Advanced Flame Spray Pyrolysis for Engineering Multifunctional Nanostructures

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1		Christos Dimitriou	--	4338	2023-11-27 11:26:46	\|
2	update references and layout	Rita Xu	-122 word(s)	4216	2023-11-28 02:24:37	\|

This entry is adapted from the peer-reviewed paper 10.3390/nano13233006

Flame spray pyrolysis (FSP) is an industrially scalable technology that enables the engineering of a wide range of metal-based nanomaterials with tailored properties nanoparticles.

flame spray pyrolysis complex assemblies multifunctional nanomaterials/nanodevices

1. Introduction

Flame spray pyrolysis (FSP) is an industrially scalable technique for the synthesis of nanomaterials, which allows to produce many types of metal, or metal-oxide nanoparticles with tailored physicochemical properties ^[1]. At the heart of the FSP technology lies an intricate process design, involving precursor atomization, combustion, and nanoparticle formation ^[2]. This enables swift, single-step synthesis, eliminating the need for post-production treatments commonly required in other methodologies. Recent advancements have further elevated the prominence of FSP in the development of nanodevices, i.e., where nanoparticles can be in situ incorporated in complex functional arrays ^[3]. Thus, FSP not only revolutionizes nanomaterials’ production but, with recent innovations, also paves the way for the next generation of nanodevices ^[4].

In the domain of nanotechnology, nanostructure synthesis represents a critical research area, encompassing a diverse range of methodologies alongside FSP. These alternative techniques, including, but not limited to, chemical vapor deposition, sol-gel processing, and electrospinning, offer unique properties in terms of particle size control, morphology, and chemical composition. The selection of an appropriate synthesis method is contingent upon a set of criteria closely related to the intended application of the nanostructures. Factors such as material versatility, environmental impact, synthesis time, and temperature range play a pivotal role in determining the suitability of a technique for specific applications, which vary from drug delivery systems to photovoltaic devices

1.1. Integration of Flame Spray Pyrolysis into the Technology Readiness Level (TRL) Scale for Nanomaterial Production

FSP stands out as an innovative and advanced methodology for the synthesis of nanomaterials, which highlights its vital role in producing a wide array of metal oxide nanoparticles with tailored morphologies and compositions ^[5]. Inherent in FSP, synthesis at elevated temperatures enhances both the crystallinity and physicochemical attributes of the nanoparticles. By adeptly adjusting operational parameters, such as precursor solution concentration, solvent type, flame temperature, oxygen-to-fuel ratio, and particle residence time in the flame zone, researchers can effectively control the nanoparticle size, distribution, and phase composition. Regarding the technology readiness level (TRL) spectrum, FSP for device applications aligns with TRL 6–8 ^[6]. This placement signifies FSP’s evolution beyond foundational laboratory research ^[1]. Given the adoption of FSP by certain industries, this positions FSP in the late stages of development and early stages of commercialization, placing it in the TRL 7–8 range (see Figure 1) ^[6].

Figure 1. Figure illustrating the complex assemblies in flame spray pyrolysis (FSP). These include double-nozzle, sequential deposition, oxygen-deficiency process, ring deposition, sequential/thin-film deposition, and scale-up methods. The resultant advanced nanomaterials/nanodevices encompass perovskites, non-oxides, quantum dots, plasmonics, nanofilms, and sensors.

1.2. Process Design

The process design of FSP ^[7] begins with the preparation of a metal or metal-organic precursor solution, typically dissolved in an appropriate solvent. This solution undergoes atomization, often facilitated by a high-pressure nozzle, to form a fine spray of droplets ^[2]. Subsequent ignition of these droplets, often with the aid of an auxiliary flame, leads to the combustion of the solvent and the eventual decomposition of the metal precursors. Within this flame environment, characterized by elevated temperatures, the precursor decomposes, and metal or metal-oxide nanoparticles nucleate and grow ^[8]. The characteristics of the resultant nanoparticles—size, morphology, crystallinity, and phase composition—can be controlled by diligent choice of the metal precursor, its concentration in the solution, the solvent’s nature, the atomization method, flame temperature, oxygen-to-fuel ratio, and the residence time of particles within the flame. Both inorganic and organic metal salts—including nitrates, acetates, and 2-ethylhexanoates—as well as metalorganic compounds such as acetylacetonates or alkoxides, serve as prevalent precursors ^[9]. These compounds are soluble in organic solvents, notably xylene (with a standard enthalpy change in combustion ^[10],

Δ_{c} H_{298}^{Ø}

= −4550 kJ mol⁻¹), toluene (−3910 kJ mol⁻¹), ethanol (−1376 kJ mol⁻¹), acetonitrile (−1256 kJ mol⁻¹), etc. Another pivotal aspect of the FSP design is its continuous mode of operation that enhances its scalability potential, making it an attractive proposition for industrial applications.

