Spray-Flame Synthesis of LaFexCo1-xO3 (x = 0.2, 0.3) Perovskite Nanoparticles for Oxygen Evolution Reaction in Water Splitting: Effect of Precursor Chemistry (Acetates and Nitrates)

The product properties of mixed oxide nanoparticles generated via spray-flame synthesis depend on an intricate interplay of solvent and precursor chemistries in the processed solution. The effect of two different sets of metal precursors, acetates and nitrates, dissolved in a mixture of ethanol (35 Vol.%) and 2-ethylhexanoic acid (2-EHA, 65 Vol.%) was investigated for the synthesis of LaFe_xCo_1-xO₃ (x = 0.2, 0.3) perovskites. Regardless of the set of precursors, similar particle-size distributions (d_p = 8-11 nm) were obtained and a few particles with sizes above 20 nm were identified with transmission electron microscopy (TEM) measurements. Using acetates as precursors, inhomogeneous La, Fe, and Co elemental distributions were obtained for all particle sizes according to energy dispersive X-ray (EDX) mappings, connected to the formation of multiple secondary phases such as oxygen-deficient La₃(Fe_xCo_1-x)₃O₈ brownmillerite or La₄(Fe_xCo_1-x)₃O₁₀ Ruddlesden-Popper (RP) structures besides the main trigonal perovskite phase. For samples synthesized from nitrates, inhomogeneous elemental distributions were observed for large particles only where La and Fe enrichment occurred in combination with the formation of a secondary La₂(Fe_xCo_1-x)O₄ RP phase. Such variations can be attributed to reactions in the solution prior to injection in the flame as well as precursor-dependent variations in in-flame reactions. Therefore, the precursor solutions were analyzed by temperature-dependent attenuated total reflection Fourier-transform infrared (ATR-FTIR) measurements. The acetate-based precursor solutions indicated the partial conversion of, mainly La and Fe, acetates to metal 2-ethylhexanoates. In the nitrate-based solutions, esterification of ethanol and 2-EHA played the most important role. The synthesized nanoparticle samples were characterized by BET (Brunauer, Emmett, Teller), FTIR, Mössbauer, and X-ray photoelectron spectroscopy (XPS). All samples were tested as oxygen evolution reaction (OER) catalysts, and similar electrocatalytic activities were recorded when evaluating the potential required to reach 10 mA/cm² current density (∼1.61 V vs reversible hydrogen electrode (RHE)).

Ultra-mild synthesis of nanometric metal chalcogenides using organyl chalcogenide precursors

Bis(trialkylsilyl) monochalcogenides and diorganyl dichalcogenides, (R₃Si)₂E and R₂E₂ (E = S, Se or Te and R = alkyl, aryl or allyl group), have emerged in the past decade as excellent reagents for the synthesis of metal chalcogenide nanoparticles (NPs) and clusters owing to their ability to transfer the chalcogenide anion (E^2-) under ultra-mild conditions and versatility in reacting even with non-conventional metal reagents or being employed in a variety of synthetic methods. In comparison, the related non-silylated diorganyl monochalcogenides R₂E have received attention only recently for the solution phase synthesis of metal chalcogenide NPs. In spite of sharing many similarities, these three families of organyl chalcogenides are different in their coordination ability and decomposition behavior, and therefore in reactivities towards metal reagents. This feature article provides a concise overview on the use of these three families as synthons for the ultralow-temperature synthesis of metal chalcogenide nanomaterials, deliberating their different decomposition mechanisms and critically assessing their advantages for certain applications. More specifically, it discusses their usefulness in (i) affording molecular precursors with different kinetic and thermal stabilities, (ii) isolating reactive intermediates for comprehending the mechanism of molecule-to-nanoparticle transformation and, therefore, achieving fine control over the synthesis, (iii) stabilizing isolable metastable or difficult-to-achieve phases, and (iv) yielding complex ternary nanoparticles with controlled stoichiometry or composites with sensitive materials without modifying the characteristics of the latter. Besides providing a perspective on the low-temperature synthesis of nanomaterials, this overview is expected to assist further progress, particularly in the field of R₂E, leading to interesting materials including metastable ones for new applications.

