Advanced Flame Spray Pyrolysis (FSP) Technologies for Engineering Multifunctional Nanostructures and Nanodevices

Abstract

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. In the present review, we discuss the recent state-of-the-art advances in FSP technology with regard to nanostructure engineering as well as the FSP reactor setup designs. The challenges of in situ incorporation of nanoparticles into complex functional arrays are reviewed, underscoring FSP's transformative potential in next-generation nanodevice fabrication. Key areas of focus include the integration of FSP into the technology readiness level (TRL) for nanomaterials production, the FSP process design, and recent advancements in nanodevice development. With a comprehensive overview of engineering methodologies such as the oxygen-deficient process, double-nozzle configuration, and in situ coatings deposition, this review charts the trajectory of FSP from its foundational roots to its contemporary applications in intricate nanostructure and nanodevice synthesis.

Keywords: TRL; complex assemblies; double nozzle; flame spray pyrolysis; multifunctional nanomaterials/nanodevices; nanofilms; non-oxides; oxygen-deficiency process; perovskites; plasmonics; quantum dots.

Figure 13

(a) Predicted normalized gas-phase mass fractions, the ZrO₂ formation profile (brown), and volume concentration (green). (b) Schematic of FSP two-phase atomizer geometry. Reprinted (adapted) with permission from [117]. Copyright 2014 American Chemical Society. (c) A flame spray pyrolysis (FSP) pilot plant is designed to produce multiple kg h⁻¹ of nanoparticles, incorporating a baghouse filter for nanoparticle collection with an approximate filtration area of $~$50 m². Reprinted from [108].

Figure 16

(a,b) TEM images illustrating necked particles of BiVO₄. Reprinted (adapted) with permission from [122]. Copyright 2011 American Chemical Society. (c) TEM images of necked-sintered BiFeO₃ particles. Reprinted from [124], with permission from Elsevier. (d–f) HRTEM micrographs of SrTi_1−xCo_xO₃ and the miller planes. (g) EDX element mapping of SrTi_1−xCo_xO₃ for the Ti, Co, and Sr atoms. Reprinted (adapted) with permission from [129]. Copyright 2021 American Chemical Society.

Figure 19

(a) A diagrammatic representation of the diverse nanoscale structures that can be engineered by the FSP method. (b) TEM depicts the hexagonal FSP-made NaYF₄:Yb,Tm nanostructures. (c) XRD data of NaYF₄:Yb,Tm nanoparticles obtained under varying fuel/oxygen flow ratios. Reprinted from [145].

Figure 28

(a) A photodetector substrate and (b,c) the synthesis via flame spray pyrolysis and subsequent aerosol self-assembly, resulting in (d) ultraporous films composed of electron-depleted ZnO nanoparticles at a 20 cm height above the burner (HAB). These films are uniformly structured and predominantly consist of spherical particles with an average TEM size of 19 nm (d). Reproduced with permission from ref. [93] Copyright 2015 Wiley-VCH. (e) SEM cross-sectional (i) and top views (ii–iv) of a SnO₂ nanoparticle film, aerosol-deposited with a precursor concentration of 0.5 mol/L, for a duration of 4 min. Reproduced with permission from ref. [98] Copyright 2012 Wiley-VCH. (f,g) SEM images of an as-deposited LSC thin film produced by flame spray deposition on a sapphire substrate: (f) top view and (g) cross-sectional view. The film was deposited over a duration of 15 min at 200 °C during the flame spray deposition process. Reprinted from [223], with permission from Elsevier.

