# Solution-Based Synthesis Routes for the Preparation of Noncentrosymmetric 0-D Oxide Nanocrystals with Perovskite and Nonperovskite Structures

Géraldine Dantelle, Sandrine Beauquis, Ronan Le Dantec, Virginie Monnier, Christine Galez, Yannick Mugnier

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Géraldine Dantelle, Sandrine Beauquis, Ronan Le Dantec, Virginie Monnier, Christine Galez, et al. Solution-Based Synthesis Routes for the Preparation of Noncentrosymmetric 0-D Oxide Nanocrystals with Perovskite and Nonperovskite Structures. Small, 2022, 18 (30), pp.2200992. 10.1002/smll.202200992. hal-03693787

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# Solution-Based Synthesis Routes for the Preparation of Noncentrosymmetric 0-D Oxide Nanocrystals with Perovskite and Nonperovskite Structures

Géraldine Dantelle, Sandrine Beauquis, Ronan Le Dantec, Virginie Monnier, Christine Galez, and Yannick Mugnier*

With the miniaturization of electronic-based devices, the foreseen potential of new optical nanoprobes and the assessment of eventual size and shape effects, elaboration of multifunctional noncentrosymmetric nanocrystals with ferroelectric, pyroelectric, piezoelectric, and nonlinear optical properties are the subject of an increasing research interest. Here, the recent achievements from the solution-based methods (coprecipitation in homogeneous and nanostructured media, sol-gel processes including various chemistries and hydro/solvothermal techniques) to prepare 0-D perovskite and nonperovskite oxides in the 5–500 nm size range are critically reviewed. To cover a representative list of covalent- and ionic-type materials, BaTiO₃ and its derivatives, niobate compounds (i.e., K/Na/LiNbO₃), multiferroic BiFeO₃, and crystals of lower symmetry including KTiOPO₄ and some iodate compounds such as Fe(IO₃)₃ and La(IO₃)₃ are systematically in focus. The resulting size, morphology, and aggregation state are discussed in light of the proposed formation mechanisms. Because of a higher complexity related to their chemical composition and crystalline structures, improving the rational design of these multifunctional oxides in terms of finely-tuned compositions, crystalline hosts and structure–property relationships still need in the future a special attention of the research community to the detailed understanding of the reaction pathways and crystallization mechanisms.

all, or only some, of the corresponding functional properties that include ferroelectricity, pyroelectricity, piezoelectricity and Second Harmonic Generation (SHG), optical activity and enantiomorphism.[1,2] A very wide range of applications covering for instance electrical-energy storage in capacitors, nonvolatile memories, sensors, actuators, and piezoelectric-based energy harvesters thus depends on these symmetry requirements resulting in 20 different crystal classes for piezoelectricity (and SHG) and only 10 possible ones for polar pyroelectric materials that may also exhibit a ferroelectric behavior. With the advent of nanoscience and nanotechnologies, the last two decades have witnessed an exponential increase in the studies aiming at preparing and characterizing some of these multifunctional NCO at the nanoscale. As in many other nanomaterials like metals, carbon nanotubes, 2-D materials, magnetic nanoparticles, fluorescent quantum dots, ..., understanding and assessing eventual size effects in these multifunctional oxides at the nanoscale have both been driven by fun-

damental and technological standpoints, following the current trend in the miniaturization of electronic and smart devices. The scaling limit of the ferroelectric order in monocrystalline nanoparticles,[3,4] the temperature transition between the ferroelectric and paraelectric phases,[5] the polarization decrease

## 1. Introduction

Among the 32 possible crystal classes (or point groups), polar and nonpolar noncentrosymmetric oxides (NCO) belong to one of the 21 noncentrosymmetric crystal classes and as such exhibit

G. Dantelle
Université Grenoble Alpes
CNRS
Grenoble INP
Institut Néel
Grenoble 38000, France

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202200992.

© 2022 The Authors. Small published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/smll.202200992

S. Beauquis, R. Le Dantec, C. Galez, Y. Mugnier
Université Savoie Mont Blanc
SYMME
Annecy F-74000, France
E-mail: yannick.mugnier@univ-smb.fr

V. Monnier
Univ Lyon
Ecole Centrale de Lyon

CNRS
INSA Lyon
Université Claude Bernard Lyon 1
CPE Lyon
CNRS
INL
UMR5270, Ecully 69130, France
associated with a reduction in the spatial coherence of the ferroelectric order below 10 nm[6–8] and the rational design of hierarchical structures to enhance the collective interactions between individual nanocrystals[9] are in this sense key illustrative examples of size effects in polar NCO at the nanoscale. Similarly, the piezoelectric response has been evidenced to be reduced in sub-20 nm perovskite-type lattices[3] and deviations from the bulk SHG responses originating from surface effects have also been suggested in two different nanomaterials, BaTiO₃ and LiNbO₃[10,11] in the Rayleigh regime (<10 nm).[12] On the contrary, in the 20–150 nm size range, the scattered second harmonic intensity collected from individual nanocrystals and colloidal suspensions scales with the square of the nanocrystal volume. If size and shape effects are still not clearly demonstrated above ≈20 nm (although crystalline heterogeneity can be present[13]), the reduced dimensions of 0-D nanomaterials remarkably come into play though, since generation of harmonic signals (SHG and higher harmonic contributions) are no longer hindered by the severe constraint of phase-matching conditions.[14] NCO nanocrystals thus behave as nonlinear point-like dipoles and their use in the biomedical imaging field further pushes some of the limits of the traditional fluorescence microscopy based on dyes, proteins, quantum dots, and up-converting nanoparticles: low penetration depth due to tissue/living cell absorption and autofluorescence, optically resonant conditions, photobleaching and blinking. Harmonic signals from NCO nanocrystals used as exogenous bio-labeling probes thus hold great promise in the bioimaging and theranostic fields[15–20] and especially for very long term and fast in vivo imaging in the four NIR biological transparency windows.[21–24] As nonlinear optics is concerned, both Rayleigh and Mie scattering from all-dielectric nanostructures[25,26] and metal-NCO core–shell cavities[27,28] have also stimulated a lot of interest for the tailored and on-demand enhancement of the wavelength-dependent emission of harmonic signals when size control of these (hybrid)-nanostructures is achieved.[29] Likewise, preparation of thin dielectric films from NCO colloidal suspensions under mild chemical conditions relying on the heterogeneous mixing with a monomer paved the way for the development of new printable and spin-coated functional devices.[30–32] Finally, the reduced dimensions of simple and complex 0-D perovskite nanomaterials and their concomitant high specific surface area have also shown promising performances in the field of photocatalysis[33,34] even if more studies are needed to elucidate the structure–property relationships and the interplay between the above-mentioned functional properties and the resulting photocatalytic activity.[35] On the other hand, if this review is exclusively dedicated to the dielectric properties of 0-D NCO nanocrystals, it is worth noting that manganite-based perovskites have also been the subject of intensive research interests aiming at assessing their magnetic properties such as the large magneto-resistance effect and the influence of the material composition on the (local) crystalline structure, Jahn–Teller distortions and transition temperatures.[36–38] Note that scaling effects have also been noticed in the magnetization of, for instance, 25 nm ferromagnetic manganite nanocrystals.[39]

In terms of materials, among the historically well-known I–V, II–IV, and III–III perovskite and nonperovskite ABO₃ crystalline lattices, BaTiO₃ and its derivatives after isovalent

chemical substitution of Ba²⁺ for Sr²⁺ or Pb²⁺, and of Ti⁴⁺ for Zr⁴⁺, have certainly attracted the most intense research efforts and have already been the subject of several comprehensive reviews published before 2015 for most of them.[40–46] It turns out that the observed modifications of the physical properties when reducing the material size are not only related to the intrinsic nanomaterial size and shape but also depend on additional factors like the crystalline quality, chemical homogeneity and composition, point defects and the potential presence of an amorphous surface layer and of nanodomains whose spatial coherence can be strongly reduced comparatively to their bulk counterpart. Synthesis methods and in particular solution-based techniques have thus been continuously refined with the aim of improving the above parameters for a better understanding of the resulting structure-property relationships. Recent progresses in the different wet chemical routes are specifically addressed in this review. As it will be discussed in Section 2, the conventional crystal growth techniques and high-temperature solid-state processing of ceramics are very restricted so as to produce monodisperse, nonaggregated and monocrystalline NCO nanocrystals with size and shape control in the 5–500 nm size range. Of utmost importance for the large-scale promotion of nanomaterial-based applications, the solution-based techniques described in the following seem unavoidable but they also are to be considered in terms of precursor and processing costs, scalability, safety, simplicity, and environment-friendly aspects. In addition, if a range of chemical processes leading to the reproducible preparation of these functional NCO nanocrystals has already been proposed, future developments and improvements in the large-scale production of these nanomaterials of tailored size, shape, and morphology, ultimately rely on the elucidation of the reaction pathways and crystallization mechanisms, and of their proper kinetics. Effects of the initial precursor concentrations, composition of the reactive medium, and processing parameters (temperature, pressure, heating rate, etc.) must be related to the formation of the different intermediate species (clusters, nuclei, amorphous entities, and nanocrystals) thus requiring the use of advanced experimental techniques to study the possible partial time overlay of the different formation steps or, on the contrary, the existence of well-separated steps during the formation process (namely the cluster occurrence, nucleation burst, nanocrystal growth and coarsening).[47–50] Interestingly, crystallization and coarsening may result from several, possibly combined, phenomena including aggregation-induced crystallization, oriented-attachment process, dissolution-precipitation mechanism (Ostwald ripening), and coalescence.[51–55] In practice, such a detailed understanding of the growth mechanisms requires extensive space- and time-resolved measurements and unfortunately, there is no general rule for the NCO nanocrystals as different crystallization pathways have been demonstrated for a given material depending on the solution-based route applied.

The above kinetic aspects and growth mechanisms, when reported in the literature, are indicated in the next sections of this review only dedicated to 0-D dielectric nanomaterials and whose global outline is largely inspired from the excellent contributions of Charles et al.[56] and Schnepp et al.[57] Very notably, the overall complexity of NCO nanocrystals has recently been increased in terms of chemical compositions with
the synthesis of three- and four-cation perovskites belonging to high-symmetry point groups,[8,58] in terms of new functional optical properties with the doping by fluorescent rare-earth ions,[59–61] in terms of crystalline structures with new hosts belonging to low-symmetry point groups[62,63] and finally, in terms of surface functionalization for bioimaging[64,65] and for the two-photon-triggered photorelease of caged compounds in advanced drug-delivery theranostic applications.[66,67] In parallel, the experimental techniques (TEM imaging, synchrotron radiations, pair distribution function analysis of the Bragg and diffuse scattering components, SAXS, real-time UV–Vis, FTIR, DLS, SHG, and AFM-based experiments, etc.) comparatively to the bulk characterization methods[68] have also witnessed a similar rise in complexity. Importantly, because of the reduced material size and associated broadening of the XRD peaks, analysis by Raman spectroscopy is very often necessary to identify the actual crystalline modification of several NCO nanocrystals. Barium titanate and its derivatives and some nio-bate compounds are, for instance, well known to exhibit different polymorphs. Regarding the solution-based techniques, new chemical precursors, chelating agents, processing techniques including the solvothermal conditions with or without microwaves are also becoming more available and affordable as reviewed in the following. Section 3 deals with coprecipitation reactions in homogeneous and nanostructured reaction media (microemulsions and bioinspired templates). The different sol-gel approaches (Section 4) leading to NCO nanocrystals are then discussed starting from the traditional chemistry route leading to metal oxides from only alkoxide precursors (Section 4.1).[69,70] In Section 4.2, the “small molecule approach” initially developed with citric acid is described. It is actually based on several chelating agents and on a variety of low-cost water-soluble metal salts to circumvent some of the limitations of the alkoxide-based route and to effectively ensure stabilization of different metal complexes of a given composition. Section 4.3 describes the controlled formation of mixed-metal oxides from the so-called Pechini method,[71] i.e., when a polymerizer like ethylene glycol is added to initiate polyesterification of homogeneously dispersed metal precursors. Alternatively, polymers can also be directly used to stabilize metal ions leading to gels upon heating according to the polymer method addressed in Section 4.4. Finally, Section 5 is devoted to the increasing use of hydrothermal/solvothermal conditions that have proven to val-uable extend the above sol-gel approaches for the preparation of colloidal suspensions of well-crystallized NCO nanocrystals with size and shape control. A special paragraph is dedicated to microwave-assisted solvothermal conditions, which appear to be very promising to shorten the overall synthesis duration while offering an extra control of the nucleation and crystallization steps due to the rapid heating rates comparatively to conventional furnaces. Note that the use of supercritical conditions with non-toxic solvents like water and alcohols is not addressed in this review despite the scalability and very short synthesis times associated with the corresponding continuous hydrothermal flow synthesis processes. So far, the preparation of NCO nanocrystals from this technology mainly concerns BaTiO₃ as outlined in two recent comprehensive reviews.[44,72] In Section 6, a table summarizing the main advantages and drawbacks of the different solution-based routes previously

discussed is included to highlight that preparation of colloidal inorganic 0-D NCO nanocrystals with size and shape control in a large range is rarely predictable. It turns out that improving, in the near future, the rational design of these multifunctional nanomaterials in terms of finely-tuned compositions, crystalline hosts and structure–property relationships still needs a special attention of the research community to the detailed understanding of the reaction pathways and crystallization mechanisms. Upscaling the production and elucidating the formation mechanisms are thus a prerequisite for the future development of new devices and industrial applications. To that end, in spite of the higher complexity of acentric nanocrystals in terms of chemical composition and crystalline structures, recent illustrative studies dealing with the reaction and crystallization pathways of centrosymmetric metal and metal oxide nanoparticles (along with preliminary results recently demonstrated for LiNbO₃ nanocrystals) are purposely given in this special section as a source of inspiration. Also, note that in all of these sections, the synthesis methods leading to representative nanoscale materials belonging to high- and low-symmetry point groups are detailed with a special focus on the perovskite-type BaTiO₃ and its derivatives after isovalent chemical substitution of Ba²⁺ and/or Ti⁴⁺, on niobate materials like K/NaNbO₃ and the largely investigated nonperovskite LiNbO₃, on the well-known multiferroic pseudo-perovskite BiFeO₃[73] and on low-symmetry compounds such as KTiOPO₄ (KTP) and Fe/La(IO₃)₃. BaTiO₃ has always been considered as a model material in terms of its functional and crystalline properties. Its lattice system presents several polymorphs with, in particular, a centrosymmetric cubic structure above the Curie temperature and a noncentrosymmetric tetragonal structure (P4mm) stable at room temperature. Similarly, potassium niobate (KNbO₃) has different polymorphs including a noncentrosymmetric orthorhombic structure (space group Amm2, Z = 2) stable at room temperature. Regarding La(IO₃)₃, several polymorphs have also been identified, with the most efficient in terms of non-linear optical response being α-La(IO₃)₃.[62,74]

## 2. Why Solution-Based Techniques Are Needed to Produce NCO at the Nanoscale?

After the discovery in the forties of ferroelectricity in the tetragonal BaTiO₃ modification and in the nonperovskite LiNbO₃ structure, noncentrosymmetric mixed-metal oxides have attracted a considerable interest because of the greater richness of their functional properties comparatively to the already used non-polar quartz for oscillators. Numerous applications have thus emerged for both the very strategic military field (such as the sonar) and the civil one. One can mention the use of piezoelectricity in electronics and medical imaging (diagnostic imaging with ultrasounds), and more recently, the development of various integrated devices for the semiconductor industry.[75] With the aim of improving the overall energy-conversion in these smart materials, a very large panel of chemical compositions had already been identified by 2000,[2,76] either in the form of bulk single crystals or ceramics. With the current trend in miniaturization, advanced functional thin films and nanometer-sized crystals have also received a considerable
interest.[77] High-temperature long-time processing methods and various deposition techniques under vacuum have thus been developed. Among them, the crystal-growth techniques like the Czochralski and Bridgman methods are very limited, if not adequate at all, at the nanoscale although $KTiOPO_4$ (KTP) nanocrystals could be collected as byproducts after centrifugation of the remaining raw powder in the growth baths.[78] Solid-state chemistry approaches also suffer from several insurmountable drawbacks beyond the relatively high degree of aggregation in the final products. Note that the term “aggregation” is used in this review to define particles that form aggregates, i.e., an ensemble of particles strongly bonded together, as defined by the IUPAC. In opposition, the term “agglomeration” is used to characterize particles assembled together in a reversible way.[79,80] After the mixing of carbonate and oxide powders, the high-temperature calcination step is not only time- and energy-consuming but results in powders showing a very large size distribution along with phase impurities from the unreacted precursors and secondary phases due to the prolonged treatment at elevated temperature. Besides the absence of any size and shape control, an accurate adjustment of the local stoichiometry is particularly hard to achieve even for the simplest bimetallic compositions[44] because of the very slow diffusion of ion species between the initial solid precursors. Among other solid-state alternatives, the high-energy ball milling process has been applied successfully to several perovskites after mixing of the necessary oxide precursors thus resulting in narrower size distributions after annealing but still with a high degree of aggregation and phase impurities.[81,82] Absence of any shape control is to be noticed as also evidenced after the ball-milling of bulk crystals of $BaTiO_3$ and $KNbO_3$.[18,83] Aqueous colloidal nanocrystal suspensions could be achieved through after sonication, either from an adjustment of the pH and polymer adsorption to promote steric repulsions or sedimentation of the larger aggregates.[84] As a consequence, the total reaction yield for the preparation of colloidal suspensions is far below 100% and the exact mass concentration is difficult to estimate for any subsequent physicochemical analysis and any further chemical surface modification.