Furthermore, the high-temperature synthesis environment ensures rapid crystallization of particles, obviating the need for post-process annealing. Meierhofer et al. ^[1] delineated the relationship between temperature and process residence time during each phase of the droplet-to-particle formation, as represented by the red line in Figure 2a and the flame temperature profile in Figure 2b. At the nozzle’s apex, temperature fluctuations range from 500 to 400 °C within the initial 10 μs. Adjacent to the capillary tip, the flame’s core registers the peak temperatures, oscillating between 3500 and 2500 °C (Figure 2). At this juncture, the precursor solution vaporizes, initiating the nucleation of the primary particles. Following initial particle formation, particles fuse cohesively within the temperature range of 1700–600 °C during coagulation and sintering processes. Driven by Brownian motion, these particles collide and coalesce, forming larger entities. As the sintered particles move further through the flame and into cooler regions (<600 °C), they can stick together into agglomerates, forming loose clusters. This clustering is due to physical forces, such as van der Waals interactions. Subsequently, these agglomerates transform into aggregates, binding more firmly through chemical (covalent) bonds in the temperature range of 400–200 °C. The FSP setup typically comprises components like liquid atomizers, combustion chambers, flame torches, and substrate holders for potential direct deposition of nanoparticles.

Figure 2. (a) Temporal scales in the fabrication of ZrO₂ nanoparticles via FSP. A time-evolving analysis encompasses the dynamics of the droplet mass ratio, the rate of product formation, nanoparticle diameter, and gas temperature, serving to demarcate distinct phases within the manufacturing process. (b) Visualization of actual FSP flame, depicting the synthesis parameters (pilot flame, precursor solution, dispersion, sheath gas). Concurrently, a graphical representation of the flame’s temperature distribution, congruent with that depicted in (a), is presented. Below the flame, a comprehensive elucidation of the droplet-to-particle transformation process in the production of nanoparticles is provided.

1.3. Recent Advancements in Product/Nanodevice Development

Figure 3 provides a chronological tracing of the literature articles related to FSP, highlighted by pertinent reviews. Introduced in the 1970s by Sokolowski et al. ^[11], FSP was utilized for synthesizing Al₂O₃ nanoparticles from an aluminum acetylacetonate precursor in a benzene-ethanol solution via an ultrasound nozzle. Despite the initial decline in interest, the technique was refined in the 1990s by Laine and colleagues at the University of Michigan ^[12]^[13]. In a pilot-scale FSP reactor, a double-alkoxide (Mg-Al) precursor in an alcoholic solution was employed to yield spinel MgAl₂O₄ nanoparticles at rates between 50 and 100 g/h. Concurrently, scientists at Tampere University of Technology employed the FSP method for various metal oxide syntheses and conducted detailed optical diagnostics on the produced aerosols ^[14]. By the commencement of the 21st century, Pratsinis’s team at the Swiss Federal Institute of Technology in Zürich further adapted FSP, highlighting its potential in catalytic material development ^[15].

Figure 3. Timeline of the flame spray pyrolysis (FSP) technology, and some pertinent review articles. The bar graph depicts the annual publication frequency (1365 documents in total) from 1977 to 2023.

