Some trends in the design of homo- and heterometallic molecular precursors of high-tech oxides

Synthetic Routes to Crystalline Complex Metal Alkyl Carbonates and Hydroxycarbonates via Sol–Gel Chemistry—Perspectives for Advanced Materials in Catalysis

Metal alkoxides are easily available and versatile precursors for functional materials, such as solid catalysts. However, the poor solubility of metal alkoxides in organic solvents usually hinders their facile application in sol–gel processes and complicates access to complex carbonate or oxidic compounds after hydrolysis of the precursors. In our contribution we have therefore shown three different solubilization strategies for metal alkoxides, namely the derivatization, the hetero-metallization and CO2 insertion. The latter strategy leads to a stoichiometric insertion of CO2 into the metal–oxygen bond of the alkoxide and the subsequent formation of metal alkyl carbonates. These precursors can then be employed advantageously in sol–gel chemistry and, after controlled hydrolysis, result in chemically defined crystalline carbonates and hydroxycarbonates. Cu- and Zn-containing carbonates and hydroxycarbonates were used in an exemplary study for the synthesis of Cu/Zn-based bulk catalysts for methanol synthesis with a final comparable catalytic activity to commercial standard reference catalysts.

Molecular Level Synthesis of InFeO3 and InFeO3/Fe2O3 Nanocomposites

New heterometallic In-Fe alkoxides [InFe(O^tBu)₄(PyTFP)₂] (1), [InFe₂(O^neoPen)₉(Py)] (2), and [InFe₃(O^neoPen)₁₂] (3) were synthesized and structurally characterized. The arrangement of metal centers in mixed-metal framework was governed by the In:Fe ratio and the coordination preferences of Fe(III) and In(III) centers to be in tetrahedral and octahedral environments, respectively. 3 displayed a star-shaped so-called "Mitsubishi" motif with the central In atom coordinated with three tetrahedral {Fe(O^neoPen)₄}^- anionic units. The deterministic structural influence of the larger In atom was evident in 1 and 2 which displayed the coordination of neutral coligands to achieve the desired coordination number. Thermal decomposition studies of compounds 1-3 under inert conditions with subsequent powder diffraction studies revealed the formation of Fe₂O₃ and In₂O₃ in the case of 3 and 2, whereas 1 intriguingly produced elemental In and Fe. In contrary, the thermal decomposition of 1-3 under ambient conditions produced a ternary oxide, InFeO₃, with additional Fe₂O₃ present as a secondary phase in a different stoichiometric ratio predetermined through the In:Fe ratio in 2 and 3. The intimate mixing of different phases in InFeO₃/Fe₂O₃ nanocomposites was confirmed by transmission electron microscopy of solid residues obtained after the decomposition of 1 and 2. The pure InFeO₃ particles demonstrated ferromagnetic anomalies around 170 K as determined by temperature-dependent field-cooled and zero-field-cooled magnetization experiments. A first-order magnetic transition with an increase in the ZFC measurements was explained by temperature-induced reduction of the Fe-Fe distance and the corresponding increase in superexchange.

Low-Temperature Solution Approaches for the Potential Integration of Ferroelectric Oxide Films in Flexible Electronics

This technical review presents the state of the art in low-temperature chemical solution deposition (CSD) processing of ferroelectric oxide thin films. To achieve the integration of multifunctional crystalline oxides with flexible and semiconductor devices is, today, crucial to meet the demands of coming electronic devices. Hence, amorphous metal-oxide-semiconductors have been recently introduced in thin-film electronics. However, their benefits are limited compared with those of ferroelectric oxides, in which intrinsic multifunctionality would make possible multiple operations in the device. However, ferroelectricity is linked to a noncentrosymmetric crystal structure that is achieved, in general, at high temperatures, over 500 °C. These temperatures exceed the thermal stability of flexible polymer substrates and are not compatible with those permitted in the current fabrication routines of Si-based devices. In addition, the manufacturing of flexible electronic devices not only calls for low-temperature fabrication processes but also for deposition techniques that scale easily to the large areas required in flexible devices. In this regard, CSD processes are the best positioned today to integrate metal oxide thin films with flexible substrates as a large-area, low-cost, high-throughput fabrication technique. Here, we review the progress made in the last years in fabricating at low-temperature crystalline ferroelectric oxide thin films via CSD methods, highlighting the recent work of our group in the preparation of ferroelectric oxide thin films on flexible polyimide substrates.

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.

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.

