Splat formation and microstructure of solution precursor thermal sprayed Nb-doped titanium oxide coatings

Solution precursor thermal spray can become a breakthrough technology for the deposition of coatings with novel chemistries; however, the understanding of the process that the feedstock material undergoes is still lacking when compared to more traditional presentations (i.e. powder and suspension). In this paper, niobium-doped TiO2 coatings were deposited by solution precursor high velocity oxy-fuel thermal spraying, studying its microstructure and phase. It was reported that a lower flame temperature produced a highly porous coating, while the porosity was reduced at higher flame temperature. Investigation of the phase content showed that, contrary to our current understanding, a higher flame power implied an increase of the anatase phase content for solution precursor spray. Three methods were used: Rietveld refinement, peak height and peak area of the x-ray diffraction patterns. Additionally, single splats were analysed, showing that as the precursor travels through the flame, pyrolysis and sintering takes place to form the solid material. These results were used to derive a model of the physico-chemical transformation of the solution precursor. This work proves that solution precursor thermal spray is a promising technique for the deposition of doped ceramic coatings, being the microstructure and phase content controllable through the spraying parameters

A Review on Metal Oxide-Graphene Derivative Nano-Composite Thin Film Gas Sensors

Most of the available commercial solid-state gas/vapor sensors are based on metal oxide semiconductors. Metal oxides (MOs) change their conductivity while exposed to gas or vapors ambient can be utilized as gas or vapor sensing materials. In recent days, graphene has attracted tremendous attention owing to its two-dimensional structure with an extremely high surface to volume ratio, electron mobility, and thermal conductivity. However, intrinsic graphene is relatively inefficient for the adsorption of gas/vapor molecules. In this regard, graphene oxide (GO) and reduced graphene oxide (rGO), which are graphene species functionalized with different oxygen groups that offer a higher amount of adsorption sites improving the sensitivity of the film. Up to now, many research groups across the globe have reported the promising performance towards gas detection using various GO/rGO-metal oxide nanocomposites. This chapter reviews the composites of graphene oxide or reduced graphene oxide and metal oxides in nanoscale dimensions (0-D, 1-D, 2-D, and 3-D) for gas sensing applications considering two specific focus areas, that is, synthesis of nanocomposites and performance assessment for gas/vapor sensing

Effect of Nb doping on structural, optical and photocatalytic properties of flame-made TiO2 nanopowder

TiO2:Nb nanopowders within a dopant concentration in the range of 0.1-15at.% were prepared by one-step flame spray synthesis. Effect of niobium doping on structural, optical and photocatalytic properties of titanium dioxide nanopowders was studied. Morphology and structure were investigated by means of Brunauer-Emmett-Teller isotherm, X-ray diffraction and transmission electron microscopy. Diffuse reflectance and the resulting band gap energy were determined by diffuse reflectance spectroscopy. Photocatalytic activity of the investigated nanopowders was revised for the photodecomposition of methylene blue (MB), methyl orange (MO) and 4-chlorophenol under UVA and VIS light irradiation. Commercial TiO2-P25 nanopowder was used as a reference. The specific surface area of the powders was ranging from 42.9m2/g for TiO2:0.1at.% Nb to 90.0m2/g for TiO2:15at.% Nb. TiO2:Nb particles were nanosized, spherically shaped and polycrystalline. Anatase was the predominant phase in all samples. The anatase-related transition was at 3.31eV and rutile-related one at 3.14eV. TiO2:Nb nanopowders exhibited additional absorption in the visible range. In comparison to TiO2-P25, improved photocatalytic activity of TiO2:Nb was observed for the degradation of MB and MO under both UVA and VIS irradiation, where low doping level (Nb < 1at.%) was the most effective. Niobium doping affected structural, optical and photocatalytic properties of TiO2. Low dopant level enhanced photocatalytic performance under UVA and VIS irradiation. Therefore, TiO2:Nb (Nb < 1at.%) can be proposed as an efficient selective solar light photocatalys

Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented

Eco-friendly facile synthesis of Co3O4-Pt nanorods for ethylene detection towards fruit quality monitoring

Ethylene, a biomarker widely employed for evaluating fruit ripening during storage, exists at extremely low concentrations. Therefore a gas sensor with high sensitivity and a sub-ppm detection limit is needed. In this work, porous Co3O4 nanorods were synthesized through a hydrothermal method involving Co(NO3)2, Na2C2O4, H2O and ethylene glycol (EG), followed by annealing at 400 degrees C in air. The surface of the porous Co3O4 nanorods was functionalized with Pt nanoparticles to enhance the ethylene sensing performance. The effect of Co3O4 surface functionalisation with Pt nanoparticles was investigated by adding different amounts of nanoparticles. The sensor's outstanding performance at the optimum working temperature of 250 degrees C is attributed to the synergy between the high catalytic activity of Pt nanoparticles and the extensive surface area of the porous Co3O4 nanorods. Compared to pure Co3O4, the 0.031 wt% Pt sensor showed better ethylene sensing performance with a response 3.4 times that of pristine Co3O4. The device also demonstrated high selectivity, repeatability, long-term stability and a detection limit of 0.13 ppm for ethylene, which is adequate for fruit quality monitoring. The gas sensing mechanism of porous Co3O4 nanorods and the influence of Pt decoration on sensor performance are discussed

