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A general strategy for nanocrystal synthesis[J].

Schematic illustration for the synthesis of porous upconversion nanocrystals in the hydrothermal system.

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A general strategy for nanocrystal synthesis.

Abstract:
Heterogeneous photo-catalysis is an advanced oxidation process (PAO), which has been the subject of numerous studies and applications, particularly using the commercial oxide of TiO2 (P25, Evonik). Zinc oxide (ZnO) has often been considered a valid alternative to TiO2 due to its good opto-electronic, catalytic and photochemical characteristics along with its low cost. In order to improve the photocatalytic performance of ZnO for practical applications, various types of synthetic approaches have been developed, including, among others, the hydrothermal / solvothermal growth method, sol-gel method, ultrasonic assisted method, deposition chemistry in vapor phase, etc. with the aim of preparing ZnO particles with different sizes and morphologies. However, all of these methods require relatively severe reaction conditions such as high temperature, sophisticated techniques, high purity of gases, adjustable gas flow, expensive raw materials, etc. Therefore, it is important to find a simple and cost-effective method for the synthesis of crystalline nano-particles of ZnO. For this reason, in the present work, the ZnO has been synthesized by three different procedures: conventional aqueous precipitation method, hydrothermal method (H) and microwave assisted method (MW). In all three processes, the same material is obtained, hydrocincite [Zn5(CO3) 2(OH)6], which evolves to crystalline ZnO after calcination thermal treatments. We investigated the effect of the calcination temperature, at the same time (2 h), on the optical, textural and structural properties. Photo-catalytic studies were performed using two selected substrates, Methyl Orange and Phenol, as toxic model substrates (one colorant and the other transparent). The catalysts prepared were characterized by several techniques: DRX, SBET, FE-SEM, TEM and UV-Vis (in diffuse reflectance mode).From the results of XRD, it has been possible to establish that a minimum difference between the relative intensities of exposed faces (I100 and I002) is a crucial factor to obtain good photocatalytic properties. This minimum difference is achieved, in our cases by thermal treatments of calcination at 400ºC, 2 h. When this temperature is chosen, there is no appreciable variation between the photocatalytic activities of the oxides of zinc obtained by the three processes, and there are small differences depending on the nature of the substrate chosen, which can be attributed to the textural differences between the oxides. In any case, the obtained zinc oxides show, for each substrate, photo-catalytic activities in the UV that are superior to those presented by the widely used commercial oxide TiO2 (P25) used as reference.

 21. Wang X, Li YD. A general strategy for nanocrystal synthesis.  2005;437:121-24

32. He M, Huang P, Zhang C, Chen F, Wang C, Ma J. . A general strategy for the synthesis of upconversion rare earth fluoride nanocrystals via a novel OA/ionic liquid two-phase system. 2011;47:9510-2

A GENERAL STRATEGY FOR NANOCRYSTAL SYNTHESIS …

17. Wang X, Zhuang J, Peng Q, Li Y. A general strategy for nanocrystal synthesis.  2005;437:121-4

Organometallic halide perovskites have quickly become one of the most interesting semiconductors for photovoltaics, with solar cells made of these materials reaching power conversion efficiencies of over 22%. We were among the first to synthesize colloidal CHNHPbX (X = I, Br) nanocrystals with different morphologies (dots, rods, wires, sheets), and showed that CHNHPbI nanostructures monitored at the single particle level show shape-correlated PL emission across whole particles, with little photobleaching observed and very few off periods. We are working to exploit the potential of such low-dimensional organometal halide perovskite semiconductors in constructing new porous and nanostructured solar cell architectures, as well as in light-emitting devices and single particle imaging and tracking. We have also been particularly interested in mixed-halide perovskites such as CHNHPbXX' (X, X' = I, Br, Cl) because of their enhanced moisture stability, and band gap tunability. We have used a combination of optical absorption spectroscopy, powder X-ray diffraction (XRD) and, for the first time, 207Pb solid state nuclear magnetic resonance (ssNMR), to probe the extent of alloying and phase segregation in these materials. Because 207Pb ssNMR chemical shifts are highly sensitive to local coordination, electronic structure, and vary linearly with halogen electronegativity and band gap, this technique gave us the true chemical speciation and composition of samples made by three different preparative methods: solution phase synthesis, thermal annealing, and solid phase synthesis. 207Pb ssNMR revealed that nonstoichiometric dopants and semicrystalline phases are prevalent in samples made by solution phase synthesis. We have shown that these nanodomains are persistent after thermal annealing up to 200 C. Further, our novel solid phase synthesis, starting from the parent, single-halide perovskites can suppress phase segregation but not the formation of dopants. These observations are consistent with the presence of miscibility gaps and spontaneous spinodal decomposition of the mixed-halide perovskites at room temperature, and underscore how strongly different synthetic procedures impact the nanostructuring and composition of organolead halide perovskites. We believe better optoelectronic properties and improved device stability and performance will be achieved through careful manipulation of the different phases and nanodomains present in these materials.

Colloidal chemistry offers an assortment of synthetic tools for tuningthe shape of NCs, but some nanoparticle morphologies require alternativeprocessing strategies.