Figure 32

(a) Schematic representation of a photoelectrochemical (PEC) cell’s fundamental mechanism, featuring an n-type semiconductor photoanode for oxygen evolution and a platinum sheet photocathode for hydrogen generation in water splitting. (b) Optical (i) and SEM (ii–iv) imaging of flame-made BiVO₄ films on FTO substrates demonstrate variations based on differing HAB settings, with cross sections at (ii) 15 cm, (iii) 10 cm, and (iv) 6 cm HAB corresponding to 60, 20, and 5 s deposition times, ensuring uniform absorbance in each film. (c–f) BiVO₄ photoanode PEC metrics were assessed in relation to (c) porosity, (d) thickness during oxidation in air-saturated 1 M KB (pH 9.3) with 0.2 M Na₂SO₃ as a hole scavenger under FTO-side (solid), and BiVO4-side (dashed) illumination. (e) PEC responses in 1 M KB (pH 9.3) were measured with (solid) and without (dashed) FeOOH/NiOOH electrocatalyst modification under FTO-side illumination. The SEM inset of (e) displays the morphology post-FeOOH/post FeOOH/NiOOH application on a 12% porosity sample. Dark current densities were logged for the 12% porosity sample with (dark dot) and without (yellow dot) electrocatalyst overlay. Measurements were taken via voltammetry at 0.010 V s⁻¹ ascending potential in 1 sun equivalent light (AM 1.5 G, 100 mW cm⁻²). (f) Longevity trials were conducted on FeOOH/NiOOH-modified samples in 1 M KB (pH 9). Reproduced with permission from ref. [227]. Copyright 2019 Wiley-VCH. (g–i) CO₂RR efficacy of f-Bi₂O₃ catalysts was evaluated in CO₂-saturated 0.1 M KHCO₃. (g) f-Bi₂O₃ exhibited specific voltammetry profiles at 5 mV s⁻¹, with comparative data for filter-collected Bi₂O₃. (h) The current density for formate production on f-Bi₂O₃ and Bi₂O₃ varied with potential during CO₂RR. (i) The Faradaic efficiency for formate generation during CO₂RR correlated with aerosol deposition duration, with measurements taken at −1.2 V vs. RHE. Reproduced with permission from ref. [251]. Copyright 2019 Wiley-VCH.

Figure 33

(a) Simplified fuel cell’s schematic representation. (b) Methanol oxidation and (c) CO stripping processes utilizing carbon-supported Pt-Ru catalyst produced via FSP in contrast to the performance of standard E-TEK catalysts. (d) Schematic figure depicting the apparatus used for the flame aerosol synthesis of Pt-Ru/C catalysts, with an accompanying TEM image of the synthesized Pt-Ru/C catalysts in the inset. Reprinted from [254], with permission from Elsevier. (e) Schematic of a Li-ion battery illustrating its constituent components. Reprinted from [255]. (f) The LTO battery cell fabrication involves setting KaCu substrates in an FSP reactor for LTO deposition, followed by compression and transfer to a glove box for sputtering LiPON, lithium, and copper, leading to the final cell assembly. (g,h) Nyquist diagrams for battery cells featuring fully lithiated rock-salt-type Li₇Ti₅O₁₂ (g) and completely delithiated (initial state) spinel Li₄Ti₅O₁₂ (h) thin films. (i) Morphological analysis of compressed LTO thin films includes (i) photographs, (ii) SEM imaging on KaCu substrates, (iii) 2D LSM color and laser imaging, and (iv) 3D surface profiling. Reprinted from [256], with permission from Elsevier.

Figure 1

Figure illustrating the complex assemblies in flame spray pyrolysis (FSP) discussed in this review. 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.

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. Reprinted (adapted) with permission from [1]. Copyright 2021 American Chemical Society. (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.

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, sourced from Scopus using the keyword ‘Flame Spray Pyrolysis’ [1,2,30,31,34,35,36,37,38,39,40,41,42,43].

Figure 4

(a) Conventional FSP (left) and our 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 [45]; 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. Reproduced with permission from ref. [47,48]. Copyright 2007 Wiley-VCH.

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. Reprinted from [52], with permission from Elsevier. (b) The concept of the novel anoxic FSP, as developed by our lab, for ZrO_2−x production. Reprinted from [54]. (c) (i) Schematic representation of anoxic FSP reactor used for the synthesis of C@Cu₂O/Cu⁰ nanoparticles. Reprinted from [55]. (ii) Anoxic FSP reactor configuration utilized for creating CuO and Cu₂O nanomaterials. Reprinted from [55,56].

Figure 6

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

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. Reprinted from [58]. (d) Example of asymmetrical DN-FSP.

Figure 8

(a) TEM images revealing the local distribution of cobalt and oxygen for Pt-Al₂O₃/Co₃O₄, (b) EDX measurements for chemical composition. Reprinted from [64]. 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. Reproduced with permission from ref. [65]. Copyright 2022 Wiley-VCH. (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⁺. Reprinted from [66], with permission from Elsevier.