Interestingly, crystallization of $BaTiO_3$ nanoparticles after mixing of the raw powder precursors (for the Ba and Ti sources) under hydrothermal conditions led to different published scenarios, namely the “in-situ reaction mechanism” involving the surface reaction and the diffusion of $Ba^{2+}$ ions into $TiO_2$ particles and the “dissolution-precipitation mechanism” after complete dissolution of the two solid precursors. This latter scenario was finally supported by in situ time-resolved neutron diffraction experiments,[85] again evidencing that elucidation of the reaction and growth mechanisms for the advanced synthesis of nanoscale NCO may be an issue especially when solid-state precursors of different solubilities are mixed.

For all these reasons, the so-called wet chemical routes or solution-based techniques have been increasingly preferred. First, a homogeneous, stoichiometric metal precursor containing two or more cations is more likely to be obtained from the liquid state by ensuring the intimate mixing of the metal precursors at the atomic/molecular scale. Trapping the “randomness of the solution state”[86] indeed allows the rapid synthesis of inorganic quaternary and even quinary oxides with

the desired stoichiometry.[8,58] Obviously, this “molecular” stoichiometry and homogeneity in the initial precursor solution is more easily achieved in the liquid state than from a mixture of dry powders. Preparation of inorganic solids by sol-gel chemistry then involves the transformation of a liquid precursor to a sol and to one of the 5 types of gel as illustrated in Figure 1.[57,86–88] Second, for the sol-gel chemistry route with alkoxide precursors, kinetics of hydrolysis and condensation reactions are catalyst-dependent, thus offering an accurate way of nanostructuring the reactive medium at the initial stage of the oxide formation.[87] More importantly, the oxide network of (mixed)-metal oxides is directly “self-organized” from the molecular precursors till the final oxide material and sometimes without the need of a high-temperature annealing step to promote crystallization.[70] Note also that in the case of monometallic oxides, purely chemical parameters like the solvent and nature of the alkoxy group (primary, secondary or tertiary) strongly affect the inorganic polymerization reaction and thus the nucleation/growth kinetics and as a consequence, the final particle size and morphology. As discussed later, a series of NCO perovskite nanocrystals has been successfully produced at (or near) room temperature and it turns out that the crystalline structure also comes into play since the isotropic cubic and pseudocubic symmetries are more easily obtained than the more anisotropic orthorhombic lattice.[58] Among other considerations, it is worth noting that the alkoxide route may be limited by the availability and cost of the metal (and even rare-earth) elements and by their potential high degree of reactivity with water. In addition, the initial hydrolysis and condensation reactions that are supposed to lead to the desired stoichiometry actually depend on several experimental parameters including the element electronegativity, the amount of water and its pH, the solvent polarity, the nature of the alkyl group, and the possible use of additional chelating agents.[89,90]

To circumvent some of the inherent limitations of the alkoxide route, both the small molecule and Pechini methods make use of a range of chelating agents (with or without additional hydroxyl groups) with the aim of stabilizing metal complexes from water-soluble metal salts. Contrary to the alkoxide route, polymerization and self-organization of the homogeneous metal salt solution leading to the oxide phase do not occur and evaporating the solvent only results in the precipitation of amorphous oxides/hydroxides. Stabilization of metal complexes with the help of citric acid or other chelating agents like dicarboxylic acids and ethylenediaminetetraacetic acid (EDTA) is a prerequisite before the solvent evaporation and formation of a dried resin. If the stoichiometry at the molecular scale is preserved between the two (or more) initial metal cations, the subsequent annealing step may result in the desired composition. As expected, the molecular structure of the chelating agent and the possible presence of hydroxyl groups have a huge impact on the final phase purity and on the lowest annealing temperature promoting crystallization and possibly Ostwald ripening.[91] In the same way, the subtle interplay between these purely chemical parameters and the physical ones (annealing temperature, heating rate) is expected to be changed in the Pechini and polymer methods after the formation of an initial stoichiometric resin. For the former, a polymerizer such as ethylene glycol is added to
<table><caption>**Figure 1.** The 5 types of gels leading to the formation of nanostructured ABO₃ materials by a sol-gel approach. Image adapted with permission.[57]
Copyright 2016, The Royal Society of Chemistry.</caption><thead><tr><th>Type of gel</th><th>Bonding</th><th>Source</th><th>Gel schematic</th></tr></thead><tbody><tr><td>Colloidal or particulate [86-88]</td><td>Particles connected by Van der Waals or hydrogen bonding</td><td>Mainly amorphous oxides or hydroxides from hydrolysis of inorganic precursors</td><td></td></tr><tr><td>Inorganic polymers [86-88]</td><td>Interconnected via covalent or intermolecular bonding</td><td>Hydrolysis and condensation of metal or non-metal alkoxides</td><td></td></tr><tr><td>Metal-chelate ('small molecule approach') [86]</td><td>Weakly-interconnected metal complexes</td><td>Gel or resins after drying from water-soluble metal chelates</td><td></td></tr><tr><td>In situ polymerizable complex ('Pechini method') [71]</td><td>Organic polymers interconnected by covalent and coordinate bonding</td><td>Polyesterification between polyhydroxy alcohol and carboxylic acid with metal complex</td><td></td></tr><tr><td>Polymer solution method [86]</td><td>Organic polymers interconnected by coordinate and intermolecular bonding</td><td>Metal salt solution and coordinating polymer</td><td></td></tr></tbody></table>

initiate polyesterification upon heating, while, for the latter, specific polymers can help in the nanostructuration of the gel before the calcination step.[57] Noteworthy, the last three approaches are easily scalable and are based, most of the time, on cheap precursors such as metal nitrates and standard organic additives.

Pressure is another important physical parameter for the processing of the sols and gels resulting from one of the above-mentioned wet chemical routes. Typical mild hydrothermal and solvothermal conditions—not involving the supercritical state—are associated with moderate temperatures, which can reach 250 °C in the most popular Teflon-lined stainless steel autoclave (350 °C in the case of a quartz vessel), and a pressure of a few tens of bar if Teflon is the liner.[92] Such conditions increase the solubility and reactivity of metal salts and complexes at relatively low temperatures and pressures, and more importantly can lead to the formation of crystalline materials, in solution, without a subsequent annealing treatment. A very large variety of crystallized inorganic (nano)-materials have thus been obtained at significantly lower temperatures comparatively to the solid-state processing route.[56,93–95] As expected, the

choice of an appropriate solvent (with or without cosolvent) and of a possible chelating agent has a considerable effect on the resulting nanocrystal size and morphology through modification of the nucleation rate, crystallization pathway, and growth mechanisms. In the same way, microwave-assisted heating through absorption by the solvent and/or reactive species of the microwave energy has several advantages in comparison to the conventional furnaces.[96,97] Among them, a higher heating rate, a better control of the reaction parameters and selectivity against side reactions as well as the selective heating in the case of a heterogeneous mixture are additional key parameters to consider when changing the overall nucleation and growth kinetics is necessary for adjusting the final nanocrystal size and shape.

In the following, with the aim of better controlling the final nanocrystal size, morphology, crystalline quality and aggregation state, the formation and crystallization mechanisms for different synthesis methods will be overviewed regarding the elaboration of representative NCO nanocrystals such as BaTiO₃, K/Na/LiNbO₃, BiFeO₃, KTP and Fe/La(IO₃)₃, NCO nanocrystals.
## 3. Water-Based Coprecipitation Reactions in Homogeneous and Nanostructured Reaction Media

Formation of sparingly soluble solid phases from coprecipitation reactions always involves conditions of high supersaturation usually achieved after a simple mixing of precursor solutions or from a chemical reaction leading to products with low solubilities. Chemical reduction, hydrolysis, and oxidation are typically the chemical reactions employed for the preparation of nanoparticles although precipitation can also result from a rapid change in temperature and concentration (from a simple mixing as already stated) since these two parameters directly affect the nucleation rate.[56] Comparatively to simple metals and binary oxide compounds, formation of mixed-metal oxides from precipitation reactions is however less straightforward, especially near room temperature, since intermediate phases such as amorphous products containing mixture of hydroxides/oxides are typically obtained. One thus has to separate the precipitation reactions directly leading to the formation of the expected NCO nanocrystals from the most common cases corresponding to amorphous intermediates that require a calcination step to promote crystallization. As already outlined in the case of mixed-metal oxides, this latter scenario very often requires the use of additional small molecules (the so-called oxalate, tartrate, and citrate routes, see Section 4.2) to stabilize intermediates with the expected stoichiometry so as to circumvent differential precipitation arising from the different chemistries of the metal cations in solution.[86] In addition, without a chelating/capping agent, aggregation and coarsening are then very likely to occur. Another limitation of the coprecipitation reactions in homogeneous solutions is that nucleation and growth may occur simultaneously thus resulting in particles showing broad size and shape distributions. On the good side, precipitation is generally characterized by its simplicity in terms of laboratory equipment and chemical reactions from cheap precursors since a direct mixing of dissolved salts can lead to the desired compound. Similarly, these general comments also apply for nanostructured reaction media, i.e., after mixing of two reactive microemulsions containing, for instance, the respective metal precursors in their aqueous phase. If a lower reaction yield is to be mentioned comparatively to the use of homogeneous aqueous solutions, reverse micelles are known to behave as nanoreactors of adjustable sizes[98–101] possibly leading to size and shape control as well as to other phenomena like oriented-attachment, self-assembly, and aggregation-induced crystallization.[51,53,54,102]

### 3.1. Precipitation from Homogeneous Solutions

Studies involving simple mixing of homogeneous solutions free from either organic precursors or other organic intermediates such as tartrates, citrates, and oxalates are relatively sparse as far as precipitation of NCO nanoparticles is concerned. Among them, the simple pouring of $(NH_4)_2TiO_4$ aqueous solution to an acid solution containing dissolved $Bi^{3+}$ ions from a nitrate precursor results in $Bi_4Ti_3O_{12}$ nanoparticles.[103] Crystallization is then obtained upon annealing between 520 °C and

680 °C, leading to an increase in the mean nanocrystal size from 15 to 56 nm. Interestingly, variations in the Curie temperature not only evidence a size effect but also the influence of defect content related to the oxygen vacancies. Similarly, several iodate compounds can be easily produced after dissolving iodic acid in water and mixing with metal cations provided that the solubility of the expected phase is lower than the one of $HIO_3$ in water. In fact, in the seventies, a large series of SHG-active iodate materials had already been produced, for instance in the Bell Laboratories.[104,105] For 0-D NCO nanocrystals, iron iodate ($Fe(IO_3)_3$), has first been produced from a simple mixing of a dissolved iron nitrate precursor with an iodic acid solution thus resulting, after filtering of the aged precipitate maintained at 80 °C for 2 d, in 30–40 nm nanocrystals.[106,107] If the total reaction yield is obviously below 100% after filtering and if no size and shape control is achieved, this simple and very cheap precipitation reaction has the merit to produce SHG-active nanocrystals in a scalable way. Similarly, nanoparticles of $LiAl(IO_3)_4$ belonging to the low-symmetry $P2_1$ space group have also been obtained after a simple coprecipitation in water and solvent evaporation thus leading to partially agglomerated but well-crystallized NCO nanomaterials.[63] However, for iodate compounds that easily precipitate in acidic solutions, solubility issues and long-term chemical stability are in this case to be considered before any further surface chemical modifications in water-based media. Regarding functional nanocomposites, nonlinear optical thin films of $SiO_2$ (and of Laponite) with embedded SHG-active $\alpha$-$LiIO_3$ nanocrystals can also be readily prepared from dip-coated solution precursors containing the dissolved lithium iodate salt. Annealing below 200 °C then results in the in situ formation and crystallization within the matrix of the functional SHG additives with a crystallite size in the 50–200 nm size range.[108,109]

To the best of our knowledge, all the other well-known III–III, II–IV, and I–V perovskite and nonperovskite materials belonging to the titanate, zirconate, niobate, and tantalate families could not be obtained from such simple routes because of a different chemistry in water of their constitutive metal cations. For instance, in the case of $BaTiO_3$, coprecipitation from chloride (or nitrate) salt solutions produces a mixture of hydrated $TiO_2$ and $BaCO_2$ whose calcination at 1000°C leads to different titanate phases (such as $BaTi_4O_9$ and $Ba_2TiO_4$).[110] Similarly, for $LiNbO_3$, a mixture of lithium carbonate and niobium hydroxide precursors can be precipitated at room temperature after raising the pH of an acidic solution used to dissolve $Nb_2O_5$ but only calcination above 700 °C results in the desired phase, whereas a higher crystallization temperature is necessary for the conventional solid-state method.[111,112] A direct precipitation at room temperature in homogeneous solutions of NCO phases is thus not reported even if different water-soluble alkali salts containing the hexaniobate Lindqvist ion [$Nb_6O_{19}$]$^{8-}$ have long been produced.[113] Only solvothermal conditions can result in the formation of the orthorhombic and of the tetragonal $KNbO_3$ polymorphs in highly alkaline solutions[114] as also evidenced for the centrosymmetric tetragonal phase of $NaNbO_3$.[115] For $KNbO_3$, the reaction mechanisms involve decomposition of [$Nb_6O_{19}$]$^{8-}$ ions into monomeric [$NbO_6]$$^7-$ species. Similar intermediates were also recently evidenced when different niobium oxides are solvothermally treated to produce $KNbO_3$
<figure><img src="image_3.png" /><figcaption>**Figure 2.** a) Nanocrystals of BiFeO₃ after a 3 min short annealing step at 720 °C of primary amorphous powders precipitated at 3 m KOH. Image reproduced with permission.[118] Copyright 2018, Hindawi. KTP nanocrystals obtained after precipitation of Ti-butoxide with KH₂PO₄ in presence of KCl; b) Hydrodynamic diameter before and after centrifugation, c) Zeta potential, d) TEM image, and e) SAED pattern. Reproduced with permission.[131] Copyright 2013, The Royal Society of Chemistry.</figcaption></figure>

nanocrystals of the orthorhombic form in KOH solutions.[116]
Note that for the nonperovskite LiNbO₃ crystalline structure,
a similar aqueous route is unsuccessful since the rock-salt
Li₃NbO₄ structure is typically obtained with another nonidenti-
fied hydrous phase according to the initial Li content.[117] In that
latter case, the prerequisite of a homogeneous dispersion of the
metal precursors at the atomic scale from the Nb-rich hexanio-
bate species that do not decompose into monomeric [NbO₆]⁷⁻
ions might explain why the formation of the functional LiNbO₃
phase is prevented.

