Spray Synthesis Research Articles

Exploring formation path of Cr2AlC phase during plasma spraying Cr/Al/Cr3C2 agglomerates: Water quenching and droplet deposition

In recent years, plasma spraying for the preparation of high-content Cr2AlC coatings has been demonstrated to be feasible. However, the formation mechanism of Cr2AlC during the spraying process remains uncertain. This study intuitively investigated the intermediate phases present during the formation process of Cr2AlC by spraying Cr/Al/Cr3C2 agglomerates. Water-quenching experiments were employed to directly analyze the evolution of agglomerated powders throughout the spraying process. As the quenching distance increased from 200 to 350 mm, a notable growth of molten powders was observed. Additionally, XRD results of agglomerates collected at varying quenching distances revealed the presence of CrC, Al8Cr5 and (Cr, Al)2Cx during the spraying process. To closely emulate the actual spraying conditions, droplets were deposited at different spraying distances. The presence of CrC, Al8Cr5 and (Cr, Al)2Cx in the spray synthesis of Cr2AlC was further substantiated by the observation of the droplets. This study proposed an innovative approach to exploring the formation mechanism of Cr2AlC during the spraying process.

Recycling of hydrogen tolerant La0.6Ca0.4Co0.2Fe0.8O3–d oxygen transport membranes with integrated life cycle assessment for plasma-assisted CO2-conversion

In this study, a recycling approach was adapted for the hydrogen tolerant La0.6Ca0.4Co0.2Fe0.8O3–d (LCCF_6428) oxygen transport membranes that have great potential in plasma-assisted CO2 conversion techniques for producing industrial fuels such as methanol. The major focus was the incorporation of sustainability measures such as integrating life cycle assessment (LCA) into the materials development at an early stage to study and compare the environmental feasibility of the recycled membrane with the primary membrane. The aim was also to ensure reduced resource depletion of critical raw materials such as cobalt and lanthanum by means of recycling. It consisted of microwave-assisted dissolution of the membrane followed by ultrasonic spray synthesis. The recycled membrane exhibited at least 83 % of the oxygen permeability of the primary membrane and maintained hydrogen tolerance up to 600 °C for 25 h which is a remarkable result for LCCF_6428 in terms of potentially enhancing its life span. As per the LCA, recycling did result in lower resource depletion. However, the recycled LCCF had a higher overall environmental impact compared to the primary LCCF, mainly due to increased electricity consumption during recycling. These results accentuate the need for a transition towards more efficient processes accompanied by cleaner and renewable sources of energy and critically indicate integration of LCA into materials development to establish the sustainability profile of materials.

Comparative study between chemical precipitation and chemical precipitation by spraying for the recovery of nanometric hydroxyapatite

The aim of this work was to develop a new method of assisted precipitation by spraying to obtain nanometer-sized hydroxyapatite and to compare it with conventional chemical precipitation by droplets. Scanning electron microscopy showed the morphological characteristics flake-like of HAp particles synthesized by the spray method at different reaction times and the formation of HAp from the first reaction time, which is different from the droplet method. X-ray diffraction confirmed the crystal structure and size of the synthesized HAp by Rietveld refinement, showing a crystallite size of about 3.77 nm for the spray synthesis and 18 nm for the droplet synthesis. FTIR and Raman were used to analyze the active normal vibrational modes of the synthesized HAp. The results showed that in the spray system, HAp formation occurs at the beginning of the reaction, a phenomenon that was not observed in the droplet method. In particular, FTIR analysis showed the presence of phosphate and carbonate modes of HAp as early as 9 minutes, which persisted throughout the reaction. This Furthermore, dynamic light scattering measurements show that the size distribution of HAp particles synthesized using the novel spray system is between 2 and 5 μm. This shows that the distribution with the spray method is more stable than with the droplet method, which shows the presence of irregular particles with a size distribution at 3 μm. The results emphasize the effectiveness of the proposed method in producing HAp with desirable properties for various biomedical applications.

