Screening Precursor-Solvent Combinations for Li4Ti5O12 Energy Storage Material Using Flame Spray Pyrolysis

Flame emission spectroscopy of single droplet micro explosions

Nanoparticles exhibit superior physical and chemical properties, making them highly desirable for various applications. Flame spray pyrolysis (FSP) is a versatile technique for synthesizing size and composition-controlled metal oxide/sulfide nanoparticles through a gas-phase reaction. To understand the fundamental mechanisms governing nanoparticle formation in FSP, simplified single-droplet experiments have proven to unravel the physicochemical mechanisms of liquid metal precursor combustions. This work introduces a novel method using flame emission spectroscopy and high-speed imaging to analyze combustion species and metal release during metalorganic single droplet combustions, with the example of the 2-ethylhexanoci acid (EHA)-tetrahydrothiophene (THT)-mesitylcopper (MiCu) precursor system. The method enables the tracing of precursor components released from droplet into the flame by spatial and temporal resolved emission tracking from combustion species (OH*, CH*, C2*, CS*, CS2*) and atomic spectral lines (Cu I). The tracking of metal emission enables the direct observation of the particle formation route, offering novel insights into the metalorganic precursor combustions. The findings of this work show a direct correlation between micro-explosions and nanoparticle formation through the gas-to-particle route. The release of copper emissions is observed with the micro-explosion event, marking the micro-explosions as the critical mechanism for the metal release and subsequent nanoparticle formation during the combustion process. The results indicate a metalorganic viscous shell formation (THT + MiCu) leading to the micro explosion. The EHA/THT ratio significantly affects the combustion behavior. Lower ratios lead to a gradual copper release before the micro explosion; higher ratios shorten the copper release and delay the micro explosion - the highest ratio results in two distinct burning stages.

Oxalic acid-assisted preparation of LTO-carbon composite anode material for lithium-ion batteries

This study presents an oxalic acid-assisted method for synthesizing spinel-structured lithium titanate (Li4Ti5O12; LTO)/carbon composite materials. The Ag-doped LTO nanoparticles (NPs) are synthesized via flame spray pyrolysis (FSP). The synthesized material is used as a precursor for synthesizing the LTO-NP/C composite material with chitosan as a carbon source and oxalic acid as an additive. Oxalic acid improves the dissolution of chitosan in water as well as changes the composition and physical and chemical properties of the synthesized LTO-NP/C composite material. The oxalic acid/chitosan ratio can be optimized to improve the electrochemical performance of the LTO-NP/C composite material, and the electrode synthesized with a high mass loading ratio (5.44 mg cm-2) exhibits specific discharge capacities of 156.5 and 136 mAh g-1at 0.05 C- and 10 C-rate currents, respectively. Moreover, the synthesized composite LTO-NP/C composite material exhibits good cycling stability, and only 1.7% decrease in its specific capacity was observed after 200 charging-discharging cycles at 10 C-rate discharging current.

Flame Synthesis of Carbon and Metal-Oxide Nanoparticles: Flame Types, Effects of Combustion Parameters on Properties and Measurement Methods

Carbon and metal-oxide nanoparticles (NP) are currently synthesized worldwide for various applications in the solar-energy, optical, pharmaceutical, and biomedical industries, among many others. Gas phase methods comprise flame synthesis and flame spray pyrolysis (FSP), which provide high efficiency, low cost, and the possibility of large-scale applications. The variation of combustion operation parameters exerts significant effects on the properties of the NPs. An analysis of the latest research results relevant to NP flame synthesis can provide new insight into the optimization of these methods and the development of these techniques for a large scale. This review offers insight into the current status of flame synthesis for carbon and metal-oxide NPs-specifically containing analysis and comparison of the most common carbon and metal-oxide NP production techniques. The burner configurations used at the laboratory scale and large scale are also discussed, followed by the assessment of the influence of combustion parameters on the properties of NPs. Finally, the features of the measurement techniques applied for determining NP properties were described.

Investigating spray flames for nanoparticle synthesis via tomographic imaging using multi-simultaneous measurements (TIMes) of emission

Tomographic imaging using multi-simultaneous measurements (TIMes) of spontaneous light emission was performed on various operating conditions of the SpraySyn burner to analyse the flame morphology and its potential impact on spray flame pyrolysis. Concurrent instantaneous and time-averaged three-dimensional measurements of CH* chemiluminescence (flame front indicator) and atomic Na emission from NaCl dissolved in the injected combustible liquid (related to hot burnt products of the spray flame) were reconstructed employing a 29-camera setup. Overlapping regions of CH* and Na are presented using isosurface visualisation, local correlation coefficient fields and joint probability distributions. The instantaneous results reveal the complex nature of the reacting flow and regions of interaction between the flame front with the hot gases that originate from the spray stream. The averaged reconstructions show that the spray flames tested are slightly asymmetric near the burner exit but develop into symmetric bell-shaped distributions at downstream locations. The changes in the flame structure for different operating conditions are analysed in light of previous studies, helping in the better understanding of the nanoparticle synthesis process. Furthermore, the importance of using measurements from two views for significantly improved alignment of the burner based on the originally proposed procedure are discussed in light of the reconstructions. This is an important aspect since the SpraySyn is intended for use as a well-defined standardised burner for nanoparticle synthesis, which is being investigated numerically and experimentally across different research groups.

