Fast and Tunable Synthesis of ZrO2 Nanocrystals: Mechanistic Insights into Precursor Dependence

From Gel to Crystal: Mechanism of HfO2 and ZrO2 Nanocrystal Synthesis in Benzyl Alcohol

Nonaqueous sol-gel syntheses have been used to make many types of metal oxide nanocrystals. According to the current paradigm, nonaqueous syntheses have slow kinetics, thus favoring the thermodynamic (crystalline) product. Here we investigate the synthesis of hafnium (and zirconium) oxide nanocrystals from the metal chloride in benzyl alcohol. We follow the transition from precursor to nanocrystal through a combination of rheology, EXAFS, NMR, TEM, and X-ray total scattering (PDF analysis). Upon dissolving the metal chloride precursor, the exchange of chloride ligands for benzylalkoxide liberates HCl. The latter catalyzes the etherification of benzyl alcohol, eliminating water. During the temperature ramp to the reaction temperature (220 °C), sufficient water is produced to turn the reaction mixture into a macroscopic gel. Rheological analysis shows a network consisting of strong interactions with temperature-dependent restructuring. After a few minutes at the reaction temperature, crystalline particles emerge from the gel, and nucleation and growth are complete after 30 min. In contrast, 4 h are required to obtain the highest isolated yield, which we attribute to the slow in situ formation of water (the extraction solvent). We used our mechanistic insights to optimize the synthesis, achieving high isolated yields with a reduced reaction time. Our results oppose the idea that nonaqueous sol-gel syntheses necessarily form crystalline products in one step, without a transient, amorphous gel state.

Stabilizing tetragonal ZrO2 nanocrystallites in solvothermal synthesis

Phase-pure tetragonal ZrO2 nanoparticles have been prepared under simple solvothermal synthesis conditions using different types of alcohols as solvents and studied using in situ X-ray scattering. The variation of tetragonal/monoclinic phase ratios within the produced powders was directly correlated with the amount of in situ generated water from solvent dehydration during the syntheses. By controlling the dehydration kinetics, either choosing primary alcohols of varying thermal stability or by changing synthesis temperatures, it is possible to selectively tune this tetragonal/monoclinic phase ratio.

Mechanistic Insight into the Precursor Chemistry of ZrO2 and HfO2 Nanocrystals; towards Size-Tunable Syntheses

One can nowadays readily generate monodisperse colloidal nanocrystals, but a retrosynthetic analysis is still not possible since the underlying chemistry is often poorly understood. Here, we provide insight into the reaction mechanism of colloidal zirconia and hafnia nanocrystals synthesized from metal chloride and metal isopropoxide. We identify the active precursor species in the reaction mixture through a combination of nuclear magnetic resonance spectroscopy (NMR), density functional theory (DFT) calculations, and pair distribution function (PDF) analysis. We gain insight into the interaction of the surfactant, tri-n-octylphosphine oxide (TOPO), and the different precursors. Interestingly, we identify a peculiar X-type ligand redistribution mechanism that can be steered by the relative amount of Lewis base (L-type). We further monitor how the reaction mixture decomposes using solution NMR and gas chromatography, and we find that ZrCl₄ is formed as a by-product of the reaction, limiting the reaction yield. The reaction proceeds via two competing mechanisms: E1 elimination (dominating) and S_N1 substitution (minor). Using this new mechanistic insight, we adapted the synthesis to optimize the yield and gain control over nanocrystal size. These insights will allow the rational design and synthesis of complex oxide nanocrystals.

Nonaqueous Chemistry of Group 4 Oxo Clusters and Colloidal Metal Oxide Nanocrystals

We review the nonaqueous precursor chemistry of the group 4 metals to gain insight into the formation of their oxo clusters and colloidal oxide nanocrystals. We first describe the properties and structures of titanium, zirconium, and hafnium oxides. Second, we introduce the different precursors that are used in the synthesis of oxo clusters and oxide nanocrystals. We review the structures of group 4 metal halides and alkoxides and their reactivity toward alcohols, carboxylic acids, etc. Third, we discuss fully condensed and atomically precise metal oxo clusters that could serve as nanocrystal models. By comparing the reaction conditions and reagents, we provide insight into the relationship between the cluster structure and the nature of the carboxylate capping ligands. We also briefly discuss the use of oxo clusters. Finally, we review the nonaqueous synthesis of group 4 oxide nanocrystals, including both surfactant-free and surfactant-assisted syntheses. We focus on their precursor chemistry and surface chemistry. By putting these results together, we connect the dots and obtain more insight into the fascinating chemistry of the group 4 metals. At the same time, we also identify gaps in our knowledge and thus areas for future research.

