Flame spray pyrolysis: An enabling technology for nanoparticles design and fabrication

Abstract

Combustion of appropriate precursor sprays in a flame spray pyrolysis (FSP) process is a highly promising and versatile technique for the rapid and scalable synthesis of nanostuctural materials with engineered functionalities. The technique was initially derived from the fundamentals of the well-established vapour-fed flame aerosols reactors that was widely practised for the manufacturing of simple commodity powders such as pigmentary titania, fumed silica, alumina, and even optical fibers. In the last 10 years however, FSP knowledge and technology was developed substantially and a wide range of new and complex products have been synthesised, attracting major industries in a diverse field of applications. Key innovations in FSP reactor engineering and precursor chemistry have enabled flexible designs of nanostructured loosely-agglomerated powders and particulate films of pure or mixed oxides and even pure metals and alloys. Unique material morphologies such as core–shell structures and nanorods are possible using this essentially one step and continuous FSP process. Finally, research challenges are discussed and an outlook on the next generation of engineered combustion-made materials is given.

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Wey Yang Teoh

Wey Yang Teoh is an ARC Australia Postdoctoral Fellow (APD). He obtained his doctorate from the School of Chemical Engineering, University of New South Wales in 2007 and spent a brief attachment at the Swiss Federal Institute of Technology (ETH) Zürich in 2003. He continued his postdoctoral work at the laboratory of Prof. Rose Amal, where he is a Program Leader in flame spray pyrolysis with focusing on energy, environmental and bioapplications. He will assume the role of Assistant Professor at the School of Energy and Environment, City University of Hong Kong.

Rose Amal

Rose Amal is an ARC Australian Professorial Fellow (APF) and University of New South Wales (UNSW) Scientia Professor. She heads a group of over 30 research members at the Particles and Catalysis Research Group, School of Chemical Engineering, UNSW. She is the Director of the ARC Centre of Excellence for Functional Nanomaterials, a multi-institution research centre. She is also the inaugural Director of the Centre for Energy Research and Policy Analysis (CERPA), the new Energy Research Institute at UNSW. Her research interests are in particle technology and nanomaterials, spreading over a wide range of applications from energy, environmental to bioapplications.

Lutz Mädler

Lutz Mädler (Dr -Ing. Univ. Freiberg/Sa., Germany in 1999) was born in Zwickau, Germany. He received his Habilitation in 2003 at Swiss Federal Institute of Technology (ETH Zurich). He was Senior Lecturer at the Department of Chemical Engineering, University of California, before he joined the Department of Production Engineering at the University of Bremen leading the Particle and Process Technology division in 2008. He is also Director at the Foundation Institute of Materials Science. His research focuses on integrated aerosol processes for nanomaterials and surface films for sensors, catalysts, and optical devices, and on nano-bio-interactions.

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1. Introduction

The large scale production of nanoparticles by flame aerosol technology is a critical aspect contributing to the progress of a wider Nanotechnology. In fact, the technology existed long before the “Nano” buzz, but has since rapidly evolved. Industry leaders such as Cabot, Cristal (formerly SCM and Millenium Chemicals), DuPont, Evonik (formerly Degussa) and Ishihara, manufacture flame-made materials in millions of tons, valued at more than $15 billion/yr.¹ This includes most notably, carbon blacks, fumed SiO₂, TiO₂, Al₂O₃ as well as other ceramic nanoparticles. While many of these nanoparticles are used in commodity applications such as reinforcing agents (carbon blacks, SiO₂, carbon-coated SiO₂), pigments (TiO₂) and flowing aids (SiO₂), emerging niche markets requiring more complex and functional materials are expected to spearhead the next phase of growth. This includes for example, polishing agents in microelectronics (doped CeO₂), catalysts (Pt/Al₂O₃, V₂O₅/TiO₂, Ru/Co₃O₄–ZrO₂), dental fillers (Ta₂O₅–SiO₂) and other specialty applications – see ref. 2 and references therein, many of which are multielements in nature. Given their complexities and the variety of available permutations, the design of sophisticated and functional flame materials is far from trivial and in fact system specific. Nevertheless, their importance and lucrativeness continue to drive the evolution of flame synthesis.

Much is to be learned from the evolution of flame aerosol technology, developed through years of industrial significance: from reactor design to process sophistication and versatility. A classic example is the industrial manufacture of carbon blacks which evolved from the “smouldering” process to the current norm of jet-like flame aerosol reactors.³ Formed via the pyrolysis of hydrocarbon sprays, the carbon black syntheses take place under fuel rich conditions where about half the hydrocarbons are burnt to supply the energy that pyrolyses the other half to carbon black.⁴ Modern aerosol reactors for the production of ceramic oxides bear a strong resemblance to carbon black jet furnaces.⁵ The fundamental differences between the two, besides the choice of metal elements present in the precursors, lies in the combustion regime in which they operate: fuel rich to obtain carbon particles and lean oxidising conditions to obtain metal oxides. In some cases, combination of the two is also possible (e.g. C/TiO₂, C/SiO₂), but this is governed by the combustion engineering in tandem with the corresponding particle formation, materials chemistry and thermodynamics of the system.

Depending on the precursor state (aqueous-based, solvent-based, vapour) and combustion conditions, flame aerosol syntheses fall into three general categories:^6,7 (a) vapour-fed aerosol flame synthesis (VAFS) where a metal precursor is supplied in the form of vapour like SiCl₄ and TiCl₄ to make most of today's ceramic commodities;⁸ (b) flame-assisted spray pyrolysis (FASP) where precursor is supplied in the state of low combustion enthalpy solution (<50% of total combustion energy) usually in aqueous solvent, and because of this, its combustion needs to be assisted by an external hydrogen or hydrocarbon flame;⁹ and (c) flame spray pyrolysis (FSP) where the precursor is also in liquid form, but with significantly higher combustion enthalpy (>50% of total energy of combustion), usually in an organic solvent.¹⁰ FSP (the focus of this review), which is the youngest of the three processes, has important technological elements such as self-sustaining flame, usage of liquid feeds and less volatile precursors, proven scalability, high temperature flames and large temperature gradients. It is also the most heavily explored in recent years for the production of complex and functional nanomaterials. Some of the most notable applications are in the areas of catalysis (as reviewed by Strobel et al.⁷ on general flame aerosols), optics and photonics,^11–15 sensors,^16–18 health care,^19–21 magnetic materials,^22–25 electroceramics for fuel cells^26,27 and composite materials.^28–33 A key feature in the use of FSP as a convenient tool in synthesising these nanomaterials is the ability to upscale its production, while closely preserving its tailored properties.

Despite being only at the dawn of FSP research, the applications and markets for these nanoparticles are so encouraging that pioneer start-ups based on the FSP process are already in operation. Quite recently, Johnson Matthey Co., a major manufacturer of catalysts, announced their systematic exploration of FSP-made catalysts at its Research Center in UK.³⁴ Many opportunities await discovery, both in particles design as well as their applications. The key element lies in the continuous integration and bridging of knowledge between combustion engineering, aerosol technology and materials science. An inevitable complexity lies in the specificity of every new materials system.

Hence this review presents up-to-date knowhow in FSP, highlighting the many innovations in this area, from reactor design (Section 2) to particles design and synthesis (Section 4). Through this, it aims to provide a clearer, and more importantly, a holistic view on the relationship between synthesis properties and the resultant particle properties (see Fig. 1). Intrinsic properties specific to the precursor formulation (covered in Section 3) such as enthalpy of combustion, solvent–metal precursor specificity and combustion environment (excess or restrictive oxygen) are some of the governing factors, while externally tunable parameters such as the rate of enthalpy of combustion, quenching rate as mediated by natural heat loss or through external quenching unit, thermophoretic gradient, mixing stages of precursors etc. define the extrinsic properties. Understanding the intricate inter-relationship between synthesis properties and the limiting boundaries of flame profile, particle formation, materials chemistry and thermodynamics is critical towards the fabrication of functional nanomaterials with specific characteristics. These include the product particles size, homogeneity, morphologies, configurations and crystallite properties.

Fig. 1 General paradigm and strategy for the synthesis of functional nanoparticles by FSP, from conceptual design to targeted end applications, taking into account the tunable synthesis parameters and their respective boundary conditions in achieving the product particles with desirable characteristics.

It should be reiterated here that the current manuscript reviews exclusively FSP, with only brief accounts of VAFS and FASP that will be provided in Section 2, in view of their relevance to FSP. Readers interested in further information on other flame aerosol techniques are referred to reviews by Pratsinis⁵ which covered extensively the fundamentals and the then state-of-the-art in VAFS of ceramics nanoparticles, and Mädler⁶ on the spray aerosol techniques (with and without flame). Since the general applications of flame-made materials have been reviewed earlier by Strobel et al.,^2,7 they shall not be repeated here. Rather, this review focuses on the general “art and science” of nanoparticle fabrication by FSP based on the most current knowledge in the area.

2. Flame aerosol reactors and flame spray pyrolysis

The origin of flame aerosols for useful materials can be traced as far back to ancient lampblacks production by which vegetable oils were combusted in oxygen-starved flames, yielding carbon soot fume.³ For centuries, this was the dominant form of carbon black, a rather coarse material probably due to prolonged and uneven residence times⁵ in these rather primitive pyrolysis reactors. Much later, the discovery of natural gas in US and gasification of tars in Europe enabled the production of carbon blacks with much finer texture to meet the demand as tire reinforcing agents.⁴ The so-called gas black or channel black was made by a VAFS process whereby gaseous hydrocarbons (natural gas) were combusted in gas jets and soot collected on water-cooled channel surfaces – justifying the name of “channel blacks” (Fig. 2, I). The high demand for carbon blacks driven by the growing tire market before and during World War II further motivated the development and scale-up of today's FSP “furnace process”. Here, liquid hydrocarbon fuel sprays, rather than natural gas, were combusted within a closed furnace, followed by control-quenching of the oxygen-starved flames by water sprays to yield “furnace blacks”⁴ (Fig. 2, III).