Double Flame-Fabricated High-Performance AlPO4/LiMn2O4 Cathode Material for Li-Ion Batteries

The spinel LiMn₂O₄ (LMO) is a promising cathode material for rechargeable Li-ion batteries due to its excellent properties, including cost effectiveness, eco-friendliness, high energy density, and rate capability. The commercial application of LiMn₂O₄ is limited by its fast capacity fading during cycling, which lowers the electrochemical performance. In the present work, phase-pure and crystalline LiMn₂O₄ spinel in the nanoscale were synthesized using single flame spray pyrolysis via screening 16 different precursor-solvent combinations. To overcome the drawback of capacity fading, LiMn₂O₄ was homogeneously mixed with different percentages of AlPO₄ using versatile multiple flame sprays. The mixing was realized by producing AlPO₄ and LiMn₂O₄ aerosol streams in two independent flames placed at 20° to the vertical axis. The structural and morphological analyses by X-ray diffraction indicated the formation of a pure LMO phase and/or AlPO₄-mixed LiMn₂O₄. Electrochemical analysis indicated that LMO nanoparticles of 17.8 nm (d _BET) had the best electrochemical performance among the pure LMOs with an initial capacity and a capacity retention of 111.4 mA h g^-1 and 88% after 100 cycles, respectively. A further increase in the capacity retention to 93% and an outstanding initial capacity of 116.1 mA h g^-1 were acquired for 1% AlPO₄.

Control of Porous Layer Thickness in Thermophoretic Deposition of Nanoparticles

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.

Flame-made Particles for Sensors, Catalysis, and Energy Storage Applications

Flame spray pyrolysis of precursor-solvent combinations with high enthalpy density allows the design of functional nanoscale materials. Within the last two decades, flame spray pyrolysis was utilized to produce more than 500 metal oxide particulate materials for R&D and commercial applications. In this short review, the particle formation mechanism is described based on the micro-explosions observed in single droplet experiments for various precursor-solvent combinations. While layer fabrication is a key to successful industrial applications toward gas sensors, catalysis, and energy storage, the state-of-the-art technology of innovative in situ thermophoretic particle production and deposition technology is described. In addition, noble metal stabilized oxide matrices with tight chemical contact catalyze surface reactions for enhanced catalytic performance. The metal-support interaction that is vital for redox catalytic performance for various surface reactions is presented.

The gas-phase formation of tin dioxide nanoparticles in single droplet combustion and flame spray pyrolysis

Tin dioxide (SnO₂) nanoparticles synthesized via flame spray pyrolysis (FSP) have promising applications for gas sensors. The formation of SnO₂ nanoparticles in the gas-phase has been investigated using single droplet combustion and FSP. Precursor solutions of Tin (II) 2-ethylhexanoate dissolved in Xylene with varying Sn concentrations were selected as the precursor-solvent system. The selected precursor-solvent system has its stability and ability to synthesize homogeneous nanoparticles, compared to metal nitrate based precursor solutions. The precursor-solvent system was studied using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and thermogravimetric analysis (TGA). The SnO₂ nanoparticles were characterized using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and transmission electron microscopy (TEM). Droplet surface micro-explosions were observed during the single droplet combustion of the precursor solutions. It is because of the heterogeneous vapor-phase nucleation, which is beneath the liquid droplet surface and caused by precursor thermal decomposition. The results show that the size of nanoparticles obtained both from FSP and single droplet combustion increases with increasing metal-precursor concentration. The TEM images of the particles from such droplet combustion reveal two types of nanoparticles with different sizes and morphologies. The current work provides fundamental understanding of precursor decomposition and particle formation during single droplet combustion, which help in-depth understanding of the flame spray pyrolysis.

Redox Activity and Nano-Bio Interactions Determine the Skin Injury Potential of Co3O4-Based Metal Oxide Nanoparticles toward Zebrafish

Redox-active metal oxide nanoparticles show varying oxidizing capacities and injury potentials toward biological systems. Here, two metal oxide libraries including transition-metal-doped Co₃O₄ and PdO-Co₃O₄ with strong chemical contacts were design-synthesized and used to investigate their biological injury potential and mechanisms using zebrafish as a model organism. Among different dopants, Cu significantly increased the oxidizing capacity of Co₃O₄. An increased amount of PdO resulted in higher density of heterojunctions, which also led to higher oxidizing capacity. The oxidizing capacity of these nanoparticles was positively correlated with higher mortality of dechorionated embryos and severe larval skin injury upon exposure. Using transgenic zebrafish Tg(LysC:eGFP), we show in real time that the redox-active nanoparticles induced skin injury and activated the infiltration of immune cells. Such inflammatory response was confirmed by the increased mRNA expression level of Nrf2a, HO-1, IL-1β, and IL-6 genes. Although the exposure to the nanoparticles alone was not lethal, the skin injury did lower the tolerance level against other environmental contaminants. More importantly, after withdrawing from the nanoparticle exposure, larvae with skin injury could recover within 24 h in uncontaminated medium, indicating such injury was transient and recoverable.

Chemistry of iron nitrate-based precursor solutions for spray-flame synthesis

Understanding the chemistry of precursor solutions for spray-flame synthesis is a key step to developing inexpensive and large scale applications for tailored nanoparticles.