Flame Spray Pyrolysis Technique Research Articles

Use of flame spray pyrolysis technique to synthesize the nanoparticles of titanium dioxide: Application on dye-sensitized photovoltaic cells

In this work, Flame Spray Pyrolysis (FSP) technique has been used to synthesize the nanoparticles of titanium dioxide (TiO2) for dye sensitized photovoltaic cells (DSPVs). X-ray diffraction (XRD) has been employed to characterize the phase structure and ratios of TiO2 made by FSP. The dye used to fabricate cells is a commercial dye N719. In particular, a reference DSPV (REF) has been constructed utilizing the reference titanium electrode which is sold by Solaronix S.A, in order to compare the photovoltaic (PV) performance with the cell made by FSP powder. As a result, the power conversion efficiency of cells are 2.47% and 7.66% for DSPV made by FSP powder and REF DSPV, respectively.

Flame-Made La2O3-Based Nanocomposite CO2 Sensors as Perspective Part of GHG Monitoring System.

Continuous monitoring of greenhouse gases with high spatio-temporal resolution has lately become an urgent task because of tightening environmental restrictions. It may be addressed with an economically efficient solution, based on semiconductor metal oxide gas sensors. In the present work, CO2 detection in the relevant concentration range and ambient conditions was successfully effectuated by fine-particulate La2O3-based materials. Flame spray pyrolysis technique was used for the synthesis of sensitive materials, which were studied with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) and low temperature nitrogen adsorption coupled with Brunauer–Emmett–Teller (BET) effective surface area calculation methodology. The obtained materials represent a composite of lanthanum oxide, hydroxide and carbonate phases. The positive correlation has been established between the carbonate content in the as prepared materials and their sensor response towards CO2. Small dimensional planar MEMS micro-hotplates with low energy consumption were used for gas sensor fabrication through inkjet printing. The sensors showed highly selective CO2 detection in the range of 200–6667 ppm in humid air compared with pollutant gases (H2 50 ppm, CH4 100 ppm, NO2 1 ppm, NO 1 ppm, NH3 20 ppm, H2S 1 ppm, SO2 1 ppm), typical for the atmospheric air of urbanized and industrial area.

Mixed potential type ammonia sensor using Fe-substituted LaCoO3 sensing electrode prepared by flame spray pyrolysis

NH3 detection is of great significance for precise control of the SCR system. In this paper, mixed potential type ammonia sensor using LaCo1-xFexO3 sensing electrode, YSZ electrolyte, and Pt reference electrode was developed. LaCo1-xFexO3 sensing materials were made by flame spray pyrolysis technique, and Fe B-site substitution was implemented to enhance the sensing performance. It was observed that in the concentration range of 20–70 ppm at 475 °C, the sensor using LaCo0.9Fe0.1O3 as sensing electrode exhibited the highest sensing signal and sensitivity to ammonia among the LaCo1-xFexO3-SE sensors (x = 0, 0.05, 0.1, 0.2, 0.3). A sectional linear relationship between the response value change(ΔV) of that sensor and the logarithm of ammonia concentration within the range of 2–15 ppm and 15–70 ppm was noticed, and the sensitivities were 17.52 and 87.22 mV/decade, respectively. Its lower detection limit reached 2 ppm at 475 °C. Its excellent sensing properties are ascribed to enriched oxygen vacancies and high electrocatalytic performance of the perovskite electrode. Moreover, the sensor exhibited good selectivity against CH4, CO, NO, NO2, and good stability against oxygen and water vapor concentration fluctuation. Besides, the sensing response to 50 ppm ammonia has been tested for 22 days and only a slight fluctuation within 7 % was observed. Furthermore, the mixed potential sensing mechanism of the sensor was discussed.