Fig. 2 Evolution of aerosol flame reactors, including FSP reactor and its modifications. (I) 1940s: Early industrial-scale production of carbon blacks as commodity particles – “channel black process”. (II) 1940–50s: Production of fumed SiO₂ (1943) from the flame hydrolysis of metal halide vapours (picture showing the first Aerosil production plant,³⁶ source: courtesy of Evonik Degussa GmbH). The process further led to the manufacture of other commodity oxide ceramics such as Al₂O₃ (1953) and TiO₂ (1954) using the same vapour-type flame technique. (III) 1970s: Furnace black process using a modern “jet-type” flame reactor. Figure adapted from ref. 36. (IV) 1977: First reported development of ultrasonic-assisted Flame Spray Pyrolysis by Sokolowski et al.,¹⁰ where metal acetylacetonate in organic solvent was used as the liquid precursor. (V) 1996/97: Gas-assisted atomiser FSP reported by Bickmore et al.²⁸ and Karthikeyan et al.⁶⁴ mark the pioneering of systematic studies in FSP. Figure adapted with permission from ref. 28. (VI) 2002: Pressure-assisted FSP reported by Mädler et al.⁶⁵ that further led to systematic studies of various simple to complex metal oxides and metal/metal oxide systems using similar type reactor. (VII) 2004: A one-step film deposition technique by thermophoretic coating that further led to a series of supported catalysts and gas-sensor fabrications.⁷⁷ (VIII) 2005: A flame height-selective rapid-quenching technique to control precisely the metal deposit size on metal oxide support.⁷⁰ (IX) 2006: Reducing FSP by limiting the ambient O₂ in flame reactor that allows production of metallic and carbon-coated nanoparticles.²⁴ (X) 2006: A multi-nozzle FSP to synthesise multi-component particles with controllable phase segregation.⁸⁹ (XI) 2008: In situ sequential coating of preformed FSP nanoparticles by introducing secondary metal vapour via a torus pipe ring with multiple hollow openings.⁹⁴ Note: Figures are adapted with permissions from the respective references unless otherwise stated.

At about the same time, the production of fumed SiO₂ by VAFS was conceived where the vapour phase SiCl₄ was hydrolysed in an oxy-hydrogen flame yielding pristine silica aerosols (Fig. 2, II).³⁵ This patented process is known as the Aerosil process, after its commercial product name. Within 10 years after the first fumed SiO₂ was produced, the manufacture of fumed Al₂O₃ (from AlCl₃) and TiO₂ (from TiCl₄, chloride process) by VAFS followed, demonstrating the versatility of the flame process for product ceramic particles.³⁶ A very important innovation of the Aerosil-like process is the commercial manufacture of lightguides for telecommunications, where SiO₂–GeO₂ particles (from SiCl₄ and GeCl₄) were flame deposited directly onto and into substrate rods and subsequently extruded into continuous thin optical fibers of extremely high purity.³⁷ Synthesis of other ceramic oxide particulates by VAFS e.g. UO₂, SnO₂, Fe₂O₃, ZrO₂, ZnO and its composites,^3,38 carbon-coated oxides^39,40 and noble metal/metal oxides⁴¹ have also been reported from their respective halide, carbonyl or acetylacetonate vapours.

Despite its early industrial significance, very little was understood regarding particle formation within VAFS. It was only in the late 80s that Dobbins and Megaridis⁴² introduced the concept of thermophoretic sampling and essentially opened up the “black box” of particle formation in vapour-fed flames. The rapid insertion and retraction of a TEM (transmission electron microscopy) copper grid into the particle-laden flame allows direct sampling of the evolution of particle morphology along the flame axis, from spherical particles to fractal-like aggregates. From there, quantitative description of the flame particles growth by coagulation and sintering, including population balance and coalescence models⁴³ contributed decisively to the development of computational tools for the design of VAFS reactors – to the point that particle sizes could be predicted from first principles without any adjustable parameters in premixed flames.⁴⁴ Further accounting for the detailed particle residence time distribution by interfacing computational fluid mechanics with particle dynamics in diffusion flames⁴⁵ allowed for quantitative understanding of a number of experimental observations: The effect of process variables (flame temperature, reactant concentration, mixing or flowrate) on product particle size distributions (example see ref. 46).

While VAFS was undoubtedly a milestone achievement in terms of flame aerosol synthesis, its usage of metal vapours is rather demanding. The continuous demands for process versatility require the replacement of metal vapour precursor, which can often be costly, difficult to handle or in many cases, simply not available. This prompted the spraying of liquid precursors directly into the flame i.e. flame-assisted spray pyrolysis (FASP). The strategy uses less volatile and economical precursors, particularly those of aqueous metal salts (e.g. nitrates and acetates). Since the precursor exists as liquid, an additional dispersion nozzle feature by ultrasonication or pressure-assistance, is necessary prior to combustion. Many of these precursors, especially aqueous nitrates have low combustion enthalpy or endothermic in nature and as such do not form self-sustaining flames. Hence an external flame source, usually oxy-hydrogen or oxy-hydrocarbon, is required to support the combustion.⁵ One of the disadvantages of FASP is the yield of submicron particles with inhomogeneous morphology and size especially when operating at low rates of combustion enthalpy (defined as the ratio of combustion rate (kJ min⁻¹) to total gas flow (g_gas min⁻¹), <4.7 kJ g⁻¹_gas),⁴⁷ where there is insufficient energy for the formation of metal vapour (see Section 3). In such cases, dense particles are formed as a result of drying and solid-state densification⁴⁸ while hollow particles are due to precipitation on surface droplets prior to evaporation of the carrier liquids.^49–52 The many types of FASP-synthesised particles include simple metal oxides – TiO₂,⁵³ ZnO,⁵⁴ perovskites – SrMnO₃, NiMn₂O₄,⁵⁵ La_1−xCe_xCoO₃,⁵⁶ La(Co, Mn, Fe)O₃^57,58 and doped-metal oxides Y₂O₃ : Eu³⁺,^{14,50,52,59–61} BaMgAl₁₀O₁₇ : Eu²⁺.^62,63

In an FSP process, the exclusive use of highly exothermic liquid precursors (hereby referred to the mixture of metal precursor and solvent), particularly those based on organic solvents gives rise to self-sustaining flames. First introduced by Sokolowski et al.¹⁰ (Fig. 2, IV) who reported the synthesis of Al₂O₃ nanoparticles by combusting an ultrasonically-dispersed spray of aluminium acetylacetonate in benzene–ethanol. The concept was only further developed two decades later by Bickmore et al.²⁸ who used a single pilot-scale FSP to synthesise spinel MgAl₂O₄, while Karthikeyan et al.⁶⁴ reported the synthesis of Al₂O₃, Mn₂O₃, ZrO₂ and Y₂O₃–ZrO₂ using a multiple diffusion-flamelet spray gun (Fig. 2, V). Pressure-assisted FSP ignited by six surrounding oxy-methane flamelets was introduced by Mädler et al.⁶⁵ and was very quickly superseded by an oxy-methane ring.⁶⁶ A variant of the more general FSP (or FASP, depending on its combustion enthalpy criteria), the emulsion combustion (ECM) utilises an organic solvent–water metal precursor stabilised in an emulsion phase.^6,67 The presence of incombustible water phase in the ECM precursor not only lowers the combustion enthalpy density, but may also result in poorer gas-phase mixing and reaction as compared to a homogenous solvent-based only precursor, thereby affecting the particles homogeneity.⁶⁷

An integral part of the FSP reactor design is the aerosols collection, which can be accomplished either by electrostatic precipitation or vacuum-assisted filtration. In the latter case, fume nanoparticles are collected on a range of high temperature resistant porous media, ranging from glass fiber⁶⁵ or PTFE membrane⁶⁸ sheet filters, to Gore-Tex⁶⁷ or PTFE-coated fiber⁶⁹ bag type filters.

While the design of FASP and FSP reactors bear many similarities (and in fact may be inter-transferable), the core difference between the two classifications does not exactly lie in the reactor design, but more essentially in the origin of majority combustion enthalpy content of the system. The bulk of flame enthalpy in FSP originates from the combustion of liquid precursor, whereby the external flame serves only as an ignition source. This provides direct control over primary particle size by controlling the combustion enthalpy rate and metal concentration of the precursor alone.^65,66 More details on particle formation will be presented in Section 3, considering also the possibilities of different types of particle configurations.

Because of the highly exothermic nature of FSP liquid precursors, flame temperatures of up to 2600 K^65,70 or 2800 K⁷¹ have been measured. At the same time, the high gas velocities of the FSP induce radial entrainment of surrounding gas.⁷² Coupled with the radiation heat loss, this gives rise to extremely short residence times (milliseconds) with high temperature gradients (170 K cm⁻¹) along the flame axis.⁷⁰ The interplay between high temperature and large temperature gradient is one of the most important features in FSP: the high local temperature promotes the formation of homogeneous and highly crystalline materials and also promotes particle growth by sintering and coalescence,⁶⁵ while the large temperature gradient (and short residence time) preserves the nanoscale feature of the particles. In fact, the rapid thermal quenching during FSP has resulted in the formation of many metastable materials such as amorphous Ta₂O₅ up to 45 wt.% in silica composites,²⁰ Y₃Al₅O₁₂,⁷³ monoclinic BaCO₃,⁷⁴ cubic and tetragonal γ-Fe₂O₃.^22,23 On an industrial scale, although the FSP process for ceramic materials is yet to see an equally large-scale implementation as VAFS with productions of tons h⁻¹, it is making its way into the market. Scalability and reproducibility have been demonstrated at pilot scale production rates of 300 to 1100 g h⁻¹ (SiO₂,⁶⁹ ZrO₂^71,75,76), although FSP industrial units might already run at higher rates.

Among the extensions to the basic FSP synthesis of ceramic oxide nanoparticles, direct deposition of particulate films is perhaps one of the most extensively explored. This technique was demonstrated by Karthikeyan et al.⁶⁴ by the natural deposition of flame aerosols (i.e. absence of phoresis) onto grit-blasted stainless steel substrate. Thermophoretic deposition was later developed for the direct fabrication of functional porous layers as Au/TiO₂ catalytic microreactors⁷⁷ and SnO₂-based gas sensors¹⁷ (Fig. 2, VII). Here, deposition is driven by the temperature difference between the cooled masked substrate⁷⁸ and the gas temperature at which the preformed open aerosol agglomerates are suspended. One of the processing advantages of this in situ technique is the independent nanoparticle synthesis and the film thickness control, where the former depends on the flame synthesis properties while the latter is a function of deposition time. Strictly speaking, this is different conceptually from the combustion chemical vapour deposition (CCVD) where particles are formed upon nucleation of metal vapour onto a temperature-controlled substrate, much alike a conventional CVD process, except the metal vapour was generated through the combustion of liquid or gaseous precursors.⁷⁹ In CCVD, the substrate properties i.e. temperature and material affect strongly the nucleation and subsequent particle growth, while particles are preformed in the in situ FSP deposition. Typically, the FSP-deposited particulate films consist of agglomerates with D_f ∼ 1.7–1.8 (as a result of Brownian coagulation) and are highly porous (>95% porosity). It has been shown theoretically that it is possible to manipulate the film profile, which is an interdependency of film thickness and porosity, by varying primary particle size, aggregate size, morphology and thermophoretic temperature difference.⁸⁰ The reasons are the depth of penetration into the film of newly arriving particles during deposition, which is a function of the migration velocity and the particle diffusion coefficient as well as the particle/aggregate structure.