Characterization of divalent metal soaps of Cannabis sativa seed oil

In this study, the structural characteristics and thermal degradation behaviour of barium, calcium and zinc soaps of hemp seed oil prepared using the metathesis method are reported. Scanning electron micrographs of the metal soaps showed similar plate-like microstructures and are indicative of an orderly packed particulate structure. The thermal decomposition consisted of one single step on the weight loss – temperature curves located around 250 – 450 oC for all the divalent metal soaps at the various heating rates and the weight loss occurring at temperatures up to 200 oC was less than 10%. The average values of the apparent activation energy obtained for the thermal decomposition of metal soaps of HSO were 18.73 kJ/mol for Zn-HSO, 13.77 kJ/mol for Ca-HSO and 8.48 kJ/mol for Ba-HSO.

The kinetics of metal soap crystallization in oil polymers

The crystallization of metal soaps in oil paint is an important chemical phenomenon that affects the appearance and structural stability of many works of art. A deep understanding of the structural transitions that occur during crystallization and their kinetics will help to support conservation decisions that minimize future detrimental change to paintings. We have used a method based on attenuated total reflection Fourier transform infrared spectroscopy and detailed spectrum analysis to quantitatively monitor all relevant metal soap structures during crystallization in a linseed oil matrix with varying degrees of polymerization. It was found that zinc soap crystallization behaviour is strongly influenced by the properties of the oil matrix, slowing down drastically with increasing polymerization, forming crystalline polymorphs in varying ratios, and demonstrating two-stage kinetics. In contrast, lead soap crystallization was invariably fast, but the degree of disorder in the crystallized phases was increasing with matrix polymerization. Besides fundamental insight into the mechanisms of metal soap crystallization, the results lay foundations for improved risk assessment during conservation treatment of oil paintings.

The kinetics of metal soap crystallization and the structures that form are highly dependent on polymer matrix properties.

Analysis of Solid-State Reaction Mechanisms with Two-Dimensional Fourier Transform Infrared Correlation Spectroscopy

The utility of two-dimensional generalized correlation spectroscopy (2D-COS) for tracking complex solid-state reactions is demonstrated using infrared spectra acquired during a photochemically induced decomposition reaction. Eleven different thin films, consisting of six monometallic and five bimetallic 2-ethylhexanoate complexes, were tracked as a function of photolysis time. Overlapping peaks in the infrared fingerprint region are readily discriminated using 2D-COS, enabling individual vibrational components to be used to distinguish whether carboxylate ligands are free/ionic or bound in a chelating, bridging, or monodentate fashion. This classification enables the decomposition mechanism to be tracked for all 11 samples, revealing that ligands bound in monodentate and bridging fashions are first converted to chelates before being lost as volatile products for all samples. The magnitude of the measured first-order rate constants for loss of chelated ligands is found to correlate linearly to the asymmetric stretching frequency of monodentate ligands but exhibits a V shape when plotted against the electronegativity of the metal center. We propose that loss of chelated ligands proceeds via C-O scission for highly electronegative transition metals but M-O scission for transition metals with low electronegativity. These results establish 2D-COS as a powerful tool to deconvolute and correlate individual components, enabling mechanistic analysis of complex chemical reactions.

Designed sol–gel precursors for atomically dispersed Nb and Pb within TiO2 as catalysts for dihydroxyacetone transformation

New N-methyldiethanolamine-modified metal alkoxides were synthesized and employed as sol–gel precursors to obtain atomically dispersed catalysts with high surface area and tunable acid–base properties.