Figure 9

(a) Overhead perspective of the stainless steel segment in the quench ring according to the research work of Hansen et al. [74]; every nozzle is angled at 10° compared with a hypothetical line passing through the central axis. Reproduced with permission from ref. [74]. Copyright 2001 Wiley-VCH. (b) Experimental configuration for in situ SiO₂ coating of TiO₂ nanoparticles produced by Teleki et al. [75], using a toroidal pipe ring with 16 gas exits to inject HMDSO-laden N₂. At burner ring distances (BRD) of (c) 5 cm and (d) 30 cm, this leads to distinct SiO₂/Al₂O₃/TiO₂ or SiO₂-layered Al/TiO₂ particles, each containing 4 wt% Al₂O₃ and 20 wt% SiO₂, respectively. Reprinted (adapted) with permission from [75]. Copyright 2008 American Chemical Society. (e) Toroidal pipe ring equipped with 8 outlets for injection of the HMDSO-laden N₂. (f) Impact of the ring N₂ flow rate (along with the associated jet Reynolds number at 300 K) under standard coating conditions on SSA (depicted by circles), rutile weight percentage (represented by triangles), anatase (shown as squares), and rutile (illustrated by diamonds) crystallite sizes of 20Si-coated Al/TiO₂. (g) Graphical representations of the (a) N₂ volume percentage for 16 jets with a combined volume flow of 15.8 L/min N₂ (v₀ = 58 m/s). The related cross-sectional views are displayed at heights of 0 (b), 0.2 (c), 1 (d), and 3 cm (e) above the outlet level. The logarithmic color gradient extends from <1 (in blue) to 100 (in red) % v/v of N₂. Reprinted (adapted) with permission from [77]. Copyright 2009 American Chemical Society.

Figure 10

(a) (i) Illustration of the enclosed FSP setup for producing Ag@SiO₂ nanoparticles, originally developed by Sotiriou et al. [78]. The ring situated between the two tubes aids in swirling the SiO₂ precursor vapor, ensuring the precise control of the SiO₂ core. (ii) Adjustment of the crystallite size of nanosilver can be achieved by varying the SiO₂ content in the final nanosilver particles and by altering the injection height of the SiO₂ precursor vapor to 30 (circles), 25 (triangles), or 20 cm (squares) above the flame spray burner. Enhancing the SiO₂ content helps to prevent the agglomeration and crystal growth of Ag nanoparticles. Additionally, introducing HMDSO (the SiO₂ precursor) at reduced heights cools the flame aerosol, further inhibiting Ag crystal growth. Reproduced with permission from ref. [78]. Copyright 2010 Wiley-VCH. (b) (i) Schematic depiction of the enclosed FSP reactor where the one-step Ag coating SiO₂ particles occur in-flight [80]. Contrary to Sotiriou et al.’s findings [78], the metal ring was at the top of the FSP-enclosed flame. (ii) XRD patterns of SiO₂@Ag⁰ nanoparticles with variations in size and shell thickness. Reprinted (adapted) with permission from [80]. Copyright 2019 American Chemical Society. (c) Schematic illustration of FSP reactor configurations: (i) designed for high-temperature nano-SiO₂ production, (ii) tailored for the hybrid rSiO₂@SiO₂ nanosilica, and (iii) set up for low-temperature nano-rSiO₂ synthesis. Reprinted (adapted) with permission from [82]. Copyright 2022 American Chemical Society. (d) FSP apparatus for creating core-shell CuO_x@SiO₂ nanoparticles. Reprinted from [85], with permission from Elsevier.

Figure 11

(a) The concept of sequential deposition technique, as originally exemplified by Sahm et al. [89], for the engineering of multilayer films. Cross-sectional SEM images of a SnO₂ layer are shown (i); a Pd/Al₂O₃ layer over a SnO₂ layer (ii); a Pd/SnO₂ layer (iii); and a Pd/Al₂O₃ layer on the top of a Pd/SnO₂ layer (iv)—all of which were deposited on ceramic bases. Reprinted from [89], with permission from Elsevier. (b) A schematic depiction of the FSP process employed for the deposition of Pd onto the TiO₂ surface in two stages (sequential deposition, SD-FSP). Reprinted (adapted) with permission from [91]. Copyright 2020 American Chemical Society.