Al2O3 Thin Layer Formed inside Porous Membrane Using Spray Synthesis Method and Its Application

Aluminum oxide (Al2O3) films have been investigated for use in various applications, and numerous deposition techniques have been reported. The spray synthesis method has the advantage of forming a thin layer of crystal at low temperatures using the appropriate precursors. A precursor prepared by diluting Methylaluminoxane with N-methyl pyrrolidone was sprayed onto a porous membrane while varying conditions such as the substrate temperature, feeding speed, and spray amount. The solution penetrated the film during spray application, and the ultra-thin layers deposited on the side wall of the internal pores were observed using a cross-sectional transmission electron microscope (XTEM). The lattice image obtained using the TEM and the composition analysis conducted using a scanning TEM and an energy-dispersive X-ray spectroscope suggest that this thin layer is a layer of Al2O3. The formation of Al2O3 occurred at lower temperatures than in previous reports. This is a major advantage for applications with low-melting-point materials. The most suitable spraying conditions were determined based on the state of deposition on the surface and inside the membrane. These conditions were applied to a three-layer separator for lithium-ion batteries and their effect on thermal stability was investigated. Through heating experiments and XRD analysis, it was confirmed that the shrinkage and melting of the separator are suppressed by spraying. This process can be expected to have wide applications in low-melting-point materials such as polyolefin.

Insights into the Spray Synthesis of UiO-66 and UiO-66-NH2 Metal–Organic Frameworks: Effect of Zirconium Precursors and Process Parameters

UiO-66, a zirconium-based metal–organic framework, was synthesized using a one-step spray synthesis method to investigate the effects of preheating the precursor solution and Zr sources on crystallinity. Using ZrCl4 with water as a modulator requires preheating at 80 °C for 120 min or 120 °C for 30 min for the spray synthesis of UiO-66 to form secondary building units (SBUs). By contrast, the use of Zr(OnPr)4 with acetic acid (AcOH) as a modulator allowed the spray synthesis of UiO-66 and UiO-66-NH2 without preheating because of the rapid formation of SBUs with AcOH. The spray-synthesized UiO-66 using Zr(OnPr)4 exhibited a BET surface area of 1258 m2/g and a CO2 adsorption capacity of 3.43 mmol/g at 273 K and 1 bar, while UiO-66-NH2 exhibited a BET surface area of 1263 m2/g and a CO2 adsorption capacity of 6.11 mmol/g under the same conditions.

Insights into the Structure Sensitivity of Fe-Based Materials for the Oxygen Evolution Reaction

The conversion of renewable energy into storable fuels is fundamental to the meeting of the net-zero CO2 emissions objective by 2050. However, the sluggish kinetics of the oxygen evolution reaction (OER) and the poor stability of electrocatalysts at industrial-level currents remain a problem for electrolyzers 1. Using platinum group metal-free electrocatalysts in anion exchange membrane electrolyzers is an appealing strategy to overcome some of the problems associated with PEM electrolyzers. Fe-based materials offer the possibility for the development of active, stable, and cost-effective electrocatalysts for the OER in alkaline media.In this work, we explore the influence of structure, composition, or crystalline phase on different Fe-based electrodes for the OER. Different fabrication procedures such as additive manufacturing 2, flame spray synthesis, or pulse laser ablation 3 allow us to have a wide range of Fe-based materials with a well-defined structure/composition/phase. The materials have been extensively characterized by physicochemical methods (XPS, Raman, XRD, ICP) and their electrocatalytic activity evaluates following careful procedures in order to obtain reliable results. The results show that the activity of Fe-rich electrodes is poor. However, if Fe is alloyed with other materials or its surface structure or crystallinity degree is modified, it results in a more active material. This information could guide the synthesis of more active and stable water-splitting materials. References S. Cherevko, S. Geiger, O. Kasian, N. Kulyk, J.-P. Grote, A. Savan, B. R. Shrestha, S. Merzlikin, B. Breitbach, A. Ludwig, and K. J. J. Mayrhofer, Catalysis Today, 262 170-180 (2016). J. Wegner, R. Martínez-Hincapié, V. Čolić, and S. Kleszczynski, Advanced Materials Interfaces, n/a (n/a), 2202499 (2023). J. Johny, Y. Li, M. Kamp, O. Prymak, S.-X. Liang, T. Krekeler, M. Ritter, L. Kienle, C. Rehbock, S. Barcikowski, and S. Reichenberger, Nano Research, 15 (6), 4807-4819 (2022).

