.

Pure titanium dioxide (TiO2) thin films were deposited and investigated as gas sensors for carbon monoxide (CO) detection. TiO2 thin films were deposited by ultrasonic spray pyrolysis technique, starting from titanium(IV) oxyacetilacetonate, onto soda-lime glass substrates. Structural, morphological, and compositional properties of the TiO2 films were obtained utilizing X-ray diffractometry, energy dispersive X-ray, scanning electron microscopy, and secondary ion mass spectroscopy techniques, respectively. Films deposited above 400 °C were polycrystalline, and its X-ray pattern fit well to the TiO2 anatase structure, and no other phases were detected, whereas films deposited at lower temperatures presented an amorphous structure. The sensitivity of the TiO2 films is analyzed by varying both thickness and deposition temperature. In addition, the sensor response was measured for CO concentrations from 1 to 300 ppm at different operation temperatures, 100, 200, and 300 °C. The highest sensitivity (~ 300) was obtained for TiO2 thin films deposited with the lowest film thickness, at a substrate temperature of 350 °C. The results obtained in this work show that the TiO2 films processed by ultrasonic spray pyrolysis exhibit very promising results for detection of CO.

An enhancing awareness about the hazardous gaseous environment in the domestic and industrial sectors is on the rise across the globe. Thus, the necessity to efficiently detect and monitor the potentially hazardous gases, mainly that are toxic and flammable, is widely being researched. Propane, a liquefied petroleum gas (LPG), is highly inflammable and explosive when it comes in contact with an ignition source. Therefore, it is highly important to develop an upgraded propane gas sensor that could be used in various places such as household appliances, liquid natural gas (LNG) and petrochemical industry, automobile and aerospace industry, and relevant sectors where the propane is used. In the present work, ultrathin intrinsic zinc oxide (i-ZnO) films were deposited by the radio frequency (RF) sputtering technique at various working pressures (2–8 mTorr) at constant 100 W for gas sensing applications. Thus, deposited films were characterized by various techniques such as X-ray diffractometry (XRD), field-emission scanning electron microscopy (FESEM), ultra-violet spectrophotometry (UV–Vis), and finally by a gas sensing technique for their structural, morphological, optical, and sensing characteristics. XRD pattern confirms the formation of hexagonal phase of ZnO with a preferred orientation along the (002) plane. The bandgap of the deposited films was determined to be between 3.19 and 3.21 eV as measured from UV–Vis spectra. The scanning electron micrographs revealed the formation of vertically aligned and cross-linked nanowall structures at lower working pressures. Finally, the gas sensing properties of the films were exclusively studied, at various operating temperatures (100, 200, and 300 °C), for different propane gas concentrations. The film deposited at 2 mTorr exhibited a gas sensitivity of 0.998 and almost equal to 30 s of response time and 35 s of recovery time at an operating temperature of 300 °C for the propane gas concentration of 500 ppm, which implies the potentiality of using this film as a propane gas sensor.

Aluminium and indium co-doped zinc oxide (AIZO) thin films were deposited using ultrasonic spray pyrolysis. Depositions were performed by varying the acetic acid and water content in the spraying solution which resulted in the formation of different nanostructures like hexagons, flowers, chisels, curved nanostructures, hexagonal pyramids, super grown hexagons, and inter-connected nanostructures. Further, the physical properties such as structural, optical, electrical, and surface texture parameters were examined. The structural studies showed that films were of crystalline nature, with different crystallite sizes and grown with a preferential orientation along (002) plane. The optical transmittance assessments proved that films were highly transparent (> 80%) in the visible region. The electrical sheet resistance was found to be in the range 29–1K Ω/□. Surface parameters like average roughness, root mean square roughness, and peak-valley height values helped to understand the homogeneity of the thin films. Finally, the suitability of AIZO films for transparent conductive oxide applications were tested by estimating the figure of merit (FOM). Among the different solution conditions, films fabricated using a starting solution containing 25 ml of acetic acid and 25 ml of water exhibited the lowest resistivity (2.47 ± 0.03 × 10−3 Ω-cm) along with the highest FOM (5.83 ± 0.42 × 10−3/Ω).

