XPS Characterization of reduced LaCoO3 perovskite

Three samples of LaCoO₃ were prepared by two different methods and calcined at 800 or 1000 °C. They had BET areas of 1, 12, and 16 m²/g and all of them showed pure perovskite X-ray diffraction patterns with identical unit cell dimensions. In a series of experiments the oxide, having larger surface area, was stepwise reduced in hydrogen at temperatures between 60 and 500 °C. The XPS spectra, taken at room temperature after evacuation at 400 °C at each reduction level, showed that the surface concentration of Co° was very low up to 300 °C but increased sharply between 300 and 350 °C (9–75%). This concentration further increased to 100% after 10 min reduction at 450 °C, but upon heating in hydrogen for an additional 10 min at 500 °C it decreased again to 75%. In another series of experiments the mixed oxide was reoxidized after each reduction. A fresh sample was reduced to 350 and 400 °C by contacting with hydrogen for 1 hr and evacuated at temperatures between 400 and 500 °C. Both high evacuation temperatures and reduction at 400 °C during 1 hr produced a sharp decrease in Co° surface concentration. These results are consistent with the catalytic behavior of this perovskite reported earlier by E. A. Lombardo et al. (4–7). A model is proposed to interpret the reduction of LaCoO₃.     

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.     

A novel CO and C3H8 sensor made of CuSb2O6 nanoparticles

Trirutile-type CuSb₂O₆ nanoparticles were synthesized by a simple and economical route, starting from copper nitrate, antimony chloride, ethylenediamine, and ethyl alcohol as solvent. The latter was evaporated by microwave radiation at 140 W. The precursor material was calcined at 200, 300, 400, 500, and 600 °C, and analyzed by powder XRD. The oxide phase was obtained at the last calcination step (600 °C), whose powders were analyzed by field-emission scanning electron (FE-SEM) and transmission electron (TEM) microscopies. Microrods, hexagonal microplates, and nanoparticles with an average size of ~ 51.2 nm were observed. A forbidden bandwidth of 3.41 eV was detected for the direct transition with UV–vis. Tests were carried out on pellets made of the powders in carbon monoxide (CO) and propane (C₃H₈) atmospheres at different concentrations and operating temperatures, obtaining high response at 300 ppm of CO and 500 ppm of C₃H₈, both at 300 °C.     

High temperature stiffening of ferroelastic LaCoO3

The cyclic ferroelastic hysteretic behavior of pure LaCoO₃ perovskite ceramic has been studied at different temperatures in four point bending. The stress-strain deformation behavior of LaCoO₃ was analyzed both in the term of the maximum stress in the cycle and in terms of the temperature used when the cyclic testing was performed. The characteristics of the stress-strain hysteresis loops, such as hysteresis loop area and irreversible strain, as well as effective Young’s modulus, were analyzed, and it was established that both the loading and the temperature history have a significant influence on the mechanical behavior of LaCoO₃. Young’s modulus values are reported to be much higher in the 700–900 °C temperature range as compared to the measurements performed in the RT-400 °C temperature range.     

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.     

Synthesis and Characterization of Perovskite-Type Oxides La1−xMxCoO3 (M = Ce,Sr) for the Selective CO Oxidation (SELOX)

Perovskite-type oxides La_1?xM_xCoO₃ (M = Ce, Sr) were prepared by citrate method, characterized and evaluated in the selective CO oxidation (SELOX-CO). The insertion of low Cerium or Strontium content generated solids with a single phase related LaCoO₃ perovskite. For higher contents we observed segregation of CeO₂ and SrCO₃. The iso-structural substitution favors the formation of vacancies. The SELOX-CO showed 100 % CO conversion at 200 °C. Higher temperatures favored hydrogen oxidation and methanation.     