Operando Investigations of Reversible and Irreversible Transformations of Metal Organic Framework Based Catalysts during the Oxygen Evolution Reaction

Intermittency issues due to the growth of renewable energy usage lead to an increasing interest in energy storage systems. Thereby, one can use the excess energy for the production of hydrogen from water by anion exchange membrane (AEM) water electrolysis using earth-abundant, non-noble metal catalysts. However, the kinetics of the oxygen evolution reaction (OER) limit the activity of the catalyst and hence, the development of performance stable and active OER catalysts is of great importance for the commercialization of AEM water electrolyzers. In metal organic frameworks (MOFs), a porous structure is created by linking metal atoms/clusters with other metal centres using organic ligands. The resulting structure with a high surface area and dispersed metal centres is a promising catalyst for OER.MOF catalysts with Ni and Co metal centres show impressive OER activity. Both, Co-MOF-74 and Ni-MOF-74 exhibit higher OER activity than their oxide counterparts produced by flame spray synthesis in form of nanoparticles (Fabbri (2017) and Abbott (2018)).1,2 Thereby, an increasing OER activity of Ni-MOF-74 during rotating disk electrode stability tests, indicates that the catalytic species undergoes favorable electronic and structural transformations.Using an in-house-developed spectro-electrochemical flow cell, these transformations were studied by operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). Operando XAS enabled us to monitor the changes occurring in the Ni metal centers while performing cyclic voltammetry (CV) from 1 VRHE up to potentials above the OER onset potential with a time resolution of 5 sec. The operando XAS measurements show that Ni-MOF-74 develops into a highly OER active and stable catalytic species. However, operando XRD measurements prove that the crystalline MOF structure changes into an amorphous structure during OER. Monitoring the electronic and structural transformations due to potential cycling, three different structures were extracted: an initial state, which disappeared within the first eight CV cycles, and two novel electronic/local structures appearing at low and high potential, respectively. The transformations in the electronic and local structure of the Ni metal centers occurring between low and high potential appear to be reversible, as the Ni metal centers return to their initial state during long-term storage in air. However, XRD measurements indicate that this further transformation does not affect the structure of the catalyst: Even though the Ni centers return to their original local structure and oxidation state after long-term storage in air, the long-range structure stays amorphous.The study focuses on elucidating the structure-performance relations of Ni-MOF-74 based OER catalysts providing the key structural parameters that lead to the formation of the highly OER active species. The mechanism of reversible and irreversible transformations occurring in Ni-MOF-74 catalysts allows the extraction of activity descriptors for the development of highly OER active and stable non-noble metal catalysts for AEM water electrolysis.

Spray synthesis of hierarchical porous metal-organic framework HKUST-1 with soft templates and methylene blue adsorption performance

We synthesize hierarchical porous HKUST-1 (HP-HKUST-1) with micropores, mesopores, and macropores using the spray synthesis and soft template methods. We utilize two types of structure-directing agents (SDAs), namely cetyltrimethylammonium bromide and citric acid for C-HKUST-1 and triblock copolymer P123 for P-HKUST-1. Paq-HKUST-1 is successfully synthesized using only water as the solvent. The volumes of the mesopores and macropores are changed by the amounts of SDAs. The highest values are: 0.78 and 0.30 cm3/g for C-HKUST-1; 0.44 and 0.38 cm3/g for P-HKUST-1; and 0.30 and 0.23 cm3/g for Paq-HKUST-1, respectively. HP-HKUST-1 shows high methylene blue (MB) adsorption rates and capacities of over 700 mg/g owing to its HP structure. In particular, C-HKUST-1 exhibits a rate constant of 19.7 mg/mg min for MB adsorption based on the pseudo-second-order adsorption kinetic model. This rate constant is 10 times higher than that of batch-synthesized HKUST-1. Furthermore, HP-HKUST-1 exhibits good stability for reuse as an MB adsorbent.