Finally, in the case of the pseudo-perovskite III–III BiFeO₃
compound, a simple low-cost precipitation followed by a short
annealing step, resulting in crystallized nanoparticles, was also
recently reported.[118] Metal nitrate salts are first dissolved in
an acidic solution and then slowly added into freshly prepared
KOH solution of varying concentrations. If a short annealing
step of 3–4 min at 720 °C allows obtaining nanocrystals with
small coherence lengths in the 30–40 nm range, the absence
of any morphology control and a certain aggregation state
are to be noted as depicted in **Figure 2a**. This is expected as
no chelating agent has been used. Regarding the reaction
mechanisms, a similar room temperature solubility of the Bi
and Fe species in a small range of initial KOH concentrations
could explain the precipitation of stoichiometric amorphous
hydroxide precursors with a Bi/Fe ratio close to one and whose
rapid calcination leads to almost phase-pure, easily redispers-
ible BiFeO₃ nanocrystals.

Aside of the above described very simple precipitation proce-
dures involving no organic compound, hydrolysis of an alkoxide
precursor used as the source of one of the two metal cations
is very likely the oldest one for the preparation of mixed-metal
oxides. Note that Section 4.1 reports the use of only alkoxide
precursors without nanostructured reaction media. As BaTiO₃
is concerned, numerous research papers and patents for the
preparation of ultrafine powders (with the aim of improving the
resulting ceramic properties) date from the sixties and seven-
ties as already reviewed in 1990.[119] The first aqueous synthesis
of fine barium titanate powders was actually reported as early
as 1955 using different Ti alkoxide precursors (also called tita-
nium esters) added dropwise to a stirred basic solution heated

above 80 °C and containing barium hydroxide.[120] The chem-
istry of various titanium organic compounds being already
well established,[121] stoichiometry has then been improved
and a first example of particle size control from 100 nm down
to 8 nm was later achieved when tetraethyl (or tetraisopropyl)
titanate is replaced by the less hydrolyzable triethanolamine
chelate of tetraisopropyl titanate.[122] To promote nucleation and
prevent crystal growth, a rapid addition in the alkaline solu-
tion is crucial. According to the authors, the drastic change in
the nanocrystal size when variable amounts of isopropanol are
added in the reaction medium is to be related to higher super-
saturation values and lower solubilities of BaTiO₃ at high iso-
propanol content. Later, this alkoxide-hydroxide route was also
proven to be suitable for the preparation of ternary and qua-
ternary oxides in the (BaₓSr₁-x)(TiₓZr₁-x)O₃ system starting with
butoxide precursors.[123] The organic solution containing Ti-
and/or Zr-butoxides stabilized in butanol is hydrolyzed by an
alkaline aqueous solution of Ba and/or Sr hydroxides. A nucle-
ophilic attack process results in the replacement of the four
butoxy groups of the M(OR)₄ (M = Ti or Zr) precursor and the
formation of M(OH)₂⁻ species after reaction of four water mole-
cules and two hydroxyl groups with the metal center. Subse-
quent water elimination and condensation induced by the avail-
able divalent cations then lead to precipitation of the expected
perovskite phase. When Ti-isopropoxide is used in place of Ti-
butoxide, dissolution of Ba(OH)₂·8H₂O under stirring results
in the formation of a gel already containing at 60 °C the local
organization of the perovskite structure.[110] Such low-tempera-
ture syntheses like other non-aqueous routes[124] result though
in the cubic form of BaTiO₃ and, as already discussed,[119,125,126]
only sintering temperatures above 800 °C can induce a phase
transformation from the cubic structure to the multifunctional
noncentrosymmetric tetragonal one after cooling down again to
room temperature. Impurity ions at the surface of the growing
nanoparticles and/or their high surface energy would create a
strong enough depolarization field thus preventing the growth
of the tetragonal form near room temperature.[43,127-129]

Noteworthy, the simple precipitation approach involving
no alkoxide precursor is also possible in aqueous solutions
for materials with more complex chemical compositions that
belong to lower symmetry point groups comparatively to the (pseudo)-cubic perovskites. This is the case of KTP nanocrystals that can be simply obtained from an aqueous solution of potassium dihydrogen phosphate (KH₂PO₄) added drop-wise into a stirred solution of titanyl chloride (TiOCl₂). Calcination of the amorphous precipitate above 500 °C is necessary to promote crystallization thus resulting in particle size of ≈200–280 nm with no shape control. The particle size can however be reduced to 25–50 nm after the stoichiometric addition of a chelating agent, namely triethanolamine, in the initial solution of titanyl chloride.[130] Optimization of this initial approach and preparation of colloidal suspensions was later performed by Mayer et al.[131] after dilution of Ti-butoxide in a HCl solution followed by addition of KH₂PO₄ and K₂CO₃ to raise the pH to 6. The obtained precipitate is a mixture of an amorphous compound and crystallized KCl. Upon heating for 2 h between 550 °C and 700 °C, the KTP phase is formed with a crystal coherence length varying from 75 ± 25 nm to 200 ± 50 nm depending on the annealing temperature. Subsequent washings in water allow removing the unwanted KCl phase. Importantly, KCl plays a key role as it allows a space-confined nucleation and growth of KTP nanocrystals thus preventing their sintering and aggregation. Successive centrifugations of the obtained KTP suspension allow narrowing down their average size from ≈150 to 80 nm (Figure 2b–e). With a zeta-potential of -40 mV at neutral pH, these KTP nanocrystals present a high colloidal stability, which opens the door to applications in the field of bioimaging.[131]

## 3.2. Precipitation from the Microemulsion Route

Simple mixing of two water-in-oil microemulsions leading to precipitation reactions without organic precursors as a source for the metal ions has also been demonstrated from Fe³⁺ and IO₃⁻ ions initially dispersed in their respective reverse micelle solutions. After formation at/or near room temperature of spherical amorphous precipitates, an aggregation-induced crystallization mechanism results in the appearance and growth of Fe(ΙΟ₃)₃ nanorods.[49,132] Interestingly, because of the non-centrosymmetric crystalline structure of iron iodate, second harmonic scattering (SHS) allows a real-time and in situ monitoring of the crystallization and growth kinetics of nanorods that strongly depend on the reaction time, water content, nature of the oil phase, temperature and surfactant concentration (Figure 3a–d). Upon addition of short-length poly(ethylene) glycol molecules within the water phase to adjust the binding interactions between the anionic headgroups of surfactant molecules and the initial amorphous precursors, several hybrid structures depicted in Figure 3e can be obtained ranging from spherical nanocrystals to bouquet-like and urchin-like microstructures after self-assembly of reaction intermediates.[133]

Regarding BaTiO₃ and its solid solutions after substitution of Ba²⁺ with Sr²⁺, the microemulsion route has also been proven valuable. Contrary to the simple precipitation approach without organic precursors, aqueous solutions of the respective chloride salts, combined with cyclohexane and emulsifiers, can lead, after the controlled addition of NaOH, to the direct formation of ferroelectric nanocrystals of adjustable size at a temperature as low as 80 °C.[126] Cyclohexane as the oil phase with Triton X-100 and

n-butanol as surfactants were used to prepare a microemulsion containing Ti⁴⁺ ions within acidic water droplets of different sizes. At this stage, formation of various Ti-butoxides cannot be ruled out. Barium and/or strontium chloride salts dissolved in deionized water are then added under continuous stirring. Interestingly, for Ba₀.₇Sr₀.₃TiO₃ spherical nanocrystals, a simple change in the initial water content allows a readily and fine size tuning from 50 nm down to 10 nm.[126] For the microemulsion route, choice of the initial reactants and surfactants is however vital for the direct formation of a NCO phase. Indeed, when barium nitrate and titanium oxynitrate are employed, both the direct precipitation in oxalic acid and the mixing of two reactive microemulsions prepared from nonylphenol anionic surfactants result in amorphous precursors. To obtain the perovskite structure, the calcination temperature can be reduced though from 700 °C to 550 °C for the microemulsion approach but the tetragonal form is only obtained after cooling from 700 °C and above. Highly aggregated nanopowders consisting in sub-50 nm nanocrystals showing no size and shape control are obtained in each case.[134]

Similarly, uniform BiFeO₃ nanocrystals were produced by a quaternary microemulsion method, starting from bismuth and ferric nitrate aqueous droplets stabilized in a microemulsion composed of octane, CTAB (cetyltrimethylammonium bromide) and butanol.[135] After mixing with a second microemulsion containing oxalic acid a Bi, Fe-oxalate is precipitated, resulting after calcination at 400 °C in pure, partially aggregated BiFeO₃ nanocrystals due to the relatively low crystallization temperature.

## 3.3. Precipitation in Presence of Biomolecules

Several highly innovative biomimetic synthesis methods were reported, paving the way to new and energy-efficient room-temperature approaches for the formation of small tetragonal BaTiO₃ nanocrystals.[129] Bansal et al. reported the formation of spherical 4–5 nm sized BaTiO₃ nanocrystals after the incubation of barium acetate and K₂TiF₆ by a pathogenic fungus at room temperature for one day.[136] The hydrolytic enzymes in the fungus play a key role in the hydrolysis of TiF₆²⁻ entities, allowing the control of the nucleation and growth processes. In addition, it is believed that the presence of a protein corona at the surface of the nanocrystals stabilizes the ferroelectric phase. Another bioassisted approach consists in using peptides as nano-templates. Tetragonal BaTiO₃ nanocrystals with a controlled size between 6 and 12 nm were formed from the room-temperature hydrolysis of a mixed Ba,Ti alkoxide inside peptide nanorings[125] which play the role of templates and confine the growth of BaTiO₃ nanocrystals (Figure 4a). The presence of peptides at the surface of the nanocrystals is likely to favor the unusual crystallization of BaTiO₃ in its tetragonal form at room temperature for this sol-gel approach in that case (proposed mechanism illustrated in Figure 4b). Finally, tetragonal BaTiO₃ nanocrystals (50–100 nm) were also produced at room temperature by Ahmad et al.[137] by the action of peptides with specific amino acids on an aqueous solution of barium acetate and potassium bis(oxalato) oxotitanate (Figure 4c). These peptides are believed to bind strongly to some inorganic compounds and even induce their crystallization.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Real-time and in situ monitoring of the crystallization and growth kinetics of Fe(ΙΟ₃)₃ nanorods probed by SHS measurements at a) different temperature and b–d) for different compositions of the reactive medium. Reproduced with permission.[49] Copyright 2011, American Chemical Society. e) Growth mechanisms of very different Fe(ΙΟ₃)₃ hybrid structures and higher-order complex forms after self-assembly following precipitation of Fe³⁺ and IO₃⁻ in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) microemulsions. Reproduced with permission.[133] Copyright 2012, American Chemical Society.</figcaption></figure>

# 4. Sol-Gel Methods

## 4.1. The Alkoxide Route

The traditional sol-gel chemistry route leading to monometallic oxide materials[69] from alkoxide precursors has been successfully extended to bimetallic oxides such as traditional perovskites[90] and even LiNbO₃ powders and films.[138,139] For this

approach, the availability and cost of metallic elements that can be used as alkoxide precursors and their high degree of reactivity with water must be taken into consideration. To decrease the sensitivity of metal alkoxides to moisture, the ligand exchange strategy was applied, for instance, between titanium isopropoxide and acetic acid groups resulting in Ti-acrylate that can be mixed with the cheap barium acetate precursor for a simple precipitation reaction in alkaline solution of sub-100 nm

<figure><img src="image_5.png" /><figcaption>**Figure 4.** a) TEM image of BaTiO₃ inside a peptide nanoring and b) corresponding scheme to illustrate the spatially-confined reaction mechanism in a nanopeptide ring. Reproduced with permission.[125] Copyright 2006, Wiley and Sons. c) TEM image of ferroelectric (tetragonal) BaTiO₃ nanocrystals obtained thanks to the room-temperature action of specific peptides on an aqueous precursor solution of barium acetate and potassium bis(oxalato) oxotitanate(IV). Reproduced with permission.[137] Copyright 2008, American Chemical Society.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 5.** a–d) Illustration of BST nanocrystals produced after hydrolysis and formation of a self-assembled nanocrystal monolith, adapted with permission.[30] Copyright 2009, Royal Society of Chemistry. e) LaMer model,[140] scheme reproduced with permission.[142] Copyright 2019, American Chemical Society.</figcaption></figure>

powder.[119] In fact, the initial hydrolysis and condensation reactions that are expected to lead to the desired ABO₃ stoichiometric phase depend on several experimental parameters including the electronegativity of the element, the amount of water, its pH, and the polarity of the solvent.

The detailed growth mechanisms are not always well understood, but a series of very convincing results have recently been reported for BaTiO₃ and its derivatives of the BaₓSr₁₋ₓTiO₃ (BST) type.[30] In that case, water diluted in ethanol is simply added to a stoichiometric mixture of titanium, barium, and strontium isopropoxides. The mixture is then sealed and removed from the glove box for an aging step allowing the preparation of a primary colloidal gel (Figure 1, first row). After syneresis around room temperature (between 15 °C and 55 °C), this leads after a few hours to the formation of a monolith containing nanocrystals from 7 to 12 nm depending on the amount of water added (Figure 5a–d). LaMer's model[140] comprising rapid nucleation and slow growth (without any further nucleation step) is then proposed to explain the growth mechanism (Figure 5e). It should be reminded that the crystal structures of BaTiO₃, SrTiO₃, and BaZrO₃ nanocrystals all exhibit cubic or pseudo-cubic symmetry at room temperature, while that of SrZrO₃ is orthorhombic. From this perspective, the increasing complexity of the synthetic procedures may be reflecting the difficulty of stabilizing crystal structures exhibiting large deviations from cubic or pseudocubic symmetries under the experimental conditions explored in this work. Note that when two alkoxides are used as precursors, namely Ba-isopropoxide and Ti-amyloxide, the pioneering work of Smith et al. already evidenced in 1969 that slow hydrolysis of the mixed precursors dissolved in isopropanol results in the room temperature production of pure BaTiO₃ nanocrystals in the 5–15 nm size range.[141] The mean nanocrystal size is increased to 30 nm after calcination at 700 °C and very notably, sintering above 1300 °C leads to translucent ceramics.

Mixing the sol-gel and the reverse micelle approaches have also long been tested to produce, for instance, a series of titanate and zirconate compounds after slow hydrolysis of metal alkoxides mixtures by microemulsions containing water. Interestingly, calcination was not necessary after reaction close to room temperature for deriving sub-20nm Ba/SrTiO₃ nanocrystals.[143]

Formation of monodisperse cubic BaTiO₃ nanoparticles was also demonstrated from a single bimetallic alkoxide molecular precursor.[144] The method consists in injecting under Ar or N₂ the mixed precursor, barium titanium ethyl hexano-isopropoxide, in diphenyl ether mixed with oleic acid. After cooling from 140 °C to 100 °C, an aqueous solution of hydrogen peroxide is then added to promote hydrolysis and condensation. No additional annealing step is required. Depending on the alkoxide/oleic acid ratio, the cubic BaTiO₃ nanoparticle size can be modulated between 4 and 12 nm.