Assessment of the Morphological, Optical, and Photoluminescence Properties of HfO2 Nanoparticles Synthesized by a Sol-Gel Method Assisted by Microwave Irradiation

In this work, we optimized the synthesis of HfO₂ nanoparticles (NPs) with a nonaqueous sol-gel method assisted by microwave heating, with a direct surfactant-free extraction and stabilization in water. To tune the structural, morphological, and photophysical properties, we explored the influence of reaction time, heating temperature, and type and concentration of a salt precursor. The controlled size, shape, crystallinity associated with high stability, a good yield of production, and stabilization in water without any surfactant modification of these HfO₂ NPs open possibilities for future optoelectronic and biomedical applications. The investigation of their optical properties, revealed a high absorption in the UV range and the presence of a large band gap, originating in transparency at visible wavelengths. Under UV excitation, photoluminescence (PL) shows three emission bands centered at 305, 381, and 522 nm and are assigned to the vibronic transition of an excited OH^•* radical or to a self-trapped exciton, to threefold oxygen vacancies V_O3 with recombination to the valence band, and to defect level, respectively. The presence of oxygen vacancies associated with PL properties is particularly attractive for optoelectronic, photocatalysis, scintillator, and UV photosensor applications. Finally, by changing the nature of the hafnium precursor salt, using hafnium ethoxide or hafnium acetylacetonate, low-crystallized and aggregated NPs were obtained, which requires further investigation.

An Evaluation of Nanoparticle Distribution in Solution-Derived YBa2Cu3O7−δ Nanocomposite Thin Films by XPS Depth Profiling in Combination with TEM Analysis

This work discusses the development of an analysis routine for evaluating the nanoparticle distribution in nanocomposite thin films. YBa2Cu3O7−δ (YBCO) nanocomposite films were synthesized via a chemical solution deposition approach starting from colloidal YBCO solutions with preformed nanoparticles. The distribution of the nanoparticles and interlayer diffusion are evaluated with X-ray photoelectron spectroscopy (XPS) depth profiling and compared with cross-sectional transmission electron microscopy (TEM) images. It is shown that the combination of both techniques deliver valuable information on the film properties as nanoparticle distribution, film thickness and interlayer diffusion.

Silica@zirconia Core@shell Nanoparticles for Nucleic Acid Building Block Sorption

The development of delivery systems for the immobilization of nucleic acid cargo molecules is of prime importance due to the need for safe administration of DNA or RNA type of antigens and adjuvants in vaccines. Nanoparticles (NP) in the size range of 20-200 nm have attractive properties as vaccine carriers because they achieve passive targeting of immune cells and can enhance the immune response of a weakly immunogenic antigen via their size. We prepared high capacity 50 nm diameter silica@zirconia NPs with monoclinic/cubic zirconia shell by a green, cheap and up-scalable sol-gel method. We studied the behavior of the particles upon water dialysis and found that the ageing of the zirconia shell is a major determinant of the colloidal stability after transfer into the water due to physisorption of the zirconia starting material on the surface. We determined the optimum conditions for adsorption of DNA building blocks, deoxynucleoside monophosphates (dNMP), the colloidal stability of the resulting NPs and its time dependence. The ligand adsorption was favored by acidic pH, while colloidal stability required neutral-alkaline pH; thus, the optimal pH for the preparation of nucleic acid-modified particles is between 7.0-7.5. The developed silica@zirconia NPs bind as high as 207 mg dNMPs on 1 g of nanocarrier at neutral-physiological pH while maintaining good colloidal stability. We studied the influence of biological buffers and found that while phosphate buffers decrease the loading dramatically, other commonly used buffers, such as HEPES, are compatible with the nanoplatform. We propose the prepared silica@zirconia NPs as promising carriers for nucleic acid-type drug cargos.