Thermosensitive folic acid-targeted poly (ethylene-<Emphasis Type="Italic">co</Emphasis>-vinyl alcohol) hemisuccinate polymeric nanoparticles for delivery of epirubicin to breast cancer cells

A novel thermosensitive folic acid (FA)-targeted succinylated poly (ethylene-co-vinyl alcohol) (EVOH) (EVOHS-FA) nanocarrier was synthesized for the specific delivery of epirubicin (EPI) to MCF-7 breast cancer cell line. Three different ratios of synthesized EVOH-Suc were reacted with FA. The structure of the desired products (EVOHS₄₀-FA, EVOHS₆₀-FA and EVOHS₈₀-FA) was confirmed by ¹H NMR and FTIR techniques. Nanoparticles were obtained by nano-precipitation procedure using DMSO/H₂O as solvent/anti-solvent. The particle size, zeta potential, entrapment efficacy and in vitro release profile of the final formulations in different temperatures were measured. The optimized nanoparticles had the particle size of 214 ± 8.5 nm, zeta potential of ?29.6 mV, PDI of 0.198 ± 0.04, and a high encapsulation efficiency that released the drug efficiently within 450 h at the temperature of 40 °C compared to 37 °C. The morphology of nanoparticles was studied by scanning electron microscopy. The in vitro cytotoxicity was evaluated using the MTT assay on MCF-7 cell lines in response to temperatures of 37 and 40 °C. The MTT assay indicated that the targeted nanoparticles carrying EPI were significantly more cytotoxic than the non-targeted nanoparticles and the free drug at 40 °C.     

Synthesis and characterization of TiO2 nanoparticles

TiO₂ nanoparticles were synthesized by thermal decomposition of a precipitate obtained from a precursor solution of titanium isopropoxide (IV) and isopropyl alcohol. The as-prepared precipitate was heated at various temperatures and the obtained samples were morphologically, texturally and structurally characterized using TGA–DTA, gas adsorption, SEM, XRD and FTIR. The UV–vis radiation absorption and the photocatalytic activity also were verified. The TiO₂ sample heated at 300 °C shows the best results to be applied as blocker in solar skin protector.     

Observation of low- and high-temperature CuFe2O4 phase at 1100 °C: The influence of Fe3+ ions on CuFe2O4 structural transformation

Samples containing CuFe₂O₄ and CuO nanoparticles were synthesized by coprecipitation method and subjected to thermal treatment at 300?°C, 500?°C, 700?°C, 900?°C and 1100?°C. Depending on the synthesis conditions, high-temperature treatment of the prepared samples brought to the formation of CuFe₂O₄ with a tetragonal or cubic structure, which was confirmed by X-ray diffraction analysis. The influence of formation mechanism onto the crystallite sizes of nanoparticles and structural transformation of CuFe₂O₄ is discussed. For the first time it was shown that variations of the initial amount of Fe³⁺and Cu²⁺ ions precursors resulted in the formation of low- and high-temperature CuFe₂O₄ phase at the same annealing temperature, 1100?°C.     

Spray-flame synthesis of La(Fe,Co)O3 nano-perovskites from metal nitrates

Nano-sized perovskites were synthesized in a spray flame from nitrate precursors dissolved in ethanol and in ethanol/2-ethylhexanoic acid (2-EHA) mixtures. Experiments with ethanol led to a broad particle-size distribution and to the formation of undesired phases such as La₂CoO₄, La₂O₃, and Co₃O₄. The addition of 2-EHA can initiate micro explosions of the burning droplets and has been systematically investigated toward the formation of single-phase, high-surface-area LaCoO₃ and LaFeO₃ with a narrow size distribution. To investigate the effect of 2-EHA, temperature-dependent changes of the chemical composition of the precursor solutions were analyzed with ATR-FTIR between 23 and 70°C. In all cases, the formation of esters was identified while in the solutions containing iron, additional formation of carboxylates was observed. The synthesized materials were characterized by BET SSA, XRD, SAED and EDX-TEM and their catalytic activity was analyzed, reaching 50% CO conversion at temperatures below 160 and 300°C for LaCoO₃ and LaFeO₃, respectively.     