Finally, still regarding BaTiO₃ synthesis, an original method based upon the kinetically controlled vapor-phase delivery of water to promote the slow hydrolysis of a bimetallic alkoxide (such as BaTi (OCH₂CHCH₃(OCH₃))₆) was developed by Brutchey and Morse.[145] The principle of this approach relies on the production of liquid water at the interface between the alcohol solution and the vapor phase, enabling the slow and controlled hydrolysis of the alkoxides and then the crystallization, at room temperature, of cubic BaTiO₃ nanocrystals. In a similar manner, sub-30 nm nanoparticles of BaZrₓTi₁₋ₓO₃ and Ba₁₋ₓSrₓTi₁₋ᵧZrᵧO₃ solid solutions were produced with higher volume fraction of noncentrosymmetric regions at 10–20 mol% of Ti⁴⁺ (Figure 6).[146]

To our knowledge and surprisingly, no report details the sol-gel synthesis of BiFeO₃ or KTP nanoparticles from the alkoxide route, despite the availability of commercial precursors, the already well-known chemistry of bismuth alkoxides[147,148] and the fact that iron alkoxide has for instance been used to produce porous Fe₃O₄ nanostructures. Note that this sol-gel approach was however employed to elaborate silica-based glass embedding KTP nanoparticles, namely glass-ceramics.[149,150] The typical process includes mixing of tetraethoxysilane, titanium butoxide, potassium acetate and PO(OC₄H₉)₃ in a 2-ethoxyethanol and then stirring the solution in the presence of a small amount of water at 50 °C for hydrolysis. The obtained gel is dried and then annealed between 630 °C and 1000 °C to promote the crystallization of KTP nanoparticles within the glassy matrix. On the other hand, this sol-gel approach based on metal alkoxides for the production of ABO₃ compounds has also been successfully coupled with treatments under solvent-thermal conditions and will be in this case detailed in Section 5.
<figure><img src="image_7.png" /><figcaption>**Figure 6.** a) TEM images and size histogram of BaZr~0.1~Ti~0.9~O~3~ nanocrystals elaborated from a modified vapor diffusion sol-gel method. Reproduced with permission.[146] Copyright 1996, Royal Society of Chemistry. b) Illustration of the hydrolysis occurring at the liquid/vapor interface. Reproduced with permission.[129] Copyright 2008, Wiley and Sons.</figcaption></figure>

In addition, when alkoxide precursors are replaced by dissolved metal salts in the presence of chelating agents (with or without polymerizer), formation of metal complexes and their subsequent heat treatment is another sol-gel approach[57] for the preparation of NCO at the nanoscale, as discussed hereafter.

## 4.2. The Small Molecule Sol-Gel Approach

An extension of the sol-gel approach involves the use of small (often chelating) molecules (oxalic acid, acetic acid, citric acid, EDTA, etc.) to stabilize one or more alkali or transition metal precursors in solution, inducing a better homogeneity of the starting precursors and thus leading to a better control of the nanocrystal size and shape upon elaboration.[57] Several chelating agents are thus effective in stabilizing metal complexes (line 3 of Figure 1) from water-soluble low-range metal salts to overcome some of the limitations inherent to the alkoxide-based sol-gel pathway.

The chelating-agent assisted synthesis of BaTiO~3~ was reported several decades ago and was, since then, largely reproduced, in particular with the aim of elaborating new multifunctional transition metal or rare-earth doped BaTiO~3~ nanocrystals that combine, for instance, SHG and up-conversion emission properties.[151,152] However, if the synthesis method efficiently produces BaTiO~3~ nanocrystals, the required thermal annealing inevitably induces particle aggregation. Among the oldest papers, one can mention the synthesis of cubic and tetragonal BaTiO~3~ nanocrystals in the 50–300 nm size range that can be easily obtained after a thermal decomposition above 550°C of barium titanyl oxalate (BaTiO(C~2~O~4~)~2·4H~2~O).[122] Sub-micron BaTiO~3~ particles were obtained from the reaction between TiO~2~ nanoparticles and BaCO~3~ in presence of oxalic acid in an alcoholic medium and further annealing at 1050 °C.[153] Kuo et al.[154] reported the formation of BaTiO~3~ thin films (<1.2 µm) from barium acetate and titanium isopropoxide in presence of acetylacetonate and after calcination at 600 °C. More recently, cubic BaTiO~3~ crystals were obtained by the controlled hydrolysis of titanium isopropoxide in presence of barium carbonate

dissolved in acetic acid.[155,156] The obtained precipitate was calcinated at relatively high temperature (800 °C–1000 °C) for a few hours (2 to 4 h), leading to nanometer-sized but aggregated particles (coherence length ≈ 50 nm). The addition of oxalic acid in the precursor solution enables to decrease the crystallization temperature of BaTiO~3~ thanks to a better mixing of the reactants and a higher homogeneity in the initial solution. As a consequence, the crystallization of BaTiO~3~ is observed at 700 °C, leading to nanostructured, spherical but still aggregated nanopowders.[151] Very interestingly, a recent study demonstrates the importance of time-resolved in situ infrared and XRD measurements to elucidate the formation mechanism of BaTiO~3~ nanocrystals after drying at 150 °C of a mixture of Ti- and Ba-solutions stabilized with citric acid and EDTA. If crystallization above 500 °C results from the reaction between two poorly crystallized, intermediate metastable phases (namely BaTi~4~O~9~ and a calcite-type BaCO~3~), lowering the CO~2~ partial pressure has been found key to avoid the formation of stable carbonates,[157]

The synthesis of KNbO~3~ and NaNbO~3~ nanocrystals was also developed after several steps and the use of KOH together with oxalic and citric acids as chelating agents.[158] Starting from cheap Nb~2~O~5~ and K~2~CO~3~ precursors, two distinct aqueous solutions were prepared, the first contains the Nb(V) precursor in a mix of oxalic and citric acids while K~2~CO~3~ is dissolved in citric acid for the second. Mixing these solutions leads to a sol and formation of KNbO~3~ upon calcination above 650 °C. Interestingly, when niobium hydroxide is used instead of Nb~2~O~5~, [159] the transparent gel obtained at 100 °C is transformed into size- and shape-polydisperse ≈350 nm ferroelectric KNbO~3~ particles after annealing at 750 °C while the same approach leads to faceted and almost monodisperse sub 50-nm antiferroelectric NaNbO~3~ nanocrystals at a lower crystallization temperature fixed at 650 °C (Figure 7a–f).

A similar crystallization mechanism was also evidenced for the preparation of LiNbO~3~ nanocrystals after dissolution of Nb~2~O~5~ and LiNO~3~ in HF and in water, respectively.[160,161] After mixing and addition of citric acid in stoichiometric amounts, the formation of a gel after drying above 100 °C is mentioned
<figure><img src="image_8.png" /><figcaption>**Figure 7.** a) TEM, b) HRTEM and c) SAED pattern of polydisperse $KNbO_3$ nanocrystals; d) TEM, e) HRTEM and f) SAED pattern of monodisperse $NaNbO_3$ nanocrystals both resulting from a small molecule sol-gel synthesis using oxalic and citric acids. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.[159] Copyright 2018, the Author(s), Published by Springer Nature. g) Single crystalline cubic-shaped $BiFeO_3$ nanocrystals by a solvent evaporation method using mucic acid as a chelating agent and excess of NaCl. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.[91] Copyright 2018, the Author(s), Published by Springer Nature.</figcaption></figure>

in both studies but different crystallized intermediates are detected upon calcination. The minimum crystallization temperature at 650–700 °C then results in sub-100 nm nanocrystals that do not show size or shape control and, as expected, a significant aggregation state. Interestingly, when a similar procedure is applied under hydrothermal conditions for 24 h at 230 °C, a net improvement in the resultant (almost cubic) crystal morphology showing {012} facets is observed together with an increase in the particle size distribution ranging from 300 nm to 1 µm.[162] More recently, $Nb_2O_5$ was replaced by niobium ethoxide dissolved in benzyl alcohol and solvothermally treated before a second treatment for 3 d in presence of $LiNO_3$[163] to adjust the decomposition rate of both precursors and thus to improve the resulting phase purity. This nonaqueous solvothermal route involving ligand exchange between the alkoxide precursor and benzyl alcohol[164] results after ether elimination in nearly monodisperse nanocrystals of 500 nm.

In the case of $BiFeO_3$, this so-called “small molecule” approach[57] was initiated by Ghosh et al.[165] and Selbach et al.[166] with citric acid and a series of carboxylic acids (tartaric, malic, malonic, succinic ...) with or without deliberate addition of additional hydroxyl groups (such as glycerol or ethylene glycol). Pure compounds but also mixtures of different phases can be obtained depending on the acid used. The average crystallite size increases with the minimum crystallization temperature (which can vary from 400 °C to 600 °C depending on the chelating agent used) as expected for a coalescence process and the formation of nanocrystal aggregates. Note that this solvent evaporation route with metal complexes has recently been revisited with mucic acid and excess of NaCl to promote the formation of individual $BiFeO_3$ nanocrystals (Figure 7g).[91] In this case, NaCl plays the role of a matrix increasing the spatial separation between primary nanocrystals so as to reduce their aggregation as already discussed for KTP (see Section 3.1).

### 4.3. The Pechini Method

The controlled formation of mixed metal NCO nanocrystals from metal ions homogeneously dispersed in a polymer precursor can also be achieved by the Pechini method (Figure 1, line 4). Typically, citric acid is used as a chelating agent to form metal citrate complexes as before, but a polymerizer such as ethylene glycol is then added to initiate polyesterization upon

heating. It is expected that the organic covalent network thus obtained will then be more stable during annealing, giving another possibility to control the crystallization and growth mechanisms. There are many carboxylic acids and polyols available that can be used in place of citric acid and ethylene glycol.

The Pechini synthesis of $BaTiO_3$ nanocrystals is widely reported in the literature. Typically, after a polyesterification reaction in presence of citric acid and polyethylene glycol, a calcination temperature of 720 °C is required to crystallize pure $BaTiO_3$ phase, with a crystallite size of ca. 25 nm but a strong aggregation state.[167,168] Anisotropic perovskite $KNbO_3$ nano-platelets were also obtained from the Pechini approach after a two-step crystallization. A primary amorphous gel obtained from the reaction of metal-citrate complexes with ethylene glycol is first annealed at $\approx 460$ °C resulting in the highly anisotropic $K_4Nb_6O_{17}$ crystalline intermediate. A template crystallization is suggested since calcination at 600°C then leads to $KNbO_3$ nanostructures with the same morphology as the crystalline intermediate.[169] 50 to 150 nm sized $Na_{0.5}K_{0.5}NbO_3$ nanocrystals have also been produced[170] from a water-soluble ammonium oxalate niobium complex mixed with sodium and potassium nitrates to prepare a gel that is thermally decomposed above 700 °C.

For $LiNbO_3$, the use of inexpensive reagents such as lithium carbonate ($Li_2CO_3$) and ammonium oxalate niobate (V) hydrate ($C_4H_4NNbO_9 \cdot xH_2O$) with citric acid and ethylene glycol, leads to aggregated nanocrystals after calcination at a temperature above 450 °C of the gel-like precursor (Figure 8a–d).[171] As the crystallization temperature is raised from 500 °C to 800 °C, the crystallite size is increased from 20 to 250 nm, respectively. Alternatively, a similar but less straightforward study was also reported using $Nb_2O_5$ firstly dissolved in HF. In that case, a careful optimization of the amounts of citric acid and ethylene glycol had to be found to avoid impurity phases and to produce sub-100 nm $LiNbO_3$ after calcination at 700 °C.[172]

For $BiFeO_3$, the lowest possible crystallization temperature strongly depends on the structure of the chelating agent and whether glycerol or ethylene glycol are added to promote polyesterification with the different (di)-carboxylic acids mentioned above. Impurity phases and the absence of any size and shape control are to be deplored after combustion of the gel.[91,166] It is likely that local thermodynamic conditions are poorly defined and that temperature is not uniform during the combustion stage. When the complexing agent is replaced by a
<figure><img src="image_9.png" /><figcaption>**Figure 8.** SEM images of aggregated nanocrystals of $LiNbO_3$ obtained with citric acid and ethylene glycol, and after air calcination of the precursor at a) 500 °C, b) 600 °C, c) 700 °C and d) 800 °C. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.[171] Copyright 2016, the Author(s), Published by Springer Nature. e) Example of $BiFeO_3$ nanocrystals obtained by self-combustion according to the Pechini method by replacing citric acid with 2-amino-2-hydroxymethyl-1,3-propanediol. Reproduced from with permission.[173] Copyright 2014, AIP Publishing.</figcaption></figure>

2-amino-2-hydroxymethyl-1,3-propanediol type fuel leading to self-combustion of the precursor, the morphology of the nanocrystals is not controlled at all (Figure 8e) and the presence of several impurities such as bismuth and iron oxides has been observed in some cases.[173] In other studies,[174,175] cheap nitrate precursors of Fe and Bi were dissolved in citric acid with ethylene glycol before drying the as-obtained gel. After grinding and calcination in air between 200 °C and 500 °C, a detailed chemical analysis of the pyrolysis products evidenced formation of several organic and nitrogen species below 300 °C and the formation of sub-10 nm $Bi_2(CO_3)O_2$ and $Fe_2O_3$ nanocrystals embedded in an amorphous matrix.[174] A solid-state reaction is demonstrated between these crystalline intermediates that finally results in the complete formation of $BiFeO_3$ at 450 °C for this easy scalable route although the final particle size and aggregation state are not specified. Interestingly, a higher crystallization temperature of around 700 °C is mentioned in ref. [175] for a very similar route showing that minor changes in the experimental protocol such as the drying step, the sol composition, and grinding of the dried gel have to be carefully assessed.

Finally, KTP nanocrystals have also been synthesized by a modified Pechini method, using ethylenediaminetetraacetic acid (EDTA) instead of citric acid, as it is known for its higher chelating ability for metallic ions and in presence of ethylene glycol as an esterification agent.[176,177] The resulting KTP nanocrystals present an average size smaller than 100 nm, but a strong aggregation state and no size nor shape control due to the high calcination temperature (>700 °C) required to induce crystallization of the KTP phase.

### 4.4. The Polymer Method

In the Pechini method, the in situ formation of a polymer is aimed to promote homogeneous stabilization by ensuring a stoichiometric mixing of the metal ions in a covalent matrix or “gel.” The direct use of polymers with metal salts to produce sol-gel precursors (Figure 1, line 5) corresponds to the “polymer method” that has been developed with many polymers showing strong interactions with metal ions. Among the most widely

used, green biopolymers (dextran, chitosan, cellulose, etc.) and synthetic polyvinyl alcohol (PVA)-type polymers can lead to homogeneous precursor solutions containing, for example, metal ions from water-soluble nitrate salts and to a gel after heating to ≈80 °C.

Monodisperse cubic $BaTiO_3$ nanocrystals were thus synthesized in a scalable way from titanium chloride, barium chloride, polyvinylpyrrolidone (PVP), and KOH as a mineralizer after a careful optimization of the refluxing temperature and composition of the reaction medium.[178,179] At high KOH concentration, smaller $BaTiO_3$ nanocrystals are obtained at ≈50 nm (Figure 9a) due to the supersaturation regime since, according to the authors, KOH promotes dissolution of the Ti precursor. Note that the proposed dissolution-precipitation mechanism is also consistent with a lower solubility of $BaTiO_3$ at high KOH content. Interestingly, the amount of adsorbed PVP is also KOH-dependent with a huge effect on the aggregation state as illustrated in Figure 9b.

