Nanostructured perovskite oxides – LaMO3 (M=Al,Co, Fe) prepared by co-precipitation method and their ethanol-sensing characteristics

Nanostructured La-based perovskite oxides − LaMO₃ (M=Al, Co, Fe) were synthesized by a new co-precipitation procedure using metal nitrate and carbonate salts as starting materials. X-ray diffraction and energy dispersive X-ray spectroscopic results confirmed the formation of single-phase nanocrystalline perovskite oxides with high purity. Characterizations by scanning/transmission electron microscopy and nitrogen adsorption revealed that LaAlO₃ was produced in the form of rectangular porous nanorods exhibiting much larger surface area and porosity compared with densely aggregated LaCoO₃ particles and loosely clustered LaFeO₃ nanoparticles with cracked-egg morphologies. The materials were characterized for gas sensing towards ethanol at 200–350 °C. From gas-sensing results, the LaAlO₃ sensor displayed n-type gas-sensing behaviors with considerably higher ethanol response than p-type LaFeO₃ and LaCoO₃ sensors, respectively. In particular, the LaAlO₃ sensor exhibited a high response of 16.45–1000 ppm ethanol and excellent ethanol selectivity against NO₂, SO₂, CO and H₂ at 350 °C. The superior gas-sensing performances could be attributed to the effective receptor function, transducer function and utility factor of LaAlO₃ nanorod structures prepared by the co-precipitation method.     

NO2 gas sensing responses of In2O3 nanoparticles decorated on GO nanosheets

Nanomaterials offer a wide range of applications in environmental nanotechnology. Hazardous pollutants in the environment are needed to be detected and controlled effectively to avoid human health risks. In this paper, we described the fine-controlled growth of In₂O₃ nanoparticles embedded on GO nanosheets by a facile precipitation method. The In₂O₃@GO nanocomposites exhibited outstanding gas sensing performance as compared with pure In₂O₃ nanoparticles towards NO₂. At 225 °C, the sensor displayed high selectivity, best response (78) to 40 ppm NO₂, quick response, and recovery times of 106s/42s. The improved sensing performances of the nanocomposite were attributed to large surface area, high gas adsorption-desorption capability, and the formation of p-n heterojunctions between In₂O₃ nanoparticles and GO nanosheets. The excellent gas detecting activities validate In₂O₃@GO nanocomposites as a promising candidate in the NO₂ gas sensor industry.     

Fast Response NO2 Gas Sensor Based on In2O3 Nanoparticles

In this research, hydrothermal‐calcination route was applied to synthesize In₂O₃ nanoparticles for gas sensor application. Hydrothermal synthesis with duration of 5 h at 180°C resulted in In(OH)₃ nanorods. Then, in the calcination step, considering controlled rate of heating and temperature, In₂O₃ nanoparticles with rough surfaces were obtained. In the next step, these nanoparticles were deposited by low frequency AC electrophoretic deposition between the interdigitated electrodes to fabricate gas sensor. Deposition in the frequency of 10 kHz resulted in the chained nanoparticles in the interelectrode space. At the end, gas sensitivity measurements were conducted at 150°C–300°C and revealed that fabricated sensor had fast response and recovery times to NO₂ gas.     

Facile synthesis of In2O3 nanoparticles with high response to formaldehyde at low temperature

In₂O₃ nanoparticles with uniform particle size (10-25 nm) were obtained using the facile precipitation strategy at room temperature with following calcined treatment. The gas-sensing performance of In₂O₃ nanoparticles with different calcined temperatures was investigated. The results demonstrated that the In₂O₃ nanoparticles calcined at 500°C exhibited highest sensing response (R_a/R_g = 68.1) to 10 ppm HCHO at 100°C with good selectivity, stability, reproducibility, and ultra-low limit of detection (1 ppm). The results of XPS, UV, and other characterizations indicated that In₂O₃-500 possessed the most absorbed oxygen species, the highest carrier mobility, and lowest band gap energies. Our work offers new insights into the development of sensing materials to the detection of volatile organic compounds (VOCs).     