LaCoO3 ceramics obtained from reactive powders

The aim of the present study was to establishing the correlation between the structure and properties of the LaCoO₃ powders obtained by aqueous sol–gel method with citric acid and their sintering behavior in order to obtain fully densified ceramics with perovskite structure. Two types of cobalt and lanthanum reagents were used in synthesis, namely nitrates and acetates. The sintering was realized at temperatures ranging between 800 and 1200 °C for 2 h. The sintered samples were investigated by classical ceramic methods (shrinkage, density, porosity) and by structural and morphological investigations: XRD, SEM, AFM and XPS. The electrical properties of the samples were determined by impedance spectroscopy. The ceramics obtained with powders starting with acetates have presented a lower sintering ability as compared with the samples obtained from powders starting with nitrates. LaCoO₃ ceramics with best properties was obtained from powders starting with nitrates sintered at 1100 °C.     

Designing zinc oxide nanostructures (nanoworms,nanoflowers, nanowalls,and nanorods) by pulsed laser ablation technique for gas-sensing application

In this study, pulsed laser ablation technique, also known as pulsed laser deposition (PLD), is used to design and grow zinc oxide (ZnO) nanostructures (nanoworms, nanowalls, and nanorods) by template/seeding approach for gas-sensing applications. Conventionally, ZnO nanostructures used for gas-sensing have been usually prepared via chemical route, where the 3D/2D nanostructures are chemically synthesized and subsequently plated on an appropriate substrate. However, using pulsed laser ablation technique, the ZnO nanostructures are structurally designed and grown directly on a substrate using a two-step temperature-pressure seeding approach. This approach has been optimized to design various ZnO nanostructures by understanding the effect of substrate temperature in the 300-750°C range under O₂ gas pressure from 10-mTorr to 10 Torr. Using a thin ZnO seed layer as template that is deposited first at substrate temperature of ~300°C at background oxygen pressure of 10 mTorr on Si(100), ZnO nanostructures, such as nanoworms, nanowalls, and nanorods (with secondary flower-like growth) were grown at substrate temperatures and oxygen background pressures of (550°C and 2 Torr), (550°C and 0.5 Torr), and (650°C and 2 Torr), respectively. The morphology and the optical properties of ZnO nanostructures were examined by Scanning Electron Microscope (SEM-EDX), X-ray Diffraction (XRD), and photoluminescence (PL). The PLD-grown ZnO nanostructures are single-crystals and are highly oriented in the c-axis. The vapor-solid (VS) model is proposed to be responsible for the growth of ZnO nanostructures by PLD process. Furthermore, the ZnO nanowall structure is a very promising nanostructure due to its very high surface-to-volume ratio. Although ZnO nanowalls have been grown by other methods for sensor application, to this date, only a very few ZnO nanowalls have been grown by PLD for this purpose. In this regard, ZnO nanowall structures are deposited by PLD on an Al₂O₃ test sensor and assessed for their responses to CO and ethanol gases at 50 ppm, where good responses were observed at 350 and 400°C, respectively. The PLD-grown ZnO nanostructures are very excellent materials for potential applications such as in dye-sensitized solar cells, perovskite solar cells and biological and gas sensors.     

Development of MgO:TiO2 thin films for gas sensor applications

In this study, structural, morphological and optical properties, and gas sensor performance of magnesium oxide (MgO) doped titanium dioxide (TiO₂) thin films were investigated in detail. Gas sensor metallic patterns were fabricated on Si substrate using traditional photolithographic technique. MgO doped TiO₂ thin films were deposited on formed Pt electrode surface by confocal sputtering (co-sputtering) system as the active layer. Thin film characterizations were realized by using secondary ion mass spectroscopy (SIMS), atomic force microscope (AFM) and UV–Vis Spectrometer (UV–Vis). Gas sensing measurements were performed by gas sensing test system against methane gas at working temperature of 300?°C. To evaluate deposition and thermal annealing effects on the sensing performance, sensors were tested under gas. The sensitivity and response/recovery time of gas sensors were measured in 1000?ppm. MgO doped TiO₂ based sensor at substrate temperature of 100?°C has high sensitivity and short response/recovery time.     