Enhancing the Utilization of Porous Li4Ti5O12Layers for Thin-Film Lithium-Ion Batteries

Thin-film lithium-ion batteries (TFBs) are promising components for powering rigid and mechanically flexible electronic devices. The production of such all-solid-state power units is yet complex and cost-intensive, and the performance is to be improved. Because binders are often absent in TFBs, it is a key issue to retain the mechanical stability and reversible discharge capacities upon mechanical bending. One approach to reduce contact losses upon cell-bending may be the fabrication of electrodes with controlled residual porosities. The remaining voids could buffer occurring volume changes during cycling of typical cathode materials. To systematically approach the fabrication of long-living high-performance flexible TFBs, this work addresses the relationship between different residual porosities of pure TFB electrodes and their electrochemical performance in flat TFB systems. Crystalline and phase-pure Li4Ti5O12 (LTO) thin-film electrodes were deposited in situ with the flame spray pyrolysis technique and compressed at two different pressures. Zero-strain LTO has been chosen as a model material because it enabled us to understand the sole impact of porosity on electrochemical performances without interfering volume changes. Our results showed that denser LTO particle networks were accompanied by a larger number of LTO particle contacts. Also, the LTO contact densities to the neighboring interfaces in a TFB cell have increased. The improved electrode contacting led to reduced electrical sheet resistivities and significantly increased practicable capacities. Future projects will require the substitution of LTO with high-voltage cathode materials to maximize energy densities. Cycling under statically and dynamically bent condition will answer whether tuned residual porosities are useful for the production of long-living flexible TFBs.

Enhanced VOCs Detection By the Co3O4/ZnO Nanocomposites, Obtained By Single Step Flame Spray Pyrolysis

Introduction The detection of volatile organic compounds (VOCs), such as acetone, ethanol or formaldehyde is a relevant practical task [1]. Enhanced sensitivity towards such gases has been reported periously for ZnO based nanomaterials, modified with Co3O4 phase [1-3]. However, the reported techniques of such nanocomposites fabrication are usually involve several stages, are time and labor consuming and barely scalable [1-5].In this work we demonstrate facile single step synthesis and improved sensitivity towards wide range of gases in dry and humid air conditions of ZnO-Co3O4 nanocomposites. Long term stability of obtained nanoparticle materials gas sensor response is demonstrated. Materials synthesis, characterization and sensor measurements A series of ZnO based materials were obtained by flame spray pyrolysis technique, described elsewhere [6]. Zn (II) and Co (II) 2-ethylhexanoates were taken as a precursors, which were diluted in toluene in 1:4 ratio by volume. Toluene also served as a fuel. Co (II) 2-ethylhexanoate has been taken in a calculated amount to form 1, 5 and 20% mol Co containing nanocomposite based on ZnO. Mixture of precursors and fuel was fed with 3 ml/min pace to a spray nozzle, where it was dispersed with 3 ml/min oxygen flow at a pressure drop of 3 bar. The nano-powders, forming during synthesis, were collected on a glass fiber filter 75 cm above the nozzle with the use of vacuum pump. During separate synthetic procedure the obtained materials were directly deposited on ceramic alumina plates, fixed at 19 cm above the nozzle on water-cooled surface. Plates were equipped with platinum contacts on one side and platinum heating element on the other, used by us in the previous work [6]. Micro-hotplates with the deposited sensitive element were welded in TO-8 case and used in further gas sensor measurements as is. One sample of alumina plate with deposit for each material were passed through 24h annealing at 500 oC and then used in sensor measurements alongside the sensors with as prepared sensitive films. Powders of synthesized materials were investigated by XRD, BET, TEM with EDX mapping, EPR techniques. Real chemical composition of obtained samples was investigated by ICP-MS method. The interaction of obtained nanoparticulate matter with VOC molecules was studied by in situ DRIFTs and Raman spectroscopy, as well as thermo-programmed desorption, coupled with mass-spectrometry. Results and Conclusions Increase in Co content leads to increase in sensor response towards VOCs, particularly oxygen-containing acetone and methanol (Fig. 1). At the same time this effect is observed only for the material with lowest Co content in case of hydrogen detection. Moreover, the sensor response towards methanol and propane decreases once sensor working temperature exceeds 400 oC in case of nanocomposite with highest Co content. TEM micrograph with EDX mapping shows uniform distribution of Co over ZnO matrix. XRD analysis indicates formation of separate Co3O4 phase in case of material with 20 % mol Co content, while EPR results give a cue for formation of both Co (II) substitutional defect in ZnO matrix as well as Co3O4 phase formation. This allows to conclude that introduction of Co in ZnO based nanocomposite has both electrical and chemical beneficial effects on metal oxide gas sensor performance, especially towards oxygen-containing VOCs. Thermal annealing at 500 oC prior to gas sensor measurements leads to response decrease of thus prepared sensitive layers, however, the resulting response is preserved over long-term operation even in high humidity conditions. Gas sensor, fabricated on the basis of sensitive layers without pre-annealing step, demonstrate slow drift of response value during long-term operation.

