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Flame Synthesis and Characterization of Oxide Nanoparticles

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Advanced Nanomaterials and Nanotechnology".

Deadline for manuscript submissions: closed (20 May 2022) | Viewed by 26846

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Dr. Silvana De Iuliis

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Institute of Condensed Matter Chemistry and Technologies for Energy, Milano Unit – National Research Council (CNR), Milano, Italy
Interests: optical diagnostic techniques; laser-induced incandescence (LII); laser-induced breakdown spectroscopy (LIBS); synthesis via flame spray pyrolysis of oxide nanoparticles and characterization; in-flame synthesis of carbon nanoparticles and characterization

Special Issue Information

Dear Colleagues,

In recent decades, oxide nanoparticles have drawn the interest of the scientific community for their peculiar properties, essentially related to their crystalline phase, degree of aggregation, surface area, and porosity. These properties make them attractive in a wide range of application fields, including photocatalysis, gas sensors, and energy storage.

Among the different synthesis methods employed, flame synthesis of oxide nanoparticles is a particularly interesting bottom-up approach for the production of tailored highly functionalized nanoparticles with specific and unique properties. In fact, changing the experimental conditions in the flame, changes in temperature field, gas velocity, and, consequently, residence time will occur, which can affect nanoparticles properties.

Although flame synthesis is the commercially used method for nanoparticle production, a lot of work is still needed to understand the chemical and physical processes of nanoparticle synthesis in relation to the properties required in a specific application. In this context, it is of particular interest to develop and apply diagnostic techniques for the investigation and monitoring of the synthesis process. Characterization of the oxide nanoparticles during and after the synthesis is important for the specific application considered. Moreover, the development of modeling and simulation tools will help to gain a whole comprehension of the processes involved.

This Special Issue on “Flame Synthesis and Characterization of Oxide Nanoparticles” will publish high-quality research articles on the broad area of synthesis, modeling, and characterization of oxide nanoparticles produced in flames.

The topics of interest include but are not limited to the following:

Oxide nanoparticles formation and growth in flames
Diagnostics
Modeling
Applications

Dr. Silvana De Iuliis
Guest Editor

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Keywords

oxide nanoparticles
diagnostics
flame synthesis
nanoparticle characterization

Published Papers (12 papers)

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Research

9 pages, 2634 KiB

Open AccessArticle

Cobalt Oxide Synthesis via Flame Spray Pyrolysis as Anode Electrocatalyst for Alkaline Membrane Water Electrolyzer

by Alfonso Pozio, Francesco Bozza, Nicola Lisi, Rosa Chierchia, Francesca Migliorini, Roberto Dondè and Silvana De Iuliis

Materials 2022, 15(13), 4626; https://doi.org/10.3390/ma15134626 - 01 Jul 2022

Cited by 3 | Viewed by 1396

Abstract

Nanostructured cobalt oxide powders as electro catalysts for the oxygen evolution reaction (OER) in an alkaline membrane electrolysis cell (AME) were prepared by flame spray synthesis (FS); an AME’s anode was then produced by depositing the FS prepared cobalt oxide powders on an AISI-316 sintered metal fiber by the electrophoretic deposition (EPD) method. FS powders and the composite electrode were characterized by SEM, XRD, and XPS analysis. The electrode showed an increase in the OER catalytic activity in a KOH 0.5 M solution with respect to commercial materials commonly applied in alkaline electrolysis, demonstrating that the flame spray synthesis of nanoparticles combined with the electrophoretic deposition technique represent an effective methodology for producing an anodic catalyst for alkaline membrane electrolyzers. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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15 pages, 1997 KiB