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by the ultrasonic spray pyrolysis technique (USP), starting from zinc acetylacetonate. A novelty process was implemented to the Zn precursor, previous to the solution preparation, namely, the Zn precursor was milled in a planetary ball milling equipment at different speed and time conditions, in order to generate new chemical species. The starting solutions were prepared by dissolving the milled precursor in a mix of methanol, acetic acid, and water. ZnO thin films were deposited at different substrate temperatures, 375, 400 and 450 °C and constant deposition time, 6 min. Sprayed ZnO films were characterized by X-ray diffraction, XRD, UV–Vis spectrophotometry and scanning electron microscopy, SEM. All films presented a wurtzite hexagonal phase confirmed from XRD, and corroborated by SEM analysis. Films exhibited a maximum optical transmittance oscillating between 80 and 85%. A band gap varying between 3.3 and 3.4 eV was estimated from the transmittance spectra. The photocatalytic response of ZnO thin films was tested by monitoring the bleaching of methylene blue (MB) dye at different time periods, under UV light irradiation. It was observed an increase in the degradation performance of the dye in all the films deposited from milled precursor, as compared with those deposited from unmilled precursor, since the MB degradation increased from 40 to 99% in 3 h. Novel mechanochemical process implemented to conventional chemical techniques enhanced the photocatalytic response of ZnO films.

In this work, we have studied the effect of aluminium dopant precursor type and doping concentration on the structural, morphological, optical and electrical properties of Al and In co-doped zinc oxide (AIZO) thin films deposited by ultrasonic spray pyrolysis. Zinc acetate dihydrate and indium acetate were used as zinc and indium precursors, respectively. Aluminium chloride and aluminium sulphate were used as aluminium precursors. The doping concentrations of Al (1 to 3 at%) and In (1 to 3 at%), were varied equally and the physical properties were analyzed. X-ray diffraction examinations confirmed that AIZO films were poly crystalline and grown as a hexagonal wurtzite structure. Scanning electron microscopy observations revealed that thin films were grown with different types of hexagonal nanostructures. From the optical and electrical measurements, the figure of merit was estimated. The values were 4.09 × 10− 3/Ω and 0.75 × 10− 3/Ω for the AIZO thin films deposited using aluminium chloride and aluminium sulphate respectively.

Indium and gallium co-doped zinc oxide (IGZO) thin films were deposited on glass substrates by ultrasonic spray pyrolysis . Physical properties such as structural, morphological, optical and electrical properties were examined on IGZO thin films with respect to the changes in the substrate temperature (425, 450 and 475 °C). Structural results showed that IGZO films were crystalline and presented hexagonal wurtzite structure. Morphological studies proved that the substrate temperature changed the sizes of hexagonal nanostructures of IGZO. Optical transmittance in the UV–vis region and electrical measurements confirmed that IGZO films were transparent (>70%) with a minimum electrical resistivity ~10.5×10 −3 Ω cm.

Micro- and nanoparticles of NiSb2O6 were synthesized by the microwave-assisted colloidal method. Nickel nitrate, antimony chloride, ethylenediamine, and ethyl alcohol were used. The oxide was obtained at 600°C and was analyzed by X-ray diffraction (XRD) and Raman spectroscopy, showing a trirutile-type structure with cell parameters a = 4.641 Å, c = 9.223 Å, and a space group P42/mnm (136). Average crystal size was estimated at ~31.19 nm, according to the XRD-peaks. The microstructure was scrutinized by scanning electron microscopy (SEM), observing microrods measuring ~3.32 μm long and ~2.71 μm wide, and microspheres with an average diameter of ~8 μm; the size of the particles shaping the microspheres was measured in the range of ~0.22 to 1.8 μm. Transmission electron microscopy (TEM) revealed that nanoparticles were obtained with sizes in the range of 2 to 20 nm (~10.7 nm on average). Pellets made of oxide’s powders were tested in propane (C3H8) and carbon monoxide (CO) atmospheres at different concentrations and temperatures. The response of the material increased significantly as the temperature and the concentration of the test gases rose. These results show that NiSb2O6 may be a good candidate for gas sensing applications.