The Identification of Multiple Crystalline Zinc Soap Structures Using Infrared Spectroscopy

The formation of crystalline zinc soaps (zinc salts of fatty acids) in oil paint layers is a common sign of paint degradation. In this study, we have used infrared spectroscopy to systematically identify differences in structure and composition of crystalline zinc soap phases, and report data analysis methods for structure attribution in challenging oil paint samples. Supported by reported crystal structures, it was possible to distinguish two distinct types of zinc soap geometry: a highly symmetrical packing for long-chain saturated soaps (type B) and an alternating packing for zinc soaps with short, unsaturated, or dicarboxylic chains (type A). These two types of packing can be identified by a single or split asymmetric COO stretch vibration band. With this new information, we studied the structure and composition of zinc soaps formed in a zinc white model paint and in a cross-section from the painting Equations in Space by Lawren Harris. Using non-negative matrix factorization, band integration and band position maps, it was possible to clearly identify zinc azelate in the model paint and map its spatial distribution. The same methods showed that the paint cross-section contained both types of zinc soap structure within the same paint layer, with the less symmetrical structure appearing only at the interface with the ground layer. The results give valuable information on the internal chemistry of oil paint layers, and the demonstrated methods can find widespread application for in-depth analysis of infrared microscopy data.

Molecular Engineering of Metal Alkoxides for Solution Phase Synthesis of High-Tech Metal Oxide Nanomaterials

The 'bottom-up' synthesis of inorganic nanomaterials with precision at the atomic/molecular level offers many opportunities for the design and improvement of the nanomaterials for various applications. Molecular engineering during soft chemical processing for the synthesis of functional nanomaterials enables the desired chemical and physical properties of the precursors, such as solubility or volatility, clean decomposition, control of stoichiometry for multimetallic species to name a few, and leads to easy control of uniform particle size distribution, stoichiometry…. This Minireview illustrates some important aspects of the molecular engineering in light of some recent developments from the molecular synthesis of nanomaterials involving non-silicon metal alkoxide systems for high-tech applications.

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

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

Heteroleptic Tin(IV) Aminoalkoxides and Aminofluoroalkoxides as MOCVD Precursors for Undoped and F-Doped SnO2 Thin Films

A series of asymmetric and potentially bidentate amino alcohols and amino fluoro alcohols (R_NOH) having a different number of methyl/trifluoromethyl substituents at the α-carbon atom, [HOC(R¹)(R²)CH₂NMe₂] (R¹ = R² = H (dmaeH); R¹ = H, R² = CH₃ (dmapH); R¹ = R² = CH₃ (dmampH); R¹ = H, R² = CF₃ (F-dmapH); R¹ = R² = CF₃ (F-dmampH)) have been used to develop new monomeric and heteroleptic tin(IV) amino(fluoro)alkoxides [Sn(OR)₂(OR_N)₂] (R = Et, Prⁱ, Bu^t). These new complexes, which were thoroughly characterized by spectroscopy (IR and multinuclei NMR (¹H, ¹³C, ¹⁹F, and ¹¹⁹Sn)) as well as single-crystal X-ray studies on representative samples, were investigated for their thermal behavior to determine their suitability as MOCVD precursors for the deposition of metal oxide thin films. The two most suitable compounds, [Sn(OBu^t)₂(dmamp)₂] and [Sn(OBu^t)₂(F-dmamp)₂], were used in a direct liquid injection chemical vapor deposition (DLI-CVD) process to deposit undoped SnO₂ and F-doped SnO₂ thin films, respectively, on silicon and quartz substrates. Film growth rates at different temperatures (from 400 to 700 °C), film thickness, crystalline quality, and surface morphology were investigated. The films deposited on quartz showed high transparency (above 80%) in the visible region and low carbon contamination on the surface (11-13% from XPS), which could easily be removed completely with 2 min of Ar⁺ sputtering.

