Gas sensing performance of hydrothermally grown CeO2–ZnO composites

The sensors based on cerium oxide–zinc oxide (CeO₂–ZnO) composites were fabricated by using thick-film screen printing of hydrothermally grown powders. The structural, morphological investigations were carried out by using XRD, FESEM and TEM and these studies revealed that the synthesized products were grown in high-density and possessed well-crystallinity. Furthermore, the gas responses were evaluated towards the ethanol, acetone, liquid petroleum gas (LPG) and ammonia gases. The 2 wt% CeO₂–ZnO composite exhibited excellent response of 94% at 325 °C and better selectivity towards ethanol with low response and recovery time as compared to pure ZnO and can stand as reliable sensor element for ethanol sensor related applications.     

Hierarchical structured TiO2 nano-tubes for formaldehyde sensing

Hierarchical structured TiO₂ nano-tubes were prepared following a two-step method: the highly ordered uniform TiO₂ nanotube arrays were first grown by the conventional electrochemical anodization of the Ti metal sheet followed by mechanical milling of the as-fabricated TiO₂ nanotube arrays. The obtained nanotubes with a length around 400 nm and opening diameter ∼100 nm were formed mixed with the spherical TiO₂ single crystals with a diameter around 10 nm indicating hierarchical nanostructure. The as-synthesized TiO₂ hierarchical nanotubes based resistive-type chemical sensor exhibits good sensitivity to formaldehyde at room temperatures with or without UV-irradiation. The response of the sensor increased almost linearly as a function of the concentration of formaldehyde from 10–50 ppm under UV irradiation. The response of the sensor to different relative humidity and other possible interferents such as ammonia, methanol and alcohol was investigated. The larger response of the sensor to formaldehyde relative to these interferents is suggested to be due to the deeper diffusion of formaldehyde into the TiO₂ nanotubes.     

Studies on the resistive response of nickel and cerium doped SnO2 thick films to acetone vapor

Undoped and Ni, Ce-doped nanocrystalline tin oxide were synthesized by co-precipitation route. Doped as well as undoped SnO₂ compositions revealed single phase structure without any impurity. The lattice constant of SnO₂ increases and the grain size decreases with doping of Ni and Ce. The responses of the sensing elements are evaluated by measuring the resistance change upon exposure to various test gases such as liquid petroleum gas (LPG), acetone, ethanol and ammonia. In comparison to LPG, ethanol, and ammonia the response towards acetone vapor increases markedly on simultaneous doping of Ni and Ce. For acetone vapors with 500 ppm at 300 °C, the undoped SnO₂ shows 31% response, while with individual Ni or Ce doping it increases to 38 and 60%, respectively, however with simultaneous doping of Ni and Ce there is a significant enhancement up to 92%. The results of gas sensing measurements reveal that the thick films deposited on alumina substrates using screen printing technique give selectively a high response of (87%) with fast recovery (∼1 min) towards 100 ppm acetone at 300 °C.     

Characterization and gas sensing properties of bead-like ZnO using multi-walled carbon nanotube templates

We report bead-like ZnO nanostructures for gas sensing applications, synthesized using multi-walled carbon nanotube (MWCNT) templates. The ZnO nanostructures are grown following a two-step process: in the first, ZnO nanoparticles are synthesized on MWCNTs by thermal evaporation of a Zn powder; and in the second, the hybrid nanostructures are heat-treated at 800 °C. Scanning and transmission electron microscopy images indicate that the bead-like ZnO nanostructures have surface protuberances with nanoparticle sizes ranging from 20 to 60 nm, and a well-crystallized hexagonal structure. Gas sensors based on multiple-networked bead-like ZnO showed considerably enhanced electrical responses and better stability to both oxidizing (NO₂) and reducing (CO) gases compared with previously reported nanostructured gas sensors, even if the response to CO gas was slow to increase. Both the NO₂ and CO gas sensing properties increased dramatically when the working temperature was increased up to 300 °C. The response sensitivities measured were 2953%, 5079%, 9641%, 3568%, and 3777% to 20 ppm NO₂ at 200, 250, 300, 350 and 400 °C, respectively. For CO gas on the other hand, the response sensitivities were 107%, 110%, 114%, 118%, and 122% at 5, 10, 20, 50, and 100 ppm concentrations, respectively. For concentrations between 5 and 20 ppm, the recovery time of the oxidizing gas was much shorter than the response time. The origin of the NO₂/CO gas sensing mechanism of the bead-like ZnO nanostructures is discussed.     

ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe₂O₃ nanorods and Fe₂O₃–ZnO core–shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core–shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5–200 ppm) at different temperatures (150–250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe₂O₃–ZnO core–shell nanorods, as well as their uniformity in terms of shape and size. The Fe₂O₃–ZnO core–shell nanorod sensor showed a stronger response to ethanol than the pristine Fe₂O₃ nanorod sensor. The response (i.e., the relative change in electrical resistance R_a/R_g) of the core–shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe₂O₃–ZnO core–shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core–shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe₂O₃–ZnO core–shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe₂O₃–ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe₂O₃–ZnO interface.     

Synthesis of flower-like 3D ZnO microstructures and their size-dependent ethanol sensing properties

Flower-like 3D ZnO microstructures constructed from nanorods of different sizes were prepared by a microwave hydrothermal (MH) process in the presence of o-, m- and p-nitrobenzoic acid, respectively. Well-crystallized flower-like ZnO microstructures were obtained after 10 min MH treatment. The X-ray powder diffraction (XRD) test indicated that all the products were consistent with the hexagonal ZnO phase, and scanning electron microscopy (SEM) investigation revealed that the flower-like 3D ZnO microstructures were built with sword-like nanorods 60-100 nm in width and several micrometers in length. The formation mechanism of these flower-like 3D ZnO microstructures is discussed briefly. The gas sensitivity of the as-prepared ZnO microstructures to ethanol at different operation temperatures and concentrations was also studied. The results indicated that the gas sensitivity of the ZnO microstructures was influenced by the particle size and microcosmic configuration, the larger particles with crowded nanorods having higher gas sensitivity.     

Selective catalytic reduction of NOX with ammonia over Mn-Ce-OX/TiO2-carbon nanotube composites

Mn-Ce-O_X catalysts loaded on TiO₂-carbonaceous materials were prepared by sol-gel method. Selective catalytic reduction of NO_X was conducted in a fixed-bed flow-reactor over catalysts coated on aluminum plates. A de-NO_X efficiency of more than 90% was obtained over the Mn-Ce-O_X/TiO₂-carbon nanotubes (CNTs) catalyst between 75 °C and 225 °C under a gas hourly space velocity (GHSV) of ~ 36,000 h⁻¹. This activity improvement is attributed to the increase of the BET surface area, and the occurrence of reaction between adsorbed NO_X and NH₃. Moreover, the de-NO_X efficiency was increased to 99.6% by adding 250 ppm SO₂ between 100 °C and 250 °C.     

Gas sensing properties of multiple networked GaN/WO3 core-shell nanowire sensors

GaN nanowires and GaN-core/WO₃-shell nanowires were synthesized by the thermal evaporation of GaN powders followed by the sputter-deposition of WO₃ and their gas sensing properties were examined. The multiple networked pristine GaN nanowire sensors showed responses of approximately 125%, 140%, 146%, 159%, and 183% to 1, 2, 3, 4, and 5 ppm NO₂ gases, respectively. These responses are comparable to those obtained previously using metal oxide semiconductor one-dimensional nanostructure sensors. The responses of the nanowires to 1, 2, 3, 4, and 5 ppm NO₂ gases were improved 1.3, 1.4, 1.6, 1.7 and 1.8 fold, respectively, further through the encapsulation of GaN nanowires with a WO₃ thin film. The improvement in the response of GaN nanowires to NO₂ gas by encapsulation is attributed to the modulation of electron transport at GaN–WO₃ heterojunction. The electron transport in the core-shell nanowires is modulated by the heterojunction with an adjustable energy barrier height, resulting in an enhanced sensing property of the core-shell nanostructures.     