Figure 12

(a) Schematic illustration of the creation of porous or solid films through the flame deposition of droplets, vapors, or particles. (b) Illustrative diagram of the flame spray pyrolysis deposition setup; the embedded image displays an external mix atomizer for film deposition. Reprinted from [96], with permission from Elsevier. (c) FSP burner combined with a temperature-regulated substrate holder designed for nanofilm creation and deposition. Reproduced with permission from ref. [98] Copyright 2012 Wiley-VCH. (d) Diagrammatic representation: (i) Flame-made nanoparticle layers, exhibiting a high porosity of 98%, are methodically deposited onto a silicon wafer using a shadow mask. Subsequently, (ii) these layers undergo in situ mechanical stabilization via an impinging xylene flame devoid of particles. Reproduced with permission from ref. [99] Copyright 2008 Wiley-VCH.

Figure 14

The ideal cubic perovskite structure exemplified by SrTiO₃. (a) The cubic structure with the Ti⁴⁺ at the cell center. (b) The octahedral polyhedron structure TiO₆. (c) Sr²⁺ at the cubic center with the octahedral structure surrounding the strontium. Different perovskite structures: (d) the ideal cubic perovskite, axis X, Y, and Z of the octahedral BX₆ have a 180-degree separation. (e) Tetragonal perovskite structure, the octahedral BX₆ tilted in only one of the three axes at an angle Θ1. (f) The orthorhombic structure, the octahedral, is titled in all three of the axes, in accordance with the angles Θ1, Θ2, and Θ3.

Figure 15

FSP formation of perovskite structure ABO₃ requires avoidance of the formation of the two separate oxides (A-oxide, B-oxide).

Figure 17

(a) Reducing FSP configuration for engineering Fe₃C or C/Fe₃C nanostructures. (b) Controlling the process atmosphere in flame spray synthesis enables the production of diverse iron-based nanoparticles. The resultant composition aligns with the relevant phase diagrams for Fe/O (left side) under oxidizing conditions. Subsequent reduction processes yield iron and iron carbide nanoparticles in accordance with the predictions derived from the Fe/C phase diagram (right side). Reprinted (adapted) with permission from [140]. Copyright 2009, American Chemical Society.

Figure 18

(a) In a one-step process, a liquid precursor containing lead, titanium, and sulfur is transformed into composite nanoparticles using a high-temperature flame reactor. Precise control of oxygen content enables fine-tuning of the system’s chemistry, facilitating the selective formation of titania support particles (oxides) and lead sulfide quantum dots (sulfides). (b) TEM analysis of a PbS−TiO₂ heterojunction. The light contrast TiO₂ nanoparticles serve as a support for PbS (darker contrast). Reprinted (adapted) with permission from [142]. Copyright 2012 American Chemical Society. (c) A schematic representation is presented depicting the operation of enclosed single-droplet (SD) combustion and enclosed reactive spray (RS) flame reactors for the production of metal sulfide particles. Emphasis is placed on elucidating the involvement of micro-explosions in the gas-to-particle pathway, observed in both SD and RS configurations. (d) From top to bottom in the column, the images depict the following: an overview of the particles, a high-resolution image of the particles, and a representative single crystalline particle—each of Cu₂S, CoS, In₂S₃, Ag₂S, MnS, and ZnS. Reproduced with permission from ref. [143]. Copyright 2023 Wiley-VCH.

