Formation mechanisms of uniform nanocrystals via hot-injection and heat-up methods

Synthesis of uniform nanocrystals is very important because the size uniformity of an ensemble of nanocrystals is directly related to the homogeneity of their chemical and physical properties. Classical theory suggests that burst nucleation and diffusion-controlled growth are the most important factors for the control of the size distribution in colloidal synthesis. In the last two decades, the numerous reports on the synthesis of uniform nanocrystals have popularized two major synthetic methods, namely, hot-injection and heat-up, to obtain uniform nanocrystals of various materials including metals, semiconductors, and oxides. Mechanistic studies on how such uniform nanocrystals are obtained in those two methods are reviewed and theoretical explanations are provided in the current review.     

Two-phase solvothermal synthesis of rare-earth doped NaYF4 upconversion fluorescent nanocrystals

The synthesis, characterization, and fluorescent spectra of rare-earth doped NaYF₄ upconversion nanocrystals are introduced in this paper. The nanocrystals were synthesized in the water-ethanol-oleic acid system via a two-phase solvothermal approach, by using rare-earth stearate as the precursor. The as-prepared nanocrystals were of hexagonal phase, strong UC fluorescent intensity, with an average size of about 25 nm, which have been characterized by TEM, SAED, powder XRD, and luminescence spectroscopy. The possible mechanism of this synthesis that the nucleation and growth of nanocrystals occurred at the solid-liquid interface was also discussed. The nanocrystals are hydrophilic, and expected to fulfil the demand for biological applications via further modification.     

Characterization of CdTe nanocrystals during their synthesis in liquid paraffin: optical properties and particle growth

This article presents a systematic investigation of the optical properties and the growth process of CdTe nanocrystals during their hot-injection-based synthesis in liquid paraffin. Cadmium(II) stearate and tributylphosphine telluride are used as precursors. The as-obtained nanocrystals are characterized by transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), UV–vis absorbance, and fluorescence spectroscopy. The changes in optical absorbance and fluorescence during the nanocrystal synthesis are studied. The average nanocrystal size and the mean nanocrystal concentration are derived from the optical spectra and their changes during the synthesis are investigated. It is found that synthesis at lower temperature (150 °C) favors the continuous nucleation and leads to the formation of relatively smaller nanocrystals (~3 nm in size), whereas the nanocrystal concentration is relatively constant during synthesis at higher temperature (250 °C) thus leading to the formation of larger nanocrystals (~5 nm in size).     

Synthesis of platinum single-twinned right bipyramid and {111}-bipyramid through targeted control over both nucleation and growth using specific peptides

Shape-controlled synthesis requires rigorous kinetic control over both nucleation and growth. For platinum (Pt), to date it is still challenging to generate twinned seeds in nucleation in a controllable fashion. Here, we report that a specific Pt binding peptide BP7A is able to mediate and stabilize single-twinned seeds formation at the nucleation stage under mild conditions. Importantly, it targets the control over nucleation directly. Combining with control over growth, we further demonstrate the rational design and synthesis of single-twinned structures, right bipyramid and {111}-bipyramid, by incorporating targeted facet stabilization over {100} facet and {111} facet, respectively. To the best of our knowledge, this is the first report on the successful synthesis of single-twinned bipyramids for Pt nanocrystals (NCs) with high yields. The work here demonstrates the power of biomolecules in recognizing and mediating inorganic nanomaterials synthesis, guiding the formation of material structures that are otherwise unconventional, and hence presenting one step further toward predictable and programmable biomimetic synthesis.     

Synthesis and magnetic properties of nickel nanocrystals

Size-controlled synthesis of nickel nanocrystals by the decomposition of nickel acetylacetone in a noncoordinating reagent, oleylamine, was studied. Three processes were employed to synthesize nanocrystals with diameters in the range of 20–60 nm. The resultant nanoparticles were characterized to be single-phase nanocrystalline nickel with a face-centered cubic type. Mechanism of the size-controlled nanocrystals formation was studied and found that relative monodispersed nanocrystals could be formed by the separation of nucleation and growth process while the size could be controlled through modulating growth period. Magnetic measurements showed that samples were still ferromagnetic, and that their saturation magnetization and coercivity are size-dependent.     