Open AccessArticle

Characterization of Fractal Structures by Spray Flame Synthesis Using X-ray Scattering

by Mira Simmler, Manuel Meier and Hermann Nirschl

Materials 2022, 15(6), 2124; https://doi.org/10.3390/ma15062124 - 14 Mar 2022

Cited by 8 | Viewed by 1824

Abstract

In this work, we take on an in-depth characterization of the complex particle structures made by spray flame synthesis. Because of the resulting hierarchical aggregates, very few measurement techniques are available to analyze their primary particle and fractal properties. Therefore, we use small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) to investigate the influence of the precursor concentration on the fractal structures of zirconia nanoparticles. The combination of information gained from these measurement results leads to a detailed description of the particle system, including the polydispersity and size distribution of the primary particles. Based on our findings, unstable process conditions could be identified at low precursor concentrations resulting in the broadest size distribution of primary particles with rough surfaces. Higher precursor concentrations lead to reproducible primary particle sizes almost independent of the initial precursor concentration. Regarding the fractal properties, the typical shape of aggregates for aerosols is present for the investigated range of precursor concentrations. In conclusion, the consistent results for SAXS and TEM show a conclusive characterization of a complex particle system, allowing for the identification of the underlying particle formation mechanism. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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9 pages, 9737 KiB

Open AccessArticle

Using In Situ Measurements to Experimentally Characterize TiO₂ Nanoparticle Synthesis in a Turbulent Isopropyl Alcohol Flame

by Benedetta Franzelli, Philippe Scouflaire and Nasser Darabiha

Materials 2021, 14(22), 7083; https://doi.org/10.3390/ma14227083 - 22 Nov 2021

Cited by 2 | Viewed by 1441

Abstract

The objective of the present work is to show the potential of in situ measurements for the investigation of nanoparticles production in turbulent spray flames. This is achieved by considering multiple diagnostics to characterize the liquid break-up, the reactive flow and the particles [...] Read more.

_{2}

nanoparticle synthesis. The considered liquid fuel is a solution of isopropyl alcohol and titanium tetraisopropoxide (TTIP) precursor. Measurements show that shadowgraphy can be used to simultaneously localize spray and nanoparticles, light scattering allows to characterize the TiO

_{2}

nanoparticles distribution in the flame central plane, and spontaneous CH* and OH* chemiluminescences, as well as global light emission results, can be used to visualize the reactive flow patterns that may differ with and without injection of TTIP. Concerning the liquid, it is observed that it is localized in a small region close to the injector nozzle where it is dispersed by the oxygen flow resulting in droplets. The liquid droplets rapidly evaporate and TTIP is quasi-immediately converted to TiO

_{2}

nanoparticles. Finally, results show high interactions between nanoparticles and the turbulent eddies. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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18 pages, 3219 KiB

Open AccessArticle

In Situ Determination of Droplet and Nanoparticle Size Distributions in Spray Flame Synthesis by Wide-Angle Light Scattering (WALS)

by Simon Aßmann, Bettina Münsterjohann, Franz J. T. Huber and Stefan Will

Materials 2021, 14(21), 6698; https://doi.org/10.3390/ma14216698 - 07 Nov 2021

Cited by 11 | Viewed by 2052

Abstract

The investigation of droplet and nanoparticle formation in spray flame synthesis requires sophisticated measurement techniques, as often both are present simultaneously. Here, wide-angle light scattering (WALS) was applied to determine droplet and nanoparticle size distributions in spray flames from a standardized liquid-fed burner setup. Solvents of pure ethanol and a mixture of ethanol and titanium isopropoxide, incepting nanoparticle synthesis, were investigated. A novel method for the evaluation of scattering data from droplets between 2 µm and 50 µm was successfully implemented. Applying this, we could reveal the development of a bimodal droplet size distribution for the solvent/precursor system, probably induced by droplet micro-explosions. To determine nanoparticle size distributions, an appropriate filter and the averaging of single-shot data were applied to ensure scattering from a significant amount of nanoparticles homogeneously distributed in the measurement volume. From the multivariate analysis of the scattering data, the presence of spherical particles and fractal aggregates was derived, which was confirmed by analysis of transmission electron microscopy images. Monte Carlo simulations allowed determining the distribution parameters for both morphological fractions in three heights above the burner. The results showed relatively wide size distributions, especially for the spherical fraction, and indicated an ongoing sintering, from fractal to spherical particles. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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13 pages, 2997 KiB

Open AccessArticle

Resistive Switching Phenomenon Observed in Self-Assembled Films of Flame-Formed Carbon-TiO₂ Nanoparticles

by Mario Commodo, Gianluigi De Falco, Ettore Sarnelli, Marcello Campajola, Alberto Aloisio, Andrea D’Anna and Patrizia Minutolo