Mädler’s review in 2004 ^[16] emphasized the increasing use of liquid-fed reactors for the aerosol-based synthesis of nano-sized powders. The rising interest in aerosol processes and the growing demand for various functional metal oxides have accelerated the R&D of these reactors. He examined four primary spray techniques: spray pyrolysis in a tubular reactor (SP), vapor flame reactor spray pyrolysis (VFSP), emulsion combustion method (ECM), and flame spray pyrolysis (FSP), comparing their energy sources and reaction mechanisms. He also outlined methods to produce consistent products and their specific applications ^[16]. In 2010, Teoh and colleagues ^[2] presented an exhaustive review focusing on FSP as a method for nanoparticle synthesis, spanning from mixed oxides to pure metals and encompassing specialized morphologies, such as core-shell structures, with minimal references to VAFS and FASP. Conversely, Camenzind and associates ^[17] delve into the surface functionalization of nanoparticles generated through flame synthesis and the incorporation of metal oxide within polymer composites. Moreover, in 2013, Sotiriou ^[18] provided an in-depth review emphasizing the plasmonic properties of flame-synthesized silica-coated nanoparticles and their potential applications in anticancer drug delivery.

Koirala et al. in 2016 ^[19] conducted a thorough examination of catalysts produced through single, double, and enclosed FSP techniques. In the same year, Li and his colleagues ^[20] detailed advancements in FSP, encompassing substrate usage, applications of external fields, innovative spray methodologies, and the construction of functional apparatus, supplemented by in situ diagnostics and multi-scale simulations. In 2019, Schneider et al. ^[21] presented the SpraySyn burner as a benchmark instrument for the spray-flame synthesis of nanoparticles. This apparatus offers conditions amenable to simulation and accommodates a variety of precursors. One year later, Pokhrel and Mädler’s review ^[22] outlined recent advancements in nanoparticles produced through FSP for sensing, catalysis, and energy storage applications, while Meierhofer and Fritsching in 2021 ^[1] provided a detailed analysis of FSP’s historical context, design, growth mechanisms, and diagnostic methodologies, emphasizing its prospective opportunities and challenges.

Meanwhile, Venkatesan et al. ^[23] highlighted that FSP offers a scalable and proficient approach to address the complexities of electrocatalyst synthesis for polymer electrolyte membrane fuel cells (PEMFCs) and solid oxide cells (SOCs), streamlining conventional multistage procedures. In 2022, Tran-Phu et al. ^[24] presented a review on Power-to-X (P2X) technologies, underscoring the significance of sustainable energy storage with zero CO₂ emissions. Within this context, FSP is identified as a crucial technique for enhancing the production of efficient (photo)electrocatalysts. Ultimately, within that year, John and Tricoli’s review ^[25] probed the particle formation mechanism, drawing insights from micro-explosions in single droplet experiments across diverse precursor–solvent pairs. The discussion emphasizes the importance of layer fabrication for industrial applications, including gas sensors, catalysis, and energy storage.

2. Engineering of Complex Nanoassemblies by Flame Spray Pyrolysis

2.1. Oxygen-Deficient FSP Process

The concept of oxygen-deficient synthesis can pertain to anoxic or reduced metal oxides. In the literature, these are referenced as M_KO_L_−x where K and L are the stoichiometry coefficients that determine the stable crystal phase M_KO_L. In this terminology, x signifies the O-deficiency coefficient.

Here, for the sake of the discussion, researchers classify these materials in three cases:

[i]: O-vacancies generation with no change in the crystal phase: lack of O atoms from the lattice, compared to the formal stoichiometry of the nominal crystal phase, with no modification of the crystal phase.
[ii]: Generation or reduced metal atoms with no change in the crystal phase: lack of O atoms from the lattice can stabilize lower-oxidation states of the metal atoms.

Often, cases [i] and [ii] are interlinked since the reduction in individual metal atoms in the lattice can be triggered thermodynamically from the generation of one or more O-vacancies in its immediate vicinity.

[iii]: Stabilization of a reduced crystal phase via lack of O atoms: certain O-deficient metal oxides can stabilize reduced phases. This occurs when a significant fraction part of the metal atoms is reduced. For example, magnetite Fe₃O₄, which contains one Fe²⁺ and two Fe³⁺, can be formed from Fe₂O₃ (two Fe³⁺) when 1/3 of the Fe³⁺-atoms is reduced to Fe²⁺. Further reduction in all Fe atoms to Fe²⁺ forms the FeO phase, while further reduction to Fe⁰-atoms forms the metallic, zero-valent-iron material. Similarly, Cu₂O (SnO) is formed when all Cu²⁺ (Sn⁴⁺) atoms in CuO (SnO₂) are reduced to the Cu¹⁺ (Sn²⁺) state.