Sequential deposition of different aerosol materials gives rise to multilayer films,^81,82 each of which can be in principle designed with controlled thickness and morphologies (Fig. 3). The direct immobilisation technique has also been extended to vacuum-assisted flame coating of aerosols onto the external and internal surfaces of porous substrates e.g. ceramic foams.⁸³ Impinging flame-annealing is a film modification technique. Depending on the thermal stability of the inter-particulate bridge, direct annealing by the flame sinters the morphology of the lacy-like porous film to one consisting of dense cauliflower-like aggregates (porosity 62%).⁷⁸ The wider gaps between cauliflowers were especially beneficial for the enhancement of analyte diffusion in gas sensor devices.⁷⁸ Similar films morphology could also be achieved using high temperature CCVD⁸⁴ and impinging VAFS techniques,⁸⁵ with similar theoretical concepts.

Fig. 3 Directly fabricated single- (A,C) and multi-layer sensing films (B,D) on ceramic substrate by FSP. The technique offers high flexibility in terms of layers composition as well as film thickness. Figure adapted with permission from ref. 82.

While most of FSP particles are synthesised in the form of metal oxides, the synthesis of metallic nanoparticles is possible under restrictive oxygen flame conditions.^24,86 To achieve this, the amount of O₂ present must be kept minimal (in inert gas-filled glove box), but sufficient for the complete combustion of hydrocarbon fuels (Fig. 2, IX). Further restricting the O₂ partial pressure can result in the formation of carbonaceous coating.²⁵ To date, the synthesis of metallic nanoparticles have only been restricted to highly electronegative metallic particles such as Bi (but unstable in air)⁸⁶ or particles which are protected by surface oxide or carbonaceous layer against further oxidations, such as C/Co,^24,25 C/Cu,⁸⁷ Co₃O₄/Co²⁴ or even Ni/Mo alloy with oxide shell.⁸⁸

Flame spraying of multicomponent elements in a single nozzle FSP configuration often limits the flexibility in obtaining a certain desirable particle configuration. For instance, uniformly dispersed Ba species within an Al₂O₃ matrix, rather than discrete oxide particles, were obtained when spraying both elements simultaneously in a single nozzle.⁸⁹ To achieve phase separation (i.e. reduced solubility), these aerosol particles (e.g. Ba and Al) have to be co-precipitated in separate flames and interfaced only at a certain angle for even interparticle aerosol mixing (Fig. 2, X). The flame temperature and remaining residence time at the point of mixing is critical to either induce delayed solid-state reactions or suppress any reactions between these discrete phases altogether. A key advantage of the concept is the independent control over particle synthesis (in terms of elements as well as their physical characteristics) at each nozzle. Particle systems that have been synthesised using the multi-nozzle, besides BaCO₃/Al₂O₃,⁸⁹ include Pt/Ba/Ce_xZr_1−xO₂⁹⁰ and carbon-supported Pt nanoparticles.⁹¹

In other circumstances, induced delay and sequential precipitation (rather than co-precipitation) of one of the metal components is required, which in this sense would render the multi-nozzle unsuitable. To do so, it is required that the secondary metal component be introduced into the flame, for example as vapour phase precursor – similar to VAFS, only after the precipitation of the primary phase particles (Fig. 2, XI).^92–94 The technique circumvents the high temperature solid-state reaction dependencies of multiple components in a co-oxidation configuration. More importantly, it allows the design of thin coating and even encapsulation by the secondary metal oxides phase, e.g. SiO₂-coated TiO₂⁹² and SiO₂-coated γ-Fe₂O₃.⁹⁵ As in the case of multi-nozzle configuration, the flame temperature and remaining residence time at which mixing of secondary component (i.e. coating precursor) is introduced determines the fate of this phase. Point of mixing at excessively high temperature and long residence time may induce solid-state reaction and uneven segregation of the two components,⁹⁴ while an excessively low temperature and insufficient residence time may result in incomplete formation of the oxide coating layer. In principle, the technique could also be applied to multi-layer coating by introducing more sequential stages.

3. Designing of liquid precursors

Formulation of liquid precursors is one of the more important steps to FSP particle synthesis. Selection of metal precursors and solvents with suitable combustion enthalpies, melting/decomposition temperatures, miscibility and chemical stability are intrinsic to the overall particles formation in the flame which in turn determine the resultant particle properties. Early formulations of FSP precursors were based on solid nitrates,^64,96,97 acetates^64,66 and acetylacetonates^98,99 as these were natural choices in direct extensions of FASP. While many of these precursors were economical and readily available commercially, they do not always yield the homogeneous morphology comprising of fine and dense particles. The low combustion enthalpy of some of these precursors coupled with their high melting/decomposition points can be disadvantageous, but as discussed below, some of these disadvantages can be circumvented by implementing the right processing conditions.

The combination of low combustion enthalpy density (<4.7 kJ g⁻¹_gas) and higher metal precursor melting/decomposition point relative to the solvent boiling point i.e. T_bp (solvent)/T_d/mp(precursor) < 1 during FSP results in the formation of inhomogeneous particles (Fig. 4).⁵⁴ Under these conditions, both micron- and nanoparticles co-exist (Fig. 4a,b) due to particle formation through the droplet-to-particle¹⁰⁰ and gas-to-particle routes,^5,101–104 respectively. The former arises from incompletely evaporated droplets as in classic spray pyrolysis,⁴⁸ while the latter from the supersaturation of metal vapour.⁹⁶ Hollow particles can even be formed if the metal precursor precipitates at the droplet surface during solvent evaporation. In the case where an impermeable shell is formed, internal pressure build-up during the evaporation of trapped solvent leads to fragmentation of the spherical shells.⁹⁶ Subsequent solid-state reactions and densification take place forming micron-sized metal oxides. In the case when FSP is operated at high combustion enthalpy densities, sufficient heat is provided to evaporate the precursor intermediate products to yield homogeneous fine particles, even at T_bp (solvent)/T_d/mp(precursor) < 1 (Fig. 4d).

Fig. 4 Morphology map of particles made by spray flames adapted and modified from ref. 47. The hollow symbols, within the broken line box, represent inhomogeneous and hollow particles: (a) CeO₂;⁷³ (b) Bi₂O₃,⁹⁶ while solid symbols represent solid and homogeneous particles: (c,f) SiO₂,⁴⁷ (d) CeO₂,⁷³ (e) Bi₂O₃.⁹⁶ All images are adapted with permissions from the respective references.

By definition, the criteria for obtaining dense and homogenous fine particles can be achieved as long as the metal precursor, in its pure form i.e. not dissolved in solvent, exists in liquid phase at ambient temperature i.e. T_mp(precursor) < T_bp(solvent) (Fig. 4c,f). Most metal alkoxides fall in this category, with the exception of some precursors e.g. aluminium propoxide, which exists as solid crystals. In general, metal alkoxides in the form of methoxides to pentoxides, are commonly used in FSP for a number of reasons: high volatilities, high combustion enthalpies, miscibility in many organic solvents, low viscosity and commercial availability (see Table 1). One drawback, besides price, is the general moisture sensitivity of metal alkoxides, rendering their handling and preparation in a humidity-free environment necessary. Very recently, a facile conversion of metal chloride (as WCl₄ and WCl₆) to moisture tolerant alkoxide was demonstrated by reacting with benzyl alcohol at room temperature and pressure.¹⁷⁹ Gaseous hydrogen chloride evolves during the alkoxylation reaction, thereby removing the chloride content.

Table 1 Selection of metal precursors reported for the FSP preparation of various oxide and non-oxide ceramic particles