In this work, we report the effect of deposition time on the physical properties such as structural, morphological, optical and electrical properties of aluminium and indium co-doped ZnO (AIZO) thin films. AIZO films were deposited by ultrasonic spray pyrolysis technique on glass substrates at different deposition times (10, 12 and 15 min) using ball milled zinc precursor. A change in crystalline nature was observed from structural analysis with respect to growth time. AIZO thin films with hexagonal nanostructures and optical transmittance higher than 70% were confirmed from the morphological and optical studies. AIZO films showed an electrical resistivity varying in the range of 2.35–4.59×10−3 Ω cm. Finally, when we compared these results with AIZO films deposited using unmilled zinc precursor; we found that ball milling the precursor has a beneficial effect in enhancing the physical properties.

In this work, we reported a chemical approach to prepare aluminium and indium co-doped ZnO thin films (AIZO) by ultrasonic spray pyrolysis. Film depositions were carried out on soda lime glass substrates at 425 °C by using a spray solution containing zinc acetate as zinc precursor, aluminium acetylacetonate as Al dopant source and indium (III) acetate as In dopant source. Physical properties such as structural, morphological, optical and electrical properties were studied with respect to the equal variations in co-dopants concentration (0.5–3 at%). X-ray diffraction patterns proved that films are poly crystalline with (002) preferential orientation. Scanning electron microscopy analysis showed that AIZO films grown like hexagonal nanopyramids, elongated grains and irregular trigonals. Optical transmittance ~85% and a minimum resistivity of 1.3×10−3 Ω cm, are achieved for films when co-doped with 1.5 at% of Al and 1.5 at% of In, confirm that AIZO films are suitable for transparent conductive oxide (TCO) applications.

Undoped zinc oxide (ZnO) thin films were reactively deposited on soda-lime glass substrates by radio frequency (rf) sputtering at room temperature. We have studied the importance of substrate rotation speed on the structural, optical and morphological properties. The rotation speeds used were 0, 20, 40, 60, and 80 rpm. The XRD measurements confirmed that films were grown with (002) plane preferential orientation. The average transmittance was above 90% in the UV–vis region. The SEM images revealed that films deposited with substrate rotation are more homogeneous than the films deposited without rotation.

Spinel-type ZnMn2O4 nanoparticles were synthesized via a simple and inexpensive microwave-assisted colloidal route. Structural studies by X-ray diffraction showed that a spinel crystal phase of ZnMn2O4 was obtained at a calcination temperature of 500 °C, which was confirmed by Raman and UV-vis characterizations. Spinel-type ZnMn2O4 nanoparticles with a size of 41 nm were identified by transmission electron microscopy. Pellet-type sensors were fabricated using ZnMn2O4 nanoparticles as sensing material. Sensing measurements were performed by exposing the sensor to different concentrations of propane or carbon monoxide at temperatures in the range from 100 to 300 °C. Measurements performed at an operating temperature of 300 °C revealed a good response to 500 ppm of propane and 300 ppm of carbon monoxide. Hence, ZnMn2O4 nanoparticles possess a promising potential in the gas sensors field.

In this work, we report the synthesis of Cu, Pt and Pd doped SnO₂ powders and a comparative study of their CO gas sensing performance. Dopants were incorporated into SnO₂ nanostructures using chemical and impregnation methods by using urea and ammonia as precipitation agents. The synthesized samples were characterized using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM). The presence of dopants within the SnO₂ nanostructures was evidenced from the HR-TEM results. Powders doped utilizing chemical methods with urea as precipitation agent presented higher sensing responses compared to the other forms, which is due to the formation of uniform and homogeneous particles resulting from the temperature-assisted synthesis. The particle sizes of doped SnO₂ nanostructures were in the range of 40-100 nm. An enhanced sensing response around 1783 was achieved with Cu-doped SnO₂ when compared with two other dopants i.e., Pt (1200) and Pd:SnO₂ (502). The high sensing response of Cu:SnO₂ is due to formation of CuO and its excellent association and dissociation with adsorbed atmospheric oxygen in the presence of CO at the sensor operation temperature, which results in high conductance. Cu:SnO₂ may thus be an alternative and cost effective sensor for industrial applications.