Water Formation in Non-Hydrolytic Sol-Gel Routes: Selective Synthesis of Tetragonal and Monoclinic Mesoporous Zirconia as a Case Study

Several non-hydrolytic sol-gel syntheses involving different precursors, oxygen donors, and conditions have been screened aiming to selectively produce mesoporous t-ZrO2 or m-ZrO2 with significant specific surface areas. The in situ water formation was systematically investigated by Karl Fisher titration of the syneresis liquids. XRD and nitrogen physisorption were employed to characterize the structure and texture of the ZrO2 samples. Significant amounts of water were found in several cases, notably in the reactions of Zr(OnPr)4 with ketones (acetone, 2-pentanone, acetophenone), and of ZrCl4 with alcohols (benzyl alcohol, ethanol) or acetone. Conversely, the reactions of Zr(OnPr)4 with acetic anhydride or benzyl alcohol at moderate temperature (200 °C) and of ZrCl4 with diisopropyl ether appear strictly non-hydrolytic. Although reaction time and reaction temperature were also important parameters, the presence of water played a crucial role on the structure of the final zirconia: t-ZrO2 is favored in strictly non-hydrolytic routes, while m-ZrO2 is favored in the presence of significant amounts of water. 1 H and 13 C NMR analysis of the syneresis liquids allowed us to identify the main reactions responsible for the formation of water and of the oxide network. The morphology of the most interesting ZrO2 samples was further investigated by electron microscopy (SEM, TEM).

Study of the methylene blue adsorption mechanism using ZrO2/Polyaniline nanocomposite

ZrO2 (Zirconia) nanoparticles (NPs), PANI (polyaniline), and ZrO2/PANI nanocomposites (NCs) were successfully synthesized using CTAB (Hexadecyltrimethylammonium bromide) and SDS (Sodium dodecyl sulfate) surfactants by following the co-precipitation method. The structural phase analysis of as-prepared, annealed nanoparticles, and nanocomposites was done using the XRD (x-ray diffraction) technique. The crystallite size of pure SDS and CTAB assisted ZrO2 NPS comes out to be 19 and 17 nm, respectively. After the formation of NCs, the size has been reduced to 15.7 and 15.9 nm, respectively for the same samples. The effect of surfactants on the dye adsorption mechanism was studied using XRD and UV–vis spectroscopy. The prepared NPs and NCs were utilized as an adsorbent for the removal of organic dye methylene blue (MB) which is used as a model compound. UV–vis spectra of the supernatant solution were taken and studied to detect the relative decrease in the dye concentration with time. The as-prepared CTAB assisted ZrO2/PANI NCs show higher adsorption activity than annealed CTAB assisted ZrO2/PANI whereas a reversal trend in the adsorption activity was observed for SDS-assisted ZrO2/PANI NCs. Various kinetic models were implemented and correlated to the experimental data to elucidate the working mechanism for dye adsorption and to set up, a relation in the adsorption activity of surfactant modified NPs and NCs.

Anisotropic morphology, formation mechanisms, and fluorescence properties of zirconia nanocrystals

ZrO₂ nanocrystals with spheres and elongated platelets were systemically prepared through a simple hydrothermal method by the use of ZrOCl₂·8H₂O and CH₃COOK as raw materials. The anisotropic morphology and formation mechanism of the monoclinic and/or tetragonal ZrO₂ were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscope, and high-resolution transmission electron microscope techniques. The uniform elongated platelets and star-like structures were composed of short nanorods with a diameter of approximately 5 nm and a length of approximately 10 nm. The different morphologies were formed due to the different contents of CH₃COO^- and Cl^- and their synergy. The fluorescence band position and the band shape remained about the same for excitation wavelengths below 290 nm and the different morphologies of the nanocrystals.