Element	Metal precursor	Solvent	Particles	Additional remark	Reference
Li	Lithium–sodium–alumatrane–glycolate	Ethanol	Li-doped Na2O·xAl2O3	Refluxing of alumatrane + sodium hydroxide + lithium hydroxide hydrate (1 h), followed by N2 distillation to remove water and excess ethylene glycol	Sutorik et al.^105
	Lithium tert-butoxide	Tetrahydrofuran–toluene	Li–ZnO		Height et al.^106
		Tetrahydrofuran–xylene	LiMn2O4, Li4Ti5O12, LiFe5PO8		Ernst et al.^26
B	Tributyl boride	2-Ethylhexanoic acid	Borosilicate glass		Brunner et al.^107
					Vollenweider et al.^26
C	Organic solvent	Ethanol	BaCO3		Strobel et al.^74
		Mineral spirit–tetrahydrofuran	C–Co	Restricted oxygen flame, C2H2	Grass et al.^25
		Tetrahydrofuran	C–Cu	Restricted oxygen flame	Athanassiou et al.^87,88
		Tetrahydrofuran	C–Cu	Restricted oxygen flame, C2H2	Athanassiou et al.^109
		Xylene	Pt/C and C/Pt	Twin nozzles	Ernst et al.^91
		Xylene–2-ethylhexanoic acid	CaCO3		Osterwalder et al.^110
F	Trifluoroacetic acid	Xylene	Ca10(PO4)6(OH)2−xFx		Loher et al.^19
	Hexafluorobenzene	2-Ethylhexanoic acid	Bioglass		Brunner et al.^107
		Xylene	CaF2, SrF2, BaF2		Grass et al.^111
		Xylene	F-MOx (MOx = TiO2, ZrO2)		Teoh et al.,^112 Emeline et al.^113
Na	Lithium–sodium–alumatrane–glycolate	Ethanol	Li-doped Na2O·xAl2O3	See Li	Sutorik et al.^105
	Sodium 2-ethylhexanoate	2-Ethylhexanoic acid–xylene	NaCl	Sodium hydrogencarbonate + ethylhexanoic acid	Grass and Stark^111
		Acetonitrile, HMDSO, tributyl phosphate, tributyl borate, fluorobenzene	Soda-lime glass		Brunner et al.,^107 Vollenweider et al.^108
Mg	Magnesium 2-ethylhexanoate	2-Ethylhexanoic acid	Mg–Ca3(PO4)2	Magnesium oxide + 2-ethylhexanoic acid	Loher et al.^19
	magnesium acetate tetrahydrate	Methanol	Ni:MgO–SiO2		Suzuki et al.^114
		Methanol–butanol	Mg2SiO4 : Cr		Tani et al.^115
		Methanol–water	MgO	Large and inhomogeneous MgO at high water content	Tani et al.^116
	Magnesium acetylacetonate	Tetrahydrofuran–ethanol	MgO–Al2O3	Dissolve metal precursor in solvent followed by filtration to remove ∼0.5 wt% of residual solids	Hinklin and Laine^33
	Magnesium pentanedionato	Tetrahydrofuran–ethanol	MgO–Fe2O3, MgO–Al2O3		Hinklin et al.^117
Al	Alumatrane	Ethanol	3Al2O3.2SiO2 (mullite)	Aluminium hydroxide hydrate + triethanolamine + ehtylene glycol (N2 distillation, 3–4 h to remove water and ethylene glycol). Product was recovered by vacuum filtration	Baranwal et al.^29
			Al2O3:(Ce,Pr)		Williams et al.^118
			CoOx–Al2O3		Azurdia et al.^31
			NiO–Al2O3		Azurdia et al.^32
			ZrO2–Al2O3		Kim and Laine^119
			(CeOx)x(Al2O3)1−x		Kim et al.^120
		Ethanol–tetrahydrofuran	Al2O3		Hinklin et al.^121
			MgO–Al2O3		Hinklin and Laine^33
		Ethanol, tetrahydrofuran–ethanol, aqueous ethanol	Y3Al5O12		Marchal et al.^122
		Methanol, butanol	TiO2/Al2O3		Kim et al.^123
	Aluminium 2-ethylhexanoate	2-Ethylhexanoate–toluene	Al2O3/ZrO2	Aluminium 2-ethylhexanoate basic + 2-ethylhexanoic acid followed by N2 distillation to remove water	Jossen et al.^76
		Acetic acid anhydride–2-ethylhexanoic acid	Al2O3/CexZr1−xO2	Aluminium 2-ethylhexanoate (16.5% Al2O3) + acetic acid/2-ethylhexanoic acid (N2 reflux, 2 h) followed by distillation to remove acetic acid	Schulz et al.^124
		Tetrahydrofuran	Hastelloy		Athanassiou et al.^88
	Aluminium acetylacetonate	Ethanol–benzene	Al2O3		Sokolowski et al.^10,98
		Ethanol–tetrahydrofuran	Al2O3	Heating to 50 °C for 1 h, and kept at ∼50 °C	Hinklin et al.^121
		Methanol–acetic acid	CuaMgbAlcOx		Haider and Baiker^125
			M/Al2O3 (M = Rh, Pt–Rh–Ru, Rh–Ru or Pt–Rh)		Hannemann et al.,^126 van Vegten et al.^127
	Aluminium chloride	Tetrahydrofuran–ethanol	Al2O3		Hinklin et al.^121
	Aluminium sec-butoxide	Butanol–2-propanol	Al2O3		Tani et al.^97
		Diethylene glycol monobutyl ether/acetic anhydride	Pt–Ba/Al2O3		Strobel et al.,^89 Piacentini et al.^128
		Xylene	Pd/Al2O3		Strobel et al.^129
			Pd/Al2O3 (film)	In situ deposition	Sahm et al.^81,82
			Pd/La2O3/Al2O3		Strobel et al.^130
			Pt–Rh–Ru/Al2O3		Hannemann et al.^126
			SiO2-coated Al–TiO2, SiO2/Al2O3/TiO2	In situ sequential flame coating	Teleki et al.^92–94
		Xylene–acetonitrile	Pt–Pd/Al2O3		Strobel et al.^131
	Aluminium isopropoxide	Isopropanol	Al2O3
		Xylene–ethylacetate	Pt/Al2O3	Maintained at 50 °C to prevent precipitation	Strobel et al.^132
	Aluminium nitrate nanohydrate	2-propanol–methanol	Al2O3		Tani et al.^97
		Deionised water–kerosene–surfactant (hexa(2-hydroxy-1,3-propylene glycol) diricinoleate)	Al2O3	Emulsion precursor	Tani et al.^97
		Ethanol, butanol	Y3Al5O12		Marchal et al.^122
		Isopropanol	Al2O3		Karthikeyan et al.,^64 Tikkanen et al.^133
		Tetrahydrofuran–ethanol	Al2O3		Hinklin et al.^121
	Lithium–sodium–alumatrane–glycolate	Ethanol	Li-doped Na2O·xAl2O3	see Li	Sutorik et al.^105
	Magna-alumatrane glycolate	Ethanol	MgO–Al2O3	Aluminium hydroxide hydrate + Magnesium hydroxide hydrate + triethanolamine + ehtylene glycol (200 °C distillation to remove ethylene glycol and water).	Bickmore et al.^28
	Al2O3 (predominantly sigma, gamma, theta-phase)	Ethanol	alpha-Al2O3	Flame spraying of slurries	Laine et al.^134
Si	Hexamathyldisiloxane (HMDSO)	2-Ethylhexanoic acid	Bioglass		Brunner et al.,^107 Vollenweider et al.^108
		Acetic acid–methanol	SiO2, SiO2/ZnO		Mädler et al.,^135 Tani et al.^136,137
		Deionised water–kerosene–hexa(2-hydroxy-1,3-propylene-gricol) diricinoleate	SiO2, SiO2/ZnO	Emulsion combustion (ECM), mixing at 10 000 rpm for 10 min	Tani et al.^67
		Ethanol	SiO2	Pilot-scale FSP	Mueller et al.^71
		Ethanol, iso-octane, methanol	SiO2		Mädler et al.^65
		Methanol	Ni:MgO–SiO2		Suzuki et al.^114
		Methanol–butanol	Mg2SiO4 : Cr		Tani et al.^115
		Methanol–hydrogen peroxide	SiO2, SiO2/ZnO		Tani et al.^67
		Toluene–2-ethylhexanoic acid	Ag/SiO2		Loher et al.^138
		Xylene	SiO2/Al2O3/TiO2		Teleki et al.^92–94
			SiO2/SnO2	In situ flame deposition-annealing	Tricoli et al.^139
		Xylene, pentane, hexane, octane, decane,dodecane, tetradecane, hexadecane	SiO2		Jossen et al.^47
	SiO2 sol	Deionised water–kerosene–hexa(2-hydroxy-1,3-propylene-gricol) diricinoleate	SiO2, SiO2/ZnO	Emulsion combustion (ECM), mixing at 10 000 rpm for 10 min	Tani et al.^67
	TEA–Si–egH	Ethanol	3Al2O3.2SiO2 (mullite)	Silica + triethanolamine + ethylene glycol (N2 distillation, 3–4 h). TEA–Si–egH is filtered and washed with acetone. The product is mixed with alumatrane and further distilled (N2, 3–4 h) to remove excess ethylene glycol	Baranwal et al.^29
	Tetraethyl siloxane (TEOS)	2-Ethylhexanoic acid–toluene	SiO2/CexZr1−xO2		Schulz et al.^124
		2-Ethylhexanoic acid–toluene	SiO2/BiVO4		Strobel et al.^140
		Acetonitrile–acetic anhydride, xylene–acetic anhydride/2-ethylhexanoic acid	Yb2O3/SiO2		Mädler et al.^141
		Ethanol	Y2SiO5 : Eu^3+		Qin et al.^142
		Methanol–AcOH	ZnO–SiO2		Ramin et al.^143
		Pentane, hexane, dodecane, xylene, 2-ethylhexanoic acid–toluene (all under N2)	Ta2O5/SiO2		Schulz et al.^20
		Toluene	SiO2–V2O5–WO3–TiO2		Jossen et al.^144
			SiO2/ZrO2		Jossen et al.^83
		Xylene	M/SiO2(M = Au, Ag or Au–Ag)		Hannemann et al.^145
		Xylene–acetonitrile	SiO2/Fe2O3		Li et al.^22,146
	Tetramethylsilane (TMS), HMDSO, tetramethyl orthosilicate (TEMOS), tetrepropyl orthosilicate (TEPOS)	Xylene	SiO2		Jossen et al.^54
P	Tributyl phosphate	2-Ethylhexanoic acid	Ca10(PO4)6(OH)2−xFx, Ca3(PO4)2, M–Ca3(PO4)2 (M = Mg, Zn)		Loher et al.^19,147
		2-Ethylhexanoic acid–toluene	Ag/Ca3(PO4)2		Loher et al.^138
		Xylene	FePO4		Rohner et al.^21
S	Dimethyl sulfoxide	Xylene	CaSO4		Osterwalder et al.^110
Cl	Hexachlorobenzene	2-Ethylhexanoic acid	NaCl		Grass and Stark^111