Metal Oxide Gas Sensors Signal Shape Processing for Selective Detection of Hydrocarbons in Realistic Air Conditions

Introduction The selective detection with high spatio-temporal resolution of hydrocarbons leakage as a result of pipelines inconsistency is a valid industrial demand [1]. Perspectives are associated with deployment of distributed networks of chemical sensors, transferring data on ambience pollution to the data processing center [2]. Miniature micromachined metal oxide semiconductor gas sensors possess a great perspective of practical use in this regard, however their long term operation in real atmosphere conditions requires improvement [3]. In this work we demonstrate improved selectivity of propane vs. methane detection in low concentrations in the ambient air of highly urbanized location by the SnO2-based semiconductor gas sensors with the modulated working temperature, using statistical analysis of “sensor response/sensor working temperature” shape representation. The obtained results of gases discrimination with artificial neural network (ANN) machine learning algorithm, used such pre-treated data as input samples, demonstrate advantages over similar classification methods without signal pre-processing, or pre-processing with previously reported methods of PCA, wavelet transformation and polynomial curve approximation. Sensor fabrication, data collection and processingThree SnO2-based materials have been used – pure SnO2, gold-modified SnO2 and bimetallic Au and Pd modified SnO2. The gas sensing materials synthesis via single step flame spray pyrolysis technique and sensors fabrication has been reported in a recent paper [4]. A fixed air flow from outside of the building of the Department of Chemistry of Moscow State University through PTFE sensor chamber was used as a background. Methane or propane were admixed to the background air flow through precise mass-flow controllers in three different concentrations from certified gas bottles. Sensors were operated in a working temperature modulation mode between 500 oC and 150 oC with period length of 60 sec. Sensitive layer resistance was measured with the 10 Hz frequency. Collected data was separated in two parts: first, collected in December 2018 was used for data processing algorithms training. Second, collected during January and February 2019, was used for the testing of elaborated data processing models. The total data set of 22440 measurement cycles,11220 of which represent an ambient air, while two data subsets of 5610 measurement cycles represent same air with admixture of methane or propane, were used for processing. The multilayer perceptron ANN-model with 2 hidden dense layers has been used for data processing model development and gases discrimination. The batch normalization and dropout techniques were implemented to avoid model overfitting.According to statistical shape analysis approach, each characteristic point of sensor response is characterized by certain gas sensor working temperature and sensitive layer resistance. All characteristic points of each single measurement cycle during analysis are converted into Kendall’s pre-shape space with removed translation and scaling factor, but with remaining rotation as well as the shape. During further processing the data samples are projected onto the unit-sphere with a pole of mean shape of sensor responses towards air without additives. Later inverse transformation of data samples leads to icon representation of response shape, eliminating possible effects of sensor baseline and response amplitude drift. Results and Conclusions Direct application of collected data to artificial neural network algorithm for methane vs. propane selective detection without any pre-processing gives a decent level of accuracy (Fig.1). However, it can be seen by the rise of discrimination error after normalization of data samples, that the main component of the sensors response, used by algorithm for gases discrimination, is of amplitude nature. It means, that the developed signal processing model may work in unstable manner when any additional gas is involved in the measurements. The use of statistical shape analysis allows to significantly improve the accuracy of discrimination of methane vs. propane in comparison with other pre-processing techniques. The remained error of gases identification is mostly related to the single day of measurements, which is associated with urban air pollution during winter heating season.