The concept of using an oxygen-lean FSP was pioneered by Grass et al. to produce oxygen-deficient metal-oxide particles ^[26] by placing the FSP nozzle inside a glove box filled with inert nitrogen and regulating the intake of oxidizing gas as illustrated in Figure 4b,c. The dispersion gas mixture in the flame can shift from a CO₂/H₂O composition (representing traditional, oxidizing flames, see Figure 4a) to a CO/H₂/H₂O mixture (under reducing conditions) ^[26]. Noble metal nanoparticles, including Pt, Au, Ag, and their alloys, can typically be produced even in oxygen-rich FSP, i.e., due to the thermodynamic preference of the metal state vs. the oxide state by the noble metal atoms. However, creating non-noble metals necessitates a reductive environment. When cobalt or bismuth organic precursors ^[27], such as cobalt(II)- and bismuth(III)-2-ethylhexanoate, are burned in a controlled atmosphere (with O₂ levels less than 100 ppm) and with a high fuel-to-oxygen ratio (see Figure 4b), it enables the swift production of pure Co and Bi metal nanoparticles, enhancing the conventional flame process. With this experimental setup, Stark et al. have explored the creation of metallic bismuth nanoparticles ensuring no soot formation ^[28]. While the reducing environment might be beneficial for producing metallic particles on a large scale ^[27]^[29], it comes with the risks of incomplete combustion ^[1]. In the case where the oxygen supply is further constrained, a fine carbonaceous layer tends to form on these metal nanoparticles ^[29]^[30]. Using this experimental setup, NiMo nanoalloys ^[31] and ZnS nanocompounds ^[32] have been reported.

Figure 4. (a) Conventional FSP (left) and reducing FSP (right), where the anoxic flame is produced by in situ introduction of reducing dispersion gas, e.g., CH₄. (b) An anoxic FSP reactor, used by Stark, with the whole reactor enclosed in a glove box filled with an inert atmosphere. By adjusting the gas flow rates, it is possible to achieve highly reduced conditions (O₂ < 100 ppm). Used with permission of Royal Society of Chemistry from ^[27]; permission conveyed through Copyright Clearance Center, Inc. (c) Schematic depiction of the step-by-step transformation from precursor to oxide, metal, and carbon-coated metal nanoparticles during the reducing flame synthesis process: Initially, the precursor undergoes evaporation and combustion, resulting in oxide nanoparticles. These particles can then be further reduced to their metallic form by H₂ and CO. Throughout this procedure, the nanoparticles increase in size due to aggregation and sintering. By introducing acetylene, these metal nanoparticles can acquire a carbon coating layer.

Strobel and Pratsinis used an oxygen-deficiency FSP process ^[33] in order to synthesize Fe₂O₃, Fe₃O₄, and FeO nanoparticles. Their setup featured an FSP nozzle with a metal tube (4 cm in diameter and 40 cm in length) positioned directly above it (as shown in Figure 4b). Situated 20 cm above the FSP nozzle and angled at 45°, an internal mix spray nozzle was directed downward. This nozzle delivered deionized water at a rate of 10 mL/min, dispersed using 5 L/min of N₂. A different oxygen-deficiency FSP setup for the production of Fe₃O₄ nanoparticles may be the utilization of a laminar, inverse diffusion flame ^[34]. This method takes advantage of the properties of the inverse flame, created when an oxidizer is injected into a flow of surrounding fuel ^[35]. Contrary to conventional flame approaches, this setup ensures that the iron particle formation occurs in a predominantly reducing atmosphere. As illustrated in Figure 5a, the burner features two concentric brass tubes with specific outer diameters, enclosed within an 11.4 cm diameter acrylic chamber. This chamber is crucial for protecting the flame from ambient air, preventing additional particle oxidation and potential secondary diffusion flame formation due to excess fuel reacting with room air. The oxidizer, either pure O₂ or an O₂-Ar mixture, is released from the innermost tube and is encircled by a blend of fuel (methane or ethylene), argon, and iron precursor vapor. A N₂ flow enveloped the resulting inverse flame.