For the alkali niobates such as $KNbO_3$, $NaNbO_3$, and $LiNbO_3$, a similar method (Figure 10) has been successfully applied using $KNO_3$, $NaNO_3$, and $LiNO_3$ salts and hydrated ammonium oxalate-niobate ($NH_4-NbO(C_2O_4)_2 \cdot xH_2O$).[180] PVA and sucrose in a molar ratio of 1:10 are then added and the aqueous mixture is heated to 90 °C to dissolve all reagents. The highly viscous mixture is then heated for 30 min at 220 °C and calcined at 530 °C. For $LiNbO_3$, this leads to a complete reaction and crystallite sizes at about 36 nm but the resulting nanocrystals are strongly aggregated (Figure 10).

For $BiFeO_3$, the molar ratio of all trivalent ions ($Bi^{3+}$ and $Fe^{3+}$) and hydroxyl groups of the PVA was adjusted between 1:1 and 4:1.[181] After calcination above 400 °C, phase-pure $BiFeO_3$ nanocrystals from X-ray diffraction studies (i.e., without apparent impurities such as $Bi_2Fe_4O_9$ and/or $Bi_{24}Fe_2O_{39}$) are only obtained when the molar ratio is 2:1 (Figure 11a). It can be noted that the minimum crystallization temperature is 400 °C with, in this case, formation of 30–40 nm nanocrystals according to the observed broadening of the (hkl) reflections. SEM imaging also reveals formation of highly aggregated nanocrystals showing a size increase after coalescence when the crystallization temperature is raised at 600 °C (Figure 11b,c).
<figure><img src="image_10.png" /><figcaption>**Figure 9.** a) FE-SEM imaging showing the size variation of cubic BaTiO₃ nanocrystals as a function of the initial KOH concentration and b) illustration of the amount of adsorbed PVP on the final aggregation state. Reproduced with permission.[179] Copyright 2016, The Ceramic Society of Japan and the Korean Ceramic Society, Published by Taylor and Francis.</figcaption></figure>

**5. Hydrothermal and Solvothermal Conditions**

As detailed in the previous sections, coprecipitation and sol-gel methods often involve the formation of amorphous intermediates in the preparation of most NCO nanocrystals. A subsequent calcination step is therefore mandatory to induce crystallization thus resulting in aggregated nanopowders which are then difficult to redisperse. In several studies, an increase in the crystallite size is also observed (through coalescence) according to the annealing temperature and the absence of any shape control is to be deplored. Previous sol-gel approaches leading to a homogenous stoichiometry at the molecular scale are in this sense poorly adapted for the preparation of colloidal NCO nanocrystal suspensions. In this regard, solvothermal conditions are much more appropriate since the solution-mediated crystallization under mild conditions is more prone to result in easily re-dispersible nanocrystals as illustrated below for representative materials belonging to the (pseudo)-cubic, orthorhombic and hexagonal symmetries. Use of specific solvents and chelating agents has also proven its efficiency toward size and shape control.

**5.1. Classical Hydrothermal/Solvothermal Method**

Because of its widespread use in electronic devices, preparation of BaTiO₃ fine powders under solvothermal conditions has already been intensively discussed.[40,182] It turns out that the reaction and crystallization pathways are very sensitive to the choice of the initial precursors and synthesis conditions. We thus focus in the following on impressive recent works in terms of size/shape control from alkoxide precursors regarding BaTiO₃ even if this approach is very likely suited to other perovskite structures like BaZrO₃, PbTiO₃, and SrTiO₃. Interestingly, it should be noted that already in 1940, barium hydroxide solutions and titanium alkoxides were already known to produce BaTiO₃ at a very moderate temperature of 80 °C.[183] Nowadays, in a typical synthesis, barium nitrate (Ba(NO₃)₂) and NaOH are dissolved in water and then mixed with a butanol solution containing titanium butoxide (Ti(OBu)₄) and oleic acid (C₁₇H₃₃COOH)). Solvothermal treatment at 135 °C for 18 h then leads to very regular cubic nanocrystals with a mean side length of 22 nm. Stabilization of the NCO nanocrystals (tetragonal modification) with oleic acid also promotes their self-organization (Figure 12a,b) into ordered arrangements.[184]

<figure><img src="image_11.png" /><figcaption>**Figure 10.** Illustration of the polymer method for LiNbO₃ with PVA and sucrose, and corresponding SEM imaging of the nanocrystals after calcination at 530 °C. Adapted with permission.[180] Copyright 2006, American Chemical Society.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 11.** a) X-ray diffraction profiles according to the calcination temperature for a sol-gel precursor of Bi<sup>3+</sup> and Fe<sup>3+</sup> ions stabilized by PVA. b,c) Corresponding SEM imaging after calcination at 400 °C (top) and 600 °C (bottom) demonstrating the aggregation state. Adapted with permission.[181]
Copyright 2010, American Ceramic Society.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 12.** a,b) TEM image of cubic-shaped NCO BaTiO<sub>3</sub> nanocrystals and their self-organization after solvothermal treatment of Ti(OBu)<sub>4</sub> and Ba(NO<sub>3</sub>)<sub>2</sub> in the presence of water in alkaline medium and oleic acid diluted in butanol. Reproduced with permission.[184] Copyright 2010, American Chemical Society. c–e) BaTiO<sub>3</sub> nanocrystals of cubic morphology when butanol is replaced by a butanol and cosolvent mixture of low permittivity. f–h) BaTiO<sub>3</sub> nanocrystals of spherical morphology when butanol is replaced by a butanol and co-solvent mixture of high permittivity. Reproduced with permission.[9] Copyright 2015, Royal Society of Chemistry.</figcaption></figure>
To date, the growth mechanisms are not fully understood but the authors point out that $M(OR)_4$ alkoxides ($M = Ti, Zr$) behave like Lewis acids which are highly reactive with water, leading to the spontaneous precipitation of metal hydroxides. To better control the nucleation and growth of the nanoparticles, $Ti(OBu)_4$ is first mixed with oleic acid in butanol which leads to a color change corresponding to a ligand exchange between the butoxy and carboxylate groups from the excess of oleic acid. Nucleation would then take place at the water (containing $Ba^{2+}$) – oil interface (where the $Ti^{4+}$ is stabilized by the oleates) which would lead to the formation of cubic nanoparticles when lamellar phases are present in the microemulsion thus prepared. These lamellar phases which are formed in the presence of oleic acid have been demonstrated by in situ SAXS and WAXS measurements during the solvothermal synthesis of $SrTiO_3$ nanocubes.[185] More or less spherical particles are then obtained in the case of reverse water-in-oil microemulsions. This structuring aspect of the microemulsion comprising lamellar phases or reverse micelles is however not the only factor responsible for the final particle morphology. In a recent study,[9] polarity of the cosolvent has also been found to have a huge influence on the final particle morphology since cubic particles are obtained in a reaction medium of constant composition (and corresponding to a supposed lamellar structure) when decanol in a 1:1 ratio with butanol is replaced by toluene, diphenyl ether or dibenzyl ether (Figure 12c–e). On the other hand, if decanol is replaced by cosolvents of higher permittivity such as diethylene glycol or benzyl alcohol, rather spherical nanoparticles are obtained (Figure 12f–h). If effect of the initial composition on the final morphology is not fully understood, additional in situ measurements (FTIR, DLS, neutron diffraction) although difficult to achieve with Parr instruments autoclaves, would be necessary. In this study, however, influence of the initial precursor concentration is fairly well understood, in line with LaMer's model, since an increase in the reactant concentration promotes nucleation and leads to smaller nanocrystals of 5 nm in size. Conversely, when the concentration decreases from $2 \times 10^{-3}$ M to $0.5 \times 10^{-3}$ M, the nanocrystal size increases to 20 nm. Noteworthy, when the oil-water two-phase solvothermal route is applied with similar precursors dispersed in the oil (hexadecane) and a mineralizer (NaOH) in water, nucleation, and crystallization at the interface results in even smaller nanocrystals with a mean size at 3.7 nm.[31] Alternatively, a one-pot nonhydrolytic solvothermal route was also developed, leading to the formation of individual and uniform $BaTiO_3$, $SrTiO_3$, and mixed $Ba_{0.5}Sr_{0.5}TiO_3$ nanocrystals of 5 nm in size. Alkaline earth metals and titanium isopropoxide were dissolved in benzyl alcohol for a solvothermal process at 200 °C for 48 h. Benzyl alcohol appears to be a good solvent as it strongly coordinates Ti alkoxide and further leads to Ti–O–Ti condensation via alcohol elimination. The resulting nanocrystals exhibit high crystal quality and strong colloidal stability resulting from the presence of nonreacted Ti–O–R groups.[186]

Regarding the niobate family, in a recent study by Lombardi et al.,[187] niobium ($Nb(OEt)_5$) and potassium ($K(OEt)$) ethoxides are mixed in ethanol under $N_2$ and hydrolyzed by adding water dropwise under magnetic stirring. The precipitate thus formed in the suspension is then treated for 48 h at 180 °C, resulting in an amorphous powder which must then be heat-treated again at

a minimum temperature of 525 °C to promote crystallization of ≈20 nm, strongly aggregated, $KNbO_3$ nanocrystals (Figure 13a). More recently, the hydrothermal synthesis of $NaNbO_3$ was also reported, starting from NaOH and amorphous or crystallized niobium oxides from different phases.[188] The resulting particles are slightly polydispersed and aggregated but all present sub-100 nm crystallite sizes. The formation of the $NaNbO_3$ phase depends on the crystallinity of the starting material and reveals a formation mechanism based on the migration of the $Na^+$ cation within the niobate network. For such synthesis, the size of the cation is a key parameter. In the case of $LiNbO_3$, the $Li^+$ cation is too small to stabilize the crystalline intermediate preventing the formation of lithium niobate under similar experimental conditions.

In addition, $KNbO_3$ nanoneedles were also obtained by hydrothermal treatment of $Nb_2O_5$ and KOH dissolved in water.[190–192] The length of the needles can reach several microns and could serve as waveguides for SHG signals. In terms of growth mechanisms, since these $KNbO_3$ needles are synthesized from $Nb_2O_5$, Magrez et al. proposed a dissolution-precipitation reaction of $Nb_2O_5$ into $[Nb_6O_{19}]^{8-}$ ions and then into $[NbO_6]^{7-}$ ions.[192] The precursor dissolution and subsequent precipitation of $KNbO_3$ nanoneedles are optimal only in a highly alkaline medium ($38.7$ wt% KOH, $0.4$ wt% $Nb_2O_5$ and $60.9$ wt% $H_2O$) leading to a preferential growth along the [011] direction corresponding to the largest interplanar spacing $d_{011} = 4.03Å$. Sides of the nanoneedles show the (100) lattice planes ($d_{100} = 3.97Å$)[190] but the growth mechanisms are not clearly elucidated. It can be noted, however, that the [011] direction of the orthorhombic lattice corresponds to the [001] direction of the high-temperature tetragonal phase (P4mm, Z = 1) which is polar in the c-direction and with a perovskite structure. Besides, when a surfactant-assisted process is implemented at 180 °C, the reaction time can be significantly decreased to one day whereas time-dependent measurements clearly evidenced formation of primary cubic-shaped nanocrystals of 5–10 nm (with presumably the tetragonal modification) that further transform into nanorods and nanowires from mesoscale self-assembly and dominant oriented attachment along the polar direction.[189] After cooling down to room temperature, the as-obtained orthorhombic nanowires have a lower transition temperature to the tetragonal phase comparatively to its bulk counterpart (=190 °C instead of 225 °C). On the other hand, although $KNbO_3$ and $NaNbO_3$ needles can be obtained, it seems that this approach remains unsuitable for the preparation of $LiNbO_3$ nanoneedles as the solubility of LiOH in water is low (12.8 g/100 mL) compared to the ones of KOH and NaOH (110 g/100 mL).[193] Importantly, hydrothermal preparation of potassium niobates has been shown to strongly depend on the starting precursors, alkalinity of the reaction medium, and temperature. For instance, when the potassium salt of the Lindquist hexaniobate $[Nb_6O_{19}]^{8-}$ ion is used, its decomposition in various transient structural units is controlled by the initial KOH concentration thus resulting in several polymorphs of well-different morphology.[114] Similarly, the orthorhombic and monoclinic forms of $KNbO_3$ were prepared after simply changing the niobium precursor from several crystallized and amorphous $Nb_2O_5$ powders and from niobium oxalate and $NbCl_5$.[116] When the Amm2 phase is obtained, the excitation
<figure><img src="image_14.png" /><figcaption>**Figure 13.** a) TEM image of KNbO₃ nanocrystals obtained after the calcination at 525 °C of amorphous precipitates prepared under solvothermal conditions from metal alkoxides. Image reproduced with permission.[187] Copyright 2016, the Royal Society of Chemistry. b) SEM image of KNbO₃ nanorods after 48 h at 180 °C for an optimal content of surfactants and TEM image obtained at 12 h after self-assembly of the primary nanocubes indicated by the white lines. c) Corresponding schematic illustration of the growth mechanisms. Images adapted with permission.[189] Copyright 1999, the Royal Society of Chemistry,</figcaption></figure>

tunability of the SHG response of colloidal suspensions of KNbO₃ nanocrystals was also demonstrated. As already pointed out, the absence of local stoichiometry in the precursors can severely complicate the exact derivation of a crystallization mechanism without an extensive use of sophisticated in situ time- and temperature-resolved experiments. Following the original study of Frost et al.[115] Skjærvø et al.[194,195] could indeed identify additional intermediate phases following the fragmentation of the initial [NbO₆]⁷⁻, [NbO₇]⁹⁻ and [NbO₄]³⁻ units contained in the T-Nb₂O₅ precursor during the solvothermal preparation of two centrosymmetric phases of orthorhombic NaNbO₃ nanocrystals. Formation and rearrangement of the transient [Nb₆O₁₉]⁸⁻ and [Nb₄O₁₆]¹²⁻ building units are accompanied by the crystallization of several known and unknown intermediates like polyoxoniobate and hydrated phases whose structures depend on the NaOH concentration (Figure 14). They also evidenced two different crystallization regimes. Below 285 °C, NaNbO₃ is formed through the direct conversion of the intermediate Na₂Nb₂O₆·H₂O phase, the crystallite size at 35–50 nm remaining constant whatever the reaction temperature or NaOH concentration. Above 285 °C, a dissolution/recrystallization mechanism accounts for the synthesis of larger crystallite sizes.[195]

For LiNbO₃, the use of Li and Nb alkoxides in ethanol shows that the dropwise addition of water in a mixture maintained under magnetic stirring leads to the formation of a hydrated precipitate rich in ethoxy groups[196] and whose elemental chemical analysis after drying is LiNbO₂·9(OC₂H₅)₀.₂·1.1H₂O. The complete removal of water and organic residues then requires an annealing temperature higher than 600 °C, whereas similar conditions are suited for the room temperature synthesis of pseudo-cubic NCO BaTiO₃ nanocrystals as discussed in Section 3.1. Interestingly, the pioneering works of Hirano and Kato[197–199] dedicated to powders and dip-coated films have evidenced the formation of a double alkoxide after simple mixing of separate Li- and Nb-ethoxide under dry N₂. After 24 h of refluxing and slow hydrolysis with water diluted in ethanol resulting in amorphous precipitates, an annealing temperature as low as 250 °C in flowing water and oxygen is enough to produce crystallized powders of stoichiometric lithium niobate. The size of the crystallites thus obtained is however not specified either from the X-ray diffraction profile or from electron microscopy images. In another reference study, thermal decomposition of niobium alkoxide and lithium acetate in 1,4 butanediol (for the so-called “glyothermal approach”) at 300 °C for 2 h in an autoclave leads to the formation of crystallized