Synthesis and ethanol sensing properties of CuO nanorods coated with In2O3

CuO/In₂O₃ core–shell nanorods were fabricated using thermal evaporation and radio frequency magnetron sputtering. X-ray diffraction and transmission electron microscopy showed that both the cores and shells were crystalline. The multiple networked CuO/In₂O₃ core–shell nanorod sensors showed responses of 382–804%, response times of 36–54 s and recovery times of 144–154 s at ethanol (C₂H₅OH) concentrations ranging from 50 to 250 ppm at 300 °C. These responses were 2.3–2.8 times higher than those of the pristine CuO nanorod sensor over the same C₂H₅OH concentration range. The origin of the enhanced ethanol sensing properties of the core–shell nanorod sensor is discussed.     

Ethanol sensing properties of networked In2O3 nanorods decorated with Cr2O3-nanoparticles

In₂O₃ nanorods decorated with Cr₂O₃ nanoparticles were synthesized by thermal evaporation of In₂S₃ powder in an oxidizing atmosphere followed by solvothermal deposition of Cr₂O₃ and their ethanol gas sensing properties were examined. The pristine and Cr₂O₃-decorated In₂O₃ nanorods exhibited responses of ~524% and ~1053%, respectively, to 500-ppm ethanol at 200 °C. The Cr₂O₃-decorated In₂O₃ nanorod sensor showed stronger electrical response to ethanol gas at 200 °C than the pristine In₂O₃ nanorod counterpart. The former also showed faster response and recovery than the latter. The pristine and Cr₂O₃-decorated In₂O₃ nanorod sensors showed the strongest response to ethanol gas at 250 and 200 °C, respectively. The Cr₂O₃-decorated In₂O₃ nanorod sensor showed selectivity for ethanol gas over other reducing gases. The underlying mechanism for the enhanced response, sensing speed and selectivity of the Cr₂O₃-decorated In₂O₃ nanorod sensor for ethanol gas is discussed.     

Effect of raw material composition on microstructures and properties of microporous MgO–Mg(Al,Fe)2O4 refractory aggregates

Microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates were prepared by the in-situ decomposition synthesis method using the magnesite, Al(OH)₃ and Fe₂O₃ as raw materials. The effect of raw material composition (theoretical Mg(Al, Fe)₂O₄ contents were 0–55 wt %) on their microstructure and strengths was investigated. When the theoretical Mg(Al, Fe)₂O₄ contents were relatively low (0–5.5 wt %), the number of neck connections between the particles in the microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates was small. As the theoretical Mg(Al, Fe)₂O₄ contents increased to be 11–22 wt %, the number of neck connections increased and the compressive strengths were enhanced. When the theoretical Mg(Al, Fe)₂O₄ contents increased to be excessive (33–55 wt %), the inter-particle pore size further increased due to the increase of volume expansion caused by the formation of more spinel, resulting in a decrease of compressive strength. Overall, when the theoretical Mg(Al, Fe)₂O₄ contents were 11–22 wt %, the microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates showed the excellent performances with the median pore sizes of 17.37–25.46 μm, the apparent porosities of 23.4–28.1%, the bulk densities of 2.57–2.79 g/cm³ as well as the compressive strengths of 41.2–75.8 MPa.     

Fabrication,structural, optical,physical and radiation shielding characterization of indium (III) oxide reinforced 85TeO2-(15–x)ZnO-xIn2O3 glass system

This work aimed to evaluate the structural, optical, and physical features of several types of glasses based on 85TeO₂-(15–x)ZnO-xIn₂O₃ (x = 2, 4, 6, and 8 mol%) system. As a result, five different samples were synthesized utilizing the melting-annealing technique. The Archimedes method was used to calculate the densities of the synthesized glasses. The structural, optical, physical, and radiation interaction characteristics of the sample were determined using XRD investigations, Raman spectra, and advanced modelling methods, producing optical band gap, refractive index, and Urbach energy values. The glass densities increased from 5.6091 g cm⁻³ to 5.6754 g cm⁻³ by increasing In₂O₃ reinforcement from 2 to 8 mol %. Urbach energies increased consistently from 0.1399 to 0.1439 eV as In₂O₃ concentration increased, apart from a drop to 0.1345 eV at x = 8. The optical transmittance and absorption characteristics altered nearly monotonically with increasing In₂O₃ ratios, showing that these characteristics may be estimated and controlled using In₂O₃ additive. By substituting ZnO with In₂O₃ within the structure, the optical band gap was dramatically enlarged. Additionally, at simulated energies greater than 0.02 MeV, the gamma-ray mass attenuation coefficient grows monotonically with In₂O₃ reinforcement. As a result, it can be stated that the high concentration In₂O₃ to TeO₂–ZnO glass combination is a good synergetic tool for integrating structural, optical, and radiation properties.     