Materials 2021, 14(16), 4672; https://doi.org/10.3390/ma14164672 - 19 Aug 2021

Cited by 2 | Viewed by 1419

Abstract

Nanostructured films of carbon and TiO₂ nanoparticles have been produced by means of a simple two-step procedure based on flame synthesis and thermophoretic deposition. At first, a granular carbon film is produced on silicon substrates by the self-assembling of thermophoretically sampled carbon nanoparticles (CNPs) with diameters of the order of 15 nm. Then, the composite film is obtained by the subsequent thermophoretic deposition of smaller TiO₂ nanoparticles (diameters of the order of 2.5 nm), which deposit on the surface and intercalate between the carbon grains by diffusion within the pores. A bipolar resistive switching behavior is observed in the composite film of CNP-TiO₂. A pinched hysteresis loop is measured with SET and RESET between low resistance and high resistance states occurring for the electric field of 1.35 × 10⁴ V/cm and 1.5 × 10⁴ V/cm, respectively. CNP-TiO₂ film produced by flame synthesis is initially in the low resistive state and it does not require an electroforming step. The resistance switching phenomenon is attributed to the formation/rupture of conductive filaments through space charge mechanism in the TiO₂ nanoparticles, which facilitate/hinder the electrical conduction between carbon grains. Our findings demonstrate that films made of flame-formed CNP-TiO₂ nanoparticles are promising candidates for resistive switching components. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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13 pages, 521 KiB

Open AccessArticle

A Monodisperse Population Balance Model for Nanoparticle Agglomeration in the Transition Regime

by Georgios A. Kelesidis and M. Reza Kholghy

Materials 2021, 14(14), 3882; https://doi.org/10.3390/ma14143882 - 12 Jul 2021

Cited by 5 | Viewed by 2632

Abstract

Nanoparticle agglomeration in the transition regime (e.g. at high pressures or low temperatures) is commonly simulated by population balance models for volume-equivalent spheres or agglomerates with a constant fractal-like structure. However, neglecting the fractal-like morphology of agglomerates or their evolving structure during coagulation [...] Read more.

d_{m}

, by up to 93 or 49%, repectively. Here, a monodisperse population balance model (MPBM) is interfaced with robust relations derived by mesoscale discrete element modeling (DEM) that account for the realistic agglomerate structure and size distribution during coagulation in the transition regime. For example, the DEM-derived collision frequency,

β

, for polydisperse agglomerates is 82 ± 35% larger than that of monodisperse ones and in excellent agreement with measurements of flame-made TiO₂ nanoparticles. Therefore, the number density,

N_{A g}

, mean,

d_{m},

and volume-equivalent diameter,

d_{v}

, estimated here by coupling the MPBM with this

β

and power laws for the evolving agglomerate morphology are on par with those obtained by DEM during the coagulation of monodisperse and polydisperse primary particles at pressures between 1 and 5 bar. Most importantly, the MPBM-derived

N_{A g}

d_{m}

, and

d_{v}

are in excellent agreement with the data for soot coagulation during low temperature sampling. As a result, the computationally affordable MPBM derived here accounting for the realistic nanoparticle agglomerate structure can be readily interfaced with computational fluid dynamics in order to accurately simulate nanoparticle agglomeration at high pressures or low temperatures that are present in engines or during sampling and atmospheric aging. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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13 pages, 3071 KiB

Open AccessArticle

Spray Flame Synthesis (SFS) of Lithium Lanthanum Zirconate (LLZO) Solid Electrolyte

by Md Yusuf Ali, Hans Orthner and Hartmut Wiggers

Materials 2021, 14(13), 3472; https://doi.org/10.3390/ma14133472 - 22 Jun 2021

Cited by 7 | Viewed by 3051

Abstract

A spray-flame reaction step followed by a short 1-h sintering step under O₂ atmosphere was used to synthesize nanocrystalline cubic Al-doped Li₇La₃Zr₂O₁₂ (LLZO). The as-synthesized nanoparticles from spray-flame synthesis consisted of the crystalline La₂Zr₂O₇ (LZO) pyrochlore phase while Li was present on the nanoparticles’ surface as amorphous carbonate. However, a short annealing step was sufficient to obtain phase pure cubic LLZO. To investigate whether the initial mixing of all cations is mandatory for synthesizing nanoparticulate cubic LLZO, we also synthesized Li free LZO and subsequently added different solid Li precursors before the annealing step. The resulting materials were all tetragonal LLZO (I4₁/acd) instead of the intended cubic phase, suggesting that an intimate intermixing of the Li precursor during the spray-flame synthesis is mandatory to form a nanoscale product. Based on these results, we propose a model to describe the spray-flame based synthesis process, considering the precipitation of LZO and the subsequent condensation of lithium carbonate on the particles’ surface. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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12 pages, 3531 KiB