A general strategy for nanocrystal synthesis

Download Free Full-Text of an article A GENERAL STRATEGY FOR NANOCRYSTAL SYNTHESIS

Colloidal nanocrystals exhibit a wide range of size- and shapedependent properties and have found application in myriad fields, incuding optics, electronics, mechanics, drug delivery and catalysis, to name but a few. Synthetic protocols that enable the simple and convenient production of colloidal nanocrystals with controlled size, shape and composition are therefore of key general importance. Current strategies include organic solution-phase synthesis, thermolysis of organometallic precursors, sol–gel processes, hydrothermal reactions and biomimetic and dendrimer templating. Often, however, these procedures require stringent experimental conditions, are difficult to generalize, or necessitate tedious multistep reactions and purification. Recently, linear amphiphilic block co-polymer micelles have been used as templates to synthesize functional nanocrystals, but the thermodynamic instability of these micelles limits the scope of this approach. We have been developing a general strategy for crafting a large variety of functional nanocrystals with precisely controlled dimensions, compositionsand architectures by using star-like block copolymers as nanoreactors. This new class of co-polymers forms unimolecular micelles that are structurally stable, therefore overcoming the intrinsic instability of linear blockco-polymer micelles. Our approach enables the facile synthesis of organic solvent- and water-soluble nearly monodisperse nanocrystals with desired composition and architecture, including core–shell and hollow nanostructures. We demonstrate the generality of our approach by describing, as examples, the synthesis of various sizes and architectures of metallic, ferroelectric, magnetic, semiconductor and luminescent colloidal nanocrystals.

1. X. Pang, L. Zhao, W. Han, X. Xin, and Z. Lin*, "A general and robust strategy for the synthesis of nearly monodisperse colloidal nanocrystals”, Nature Nanotechnology, 8, 426 (2013).

A general strategy for nanocrystal synthesis.
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Synthesis and characterization of nearly monodisperse …

Nanocrystals of metal phosphides, which can have useful catalytic, electronic, and magnetic properties, are known to be accessible by using trioctylphosphine (TOP) as a highly reactive phosphorus source. Here we report a general strategy for synthesizing transition metal phosphides, including those with 4d and 5d transition metals that have not previously been reported as unsupported nanocrystals. Unlike previously reported methods that involve direct decomposition of organometallic precursors, our method utilizes preformed metal nanoparticles as templates for generating metal phosphide nanocrystals. Metal nanoparticles are reacted with TOP in a hot solvent (290−360 °C) to form transition metal phosphides such as Ni2P, PtP2, Rh2P, PdP2, Pd5P2, and Au2P3. Furthermore, nanostructures such as hollow spheres can be easily made using a Kirkendall-type mechanism, which utilizes metal nanoparticles as reactive templates.

General & Introductory Chemistry

The capacity to self-generate a micellar surface coating is highly sensitive to the chemical nature of the reaction solvent. Traditional nonpolar solvents, like dioctylether (DOE) and octadecene (ODE) prevent micellar encapsulation of nanocrystals. The use of PEG as a reaction solvent is important because of its ‘amphibious’ nature (that is, soluble in both polar and nonpolar solvents). The only solubility exception for the nanocrystals is hexane (), in which PEG is insoluble. However once PEG is removed from the nanocrystals (see below), they become soluble in aliphatic hydrocarbons. It is thus surprising that the nanocrystals are instantly soluble in diethyl ether, a solvent in which PEG does not disperse. We believe that this feature is a result of the amphiphilicity of amphipol, which can solubilizes a large amount of PEG in ether, even when present in small quantities.

Surface modification, functionalization and …

This phenomenon of spontaneous encapsulation is related to the nature of the amphiphilic coordinating ligand, as well as the reaction solvent. We have independently evaluated the contributions of both of these reaction components, as summarized in . The multidentate, amphiphilic structure of the amphipol is crucial for attaining the dual functionality of coordination and encapsulation. Traditional hydrophobic coordinating ligands used in high-temperature nanocrystal reactions, such as oleic acid, can be used to prepare stable, monodisperse colloids in nonpolar solvents. However these ligands are poor surfactants, and cannot stabilize nanocrystals in polar solvents. We have found that achieving efficient encapsulation from a coordinating ligand requires a balanced ratio of coordinating groups to hydrophobic groups. That is, too many coordinating groups yield poor encapsulation efficiency, whereas ligands containing too many hydrophobic domains cannot stabilize the nanocrystals during growth. In general, we find that alkylation ratios of 30-60% work well for aliphatic chain lengths from 8 to 14 carbons. Interestingly, spatial or structural ordering of these domains is not necessary, as amphipols with ordered structures (PMAT, ) and randomly grafted structures (PAA-OA0.4) yield nearly identical particles. On the other hand, the use of a linear, graft-like polymer backbone is crucial for the success of this procedure, as it allows directional orientation of the hydrophilic and hydrophobic domains while preventing crosslinking. Performing this same procedure with a di-carboxy PEG ligand resulted in complete precipitation of the nanocrystals after nucleation due to ligand-induced crosslinking.

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