Synthesis Of Metal Oxide Nanocrystals Research Articles

Ligand Conversion in Nanocrystal Synthesis: The Oxidation of Alkylamines to Fatty Acids by Nitrate.

Ligands are a fundamental part of nanocrystals. They control and direct nanocrystal syntheses and provide colloidal stability. Bound ligands also affect the nanocrystals’ chemical reactivity and electronic structure. Surface chemistry is thus crucial to understand nanocrystal properties and functionality. Here, we investigate the synthesis of metal oxide nanocrystals (CeO2-x, ZnO, and NiO) from metal nitrate precursors, in the presence of oleylamine ligands. Surprisingly, the nanocrystals are capped exclusively with a fatty acid instead of oleylamine. Analysis of the reaction mixtures with nuclear magnetic resonance spectroscopy revealed several reaction byproducts and intermediates that are common to the decomposition of Ce, Zn, Ni, and Zr nitrate precursors. Our evidence supports the oxidation of alkylamine and formation of a carboxylic acid, thus unraveling this counterintuitive surface chemistry.

Simultaneous Size- and Phase-Controlled Synthesis of Metal Oxide Nanocrystals through Esterification Reactions

Here, we report on the synthesis of metal oxide (manganese oxide) nanocrystals with simultaneous size- and phase-controlled manners through thermal decomposition of metal oleate. Bulk manganese oxide powders were applied as precursors, which react with oleic acid to yield manganese oleate at elevated temperature. In the presence of long-chain alkyl alcohols, these as-formed manganese oleates would react with alcohol to yield esters, which consequently leads to manganese hydroxide (oxide) (MnO or Mn3O4) nanocrystals. In this way, the size of manganese oxide nanocrystals can be tuned by different amounts of alkyl alcohols. Furthermore, the phase of manganese oxide nanocrystals can also be simultaneously controlled, where a small amount and large amount of alkyl alcohols lead to the formation of MnO and Mn3O4 phase, respectively. These high-quality manganese oxide nanocrystals are able to self-assemble into ordered nanocrystal superlattices.

Group 13 Lewis acid catalyzed synthesis of metal oxide nanocrystals via hydroxide transmetallation.

A new transmetallation approach is described for the synthesis of metal oxide nanocrystals (NCs). Typically, the synthesis of metal oxide NCs in oleyl alcohol is driven by metal-based esterification catalysis with oleic acid to produce oleyl oleate ester and M-OH monomers, which then condense to form MxOy solids. Here we show that the synthesis of Cu2O NCs by this method is limited by the catalytic ability of copper to drive esterification and thus produce Cu+-OH monomers. However, inclusion of 1-15 mol% of a group 13 cation (Al3+, Ga3+, or In3+) results in efficient synthesis of Cu2O NCs and exhibits size/morphology control based on the nature of M3+. Using a continuous-injection procedure where the copper precursor (Cu2+-oleate) and catalyst (M3+-oleate) are injected into oleyl alcohol at a controlled rate, we are able to monitor the reactivity of the precursor and M3+ catalyst using UV-visible and FTIR absorbance spectroscopies. These time-dependent measurements clearly show that M3+ catalysts drive esterification to produce M3+-OH species, which then undergo transmetallation of hydroxide ligands to generate Cu+-OH monomers required for Cu2O condensation. Ga3+ is found to be the "goldilocks" catalyst, producing NCs with the smallest size and a distinct cubic morphology not observed for any other group 13 metal. This is believed to be due to rapid transmetallation kinetics between Ga3+-OH and Cu+-oleate. These studies introduce a new mechanism for the synthesis of metal oxides where inherent catalysis by the parent metal (i.e. copper) can be circumvented with the use of a secondary catalyst to generate hydroxide ligands.

Evolution of Atomic-Level Structure in Sub-10 Nanometer Iron Oxide Nanocrystals: Influence on Cation Occupancy and Growth Rates.