<figure><img src="image_15.png" /><figcaption>**Figure 14.** Schematic of the different transient species and structures leading to NaNbO₃ from T-Nb₂O₅ under hydrothermal conditions. Niobium in green and oxygen in red. Reproduced with permission.[195] Copyright 2021, American Chemical Society.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Figure 15.** a) SEM imaging of $LiNbO_3$ particles after thermal decomposition of Nb-ethoxide and lithium acetate in butanediol. Reproduced with permission.[138] Copyright 1993, Elsevier B.V. b) TEM imaging of $LiNbO_3$ nanocrystals after a 4 d solvothermal treatment at 220 °C. Reproduced with permission.[200] Copyright 2004, Wiley-VCH.</figcaption></figure>

particles with a flattened shape and an average diameter of 1 µm (Figure 15a).[138]

Since these first proofs of principle, the treatment of simple or double alkoxides under solvothermal conditions has been the subject of several studies, starting with the original work of Niederberger et al.[200] The use of $Nb(OEt)_5$ and metal Li dissolved in benzyl alcohol leads, after a 4 d treatment at 220 °C, to $LiNbO_3$ nanocrystals with a size between 20 and 50 nm (Figure 15b). According to the X-ray diffraction profile, the reflection (110) at ≈33° is more intense than expected and of smaller FWHM than the (006) one at ≈39°, suggesting a platelet morphology. In 2012, Mohanty et al. further improved the resulting size and shape polydispersity with the preparation of almost spherical and cubic-like nanocrystals from a commercial double alkoxide precursor ($LiNb(OEt)_6$) diluted in its parent alcohol and different amounts of 1,4-butanediol.[201] More recently, this ligand exchange strategy initially suggested by Livage et al. for metal oxides[70] has been thoroughly investigated with the aim to better control the final morphology and to understand the reaction pathways and crystallization mechanisms after a systematic change of the reaction parameters.[55,202–204] For instance, the solvothermal aqueous route developed by Ali and Gates makes use of $Nb(OEt)_5$ and benzyl alcohol but with triethylamine as a surfactant and stoichiometric amount of $LiOH \cdot H_2O$ for the lithium source.[55] For this low hydrolysis rate, time-dependent solvothermal treatments demonstrate that the optimal reaction time is 36 h at 220 °C for the preparation of monocrystalline and uniform $LiNbO_3$ nanocrystals of mean size ≈30 nm according to TEM images and the broadening of several (hkl) XRD peaks. Oriented attachment and Ostwald ripening are proposed to account for the initial growth step since sub-10 nm, poorly crystallized intermediates are observed at short reaction times (<30 h). After 96 h, the particle size (and size-polydispersity) has strongly increased but for a constant XRD crystallite size suggesting aggregation of the triethylamine-passivated primary nanocrystals and formation of polycrystalline nanoparticles (Figure 16). Interestingly, in another study, the acentric structure of $LiNbO_3$ is preserved as demonstrated by SHG measurements from very small, monodisperse nanocrystals of mean size at 7 nm.[203] Here, a nonaqueous sol-gel route with niobium ethoxide and lithium acetylacetonate dissolved in benzyl alcohol was investigated

leading, according to the authors, to formation of hydroxide species. The bidentate “acac” that is coordinated to $Li^+$ ions would indeed control the solvolysis rate although identification of byproducts in the reaction medium is not provided after the four day solvothermal treatment at 220 °C. The proposed reaction pathways and growth mechanism involving solvolysis of the precursors and condensation reactions would thus require more space- and time-resolved measurements since crystalline intermediates below 5 nm are observed in agreement with their previous study[55] and, more importantly, with the 6.5 Å hydrous phase already detected by Nyman et al. (Figure 17a) when Lindqvist ions are used as starting precursors.[117] More recently, another systematic study regarding the influence of the nature of the Li source was also nicely demonstrated on the resulting morphology, purity, and crystallinity of $LiNbO_3$ nanocrystals prepared from the non-aqueous sol-gel route.[204]

Note that a systematic investigation of the reaction medium composition for a very similar nonaqueous route has also been proposed by Urbain et al. for the preparation of sub-50 nm $LiNbO_3$ nanocrystals of adjustable mean size and shape anisotropy according to the chain length of the different glycols used as co-solvent.[202] The reaction pathway is composed of several steps including ligand exchange with the initially mixed lithium niobium ethoxide diluted in its parent alcohol, thermal dissociation of the modified precursor leading to the formation of a $Nb=O$ double bond[205] and of positively charged carbocations whose stability directly influences the formation rate of monomers and precipitation of smaller nanocrystals with glycols of longer carbon chain (Figure 18a). The nanocrystal growth assumes addition, by a nucleophilic attack, of new monomers containing a $Nb=O$ double bond to the primary nanocrystals resulting in particles of very variable shape anisotropy and the release of ethers. Such a flattened morphology along the polar c-axis was not observed for materials exhibiting cubic or pseudocubic crystal structures[58] but is also obvious for micrometer particles produced by the glythermal method (Figure 17a). Both the specific nonperovskite crystalline structure of $LiNbO_3$ comprising an unfavorable arrangement of face-sharing oxygen octahedra along the polar direction and the build-up of a ferroelectric order lowering the addition of new monomers on (00-1) planes, could account for the slower growth velocity along [001].[202] The mean size $S_{012}$ defined along the [012] direction
<figure><img src="image_17.png" /><figcaption>**Figure 16.** Time-dependent TEM images showing primary crystalline intermediates obtained after a reaction time of a) 30 h and after b) 36 h and d) 96 h evidencing the growth through Ostwald ripening and/or aggregation of primary $LiNbO_3$ nanocrystals obtained from the solvothermal treatment of Nb(OEt)$_5$ and LiOH.H$_2$O in benzyl alcohol with triethylamine as a surfactant. Adapted with permission.[55] Copyright 2018, American Chemical Society.</figcaption></figure>

and the anisotropic ratio $S_{110}/S_{006}$ derived from XRD refinements along [110] and [006] indeed significantly decrease with the chain length of the glycol used as cosolvent (Figure 18b-e).

Importantly, all the above sol-gel routes investigated for the preparation of $LiNbO_3$ nanocrystals require the preparation of a homogeneous and stoichiometric precursor solution at the molecular (or atomic) scale as demonstrated by Nyman et al. who compared an aqueous and a nonaqueous approach corresponding to a thermal decomposition of Nb(OEt)$_5$ and hydrated lithium acetate in butanediol.[117] In the latter case, the results are very similar to the original study of Kominami et al.[138] with the formation after 3–5 d at 225 °C of “flattened” particles of ≈1 µm diameter (Figure 15a) indicating that nucleation is less favored in butanediol than in a mixture of ethanol–butanediol. For the former hydrothermal route, aqueous dissolution of the lithium-rich hexaniobate salts ($Li_8[Nb_6O_{19}]·22H_2O$) in Lindqvist ions of the type $[Nb_6O_{19}]^{8-}$ results in a mixture of the centrosymmetric $Li_3NbO_4$ phase and of a hydrated compound whose crystalline structure has not been identified but whose first diffraction peak corresponds to a lattice spacing of ≈6.5 Å (Figure 17a). Interestingly, dissolution of the $[Nb_6O_{19}]^{8-}$ ions into $[NbO_6]^{7-}$ ions might be here prevented contrary to what is observed for the preparation of $KNbO_3$ and $NaNbO_3$ as already mentioned above.[193]

Regarding the synthesis of bismuth ferrite nanocrystals, a considerable amount of work has been paid to the use of

solvothermal conditions for the preparation of micro- and nanocrystals with several of the solution-based routes previously described. An interesting approach consists in producing sub-micron $BiFeO_3$ nanocrystals from the treatment of amorphous $Fe(OH)_3$ and $Bi(OH)_3$ precipitates involving a dissolution/recrystallization process.[206] For temperature above 200 °C and at high mineralizer (KOH, NaOH) concentration, the amorphous precursors are dissolved and further crystallize to form pure $BiFeO_3$ crystals (Figure 19a). The mineralizer concentration is critical and must be finely adjusted as a too high KOH concentration promotes the formation of parasitic phases (namely $Bi_2Fe_4O_9$ and $Bi_{25}FeO_{40}$) through the formation of Bi–O–Bi bridges, whereas a low KOH concentration would prevent the dissolution of the amorphous ferrite and bismuth hydroxide precipitates. Similarly, the reaction temperature is to be set above 150 °C to allow crystallization of the targeted $BiFeO_3$ phase. Interestingly, upon addition of urea in the initial solution, Wei et al.[207] succeeded in producing smaller $BiFeO_3$ nanocrystals at a lower temperature (120 °C) although the morphology and aggregation state are still poorly controlled (Figure 19b). Above 60 °C, decomposition of urea leads to the in situ production of $CO_2$ (gas) and hydroxyl groups thus increasing the autogenous pressure and $OH^-$ concentration which promotes, at lower temperatures, the overall dissolution/crystallization mechanism. This latter scenario is also supported by the study of Cho et al.[208] since the addition of

<figure><img src="image_18.png" /><figcaption>**Figure 17.** a) X-ray diffraction pattern showing a mixture of $Li_3NbO_4$ and of a hydrous phase after hydrothermal treatment of lithium hexaniobate salts. For this aqueous route, the structure of $[Nb_6O_{19}]^{8-}$ ions is also shown. b) SEM image of micrometric particles of $LiNbO_3$ after 3–5 d of solvothermal treatment at 225 °C of a mixture of Nb(OEt)$_5$ and lithium acetate hydrate in butanediol. Adapted with permission.[117] Copyright 2009, American Chemical Society.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Figure 18.** a) Reaction pathway leading to $LiNbO_3$ nanocrystals with different shape anisotropy. GlyO and EtO are glyoxy and ethoxy groups corresponding to $-OC_4H_8OH$ and $-OC_2H_5$, respectively. In the lower panel, the blue oxygen octahedra surrounding $Nb^{5+}$ is illustrated together with the spontaneous polarization Ps (green arrow) pointing towards the [001] direction. (Right) TEM images showing the mean size decrease of $LiNbO_3$ nanocrystals obtained with b) pure ethanol and upon addition of c) 1,3-propanediol, d) 1,4-butanediol and e) 1,5-pentanediol. The corresponding anisotropic ratios defined as $S_{110}/S_{006}$ are > 8.3, 2.9, 2.6, and 2.6, respectively. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.[202] Copyright 2021, MDPI.</figcaption></figure>

triethanolamine and KOH in the initial mixture of dissolved nitrate salts results in a Bi-rich precipitate while iron is still in

solution due to the formation of Fe complexes. At 130 °C, dissolution of the solid intermediates and reprecipitation of $BiFeO_3$

<figure><img src="image_20.png" /><figcaption>**Figure 19.** a) $BiFeO_3$ nanocrystals resulting from a hydrothermal treatment of $Fe(OH)_3$ and $Bi(OH)_3$ precipitates at 200 °C (Reproduced with permission.[206] Copyright 2006, Elsevier) and b) upon addition of urea at a lower soaking temperature of 120 °C. Reproduced with permission.[207] Copyright 2016, Elsevier B.V. c) Addition of PVA not only allows reducing the crystallization temperature at 160°C but also the size-polydispersity and the mean nanocrystal size at about 10 nm. Reproduced with permission.[209] Copyright 2008, Elsevier Ltd.</figcaption></figure>
is thus facilitated leading to sub-100 nm nanocrystals with effi-
cient visible-light photocatalytic activity. Note that the size (and
size-polydispersity) can be remarkably reduced down to ≈10 nm
(Figure 19c) with the polymer method, i.e., when the initial
precipitate is mixed with a high content of poly(vinyl alcohol)
(PVA) and KOH before the hydrothermal treatment at 160 °C
for 9 h.[209]

**5.2. Microwave (MW)-Assisted Solvothermal Route**

MW-assisted liquid phase approaches appear particularly inter-
esting for the production of small NCO nanocrystals with
homogeneous size and more generally in the field of inorganic
material synthesis.[96,97] Contrary to conventional heating in an
oven or in a water/oil bath that involves radiation, conduction,
or convection processes and thus large temperature gradients
within the reaction medium and between the surface and the
material core, MW radiations indeed induce almost instant
and homogeneous heating through the local absorption by
the solvent and monomers of the electromagnetic excitation.
This rapid heating promotes a nucleation burst and the simul-
taneous appearance of nuclei that are more prone to grow at
the same rate in the entire reaction medium thus leading to
more monodisperse nanocrystals. With the recent availability of
dedicated, sophisticated but relatively expensive equipment and
the possibility to combine MW-heating with several of the solu-
tion-based routes previously discussed, a large panel of nano-
materials has already been produced by 2010 and, among them,
several representative NCO bimetallic oxides.[96,97] Most of the
reported MW-assisted solvothermal routes use a microwave
oven operating at a 2.45 GHz frequency for a typical power
in the 200–1400 W range. The recent and significant develop-
ments are reported hereafter for the production of BaTiO₃, sev-
eral niobate compounds, BiFeO₃ and α-La(IO₃)₃.

BaTiO₃ nanocrystals have been synthesized from several
barium precursors such as metal barium, barium acetate,
BaCl₂ or Ba(OH)₂ and titanium alkoxides dispersed in water
with NaOH or KOH as a mineralizer to adjust the pH above
12.[210–215] Increasing the complexity in terms of chemical
composition with the incorporation of multifunctional rare-
earth ions has also been demonstrated with the preparation
of spherical 65 nm Nd-doped BaTiO₃ nanocrystals after a
short heating treatment at 150 °C for 2 h.[212] Both well-faceted
cubic-shaped and spherical BaTiO₃ nanocrystals with a partial

tetragonal order were obtained[214] for prolonged treatment of
10 and 20 h at 170 °C (Figure 20a,b). Comparison with con-
ventional heating was also demonstrated to result in a signifi-
cant reduction of the size-polydispersity under MW heating.
Note that small variations of the MW excitation frequency
from the conventional 2.45 GHz value to 5.5 GHz, band-
width sweep times and reaction duration have a considerable
impact on the nanocrystal morphology, purity, and porosity.[214]
Regarding their shape, a low MW frequency is suggested to
account for a slower nucleation rate leading to cubic-shaped
nanocrystals due to the dominant growth rate whereas a faster
nucleation at higher MW frequency promotes the formation of
smaller spherical particles. Temperature is another important
processing parameter. Only 20 min are necessary to produce
tetragonal BaTiO₃ nanocrystals of ≈50 nm in size at 180 °C.[213]
In this study, the initial KOH concentration severely affects
the final nanoparticle size and morphology since aggregation
and Ostwald ripening result in hollow structures at high alka-
linity. Noteworthy, when similar precursors are dispersed in
anhydrous benzyl alcohol, a mildly coordinating solvent with
a high boiling point (205 °C) and a greater dissipation factor
(tan δ = 0.66) than water,[216] a higher synthesis temperature
of 200 °C can be reached leading to well crystallized and easily
redispersable BaTiO₃ nanocrystals for an MW-assisted treat-
ment shorter than 3 min (Figure 20c). If identification of the
cubic or tetragonal form of barium titanate is not provided, a
narrow size range (6–18 nm) is to be noticed.[210]