Characterization of hydrogenation active sites on LaCoO3 perovskite

LaCoO₃ becomes active for hydrogenation of ethene upon reduction in hydrogen at temperatures between 300 and 490 °C. Several aspects of the reacting system were studied in order to ascertain the nature of the active sites generated in this manner. Catalyst deactivation was evaluated by comparing rates between two successive experiments. An upper limit was estimated for the amount of polymeric residues formed after a single run: 1.1 ± 0.5 × 10¹⁴ molecules of C₂ per square centimeter. Reduced LaCoO₃ also catalyzed the self-hydrogenation of ethene. When a mixture of C₂H₄:D₂ = 1:1 was reacted over LaCoO₃ reduced to varying extents multiple-exchanged ethenes and ethanes were formed. The exchange patterns were almost unaffected by the extent of reduction. The effect of pretreatment temperatures was also evaluated. The solid in its reduced form was particularly sensitive to high-temperature treatments. The amounts of CO chemisorbed when plotted vs extent of reduction gave curves that were almost identical to the activity plots. The results reported here, discussed in terms of the current literature, are consistent with a model in which finely dispersed Co⁰, formed in the oxide matrix upon reduction, is the locus of hydrogenation activity.     

LaFeO3 ceramics as selective oxygen sensors at mild temperature

In this study, an investigation about the oxygen sensing properties of lanthanum orthoferrite (LaFeO₃) ceramics is reported. LaFeO₃ nanoparticles were synthesized by using tartaric sol-gel route and annealed in air at different temperatures (500, 700 and 900 °C). The samples have been characterized by using thermal analysis (TA), BET surface area and porosity, Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Results of sensing tests indicate that LaFeO₃ nanoparticles exhibit good response to oxygen at mild temperatures (300–450 °C). The effect of annealing temperature on gas sensing performance was investigated, demonstrating that LaFeO₃ ceramics obtained after annealing at 500 °C display better characteristics with respect to others. The oxygen sensor developed shows also high stability in humid environment and excellent selectivity to oxygen over other interfering gases such as CO, NO₂, CO₂, H₂ and ethanol.     

PtO2-nanoparticles functionalized CuO polyhedrons for n-butanol gas sensor application

The CuO polyhedrons functionalized with different amounts of PtO₂ nanoparticles were synthesized by simple two-step method. The gas sensing properties of the sensors prepared by PtO₂ functionalized CuO polyhedron were studied and compared with pure CuO sensors. The electrical sensitivity values show that the response of S2-CuO (3.5%_wt PtO₂-CuO) polyhedron is higher than that of pure CuO polyhedron in n-butanol/air atmosphere. The sensor showed excellent reproducibility and good selectivity to n-butanol gas, and its working temperature was relatively low (180?°C), and the reaction time quickly reached 2.4?s. The enhanced properties are attributed to the structure of the CuO polyhedron and the synergistic effect of CuO and PtO_2.     

Influence of carbon precursor on porosity,surface composition and catalytic behaviour of CMK-3 in oxidative dehydrogenation of propane to propene

Ordered mesoporous CMK-3 carbon replicas were synthesized by infiltration of mesopores present in a SBA-15 silica template with two different carbon precursors, i.e. sucrose and poly(furfuryl alcohol). The obtained composites were carbonized under an inert gas atmosphere at 550, 650, 750 and 850?°C, and the template was etched with a HF solution. The final carbon replicas were analyzed by various physicochemical techniques, including low-temperature N₂ adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, and tested as catalysts in the oxidative dehydrogenation of propane (ODP) at 450?°C. Both series of materials differed strongly with respect to their porosity, but showed very similar surface composition determined by XPS. Higher porosity of CMK-3 prepared using the sucrose precursor influenced propane conversion and selectivity to propene. Furthermore, oxygen containing groups (e.g. carbonyl-type) were found to be less sensitive to the type of carbon precursor than to the ODP reaction conditions.     