Figure 20

(a) Illustration of an enclosed FSP configuration, comprising a particle formation zone (1), an acetylene carbon black (ACB)-coating region (2), and a quenching zone (3). The precise regulation of O₂ stoichiometry within the ACB-coating area is achieved by enclosing the unit with quartz tubes. Subsequently, the coated particles are subjected to cooling using nitrogen (N) at the conclusion of the coating zone to prevent carbon black combustion upon exposure to ambient air during the filtration process. (b) TEM image of as-prepared segregated ACB and LiFePO₄ nanostructures. Reprinted from [149], with permission from Elsevier. (c) The XRD pattern of the calcium phosphate (CaP) nanoparticles is presented. Depending on the FSP synthesis conditions employed, the resultant nanoparticles manifest either crystalline or amorphous characteristics. Predominant diffraction peaks are attributed to hydroxyapatite; Ca₅(PO₄)₃OH, though the presence of CaO, is also detected. TEM micrographs of the freshly synthesized (d) CaP_L and (e) CaP_S materials are provided. The CaP_L nanoparticles display a spherical morphology characterized by a loosely agglomerated structure. In contrast, the CaP_S particles are evidently fused, with discernible sintered necks. Reprinted from [153]. (f,g) TEM images for as-synthesized vanadium phosphate (VOPO₄) particles from (f) sucrose-based solutions and (g) DMF-based solutions. Reprinted from [146].

Figure 21

(a) XRD pattern of the as-synthesized barium carbonate (BaCO₃) is presented. Reflections not labeled are attributable to the monoclinic phase of BaCO₃. Only minute traces of the orthorhombic phase (O) have been detected. (b) TEM showcasing nanoparticles of BaCO₃ synthesized via flame-based methods. The inset provides a representation of the corresponding electron diffraction pattern. Reprinted from [155], with permission from Elsevier.

Figure 22

(a) Illustration depicting the experimental arrangement employed for FSP synthesis of magnetic nanoparticles. Specifically, the reducing flame spray synthesis (RFSP) was executed under a controlled nitrogen atmosphere, with the pilot flame constituted of an O₂/H₂/air mixture. (b) TEM image of flame-made Fe/Fe₃O₄ nanostructures. Reprinted from [156], with permission from Elsevier.

Figure 23

(a) A HR-TEM image of as-prepared mixed silica-ZnO crystallites. This image prominently displays the crystalline lattice structure of the ZnO component. Additionally, an inset is provided, offering a lower magnification view that elucidates the overall morphology of the powder under investigation. Reprinted from [188], with the permission of AIP Publishing. (b) Combustion flame of the SiO₂-metal oxide solution with all the produced colored metal oxide QDs and the TEM image of CuO QDs. Reprinted from [189]. (c) TEM analysis of CuO-SrTiO₃ nanostructures, where the CuO QDs are distinctly delineated by dashed yellow circles. Inset: elemental mapping of CuO QDs within a specific region of interest. Reprinted (adapted) with permission from [130]. Copyright 2021 American Chemical Society. (d,e) HR-TEM images and the locally enlarged HR-TEM images of CQDs/TiO₂-C. Reprinted from [190], with permission from Elsevier.

Figure 24

(a) STEM image of the 2Ag/SiO₂ composite material EDX analysis: area 1, containing silver, and area 2, representing pure SiO₂. (b) Diffuse reflectance ultraviolet/visible (UV/vis) spectra for various xAg/SiO₂ compositions, where x denotes the silver concentration. A consistent plasmon absorption band of Ag metal at 410 nm was observed in all Ag-containing samples. TEM image, shown as an inset, depicted Ag nanoparticles (dark dots) dispersed on nanostructured silica support (gray) in the 6Ag/SiO₂ nanostructure. Reprinted (adapted) with permission from [202]. Copyright 2010, American Chemical Society. (c,d) TEM images of the nanosilver coated with 7.8 wt% SiO₂ are presented. Reproduced with permission from ref. [78]. Copyright 2010 Wiley-VCH. (e,f) TEM images of (e) 5–25 SiO₂@Ag⁰ NPs (SiO₂ thickness = 5 nm), and (f) 1–25 SiO₂@Ag⁰ NPs (SiO₂ thickness = 1 nm). (g) UV/vis spectra were recorded for suspensions of SiO₂@Ag⁰ NPs with uniform particle size across three distinct variants: 1–15, 25 (SiO₂: 1 nm), 3–15, 25 (SiO₂: 3 nm), and 5–15, 25 (SiO₂: 5 nm). The inset of the figure presents photos of these particle suspensions. Additionally, schematic representations of the particles are provided to visually convey the influence of shell thickness. Reprinted (adapted) with permission from [80]. Copyright 2019 American Chemical Society.