In addition, chemical analysis of the byproducts can be
tedious but necessary to derive the exact reaction pathway.
This has been done for ZnO for which the MW-assisted prepa-
ration is not only much faster (a few minutes) comparatively
to the standard autoclave treatment (several hours) but also
involves ester elimination, namely benzyl acetate, when the
corresponding Zn acetate is used as precursor.[210] Thermal
effects are thus not the only parameter responsible for the
shorter reaction time in the MW-assisted routes since non-
thermal effects associated with the better stabilization of highly
polar transient species are to be considered as well.[217] On
the other hand, when hydrated BaCl₂ and titanium oxide are
used as precursors, time-dependent studies also demonstrated
that the formation of the perovskite is much quicker at 170 °C
with the MW heating (≈25 min) while several hours are
required for the conventional heating. Sintering for a few hours
at 850 °C is however necessary to derive the tetragonal modifi-
cation from both approaches.[218]

<figure><img src="image_21.png" /><figcaption>**Figure 20.** SEM images of BaTiO₃ nanocrystals prepared by hydrothermal routes assisted by MW heating at 170 °C for 20 h at a) 2.45 GHz and a) 5.5 GHz frequency for the cubic-shaped and spherical nanocrystals, respectively. Images adapted with permission.[214] Copyright 2008, American Chemical Society. c) TEM images of BaTiO₃ nanocrystals produced at 200 °C for a few seconds. Reproduced with permission.[210] Copyright 1996, Royal Society of Chemistry.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Figure 21.** SEM or TEM images of BFO nanocrystals prepared by MW-assisted hydrothermal processes, under different conditions: a) at 180 °C for 30 min,[226] b) at 180 °C for 60 min[227] and c) at 180 °C for 1 h.[228] Adapted with permission.[226] Copyright 2008, AIP Publishing. Adapted with permission.[227] Copyright 2011, The American Ceramic Society and Adapted with permission.[228] Copyright 2011, Elsevier Ltd.</figcaption></figure>

Regarding lead-free niobate compounds, several works evidenced that very short synthesis times are also suited to prepare several compositions of functional materials including NaNbO₃, KNbO₃, and K₀.₅Na₀.₅NbO₃ from Nb₂O₅ and different amounts of KOH/NaOH.[219–222] If the detailed reaction pathways are not fully discussed, these studies all have in common the preparation of the desired phase but without any particle size control in the nanoscale range and an aggregation state that prevents further preparation of colloidal suspensions. Use of other niobium sources with additional chelating agents would thus be needed to better adjust the final morphologies except if highly hierarchical porous structures are to be prepared for electrode materials. This was recently achieved by a MW-induced autocombustion process of a gel-like precursor prepared from drying a mixture of (NH₄)₃NbO(C₂O₄)₃, CH₃COOLi, and citric acid. After a few minutes of irradiation, this latter is ignited leading to the release of decomposed gas and a very porous morphology in the resulting aggregates composed of 10–20 nm LiNbO₃ composites.[223] Interestingly, Carreno et al. later evidenced that ball milling of Nb₂O₅, Li₂CO₃, and cellulose with additional use of parafilm as a fuel also result in a very short, energy-efficient, and clean synthesis of catalyst-active, aggregated sub-50 nm LiNbO₃ nanoparticles after auto-combustion of the mixture upon MW irradiation for only 25 s. After ignition within 10–15 s, the reaction is finally completed during a burning period for 40–50 s during which very high temperatures (>1200 °C) can be easily reached. The total reaction time is thus below 5 min after the necessary cooling step of 3 min for the multiwave Anton Paar oven equipment used that also allows production of relative high amounts (hundreds of mg) of nanopowders per batch.[224] Very recently, it is worth mentioning another study for its chemical strategy.[225] Starting from Nb₂O₅, niobic acid (Nb₂O₅·nH₂O) is first prepared for its higher solubility while citric acid is then added to chelate the Nb⁵⁺ ions. Upon addition of LiOH.H₂O, reaction between the LiOH and Nb(OH)₅ species at 260 °C and pH = 9 for only 2 and 3 h results in polydisperse, but easily redispersible nanocrystals of mean size 42 and 65 nm, respectively.

In the case of BiFeO₃, nanocrystals have been successfully synthesized through MW-assisted hydrothermal routes.[226–230] Typically, nitrate hydrate precursors (Bi(NO₃)₃ and Fe(NO₃)₃) were dissolved in water in an equimolar ratio. In some cases, nitric acid[227] or ultrasounds[228] are employed to improve the precursor solubility. Different mineralizers including NaOH, KOH, or a mixture of KOH/Na₂CO₃ are then added to form a precipitate which is then placed in a closed vessel and heat

treated at 180 °C to promote crystallization for a typical duration fixed between 1 and 60 min. During the solvothermal process, reaction between the Bi³⁺ and FeO₃³⁻ ions would result in the formation of BiFeO₃ nanocrystals[229] but overall, the phase purity (i.e., without Bi₂Fe₄O₉ or Bi₂₅FeO₄₀), mean nanocrystal size, morphology and resulting functional properties strongly depend on several experimental parameters including alkalinity of the reaction medium, nature of the mineralizer, excitation frequency, stirring, concentration of metal species and the synthesis temperature and duration.[97,231] Single crystalline cubic-shaped nanocrystals with a typical size ranging from 50 to 200 nm (Figure 21a) have for instance been prepared with a given amount of Na₂CO₃ and KOH and at 180 °C for 30 min.[226] Interestingly, for very similar experimental conditions, Zhu et al. obtained after 60 min at 180 °C, almost monodisperse spherical nanocrystals in the 30–40 nm size range (Figure 21b) while a sillenite phase (Bi₂₅FeO₄₀) is produced without Na₂CO₃.[227] In another time-dependent study, phase-pure submicronic BiFeO₃ crystals (Figure 21c) have been prepared after 1 h with KOH as the only mineralizer[228] but several crystalline intermediates are mentioned for shorter heat treatments.[229] This again points out that subtle changes in the experimental parameters lead to very different morphologies, chemical compositions, and aggregation states (as already outlined before for BaTiO₃[97]) and that less is known at this stage on the underlying detailed reaction pathways and growth mechanisms. For instance, Li et al. also demonstrated that precipitation of the initial amorphous precursor in presence of ethylene glycol prior to the MW-treatment in NaOH results in the formation of the crystalline sillenite intermediate and finally, at longer reaction times, in BiFeO₃ nanoplates with exposed {100}c facets when the structure is described in the pseudo-cubic lattice system.[230] Adsorption of OH⁻ ions and a dissolution/precipitation could account for such a 2D morphology. Note that the radiation power was varied but the reaction temperature is not specified making it difficult the comparison with previous studies.

Impact of small changes in the experimental parameters was also clearly evidenced for the synthesis of α-La(IO₃)₃ after simple coprecipitation in water between LaCl₃ and HIO₃ and heat-treatment for a few minutes around 220 °C of the amorphous intermediate.[232] The subtle interplay between the synthesis temperature, radiation power, precursor concentration already discussed in ref. [233] and molar ratios between La³⁺ and IO₃⁻ ions is summarized below (Figure 22a). Different lanthanum iodate phases (La(IO₃)₃(H₂O), La(IO₃)₃(HIO₃), La(IO₃)₂.₆₆(OH)₀.₃₃) can be produced below 220 °C while above,
<figure><img src="image_23.png" /><figcaption>**Figure 22.** a) TEM image of α-La($IO_3)_3$ nanocrystals synthesized at 250 °C for 10 min, with associated size distribution measured by DLS. b) Illustration of the reaction scheme and phase transformations according to the experimental conditions. Adapted with permission.[232] Copyright 2020, the Royal Society of Chemistry.</figcaption></figure>

the SHG active α-La($IO_3)_3$ compound is obtained without phase impurity. Interestingly, the as-prepared 40 nm α-La($IO_3)_3$ nanocrystals (Figure 22b) show a narrow size distribution (±10 nm) while composition can be readily adjusted with the incorporation of Er$^{3+}$/Yb$^{3+}$ rare-earth ions thus complementing their inherent nonlinear optical properties with upconversion functionalities and nanothermometry capabilities.[62,234,235]

**6. Why Elucidation of the Formation Mechanisms Is a Prerequisite?**

Adjusting the size, morphology, crystalline quality, aggregation state, colloidal stability and surface composition of NCO nanocrystals is of paramount importance in the nanomaterial science as their multifunctional properties, energy conversion efficiencies, possible size- and shape effects, and finally, their potential for new applications directly result from a rational control of these parameters. The main advantages and drawbacks of the solution-based routes previously discussed are summarized in **Table 1** and it clearly appears that none of the approaches satisfies all the criteria in terms of size and shape control in a large range of morphologies, environment-friendly aspects, low degree of aggregation state, simplicity, scalability, precursor and equipment costs, and, more importantly, of the elucidation of the reaction pathways and growth mechanisms through in situ and real-time measurements of the nucleation and crystallization events allowing identification of the transient monomers/phases and possible byproducts.

If such general considerations also stand for centrosymmetric nanomaterials like metals and simple binary metal oxide nanoparticles, one has to keep in mind that these latter i) most often belong to higher symmetry point groups with ii) a lower number of constitutive elements and iii) that the chemistry of noble metal precursors in solution is very specific. If a much higher complexity in terms of crystalline structures and compositions is to be considered for the rational design of NCO nanocrystals, the above-described solution-based routes have already been applied with success for numerous centrosymmetric compounds while several reaction pathways and crystallization/growth mechanisms, and combination of, have also been described and discussed.[236-238] If many experimental studies have been conscientiously performed, the available ranges of NCO nanocrystal size and morphology together with their concomitant size/shape distributions are, as already mentioned,

much more restricted comparatively to, for instance, metal nanoparticles (Au, Ag, Pd, ...)[239-243] whose crystallization mechanisms have been extensively studied and optimized. Indeed, for these latter, the successive formation of clusters and nuclei, and subsequent nanocrystal growth, can be influenced by several experimental parameters so as to adjust the nucleation step and to promote (or inhibit) particular growth directions from, for instance, overgrowth strategies after careful consideration of the surface tension energies of the low-index crystallographic facets or after proper adsorption of foreign ions or chelating agents. Similarly, the formation mechanisms of semiconducting quantum dots from the hot-injection method are particularly well-understood.[244-246] An abundant literature and impressive studies have also been devoted to centrosymmetric oxide materials (sulfates, carbonates, ...) and to bioinspired routes evidencing that organic-inorganic interactions between organic additives and primary solid intermediates can result in the so-called "nonclassical crystallization mechanisms."[51-54,237,238,247,248] The aggregation-mediated crystallization and oriented-attachment process leading to the formation of mesocrystals from nonspherical building units differ in that way from classical concepts of crystalline growth based on the simple surface addition of ions/atoms, dissolution-precipitation phenomena (Ostwald ripening), coarsening of crystalline intermediates resulting from coalescence[249] and intraparticle coarsening causing a morphology change in the final growth stage at low monomer concentration.[250] Note that examples of ordered nanocrystal superstructures have already been evidenced for several titanate and niobate NCO compounds.[251] This strategy has also been successfully illustrated by Wood et al. for the preparation of anisotropic $LiNbO_3$ nanostructures.[252] The nonhydrolytic decomposition of mixed Li, Nb isopropoxide ($LiNb(OPr')_6$) in presence of triphenylphosphine oxide (TPPO) at 360 °C leads to the formation of crystalline $LiNbO_3$ seeds stabilized by TPPO ligands and whose morphology might result from a lower crystalline growth along the [012], [104], and [006] directions. Absence of coalescence between the primary seeds clearly indicates that surface-anchored TPPO ligands stabilize the sub-10 nm $LiNbO_3$ seeds while their polar structure could be responsible for the directed-aggregation of elongated superstructures along the c-axis and formation of complex sheaf-like morphologies (Figure 23).

If mesocrystals demonstrate new promising chemical and physical properties resulting from the ordered nanoparticle self-assembly,[248,251] they remain though out of the scope of
<table><caption>**Table 1.** Overview of the main advantages and drawbacks for the preparation of noncentrosymmetric 0-D oxide nanocrystals from solution-based synthesis routes.</caption><thead><tr><th></th><th>Advantages</th><th>Drawbacks</th></tr></thead><tbody><tr><td>Water-based coprecipitation</td><td>Simple &amp; harmless<br>Low-cost precursors</td><td>Often requires a calcination step that promotes crystallization &amp; aggregation</td></tr><tr><td>From homogeneous<br>solution (no additive)</td><td>High reaction yield<br>Versatile &amp; Green</td><td>Broad size and shape distributions<br>Phase purity and stoichiometry<br>not always achieved</td></tr><tr><td>Within microemulsions</td><td>Simple and cheap<br>Good size/shape control in<br>a narrow range</td><td>Low reaction yield<br>Use of large amounts of organic solvents and surfactants</td></tr><tr><td>With bio-molecules</td><td>Size/shape control demonstrated<br>Room temperature (energy-efficient) synthesis of polar NCO</td><td>Nanostructuration effects and crystallization mechanisms poorly understood<br>Only for BaTiO<sub>3</sub> so far</td></tr><tr><td>Sol-gel routes</td><td>Phase purity and stoichiometry<br>Abundant literature</td><td>Often requires a calcination step that promotes crystallization &amp; aggregation</td></tr><tr><td>From alkoxides</td><td>Size/shape control demonstrated<br>for (pseudo)cubic lattices<br>Adapted to the microemulsion<br>route and solvothermal conditions</td><td>Cost, availability, and reactivity of precursors<br>Reaction pathways highly sensitive to the hydrolysis rate, pH, and solvent</td></tr><tr><td>Small molecules, Pechini<br>and polymers routes</td><td>Large range of compositions thanks to the homogeneous precursor mixing at the molecular scale<br>High reaction yield and scalable<br>Size control according to the annealing temperature</td><td>Large amounts of solvent and organics are to be evaporated from the initial solution and upon calcination<br>Crystallization temperature largely dependent on the precursors and additives<br>Not harmless for some conditions resulting in the gel combustion</td></tr><tr><td>Hydro/solvothermal conditions</td><td>Size and shape control<br>Calcination step not required to promote crystallization<br>Low degree of aggregation<br>Mild conditions<br>Green</td><td>«Black box», real-time and in situ measurements hard to achieve to elucidate the formation mechanisms<br>Complex reaction pathways highly sensitive to the reactive medium composition and processing parameters</td></tr><tr><td>Classical heating</td><td>Cheap autoclaves<br>Abundant literature</td><td>Long thermal treatments are often required</td></tr><tr><td>MW-assisted</td><td>Rapid<br>Narrow size distribution and ultrasmall nanoparticles (&lt;5 nm)<br>achievable</td><td>Final size and shape very sensitive to the processing parameters<br>Expensive equipment<br>Not yet very documented for NCOs</td></tr></tbody></table>

this review mainly dedicated to individual, nonagglomerated 0-D nanocrystals. Controlling the formation and stabilization by organic additives of preformed building blocks is however a very appealing approach to optimize the rational design of NCO nanocrystals. This can be done through the aggregation-mediated crystallization mechanism since the variation of the binding interaction between the amorphous inorganic intermediates and the surface-adsorbed organic additives are known to alter the final morphology[51,53] as already highlighted for Fe(IO₃)₃ (Figure 3e). Local nonequilibrium conditions within the initial loosely packed amorphous intermediates not only result in crystallization of nanofilaments or nanoplates but also in possible higher-order hybrid structures after rearrangements of the surface-anchored organic molecules and/or from dipolar interactions between the metastable crystalline intermediates. Note that such architectures have also been observed in the case of ZnO mesocrystal microspheres self-assembled from primary hexagonal nanoplatelets.[253] Interestingly, if a new thermodynamic equilibrium is reached after the primary aggregation-mediated crystallization step, individual nanocrystals can be derived instead of complex hybrid superstructures.