Mesoporous In2O3 materials prepared by solid-state thermolysis of indium-organic frameworks and their high HCHO-sensing performance

Mesoporous In₂O₃ materials were synthesized by calcining indium-organic frameworks (InOFs, CPM-5 and MIL-68), which were further successfully utilized to detect toxic HCHO vapor. By taking the intrinsic structural features of two InOF precursors into account, the surface areas of produced indium oxides were well investigated via controlling the calcination temperature. The influence of surface area on the gas sensing performance was studied in detail. Porous In₂O₃ prepared by heat treatment at 650 °C showed the highest responses to 50 ppm HCHO (Rg/Ra = 31.8 and 38.0, respectively; Rg, resistance in gas; Ra, resistance in air) at 210 °C, which surpass the values of all the reported In₂O₃ materials to date under the similar conditions. The promising HCHO-sensing properties enable these InOF-templated mesoporous In₂O₃ materials to be competitive candidates for detecting poisonous formaldehyde in practice.     

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.     

Sintering behavior of IZO ceramic targets prepared using hydrothermally synthesized Zn-doped In2O3 nanoparticles

Zn-doped In₂O₃ composite nanoparticles were synthesized by precipitation/hydrothermal processing in the pH range of 5–11 and at the reaction temperature of 180 °C. The precursor solution used to prepare these nanoparticles was composed of In(NO₃)₃ and ZnSO₄·7H₂O, mixed at the In:Zn atomic ratios of 1:1, 1.5:1, and 2.5:1. The effects of In:Zn atomic ratio and pH on the morphology and phase composition of the composite nanoparticles were investigated in detail using a scanning electron microscope (SEM), a transmission electron micrograph (TEM), and an X-ray diffraction (XRD) system. The obtained results show that under alkaline conditions (pH 9–11), the nanoparticles exhibit cubic morphology. However, at pH 7, the morphology is sheet-like. Based on XRD analyses, the Zn content in hydrothermally processed nanoparticles is highest when the pH of the precursor solution is 7. Assessments of sintering behavior demonstrate that the IZO ceramic targets sintered at 1550 °C for 20 h are composed of In₂O₃ and Zn₃In₂O₆ phases, with uniform grain morphology and size distribution.     

Preparation and characterization of single crystalline In2O3 films deposited on MgO (110) substrates by MOCVD

Epitaxial indium oxide (In₂O₃) films have been prepared on MgO (110) substrates by metal-organic chemical vapor deposition (MOCVD). The deposition temperature varies from 500 °C to 700 °C. The films deposited at each temperature display a cube-on-cube orientation relation with respect to the substrate. The In₂O₃ film deposited at 600 °C exhibits the best crystalline quality. A clear epitaxial relationship of In₂O₃ (110)|MgO (110) with In₂O₃ [001]|MgO [001] has been observed from the interface area between the film and the substrate. The average transmittance of the prepared films in the visible range is over 95%. The band gap of the obtained In₂O₃ films is about 3.55–3.70 eV.     

Fabrication and NO2 gas sensing performance of TeO2-core/CuO-shell heterostructure nanorod sensors

TeO₂-nanostructured sensors are seldom reported compared to other metal oxide semiconductor materials such as ZnO, In₂O₃, TiO₂, Ga₂O₃, etc. TeO₂/CuO core-shell nanorods were fabricated by thermal evaporation of Te powder followed by sputter deposition of CuO. Scanning electron microscopy and X-ray diffraction showed that each nanorod consisted of a single crystal TeO₂ core and a polycrystalline CuO shell with a thickness of approximately 7 nm. The TeO₂/CuO core-shell one-dimensional (1D) nanostructures exhibited a bamboo leaf-like morphology. The core-shell nanorods were 100 to 300 nm in diameter and up to 30 μm in length. The multiple networked TeO₂/CuO core-shell nanorod sensor showed responses of 142% to 425% to 0.5- to 10-ppm NO₂ at 150°C. These responses were stronger than or comparable to those of many other metal oxide nanostructures, suggesting that TeO₂ is also a promising sensor material. The responses of the core-shell nanorods were 1.2 to 2.1 times higher than those of pristine TeO₂ nanorods over the same NO₂ concentration range. The underlying mechanism for the enhanced NO₂ sensing properties of the core-shell nanorod sensor can be explained by the potential barrier-controlled carrier transport mechanism.