CO2 hydrogenation over FSP-made iron supported on cerium modified alumina catalyst

Nanocrystalline Fe/Al2O3 and Fe/Ce-Al2O3 catalysts have been synthesized by using one-step flame spray pyrolysis technique (FSP). For comparison purposes, the similar catalysts were also prepared by the conventional impregnation method. The FSP-made Fe/Al2O3 and Fe/Ce-Al2O3 with 3 and 5 wt % Ce exhibited the crystal structure mainly of FeAl2O4 spinel phase and small trace of alumina without the presence of other phases such as iron oxide or cerium oxide phases. However, addition of 7 wt % Ce, the additional peaks of both CeO2 and Fe2O3 were observed. The XPS and TPR results suggest the presence of Fe2O3 cluster on the FSP-made catalyst surface. The catalytic performance of the FSP-made catalysts in terms of both CO2 reaction rate and selectivity to hydrocarbon were much better than those prepared by impregnation method. Addition of Ce also promoted the CO2 hydrogenation rate and hydrocarbon selectivity. Improvement of catalytic properties of FSP-made catalysts was probably due to the high interaction of Fe and Al2O3, which led to the high dispersion of Fe2O3 on catalyst surface. Addition of Ce promoted the formation of iron carbide, which was the active site for hydrocarbon formation, however, excessive amount of Ce doping could resulted in the coverage of the active sites, resulting in the decrease of catalytic performances.

Development of highly sensitive [Ru(bpy)3]2+ - Based optical oxygen sensing thin films in the presence with Fe3O4 and Fe3O4@Ag NPs

Optical sensors based on chemically sensitive matrices/dyes have been used for accurate measurements of oxygen levels. In this study, the intensity-based response of highly sensitive tris(2,2′-bipyridyl) ruthenium(II) chloride [Ru(bpy)3]2+ dye to oxygen (O2) was analyzed in the polymeric ethylcellulose matrix (EC) in the presence of two magnetite forms; Fe3O4 and Fe3O4@Ag nanoparticles (NPs). Fe3O4 NPs were synthesized by flame spray pyrolysis (FSP) and then Fe3O4@Ag NPs were prepared by a core-shell coating method. Iron oxide polymorphs were characterized by using X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It was measured that the particles synthesized by the FSP technique have an average size of ˂60 nm Fe3O4, Fe3O4@Ag NPs, and 1-butyl-3-methylimidazolium tetrafluoroborate; ([BMIM][BF4]) were used as additives for enhancement of the sensitivity of the [Ru(bpy)3]2+ dye to O2 gas. The effects of these additive materials were examined by photoluminescence spectroscopy (PL). [Ru(bpy)3]2+ dye doped nanofiber in the presence with the Fe3O4@Ag NPs additive yielded higher Stern-Volmer constant (KSV) and extended linear response range when compared with the sensing slides in the presence with Fe3O4 NPs and the additive-free forms. The higher response ratio (I0/I100) was found as 3.45 for the ruthenium dye doped EC nanofiber form in the presence with Fe3O4@Ag NPs. Also, KSV values were calculated as 2.41 × 10−2 between the concentration range of 0–100% [O2]. According to our knowledge, it is the first time Fe3O4 and Fe3O4@Ag NPs were used for the enhancement of oxygen sensitivity of the ruthenium dye.

Characterization and evaluation of Fe–N–C electrocatalysts for oxygen reduction directly synthesized by reactive spray deposition technology

A novel method for synthesizing iron–nitrogen–carbon (Fe–N–C) electrocatalysts using a modified flame spray pyrolysis technique called reactive spray deposition technology (RSDT) is described. The physicochemical properties of the RSDT-synthesized catalyst are quantified through a series of experiments including nitrogen adsorption, electron microscopy, thermogravimetry, spectroscopy and electrochemical analysis with the overall aim of exploring opportunities to use RSDT as a single-step, scalable alternative to multi-step, energy-intensive furnace-based methods for synthesizing PGM-free electrocatalysts for oxygen reduction. The Fe–N–C is synthesized by pyrolyzing a liquid solution precursor mixture under oxygen lean conditions without the use of additional support material or heat treatment steps. Properties of critical importance to the performance of the Fe–N–C catalyst are discussed in detail with respect to properties of similar catalysts synthesized by multi-step methods reported in the literature. Materials characterization shows evidence of ORR active Fe–Nx sites, a high fraction of pyridinic nitrogen and carbon-encapsulated iron-rich particles. The existence of undesired amorphous carbon mixed with the catalytically active material is also observed and may require process development in future to remove. Rotating disk electrode analysis in alkaline media of the RSDT-synthesized Fe–N–C confirmed catalytic activity toward oxygen reduction, which is shown to follow a two-step reaction mechanism. While the activity of the Fe–N–C catalyst is lower than that of commercial Pt/C, it shows superior stability with a decrease in half-wave potential of only 5 mV after 4000 cycles in alkaline media, encouraging further investigation of this alternative flame-based synthesis route.