Multi-component precursor droplet evaporation in spray synthesis of cathode materials

Spray synthesis methods are highly attractive for manufacturing materials for energy storage and conversion owing to their scalability and controllability. The key to achieving high controllability in spray synthesis is understanding the complex relationship between the precursor spray droplet evaporation process and the final product structure. Although several studies on precursor droplet evaporation have been reported, very few consider multi-component droplets, which are relevant in practical applications. In this study, we analysed the evaporation of multi-component single precursor droplets used in flame-assisted spray pyrolysis (FASP) of Li(Ni0.8Co0.1Mn0.1)O2 (NCM811) cathode materials. The product particles of FASP were characterized using SEM, EDS and EELS to determine the structure and morphology of particles obtained from precursors with and without urea additives. The qualitative and quantitative changes during evaporation were analysed using high-speed visualization of suspended single droplets on cross-wires. An in-house developed numerical model was used to track the trajectory of the solutes within the evaporating droplet and elucidate the connection between the evaporation process and the final product structure. It was found that evaporating droplets with multiple precursor solutes (Li, Ni, Co, and Mn) formed product particles with gradient element distribution in the radial direction. However, the addition of a small amount of urea (2.5 wt%) caused significant disturbances in the droplet evaporation process triggered by the urea decomposition reaction leading to uniformly distributed elements in the final product particle. The study contributes towards enhancing fundamental knowledge on multi-component droplet evaporation leading to improved design of spray drying and synthesis processes.

Hydrogen-Tolerant La0.6Ca0.4Co0.2Fe0.8O3-d Oxygen Transport Membranes from Ultrasonic Spray Synthesis for Plasma-Assisted CO2 Conversion.

La0.6Ca0.4Co1-xFexO3-d in its various compositions has proven to be an excellent CO2-resistant oxygen transport membrane that can be used in plasma-assisted CO2 conversion. With the goal of incorporating green hydrogen into the CO2 conversion process, this work takes a step further by investigating the compatibility of La0.6Ca0.4Co1-xFexO3-d membranes with hydrogen fed into the plasma. This will enable plasma-assisted conversion of the carbon monoxide produced in the CO2 reduction process into green fuels, like methanol. This requires the La0.6Ca0.4Co1-xFexO3-d membranes to be tolerant towards reducing conditions of hydrogen. The hydrogen tolerance of La0.6Ca0.4Co1-xFexO3-d (x = 0.8) was studied in detail. A faster and resource-efficient route based on ultrasonic spray synthesis was developed to synthesise the La0.6Ca0.4Co0.2Fe0.8O3-d membranes. The La0.6Ca0.4Co0.2Fe0.8O3-d membrane developed using ultrasonic spray synthesis showed similar performance in terms of its oxygen permeation when compared with the ones synthesised with conventional techniques, such as co-precipitation, sol-gel, etc., despite using 30% less cobalt.

Enhancement of Hydrogen Adsorption on Spray-Synthesized HKUST-1 via Lithium Doping and Defect Creation.

We prepared HKUST-1 (Cu3BTC2; BTC3- = 1,3,5-benzenetricarboxylate) using a spray synthesis method with Li doping and defect created via partial replacement of H3BTC with isophthalic acid (IP) to enhance the H2 adsorption capacity. Li-doping was performed by incorporating LiNO3 in HKUST-1 via spray synthesis and subsequent thermal treatment for decomposing NO3-, which enhances H2 uptake at 77 K and 1 bar per unit mass and per unit area from 2.37 wt% and 4.16 molecules/nm2 for undoped HKUST-1 to 2.47 wt% and 4.33 molecules/nm2, respectively. Defect creation via the replacement of the BTC3- linker with the IP2- linker slightly in HKUST-1 skeleton did not affect H2 uptake. Both Li-doping and defect creation significantly enhanced H2 uptake to 3.03 wt%, which was caused by the coordination of Li ions with free carboxylic groups of the created defects via IP replacement.