Open AccessArticle

Nano-TiO₂ Coating Layers with Improved Anticorrosive Properties by Aerosol Flame Synthesis and Thermophoretic Deposition on Aluminium Surfaces

by Gianluigi De Falco, Giuseppe De Filippis, Carmela Scudieri, Luca Vitale, Mario Commodo, Patrizia Minutolo, Andrea D’Anna and Paolo Ciambelli

Materials 2021, 14(11), 2918; https://doi.org/10.3390/ma14112918 - 28 May 2021

Cited by 5 | Viewed by 2161

Abstract

TiO₂ in the form of nanoparticles is characterized by high photocatalytic activity and high resistance to oxidation, making it an excellent candidate to realize coatings for improving the corrosion resistance of aluminium surfaces. Different coating technologies have been proposed over the years, which often involve the use of toxic compounds and very high temperatures. In this work, an alternative and novel one-step method for the coating of aluminium alloy surfaces with titania nanoparticles is presented. The method is based on the combination of aerosol flame synthesis and direct thermophoretic deposition and allows to produce nanostructured thin coating layers of titania with different features. Specifically, 3.5 nm anatase nanoparticles were synthesized and deposited onto aluminium alloy AA2024 samples. The thickness of the coating was changed by modifying the total deposition time. A thermal annealing treatment was developed to improve the adhesion of nano-titania on the substrates, and the morphology and structures of the coatings were characterized using (ultra violet) UV-vis absorption, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The corrosion resistance behavior of the coatings was evaluated by means of electrochemical polarization measurements, coupled with a numerical analysis using COMSOL software. Both the experimental and numerical electrochemical polarization curves showed a significant increase in the corrosion potential of coated substrates with respect to the bare aluminium and a decrease in the current density. The coatings obtained with higher deposition time and greater thickness showed the best performances in terms of the resistance of the aluminium surfaces to corrosion. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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7 pages, 1473 KiB

Open AccessArticle

Effect of External Charging on Nanoparticle Formation in a Flame

by Elena Fomenko, Igor Altman and Igor E. Agranovski

Materials 2021, 14(11), 2891; https://doi.org/10.3390/ma14112891 - 28 May 2021

Cited by 1 | Viewed by 2600

Abstract

This paper attempts to demonstrate the importance of the nanoparticle charge in the synthesis flame, for the mechanism of their evolution during formation processes. An investigation was made of MgO nanoparticles formed during combustion of magnesium particles. The cubic shape of nanoparticles in an unaffected flame allows for direct interpretation of results on the external flame charging, using a continuous unipolar emission of ions. It was found that the emission of negative ions applied to the flame strongly affects the nanoparticle shape, while the positive ions do not lead to any noticeable change. The demonstrated effect emphasizes the need to take into account all of the phenomena responsible for the particle charge when modeling the nanoparticle formation in flames. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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14 pages, 12076 KiB

Open AccessArticle

Control of Porous Layer Thickness in Thermophoretic Deposition of Nanoparticles

by Malte Schalk, Suman Pokhrel, Marco Schowalter, Andreas Rosenauer and Lutz Mädler