Structural and optical properties of SrAl2O4: Eu2+/Dy3+ phosphors synthesized by flame spray pyrolysis technique

Abstact In this study Sr0.985Al2O4:0.005Eu2+/0.01Dy3+ luminescent materials with long afterglow and high intensity were synthesized by flame spray pyrolysis (FSP) method for the first time. Precursor solution was divided into two separate part. One of the parts was used FSP process, the other one was dried to simulate sol-gel process. Thermal properties and organic content of FSP derived particles and dried gel were compared using DTA-TG and FTIR devices. FSP derived particles were sintered in argon medium at 1200 oC. Phase and elemental analysis of the final phosphors were carried out by X-Ray Diffraction (XRD) and X-Ray Photo electron Spectroscopy (XPS) measurements. Scanning electron microscopy (SEM) images were obtained to observe particle morphology. Optical properties of phosphor were determined by Photoluminescence (PL), thermoluminescence (TL) and radioluminescence (RL) measurements. The emission spectra of Sr0.985Al2O4:0.005Eu2+/0.01Dy3+ were composed by a green band at 525 nm at room temperature when it was excited by 350 nm light, assigned to anomalous luminescence originated by Eu2+ and Dy3+ in the host lattice. There were estimated peaks at around 90, 130, 170 and 200 oC under the main glow curve in the sample irradiated by X-ray and He-Ne laser (λexc = 632 nm). RL spectrum of Sr0.985Al2O4 :0.005Eu2+/0.01Dy3+ powders have four distinct emission peaks at around 527 nm, 576 nm, 667 nm, 753;nm and a comparatively weak emission band at about 846 nm.

Flame aerosol deposited Li4Ti5O12 layers for flexible, thin film all-solid-state Li-ion batteries

Flexible thin film all-solid-state Li-ion batteries are considered as promising candidates to power a multitude of flexible and miniaturized electronic devices. The production of crystalline battery active materials generally involves high process temperatures above 500 °C. One current challenge in mechanically flexible thin film electrode fabrication is the direct deposition of such crystalline active materials onto temperature sensitive substrates. In the current work we have made a paradigm shift depositing highly pure crystalline Li4Ti5O12 nanoparticles onto a flexible polyimide foil in a single step using flame spray pyrolysis technique. The Li4Ti5O12 films were mechanically compressed at room temperature to 0.55 µm thin layers, to enhance their adhesion to the substrates, i.e. to increase mechanical stability. The smooth Li4Ti5O12 electrodes were covered with a solid electrolyte and tested against lithium metal electrodes. Stable electrochemical cycling behavior of the battery cells demonstrated the feasibility of the proposed technique for LTO thin film electrode fabrication on temperature sensitive and mechanically flexible polyimide substrates. Fundamental data on possible electrode cyclability upon electrode bending was obtained by successful cycling of LTO flex-TFBs in statically bent condition. This study could initialize a new branch for facile manufacturing of flexible thin film battery cells.

ZnO nanoparticle preparation route influences surface reactivity, dissolution and cytotoxicity.