Ca	Calcium 2-ethylhexanoate	2-Ethylhexanoic acid	Ca10(PO4)6(OH)2−xFx, Ca3(PO4)2, M–Ca3(PO4)2 (M = Mg, Zn)	Calcium oxide + 2-ethylhexanoic acid	Loher et al.^19,146
				Calcium hydroxide + 2-ethylhexanoic acid	Brunner et al.^147
		2-Ethylhexanoic acid–toluene	Ag/Ca3(PO4)2	Calcium hydroxide + 2-ethylhexanoic acid	Loher et al.^138
		2-Ethylhexanoic acid–xylene	CaO, CaCO3, CaSO4	Calcium hydroxide + 2-ethylhexanoic acid (140 °C, 3 h)	Osterwalder et al.^110
			Glass and bioglass		Brunner et al.,^107 Vollenweider et al.^108
Ti	Titanatrane		TiO2	TiO2 in ethylene glycol (EG) + triethanolamine (200 °C, 8 h) followed by distillation to remove EG. Resultant precursor is hydrolytically stable	Bickmore et al.^149
		Methanol, butanol	TiO2/Al2O3		Kim et al.^123
	Titanium tetraisopropoxide (TTIP)	Propionic acid, 2-ethylhexanoic acid, methanol, xylene, pyridine	Au/TiO2		Chiarello et al.^150
		Toluene	SiO2–V2O5–WO3–TiO2		Jossen et al.^144
		Xylene	Nb–TiO2, Cu–TiO2		Teleki et al.^151
			M/TiO2 (M = Au, Ag or Au–Ag)		Hannemann et al.^145
			SiO2-coated Al–TiO2, SiO2/Al2O3/TiO2	In situ sequential flame coating	Teleki et al.^92–94
		Xylene–acetonitrile	TiO2		Teoh et al.,^151–154 Teleki et al.^155
			Pt/TiO2		Schulz et al.,^70 Teoh et al.^152,154
			Fe–TiO2		Teoh et al.^156
			V2O5/TiO2		Schimmoeller et al.^83
			Ag/TiO2		Gunawan et al.^157
		Xylene–tetrahydrofuran	Li4Ti5O12		Ernst et al.^26
V	Vanadium oxo-triisopropoxide	Toluene	SiO2–V2O5–WO3–TiO2		Jossen et al.^144
		Xylene–acetonitrile	V2O5/TiO2		Schimmoeller et al.^83
	Vanadyl naphthenate	Toluene–2-ethylhexanoic acid	BiVO4 and SiO2/BiVO4		Strobel et al.^140
Cr	Chromium 2-ethylhexanoate	Mineral spirit–tetrahydrofuran	Hastelloy	Restricted oxygen flame	Athanassiou et al.^88
	Chromium acetate	Methanol/butanol	Mg2SiO4 : Cr		Tani et al.^115
	Chromium acetylacetonate	Diethylene glycol monbutyl ether/ethanol	Cr–WO3		Wang et al.^158
Mn	Manganese 2-ethylhexanoate	Mineral spirit–tetrahydrofuran	Hastelloy	Restricted oxygen flame	Athanassiou et al.^88
		Xylene–tetrahydrofuran	LiMn2O4		Ernst et al.^26
	Manganese acetylacetonate	Xylene–tetrahydrofuran	LiMn2O4		Ernst et al.^26
	Manganese nitrate	Isopropanol	Mn2O3		Karthikeyan et al.,^64 Tikkanen et al.^133
Fe	Iron 2-ethylhexanoate	Tetrahydrofuran	Hastelloy	Restricted oxygen flame	Athanassiou et al.^88
	Iron acetylacetonate	Xylene	FePO4		Rohner et al.^21
		Xylene–acetonitrile	Fe2O3, SiO2/Fe2O3		Li et al.^22,23,146
	Iron naphthenate	Mineral spirit–xylene	M/Fe2O3/Fe3O4 (M = Au, Ag or Au–Ag)		Hannemann et al.^145
		Mineral sprit–xylene/acetonitrile	Fe–TiO2		Teoh et al.^156
		Mineral sprit–xylene–tetrahydrofuran	LiFe5PO8		Ernst et al.^26
	Iron(III) nitrate	Ethanol	Nd : Co : Fe2O3		Dosev et al.^159
	Iron propionate	Ethanol	(MgO)x(Fe2O3)1−x		Hinklin et al.^117
Co	Cobalt 2-ethylhexanoate	2-Ethylhexanoic acid	Co-soda lime glass		Brunner et al.^107
		Mineral spirit–tetrahydrofuran	Co3O4, CoO, fcc–Co	Restricted oxygen flame	Grass and Stark^24
			C–Co	Restricted oxygen flame, C2H2	Grass et al.^25
			Co nanowire	External magnetic field	Athanassiou et al.^109
		Xylene	Co3O4, Co3O4–ZrO2, Ru–Co3O4–ZrO2		Teoh et al.^160
	Cobalt acetate tetrahydrate	Carboxylic acids (acetic acid, propionic acid, butyric acid, hexanoic acid, octanoic acid or 2-ethylhexanoic acid)	LaCoO3		Chiarello et al.^161–163
		Propionic acid–propanol–water	Pd/LaCoO3	Vigorous stirring at 60 °C	Chiarello et al.^164,165
	Cobalt nitrate tetrahydrate	Ethanol	Nd : Co : Fe2O3		Dosev et al.^159
		Propionic acid + alcohol (methanol, ethanol, 1-propanol)	LaCoO3		Chiarello et al.^166
	Cobalt propionate	Ethanol	Co3O4, NiO–Co3O4	Reactive distillation of cobalt nitrate hexahydrate + propionic acid under N2 sparging (150 °C, 6 h) to remove water, propionic acid and NOx	Azurdia et al.^167
		Ethanol–propionic acid	CoOx–Al2O3	Cobalt nitrate hydrate + propionic acid under N2 sparging followed by distillation (150 °C, 6 h) to remove water and NOx	Azurdia et al.^31
Ni	Nickel 2-ethylhexanoate	Tetrahydrofuran	Ni/Mo alloy		Athanassiou et al.^88
	Nickel acetate	Methanol	Ni:MgO–SiO2		Suzuki et al.^114
	Nickel propionate	Ethanol	NiO, NiO–MOx (Mox = Co3O4, MoO3, CuO)	Nickel nitrate hexahydrate + propionic acid under N2 sparging and distillation (150 °C, 4h) to remove water, propionic acid and NOx	Azurdia et al.^167
		Ethanol, ethanol–methanol, ethanol–butanol	NiO–Al2O3	Reactive distillation of nickel nitrate hexahydrate + propionic acid under N2 sparging (150 °C, 6 h) to remove water, propionic acid and NOx	Azurdia et al.^32
Cu	Copper 2-ethylhexanoate	Tetrahydrofuran	C–Cu	Restricted oxygen flame	Athanassiou et al.^87
		Xylene	Cu–TiO2		Teleki et al.^151
			Cu–SiO2		Height and Pratsinis^168
		Xylene, xylene–2-ethylhexanoic acid	CuO, Cu–MOx (MOx = SiO2, Al2O3, TiO2, CeO2,ZrO2)		Kydd et al.^169,170
	Copper nitrate trihydrate	Acetic acid–methanol	CuaMgbAlcOx		Haider and Baiker^125
	Copper propionate	Ethanol	CuO, NiO–CuO	Reactive distillation of copper nitrate hydrate + propionic acid under N2 sparging (150 °C, 6 h) to remove water, propionic acid and NOx	Azurdia et al.^167
Zn	Zinc acetate dihydrate	Methanol–water	ZnO	Large and inhomogeneous ZnO at high water content	Tani et al.^116
	Zinc acetylacetonate	Methanol–AcOH	ZnO–SiO2		Ramin et al.^143
		Colloidal SiO2 (Ludox)–Methanol–AcOH	ZnO–SiO2	Flame spraying of slurries	Ramin et al.^143
	Zinc acrylate	Acetic acid–methanol	SiO2/ZnO		Mädler et al.,^135 Tani et al.^136,137
			ZnO		Tani et al.^171
	Zinc napthenate	2-Ethylhexanoic acid	Zn–Ca3(PO4)2		Loher et al.^19
		Mineral spirit–ethanol	ZnO, Ag/ZnO		Height et al.^172
		Mineral spirit–toluene	ZnO, M–ZnO (M = In, Sn, Li)		Height et al.^106
Y	Yttrium 2-ethylhexanoate	2-Ethylhexanoic acid–ethanol	Y2O3/ZrO2	Yttrium nitrate hexahydrate + ethanol + 2-ethylhexanoic acid (N2 distilled to remove ethanol, water, NO)	Jossen et al.^76
		2-Ethylhexanoic acid–toluene	Y2O3/ZrO2	Yttrium nitrate hexahydrate + ethanol + 2-ethylhexanoic acid (120 °C distillation to remove ethanol, water, NO)	Jossen et al.^75
		Toluene	Y2O3 : Eu^3+	Yttrium nitrate hexahydrate + aqueous ammonia to form Y(OH)3 followed by refluxing with 2-ethylhexanoic acid + acetic acid (65 °C, 4 h)	Camenzind et al.^12,13
		Tetrahydrofuran–ethanol	Y3Al5O12		Marchal et al.^122
	Yttrium acetate	Aqeuous acetic acid	Y2O3/ZrO2		Karthikeyan et al.^64
	Yttrium acetylacetonate	Ethanol	Y3Al5O12		Marchal et al.^122
	Yttrium butoxide	Toluene	Y2O3/ZrO2		Jossen et al.^76
	Yttrium methoxyacetate	Aqueous ethanol	Y3Al5O12	Yttrium trichloride + methoxyacetic acid (reflux under N2,135 °C, 2 h)	Marchal et al.^122
	Yttrium nitrate hexahydrate	Ethanol	Y2O3/ZrO2	Inhomogeneous and segregated Y2O3	Jossen et al.^76
			Y2O3 : Eu^3+		Dosev et al.^14
			Y2SiO5 : Eu^3+		Qin et al.^142
		Ethanol, butanol	Y3Al5O12		Marchal et al.^122
		Ethanol–acetic anhydride	Y2O3/ZrO2	Slow addition of acetic anhydride under N2. Resulting NOx is bubbled through NaOH. Inhomogeneous and segregated Y2O3	Jossen et al.^76
	Yttrium propionate	Ethanol, tetrahydrofuran	Y3Al5O12	Yttrium nitrate hexahydrate + propionic acid (N2,145 °C)	Marchal et al.^122
Zr	Zirconyl 2-ethylhexanoate	2-Ethylhexanoic acid	Pt/CexZr1−x		Stark et al.^173
		2-Ethylhexanoic acid–toluene	MOx/CexZr1−xO2(MOx = SiO2, Al2O3)	Acetic acid digested zirconium carbonate + 2-ethylhexanoic acid + cerium acetate hydrate (N2 distilled to remove water and acetic acid)	Schulz et al.^124
			Pt/Ba/CexZr1−xO2	Twin nozzles	Strobel et al.^131
		Ethanol	Y2O3/ZrO2	Zirconium carbonate hydroxide oxide dissolved in acetic acid (50 °C) + 2-ethylhexanoic acid followed by distillation (120 °C) to remove water and acetic acid	Jossen et al.^76