Fuel Gas Upgrading Over La1−xCexCoO3 Mixed Oxide with Toluene as Model Compound

Perovskite-type mixed oxides La_1?xCe_xCoO₃ (x = 0, 0.2, 0.4) were synthesized by sol–gel method via polyvinyl alcohol (PVA) as gelating agent. LaCoO₃ and CeO₂ phases were presented while intermediate phase, La(OH)₃, disappeared during LaCoO₃ transformation. Introduction of Ce decreased crystal size of catalyst from 22.18 to 13.38 nm. Particle size and specific surface area were in the range of 9.58–13.72 μm and 6.03–9.23 m²/g, respectively. The addition of Ce increased the reduction temperature which indicated the strong interaction between Co and perovskite structure. Catalytic activity was investigated by steam reforming of toluene at 500–800 °C. Conversions of carbon and hydrogen to CO, H₂, and CH₄ at steam per carbon ratio (S/C) of 2:1 were clearly higher than 4:1. Increasing S/C ratio to 4:1 inhibited syngas production efficiency by combustion and water–gas shift reaction. The presence of Ce in the catalyst did not improve activity of catalyst significantly. However the metal enhanced stability by promoting the formation of filamentous carbon on the surface such that the active sites are still accessible to the active gas during the experiment. After reforming, catalysts were transformed to La(OH)₃, Co⁰, and Ce₄O₇ phases and no significant deterioration in catalytic performance was detected after 6 h. In this study steam reforming of toluene over La_0.6Ce_0.4CoO₃ at 800 °C with S/C 2:1 yielded highest carbon conversion as CO and hydrogen conversion as H₂ of 64.42 and 63.23 %. The LHV and H₂/CO of produced gas at this optimum condition are 4.22 MJ/N m² and 2.91, respectively.     

WO3 nanomaterials synthesized via a sol-gel method and calcination for use as a CO gas sensor

Carbon monoxide is a poisonous and hazardous gas and sensitive sensor devices are needed to prevent humans from being poisoned by this gas. A CO gas sensor has been prepared from WO₃ synthesized by a sol-gel method. The sensor chip was prepared by a spin-coating technique which deposited a thin film of WO₃ on an alumina substrate. The chip samples were then calcined at 300, 400, 500 or 600 °C for 1 h. The sensitivities of the different sensor chips for CO gas were determined by comparing the changes in electrical resistance in the absence and presence of 50 ppm of CO gas at 200 °C. The WO₃ calcined at 500 °C had the highest sensitivity. The sensitivity of this sensor was also measured at CO concentrations of 100 ppm and 200 ppm and at operating temperatures of 30 and 100 °C. Thermogravimetric analysis of the WO₃ calcined at 500 °C indicated that this sample had the highest gas adsorption capacity. This preliminary research has shown that WO₃ can serve as a CO gas sensor and that is should be further explored and developed.     

Characteristics and gas sensor applications of ZnO-Perovskite heterostructure

In this study, zinc oxide (ZnO) nanofibers were prepared using the electrospinning method, and the effects of different spinning voltages and annealing temperatures on the fiber structure were tested. La_0.8Sr_0.2FeO₃ (LSFO) perovskite film was prepared by a sol-gel method. Then we dip LSFO on ZnO nanofiber and grow it on the interdigital gold electrode substrate for gas sensors. The results show that the ZnO/LSFO heterostructure gas sensor has a good sensing response to H₂S gas and exhibits good gas selectivity. The best gas response is 52.17% under 4 ppm H₂S and work temperature 200°C, which has good recovery and reproducibility.