Droplet‐Based Microreactors for Continuous Production of Palladium Nanocrystals with Controlled Sizes and Shapes

Droplet‐based microreactors are used for the continuous production of Pd nanocrystals. Specifically, commercially available polytetrafluoroethylene (PTFE) tube and silica capillaries are utilized to fabricate a fluidic device capable of generating water‐in‐oil droplets. In addition to the feasibility of using such droplets as microreactors for conducting a synthesis, the ability to control the composition and concentration of reagents by adjusting the flow rates is demonstrated; reagents are mixed by periodically pinching the PTFE tube, and nanocrystals are produced with uniform size distribution in a continuous fashion. The capability to tailor the size and shape of the resultant nanocrystals is further demonstrated by introducing the reducing agent and capping agent at different flow rates to control the nucleation and growth processes. The ability to transform a bulk synthesis into a droplet‐based system holds great potential for the development of a new route to the high‐volume production of nanocrystals.     

胶体纳米晶合成与形貌控制策略及机理EI北大核心CSCD

胶体纳米晶合成与控制策略主要从动力学方面考虑,一般要结合液相胶体成核生长理论和晶体生长理论来分析。本文首先从成核阶段、生长阶段和熟化阶段的控制方面阐述了胶体纳米晶形貌合成与控制。然后对于经典晶体学理论解释不了的现象,阐述了选择吸附机理、有效单体机理、取向连接机理等用来解释胶体纳米晶的合成机理。本文还对近年来发展的其他一些合成纳米材料的新机理或多种机理共同作用做了简要介绍。最后,对纳米晶合成与形貌控制的前景作了概述,认为定量和精准结构控制是纳米晶形貌合成与控制面临的巨大挑战和发展趋势。     

Dimension-controlled synthesis of CdS nanocrystals: from 0D quantum dots to 2D nanoplates

The dimension-controlled synthesis of CdS nanocrystals in the strong quantum confinement regime is reported. Zero-, one-, and two-dimensional CdS nanocrystals are selectively synthesized via low-temperature reactions using alkylamines as surface-capping ligands. The shape of the nanocrystals is controlled systematically by using different amines and reaction conditions. The 2D nanoplates have a uniform thickness as low as 1.2 nm. Furthermore, their optical absorption and emission spectra show very narrow peaks indicating extremely uniform thickness. It is demonstrated that 2D nanoplates are generated by 2D assembly of CdS magic-sized clusters formed at the nucleation stage, and subsequent attachment of the clusters. The stability of magic-sized clusters in amine solvent strongly influences the final shapes of the nanocrystals. The thickness of the nanoplates increases in a stepwise manner while retaining their uniformity, similar to the growth behavior of inorganic clusters. The 2D CdS nanoplates are a new type of quantum well with novel nanoscale properties in the strong quantum confinement regime.     

Laser ablation in liquids: Applications in the synthesis of nanocrystals

This work presents a survey on the recent progress in laser ablation of a solid target in a confining liquid for the synthesis of nanocrystals with focus on the mechanism of nanocrystal growth. The effects of liquid confinement, thermodynamic nucleation, phase transition, and kinetic growth of the nanostructures are discussed in detail. Besides, a variety of applications of the laser ablation is reviewed, including surface patterning, surface cleaning, and surface coating. Experimental results and theoretical analysis indicate that laser ablation of a solid target in a confining liquid provides an effective means to synthesize nanocrystals, especially for the metastable nanocrystals such as diamond and carbon related materials, immiscible alloys, etc. The laser ablation in liquids has demonstrated the following advantages: (i) a chemically “simple and clean” synthesis, (ii) an ambient conditions not extreme temperature and pressure, and (iii) the new phase formation of nanocrystals may involve in both liquid and solid. These advantages allow us to combine selected solid targets and liquid to fabricate compound nanostructures with desired functions.     

A facile and general approach to polynary semiconductor nanocrystals via a modified two-phase method

Cu(2)ZnSnS(4) nanocrystals were synthesized through a modified two-phase method and characterized with transmission electron microscopy (TEM), powder x-ray diffraction (XRD) and UV-vis spectroscopy. Inorganic metal salts were dissolved in the polar solvent triethylene glycol (TEG) and then transferred into the non-polar solvent 1-octadecene (ODE) by forming metal complexes between metal ions and octadecylamine (ODA). Since nucleation and growth occur in the single phase of the ODE solution, nanocrystals could be produced with qualities similar to those obtained through the hot-injection route. Balancing the reactivity of the metal precursors is a key factor in producing nanocrystals of a single crystalline phase. We found that increasing the reaction temperature increases the reactivity of each of the metal precursors by differing amounts, thus providing the necessary flexibility for obtaining a balanced reactivity that produces the desired product. The versatility of this synthesis strategy was demonstrated by extending it to the production of other polynary nanocrystals such as binary (CuS), ternary (CuInS(2)) and pentanary (Cu(2 - x)Ag(x)ZnSnS(4)) nanocrystals. This method is considered as a green synthesis route due to the use of inorganic metal salts as precursors, smaller amounts of coordinating solvent, shorter reaction time and simpler post-reaction treatment.     