Hybrid approach to obtain high-quality BaMO3 perovskite nanocrystals

A novel hybrid solvothermal approach for perovskite nanocrystal formation via accurate control of the hydrolytic process is reported. This new synthetic methodology sets a whole general route to successfully tune the sizes of high-quality BaMO3 (M = Ti4+, Zr4+, and Hf4+) perovskite nanocrystals. Purely cubic-phase nanocrystals (stable in alcohol media) were obtained using controlled water amounts, combining the well-known aqueous sol-gel process with the classic solvothermal method. Exhaustive optimizations revealed feasibility of a fast (1 hour) and reproducible synthesis with small variations in the crystal size or agglomeration parameters. The study also reveals water content as the pivotal factor to achieve this wide range of sizes through a controlled hydrolytic step. Finally, the study of the hydrolytic process made it possible to shed some light on mechanistic insights of this synthetic route.

Designed single-phase ZrO2 nanocrystals obtained by solvothermal syntheses

Crystal growth pathways controlled by the acidity, type and concentration of the capping agent lead to different nanostructures and crystalline phases.

Detrimental Effect and Neutralization of in Situ Produced Water on Zirconia Nanoparticles Obtained by a Nonaqueous Sol-Gel Method

In this work, the phase purity and size of zirconia nanocrystals samples were studied in terms of zirconium concentration, added water content, and subsequent use of a post solvothermal treatment. The progressive tetragonal-to-monoclinic transformation of zirconia sample was observed to be strongly related to the water content of the alcoholic medium. But more surprisingly, it has been shown that even under initially anhydrous conditions and for particle size below 5 nm, the phase purity of the samples was deteriorated by a side-reaction of alcohol dehydration catalyzed by the surface of the nanoparticles (NPs). Since the phase transformation is essentially driven by the water content of the reaction mixture, we have shown that it was possible to recover an excellent phase purity without the help of any usual dopants by adding a strong alkaline desiccating agent. Provided that a sufficient sodium to zirconium ratio was ensured, the formation of the monoclinic phase was not observed whatever the zirconium precursor concentration. The effectiveness of this cure was related to the ability of sodium metal to generate reactive alkoxide able to neutralize water and to catalyze an alternative sol-gel mechanism leading to the formation of the t-ZrO₂ NPs.

Tailoring the Synthesis of LnF3 (Ln = La-Lu and Y) Nanocrystals via Mechanistic Study of the Coprecipitation Method

Here, 15 LnF₃ nanocrystals are synthesized using coprecipitation method with citrate stabilization to allow the fast, easy, and reproducible synthesis of several nanoscaled structures in water. General trends related to the behavior of LnF₃ nanocrystals are highlighted due to their broad range of application in several fields (e.g., medical applications). The same nature for all Ln³⁺ cations is expected due to the internal role of f orbitals. However, we found that the use of different lanthanide elements is crucial in the final size, shape, assembly, and crystalline structure. In addition, the decrease of the cation size of the lanthanide series changes the behavior of these compounds, resulting in hexagonal, orthorhombic, and cubic crystalline structures. In addition, we are able to tune the cubic crystalline phase to pure orthorhombic by modifying the pH of the system using HBF₄ instead of tetramethylammonium citrate. Via ¹¹B NMR, we demonstrated the mechanism of HBF₄ as fluorinating agent if an additional source of F^- is not added during the synthesis. ¹H NMR and IR techniques were performed to unravel the picture of the surface chemistry of the two representative metal cations (Y and La). Finally, HRTEM and SAED were performed to uncover the shape of the obtained nanocrystals and the preferential orientation of the assembled particles, giving crucial information on the involved mechanisms. This study reveals not only the dependence of the crystalline structure on the used metal and pH but also ability to achieve LnF₃ assembled particles depending on the final shape and temperature.