PACS

61.46. + w; 07.07.Df; 73.22.-f     

Electric field enhancements in In2O3-coated single-walled carbon nanotubes

In₂O₃ nanoparticles are coated on the surfaces of single-walled carbon nanotubes (SWCNTs) by a successive ionic layer adsorption and reaction process. The thickness of the In₂O₃ nanoparticle film is tuned by controlling the number of coating cycles. The electric field around the In₂O₃-coated SWCNTs is compared with that of pristine SWCNTs. Field enhancement of the In₂O₃-coated SWCNTs is confirmed by conductive atomic force microscopy at low electric field (contact mode: 1 V to −1 V) and also field emission (FE) analysis at high electric field (0–4.2 V/μm). The uniformity and emission stability are also measured via FE analysis. Near infrared and X-ray photoemission spectroscopy data are suggested to explain the charge transfer, bandgap change between the In₂O₃ nanoparticles and SWCNTs, and the electric field enhancements in the In₂O₃-coated SWCNTs at both low and high electric field.     

Sr and Fe co-doped Ba2In2O5 as a new proton-conductor-derived cathode for proton-conducting solid oxide fuel cells

BaSrInFeO₅ (BSIF), a new cathode material for proton-conducting solid oxide fuel cells (SOFCs), is designed based on the modification of the Ba₂In₂O₅ proton conductor with Sr and Fe cations. Compared with the Ba₂In₂O₅ proton conductor tailored with only Fe cations (Ba₂InFeO₅, BIF), doping Sr can improve the chemical stability and also benefit the formation of oxygen vacancies. The proton mobility is also improved with Sr-doping, which is confirmed by first-principles calculations and experimental studies. An H-SOFC using the BSIF cathode generates a relatively high peak power density of 1192 mW cm^-2 at 700 ^oC, which is superior to many cells in previous reports. First-principles calculations find that the cathode oxygen reduction reaction (ORR) energy barrier for BSIF is significantly lower than that for BIF. Although Ba₂In₂O₅ is less studied, the derived cathode materials can still present decent performance, probably offering new material selections for H-SOFCs.     

Effect of polyvinylpyrrolidone on morphology and structure of In2O3 nanorods by hydrothermal synthesis

Indium oxide (In₂O₃) nanorods were hydrothermally synthesized from aqueous InCl₃ solution in urea with addition of polyvinylpyrrolidone (PVP) as a steric stabilizer. Indium hydroxide, In(OH)₃, was precipitated at 60 °C and was changed into a transient InOOH phase upon calcination at 250 °C in air. X-ray diffractometry revealed that the existence of PVP delays the phase transformation of InOOH. Cubic-structured In₂O₃ phase was then formed when temperature was raised to 350 °C, regardless of the PVP concentration. The In(OH)₃ phase without the PVP showed a rod-based, flower-like morphology of polycrystalline character. Minor addition of the PVP, i.e., 0.1–2 wt.%, resulted in a pronounced evolution in morphology from the three-dimensional, flower-like form to discrete, one-dimensional nanorods aligned in planar form. Both the flower-like and discrete nanorod morphologies were preserved after heat treatments at 250 and 350 °C. This reveals that the morphological change is attributable to preferential adsorption of the PVP molecules on the In(OH)₃ crystallite surface, so that the aggregate attachment responsible for the multipod growth is inhibited.     

Design and self-assembly of metal-organic framework-derived porous Co3O4 hierarchical structures for lithium-ion batteries

Porous metal oxides hierarchical structures with controlled morphologies have received great attention because of their promising applications in catalysis, energy storage, gas sensing, etc. Porous Co₃O₄ hierarchical structures with controlled morphologies were synthesized on the basis of a pyrolytic conversion of Co-based metal-organic frameworks (Co-MOFs), which were initially grown in solutions containing Co(NO₃)₂·6H₂O, 1,3,5-Benzentricarboxylic acid and pyrazine as solute and N,N-dimethylformamide (DMF) as solvent under a solvothermal condition. Porous Co₃O₄ with twin hemispherical and flower-like structures were obtained with the assistance of PVP by adjusting the amount of pyrazine. The results of nitrogen adsorption–desorption indicate the BET surface area (22.6 m² g⁻¹) of twin hemispherical Co₃O₄ structures is lower than that (33.3 m² g⁻¹) of flower-like Co₃O₄ structures. However, the pore size of twin hemispherical Co₃O₄ structures is smaller, which is centered at about 2.5, 4.0 and 20.0 nm. The Co₃O₄ with twin hemispherical structures exhibit more excellent electrochemical performance as anode materials for lithium ion batteries than that of flower-like Co₃O₄ structures, which may be attributed to the smaller particle size and compact porous structures with suitable pore size.     