ZnO nanoparticles (nZnO) are commonly used in nanotechnology applications despite their demonstrated cytotoxicity against multiple cell types. This underscores the significant need to determine the physicochemical properties that influence nZnO cytotoxicity. In this study, we analyzed six similarly sized nZnO formulations, along with SiO2-coated nZnO, bulk ZnO and ZnSO4 as controls. Four of the nZnO samples were synthesized using various wet chemical methods, while three employed high-temperature flame spray pyrolysis (FSP) techniques. X-ray diffraction and optical analysis demonstrated the lattice parameters and electron band gap of the seven nZnO formulations were similar. However, electrophoretic mobility measures, hydrodynamic size, photocatalytic rate constants, dissolution potential, reactive oxygen species (ROS) production and, more importantly, the cytotoxicity of the variously synthesized nZnO towards Jurkat leukemic and primary CD4+ T cells displayed major differences. Surface structure analysis using FTIR, X-ray photoelectron spectroscopies (XPS) and dynamic light scattering (DLS) revealed significant differences in the surface-bound chemical groups and the agglomeration tendencies of the samples. The wet chemical nZnO, with higher cationic surface charge, faster photocatalytic rates, increased extracellular dissolution and ROS generation demonstrated greater cytotoxicity towards both cell types than those made with FSP techniques. Furthermore, principal component analysis (PCA) suggests that the synthesis procedure employed influences which physicochemical properties contribute more to the cytotoxic response. These results suggest that the synthesis approach results in unique surface chemistries and can be a determinant of cellular cytotoxicity and oxidative stress responses.

Effect of doping Fe/Cu/Ti on WO3 on furfural degradation

This research improved tungsten oxide catalysts to increase efficiency in photocatalytic degradation of furfural under visible light. The aim of this research was to compare the efficiency of modified tungsten oxide with undoped and commercial tungsten oxide. Tungsten oxide nanoparticles were doped with 3 single metals, which were Fe/Cu/Ti at 1%wt, 2%wt, and 3%wt, synthesized by flame spray pyrolysis technique (FSP) and then characterization by X-Ray Diffraction (XRD), N2 adsorption/desorption (BET surface area analysis), UV-Vis Spectroscopy (UV-Vis). Photocatalytic degradation experiments using doped WO3 were carried out with 5 ppm initial concentration of furfural solution using 0.6 M catalyst concentration under visible light. From the results, FSP-synthesized WO3 has better efficiency in furfural degradation than the commercial WO3. All catalysts have mesoporous structure because an average pore size is in the range of 6-10 nm. Among all synthesized and doped WO3, it can be concluded that 3%wt Fe-doped tungsten oxide provides the highest acceleration rate in photocatalytic degradation of furfural.

Synthesis, Physicochemical Characterization, and Cytotoxicity Assessment of CeO2 Nanoparticles with Different Morphologies

With the growing use of nanomaterials, it is essential to carefully determine whether they represent a risk for potential users. So far, validated stand‐alone methods that allow a proper risk assessment are still rare. In the present study, the cytotoxicity of CeO2 nanoparticles has been assessed. For this purpose, a variety of well‐defined CeO2 nanoparticles has been prepared by using either hydrothermal synthesis or flame spray pyrolysis (FSP), resulting in nanoparticles of different morphologies and sizes. The FSP technique is known to produce particles of a very small size (in the range of nanometers), which can easily become airborne. We employed a characterization procedure that makes use of physicochemical techniques, comprising N2 physisorption, XRD, TEM, as well as ζ‐potential and surface‐charge measurements. The cytotoxicity of the nanoparticles was evaluated in vitro on two different human lung cell lines (A549 and MRC‐5). The tests showed that, despite the differences in surface properties, size, and morphologies, neither of the CeO2 samples gave rise to a cytotoxic response.

Synthesis of Nanoparticles through Flame Spray Pyrolysis: Experimental Apparatus and Preliminary Results

Nanostructured materials represent nowadays a wide, and probably still largely unexplored, field of potential applications. In fact, this is a research topic in high and rapid development, both at a basic level and under the point of view of possible practical applications, leaving large space for a thorough scientific analysis, which requires with no doubt long time for ultimate conclusions. This paper deals with the preliminary work performed in the field of FSP (Flame Spray Pyrolysis) synthesis for nanoparticles, using an external mixing gas assisted nozzle. Particularly, an experimental apparatus has been designed, realized and characterized for the controlled synthesis of nanoparticles by the flame spray pyrolysis technique. The apparatus consists of a gas-assisted spray for droplet generation and dispersion in a secondary pilot flame. By dissolving suitable precursors in a liquid fuel, different types of nanoparticles have been produced. In the preliminary tests SiO2 and TiO2 have been synthesized and characterized by TEM analysis and XRD, respectively, as a function of the main operating conditions (for instance, precursor concentration) of the experimental apparatus. The designed set-up shows good stability and reproducibility of the reaction flame and, therefore, of the material produced and the obtained results, even if preliminary, encourage the use of the experimental apparatus for the controlled synthesis of nanostructured materials.