The use of mixed-metal single source precursors for the synthesis of complex metal oxides

This Feature Article highlights the use of mixed-metal single source precursors to directly access useful complex metal oxide materials.

Compartmentalization of Alkaline-Earth Metals in Salen-Type Cu- and Ni-Complexes in Solution and in the Solid State

The precise arrangement of metal ions in type and number by a ligand represents an important challenge in biology as well as in materials science. The preorganization of different metal ions such as alkaline-earth and transition-metal ions is of particular interest for the design of catalysts or precursors of oxides. This study is based on a Ω-shaped salen-derived ligand comprising N₂O₂ and O₂O₂ coordination sites. The selective binding of Cu(II) and Ni(II) and alkaline-earth-metal ions is influenced by many factors such as the size of the cation, the solvent, or the counterion. UV-vis and ¹H NMR titrations and single-crystal X-ray structures reveal that the obtained complexes tend to adopt different structures in solution compared to the solid state. Mainly discrete motifs with a stoichiometry 1:1 (LM₁ to alkaline-earth-metal ions) have been shown to form in the solid state, whereas in solution, the 2:1 complexes are predominant.

Bismuth vanadate-based semiconductor photocatalysts: a short critical review on the efficiency and the mechanism of photodegradation of organic pollutants

The number of publications on photocatalytic bismuth vanadate-based materials is constantly increasing. Indeed, bismuth vanadate is gaining stronger interest in the photochemical community since it is a solar-driven photocatalyst. However, the efficiency of BiVO₄-based photocatalyst under sunlight is questionable: in most of the studies investigating the photodegradation of organic pollutants, only few works identify the by-products and evaluate the real efficiency of BiVO₄-based materials. This short review aims to (i) present briefly the principles of photocatalysis and define the photocatalytic efficiency and (ii) discuss the formation of reactive species involved in the photocatalytic degradation process of pollutants and thus the corresponding photodegradation mechanism could be determined. All these points are developed in a comprehensive discussion by focusing especially on pure, doped, and composite BiVO₄. Therefore, this review exhibits a critical overview on different BiVO₄-based photocatalytic systems with their real efficiency. This is a necessary knowledge for potential implementation of BiVO₄ materials in environmental applications at larger scale than laboratory conditions.

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.

From iron coordination compounds to metal oxide nanoparticles

Various types, shapes and sizes of iron oxide nanoparticles were obtained depending on the nature of the precursor, preparation method and reaction conditions. The mixed valence trinuclear iron acetate, [Fe₂^IIIFe^IIO(CH₃COO)₆(H₂O)₃]·2H₂O (FeAc1), μ₃-oxo trinuclear iron(III) acetate, [Fe₃O(CH₃COO)₆(H₂O)₃]NO₃∙4H₂O (FeAc2), iron furoate, [Fe₃O(C₄H₃OCOO)₆(CH₃OH)₃]NO₃∙2CH₃OH (FeF), iron chromium furoate, FeCr₂O(C₄H₃OCOO)₆(CH₃OH)₃]NO₃∙2CH₃OH (FeCrF), and an iron complex with an original macromolecular ligand (FePAZ) were used as precursors for the corresponding oxide nanoparticles. Five series of nanoparticle samples were prepared employing either a classical thermal pathway (i.e., thermal decomposition in solution, solvothermal method, dry thermal decomposition/calcination) or using a nonconventional energy source (i.e., microwave or ultrasonic treatment) to convert precursors into iron oxides. The resulting materials were structurally characterized by wide-angle X-ray diffraction and Fourier transform infrared, Raman, energy-dispersive X-ray, and X-ray fluorescence spectroscopies, as well as thermogravimetric analysis. The morphology was characterized by transmission electron microscopy, atomic force microscopy and dynamic light scattering. The parameters were varied within each route to fine tune the size and shape of the formed nanoparticles.

Photochemical solution processing of films of metastable phases for flexible devices: the β-Bi2O3 polymorph

The potential of UV-light for the photochemical synthesis and stabilization of non-equilibrium crystalline phases in thin films is demonstrated for the β-Bi₂O₃ polymorph. The pure β-Bi₂O₃ phase is thermodynamically stable at high temperature (450-667 °C), which limits its applications in devices. Here, a tailored UV-absorbing bismuth(III)-N-methyldiethanolamine complex is selected as an ideal precursor for this phase, in order to induce under UV-light the formation of a -Bi-O-Bi- continuous network in the deposited layers and the further conversion into the β-Bi₂O₃ polymorph at a temperature as low as 250 °C. The stabilization of the β-Bi₂O₃ films is confirmed by their conductivity behavior and a thorough characterization of their crystal structure. This is also supported by their remarkable photocatalytic activity. Besides, this processing method has allowed us for the first time the preparation of β-Bi₂O₃ films on flexible plastic substrates, which opens new opportunities for using these materials in potential applications not available until now (e.g., flexible photocatalytic reactors, self-cleaning surfaces or wearable antimicrobial fabrics). Therefore, photochemical solution deposition (PCSD) demonstrates to be not only an efficient approach for the low temperature processing of oxide films, but also an excellent alternative for the stabilization of metastable phases.