Comparison on photocatalytic degradation of gaseous formaldehyde by TiO2, ZnO and their composite

In order to compare the photocatalytic properties of TiO₂, ZnO and their composite in the gas phase pollutant environment, nanocomposite with different mole ratios of TiO₂/ZnO were designed to degrade gaseous formaldehyde. The results showed that the rate constant of TiO₂ for formaldehyde degradation was 0.05 min^?1 which was two orders of magnitude larger than that of ZnO in our experiment. Through comprehensive analysis of UV–vis diffuse reflectance (UV–vis) spectra, photoluminescence spectra (PL) and energy band diagram, it was found that the differences of photocatalytic properties between ZnO and TiO₂ may mainly originate from the increased recombination of photoinduced charges in ZnO. The photocatalytic properties of TiO₂/ZnO composite for formaldehyde degradation were much worse than those of TiO₂, while better than those of ZnO. The addition of a small amount of ZnO weakened the photocatalytic properties of TiO₂. It may be attributed to that the recombination action of photoinduced electron–hole pairs in ZnO.     

Gas sensing properties of asymmetrically reduced Na1/2Bi1/2TiO3–based ceramics

We report the gas sensing properties of a type of new materials, Na_1/2Bi_1/2TiO₃ (NBT)-based ceramics. After the NBT-based ceramics were asymmetrically reduced and coated with Au electrodes, the materials exhibit relatively large electrical responses when exposed to oxygen and some oxidizable gases at a relatively low temperature (≤300 °C). An electric voltage ～60 mV is measured in the mixture of O₂ and N₂ (1% O₂). In oxidizable gases, a negative response can be obtained. The measured voltages are ?45 mV and ?98 mV in the mixtures of H₂/air (1000 ppm H₂) and C₂H₅OH/air (1000 ppm C₂H₅OH), respectively. The electrical responses are proportional to the logarithm of the concentrations of the analyzed gases. Also, the electrical responses to oxygen and oxidizable gases have opposite signs, and the model of mixed-potential is proposed to explain the gas sensing phenomenon. This study provides a new material and a simple design for gas sensors. The proposed gas sensor comprises a reduced NBT-based ceramic wafer with the same electrodes on the opposite surfaces. Additional components in traditional gas sensors, such as sensing or reference electrode, are unnecessary.     

Electrochemical decomposition of CO2 and CO gases using porous yttria-stabilized zirconia cell

Electrochemical decomposition of CO₂ and CO gases using a porous cell of Ru-8 mol% yttria-stabilized zirconia (YSZ) anode/porous YSZ electrolyte/Ni–YSZ cathode system at 400–800 °C was studied by analyzing the flow rate and composition of outlet gas, current density, and phases and elementary distribution of the electrodes and electrolyte. A part of CO₂ gas supplied at 50 ml/min was decomposed to solid carbon and O₂ gas through the cell at the electric field strengths of 0.9–1.0 V/cm. The outlet gas at a flow rate of 3 ml/min included 61–63% CO₂ and 37–39% O₂ at 700–800 °C and the outlet gas at a flow rate of 50 ml/min included 73–96% (average 85%) CO₂ and 4–27% (average 15%) O₂ at 800 °C. On the other hand, the supplied CO gas was also decomposed to solid carbon, O₂ and CO₂ gases at 800 °C. The fraction of outlet gas at a flow rate of 50 ml/min during the CO decomposition at 800 °C for 5 h was 11–36% CO, 59–81% O₂ and 2–9% CO₂. The detailed decomposition mechanisms of CO₂ and CO gases are discussed. Both Ni metal in the cathode and porous YSZ grains under the DC electric field have the ability to decompose CO gas into solid carbon and O²⁻ ions or O₂ gas.     

A novel ethanol sensor with high response based on one-dimensional YFeO3 nanorods

As the cognition of metal oxide semiconductor becomes deeper and deeper, their excellent sensing ability has also been demonstrated. The gas sensors with metal oxide semiconductor as basis materials have become a hot topic at present. Enhancing the sensitivity and reducing the test limit of the sensor are exceedingly important topic. It is crucial to regulate the morphology of metal oxide semiconductor materials to improve the gas sensing performance. Low-dimensional materials such as quantum dots, one-dimensional nanowires and nanorods usually show the excellent gas-sensitive properties. In this work, one-dimensional YFeO₃ nanorods were synthesized by electrospinning technology. The one-dimensional rod-like structure enables more active sites to be exposed on the surface of materials, which can effectively promote the adsorption process of the YFeO₃ nanorods to the test gases, so as to improve the gas sensing performance. Found by testing the gas sensitivity, YFeO₃ nanorods responds far better to ethanol than other tested gases. The response and recovery time of YFeO₃ nanorods to 100 ppm ethanol at 350 °C was approximately 19 s and 9 s, respectively. It indicates that the response and recovery ability of YFeO₃ nanorods to ethanol were excellent. The study can provide technical reference for subsequent preparation of remarkable performance ethanol sensor and enrich the materials category of gas sensor fields.     