Figure 25

(a) HR-TEM image depicting nanoparticles consisting of Au/Fe₂O₃ cores enveloped by amorphous SiO₂ shells, measuring 2.6 nm in thickness with the SiO₂ content in these shells quantified at 5.7 wt%. Inset: Elemental EDXS mapping for all three elements (Au, Fe, Si) together in a merged image. Reproduced with permission from ref. [203]. Copyright 2014 Wiley-VCH. (b) A HAADF-STEM image, characterized by Z-contrast, displaying the uncoated 50Ag/Fe₂O₃ sample. In addition, TEM images are presented for (c) the SiO₂-coated 10Ag/Fe₂O₃ sample and (d) the SiO₂-coated 35Ag/Fe₂O₃ sample. Above these visual representations, schematic diagrams are included, illustrating the structural characteristics of both uncoated and SiO₂-coated particles. Reprinted (adapted) with permission from [204]. Copyright 2011 American Chemical Society.

Figure 26

(a) A schematic figure illustrating the FSP setup employed in the synthesis of silica-coated silver nanoparticles. (b) The Raman spectra analysis encompassed two distinct samples: pure rhodamine R6G powder and R6G molecules at a concentration of 10⁻⁶ mol L⁻¹, both with and without the presence of 5 wt% SiO₂-coated Ag nanoparticles. The presence of plasmonic particles induces the surface-enhanced Raman scattering (SERS) effect. Reprinted (adapted) with permission from [211]. Copyright 2013, American Chemical Society.

Figure 27

(a) TEM image of 20Ag/TiO₂, synthesized under the condition X/Y = 8/5 (precursor feed rate/dispersion gas) accompanied by selected (b) high-resolution images for detailed examination. Notably, disordered titanium oxide, observable on both the nanosilver and TiO₂, is highlighted using blue arrows and red dashed lines. (c) An illustrative diagram is included, depicting the formation of titanium suboxide (Magnéli phases) on nanosilver and TiO₂, resulting from robust metal–support interactions (SMSI). Reprinted from [212], with permission from Elsevier.

Figure 29

(a) Using flame aerosol deposition and mechanical stabilization (in situ annealing), nanostructured films are produced in a single step. Nanoparticles formed in the flame are thermophoretically deposited onto substrates like Si, glass, or polymer-coated materials. By infusing a polymer, such as through spin coating, these films gain mechanical stability. Incorporating a sacrificial layer, like polyvinylpyrrolidone (PVP), enables the creation of free-standing polymer nanocomposite films. (b) HRTEM image of Ag/TiO_x nanoparticles illustrates the pronounced crystallinity of both Ag and TiO_x within the synthesized nanoparticles. (c) Cross-sectional scanning electron microscopy (SEM) representation of the Ag/TiO_x polymer nanocomposite film with a deposition time (t_d) of 15 s and a composition of 50% Ag/Ti. Reprinted from [230].

Figure 30

(a) HCHO-sensing models of SnO₂ nanoparticles with AgO_x-doping at a moderate content (0.2 wt%), and the response histogram of the AgO_x-doped SnO₂ sensors with different Ag contents (S-0 to S-1Ag) for toxic gases (NH₃ and NO), flammable gases (C₂H₂, C₂H₄, H₂ and CH₄), and VOCs (C₃H₆O, C₆H₆, C₂H₅OH, HCHO, CH₃OH, C₇H_8, and C₈H₁₀) at 350 °C. Reprinted from [236], with permission from Elsevier. (b) H₂-sensing models of PtO_x-loaded Zn₂SnO₄ nanoparticles with optimum Pt contents, and the sensor response to 10,000 ppm H₂ of 0–3 wt% PtO_x-loaded Zn₂SnO₄ (S-0 to S-3Pt) as a function of operating temperature in the range of 200–400 °C. Reprinted from [237], with permission from Elsevier.

Figure 31

(a) Schematic diagrams for FSP synthesis of La₂O₃-loaded WO₃ nanoparticles. (b) Areal-view SEM images of S-2La (two spin-coating cycles). (c) Sensor responses toward 5000 ppb NO₂ of 0–2 wt% La₂O₃-loaded WO₃ two-cycle spin-coated films (S-0 to S-2La) in terms of temperature (25–350 °C). Reprinted (adapted) with permission from [242]. Copyright 2023, American Chemical Society.