Synthesis of symmetrical hexagonal-shape PbO nanosheets using gold nanoparticles

Anisotropic growth of PbO with symmetrical hexagonal-shape nanosheet morphology was demonstrated for the first time via solution phase synthesis in the presence of Au nanoparticles at room temperature. Au NPs play a critical role in the formation of PbO nanosheets. No nanosheets were formed in absence of Au NPs. The effect of Au NPs appears to result from their ability to provide nucleation sites to seed anisotropic growth of the PbO nanocrystals and later the nanocrystals aggregated to form nanosheet structure. The method demonstrated here provides a facile room temperature colloidal method of producing high-quality and yield of high-symmetrical hexagonal-shape PbO nanosheets with controlled edge length.     

Statistics of nucleation associated with the growth of whisker nanocrystals

A theoretical investigation of the statistics of nucleation of the next whisker-nanocrystal layer in the monocentric mode is performed. It is shown that anticorrelation of nucleation processes because of a small nanodrop size suppresses Poisson’s scatter in the length of whisker nanocrystals. The dependence of the variance of whisker nanocrystals in length on the rate of their growth and their radius is obtained.     

Controllable Crystallization of Two-Dimensional Bi Nanocrystals with Morphology-Boosted CO2 Electroreduction in Wide pH Environments

Two-dimensional low-melting-point (LMP) metal nanocrystals are attracting increasing attention with broad and irreplaceable applications due to their unique surface and topological structures. However, the chemical synthesis, especially the fine control over the nucleation (reduction) and growth (crystallization), of such LMP metal nanocrystals remains elusive as limited by the challenges of low standard redox potential, low melting point, poor crystalline symmetry, etc. Here, a controllable reduction-melting-crystallization (RMC) protocol to synthesize free-standing and surfactant-free bismuth nanocrystals with tunable dimensions, morphologies, and surface structures is presented. Especially, ultrathin bismuth nanosheets with flat or jagged surfaces/edges can be prepared with high selectivity. The jagged bismuth nanosheets, with abundant surface steps and defects, exhibit boosted electrocatalytic CO₂ reduction performances in acidic, neutral, and alkaline aqueous solutions, achieving the maximum selectivity of near unity at the current density of 210 mA cm^–2 for formate evolution under ambient conditions. This work creates the RMC pathway for the synthesis of free-standing two-dimensional LMP metal nanomaterials and may find broader applicability in more interdisciplinary applications.     

Synthesis of Nanocomposite Materials Using the Reprecipitation Method

Room temperature solution-based synthetic methods are an important option for the production of a wide range of nanomaterials. These methods often rely on self-assembly or self-organization of molecular precursors, with specific control of their nucleation and growth properties. We are developing strategies for the creation of multifunctional composite nanoparticles as well as models for predicting the bulk properties from the individual components and parameters of the processing conditions. One method of synthesis is a reprecipitation technique in which nanoparticle nucleation and growth is induced by the rapid injection of a molecular solution into a miscible non-solvent. Here we demonstrate that this method can be extended to multiple components, enabling the preparation of nanoparticles with multiple functionalities as well as materials with new properties arising from cooperative effects between the constituents. We synthesized nanocrystals of the charge transfer complex perylene:TCNQ (tetracyanoquinodimethane). Using the increase of the charge transfer band in the near infrared and dynamic light scattering data we try to develop a model for the formation and growth of these nanocrystals. By freeze drying the samples during growth and imaging with scanning electron microscope, we observe the morphologies of the crystals during their growth.     

Ordered array of self-assembled SiC nanocrystals fabricated by selective oxide desorption and nanosphere lithography

We have developed a novel technique based on the selective desorption of an oxide film in order to grow ordered arrays of silicon carbide nanocrystals on a silicon surface. These nanocrystals form as a byproduct of void nucleation in the oxide during the high-temperature vacuum annealing of silicon, a process which normally produces a random distribution of nanocrystals across the silicon surface after its oxide layer has been fully desorbed. By the pre-deposition of a thin layer of excess silicon on the oxide surface through a patterned lithography mask, site-specific nucleation of voids in the silicon oxide can instead be achieved during the annealing step, leading to the growth of silicon carbide nanocrystals in regular patterns over the silicon surface.     