NO2 gas sensing properties of Au-functionalized porous ZnO nanosheets enhanced by UV irradiation

Porous ZnO nanosheets were synthesized by thermal evaporation. The morphology, crystal structure, and sensing properties of the ZnO nanosheets to NO₂ gas at room temperature under UV illumination were examined. Au nanoparticles with diameters of a few tens of nanometers were distributed over the ZnO nanosheets. The responses of the multiple networked nanosheet gas sensors were improved 1.8–3.3 fold by Au functionalization at NO₂ concentrations ranging from 1 to 5 ppm. Furthermore, the Au-functionalized ZnO nanosheet gas sensors showed a considerably enhanced response at room temperature under ultraviolet (UV) illumination. In addition, the mechanisms through which the gas sensing properties of ZnO nanosheets are enhanced by Au functionalization and UV irradiation are discussed.     

Low-temperature preferential oxidation of CO in a hydrogen rich stream (PROX) over Au/TiO2: Thermodynamic study and effect of gold-colloid pH adjustment time on catalytic activity

By simulating CO and H₂ oxidations at thermodynamic equilibrium and studying the catalytic oxidations over Au/TiO₂, preferential oxidation of CO in a H₂ rich stream (PROX) was investigated. During the simulation, at least two cases under different gaseous feeds, H₂/CO/O₂/N₂ = 50/1/0.5/48.5 or 50/1/1/48 (vol.%) were examined under the assumption of an ideal gas and one atmosphere pressure in the reactor. It was found that the addition of 1% O₂ (the latter case) effectively reduced CO concentration to less than 100 ppm in the temperature range between 0 and 90 °C. This range narrowed to between 0 and 50 °C with the addition of 3% H₂O and 15% CO₂ in the feed. The thermodynamic study suggests that 1% CO in a H₂ rich system can be decreased to below 100 ppm within those low temperature ranges, if there is no substantial adsorptions onto the catalyst surface and the reactions rapidly reach equilibrium. During the catalysis reaction study, a well-pH adjusted Au/TiO₂ catalyst was found very active for PROX. CO conversions at the reactor outlet were close to those at equilibrium. Au/TiO₂ used in this work was prepared via deposition-precipitation (DP) method. The influence of gold colloid pH (at 6) adjustment time on gold loading, gold particle size and chloride residue on TiO₂ surface was detected by atomic absorption (AA), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A pH adjustment time of at least 6 h for the preparation of gold colloids at room temperature was demonstrated to be essential for the high catalytic activity of Au/TiO₂. This was attributed to the smaller gold particle and the less chloride residue on the catalyst surface.     

Formation of TiO2 nano fibers on a micro-channeled Al2O3-ZrO2/TiO2 porous composite membrane for photocatalytic filtration

In this study, needle-shape TiO₂ fibers were successfully fabricated inside a micro-channeled Al₂O₃-ZrO₂ composite porous membrane system using sol-gel method. The micro-channeled Al₂O₃-ZrO₂ composite was fabricated using the fibrous monolithic (FM) process. Pure anatase phase TiO₂ was crystallized from the as-coated amorphous phase during calcination at 510 °C. The TiO₂ fibers grew on the surface frame of the micro-channeled Al₂O₃-ZrO₂ composite membrane and fully covered the inside of the micro-channeled pores. The specific surface area of the TiO₂ coated membrane system was dramatically increased by over 100 fold compared to that of the non-coated system. The photocatalytic activity of the membrane was also assessed and was shown to very effectively convert organic materials. Thus, this novel membrane holds promise for use as an advanced filtration system.     

Photocatalytic activity of transparent porous glass supported TiO2

A porous glass tube with a composition of 96SiO₂·4B₂O₃ (wt%) supported TiO₂ shows high photooxidation activity due to its transparency and large surface area. The surface area of the porous glass tube supported TiO₂ is 10,000 times larger than that of conventional materials. TiO₂ crystals supported are anatase type. Transparency of the porous glass tube is very important. Herein, sol–gel and chemical vapor deposition (CVD) processes were employed as TiO₂ supporting processes. CVD process is more effective. For instance, an aqueous methylene blue solution with 1 ppm concentration almost thoroughly decomposes at a contact time of 300 s using porous glass tube supported TiO₂ prepared by CVD process under irradiating with 10 W low-pressure mercury lamp, on the other hand, opaque porous alumina tube supported TiO₂ was only 25%. The smaller the pore size of the porous glass tube, the larger the transparency and the permeation resistance through porous glass tube. Hence, porous glass tube with ca. 40 nm pore diameter is suitable from the standpoint of a practical use.     