Review—Non-Invasive Monitoring of Human Health by Exhaled Breath Analysis: A Comprehensive Review

Exhaled human breath analysis is a very promisingfield of research work having great potential for diagnosis of diseases in non-invasive way. Breath analysis has attracted huge attention in thefield of medical diagnosis and disease monitoring in the last twodecades. VOCs/gases (Volatile Organic Compounds) in exhaled breath bear thefinger-prints of metabolic and biophysicalprocesses going on in human body. It’s a non-invasive, fast, non-hazardous, cost effective, and point of care process for diseasestate monitoring and environmental exposure assessment in human beings. Some VOCs/gases in exhaled breath are bio-markers ofdifferent diseases and their presence in excess amount is indicative of un-healthiness. Breath analysis has the potential for earlydetection of diseases. However, it is still underused and commercial device is yet not available owing to multiferrious challenges.This review is intended to provide an overview of major biomarkers (VOCs/gases) present in exhaled breath, importance of theiranalysis towards disease monitoring, analytical techniques involved, promising materials for breath analysis etc. Finally, relatedchallenges and limitations along with future scope will be touched upon.will be touched upon

Photocatalytic Conversion of Carbon Dioxide To Fuel

Carbon dioxide (CO2) is a harmful greenhouse gas that is being increased in the atmosphere. As CO2 emissions build up in the atmosphere, it tends to warm the climate leading to many adverse changes in atmosphere and thus causes various irreparable damages on our environment. This critical juncture compelled researchers to work upon converting this emitted CO2 into fuel like methanol, butanol or even gasoline which can reduce the emission of GHG in the atmosphere. Photocatalytic approach to convert CO2 into methanol in the presence of TiO2, a photocatalyst has been very promising till date and discussed in this work. TiO2. TiO2 synthesis was done by two routes, hydrolysis and sol-gel. In hydrolysis method, various concentration from 1 mol.dm-3 to 3 mol.dm-3 and volume from 50 ml to 200 ml. Anatase phase increased with the increase in volume. The same was the trend in increase of photoluminescence. Sol-gel proved to be a better photocatalyst with a lower size range of 40-60 nm than hydrolysis method that range from 40-90 nm. The photo reduction of CO2 with H2O was efficaciously demonstrated in a laboratory scale photoreactor with TiO2 and TiO2-Nb catalysts. The maximum methanol yield amongst all the catalysts was 0.15 μgm/ml by using TiO2(sol-gel) under UV irradiation. Compared with other catalysts proved to be a better photocatalyst. Doping had no positive impact on photo reduction of CO2to methanol. All in all, asuccessful demonstration of material synthesis and its application in photo reduction of CO2 was achieved

Insights into the Sensing Mechanism of a Metal-Oxide Solid Solution via Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Recently, the influence of Nb addition in the oxide solid solution of Sn and Ti was investigated with regard to the morphological, structural and electrical properties for the production of chemoresistive gas sensors. (Sn,Ti,Nb)xO2-based sensors showed promising features for ethanol monitoring in commercial or industrial settings characterized by frequent variation in relative humidity. Indeed, the three-metal solid solution highlighted a higher response level vs. ethanol than the most widely used SnO2 and a remarkably low effect of relative humidity on the film resistance. Nevertheless, lack of knowledge still persists on the mechanisms of gas reaction occurring at the surface of these nanostructures. In this work, operando Diffuse Reflectance Infrared Fourier Transform spectroscopy was used on SnO2- and on (Sn,Ti,Nb)xO2-based sensors to combine the investigations on the transduction function, i.e., the read-out of the device activity, with the investigations on the receptor function, i.e., compositional characterization of the active sensing element in real time and under operating conditions. The sensors performance was explained by probing the interaction of H2O and ethanol molecules with the material surface sites. This information is fundamental for fine-tuning of material characteristics for any specific gas sensing applications

Semiconductor Gas Sensors: Materials, Technology, Design, and Application

This paper presents an overview of semiconductor materials used in gas sensors, their technology, design, and application. Semiconductor materials include metal oxides, conducting polymers, carbon nanotubes, and 2D materials. Metal oxides are most often the first choice due to their ease of fabrication, low cost, high sensitivity, and stability. Some of their disadvantages are low selectivity and high operating temperature. Conducting polymers have the advantage of a low operating temperature and can detect many organic vapors. They are flexible but affected by humidity. Carbon nanotubes are chemically and mechanically stable and are sensitive towards NO and NH3, but need dopants or modifications to sense other gases. Graphene, transition metal chalcogenides, boron nitride, transition metal carbides/nitrides, metal organic frameworks, and metal oxide nanosheets as 2D materials represent gas-sensing materials of the future, especially in medical devices, such as breath sensing. This overview covers the most used semiconducting materials in gas sensing, their synthesis methods and morphology, especially oxide nanostructures, heterostructures, and 2D materials, as well as sensor technology and design, application in advance electronic circuits and systems, and research challenges from the perspective of emerging technologies. © 2020 by the authors. Licensee MDPI, Basel, Switzerland