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.

Volatile Heterobimetallic Complexes from PdII and CuII β-Diketonates: Structure, Magnetic Anisotropy, and Thermal Properties Related to the Chemical Vapor Deposition of CuPd Thin Films

A novel approach for preparing volatile heterometallic complexes for use as precursors for the chemical vapor deposition of various materials is reported. New CuPd complexes based on β-diketonate units were prepared, and their structures and compositions were determined. [PdL₂ *CuL₂ ] (1) and [PdL₂ *Cu(tmhd)₂ ] (2) (L=2-methoxy-2,6,6-trimethylheptane-3,5-dionate; tmhd=2,2,6,6- tetramethylheptane-3,5-dionate) are 1D coordination polymers with alternating metal complexes, which are connected through weak interactions between the Cu atoms and the OCH₃ groups from the ligand of the Pd complexes. The volatility and thermal stability were studied using thermogravimetric and differential thermal analyses and mass spectrometry. Compound 1 vaporizes without decomposition into monometallic complexes. It exhibits magnetic anisotropy, which was revealed from the angular variations in the EPR spectrum of a single crystal. The vapor thermolysis process for 1 was investigated using mass spectrometry, allowing the process to be framed within the temperature range of 200-350 °C. The experimental data, supported by QTAIM calculations of the allowed intermolecular interactions, suggest that 1 likely exists in the gas phase as bimetallic molecules. Compound 1 proved to be suitable as a single-source precursor for the efficient preparation of CuPd alloy films with tunable Cu/Pd ratio. A possible mechanism for the film growth is proposed based on the reported data.

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.

Synthesis and characterisation of thioether functionalised gallium and indium alkoxides

The first coordination of thioethers in metal alkoxides is described and the bonding compared to thioether-containing aminoalcoholate derivatives.

Characterization and photocatalytic study of tantalum oxide nanoparticles prepared by the hydrolysis of tantalum oxo-ethoxide Ta8(μ3-O)2(μ-O)8(μ-OEt)6(OEt)14

Ta8(μ3-O)2(μ-O)8(μ-OEt)6(OEt)14 (1) was obtained by the controlled hydrolysis of tantalum ethoxide Ta(OEt)5 in the presence of ammonia. Compound 1 is considered as the intermediate building block in the sol-gel polymerization of Ta(OEt)5. Further hydrolysis of compound 1 yielded nanoparticles of Ta2O5, which were characterized by various techniques such as TGA-DTA-DSC, UV-vis DRS, XRD, SEM, TEM, particle size analyzer (DLS) and the Brunauer-Emmett-Teller (BET) method. The band gap of the particles was calculated by using the Tauc plot. The photocatalytic activity of Ta2O5 nanoparticles was tested by the degradation of the organic dye rhodamine B.

Hybrid Materials Based on the Embedding of Organically Modified Transition Metal Oxoclusters or Polyoxometalates into Polymers for Functional Applications: A Review

The covalent incorporation of inorganic building blocks into a polymer matrix to obtain stable and robust materials is a widely used concept in the field of organic-inorganic hybrid materials, and encompasses the use of different inorganic systems including (but not limited to) nanoparticles, mono- and polynuclear metal complexes and clusters, polyhedral oligomeric silsesquioxanes (POSS), polyoxometalates (POM), layered inorganic systems, inorganic fibers, and whiskers. In this paper, we will review the use of two particular kinds of structurally well-defined inorganic building blocks, namely transition metals oxoclusters (TMO) and polyoxometalates (POM), to obtain hybrid materials with enhanced functional (e.g., optical, dielectric, magnetic, catalytic) properties.

Partially fluorinated oxo-alkoxide tungsten(vi) complexes as precursors for deposition of WOx nanomaterials

Volatile partially fluorinated tungsten(vi) oxo-alkoxides yield tungsten oxide nanorods upon chemical vapour deposition.