Mesoporous silica particles by aerosol spray synthesis; solid core and porous shell structure study

Mesoporous silica core shell particles are materials of high interest especially due to pharmaceutical and catalytic applications. In the current work we presented the synthesis of uniformly porous and solid core porous particles by aerosol spray pyrolysis route according to the principles of EISA, by a one step process. The particles are evaluated morphologically by TEM and by nitrogen adsorption and desorption isotherms for pore size distribution and pore connectivity, validating the effect of synthesis parameters on the core size, surface area and porosity of the uniformly porous and core-shell structures. Three different surfactants Brij-58, CTAB, P123 at various concentrations and combinations as well as three different core type compositions of NaCl, CeO2 and, Ce0.8Zr0.2O2 have been evaluated for the controlled synthesis of silica core-shell particles by a scalable synthesis method.

Spray synthesis of rapid recovery ZnO/polyaniline film ammonia sensor at room temperature

Spray synthesis of rapid recovery ZnO/polyaniline film ammonia sensor at room temperature

Spray synthesis of Pd nanoparticle incorporated HKUST-1, and its catalytic activity for 4-nitrophenol reduction

In this study, we synthesized Pd nanoparticles incorporated into a HKUST-1 metal–organic framework (Pd/HKUST-1) via a recently developed spray synthesis method and evaluated their catalytic activity. Spray synthesis is a method for obtaining MOFs in a short time by MOF precursor solution spraying and subsequent heating. The synthesized Pd/HKUST-1 is characterized via transmittance electron microscopy (TEM) with energy dispersive X-ray spectrometry, inductively coupled plasma atomic emission spectrometry, X-ray diffraction, and N2 adsorption measurements. TEM results revealed that Pd nanoparticles were well dispersed in HKUST-1. Pd/HKUST-1 with varying Pd nanoparticle concentrations were obtained by varying the number of commercial Pd nanoparticles in the HKUST-1 precursor solutions. The BET surface area of Pd/HKUST-1 was up to 1069 m2/g. The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with NaBH4 was performed using Pd/HKUST-1 as a catalyst. Moreover, the reaction rate of the 4-NP reduction is up to 160 s–1 g−1, which is the highest rate constant among the Pd/MOFs.

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

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.

Particle interactions and their effect on magnetic particle spectroscopy and imaging.

Signal stability is crucial for an accurate diagnosis via magnetic particle imaging (MPI). However, MPI-tracer nanoparticles frequently agglomerate during their in vivo applications leading to particle interactions altering the signal. Here, we investigate the influence of such magnetic coupling phenomena on the MPI signal. We prepared Zn0.4Fe2.6O4 nanoparticles by flame spray synthesis and controlled their inter-particle distance by varying SiO2 coating thickness. The silica shell affected the magnetic properties indicating stronger particle interactions for a smaller inter-particle distance. The SiO2-coated Zn0.4Fe2.6O4 outperformed the bare sample in magnetic particle spectroscopy (MPS) in terms of signal/noise, however, the shell thickness itself only weakly influenced the MPS signal. To investigate the importance of magnetic coupling effects in more detail, we benchmarked the MPS signal of the bare and SiO2-coated Zn-ferrites against commercially available PVP-coated Fe3O4 nanoparticles in water and PBS. PBS is known to destabilize nanoparticle colloids mimicking in vivo-like agglomeration. The bare and coated Zn-ferrites showed excellent signal stability, despite their agglomeration in PBS. We attribute this to their process-intrinsic aggregated morphology formed during their flame-synthesis, which generates an MPS signal only little affected by PBS. On the other hand, the MPS signal of commercial PVP-coated Fe3O4 strongly decreased in PBS compared to water, indicating strongly changed particle interactions. The relevance of this effect was further investigated in a human cell model. For PVP-coated Fe3O4, we detected a strong discrepancy between the particle concentration obtained from the MPS signal and the actual concentration determined via ICP-MS. The same trend was observed during their MPI analysis; while SiO2-coated Zn-ferrites could be precisely located in water and PBS, PVP-coated Fe3O4 could not be detected in PBS at all. This drastically limits the sensitivity and also general applicability of these commercial tracers for MPI and illustrates the advantages of our flame-made Zn-ferrites concerning signal stability and ultimately diagnostic accuracy.