Nanometric NaYF4 as an Unconventional Support for Gold Catalysts for Oxidation Reactions

The metal-support interaction plays an important role in gold catalysis. We employ here crystalline cubic (α-) and hexagonal (β-) phases of heterometallic fluoride NaYF₄ nanoparticles (NPs), obtained by the decomposition of a single source precursor [NaY(TFA)₄(diglyme)] (TFA = trifluoroacetate), as nonoxide supports for gold catalysts. Using an isostructural gadolinium analogue, we also obtained doped α-NaYF₄:Gd³⁺ and β-NaYF₄:Gd³⁺ NPs. A successful deposition of ∼1% by weight gold NPs of average size 5-6.5 nm on these doped and undoped metal fluorides using HAuCl₄·3H₂O afforded Au/NaYF₄ catalysts which were thoroughly characterized by using several physicochemical techniques such as X-ray diffraction, Brunauer-Emmett-Teller analysis, high-resolution transmission electron microscopy, energy-dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy. A comparative study of the above catalysts for different oxidation reactions show that while for the aerobic oxidation of trans-stilbene in solution phase, they are either better (in terms of stilbene conversion) or at par (in terms of trans-stilbene oxide yield) in comparison to the reference catalyst Au/TiO₂ of the World Gold Council, their activity toward CO oxidation reactions in gas phase remains much less than that of gold catalysts supported on metal oxides.

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

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

The crystallization of metal soaps and fatty acids in oil paint model systems

The formation and crystallization of metal soaps in oil paint layers is an important issue in the conservation of oil paintings. The chemical reactions and physical processes that are involved in releasing metal ions from pigments and fatty acids from the oil binder to form crystalline metal soap deposits have so far remained poorly understood. We have used a combination of differential scanning calorimetry (DSC) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) on model mixtures of palmitic acid, lead palmitate or zinc palmitate and linseed oil to study the transition from amorphous material to crystalline fatty acid or metal soap. This transition forms the final stage in the cascade of processes leading to metal soap-related oil paint degradation. Palmitic acid as well as the metal soaps showed nearly ideal solubility behavior. However, it was found that, near room temperature, both lead and zinc palmitate are practically insoluble in both liquid and partially polymerized linseed oil. Interestingly, the rate of metal soap and fatty acid crystallization decreased rapidly with the degree of linseed oil polymerization, possibly leading to systems where metal soaps are kinetically trapped in a semi-crystalline state. To explain the various morphologies of metal soap aggregates observed in oil paint layers, it is proposed that factors affecting the probability of crystal nucleation and the rate of crystal growth play a crucial role, like exposure to heat or cleaning solvents and the presence of microcracks.

Asymmetrically substituted triazenes as poor electron donor ligands in the precursor chemistry of iron(ii) for iron-based metallic and intermetallic nanocrystals

First as precursors: Heteroleptic Fe^II complexes derived from new asymmetric triazene ligands t-BuNN-NHR (R = Et, i-Pr, n-Bu) show interesting molecular and precursor chemistry.

Hybrid Fibrillar Xerogels with Unusual Magnetic Properties

We report on the preparation of a hybrid nanomaterial made up of 1D filaments of an antiferromagnetic self-assembling bicopper complex encapsulated in polymer nanofibrils. The encapsulation process is achieved through the heterogeneous nucleation of the growth of polymer fibrils obtained by thermoreversible gelation as shown by calorimetry experiments. Neutron scattering experiments confirm that the filaments of a bicopper complex retain their 1D character after encapsulation in the fibrils. Superconducting quantum interference device experiments show that the bicopper complex, originally in the gapped spin state in the 3D bulk mesophase, displays a gapless behavior once encapsulated. Extended absorption fine structure and infrared results further highlight the difference in the molecular arrangement of the bicopper complex between the bulk mesophase and the encapsulated state, which may account for the magnetic behavior. This material, which is largely disordered, differs totally from the usual magnetic systems where this effect is observed only on highly crystalline systems with long-range order. Also, this hybrid material is very easy to prepare from its basic constituents and can be further processed in many ways.

Structural isomers of iron(iii) N-methyl diethanolaminate as sol–gel precursors for iron-based oxide nanomaterials

New aminoalkoxides of Fe(iii) were synthesized, evaluated for different sol–gel parameters to stabilize nano-sized colloidal suspensions, and employed for the elaboration of magnetic Fe₂O₃ and BiFeO₃ NPs and thin films.