Synthesis and dielectric properties of polyaniline/titanium dioxide nanocomposites

Two series of polyaniline–TiO₂ nanocomposite materials were prepared in base form by in situ polymerization of aniline with inorganic fillers using TiO₂ nanoparticles (P25) and TiO₂ colloids (Hombikat), respectively. The effect of particle sizes and contents of TiO₂ materials on their dielectric properties was evaluated. The as-synthesized polyaniline–TiO₂ nanocomposite materials were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR), thermal analysis (DTA/TGA), and X-ray diffraction (XRD). Dielectric properties of polyaniline–TiO₂ nanocomposites in the form of films were measured at 1 KHz–1 MHz and a temperature range of 35–150 °C. Higher dielectric constants and dielectric losses of polyaniline–TiO₂ nanocomposites than those of neat PANI were found. PANI–TiO₂ nanocomposites derived from P25 exhibited higher dielectric constants and losses than those from Hombikat TiO₂ colloids. Electrical conductivity measurements indicate that the conductivity of nanocomposites is increased with TiO₂ content. The dielectric properties and conductivities are considered to be enhanced due to the addition of TiO₂, which might induce the formation of a more efficient network for charge transport in the base polyaniline matrix.     

Influence of ball milling on the sintering behaviour of ZnO powder

A high purity ZnO powder was milled with either YSZ or Al₂O₃ balls. The weight losses of YSZ and Al₂O₃ balls after milling for 4 h are 10 and 40 ppm, respectively. The debris of the milling media acts as sintering aid to the ZnO powder. As a result, the grain size of the sintered ZnO specimens is reduced. The ratio of the grain boundary energy over surface energy is also decreased.     

Preparation of high solids content nano-titania suspensions to obtain spray-dried nanostructured powders for atmospheric plasma spraying

Nanosized TiO₂ powder with an average primary size of ∼20 nm and surface area of ∼50 m²/g (Aeroxide^® P25, Degussa-Evonik, Germany) was used as starting material. A colloidal titania suspension from the same supplier was also used (W740X). The dispersing conditions were studied as a function of pH, dispersant content, and solids loading. Well-dispersed TiO₂ nanosuspensions with solids contents up to 30 vol.% (62 wt%) were obtained by dispersing the powder with 4 wt% PAA. Suspensions with solids contents as high as 35 vol.% were prepared by adding the TiO₂ nanoparticles to the TiO₂ colloidal suspension under optimised dispersing conditions.TiO₂ powder reconstitution was performed by spray drying both types of nanosuspensions to obtain free-flowing micrometre-sized nanostructured granules. The spray-dried nanostructured TiO₂ granules were deposited on austenitic stainless steel coupons using atmospheric plasma spraying. Coating microstructure and phase composition were characterised using scanning electron microscopy and X-ray diffraction techniques.     

Sonosynthesis of nanostructured TiO2 doped with transition metals having variable bandgap

Here is described a sonosynthesis method to produce nanostructured TiO₂ pure and doped (Al, C, Co, Fe and Rh). The synthesized TiO₂ is amorphous and is transformed to anatase, brookite or rutile by heat treatments at temperatures between 100 and 300 °C. Pure TiO₂ can be partially transformed to brookite between 100 and 300 °C. The band gap in all heat treated samples from 100–600 °C is relatively constant, 3.2 eV, except for those doped with Fe. This effect on the band gap is the results of a bi/tri-crystal (anatase:brookite:rutile) framework. Rhodium is the most effective dopant to narrow the band gap, the opposite effect is observed with C. In single phase frameworks the bandgap can be modified ranges from 2.38 to 4.10 eV depending on the dopant. TiO₂ lattices are rigid enough to promote an outwards diffusion of the dopants to the surface of the particles forming nanostructured precipitates. The precipitates develop a network of quantum-dots with sizes between 5 and 10 nm.