Figure 5. (a) Experimental setup of laminar, inverse diffusion flame stabilized on a burner for the synthesis of magnetic iron oxide nanoparticles with reduced oxidation state. (b) The concept of the novel anoxic FSP, for ZrO_2−x production. (c) (i) Schematic representation of anoxic FSP reactor used for the synthesis of C@Cu₂O/Cu⁰ nanoparticles. (ii) Anoxic FSP reactor configuration utilized for creating CuO and Cu₂O nanomaterials.

Recently, researchers have exemplified a novel anoxic FSP process, to engineer ZrO_2–x (see Figure 5b) ^[36] and C@Cu₂O/Cu⁰ (see Figure 5c) ^[37] nanoparticles. The anoxic FSP concept relies on the combustion of CH₄ in the dispersion gas. This introduces reducing agents that can modify the primary Zr particle by creating oxygen vacancies (V_O). XPS and EPR confirm that the increased dispersion of the CH₄ promotes the formation of oxygen vacancies ^[36]. A more complicated oxygen-deficiency FSP setup, which includes a dispersion feed consisting of {oxygen (O₂)–methane (CH₄)} mixture, in tandem with enclosed FSP flame with radial N₂, is necessary for the synthesis of non-graphitized carbon/Cu₂O/Cu⁰ heterojunction (see Figure 5c) ^[37]. The modification in the dispersion gas mixture leads to increased temperatures and generates reducing agents for the controlled phase transformation from CuO to Cu₂O and Cu⁰ (see Figure 5c).

2.2. Double-Nozzle FSP Configuration

In the case of mixed structures, e.g., heterojunctions, core-shell compositions, etc., the application of two FSP nozzles that operate in tandem offers advantages. Typical examples include the cases where a nanomaterial (NP1) and a cocatalytic nanomaterial (NP2) are combined. In the conventional single-nozzle FSP, a single precursor contains both the elements of nanomaterial (NP1) and nanomaterial (NP2) and produces the combined material in a single flame (see Figure 6a).

Figure 6. Symmetric and asymmetric DN-FSP configuration for two particle formation regarding the (i) atomic, (ii) particle, or (iii) agglomeration scale.

Double-nozzle FSP entails two independent spray flames, with the precursor of NP1 inserted in a different flame than NP2 (see Figure 6). This method unlocks several options for independent size control, mixing, and specific deposition for the two nanomaterials by altering the primary geometrical parameters of distance and intersection of the flames. As shown in Figure 6: (i) At a small flame-intersection distance, where the centers of the flames are in contact, the atoms are in the preliminary stages of crystallization, producing well-mixed particles, tending to be similar to the single-nozzle FSP. In this case, the second flame substantially increases the synthesis overall temperature. (ii) When the intersection occurs after the endpoints of the flames, the materials are well crystallized, resulting in well-mixed primary particles of NP1 and NP2. (iii) At increased intersection distance, the two materials mix at their sintering stage or bigger distances at the agglomeration stage.

Thus, by changing the geometrical disposition of the two flames via the parameters a, b, d, Φ₁, Φ₂, and Z (see Figure 7b), the symmetrical/asymmetrical DN-FSP configuration offers a versatile technology that allows for the control of composite configurations at different synthesis stages, i.e., at the atomic scale, at the particle scale, or the aggregate’s scale (see Figure 6).

Figure 7. (a) Schematic example of SN-FSP where two precursors are mixed before being fed to the flame. (b) Geometry parameters of DN-FSP. (c) Example of a symmetrical DN-FSP, used for engineering of La-doped SrTiO₃, with surface deposition of CuO. (d) Example of asymmetrical DN-FSP.