Spinel iron oxide nanocrystals (NCs) have been reported to have atomic-level core and surface structural features that differ from those of the bulk material. Recent advances in a continuous growth synthesis of metal oxide NCs make it possible to prepare a series of NCs with subnanometer control of size with diameters below 10 nm that are well-suited for investigating size-dependent structure and reactivity. Here, we study the evolution of size-dependent structure in spinel iron oxide and determine how nanoscale structure influences the growth of NCs. We synthesized spinel iron oxide NCs via a continuous growth method that permits layer-by-layer control of size in order to monitor nanoscale structure over 16 core sizes between 3 and 10 nm. X-ray total scattering data were collected and analyzed with pair distribution function (PDF) analysis in order to refine quantitative structural features including cation occupancies that could be used to detect changes both in the oxidation state and the presence of tetrahedrally coordinated cation vacancies in the NCs. We find that the average iron oxidation state increases as core diameters decrease from 8 down to 3 nm. The trend in iron oxidation state can be explained by the oxidation of surface layers in the NCs. For samples exposed to air for several weeks, oxidation appears to cease when a volume equivalent to that of an ∼1.3 nm shell is converted to the more oxidized maghemite. The number of tetrahedrally coordinated cation vacancies also increases as the NC core size decreases. The correlation between the number of these vacancies and the faster growth for smaller NCs suggests that these reactive vacancies may be responsible for the rapid growth observed for nanocrystals with diameters smaller than 8 nm.

Secondary Particle Formation during the Nonaqueous Synthesis of Metal Oxide Nanocrystals.

This study aims to elucidate the aggregation and agglomeration behavior of TiO2 and ZrO2 nanoparticles during the nonaqueous synthesis. We found that zirconia nanoparticles immediately form spherical-like aggregates after nucleation with a homogeneous size of 200 nm, which can be related to the metastable state of the nuclei and the reduction of surface free energy. These aggregates further agglomerate, following a diffusion-limited colloid agglomeration mechanism that is additionally supported by the high fractal dimension of the resulting agglomerates. In contrast, TiO2 nanoparticles randomly orient and follow a reaction-limited colloid agglomeration mechanism that leads to a dense network of particles throughout the entire reaction volume. We performed in situ laser light transmission measurements and showed that particle formation starts earlier than previously reported. A complex population balance equation model was developed that is able to simulate particle aggregation as well as agglomeration, which eventually allowed us to distinguish between both phenomena. Hence, we were able to investigate the respective agglomeration kinetics with great agreement to our experimental data.

A feasible strategy to balance the crystallinity and specific surface area of metal oxide nanocrystals

Practical, efficient synthesis of metal oxide nanocrystals with good crystallinity and high specific surface area by a modified polymer-network gel method is demonstrated, taking ZnO nanocrystals as an example. A novel stepwise heat treatment yields significant improvement in crystal quality. Such nanophase materials can effectively degrade common organic dyes under solar radiation and can perform very well in photo-assisted detection of NO2 gas. Other typical metal oxide nanocrystals with good crystallinity and high specific surface area were also synthesized successfully under similar conditions. This work provides a general strategy for the synthesis of metal oxide nanocrystals, balancing the crystallinity and specific surface area.

Growth Mechanism and Surface Chemical Characteristics of Dicarboxylic Acid-Modified CeO2 Nanocrystals Produced in Supercritical Water: Tailor-Made Water-Soluble CeO2 Nanocrystals

Tailor-made surface-modified metal oxide nanocrystals enable various applications including medical, electronic, magnetic, and photovoltaic devices. Both the synthesis and application of surface-modified metal oxide nanocrystals rely on the interaction between organic molecules and the surface of metal oxides. From this viewpoint, we have focused on the synthesis of metal oxide nanocrystals using supercritical water in the presence of organic molecules as a surface modifier. Here, we describe the use of dicarboxylic acids with various chain lengths as the modifiers of CeO2 nanocrystals. The morphology and displayed crystallite plane of CeO2 nanocrystals could be controlled by the length of dicarboxylic acids. Long dicarboxylic acids produced cuboctahedral or cubic CeO2 nanocrystals, possibly because of the decreased growth rate of the {200} plane. The growth mechanism of the CeO2 nanocrystals is discussed in detail. Furthermore, dicarboxylic acids on the surface of the CeO2 nanocrystals changed the isoele...

Direct Thermal Decomposition of Metal Nitrates in Octadecylamine to Metal Oxide Nanocrystals

We have developed a method for the synthesis of metal oxide nanocrystals with controllable shape and size, which is based on the direct thermal decomposition of metal nitrates in octadecylamine. Mn3O4 nanoparticles and nanorods with different lengths were synthesized by using manganese nitrate as the decomposition material. Other metal oxide nanocrystals such as NiO, ZnO, CeO2, CoO, and Co3O4 were also prepared by this method. These nanocrystals were then assembled into 3D colloidal spheres by a surfactant-assisted self-assembly process. Subsequently, calcination was carried out to remove the surfactants to obtain mesoporous metal oxides, which show large pores, good crystallization, thermally stable pore mesostructures, and potential applications in various fields, especially in catalysis and lithium-ion batteries.