A mild phosphine-free synthesis of alkylamine-capped CdSe nanocrystals

A new phosphine-free approach has been developed to synthesize high-quality cadmium selenide (CdSe) nanocrystals with cubic zinc-blende structure, by using the highly reactive selenium (Se) precursor at milder temperature than that used in the traditional phosphine route. This Se precursor was obtained from the reduction of Se powder by sodium borohydride in N,N-dimetbylformamide, in the absence of phosphine. Without the addition of other long-chain coordinating substances in this approach, the alkylamines such as dodecylamine (DDA) and octylamine (OA) were used as reaction solvents, and they also acted as surface capping reagents to produce DDA-capped and OA-capped CdSe NCs, respectively. The rapid nucleation and slow growth were observed by ultraviolet-visible absorption spectrum. The resulting OA-capped CdSe NCs grew faster compared with DDA-capped CdSe NCs under the same other conditions. These as-synthesized CdSe nanocrystals showed relatively narrow size distribution and high photoluminescence quantum efficiency (up to 9.4% for OA-capped CdSe NCs). This mild approach is low cost, relatively low danger and high production yield (approximately 80%), indicating that it is very effective for the phosphine-free synthesis of alkylamine-capped CdSe nanocrystals.     

A Universal Synthesis Strategy for Lanthanide Sulfide Nanocrystals with Efficient Photocatalytic Hydrogen Production

As an important lanthanide (Ln)-based functional materials, the Ln chalcogenides possess unique properties and various applications. However, the controllable synthesis of Ln chalcogenide nanocrystals still faces great challenges because of the rather poor affinity between Ln and chalcogenide ions (S, Se, Te) as well as strong preference of combination with existed oxygen. Herein, a facile but general heterogeneous nucleation synthetic strategy is established toward a series of colloidal ternary Cu Ln sulfides nanocrystals using the Ln dithiocarbamates and CuI as precursors. To extend this synthetic protocol, similar strategy is used to prepare six kinds of high quality CuLnS₂ nanocrystals, while the bulk ones are only obtained by the traditional solid-state reaction at rigorous condition. Importantly, high-entropy nanocrystals CuLnS₂ and CuEu_xLn_2-xS₃ which contain six Ln elements (Nd, Sm, Gd, Tb, Dy) are readily obtained by the co-decomposed process attributed to their similar diffusion speed. As a proof-of-concept application, CuEu₂S₃ nanocrystals showed efficient photocatalytic hydrogen production properties.     

Synthesis of platinum multipods: an induced anisotropic growth

This paper reports a highly effective synthesis of platinum multipods from platinum 2,4-pentanedionate in organic solvents. A trace amount of silver acetylacetonate is used to trigger the nucleation and the anisotropic growth of Pt nanocrystals. The morphologies of Pt multipods made include I- and V-shaped bipods, various types of tripods, and planar and three-dimensional (3D) tetrapods. The 3D Pt tetrapods can be well-defined, resembling those observed for II-VI semiconducting materials, such as CdS and CdSe. Control of morphology and the multipod-to-sphere transitions under various conditions have been systematically studied. A mode of formation based on the induced kinetically controlled growth has been discussed.     

Orientation dependent growth of SiC nanocrystals at the SiO2/Si interface

Cubic SiC nanocrystals are formed epitaxially and void-free on single crystal Si substrate by reactive annealing in CO. In this study characterization of the nucleation, growth and morphology is presented on differently oriented single crystal Si substrates. It is found that SiC nanocrystals of various shape can be grown in different densities on the (100), (110) and (111) Si surfaces with an average size of 30-60 nm. Effect of annealing time, CO concentration, substrate orientation and crystal size on crystallite growth is discussed. Parameters to obtain increased SiC nucleation density are determined.     

ZnO-Au hybrid nanocrystals via a high temperature reflux route

ZnO-Au hybrid nanocrystals with good dispersibility and uniform size are prepared via a high temperature reflux route. The structure, morphology, interface structure and optical properties were characterized by X-ray diffraction high resolution transmission electron microscope and UV-vis absorption spectrophotometer. The formation of hybrid nanocrystals was believed to arise from the heterogeneous nucleation and epitaxial growth, high temperature and surfactant are proposed as the key factors for producing fine and uniform hybrid nanocrystals.