		Xylene	Rh/CexZr1−xO2		Hotz et al.^174
	Zirconium acetate	Aqueous acetic acid	Y2O3/ZrO2		Karthikeyan et al.^64
	Zirconium butoxide	n-Butanol	ZrO2		Karthikeyan et al.^64
	Zirconium propionate	Ethanol	ZrO2–Al2O3	Zirconium carbonate + 2-ethylhexanoic acid followed by distillation (120 °C, 2 h) to remove water and propionic acid	KimandLaine^119
	Zirconium propoxide	Isopropanol–ethanol	Y2O3/ZrO2		Jossen et al.^76
		Isopropanol–toluene	MOx/ZrO2 (MOx = SiO2, La2O3, CeO2, Y2O3, Al2O3)		Jossen et al.^75
		Isopropanol–xylene	ZrO2, Co3O4–ZrO2,Ru–Co3O4–ZrO2		Teoh et al.^160
	Zirconium tetraacetylacetonate	Isooctane/acetic acid/butanol	CeO2/ZrO2	Low boiling point solvent resulting in segregated ZrO2 and CeO2 phase	Stark et al.^175
		lauric acid/acetic acid	CexZr1−xO2	Heating to full dissolution	Stark et al.^175
Nb	Niobium 2-ethylhexanoate	Xylene	Nb–TiO2		Teleki et al.^151
Mo	Ammonium molybdate	Lactic acid	MoO3, NiO–MoO3	Dissolve ammonium molybdate in aquoeus lactic acid (heating >100 °C, 2h) followed by removal of excess water and acid in rotary evaporator	Azurdia et al.^167
	Molybdenum 2-ethylhexanoate	Mineral spirit–tetrahydrofuran	Ni/Mo alloy		Athanassiou et al.^88
Ru	Ruthenium acetylacetonate	Xylene	Ru–Co3O4–ZrO2		Teoh et al.^160
	Ruthenocen	Xylene, methanol–acetic acid	Pt–Rh–Ru/Al2O3, Rh–Ru/Al2O3		Hannemann et al.^126
Rh	Rhodium 2-ethylhexanoate	Xylene	Rh/CexZr1−xO2		Hotz et al.^174
	Rhodium acetylacetonate	Xylene, methanol–acetic acid	M/Al2O3 (M = Rh, Pt–Rh–Ru, Rh–Ru or Pt–Rh)		Hannemann et al.,^126 van Vegten et al.^127
Pd	Palladium acetate	Propionic acid–propanol–water	Pd/LaCoO3	Vigorous stirring at 60 °C	Chiarello et al.^164,165
	Palladium acetylacetonate	Xylene	M/Al2O3 (M = Pd, Pt–Pd)		Strobel et al.,^129,131 Sahm et al.^81,82
			Pd/La2O3/Al2O3		Strobel et al.^130
Ag	Silver acetate	2-Ethylhexanoate–toluene	Ag/Ca3(PO4)2, Ag/SiO2		Loher et al.^138
	Silver benzoate	Pyridine	Ag/MOx (MOx = SiO2, TiO2, Fe2O3/Fe3O4)		Hannemann et al.^145
		Xylene–TTIP	Ag/TiO2		Gunawan et al.^157
	Silver nitrate	Ethanol	Ag/ZnO		Height et al.^172
			Ag/TiO2		Keskinen et al.^176,177
In	Indium acetylacetonate	Toluene	In–ZnO		Height et al.^106
Sn	Tin 2-ethylhexanoate	Ethanol	SnO2 (film)		Sahm et al.^16
		Toluene	SnO2, Pd/SnO2, Pt/SnO2 (films)	In situ flame deposition	Mädler et al.,^17,18 Sahm et al.^81,82
			Sn–ZnO		Height et al.^106
		Xylene	SnO2, SiO2/SnO2	In situ flame deposition-annealing	Kuhne et al.,^178 Tricoli et al.^78,139
Ba	Barium(II) 2-ethylhexanoate	Ethanol	BaCO3		Strobel et al.^74
			Pt–Ba/Al2O3	Twin nozzles	Strobel et al.,^89 Piacentini et al.^128
		2-Ethylhexanoic acid–toluene	Pt/Ba/CexZr1−xO2	Twin nozzles	Strobel et al.^90
La	Lanthanum 2-ethylhexanoate	2-Ethylhexanoic acid–toluene	La2O3/ZrO2	Lanthanum(III) acetylacetonate hydrate + acetic anhydride + 2-ethylhexanoic acid (distilled under N2 to remove acetic acid, acetylacetonate, water)	Jossen et al.^75
	Lanthanum acetate dihydrate	Carboxylic acids (Acetic acid, propionic acid, butyric acid, hexanoic acid, octanoic acid, 2-ethylhexanoic acid)	LaCoO3		Chiarello et al.^161–163
		Propionic acid–propanol–water	Pd/LaCoO3	Vigorous stirring at 60 °C	Chiarello et al.^164,165
	Lanthanum isopropoxide	Xylene	Pd/La2O3/Al2O3		Strobel et al.^130
Ta	Tantalum ethoxide, tantalum butoxide, tantalum tetraethylacetonate	Pentane, hexane, dodecane, xylene, 2-ethylhexanoic acid–toluene (all under N2)	Ta2O5/SiO2		Schulz et al.^20
W	Ammonium tungstate hydrate	Diethylene glycol monbutyl ether–ethanol	WO3, Cr–WO3		Wang et al.^158
	Tungsten benzyl alkoxide	Benzyl alcohol	WO3	Vigorous stirring of WCl4 or WCl6 in benzyl alcohol for 8 h at 20 °C until pH 4.5. Gaseous HCl is liberated.	Pokhrel et al.^179
	Tungsten carbonyl	Tetrahydrofuran	WO3		Pokhrel et al.^179
	Tungsten ethoxide	Toluene	SiO2–V2O5–WO3–TiO2		Jossen et al.^144
Pt	Platinum acetylacetonate	2-Ethylhexanoic acid	Pt/CexZr1−xO2		Stark et al.^173
		2-Ethylhexanoic acid–toluene	Pt/Ba/CexZr1−xO2	Twin nozzles	Strobel et al.^131
		Ethanol	Pt–Ba/Al2O3	Twin nozzles	Strobel et al.,^89 Piacentini et al.^128
		Toluene	Pt/SnO2 (films)		Mädler et al.^17,18
		Xylene	Pt/C and C/Pt	Twin nozzles	Ernst et al.^91
		Xylene–acetonitrile	Pt/TiO2		Teoh et al.^152,154
		Xylene–ethyl acetate	Pt/Al2O3		Strobel et al.^132
		Xylene, methanol/acetic acid	M/Al2O3 (M = Pt–Rh–Ru, Pt–Rh)		Hannemann et al.^126
Au	Dimethyl gold(III)-acetylacetonate	Xylene	Au/MOx, Au–Ag/MOx (MOx = TiO2, SiO2, Fe2O3/Fe3O4)	Extremely moisture- and thermal-sensitive precursor	Hannemann et al.^145
		Xylene–pyridine	Au/TiO2		Chiarello et al.^150
	Gold chloride	Acetonitrile–2-ethylhexanoic acid	Au-soda lime glass		Brunner et al.^107
		Xylene–acetonitrile	Au/MOx (MOx = TiO2, SiO2)		Mädler et al.^180
	Gold-triphenylphosphine-nitrate	Tetrahydrofuran/isooctane	Au/TiO2 (film)		Thybo et al.^77
Bi	Bismuth 2-ethylhaxanoate	2-Ethylhexanoic acid–toluene	BiVO4 and SiO2/BiVO4		Strobel et al.^140
		Mineral spirit–tetrahydrofuran	Bi	Restricted oxygen flame	Grass and Stark^86
			CeO2/Bi		Grass et al.^181
	Bismuth acetate	Acetic acid, nitric acid and ethanol	Bi2O3	Bismuth trinitrate pentahydrate + acetic acid in an ethanol/nitric acid medium	Mädler and Pratsinis^96
	Bismuth trinitrate pentahydrate	Nitric acid–alcohol (alcohol = methanol, ethanol, methoxy propanol, ethoxy ethanol, propylene glycol propylether, diethylene glycol-monoethylether	Bi2O3		Mädler and Pratsinis,^96 Jossen et al.^47
Ce	Cerium 2-ethylhexanoate	2-Ethylhexanoic acid	Pt/CexZr1−xO2		Stark et al.^173
		2-Ethylhexanoic acid–toluene	Pt/Ba/CexZr1−xO2	Twin nozzles	Strobel et al.^131
		Xylene	Rh/CexZr1−xO2		Hotz et al.^174
	Cerium acetate hydrate	2-Ethylhexanoic acid–toluene	MOx/CexZr1−xO2 (MOx = SiO2, Al2O3)	Acetic acid digested zirconium carbonate + 2-ethylhexanoic acid + cerium acetate hydrate (N2 distilled to remove water and acetic acid)	Schulz et al.^124
		Acetic acid	CeO2	Inhomogeneous particle size	Mädler et al.^66
		Acetic acid–isooctane–butanol	CeO2	Homogeneous particle size	Mädler et al.^66
			CeO2/ZrO2	Low boiling point solvent resulting in segregated ZrO2 and CeO2 phase	Stark et al.^175
		Lauric acid–acetic acid	CexZr1−xO2	Heating to full dissolution	Stark et al.^175
	Cerium(III) nitrate	Ethanol	Al2O3 : Ce		Williams et al.^118
	Cerium(III) octoate	2-Ethylhexanoic acid–toluene	CexZr1−xO2		Jossen et al.^75
		Tetrahydrofuran	CeO2/Bi	Restricted oxygen flame	Grass et al.^181
	Cerium propionate	Ethanol	(CeOx)x(Al2O3)1−x	Reaction distillation of cerium carbonate with propionic acid (120 °C, 2 h, flowing N2)	Kim et al.^120
Pr	Praseodymium(III) nitrate	Ethanol	Al2O3 : Pr		Williams et al.^118
Nd	Neodymium(III) nitrate	Ethanol	Nd : Co : Fe2O3		Dosev et al.^159
Eu	Europium 2-ethylhexanoate	Toluene	Y2O3 : Eu^3+		Camenzind et al.^12,13
	Europium(III) nitrate	Ethanol	Y2O3 : Eu^3+		Dosev et al.^14
			Eu : Gd2O3		Goldys et al.^182
			Y2SiO5 : Eu^3+		Qin et al.^142
		Nd : Co : Fe2O3 in ethanol slurry	Eu : Gd2O3/Nd : Co : Fe2O3	Flame spraying of slurry	Dosev et al.^159
Gd	Gadolinium(III) nitrate	Ethanol	Eu : Gd2O3		Goldys et al.^182
		Nd : Co : Fe2O3 in ethanol slurry	Eu : Gd2O3/Nd : Co : Fe2O3	Flame spraying of slurry	Dosev et al.^159
Ho	Holmium 2-ethylhexanoate	2-Ethylhexanoic acid	Ho–BaF2	Holmium oxide in 2-ethylhexanoic acid	Grass and Stark^111
Yb	Ytterbium 2-ethylhexanoate	Xylene	Yb2O3/SiO2	Reactive distillation of yttrium nitrate + 2-ethylhexanoic acid (N2, 107 °C)	Mädler et al.^141
	Ytterbium nitrate pentahydrate	Acetonitrile–acetic anhydride–acetic acid	Yb2O3/SiO2		Mädler et al.^141