Production of Carbon Black in Turbulent Spray Flames of Coal Tar Distillates

Conventional carbon black production occurs by pyrolysis after heavy aromatic feedstock is injected into the post-combustor region of furnace black reactors. The current work examines the conversion of the coal tar distillate in turbulent spray flames to demonstrate a more compact reactor configuration. Coal tar distillates diluted in toluene is atomized and burned in a standardized flame spray synthesis configuration, known as SpraySyn. Flame conditions are characterized by thermocouple, soot pyrometry and image analysis and product particle properties are examined by TEM and Raman spectroscopy. The measured flame temperature corresponds to the range of temperatures used in the furnace black process, but the current synthesis includes oxidizing conditions and faster residence times. The resulting carbon black particles are aggregates with primary particle sizes on the small end of the carbon black size spectrum, according to analysis of TEM images. Carbon black, formed under a range of flame temperatures, show Raman spectra with features resembling typical carbon black materials. Conversion of coal tar distillate to carbon black by direct flame synthesis may be a scalable method to produce high-surface area grades without a conventional pyrolysis reactor stage.

Crystallographic phase formation of iron oxide particles produced from iron nitrate by liquid flame spray with a dual oxygen flow

AbstractWe fabricated iron oxide particles from iron(III) nitrate in a liquid flame spray synthesis. Unlike in most liquid flame spray studies, we implemented a secondary oxygen flow. The effect of the gas flow setup and two additives to the precursor solution, oxalic acid and citric acid, on the resulting particles was studied, with the focus on crystallographic phase composition. The synthesis yielded either pure maghemite or maghemite/hematite mixed phase powders. For solutions without additives, the maghemite fraction was almost linearly dependent on the equivalence ratio. The specific surface area was highest for the smallest equivalence ratios, then decreased, and increased again for the highest values. Some variation was observed between samples with equal equivalence ratios but the total oxygen flow divided differently between the two oxygen channels, a higher atomization flow promoting larger hematite fraction, and higher specific surface area. Both additives reduced the amount of hematite in the powder samples, citric acid being the more efficient one. Citric acid slightly raised the specific surface area, whereas oxalic acid dropped it in half.

Improvement of production efficiency of spray-synthesized HKUST-1

Spray synthesis of metal–organic frameworks (MOFs) is desirable for scaled production. In this study, we designed a new apparatus for spray-synthesizing MOFs, wherein an upward spraying and swirling air flow was applied to prevent loss of the precursor solution. We evaluated the effects of the flow rate and temperature of swirling air, temperature of the reactor tube, initial feed rate of the precursor solution, and precursor concentration on the yield, purity, space–time yield (STY), BET surface area, and average particle size. The swirling air flow along with re-spraying of the solution accumulated on the upward spray nozzle improved yield significantly. The highest STY was 45.7 kg/m3/day obtained at a precursor feed rate of 5 mL/min; the highest surface area was 1,872 m2/g obtained at a precursor concentration of 1.38 mol/L. The sample with the highest surface area exhibited 2.60 wt% of hydrogen adsorption capacity at 77 K and 1 bar, with 7.6–6.1 kJ/mol of heat of hydrogen adsorption.

Spray Synthesis of CuFeO2 Photocathodes and In-Operando Assessment of Charge Carrier Recombination

Spray Synthesis of CuFeO₂ Photocathodes and <i>In-Operando</i> Assessment of Charge Carrier Recombination