Al₂O₃: DN-FSP was first implemented by Strobel et al. ^[38], producing in one nozzle Al₂O₃ and in the second nozzle Pt/BaCO₃, thus forming individual Al₂O₃ and monoclinic BaCO₃ nanoparticles. Increasing the internozzle distance delayed flame product mixing, increasing the crystallinity of BaCO₃. In contrast, the single-nozzle process yielded Al₂O₃ particles with amorphous Ba species. The two-nozzle process enhanced NO_x storage behavior, while the single-nozzle approach showed negligible NO_x retention ^[38]. Following this successful novelty method, a series of Al₂O₃-based articles were published, herein chronologically presented: Minnermann et al. ^[39] produced in one nozzle Al₂O₃ and in the other pure oxide or mixed CoO_x. Single flame synthesis is inadequate for producing an effective Al₂O₃/Co FT catalyst due to inadequate reducible cobalt oxide support particle size. The DN-FSP geometry significantly influences the resulting catalyst, yielding smaller alumina particles as the intersection distance increases, resulting in good adhesion of the two oxides and good stabilization. Høj et al. ^[40] produced Al₂O₃/CoMo by DN-FSP, and varying flame mixing distances (81–175 mm) minimized the formation of CoAl₂O₄, detectable only at short flame distances. Notably, employing DN-FSP synthesis achieved superior promotion of the active molybdenum sulfide phase, potentially attributed to reduced CoAl₂O₄ formation, consequently enhancing Co availability for promotion. Schubert et al. ^[41], through DN-FSP, produced Al₂O₃/Co enhanced with Pt (0.03, 0.43 wt%) deposition in the first nozzle and other materials in the second nozzle. Noble metals enhance catalyst reducibility, yielding abundant metallic Co sites. Due to their high cost, optimizing synthetic strategies for low concentrations is essential. Regardless of the preparation approach, adding 0.03 wt% Pt significantly improves catalytic activity in CO₂ methanation, and 0.43 wt% Pt marginally increases the catalyst reduction. Using DN-FSP, Horlyck et al. ^[42] produced Al₂O₃/Co with Lanthanum doping (0–15 wt%). Increased La content and wider nozzle distance suppressed undesirable CoAl₂O₄ spinel phase, promoting easily reducible Co species. La addition enhanced carbon resistance, ensuring maximum methane conversions at 15 wt% La without catalyst deactivation or carbon formation. Stahl et al. ^[43] used DN-FSP to produce Co/Al₂O₃; in the nozzle of Al₂O₃, one additional particle—SmO_x, ZrO_x, or Pt—was formed contributing different cocatalytic effects, enhancing surface hydrogen or carbon oxide concentrations (see Figure 8a,b). All catalysts had consistent morphology with interconnected 12 nm alumina oxides and

~

8 nm cobalt oxides. For CO₂ methanation, Pt and zirconia proved optimal, aligning with Pt-enhanced H₂ adsorption and zirconia’s higher CO₂ adsorption due to oxide sites with medium basicity.

Figure 8. (a) TEM images revealing the local distribution of cobalt and oxygen for Pt-Al₂O₃/Co₃O₄, (b) EDX measurements for chemical composition. DΝ-FSP-prepared (c) SiO₂/Co, (d) SiO₂-TiO₂/Co, (e) and TiO₂/Co; left images show STEM-HAADF and right images show EDX mappings of the elements Co (blue), Si (red) and Ti (yellow). (f) Particle size distributions of Co₃O₄ for the materials SiO₂, SiO₂-TiO₂, and TiO₂. (g) STEM-HAADF of the nano-mixed CeO₂:Eu³⁺/Y₂O₃:Tb³⁺ and its elemental mapping for Ce in red and Y in green, (h) d_TEM distribution of CeO₂:Eu³⁺ and Y₂O₃:Tb³⁺.

TiO₂: Grossmann et al., through the utilization of DN-FSP, produced TiO₂ with deposited Pt particles ^[44]. Geometric configurations in DN-FSP strongly influenced Pt particle size and distribution on TiO₂. Larger intersection distances and smaller angles result in nonuniform large and broadly distributed Pt clusters on TiO₂. Conversely, smaller distances and larger angles enhance Pt dispersion and a uniform mixing, akin to single flame; however, DN-FSP allows for individual tuning of compound particle sizes. Solakidou et al. produced {TiO₂-Noble metal} nanohybrids, with deposition of Pt⁰, Pd⁰, Au⁰, or Ag⁰ ^[45]. As shown, DN-FSP is superior vs. single-nozzle-FSP for finely dispersing noble metals on TiO₂ support, achieving a narrower size distribution ^[32]. DN-FSP promoted intraband states in TiO₂/noble metal, reducing the band gap. Efficient H₂ generation presented the following trend: Pt⁰ > Pd⁰ > Au⁰ > Ag⁰, in line with a higher Schottky barrier upon TiO₂ contact ^[32]. Gäßler et al. produced SiO₂, TiO₂, and SiO₂-TiO₂ mixture with DN-FSP deposition of Co₃O₄ (see Figure 8c–f) ^[46]: titania, comprising anatase and rutile phases, the SiO₂-TiO₂ mixed support, with separate anatase and silica phases. H₂O adsorption varies significantly based on the support: SiO₂ < SiO₂-TiO₂ < TiO₂. CH₄ formation rate increased with higher TiO₂ fractions, while CO formation rate peaked in the mixed support. Psathas et al. used DN-FSP to engineer heterojunctions of perovskite SrTiO₃ with deposited CuO nanoparticles (0.5 to 2 wt%) ^[47]. Higher CuO deposition led to larger SrTiO₃ particle sizes due to increased enthalpy from the second flame ^[22]. Scanning TEM depicted small CuO particles (<2 nm), mainly found on the surface of SrTiO₃. The dopant concentration significantly controlled the selective production of H₂ or CH₄ from H₂O/CH₃OH. CuO incorporation drastically shifted production to CH₄, achieving a rate of 1.5 mmol g⁻¹ h⁻¹ for the La:SrTiO₃/CuO catalyst (0.5 wt%) ^[47].