---

Low cost chemical conversion of metal oxides and hydroxides to soluble “trane” complexes is a robust alternative to preparing moisture insensitive metal precursors. Introduced by Laine and co-workers, the procedure involves moderate heating-distillation of solid metal oxides and/or hydroxides in triethanolamine (trane)/ethylene glycol under inert environment.²⁸ In the past, a range of trane complexes ranging from Ti,¹⁴⁹ Li, Na,¹⁰⁵ Ca, Ba, Sr,¹⁸³ Si,²⁹ Ce, Zr, Y, Yb, Er, Tm to Pr,^11,118 or even multi-metal tranes such as Sr–Si–Al, Ba–Si–Al and Al–Mg²⁸ were synthesised. A similar polymerisable complex or Pechini method is another promising technique for the conversions of metal oxide, carbonates or salts to liquid precursors.^184–188 It involves dissolving of single or multi-metal components in an acetic acid (as chelating agent)–ethylene glycol–methanol mixture followed by heating to polymerise the acetic acid and ethylene glycol. Both the trane and polymerised complex can be readily dissolved in alcohols.

Metal carboxylates¹⁸⁹ are another group of metal precursors with proven versatility and ease in handling (hydrolytically insensitive). The simplicity of carboxylic acids in forming favourable ligand exchange, especially with metal nitrates, is attractive.⁹⁶ Although short chain carboxylates such as acetates have low combustion enthalpy, its calorific value can generally be increased by increasing the carbon chain length. This prompted the synthesis of Co, Cu, Ni,¹⁶⁷ Y¹²² propionates or Y^12,13,75 and Yb¹⁴¹ 2-ethylhexanoates by reaction of the long chain alkanoic acids with metal nitrates (Table 1). Heating or refluxing of the metal nitrate–alkanoic acid mixture promotes ligand exchange. Besides reacting with metal nitrates, the reaction with 2-ethylhexanoic acid can also be extended to the conversion of other low cost materials such as acetates (Na, Ba, Sr, Bi),¹¹¹ oxides (Ca,^19,147 Mg,¹⁹ Ho¹¹¹), hydroxides (Ca,^110,148 Al^76,124) and even carbonates (Na,¹¹¹ Zr⁷⁶). The higher viscosity of 2-ethylhexanoic acid compared to water or ethanol has to be accounted for in nozzle design and liquid pump selection or dilution with e.g. xylene.

4. Particles formation and configurations by FSP

In the previous section, we identified the relevance of liquid precursor design to the formation of particles through the droplet-to-particle or gas-to-particle routes, the latter ensures homogeneous morphologies and sizes. In this section, we further review the different categories of particle configurations that may form in the gas-to-particle route in FSP. These particles form in sequential stages of: (1) precursor spray evaporation/decomposition forming metal vapour; (2) nucleation as a result of supersaturation; (3) growth by coalescence and sintering; and (4) particles aggregation (forming hard agglomerates by chemical bonds) and agglomeration (forming soft agglomerates by mainly physical bonds) (Fig. 5). Since the general gas-to-particle mechanism has been well-studied and reviewed in the past, it will not be repeated here, but readers are referred to previous literature for a more detailed understanding.^5,101–103 Despite the common route to particle formation under the classification of gas-to-particle, the uniqueness of every material system prohibits generalisation of each particle configuration predictably. Prior insights of the materials properties and thermodynamic need to be taken into account in relevance to the flame conditions. Fig. 5 summarises all the particle configurations (and combinations) obtained by FSP to date. It must be mentioned that the details of particles formation on most of these routes were based only on the most current conceptual understanding, rather than proven experimentally.

Fig. 5 The formation of different particle configurations via the gas-to-particle mechanism for single and multicomponent systems. Please refer to the text for details. Although not shown in the Figure, the stage of droplet spray should in the context of FSP, precede the metal vapour stage.

Flame spraying of single metal component precursors in open ambient (air) condition produces of simple oxides or carbonates in the case of alkaline metals through reaction with the CO₂-laden flame (Fig. 5, Route I). A wide range of elements across the periodic table, from MgO to Yb₂O₃, have been synthesised via this route (see Fig. 6 and Table 1). The high entrainment of ambient air⁷² during particle synthesis provides a continuous and excess supply of O₂ to the flame (oxidizing). Coupled with high flame temperatures, these are the reasons that minimal amount of carbon soot formed on the metal oxide surfaces in this process. The high temperature formation in the flame renders high thermal stability of the as-prepared particles as compared to other lower temperature wet techniques.^66,190,191

Fig. 6 Examples of as-prepared metal oxides of different morphologies made by FSP, varying from: (a) hexagonal/octagonal platelet γ-Fe₂O₃; (b) slightly oblonged ZnO; (c) spherical TiO₂; and (d) rhomboid-shaped CeO₂ with sharp edges.

Additionally, highly crystalline nanoparticles are often produced by FSP, as induced by the high flame temperature crystallisation. The polymorphic structures of some oxides, such as TiO₂,^152,192 Y₂O₃¹² and WO₃,^158,179 can be designed by capitalising the inter-relationship of flame synthesis and materials characteristics. Only in a few instances are amorphous materials such as that in SiO₂^65,135 and C⁹¹ obtained due to insufficient residence time and/or flame temperatures. Extension of the high temperature flame regime is achievable by enclosing the FSP flame in a quartz tube, where external entrainment of ambient air is prevented while conserving the convection heat within the tube.¹⁵⁵ Doing so extends the residence time at which aerosol formation and growth takes place. Besides obtaining larger particle sizes, the technique is unique in the reduction of crystal defects such as those achieved in enhanced magnetic moment of γ-Fe₂O₃^23,146 as well as the decreased electron capacitance of TiO₂ nanoparticles in dye-sensitised solar cells due to elimination of charge trapping sites.¹⁹¹

Oxygen containing anions such as phosphates^19,21 and sulfates (currently limited only to group II metal i.e. Ca²⁺)¹¹⁰ can also form in the presence of P and S, respectively. The high reactivity of group II metals is particularly interesting as it allows the formation of fluorides (BaF₂, CaF₂, SrF₂) even in the presence of air,¹¹¹ due to the higher affinity of F⁻ towards this group of metals compared to oxygen anionic species (i.e. oxides and carbonates). Likewise, NaCl salts can be synthesised in the presence of Cl⁻.¹¹¹

Enclosure of FSP in an inert environment in practice suppresses the amount of external O₂ being entrained (limiting O₂ solely from internally delivered sources i.e. dispersant and sheath gases), and depending on the oxidising affinity of the carbon precursor and that of the metal compound, could yield metallic nanoparticles (Fig. 2 IX).²⁴ To date however, only the FSP synthesis of metallic Co,^24,109 Bi⁸⁶ and Cu^87,193 have been reported, where metallic Bi must be kept under anaerobic conditions to prevent rapid oxidation to Bi₂O₃, while metallic Co and Cu nanoparticles are protected either by a surface oxide or carbon layer (Fig. 5, Route II). The latter is formed when the total amount of O₂ provided is less than needed for total combustion of all carbon sources, allowing deposition of carbonaceous layers in the form of soot, graphene or graphite. In some cases C₂H₂, CO or H₂ can be provided externally to the spray flame to induce higher degree of reduction and yield highly ordered carbonaceous deposition layers.^25,193

The particle formation paths in multicomponent systems are far more complex than single components. A wide range of particle configurations and their permutations are possible, dictated by the kinetics and thermodynamics of the component mixture. Hereby, we present the most general classes of particle configurations for multicomponents (Fig. 5, Route II–VI). In highly complex systems, particularly those comprising a large number of elements with different properties, combinations of two or more configurations from Route I–IV are indeed possible.

Direct encapsulation of a secondary component metal or metal oxide shell or even carbonaceous coating is a demanding process due to the difficult entropic requirements. The possibilities exist for the formation of composite materials (e.g. complex oxides, solid solutions) as well as interparticle phase segregations. To maintain an even coating morphology, favourable and strong interfacial interaction between the two components, besides higher amount of the coating component than the solid-state solubility limit, are required at the stage where the secondary component is precipitated. Phase segregation could result if the temperature-dependent interfacial interaction is weaker than that required to maintain homogeneous coating. To date, only a small number of such systems prepared by direct flame spraying of the components mixture have been achieved i.e. SiO₂¹²⁴ and Al₂O₃^124,194 coatings on Ce_xZr_1−xO₂, as well as TiO₂ on CeO₂.¹⁹⁵In situ sequential coating technique (Fig. 2, XI and Section 2) capitalizes on the introduction of the secondary component at a stage where the primary component is fully formed, and which temperature and remaining residence time is sufficiently low to avoid segregation of the homogeneous encapsulation layer.^92–95

In the case where full encapsulation is not attainable, either due to insufficient amount of the secondary component or occurrence of phase segregation (as small deposits), a supported metal/metal oxide on another metal oxide component can result (Fig. 5, Route III). Most notably, deposition of the secondary component as fine noble metals (Ru, Rh, Pd, Ag, Pt to Au) or even alloys on various metal oxide supports such as Al₂O₃, TiO₂, ZnO, SiO₂, Ce_xZr_1−xO₂, etc. have been reported by FSP (see Fig. 7, Table 1). Such morphologies have been largely explored for catalytic applications,^7,196 while others such as gas sensors^18,81,82,197 and antimicrobial^138,168,157 applications also exist. In many cases, the low boiling/sublimation points of the noble metals relative to that of the oxide supports allow sequential nucleation and growth along the concentration gradient of the spray flame.^{70,129,132,152} The support metal oxide is first nucleated at high temperature followed by that of noble metals at cooler downstream gas temperature. The rapid quenching and short residence time of the spray flame is advantageous here as it prevents significant sintering of the noble metals upon deposition on support and therefore maintains high surface dispersion of deposits (Fig. 7). Strong metal (or deposit)–support interaction is another factor governing its dispersion, as the strong interaction limits the mobility (and hence sintering) of the deposits at high flame temperatures. See for example ref. 130–132 on the effect of Pt–Pd alloy deposits on Al₂O₃, as compared to deposits in their pure forms.