Thermoelectricity and antivibration properties of screen-printed nanodoped In1.35ZnO2.11/In2O3 thin-film thermocouples on alumina substrates

Owing to the low flow field disturbances and fast response, thin-film thermocouples (TFTCs) are used to measure the service temperature of aero-engines. Indium tin oxide (ITO) and In₂O₃ are widely used in high–temperature measurements. However, ITO undergoes phase transition and consequent thermoelectric failure at above 1300 °C. In this study, In_1.35ZnO_2.11/In₂O₃ TFTCs were prepared on alumina substrates via screen printing method through introduction of ZnO nanopowder followed by annealing treatment. Results show that prepared TFTCs exhibited good thermoelectric properties at 1500 °C. The morphology, structure, and electrical coefficients of TFTCs were investigated. The average Seebeck coefficient was 39.8 μV/°C at 1500 °C with a drift rate (DT) of 0.84 °C/h, which was significantly improved with respect to that of ITO/In₂O₃, corresponding to 44.5 μV/°C at 1270 °C with a DT of 5.44 °C/h and failed at higher temperature. The result of lumped capacity method test show that the response time was 4.8 ms at 100 °C. Preliminary engine gas temperature measurements with a heat load of 1000 °C at 1 Mach show that these TFTCs are promising candidates for engineering applications. Finally, the structural reliability under high-magnitude vibration and impact tests (10–2000 Hz/20 g and 100 g/11 ms) was also investigated. As a result of the excellent bonding strengths of 47.36 and 59.83 N between the film and the substrate for both In_1.35ZnO_2.11 and In₂O₃, respectively, destructive cracking and peeling of the film were not observed, and no change in the Seebeck coefficient of the sample occurred after impact and vibration tests. These results provide an important basis for the potential application of In_1.35ZnO_2.11/In₂O₃ TFTCs in aero-engine high-temperature measurements of flow channel components.     

Hetero-epitaxy growth of cobalt oxide/nickel oxide nanowire arrays on alumina substrates for enhanced ethanol sensing characteristics

In this work, Co₃O₄ nanowire arrays are prepared in-situ on the flat alumina substrates via a simple hetero-epitaxial growth without adding seed layers. It is found that the density of Co₃O₄ nanowires can be regulated by varying the concentration of NH₄F that acts as substrate activation promoting the formation of nuclei on alumina substrate. In order to improve the gas-sensing performance, porous NiO nanosheets are anchored on the surface of Co₃O₄ nanowire arrays to form a novel heterostructure (Co₃O₄/NiO). Gas-sensing tests indicate a higher response value of these array composites towards ethanol (R_g/R_a = 4.26) than that of pristine Co₃O₄ nanowire arrays (R_g/R_a = 1.27) and NiO nanosheets (R_g/R_a = 1.89). The improved gas-sensing performances resulting from the special array structures and novel heterojunctions can provide abundant diffusion channels for gas molecules as well as a synergistic effect between Co₃O₄ and NiO.     

Selective preparation of nanorods and micro-octahedrons of Fe2O3 and their catalytic performances for thermal decomposition of ammonium perchlorate

Nanorods and micro-octahedrons of α-Fe₂O₃ were selectively synthesized through one-step hydrothermal treatment of iron(III) chloride in aqueous formamide solution for different time. The resulting samples were characterized with XRD, FE-SEM, TEM, and FT-IR. It was found that monodispersed nanorods were 10-25 nm in diameter and 50-100 nm in length, while the octahydrons were 100-400 nm in size. These α-Fe₂O₃ nanorods and micro-octahedrons exhibited quite different catalytic performances on thermal decomposition of ammonium perchlorate. The nanorods significantly reduced the decomposition temperature of ammonium perchlorate, but the micro-octahedrons did not. This study provides alternative choice of good burning rate catalysts for composite solid propellants in solid fueled rockets.