Materials 2021, 14(9), 2395; https://doi.org/10.3390/ma14092395 - 04 May 2021

Cited by 2 | Viewed by 1685

Abstract

The film thickness plays an important role in the performance of materials applicable to different technologies including chemical sensors, catalysis and/or energy materials. The relationship between the surface and volume of the functional layers is key to high performance evaluations. Here we demonstrate the thermophoretic deposition of different thicknesses of the functional layers designed using flame combustion of tin 2-ethylhexanoate dissolved in xylene, and measurement of thickness by scanning electron microscopy and focused ion beam. The parameters such as spray fluid concentration (differing Sn²⁺ content), substrate-nozzle distance and time of the spray were considered to investigate the layer growth. The results showed ≈ 23, 124 and 161 μm thickness of the SnO₂ layer after flame spray of 0.1, 0.5 M and 1.0 M tin 2-EHA-Xylene solutions for 1200 s. While Sn²⁺ concentration was 0.5 M for all the flame sprays, the substrates placed at 250, 220 and 200 mm from the flame nozzle had layer thicknesses of 113, 116 and 132 µm, respectively. Spray time dependent thickness growth showed a linear increase from 8.5 to 152.1 µm when the substrates were flame sprayed for 30 s to 1200 s using 0.5 M tin 2-EHA-Xylene solutions. Changing the dispersion oxygen flow (3–7 L/min) had almost no effect on layer thickness. Layers fabricated were compared to a model found in literature, which seems to describe the thickness well in the domain of varied parameters. It turned out that primary particle size deposited on the substrate can be tuned without altering the layer thickness and with little effect on porosity. Applications depending on porosity, such as catalysis or gas sensing, can benefit from tuning the layer thickness and primary particle size. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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8 pages, 8543 KiB

Open AccessArticle

Effect of Laser Irradiation on Emissivity of Flame-Generated Nanooxides

by Silvana De Iuliis, Roberto Dondè and Igor Altman

Materials 2021, 14(9), 2303; https://doi.org/10.3390/ma14092303 - 29 Apr 2021

Cited by 3 | Viewed by 2076

Abstract

The application of pyrometry to retrieve particle temperature in particulate-generating flames strictly requires the knowledge of the spectral behavior of emissivity of light-emitting particles. Normally, this spectral behavior is considered time-independent. The current paper challenges this assumption and explains why the emissivity of oxide nanoparticles formed in flame can change with time. The suggested phenomenon is related to transitions of electrons between the valence and conduction energy bands in oxides that are wide-gap dielectrics. The emissivity change is particularly crucial for the interpretation of fast processes occurring during laser-induced experiments. In the present work, we compare the response of titania particles produced by a flame spray to the laser irradiation at two different excitation wavelengths. The difference in the temporal behavior of the corresponding light emission intensities is attributed to the different mechanisms of electron excitation during the laser pulse. Interband transitions that are possible only in the case of the laser photon energy exceeding the titania energy gap led to the increase of the electron density in the conduction band. Relaxation of those electrons back to the valence band is the origin of the observed emissivity drop after the UV laser irradiation. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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13 pages, 3039 KiB

Open AccessArticle

Room-Temperature Catalyst Enables Selective Acetone Sensing

by Ines C. Weber, Chang-ting Wang and Andreas T. Güntner

Materials 2021, 14(8), 1839; https://doi.org/10.3390/ma14081839 - 08 Apr 2021

Cited by 7 | Viewed by 2672

Abstract

Catalytic packed bed filters ahead of gas sensors can drastically improve their selectivity, a key challenge in medical, food and environmental applications. Yet, such filters require high operation temperatures (usually some hundreds °C) impeding their integration into low-power (e.g., battery-driven) devices. Here, we reveal room-temperature catalytic filters that facilitate highly selective acetone sensing, a breath marker for body fat burn monitoring. Varying the Pt content between 0–10 mol% during flame spray pyrolysis resulted in Al₂O₃ nanoparticles decorated with Pt/PtO_x clusters with predominantly 5–6 nm size, as revealed by X-ray diffraction and electron microscopy. Most importantly, Pt contents above 3 mol% removed up to 100 ppm methanol, isoprene and ethanol completely already at 40 °C and high relative humidity, while acetone was mostly preserved, as confirmed by mass spectrometry. When combined with an inexpensive, chemo-resistive sensor of flame-made Si/WO₃, acetone was detected with high selectivity (≥225) over these interferants next to H₂, CO, form-/acetaldehyde and 2-propanol. Such catalytic filters do not require additional heating anymore, and thus are attractive for integration into mobile health care devices to monitor, for instance, lifestyle changes in gyms, hospitals or at home. Full article

(This article belongs to the Special Issue Flame Synthesis and Characterization of Oxide Nanoparticles)

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