Mesoporous ZrO2 Nanoframes for Biomass Upgrading

The rational design and preparation of a high-performance catalyst for biomass upgrading are of great significance and remain a great challenge. In this work, mesoporous ZrO₂ nanoframe, hollow ring, sphere, and core-shell nanostructures have been developed through a surfactant-free route for upgrading biomass acids into liquid alkane fuels. The obtained ZrO₂ nanostructures possess well-defined hollow features, high surface areas, and mesopores. The diversity of the resultant ZrO₂ nanostructures should arise from the discrepant hydrolysis of two different ligands in zirconocene dichloride (Cp₂ZrCl₂) as the zirconium precursor. The time-dependent experiments indicate that Ostwald ripening and salt-crystal-template formation mechanisms should account for hollow spheres and nanoframes, respectively. Impressively, compared with the hollow sphere, commercial nanoparticle, and the ever-reported typical results, the ZrO₂ nanoframe-promoted Ni catalyst exhibits greatly enhanced catalytic activity in the upgrading of biomass acids to liquid alkane fuels, which should be ascribed to the hollow feature, large active surface area, highly dispersed Ni, and strong metal-support interactions arising from the structural advantages of nanoframes. The nanoframes also possess excellent solvothermal and thermal stability. Our findings here can be expected to offer new perspectives in material chemistry and ZrO₂-based catalytic and other applications.

Solution Species and Crystal Structure of Zr(IV) Acetate

Complex formation and the coordination of zirconium with acetic acid were investigated with Zr K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) and single-crystal diffraction. Zr K-edge EXAFS spectra show that a stepwise increase of acetic acid in aqueous solution with 0.1 M Zr(IV) leads to a structural rearrangement from initial tetranuclear hydrolysis species [Zr₄(OH)₈(OH₂)₁₆]⁸⁺ to a hexanuclear acetate species Zr₆(O)₄(OH)₄(CH₃COO)₁₂. The solution species Zr₆(O)₄(OH)₄(CH₃COO)₁₂ was preserved in crystals by slow evaporation of the aqueous solution. Single-crystal diffraction reveals an uncharged hexanuclear cluster in solid Zr₆(μ₃-O)₄(μ₃-OH)₄(CH₃COO)₁₂·8.5H₂O. EXAFS measurements show that the structures of the hexanuclear zirconium acetate cluster in solution and the solid state are identical.

Amino Acid-Based Stabilization of Oxide Nanocrystals in Polar Media: From Insight in Ligand Exchange to Solution ¹H NMR Probing of Short-Chained Adsorbates

Ligand exchange is a crucial step between nanocrystal synthesis and nanocrystal application. Although colloidal stability and ligand exchange in nonpolar media are readily established, the exchange of native, hydrophobic ligands with polar ligands is less systematic. In this paper, we present a versatile ligand exchange strategy for the phase transfer of carboxylic acid capped HfO2 and ZrO2 nanocrystals to various polar solvents, based on small amino acids as the incoming ligand. To gain insight in the fundamental mechanism of the exchange, we study this system with a combination of FTIR, zeta potential measurements, and solution (1)H NMR techniques. The detection of surface-associated, small ligands with solution NMR proves challenging in this respect. Tightly bound amino acids are undetectable, but their existence can be proven through displacement with other ligands in titration experiments. Alternatively, we find that methyl moieties belonging to bound species can circumvent these limitations because of their more favorable relaxation properties as a result of internal mobility. As such, our results are not limited to amino acids but to any short-chained ligand and will therefore facilitate the rigorous investigation and understanding of various ligand exchange processes.

Excellent performance in hydrogenation of esters over Cu/ZrO2 catalyst prepared by bio-derived salicylic acid

A variety of bio-derived organic carboxylic acids are used to prepare Cu/ZrO₂. In particular, salicylic acid provides superb control over the distribution of small Cu particles on t-ZrO₂, which affords enhanced catalytic activity.

From ligands to binding motifs and beyond; the enhanced versatility of nanocrystal surfaces

Surface chemistry bridges the gap between nanocrystal synthesis and their applications.

Dopant and excitation wavelength dependent color-tunable white light-emitting Ln3+:Y2WO6 materials (Ln3+ = Sm, Eu, Tb, Dy)

Color-tunable Y₂WO₆ microstructures doped or co-doped with Sm³⁺, Eu³⁺, Tb³⁺, and Dy³⁺ were synthesized in the presence of glycerol both as a solvent and a structure directing agent for obtaining white light emitting materials.