Other particles: Tada et al., using DN-FSP, produced a ZrO₂/CuO heterostructure ^[48]. Changing the geometrical parameters of DN-FSP altered the proportion of interfacial sites vs. copper surface sites. As active sites are primarily at the metal–oxide interface, ZrO₂/CuO with smaller CuO clusters exhibited higher activity in methanol synthesis via CO₂ hydrogenation. Gockeln et al., by a combination of DN-FSP and a lamination technique ^[49], synthesized in situ carbon-coated nano-Li₄Ti₅O₁₂ Li-ion battery electrodes. Li et al. synthesized LiMn₂O₄ spinel as a cathode material for Li-ion batteries via screening 16 different precursor–solvent combinations ^[50]. To overcome the drawback of capacity fading, the deposition of AlPO₄ (1–5%) via DN-FSP was homogeneously mixed with LiMn₂O₄. The optimal 1% AlPO₄ with LiMn₂O₄ demonstrated an energy density of 116.1 mA h g⁻¹ at 1 C (one-hour discharge). Henning et al. used DN-FSP to engineer luminescent biosensors CeO₂:Eu³⁺/Y₂O₃:Tb³⁺ ^[51]. CeO₂:Eu³⁺ nanoparticles (6 nm, 22 wt%) and Y₂O₃:Tb³⁺ nanoparticles (32.5 nm, 78 wt%) were shown to function as robust optical-based ratiometric H₂O₂ biosensors (see Figure 8g,h). Based on the collective effect, H₂O₂ caused significant luminescence quenching in CeO₂:Eu³⁺ nanocrystals, but Y₂O₃:Tb³⁺ nanoparticles were unaffected ^[30].

Asymmetric Double Flame: Lovell et al. utilized asymmetric-DN-FSP geometry to control the SiO₂ interaction with Ce_0.7Zr_0.3O₂ nanoparticles ^[52]. Tuning the intersection distance during DN-FSP (18.5 to 28.5 cm) prevented silica coating. Short intersection distances led to high surface-area silica encapsulating ceria-zirconia, while longer distances suppressed this encapsulation. The material at longer intersection distances, used as Ni support for dry methane reforming, showed enhanced oxygen storage capacity and basicity, yielding a highly selective catalyst. Psathas et al. used asymmetrical-DN-FSP-deposited NiO or Pt⁰ nanomaterials on the surface of Ta₂O₅ or the perovskite NaTaO₃ ^[53]. Single-step synthesis of the smallest produced NaTaO₃ (<15 nm), with finely dispersed NiO or Pt⁰ (<3 nm). NaTaO₃/NiO produced from FSP had half the photocatalytic hydrogen production than those from DN-FSP. Also, DN-FSP had a ten times higher yield than the conventional deposition of wet-impregnated NiO. Similar results were found for the photocatalytic efficiency of NaTaO₃/Pt⁰, which was 30% more photocatalytically active than the conventional liquid-Pt photo-deposition method ^[36].

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Subjects: Nanoscience & Nanotechnology

Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :

Christos Dimitriou

Pavlos Psathas

Maria Solakidou

Yiannis Deligiannakis

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Update Date: 28 Nov 2023

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