Fig. 7 Examples of FSP-made supported noble metals on various metal oxide supports prepared in a single step, as illustrated in Route III – (a) Au/CeO₂, (b) Pt/TiO₂, (c) Au/SiO₂/γ-Fe₂O₃ and (d) Ag/TiO₂.

Despite the elegance of Route III synthesis, it is somewhat limited by the thermodynamic properties of the deposit-support, where besides the difference in boiling points, solid-state phase miscibility of deposit-support materials should also be taken into consideration. This is particularly important when the deposits are nucleated at high temperature where solid-state reaction, e.g. phase miscibility, formation of complex oxides, with support materials is non-negligible. To circumvent this limitation, the support and deposit materials can be precipitated separately in a twin-nozzle configuration (Fig. 2 X), interfacing both flames at a point with desirable extent of solid-state reactions.⁹¹ Rapid quenching of the flame at the stage of post-precipitation of noble metal deposits is another effective technique in limiting the deposit/support sintering⁷⁰ (Fig. 2, VIII).

Because the deposit/support particles are formed at high temperature, they (i.e. dispersion of metal deposits, Fig. 5, Route III) are often presumed to exhibit high thermal stability. While this may be true,¹³² but depending on the metal-support interactions, they may be subjected to significant loss in metal dispersion when overly exposed to very high temperatures. For instance, the initially highly-dispersed Pd deposits (on Al₂O₃) sinter significantly from 5 nm to 50–150 nm when exposed to 1000 °C,¹³⁰ rendering them indifferent from other lower temperature preparations. Besides surface dispersion, the oxidation state of the as-prepared metal deposits is an important characteristic that should not be overlooked. It has direct influence on some targeted end applications, including, catalytic hydrogenation,¹³² photocatalysis¹⁵⁴ and antimicrobial.¹⁵⁷ Other non-precious metals deposit/support structures that have been synthesised via Route III are V₂O₅/TiO₂^83,197 and CuO/CeO₂.^169,198

In some multicomponent systems, the different particle phases tend to segregate completely with little miscibility allowing only physical inter-particle mixing (Fig. 5, Route IV). Achieving such configuration during co-precipitation in a single flame is usually rare as multicomponent solid state reaction or mixing at an atomic level is usually inevitable at high temperature. CeO₂/Bi prepared in a restrictive oxygen flame is the only such system reported so far.¹⁸¹ By adopting a more engineering approach such as that in the twin nozzle synthesis of Pt/Ba/Al₂O₃, the interaction of Pt/BaCO₃ and Al₂O₃ was deliberately kept apart by separate precipitations in independent spray flames and only mixed at a point where the flame temperature was low enough to avoid solid-state reactions between the two phases.⁸⁹ Formation of BaAl₂O₄ is detrimental to the NO_x storage capacity during catalytic deNO_x applications.^89,128 The same concept was adopted for the flame mixing of Pt/BaCO₃ and Ce_xZr_1−xO₂.⁹⁰

Segregation of phases could also take place within a single particle host matrix (Fig. 5, Route V) such as that of SiO₂/ZnO^135–137 and SiO₂/γ-Fe₂O₃^22,146 (Fig. 8). In these systems, molten SiO₂ is thought to envelope the precipitated particles i.e. ZnO or γ-Fe₂O₃ producing a multicore structure (see Fig. 8b,c), while the encapsulation layer acts as diffusion barrier, retarding the agglomeration and sintering of the core particles.^22,199 One of the features of such configuration is the phase immiscibility or limited solid-state reactions between the core particles and the encapsulation layer, despite being in intimate contact. This could possibly arise from the limited thermodynamic miscibility of the two phases at the encapsulation temperature, or the formation of mixed oxide layers at the phase boundary that further suppressed the miscibility.

Fig. 8 Examples of embedded core particle systems prepared by a single-step FSP as depicted in Route V – (a) single core and (b) multicore γ-Fe₂O₃ in a single SiO₂ matrix and (c) ZnO cores in amorphous SiO₂ matrix. Figure (c) adapted with permission from ref. 136.

For systems where general homogeneous mixing between multicomponents at intra-particle level are preferred and depending on the states as well as their nature of mixing, a wide variety of configurations could result, ranging from substitutionally-doped systems, solid solutions, dispersed mixed oxides, complex metal oxides (perovskite, spinel, etc.) to metal alloys. Such particle configuration in route VI (Fig. 5) is exclusive to highly miscible multi-elements. The bottom-up synthesis and high synthesis temperature in FSP can be advantageous in enhancing substitutional dopant concentrations, while its steep temperature gradient and short residence time prevent segregation of the dopants.¹⁵⁶ A classic example of perfect solid solutions by FSP is that of Ce_xZr_1−xO₂, which was obtainable when using high boiling point solvents (i.e. gas-to-particle route) to ensure good distribution of precursor in the flame.¹⁷⁵ Unlike solid solutions, Ta₂O₅/SiO₂²⁰ and Co₃O₄/ZrO₂¹⁶⁰ retain the individual crystallite structures, while still achieving physical dispersity within a single particle matrix. A large family of complex metal oxides involving the oxide of two or more metals also fall in this route VI configuration: MgAl₂O₄,^28,33 3Al₂O₃ · 2SiO₂,²⁹ Y₃Al₅O₁₂,¹²² LaCoO₃,^161–163 CoO_x–Al₂O₃,³¹ NiO–Al₂O₃,³² LiMn₂O₄, Li₄Ti₅O₁₂, LiFe₅O₈,²⁶ BiVO₄.^140,200,201 In the event of multicomponent synthesis in restrictive oxygen flames, even alloys such as Ni/Mo could be achieved.⁸⁸

5. Concluding remarks and perspectives

The advancement of FSP currently lies at the intersection of combustion science, aerosols technology and materials chemistry. As such, the interdisciplinary knowledge of all intersecting fields is not only inevitable but is becoming increasingly important in the design of flame-made nanoparticles. As in the past decade, innovations in FSP technology, in terms of reactor engineering and particles design, will continue to be applications-driven. In terms of industrial applications, FSP is a proven scalable and high throughput process unmatched by other wet chemical techniques.^67,71,75 Formulation of liquid precursors, which was previously perceived to be a “stumbling block” requiring expensive and moisture-sensitive chemicals, is now reasonably well-understood. In fact, high quality metal precursors can now be synthesised from cheap raw materials through various routes, even though more work is still required in the area of precious metals.

In terms of particle homogeneity, it is often limited by the so-called self preserving size distributions (SPSD), which forms the asymptotic limit of polydispersity for particles growth by Brownian coagulation, giving number geometric standard deviation of ∼1.46.⁵ However, there is increasing demand especially in bioapplications for particles with much narrower size distribution (high monodispersity), whilst maintaining high particle purity and crystallinity. Exclusive particle growth by surface condensation rather than coagulation is one possibility¹⁰² but the implementation of such restrictive system is highly challenging, especially within the harsh spray flame conditions. Manipulations of flame combustion, fluid and aerosol dynamics is necessary to achieve this task.

As for the production of non-oxygen containing ceramics, only group II metal fluorides have been reported by FSP to date.¹¹¹ The synthesis of metal carbides, nitrides, phosphides and sulfides are still elusive in FSP, while the synthesis of SiC and Si₃N₄ has been shown to be possible by VAFS, using SiH₄ precursors.^202,203 A requirement for such systems would be to restrict the presence of O₂ which would otherwise result in the formation of metal oxides, carbonates, phosphates or sulfates. Creation of anisotropic materials or particle shape control by FSP is also lacking. Besides CeO₂ rhomboids,⁶⁶ BaF₂ cubes,¹¹¹ (In, Sn)-doped ZnO nanorods¹⁰⁶ and γ-Fe₂O₃ hexagonal/octagonal platelets,^22,23 most other FSP-derived particles are spherical. Moreover, most of these non-spherical shapes are naturally occurring in the flame thereby providing little flexibility in terms of shape manipulation. Magnetically-assisted formation of metallic Co nanowires by FSP reported recently marks the beginning of externally-induced shape control by FSP.¹⁰⁹ More work in this direction is required, for example, template-assisted, thermophoretically- or electrophoretically-induced flame aerosols alignment and assemblies.

In the last few years, we have seen successful combinations of FSP-derived inorganic particles with organic materials particularly for bioapplications, namely dental fillers,^20,141 bioactive polymer films¹⁴⁷ and magnetic particles with drug delivery^204,205 as well as magnetic resonance imaging^205,206 functions. This requires the coupling of predesigned flame particles with functional organic molecules such as DNA, protein, polymers and surfactants. More developments in this regard, encompassing various areas of applications from functional pigments, sensors to bioelectronics are expected to take place in the near future. Of equal importance is the fabrication of flame particles-based devices as readily demonstrated for gas sensors.^17,85,178 Other potential devices such as solar cells, fuel cells, batteries, lab-on-a-chip etc. are also possible. The ability to create short-range and long-range ordering of these particles as-prepared via FSP, with or without templating, and their further assemblies in devices would also be of tremendous benefit.

Acknowledgements

The authors gratefully acknowledge Prof. Sotiris E. Pratsinis (ETH Zürich) and Prof. Jan-Dierk Grunwaldt (Technical University of Denmark) for their insightful comments and discussion on the manuscript. The authors also thank Dr Dan Li, Dr Richard Kydd and Yung Kent Kho (UNSW) and Dr Frank Krumeich (ETH Zürich) for many of the electron microscopy images presented in this review. W. Y. T. and R. A. thank the Australian Research Council (ARC) for financial support on various FSP-related projects.

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