Such a scenario has recently been demonstrated for the preparation of LiNbO₃ nanocrystals of adjustable mean size from an aqueous sol-gel route under solvothermal conditions.[254] The double metal alkoxide LiNb(OEt)₆ is obtained upon simple mixing of LiOEt and Nb(OEt)₅ in the parent alcohol while ligand exchange is promoted by the addition of alcohols or glycols of different chain lengths. Condensation of the first species is then triggered under magnetic stirring at low hydrolysis rates of the monomer solution and evidenced by DLS measurements showing the likely presence of ≈3 nm oxoclusters containing a mixed-metal oxide core surrounded by surface hydroxy and alkoxy groups (Figure 24a). This is supported by the existence of dimeric species (LiNbO(OEt)₄EtOH)₂ already evidenced under nonhydrolytic conditions[255,256] and the pioneering works of Eichorst[257,258] although more detailed chemical analysis of these transient oligonuclear oxo-species is still clearly required. For such low hydrolysis rates, partial substitution of the surface alkoxy chains by hydroxyl groups allows further condensation and formation of clusters of increasing size, eventually resulting in the first nuclei and, subsequently, particle growth.[259,260] The identification, structure, and use
<figure><img src="image_24.png" /><figcaption>**Figure 23.** Proposed reaction pathway and growth mechanism for the synthesis of anisotropic sheaf-like $LiNbO_3$ superstructures after directed aggregation along the polar [001] direction of the primary surface-passivated nanocrystals. Reproduced with permission.[252] Copyright 2008, Wiley-VCH.</figcaption></figure>

of, for instance, various titanium oxo-clusters have been extensively discussed for the preparation of advanced functional (nano)-materials.[261–264] Formation of the primary solid intermediates is here no longer seen as the result of kinetically controlled hydrolysis and condensation reactions but as a one-step hydrolysis-condensation event producing first well-defined oligonuclear oxo-alkoxide species and, second, nuclei after their coalescence. In the case of $LiNbO_3$ and contrary to what is observed with barium titanate-based nanocrystals (Figure 5a–d), only a viscous, nontransparent light-scattering gel containing ≈5 nm amorphous nuclei is obtained at room temperature thus requiring a solvothermal treatment to promote crystallization. Importantly, the variable hydrophobic interactions between the pendent organic chains, after ligand exchange of the primary ethoxy groups with alcohols of longer carbon chain or diols, alter the gelation time (varying between

a few seconds for primary alcohols to hours in the case of diols) and the formation of a colloidal gel composed of nuclei connected by hydrogen bonding (Figure 1, first row). Extensive TEM imaging and structural analysis of the time- and temperature-dependent experiments first demonstrate the presence of surface-passivated crystalline intermediates of 4–6 nm (corresponding to the XRD profiles in Figure 17a[55,203]) whose aggregation (Figure 24b,c) leads, after rearrangement of the surface ligands, to $LiNbO_3$ nanocrystals. After 24 h of solvothermal treatment, the mean $LiNbO_3$ nanocrystal size is then adjustable between 15 and 70 nm (Figure 24c). For instance, when 1,4-butanediol is used instead of butanol, higher binding interactions between the crystalline intermediates and the surface glycoxy groups are expected due to different inductive effects. Mesoscale transformations within primary aggregates leading to the $LiNbO_3$ phase are thus frustrated and occur at a higher

<figure><img src="image_25.png" /><figcaption>**Figure 24.** a) Optical images and schematic of the transient species evidenced at room temperature after ligand exchange with the mixed ethoxide precursors and hydrolysis of the reaction medium at *t* = 0. b) Time-dependent annealing treatments demonstrate the formation after 6 h of aggregates of different sizes according to the alkoxy (left, butanol case) and glycoxy groups (right, 1,4-butanediol case) anchored at the surface of the passivated 5 nm crystalline intermediates (in blue). c) After 24 h at 235 °C, the aggregation-mediated crystallization results in smaller $LiNbO_3$ nanocrystals (in red) when butanol is used as a cosolvent (right) due to lower binding interaction with the crystalline intermediate cores comparatively to 1,4-butanediol (right).[254]</figcaption></figure>
temperature within larger aggregates, comparatively to the
butanol case, thus resulting in bigger nanocrystals.

In the future, one can expect that a detailed identification
and systematic change of the transient oxo-clusters available for
the synthesis of 0-D NCO nanocrystals will further expand the
accessible size and morphology ranges. Note that the library of
simple alkoxides and even heterometallic alkoxides[89,256,265-267]
available for the preparation of I–V, II–IV, and III–III perovskite
and nonperovskite compounds has long been investigated,
for instance, for the classical sol-gel process and MOCVD
technique, thus opening a new avenue to prepare 0-D NCO
nanocrystals with improved performances. Use at low supersat-
uration level of molecular species already containing preformed
stoichiometric moieties is indeed another source of inspiration.
This has recently been demonstrated from heterometallic tri-
nuclear oxo-centered clusters leading, under mild solvothermal
conditions, to the formation of ternary metal oxides.[268] Sev-
eral centrosymmetric spinel-like metal ferrite nanocrystals
of general composition MFe₂O₄ (M = Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺,
and Zn²⁺) were thus obtained with a narrower size distribu-
tion and a better control of composition when heterometallic
oxoclusters are used as single precursors instead of separate
metal sources coming from nitrate salts or acetates. In line
with the general requirement of a given stoichiometry at the
molecular scale amenable with the solution-based approaches,
the presence here of both metal ions (covalently bound via an
oxo-bridge within the starting oxoclusters) promotes nucleation
of pure ternary metal oxides instead of transient phase impu-
rities. In addition, the electron donor feature of the bridging
carboxylate ligands within the heterometallic oxocluster precur-
sors strongly influences the resulting nanocrystal size with a
decrease in size for electron donor groups.[268]

On the other hand, for this molecular approach, new reaction
pathways and crystallization mechanisms are to be expected
again highlighting some limitations of the classical nucleation
and growth theories. For instance, a “continuous growth” from
tri-iron-oxo clusters stabilized with carboxylate ligands into
larger molecular clusters, through continuous esterification of
the surface oleate groups with solvent molecules, was clearly
evidenced before the formation of the primary iron oxide
(Fe₂O₃) nanocrystals.[269] Again, a combination of several exper-
imental techniques including NMR and mass spectroscopies,
X-Ray scattering, advanced electron microscopy, and chemical
analysis of byproducts from time-dependent measurements
were necessary to address the nanocrystallization pathway. In
the literature, such a detailed investigation of the prenucleation
stage, also called induction period, aims at identifying i) the
different reaction intermediates (including for example com-
plexes, molecular clusters, polyoxometallates, oxo-clusters, and
aggregates), ii) the precursor reaction rates and iii) the solution
structures containing solute-rich and solute-poor liquid phases
when spinodal decomposition is involved, has first been demon-
strated for the preparation of metal nanoparticles and semicon-
ductor quantum dots.[238,246,270,271] Obviously, the increasing use
of space- and time-resolved measurement techniques to better
address and understand the molecular reaction pathways and
nanoparticle formation mechanisms is also expected to further
improve the understanding of the formation mechanisms and
the rational design of NCO nanocrystals. As already mentioned,

they most often belong to lower symmetry point groups and
contain a higher number of constitutive elements so that their
higher degree of local anisotropy will very likely help to derive a
better overview of the diverse nonclassical pathways to crystal-
lization and eventual morphological transformations.[237]

7. Conclusion and Prospects

Finally, the tentative list below is aimed to summarize some
of the main concepts previously discussed and to give possible
new directions so as to better improve and understand/control
the structure-property relationships of 0-D NCO nanocrystals
through the exact determination of the reaction pathways and
crystallization mechanisms.

- Due to the higher complexity of NCO materials in terms of structures and compositions, the simple room-temperature coprecipitation method is very limited and crystallization is barely possible without a calcination step thus preventing the formation of easily redispersible nanometer-sized crystals. The aggregation state can however be limited if the initial amorphous precipitate is intimately mixed within an inert salt of high melting point such as KCl and NaCl, thus ensuring a confined nucleation and growth.[91,131]

- For the preparation of well-defined nanocrystals in terms of size and morphology, a subtle interplay between the physical processing parameters (temperature, pressure, heating rates, and reaction duration) and chemical considerations (composition of the reaction medium, starting precursors, local stoichiometric monomers, transient species, use of organic additives ...) is to be found most of time. To go further for these complex oxides and to avoid time-consuming trial-and-error approaches, modeling approaches are increasingly needed to better address the interplay between solvation effects, surface tensions, and growth velocities along the [hkl] directions and the possible selective adsorption of specific ions or organic additives onto specific crystalline planes.

- As discussed in Section 2, if solid-state approaches are in-
appropriate for the synthesis of noncentrosymmetric mixed-
metal oxides at the nanoscale, use of partially dissolved solid
precursors (like simple oxides or metals) can also consid-
erably limit the determination of an exact crystallization
mechanism despite the increasing use of mild solvothermal
conditions. Trapping the “randomness of the solution state”
appears more and more as a prerequisite for the synthesis of
0-D NCO nanocrystals with increased complexity and multi-
functionality, such as ternary, quaternary, and even quinary
oxides. Importantly, if stoichiometry at the molecular scale
is a well-established strategy, use of preformed stoichiomet-
ric heteroatomic moieties already containing oxobridges be-
tween different metal cations has also proven its efficiency.
Temperature- and pressure-induced decomposition of poly-
oxometallate ions for the preparation of niobate compounds,
for instance, may result in the desired composition but sev-
eral transient species and crystalline intermediates are typi-
cally observed.[195]

- If a reproducible synthesis of nanocrystals can be expected
from most solution-based approaches, there are also several
examples highlighting that very small changes in the pro-
cessing parameters, and not only for the microwave-assisted
approach, have huge effects on the formation mechanisms.
When possible, a systematic analysis of byproducts com-
bined with space- and time-resolved experiments is necessary
to derive the exact reaction and crystallization pathways. This
includes identification of the transient species, monitoring
of the nucleation step, and nanocrystal growth. To that end,
scattering of polarization-resolved nonlinear optical signals
might not only help to get the formation kinetics but also the
real-time size and shape together with local structural infor-
mation on point group symmetry.[⁴⁹,⁵⁰] Determination of pos-
sible templating and nanostructuration effects through the
appearance of lamellar phases or reverse microemulsions is
also key as recently discussed.[⁹,¹⁸⁵]

- Studies dealing with the preparation of metal and oxide nanocrystals from solution-based approaches are a great source of inspiration in terms of formation and crystallization mechanisms. Indeed, the large panel of growth mechanisms already identified and intensively discussed[²³⁷] is useful to derive a better understanding of the formation mechanisms of several 0-D NCO nanocrystals as outlined in this review. On the contrary, the higher complexity of these latter in terms of composition, possible crystal classes, and reticular systems is expected to give new insights into the crystallization pathways, interfacial phenomena, and the influence of the build-up of a ferroelectric order at the nanoscale. Besides, new synthesis strategies have been implemented for the production at room temperature of well-defined NCO nanocrystals through the control hydrolysis at a vapor/liquid interface[⁴⁶] or the use of biomolecules[¹²⁵] to better controlled the nanocrystal size and morphology while promoting the polar order of a given structure.

- Although many solution-based synthesis routes have been discussed in this review, heat-up methods (thermal decomposition method, hot injection), that are largely used for the elaboration of heterometallic fluoride nanocrystals (NaYF₄, KGdF₄),[²⁷²] metal nanoparticles (Ag, Pt, ...),[²⁷³] oxide nanocrystals (Fe₂O₃)[⁴⁷] and quantum dots[²⁴⁴,²⁴⁵] have not been mentioned as they are very scarcely employed for NCO preparation. For mixed-metal oxides, and to the best of our knowledge, only two studies are dedicated to the cubic modification of size-controlled BaTiO₃ nanocrystals[¹⁴⁵,²⁷⁴] as recently discussed[⁴⁵] whereas this approach is well suited for the preparation of various metal halide perovskite nanostructures.[²⁷⁵] These simple and efficient methods, involving high reaction temperature (up to 350 °C) and use of organic solvents, lie on the controlled decomposition of organometallic complexes in presence of surfactants, with the aim to produce monodisperse nanocrystals by decoupling the nucleation and growth steps through a ligand-assisted process. However, this method does not seem appropriate for the large-scale production due to its high cost and acute toxicity of the organometallic precursors.

- Once NCO nanocrystals with adequate size, targeted mor-
  phology, crystal quality, and colloidal stability are obtained in
  a controlled way, upscaling the production may be required.
  Despite the larger processed volume, the difficulty here lies
  in maintaining the solution homogeneity and the identi-
  fied physical and chemical parameters leading to a given

formation mechanism. One could rely on the design and de-
velopment of inventive equipment, such as the use of specific
autoclaves equipped with stirring blades for a better mixing
of the initial precursor solution.

- Finally, there is also a growing demand for developing multi-component and multifunctional nanocrystals combining for instance several contrast agent opportunities. This trend in the nanomedicine field led to the preparation of nanoparticles with bimodal and trimodal capabilities thus allowing the use of different imaging techniques combining for instance high resolution (multiphoton microscopy) or high penetration depths (MRI, PET, CT, ultrasound, photoacoustic)[²⁷⁶,²⁷⁷] and the development of theranostic applications.[²⁷⁸] Regarding NCO nanocrystals, such an increase in multifunctionality has recently been demonstrated through simple NCO doping with rare-earth luminescent ions,[⁶²,²³⁵,²⁷⁹] advanced modification of their surface composition[⁶⁶,⁶⁷,²⁸⁰] (for the on-demand photo-triggered release of chemotherapeutics or for dual multiphoton and MRI contrasts) and the formation of core/shell nanostructures[⁶⁰] by heterogeneous nucleation.

Acknowledgements

This work was supported by the French Agence Nationale de la
Recherche (projects RACINE and DARE), the 2015–2020 Contrat Plan
État Région (project E-TIME), and the French-Swiss Interreg V program
(project OncoNanoscreen). The authors also want to kindly thank
several Ph.D., graduate, and undergraduate students including Florian
Riporto, Mathias Urbain, Sylvain Regny, Gareth Clarke, Sebastian
Schwung, Moustafa El Kass, Oisin Phelan, Sian Sweeney, Rhiagh Cleary,
Théo Tytus, Nicolas Anspach, Camille Jenny, Paul Emin, Lara Bouvier-
Garzon, and Lou Gaucher for their involvement during their stay in the
SYMME and Institut Néel Laboratories.

Conflict of Interest

The authors declare no conflict of interest.

Keywords

multifunctional nanocrystals, noncentrosymmetric oxides, nucleation/
growth mechanisms, solution-based synthesis

Received: February 15, 2022

Revised: May 21, 2022

Published online: June 12, 2022

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<figure><img src="image_26.png" /></figure>

**Géraldine Dantelle** earned her Ph.D. in Materials Science at Sorbonne Université, Paris, France in 2006. She became a postdoctoral researcher in the Quantum Information Processing group in the Materials Science Department at the University of Oxford, UK. In 2009, she was hired as a researcher at CNRS - French National Center for Scientific Research - in Paris-Saclay. In 2015, she moved to the Institut Néel, Grenoble, France, to pursue her studies as a research director of luminescent and SHG-active nanocrystals for LED lighting, bio-imaging, and nanothermometry.

<figure><img src="image_27.png" /></figure>

**Yannick Mugnier** is an associate professor leading the group of Functional Nanomaterials at the SYMME laboratory of the Savoie Mont-Blanc University, Annecy, France. In 2000, he completed his Ph.D. in Material Science under the supervision of Prof. Jacques Bouillot before a post-doc in optical microscopies at the Geneva University. His habilitation in 2014 and his current research activities are focused on the synthesis of various noncentrosymmetric oxide nanocrystals from several chemical routes and on their non-linear optical characterizations for advanced biomedical applications.