Spinel ZnCo2O4 nanoparticles were synthesized by means of the microwave-assisted colloidal method. A solution containing ethanol, Co-nitrate, Zn-nitrate, and dodecylamine was stirred for 24 h and evaporated by a microwave oven. The resulting solid material was dried at 200 °C and subsequently calcined at 500 °C for 5 h. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy, confirming the formation of spinel ZnCo2O4 nanoparticles with average sizes between 49 and 75 nm. It was found that the average particle size decreased when the dodecylamine concentration increased. Pellets containing ZnCo2O4 nanoparticles were fabricated and tested as sensors in carbon monoxide (CO) and propane (C3H8) gases at different concentrations and temperatures. Sensor performance tests revealed an extremely high response to 300 ppm of CO at an operating temperature of 200 °C.

Pure and copper (Cu)-incorporated tin oxide (SnO2) pellet gas sensors with characteristics provoking gas sensitivity were fabricated and used for measuring carbon monoxide (CO) atmospheres. Non-spherical pure SnO2 nano-structures were prepared by using urea as the precipitation agent. The resultant SnO2 powders were ball milled and incorporated with a transition metal, Cu, via chemical synthesis method. The incorporation is confirmed by high-resolution transmission electron microscope (HRTEM) analysis. By utilizing Cu-incorporated SnO2 pellets an increase in the CO sensitivity by an order of three, and a decrease in the response and recovery times by an order of two, were obtained. This improvement in the sensitivity is due to two factors that arise due to Cu incorporation: necks between the microparticles and stacking faults in the grains. These two factors increased the conductivity and oxygen adsorption, respectively, at the pellets’ surface of SnO2 which, in turn, raised the CO sensitivity.

Bystromite (MgSb2O6) nanorods were prepared using a colloidal method in the presence of ethylenediamine, after a calcination step at 800 °C in static air. From X-ray powder diffraction analyses, a trirutile-type structure with lattice parameters a = 4.64 Å and c = 9.25 Å and space group P42/mnm was identified. Using scanning electron microscopy (SEM), microrods with sizes from 0.2 to 1.6 μm were observed. Transmission electron microscopy (TEM) analyses revealed that the nanorods had a length of ~86 nm and a diameter ~23.8 nm. The gas-sensing properties of these nanostructures were tested using pellets elaborated with powders of the MgSb2O6 oxide (calcined at 800 °C) at temperatures 23, 150, 200, 250 and 300 °C. The pellets were exposed to different concentrations of carbon monoxide (CO) and propane (C3H8) at these temperatures. The results showed that the MgSb2O6 nanorods possess excellent stability and high sensitivity in these atmospheres.

A study on the propane gas-sensing properties of Cu-doped ZnO thin films is presented in this work. The films were deposited on glass substrates by sol-gel and dip coating methods, using zinc acetate as a zinc precursor, copper acetate and copper chloride as precursors for doping. For higher sensitivity values, two film thickness values are controlled by the six and eight dippings, whereas for doping, three dippings were used, irrespective of the Cu precursor. The film structure was analyzed by X-ray diffractometry, and the analysis of the surface morphology and film composition was made through scanning electron microscopy (SEM) and secondary ion mass spectroscopy (SIMS), respectively. The sensing properties of Cu-doped ZnO thin films were then characterized in a propane atmosphere, C₃H₈, at different concentration levels and different operation temperatures of 100, 200 and 300 °C. Cu-doped ZnO films doped with copper chloride presented the highest sensitivity of approximately 6 × 10⁴, confirming a strong dependence on the dopant precursor type. The results obtained in this work show that the use of Cu as a dopant in ZnO films processed by sol-gel produces excellent catalysts for sensing C₃H₈ gas.