Flame-spray-made Zn[sbnd]In[sbnd]O alloyed nanoparticles for NO2 gas sensing

In this work, ZnInO nanoparticles with different Zn metallic contents (100·Zn/(Zn + In)) were produced by one-step flame spray pyrolysis technique and systematically characterized for NO2 sensing. Structural characterizations by x-ray diffraction, transmission and scanning electron microscopy indicated that Zn and In form solid-solution oxide with smaller particle size and lower crystallinity compared with undoped ZnO and In2O3 ones. In addition, energy-dispersive and X-ray photoemission spectroscopic analyses revealed that materials prepared with Zn content of 25% and 33% were In-rich oxides while those prepared with Zn contents of 40% and 50% were actually Zn-rich oxides. From gas-sensing measurement, only ZnInO sensors with the 50% input Zn content showed improved NO2 response while other ZnInO sensors displayed inferior performances compared with undoped In2O3 sensors. The roles of Zn content on NO2-sensing mechanisms of ZnInO sensor were explained based on particle/grain size and doping effects of solid-solution oxide. The ZnInO oxide with 50% input Zn content exhibited the optimal sensor response of 1476 to 5 ppm NO2 at 250 °C. In addition, it still had good responses of 17 to low NO2 concentrations of 0.125 ppm and good selectivity against NO, H2, H2S and CO. Therefore, the flame-made ZnInO sensor is another promising candidate for sensitive and selective NO2 detections.

Pt-doped In2O3 nanoparticles prepared by flame spray pyrolysis for NO2 sensing

Undoped In2O3 and 0.25–1.00 wt% M (M=Pt, Nb, and Ru)-doped/loaded In2O3 nanoparticles were successfully synthesized in a single-step flame spray pyrolysis technique using indium nitrate, platinum (II) acetylacetonate, niobium ethoxide, and ruthenium (III) acetylacetonate precursors. The undoped In2O3 and M-doped In2O3 nanoparticles were characterized by Brunauer–Emmett–Teller (BET) analysis, X-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM & TEM). The BET average diameter of spherical nanoparticles was found to be in the range of 10.2–15.2 nm under 5/5 (precursor/oxygen) flame conditions. All XRD peaks were confirmed to correspond to the cubic structure of In2O3. TEM images showed that there is no Pt nanoparticle loaded on In2O3 surface, suggesting that Pt should form solid solution with the In2O3 lattice. Gas sensing studies showed that 0.5 wt% Pt doping in In2O3 nanoparticles gave a significant enhancement of NO2 sensing performances in terms of sensor response and selectivity. 0.5 wt% Pt/In2O3 exhibited a high NO2 response of ~1904 to 5 ppm NO2 at 250 °C and good NO2 selectivity against NO, H2S, H2, and C2H5OH. In contrast, Nb and Ru loading resulted in deteriorated NO2 response. Therefore, Pt is demonstrated to be an effective additive to enhance NO2 sensing performances of In2O3-based sensors.

General Flame Approach to Chainlike MFe2O4 Spinel (M = Cu, Ni, Co, Zn) Nanoaggregates for Reduction of Nitroaromatic Compounds

Unique chainlike MFe2O4 (M = Cu, Ni, Co, and Zn) nanoaggregates (NAs) have been prepared by facile flame spray pyrolysis of nitrates/ethanol precursor. Synthesized MFe2O4 are composed of primary nanocrystallites mainly ranging from 8 to 20 nm and show uniform aggregations with size of 200–400 nm. As heterogeneous catalysts, it is clearly noted that CuFe2O4 NAs have the highest catalytic activity for the reduction of nitroaromatic compounds compared to NiFe2O4, CoFe2O4, and ZnFe2O4. The highest rate constant of 36.17 min–1·g–1 is achieved at a low catalyst usage (10 μL, 0.2 mg·mL–1). The remarkably enhanced catalytic performance of CuFe2O4 NAs is mainly attributed to the promoted electron transfer on Cu2+ active sites, which is facilitated by unique spinel structures and chainlike aggregate morphology. It is demonstrated that flame spray pyrolysis technique is an effective route to produce binary or complex oxides for potential industrial use.