A convenient and quantitative route to Sn(iv)–M [M = Ti(iv), Nb(v), Ta(v)] heterobimetallic precursors for dense mixed-metal oxide ceramics

Simplicity is beauty. Reactions of SnCl₄ with M(OR)_x yield novel heterobimetallics with simple addition formula.

Comparison and functionalization study of microemulsion-prepared magnetic iron oxide nanoparticles

Magnetic iron oxide nanoparticles (MION) for protein binding and separation were obtained from water-in-oil (w/o) and oil-in-water (o/w) microemulsions. Characterization of the prepared nanoparticles have been performed by TEM, XRD, SQUID magnetometry, and BET. Microemulsion-prepared magnetic iron oxide nanoparticles (ME-MION) with sizes ranging from 2 to 10 nm were obtained. Study on the magnetic properties at 300 K shows a large increase of the magnetization ~35 emu/g for w/o-ME-MION with superparamagnetic behavior and nanoscale dimensions in comparison with o/w-ME-MION (10 emu/g) due to larger particle size and anisotropic property. Moringa oleifera coagulation protein (MOCP) bound w/o- and o/w-ME-MION showed an enhanced performance in terms of coagulation activity. A significant interaction between the magnetic nanoparticles and the protein can be described by changes in fluorescence emission spectra. Adsorbed protein from MOCP is still retaining its functionality even after binding to the nanoparticles, thus implying the extension of this technique for various applications.

Photoreactive surfactants: a facile and clean route to oxide and metal nanoparticles in reverse micelles

A new class of photoreactive surfactants (PRSs) is presented here, consisting of amphiphiles that can also act as reagents in photochemical reactions. An example PRS is cobalt 2-ethylhexanoate (Co(EH)(2)), which forms reverse micelles (RMs) in a hydrocarbon solvent, as well as mixed reversed micelles with the standard surfactant Aerosol-OT (AOT). Small-angle neutron scattering (SANS) data show that mixed AOT/PRS RMs have a spherical structure and size similar to that of pure AOT micelles. Excitation of the ligand-to-metal charge transfer (LMCT) band in the PRSs promotes electron transfer from PRS to associated metal counterions, leading to the generation of metal and metal-oxide nanoparticles inside the RMs. This work presents proof of concept for employing PRSs as precursors to obtain nearly monodisperse inorganic nanoparticles: here both Co(3)O(4) and Bi nanoparticles have been synthesized at high metal concentration (10(-2) M) by simply irradiating the RMs. These results point toward a new approach of photoreactive self-assembly, which represents a clean and straightforward route to the generation of nanomaterials.

Novel heteroleptic heterobimetallic alkoxide complexes as facile single-source precursors for Ta(5+) doped TiO(2)-SnO(2) nanoparticles

The nanometric mixed tin-titanium oxide doped with a M(5+) cation was recently shown as a promising thermoelectric material. We report here synthesis of novel molecular precursors for above material using a convenient approach of reacting a metal chloride with a metal alkoxide. Heterometallic complexes with simple addition formula [(EtOH)(2)(OEt)(2)Ti(μ-OEt)(2)SnCl(4)] (1·EtOH) and [(EtOH)(OEt)(3)Ta(μ-OEt)(2)SnCl(4)] (2) were isolated in quantitative yield, which on recrystallization from isopropanol afforded mixed-alkoxide complexes [(Pr(i)OH)(2)(OPr(i))(2)Ti(μ-OEt)(2)SnCl(4)] (3) and [(Pr(i)OH)(OPr(i))(3)Ta(μ-OEt)(2)SnCl(4)] (4), respectively, thus indicating the robustness of the heterometallic M(μ-OEt)(2)Sn core in the solution phase. Facile conversion of these precursors to halide-free spinodal form of Ta(5+)-doped TiO(2)-SnO(2) as a potential thermoelectric material is reported.

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

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.