Metallic iron nanoparticles: Flame synthesis, characterization and magnetic properties

Metallic iron (Fe) nanoparticles (NPs) with a typical core–shell structure have been prepared by a simple and continuous flame spray pyrolysis (FSP) method, which are stabilized by the corresponding Fe3O4 shell with a thickness of 4–6nm. The size of metallic Fe cores is about 30–80nm. The core–shell structured iron NPs show an air stability as long as one month as a result of the protection of oxide shell. Through the control of the residence time of materials in flame and flame atmosphere, metallic Fe and iron oxides are obtained, showing a better external magnetic field responsibility. It is concluded that the evolution of morphology and composition of flame-made magnetic NPs could be attributed to the competition mechanism between reduction and oxidation reactions of in situ flame combustion, which offers more choices and better effective design strategy for the synthesis of advanced functional materials via FSP techniques.

Low-temperature selective catalytic reduction of NO with NH3 over V/ZrO2 prepared by flame-assisted spray pyrolysis: Structural and catalytic properties

A series of V/ZrO2 (V/Zr atomic ratio=0.05, 0.11, 0.17, 0.25, 0.33, 0.42), V-WOx(0.66)/ZrO2, and V-MoOx(0.66)/ZrO2 were synthesized by adopting a one-step flame spray pyrolysis technique and investigated for the low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. Our XRD results suggest that the vanadia species are in amorphous state as dominant surface monomeric VOx species on zirconia support in V/ZrO2 samples with V/Zr≤0.17. Further increase in vanadia content led to the formation of ZrV2O7 solid solution as a result of zirconia migration into the V2O5 crystallites. H2-TPR data results are highly consistent with the XRD results that the low temperature shifts in the reduction peaks of 15% V/ZrO2 catalyst attributed to the presence of more easily reducible dominant surface monomeric VOx species. The evolution of a new reduction peak at high temperature indicates the formation of dominantly isolated polymeric V oxides species in V/ZrO2 samples with V/Zr≥0.25. Our H2-TPR profiles for tungsten-promoted V/ZrO2 catalyst revealed the shift of (T1) peak position to lower temperatures, suggesting that the reduction potential of vanadium oxide species is increased compared to V/ZrO2 catalyst. This occurrence signifying that the addition of tungsten to V/ZrO2 promoted the catalyst for the formation of monomeric surface vanadia species, whereas the addition of Mo promoted the formation of polymeric VOx species and thus inhibit the SCR activity. An intense sharp band characteristic of coordinatively held NO2 species is evident at 1630cm−1, no gaseous N2O species were detected at 2224cm−1, 1286cm−1 in the in-situ FT-IR spectra of NO adsorbed over the V/ZrO2. Our in-situ FT-IR studies of NO+NH3 co-adsorption demonstrate that the strong signals at 1435 and 1714cm−1 (bending vibration of NH4+) have seen to decrease with respect to temperature. This observation reveals that the NH4+ species bound to Brönsted acid sites are responsible for the enhancement in the SCR reaction over the vanadia-zirconia surfaces. The low temperature NO catalytic reduction activity would indicate that an optimal dispersion of monomeric VOx species on zirconia is attained with the 15% of V in the V/ZrO2 catalyst. The introduction of Zr (seven fold coordination) into V2O5 may lead to the reduction of elemental number in crystal grain and deviation of adjacent oxygen atoms. The change in lattice parameter, crystal anisotropy and negative thermal expansion can block the release of labile oxygen, this seems to be the reason for the decrease in catalytic activity in V/ZrO2 catalysts with V/Zr≥0.25. The addition of tungsten has a strong influence on the NO conversion, since tungsten loading with the WOx/VOx atomic ratio=0.66 exhibits a maximum conversion of ∼98% in the temperature range 180–240°C, whereas the molybdenum loading showed an inhibition effect on the SCR activity of V/ZrO2 catalyst.